Search Results - (Author, Cooperation:D. Yoon)

2011-07-02

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2018-05-03

American Physical Society (APS)

0031-9007

1079-7114

Physics

Atomic, Molecular, and Optical Physics

2018-06-02

The Society of Nuclear Medicine (SNM)

0022-3123

Medicine

2018-11-24

American Association for the Advancement of Science (AAAS)

2375-2548

Natural Sciences in General

1089-7550

AIP Digital Archive

Physics

We have studied second-order nonlinear optical properties and dielectric relaxation characteristics for two amorphous epoxy polymers based on bisphenol-A with nitroaniline-type nonlinear optical moieties covalently attached as part of the main chain. The nonlinearities, rather large immediately after corona poling, undergo a slow decay at ambient temperature (e.g., ∼29% reduction in the nonlinear coefficient d33 in 28 days), even though the nominal glass transition temperature Tg is ∼80 °C. In dielectric relaxation measurements, both the unpoled and poled samples exhibit two relaxation modes: an α relaxation attributed to glass transition and a β relaxation due to local segmental motions of the main chain. Surprisingly, the critical temperature T∞ associated with the glass transition, obtained by a WLF [M. L. Williams, R. F. Landel, and J. P. Ferry, J. Am. Chem. Soc. 77, 3701 (1955)] fit of the α relaxation characteristics, shows a significant decrease (as much as 40 °C) with increasing poling field. Moreover, the relaxation times of temporal decay of field-induced alignment at ambient temperature, as determined from the stretched exponential function fit of birefringence changes, are found to be consistent with extrapolations from the the dielectric α relaxation characteristics of poled samples.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A new linear epoxy polymer containing 4-amino-4'-nitrotolane chromophores attached to the chain backbone was synthesized in an attempt to enhance both the poling-induced nonlinear optical susceptibility and its long-term stability. The polymer films poled by corona discharge showed very large nonlinear optical doubling coefficients of d33 (approximately-equal-to) 89 pm/V and d31(approximately-equal-to)25 pm/V for incident light of 1.06 μm wavelength, and a linear electro-optic coefficient r13(approximately-equal-to)8 pm/V at 633 nm wavelength. Poling-induced alignment of the nonlinear optical moieties showed no detectable decay in two weeks at ambient temperature, as studied by the birefringence. Even at 100 °C the relaxation time estimated from the birefringence decay was ca. 450 h, consistent with roughly 16% decrease in dij coefficients in 20 h at this temperature and also the extrapolation from the dielectric α relaxation characteristics. The dielectric α relaxation temperatures of the poled polymer samples exhibited higher values than that of the unpoled counterpart. This unusual behavior may be due to a significant contribution of Maier–Saupe thermotropic interactions among the tolane moieties that favor parallel alignment.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Stochastic dynamics simulations of n-alkane melts (C13H28 and C28H58 ) have been performed in order to calculate their static properties. The system consists of linear chains of mass points, which are connected by fixed bond lengths. Each point of a chain is subjected to valence bond and torsional forces. Additionally, nonbonded interactions are taken into account by a truncated, Lennard-Jones potential. The mean-square end-to-end distance and the radius of gyration of the chains as well as the average fraction of trans conformations were calculated and compared with the results obtained by Monte Carlo simulations and by unperturbed rotational isomeric states model calculations. The comparison exhibits very good agreement between the results of the different approaches, and demonstrates the potential advantages of stochastic dynamics simulations for polymeric systems. An investigation of different torsional potentials exhibited the subtle effect of the curvature of the torsional potential on bulk structures.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

