Films and Membranes

MIP films are polymerized on flat surfaces. Surfaces derivatized with polymerizable groups are preferred since they allow covalent attachment to the surface. MIP films are attractive as sensing elements in MIP based sensors [112]. 

Molecularly imprinted membranes can be prepared either as thick films or as composites in the pores of base-membranes. In composite membranes, the selective properties of the imprinted material are combined with the properties of the base-membrane. Membranes can also be prepared by phase inversion polymerization. The selective nature of MIPs makes it possible to prepare membranes with selective permeability [113, 114], 

Films and membranes include a formidable array of materials that are widely used in a range of industrial applications. Films find application as coatings and packaging materials such as food wraps, and membranes are used for separations, controlled release, coating and packaging barriers and contact lenses. Structural studies of films fall into two major categories model film studies and the study of commercial films. Model studies are generally. conducted in university research laboratories where the thin, flat structure of a melt cast or drawn film provides an ideal speci- 

If the TEM is used, interpretation always requires care, even for this easier case. Problems of radiation damage of the polymer [61] (Chapter 3) cannot be removed by using thin specimens. 

Model film studies are more appropriate and easier to relate to commercial films in cases where the orientation is low or primarily uniaxial, as biaxial orientation is difficult to mimic on a small scale. Films with large second phase particles also cannot be studied by this method as the true structure cannot be reproduced in a thin film. Nevertheless, many of the basic concepts used in describing microstructure and its development in films, particularly semicrystalline films, have been derived from model film studies. 

Structures in very thin films are clearer because they do not overlap in the transmission image. 

Model cast films are made with no possibility of damage or deformation during specimen preparation by microtomy or fracture. 

Structures in model semicrystalline films may be larger and better defined than in commercial materials because of lower nucleation densities or lower quenching rates. 

STM is much more reliable in producing images with atomic resolution, but only for conducting samples. Bulk polymers must be coated with a thin layer of conducting metal. Even a very thin layer will obscure detail less than 1 nm in size, allowing a quantitative measurement of fibrillar or lamellar profiles, but hiding molecular structures [74]. 

Model studies have helped considerably in understanding blown PE films. The x-ray diffraction patterns from these films could not be interpreted unambiguously and their microstructure was unclear until Keller and Machin [76] produced the same structures in model drawn films. They described the structure and showed how it was formed during crystallization from an oriented melt. This helped in many ways as, for example, the effect of molecular weight distribution on film properties could be understood by its effect on the structure formation. The orienta- 

Membranes can be thought of as special types of films that provide specific end use characteristics. Membrane technology has replaced some conventional techniques for separation, concentration or purification [78]. Applications include desalination, dialysis, blood oxygenators, controlled release drug delivery systems and gas separation. Processing of polymer films and membranes is well known to affect the morphology, which in turn affects the physical and mechanical properties. As is true for all films, membrane separation properties are based on both the chemical composition and the structure resulting from the process. Membranes are produced in two major forms, as flat films and as porous hollow fibers, both of which will be discussed in this section. 

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As previously discussed, solvents that dissolve cellulose by derivatization may be employed for further functionahzation, e.g., esterification. Thus, cellulose has been dissolved in paraformaldehyde/DMSO and esterified, e.g., by acetic, butyric, and phthalic anhydride, as well as by unsaturated methacrylic and maleic anhydride, in the presence of pyridine, or an acetate catalyst. DS values from 0.2 to 2.0 were obtained, being higher, 2.5 for cellulose acetate. H and NMR spectroscopy have indicated that the hydroxyl group of the methy-lol chains are preferably esterified with the anhydrides. Treatment of celliflose with this solvent system, at 90 °C, with methylene diacetate or ethylene diacetate, in the presence of potassium acetate, led to cellulose acetate with a DS of 1.5. Interestingly, the reaction with acetyl chloride or activated acid is less convenient DMAc or DMF can be substituted for DMSO [215-219]. In another set of experiments, polymer with high o -celliflose content was esterified with trimethylacetic anhydride, 1,2,4-benzenetricarboylic anhydride, trimellitic anhydride, phthalic anhydride, and a pyridine catalyst. The esters were isolated after 8h of reaction at 80-100°C, or Ih at room temperature (trimellitic anhydride). These are versatile compounds with interesting elastomeric and thermoplastic properties, and can be cast as films and membranes [220]. 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

Polymer complexes associated with two or more complementary polymers are widely used in potential applications in the form of particles, hydrogels, films, and membranes. In particular, a polyion complex (PIC) can be easily formed when oppositely charged polyelectrolytes are mixed in aqueous solution and interact via... 

V. Krishnan, A.L. Xidis, and V.D. Neff, Prussian blue solid-state films and membranes as potassium ion-selective electrodes. Anal. Chim. Acta 239, 7-12 (1990). 

Royce W. Murray is Kenan Professor of Chemistry at the University of North Carolina at Chapel Hill. He received his B.S. from Birmingham Southern College in 1957 and his Ph.D. from Northwestern University in 1960. His research areas are analytical chemistry and materials science with specialized interests in electrochemical techniques and reactions, chemically derivatized surfaces in electrochemistry and analytical chemistry, electrocatalysis, polymer films and membranes, solid state electrochemistry and transport phenomena, and molecular electronics. He is a member of the National Academy of Sciences. 

