Segmental Dynamics, Fragility Index, and Free-Volume

Theoretical Approaches to Calculating the Class-Rubber Relaxation Temperature 

The T of polymer blends can be related to the blend composition by different equations, as shown in Table 12.2. The T of miscible blends of poly(p-dioxa-none) with poly(vinyl phenol) (PPDO/PVPh) [74], as studied using the Fox [75], Gordon-Taylor [76], Couchman-Karasz [77] and Kwei [78] models, showed that the experimental data lay below the Fox equation, suggesting that the free volume of the blends was larger than predicted, assuming free volume additivity. On the other hand, the Gordon-Taylor and the simplified Kwei equations fitted the experimental T values appropriately. Other studies have shown that the Ta of SAN/PMMA blends was also effectively approached by the Gordon-Taylor relationship [79,80]. 

Segmental Dynamics, Fragility Index, and Free-Volume 

The effectiveness of blending can be also assessed from the point of view of segmental mobility, by measuring either the free-volume coefficient or the thermal expansion coefficient in the rubbery state. This can be considered not only as an indicator of the degree of miscibUity, with its consequent viscoelastic performance, but also as a technological parameter for applications where the transport properties are crucial, as in the case of membranes for gasoline [89]. The free-volume coefficient can be drawn from an analysis of the E spectra, taking into account the empirical expression formulated by Doolittle and Doolittle [90], where the relaxation times of the viscoelastic mechanism were found to behave as follows  

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