Measuring molecular order in poly(3-alkylthiophene) thin films with polarizing spectroscopies

We measured the molecular order of poly(3-alkylthiophene) chains in thin films before and after melting through the combination of several polarized photon spectroscopies: infrared (IR) absorption, variable angle spectroscopic ellipsometry (SE), and near-edge X-ray absorption fine structure (NEXAFS). The data from the various techniques can be uniformly treated in the context of the dielectric constant tensor epsilon for the film. The combined spectroscopies allow determination of the orientation distribution of the main-chain axis (SE and IR), the conjugated pi system normal (NEXAFS), and the side-chain axis (IR). We find significant improvement in the backbone order of the films after recrystallization of the material at temperatures just below the melting temperature. Less aggressive thermal treatments are less effective. IR studies show that the changes in backbone structure occur without significant alteration of the structure of the alkyl side chains. The data indicate that the side chains exhibit significant disorder for all films regardless of the thermal history of the sample.     

Chain-length dependence of the conformational order in self-assembled dialkylammonium monolayers on mica studied with soft X-ray absorption

Self-assembled alkyl chain based monolayers on mica are important for industrial and technological processes since they can be employed for an organic modification of the inorganic substrate. The conformational structure and orientational order of the films determine the interaction of the modified substrate with the environment and the chemical character and stability of its surface. We have studied the conformational order in ion exchanged dialkylammonium monolayers adsorbed on mica depending on the length of the alkyl chains systematically with near-edge X-ray absorption fine structure spectroscopy (NEXAFS). In addition, films were characterized by water contact angle measurements. The experimentally determined average tilt angles of the chains are discussed in terms of the degree of order. It was found that the absolute number of gauche defects in the films increases with decreasing chain length.     

Surface structure and chemical switching of thioctic acid adsorbed on Au(111) as observed using near-edge X-ray absorption fine structure

Thioctic acid (alpha-lipoic acid) is a molecule with a large disulfide-containing base, a short alkyl chain with four CH2 units, and a carboxyl termination. Self-assembled monolayer (SAM) films ofthioctic acid adsorbed on Au(111) have been investigated with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS) to determine film quality, bonding, and morphology. Using standard preparation protocols for SAMs, that is, dissolving thioctic acid in ethanol and exposing gold to the solution, results in poor films. These films are highly disordered, contain a mixture of carboxyl and carboxylate terminations, have more than monolayer coverage, and exhibit unbound disulfide. Conversely, forming films by dissolving 1 mmol thioctic acid into 5% acetic acid in ethanol (as previously reported with carboxyl-terminated alkanethiols) forms ordered monolayers with small amounts of unbound sulfur. NEXAFS indicates tilted over endgroups with the carboxyl group normal on average 38 degrees from the surface normal. Slight angle-dependent intensity modulations in other features indicate alkyl chains statistically more upright than prostrate on the surface. Reflection-absorption Fourier transform infrared (RA-FTIR) spectra indicate hydrogen bonding between neighboring molecules. In such well-formed monolayers, a stark reorientation occurs upon deprotonation of the endgroup by rinsing in a KOH solution. The carboxylate plane normal is now about 66 degrees from sample normal, a much more upright orientation. Data indicate this reorientation may also cause a more upright orientation to the alkyl portion of the molecules.     

Structure-function relationships in high-density docosylsilane bonded stationary phases by Raman spectroscopy and comparison to octadecylsilane bonded stationary phases: effects of aromatic compounds

The effects of four aromatic compounds on alkyl chain conformational order for a series of high-density docosylsilane (C22) stationary phases with surface coverage ranging from 3.61 to 6.97 micromol/m2 are investigated using Raman spectroscopy. Aromatic compounds studied include benzene-d6, toluene-d8, aniline-d7 and anisole-d8. In general, these aromatic solvents decrease the conformational order of the C22 phases relative to air suggesting partitioning of the aromatics into the alkyl chains of these stationary phases. Changes in alkyl chain conformational order are linearly dependent on the solvent hydrophobicity parameter, log K ow, and are also dependent on stationary phase properties (i.e. polymerization method and surface coverage). A comparison is made between C22 and C18 bonded phase systems. The conformational order of the alkyl chains in a mixed solute/mobile phase system is also studied using 80% methanol/water as the mobile phase and aniline-d7, anisole-d8 or toluene-d8 as solutes. Collectively, the Raman spectroscopic evidence at the molecular level suggests interaction of these aromatic species with the bonded alkyl chains through partitioning.     

