The Tensile Properties of Polyethylene Single Crystals.

A new, very sensitive, instrument called the nanotensilometer was employed at room temperature to measure the stress -strain characteristics of individual, solution-grown, polyethylene crystals having a thickness of about 10 nm and planar dimensions of 15 (mu)m. Orientation studies were performed of the crystal angle and offset relative to the position of a 3 (mu)m controllable gap which the crystals spanned. Only the low strain modulus was dependent on angle where E(,a) = 0.44 x 10('10) and E(,b) = 1.7 x 10('10) dynes/cm('2) for strain rates of 1 nm/sec. At least some of this anisotropy is probably a result of corrugations of the crystal lamella. No correlation of forces or moduli with respect to the orientation of the {110} planes was observed. Studies were made at nominal strains between about 10% and 300% where microfibrils determine the forces. Under these conditions, the sample stress and modulus exhibited the same inverse relationship as a function of elongation. With data from electron microscopy studies, the modulus of the fibrils was calculated. The measured work agreed with a surface energy calculation suggesting that filbrils are constructed of intact segments pulled from the crystal's edge. Crystals were stretched and compressed repeatedly over a fixed elongation interval at strains as high as 25%. By pausing at the ends of the interval, the stress relaxation modulus was determined to have a time constant averaging 11 seconds but decreasing with elongation. A single Maxwell element provided a concise description of the data. The dynamic modulus of this model contained 90% of the strength of the crystal. Both moduli in this model increased 8% per cycle showing a strain hardening effect. Freshly created crystal surfaces were observed to be very "sticky", being able to bond to another part of the crystal with a strength comparable to that of the crystal body. This ability was diminished after an hour's exposure to air.