Conformations of Ring Polystyrenes in Semidilute Solutions and in Linear Polymer Matrices Studied by SANS

Conformations of highly purified ring polystyrene, R-70, with the molar mass of 70 kg/mol, in a good solvent and in linear polymer homologue matrices were examined by small-angle neutron scattering (SANS) measurements. The radii of gyration R_g of R-70 were estimated by the Guinier's approximation from the SANS profiles obtained, and the polymer volume fraction φ dependence of R_g ² was discussed. In deuterated toluene as a good solvent, R-70 exhibits the R_g ² ∼ φ^-0.29±0.01 dependence at high φ above the overlap volume fraction, φ₀∗ (i.e., 1 < φ/ φ₀∗ < 20). This exponent -0.29 shows stronger φ dependence than that for semidilute solutions of linear polymers, -0.25, predicted from the scaling theory, suggesting that the ring expands more sensitively than linear chains when φ decreases in semidilute regime. In contrast, the φ dependence of R_g ² of R-70 is evidently weaker than that of the recent simulation for ring polymer solutions (R_g ² ∼ φ^-0.59) by Reigh et al. This difference is thought to originate from the difference in the ring chain length; i.e., the simulation treated much longer rings than the ring adopted in this study. Therefore, it is expected that the exponent -0.29 for the ring polymer solutions obtained in this study is not a limiting value but is a transit one toward higher φ/ φ₀∗ region. The size of R-70 is also increased when the ring was diluted with linear polystyrenes. However, the degree of expansion of the rings in linear polymer matrices is considerably lower than that in toluene solutions. Moreover, the molar masses of the linear chains added hardly effect the expansion behavior of the rings. In fact, the dimension of rings gets closer to that of the Gaussian rings as a larger amount of linear chains is added.