Polymers permeation properties

The wide range of polymer permeation properties were first observed and modelled over a century ago. However, little further practical or theoretical work was undertaken until after the turn of the century when the need for improved pneumatic tyres developed. This led to the replacement of natural rubber with butyl rubber due to its lower permeability. Subsequently, the need for materials with ever-improved permeation properties has grown unabated. For instance, polymers which are relatively impermeable are used in food packaging and encapsulation of electronic circuits, whilst polymers which are very selectively permeable are also used in food packaging as well as in gas and liquid separation processes. Applications also exist for polymers which can release drugs or pesticides at a controlled rate. 

The wide use of polymers in the electrical industry arises from their excellent electrical insulation and dielectric isolation properties. However, these are dependent upon the polymer permeation properties. For instance, the sheathing of cables and wires is sensitive to the presence of water in the polymer. It has been shown that the most common form of insulation breakdown arises from electrochemical treeing. This arises from the simultaneous effects of an electric field and moisture present in the shielding, either initially present or by permeation. 

The chemical modification of poly (2,6-dimethyl-l,4-phenylene oxide) (PPO) by several polymer analogous reactions is presented. The chemical modification was accomplished by the electrophilic substitution reactions such as bromination, sulfonylation and acylation. The permeability to gases of the PPO and of the resulting modified polymers is discussed. Very good permeation properties to gases, better than for PPO were obtained for the modified structures. The thermal behavior of the substituted polymers resembled more or less the properties of the parent polymer while their solution behavior exhibited considerable differences. 

In our research, three chemical modification approaches were investigated bromination, sulfonylation, and acylation on the aromatic ring. The specific objective of this paper is to present the chemical modification on the PPO backbone by a variety of electrophilic substitution reactions and to examine the features that distinguish modified PPO from unmodified PPO with respect to gas permeation properties, polymer solubility and thermal behavior. 

The solution properties of the brominated PPO derivatives were similar to those of the parent polymer. The gas permeation properties of PPO and PPO containing between 6.5% to 100% Br groups per repeat unit are summarized in Table I. 

Chemical modifications of PPO by electrophilic substitution of the aromatic backbone provided a variety of new structures with improved gas permeation characteristics. It was found that the substitution degree, main chain rigidity, the bulkiness and flexibility of the side chains and the polarity of the side chains are major parameters controlling the gas permeation properties of the polymer membrane. The broad range of solvents available for the modified structures enhances the possibility of facile preparation of PPO based membrane systems for use in gas separations. 

Sakai, T., Takenaka, H., Wakabayashi, N., Kawami, Y. and Torikai, E. 1985. Gas permeation properties of solid polymer electrolyte (SPE) membranes. Journal of... 

Hietala, S., Skou, E. and Sundhokn, F. 1999. Gas permeation properties of radiation-grafted and sulfonated poly-(vinylidene fluoride) membranes. Polymer 40 5567-5573. 

Primary interest was in the barrier properties obtained from plasma organo-silicones and from inorganic "SIN" coatings. Spectral grade HMDSO was used in the former case, while mixtures of SiH and NH were used to produce the SIN structures. The substrate in much or the work was DuPont Kapton type H polylmide film, 51 pm thick. Substrate temperatures extended to 450 C, as described earlier (6). The thickness of plasma-polymer deposits was about 0.5 pm. Moisture permeation was evaluated by the routine of ASTME-96-53 T (water vapor transmission of materials in sheet form). Additional, more precise data, were obtained with both a Dohrmann Envirotech Polymer Permeation Analyser, modified as previously described (6), and a Mocon "Permatran W" moisture permeation apparatus. 

The solubility of a gas is an integral part for the prediction of the permeation properties. Various models for the prediction of the solubility of gases in elastomeric polymers have been evaluated (57). Only a few models have been found to be suitable for predictive calculations. For this reason, a new model has been developed. This model is based on the entropic free volume activity coefficient model in combination with Hildebrand solubility parameters, which is commonly used for the theory of regular solutions. It has been demonstrated that mostly good results are obtained. An exception... 

Figures 2.20 and 2.21 show the significant difference between diffusion in liquids and in rubbery and glassy polymers. A great deal of work has been performed over the last two decades to achieve a quantitative link between the structure of polymers and their permeation properties. No such quantitative structure-property relationship is at hand or even in sight. What has been achieved is a set of semiempirical rules that allow the permeation properties of related families of polymers to be correlated based on small changes in their chemical structures. The correlating tool most generally used is the polymer s fractional free volume v/ (cm3/cm3), usually defined as...

