Letter pubs.acs.org/Langmuir

Interfacial Segregation in Polymer Blends Driven by Acid−Base Interactions Shishir Prasad,†,§ He Zhu,† Anish Kurian,†,∥ Ila Badge,† and Ali Dhinojwala*,† †

Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: Infrared-visible sum frequency generation spectroscopy (SFG) was used to measure the interfacial concentrations of poly(methyl methacrylate) (PMMA)/ polystyrene (PS) blends next to a sapphire substrate. The acid−base interactions of carbonyl groups of PMMA with the hydroxyl groups on the sapphire drive the interfacial segregation of PMMA next to the sapphire substrate. Using the shift of sapphire surface OH peaks, we have determined the difference in interfacial energy between the PMMA/ sapphire and the PS/sapphire to be ∼44−45 mJ/m2. These results highlight the importance of acid−base interactions and their role in controlling the interfacial segregation next to solid substrates in polymer blends.



INTRODUCTION In polymer blends, it is expected that molecules with lower surface energy will preferentially segregate at the air surface.1−6 In addition to the difference in surface energy, molecular architecture, molecular weight, and solvents used for spin coating may also play an important role in interfacial segregation.7−12 For segregation next to solid substrates, an additional effect due to interactions of the chemical groups with the substrate could play an important role.13,14 For example, in polystyrene (PS)/polymethylmethacrylate (PMMA) blends in contact with sapphire, the carbonyl groups in PMMA have much stronger interactions than the phenyl groups in PS with the sapphire surface OH. On the basis of the differences in interaction energies, it is expected that PMMA will segregate next to the sapphire substrate.15 Techniques such as neutron reflectivity, forward-recoil spectrometry, and secondary ion mass spectroscopy have been used in previous studies,2,3,13 and some of these techniques require deuteration of one component to improve the contrast. However, determining concentration of segregated molecules next to solid surfaces, particularly in molecular contact with the substrate, is very challenging, particularly at low concentrations of one of the components in the polymer blends.2,3,16 Interfacial concentrations play an important role in many areas, such as wetting, adhesion, friction, mechanical properties of polymer composites, and thermal and electrical transport across interfaces. Here, we used SFG spectroscopy to directly measure the interfacial concentration for PS/PMMA blends in contact with the sapphire substrates. SFG spectra provide information on the structure and chemical composition of interfacial molecules. SFG has been used to study PS and PMMA structure at solid interfaces17−19 and polymer blends.6,20 The direct calculation of interfacial or surface concentrations is difficult because the SFG signals are directly proportional to both surface concentrations © 2013 American Chemical Society

and the orientation of the interfacial molecules. We recently used SFG to study the interactions between the surface OH groups on the sapphire substrate (acidic) with different polymers. We showed that the shift of the OH vibrational peak was proportional to the strength of the acid−base interaction.15 Here, we exploited the differences in the OH vibration peak in contact with PMMA or PS to measure the interfacial concentrations of PMMA/PS blends in contact with the sapphire substrates. The shifts of the surface OH peak are used to calculate the difference in interfacial energy between PMMA/sapphire and PS/sapphire (∼ 44 mJ/m2), and this difference is much larger than the difference in surface energies between PMMA/air and PS/air surfaces (≤1 mJ/m2 at 125 oC21). The differences in the interfacial energies demonstrated a strong interfacial segregation driven by the strength of the acid−base interactions.



EXPERIMENTAL SECTION

Sample Preparation: PMMA of molecular weight (Mw) 470 kg/mol and polydispersity of 1.06 was purchased from Scientific Polymer Products. The glass transition temperature (Tg) of PMMA is 133 oC, measured by using differential scanning calorimetry (DSC) at a heating rate of 10 oC/min. The proton NMR showed that the PMMA was 92% syndiotactic. PS of Mw = 120 kg/mol and polydispersity of 1.05 was purchased from Polymer Source. The Tg of PS supplied by Polymer Source was 100 oC measured at a heating rate of 10 oC/min. Toluene was purchased from Alfa Aesar of 99.7% minimum purity and was used as received. A 2 wt % solution of polymer blends in toluene and a spin speed of 2000 rpm for 1 min was used to prepare thin films on solid substrates. The sapphire prisms were used for the SFG experiments, and the flat silicon or sapphire plates were used for X-ray Received: September 12, 2013 Revised: October 16, 2013 Published: December 6, 2013 15727

