A Study of Deformation Mechanism of High Density Polyethylene and Polypropylene by Simultaneous Measurement of Stress, Strain and Infrared Dichroism

Abstract

In the previous paper very efficient use was made of simultaneous measurement of stress, strain and infrared dichroism in interpreting the stress-strain curve of low density polyethylene. The stretcher employed in the previous studies has been improved so that the upper and lower crossheads can move up and down at the same rate, attaining the total stroke of 30cm. A drawtube has also been furnished in the way of an oven to control measuring temperature by circulating air at a controlled temperature.For high density polyethylene, the dichroic ratio D(⊥/||)for 730 and 720cm-1 bands has been measured as a function of strain at a constant rate of elongation of 30%/min, and the orientation functions of a-, b- and c-axes, Fα, Fβ and Fε, have been evaluated. These orientation functions plotted against strain give similar curves to those for low density polyethylene reported previously, indicating that quite the same explanation can be given to the stress-strain curve of the high density polyethylene sample as in the case of low density polyethylene.For polypropylene, on the other hand, the orientation function Fε for c-axis evaluated from the dichroic ratio D(||/⊥) for 998 and 840cm-1 bands takes negative values at first and then positive values as the strain increases, indicating that molecular chains in crystalline regions orient at first perpendicularly and then in parallel in the direction of elongation. This negative dichroic ratio is consistent with negative birefringence observed at small strains for the same material. The strain at which Fε takes its minimum coincides with the strain at the yield point. From these observations, the following deformation processes can be considered to occur during the stress-strain measurement: Below the yield point, spherulites or superstructures consisting of lamellae show elastic deformation, in which the lamellae rotate their axes in the direction of elongation. Beyond the yield point, the spherulites or superstructures show irrecoverable deformation due to the unfolding of folded chains which can orient in the direction of stretching as the strain increases. In this case, the strain at which the necking of the sample film can be observed coincides with the strain at which the unfolding takes place.