Interrelation Between Morphology and Relaxation Behavior in Self-Assembled Poly((styrene-stat-butadiene)-Block-Butadiene) Copolymers

Polymer blends are commonly used to improve the performance of composite materials for tire treads. The relaxation behavior of such composites can be systematically influenced since contributions of both polymeric components superimpose due to a phase separation tendency. This can be used to optimize wet grip and rolling resistance. A remaining disadvantage of this approach is, however, that the morphology of the polymer matrix is determined by the processing step. An alternative concept is to use self-assembled systems like block copolymers as a polymer matrix. In order to check their potential, diblock copolymers combining a hard styrene-stat-butadiene (SB) block with a soft butadiene (B) block are polymerized anionically. Two series of samples with different volume fraction as well as styrene contents in the random SB block are synthesized. The influence of these parameters and processing on the microphase-separated state, morphology and miscibility is studied by atomic force microscopy and small angle X-ray scattering. The diblock copolymers were processed following standard procedures and vulcanized with sulfur. The relaxation behavior of the cross-linked block copolymers is investigated by dynamic shear measurements and dielectric spectroscopy in a broad frequency-temperature range. The temperature-dependence of the relaxation times t_αB (soft phase) and t_αSB (hard phase) is quantified using the VFT equation and is compared to the situation in related homopolymers. The influence of morphology and segregation strengths on the relaxation behavior is considered. The implications for the use of P(SB-block-B) copolymers in composites for tire treads are discussed.