Crosslinked and noncrosslinked diisocyanate‐linked elastomers containing substituted urea groups

Abstract

AbstractA study was made of the preparation and properties of diisocyanate‐linked elastomers prepared from polyoxypropylene glycol (POPG), toluene‐2,4‐diisocyanate (TDI), and toluene‐2,4‐diamine (TDA). The chemical structure and gelation of a series of prepolymers were investigated first. Next, many series of elastomers were prepared at 60°C. in open molds and at 110°C. in closed molds from mixtures in which values of the ratio of isocyanate groups to the sum of the hydroxyl groups and amino groups (R1) and the ratio of the amino groups to hydroxyl groups (R2) were systematically varied. Cure at 60°C. gave tough, noncrosslinked elastomers, the properties apparently resulting from interchain attractive forces, while cure at 110°C. gave covalently crosslinked elastomers having rather similar properties. Glass‐transition temperatures increased linearly with the concentration of substituted urethane groups but were independent of the concentration of substituted urea groups. An explanation for the surprising behavior is presented. The noncrosslinked elastomers were soluble in N,N‐dimethylacetamide (DMA) and similar solvents. Intrinsic viscosities in DMA were found to increase with R1 at constant R2. Young's modulus, tensile strength, and ultimate elongation were determined between −23 and 82°C. At elevated temperatures, the crosslinked elastomers showed near‐equilibrium moduli, while the noncrosslinked ones did not. The number of chains per milliliter for the crosslinked elastomers was calculated from the quantities of excess isocyanate and urea hydrogens, along with an assumed topology for the network. The moduli for both types of elastomers were related to chemical composition. Various relationships between the ultimate properties and equilibrium moduli were evaluated. Extensions up to 5000% with elastic recoveries of better than 90% were noted.