Prediction and measurement of the thermal expansion coefficient of crystalline polymers

Abstract

Abstract Among the increased structural demands now being made on both unfilled and reinforced plastics is that of dimensional stability under various performance environments. Crystalline polymers are heterogeneous materials consisting of two distinct phases and, as such, can be treated as molecular versions of engineering composites. This paper first outlines the general physical model whereby a crystalline polymer is considered to be a multi‐ply laminate of unidirectionally reinforced plies. The calculational format is then detailed for the prediction of the stiffness and thermal expansion coefficient of an isotropic sheet of crystalline polymer and a sample calculation is given for quenched high density polyethylene. A data base is presented for the stiffness and thermal expansion coefficient of low and high density polyethylene having quenched, slowcooled, and annealed thermal histories. Comparison between experimental and predicted results yields good agreement in all cases to better than 25 percent. Implications and limitations of the predictive technique are discussed.