Ziad Ali-Adib

Molecular assemblies of novel amphiphilic phthalocyanines: an investigation into the self-ordering properties of complex functional materials

The synthesis and liquid crystalline properties of unsymmetrically substituted Pcs which contain both alkyl and hydroxy terminated tetra(ethyleneoxy) side-chains are described. Despite poor Langmuir-Blodgett film forming properties, the amphiphilic Pcs are of interest as they display a variety of columnar mesophases and the ability to form self-ordered lamellar films by simple solvent casting. Lamellar order can be obtained in spin-coated films of these derivatives by annealment at a temperature at which the Pc is liquid crystalline. These self-ordering spin-coated films are an alternative to the laborious Langmuir-Blodgett technique for the fabrication of highly ordered and uniform films of functional molecular materials.

Second-harmonic generation from thick all-polymeric Langmuir–Blodgett films prepared using polyurethanes

A polyurethane containing 4-dialkylamino-4′-nitroazobenzene (Disperse Red) chromophores has been used as the ‘active’ layers in alternating Y-type Langmuir-Blodgett films for second-harmonic generation from light of wavelength 1.06 µm. The choice of ‘passive’ polymer greatly affects the intensity of the second-harmonic light, I2ω, obtained. With the best ‘passive’ polymer investigated, the square root of I2ω increases in proportion to the number of bilayers in the film up to a thickness of ca. 75 bilayers. The value of I2ω decreases significantly over a period of months when the films are stored at 20 °C.

Efficient second-harmonic generation from all-polymeric Langmuir-Blodgett “AB” films containing up to 600 layers

Abstract Alternating Langmuir-Blodgett films were prepared for second harmonic (SH) generation from an “active” polymer containing a hemicyanine chromophore and various “passive” polymers. With the best “passive” polymer, films containing up to 300 active layers (600 layers in total with a total thickness of 1.5 μm) had the intensity of the SH signal produced from incident light of wavelength 1.064 μm proportional to the square of the number of active layers. The intensity of the SH signal was unchanged after the film had been left in the dark at 20 °C for 8 months. Over a period of 4 days the intensity of the SH signal was stable at 60 °C, but unstable at 80 °C.

AFM and X-ray studies of films formed by thermal evaporation and by the LB technique

Abstract The object of this research was to compare multilayers formed by thermal evaporation and the LB technique and to investigate their structure by atomic force microscopy (AFM) and X-ray diffraction. Films of a wide range of amphiphilic materials were formed by thermal evaporation in vacuo onto substrates maintained at temperatures chosen to maximise order as evidenced by low angle X-ray diffraction. Where it was possible multilayers of these material were also formed by the Langmuir-Blodgett (LB) technique. The X-ray diffraction patterns obtained from films formed in the two different ways are very similar. Studies of these films made by AFM show that, in most cases, there is substantial surface structure and terracing in the case of films formed by both techniques. We have made particular studies of films of cadmium stearate. Multilayers of this material made by thermal evaporation appear to consist of platelets having a Y-structure whose planes are parallel to the substrate. LB films of this material show terracing but also flat areas. Even these areas contain circular defects less than a micron in diameter which tend to multiply and grow with time. The best thermally evaporated films we obtained were formed from N -methylstearamide. These films had relatively smooth surfaces showing only small steps and had a good Y-layer structure as evidenced by X-ray diffraction.

Solvent cast films derived from amphiphilic phthalocyanines: an alternative to the Langmuir–Blodgett technique for the preparation of ordered multilayer films

Solvent cast films of novel amphiphilic phthalocyanine derivatives are described which possess highly ordered multilayer structures analogous to Langmuir–Blodgett films.

