Catalyst‐Free Synthesis of Covalent Adaptable Network (CAN) Polyurethanes from Lignin with Editable Shape Memory Properties

Abstract

Here a new strategy of catalyst-free direct synthesis of covalent adaptable network polyurethanes (LPUs) from lignin with editable shape memory effect is reported. Using unmodified lignin, PEG, and isocyanate under the condition of the isocyanate index less than 1.0 (NCO/OH < 1.0), a variety of LPUs are obtained. When NCO/OH = 0.8, a stable cross-linked network can be formed (ex. the gel content of LPU50-0.8 was 98 � 0.3 %). The activation energy (E a ) value of LPUs is similar to that of polyhydroxyurethanes (PHUs), at around 110 kJ mol À 1 . With an increase of lignin content, the LPUs show a transition from ductile fracture to brittle fracture mode. And the mechanical properties of LPUs are significantly enhanced after extrusion processing, with the maximum modulus reaching 649 � 26 MPa and the maximum toughness up to 9927 � 111 kJ m À 3 . The improvement in mechanical properties is due to the homogenization of complex cross-linked network under the powerful external force of the extruder and the lignin that originally was free in the system participated in the exchange reactions. Moreover, LPUs can also be prepared continuously in one step by using an extruder as the reactor. In addition, LPU50-0.8 has an editable shape memory effect. This study develops a novel method for the synthesis of LPU from lignin with NCO/OH < 1.0, showcasing new possibilities for value-added utilization of lignin, and expands the bio-based products portfolio from biomass feedstock to help meet future green manufacturing demands.

Lignin-Based Covalent Adaptable Network Polymers─When Bio-Based Thermosets Meet Recyclable by Design

“…Lignin has been also exploited in polyurethane networks with CAN properties. 118 characterized with excess hydroxyl groups that participated in the exchange reactions involving urethane bonds (Scheme 22). When lignin content was low (<40%), the modulus and tensile strength of thermosets decreased with the increase of the isocyanate index (NCO/OH).…”

“…Lignin has been also exploited in polyurethane networks with CAN properties . Specifically, EHL was treated with poly ethyleneglycol with a molecular weight of 2000 (PEG 2000) and hexamethylene­diisocyanate (HDI).…”

High‐Performance, Light‐Stimulation Healable, and Closed‐Loop Recyclable Lignin‐Based Covalent Adaptable Networks

“…Although those LPUs had excellent bond exchange characteristics and processibilities, their mechanical properties were poor with a maximum toughness of around 9.93 ± 0.11 MJ m −3 . [17] Zhang et al reported a fully bio-based vitrimer with a high lignin content and the bio-based vitrimer showed excellent shape memory and repairing properties at elevated temperatures. However, the bio-based vitrimer had tensile strength of only 12.9 ± 1.5 MPa, with an elongation value of 10.8 ± 1.0%.…”

“…[ 16 ] Moreover, due to the complex structure and undefined molecular weight, most lignin‐based CANs reported in the literature possessed low mechanical properties. In an earlier study, [ 17 ] we prepared lignin‐based polyurethanes (LPUs) containing dynamic bonds using unmodified enzymatic hydrolysis lignin as the raw material via a new strategy of utilizing a low isocyanate index (NCO/OH < 1.0). Although those LPUs had excellent bond exchange characteristics and processibilities, their mechanical properties were poor with a maximum toughness of around 9.93 ± 0.11 MJ m −3 .…”

“…Our previous study [ 17 ] showed that, at NCO/OH < 1.0, there were excess hydroxyl groups available in the network to participate in the formation of coordination sacrificial bonds, [ 22 ] which helped dissipate energy when external forces were applied and significantly improved the mechanical properties of LPU materials. [ 23 ] In addition, due to existence of a large number of conjugated molecular structures, lignin has excellent light absorption capacity.…”