Self-assembled structure and relaxation dynamics of diblock copolymers made of polybutadiene and styrene/butadiene rubber

Structural features and relaxation dynamics of two series of crosslinked poly(butadiene-block-(styrenestat-butadiene)) diblock copolymers with systematically varied segregation strength and morphology are studied by atomic force microscopy (AFM) and dynamic shear measurements. Series I contains symmetric diblock copolymers with variable styrene content in the SBR block. In Series II the volume fraction of the SBR block Phi(SBR) is varied while keeping the styrene content in the SBR block almost constant. AFM results indicate that symmetric diblock copolymers of Series I with styrene contents >= 35 mol% in the SBR block are well microphase-separated in accordance with thermodynamic models using an effective interaction parameter chi(eff). The samples of Series II with volume fractions in the range 30 vol% <= Phi(SBR) <= 60 vol% show cylindrical and lamellar morphologies as expected based on thermodynamic equilibrium concepts. This shows that the diblock copolymer morphologies existing at the crosslinking temperature are fixed but not significantly altered by the vulcanization procedure. Depending on the segregation of PB and SBR blocks in the crosslinked state, there are three different relaxation scenarios: (i) two well separated dynamic glass transitions alpha(PB) and alpha(SBR) for strongly segregated samples consisting mainly of two pure phases, PB and SBR, (ii) significant relaxation modes located between the alpha relaxation processes of pure PB and SBR for weakly segregated systems with a lot of interfacial material and (iii) one single dynamic glass transition alpha at intermediate temperatures for practically miscible systems. These three situations are explained based on the chemical composition of subsystems having typical dimensions of about 1-3 nm consistent with the characteristic lengths xi(alpha) of the glass transition quantifying the dimensions of cooperative rearranging regions (CRRs).