Adsorption, polymer interfaces

S. Granick. Dynamics of adsorption and desorption at polymer/solid interfaces. In Polymer Interfaces and Adhesion. Boston Butterworth-Heinemann, 1992, p. 227. 

As has been depicted in Fig. 1, various conformations are possible for adsorbed polymers, depending on polymer-polymer, polymer-solvent, and polymer-interface interactions and the flexibility of polymers. To determine experimentally the conformation of adsorbed polymers only adsorption isotherm data are insufficient. The average thickness of the adsorbed polymer layer, the segment density distribution in this layer, the fraction of adsorbed segments, and the fraction of surface sites occupied by adsorbed segments must be measured. Recently, several unique techniques have become available to measure these quantities. 

In the Current State of the Art we will review some of the recent SANS and reflectivity data from ISIS, which also serve to point to future directions and opportunities. Recent reflectivity measurements, on the adsorption of polymers and polymer/surfactant mixtures at interfaces, surface ordering in block copolymer systems, time dependent inter-diffusion at polymer-polymer interfaces, and the contribution of capillary waves to interfacial widths, will be described. The use of SANS to investigate the dynamic of trans-esterification of polyester blends, the deformation of copolymers with novel morphologies, and the use of diffraction techniques to determine the structure of polymeric electrolytes, will be presented. 

A pervaporation system consists of equilibria at both sides of the membrane. One side of the membrane is in contact with the feed liquid mixture, while the other side is exposed to the permeate vapor at low pressure. It is considered that equilibria are established locally at both sides of the membrane. Adsorption equilibrium at the liquid-polymer interface must be established on the feed side, while an adsorption equilibrium at the vapor-polymer interface must be established on the permeate side. Further, both sides of the membrane are connected by liquid phase and gas phase diffusions of permeant molecules in the polymer. Therefore, adsorption equilibria at both liquid-polymer and vapor-polymer interfaces must be studied to fully discuss pervaporation phenomena. This aspect is neglected in many pervaporation papers. Although adsorption at the liquid-polymer interface can be studied by inverse phase liquid chromatography (20,21), this paper shows that adsorption at the vapor-polymer interface can be studied by IGC. 

It is worth repeating that one of the real advantages of preparing interfaces by simple adsorption of a preformed polymer is ease of fabrication. Another is chemical versatility which is limited only by the synthesis of new polymers. One recent synthetic development is reductive loss of Cl" followed by re-oxidation and pyridyl incorporation, all of which can be made to occur within a preformed electrode-polymer interface, as shown on the next page. The work is described in another paper in this volume (34). Another development is the preparation of an extended series of related PVP polymers based on Ru-bpy chemistry, -(py)Ru(bpy)2X2+... 

This extends the previous work (I ) In which the Lennard-Jones type surface potential function and the frictional function representing the Interfaclal forces working on the solute molecule from the membrane pore wall were combined with solute and solvent transport through a pore to calculate data on membrane performance such as those on solute separation and the ratio of product rate to pure water permeation rate in reverse osmosis. In the previous work (1 ) parameters Involved in the Lennard-Jones type and frictional functions were determined by a trial and error method so that the solutions in terms of solute separation and (product rate/pure water permeation rate) ratio fit the experimental data. In this paper the potential function is generated by using the experimental high performance liquid chromatography (HPLC) data in which the retention time represents the adsorption and desorption equilibrium of the solute at the solvent-polymer interface. 

Improved understanding of the mechanism, energetics, and structure of the bonding of water to surfaces is needed. Such information is a key to fundamental clarification of the interfacial structure at solid-liquid surfaces. Poor understanding of the thermodynamics of polymer adsorption at interfaces is impeding scientific progress on corrosion inhibition, colloidal stability, alteration of membrane selectivity, and electrocrystallization additives. 

Fig. 6.3. a Polymer adsorption on interface showing the surface adsorption at low concentration of polymer (q) and at higher concentration of polymer (c2). 6 is adsorbed layer thickness, b Full coverage of the interface by the adsorbed copolymer. The hydrophobic part of the copolymer is shown at the interface (A) and the hydrophilic part protruding in H20 (B). c Schematic showing the segment density distribution for polymer segments attached to the interface... 

