Melt crystallized polyamide 6.6 and its copolymers, Part II. Crystallization mechanisms in the homopolymer

Abstract

The crystallization kinetics of polyamide 66 have been studied using polarized optical microscopy. The growth rate data for positively birefringent spherulites in polyamide 66 show a distinct change of slope, which would be identified as a regime I/II transition based on secondary nucleation theory. However, recent data for the same specimens crystallized isothermally, from small angle X-ray scattering found the lamellar thickness to be constant at approximately 2.0 chemical repeat units, but with an internal crystalline core and a substantial innerlayer. The crystal core increases in size to 2 chemical repeat units with both time and temperature at the expense of the inner layer. This evidence is totally inconsistent with secondary nucleation theory, where a lamellar thickness which varies significantly with supercooling is an integral part of the derivations. A calculation of the dimensions of the critical nucleus is reported here, using surface free energies, which found it to be impossibly large at a value between 14 and 360 stems in size, further suggesting that another crystallization mechanism is operating. Calculations of the surface free energy of the hydrogen-bonded surface suggest that it is the high energy surface, rather than the folded surface, which normally occurs as the high energy surface in polymers. As the high energy surface, the hydrogen-bonded surface would be expected to be the growth face, as occurs in non-polymeric materials. An earlier model of Lovinger, which placed the fold direction into the melt, generating a rough surface, is consistent with these results. It is suggested that crystallization in polyamide 66, if not in all polyamides, occurs through a surface roughening mode of growth. This is a natural consequence of the presence of H-bonding in the direction of growth. In one sense, polyamide 66 is conventional in its growth behavior, relative to non-polymeric materials, as the growth face is the highest energy surface. As such, the lamellar thickness would no longer be a morphological variable related to the supercooling in any direct way as an essential part of any crystallization theory for polymers. Such behavior is impossible in other polymeric systems as the fold surface is the highest energy surface and the presence of folds prohibits growth on that surface. However, models of surface roughening, which were developed as an alternative explanation for the behavior of, principally, polyethylene, predict the conventional lamellar thickness – supercooling relation to exist, which is inconsistent with the observed existence of a constant lamellar thickness with variable crystal core dimensions. It is suggested that polyamide 66 be taken as a paradigm for a different kind of polymer crystallization than that normally encountered. That is crystallization in which the growth face is the highest energy surface, not the folded chain surface, having much in common with the behavior of non-polymeric materials. The energetic changes occurring in this case, however, are governed by a combination of entropic and enthalpic barriers to crystallization, rather than being dominated by enthalpic considerations, as in metals. This is a direct result of the entropic effects of the long chain nature of polymers combined with the enthalpic effects of the hydrogen-bonding.