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Figure 1. SFG spectra for PMMA-PS blends, at different mole fractions of PMMA, collected in the (A) hydrocarbon and (B) hydroxyl regions. The solid lines in Figures 1(A) and (B) are the fits using eq 1. All the spectra were measured using the PPP polarization.

to interaction of the phenyl groups with the surface OH.17 For PMMA, the carbonyl groups have a strong interaction with the surface OH groups.18 The peak assignments are summarized in Table S3 of the Supporting Information. The main peaks in the SFG spectra at 2950−55 cm−1 and 3000 cm−1 are assigned to the symmetric and asymmetric methyl groups (−OCH3), respectively. The peaks at 3025 cm−1, 3060−3065 cm−1, and 3080 cm−1 are assigned to the ν20b, ν2, and ν20a phenyl vibrational modes of PS, respectively. The broad peak between 3400 and 3800 cm−1 is assigned to surface OH groups on the sapphire substrate.15,22 We have shown previously that the position of the surface OH peak shifts to lower wavenumbers because of the acid−base interactions.15 The SFG spectra for the blank sapphire substrates are shown in Figure S5 of the Supporting Information,15 and the stronger the acid−base interactions, the larger the shift. The surface OH peak shifted to 3580 cm−1 and 3645 cm−1 for PMMA and PS, respectively. For PMMA-PS blends, as the concentration of PMMA increased, the broad OH peaks shifted to lower wavenumbers. This broad peak can be fitted with two peaks. One broad peak centered at 3580 cm−1, which corresponds to PMMA, and the other broad peak centered at 3645 cm−1, which corresponds to PS. Interestingly, even at 0.0005 bulk mole fraction of PMMA, we observed SFG peaks in the hydrocarbon and OH region assigned to PMMA units, indicating strong segregation of PMMA next to the sapphire substrate. We have also measured the SFG spectra using SSP polarization (data shown in Figure S6 of the Supporting Information) in the hydrocarbon region that supports the conclusions drawn from the PPP analysis. The OH peaks in SSP polarization were weak and were not used for the analysis reported in this paper. The hydroxyl peaks in the SFG spectra can be modeled using two peaks in the Lorentzian equation (eq 1). These two peaks are assigned to the surface OH groups interacting with either PMMA or PS basic groups. There are three main unknowns in the fitting procedure: Aq (amplitude strength), ωq (position of the peak), and Γq (width). Both the width and the peak position were determined by fitting the SFG spectra for

photoelectron spectroscopy (XPS). The substrates (prism and plates) were sonicated twice in toluene followed by an air plasma cleaning for 5 min. The thickness of the polymer films were on average 120 ± 10 nm for all the different blend composition used in this study. Ellipsometer was used to measure the film thickness (Supporting Information). The films were annealed in a vacuum oven at a temperature of 125 oC and the vacuum of one inch of Hg for a period of 12 h. The annealing temperature is a little below the Tg of PMMA. A similar experiment with the annealing temperature above the Tg of PMMA at 150 oC gives a comparable interfacial concentration of PMMA. SFG spectra were fitted using the following Lorentzian equation:17

I(SFG) ∝ |χeff,NR +

∑ (Aqeiϕq/(ωIR − ωq − i Γq))|2 q

(1)

The terms Aq, Γq, ϕq, and ωq, are the strength, damping constant, relative phase of the resonant mode with respect to the nonresonant term (χeff,NR), and the resonant frequency of the molecular vibration, respectively. The amplitude strength (Aq) is proportional to the orientation and the concentration of the interfacial molecules. The resonant frequency (ωq) is related to the infrared and Raman active modes of the molecules. The following procedure was followed for the OH analysis. First, the homopolymer spectra were fitted using the Lorentzian equation, and all the parameters were varied to obtain the best fit. The blend data were fitted by fixing the ωq and Γq values obtained from fitting the homopolymer spectra and varied Aq to obtain the best fits. Detailed SFG and XPS experiment methods can be found in the Supporting Information.



