Glass transition temperature, phase polymers

While the determination of the glass transition temperature of polymer stationary phases by inverse gas chromatt t lqf seems well-establiriied, several authors 11,15, 2 7) reported the failure of the technique to detect known transitions. In-... 

The data is recorded and can be plotted at the computer. From these DSC plots, thermal events such as melting points, phase change temperatures, chemical reaction temperatures and glass transition temperature of polymers can be determined. An endothermic peak is plotted in the upward direction and an exothermic peak in the downward direction... 

The temperature has a very complicated influence on the E-M performance of polymers. The Young s modulus of polymer decreases with the temperature, but the temperature dependence of its dielectric properties can be very different. For some of the piezoelectric and electrostrictive polymers, there is a phase transition at temperatures close to room temperature. At the phase transition temperature, the dielectric permittivity reaches its maximum and the polymer exhibits a high dielectric loss. Additionally, at temperatures around the glass transition temperature, all polymers exhibit some dielectric relaxation and elastic relaxation, resulting in a frequency dependence of material properties and a high dielectric and elastic loss. 

The effect of the flexibility of the spacer on the glass transition temperature and formation of the mesophase is illustrated in Table 6.3 on the example of a comparison of aliphatic and oxyethylene groups with the same length and identical mesogenic groups. For close values of the glass transition temperatures of polymers 1,2, and 3 and 4, the LC phase is not realized for the polymers with the more flexible spacers (polymers 1 and 3). In this case, the hypothetical Tol is apparently below the experimentally found 7g (cf. Fig. 6.4). However, if Tg is decreased even more (polymer 5) so that Tg is below 7 cl. then formation of the mesophase is possible. 

This process has been illustrated by Duda et al. for a variety of polymers [59] and Figine 13.12 shows the predictive abilities of their theory for the toluene-polystyrene system. Parameter values used are listed in Table 13.1. The results obtained are excellent. These authors also note that the diffusion coefficient given by Eq. (13.6.11) is insensitive to polymer molecular weight, and there is, therefore, no influence of polydispersity. Furthermore, for semicrystalline polymers above the glass transition temperature, the polymer may be considered to be made up of two phases—one of which has a zero diffusivity [51]. Thus, if the volume fiaction of the crystalline phase is 4>, the effective diffusivity of the polymer is D(j). Finally, Kulkami and Mashelkar [63] have proposed an altered fi-ee-volume-state model that seeks to provide a unified... 

In particular, blends of PVDF with a series of different polymers (polymethylmethacrylate [100-102], polyethylmethacrylate [101], polyvinyl acetate [101]), for suitable compositions, if quenched from the melt and then annealed above the glass transition temperature, yield the piezoelectric [3 form, rather than the normally obtained a form. The change in the location of the glass transition temperature due to the blending, which would produce changes in the nucleation rates, has been suggested as responsible for this behavior. A second factor which was identified as controlling this behavior is the increase of local /rans-planar conformations in the mixed amorphous phase, due to specific interactions between the polymers [102]. 

For the case of the crystallization from the amorphous phase, the blending with PPO for lower contents (less than 30wt%) favours the obtainment of the a" ordered modification with respect to the a disordered modification, which is obtained for the unblended polymer. For higher PPO contents the obtainment of the p form is favored [105]. This behavior would be simply due to the increases of the glass transition temperature, and hence of the crystallization temperature on heating, which correspond to increased PPO contents in the blends [105],... 

This variation in the properties of polymers along their interfaces with inclusions is extended to layers of a sometimes significant thickness. This follows from the fact that, if only a thin surface-layer of the polymer was affected by its contact with the other phase, then the change in Tg should be insignificant, since the level of the glass transition temperature is associated with the bulk of the polymer, or, at least, with a large portion of it. 

---