A Facile Route to ZnS–Polymer Nanocomposite Optical Materials with High Nanophase Content via γ‐Ray Irradiation Initiated Bulk Polymerization

Abstract

Studying the fabrication and composition of semiconductor nanoparticles in polymer matrices has attracted the interest of many scientists because nanoparticle–polymer composites may find applications in high-refractive-index materials, light-emitting diodes, photocatalysts, photovoltaic solar cells, and nonlinear optical devices. Control of the nanoparticle size and size distribution as well as the dispersion homogeneity in the polymer matrix are critical prerequisites for controlling the properties of the composites. However, nanoparticles are prone to aggregation in the polymer because of their high specific surface energies and inherently hydrophilic character. Therefore, it is still a technological challenge to incorporate inorganic nanoparticles into polymer matrices and thus to prepare transparent bulk nanocomposites with high nanoparticle content. Far fewer studies of nanoparticle–polymer bulk materials have been reported than of nanocomposite films. Semiconductor nanoparticles (PbS, CdSe, CdTe) have been introduced into bulk polymer matrices to prepare bulk nanocomposites, although the content of inorganic particles was low (< 5 wt %). In these studies, two main approaches have been developed: in situ formation of nanoparticles in a presynthesized polymer and bulk polymerization of an organic monomer in the presence of premade nanoparticles. The latter provides full synthetic control over both the nanoparticles and the matrices, and is a more effective and practical route for fabricating bulk polymer nanocomposites on a large scale. We have utilized UV radiation curing to prepare transparent ZnS–polymer nanocomposite films with high ZnS contents from a solution mixture of premade ZnS particles and acrylate macromers. This is an effective method for preparing nanocomposite films with nanoparticles dispersed homogeneously horizontally and vertically throughout the entire polymer matrix, but it is unsuitable for preparing thick bulk nanocomposites. In addition, the ZnS nanoparticles obtained by the usual method cannot be redispersed in monomer or solvent because of their incomplete surface modification with organic molecules. The application of c-ray irradiation in the preparation of bulk polymer nanocomposites has remarkable advantages, such as processing under ambient pressure at room temperature and quick polymerization of monomers. These valuable properties can be put to use in the fabrication of nanocomposites with homogeneously dispersed inorganic nanoparticles. In this communication, we report a novel, facile route for the preparation of transparent bulk nanocomposites with high ZnS content via c-ray irradiation initiated polymerization (Fig. 1). Our strategy involves the design and optimization of the nanoparticle surface and polymeric monomer as well as the selection of a suitable polymerization technology. First, we reasoned that the compatibility between nanoparticles and the polymer matrix is a prerequisite for synthesizing a bulk nanocomposite with high nanophase content. So, it is necessary to tailor the corresponding polymer matrix by the decoration of the nanoparticles with organic molecules. Furthermore, it is highly important that the monomer should act as both the ligand and solvent for the inorganic nanoparticles. It has been reported that polar organic solvent molecules, such as N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO), co-ordinate to the surface of ZnS or CdS nanocrystallites and can effectively stabilize them. Thus, it is expected that a monomer with a similar structure to DMF can be selected as both the solvent and ligand for the nanoparticles. Here, we have selected N,N-dimethylacrylamide (DMAA), which can effectively disperse and stabilize ZnS particles, as the monomer. Finally, the c-ray irradiation technique is another crucial factor in the fabrication of a bulk polymer nanocomposite with high particle content. This polymerization method can induce a mild and rapid gelation process for the bulk polymeric system containing a high content of nanoparticles uniformly dispersed inside. We present a simple approach for the large-scale production of mercaptoethanol (ME)-capped ZnS nanoparticles in DMF from Zn(OAc)2 (Ac: acetate) and thiourea. The MEC O M M U N IC A TI O N S