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Cite this: DOI: 10.1039/c4cp01849d
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Glass transition of poly(methyl methacrylate) nanospheres in aqueous dispersion Shuo Feng,a Yuenan Chen,a Biyun Mai,a Wanchu Wei,a Caixia Zheng,a Qing Wu,ab GuoDong Liang,ab HaiYang Gaoab and FangMing Zhu*ab Surfactant-free nanospheres and latex nanospheres of poly(methyl methacrylate) (PMMA) with diameter ranging from 20 to 220 nm are prepared by atom transfer radical polymerization (ATRP) in microemulsions and subsequent dialysis against deionized water. The glass transitions of these PMMA nanospheres are characterized using nano differential scanning calorimetry (nano-DSC) in aqueous dispersions. The glass transition temperature (Tg) of the surfactant-free PMMA nanospheres and
Received 29th April 2014, Accepted 12th June 2014
nonionic PMMA latex nanospheres with diameters below 150 nm is less than that of the PMMA bulk, and
DOI: 10.1039/c4cp01849d
is size-independent and is near to that of the PMMA bulk. The influence of the environment surrounding
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the PMMA nanospheres on glass transitions as well as comparisons to our prior studies with polystyrene (PS) nanospheres in aqueous dispersions are discussed.
Tg decreases with the decrease of the diameter. In contrast, Tg of the anionic PMMA latex nanospheres
Introduction The glass transition of nano-sized polymers has attracted growing attention as a result of the deviation from bulk values of the glass transition temperature (Tg) that possess a wide variety of potential applications in nanoscience and nanotechnology.1–27 Moreover, the deviation could be explained in terms of the difference of segmental dynamics in the polymer surface from that in the bulk.28–35 In the polymer bulk, if the chemical structure of the polymer chains is not considered, the segmental dynamics is only impacted by the interactions among the chain segments. Whereas the segmental dynamics in the surface is not only influenced by the interactions among the chain segments but also by the surrounding environment of the polymer.4,6,7,9,13,36–44 However, it is hard to explore the glass transition of the polymer surface due to the disturbance from the polymer bulk. Note that since the discovery of the deviation from bulk values of Tg of nano-sized polymers, there has been intense interest in demonstrating and understanding the glass transition of the polymer surface from another angle.3,12,28,29 For ultrathin polymer films on substrates, glass transition has been reported to depend upon not only the thickness of the film but also the identities of the substrate and the polymer. Keddie et al.4 reported that Tg of poly(methyl methacrylate) a
Key Lab for Polymer Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China. E-mail: [email protected]; Fax: +86-20-84114033; Tel: +86-20-84113250 b DSAPM Lab, Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, 510275, China. E-mail: [email protected]; Fax: +86-20-84114033; Tel: +86-20-84113250
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(PMMA) standing on native oxide of silicon increased with decreasing film thickness, while a decrease in Tg with decreasing film thickness on gold. For films of PMMA on the native oxide of silicon, hydrogen bonding at the interface between PMMA films and the substrate was speculated to restrict segmental dynamics leading to an increase in Tg. Recently, some studies on the glass transition of polymer nanospheres have been taken into account because they can be dispersed in water, a soft environment.20,24,25 Ediger et al.20 found that decreasing the size of polystyrene (PS) nanospheres suspended in aqueous dispersion leads to a reduction of heat capacity (DCp) instead of an evident deviation of Tg from the bulk. In addition Tg of the PS nanospheres prepared from surfactant-free emulsion polymerization was found to reduce with decreasing nanosphere diameter. Moreover,24 when the PS nanospheres were capped with silica, the hard interface leads to a size invariant Tg. We25 have previously explored the glass transition of three PS nanospheres including surfactant-free, nonionic latex and anionic latex nanospheres in aqueous dispersions. An apparent size-dependent glass transition on the surfactant-free PS nanospheres and nonionic PS latex nanospheres was observed, and Tg decreases with the reduction of size for PS nanospheres. However, the extent of Tg deviation of nonionic PS latex nanospheres was less than that of the surfactant-free PS nanospheres. As for the anionic PS latex nanospheres, no size-dependence but a slight deviation (B5 K) of Tg from the PS bulk was observed. Herein, we focused on glass transition of surfactant-free nanospheres and latex nanospheres of PMMA with diameter ranging from 20 to 220 nm in aqueous dispersion. These spheres were prepared by atom transfer radical polymerization (ATRP) in microemulsions and
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subsequent dialysis against deionized water. The unique glass transition of PMMA nanospheres in aqueous dispersion was probed by nano differential scanning calorimetry (nano-DSC). Moreover, the influence of the environment surrounding the PMMA nanospheres on glass transitions as well as comparisons to our prior studies with PS nanospheres in aqueous dispersions are discussed.
