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The processing temperature of thermotropic liquid crystalline polymers (LCPs) can be reduced by increasing the flexibility of the backbone, simultaneously decreasing the relaxation time and resulting in a decrease in the mechanical properties in the processed polymer. Here, we report on the development of thermotropic LCPs with low processing temperatures, an increased relaxation time, and excellent mechanical performance via a versatile two-step synthesis. These LCPs can be processed at temperatures below 200 degrees C into injection-molded parts which have a mechanical performance comparable to that of commercially available grades with Young's modulus and a tensile strength over 12.5 GPa and 200 MPa, respectively. These LCPs are synthesized using carboxylic acid-terminated thermotropic prepolymers that are produced via a standard acidolysis polycondensation at 220 degrees C, a temperature that allows incorporation of monomers having lower thermal stability. Subsequently, the prepolymers are subjected to a chain-extension reaction with bis(2-oxazoline)s in a reactive extrusion step. The facile two-step method provides excellent control over the chemical composition, molecular weight, and chain architecture of the resulting polymer. As the molecular weight of the LCPs increases, they display interesting and uncharacteristic viscoelastic behavior. The viscoelastic response of the LCPs changes from that of an unentangled melt, typically observed in rigid LCPs, toward a physically constrained melt indicated by the appearance of a crossover point with an increase in molar mass. Such a response is normally not observed in commercial thermotropic LCPs even with an increase in molar mass. We attribute the change in the viscoelastic response to the chain-extension reaction. This fundamental change in the state of the nematic melt greatly increases the relaxation timescales of both the interchain orientation and the nematic texture, providing further evidence toward their interrelation. The change in the relaxation mechanism of the nematic phase, with increasing molar mass, is followed by time-resolved X-ray diffraction and optical microscopy.

Thermoplastic composites based on thermotropic liquid crystalline polymer (LCP) materials are interesting candidates for reinforced composite application due to their promising mechanical performance and potential for recyclability. In combination with a societal push toward the more sustainable use of materials, these properties warrant new interest in this class of composites. Though numerous studies have been performed in the past, a coherent set of design rules for LCP design for the generation of injection-molded reinforced thermoplastic composites is not yet available, likely due to the complex interplay between LCP and matrix components. In this study, we report on the processing of poly(L-lactide) with two different LCPs, at relatively low processing temperatures. The study focuses on critical parameters for the morphological development and mechanical performance of LCP-reinforced composites. The influence of blend composition and the processing conditions, on the mechanical response of the composites, is investigated using rheology, wide-angle X-ray diffraction, mechanical analysis, and microscopy techniques. The study conclusively demonstrates that both the matrix viscosity and viscosity ratio between the dispersed and matrix phase, determine the deformation and breakup of the dispersed LCP droplets during extrusion. In addition, the thermal dependence of the viscosity ratio appears to be a critical parameter for the composite performance after injection molding. For example, during injection molding, stretching and molecular orientation of the LCP phase into highly oriented fibrils are prevented when the viscosity ratio increases rapidly upon cooling. In contrast, melt drawing proves to be a more effective processing route as the extensional flow field stabilizes elongated droplets, independent of the viscosity ratio. Overall, these findings provide valuable insights in the morphological development of LCP-reinforced blends, highlighting the importance of the development of viscoelastic properties as a function of temperature, and provide guidelines for the design of new LCP polymers and their thermoplastic composites.

Molecules and atoms at material interfaces have properties that are distinct from those found in the bulk. Distinguishing the interfacial species from the bulk species is the inherent difficulty of interfacial analysis. For organic photovoltaic devices, the interface between the donor and acceptor materials is the location for exciton dissociation. Dissociation is thought to occur via a complex route effected by microstructure and the electronic energy levels. The scale of these devices and the soft nature of these materials create an additional level of difficulty for identification and analysis at these interfaces. The transmission electron microscope (TEM) and the spectroscopic techniques it incorporates can allow the properties of the donor-acceptor interfaces to be revealed. Cross-sectional sample preparation, using modern focused ion beam instruments, enables these buried interfaces to be uncovered with minimal damage for high resolution analysis. This powerful instrument combination has the ability to draw conclusions about interface morphology, structure and electronic properties of organic donor-acceptor interfaces at the molecular scale. Recent publications have demonstrated these abilities, and this article aims to summarise some of that work and provide scope for the future.

