Concentration Of Spreading Solution Research Articles

Effects of spreading and subphase conditions on the interfacial behavior of an amphiphilic copolymer poly(n-butylacrylate)-b-poly(acrylic acid)

Aggregation behavior of an amphiphilic diblock polyelectrolyte poly(n-butylacrylate)-b-poly(acrylic acid) (PnBA-b-PAA) at the air/water interface and its Langmuir−Blodgett (LB) films were respectively characterized by the Langmuir film balance technique and atomic force microscopy. Effects of spreading solution concentration and volume, and subphase temperature and ionic strength on the interfacial aggregation behavior of PnBA-b-PAA under alkaline condition were systematically studied. With the increase of spreading concentration, surface pressure−molecular area isotherms shift to small molecular areas at the corresponding spreading volumes. With the rise in subphase temperature, the isotherms shift to small molecular areas due to the decreased pKa value and increased solubility of PAA blocks. With the increase of subphase ionic strength, the isotherms shift to large molecular areas because of the predominant salting out effect. The LB films transferred from alkaline subphase with different ionic strengths are more likely to exhibit isolated circular micelles. With the increase of spreading concentration, diameters of micelle cores first decrease and then increase, a behavior which is attributed to the gradually increased chain entanglements and surface polymer concentration, respectively. For small spreading volume, core diameters in the LB films first decrease and then increase with the rise in subphase temperature. For large spreading volume, however, core diameters increase with temperature due to the higher mobility and weaker entanglements of PAA blocks underwater. With the increase of subphase ionic strength, furthermore, core diameters first increase and then decrease, which is attributed to the predominant electrostatic shielding and salting out effects, respectively.

Effects of Spreading Conditions on the Aggregation Behavior of a Symmetric Diblock Copolymer Polystyrene- block-poly(methyl methacrylate) at the Air/Water Interface.

Langmuir monolayers and Langmuir-Blodgett (LB) films of a symmetric diblock copolymer polystyrene- block-poly(methyl methacrylate) (PS- b-PMMA) were characterized by the film balance technique and tapping mode atomic force microscopy, respectively. Effects of both the spreading solution concentration and the surface concentration on the aggregation behavior of PS- b-PMMA at the air/water interface and the morphologies of its LB films were studied in detail. When the monolayers spread in different concentrations (≤0.50 mg/mL), all their initial morphologies exhibit tiny circular micelles because of the long hydrophilic PMMA block in the copolymer. The initial tiny circular micelles form spontaneously and then aggregate into small ones upon compression, which can further coalesce into rodlike aggregates or large micelles depending on the spreading concentrations. The LB films of PS- b-PMMA usually exhibit various mixed structures of rodlike aggregates and circular micelles, which can further transform into labyrinth patterns under some special spreading conditions. Besides spreading concentration and volume, we discover that the detailed spreading process should also be responsible for the initial and final morphologies of the LB films. Furthermore, the LB films prepared under different spreading conditions can be regarded as in the equilibrium or nonequilibrium structures because of the kinetic effect difference resulting from the different PS chain entanglement degrees.

Preparation of Patterned Ultrathin TiO2 Films by Langmuir–Blodgett Method Using Mixed Monolayers of Octadecylamine and 1H,1H-Perfluoro-1-dodecanol

An approach for the preparation of patterned ultrathin TiO2 films is demonstrated by using phase-separated Langmuir monolayers, wherein the mixed monolayer of long-chain alkylamine and perfluoroalcohol is formed on an aqueous subphase containing potassium titanium oxalate, followed by deposition onto a solid substrate as a single-layer Langmuir–Blodgett (LB) film. Atomic force microscope observation of the LB films shows that ring-like TiO2 domains are formed on the surface of the substrate and that the size and shape of the TiO2 domains can be tuned by concentration of spreading solution as well as mixing ratio of the two film materials. Subsequent heat-treatment at 500 °C results in TiO2 films of 1.6 nm in thickness maintaining the ring-like pattern.

