Isotope effect on the melting temperature of non-polar polymers.

The isomorphous monoclinic-(011) forms of "normal" (hydrogenated) and perdeuterated hexatriacontanes (n-C sub (36) H sub (74) and n-C sub (36) D sub (74) have been examined by X-ray diffraction at room temperature. Substitution of deuterium for hydrogen results in slight reduction in the parameters a sub a = a sin beta and b of the orthorhombic subcell by 0.33 +- 0.02% and 0.19 +- 0.02% respectively (within experimental error the c parameter appears to be unaffected). This reduction in cross sectional area of the unit cell as seen in projection along the chain axis is primarily a consequence of C-D bonds being slightly shorter than C-H bonds. Based upon the change in molecular volume, and an associated change in molecular polarizability, a relationship is proposed to account for the differences in melting temperatures of normal and perdeuterated hexatriacontane. The relationship also predicts successfully corresponding differences in melting temperature occasioned by perdeuteration of polyethylene and isotactic polystyrene, but fails to do so in the case of isotactic polypropylene. We speculate that, in the latter case, isotope substitution influences rotational degrees of freedom of pendant methyl groups, leading to an isotope dependent entropy of melting not experienced in the other species cited.