Diblock copolymers swollen with compressible fluids: Fundamentals and applications
The phase behavior of block copolymers has been previously studied both in the melt and in liquid solution, however the influence of compressible solvents has not been explored. Here, the effect of carbon dioxide on the phase behavior of block copolymers is investigated. The effect that CO 2 sorption has on the phase behavior of block copolymers depends on the type of transition: upper order-to-disorder transition (UODT) or lower disorder-to-order transition (LDOT). Poly(styrene-block-isoprene) and poly(stryrene-block-hexyl methacrylate) were examined as UODT-type systems. CO2 sorption increases the copolymer miscibility in a manner consistent with liquid diluents. Conversely for poly(styrene- block-n-butyl methacrylate), which exhibits LDOT-type behavior, CO 2 sorption induces microphase separation, in some cases depressing the transition by hundreds of degrees. This behavior is in stark contrast to the effect of liquid diluents on this system, which slightly increase the miscibility of the segments.
To explain these differences in phase behavior, a series of light n-alkanes (C1–C4) were used as the solvent in place of CO2. This homologous series can be used to directly observe the effect of polymer phase compressibility on the phase behavior. The enthalpic interactions with the polymer between the series are similar, whereas the effect the sorption of these alkanes have on the compressibility of the polymer phase is highly dependent on chain length for light alkanes. At identical solvent loading, the location of the UODT temperature was invariant with choice of n-alkane, indicating that the phase behavior in this case is dominated by enthalpic effects. However for LDOT-type systems, a systematic effect of chain length on the depression in the transition was found. The compressibility increase of the polymer with fluid sorption is the driving force for the induction of phase separation.
One application of CO2 swollen diblock copolymers is the synthesis of mesoporous silica. The synthesis involved the infusion and selective condensation of tetraethylorthosilicate (TEOS) inside a block copolymer film swollen with carbon dioxide (CO2) by an acid catalyzed reaction. The amphiphilic copolymers used for the templating were poly(styrene-block-ethylene oxide). TEOS reacts selectively in the hydrophilic phase to form silica while the styrene phase remains unmodified.