Glass-to-rubber transition temperature

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. 

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). 

Polymer networks are conveniently characterized in the elastomeric state, which is exhibited at temperatures above the glass-to-rubber transition temperature T. In this state, the large ensemble of configurations accessible to flexible chain molecules by Brownian motion is very amenable to statistical mechanical analysis. Polymers with relatively high values of such as polystyrene or elastin are generally studied in the swollen state to lower their values of to below the temperature of investigation. It is also advantageous to study network behavior in the swollen state since this facilitates the approach to elastic equilibrium, which is required for application of rubber elasticity theories based on statistical thermodynamics. ... 

DMA spectra are used to probe the so-called viscoelastic nature of materials. The properties of polymeric resins, for example, change dramatically near their glass-to-rubber transition temperature (Tg). A change of several orders of magnitude in the storage modulus E, which characterizes the stiffness of the material, is a common occurrence for polymers around Tg. 

There is, however, one troublesome discrepancy between the experimentally observed results and the above model. The glass-to-rubber transition temperature of atactic polystyrene measured with DMA is about 100 On the other hand,... 

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. 

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. 

DSC curves of the PLA samples, represented in Fig. 11, clearly show the presence of glass transition temperature around 60 °C. It is important to point out that this glass-to-rubber transition of amorphous component agrees well with the change in the elongation observed in the TMA. More importantly, the samples also provide obvious crystallization peak around 110 °C. The crystallization peak shows gradual increase by the inclusion of the clay content, suggesting quantitative increase in the amount of the crystalline structure. Thus, it... 

Modulus-temperature relations for amorphous polymers in static test reveal a sharp drop of modulus in the glass-to-rubber transition region (see Figure 1.19). Since the storage modulus G (oj) behaves like a modulus measured in a static test, it decreases in the glass transition region. However, the loss modulus G"((jj) and tan 6 go through a maximum under the same conditions. 

For solids which are purely elastic tan S equals zero. Metals, for example, conform fairly closely to this ideal another good example of a low-damping solid is quartz. Polymers on the other hand have values of S of the order of several degrees in certain temperature ranges (for instance the glass to rubber transition) 8 may approach 30°. This very high damping is frequently of use technically (4.N.3). 

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