Shear relaxations in partially crystalline polymers

The repeat unit of the PET molecule, the product of a condensation reaetion of terephthalic acid and ethylene glycol, was shown in Fig. 2.3 and is repeated here in Fig. 5.20. For their study, Illers and Breuer chose a commercial product with no detectable initial crystallinity, and removed all traces of residual stress as well as any previously existing water by appropriate prolonged thermal treatments above the glass-transition temperature of 67 °C. Different levels of crystallinity in samples were then obtained by nine separate isothermal crystallization protocols at temperatures ranging from 70 to 245 °C for pre-selected times until equilibrium crystallinities ranging from 0 to 46% were achieved in each case. Only samples subjected to temperatures above 86 °C showed X-ray evidence of crystallinity. 

The curves in Fig. 5.21(a) show that in samples with crystallinity less that 2%-3% the storage moduli // do not differ from that of fully glassy PET. In these, the moduh drop quite precipitously near 70 °C down to very low levels typical of uncross-linked linear-chain polymers of around 1.0 MPa. Samples with crystallinities above 16% show more gradual drops in //, which are also moved out to the temperatures of their initial crystalhzation treatments, beyond which the drops become much steeper. Clearly, while there is a definite decrease in storage modulus down to levels of around 100 MPa, this decrease is pushed far out to temperatures reaching 240 °C by the reinforcing effect of the crystalline fraction. 

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