Crystalline phase impermeability

In another model, Harland and Peppas [159] considered the diffusion of solutes through semicrystalline hydrogel membranes. These types of membranes were assumed to consist of a crosslinked, swollen (amorphous) phase through which solute diffusion occurred and an impermeable, crystalline phase. A simplified form of the model assumes uniform amorphous regions. With this assumption, the diffusion coefficient through a semi-crystalline membrane, Dc, was written as... 

In order to inhibit the oxidation of polymers, the antioxidant has to be present in sufficient concentration at the various oxidation sites. In this respect, both the distribution of antioxidants and the morphology of the host polymer assume greater significance. Examination of the distribution of photo-antioxidants in typical commercial semi-crystalline polymers, such as polyolefins, has shown " " " that they are rejected into the amorphous region on the boundaries of spherulites. Such nonuniform distribution of antioxidants leads to an increase in their concentration in the amorphous region, which is most susceptible to oxidation (the crystalline phase is normally impermeable to oxygen). However, in the case of polymer blends, a nonuniform distribution of antioxidants can undermine the overall stability of the blend, especially when the more oxidizable component of the polymer blend is left unprotected. 

In the lamellar crystals of semicrystalline materials and the extended chain structure of oriented polymers, chain packing is usually much more efficient than in the amorphous, isotropic state. The efficiency of chain packing in the crystalline phase reduces the free volume available for transport to such an extent that, as a first approximation, the crystalline phase may be regarded as impermeable relative to the amorphous phase. 

Transport in semi-crystalline polymers can, thus, be treated on the basis of a simple two phase model, consisting of a dispersed impermeable crystalline phase in a permeable amorphous matrix. Lasoski and Cobbs (12) attempted to correlate the solubility of water vapor in unoriented poly(ethylene terephthalate) and nylon 610 with... 

Vieth and Wuerth (2Ji) found negative deviations from the simple two phase model for semicrystalline polypropylene suggesting that the presence of crystallites in some way reduces the sorptive capacity of the amorphous phase. However, analysis of samples using x-ray diffraction revealed the presence of a less stable crystalline phase having a lower density. Since the crystalline volume fraction is commonly determined from density measurements, the presence of a second, less dense (however, still impermeable) crystalline phase would seem... 

We assume that there were only two phases, impermeable PA crystalline phase and permeable molten polyether phase, which are both continuous (percolated phases). The model in this case consists of two continuous phases in parallel paths of permeation. The overall permeability for such a case in the resistance model is given by ... 

Equations accounting for the effects of deviation from planar orientation of flakes on film permeability in the presence of a second impermeable crystalline phase [21]), and in the case of randomly oriented layered clay platelets [28, 29]. 

In addition to particle aspect ratio and concentration, permeability is affected by several other factors including polymer crystallinity, particle orientation, and adhesion. Modified equations to accommodate misalignment effects and the presence of the (assumed) impermeable to oxygen crystalline phase of HOPE in mica/HDPE films have been proposed [37]. 

Vittoria et al. [31,73] studied the permeability to dichloromethane vapors of hlms of SPS thermally crystallized from the amorphous phase in a form. Results showed that, for each degree of crystallinity, the fraction of impermeable phase is higher than the crystalline phase, at low vapor activity. Therefore, Vittoria and colleagues suggested that this was due to the presence of a mesomorphic phase, in addition to the crystalline and the amorphous. This mesophase, characterized by conformational order not sufficient to give rise to discrete X-ray reflections, is less permeable than the amorphous sample to the vapors at low activity and became permeable at higher activity (a > 0.6). 

In crystalline polymers, the crystalline areas act as impermeable barriers to permeating molecules and have the same effect as inert fillers, i.e. they force the permeant molecules to diffuse along longer path lengths. " Permeant solubility is proportional to the product of the amorphous volume fraction (f) and the solubility S of the permeant in the amorphous phase. Figure 11 shows the effect of increasing crystallinity, and hence density, on nitrogen permeability. The thermal history of a crystallizable polymer can profoundly affect the permeation properties, since this can affect the number and size of crystallites present. 

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