Doubly crosslinked pH-responsive microgels prepared by particle inter-penetration: Swelling and mechanical properties

The structure and properties of conventional, singly crosslinked, pH-responsive microgel particles (SX microgels) have been extensively studied. Recently, doubly crosslinked microgels (DX microgels) have been reported. These are a new type of hydrogel that are constructed from covalently linked SX microgels. In this study we report for the first time an investigation of a new class of DX microgels which are pH responsive. The DX microgels were prepared using covalent linking of physically gelled dispersions of inter-penetrating, vinyl-functionalised microgels. The pH-responsive SX microgels used were poly(MMA/MAA/EGDMA) (methyl methacrylate, methacrylic acid and ethyleneglycol dimethacrylate) and poly(EA/MAA/BDDA) (ethylacrylate and butanediol diacrylate). The two microgel types considered (abbreviated as M-EGD and E-BDD) were functionalised with glycidyl methacrylate (GM). pH-triggered swelling of concentrated dispersions and free-radical coupling of the vinyl groups was used to prepare the DX microgels. The relationships between DX microgel composition and mechanical properties are investigated using dynamic rheology and swelling experiments. The DX microgels had storage modulus values of up to 20 kPa at a particle volume fraction (p) of 0.10. The yield strains (γ*) could be varied between 5 and 65%. The ability to tune the mechanical properties of the DX microgels using the degree of functionalisation of the parent GM-functionalised microgel, p and pH is demonstrated. We show that control of intra- and inter-particle crosslinking can be achieved using preparation conditions. The results are explained using a general relationship between the storage modulus and γ*. The new DX microgels have potential application as injectable gels for soft and load-bearing tissue repair. © 2011 The Royal Society of Chemistry.