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Experimental section Materials Methyl methacrylate (MMA) was purified twice by passing through a basic alumina column to remove the inhibitor, and stored at 268 K. 2,2 0 -azobisisobutyronitrile (AIBN) was purified by recrystallization from ethanol at 313 K and dried under vacuum. CuBr2 (99.999%, Aldrich), sodium dodecyl benzene sulfonate (SDBS, Aladdin Reagent), polyoxyethylene (20) oleyl ether (Brij 98, Aladdin Reagent), n-hexanol (Aladdin Reagent) and 2,2 0 -bipyridine (bpy, J&K Scientific Ltd) were used as received. Deionized water was used for all experiments. All solvents and reagents if not specified were purchased from Sinopharm Chemical Reagent Co. Ltd and used as received. Preparation of PMMA nanospheres in aqueous dispersion Uniform aqueous dispersed PMMA nanospheres surrounded by different surfactants were prepared by ATRP in microemulsions. Two aqueous dispersions of anionic and nonionic PMMA latex nanospheres were prepared via ATRP in microemulsions using, respectively, sodium dodecyl benzene sulfonate (SDBS) and polyoxyethylene (20) oleyl ether (Brij 98) as surfactants. The third aqueous dispersion of surfactant-free PMMA nanospheres was obtained by the dialysis of aqueous dispersions of anionic PMMA latex nanospheres against deionized water. Briefly, in a typical preparation of PMMA latex nanospheres, deionized water and SDBS or Brij 98 were mixed by high cuts of the isotropic mulser (Gongyiyuhua FA-20) to achieve a surfactant solution and n-hexanol was used as a co-surfactant for SDBS. FPT (freeze pump thaw) cycles were carried out on the resulting surfactant solution 3 times under a nitrogen atmosphere before the polymerization of MMA. All manipulations of ATRP in microemulsion were carried out under a pure nitrogen atmosphere using the standard Schlenk techniques. MMA, CuBr2 and bpy were added into a Schlenk flask under magnetic stirring. The reaction mixture was stirred at 333 K for 1 h to form a Cu(II) complex. The AIBN initiator was dissolved in the SDBS or Brij 98 aqueous solution and stirred for 30 min under a pure nitrogen atmosphere. The resultant complex solution was slowly injected into the surfactant solution at room temperature and then the polymerization was initiated at 343 K. The microemulsion polymerization was carried out under magnetic stirring for designed reaction time. Desired molecular weight of PMMA and size of nanospheres can be archived by adjusting the ratio of the surfactant to the monomer, the monomer to the initiator and polymerization time. Dowex MSC-1 ion exchange resin was stirred together with the aqueous suspensions after polymerization. The monomer (MMA) was completely removed by vacuum
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evaporation at 318 K. The polymer was precipitated in excess methanol and reprecipitated twice using toluene–methanol and dried in a high vacuum oven. In order to acquire an aqueous dispersion of surfactant-free PMMA nanospheres, dialysis against deionized water was carried out on aqueous dispersions of anionic PMMA latex nanospheres repeatedly to remove the surfactant of SDBS. The amount of SDBS remaining on the PMMA nanosphere was determined by elemental analysis (EA) on a Elementar Vario EL elemental analyzer after dialysis.20,25 When the amount of SDBS remaining on the PMMA nanospheres could not be determined by EA, the PMMA nanospheres were not emulsified by SDBS, namely, surfactant-free PMMA nanospheres. Characterization Molecular weight and molecular weight distribution (Mw/Mn) of PMMA were determined by gel permeation chromatography (GPC) against narrow molecular weight distribution polystyrene standards on a Waters 2414 refractive index detector at room temperature with THF as a solvent. Dynamic light scattering (DLS) measurements were conducted on a Brookhaven BI-200SM apparatus with a BI-9000AT digital correlator and a He–Ne laser at 532 nm. The samples were placed in an index-matching decaline bath with temperature controlled within 0.2 K. Data were analyzed by the CONTIN algorithm, while the average hydrodynamic diameter (hDhi) and size polydispersity of the particles were obtained by a cumulant analysis of the experimental correlation function. In DLS, the Laplace inversion of each measured intensity–intensity time correlated function in a dilute solution can result in a characteristic line width distribution G(G). For a purely diffusive relaxation, G(G) can be converted to a hydrodynamic radius distribution f (Dh) by using the Stokes–Einstein equation. The morphological observation of PMMA nanospheres was performed on a scanning electron microscopy (SEM) (Hitachi S-4800) with an accelerating voltage of 10.0 kV at a 8.0 mm working distance. A drop from the previously prepared aqueous dispersion of the PMMA nanosphere was deposited onto a cover glass. The cover glass was dried at room temperature and atmospheric pressure for several hours before examination in SEM. Glass transition of PMMA nanoshperes in aqueous dispersion was determined using nano-DSC (TA Instruments). The calibration was typically performed with 1 mol kg1 NaCl(aq) using calibration mode.45 Equal volumes (0.5 mL) of the PMMA nanosphere in aqueous dispersion and reference solution of the surfactant were injected into the sample and reference cells, respectively. The concentration of PMMA nanospheres in aqueous dispersion was 8 mg mL1 which was determined gravimetrically on the solid after demulsification. Nano-DSC heating and cooling scans were performed at 2 K min1 over a temperature range of 298 to 403 K at 6 atm. During each scan, the heat capacity difference between the sample cell and the reference cell was plotted as a function of temperature. Glass transition of the PMMA bulk was measured on a Perkin-Elmer DSC 8500 instrument. The calibration was performed with indium and tin, and all tests were run employing ultra pure nitrogen as purge gas. Sealed pans were utilized for all tests in conventional DSC
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measurements. The heating and cooling scans were performed at 5 K min1 over a temperature range of 298 to 423 K.
Results and discussion
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Preparation and characterization of PMMA nanospheres Table 1 lists the parameters of PMMA molecular chains and PMMA nanospheres. PMMA with narrow molecular weight distribution (Mw/Mn o 1.55) and PMMA nanospheres with narrow size distribution were successfully prepared using ATRP in microemulsion. In this report, we focus on the preparation of anionic (sample 1–9) and nonionic (sample 10–14) PMMA latex nanospheres using, respectively, SDBS and Brij 98 as surfactants. The third aqueous suspensions of surfactant-free PMMA nanospheres were obtained through dialysis on PMMA microemulsions using SDBS as a surfactant against a large volume of deionized water to entirely remove SDBS.
Fig. 1 SEM images of 50.3 nm diameter surfactant-free PMMA nanospheres before (a) and after (b) DSC heating scan.
Glass transition of PMMA nanospheres in aqueous dispersions Fig. 1a provides the representative SEM images of surfactantfree PMMA nanospheres with an average diameter (hDi) of B50 nm obtained from their aqueous suspensions before the measurement of glass transition. Clearly from the image, smooth and fairly monodisperse PMMA nanospheres have been prepared, revealing that the microemulsion polymerization is well controlled. The surfactant-free PMMA nanospheres in aqueous dispersion appeared as a translucent and stable liquid. Fig. 1b shows the size and morphology of surfactant-free PMMA nanospheres after the measurement of glass transition by nano-DSC in the temperature range from 298 to 403 K. The average size and morphology of the surfactant-free PMMA nanospheres during heating scan performed by nano-DSC are not obviously altered. As exhibited in Fig. 2, the Dh distribution of the representative anionic (hDhi B 18 nm) and nonionic Table 1 The parameters of PMMA molecular chains and PMMA nanospheres
PMMA nanospheres
PMMA molecular chains
Samplea
hDhib (nm)
hDic (nm)
Mwd (kg mol1)
Mw/Mnd
1 2 3 4 5 6 7 8 9 10 11 12 13 14
18.3 25.5 50.3 77.6 138.0 221.1 21.7 22.2 26.8 27.3 46.1 85.5 117.3 154.8
22.4 28.3 44.6 74.5 134.4 235.2 23.4 25.7 26.2 n.d. n.d. n.d. n.d. n.d.
