Glass-rubber transition region

The temperature dependence of the compliance and the stress relaxation modulus of crystalline polymers well above Tf is greater than that of cross-linked polymers, but in the glass-to-rubber transition region the temperature dependence is less than for an amorphous polymer. A factor in this large temperature dependence at T >> TK is the decrease in the degree of Crystallinity with temperature. Other factors arc the reciystallization of strained crystallites ipto unstrained ones and the rotation of crystallites to relieve the applied stress (38). All of these effects occur more rapidly as the temperature is raised. 

The temperature region in which a polymer can be used, is limited at the low as well as at the high side. The most serious limitation at low temperatures is, as a matter of fact, the glass - rubber transition for rubbers, below Tg they lose their rubbery nature and pass into the glassy phase. Besides, for thermoplastics, the principle limitation is that they become brittle at low temperatures, and thus lose their impact strength. The other properties are not affected, but, in most cases, (E-modulus, tensile strength), improved upon decrease of temperature. First of all we shall consider at which temperature cold-brittleness appears. 

Brittleness is found with semi-crystalline polymers below their glass-rubber transition Tg. An example is PP, which becomes brittle at about T -10 °C. PE retains its ductile nature down to very low temperatures. Other polymers have a Tg of some tens of °C above room temperature, such as polyamides and thermoplastic polyesters. Various mechanisms are responsible for a reasonable impact strength at room temperature for polyamides this is, for instance, the absorption of water also secondary transitions in the glassy region may play a role. 

The value of Tg is important, since rigidity decreases in the glass transition region, Tg is closely related to the structure and crosslinking density of the cured resin. The structure of the cured resin can be derived from the initial starting materials, the epoxy prepolymer and hardener, and reaction conditions. However, the structures of many cured resins is still unclear which prevents to establish the structure-mechanical properties relationships. Further studies are needed. Furthermore, the commercial epoxy formulations may contain several components and also diluents, plasticizers, liquid rubbers, etc. which makes a prediction of Tg and mechanical properties even more difficult. 

The temperature dependence of the relaxation modulus at 500 seconds of polycarbonate (7), polystyrene (8), and their blends (75/25, 50/50, and 25/75) was obtained from stress-relaxation experiments (Figure 4, full lines). In the modulus-temperature curves of the blends, two transition regions are generally observed in the vicinity of the glass-rubber transitions of the pure components. The inflection temperatures Ti in these transition domains are reported in Table I they are almost independent on composition. The presence of these two well-separated transitions is a confirmation of the two-phase structure of the blends, deduced from microscopic observations. 

Increasing filler loading broadens the relaxation spectrum of the cure reaction. Broadening the relaxation spectrum by filler loading also has been found in the mechanical spectrum of cured rubber from the glass transition region to rubbery plateau region [15]. 

As the temperature is raised the thermal agitation becomes sufficient for segmental movement and the brittle glass begins to behave in a leathery fashion. The modulus decreases by a factor of about 10- over a temperature range of about I0-20°C in the glass-to-rubber transition region. 

As we have seen above, the transition that separates the glassy state from the viscous state is known as the glass-rubber transition. This transition attains the properties of a second-order transition at very slow rates of heating or cooling. In order to clearly locate the region of this transition and to provide a broader picture of the temperature dependence of polymer properties the principal regions of viscoelastic behavior of polymers will be briefly discussed. 

Another major discrepancy between theory and experiment is exemplified in Figure 3-21. In this figure, the predicted relaxation according to the Rouse theory is compared with an experimental result for polystyrene in the primary transition region. It is clear that polystyrene undergoes its glass-to-rubber... 

---