Radiation crosslinking and liquid-liquid transitions in unsaturated polyesters

Ionizing radiation offers clean and easily regulated way of initiating polymerization and crosslinking reactions, independent of the reaction temperature but only very few methods can be used for in-situ reaction monitoring. The advantages and limitations of electrical conductivity as a method for reaction monitoring are discussed on crosslinking of unsaturated polyester (UP) resin as an example. The results are compared to non electrical methods like extraction analysis and differential scanning calorimetry (DSC). The knowledge gathered over the years about the various aspects of UP crosslinking by us and other authors is reviewed and reinterpreted according to newer approaches, taking into account not only the influence of microgel formation in various reaction conditions but also reaction induced phase separation, its influence has been shown to depend on polyester composition. The importance of determining the extent of post-irradiation crosslinking that is rarely studied is shown. The correct approach to interpretation of reaction conductivity data is presented and the need to avoid use of logarithms of original data if not based on a particular physical background is stressed. The application of electrical conductivity method to study of crosslinking of UP resins led to discovery of liquid-liquid transitions that were investigated in greater detail. In addition to viscometry and DSC, the structural background of those transition, the decrease of short-range or local order based on inter-and intramolecular hydrogen bonding was confirmed by spectroscopy, particularly near-infrared spectroscopy. The details on those transitions, mostly on less known upper liquid-liquid transition, that should be investigated in all polymers, especially those with lower glass transition temperature, are presented.