M. Tyagi

Novel way of making high refractive index plastics; metal containing polymers for optical applications

Abstract Plastics are being preferred in almost all possible applications of materials. Several new applications including optical devices are being developed using plastics replacing conventional materials like inorganic glass etc. For the optical applications, the most important properties of plastics essential for their desired performance include refractive index, Abbe number, optical clarity, etc. The biggest challenge in developing suitable materials for optical applications has always been to meet the criteria of high refractive index along with a high Abbe number. Normally, if the refractive index increases, the Abbe number automatically decreases. The researchers have tried several approaches to deal with this typical challenge without which it is not possible to develop novel optical plastics. Presently the most popularly known optical plastics includes polymers such as polymethacrylates, polyurethanes, polycarbonates, polystyrene and diethylene glycol bis allyl carbonate. The latest material of high refractive index plastics with a refractive index of 1.67 belongs to the polythiourethanes chemistry. Several approaches are being tried world over, to develop materials of high refractive index. One of the approaches being pursued for enhancement of refractive index of existing monomers pertain to the incorporation of metals or metal salts in the matrices. The other commonly tried but difficult to achieve approaches pertain to the preparation of nanoparticles or nanocomposites.

Growth and characterization of lithium yttrium borate single crystals

Single crystals of 0.1% Ce doped Li6Y(BO3)3 have been grown using the Czochralski technique. The photoluminescence study of these crystals shows a broad emission at ∼ 420 nm corresponding to Ce3+ emission from 5d→4f energy levels. The decay profile of this emission shows a fast response of ∼ 28 ns which is highly desirable for detector applications.

Novel Nanocomposite Optical Plastics: Dispersion of Titanium in Polyacrylates

Polyacrylates have become the preferred materials for optical applications replacing the conventionally used glass due to their superior optical clarity. The major disadvantage with polyacrylates is their low (1.40–1.50) refractive index besides their poor impact resistance. The improvements in refractive index as well as mechanical properties can be achieved by way of incorporation of metals or metal compounds in the matrix. A novel methodology for the incorporation of high refractive index metals into low refractive index polymeric materials to improve the refractive index and impact resistance of the latter has been developed. With the in-situ formation of nanoparticles of , the refractive index of polyacrylates improved from 1.45 to 1.53 and the Abbe number increased from 40 to 57. One of the interesting dimension of this study pertains to the possibility of tailor-making of the two key optical properties of materials by way of varying the amount of being formed in-situ. Thermal stability and impact resistance of nano dispersed (4.3% by wt. of Ti) polyacrylates are found to be better than the neat polyacrylates. Moreover, -containing polyacrylate is of light weight. TEM, SEM, and IR analysis confirms the in-situ formation of nanoparticles of . Gamma irradiation has been used as an eco-friendly technique for polymerization. The developed compositions can be cast polymerized into clear and bubble free material for optical applications.

Studies on Development of Polymeric Materials Using Gamma Irradiation for Contact and Intraocular Lenses

For the development of materials for contact lenses and intraocular lenses, the selection criteria is based on the (i) capacity to absorb and retain water, (ii) hydrophilicity and hydrophobicity, (iii) refractive index and (iv) hardness besides the other essential properties. Various monomers are being studied to develop suitable materials for such applications. Selection of suitable monomers that can be converted into optical materials of desired characteristics is the most essential step. In the present paper, an attempt has been made to develop suitable optical polymers based on 2-hydroxy ethyl methacrylate (HEMA), N-vinyl pyrrolidone (NVP), methyl methacrylate (MMA), methacrylic acid (MAA), and styrene. Compositions were prepared in such a way that polymers of varying hydrophilicity or hydrophobicity could be obtained keeping HEMA as the base (main) monomer. For polymerization, gamma irradiation (Co-60 as a source) was used. The results of the study showed that: (i) an increase in NVP and MAA content brought in an increase in hydrophilicity of polymerized HEMA (pHEMA), while the addition of styrene and MMA decreased hydrophilicity of polymerized HEMA (pHEMA), (ii) polymers for contact lenses with water retention capacity as high as >50 wt.% and as low as <10 wt% with varying content of suitable comonomers can be designed, (iii) polymeric materials for contact lenses can be made by using radiation processing such as Co-60 and (iv) a dose of 40 kGy was found to be ideal for purpose.

Czochralski Growth of NaY(WO4)2 Single Crystals

Single crystal growth of NaY(WO4)3 was carried out by the Czochralski technique using a starting charge rich in Y2(WO4)3 composition. This off‐stoichiometric starting composition used for the growth has been found to yield reasonable size single crystals. The XRD measurements carried out on the powder from different positions of the grown crystal confirmed the presence of desired single phase NaY(WO4)2. Grown crystals were found to have good transparency.

Photoluminescence Studies on NaBi(WO4)2 Single Crystals at Low Temperatures

Photoluminescence properties of NaBi(WO4)2 crystals were studied near liquid nitrogen temperatures. The excitation and emission properties were found to be similar to an iso‐structural Scheelite PbWO4. A metal‐to‐metal charge‐transfer character (Bi3+→W6+) is found responsible for the emission process. The thermal activation energy for quenching of the luminescence was found to be about 160 meV.

Challenges in preparation of metal-containing nanocomposites; dispersion of titanium into plastics

Abstract Metal containing polymers are of great interest as they have proven to be the most promising materials for applications such as microelectronics, holography and magnetic recording of information. Metal containing polymers also provide an alternative to even the specialty glass materials conventionally used for optical applications. Metals such as barium, lead and lanthanum have been used to improve the optical properties such as refractive index and mechanical properties such as hardness of optical plastics. The metal-containing optical plastic materials with improved refractive index of as much as 1.60 along with the Abbe number of more than 30 have been successfully designed using these metals by the authors of this paper. To further increase the refractive index without any drop in Abbe number and to improve the hardness of the polymer matrix, titanium would be the most promising metal. Incorporation of titanium in plastic matrices to form homogeneous and optically clear compositions is a challenging task. This paper reviews the state-ofthe- art by which titanium can be dispersed in polymer matrices to form metal containing composites as optical plastics.