Molecular weight markers for electrophoresis and hybridization techniques are widely used. These markers provide information in regard to molecular weights of rearranged bands. These techniques are useful in monitoring patients for relapse or residual disease status. The 32P-labeled and biotinylated DNA molecular weight markers provide visualization on the film and membrane. Hardware systems with band size computation capabilities are available. 

Brack, H.-P, Ruegg, D., Biihrer, H., Slaski, M., Alkan, S. and Scherer, G. G. 2004. Differential scanning calorimetry and thermogravimetric analysis investigation of the thermal properties and degradation of some radiation-grafted films and membranes. Journal of Polymer Science Part B Polymer Physics 42 2612-2624. 

Arcella, V., Ghielmi, A. and Tommasi, G. 2003. High-performance perfluoropoly-mer films and membranes. Annals of the New York Academy of Sciences 984 226-244. 

Visions of various two dimensional surfaces, films and membranes. 

Many polyphosphazenes form strong films and membranes when solutions are spread on a flat surface and the solvent is allowed to evaporate. Some of these films have special properties such as resistance to UV radiation, and are thus of interest as coatings for solar cells, aircraft, or space craft. The polymer [NP(OCH2CF3)2] and related mixed-substituent derivatives have been studied from this viewpoint.1... 

Recently several groups have tried to improve the properties of anisotropic gas separation membranes by chemically modifying the surface selective layer. For example, Langsam at Air Products and Paul et al. at the University of Texas, Austin have treated films and membranes with dilute fluorine gas [66-71], In this treatment fluorine chemically reacts with the polymer structure. By careful... 

Membranes and composites from cellulose and cellulose esters are important domains in the development and application of these polymer materials. The most important segment by volume in the chemical processing of cellulose contains regenerated cellulose fibers, films, and membranes, hi the case of the cellulose esters mainly cellulose nitrate and cellulose acetate as well as novel high-performance materials created therefrom are widely used as laminates, composites, optical/photographic films and membranes, or other separation media, as reviewed in [1], The previously specified nanocelluloses from bacteria and wood tie in with these important potentials and open novel fields of application. 

Spiropyrans show promise for optical recording, three-dimensional optical memories,214 and holography.215 The dyes currently under study for these applications very probably will not be used merely dissolved in a bulk polymer matrix, but will be oriented in films and membranes, or adsorbed or vapor deposited on solid substrates to take advantage of the nonlinear optical properties of the colored forms. For example, thick (0.5 mm) PMMA films of 6-nitro-thiaBIPS can be used to record wavelength-multiplexed volume holograms with an infrared diode laser. This system is impractical at present because of fatigue and poor diffraction efficiencies.216... 

The formation of microvoids (or micropores) in drawn /3-PP is a well-known feature, used successfully to produce breathable films and membranes. Macroscopically their occurrence is correlated with an opacification (i.e. whitening) of the specimen. Their amount can, for example, be measured at given strain rate/draw ratio by liquid porosimetry (e.g. mercury intrusion), gas absorption or by the determination of the density of the deformed sample [129,177,178]. 

Figure 3.63. Examples of fluorophores used in the study of monolayers, LB films and membranes. NBD-DPPC dipalmitoylphosphatidylcholine, labeled with NBD (7-nitrobeno-2-oxa-1,3-diazole) DPHpPC a diphenylhexatriene-labeled phosphatidylcholine.

The different phase behaviours which are realised with representative molecular architectures are shown in Fig. 8.5 (a) to (c). Such mixtures produce solid polymeric structures such as films and membranes with distinctive morphologies. 

The SBA-2 crystal growth is plate-like and excellent for making thin films and membranes with the six-fold axis normal to the sheet direction. As expected for this geometry, the unit-cell parameter c a ratio is about 1.62. After calcination, the large... 

In the past decades, research on porous materials has increased considerably because of their wide-ranging applications (e.g., sensing, gas storage, catalysis, energy transformation and storage, among others). The term porous materials applies to a wide variety of substances, from clay minerals and silicates to metal oxides, metal-organic frameworks, or even thin films and membranes. Porous metals and carbons can also be included under such systems. 

Zeolites are currently manu ctured as micron size crystals and compacted into millimeter size pellets for applications as packed beds. In many catalytic and adsorptive applications, mass and heat transfer properties could potentially be improved by structuring the zeolite in a different way. Research in this area led already to significant achievements. Alternatively structured zeolite matter are e.g. delaminated zeolites [1], supported zeolite films and membranes [2], hybrid structures with microporosity in walls of ordered mesoporous materials [3-6] and nanosized zeolites such as those synthesized in confined space [7]. The common property of these alternative zeolites is that at least in one direction, the zeolite framework has a dimension of around a nanometer. 

An important feature of the mosaic or subunit model of the molecular organization of lipid and lipid-protein films and membranes (4, 5,12) is the cohesion between subunits and between molecules within each subunit (2, 13). The latter can be measured as microviscosity by polarization of fluorescence (14) of the surface film the former is measured as surface viscosity by various techniques (15,16). The relevance of this parameter to the exploration of biological surfaces cannot be overemphasized. Surface viscosity, in fact, has also been invoked as a force stabilizing pulmonary alveoli (6, 7,13, 16). 

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