Side‐chain crystallinity. II. Heats of fusion and melting transitions on selected copolymers incorporating n‐octadecyl acrylate or vinyl stearate

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Abstract
The heats of fusion and the melting transitions of the crystallinity present in the side chains were determined for selected copolymers incorporating n‐octadecyl acrylate or vinyl stearate. A major purpose of this investigation was to ascertain the effect of interrupting the long ordered 18‐carbon side chains by randomly interspersed amorphous side chains of various lengths. For this purpose the lower acrylate homologs (C1 through C8 and including oleyl, C8) were copolymerized over the composition range with n‐octadecyl acrylate. It was found that simple dilution of the crystalline component (from comonomer b) by the amorphous component (from comonomer a) governed the decline in the heats of fusion and the fraction of crystallinity present. High crystallization rates were encountered because equilibrium crystallinity was nearly achieved for most of the copolymers. Melting point depression was less than theory in copolymers having short amorphous comonomer side chain lengths, but approached the theoretical depression as these side chains became very long. Thus the outer methylene sequences (the crystalline sequences) of the fatty co‐units could bridge the smaller amorphous a units, giving rise to larger crystal sizes than theory specified. Main‐chain stiffness, when present in the melt, had a small effect on the distribution of crystallite sizes but exhibited a much larger influence in preventing the attainment of equilibrium crystallinity, especially at high amorphous comonomer compositions. However, crystallinity was still high compared with that of copolymers described in the literature crystallizing through their main‐chain units. When long blocks of crystalline segments were present (as in compositionally heterogeneous vinyl stearate copolymers), melting point depression was small and followed the theoretical probability sequence function.

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