New rheo-optic measurements of the orientation of magnetic particles (used in particulate recording media) in suspension under imposed external fields (hydrodynamic and/or magnetic) are presented. The apparatus consists of a rheo-optical device1 employing polarization modulation technology, and the external magnetic and flow field generating equipment (plane Poiseuille flow cell and quadrupole electromagnet). The polarization modulation dichroism measurement technique, which has the advantage of improved sensitivity and precision over conventional optical measurements, is able to characterize uniaxial single domain magnetic particle suspensions under external magnetic and hydrodynamic fields. Linear dichroism measurements, which reflect the degree of particle alignment about the field axis, are shown to be sensitive indicators of particle magnetic interactions, of effects of particle intrinsic magnetic properties, and of effects of particle shape. In this paper we focus on the comparison between effects of imposed magnetic and hydrodynamic fields on particle orientation for rodlike γ-Fe2O3, CrO2, and plate-like Ba ferrites. Relationships between the particle orientation due to the two external fields are established through dimensional analysis, and the "equivalent magnetic field,'' which yields the same magnitude of dichroism obtained under a given flow field, is measured. The flow field and the equivalent magnetic field show a nearly linear relationship. Once the equivalent magnetic field plot is obtained experimentally, one can conveniently transform hydrodynamic field to applied magnetic field and then determine the particle orientation. Preliminary results indicate that this device represents the best technique for systematic investigation of magnetic particle orientation during simultaneous imposition of magnetic and hydrodynamic fields.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Planar and channel optical waveguides have been produced in Y-cut LiTaO3 substrates by zinc diffusion from the gaseous phase at 800 °C. Prism coupler measurements on planar waveguides diffused for 6 h indicate effective index changes of 0.0063 and 0.0033 for TE and TM polarizations, respectively, with effective diffusion depths of 3.5 and 3.9 μm for the two polarizations. Losses in channel waveguides at 0.633 μm wavelength were 1.0±0.2 dB/cm for TE polarized fields, as compared with 4.7±1.0 dB/cm measured on companion samples produced by the conventional Ti indiffusion process at 1150 °C. Electro-optic modulation has been demonstrated in channel waveguide interferometers produced by this technique. Based on electron microprobe analysis, a surface Zn concentration of ∼1.3×1021 cm−3 has been determined.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Extending the novel method of Eich et al. [J. Appl. Phys. 66, 3241 (1989)] to prepare highly stable second-order nonlinear optical (NLO) polymers via chemical cross-linking under electric field, we have obtained a new polymer exhibiting large and stable second-order optical nonlinearities after relaxation at 80 °C. This was achieved by forming a network polymer from two NLO-active monomers, bifunctional N,N-(diglycidyl)-4-nitroaniline and trifunctional N-(2-aminophenyl)-4-nitroaniline. Here, every NLO moiety is connected to the network by a single covalent bond. After full cure under corona poling at 120 °C, the sample exhibited at ambient conditions d33(approximately-equal-to)50 pm/V and d31(approximately-equal-to)16 pm/V at 1064 nm fundamental wavelength, as estimated from the Maker fringe data. Upon heating to 80 °C, the nonlinearities decreased somewhat initially, but leveled off and remained stable at 80 °C. This stable sample gave d33(approximately-equal-to)42 pm/V and d31(approximately-equal-to)14 pm/V at ambient conditions. Furthermore, the linear electro-optical coefficient of this sample, measured with a Mach–Zehnder interferometer, was r13(approximately-equal-to)6.5 pm/V at 530.9 nm. From this r13 coefficient one estimates d31(approximately-equal-to)11 pm/V at 1064 nm, in good agreement with the Maker fringe value.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The electrical and interfacial properties of gold-polyimide-silicon structures were studied experimentally by measuring the capacitance-voltage (C-V) characteristics. Two different polyimides of PMDA-ODA (synthesized from pyromellitic dianhydride and oxydianiline) and BTDA-ODA (from benzophenone tetracarboxylic dianhydride and oxydianiline), respectively, were employed. Polyimide coatings on n-type silicon (with a doping level N=5×1014 cm−3) were prepared by spin coating followed by thermal cure to range from 0.4 to 2.1 μm in thickness. The resulting high frequency C-V plots of these structures were well defined and could be described by the basic theory on metal-insulator-semiconductor (MIS) devices. Distinct differences in the C-V characteristics were observed, depending upon the polyimides. PMDA-ODA dielectrics resulted in C-V hysteresis, believed to be caused by charge injection. On the other hand, BTDA-ODA showed much smaller hysteresis, but some stretch-out in the C-V curve. These differences are attributed to the different interfacial properties of these polyimides in contact with silicon substrates.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Electrical conduction phenomena in polyimide (Kapton) films were studied with particular attention devoted to the separation of interface and bulk phenomena. The measurements were carried out with a variety of methods in the temperature range of 50 to 270 °C at electrical fields of 104 to 6×105 V/cm and at time intervals of up to 2×104 s after voltage application. Biased, two-side metallized samples yield, after sufficiently long voltage application, interface-controlled steady-state currents described by Schottky injection, modified by space-charge layers in the vicinity of the electrodes. The effective work function for aluminum-polyimide is estimated to be 1.7 eV in the temperature range between 100 and 270 °C. A distinct dependence of these currents on electrode material is observed. Bulk phenomena were studied on one-side metallized samples subject to positive-corona charge injection. At temperatures below 200 °C, significantly larger currents than those for biased, two-side metallized samples were observed. The current-voltage characteristics are ohmic at low fields and space-charge limited at high fields. From these data, trap modulated mobilities for positive carriers of 4×10−12 cm2/V s at 50 °C and 10−9 cm2/V s at 200 °C, ohmic bulk conductivities of 10−16 (Ω cm)−1 at 100 °C and 10−14 (Ω cm)−1 at 200 °C and an intrinsic carrier density of 5×1013 cm−3 independent of temperature are obtained. Activation energies for the mobility are between 0.2 and 0.8 eV for the temperature range between 50 and 200 °C.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

