抄録
For all numerous studies, the mechanical properties of crystalline polymers have not been well-understood when compared with those of amorhous polymers which have been much clarified theoretically and experimentally. For example, such a fundamental problem as which phase in crystalline or non-crystalline phases contributes to what extent to the deformation processes of crystalline polymers, has been left unsolved.
In this paper, the elastic recovery of a low density polyethylene stretched to various degree of % elongation is explained quantitatively in terms of the response of internal structures manifested by the changes of orientation factors of crystalline and non-crystalline phases and of degree of crystallinity, which are evaluated from simultaneous measurements of X-ray diffraction and of optical birefringence in the stretched and released states.
The sample polymer used is a low density polyethylene (Mitsubishi Yukalon K-3202) having melt index of 3.5 and branched degree of 2.0 CH3/100 carbon atoms, and the original test specimen has been cast from melt into thin film of about 0.2mm thick by a hot press and annealed. The original specimen thus prepared is of a random nature without any preferential orientation of crystal axis.
Within the elastic limit, the elastic recovery of test specimen is accompanied by the complete recovery of orientation factors to 0 either in crystal or in non-crystal phase. From the plastic region just beyond the yielding point, the recovery of orientation factors of three crystallographic axes is not complete, especially, showing such a peculiar behavior of b-axis orientation factor as going from negative to positive value, and not to 0, which might be explained in terms of some preferential collapse of crystal texture. From the plastic region near the beginning of necking the recovery of orientation factors of the three crystallographic axes is still fairly good, from the plastic region near the end of necking (complete of fibre-structure) the recovery becomes very poor, and from the region of strecthed fibre-structure the recovery is somewhat improved, again.
From the plastic regions as classified above the recovery of non-crystalline phase evaluated from the change of the orientation factor of methylene unit is generally better than that of crystalline phase, while from the stretched fibre-structure the recovery is extremely superior.
The elastic recovery of test specimen is also accompanied by the change of degree of crystallinity, usually showing the decrease of the degree with the elastic recovery, except for the regions within the yielding point and of stretched fiber-structure where the degree increases with the elastic recovery in contrast to the above.
