Negative thermal expansion of a polydiacetylene single crystal

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Abstract
Macroscopic thermal expansion in the chain direction has been measured for the first time on organic polymeric single crystals. Negative linear thermal expansion coefficients αM are reported and related to chain torsional motion and equilibrium point‐defect formation for a solid‐state polymerized phase of 2,4‐hexadiyne‐1,6‐diol bisphenylurethane (HDU) which contains crystallographically located interstitial dioxane and for a dioxanefree phase obtained by thermal annealing. Data for as‐polymerized single crystals (which are probably of extended chain morphology) between −50 and 100°C give αM = −(1.686 ± 0.039) × 10−5 − (1.35 ± 0.18) × 10−7 t with t in °C. During volatilization of 11.7 ± 1.0 wt‐% interstitial dioxane and a resulting crystal structure change, the as‐polymerized fibers fibrillate and shrink irreversibly by 0.16 ± 0.04%. Although dichroism and diffraction measurements indicate both a high degree of crystallinity and chain alignment for the dioxane‐free phase, the average thermal expansion coefficient, (−3.0 ± 1.0) × 10−6 °C−1 between −50 and 150°C, is about an order of magnitude less than for the as‐polymerized single crystals.