Bulk and Structure Modification of Polymers
Most polymeric materials used in gas sensors are not a single polymer, and they contain chemicals that modify some physical and/or chemical behavior. These additives such as solvents (porogens), cross-linkers, initiators and plasticizer are being generall
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Bulk and Structure Modification of Polymers
24.1
Modifiers of Polymer Structure
Most polymeric materials are not a single polymer but they contain chemicals that modify some physical and/or chemical behavior. These additives are generally added to modify properties, assist in processing, and introduce new properties to a material (Carraher 2008). Some of these additives are present in minute amounts and others are major amounts of the overall composition.
24.1.1
Solvents (Porogens)
It is known that gas penetrability and specific surface area are two of the most important parameters of polymers intended for application in gas sensors. This means that polymers synthesized should be porous. Experiments have shown that this parameter of polymers is controlled mainly by the solvent (Pichon and Chapuis-Hugon 2008). The solvent serves to bring all the components in the polymerization—the template, the functional monomer(s), the cross-linker, and the initiator—into one phase, and therefore just the physical and chemical characteristics of the solvent determine both the accuracy of the assembly between the template and the monomer and the creation of the pores in polymers (Cormack and Elorza 2004). For this reason, the solvent is commonly referred to as the “porogen.” When polymerization occurs, solvent molecules occupy space in the polymer network and create the pores required to allow the diffusion of the template out of the network, and its subsequent diffusion back into the polymer, during recognition. This means that the porosity of the polymers can be determined by the overall concentration of monomers and cross-linkers in the solution. A favorable solvent for molecular imprinting polymers (MIPs) will create well-developed pores within the network and increase the total pore volume. However, a large amount of solvent can ultimately lead to the formation of microspheres and nanospheres instead of a large and stable cross-linked network (Bergmann and Peppas 2008). Aside from its dual role as a solvent and a pore-forming agent, the solvent in a covalent polymerization must also be judiciously chosen so that it simultaneously maximizes the likelihood of complex formation between the template and the functional monomers. More specifically, the use of a highly thermodynamic solvent tends to result in polymers with well-developed pore structures and high specific surface areas, while the use of a thermodynamically poor solvent leads to polymers with poorly developed pore structures and low specific surface areas. Historically, chloroform has been used as a highly thermodynamic solvent; other solvents that have been investigated include dimethyl G. Korotcenkov, Handbook of Gas Sensor Materials, Integrated Analytical Systems, DOI 10.1007/978-1-4614-7388-6_24, © Springer Science+Business Media New York 2014
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Bulk and Structure Modification of Polymers
Table 24.1 Solvents typically used for preparing polymer films Solvent
Polymer
Toluene/ethanol
Ethyl cellulose (EC); poly(vinyl chloride-co-isobutyl vinyl
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