Hydrodynamically induced crystallization of polymers from solution. V. Tensile properties of fibrillar polyethylene crystals

Abstract

AbstractA study has been made on the tensile properties of mats of fibrillar polyethylene crystals grown at various temperatures from stirred xylene solutions in a Couette‐type instrument. The stress‐strain behavior of fibrillar crystals precipitated above 100°C was found to be essentially Hookean, and the maximum values determined for Young's modulus and tensile strength at break amounted to 27 × 104 kg/cm2 and 10 × 103 kg/cm2, respectively. The elongation at break for these high temperature fibers was 6%. Stress‐strain diagrams for fibrillar polyethylene crystals grown at temperatures below 100°C were curved and the elongations at break amounted to roughly 12%. Both the Young's modulus and the tensile strength were found to increase strongly with the crystallization temperature; this trend is attributed mainly to the decrease in the amount of lamellar overgrowth and the improvement in orientation.Microscopic examinations have shown that the mats can be regarded as assemblies of macrofibers having diameters in the micron range. Homogeneous fracturing is found in samples with extensive lamellar overgrowth, whereas carefully washed samples fractured into fibrils. Electron micrographs reveal that the elementary fibrils (the shish‐kebabs) develop brittle fractures and that the lamellar overgrowth is capable of transmitting stresses normal and parallel to the backbones. Digestion of the fibrillar crystal material with fuming nitric acid causes a decrease in tensile strength and elongation at break, but the Young's modulus remains approximately unaltered.