title: Transient solidlike behavior near the cylinder/disorder transition in block copolymer solutions.
abstract: A nearly symmetric polystyrene-block-polyisoprene diblock copolymer dissolved at a concentration of 40% in styrene-selective solvents exhibited a cylinder-to-disorder transition upon heating. The solvents used were diethyl phthalate (DEP) and 75:25 and 50:50 mixtures of DEP with di-n-butyl phthalate (DBP). In DEP, the most styrene-selective of the three solvents, rheological measurements indicated a distinct plateau in the temperature-dependent elastic modulus across the 8 degrees C interval above the order-disorder transition temperature, T(ODT) = 116 degrees C. Previous small-angle neutron scattering measurements in this regime indicated the equilibrium phase to be a liquidlike solution of approximately spherical micelles. An isothermal frequency sweep in this regime indicated a very long relaxation time. Annealing eventually led to the recovery of liquidlike rheological response, over a time scale of hours. Qualitatively similar phenomena were also observed in 75:25 DEP/DBP and 50:50 DEP/DBP solutions, except the fact that the temperature window of the transient response is narrow and the time scale for the recovery diminishes significantly. Neither small-angle X-ray scattering nor static birefringence gave any clear signature of the transient structure. The structure that leads to the transient rheological response is attributed to micellar congestion due to the slow relaxation of anisotropic micelles into an equilibrium distribution of micelles. Possible origins of the remarkable solvent selectivity dependence are also discussed.
authors.affiliation: School of Chemical and Biological Engineering and NANO Systems Institute, National Core Research Center, Seoul National University, Seoul 151-744, South Korea.
authors.forename: Moon Jeong
authors.initials: MJ
authors.lastname: Park
authors.forename: Kookheon
authors.initials: K
authors.lastname: Char
authors.forename: Timothy P
authors.initials: TP
authors.lastname: Lodge
authors.forename: Jin Kon
authors.initials: JK
authors.lastname: Kim
mesh_terms: D002070:Butadienes / Q000737:chemistry*; D003993:Dibutyl Phthalate / Q000737:chemistry; D010420:Pentanes / Q000737:chemistry*; D044367:Phase Transition; D010795:Phthalic Acids / Q000737:chemistry; D011108:Polymers / Q000737:chemistry*; D011137:Polystyrenes / Q000737:chemistry*; D012212:Rheology; D053838:Scattering, Small Angle; D012996:Solutions / Q000737:chemistry; D013696:Temperature; D013997:Time Factors; D014961:X-Ray Diffraction
chemical_list: D002070:Butadienes; D010420:Pentanes; D010795:Phthalic Acids; D011108:Polymers; D011137:Polystyrenes; D012996:Solutions; C522518:polystyrene-block-polyisoprene; D003993:Dibutyl Phthalate; C007379:diethyl phthalate