Polymers, monolayer studies

Koutsos V, van der Vegte E W and Hadziioannou G 1999 Direct view of structural regimes of end-grafted polymer monolayers a scanning force microscopy study Macromolecules 32 1233-6... 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

Although monolayers at the gas-water interface are useful to study adsorption phenomena of e.g. proteins at membranes they are not a very good model, since they represent only one half of a biological membrane. Attempts have therefore been made to extend this concept of polymer monolayers to bilayers and particularly to liposomes. It was to prove, whether the monomers (Table I.) could form bilayers and whether a polyreaction within these bi-layers was possible under retention of the structure and the orientation of the molecules. 

Studies on polymer monolayers spread at air - water interface to characterize their physical properties, as surface pressure, -it, surface potential, AV, surface viscosity, t S, and surface rigidity [39,40] have been reported abundantly. 

Polymer monolayers on the aqueous subphase are studied using the Langmuir technique as is shown in Scheme 3.2. 

As it has been remarked studies on polymer monolayers spread at the air -water interface to characterize their surface behavior as a function of area, of... 

It is very well known that different macromolecular arrangements may be induced either by changing the nature of the subphase [44, 68, 69] or by changing the spreading solvent [70], However, only a few studies describing these effects on polymer monolayer surface pressure behavior have been reported [71-73]. 

Most of the studies of insoluble polymer Langmuir monolayers have been devoted to neutral polymers, and less effort have been devoted to polymer monolayers of polyelectrolytes. 

Monolayers of micro- and nanoparticles at fluid/liquid interfaces can be described in a similar way as surfactants or polymers, easily studied via surface pressure/area isotherms. Such studies provide information on the properties of particles (dimensions, interfacial contact angles), the structure of interfacial layers, interactions between the particles as well as about relaxation processes within the layers. Such type of information is important for understanding how the particles stabilize (or destabilize) emulsions and foams. The performed analysis shows that for an adequate description of II-A dependencies for nanoparticle monolayers the significant difference in size of particles and solvent molecules has be taken into account. The corresponding equations can be obtained by using a thermodynamic model developed for two-dimensional solutions. The obtained equations provide a satisfactory agreement with experimental data of surface pressure isotherms in a wide range of particle sizes between 75 pm and 7.5 nm. Moreover, the model can predict the area per particle and per solvent molecule close to real values. Similar equations were applied also to protein monolayers at liquid interfaces. 

E. Sheppard and N. Tcheurekdjian, Monolayer studies. 3. Spreading of polystyrene latexes at water/air interface, Kolloid Z. Z. Polym. 225, 162-170 (1968). 

Ruths, J., Essler, F., Decher, G., Riegler, H. (2000). Polyelectrolytes I polyanion/polycation multilayers at the air/ monolayer/water interface as elements for quantitative polymer adsorption studies and preparation of hetero-superlattices on solid surfaces. Langmuir 16 8871-8. 

Poly(y-w-decyl-L-glutamate). Monolayers of this polymer were spread from solution in chloroform in which it is freely soluble. The surface pressure-area isotherm (Figure 1) has several notable features. In contrast to most polymers previously studied, there is very little tail at low pressures, and there is an almost linear rise to the commencement of the plateau just below 30 A /residue. There is very little indication of hysteresis, shown by a small hump at the beginning of the plateau with certain other polymers, and the plateau extends almost completely level down to 12.5 A. The pressure then rises, and final collapse occurs at a pressure well below that of most other polymers. 

Naumann CA, Knoll W, Frank CW (2001) Hindered diffusion in polymer-tethered membranes a monolayer study at the air-water interface. Biomacromol 2 1097-1103... 

Borve KGN, Sjoblom J, Stenius P. Water in crude oil emulsions from the Norwegian continental shelf. 5. A comparative monolayer study of model polymers. Colloids Surf 1992 63 241-251. 

Studies on polymer monolayers spread at the air-water interface are now in progress in our laboratory. Biocompatible and biodegradable polymers used as nanoparticles carrying biologically active substances are characterized using the surface balance, surface potential and protein adsorption/desorption measurements. The combined data of all these measurements provide information on drug and protein penetration/delivery with these polymers. 

