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Magnetically tunable composite hydrogels with cellulose nanocrystals

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Title: Magnetically tunable composite hydrogels with cellulose nanocrystals
Author(s): Srbova, Linda
Contributor: University of Helsinki, Faculty of Science, none
Discipline: none
Degree program: Master's Programme in Chemistry and Molecular Sciences
Specialisation: Polymer Materials Chemistry
Language: English
Acceptance year: 2020
In this work, a series of biocompatible nanocomposite hydrogels was prepared by UV-initiated polymerization based on 2-hydroxyethyl methacrylate (HEMA), using ethylene glycol dimethacrylate (EGDMA) as a crosslinker and 2-hydroxymethyl-2-methylpropiophenone as a photoinitiatior, containing liquid crystals of cellulose nanocrystals (CNCs) doped with magnetic nanoparticles. The formation of liquid crystals was achieved thanks to the intrinsic property of CNCs to self-assemble above a critical aqueous concentration. By varying the preparation conditions, allowing different times for phase-separation between the nanoparticles and CNCs and exposing the polymerization mixture to small magnetic field, films with different size and orientation of CNC liquid crystal domains were synthesized. Subsequently, the hydrogel films were studied by dynamic mechanical analysis (DMA) to evaluate the effect of these parameters on the mechanical properties, specifically the Young’s modulus and the ultimate strength. Also, the microstructure of the films was studied via polarized optical microscopy (POM) and scanning electron microscopy (SEM). The water uptake capacity was also analyzed. The results indicate that the presence of cellulose nanocrystals modulates the architecture of the prepared hydrogels. Cholesteric microdomains were embedded in PHEMA matrix and their uniaxial alignment was achieved upon exposure to small static magnetic field, already after several hours. Moreover, structural gradient in the distribution of the liquid crystalline microdomains, in dependence on their size, was obtained within the material. This originated from the direct proportionality between the size and the density of liquid crystals. Finally, it was shown that cellulose nanocrystals act as reinforcing structures of the hydrogels, when the degree of their self-assembly is sufficient, and therefore the resulting hydrogel exhibits both higher resistance to elastic deformation and also higher ultimate strength. Finally, we showed that mechanical performance of these nanocomposites can be enhanced by systematic orientation of the liquid crystalline domains.
Keyword(s): cellulose nanocrystals magnetic nanoparticles hydrogel liquid crystal PHEMA

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