Thermo-responsive discotic nematic hydrogels†
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 (ϕZrP) 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 ϕZrP 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 (ϕI).