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.


Section 1. Materials and General Methods
All reagents were commercially available and used as supplied without further purification. The following chemicals and reagents employed were purchased commercially as follows.

Fourier Transform Infrared Spectroscopy
2 Fourier transform infrared spectroscopy was conducted using a Nicolet iS10 Fourier-transform infrared spectrometer with an attenuated total reflection (ATR) attachment, and the scan ranges from 600 to 4000 cm -1 with 32 scans at a spectral resolution of 4.0 cm -1 .

Thermogravimetric Analysis
The thermogravimetric analysis (TGA) and cross-linking behaviors of CO-PIMs were performed on using a Netzsch simultaneous TG-DSC (449 F3) under N2 flow (30 mL/min). [S1] Samples (5~10 mg) were heated from 50 to 900 °C with a rate of 20 °C/min. To observe the morphology of CO-PIM-75 samples after TGA-DSC tests, the aluminium crucible after tests was opened.

Differential Scanning Calorimetry
Differential scanning calorimetry (DSC) measurements were carried using on a NETZSCH DSC 214 instrument. Approximately 8 mg of samples was placed in a pierced aluminum pan, and the test was performed from 30 to 300 °C under a N2 flow with a heating rate of 10 °C/min. Glass transition temperature (Tg) was obtained from the change in slope during the first heating curve.

Degradation Experiment
Around 80 mg of CO-PIM-75 vitrimer was immersd into a solution of 0.1 M acid (HCl+H2O) and 0.1 M alkali (NaOH+H2O) in a glass vial, respectively. The degradation behavior of samples was observed at ambient temperature in one week. [S2]

Swelling Ratio and Gelation Content Experiment
The dry CO-PIMs (~45 mg) were weighed (m0) and soaked into glass vials with EtOH (~2.5 g) under ambient temperature. Then the samples were taken out after 72 h followed by wiping the residual solvent on the surface, and the mass was weighed (ms). Finally, the samples were dried in a vacuum oven at 70 °C until the weight (md) remained unchanged. The swelling ratio (SR) and gelation content (G) was calculated as following equations: [S3] = m s −m 0 m 0

Dynamic Mechanical Analysis
Dynamic mechanical analysis (DMA) was conducted on a DMA Q800 apparatus (TA Instruments). Tests were done in a tension film mode with a fixed frequency of 1 Hz, a strain amplitude of 20 μm, a preload force of 0.01 N, and 125% force track. Samples were scanned from -20 to 250 °C at a rate of 5 °C/min. The cross-linking density ( ) was calculated by the equation of ′ = 3 e , where ′ is the modulus of rubbery plateau by DMA, R is the ideal gas contant (8.314 J mol -1 K -1 ), and T refers to the absolute temperature with (Tg + 40) °C. [S4]

Dilatometry Experiments
Dilatometry and the value of topology freezing transition temperature (Tv) were determined using a DMA Q800 (TA Instruments) apparatus in a tension film geometry. Loaded with a weak elongational force of 11 kPa, the rectangular specimen with a dimension of ~10×4×0.2 mm 3 was scanned from -20 to 300 °C with a heating rate of 5 °C/min. [S5] Tensile Tests of Dry Samples The mechanical measurements were performed using a tensile tester (Zwick/Roell Z2005 universal testing machine with a 200 N sensor) at 25 °C with the relative humidity of 46%.
Unless otherwise noted, all the test samples were dumbbell shaped (effective length: 12 mm, width: 2 mm, and measured thickness around 0.3 mm). The tensile curves were measured at a constant rate of 2 mm/min and at least five specimens were tested for each sample. The elastic modulus of the films was determined by calculating the slope of the initial position of monotonic tensile stress-strain curves. Stress relaxation tests for the CO-PIMs were performed with a predefined 0.2 mm strain at a deformation rate of 2 mm/min, and then the strain was kept constant at ambient temperature. The cyclic tensile test of CO-PIM-75 was repeated five times by stretching the films to 0.2 mm at a rate of 2 mm/min and then unloading it.
The tensile toughness (τ), a parameter that characterizes the work required to fracture the sample per unit, was calculated by the area surrounded by stress (σ)-strain (ε) curves, using the following equation: [S6] τ=∑ = =0 ( 3) where is the tensile stress, is the elongation at break.

The Contact Angle Measurement
The hydrophobicity of the films was studied on a contact angle analyzer (DSA100, Germany).
The change of contact angle measurement with time (0 min, 3min, 5min, 10min) was recorded.

