Article pubs.acs.org/Macromolecules
Effect of Molecular Chain Architecture on Dynamics of Polymer Thin Films Measured by the Ac-Chip Calorimeter Jiao Chen, Linling Li, Dongshan Zhou, Jie Xu, and Gi Xue* Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Key Laboratory of High Performance Polymer Materials and Technology (Nanjing University), Ministry of Education, Nanjing University, Nanjing 210093 P. R. China S Supporting Information *
ABSTRACT: It was reported that glass transition temperature (Tg) measured by differential alternating current (ac) chip calorimetry showed little thickness dependence for polymer films. Here we demonstrate the detection of Tg in thin films by ac-chip calorimeter and show that Tg is decreased as the thickness is reduced for oligomers and star-shaped polymers, as compared with their long linear analogues. The deviation range is a few to more than ten Kelvin. Such a depression in Tg is quite pronounced for ac-chip calorimetric measurement at a high frequency of 10 Hz. We argue that the perturbation in the increased interfacial free volume for spin-cast oligomers and dendrimers is the major reason for increasing segmental dynamics for ultrathin films.
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INTRODUCTION The physical theory of glass transition still remains a challenging and unsolved problem. Up to now, there have been so many reports about the dependence of glass transition temperature (Tg) with geometrical size in nano scales. The results on various systems show quite different Tg behaviors with decreasing film thickness.1−10 It depends on various factors, such as the sample properties, process of preparation, and measurement methods. In most cases, Tgs of ultrathin polymer films decrease under geometrical confinement. However, several measurements claimed no thickness dependence of Tg, among which ac-chip calorimetry and dielectric measurement attract more attention.11−14 Although so many investigations have been reported, the physical meaning for Tg changes seems to be somewhat unclear. Calorimetry is a classical method to detect Tg, and it takes a special place compared with other techniques due to its universality and simplicity. However, it is difficult to detect the heat capacity data of ultrathin films as commercial DSC equipments have limited sensitivities. In some recent published works, Simon reported the thickness dependence of calorimetric Tg for high molecular weight polystyrene detected by the use of a flash DSC.15 But it is still difficult to investigate an ultrathin film as the thickness is reduced to several nanometers.16 Schick et al. originally developed the ac-chip calorimeter, which can measure thermal properties of very minor samples at thermodynamic equilibrium.12,14,16 Previously, we measured the Tg of poly(2,6-dimethyl-1,5-phenylene oxide) films with thickness ranging from about 6 to 300 nm by ac-chip calorimetry, and there was no depression in Tg for the investigated films.14 It is still necessary to deeply investigate the relationship between the thickness dependence and the intrinsic properties of the sample geometries and dimensions. © 2014 American Chemical Society
As is well-known that polymer chain is not stiff and straight, and it usually twists and bends around to form a tangled mess. So polymer chains tend to entangle with each other, especially when the molecular weight is above the critical molecular weight (Mc). The investigation of entanglements for long polymer chains is important for both fundamental study for chain conformation and the mechanical applications as the industrial materials.17−21 Mark and Tobolsky first mentioned entanglement and the computation of entanglement spacings.18 Entanglement characteristics and spacings can be observed by many methods. The tensile stress in thin film is caused by the spin-coating, and the entangled polymer chains would be stretched by the stress. So chain entanglement is expected to have some effects on the interfacial stress and coupling for spincast polymeric films. The interfacial effect is very important on the Tg changes for polymer film study.22 Russell23 pointed out that when the size of an object decreases, the surface/volume ratio increases and interfaces become even more important. The surface or interface can influence the chain configuration and the segmental or long-range dynamics. Forrest suggested that the free surfaces contribute to the decrease of Tg.24 Recently, Dalnoki-Veress et al. directly investigated this effect by controlling the number of free surfaces on Tg deviation for polymer films using ellipsometry.25 They found a significant Tg reduction for a freestanding thin film. After transferring it to a silica substrate the reduction disappeared and the Tg recovered to the bulk values.25 Simon et al. placed PS thin films onto a layer of inert oil/grease to reduce the substrate surface energy, Received: January 24, 2014 Revised: March 3, 2014 Published: May 12, 2014 3497
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Figure 1. Temperature dependence of amplitude of the differential voltage in cooling process using ac-chip calorimetry for PS and PMMA films with high molecular weight. The molecular weight is 60 and 100 kg/mol, respectively.
