Soft Matter View Article Online
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
PAPER
Cite this: Soft Matter, 2015, 11, 3994
View Journal | View Issue
Mechanisms of criticality in environmental adhesion loss† Christopher White,*a Kar Tean Tan,‡a Donald Hunston,a Kristen Steffens,a Deborah L. Stanley,a Sushil K. Satija,b Bulent Akgun§b and Bryan D. Vogt*c Moisture attack on adhesive joints is a long-standing scientific and engineering problem. A particularly interesting observation is that when the moisture level in certain systems exceeds a critical concentration, the bonded joint shows a dramatic loss of strength. The joint interface plays a dominant role in this phenomenon; however, why a critical concentration of moisture exists and what role is played by the properties of the bulk adhesive have not been adequately addressed. Moreover if the interface is crucial, the local water content near the interface will help elucidate the mechanisms of criticality more than the more commonly examined bulk water concentration in the adhesive. To gain a detailed picture of this criticality, we have combined a fracture mechanics approach to determine joint strength with neutron reflectivity, which provides the moisture distribution near the interface. A well-defined model system, silica glass substrates bonded to a series of polymers based on poly(n-alkyl methacrylate), was utilized to probe the role of the adhesive in a systematic manner. By altering the alkyl chain length, the molecular structure of the polymer can be systematically changed to vary the chemical and physical properties of the adhesive
Received 8th December 2014, Accepted 10th April 2015
over a relatively wide range. Our findings suggest that the loss of adhesion is dependent on a combination
DOI: 10.1039/c4sm02725f
from water absorption, and water-induced weakening of the interfacial bonds. This complexity explains
of the build-up of the local water concentration near the interface, interfacial swelling stresses resulting the source of criticality in environmental adhesion failure and could enable design of adhesives to
www.rsc.org/softmatter
minimize environmental failure.
Introduction Moisture attack on polymeric adhesive joints has been extensively investigated as environmental moisture can limit their durability and service life.1,2 A common strategy to study the susceptibility to moisture is to vary the water content by swelling a series of specimens to equilibrium at different relative humidities (RH). The equilibrium moisture content and the joint strength can be correlated. However, the joint strength for some adhesive–substrate a
Materials and Structural Systems Division, National Institute of Standards and Technology, Gaithersburg, MD, USA. E-mail: [email protected]; Tel: +1-301-975-6016 b Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD, USA c Department of Polymer Engineering, University of Akron, Akron, OH, USA. E-mail: [email protected]; Tel: +1-330-972-8608 † Electronic supplementary information (ESI) available: ATR-FTIR, XPS, raw SLBT data, raw NR data with fits, interfacial moisture profiles at multiple humidities for PMMA and PBMA, and Gc for PPMA as a function of RH. See DOI: 10.1039/ c4sm02725f ‡ Present address: PPG Industries, Coating Innovation Center, 4325 Rosanna Drive, Allison Park, PA, 15101, USA. ˘aziçi University, Bebek 34342, § Present address: Department of Chemistry, Bog Istanbul, Turkey.
3994 | Soft Matter, 2015, 11, 3994--4001
systems3–9 decreases abruptly over a small incremental increase in RH, which is often called the critical RH. For moisture concentrations below the critical level, only a minor decrease in joint strength is observed with increasing water content. Almost all of the change in the joint strength occurs in narrow moisture content near the critical RH with further increases in the moisture content beyond the critical RH generally not significantly impacting the joint strength. Under these high humidity conditions, the joint strength is often very low and not technologically useful. Based on experimental observations, it is generally agreed that the abrupt drop in adhesion strength is an interfacial effect.1–3 First, the failure of the joint shifts from cohesive below the critical RH to adhesive when the critical moisture content is exceeded. Second, calculations of the thermodynamic interfacial free energy for many adhesive/substrate interfaces reveal a shift from favorable in dry conditions to weakly favorable or unfavorable in the presence of water. Finally, and perhaps most importantly, the dramatic decrease in joint strength at the critical RH can frequently be mitigated through appropriate surface treatment (e.g., coupling agents). Although these arguments make a strong case for the importance of the interface, they do not explain why the drop in strength occurs over a very narrow range of water concentrations or what determines the critical moisture level.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Paper
In addition to the interface, several ideas have been advanced related to the role of the bulk properties of the adhesive in determining the critical RH.10–12 Chemical bonding of the adhesive to the substrate should inhibit adsorption of water, while the swelling of the reminder of the adhesive will be constrained from expansion at the interface in the directions parallel to the interface. This constrained swelling will produce stresses particularly localized at the edges of the interface, which could reduce the strength of the bonded joint. This effect increases with increasing water content in the adhesive. However, the situation is complex because these stresses may also relax with time; this behavior not usually considered when addressing the effect of moisture on bond strength. Moreover, this swelling argument does not provide an explanation to why the bond strength exhibits a sudden drop over a very narrow range of RH. Additionally, the chemical structure of the polymer itself5,6 has been suggested to impact the critical RH through control of the water content in the polymer at a given RH. At low RH, an approximately linear correlation between the RH and water content consistent with Henry’s law was observed, but further increasing the RH resulted in a positive deviation in the water concentration from Henry’s law. Intriguingly, this change in slope occurred at approximately the same RH as the rapid loss of strength in the adhesive joint. In light of the good agreement, a cause and effect relationship has been suggested with the change in slope for the absorption curve being attributed to aggregation of water molecules in the polymer matrix. This proposed correlation provides a possible molecular level explanation for the critical moisture concentration (or critical RH) for adhesion based on the aggregation of water molecules. Although this hypothesis is a very attractive idea, the bulk absorption effect in isolation cannot explain the critical importance of the interface as discussed previously. Another complication is that the local water concentration near the interface is different from the water content away from the interface in the same polymer.13–15 Since the failure shifts from cohesive to adhesive at the critical RH, the bulk absorption hypothesis must address how this change in the bulk water content impacts the local water concentration near the interface in order to assess the relevance of this hypothesis. Our previous work began to examine these issues by combining bulk and interfacial characterization techniques to examine a single model system of poly(methyl methacrylate) (PMMA) supported on a glass substrate.9 For this system, the critical RH corresponded with a change in the slope of the bulk absorption curve (hence swelling), but simultaneously the local excess water concentration near the interface extended into the film (perpendicular to the buried interface) at the critical RH. The combination of these two effects suggests a subtle interplay been bulk and interfacial effects. A mechanism based upon interfacial stress imposed by swelling combined with a weakening of the interface bond by the high local moisture content was proposed. However, this proposed mechanism has not been tested beyond a single system. To provide insight into the mechanism of criticality, we extend this study to two additional members of the poly(n-alkyl
This journal is © The Royal Society of Chemistry 2015
Soft Matter
methacrylate) (PAMA) family: poly(n-butyl methacrylate) (PBMA), and poly(ethyl methacrylate) (PEMA), while using glass as a common support. As the alkyl chain length is increased, the glass transition and elastic modulus for the polymer decrease significantly. These features allow for a detailed inspection of the controlling factors for durability of adhesive joints in humid environments. The study focuses on addressing three important questions. First, what determines the RH where the adhesion strength drops for a particular system? Second, how is the water distributed in the polymer at various RH and is this directly related to adhesion strength? Finally, what role, if any, is played by the bulk properties of the polymer?
Experimental section Materials and joint preparation The geometry utilized was a polymer film supported on a glass substrate, rather than a full adhesive joint, to promote equilibration at different RH levels. A commercially available (Scientific Polymer Products) homologous series of PAMAs with known molecular mass, Mw, and glass transition temperature, Tg: PMMA (Mw = 120 kg mol1, Tg = 105 1C), PEMA (Mw = 250 kg mol1, Tg = 63 1C), and PBMA (Mw = 180 kg mol1, Tg = 15 1C); were used in this study. PMMA and PEMA are glassy while the PBMA is rubbery at ambient temperature. The substrates were either silicon wafers with a thermal oxide layer for neutron reflectivity (NR) experiments or borosilicate glass disks with an 8 mm diameter hole for fracture mechanics measurements. In both cases, the surface chemistry is SiOx glass. Each substrate was cleaned using acetone. The fracture mechanics specimen was multi-layered and prepared using the following procedure. First, the hole in the glass substrate was covered by adhering a circular piece of Kapton tape (diameter = 9.5 mm, thickness = 25 mm) to the substrate using a 3.75 mm thick acrylic adhesive. The adhesive is just strong enough to hold the Kapton but offers little resistance during the fracture test. This tape also served as a precrack (0.75 mm long) in the fracture test since it is very thin. A nominally 15 mm thick PAMA (methyl, ethyl, propyl or butyl methacrylate) film was coated on the glass substrate and the Kapton tape by spin-casting at 104.7 rad s1 for 20 s from 20 mass% solution in toluene. On top of the PAMA, 1.1 mL of diglycidyl ether of bisphenol A resin (DER 332, Dow Chemical) containing 43 per hundred part by mass resin polyether triamine curing agent (Jeffamine T-403) was spun-cast at 104.7 rad s1 for 20 s. A 50 mm thick piece of Kapton E film was then placed on top of the uncured epoxy resin to act as a mechanical reinforcing layer for the PAMA coating. Without the epoxy and Kapton layer, the loading shaft may push through the PAMA film. The resulting adhesive layer was therefore a multi-layered film consisting of the precrack Kapton tape, PAMA, epoxy and Kapton film, as shown in Fig. 1. The fabricated joint was dried at ambient temperature for 48 h followed by curing at 60 1C for 1 h. For neutron reflectivity samples, the PAMA was spun coated from toluene solution directly onto the clean silicon wafer and then subjected to this same heat treatment.