Raman spectroscopy of octadecylsilane stationary phase conformational order. Effect of solvent

The effect of solvent on the conformation of alkyl chains of two octadecysilane-based stationary phases is probed using Raman spectroscopy. Spectral data indicate that the alkyl chains of commercially available polymeric and monomeric solid-phase extraction stationary phases are disordered to a varying extent by solvents of different polarity. For the polymeric octadecylsilane stationary phase, the polar solvents water, acetonitrile, methanol, acetone and isopropanol have little impact on the conformational order of the octadecylsilane bonded phase relative to air. However, the alkyl portion of this stationary phase is substantially disordered in the low-polarity solvents tetrahydrofuran, chloroform, benzene, toluene and hexane. The monomeric octadecylsilane stationary phase is less susceptible to disordering by solvents, although more disorder in the less polar solvents is also observed for this system. These results are interpreted in terms of the local surface bonding density and interchain spacing of these two stationary phases, and the ability of the solvent to penetrate the chains as a function of polarity. The results clearly demonstrate the ability of Raman spectroscopy to precisely indicate subtle changes in conformational order of alkylsilane stationary phases.     

Molecular recognition of cytosine- and guanine-functionalized nucleolipids in the mixed monolayers at the air-water interface and Langmuir-Blodgett films

Molecular recognition of mixed nucleolipids of 1-(2-octadecyloxycarbonylethyl)cytosine and 7-(2-octadecyloxycarbonylethyl)guanine in the monolayers at the air-water interface and Langmuir-Blodgett (LB) films has been investigated in detail using surface pressure/potential-area isotherms, infrared reflection-absorption spectroscopy (IRRAS), and Fourier transform infrared (FTIR) transmission spectroscopy, respectively. Prior to molecular recognition, the cytosine moieties in the monolayer were hydrogen bonded with an almost flat-on orientation, the alkyl chains were uniaxially oriented with respect to the film normal, the guanine moieties in the monolayer were stacked probably through pi-pi interaction with an end-on orientation, and the C-C-C planes of the alkyl chains were preferentially oriented parallel to the water surface. In the monolayer of equimolar mixture, molecular recognition between the cytosine and guanine moieties occurred together with the ring planes of base pairing and the C-C-C planes of the alkyl chains favorably oriented parallel to the water surface. The guanine moieties underwent an orientation change from an end-on mode before molecular recognition to a flat-on one after molecular recognition. The base pairing between the cytosine and guanine moieties in the monolayers was achieved since the N7-substituted guanine derivatives suppressed the formation of guanine tetramers. Both the IRRAS spectra of the monolayers and the FTIR spectra of the LB films presented the exact sites in the cytosine and guanine moieties for the formation of triple hydrogen bonds. The base pairing resulted in a change in molecular orientation and interaction, and the corresponding LB film exhibited a different phase transition behavior from a typical crystal transition for the cytosine-functionalized nucleolipids and an analogous glass transition for the guanine-functionalized nucleolipids. The thermal stability of the mixed LB film was improved in comparison to the LB films of pure components.     

