Isoprene/Styrene Tapered Multiblock Copolymers with up to Ten Blocks: Synthesis, Phase Behavior, Order, and Mechanical Properties

The living anionic copolymerization of isoprene and styrene in cyclohexane affords tapered block copolymers due to the highly disparate reactivity ratios of rI = 12.8 and rS = 0.051. Repeated addition of a mixture of these monomers was exploited to generate tapered multiblock copolymer architectures of the (AB)n type with up to 10 blocks (1 ≤ n ≤ 5), thereby subdividing the polymer chains in alternating flexible polyisoprene (PI) and rigid polystyrene (PS) segments. Three series of well-defined tapered multiblock copolymers with approximate molecular weights of 80, 240, and 400 kg/mol were prepared on the 100 g scale. Via this synthetic strategy polymer chains were divided in di-, tetra-, hexa-, octa-, and decablock tapered multiblock structures. Because of the living nature of the polymerization, low dispersities in the range 1.06–1.28 (decablock) were obtained. To ensure full monomer conversion prior to the addition of the isoprene/styrene mixture, kinetic Monte Carlo simulation was employed, permitting to simulate chain growth in silico by employing the known polymerization rates and rate constants kp. The synthesized tapered multiblock copolymers were characterized via SEC and selected samples via oxidative degradation of the polyisoprene block in solution, confirming the well-defined nature of the PS segments. Subsequently, the question was addressed, to which extent the tapered multiblock copolymers are capable of forming ordered nanosegregated morphologies. Detailed thermal, structural, and rheological investigations showed that the tapered multiblock copolymers with a molecular weight of 240 kg/mol formed ordered phases with the expected lamellar morphology. However, X-ray scattering data and transmission electron microscopy (TEM) images of the octablock and decablock copolymers reflect weakly ordered structures at ambient temperature. The domain spacing, d, was found to scale as d ∼ N0.62, where N is the total degree of polymerization, suggesting stretching of chains and nonideal configurations. Following the structure factor, S(q), as a function of temperature revealed that the tapered multiblock copolymers undergo a fluctuation-induced first-order transition at the respective order-to-disorder transition temperature, TODT. The viscoelastic response of the tapered copolymers was controlled by the nanodomain structure, the degree of segregation, nanodomain-bridging configurations of blocks, and also the proximity to the glass temperature of the vitrified PS domains. Tapered hexablock copolymers were found to best combine structural integrity and mechanical toughness, while maintaining a large strain at break (>900%)