Rheological and dielectric behavior of dipole-inverted (SIS)p-type multiblock copolymers: Estimates of bridge/loop fractions for respective I blocks and effect of loops on high extensibility of bridges

摘要

A series of symmetric styrene- (S-) cis-isoprene (I) multiblock copolymers of (SIS)(p)-type (p = 1, 2, 3, and 5 corresponding to tri-, penta-, hepta-, and undecablock) was synthesized with an anion-coupling method and their rheological and dielectric behavior was examined in n-tetradecane (C14), a solvent dissolving the I block and precipitating the S block. The molecular weights of the constituent blocks were almost identical for these copolymers, and a particular I block(s) of respective copolymers had symmetrically once-inverted type-A dipole while the remaining I blocks had noninverted dipoles. At 20 degrees C, the (SIS)(p)/C14 systems with the (SIS)(p) concentration of 40 wt % formed a bee lattice of glassy, spherical S domains (as confirmed from SAXS) and exhibited the gel-like elasticity. This elasticity was sustained mainly by the bridge-type I blocks connecting the S domains and partly by the coexisting loop-type I blocks. The dielectric loss (epsilon") data exclusively detected the fluctuation of the midpoint of the dipole-inverted I block having either bridge or loop conformation. (Since the ends of the I blocks were immobilized on the surface of the glassy S domains, the motion of the I blocks having noninverted dipoles was dielectrically inert.) A moderate difference was noted for the epsilon" data of the (SIS)p copolymers normalized by the volume fractions of respective dipole-inverted I blocks. This difference unequivocally indicated that the bridge- and loop-type I blocks exhibited different dielectric responses and that the bridge fraction phi(bridge) moderately changed with the location of the dipole-inverted I block in the (SIS)p copolymer backbone as well as the number of repeating SIS units, p. On the basis of a hypothesis of dielectric similarity between the loop and its half-fragment tail under an osmotic constraint, phi(bridge) was estimated from the epsilon" data of the (SIS)p copolymers and their precursor SI diblock copolymer (having the tail-type I block). phi(bridge) was a little larger for the I block located at the center of the copolymer backbone than for the off-center I block and phi(bridge) of the center I block decreased slightly with increasing p. The (SIS)5 undecablock system was extensible up to the stretch ratio of lambda congruent to 30, and the loops therein appeared to osmotically stretch the bridges thereby helping the bridges bear such a high extensibility.