Mechanistic insights on irradiation induced fabrication of nanoporous guar gum/chitosan pH responsive hydrogels: a combined DFT and spectroscopic study.

Gamma (γ) irradiation induced modification of biopolymers has emerged as a sustainable alternative to fabricate stimulus responsive hydrogels. Even though the energy driven technique promises an efficient and scalable method that can minimize chemical usage, utilization of renewable biopolymers like guar gum not only creates a circular economy, but also delivers a sustainable alternative to synthetic polymers. Although guar gum, is widely studied due to its biocompatible, biodegradable nature and exceptional pH- and temperature-dependent gelling properties, limitations due to uncontrolled swelling and low mechanical strength make it crucial to modify its polymer chains to create a functional material. To address these challenges, the current work establishes a clear mechanistic understanding of γ irradiation of guar gum as a crosslinker in guar gum-chitosan (GC) matrixes, which was supported through DFT and spectroscopic studies. This was done by pre-irradiating guar gum with an energy source (γ rays), which created reactive oxygen sites that could introduce crosslinks through radical recombination, which affected the swelling and mechanical properties of the hydrogel. Spectroscopic and computational studies showed that crosslinking is concentrated at the C6 and C4 positions of the mannose chain in irradiated guar gum units, where glycerol is incorporated as a flexible crosslinking bridge (R-O-R) at higher concentrations of γ-G rather than solely acting as a plasticizer. DFT calculations showed a favorable binding energy (-9.516 eV) for water elimination at the C6 position of the mannose unit which supported the feasibility of the reaction. The varying concentrations of γ-G (100, 200, 300, and 400 ppm) resulted in non-monotonic variance, where surpassing the critical concentration of γ-G in GCγ-400 led to the formation of a flexible, homogeneous network that controlled pH-responsive behavior and enhanced mechanical properties, unlike the rigid, brittle network dominated by backbone-backbone (R-R) crosslinks formed in GCγ-300. While the pore radii remained comparable across the formulations (∼1.5 to 2.2 nm), GCγ-400 showed the greatest increase in surface area (15.9 to 33.7 m2 g-1) and pore volume (0.024 to 0.049 cm3 g-1) relative to pristine GC, suggesting its potential for stimulus responsive based applications like sensors, actuators, drug delivery systems etc. Therefore, the study demonstrates that pre-irradiated polysaccharide can function as a macromolecular radical crosslinking initiator where a varying concentration of γ-G modulated radical recombination induced crosslinking within the respective polymer matrix.