Soft Matter, 2015, 11, 3994--4001 | 3995
View Article Online
Soft Matter
Paper
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Fracture mechanics Adhesion measurements were performed using the shaft-loaded blister technique performed on a tensile-testing machine at a crosshead displacement rate of 5 mm s1. Details of the experimental procedure are provided elsewhere.9,13 The corresponding fracture energy, Gc, is given by16 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 P4 Gc ¼ (1) 16p4 a4 Eh where P is the applied load, a is the crack length (radius of the blister), h is the total thickness of the adhesive layer and E is the Young’s modulus of the multi-layer film: the PAMA coating, the adhesive layer and the Kapton backing. E is determined from the rule of mixture: E¼
i X
vi Ei
(2)
i¼0
where Ei and vi are the modulus and volume fraction of the ith component, respectively. The joints were preconditioned at a constant RH (0% to E 100%) at 23 1C for 72 h and then tested at ambient temperature. Elapsed time between the joint removal from the chamber and testing was o5 min. An instron tensile test machine was used to press the shaft into the coating through the hole (see Fig. 1). A video camera was focused on the coating side to measure the radius of the debonded region (blister) as it grew. The measured load, P, from the instron and the blister radius, a, from the video were used in eqn (1) to calculate the fracture energy. Joints were tested in triplicate at each relative humidity and the error bars represent plus or minus one standard deviation from the mean value. Water sorption isotherms PAMA free-standing films (5 mm 5 mm 0.55 mm) were prepared using a drawdown technique, in which a solution containing 20% mass fraction of the PAMA in toluene was deposited onto release paper. Excess solution was removed by firmly drawing a metal bar across the paper. The resulting films were then heat treated as described previously. They were dried extensively until a constant mass was obtained. Adsorption and desorption isotherms were then measured at 23 1C 0.1 1C
using a moisture sorption analyzer equipped with a microbalance having a mass resolution of 0.1 mg. The instrument can accurately control the environmental chamber to within 0.5% of the preset values of RH for the entire RH range of the present study. Duplicate measurements indicated that the isotherm data were highly reproducible. Spectroscopy Failure surfaces from the fracture mechanics tests were examined using attenuated total-reflection (ATR)-FTIR (Nicolet Nexus FTIR Spectrometer equipped with a mercury–cadmium–telluride detector and a SensIR Durascope attenuated total reflectance (ATR) accessory) and X-ray photoelectron spectroscopy (XPS) to determine the loci of joint failure. Fresh PAMA free-standing films and cleaned substrate were analyzed as controls. Consistent pressure on the specimens was applied using the force monitor on the Durascope, and the sampling area was approximately 1 mm in diameter. All spectra were collected in the range from 650 cm1 to 4000 cm1 at a nominal resolution of 4 cm1, and averaged over 128 scans. Five different locations on each specimen were analyzed. XPS was performed on a Kratos Axis Ultra DLD spectrometer using monochromated Al Ka radiation with an analysis area of 300 mm 700 mm. Survey spectra were performed with pass energy of 160 eV and high resolution examination used a pass energy of 40 eV. Contact angle measurements Contact angle measurements were performed on fresh PAMA freestanding films and cleaned substrates by the sessile drop technique using a Kruss G2 contact angle instrument with deionized water and diiodomethane. The geometric mean method was used to calculate the surface energy components17 and the corresponding thermodynamic work of adhesion was computed.8 Hygroscopic swelling measurements Hygroscopic swelling was examined using a measurement technique adapted from an approach reported by Wong et al.18 involving exposure of two identical samples (fresh PAMA free-standing films) to 100% RH for 3 days to reach saturation. Changes in length and mass as a function of time during isothermal desorption at room temperature were then monitored. Length change was monitored with dynamic mechanical thermal analysis (DMTA), while a gravimetric analysis instrument with an automatic balance was used to quantify mass change. Neutron reflectivity
Fig. 1 Schematic of the shaft-loaded blister test joint (not to scale). The arrow indicates the direction of loading.
3996 | Soft Matter, 2015, 11, 3994--4001
Specimens for NR were exposed to deuterium oxide, D2O (Aldrich, 99.9%) in place of common protonated water vapor. The deuterium labeling provides significant contrast in neutron reflectivity due to the large difference in scattering cross-section between D and 1H. The sample was held at ambient (23 1C 2 1C) temperature for the neutron reflectivity under vacuum (B107 Torr). The partial pressure of D2O into this vacuum was controlled by the temperature (7–22 1C) of a degassed reservoir containing D2O. The sample was allowed to equilibrate for at least 30 min prior to NR measurements. The saturation vapor pressure of
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Paper
D2O at 23 1C is 0.02516 bar.19 NR measurements were performed at the Center for Neutron Research on the NG-7 reflectometer at the National Institute of Standards and Technology (Gaithersburg, MD) in the following configuration: ~ = 0.025. wavelength, l = 0.4768 nm and wavelength spread, lDl NR is capable of probing the neutron scattering length density (NSLD) at depths of up to several hundred nanometer, with an effective depth resolution of several angstroms. The reflectivity profiles were fit with a recursive algorithm to quantify the distribution of water within the polymer film from the NSLD profiles. The fits utilized a standard Parratt recursive algorithm,20 which uses multiple layers to describe the interfaces, in the REFLPAK software package.21 The interfaces are assumed to be in the form of error function for the fits. The REFLPAK software was used for both data reduction and data fitting.20 The model utilized in all cases for fitting the NR data consists of 3 layers: silicon substrate (fixed NSLD = 2.07 106 Å2), silicon oxide (NSLD B 3.5 106 Å2) and polymer (varied NSLD). Heterogeneity in the D2O (NSLD = 6.33 106 Å2) concentration near the silicon oxide–polymer interface can be assessed by an apparent increase in the silicon oxide thickness and the width of the interface between silicon oxide and polymer. Prior studies have demonstrated that metal oxide is not swollen by the D2O15 and that the excess (in equilibrium) water near the inorganic–polymer interface determined from NR can quantitatively describe thickness dependent swelling.22
Results and discussion Fig. 2 shows the fracture energies of all PAMA–glass systems as a function of RH. Three distinct regions of crack growth behavior may be identified for PMMA and PEMA, which are denoted as regions I, II and III. In the region I (o60% RH), the adhesion was relatively insensitive to moisture and the crack propagation was found to occur in a stick-slip manner. ATRFTIR and XPS revealed that the failure path was cohesive in the polymer layer (see Fig. S1 and S2 in ESI†). As the RH was raised from 60% to 70% (region II), analogous stick-slip crack propagation was observed for the PMMA and PEMA joints but the
Fig. 2 The fracture energy, Gc, for different polymers (PMMA , PEMA , and PBMA, ’) on silica surfaces over a wide range of RH: shaft-loaded blister tests were conducted at (23 2) 1C. Error bars represent two standard deviations.
This journal is © The Royal Society of Chemistry 2015
Soft Matter
fracture energies dropped dramatically (nearly two orders of magnitude) over a small change in RH. The loci of failure were complex involving a mixture of adhesive and cohesive failure. In region III (470% RH), the fracture energies of PMMA and PEMA joints were now extremely low and exhibited a transition from a stick-slip fracture to a stable crack growth (see Fig. S3 in ESI†). XPS showed that the joints in region III failed predominantly at the polymer/substrate interface. While the Gc-RH relationship of PBMA joints did not show these three-region characteristics, four unique features in their fracture behaviors are worth noting. First, the fracture energies seemed to show a gradual linear decrease as the RH is increased although the change is within experimental uncertainty. The criticality in RH is clearly absent. Second, the failure is cohesive (in the polymer) with stick-slip crack growth over the entire range of RH levels. Third, among the systems examined, the PBMA joints exhibited the worst adhesion to glass in dry conditions, but showed the best adhesion strength of the PAMAs examined in wet conditions (470% RH). Moreover, the fracture energies in dry state are only slightly higher than that in wet conditions for the PBMA joints, suggesting that moisture has little impact on the adhesion of this system. It is quite interesting that a small change in the chemistry of the polymer (ethyl to butyl) can dramatically impact the response of the joint strength to RH. To elucidate the origins of the observed criticality in adhesion loss, we will examine 4 ideas that have been suggested as possible contributing factors: bulk sorption behavior, interface water distribution, free energy change associated with adhered and delaminated joint, and swelling behavior. We will begin with the possible contribution of bulk sorption properties of the adhesives. Fig. 3 illustrates water sorption isotherms, the equilibrium moisture mass concentration (Mc) for samples exposed at different RH values. In the case of PMMA and PEMA, the bulk sorption curves show a change in slope at approximately the same RH as the drop in strength of the adhesive joints, which is consistent with the notion that the two behaviors are related. The result for PBMA, however, yields a different conclusion. This system does not show a sudden drop in strength for the adhesive joint as the RH is increased even though the sorption curve does exhibit a deviation from Henry’s law at approximately 60% RH. Note that the water concentrations in bulk PEMA and PBMA are less than that in PMMA at the same RH value by a factor of 1.8 and 3, respectively. If the data are adjusted for this difference, the curves overlap perfectly (inset in Fig. 3) so there is no significant difference in the shapes of these isotherms. This result suggests that a simple change of slope in the bulk sorption isotherm curve cannot alone explain the critical RH. It is noteworthy, however, that the water content in PBMA is very low so it might be argued that although there is a change in slope at 60% RH, there simply is not enough water present to cause the drop in bond strength. With PMMA and PEMA the water content by mass at the critical RH is about 0.5% and 0.28%, respectively. PBMA reaches 0.28% water content at a RH of about 80%, and yet even here there is no drop in adhesive strength for the PBMA bonds. Consequently, the total amount
Soft Matter, 2015, 11, 3994--4001 | 3997
View Article Online
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Soft Matter
Paper
Fig. 3 Moisture sorption isotherms for ( ) PMMA, ( ) PEMA, and ( ) PBMA in terms of mass% water sorbed. (inset) Shifted moisture sorption isotherm data to adjust for the difference in equilibrium water concentration between PMMA and two other PAMAs by multiplication by 1.8 and 3 for PEMA and PBMA, respectively. Experimental uncertainty is less than the size of the symbols.
