Melting transition temperature structural regularity, effects

The stmctural dependence of the crystalline melting temperature is essentially the same as that for the glass transition temperature. The only dilTerence is the effect of structural regularity, which has a profound influence on crystallizability of a polymer. T is virtually unaffected by structural regularity. From a close examination of data for semicrystalline polymers it has been established that the ratio Tg/T , (K) ranged from 0.5 to 0.75. The ratio is formd to be closer to 0.5 in symmetrical polymers (e.g., polyethylene and polybutadiene) and closer to 0.75 in unsymmetrical polymers (e.g., polystyrene and polychloro-prene). This behavior is shown in Figure 4.9. 

The thermal behaviour of materials can also provide important information about the structure and morphology of a material. For example, while most synthetic polymers have a glass transition temperature (Tg) associated with amorphous structure in the material, only polymers with regular chain architecture can crystalhse and so have a melting temperature (IJn). These in turn can have a direct effect on the mechanical performance of the materials since below Tg polymers tend to be glassy and become more rubbery above 7. These thermal properties can also be used to identify or verify the nature of the composition. For example, random copolymers will only exhibit one Tg that will be somewhere in between the 7 s of the individual homopolymers, whereas block copolymers will exhibit 7 s characteristic of each homopolymer but will be slightly shifted due to imperfect phase separations. Similarly this can be applied to polymeric blends, which are essentially two polymeric systems mixed together. 

The authors distinguish between three different cases for the dewetting mechanism (Figure 9). In thin films as well as in the bulk, a regular bilayer morphology is developed. Disordering into an isotropic melt occurs in two steps. A smectic mesophase is formed before the layered structure finally breaks up at elevated temperatures. This transition is characteristically effected by the interfaces in thin films on a flat substrate and, as a consequence, a peculiar self-dewetting is observed. 

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