Effect of solvent on aggregation structure and responsive properties of copolymeric gel with side chain crystals

The copolymeric gel films were prepared by a radical copolymerization of stearyl acrylate(SA), acrylic acid(AA) and N,N′-methylenebisacrylamide (MBAA) as a cross-linking agent. The copolymeric (molar ratio SA/AA/MBAA: 24.7/74.3/1.0) gel films with crystalline side chains were swollen in three different kinds of solvent and their aggregation structure and responsive properties have been investigated. In order to control the SA side chains-solvent interaction and to dissociate carboxyl groups in AA, solvents such as dimethyl sulfoxide (DMSO), (1-hexanol/DMSO) mixed solvents and water with different pH values were used. The responsive behaviors of the (SA/AA/MBAA) gel film swollen in different solvents were discussed on the basis of the weight swelling ratio. The X-ray diffraction study of the gel film revealed that the long period corresponding to a layer distance for the (SA/AA/MBAA) gel film swollen in DMSO increased with temperature or the swelling ratio, and the interchain distance of alkyl side chains remained constant with the variation of temperature up to the melting temperature of SA side chain crystals. The swelling ratio of the gel film in (1-hexanol/DMSO) mixed solvent exhibited an abrupt increase in the case of about 40wt% of 1-hexanol, because the SA side chain crystals of the gel film in the mixed solvents were dissolved due to an increase in SA-solvent interaction. On the other hand, the gel film in the water with different pH values showed a sharp increase in the swelling ratio above pH = 11, because the ionic repulsive force among the AA groups became greater than the aggregation one among alkyl side chains. These results indicate that the swelling ratio of the (SA/AA/MBAA) gel film can be controlled by temperature, pH and the magnitude of solubility parameter of swelling solvent.     

具有不同取代链长的卟啉衍生物LB膜的结构研究

本文研究了三种羧酸取代的四苯基卟啉衍生物在空气/Cd^2^+水溶液界面上所形成的单层膜及LB膜。这三种卟啉衍生物中, 一种没有脂链, 另外两种具有不同长度的脂链。由π-A等温线得到的平均表观分子面积相差很大。紫外-可见光谱表明, LB膜中卟啉的Soret吸收带相对于溶液的吸收均红移, 但红移程度不同。LB膜的偏振紫外-可见光谱表明, LB膜中三种卟啉衍生物的卟啉环具有基本一致的取向。运用亚相降低法得到了三种卟啉衍生物单层LB膜, 其紫外-可见光谱与用垂直提拉法得到的LB膜的紫外-可见光谱具有一致的特征。这些结果表明: 卟啉衍生物有无取代链及取代链长的不同对平均表观分子面积的大小和膜中环间的距离有影响, 但对环的取向没有影响。环的取向由环本身及环上的亲水取代基来确定。气/液界面上三种卟啉衍生物的单层膜中环也具有一致的取向, 且与LB膜中环的取向相差不大。提拉不会对膜中环的取向及膜的结构造成大的改变。     

NO2 gas detection by Langmuir-Blodgett films of copper phthalocyanine multilayer structures

Copper (II) phthalocyanine derivatives with four alkyl chains were synthesized. Langmuir-Blodgett (LB) films of the following two compounds were deposited: Cu(II)[tetrakis(3,3-dimethyl-butoxycarbonyl)] phthalocyanine and Cu(II)[tetrakis(n-butoxycarbonyl)] phthalocyanine. Moreover, LB films from a 1:1 mixture of the two compounds were prepared. All isotherms and optical data were consistent with the molecules being stacked nearly edge-on to the substrate, with the molecular faces having a preferred orientation perpendicular to the dipping direction. The dynamic response characteristics of the electrical conductance of the LB films to different NO₂ concentrations in dry air are described. A relationship has been observed between the anisotropic molecular orientation and the electrical conductivity parallel to the substrate.     

Control of surface properties using fluorinated polymer brushes produced by surface-initiated controlled radical polymerization

