Polymers hydroperoxide determination

Polymer immobilization. Mo-peroxide, 427 Polymerization agents, 621, 622 peroxide value, 661, 662 peroxycarboxyUc acids, 698 radical polymerization, 697, 707 styrene, 697, 720 sulfonyl peroxides, 1005 thermochemistry, 155 Polymers aging, 685 autoxidation, 623 hydroperoxide determination, 685 Poly(methacrylonitrile peroxide)... 

Hydroperoxide determination by iodometric titrations are quite common in the literature, the essential differences in the methods lies in the end point determination [Mielewski et al., 1989]. The propagation mechanism in the standard UV degradation process involves the production of hydroperoxides by the abstraction of a hydrogen atom from the polymer by the peroxy radical. 

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. 

The water (moisture) content can rapidly and accurately be determined in polymers such as PBT, PA6, PA4.6 and PC via coulometric titration, with detection limits of some 20 ppm. Water produced during heating of PET was determined by Karl Fischer titration [536]. The method can be used for determining very small quantities of water (10p,g-15mg). Certified water standards are available. Karl Fischer titrations are not universal. The method is not applicable in the presence of H2S, mercaptans, sulfides or appreciable amounts of hydroperoxides, and to any compound or mixture which partially reacts under the conditions of the test, to produce water [31]. Compounds that consume or release iodine under the analysis conditions interfere with the determination. 

The progress of polymer degradation may be followed by a wide variety of techniques, some of them being mentioned at the right column in the Bolland-Gee scheme (Scheme 2). There are techniques that directly monitor some of the elementary reaction steps such as, for example, oxygen absorption (reaction 2), differential scanning calorimetry (DSC) (reaction 3), chemiluminescence (reaction 11) analytical and/or spectral methods of determination of hydroperoxides, etc. 

Abstract The oxidation of polymers such as polypropylene and polyethylene is accompanied by weak chemiluminescence. The development of sensitive photon counting systems has made it comparatively easy to measure faint light emissions and polymer chemiluminescence has become an important method to follow the initial stages in the oxidative degradation of polymers. Alternatively, chemiluminescence is used to determine the amount of hydroperoxides accumulated in a pre-oxidised polymer. Chemiluminescence has also been applied to study how irradiation or mechanical stress affects the rate of polymer oxidation. In recent years, imaging chemiluminescence has been established as a most valuable technique offering both spatial and temporal resolution of oxidation in polymers. This technique has disclosed that oxidation in polyolefins is non-uniformly distributed and proceeds by spreading. 

Baker anhydrous sulfur dioxide of 99.98% purity was used without further purification. Sulfur dioxide was passed through the respective monomers, kept at 0-5°C., and the dissolved S02 content was determined iodometrically in each case, running a control experiment side by side. tert-Butyl hydroperoxide (TBHP), supplied by the Monomer-Polymer Laboratories of the Bordeh Chemical Co., was used as obtained without further purification. 

Plasma vs. Corona Treatment of Polypropylene (PP1. Corona treatments of polyolefins to modify their surfaces are very common in the polymer industry. The chemistry at such surfaces has been widely studied by XPS (4). It is generally assumed that corona treatments create abundant amounts of radicals which react with oxygen to form a hydroperoxide. This reacts further to eventually form crosslinks, oxidized products (ranging from hydroxyls to esters) with and without chain scission. The latter process is believed to lead to low-molecular weight material. There is some controversy over this material. Its role in determining the surface properties of the modified polymer is not completely understood. Its formation cannot be demonstrated directly by XPS, but only by comparing spectra before and after washing. 

Anton, Determination of Hydroperoxides in ultraviolet irradiated nylon 66IIJ. Appl. Polym Sci. (1965), v. 9, No 5, 1631-1639. 

Hence, it results in an unstable hydroperoxide RiOOH and the new free radical, formed on another polymer molecule. In other words, the chain of events so far yields in fracture of the first polymer chain and a damage brought to the second polymer chain. The conversion of peroxides (ROO ) to hydroperoxides (ROOH) is the rate-determining step for the chain reaction. 

This prompted us to review the work on the decomposition of hydroperoxides and in particular to determine what portion of the hydroperoxides that decomposes leads ultimately to bond scission. It is important to point out in this context that it is possible in principle for a polymer to oxidize extensively without losing its physical properties. The most important process in the breakdown of physical properties is bond scission in the backbone of the polymer chain. This can be best determined by measurements of the molecular weight of a photooxidizing or photodegrading polymer. In our laboratories we have studied this process by automatic viscometry which permits a very precise measurement of bond scission in polymers the apparatus has been described previously (12). For these studies we prepared a singlet-oxygen adduct of cis-polyisoprene by the following scheme ... 

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