Concentration-Driven Self-Assembly of PS-b-PLA Bottlebrush Diblock Copolymers in Solution

Abstract

Bottlebrush polymers are a class of semiflexible, hierarchical macromolecules with unique potential for shape-, architecture-, and composition-based structure–property design. It is now well-established that in dilute to semidilute solution, bottlebrush homopolymers adopt a wormlike conformation, which decreases in extension (persistence length) as the concentration and molecular overlap increase. By comparison, the solution phase self-assembly of bottlebrush diblock copolymers (BBCP) in a good solvent remains poorly understood, despite critical relevance for solution processing of ordered phases and photonic crystals. In this work, we combine small-angle X-ray scattering, coarse-grained simulation, and polymer synthesis to map the equilibrium phase behavior and conformation of a set of large, nearly symmetric PS-b-PLA bottlebrush diblock copolymers in toluene. Three BBCP are synthesized, with side chains of number-averaged molecular weights of 4500 (PS) and 4200 g/mol (PLA) and total backbone degrees of polymerization of 100, 255, and 400 repeat units. The grafting density is one side chain per backbone repeat unit. With increasing concentration in solution, all three polymers progress through a similar structural transition: from dispersed, wormlike chains with concentration-dependent (decreasing) extension, through the onset of disordered PS/PLA compositional fluctuations, to the formation of a long-range ordered lamellar phase. With increasing concentration in the microphase-separated regimes, the domain spacing increases as individual chains partially re-extend due to block immiscibility. Increases in the backbone degree of polymerization lead to changes in the scattering profiles which are consistent with the increased segregation strength. Coarse-grained simulations using an implicit side-chain model are performed, and concentration-dependent self-assembly behavior is qualitatively matched to experiments. Finally, using the polymer with the largest backbone length, we demonstrate that lamellar phases develop a well-defined photonic band gap in solution, which can be tuned across the visible spectrum by varying polymer concentration.

Theoretical advances in molecular bottlebrushes and comblike (co)polymers: solutions, gels, and self-assembly

“…[70][71][72][73][74][75][76][77] Self-assembly of such block copolymers in selective solvents produces diverse nanostructures with dimensions and morphology controlled by the branched architecture of the blocks. [78][79][80][81][82][83][84][85][86][87][88][89] Moreover, structural properties of molecular aggregates may exhibit unusual dependences on the degrees of polymerization of the blocks that differ from those predicted and experimentally observed for micelles composed of linear block copolymers. 90,91 Swelling of covalently crosslinked networks with brush-like strands in a good solvent leads to ''haired gels''.…”

“…In concentrated solutions, the equilibrium dimensions of micelles attain the molecular mass dependences characteristic for melts of microphase segregated diblock copolymers, R ∼ H ∼ N 2/3 . Depending on selectivity of the solvent, self-assembled aggregates of AB block copolymers can retain their shape or undergo morphological transformations 86,100,116 upon increasing solution concentration c .…”

“…In these regimes, the ratio of the corona thickness H C a(QN A /c) 1/3 to the core radius R C a(QN B /t B ) 1/3 decreases as H/R B c À1/3 irrespective of specific architectures of blocks A and B, making R and H closer in size upon increasing c. In concentrated solutions, the equilibrium dimensions of micelles attain the molecular mass dependences characteristic for melts of microphase segregated diblock copolymers, R B H B N 2/3 . Depending on selectivity of the solvent, self-assembled aggregates of AB block copolymers can retain their shape or undergo morphological transformations 86,100,116 upon increasing solution concentration c.…”

Relationship between Graft Density and the Dilute Solution Structure of Bottlebrush Polymers: An Inter-chemistry Comparison and Scaling Analysis

“…There have been a large number of studies interrogating relationships between parameters such as backbone and side chain length, – but only recently has the impact of graft density come into focus, and its influence on systems with a polynorbornene backbone prepared by ring-opening polymerization (ROMP) of a norbornene-terminated macromonomer (side chain) has yet to be examined. This grafting-through ROMP approach has become ubiquitous for its ability to generate controlled bottlebrush architectures. – There are now sufficient data on the influence of graft density on the dilute solution conformation of other chemistries that universal scaling relationships can be probed, and a comparison among them could yield useful information on how to tune properties through tailoring both architecture and chemistry.…”

Hairy Gels: A Computational Study

“…Self-assembly of bottlebrush block copolymers in solutions remains a topic of intensive experimental [ 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 ] and theoretical [ 42 , 65 , 66 ] research. It was demonstrated that branched architecture of soluble blocks leads to a variety of self-assembling aggregates that could potentially serve as precursors of gels with bottlebrush strands.…”