Control of the interfacial structures of ionic liquids (ILs) at charged interfaces is important to many of their applications, including in energy storage solutions, sensors and advanced lubrication technologies utilising electric fields. In the case of the latter, there is an increasing demand for the study of non-halogenated ILs, as many fluorinated anions have been found to produce corrosive and toxic halides under tribological conditions. Here, the interfacial structuring of a series of four imidazolium ILs ([C(n)C(1)Im]) of varying alkyl chain lengths (n = 5, 6, 7, 10), with a non-halogenated borate-based anion ([BOB]), have been studied at charged interfaces using sum frequency generation (SFG) spectroscopy and neutron reflectivity (NR). For all alkyl chain lengths, the SFG spectra show that the cation imidazolium ring responds to the surface charge by modifying its orientation with respect to the surface normal. In addition, the combination of SFG spectra with electrochemical NR measurements reveals that the longest alkyl chain length (n = 10) forms a bilayer structure at all charged interfaces, independent of the ring orientation. These results demonstrate the tunability of IL interfacial layers through the use of surface charge, as well as effect of the cation alkyl chain length, and provide valuable insight into the charge compensation mechanisms of ILs.

We report an investigation of structure and photophysics of thin layers of cibalackrot, a sturdy dye derived from indigo by double annulation at the central double bond. Evaporated layers contain up to three phases, two crystalline and one amorphous. Relative amounts of all three have been determined by a combination of X-ray diffraction and FT-IR reflectance spectroscopy. Initially, excited singlet state rapidly produces a high yield of a transient intermediate whose spectral properties are compatible with charge-transfer nature. This intermediate more slowly converts to a significant yield of triplet, which, however, does not exceed 100% and may well be produced by intersystem crossing rather than singlet fission. The yields were determined by transient absorption spectroscopy and corrected for effects of partial sample alignment by a simple generally applicable procedure. Formation of excimers was also observed. In order to obtain guidance for improving molecular packing by a minor structural modification, calculations by a simplified frontier orbital method were used to find all local maxima of singlet fission rate as a function of geometry of a molecular pair. The method was tested at 48 maxima by comparison with the ab initio Frenkel-Davydov exciton model. Published under license by AIP Publishing.

© the Owner Societies. Crystalline organic semiconducting materials are much in demand for multiple electronic and optoelectronic device applications. Here, solution grown ultrathin rhombic crystals of a trimethine carbocyanine anionic dye are used to establish relationships between structural and optical properties. The dye crystallized in the monoclinic space group P21/c featuring alternating layers of molecules in two different herringbone type patterns, with perchlorate counterions located mostly within one of the two layers. Micro transmittance spectroscopy revealed a broadened spectrum compared to those obtained in solution and in an amorphous thin film. Using polarized light, transmission spectroscopy revealed strong low-energy and weak high-energy bands polarized along the crystallographic b- and c-axis, respectively. Using the extended dipole approximation, significant exciton couplings are predicted between neighboring molecules in the crystal, of the order of the intrinsic monomer reorganization energies associated with nuclear relaxation after excitation, depicting a complex spectral scenario. The exciton coupling pattern explains the relative energies of the b- and c-polarized components but the observed intensities are opposite to expectations based on chromophore alignment within the crystal.

