Mechanically Robust, Recyclable, and Self‐Healing Polyimine Networks

Abstract

To achieve energy saving and emission reduction goals, recyclable and healable thermoset materials are highly attractive. Polymer copolymerization has been proven to be a critical strategy for preparing high‐performance polymeric materials. However, it remains a huge challenge to develop high‐performance recyclable and healable thermoset materials. Here, polyimine dynamic networks based on two monomers with bulky pendant groups, which not only displayed mechanical properties higher than the strong and tough polymers, e.g., polycarbonate, but also excellent self‐repairing capability and recyclability as thermosets are developed. Owing to the stability of conjugation effect by aromatic benzene rings, the final polyimine networks are far more stable than the reported counterparts, exhibiting excellent hydrolysis resistance under both alkaline condition and most organic solvents. These polyimine materials with conjugation structure can be completely depolymerized into monomers recovery in an acidic aqueous solution at ambient temperature. Resulting from the bulky pendant units, this method allows the exchange reactions of conjugation polyimine vitrimer easily within minutes for self‐healing function. Moreover, the introduction of trifluoromethyl diphenoxybenzene backbones significantly increases tensile properties of polyimine materials. This work provides an effective strategy for fabricating high‐performance polymer materials with multiple functions.

High-Performance Poly(imide-imine) Vitrimers Derived from Semiaromatic Bisimide Diamines

“…To make this conversion visible, TGA-DSC synchronous thermal analysis has been performed on the resulting PIIH films from room temperature to 800 °C in nitrogen, during which the heat flow resulting from the imine self-cross-linking was monitored by the DSC accessory. As shown in Figures S27–S31 (Supporting Information), an evident exothermic peak can be observed at around 230–350 °C along with the insignificant weight loss for all PIIH samples from their DSC and TGA curves, possibly resulting from the self-cross-linking reaction between aromatic imine linkages to form cyclic dimer or trimer in nitrogen flow (Figure S32, Supporting Information), which have been confirmed in several reported investigations. ,– Additionally, the enthalpy (Δ H ) for imine cross-linking varies from 168.7 to 187.9 J·g –1 upon varying the quantity and substitution position of the substituents, suggesting that the imine cross-linking behavior is substituent dependent. The TGA and corresponding derivative thermogravimetry (DTG) curves obtained from TGA-DSC measurements are also given in Figures S27–S31.…”

“…The glass transition temperature ( T g ) of the resulting PIIH s was measured using DMA, the results of which are shown in Figure b and listed in Table . It can be seen that T g of the prepared PIIH s (211–306 °C) is significantly improved compared with those reported in the literature ( T g = 25–120 °C). ,,,,,, It is worth noting that m -MPIIH and o -MPIIH show two obvious glass transitions, respectively, at 211–216 and 270–280 °C, which usually appears in polymers or composites with two incompatible phases, while this phenomenon is significantly restrained for o -DMPIIH and m -3FPIIH . Additionally, an evident subglass transition appears at around 200 °C for o -DMPIIH and Un -PIIH , whereas it shifts to a much higher temperature (∼270 °C) for m -3FPIIH .…”

“…The thermal decomposition temperature of the resulting PIIH films was tested using the TGA method in nitrogen. As listed in Table , the 5% weight loss temperature ( T d5% ) of all samples is above 340 °C and 10% weight loss temperature ( T d10% ) is around 380 °C, much higher than those of a few reported polyimines whose T d5% is commonly in the range of 200–300 °C, ,,,,, indicating that the inclusion of the rigid imide linkages favors higher heat resistance albeit some thermo-susceptible groups like methyl substitutes and alicyclic moiety have been introduced. Besides, it is found that T d5% for all PIIH films is close regardless of the main chain compositions, indicating that the substitution position, type, and quantity of the substitutes exert negligible effect on the pyrolysis behavior of the resulting PIIH s when the molecular structures of the bisimide diamines are similar.…”

“…These tensile properties are significantly higher than those of a majority of reported polyimines whose tensile strength is typically in the range of 20–50 MPa with tensile modulus less than 1.5 GPa, ,,,,,,, and are even comparable to those of some traditional high-performance polyimides (tensile strength: 90–120 MPa, tensile modulus: 2–3 GPa). – ,– On the one hand, the prepared PIIH films contain rigid and large polar imide linkages (Figure S39, Supporting Information), which increases the polymer backbone rigidity and the intermolecular interactions as well as the cohesive energy density, thus endowing the resulting PIIH films with high tensile modulus and tensile strength. On the other hand, the as-synthesized bisimide-containing diamines possess flexible ether linkages, and the cross-linker, tri­(2-aminoethyl) amine, also contains elastic methylene units (Figure S39, Supporting Information), which could facilitate the single bond rotation and conformational rearrangement of the polymer network when subjected to external stress, consequently increasing the toughness and robustness of the PIIH networks. ,,,, By contrast, when o -MHBIDA and m -3FBIDA are used to fabricate polyimines, the tensile strength slightly decreases to 88.3 MPa for o -MPIIH and 83.6 MPa for m -3FPIIH . However, an evident reduction in tensile properties is observed for Un -PIIH (tensile strength 71.5% of that of o -DMPIIH and tensile modulus 63.3% of that of o -DMPIIH ) when all substituents are eliminated from the starting diamine monomer Un -HBIDA .…”

“…It has been reported by Zeng and Yu et al ,– that aromatic imine linkages are prone to self-cross-linking when thermally treated at the temperature of above 200 °C, accompanied by –CN– (imine linkage) converting to −C–N– (aminal linkage). To make this conversion visible, TGA-DSC synchronous thermal analysis has been performed on the resulting PIIH films from room temperature to 800 °C in nitrogen, during which the heat flow resulting from the imine self-cross-linking was monitored by the DSC accessory.…”