Thermodynamics of rubbery behavior

together with the Tg of 200 K of polyisoprene, suggests that at 270 K, at which temperature a fully developed rubbery response is obtained in polyisoprene, the configurational relaxation period must be of the order of 1.0 s, which is quite consistent with expectations. 

For the purpose of considering both rubbers with fully recoverable deformation and glassy polymers that exhibit rubbery behavior above their Tg through the presence of entanglement networks, we distinguish two separate families of material (a) rubbers that have been cross linked through vulcanization and (b) some glassy polymers that show prominent rubbery behavior in their mechanical responses above their Tg for short periods. 

Some of the prominent glassy polymers that we consider in the second family that can exhibit rubbery behavior above their Tg are polystyrene (PS), polypropylene (PP), polyvinylchloride (PVC), and polymethyl methacrylate (PMMA). These are shown in Fig. 2.1 in Chapter 2. 

The elastic deformation resistance of rubbers and their temperature dependence must be viewed on a thermodynamic basis. Moreover, the mechanistic response of rubbers is also best understood in a principal-axis frame of reference. 

The first law of thermodynamics states that under isothermal conditions the internal energy change AU of any closed system is positive on absorption of an increment of heat dg and of work AW, i.e., 

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