High-temperature relaxations

We can therefore conclude that differences in the structural relaxation between bead-spring and chemically realistic models can be attributed to either the differences in packing that we discussed above or the presence of barriers in the dihedral potential in atomistic models. To quantify the role of dihedral barriers in polymer melt dynamics, we now examine high-temperature relaxation in polymer melts. 

According to a dynamic mechanical study by Saba, Sauer, and Woodward (1963) for PPO and a dilatometric study by Daane and Barker (1964) for cellulose acetate, the high-temperature relaxations of PPO, CDA, and CTA are primary relaxations which arise from segmental motion in the amorphous phase and the low-temperature relaxations are presumably ascribed to a more local motion in the amorphous phase. 

The high-temperature relaxation process is typical for amorphous polymers and can be assigned to the a-relaxation that appears in the whole frequency range and in the temperature interval from 50 to 100°C. This process is well observed for all samples. It corresponds to the glass-rubber transition of the amorphous phase. 

High Temperature Relaxation The high temperature relaxation arises at 80 °C until the onset of degradation 210 °C (Figs. 2.13a and b). It can be well described by the Arrhenius model and is present in both neutralized and nonneutralized CS. The slope of these curve represent the activation energy of each process. The temperature dependence of dc conductivity and relaxation time are Arrhenius type. For this relaxation, nonneutralized CS films seem to be more sensitive to water, since in the dry state... 

Pizzoli et al. [85] by DMA and dielectric measurements observed a high temperature relaxation near 140 °C the calculated activation energy was 100 kJ/mol, and this value is in agreement with the activation energy of the... 

The initial published reports on high density polyethylene were dynamic mechanical studies, but before considering them it is necessary to compare the mechanical relaxations in isotropic material with those observed in unoriented low density polyethylene. From the schematic curve of tan S v. temperature f Fig. 7(b)] it can be seen that the p relaxation, which was ascribed to branch point mobility, is not present, and that the high temperature relaxation is frequently resolvable into a and a peaks. 

Men Y, Rieger J, Strobl G (2003b) Role of the entangled amorphous network in tensile deformation of semicrystalline polymers. Phys Rev Lett 91 955021-955024 Men Y, Strobl G (2002) Evidence for a mechanically active high temperature relaxation process in syndiotactic polypropylene. Polymer 43 2761-2768 Plazek DJ, Chay I, Ngai KL, Roland CM (1995) Visoelastic properties of polymers. 4. Thermo-rheological complexity of the softening dispersion in polyisobutylene. Macromolecules 28 6432-6436... 

Figures 10 and 11 show the TSDC curves at 60 °C polarization temperature of the EVA before and after exposure at CDER and URAER sites respectively. There are differences in relative magnitudes of the low and high temperature relaxation peaks. These results suggest that prolonged exposure selectively affects the poly (vinyl acetate) rich phase, with much less impact on the polyethylene rich phase. This is due to the progress of EVA crosslinking reaction such as temperature increase by long-term exposure. We found also, that the aged EVA after exposure showed considerable decrease in current intensity for the high temperature polarization due to secondary melting peaks as it will be revealed by DSC technique.

Some generalizations can be made about these types of relaxation behavior (Boyd 1984,1985). Inherently low crystallinity polymers show no crystalline high-temperature relaxation process (but do possess a well-developed amorphous fraction glass transition, in this case denoted a). Inherently easily crys-tallizable, high-crystallinity polymers show both and p relaxations, where P is the glass transition. However, in these materials the p process does not tend to be very prominent because the amorphous phase is the minor phase. All crystalline polymers show the low-temperature y process (referred to as P when a is the glass transition). 

Williamswho investigated the plasticization of polystyrene. When this dielectrically rather inert polymer is added to dibutyl phthalate, the single relaxation process of the pure ester is initially broadened, but at polystyrene concentrations above 40%, two broad relaxations develop. Analysis suggests that the low temperature process involves ester molecules which are relatively free to relax their dipoles in whole or in part, whilst the high temperature relaxation involves the cooperative motion of polymer chain and ester dipoles. This latter process thus being responsible for plasticization. 

Two or three relaxation processes occur in semicrystalline polymers. The low-temperature (y or P) process is a subglass process occurring in the amorphous phase. The medium or high temperature process (p or a,) is associated with the glass—rubber transition of the amorphous component. The glass transition is very weak, and in many cases difficult to find, in highly crystalline polymers like linear polyethylene. A certain class of polymers shows a high-temperature relaxation process denoted a, which is a combined crystalline and amorphous process. Reorientation of the chain by a 180° twist of the molecule in the crystals and a certain axial... 

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