of water in the polymer does not seem to be directly correlated with the critical RH. The second possible factor that might contribute to the drop in adhesion is a higher local water concentration near the interface. While the bulk sorption isotherms are expected to reflect the general water concentration in free-standing film, these isotherms do not necessarily tell us about the concentrations near the PAMA–silicon oxide interface. Information on interfacial water concentration is of fundamental importance because adhesive failure is observed at high RHs for PMMA and PEMA. To assess differences in interfacial moisture between glassy (PMMA) and rubbery (PBMA) polymers, NR profiles were fit using a recursive algorithm based on the method described by Parratt using the methods described by Yu et al.20 The reflectivity data are shown in Fig. 4 as a function of the momentum transfer vector, q, which is normal to the film surface for these specular reflectivity data. The fits quantitatively describe features in the reflectivity spectra over the entire q-range examined in all cases. These fits provide profiles that describe the thickness and interfacial width of the silicon oxide and polymer layers in terms of the neutron scattering length density (NSLD) profile through the thickness of the film as shown in Fig. S4 (ESI†). The NSLD is not dependent on the bonding in the system, only the density and atomic nuclei present. For an ideal mixture (no volume change on mixing), the NSLD is simply the volume fraction averaged NSLD of the individual components. The interfacial water (D2O) concentration profiles for glassy (PMMA) and rubbery (PBMA) polymers were determined from the fits of the NR data (see Fig. S4–S6, ESI† for more details) and are shown in Fig. 5. A general feature to be noted is that water molecules are not uniformly distributed throughout the interfacial region (over distances of a few nm). Enhanced water adsorption is observed in the proximity of the glass oxide. This excess is likely the result of the hydrophilic character of the
3998 | Soft Matter, 2015, 11, 3994--4001
Fig. 4 NR data (points) and associated recursive fits (lines) for (a) PMMA and (b) PBMA films in the dry (red) and hydrated (blue) states. A clear shift to lower q in the reflectivity can be observed at high q for the hydrated films.
silanol-rich surface of the glass, which has concentrations of hydroxyl groups of up to eight per nm2.23 The apparent drop in water concentration very close to the interface is a reflection of the surface roughness in the oxide layer, which is on the order of 0.5 nm, as water cannot penetrate into the silicon oxide. In this case, the material contains polymer, D2O, and silicon oxide, which accounts for the overall decrease in concentration as the silica content increases (as shown by the black lines in Fig. 5). As the D2O and H2O exhibit similar, but slightly different properties, we report all the sorption data in terms of the activity of the D2O, which is quotient of the partial pressure of D2O and saturation pressure of D2O at that temperature. In an ideal case, the sorption of D2O and H2O should be identical at the same activity (relative humidity). The accuracy of the concentrations calculated from the NSLD improves as the concentration increases due to the volume fraction weighting in the calculation. We estimate the uncertainty to be 1%. At low D2O activity, (ocritical RH) (Fig. 5a), the concentration profiles for PMMA and PBMA are virtually identical (within the uncertainty). D2O concentration is decreased in PBMA when the distance from the interface is greater than B2 nm, but not outside of experimental error. This difference in solubility is consistent with the sorption in free-standing films (cf. Fig. 3). These data suggest that the local water concentration near the interface is influenced by the hydrophilicity of glass oxide while the bulk polymer properties play a minor role. However, there is a substantial increase in the interfacial D2O concentration for PMMA relative to PBMA at a high D2O activity (4critical RH), as shown in Fig. 4b. The water concentration in the bulk of the film (42 nm from the glass oxide surface) is simply governed by the hydrophobicity
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Paper
Fig. 5 Water distribution (volume fraction, fD2O) near the interface for PMMA and PBMA at (a) low D2O activity = 0.32 (PD2O = 0.008 bar) and (b) high D2O activity = 0.62 (PD2O = 0.016 bar) obtained from NR experiment. At low RH, the interfacial moisture is almost indistinguishable, while there is more water at the interface with PMMA at high activity.
of polymers; due to increasing alkyl side chains, the bulk D2O concentration decreases from PMMA relative to PBMA. To compare the interfacial water concentration curves for PMMA and PBMA, two features will be examined: the height of the peaks (maximum water concentration near the interface) (Fig. 6a), and the peak width at half height (thickness of the water rich layer distributed across the interface between the glass oxide and bulk polymer) (Fig. 6b). The maximum concentration is found to increase with increasing RH, but there is no discontinuity in this maximum concentration between the 60% and 70% RH. The values for peak width at half maximum are shown in Fig. 6b. Below the critical RH, the average thickness of the water rich layer near the interface is between one and two diameters of a water molecule and does not change with D2O activity. In the case of PBMA, this behavior continues over the entire RH range, but for PMMA, there is a sudden increase in thickness of the interfacial water layer at approximately the same RH as the drop in adhesion strength. The potential consequences on adhesion are twofold. First, an increase in thickness of the interfacial water may decrease the contact area between the adhesive and substrate. Second, multiple layers of water molecules cannot sustain any significant stresses. Therefore, an increase in water film thickness might be expected to disrupt interfacial bonding, which underlines the high relevance of this observation to sudden debonding. A third possible factor for the critical RH in adhesion is if the free energy associated with the interface is favorable. The work of adhesion, WA, indicates the thermodynamic interactions between the adhesive and the substrate across an idealized flat surface. Calculating the value of this parameter in a dry environment, WA, and in the presence of moisture, WAL,
This journal is © The Royal Society of Chemistry 2015
Soft Matter
Fig. 6 A comparison is made between interfacial water absorption behaviors of two polymers obtained from NR experiments as a function of the activity (partial pressure) of D2O: (a) maximum in water concentration near interface for PMMA/glass and PBMA/glass joints. (b) Width at half height for water peak near interface in PMMA/glass and PBMA/glass joints.
