Dynamic materials: exploring covalent adaptable networks in bioinspired composites
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Abstract
Covalent adaptable networks (CANs) are crosslinked polymer materials that possess advanced dynamic properties based on their ability to undergo covalent bond exchanges. These dynamic properties allow for various applications, including improvements in terms of recycling. While intriguing on their own, they can also be integrated into other polymer-based materials that rely on other phases as well. Such composites are widely used, benefiting from their reinforcing phases and hierarchical architectures to achieve properties beyond those of pure polymers. Bioinspired composites are particularly unique: They utilize design elements based on natural or biological materials to achieve advanced properties or features. However, most composites, and certainly bioinspired composites, suffer greatly from complex preparation methods and an inability to be recycled.
We show that incorporating CANs as the polymeric phase in bioinspired composites can significantly enhance recyclability. This includes the ability to mill used composites and perform compression molding, enabling the formation of new samples, as well as the potential for healing, allowing for the reuse of materials that would otherwise be discarded. Moreover, we show that the benefits of dynamic properties can extend beyond recyclability alone. The dynamic chemistries of CANs can facilitate enhanced interphase interactions and synergies, leading to improvements in mechanical performance. Additionally, we demonstrate that unique functions of polymer phases, such as shape memory behavior, can be extended to the overall composite through stress transfer between the compatible phases. Lastly, we show that (re)shaping of bioinspired composites can be a remarkably impactful feature of dynamic properties. The ability to fabricate complex geometries significantly expands the functional range of composite materials and can enable them to replace metals or polymers in applications where they were previously not viable on account of limited options for sample shapes.
In this thesis I demonstrate these concepts by showing three self-contained bioinspired (nano)composite designs:
1. Chapter 2 introduces waterborne CANs that feature internal catalysis, dissociative bond exchange and customizable properties through selection of different crosslinkers. Random copolymers based on aspartic acid can catalyze the exchange of dynamic covalent ester bonds when combined with nucleophilic crosslinkers. We also prepare advanced weldable and shapeable nacre-mimetic nanocomposites by combining the CAN with sodium fluorohectorite nanoplatelets.
2. Chapter 3 demonstrates the incorporation of epoxy-based CANs in wood-based composites. These components demonstrate their compatibility via impregnation of the CAN into the lumens and intercellular spaces of delignified wood, yielding a flexible reinforced composite material. The transparent composites can be shaped to high curvatures and in three dimensions based on partial cellulose dissolution induced by the transesterification catalyst utilized in the CAN phase.
3. Chapter 4 introduces a variety of dynamic polyurethanes that are first dispersed in aqueous media, which allows further combination with cellulose nanofibrils in aqueous conditions to prepare bioinspired nanocomposites. Using both siloxane and disulfide chemistries imbues crosslinked polyurethanes with dynamic properties that we use to allow for shaping and laminate formation in the context of the composites. Because of the nature of the aqueous polyurethane dispersions, they exhibit unique synergy with the cellulose nanofibrils, leading to mechanical properties that surpass those of pure cellulose nanofibrils.
This thesis explores a range of approaches to incorporate dynamic polymer systems into bioinspired composite materials, based on different types of CAN polymers as well as variations of the reinforcing phase.