Crystalline polymer permeability

Compared with most other crystalline polymers the permeability of P4MP1 is rather high. This is no doubt due to the ability of gas molecules to pass through the open crystal structure with the large molecular spacing. 

They are generally more permeable to gases than crystalline polymers, more so above T, than below (see later). 

Oxidation is also dependent on the permeability of the polymer to oxygen. Table 10.1 lists the permeabilities of selected polymers to oxygen. Because bulk oxidations are dependent on the permeability to oxygen, crystalline polymer forms are more resistant to oxidation than amorphous forms. Also, the very nature of the molecules present in the chains affects the tendencies toward oxidation. Thus the fluorine atom in polytetrafluoroethylene (ptfe)... 

Generally, polymers that crystallize are not considered good candidates for membrane materials however, there are some exceptions [26, 27]. The presence of crystallinity reduces permeability [28,29] and good membranes should be capable of high fluxes. The usual physical picture is to think of a semicrystalline polymer in terms of a simple two-phase model one phase being amorphous and the other being crystalline. In the typical case, the crystals do not sorb or transmit penetrant molecules the following relationship has been proposed [28, 29] to describe the extent to which crystallinity reduces permeability from that if the polymer were amorphous... 

As an illustration, Fig. 18.6 shows the permeability of nitrogen (at room temperature) for a great variety of polymers (elastomers, semi-crystalline polymers and glassy polymers). It can be seen that the values of P vary by a factor of nearly one million if silicone rubber on the one hand is compared with poly(vinylidene chloride) on the other ... 

Acetal translucent crystalline polymer is one of the stiffest TPs available. It provides excellent hardness and heat resistance, even in the presence of solvents and alkalies. Its low moisture sensitivity and good electrical properties permit direct competition with die-cast metal in a variety of applications. In addition, acetal has extremely high creep resistance and low permeability. Acetal is also available as a copolymer (Hoechst Celanese Corp. s Celcon) for improved processability. The homopolymer (DuPont s Delrin) has a very low coefficient of friction and its resistance to abrasion is second only to nylon 6/6. Acetals are frequently blended with fibers such as glass or fluorocarbon to enhance stiffness and friction properties. Acetal is not particularly weather-resistant, but grades are available with UV stabilizers for improved outdoor performance. Acetal, whether homopolymer or copolymer, is not used to any significant degree in forming structural foams. 

PE is a crystalline polymer with many grades, with a wide variety of crystallinity and molecular weight. A porous PE membrane is more drug-permeable than non-porous membranes of low-crystalline PE, both of which are available for the reservoir system. EVA, as a copolymer of ethylene and vinyl acetate with 9-40 wt.% vinyl acetate, is favorably used for the reservoir membrane. However, it should be noted that EVA, especially the copolymer with high vinyl acetate content, is resistant to hydrophilic liquid substances such as water and glycerin. But this copolymer swells and deforms itself in lipophilic liquid substances. Hence, paraffin, squalene, and IPM could not be used as reservoir liquids in combination with EVA. 

The unique molecular packing of rod-like chains in liquid crystalline polymers (LCP) closely resembles the extended chain structure of highly oriented flexible chain polymers, suggesting that these materials are good candidates for barrier applications. Thermotropic LCP s, first developed in the early 1970 s, have been the object of much interest because of their excellent mechanical properties and ease of product fabrication. Preliminary observations have shown that a commercially available wholly aromatic thermotropic copolyester has gas permeability coefficients that are lower than those of polyacrylonitrile (4.). These results raise some fundamental questions as to the nature of the mechanism for transport of small molecules through a matrix of ordered rigid rod-like chains. 

The potential advantage of crystalline polymers for barrier applications was recognized by Morgan (1) although, previous investigators had reported reduced permeabilities in semi-crystalline polymers (2,2) As will be discussed, the equilibrium concentration of sorbed penetrant and the diffusion process are affected by crystallinity in uniquely different ways. 

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... 

Crystals or filler particles that do not sorb the penetrant will obviously be Impermeable to them and, thus, reduce transport rates in the composite. Figure 4 illustrates this effect for matrices in which such particles are arranged in an ordered and in a random manner. The rate of transport in such systems will be slower than when such particles are absent because of the reduced area for transport and the resulting more tortuous path for permeation. The effective diffusion coefficient for such cases will be a factor K smaller than in the pure amorphous material D. The permeability coefficient in a crystalline polymer should then be given by... 

The permeability of a crystalline polymer to gases depends on its crystallinity index and degree of order. These depend on processing conditions, so the preparation conditions and thermal history of the samples to be measured must be clearly stated. 1... 

For semi-crystalline polymers above Tg, the permeability is proportional to the nth power of the amorphous volume fraction n lies between 1.2 and 2. The gas must diffuse between the lamellar crystals, and the detailed morphology depends on the polymerisation route, thermal history and whether orientation is present. The permeability of gases of molecular weight M is approximately inversely proportional to y/M. However, Table 11.2 shows that the ratio of CO2 to O2 permeability in glassy polymers is higher than in semi-crystalline polymers. 

The influence of chain packing (Le. free volume) on solubility, diffusivity and permeability in liquid crystalline polymers can be studied by comparing properties of LCPs in the disordered, isotropic state with those in the ordered, liquid crystalline state. HIQ-40 is a random, glassy, thermotropic, nematogenic terpolymer synthesized from 40 mole percent p-hydroxybenzoic acid and 30 mole percent each of isophthalic acid and hydroquinone. The chemical structures of the constituent monomers for fflQ-40are ... 

Table I. Oxygen permeability in glassy barrier polymers and liquid crystalline polymers at 35 C...

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