Cross-linking monomers polymer properties affected

Other urgent improvements concern the mass transfer properties of the materials and bleeding of residual traces of template remaining entrapped in the material.4 The latter properties are influenced by the polymer morphology, which in turn is affected by the type and amount of cross-linking monomer, and the type of porogenic solvent. 

A polymer is a macromolecule that is constructed by chemically linking together a sequent of molecular fragments. In simple synthetic polymers such as polyethylene or polystyrer all of the molecular fragments comprise the same basic unit (or monomer). Other poly me contain mixtures of monomers. Proteins, for example, are polypeptide chains in which eac unit is one of the twenty amino acids. Cross-linking between different chains gives rise to j-further variations in the constitution and structure of a polymer. All of these features me affect the overall properties of the molecule, sometimes in a dramatic way. Moreover, or... 

Polymers are macromolecules built of smaller units called monomers. The process by which they are formed is called polymerization. They may be synthetic (nylon, Teflon, and Plexiglas) or natural (such as the biopolymers starch, cellulose, proteins, DNA, and RNA). Homopolymers are made from a single monomer. Copolymers are made from two or more monomers. Polymers may be linear, branched, or cross-linked, depending on how the monomer units are arranged. These details of structure affect polymers properties. 

In both of these systems, various monomer systems can be combined to add toughness, flexibility, or other desired properties. The resulting polymer is a three-dimensional network with short links between chains. The cross-link density, how close together the cross-links occur in the backbone of the polymer, significantly affects polymer properties. Increasing the cross-link density causes thermoset polymers to become stiffer and more brittle. For example, the chemical composition of rubber bowling balls and rubber tires is very similar, but they differ substantially in cross-link density, and therefore in behavior. 

Because of the versatility of the polyurethane system it is possible to introduce comonomers which can affect the physical properties of the derived polymers. For example, photo cross-linkable polyurethanes are formulated using 2,5-dimethoxy-2,4 -diisocyanato stilbene as a monomer (76). Comonomers, having an azoaromatic chromophore, are used in optical bleaching applications (77), or in the formation of photorefractive polymers (78). The latter random poljnners have second-order nonlinear optical (NLO) properties. Linear poljnners are also obtained from HDI/PTMG and diacetylenic diols. These polymers can be cross-linked through the acetylenic linkages producing a network polymer with properties similar to poly(diacetylenes) (79). 

The structure of nonfluorinated monomers and their ratio to the fluorinated monomer affect repellency as well as other properties of the polymer such as melt flow and hardness. Comonomers with a cross-linking function, such as a hydroxyl, epoxy, or vinyl group, are used to increase the durability of the repellent polymer. Hybrid fluoropolymers, consisting of hydrophobic and hydrophilic segments, are discussed in Chapter 13. 

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