Scaling Behavior of Swollen Chains in Bimodal Molecular Weight Blends

Abstract

The phenomenon of swelling of several long polymer chains (length N), mixed at concentrations below c* in a series of shorter chain “solvents” (length P), has been quantitatively studied by small-angle neutron scattering. The blends consist of perdeuterated polystyrene guest chains (Mw = 87 000−2 950 000), in hydrogenous PS hosts (Mw= 2 530−2 000 000). Scaling relationships have been established for the long chain's radius of gyration Rg and second virial coefficient A2. For bimodal blends where the long chain is in the unswollen regime (P > N1/2), the scaling is that of a random walk chain, with Rg ∼ N0.504±0.009 and A2 ∼ P-0.95±0.24. For the swollen regime, one observes Rg ∼ N0.59±0.02P-0.10±0.02 and A2 ∼ N-1/5P-0.59±0.10. The weaker than predicted dependence of Rg on P, −1/10 vs −1/5 from theory, implies that in the swollen regime, the screening blob size ξ depends on P1/2 rather than P as originally hypothesized. The approximate crossover point at qRg(short) ≈ 1 in the static structure factor of the long chain from S(q) ∼ q-1.7 for large distances (excluded volume) to ideal chain statistics S(q) ∼ q-2 at smaller scales is seen as a confirmation of the screening effect of the short chains.