Glassy polymers glass transition

Fig. 19. Generalized modulus—temperature curves for polymeric materials showing the high modulus glassy state, glass-transition regions for cured and uncured polymers, plateau regions for cross-linked polymers, and the dropoff in modulus for a linear polymer.

As already mentioned in Chap. 2.2. one of the most obvious features of the l.c. side chain polymers is their ability to become glassy. The glass transition can be observed by cooling nematic, cholesteric and smectic polymers depending on the chemical constitution of the system and is indicated e.g. by a bend in the V(T) curves as schematically shown in Fig. 8. Two questions are of interest which will be discussed in this chapter ... 

Since the 1980s, the polymer science approach to the study of the glassy state, glass transitions, and their importance for structures, properties and water relationships in food materials, products, and processes was recognized by a growing number of food scientists and technologists [6.4.3], As a result, the following questions arose (Chapter 3 in [B.53]) ... 

Two of these are under extensive investigation and are currently being studied for gas separation on a pilot scale. These are DuPonfs 2,2-bistrifluoromethyl-4,5-difluoro-l,3-dioxole/tetrafluorethylene copolymer (Teflon AF 2400 ) and poly(4-methyl-2-pentyne) (PMP). All three polymers, PTMSP, PMP and Teflon AF2400, are glassy with glass transition above 230°C and have a very high fractional free volume (FFV). Figure 7.4 shows the chemical structure and fractional free volume of these three polymers. 

This chapter first will present the principles of dynamic mechanical analysis and some of its facets in polymer investigations and then give an overview of trends in viscoelastic parameters of multiphase polymeric systems in the glassy region, glass transition region, and the rubbery plateau. 

The diffusion behavior of simple gases (i.e., gases above the critical temperature) in glassy polymers is often quite different from the behavior of the same gases in the same polymers but above the polymer glass transition temperature [64], In particular, gas solubUity shows negative deviations from Henry s law, and the dissolution process is much more exothermic in glassy polymers. These and other... 

In the case of polymer molecules where the dipoles are not directly attached to the main chain, segmental movement of the chain is not essential for dipole polarisation and dipole movement is possible at temperatures below the glass transition temperature. Such materials are less effective as electrical insulators at temperatures in the glassy range. With many of these polymers, e.g., poly(methyl methacrylate), there are two or more maxima in the power factor-temperature curve for a given frequency. The presence of two such maxima is due to the different orientation times of the dipoles with and without associated segmental motion of the main chain. 

Glass transition Transition region or state in which an amorphous polymer changed from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. Transition occurs over a narrow temperature region similar to solidification of a liquid to a glassy state. This transformation causes hardness, brittleness, thermal expansibility, specific heat, and other properties to change dramatically. 

It must be above its glass transition temperature, which means that the polymer chains have sufficient thermal energy to move freely. Many rubbery materials have glass transition temperatures around 200 K, below which they are glassy, like plastics. 

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