Langmuir-Blodgett-Kuhn Research Articles

Ultrathin Conjugated Polymer Network Films of Carbazole Functionalized Poly(p-Phenylenes) via Electropolymerization

Ultrathin films of a cross-linked and chemically distinct conjugated poly(p-phenylene) network via electropolymerization are described. The amphiphilic network precursor was synthesized by incorporating the alkoxy carbazole group (-O(CH2)5Cb) to a poly(p-phenylene) (C6PPPOH) backbone. In order to investigate the combined thin film electrochemical and photophysical properties of poly(p-phenylene)s and polycarbazole conjugated polymers, C6PPPC5Cb was deposited on substrates using the Langmuir Blodgett Kuhn (LBK) method. The monolayer isotherm of the polymer, C6PPPC5Cb, showed a liquid expanded region slightly different from the parent polymer C6PPPOH. Multilayers (up to 30 layers) were transferred to different substrates such as quartz, gold coated LaSFN9 and ITO substrates for analysis. For conversion to a conjugated polymer network (CPN) film, the electroactive carbazole group was electropolymerized using cyclic voltammetry (CV) resulting in polycarbazole linking units. The differences in the film properties and corresponding changes in the electrochemical behavior indicate the importance of film thickness and electron/ion transport process in cross linked network films. From the electrochemical studies, the scan rate was found to have a considerable effect on electropolymerization with higher oxidation and reduction peak values found for the rigid network polymer compared to the uncrosslinked polymers.

Fabrication and Characterization of Multilayer Films from Amphiphilic Poly(p-phenylene)s

Over the past two decades, considerable efforts have been devoted to the development of conjugated polymeric materials for electronic applications due to the tunability of their properties through variation of their chemical structure. The LB technique is one of the most effective and precise methods for controlling the organization and thereby the properties of polymer films at the nanoscale for device fabrication. A detailed study was performed on the Langmuir-Schaefer (LS) monolayer and Langmuir-Blodgett-Kuhn (LBK) multilayer formation of newly designed conjugated poly(p-phenylene)s (C(n)PPPOH), incorporated with alkoxy groups with different chain lengths (C(6)H(13)O-, C(12)H(25)O-, and C(18)H(37)O-) and hydroxyl groups on the polymer backbone. The monolayer formed at the air-water interface was characterized using surface pressure-area isotherms, including hysteresis measurements. The films were then transferred to different hydrophilic solid substrates and analyzed using surface plasmon resonance spectroscopy, UV-vis spectroscopy, fluorescence spectroscopy, and AFM measurements. The results showed that the polymer with a short alkoxy chain (C(6)PPPOH) forms uniform monolayers at the air-water interface and can be transferred as multilayer films compared to C(12)PPPOH and C(18)PPPOH. The observed film thicknesses measured by SPR and AFM studies were similar to the theoretical value obtained in the case of C(6)PPPOH, whereas this was not the case with the other two polymers. The present study shows that the polymer C(n)PPPOH with short alkoxy chain can be transferred onto different solid substrates for device fabrication with molecular level control.

Langmuir−Blodgett−Kuhn and Self-Assembled Films of Asymmetrically Substituted Poly(paraphenylene)

Asymmetrically substituted poly(paraphenylene) (PhPPP) with hydrophilic and hydrophobic side chains was investigated. The polymer behavior at the air-water interface was studied on the basis of surface pressure-area (pi-A) isotherms and compression/expansion hysteresis measurements. PhPPP can form stable monolayers with an area per repeat unit of A=0.20+/-0.02 nm2 and a collapse pressure in the range of pi=25 mN/m. Then, Langmuir-Blodgett-Kuhn (LBK) films of PhPPP were prepared by horizontally and vertically transferring the Langmuir monolayers onto hydrophilic solid substrates at pi=12 mN/m. Cross-section analysis of the AFM tapping-mode topography images of a single transferred monolayer reveals a thickness of d0=0.9+/-0.1 nm. Taking into account the obtained monolayer thickness, curve-fitting calculations of angular scan data of LB monolayers measured using surface plasmon resonance (SPR) spectroscopy lead to a value for the refractive index of n=1.78+/-0.02 at lambda=632.8 nm. Next, the spontaneous formation of a PhPPP monolayer by adsorption from solution was studied ex situ by atomic force microscopy and UV-vis spectroscopy and in situ by using SPR spectroscopy. Stable self-assembled monolayers of PhPPP can be formed on hydrophilic surfaces with a thickness similar to that of the monolayer obtained using the LB method. The characterization results confirmed the amphiphilic character and the self-assembly properties of PhPPP, as well as the possibility of preparing homogeneous monolayer and multilayer films.

