Glass transition in ultrathin polymer films: A thermal expansion study

Bhattacharya, M.; Sanyal, M. K.; Geue, Th.; Pietsch, U.

doi:10.1103/physreve.71.041801

Cited by 38 publications

(33 citation statements)

References 31 publications

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“…15 Contrary to the thicker films and bulk samples, a melt of ultrathin films (h 0 ¼B6 and 4 nm, that is h 0 o2R g ) measured at a heating rate of 0.01 1C min À1 are 5.6Â10 À3 1C À1 (inset of Figure 6d) and 9.8Â10 À3 1C À1 (inset of Figure 6f; not shown in Table 1), indicating much larger values than those of bulk samples. Such a large a melt of ultrathin PS films is also reported in another XR study collected at conventional heating rate, 27 where the maximum a melt was estimated to be 5.6Â10 À3 1C À1 . In an ultraslow cooling measurement recorded at the rate of 0.01 1C min À1 , the values of a glass and a melt of a 6-nm-thick film are obtained as 1.2Â10 À4 and 3.7Â10 À4 1C À1 , respectively; these expansivities are in striking contrast to those of the film, with corresponding thickness measured at the ultraslow heating, although there is no significant difference between the width w at ultraslow heating (w¼6.0±1.0 1C) and that at ultraslow cooling (w¼4.5 ± 1.0 1C).…”

Section: Discussionmentioning

confidence: 80%

“…The value h c (q) corresponds to a critical thickness below which the narrowing or broadening of the glass transition occurs. For a 36-nm-thick PS film, the difference between T g in the interfacial layer and T g in the surface layer was estimated to be 11 1C, 8 Because the previously estimated values of h c for films with different molecular weights demonstrating a broadening of the glass transition is reported to be on the order of 20-30 nm, [15][16][17]27,28 which is totally consistent with the values obtained in this study, there is a possibility that equation (2) still holds for glassy polymer films that show a narrowing of the glass transition. The w of the PS film with M w ¼8.35Â10 5 g mol À1 measured at a cooling rate of 0.01 1C min À1 ('Â') may indicate a more pronounced narrowing behavior that certainly deviates from the solid line fitted by the data of M w ¼5.2Â10 4 g mol À1 films obtained at the same rate.…”

Section: Resultsmentioning

confidence: 99%

“…In previous studies, 17,27,29 w is defined as T + ÀT À . Although we already obtained w from the fit to the data by adopting equation (1), we separately determined T + and T À using h(T) curves as the Broadening, no broadening and narrowing of glass transition C Yang and I Takahashi temperatures at which the fitted curve of equation (1) deviates from linear fits in rubbery state or glassy state by 2%; the linear fits and T + and T À are indicated by dashed lines and arrows, respectively, in Figures 2-6 (arrows are shown only in Figure 6).…”

Section: Discussionmentioning

confidence: 99%

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Broadening, no broadening and narrowing of glass transition of supported polystyrene ultrathin films emerging under ultraslow temperature variations

Yang

Takahashi

2011

Polym J

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We report X-ray reflectivity measurements of polystyrene thin films supported on Si substrates at various heating and cooling rates. At a heating rate of 0.05 1C min À1 , the width of the glass transition w does not show any noticeable thickness dependence. However, at faster (0.5 1C min À1 ) and slower (0.01 1C min À1 ) heating rates, w shows remarkable thickness dependence: it broadens at a faster rate and narrows at a slower rate with decreasing film thickness. Our results suggest that the heterogeneous character of the dynamics can be transformed into a homogeneous one in a glassy film with thickness comparable to a critical thickness that depends on the rate of temperature variation. Polymer Journal (2011) 43, 390-397; doi:10.1038/pj.2010.145; published online 19 January 2011Keywords: confinement effects; glass transition; polymer thin films; transition breadth; transition broadening; X-ray reflection INTRODUCTIONThe molecular dynamics in liquid, especially close to and below the glass transition temperature T g , are very heterogeneous. 1 From a theoretical viewpoint in which the mean structural relaxation in glassy systems occurs through rearrangements of correlated particles, the spatial particle region surrounded by other subsystems composed of particles with differing mobilities is called a cooperatively rearranging region. 2,3 Because of the dynamic heterogeneity, temporal relaxations depend on the subsystems, and the glass transition generally occurs within a certain temperature range called the width of glass transition w. Polymers confined in a particular size of cooperatively rearranging region can be good candidates for investigating the spatial size of dynamic heterogeneities. A striking result in confined systems is that T g of thin films often exhibits a remarkable thickness dependence. 4,5 This result strongly suggests that interfaces in a confined system have an important role. Many studies have revealed the properties of interfacial regions and have provided extensive evidence that the molecular mobility is much higher in the free surface region than in the bulk region at the same temperature; 6-9 dynamics near the free surface are very fast. On the other hand, the interaction between a polymer and a solid substrate can suppress molecular relaxation in an interfacial region, leading to peculiar slower dynamics in this region. [9][10][11][12][13] Measurements of the relaxation rate using a 20-nm-thick fluorescently labeled layer that has been incorporated into polymer films have revealed that the rate of dynamics depends on the distance from both the free surface and the interface. These dynamics vary continuously, exhibiting bulk-like characteristics toward the interior layer sandwiched between the surface and the interfacial layers. 9

