Polymer interaction with organic vapors

Microwave or radio frequencies above 1 MHz that are appHed to a gas under low pressure produce high energy electrons, which can interact with organic substrates in the vapor and soHd state to produce a wide variety of reactive intermediate species cations, anions, excited states, radicals, and ion radicals. These intermediates can combine or react with other substrates to form cross-linked polymer surfaces and cross-linked coatings or films (22,23,29). 

Equations 1-7 were developed for gas permeation through rubbery polymers. They must be used with caution with glassy polymers and/or organic vapors that interact strongly with the barrier. 

The diffusion of larger organic vapor molecules is related to absorption. The rate of diffusion is dependent on the size and shape of the diffusate molecules, their interaction with the polymer molecules, and the size, shape, and stiffness of the polymer chains. The rate of diffusion is directly related to the polymer chain flexibility and inversely related to the size of the diffusate molecules. 

Organic polymers comprise the most common type of coating used with AW sensors due to their capability to reversibly sorb vapors and liquids. For those polymers whose interactions with AWs can be treated as perfectly elastic, the fact that A is invariably larger than ft means that the value of the term (A + fi)/( + 2ft) is constrained between 0.67 and 1 thus, this ratio can be approximated using a value of 0.84. The magnitude of a purely elastic perturbation is then proportional to the product of shear modulus and thickness, with no more than a 16% error. 

No deposition of materials occurs in most cases however, the deposition of plasma polymer could occur depending on the nature of substrate polymer. Such a deposition of materials can be viewed as PP of organic vapors, which emanated from the substrate, by the interaction with plasma. Because the major player is the luminous gas phase, the surface treatment is included in this book under the term luminous chemical vapor treatment (LCVT). 

Thermoplastics, in contact with organic liquids or vapors, will fail at lower stress or strain even if the interacting chemical is not ordinarily considered to be a solvent for the polymer. The effect of these chemicals is believed to be because of localized plasticization that reduces the effective Tg, and thus increases the localized mobility of polymer chains and promotes craze and crack development. 

The transport of caibon dioxide in polymers has historically been analyzed in the same manner as other simple gases (1). Recent studies have shown, however, that the effects of CX>2 on polymers include scrnie features commonly associated with organic solvents, including swelling (2-5). and depression of glass transition temperatures, i.e., plasticization (6-8). Moreover, CO2 can be handled as a liquid at room temperature under ratho moderate pressures its critical temperature is 31 0 and its saturated vapor pressure at 25" C is 64.6 atm (950 psi). For these reasons it seems appropriate to consider near-critical CX>2 as a highly volatile solvent, rather than as a gas, in its interactions with polymers. 

Sorption and diffusion in polymers are of fundamental and practical concern. However, data acquisition by conventional methods is difficult and time consuming. Again, IGC represents an attractive alternative. Shiyao and co-workers, concerned with pervaporation processes, use IGC to study adsorption phenomena of single gases and binary mixtures of organic vapors on cellulosic and polyethersulfone membrane materials (13). Their work also notes certain limitations to IGC, which currently restrict its breadth of application. Notable is the upper limit to gas inlet pressure, currently in the vicinity of 100 kPa. Raising this limit would be beneficial to the pertinent use of IGC as an indicator of membrane-vapor interactions under conditions realistic for membrane separation processes. 

Studies of the diffusion of benzene in natural rubber represent some of the earliest detailed examinations of the interaction of an organic solvent with a polymer. Hayes and Park carried out measurements at low concentrations by the vapor sorption method (1), and at higher concentrations by determining the concentration distribution using an interferometric method (2). Complementary measurements by vapor transmission to determine the diffusion coefficient from time-lag data were carried out at low concentrations by Barrer and Fergusson (3). The main results of these studies have been summarized in Fujita s review (4) of organic vapor diffusion in polymers above the glass transition temperature. However, the problems with these measurements were not referenced. 

SAW device, when coated with polymer coating behaves as a SAW sensor. SAW devices as a vapor sensor are attractive because of their small size (< 0.1 cm ), ruggedness, low cost, electronic output, sensitivity and adaptability to a wide variety of vapor phase analytical problems. The presence of thin organic film on the SAW device surface can potentially cause attenuation of the wave through interaction with both longitudinal and the vertical shear components. Attenuation increases with the length of the SAW device and also with the inverse of the wavelength... 

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