Mechanochemical nanoparticle functionalization for liquid crystal nanocomposites based on COOH-pyridine heterosynthons

Nanoparticle/liquid crystal (NP/LC) composites can be stabilized by hydrogen bonding, a relatively strong, yet reversible interaction allowing for the annealing of defects. Previously, nanocomposites based on 4-hexylbenzoic acid ( 6BA ) and ZrO 2 NPs with pendant carboxylic acid groups were investigated, leading to problems of particle aggregation due to intra- and inter-particle hydrogen bonding. Here we report the synthesis of NP-LC composites based on different hydrogen bond acceptor and donor groups, promoting NP-LC coupling and reducing aggregation due to NP-NP interactions. Specifically, we developed a mechanochemical, solvent-free approach for efficient functionalization of ZrO 2 NPs with (3-(pyridin-4-yl)propyl)phosphonic acid ( 3-PPA ), leading to NPs with pendant pyridine groups that act as hydrogen bond acceptors only. This is the first example of using hydrogen bonded heterosynthons to tune the LC-NP interactions. The miscibility of the pyridine-functionalized NPs with 4-hexylbenzoic acid ( 6BA ), a strong hydrogen bond donor, versus trans -4-butylcyclohexanecarboxylic acid ( 4-BCHA ), a weaker hydrogen bond donor, was characterized by polarized optical and fluorescence microscopies. The specificity of the NP/LC acceptor/donor hydrogen bonds improves the miscibility over the NP/LC dispersions based on COOH dimer interactions only. The different effect of the NPs on the 4-BCHA properties as compared to 6BA can be related to the relative strengths of the COOH-pyridine hydrogen bonds. Nanoparticle/liquid crystal (NP/LC) composites can be stabilized by hydrogen bonding, a relatively strong, yet reversible interaction allowing for the annealing of defects.