Particle laden fluid interfaces: Dynamics and interfacial rheology

We review the dynamics of particle laden interfaces, both particle monolayers and particle + surfactant monolayers. We also discuss the use of the Brownian motion of microparticles trapped at fluid interfaces for measuring the shear rheology of surfactant and polymer monolayers. We describe the basic concepts of interfacial rheology and the different experimental methods for measuring both dilational and shear surface complex moduli over a broad range of frequencies, with emphasis in the micro-rheology methods. In the case of particles trapped at interfaces the calculation of the diffusion coefficient from the Brownian trajectories of the particles is calculated as a function of particle surface concentration. We describe in detail the calculation in the case of subdiffusive particle dynamics. A comprehensive review of dilational and shear rheology of particle monolayers and particle + surfactant monolayers is presented. Finally the advantages and current open problems of the use of the Brownian motion of microparticles for calculating the shear complex modulus of monolayers are described in detail.     

Light-scattering study of monolayer viscoelasticity

Monolayer viscoelasticity has been investigated through light-scattering techniques. We have studied several insoluble monolayers: myristic and stearic acids, propyl stearate, and polymer films: polyvinyl acetate, polymethyl metacrylate. We obtain information about compressibility and shear moduli, and dilational and shear surface viscosities. We found it necessary to introduce a relaxation phenomenon associated with monolayer vertical motion and a new surface viscosity coefficient related to this kind of motion.     

Adsorption of water-soluble polymers with surfactant character. Dilational viscoelasticity

A brief summary of dilational surface viscoelatic properties of spread and adsorbed surfactant polymer films at the air-water interface is reported. The viscoelastic moduli have been measured as a function of frequency and surface pressure. The combination of several techniques, oscillating drop and barrier experiments and electrocapillary waves (ECW), has allowed us to investigate a broad frequency range. The dynamic elasticity epsilon shows a slight change with frequency and a noticeable pressure dependence for both kinds of monolayers. In the spread films, elasticity increases steeply with surface pressure, and reaches a constant value before the polymer begins to dissolve into the bulk. On the other hand, the adsorbed films exhibit a pronounced elasticity maximum, followed by a considerable decay when a loose surface structure is formed. The position of the maximum depends on the polymer chemical composition and molecular weight. The results on the overlapping surface pressure range confirm the dynamic equivalence of spread and adsorbed monolayers. At low surface concentration, the agreement between static and dynamic elasticity is quite satisfactory, but the values diverge considerably at higher surface pressures. The loss modulus omegakappa decreases monotonically with increasing omega, becoming zero (it can even take apparent negative values) for the highest frequencies. The frequency dependence of the elasticity has been well described by the diffusive control model of Lucassen-van den Tempel (LVT). However, its predictions for omegakappa do not coincide with the experimental data. The differences between experimental and theoretical values increase at low frequencies.     

Self-organization of polymer particles on hydrophobic solid substrates in aqueous media. I. Self-organization of cationic polymer particles on alkylated glass plates

Monodisperse, cationic polymer particles bearing quaternary ammonium groups effectively self-organized on hydrophobic solid substrates such as alkylated glass plates and polymer films to form particle monolayers. With an increase of the particle surface charge density, the surface coverage decreased and the morphology of particle monolayers changed from aggregated type to dispersed type. The dispersed type of particle monolayers having a relatively regular particle distance was formed at higher temperature. The self-organization behaviors on alkylated glass plates were different from those on unmodified glass plates through electrostatic interaction. The formation of particle monolayers on alkylated glass plates occurred only over a certain latex concentration range in contrast with that on unmodified glass plate. The adhesive strength of particle monolayers was enhanced by annealing at temperatures above the glass transition temperature (T _g) of the particles. Lens-shaped particle monolayers were fabricated by annealing the dispersed type of particle monolayers.     