Understanding and Controlling Morphology Formation in Langmuir–Blodgett Block Copolymer Films Using PS-P4VP and PS-P4VP/PDP

This contribution offers a comprehensive understanding of the factors that govern the morphologies of Langmuir-Blodgett (LB) monolayers of amphiphilic diblock copolymers (BCs). This is achieved by a detailed investigation of a wide range of polystyrene-poly(4-vinyl pyridine) (PS-P4VP) block copolymers, in contrast to much more limited ranges in previous studies. Parameters that are varied include the block ratios (mainly for similar total molecular weights, occasionally other total molecular weights), the presence or not of 3-n-pentadecylphenol (PDP, usually equimolar with VP, with which it hydrogen bonds), the spreading solution concentration ("low" and "high"), and the LB technique (standard vs "solvent-assisted"). Our observations are compared with previously published results on other amphiphilic diblock copolymers, which had given rise to contradictory interpretations of morphology formation. Based on the accumulated results, we re-establish early literature conclusions that three main categories of LB block copolymer morphologies are obtained depending on the block ratio, termed planar, strand, and dot regimes. The block composition boundaries in terms of mol % block content are shown to be similar for all BCs having alkyl chain substituents on the hydrophilic block (such as PS-P4VP/PDP) and are shifted to higher values for BCs with no alkyl chain substituents (such as PS-P4VP). This is attributed to the higher surface area per repeat unit of the hydrophilic block monolayer on the water surface for the former, as supported by the onset and limiting areas of the Langmuir isotherms for the BCs in the dot regime. 2D phase diagrams are discussed in terms of relative effective surface areas of the two blocks. We identify and discuss how kinetic effects on morphology formation, which have been highlighted in more recent literature, are superposed on the compositional effects. The kinetic effects are shown to depend on the morphology regime, most strongly influencing the strand and, especially, planar regimes, where they give rise to a diversity of specific structures. Besides film dewetting mechanisms, which are different when occurring in structured versus unstructured films (the latter previously discussed in the literature), kinetic influences are discussed in terms of chain association dynamics leading to depletion effects that impact on growing aggregates. These depletion effects particularly manifest themselves in more dilute spreading solutions, with higher molecular weight polymers, and in composition regimes characterized by equilibrium degrees of aggregation that are effectively infinite. It is by understanding these various kinetic influences that the diversity of structures can be classified by the three main composition-dependent regimes.

Aggregation behavior of polystyrene-b-poly(acrylic acid) at the air–water interface

The aggregation behavior of amphiphilic block copolymer polystyrene-b-poly(acrylic acid) (PS-b-PAA) at the air–water interface was investigated through surface pressure measurements (isotherms, compression–expansion hysteresis and compression relaxation experiments), Brewster Angle Microscopy (BAM) imaging and Atomic Force Microscopy (AFM) imaging. It is found that PS-b-PAA (Mn = 11490 g mol−1, PAA wt%∼62%) forms a stable Langmuir monolayer on the water surface (pH = 7) by using N,N-dimethylformamide (DMF) as the spreading solvent. The aggregation of block copolymer is induced by an initial diffusion of DMF into water from the interface. Upon compression of the film, the pseudoplateau observed in the Langmuir isotherm corresponds to a “pancake-to-brush” transition with the PAA chains gradually dissolving in the water subphase and stretching underneath the PS cores. Based on the isotherms and the BAM images, it is suggested that the polymer chain dynamics in spreading solutions with different concentrations at the time of solvent diffusion influence the interfacial behavior of block copolymers significantly. The Langmuir–Blodgett (LB) films prepared at different surface pressures from different spreading solution concentrations were scanned by AFM. A variety of morphologies such as wormlike, porous and reticulate structures, and dots were observed. The isotherms and AFM images show the spreading solution concentration and surface pressure dependence of the aggregation behavior of PS-b-PAA copolymer at the air–water interface.

