CHAIN ORIENTATION AND CRYSTALLIZATION

Abstract

The article starts with the assertion that in order to obtain fibrous crystals with extended chain type characteristics the molecules have to be extended prior to crystallization otherwise chain folded lamellae will result. Further it asserts that in the case of non-homogeneous chain extension those chains of the assembly which are inadequately extended will give rise to lamellar crystals nucleated along the fibrous ones already formed. The main body of the article provides substantiation of this generalization and demonstrates its consequences in a number of diverse examples. It is divided in two parts according to the two broad categories of chain extension. (1) Static elongation which is applicable to crosslinked melts (elastomers) and (2) chain extension by flow achieved by elongational flow in solutions and melts. The part dealing with (1) is basically a review centred largely on past works by one of us while the part concerned with (2) is a preliminary account of work in progress. The latter in particular describes how characterized elongational flow fields can be purposefully created and how the resulting chain extension and the ensuing crystallization can be studied in situ as a function of a number of variables. In the course of it reference is made amongst others to criteria for complete chain extension, to the role of entanglements and to the role of localized flow fields in the vicinity of the solidified portions of the flowing system. Attention is drawn to the diversity of effects which all arise from the same few basic principles and to the numerous fundamental and technological implications.