RESULTS AND DISCUSSION Figure 1 shows the SFG spectra collected for PMMA/PS blend/sapphire interfaces using PPP polarization (p-polarized SFG, visible, and IR beams). The SFG spectra were collected by scanning the IR laser from 2700 to 3800 cm−1. The hydrocarbon and surface OH peaks are observed between 2700 and 3100 cm−1 and 3400−3800 cm−1, respectively. The peak assignments and orientational analysis of the side groups for PMMA and PS have been discussed previously.17,18 In the case of PS, the phenyl groups are oriented parallel to the surface due 15728

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As a comparison, we used X-ray photoelectron spectroscopy (XPS) to measure the surface composition near the air surface. Figure 3 (panels A and B) show the XPS intensity as a function of binding energy in C1s and O1s scan regions, respectively. The ratios of the oxygen peak intensity to the carbon peak intensity are used to calculate the surface mole fraction of PMMA, and the results are shown in Figure 2. The XPS data indicate that there is a preference for PS segments at the air surface. However, at the bulk mole fraction of less than 0.04, the surface compositions are similar to those in the bulk. We were unable to use SFG to calculate surface excess because the SFG spectra have only hydrocarbon peaks6 for analysis (the OH peak is absent), and the analysis depends on absolute values of molecular hyperpolarizability and orientation. The differences between the segregation at the sapphire interface and the air surface are striking. The differences in the surface energies of PMMA (σA ∼ 33 mJ/m2 at 125 oC) and PS (σB ∼ 33 mJ/m2 at 125 oC) are small. Nonetheless, the interfacial segregation is strongly affected by the acid−base interactions at the sapphire substrates. The shifts in the surface OH peak can be related to the interfacial interaction energy, ΔH, using the Badger−Bauer equation.15,24 For PS and PMMA, the ΔH is 0.85 ± 0.04 and 1.56 ± 0.03 kcal/mol, respectively (measured using films consisting of either 100% PS or PMMA).15 We can use the ΔH values to estimate the differences in the interfacial energy between the PMMA and the PS homopolymers in contact with the sapphire substrate. The interfacial energy can be expressed as follows.25

homopolymers (PMMA and PS). These two parameters (ωq and Γq for both PMMA and PS) were kept constant when fitting the data for polymer blends and only the amplitude strengths (for both PS and PMMA fraction) were determined from the fits. The amplitude strength in the SFG spectra depends on the molecular hyperpolarizability, orientation, and the concentration of the interfacial molecules.23 It is likely that surface OH groups may have different orientations. However, we do not anticipate a specific subset of orientated OH interacting differently with PMMA than PS, and the orientation of surface OH does not depend on the concentration of the blend. Because of these reasons, we can use the Aq values to calculate the mole fraction of PMMA repeat units next to the surface, Aq,PMMA/(Aq,PMMA + Aq,PS). We refer to “mole fraction” as a mole fraction of the repeat unit. For these analyses, we assume that one acid group is interacting with one basic group, which is reasonable considering the nature of acid−base interactions. The interfacial concentrations calculated using the amplitude strength are plotted as a function of the PMMA mole fraction in the PMMA-PS blends (Figure 2).

AB σAS = σA + σS − WAS − 2 σAdσSd

(2)

AB σBS = σB + σS − WBS − 2 σBdσSd

(3)

In eqs 2 and 3, the symbols A, B, S, and AB are used to denote PMMA, PS, sapphire, and acid−base interactions, AB respectively. The terms WAB AS and WWBS represent the energy/ area due to acid−base interactions for PMMA/sapphire and PS/sapphire, respectively, and can be related to ΔH as follows:25

Figure 2. Interfacial PMMA mole fraction (with error bars) as a function of the bulk PMMA mole fraction calculated using the SFG spectra in the hydroxyl (red ○). The ratio of Aq,PMMA/(Aq,PMMA + Aq,PS) calculated using the 2955 cm−1 PMMA and 3025 cm−1 PS peaks are shown in Figure 2 (data collected using the PPP polarization are shown as blue • and the SSP polarization as green ■). As a comparison, the data for surface PMMA mole fraction (measured using XPS results shown in Figure 3) as a function of the bulk PMMA mole fraction are also shown in Figure 2 (black △).

