The strength and stiffness of polymers

Abstract

Fully alined long chain polymers would have a Young modulus in the alinement direction similar to that of steel. Practically available polymeric materials have moduli less than one-tenth, and usually less than one-fiftieth of this, even with a high degree of molecular alinement. The paradox is of course resolved in principle by recognizing the predominance of chain folding in polymer crystallization, which allows strong alinement to occur with little or no extended chain continuity in the alinement direction. Questions which arise from this are: (1) what are the actual mechanisms of compliance in the material of relatively low modulus? and (2) by what means may it be possible to achieve full extension and alinement of a high proportion of the chains? Some tentative answers can be given in the light of current researches. Fully extended alinement for a small proportion is obtainable. Extension of a substantial proportion, rather than all, is the desideratum, since the folded chains contribute toughness. It can be useful to think of polymers as intrinsically composite materials, even when chemically homogeneous.