Osamu Urakawa

Dielectric study of concentration fluctuations in concentrated polymer solutions

Abstract Dielectric relaxation of the segmental mode in concentrated solutions of poly(vinyl acetate) (PVAc) in toluene (Tol) and in 1-methylnaphthalene (MN) and those of poly(vinyl octanoate) (PVOc) in Tol were studied. It was found that the distributions of relaxation times in concentrated solutions were much broader than those in dilute solution and in the undiluted bulk polymer. The broad spectra were ascribed to local heterogeneities. The half width Λ increased with decreasing temperature and this behavior was explained by assuming that Λ is proportional to the amplitude of the local concentration fluctuation Δφ times the slope of the φ dependence curve of the average relaxation time τ . This assumption leads to a linear relationship between Λ and Δφ /( T − T 0 ) 2 where T 0 is the Vogel critical temperature. Using this relation, we attempted to estimate Δφ .

Dielectric study on dynamics and conformation of poly(d,l-lactic acid) in dilute and semi-dilute solutions

Abstract Fractionated samples of d , l -poly(lactic acid) (PLA) were prepared and the dielectric normal mode relaxation was studied for dilute and semi-dilute solutions of the PLA in a good solvent benzene. Results indicate that in the dilute regime the normal mode relaxation time is proportional to [η]Mw in agreement with the Rouse–Zimm theory, where [η] and Mw denote the intrinsic viscosity and weight average molecular weight, respectively. The dielectric relaxation strength which is proportional to the mean square end-to-end distance 〈r2〉 increases with increasing Mw with the power of 2ν, where ν is the excluded volume parameter determined from [η]. The relaxation time in the semi-dilute regime increases with increasing concentration C due to increases of the entanglement density and the friction coefficient. The relaxation time corrected to the iso-friction state agrees approximately with the dynamic scaling theories. The relaxation strength decreases with increasing concentration indicating that 〈r2〉 decreases on account of the screening of the excluded volume effect. The concentration dependence of 〈r2〉 agrees approximately with the scaling theory proposed by Daoud and Jannink.

Dielectric relaxation of guest molecules in a clathrate structure of syndiotactic polystyrene.

Structure and dynamics of semicrystalline polymer films composed of syndiotactic polystyrene (sPS) and 2-butanone were examined through X-ray diffraction, polarized FTIR, and dielectric relaxation measurements. The X-ray and FTIR measurements revealed its crystal structure to be δ-clathrate containing 2-butanone molecules inside. The carbonyl group of 2-butanone in the crystal was found to orient preferentially parallel to the ac plane of the crystal through the polarized ATR FTIR measurements. Dielectric measurements were also conducted on these film samples to see only the relaxation dynamics of 2-butanone thanks to the high dielectric intensity of 2-butanone compared to sPS. Two relaxation modes denoted by slow and fast modes appeared. The former was assigned to the motion of 2-butanone molecules entrapped in the cavities of the crystalline (δ-form) and the latter to those in the amorphous region. We focused on the slow mode in order to elucidate the specific dynamics of the guest molecule confined in the crystalline region. The relaxation time of the slow mode was about 4 orders of magnitude longer than that of liquid 2-butanone. This suggests that the dynamics of guest molecules is highly restricted due to the high barrier to conformational and/or orientational change of the guest molecule in the cavity of δ-crystal. Furthermore, the dielectric intensity Δε of the slow mode was much smaller than the one calculated from that of bulk liquid 2-butanone and the guest concentration in the crystalline region (the intensity was only 10% of the estimated value from the bulk liquid data). This result also indicates that the free rotational motion of 2-butanone molecules is restricted inside the crystal. This will be consistently related to the weak uniplanar orientation of the carbonyl group of 2-butanone parallel to the ac plane revealed by the X-ray and polarized ATR FTIR measurements.

Self‐diffusion and Spatially Heterogeneous Dynamics in Supercooled Liquids Near Tg

In spite of the fundamental and practical importance of the self‐diffusion coefficient, values of this quantity are not generally available near the glass transition temperature Tg. Here we describe experimental results for three different single component glass‐forming systems. In tris‐naphthylbenzene, the self‐diffusion coefficient at Tg is 400 times larger than the value anticipated by the Stokes‐Einstein equation. Preliminary results on ortho‐terphenyl indicate similarly enhanced translational diffusion near Tg. For oligomers of polystyrene (20‐mers), the temperature dependence of the terminal relaxation times completely tracks that of the self‐diffusion coefficient.

Component Dynamics in Polystyrene/4‐Pentyl‐4′‐Cyanobiphenyl Blend

We studied the dielectric and viscoelastic relaxation behaviors of polystyrene (PS)/4‐pentyl‐4′‐cyanobiphenyl (5CB) miscible blend, and examined the relationship between segmental motion of PS and rotational motion of 5CB in the blend. Viscoelastic measurements could detect the global motion of PS‐chain, and the longest relaxation time τL of PS was determined. On the other hand, 5CB motion was detected by dielectric measurement, because of much larger electric dipole of cyano group in 5CB than that of PS. The dielectric spectra of the blends were bimodal, indicating the existence of two distinct dynamical modes related to the 5CB motion. The relaxation times of these two modes are named τslow and τfast. Temperature dependences of τslow and τL were found to be almost the same, and τslow was close to the segmental relaxation time τseg of PS estimated from the τL values. This means that these two motions, PS‐segmental motion and slow mode of 5CB are cooperative. On the other hand, temperature dependence of τ...

Relationship between global and segmental dynamics of poly(butylene oxide) studied by broadband dielectric spectroscopy

Relationship between segmental relaxation and normal-mode relaxation has been studied for molten poly(butylene oxide)s having various molecular weights by broadband dielectric spectroscopy over a wide temperature (T) range. We found that T dependence of the segmental relaxation time, τs, was weaker than the normal mode time, τn, at high T(>250 K ∼ Tg + 50 K), and the τn/τs ratio systematically decreased with increasing temperature. This high temperature complexity, whose mechanism has not been discussed in detail so far, was quantitatively explained by assuming the two step processes: local conformation change of polymers (elementary process) occurs first, and then the motion of a segment unit (second process) occurs. It was also found that the elementary process was strongly correlated with the experimentally observed β-relaxation.