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Rheological and structural properties were examined for a 30 wt % solution of a 1,4-butadiene-styrene-1,4-butadiene (BSB) triblock copolymer (MS ) 108 × 10 3 , MB ) 13.3 × 10 3 for each B block) in an S-selective solvent, dibutyl phthalate (DBP). At equilibrium at 25 °C, a bcc lattice of the unsolved, soft (rubbery) B domains connected by the middle S blocks (a lattice-type network) was formed to exhibit the elastic behavior against small strains. This lattice-type network was disrupted under steady shear to lose its elasticity, and the strongest disruption occurred at an intermediate shear rate (γ ˘) close to a frequency of B/S concentration fluctuation. This disruption behavior was similar to that of a BS/DBP diblock micellar lattice system. However, for the BSB/DBP system, an order of magnitude increase of γ ˘resulted in a minor change (by a factor <30%) of a time t r ∞ required for the full recovery of the elasticity during a quiescent rest after the preshear. This result, quantitatively different from that seen for the BS/DBP diblock system, was related to the bridge-type configuration of the middle S blocks of BSB: Under the steady shear, a large fraction of the bridges in the flowing defect region would be converted into loops. Thus, the re-formation of the bridges in this region (a process absent in the BS/ DBP system) should be required for the full recovery of the elasticity of the BSB/DBP system. This bridge re-formation, leading to a recovery of the order of the B domain arrangement and allowing the coexisting loops to exhibit the elasticity, should be accompanied by a transient mixing of the B and S block and thus gave the very weakly γ ˘-dependent t r ∞ determined by the mixing enthalpy.

ABSTRACT:Polystyrene-polybutadiene (PS-SB) and polystyrene-polyisoprene (PS-PI) diblock copolymers form spherical micelles with PS cores and PB/PI corona in a PB/PI-selective solvent, n-tetradecae (C14). At high concentrations, the micelles are further arranged on bcc lattices. Structural factors governing the equilibrium modulus G e of these PS-PB and PS-PI micellar lattices are discussed in this study. It turned out that G e is primarily determined by the number density of the corona blocks corona and proportional to corona while the number density of the bcc lattice cell, D À3 with D being the lattice spacing (determined from SAXS), has only secondary effects on G e . The proportionality between G e and corona reflects the thermodynamic origin of the lattice elasticity, the entropic elasticity of the corona PB/PI blocks having osmotically correlated conformations. These results suggest a necessity of re-examination of a previous hypothesis that the cell number density primarily determines G e of bulk copolymer systems forming cubic phases [Kossuth et al., J. Rheol., 43, 167 (1999)]. Furthermore, the G e = corona ratio was found to be moderately different for the PS-PB/C14 and PS-PI/C14 micellar lattices. This difference can be related to a difference in the magnitude of the osmotic correlation for the PB and PI corona blocks due to a slight difference of their solubilities in C14. [DOI 10.1295/polymj.36.430] KEY WORDS Diblock Copolymer Micellar Lattice / Equilibrium Modulus / Osmotic Requirement / Lattice Stability / Diblock copolymers form micelles with precipitated cores and solvated corona in selective solvents. These micelles form cubic lattices at high concentrations where the corona blocks of neighboring micelles are overlapping each other. [1][2][3][4][5][6] This lattice formation results from a compromise of contradicting thermodynamic requirements, an elastic requirement of randomizing corona conformation and an osmotic requirement of reducing the spatial gradient of the corona concentration.1,4-6 Under these requirements, the corona blocks of neighboring micelles are forced to have mutually correlated conformations. The micelles are arranged on cubic lattices so as to satisfy the thermodynamic requirements and minimize this correlation.The micellar lattice is the thermodynamically stable structure and exhibits equilibrium elasticity.1-6 For polystyrene-polybutadiene (PS-PB) copolymers in a PB-selective solvent, n-tetradecane (C14), the equilibrium modulus G e was found to be proportional to the number density corona of the corona PB blocks and the proportionality constant (G e = corona ratio) was smaller than the constant k B T expected for the simplest case of the entropic elasticity of independent corona blocks (k B = Boltzmann constant and T = absolute temperature). [4][5][6] This proportionality reflects the thermodynamic origin of the lattice. Namely, the osmotically correlated corona blocks sustain the lattice and thus their number density is the primary factor determining G e . At the same ...