Non-linear mechanical behaviour in a highly oriented liquid crystal copolyester

Abstract

Oriented liquid crystal copolyesters are mechanically non-linear at surprisingly low levels of strain when stretched in the orientation direction. As the static strain increases the axial Young's modulus also increases in an asymptotic fashion, the effect being most marked above the glass transition. Several models, each with its own distinct philosophy, have been advanced to account for this behaviour. In order to evaluate critically the various models we have studied the non-linear behaviour of a highly oriented random copolyester film over a wide temperature range. We have also measured the shear modulus of the film and similar fibres (where a wider temperature range can be covered) since this is a key parameter in the various theories. On the basis of comparisons between measured and predicted shear moduli we conclude that the models proposed by Allen and Roche, and by Zhang, Davies and Ward describe the non-linearity best, especially at high temperatures where the units of structure are free to rotate, while the model of Northolt and van der Hout is unsatisfactory. None of the models is applicable well below the glass transition. We also note that all the models are exponential in character, whereas better fits to the data are obtained with other empirical asymptotic expressions.