Double Glass Transition Temperatures of Poly(methyl methacrylate

Dec 26, 2013 - Department of Polymer Material Science and Engineering, School of Material Science and Engineering, Tianjin University, Tianjin,. 30007...
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Double Glass Transition Temperatures of Poly(methyl methacrylate) Confined in Alumina Nanotube Templates Linling Li,† Dongshan Zhou,† Dinghai Huang,‡ and Gi Xue†,* †

Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructure, Nanjing University, Nanjing, 210093, P. R. China ‡ Department of Polymer Material Science and Engineering, School of Material Science and Engineering, Tianjin University, Tianjin, 300072, P. R. China S Supporting Information *

ABSTRACT: Recently, confinement of polymers with different geometries has become a research hotspot. Here, we report the dramatic deviation of glass transition behaviors of poly(methyl methacrylate) (PMMA) confined in cylindrical nanopores with diameter significantly larger than chain’s radius of gyration (Rg). Fast cooling a PMMA melt in the nanopores results in a glass with one single glass transition temperature (Tg). But two distinct Tgs are detected after slow cooling the melt. The deviation in Tg could be as large as 45 K. This phenomenon is interpreted by a two-layer model. During vitrification under slow cooling two distinct layers are formed: a strongly constrained interfacial layer showing an increased Tg as compared to that of the bulk polymer and a core with a decreased Tg. By thermal annealing experiments, we find that these two Tgs are inherently correlated. In addition, the deviation of Tg for PMMA confined in nanopores reveals a dependence on molecular weight.

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extension of the case of 1D confinement.19,22,25 Both the size effect and interfacial interaction effect are generally considered important for the glass transition behavior of polymers under nanoconfinement. It has been well accepted that a liquid-like layer with enhanced mobility exists at the surface of a polymer glass, and the effect of free-surface on the Tg reductions has been widely studied.28−32 However, the effect of an irreversibly adsorbed layer at the substrate interface on the Tg behavior is much less investigated. Using X-ray photon correlation spectroscopy (XPCS), Koga et al. have shown that the irreversibly adsorbed layer induces a significant increase in the viscosity with decreasing the distance from the substrate.33 Essentially, such confinement effects should be related to the specific chain configurations, adopted by the polymer molecule in the attempt to reach thermodynamic equilibrium or to minimize their free energy under the perturbations in the molecular potential induced by interfaces.34,35 Recently, Napolitano et al. have proposed that the local free volume at buried polymer/substrate interfaces shows a strong correlation with the changes in Tg in ultrathin films, the deviation from bulk behavior should also attribute to the perturbation in the free volume arising from chain adsorption.36−38 It indicates the possibility to alter the glass transition behavior of polymers

INTRODUCTION In past decades, the glass transition behaviors of polymers confined to nanoscales have attracted considerable scientific and technological interest. However, there is an ongoing debate on whether and how the glass transition temperature (Tg) of a polymer changes under nanoconfinement. The glass transition temperature of polymers under nanoconfinement has been reported to decrease, increase or remain constant compared with their bulk value, depending on the details of the sample, preparation and measurement techniques.1−9 It is worth noting that few studies reveal a double glass transitions behavior for polymers under nanoconfinement.10,11 Tsagaropoulos and Eisenberg observed two Tgs for several polymers filled with nanosized silica particles, and the difference between the two Tgs is in the range 60−100 K.10 Such complex Tg dynamics is more commonly observed for glass forming liquids confined in nanopores.12−18 However, the origin of such double glass transitions behavior under nanoconfinement is still unclear and deserves more attention. Previous nanoconfinement experiments of polymers are mainly focused on thin films, which are considered as onedimensional (1D) confinement. Recently, the glass transition behaviors of polymers under higher geometrical confinements have attracted more and more attentions, such as polymers confined in nanopores (2D confinement)19−21 as well as nanoparticles,22,23 nanoglobules24 and nanocomposites (3D confinement),25−27 which are usually considered as the © 2013 American Chemical Society

Received: September 26, 2013 Revised: December 17, 2013 Published: December 26, 2013 297

