INFLUENCE OF COMPOSITION AND MOLECULAR-STRUCTURE ON THE MICROHARDNESS OF LIQUID-CRYSTALLINE COPOLYMERS

The mechanical properties of copolyesters of p-hydroxybenzoic acid (PHB) and poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) and of PHB and poly(ethylene terephthalate) (PET) were investigated over a wide range of compositions using the microhardness technique. The mechanical behaviour has been interpreted in terms of an aggregate model of additive units of structure. On this basis the changes in microhardness with composition can be related to the following factors: degree of crystallinity, crystal thickness and polymorphic crystal forms. In the case of the amorphous materials the increase of the microhardness with increasing number of flexible chain sequences (PET, PEN) has been discussed in terms of a simple additive behaviour of single components PET/PHB and PEN/PHB. In the case of the crystallized materials it is shown that the hardness of the PET and PEN crystals is an increasing function of the crystallite dimensions (flexible chain domains).