Crosslink scission

These processes compete with radiation-induced crosslinking, scission and, for case (c), polymerization. 

Polymer-melamine crosslinks are also broken during exposure to UV light. Although some crosslink scission occurs in the dark by direct hydrolysis, the rate of scission (as measured by the rate of disappearance of residual methoxy functionality) Increases dramatically with Increasing UV light Intensity (11). Scission occurs with UV light in the absence of humidity but at a much reduced rate. 

To summarize the sol-gel and ESR data, the accelerated creep during irradiation is not caused by reactions which would significantly change the rate of crosslinking, scission, concentration, or type of free radicals. 

The amounts of crosslinking, scission, and number and type of free radicals produced in unstressed polystyrene samples by the radiation agreed well with literature values. Stressed samples gave the same results as unstressed samples, with the possible exception of a slightly lower amount of chain scission for stressed samples. This is opposite... 

EDPE/PP 1 5 RT vac X 300-1500 Crosslinking, scission, crystallization, thermal analysis, density 5... 

Some chain scission does however occur but typically the crosslink scission ratio is of the order of 10 1. 

Keywords Radiation, Medical polymers. Mechanical properties, Crosslinking, Scission, Aging,... 

It is generally accepted that the other polyacrylates have mechanisms of breakdown in which random scission is important (Scheme 8). In contrast to the situation in the cases of PMMA, PS and PAMS, where one of the two macroradicals depolymerizes to monomer, the radicals produced by homolysis in PMA do not behave in this way. It may be assumed that the more reactive primary radical (B) stabilizes itself by intermolecular and intramolecular transfer of a tertiary H atom and radical (A) may also be involved in this way. The product in either case is a tertiary radical (C) which provides a site for various possible reactions, including scission and crosslinking. Scission leads to an unsaturated chain end and radical (A). 

Verbruggen and co-workers [13] prepared a model NR gumstock (i.e., cure system only, no fillers or other additives) compound and subjected a thin film (300 pm thick) of it to DPDS at 200 "C. Solvent extraction (with acetone and tetrahydrofuran), swelling experiments (in toluene) and chemical analysis by GPC and nuclear magnetic resonance (NMR) spectroscopy were used to assess the degree of devulcanisation and to characterise the samples produced. It was found that complete network breakdown was obtained with 2.4% w/w of DPDS after 2 h of heating, but that both crosslink scission and main-chain scission had occurred. 

A novel reactor for pyrolysis of a PE melt stirred by bubbles of flowing nitrogen gas at atmospheric pressure permits uniform temperature depolymerisation. Sweep-gas experiments at temperatures 370-410 C allowed pyrolysis products to be collected separately as reactor residue (solidified PE melt), condensed vapour, and uncondensed gas products. MWDs determined by GPC indicated that random scission and repolymerisation (crosslinking) broadened the polymer-melt MWD. 19 refs. USA... 

SCHEME 31. S (a) Oxidation, dehydrofluorination and crosslinking reactions as a result of EB irradiation of poly (vinylidenefluoride-co-hexafluoropropylene-co-tetrafluoroethylene). (b) Chain scission reactions as a result of EB irradiation of poly (vinylidenefluoride-co-hexafluoropropylene-co-tetrafluoroethylene). 

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