Search

A study was carried out on the effects of thermal aging on specific volume V and specific enthalpy H of polystyrene below the glass transition temperature. The dependence of V and H on annealing time was similar. No significant effect of the distribution of molecular weight on the relaxation behavior was found. The behavior of V and H reaching a maximum after a double-step temperature-jump was also investigated. The time for the maximum V and H varied in proportion to the period of pre-annealing. The time for the maximum volume was slightly longer than that for the maximum enthalpy. These facts indicate that the dependence of V and H on the order parameters specifying the glassy state are similar but not exactly the same. From the enthalpy and volume relaxation curves, the enthalpy required for creating free volumes in polystyrene was calculated to be 2.0 kJ cm-3. The behavior of the volume relaxation was discussed by assuming a wide distribution of relaxation time.

The dielectric properties of poly(ethylene oxide) (PEO) were examined for 10 samples ranging in molecular weight, M, from 190 to 2×106. Two dielectric relaxations were observed. The one observed at high temperature and low frequency is called a process, and the other at low temperature and high frequency is called ß one. The ß process is due to the local motion of amorphous PEO segments, and has a loss maximum temperature, Tmax, and an apparent activation energy, Ea, independent of either M or degree of crystallinity, Xc. On the other hand, the a process, which is due to a large scale conformational rearrangement of amorphous PEO segments, is dependent on both M and Xc. The relaxation strength and Tmaxvary with Xc, while the Eaand the relaxation width do not depend on Xcbut exhibit an abrupt increase at M of about 104. These results may be due to change not only in Xcbut also in the crystalline morphology with M.

Relaxation of volume, enthalpy, and dielectric polarization have been studied for poly(vinyl acetate) and polystyrene around a glass-transition point. It has been concluded that the correlation time of microbrownian motion in glassy polymers shifts with time during the period of volume relaxation. The time dependence of dielectric relaxation time ?Dhas been represented approximately bylogtD=alogt-b where a and b denote constants. By application of this equation to the volume and enthalpy relaxation processes, the time dependence of the specific volume V(t) or enthalpy H(t) can be represented in the form V(t)-V(8)=ct-Bwhere B and c are constants. The values of B for the volume relaxation agree well with those for the enthalpy.

The influences of diluent on molecular motions and glass transition were studied on the system poly(methyl acrylate)—toluene by means of dielectric and thermal measurements over a wide concentration range. Three relaxation processes assigned respectively to the segmental motion of the polymer chain, rotation of the side group, and local motion were recognized between 80 and 300K with decreasing temperature. Two step-wise changes in DTA diagram were observed and can be assigned as glass transition points TgIand TgII. The magnitude of dielectric dispersion, activation plot, and glass transition temperature are discussed by comparing them with those of systems reported previously. Coalescence of the primary and secondary loss peaks was observed and are explained in terms of the correlation functions for motion of the dipole.

The dielectric normal mode process was investigated on semi-dilute and concentrated solutions of poly(2,6-dichloro-1,4-phenylene oxide)(PDCPO-M) in a good solvent, chlorobenzene, at 300 K. Four linear, short chain samples with molecular weight M less than 20×103, two nearly linear samples of intermediate M, and a high M, randomly branched sample (PDCPO-380) of M=380×103were used. The dielectric relaxation time t for 40 wt% solutions of the linear samples was approximately proportional to M2, indicating Rouse behavior of non-entangled chains. The loss curves of the PDCPO-17 solutions were symmetrical in shape, and the half width A increased slightly with increasing concentration C due presumably to the increase in the local friction constant ?. On the other hand, for the branched, high M PDCPO-380 sample, its loss curve not only became broader with increasing C, but split into two peaks at about C=30 wt%. This is presumably because the relaxation time of longer branches of the sample were retarded more strongly by entanglement. From the relaxation strength, the mean square end-to-end distance ‹r2› was determined as a function of C for PDCPO-17 solutions. The slope of the log‹r2› vs. logC plot was approximately -0.08 in the range of 0.2

The dielectric behavior of six-arm star-shaped cis-polyisoprenes (6S-PI) with the arm molecular weight Mpranging from 3.5×103to 21×103was compared with that of corresponding linear cis-polyisoprenes (L-PI) which were precursors for the arms. In the entangled regime, the normal mode relaxation time tnof 6S-PI was longer and its dependence on molecular weight was stronger for 6S-PIs than for L-PIs. However, an exponential increase of tnwith Mpwhich is characteristic of a highly entangled star chains was not clearly observed for the 6S-PIs examined, because Mpwas not large enough. In the non-entangled regime, the relaxation time tn?at an iso-friction state of 6S-PI was close to that of corresponding L-PI, as suggested by the Rouse-Ham theory. However, the relaxation mode distribution for 6S-PI was broader than that of L-PI. A model calculation using Rouse-Ham dynamics suggested that the distribution of arm length could possibly lead to a substantial broadening for the relaxation mode distribution.

