The role of crystallinity in the deformation of main-chain liquid-crystalline copolyesters

The importance of crystallinity in the deformation processes of main-chain liquid-crystalline copolyesters is examined. Two materials are studied: one a copolymer of hydroxybenzoic acid (HBA) with 2-hydroxy-6-naphthoic acid ( HNA); the other a copolymer of HBA with isophthalic acid (IA) and hydroquin one (HQ). In both cases it is observed that the transverse compressive yiel d stress and modulus of the polymer are optimised by annealing at about 20 degrees C below the melting temperature. In the case of the copolymer conta ining HBA and HNA, it is found that deformation of unannealed samples resul ts in a pronounced loss of crystallinity. However, the effect is less marke d in annealed samples, and the crystallinity is found to recover after mode st annealing close to the glass transition temperature. In both polymers, e vidence is found for a stress-induced solid-state phase transition, which m ay be similar to the martensitic phase transition proposed for polyethylene . (C) 2000 Elsevier Science Ltd. All rights reserved.