Styrene-dimethylsiloxane (S-DMS) di- and triblock copolymers, whose overall molecular weight ranged from 3,800 to 4.7 ×\times 10\sp5, and styrene contents from 4 to 96%, were studied using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy. Blends of the S-DMS diblock copolymer and PS homopolymer with PS weight fraction ranging from 0.04 to 0.96 for two different PS to S block molecular weight ratios, 30:1 and 1:50, were also studied by DSC. Comparisons of calculated and the experimental specific heat, C\sb{\rm p}, were made to estimate the relative error in C\sb{\rm p} measurements and the percent mixing of block chains in their opposite microphase. The occurrence of DMS microphase crystallization depends on the DMS block molecular weight, but the degree of crystallinity depends both on weight percent of DMS and on the cooling rate. The critical DMS block molecular weight at which the DMS microphase crystallize under the experimental conditions used in this work is between 3,800 and 15,400. The glass transition temperature, T\sb{\rm g}, of fully amorphous DMS microphases is influenced by the overall composition of each sample. The T\sb{\rm g} of semicrystalline DMS microphases is affected by the degree of crystallinity of the DMS microphases. The variation in DMS microphase T\sb{\rm g} were attributed to a thermal stress effect. The number of styrene T\sb{\rm g}'s in each blend was used to characterize the mixing of PS and the S blocks. The weight fraction of styrene segments in each styrene phase was calculated from the heat capacity change at T\sb{\rm g}. In styrene phase separated blends, the weight fraction of each styrene phase varied with the annealing temperature. A phase diagram was constructed for the blends with 30:1 PS to S block molecular weight ratio

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