Mass transport barrier

The intent of this chapter is to establish a comprehensive framework in which the physicochemical properties of permeant molecules, hydrodynamic factors, and mass transport barrier properties of the transcellular and paracellular routes comprising the cell monolayer and the microporous filter support are quantitatively and mechanistically interrelated. We specifically define and quantify the biophysical properties of the paracellular route with the aid of selective hydrophilic permeants that vary in molecular size and charge (neutral, cationic, anionic, and zwitterionic). Further, the quantitative interrelationships of pH, pKa, partition... 

In Section III, emphasis was placed on flux kinetics across the cultured monolayer-filter support system where the passage of hydrophilic molecular species differing in molecular size and charge by the paracellular route was transmonolayer-controlled. In this situation, the mass transport barriers of the ABLs on the donor and receiver sides of the Transwell inserts were inconsequential, as evidenced by the lack of stirring effects on the flux kinetics. In this present section, the objective is to give quantitative insights into the permeability of the ABL as a function of hydrodynamic conditions imposed by stirring. The objective is accomplished with selected corticosteroid permeants which have been useful in rat intestinal absorption studies to demonstrate the interplay of membrane and ABL diffusional kinetics (Ho et al., 1977 Komiya et al., 1980). 

The whole-cell biocatalysis approach is typically used when a specific biotransformation requires multiple enzymes or when it is difficult to isolate the enzyme. A whole-cell system has an advantage over isolated enzymes in that it is not necessary to recycle the cofactors (nonprotein components involved in enzyme catalysis). In addition, it can carry out selective synthesis using cheap and abundant raw materials such as cornstarches. However, whole-cell systems require expensive equipment and tedious work-up because of large volumes, and have low productivity. More importantly, uncontrolled metabolic processes may result in undesirable side reactions during cell growth. The accumulation of these undesirable products as well as desirable products may be toxic to the cell, and these products can be difficult to separate from the rest of the cell culture. Another drawback to whole-cell systems is that the cell membrane may act as a mass transport barrier between the substrates and the enzymes. 

Further, incorporation of nanoclays tends to increase the T of PLA to 275 and 260"C respectively for PLA/C20A and PLA/C93A nanocomposites. On the other hand, nanocomposite prepared using C93A and C30B nanoclays exhibited T. of 306 and 307 C respectively. The nanoclays act as a heat barrier over matrix macromolecules, which enhances the thermal stability in the layered silicate nanocomposites as well as assists in the formation of char after thermal decomposition. PLA/OMMT nanocomposite showed the optimimi T. of 320 C, which confirms the improved capacity of exfoliated silicate layers as a superior heat insulator and mass transport barrier to the volatile products generated during the thermal decomposition. 

The thermal stability of some thermosetting nanocomposites obtained by thermal cationic cure of diglycidilether of bisphenol A and y-valerolactone using rare earth metal triflates (trifluoromethanesulfonate) as initiators and containing different types of Cloisite was studied. The addition of clay into the polymeric matrix was found to increase the thermal stability, acting as a superior insulator and mass transport barrier to the volatile products evolved during thermal decomposition [67]. 

The comprehensive flame retardation of polymer-clay nanocomposite materials was reported by Dr. Jeff Gilman and others at NIST [7]. They disclosed that both delaminated and intercalated nanoclays improve the flammability properties of polymer-layered silicate (clay) nanocomposites. In the study of the flame retardant effect of the nanodispersed clays, XRD and TEM analysis identified a nanoreinforced protective silicate/carbon-like high-performance char from the combustion residue that provided a physical mechanism of flammability control. The report also disclosed that The nanocomposite structure of the char appears to enhance the performance of the char layer. This char may act as an insulation and mass transport barrier showing the escape of the volatile products generated as the polymer decomposes. Cone calorimetry was used to study the flame retardation. The HRRs (heat release rates) of thermoplastic and thermoset polymer-layered silicate nanocomposites are reduced by 40% to 60% in delaminated or intercalated nanocomposites containing a silicate mass fraction of only 2% to 6%. On the basis of their expertise and experience in plastic flammability, they concluded that polymer-clay nanocomposites are very promising new flame-retarding polymers. In addition, they predict that the addition... 

This product acts as a separator or mass transport barrier between the cathode and the anode to limit electrochemical self-discharge. If the integrity of this separator is breached, the battery can experience a thermal runaway condition, whereby the active electrochemical components are chemically consumed with accompanying generation of large amounts of excess heat. At the same time, if battery conditions are such that alloy formation exceeds usage, the excess alloy can cause periodic shorting, the alloy noise sometimes seen in cold-stored batteries. 

Chrisaffis et al. evaluate the decomposition mechanism by comparing experimental data with a theoretical model, with a principal assumption that the process is describ-able by a single mechanism. Due to the higher thermal stability of the nanocomposites, an increase in activation energy for both sPS/MWCNT and sPS/ND is observed. A discussion of the nanoparticle rich areas as mass transport barriers, as well as local... 

The thermal stability of a material is characterized mainly by thermogravi-metric analysis (TGA), where the sample mass loss due to volatilization of degraded by-products is monitored as a function of temperature. Usually, polymer-LDH nanocomposites have enhanced thermal stability compared with virgin polymers and conventional composites because the well-dispersed LDH layers can act as a superior thermal insulator and mass transport barrier to the volatile products generated during decomposition. 

Figure 12.8 shows the TGA results for PS-DCTBAB-MMT and PS-PCDBAB-MMT nanocomposites and Figure 12.9 shows those for PS-co-BA-PCDBAB-MMT and PS-co-BA-DCTBAB-MMT. Only a slight improvement in the thermal stability of PCNs was observed above 50% degradation, relative to the neat polystyrene (see Figure 12.8). Jan et al. also reported that epoxy-clay nanocomposites only showed enhanced thermal stability from 40 to 50% weight degradation. The thermal stability of PS-CNs was also found to increase slightly when the clay loading increased. This has been a characteristic feature of different PCNs, irrespective of their preparation route.The formation of clay char, which acts as a mass transport barrier and insulator between the polymer and the superficial zone where the polymer decomposition takes place, is the cause of the improvements in the thermal stability of Concurrently, the restricted thermal motion...

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