Mechanically stressed polymer systems

The Eyring analysis does not explicity take chain structures into account, so its molecular picture is not obviously applicable to polymer systems. It also does not appear to predict normal stress differences in shear flow. Consequently, the mechanism of shear-rate dependence and the physical interpretation of the characteristic time t0 are unclear, as are their relationships to molecular structure and to cooperative configurational relaxation as reflected by the linear viscoelastic behavior. At the present time it is uncertain whether the agreement with experiment is simply fortuitous, or whether it signifies some kind of underlying unity in the shear rate dependence of concentrated systems of identical particles, regardless of their structure and the mechanism of interaction. 

This point of view concerning the mechanism of mechanochemical destruction of macromolecular compounds is based on two factors. One is the gradual character of this process, as reported by us earlier (JO). The other factor is the increase in potential energy and, consequently, the chemical reactivity of polymer systems that are placed in a stressed state mechanically. 

Such a view leads to the idea that it is possible to extend mechano-chemical synthesis, using practically all of the methods of macromolecular chemistry and mechanical energy as activation factors. With that end in mind, judicious elaboration of the reaction systems, the choice of an adequate kind of mechanical stress (which must agree with the physical state of the initial polymer), and the use of a macromolecular reaction partner are required. 

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. 

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. 

It is well known that the polymer systems undergo various mechanical treatments during their exploitation. This leads to the change of component compatibility and to the shift of boundary curves. The theory of this phenomenon has been worked out. The present article is dealing with the study of influence of the shear rates (j) and stresses (d) on phase transitions in the systems PDMS (1 =2.7 105) -MEK and CA-I (Mn=6.1 lC)4 > degree of substitution (DS)2.4)-dioxane-water,... 

Mechanochemical preparation of block copolymers is of historic interest. In a polymer blend, mechanical stress causes degradation, producing macroradicals which in turn lead to the formation of a large number of block and graft copolymers. Berlin assumes [269] that the radicals predominantly attack stress-activated backbone C—C bonds. Radical combination is not regarded as important as radical concentration in the system is negligible. 

While ionomers of many types have been made and characterized [1,2,3], there is little work on the overall relaxation mechanisms. For polymers with low ionic concentrations, there is general agreement on the fundamental relaxation step. The stress relaxes by detachment of an ion pair from one cluster and reattachment to another. For the styrene/methacrylic acid Na salt (ST/-MAA-Na) system, there is a secondary plateau in the relaxation modulus which depends on the ionic content and can be described as a rubbery modulus [4], While a rubbery modulus with stress relaxation due to ionic interchange has been invoked earlier, it does not adequately describe the relaxation curves. A different approach is taken here. 

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