Mechanism of Nonlinear Energy Dissipation in Styrene–Isoprene–Styrene Triblock Elastomer Containing an Associative Network

Styrene–isoprene–styrene (SIS) triblock copolymers with a low S content form spherical S domains and behave as elastomers at room temperature because the I blocks elastically bridge the glassy S domains. This study focused on SIS carrying acid (COOH) or salt (COOCs) groups in the I block and examined their energy dissipation in cyclic tensile tests at 25 °C. The stretch rate $\dot{\lambda }$ defined with respect to the initial length L^[1](0) of the specimen was set constant (=0.1 and 0.01 s^–1), and the maximum stretch ratio with respect to this L^[1](0) was chosen to be 4. The specimen length at the end of the n-th cycle (n = 1–5) was tuned in a way that the engineering stress σ_eng just vanished at the cycle end (which avoided wrinkling/bending of the specimen). In these conditions, the neat SIS showed no significant hysteresis of σ_eng in the second and following cycles, which reflected the lack of mechanical rupture of the glassy S domains gripping the I block ends. The SIS copolymer having the COOH group behaved similarly to the neat SIS sample because the hydrogen bonds of those groups (dynamic crosslinks) thermally dissociated quickly before being broken mechanically. In contrast, the SIS copolymer having the COOCs group exhibited significant energy dissipation characterized by a large, $\dot{\lambda }‐$insensitive hysteresis in the first cycle. In later cycles, this hysteresis did not vanish and still gave a closed σ_eng-λ loop. The thermal dissociation of the COOCs aggregates was much slower than the stretch, so that the aggregates were mechanically broken and then thermally reformed during the cycle. This breakage–reformation appeared to be the main mechanism underlying the large hysteresis. Further details of this mechanism were analyzed within a modified neo-Hookean framework. This analysis adopted the three-chain treatment considering the network strands oriented only in the stretch and other two perpendicular directions and assumed the COOCs aggregates to be mechanically broken under high stretch of the strands but also reformed in the perpendicular directions through rapid thermal motion of the strands carrying broken fragments of the salt aggregates. This breakage followed by the reformation, resulting in an anisotropy of the strand number density in the stretch and two other directions, appeared to be the structural origin of the large, $\dot{\lambda }‐$insensitive hysteresis mentioned above. This assignment of the hysteresis was in harmony with spontaneous recovery of the specimen length that slowly occurred in a load-free state after the cyclic test.