Comparative study of self‐assembly of a range of monofunctional aliphatic molecules on magnetron‐sputtered aluminium

The adsorption of a range of organic molecules from toluene onto the oxidized surface of magnetron‐sputtered aluminium metal is studied using sessile drop water contact angle measurements. Molecules with different head group functionalities and various chain lengths are considered, including alkyl carboxylic acids, alkyl phosphonic acids, alkyl amines, alkyl trimethoxysilanes, alkyl trichlorosilanes and epoxy alkanes. Alkyl phosphonic and carboxylic acids are identified as readily forming the most well‐packed monolayers on the aluminium surface, whereas the others adsorb less well and the chlorosilanes polymerize as a result of combination with moisture to form a thick deposit. The high‐adsorption‐density monolayers of alkyl phosphonic and carboxylic acids were studied using polarization modulation infrared reflection–absorption spectroscopy (PM‐IRRAS) and x‐ray photoelectron spectroscopy (XPS): PM‐IRRAS reveals relatively poorer ordering of the C₁₀ alkyl carboxylic acid monolayer compared with that formed from the phosphonic acid, and XPS data suggest that this is likely to relate to a lower ability to displace preadsorbed volatile organic compounds. Copyright © 2004 John Wiley & Sons, Ltd.     

The effect of polymerizable surfactants on morphology and electro-optical of PDLCs films based on epoxy acrylate system

Polymer dispersed liquid crystals (PDLCs) have been extensively studied for various excellent electro-optical applications. The anchoring interaction of liquid crystals (LCs) molecules on the surface of the polymer cavity surrounding an LCs droplet has a crucial effect on the electro-optical performance of the PDLCs. The effect of polymerizable surfactants on the electro-optical properties of PDLCs films was studied in detail. The active double bonds were polymerized with prepolymer to stabilize the performance of polymer matrix. The experimental results showed that polymerizable surfactants could effectively reduce the driving voltage. The speed of polymerization was monitored by real-time transmittance. The electro-optical properties of PDLC films were measured by Polarimeter (PerkinElmer Model 341). The driving electric field was reduced from 3.9 V/μm to about 2.8 V/μm for doping undec-10-enoic acid at curing temperature 80?°C. The surfactants containing polymerizable functional groups, polarity, and alkyl chain weakened the surface anchoring between LCs droplets and polymer interface. The morphologies of PDLCs films were also investigated by polarizing optical microscopy (POM) and Fourier transform infrared (FTIR) images. The LC droplets were encapsulated by polymerizable surfactant according to FTIR images.     

Bifunctional Adhesion Promoter for Grafting Polypyrrole Films on Metal/Metal Oxide Surfaces

A new class of pyrrole derivatives, ω-[(3-phenyl) pyrrol-1-yl] alkyl phosphonic acids with long chains of 10 and 12 carbon atoms were synthesised to graft polypyrrole layers on metal/metal oxide surfaces. These compounds are bifunctional containing two reactive moieties, pyrrole as the polymerisable group and phosphonic acid as the anchoring group. Contact angle measurements and X-ray photoelectron spectroscopy (XPS) confirmed adsorption with phosphonic acid group attached to the surface. Surface plasmon resonance (SPR) spectroscopy indicated that adsorption starts in seconds and is completed in few hours. Adsorption is followed by surface induced polymerisation with further monomer. We obtained dense and homogeneous polypyrrole films, which were characterised for their morphology and thickness by atomic force microscopy (AFM). The derivatives form a strongly bonded composite of metal with polymer.     

Preparation and characterization of humidity sensor based on poly(3-alkyl)pyrrole containing phosphonic acid groups

Poly(3-alkyl)pyrroles containing phosphonic acid groups with different alkyl segment lengths were chemical synthesized and the properties were measured by FTIR and UV-Vis spectroscopy. FTIR and UV-Vis results showed that the synthesized polymers were in a low doping level through chemical polymerization. By spin-coating on the surface of substrates, the polymer can be used as a humidity sensor. The change of DC electric current of the polypyrroles varies with the chain length of the alkyl substituents. The capacitance increases with the increase of the humidity and resistance decreases with the increase of humidity. This result is different from that of polypyrrole without alkyl substituents due to the influence of the phosphonic acid group. The sensor showed the resistive-type at high relative humidity, and the capacitivetype at the low relative humidity. The sensor exhibited very fast response to the change of environment humidity.     

