Order−Order Transition in a Rod−Coil Diblock Copolymer Induced by Supercritical CO2

We synthesized a series of well-defined rod−coil block copolymers (BCPs), poly(dimethylsiloxane)-b-poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PDMS-b-PMPCS), containing a CO2-philic block PDMS, and we studied their phase structures in bulk and after treatment with supercritical CO2 (scCO2) using small-angle X-ray scattering. The nanostructure of the BCP changed from lamellae (LAM) to hexagonally packed cylinders (HEX) when the volume fraction of PDMS (f PDMS) decreased to ∼18%. Although the glass transition temperature (T g) of the PMPCS block in scCO2 could not be measured directly in our experiment, it significantly decreased in scCO2 at 110 bar as estimated from SAXS results. More significantly, we observed a HEX−LAM order−order transition induced by scCO2 in a HEX-structured BCP which was located close to the HEX−LAM phase boundary in the BCP phase diagram under our experimental conditions. The OOT could be attributed to increases in both the volume fraction of the coil block and the effective interaction parameter. The lamellar structure was fixed by the rigid-rod-like block during depressurization. It was stable below the high T g of the rod block and could change back to the original HEX structure by thermal annealing above the T g of the rod block. For other BCPs which were located far away from the HEX−LAM phase boundary, the structures did not change, but the periodic sizes increased following scCO2 treatment.