Polymer Structure

The viscoelastic deviations from ideal elasticity or purely viscous flow depend on both the experimental conditions (particularly temperature with its five temperature regions and magnitudes and rates of application of stress or strain). They also depend on the basic polymer structure particularly molecular weight (MW), molecular weight distribution (MWD), crystallinity, crosslinking, and branching (Chapter 1).

High MW glassy polymer [an amorphous polymer well below its glass transition temperature (Tg) value (Chapter 1)] with its very few chain motions are possible so the material tends to behave elastically, with a very low value for the creep compliance of about 10"9 Pa-1. When well above the Tg value (for an elastomer polymer) the creep compliance is about 10"4 Pa1, since considerable segmental rotation can occur.

The intermediate temperature region that corresponds to the region of the Tg value, is referred to as the viscoelastic region, the leathery region, or the transition zone. Well above the Tg value is the region of rubbery flow followed by the region of viscous flow. In this last region flow occurs owing to the possibility of slippage of whole polymer molecular chains occurring by means of coordinated segmental jumps.

These five temperature regions give rise to the five regions of viscoelastic behavior. Light crosslinking of a polymer will have littie effect on the glassy and transition zones, but will considerably modify the flow regions.

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