Effect of the Microstructure of Polydiene Block on Order−Disorder Transition in Diblock Copolymers Containing Polystyrene Block or Poly(vinylcyclohexane) Block

Abstract

The effect of the microstructure of polydiene blocks on order-disorder transition (ODT) in diblock copolymers containing polystyrene (PS) or poly(vinylcyclohexane) (PVCH) blocks was investigated. For the study, a series of polystyrene-block-polybutadiene (SB diblock) copolymers with varying microstructures in the polybutadiene (PB) block and polystyrene-block-polyisoprene (SI diblock) copolymers with varying microstructures in the polyisoprene (PI) block were synthesized. Subsequently, either only the PB block in an SB diblock copolymer was selectively saturated to obtain PS-block-poly(ethylene-co-1-butene) (PEB) copolymer or both blocks were saturated to obtain PVCH-block-PEB copolymer. Also, either only the PI block in an SI diblock copolymer was selectively saturated to obtain PS-block-poly-(ethylene-alt-propylene) (PEP) copolymer or both blocks were saturated to obtain PVCH-block-PEP copolymer. The ODT temperature (T ODT ) of each block copolymer was determined from oscillatory shear rheometry. It has been found that (i) the TOOT of an SB diblock copolymer with predominantly 1,2-addition in the PB block is much higher than that of an SB diblock copolymer with predominantly 1,4-addition in the PB block, (ii) the TOOT of an SI diblock copolymer with high vinyl content (predominantly 3,4- and 1,2-additions) in the PI block is much higher than that of an SI diblock copolymer with predominantly 1,4-addition in the PI block, (iii) the T ODT s of PS-block-PEB and PS-block-PEP copolymers are considerably higher (>100 °C) than those of the unsaturated precursors regardless of the microstructures of polydiene block, and (iv) the T ODT s of PVCH-block-PEB and PVCH-block-PEP copolymers are higher or lower than those of the unsaturated precursors depending upon the microstructure of polydiene block. By curve fitting the experimental phase diagrams, which were obtained from cloud point measurement, to the theoretical phase diagrams based on the Flory-Huggins theory, temperature-dependent segment-segment interaction parameters (χ) were determined for PS/PB, PS/PEB, PVCH/PEB, PS/PI, PS/PEP, and PVCH/ PEB pairs with varying microstructures of polydienes. It is found that while values of χ for the polymer pairs thus determined explain satisfactorily the experimentally determined T ODT s of all the block copolymers investigated, the conformational asymmetry parameter (e) explains the experimentally determined T ODT s of the completely saturated PVCH-block-PEB and PVCH-block-PEP copolymers, but not of the partially saturated PS-block-PEB and PS-block-PEP copolymers. Further, it is found that values of e do not explain the effect of microstructures of PB block on the T ODT of SB diblock copolymer with varying microstructures. Thus, we conclude that a correlation between e and χ seems to exist for completely saturated block copolymers, but not for partially saturated block copolymers.