Mechanical Relaxation in Solid-state Polymerized Poly(ethylene terephthalate)

Abstract

The dynamic mechanical tensile properties, storage modulus E′ and loss modulus E″, of the amorphous and semi-crystalline PET samples, ranging in molecular weight from 15,000 to 300,000 g/mol, were measured over temperature T range from 150°C to +250°C at four frequencies v=3.5, 11, 35 and 110 Hz. The samples with molecular weight larger than 15,000 were produced by solid-state polymerization in high vacuum. An increase in the height of loss β-peak, at T= −60… -30°C, with an increase in molecular weight was found both in the amorphous and semi-crystalline PET. On the other band, the height of loss β-peak for the amorphous samples appeared to be smaller in comparison with that for the semi-crystalline samples. By contrast, the height of loss δ-peak, at T= + 90. +110°C, for the amorphous samples was larger than for the semicrystalline samples. An increase in the E value with an increase in the molecular weight of the amorphous polymer was accompanied by an increase in the E″ value. This behavior was explained by the effective interpenetrated network of the high-molecular-weight polymer and better short-range ordering in the low-molecular-weight polymer. The intensity of the β-process was found to weaken with an increase in the chain ends concentration in both the amorphous and semi-crystalline samples. This result indicates that there is no contribution of the chain ends to the process of β-relaxation in PET.