Enthalpic Relaxation in Polymer Blends

Physical aging studies have been performed on several polymer-polymer mixtures Tables 6.1 and 6.2 list the systems investigated. Most of these studies have made use of DSC and enthalpic data to assess physical aging behavior in blends. 

Early enthalpy relaxation studies of PMMA/SAN blends carried out by Naito et of. [3 3] indicated a single endothermic aging peak for the 50/50 blend and the pure 

Since x is related to x, A , and Tq [Eq. (6.6)], even if the two blend components have a similar Tg, when these kinetic parameters differ so will the enthalpy relaxation at %. Therefore it is the difference in the enthalpy relaxation of the single components that is crucial when investigating polymer-polymer miscibility. 

The ability to discriminate between misdble and immisdble blends using DSC has also been demonstrated by Jorda and Wilkes [38] who showed, after aging, that blends of racemic polylactide Tg = 53 °C) and its optically active L-form (Tg = 65 °C) were two-phase, a convindng demonstration that two stereoregular forms of a polymer may be immisdble. 

The sequential aging theory proposed by Chai and McCrum [69] may provide an alternative explanation to the PS/PVME results. These authors postulated that at a given Ta and ta the viscoelastic elements with relaxation times equivalent to ta wUl be aging, but that elements with t ta will already have reached equilibrium and those with t ta will not yet have begun to move towards equilibrium. Thus, in the PVME/ PS case, the more flexible PVME, which will also be closer to its own Tg, will possess more elements with shorter t than the PS. Consequently, the PVME relaxation spectrum will tend to move more rapidly towards equilibrium than the PS and so would age more rapidly. 

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