Development of acrylic nanocomposites for optical applications

Abstract

Development of optical plastics happened solely because of the fact that the conventionally used materials i.e. glass has several drawbacks in spite of exhibiting the desired optical properties. A range of optical plastics with different characteristics have already been in the market for quite some time. When the issues like scratch resistance, mechanical strength, etc. go against the plastic materials, their optical properties plus their low density are the favorable parameters. At present, plastics have already become the preferred materials for several applications especially in the biomedical field. The users are able to use light weight optical lenses, thanks to the emergence of optical plastics. The two major criteria i.e. refractive index and Abbe number are the ones based on which the optical plastics are evaluated for their suitability. Attempts have been made to maneuver the refractive index and Abbe number in a manner that materials of high Abbe number along with high refractive index can be designed. Of the materials being marketed so far, polythiourethanes are the best exhibiting a refractive index of 1.67. But the chemistry of polythiourethane does not allow the possibility of achieving a high Abbe number at the same time. On the other hand, there are optical plastics which show high Abbe number but they suffer from low refractive index. To be able to develop new chemistries that meet with the requirements has been a difficult and time consuming task. The novel ways of producing low- weight optical plastics of high refractive index and at the same time optimum Abbe number are also being attempted. Methodologies like the hybrid materials involving organic matrix with the inorganic (dispersed) phase has been seen as the most prominent possibility to resolve such challenges. This thesis is the research work on the very subject of incorporating the inorganic materials in an organic matrix to produce optical plastics materials of desired properties. Taking cue from the experience of using metals of different types into glass to improve its optical and mechanical properties, the idea of incorporating same type of metals or metallic salts into monomers like acrylic acid has been pursued for the thesis work presented here. An attempt has been made to transform a basic material like acrylic acid into a highly valuable optical plastic. Several metal salts (barium hydroxide, lanthanum oxide, titanium tetrachloride and organic titanates) have been incorporated in acrylic acid matrix to produce the optical plastics of desired properties. In order to achieve the mechanical as well as other related properties, certain monomers were also used along with the cross-linkers. It has been a tough task especially in the case of incorporation of lanthanum and titanium in acrylic acid. The novelty of this work is the in- situ incorporation of these metals to produce optically clear materials having both Abbe number and refractive index within the desired limits. In fact the refractive index much higher than that of the most popular optical plastic i.e. polycarbonate has been achieved by using the methods described in this thesis. Use of co-monomers and cross-linkers were also found to be useful to produce optical plastics for ophthalmic applications. The materials developed from this work were characterized and evaluated for the essential properties of an ophthalmic lens. The results are encouraging and they show the path forward for the optical industry to adopt the new kind of hybrid (nanocomposite) materials. The thesis has been presented in 8 chapters. The brief about each chapter is given as follows: Chapter 1: Chapter 1 is the Literature Survey which deals with the complete survey of literature about the research undertaken by various national and international groups on developing nanocomposite materials for optical applications. State-of-the-art work pertaining to the incorporation of metals and metal salts (nanoscale) in organic polymer matrices has been thoroughly reviewed in this chapter. The development of materials for the use of optical industry has been the key focus highlighting the national and international status. The whole subject of literature survey is covered under the following headings: 1. a) Optical properties 2. b) Optical materials 3. c) Acrylate: their chemistry, structure and behavior 4. d) Acrylates for optical applications 5. e) Nanomaterials and nanocomposites 6. f) Challenges in making optical nanocomposites 7. g) Recent developments in metal-containing nanocomposites for optical applications 8. h) How metal-containing acrylate nanocomposites can be useful in resolving many of the existing drawbacks of the optical plastics 9. i) State-of-the-art and 10. j) Path forward With the available knowledge so far, there are many gap areas which need to be covered and a lot more needs to be done. No doubt that plastics are considered as the most ideal materials for optical applications, and that the development of nanocomposites pave the way for meeting the challenges and growing demand of the present world, the technology is still far from the hands of a common user. With the potential of being one of the most promising technologies, nanotechnology is at a nascent stage and needs to be exploited further for optical applications Chapter 2: Chapter 2 deals with the description of the materials used in the present thesis work. Use of various metal salts, dispersion matrices, co- monomers and cross-linkers have been described in detail. Amongst the metal salts, barium hydroxide, titanium tetrachloride, organic titanates (tetraisopopyl titanate, triethanolamine titanate, titanium acetylacetonate, polybutyl titanate, tetrabutyl titanate and tetra 2-ethylhexyl titanate) and lanthanum oxide have been used to prepare metal-containing novel nanocomposites. As dispersion matrices, acrylic acid and 2-hydroxyethylmethacrylate have been used for the dispersion of the above mentioned metal salts. Cinnamic acid was added as a co-monomer to enhance the optical properties of metal acrylate compositions. Styrene was used to achieve a cross-linked network structure of metal acrylate. Styrene has been used to impart hardness and clarity to the nanocomposites. Chapter 3: Chapter 3 describes the methodology and various techniques adopted for the synthesis of metal-containing acrylic nanocomposites. Approaches followed for the preparation of barium-containing nanocomposites, titanium-containing nanocomposites and lanthanum- containing nanocomposites have been discussed in detail. Salts of metals such as barium, titanium and lanthanum were dispersed in acrylate matrices using both in-situ and ex-situ approaches. It has been concluded during the discussion that in-situ synthesis and dispersion of nanoparticles of metal salts in the acrylate matrix is an ideal way to achieve clear and homogeneous