Although ellipsometry is well established as an experimental technique for the investigation of adsorbed layers, the number of studies at fluid/liquid interfaces is relatively small. Ellipsometry was used for investigation of the layer thickness between two immiscible liquids near the critical point (254, 255). This technique was also quite often used for in situ studies of the adsorption kinetics at an air/protein solution surface or polymer monolayers at an air/water interface (251, 256). It was also shown that ellipsometric re-... 

The most topical theoretical problems for the formation of clusters and nanoparticles in polymeric matrices involve questions about structural-morphological and spatial organization at the local, molecular, and supramolecular levels. Among such problems are the thermodynamic peculiarities of cluster and nanoparticle formation. An analysis has recently been performed using macrochelates. The most important problem seems to be the nanoparticle stabilization by polymer monolayers and LBFs. A insufficient munber of studies have been carried out on this topic. Thus the natme of the adhesion at the interfaces formed still remains obscure. 

This technique has been used in the examination of structural details, and in some cases molecular weight determination of various types of polymers, and is extremely useful in polymer snrface studies (see Chapter 4). The technique has also been used to provide new information and establish relations between species in monolayers, e.g., in the bonding of plastic films to metal or glass. 

It needs to be mentioned here that many other experimental techniques are available for studying monolayers at the air-water interface. Most frequently, surface potential is measured to evaluate the molecular orientation of amphiphiles at the interface. This method is, however, better suited to the study of small molecules. Polymeric amphiphiles, due to their conformational dynamics, are difficult to analyze and simple dielectric layer models do not apply, or produce large errors. Grazing incidence X-ray diffraction provides information on molecular packing, and spectroscopic methods are used to study molecular interactions and the structural changes of molecules upon compression. Fluorescence microscopy is useful for studying two-dimensional organization of small molecular mass amphiphiles however, it is not applied to polymer monolayers. For a more comprehensive overview of experimental methods used to study monolayers at the air-water interface, the reader is referred to more specialized articles, e.g. [18]. 

Partly soluble triblock copolymers are also sometimes used for monolayer studies. Such investigations could provide data on desorption kinetics, and allow for comparison of the film structure, whether spread or adsorbed. However, attention should be paid to data interpretation in such cases because intricate equilibriums take place in such systems. A somewhat confusing study has been presented concerning the monolayer miscibility between PLA and PEO-PPO-PEO (also known as Pluronic) in monolayers [53]. The authors attempted to discuss interactions between the triblock copolymer and a homopolymer (PLA) on the basis of Langmuir monolayer experiments however, the results show unrealistic values for molecular areas, and therefore conclusions from those measurements cannot be quantitative. In particular, surface pressure-area isotherms for pure polymers and their mixtures reveal, in the compressed state, areas per monomer unit of the order of 3 h and below. Such low values cannot be real and most probably result either from material dissolution in the subphase or poor spreading at the air-water interface. Indeed, the isotherms do not appear smooth, which suggests low film stability and difficulties in forming a true monolayer. 

Real polymers live in spatial dimension d= > (ordinary polymer solutions) or in some cases in / = 2 (polymer monolayers confined to an interface" " ). Nevertheless, it is of great conceptual value to define and study the mathematical models— in particular, the SAW— in a general dimension d. This permits us to distinguish clearly between the general features of polymer behavior (in any dimension) and the special features of polymers in dimension d= 3. The use of arbitrary dimensionality also makes available to theorists some useful technical tools (e.g., dimensional regularization) and some valuable approximation schemes (e.g., expansion in [Pg.51]

Sato,N., Sugiura, K and Ito, S. (1997), Molecular motion in polymer monolayers at the airAvater interface. A time-resolved study of fluorescence depolarization. Langmuir. 13(21) pp. 5685-5690. 

In this figure OTS refers to octadecyltrichlorosilane and FDS to an incomplete monolayer coating of fluorinated surfactant that appeared to inhibit slip. The results were analysed in terms a slip length model. This model is widely used in polymer flow studies and assumes that slip at the wall produces a flow enhancement equivalent to displacing the solid wall by a fixed slip length, b into the slip surface. This length is also called the Navier length . The model is equivalent to... 

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