Azobenzene-Containing Polyamic Acid with Excellent Langmuir−Blodgett−Kuhn Film Formation Behavior Suitable for All-Optical Switching

To generate command surfaces for all-optical switching, highly ordered polymeric Langmuir-Blodgett-Kuhn (LBK) multilayers were fabricated onto silicon substrates and gold-coated optical glass slides from novel azobenzene-bearing polyamic acid systems. Pronounced Bragg peaks and well-defined Kiessig fringes observed in the X-ray reflectivity measurement for samples on silicon substrates indicate that these films possess a regularly repeated Y-type LBK multilayer structure and ultrasmooth surfaces. Fourier transform infrared (FT-IR) spectra taken by grazing incidence reflection suggest specific orientations of the functional groups in the layers. The excellent film-forming properties of the polyamic acid allow for a smooth buildup of several hundreds of layers of the LBK films onto gold-coated glass slides, which in turn allows for determining the geometrical thickness and the anisotropic refractive indices of the films by using optical waveguide spectroscopy. Interestingly, the probe laser beam induced a distinct fluorescence signal from the films, which remained even after the film underwent a thermal imidization process at 160 degrees C for 8 h in vacuo. LBK films fabricated from these compounds can be successfully applied for all-optically switching the alignment of liquid crystals by irradiation with light of different wavelengths.

Novel azobenzene-containing polyamic acids as Langmuir–Blodgett–Kuhn multilayer films and for liquid crystal alignment switching

New azobenzene-containing polyamic acids (PAAs) with systematically tailored architectures were synthesized and assembled as Langmuir–Blodgett–Kuhn (LBK) multilayers. Surface plasmon resonance (SPR) spectroscopy was employed to measure the film thickness and its dependence upon the number of layers deposited. The pronounced Bragg peak in the X-ray reflectivity scans indicated the films' regularly repeated internal layer structure. Photoisomerization studies confirmed the excellent reversibility of the trans– cis and cis– trans isomerization reactions of the azobenzene sidegroups within the multilayer structures. Further, these films were found to be able to switch the orientation of liquid crystalline layers between homeotropic and planar alignment by the reversible photoisomerization of the azobenzene moieties.

Spontaneous desorption time of flight mass spectrometry for the analysis of chemical reactions in thin solid films of a few monolayers and up to the micrometer regime

We demonstrate spontaneous desorption time of flight mass spectrometry investigations on chemical reactions within thin solid films with a thickness of a few monolayers deposited by the Langmuir–Blodgett–Kuhn (LBK) technique and in the μm regime by spincoating. The photo induced decomposition of the polymeric system of a dye, disperse red 1, covalently bound to polymethylmethacrylate is one of the systems under investigation. Films of a thickness, which are typical for channel waveguides, were illuminated with UV and annealed subsequently. The decomposition of the dye after the illumination could be proven by the decrease of the mass peaks of intact dye and the increase of dye fragments. The annealing process leads to an evaporation of the loosened dye, its fragments and fragments of the polymer backbone. The results are compared with in situ bleaching experiments observed by time dependent photo thermal beam deflection. The second system consists of a poly[( tert-butylacrylate)-co-CN(3-methacroylpropyl)-3,4-dimethylmaleicacidanhydride)] multilayer LBK sample that is cross-linked via UV irradiation and saponified by wet HCl treatment to achieve a thin film poly(methacrylic acid) network. The time development of the saponification is observed by the mass spectrometry and Fourier Transform Infrared Spectroscopy.

Influence of the Polymethacrylate Chain Conformation in Langmuir–Blodgett–Kuhn Films on the Glass-Transition Temperature

Langmuir–Blodgett–Kuhn (LBK) multilayer assemblies of poly(ethyl methacrylate) (PEMA) were analyzed by X-ray- (XR) and neutron reflectometry (NR). The preparation of alternating stacks of perdeuterated and protonated layers allowed for the determination of the inner layer structure via NR. The depression of the glass-transition temperature Tg of these thin films was examined by surface plasmon spectroscopy (SPS) and waveguide spectroscopy (WGS). The depression of a relaxation temperature of the chains in LBK film was measured and related to the Tg depression of spincoat systems. The Tg depression appeared to be independent of the conformation of the polymer chains.

