pH-responsive microgels containing hydrophilic crosslinking co-monomers: shell-exploding microgels through design

摘要

pH-responsive microgels are crosslinked polymerncolloids that swell when the pH approaches the pKa ofnthe particles. They have potential application for injectablengels for tissue repair and drug delivery systems. This studynfocuses on the pH-triggered gelation behaviour of a seriesnof poly (EA/MAA/X) microgels. EA and MAA are ethylacrylatenand methacrylic acid. Here, we investigate theneffect of crosslinking monomer type (X) on microgelnproperties. The crosslinking monomers used were polyn(ethyleneglycol) dimethacrylate (PEGD), ethyleneglycolndimethacrylate (EGD) and butanediol diacrylate (BDD).nThe microgel containing PEGD (m-PEGD) is a newnsystem. The microgel containing BDD (m-BDD) was usednas a control system. The concentrated microgel dispersionsnformed physical gels when the pH was increased to 5.3–n6.7, and the polymer volume fractions (fp) were abovenabout 0.05. Evidence from photon correlation spectroscopyn(PCS) and dynamic rheology was presented for abrupt pHtriggerednincreases, and then decreases of the hydrodynamicndiameters for m-PEGD and the microgel prepared usingnEGD (m-EGD). This appears to be tuneable throughncrosslinker structure. An unexpected gelation behaviour,nwhich may involve a new gel state for microgels, wasnfound for m-PEGD dispersions. Uniquely, those dispersionsnformed gels at pH values less than the microgel's pKa. Thisnbehaviour was linked to an outer-shell electrostatic repulsiveninteraction. The data point to a phenomenon, wherebynthe m-PEGD shells appear to explode at pH values aboven7.0. The control microgel prepared, using BDD (m-BDD),ndid not show any evidence of shell fragmentation at anynpH. That microgel has potential as a model pH-responsivenmicrogel system in that the properties measured by PCSnand rheology agreed well. To probe that system in morendetail, the rheological data for m-BDD was analysed usingnscaling theory. The variation of the storage modulus (G')nwith fp gave a scaling exponent of 2.0.