Polymers 10(9), 935 (2018). doi:10.3390/polym10090935 special issue: "Liquid Crystalline Polymers", In this study, we report on the visco-elastic response during start-up and cessation ofshear of a novel bio-based liquid crystal polymer. The ensuing morphological changes are analyzedat different length scales by in-situ polarized optical microscopy and wide-angle X-ray diffraction.Upon inception of shear, the polydomain texture is initially stretched, at larger strain break upprocesses become increasingly important, and eventually a steady state texture is obtained. The shearstress response showed good coherence between optical and rheo-X-ray data. The evolution of theorientation parameter coincides with the evolution of the texture: the order parameter increases as thetexture stretches, drops slightly in the break up regime, and reaches a constant value in the plateauregime. The relaxation of the shear stress and the polydomain texture showed two distinct processeswith different timescales: The first is fast contraction of the stretched domain texture; the second is theslow coalescence of the polydomain texture. The timescale of the orientation parameter’s relaxationmatched with that of the slow coalescence process. All processes were found to scale with shear ratein the tested regime. These observations can have far reaching implications for the processing ofliquid crystal polymers as they indicate that increased shear rates during processing can correspondto an increased relaxation rate of the orientation parameter and, therefore, a decrease in anisotropyand material properties after cooling., Published by MDPI, Basel

Water-based cellulose cholesteric liquid crystalline phases at rest can undergo structural changes induced by shear flow. This reflects on the deuterium spectra recorded when the system is investigated by rheo-nuclear magnetic resonance (rheo-NMR) techniques. In this work, the model system hydroxypropylcellulose (HPC)+water is revisited using rheo-NMR to clarify unsettled points regarding its behavior under shear and in relaxation. The NMR spectra allow the identification of five different stable ordering states, within shear and relaxation, which are well integrated in a mesoscopic picture of the system’s structural evolution under shear and relaxation. This picture emerging from the large body of studies available for this system by other experimental techniques, accounts well for the NMR data and is in good agreement with the three distinct regions of steady shear flow recognized for some lyotropic LC polymers. Shear rates in between 0.1 and 1.0 s–1 where investigated using a Taylor–Couette flow and de...

The role of additives and thermal treatment in the formation of donor–acceptor copolymer organic films of PFO–DBT (poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole]) with increased transport properties is addressed by resonant Auger photoemission and core-hole clock spectroscopy, which allows an analysis of the competition between the inner shell core-hole lifetime and the motion of photoexcited electrons on a femtosecond time scale. From the branching of the competing core-hole decay channels, we study the delocalization dynamics of excited electrons over empty molecular orbitals. We find evidence of ultrafast charge-carrier transfer from specific orbitals (LUMO+1) and increased coupling in copolymer assemblies when a solvent additive (1,8 diiodooctane) is added and samples are post-treated with thermal annealing. Relative conformational energies and core-hole spectra were calculated by time-dependent density functional theory.

Here, a comprehensive study of the influence of polymer:fullerene mixing behavior on the performance, thin-film microstructure, photophysics, and device physics of polymer solar cells is presented. In particular, blends of the donor polymer PBDTTT-EFT with the acceptor PC71BM that exhibit power conversion efficiencies over 9% are investigated. Through tuning of the fullerene concentration in PBDTTT-EFT:PC71BM blends, the impact of fullerene mixing behavior is systematically investigated via a combination of synchrotron-based X-ray scattering and spectroscopy techniques. The impact of fullerene loading on photophysics and device physics is further explored with steady-state photoluminescence measurements, ultrafast transient absorption spectroscopy, and transient photovoltage measurements. In the low fullerene concentration regime (70 wt %), large fullerene domains result in incomplete PC71BM exciton harvesting with the presence of fullerene molecules also disrupting the molecular packing of polymer crystallites. The optimum fullerene concentration of similar to 60-67 wt % balances the requirements of charge generation and charge collection. These findings demonstrate that controlling the fullerene concentration in the mixed phase and optimizing the balance between pure and mixed phases are critical for maximizing the efficiency of highly mixed polymer/fullerene solar cells.

