Poly[alkyl methacrylate-b-butadiene-b-alkyl methacrylate] triblock copolymers : synthesis, morphology, and mechanical properties at high temperatures

Block copolymers of the ABA type, where B is polybutadiene (PBD) and A is poly(ethyl methacrylate) (PEMA), poly(tert-butyl methacrylate) (P-t-BMA), poly(methyl methacrylate) (PMMA), or poly(isobornyl methacrylate) (PIBMA), have been successfully synthesized by sequential anionic polymerization of butadiene and methacrylates with the diadduct of tert-butyllithium to m-diisopropenylbenzene as a difunctional initiator. Block copolymers of a narrow molecular weight distribution (1.10) have been analyzed by differential scanning calorimetry, transmission electron microscopy, and dynamic mechanical analysis. These materials are phase-separated and have high mechanical performances. Special attention has been paid to the service temperature of these thermoplastic elastomers in comparison with a styrene−butadiene−styrene (SBS) triblock copolymer. The upper service temperature (UST) has been estimated from the temperature dependence of the tensile properties in the 25−150 °C range and found to change with the outer blocks. Polystyrene (Tg = 100°C) is at the origin of the lower UST, which is however comparable to PEMA (Tg = 90 °C) and P-t-BMA (Tg = 116 °C) containing triblock copolymers of similar molecular weight and composition. PMMA (Tg = 132 °C) outer blocks increase the UST of the triblocks, which is further increased by hydrogenation of the PBD midblock. The higher UST has been found for a PIBMA (Tg = 202 °C)-containing triblock copolymer that shows an ultimate tensile strength higher than 2 MPa at 150 °C.