Yoshihisa Miyamoto

Glass transitions and dynamics in thin polymer films: Dielectric relaxation of thin films of polystyrene

The glass transition temperature T(g) and the temperature T(alpha) corresponding to the peak in the dielectric loss due to the alpha process have been simultaneously determined as functions of film thickness d through dielectric measurements for polystyrene thin films supported on glass substrate. The dielectric loss peaks have also been investigated as functions of frequency for a given temperature. A decrease in T(g) was observed with decreasing film thickness, while T(alpha) was found to remain almost constant for d>d(c) and to decrease drastically with decreasing d for d<d(c). Here, d(c) is a critical thickness dependent on molecular weight. The relaxation time tau(alpha) of the alpha process, which was measured as the frequency at which the dielectric loss realizes its peak value at a given temperature, was found to have a d dependence similar to that of T(alpha). The relaxation function for the alpha process was obtained by using the observed frequency dependence of the peak profile of the dielectric loss. The exponent beta(KWW), which was obtained from the relaxation functions, decreases as thickness decreases. This suggests that the distribution of relaxation times for the alpha process broadens with decreasing thickness. The thickness dependence of T(g) is directly related to the distribution of relaxation times for the alpha process, not to the relaxation time itself. The value of the thermal expansion coefficient normal to the film surface was found to increase with decreasing film thickness below T(g), but to decrease with decreasing film thickness above T(g). These experimental results are discussed in the context of a three-layer model in which within thin films there are three layers with different mobilities and glass transition temperatures.

Growth shape of isotactic polystyrene crystals in thin films

The crystal growth of isotactic polystyrene (it-PS) is investigated in very thin, 11 nm thick films. The it-PS crystals grown in the thin films show quite different morphology from that in the bulk. With decreasing crystallization temperature, the branching morphology in a diffusion field appears: dendrites and compact seaweed. The branching morphology is formed through a morphological instability caused by the gradient of film thickness around a crystal; the thicker the film thickness, the larger is the lateral growth rate of crystals. Regardless of the morphological change, the growth rate as well as the lamellar thickness depends on the crystallization temperature as predicted by the surface kinetics.

Glass transition temperature and dynamics of α-process in thin polymer films

The glass transition temperature Tg and the temperature Tα corresponding to the peak in the dielectric loss due to the α-process have been simultaneously determined as functions of the film thickness d through dielectric measurements for thin films of polystyrene. A decrease of Tg was observed with decreasing film thickness, while Tα was found to remain almost constant for d > dc and decrease drastically for d < dc. Here, dc is a critical thickness dependent on molecular weight. The thickness dependence of Tg is related to the distribution of the relaxation times of the α-process, not to the relaxation time itself.

Dynamics of alpha and beta processes in thin polymer films: poly(vinyl acetate) and poly(methyl methacrylate).

Dynamics of thin films of poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) have been investigated by dielectric relaxation spectroscopy in the frequency range from 0.1 Hz to 1 MHz at temperatures from 263 to 423 K. The alpha process, the key process of glass transition, is observed for thin films of PVAc and PMMA as a dielectric loss peak at a temperature T(alpha) in temperature domain with a fixed frequency. For PMMA, the beta process is also observed at a temperature T(beta). For PVAc, T(alpha) decreases gradually with decreasing thickness, and the thickness dependence of T(alpha) is almost independent of the molecular weight (Mw< or =2.4x10(5)). For PMMA, T(alpha) remains almost constant as thickness decreases down to a critical thickness dc, at which point it begins to decrease with decreasing thickness. Contrastingly, T(beta) decreases gradually as thickness decreases to dc, and below dc it decreases drastically. For both PVAc and PMMA, the broadening of the distribution of the relaxation times in thinner films is observed and this broadening is more pronounced for the alpha process than for the beta process. It is also observed that the relaxation strength is depressed as the thickness decreases for both the polymers.

