Artificial Intelligence Agents for Materials Sciences.

The artificial intelligence (AI) tools based on large-language models may serve as a demonstration that we are reaching a groundbreaking new paradigm in which machines themselves will generate knowledge autonomously. This statement is based on the assumption that the ability to master natural languages is the ultimate frontier for this new paradigm and perhaps an essential step to achieving the so-called general artificial intelligence. Autonomous knowledge generation implies that a machine will be able, for instance, to retrieve and understand the contents of the scientific literature and provide interpretations for existing data, allowing it to propose and address new scientific problems. While one may assume that the continued development of AI tools exploiting large-language models, with more data used for training, may lead these systems to learn autonomously, this learning can be accelerated by devising human-assisted strategies to deal with specific tasks. For example, strategies may be implemented for AI tools to emulate the analysis of multivariate data by human experts or in identifying and explaining patterns in temporal series. In addition to generic AI tools, such as Chat AIs, one may conceive personal AI agents, potentially working together, that are likely to serve end users in the near future. In this perspective paper, we discuss the development of this type of agent, focusing on its architecture and requirements. As a proof-of-concept, we exemplify how such an AI agent could work to assist researchers in materials sciences.

Low-cost screen-printed electrodes based on electrochemically reduced graphene oxide-carbon black nanocomposites for dopamine, epinephrine and paracetamol detection.

A green approach for the preparation of carbon black (CB) and electrochemically reduced graphene oxide composite (ERGO) is described based on screen printed carbon electrodes (SPCEs) fabricated on poly(ethylene terephthalate) (PET) as electrochemical sensors. This approach leads to a heterogeneous hydrophilic surface with high concentration of defect sites according to scanning electron microscopy, contact angle and Raman spectroscopy measurements. The SPCE/CB-ERGO sensor was tested with dopamine (DA), epinephrine (EP) and paracetamol (PCM), exhibiting an enhanced electrocatalytic performance compared to the bare SPCE. It displayed a wider linear range, lower limit of detection and a remarkably higher analytical sensitivity, viz. 1.5, 0.13 and 0.028 A L mol-1 for DA, EP and PCM, respectively, being also capable of simultaneous determination of the three analytes. Such high performance is demonstration that SPCE/CB-ERGO may serve as generic platform for cost-effective flexible electrochemical sensors.

Challenges in Application of Langmuir Monolayer Studies To Determine the Mechanisms of Bactericidal Activity of Ruthenium Complexes.

The effects induced by antibiotics on the bacterial membrane may be correlated with their bactericidal activity, and such molecular-level interactions can be probed with Langmuir monolayers representing the cell membrane. In this study, we investigated the interaction between [Ru(mcbtz)2(PPh3)2] (RuBTZ, mcbtz = 2-mercaptobenzothiazoline) and [Ru(mctz)2(PPh3)2] (RuCTZ, mctz = 2-mercaptothiazoline) with Langmuir monolayers of a lipid extract of Escherichia coli, an extract of lipopolysaccharides (LPSs), and a zwitterionic phospholipid, dioleoylphosphatidyl choline (DOPC). RuBTZ and RuCTZ had little effects on DOPC, which is consistent with their negligible toxicity toward mammalian cells that may be approximated by a zwitterionic monolayer. Also little were their effects on LPSs. In contrast, RuBTZ and RuCTZ induced expansion in the surface pressure isotherms and decreased the compressional modulus of the E. coli lipid extract. While the more hydrophobic RuBTZ seemed to affect the hydrophobic tails of the E. coli extract monolayer to a larger extent, according to polarization modulation infrared reflection absorption spectroscopy results, evidence of a stronger RuBTZ interaction could not be confirmed unequivocally. Therefore, the interaction with the E. coli cell membrane cannot be directly correlated with the observed higher bactericidal activity of RuBTZ, in comparison to that of RuCTZ. This appears to be a case in which Langmuir monolayer studies do not suffice to determine the mechanisms responsible for the bactericidal activity.

Impedance spectroscopy for monosaccharides detection using responsive hydrogel modified paper-based electrodes.