The foreign body reaction occurring around soft tissue implants and thrombosis on surfaces in contact with blood are the major reactions encountered with implants. Both reactions involve the interaction of cells with the implant, especially in the later stages, and much previous study has therefore emphasized cellular events in the biocompatibility process. However, cells encounter foreign polymer implants under conditions that ensure the prior adsorption of a layer of protein to the polymer interface. The properties of the adsorbed layer are therefore important in mediating cellular response to the material. 

In recent years, a great deal of effort has been devoted to the study of antithrombogenic polymers (1-3).It has been shown in the present authors laboratory (4-7) that modified insoluble polystyrenes substituted either with sulfonate or amino acid sulfamide groups, exhibit anticoagulant activity, when suspended in plasma. This property can be attributed to the adsorption of thrombin and antithrombin III, at the plasma-polymer interfaces (7-9). 

Adhesion interactions at the solids/polymers interfaces are first and foremost adsorption interactions between the sofid surface and polymer molecules [1—11]. After polymerization there is a low molecular-weight fraction of coupling agents, which can decrease the cohesion and adhesion of the polymer film. If the molecules from this fraction interact with the filler particles preferentially (which can be reached due to the filler surface modification) instead of with the material surface covered, then the boundary layer of the film can be free from this fraction and adhesion increases as strengthening the boundary layer of the coating leads to stronger adhesion of the coating to the covered surfaces [46]. 

The interpenetration between the polymer chains and the mucin chains in the mucosa is believed to be the main physical mechanism for mucoadhesion, and together with the adsorption theory is the most accepted in the literature. This mechanism was first proposed in the polymer-polymer interface" " but was later applied to mucoadhesion due to the polymeric nature of mucin. Basically, during interpenetration the molecules of the mucoadhesive and the mucin molecules in the mucosa are brought into contact, and due to the concentration gradient the polymer chains penetrate into the mucin network with specific diffusion coefficients (Figure 52.4). 

There is no unified theory to explain the process of mucoadhesion. The total phenomenon of mucoadhesion is a combined result of all these theories. First, the polymer gets wet and swells (wetting theory) followed by the noncovalent (physical) bonds created within the mucus-polymer interface (electronic and adsorption theory). Then, the polymer and protein chains interpenetrate (diffusion theory) and entangle together to form further noncovalent (physical) and covalent... 

From a theoretical point of view, adsorption at interfaces, either solid-liquid (S/L). liquid-vapor (L/V>, or liquid-liquid (L/L), is one of the topic.s of highest interest in the physical chemistry of colloidal systems due to the increasing number of important technological areas in which it finds application. We will only be here concerned with solid-liquid interfaces and. in particular, with the two main ways of favoring stabilization of pharmaceutical suspensions, which are the adsorption of surfactants and of polymers as additives in suspension. Let us first consider the fundamentals of these processes and later some results will be discussed. 

Fig. 11.3 The amount of dissolved gas determines the nature and extent of the surface structuration. 300 nm thick 250 kDa PS films were immersed during 10 min in water at pH 1.5 and then studied by AFM in air. Before treating the surfaces, the aqueous soiutirai was in contact with air during different periods of time after degassing as indicated, to change the amount of gas dissolved, (a) Shortly after degassing the preferential adsorption of ions at the water/polymer interface is likely to be responsible for the observed self-assembled nanostructure. Increasing amounts of gas in the solution move the ions away from the polymer surface limiting the structuration below the bubbles (b), or the low-density layer (c). Adapted with permission from [21]. Copyright (2011) American Chemical Society...

The QCh study performed has provided a large bulk of results enabling one to highlight the main peculiarities of the intermolecular interaction at the fumed silica/ PDMS polymer interface from getting reliable values of the adsorption energy to explaining a particular feature related to the optical spectra behavior. 

The subject of the mechanisms and degree of polymer adsorption at interfaces is also discussed in more detail in Chapter 14. For now, suffice it to say that macromolecular additives to emulsion systems constitute a major pathway for attaining workable, long-lived practical emulsions. In fact, their use is essential to many important product types, not the least of which are food colloids, inks, pharmaceuticals, and the photographic industry. 

Henry, M., Bertrand, P. (2004) Influence of polymer surface hydrophilicity on albumin adsorption. Surf. Interface Anal, 36,729-732. 

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