Segmental chain mobility

The third type of cross-linking involves formation of crystalline portions within an amorphous grouping (Figure 2.21). For vinyl polymers, the temperature that allows segmental chain mobility to occur is well below room temperature, but the temperature to disrupt crystalline formations is typically well above room temperature. Within mixtures of crystalline and amorphous structures below the T, the crystalline portions act to tie in or connect the surrounding areas acting as cross-links. 

Polymers can exhibit a number of different conformational changes with each change accompanied by differences in polymer properties. Two major transitions occur at Tg, which is associated with local, segmental chain mobility in the amorphous regions of a polymer, and the melting point (Tjj), which is associated with whole chain mobility. The Tn is called a first-order transition temperature, and Tg is often referred to as a second-order transition temperature. The values for Tjj are usually 33 to 100% greater than for Tg, and Tg values are typically low for elastomers and flexible polymers and nigher for hard amorphous plastics. 

Interestingly, a decrease in permeability with DS for HMW polymer is faster than for LMW polymer. Considering that for a given DS, densities of LMW HSPPO and HMW HSPPO are comparable, a faster decrease in permeability for the latter polymer should be a result of more significant decrease in its segmental chain mobility. In other words, an increase in Tg... 

Diffusion of small molecular penetrants in polymers often assumes Fickian characteristics at temperatures above Tg of the system. As such, classical diffusion theory is sufficient for describing the mass transport, and a mutual diffusion coefficient can be determined unambiguously by sorption and permeation methods. For a penetrant molecule of a size comparable to that of the monomeric unit of a polymer, diffusion requires cooperative movement of several monomeric units. The mobility of the polymer chains thus controls the rate of diffusion, and factors affecting the chain mobility will also influence the diffusion coefficient. The key factors here are temperature and concentration. Increasing temperature enhances the Brownian motion of the polymer segments the effect is to weaken the interaction between chains and thus increase the interchain distance. A similar effect can be expected upon the addition of a small molecular penetrant. 

The temperature at which local segmental mobility occurs is called the Eg and that at which wholesale polymer chain mobility occurs is called the T -... 

The activation energy for polystyrene creep as given by Figure 9 is approximately 8 kcal. This relatively low activation energy probably indicates that the flow units—e.g., chain segments—whose mobility causes the creep are relatively short. 

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