Dielectric properties of solid films of poly(dichloro phosphazene) (PCPN), poly(diphenoxy phosphazene) (PPPN), and poly(difluoroethoxy phosphazene) (PFPN) were studied in the frequency range from 0.1 to 100 kHz. PCPN exhibited only one relaxation peak (named as a process), while PPPN and PFPN exhibited two loss maxima (a and ß) in the dielectric loss e? curve within the temperature range between 90 and 460 K. The a process for the each sample was seen just above its glass transition temperature Tg, while the ß relaxation was observed only for PPPN and PFPN in the region far below their Tg. This result indicates that the ß process is due to rotation of the side groups. Based on the simplest two site model, we estimated the amplitude of the rotational oscillation of the side groups and the energy difference between the two sites. Besides these a and ß relaxations, PPPN and PFPN showed the mesophase transition at 400 and 345 K, respectively, where the dielectric constant increased step-wise. The dielectric constant in the mesophase was partly attributable to the polarization of the type-A dipoles.

Dielectric studies were performed on acrylonitrile–butadiene random copolymer with intermolecular carbon–carbon cross-links by dicumyl peroxide. The effects of cross-linking and of stretching on the a-relaxation were examined on the polymer coated with evaporated-gold electrodes. With increasing cross-linking density the strength of dielectric relaxation decreased, the position of loss maximum shifted to a higher temperature, and the half width of the absorption curve increased. The apparent activation energy was virtually unchanged. On the other hand, the effect of stretching on the a-relaxation could not be observed in the range of the elongation ratio upto 3 examined in these experiments. These results may be interpreted as due to the decrease in free volume, especially in those regions close to the cross-links and hence, to the broadening of the distribution of free volume.

Dielectric and thermal behavior of toluene solutions of an isotactic-rich. and a syndiotactic-rich poly(methyl methacrylate) have been investigated over wide ranges of concentration and temperature to examine the effect of stereoregularity. Three dielectric relaxations designated as a, ß, and ? were observed in both systems. These processes can be assigned, respectively, to the process due to segmental motion of the polymer chains, rotation of toluene molecules, and secondary motion of toluene coupled with those of the polymer. For both systems, plots of loss maximum frequency versus reciprocal temperature for the a process conform to the Vogel–Tamman equation. The Vogel–Tamman parameter B for solutions of the syndiotactic polymer is larger than that for the isotactic polymer, while the other two parameters A and T0are nearly independent of stereoregularity. Differential thermal analysis curves for solutions with low to medium concentrations each exhibits a two-step change, which can be assigned to the two glass. transitions, TgIand TgII. The TgIfor the solutions of the syndiotactic polymer is higher than that of the isotactic polymer, while the TgIIis the same for both polymers.

The contributions of water occluded in polymeric media to dielectric relaxation were examined in detail on hydrophobic polymer specimens exposed to 18.7 torr humidity (78.6% relative humidity) at 25°C. The polymers examined were polyethylenes, oxidized polyethylenes, polystyrene, poly(styrene-co-acrylonitrile), poly(methyl methacrylate), and poly(vinyl acetate). In these polymers, except for the polyethylenes, the dielectric water relaxation peak usually appeared in a temperature range around 200 K (at 1 kHz) with an activation energy of about 40 to 50 kJ mol-1. The magnitude of dielectric dispersion due to water was calculated by the Onsager equation, assuming that occluded water molecules can rotate freely. The calculated values are about twice as large as the observed values for each system. This implies that the motion of water molecules is subject to certain hindrances from the polymeric media. In polyethylenes, especially in oxidized polyethylenes, the behavior of water molecules was markedly different from that in other polymers. Low- and medium-density polyethylenes absorbed virtually no water, while high-density polyethylene and oxidized polyethylenes absorbed a little. These polyethylenes exhibited two loss peaks at about 230 and 270K with the activation energies of about 120 and 316 kJ mol-1, respectively.

Heat evolution from and mechanical work done on films of a styrene–butadiene–styrene (SBS) triblock copolymer were investigated under tensile strain. The relationship between microphase structure and thermomechanical behavior was examined on SBS films having lamellar, cylindrical, and spherical morphology which were cast from solutions in three mixed solvents. For SBS films having lamellar and cylindrical morphology, the enthalpy changes due to the plastic-to-rubber transition were found to be 0.24 and 0.17J cm-3, respectively. These values correspond to the enthalpy required for irreversible destruction of the polystyrene domains. On the other hand, an SBS film having spherical polystyrene domains exhibited behavior similar to that of a vulcanized rubber.