Polyethylene oxygen solubility

Polyethylene terephthalate) (PET), with an oxygen permeability of 8 iiiuol/(ius-GPa), is not considered a barrier polymer by die old definition however, it is an adequate barrier polymer for holding carbon dioxide in a 2-L bottle for carbonated soft drinks. The solubility coefficients for carbon dioxide are much larger than for oxygen. For the case of the PET soft drink bottle, the principal mechanism for loss of carbon dioxide is by sorption in the bottle walls as 500 kPa (5 atm) of carbon dioxide equilibrates with the polymer. For an average wall thickness of 370 pm (14.5 mil) and a permeabdity of 40 nmol/(m-s-GPa), many months are required to lose enough carbon dioxide (15% of initial) to be objectionable. 

Measurements of the permeability and diffusivity have been undertaken for helium and oxygen for a range of highly oriented polyethylene films The deduced solubilities for both gases are proportional to the amorphous volume fraction showing that the non-crystalline regions are the transport medium in all instances. The solubility obtained for oxygen is about ten times that for helium. This result is consistent with the proposal of Michaels and Bixler which relates the amorphous phase... 

Such a substrate effect is observed in a wide variety of hydrogels prepared from water-soluble vinyl monomers (e.g., the sodium salt of styrene sulfonate, acrylic acid, and acrlyamide), and on various hydrophobic substrates, such as Teflon, polyethylene, polypropylene, PVC, and polymethyl methylacrylate (PMMA) [52]. This template effect is due to retardation of the radical polymerization near the rough and hydrophobic substrates that trap oxygen at the solid surface [82]. 

This catalytic system consisted of an aqueous acidic solution of a palladium salt with a cop-per(I) salt and oxygen or air as an oxidant however, application of the Wacker process to longer chain alkenes has been a challenge due to their low solubility in aqueous media. This has led to research in biphasic systems using tetraalkylammonium salts,93 polyethylene glycols,94 as well as cyclodextrins.95,96... 

Some years ago, Malmstrom etal. synthesized water-soluble metal phosphine complexes based on water-soluble polymers [41], In order to have solubility in both an acidic and a basic medium, they prepared two different water-soluble polymers. For the first, they made methyl [4-(diphenylphosphino)benzyl]amine (PNH) react with poly(acrylic acid) (PAA) using dicydocarbodiimide (DCC) as the coupling agent, under strict exclusion of oxygen (25). For the second, they reacted (4-carboxy-phenyl)diphenylphosphine with polyethylene imine (PEI) at room temperature (26). The reduction by sodium borohydride was made in situ, followed by the addition of methanesulfonic add and diethyl ether. Then, the methanesulfonic salt of phosphinated polyethylenimine was predpitated. 

It is important to note that the rates of reactions in solid polymers will be controlled not only by the rate of diffiision but also by the solubility of the permeant in the polymer. For example, the diffiision constant for oxygen is quite large in many polymers, but usually the solubility is very low, and as a result, rates of oxidation tend to be quite small. Experimental values of the permeability P and diffiision constant D for various organic permeants and oxygen in low-density polyethylene (8) illustrate this point (see Table 2). 

As indicated, a material that is a good barrier has a low value of the combined diffusion coefficient and solubility coefficient values for a particular penetrant. Preferably, both D and S should be low. For instance, polyethylene is an excellent barrier to water because water has very low solubility and diffusion coefficient values in polyethylene, but it has a relatively high P for oxygen because O2 has higher solubility than water. A permeability coefficient value is valid only for a particular polymer/permeant pair, and as in the case of polyethylene, a structure may be a good barrier for a certain permeant and only a fair or poor barrier for a different one. 

In the absence of oxygen, the chlorination of polyethylene, with or without a catalyst, can be controlled to provide products with varying chlorine content. The chlorination process is statistically random so that chlorination of polyethylene to the same chlorine content as poly(vinyl chloride) (60%) gives a product that is chemically different from PVC yet fiilly compatible with it. This random chlorination of polyethylene destroys its crystallinity. At a degree of chlorination corresponding to the loss of all its crystallinity, the chlorinated product becomes soluble at room temperature. The p-bromination of polyethylene follows a similar course to yield a rubberlike polymer at 55% bromine content. 

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