Abstract
In this study, a series of poly (4-vinylpyridine)-b-Poly (L-lactide) (P4VP-PLLA) chiral block copolymers (BCPs*) were synthesized by ring-opening and atom transfer radical polymerization in sequence. A unique helical phase could be obtained from the self-assembly of P4VP-PLLA BCPs* with P4VP-rich fractions in bulk whereas a typical phase, a cylinder phase, was found in PLLA-rich fractions. The formation of the helical phase is attributed to the chiral effect on the self-assembly of the P4VP-PLLA BCPs*. To examine the origins and morphological evolution of the self-assembly of BCPs* for helical architecture, the P4VP-PLLA BCPs* was self-assembled in solution. For P4VP-rich P4VP-PLLA, helical superstructure was found at the early state of self-assembly and no significant crystallinity was identified; it indicates that the formation of the helical superstructure is attributed to the microphase separation instead of crystallization. Note that the PLLA is a crystallizable polymer. The self-assembled morphology might be the result of microphase separation and crystallization. With the progress of self-assembly, tubular superstructure was obtained. The observed results are consistent with the prediction from TCLB (tilted chiral lipid bilayer) theory, suggesting that the twisting and bending forces for the forming helical superstructure is attributed to chiral effect for self-assembly. The tubular superstructure is resulted from the scrolling of the helical superstructure to reach the most stable Gibbs free energy state. By contrast, BCP single crystals were found in the self-assembly of PLLA-rich P4VP-PLLA. The P4VP-PLLA single crystal appears as layer-by-layer amorphous P4VP and crystalline PLLA texture. As a result, the formation of the final morphology for block copolymer self-assembly is strongly dependent upon the competition between microphase separation and crystallization. In the case of P4VP-rich fractions, the crystallization event is significantly constrained by microphase separation. By contrast, for PLLA-rich fractions, the self-assembled morphology is mainly resulted from crystallization. We hypothesize that the discrepancies between the self-assembly of P4VP-rich and PLLA-rich fractions is attributed to the variation in the energy barrier for the formation of microphase separation and crystallization so as to justify the final morphology from self-assembly. Also, by taking advantage of forming various superstructures, the decoration of functional inorganic nanoparticles could be achieved by using those superstructures as templates for hybridization through the association with the P4VP. In this study, silver (Ag) system was utilized due to the easily decorated property and its potential applications. As demonstrated, Ag nanoparticles could be well distributed on the surface of the superstructures.