Ordering by flow near the disorder-order transition of a triblock copolymer styrene-isoprene-styrene

Abstract

An oriented morphology has been generated by cooling a triblock copolymer styrene-isoprene- styrene (SIS) below ita order-disorder transition (ODT) temperature and annealing it there at constant quench depth AT while simultaneously shearing it. The polymer has an equilibrium microphase-separated morphology of hexagonally packed cylinders of polystyrene in a polyisoprene matrix. The evolving structure was investigated with rheology and small-angle X-ray scattering (SAXS). SAXS showed that mirophase separation occurred rapidly upon cooling below the ODT temperature while the growth of large-scale spatial order (-1-pm scale) needed long annealing times as shown with low-frequency dynamic mechanical measurements. The slow part of the structuring process gave sufficient time for flow alignment of microphase- separated domains during their growth phase. Near morphology was obtained through large- amplitude oscillatory shear at -25 K below ODT. The low-frequency linear viscoelastic properties of the single crystal structure were found to be affected by the domain alignment in the flow direction. The quench depth AT is an important parameter: at a small quench depth of AT 10 K, microphase separation was found to be suppressed by the oscillatory shear.