Polymer membranes, microchannels

FIGURE 3.22 General idea of polymer membrane formation at the interface of a two-phase organic/aqueous flow in an X-shaped microchannel layout [435]. Reprinted with permission from the American Chemical Society. 

Scheme 4.87 Formation of enzyme—polymer membranes in a microchannel. Reprinted with permission from [344]. Copyright 2005 The Royal Society of Chemistry.

Pervaporation can be used in microfluidic devices for sample concentration. Microchannels 500 nm high, 4 mm long, and 2 to 30 pm thick have been fabricated with polyimide using thin film deposition [263]. Pervaporation occurs through the polyimide polymer membrane (Fig. 7.18). In this study, water was... 

Besides the synthesis of bulk polymers, microreactor technology is also used for more specialized polymerization applications such as the formation of polymer membranes or particles [119, 141-146] Bouqey et al. [142] synthesized monodisperse and size-controlled polymer particles from emulsions polymerization under UV irradiation in a microfluidic system. By incorporating a functional comonomer, polymer microparticles bearing reactive groups on their surface were obtained, which could be linked together to form polymer beads necklaces. The ability to confine and position the boundary between immiscible liquids inside microchannels was utilized by Beebe and coworkers [145] and Kitamori and coworkers [146] for the fabrication of semipermeable polyamide membranes in a microfluidic chip via interfacial polycondensation. 

Single and parallel dual-membrane structures are successfully prepared by using multilayer flow such as organic/aqueous two-layer flow and organic/aqueous/organic three-layer flow inside a microchannel, and this method can be applied to the preparation of surface-modified polymer membranes (Fig. 42). For example, horseradish peroxidase is immobilized on one side of the membrane surface, and this enzyme-modified membrane realizes substrate permeation and a subsequent reaction. 

Fig. 41 Polymer membrane formation under organic/aqueous two-phase flow in an X-shaped microchannel...

Figure 7.16 Design and synthesis of a chemically functional polymer membrane by an interfacial polycondensation reaction and multilayer flow inside a microchannel.

Stretched Polymers MF membranes may be made by stretching (Fig. 20-68). Semicrystalline polymers, if stretched perpendicular to the axis of crystallite orientation, may fracture in such a way as to make reproducible microchannels. Best known are Goretex produced from Teflon , and Celgard produced from polyolefin. Stretched polymers have unusually large fractions of open space, giving them very high fluxes in the microfiltration of gases, for example. Most such materials are very hydrophobic. 

Wang,Y.X., Cooper, J.W., Lee, C.S., DeVoe, D.L., Efficient electrospray ionization from polymer microchannels using integrated hydrophobic membranes. Labchip 2004, 4, 363-367. 

A method of microfahrication based on multicomponent laminar flow inside microchannels was developed by Whitesides, Kenis and colleagues [257, 258]. Laminar streams of solutions enable a reaction at the interface between streams to make membranes inside the microchannel. For example, the reaction at the interface between two aqueous phases crmtaining the oppositely charged polymers poly (sodium 4-styrenesulfonate) and hexadimethrine bromide flowing laminarly in parallel produces a polymeric structure (membrane) deposited on glass at the laminar flow interface (Fig. 40). 

The plane electrodes are separated by isolating spacers, which may lead to the formation of parallel flow channels. In any case, the electrodes are plane sheets which can be replaced and thus made out of any plain material, e.g. nickel, lead, glassy carbon or graphite. Recent technolo cal developments made at the Institute of Microtechniques, Mainz [6, 7], have led to the construction of versatile microchannel electrochemical reactors. Indeed, the pressure can be elevated to up to 35 bar and the electrodes can be stacked in order to increase the overall electrode area. Moreover, polymer electrolyte membranes can be inserted, separating anodic and cathodic compartments if necessary, and finally heat exchangers may be integrated. 

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