D. K. Oh

Reversible polarization gratings on thin films of polyoxetanes bearing 4-(N,N-diphenyl)amino-4′-nitroazobenzene chromophores

Polarization gratings were fabricated by the two-beam coupling method on the surface of a series of polyoxetanes containing 4-(N,N-diphenyl)amino-4′-nitroazobenzene pendants with the azobenzene chromophores either side-on attached or end-on attached through short or long spacers to the main chain. The dynamics of formation of the gratings or writings was studied in relation to the diffraction efficiency. The erasing behavior of the gratings was also examined, irradiating a linearly polarized single beam. The temporal stability of the diffraction efficiency was further studied after removing the coupled excitation beam. Both the mode of attachment of the chromophore to the main chain and the length of the side-chain spacer greatly influenced the dynamic properties of the diffraction gratings. The presence of a long side-chain spacer and side-on attachment of the chromophore resulted in faster writing and erasing behavior. A preliminary study by solid-state 13C NMR of the relaxational behavior of the main-chain CH2 groups and the pendant chromophores provided a reasonable picture to help explain the photo-induced reorientation and relaxation of the azo chromophores at the molecular level.

Dimeric compounds containing malonic acid as the central linking unit: synthesis and mesomorphic properties

Eight new dimeric compounds containing malonic acid as the central linking unit (series M- n−X) have been synthesized and their mesomorphic properties studied. All these compounds are found to be liquid crystalline, forming mainly nematic and smectic phases. We have carried out microscopic textural observations, X-ray diffractometry as well as solid state NMR studies on these compounds.

Critical chain dynamics near the interdigitated–noninterdigitated chain configurational phase transition in decylammonium chloride

We have carried out a 1H nuclear magnetic resonance line shape analysis near the irreversible interdigitated-to-noninterdigitated chain configurational phase transition in decylammonium chloride (C10H21NH3Cl), a model biomembrane. The splitting parameter corresponding to the nematic order of the hydrocarbon chain shows a two-dimensional Ising behavior and indicates a chain defect motion in addition to the chain reorientational motion. The origin of the low-frequency critical fluctuation is attributed to a critical slowing down of the collective gear-like reorientation of the rigid hydrocarbon chain.

Conductivity in the I2-doped PBMPV conducting polymers

Abstract A series of I 2 -doped poly[2-buthoxy-5-methoxy-1,4-phenylenevinylene] (PBMPV) conducting polymers were synthesized and systematically studied by means of electrical conductivity measurements. The conductivity vs degree of doping as well as in situ and aging measurements reveal a peculiar behavior attributable to the dopant kinetics.

Luminescence properties of poly[1,4‐phenylenevinylene‐co‐(2,3,5,6‐tetramethyl‐1,4‐phenylenevinylene)] – Improved EL efficiency through partial disruption of main chain π‐conjugation by substituents

Two copolymers of poly(p-phenylenevinylene) (PPV) containing low levels (0.8 and 3.6 mole %) of the 2,3,5,6-tetramethyl-I 4-phenylenevinylene (TMPV) units were prepared via the sulfonium salt precursor route, The luminescence properties of the two copolymers were studied in detail mainly for photoluminescence (PL) and electroluminescence (EL) characteristics in comparison with those of PPV itself. Inclusion of low levels of TMPV units improves EL efficiency by more than 10 times when a single layer device was constructed using ITO coated glass anode and aluminum metal cathode. This was explained by the possible partial disruption of main chain π-conjugation by the TMPV units which force its phenylene or vinylene structure to twist out of the coplanar structure in order to avoid of the steric repulsion between the methyl and vinyl protons in the TMPV units. The UV-vis, PL and EL spectra of the copolymers support this supposition.

Piezoelectric resonance study of urea

Abstract Piezoelectric resonance study of polycrystalline urea (OC(NH2)2) has been made by the newly developed inductive method employing a standard pulsed NMR spectrometer. It is shown that the temperature and DC electric field dependence of the piezoelectric resonance parameters proves to be a very sensitive indicator of the electromechanical changes in piezoelectric materials

Detection of DC voltages in multi-layered magnetic thin films undergoing ferromagnetic resonance

Summary form only given. In 1996, Berger (Phys. Rev. B vol. 54, p. 9354, 1996) predicted that a DC current crossing an interface between the normal and ferromagnetic layer would induce a precession of the magnetization in the ferromagnetic layer. Slonczewski (J. Magn. Magn. Mater. vol. 159, p. L1, 1996) has proposed a similar current-induced spin precession, as well as a switching of the magnetization between two different static directions. Tsoi et al. (Phys. Rev. Lett. vol. 80, p. 4281, 1998) have also obtained experimental evidence of such spin precession, in a Cu/Co multilayer traversed by a current normal to layers at 4 K. Recently, the inverse effect has been theoretically predicted by Berger and Slonczewski independently (L. Berger, Phys. Rev. B vol. 59, p. 11465, 1999). When an external microwave is absorbed under a ferromagnetic resonance (FMR) condition, precession of the magnetization will generate a DC voltage across the normal/ferromagnetic multilayer. In this work, we report the experimental results supporting such DC voltage generation.

Pulsed piezoelectric resonance of urea

Abstract Piezoelectric resonance study of polycrystalline urea (OC(NH 2 ) 2 ) has been made by an inductive method employing a standard pulsed NMR spectrometer. As a result, previously unknown anomalous behaviors in the d.c. electric field dependence of the piezoelectric resonance parameters were observed.