Structure of Polymer Rings in Linear Matrices: SANS Investigation

We have presented a systematic small angle neutron scattering investigation of ring–linear blends, where we vary either the ring volume fraction for a given ring in a long linear matrix or the matrix length at a low ring fraction. We found that the ring radius of gyration (Rg) shrinks with increasing ring fraction and nearly reaches the Rg of the ring melt at a volume fraction of ϕR = 0.5. At the same time, the fractal dimension, which is close to a Gaussian conformation at low ϕR, decreases to the value of the ring melt. Aside from very short matrices, the ring size is independent of the host length. Following a random phase approximation treatment, the effective Flory–Huggins parameter (χF,RL) is negative and independent of ϕR, signifying ring–linear attraction that leads to ring–ring repulsion. χF,RL decreases with decreasing ring size, which might be related to the decreasing possibility of threading events, when the ring size becomes smaller. The ϕR-dependent data in the cross-over regime between low Q Porod scattering ∼ Q–4 and polymer RPA contribution displays an intermediate Q–2 regime, the intensity of which increases proportional to ϕR. The origin of this Q–2 contribution is not clear but may be related to critical fluctuations or microphase separation induced by a nonlocal positive contribution to χF,RL. The experimental results fit well into the general picture that is mainly based on simulations, even though in detail, quantitative differences are obvious. Finally, our results are in qualitative agreement with earlier studies on polystyrene ring–linear blends.