On the origins of giant screw dislocations in polymer lamellae

Giant screw dislocations in chain-folded polymer crystals, having axes necessarily parallel to chain stems, traverse lamellar thickness and, although without hollow cores, have large Burgers vectors, often in excess of 100 إ. From a geometrical standpoint, their mode of formation was recognized four decades ago by Geil and Reneker. On a molecular scale, however, mechanisms by which shear forces, and necessary planes of easy shear oblique to growth faces (effectively tears or slits), arise during crystallization by chain folding have long remained obscure. For some time shear has credibly been associated with lamellar warping under the influence of inhomogeneously distributed congestion in fold surfaces. A study of dendritic crystals in polyethylene has now shown how on occasion impingements inherent in layer spreading by folded chains, combined with kinetic barriers of entropic character, can produce defects that permit shear across nominal fold planes with little or no encumbrance from bridging folds. Evidence for this mechanism is particularly clear for dislocations initiated at reentrant corners and, although not readily amenable to direct observation, similar behavior likely underlies marked increase in dislocation densities observed with transition from Regime I to Regime II in crystallization from the melt. Proposed mechanisms do not rely upon specific characteristics of the experimental system studied.