In Situ Synthesis of Silver and gold-epoxy Nanocomposites by Photoinduced Electron Transfer and Cationic Polymerization Processes

Metallic nanoparticles have special properties that may allow their use in a number of advanced functional applications. As a result, the fabrication of polymer-metal nanocomposites has attracted a great deal of attention in recent years.1-5 Nanocomposite materials consisting of dispersed metal nanoclusters in a polymeric matrix show extraordinary physical properties and they have been proposed for optical,6 electrical7 and medical8 applications as well as for data storage. 9 In particular silver and gold nanoparticles are very important for their excellent electrical conductivity,10 antimicrobial effect11 and optical properties.12 Nevertheless, the homogeneous dispersion of these metal nanoparticles is a key challenge due to their easy agglomeration arising from their high surface free energy. Nobel metals nanoparticles can be readily produced by reductive processes in homogenous or micellar solution or in preformed polymer films. However, until recently in situ formation of well dispersed metallic nanoparticle polymer composites have been scarcely investigated. Kim et al13 reported simultaneous synthesis of silver nanoparticles and the polymer film constituting the nanocomposite film through an in situ electron transfer reaction and the copolymerization of styrene and amphiphilic urethane acrylate nonionomer, which contains hydrophobic poly(propylene oxide) segments and hydrophilic poly(ethylene oxide) segments along the same backbone. Many photolytically formed radicals can be oxidized14 by suitable oxidants such as onium salts. The cations thus generated are used as initiating species for cationic polymerizations.15 Being photolyzed with fairly high quantum yields (0.41 for benzoin)16, benzoin derivatives are so far the most effective photoinitiators. The photolysis of a typical benzoin derivative, 2,2-dimethoxy-2- phenyl acetophenone (DMPA), results in the generation of strong electron donor radicals. The efficiency of oxidizing agents is related to their electron affinity. The higher the oxidation power of the oxidant, the higher (more positive) is the reduction potential E1/2red .17,18 Diphenyliodonium and Nalkoxy pyridinium salts have a relatively high reduction potential. Being very suitable for the oxidation of free radicals, these salts have been most frequently used for the oxidation of photogenerated free radicals.19-21 In contrast, triphenylsulphonium salts have only limited potential for radical-induced cationic polymerizations due to their low reduction potential. Besides onium salts, silver salts with nonnucleophilic counter anions such as SbF- 6, PF- 6, AsF6
- and BF4 - are very suitable oxidants in radical promoted cationic polymerization.22 As part of our ongoing research efforts in the design and development of photoinitiating systems for the preparation of complex macromolecular architectures, we report herein an efficient synthetic methodology for in situ generation of silver nanoparticles during photoinduced cationic curing of bisepoxides. Our approach toward the preparation of silver nanocomposites is unique in the way that silver nanoparticles and initiating cations are formed in a single redox process.