To obtain polystyrene grafted to mica, a free-radical initiator 2,2'-azobis(isobutyramidine hydrochloride) (AIBA) was attached by ion exchange to the surface of an ultrahigh specific surface area (ca. 100 m2/g) muscovite mica powder, and the resulting material was used as an initiator to polymerize styrene. Grafted polymer was indeed obtained, in amounts up to ca. 10% by weight (ca. 30% by volume) of polymer based on mica, but it was found that grafting proceeded by an unexpected mechanism. Instead of propagation of free radicals from the surface into the bulk monomer (growth from the surface), grafting took place via attack of growing chains thermally-initiated in the monomer on disproportionation products of AIBA attached to the surface (growth to the surface). The grafted polymer consisted primarily of high molecular weight chains (M ca. 10®) bound by their headgroups to a very small fraction (less than 1 per 4000) of the surface ion-exchange sites. Consistent with this mechanism, grafted polymer could also be obtained with "thermallydeactivated" AIBA-mica at rates comparable to those obtained with "fresh" AIBA-mica. Grafting was accelerated when 2,2'-azobis(isobutyronitrile) was added to the monomer as an additional initiator.
I. IntroductionGrafting of polymers to surfaces (i.e., chemically bonding the headgroups to the surface) is of potential interest in a wide variety of fields such as composite materials and adhesives,1 chromatography,2 protein immobilization,3 colloid stabilization,4 and medicine and dentistry.5 There have been a number of such polymerizations reported, and extensive references can be found in papers by Dekking,6'7 Laible and Hamann,8 Tsubokawa et al.,9 Bhattacharya et al.,10 and Boven et al.11 Most of this work has involved silicon dioxide aerosils, carbon black, or clays as the substrate. We have been particularly interested in the layered silicate mineral mica.12 Mica has a well-defined, flat, crystalline surface, and this has led to its recent widespread use as the substrate for atomic force microscopy (AFM), scanning tunneling microscopy (STM), and surface force measurement studies.13-15 It also has surface ion-exchange sites16 which can be used to attach initiators bearing cationic groups.A new process we have developed allows the preparation of highly delaminated mica powders with ultrahigh specific surface areas.17 The ultrahigh specific surface areas allow accurate determination of the number of ion-exchange sites per unit of surface area and the degree of ion exchange that takes place upon treatment with cationic organic compounds.18 These micas, therefore, seemed particularly well-suited to the study of surface-initiated polymerization.The initiator we selected was a water-soluble bicationic azo compound previously reported for the preparation of polymer-clay composites, 2,2'-azobis(isobutyramidine hydrochloride) (AIBA):6•7 AIBA-mica ion-exchange complexes could readily be prepared by simply stirring the mica with aqueous AIBA solutions.17 (For simplicity, we use the name AI...