Figure 2. Temperature dependence of amplitude of the differential voltage in cooling process using ac-chip calorimetry for PS and PMMA films with low molecular weight. The molecular weight is 6 kg/mol and 7 kg/mol, respectively.
there is a Tg depression but invariant segmental dynamics. It seems that Tg depression does not indicate such changes in segmental dynamics. To better investigate the factors that affect Tg or segmental dynamics of thin films with ac-chip measurement, we measured the Tgs of oligomers and starshaped polymers, and also found the thickness dependence even at a high frequency. We propose that these results are due to the increased interfacial free volume.
and observed the thickness dependence of Tg for supported films.15 In our recent work we also find a case that there is film thickness dependence of Tg detected using ac-chip calorimeter.16 We controlled the interface by preparing PS films containing immiscible surfactant molecules, which assembled on the substrate and resulted in a mobile interface.16 Introducing a mobile interface or reducing the surface energy can release the interfacial stress, which helps recover the thickness dependence of Tg for supported films. Napolitano26 pointed out that the geometrical size and interfacial interactions are not sufficient to predict the properties of confined polymers. An increased free volume, caused by the decrease of packing density, might accelerate the segmental dynamics and justify a depression in Tg.26 They tried to determine the local free volume in the region of polymer/ substrate interfaces, and found that the changes of Tg in ultrathin films are strongly related to such free volume. Nguyen observed further that well-annealed poly(vinyl acetate) films exposed to controlled humidity do not uptake as much moisture as films annealed at shorter times.27 So we consider that the change in free volume should be an important role when describing the deviation from bulk behavior. Boucher et al.28 found a Tg depression for PS thin films by DSC and capacitive dilatometry (CD), however no depression with ac-calorimetry and broadband dielectric spectroscopy (BDS). DSC and CD deliver information about thermodynamics marked by Tg, while ac-calorimetry and BDS investigate the segmental dynamics. So for thin films under confinement,
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EXPERIMENTAL SECTION
Materials. The monodisperse polystyrene (PS) (Mn = 214 kg/mol, d = 1.03; Mn = 60 kg/mol, d = 1.03; Mn = 6 kg/mol, d = 1.06), poly(methyl methacrylate) (PMMA) (Mn = 105 kg/mol, d = 1.09), star-shaped PS (eight-arm, Marm = 34.6 kg/mol, d = 1.02; three-arm, Marm = 109.8 kg/mol, d = 1.07) were purchased from Polymer Source Inc., Canada. The oligomer poly(methyl methacrylate) (Mn = 7 kg/ mol, d = 1.53) was prepared by the copolymerization of methyl methacrylate. Toluene was commercially available and distilled before use. We prepared thin films by spin-coating polymer solutions with different concentrations in toluene at the same spin rate (2000 r/min). All solutions were filtered before spin-coating. The film thicknesses were measured with ellipsometry by spin-coating each solution on silicon wafers. For ac-chip measurement the solution was directly spincoated onto a clean sensor. Ac-Chip Calorimetric Measurement. In this work, thin films were supported by the sensor XI392 (Xensor Integrations, NL) with a large smooth heated area (100 μm × 100 μm). The samples were measured at the frequency of 10 Hz and a heating/cooling rate of 1 K/ min. During the process of measurement, samples were protected by 3498
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nitrogen atmosphere and each sample was scanned for three times. We directly obtained the amplitude of the complex differential voltage as a function of measuring temperature. The Tg was determined as the halfstep temperature of the amplitude. Besides, we also used the first derivative of the amplitude curve to extract Tg. After the derivative, the imaginary part of complex heat capacity was fitted by a Gauss function and the peak value was considered as the Tg.13 The values are quite similar to those determined by half-step method as shown in Figure S1, S2, and S3 of the Supporting Information.
