R. D. Miller

Orientational decay in poled second‐order nonlinear optical guest‐host polymers: Temperature dependence and effects of poling geometry

The orientational decay of chemically and thermally stable high‐temperature chromophores doped into thin films made from polyimides and a variety of other polymeric hosts has been investigated. The chromophores are aligned using electric field poling and second‐harmonic generation (SHG) is used to probe the decay of the electric field poling induced alignment. The decay rate of the SHG signal from films poled using both a corona discharge and side‐by‐side in‐plane electrodes was measured. When electrodes are chosen so that the effects of charge injection are minimized, little difference has been observed between the orientational decays from films poled using the two methods for either an amorphous preimidized polyimide host or a highly anisotropic film poled during imidization. The films imidized during poling showed significant orientational stability at 250 °C for over 15 h after a fast initial partial decay. In addition, the decay of the SHG signal was measured as a function of temperature below the g...

Polymer chain organization and orientation in ultrathin films: A spectroscopic investigation

The effect of confinement on the crystallinity and chain orientation of ultrathin poly(di-n-hexylsilane) films has been investigated using UV absorption, fluorescence and IR spectroscopies. UV absorption measurements in a series of poly(di-n-hexylsilane) films having thicknesses between 50 and 3500 A have shown that, for thicknesses less than 500 A, the polymer backbone disorders and extensive crystallization of the films is hindered irrespective of molecular weight or surface hydrophobicity. Fluorescence studies showed that rapid energy transfer occurs from the disordered chain segments to the crystalline ones. The orientation of both the polymer backbone and side chains was probed with IR absorption and grazing incidence reflection measurements. The side chains are extended, although not completely in the all-trans conformation, with their carbon plane mostly perpendicular to the substrate. The backbone lies extended, with the polymer axis parallel to the plane of the film. The hexyl side-chains disorder in films less than 2000 A thick and this disordering occurs through the introduction of gauche bonds. Our findings suggest the possibility of using thickness to control the chain organization and morphology of a polymer thin film.

Re‐evaluation of the thermal stability of optically nonlinear polymeric guest‐host systems

Guest‐host polymer systems with potential use in electro‐optic devices are discussed. The polymer host is a polyimide and the guest chromophores are 2,4,5‐triarylimidazoles (lophines). Poling stabilities have been obtained by extrapolating the second harmonic generation decay using a stretched exponential function and extrapolated lifetimes greater than a year at 80 °C have been obtained. In addition, an apparent relationship between the stability of poled order and the glass transition temperature is discussed.

Pore size distributions in nanoporous methyl silsesquioxane films as determined by small angle x-ray scattering

Small angle x-ray scattering (SAXS) measurements were performed on nanoporous methyl silsesquioxane films that were generated by the incorporation of a sacrificial polymeric component into the matrix and subsequently removed by thermolysis. The average pore radii ranged from 1 to 5 nm over a porosity range of ∼5–50%. The distribution in pore size was relatively broad and increases in breadth with porosity. The values and observations obtained by SAXS are in good agreement with field emission scanning electron microscopy.

The design, synthesis, and evaluation of chromophores for second‐harmonic generation in a polymer waveguide

In order to develop nonlinear optical chromophores for frequency doubling applications, it is important to understand the magnitudes of the relevant chromophore properties that will be required. In this paper the use of organic polymers with attached optically nonlinear chromophores to form waveguide second‐harmonic‐generation devices will be discussed. In particular, the use of such a frequency doubling device in optical storage applications will be considered. To this end limits on the quantity μβ are obtained where μ is the dipole moment and β the hyperpolarizability. Experimental values for electron donor/acceptor‐substituted benzenes, stilbenes, and tolanes are compared to these requirements. None of the chromophores treated here have both sufficiently high optical nonlinearity and sufficiently low optical absorption at a doubled optical frequency of 400 nm to be practical in the specific application described.

Crosslinked networks based on trimethoxysilyl functionalized poly(amic ethyl ester) chain extendable oligomers

Abstract We report here the synthesis and characterization of poly(amic ethyl ester) oligomers functionalized with trimethoxysilyl end-groups. The oligomers were prepared via the condensation of 1,1-bis(4-aminophenyl)-1-phenyl-2.2,2-trifluoroethane, diethyldichloromellitate and p -aminophenyltrimethoxysilane. The p -amino-phenyltrimethoxysilane was used to control both molecular weight and end-group functionality. These oligomers were thermally cured to form crosslinked networks. The resulting networks possesed T g in excess of 450°C and thermal decomposition temperatures in the proximity of 500°C. The formation of networks was confirmed by swelling studies. These oligomers offer an attractive processing alternative as low viscosity precursors to crosslinked polyimide networks.