Correlation of the permeation properties of a wide variety of polymers with their free volume is not possible [32], But, within a single class of materials, there is a correlation between the free volume of polymers and gas diffusion coefficients an example is shown in Figure 2.24 [33], The relationship between the free volume and the sorption and diffusion coefficients of gases in polymers, particularly glassy polymers, has been an area of a great deal of experimental and theoretical work. The subject has recently been reviewed in detail by Petropoulos [34] and by Paul and co-workers [35,36],... 

N. Tanihara, H. Shimazaki, Y. Hirayama, N. Nakanishi, T. Yoshinaga and Y. Kusuki, Gas Permeation Properties of Asymmetric Carbon Hollow Fiber Membranes Prepared from Asymmetric Polymer Hollow Fibers, 7. Membr. Sci. 160, 179 (1999). 

Various chitosan derivatives of enhanced solubility, mucoadhesive, and permeation properties were developed. V-Trimethyl chitosan chloride (TMC) is a quater-nized derivative of chitosan with superior aqueous solubility over a broader pH range and penetration-enhancing properties under physiological conditions [78]. Carboxymethylated chitosan (CMChi) is a polyampholytic polymer able to form viscoelastic gels in aqueous environments. CMChi appears to be less potent compared with the quaternized derivative. Neither TMC nor CMChi have been found to provoke damage of the cell membrane, and therefore, they should not alter the viability of nasal epithelial cells [79],... 

Hirayama Y, Kase Y, Tanihara N, Sumiyama Y, Kusuki Y, and Haraya K. Permeation properties to CO2 and N2 of poly(ethylene oxide)-containing and crosslinked polymer films. J. Membr. Sci. 1999 160 87-99. 

Pinnau I, Casillas CG, Morisato A, and Freeman BD. Long-term permeation properties of poly(l-trimethylsilyl-propyne) membranes in hydrocarbon-vaport environment. J. Polym. Sci. PartB Polym. Phys. 1997 35 1483-1490. 

Mucus also appears to be a barrier to the permeation enhancing effect of polymeric or monomeric absorption enhancers. In the aforementioned TMC studies, the enhancement effect (enhancement ratio = permeation rate of the drug in the presence of polymer vs. permeation rate of the drug alone) was higher in vitro (Caco-2 cells no mucus secretion) than the absorption enhancement in vivo. Meaney and O Driscoll studied the effect of mucus on the permeation properties of a micellar system consisting of sodium taurocholate in a coculture of Caco-2 and Ht29GlucH (mucin-secreting) cells. They found that the effect of bile salts on the permeation of hydrophilic paracellular markers was increased in the cocultures that were pretreated with the mucolytic compound A/-acetylcysteine. 

The most common rubber polymers used in SVI closures are natural and butyl rubber (Table 9). Silicone and neoprene also are used but less frequently in sterile products. Butyl rubber has great advantages over natural rubber in that butyl rubber requires fewer additives, has low water vapor permeation properties, and has good characteristics with respect to gaseous (e.g., oxygen) permeation and reactivity with the active ingredient. 

While the above three properties of polymers affect the permeation properties of the membrane directly, the effect of the molecular weight on permeation properties is still controversial, although there are some evidences that an increase in molecular weight enhances the permeation rate owing to an increase in chain stiffness. 

The structure of an amorphous polyamide prepared from hexamethylenediamine and isophthalic/tere-phthalic acids was modified in order to determine the effect of chemical structure on the oxygen permeation properties. The greatest increase in permeation was obtained by lengthening the aliphatic chain. Placement of substituents on the polymer chain also led to increased permeation. Reversal of the amide linkage direction had no effect on the permeation properties. Free volume calculations and dielectric relaxation studies indicate that free volume is probably the dominant factor in determining the permeation properties of these polymers. 

In order to make meaningful comparisons of permeation properties, it was necessary to insure that no complicating factors were present in the polymers under study. The major precaution was to... 

Factors Affecting Polyamide OPV. In order to determine the polymer properties which affect polyamide permeation properties, the n-I series was studied in more detail. Figure 3 shows the... 

Through systematic modification of the polymer backbone, the effects of chemical structure upon the oxygen permeation properties of aliphatic-aromatic amorphous polyamides were determined. In this class of polymers, the greatest effects were obtained by alteration of the chain length and disruption of the amide hydrogen bonding by N-alkylation. It is remarkable that reversal of the amide linkage has no effect whatsoever on the permeation properties of the examples studied. 

Poly(ethylene terephthalate) Poly(ethylene terephthalate) is a widely used semicrystalline polymer. The macroscopic properties of PET such as thermal, mechanical, optical, and permeation properties depend on its specific internal morphologies and microstructure arrangement. It can be quenched into the completely amorphous state, whereas thermal and thermomechanical treatments lead to partially crystallized samples with easily controlled degrees of crystallinity. The crystallization behavior of thermoplastic polymers is strongly affected by processing conditions [91-93]. 

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