Blends of PDMS and random copolymers of dimethylsiloxane and methylphenylsiloxane: Phase separation in the quiescent state and under shear

The miscibility of random copolymers (COP), consisting of dimethylsiloxane and methylphenylsiloxane units, with poly(dimethylsiloxane)s (PDMS) was stu died in the absence and in the presence of shear experimentally as well as theoretically. Blends of COP0.86 28 with PDMS 33 (subscripts: volume fracti on of DMS in the copolymer, numbers after thc abbreviations: weight average molar masses in kg/mol) were investigated far from critical conditions on the PDMS side of the phase diagram. According to these experiments the two phase regime increases by shear without exception and the maximum effects: grow from 3 to 12 K as the PDMS concentration increases. Theoretical calcul ations were performed under the: premise that shear destroys clusters of li ke segments formed under equilibrium conditions, The effects ts calculated in this manner are of the comet order of magnitude, but their concentration dependence contradicts the measurements. Blends of COP0.71 7 With PDMS 27, PDMS 33, or PDMS 38 exhibit critical concentrations at approx. 23 wt.-% PD MS. For sufficiently low PDMS contents shear reduce's the miscibility again according to experiment and theory. However, measurements demonstrate that the susceptibility of the blends towards shear decreases as the concentrat ion of PDMS increases until the effect changes sign and the homogeneous reg ion expands as the systems flow, in contrast to the calculations which yiel d a monotonous increase of shear effects. Possible reasons for the observed discrepancies an discussed.