Herein we present a novel sensor for the detection of monosaccharides (e.g. glucose, fructose) in solution, using electrical impedance spectroscopy. The sensor is based on carbon interdigitated electrodes, printed on paper using screen printing. The surface of the electrodes was modified with a thin layer of hydrogel containing acrylamide copolymerised with 20 mol% 3-(Acrylamido)phenylboronic acid (PBA). It was observed that the hydrogel layers containing 20 mol% PBA swell considerably in the presence of glucose and fructose. This in turn changes the measured impedance across the electrodes, making it a suitable sensor for the quantitative detection of saccharides. We investigated the impedance and capacitance variations with different concentrations of glucose and fructose (0-5 mM) in aqueous phosphate buffer solutions. Variations in impedance were attributed to changes in the dielectric properties of the hydrogel under an applied electric field, due to swelling of the hydrogel layer induced by uptake and binding of sugar molecules to the boronate species within the gel. Impedance measurements at 1 kHz demonstrated that hydrogel swelling leads to an increased mobility of ions within the swollen hydrogel layer. The impedance decreased with increasing sugar concentration and the relative capacitance curves are markedly different for fructose and glucose, as the hydrogel exhibits greater swelling in the presence of fructose than glucose over the same concentration range. As the proposed sensor was shown to be suitable for the detection of glucose at concentration levels found in human sweat, future work will focus on the incorporation of these modified paper-based electrodes into wearable skin patches for non-invasive sugar monitoring in sweat.

Terminação de novilhos em pastagem de aveia branca consorciada com leguminosa ou associada à suplementação energética / Finishing steers on oat pasture intercropped with legumes or receiving energetic supplementation

O objetivo do experimento foi avaliar a associação da ervilhaca (Vica sativa) ou da suplementação energética em pastagem de aveia branca (Avena sativa) sobre o desempenho de novilhos de corte na fase de terminação. Foram utilizados 18 novilhos castrados, 1/4 Marchegiana 1/4 Aberdeen Angus 2 / 4 Nelore, com peso vivo e idade média de 360kg e 19 meses, respectivamente, divididos em três tratamentos: pastagem de aveia branca, pastagem de aveia branca consorciada com ervilhaca e pastagem de aveia branca com suplementação energética (milho grão triturado, 1% do peso vivo por dia, na matéria seca)...

The aim of this experiment was to evaluate the association of vetch (Vica sativa) or energy supplementation in oat (Avena sativa) on the performance of steers in the finishing phase. The experiment was performed in 18 steers, quarter Marchegiana, quarter Aberdeen Angus and 2/4 Nellore genetic groups, with the live weight and age of 360kg and 19 months, respectively, divided into three treatments: pasture white oat, pasture white oat intercropped with vetch and oat pasture with energetic supplementation with the basis of ground corn (1% body weight per day in dry matter)...

Terminação de novilhos em pastagem de aveia branca consorciada com leguminosa ou associada à suplementação energética / Finishing steers on oat pasture intercropped with legumes or receiving energetic supplementation

O objetivo do experimento foi avaliar a associação da ervilhaca (Vica sativa) ou da suplementação energética em pastagem de aveia branca (Avena sativa) sobre o desempenho de novilhos de corte na fase de terminação. Foram utilizados 18 novilhos castrados, 1/4 Marchegiana 1/4Aberdeen Angus2/4 Nelore, com peso vivo e idade média de 360kg e 19 meses, respectivamente, divididos em três tratamentos: pastagem de aveia branca, pastagem de aveia branca consorciada com ervilhaca e pastagem de aveia branca com suplementação energética (milho grão triturado, 1% do peso vivo por dia, na matéria seca). O delineamento experimental utilizado foi o de blocos ao acaso com três tratamentos e três repetições. Houve interação significativa entre o período de avaliação e os tratamentos para participação do componente folha da aveia na pastagem. A participação da ervilhaca aumentou no decorrer do experimento de 3,51% para 12,21% consequentemente a participação da aveia diminuiu no decorrer do experimento. Não houve diferença significativa (P>0,05) para massa de forragem, carga animal, oferta de forragem e taxa de acúmulo diário entre os tratamentos. Os animais que receberam suplementação apresentaram maior (P<0,05) ganho de peso médio diário (1,08kg/dia), ganho de peso vivo por hectare (4,47kg/ha/dia) e ganho de peso vivo total (242,82kg) em comparação com aqueles que receberam somente a pastagem de aveia (0,68kg; 2,50kg/ha/dia; 123,49kg, respectivamente). Já os animais que permaneceram em pastagem consorciada de aveia e ervilhaca apresentaram resultados intermediários (0,89kg/dia; 2,99kg/ha/dia e 154,26kg, respectivamente). O melhor peso final foi para o tratamento com suplementação (424,44kg). A suplementação energética em pastagem de aveia branca apresentou melhores resultados de desempenho animal. No entanto, a consorciação de aveia branca e ervilhaca é uma alternativa quando o suplemento (grão de milho) está com um custo elevado, uma vez que o desempenho, apesar de menor, não foi diferente dos animais suplementados.(AU)