Growing interests have been devoted to the design of polymer acceptors as potential replacement for fullerene derivatives for high-performance all polymer solar cells (all-PSCs). One key factor that is limiting the efficiency of all-PSCs is the low fill factor (FF) (normally

Thermal self-assembly of C-60 on (1 x 2)-Pt(110) following room-temperature deposition has been studied by means of scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density functional theory (DFT) calculations. Two distinct C-60 chemisorption phases have been identified and characterized as a function of the annealing temperature. After a thermal treatment at 700 K, islands of a C-60 quasi-hexagonal lattice form. These islands are characterized by the highest surface density so far reported for a two-dimensional surface-supported fullerene phase (the surface area per molecule is 87.0 angstrom(2)), with intermolecular nearest-neighbor distances equal to 9.6 +/- 0.1 angstrom. Embedded nanowires of fullerene dumbbell dimers (with an intermolecular distance equal to 9.2 +/- 0.1 A) occasionally nucleate within this high-density phase following a "molecular zip" mechanism. Highly site selective chemisorption driven by the particularly strong overlayer-substrate bonding is proposed to be responsible for the first reported example of surface-templated chemical bond formation between fullerene molecules. After annealing at 850 K, an oblique C-60 chemisorption phase forms. This is characterized by a tetramolecular basis associated to each lattice point of the two-dimensional superstructure and by a peculiar bright-dim contrast in STM images acquired at positive sample bias values. The combination of high-resolution STM images with LEED data and DFT calculations leads to the conclusion that the (1 x 2) substrate reconstruction is lifted at the fullerene-platinum interface and also provides a detailed description of the molecular bonding sites and orientations found within the phase. It is proposed that the main factor ruling the interconversion of chemisorption phases is the variation of substrate atom mobility as a function of temperature.

International audience; Polarization resolved nonlinear microscopy (PRNM) is a powerful technique to gain microscopic structural information in biological media. However, deep imaging in a variety of biological specimens is hindered by light scattering phenomena, which not only degrades the image quality but also affects the polarization state purity. In order to quantify this phenomenon and give a framework for polarization resolved microscopy in thick scattering tissues, we develop a characterization methodology based on four wave mixing (FWM) process. More specifically, we take advantage of two unique features of FWM, meaning its ability to produce an intrinsic in-depth local coherent source and its capacity to quantify the presence of light depolarization in isotropic regions inside a sample. By exploring diverse experimental layouts in phantoms with different scattering properties, we study systematically the influence of scattering on the nonlinear excitation and emission processes. The results show that depolarization mechanisms for the nonlinearly generated photons are highly dependent on the scattering center size, the geometry used (epi/forward) and, most importantly, on the thickness of the sample. We show that the use of an un-analyzed detection makes the polarization-dependence read-out highly robust to scattering effects, even in regimes where imaging might be degraded. The effects are illustrated in polarization resolved imaging of myelin lipid organization in mouse spinal cords

The rheological behavior of different precursor poly(p-phenylene vinylene) (prec-PPV) monolayers at the air-water interface was investigated using an interfacial stress rheometer (ISR). This device nicely reveals a transition of the precursor poly(2,5-dimethoxy-1,4 phenylene vinylene) (prec-DMePPV) monolayer from Newtonian to elastic behavior with increasing surface pressure. The transition is accompanied by an increase in the modulus. This behavior coincides with the coagulation of different 21) condensed domains as revealed by Brewster angle microscopy (BAM). However, partly converted prec-DMePPV monolayers show elastic behavior even at low surface pressures, although a sudden increase of the moduli does occur. This phenomenon is attributed to enhanced hydrophobic interactions between the conjugated moieties in the partly converted polymers. The latter also explains the stretching behavior of the partly converted prec-DMePPV upon transfer in Langmuir-Blodgett-type vertical dipping. The increase of the moduli which is observed is much more gradual in the precursor poly(2,5-dibutoxy-1,4-phenylene vinylene), prec-DBuPPV, a monolayer which is in agreement with the expected expanded state of the latter monolayer.

