Barrier Property Analysis

When measuring the barrier properties of polymers we are generally interested in liquid or vapor permeability depending on the specific application. 

We measure the permeation rate of liquids through bottles by filling them with the liquid of interest and placing them in a controlled atmosphere chamber. At intervals we remove the bottles, weigh them and return them to the chamber. We repeat this procedure over a period of days, or even weeks, until their rate of weight loss reaches a steady value. We calculate the permeability factor from Eq, 8.10, 

Pj = permeability factor R mass loss per day T = average wall thickness of bottle A = surface area of bottle 

We test the permeability of polymer films or sheets to various vapors and gases by mounting the polymer between chambers that contain different concentrations of the migrant molecules. We can determine the permeability from pressure changes, volumetric changes, or by microanalytical techniques that measure the concentration of the migrant molecules in a stream of gas flowing across the low concentration side of the barrier. 

When determining the permeability of films to water vapor, we seal a desiccant into a small cup with the polymer covering the opening. We weigh the cup before placing it in an oven at controlled temperature and humidity. After a given period of time we remove it and weigh it a second time. We calculate the film s water vapor transmission rate based on the area of the cup s mouth and the time that it was in the oven. 

---

There are several difficulties in the application of this technique to the analysis of sodium barrier properties of these polyimide films. First, as we have seen above, large shifts in the surface potential characteristics of MPOS structures can be associated with electronic conduction in the polyimide and charging of the polyimide-oxide interface. These shifts are not readily separable from any that might be caused by the inward drift of sodium ions. Second, the effect of the electronic charging process is to buck out the electric field in the polyimide which is needed to drive the ion drift mechanism. As seen in Figure 6, the electric field is reduced to very small values in a matter of minutes or less, particularly at the higher temperatures where ion drift would normally be measured. 

Bowser, P.A. and White, R.J. Isolation, barrier properties and lipid analysis of stratum compactum a discrete region of the stratum corneum, Br. J. Derm., 112, 1-14, 1985. 

The dynamic behavior of the repository depends on the physical and chemical properties of the system, their change with time, the interactions between the waste and the geologic medium, and the processes involved in loss of barrier stability. Analysis of these phenomena requires models and data such as are discussed in this symposium. 

Occludin phosphorylation may provide a molecular mechanism to control barrier properties. Studies from our group have demonstrated that both VEGF and shear stress induce permeability across endothelial monolayers associated with a rapid phosphorylation of occludin (67,68). The occludin phosphorylation was attenuated by a non-hydrolyzable cAMP analog that also inhibits shear-induced permeability (68). This phosphorylation of occludin appears to be serine or threonine directed since immunoprecipitation of occludin and phosphotyrosine blotting did not reveal any evidence of occludin tyrosine phosphorylation in this cell system (unpublished observation). However, in epithelial cells, evidence of occludin tyrosine phosphorylation exists (69). In addition, others have identified occludin phosphorylation in response to histamine (70) and use of brain extracts has helped identify casein kinase II as an occludin kinase (71). Collectively, this work demonstrates a close association of occludin phosphorylation with permeability. Future studies identifying specific occludin phosphorylation sites, followed by mutational analysis, should reveal the functional significance of occludin phosphorylation. 

Silver nanoparticles were incorporated into PIA-based films which were obtained by casting from a CF solution. To accelerate the release of silver, composites were prepared by adding pure or surfactant-modified cellulose nanoctystals. The TEM images showed that silver and cellulose particles were distributed homogenously, results which were confirmed by the improvement of barrier properties to o g gen and water. However, the migration from the films and the release of silver increased with the presence of cellulose, especially when cellulose was modified with a surfactant. A potential interaction between the surfactant and PLA that resulted in degradation of the polymeric matrix might be the cause of such an effect. This report did not include the antimicrobial activity analysis of the prepared films. ... 

BenBettaieb et al. [188] displayed the effect of electron beam accelerator doses (0, 20, 40 and 60 kGy) on structural, mechanical and barrier properties of edible lightly plasticized chitosan-fish gelatin blend film. UV-vis analysis showed that all films displayed an absorbance peak between 280 and 385 nm. These peaks are shifted toward higher wavelengths after irradiation. This clearly showed some modifications in the interactions (hydrogen bonds, amide groups) between polymer chains induced by the irradiation. 

Garbassi and co-workers [15] have the reviewed the subject of polymer surface analysis including surface characterisation techniques using a wide variety of spectroscopies and measurement of contact angles and surface force. Also, studies on surfaces and applications of surface science, including wettability, adhesion, barrier properties, biocompatibility, reduction of friction, and wear resistance, were carried out. 

But we aU know that chemical compatibility plays a significant role in diffusion and barrier properties. Although the above analysis is correct about diffusion constants, diffusion requires a concentration gradient to drive it. If the chemical and PLA are not compatible, then the concentration of the chemical in the surface layer of the polymer will be low and so will be the rate of diffusion. So, solubility (and therefore solubility parameters) is very important. For the packaging industry in particular, the permeabiUty is the most used measure and this neatly combines both issues because permeability = diffiisivity x solubility in the large number of cases where Fickian diffusion is the governing phenomenon. 

---