AB AB WAS = ΔHA × nOH and WBS = ΔHB × nOH

(4)

In eq 4, ΔHA and ΔHB are the acid−base interaction energies for PMMA and PS in the units of energy/mol. The term nOH is moles of surface OH groups per unit area. We have used the approach described by McCafferty26 to characterize the OH group density on the sapphire surface using XPS. Our calculations based on the XPS results show that there are approximately nine surface OH groups per nm2 (the detailed calculations are provided in the Supporting Information). This density of surface hydroxyl groups corresponds to nOH ∼ 1.5 × 10−5 mol/m2. In eqs 2 and 3, σdA, σdB, and σdS are the dispersion components of the surface energy σA, σB, and σS, respectively. The differences in surface energies between PMMA and PS are small, and we can simplify the eqs 2 and 3 and write the differences in the interfacial energies (σAS − σBS) as follows:

The changes in the spectral features in the SFG spectra in the hydrocarbon region also support the strong segregation of PMMA next to the sapphire substrate. However, it is not possible to use this region to determine the interfacial concentration in this case because the molecular hyperpolarizability and the orientation contributions are unknown. Here, we have analyzed the ratio of the amplitude strength of PMMA with respect to the total amplitude strength, Aq,PMMA/ (Aq,PMMA + Aq,PS) to qualitatively discuss the segregation of PMMA. We used the most dominant peaks (2955 cm−1 peak for PMMA and the 3025 cm−1 peak for the PS) in the SFG spectra to estimate the relative interfacial concentration of PMMA as a function of bulk concentration (Figure 2). The results from the hydrocarbon region are plotted in Figure 2 using the right-hand side y axis. We have also included the data points analyzed using the SSP data showed in the Supporting Information. The similarity in the trend of the results from OH and hydrocarbon analysis suggests that the changes in the ratio of the amplitude strength correlates with the interfacial concentration calculated using the analysis of the OH peaks.

(σAS − σBS) ≈ nOH(ΔHA − ΔHB)

(5)

Using eq 5, we calculated that the value of (σAS − σBS) is ∼ −44 mJ/m2. This calculation assumes that all the surface OH groups are participating in the acid−base interactions. The amplitude strength of the nonhydrogen bonded peak observed in the SFG spectra is much smaller than the broad peaks associated with the acid−base interactions with PMMA or PS 15729

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Figure 3. XPS spectra measured for PMMA-PS blends at different mole fraction plotted as a function of the binding energy in the regions corresponding to the (A) C1s peak and (B) O1s peak.

solid surface, and the interface may reach a steady-state situation much earlier while the rest of the film continues to change. We also anticipate that the strong acid−base interactions may speed up the segregation of the PMMA chains next to the solid interface. This observation is also supported by our preliminary SFG results for low molecular weight PMMA which also show similar trends, in this case the diffusion process should be even faster. On the basis of the annealing time and film thickness, one could estimate that the diffusion constant has to be faster than ≈10−16 cm2/s to reach the equilibrium state. In comparison to the diffusion constants measured for PS of similar molecular weights, these times are sufficient for chains to diffuse and reach equilibrium concentration at the sapphire interface (assuming diffusion is controlled by fast moving chains).27 One could also anticipate that the segregation profile could have evolved rapidly in the spin coating and subsequent drying process. In this case, the concentration of PMMA chains next to the sapphire interface may be higher than the equilibrium concentration, and the attached chains may not have enough time to diffuse away from the surface (these diffusion constants can be 2 orders of magnitude slower than the bulk diffusion constants),28,29 and this may lead to nonequilibrium composition next to the solid surface. Even if this hypothesis is possible, it is clear that the strong acid−base interactions are again playing an important differential role in the attachment and diffusion of the PMMA chains.

