Synthesizing Self-Healing and Recyclable Silicones Using the Diels-Alder Reaction as a Cross-Linker: Investigation of Various Dienes and Dienophile Systems
This thesis focuses on the synthesis of recyclable and self-healing polysiloxane elastomer networks. These features were achieved through the use of thermally reversible Diels-Alder (DA) and retro-Diels-Alder (rDA) reactions. In this work, for the model system, two different dienes (3 and 8) and six dienophile were explored, of which five of the dienophiles are commercially available and one of them was synthesized in the lab (13) to produce a series of model DA adduct. Model systems consisting of diene-functionalized trisiloxanes and bismaleimides as dienophiles were utilized to develop a fundamental understanding of how the electronic differences in the coupling systems would influence the efficiency of the overall reaction. Then for the elastomers, three different methylhydrosiloxane-dimethylsiloxane copolymer, trimethylsiloxane terminated (PDMS) with different molecular weights and Si-H group mole percentages [32 a = 3-4% Si-H and 13000 g/mol; 32 b = 7-9% Si-H and 5500-6500 g/mol; 32 c = 25-30% Si-H and 2000-2600 g/mol] were used and functionalized with two different dienes (3 and 8) to produce six polymeric diene systems (33 a, 33 b, 33 c, 34 a, 34 b and 34 c). After analyzing the model systems, the optimal temperature for adduct formation was determined to be between 60 °C – 70 °C, while the rDA reactions occur were found to occur between 90 °C and 110 °C , depending on the system. The tensile strengths of the elastomer systems correlated well with the cross-link densities of individual elastomers (elastomers were elongated between 0.3 cm and 2.54 cm). Furthermore, the hardness of the elastomers also correlated with the cross-link density of the elastomer (Shore 00 values ranged from 32 to 8 ). However, all of the elastomers displayed a decrease in their Shore 00 values after being damaged and healed. Of particular note in this study are elastomers 35 b and 35 c. Not only were these the only examples of translucent and colourless materials, the elastomers fully cured at room temperature in only 5 h. After mechanical damage the elastomers were heated to 80 °C to induce mobility in the polymer chains, complete healing of the mechanical damage was observed to occur in approximately 3 min and upon cooling to room temperature it cured and got solid again.