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RESULTS AND DISCUSSION Anodic aluminum oxide (AAO) templates with uniform arrays of straight cylindrical nanopores can be easily infiltrated with polymer melts. Such AAO templates were used to confine PMMA. When the molten polymer is in contact with the porous template, wetting and complete filling of pores usually take place on different time scales, resulting in the formation of very different one-dimensional polymeric nanostructures (nanotubes or nanorods).40,41 To ensure that the polymeric nanorod structure was formed, the infiltration process was allowed to continue for long enough time (24 h) and the sample was cooled back to room temperature slowly. Figure 1

under confinement by controlling the local free volume at the interface. In the present work, we investigate Tg for poly(methyl methacrylate) (PMMA) confined in cylindrical nanopores by differential scanning calorimetry (DSC). We observe two Tgs for PMMA confined in larger nanopores (diameter 80 nm, which is much larger than the radius of gyration (Rg) of the polymer chain). The experimental results are interpreted by a two-layer model. Because of the strong interfacial interaction, polymer chains in the interfacial layer show an increased Tg, while the loose packing of polymer chains in the core volume induces a decreased Tg. On the basis of previous simulations results, we assume that the two Tgs are inherently correlated, which has been demonstrated by the fast cooling and thermal annealing experiments. In addition, we also investigate the influence of molecular weight on the glass transition behavior of PMMA confined in nanopores.

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Article

EXPERIMENTAL SECTION

Materials. Two kinds of monodisperse PMMA samples with number-average molecular weights Mn = 6 and 35 kg/mol (denoted as PMMA6K and PMMA35K, respectively), stereoregular composition 6% isotactic, 38% atactic, and 56% syndiotactic, were purchased from Polymer Source Inc. (Dorval, Canada). The anodic aluminum oxide (AAO) templates with an average pore diameter of 80 nm and ∼100 μm of length were purchased from PUYUAN NANO Co. (Hefei, China), which were prepared via a two-step anodization process. The membranes were rinsed thoroughly with chloroform and methanol to remove possible impurities on surfaces and then annealed at 150 °C for 2 h in vacuum before use. Preparation of PMMA-Filled AAO Sample. The PMMA sample was dissolved in toluene and a film with thickness of about 100 μm was prepared by solution-casting onto a clean cover glass. After drying the film under ambient conditions for several days, it was dried under vacuum for 24 h at 150 °C. Then, the dried film was placed on top of the AAO template, and was heated at 160 °C (for PMMA6K) and 190 °C (for PMMA35K) under vacuum (100 mbar) for 24 h. The thermal stability of PMMA was checked by thermogravimetry, as shown in Figure S1 of the Supporting Information. The PMMA melt was drawn into the AAO nanopores via capillary forces and eventually formed the PMMA nanorods inside the nanopores after slow cooling to room temperature. Prior to analysis, excess PMMA was carefully removed from the surface of the AAO template with a sharp razor blade. The PMMA nanorods were finally obtained by immersing the PMMA-filled AAO sample in a sodium hydroxide aqueous solution (1.0 mol/L) for 24 h to remove the AAO matrix.39 Characterization. Scanning electron microscopic (SEM) micrographs were recorded using a scanning electron microscope HITACHI S-4800 employing an acceleration voltage of 20 kV. DSC measurements were performed on a Mettler-Toledo DSC1 STARe differential scanning calorimeter under dry nitrogen atmosphere. Temperature calibration was performed before experiments using indium as a standard. The mass of the PMMA-filled AAO sample for the DSC measurements was about 20 mg, and the heating rate was 10 K/min to minimize the influence of the thermal lag. The amount of PMMA filled in the AAO template was determined by thermogravimertic analysis (TGA), and the measurement was carried out on the Perkin-Elmer TGA-Pyris system. The samples were heated from room temperature to 700 °C at 10 K/min under dry nitrogen. As shown in Figure S1, the mass fractions of PMMA that infiltrated into the AAO template are about 25 wt % for PMMA6K and 14 wt % for PMMA35K. Besides, the thermal decomposition temperatures of PMMA confined in 80 nm AAO template increase by 12−22 K compared to their bulk values.

