New Building Blocks for Self-Healing Polymers

Abstract

The healing efficiency in self-healing materials is bound by the ability to form blends between the prepolymer and curing agent. One of the problems in the development of self-healing polymers is the reduced affinity of the bismaleimide curing agent for the elastomeric furan-containing matrix. Even when stoichiometric amounts of both components are applied, incompatibility of components can significantly reduce the effectiveness of self-healing, and lead to undesirable side effects, such as crystallization of the curing agent, in the thickness and on the surface. This is exactly what we have seen in the development of linear and cross-linked PUs using BMI as a hardener. In this work, we present a new series of the di- and tetrafuranic isocyanate-related ureas-promising curing agents for the development of polyurethanes-like self-healing materials via the Diels-Alder reaction. The commonly used isocyanates (4,4'-Methylene diphenyl diisocyanate, MDI; 2,4-Tolylene diisocyanate, TDI; and Hexamethylene diisocyanate, HDI) and furfurylamine, difurfurylamine, and furfuryl alcohol (derived from biorenewables) as furanic compounds were utilized for synthesis. The remendable polyurethane for testing was synthesized from a maleimide-terminated prepolymer and one of the T-series urea. Self-healing properties were investigated by thermal analysis. Molecular mass was determined by gel permeation chromatography. The properties of the new polymer were compared with polyurethane from a furan-terminated analog. Visual tests showed that the obtained material has thermally induced self-healing abilities. Resulting polyurethane (PU) has a rather low fusing point and thus may be used as potential material for Fused Deposition Modeling (FDM) 3D printing.

Keywords: Diels–Alder reaction; mechanical properties; polyurethane; self-healing; thermal properties.

Figure 1

Schematic representation of the strategy used for the synthesis of self-healing polyurethanes.

Scheme 1

Synthesis of the Fu-containing curing agents.

Scheme 2

(a) Synthesis of recyclable polymer (PU-DA); (b) Schematic view of reversible covalent network formed by DA reaction between H-bonding hard segments and Fu-containing urethane derivative.

Figure 2

Self-healing properties of PU-DA sample demonstrated by two-stage thermal cycle.

Figure 3

NMR (a) and IR (b) spectra of prepolymer, PU-DA, and FA-T.

Figure 4

TGA and DTG (a) curves, DSC curves (b) of PU-DA.

Scheme 3

Classic and modified synthetic approach for remendable PUs.

Figure 5

Representative loading diagrams for pristine and healed samples of PU-DA.