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Mobility Gradient of Polystyrene in Films Supported on Solid Substrates, by Yoshihisa Fujii, Hiroshi Morita, Atsushi Takahara and Keiji Tanaka Probing Properties of Polymers in Thin Films Via Dewetting, by Günter Reiter Heterogeneous and Aging Dynamics in Single and Stacked Thin Polymer Films, by Koji Fukao, Takehide Terasawa, Kenji Nakamura, Daisuke Tahara Heterogeneous Dynamics of Polymer Thin Films as Studied by Neutron Scattering, by Rintaro Inoue and Toshiji Kanaya



Mobility Gradient of Polystyrene in Films Supported on Solid Substrates

In this review, we show the distribution of glass transition temperature (T g) in monodisperse polystyrene (PS) films coated on silicon oxide layers along the direction normal to the surface. Scanning force microscopy with a lateral force mode revealed that surface T g (\( T_{\rm{g}}^{\rm{s}} \)) was lower than the corresponding bulk T g (\( T_{\rm{g}}^{\rm{b}} \)). Interestingly, the glass transition dynamics at the surface was better expressed by an Arrhenius equation than by a Vogel–Fulcher–Tamman equation. Interdiffusion experiments for PS bilayers at various temperatures, above \( T_{\rm{g}}^{\rm{s}} \) and below \( T_{\rm{g}}^{\rm{b}} \), enabled us to gain direct access to the mobility gradient in the surface region. T g at the solid substrate was examined by fluorescence lifetime measurements using evanescent wave excitation. The interfacial T g was higher than the corresponding \( T_{\rm{g}}^{\rm{b}} \). The extent of the elevation was a function of the distance from the substrate and the interfacial energy. The T g both at the surface and interface was also studied by the coarse-grained molecular dynamics simulation. The results were in good accordance with the experimental results. Finally, dynamic mechanical analysis for PS in thin and ultrathin films was made. The relaxation time for the segmental motion became broader towards the faster and slower sides, due probably to the surface and interfacial mobility.
Graphical Abstract
Yoshihisa Fujii, Hiroshi Morita, Atsushi Takahara, Keiji Tanaka

Probing Properties of Polymers in Thin Films Via Dewetting

In our quest to make functional devices smaller, the thickness of polymer films has reached values even smaller than the diameter of the unperturbed molecule. Many experimental studies have been devoted to the determination of the behavior of such thin films as a function of film thickness and temperature. However, despite enormous efforts over the last few decades, our understanding of the origin of some puzzling properties of such thin films is still not satisfactory and several peculiar observations remain rather mysterious. In this context, we explore the consequences of film preparation, i.e., the transition from a dilute polymer solution to the glassy state, with respect to the properties of polymers in thin films. This transition is likely to result in residual stresses arising from out-of-equilibrium chain conformations due to rapid solvent loss. Consequently, depending on thermal history and ageing time, such films exhibit significant changes even in the glassy state, which we can quantify by performing detailed studies of visco-elastic dewetting of thin polystyrene films on solid substrates. We explore relaxation times, residual stresses, and temporal changes to the stability of non-equilibrated thin films as they progress toward stable equilibrium behaviors. We present some tentative ideas on the relation between the observed atypical mechanical and relaxational behavior and the metastable states introduced by sample preparation.
Graphical Abstract
3D view (measured by atomic force microscopy) of a typical hole obtained by dewetting a polystyrene film on a PDMS-coated substrate at temperatures close to the glass transition of polystyrene.
h(x,t) is the profile of the film, h 0 is the initial height of the film, H(t) is the height of the front, L(t) is the dewetted distance, W(t) is the width of the rim, and v(x,t) is the velocity of the film. Typical 3D cross-sections show the asymmetric shape of the rim at early stages and comparatively low temperatures (left) and the more symmetric shape at late stages or at high temperatures (right).
Günter Reiter

Heterogeneous and Aging Dynamics in Single and Stacked Thin Polymer Films

The glass transition and the α-process above the glass transition temperature, T g, and the aging dynamics below T g were investigated for single and stacked thin polymer films. First, the glass transition dynamics of stacked thin films of polystyrene (PS) and poly(2-chlorostyrene) (P2CS) were measured using differential scanning calorimetry and dielectric relaxation spectroscopy. The T g for as-stacked thin PS films is much less than that of the bulk sample. However, after annealing at high temperatures above T g, the stacked thin films exhibit glass transition at a temperature almost equal to the T g of the bulk system. The α-process dynamics of stacked thin films of P2CS show a very slow time evolution from single thin film-like dynamics to bulk-like dynamics during the isothermal annealing process. The temperature dependence of the relaxation time for the α-process changes from Arrhenius-like to a Vogel–Fulcher–Tammann dependence with an increase in annealing time. Secondly, the aging dynamics of P2CS ultrathin films with thicknesses less than 10 nm were investigated using dielectric relaxation spectroscopy. The imaginary part of the dielectric susceptibility, ε″, for P2CS ultrathin films with a thickness of 3.7 nm increases with an increase in isothermal aging time, but this is not the case for P2CS thin films thicker than 9.0 nm. This anomalous increase in ε″ for the ultrathin films is strongly correlated with the presence of a mobile liquid-like layer within the thin films.
Graphical Abstract
Koji Fukao, Takehide Terasawa, Kenji Nakamura, Daisuke Tahara

Heterogeneous Dynamics of Polymer Thin Films as Studied by Neutron Scattering

This review covers recent progress in studies on the dynamics of polymer thin films in the glassy state and near the glass transition temperature, as revealed by neutron scattering. First, the glassy dynamics including low energy excitation (Boson peak) and the fast localized process of polymer thin films at the picosecond timescale was studied to reveal the effect of film thickness. The dynamic heterogeneity of polymer thin films in the glassy state was also evaluated in terms of non-Gaussian parameters. Second, the glass transition temperature of polymer thin films was investigated using inelastic neutron scattering with high-energy resolution to clarify the mechanism of its dependence on film thickness. Finally, neutron reflectivity was used to study the distribution of glass transition temperature in a multilayered thin film consisting of deuterated polymer and hydrogenated polymer.
Graphical Abstract
Rintaro Inoue, Toshiji Kanaya


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