PREPARATION AND PERFORMANCE OF pH-RESPONSIVE P(MMA-co-MAA-co-HEMA) MICROGELS

pH-responsive microgels are cross-linked polymer colloids that swell when the pH approaches the pKa of the particles. They have potential application for injectable gels for tissue repair. They can undergo volume phase transition as a function of pH and concentration. However, a major drawback of such materials is the lower mechanical strength. Pre-emulsion technology was used to prepare the pH-responsive poly (methyl methacrylate-co-methacrylic acid-co-2-hydroxyethyl mthacrylate) (P (MMA-co-MAA-co-HEMA) microgels. The morphology, particle size as well as the swelling ratio were investigated by transmission electron microscopy (TEM) and dynamic light scattering (DLS). The TEM data showed the number-average size of the microgel particles was about 90 nm when the mole fraction of MAA was 25%, numbered G(HEMA-25) or G(MAA-25). When keeping the mass of MMA and HEMA the same, the particle size increased with the increasing of the MAA content. The maximum swelling ratio (q) of G(MAA-25) can be up to 1371. The microgels exhibit good pH responsibility and swelling properties. Fluid-to-gel phase diagrams for the G(HEMA-25),G(HEMA-16) and G(HEMA-4.5) dispersions were determined as a function of concentration (c(p)) and pH using tube-inversion measurements. The results showed that the critical minimum concentration (c(p),(min)) increased and the critical maximum pH (pH(c,max)) decreased when gelatinization occured. The storage modulus of the gelled dispersions was investigated by using rheometry. The results show that storage modulus of gels increases with. the increasing of HEMA contents. Because hydrophilic hydroxyl groups of HEMA may increase the hydrogen bond sites, which not only improve mechanical properties of microgels, but also make the network structure more stable. The storage modulus also depends on the MAA content. The storage modulus increased when the MAA mole fraction increased from 4.5 mol% to 10 mol% and then decreased. The storage modulus decreased with increasing temperature. When the mole fraction of MAA was 25% based on the total monomer and the concentration of G (HEMA-25) was 15 wt%, the storage modulus of colloidal gel reached 20000 Pa at scanning frequency (omega) = 100 rad/s.