The aim of this experiment was to evaluate the association of vetch (Vica sativa) or energy supplementation in oat (Avena sativa) on the performance of steers in the finishing phase. The experiment was performed in 18 steers, quarter Marchegiana, quarter Aberdeen Angus and 2/4 Nellore genetic groups, with the live weight and age of 360kg and 19 months, respectively, divided into three treatments: pasture white oat, pasture white oat intercropped with vetch and oat pasture with energetic supplementation with the basis of ground corn (1% body weight per day in dry matter). The experimental design was a randomized block design with three treatments and three replications. The interaction was observed by the period of evaluation and treatments for the participation of oat leaf in grassland. The participation of vetches increased in the course of the experiment from 3.51% to 12.21% as opposed to the participation of oat, which decreased in the course of the experiment in all treatments. No significant difference (P>0.05) was found for the forage mass, stoking rate, forage offer and rate of daily accumulation between treatments. The animals that received supplementation showed higher (P <0.05) average daily weight gain (1.08kg/day), live weight gain for ha (4.47kg/ha/day) and total live weight gain (242.82kg) compared with those animals that received only oat (0.68kg, 2.50kg/ha/day; 123.49kg, respectively). While the animals that have remained in pasture with oat and vetch showed intermediate results (0.89kg/day, 2.99kg/ha/day and 154.26kg, respectively) the final weight was better in the treatment with supplementation (424.44kg). The energy supplementation with oat pasture is presented as the best alternative to improve animal performance. Nevertheless, the oat plus vetch is an alternative when the supplement (corn grain) has a high cost, once the animal performance was lower, but it was not different from the energetic supplementation.(AU)

Correlation of [RuCl3(dppb)(VPy)] cytotoxicity with its effects on the cell membranes: an investigation using Langmuir monolayers as membrane models.

One of the major challenges in drug design is to identify compounds with potential toxicity toward target cells, preferably with molecular-level understanding of their mode of action. In this study, the antitumor property of a ruthenium complex, mer-[RuCl3(dppb)(VPy)] (dppb = 1,4-bis(diphenylphosphine)butane and VPy = 4-vinylpyridine) (RuVPy), was analyzed. Results showed that this compound led to a mortality rate of 50% of HEp-2 cell with 120 ± 10 µmol L(-1), indicating its high toxicity. Then, to prove if its mode of action is associated with its interaction with cell membranes, Langmuir monolayers were used as a membrane model. RuVPy had a strong effect on the surface pressure isotherms, especially on the elastic properties of both the zwitterionic dipalmitoylphosphatidylcholine (DPPC) and the negatively charged dipalmitoylphosphatidylglycerol (DPPG) phospholipids. These data were confirmed by polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). In addition, interactions between the positive group from RuVPy and the phosphate group from the phospholipids were corroborated by density functional theory (DFT) calculations, allowing the determination of the Ru complex orientation at the air-water interface. Although possible contributions from receptors or other cell components cannot be discarded, the results reported here represent evidence for significant effects on the cell membranes which are probably associated with the high toxicity of RuVPy.

Comparative study of liponucleosides in Langmuir monolayers as cell membrane models.

Liponucleosides may assist the anchoring of nucleic acid nitrogen bases into biological membranes for tailored nanobiotechnological applications. To this end precise knowledge about the biophysical and chemical details at the membrane surface is required. In this paper, we used Langmuir monolayers as simplified cell membrane models and studied the insertion of five lipidated nucleosides. These molecules varied in the type of the covalently attached lipid group, the nucleobase, and the number of hydrophobic moieties attached to the nucleoside. All five lipidated nucleosides were found to be surface-active and capable of forming stable monolayers. They could also be incorporated into dipalmitoylphosphatidylcholine (DPPC) monolayers, four of which induced expansion in the surface pressure isotherm and a decrease in the surface compression modulus of DPPC. In contrast, one nucleoside possessing three alkyl chain modifications formed very condensed monolayers and induced film condensation and an increase in the compression modulus for the DPPC monolayer, thus reflecting the importance of the ability of the nucleoside molecules to be arranged in a closely packed manner. The implications of these results lie on the possibility of tuning nucleic acid pairing by modifying structural characteristics of the liponucleosides.

Interaction of oligonucleotide-based amphiphilic block copolymers with cell membrane models.