Intermediate temperature proton conductors for PEM fuel cells based on phosphonic acid as protogenic group: a progress report

The melting behaviour and transport properties of straight chain alkanes mono- and difunctionalized with phosphonic acid groups have been investigated as a function of their length. The increase of melting temperature and decrease of proton conductivity with increasing chain length is suggested to be the consequence of an increasing ordering of the alkane segments which constrains the free aggregation of the phosphonic acid groups. However, the proton mobility is reduced to a greater extent than the proton diffusion coefficient indicating an increasing cooperativity of proton transport with increasing length of the alkane segment. The results clearly indicate that the "spacer concept", which had been proven successful in the optimization of the proton conductivity of heterocycle based systems, fails in the case of phosphonic acid functionalized polymers. Instead, a very high concentration of phosphonic acid functional groups forming "bulky" hydrogen bonded aggregates is suggested to be essential for obtaining very high proton conductivity. Aggregation is also suggested to reduce condensation reactions generally observed in phosphonic acid containing systems. On the basis of this understanding, the proton conductivities of poly(vinyl phosphonic acid) and poly(meta-phenylene phosphonic acid) are discussed. Though both polymers exhibit a substantial concentration of phosphonic acid groups, aggregation seems to be constrained to such an extent that intrinsic proton conductivity is limited to values below sigma = 10(-3) S cm(-1) at T = 150 degrees C. The results suggest that different immobilization concepts have to be developed in order to minimize the conductivity reduction compared to the very high intrinsic proton conductivity of neat phosphonic acid under quasi dry conditions. In the presence of high water activities, however, (as usually present in PEM fuel cells) the very high ion exchange capacities (IEC) possible for phosphonic acid functionalized ionomers (IEC >10 meq g(-1)) may allow for high proton conductivities in the intermediate temperature range (T approximately 120 -160 degrees C).     

Nanoparticle assemblies as probes for self-assembled monolayer characterization: correlation between surface functionalization and agglomeration behavior

The ordering of dodecyl chains has been investigated in mixed monolayers of phosphonic acid capping agents on the surface of hydrothermally prepared zirconia nanocrystals. Methyl-, phenyl-, pyryl-, and tert-butylphosphonic acids have been used to investigate series with different mixing ratios with dodecylphosphonic acid as the cocapping agent for the mixed monolayer formation. Fourier transform infrared (FTIR) studies revealed that an increasing amount (different for each type) of coadsorbed capping agent reduces the ordering of the dodecyl chains significantly. Small-angle X-ray scattering (SAXS) verified that with increasing amount of cocapping agent the agglomeration of the particles decreases. The strong correlation of the agglomeration behavior with the ordering of the surface-bound alkyl chains leads to the conclusion that interparticle bilayers, formed via long alkyl chain packing, are responsible and can be controlled on a molecular level by coadsorbing various molecules. On the basis of this correlation, nanoparticles can be used as probes for self-assembled monolayer investigation by an indirect structural method (SAXS) and correlated with the routine spectroscopical method for the chemical analysis of surface groups (FTIR).     

Polar and azimuthal alignment of a nematic liquid crystal by alkylsilane self-assembled monolayers: effects of chain-length and mechanical rubbing

Alkylsilane self-assembled monolayers (SAMs) on oxide substrates are commonly used as liquid crystal (LC) alignment layers. We have studied the effects of alkyl chain length, photolytic degradation, and mechanical rubbing on polar and azimuthal LC anchoring. Both gradient surfaces (fabricated using photolytic degradation of C18 SAMs) and unirradiated SAMs composed of short alkyl chains show abrupt transitions from homeotropic to tilted alignment as a function of degradation or chain length. In both cases, the transition from homeotropic to tilted anchoring corresponds to increasing wettability of the SAM surfaces. However, there is an offset in the critical contact angle for the transition on gradient vs unirradiated SAMs, suggesting that layer thickness is more relevant than wettability for LC alignment. Mechanical rubbing can induce azimuthal alignment along the rubbing direction for alignment layers sufficiently near the homeotropic-to-planar transition. Notably, mechanical rubbing causes a small but significant shift in the homeotropic-to-tilted transition, e.g., unrubbed C5 SAMs induce homeotropic anchoring, but the same surface after rubbing induces LC pretilt.     