Tensile Tests of the CO-PIMs Films with Moisture
Pristine polyimine films with dumbbell shaped (effective length: 12 mm, width: 2 mm, and measured thickness around 0.3 mm) were prepared in the manner described above. At ambient temperature, the pre-tared samples were immersed into sample bottles filled with deionized water. After 24h, upon removal of the water from the surface of samples within 1 min, the wet tensile samples were measured at 25 °C with the relative humidity of 46% immediately. [S7] The 4 strength retaining efficiency (η1) of wet sample was calculated by the ratio of tensile strength of the recycled sample (σw) to that of the original sample (σO). [S8]

Solubility and Chemical Resistance Tests
The rectangular CO-PIM-75 samples (10~20 mg) were separately immersed in different solvents (including MeOH, EtOH, THF, DCM, EtOAc, DMF and NMP) at ambient temperature for 7 days. Afterward, the solid and liquid phase were separated. Upon removal of the solvent from the surface of samples, the remaining mass was determined. The mass change rate (D%) was calculated according to the following equation: is the initial mass of the CO-PIMs before soaking, and 2 is the mass of the CO-PIMs after soaking for 7 days in the ambient environment.

Reshaping of Thermoset
A simple experiment was conducted by heating at 80 °C and cooling to fix the deformed shape of strip.

Self-healing Tests
The images of optical microscopy (CPV-601C) were used for assessing the self-healing

Thermal Recyclability Experiments
A plate vulcanizer (QLB-50T) was used as the reprocessing recycle. The virgin films were cut into small pieces and placed between two steel sheets covered with two polyimide (PI) films.
The reprocessing procedure was hot pressed with 10 MPa of pressure at 150 °C for 10 min to obtain a newly testable film. [S9] Afterwards, the tensile measurements of thermoset CO-PIMs with one round of reprocessing recycling were performed on a tensile tester (Zwick/Roell Z2005 universal testing machine with a 200 N sensor). The strength recycling efficiency (η2) was calculated by the ratio of tensile strength of the recycled sample (σR) to that of the original sample (σO). [S8] The elastic modulus recycling efficiency (η3) was calculated by the ratio of elastic modulus of the recycled sample (ER) to that of the original sample (EO). The toughness recycling efficiency (η4) was calculated by the ratio of toughness of the recycled sample ( R ) to that of the original sample ( O ).

Chemical Recyclability Experiments
For chemical recyclability experiments, the recycling solution was composed of 15.0 mg of 2, 4-ODA, 96.3 mg of 6FAPB and 36 g of NMP ( Figure S14). Then the goldfish-like CO-PIM-75 plastic was added in the recycling solution, and the mixture was stirred at 30 °C until complete dissolution. [S10] Then, 80.55 mg of TA was added into the solution to consume the free amine groups. After stirring for 0.5 h, 29.3 mg of TREN was added into the solution for 30 s resulting in the formation of the chemical recycled polyimine solution. Finally, the chemical recycled polyimine solution was poured into silicone abrasives with Chinese character "福" shape, and solvent evaporation with drying oven of 70 °C for 48 h.

Section 2. Synthesis and Characterization of Model Compounds (a) Typical Procedure for the Preparation of CO-PIMs
It was synthesized via condensation polymerization, in which a CAN based on two diamine monomers and TREN. The general scheme adopted for the synthesis of the polyimine copolymer is shown in Scheme 1. Terephthalaldehyde and 1-Methyl-2-pyrrolidinone in a sample bottle was stirred at ambient temperature for dissolving completely. To determine the effect of copolymerization on properties, a certain amount of 2, 4-ODA and 6FAPB were added under 2 h of stirring until they were dispersed and reacted completely. Then, we obtained a black-to-orange and homogeneous solution with the increased content of 6FAPB. TREN and the residual NMP with solid content of 10 wt% were added to the vial containing the prepolymer under accelerating stirring for 30 s. The solution was then poured into glass disc quickly. After solvent evaporation at 70 °C for 2 days. A yellow thin film was obtained after peeling it off from the glass disc upon cooling to ambient temperature. According to our latest work, [S11] postcuring temperature is an important parameter in the field of conjugation polyimine materials.
Thus, the target films in this work were cut into a dumbbell-shaped spline, and heated for 1 h under reduced pressure at 150 °C for subsequent testing. The molar ratio of CO-PIM-0, CO-6 PIM-25, CO-PIM-50, and CO-PIM-75 were listed in Table S1. The films with different mass ratios of 2, 4-ODA to 6FAPB were denoted as CO-PIM-0, CO-PIM-25, CO-PIM-50 and CO-PIM-75, respectively. Therefore, all the yields of the homogeneous and red brown films for CO-PIM-0, CO-PIM-25, CO-PIM-50 and CO-PIM-75 were over 95%. Meanwhile, the thickness of the CO-PIMs films can be adjusted by controlling the concentration of copolyimine networks in the NMP solvent. [S12]

ODA
The aldehyde groups terminated polyimine oligomer by using TA and symmetrical 4, 4'-ODA was prepared according to the method of CO-PIM-0 material, and the dosage of TA and 4, 4'-ODA monomers was the same as the TA and 2, 4-ODA monomers of CO-PIM-0, respectively. Table S1. Constitutions of various CO-PIMs vitrimers with copolymerization.