Phase behavior of blends of styrenic triblock copolymers with polyphenylene oxid

The degree of solubilization of poly(2,6-dimethyl-1,4-phenylene oxide), PPO, homopolymer in the polystyrene, PS, microphases of triblock copolymers has been investigated by differential scanning calorimetry while independently varying the PS block molecular weight and the PPO molecular weight. Dynamic mechanical thermal analysis and scanning transmission electron microscopy were additional methods used to investigate the phase behavior of a particular PPO/triblock copolymer system. The exothermic heat of mixing for PPO and PS units causes a dramatic increase in the extent of homopolymer solubilization relative to that found previously by others when the homopolymer was polystyrene. While the molecular weight of the PS block is a major factor determining the extent to which PPO and PS segments mix, the molecular weight of the PPO has little or no effect over the range investigated, contrary to what has been observed for blends of homopolystyrene with such copolymers. This appears to be a direct consequence of the additional driving force for solubilization provided by the exothermic mixing of PPO and PS. Effects of the rubber block and brief annealing are discussed.;A model is developed for estimating the extent to which a homopolymer, H, can form a mixed phase with the A segments of a block copolymer when there is a favorable enthalpy of mixing A and H. The model assumes a lamellar domain morphology and ignores any interfacial effects. Enthalpic and entropic terms in the free energy expression favor mixing of H and A; whereas, conformational changes for both A and H chains oppose mixing. An equilibrium saturation composition of homopolymer in the mixed phase results from balancing these opposing effects. An exothermic interaction dramatically alters the phase behavior from that of an athermal system, e.g., homopolystyrene blends with styrene-based block copolymers. Model predictions are compared with the experimental results obtained in this study with good semi-quantitative agreement on the extent of homopolymer incorporation into the copolymer domains and the relative unimportance of homopolymer molecular weight on solubilization. Limitations of the model are discussed.