Synthesis of methacrylate-functionalized phosphonates and phosphates with long alkyl-chain spacers and their self-aggregation in aqueous solutions

Abstract  Polymerizable amphiphilic organophosphorous compounds were synthesized and their self-aggregation behavior was investigated. The studied molecules contain a hydrophilic phosphorus end group, an alkyl chain spacer with a variable length from 3 to 11 CH₂ groups and a polymerizable methacrylic group at the other chain end. Thus, the molecules represent a class of polymerizable surfactants. Two different reaction methods were used; either unsaturated alcohols or bromine-containing alcohols were applied as starting compounds for the preparation of the organophosphorous surfactants. The self-aggregation and micelle formation of the prepared compounds were investigated in aqueous solution by dynamic light scattering measurements. The critical micelle concentration of the P-containing amphiphiles was in all cases smaller than 0.040 mol/l and strongly dependent on the polarity of the phorphorous head group and the chain length of the spacer. Graphical abstract   The synthesis of organophosphorous amphiphiles as surface active monomers for the modification of metal oxide surfaces is presented. The spacer between the phosphorous head group and the methacrylate group was varied with regard to their length and composition. The self-aggregation behavior of these methacrylate-functionalized phosphates and phosphonates surfactants was investigated.     

Alkyl chains of surface ligands affect polytypism of cdse nanocrystals and play an important role in the synthesis of anisotropic nanoheterostructures

We show that the length of the alkyl chain of surface ligands can shift the equilibrium between the wurtzite and zinc blende polytypes of CdSe nanocrystals. In-situ wide-angle X-ray scattering measurements reveal that short-chain (e.g., propyl) phosphonic acids stabilize CdSe nanocrystals with the zinc blende phase whereas octadecylphosphonic acid stabilize nanocrystals with the wurtzite phase. We also demonstrate how this effect can be used to improve the shape selectivity in the synthesis of anisotropic CdSe/CdS and ZnSe/CdS nanoheterostructures.     

Mechanisms of thermal decomposition of organic monolayers grafted on (111) silicon

The thermal stability of different organic layers on silicon has been investigated by in situ infrared spectroscopy, using a specially designed variable-temperature cell. The monolayers were covalently grafted onto atomically flat (111) hydrogenated silicon surfaces through the (photochemical or catalytic) hydrosilylation of 1-decene, heptadecafluoro-1-decene or undecylenic acid. In contrast to alkyl monolayers, which desorb as alkene chains around 300 degrees C by the breaking of the Si-C bond through a beta-hydride elimination mechanism, the alkyl layers functionalized with a carboxylic acid terminal group undergo successive chemical transformations. At 200-250 degrees C, the carboxyl end groups couple forming anhydrides, which subsequently decompose at 250-300 degrees C by loss of the functional group. In the case of fluorinated alkyl chains, the C-C bond located between CH2 and CF2 units is first broken at 250-300 degrees C. In either case, the remaining alkyl layer is stable up to 350 degrees C, which is accounted for by a kinetic model involving chain pairing on the surface.     

About the choice of the protogenic group in polymer electrolyte membranes: Ab initio modelling of sulfonic acid, phosphonic acid, and imidazole functionalized alkanes

The possible use of sulfonic acid, phosphonic acid, or imidazole as the protogenic group in polymer electrolyte membranes for fuel cells operating at intermediate temperature (T>100 degrees C) and very low humidity conditions is examined by comparing specific molecular properties obtained with first principles based electronic structure calculations. Potential energy profiles determined at the B3LYP/6-311G** level for rotation of imidazole, phosphonic acid and sulfonic acid functional groups on saturated heptyl chains revealed that the torsional barriers are 3.9, 10.0, and 15.9 kJ mol-1, respectively; indicating that the imidazole is clearly the most labile when tethered to an alkyl chain. Minimum energy conformations (B3LYP/6-311G**) of methyl dimers of each of the acids indicated that the binding of the pairs of the acids is greatest in the phosphonic acids and lowest for the imidazoles. Comparison of the ZPE corrected total energies of the methyl acid dimers with corresponding pairs consisting of the conjugate acid and conjugate base revealed that the energy penalty in transferring the proton (from acid to acid) was greatest for imidazole (120.1 kJ mol-1) and least for the phosphonic acid (37.2 kJ mol-1). This result is in agreement with experimentally measured proton conductivities of acid-functionalized heptyl compounds under dry conditions and further underpins the observation that phosphonic acid possesses the best amphoteric character critical in achieving proton conductivity when no solvent (i.e. water) is present. Finally, BSSE corrected binding energies were computed for the methyl acids with a single water molecule and indicated that while the magnitude of the interaction of the sulfonic and phosphonic acids with water are similar (47.3 and 44.4 kJ mol-1, respectively), the binding is much weaker to the imidazole (28.8 kJ mol-1). This result suggests that the oxo-acids will probably retain water better under very low humidity conditions and that the dynamics of the hydrogen bonding of the first hydration water molecules will be more constrained with -SO3H and -PO3H2 than imidazole.     