Interfacial dynamic properties of polymer films

Many vinyl homopolymers are surface active by virtue of their side chains being hydrophilic while backbone hydrocarbon structure being hydrophobic, thus they form remarkably stable monolayers. We report here the studies of collective dynamics of these monolayers and thin films with the technique of surface light scattering by the spontaneous capillary waves on the air/water and oil/water interfaces. The dynamics are represented by the viscoelastic parameters of the lateral dilational mode, which couples efficiently with the transverse mode in the case of air/water interface.     

Some aspects of AFM nanomechanical probing of surface polymer films

We analyzed how the approach developed for the microindentation of non-uniform elastic solids can be adapted to analyze the atomic force microscopy (AFM) probing of ultrathin (1-100 nm thick) polymer films on a solid substrate, as well as polymer films with a multilayered structure. We suggested that recent Johnson's modification of the contact mechanics model that included a viscoelastic contribution could also be utilized to analyze rate-dependent loading data for polymer surfaces. The graded model proposed for microindentation experiments was modified allowing to account not only for variable elastic moduli within different layers but also for the gradient of properties between layers within a transition zone. Two examples of a recent application of this model for molecularly thick hyperbranched polymer monolayers (<3 nm thick) and tri-layered polymer films (20-40 nm thick) tethered to a solid substrate were presented and discussed. In both cases, complex shapes of both loading curves and elastic modulus depth profiles obtained from experimental AFM data were successfully fitted by the graded model with realistic structural parameters.     

Rheology of poly(methyl methacrylate) Langmuir monolayers: percolation transition to a soft glasslike system

An experimental study of the equilibrium properties and of the surface rheology of Langmuir monolayers of poly(methyl methacrylate) (PMMA) at the air/water interface has been carried out as a function of polymer concentration (Γ) and molecular weight (M(w)). Dilational and shear complex elasticity moduli covering a frequency range from 10(-3) to 0.2 Hz have been discussed. It was found that the air∕water interface behaves as a poor solvent for PMMA monolayers, thus suggesting that the polymer coils take collapsed soft-disks (pancakes) shape at the interface. The equilibrium and dynamic results suggest a fluid-to-soft-glass transition as the polymer concentration increases above a critical packing fraction at constant temperature. This two-dimensional transition is in agreement with results previously discussed for the dilational rheology of poly(4-hydroxystyrene) [F. Monroy, F. Ortega, R. G. Rubio, H. Ritacco, and D. Langevin, J. Chem. Phys. 95, 056103 (2005)]. Furthermore, the Γ-dependence of the relaxation dynamics of the monolayers suggests that the gel state may be considered as a fragile soft glass.     

Surface dilational moduli of poly (ethylene oxide), poly (methyl methacrylate), and their blend films

Surface dilational moduli of poly (ethylene oxide) (PEO), poly (methyl methacrylate) (PMMA), and compatible PEO/PMMA blend films spread at the air-water interface were investigated as a function of surface concentration. The surface dilational modulus of an expanded PEO film increased as the surface concentration increased to 0.4mg/m(2), which corresponds to the limiting surface area of PEO. After peaking at this value, the surface dilational modulus decreased with an increase in the PEO concentration. Lissajous orbits of PEO films exhibited positive hysteresis loops for all surface concentration ranges. On the other hand, the surface dilational modulus of a condensed PMMA film steeply increased as the surface concentration increased. Lissajous orbits of PMMA films changed from positive hysteresis loops to negative loops at the surface concentration at which the surface pressure reached in the plateau region. The magnitude of the surface dilational modulus of PMMA was larger than that of PEO at a fixed surface concentration. The surface dilational moduli of the PEO/PMMA blend films increased with the total surface concentration and their magnitudes were less than those of the individual PMMA films and larger than those of the individual PEO films at fixed surface concentrations. Lissajous orbits of the PEO/PMMA blend films also changed from positive hysteresis loops to negative loops beyond the surface concentration at which the plateau surface pressure of PEO was attained.     

