The stiffness of polymers in relation to their structure

Abstract

In this paper we set out to review experimental values and theoretical estimates of the Young’s modulus of crystalline and amorphous polymers. Not unnaturally, this reveals a lack of complete numerical agreement. The disparity is worst for the transverse directions to the fibre axis when calculated theoretically. Nor is there complete agreement between experimental determinations of the moduli made by different experimental techniques. Nevertheless, an interesting and reasonably satisfactory picture emerges from which can be seen those areas which are most in need of further research. After outlining the problem in general, we give an account of the various methods available for obtaining the Young’s modulus of polymers along the chain and at right angles to the chain. These consist of macroscopic stretching methods, the use of sound waves, x-ray diffraction, Raman scattering and inelastic neutron scattering spectroscopy. The relative significance of static and dynamic measurements is also discussed. An account is given of experimentally determined values of E||, the modulus along the chain, and in the next section these values are compared with theoretical estimates. It turns out that most work has been done with polyethylene, where major discrepancies can be seen between the experimental and theoretical values. However, other polymers have received a fair amount of attention. Experimentally determined values of Elare next discussed, and are compared with theoretical estimates. The situation here is confused, but largely because elhas received less attention than E||. Finally the problem of isotropic amorphous and crystalline polymers is considered. This is the most complicated, and technologically the most important. Some progress has been made in estimating theoretically Young’s modulus from consideration of molecular stretching mechanisms, but basically the subject is still in its infancy. © 1971, Walter de Gruyter. All rights reserved.