Surface-grafted styrene-based homopolymer and diblock copolymer brushes bearing semifluorinated alkyl side groups were synthesized by nitroxide-mediated controlled radical polymerization on planar silicon oxide surfaces. The polymer brushes were characterized by X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and time-dependent water contact angle measurements. Angle-resolved XPS studies and water contact angle measurements showed that, in the case of the diblock copolymer brushes, the second block to be added was always exposed at the polymer-air interface regardless of its surface energy. Values of z*/Rg were estimated based on the radius of gyration, Rg, of the grafted homopolymer or block copolymer chains for the grafted brushes and thickness of the brush, z*. The fact that z*/Rg > 1 suggests that all these brushes are stretched. These results support the idea that after grafting the first block onto the surface the nitroxide-end capped polymer chains were able to polymerize the second block in a "living" fashion and the stretched brush so formed was dense enough that the outermost block in all cases completely covers the surface. NEXAFS analysis showed a relationship between the surface orientation of the fluorinated side chains and brush thickness with thicker brushes having more oriented side chains. Time-dependent water contact angle measurements revealed that the orientation of the side chains of the brush improved the surface stability toward reconstruction upon prolonged exposure to water.     

Cononsolvency of poly-N-isopropyl acrylamide (PNIPAM): Microgels versus linear chains and macrogels

Poly-N-isopropyl acryl amide (PNIPAM) is swollen in both pure water and pure methanol but collapses in mixtures of these solvents. In this review, this cononsolvency of PNIPAM in water/methanol mixtures is discussed. Experimental studies of linear PNIPAM chains and macrogels are compared to microgels. Theoretical studies are presented based on molecular dynamics simulation and quantum mechanical calculations as well as semi-empirical models. The different explanations for the cononsolvency available in the literature are introduced. Experiments show that all PNIPAM species collapse and re-swell at comparable methanol fraction in the mixture. Cross-linker density of macrogels and microgels has only slight influence on cononsolvency, whereas chain length of linear chains has a significant influence. Microgels provide advantages to study cononsolvency by en'abling a broader experimental approach. Furthermore, multi-sensitive microgels can be prepared, which contain compartments sensitive to different stimuli.     

Orientation of pentacene molecules on SiO2: from a monolayer to the bulk

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy is used to study the orientation of pentacene molecules within thin films on SiO2 for thicknesses ranging from monolayers to the bulk (150 nm). The spectra exhibit a strong polarization dependence of the pi* orbitals for all films, which indicates that the pentacene molecules are highly oriented. At all film thicknesses the orientation varies with the rate at which pentacene molecules are deposited, with faster rates favoring a thin film phase with different tilt angles and slower rates leading to a more bulklike orientation. Our NEXAFS results extend previous structural observations to the monolayer regime and to lower deposition rates. The NEXAFS results match crystallographic data if a finite distribution of the molecular orientations is included. Damage to the molecules by hot electrons from soft x-ray irradiation eliminates the splitting between nonequivalent pi* orbitals, indicating a breakup of the pentacene molecule.     

Ultrathin films of semiconducting polymers on water

Ultrathin films of polythiophene derivatives spread on water were studied by means of synchrotron radiation, using grazing incidence diffraction and (specular) reflectometry to obtain the molecular orientation in the films. The semicrystalline films were anisotropic, showing a strong tendency of orienting the crystalline alpha-axis perpendicularly to the water subphase. The crystalline domains extend essentially through the entire sample thickness, found to be 10-15 nm. A large expansion of the unit cell alpha-parameter was seen upon doping the films in situ. The reflectometry data were well-fitted by a model with a sinusoidal density variation being damped toward the water subphase. This indicates that the crystalline order was most developed at the polymer-air interface and deteriorated down toward the water, possibly due to the hydrophobicity of the alkyl side chains.     

Molecular orientation and composition at the surface of spin‐coated polyfluorene:Fullerene blend films

The surface composition in spin‐coated films of polyfluorene:fullerene blends was determined quantitatively by near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy. By comparing partial and total electron yield spectra, we found vertical compositional differences in the surface region. Furthermore, the orientation of the polymer chains was investigated by variable‐angle NEXAFS. Blend films of poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐co‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole] with [6,6]‐phenyl‐C61‐butyric acid methyl ester in two different blend ratios were studied. Results showed polymer enrichment of the surfaces for films with a polymer:fullerene weight ratio of 20:80 and of 50:50, spin‐coated from both chlorobenzene and chloroform solutions. The angular dependence of the NEXAFS spectra of the pure polymer films showed a preferential plane‐on orientation, which was slightly stronger in the subsurface region than at the surface. In blend films, this orientational preference was less pronounced and the difference between surface and subsurface vanished. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Characteristics of monolayer behavior and multilayer film structures of comb polymers having different kinds of fluorocarbon side-chains

