Permeability polymer film

The conventional PSP consists of sensor probe molecules contained in a transparent oxygen-permeable polymer film matrix (Fig. 3). The oxygen molecules permeate into the polymer film matrix by diffusion. In contrast, PSP consists of sensor molecules directly adsorbed onto a substrate such as an anodic oxidized aluminum plate (Fig. 4). Thus, polymers and probes are important factors for PSP application. 

Many PSPs are composed of probe dyes, such as polycyclic aromatic hydrocarbons (e.g., pyrene) and coordination compounds (e.g., platinum por-phryins and ruthenium(II) polypyridyl complexes) immobilized in various gas permeable polymer films such as silicon polymer, organic glassy polymers (e.g., poly(methylmethacrylate), polystyrene), fluorinated polymers, or cellulose derivatives such as ethyl cellulose [9,10]. As probe molecules interact with polymer matrices directly, the properties of PSPs strongly depend on the properties of polymer matrices. The oxygen permeability of polymer matrix is an especially important factor for highly sensitive PSP. 

In addition to homogenous, permeable polymer films, microporous polymeric films have been also used as membrane materials. Difficulties with their use are the possible production of microemboli due to inadvertent sparging, high water vapor flux and blood damage due to the nature of the microporous material (10). 

Although microporous membranes are a topic of research interest, all current commercial gas separations are based on the fourth type of mechanism shown in Figure 36, namely diffusion through dense polymer films. Gas transport through dense polymer membranes is governed by equation 8 where is the flux of component /,andare the partial pressure of the component i on either side of the membrane, /is the membrane thickness, and is a constant called the membrane permeability, which is a measure of the membrane s ability to permeate gas. The ability of a membrane to separate two gases, i and is the ratio of their permeabilities,a, called the membrane selectivity (eq. 9). 

Many polymer films, eg, polyethylene and polyacrylonitrile, are permeable to carbon tetrachloride vapor (1). Carbon tetrachloride vapor affects the explosion limits of several gaseous mixtures, eg, air-hydrogen and air-methane. The extinctive effect that carbon tetrachloride has on a flame, mainly because of its cooling action, is derived from its high thermal capacity (2). 

The diffusion of oxygen through polymer films has been examined by a number of workers. Guruviah measured the permeability to oxygen of films cast from five paints (Tabic 14.4) and compared the results with the... 

Theoretical aspects of mediation and electrocatalysis by polymer-coated electrodes have most recently been reviewed by Lyons.12 In order for electrochemistry of the solution species (substrate) to occur, it must either diffuse through the polymer film to the underlying electrode, or there must be some mechanism for electron transport across the film (Fig. 20). Depending on the relative rates of these processes, the mediated reaction can occur at the polymer/electrode interface (a), at the poly-mer/solution interface (b), or in a zone within the polymer film (c). The equations governing the reaction depend on its location,12 which is therefore an important issue. Studies of mediation also provide information on the rate and mechanism of electron transport in the film, and on its permeability. 

X10 cm /s. This is over 1,000 X smaller than the Hiffusion coefficient for this osmium complex diffusing freely in the acetonitrile solvent (obtained from the limiting current at the naked Pt electrode), and the observed PD corresponds to a very low permeability of the polymer film to luch bulky permeants. 

We test the permeability of polymer films or sheets to various vapors and gases by mounting the polymer between chambers that contain different concentrations of the migrant molecules. We can determine the permeability from pressure changes, volumetric changes, or by microanalytical techniques that measure the concentration of the migrant molecules in a stream of gas flowing across the low concentration side of the barrier. 

When determining the permeability of films to water vapor, we seal a desiccant into a small cup with the polymer covering the opening. We weigh the cup before placing it in an oven at controlled temperature and humidity. After a given period of time we remove it and weigh it a second time. We calculate the film s water vapor transmission rate based on the area of the cup s mouth and the time that it was in the oven. 

