Effects of end-block associating homopolymers on the thermomechanical properties and phase behavior of a triblock copolymer

The effects of low molecular weight poly(xylenyl ether) (PXE) homopolymers on the phase behavior and thermomechanical properties of a poly{styrene-b-(ethylene-co-butylene)-b-styrene} triblock copolymer (SEBS) are reported. The PXE homopolymers associate with the polystyrene (PS) end blocks of the copolymer and lead to significant property enhancements, including elevation and extension of the rubbery plateau, an increase in the room temperature shear modulus, elevation of the glass transition of the glassy microphase, and an increase in the order-disorder transition temperature. The broad nature of the loss transitions for the mixed microphase of PS and PXE indicates that solubilized PXE is heterogeneously distributed within the PS microdomains, locating preferentially at the center of the microdomain. Miscibility of the homopolymers within the end-block microdomains is limited as evidenced by the observation of macrophase formation above a critical homopolymer concentration. Experimental phase and property information is used to construct a phase diagram for the blend system. At low temperatures, the phase diagram is dominated by upper critical solution temperature behavior associated with the repulsive interactions of PXE with the EB midblock. At high temperatures, the phase diagram exhibits lower critical solution temperature behavior consistent with the known phase behavior of blends of PXE with polystyrene homopolymers.