With comb polymers with different kinds of fluorocarbon side-chains, significant effects of spreading solvents on the monolayers at the air/water interface and the molecular arrangement in the deposited films could be found by the use of trifluoroacetic acid and its mixed solvents with n-hexane, depending on the atoms, fluorine or hydrogen, substituted at the end of the fluorocarbon side-chains. These facts are probably due to intralayer hydrogen bonding cooperating with the carbonyl groups of the long-chain esters. The deposited films of solid substrates were characterized by polarized near-edge X-ray absorption fine structure (NEXAFS) spectroscopy as well as in-plane X-ray diffraction and scanning electron and atomic force microscopy.     

Surface organization, light-driven surface changes, and stability of semifluorinated azobenzene polymers

A series of polymers with 4-perfluoroalkyl-modified azobenzene side groups was investigated for its light-induced changes in surface properties. The ultraviolet (UV) light activated trans to cis isomerization of the azobenzene group, and the influence of molecular order and orientation on this process were studied using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and water contact angle measurements. Light-induced molecular reorganization in the near-surface region was studied by NEXAFS using in situ UV irradiation of polymer thin films. Differential scanning calorimetry and wide-angle X-ray scattering studies showed that sufficiently long fluoroalkyl groups formed well-ordered smectic mesophases in the bulk, as well as on the surface, which was evidenced by NEXAFS. The disruption of mesogen packing by photoisomerization was found to be influenced by the fluoroalkyl segment length. Surfaces with perfluorohexyl and perfluorooctyl groups that showed high orientational order were also highly resistant to light-induced changes. In such cases, the trans-cis isomerization resulted in greater lowering of the azobenzene phenyl ring order parameters than the perfluoroalkyl order parameters. UV exposure caused reorientation of the phenyl rings of the azobenzene group, but the terminal perfluoroalkyl segments remained more or less ordered.     

Chloroaluminum phthalocyanine thin films: chemical reaction and molecular orientation

The chemical transformation of the polar chloroaluminum phthalocyanine, AlClPc, to μ-(oxo)bis(phthalocyaninato)aluminum(III), (PcAl)₂O, in thin films on indium tin oxide is studied and its influence on the molecular orientation is discussed. The studies were conducted using complementary spectroscopic techniques: Raman spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. In addition, density functional theory calculations were performed in order to identify specific vibrations and to monitor the product formation. The thin films of AlClPc were annealed in controlled environmental conditions to obtain (PcAl)₂O. It is shown that the chemical transformation in the thin films can proceed only in the presence of water. The influence of the reaction and the annealing on the molecular orientation was studied with Raman spectroscopy and NEXAFS spectroscopy in total electron yield and partial electron yield modes. The comparison of the results obtained from these techniques allows the determination of the molecular orientation of the film as a function of the probing depth.

Adlayers of dimannoside thiols on gold: surface chemical analysis

Carbohydrate films on gold based on dimannoside thiols (DMT) were prepared, and a complementary surface chemical analysis was performed in detail by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), near-edge X-ray absorption fine structure (NEXAFS), FT-IR, and contact angle measurements in order to verify formation of ω-carbohydrate-functionalized alkylthiol films. XPS (C 1s, O 1s, and S 2p) reveals information on carbohydrate specific alkoxy (C-O) and acetal moieties (O-C-O) as well as thiolate species attached to gold. Angle-resolved synchrotron XPS was used for chemical speciation at ultimate surface sensitivity. Angle-resolved XPS analysis suggests the presence of an excess top layer composed of unbound sulfur components combined with alkyl moieties. Further support for DMT attachment on Au is given by ToF-SIMS and FT-IR analysis. Carbon and oxygen K-edge NEXAFS spectra were interpreted by applying the building block model supported by comparison to data of 1-undecanethiol, poly(vinyl alcohol), and polyoxymethylene. No linear dichroism effect was observed in the angle-resolved C K-edge NEXAFS.     