Stability of foam films and surface rheology: an oscillating bubble study at low frequencies

The dilational surface elasticity epsilon and the dilational surface viscosity eta of the two nonionic surfactants n-dodecyl-beta-d-maltoside (beta-C12G2) and tetraethyleneglycol-monodecyl ether (C10E4) were studied using the oscillating drop method. The experiments were carried out at different concentrations and frequencies with an accessible frequency range of 0.005-0.2 Hz. The results are discussed in the light of previous disjoining pressure measurements that demonstrated that the stability of thin liquid films cannot be explained solely by the magnitude of the surface forces. Indeed, a comparison of the results obtained for beta-C12G2 with those obtained for C10E4 reveals a correlation between the stability of the films and the surface dilational elasticity of the respective monolayers.     

Self-organization of polymer particles on hydrophobic solid substrates in aqueous media. II. Self-organization of cationic polymer particles on polymer films and wettability control with particle monolayers

Self-organization of cationic polymer particles through hydrophobic interaction on polymer films in aqueous system and characteristic properties of the resulting particle monolayers were investigated. Cationic polymer particles bearing quaternary ammonium groups on their surfaces effectively self-organized on polymer films. With an increase of the particle surface charge density, the surface coverage and average aggregate size (N _a) decreased. The surface coverage control was accomplished by tuning the ionic strength of the media. The wettability of polymer films for water was imparted by the formation of particle monolayers on them. Annealing of the particle monolayers resulted in the increase of the adhesive strength, while the wettability for water was lost. Further improvements of both wettability and adhesive strength of particle monolayers were achieved by the immobilization of silica colloids on the particle monolayers. This method would be effective for the hydrophilization of polymer films.     

Poly(itaconates) monolayers behavior at the air/water interface. Study at different surface concentration

Polymer monolayers spread at the air/water interface were obtained for: poly(monooctyl itaconate) (PMOI), poly(monodecyl itaconate) (PMDI), poly(monododecyl itaconate) (PMDoI), poly(monobenzyl itaconate) (PMBzI), poly(methyldodecyl itaconate) (PMeDoI) and the alternating copolymer (monooctyl itaconate-alt-maleic anhydride) (MOI-alt-MA). By monolayer compression at constant temperature, the respective Langmuir isotherms for these polymers were obtained. For all polymers the zero-pressure limiting area per repeating unit (ru) Ao, and the collapse pressure π_c were determined. At low surface polymer concentrations, the monolayers characterization was carried out according to the surface pressure expressed as a function of the surface concentration. The behavior observed was described by the virial expansion development. At the semidilute region, the surface pressure variation was expressed in terms of the scaling laws as a power function of the surface concentration.     

疏水基支链化对甜菜碱表面扩张流变性质的影响

利用悬挂滴方法研究了同分异构的直链（C₁₆PB）和支链（C₁₆GPB）十六烷基羟丙基羧酸甜菜碱的表面扩张流变性质,考察了时间、表面压、工作频率及体相浓度对扩张模量和相角的影响.研究发现,羟丙基甜菜碱分子在溶液表面上吸附时,整个亲水基团倾向于平铺在表面上,造成较高的表面扩张模量,表面膜性质由亲水基团取向变化等膜内过程控制.甜菜碱分子疏水烷基的支链化造成分子间相互作用增强,不仅能增大模量,而且在高浓度条件下出现动态模量的最大值现象,说明表面膜的强度与分子排布密切相关,并非单纯由表面分子浓度决定.     

Dynamic surface elasticity of polymer solutions

The surfaces of surfactant solutions exhibit viscoelastic dilational behavior which may be investigated by modern optical and mechanical methods. The present study focuses on the dynamic dilational properties of the polymer solution — gas interface. Linear flexible polymer chains at the surface are considered as consisting of trains, lying on the surface, and loops and tails that protrude into the liquid. The Rouse model is used to describe the inner dynamics of the trains, whereas their motion relative to their neighbors on the surface can be described with the help of the reptation conception. An expression for the complex dynamic dilational surface elasticity has been derived. Although, in general, the relaxation of the surface tension is characterized by an infinite series of relaxation times, it is shown that for many systems the dynamics of the surface layer can be described approximately by only two main relaxation times. The dispersion equation, which was obtained earlier for solutions of conventional surfactants, is shown to be fulfilled for polymer solutions also.     

