Thermal expansion of ultradrawn polystyrene

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Abstract
Measurements on the linear thermal expansivities α∥ and α⊥ parallel and perpendicular to the extrusion direction, respectively, have been carried out between −160 and 30°C for atactic polystyrene (aPS) with extrusion ratio 1 ≤λ≤ 15 and isotactic polystyrene (iPS) with 1 ≤λ≤ 7.5. For both aPS and iPS, α∥ decreases sharply with increasing λ whereas α⊥ shows only a slight increase. Below λ = 5 the anisotropy α⊥/α∥ is nearly the same for both, but α⊥/α∥ for iPS becomes much larger at higher λ. This is accompanied by an abrupt rise in crystallinity and probably results from the increase in the number and tautness of intercrystalline tie molecules. The birefringences of aPS and iPS increase with λ and have nearly the same values for λ < 5. At higher λ, however, there is a sharp rise in the birefringence of iPS which is probably associated with the sudden increase in crystallinity. With an aggregate model, the chain orientation function for aPS calculated from thermal expansivity is found to be in reasonable agreement with the corresponding value obtained from birefringence.