Rubbery region also

Experimentally, the glass transition has also manifested itself by a sharp increase in relative rigidity (measured by dynamic-mechanical methods) and a simultaneous drastic decrease in the rate constant of the autocatalytic epoxy-amine reaction The mobility or rigidity of the system is a function of reaction conversion a in the pre-gel region it can be characterized by dynamic viscosity which is proportional to M of the reacting system. Beyond the gel point, still in the rubbery region but not close to the gel point, the dynamic modulus, G, is at low frequencies proportional to " (m a 1)... 

The choice of the process depends, inter alia, on the requirements of the end product with injection molding, narrower tolerances can be achieved than with thermoforming or rotomolding, for example. The nature of the polymer also plays a role it depends, for instance, on the stability of the polymer at the high temperatures in a process. The length of the rubbery region on the temperature scale determines the ease of vacuum forming too brittle a polymer is difficult to machine, for example. 

The increase in modulus may also often be expressed in terms of the slightly different Mooney (1951) or Guth (1944)-Smallwood (1944) equations. For example, with glass-bead-filled epoxy resins, Lewis and Nielsen (1970) found agreement between predicted and observed values of modulus in the rubbery region using the Mooney (1951) equation ... 

In any case, the difference between the two equations is small at volume fractions up to, say, 0.25. Equation (12.9) was also used successfully by Landel (1958) to characterize the modulus of glass-bead-polyisobutylene composites in the long-time (rubbery) region of stress relaxation. 

It is important to note that the Hi2MDI-based polyurethane does not show constant dynamic storage modulus in the rubbery region and also that an unusual property possessed by the CHDI-based PUs is the apparently constant dynamic storage modulus ) possessed over the relatively large temperature range of + 20°C to 180°C or above depending upon types of formulation. 

The rubbery plateau also represents an indicator of miscibility (Badia et al. 2015). Linear triblock copolymers composed of poly(n-butyl acrylate) and polystyrene (PS-PBA-PS) have phase-separated morphologies exhibiting two rnbbery plateaux (Pakula et al. 2011). The first plateau lies between the Tg of the PBA blocks and the Tg of the PS blocks in this region, the glassy PS microdomains act like physical crosslinks for the flexible PBA blocks. The second rubbery plateau starts after the Tg of PS and continues until the order-disorder transition, where the microdomain strncture transforms into a disordered homogeneous phase (Han et al. 1989). The length and the storage modnlns of the rubbery plateau depend on the molecular... 

---