44 41 40 48 44 46 11 28 130 46 47 44 43 45
1.45 1.42 1.50 1.43 1.50 1.49 1.51 1.50 1.55 1.31 1.35 1.33 1.43 1.46
a Polymerization conditions: temperature 343 K; 2,20 -azobisisobutyronitrile (AIBN)–CuBr2–2,2 0 -bipyridine (bpy) used as an initiator system for samples 1–14; SDBS used as a surfactant for samples 1–9 and Brij 98 for samples 10–14. b Determined by DLS. c Determined by SEM. d Determined by GPC.
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Fig. 2 Dh distributions of the representative anionic (sample 1, Dh B 18 nm) and nonionic (sample 11, Dh B 46 nm) PMMA latex nanospheres in microemulsion before and after heating scan.
(hDhi B 46 nm) PMMA latex nanospheres in microemulsions detected by DLS is nearly invariable before and after the measurement of the glass transition. These results indicate that the three PMMA nanospheres in different aqueous dispersions do not aggregate during the measurement of the glass transition. In addition, we note that the MMA monomer in the current work was entirely inexistent in all samples, which was confirmed by gas chromatography (GC). Fig. 3 plots the size dependence of Tg Tg,bulk data for surfactant-free PMMA nanospheres, nonionic and anionic PMMA latex nanospheres in aqueous dispersion obtained by nano-DSC. In this case, nano-DSC heating and cooling scans were performed at 2 K min1 over a temperature range from 298 to 403 K at 6 atm. Note that all reported Tgs were recorded as the midpoint value from the extrapolation of the two linear regression lines which were fit to the glass and liquid line.25,46 Note that PMMA can absorb a small amount (B2 wt%) of water.47 Therefore, it is necessary to verify whether the water absorbed in PMMA may influence the Tg deviation. The results of DSC analysis for PMMA soaked in the water show that Tg was almost identical (B0.8 K) to that of dried PMMA in a nitrogen atmosphere. This result also implies that the conductivity of the medium will hardly affect Tg of PMMA. As present in Fig. 3, with the decrease of nanosphere diameter in the sub-100 nm range, there is a systematic decrease in Tg for
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Fig. 3 Tg Tg,bulk of surfactant-free PMMA nanospheres, nonionic and anionic PMMA latex nanospheres in aqueous dispersions measured by nano-DSC as a function of diameter.
the surfactant-free PMMA nanospheres in aqueous dispersions. Furthermore, Tg of the nonionic PMMA latex nanospheres also shows a similar trend in size-dependence as well as the surfactantfree PMMA nanospheres. However, the extent of Tg deviation of the nonionic PMMA latex nanospheres from the PMMA bulk was less than that of the surfactant-free PMMA nanospheres. As for the anionic PMMA latex nanospheres, no size-dependence but a slight deviation (B5 K) of Tg from the PMMA bulk is displayed in Fig. 3. Here, we suggest the mechanism for the Tg deviations from bulk for PMMA nanospheres under different confined environments in aqueous dispersions, which are schematically depicted in Fig. 4. For surfactant-free PMMA nanospheres (Fig. 4a), the nanosphere can be divided into two parts including the bulk and the surface.8,20,24,25 As illustrated in Fig. 4a, the bulk core is surrounded by the surface. In the surface, Tg gradiently increases from outside to inside and approaches to a maximum value near to that of the bulk because of lower chain entanglement density, higher free volume and enhanced segmental dynamics for the outer surface than the bulk,13,28–34,48–50 which shall give rise to the decrease of Tg.48 Furthermore, the presence of such an outer surface has been proposed for 1D ultrathin films.3,8 For nonionic PMMA latex nanospheres (Fig. 4b), the lesser extent of Tg deviation from the PMMA bulk compared with surfactant-free
Fig. 4 Mechanism for the deviations of Tg for surfactant-free PMMA nanosphere (a), nonionic PMMA latex nanosphere (b) and anionic PMMA latex nanosphere (c) with a diameter less than 100 nm under different confined environments in aqueous dispersions.
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PMMA nanospheres implies that the segmental dynamics in the outer surface may be suppressed due to the incorporation of Brij into PMMA. As reported by Robertson et al.,26 Tg of PS spherical nanodomains within a poly(styrene-co-butadiene) matrix decreased with decreasing size of the spheres, for instance, Tg Tg,bulk = 38 K for 23 nm PS spherical nanodomains. Note that the extent of Tg-deviation from bulk for PS spherical nanodomains was much less than that of surfactantfree nanospheres from literature data24 and our study,25 which is in line with lesser extent Tg-deviation for the nonionic latex nanospheres. Moreover, Robertson et al. also attributed the decrease in Tg for PS spherical nanodomains to intermixing between the phases.26 Therefore, it is believed that the reduction of the free volume and segmental dynamics results from the incorporation of Brij giving rise to the increase of Tg relative to surfactant-free PMMA nanospheres. For anionic PMMA latex nanospheres (Fig. 4c), not only the free volume in the surface is decreased but also the segmental dynamics of PMMA is further inhibited by the dodecyls which cannot move freely because they are anchored by the ionic bond. The suppression on the segmental dynamics of PMMA nanospheres as a result of the anionic surfactant is in accordance with that of PS nanospheres.25 SDBS on the surface of nanospheres can confine the glass transition of PMMA and PS. The experimental results of nano-DSC characterization are demonstrated in Fig. 5. Nano-DSC has been proved efficient in
Fig. 5 Nano-DSC curves of surfactant-free PMMA nanospheres with a hDi of B18 nm (a), 50 nm (b) and 138 nm (c), nonionic PMMA latex nanospheres with a hDi of B46 nm (d), anionic PMMA latex nanospheres with a hDi of B50 nm (e) in aqueous dispersion and the DSC curve of bulk PMMA (f).
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Fig. 6 Tg Tg,bulk of PMMA nanospheres and comparable PS nanospheres25 in aqueous dispersions measured by nano-DSC as a function of diameter.
