Linear viscoelasticity of unentangled corona blocks and star arms
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ABA-type triblock copolymers form micellar structures consisting of B-rich cores and A-rich coronas in A-selective solvents. The relaxation of A corona is known to be qualitatively similar to but quantitatively different from that of a star-shaped A chain due to the geometric (spatial) constraint by the core and the thermodynamic (osmotic) constraint. The effect of the geometric constraint on the block dynamics can be modeled by a chain with one end grafted onto an impenetrable wall. We show that the impenetrable wall slightly accelerates the end-to-end vector relaxation in a direction normal to the wall while it slightly decelerates the viscoelastic terminal relaxation. To test this prediction, we performed linear viscoelastic measurements for model systems: For polystyrene–polyisoprene–polystyrene (SIS) triblock copolymers in S-selective solvent (diethyl phthalate) forming micelles, the viscoelastic relaxation of unentangled S blocks (corona blocks) was indeed slower compared with that of star-branched S chains having the same molecular weight. Nevertheless, the deceleration was stronger than that expected from our theory, and possible reasons were discussed.
KeywordsBlock copolymer Tethered chain Linear viscoelasticity Relaxation time
Q.C. thanks the financial support from the G-COE program at the Institute for Chemical Research, Kyoto University. T.U. thanks the support by Grant-in-Aid (KAKENHI) for Young Scientists B 25800235. The authors thank Prof. Hiroshi Watanabe for continuous support and encouragement on this work. The authors thank Prof. Yo Nakamura for kindly providing star-shaped PS used in this work. The authors thank Prof. Toshiji Kanaya for kindly allowing the authors to do the SAXS measurements in his laboratory.
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