Figure 1. SEM micrographs of (a) the pristine AAO template with a pore diameter of 80 nm (from section view), (b) surface of the AAO template containing PMMA6K nanorods inside, and (c, d) the PMMA6K nanorods after removing AAO template: (c) from top view and (d) from side view.

presents SEM micrographs of the pristine AAO template, the PMMA6K-filled AAO sample and the resulting PMMA6K nanorods after removal of AAO template. The diameters of the AAO nanopores and the resulting PMMA nanofibers are about 80 nm. Calorimetry is an effective analytical tool to characterize the glass transition and phase transformations under confinement, which can reach high sensitivity and reproducibility. However, in most cases calorimetry reveals only a weak dependence of the glass transition temperature on confinement as long as the confining dimensions are above 10 nm, which contradicts many other studies applying other techniques to similar systems.3 Figure 2 shows the DSC heating traces of bulk PMMA6K and PMMA6K confined in 80 nm AAO nanopores. The bulk PMMA6K exhibits a single Tg at about 110 °C (denoted as Tg,bulk), while the glass transition behavior of PMMA6K confined in the AAO nanopores deviates dramatically from the bulk characteristics. Two distinct Tgs are observed: one is lower than the bulk value (denoted as Tg,lo) and the other is at a much higher temperature (denoted as Tg,hi). As shown in Figure 2b, the difference between the two Tgs reaches values as large as 45 K: Tg,lo is about 10 K lower than Tg,bulk and Tg,hi is about 35 K higher. To confirm that these two thermal transitions are glass transitions, sub-Tg annealing experiments were carried out. After annealing PMMA6K-filled AAO samples at Tg,lo −10 K and 298

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volume of PMMA in a given layer is proportional to the step change of the heat capacity of that layer, and a possible gradient in chain packing density along the pore radius is ignored. d [1 − (ΔCp , lo/(ΔCP , lo + ΔCP , hi))1/2 ] 2 Where ζ is the thickness of the interfacial layer and d is the diameter of the AAO nanopores, ΔCp,hi and ΔCp,lo are the changes of heat capacity at Tg,hi and Tg,lo, respectively. According to the above equation, the thickness of the interfacial layer is about 14 nm for PMMA6K confined in 80 nm AAO template, which is much larger than that reported for small glass forming liquids confined in nanopores (1−2 nm),12,18 but similar to the thickness of the interfacial immobilized layer of polymer/silica nanocomposites.11 As is well-known, the influence of the substrate wall on glass transition could propagate a much longer distance by longrange effects, and such an influence on the Tg dynamics should significantly increase with decreasing the distance from the substrate.33,42,43 Besides, the polymer confined to the higher geometrical dimensionalities (such as nanopores) should have a stronger interfacial effect due to the larger surface-to-volume ratio.22,25 The increased Tg in the interfacial layer is due to the interaction between polymer chains with the pore wall, while the decreased Tg in the core volume is unexpected. In our system, there is no free surface. Usually, faster dynamics and lower Tgs are associated with silanized surfaces, soft confinement, or the existence of an additional interfacial liquid layer. As Richert pointed out, why a more immobilized polymeric layer at the surface leads to faster dynamics within the core volume is still unclear.44 Fakhraai and Forrest have demonstrated that the reduced Tg values often reported in thin PS films display a striking cooling rate dependence.45 As the cooling rate reaches a few K/s, the film thickness-dependent reductions in Tg would be absent. In order to investigate the origin of the decreased Tg in the core volume, we studied the influence of cooling rate on the glass transition behavior of PMMA confined in AAO nanopores. Figure 3 compares the heat capacity curves of bulk PMMA6K, a hyperquenched glass of PMMA6K filled in AAO and a standard glass cooled at 10 K/min of PMMA6K filled in AAO. Formation ζ=

Figure 2. Normalized DSC traces of (a) bulk PMMA6K, (b) PMMA6K confined in 80 nm AAO template, and PMMA6K-filled AAO samples annealed at temperatures of (c) Tg,lo-10 K and (d)Tg,hi-10 K for 2 h. Both, the cooling and the heating rate equal 10 K/min. The dashed lines indicate the Tg values and the solid lines are guides to the eye to identify the changes of the enthalpy relaxation peaks after sub-Tg annealing. The inset illustrates the possible Tg distribution of PMMA confined in AAO nanopores.

