(338ae) Substituent Effects on the Hydrolysis of Acetal Bonds to Design Hydrolysable Thermosets from Renewable Feedstocks

Polymer matrix composites (PMCs) are lightweight materials which combine costly reinforcements, such as carbon or glass fibers, within a continuous polymer phase. Thermoset polymers are often employed in structural PMCs for their excellent thermal and dimensional stability, mechanical strength, and chemical resistance to enable high-performance and structural applications. However, the permanently crosslinked molecular structure of thermosets limits end-of-life disposal options for PMCs. Conventional end-of-life processes for PMCs include mechanical grinding, pyrolysis, and landfilling. This ultimately results in a loss of costly reinforcements and increases the amount of petrol-derived plastic waste in the environment.

Incorporating reversible bonds into thermoset polymer networks has been shown to create hydrolysable materials that can be recycled at the end of a product’s operational lifecycle. By this method, reversible chemistries placed within or between polymer chains provide sites for bond exchange or hydrolysis reactions to take place in response to an appropriate stimulus. This technique can effectively eliminate bulk waste and allow for high-value materials in thermoset-based products to be reobtained. In this work, bio-derived precursors containing acid-labile acetal bonds are synthesized to generate hydrolysable, high-performance, PMCs from which continuous fibers can be recovered after use. Acetal bonds are readily formed by the condensation reaction which takes place between an aldehyde with two equivalents of an alcohol component. While acetals are generally stable in neutral and basic conditions, they can rapidly degrade in acidic solutions. Moreover, the products of this hydrolytic reaction are charge-neutral and non-toxic.

It has been well-established that the structure and substituents of acetal bonds govern monomeric hydrolysis kinetics. Once incorporated into a polymer, these trends will also control the propensity of the bulk material to hydrolyze. The effects of structural variations on hydrolysis of bio-based acetal monomers are evaluated herein with the purpose of designing hydrolysable thermosets and polymer-matrix composites. Small molecule studies conducted using ¹H NMR highlighted the importance of aromatic substituents on the hydrolytic nature of acetal linkages. Half-lives (t_1/2) were calculated from the obtained kinetic curves to compare the relative hydrolysis rates (k_obs) of each synthesized compound. Both the electronic nature and proximity of aromatic substituents were found to greatly influence the degradation rates of cyclic acetal bonds. For example, the acetal of 4-hydroyxybenzaldehyde methacrylate (t_1/2 ~ 1.5 hours) hydrolyzed over 10³ times faster than 2-hydroxybenzaldehyde methacrylate (t_1/2 ~ 2025 hours). Conversely, the presence of an electron-donating methoxy group in the para position enhances the rate of acetal hydrolysis relative to that of 4-hydroyxybenzaldehyde methacrylate.

Thermosets fabricated from select monomer precursors were characterized by dynamic mechanical analysis and differential scanning calorimetry experiments to establish additional structure-property relationships. Cured polymers exhibited room temperature storage modulus values ranging from 2-3 GPa and glass-transition temperatures between 140-200°C. Hydrolysis of these materials in 1 M HCl solutions followed similar trends to those outlined in the small molecule studies. These findings indicate that the performance and hydrolytic behaviors of thermosets are tailorable by altering the structure of acetal precursors. While this work focuses on implementing structure-reactivity relationships to tune the hydrolytic nature of thermosets, these findings can also be applied to elastomers and hydrogels.