Study On The Preparation And Properties Of Novel Second-Order Nonlinear Optical Polymeric Films

Recently, various novel and high performance second-order nonlinear optical (NLO) organic polymer materials, owing to their wide application in electro-optical (E-O) modulated devices and advantages superior to inorganic materials, have been prepared and investigated by numerous scientists. It is still considered to be a challenge for researchers to prepare novel polymer lattices with macroscopically non-centrosymmetric structure and in which chromophoric dipolar molecules arranged with orientation. We can define three general approaches to achieving noncentrosymmetric lattices of nonlinear optical chromophores: electric-field poling technique, Langmuir-Blodgett film technique and Layer-by-Lay (LBL) assembly technique. Accordingly, three types of NLO materials were classified: poled polymer materials, NLO LB film materials and NLO assembly film materials. The theory and development of these types of NLO organic polymer film materials have been reviewed in chapter 1. In this dissertation, we have provided some promising attempts to fabricate novel high performance and thermal stability NLO organic polymer films in the three fields of NLO organic polymer materials, respectively. In chapter 2, we have synthesized and characterized two side-chain crosslinkable PMMA and PS types of NLO polymers through sealed-tube polymerization method. We have evidently improved the thermal stability of poly [(1-(4-nitrophenyl)-2-(4-{[2-(methacryloyloxy) ethyl] ethylamino}-phenyl)diazene (DR1M))-co-(methyl methacrylate(MMA))](P1) and poly [(DR1M)-co-(styrene(St))] (P2), respectively, by copolymerizing with epoxy glycidyl methacrylate (GMA). Studied with DSC and TGA, the glass transition temperature of the two polymers is 125℃and 124℃respectively, approximately increased 30-50℃compared with that before crosslinked. The values of electro-optic coefficient (r33) of the polymer P1, P2 after crosslinked, measured with the simple reflection technique at 1.3 μm wavelengths, was 12.50 pm/v, 8.60 pm/v and they both kept around 75% of their initial values after 100h at room temperature. Two crosslinkable second-order nonlinear optical fluorinated polymers (P3, P4) were prepared by copolymerization of 2, 3, 4, 5, 6-pentafluorostyrene, GMA and DR1M via same technique. The crosslinkable polymers were characterized concretely and have large relative molecular weight, good organosolubility, excellent film-forming property. Polymer P3 and P4 possess high glass transition temperature (106-110 oC) and high thermal decomposition temperature (290-350 oC) after crosslinked. Furthermore, the polymer P3 films possess not only high values (12-16 pm/v) of electro-optic coefficient (r33) at 1.3 μm wavelengths but also the low optical loss (1.7 dB/cm) at 1.55 μm wavelengths, which is instructive for application of electro-optical devices. The above confirmed that the thermal stability of the poled polymer P1-P4 were improved significantly, owing to the crosslinking of the epoxy group in the copolymers. In addition, the organic chromophore ((3-Triethoxysilanyl-propyl)-carbamic acid 2-{ethyl-[4-(4-nitro-phenylazo)-phenyl] -amino}-ethyl ester (SG-DANS))-inorganic matrix hybrid materials have beenfabricated via sol-gel methodology and using the obtained hybrid materials, a polymer waveguide electro-optic modulator (Mach-Zehnder Interferometer) was fabricated and studied primarily. In chapter 3, a photopolymerisable SHG active Langmuir-Blodgett (LB) film of 1-(10-{[(10-nitro)-6,7-azobenzenl]-ether}-decyl)-3-(tetracosa-12,14-diynyl)urea (N-AEDTDU) with nontraditional architecture was fabricated based on the strong molecular interactions of hydrogen bonding. Further, the noncentrosymmetric and nontraditional structures of the LB monolayer and multilayer were proved by characterization of surface pressure-area (π-A) isotherm, Atomic force microscopy (AFM), UV-vis absorption spectroscopy, contact angle as well as second harmonic generation (SHG) equipments. The study of SHG activity demonstrated that the thermal stability of nonlinear optics of LB films was improved evidently after photopolymerization via UV irradiation. (A 17-layer LB multilayer film of NAEDTDU on one side of a quartz glass after irradiation retained >55 % of the origin polar order up to 120 °C.) Most interestingly, accompanied with the falling back of the environmental temperature, we discovered all the photopolymerized LB monolayer and multilayer films exhibited unique phenomenon of SHG rebounding from decreased intensity induced by heating, which is attributed to the photopolymerization of diacetylene. In chapter 4, a promising methodology for fabricating high thermally stable organic second-order NLO thin films containing low molecular weight chromophoric (LMWC) molecules was sustained by the electric field-induced layer-by-layer assembly. The new LMWC molecule but-2-enedioic acid mono-[2-({4-[4-(2-carboxy-2-cyano-vinyl)-phenylazo]-phenyl}-methyl-amino)-eth-yl]ester(BCPE) was first designed and synthesized successfully, which possesses two negative groups at both ends and still retains the molecular polarity after ionized.