Phase Transitions in Monolayers of PS−PEO Copolymer at the Air−Water Interface

Molecular reorganization of rod-coil diblock copolymers at the air-water interface

Amphiphilic rod-coil diblock copolymers consisting of flexible poly(ethylene oxide) (PEO) and rodlike poly{(+)-2,5-bis[4'-((S)-2-methylbutoxy)phenyl]styrene} (PMBPS) with predominant hydrophobic contents formed ordered monolayers at the air-water interface. The structures of monolayers transferred to a mica substrate at different surface pressures by the Langmuir-Blodgett method were investigated by atom force microscopy (AFM). For PEO(104)-b-PMBPS(17) copolymer (the subscripts denote the number-averaged polymerization degree of each block), a complete spectrum of molecular reorganization at variable surface pressures was observed. Spherical surface aggregates of LB monolayers were spontaneously formed during the solvent evaporation after the deposition of polymer solution. Continuous compression led to the pancake-to-brush conformation transition of the copolymer that PEO chains desorbed from the air-water interface and went into the water subphase. The effective content of the rod block changed continuously from 61% at zero pressure to 93% at the start of the monolayer collapse, igniting the molecular reorganization. As a result, coalescence of individual spherical aggregates into long cylindrical aggregates with an increase of the surface pressure was observed. For a series of block copolymers PEO(104)-b-PMBPS(m)() (m = 17, 30, 45, 53), as the rod contents increased from 61% to 83%, the morphological transition from spherical aggregates to long cylindrical aggregates in orientational order developed at zero pressure, which showed a similar dependence on the effective contents of the rod block to PEO(104)-b-PMBPS(17) at different pressures. In comparison to coil-coil block copolymers PEO-b-PS, the rod-coil block copolymers PEO-b-PMBPS exhibited distinct structure reorganization behavior, in which the orientation of rod block might play an important role.

Optical properties of thermally responsive amphiphilic gold nanoparticles protected with polymers

Amphiphilic thermally responsive gold nanoparticles have been prepared by protecting the particles with both polystyrene, PS, and poly(N-isopropylacrylamide), PNIPAM, chains. Particles form a monolayer on a water surface in a Langmuir trough, and according to in situ spectroscopic measurements, the surface plasmon resonance, SPR, band undergoes a blue-shift during the monolayer compression. The compression-induced blue-shift is related to a change in the conformation of tethered PNIPAM chains; the phenomenon is discussed on the basis of Mie-Drude theory. In contrast, a red-shift in the SPR of the multilayers of the same nanoparticles transferred at different temperatures has been observed with increasing the deposition cycle, attributed to the presence of a weak interparticle coupling in the multilayer.

Polystyrene-block-poly(ethylene oxide) stars as surface films at the air/water interface

Star diblock copolymers containing polystyrene (PS) and poly(ethylene oxide) (PEO) were investigated as surface films at the air/water interface. Both classic and dendritic-like stars were prepared containing either a PS core and PEO corona or the reverse. The investigated polymers, consisting of systematic variations in architectures and compositions, were spread at the air/water interface, generating reproducible surface pressure-area isotherms. All of the films could be compressed to higher pressures than would be possible for pure PEO. For stars containing 20% or more PEO, three distinct regions appeared. At higher areas, the PEO absorbs in pancakelike structures at the interface with PS globules sitting atop. Upon compression, a pseudoplateau transition region appeared. Both regions strongly depended on PEO composition. The pancake area and the pseudoplateau width and pressure increased in a linear fashion with an increasing amount of PEO. In addition, minimum limits of PEO chain length and mass percentage were determined for observing a pseudoplateau. At small areas, the film proved less compressible, producing a rigid film in which PS dominated. Here, the film area increased with both molecular weight and the amount of PS. Comparison with pure linear PS showed the stars spread more, occupying greater areas. Among the stars, the PEO-core stars were more compact while the PS-core stars spread more. The influence of architecture in terms of the core/corona polymers and branching were also examined. The effects of architecture were subtle, proving less important than PEO chain length or mass percentage.

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.

BSA adsorption on bimodal PEO brushes

BSA adsorption onto bimodal PEO brushes at a solid surface was measured using optical reflectometry. Bimodal brushes consist of long (N=770) and short (N=48) PEO chains and were prepared on PS surfaces, applying mixtures of PS(29)-PEO(48) and PS(37)-PEO(770) block copolymers and using the Langmuir-Blodgett technique. Pi-A isotherms of (mixtures of) the block copolymers were measured to establish the brush regime. The isotherms of PS(29)-PEO(48) show hysteresis between compression and expansion cycles, indicating aggregation of the PS(29)-PEO(48) upon compression. Mixtures of PS(29)-PEO(48) and PS(37)-PEO(770) demonstrate a similar hysteresis effect, which eventually vanishes when the ratio of PS(37)-PEO(770) to PS(29)-PEO(48) is increased. The adsorption of BSA was determined at brushes for which the grafting density of the long PEO chains was varied, while the total grafting density was kept constant. BSA adsorption onto monomodal PEO(48) and PEO(770) brushes was determined for comparison. The BSA adsorption behavior of the bimodal brushes is similar to the adsorption of BSA at PEO(770) monomodal brushes. The maximum of BSA adsorption at low grafting density of PEO(770) can be explained by ternary adsorption, implying an attraction between BSA and PEO. The contribution of primary adsorption to the total adsorbed amount is negligible.

