Polymerization of Styrene with Initiator Ionically Bound to High Surface Area Mica: Grafting via an Unexpected Mechanism

Abstract

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...

Determination of the Cation-Exchange Capacity of Muscovite Mica

“…The mica fines were supplied by von Roll Isola (Breitenbach, Switzerland). The delamination procedure has been previously described (32).…”

Alkali Metals Ion Exchange on Muscovite Mica

“…The starting material, which consists of small platelets, was supplied by von Roll Isola (Breitenbach, Switzerland) and characterized in a previous study (18). The delamination procedure has been previously described (19), and the same batch of delaminated mica was used in all experiments.…”

Rheological evidence for the microstructure of intercalated polymer/layered silicate nanocomposites

“…And, this ordered structure with a large contacting interface area, even without strong interactive parts, can still alter the dynamics of the nanocomposites. There has been a lot of research on the fabrication of polystyrene (PS)/ clay nanocomposites, and the degree of intercalation according to both their molecular weight and the surface properties of the silicate layer 3,5,6) . But, there has been no report on the dynamics of the intercalated PS chains in the nanoscopically confined geometries.…”