Mechanical stress, degradation

Moreover, the strong mechanical stress degrades the individual fibers and makes them less flexible. Because of the higher washing temperatures, fabrics shrink and become more wrinkled. 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Thermoplastic polymers can be heated and cooled reversibly with no change to their chemical structure. Thermosets are processed or cured by a chemical reaction which is irreversible they can be softened by heating but do not return to their uncured state. The polymer type will dictate whether the compound is completely amorphous or partly crystalline at the operating temperature, and its intrinsic resistance to chemicals, mechanical stress and electrical stress. Degradation of the basic polymer, and, in particular, rupture of the main polymer chain or backbone, is the principal cause of reduction of tensile strength. 

The time dependent properties creep and stress relaxation can be considered as tests to monitor degradation or as degradation tests that add mechanical stress. 

Creep behaviour is a commonly used and very important measure of the effect of mechanical stress on plastics, but it is less used as a means of monitoring degradation due to environmental agents. At shorter times, the measured creep is predominantly due to physical effects and it is only at longer times that environmental effects will be apparent. It can be noted that creep tests use the same test piece at successive time intervals which is advantageous from the point of view of reproducibility. 

Stress-induced degradation - reactions that are catalyzed by subjecting the article to mechanical stress. 

In aerobic composting, an air blower distributes air under the pile and maintains most of the pile in aerobic conditions for faster degradation. The piles are turned daily to redistribute material and moisture and to maintain porosity of the pile. The mechanical stress imposed by turning the compost piles facilitates... 

For degradable polymers, the next step is to determine when and how the polymer needs to degrade. Are there preferred degradation products Does degradation need to be fast or slow What environmental factors may contribute to the degradation rate - moisture, sunlight, microbes, mechanical stresses, or cyclic weathering ... 

It is necessary to remember that the combination of several factors often has a synergistic effect a plastic resistant to a chemical in the absence of mechanical stress and at ambient temperature can crack quickly when exposed to the under load and be degraded more or less quickly in the event of a temperature rise. 

The weakly immunogenic protamine sulfate USP (1) condenses DNA to form a toroid structure of super-coiled DNA about 50 nm in diameter (2). The DNA in this form or in the preformed LPDI complex cannot be displaced from the protamine by polycations such as spermidine and histones or by other nucleic acids like genomic DNA (2). DNA in this toroid structure is transcriptionally inactive and this conformation allows for protection of DNA from enzymatic degradation by nucleases and other environmental assaults such as mechanical stress (1,2). After the liposome surrounds the toroid, the resulting homogenous LPDI nanoparticles are slightly less than... 

Mechanical syntheses are, of course, generally affected by the presence of radical acceptors (20,29). The yield on copolymers also increases with duration of mechanical stress. However, if the milling time is too long, the properties of the graft or block copolymers can be deteriorated by degradation of initial product. The nature of the balls and mill material can also influence the reaction (37) the mechanical activation of inorganic materials. The production of graft and block copolymers on freshly formed surfaces has been established in the lierature. 

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. 

There has been a prevailing theory that oxidative degradation is accelerated by mechanical stress [100]. This theory is based on fracture kinetic work by Tobolsky and Eyring [101], Bueche [102, 103, 104], and Zhurkov and coworkers [105, 106, 107]. Their work resulted in an Arrhenius-type expression [108] sometimes referred to as the Zhurkov equation. This expression caused Zhurkov to claim that the first stage in the microprocess of polymer fracture is the deformation of interatomic bonds reducing the energy needed for atomic bond scission to U=U0-yo, where U0 is the activation energy for scission of an interatomic bond, y is a structure sensitive parameter and o is the stress. 

A basic requirement of the ESR technique is the presence of molecules or atoms containing unpaired electrons. Such species can be generated in polymeric systems by homolytic chemical scission reactions or by polymerization processes involving unsaturated monomers. These reactions can be initiated thermally, photochemically, or with a free-radical initiator, and, in the case of scission, by mechanical stress applied to the system. Therefore, ESR can be used to study free-radical-initiated polymerization processes and the degradation of polymers induced by heat, light, high-energy radiation, or the application of stress. 

Fujisawa T, Hattori T, Takahashi K, Kuboki T, Yamashita A, Takigawa, M. Cyclic mechanical stress induces extracellular matrix degradation in cultured chondrocytes via gene expression of matrix metalloproteinases and interleukin-1. J Biochem (Tokyo). 1999 125 966-975. 

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