Glass-to-rubber transition

Most PHAs are partially crystalline polymers and therefore their thermal and mechanical properties are usually represented in terms of the glass-to-rubber transition temperature (Tg) of the amorphous phase and the melting temperature (Tm) of the crystalline phase of the material [55]. The melting temperature and glass transition temperature of several saturated and unsaturated PHAs have been summarized in Table 2. 

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. 

Because direct dynamic measurements of materials state within the barrel are impossible, the structural and molecular changes relevant to extrusion must be measured off-line and related to real process conditions. In principle, we need to explain not only the effect of applied physical parameters of heat, shear and pressure, but also the effect of formulation. This is not yet possible, and for reasons stated above, the behaviour of carbohydrate (starch) dominated systems will be quite different from proteinaceous systems, since their heat denaturation behaviour and glass to rubber transitions are different in detail during their conversion from a moist powder to a continuous melt. ... 

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. 

Observed Tg s vary from -123°C for polyfdimelhyl siloxane) (1-43) to 273°C for polyhydantoin (11-2) polymers used as wire enamels and to even higher temperatures for other polymers in which the main chain consists largely of aromatic structures. This range of behavior can be rationalized, and the effects of polymer structure on Tg can be predicted qualitatively. Since the glass-to-rubber transition... 

Many relatively slow or static methods have been used to measure Tg. These include techniques for determining the density or specific volume of the polymer as a function of temperature (cf. Fig. 11-1) as well as measurements of refractive index, elastic modulus, and other properties. Differential thermal analysis and differential scanning calorimetry are widely used for this purpose at present, with simple extrapolative eorrections for the effects of heating or cording rates on the observed values of Tg. These two methods reflect the changes in specific heat of the polymer at the glass-to-rubber transition. Dynamic mechanical measurements, which are described in Section 11.5, are also widely employed for locating Tg. 

The development of a maximum in tan 5 or ihe loss modulus at the glass-to-rubber transition is explained as follows. At temperatures below Tg the polymer behaves elastically, and there is little or no flow to convert the applied energy into internal work in the material. Now It, the energy dissipated as heat per unit volume of material per unit time because of flow in shear deformation, is... 

Figure 4 shows the TPA results for the bisphenol-A and the bisphenol-S linked polymers cured at 280°C for six days. Both the dynamic shear modulus and the mechanical loss factor are given as a function of temperature from -150°C to about +300°C. During a TPA run, a temperature scan covering the complete glass-to-rubber transition could not be achieved because the sample softened as the glass transition temperature, Tg, was approached. 

Elastin, which is substantially amorphous but fibrous at all levels of investigation (starting from the largest filaments which are about 6 fim in diameter and down to about 10 nm (17,18)), is a fragile, glassy substance when dry and has a glass-to-rubber transition temperature at about 200 C (19) upon hydration or solvation with appropriate solvents, it becomes a rubbery system with the glass transition below room temperature (20). 

Tg Behavior. As shown In Figures 1 and 2 all the D.E.R.331/ECrO systems exhibited single glass-to-rubber transitions the resins were also quite clear In appearance. Thus It was concluded that In these systems the epoxldlzed oil acted as a relatively miscible Internal plasticizer. At the same time, the Increasing breadth of the transition with Increasing ECrO content suggests the onset... 

The onset of the Tg is near 175°C. This composite, which is 45° carbon-fiber-reinforced, shows a dynamic storage modulus of the epoxy matrix in the glassy-state of ca. 15 GPa. At the onset of the glass-to-rubber transition (see Figure 6), the modulus drops gradually from 15 GPa (175°C) to about 3 GPa (300°C) as the rubbery plateau is reached. 

Erom the practical point of view, fundamental information on the processability of polymers is usually obtained through thermal analysis, which provides knowledge of the main polymer transitions (melting and glass-to-rubber transition to the crystalline and amorphous phases, respectively). In addition to the well-established calorimetric techniques, experimental methods capable of revealing the motional phenomena occurring in the solid state have attracted increasing attention. 

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