Effects of Helical Chain Shape on Lamellae-Forming Block Copolymer Self-Assembly

Understanding the effects of nonideal polymer chain shapes on block copolymer self-assembly is important for designing functional materials, such as biopolymers or conjugated polymers, with controlled self-assembly behavior. While helical chain shapes in block copolymers have been shown to produce unique morphologies, the details of how chain helicity influences the thermodynamics of self-assembly are still unclear. Here, we utilize model coil-coil and coil-helix block copolymers based on polypeptoids, for which the chain shape can be tuned from helix to coil via monomer chirality with otherwise constant chemistry. This model block copolymer system is used to probe the effects of chain helicity on the thermodynamics of block copolymer self-assembly. Small-angle X-ray scattering of the bulk materials shows that the block copolymers form well-ordered lamellar structures. While having identical domain spacing, the coil-helix block copolymer displays a lower order-disorder transition temperature (T _ODT ) than its coil-coil analogue. The coil-helix block copolymer is found to have a smaller enthalpic contribution to mixing, supported by a smaller effective Flory-Huggins interaction parameter (χ _eff ) determined in the disordered state. Furthermore, the helical block of the coil-helix block copolymer experiences larger chain stretching penalties in the lamellar morphology, which leads to a larger entropic gain upon disordering. The combined effects of the enthalpic and entropic contributions are likely to have lowered the T _ODT of the coil-helix block copolymer, yielding insight into the importance of different thermodynamic contributions that arise from polymer chains with nonideal shapes in block copolymer self-assembly.