Network Structure Control of Binary Mixed Langmuir Monolayers of Homo-PS and PS-b-P2VP

Our recent work showed there existed a composition window for mixed Langmuir monolayers of homopolystyrene (h-PS) and a symmetric diblock copolymer polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) to form necklace-network structures at the air/water interface. In order to study further the possible mechanism and control the network structure (i.e., surface coverage and nanoaggregate diameter), effects of spreading solution concentration and volume, subphase temperature, and transfer pressure on the network structure were studied by the Langmuir monolayer technique and tapping mode atomic force microscopy. With the increase of transfer pressure, there existed a novel nonlinear behavior for the nanoaggregate diameter first to increase, then to decrease, and finally to increase again, while the surface coverage tended to increase step by step. Moreover, with the elevation of temperature, chain motion between the adjoining nanoaggregates tended to be improved and thus the nanoaggregate diameter tended to be more uniform.

Langmuir Technique and Brewster Angle Microscope Studies of the Interfacial Behavior of Bitumens, Asphaltenes, and Maltenes at the Air−Water Interface. 2. Variable Spreading Solution Concentration Effect

Efficient breaking of water-in-bitumen emulsions that cause undesirable effects in the petrochemical and oil sands industries requires a complete knowledge of the interfacial properties of bitumen and the surface active materials of which it is composed (asphaltenes and maltenes). In this work, the capability of bitumens, asphaltenes, and maltenes to form monolayers at the air—water interface and the structural conformations formed under different compression stages and constant added volumes taken from different concentration spreading solutions are analyzed using a Langmuir trough and Brewster angle microscopy. The results presented here support the conclusions extracted from previous studies and bring new light regarding the origin of the formed aggregates. In this way, the results confirm that, independent of the amount of added surfactant, the three fractions arrange in association structures, forming true monolayers at the air—water interface at low spreading solution concentrations. Out-of-plane regions were only verified for asphaltenes before compression, for high spreading solution concentrations, or at high surface pressures, for low concentrations. The area per molecule of the different association structures noticeably depends upon the spreading solution concentration. Decreasing concentrations lead to increasing areas per molecule. Therefore, aggregates with different degrees of aggregation can form at both the air-water interface and the bulk spreading solution.

Toroid Morphology by ABC-Type Amphiphilic Rod−Coil Molecules at the Air−Water Interface

The interfacial and aggregation behavior of the ABC-type amphiphilic molecules with semirigid dumbbell-shaped core and variable length of hydrophobic branched tails (R=(CH2)nCH3 with n=5 (1), 9 (2), 13 (3)) were investigated. At low surface pressure, smooth, uniform monolayers were formed at the air-water interface by molecules 1 and 2, whereas for molecule 3 unique 2D toroid aggregates have been formed. These aggregates were relatively stable within a range of surface pressure and spreading solution concentration. Upon compression, the 2D toroid aggregates collapsed into large, round 3D aggregates. Finally, the choice of spreading solvent has a great influence on aggregation formation into 2D or 3D micelles as a result of the variable balance of the hydrophobic interactions of branched tails and the pi-pi stacking interaction between aromatic segments.

Novel Two-Dimensional “Ring and Chain” Morphologies in Langmuir−Blodgett Monolayers of PS-b-PEO Block Copolymers: Effect of Spreading Solution Concentration on Self-Assembly at the Air−Water Interface

A polystyrene-b-poly(ethylene oxide) (PS-b-PEO) (MW = 141k, 11.4 wt% PEO) diblock copolymer in the hydrophobic regime was spread from chloroform solutions of various concentrations at the air-water interface, and the resultant monolayers were transferred to glass substrates and imaged using atomic force microscopy. Monolayers prepared under identical conditions were also characterized at the air-water interface via Langmuir compression isotherms. The effects of spreading solution concentration on surface features, compressibility, and limiting mean molecular area were determined, revealing several interesting trends that have not been reported for other systems of PS-b-PEO. Spreading solutions > or = 0.50 mg/mL resulted almost exclusively in dot and spaghetti morphologies, with no observed continent features, which have been commonly found in more hydrophobic systems. For lower spreading solutions, < or = 0.25 mg/mL, we observed a large predominance of two novel surface morphologies, nanoscale rings and chains. The surface pressure (pi)-area (A) isotherms also exhibited a unique dependence on the spreading solution concentration, with limiting mean molecular areas and isothermal compressibilities of PS-b-PEO monolayers increasing below a critical concentration of spreading solution, suggesting a greater contribution from the PEO blocks. These results suggest that PS chain entanglement prior to solvent evaporation plays an important kinetic role in the extent of PEO adsorption at the air-water interface and in the morphologies of the resulting self-assembled surface aggregates.

