Polymers, chemical modification double bonds

Although, the heat resistance of NBR is directly related to the increase in acrylonitrile content (ACN) of the elastomer, the presence of double bond in the polymer backbone makes it susceptible to heat, ozone, and light. Therefore, several strategies have been adopted to modify the nitrile rubber by physical and chemical methods in order to improve its properties and degradation behavior. The physical modification involves the mechanical blending of NBR with other polymers or chemical ingredients to achieve the desired set of properties. The chemical modifications, on the other hand, include chemical reactions, which impart structural changes in the polymer chain. 

A product of this type will have over 50% of its weight derived from maleic anhydride. This very high content of reactive double bonds will lead to a very brittle solid when it is cross-linked with styrene. Without further modification, this solid material will have very high tensile moduli, probably over 600 kpsi, but a very low tensile elongation, way below 1 %. Such a brittle material obviously has only very limited applications. Thus, for most general-purpose applications, it is necessary to incorporate some chemically inert components to soften the polymer backbone. This will reduce the cross-linking density and improve the physical properties of the cured solid. 

The reactivity of conjugated double bonds is significantly different from that of isolated double bonds in the polymer backbone of polydienes. This difference can be used for selective chemical modification of the dienes with reactions such as the Diels—Alder reaction. With maleic anhydride as an enophile, selective addition to the terminal pair of conjugated double bonds in a chloroprene oligomer is complete in a few hours to give 102.391... 

The polybutene structure also contains one carbon-carbon double bond at the end of the polymer chain [40]. The nature of this double bond is important in defining the ease with which it will undergo chemical modification. Normally it is found as the CIS- and franx-trisubstituted group, but polybutenes having the more reactive disubstituted vinylidene structure are now available. Fig. 2.2. Polybutenes have good stability as lubricant components, even whilst containing the residual unsaturation. It is possible to react the double bond to produce products such as lubricant dispersants, see Chapter 7, but the reaction is achieved only under certain controlled conditions. 

Also, the method how the ablation parameters are acquired can have a pronounced influence on the results. The ablation rate can be defined either as the depth of the ablation crater after one pulse at a given fluence, or as the slope of a linear fit of a plot of the ablation depth versus the pulse number for a given fluence. Very different ablation rates can result from the two different measurement methods. This is especially the case for materials where ablation does not start with the first pulse, but after multiple pulses, or if the ablation crater depth after one pulse is too small to be measured. The process that occurs if ablation does not start with the first laser pulse is called incubation. It is related to physical or chemical modifications of the material by the first few laser pulses, which often results in an increase of the absorption at the irradiation wavelength [32,33], for example, the formation of double bonds in poly (methylmethacrylate) (PMMA). Incubation is normally observed only for polymers with low absorption coefficients at the irradiation wavelength. 

Impregnation of reactive species into a host polymer provides a method for polymer modification via a chemical reaction. TTiis work has been extended by showing that photolysis of W(CO)6 in polyethylene (PE) can lead to isomerization of the C=C double bonds, an interesting variation of Wrighton s work on catalytic alkene isomerization [40]. The method can be used to prepare iso-merized PE because the W(CO)e can be removed entirely after the isomerisation has taken place [5,41]. Ultraviolet photolysis of Fe(CO)5 in PE under a pressure of H2 leads to reduction of up to 80% of the C=C bonds [41], while photolysis under an atmosphere of O2 generates an oxide, most probably Fe203, within the PE matrix [41]. 

Chemical Unsaturation Presence of double or triple bonds in a chain organic molecule such as that of some polymers, usually between carbon atoms. Unsaturation makes the molecule more reactive, espeeially in free-radical addition reactions such as addition polymerization, and polymers more susceptible to degradation, eross-linking, and chemical modification. Also called Polymer Chain Unsaturation. 

The well-defined surface functionalization using classical organic reactions plays an important role in membrane development. In order to be susceptible to reaction, the polymer chain should contain double bonds, hydroxyl groups or benzene rings. An example is the modification of polysulfone by reaction with different chemicals to increase hydrophilicity. The surface modification of polysulfone membranes has been reported by several authors [120, 121]. 

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