Rheologica Acta

The long-time stress relaxation was examined for poly(styrene)-poly(ethylene-co-butylene)-polystyrene triblock copolymer with asymmetric composition in the microphase separated state. Stress relaxation data from a parallel plate rheometer were extended to very long times by using a newly built compression instrument. Three properties were measured, the relaxation time spectrum, the equilibrium modulus, and the instantaneous modulus. Relaxation experiments over several months (without zero drift) allowed determination of extremely long relaxation times as found in soft solids and in liquids near the gel point. In this way, we can distinguish solid from liquid behavior within the experimental time frame. The BCC lattice structure of the test polymer behaves as liquid except at low temperature where an equilibrium modulus was observed in an experiment that lasted over a period of four months. The corresponding instantaneous modulus was observed to be constant for the same material and for the same time period