metal-containing nanocomposites. For the polymerization of metal-containing monomeric compositions, a gamma irradiation technology has been adopted and established as a novel and eco-friendly and processing techniques. Using this safe irradiation technology provides the advantage of eliminating the use of initiators during polymerization thus proving to be a cost effective and safe- to-use technology. After polymerization, various instrumental state-of-the-art techniques have been used for the characterization of metal-containing nanocomposites. State-of-the-art instrumental techniques have been used for the analysis of basic parameters such as refractive index, Abbe number, transmittance (%), particle size etc to confirm the formation of nanocomposites. Chapter 4: Chapter 4 pertains to the development of barium-containing nanocomposites based on acrylates and cinnamates for optical applications. Synthesis and dispersion of barium nanoparticles (in-situ) in acrylic matrix has been carried out in a novel and cost-effective manner. The chapter describes various approaches that have been followed to design barium acrylate nanocomposites for optical applications. It should be highlighted that the use of eco-friendly gamma irradiation technology leads to the formation of transparent and hard nanocomposites. In the case of development of barium-containing nanocomposites, three approaches have been followed. a) Preparation of barium acrylate nanocomposites by the dispersion of barium hydroxide in acrylic acid followed by its polymerization using styrene. The resulting nanocomposites were transparent and hard exhibiting high refractive index and Abbe number. They were also found to be better in terms of physico-mechanical properties when compared to conventionally available materials. b) Preparation of barium cinnamate compositions by the dispersion of barium hydroxide in cinnamic acid. The compositions were characterised using different instrumental techniques. This was followed by polymerization using gamma irradiation. c) Preparation of barium cinnamate-barium acrylate nanocomposites by the dispersion of barium cinnamate in barium acrylate. Varying amounts of barium cinnamate were dispersed in barium acrylate resulting in clear and uniform dispersions. This was followed by the addition of styrene and polymerization by gamma irradiation. This resulted in transparent and high refractive index nanocomposites. For the purpose of comparison of properties, cinnamic acid-acrylic acid polymer was prepared a standard reference material. Dispersion of varying amounts of cinnamic acid was carried out in acrylic acid followed by addition of styrene and polymerization using gamma irradiation. Evaluation of the nanocomposites has been carried out with respect to key optical properties such as refractive index, Abbe number, transmittance, etc. Thermal stability and physico-mechanical properties has been studied and compared with the conventional optical plastic material. Overall, this chapter gives a description of developing high refractive index barium- containing nanocomposites for optical applications. Chapter 5: Chapter 5 describes the preparation of titania acrylate nanocomposites using an inorganic salt of titanium. Titanium tetrachloride was used as a titanium precursor for dispersion in 2-hydroxyethyl methacrylate. The addition of titanium tetrachloride in 2-hydroxyethyl methacrylate led to the dispersion of nanoparticles of titania resulting from the hydrolysis of titanium tetrachloride. Titania acrylate compositions were studied for various parameters such as IR and UV-Visible spectroscopy, particle size, thermal stability, etc. Using the novel methodology of gamma irradiation, titania acrylate compositions were cast polymerized into lenses. The resulting nanocomposites were transparent, hard and exhibited high refractive index, higher than poly (2-hdroxyethyl methacrylate) as such. The physico-mechanical properties of titania acrylate nanocomposites were also improved. It should be noted here that incorporation of titanium into polymer matrices for optical applications has been done for the first time. The nanocomposites exhibit better properties than the conventional optical plastic materials used so far. This paves the way for further exploitation of novel metal containing optical polymers. Chapter 6: Chapter 6 details the development of novel titanium-containing nanocomposites using organic titanates. Acrylic acid has been used as the dispersion matrix and six different organic titanates have been used for the preparation of titanate acrylate compositions. Cinnamic acid as a co- monomer was added to improve the optical properties of the titanate acrylate compositions. Finally styrene was added as a cross-linker followed by polymerization through gamma irradiation. The nanocomposites obtained by the dispersion of titanates in acrylic acid and polymerization by gamma irradiation have been evaluated for key optical properties such as refractive index and Abbe number. Clear and transparent titanate acrylate nanocomposites were obtained having superior optical and physico- mechanical properties than the conventional optical polymers. Chapter 7: Chapter 7 describes the development of lanthanum acrylate nanocomposites involving in-situ synthesis and dispersion of lanthanum oxide nanoparticles in acrylic acid. A novel methodology for the in-situ preparation and dispersion of nanoparticles in acrylic acid has been developed. Key optical properties such as refractive index and Abbe number have been improved. Use of cinnamic acid and styrene have helped to obtain transparent and clear high refractive index nanocomposites. Chapter 8: In Chapter 8, conclusions of the present thesis work have been presented. The process technology for making lanthanum acrylate nanocomposites of varying refractive index by employing in-situ dispersion of nanoparticles of lanthanum has been developed. The technology developed for the preparation of lanthanum acrylate nanocomposites for making optical plastics of high refractive index is an indigenous, cost effective and novel technology with good potential for commercialization. The present work has great relevance for the growth of optical plastics industry in India as much as for the society looking for an indigenous product with an affordable cost. The outcome of this entire novel study has been discussed and the path forward for several applications has been described. The author has developed a novel technology for the incorporation of metals in acrylic acid and casting them into transparent and clear high refractive index lenses. Gamma irradiation has been used as a novel technique for cast polymerization of metal acrylate compositions. The work carried out by the author has resulted in two publications in international reputed journals, one chapter in an international reputed book and several awards at national and international platforms.