Fig. 7 Change in Tg with respect to diameter for surfactant-free PMMA and PS nanospheres in aqueous dispersions. Inset schematically compares surfactant-free PMMA and PS nanospheres as a core–shell model.
measuring the thermal transition of polymers in aqueous dispersions for its high sensitivity and excellent resolution.25,51,52 As the hDi increases from 18 to 50 nm, Tg of the surfactant-free PMMA nanospheres is observed to substantially increase. Meanwhile, the distribution of glass transition is found to broaden. This is because when the radius of PMMA nanospheres is more than the thickness of the outer surface consisting of different mobile layers, a very broad distribution of glass transition attributed by the core and various mobile layers should be displayed in the DSC traces. Note that the DSC traces can only show the glass transition of major mass fraction in PMMA nanospheres due to the accuracy of the instrument. For example, only the glass transition of the outer surface or the bulk core could be observed when the radius of PMMA nanospheres was small or big enough, such as hDi B 18 nm or B138 nm. In addition, reduction of the heat capacity jump (DCp) for the surfactant-free PMMA nanospheres and nonionic PMMA latex nanospheres implies the enhanced segmental dynamics in the outer surface as we clarified above.20 Fig. 6 shows the size dependence of Tg Tg,bulk data collected for surfactant-free PMMA nanospheres, nonionic and anionic PMMA latex nanospheres and comparable PS nanospheres25 in aqueous dispersion. Apparently, the trend in size-dependent Tg of surfactant-free PMMA nanospheres and nonionic PMMA latex nanospheres is quite similar to the comparable PS nanospheres.25 Tg decreases with the decrease of the diameter. As for the anionic PMMA and PS latex nanospheres, no sizedependent Tg from the bulk has been observed. In spite of the qualitative similarities between Tgs of the PMMA and comparable PS nanospheres, there are some considerable quantitative differences needed to address. According to Fig. 7, a quantitative comparison of Tg deviation with respect to diameter between the surfactant-free PMMA and PS nanospheres in aqueous dispersions was drawn. The Tg–Dh data of the surfactant-free PS nanospheres were obtained formerly utilizing the same nano-DSC and the same experimental procedure. Tg of the surfactant-free PS nanospheres shows greater deviation from the bulk than that of the surfactant-free PMMA.
In other words, the slope (dTg/dDh) for surfactant-free PS nanospheres is much larger than that for surfactant-free PMMA nanospheres. For instance, the surfactant-free PS nanosphere with a diameter of B54 nm, Tg Tg,bulk = 50 K while for surfactant-free PMMA nanosphere with a diameter of B50 nm, Tg Tg,bulk = 20 K. Moreover, the threshold nanosphere diameter below which Tg deviation from the bulk observed for the surfactant-free PMMA nanosphere is obviously less than that for the surfactant-free PS nanosphere. As shown in Fig. 7, Tg of the surfactant-free PMMA nanospheres with diameter below 150 nm is less than that of the PMMA bulk. However, for the surfactant-free PS nanospheres, the threshold nanosphere diameter below which Tg deviates from bulk is 200 nm. The Fig. 7 inset schematically illustrates the mechanism of Tg-deviation of surfactant-free PMMA and PS nanospheres as a core–shell model. In fact, the hydrogen bond between the hydroxy groups in water and the carbonyl groups in PMMA is feasible.4,53,54 The hydrogen bond between PMMA segments and water may somehow hinder the segmental dynamics in the surface as shown in the Fig. 7 inset,4,53–55 which we assume is the reason for the smaller Tg deviation from the bulk for surfactant-free PMMA nanospheres than surfactant-free PS nanospheres. The similar result has been found in other studies on Tg deviation from the bulk of PMMA ultrathin films.4,53–55 A smaller Tg deviation from the bulk as a result of the hydrogen bond between hydroxyl groups in the substrate and the ester groups in the polymers was reported for PMMA ultrathin films.4,53–55 In other words, the confinement to PMMA chains caused by the surrounding water environment is much stronger than to PS. To confirm further the impact of the hydrogen bond on Tg of surfactant-free PMMA nanospheres in aqueous dispersions, the investigation on Tg of surfactantfree PMMA nanospheres with the same diameter suspended in n-heptane without the hydrogen bond was performed. The freeze dried surfactant-free PMMA nanospheres were dispersed in n-heptane under ultrasonic irradiation. Tg of the 50 nm PMMA nanospheres dispersed in n-heptane was measured by nano-DSC and the Tg Tg,bulk = B60 K. The Tg deviation from
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latex nanospheres show a size-independent and bulk-like Tg, which was consistent with the results for anionic PS latex nanospheres. All the results above certify that the environmental impact is the key factor governing the deviation of Tg of polymer nanospheres in aqueous dispersions.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (21174167), the NSF of Guangdong Province (S2013030013474) and the Guangdong Province Higher School Science and Technology Innovation Key Project.
Fig. 8 Tgs of PMMA and PS nanospheres with a diameter of B25 nm in aqueous dispersions as a function of molecular weight.
bulk was quite greater than that of the same surfactant-free PMMA nanospheres in aqueous dispersions. The results strongly implied that the hydrogen bond between PMMA segments and water is the main reason for smaller Tg deviation from the bulk of surfactant-free PMMA nanospheres. In addition, we investigated Tgs of PMMA and PS nanospheres with a diameter of B25 nm in aqueous dispersion as a function of molecular weight. As shown in Fig. 8, there is no apparent influence of molecular weight on Tg both for PMMA and PS nanospheres. Consequently, different chemical structure of polymer chains can result in various degrees of interaction with the surrounding environment, which may further affect Tg of nano-sized polymers. Therefore, there is reason to believe that environmental impacts surrounding PMMA and PS nanospheres in aqueous dispersions are the key factors governing the Tg deviation.
