Transport of plasticizing penetrants

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... 

Transport in hydrocarbon polymers exposed to organic v mrs or hydrophilic po rmers expoUd to water vapor ideally is characterized by diffusion coefficients that increase exponentially widi die concentradon of plasticizing penetrant. " Such a relationship is represented graphically in Fig. 20.3-6a. 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,... 

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 ... 

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). 

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. 

The dissolution of a polymer in a penetrant involves two transport processes, namely penetration of the solvent into the polymer, followed by disentanglement of the macromolecular chains. When an uncrosslinked, amorphous, glassy polymer is in contact with a thermodynamically compatible liquid (solvent), the latter diffuses into the polymer. A gel-like layer is formed adjacent to the solvent-polymer interface due to plasticization of the polymer by the solvent. After an induction time, the polymer is dissolved. A schematic diagram of solvent diffusion and polymer dissolution is shown in Fig. 1. However, there also exist cases where a polymer cracks when placed in a solvent. 

OJ The worldwide consumption of plastics by market in 1992. Packaging is the dominant market, taking approximately one-third of aU plastic produced. The hi-tech areas penetrated by the engineering plastics are mainly in mechanical engineering, transport (aerospace, autonrative) and electrical engineering. 

The local solubility and diffusivity must be considered to see when true transport plasticization occurs. For this it is more usefiil to consider the local mutual diffusion coefficient Dap(C) rather than the average value assessed from equation 30. The local mutual diffusion coefficient is a measure of the ability of a penetrant to move through the membrane at a point where the local conditions are well defined in terms of the local penetrant concentration Ca, volume fraction weight fraction cop, or penetrant fugacity, whichever is most convenient. 

Although extensive data for mixed-gas permeation in the plasticization regime are not available, good pure-gas values are available and illustrate some important points. The permeability of CO2 in various substituted polycarbonates shown in Figure 28 (111) increases with pressure above a certain characteristic value for each of the polymers. The shapes of the permeability curves are like the curve shown schematically in Figure 21. Plasticization occurs despite the fact that the downstream pressure is maintained at a vacuum of less than 1330 Pa (10 mm Hg). To determine when true transport plasticization occurs, the local diffusion coefficient Dap(Ca), which measures the ability of a penetrant to move through the membrane at a point where the local concentration of penetrant is equal to Ca, must be considered. The sorption isotherms for CO2 in the various materials are shown in Figure 29 (111), and application of the standard... 

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