A liquid-crystal window has been considered as an optical shutter to control solar energy. Transmittance of dynamic-scattering, twisted-nematic, and phase change guest-host (PCGH) cells having 6, 9, and 12 μm thicknesses has been measured and compared. A PCGH cell with black dye exhibits an anomalous transmission dip in the region of cholesteric to nematic or vice versa phase transformation for incident incoherent light of λ〈500 μm. We attribute this dip to voltage-induced inhomogeneities generated during the phase transition. Transmittance controllability as high as 50% has been achieved with a 6-μm-thick PCGH cell with one polarizer. A comparison of the transmittances of the three types shows that PCGH material is a suitable candidate for large size window fabrication.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Thin films ranging from 400 nm to 4 μm thickness of poly( p-phenylene biphenyltetracarboximide) (BPDA-PDA), prepared by thermal imidization of the precursor poly(amic acid) on substrates, have been investigated by the optical waveguide spectroscopy. These polyimide films, most prominent for potential applications as the interlevel dielectrics in multilevel interconnect technologies owing to their low coefficients of thermal expansion and excellent thermal/mechanical properties, are found to exhibit an extraordinarily large anisotropy in the refractive index, with the measured in-plane refractive index n(parallel)(approximately-equal-to)1.852 and the out-of-plane value n⊥(approximately-equal-to)1.612 at 632.8 nm wavelength, nearly independent of the film thickness. This large optical anisotropy indicates a very strong preference of polymer chains to orient along the film surface, and suggests a considerably larger (by ca. 27%) dielectric constant in the film plane than that along the film thickness. Moreover, there is some evidence for the existence of a very thin polyimide layer of slightly lower density and higher anisotropy adjacent to the substrate.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Thin polyimide films with dispersed nano-foam morphology have been prepared for the purpose of obtaining low dielectric polymer insulators for microelectronic applications. They were obtained by utilizing micro phase-separated triblock copolymers where the thermally stable polyimide matrix component was derived from pyromellitic dianhydride (PMDA) with 1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane (3F) and a thermally labile poly(propylene oxide)(PO) component comprised the outside block of the ABA triblock architecture. TEM studies show that the initial irregular nanoscale phase-separated morphology of polyimide triblock copolymers are mostly maintained in the final nano-foam films upon thermal decomposition of the dispersed PO component. The nano-foam polyimide films exhibit significantly lower dielectric constants ε′ (e.g., 2.3 at 19% porosity) as compared with ε′≈2.9 for the homopolymer, as predicted by Maxwell–Garnett theory, with the nano-pore structures remaining stable at 350 °C. © 1996 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Thin films for optical second-harmonic generation (SHG) were prepared from a newly designed and synthesized amorphous polymer that incorporated a high density of active nonlinear optical groups (p-nitroaniline as attached side groups). For alignment of the nonlinear groups a very high electric field was applied by a corona discharge to the polymer films above Tg (125 °C). The subsequent freezing process resulted in a polymer film initially exhibiting a very high second-order nonlinear coefficient, d33=31 pm/V, measured by the Maker-fringe technique, plus excellent thermal, mechanical, and optical properties. The dynamics of polar alignment and decay, studied by in situ poling (or depoling) and SHG measurements, indicated a multiple exponential behavior with the average relaxation time somewhat longer than expected from extrapolation of the dielectric relaxations data according to the Williams–Landel–Ferry equation. The frozen-in SHG behavior at room temperature (∼100 °C below Tg) relaxed after 5 days to a stabilized value of 19 pm/V for d33.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Stochastic dynamics simulations of polymethylene chains (CnH2n+2 for n=13, 28, 60) represented by united atom models have been carried out in order to investigate both the equilibrium and dynamic properties of polymer melts confined between flat solid surfaces. The packing manner of monomer segments, segment orientation and local conformations of chains are found to be independent of chain length, and agree very well with the results of the previous Monte Carlo simulations of C13H28 melts. The chains are extremely flattened close to the surface and many molecules assume essentially two-dimensional train configurations even in the case of C60H122 melts. However, the local chain conformations are only slightly affected by the surfaces. The apparent self diffusivities are found to be independent of the distance from the surface as the result of a reduced mobility normal to the surface and an enhanced mobility parallel to the surface. The absolute magnitudes of self-diffusion constants are larger than experimental values, most likely due to the application of united atom model.