In the present communication one monolayer Langmuir-Blodgett (LB) films of cadmium arachidate (CdA), lead arachidate (PbA), calcium arachidate (CaA), zinc arachidate (ZnA) and barium arachidate (BaA) have been investigated using core level loss spectroscopy, electron energy loss spectroscopy (EELS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS), The core level loss spectra and electron energy loss spectra of all the films were largely similar. This has been attributed to the similarity of the band structure of these films. which depends on the hydrocarbon chains rather than the cation incorporated. The NEXAFS spectra show a large angular dependence for the PbA film and barely any angular dependence for BaA. A possible explanation for this observation has been offered on the basis of electronegativities of the cations. (C) 1998

An X-ray flexure-based microgap rheometer (X-FMR) has been designed for combining rheology and in situ small-angle X-ray scattering from the vorticity plane. The gap distance can be varied continuously from 500 μm down to several μm, which provides the unique possibility to generate a strong confinement for many complex fluids. A singular advantage of this setup is the possibil- ity to directly probe the vorticity direction of the flow field with a microfocus X-ray beam and to probe the structural response of the fluid to combined shear and confinement in the vorticity plane. The sliding-plate setup operates over a wide range of shear rates of γ ̇ = 10^−3 – 10^3 s^−1 and strains in the range of 10^−4 – 10^2. The flexure-based bearing maintains the plate parallelism within 10−5 rad. The X-FMR requires very small sample volumes on the order of 10 μl. The applicability of the device is demonstrated here with limited examples of a nematic suspension of fd virus (rods), and a crystalline suspension containing sterically stabilized polystyrene-butylacrylate latex particles. ispartof: Review of Scientific Instruments vol:85 issue:6 ispartof: location:United States status: published

Yarns of different inherent viscosities, in the range 0.6-1.1 dL/g, spun in industrial plants, and drawn at room temperature to obtain mesomorphic samples, have been characterized. The evolution from the mesomorphic form toward the triclinic crystalline form has also been studied by combined differential scanning calorimetry (DSC), dynamic mechanical analyses (DMA), and accurate wide angle X-ray diffraction experiments. The DMA analysis of the mesomorphic samples allows better resolution of the glass transition and crystallization phenomena, which are superimposed in the DSC scans. The degree of molecular orientation in the mesomorphic samples, and the temperature of crystallization from the mesomorphic form (60-80-degrees-C), are essentially independent of the polymer molar mass. (C) 1994 John Wiley & Sons, Inc.

We report on the second-order nonlinear optical susceptibility chi(2)(2omega,omega,omega) of dye-doped Langmuir-Blodgett monolayers. Chi(2) is found to exhibit a nonlinear dependence on surface density, which is attributed to microscopic local-fields. In order to calculate the microscopic local-field for the disordered systems, a novel Monte-Carlo type of calculation is described, in which the tilt angle of the dye unit is a sensitive parameter. It is shown also that the tilt angle of rod-shaped chromophores in Langmuir-Blodgett films can be measured by angular-dependent absorption measurements.

The azimuthal helix orientation of the rigid-rod amphiphilic diblock copolypeptides (PLGA-b-PMLGSLGs) of poly(alpha-L-glutamic acid) (PLGA) and poly(gamma-methyl-L-glutamate-ran-gamma-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) in Langmuir-Blodgett (LB) monolayers was investigated using polarized transmission Fourier transform infrared spectroscopy. The relative position of dipping with respect to the previous transfer position can be used to manipulate the azimuthal orientation of the helices parallel to or tilted by an angle of 450 with respect to the dipping direction in the transferred films. The study of the azimuthal order for the LB monolayers of PLGA-b-PMLGSLGs of various block lengths revealed that the observed effect arises mainly from the deformation of the PMLGSLG top brush layer, induced by the flow Orientation around the transfer region. In those cases where the PMLGSLG block is tilted by a sufficiently large angle with respect to the surface normal, high azimuthal order parameters of 0.5-0.75 were obtained.

