Viscoelastic properties of arborescent polystyrene-graft-polyisoprene copolymers

Arborescent polymers are highly branched macromolecules obtained from successive grafting reactions according to a generation-based scheme. Grafting side chains onto a linear substrate randomly functionalized with coupling sites thus yields a generation zero (G0) polymer, with a comb-branched structure. Further cycles of substrate functionalization and grafting lead to arborescent polymers of generations G1, G2, etc., with a dendritic architecture. Arborescent copolymers can also be obtained when side chains with a chemical composition different from the substrate are used in the last grafting cycle. Copolymers of generations G0-G3 were thus obtained by grafting polyisoprene chains with a high cis-1,4-unit content onto polystyrene substrates. The dynamic mechanical behavior of the arborescent polystyrene-graft-polyisoprene copolymers was investigated as a function of side-chain length and generation number. The zero-shear viscosity of the copolymers was lower than for linear polyisoprenes of comparable molecular weight, but its scaling behavior as a function of molecular weight was consistent with entanglement formation. The zero-shear recoverable compliance increased with the overall molecular weight of the copolymers to reach values up to 10 times larger than for the linear analogues. Isoprene copolymers of generations G0 and G1 displayed viscous flow behavior at low shear rates, their terminal relaxation time increasing with the molecular weight of the side chains. A relaxation at intermediate frequencies, associated with the polyisoprene chains, appeared at similar frequencies for copolymers of different generations with side chains of comparable molecular weights. The frequency dependence of the modulus observed for highly branched (G2 and G3) isoprene copolymers was analogous to microgels and filled polymer systems, the modulus-frequency master curves displaying thermorheological complexity with breakdown of the time-temperature superposition principle for copolymers with short side chains.