indicates the thermodynamic impact of water on the hydrolytic stability of the interface bonds. This behavior is of particular interest here since the glass surface has relatively low roughness. Table 1 shows how the work of adhesion varies with hydration for the three systems tested here. In all cases, there are large differences between wet and dry values. The very low work of adhesion in a moist environment indicates that water molecules appreciably weaken the interface but the joint is still thermodynamically favored as the interfacial PAMA/glass interaction is stronger than the PAMA/water and glass/water.24 This change in the work of adhesion can be rationalized in terms of the interactions between glass and PAMA. One to two silanol groups per nm2 can interact with the adsorbed PAMA depending on the concentration of carbonyl groups near the surface and the distribution of silanol groups.25 The affinity of PMMA towards the acidic glass oxide produces a significant increase in Tg of PMMA thin films supported on silica.26,27 Consequently, the work of adhesion calculations suggests that water weakens the interface, but does not explain why the behavior of PBMA is different. The final factor potentially influencing the critical RH in adhesion is swelling induced stresses. Water molecules can adsorb into PAMAs and increase the spacing between chains by entering voids or free volume (free water) or via hydrogen bonding with carbonyl groups (bound water molecules), which disrupts the hydrogen bonding.28 This expansion can induce significant stresses at the geometrically confined polymer/substrate interface to adversely impact joint strength. However, quantifying
Soft Matter, 2015, 11, 3994--4001 | 3999
View Article Online
Soft Matter Table 1
Properties of PAMA/glass interfaces
PMMA/glass PEMA/glass PBMA/glass
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Paper
WA (mJ m2)
WAL (mJ m2)
Virgin Gc (J m2)
Recovered Gc (J m2)
91 2 79 1 81 2
7.0 0.3 5.0 0.3 5.0 0.4
191 46 190 37 68 18
109 17 117 27 57 4
the impact of bulk swelling at the confined interface is not trivial and it is further complicated by the heterogeneous water concentration as the interface is approached. Moreover, the material properties in the interfacial region may differ from the bulk polymer. Nonetheless, since the load-transfer efficiency is a strong function of the polymer’s modulus, the confined glassy polymer/substrate interface will be stressed with larger swellinginduced stresses compared to the rubber/substrate interface. The bond failure measurements involve evaluating the energy required to propagate a crack, and it is possible that the swellinggenerated stresses contribute to the crack tip stress field, which reduces the external load that needs to be applied for crack propagation. This postulate is in agreement with work of Jing et al. who reported that supported PMMA films could be debonded from substrates simply by water-induced swelling stresses alone.29 Their theoretical analysis based on a two-layer model showed that the adsorbed water layer induces misfit strain between the swollen layer and the interior dry layer of PMMA, thus inducing bending stress in PMMA layers. The film detachment occurs when the swelling stress exceeds the PMMA/substrate interfacial strength. Notably, this debonding process is reversible. This stress-assisted mechanism would also explain why the interfacial PMMA/glass oxide and PEMA/glass oxide bonds, which are predicted as being stable against moisture intrusion from thermodynamic arguments, effectively rupture upon exposure to high RHs. However, a sole contribution from bulk swelling is insufficient for the following reasons. First, interfacial failure was only observed at high RHs. Second, the adhesion of PMMA with various types of substrates under 100% RH conditions can be tuned between adhesive and cohesive failures through modification of the substrate surface energy.30 There is one further experiment that can provide some insight into this adhesive joint failure behavior: recovery tests. Joint specimens of all three polymers were exposed to 100% RH for 72 h followed by extensive drying in desiccators for 720 h. The fracture behavior was then measured on the dried samples. As indicated in Table 1, PBMA exhibits full adhesion reversibility after extensive drying, while only partial recoverability was observed for PMMA and PEMA counterparts. Moreover, ATRFTIR and XPS confirmed that the fracture paths for the latter joints are mixed cohesive and adhesive failures. One might speculate that water disruption of the interface between the PAMA and glass and swelling stresses taken individually might be largely reversible. Since the adhesive and substrate are still in close proximity, the interactions between PAMA and glass can reform when the water is removed. When both mechanisms are present, however, the swelling stresses may deform
4000 | Soft Matter, 2015, 11, 3994--4001
the material when the interface is weakened so reformation of the interactions between PAMA and glass is hindered when the sample is dried. Thus the recovery of adhesive strength of the joint is only partial. From all the observations discussed above, we can conclude that, in analogy to stress-corrosion cracking, the sudden adhesion loss is dictated by a combination of the interfacial water accumulation and swelling-stress built-up at the polymer/substrate interfaces. Contrary to glassy PMMA and PEMA that exhibit critical adhesion loss, the rubbery polymer, PBMA, is able to suppress interfacial water accumulation (due to its more hydrophobic character) and to relieve swelling stresses (due to its ability to creep and lower modulus), leading to the absence of a critical RH. To further test this hypothesis, we examined the adhesive strength of a poly(n-propyl methacrylate) (PPMA)/glass joint as a function of relative humidity. The Tg (37 1C) is very near the testing temperature (23 1C). This results in a significant decreased modulus31 that relaxes the stresses developed by swelling. As shown in Fig. S7 (ESI†), the PPMA does not exhibit a catastrophic critical humidity and the joint performance mimics that of PBMA. The critical RH behavior can likely be avoided by minimizing the interfacial water accumulation through substrate modification28 and/or avoiding swelling-stress built-up at the polymer/substrate interfaces.
Conclusions The present study investigated the effects of moisture on the properties of three model adhesive systems based on glass and poly(n-alkyl methacrylate). For PMMA and PEMA, a dramatic drop in adhesive strength is observed for samples exposed to high relative humidity (460% RH). The other system (PBMA) has much lower strength when dry, but shows little loss in strength when exposed to relative humidities approaching 100%. This difference in adhesion behavior cannot be explained based on the thermodynamic work of adhesion since all three systems show similar drops when water is present. Another factor, which does not correlate with the loss of adhesion strength, is the change in slope for the sorption curves. All three systems show slope changes between 60% RH and 70% RH. PBMA has a lower water content that the other systems when exposed to relative humidities between 60% and 70%; however, PBMA does not show a drop in adhesion strength even when the water content exceeds the level where PEMA exhibits the loss of joint strength. Consequently, a mechanism based on the deviation from Henry’s law in the sorption isotherm curves and/or the total water content in the adhesive does not provide an explanation for the adhesion behavior. On the other hand, one factor that does show a correlation with adhesion behavior is the thickening of the local water rich layer near the interface. In the case of PMMA, there is a sudden increase in thickness of this region at the critical RH for adhesion while PBMA does not show this increase or a critical RH. This interfacial water accumulation could result in displacement of interfacial bonds, leading to abrupt adhesion loss at the critical RH. Finally, PBMA has a much lower modulus
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Paper
and exhibits significant creep, which should reduce stresses that arise from swelling induced by water. This creep behavior is not available for the much stiffer PMMA or PEMA. Experiments indicate that this produces significant differences in swelling stresses, which might contribute to the drop in bond strength. Based on these results, we propose that the origin of the criticality in adhesion loss might be attributed to the combination of interface weakening by ingress of water molecules assisted by moisture-induced swelling stresses. Experiments show that the loss of adhesion strength is partially reversible when the specimen is dried.
Notes and references 1 A. J. Kinloch, Adhesion and Adhesives: Science and Technology, Chapman and Hall, London, 1987. 2 K. T. Tan, C. C. White, D. L. Hunston, B. D. Vogt and A. P. Haag, in Advances in Structural Adhesive Bonding, ed. D. A. Dillard, Woodhead, Cambridge, UK, 2010, pp. 547–568. 3 D. M. Brewis, J. Comyn, A. K. Raval and A. J. Kinloch, Int. J. Adhes. Adhes., 1990, 10, 247–253. 4 J. Cognard, Int. J. Adhes. Adhes., 1988, 8, 93–99. 5 D. R. Lefebvre, K. M. Takahashi, A. J. Muller and V. R. Raju, J. Adhes. Sci. Technol., 1991, 5, 201–208. 6 D. R. Lefebvre, K. M. Elliker, K. M. Takahashi, V. R. Raju and M. L. Kaplan, J. Adhes. Sci. Technol., 2000, 14, 925–937. 7 J. Comyn, C. L. Groves and R. W. Saville, Int. J. Adhes. Adhes., 1994, 14, 15–20. 8 R. A. Gledhill and A. J. Kinloch, J. Adhes., 1974, 6, 315–330. 9 K. T. Tan, B. D. Vogt, C. C. White, K. L. Steffens, J. Goldman, S. K. Satija, C. Clerici and D. L. Hunston, Langmuir, 2008, 24, 9189–9193. 10 Y. Weitsman, J. Compos. Mater., 1977, 11, 378–394. 11 J. P. Sargent and K. H. G. Ashbee, J. Phys. D: Appl. Phys., 1981, 14, 1933–1938. 12 C.-S. Jou, S. T. Sackinger and R. J. Farris, J. Coat. Technol., 1995, 67, 71–77.
This journal is © The Royal Society of Chemistry 2015
Soft Matter
13 T. Nguyen, E. Byrd, D. Bentz and C. Lin, Prog. Org. Coat., 1996, 27, 181–193. 14 W.-L. Wu, W. J. Orts, C. J. Majkrzak and D. L. Hunston, Polym. Eng. Sci., 1995, 35, 1000–1004. 15 B. D. Vogt, V. M. Prabhu, C. L. Soles, S. K. Satija, E. K. Lin and W.-l. Wu, Langmuir, 2005, 21, 2460–2464. 16 K. T. Wan and Y. W. Mai, Int. J. Fract. Mech., 1995, 74, 181–197. 17 D. K. Owens and K. C. Wendt, J. Appl. Polym. Sci., 1969, 13, 1741. 18 E. H. Wong, R. Rajoo, S. W. Koh and T. B. Lim, Trans. ASME, 2002, 124, 122–126. 19 G. Jakli and W. A. Van Hook, J. Chem. Eng. Data, 1981, 26, 243–245. 20 C.-Y. Yu, E. Kim and J.-Y. Kim, J. Nanosci. Nanotechnol., 2011, 11, 4624–4628. 21 P. A. Kienzle, K. V. O’Donovan, J. F. Ankner, N. F. Berk and C. F. Majkrzak, 2000–2006, http://www.ncnr.nist.gov/ reflpak. 22 B. D. Vogt, C. L. Soles, R. L. Jones, C.-Y. Wang, E. K. Lin, W.-l. Wu, S. K. Satija, D. L. Goldfarb and M. Angelopoulos, Langmuir, 2004, 20, 1453–1458. 23 L. T. Zhuravlev, Colloids Surf., A, 2000, 173, 1–38. 24 F. M. Fowkes, in Physicochemical Aspects of Polymer Surfaces, ed. K. L. Mittal, Plenum Press, New York, 1981, pp. 583–603. 25 J. M. Berquier and H. Arribart, Langmuir, 1998, 14, 3716–3719. 26 C. E. Porter and F. D. Blum, Macromolecules, 2002, 35, 7448–7452. 27 C. E. Porter and F. D. Blum, Macromolecules, 2000, 33, 7016–7020. 28 D. T. Turner, Polymer, 1982, 23, 197–202. 29 B. Jing, J. Zhao, Y. Wang, X. Yi and H. Duan, Langmuir, 2010, 26, 7651–7655. 30 E. P. O’Brien, C. C. White and B. D. Vogt, Adv. Eng. Mater., 2006, 8, 114–118. 31 P. Panagos and G. Floudas, J. Non-Cryst. Solids, 2015, 407, 184–189.
