ANISTROPIC THERMAL-EXPANSION OF ORIENTED CRYSTALLINE POLYMERS

The linear thermal expansitivity of oriented high-density polyethylene (HDPE) and polypropylene (PP) with draw ratio between 1 and 18 has been measured between 120 and 300 K. The expansivity perpendicular to the draw direction z ̂(α⊥) increases with λ, because of the alignment of the crystallite chain axes along ẑ; the expansivity parallel to z ̂(α) decreases very sharply with λ, becoming negative at about λ = 3 for HDPE and λ = 7 for PP - a consequence of the negative expansivity of the crystalline phase along its chain axis and the constraining effect of the stiff intercrystalline bridges. A model treating the drawn crystalline polymer as a composite consisting of partly aligned crystallites embedded in an isotropic amorphous matrix is adequate for explaining the behaviour of α⊥, whereas a parallel-series model can give reasonable estimates of α for ultra-oriented samples, which is largely independent of λ and decreases with increasing temperature. However, neither model can account for the behaviour of α at low draw ratio. © 1979.