Surface modification of thermally expandable microspheres by grafting poly(glycidyl methacrylate) using ARGET ATRP

This study demonstrates the surface modification of thermally expandable core/shell microspheres by grafting glycidyl methacrylate (GMA) using activators regenerated by electron transfer (ARGET) ATRP. To retain the expansion properties it was essential to minimize the shear forces, use solvents compatible with the microspheres and keep the reaction times short (three hours or less). Using microspheres with hydroxyl groups on the surface, it was found that after converting these to α-bromo esters, GMA could be grafted by ARGET ATRP using only 50 ppm of copper catalyst in toluene at 30 °C. Decent control of the polymerization was achieved with PMDETA as ligand reaching PDIs of 1.4 for the solution polymerization of GMA. When microspheres were present, the polymerization was less controlled with higher PDIs. The epoxide groups of the grafted microspheres were hydrolyzed by HCl in THF providing a hydrophilic surface of the microsphere. The expansion property of the microspheres was studied after each reaction step by thermal mechanical analysis, and it was found that the expansion capacity was well preserved with only limited negative effect on the microspheres.