Equilibrium melting of flexible linear macromolecules

Abstract The extensive literature on equilibrium melting of flexible linear macromolecules has been reviewed during the last year. The discussion in this paper is based on experiments on actual, large, extended chain crystals and extrapolations on metastable crystals. A series of 32 complete sets of melting temperatures, heats of fusion, volume changes on fusion, and entropies of fusion have been collected. These data are compared to melting data on rigid macromolecules and small macromolecules. Only a relatively simple model of rotational isomerism is needed to understand the entropy of fusion of flexible linear macromolecules which is 8‐12 J/K/mole of rigid backbone units whenever there is no mobility in the crystal at the time of melting. This is much less information on detailed conformational analysis than is generally assumed to be necessary to interpret melting. Instead, it is of importance to consider for the discussion of melting also the change in packing fraction on fusion, which is high for crystals with planar zig‐zag conformation, and low for helices. Both types of crystals lead to similar packing fractions in the melt. Higher packing fractions, both in the melt and crystal, are found for polyesters, polyamides, and polyoxides. The heats of fusion which make up the third group of melting parameters considered are more connected with molecular size (surface area) and cohesive energy density than mobility, so that larger mobile backbone units and higher cohesive energy densities lead to higher melting temperatures. A further addition to the heat of fusion comes from the presence of high energy rotational isomers in the melt, which again increases the melting temperature. Further refinement of this initial discussion is possible, but all aspects of melting must be taken into account rather than concentrating on single aspects as has been done in the past.