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RESULTS AND DISCUSSION According to previous reports, there is no depression in Tg for thin films detected with ac-chip calorimeter.5,11−14 Here we detected the Tg of films of PS and PMMA using this method. Figure 1 shows the Tg for PS and PMMA films with high molecular weight, that is 60 and 100 kg/mol, respectively. No Tg deviation between thin (15 nm) and thick (150 nm) films is observed for both samples. This is totally consistent with previous studies.5,11−14 To investigate the effect of molecular weight on the thickness dependence of Tg, we also took the similar experiments for oligomer PS and PMMA. The molecular weight is 6 and 7 kg/ mol, respectively, which are both below their critical entanglement molecular weight. It is interesting that Tg deviations between thin and thick films occur here. The reduction is 12 °C for PS and 6 °C for PMMA, as shown in Figure 2. According to Figure 2, it seems that the deviation value is not so large as that measured by other techniques. This is partially because the Tg was measured at a relative high frequency (10 Hz), such a Tg deviation has already been quite significant for ac-chip calorimeter. On the basis of the results above, we obtain the molecular weight dependency of Tg deviation, as shown in Figure 3. Ac-chip calorimetric technique for glass transition studies of nanometer thin films covers 3 orders of magnitude in frequency.13 In our experiment, we choose 10 Hz as the test frequency because it is a proper frequency to obtain good signal according to previous experiences. There is a report about temperature dependence of the deviations from bulk behavior in ultrathin polymer films.29 They performed the measurements of the complex dielectric function in the frequency ranging from 10 mHz to 10 MHz.29 Frequency of the maximum of the structural relaxation process and the temperature fit the VogelFulcherTammann (VFT) equation. For different film thicknesses, there are deviations from the VFT behavior, which means there are Tg deviations. As the frequency increases, the deviation becomes smaller. The cooling rate has the same effect. Forrest et al. reported that the reduced Tg values display a striking cooling rate dependence.30 This effect is so pronounced that for cooling rates of only a few K/s, we would not expect to see any reductions in Tg with decreasing film thicknesses. Similar results can also be found in other reports.15 Both the frequency and cooling rate are related to the relaxation times which influence the Tg. So the measurements at high frequencies may provide reduced confinement effects. The potential large stress between chain molecules is another factor that contributes to the segmental relaxation in spin-cast thin film. Ngai suggested that the intermolecular coupling should be especially important when considering the local segmental dynamics and controlling the final properties of the polymeric materials.17,22,31 Tsui et al. studied surface mobility and the dynamics in short-chain PS supported film and found that the thickness dependence of Tk studied by viscosity is in
Figure 3. Molecular weight dependency of Tg deviation for PS and PMMA thin films with h = 30 or 15 nm.
good agreement with that of Tg detected by thermal expansivity.10 Here, we demonstrate a chain length dependence of the reduction of Tg in PS films with h = 30 nm and h = 15 nm, respectively. Figure 3 shows that for PS with Mwt > 60 kg/ mol, the calorimetric ac-chip measured Tg does not change from the bulk values. However, for short chain PS with Mwt = 6 kg/mol, a significant reduction occurs for calorimetric Tg. For the PS film with h = 15 nm, Tg is reduced by 12 °C from the bulk, as shown in Figure 2. We find that low Mwt PMMA also shows dependence of calorimetric ac-chip measured Tg on film thickness. Here we notice that the reduction for PS is more than PMMA. The dependence of calorimetric Tg on film thickness for low molecular weight PS shown in Figure 3 is remarkable as compared to that no thickness dependence of Tg in high molecular weight PS (Figure 1). We argue that the Tg deviation between ultrathin film and bulk sample for short chain PS is due to the release of the interfacial coupling for spin-cast film. Polymer samples can be in different states of overlap of chains by specific preparation methods.32 However, it is still unavailable to control and detect the interchain proximity on short length scales, and entirely understand its effect on chain dynamics. Recently, we observed that controlling interchain proximity for polystyrene glass could strongly influence the local segmental motion.31 Napolitano26 reported that the depression of Tg measured in supported thin films under confinement is proportional to the strength of adsorption and, thus, to the free volume at the interfacial region. Packing frustration can increase the free volume, and results in an acceleration of the segmental 3499
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Figure 4. Temperature dependence of amplitude of the differential voltage in cooling process using ac-chip calorimetry for eight-arm PS and threearm PS films. The arm molecular weight of the star-shaped PS is 34.6 and 109.8 kg/mol, respectively.
