Phase behavior for blends of styrene containing triblock copolymers with poly(2,6‐dimethyl‐1,4‐phenylene oxide)

Abstract

AbstractThe extent to which the styrene end‐blocks of three commercially available triblock copolymers can mix with a particular poly(2,6‐dimethyl‐1,4‐phenylene oxide) (Mn = 22,600 and Mw = 34,000) or PPO has been examined by investigation of the glass transition behavior of the PPO and polystyrene (PS) portions of the blends using differential scanning calorimetry. Each block copolymer has a butadiene‐based mid‐block which was hydrogenated for two of these materials, but not the third. The three copolymers differ substantially in overall molecular weight and in molecula weight of the blocks. However, in analogy with the literature on blends of homopolymer polystyrene with styrene‐based block copolymers, the molecular weight of the PS block should be the principal factor affecting the phase behavior in the present blends. Mixtures of the PPO with the block copolymers having PS blocks with M = 14,500 (nonhydrogenated midblock) and with M = 29,000 (hydrogenated mid‐block) exhibited single composition‐dependent Tgs for the hard phase, indicating complete mixing of PS segments with the PPO, for all proportions. On the other hand, the block copolymer having a PS block with M = 7,500 and a hydrogenated mid‐block exhibited two separate hard phase Tgs corresponding to an essentially pure PPO phase and a PS‐rich phase. For blends of homopolymer PS with styrene‐based block copolymers, the similar two‐phase behavior of the glassy portion can be readily explained by entropic considerations. For the present case, the favorable enthalpic contribution for mixing PPO and PS is an additional factor which seems to influence the restrictions on molecular weight for complete mixing; however, additional work is needed to develop a more quantitative assessment of this new issue.