Implicit Side-Chain Model and Experimental Characterization of Bottlebrush Block Copolymer Solution Assembly

Bottlebrush polymers are a class of macromolecules with linear backbones and densely grafted side chains, possessing unique physical properties due to strong steric interactions between the side chains. These interactions serve to both stiffen the molecular contour and impart a molecular width, leading to material properties distinct from linear counterparts. This is especially important for the case of diblock bottlebrush copolymers, where side-chain-induced stiffness leads to assembly on large length scales while simultaneously suppressing molecular entanglements that would otherwise hinder self-assembly. The resulting microphase-separated structure is thus ideal for photonic crystals, due to the formation of long-range order with features on length scales commensurate with the wavelength of visible light. Despite the usefulness of block copolymer bottlebrush materials, it remains challenging to model their self-assembly in solution due to the additional side-chain degrees of freedom, which are computationally expensive to resolve. In this paper, we develop a simulation model for bottlebrush block copolymer solution self-assembly that adapts a recently developed implicit side-chain model, accounting for the effect of side chains within a simplified, bead-rod polymer model. We further extend this model by deriving an intermolecular interaction potential from scaling arguments and show that we can predict bottlebrush self-assembly over a range of block fractions and polymer concentrations. For all architectures studied, we observe an order-disorder transition to lamellar morphologies and regions of significant compositional fluctuations in the disordered phase. We compare computational predictions of self-assembly to experiments, using diblock bottlebrush copolymers synthesized with poly(norbornene) backbones and polystyrene side chains and poly(lactic acid) side chains on each block. We show that the simulation structure factors match well with experimental scattering and indicate the emergence of self-assembly at high concentrations.