Marilyn R. Unroe

Influence of two‐photon absorption on third‐order nonlinear optical processes as studied by degenerate four‐wave mixing: The study of soluble didecyloxy substituted polyphenyls

We have investigated the influence of two‐photon absorption on the third‐order nonlinear optical properties of model organic molecules using the technique of degenerate four‐wave mixing (DFWM). A theoretical formulation developed here shows that the presence of two‐photon absorption, which is related to the imaginary part of the third‐order susceptibility χ(3), leads to an enhancement of the effective third‐order nonlinearity and to the appearance of effects caused by the formation of two‐photon generated excited states. The dynamic behavior of the nonlinearity is then governed by the properties of excited molecules. The nonlinear effects also involve contributions which depend on the fifth power of the electric field. We have performed a systematic study of third‐order nonlinear optical properties of alkoxy (–C10H21OCH) substituted p‐polyphenyl oligomers using the technique of time‐resolved degenerate four‐wave mixing with subpicosecond pulses at 602 nm. Experimentally determined values of the second‐ord...

SOME CLAY NANOCOMPOSITES BASED ON A HIGH-TEMPERATURE, HIGH-PERFORMANCE POLYMER

The techniques of reinforcing relatively tractible polymers by means of intercalated and exfoliated clay particles are applied here to the more challenging case of a necessarily less tractible high-performance polymer. The polymer, poly(biphenyl ether triphenylphosphate), was chosen because of its earlier characterization in U.S. Air Forceprograms developing such composites. Conditions were found in which the polymer intercalated and exfoliated a montmorillonite clay, and the resulting nanocomposites were characterized with regard to their structures, thermal properties, and mechanical properties. As was found in previous investigations of high-performance materials, it was possible to improve a few properties, but then only at the cost of undesirable changes in other properties. In the present case, it was at least somewhat possible to increase thermal stability and to significantly increase extension moduli.

Toughening of a high-temperature polymer by the sol–gel, in situ generation of a rubbery silica–siloxane phase

A phenyl ether phenyl phosphate polymer, specifically poly(biphenyl ether triphenyl phosphate), was modified to increase its tractability and to improve its toughness. The first goal was achieved by increasing its solubility by sulfonation of the chain, and the second, by the in situ generation of a rubbery phase. This phase was generated by a modification of the usual sol-gel reaction (which usually generates a hard silicalike material by the hydrolysis of a tetrafunctional organosilicate). In this case, a difunctional silicate was included, thus introducing some softening organic groups into the dispersed phase. A bonding agent, N,N-diethylaminopropyltrimethoxysilane, was also included to improve the bonding between the two phases in this organic-inorganic composite. As expected, the glass transition temperatures generally increased slightly upon sulfonation, but decreased significantly upon introduction of the rubbery phase. Most important, the toughness of the polymer was successfully increased with, for example, only 8 wt % of the rubbery phase, quadrupling extensibility to 20 % and markedly increasing the toughness.

Synthesis of substituted thiophene-benzobisthiazole oligomers for molecular weight: third-order NLO property correlations

In an effort to better understand the relationship between molecular weight and third-order nonlinear optical activity for condensation polymers, a series of oligomeric thiophene containing benzobisthiazoles were synthesized via a trimethylsilylpolyphosphate catalysed condensation of a bis-o-aminothiophenol monomer and a didecyloxythiophene dicarboxylic acid. The physical and chemical characterization of these oligomers, including molecular weight determinations, are summarized. The enhancement of the bulk susceptibility and second molecular hyperpolarizability are demonstrated to increase with increasing oligomer length. Based on the data from femtosecond degenerate four-wave mixing experiments the second hyperpolarizability of the oligomers is enhanced by two-photon resonance.