Quadrupolar and polar anisotropy in end-grafted α-helical poly(γ-benzyl-L-glutamate) on solid substrates

Using grazing incidence reflectance Fourier transform infrared spectroscopy (GIR-FTIR) and electro-optic (EO) measurements, we have determined the degree of quadrupolar and polar anisotropy in end-grafted α-helical poly(γ-benzyl-L-glutamate) (PBLG) chains. The results are compared to data obtained on spin-cast and on Langmuir–Blodgett–Kuhn (LBK) films. End-grafted films were prepared by a vapor-deposition-polymerization (VDP) scheme and have thicknesses of up to 70 nm. The quadrupolar order of VDP films, as estimated by the nematic order parameter S, is higher than in spin-cast and LBK films. This result indicates a preferentially perpendicular alignment of PBLG chains in VDP films. Furthermore, after the removal of the physisorbed chains from the grafted films by intensive washing with solvent, the quadrupolar order is lowered while the polar order increases significantly, suggesting that the physisorbed chains might form anti-parallel pairs with the surface-grafted chains.

Polymer-Dispersed Liquid Crystal Monolayers

The first examples of two-dimensional polymer-dispersed liquid crystals (PDLCs) have been prepared via the Langmuir−Blodgett technique and analyzed using tapping mode atomic force microscopy (TMAFM). Characterization of the thermodynamics, polymerization kinetics, and topology of mixtures of 2-pentadecylaniline (2PDA) and a ferroelectric liquid crystal termed 10PPB2 was investigated. Systematic pressure versus area isotherms indicate that the mixed monomer/liquid crystal exists as a phase-separated monolayer after the evaporation of the spreading solvent, typical of a solution-induced phase separation (SIPS). Interfacial polymerization of the monomer in the presence of the liquid crystal has been accomplished. Langmuir−Blodgett−Kuhn (LBK) films obtained by transferring the polymer/liquid crystal system on freshly cleaved mica surfaces revealed a polymer-dispersed liquid crystalline system with circular liquid crystalline domains trapped within a poly(alkylaniline) matrix. Statistical characterization of domain parameters (i.e. domain density, mean diameter, and polydispersity) was performed, and the effect of monolayer and polymerization conditions on these parameters was investigated. From these studies it was found that domain morphology could be controlled by the manipulation of monolayer spreading and compression rates. The high anisotropy and controlled symmetry of these monolayer liquid crystal domains make them attractive models of bulk PDLCs.

Supramolecular Assemblies for Second Order Nonlinear Optics Stabilized by Electrostatic Interaction

Electrostatic interactions are being used instead of covalent linkage to stabilize orientational order in novel materials for second order nonlinear optics. In complex supramolecular architectures, this simplifies the variation of chromophores or polymeric backbones. Other molecules can readily be inserted into the system in order to tune optical or thermodynamic properties without synthetic effort. Noncentrosymmetric ultrathin films consisting of highly ordered "active-passive" AB-Y-type multilayers of NLO active ionic amphiphiles and an ionic polymeric buffer material have been obtained by the Langmuir-Blodgett-Kuhn (LBK) technique. Combination with self-assembly by adsorption from solution is demonstrated.

High glass transition temperature syndioregic second-order non-linear optical polymer in Langmuir–Blodgett–Kuhn films

We report here recent progress toward the goal of processing polymers near room temperature to produce all-polymeric non-linear optical films which may ultimately find application as active waveguides. Polymers designed for spin-casting were found to be processable by the Langmuir–Blodgett–Kuhn (LBK) technique. The T g of the bulk polymer is about 240°C. The temporal stability of non-centrosymmetric order in LBK films at elevated temperatures was followed by measurement of the second harmonic generated (SHG) light intensity. The SHG signal was found to be constant in LBK films heated at 95°C for 10 h.

Supramolecular thin films via the Langmuir-Blodgett-Kuhn (LBK) technique

Supramolecular organic thin films can be made by combining the Langmuir-Blodgett-Kuhn (LBK) technique and a host of self-assembly processes. Studies have been made on the formation of ‘passive-active’ AB, Y-type multilayers with a class of polyelectrolytes called ionenes and ionic NLO active amphiphiles. These films were investigated using several ultrathin film sensitive techniques to determine multilayer structure and ordering. Foremost of these techniques is surface plasmon spectroscopy (SPS), which is highly sensitive to thickness and refractive index changes of an adsorbed dielectric film. Unique layered architectures were only possible via deposition using an alternate three-compartment dipping trough. Two main factors were determined necessary for film integrity: electrostatic interaction in the hydrophilic layers and stability of the hydrophobic barrier layer. Combined with polymer self-assembly solution adsorption, film architectures from AXB to AXYZB types were realized. At the same time, the uniqueness of the approach allowed the preparation of systems suitable for in situ polymer solution adsorption investigations.

