A new polythiourethane thermosetting system based on a diisocyanate and a trithiol was studied. After characterization of the reactive species, two critical temperatures, namely T g∞ (maximum glass transition temperature of the thermosetting system) and gel T g (T g of the material at the gel point), were determined. The conversion at gel point was also determined and compared to the theoretical prediction. Two different characteristics of the evolution of a reactive system, T g and conversion x as a function of time, were related. This is of particular interest for understanding the curing process, especially when non-isothermal cure schedules are used. Viscoelastic and plastic properties were investigated, with the aim of establishing connections between the chemical structure and properties through detailed analysis of the polymer chain motions (β and α relaxations). Measurements of the storage modulus at the rubbery plateau and of the critical strain intensity factor K Ic complete the study.
Both viscoelastic behavior and plastic deformation in compression were investigated on new polythiourethane networks based on diisocyanates and tri-or tetra-thiols. Dynamic mechanical analysis was used to characterize the mechanically-active a and b relaxations as a function of crosslink density and nature of the diisocyanate. Data molecular analysis on these novel materials led to conclusions in good agreement with earlier statements on the well-known epoxy-amine networks. Besides, consideration of the relaxational behavior, and especially in the b relaxation region, allowed one to interpret the plastic deformation properties of the systems under study and to predict their poor resistance to fracture. By the way, analysis of the energy required to yield permitted determination of a new characteristic temperature, T y , which can be compared to the thermomechanical main transition temperature, T a , and to the glass transition temperature, T g .
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