Conclusions In this report, we used nano-DSC to explore the glass transition of surfactant-free, nonionic and anionic PMMA latex nanospheres in aqueous dispersions. The three aqueous dispersed PMMA nanospheres were prepared by ATRP in microemulsions and subsequent dialysis against deionized water. Substantial deviations in Tgs of surfactant-free PMMA nanospheres and nonionic PMMA latex nanospheres relative to bulk were observed, and Tg decreases with the reduction of nanosphere diameter. In fact, Tg behavior of the surfactant-free and nonionic PMMA is qualitatively similar to that of the comparable PS nanospheres. However, compared to the surfactant-free PS nanospheres, Tg of the surfactant-free PMMA nanospheres shows lesser deviation from the bulk. The environmental impact on the glass transition of surfactant-free PMMA nanospheres is greater by contrast with surfactant-free PS nanospheres due to the hydrogen bond between PMMA and water. The nano-DSC results of surfactant-free polymer nanospheres in aqueous dispersions verify the gradiently layered model. Moreover, the anionic PMMA
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Cite this: DOI: 10.1039/c4cp01849d
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Glass transition of poly(methyl methacrylate) nanospheres in aqueous dispersion Shuo Feng,a Yuenan Chen,a Biyun Mai,a Wanchu Wei,a Caixia Zheng,a Qing Wu,ab GuoDong Liang,ab HaiYang Gaoab and FangMing Zhu*ab Surfactant-free nanospheres and latex nanospheres of poly(methyl methacrylate) (PMMA) with diameter ranging from 20 to 220 nm are prepared by atom transfer radical polymerization (ATRP) in microemulsions and subsequent dialysis against deionized water. The glass transitions of these PMMA nanospheres are characterized using nano differential scanning calorimetry (nano-DSC) in aqueous dispersions. The glass transition temperature (Tg) of the surfactant-free PMMA nanospheres and
Received 29th April 2014, Accepted 12th June 2014
nonionic PMMA latex nanospheres with diameters below 150 nm is less than that of the PMMA bulk, and
DOI: 10.1039/c4cp01849d
is size-independent and is near to that of the PMMA bulk. The influence of the environment surrounding
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the PMMA nanospheres on glass transitions as well as comparisons to our prior studies with polystyrene (PS) nanospheres in aqueous dispersions are discussed.
Tg decreases with the decrease of the diameter. In contrast, Tg of the anionic PMMA latex nanospheres
Introduction The glass transition of nano-sized polymers has attracted growing attention as a result of the deviation from bulk values of the glass transition temperature (Tg) that possess a wide variety of potential applications in nanoscience and nanotechnology.1–27 Moreover, the deviation could be explained in terms of the difference of segmental dynamics in the polymer surface from that in the bulk.28–35 In the polymer bulk, if the chemical structure of the polymer chains is not considered, the segmental dynamics is only impacted by the interactions among the chain segments. Whereas the segmental dynamics in the surface is not only influenced by the interactions among the chain segments but also by the surrounding environment of the polymer.4,6,7,9,13,36–44 However, it is hard to explore the glass transition of the polymer surface due to the disturbance from the polymer bulk. Note that since the discovery of the deviation from bulk values of Tg of nano-sized polymers, there has been intense interest in demonstrating and understanding the glass transition of the polymer surface from another angle.3,12,28,29 For ultrathin polymer films on substrates, glass transition has been reported to depend upon not only the thickness of the film but also the identities of the substrate and the polymer. Keddie et al.4 reported that Tg of poly(methyl methacrylate) a
Key Lab for Polymer Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China. E-mail: [email protected]; Fax: +86-20-84114033; Tel: +86-20-84113250 b DSAPM Lab, Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, 510275, China. E-mail: [email protected]; Fax: +86-20-84114033; Tel: +86-20-84113250
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(PMMA) standing on native oxide of silicon increased with decreasing film thickness, while a decrease in Tg with decreasing film thickness on gold. For films of PMMA on the native oxide of silicon, hydrogen bonding at the interface between PMMA films and the substrate was speculated to restrict segmental dynamics leading to an increase in Tg. Recently, some studies on the glass transition of polymer nanospheres have been taken into account because they can be dispersed in water, a soft environment.20,24,25 Ediger et al.20 found that decreasing the size of polystyrene (PS) nanospheres suspended in aqueous dispersion leads to a reduction of heat capacity (DCp) instead of an evident deviation of Tg from the bulk. In addition Tg of the PS nanospheres prepared from surfactant-free emulsion polymerization was found to reduce with decreasing nanosphere diameter. Moreover,24 when the PS nanospheres were capped with silica, the hard interface leads to a size invariant Tg. We25 have previously explored the glass transition of three PS nanospheres including surfactant-free, nonionic latex and anionic latex nanospheres in aqueous dispersions. An apparent size-dependent glass transition on the surfactant-free PS nanospheres and nonionic PS latex nanospheres was observed, and Tg decreases with the reduction of size for PS nanospheres. However, the extent of Tg deviation of nonionic PS latex nanospheres was less than that of the surfactant-free PS nanospheres. As for the anionic PS latex nanospheres, no size-dependence but a slight deviation (B5 K) of Tg from the PS bulk was observed. Herein, we focused on glass transition of surfactant-free nanospheres and latex nanospheres of PMMA with diameter ranging from 20 to 220 nm in aqueous dispersion. These spheres were prepared by atom transfer radical polymerization (ATRP) in microemulsions and
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subsequent dialysis against deionized water. The unique glass transition of PMMA nanospheres in aqueous dispersion was probed by nano differential scanning calorimetry (nano-DSC). Moreover, the influence of the environment surrounding the PMMA nanospheres on glass transitions as well as comparisons to our prior studies with PS nanospheres in aqueous dispersions are discussed.
