Rheological and Dielectric Behavior of a Styrene−Isoprene−Styrene Triblock Copolymer in Selective Solvents. 2. Contribution of Loop-Type Middle Blocks to Elasticity and Plasticity

Abstract

Rheological and dielectric behavior was examined for concentrated solutions of a styreneisoprene-styrene (SIS) triblock copolymer in monomeric and polymeric I-selective solvents, n-tetradecane (C14) and a low-M homopolyisoprene (I-1; M ) 1.4K). The I blocks had symmetrically once-inverted dipoles along the block contour, and their midpoint motion was dielectrically detected. The SIS solutions exhibited rubbery, plastic, and viscous behavior at low, intermediate, and high temperatures (T). Dielectric and viscoelastic data strongly suggested that the S and I blocks were more or less homogeneously mixed in the viscous regime. In the rubbery and plastic regimes, the S blocks were segregated to form spherical domains, and the I blocks took either the loop or bridge conformation. In these regimes, the inverted dipoles of the I blocks enabled us to dielectrically estimate the loop fraction, φ1 = 60% in C14 and I-1. These loops, having osmotically constrained conformations, strongly affected the rheological properties of the SIS solutions. A strong osmotic constraint in C14 resulted in almost equal contributions of the loops and bridges to the equilibrium modulus. The loop contribution became less significant in I-1 that (partly) screened this constraint. Similarly, the yield stress σy in C14 was essentially determined by dangling (noninterdigitated) loops at relatively high T where the S/I mixing barrier was rather small, while the bridges and interdigitated loops had a large contribution when this barrier was enhanced, i.e., at lower T and/or in I-1 (a poorer solvent for the S blocks than C14).

Silica reinforced triblock copolymer gels

“…The disordering of the network (T ODT ), on the other hand, sets-in at the crossover of G′ and G″. Both transitions can be interpreted in terms of the stability of the microstructure of the system [30][31][32][33]. In the plastic fluid regime, the polystyrene domains become softer and stress can cause the pullout of styrene blocks.…”

“…Rheological studies [30][31][32][33][34] of polystyrene-poly(ethylene/butylene)-polystyrene (PS-PEB-PS) triblock copolymer gels showed that the gels are in a rubber-like viscoelastic state at ambient temperature, which means that the polystyrene domains act as permanent physical crosslinks. Above the glass transition temperature (T g ) of the PS nanodomains, there is a change from a rubbery gel to a plastic fluid.…”

Phase behavior and viscoelastic properties of entangled block copolymer gels

“…1(b)], the bridging chains produce an association, which can lead to phase-separated solutions at low polymer contents 2,4 and physical networks at high polymer contents. [5][6][7][8][9][10][11][12][13][14] Watanabe and coworkers 9,10 conducted extensive investigations of styrene-isoprene-styrene (SIS) triblocks in the midblock-selective solvent tetradecane at various polymer concentrations. The fraction of bridges was directly determined by an elegant dielectric technique and was shown to decrease as the triblock was diluted.…”

Further Investigation of Equilibrium Modulus of Diblock Copolymer Micellar Lattice

“…3 ) revealed the swelling of this block with C14 that decreased the glass transition temperature T g to 30 C. 13 At the same time, it should be emphasized that our PS block is longer and should have T g > 30 C to behave as a rigid (glassy) anchor for the PI corona at our experimental temperature, 25…”