The Mechanical Properties and Structure of Oriented Polymers

Abstract

First considerations of the mechanical properties of oriented polymers require an understanding of the definition of anisotropic behaviour at a macroscopic level. This is obtained by a generalization of Hooke’s law to anisotropic materials. From this starting point, experimental measurements can be undertaken by a wide range of techniques from dead-loading creep to dynamic mechanical and ultrasonic measurements.

The interpretation of mechanical anisotropy requires consideration of the molecular structure in the first instance. This leads to examination of crystal moduli, either from theoretical calculations or experimental data. Next, there is the influence of orientation and morphology, both of which may be important in particular systems. Attempts to undertake quantitative analyses of mechanical anisotropy have two simple starting points. These are the aggregate model, which deals with molecular orientation and the Takayanagi model which recognises the composite nature of crystalline polymers. Developments based on these two approaches will be discussed.

The anisotropic mechanical behaviour also reflects relaxation processes, which are related to orientation and morphology. Some definitive situations will be considered, including both reversible relaxation behaviour and plastic flow.