Surface force measurements, polymer

Fig. 3 a-c. Summary of data from different laboratories, obtained by surface force measurement, on the average layer thickness L as a function of tethered chain length for flat, tethered layers constructed by adsorption of amphiphilic polymers on mica. Adapted from Ref. 21. (a) Data of reference 20 on poly-tert-butylstyrene chains anchored by adsorbing blocks of poly-2-vinylpyridine. (b) Data of references 11 and 12 on polystyrene chains anchored by adsorbing blocks of poly-2-vinylpyridine. (c) Data of references 13 and 14 on polystyrene chains anchored by adsorbing zwitterionic groups [13] or by small adsorbing blocks of polyethyleneoxide [14]... 

The process of adsorption of polyelectrolytes on solid surfaces has been intensively studied because of its importance in technology, including steric stabilization of colloid particles [3,4]. This process has attracted increasing attention because of the recently developed, sophisticated use of polyelectrolyte adsorption alternate layer-by-layer adsorption [7] and stabilization of surfactant monolayers at the air-water interface [26], Surface forces measurement has been performed to study the adsorption process of a negatively charged polymer, poly(styrene sulfonate) (PSS), on a cationic monolayer of fluorocarbon ammonium amphiphilic 1 (Fig. 7) [27],... 

The nanometer level of characterization is necessary for nanochemistry. We have learned from the history of once-new disciplines such as polymer science that progress in synthesis (production method) and in physical and chemical characterization methods are essential to establish a new chemistry. They should be made simultaneously by exchanging developments in the two areas. Surface forces measurement is certainly unique and powerful and will make a great contribution to nanochemistry, especially as a technique for the characterization of solid-liquid interfaces, though its potential has not yet been fully exploited. Another important application of measurement in nanochemistry should be the characterization of liquids confined in a nanometer-level gap between two solid surfaces, for which this review cites only Refs. 42-43. 

Up to date, besides the SFA, several non-interferometric techniques have been developed for direct measurements of surface forces between solid surfaces. The most popular and widespread is atomic force microscopy, AFM [14]. This technique has been refined for surface forces measurements by introducing the colloidal probe technique [15,16], The AFM colloidal probe method is, compared to the SFA, rapid and allows for considerable flexibility with respect to the used substrates, taken into account that there is no requirement for the surfaces to be neither transparent, nor atomically smooth over macroscopic areas. However, it suffers an inherent drawback as compared to the SFA It is not possible to determine the absolute distance between the surfaces, which is a serious limitation, especially in studies of soft interfaces, such as, e.g., polymer adsorption layers. Another interesting surface forces technique that deserves attention is measurement and analysis of surface and interaction forces (MASIF), developed by Parker [17]. This technique allows measurement of interaction between two macroscopic surfaces and uses a bimorph as a force sensor. In analogy to the AFM, this technique allows for rapid measurements and expands flexibility with respect to substrate choice however, it fails if the absolute distance resolution is required. 

Let us mention at this point that a number of studies have used block copolymers in order to test the Alexander-de Gennes description of polymer brushes. For example, surface force measurements have provided some global character-... 

We have seen above that surface-force measurements provide important information about interactions between solid hydrophobic surfaces coated with surfactants and polymers, and that some of the informa tion obtained is directly relevant for oil-in-water emulsions. However, the details of the interaction pro files are expected to be different for liquid hydrocarbon droplets coated with the same molecules as the model solid surfaces. In particular, the coalescence behavior of the emulsion droplets cannot be modelled. It is even more difficult to make a solid model surface that mimieks the behavior of water-in-oil emulsions. At present, the best one can do is to use a polar surface that attracts the polar part of the emulsifier. In fliis way the orientation of the emulsifier on the model sur face and at the water-in-oil emulsion surface will be the same. This will allow us to draw some eonclusions about how polar solid surfaces coated with emulsifiers interact across oil, but care should be taken when using such results to draw conclusions about water-in-oil emulsions. 

Another technique that was used to estimate the solvent content and the number of solvent molecules per EG monomers of PLL- -PEG coatings was recently developed by Pasche et al. and involves coUoidal-probe APM surface force measurements. The main assumption made in this technique is that the unperturbed PEG layer is compressed by the colloidal probe from a fully solvated state to a solvent-free, dry state. Thus, the decrease in the layer thickness upon compression is likely to reflect the amount of solvent absorbed within the polymer brush. The results of that study are in reasonable agreement with the findings of the present work. 