Interactions between hen egg-white lysozyme, PEG2,000, and PLA50 at the air–water interface

In this paper, we compared the efficiency of polymer films, made of a poly(ethylene glycol) (PEG2,000)/poly(d,l-lactide) (PLA50) mixture, or a PEG2,000-PLA50 copolymer, to prevent adsorption of a model protein, the hen egg-white lysozyme (HEWL), at the air-water interface. This was achieved by analyzing the surface pressure/surface area curves, and the X-ray reflectivity data of the polymer films spread on a Langmuir trough, obtained in absence or in presence of the protein. For both the mixture and the copolymer, the amount of protein adsorbed at the air-water interface decreases when the density of the polymer surface coverage increases. It was shown that even in a condensed state, the polymer film made by the mixture can not totally prevent HEWL molecules to adsorb and penetrate the polymer mixed film, but however, protein molecules would not be directly exposed to the more hydrophobic phase, i.e. the air phase. It was also shown that the configuration adopted by the copolymer at the interface in its condensed state would prevent adsorption of HEWL molecules for several hours; this would be due in particular to the presence of PEG segments in the interfacial film.

Thin films of hydrophobically modified poly(N,N-dimethylacrylamide)

The behavior of a poly(N,N-dimethylacrylamide) hydrophobically modified by incorporating 0.33 mol % of a pyrenyl derivative, [4-(1-pyrenyl)butyl]amine hydrochloride (PY) and 3.56 mol % of dodecylamine (DO) has been studied at the air/water interface. Surface pressure-area isotherm measurements show that the film is initially anchored by the hydrophobic groups at the air-water interface with a pancake-like structure and, with increasing surface pressure, evolves to a quasi mushroom structure, finally reaching a brush configuration at high pressures. Monolayers of this polymer were transferred to silica substrates using the Langmuir-Blodgett (LB) technique at 5, 15, and 25 mN.m(-1). The properties of the LB films were studied by steady-state and time-resolved fluorescence as well as by atomic force microscopy. The results show that the aggregates formed at low pressures are disrupted by pressure increase, while the water-soluble poly(N,N-dimethylacrylamide) becomes dissolved in the water subphase.

AFM study of micelle chaining in surface films of polystyrene-block-poly(ethylene oxide) stars at the air/water interface

A series of three-arm star block copolymers were examined using atomic force microscopy (AFM). These stars consisted of a polystyrene core composed of ca. 111 styrene units/branch with poly(ethylene oxide) (PEO) chains at the star periphery. Each star contained different amounts of PEO, varying from 107 to 415 ethylene oxide units/branch. The stars were spread as thin films at the air/water interface on a Langmuir trough and transferred onto mica at various surface pressures. Circular domains representing 2D micelle-like aggregated molecules were observed at low pressures. Upon further compression, these domains underwent additional aggregation in a systematic manner, including micellar chaining. At this point, domain area and the number of molecules/domain increased with increasing pressure. In addition, it was found that longer PEO chains led to greater intermolecular separation and less aggregation. These AFM results correspond to attributes seen in the surface pressure-area isotherms of the stars. In addition, they demonstrate the viability of AFM as a quantitative characterization technique.

Nanostructure of a "carpet"-like dense layer/polyelectrolyte brush layer in a block copolymer monolayer at the air-water interface

The "carpet"/brush double layer structure in the polyelectrolyte layer in the amphiphilic diblock copolymer monolayer at the air-water interface was quantitatively studied by in situ neutron reflectometry in addition to X-ray reflectivity measurements. As a result of the higher contrast between polyelectrolyte [poly(methacrylic acid)] and solvent (D(2)O) for the neutron, the brush structure could be estimated more accurately as a function of surface pressure, that is, brush density. The thickness of the carpet layer, which is thought to be formed to reduce the interfacial free energy between water and the hydrophobic layer, was almost constant at 10-20 A at any surface pressure studied. Growth was clearly observed in the whole brush length with increasing surface pressure, and it was estimated to be almost 60% of the full-stretch length of the ionic polymer chain. Furthermore, by the comparison of density profiles by neutron and X-ray reflectometry, an anomalous hydration was suggested.