Fracture energy of gels

Abstract:To clarify effects of crack speed and cross-link density on the fracture energy of acrylamide gels, we evaluated the roughness of the fracture surface and measured the fracture energy taking into account the roughness. The fracture energy increases linearly with crack speed Vin a fast crack speed region, and the increasing rate of fracture energy with V decreases with increasing cross-link density in the gels. In a slow crack speed region the fracture energy depends on crack speed more strongly than in the fast crack speed region. This indicates that a qualitative change exists in the fracture process of the gels.

Molecular dynamics simulation of polymer crystallization from an oriented amorphous state.

The molecular process of crystallization from an oriented amorphous state was reproduced by molecular dynamics simulation for a realistic polyethylene model. The initial oriented amorphous state was obtained by uniaxially drawing an isotropic glassy state at 100 K. By the temperature jump from 100 K to 330 K, there occurred crystallization into the fiber structure, during the process of which we observed the developments of various order parameters. The real-space image and its Fourier transform revealed that a hexagonally ordered domain was initially formed, and then a highly ordered crystalline state with stacked lamellas developed after further adjustment of the relative heights of the chains along their axes.

CRYSTAL GROWTH OF ISOTACTIC POLYSTYRENE IN ULTRATHIN FILMS: FILM THICKNESS DEPENDENCE

The film thickness dependence of crystal growth is investigated for isotactic polystyrene (it-PS) in thin films for thicknesses from 20 down to 4 nm. The single crystals of it-PS grown at 180°C in the ultrathin films show a morphology typical of diffusion-controlled growth: dense branching morphology and fractal seaweed. The characteristic length of the morphology, i.e., the width of the branch, increases with decreasing film thickness. The thickness dependence of the crystal growth rate shows a crossover around the lamellar thickness of 8 nm. The thickness dependences of the growth rate and morphology are discussed in terms of the diffusion of chain molecules in thin films.

Growth Rate of Isotactic Polystyrene Crystals in Thin Films

The lateral crystal growth rate of isotactic polystyrene lamellae developing in thin films (ranging from 20 nm to 500 nm) decreases with decreasing film thickness. The decrease in growth rate is inversely proportional to the film thickness: G ( D )= G (∞)(1- d / D ), where G ( D ) is the growth rate in a film of thickness D , G (∞) is the growth rate in the bulk, and d is a constant with a value of approximately 6 nm. Additionally, d is observed to be independent of the crystallization temperature; molecular weight and substrate material used in this study. It is proposed that the constant value of d corresponds to the tube diameter in the reptation model of polymer dynamics, and that near the substrate this diameter is reduced, thereby reducing the mobility of the molecules, and consequently the crystal growth rate.

The morphology of isotactic polystyrene crystals grown in thin films: the effect of substrate material

Thin isotactic polystyrene films (∼50 nm thick) have been crystallized from the melt on various substrate materials (carbon, glass, mica, polyimide sheet and silicon). The morphology of the crystals has been examined using atomic force microscopy, and was found to be dependent on the nature of the substrate, with two basic types of crystal forming. Crystals either develop around giant screw dislocations, or around small bundles of lamellae growing perpendicular to the substrate surface. It has further been observed that the number of screw dislocations generated in the lamellae is also dependent on the substrate, as is the growth rate of the spiral terraces. These effects are interpreted in terms of interactions between the molecules in the melt and the substrate surface.

Crystal Growth of Isotactic Polystyrene in Ultrathin Films: Thickness and Temperature Dependence

The growth rate and morphology of isotactic polystyrene crystals grown in ultrathin films have been examined experimentally in terms of the dependences both on the film thickness and on the crystallization temperature. We have found that the thickness dependence of growth rate, G, shows a crossover change when the film thickness becomes comparable with the lamellar thickness of the polymer crystals, irrespective of the temperatures. The morphology of crystals grown in ultrathin films shows a branching typical of dendrites, the growth of which is supposed to be controlled by a diffusion field. The change in the tip width of the dendrites with crystallization temperature follows the expected dependence of the Mullins–Sekerka stability length, ℓMS ∝ (D/G)1/2, determined by the diffusion coefficient, D, and the growth rate. The results confirm that a diffusion field plays an essential role in the evolution of the structure.