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Section: Discussionmentioning

confidence: 80%

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Broadening, no broadening and narrowing of glass transition of supported polystyrene ultrathin films emerging under ultraslow temperature variations

Yang

Takahashi

2011

Polym J

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“…Whereas the segment between A and B 0 can be regarded as a bridge-like configuration across the film (bridge AB 0 ). 19,20 In Figure 5, several dash dotted lines parallel to the two interfaces divide the film into several pseudolayers where the local T g 's in each pseudolayer increases with increasing distance from the free surface as in a pseudolayer model proposed in ref 21. During the sliding motion considered to be dominant in ultrathin films, source of the enhanced driving force for the sliding motion may be in the loop AB embedded in the unfrozen pseudolayers at a temperature the inner pseudolayers are still frozen.…”

Section: Resultsmentioning

confidence: 99%

Thickness Anomalies in Supported Polystyrene Films with Thicknesses Comparable to the Radius of Gyration

2009

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We report on X-ray reflectivity of ultrathin polystyrene films with thickness h, which is comparable to the radius of gyration R g . Tiny jumps in thickness, accompanying an increase in surface roughness and decrease in electron density, are reproductively observed at around 80 C and 92 C only with increasing temperature. For films thinner than R g , the jump at 92 C disappears. However, films of 2R g thick, the jump at 80 C is hardly observed with a faint trace in slope at this temperature. For much thicker films, neither jump appears in heating experiments. The thickness dependent complex behavior observed in non-equilibrium condition is considered to be some indication of surface and confined effects, because the characteristic film thicknesses coincide with the order of R g .KEY WORDS: Ultrathin Polystyrene Film / Glass Transition / Thickness Jump / X-Ray Reflectivity / Physical properties of systems are strongly affected by symmetry, dimensionality, and the number of constituent particles. Part of such dependences comprises an interesting field of physics, widely known as confined effects. Numerous experimental and theoretical studies have reported many intriguing confined effects on the glass transition of polymeric materials, and therefore glass-forming polymers are recognized to be an excellent example of such systems. As reproducible experimental results are easily obtained, polystyrene (PS) thin films with atomically flat surfaces have been used as standard glass-forming polymer films. For substrate-supported PS films, a large reduction in the glass transition temperature (T g ) dependent on the film thickness is observed for films less than 40 nm thick.1,2 More interestingly, the so-called free-standing PS films showed T g reduction much larger than those of supported films.3,4 Since the free-standing PS films are free from the interaction between the solid substrates, the strong reduction in T g is considered to reflect enhanced mobility inherent in the surface region exposed to air. The positron annihilation technique was used to estimate the free volume in the surface region, and indicated that the effective T g of PS was T g (bulk)-25 C and T g (bulk)-43 C for the surface region to a depth of 5 nm and 2 nm, respectively.5,6 Scanning viscoelasticity microscopy 7 showed enhanced mobility on the PS surface, and subsequent atomic force microscopy measurements confirmed a soft surface region 3-4 nm thick with low T g . 8 Regarding thermodynamical considerations, concentration of the ends of molecules in the surface region was pointed out, 9 suggesting a decrease in polymer density on the surface, subsequently confirmed by simulations. 10,11 Since the reduced density naturally leads to an increase in the free-volume and enhanced mobility of chain segments, several experimental studies on the surface glass transition of PS successfully employed a two-layer 1 or three-layer 5 model where the surface layer is characterized by constant thickness and peculiar physical properties (e.g., density, and viscosity...

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“…However, it is worth to note that the change of the specific heat capacity in the glassy and liquid state might also be explained by a three layer model which also applies here. 81 To differentiate between both possibilities, additional investigations on a broader range of samples and different polymers are required. Such studies are under preparation, which will also including a more quantitative discussion.…”

Section: Derivative Analysis Of Specific Heat Spectroscopy Datamentioning

confidence: 99%

Calorimetric evidence for a mobile surface layer in ultrathin polymeric films: poly(2-vinyl pyridine)

et al. 2015

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Specific heat spectroscopy was used to study the dynamic glass transition of ultrathin poly(2-vinyl pyridine) films (thicknesses: 405-10 nm). The amplitude and the phase angle of the differential voltage were obtained as a measure of the complex heat capacity. In a traditional data analysis, the dynamic glass transition temperature T g is estimated from the phase angle. These data showed no thickness dependency on T g down to 22 nm (error of the measurement of AE3 K). A derivative-based method was established, evidencing a decrease in T g with decreasing thickness up to 7 K, which can be explained by a surface layer. For ultrathin films, data showed broadening at the lower temperature side of the spectra, supporting the existence of a surface layer. Finally, temperature dependence of the heat capacity in the glassy and liquid states changes with film thickness, which can be considered as a confinement effect.

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Glass transition in ultrathin polymer films: A thermal expansion study

Cited by 38 publications

References 31 publications

Broadening, no broadening and narrowing of glass transition of supported polystyrene ultrathin films emerging under ultraslow temperature variations

Broadening, no broadening and narrowing of glass transition of supported polystyrene ultrathin films emerging under ultraslow temperature variations

Thickness Anomalies in Supported Polystyrene Films with Thicknesses Comparable to the Radius of Gyration

Calorimetric evidence for a mobile surface layer in ultrathin polymeric films: poly(2-vinyl pyridine)

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