Surfaces polymer films

When the spectral characteristics of the source itself are of primary interest, dispersive or ftir spectrometers are readily adapted to emission spectroscopy. Commercial instmments usually have a port that can accept an input beam without disturbing the usual source optics. Infrared emission spectroscopy at ambient or only moderately elevated temperatures has the advantage that no sample preparation is necessary. It is particularly appHcable to opaque and highly scattering samples, anodized and painted surfaces, polymer films, and atmospheric species (135). The interferometric... 

Most SECM measurements are carried out with the sample under a thick liquid layer, and thus the tip must be sheathed in an insulator to achieve high resolution. SECM measurements can also be carried out within a thin layer of water that forms on the surface of a sample in air. In this case very high resolution can be attained using tips without insulation (e.g., the usual W or Pt-Ir STM-type tips) because the tip area is defined by the small part of the tip that touches the liquid layer (34,35). Studies of mica surfaces, polymer films, and some biological samples as described below are possible by this technique. With this mode it is also possible to fabricate small metal structures in polymer films as demonstrated previously (36). High-resolution electrochemical deposition of silver nanostructures on mica surfaces in humid air was also achieved (35). For detailed discussion on SECM applications for fabrication, see Chapter 13. 

This approach has been used in order to describe the electron propagation through surface polymer films [19-23]. In these models it was assumed that transport occurs as a sequence of successive steps between adjacent redox centers of different oxidation states. The electron hopping has been described as a bimolecular process in the direction of the concentration gradient. 

In the course of electrochemical oxidation or reduction of the surface polymer films or membranes, the overall electroneutrality of the polymer phase is retained by ion-exchange processes between the polymer film and the bulk electrolyte solution [3, 72]. Not only has ion transport to be considered, but solvent and other neutral molecules may also enter or leave the film during the charging/discharging processes [73-78]. In order to maintain electroneutrality in the simplest case either counterions enter the film or coions leave it. The relative contributions of the ions carrying different charges to the overall charge transport may depend on their physical properties (e.g., size) and/or on their chemical nature (e.g., specific interactions with the polymer), as well as on other parameters (e.g., potential) [1-4, 73-92]. 

In situ conductivity measurements [52, 53, 164, 189, 195-202], luminescence techniques [203], different spectroscopies in combination with electrochemical methods [35-39, 82, 114, 115, 153-156, 204-208], and surface plasmon resonance [209] have also supplied valuable information about the ionic and electronic charge-transfer and charge transport processes. Microscopies have given new insights into the structure and morphology of surface polymer films, as well as their changes with the experimental conditions [151-159, 210-218]. The successful... 

One interesting new field in the area of optical spectroscopy is near-field scaiming optical microscopy, a teclmique that allows for the imaging of surfaces down to sub-micron resolution and for the detection and characterization of single molecules [, M]- Wlien applied to the study of surfaces, this approach is capable of identifying individual adsorbates, as in the case of oxazine molecules dispersed on a polymer film, illustrated in figure Bl.22,11 [82], Absorption and emission spectra of individual molecules can be obtamed with this teclmique as well, and time-dependent measurements can be used to follow the dynamics of surface processes. 

The cleaning or depassivation eflect is of great importance in sonoelectrochemistry, as it can be employed to wash off surface-adsorbed species and reduce blocking of the electrode by adsorption of reaction products. This eflect has been reported, for example, for the depassivation of iron electrodes and for the removal of deposits and in the presence of polymer films on the electrode surface. However, damage of the electrode surface, especially for materials of low hardness such as lead or copper, can also occur under harsh experimental conditions and applied intensities [70, Tf, 80]. 

Figure 5.7 Sketch of an interfacial polymerization with the collapsed polymer film being withdrawn from the surface between the immiscible phases. [Redrawn with permission from P. W, Morgan and S. L. Kwolek, J. Chem. Educ. 36 182 (1959) copyright by the American Chemical Society.]...

Fig. 1. The hthographic process. A substrate is coated with a photosensitive polymer film called a resist. A mask with transparent and opaque areas directs radiation to preselected regions of the resist film. Depending on resist characteristics, exposed or unexposed portions of the film are removed using a developer solvent. The resulting pattern is then transferred to the substrate surface and the resist is stripped.

In 1954 the surface fluorination of polyethylene sheets by using a soHd CO2 cooled heat sink was patented (44). Later patents covered the fluorination of PVC (45) and polyethylene bottles (46). Studies of surface fluorination of polymer films have been reported (47). The fluorination of polyethylene powder was described (48) as a fiery intense reaction, which was finally controlled by dilution with an inert gas at reduced pressures. Direct fluorination of polymers was achieved in 1970 (8,49). More recently, surface fluorinations of poly(vinyl fluoride), polycarbonates, polystyrene, and poly(methyl methacrylate), and the surface fluorination of containers have been described (50,51). Partially fluorinated poly(ethylene terephthalate) and polyamides such as nylon have excellent soil release properties as well as high wettabiUty (52,53). The most advanced direct fluorination technology in the area of single-compound synthesis and synthesis of high performance fluids is currently practiced by 3M Co. of St. Paul, Minnesota, and by Exfluor Research Corp. of Austin, Texas. 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Surface properties are generally considered to be controlled by the outermost 0.5—1.0 nm at a polymer film (344). A logical solution, therefore, is to use self-assembled monolayers (SAMs) as model polymer surfaces. To understand fully the breadth of surface interactions, a portfoHo of chemical functionahties is needed. SAMs are especially suited for the studies of interfacial phenomena owing to the fine control of surface functional group concentration. 

Macroscopically, the solvent and precipitant are no longer discontinuous at the polymer surface, but diffuse through it. The polymer film is a continuum with a surface rich in precipitant and poor in solvent. Microscopically, as the precipitant concentration increases, the polymer solution separates into two interspersed Hquid phases one rich in polymer and the other poor. The polymer concentration must be high enough to allow a continuous polymer-rich phase but not so high as to preclude a continuous polymer-poor phase. 

---