Paolo Maltese

Synthesis of alkyl and N-alkyl-substituted polyanilines: A study on their spectral properties and thermal stability

Abstract Polyaniline (PANI), N-methyl- and N-ethyl-PANI, 2- and 3-ethyl-PANI as well as 2,6-dimethyl- and 3,5-dimethyl-PANI have been synthesized using ammonium persulfate as catalyst. These polymers have been studied in the undoped and doped state by FT-IR and electronic spectroscopy in the range from the UV–VIS to the NIR portion of the spectrum. As doping agent camphorsulfonic acid was used. The chemical structures of the N-methyl- and N-ethyl-PANI have been found to contain quinoneimine units along the polymer chain. In the doped state the N-alkyl-PANIs show the polaronic band transition in the NIR part of the spectrum, at longer wavelength than doped unsubstituted PANI. A similar but less pronounced phenomenon is observed in the case of ethyl-substituted phenyl rings of PANI. Dimethyl-substitution at the phenyl ring of PANI hinders the formation of high molecular weight polymer. Only oligomers are formed. The thermal stability of unsubstituted PANI in the undoped state is very high and N-alkyl substitution and phenyl ring substitution lowers the thermal stability of PANI. Doped samples show a significantly worse thermal resistance in comparison to the corresponding undoped samples, this is due essentially to the volatility and the decomposition of the protonic acid used as dopant. Good general agreement has been found between the predicted thermal stability on the basis of increment groups calculations and the experimental results. The thermal decomposition of unsubstituted and undoped PANI has been followed by FT-IR spectroscopy.

Addressing cycles for fast settling grey shades in ferroelectric liquid crystal matrices

Abstract The possible techniques to obtain grey levels in surface-stabilized ferroelectric liquid crystal cells of practical construction are discussed. The observation is reported of collateral phenomena of slow memory and high-frequency high-voltage effects which influence the matrix-addressing capability. A two-slot addressing scheme, devised to obtain grey shades in a continuously refreshed panel, is presented. It makes use of hf bias and multiple erasure steps in time-overlapping row selection sequences. For time-changing grey levels, the optical response follows within a single addressing cycle.

An addressing effective computer model for surface stabilized ferroelectric liquid crystal cells

Abstract A simple uniform director model is discussed which provides, for the first time, an agreement with experimental data for the dynamic optical behaviour of SSFLC cells subject to matrix addressing waveforms. Its use in fast running, interactive software has enabled us to design new, high speed matrix addressing schemes which have been experimentally tested.

Measurements of image sticking and hysteresis in SSFLC cells

Abstract In order to evaluate the switching properties of FLC mixtures in their relation to addressing waveforms, alignment layers and general cell construction we have built an experimental setup which allows for dynamic measurements of the optical response of a test cell under real addressing conditions. In synchronisation with a selection voltage different data pulses can be sequentially applied and the corresponding optical response of a test cell can be recorded under computer control. Thank to this equipment we are able to record hysteresis curves of the response. On this basis we introduce some useful parameters (switching amplitude, image sticking) which can be measured and compared for different cells. The setup can provide several selection and data waveforms so that influence of their choice can also be studied.

Optical characterization at wavelengths of 632.8 nm and 1549 nm of POLICRYPS switchable diffraction gratings

Optical characterization at the wavelengths of 632.8 nm and 1549 nm of an electrically switchable POLICRYPS grating sample is reported. The sample consists of a 10 m thick cell made of ITO coated glass filled with a mixture of 30% nematic liquid crystal 5CB and pre-polymer NOA61. The grating with a period of 1.34 m was written by laser curing at wavelength of 351 nm. A self-aligning setup both for visible and for infrared was used for the characterization. At the Bragg angle the grating starts to be switched off by applying an electric field of 2.5 V/m and complete switching occurs at about 5 V/m both at visible and at infrared. The grating shows a switching time of about 4 ms for both wavelengths. A FWHM of about 7 was measured at 1549 nm, corresponding to an optical bandwidth of about j u =266 nm.

