CaCO₃ mineralization in polymer composites with cellulose nanocrystals providing a chiral nematic mesomorphic...

CaCO₃ mineralization was carried out using cellulose nanocrystal (CNC)/polymer composites wherein a chiral nematic structure of CNC assembly was immobilized in advance via a polymerization process of the precursory aqueous CNC/vinyl monomer lyotropics (7-11 wt% CNC in feed). Two series of polymer composites were prepared: CNC/poly(2-hydroxyethyl methacrylate) (PHEMA) and CNC/poly(2-hydroxyethyl methacrylate-co-acrylic acid) (P(HEMA-co-AA), HEMA:AA = 95:5-70:30 in mol). The mineralization was allowed to proceed solely by soaking the composite films in a salt solution containing Ca²⁺ and HCO₃^- under a low-basic condition (pH ≤ 9). Polymorphism of CaCO₃ deposited inside the films was examined by X-ray diffractometry as a function of the soaking time (1-5 day) and also of the matrix composition. In the CNC/PHEMA series, the polymorphic form changed from amorphous calcium carbonate (ACC) (1-day soaking) to metastable crystalline vaterite (3-day soaking) and then to a mixture of vaterite and aragonite (5-day soaking). In the mineralization of the CNC/P(HEMA-co-AA) series, the formation of stable calcite was prominent besides minor appearance of vaterite. It was deduced that the mesofiller CNC and the AA unit in the vinyl polymer, both bearing an anionic group (-SO₃^- or -COO^-), contributed to capturing Ca²⁺ to facilitate the CaCO₃ deposition in the swollen film matrix. The pre-invested chiral nematic organization was kept in any of the mineralized films (dried); however, the helical pitch was appreciably reduced relative to that observed before the mineralization, attributable to the increase of ionic strength in the CNCs' surroundings accompanied by the wet process. Thermogravimetry showed that the mineralization definitely improved the thermal performance (heat/flame resistance) of the mesomorphic order-retaining CNC/polymer composites.