Liquid crystals aligned to enhance visibility

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  • Nakanishi wins Welch Award in Chemistry

    Koji Nakanishi, Centennial Professor of Chemistry at Columbia University, will receive the 1996 Robert A. Welch Award in Chemistry, honoring his lifetime achievements in bioorganic chemistry. The award$300,000 and a gold medallionwill be presented to Nakanishi in October by the Welch Foundation, Houston.

    Nakanishi's career has focused on

    natural bioactive compoundseluci-dating their structures, explaining their modes of action, and exploring their use to solve human problems. His research groups, notable for multi-disciplinary and often international collaborations, have characterized more than 180 natural products, in-cluding metal-sequestering com-pounds from sea squirt blood, toxins from wasps, and antimutagens from plants. Their work on visual pigments is helping to reveal the mechanism of vision.

    Many of these compounds are avail-able only in minute amountsor ex-ist only fleetingly. So his work has stressed isolation and purification, as well as innovative approaches to structure elucidation, notably circular dichroic spectroscopy.

    The Welch Award adds to Naka-nishi's long list of honors, which in-clude, in 1990, the Imperial Prize of the Japan Academy and the Ameri-can Chemical Society's Arthur C. Cope Award, and in 1994, the Na-tional Academy of Sciences Award in Chemical Sciences.

    Maureen Rouhi

    800,000-metric-ton-per-year ethylene unit will bring the site's total annual ethylene capacity to more than 1.6 mil-lion metric tons. Expected to start up in early 2000, the new unit will use natu-ral gas liquidspropane and light naphthaas feedstocks.

    The expansion will also add 535,000 metric tons per year of polyethylene ca-pacity and 410,000 metric tons per year of ethylene glycol capacity. And Yan-pet will build 260,000 metric tons per year of polypropylene capacity, using the propylene by-product from the new ethylene unit. The expansion will bring the site's annual ethylene-deriva-tive products capacity to more than 2 million metric tons per year.

    Meanwhile, Mobil and another Sau-di partner, Petromina state-owned firmare building a 2 million-barrel-per-year lubricant basestock refinery at the Yanbu site. The refinery will go on-line in the first quarter of 1997.

    George Peaff

    Liquid crystals aligned to enhance visibility A team of Swiss researchers has devel-oped a photopolymerization technique that improves the visibility of liquid-crystal displays by controlling the tilt-ing and alignment of the liquid-crystal molecules.

    Physicists Martin Schadt and Hubert Seiberle and organic chemist Andreas Schuster at liquid-crystal R&D firm Rolic Ltd., Basel, use linearly polarized ultraviolet radiation to simultaneously align and cross-link the polymer mole-cules of the thin film that coats sub-strates that sandwich liquid-crystal molecules in displays.

    The resulting photoaligned polymer film orients the adjacent layer of liquid-crystal molecules parallel to the UV ra-diation's polarization direction. The film also tilts the long axes of the liq-uid-crystal molecules at an angle to the plane of the substrate surface [Nature, 381, 212 (1996)].

    The technique is an important break-through, says Jos van Haaren, research scientist at Philips Research Laborato-ries, Eindhoven, the Netherlands. "The Rolic group has solved the problem of tilting liquid-crystal molecules," he tells C&EN. "This was lacking in previ-ous photoalignment techniques." The

    method may find industrial uses, he notes.

    "We developed normal photopoly-mers that are sensitive to polarized light," says Schadt, Rolic's chief execu-tive officer. "The molecules induce alignment in the liquid crystals adja-cent to them. The direction depends on how we cross-link [the photopolymer molecules] with the light."

    He adds that the method can align twisted nematic displays used in TV and computer screens, where the rod-like organic liquid-crystal molecules are arranged in a spiral-staircase con-figuration between the two substrates. The molecules in each picture ele-mentpixelof the display transmit linearly polarized light. When an elec-tric field is applied perpendicular to the pixel substrates, the liquid-crystal molecules change orientation, cutting light transmission. To control this change, the molecules next to the sub-strate surface must be aligned and tilt-ed out of plane before the electric field is switched on.

    In current industrial uses, liquid-crystal molecule alignment and tilting are controlled by mechanically brush-ing the thin polymer films on the sub-strates with a velvet cloth. But this gen-erates dust and can cause electrostatic

    damage to transistors in the displays, van Haaren notes in a commentary in the same issue of Nature. The new tech-nique overcomes these problems.

    The mechanical alignment method also results in restricted viewing angles for liquid-crystal displays. The pho-toalignment technique can be used to widen the field of view by dividing a single liquid-crystal display pixel into four subpixels, each with a different twisted nematic configuration.

    The researchers point out that pho-toaligned liquid-crystal displays are stable to both heat and light. They sug-gest that linear photopolymerization technology not only improves the per-formance of existing liquid-crystal dis-playssuch as viewing properties but it also opens the way to new dis-play configurations and other types of optically anisotropic devices.

    "The next step is to see if the pho-toalignment technique is technological-ly compatible with existing processes for making these displays," points out van Haaren. "For instance, the pho-topolymer has to be transparent, and it has to have a low contamination level of the liquid-crystal layer. Otherwise, it may adversely affect the switching performance."

    Michael Freemantle

    MAY 20,1996 C&EN 9

    Liquid crystals aligned to enhance visibility

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