Melting temperature of networks formed from randomly arranged crystallites

5 Melting temperature of networks formed from randomly arranged crystallites 

Since networks with unique properties result from cross-linking highly axially oriented polymer chains, the question naturally arises as to what limits there are, if any, to the degree of intermolecular order required to observe these effects. Partially crystalline undeformed polymers possess a large amount of intermolecular order, since significant portions of the polymer molecules are constrained to lie parallel to one another in three-dimensional array. This order is only on a microscopic scale since the crystalline regions are randomly arranged relative to one another. The question as to whether the presence of such order influences the properties of the resultant isotropic network is a matter to be decided by experiment. 

The greater stability of the crystalline state of networks formed from unoriented but crystalline chains compared with networks formed from amorphous polymers, can be explained in the same way as for networks formed from axially oriented natural rubber. Although prior to network formation the crystallites are randomly arranged relative to one another, portions of chains are still constrained to lie in parallel array. The cross-linking of the predominantly crystalline polymer cannot, therefore, involve the random selection of pairs of units. The units that can be paired are limited by the local chain orientation imposed by the crystalline structure. An increase in the isotropic melting temperature of such networks would therefore be expected. It can be concluded that orientation on a macroscopic scale is not required for partial order in the liquid state to develop. Concomitantly a decrease in the entropy of fusion will result, which reflects the increase in molecular order in the melt. This is an important concept that must be kept in mind when studying the properties of networks formed in this manner. This conclusion has important implications in studying the properties of networks formed from unoriented crystalline polymers. 

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