Dizon, Glenieliz
(2022)
Sugar-derived compounds for gelation of various solvents and forming supramolecular dual-network materials for 3D printing applications.
PhD thesis, University of Nottingham.
Abstract
Low molecular weight gelators (LMWGs) are a class of organic compounds with molecular weights of less than 2000 Da that show gelation behaviour in a solvent. In recent years, LMWGs have been the subject of intensive research from material, synthetic, supramolecular, and biological chemists. They are an essential class of functional materials, finding applications in various industries such as cosmetics, food processing, healthcare and many more. During the development of soft material systems inspired by green chemistry, this research shows that naturally occurring starting materials can prepare benzylidene sorbitol and xylitol derivatives. These compounds gelate a range of organic solvents, aqueous mono- and divalent salt solutions, photocurable monomers, DMSO/water solutions, and ethanol-water solutions, with the equimolar mixture of two of the gelators gelling all compositions from 100% ethanol to 100% water (something neither of the individual components does). We explored the influence of modifications to the acetal substituents on the formation of the compounds and the impact of steric bulk on the self-assembly properties of the gelators. The effect of solvent on the self–assembly, morphology, and rheology of the 1,3:2,4-di(4-isopropylbenzylidene)-D-sorbitol (DBS-iPr), 2,4(4-isopropylbenzylidene)-D-sorbitol (MBS-iPr) and the equimolar multicomponent (DBS-MBS-iPr) gels have been investigated using scanning electron microscopy (SEM). By virtue of lamellar segregation of hydrophilic and hydrophobic regions of the molecules, DBS-iPr gelates polar solvents to form smooth flat fibres. In non-polar solvents such as cyclohexane, aided by strong hydrogen bonding interactions, it results in helical fibres where the chirality is determined by the stereochemistry of the sugar. Oscillatory rheology revealed that MBS-iPr gels have appreciable higher strength and elasticity than DBS-iPr gels, regardless of the solvent medium employed. Powder X-ray diffraction was used to probe the arrangement of the gelators in the xerogels they form, and two single-crystal X-ray structures of related MBS derivatives give the first precise structural information concerning layering and hydrogen bonding in the monobe highernzylidene sorbitol compounds. This kind of layering could explain the apparent self-sorting behaviour of the DBS-MBS-iPr multicomponent gels.
Furthermore, thixotropic and 'self-healing' behaviour were observed on some gels formed in DMSO:H2O ratios and an alkyl monomer, ethyl acrylate, by recovering their 3D network after strong mechanical shaking. Few of the monomer gels were photo polymerised to form dual-network materials. These composites were characterised using infrared spectroscopy and dynamic mechanical analysis to identify how the gelators affect the polymer material. Depending on the monomer used, some gelators act as good rheological modifiers for the photocurable monomers, improving the material's storage modulus. A xylitol-based di-acid gelator, DBX-CO2H, showed instant gelation abilities in water-miscible photocurable monomers via pH switch. A formulation of DBX-CO2H in 2-hydroxyethyl acrylate(HEA):H2O improved the surface adhesion and increased the viscosity of HEA. With these results, success was found in identifying a printable photocurable HEA ink for reactive inkjet 3D Printing. After a few optimisations, a pattern was 3D printed layer-by-layer using the di-acid gelator as a significant rheological modifier. The combination of sorbitol/xylitol-derived gelators reported in this work could find potential applications as a single or multicomponent system in different fields such as soft materials for personal care products, polymer nucleation/clarification, and energy technology.
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