Microstructural diagnosis of block copolymer nonlinear mechanical properties I. Uniaxial stress/strain

An earlier study on the linear dynamic properties of triblock copolymers was continued into the nonlinear property regime. Uniaxial tensile tests were performed on styrene-butadiene-styrene block copolymers. The three polymers, having polystyrene (PS) fractions 0.27, 0.29, and 0.48, also represented three molecular weights (0.49, 0.58, and 1.0 × 105). Specimens were prepared by casting from three solvents (methyl ethyl ketone, MEK; cyclohexane; and a 9:1 mixture of tetrahydrofuran and MEK). This wide range of variables led to a correspondingly wide spectrum of properties, including the “plastic-to-rubber” transition in some cases. Stress-strain curves were characterized by modulus, yield stress, general elevation, and fracture stress and elongation. Results were interpreted in terms of microstructural parameters obtained from the earlier linear tests, including interphase properties, and molecular features as well. Special insight emerged from comparison of first-cycle (to 300 percent elongation) and second-cycle tests, including changes in modulus, yielding, level, and the appearance of set. Additional data on sample preparation influences and PS continuity effects on sample properties were also obtained.

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