Mechanically Robust and Chemically Recyclable Polyhydroxyurethanes from CO2-Derived Six-Membered Cyclic Carbonates

Polyphenol lignin derived non-isocyanate polyurethane with mechanically robust and photothermal conversion performances

The development of lignin-based non-isocyanate polyurethane (LNIPU) to replace petrochemical polyurethane (PU) has attracted attention, but is limited by the low reactivity and heterogeneity of lignin. Herein, high reactive polyphenol lignin was employed to prepare LNIPU via cyclic carbonation of lignin and subsequently reacting with diamine. The integration of polyphenol lignin constructed hydroxylaminoformate structures in polymeric hard segments, promoting the formation of intermolecular hydrogen bond networks and dynamic hard domains. The resultant LNIPU possessed tunable hard-soft nanophase separation structures by altering cyclic carbonated lignin amounts. And LNIPU behaved fantastic thermostability with the initial degradation temperature (T5%) of 200 °C. The tensile strength, Young's modulus and fracture energy of LNIPU were 5.96 MPa, 33.5 MPa and 0.25 MJ/m3, increased by 7.6, 5.1 and 2.8 times compared with NIPU, respectively. Due to the dual dynamic covalent and noncovalent networks, LNIPUs had relatively low activation energy (77.32-107.25 J/mol) and high molecular motility, enabling 100 % self-healing nature. Moreover, abundant aromatic conjugated structures in LNIPUs with polyphenol lignin enabled outstanding photothermal conversion ability. Under NIR irradiation with a power density of 1.0 W/cm2, the surface temperature of LNIPU 30 % quickly increased to 200 °C after 120 s, and displayed excellent stability undergoing 5 heating-cooling cycles. Therefore, this work not only provides a green and facile strategy for developing lignin-derived non-isocyanate polyurethan, but also provides a promising green engineering material with high strength, toughness and photothermal conversion property.

Comparing Surface and Bulk Curing Processes of an Epoxy Vitrimer

We used atomic force microscopy-based infrared spectroscopy (AFM-IR) and nanomechanical mapping (AFM-NM) to image the surface of a vitrimer, specifically dicarboxylic acid-cured diglycidyl ether of bisphenol A (DGEBA), to assess the curing process of a surface layer and compared this to the process in the bulk. We identified the β-hydroxy esters with various functionalities that are the key to form the cross-links for a system, including difunctional DGEBA and carboxylic acids. The IR peaks of the carbonyl group in generated ester groups are distinguished clearly from those in acids, allowing us to quantitatively assess the curing process at the surface and in the bulk. The initial curing at the surface exhibits a gradual cross-linking and is found to be lower than a rapid cross-linking in the bulk due to a relatively lower concentration of the β-hydroxy esters with high functionalities. This curing process leads to a smaller chemically and mechanically heterogeneous nanostructure at the surface relative to the bulk. After multiple reprocessings, a substantial number of esters lacking dynamic exchange capability form in the bulk, which decrease the flowability and reprocessability of the vitrimers and therefore the mechanical properties.