Soft Matter, 2015, 11, 3994--4001 | 4001
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
PAPER
Cite this: Soft Matter, 2015, 11, 3994
View Journal | View Issue
Mechanisms of criticality in environmental adhesion loss† Christopher White,*a Kar Tean Tan,‡a Donald Hunston,a Kristen Steffens,a Deborah L. Stanley,a Sushil K. Satija,b Bulent Akgun§b and Bryan D. Vogt*c Moisture attack on adhesive joints is a long-standing scientific and engineering problem. A particularly interesting observation is that when the moisture level in certain systems exceeds a critical concentration, the bonded joint shows a dramatic loss of strength. The joint interface plays a dominant role in this phenomenon; however, why a critical concentration of moisture exists and what role is played by the properties of the bulk adhesive have not been adequately addressed. Moreover if the interface is crucial, the local water content near the interface will help elucidate the mechanisms of criticality more than the more commonly examined bulk water concentration in the adhesive. To gain a detailed picture of this criticality, we have combined a fracture mechanics approach to determine joint strength with neutron reflectivity, which provides the moisture distribution near the interface. A well-defined model system, silica glass substrates bonded to a series of polymers based on poly(n-alkyl methacrylate), was utilized to probe the role of the adhesive in a systematic manner. By altering the alkyl chain length, the molecular structure of the polymer can be systematically changed to vary the chemical and physical properties of the adhesive
Received 8th December 2014, Accepted 10th April 2015
over a relatively wide range. Our findings suggest that the loss of adhesion is dependent on a combination
DOI: 10.1039/c4sm02725f
from water absorption, and water-induced weakening of the interfacial bonds. This complexity explains
of the build-up of the local water concentration near the interface, interfacial swelling stresses resulting the source of criticality in environmental adhesion failure and could enable design of adhesives to
www.rsc.org/softmatter
minimize environmental failure.
Introduction Moisture attack on polymeric adhesive joints has been extensively investigated as environmental moisture can limit their durability and service life.1,2 A common strategy to study the susceptibility to moisture is to vary the water content by swelling a series of specimens to equilibrium at different relative humidities (RH). The equilibrium moisture content and the joint strength can be correlated. However, the joint strength for some adhesive–substrate a
Materials and Structural Systems Division, National Institute of Standards and Technology, Gaithersburg, MD, USA. E-mail: [email protected]; Tel: +1-301-975-6016 b Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD, USA c Department of Polymer Engineering, University of Akron, Akron, OH, USA. E-mail: [email protected]; Tel: +1-330-972-8608 † Electronic supplementary information (ESI) available: ATR-FTIR, XPS, raw SLBT data, raw NR data with fits, interfacial moisture profiles at multiple humidities for PMMA and PBMA, and Gc for PPMA as a function of RH. See DOI: 10.1039/ c4sm02725f ‡ Present address: PPG Industries, Coating Innovation Center, 4325 Rosanna Drive, Allison Park, PA, 15101, USA. ˘aziçi University, Bebek 34342, § Present address: Department of Chemistry, Bog Istanbul, Turkey.
3994 | Soft Matter, 2015, 11, 3994--4001
systems3–9 decreases abruptly over a small incremental increase in RH, which is often called the critical RH. For moisture concentrations below the critical level, only a minor decrease in joint strength is observed with increasing water content. Almost all of the change in the joint strength occurs in narrow moisture content near the critical RH with further increases in the moisture content beyond the critical RH generally not significantly impacting the joint strength. Under these high humidity conditions, the joint strength is often very low and not technologically useful. Based on experimental observations, it is generally agreed that the abrupt drop in adhesion strength is an interfacial effect.1–3 First, the failure of the joint shifts from cohesive below the critical RH to adhesive when the critical moisture content is exceeded. Second, calculations of the thermodynamic interfacial free energy for many adhesive/substrate interfaces reveal a shift from favorable in dry conditions to weakly favorable or unfavorable in the presence of water. Finally, and perhaps most importantly, the dramatic decrease in joint strength at the critical RH can frequently be mitigated through appropriate surface treatment (e.g., coupling agents). Although these arguments make a strong case for the importance of the interface, they do not explain why the drop in strength occurs over a very narrow range of water concentrations or what determines the critical moisture level.
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Paper
In addition to the interface, several ideas have been advanced related to the role of the bulk properties of the adhesive in determining the critical RH.10–12 Chemical bonding of the adhesive to the substrate should inhibit adsorption of water, while the swelling of the reminder of the adhesive will be constrained from expansion at the interface in the directions parallel to the interface. This constrained swelling will produce stresses particularly localized at the edges of the interface, which could reduce the strength of the bonded joint. This effect increases with increasing water content in the adhesive. However, the situation is complex because these stresses may also relax with time; this behavior not usually considered when addressing the effect of moisture on bond strength. Moreover, this swelling argument does not provide an explanation to why the bond strength exhibits a sudden drop over a very narrow range of RH. Additionally, the chemical structure of the polymer itself5,6 has been suggested to impact the critical RH through control of the water content in the polymer at a given RH. At low RH, an approximately linear correlation between the RH and water content consistent with Henry’s law was observed, but further increasing the RH resulted in a positive deviation in the water concentration from Henry’s law. Intriguingly, this change in slope occurred at approximately the same RH as the rapid loss of strength in the adhesive joint. In light of the good agreement, a cause and effect relationship has been suggested with the change in slope for the absorption curve being attributed to aggregation of water molecules in the polymer matrix. This proposed correlation provides a possible molecular level explanation for the critical moisture concentration (or critical RH) for adhesion based on the aggregation of water molecules. Although this hypothesis is a very attractive idea, the bulk absorption effect in isolation cannot explain the critical importance of the interface as discussed previously. Another complication is that the local water concentration near the interface is different from the water content away from the interface in the same polymer.13–15 Since the failure shifts from cohesive to adhesive at the critical RH, the bulk absorption hypothesis must address how this change in the bulk water content impacts the local water concentration near the interface in order to assess the relevance of this hypothesis. Our previous work began to examine these issues by combining bulk and interfacial characterization techniques to examine a single model system of poly(methyl methacrylate) (PMMA) supported on a glass substrate.9 For this system, the critical RH corresponded with a change in the slope of the bulk absorption curve (hence swelling), but simultaneously the local excess water concentration near the interface extended into the film (perpendicular to the buried interface) at the critical RH. The combination of these two effects suggests a subtle interplay been bulk and interfacial effects. A mechanism based upon interfacial stress imposed by swelling combined with a weakening of the interface bond by the high local moisture content was proposed. However, this proposed mechanism has not been tested beyond a single system. To provide insight into the mechanism of criticality, we extend this study to two additional members of the poly(n-alkyl
This journal is © The Royal Society of Chemistry 2015
Soft Matter
methacrylate) (PAMA) family: poly(n-butyl methacrylate) (PBMA), and poly(ethyl methacrylate) (PEMA), while using glass as a common support. As the alkyl chain length is increased, the glass transition and elastic modulus for the polymer decrease significantly. These features allow for a detailed inspection of the controlling factors for durability of adhesive joints in humid environments. The study focuses on addressing three important questions. First, what determines the RH where the adhesion strength drops for a particular system? Second, how is the water distributed in the polymer at various RH and is this directly related to adhesion strength? Finally, what role, if any, is played by the bulk properties of the polymer?
Experimental section Materials and joint preparation The geometry utilized was a polymer film supported on a glass substrate, rather than a full adhesive joint, to promote equilibration at different RH levels. A commercially available (Scientific Polymer Products) homologous series of PAMAs with known molecular mass, Mw, and glass transition temperature, Tg: PMMA (Mw = 120 kg mol1, Tg = 105 1C), PEMA (Mw = 250 kg mol1, Tg = 63 1C), and PBMA (Mw = 180 kg mol1, Tg = 15 1C); were used in this study. PMMA and PEMA are glassy while the PBMA is rubbery at ambient temperature. The substrates were either silicon wafers with a thermal oxide layer for neutron reflectivity (NR) experiments or borosilicate glass disks with an 8 mm diameter hole for fracture mechanics measurements. In both cases, the surface chemistry is SiOx glass. Each substrate was cleaned using acetone. The fracture mechanics specimen was multi-layered and prepared using the following procedure. First, the hole in the glass substrate was covered by adhering a circular piece of Kapton tape (diameter = 9.5 mm, thickness = 25 mm) to the substrate using a 3.75 mm thick acrylic adhesive. The adhesive is just strong enough to hold the Kapton but offers little resistance during the fracture test. This tape also served as a precrack (0.75 mm long) in the fracture test since it is very thin. A nominally 15 mm thick PAMA (methyl, ethyl, propyl or butyl methacrylate) film was coated on the glass substrate and the Kapton tape by spin-casting at 104.7 rad s1 for 20 s from 20 mass% solution in toluene. On top of the PAMA, 1.1 mL of diglycidyl ether of bisphenol A resin (DER 332, Dow Chemical) containing 43 per hundred part by mass resin polyether triamine curing agent (Jeffamine T-403) was spun-cast at 104.7 rad s1 for 20 s. A 50 mm thick piece of Kapton E film was then placed on top of the uncured epoxy resin to act as a mechanical reinforcing layer for the PAMA coating. Without the epoxy and Kapton layer, the loading shaft may push through the PAMA film. The resulting adhesive layer was therefore a multi-layered film consisting of the precrack Kapton tape, PAMA, epoxy and Kapton film, as shown in Fig. 1. The fabricated joint was dried at ambient temperature for 48 h followed by curing at 60 1C for 1 h. For neutron reflectivity samples, the PAMA was spun coated from toluene solution directly onto the clean silicon wafer and then subjected to this same heat treatment.
