engleski

Effect of Structure Change on Radiation Crosslinking of Unsaturated Polyesters

During the course of crosslinking of unsaturated polyesters reacting system, that was liquid prior to reaction, gels, and becomes solid. Crosslinking reaction begins to be controlled by the change of the physical state of the system at an early stage of reaction. The kinetics can not be studied by the usual kinetical methods. In-source 60Co gamma rays induced crosslinking of unsaturated polyester with styrene was followed directly and continuously by measuring electrical conductivity change. The results of extraction analysis proved good correlation between the change of electrical conductivity and the extent of curing. The gel content was inversely proportional to conductivity and free styrene content directly proportional to conductivity. DC-electrical conductivity has shown high sensitivity toward structural changes and enabled us to detect liquid-liquid transitions in unsaturated polyester. The upper liquid-liquid transition (T lr) is less known transition caused by a stepwise decrease of intramolecular short-range local order that remains above the glass and lower liquid-liquid transitions. The local order is based on secondary valent interactions and is enhanced by hydrogen bonding. The linear temperature dependence of the viscosity and dc electrical conductivity of unsaturated polyesters showed a change of slope caused by the Tlr. Those changes were the result of the diminishing of the local order (which includes several bond lengths) caused by breaking of the intramolecular interactions. The intramolecular nature of the Tlr in the polyesters under consideration was proved by its insensitivity to crosslinking and dilution with solvents. In the corresponding temperature range, DSC thermograms shoved expected endothermic changes. The structure changes related to the Tlr in the investigated polyesters were determined by 1H NMR and NIR spectroscopy. The proton NMR indicated that the stepwise change in hydrogen bonding occurred in the corresponding temperature interval. More detailed results were achieved by NIR spectroscopy. Sharp changes of absorptions related to hydrogen bonding were detected in the temperature range of the Tlr temperature. The crosslinking reaction rates were calculated from the slope of conductivity data plotted against the reaction time. The Arrhenius plot of the reaction rates showed a breaking point in the temperature range of the upper liquid-liquid transition. The reaction had lower activation energy in the temperature range above the Tlr because the breaking of the intramolecular interactions allowed more styrene molecules to penetrate the coil and more double bonds became available to crosslinking thus increasing its rate. The Tlr transition also influenced the dose at which vitrification was achieved.

radiation crosslinking; unsaturated polyesters; electrical conductivity; liquid-liquid transitions; NIR spectroscopy; hydrogen bonding

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