Sol–Gel Condensations to Form Polytetrahydrofuran Networks and Their Elastomeric Behavior

Abstract

A cationic ring‐opening polymerization was used to prepare hydroxyl‐terminated polytetrahydrofuran (PTHF) [–(CH2)4O–] having a number‐average molecular weight of 20,700 g mol−1, and a relatively narrow molecular weight distribution. Elastomeric tetrafunctional networks were prepared by hydrolysis–condensation reactions on this polymer and on two others that had been commercially obtained. The synthesis was carried out at 60°C with 3‐isocyanatopropyltriethoxysilane as an end‐capping molecule and Sn‐octoate as a catalyst. Completeness of the end‐capping reaction was followed by infrared absorption spectroscopy, which showed gradual decreases of peaks at 2270.7 and 3475 cm−1 from the disappearances of isocyanato and hydroxyl groups, respectively, and the appearance of a peak at 3350 cm−1 from newly formed NH groups. Analysis of the kinetic data suggest that the –NCO, –OH, and –NH– peaks tend to exhibit first‐order kinetics at the beginning of the reaction and reach asymptotic values at extended times. The elastomers thus obtained were studied with regard to their equilibrium swelling in toluene at 25°C, and their stress–strain isotherms in elongation. For some of the samples, high elongations seemed to bring about highly desirable strain‐induced crystallization, as evidenced by upturns in the modulus. Swelling of these samples with increasing amounts of the non‐volatile diluent dibutyl phthalate caused the upturns to gradually disappear, proving that they did indeed result from strain‐induced crystallization rather than limited chain extensibility.