Electronic Resource

1089-7690

AIP Digital Archive

Physics

Chemistry and Pharmacology

Biphasic equilibria of lyotropic solutions of semiflexible polymers are calculated using the worm-like chain model with limiting curvature. The threshold concentration at incipient phase separation varies gradually with molecular weight to reach a finite value at infinite chain length. This limiting concentration is found to depend on the axial ratio of the persistence length and on the temperature. The theory is then applied specifically to poly(n-hexyl isocyanate) (PHIC) solutions. Deriving the configurational parameters from measurements on chain dimensions in dilute solutions, theoretical calculations are found to satisfactorily match recent experimental results on the molecular weight dependence of the threshold concentration of PHIC in toluene.

Electronic Resource

1365-2559

Blackwell Publishing Journal Backfiles 1879-2005

Medicine

Aims:  Maspin is a recently described member of the serpin family or protease inhibitors that is known to be a tumour suppressor gene product. Loss of maspin expression has been found in most breast cancer cases and is correlated with cell motility and tumour invasiveness. However, its precise role in human breast cancer remains to be discovered. We aimed to evaluate the role of maspin in early-stage breast cancer.Methods and results:  We analysed the expression of maspin in 192 stage I and II primary breast cancers by immunohistochemistry. Of these cases, 34.4% showed maspin expression. Maspin expression was more frequently found in invasive ductal carcinoma (36.4%) than in invasive lobular carcinoma (7.1%). High maspin expression was demonstrated in breast cancers showing high histological grade or lymphocyte-rich stroma (P 〈 0.05). Maspin expression was not associated with overall and disease-free survival rate of breast cancer.Conclusions:  The results indicate that different biological mechanisms may be responsible for maspin expression in histologically distinct types of breast cancer. Our survey suggests that maspin expression in breast cancer might have a compensatory role rather than prognostic one.

Electronic Resource

0022-0248

Elsevier Journal Backfiles on ScienceDirect 1907 - 2002

Chemistry and Pharmacology

Geosciences

Physics

Electronic Resource