Plasticizing penetrants, transport

Transport In hydrocarbon polymers exposed to organic vapors or hydrophilic polymers exposed to water vapor typically is characterized by diffusion coefficients that increase exponentially with the concentration of plasticizing penetreat.10 13 Such a relationship is represented graphically in Fig. 20.3-6g. At relatively low concentrations of a plasticizing penetrant, the linear relationship between D(C) and C shown in Fig. 

The kinetics of transport depends on the nature and concentration of the penetrant and on whether the plastic is in the glassy or rubbery state. The simplest situation is found when the penetrant is a gas and the polymer is above its glass transition. Under these conditions Fick s law, with a concentration independent diffusion coefficient, D, and Henry s law are obeyed. Differences in concentration, C, are related to the flux of matter passing through the unit area in unit time, Jx, and to the concentration gradient by,... 

At such extraordinarily low penetrant concentrations, plasticization of the overall matrix is certainly not anticipated. Motions involving relatively few repeat units are believed to give rise to most short term glassy state properties. In rubbery polymers, on the other hand, longer chain concerted motions occur over relatively short time scales, and one expects plasticization to be easier to induce in these materials. Interestingly, no known transport studies in rubbers have indicated plasticization at the low sorption levels noted above for PVC and PET. 

In an attempt to justify the assumption of plasticization put forth in their interpretation of 3 in Eq (A-2), Raucher and Sefcik compare transport data and NMR data for the C02/pvC system This comparison has several questionable aspects To relate local molecular chain motions to the diffusion coefficient of a penetrant, one should use the so-called local effective coefficient, Deff O such as shown in Figure 5 rather than an average or "apparent" diffusion coefficient as was employed by these authors Deff(C) describes the effects of the local sorbed concentration on the ability of the average penetrant to respond to a concentration or chemical potential gradient in that region ... 

Large amounts of nitrosamines leak into the environment from the pharmaceutical and food industries, plastics industry, textile industry, waste transport (motor vehicles), industrial effluents (dyes, lubricants, mbber), and the production of solvents. Fuel manufacturing plants and oil refineries are also important emitters of nitrosamines, as well as landfills and fossil fuel combustion processes (to produce heat and power). These compounds naturally penetrate the environment through animal droppings. 

Many food processes, which affect food quality and stability, are diffusion controlled (Karel et al., 1994 Roos, 1995). Transport of key penetrants such as water into or out of a polymeric food matrix can play a critical role in food quality and stability. Water is one of the major components and a very good plasticizer in foods. The quality and stability of dehydrated products, multi-domain foods, and the performance of biofilms and encapsulation and controlled release technologies are affected by moisture transport. The rates of molecular mobility and diffusion-limited reactions strongly depend on the factors surrounding the food. Temperature and water activity (fl ) pl y significant roles in penetrant diffusion. The physical state of the carrier matrix, chemistry, size, and structure of diffusing molecule and specific... 

Other areas of technology where the transport of small molecules through polymers plays a key role include foams (where small molecules are used as blowing agents for foam expansion [9-11] and any gas trapped in the cells of a closed-cell foam affects key properties such as the thermal conductivity [12]), plasticization [13,14], removal of process solvents, residual monomers or other impurities by techniques such as supercritical fluid extraction [15,16], biosensors, drug implants, and polymer electrolytes (where the penetrants are ionic). 

Barrier plastics will be used as examples in this paper. Barrier and selectivity are two sides of the same coin. Both properties are determined by the same types of transport phenomena. [3] Similar techniques can, therefore, also be applied to study separation membranes. The same approximations are valid if the mixture flowing through the membrane is sufficiently dilute that (i) it does not significantly affect the structure and properties of the membrane, and (ii) the components of the mixture can be treated as independent penetrants. The same general approaches can also be applied to concentrated mixtures, but only provided that certain simplifying approximations are not made. 

Figure 1. Schematic illustration of how the results of three different types of calculations, each one providing a perspective at a different scale, can be combined synergistically, to construct a unified physical model for the transport of penetrant molecules in plastics.

A study is in progress on the transport of penetrant molecules in barrier plastics, utilizing a synergistic combination of techniques. 

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