Development of Sustainable Hydrogels with Enhanced Hydrolytic and Thermal Stability


Student thesis: Doctoral Thesis

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Award date25 Aug 2021


The prevailing energy and environment challenges have led to the exploration of hydrogels as a suitable substitute for non-eco-friendly engineering matrices. Hydrogels are 3-D, resilient, biodegradable, stimuli-responsive, functional nanostructures that can be tuned to meet specific functions. An in-depth study of hydrogels aimed at understanding the interplay between the formation of well-defined microstructure and composition is crucial for expanding hydrogel technology, as is exploring environmentally sustainable materials and methods. In this thesis, carboxymethyl cellulose-based, functional hydrogels are successively designed, fabricated, and comprehensively characterized. A detailed evaluation of the effect of crosslinker and polymer composition on hydrogel microstructure and stability to water and heat is conducted. Mathematical models that relate hydrogel polymer ratio and microstructure parameters (namely, molecular weight between crosslinks, mesh size and crosslinking density) to its swelling for the optimum hydrogel – carboxymethyl cellulose (CMC)/chitosan (CSN)/fumaric acid (FU) hydrogels – are developed. This research provides insight into the tuning of zinc ion (Zn2+) concentration, selection of carboxylic acids as crosslinkers and varying CMC and CSN ratio in developing CMC-CSN-based hydrogels. It also provides some theoretical and practical support for advancing hydrogel technology for wearable energy applications. Overall, this study is promising for the substitution of depleting petroleum-based polymers with natural polymers for composite hydrogels.

First, single polymer-based hydrogels were prepared using CMC/fumaric acid (FU) crosslinked with Zn2+ and characterized. The objective was to determine Zn2+ concentration that gives the most stable hydrogel with optimal properties. The inspiration was that divalent ions such as Zn2+ react in unconventional patterns with anionic polysaccharides like CMC to form 3D matrices. Thus, incorporation in varying amounts will influence hydration and crystal lattice deformation, leading to crosslinking in unique patterns. Besides, Zn2+ can enhance electrical, optical, and antibacterial properties; and the effect of Zn2+ amount on CMC-based hydrogels have not been reported. Unlike the conventional trend where crosslinking increases as swelling decreases, the results show that increasing Zn2+ increases swelling and crosslinking. At 0.5 M, Zn2+ facilitates the formation of a super-absorbent yet stable, crystalline (79% crystallinity index (CI)) FA-0.5, having water-absorbency (2259%) and stability toward water (51%) and heat (-3.17 mW and 0.9% degradation at 50 ℃)) ascribable to modifications associated with chain reconfiguration and functional groups interaction. Also, when applied on fabric, the thermal stability (0% and 0.07% mass losses at 50 ℃ and 100 ℃, respectively) and enhanced absorbance after five wash cycles show the potential of FA-0.5 as a good host system for wearable applications. It was concluded that (1) 0.5M Zn2+ produces the optimum microstructure and performance for CMC~FU~Zn2+ hydrogels, and (2) the formation of a well-defined CMC~Zn2+ microstructure depends not only on Zn2+ but on its amount and chemical interaction with other hydrogel components.

Second, bi-polymer natural composite hydrogels were synthesized using CMC, CSN, and four crosslinkers (citric acid (CA), FU, tartaric acid (TA), and Zn2+) and characterized. The effect of crosslinker type on microstructure was evaluated. Crosslinking densities, mesh sizes and molecular weights between crosslinks were also evaluated. This study was inspired from the first. The focus was to improve the hydrolytic and thermal stability of the CMC-based hydrogel. Besides, it was anticipated that the carboxylic acids would introduce hydrophilic functional groups in different amounts which can interact with complexed CMC-CSN to alter hydrogel chemistry, consequently crosslinking density and microstructure formation yielding hydrogels with varying hydro properties. It was also crucial that non-toxic crosslinkers be explored to meet environmental and laboratory-to-industrial translation demands. With respect to hydrolytic properties, the results show that while CMC~CA~CSN is superabsorbent, CMC~FU~CSN absorbs water yet exhibits well-defined microstructure (155% swelling, 79% water absorbency and 82% gel fraction) superior to the other crosslinked hydrogels and the non-crosslinked composite, CMC~CSN. However, its gel fraction was within the same range as CMC~CSN (332% swelling, 91% water absorbency and 79% gel fraction). Other characterization results confirm that CMC~FU~CSN has the optimum microstructure (3 nm mesh size, 261 g/mol molecular weight between crosslinks and 0.01 μmol/cm3 effective crosslinking density), thermal stability (2% mass loss at 50℃ against 6% at 50℃ for CMC~CSN), semi-crystalline (56% CI) structure and chemical properties. CMC~FU~CSN microstructure is attributable to fewer ionic groups, which limits electrostatic repulsion and osmotic pressure. It was concluded that (1) complexation of CMC with CSN improves hydrolytic and thermal stability and microstructure of the composite hydrogels; and (2) the lower the amount of active hydrophilic groups in a crosslinker, the denser the crosslinks, the more substantial the bonding strength, and the more resistant the hydrogel to heat and water.

Third, the effect of polymer ratio on the microstructure of the CMC~FU~CSN hydrogels was investigated using mathematical modelling and experiment. A series of hydrogels were synthesized by varying the polymer ratio of CMC and CSN between 1 and 5 and characterized for swelling and microstructure parameters responses. Further, best fit values for the responses were obtained and predictive models were generated and validated. The optimum microstructure was then loaded with polyethylene glycol (PEG) and bismuth telluride (Bi2Te3) and coated on fabric for imparting thermal sensitivity. The results show that (1) optimum microstructure (26 nm mesh size, 116 μmol/cm3 effective crosslinking-density, 348% swelling, and 63% gel fraction) is found at CMC:CSN=1:3 for G3; (2) the model shows good agreement with experimental data demonstrating potential for estimating hydrogel swelling and microstructure; and (3) G3/PEG and G3/PEG/Bi2Te3 enhance the thermal conductivity of fabric at ambient, body, and elevated temperatures. It was concluded that for CMC~FU~CSN hydrogels (1) optimum microstructure is achievable at CMC:CSN=1:3; (2) the model can be used to predict swelling for CMC and CSN ratio varying between 1 and 5; and (3) G3 is a suitable host system for wearable applications.

Finally, the effect of CMC and CSN ratio on the microstructure of the superabsorbent CMC~CA~CSN was explored by varying CMC:CSN by 1:1, 1:2 and 2:1. Hydrolytic, thermal, and structural properties of the hydrogels were studied. The hydrogels were functionalized with beta-cyclodextrin (β-CD), and the effect of β-CD on microstructure and antibacterial activity was examined. It was found that (1) polymer ratio influences microstructure formation and (2) optimum ratio is at CMC:CSN=1:1, at which hydrolytic stability increased from 63% to 82% and thermal stability improved from 4% to 2% mass loss when β-CD was added. It was concluded that (1) optimal microstructure for the CMC~CA~CSN composite hydrogel is at CMC:CSN=1:1 in which thermally stable, porous, semi-crystalline M1 exhibits optimal hydrolytic properties (495% swelling, 177% water absorbency and 63% gel fraction) and (2) inclusion complexation of CMC~CA~CSN with β-CD enhances composite hydrogel microstructure and improves the hydrolytic and thermal stability.

    Research areas

  • Natural polymer-based composite hydrogels, Carboxymethyl cellulose, Chitosan, Hydrogel matrices, Environmental sustainability, Non-toxicicity, Affordability, Microstructure, Flory Rhener, Mathematical models for predicting microstructure parameters, Fumaric acid, Non-cytotoxic crosslinkers