Viscoelastic theory for nematic polymer interfaces

A macroscopic theory for the dynamics of compressible nematic polymer‐viscous fluid interfaces is developed from first principles. The theory is used to define and characterize the basic interfacial viscoelastic material properties of the ordered interfaces. The theory is based on a decomposition of the kinematic fields and nematic tensor order parameter that takes into account the symmetry breaking of the interface. The interfacial rate of entropy production used to identify the interfacial viscoelastic modes is given in terms of surface rate of deformation tensor and the surface Jaumann derivative of the tangential component nematic tensor order parameter. The derived surface viscous stress tensor is asymmetric and thus describes surface flow‐induced changes in the tensor order parameter. Consistency with the Boussinesq surface fluid appropriate for Newtonian interfaces is established. The interfacial material functions are identified as the dynamic surface tension, the interfacial dilational viscosities, and the interfacial shear viscosities. The interfacial material functions depend on the surface tensor order parameter and as a consequence anisotropy is their characteristic feature. Two characteristic interfacial tensions and two dilational viscosities are predicted depending on the director orientation. In addition six interfacial shear viscosities arise as the directors sample the velocity, velocity gradient, and vorticity directions. Finally the theory provides for the necessary theoretical tools needed to describe the interfacial dynamics of nematic polymer interfaces, such as capillary instabilities, Marangoni flows, and wetting phenomena.     

Monolayer properties of a fuzzy rod polymer: Poly(γ-stearyl α, L-glutamate)

Static and dynamic properties, and surface morphologies of monolayers at the air-water interface of a fuzzy rod polymer, poly(γ-stearyl α, L-glutamate), PSLG, have been examined by the Wilhelmy plate method for surface pressure, electrically induced capillary wave diffraction (ECWD), epi-fluorescence microscopy, and atomic force microscopy (AFM). The monolayers were first formed by spreading polymer solutions at the air-water interface and allowing the solvent to evaporate to obtain polymer films, i.e., spread monolayers. The surface mass density was varied by either successive additions of more solutions on a given surface area or step-wise compression of the surface barrier on a Langmuir trough. Surface pressure isotherms at 23–;60°C were confirmed to be reversible and reproducible, and an abrupt change at approximately 60°C was observed, which is reported as the melting point of crystalline stearyl side chains. By AFM, the monolayer director n by surface alignment was confirmed as perpendicular to the compression direction and certain islands of departure from the monolayer state were visualized upon transferring the monolayers horizontally to silicon wafers. Macroscopic anisotropy in the surface alignment was probed by the electrocapillary waves propagated perpendicular (⟂) and parallel (∥) to the director n; the surface tension anisotropy amount to about 7% difference, σ_⟂/σ_⟂ < 0.07, where σ is the surface tension deduced from the wave propagation characteristics. Multidomain morphologies of the monolayers were imaged by epi-fluorescence microscopy and they were found to differ according to the method of monolayer mass density variation, i.e., the successive addition and step-wise compression. © 1996 John Wiley & Sons, Inc.     

Enhancing electroluminescence performance of MEH-PPV based polymer light emitting device via blending with organosoluble polyhedral oligomeric silsesquioxanes

Organosoluble polyhedral oligomeric silsesquioxanes (POSS) blending effect on electroluminescence properties of MEH-PPV based polymer light emitting device was investigated. Excellent compatibility and surface morphology of organosoluble POSS and MEH-PPV based composite films were observed using AFM spectroscopy. The surface roughness of POSS:MEH-PPV composite film increased with increasing POSS content. Interfacial area between the light-emitting layer and cathode was favorably enhanced for cathode electron-injection. MEH-PPV blended with POSS would create a better balance between the electron and hole fluxes for POSS:MEH-PPV composite film based devices. This led to greater current efficiency of the POSS:MEH-PPV composite film based device as compared to one with a light emitting layer of MEH-PPV. Organosoluble POSS concentration effects on the PL spectra and EL performances were also studied for the POSS:MEH-PPV composite film based polymer light emitting devices.     

