Ryusuke Nozaki

Dielectric relaxation measurements of poly(vinyl acetate) in glassy state in the frequency range 10−6–106 Hz

Dielectric relaxation measurements covering a wide frequency range extending from 10−6 to 106 Hz were made on poly(vinyl acetate) with conventional glass transition temperature Tg of 31 °C at temperatures between 26.85 and 84.77 °C. It was found that temperature dependence of frequency of the maximum dielectric loss below Tg cannot be described by the Williams–Landel–Ferry equation which gives a complete explanation to the dependence in the temperature range sufficiently higher than Tg, but by a simple expression of Arrhenius type with activation energy 138 kcal/mol. It was also found that distribution of the relaxation times changes abruptly in the vicinity of Tg. Potential barrier height for the chain motion has a distribution to some extent below Tg.

Dielectric relaxation of poly(vinyl acetate)

Dielectric relaxation measurements of undiluted and diluted poly(vinyl acetate) PVAc have been made at temperatures 288<T<366 K over an extremely wide frequency range from 1 μHz to 150 MHz. The complex permittivity can be described quantitatively by the Havriliak–Negami equation e*=e∞+Δe[1+(jωτ0)β]−α, 0<α, β≤1. It has been found that the parameter α is given by a linear equation of dipole moment ratio g as α=1.09–0.91g. This observation suggests that α is connected closely with local chain conformations. On the other hand, β is nearly constant ∼0.87 above the glass transition temperature and is independent of effect of the diluent. This suggests that the constant value of β is an inherent characteristic of the relaxation spectrum of PVAc.

Broadband dielectric relaxation study in a ferroelectric liquid crystal by time domain reflectometry

Abstract Complex permittivity of a ferroelectric liquid crystal, 4-[(S,S)-2,3-epoxyhexyloxy]-phenyl 4-(decyloxy)-benzoate, has been measured in the frequency range between 10MHz and 4GHz, at temperatures from 112°C down to 53°C by using a time domain reflectometry. Spectral decomposition of the experimental data showed that two types of relaxation are involved. The faster relaxations are active in the ferroelectric phase whereas the slower ones are very much hindered. It can be concluded that this hindered relaxation may correspond to the slowing down of the molecular motion around the long axis, which has been considered as leading to a resultant macroscopic polarization.

Evaluation of dielectric relaxation spectrum of phospholipids in solution by time domain reflectometry

Time domain reflectometry (TDR) was applied to dielectric relaxation measurements and relaxation spectra of four kinds of phospholipids (dipalmitoyl phosphatidylcholine, egg lecithin, phosphatidyl ethanolamine, and sphingomyelin) and were evaluated in chloroform solutions. Fast measurements covering a wide frequency range from 1 MHz to 10 GHz could be done easily for materials having multiple relaxations and dc conductivity. A procedure of extracting individual spectra from the complicated complex permittivity has been presented. Reasonable assignment of molecular motion could be given to each spectrum. The first spectrum observed at about 1.5 ns is caused by an overall rotation of the whole molecule in the solution. The second spectrum at 0.2–0.4 ns is related to a motion of the zwitterionic head group. The third spectrum observed in lecithin at 50–80 ps is mainly caused by internal motions of the ester groups. It is concluded that the TDR method is quite suitable for the investigation of molecular dynam...

Dielectric permittivity measurements in the ultralow frequency region

A new dc transient method is described for fast measurements of dielectric permittivity in the μHz frequency region. A square‐wave voltage pulse is applied to the dielectric instead of the usual step voltage pulse. The whole transient current is measured two times faster than in the conventional method using the step voltage.

Evaluation of dielectric permittivity by dc transient current

Inverse Laplace transformation of the Havriliak–Negami (HN) equation of complex dielectric permittivity has been obtained analytically both in short time and long time regions. A convenient procedure for determining the relaxation parameters in the HN equation from the dc transient current has been presented. Even if the current measurement is not carried out over the whole time range but only in a restricted time range, this procedure gives satisfactory values to the relaxation parameters.

Dielectric relaxation of amorphous poly(propylene oxide)s at gigahertz frequencies

Abstract Dielectric relaxation properties of amorphous poly(propylene oxide)s with nominal molecular weights of 1000 and 2000 have been studied as a function of temperature at gigahertz frequencies using a time-domain reflectometry method. Two relaxation processes were observed in the dielectric spectra. The dielectric relaxation parameters were quantitatively estimated by the Havriliak-Negami equation. At the lower frequency side of the main dispersion, a normal-mode relaxation was seen and the relaxation times obtained from the spectra were found to be approximately one decade longer than the values calculated assuming the Rouse-Zimm theory. The hydrogen-bonded structures in the system are discussed. The relaxation strength is also discussed in terms of chain conformation. The relaxation times of the higher frequency process were found to be independent of the molecular weight. The shortest possible relaxation times estimated by the Vogel-Fucher-Tamman equation were in good agreement with those reported before.

Relaxation of isolated dipoles in polymer chain

Abstract High-precision dielectric measurements were performed on poly(4-chlorostyrene, 4-methylstyrene) copolymers with various mole fractions, x, of 4CA in benzene solutions in a frequency range 1–150 MHz at temperatures between 279–318 K. Dielectric dispersion extrapolated to x = 0 reflects the orientation of isolated dipoles of 4CS units in the non-polar polymer chain. The dispersion and absorption calculated by the Kohlrausch-Williams-Watts (KWW) response function fit the data well. The autocorrelation function in the polymer chain is thus given directly by the KWW function with βk = 0.58.

Temperature coefficient of the dipole moment of poly(4‐chlorostyrene, 4‐methylstyrene) copolymers in benzene solutions

Mean square values of the dipole moments of poly(4‐chlorostyrene) and copolymers of poly(4‐chlorostyrene, 4‐methylstyrene) have been determined at up to five different temperatures. There is a significant positive temperature coefficient of the mean square dipole moment. Curves of the dipole moments and of the slopes, normalized to unity at P4CS, have essentially the same shapes. The copolymers in benzene solutions lead to values of the mean square dipole moments that are about 20% larger than measurements in p‐xylene.