Novel Nanostructures from Self‐Assembly of Chiral Block Copolymers

Abstract

A diblock copolymer system constituting both achiral and chiral blocks, polystyrene-block-poly(L-lactide) (PS-PLLA), was designed for the examination of chiral effects on the self-assembly of block copolymers (BCPs). A unique phase with three-dimensional hexagonally packed PLLA helices in PS matrix, a helical phase (H*), can be obtained from the self-assembly of PS-rich PS-PLLA with volume fraction of PLLA f PLLAv = 0.34, whereas no such phase was found in racemic polystyrene-block-poly(D.L-lactide) (PS-PLA) BCPs. Moreover, various interesting crystalline PS-PLLA nanostructures can be obtained by controlling the crystallization temperature of PLLA (T(c,PLLA) ), leading to the formation of crystalline helices (PLLA crystallization directed by helical confined microdomain) and crystalline cylinders (phase transformation of helical nanostructure dictated by crystallization) when T(c,PLLA) < T(g,PS) (the glass transition temperature of PS) and T(c,PLLA) ≧ T(g,PS) , respectively. As a result, a spring-like behavior of the helical nanostructure can be driven by crystallization so as to dictate the transformation (i.e., stretching) of helices and to result in crystalline cylinders. For PS-PLLA with PLLA-rich fraction (f PLLAv = 0.65), another unique phase, a hexagonally packed core-shell cylinder phase with helical sense (CS*), in which the PS microdomains appear as shells and PLLA microdomains appear as matrix and cores, can be found in the self-assembly of PLLA-rich PS-PLLA BCPs. The formation of those novel phases: helix and core-shell cylinder is attributed to the chiral effect on the self-assembly of BCPs, so we named this PS-PLLA BCP as chiral BCP (BCP*). For potential applications of those materials, the spring-like behavior with thermal reversibility might provide a method for the design of switchable nanodevices, such as nanoscale actuators. In addition, the PLLA blocks can be hydrolyzed. After hydrolysis, helical nanoporous PS bulk and PS tubular texture can be obtained and used as templates for the formation of nanocomposites.

Chiral transfer‐dictated self‐assembly of chiral block copolymers

“…The most studied BCPs* in bulk/films belonged to PS-b-PLLA, where single helixes*, networks, core-shell cylinders, and concentric/roll cake-like spiral lamellae were obtained from a manipulation on the volume fraction of PLLA (f PLLA ) and the overall molecular mass. [75,76] The helix* phase from PS-b-PLLA/PS-b-PDLA BCPs* were systemically characterized by Ho et al [35,38,[55][56][57][58] In a combination of transmission electron microscopy images (TEM) and small-angle X-ray scattering (SAXS) results, it was evidenced that this intriguing helix* phase contained PLLA/PDLA spring-like single helixes* embedded in a PS matrix, with an interdigitated and hexagonal fashion (Figure 3A). [35] This unique feature was verified by 3D TEM tomography, as well as the morphological chirality.…”

Precise Synthesis and Thin Film Self-Assembly of PLLA-b-PS Bottlebrush Block Copolymers

“…In this article, we report a route to synthesize BBCPs containing PLLA and PS, and detail their rapid self-assembly and morphological transitions depending on the PLLA block volume fraction using solvent vapor annealing. PLLA was chosen as one of polymeric side chains in our attempts to obtain various nanostructures taking advantage of the semi-crystallinity of PLLA [26][27][28]. In addition, selective etching of the PLLA block can create porous thin films, which can be used as sacrificial templates in the field of nanotechnology [29,30].…”

Combining Living Anionic Polymerization with Branching Reactions in an Iterative Fashion to Design Branched Polymers

“…[1][2][3][4][5][6] In the case of branched polymers consisting of chemically different chains, these chains are generally thermodynamically immiscible to exhibit unique phase and morphological behavior due to the characteristic branched architectures, resulting in the formation of periodically ordered nanostructures and molecular assemblies that have many potential applications in the fields of nanoscience and technology, similar to cases using block copolymers. [7][8][9][10][11] In order to fundamentally understand the effect of chain branching on polymer properties, behavior, and morphology, the synthesis of branched polymers with with welldefined structures and low degrees of compositional heterogeneity is essential. Unfortunately, however, the synthesis of well-defined branched polymers has been difficult even now and met limited success.…”