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花花太岁

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虫号: 480053
注册: 2007-12-15
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专业: 配位化学

[交流] 求助翻译，非常感谢（高分子物理结晶方向）

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 A ˚ . From a geometrical standpoint, their mode of formationwas recognized four decades ago by Geil and Reneker. On a molecular scale, however, mechanisms by which shear forces, and necessaryplanes of easy shear oblique to growth faces (effectively tears or slits), arise during crystallization by chain folding have long remainedobscure. For some time shear has credibly been associated with lamellar warping under the inﬂuence of inhomogeneously distributedcongestion 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.

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