NONLINEAR MECHANICAL-BEHAVIOR IN A HIGHLY ORIENTED LIQUID-CRYSTAL COPOLYESTER

Oriented liquid crystal copolyesters are mechanically non-linear at su rprisingly low levels of strain when stretched in the orientation dire ction. As the static strain increases the axial Young's modulus also i ncreases in an asymptotic fashion, the effect being most marked above the glass transition. Several models, each with its own distinct philo sophy, have been advanced to account for this behaviour. In order to e valuate critically the various models we have studied the non-linear b ehaviour of a highly oriented random copolyester film over a wide temp erature 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 comp arisons between measured and predicted shear moduli we conclude that t he models proposed by Allen and Roche, and by Zhang, Davies and Ward d escribe 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.