Glass-rubber transition, poly methyl

Amorphous polymers of commercial importance include polymers which are glassy or rubbery at room temperature. Many amorphous thermoplastics, such as atactic polystyrene and poly (methyl methacrylate), form brittle glasses when cooled from the melt. The glass transition temperature, Tg or glass-rubber transition, is the temperature above which the polymer is rubbery and can be elongated and below which the polymer behaves as a glass. Thermal analysis of amorphous polymers shows only a glass transition temperature whereas crystalline poly-... 

Mina, M. R, Ania, R, Rluy, T. A., Michler, G. H., and Balta Calleja, P. J. 2004. Micromechanical behavior and glass transition temperature of poly(methyl methacrylate)-rubber blends. Journal of Macromolecular Science B Physics 43(5) 947-961. 

Measurements as a function of pressure have been made in a few cases. For poly(methyl methacrylate) (128), the hysteresis shear absorption at 1 GPa is about one-fourth of that at ambient pressure. For plasticized poly(vinyl chloride) (125), pol5dsobutylene (123), and natural rubber vulcanizate (124), upon the application of pressure, the glass transition peak shifts to higher temperature with a lower but broader peak. 

The glass transition temperature of the polymer has a very marked influence on the diffusion coefficient. This is most clearly shown by the increase in Dq, the diffusion coefficient extrapolated to zero permeant concentration, with decreasing as shown in Figure 8. The large increase in Dq with decreasing value of Tg also leads to a significant decrease in the concentration dependence of the diffusion coefficient. For instance, the diffusion coefficient of benzene in natural rubber, poly(ethyl acrylate) and poly(methyl acrylate), with values of 200 K, 250 K and 280 K respectively, increases by 2.9, 20 and 340 times on increasing the volume fraction of benzene from 0 to 0.1. 

Mina MF, Ania F, Huy TA, Michler GH, Balta Calleja FJ. Micromechanical behaviour and glass transition temperature of poly(methyl methacrylate) - rubber blends. / Macromol Sci B... 

The second example of a polymer reaction is the industrial cross-linking of rubber by vulcanization sketched in Fig. 3.50. The process was invented already in 1839 by C. N. Goodyear without knowledge of its chemical stracture. Natural rubber is cis-poly(l-methyl-1-butenylene) or polyisoprene with a low glass transition temperature of about 210 K. Its structure and those of other rubbers are given in Fig. 1.15. The addition of sulfur in the form of Sg rings and heating causes the vulcanization. Of the listed cross-hnks in Fig. 3.50, only the left example is an efficient network former. The sulfur introduces about 1 cross-link for each of 50 S-atoms used. Modem vulcanization involves activators and accelerators for increased efficiency. The detailed mechanism is rather complicated and not fully understood. 

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