Microphase Separation of Linear and Cyclic Block Copolymers Poly(styrene-b-isoprene):  SAXS Experiments

We have studied the effect of cyclization of a linear poly(styrene-b-isoprene) PS-b-PI copolymer system on the morphology of the organized nanostructures. The linear and the cyclic copolymers have exactly the same degree of polymerization, i.e., 290 for PS and 110 for PI, corresponding to a volume fraction ΦPS = 0.78. We used small-angle X-rays scattering to characterize the microphase separation and to highlight the difference in the microstructure between linear (several scattering peaks) and cyclic copolymer systems (single scattering peak). Our results show that the linear PS−PI is microphase-separated and organized in a 2-d hexagonal packing (reflections 1:√3:√4:√7:√9:√12: etc.) as expected at this volume fraction. In contrast, the morphology adopted by cyclic copolymer chains is a liquidlike micellar phase (single scattering peak). Both the domain spacing as well as the temperature effects were found very sensitive to the cyclization of the linear diblock copolymer. This results shows that the cyclization of a linear block copolymer induces remarkable changes in the morphology of the organized nanostructure.