Tg,hi −10 K for 2 h, the typical endothermic enthalpy relaxation peaks appear in the DSC curves shown in Figure 2, parts c and d, respectively. Some studies have reported a core−shell or two-Tgs picture regarding the dynamics of material within porous glasses.12−18 Park and McKenna12 reported two Tgs for the small molecule liquid o-terphenyl and its polystyrene solution confined in controlled pore glasses. A two-layer model was proposed, in which a “core” liquid with lower Tg in the center is surrounded by a layer interacting with the pore wall showing a higher Tg, as illustrated in the inset in Figure 2. In our system, PMMA chains near to the pore wall have a decreased mobility due to interfacial interactions by hydrogen bonding with the pore walls,21 and they form an interfacial layer with increased Tg. PMMA chains in the core volume seem to have an increased mobility and show a decreased Tg. The length scale of the interfacial layer (ζ) can be estimated by assuming that the

Figure 3. (a) Normalized DSC traces of bulk PMMA6K (black), hyperquenched glass of PMMA6K filled in 80 nm AAO (red) and standard glass of PMMA6K filled in 80 nm AAO (blue). All samples are heated at 10 K/min. (b) Temperature derivative of the heat capacity curves shown in part (a). 299

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different Tas for 60 min, the Tg evolution differs remarkably for different Tas. At temperatures below the bulk Tg of PMMA6K (Tg,bulk = 113 °C), a single Tg with enthalpy relaxation peak is observed; while, at temperatures above Tg,bulk, two Tgs are displayed. The higher Tg (Tg,hi) increases with increasing Ta, and the lower Tg (Tg,lo) decreases. The homogeneous hyperquenched PMMA6K glass filled in AAO nanopores is under a nonequilibrium state, it would relax to a more stable state with heterogeneous structures if the polymer chains have enough mobility to move from the core volume to the interfacial layer. After such transfer, the local packing density in the interfacial layer would increase, and that in the core volume would decrease correspondingly. The glass transition behavior recovers back to the two-Tgs picture. For temperatures below Tg,bulk, all the polymer chains are frozen and the enthalpy relaxation behavior happens; for temperatures above Tg,bulk, increasing the annealing temperature would enhance the chain mobility and accelerate the transfer process, which induces a more heterogeneous Tg dynamics in the nanopores (that is the higher Tg,hi increases and the lower Tg,lo decreases). Second, we investigate the influence of annealing time (ta) on the Tg’s evolution. From Figure 5, we can see that the relaxation rates to heterogeneous structure are too slow at temperatures close to Tg,bulk (such as 120 °C), and too fast at temperatures far above Tg,bulk (such as 160 °C). So, we choose the temperatures between 130 and 150 °C as the annealing temperatures in our experiment. Figure 6 shows the heat capacity curves of hyperquenched PMMA6K-filled AAO samples annealed at 140 °C for different annealing times, and other two annealing experiments at 130 and 150 °C can be seen in Figure S2 of Supporting Information. From Figure 6, it is found that Tg,hi increases and Tg,lo decreases gradually with increasing annealing time, which is similar to the phenomenon for increasing the annealing temperature. Essentially, the increase of annealing temperature or annealing time could both induce more chains to transfer from core volume to the interfacial layer. Meanwhile, the width of Tg,hi’s peak in Figure 6 (b) gradually increases, which suggests that the distribution of relaxation times in the interfacial layer becomes broader during the annealing process. Demonstrated by the results of Figure 5 and 6, we can see that the two Tgs observed for the polymer confined in nanopores are inherently correlated. The change of one would cause the variation of the other, inevitably. Moreover, we show that the glass transition behavior of the polymer confined in nanopores could be controlled by different thermal annealing processes. Napolitano et al. have proven that Tg can be tuned without affecting the interfacial chemistry (constant film thickness, same polymer/substrate) by changing packing density upon controlled annealing (chain adsorption).36 Our results are consistent with their conclusions. Here, we take the difference of Tg,hi and Tg,lo (ΔTg =Tg,hi - Tg,lo) as the parameter to monitor the Tg’s evolutions during annealing. The ΔTg represents the heterogeneity of Tg dynamics of PMMA confined in AAO nanopores. The ΔTg’s evolutions of hyperquenched PMMA6K-filled AAO samples with annealing time at different Tas are shown in Figure 7. For all Tas, the ΔTg increases dramatically in the earlier times and gradually approaches a limiting value (ΔTg,∞) at later times, which indicates that there may exist two different processes during annealing. The insert graph shows the dependence of ΔTg,∞ on Ta. ΔTg,∞ increases linearly as Ta increases, and the extended line intersects the x axis at a point, which is close to the Tg of the hyperquenhed sample before annealing. It means that

