The effect of end groups on thermodynamics of immiscible polymer blends. 2. Cloud point curves

The effect of chain end groups on the thermodynamics of polymer blends is investigated by the collection of cloud point curves for low molecular weight blends of poly(isoprene) (PIP) with three different am-functionally-terminated poly(dimethyl siloxane) (PDMS) materials: alpha,omega -trimethylsilyl PDMS (PDMS-CH3), alpha,omega -propylamino PDMS (PDMS-NH2,) and alpha,omega -propylcarboxy PDMS (PDMS-COOH). The cloud point curves for these blends differ radically, demonstrating the possibility of controlling miscibility of low molecular weight blends by modification of polymer chain ends. The apparent critical points for equivalent molecular weight blends of PIP/PDMS-CH3, PIP/PDMS-COOH, and PIP/PDMS-NH2 are found at temperatures of 250, 190, and 85 degreesC, respectively, scaling inversely with the polarity of the PDMS end group. Cloud point curves, calculated from the Flory-Huggins (F-H) rigid lattice model and a quasi-binary extension of the F-H model, are in good agreement with the experimental cloud point data, and are used to estimate the interaction parameters for PIP/PDMS-CH3 and PIP/PDMS-NH2 blends. The interaction parameters for both blend types are found to be insensitive to the molecular weight of the PDMS component and to scale inversely with the polarity of the end group. The total interaction parameters for the PDMS-NH2 blends are also calculated from the summation of binary interactions between blend components, PIP, PDMS, and end groups. Incorporation of amine end groups onto PDMS causes a decrease in the total interaction parameter calculated in this manner, consistent with the increased miscibility observed experimentally in blends containing PDMS-NH2 in comparison to those containing PDMS-CH;. In addition, the calculated binary interaction parameters show the following trend: X-DMS/NH2 > X-IP/DMS > X-IP/NH2. This suggests that the observed compatibilization effect in blends with PDMS-NH2 is driven by the repulsive nature of the interactions between the PDMS backbone and its end groups. (C) 2001 Published by Elsevier Science Ltd.