Polymer oxide

Electrogenerated conducting polymer films incorporate ions from the electrolyte medium for charge compensation (182). Electrochemical cycling in an electrolyte solution results in sequential doping and undoping of the polymer film. In the case of a -doped polymer, oxidation of the film results in the... 

She et al. [128] used rolling contact to estimate the adhesion hysteresis at polymer/oxide interfaces. By plasma oxidation of the cylinders of crosslinked PDMS, silica-like surfaces were generated which could hydrogen bond to PDMS r olecules. In contrast to unmodified surfaces, the adhesion hysteresis was shown to be larger and proportional to the molecular weight of grafted polymer on the substrate. The observed hysteresis was interpreted in terms of the orientation and relaxation of polymer chains known as Lake-Thomas effect. 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

Scheme 1 Free radical chain process involved in polymer oxidation.

The flow of a current through an electrochemical system demonstrates the main difference between material based on conducting polymers and all the other industrial nonconducting polymers conducting polymers oxidize and reduce electrochemically in a reverse way, as do metals or redox couples ... 

Figure 16. Evolution of the population of the polaronic and bipolaronic bands during polymer oxidation. CB, conducting band, P.B., polaronic band, V.B., valence band, B.P.B., bipolaronic band.

Steps 1 and 2 of polymer oxidation described in the previous section can be considered as a relaxation step. Then the oxidation is completed by swelling184 186 under diffusional control. The electrochemically stimulated conformational relaxation, swelling, and oxidation of a conducting polymer is shown in Fig. 35. 

This is the relaxation time of the polymer oxidation under electro-chemically stimulated conformational relaxation control. So features concerning both electrochemistry and polymer science are integrated in a single equation defining a temporal magnitude for electrochemical oxidation as a function of the energetic terms acting on this oxidation. A theoretical development similar to the one performed for the Butler-Volmer equation yields... 

The relative growth rate per cycle v is calculated from the peak current of the respective polymer oxidation using Eq. (3) ... 

Derivitization reactions have previously been employed to extend the sensitivity and resolution of IR, ultraviolet and X-ray photo-electron spectroscopy (7-13). Yet no proposed method has the range to accommodate the major oxidation products from polyolefins. As part of an ongoing study of polymer oxidation and stabilization, we discuss here a series of reactions with small, reactive gas molecules. The products from these reactions can be rapidly identified and quantified by IR. Some of these reactions are new, others have already been described in the literature, although their products have not always been fully identified. 

Synergy between primary and secondary anti-oxidants occurs and often a mixture is employed. Also included are metal complexing agents, e.g., EDTA (ethylenediaminetetraacetic acid), citric acid, the purpose of which is to deactivate extraneous metal ions that catalyse polymer oxidation. 

The carbonyl index is not a standard technique, but is a widely used convenient measurement for comparing the relative extent and rate of oxidation in series of related polymer samples. The carbonyl index is determined using mid-infrared spectroscopy. The method is based on determining the absorbance ratio of a carbonyl (vC = 0) band generated as a consequence of oxidation normalised normally to the intensity of an absorption band in the polymer spectrum that is invariant with respect to polymer oxidation. (In an analogous manner, a hydroxyl index may be determined from a determination of the absorbance intensity of a vOH band normalised against an absorbance band that is invariant to the extent of oxidation.) In the text following, two examples of multi-technique studies of polymer oxidation will be discussed briefly each includes a measure of a carbonyl index. 

The formation and role of hydroperoxide groups, particularly in the early stages of polymer oxidation is well discussed in the introduction to the next chapter and also features in many of the references cited in this chapter. Their detection and quantification is therefore important. Although this can be done directly or implicitly through many of the instrumentation techniques discussed in this chapter, there are several tests that have been developed, some of which are still widely used, that are based more on chemical methods, titration or staining. The majority have been applied to polyolefins, especially polyethylene. 

Isothermal Kinetics of Polymers Oxidation and Its Relation to the Concentration of Oxygen in Surrounding Atmosphere 487... 

Some traces of metal and metal ions may initiate the decomposition of hydroperoxides even at room temperature. Traces of metal ions are present in almost all polymers and they may affect considerably the polymer oxidation and its subsequent degradation. The sequence of efficiency of metal ions to enhance degradation depends on its valence state and the type of its ligand, but may be postulated as follows ... 

As was already indicated, Scheme 5, which had been considered as the origin of the light emission arising from polymer oxidation, is not a unique mechanism. 

A direct oxidation of polymer additive, which may occasionally give a much stronger signal than the oxidation of polymer itself. This is very important as it may lead to an erroneous relation between the rate of polymer oxidation and chemiluminescence intensity. 

Comparison of chemiluminescence isothermal runs with oxygen uptake and DSC measurements has been at the centre of interest since practical industrial applications of the chemiluminescence method were attempted. It is a fact that the best comparison may be achieved when studying polymers that give a distinct induction time of oxidation typical for autoaccelerating curves of a stepwise developing oxidation. This is the particular case of polyolefins, polydienes and polyamides. The theoretical justification for the search of a mutual relationship between the oxidation runs found by the various methods follows directly from the kinetic analysis of the Bolland-Gee scheme of polymer oxidation. 

ISOTHERMAL KINETICS OF POLYMERS OXIDATION AND ITS RELATION TO THE CONCENTRATION OF OXYGEN IN SURROUNDING ATMOSPHERE... 

Figure 5.8 GALDI mass spectra of mastic with variable amounts of polymer, artificially aged for 880 h (36] (a) without polymer (b) normal mastic and (c) with added polymer. Oxidation and decomposition is enhanced with lower amounts of polymer, thus the mastic polymer retards oxidation of the triterpenoids...

The overall results obtained from the HBCD mixture experiments were interpreted in terms of a catalytic effect of strong Lewis acids such as SbBr3 or BiBr3 on the degradation of the HBCD by facilitating the loss of halogen. In the polymer/oxide/DBDPO ternary mixtures the in-situ formation of these species should produce a similar effect on the halogen loss from the system. 

CL accompanies many reactions of the liquid-phase oxidation of hydrocarbons, ketones, and other compounds. It was discovered in 1959 for liquid-phase ethylbenzene oxidation [219,220]. This phenomenon was intensively studied in the 1960s and 1970s, providing foundation for several methods of study of oxidation, decay of initiators, and kinetics of antioxidant action [12,17,221], Later this technique was effectively used to study the mechanism of solid polymer oxidation (see Chapter 13). 

Peroxyl radicals were identified as products of hydrocarbon and polymer oxidation by an 03 02 mixture and were proved by EPR spectroscopy [118,119]. 

The encounter of two free valences as a result only of diffusion of segments of a macromolecule. Since the radius of segmental diffusion is limited in the real time, this mechanism can be efficient at the high initiation rate and intense mobility of the polymer segments. Under the conditions of polymer oxidation, this mechanism is possible at the chain length close to unity. Some examples are given in Table 13.2. 

Polymer oxidation is similar to oxidation of low-molecular-weight analogs in the liquid phase and has several peculiarities caused by the specificity of solid-phase free radical reactions of macromolecules. Several monographs are devoted to this field of chemistry [11,12,33-41],... 

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