THE ROLE OF MOLECULAR NETWORKS IN THE DEFORMATION-BEHAVIOR OF SOLID POLYMERS

In early research on polyethylene terephthalate, studies of shrinkage force, shrinkage and birefringence on melt spun fibres confirmed the existence of a molecular network which was of key importance in determining the strain hardening behaviour and hence the natural draw ratio. These ideas were translated to the ultra drawing of polyethylene, where it was found that molecular weight and initial morphology determined the ultimate extensibility of the polymer. The existence of the network in the drawn polymer was confirmed by measurements of thermal expansion and shrinkage force. Recently this research has been extended to define the network which forms in the spinning threadline for very high wind up speeds and how this subsequently affects the drawing behaviour and the final properties of the drawn yarns. Specific issues addressed have included the effect of molecular weight on the behaviour of polyethylene, terephthalate fibres and the extension of the studies to polybutylene terephthalate where changes in the crystalline structure can occur. In films, a comparison has been made between uniaxial and uniaxial planar (constant width) films. Refractive index and infrared spectroscopic measurements have been undertaken to determine the chain axis orientation and conformational changes. It has been shown that a quantitative description can be obtained in terms of the deformation of a rubber-like network. More recently, studies have been undertaken of two-stage stretching, with the additional objective of determining the factors influencing the development of crystalline orientation in the second draw direction.