dynamics which reflects a reduction of Tg.26 They pointed out that the size effect and interfacial interactions are not sufficient to fully predict the dynamics of polymers under confinement. Perturbation in the free volume should also be considered when describing the deviation from bulk behavior in ultrathin polymer films. Confined at the nanoscale level, poly(tertbutylstyrene) films capped in between aluminum layers (no free surfaces) show a Tg reduction resulting from the presence of an excess in free volume located in between the interfacial immobilized layer and the bulk core.33,34 This result shows that the magnitude of Tg depression and interfacial free volume are linearly correlated. Here in our experiment, there is thickness dependence of Tg for oligomers. We speculate that there is more interfacial free volume in oligomers due to the absence of entanglement. Chains are not entangled with each other and there are more chain ends. During the process of spin-coating, chain ends can run off and increase the interfacial free volume. However, for high molecular weight polymers, chains are strongly entangled and there is no significant difference of interfacial free volume between thin and thick films. We also took the ac-chip experiment for star-shaped PS. For eight-arm PS (Marm = 34.6 kg/mol), there is a 5 °C Tg deviation between thin and thick films (Figure 4). It is a obvious deviation as the measurement is undertaken at such a high frequency (10 Hz). While for the three-arm PS (Marm = 109.8 kg/mol), the deviation is less obvious. The architecture of the macromolecule has an important role on the thickness dependence of Tg and the interfacial Tg.35 So here the occurring of Tg deviation is possible due to the special molecular architecture of dendrimers. With the increasing number of arms, chains are not easy to entangle with each other, which means that there is more free volume. During the process of spin-coating, there is more interfacial free volume in thin films, which would induce a larger reduction in Tg. As we say that the molecular architecture contributes to the thickness dependence of Tg, for the three-arm PS, the resulting interfacial free volume is not enough to reduce the Tg, so the deviation is less pronounced. Foster pointed that branching has a significant effect on confinement. They found that the fragilities of the branched chains are decreased in thin films relative to the bulk values, while the fragility of the linear chain is unchanged.36 It is well-known that the fragility is directly related to Tg, so our result is consistent with their viewpoint. For polymers with same molecular weight, dendrimers are easier to result in
thickness dependence of Tg compared with linear polymers. We speculate that the geometric confinement behavior of high molecular weight dendrimers is similar to that for linear low molecular weight polymers.
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CONCLUSION In this article, we detect the Tg of polymers by ac-chip calorimeter and find that Tg for ultrathin films of oligomers and star-shaped polymers decreases compared with thick films. The architecture of the macromolecule takes an important role on the thickness dependence of Tg and the interfacial Tg. The absence of entanglement in oligomers and increasing arm numbers of dendrimers can both change the architecture of polymers. We suggest that such Tg deviations with ac-chip measurement for thin films are due to the increased interfacial free volume for oligomers and dendrimers processed by spincoating. So the deviation from bulk behavior can be correlated to the change of interfacial free volume.
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ASSOCIATED CONTENT
S Supporting Information *
First derivative of the amplitude curve and Gauss fitting for all the samples. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*(G.X.) E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully appreciate Prof. Christoph Schick in University of Rostock for his help in building the ac-chip calorimetry in our laboratory. We also appreciate the financial support from National Basic Research Program of China (973 Program, 2012 CB 821503) and Chinese National Natural Science Foundation (Nos. 21274060 and 51133002).
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REFERENCES
(1) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Faraday Discuss. 1994, 98, 219−230. (2) Wallace, W. E.; van Zanten, J. H.; Wu, W. L. Phys. Rev. E 1995, 52, R3329−R3332.
