Synthesis And Nonlinear Optical Properties Of Crosslinkable Hyperbranched Polymers Containing Pendant Azobenzene Chromophores

Due to high nonlinear coefficient, short response time, large operational bandwidth, ease of processing, low dielectric constant, polymeric nonlinear-optical materials have received considerable attention in the past 30 years for their potential applications in photonics. One of the main challenges still facing in this field is how to effectively translate high molecular optical nonlinearities of chromophores into large macroscopic electro-optic (EO) coefficients and maintain good temporal stability. The involved problem is that the chromophoric aggregation resulted from strong intermolecular electrostatic interactions often quenches the EO activity. The recent theoretical analyses suggest that, due to the steric effect, the spherical molecular shape can minimize the intermolecular electrostatic interaction, resulting in maximum macroscopic nonlinearity. Hyperbranched polymers are globular macromolecules, can thus minimize the intermolecular electrostatic interactions with their site-isolation effect, and optical loss in the NLO process with their large void-containing highly branched structure. Moreover, hyperbranched polymers are easy to synthesize via "one-step" approach. Hyperbranched polymers, therefore, are promising candidates for NLO materials with good macroscopic EO activity. To develop NLO polymers with high macroscopic nonlinearity and thermal stability, in this thesis, a series of NLO hyperbranched polymers were designed and synthesized. Their NLO properties were investigated.1. Synthesis and nonlinear optical properties of NLO hyperbranched poly(aryl ester)sTwo hyperbranched poly(aryl ester)s with methyl ester (P1) and epoxy (P2) terminal groups containing pendant azobenzene chromophores were prepared through an "A₂ + B₃" approach used for second-order nonlinear optical materials. Their chemical structures were characterized by NMR and GPC analysis. The polymers have good solubility in common organic solvents and film-forming ability. The pure films were fabricated successively without doping into other matrices. The poled films exhibit high second-harmonic-generation coefficients (> 50 pm/V) due to the three-dimensional spatial isolation effect resulting from their highly branched structures. The optical nonlinearity of the poled P2 film is more thermally stable than that of P1 due to the cross-linking of epoxy groups with carboxylic acid groups in the former during poling. The onset decay temperature of SHG intensity of P2 was determined to be around 155℃, which was 20℃higher than that of P1.2. Synthesis and Characterization of Hyperbranched Polytriazole via an "A₂ + B₃" Approach Based on Click ReactionThe hyperbranched polytriazole (hb-PTA) was synthesized through "A₂ + B₃" approach using "click reaction". 4-N,N'-dis(2-azidoethyl)amino-4'-nitro-azobenzene and 1,3,5-tris(alynyloxy)benzene were synthesized to be used as A₂ and B₃ monomer, respectively. The polymerization was carried out via "one-pot" and "slow-addition" methods. The obtained hb-PTA was soluble in common organic solvents. The molecular structure was characterized by ¹H NMR, FTIR, and GPC. The degree of branching of hb-PTA was determined to be around 0.50. The obtained hb-?TA exhibits high thermal stability.3. Synthesis and nonlinear optical properties of hyperbranched polytriazole containing second-order nonlinear optical chromophoreThe hyperbranched polytriazole (hb-PTA) containing second-order nonlinear optical chromophore was synthesized through "A₂ + B₃" approach based on "click reaction". Its corresponding linear analogue (l-PTA) was prepared for comparison. The hb-PTA has better solubility in common organic solvents than the l-PTA. Both the polymers exhibit good thermal stability with 5% weight loss temperatures over 260℃. The poled film of hb-PTA exhibits much higher second-harmonic coefficient (96.8 pm/V) than that of l-PTA (23.5 pm/V). The onset decay temperature of SHG intensity of hb-PTA was determined to be around 165℃, which was 42℃higher than that of l-PTA. The three-dimensional spatial isolation effect resulting from the highly branched structure and the cross-linking of the terminal acetylene groups at moderate temperature play important roles in the enhancement of optical nonlinearity.