Self-assembled carbon nanotubes on gold: polarization-modulated infrared reflection-absorption spectroscopy, high-resolution X-ray photoemission spectroscopy, and near-edge X-ray absorption fine structure spectroscopy study

Recently we reported noncovalent functionalization of nanotubes in an aqueous medium with ionic liquid-based surfactants, 1-dodecyl-3-methylimidazolium bromide (1) and 1-(12-mercaptododecyl)-3-methylimidazolium bromide (2), resulting in positively charged single-wall carbon nanotube (SWNT)-1,2 composites. Thiolation of SWNTs with 2 provides their self-assembly on gold as well as templating gold nanoparticles on SWNT sidewalls via a covalent -S-Au bond. In this investigation, we studied the electronic structure, intermolecular interactions, and packing within noncovalently thiolated SWNTs and also nanotube alignment in the bulk of SWNT-2 dried droplets and self-assembled submonolayers (SAMs) on gold by high-resolution X-ray photoemission spectroscopy (HRXPS), C K-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS). HRXPS data confirmed the noncovalent nature of interactions within the nanocomposite of thiolated nanotubes. In PM-IRRAS spectra of SWNT SAMs on gold, the IR-active vibrational SWNT modes have been observed and identified. According to PM-IRRAS data, the hydrocarbon chains of 2 are oriented with less tilt angle to the bare gold normal in a SAM deposited from an SWNT-2 dispersion than those of 1 deposited from an SWNT-1 dispersion on the mercaptoethanesulfonic acid-primed gold. For both the dried SWNT-2 bulk and the SWNT-2 SAM on gold, the C K-edge NEXAFS spectra revealed the presence of CH-pi interactions between hydrocarbon chains of 2 and the pi electronic nanotube structure due to the highly resolved vibronic fine structure of carbon 1s --> R*/sigma*C-H series of states in the alkyl chain of 2. For the SWNT-2 bulk, the observed splitting and upshift of the SWNT pi* orbitals in the NEXAFS spectrum indicated the presence of pi-pi interactions. In the NEXAFS spectrum of the SWNT-2 SAM on gold, the upshifted values of the photon energy for R*/sigma*C-H transitions indicated close contact of 2 with nanotubes and with a gold surface. The angle-dependent NEXAFS for the SWNT-2 bulk showed that most of the molecules of 2 are aligned along the nanotubes, which are self-organized with orientation parallel to the substrate plane, whereas the NEXAFS for the SWNT-2 SAM revealed a more normal orientation of functionality 2 on gold compared with that in the SWNT-2 bulk.     

Comparison of molecular orientation and phase transition behaviors in the two kinds of ordered ultrathin films of reversed duckweed polymer ES-3 studied by infrared grazing reflection-absorption spectroscopy

A multilayer LB film and a casting film of reversed duckweed polymer ES-3 on Au-evaporated glass slides were investigated by Fourier Transform infrared grazing reflection-absorption spectroscopy. It is found that the two kinds of ordered ultrathin films have different orientation of alkyl chains, nearly perpendicular to the substrate surface for the LB film while rather tilted for the casting film. The studies on their thermal transition behaviors indicate that both of the films have three phase transition processes, respectively, occurring near 65, 105 and 140 degrees C for the former while near 80, 105 and 140 degrees C for the latter, but show different transition behavior in the each corresponding transition process. It is referred that at room temperature there are island-like domain structures formed in the LB film, but no ones in the casting film; however, the latter can form the domain structures between the first two transition points due to the desorption of solvents. The formation of domain structure seems to play two important roles, one of which is to make alkyl chains more perpendicular to the substrate surface, and the other to make alkyl chains more packed closely. Thermal cyclic experiments reveal that neither of the films could return to its original state after thermal cyclic treatment up to the temperature, which is above the third transition point, although its alkyl chain becomes highly ordered again.