The synthesis of amphiphilic diblock copolypeptides consisting of poly(alpha-L-glutamic acid) (PLGA) and poly(gamma-methyl-L-glutamate-ran-gamma-stearyl-L-glutamate) with 30 mol % of stearyl substituents (PMLGSLG) and their monolayer behavior at the air-water interface have been studied. PLGA-b-PMLGSLG was synthesized via a diblock copolymer precursor consisting of poly(gamma-tert-butyl-L-glutamate) (PtBuLG) and PMLGSLG blocks, with the tert-butyl group as a mild acid-labile protecting group for the carboxylic acid. The polymerization conditions were found to influence the alpha-helix to beta-sheet content ratio and can be tuned to significantly enhance the cliblock copolypeptide helicity. Purely alpha-helical PtBuLG-b-PMLGSLG diblock copolymers were successfully prepared. After removal of the tert-butyl group, the study of the PLGA-b-PMLGSLG amphiphilic cliblock copolymers in Langmuir monolayers; and Langmuir-Blodgett films demonstrated the formation of a stable alpha-helical double-brush structure, with the helices tilted away from the substrate surface. These double-brush monolayers combine the unique properties arising from the unidirectionally aligned helix macrodipole and the liquid-like features of the side chain mantle of the PMLGSLG block. Such systems are promising for thin film applications requiring incorporation and orientation of bio- and optical molecules. (C) 2010 Elsevier Ltd. All rights reserved.

In molecular devices, the importance of interfaces cannot be neglected as they determine charge injection and charge flow and, therefore, the device performance. Herein we report on the interaction of one single layer of Zn-tetraphenyl-porphyrin with Ag(110) and Si(111). Photoemission, near-edge X-ray absorption, and resonant photoemission are used to study the bonding nature, the adsorption geometry as well as the dynamics of electron transfer between the molecules and the metal or semiconductor surfaces. Molecule-substrate charge transfer is driven by the overlap with the molecular pi orbitals. In particular, the coupling of the phenyl legs with the substrate and the relative excited charge injection are dramatically different for the two surfaces considered.

The surface potentials and effective dipole moments of alpha-helical amphiphilic diblock copolypeptides during monolayer compression at the air water interface are reported. Amphiphilic diblock copolypeptides (PLGA-b-PM LGSLGs) of poly(alpha-L-glutamic acid) (PLGA) and poly(gamma-methyl-L-glutamate-ran-gamma-stearyl-L-glutamate) with 30 mol% of stearyl substituents (PMLGSLG) of various block lengths were studied during the double-brush formation process at the water surface. Upon monolayer spreading of PLGA-b-PMLGSLGs, surface potentials of hundreds of millivolts were recorded, attributed to the dipole moments of water molecules reorienting due to interactions with the monolayers. Upon compression, the effective dipole moments derived from the surface potentials of the PLGA-b-PMLGSLG monolayers decrease gradually, most likely as a result of the immersion of the hydrophilic block in water and cancellation of the interactions between the hydrophobic block and the underlying water molecules. The polypeptide macrodipole moment immersed in water was apparently effectively screened out. The remaining effective dipole moment of the monolayer contributes mainly to the hydrophobic block, and upon tilting away from the water surface toward the surface no it was found to increase with the hydrophobic block length, indicating the gradual formation of unidirectional aligned polypeptide molecules in the double-brush monolayer.

Corannulene (COR) buckybowls were proposed as near ideal hosts for fullerene C-60, but direct complexation of C-60 and COR has remained a challenge in supramolecular chemistry. We report the formation of surface-supported COR-.C-60 host-guest complexes by deposition of C-60 onto a COR lattice on Cu(110). Variable-temperature scanning tunneling microscopy studies reveal two distinctly different states of C-60 on the COR host lattice, with different binding energies and bowl-ball separations. The transition from a weakly bound precursor state to a strongly bound host-guest complex is found to be thermally activated. Simple model calculations show that this bistability originates from a subtle interplay between homo- and heteromolecular interactions.

We measured NEXAFS spectra on both C and N K-edges of an ordered monolayer of lead phthalocyanine adsorbed onto an InSb(001) 4 × 2/c(8 × 2) substrate. We compared the spectra with the calculated density of unoccupied states of an isolated molecule. We demonstrate that molecules lay flat on the surface, determine the energy position of the lowest excited state, and show that it is site dependent. Finally, we compare the core exciton structure with the valence exciton structure previously measured by two different valence spectroscopic techniques and discuss their differences in terms of electron−nuclear and electron−hole Coulomb interactions.

