Preparation and applications of ion exchange membranes by radiation-induced graft copolymerization of polar monomers onto non-polar films

This paper reviews methods of modifying polydimethylsiloxane (PDMS) polymers to improve their properties for biomedical applications. The modification methods are discussed under three different categories: bulk, surface and other modification techniques. Surface modification techniques include physical and chemical techniques to modify polymer surfaces. Bulk modification techniques include blending, copolymerization, interpenetrating polymer networks (IPNs) and functionalization. The third category includes less common modification techniques.

© 2001 Society of Chemical Industry

Modification of polysiloxane polymers for biomedical applications: a review

Radiation-grafted anion-exchange membrane (RG-AEM) research has predominantly focused on the chemical stability of the polymer-bound positively-charged head-groups that enable anion conduction. The effect of the backbone polymer chemistry, of the precursor film, on RG-AEM stability has been studied to a lesser extent and not for RG-AEMs made from pre-irradiation grafting of polymer films in air (peroxidation). The mechanical strength of polymer films is generally weakened by exposure to high radiation doses (e.g. from a high-energy e−-beam) and this is mediated by chemical degradation of the main chains: fluorinated films mechanically weaken at lower absorbed doses compared to non-fluorinated films. This study systematically compares the performance difference between RG-AEMs synthesised from a non-fluorinated polymer film (low-density polyethylene – LDPE) and a partially-fluorinated polymer film (poly(ethylene-co-tetrafluoroethylene) – ETFE) using the peroxidation method (pre-irradiation in air using an e−-beam). Both the LDPE and ETFE precursor films used were 25 μm in thickness, which led to RG-AEMs of hydrated thicknesses in the range 52–60 μm. The RG-AEMs (designated LDPE-AEM and ETFE-AEM, respectively) all contained identical covalently-bound benzyltrimethylammonium (BTMA) cationic head-groups. An LDPE-AEM achieved a OH− anion conductivity of 145 mS cm−1 at 80 °C in a 95% relative humidity environment and a Cl− anion conductivity of 76 mS cm−1 at 80 °C when fully hydrated. Alkali stability testing showed that the LDPE-AEM mechanically weakened to a much lower extent when treated in aqueous alkaline solution compared to the ETFE-AEM. This LDPE-AEM outperformed the ETFE-AEM in H2/O2 anion-exchange membrane fuel cell (AEMFC) tests due to high anion conductivity and enhanced in situ water transport (due to the lower density of the LDPE precursor): a maximum power density of 1.45 W cm−2 at 80 °C was achieved with an LDPE-AEM alongside a Pt-based anode and cathode (cf. 1.21 mW cm−2 for the benchmark ETFE-AEM). The development of more mechanically robust RG-AEMs has, for the first time, led to the ability to routinely test them in fuel cells at 80 °C (cf. 60 °C was the prior maximum temperature that could be routinely used with ETFE-based RG-AEMs). This development facilitates the application of non-Pt catalysts: 931 mW cm−2 was obtained with the use of a Ag/C cathode at 80 °C and a Ag loading of 0.8 mg cm−2 (only 711 mW cm−2 was obtained at 60 °C). This first report on the synthesis of large batch size LDPE-based RG-AEMs, using the commercially amenable peroxidation-type radiation-grafting process, concludes that the resulting LDPE-AEMs are superior to ETFE-AEMs (for the intended applications).

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Non-fluorinated pre-irradiation-grafted (peroxidated) LDPE-based anion-exchange membranes with high performance and stability

Preparation of ion-exchange membranes by grafting acrylic acid into pre-irradiated polymer films—1. grafting into polyethylene

Polyolefins such as polyethylene, polypropylene and their copolymers have excellent bulk physical/chemical properties, are inexpensive and easy to process. Yet they have not gained considerable importance as speciality materials due to their inert surface. Polyethylene in particular holds a unique status due to its excellent manufacturer- and user-friendly properties. Thus, special surface properties, which polyethylene does not possess, such as printability, hydrophilicity, roughness, lubricity, selective permeability and adhesion of micro-organisms, underscore the need for tailoring the surface of this valuable commodity polymer. The present article reviews some of the existing and emerging techniques of surface modification and characterisation of polyethylene. Surface modification of polymers, polyethylene in particular, has been extensively studied for decades using conventional tools. Although some of these techniques are still in use, they suffer from distinct shortcomings. During the last two decades, different means of surface modification have been thoroughly explored. The increasing expectancy for smart materials in daily life has, of late, sharply influenced research in the area of surface modification. Technologies that involve surface engineering to convert inexpensive materials into valuable finished goods have become even more important in the present scenario. In this review article we have attempted to broadly address almost all conventional and modern techniques for the surface modification of different physical forms and chemical compositions of polyethylene. This article will hopefully stimulate further research in this area and result in the development of polyolefins with multi-functional and responsive surfaces, which would ultimately lead to the commodities of polyolefins with smart surfaces.

Surface modification of polyethylene

In the grafting of acrylic acid onto polyethylene by preirradiation method, the irradiation in air gave a higher rate of grafting than in N/sub 2/, since the diffusion rate of monomer is larger for less cross-linked polyethylene. The rate of grafting decreased with increasing time of storaging polyethylene because of the decay of trapped radicals. The effects of storage conditions on the grafting activity was reasonably interpreted by assuming that grafting is predominantly controlled by both concentration of trapped radicals and monomer diffusibility in the polymer matrix. Mohr's salt was found to depress the homopolymerization of acrylic acid without marked change of grafting rate. The rate of grafting was increased by the addition of ethylene dichloride due to the increase in monomer diffusion.

Graft polymerization of acrylic acid onto polyethylene film by preirradiation method. II. Effects of oxygen at irradiation, storage time after irradiation, mohr's salt, and ethylene dichloride

Synthesis of ion exchange membrane by radiation grafting of acrylic acid onto polyethylene

PURPOSE: An organic high polymer is, after exposed to radiation, kept at low temperature and continuously dipped in a solution of reactive monomers kept at higher temperature than that of the polymer to effect the reaction in an atmosphere of inert gas to produce uniform graft polymer suitably used as a diaphragm for electrolysis. CONSTITUTION: A roll sheet of organic high polymer such as polyethylene is irradiated with 0.1W90 Mrad of electron or gamma rays and set in an unrolling chamber which is kept below 0°C in an atmosphere of inert gas such as nitrogen so that the decrease of radicals formed is prevented. Then, the rolled sheet is unrolled and dipped in a solution of a reactive monomer such as acrylic acid that is kept less than 5ppm in the oxygen content by use of an inert gas, preferably less than 2ppm at 10W60°C to produce the objective graft polymer. COPYRIGHT: (C)1982,JPO&Japio

Takashi Takayama

Papers

Graft polymerization of acrylic acid onto polyethylene film by preirradiation method. II. Effects of oxygen at irradiation, storage time after irradiation, mohr's salt, and ethylene dichloride

Synthesis of ion exchange membrane by radiation grafting of acrylic acid onto polyethylene

Production of graft polymer