Accordion polymers for nonlinear optical applications

The design and synthesis of mainchain chromophore polymers in a syndioregic (head-to-head) configuration is described. Polymers were designed for both electric-field poling and Langmuir-Blodgett-Kuhn (LBK) deposition. LBK deposition gives intrinsically polar films (no E-field poling) as evidenced by nonlinear optical (χ (2)) properties.

Organized photochromic azo-polymeric structures: self-assembled and Langmuir—Blodgett—Kuhn layers

Self-assembled monolayers (SAMs) of azo-silane molecules exhibit persistent dichroism upon linearly polarized light irradiation. This dichroism could be both written and erased by irradiation with light of an appropriate wavelength. In addition, the sign of this dichroism can be inverted by choosing the appropriate polarization of the irradiating light, thus showing a ‘smart communication’ between the light polarization and the ultra-thin photochromic layers. Langmuir—Blodgett—Kuhn (LBK) multilayer assemblies of ‘hairy rod’ polyglutamates with stiff main chains and flexible side chains containing photochromic azo units exhibit a highly optically anisotropic structure when the azo molecules are in the trans form, and a nearly optically isotropic structure when the azo molecules are in the cis form. The trans ⇔ cis photoisomerization of the azo molecules switches them between a highly oriented trans configuration and a bent cis configuration, thus turning the anisotropy ‘on’ and ‘off’. In contrast to spun-cast polymer films containing azobenzene units, photoselection under polarized light irradiation does not occur in these LBK structures.

In Situ Investigations of Polymer Self-Assembly Solution Adsorption by Surface Plasmon Spectroscopy

In this study, we report the use of surface plasmon spectroscopy (SPS) to investigate the self-assembly polymer solution adsorption process in situ. A sealed sample cell made of Teflon was assembled with the substrate initially coated with a positively charged surface using the Langmuir-Blodgett-Kuhn (LBK) deposition technique. The cationic and anionic polymer combinations were comprised mainly of a class of polyelectrolytes called ionenes and several anionic polymers. The solution adsorption process generally showed linear behavior except were the deviation is attributed to a poorly charged initial layer. The adsorption behavior is postulated to be due to conformational requirements and charge distribution of the individually adsorbed polymers. The time of adsorption for each polymer pair was also investigated. The technique promises to be an important tool in determining the mechanism for these types of adsorption processes.

Langmuir-Blodgett-Kuhn Multilayers of Polyglutamates with Azobenzene Moieties: Investigations of Photoinduced Changes in the Optical Properties and Structure of the Films

Langmuir-Blodgett-Kuhn (LBK) multilayer assemblies of hairy-rod polyglutamates with stiff main chains and flexible side chains containing photochromic azo units exhibit a highly optically anisotropic structure when the azo molecules are in the trans form and a nearly optically isotropic structure when the azo molecules are in the cis form. The trans ↔ cis photoisomerization of the azo molecules switches them between a highly oriented trans configuration and a bend cis configuration, thus turning the anisotropy on and off' ; the corresponding optical order parameter changes by about 1 order of magnitude. In contrast to spun-cast polymer films containing azobenzene units, photoselection under polarized light irradiation does not occur in these LBK structures. The study of the electronic density distribution of these films shows that a remarkable variation in the LBK structures' stability comes with a seemingly small change in the side-chain structure. A stable LBK double-layer structure has been prepared, and its physical origin has been discussed.

Time-of-Flight Secondary Ion Mass Spectrometric Measurements of Molecular Weight Distributions for Functionally Terminated Oligomers and Transferred Langmuir-Blodgett-Kuhn Monolayers

We report the first direct measurements of the molecular weight distribution in transferred condensed monolayers of amphiphilic oligomers determined by time-of-flight secondary ion mass spectrometry (TOFSIMS). Two α,ω-functional oligomers of poly(dimethylsiloxane) are investigated, comprising pentylamine and propylcarboxy end groups. Measurements are performed directly on condensed monolayer films transferred to silver-coated substrates using the Langmuir-Blodgett-Kuhn (LBK) technique. These results are compared to the original distributions measured on submonolayer thin films of the original oligomers prepared by spin coating onto silver-coated substrates and to molecular weight determinations provided by end group titration and size exclusion chromatography analyses. Different families of ions are found for the two different thin film preparation methods. A number of tentative assignments are proposed for these masses, based upon consideration of the effects of the LBK film deposition process and the influence of interactions between the end groups and the substrate. The molecular weight distributions for LBK films of both oligomers is found to be narrower and shifted to higher molecular weights than are those for the corresponding spin-coated films. The changes in molecular weight distribution observed are attributed to dissolution of lower molecular weight species into the aqueous subphase during the LBK film deposition process.

