Mechanism of Deformation of Crystalline Polymers

Abstract

The elemental mechanism of deformation of crystalline polymers can be classified into the following types:
(1) deformation of chain molecules in the amorphous state (to some extent, rubbery) (2) rotation of crystallites or lamelae (3) inter-lamellar slip (4) intra-lamellar slip (inter-ribbon slip within the lamelae) (5) unfolding (ridding the chain of the folded form).
In view of the geometrical complexity in dealing with the deformation of three dimensional spherulites, the deformation of two dimensional spherulites, and samples having the“row structure” have been discussed in terms of the elemental mechanism mentioned above. A comprehensive study of the molecular orientation and crystalline texture in drawn and rolled polyethylene on consecutive stages of heat annealing has enabled a picture of the structural change and reorientation process activated by rubber elastic compression to be outlined. It has been shown that unfolding would not occur so often on elongating the spherulite as usually considered. The inter- and intra-lamellar slips seems to account for the yield in the stress-strain curve of unisotropic films the texture of which is assumed to be of“row structure”which we visualise as consisting of stacks of lamellae. The picture drawn is also consistent with the remarkable anisotropy of the film exhibited by the stress-strain curves. During the annealing process of the drawn and rolled branched polyethylene, the lamellae first begin to rotate around the b axis and keep the molecular orientation within them unaltered until the lamellar become perpendicular to the originally drawn line and the molecular axis inclines to the drawn direction. In the next stage (i.e., at higher temperatures) the compression does not affect the lamellar surfaces orientation but the molecular inclination within them increases until the lamellae become disrupt (the intra-lamellar slip). The lamellae then reform with a distribution in orientation about the axis normal to the film plane. Finally randomisation occurs around the b axis before melting (indicating stability of b axis) and complete disorientation takes place. All the above findings indicate at least qualitatively that deformation occurs in the order of (1)∼(4) mentioned previously. Of course, some of the mechanism is accompanied by some others and the order of the elementary mechanisms to be realized should be dependent on the experimental conditions, especially on temperature. The individuality of the lamellae-requiring an appreciable amount of chain folding even in drawn samples-is nevertheless borne out. The necking occurring in the “cold draw”may be attributable to inter- and intra-lamellar slips.