Amorphous glassy ones

The chains that make up a polymer can adopt several distinct physical phases the principal ones are rubbery amorphous, glassy amorphous, and crystalline. Polymers do not crystallize in the classic sense portions of adjacent chains organize to form small crystalline phases surrounded by an amorphous matrix. Thus, in many polymers the crystalline and amorphous phases co-exist in a semicrystalline state. 

As polymers solidify from the molten state, their free volume decreases and their organization increases. Solid polymers fall into one of three classes rubbery amorphous, glassy amorphous, and semicrystalline, which we introduced in Chapter 1. 

Solid polymers can adopt a wide variety of structures, all of which are derived from the three basic states rubbery amorphous, glassy amorphous, and crystalline. Either of the amorphous states can exist in a pure form. However, crystallinity only occurs in conjunction with one of the amorphous states, to form a semicrystalline structure. 

Food materials (ingredients or whole systems) can be composed of matter in one, two, or all three physical states solid (crystalline or amorphous or a combination of both), liquid, and gas. The crystalline state is an equilibrium solid state, whereas the amorphous glassy state is nonequilibrium solid state. The main transitions that occur between the physical states of materials of importance to foods are summarized by Roos and Karel (1991) and Roos (2002). The most important parameters affecting the physical state of foods, as well as their physicochemical properties and transition temperatures, are temperature, time, and water content (Slade and Levine, 1988 Roos, 1995). Pressure is not included in this list, as food materials usually exist under constant pressure conditions. 

What about semi-crystalline polymers On the basis of the arguments we have made so far we would expect that highly crystalline materials should be very stiff. But polymers are semi-crystalline. We therefore have to consider two situations, one where the polymer has a low Tg and one where it has a high T. In the latter case we would expect the material to be relatively stiff (as polymers go), as it would be some combination of the covalent bonds and intermolecular forces that would respond to a stress, as in amorphous glassy polymers. On the other hand, we would anticipate that polymers where the amorphous domains are well above the Tg at the temperature of use would have a lower modulus and be more flexible. And indeed they are. However, conformational freedom in the amorphous domains is restricted by the crystalline domains, so that the modulus depends significantly on the degree of crystallinity (Figure 13-25). 

In previous examples of substituted polysiloxanes, the relaxation of the side chains with the rigid backbone was assumed to describe the polymer chain in the amorphous glassy state. This assumption, relaxation of side chains only, can be used to study the crystalline states of PDES. Certainly, this simplification is extreme, but it can be usefiil to understand the available orientation of the pendant groups when the polymer chains undergo transition from one crystalline form to another. The present approach does not address the chain reorientation or the interchain interactions in the crystalline state. 

As shown in Fig. 20.4-3, this added effect can improve helium selectivity relative to methane by a factor of 1.73. At the same time, a factor of 2.0 loss in helium solubility and a factor of2.S [eduction in heiium diffusivjty due to the crystallinity produces a 5,6-fold reduction in helium productivity relative to the hypothetical amorphous film case. Interestingly, the effects of chain restriction on different sizes of peaetranls ware found by Michaels to be effective only for rubbery materials, not glassy ones. A logical explanation for this fact was ndvanced in terms of the already low mobility of chain segments in the glass compared to the rubbery stale ... 

The uncertainty over the modulus of the amorphous phase E , represents another difficulty in estimating q. One approach to the problem of calculating this figure is that of Bowden, but at best this gives a very approximate answer. He considers an amorphous glassy polymer, which typically has a Young s modulus of the order of 3-4 x 10 Nm . If this modulus is controlled by secondary intermolecular forces, a reasonable estimate of the modulus can be made from the interatomic potential. 

When rubbery materials are cooled, two typical outcomes are obtained (1) the material partially crystallizes to become a flexible semicrystalline substance or (2) the material does not crystallize and, at sufficiently low temperature, becomes an amorphous glassy solid. One of the most confusing aspects of the behavior of macromolecules during the 19th century was the observation of crystallization. It was believed that polymers could not crys-... 

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