and this supports this assumption. Nevertheless, the large differences in interfacial energies explain the strong segregation of PMMA at the sapphire interface. There are potentially other reasons that may also be responsible for strong segregation of PMMA at the solid interface. One driving force could be differential solubility of PS and PMMA in the solvent used for spin coating (toluene). As PS is more soluble in toluene, the solvent effect may induce surface preferential segregation of PS. However, this segregation can be reduced or removed by annealing the sample above Tg for a sufficient amount of time.5,6 In addition, the solvent effect is expected to be less pronounced at the polymer/substrate interface compared to the segregation driven by interactions with the substrate.14 Also, it is likely that for a certain range of concentration, the two components of the blends may have phase separated, making the films heterogeneous with the PMMA phase next to the sapphire surface. However, these cannot be the reasons for the strong segregation of PMMA at the sapphire interface at bulk concentrations as low as 0.001 mol fraction, where the blends are expected to be miscible (Figure S3 of the Supporting Information). It is also expected that molecular weights used in this study (470 kg/mol of PMMA and 120 kg/mol for PS) may influence the interfacial segregation. However, similar experiments using different molecular weights of 21.1 and 101 kg/ mol PMMA show that this effect is small and is not the main driving force for segregation. Therefore, we conclude that the strong acid−base interaction between the PMMA and sapphire substrate is the main driving force for interfacial segregation. Finally, we would like to reflect whether the interfacial segregation reflects an equilibrium condition. If the PMMA and PS were dispersed uniformly after the spin-coating process (this condition corresponds to the interfacial concentration of PMMA to be less than what will be expected for the equilibrium state) then the question is whether the annealing times were sufficient for PMMA and PS chains to diffuse and reach equilibrium conditions. SFG is only measuring the concentration of molecules in the immediate vicinity of the



CONCLUSIONS In conclusion, we have shown that the strong acid−base interactions between the carbonyl groups of PMMA and the surface OH groups of sapphire are responsible for a very strong interfacial segregation in thin films of PS/PMMA blends. Even with bulk mole fraction of PMMA of 0.01, the interfacial concentration of PMMA is as large as 50−60%. The difference in the interfacial energies calculated between PS and PMMA is as large as 44 mJ/m2, and this plays an important role in interfacial segregation. Even though qualitatively we can explain 15730