Photoreactive Langmuir–Blodgett–Kuhn multilayer assemblies from functionalized liquid crystalline side chain polymers. II. Structural characterization of homo‐ and copolymers containing azobenzene and phenylbenzoate moieties

AbstractLiquid crystalline side chain polymers containing photoreactive azobenzene chromophores in the dark‐adapted trans form and phenylbenzoate groups as co‐mesogens have been prepared and investigated as monomolecular layers at the water/air interface and as multilayer assemblies deposited onto solid supports by the Langmuir–Blodgett–Kuhn (LBK) technique. Pressure area isotherms at variable temperatures together with reflection absorption spectra and Brewster angle microscopic pictures have been taken and analyzed (i) to derive information about the structural order and the packing properties of the mesogenic side groups within the monolayers of different homo‐ and co‐polymeric systems and ii) to optimize the conditions for the transfer of the monolayers.The resulting multilayer assemblies were primarily characterized by X‐ray reflectometry studies, which yield the film thickness, the electron density modulations indicative of any internal (double) layer periodicity, and two roughness parameters characterizing the substrate/film and the film/air interfaces, respectively. These investigations were complemented by surface plasmon, UV‐vis and IR spectroscopic measurements which provided further information as to the optical/structural properties of the samples.It was found that those systems containing only azobenzene side groups (or a high percentage of them) are composed of metastable double layer subunits which can be easily assembled to thicker films even capable of carrying waveguide modes. The azobenzene chromophores forming various types of aggregates have an average tilt angle of 36° relative to the film normal presumably with a broad angular distribution.If the azobenzene mesogens are diluted by phenylbenzoate co‐mesogens in the radio 1:2, only the first few dipping cycles show a good monolayer transfer so that only thin multilayer assemblies can be built up. Moreover, all these preparations though deposited in the Y‐type mode show no indication of a doublelayer structure in the transferred film, but instead exhibit the so‐called mesogen peak with a smectic periodicity corresponding to only a single monolayer. A structural model with interdigitating side groups is proposed to account for these experimental results.

Langmuir-Blodgett-Kuhn multilayer assemblies of liquid-crystalline azo-dye side chain homo- and copolymers

We report on the characterization of Langmuir-Blodgett-Kuhn (LBK) multilayer assemblies of novel liquid-crystalline (LC) side chain polymers containing azo dyes. The structure and optical properties of these materials were determined. Persistent and reversible optical data storage and possible molecular mechanism are discussed in this paper.

Photoreactive Langmuir–Blodgett–Kuhn multilayer assemblies from functionalized liquid‐crystalline side chain polymers, I. Homopolymers containing azobenzene chromophores

AbstractStable monomolecular layers of a liquid‐crystalline homopolymer bearing azobenzene chromophores in the mesogenic side groups could be prepared at the water–air interface by slow compression at room temperature. From the area per monomer unit of A0≈0.25 nm2 a structural model with tightly packed azobenzene groups oriented nearly parallel to the surface normal was deduced. After transfer to a solid support in a Y‐type mode the obtained Langmuir–Blodgett–Kuhn (LBK) multilayer assemblies of varying thicknesses (from a double layer to a thick film of d = 238 nm, capable of guiding optical modes) were prepared and analyzed by X‐ray reflection and optical evanescent wave techniques. A monolayer thickness of d0 = 2.52 nm, as well as the anisotropic index of refraction of these layers, could thus be evaluated.Photo‐isomerization by UV‐irradiation (at λ = (360±30)nm) was examined by UV‐VIS absorption spectroscopy. X‐ray reflectivity data of samples illuminated for 30 min showed a complete loss of the layered structure of the LBK samples. As a result strongly light‐scattering films were obtained. Surface plasmon microscopy was used to demonstrate the usefulness of this photo‐induced order–disorder transition for optical information storage.