The Effect of Hydrostatic Pressure on the Lower Critical Ordering Transition in Diblock Copolymers

Abstract

The effect of hydrostatic pressure on the lower critical ordering transition (LCOT) was investigated by in situ small angle neutron scattering on symmetric and asymmetric diblock copolymers of perdeuterated polystyrene and poly(n-butyl methacrylate). These systems exhibit a transition from the disordered to ordered state upon heating. Similar to the lower critical solution transition (LCST) in polymer mixtures, the LCOT is entropically driven and is accompanied by an increase in volume on demixing of the copolymer blocks. As a consequence, application of hydrostatic pressure markedly increases the temperature at which the transition from the disordered to the ordered state occurs. Small angle neutron scattering studies as a function of temperature and pressure show that the pressure dependence of the LCOT, ΔT LCOT/ΔP, is up to +147 °C/kbar (1.45 ± 0.07 °C/MPa), roughly 1 order of magnitude greater than that seen at elevated pressures for diblock copolymers exhibiting an upper critical ordering transition (UCOT). Additionally, SANS data obtained at various pressures were superimposed to generate master curves for the peak intensity, peak position, and full width at half-maximum (fwhm). This suggests an equivalence between temperature and pressure of the thermodynamic behavior of systems that exhibit the LCOT.

Thermodynamics and Kinetic Processes of Polymer Blends and Block Copolymers in the Presence of Pressurized Carbon Dioxide

“…The net result is that Equation 9 is positive and the LCST/LDOT generally increases with increasing P, as has been confirmed experimentally [84][85][86] and theoretically. [86][87][88][89] This pressure dependence can be envisaged as "squeezing" the component polymers together, which counters the effect of thermally-weakened specific interactions or tempers the compressibility difference responsible for the LCST/LDOT behavior.…”

“…The sorption of CO 2 preferentially occurs within the PnBMA block, wherein CO 2 exacerbates the existing free-volume effect and lowers the LDOT. As with the PS/PVME blend, exposure of a PS-b-PnBMA copolymer to hydrostatic pressure alone increases [85] the LDOT by ca. 1.5°C MPa…”

Self-assembled block copolymers: Bulk to thin film

“…This temperature is referred to as the order-to-disorder transition temperature (T ODT ). [15][16][17][18][19][20] Some block copolymers exhibit the disorder-to-order transition (LDOT) with increasing temperature, [21][22][23][24][25][26][27][28] and a very few show the closed-loop (or immiscibility loop) phase behavior. [29][30][31][32][33][34][35][36][37] Also, the order-toorder transition (OOT) has been reported for many block copolymers when temperature is changed.…”

Miscibility behavior and single chain properties in polymer blends: a bond fluctuation model study

“…Moreover, the v parameter often depends on composition and chain length as well. More recently the effect of pressure on the incompatibility v of homopolymer blends and the ordering in diblock copolymers has attracted attention 19) . (ii) According to the original mean field theory, the demixing temperature is independent of the molecular architecture.…”