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molecular-weight polystyrene/low-molecular-weight poly(methyl methacrylate) blend films. Macromolecules 1998, 31, 863−869. (11) Foster, M. D.; Greenberg, C. C.; Teale, D. M.; Turner, C. M.; Corona-Galvan, S.; Cloutet, E.; Butler, P. D.; Hammouda, B.; Quirk, R. P. Effective χ and surface segregation in blends of star and linear polystyrene. Macromol. Symp. 2000, 149, 263−268. (12) Kweskin, S.; Komvopoulos, K.; Somorjai, G. Molecular restructuring at poly (n-butyl methacrylate) and poly (methyl methacrylate) surfaces due to compression by a sapphire prism studied by infrared-visible sum frequency generation vibrational spectroscopy. Langmuir 2005, 21, 3647−3652. (13) Russell, T. P.; Coulon, G.; Deline, V.; Miller, D. Characteristics of the surface-induced orientation for symmetric diblock PS/PMMA copolymers. Macromolecules 1989, 22, 4600−4606. (14) Walheim, S.; Böltau, M.; Mlynek, J.; Krausch, G.; Steiner, U. Structure formation via polymer demixing in spin-cast films. Macromolecules 1997, 30, 4995−5003. (15) Kurian, A.; Prasad, S.; Dhinojwala, A. Direct measurement of acid-base interaction energy at solid interfaces. Langmuir 2010, 26, 17804−17807. (16) Harton, S. E.; Stevie, F. A.; Ade, H. Investigation of the effects of isotopic labeling at a PS/PMMA interface using SIMS and mean-field theory. Macromolecules 2006, 39, 1639−1645. (17) Gautam, K. S.; Schwab, A. D.; Dhinojwala, A.; Zhang, D.; Dougal, S. M.; Yeganeh, M. S. Molecular structure of polystyrene at air/polymer and solid/polymer interfaces. Phys. Rev. Lett. 2000, 85, 3854−3857. (18) Rao, A.; Rangwalla, H.; Varshney, V.; Dhinojwala, A. Structure of poly (methyl methacrylate) chains adsorbed on sapphire probed using infrared-visible sum frequency generation spectroscopy. Langmuir 2004, 20, 7183−7188. (19) Lu, X.; Shephard, N.; Han, J.; Xue, G.; Chen, Z. Probing molecular structures of polymer/metal interfaces by sum frequency generation vibrational spectroscopy. Macromolecules 2008, 41, 8770− 8777. (20) Kweskin, S. J.; Komvopoulos, K.; Somorjai, G. A. Entropically mediated polyolefin blend segregation at buried sapphire and air interfaces investigated by infrared-visible sum frequency generation vibrational spectroscopy. J. Phys. Chem. B 2005, 109, 23415−23418. (21) Wu, S. Surface and interfacial tensions of polymer melts. II. Poly (methyl methacrylate), poly (n-butyl methacrylate), and polystyrene. J. Phys. Chem. 1970, 74, 632−638. (22) Anim-Danso, E.; Zhang, Y.; Alizadeh, A.; Dhinojwala, A. Freezing of water next to solid surfaces probed by infrared-visible sum frequency generation spectroscopy. J. Am. Chem. Soc. 2013, 135, 2734−2740. (23) Shen, Y. The principles of nonlinear optics; Wiley: Hoboken, NJ, 1984; Vol. 1. (24) Badger, R. M.; Bauer, S. H. Spectroscopic studies of the hydrogen bond. II. The shift of the O-H vibrational frequency in the formation of the hydrogen bond. J. Chem. Phys. 1937, 5, 839−851. (25) Chaudhury, M. K. Interfacial interaction between low-energy surfaces. Mater. Sci. Eng., R 1996, 16, 97−159. (26) McCafferty, E.; Wightman, J. Determination of the concentration of surface hydroxyl groups on metal oxide films by a quantitative XPS method. Surf. Interface Anal. 1998, 26, 549−564. (27) Anderson, J.; Jou, J. Small-angle neutron scattering studies of diffusion in bulk polymers: Experimental procedures. Macromolecules 1987, 20, 1544−1549. (28) Liu, Y.; Reiter, G.; Kunz, K.; Stamm, M. Investigation of the interdiffusion between poly (methyl methacrylate) films by marker movement. Macromolecules 1993, 26, 2134−2136. (29) Lin, E. K.; Wu, W.; Satija, S. K. Polymer interdiffusion near an attractive solid substrate. Macromolecules 1997, 30, 7224−7231.

the segregation, there is no quantitative thermodynamic or kinetic models to explain these results and will be the focus of our future work.



ASSOCIATED CONTENT

S Supporting Information *

SFG and XPS experiment method, DSC scan for PMMA, PMMA tacticity measurement, sapphire surface OH density measurement, SSP spectra of hydrocarbon region, calculated phase diagram of PMMA-PS blend, SFG peak assignments, and fitting parameters. This material is available free of charge via the Internet at http://pubs.acs.org.



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses §

BD Medical, Becton Dickinson and Company, Franklin Lakes, NJ 07417. ∥ 3M, 3 M Center, MN 55144−1000. Author Contributions ‡

H. Z., S.P., and A.K. contributed equally to this work

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We acknowledge the financial support from the National Science Foundation (Grants DMR-0512156 and DMR1105370), and S.P. acknowledges the financial support of Air Force Research Laboratory under the Collaborative Research and Development III contract FA 8650-07-D-5800, 0056. We also appreciate the help from Mr. Jukuan Zheng for the NMR characterization.



REFERENCES

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Interfacial segregation in polymer blends driven by acid-base interactions.

Infrared-visible sum frequency generation spectroscopy (SFG) was used to measure the interfacial concentrations of poly(methyl methacrylate) (PMMA)/po...
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