Soft Matter, 2015, 11, 3994--4001 | 3995
View Article Online
Soft Matter
Paper
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Fracture mechanics Adhesion measurements were performed using the shaft-loaded blister technique performed on a tensile-testing machine at a crosshead displacement rate of 5 mm s1. Details of the experimental procedure are provided elsewhere.9,13 The corresponding fracture energy, Gc, is given by16 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 P4 Gc ¼ (1) 16p4 a4 Eh where P is the applied load, a is the crack length (radius of the blister), h is the total thickness of the adhesive layer and E is the Young’s modulus of the multi-layer film: the PAMA coating, the adhesive layer and the Kapton backing. E is determined from the rule of mixture: E¼
i X
vi Ei
(2)
i¼0
where Ei and vi are the modulus and volume fraction of the ith component, respectively. The joints were preconditioned at a constant RH (0% to E 100%) at 23 1C for 72 h and then tested at ambient temperature. Elapsed time between the joint removal from the chamber and testing was o5 min. An instron tensile test machine was used to press the shaft into the coating through the hole (see Fig. 1). A video camera was focused on the coating side to measure the radius of the debonded region (blister) as it grew. The measured load, P, from the instron and the blister radius, a, from the video were used in eqn (1) to calculate the fracture energy. Joints were tested in triplicate at each relative humidity and the error bars represent plus or minus one standard deviation from the mean value. Water sorption isotherms PAMA free-standing films (5 mm 5 mm 0.55 mm) were prepared using a drawdown technique, in which a solution containing 20% mass fraction of the PAMA in toluene was deposited onto release paper. Excess solution was removed by firmly drawing a metal bar across the paper. The resulting films were then heat treated as described previously. They were dried extensively until a constant mass was obtained. Adsorption and desorption isotherms were then measured at 23 1C 0.1 1C
using a moisture sorption analyzer equipped with a microbalance having a mass resolution of 0.1 mg. The instrument can accurately control the environmental chamber to within 0.5% of the preset values of RH for the entire RH range of the present study. Duplicate measurements indicated that the isotherm data were highly reproducible. Spectroscopy Failure surfaces from the fracture mechanics tests were examined using attenuated total-reflection (ATR)-FTIR (Nicolet Nexus FTIR Spectrometer equipped with a mercury–cadmium–telluride detector and a SensIR Durascope attenuated total reflectance (ATR) accessory) and X-ray photoelectron spectroscopy (XPS) to determine the loci of joint failure. Fresh PAMA free-standing films and cleaned substrate were analyzed as controls. Consistent pressure on the specimens was applied using the force monitor on the Durascope, and the sampling area was approximately 1 mm in diameter. All spectra were collected in the range from 650 cm1 to 4000 cm1 at a nominal resolution of 4 cm1, and averaged over 128 scans. Five different locations on each specimen were analyzed. XPS was performed on a Kratos Axis Ultra DLD spectrometer using monochromated Al Ka radiation with an analysis area of 300 mm 700 mm. Survey spectra were performed with pass energy of 160 eV and high resolution examination used a pass energy of 40 eV. Contact angle measurements Contact angle measurements were performed on fresh PAMA freestanding films and cleaned substrates by the sessile drop technique using a Kruss G2 contact angle instrument with deionized water and diiodomethane. The geometric mean method was used to calculate the surface energy components17 and the corresponding thermodynamic work of adhesion was computed.8 Hygroscopic swelling measurements Hygroscopic swelling was examined using a measurement technique adapted from an approach reported by Wong et al.18 involving exposure of two identical samples (fresh PAMA free-standing films) to 100% RH for 3 days to reach saturation. Changes in length and mass as a function of time during isothermal desorption at room temperature were then monitored. Length change was monitored with dynamic mechanical thermal analysis (DMTA), while a gravimetric analysis instrument with an automatic balance was used to quantify mass change. Neutron reflectivity
Fig. 1 Schematic of the shaft-loaded blister test joint (not to scale). The arrow indicates the direction of loading.
3996 | Soft Matter, 2015, 11, 3994--4001
Specimens for NR were exposed to deuterium oxide, D2O (Aldrich, 99.9%) in place of common protonated water vapor. The deuterium labeling provides significant contrast in neutron reflectivity due to the large difference in scattering cross-section between D and 1H. The sample was held at ambient (23 1C 2 1C) temperature for the neutron reflectivity under vacuum (B107 Torr). The partial pressure of D2O into this vacuum was controlled by the temperature (7–22 1C) of a degassed reservoir containing D2O. The sample was allowed to equilibrate for at least 30 min prior to NR measurements. The saturation vapor pressure of
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Paper
D2O at 23 1C is 0.02516 bar.19 NR measurements were performed at the Center for Neutron Research on the NG-7 reflectometer at the National Institute of Standards and Technology (Gaithersburg, MD) in the following configuration: ~ = 0.025. wavelength, l = 0.4768 nm and wavelength spread, lDl NR is capable of probing the neutron scattering length density (NSLD) at depths of up to several hundred nanometer, with an effective depth resolution of several angstroms. The reflectivity profiles were fit with a recursive algorithm to quantify the distribution of water within the polymer film from the NSLD profiles. The fits utilized a standard Parratt recursive algorithm,20 which uses multiple layers to describe the interfaces, in the REFLPAK software package.21 The interfaces are assumed to be in the form of error function for the fits. The REFLPAK software was used for both data reduction and data fitting.20 The model utilized in all cases for fitting the NR data consists of 3 layers: silicon substrate (fixed NSLD = 2.07 106 Å2), silicon oxide (NSLD B 3.5 106 Å2) and polymer (varied NSLD). Heterogeneity in the D2O (NSLD = 6.33 106 Å2) concentration near the silicon oxide–polymer interface can be assessed by an apparent increase in the silicon oxide thickness and the width of the interface between silicon oxide and polymer. Prior studies have demonstrated that metal oxide is not swollen by the D2O15 and that the excess (in equilibrium) water near the inorganic–polymer interface determined from NR can quantitatively describe thickness dependent swelling.22
Results and discussion Fig. 2 shows the fracture energies of all PAMA–glass systems as a function of RH. Three distinct regions of crack growth behavior may be identified for PMMA and PEMA, which are denoted as regions I, II and III. In the region I (o60% RH), the adhesion was relatively insensitive to moisture and the crack propagation was found to occur in a stick-slip manner. ATRFTIR and XPS revealed that the failure path was cohesive in the polymer layer (see Fig. S1 and S2 in ESI†). As the RH was raised from 60% to 70% (region II), analogous stick-slip crack propagation was observed for the PMMA and PEMA joints but the
Fig. 2 The fracture energy, Gc, for different polymers (PMMA , PEMA , and PBMA, ’) on silica surfaces over a wide range of RH: shaft-loaded blister tests were conducted at (23 2) 1C. Error bars represent two standard deviations.
This journal is © The Royal Society of Chemistry 2015
Soft Matter
fracture energies dropped dramatically (nearly two orders of magnitude) over a small change in RH. The loci of failure were complex involving a mixture of adhesive and cohesive failure. In region III (470% RH), the fracture energies of PMMA and PEMA joints were now extremely low and exhibited a transition from a stick-slip fracture to a stable crack growth (see Fig. S3 in ESI†). XPS showed that the joints in region III failed predominantly at the polymer/substrate interface. While the Gc-RH relationship of PBMA joints did not show these three-region characteristics, four unique features in their fracture behaviors are worth noting. First, the fracture energies seemed to show a gradual linear decrease as the RH is increased although the change is within experimental uncertainty. The criticality in RH is clearly absent. Second, the failure is cohesive (in the polymer) with stick-slip crack growth over the entire range of RH levels. Third, among the systems examined, the PBMA joints exhibited the worst adhesion to glass in dry conditions, but showed the best adhesion strength of the PAMAs examined in wet conditions (470% RH). Moreover, the fracture energies in dry state are only slightly higher than that in wet conditions for the PBMA joints, suggesting that moisture has little impact on the adhesion of this system. It is quite interesting that a small change in the chemistry of the polymer (ethyl to butyl) can dramatically impact the response of the joint strength to RH. To elucidate the origins of the observed criticality in adhesion loss, we will examine 4 ideas that have been suggested as possible contributing factors: bulk sorption behavior, interface water distribution, free energy change associated with adhered and delaminated joint, and swelling behavior. We will begin with the possible contribution of bulk sorption properties of the adhesives. Fig. 3 illustrates water sorption isotherms, the equilibrium moisture mass concentration (Mc) for samples exposed at different RH values. In the case of PMMA and PEMA, the bulk sorption curves show a change in slope at approximately the same RH as the drop in strength of the adhesive joints, which is consistent with the notion that the two behaviors are related. The result for PBMA, however, yields a different conclusion. This system does not show a sudden drop in strength for the adhesive joint as the RH is increased even though the sorption curve does exhibit a deviation from Henry’s law at approximately 60% RH. Note that the water concentrations in bulk PEMA and PBMA are less than that in PMMA at the same RH value by a factor of 1.8 and 3, respectively. If the data are adjusted for this difference, the curves overlap perfectly (inset in Fig. 3) so there is no significant difference in the shapes of these isotherms. This result suggests that a simple change of slope in the bulk sorption isotherm curve cannot alone explain the critical RH. It is noteworthy, however, that the water content in PBMA is very low so it might be argued that although there is a change in slope at 60% RH, there simply is not enough water present to cause the drop in bond strength. With PMMA and PEMA the water content by mass at the critical RH is about 0.5% and 0.28%, respectively. PBMA reaches 0.28% water content at a RH of about 80%, and yet even here there is no drop in adhesive strength for the PBMA bonds. Consequently, the total amount
Soft Matter, 2015, 11, 3994--4001 | 3997
View Article Online
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Soft Matter
Paper
Fig. 3 Moisture sorption isotherms for ( ) PMMA, ( ) PEMA, and ( ) PBMA in terms of mass% water sorbed. (inset) Shifted moisture sorption isotherm data to adjust for the difference in equilibrium water concentration between PMMA and two other PAMAs by multiplication by 1.8 and 3 for PEMA and PBMA, respectively. Experimental uncertainty is less than the size of the symbols.
