On the Annealing of Polyethylene Crystals Formed from Dilute Solution
On the Annealing of Polyethylene Crystals Formed from Dilute Solution
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DOI：
10.1021/ma60012a016
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发表时间：
1969-11
期刊：
影响因子：
5.5
通讯作者：
L. Mandelkern;R. Sharma;J. Jackson
中科院分区：
文献类型：
--
作者：
L. Mandelkern;R. Sharma;J. Jackson
The critical annealing temperature, defined as the temperature at which crystals formed in dilute solution and then dried undergo major altei ations in thermodynamic and morphological properties, has been accurately determined as a function of the initial crystallite thickness. This temperature is shown to obey the theory for the melting of crystals of small size and thus can be identified with the melting temperature. It is, therefore, not necessary, as has been proposed, toendow the chain units within the interior of the crystals with any extraordinary mobility. The well-known changes in properties that are observed upon annealing are merely con-sequences of the fusion process and recrystallization. t is well established that when crystals prepared from dilute solutions of long-chain molecules are sepa-rated from the mother liquor, major alterations occur in morphological and thermodynamic properties when the samples are heated above a certain temperature. 2-6 These changes in properties include as much as a three-to fourfold increase in crystallite thickness as well as a substantial elevation of the dissolution temperature4 (melting temperature in a dilute solution.) Two quite different points of view have been expounded to explain these observations. On the one hand, it has been pro-posed7-10 that a regularly folded chain type of interfacial structure is initially present and maintained dur-ing the heating (annealing) process. Therefore, in order to explain the increase in thickness, the polymer molecule must possess an extraordinarily highand co-ordinated mobility within the crystalline phase. 2· 7-10 Such mobility would be an extraordinary characteristic of a crystalline polymer. In contrast, it has also been proposed4· 5·*** 9* 11· 12 that the observations in question are a consequence of either melting or of partial melting followed by recrystallization from the molten state. Measurements of the changes in density, birefringence, and wide angle X-ray diffraction patterns during iso-thermal annealing4 support this concept. Further confirmation of this hypothesis is found in the calorimetric properties of these crystals during fusion12 and the observation of Nagai and Kajikawa13 that