Effect of Processing Conditions on the Formation of Aggregates and Phase Domains in Monolayers of the Hemicyanine Dye, 4-(4-(Dihexadecylamino)styryl)-N-methylpyridinium Iodide

Epifluorescence microscopy and tapping mode AFM measurements were used to study the morphology of Langmuir−Blodgett monolayers of the hemicyanine dye, 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide, I, prepared under conditions of varying spreading solution concentration, compression speed, aging, subphase composition, and deposition pressure. The morphology, which is the result of coexisting liquid-expanded (LE) and liquid-condensed (LC) phases, is very sensitive to film preparation conditions but relatively insensitive to deposition pressure for a given film preparation protocol. Emission images reveal the following: domain size and shape are unaffected by deposition pressure; increasing spreading solution concentration while maintaining a constant number of molecules on the surface results in increased domain size; emission from the LC domains is more highly variegated as compression speed increases; and incorporation of I- in the subphase results in an entirely new morphology, in contrast to Cl- which only reduces domain size slightly. Far-field fluorescence from films of I is nearly constant as a function of pressure, indicating that aggregation occurs long before the compression process, perhaps even persisting from the spreading solution. This suggests that for monolayers of I, unlike monolayers of fatty acids, both aggregates and phase domains are seeded early in the Langmuir process. This is corroborated by the observation of aggregate formation at the air−water interface before compression takes place. A third phase was also observed and shown to be tens of monolayers in thickness and ca. 1 μm in lateral size by atomic force microscopy. This phase is likely associated with local film collapse and the presence of small bright regions in the epifluorescence images.

Study on monolayer behavior of C 60 using BAM and π– A isotherms

The influence of the experimental conditions (concentration, subphase, volume, waiting time, compressing speed and compression–expansion cycle, etc.) and UV-light irradiation on the monolayer behavior of C 60 was studied in details by using pressure–area ( π– A) isotherms and Brewster angle microscopy (BAM) images. The concentration of spreading solution and the subphase were the most important factors in the formation of relatively homogenous C 60 monolayers and Langmuir–Blodgett (LB) films. A concentration of 10 −5 M and the phenol subphase were preferred.

Langmuir films of P(VDF-TrFE) copolymers

Langmuir monolayers produced from a ferroelectric material, poly(vinylidenefluoride-trifluoroethylene (P(VDF-TrFE) copolymer (70:30) have been investigated through surface pressure and surface potential isotherms. The influence of spreading solvents, concentration of spreading solution and compression speed has been examined. In contrast to monolayers of traditional amphiphilic materials, the surface potential decreased upon film compression.

Structure and polymerisation behaviour of cadmium 10,12-pentacosadiynoate Langmuir-Blodgett films

Two different types of Y-films (a Y-film from Z-type deposition and a Y-film from Y-type deposition) have been observed during the Langmuir-Blodgett (LB) film deposition of metal diynoates, but their structural characterisation has not received much attention. In the present work, a study of LB film deposition of cadmium 10,12-pentacosadiynoate, for evaluating the effects of monolayer aging and concentration of spreading solution, has been carried out and three different types of Y-films of cadmium 10,12-pentacosadiynoate have been obtained. Structural characterisation and study of the topochemical polymerisation of different types of films show remarkably large differences in their bilayer spacing and diacetylene polymerisation behaviour. This study shows that Y-films obtained by overturning in the Z-type build-up process are structurally quite different from the Y-films obtained from the Y-type build-up process.