of the hyperquenched glass was achieved by rapidly transferring the molten sample contained in an aluminum pan into liquid nitrogen, the estimated cooling rate of the processing being ∼120 K/s.24 Interestingly, the hyperquenched PMMA glass recovers to the bulk characteristic. It is known that the relaxation times of confined and bulk liquids are equal at high temperature far above Tg. So, if the cooling rate is high enough, the polymer chains in the nanopores would be frozen and maintain the conformations of the melt state in a nearly homogeneous state with bulk characteristics. We propose that the variation of Tg is associated with the slight fluctuations in chain conformations or local packing density. The simulation results of polymer melts confined between two flat hard walls reveal that the density profile of the polymer is heterogeneous, and strongly depends on the distance from the solid wall.46−50 The packing density of the polymer chains close to hard wall is enhanced, and it decays toward the bulk value as approaching the center. When the polymer is in the melt state, the decay length scale for the density is smaller than the radius of gyration (Rg). With increasing supercooling the decay length becomes much larger than Rg. As the polymer confined in the nanopores is under isochoric glass formation condition, so the interfacial layer with enhanced packing density is formed during a slow cooling process, the packing density of the polymer chains in the core volume would be slightly reduced, subsequently. It indicates that the two Tgs observed for polymer confined in nanopores should be inherently correlated due to the mass conservation. Between the hyperquenched and slow cooling rates, there should be a critical cooling rate at which the glass transition behavior changes from single Tg to two Tgs. Unfortunately, it is difficult to control faster cooling for such big samples. Here we design a thermal procedure as illustrated in Figure 4 to

Figure 4. Schematic of the thermal procedure to investigate the Tg evolutions of hyperquenched PMMA6K-filled AAO samples.

investigate the Tg’s evolutions of hyperquenched PMMA6Kfilled AAO samples upon annealing. We first hyperquench the sample to maintain the conformations of melt state; then heat the hyperquenched sample to the desired temperature (Ta) and anneal for certain times (ta); after cooling back to room temperature, subsequent heating at 10 K/min is carried out to determine the values of Tg. First, we investigate the influence of annealing temperature (Ta) on the Tg’s evolution. As shown in Figure 5, the hyperquenched PMMA6K-filled AAO samples are annealed at 300

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Figure 5. (a) Normalized DSC traces of hyperquenched PMMA6K-filled 80 nm AAO samples annealed at different annealing temperatures (Ta) for 60 min. For comparison, the orange line, which corresponds to the sample cooled from melt state at 10 K/min, is shown at the bottom. All samples are heated at 10 K/min. (b) Temperature derivative of the heat capacity curves shown in part a.

