Malleable and recyclable imide-imine hybrid thermosets: influence of imide structure on material property

Although thermosetting polyimides have been widely used in many fields, it is still a challenging task to realize their repairability, reprocessibility, and recyclability, which are highly desirable in mitigating the cost and environmental concern. In this work, a series of malleable imide-imine hybrid thermosets were prepared through imine condensation of amino-terminated imide macromonomers with a dialdehyde and triamine crosslinker. Such novel hybrid materials exhibit rehealability and recyclability enabled by the dynamic imine bonds, while retaining the excellent mechanical and thermal properties of polyimide. Success here not only expands the library of building blocks for preparation of repairable and recyclable polyimides targeting different applications, but also opens new possibilities for reprocessing thermosetting polymers and developing high-performance dynamic covalent hybrid polymeric materials. applications in liquid crystal 30 , separation film 31 , flexible printed circuit 32 , battery 33 , 34 ,


Introduction
A hybrid material consisting of two or more constituents blended on the molecular level usually shows characteristics in between those of the original constituents 1 , allowing "performance integration" in a single material. [2][3][4] With the need and advent of various smart/multifunctional materials, efficient and feasible hybridization strategies have attracted increasing attention 5,6 . Particularly, hybridization of classic thermosetting polymers (i.e., highly cross-linked covalent network polymers) by incorporating dynamic covalent bonds represents a rapidly emerging and practical chemical approach, which can lead to the formation of a novel class of polymers referred as "covalent adaptable networks" (CANs). Consisting of dynamic covalent crosslinks, CANs combine excellent mechanical properties of thermosets with reprocessability, rehealability, and recyclability of thermoplastics [7][8][9] . Since the first example of Diels-Alder reaction based CAN was reported in 2002 10 , various reversible reactions such as transesterification [11][12][13] , olefin metathesis [14][15][16] , disulfide chemistry [17][18][19] , boronic acid chemistry [20][21][22] and Schiff base reaction 23,24 have been employed for the CAN preparation. Among them, polyimines (PIs) prepared via Schiff base reactions are of particular interest due to readily available starting materials, straightforward synthesis, and excellent rehealability, reprocessability, and recyclability [24][25][26][27][28] .
Commercially available thermosetting polyimides (PIms) are a class of widely used polymers, which contain cyclic imide moieties in their backbones 29 . Their rigid backbones give them excellent mechanical properties, dielectric properties, and resistance to radiation and high temperature, thus leading to a broad range of applications in liquid crystal 30 , separation film 31 , flexible printed circuit 32 , battery 33 , organic light-emitting diode 34 , optical fiber 35 , and other fields. However, at present, PIms are mainly produced as thermosetting materials. Therefore, any damages or structural defects cannot be repaired, and it is also challenging to reprocess and recycle these thermosets, which strongly impede their sustainable development. To overcome these challenges, hybridization of PIms by incorporating dynamic covalent bonds into the polymer backbone represents a promising solution.
Previously, as a proof-of-concept study, we demonstrated the excellent mechanical properties, rehealability and recyclability of an imide-imine hybrid polymer, which proved the feasibility of the hybridization strategy 36 . The strategy introduces dynamic imine bonds into the PIm backbone to realize the hybridization of PI and PIm at the molecular level. Consequently, the resulting organic hybrid materials exhibit the advantages of both PI (reprocessability, self-healing, and recycling properties) and PIm (excellent mechanical and thermal properties). However, the scope of such bottom-up hybridizing approach and the structure-property relationship of this novel class of hybrid malleable thermosets are still not clear, thus lacking practical guidelines for rational structural design. Given the easy accessibility of the imides and imines, such hybridization strategy could pave a way toward malleable PIm thermosets with a wide range of mechanical and thermal properties. We herein report a systematic study on the scope of this hybridization strategy to demonstrate its general applicability and the tunability of mechanical and thermal properties of the novel imine-imide hybrid thermosets.

Results and discussion
The poly(imide-imine) (PIm-PI) hybrid materials were prepared as thin films instead of powder samples to facilitate the subsequent sample characterization. Three Both FT-IR (Fig. S3, ESI †) and solid-state 13 C NMR spectroscopy (Fig. S4, ESI †) analysis support the anticipated chemical structures of the six hybrid poly(imideimine)s. The disappearance of the stretching vibration peak of the aldehyde groups and the appearance of the imine bond stretching band confirm the efficient polymerization process. Both the band at 1375 cm -1 due to C-N-C stretching vibration, and the ones at ~1718 cm -1 and 1782 cm -1 originated from the imide carbonyl groups confirm the With a series of hybrid materials in hand, we next tested and compared the mechanical properties of the as-obtained PIm-PIs films. All the tensile tests were performed at a tensile speed of 2 mm min -1 and each test was repeated at least 3 times with different samples (Fig. 1). Table 1 shows the performance of the six hybrid films with different imide fragments. To make a fair comparison, the molar ratio of imide to imine moiety was fixed to 1:4, and the degree of crosslinking was set to 50% (ESI †).
All hybrid films exhibit good thermal properties with glass transition temperature (T g ) higher than 130 °C and thermal decomposition temperature higher than ~200 °C. Besides the mechanical properties, the thermal properties of the imide-imine hybrid polymers were also assessed through DMA and TGA characterizations (Fig. S7, S8, Table S1, ESI †), and the relevant parameters are summarized in Table 1 24,28,46 . The repairability and recyclability of PIm-PIs were thus explored. Following a similar procedure for polyimines reported by our team 28,47 , a piece of PIm-PI was cut into two pieces, which were then put together in contact with a crack width of ~400 µm. To the contact area and 66-103%, respectively (Table S2).

Conclusions
In summary, by integrating dynamic imine bonds with imide building blocks, a series was allowed to stand for 1 h until the solid completely precipitated out. The precipitates were collected by centrifugation and suction filtration. The collected dark brown solid was dried under vacuum at 65 °C, and finally separated and purified by column chromatography (eluent: CH 2 Cl 2 /CH 3 OH=180:1) to obtain the pure product.

Preparation of PIm-PIs films:
The molar ratio of imide: imine groups was fixed to 1:4 in all the polymers. The molar percentage of the crosslinking amine moieties in the total primary amines in the resulting polymer network (denoted as crosslinking density) was fixed to 50%. Imide monomers Im, tris(2-aminoethyl)amine (TREN) and diethylenetriamine (DETA) were uniformly dispersed in DMF in 3:3:4 equivalents. A solution of TPA in DMF was added into the above solution. The solution was poured into a dust-free glass dish. After two hours, the solution turned into a completely opaque gel state. The gel was kept at room temperature for 2 h, and the resulting film was then heated on a flat heating device at 50 °C for 12 h, and kept at 60 °C, 80 °C, 100 °C and 120 °C for 2 h at each temperature. After soaking in deionized water at room temperature to demold, the film was transferred to a vacuum oven and kept at 65 °C overnight. The obtained films were finally heat-pressed at 65 °C for 2 h and 75 °C for 2 h to give a smooth and transparent organic hybrid film.

Recycling of hybrid films:
The recycling procedure reported previously was followed. 24, 36

Conflicts of interest
The authors declare no conflicts of interest.