Poly , size effects diffusion

Table n. Size Effects on Diffusion in Poly(Methyl Methacrylate)... 

Interpretation of pubhshed data is often comphcated by the fact that rather complex catalytic materials are utilized, namely, poly disperse nonuniform metal particles, highly porous supports, etc., where various secondary effects may influence or even submerge PSEs. These include mass transport and discrete particle distribution effects in porous layers, as confirmed by Gloaguen, Antoine, and co-workers [Gloaguen et al., 1994, 1998 Antoine et al., 1998], and diffusion-readsorption effects, as shown by Jusys and co-workers for the MOR and by Chen and Kucemak for the ORR [Jusys et al., 2003 Chen and Kucemak, 2004a, b]. Novel approaches to the design of ordered nanoparticle arrays where nanoparticle size and interparticle distances can be varied independently are expected to shed hght on PSEs in complex multistep multielectron processes such as the MOR and the ORR. 

The release from microcap systems is more complicated. First the drug has to dissolve in dissolution fluid which has diffused into the tablets via pores and then between the plates of poly (DL-lactic acid) forming the walls of the microcapsules. This drug solution then has to diffuse out of the tablet via the same route. The effect of compression on the release has more significance in the simple matrix tablets than the microcap systems, because of the above mechanism of release. Higher compressions reduce the size of the pores between the poly(DL-lactic acid) plates, which extends the release. 

Many computational studies of the permeation of small gas molecules through polymers have appeared, which were designed to analyze, on an atomic scale, diffusion mechanisms or to calculate the diffusion coefficient and the solubility parameters. Most of these studies have dealt with flexible polymer chains of relatively simple structure such as polyethylene, polypropylene, and poly-(isobutylene) [49,50,51,52,53], There are, however, a few reports on polymers consisting of stiff chains. For example, Mooney and MacElroy [54] studied the diffusion of small molecules in semicrystalline aromatic polymers and Cuthbert et al. [55] have calculated the Henry s law constant for a number of small molecules in polystyrene and studied the effect of box size on the calculated Henry s law constants. Most of these reports are limited to the calculation of solubility coefficients at a single temperature and in the zero-pressure limit. However, there are few reports on the calculation of solubilities at higher pressures, for example the reports by de Pablo et al. [56] on the calculation of solubilities of alkanes in polyethylene, by Abu-Shargh [53] on the calculation of solubility of propene in polypropylene, and by Lim et al. [47] on the sorption of methane and carbon dioxide in amorphous polyetherimide. In the former two cases, the authors have used Gibbs ensemble Monte Carlo method [41,57] to do the calculations, and in the latter case, the authors have used an equation-of-state method to describe the gas phase. 

Poncet et al. (1999) monitored frequency-dependent dielectric measurements to examine the phase-separation process in poly(2,6-dimethyl-1,4-phenylene ether) (PPE) in a DGEBA-MCDEA resin. Dielectric measurements measured the build up in Tg both in the PPE-rich continuous phase and in the epoxy-rich occluded phases for 30-60-wt.% PPE mixtures. In the 30% PPE mixmre, the rate of reaction of the thermoset phase is equivalent to that of the neat system due to two opposing effects, namely a slower reaction rate due to dilution and a low level of conversion at vitrification due to the presence of high-Tg PPE. In the 60-wt.% mixture the dilution effect of the PPE has a large effect of decreasing the reaction rate. The continuous thermoplastic-rich phase vitrifies first, followed by the thermoset occluded phase. The final morphology (size of occluded particles and composition of continuous phase) is affected by kinetics, diffusion and viscosity during phase separation. 

Considering the complexity of the sorption behavior of the bulk resin, the almost classical Fickian behavior of the composites comes as a surprise and represents a welcome simplification. At the present time the only explanation that can be made is by analogy with the size dependence of relaxation controlled diffusion effects reported by Berens (13) for diffusion of vinyl chloride in poly(vinylchloride). It was noted that as the particle size and, therefore, the path length diminished, the contribution of the... 

Figure 6. Effect of penetrant size, expresssed as van der Waals volume, on diffusion coefficient in poly(vinyl chloride) at 30 °C. (Reproduced with permission from Reference 20.)...

Core/wall ratio cannot only decide the wall thickness of a microcapsule but also the effectiveness of the microencapsulation." The work" on the microencapsulation of xylitol by poly (urethane-urea), which was performed in a water-in-oil system, indicates that the most proper core/wall ratio is 77/23 in terms of high encapsulation yield and xylitol loading content. Another work on the microencapsulation of perfume by polyurea, which was performed in a oil-in-water system, found that with the decrease of the core/wall ratio, the size of the formed microcapsule increased even the original droplet size is roughly the same. An outward diffusion mechanism was proposed to explain... 

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