Figure 6. (a) Normalized DSC traces of hyperquenched PMMA6K-filled 80 nm AAO samples annealed at 140 °C for different annealing times. All samples are heated at 10 K/min. (b) Temperature derivative of the heat capacity curves shown in part a.

annealing near or below Tg,bulk shows little impact on Tg’s evolutions, which is consistent with the result of Figure 5. From the results of the annealing experiments, we can see that under controlled thermal treatments, PMMA glass prefers to form a heterogeneous structure rather than a homogeneous one in the AAO nanopores. As illustrated in Figure 8, there are two routes resulting in a heterogeneous Tg dynamics. One is slow cooling from the melt, and the other is annealing the hyperquenched glass at high temperatures. Both of the two routes should involve increasing packing density near the pore wall. The critical entanglement molecular weight (Mc) for PMMA is 29.5 kg/mol.51 The experimental results in Figures 1−8 were recorded from a PMMA oligomer with molecular weight of 6 kg/mol. The effect of molecular weight on the glass transition behavior of PMMA confined in AAO nanopores was also investigated. We choose a PMMA with molecular weight of 35 kg/mol as the sample. Figure 9 compares the heat capacity curves of bulk PMMA35K and PMMA35K confined in 80 nm AAO nanopores. The PMMA35K-filled AAO sample cooled from the melt at 10 K/min reveals a single Tg. It means that the

cooling rate of 10 K/min is fast enough to quench the polymer chains to form a homogeneous polymer. However, from the results shown in Figures 1−8, the cooling rate (10 K/min) was fast enough to separate the two glass transitions for PMMA6K in the AAO. The difference may be caused by the entanglement effect. The entanglement between longer polymer chains would significantly increase the interchain constraints and thus prevent chains transferring toward the interfacial layer. Besides, the broad exothermic peak before Tg is induced by some level of structural instability during the cooling process.24 However, after annealing PMMA35K filled in AAO at 150 °C for 2 and 8 h, two distinguished Tgs appear, as shown in Figure 9. It means that entangled chains need longer time to adapt the equilibrium structure in the pores and to show the two glass transitions. The difference between the two Tgs is about 45 K after annealing PMMA35K for 8 h at 150 °C.

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CONCLUSIONS In this study, the glass transition behaviors of PMMA confined in AAO nanopores with a diameter larger than the polymer chain’s Rg were investigated. The calorimetric results reveal a 301

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increased Tg due to the strong interfacial interaction, which can propagate into the center part during the slow vitrification process and form the interfacial layer; meanwhile, due to the isochoric glass formation conditions, the local packing density in the core volume would be reduced, which induces a decreased Tg. In our system, the polymer glass confined in nanopores prefers to form a heterogeneous structure rather than a homogeneous one, and the two Tgs observed for the polymer confined in nanopores are inherently correlated, which has been proven by annealing experiments. In addition, the kinetics of the formation of the core−shell structure of the polymer confined in nanopores reveals a dependence on molecular weight. The higher molecular weight PMMA needs longer annealing to form a two-layer structure with two Tgs.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 7. ΔTg’s evolution of hyperquenched PMMA6K-filled AAO samples with annealing time at different annealing temperatures: 130 (black), 140 (red), and 150 °C (blue). The solid lines are polynomial fits to the experimental data. The inset shows the linear relationship of ΔTg,∞ with annealing temperature, the star symbol represents the Tg of the hyperquenhed sample before annealing.

Thermogravimetric analysis for PMMA samples and the heat capacity curves of hyperquenched PMMA6K-filled AAO samples annealed at 130 and 150 °C for different annealing times. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (G.X.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author appreciates the financial support of National Basic Research Program of China (973 program, 2012CB821503) and Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT). This work was also supported by the NSF of China (51133002, 21174062, 21274060, 21274059 and 21027006).

Figure 8. Schematic graph of the two routes to yield the heterogeneous Tg dynamics in nanopores. The red color represents the interfacial layer with higher Tg, yellow color represents the core volume with lower Tg and orange color represents the polymer with bulk characteristics.

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double glass transition behavior: one Tg is lower than the bulk value and the other is much higher. A two-layer model is proposed: polymer chains near the pore walls show the

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Figure 9. (a) Normalized DSC traces of bulk PMMA35K (black line), PMMA35K confined in 80 nm AAO cooled from the melt at 10 K/min (red line) and annealed at 150 °C for 2 h (blue line) and 8 h (olive line), respectively. All samples are heated at 10 K/min. (b) Temperature derivative of the heat capacity curves shown in part (a). 302

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