Using click chemistry to dial up the modulus of doubly crosslinked microgels through precise control of microgel building block functionalisation

Doubly crosslinked microgels (DX MGs) are hydrogels constructed by covalently interlinked vinyl-functionalised microgel particles. Until now it has not been possible to precisely control the extent of vinyl functionalisation of the microgel (MG) particles which act as the colloidal building blocks for hydrogel assembly. Furthermore, the range of DX MGs prepared to date has been modest. This study addresses both of these challenges by constructing a new class of DX MG using MG particles that were vinyl functionalised by copper catalysed azide-alkyne cycloaddition (CuAAC). Here, poly(2-vinylpyridine-co-propargylacrylate) (PVP-PA) MG particles were prepared and vinyl functionalised by CuAAC using azidopropyl methacrylate (AZPMA) to give PVP-PA(x)-PMA(y) MGs. Values for the extent of vinyl functionalisation (y) were varied from 0 to 7.0 mol% in a precisely controlled manner. Concentrated dispersions were transformed from fluids to physical gels at pH values of less than or equal to 3.0 and covalently inter-linked to give PVP-PA(x)-PMA(y) DX MGs by free-radical coupling of the vinyl groups of neighbouring particles. The storage modulus of the DX MGs varied linearly with both MG concentration and the value for y. Not only did the new DX MGs studied here enable precise control of MG functionalisation and DX MG mechanical properties, they also showed evidence of colloidal crystallinity which may lead to future photonic gel applications. Our CuAAC-based approach should be versatile and is expected to enable a range of new DX MGs to be prepared.