Koji Fukao

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.

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.

Glass transition and α-relaxation dynamics of thin films of labeled polystyrene

The glass transition temperature and relaxation dynamics of the segmental motions of thin films of polystyrene labeled with a dye, 4-[N-ethyl-N-(hydroxyethyl)]amino-4-nitroazobenzene (Disperse Red 1, DR1) are investigated using dielectric measurements. The dielectric relaxation strength of the DR1-labeled polystyrene is approximately 65 times larger than that of the unlabeled polystyrene above the glass transition, while there is almost no difference between them below the glass transition. The glass transition temperature of the DR1-labeled polystyrene can be determined as a crossover temperature at which the temperature coefficient of the electric capacitance changes from the value of the glassy state to that of the liquid state. The glass transition temperature of the DR1-labeled polystyrene decreases with decreasing film thickness in a reasonably similar manner to that of the unlabeled polystyrene thin films. The dielectric relaxation spectrum of the DR1-labeled polystyrene is also investigated. As the thickness decreases, the alpha -relaxation time becomes smaller, and the distribution of the alpha -relaxation times becomes broader. These results show that thin films of DR1-labeled polystyrene are a suitable system for investigating confinement effects of the glass transition dynamics using dielectric relaxation spectroscopy.

Evidence for the molecular-scale origin of the suppression of physical ageing in confined polymer: fluorescence and dielectric spectroscopy studies of polymer-silica nanocomposites

Fluorescence spectroscopy was used to characterize the rate of physical ageing at room temperature in nanocomposites of silica (10?15?nm diameter) nanoparticles in poly(methyl methacrylate) (PMMA). The physical ageing rate was reduced by more than a factor of 20 in 0.4?vol% silica?PMMA nanocomposites relative to neat PMMA. The molecular-scale origin of this nearly complete arresting of physical ageing was investigated with dielectric spectroscopy. The strength of the ? relaxation process was reduced by nearly 50% in the nanocomposite relative to neat PMMA. This reduced strength of the ? process results from dipoles (ester groups) having hindered motions or being virtually immobile on the timescale being probed at a frequency of 100?Hz. This hindered mobility results from hydrogen bonding between PMMA ester side groups and hydroxyl units on the surface of the silica nanoparticles. In contrast, no reduction in physical ageing rate was observed upon addition of silica to polystyrene, which cannot form hydrogen bonds with the silica surfaces. Thus, the molecular origin of the suppressed physical ageing in silica?PMMA nanocomposites is the interfacial hydrogen bonding, which leads to a major reduction in the strength of the ? process, i.e., the ? process is largely responsible for the observed physical ageing.

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.

New Evaluation Method of Evaporated Organic Thin Films by Energy Dispersive X-Ray Diffractometer

A new method was developed to obtain clear diffraction patterns of organic thin films evaporated on substrates such as Au. In this method, total reflection on substrates was utilized to most efficiently collect the diffracted X-rays from organic thin films. The X-rays were detected with a pure Ge solid-state detector set at a constant scattering angle and analyzed with a multichannel analyzer. Excellent performance of this system was demonstrated for a thin stearic acid film evaporated on Au and Cu-phthalocyanine on glass.

Molecular dynamics studies on local ordering in amorphous polyethylene

A molecular dynamics (MD) simulation was performed on a linear poly(ethylene) model under bulk amorphous conditions. A molecule was composed of united atoms (CH2 and CH3 unit), and the pressure of the MD cell was controlled to be 1 atm. The system was stepwise cooled from 600 to 120 K. At each temperature 2 ns simulation was performed. Local structural ordering proceeded as the temperature was lowered. The mean length of the trans sequences increased with decreasing temperature according to the Arrhenius law above 300 K. The correlation volume abruptly increased between 300 and 330 K, which is the measure of parallel ordered structure of the model chain used by Rigby and Roe. Fluctuations of the density, the length of the trans sequences, and the correlation volume developed toward the maximum values between 300 and 330 K. Then the fluctuations decreased with further cooling and the glass transition occurred at 230 K (≡Tg). A cluster was observed below Tg, which consisted of many planar-zigzag segments or...

Relaxation dynamics in thin supported polymer films

Abstract The glass transition and relaxation dynamics of thin polymer films supported on a substrate have been investigated for three different polymers, atactic polystyrene, poly(vinyl acetate), and atactic poly(methyl methacrylate) by using dielectric relaxation and thermal expansion spectroscopy. The temperature, T α , at which the imaginary component of the susceptibility has a peak due to the α-process at a fixed frequency, is measured as a function of thickness and the molecular weight of the polymers. The observed thickness and molecular weight dependence of T α is discussed in comparison with that of T g observed in the freely standing films of polystyrene.