Thermo-responsive discotic nematic hydrogels

Abstract

We synthesized a novel thermo-responsive discotic nematic hydrogel. This liquid crystalline polymer nanocomposite was fabricated using extremely high-aspect-ratio (ξ = diameter/thickness ∼ 10³) zirconium phosphate (ZrP) nano-sheets embedded into a polymeric network. These nano-sheets were immersed into a solution of constituent monomers of N-isopropylacrylamide (NIPAm) and acrylamide (AAm) with a subsequent polymerization in situ to obtain a hydrogel embedded with the nematic liquid crystal of the nano-sheets. We analyzed the effect of initiator (APS) and catalyst (TEMED) concentration on the nematic domains at ZrP nano-sheet volume fractions (ϕ_ZrP) near the isotropic–nematic (I–N) transition. The mixing during hydrogel preparation allowed the aligning of the nano-sheets into highly ordered domains, which were eventually disturbed by the effect of APS and TEMED on the polymer matrix. The effect of ZrP nano-sheet concentration and temperature on the nematic hydrogels was also studied. By increasing the temperature above the LCST of PNIPAm, the hydrogels entered into a hydrophilic-to-hydrophobic transition. Interestingly, hydrogels containing ZrP nano-sheets presented increased hydrogel water retention. In addition, we observed that the variation of the nematic ordering (S₂) and ϕ_ZrP within the system manipulated the birefringent color patterns of the hydrogels. Finally, a nematic hydrogel placed into a thin-rectangular capillary from 298 K to 333 K presented edge-wrinkling. An isotropic-to-nematic transition guided by the increment in the ZrP nano-sheet nematic ordering was observed at a nano-sheet concentration slightly below the isotropic concentration (ϕ_I).