Phase Behavior and Block Sequence Effects in Lithium Perchlorate-Doped Poly(isoprene-b-styrene-b-ethylene oxide) and Poly(styrene-b-isoprene-b-ethylene oxide) Triblock Copolymers

Abstract

We compare the phase behavior of a series of lithium perchlorate-doped poly(isoprene-b-styrene-b-ethylene oxide) (ISO) and poly(styrene-b-isoprene-b-ethylene oxide) (SIO) triblock copolymers. Complete phase diagram isopleths (fI = fS) are described for the lithium-doped ISO system and the previously reported lithium-doped SIO system. We examine the effects of block sequencing on the microstructures present in the doped SIO and ISO diagrams using small-angle X-ray scattering. Three phases were identified in the doped ISO phase diagram isopleth: two-domain lamellae (LAM2), core−shell cylinders (CSC), and three-domain lamellae (LAM3). The order−disorder transition temperatures increase dramatically upon doping and salt loadings up to [O]:[Li] concentrations of 3:1 are accomplished without macrophase separation. The increase in segregation strength in the ISO system upon doping leads to a disappearance of the more geometrically elaborate network phase in favor of the CSC phase as a result of the imbalance in interaction parameters. In addition, we used differential scanning calorimetry (DSC) to determine the effect of having either a glassy block or a rubbery block adjacent to the poly(ethylene oxide) (PEO) block in both neat and doped samples. Melting temperatures for a given domain thickness are lower in the ISO triblock copolymers relative to the SIO counterparts. This temperature discrepancy is associated with confinement by the glassy polystyrene block, which reduces the crystallite size, leading to PEO melting temperatures that are closer to room temperature in ISO triblock copolymers. Through comparative phase diagram and DSC studies, we have gleaned the effects of connectivity and lithium perchlorate concentration on triblock copolymer microstructure.