Preparation And Properties Of Side Chain PI-based 2^nd-Order NLO Materials

Future generations of optoelectronic devices for information storage, telecommunication, optical switch and signal processing are predicted to be highly dependent on the development of materials with excellent nonlinear optical (NLO) properties. Recent studies have shown that polymer systems superior to the conventional inorganic materials in large NLO coefficient, ultra-fast response time, low dielectric constant, intrinsic architectural tailor-ability and process-ability and low cost.Azo-chromophore functionalized aromatic polyimides (PI) have shown promising potential in electro-optical (EO) applications because of their higher temperature alignment stability, better mechanical properties and better process-ability than other polymer systems due to the azo chromophores? reorientation for photo induced cis-trans isomerization and high T_gs of PIs matrix. Hence, they especially have potential applications in information storage and optical switch.Prior studies of Pi-based poled films focuse in the molecular-design and synthesis of chromophore with large NLO coefficient and PI with high T_g. The conventional strategy is two-step method. Firstly, a chromophore-functionalized diamine is synthesized by the chemical modification of a diamine. Then the functionalized diamine is condensated to a dianhydride to form poly (amic acid) (PAA) and then cyclization and poling. The other strategy is a mild Mitsunobu reaction of chromophore toward functionaliz'ed PAA or soluble PI directly. The former has more steps and need to purify the functionalized diamine. And the later usually has too lower T_g to satisfy the application demands. Furthermore, EO effects on the basis of field-induced and thermal optic (TO) effects on the basis of temperature-affected are getting more and more joint in EO device and materials. Because the temperature dependent refractive change dn/dT of polymer is bigger than that of inorganic materials by one digit, it is very essential to investigate the TO coefficients and their polarization dependence dΔn/dT for EO application. Although the interference of EO and TO effects can be reduced by matching of polymer, basis and buffer layers, the control TO effects by material self should be investigated.In this paper, a new synthetic strategy was developed to prepare the side chain PIs with 2nd-order NLO azobenzene chromophores. These PIs were thermal imidized from their precursors, PAAs were condensed from 2,2? bis(3,4-dicarboxy-phenyl) hexafluropropane dianhydride (6FDA)/pyromellitic dianhydride (PMDA) and m-phenylene diamine (MPD), and then a post-azo-coupling reaction of p-Nitrophenyldiazonium fluoborate (NBDF) toward the arylamine ring of PAAs. The poled films were prepared by spin-coating on lenses substrates and in-situ poling and simultaneous imidization of side-chain PAAs. To optimize the synthesis and processing technology, the structure-properties and other film parameter from prior-azo-coupling reaction and post-azo-coupling reaction respectively were studied in detail. Effects of chromophore concentration, film thickness and processing condition on dnldT and dΔn/dT of 6FDA-PMD poled films were studied.Chapter 1 gives a brief survey of the science and technology of NLO polymeric materials based on side chain azobenzene chromophores and high thermal stable PIs. The following topics are reviewed in detail: introduction of principle and origin of NLO, the preparation and measurement techniques for poled polymers, recent development of NLO polymers based on azobenzene chromophore functionalized side-chain PIs.The synthesis, structure and properties of two series of PIs containing side chain azobenzene chromophores from PMDA and 6FDA are studied in chapter 2. The PIs possess a high NLO density (>90mol %), without phase separation and crystallization. Comparing to the prior-azo-coupling reaction, the values of [η] (0.788dL/g and 0.829dL/g, respectively) and T_g (265.7℃and 236.0℃, respectively) of PIs via a post-azo-coupling reaction are improved. The PAAs exhibit good solubility in polar solvents such as NMP and DMF. 6FDA-MPD PAAs process better solubility in common organic such as cyclohexanone.Effects of dissolve, spin-coating, drying, thermal imidization and corona poling on the NLO properties and other film parameters of poled films are studied in Chapter 3. The values of d₃₃ of the two poled films are 12.21 and 17.44 pm/V@1064nm by a SPR-SHG technique, respectively. The value ofγ₃₃ for 6FDA-based poled films with good optical quality is 25.54 pm/V @632.8nm by an ATR technique. The optical loss (4.3dB) of 6FDA-based poled films is lower than that (7.2dB) of PMDA-based poled films due to the introduction of F-group. The order parameters of two serials of poled film are 24% and 26%. Their higher temperature alignment stabilities are confirmed by a depolarization method. Their values of d₃₃ are stable up to the temperature of 235℃and 210℃, respectively.The fabrication of ATR polymeric EO modulators with 6FDA-MPD poled films is studied in chapter 4. Its modulation depth is about 60%. The maximum bandwidth reaches 1.47GHz. Its ratio of power dissipation index to bandwidth is about 700Mw/GHz. Its insertion loss is 4.57dB. The half-wave potential is 6.89V. Wireless video communications via sine wave, triangular wave and sawtooth wave were performed successfully. Their linear responses are excellent.The effects of chemical structure, film thickness and processing condition on TO coefficients and their polarized dependent of 6FDA-MPD poled films are studied in chapter 5. TO coefficients in TE- and TM-polarized lights (dn_TE/dT and dn_TM/dT, respectively) were measured by ATR configuration. Results confirm that the values of coefficients of thermal expansion (CTE), TO and their polarization dependence dΔn/dT increased linearly with the growth of chromophore concentration, increase obviously with the growth of thickness, but varied very small. It causes the reduction of dΔn/dT witch can be explained by the mechanism of collapse out-of-plane due to the molecular-orientation. The boiling point of solvent, condition of spin-coating and drying, thermal imidization and poling have effects on TO effects too. To prioritization of TO properties and their polarization dependence, strategies of adjusting chemical structures, film thickness and processing conditions were suggested.The content of this paper is summarized in chapter 6 and some future works are recommended.