Enhancing Mechanical Properties in Injectable Nanocomposite Hydrogels through Dynamic Covalent Bonding at the Polymer-Nanoparticle Interface
Bruno F. Urbano
Departamento de Polímeros, Facultad de Ciencias Químicas, Universidad de Concepción, Edmundo Larenas 129, Concepción, Chile.
The incorporation of nanoparticles into hydrogels to form nanocomposite hydrogels (NCgels) has emerged as a promising approach for enhancing both the physical and mechanical properties of polymers, while simultaneously introducing new functionalities. The mechanical reinforcement of hydrogels is generally attributed to the effective transfer of stress from polymer chains to nanoparticles, which typically possess a higher modulus and thus serve as reinforcing agents. However, a significant disparity in mechanical properties, such as Young’s modulus, between the polymer matrix (measured in kPa) and the nanoparticles (measured in GPa) can result in localized stress concentrations. These concentrations often occur at the interface between the polymer and the nanoparticles, leading to challenges such as particle-matrix debonding, void formation, and, ultimately, material failure[1].
In this study, we examined the polymer-nanoparticle interface within hydrogels by incorporating dynamic covalent bonds (DCBs) based on boronic acid/boronate esters. These DCBs serve as energy-dissipating sites to help reduce stress concentrations. To achieve this, a Wulff-type boronic acid-based and phenyl boronic acid monomer was grafted onto the surface of SiO2 nanoparticles using reversible addition-fragmentation chain transfer (RAFT) polymerization. Afterwards, these functionalized nanoparticles were incorporated into boronic acid-modified alginate at concentrations of 2.0 wt% and up to 0.5 wt%, respectively. The resulting hydrogels displayed liquid-like behavior below pH 7.6. They transitioned to a gel state above this pH, a change caused by the formation of more stable esters through boron hybridization.
The hydrogels were characterized using rheometry to evaluate their viscoelastic properties. Incorporating functionalized nanoparticles caused a significant increase in the storage modulus compared to two control hydrogels: one without nanoparticles and another loaded with non-functionalized nanoparticles. This enhancement is due to the formation of boronate esters between the nanoparticle surface and the polymer matrix. Strain-stiffening behavior was also observed as the material approached its yield point[2]. Additionally, cyclic deformation tests were performed to assess the reversibility of the hydrogels under both low and high strain rates. The hydrogels maintained their storage modulus under low deformation after three cycles, while a notable decrease in the storage modulus was seen at high deformation rates.
Acknowledgements: The authors thank ANID, FONDECYT Regular 1211450 and 1252146
References
[1] M. Quaresimin, et al. Composites Science and Technology 123 (2016) 187e204
[2] Rachel Ollier, et al. Chemical Science, 2023,14, 4796-4805.
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