The effect of pressure on the liquid-liquid phase equilibrium of two polydisperse polyalkylsiloxane blends

The effect of pressure-induced immiscibility in polymer blends is investigated by experimental and theoretical methods. Experimental data of cloud point curves and critical points are obtained by turbidity measurements. The chosen system is a mixture of polydimethylsiloxane and polyhexylmethylsiloxane which is one of the very few polymer blends exhibiting pressure-induced immiscibility. This unusual behaviour is related to a critical temperature minimum of the critical curve and cloud point isopleths at positive pressure in the temperature pressure diagram. The effect of the chain length on the critical temperature minimum is investigated here based on theoretical models. The effect of different attraction strength between the polymer segments on the critical temperature minimum is traced in a global phase diagram. In addition, we investigate the appearance of a small closed-loop of miscibility caused by the combination of the critical temperature minimum at positive pressure with a double-humped or bimodal cloud point curve. Such miscibility window can make it possible to process composit materials from incompatible polymers.