Cholesteric Behavior of Poly(γ-benzyl-l-glutamate)-Functionalized Nanoparticles

Following earlier work showing that nanoparticles (NPs) with semiflexible polymer ligands can form lyotropic nematic liquid crystals (LCs), this strategy was expanded to explore whether other LC phases are accessible by using synthetic polypeptides. The key finding of this work was that ZrO₂ NPs grafted with poly(hexyl-isocyanate) ligands can exhibit liquid crystalline properties independently, without needing to be dispersed in a LC matrix. Following this result, poly(γ-benzyl-l-glutamate (PBLG) was chosen due to its well-understood cholesteric LC properties. Phosphonic acid-functionalized PBLG ligands of different molecular weights above the threshold to form lyotropic phases were synthesized and grafted to 4 nm metal oxide NPs and assessed for LC behavior. Cholesteric lyotropic phases, gels, and fibrils were observed and characterized by optical microscopy and small-angle X-ray scattering. Pinning of PBLG chains to NPs had the largest effect for low molecular weight PBLG with a significant decrease in the critical concentration required to form a lyotropic phase. This trend diminished as the molecular weight of the PBLG ligands increased. Structure wise, the NP contribution enlarges the cholesteric structure with an increased cholesteric pitch owing to the steric requirements of the tethered PBLG ligands. In general, the PBLG-NPs behave like the free polymer, indicating that NPs with synthetic polypeptide ligands, given their relatively simple synthesis, are promising building blocks to construct biocompatible, stimulus-responsive nanocomposites as well as for NP-based biomedical applications.