Transition, in glassy polymers

Secondary Transitions in Glassy Polymers and Methods of Their Determination 120... 

Secondary transitions in glassy polymers are closely associated with limited molecular mobility, i.e. with the rotational and vibrational motions of relatively short chain sections. The motional units may be identified with sequences of the main chains consisting of four to six groups, or with side drains and their parts. Generally, it is believed that... 

Of the thermodynamic quantities just mentioned, only the determination of the expansion coefficient or other quantities reflecting its change have assumed practical importance for the identification of secondary transitions in glassy polymers. The most efficient methods for the investigation of the dynamics and intensity of molecular motions have so far been those based on the interference between molecular motion and the oscillating magnetic, electric or mechanical force field. In recent years, methods which employ various probes or labels in the study of molecular mobility have increasingly been used. 

Kambour, R. P. and Bernier, G. A. (1968) Craze formation yield stress and the so-called ductile-brittle transition in glassy polymers, Macromolecules, 1, 190-191. 

This approach offers a deeper understanding of the brittle-ductile transition in glassy polymers in terms of competition between crazing and yielding. Both are activated processes, in general with different temperature and strain rate sensitivities, and one will be favoured over the other for some conditions and vice versa for other conditions. An additional complexity can arise from the nature of the stress field that may favour one process rather than the other, but the latter consideration does not enter into our discussion of the... 

All the results presented in this chapter show that FIR spectroscopy allows us to clarify molecular mechanisms of relaxational S, P and y-transitions in glassy polymers and to establish the relations of these processes with molecular characteristics of polymers such as the structure of a monomer unit, the cohesion energy, and thermodynamic chain rigidity. 

Experimental data collected so far show that most secondary relaxations (transitions, dispersions) in glassy polymers are a consequence of the conformational isomerization of short sections of main or side chains and that their kinetics may be satisfactorily described by means of the site model in which stable conformations are separated by a potential... 

There have been proposed several types of short-range molecular motions to account for the various transitions. Following Heijboer (1977), Fig. 13.30 illustrates schematically possible modes of molecular motion in secondary relaxation in glassy polymers. The classification is outlined below. 

As the loading rate increases, thermal effects need to be accounted for and the analysis is extended to a coupled thermomechanical framework. Evidence of a temperature effect in glassy polymer fracture is found (e.g., in [2,3]) with a temperature increase beyond the glass transition temperature Tg. The influence of thermal effects on the fracture process is also reported. 

Fundamental studies of gas transport in polymers other than rubbers began with the classical work of Meares in 1954 He was the first to demonstrate and theorize about the now well-known inflection in the Arrhenius plots of D near the ass transition temperature. He also speculated abcut two modes of sorption in glassy polymers. Later studies were initiated with many polymers by Barrer, Michaels and their coworkers together with important contributions by Brandt, Stern, Stannett and many others ... 

The basic transport mechanism through a polymeric membrane is the solution diffusion as explained in Section 4.2.1. As noted, there is a fundamental difference in the sorption process of a rubbery polymer and a glassy polymer. Whereas sorption in a mbbery polymer follows Henry s law and is similar to penetrant sorption in low molecular weight liquids, the sorption in glassy polymers may be described by complex sorption isotherms related to unrelaxed volume locked into these materials when they are quenched below the glass transition temperature, Tg. The various sorption isotherms are illustrated in Figure 4.6 [47]. 

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