Birefringence of high-pressure injection-molded high-density polyethylene at 70.5 μm wavelength

A newly constructed far-infrared laser interferometer was used to measure the birefringence of 1-mm-thick injection-molded high-density polyethylene test bars, manufactured at unusually high molding pressures. The long wavelength used, 70.5 μm, allowed nondestructive tests to be made, demonstrating the usefulness of far-infrared techniques for probing crystalline polymers. The birefringence was shown to increase with increasing molding pressure, supporting the belief that molecular orientation increases with increasing molding pressure. The tensile modulus was also measured and was found to increase linearly with molding pressure.