Phosphorus-containing oligoamides obtained by a novel one-pot degradation of polyamide-6

This paper describes a novel strategy for the recycling of polyamide materials and their transformation into functional reactive oligomers with new properties. The method is illustrated by the heating of polyamide-6 (20,000 Da) in the presence of diesters of the phosphonic acid, (RO)₂P(O)H, where R could be -CH₃, -C₂H₅ or -C₆H₅. It is found that the reaction proceeds in several parallel processes: (i) phosphorylation of the amide group by the alkyl esters of the phosphonic acids and (ii) degradation of the main chain through an exchange reaction between the amide and phosphonic acid ester groups. Alternatively the depolymerization could be induced via a radical reaction with the participation of the polyamide moieties and the P-H group. The proceeding of the abovementioned reactions and the structure of the phosphorus-containing oligoamides are confirmed by ³¹P, ¹H and ¹³C NMR spectroscopy. Their molecular weights are determined by size exclusion chromatography.     

Investigation of surface anchoring strength and pretilt angle in NLC on rubbed polythiophene surfaces with alkyl chain lengths

A high pretilt angle for the nematic liquid crystal, 4-n-pentyl-4'-cyanobiphenyl (5CB), was observed on rubbed polythiophene (PTP) surfaces having alkyl chains with more than ten carbon atoms. We consider that this is due to a surface-excluded volume effect caused by the long alkyl chains between the LC molecules and the PTP surfaces. The polar anchoring strength in 5CB on rubbed PTP surfaces with long alkyl chains has also been successfully evaluated. The extrapolation length d_e of 5CB increases with increasing alkyl chain lengths above the seven carbons of alkyl chain R7; that is, it may be attributed to the high pretilt angle. An extrapolation length of 5 nm is observed in 5CB for the seven carbons alkyl chain R7 on the PTP surface; this indicates high anchoring strength.     

Alignment and ordering mechanisms at a liquid crystal–solid interface

Abstract

The role of surface coupling agents on the aligning and ordering mechanisms at a liquid crystal–solid interface are examined with deuterium nuclear magnetic resonance. The cylindrical channels of alumina membranes 0·2 μm in diameter are chemically modified using an aliphatic acid (C _n H_2n+1 COOH) as a surface coupling agent and filled with the liquid crystal compound 4′-pentyl-4-cyanobiphenyl deuteriated in the α position of the hydrocarbon chain (5CB-αd ₂). The preferred anchoring direction at the cavity wall and its strength are found to depend on the length of the aliphatic chain of the surface coupling agent which determine the nematic director field in the pores. The planar polar configuration with homeotropic anchoring conditions is stable for agents with n ≥7 while chain lengths n ≤6 support a uniform axial configuration with planar anchoring at the cavity wall. The pretransitional orientational ordering at the cavity wall above the clearing temperature is strongly reflected in the spectra. The radical changes in the quadrupole splitting as the length of the aliphatic chain of the surface coupling agent is varied indicates strong coupling between the 5CB molecules and the n = 15 surface, while shorter chain lengths reveal substantially reduced degrees of coupling.     

HPLC separation of positional isomers on a dodecylamine-N, N-dimethylenephosphonic acid modified zirconia stationary phase

The chromatographic performance of a new zirconia stationary phase (DPZ) modified with dodecylamine-N,N-dimethylenephosphonic acid (DDPA) is studied by using positional isomers as probes. The DDPA modified zirconia via one phosphonic group has a polar inner-layer and a non-polar outer-layer on its surface. The alkyl chain of outer-layer provides the hydrophobic interaction, while the polar inner-layer that consists of an amine group and a free phosphonic group provided dipolar and ion-exchange/columbic repellent interaction sites. The effects of methanol content, ionic strength and pH of mobile phase on capacity factors of the solutes are studied in detail, and baseline separations of toluidine, nitroaniline, aminophenol, dihydroxybenzene, and nitrophenol isomers were achieved on the new zirconia stationary phase. In addition, retention mechanism of the isomers on the DDPA-modified zirconia stationary phase is also proposed.     

