Negative thermal expansion in oriented crystalline polymers

Measurements on the thermal expansivity α ∥ and α ⟂ (along and normal to the draw direction, respectively) have been carried out for a series of oriented polymers with widely different crystallinities (0.36–0.81) and draw ratios (1–20) and over large temperature ranges covering the major amorphous transitions in each case. While α ⟂ increases with temperature, α ∥ tends to decrease sharply above the transition temperature. For highly crystalline polymers, α ∥ decreases to values typical of polymer crystals (−1 × 10 −5 K −1 ) and this can be attributed to the constraining effect of the crystalline bridges connecting the crystalline blocks. However, for polymers of lower crystallinity, α ∥ may become an order of magnitude more negative and this remarkable phenomenon is attributed to the rubber–elastic contraction of taut tie‐moleucles. Since taut tie‐molecules and bridges have drastically different effects on α ∥ at high temperatures, this allows a rough determination of their relative fractions.