Oligonucleotides have unique molecular recognition properties, being involved in biological mechanisms such as cell-surface receptor recognition or gene silencing. For their use in human therapy for drug or gene delivery, the cell membrane remains a barrier, but this can be obviated by grafting a hydrophobic tail to the oligonucleotide. Here we demonstrate that two oligonucleotides, one consisting of 12 guanosine units (G(12)), and the other one consisting of five adenosine and seven guanosine (A(5)G(7)) units, when functionalized with poly(butadiene), namely PB-G(12) and PB-A(5)G(7), can be inserted into Langmuir monolayers of dipalmitoyl phosphatidyl choline (DPPC), which served as a cell membrane model. PB-G(12) and PB-A(5)G(7) were found to affect the DPPC monolayer even at high surface pressures. The effects from PB-G(12) were consistently stronger, particularly in reducing the elasticity of the DPPC monolayers, which may have important biological implications. Multilayers of DPPC and nucleotide-based copolymers could be adsorbed onto solid supports, in the form of Y-type LB films, in which the molecular-level interaction led to lower energies in the vibrational spectra of the nucleotide-based copolymers. This successful deposition of solid films opens the way for devices to be produced which exploit the molecular recognition properties of the nucleotides.

Properties of lipophilic nucleoside monolayers at the air-water interface.

The capability of self-assembly and molecular recognition of biomolecules is essential for many nanotechnological applications, as in the use of alkyl-modified nucleosides and oligonucleotides to increase the cellular uptake of DNA and RNA. In this study, we show that a lipophilic nucleoside, which is an isomer mixture of 2'-palmitoyluridin und 3'-palmitoyluridin, forms Langmuir monolayers and Langmuir-Blodgett films as a typical amphiphile, though with a smaller elasticity. The nucleoside may be incorporated into dipalmitoyl phosphatidyl choline (DPPC) monolayers that serve as a simplified cell membrane model. The molecular-level interactions between the nucleoside and DPPC led to a remarkable condensation of the mixed monolayer, which affected both surface pressure and surface potential isotherms. The morphology of the mixed monolayers was dominated by the small domains of the nucleoside. The mixed monolayers could be deposited onto solid substrates as a one-layer Langmuir Blodgett film that displayed UV-vis absorption spectra typical of aggregated nucleosides owing to the interaction between the nucleoside and DPPC. The formation of solid films with DNA building blocks in the polar heads may open the way for devices and sensors be produced to exploit their molecular recognition properties.

Exploiting distinct molecular architectures of ultrathin films made with iron phthalocyanine for sensing.

The possibility of generating distinct film properties from the same material is crucial for a number of applications, which can only be achieved by controlling the molecular architecture. In this paper we demonstrate as a proof-of-principle that ultrathin films produced from iron phthalocyanine (FePc) may be used to detect trace amounts of copper ions in water, where advantage was taken of the cross sensitivity of the sensing units that displayed distinct electrical properties. The ultrathin films were fabricated with three methods, namely physical vapor deposition (PVD), Langmuir-Blodgett (LB), and electrostatic layer-by-layer (LbL) techniques, where for the latter tetrasulfonated phthalocyanine was used (FeTsPc). PVD and LB films were more homogeneous than the LbL films at both microscopic and nanoscopic scales, according to results from micro-Raman spectroscopy and atomic force microscopy (AFM), respectively. From FTIR spectroscopy data, these more homogeneous films were found to have FePc molecules oriented preferentially, tilted in relation to the substrate surface, while FeTsPc molecules were isotropically distributed in the LbL films. Impedance spectroscopy measurements with films adsorbed onto interdigitated gold electrodes indicated that the electrical response depends on the type of film-forming method and varies with incorporation of copper ions in aqueous solutions. Using principal component analysis (PCA), we were able to exploit the cross sensitivity of the sensing units and detect copper ions (Cu(2+)) down to 0.2 mg/L, not only in ultrapure water but also in distilled and tap water. This level of sensitivity is sufficient for quality control of water for human consumption, with a fast, low-cost method.

Doping in poly(o-ethoxyaniline) nanostructured films studied with atomic force spectroscopy (AFS).

