Preparation of pH-sensitive hydrogel microspheres of poly(acrylamide-co-methacrylic acid) with sharp pH-volume transition

Monodisperse hydrogel microsphere of polyacrylamide (AAm)-methacrylic acid (MAc) cross-linked by N,N'-methylene-bis(acrylamide) (MB) with sharp pH-volume transition was prepared in ethanol. The dynamic light scattering (DLS) was employed to evaluate the pH sensitivity of these microspheres. The effects of main factors: composition of copolymer, cross-linked degree, and initial total concentration or solid content of comonomers were investigated. Osmotic pressure and deformation of cross-linked polymer network were considered as the two dominant factors influencing the characteristics of pH-volume transition. High content of MAc and cross-linked degree increased the osmotic pressure, thereby moving the onset of pH-volume transition to higher pH. Association/dissociation of poly-MAc segments in the domains contributed to the free energy of hydrogel-solvent mixing. As soon as pH was high enough to overcome the osmotic pressure, the dissociated poly-MAc segments simultaneously decreased the osmotic pressure and free energy of hydrogel-solvent mixing, thereby allowing the sharp and large volume transition. As a result, microspheres were prepared with pH-volume transition of almost 12 times to their original volume within a narrow range of pH variation, ca. 0.5.