Penetrant molecules through polymers

Ep Activation energy for permeation of small penetrant molecules through polymers. 

The study of the transport of penetrant molecules through polymers [1,2] is important in many areas of technology. There are two types of industrially important polymeric systems for which such transport phenomena are crucial ... 

Other areas of technology where the transport of small molecules through polymers plays a key role include foams (where small molecules are used as blowing agents for foam expansion [9-11] and any gas trapped in the cells of a closed-cell foam affects key properties such as the thermal conductivity [12]), plasticization [13,14], removal of process solvents, residual monomers or other impurities by techniques such as supercritical fluid extraction [15,16], biosensors, drug implants, and polymer electrolytes (where the penetrants are ionic). 

Permeation of small molecules through polymers usually occurs by the solution-diffusion mechanism, which has two key steps. The penetrant molecule is first "sorbed" by the polymer, i.e., dissolves in the polymer. It then crosses the specimen by a succession of diffusive "jumps". P is therefore equal to the product of the diffusivity (or diffusion coefficient) D and solubility S ... 

Reports in the literature on the transport of penetrant molecules through NR based blends and IPN s are few when compared to the existing literature on most common/commercial polymers/blends. The sorption and diffusion of aromatic penetrants into different NR blends such as NR/BIIR, NR/CIIR, NR/neoprene, NR/EPDM, NR/polybutadiene, and NR/SBR were studied by Siddaramaiah et al. The diffusion coefficient (D) of the penetrants was found to range from 6.8 to 84.3 x 10 cm /s at a temperature range of 25-60 °C. Results indicated that the transport data were affected by the nature of the interacting solvent molecule rather their sizes, and also by the structural variations of the elastomers blended with NR. The activation parameters for the diffusion of the penetrants ranged from 4.16 to 30.48 kJ/mol. 

The permeability of small penetrant molecules through an organic matrix is determined by the solubility and diffusivity of the small molecule in the matrix as well as by the mean-square displacement (total path length traveled) divided by the sample thickness. In principle, the addition of a filler in the polymer matrix is expected to affect the solubility and diffusivity of a penetrant molecule, especially in the vicinity of the filler (i.e in the filler-polymer interfacial region and at least one polymer Rg away from the filler surface). Also, it is expected that fillers will affect the path tortuosity (mean-square displacement of penetrant versus film thickness) directly, when penetrants are forced to travel around impermeable fillers, and indirectly, when fillers induce polymer chain aUgnment or alignment and modification of polymer crystallites. ... 

As it is known [50-52], introduction of organoclay in a polymeric matrix results in essential reduction of permeability to gas of the nanocomposites obtained by such a mode in comparison with a matrix polymer. As a rule, such permeability to gas reduction is explained by an increase in the meandering trajectory of the gas-penetrant molecules through the nanocomposite by virtue of the availability of organoclay anisotropic particles within it [50, 52]. So, the relative permeability to gas characterising a reduction in this parameter for nanocomposites in comparison with a matrix polymer, is defined as follows [50] ... 

The efficacy of polymers when used to protect metals from corrosive environments is influenced by their efficiency as barrier materials. When applied to metals by some techniques, such as fluidised bed coating, there is always the danger of macro-diffusion through pinholes which are gross imperfections in the surface and which do not have to be visible to be very much greater than the dimension of penetrating molecules. 

Saleem, M. Asfour, A.A. De Kee, D. 1989, Diffusion of organic penetrants through low density polyethylene (LDPE) films Effect of size and shape of the penetrant molecules. J. Appl. Polym. Sci. 37 617-625. 

The literature contains a very large amount of both experimental and theoretical information on the diffusion of small molecules, especially gases, in polymers. Since the pioneering works (1,2) on the diffusion of gases through rubber septa interest in diffusion phenomena in polymers has continously increased and diversified. The considerable interest and the concentrated academic and industrial research efforts in the study of diffusion in polymers arises from the fact that important practical applications for these materials depend to a great extent on diffusion phenomena. In the last five decades a series of classic books and reviews have been devoted to the presentation of the main topics, experimental results, theories and applications for the diffusion of small penetrants in and through polymers (3-19). An interested reader will most certainly find in one of these references information which apply to her/his special area of interest. 

To construct the model, it has been assumed that the amorphous polymer regions posses an approximately paracrystalline order with chain bundles locally parallel. A penetrant molecule may diffuse through the matrix of the polymer by two modes of motion, Fig. 5-2. 

The penetration of a solvent, usually water, into a polymeric implant initiates dmg release via a diffusion process. Diffusion of dmg molecules through non-porous polymer membranes depends on the size of the dmg molecules and the spaces available between the polymeric chains. Even through the space between the polymer chains may be smaller than the size of the dmg molecules, dmg can still diffuse through the polymer chains due to the continuous movement of polymer chains by Brownian motion. 

The sizes and shapes of the penetrant molecules, and their solubilities, are the key properties determining their relative permeabilities in a given polymer, i.e., the selectivities of polymers between them, when permeation occurs by the solution-diffusion mechanism. For example, the activation energy for diffusion typically increases with increasing size of penetrant molecules of identical shape permeating through a given polymer. 

For nonspherical penetrants, such as the linear alkanes or the flat aromatic molecules, the orientation of the penetrant in diffusing through the polymer results in an effective diffusional dimension related to the cross-sectional area of the molecule. The increase of this effective diffusional dimension in a series of such molecules is much slower than the increase of the van der Waals volume. Furthermore, even among small penetrant molecules, some (such as CO2) are much less spherical in shape than others (such as C>2)-... 

Copolymers of vinylidene chloride and vinyl chloride have been used as test cases. Permeation of penetrant molecules is generally believed to occur through the amorphous regions of these semicrystalline polymers. Their study is, however, complicated by the necessity to understand the effects of the presence of the crystallites in addition to the amorphous regions. 

Small molecules can penetrate and permeate through polymers. Beeause of this property, polymers have found widespread use in separation technology, protection coating, and controlled delivery [53]. The key issue in these applications is the selective permeability of the polymer, which is determined by the diffusivity and the solubility of a given set of low-molecular-weight compounds. The diffusion of a small penetrant oeeiu s as a series of jitmps... 

Permeability of Plasma-Treated Gas-Separating Polymer Membranes. Analyzing relation (9-97), compare the influence of sizes of penetrating molecules and cross-links on the permeability of plasma-treated gas-separating polymer membranes. Why do these two sizes appear in relation (9-97) for gas permeability through the membrane in a non-symmetric way with significantly different powers. 

Films of hydrophilic polymers are the traditional objects of research into electrolyte transfer in polymers. For a long period this process was described as diffusion of water molecules through pores in the films. Selectivity of membranes towards the penetrants was attributed to the presence of charges on the pore walls impeding diffusion of similarly charged ions. 

---