Expansivity rubbery-state

Figure 10.8 Expansion coefficient in rubbery state for amine-crosslinked epoxies, E (O), and for crosslinked aliphatic polyesters, P (<0>), versus crosslink density (n).

Thermal mechanical analysis was utilized by Ophir 174) to study the densification of Bisphenol-A-based epoxies. The glass transition temperature can easily be characterized by a slope change as the resin transits from the glassy state to the rubbery state (see Fig. 25). Hence, in glassy material, it is typically represented by two thermal expansivity parameters, one below T (glassy thermal expansivity) and one... 

Figure 28 shows the thermal expansivity of epoxy above its Tg as a function of thermal history. Rubbery-state expansivity is generally an order of magnitude larger compared to the glassy-state expansivity (see Table 2). As-cast epoxy has an expansivity above Tg of 3.22 x 10-4 K-1. With postcuring and quenching, this parameter tends... 

Fig. 28. Rubbery state thermal expansivity of Fiberite934 as a function of thermal history...

At the glass transition temperature (Tg), a thermoplastic material changes from a glassy state to a rubbery state. The properties of the material also change significantly. Tg values most often listed for polymers correspond to stiffening temperatures [3], The coefficient of thermal expansion usually doubles below Tg for these materials. Materials above the 7 , may be functional, but the performance may become unpredictable because most thermoplastic components are designed based on properties tested below 7 ,. 

Glassy-state expansivity decreases rubbery-state expansivity increases... 

Glass transition temperature (Tg) The temperature where the polymer transits from a rubbery state to a glassy state. The thermal expansion coefficient in the rubbery state is two to three times greater than in the glassy state because of greater molecular (chain) mobility. Tg varies between —120 and +130°C, depending on the type of polymer. Rubbery polymers such as elastomers have —ve Tg, i.e. Tg is well below their use temperature, whereas common plastics such as polyvinyl chloride (PVC) polymers have + ve Tg, i.e. Tg is well above their use temperature. However, if used above its Tg, PVC would display the usual rubber-like behaviour. 

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