Polymers grafted on mica by radical chain growth from the surface

Mica is a popular model substrate for surface studies due to its availabili ty in sheet form and its molecularly flat surface. As mica has ion-exchange able surface ions, a variety of organic moities bearing cationic groups can readily be attached to the surface via ion exchange at very high surface d ensities (similar to 1 per 50 Angstrom(2)). We wished to investigate the pr eparation of polymers grafted to mica by free radical polymerization initia ted from the surface. Mica powders with ultrahigh specific surface areas (s imilar to 100 m(2) g(-1)) were treated with aqueous solutions of a peroxide radical initiator bearing one cationic group to exchange the mica surface ions with initiator cations. The resulting materials were used to initiate the polymerization of styrene. Following reaction, it was found that polyst yrene chains were bound to the surface. The individual chains could be imag ed in the form of 'molecular droplets' using scanning electron microscopy ( SEM). The observed kinetics of bound chain growth support the view that cha in growth proceeds by propagation of surface free radicals into the liquid monomer phase, i.e. 'growth from the surface'. The results obtained with th e monocationic, surface-bound peroxide initiator strongly differ from those obtained with a dicationic surface-bound azo initiator, where growth has b een shown to occur 'to the surface'. At sufficiently high density of attach ed chains, the droplets overlap and thin films result. (C) 1999 Elsevier Sc ience B.V. All rights reserved.