Fluorescence behaviour of pyrene substituted tri- and hexa-silanes

Abstract The absorption and emission spectra of pyrene substituted tri- and hexa-silanes are reported. σ (SiSi)→π * (pyrene) charge-transfer fluorescence is observed for 1-(1-pyrenyl) tridecamethylhexasilane but not for 1-(1-pyrenyl) hepta-methyltrisilane. This is explained by the difference in oxidation potential of the tri- and hexa-silane chain. The emission of the bichromophoric systems 1,6-di (1-pyrenyl) dodecamethylhexasilane and 1-(4-N,N-dimethylanilino)-6-(1-pyrenyl) dodecamethylhexasilane is mainly determined by the σ→π * charge transfer. 1,3-di-(1-pyrenyl) hexamethylhexasilane and 1-(4,N,N-dimethylanilino)-3-(1-pyrenyl) hexamethyltrisilane show respectively excimer and exciplex emission.

Characterization of porous low-k films using variable angle spectroscopic ellipsometry

Variable angle spectroscopic ellipsometry (VASE™) is used as a tool to characterize properties such as optical constant, thickness, refractive index depth profile, and pore volume fraction of single and bilayer porous low-k films. The porous films were prepared using sacrificial pore generator (porogen) approach. Two sets of porous films with open- and closed-pore geometries were measured. Three models were used for data analysis: Cauchy, Bruggeman effective medium approximation (BEMA), and graded layer. Cauchy, a well-known model for transparent films, was used to obtain thickness and optical constant, whereas BEMA was utilized to calculate the pore volume fraction from the ellipsometric data. The Cauchy or BEMA models were then modified as graded layers, resulting in a better fit and a better understanding of the porous film. The depth profile of the porous film implied a more porous layer at the substrate-film interface. We found 3%–4% more porosity at the interface compared with the bulk for both film...

Molecular probe techniques for studying diffusion and relaxation in thin and ultrathin polymer films

Two optically based, molecular probe techniques are employed to study relaxation and small-molecule translational diffusion in thin and ultrathin (thicknesses < ∼200 nm) polymer films. Second harmonic generation (SHG) is used to study the reorientational dynamics of a nonlinear optical chromophore, Disperse Red 1 (DR1) ( previously shown to be an effective probe of α-relaxation dynamics) either covalently attached or freely doped in polymer films. Our studies on films ranging in thickness from 7 nm to 1 μm show little change in T g with film thickness; however, a substantial broadening of the relaxation distribution is observed as film thickness decreases below approximately 150 nm. Experimental guidelines are given for using fluorescence nonradiative energy transfer (NRET) to study translational diffusion in ultrathin polymer films. Appropriate choice of a fluorescence donor species is important along with ensuring that diffusion is slow enough to be measured appropriately. Initial results on the diffusion of a small-molecule probe, lophine, in poly(isobutyl methacrylate) indicates that there is little change in probe diffusion coefficients in films as thin as 90 nm as compared to bulk films.

Transmission electron microscopy of 3F/PMDA-polypropylene oxide triblock copolymer based nanofoams

Transmission electron microscopy was performed on a polymeric nanofoam material, derived from a triblock copolymer composed of a fluorinated polyimide center block, 3F/PMDA (derived from pyromelletic dianhydride (PMDA) and 1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane (3F)) and polypropylene oxide (PO) end blocks. The cast and imidized polymer exhibits a microphase-separated morphology consisting of PO microdomains within a polyimide matrix. The final nanofoam material is obtained by decomposing PO microdomains into low molecular weight products, which diffuse out of the polyimide matrix leaving nanometer length scale voids. Ruthenium tetroxide staining prior to microscopy was used to enhance the contrast between the 3F/PMDA matrix and the PO microdomains or voids, which permitted a more detailed view of the microstructure of both the foamed and unfoamed materials. From the power spectra of the micrographs, spatial correlation between the PO microdomains in the unfoamed material and between the voids in the foam were found. An interdomain separation distance of ca. 37 nm was observed. Analysis of the image yielded an average area of 411 nm2 for the PO domains. The analysis indicated that the PO domains were oblong, having average major and minor dimensions of 35 and 12.5 nm, respectively. An autocorrelation of the image showed that the domain center of masses were positioned 41 nm apart, in close agreement with the domain spacing (ca. 37 nm) found as described above.