Sequence Effects on Block Copolymer Self-Assembly through Tuning Chain Conformation and Segregation Strength Utilizing Sequence-Defined Polypeptoids

Polymers with sequence control offer the possibility of tuning segregation strength with comonomer sequence instead of chemical identity. Here, we have synthesized polystyrene-b-polypeptoid diblock copolymers that differ only in the sequence of comonomers in the polypeptoid block, where nonpolar phenyl side chains are incorporated to tune compatibility with polystyrene. Using small-angle X-ray scattering, we see that these materials readily self-assemble into lamellae, with domain spacings and order-disorder transition temperatures varying with sequence, despite identical composition. The ordered state is likely governed by chain conformational effects that localize compatibilizing comonomers at the block-block interface. These altered chain conformations are supported by simulations with self-consistent field theory (SCFT) and lead to the observed changes in domain spacing. However, the trends seen in the order-disorder transition are not captured by SCFT simulations or effective χ parameters, measured in the disordered phase by approximating the copolypeptoid as a uniform block. The disagreement between measured thermodynamic properties and coarse-grained approaches like SCFT and effective χ points to the importance of molecular-scale effects in sequence-defined materials. Additionally, a reversal in relative disordering temperatures between forward and inverse taper sequences is observed compared to previous studies, likely due to a combination of sequence definition at the monomer length scale and the use of a "styrene-like" compatibilizing side chain rather than a true polystyrene repeat unit. These results demonstrate that comonomer sequence tunes chain conformation and segregation strength, suggesting that sequence design could be used to target desired properties and morphologies in block copolymer materials while retaining important chemical functionalities.