ABSTRACT
An intrinsic defect of polyolefins is the presence of turbidity, which limits their use as an optical material. Only poly-4-methylpentene represents a transparent material. However, polyolefins in melts are transparent and the turbidity is due to the formation of coarse spherulite structures during the process of crystallization. At the same time, rapid cooling of polyolefin melts, i.e., "quenching," results in the preservation of the structure particular to the melt. This fact is confirmed by results obtained in polarization microscopy studies of the structure characteristics of normal and quenched polyolefins (Fig. 1). Quenched polyolefins exhibit fine-grain spherulite structure, while in normal polyolefins a coarse spherulite structure is revealed. In normal polyolefins with coarse spherulite structure, where the sizes are comparable to the wavelength of the transmitted light flux, the scattering process takes place and, consequently, the light permeability is rather low. The sizes of spherulites in quenched polyolefins are much smaller than the relative wavelength of transmitted light flux and the scattering is insignificant. An investigation into the light permeability of polyolefins upon quenching, over the wavelength range of 300-700 nm, has shown that after quenching the structure of the polyolefins has a considerable effect on light transmission. Of the polyethylene samples examined, the highest transmission has been observed for high-pressure polyethylene. Light permeability in the ultraviolet region may be as high as 70%, rising monotonically with the wavelength and reaching 95% relative to glass at the wavelength of700 nm. Light permeability of low-and medium-pressure polyethylene is practically at the same level. An increase in the wavelength leads to a linear rise in the light permeability, which reaches 70% at 700 nm. Lower light permeability of medium-and low-pressure polyethylene upon quenching is conditioned by slight branching, which re-
suits in the absence of steric barriers for the formation of spherulites in contrast to high-pressure polyethylene. Nonquenched polyethylene samples possess negligible light permeability. The highest light permeability is exhibited by high-pressure polyethylene, the light transmission of which amounts to 15% at 300 nm and increases to as high as 55% at 700 nm. In instances of low-and medium-pressure polyethylene, the light permeability of nonquenched samples increased from 10% at 300 nm to 35% at 700 nm.