Examples of amphitropic polymers: monolayer film, Langmuir–Blodgett film and liquid-crystalline properties of some polymeric amphiphiles containing cholestanol moieties and those of some closely related non-polymeric amphiphiles

A range of alternating copolymers were prepared by free-radical-initiated copolymerizations of maleic anhydride with a series of α-alkenes containing cholestanyl moieties. Derivatives of these copolymers were prepared by reacting the anhydride residues with methanol, water, dimethylamine and/or morpholine. A related series of non-polymeric amphiphiles containing steroid moieties was also prepared. Isotherms were measured for monolayers of the various polymers and the various non-polymeric amphiphiles on water and, where possible, Langmuir–Blodgett (LB) multilayers were prepared. The majority of the materials gave good isotherms (relatively steep with collapse pressures >40 nM m–1) indicating that the monolayers were ordered and, as determined by the detection of Bragg peaks by X-ray diffraction, Y-type LB films with regular layer structures. Appropriate materials were also examined, by optical microscopy and differential scanning calorimetry (DSC), for possible liquid-crystalline properties. Four polymers and one non-polymeric amphiphile exhibited smectic A mesophases. Another non-polymeric amphiphile exhibited a cholesteric mesophase. Thus, examples were found of amphitropic polymers and non-polymeric amphiphiles which can form organised molecular arrangements both because they are amphiphilic and because they contain mesogens.

Preparation and crystallinity of a large unsubstituted crown ether, cyclic heptacosa(oxyethylene)(cyclo-E27, 81-crown-27), studied by Raman spectroscopy, X-ray scattering and differential scanning calorimetry

Cyclic heptacosa(oxyethyelene)(cyclo-E27, 81-crown-27) was prepared from linear α-hydro, ω-hydroxy-heptacosa(oxyethylene) by reaction with tosyl chloride under alkaline conditions, purified by preparative-scale gel permeation chromatography, and studied by laser-Raman spectroscopy, wide-angle and small-angle X-ray scattering, and differential scanning calorimetry. Comparison was made with the properties of linear oligo(oxyethylene) dimethyl ethers (including C1E27C1). The sub-cell of the crystalline cyclic oligomer was the same as that of its linear counterparts, i.e. the same as that of high-molar-mass poly(oxyethylene). However, the cyclic oligomer crystallised as a twice-folded ring, as confirmed by its long spacing and the frequency of its single-node longitudinal acoustic mode (LAM-1). Enthalpies of fusion and melting temperatures were analysed to provide estimates of the enthalpy and entropy of formation of a fold in an oligo(oxyethylene) layer crystal.

Amylose triacetate Langmuir-Blodgett films: Y, Z and alternated with other polymers

Abstract Amylose triacetate was synthesized and gel permeation chromatography used to produce three fractions having different mean molecular weights. The fraction having the largest molecular weight was used to form Langmuir-Blodgett multilayers on its own and also when alternated with two different synthetic polymers. The films were characterized using X-ray diffraction, scanning electron microscopy and, in one case, second harmonic generation. Films formed from amylose triacetate alone gave no Bragg peaks. Study of alternating layers showed that amylose triacetate monolayers were about 1 nm thick as would be expected if this material takes on a helical structure.

Surface potentials of Langmuir-Blodgett alternating layer structures

Abstract The surface potential of a Langmuir-Blodgett film formed from two distinct alternating materials has been measured and values of surface potential vs. the number of bilayers have been plotted. Two distinct components are observed. One varies linearly with the number of layers deposited and is of different sign depending whether the deposition is ABAB… or BABA… The other component is similar to that previously observed by us for Y-layer materials and arises from charge distributed in the material in a manner similar to that in a Schottky barrier. We have studied alternating layers formed from stearic acid vs. stearylamine.

Properties of polymeric Langmuir–Blodgett films containing sulphonyl-substituted azobenzene moieties for second harmonic generation

Abstract Polymeric Langmuir–Blodgett films incorporating a sulphonyl-substituted azobenzene chromophore have been studied by second harmonic generation. These films are more stable to light and more transparent than alternatives based on hemicyanine and other azo dyes that have been used previously. The initial 100 nm of polymeric LB film, nearest the substrate, exhibits a nonlinear susceptibility of χpp(2)=6.3 pm V−1 and a chromophore nonlinear vector susceptibility of magnitude β=46×10−50 C2 m−3 V−2. A loss of performance is observed for molecular layers a distance greater than 100 nm from the substrate, which is reflected in a reduced but uniform nonlinearity in the layers deposited later in the sample fabrication process.