Synthesis and Application of Thermally Reversible Polymeric Networks from Vegetable Oils

In our modern life, thermosetting polymers can be found everywhere, from tableware to building materials, from furniture to jewelries, and from household electronics to airplane parts. Thermosetting polymers are light weight and possess excellent mechanical strength, thermal, and chemical resistance. These properties are the result of the molecular architecture of the polymeric network, which is typically a 3-dimensional structure with strong connections (cross-links). A major disadvantage of the materials is the difficulty to reprocess and recycle without loss of the desired properties. Furthermore, most thermosets are produced from petroleum resources, of which the use is under pressure. The main goals of this research were to synthesize recyclable thermosets from green sources such as vegetable oils. In order to make the materials recyclable, it was decided to develop a method that allows breakup of the cross-links without irreversible degradation of the material. In chemical terminology, we've made the cross-links with a reaction that is reversible in nature. A thermally reversible Diels-Alder reaction was used for this. Specifically, this means that the cross-links break at an elevated temperature, so that the material can be reprocessed. The research has shown that this approach is successful and that thermally reversible networks can be made from renewable sources. In addition, we have identified a number of attractive application areas for these new materials

Towards Thermally Reversible Networks Based on Furan-Functionalization of Jatropha Oil

A novel biobased monomer for the preparation of thermally reversible networks based on the Diels-Alder reaction was synthesized from jatropha oil. The oil was epoxidized and subsequently reacted with furfurylamine to attach furan groups via an epoxide ring opening reaction. However, furfurylamine also reacted with the ester groups of the triglycerides via aminolysis, thus resulting in short-chain molecules that ultimately yielded brittle thermally reversible polymers upon cross-linking via a Diels-Alder reaction. A full-factorial experimental design was used in finding the optimum conditions to minimize ester aminolysis and to maximize the epoxide ring opening reaction as well as the number of furans attached to the modified oil. The optimum conditions were determined experimentally and were found to be 80 °C, 24 h, 1:1 molar ratio, with 50 mol % of LiBr with respect to the modified oil, resulting in 35% of ester conversion, 99% of epoxide conversion, and an average of 1.32 furans/triglyceride. Ultimately, further optimization by a statistical approach led to an average of 2.19 furans per triglyceride, which eventually yielded a flexible network upon cross-linking via a Diels-Alder reaction instead of the brittle one obtained when the furan-functionalization reaction was not optimized

Thermally Reversible Polymeric Networks from Vegetable Oils

Low cross-link density thermally reversible networks were successfully synthesized from jatropha and sunflower oils. The oils were epoxidized and subsequently reacted with furfurylamine to attach furan groups onto the triglycerides, preferably at the epoxide sites rather than at the ester ones. Under the same reaction conditions, the modified jatropha oil retained the triglyceride structure more efficiently than its sunflower-based counterpart, i.e., the ester aminolysis reaction was less relevant for the jatropha oil. These furan-modified oils were then reacted with mixtures of aliphatic and aromatic bismaleimides, viz. 1,12-bismaleimido dodecane and 1,10-(methylenedi-4,1-phenylene)bismaleimide, resulting in a series of polymers with Tg ranging between 3.6 and 19.8 °C. Changes in the chemical structure and mechanical properties during recurrent thermal cycles suggested that the Diels-Alder and retro-Diels-Alder reactions occurred. However, the reversibility was reduced over the thermal cycles due to several possible causes. There are indications that the maleimide groups were homopolymerized and the Diels-Alder adducts were aromatized, leading to irreversibly cross-linked polymers. Two of the polymers were successfully applied as adhesives without modifications. This result demonstrates one of the potential applications of these polymers