of water in the polymer does not seem to be directly correlated with the critical RH. The second possible factor that might contribute to the drop in adhesion is a higher local water concentration near the interface. While the bulk sorption isotherms are expected to reflect the general water concentration in free-standing film, these isotherms do not necessarily tell us about the concentrations near the PAMA–silicon oxide interface. Information on interfacial water concentration is of fundamental importance because adhesive failure is observed at high RHs for PMMA and PEMA. To assess differences in interfacial moisture between glassy (PMMA) and rubbery (PBMA) polymers, NR profiles were fit using a recursive algorithm based on the method described by Parratt using the methods described by Yu et al.20 The reflectivity data are shown in Fig. 4 as a function of the momentum transfer vector, q, which is normal to the film surface for these specular reflectivity data. The fits quantitatively describe features in the reflectivity spectra over the entire q-range examined in all cases. These fits provide profiles that describe the thickness and interfacial width of the silicon oxide and polymer layers in terms of the neutron scattering length density (NSLD) profile through the thickness of the film as shown in Fig. S4 (ESI†). The NSLD is not dependent on the bonding in the system, only the density and atomic nuclei present. For an ideal mixture (no volume change on mixing), the NSLD is simply the volume fraction averaged NSLD of the individual components. The interfacial water (D2O) concentration profiles for glassy (PMMA) and rubbery (PBMA) polymers were determined from the fits of the NR data (see Fig. S4–S6, ESI† for more details) and are shown in Fig. 5. A general feature to be noted is that water molecules are not uniformly distributed throughout the interfacial region (over distances of a few nm). Enhanced water adsorption is observed in the proximity of the glass oxide. This excess is likely the result of the hydrophilic character of the
3998 | Soft Matter, 2015, 11, 3994--4001
Fig. 4 NR data (points) and associated recursive fits (lines) for (a) PMMA and (b) PBMA films in the dry (red) and hydrated (blue) states. A clear shift to lower q in the reflectivity can be observed at high q for the hydrated films.
silanol-rich surface of the glass, which has concentrations of hydroxyl groups of up to eight per nm2.23 The apparent drop in water concentration very close to the interface is a reflection of the surface roughness in the oxide layer, which is on the order of 0.5 nm, as water cannot penetrate into the silicon oxide. In this case, the material contains polymer, D2O, and silicon oxide, which accounts for the overall decrease in concentration as the silica content increases (as shown by the black lines in Fig. 5). As the D2O and H2O exhibit similar, but slightly different properties, we report all the sorption data in terms of the activity of the D2O, which is quotient of the partial pressure of D2O and saturation pressure of D2O at that temperature. In an ideal case, the sorption of D2O and H2O should be identical at the same activity (relative humidity). The accuracy of the concentrations calculated from the NSLD improves as the concentration increases due to the volume fraction weighting in the calculation. We estimate the uncertainty to be 1%. At low D2O activity, (ocritical RH) (Fig. 5a), the concentration profiles for PMMA and PBMA are virtually identical (within the uncertainty). D2O concentration is decreased in PBMA when the distance from the interface is greater than B2 nm, but not outside of experimental error. This difference in solubility is consistent with the sorption in free-standing films (cf. Fig. 3). These data suggest that the local water concentration near the interface is influenced by the hydrophilicity of glass oxide while the bulk polymer properties play a minor role. However, there is a substantial increase in the interfacial D2O concentration for PMMA relative to PBMA at a high D2O activity (4critical RH), as shown in Fig. 4b. The water concentration in the bulk of the film (42 nm from the glass oxide surface) is simply governed by the hydrophobicity
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Paper
Fig. 5 Water distribution (volume fraction, fD2O) near the interface for PMMA and PBMA at (a) low D2O activity = 0.32 (PD2O = 0.008 bar) and (b) high D2O activity = 0.62 (PD2O = 0.016 bar) obtained from NR experiment. At low RH, the interfacial moisture is almost indistinguishable, while there is more water at the interface with PMMA at high activity.
of polymers; due to increasing alkyl side chains, the bulk D2O concentration decreases from PMMA relative to PBMA. To compare the interfacial water concentration curves for PMMA and PBMA, two features will be examined: the height of the peaks (maximum water concentration near the interface) (Fig. 6a), and the peak width at half height (thickness of the water rich layer distributed across the interface between the glass oxide and bulk polymer) (Fig. 6b). The maximum concentration is found to increase with increasing RH, but there is no discontinuity in this maximum concentration between the 60% and 70% RH. The values for peak width at half maximum are shown in Fig. 6b. Below the critical RH, the average thickness of the water rich layer near the interface is between one and two diameters of a water molecule and does not change with D2O activity. In the case of PBMA, this behavior continues over the entire RH range, but for PMMA, there is a sudden increase in thickness of the interfacial water layer at approximately the same RH as the drop in adhesion strength. The potential consequences on adhesion are twofold. First, an increase in thickness of the interfacial water may decrease the contact area between the adhesive and substrate. Second, multiple layers of water molecules cannot sustain any significant stresses. Therefore, an increase in water film thickness might be expected to disrupt interfacial bonding, which underlines the high relevance of this observation to sudden debonding. A third possible factor for the critical RH in adhesion is if the free energy associated with the interface is favorable. The work of adhesion, WA, indicates the thermodynamic interactions between the adhesive and the substrate across an idealized flat surface. Calculating the value of this parameter in a dry environment, WA, and in the presence of moisture, WAL,
This journal is © The Royal Society of Chemistry 2015
Soft Matter
Fig. 6 A comparison is made between interfacial water absorption behaviors of two polymers obtained from NR experiments as a function of the activity (partial pressure) of D2O: (a) maximum in water concentration near interface for PMMA/glass and PBMA/glass joints. (b) Width at half height for water peak near interface in PMMA/glass and PBMA/glass joints.
