In Situ Synthesis of Polymer/Clay Nanocomposites by Type II Photoinitiated Free Radical Polymerization

INTRODUCTION Polymer/clay nanocomposites, in which nanosized silicate plates of clay are uniformly dispersed in the polymer matrix, provide enhanced mechanical and thermal properties compared to conventional composites. Not all polymer and clay combinations will form nanocomposites: the compatibility and interfacial properties between polymer matrix and clay layers significantly influence the essential characteristics of materials. Generally, the clays have poor compatibility with the polymer matrix, except for water soluble polymers. Therefore, the clay must be organically modified using organic surfactants to improve compatibility. The nanocomposites can be formed by the following four principal methods, namely, solution exfoliation, melt intercalation, in situ polymerization, and template synthesis. In situ polymerization technique is the mostly used way to prepare the nanocomposites because of the types of nanofillers and polymer precursors can be varied in a wide range to achieve desired properties. In this case, the monomer together with the initiator and/or catalyst is intercalated within the silicate layers, and the polymerization is initiated by external stimulation such as thermal, photochemical, or chemical activations. Various in situ polymerization techniques, including conventional free radical polymerization, controlled radical polymerization, ring-opening polymerization, ring-opening metathesis polymerization, cationic polymerization, and anionic polymerization, have been reported and summarized in our recent review article. Among them, the conventional free radical polymerization is the most practical and simple route in such applications because of its applicability to a wide range of monomers. Photoinitiated free radical polymerization offers several advantages over thermally initiated free radical polymerization, including low-temperature conditions, solvent-free formulation, and a rapid polymerization rate. The photoinitiated free radical polymerization can be initiated by both cleavage (Type I) and H-abstraction type (Type II) photoinitiators. Type II photoinitiators undergo a bimolecular reaction where the excited state of the photoinitiator interacts with a hydrogen donor to generate free radicals. Because of their better optical absorption properties in the near-UV spectral region, these type photoinitiators are preferred in applications where the long wavelength absorption is required.