The study of intermolecular interactions at interfaces is essential for a number of applications, in addition to the understanding of mechanisms involved in sensing and biosensing with liquid samples. There are, however, only a few methods to probe such interfacial phenomena, one of which is the atomic force spectroscopy (AFS) where the force between an atomic force microscope tip and the sample surface is measured. In this study, we used AFS to estimate adhesion forces for a nanostructured film of poly(o-ethoxyaniline) (POEA) doped with various acids, in measurements performed in air. The adhesion force was lower for POEA doped with inorganic acids, such as HCl and H(2)SO(4), than with organic acids, because the counterions were screened by the ethoxy groups. Significantly, the morphology of POEA both in the film and in solution depends on the doping acid. Using small-angle X-ray scattering (SAXS) we observed that POEA dissolved in a mixture of dimethyl acetamide exhibits a more extended coil-like conformation, with smaller radius of gyration, than for POEA in water, as in the latter POEA solubility is lower. In AFS measurements in a liquid cell, the force curves for a POEA layer displayed an attractive region for pH>or=5 due to van der Waals interactions, with no contribution from a double-layer since POEA was dedoped. In contrast, for pH

Atomic force spectroscopy on poly(o-ethoxyaniline) nanostructured films: sensing nonspecific interactions.

Atomic force spectroscopy (AFS) was used to measure interaction forces between the tip and nanostructured layers of poly(o-ethoxyaniline) (POEA) in pure water and CuSO4 solutions. When the tip approach and retraction were carried out at low speeds, POEA chains could be physisorbed onto the Si3N4 tip via nonspecific interactions. We conjecture that while detaching, POEA chains were stretched and the estimated chain lengths were consistent with the expected values from the measured POEA molecular weight. The effects from POEA doping could be investigated directly by performing AFS measurements in a liquid cell, with the POEA film exposed to liquids of distinct pH values. For pH > or = 6.0, the force curves normally displayed an attractive region for POEA, but at lower pH values-where POEA is protonated-the repulsive double-layer forces dominated. Measurements in the liquid cell could be further exploited to investigate how the film morphology and the force curve are affected when impurities are deliberately introduced in the liquid. The shape of the force curves and the film morphology depended on the concentration of heavy metal in the liquid cell. AFS may therefore be used to study the interaction between film and analyte, with important implications for the understanding of mechanisms governing the sensing ability of taste sensors.

Transitions in the orientational order of liquid crystals induced by periodic patterned substrates.

The orientational order of liquid crystals (LCs) induced by periodic patterned substrates has been investigated with cells coated by azopolymer films that could be photoaligned in a controlled way. Two regimes were observed depending on the period of the patterns: (i) above 3.0 microm the LC follows the direction imposed by the patterned substrate since the energy stored in the surface potential minimizes the elastic energy of the LC medium. (ii) For periods smaller than 1.0 microm a homogeneous in-plane state was induced and the LC did not follow the orientation imposed by the surface. This in-plane transition could be explained qualitatively by a theoretical model based on the competition between the Frank-Oseen elastic energy and the phenomenological surface potential. The results also suggest an out-of-plane transition for the LC director as the period was reduced. These results agree with data in the literature for patterned substrates with completely distinct architectures. This indicates that for a particular LC sample the overall behavior depends basically on the texture period instead of the texture architecture. The textures were characterized with a scanning near-field optical microscope (SNOM), which allowed simultaneous morphological and optical images in the submicrometer range.

Phenomenological analysis of the light intensity dependence of the photoalignment process in azo-containing polymeric films.

A phenomenological model is proposed to analyze the influence of the incident light intensity on the photoinduced anisotropy of an azobenzene-containing polymer film. The optical anisotropy was generated in the films by the incidence of linearly polarized light and monitored by transmittance measurements.

Phase transition in poly(vinylidene fluoride) investigated with micro-Raman spectroscopy.

The phase transition from the non-polar alpha-phase to the polar beta-phase of poly(vinylidene fluoride) (PVDF) has been investigated using micro-Raman spectroscopy, which is advantageous because it is a nondestructive technique. Films of alpha-PVDF were subjected to stretching under controlled rates at 80 degrees C, while the transition to beta-PVDF was monitored by the decrease in the Raman band at 794 cm(-1) characteristic of the alpha-phase, along with the concomitant increase in the 839 cm(-1) band characteristic of the beta-phase. The alpha-->beta transition in our PVDF samples could be achieved even for the sample stretched to twice (2x-stretched) the initial length and it did not depend on the stretching rate in the range between 2.0 and 7.0 mm/min. These conclusions were corroborated by differential scanning calorimetry (DSC) and X-ray diffraction experiments for PVDF samples processed under the same conditions as in the Raman scattering measurements. Poling with negative corona discharge was found to affect the alpha-PVDF morphology, improving the Raman bands related to this crystalline phase. This effect is minimized for films stretched to higher ratios. Significantly, corona-induced effects could not be observed with the other experimental techniques, i.e., X-ray diffraction and infrared spectroscopy.