The effect of chain length on the helix orientation of alpha-helical diblock copolypeptides in Langmuir and Langmuir-Blodgett monolayers is reported for the first time. Amphiphilic diblock copolypeptides (PLGA-b-PMLGSLGs) of poly(alpha-L-glutamic acid) (PLGA) and poly(gamma-methyl-L-glutamate-ran-gamma-stearyl-L-glutamate) with 30 mol% of stearyl substituents (PMLGSLG) of various block lengths were studied. The tilt angle between the helices and the substrate-normal decreases upon increasing the transfer pressure coincident with "double brush" formation. The hydrophobic block length strongly affects the maximum surface chain density and thereby the helix orientation of the diblock copolypeptides. Increasing the degree of polymerization of the hydrophobic block (DP(PMLGSLG)) results in an increase in the helix tilt angle tentatively attributed to the off-axis interactions of the unscreened peptide dipoles between the parallel aligned alpha-helices. In those cases where the alkyl side chains surround the PMLGSLG helices, the smallest helix tilt angle of 29 degrees for the PMLGSLG block and 67 degrees for the PLGA block were obtained for the diblock copolymer of DP(PLGA) 37 and DP(PMLGSLG) 24. For smaller DP values of the hydrophobic block, in particular the diblock copolymer having DP(PMLGSLG) 11, the long alkyl side chains are partially expelled from the brush layer and the alpha-helices of the PMLGSLG block are oriented nearly perpendicular to the interface.

Detailed measurements of the macroscopic second-order optical nonlinearity chi(2) (2-omega,omega,omega) of Langmuir-Blodgett dye-doped monolayers are reported. The observed deviations from a linear behavior of chi-(2) with increasing surface density are shown to be due to local-field effects. In order to calculate these local-field factors for disordered systems, a novel Monte Carlo type calculation is introduced. This calculation not only accounts for density variations in the monolayers but also incorporates the effect of off-diagonal elements of the (microscopic) linear susceptibility tensor. Quantitative agreement is found between the calculations and the experimental results using only the molecular hyperpolarizability as a free parameter. A method is presented to determine the tilt angle of the chromophores in Langmuir-Blodgett monolayers from the anisotropy of the linear absorption. The tilt angle determined this way is in excellent agreement with a determination by second-harmonic generation.

In this work we report the fabrication of Langmuir and Langmuir-Blodgett (LB) films of a substituted ZnPc (octakis(oxyoctyl) phthalocyanine of zinc), and their characterization by means of several techniques. These characterization techniques include surface pressure (pi-A) and surface potential (Delta V-A) isotherms as well as UV-vis Reflection spectroscopy and Brewster Angle Microscopy (BAM) for the films at the air-water interface together with UV-vis absorption and IR spectroscopies and Atomic Force Microscopy (AFM) for the LB films. The pi-A and Delta V-A isotherms and BAM images indicate a phase transition at a surface pressure of ca. 9mN/m and a multilayer formation at surface pressures around 19-20 mN/ m; at a surface pressure around 27 mN/ m a disordered collapse of the film occurs. In addition, AFM images of LB films at p= 10mN/ mand p= 20 mN/ m show a monomolecular and a multilayered film, respectively. The comparison of the UV-vis spectrum of ZnPc in solution, the reflection spectra of the Langmuir films and UV-vis spectra of LB films reveals a significant reduction in the Q band intensity for the films, indicative of an organization of ZnPc in the Langmuir and LB films versus the randomdistribution in solution. The UV-vis Reflection spectra are also consistentwith multilayer formation at surface pressures around 19-20 mN/ m. The relative intensities of the IR spectrum bands change from the KBr pellet to the LB filmwhich is also attributable to orientation effects in the film. Cyclic voltammetric experiments of LB films incorporating the ZnPc derivative show peaks that can be correlated with redox processes occurring in the phthalocyanine ring. A small but significant influence of the surface pressure and the number of deposited layers in the electrochemical behaviour is observed. The electrochemical response of cast films exhibits some differences with respect to that of LB films which have been attributed to their different molecular organizations. (C) 2014 Elsevier B. V. All rights reserved.