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Experimental section Materials Methyl methacrylate (MMA) was purified twice by passing through a basic alumina column to remove the inhibitor, and stored at 268 K. 2,2 0 -azobisisobutyronitrile (AIBN) was purified by recrystallization from ethanol at 313 K and dried under vacuum. CuBr2 (99.999%, Aldrich), sodium dodecyl benzene sulfonate (SDBS, Aladdin Reagent), polyoxyethylene (20) oleyl ether (Brij 98, Aladdin Reagent), n-hexanol (Aladdin Reagent) and 2,2 0 -bipyridine (bpy, J&K Scientific Ltd) were used as received. Deionized water was used for all experiments. All solvents and reagents if not specified were purchased from Sinopharm Chemical Reagent Co. Ltd and used as received. Preparation of PMMA nanospheres in aqueous dispersion Uniform aqueous dispersed PMMA nanospheres surrounded by different surfactants were prepared by ATRP in microemulsions. Two aqueous dispersions of anionic and nonionic PMMA latex nanospheres were prepared via ATRP in microemulsions using, respectively, sodium dodecyl benzene sulfonate (SDBS) and polyoxyethylene (20) oleyl ether (Brij 98) as surfactants. The third aqueous dispersion of surfactant-free PMMA nanospheres was obtained by the dialysis of aqueous dispersions of anionic PMMA latex nanospheres against deionized water. Briefly, in a typical preparation of PMMA latex nanospheres, deionized water and SDBS or Brij 98 were mixed by high cuts of the isotropic mulser (Gongyiyuhua FA-20) to achieve a surfactant solution and n-hexanol was used as a co-surfactant for SDBS. FPT (freeze pump thaw) cycles were carried out on the resulting surfactant solution 3 times under a nitrogen atmosphere before the polymerization of MMA. All manipulations of ATRP in microemulsion were carried out under a pure nitrogen atmosphere using the standard Schlenk techniques. MMA, CuBr2 and bpy were added into a Schlenk flask under magnetic stirring. The reaction mixture was stirred at 333 K for 1 h to form a Cu(II) complex. The AIBN initiator was dissolved in the SDBS or Brij 98 aqueous solution and stirred for 30 min under a pure nitrogen atmosphere. The resultant complex solution was slowly injected into the surfactant solution at room temperature and then the polymerization was initiated at 343 K. The microemulsion polymerization was carried out under magnetic stirring for designed reaction time. Desired molecular weight of PMMA and size of nanospheres can be archived by adjusting the ratio of the surfactant to the monomer, the monomer to the initiator and polymerization time. Dowex MSC-1 ion exchange resin was stirred together with the aqueous suspensions after polymerization. The monomer (MMA) was completely removed by vacuum
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evaporation at 318 K. The polymer was precipitated in excess methanol and reprecipitated twice using toluene–methanol and dried in a high vacuum oven. In order to acquire an aqueous dispersion of surfactant-free PMMA nanospheres, dialysis against deionized water was carried out on aqueous dispersions of anionic PMMA latex nanospheres repeatedly to remove the surfactant of SDBS. The amount of SDBS remaining on the PMMA nanosphere was determined by elemental analysis (EA) on a Elementar Vario EL elemental analyzer after dialysis.20,25 When the amount of SDBS remaining on the PMMA nanospheres could not be determined by EA, the PMMA nanospheres were not emulsified by SDBS, namely, surfactant-free PMMA nanospheres. Characterization Molecular weight and molecular weight distribution (Mw/Mn) of PMMA were determined by gel permeation chromatography (GPC) against narrow molecular weight distribution polystyrene standards on a Waters 2414 refractive index detector at room temperature with THF as a solvent. Dynamic light scattering (DLS) measurements were conducted on a Brookhaven BI-200SM apparatus with a BI-9000AT digital correlator and a He–Ne laser at 532 nm. The samples were placed in an index-matching decaline bath with temperature controlled within 0.2 K. Data were analyzed by the CONTIN algorithm, while the average hydrodynamic diameter (hDhi) and size polydispersity of the particles were obtained by a cumulant analysis of the experimental correlation function. In DLS, the Laplace inversion of each measured intensity–intensity time correlated function in a dilute solution can result in a characteristic line width distribution G(G). For a purely diffusive relaxation, G(G) can be converted to a hydrodynamic radius distribution f (Dh) by using the Stokes–Einstein equation. The morphological observation of PMMA nanospheres was performed on a scanning electron microscopy (SEM) (Hitachi S-4800) with an accelerating voltage of 10.0 kV at a 8.0 mm working distance. A drop from the previously prepared aqueous dispersion of the PMMA nanosphere was deposited onto a cover glass. The cover glass was dried at room temperature and atmospheric pressure for several hours before examination in SEM. Glass transition of PMMA nanoshperes in aqueous dispersion was determined using nano-DSC (TA Instruments). The calibration was typically performed with 1 mol kg1 NaCl(aq) using calibration mode.45 Equal volumes (0.5 mL) of the PMMA nanosphere in aqueous dispersion and reference solution of the surfactant were injected into the sample and reference cells, respectively. The concentration of PMMA nanospheres in aqueous dispersion was 8 mg mL1 which was determined gravimetrically on the solid after demulsification. Nano-DSC heating and cooling scans were performed at 2 K min1 over a temperature range of 298 to 403 K at 6 atm. During each scan, the heat capacity difference between the sample cell and the reference cell was plotted as a function of temperature. Glass transition of the PMMA bulk was measured on a Perkin-Elmer DSC 8500 instrument. The calibration was performed with indium and tin, and all tests were run employing ultra pure nitrogen as purge gas. Sealed pans were utilized for all tests in conventional DSC
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measurements. The heating and cooling scans were performed at 5 K min1 over a temperature range of 298 to 423 K.
Results and discussion
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Preparation and characterization of PMMA nanospheres Table 1 lists the parameters of PMMA molecular chains and PMMA nanospheres. PMMA with narrow molecular weight distribution (Mw/Mn o 1.55) and PMMA nanospheres with narrow size distribution were successfully prepared using ATRP in microemulsion. In this report, we focus on the preparation of anionic (sample 1–9) and nonionic (sample 10–14) PMMA latex nanospheres using, respectively, SDBS and Brij 98 as surfactants. The third aqueous suspensions of surfactant-free PMMA nanospheres were obtained through dialysis on PMMA microemulsions using SDBS as a surfactant against a large volume of deionized water to entirely remove SDBS.
Fig. 1 SEM images of 50.3 nm diameter surfactant-free PMMA nanospheres before (a) and after (b) DSC heating scan.
Glass transition of PMMA nanospheres in aqueous dispersions Fig. 1a provides the representative SEM images of surfactantfree PMMA nanospheres with an average diameter (hDi) of B50 nm obtained from their aqueous suspensions before the measurement of glass transition. Clearly from the image, smooth and fairly monodisperse PMMA nanospheres have been prepared, revealing that the microemulsion polymerization is well controlled. The surfactant-free PMMA nanospheres in aqueous dispersion appeared as a translucent and stable liquid. Fig. 1b shows the size and morphology of surfactant-free PMMA nanospheres after the measurement of glass transition by nano-DSC in the temperature range from 298 to 403 K. The average size and morphology of the surfactant-free PMMA nanospheres during heating scan performed by nano-DSC are not obviously altered. As exhibited in Fig. 2, the Dh distribution of the representative anionic (hDhi B 18 nm) and nonionic Table 1 The parameters of PMMA molecular chains and PMMA nanospheres
PMMA nanospheres
PMMA molecular chains
Samplea
hDhib (nm)
hDic (nm)
Mwd (kg mol1)
Mw/Mnd
1 2 3 4 5 6 7 8 9 10 11 12 13 14
18.3 25.5 50.3 77.6 138.0 221.1 21.7 22.2 26.8 27.3 46.1 85.5 117.3 154.8
22.4 28.3 44.6 74.5 134.4 235.2 23.4 25.7 26.2 n.d. n.d. n.d. n.d. n.d.
