Liquid-Crystalline Phase Behavior in Polypeptoid Diblock Copolymers

Abstract

Polypeptoid homopolymers and block copolymers undergo thermal transitions in the solid state that can be detected by differential scanning calorimetry (DSC), but so far there is neither consensus on the underpinnings of the observed thermal transitions, nor consensus on the expected number of transitions. We synthesized a series of polypeptoid diblock copolymers containing hydrophobic alkyl sidechains and hydrophilic ethyleneoxide sidechains, systematically varying side-chain length (S), backbone mainchain length (N), block copolymer composition (n/m), and N-terminal group, 1 and studied their thermal transitions by a combination of X-ray scattering and DSC. The thermal transitions are largely unaffected by S, N, and n/m, but strongly affected by the N-terminal group. Block copolymers with an acetylated N-terminus exhibit two thermal transitions. The low temperature thermal transition is due to a transition from a crystalline phase to a sanidic liquid crystalline mesophase. The molecules adopt planar, board-like conformations and are arranged in a rectangular crystal lattice with extended backbones that run parallel to each other. The side-chains extend on either side and are located within the plane of the backbone. The liquid crystalline phase is characterized by conformational disorder in dimensions normal to the molecular plane. The high temperature thermal transition is due to melting of the liquid crystalline phase to give an isotropic phase. Block copolymers with a free N-terminus (non-acetylated) exhibit only one thermal transition, and similar out-of-plane conformational disorder. This disorder appears to be due to a difference in the pendant side chain display angle of the terminal nitrogen atom.

Effect of processing and end groups on the crystal structure of polypeptoids studied by cryogenic electron microscopy at atomic length scales

“…The unit cell of a variety of crystalline polypeptoids is orthorhombic cell 5,12,18,[20][21][22][23] . Following the nomenclature in ref.…”

“…22, the a dimension represents the inter-chain spacing (4.5 Å for all crystalline polypeptoids), the b dimension corresponds to the chain length in an extended conformation, and the c dimension is related to the width of the side chains. Recent studies based on X-ray scattering 5,22,23 , MD simulation 24 , NMR 25 , and high resolution electron microscopy imaging 18 , indicate that the chains adopt an all-cis conformation in the crystalline state.…”

Linking two worlds in polymer chemistry: The influence of block uniformity and dispersity in amphiphilic block copolypeptoids on their self‐assembly

“…Very recently, Greer et al . followed up this work with fascinating results of the phase behavior of relatively short amphiphilic polypeptoid diblock copolymers . Here, they state that the melting temperatures are largely unaffected by the overall chain length, the side chain length of the hydrophobic block, and the block copolymer composition but mainly affected by the N‐terminal group.…”

Recent advances in crystallization and self‐assembly of polypeptoid polymers

“…In addition, Greer and coworkers recently studied the crystallization of polypeptoid diblock copolymers with distinct end groups (Figure A,B) . These polypeptoids containing hydrophobic alkyl side chains and hydrophilic ethylene oxide side chains.…”

“…Geometric structures of the N ‐terminal Ndc monomers in the acetylated (C) and non‐acetylated polypeptoids (D). Second heating DSC trace of Ac‐Ndc 9 ‐Nte 9 (E) and second heating DSC trace of H‐Ndc 9 ‐Nte 9 . Adapted with permission from ref.…”