Melting of saturated fatty acid zinc soaps

The melting of alkyl chains in the saturated fatty acid zinc soaps of different chain lengths, Zn(C(n)H(2n+1)COO)(2); n = 11, 13, 15, and 17, have been investigated by powder X-ray diffraction, differential scanning calorimetry, and vibrational spectroscopy. These compounds have a layer structure with the alkyl chains arranged as tilted bilayers and with all methylene chains adopting a planar, all-trans conformation at room temperature. The saturated fatty acid zinc soaps exhibit a single reversible melting transition with the associated enthalpy change varying linearly with alkyl chain length, but surprisingly, the melting temperature remaining constant. Melting is associated with changes in the conformation of the alkyl chains and in the nature of coordination of the fatty acid to zinc. By monitoring features in the infrared spectra that are characteristic of the global conformation of the alkyl chains, a quantitative relation between conformational disorder and melting is established. It is found that, irrespective of the alkyl chain length, melting occurs when 30% of the chains in the soap are disordered. These results highlight the universal nature of the melting of saturated fatty acid zinc soaps and provide a simple explanation for the observed phenomena.     

Tailored interfaces in nanocomposites

Different kinds of clays based on sodium montmorillonite have been modified i) by cationic exchange of alkylammonium ions and ii) by covalent grafting of organosilane. We have prepared organophilic clays with different gallery heights (due to alkylammonium ions with alkyl chain length varying from 8 to 18 carbon atoms) and with a specific functionalisation (due to the organosilane). We have generated organoclays with different kind of organic layer properties: ionically bonded or physically adsorbed alkylammonium ions and covalently grafted organosilane. These different organoclays have shown various specific behaviours when dispersed in monomers or in a reactive mixture, proving the influence of the surfactant on the nanocomposite final properties.     

Study of the formation of self-assembled monolayers on nitinol

Shape memory alloys such as nitinol (NiTi) have gained interest due to their unique and unusual properties of thermal shape memory, superelasticity, and good damping properties. Nitinol is mainly used for medical purposes. In order to control the surface properties of this alloy, self-assembled monolayers (SAMs) were formed and characterized on the native oxide surface of nitinol for the first time. Factors which affect the formation of SAMs, such as head group functionality, chain length, and tail group functionality, were varied and analyzed. Functionalized alkyl phosphonic acid molecules (OH, COOH, and CH3) formed monolayers on the nitinol surface using a simple deposition method resulting in the molecules being ordered and strongly bound to the surface. Diffuse reflectance infrared spectroscopy (DRIFT), contact angle goniometry, atomic force microscopy (AFM), and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) were used to characterize the surfaces before and after organic modification.     

Effect of chemical structure of interface modifier of TiO2 on photovoltaic properties of poly(3-hexylthiophene)/TiO2 layered solar cells

Two classes of phosphonic acid-bearing organic molecules, 2-oligothiophene phosphonic acid and omega-(2-thienyl)alkyl phosphonic acid were adopted as interface modifiers (IMs) of the TiO(2) surface, to increase its compatibility with poly(3-hexylthiophene) (P3HT). The self-assembled monolayers of these molecules on titania surface were characterized by making contact angle measurements and X-ray photoelectron spectroscopy (XPS). Atomic force microscopic (AFM) images revealed that the adsorption of IMs effectively smooths the TiO(2) surface. Both photoluminescence (PL) spectroscopy and PL lifetime measurements were made to investigate the photoinduced properties of the TiO(2)/IM/P3HT layered-junction. The PL quenching efficiency increased with the number of thiophene rings and as the alkyl chain-length in IMs decreased. Meanwhile, the decline in the PL lifetime followed a similar trend as the PL quenching efficiency. Additionally, the power conversion efficiency (PCE) of the ITO/TiO(2)/IM/P3HT/Au devices was examined by measuring their photocurrent density-applied voltage (J-V) curves. The experimental results indicated that the short-circuit current density (J(SC)) increased with the number of thiophene units and as the hydrocarbon chain-length in IMs decreased. However, the open-circuit voltage (V(OC)) of the devices slightly fell as the energy level of the highest occupied molecular orbital (HOMO) of IM decreased. The PCE of the device with 2-terthiophene phosphonic acid was 2.5 times that of the device with 10-(2-thienyl)decyl phosphonic acid.