Barrier and membrane polymers

Free Volume and Transport Properties of Barrier and Membrane Polymers... 

Dendritic macromolecules exhibit compact globular structures which lead to their low viscosity in the melt or in solution. Furthermore, dendritic macromolecules are characterized by a very large number of available functional groups, which lead to unprecedented freedom for changing/tuning/tailoring the properties of these multivalent scaffolds via complete or partial derivatization with other chemical moieties. All these features have contributed to multidisciplinary applications of these unique macromolecular structures in recent years 6, 7). The development of efficient synthetic routes in recent years has given rise to a virtually unlimited supply of commercially available dendritic polymers, at very affordable price. The transport properties of hyperbranched and dendritic polymers have recently attracted attention as potentially new barrier and membrane materials 8-9). 

More commonly, permselective membranes create a differential in the transport rate of different species. The permeability differences may be based on physical size, solubility differences, or specific interactions between the polymer and the diffusing species. Permselective membranes are used in a variety of functions, such as diffusion barriers and membranes for interference removal. A commonly used permselective membrane is Nafion which demonstrates an extremely high affinity for organic cations, reasonable affinity for inorganic cations, and almost completely excludes both organic and inorganic anions [16]. 

Although the partition coefficient is expected to remain constant, its magnitude is important. Since this coefficient represents the concentration of drug in the membrane relative to that in the core, an excessively high partition coefficient will allow quick depletion of the core and an ineffective delivery system. For effective diffusional systems, the partition coefficient should be less than unity. If the value of this coefficient is greater than 1, the surrounding polymer does not represent a barrier, and drug release becomes first-order. 

Furthermore, another advantage of nanofillers is not only to reinforce the rubber matrix but also to impart a number of other properties such as barrier properties, flammability resistance, electrical/electronic and membrane properties, and polymer blend compatibility. In spite of tremendous research activities in the field of polymer nanocomposites during the last two decades, elastomeric nanocomposites... 

A current theme in plasmid-based delivery approaches is to mimic Nature s methods for nucleic acid delivery. To date, the best system to emulate Nature has been viral vectors. Briefly, most viral vectors escape immune surveillance, interact with cell membranes (e. g., receptor), internalize (via endocytosis), escape from endosomes, migrate to the nuclear envelope, enter the nucleus, and finally take over cellular functions. Plasmid-based systems (cationic liposomes and cationic polymers) can mimic portions of these events. This chapter will explore the barriers facing gene delivery vectors, with an emphasis of the pharmacokinetic behavior of these systems. In order to understand the in-vivo barrier, a brief review of physiology will be provided. 

Cross-section structure. An anisotropic membrane (also called asymmetric ) has a thin porous or nonporous selective barrier, supported mechanically by a much thicker porous substructure. This type of morphology reduces the effective thickness of the selective barrier, and the permeate flux can be enhanced without changes in selectivity. Isotropic ( symmetric ) membrane cross-sections can be found for self-supported nonporous membranes (mainly ion-exchange) and macroporous microfiltration (MF) membranes (also often used in membrane contactors [1]). The only example for an established isotropic porous membrane for molecular separations is the case of track-etched polymer films with pore diameters down to about 10 run. All the above-mentioned membranes can in principle be made from one material. In contrast to such an integrally anisotropic membrane (homogeneous with respect to composition), a thin-film composite (TFC) membrane consists of different materials for the thin selective barrier layer and the support structure. In composite membranes in general, a combination of two (or more) materials with different characteristics is used with the aim to achieve synergetic properties. Other examples besides thin-film are pore-filled or pore surface-coated composite membranes or mixed-matrix membranes [3]. 

Polymers for membrane preparation can be classified into natural and synthetic ones. Polysaccharides and rubbers are important examples of natural membrane materials, but only cellulose derivatives are still used in large scale for technical membranes. By far the majority of current membranes are made from synthetic polymers (which, however, originally had been developed for many other engineering applications). Macromolecular structure is crucial for membrane barrier and other properties main factors include the chemical structure of the chain segments, molar mass (chain length), chain flexibility as well as intra- and intermolecular interactions. 

An interesting pore-filled composite membrane, made by photograft copolymerization onto a solvent-stable PAN UF membrane, has been established [47]. High flux and selectivity for PV separation of organic-organic mixtures were achieved by a very thin selective barrier and prevention of swelling of the selective polymer in the pores of the barrier. 

More recently, a new series of water dispersed anionic polymers, the AQ 29D, 38D and 55D polymers were released by Eastman Kodak. Since that time, these polymers were used as electrode modifier (12, 13), as covering membrane (14) and as support for enzyme immobilization (15, 16). AQ polymers are high molecular weights (14,000 to 16,000 Da) sulfonated polyester type polymers (17, 18). Their possible structures have been recently presented (18). The AQ polymer serie shows many interesting characteristics useful for the fabrication of biosensors. They are water dispersed polymers and thus compatible with enzymatic activity. They have sulfonated pendant groups similar to Nafion and they can act as a membrane barrier for anionic interferring substances and they offer the possibility to immobilize redox mediators by ion exchange. 

Several cytotoxicity assays based on different mechanisms and measurement principles have been described in the literature. During safety assessment of new compounds, it is crucial to select a cytotoxicity assay addressing the correct mechanism. In this set of experiments, we use PEI (18-20) as model enhancer mimicking a possible effect of nanoparticles/delivery enhancers on cytotoxicity and epithelial barrier function. PEI polymers are widely used for non-viral gene delivery. For these kinds of enhancers, a cell membrane perforating mechanism is described in the literature. The rather high toxicity of PEI polymers is one of the major limiting factors especially for its in vivo use. 

The membrane in a broad sense is a thin layer that separates two distinctively different phases, i.e., gas/gas, gas/liquid, or liquid/liquid. No characteristic requirement, such as polymer, solid, etc., applies to the nature of materials that function as a membrane. A liquid or a dynamically formed interface could also function as a membrane. Although the selective transport through a membrane is an important feature of membranes, it is not necessarily included in the broad definition of the membrane. The overall transport characteristics of a membrane depends on both the transport characteristics of the bulk phase of membrane and the interfacial characteristics between the bulk phase and the contacting phase or phases, including the concentration polarization at the interface. The term membrane is preferentially used for high-throughput membranes, and membranes with very low throughput are often expressed by the term barrier. ... 

Hirotsugu Yasuda is Professor Emeritus of Chemical Engineering and Director of the Center for Surface Science and Plasma Technology, University of Missouri-Columbia. He has over. 300 publications in refereed journals and books and was a pioneer in the exploration of low-pressure plasma for surface modification of materials and deposition of nanofilms as barrier and permselective membranes in the late 1960s. He received the Ph.D. degree in physical and polymer chemistry from the State University of New York, College of Environmental Science and Forestry, Syracuse. 

In the case of a composite membrane consisting of a skinless porous substrate and a dense film, permeability and permselectivity may be determined solely by the resistance of the denser film. Different membrane polymers may therefore be employed for the thin barrier layer and the thick support structure. This permits a combination of properties which are not available in a single material. Such membranes were initially developed for desalination by reverse osmosis where they are known as thin- or ultrathin-film composites or nonlntegrally-skinned membranes. A second type of composite membrane is utilized for gas separations. It is a composite consisting of an integrally-skinned or asymmetric membrane coated by a second, more permeable skin which is used to fill skin defects. The inventors of the latter have entitled their device a resfstanee model membrane, but the present author prefers the term coated integrally-skinned composites. 

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