Vapor-polymer interface

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. 

The intrinsic structure of a liquid-vapor interface resembles the surface of a polymer liquid in contact with a nonattractive solid substrate at the pressure where the liquid coexists with its vapor. In the latter case, the system is in the vicinity of the drying transition and a layer of vapor intervenes between the substrate and the polymer liquid. There is, however, one important difference between the vapor-polymer interface and the behavior of a polymer at a solid substrate the local position of the interface can fluctuate. Let us first consider the case where the film is very thick and the solid substrate does not exert any influence on the free surface of the film in contact with its vapor. The fluctuations of the free surface are capillary waves. Neglecting bubbles or overhangs, one can use the position of the liquid-vapor interface, z = h x,y), as a function of the two lateral coordinates, x and y, parallel to the interface to describe the system configuration on a coarse scale. In this Monge representation, the free energy of the interface is given by the capillary-wave Hamiltonian " ... 

The simulations described above were performed at constant density, i.e., a volume was imposed on the system irrespective of the resulting pressure or chemical potential. MD simulations performed at constant chemical potential, where the confined liquid is in equilibrium with a vapor or bulk liquid phase, have also been performed. Simulations with free surfaces, i.e., with vapor/polymer interfaces, allow for the study of the equilibrium liquid-vapor interface structure and the calculation of the surface tension, a thermodynamic property fundamental to the understanding of the behavior of a material at interfaces. An MD study of the equilibrium liquid-vapor interface structure and surface tension of thin films of n-decane and n-eicosane (C20H42) has been performed in Ref. 26. The system studied consisted of a box with periodic boundary conditions in all directions. The liquid polymer, however, while fully occupying the x and y dimensions, occupied only a fraction of the system in the z direction, resulting in two liquid-vapor interfaces. The liquid phase ranged from about 4.0 to 7.0 nm in thickness. Simulations were performed at 400 K for both decane and eicosane, with additional decane simulations at 300 K. A similar system of tridecane molecules, using a well calibrated EA force field, has been studied at 400 K and 300 K in Ref 32. 

It is perhaps obvious that the nature of the interface between a molecular solid (polymer) and a (clean) metal surface is not necessarily equivalent to the interface formed when a metal is vapor-deposited (essentially atom-by-atom ) on to the (clean) surface of the polymer or molecular solid. Atoms of all metals are active in the form of individual atoms , even gold atoms. In the context of the new polymer LEDs, some of the works discussed in chapter 7 involve the study of the early stages of formation of the interface in the latter configuration (metal-on-polymer interfaces). Very little has been reported on conjugated polymer-on-metal interfaces, however, primarily because of the difficulties in preparing monolayers of polymer materials on well defined metal substrates appropriate for study (via PES or any other surface sensitive spectroscopy). The issues discussed below are based upon information accumulated over two decades of involvement with the surfaces of condensed molecular solids and conjugated polymers in ultra-thin form, represented by the examples in the previous chapter. 

The molar volume in these equations is difficult to assign. This was found to be a problem in the case of a polar liquid. Recently Roe (29) pointed out that, in the case of polymeric liquids, the thickness of the transition layer depends not only on the size of the repeat unit but also on the degree of correlation between successive structural units, or, in other words, on the flexibility of the polymer chain. It is, therefore, not appropriate to use the cube root of the molar volume as a measure of the thickness of the monomolecular layer at the vapor-liquid interface. 

The metal-on-polymer interface has been the most studied Interface as metals can conveniently be deposited by evaporation in situ 1n a controllable fashion in a UHV system (26-33). In the case of polyimide, Cu and Cr have been the most studied metals but other metals including N1, Co, Al, Au, Ag, Ge, Ce, Cs, and Si have been studied. The best experimental arrangement includes a UHV system with a load lock Introduction chamber, a preparation chamber with evaporators, heating capabilities, etc., and a separate analysis chamber. All the chambers are separated by gate valves and the samples are transferred between chambers under vacuum. Alternative metal deposition sources such as organometall1c chemical vapor deposition are promising and such techniques possibly can lead to different interface formation than obtained by metal evaporation(34). 

The problem of adhesion between a polymer and a metal is strongly dependent on the specific type of polymer and metal involved, as well as on the deposition process under which the interface between the two is formed. In order to improve adhesion, different pretreatment methods can be used, but the development of such techniques requires detailed information about metal-polymer interfaces. Particularly, in the case of thin metal films deposited by physical vapor deposition (PVD) in ultra high vaccum (UHV), X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) have been used to obtain chemical information about initial film growth modes,... 

In order make an effort to bring the polyimide-metal adhesion problem to an even more fundamental level, we have previously proposed that model molecules, chosen as representative of selected parts of the polyimide repeat unit, may be used to predict the chemical and electronic structure of interfaces between polyimides and metals (12). Relatively small model molecules can be vapor deposited in situ under UHV conditions to form monolayer films upon atomically clean metal substrates, and detailed information about chemical bonding, charge transfer and molecular orientation can be determined, and even site-specific interactions may be recognized. The result of such studies can also be expected to be relevant in comparison with the results of studies of metal-polymer interfaces. Another very important advantage with this model molecule approach is the possibility to apply a more reliable theoretical analysis to the data, which is very difficult when studying complex polymers such as polyimide. 

Following an overview of this volume, the first section, which consists of three chapters, focuses on methodology and instrumentation. The next three sections consider characterization of vapor—polymer systems (4 chapters), polymer—polymer systems (4 chapters), and surfaces and interfaces (6 chapters). The final two sections cover analytical applications (2 chapters) and the application of IGC in coal characterization and food science (1 chapter each). 

Fig. 11.1 (a) Craitact-mode AFM deflection images of PS in water. The presence of nanobubbles is obsCTved. Occasionally the bubbles are removed by the effect of the tip only a portion of the nanobubble appears in the image white arrows), (b) Schematic representation of a nanobubble in a water/polymer interface. The contact angle 0 is determined by the equilibrium between the horizontal forces in the triple solid-liquid-vapor contact line liquid-vapor 71.v, solid-liquid 75.1, and solid-vapor 75. interfacial tensions. The vertical component of the liquid-vapor interfacial tension, 71. sin(0), is equilibrated by a deformation of the substrate, as described in the text... 

The polymer-metal interface shown in Fig. was derived from an electron micrograph obtained by Mazur and Reich.They electrodeposited silver from a silver ion solution diffusing through a polyimide film. Particles not connected to the diffusion source were removed by computer analysis. The deposited silver particles essentially "decorate the concentration profile and permit the diffusion front to be observed. A 1000-A thin slice was used to aproximate two dimensional diffusion. The fractal dimension of this interface was determined by computer analysis to be approximately 1.7. Similar ramified interface fronts are created by vapor deposition of metal atoms on polymers and by certain ion bombardment treatments of polymer surfaces. The fractal front is fairly insensitive to the details of the concentration profile. However, strong chemical potential gradients in asymmetric interfaces may promote a more planar, less ramified structure. The fractal characteristics of polymer interfaces... 

In Section 1.3 we continue the discussion of Monte Carlo simulations of polymer blends and polymer solutions, but with the emphasis on interfaces that result in the context of phase separation interfaces between coexisting phases in the bulk (liquid-liquid interfaces in a blend, liquid-vapor-type interfaces in a solution) and at solid external walls. It will be shown how all the surface free energies entering Young s formula for the contact angle of droplets can be determined, and how one can estimate the location of wetting transitions. Coarse-grained models are the focus of this section. 

---