Crystallographic factors affecting the structure of polymeric spherulites. II. X-ray diffraction analysis of directionally solidified polyamides and general conclusions

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Abstract
The morphological diversity in even‐even nylons described in Paper I was found to be the result of crystallographic factors related to the high extent of hydrogen bonding. Among all spherulitic species in these polyamides, only positive spherulites crystallize in accordance with the phenomenological theory of Keith and Padden. However, as a result of their unit cell orientation which renders the hydrogen‐bonded sheets radial and the molecular chains almost tangential, folding occurs predominantly into the melt rather than in the usual tangential manner. The microstructure of negative spherulites departs from the norm in that long ribbonlike lamellae are absent; instead, negative fibrils are composed of short segments containing tangentially arranged stacks of hydrogen‐bonded sheets. In spherulitic aggregates the molecular chains are crystallized at the unusually acute angle of 49° to the direction of solidification. The preferred direction of fibrillar growth in these aggregates is oblique to the spherulitic radius; this could account for their variable and interrupted growth rate, their apparent coarseness of texture, as well as their extinction at 45° to the crossed polars. All spherulitic species crystallize with the α‐unit cell. A possible explanation for this is offered, relating the unilateral shear of hydrogen‐bonded planes to dipole interactions between adjacent layers. Such interaction would also help explain the positive birefringence of spherulitic aggregates which is inconsistent with their polarizabilities.