Dynamic mechanical relaxation behavior of block copolymers: Styrene-butadiene-styrene rubber, SBS

The real and imaginary parts of the complex shear compliance (J' and J") of styrene-butadiene-styrene block copolymer, SBS, have been measured isothermally over the frequency range; 0.03-30 Hz, at different temperatures ranging from - 70 to 130degreesC. SBS is taken as representative example of thermoplastic rubbers, which are not susceptible for direct construction of master curves by the usual time-temperature-superposition, tTs, technique. A method has been developed to construct master curves, which we call "step mastering technique". The step mastering was carried out by constructing different master curves at different temperatures covering both glass transitions which were then superpositioned as whole to maintain the individual shift factors of different processes unchanged. The relaxation frequency of the low temperature glass process can be obtained at any given reference temperature by fitting and extrapolating the activation curve in order to superposition its low temperature master curve (as whole) on the high temperature master one at the correct position of frequency that corresponds the new reference temperature. Accordingly, it was possible for the first time to construct a complete master curve for J' and J" for SBS at 30degreesC. The validity of the master curves obtained by the step mastering technique were confirmed by real frequency measurements in the MHz region using the Quartz method. The analysis showed that the relaxation spectrum consists of four relaxation processes. Using a molecular (meander) model, it was possible from the plateau compliance to determine the size of the structural units of the two phases, which was estimated to be 6.5 and 19.5 ran for PB- and PS-phases, respectively. A possible geometrical packing of the different domains is suggested and compared with the literature data.