The interaction of meso-tetraphenylporphyrin with phospholipid monolayers.

In this article, we investigate the interaction of meso-tetraphenylporphyrin (TPP) with phospholipid monolayers. Pure TPP molecules form films at the air-water interface with large extension of aggregation, which is confirmed by UV-vis spectra of transferred monolayers. For mixed films of TPP with dipalmitoyl phosphatidyl choline (DPPC) or dipalmitoyl phosphatidyl glycerol (DPPG), on the other hand, aggregation is only significant at high surface pressures or high concentrations of TPP (above 0.1 molar ratio). This was observed via Brewster angle microscopy (BAM) for the Langmuir films and UV-vis spectroscopy for transferred layers onto solid substrates. TPP indeed causes the DPPC and DPPG monolayers to expand, especially at the liquid-expanded to liquid-condensed phase transition for DPPC. The effects from TPP cannot be explained using purely geometrical considerations, as the area per TPP molecule obtained from the isotherms is at least twice the expected value from the literature. Therefore, interaction between TPP and DPPC or DPPG should be cooperative, so that more phospholipid molecules are affected than just the first neighbors to a TPP molecule.

Molecularly organized Langmuir-Blodgett films from a ruthenium biphosphine complex.

Langmuir-Blodgett (LB) films from a ruthenium complex mer-[RuCl3 (dppb)(4-Mepy)] (dppb = PPh2 (CH2)4PPh2; 4-Mepy = 4-methylpyridine), termed Ru-Pic, display a distinct color, which is different from the coloration exhibited by cast films or chloroform solutions. The solution and cast films are red, while the LB films are green-bluish. The manifestation of the blue color in the LB film finds its explanation in a unique absorption band at 690 nm, which is associated with the oxidation of the phosphine moieties. Fluorescence emission and absorption-reflection infrared spectroscopy measurements revealed the molecular organization in the LB films. In contrast, cast films showed a random distribution of complexes. Surface-enhanced Raman scattering was also used in an attempt to identify the main interactions in Ru-Pic.

Study of the diclofenac/phospholipid interactions with liposomes and monolayers.

The interaction of diclofenac sodium (SD) with soya phosphatidylcholine (SPC) has been studied with floating Langmuir monolayers and liposomes. SD was either introduced into the subphase of SPC monolayers or co-spread with SPC on an aqueous subphase. In both cases, SD caused the surface pressure isotherm to become more expanded, thus demonstrating the affinity between SD and SPC. The incorporation of SD caused SPC liposomes to have a decreased diameter according to light scattering experiments. When SPC liposomes were injected into an aqueous subphase, their destruction yielding surface-active monomers could be monitored by changes in surface pressure. SD-loaded liposomes displayed a much faster kinetics when the surface density of surface-active monomers was plotted against time, with rate constants increasing significantly with the SD concentration. The kinetic profile can be quantitatively analyzed by plotting ln[1 - (gamma/gamma infinity)] versus t1/2.

In situ UV-vis absorbance measurements for Langmuir films of poly[4'-[[2-(methacryloyloxy)-ethyl]ethylamino]-2-chloro-4-nitroazobenzene] (HPDR13) azopolymer.

In situ UV-vis absorbance measurements are used to investigate aggregation in Langmuir films from the azopolymer poly[4'-[[2-(methacryloyloxy)-ethyl]ethylamino]-2-chloro-4-nitroazobenzene] (HPDR13), a methacrylate derivative of DR13. The level of aggregation in a Langmuir film is intermediate between that of HPDR13 in chloroform solution and in a deposited LB film, as expected. Absorption is negligible at large areas per monomer, and starts to increase at a critical area that is the same as the one obtained in surface potential isotherms, being close to twice the area per monomer for a condensed film. This indicates that the onset for light absorption coincides with a critical packing density where monolayer structuring occurs and there is a sharp change in the effective dielectric constant of the film/water interface. Consistent with a featureless pressure-area isotherm for HPDR13, denoting no significant molecular rearrangement upon film compression, the UV-vis spectra did not vary with the surface pressure. The intensity of absorbed light increased, though, as the film was compressed owing to a higher density of chromophores. At higher subphase temperatures, larger flexibility of HPDR13 chains led to a more compact arrangement, causing the area per monomer to decrease and the absorbed light to increase-with approximately opposite trends.