Soybean oil–divinylbenzene thermosetting polymers: synthesis, structure, properties and their relationships

The cationic copolymerization of regular soybean oil, low saturation soybean oil (LoSatSoy) and conjugated LoSatSoy oil with divinylbenzene initiated by boron trifluoride diethyl etherate (BF3·OEt2) or related modified initiators provides polymers ranging from soft rubbers to hard plastics, depending on the reagents, stoichiometry and initiators used. Nuclear magnetic resonance spectroscopy and dynamic mechanical analysis (DMA) indicate that the resulting polymers are typical thermosets. The structure of the bulk polymer is that of a densely crosslinked polymer network mixed with a certain amount of less saturated free oil. The moduli of the bulk polymers are approximately 4×108–1×109 Pa at room temperature, which are comparable to those of conventional plastics. Typically, micro-phase separation occurs in soybean oil and LoSatSoy polymers as evidenced by the appearance of two glass transition temperatures α1 and α2; α1 (approximately 80°C) is the glass transition temperature of the crosslinked polymer, and α2 (approximately 0°C) corresponds to the solid–liquid transition (glass transition) of the oil-rich phase in the bulk polymer. In the conjugated LoSatSoy polymers, the crosslinked polymer network becomes more compatible with the unreacted free oil. Three distinct decomposition temperature regions of the thermosets are observed by thermogravimetric analysis (TGA): 200–400°C, 400–530°C, and above 530°C. These regions correspond to evaporation and decomposition of the unreacted free oil, degradation and char formation of the crosslinked polymer network, and subsequent oxidation of the char residues in air, respectively. The thermal stability of these polymers is found to be largely dependent on the amount of unreacted free oil in the bulk polymer. The conjugated LoSatSoy polymers have the highest moduli and thermal stabilities, since they contain the least unreacted free oil.