Molecular cooperativity glass transition

One final but very important point needs to be made regarding the glass transition. As highlighted by Ludescher et al. (2001), it must be kept in mind that the glass transition is a macroscopic manifestation of cooperative changes in molecular mobility (specifically, translational mobility) of individual molecules in a continuous amorphous phase—where the change in... 

Early investigations of crosslinked polymeric materials observed line width changes with cure. An increase in the line width is observed in vulcanized elastomers at room temperature, compared with the unvulcanized samples 13). The effect of vulcanization on line widths of butadiene-styrene copolymers in a temperature range near the glass transition (Tg), was interpreted in terms of the formation of cooperative domains, which lead to larger linewidths (Av)14). Both, an increase in Av and shift of the transition range to higher temperatures due to the restriction of molecular mobility... 

Although molecular mobility is severely restricted below the glass transition temperature, the dynamic glass transition temperature (main transition or, conventionally -relaxation) in polymers as it have been described above, is usually accompanied by subglass secondary relaxations labeled as p, y, S, relaxations. The glass transition at low temperatures is assumed to be caused by the cooperative motion of many particles, while the secondary relaxations have a more localized molecular... 

The mechanical dispersion peaks in low-Tg epoxies such as Bisphenol-A based resin (Epon 828, products from Shell Development Company) have been the subject of numerous studies 143,145148,152 "155, l59>. The alpha-dispersion peak related to the glass transition can undoubtly be attributed to the large-scale cooperative segmental motion of the macromolecules. The eta-relaxation near —55 °C, however, has been the subject of much controversy 146,153). One postulated origin of the dispersion peak is the crankshaft mechanism at the junction point of the network epoxies (Fig. 17). The crankshaft motion for linear macromolecules was first propos-ed 163 166> as the molecular origin for secondary relaxations which involved restricted motion of the main chain requiring at least 5 and as many as 7 bonds 167>. This kind of... 

Vidal Russell, E., and Israeloff, N. E., Direct observation of molecular cooperativity near the glass transition. Nature 408, 695 (2000). 

There have been several attempts to relate Kg to the critical molecular weight (Mcr, which is discussed in Chapter 13) based on considerations of the effects of polymer chain entanglements on the large-scale cooperative motions of chain segments involved in the glass transition for example, see references [97-99]. It was also suggested [100,101], from considerations of chain stiffness and the statistics of chain conformations, that Kg should be proportional to a power of... 

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