THE INFLUENCE OF SHEAR ON THE ORDERING TEMPERATURE OF A TRIBLOCK COPOLYMER MELT

The effect of shear on the ordering temperature of a triblock copolyme r melt of polystyrene-polybutadiene-polystyrene (SBS) is examined by i n situ small angle neutron scattering (SANS). Results obtained by SANS are compared to the theologically determined order-disorder transitio n temperature, T-RODT=115+/-5 degrees C. The SANS measurements from a Couette geometry shear cell are then used to construct a ''dynamical p hase diagram'' based on characteristic changes in the scattering with temperature and shear rate, y. A shear rate dependent ordering tempera ture, T-ord(y), is identified as the system is sheared isothermally fr om the disordered state, The scattering behavior is shown to be highly strain dependent. We compare our findings on the shear rate dependenc e of the ordering transition in triblock materials with previous obser vations on diblock copolymer materials and theoretical expectations fo r the shear rate dependence of the order-disorder transition temperatu re. A simple scaling argument leads to a good description of the shear rate dependence of T-ord(y) in both diblock and triblock copolymer me asurements over the range of shear rates examined. (C) 1996 American I nstitute of Physics.