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(3) Forrest, J. A.; Dalnoki-Veress, K.; Dutcher, J. R. Phys. Rev. E 1997, 56, 5705−5716. (4) Forrest, J. A.; Dalnoki-Veress, K.; Dutcher, J. R. Phys. Rev. E 1998, 58, 6109−6114. (5) Efremov, M. Y.; Olson, E. A.; Zhang, M.; Zhang, Z.; Allen, L. H. Phys. Rev. Lett. 2003, 91, 085703. (6) Ellison, C. J.; Torkelson, J. M. Nat. Mater. 2003, 2, 695−700. (7) Ellison, C. J.; Ruszkowski, R. L.; Fredin, N. J.; Torkelson, J. M. Phys. Rev. Lett. 2004, 92, 095702. (8) Alcoutlabi, M.; McKenna, G. B. J. Phys.: Condens. Matter 2005, 17, R461−R524. (9) Miyazaki, T.; Inoue, R.; Nishida, K.; Kanaya, T. Eur. Phys. J.-Spec. Top. 2007, 141, 203−206. (10) Yang, Z. H.; Fujii, Y.; Lee, F. K.; Lam, C. H.; Tsui, O. K. C. Science 2010, 328, 1676−1679. (11) Huth, H.; Minakov, A. A.; Schick, C. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 2996−3005. (12) Huth, H.; Minakov, A. A.; Serghei, A.; Kremer, F.; Schick, C. Eur. Phys. J.-Spec. Top. 2007, 141, 153−160. (13) Schick, C. Eur. Phys. J.-Spec. Top. 2010, 189, 3−36. (14) Zhou, D.; Huth, H.; Gao, Y.; Xue, G.; Schick, C. Macromolecules 2008, 41, 7662−7666. (15) Gao, S. Y.; Koh, Y. P.; Simon, S. L. Macromolecules 2013, 46, 562−570. (16) Chen, J.; Xu, J.; Wang, X.; Zhou, D.; Sun, P.; Xue, G. Macromolecules 2013, 46, 7006−7011. (17) Xu, J.; Li, D. W.; Chen, J.; Din, L.; Wang, X. L.; Tao, F. F.; Xue, G. Macromolecules 2011, 44, 7445−7450. (18) Mark, H. F.; Tobolsky, A. V. Physical Chemistry of High Polymeric Systems, 2nd ed.; Interscience Publishers Incorporated: New York, 1950. (19) Porter, R. S.; Johnson, J. F. Chem. Rev. 1966, 66, 1−27. (20) de Gennes, P. G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1979. (21) Watanabe, H. Prog. Polym. Sci. 1999, 24, 1253−1403. (22) Ngai, K. L. Eur. Phys. J. E 2003, 12, 93−100. (23) Russell, T. P. Science 2013, 341, 1351−1352. (24) Forrest, J. A.; Mattsson, J. Phys. Rev. E 2000, 61, R53−R56. (25) Baumchen, O.; McGraw, J. D.; Forrest, J. A.; Dalnoki-Veress, K. Phys. Rev. Lett. 2012, 109, 055701. (26) Napolitano, S.; Rotella, C.; Wubbenhorst, M. ACS Macro Lett. 2012, 1, 1189−1193. (27) Nguyen, H. K.; Labardi, M.; Lucchesi, M.; Rolla, P.; Prevosto, D. Macromolecules 2013, 46, 555−561. (28) Boucher, V. M.; Cangialosi, D.; Yin, H. J.; Schonhals, A.; Alegria, A.; Colmenero, J. Soft Matter 2012, 8, 5119−5122. (29) Napolitano, S.; Lupascu, V.; Wubbenhorst, M. Macromolecules 2008, 41, 1061−1063. (30) Fakhraai, Z.; Forrest, J. A. Phys. Rev. Lett. 2005, 95, 025701. (31) Mark, J.; Ngai, K.; Graessley, W.; Mandelkern, L.; Samulski, E.; Koenig, J.; Wignall, G.; Cambridge University Press: Cambridge, U.K., 2003. (32) Jiang, W.; Zuo, C.; Hu, J.; Gu, Q.; Chen, W.; Xue, G. Macromolecules 2008, 41, 5356−5360. (33) Napolitano, S.; Pilleri, A.; Rolla, P.; Wubbenhorst, M. ACS Nano 2010, 4, 841−848. (34) Napolitano, S.; Cangialosi, D. Macromolecules 2013, 46, 8051− 8053. (35) Glynos, E.; Frieberg, B.; Oh, H.; Liu, M.; Gidley, D. W.; Green, P. F. Phys. Rev. Lett. 2011, 106, 128301. (36) Wang, S. F.; Yang, S.; Lee, J.; Akgun, B.; Wu, D. T.; Foster, M. D. Phys. Rev. Lett. 2013, 111, 068303.
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