Degradation mechanochemistry

Hon DNS (1987) Mechanochemistry of lignocellulosic materials In Grassie N (ed) Develop ment m polymer degradation-7 Applied Science Publishers, Essex, England, 165-191... 

This review is organized on the basis of polymer architecture and highlights its effect on polymer mechanochemistry. The topic is restricted mostly to CST, chain degradation, and activation of mechanophores in dilute solution because more experimental and theoretical literature is available than that relating to the solid state. Solution or solid phenomena in which no CST or mechanochemical reaction occurs are therefore excluded. 

To make this review self-contained and to provide a foundatitMi for further discussion, we have included the experimental methods and theoretical models of mechanical degradation for linear chains in the second and third sections, respectively. From the fourth to seventh sections, the mechanochemistry of cyclic polymers, graft polymers, star-shaped polymers (star-shaped polymers), dendrimers, and hyperbranched polymers is summarized. In the eighth section, we survey the mechanochemistry of supramolecular aggregates and knotted polymers, where the topology constraints are temporal. We hope our overview can serve as a guideline for the future work in the field of polymer mechanochemistry. 

Bulk mechanochemistry. Unlike linear polymers, the activation of mechanophore in nonlinear macromolecules in bulk is almost blank. Recently, May found that polymers with branched architectures activate more slowly than linear counterparts in solution, yet more quickly in solid-state tensile experiments [198]. In the bulk, more factors take part in the chain degradation event, including but not limited to chain entanglements, phase separation, crystallization, and supramolecular interactions. Inspections in this direction can aid the design of mechanoresponsive materials in the solid state. 

Much of the work done in recent years on polymer mechanochemistry has made use of the high elongational strain rates observed around collapsing cavitation bubbles in sonicated solutions, as outlined in the section on mechanoluminescence [27]. In addition to the distinctive features of sonochemically-induced mechanical reactivity described above, further attention needs to be paid to the sonication conditions in the case of mechanochemical catalysis, because catalyst lifetime and turnover number are reduced by sonochemical byproducts. Implosion of cavitation bubbles is essentially an adiabatic process which leads to formation of local hotspots within the bubble in which temperature and pressure increases drastically. The content of cavitation bubbles pyrolyses under these extreme conditions and results in formation of reactive species, such as radicals and persistent secondary byproducts acidic byproducts may also form from the degradation of the substrates [75]. Chemical impurities deactivate the reactive catalyst partially if not completely. Recent studies in our group have shown that heat capacity of gas... 

The MWD resulting from mechanochemistry may not be widely different for different degradation mechanisms of 2 for the case of... 

The compound NO has been also widely used [17, 20, 101] to detect radicals spectroscopically in degraded polymer. The effect of radical acceptors in mechanochemistry is described in Section III.H. The use of a mixture of norbornene and sulfur dioxide has also been suggested as one of the most sensitive detectors of free radicals thus far discovered [137]. 

Mechanochemistry of Polyisobutylene Solutions Degradation Energy as a Function of Experimental Conditions ... 

Fig. 2.13 Mechanochemistry of polystyrene solutions degradation index DI as a function of the product of time t times the shift factor a. The symbols represent different polymers and conditions [58].

A knowledge of the effect of shear on the properties and composition of polymers is necessary from both a fundamental and practical view. Shear is indeed the prime variable in mechanochemistry. An understanding of its influence can provide a prediction of reliable processing and use conditions for polymers so that mechanochemical reactions can be controlled where they are desirable and be prevented where they lead to undesirable degradation. 

Fig. 2.18 Mechanochemistry of polyacrylamide solutions versus degrading shear stress...

Fig. 2.24 Mechanochemistry of polyacrylamide in aqueous solution bimodal degradation product shown by GPC [52].

The temperatures for maximum stability under shear in air for eight different polymers are listed in Table 3.2. There are only a few well-documented cases such as polystyrene and natural rubber, that fully reveal the general characteristics of mechanochemistry as a function of temperature. The temperature for maximum stability depends on polymer composition and its associated transitions and stability conditions. The temperature for maximum stability in shear must be above the melting and glass temperatures and below the temperature for extensive thermal and oxidative degradation. The temperatures in Table 3.2 are reduced slightly at ever higher imposed stresses. Correlations of this type for maximum stability in shear are likely to hold for the rubbery melt, and solution states. 

Temperature also influences the mechanochemistry of poisoners under uniaxial loads. This mode of fracture has been compared to degradation... 

Most workers, with few exceptions [79-81], have found a change in mechanochemistry with temperature with the maximum degradation gen-... 

Mechanochemistry of Polystyrene Degradation within the Capillary Reservoir during Extrusion for Different Polystyrenes of > l.T... 

The radical mechanism for styrene rubber mechanochemistry was demonstrated by using S-labeled l,T-dinaphthyl disulfide [21]. Good agreement was observed between the extent of degradation measured colorimetrically and by labeling. 

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