Side‐chain crystallinity. V. Heats of fusion and melting temperatures on monomers whose homopolymers have long side chains

Abstract

AbstractHeats of fusion and melting temperatures were obtained for selected monomeric n‐alkyl acrylates, N‐n‐alkylacrylamides, and vinyl esters. The corresponding thermodynamic parameters for homopolymers, derived from these monomers, had been reported previously from this laboratory. The α‐hexagonal crystal modification was indicated near the melting point for the higher n‐alkyl acrylates, but a β form was stable at low temperatures for the entire series. The magnitude of the heats of fusion indicated β polymorphs for vinyl esters in support of x‐ray diffraction analysis from the literature. Because hexagonal crystal geometry prevailed in all reported homopolymers having long side chains, greater emphasis was placed on thermodynamic data for monomers exhibiting this crystal modification. Accordingly, a convergence temperature was estimated statistically for the α‐hexagonal crystal modification of these systems and appropriate literature values of the n‐alkanes and ethyl esters. The convergence temperature was computed to be 135°C, uncorrected for the entropy of disorientation. The anomalously large interfacial end‐packing‐defect energy of the poly(n‐alkyl acrylates) and the poly‐N‐n‐alkylacrylamides was shown to be associated with a high energy barrier to molecular transport in the melt as the vitreous state was approached. In support of this conclusion, similarity of the glass and melting transition temperatures of these homopolymer homologs occurred near their critical side‐chain lengths, below which the homopolymers are amorphous. A special critical requirement of nucleus length was not indicated from rough estimations of nucleation parameters for the poly(n‐alkyl acrylates). These findings lent increased, but still not unqualified, support to an x‐ray diffraction study from the literature. The latter had specified the inclusion of the entire side chain and the main‐chain units in the crystal lattices of the higher poly(n‐alkyl acrylates).