Synthesis and characterization of elastomeric heptablock terpolymers structured by crystallization

We report the synthesis and characterization of fully saturated hydrocarbon block copolymer thermoplastic elastomers with competitive mechanical properties and attractive processing features. Block copolymers containing glassy poly(cyclohexylethylene) (C), elastomeric poly(ethylene-alt-propylene) (P), and semicrystalline poly(ethylene) (E) were produced in a CEC - P - CEC heptablock architecture, denoted XPX, by anionic polymerization and catalytic hydrogenation. The X blocks contain equal volume fractions of C and E, totaling 40% - 60% of the material overall. All the XPX polymers are disordered above the melt temperature for E (T_m,E - 95 °C) as evidenced by SAXS and dynamic mechanical spectroscopy measurements. Cooling below T_m,E results in crystallization of the E blocks, which induces microphase segregation of E, C, and P into a complex morphology with a continuous rubbery domain and randomly arranged hard domains as shown by TEM. This mechanism of segregation decouples the processing temperature from the XPX molecular weight up to a limiting value. Tensile mechanical testing (simple extension and cyclic loading) demonstrates that the tensile strength (ca. 30 MPa) and strain at break (>500%) are comparable to the behavior of CPC triblock thermoplastic elastomers of similar molecular weight and glass content. However, in the CPC materials, processability is constrained by the order - disorder transition temperature, limiting the applications of these materials. Elastic recovery of the XPX materials following seven cycles of tensile deformation is correlated with the fraction of X in the heptablock copolymer, and the residual strain approaches that of CPC when the fraction of hard blocks f_X = 0.39.