Thermal expansivity of oriented poly(ethylene terephthalate)

The linear thermal expansivity of poly(ethylene terephthalate) extruded at 50 and 90°C to extrusion ratios λ of 1–4.8 has been measured between 120 and 300 K. With increasing λ, the expansivity along the extrusion direction (α ∥ ) decreases sharply, while that in the transverse direction (α ⊥ ) shows a slight increase. For λ < 3, the large drop in α ∥ and the accompanying increase in the axial Young's modulus E ∥ can be ascribed to chain alignment in the crystalline regions and to an increase in number and tautness of intercrystalline tie chains. At higher λ, however, the crystalline orientation apparently becomes saturated, so that taut tie molecules are solely responsible for further changes in both α ∥ and E ∥ . On the other hand, α ⊥ is mainly determined by crystalline orientation for all λ, thus showing very little increase at large λ. For the highly oriented samples (λ ≥ 3), the Takayanagi model provides a reasonable description of the behavior of α ∥ and α ⊥ .