Changes induced chain scission

One can consider that the kinetics carbonyl build-up is representative of the overall oxidation kinetics, at least when considered at the molecular scale (or monomer unit). It remains to establish a relationship between structural changes at this scale and molar mass changes. For the PE polymer understudy, random chain scission is predominant. It will be assumed that the main scission process is the rearrangement of alkoxyl radical (p scission). Then, every elementary reaction generating alkoxyl radicals will induce chain scission. In the chosen mechanistic scheme, both hydroperoxide decomposition processes and the nonterminating bimolecular peroxyl combination are alkoxyl sources. Thus, the number of moles of chain scissions per mass unit (s) is given by ... 

Chemical aging or chemical degradation is distinct from physical aging in that it involves often irreversible changes to the chemical structure of the polymer. Examples include oxidative crosslinking, de-polymerization, and UV-induced chain scission. These chemical changes can alter many of the physical and chemical properties of a polymeric material and again, often occur over extended timescales. 

The results of the contact angle measurements (Table I), ATR, and mass spectroscopy (11) of the irradiated and unirradiated PNF s indicate that neither a change in the surface chemistry, per se, nor irradiation-induced chain scission occurs. 

Mechanical transduction in these systems can be at the molecular, supramolecular or the micro/macro levels. For instance, chemical and structural changes caused by mechanical stimuli, such as flow- and ultrasound-induced chain scission, are interesting phenomena that can be taken into account in the development of mechano-responsive materials. In the following section, we will look closely at the types of response to mechanical stimuli and the underlying mechanisms involved either in mechano-chemical or physical transformations. 

The 355 nm emission is sharp and intense at the start of irradiation, and the intensity decreases with prolonged irradiation time. The 440 nm emission is weak and broad, and the intensity does not change with the irradiation time. Emission spectra of PMPrS obtained at ion fluences of 0.15,0.76, and 1.53 p,C/cm2 shows emission bands at 350 nm and 440 nm. The decrease in the intensity of the main peak indicates that main chain scission (photolysis) occurs under ion beam irradiation. Intense and sharp emission at 340 nm and weak broad emission at 440 nm for PDHS at 354 K are observed at the beginning of the irradiation and decrease on further irradiation. At 313 K and 270 K, sharp intense main emissions at 385 nm are seen. The 340 nm and 385 nm emission bands are assigned to a - a fluorescence. Experimental results have shown the presence of a phase transition at 313 K for PDHS.102,103 Below 313 K, the backbone conformation of PDHS is trans-planar, and above the solid-solid phase change temperature, a disordered conformation is seen. Fluorescent a -a transitions occur at 355 nm for PMPS, 350 nm for PMPrS, and 385 nm and 340 nm for PDHS. Emissions around 440 nm are observed at all temperatures examined and are assigned to defect and network structures induced by ion beams. 

Thus, it is important to realize that just because there is no change in the spectrum does not necessarily mean no change in the overall secondary structure of the protein. For example, a polypeptide chain scission may take place inducing a break in a disordered region, leaving individual separate portions of secondary structure intact. 

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