Interfacial rheology of polyelectrolytes and polymer monolayers at the air–water interface

In this review, we describe interfacial rheology studies of polymer monolayers at the air–water interface. Since polyelectrolytes are usually soluble in water, the formation of surface monolayers requires the presence of a surfactant of opposite charge. The first part of the review is dedicated to these mixed monolayers. The second part is related to neutral monolayers that can be either adsorbed or deposited at the interface. Interfacial rheology studies of these systems are still scarce, despite a considerable interest: insoluble polymer monolayers in two dimensions are suitable model systems for the tests of polymer theories in two dimensions, such as and glass transition. The rheology of soluble polymer monolayers has important connections with the dynamic properties of dispersions stabilized with these polymers.     

Dilational surface rheology of polymer and polymer/surfactant solutions

Recent studies show strong influence of the dilational surface rheological properties on the stability and dynamics of foam and emulsions. On the other hand, the dilational dynamic surface elasticity proved to be highly sensitive to conformational transitions of macromolecules at fluid–fluid interfaces and can be used to investigate the adsorption mechanism and aggregate formation in the surface layer. The intention of this review consists in the discussion of recent progress in the dialtional surface rheology of solutions of non-ionic homopolymers, block copolymers, polyelectrolytes, polyelectrolyte/surfactant and protein/surfactant complexes.     

Crystalline Morphology and Crystallization Kinetics of Melt-miscible Crystalline/Crystalline Polymer Blends of Poly（vinylidene fluoride） and Poly（butylene succinate-co-24mol% hexamethylene succinate）

Poly（vinylidene fluoride） （PVDF） and poly（butylene succinate-co-24 mol% hexamethylene succinate） （PBHS）, both crystalline polymers, formed melt-miscible crystalline/crystalline polymer blends. Both the characteristic diffraction peaks and nonisothermal melt crystallization peak of each component were found in the blends, indicating that PVDF and PBHS crystallized separately. The crystalline morphology and crystallization kinetics of each component were studied under different crystallization conditions for the PVDF/PBHS blends. Both the spherulitic growth rates and overall isothermal melt crystallization rates of blended PVDF decreased with increasing the PBHS composition and were lower than those of neat PVDF, when the crystallization temperature was above the melting point of PBHS component. The crystallization mechanism of neat and blended PVDF remained unchanged, despite changes of blend composition and crystallization temperature. The crystallization kinetics and crystalline morphology of neat and blended PBHS were further studied, when the crystallization temperature was below the melting point of PBHS component. Relative to neat PBHS, the overall crystallization rates of the blended PBHS first increased and then decreased with increasing the PVDF content in the blends, indicating that the preexisting PVDF crystals may show different effects on the nucleation and crystal growth of PBHS component in the crystalline/crystalline polymer blends.     

Monolayers of hydrogen-bonded polymer blends at the air–water interface: poly(vinylacetate)+poly (4-hydroxystyrene)

 The surface pressure (Π) vs surface concentration (Γ_s) curves of the hydrogen-bonded polymer blend poly(vinylacetate)+ poly(4-hydro-xystyrene) (PVAc+P4HS) have been measured at 25 °C onto a water subphase at pH=2.0. While PVAc forms extended monolayers, and the free surface of water is found to be a good solvent for it, P4HS forms compressed monolayers, and the surface is a near Θ-type solvent for it. PVAc and P4HS form miscible non-ideal monolayers until near the collapse pressure through the whole concentration range. The composition dependence of the Π–Γ_s curves is rather complex. Contrary to what might be expected, the addition of PVAc to the blend does not reduce the rigidity of the monolayer until its weight fraction is larger than 0.5. The compressibility data of the P4HS-rich monolayers suggest the existence of a second maximum at high surface coverages, a result already observed in some polysiloxanes. Received: 11 March 1998 Accepted: 7 May 1998