In Situ Polymerization Methods For Preparing Semiconductor Nanoparticles/Polymer Nanocomposite Materials

Optical materials are one of the most essential materials for the modern science and technology. The synthesis and research of optical materials is one of the most important components of materials science, and it's in rapid development. The conventional optical materials consist of inorganic and organic (polymer) materials. The inorganic materials have good mechanical properties (such as high strength, high hardness and high rigidity), good optical properties (high refractive index, large Abbe's numbers, good optical transparency) and thermal stability, but it is difficult to process to fabricate optical components and devices. Polymer optical materials as the substitute of inorganic materials have been widely used in lenses, optical fibers, opticalwaveguides, optical coatings, optoelectronic devices and antireflection coatings due to their advantages of light weight, flexibility, impact resistance, easily processability, dye ability and excellent optical properties, etc. However, the polymer materials have the disadvantages of low surface hardness, narrow change range of refractive index (1.3~1.7), and low heat-resistance. Thus, one of the current leading research directions on polymer optical materials is to improve these properties of polymer materials and research the materials with more outstanding properties in order to meet the needs in high performance and high precision for optical materials. With the development of nanotechnology, more and more functional inorganic nanoparticles and nanomaterials have been developed. For example, the research on nanoparticles with photoluminescence, non-linear optical properties, photochromism, etc. have drawn many attentions of scientists from all over the world. In order to realize the function and applications of these nanoparticles and nanomaterials, the nanoparticles and nanomaterials should be incorporated with macro materials for example, organic polymers for further application on devices. So there came the nanocomposites. The approach of inorganic/organic polymer nanocomposites technology facilitates us to prepare for diverse materials with diverse properties. Compared with the pure inorganic and pure compound organic materials, inorganic/organic polymer nanocomposites have the unparalleled advantages. Optical nanocomposites consisting of inorganic and organic components with controllable length scales of inorganic nano-building blocks ranging from a few angstroms to a few tens of nanometers have attracted broad attention from both fundamental and applied research due to their distinguished physical, chemical properties and well processable properties for optical applications. The nanocomposites materials can behave the advantages of organic polymers and that of inorganic materials. In chapter one, we extensively reviewed and discussed the methods for preparing transparent nanocomposites materials by taking the high refractive index nanocomposites materials as an example. The applications of high refractive index nanocomposites were also reviewed. Besides, other transparent nanocomposites with novel properties, for example, photoluminescence, photochromism, UV-shielding, etc. were also reviewed.In this dissertation, transparent nanocomposite materials were prepared by incorporating ZnS, ZnxCd1-xS etc. with polymer matrix by in-situ polymerization method via design and tailoring the surface structure of nanoparticles and the structure of monomers for polymer matrix. The properties of nanocomposites with high refractive index, photoluminescence were also studied. In the first section of chapter Two, transparent bulk polymer nanocomposites with high contents of ZnS nanoparticles (NPs) were prepared via free radical initiated in-situ bulk polymerization of N,N-dimethylacrylamide (DMAA), styrene (St) and divinylbenzene (DVB) using 2,2′-azobisisobutyronitrile (AIBN) as initiator in the presence of 2-mercaptoethanol (ME) capped ZnS nanoparticles. Firstly, mercaptoethanol capped ZnS nanoparticles were synthesized in dimethylformamide(DMF). The ZnS nanoparticles with average size of 3nm can disperse in DMF without any precipitation. The ZnS nanoparticles powders were attained by anti-precipitation method, and it's found that it can be well dispersed in DMAA. The novelty of our strategy is that N,N-dimethylacrylamide (DMAA) we selected is as well a monomer as a solvent which can effectively disperse and stabilize ZnS NPs. The structure and properties of the nanocomposites were studied. Sphalerite ZnS NPs with average size of about 3nm were dispersed homogeneously in polymer matrices. The ME capped ZnS nanophase reached 30 wt%, while the bulk nanocomposites still exhibited good transparency in the visible light range. TGA study showed that the nanocomposite materials had excellent thermal stability. Dynamic mechanical analysis and pencil hardness studies showed that the materials had good mechanical properties. However, with the increase of the ME capped ZnS content, the glass transition temperature of the nanocomposites decreased probably due to the plasticizing effect of the ME capped ZnS NPs. Refractive indices of the nanocomposite materials increased from 1.54 for the matrix to 1.58 as increasing the weight fraction of the ME capped ZnS NPs to 30 wt%. In the second section of this charpter, nanocomposites with IPN structure were prepared by introducing ZnS nanoparticles crosslinked polyurethane. The nanocomposites with IPN structure had improved the mechanic porferance of the nanocomposites prepared in the first section of this charpter.In chapter Three, Alloyed ZnxCd1-xS nanoparticles(NPs) were synthesized by a simple one-step wet chemical route in DMF solvent using ME as capping agent as well as zinc acetate, cadmium acetate and thiourea as sources of Zn, Cd and S respectively. These ternary alloyed NPs exhibited a tunable photoluminescence from 375 nm to 568 nm by changing the Zn/Cd ratio and these free-standing ternary alloyed NPs powders showed well redispersibility in organic polar monomer such as DMAA. Furthermore, a series of transparent polymer nanocomposites with continuous controllable fluorescence from 452 nm to 568 nm can be easily obtained by in-situ bulk polymerization of organic monomers (DMAA, St, and DVB) with dispersed ZnxCd1-xS NPs. These prepared nanocomposites with tunable PL show good optical transparency and thermal stability, and may be used as optical materials for potential applications.In chapter Four, a novel method for preparing ZnS nanoparticles in ethylene glycol(EG) was invented. ZnS nanoparticles have been prepared by weak coordination control with multi- hydroxyl (ethanol) amine, for example, diethanolamine (DEA), triethanolamine (TEA) etc. in ethylene glycol. Zn(OAc)2, Zn(NO3)2, ZnCl2, can be used as the zinc source respectively, while thiourea can be used as the sulfur source. The prepared ZnS nanoparticles can be well dispersed in ethylene glycol without precipitation. We also studied the reaction mechanism for preparing ZnS nanoparticles, and the factors influencing the crystal type. With Zn(OAc)2 as zinc source, ZnS nanoparticle with cubic form can be obtained with or without DEA. However, when ZnS nanoparticles were prepared by ZnCl2, Zn(NO3)2 as zinc source without DEA, white precipitation of ZnS nanoparticles were hexagonal form; In the presence of DEA, ZnS nanoparticle with cubic form can well dispersed in EG. TGA study shows that ZnS nanoparticles have less surface-capping agent than that of reported. The photoluminescence of the ZnS nanoparticles was also studied. Besides, nanocomposite films of ZnS in PU matrix were prepared and studied. It shows that the incorporation of ZnS nanoparticles improve the refractive index.