Curving and Twisting in Self-Assembly of Triblock Terpolymers Driven by a Chiral End Block

A series of triblock terpolymers composed of a chiral end block, poly(l-lactide) (PLLA) (designated as chiral triblock terpolymers), are designed and synthesized for self-assembly. The chiral PLLA block can contribute the chirality effect for the twisting of microphase-separated domains. By attaching an asymmetric amount of the chiral block to a symmetric diblock of polyisoprene-b-polystyrene (PI-PS), compositional asymmetry can be created for the curving of microphase-separated domains. Moreover, the difference in persistence lengths of PS and PLLA blocks can give conformational asymmetry to the curving. With the combined effects of twisting and curving, the curvature at the saddle surface of microphase-separated domains might become significant to give the formation of a network phase. Consequently, a transformation from two-domain lamellae to core-shell network phases, then to a core-shell cylinder phase, and finally to a three-domain lamellar phase with undulation due to the rod-rod interaction with the chirality effect from the semiflexible PLLA, can be found in the isopleths of the triblocks. Interestingly, a core-shell double diamond phase could be found near the center of the ternary phase diagram; we speculate that the alleviation of packing frustration in the core of the self-assembled microdomain is attributed to the character of semiflexible PLLA. Accordingly, the phase window of network formation can be enlarged by the combined effects of compositional asymmetry, conformational asymmetry, and chirality.