Formation of large PEE domains in PEE212-PEO112 diblock copolymer monolayers: shift of the PEO-desorption transition

PEE212-PEO112 diblock copolymer monolayers are studied at the air/water interface. At large molecular areas, with X-ray reflectivity, PEE domains are observed, which are partly immersed into the water. The domain thickness increases on compression (28 to 40 A). With off-specular X-ray reflectivity, an average domain radius of 750 A is found, but there are also smaller domains. Due to these space constraints, most PEO blocks form a brush beneath the PEE domains. Only a few PEO blocks form a corona surrounding the domains and adsorb flatly onto the air/water interface. The PEO desorption transition is observed at the typical pressure of 9 mN/m, when the flatly adsorbed PEO is compressed at a domain fraction of 95%. It occurs at 6 A2/EO monomer, about half the value found for lipopolymers or diblock copolymers with NPEE approximately NPEO or NPEE < NPEO. Apparently, the thickness of the PEE domains is determined by the forces from the two interfaces, not by the PEO block, for which flat adsorption beneath the domain would be more favorable instead of formation of a PEO brush.

Surface behavior of amphiphilic heteroarm star-block copolymers with asymmetric architecture

We study the surface behavior of the asymmetric amphiphilic heteroarm poly(ethylene oxide) (PEO)/polystyrene (PS) star polymer on solid substrate. These star polymers differ in both architecture (four- and three-arm molecules, PEO-b-PS(3) and PEO-b-PS(2)) and in the length of PS chains (molecular weight from about 10 000 up to 24 000). We observed that, for a given chemical composition with a predominant content of hydrophobic blocks, the compression behavior of the PS domain structure controls the surface behavior and the final morphology of the monolayers. New features of the surface behavior of star-block copolymers are high stretching of the PS arms from the interface and enhanced stability of the circular PS domain structure, even at high compression. We suggest that for asymmetric star-block copolymers both architecture and chemical composition heavily favor the formation of highly curved interfaces and, thus, more stable circular domain structure with stretched PS arms.

Hydrophilic chain length dependence of the ionic amphiphilic polymer monolayer structure at the air/water interface

Detailed analysis of an interesting poly(methacrylic acid) (PMAA) brush structure in water of a diblock copolymer [(Et(2)SB(m)-b-(MMA)(n), where Et(2)SB is diethylsilacyclobutane] monolayer reported previously was performed by X-ray and neutron reflectometry and indicated that the hydrophilic layer formed with a relatively long PMAA chain is not a simple layer but is divided into two layers, that is, a "carpet"-like dense PMAA layer near the hydrophobic layer and a polyelectrolyte brush layer. The hydrophilic chain length dependence of the diblock copolymer monolayer was analyzed using m:n = 30:x polymer samples, especially of the PMAA double layer structure. With the increase in PMAA chain length in polymer samples, a carpet layer is mainly formed up to n approximately 50. With further increase in the PMAA chain length beyond n approximately 50, a well-defined brush layer appears. On the other hand, the variation in hydrophobic layer thickness with methacrylic acid unit is minimum at the critical PMAA length, that is, n approximately 50 under a constant surface pressure condition. It is strongly suggested that the two discrete layers contribute differently to surface pressure. Furthermore, from the comprehensive viewpoint, the major factor for brush formation was clarified not to be the absolute length of the PMAA chain, but the ratio of PEt(2)SB and PMAA chain length is an important factor.

NEUTRON REFLECTION FROM LIQUID INTERFACES

Recent applications of neutron reflectometry to the study of wet interfaces are described. An outline is given of the basic principles that allow the techniques to determine composition and structure in a variety of situations. These are the adsorption of surfactant molecules at air/liquid and solid/liquid interfaces, the shape of the segment-density profiles of different types of polymer, including block copolymers and polyelectrolytes, adsorption in mixed surfactant and polymer/surfactant systems, and interfacial systems of biophysical interest.

Interfacial properties of adsorbed films made of a PEG2000 and PLA50 mixture or a copolymer at the dichloromethane–water interface

Adsorption kinetics of films of poly(ethylene glycol) (PEG2000) studied by the dynamic pendant drop method showed that PEG2000 was more tensioactive at the dichloromethane (DCM)-water interface than at the air-water interface. When initially solubilized into DCM, PEG2000 segments would form an adsorbed layer with hydrophobic segments buried into the polymer chains turned toward the organic phase. Compression of this layer, accompanied by viscoelastic effects, led to expulsion of some hydrophilic tails toward the water phase. When initially dissolved in water, adsorption of PEG2000 segments led to an elastic PEG2000 layer organized on both sides of the interface. Results showed that when the PEG2000-PLA50 (poly(D,L-lactide)) copolymer film was adsorbed at the DCM-water interface, it resulted in a mixed layer exclusively turned toward DCM and its rheological properties were governed by PLA50. When adsorption at the DCM-water interface resulted from a physical mixture of PEG2000 and PLA50, rheological properties of the film were influenced by the initial localization of PEG2000 in the bulk phases. In the case of a mixed film formed by the adsorption of PLA50 from DCM and PEG2000 from water, results showed that PEG2000 segments totally pushed those of PLA50 away from the interface and exclusively influenced the behavior of the mixed film.