Finally the synthesis of inorganic-polymer composite membranes should be mentioned. Several attempts have been made to combine the high permeability of inorganic membranes with the good selectivity of polymer membranes. Furneaux and Davidson (1987) coated a anodized alumina with polymer films. The permeability increased by a factor of 100, as compared to that in the polymer fiber, but the selectivities were low (H2/O2 = 4). Ansorge (1985) made a supported polymer film and coated this film with a thin silica layer. Surprisingly, the silica layer was found to be selective for the separation mixture He-CH4 with a separation factor of 5 towards CH4. The function of the polymer film is only to increase the permeability. No further data are given. 

Polymer film Treatment time (min) Neutron activation O2 permeability N2 permeability ... 

Thus, the permeability values are high when the solubility parameters of the diffusion molecules are similar to that of the polymer film. 

No amount of sterilization wiU prevent or even slow autooxidation, and there are only two defenses removal of O2 and addition of inhibitors. Oxygen barriers in food packaging are a major topic in the engineering of polymer films. The barrier properties of various polymers are very important in food applications, and many of these are multilayer polymers that have a thin layer of an impermeable polymer (such as polyacrylonitrile and ionic polymers) on a cheaper but O2-permeable polymer such as a polyolefin, which gives mechanical strength to the fikn. 

An early example of an MIP-QCM sensor was a glucose monitoring system by Malitesta et al. (1999). A glucose imprinted poly(o-phenylenediamine) polymer was electrosynthesized on the sensor surface. This QCM sensor showed selectivity for glucose over other compounds such as ascorbic acid, paracetamol, cysteine, and fructose at physiologically relevant millimolar concentrations. A unique QCM sensor for detection of yeast was reported by Dickert and coworkers (Dickert et al. 2001 Dickert and Hayden 2002). Yeast cells were imprinted in a sol-gel matrix on the surface of the transducer. The MIP-coated sensor was able to measure yeast cell concentrations in situ and in complex media. A QCM sensor coated with a thin permeable MIP film was developed for the determination of L-menthol in the liquid phase (Percival et al. 2001). The MIP-QCM sensor displayed good selectivity and good sensitivity with a detection limit of 200 ppb (Fig. 15.7). The sensor also displayed excellent enantioselectivity and was able to easily differentiate the l- and D-enantiomers of menthol. 

The large pores (ranging from 0.01 pm to 0.1 pm) are sealed by the continuous polymer film formed in the comatrix of LMM and LMC. Consequently, they show reduced water absorption, water permeability and water vapor transmission over ordinary cement mortar and concrete and this effect increases with an increase in polymer content and polymer-cement ratio (Fig. 6.15). The improved water permeability also improves the resistance to chloride ion entry and hence corrosion mitigation [87]. 

The concept of permeability. Pm, described first in Section 4.3.2.2 also applies to membranes. Equation (4.77) relates the permeability to the diffusion coefficient and solubility. Some representative values of permeabilities for common gases in common polymer films are given in Table 4.17. The units of permeability in Table 4.17 are obtained when diffusivity is in units of m /s, and gas solubility is in units of m gas m /(m soUd-N). Note that carbon dioxide permeabilities are generally 3-4 times... 

The diffusion and the permeability are inversely related to the density, degree of crystallinity, orientation, filler concentration, and crosslink density of a polymeric film. As a general rule, the presence of plasticisers or residual solvents increases the rate of diffusion in polymers. Films cast from poor solvents have high permeability. The rate of diffusion or permeability is independent of the molecular weight of the polymer, providing the polymer has a moderately high molecular weight. 

Gas permeabilities for 02, N2 and CO2 were realised with Lyssy GPM 20 apparatus. Surface tensions of modified and unmodified polymers films were derived from contact angle measurements which were evaluated with six liquids (H20, HCONH2, Hg, CH2I2, tricresyl-phosphate and 1 -bromonaphtalene). 

When a more permeable polymer is blended with PVC, the permeability coefficient is expected to increase. The magnitude of the increase, however, will depend on the morphology of the blend. A laminar composite film containing 10% EVA would give an increase in permeability of about 10% compared with a pure PVC film (15, 18). The reported measurements for PVC with 10% EVA give an increase of about 400% (Figures 1, 2, and 3). 

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