Surface functionalized polymer particles and thereby generation of PNIPAAM grafted monoliths by shear aggregation and ATRP

Surface functionalization of the materials to achieve functionalities like thermal responsiveness has found increasing interest. In the current study, such a functionalization with poly(N-isopropylacrylamide) (PNIPAAM) polymer has been reported by controlled grafting from the surface of aggregated particles to form macroporous monoliths. The particles were first modified with a thin polymer shell of a functional monomer 2-(2-bromopropionyloxy) ethyl acrylate, BPOEA (or copolymer with styrene), which also contained a terminal atom transfer radical polymerization (ATRP) moiety. Surfactant free emulsion polymerization was carried out to generate polystyrene seed particles as well as the shell around them. Due to the colloidal unstability of BPOEA polymer chains, stable particles were formed only in presence of excess molar amount of styrene (styrene to BPOEA mole ratio of 3:1) or seed particles. Collapse of the BPOEA polymer chains on the seed particles was also enhanced by the addition of salt in the shell forming monomer batch, which did not allow the formation of stable secondary particles. The surface morphology of the particles was affected both by the kinetic collapse of the BPOEA chains on surface of the particles and subsequent polymerization as well as efficient stirring of the contents. The emulsifier free particles could not be gelated by the addition of salt and were destabilized by shear. The resulting aggregates were stable and porous which allowed simultaneous grafting of PNIPAAM polymer from the surface as well as imparted dimensional stability to the structure during the grafting process. It was also required to optimize the extent of grafting as the excess grafting could reduce the porosity of the structure.