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Thermodynamics and rheology of partially miscible polymer blends Chopra, Divya


The phase behaviour of two partially miscible binary polymer blend systems was investigated using rheological, optical, turbidimetric and light-scattering techniques. Rheological techniques included the evaluation of linear and non-linear rheology using parallel plate, sliding plate and capillary rheometry. The two systems comprised a) a model upper critical solution temperature mixture of varying M[sub w] (4.3-162 kg/mol) poly(dimethylsiloxane) and high molecular weight (M[sub w] = 83.8 kg/mol) poly(ethylmethylsiloxane) [PDMS/PEMS]; and b) a lower critical solution temperature mixture of high M[sub w] (100-200 kg/mol) poly(styrene-co-maleic anhydride) and poly(methyl methacrylate) [SMA/PMMA] with SMA component having 8%, 14% and 32% by weight maleic anhydride content (MA). The Theologically determined phase separation temperature was marked by changes in the temperature dependence of the elastic moduli at constant frequency and of the stresses at constant shear rate. Failure of time-temperature superposition in frequency sweeps in the linear region was also observed. The phase diagrams of the blends were modeled by using a temperature dependent expression for the interaction parameter of Flory-Huggins theory, based on the concept of generalized Gibbs free energy of mixing. The phase behaviour, morphology and interfacial tension of PDMS/PEMS blends was very sensitive to the molecular weight of the individual components. Miscibility increased and interfacial tension decreased as the molecular weight of PDMS decreased. SMA/PMMA blends containing 14% MA were found to be more miscible than those containing 8% or 32% MA, a finding attributed to the compositional dependence of intermolecular (SMA-SMA) and intramolecular (SMA-PMMA) interactions in the different samples. Shear-induced fibrillar type growth of SMA inclusions in the phase-separated region in the case of SMA/PMMA with 32%MA was established using rheological tools.

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