Measurement of coil contraction by total-intensity light scattering from isorefractive ternary solutions

The radius of gyration of a guest polymer (polystyrene) has been measured in the presence of a host polymer (poly(methyl methacrylate)) in a solvent (ethyl benzoate) which is good for both polymers and isorefractive with the host polymer. A range of host concentrations from dilute to semidilute has been studied for a guest and host pair with similar molecular weights. It is found that no change in coil size occurs in the dilute regime, and at host concentrations greater than c*, the guest chain contracts in the same manner as observed for chains in binary solutions by neutron scattering. There is no evidence that the small, unfavourable interaction between polystyrene and poly(methyl methacrylate) leads to a steeper contraction with increasing host concentration or to an onset of contraction at lower concentrations of host polymer. It is shown that conflicting results obtained in previous experiments of this type probably result from improper extrapolation to infinite dilution, due to non-negligible non-linear thermodynamic terms, or to small scattering contributions from the host polymer.