A study of hydrogel composites containing pH-responsive doubly crosslinked microgels

Recently, our group established a new approach for preparing injectable hydrogels using vinyl-functionalised pH-responsive microgel particles [Liu et al., Soft Matter, 2011, 7, 4696]. pH-responsive microgels swell when the pH approaches the pK(a) of the particles. Liu et al. used inter-particle crosslinking of vinyl-functionalised microgel particles to prepare hydrogels composed of doubly crosslinked microgels (D-microgels). Here, we combine vinyl-functionalised microgels with added, small-molecule, crosslinkers to prepare high modulus D-microgel/hydrogel (H-X) composites for the first time. The vinyl-functionalised microgel particles used were poly(EA/MAA/BDD)/GM; where, EA MAA, BDD and GM are ethyl acrylate and methacrylic acid, 1,4-butanediol diacrylate and glycidyl methacrylate, respectively. Two added crosslinkers were used to demonstrate the versatility of our approach. They were ethyleneglycol dimethacrylate (EGD) and N, N'-methylenebisacrylamide (BA). We compare the data to control hydrogel composites prepared using non-vinyl-functionalised singly crosslinked microgels (S-microgels). All of the composites showed pH-dependent swelling behaviours and mechanical properties. The storage modulus value for the as-made D-microgel/H-EGD composite was 0.12 MPa and is the highest reported to date for a hydrogel containing pH-responsive microgels. The as-made control S-microgel/H-X composites had high ductilities. Dynamic rheology data were used to determine the effects of vinyl functionalisation on the composite mechanical properties. All of the composites exhibited pH-dependent swelling and a "breathing in" transition occurred. The swollen D-microgel/H-X composites retained their high modulus values upon swelling; although, their ductilities decreased. Because we used two different crosslinkers and pH-responsive microgels containing carboxylic acid groups, the method introduced here for preparing high modulus hydrogel composites should be widely applicable.