44 41 40 48 44 46 11 28 130 46 47 44 43 45
1.45 1.42 1.50 1.43 1.50 1.49 1.51 1.50 1.55 1.31 1.35 1.33 1.43 1.46
a Polymerization conditions: temperature 343 K; 2,20 -azobisisobutyronitrile (AIBN)–CuBr2–2,2 0 -bipyridine (bpy) used as an initiator system for samples 1–14; SDBS used as a surfactant for samples 1–9 and Brij 98 for samples 10–14. b Determined by DLS. c Determined by SEM. d Determined by GPC.
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Fig. 2 Dh distributions of the representative anionic (sample 1, Dh B 18 nm) and nonionic (sample 11, Dh B 46 nm) PMMA latex nanospheres in microemulsion before and after heating scan.
(hDhi B 46 nm) PMMA latex nanospheres in microemulsions detected by DLS is nearly invariable before and after the measurement of the glass transition. These results indicate that the three PMMA nanospheres in different aqueous dispersions do not aggregate during the measurement of the glass transition. In addition, we note that the MMA monomer in the current work was entirely inexistent in all samples, which was confirmed by gas chromatography (GC). Fig. 3 plots the size dependence of Tg Tg,bulk data for surfactant-free PMMA nanospheres, nonionic and anionic PMMA latex nanospheres in aqueous dispersion obtained by nano-DSC. In this case, nano-DSC heating and cooling scans were performed at 2 K min1 over a temperature range from 298 to 403 K at 6 atm. Note that all reported Tgs were recorded as the midpoint value from the extrapolation of the two linear regression lines which were fit to the glass and liquid line.25,46 Note that PMMA can absorb a small amount (B2 wt%) of water.47 Therefore, it is necessary to verify whether the water absorbed in PMMA may influence the Tg deviation. The results of DSC analysis for PMMA soaked in the water show that Tg was almost identical (B0.8 K) to that of dried PMMA in a nitrogen atmosphere. This result also implies that the conductivity of the medium will hardly affect Tg of PMMA. As present in Fig. 3, with the decrease of nanosphere diameter in the sub-100 nm range, there is a systematic decrease in Tg for
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Fig. 3 Tg Tg,bulk of surfactant-free PMMA nanospheres, nonionic and anionic PMMA latex nanospheres in aqueous dispersions measured by nano-DSC as a function of diameter.
the surfactant-free PMMA nanospheres in aqueous dispersions. Furthermore, Tg of the nonionic PMMA latex nanospheres also shows a similar trend in size-dependence as well as the surfactantfree PMMA nanospheres. However, the extent of Tg deviation of the nonionic PMMA latex nanospheres from the PMMA bulk was less than that of the surfactant-free PMMA nanospheres. As for the anionic PMMA latex nanospheres, no size-dependence but a slight deviation (B5 K) of Tg from the PMMA bulk is displayed in Fig. 3. Here, we suggest the mechanism for the Tg deviations from bulk for PMMA nanospheres under different confined environments in aqueous dispersions, which are schematically depicted in Fig. 4. For surfactant-free PMMA nanospheres (Fig. 4a), the nanosphere can be divided into two parts including the bulk and the surface.8,20,24,25 As illustrated in Fig. 4a, the bulk core is surrounded by the surface. In the surface, Tg gradiently increases from outside to inside and approaches to a maximum value near to that of the bulk because of lower chain entanglement density, higher free volume and enhanced segmental dynamics for the outer surface than the bulk,13,28–34,48–50 which shall give rise to the decrease of Tg.48 Furthermore, the presence of such an outer surface has been proposed for 1D ultrathin films.3,8 For nonionic PMMA latex nanospheres (Fig. 4b), the lesser extent of Tg deviation from the PMMA bulk compared with surfactant-free
Fig. 4 Mechanism for the deviations of Tg for surfactant-free PMMA nanosphere (a), nonionic PMMA latex nanosphere (b) and anionic PMMA latex nanosphere (c) with a diameter less than 100 nm under different confined environments in aqueous dispersions.
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PMMA nanospheres implies that the segmental dynamics in the outer surface may be suppressed due to the incorporation of Brij into PMMA. As reported by Robertson et al.,26 Tg of PS spherical nanodomains within a poly(styrene-co-butadiene) matrix decreased with decreasing size of the spheres, for instance, Tg Tg,bulk = 38 K for 23 nm PS spherical nanodomains. Note that the extent of Tg-deviation from bulk for PS spherical nanodomains was much less than that of surfactantfree nanospheres from literature data24 and our study,25 which is in line with lesser extent Tg-deviation for the nonionic latex nanospheres. Moreover, Robertson et al. also attributed the decrease in Tg for PS spherical nanodomains to intermixing between the phases.26 Therefore, it is believed that the reduction of the free volume and segmental dynamics results from the incorporation of Brij giving rise to the increase of Tg relative to surfactant-free PMMA nanospheres. For anionic PMMA latex nanospheres (Fig. 4c), not only the free volume in the surface is decreased but also the segmental dynamics of PMMA is further inhibited by the dodecyls which cannot move freely because they are anchored by the ionic bond. The suppression on the segmental dynamics of PMMA nanospheres as a result of the anionic surfactant is in accordance with that of PS nanospheres.25 SDBS on the surface of nanospheres can confine the glass transition of PMMA and PS. The experimental results of nano-DSC characterization are demonstrated in Fig. 5. Nano-DSC has been proved efficient in
Fig. 5 Nano-DSC curves of surfactant-free PMMA nanospheres with a hDi of B18 nm (a), 50 nm (b) and 138 nm (c), nonionic PMMA latex nanospheres with a hDi of B46 nm (d), anionic PMMA latex nanospheres with a hDi of B50 nm (e) in aqueous dispersion and the DSC curve of bulk PMMA (f).
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Fig. 6 Tg Tg,bulk of PMMA nanospheres and comparable PS nanospheres25 in aqueous dispersions measured by nano-DSC as a function of diameter.
