Healable, Recyclable, and Programmable Shape Memory Organogels Based on Highly Malleable Catalyst-Free Carboxylate Linkages

Carbohydrate-Based Reprocessable and Healable Covalent Adaptable Biofoams

Polymeric foams derived from bio-based resources and capable of self-healing and recycling ability are of great demand to fulfill various applications and address environmental concerns related to accumulation of plastic wastes. In this article, a set of polyester-based covalent adaptable biofoams (CABs) synthesized from carbohydrates and other bio-derived precursors under catalyst free conditions to offer a sustainable alternative to conventional toxic isocyanate-based polyurethane foams is reported. The dynamic β-keto carboxylate linkages present in these biofoams impart self-healing ability and recyclability to these samples. These CABs display adequate tensile properties especially compressive strength (≤123 MPa) and hysteresis behavior. The CABs swiftly stress relax at 150 °C and are reprocessable under similar temperature conditions. These biofoams have displayed potential for use as attachment on solar photovoltaics to augment the output efficiency. These CABs with limited swellability in polar protic solvents and adequate mechanical resilience are suitable for other commodity applications.

Circularity in polymers: addressing performance and sustainability challenges using dynamic covalent chemistries

The circularity of current and future polymeric materials is a major focus of fundamental and applied research, as undesirable end-of-life outcomes and waste accumulation are global problems that impact our society. The recycling or repurposing of thermoplastics and thermosets is an attractive solution to these issues, yet both options are encumbered by poor property retention upon reuse, along with heterogeneities in common waste streams that limit property optimization. Dynamic covalent chemistry, when applied to polymeric materials, enables the targeted design of reversible bonds that can be tailored to specific reprocessing conditions to help address conventional recycling challenges. In this review, we highlight the key features of several dynamic covalent chemistries that can promote closed-loop recyclability and we discuss recent synthetic progress towards incorporating these chemistries into new polymers and existing commodity plastics. Next, we outline how dynamic covalent bonds and polymer network structure influence thermomechanical properties related to application and recyclability, with a focus on predictive physical models that describe network rearrangement. Finally, we examine the potential economic and environmental impacts of dynamic covalent polymeric materials in closed-loop processing using elements derived from techno-economic analysis and life-cycle assessment, including minimum selling prices and greenhouse gas emissions. Throughout each section, we discuss interdisciplinary obstacles that hinder the widespread adoption of dynamic polymers and present opportunities and new directions toward the realization of circularity in polymeric materials.

Closed-loop recyclable and biodegradable thioester-based covalent adaptable networks

Closed-loop recyclable and biodegradable aliphatic covalent adaptable networks (CANs) based on dynamic β-CO thioester linkages that exhibit a service temperature beyond 100 °C are reported. These CANs possessing tensile strength and modulus values of up to 0.3 and 3 MPa, respectively, effectively undergo stress relaxation above 100 °C. The samples exhibit creep resistance ability and low hysteresis loss, and are repeatedly reprocessable at 120 °C. These CANs are depolymerizable to monomers under mild conditions and lose notable mechanical strength (92.4%) and weight (76.5%) within ∼35 days under natural biodegradation conditions.

Stress-Induced Shape-Shifting Materials Possessing Autonomous Self-Healing and Scratch-Resistant Ability

Covalent adaptable networks (CANs) capable of both shape-shifting and self-healing ability offer a viable alternative to 4D printing technology to gain access to various complex shapes in a simplified manner. However, most of the reported CANs exhibit shape-shifting ability in the presence of temperature, light or chemical stimuli, which restricts their further utilization as realization of such a controlled environment is not feasible under complex scenarios. Herewith, we report a set of CANs based on a room-temperature exchangeable thia-Michael adduct, which undergoes rearrangement in network topology on application of external stress. These CANs with tensile strength (≤6 MPa) and modulus (≤71.4 MPa) adopt to any programmed shape under application of nominal stress. The CANs also exhibit stress-induced recyclability, self-welding and self-healing ability under ambient conditions. The transparency and ambient condition self-healing ability render these CANs to be utilized as scratch-resistant coatings on display items.

Bio-Based, Self-Healing, Recyclable, Reconfigurable Multifunctional Polymers with Both One-Way and Two-Way Shape Memory Properties

Shape memory polymers (SMPs) have attracted wide attention over the past few decades due to their fantastic applications in modern life. Nevertheless, excellent self-healing properties, recyclability, solid-state plasticity, and reversible shape-switching ability are necessary but can rarely be satisfied in one material. Herein, we report multifunctional SMPs by constructing a dynamic boronic ester bond cross-linking network using sustainable Eucommia ulmoides gum as a raw material. Thanks to the crystallization and wide melting temperature range, these kinds of SMPs have thermal-triggered one-way shape memory performance and show two-way shape memory properties, whether under constant stress or stress-free conditions. Owing to the dynamic nature of the boronic ester bond, it exhibits good self-healing properties (near 100% at 80 °C), shape reconfigurability, and chemical recyclability. In addition, by incorporating multiwalled carbon nanotubes, the formed composite is responsive to 808 nm near-infrared light. Its applications are further exploited, including photoresponsive actuators, vascular stents, and light-driven switches. This paper provides a simple way for fabricating multifunctional SMPs, and the as-prepared materials have potential applications in diverse fields, such as biomedicine, intelligent sensing, and soft robotics.