Michael F. Roberts

Third‐order nonlinear optical properties of thin films of poly(p‐phenylene benzobisthiazole) and its molecular composites with polyamides

The nonresonant third‐order nonlinear optical properties of thin films of poly(p‐phenylene benzobisthiazole) (PBZT) and PBZT/nylon 66 and PBZT/poly(trimethylhexamethylene terephthalmide) (PTMHT) molecular composites have been investigated using picosecond third‐harmonic generation at 1.9 μm. The measured χ(3)(−3ω,ω,ω,ω) for the pure PBZT was (1.37±0.27)×10−11 esu. The χ(3) of PBZT/nylon composites showed a linear dependence on composition. In contrast, the χ(3) of PBZT/PTMHT molecular composites significantly deviated from this linear dependence, showing enhanced values. No in‐plane anisotropy in any of the films was detected. The optical damage threshold of PBZT was measured to be ≳50 GW/cm2 for 30 ps pulses at 1.9 μm.

Lewis acid coordination complexes of polymers: 3. Poly(benzobisimidazobenzophenanthroline) ladder and semiladder polymers

Abstract The structures, solution properties and solid state properties of complexes of poly(benzobisimidazobenzophenanthroline) ladder (BBL) and its semiladder analogue (BBB) with the Lewis acid aluminium(III) chloride (AlCl3) and gallium(III) chloride (GaCl3) are reported. At a 4:1 mole ratio of Lewis acid to polymer (BBB or BBL) repeat unit, electron donor-acceptor complexes of Lewis acid-base type, which are highly soluble in organic solvents, are formed. A detailed spectroscopic investigation shows that complexation of both polymers involves coordination of the Lewis acids to the electron-rich carbonyl oxygens and the imine nitrogens in the polymer backbones. The complexes of BBL exhibit the characteristic rigid-rod behaviour of the pristine ladder polymer. BBL complexes have rigid, rod-like chain conformations in dilute solution as evidenced by their solvent independent intrinsic viscosities. The complexes form liquid crystalline solutions above 8 wt% BBL in AlCl 3 nitromethane and above 9–9.5 wt% in GaCl 3 nitromethane . The solid complexes of BBL have slightly smaller optical absorption edges than BBL and ∼50% larger absorption coefficients at λmax. D.s.c. revealed a glass transition (Tg) of the 4:1 GaCl3:BBL complex at 15°C. Dynamic mechanical experiments indicated the Tg, measured at the loss tangent peak, was in the range 29–37°C depending on frequency, with an activation energy of 448 kJ mol−1. The complexes of BBB by contrast exhibit flexible-coil behaviour due to the polymer topology. In solution, BBB complexes have intrinsic viscosities which depend on the solvent and Lewis acid used, due to differing degrees of coil expansion in different solvent media. Coil expansion in solution leads to improved conjugation of BBB complexes as evidenced by comparison of the electronic absorption spectra with those of BBL. BBB complexes, like the pure polymer, are apparently not liquid crystalline at high concentrations in solution. The solid 4:1 GaCl3:BBB complex has a Tg at 30°C, and a dynamic mechanical analysis Tg in the range 12–24°C. The Tg activation energy, at 293 kJ mol−1, is lower than that of the BBL GaCl 3 complex. The overall results of the present studies provide a basis for understanding the structure and properties of these polymers in terms of intermolecular interactions and also have implications for the processing of the polymers for diverse applications.

Nonlinear Optical Properties of Ladder Polymers and Their Model Compound

The third order nonlinear optical properties of thin films of polybenzimidazobenzophenanthroline ladder (BBL) and semi–ladder (BBB) polymers and their model compound cis–BBB were investigated by picosecond, third harmonic generation. The χ (3) (−3ω, ω ω, ω) spectrum of BBL, BBB, and cis-BBB was obtained in the wavelength range 1.0–2.4µm and shown to exhibit a three–photon peak at respectively 1.695, 1.695, and 1.50 µm. The magnitude of the three–photon enhanced χ (3) was 6.40×10 −11 , 3.15×10 −11 , and 0.77×10 −11 esu, respectively, for BBL, BBB, and cis–BBB. The results show that the χ (3) of BBL and BBB is enhanced by a factor of 50 and 17, respectively, relative to the model compound cis–BBB at 1.05µm.

Dispersion of x(3) in fused aromatic ladder polymers and their precursors probed by third-harmonic generation

Third harmonic generation (THG) experiments have been performed on a variety of conjugated organic materials over the energy range 0.5 - 1.5 eV. The measured dispersion of (chi) (3)(-3(omega) ;(omega) ,(omega) ,(omega) ) is presented and comparisons between structurally related materials are made to elucidate structure-property relationships. In the benzimidazobenzophenanthroline material series consisting of the fully fused ladder polymer BBL, the semiladder polymer BBB, and the model molecule cis-BB, the effects of polymerization and ring fusion are observed. The cis-BB displays an interference in the THG spectrum, while this feature is absent in the BBB and BBL. The difference in the nonlinearity of the BBB and BBL is attributed to the difference in the number density of the two materials. The effect of the ladder structure on the nonlinearity is demonstrated by comparing the nonlinearity of BBL with PPI, a polyazomethine possessing a similar backbone. The ladder structure increases the nonlinearity by a minimum factor of 5 throughout the energy range examined. The effect of donors on the polyazomethine backbone is examined using the hydroxy and methoxy donors. The methoxy donor increases the nonlinearity, while the hydroxy donor reduces the nonlinearity. The physical implications of these observations are discussed.

Preparation and Third-Order Nonlinear Optical Properties of Host-Guest Molecular Composites

Host-guest molecular composites of conjugated rigid-rod polymers in the matrix of flexible-chain polymers were prepared from blend solutions of the Lewis acid complexes of the component polymers. Molecular composites of poly(p-phenylene benzobisthiazole) (PBZT) with the polyamides Nylon 66 and PTMHT were prepared and their linear and nonlinear optical properties were investigated by optical absorption and third harmonic generation spectroscopy, respectively. Dispersion of PBZT in a polyamide matrix was found to result in reduced π-electron interactions as evidenced by the narrowing of the main absorption band. The χ (3) spectra of composites were measured in the wavelength range 0.8–2.4μm which spans the resonant and nonresonant regions. Nonresonant χ (3) at 1.9μtm was studied as a function of composite composition and linear and nonlinear dependences of χ (3) with mole fraction of PBZT were observed for nylon 66 and PTMHT composites respectively. Our findings that both χ (3) and χ (3) /α can be enhanced in molecular composites suggest that this is an important approach to optimize materials for nonlinear optics. Preliminary results of our studies of other new molecular composites of rigid-rod and ladder polymers are also discussed.

Effects of Complexation on the Glass Transition Temperature of Polymers

The effects of Lewis acid complexation on the glass transition temperature (T g ) of several polymers with strong intermolecular interactions was investigated. The decrease in the T g due to GaCl 3 complexation of aliphatic and aromatic polyamides was 40–600° C and 148° C, respectively, and was shown to originate from scission of the intermolecular hydrogen bonds. The reduction in the T g due to GaCl 3 complexation of rigid–chain polymers was greater that 325° C and can be explained by the mitigation of the otherwise strong van der Waals forces in the pristine polymers. Thus, the dominant effect of intermolecular interactions on the T g of several polymers has been probed by Lewis acid complexation.