Fig. 7 Change in Tg with respect to diameter for surfactant-free PMMA and PS nanospheres in aqueous dispersions. Inset schematically compares surfactant-free PMMA and PS nanospheres as a core–shell model.
measuring the thermal transition of polymers in aqueous dispersions for its high sensitivity and excellent resolution.25,51,52 As the hDi increases from 18 to 50 nm, Tg of the surfactant-free PMMA nanospheres is observed to substantially increase. Meanwhile, the distribution of glass transition is found to broaden. This is because when the radius of PMMA nanospheres is more than the thickness of the outer surface consisting of different mobile layers, a very broad distribution of glass transition attributed by the core and various mobile layers should be displayed in the DSC traces. Note that the DSC traces can only show the glass transition of major mass fraction in PMMA nanospheres due to the accuracy of the instrument. For example, only the glass transition of the outer surface or the bulk core could be observed when the radius of PMMA nanospheres was small or big enough, such as hDi B 18 nm or B138 nm. In addition, reduction of the heat capacity jump (DCp) for the surfactant-free PMMA nanospheres and nonionic PMMA latex nanospheres implies the enhanced segmental dynamics in the outer surface as we clarified above.20 Fig. 6 shows the size dependence of Tg Tg,bulk data collected for surfactant-free PMMA nanospheres, nonionic and anionic PMMA latex nanospheres and comparable PS nanospheres25 in aqueous dispersion. Apparently, the trend in size-dependent Tg of surfactant-free PMMA nanospheres and nonionic PMMA latex nanospheres is quite similar to the comparable PS nanospheres.25 Tg decreases with the decrease of the diameter. As for the anionic PMMA and PS latex nanospheres, no sizedependent Tg from the bulk has been observed. In spite of the qualitative similarities between Tgs of the PMMA and comparable PS nanospheres, there are some considerable quantitative differences needed to address. According to Fig. 7, a quantitative comparison of Tg deviation with respect to diameter between the surfactant-free PMMA and PS nanospheres in aqueous dispersions was drawn. The Tg–Dh data of the surfactant-free PS nanospheres were obtained formerly utilizing the same nano-DSC and the same experimental procedure. Tg of the surfactant-free PS nanospheres shows greater deviation from the bulk than that of the surfactant-free PMMA.
In other words, the slope (dTg/dDh) for surfactant-free PS nanospheres is much larger than that for surfactant-free PMMA nanospheres. For instance, the surfactant-free PS nanosphere with a diameter of B54 nm, Tg Tg,bulk = 50 K while for surfactant-free PMMA nanosphere with a diameter of B50 nm, Tg Tg,bulk = 20 K. Moreover, the threshold nanosphere diameter below which Tg deviation from the bulk observed for the surfactant-free PMMA nanosphere is obviously less than that for the surfactant-free PS nanosphere. As shown in Fig. 7, Tg of the surfactant-free PMMA nanospheres with diameter below 150 nm is less than that of the PMMA bulk. However, for the surfactant-free PS nanospheres, the threshold nanosphere diameter below which Tg deviates from bulk is 200 nm. The Fig. 7 inset schematically illustrates the mechanism of Tg-deviation of surfactant-free PMMA and PS nanospheres as a core–shell model. In fact, the hydrogen bond between the hydroxy groups in water and the carbonyl groups in PMMA is feasible.4,53,54 The hydrogen bond between PMMA segments and water may somehow hinder the segmental dynamics in the surface as shown in the Fig. 7 inset,4,53–55 which we assume is the reason for the smaller Tg deviation from the bulk for surfactant-free PMMA nanospheres than surfactant-free PS nanospheres. The similar result has been found in other studies on Tg deviation from the bulk of PMMA ultrathin films.4,53–55 A smaller Tg deviation from the bulk as a result of the hydrogen bond between hydroxyl groups in the substrate and the ester groups in the polymers was reported for PMMA ultrathin films.4,53–55 In other words, the confinement to PMMA chains caused by the surrounding water environment is much stronger than to PS. To confirm further the impact of the hydrogen bond on Tg of surfactant-free PMMA nanospheres in aqueous dispersions, the investigation on Tg of surfactantfree PMMA nanospheres with the same diameter suspended in n-heptane without the hydrogen bond was performed. The freeze dried surfactant-free PMMA nanospheres were dispersed in n-heptane under ultrasonic irradiation. Tg of the 50 nm PMMA nanospheres dispersed in n-heptane was measured by nano-DSC and the Tg Tg,bulk = B60 K. The Tg deviation from
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latex nanospheres show a size-independent and bulk-like Tg, which was consistent with the results for anionic PS latex nanospheres. All the results above certify that the environmental impact is the key factor governing the deviation of Tg of polymer nanospheres in aqueous dispersions.
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Acknowledgements This work was supported by the National Natural Science Foundation of China (21174167), the NSF of Guangdong Province (S2013030013474) and the Guangdong Province Higher School Science and Technology Innovation Key Project.
Fig. 8 Tgs of PMMA and PS nanospheres with a diameter of B25 nm in aqueous dispersions as a function of molecular weight.
bulk was quite greater than that of the same surfactant-free PMMA nanospheres in aqueous dispersions. The results strongly implied that the hydrogen bond between PMMA segments and water is the main reason for smaller Tg deviation from the bulk of surfactant-free PMMA nanospheres. In addition, we investigated Tgs of PMMA and PS nanospheres with a diameter of B25 nm in aqueous dispersion as a function of molecular weight. As shown in Fig. 8, there is no apparent influence of molecular weight on Tg both for PMMA and PS nanospheres. Consequently, different chemical structure of polymer chains can result in various degrees of interaction with the surrounding environment, which may further affect Tg of nano-sized polymers. Therefore, there is reason to believe that environmental impacts surrounding PMMA and PS nanospheres in aqueous dispersions are the key factors governing the Tg deviation.
Conclusions In this report, we used nano-DSC to explore the glass transition of surfactant-free, nonionic and anionic PMMA latex nanospheres in aqueous dispersions. The three aqueous dispersed PMMA nanospheres were prepared by ATRP in microemulsions and subsequent dialysis against deionized water. Substantial deviations in Tgs of surfactant-free PMMA nanospheres and nonionic PMMA latex nanospheres relative to bulk were observed, and Tg decreases with the reduction of nanosphere diameter. In fact, Tg behavior of the surfactant-free and nonionic PMMA is qualitatively similar to that of the comparable PS nanospheres. However, compared to the surfactant-free PS nanospheres, Tg of the surfactant-free PMMA nanospheres shows lesser deviation from the bulk. The environmental impact on the glass transition of surfactant-free PMMA nanospheres is greater by contrast with surfactant-free PS nanospheres due to the hydrogen bond between PMMA and water. The nano-DSC results of surfactant-free polymer nanospheres in aqueous dispersions verify the gradiently layered model. Moreover, the anionic PMMA
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