indicates the thermodynamic impact of water on the hydrolytic stability of the interface bonds. This behavior is of particular interest here since the glass surface has relatively low roughness. Table 1 shows how the work of adhesion varies with hydration for the three systems tested here. In all cases, there are large differences between wet and dry values. The very low work of adhesion in a moist environment indicates that water molecules appreciably weaken the interface but the joint is still thermodynamically favored as the interfacial PAMA/glass interaction is stronger than the PAMA/water and glass/water.24 This change in the work of adhesion can be rationalized in terms of the interactions between glass and PAMA. One to two silanol groups per nm2 can interact with the adsorbed PAMA depending on the concentration of carbonyl groups near the surface and the distribution of silanol groups.25 The affinity of PMMA towards the acidic glass oxide produces a significant increase in Tg of PMMA thin films supported on silica.26,27 Consequently, the work of adhesion calculations suggests that water weakens the interface, but does not explain why the behavior of PBMA is different. The final factor potentially influencing the critical RH in adhesion is swelling induced stresses. Water molecules can adsorb into PAMAs and increase the spacing between chains by entering voids or free volume (free water) or via hydrogen bonding with carbonyl groups (bound water molecules), which disrupts the hydrogen bonding.28 This expansion can induce significant stresses at the geometrically confined polymer/substrate interface to adversely impact joint strength. However, quantifying
Soft Matter, 2015, 11, 3994--4001 | 3999
View Article Online
Soft Matter Table 1
Properties of PAMA/glass interfaces
PMMA/glass PEMA/glass PBMA/glass
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Paper
WA (mJ m2)
WAL (mJ m2)
Virgin Gc (J m2)
Recovered Gc (J m2)
91 2 79 1 81 2
7.0 0.3 5.0 0.3 5.0 0.4
191 46 190 37 68 18
109 17 117 27 57 4
the impact of bulk swelling at the confined interface is not trivial and it is further complicated by the heterogeneous water concentration as the interface is approached. Moreover, the material properties in the interfacial region may differ from the bulk polymer. Nonetheless, since the load-transfer efficiency is a strong function of the polymer’s modulus, the confined glassy polymer/substrate interface will be stressed with larger swellinginduced stresses compared to the rubber/substrate interface. The bond failure measurements involve evaluating the energy required to propagate a crack, and it is possible that the swellinggenerated stresses contribute to the crack tip stress field, which reduces the external load that needs to be applied for crack propagation. This postulate is in agreement with work of Jing et al. who reported that supported PMMA films could be debonded from substrates simply by water-induced swelling stresses alone.29 Their theoretical analysis based on a two-layer model showed that the adsorbed water layer induces misfit strain between the swollen layer and the interior dry layer of PMMA, thus inducing bending stress in PMMA layers. The film detachment occurs when the swelling stress exceeds the PMMA/substrate interfacial strength. Notably, this debonding process is reversible. This stress-assisted mechanism would also explain why the interfacial PMMA/glass oxide and PEMA/glass oxide bonds, which are predicted as being stable against moisture intrusion from thermodynamic arguments, effectively rupture upon exposure to high RHs. However, a sole contribution from bulk swelling is insufficient for the following reasons. First, interfacial failure was only observed at high RHs. Second, the adhesion of PMMA with various types of substrates under 100% RH conditions can be tuned between adhesive and cohesive failures through modification of the substrate surface energy.30 There is one further experiment that can provide some insight into this adhesive joint failure behavior: recovery tests. Joint specimens of all three polymers were exposed to 100% RH for 72 h followed by extensive drying in desiccators for 720 h. The fracture behavior was then measured on the dried samples. As indicated in Table 1, PBMA exhibits full adhesion reversibility after extensive drying, while only partial recoverability was observed for PMMA and PEMA counterparts. Moreover, ATRFTIR and XPS confirmed that the fracture paths for the latter joints are mixed cohesive and adhesive failures. One might speculate that water disruption of the interface between the PAMA and glass and swelling stresses taken individually might be largely reversible. Since the adhesive and substrate are still in close proximity, the interactions between PAMA and glass can reform when the water is removed. When both mechanisms are present, however, the swelling stresses may deform
4000 | Soft Matter, 2015, 11, 3994--4001
the material when the interface is weakened so reformation of the interactions between PAMA and glass is hindered when the sample is dried. Thus the recovery of adhesive strength of the joint is only partial. From all the observations discussed above, we can conclude that, in analogy to stress-corrosion cracking, the sudden adhesion loss is dictated by a combination of the interfacial water accumulation and swelling-stress built-up at the polymer/substrate interfaces. Contrary to glassy PMMA and PEMA that exhibit critical adhesion loss, the rubbery polymer, PBMA, is able to suppress interfacial water accumulation (due to its more hydrophobic character) and to relieve swelling stresses (due to its ability to creep and lower modulus), leading to the absence of a critical RH. To further test this hypothesis, we examined the adhesive strength of a poly(n-propyl methacrylate) (PPMA)/glass joint as a function of relative humidity. The Tg (37 1C) is very near the testing temperature (23 1C). This results in a significant decreased modulus31 that relaxes the stresses developed by swelling. As shown in Fig. S7 (ESI†), the PPMA does not exhibit a catastrophic critical humidity and the joint performance mimics that of PBMA. The critical RH behavior can likely be avoided by minimizing the interfacial water accumulation through substrate modification28 and/or avoiding swelling-stress built-up at the polymer/substrate interfaces.
Conclusions The present study investigated the effects of moisture on the properties of three model adhesive systems based on glass and poly(n-alkyl methacrylate). For PMMA and PEMA, a dramatic drop in adhesive strength is observed for samples exposed to high relative humidity (460% RH). The other system (PBMA) has much lower strength when dry, but shows little loss in strength when exposed to relative humidities approaching 100%. This difference in adhesion behavior cannot be explained based on the thermodynamic work of adhesion since all three systems show similar drops when water is present. Another factor, which does not correlate with the loss of adhesion strength, is the change in slope for the sorption curves. All three systems show slope changes between 60% RH and 70% RH. PBMA has a lower water content that the other systems when exposed to relative humidities between 60% and 70%; however, PBMA does not show a drop in adhesion strength even when the water content exceeds the level where PEMA exhibits the loss of joint strength. Consequently, a mechanism based on the deviation from Henry’s law in the sorption isotherm curves and/or the total water content in the adhesive does not provide an explanation for the adhesion behavior. On the other hand, one factor that does show a correlation with adhesion behavior is the thickening of the local water rich layer near the interface. In the case of PMMA, there is a sudden increase in thickness of this region at the critical RH for adhesion while PBMA does not show this increase or a critical RH. This interfacial water accumulation could result in displacement of interfacial bonds, leading to abrupt adhesion loss at the critical RH. Finally, PBMA has a much lower modulus
This journal is © The Royal Society of Chemistry 2015
View Article Online
Published on 10 April 2015. Downloaded by RUTGERS STATE UNIVERSITY on 26/07/2015 18:57:03.
Paper
and exhibits significant creep, which should reduce stresses that arise from swelling induced by water. This creep behavior is not available for the much stiffer PMMA or PEMA. Experiments indicate that this produces significant differences in swelling stresses, which might contribute to the drop in bond strength. Based on these results, we propose that the origin of the criticality in adhesion loss might be attributed to the combination of interface weakening by ingress of water molecules assisted by moisture-induced swelling stresses. Experiments show that the loss of adhesion strength is partially reversible when the specimen is dried.
Notes and references 1 A. J. Kinloch, Adhesion and Adhesives: Science and Technology, Chapman and Hall, London, 1987. 2 K. T. Tan, C. C. White, D. L. Hunston, B. D. Vogt and A. P. Haag, in Advances in Structural Adhesive Bonding, ed. D. A. Dillard, Woodhead, Cambridge, UK, 2010, pp. 547–568. 3 D. M. Brewis, J. Comyn, A. K. Raval and A. J. Kinloch, Int. J. Adhes. Adhes., 1990, 10, 247–253. 4 J. Cognard, Int. J. Adhes. Adhes., 1988, 8, 93–99. 5 D. R. Lefebvre, K. M. Takahashi, A. J. Muller and V. R. Raju, J. Adhes. Sci. Technol., 1991, 5, 201–208. 6 D. R. Lefebvre, K. M. Elliker, K. M. Takahashi, V. R. Raju and M. L. Kaplan, J. Adhes. Sci. Technol., 2000, 14, 925–937. 7 J. Comyn, C. L. Groves and R. W. Saville, Int. J. Adhes. Adhes., 1994, 14, 15–20. 8 R. A. Gledhill and A. J. Kinloch, J. Adhes., 1974, 6, 315–330. 9 K. T. Tan, B. D. Vogt, C. C. White, K. L. Steffens, J. Goldman, S. K. Satija, C. Clerici and D. L. Hunston, Langmuir, 2008, 24, 9189–9193. 10 Y. Weitsman, J. Compos. Mater., 1977, 11, 378–394. 11 J. P. Sargent and K. H. G. Ashbee, J. Phys. D: Appl. Phys., 1981, 14, 1933–1938. 12 C.-S. Jou, S. T. Sackinger and R. J. Farris, J. Coat. Technol., 1995, 67, 71–77.
This journal is © The Royal Society of Chemistry 2015
Soft Matter
13 T. Nguyen, E. Byrd, D. Bentz and C. Lin, Prog. Org. Coat., 1996, 27, 181–193. 14 W.-L. Wu, W. J. Orts, C. J. Majkrzak and D. L. Hunston, Polym. Eng. Sci., 1995, 35, 1000–1004. 15 B. D. Vogt, V. M. Prabhu, C. L. Soles, S. K. Satija, E. K. Lin and W.-l. Wu, Langmuir, 2005, 21, 2460–2464. 16 K. T. Wan and Y. W. Mai, Int. J. Fract. Mech., 1995, 74, 181–197. 17 D. K. Owens and K. C. Wendt, J. Appl. Polym. Sci., 1969, 13, 1741. 18 E. H. Wong, R. Rajoo, S. W. Koh and T. B. Lim, Trans. ASME, 2002, 124, 122–126. 19 G. Jakli and W. A. Van Hook, J. Chem. Eng. Data, 1981, 26, 243–245. 20 C.-Y. Yu, E. Kim and J.-Y. Kim, J. Nanosci. Nanotechnol., 2011, 11, 4624–4628. 21 P. A. Kienzle, K. V. O’Donovan, J. F. Ankner, N. F. Berk and C. F. Majkrzak, 2000–2006, http://www.ncnr.nist.gov/ reflpak. 22 B. D. Vogt, C. L. Soles, R. L. Jones, C.-Y. Wang, E. K. Lin, W.-l. Wu, S. K. Satija, D. L. Goldfarb and M. Angelopoulos, Langmuir, 2004, 20, 1453–1458. 23 L. T. Zhuravlev, Colloids Surf., A, 2000, 173, 1–38. 24 F. M. Fowkes, in Physicochemical Aspects of Polymer Surfaces, ed. K. L. Mittal, Plenum Press, New York, 1981, pp. 583–603. 25 J. M. Berquier and H. Arribart, Langmuir, 1998, 14, 3716–3719. 26 C. E. Porter and F. D. Blum, Macromolecules, 2002, 35, 7448–7452. 27 C. E. Porter and F. D. Blum, Macromolecules, 2000, 33, 7016–7020. 28 D. T. Turner, Polymer, 1982, 23, 197–202. 29 B. Jing, J. Zhao, Y. Wang, X. Yi and H. Duan, Langmuir, 2010, 26, 7651–7655. 30 E. P. O’Brien, C. C. White and B. D. Vogt, Adv. Eng. Mater., 2006, 8, 114–118. 31 P. Panagos and G. Floudas, J. Non-Cryst. Solids, 2015, 407, 184–189.
Soft Matter, 2015, 11, 3994--4001 | 4001
