Peculiar Transition between Chiral and Achiral Networks in Self-Assembly of Chiral Block Copolymers

Herein, a peculiar phase transition between the mesochiral and achiral networks can be found in the self-assembly of chiral block copolymers. A chiral network phase, an alternating gyroid (G^A), with controlled helicity can be achieved by the self-assembly of chiral triblock terpolymers, polyisoprene-block-polystyrene-block-poly(l-lactide) (PI–PS-PLLA), and polyisoprene-block-polystyrene-block-poly(d-lactide) (PI–PS-PDLA). An interesting phase transition between chiral G^A with negative PLLA network and positive PI network within the PS matrix and core–shell double gyroid (DG) with a pair of PLLA networks but opposite handedness with PS shells in the PI matrix can be found, reflecting a phase transformation between the dispersed PI network and PI matrix with minor compositional variation. Similar results can also be found in the case of PDLA but with opposite handedness. Homochiral evolution at different length scales via self-assembly can be evidenced by vibrational circular dichroism (VCD) spectra of C═O stretching (conformational chirality due to intrachain chiral interaction) and the C–O–C parallel mode (hierarchical chirality due to interchain chiral interaction). Most interestingly, the molecular dispositions of the helical polymer chains in the self-assembled phase based on the VCD results of the C–O–C parallel mode are in line with the observed phase behaviors at which the VCD signals from G^A will be nullified after the formation of the core–shell DG. As a result, the transition mechanism can be clarified from the conformational level to the mesoscale with respect to the self-assembly process.