A region of broad nonlinear curvature is observed in DSC traces of ABA copolymers during temperature sweeps between the glass transition temperatures of the two principal microphases. This curvature is attributed to a smoothly varying composition profile through the interfacial region lying between microphases. The shape of the DSC curve is proposed to be a fingerprint for a given profile. This curvature is, however, shown to be a possible source of uncertainty in precise evaluation of Tg for the microphases and the reported enhanced breadth of these transitions. In addition, microphase separation temperatures are readily identifiable from DSC traces, and their values correspond well to the predictions of Leary‐Williams theory. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 267–274, 1999

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The nonlinear rheological character of microphase-separated ABA-type block copolymer melts was investigated in the context of stress relaxation experiments. Five grades of triblock copolymers were studied in step strain programs. End blocks were polystyrene and middle blocks either polybutadiene or polyisoprene. Molecular weights ranged from 61 300 to 140 000 and styrene fractions from 0.14 to 0.44. Test temperatures (90-110 "C) were above Tg of the styrenic microphases and below the separation temperatures of these polymers, so all were multiphase systems with each phase in the liquid state. Strain amplitudes (yo) ranged from 0.05% to 3.86%. At the highest yo, relaxation moduli dropped steadily in liquidlike fashion; at lowest yo, the stress relaxation was overtaken by a competing microstructural recovery process which caused the modulus to pass through a minimum and then increase. Results are discussed in terms of the interfacial diffuse phase (interphase), morphology, and molecular weight. IE950035L@ Abstract published in Advance ACS Abstracts, September 1, 1995.