Crystalline-amorphous features folded-chain

Appropriately, this was called the Folded Chain Theory and is illustrated in Fig. A.ll. There are several proposals to account for the co-existence of crystalline and amorphous regions in the latter theory. In one case, the structure is considered to be a totally crystalline phase with defects. These defects which include such features as dislocations, loose chain ends, imperfect folds, chain entanglements etc, are regarded as the diffuse (amorphous) regions viewed in X-ray diffraction studies. As an alternative it has been suggested that crystalline... 

The semi-crystalline structure of polymers is characterized by special features. Firstly, the crystallites are embedded in an amorphous matrix, resulting in a two-phase morphology. Secondly, most polymers form folded-chain crystals in which the chains fold back into the same crystallite. Thirdly, several crystallites stack up and form superstructures known as sphemlites [8], Fig. 1.2. Finally, chains wander from one crystallite to the next one, thereby connecting them to each other. The chain segment between two adjacent crystals is known as tie-molecule [9], Tie-molecules act as stress-transmitters [10-12]. Thus they play an important role in mechanical properties of semi-crystalline polymers. 

Prom the series of experiments reported above it is evident that chain folding at the interphase plays an important role in packing of the chains within the crystalline lattice. In this section we aim to investigate the influence of the interphase on the melting behaviour of crystals and its implications in the polymer melt. Material investigated for the purpose is a solution crystallized UHMW-PE. Salient features on the material have been summarized in the section 4 of this chapter and details have been provided in [31]. Since the solution crystallized UHMW-PE is made from dilute solutions, the number of entanglements between the crystalline and amorphous regions is reduced to an extent that the material can be drawn in the solid state (Fig. 15.5) by more than 100 times. 

In one extreme, termed the regularly folded-adjacent-re-entry stmcture, the molecular chains appear to be accordion-like, making precise hairpin turns in order to yield the optimum level of possible crystallinity. However, equally consistent with the gross morphological features is the other model illustrated. Here, there is a distinct, disordered, amorphous overlayer. This schematic representation has popularly been termed the switchboard model. Both of these interfacial stmctures, and those in between, are consistent with the electron micrographs. The reason for introducing these concepts here is that a lamellar-type crystallite is also the universal mode of crystallization of a homopolymer from the pure melt. 

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