Mechanically robust, healable, shape memory, and reprocessable biobased polymers based on dynamic pyrazole-urea bonds

Recyclable Dynamic Covalent Networks Derived from Isocyanate Chemistry: The Critical Role of Electronic and Steric Effects in Reversibility

The dynamic covalent networks (DCNs), featuring dynamic covalent bonds (DCBs) formed through isocyanate-involved chemistry, potentially contributes to a circular economy in polyurea and polyurethane industries, due to the inherent recyclability of DCNs. Over the past decade, remarkable progress has been made in the development of isocyanate-derived DCBs (IdDCBs) for the synthesis of recyclable DCNs, aiming to substitute conventional, non-recyclable materials. Herein, the fundamental aspect of the IdDCB-related chemistries reported to date is investigated, and it is found that their reversibility is governed by electronic and steric effects. This discovery encourages us to structure the review into three sections. The first section examines the reversibility of various IdDCBs through the lens of electronic and steric influences. The findings show that the reversibility of some IdDCBs is driven by a single chemical effect, with the examples of steric effect contributing to the dynamic behavior of thiourethanes and hindered ureas, while other cases of reversibility arise from a combination of two or more chemical effects. The knowledge thus established allows to categorize and discuss the technologically relevant DCNs, with particular emphasis on how these chemical effects influence their recyclability. Finally, the review concludes by highlighting several potentially impactful research directions that merit further exploration.

Biobased vitrimers: towards sustainability and circularity

In polymer science and technology, the distinction between thermoplastic and thermosetting materials has always been sharp, clear, and well-documented: indeed, the former can theoretically be reprocessed a potentially infinite number of times by heating, forming, and subsequent cooling. This cannot be done in the case of thermosetting polymers due to the presence of cross-links that covalently bind the macromolecular chains, giving rise to insoluble and infusible polymeric networks. In 2011, the discovery of vitrimers revolutionized the classification mentioned above, demonstrating the possibility of using new materials that consist of covalent adaptable networks (CANs): this way, they can change their topology through thermally-activated bond-exchange reactions. Recently, the increasing attention directed at green systems and circular economies has pushed the scientific community toward the synthesis and characterization of biobased materials, including vitrimers. Indeed, these latter represent a practical and reliable answer to the demanding issue of the eco sustainability of both materials and related technologies. The main advantage of using biobased vitrimers relies on their limited environmental impact as compared with the traditional systems deriving from fossil sources. Furthermore, biobased vitrimers exploit the same chemistries and plants already optimized for their fossil-based counterparts. The present work aims to review the current use of biobased vitrimers for advanced applications, highlighting their importance for designing novel, green, and sustainable materials that perfectly match the up-to-date circular economy concept.

Phloroglucinol-Based Antimicrobial Shape-Memory Photopolymers for Microimprint Lithography

In this study, for the first time, biobased photopolymers were synthesized from phloroglucinol tris epoxy with and without different comonomers, phloroglucinol, 1,4:3,6-dianhydro-D-sorbitol, and 1,4-cyclohexanedimethanol. The rheological, thermal, mechanical, shape-memory, and antimicrobial properties of photopolymers were investigated. The addition of comonomers reduced the photocuring rate (gel time increased from 325 s to 434-861 s) and rigidity (storage modulus decreased from 330.76 to 15.42-85.77 MPa), reduced their brittleness, and increased the flexibility (elongation at break increased from 0.9 to 1.89-4.51%), although the tensile strength of the polymers remained sufficiently high (tensile strength was reduced from 292.00 to 132.62-234.54 MPa). All polymers exhibited a thermoresponsive shape-memory behavior as they could maintain a temporary shape below their glass-transition temperature and return to the permanent shape when the temperature was raised again above the glass-transition temperature. All polymers showed high antibacterial activity against Staphylococcus aureus (90.3-96.4%) and Escherichia coli (97.8-99.6%) even after 1 h of contact with bacteria. The photoresins were tested in microimprint lithography and confirmed to accurately reproduce the shape features of the 3D printed target. Compositions prepared with 1,4-cyclohexanedimethanol were the most promising due to fast photocuring and the highest flexibility. Synthesized biobased photopolymers have a wide range of properties, making them potential candidates for the production of functional coatings, biomedical devices, or flexible electronics.

Bio-Based Thermosetting Resins: From Molecular Engineering to Intrinsically Multifunctional Customization

Recent years have witnessed a growing interest in bio-based thermosetting resins in terms of environmental concerns and the desire for sustainable industrial practices. Beyond sustainability, utilizing the structural diversity of renewable feedstock to craft bio-based thermosets with customized functionalities is very worthy of expectation. There exist many bio-based compounds with inherently unique chemical structures and functions, some of which are even difficult to synthesize artificially. Over the past decade, great efforts are devoted to discovering/designing functional properties of bio-based thermosets, and notable progress have been made in antibacterial, antifouling, flame retardancy, serving as carbon precursors, and stimuli responsiveness, among others, largely expanding their application potential and future prospects. In this review, recent advances in the field of functional bio-based thermosets are presented, with a particular focus on molecular structures and design strategies for discovering functional properties. Examples are highlighted wherein functionalities are facilitated by the inherent structures of bio-based feedstock. Perspectives on issues regarding further advances in this field are proposed at the end.

Thermo-Responsive Shape Memory Vanillin-Based Photopolymers for Microtransfer Molding

Novel thermo-responsive shape-memory vanillin-based photopolymers have been developed for microtransfer molding. Different mixtures of vanillin dimethacrylate with tridecyl methacrylate and 1,3-benzenedithiol have been tested as photocurable resins. The combination of the different reaction mechanisms, thiol-acrylate photopolymerization, and acrylate homopolymerization, that were tuned by changing the ratio of monomers, resulted in a wide range of the thermal and mechanical properties of the photopolymers obtained. All polymers demonstrated great shape-memory properties and were able to return to their primary shape after the temperature programming and maintain their temporary shape. The selected compositions weretested by the microtransfer molding technique and showed promising results. The developed thermo-responsive shape-memory bio-based photopolymers have great potential for forming microtransfered structures and devices applicable on non-flat surfaces.

Dynamic covalent polymers enabled by reversible isocyanate chemistry

Reversible isocyanate chemistry containing urethane, thiourethane, and urea bonds is valuable for designing dynamic covalent polymers to achieve promising applications in recycling, self-healing, shape morphing, 3D printing, and composites.