Mechanical Destruction of Solid Polymers

Since destruction of polymer materials is very important for practical purposes, a large number of investigations on fracture phenomena in polymers have been carried out from both the experimental and theoretical points of view. Several reports provide indirect evidence for main chain scissions, for example decreases in molecular weight or initiations of the graft or block copolymerization after mastication. Direct evidence for chemical bond scission can be obtained from ESR measurements on fractured polymer materials [21]. The high reactivity and high mobility of free radicals produced by mechanical fracture (mechano-radicals) can also be followed. The ESR application to mechanical destruction of polymer materials is presented below. Temperature-dependent ESR spectra of polymer radicals produced 

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Free Radicals Produced by Mechanical Destruction of Solid Polymers Mechano-Radicals... 

Scissions of main-chains by the mechanical destruction of the polymers are experimentally proved by the analyses of the observed ESR spectra for the various pdy-mers PE, PTFE, PB, PP and PMMA. A pair formation of the radicals, (mechano-radicals), after the milling is clearly demonstrated and this pair formation is believed to be the direct evidence for tl mechano-radicals formed primarily by the medianical actions. A model for chain rupture in an amorphous pdymer was proposed. Excess electrons produced by the triboelectricity due to the friction, diich is always accompanied with the mechanical fracture, play an important role, with coexistence of oxygen, in the thermal conversion of the mechano-radicals. The characteristic behaviors of the mechano-radicals, the hi er reactivity with oxygen, complete photoconversion of the peroxy radical, indicate that the mechano-radicak are formed and trapped on the fresh surfaces produced by cleavage in the solid polymer. The polymerizations initiated at the low temperatures by the PTFE mechano-radicals were reported and the copol5mierization is experimentally evidenced. 

ESR spectrum observed after the simultaneous fracture of PTFE with MMA at 77 K was found to be the quartet-quintet which is undoubtedly attributed to the propagating radical of PMMA. No trace of the PTFE radical was mixed in the observed spectrum. It is believed from both experimental and theoretical reasons that no radical is produced by a mechanical destruction of any solid consisting of a low molecular organic compound 31). Thus, the radicals produced by this mechanical action originate from not MMA but the PTFE polymers, although both solids of MMA and PTFE were fractured amultaneously. Accordingly the polymerization of MMA, which was proved by ESR, had been initiated not by the MMA radical but... 

It was experimentally proved by ESR that all the polymers so far investigated generated mechanoradicals by various mechanical agitations. Based on the these experimental facts it can be concluded that macroscopic fractures of solid polymers induce microscopic destruction, Le, scission of chemical bonds, although molecular mechanisms for the bond scissions caused by mechanical forces still remain unknown. Most ESR experiments for detecting the mechanoradicals were done at 77 K. The low temperature is required only for stabilization of unstable radicals. It is reasonable, therefore, to assume that mechanoradicals are formed even at room temperature in the polymers in which mechanoradical production is observed at 77 K. 

Destruction of macromolecules and distortion of the permolecular structure lead to changes in their properties, especially solubility, resistance to chemical agents, strength, fatigue and impact viscosity, as well as elasticity and plasticity. From the practical viewpoint, variations in polymer properties induced by the tribochemical destruction raise interest in two respects first, as unavoidable phenomena that accompany any mechanical effect on the polymers during their treatment or operation and, second, as a deliberate change of properties of solid pol3uners to obtain materials with specific characteristics. 

It was confirmed that the fracture of the ethylene monomers at 77 K produced no free radicals. The quintet ESR spectrum shown in Fig. 7.24 can be undoubtedly attributed to the propagating radical of polyethylene, —CpHp2 (Ca ) H 2, when both the polymers and the monomers are simultaneously fractured. The quintet is due to hyperfine splitting of two a- and two fi-hydrogen nuclei. No trace of the Pl FE radical was detected in the observed spectrum. Accordingly the polymerization of ethylene, which was proved by ESR, had been initiated not by the ethylene radicals but by the PTFE mechano radicals at as low a temperature as 77 K. This extremely high reactivity of the radicals is rather surprising because both PTFE and ethylene react in the solid state at 77 K. The mechano radicals newly created by the chain scission are surrounded by the monomer molecules because the radicals are trapped in the fresh surface formed by the mechanical destruction. 

The pnnaide of the novd ultrasonic spectro%opy utilizing a wide-band polymeric transducer and the FFT (fast Fourier transformation) analysis of a single pulse is introduced and its application to various polymraic matenals is reviewed Ultrasonic analysis of mechanical rdaxation processes and phase transitioiis m solid polymers as wdl as practical non-destructive inspection of defects m composite materials are described... 

After application, the liquid coating must be converted into a solid polymeric film (viscosity > 10 cps) in order to build up satisfactory performance properties (termed the film formation process) [74]. As water evaporates from a film of emulsion polymer, the distance of separation between the submicron particles continues to decrease and, ultimately, capillary tubes form. In a capillary tube, surface tension results in a force that tends to collapse the tube. Moreover, the smaller the diameter of the tube, the greater the destruction force. When the particles are so close to one another, the destruction force is strong enough to overcome the repulsion forces originating from either the electrostatic or steric interaction mechanism striving to push the neighboring particles apart. Coalescence of the particles to form a continuous film is thus possible. 

The mechanism of autoxidation of solid polyolefins RH has many common features with hydrocarbon oxidation polymers are oxidized by the chain route the chain develops as alternation of acts of R- with O2 and RO with RH in the absence of an inhibitor and at a sufficiently high [O2] in polymer, chain oxidation occurs with autoacceleration because the hydroperoxide groups that formed are a source of initiation. However, there are several substantial distinctions. As already mentioned in Chapter 6, reactions resulting in polymer destruction play an important role. In initiated oxidation the main source of destruction is reactions of peroxide radicals, whereas in autoxidation the contribution of alkoxy radicals, which decompose in the reaction of the type... 

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