RHEOLOGICAL INVESTIGATION OF SHEAR-INDUCED STRUCTURE CHANGES IN MULTIBLOCK COPOLYMERS

摘要

Styrene-butadiene multiblock copolymers were examined with both newly introduced and established rheological techniques and by transmission electron microscopy (TEM) to evaluate shear-induced structural changes in these polymers. Transient rheological tests (based on superposed flow principles) were developed which probed structural changes that occur in the copolymers during and at the cessation of steady shear. Data from these tests indicated that for the cylindrical morphology copolymer (SB1) there were structural changes occurring during steady shear that were recovered upon cessation of shear. The recovery process took place on time scales that could be significant in processing. The lamellar morphology material (SB2) did not exhibit this recovery behavior. Longer-term structure changes were investigated using established techniques and showed differences between the cylindrical and lamellar copolymers. When tested at 210 degrees C, peaks in tan delta occurred at 30 rad/s for SB1 and at 0.5 rad/s for SB2 with saturation strain levels of 150 strain units (SU) for SB1 and 80 SU for SB2. TEM analysis of SB2 indicated that, although rheological changes are significant up to 80 SU, better alignment of the domain morphology can be achieved at moderately low strains of 7 SU. This indicates that the copolymers' rheological changes, which occur as a result of steady shearing, may not be due entirely to domain alignment, but may also be due to more local molecular rearrangements (e.g., chain disentanglement). (C) 1996 John Wiley & Sons, Inc. [References: 13]