SLOW DIELECTRIC RELAXATION OF A STYRENE-ISOPRENE-STYRENE TRIBLOCK COPOLYMER WITH DIPOLE INVERSION IN THE MIDDLE BLOCK - A CHALLENGE TO A LOOP BRIDGE PROBLEM

摘要

Dielectric behavior was investigated for lamellar-forming polystyrene-polyisoprene-polystyrene (PS-PI-PS) triblock and PS-PI diblock copolymers, the latter being identical to the half-contour of the former. The end PI block of PS-PI had type-A dipoles parallel in the same direction along its contour, while the middle PI block of PS-PI-PS had type-A dipoles inverted at its center. Those type-A dipoles enabled us to dielectrically observe the global motion of the PI blocks connected to the glassy PS domains. The dipole inversion was an absolute requirement for this observation for the middle PI blocks having a fixed end-to-end vector R: For the middle blocks without the inversion, the polarization (proportional to R) does not fluctuate and the global motion is dielectrically inert. The end PI block of PS-PI took only a tail conformation, while the middle PI block of PS-PI-PS took either a loop or a bridge conformation. For evaluation of the bridge fraction for the latter, dielectric loss (epsilon'') curves were compared for the end and middle PI blocks before and after imposition of a large-amplitude (gamma(0)) oscillatory shear, and the shear effects on the viscoelastic properties and lamellar alignment were also examined. Before the large-gamma(0) shear, the end and middle PI blocks exhibited nearly the same dielectric relaxation mode distribution at long time scales but the relaxation intensity was larger for the former by a factor lambda(v) = 1.7. An argument of similarity of behavior of the tail and loop led us to assign the slow relaxation of the middle block to the loop relaxation, giving an estimate for the bridge fraction phi(bridge) congruent to (lambda(v) - 1)/lambda(v) congruent to 40%. Equilibrium theories successfully explained this phi(bridge) value. For PS-PI the large-gamma(0) shear significantly decreased the viscoelastic moduli, improved the lamellar alignment a little, but hardly affected the dielectric behavior. On the other hand, for PS-PI-PS the shear hardly affected the moduli and lamellar alignment but increased the dielectric relaxation intensity at long time scales. These differences between PS-PI and PS-PI-PS suggested that the shear decreased phi(bridge) for the latter because it preferred the PS-PI-PS loop generating smaller stress as compared to the PS-PI-PS bridge. Thus, the observed shear effects were in harmony with the above assignment for the slow relaxation of the middle block. [References: 40]