A MOLECULAR COMPOSITE APPROACH TO THE THERMAL-EXPANSION BEHAVIOR FOR ANISOTROPIC CRYSTALLINE POLYMERS

The dimensional stability of crystalline polymers such as polyethylene and polypropylene is a critical property in structural applications requiring known dimensional stability. It is essential. to evaluate how such structural systems will respond when exposed to thermal fluctuations. Utilizing the molecular composite approach, oriented polyethylene has been modeled as a macroscopic composite of its constituent crystalline and amorphous phases. The molecular composite model, originally developed to describe the isotropic behavior of crystalline polymers, has been refined to account for the anisotropic micromorphological behavior of oriented polyethylene solids. This refinement accounts for the deformation and orientation of both constituent phases; the variation of the amorphous phase mechanical and expansional properties due to the taut tie molecules has likewise been considered. Furthermore, a theoretical expression relating the crystalline orientation descriptors, lambda and f(c), has been developed. A detailed calculational format for the prediction of the expansivity of oriented polyethylene is outlined. The overall pattern of anisotropic dimensional behavior has been correctly predicted when compared with experimentally measured thermal expansivities of various types of polyethylene. (C) 1995 John Wiley and Sons, Inc.