Molecular scission

As we discussed in Sect. 2.3, in aU crazable polymers toughness is limited by fracture which occurs almost exclusively by the breakdown of craze matter under stress. When no extrinsic imperfections are present, craze fibrils fracture as a result of molecular level stress concentrations when load-bearing molecules fracture and set off an unstable cascade of molecular scissions. When large, micron-si d particulate... 

The main chains of the EPDM molecule have no double bonds and thus EPDM does not deteriorate due to molecular scission even after prolonged exposure to sunlight and high... 

Crystallization enthalpies of the irradiated blends were lower than predicted by the additivity rule. The crosslinking of the amorphous domains lowered the melting enthalpies. Irradiated blends showed separated crystallizations, at their characteristic temperatures of crystallization, independent of composition. On irradiation, the crystallization temperature of PE did not change, whereas that of UHMWPE slightly decreased. Separate irradiation of UHMWPE and PE favored molecular scission over the crosslinking, whereas irradiation of the blends favored crosslinking. The melting temperature, T, increased linearly with... 

The chemical resistance of PEEK is shown in Table 6.3. PEEK exhibits a remarkable chemical resistance, comparative to fluoropolymers. PEEK is approved by the FDA. PEEK undergoes crosslinking by irradiation in vacuum under stress. The tensile properties of PEEK sheets after UV radiation show a tendency to embrittlement. This is caused not only by crosslinking but also by the orientation of molecular chains resulting from the temperature rise of the specimens. Furthermore, the tensile stress applied during exposure accelerates molecular scission and disturbs the crosslinking. ... 

Some studies of unreinforced polystyrene mouldings that had been irradiated with UV while under an applied tensile stress showed that molecular scission is accelerated by the stress [43]. This could have interesting implications for weathering testing conditions which are generally unstressed. 

Molecular-scission-controlled fracture of oriented semi-crystalline poiymers... 

MALDI produces ions from large molecules with little molecular scission or fragmentation. MALDI ion sources are often coupled with a time-of-flight (TOE) mass analyzer in order to provide a virtually unlimited mass range. The MALDI technique is similar to laser desorption/ionization and the two methods have some common features. A short, intense laser pulse is fired against the sample, causing the analyte molecules to pass into the gas phase. Some of these molecules have electric charge. 

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. 

The CL analysis applied on the degradation of hydroxyl-terminated polybutadiene based polyurethane elastomers illustrates the effect of previous energetic treatment to which samples were subjected. The increase in the initial CL intensity and the shift of maximum CL intensity upon the shorter time are the effects of the initiation of oxidation by molecular scissions and, respectively, the reactions of free radicals with oxygen that leads to decrease in the material durability (Fig. 61) [06C1]. Another proof for the beginning of degradation by prior processing is the increased CL-emission at the start of the CL measurements. (Fig. 62). 

It should be pointed out at this stage that the formation of an oily degraded surface layer is not necessarily a beneficial feature. If the layer is readily removed from the rubber, then further deterioration can proceed rapidly. Indeed, if in the early stages of decomposition, the rubber is rendered softer and weaker, it will be torn away more easily and the rate of wear will be correspondingly greater than in the absence of general molecular scission. On the other hand, if the liquidlike film is viscous, tarry, and adhesive, it appears to be retained on the rubber surface to act as a protective layer. The rate of wear is then much reduced. 

The glass transition work was preceded by a study of ultrafiltration of proteins with W. J. Elford in London and was followed by a year with D. Spence, a rubber chemist who had been co-originator of accelerators in vulcanization. We groped with cross-linking and molecular scission of natural rubber by unusual reagents before these processes were clearly understood. Thereafter, back to proteins... 

Heating a fiber to progressively higher temperatures in air will lead to physical as well as chemical changes within the fiber. At sufficiently high temperatures, molecular scission, oxidation, and other complex chemical reactions associated with decomposition of the fiber will occur causing possible discoloration and a severe drop in physical and end-use properties for the fiber. 

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