Thermoreversible Order–Order Transition of a Diblock Copolymer Induced by the Unusual Coil–Rod Conformational Change of One Block

Unusual liquid crystalline (LC) behavior and interesting nanophase separation behavior in the self-assemblies of a series of diblock copolymers (BCPs), poly(dimethylsiloxane)- b -poly{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene} (PDMS- b -PBPCS), were observed. The synthesized PDMS- b -PBPCS BCPs with PBPCS blocks of different molecular weights had volume fractions of PBPCS ( f PBPCS ’s) from 77% to 90%. At the molecular scale, the BCPs with f PBPCS values of 77% and 83% were in amorphous isotropic state in the entire temperature range studied, and thus, they were in coil–coil BCPs. For the BCPs with f PBPCS values of 87% and 90%, the PBPCS blocks displayed a thermodynamically stable isotropic phase at lower temperatures and an LC phase at higher temperatures. Therefore, they exhibited a transition from the coil–coil to rod–coil BCPs with increasing temperatures. At the nanophase structure scale, all of the BCPs were coil–coil type and self-assembled into body-centered cubic (BCC) structures in the low temperature range. Moreover, some of the nanophase-separated structures of these BCPs could be thermoreversibly transformed by changing temperatures. For the BCPs with f PBPCS values between 77% and 83%, they remained BCC nanophase-separated structures during heating (up to 260 °C) as well as during subsequent cooling. For the BCPs with f PBPCS values of 87% and 90%, when the temperature was higher than the isotropic-to-LC transition temperatures of the PBPCS blocks, the new nanophase-separated hexagonally packed cylinder (HEX) structures started to form. However, only the nanophase-separated structure of the BCP with an f PBPCS value of 90% could completely transform to the HEX structure when the temperature reached 250 °C. As the temperature was reduced, the nanophase-separated structures of these two samples gradually reverted to the BCC structures below the LC-to-isotropic transition temperatures of the PBPCS blocks.