Surface Force Measurements. Another method to measure the thickness of adsorbed layers is by the surface force apparatus (SFA) (17). In this method two freshly cleaved mica sheets are glued to the surfaces of two crossed cylindrical lenses. Polymer chains are then allowed to adsorb on the mica sheets. In order to measure the thickness of the adsorbed layers the two cylinders are brought in contact and the force between them is measured as a fimction of separation. The onset of the repulsive force associated with compression of the adsorbed layer can be related to the thickness of the adsorbed layer. On the other hand, in the event of bridging between the adsorbed layers the force will be attractive. Recent advances in the instrument have made it possible to probe the effect adsorption has on the flow of fluid past a surface (18). 

Some proteins, particularly globular ones, are possibly the only true monodisperse water-soluble polymers that are available today. The globular proteins are however very different from synthetic polymers since they are built from many different types of segments and have a well-defined three-dimensional structure. Surface force measurements can provided a great deal of information... 

Dynamic aspects of adsorbed layers of PS on oxidic surfaces have also been studied by surface-force measurements [73]. The effect of compression was measured for PS adsorbed on mica from cyclopentane near conditions. If compression was performed slowly the layers seemed to become irreversibly compressed as was concluded from the absence of long-range bridging attraction. Even after several days the polymer layers did not relax. 

Surface Force Measurements. Another method to measure the thickness of adsorbed layers is hy the surface force apparatus (SFA) (90). In SFA, two freshly cleaved mica sheets carrying adsorbed polymer chains are brought together, and forces between surfaces are measured as a function of separation. The thickness of the adsorbed layer is estimated from the onset of the repulsive force as the adsorbed layers overlap. 

Kelly T W ef a/1998 Direct force measurements at polymer brush surfaces by atomic force microscopy Macromoiecuies 31 4297-300... 

Section 4.1 briefly describes some of the commonly employed experimental tools and procedures. Chaudhury et al., Israelachvili et al. and Tirrell et al. employed contact mechanics based approach to estimate surface energies of different self-assembled monolayers and polymers. In these studies, the results of these measurements were compared to the results of contact angle measurements. These measurements are reviewed in Section 4.2. The JKR type measurements are discussed in Section 4.2.1, and the measurements done using the surface forces apparatus (SFA) are reviewed in Section 4.2.2. 

Israelachvili and coworkers [64,69], Tirrell and coworkers [61-63,70], and other researchers employed the SFA to measure molecular level adhesion and deformation of self-assembled monolayers and polymers. The pull-off force (FJ, and the contact radius (a versus P) are measured. The contact radius, the local radius of curvature, and the distance between the surfaces are measured using the optical interferometer in the SFA. The primary advantage of using the SFA is its ability to study the interfacial adhesion between thin films of relatively high... 

The surface forces apparatus (Section 2.3) enables the estimation of a surface energy term, Fq (Eq. 9), providing sufficiently smooth surfaces can be produced. In recent years Chaudhury, Pocius and colleagues have made a valuable contribution to the field of adhesion by developing the technique to study energies of adhesion and of surface energies of polymers [81-85]. These SFA results provide alternatives to values based on traditional destructive tests or contact angle measurements. 

A very similar technique is atomic force microscope (AFM) [38] where the force between the tip and the surface is measured. The interaction is usually much less localized and the lateral resolution with polymers is mostly of the order of 0.5 nm or worse. In some cases of polymer crystals atomic resolution is reported [39], The big advantage for polymers is, however, that non-conducting surfaces can be investigated. Chemical recognition by the use of specific tips is possible and by dynamic techniques a distinction between forces of different types (van der Waals, electrostatic, magnetic etc.) can be made. The resolution of AFM does not, at this moment, reach the atomic resolution of STM and, in particular, defects and localized structures on the atomic scale are difficult to see by AFM. The technique, however, will be developed further and one can expect a large potential for polymer applications. 

The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

There have also been a number of simulations of more realistic models of polymers at surfaces [65-77], The behavior of these more realistic models of polymers is similar to that of the model systems discussed above with no real surprises. Of course, the use of realistic models allows a direct comparison with experiment. For example, surface forces apparatus measurements [78] show that in some branched alkanes the force is a monotonic rather than oscillatory function of the separation. This is a surprising result because these branched alkanes pack quite efficiently (in fact they crystallize under some conditions), and this would imply that the surface forces should be oscillatory. Several... 

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