Degradation of polymers mechanical

The oxidation of radicals in our experiments was carried out at 760 mm Hg oxygen pressure and at low temperatures. The macroradicals were generated by mechanical degradation of polymers in vacuo. The polymeric particles, made during the degration procedure, fell into a... 

A wide variety of techniques and apparatus have been developed for ESR studies of the mechanical degradation of polymers. These are mostly purpose built for a particular investigation. 

Mechanical degradation of polymer is much more severe at higher flow rates, longer flow distances, and lower brine permeabilities of porous media. In a lower-permeability porous medium, the average pore throat diameter is smaller, and the stress acting on the polymer is larger. Thus, it is more probable for the polymer chains to be broken and the viscosity to be more heavily reduced. Similarly, we can understand the effects of flow rate and flow distance. 

While these results certainly indicate the utility of stress MS for studying the mechanical degradation of polymers, they provide no positive support for the Zhurkov theory. Those studies which report a similarity in thermal and mechanical degradation products are suspect because the origin of the observed products is not known. Those studies in which degradation products are clearly distinguished from residual impurities, i.e., nylon 66, PAN, and PVC, apparently have different thermal and mechanical degradation mechanisms. 

Mechanical degradation of polymers, which refers to a process in which the activation energy for scission of the polymer is provided, at least in part, by mechanical action on the polymer [Hunston and Zakin, 1980b], imposes severe limitations on the use of polymers as DRAs and also exerts detrimental effects on other practical applications, such as in polymer preparation and in polymer use as viscosity index improvers for motor oils. Therefore, it has received considerable attention over the last several decades, and a large number of papers have been published, among which there are several review articles [Pollert and Sellin, 1989 Singh, 1990 Moussa and Tiu, 1994 Choi et al., 2000b]. 

Mechanical degradation of polymers has been studied for more than 70 years in several flow fields encompassing strong elongation components. In certain flow fields the streamlines are symmetric with a stagnation point. In the vicinity of the stagnation point, the dwell time of the fluid element is longer than the timescale for coU extension. Such flow is referred to as quasi-steady-state-flow (QSSF). hi most other cases the dwell time is shorter than the coil extension time and the flow is referred to as fast-transient-flow (FTF). 

Mechanical degradation of polymers often results in chain scission accompanied by the formation of free radicals. In the presence of oxygen, the final products are complex and may include peroxides and hydroperoxides and a range of oxygen - containing species formed as a result of further reaction of radicals and of hydroperoxides (20,21). 

Mechanicai degradation refers to molecular scission induced by the application of mechanical stresses. The stresses can be shear stresses, elongational stresses, or a combination of the two. Mechanical degradation of polymers can occur in the solid state, in the molten state, and in solution. An extensive review of the field of mechanically induced reactions in polymers was published by Casale and Porter [38]. In an extruder, mechanical stresses are applied mostly to the molten polymer. 

Mechanical degradation of polymers in turbulent flow has been extensively investigated and reviewed by Pollert and Sellin (148), Singh (20), Moussa and coworkers (149), Moussa and Tin (43), Gyr and Bewersdorff (23), Den Toonder and co-workers (65), Nguyen and Kausch (146), Brostow and co-workers (62-64), Choi and co-workers (141), Rho and co-workers (150), Kim and co-workers (151-155), and Choi and co-workers (156-159). 

High-density polyethylene, Sholex 6050, may serve as an example. The materials were subjected both to thermal degradation and y-irradiation together with 2,4,6-tri-tert-butyl nitrosobenzene. As in the case of mechanical degradation of polymers in the presence of nitrosobenzene, ESR spectra from nitroxide radicals were observed... 

Prevailing shear rate in injection and flooding process (besides the usual shear thinning behaviour of polymer solutions, extreme shear conditions can result in mechanical degradation of polymer chains). 

J. Sohma and M. Sakaguchi, ESR studies on mechanical degradation of polymers and reac-... 

The first theory for the mechanical degradation of polymers was developed by Frenkel [1]. A related concept was also espoused by Kauzman and Eyring [2]. Frenkel considered that sheared polymer molecules are extended in the flow direction. The bonds in the middle are the most extended while the chain ends retain more or less their coiled shape. The forces of viscous drag thus put a strain on the central bonds with a force that increases as the square of molecular length. Above a critical shear rate, the central bonds of the macromolecules are expected to be broken preferentially. Molecules with less than a critical molecular weight are considered to be stable under fixed shear conditions. For a DP (degree of polymerization) of 10, a critical shear rate, y, of 10 sec was given [1]. In retrospect, however, the results should also depend on the viscosity of the medium, which, with shear rate, controls the stress. 

Butyagin investigated the mechanical degradation of polymers with quaternary carbon atoms in the main chain. He distinguished different radical types [24]. An unpaired electron was found (1) at the chain end for poly(methyl methacrylate) and for polyisobutylene (2) in the methyl group of the main chain of poly(methyl methacrylate) and (3) in the benzene ring in poly(a-methylstyrene). 

Matia, F. P. La, Valenza, A. Thermo-Mechanical Degradation of Polymer Blends, Die Angewandte Makromoiekulare Chemie, 216(1998), p. 45 - 65... 

The results of the modification(s) on the system and equipment show that the service life of equipment more than doubled, mechanical degradation of polymer decreased more than 50%, and energy consumption decreased markedly. 

The modifications on the system and equipment and their results are shown. For example, the service life of beam pumps more than doubled mechanical degradation of polymer in the system and triplex pumps decreased more than 50 and 70%, respectively and the energy consumption of mixing equipment decreased 80%, among other results. 

In high-intendty ultrasonics in liquids, the local absorption of elastic energy is closely related to the phenomenon of cavitation. In depolymerization reactions free radicals are usuaUy formed. In this care the shear stresses not only dissociate the bonds in the main chain, but also separate the radicals formed, preventing in this way their recombination. An extensive review of the experimental woik on the mechanical degradation of polymers has been given by Porter and Casale (77). 

The topic of ESR spectroscopy in the 1986 edition of the Encyclopedia of Polymer Science and Technology (EPST) has included the classic applications of ESR, to systems in which the species containing unpaired electron spins were part of the S3 tem the detection and identification of radicals in polymerization reactions, during mechanical degradation of polymers, and in the attack of polymers by ozone, UV radiation, and... 

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