Glass transition temperature of the blend

The foam processing window of the blend systems is also controlled by the glass transition temperature of the blend phases. With regard to the neat blend system, the addition of PS continuously lowers the glass transition temperature of the PPE/PS phase, as predicted by the Couchman equation [77] (Fig. 25). For the carbon dioxide-laden case, the plastifying effect needs to be taken into account, which lowers the glass transition temperature of both PPE/PS and SAN. 

The spontaneous mixing of the two polymers will transpire at a rate which reflects the degree of miscibility of the system. As X approaches the critical value for phase separation, "thermodynamic slowing down" of the interdiffusion will occur [12]. The rate of increase of the scattering contrast reflects the proximity of the system to criticality, as well as the strong composition dependence of the glass transition temperature of the blend. Extraction of a value for either the self diffusion constants [13,14] or the interaction parameter is not feasible from the presently available data. 

A polymer blend that obeys equation (12.7) consists of equal fractions by weight of two polymers with glass-transition temperatures 50 and 190°C. Calculate the glass-transition temperature of the blend. 

In blends of PBN and poly(ether imide) (PEI), one single Tg was observed. The glass transition temperature increases monotonously with the increase of PEI content. The glass transition temperatures of the blends fit the Fox equation, indicating a mixing of the components. 

Tg is the glass transition temperature of the blend, Tg is the glass transition temperature of the pure polymer 1, Tg is the glass transition temperature of the pure polymer 2, K is the ratio of the difference between the expansivities above and below the glass transition of polymer 2 and polymer 1, and volume fraction of polymer 2. 

The density-composition curve (Fig. 7) shows increased density in all cases, maxima occurring at 80% PU. The IPN s exhibited the largest increase at all concentrations. This increase seems to indicate increased molecular mixing in full IPN s. This can be verified by the glass transition temperature behavior. Kim et al explained the increased density of IPN s qualitatively by means of chain entanglements at the domain boundaries (24). However, it is possible to quantitatively correlate it with Tg shift. As the glass transition temperatures of the blends shift inward (from... 

Figure 4.24. Diffusion coefficients as functions of the composition in the miscible blend polystyrene-poly(xylenyl ether) (PS-PXE) at a temperature 66 °C above the (concentration-dependent) glass transition temperature of the blend, measured by forward recoil spectrometry. Squares represent tracer diffusion coefficients of PXE (VpxE = 292), circles the tracer diffusion coefficients of PS and diamonds the mutual diffusion coefficient. The upper solid line is the prediction of equation (4.4.11) using the smoothed curves through the experimental points for the tracer diffusion coefficients and an experimentally measured value of the Flory-Huggins interaction parameter. The dashed line is the prediction of equation (4.4.11), neglecting the effect of non-ideality of mixing, illustrating the substantial thermodynamic enhancement of the mutual diffusion coefficient in this miscible system. After Composto et al. (1988).

The change in entropy of mixing at the glass transition temperature of the blend can be used to account for the observations. 

The equation for the glass transition temperature of the blend obtained from relating the change in the entropy of mixing to the change in pressure of the blend at the glass transition turns out to be cubic in nature. Equation (6.40) can... 

The change in entropy of mixing at the glass transition temperature of the blend can be used to account for the observations. A quadratic expression for the mixed glass transition temperature is developed from the analysis. For a symmetric blend the conditions when the solution is one root denoting the miscible blends is derived. For a symmetric... 

The first of these ratios can readily be evaluated from Eqs. (7-10). At the glass transition temperature of the blend (T g T ) the monomeric... 

There have been several miscible high temperature polymer pairs defined in the literature. Several of these pairs are miscible from solution but are immiscible when processing is attempted in the melt state. These results indicate that the blends phase separate when heated above their glass transition temperature. This further shows that kinetic factors as well as thermodynamic factors are important in the observed miscibility. Also, the role of the solvent in the observed miscibility needs to be better understood. One of the current technical challenges is to widen the temperature range between the glass transition temperature of the blend and its phase separation temperature, to allow miscible blends to be processed in the melt state. 

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