Design of an Ultrashort Directional Coupler With an SSFLC Coupling Layer

Abstract A design of a vertical directional coupler, operating at the wavelength λ= 632.8 nm, is presented. The coupler consists of a layer of the Hoechst ferroelectric liquid crystal mixture FELIX-M4851–025, homogeneously aligned and sandwiched between two single mode ion-exchanged BK7 glass waveguides. The design of the directional coupler was carried out by using the beam propagation method, taking into account also teflon alignment layer and ITO electrode contributions. Given the FLC mixture refractive indices, n o =1.611 and n o = 1.466 at λ= 632.8 nm and T= 25°C, calculations show that less than 60 μm coupling length, with an extinction ratio higher than 21 dB can be obtained by using single mode waveguides with a refractive index change at the surface Δn= 0.013, an FLC layer 2 μm thick, and cone axis tilted by about 19° from the propagation direction of the lightwave.

Use of Ptfe Alignment Layers in Passive Addressed Ssflc Displays

Abstract Polytetrafluoroethylene (PTFE or Teflon®) layers have been developed as orientation films in order to produce ferroelectric liquid crystals displays with improved addressing behaviour. PTFE layers have been friction deposited on glass substrate with ITO electrodes at various temperatures in the range 130 °C-200 °C. The PTFE layers have been used to assemble displays which were addressed and showed no hysteresis, reduced image sticking and reduced switching time with respect to standard test cells employing rubbed polymers as alignment layers. In order to assure the best electro-optical performance, PTFE films have been characterized from the point of view of uniformity and thickness as function of deposition parameters.

Influence of Charge Transfer Complex Doping of Polyamide Alignment Film on SSFLC Cell Performance

Abstract New alignment films for surface stabilized ferroelectric liquid crystals displays (SSFLCDs) have been produced by doping standard polyamide alignment films with an organic conducting charge transfer complex. The doping process exploits absorption equilibrium between a solution of the dopant and the polymeric solid phase. Dopant concentration in the solid state has been evaluated by spectrophotometric analysis in the UV-visible range. Test displays with ITO electrodes and doped alignment layers have been fabricated and their performance evaluated with new techniques. It has been found that optical response is a function of dopant concentration in the alignment layer and that display performances can improve at an optimum concentration. In fact addressing hysteresis is reduced at low dopant concentration, but increases at higher concentrations.

A construction for an analog grey scale SSFLC display

Abstract We introduce a promising low cost analog grey scale structure for SSFLC panels. The display under study consists of two glass substrates, one of which is covered as usual with ITO electrodes. The other substrate is covered with a high resistivity transparent film and with a pattern of thin, parallel metal conductors. Spacers can be formed by the photoresist stripes left on metal conductors with thickness controlled in spin coating process. The grey scale is produced due to variation of the FLC switching area beside the metal tracks according to the applied data voltage. The high resistivity coating leads to non-uniformity of the electric field which is responsible for domain creation and switching process growing gradually from the metal tracks. We also discuss addressing hysteresis and operation range problems and present our choice for FLC mixture, aligning layers and addressing waveforms. Finally we give addressing hysteresis measurements and preliminary results from our devices.

Comparative studies of the multiplexability of SSFLC devices with respect to alignment characteristics

Abstract Several series of Surface-Stabilized Ferroelectric Liquid Crystal (SSFLC) cells were prepared using different aligning techniques (obliquely evaporated SiOx, buffed polyimides and nylon). The surface structure of aligning layers were studied by Atomic Force Microscopy (AFM). The samples were filled with various commercial FLC materials from Merck, BDH and Chisso. Our investigations were focused on the correlation between the aligning layer and the electro-optic characteristics of the SSFLC samples. The strength of the elastic torque is related to the surface conditions and evaluated using a specially designed test waveform. The switching properties and image sticking phenomenon have been studied under real addressing conditions.