Doubly crosslinked poly(vinyl amine) microgels: hydrogels of covalently inter-linked cationic microgel particles

Doubly crosslinked (DX) microgels are macroscopic hydrogels comprised of covalently inter-linked singly crosslinked (colloidal) microgel particles. In this study we demonstrate for the first time that DX microgels can be prepared from concentrated dispersions of singly crosslinked (SX) poly(vinyl amine) (PVAM) microgel particles. The latter were of micrometer size, cationic and contained high primary amine contents. The DX PVAM morphologies contained extensive inter-connected porosity as determined by optical microscopy and SEM. The effective porosity ranged from 76 to 93 vol% and was tuneable through microgel particle concentration. The mechanical properties of the DX PVAM microgels were investigated using dynamic rheology. The best DX PVAM microgel had a storage modulus (G') of 41 kPa and yield strain of 46%, which are a good combination of elasticity and ductility. This gel had an internal porosity of 76 vol%. The dependence of G' on the effective volume fraction (ϕ_eff) for the DX PVAM microgels was tuneable and followed the equation: G'∼ exp(bϕ_eff), with b = 16.4. The latter value indicated low particle softness. The DX PVAM gels were also injectable and could be prepared at 37 °C. Furthermore, the gel mechanical properties after swelling for 3 days at physiological pH and ionic strength were similar to those before swelling. Because these injectable DX PVAM microgels have high primary amine contents they are well suited to functionalisation and should have potential applications in areas including catalysis, composite hydrogels and biomaterials.