Negative thermal expansion in oriented crystalline polymers
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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.