Light‐absorbing copolymers of polyimides as efficient photothermal materials for solar water evaporation

Polyimide (PI), an important engineering polymer with a rigid chemical structure, readily has excellent chemical stability, heat resistance, and electrical insulation but lacks broadband photothermal properties. Herein, we design and synthesize PI copolymers that embrace intrinsic photothermal properties by using two diamine monomers of (Z)‐2,3‐bis(4‐aminophenyl) acrylonitrile (CNDA) and 4,4‐diphenyldiamine (NDA) with strong ultraviolet (UV), and near‐infrared (NIR) absorption capabilities, respectively. Tuning the molar ratio of the two diamines can modulate UV and NIR light absorption and regulate the intrinsic photothermal properties of PIs. After condensation with pyromellitic dianhydride, the resulting PI‐0.5 with a unit molar ratio of CNDA:NDA = 1 shows the best photothermal efficiency. PI‐0.5 is used to construct 3D steam generators with vertically dried channels by a freeze‐drying method. The 3D steam generators show a good water evaporation rate and continuously operate with excellent stability under varying salinity and pH conditions. The synthetic design herein suggests that PI can be molecularly engineered to be intrinsic photothermal materials, expanding the properties and applications of existing PIs.


INTRODUCTION
Population growth and rapid modernization have caused a dramatic increase in global energy consumption. [1,2]Solar energy is an abundant renewable resource, and its effective use can mitigate climate change and the global energy crisis triggered by fossil fuels. [3,4]21][22] An ideal photothermal material should efficiently collect light across the solar spectrum and convert sunlight to heat without any other energy transformation (e.g., reradiation).[35] Despite remarkable advances, challenges persist.Consequently, highly efficient and effective photothermal materials remain to be explored.
Polyimides (PIs) have excellent physicochemical properties such as outstanding chemical stability, superior thermal stability, and excellent electrical insulation, which lead them to a remarkable prominence in aerospace, [36][37][38] microelectronics, [39,40] and separation technologies. [41,42]mong the many types of PIs, black PIs [43][44][45] have total visible light absorption and are considered good candidates for covering and light-absorbing films in optical terminators. [46]wing to their low transmittance and high light absorption, black PIs are expected to produce copious heat and show enormous potential in photothermal applications.In black PIs, the intermolecular and intramolecular charge-transfer complexes [47,48] contribute to photothermal effects.[51] For example, the previous works utilized the composite black PIs through mixing the carbon nanotubes or MXene with PI to prepare the photothermal material for solar evaporation.Clearly, this solar evaporation system utilized PI as a solar evaporation skeleton rather than a solar absorber for its excellent mechanical and heat resistance properties.However, the composite black PIs are prone to aggregation and inevitably deteriorate the mechanical properties, and all previous polyimide do not show intrinsic photothermal character.To avoid the necessity of adding light-absorbing fillers, the synthesis of black PIs that are intrinsically photothermal becomes urgently demanded.
Herein, we design a series of black PIs with highly efficient photothermal properties.The PIs were synthesized through random copolymerization of dianhydride monomer of pyromellitic dianhydride (PMDA) and two diamine monomers of 4,4-diphenyldiamine (NDA) and (Z)−2,3bis(4-aminophenyl) acrylonitrile (CNDA) with strong ultraviolet (UV) and near-infrared (NIR) light absorption, respectively (Figure 1).The resulting black PIs displayed strong solar light absorption and high photothermal efficiency.As a proof of concept, the black PIs were used to construct 3D solar water evaporators via a directed freeze-drying technique, showing a high water evaporation rate of 2.31 kg m −2 h −1 .The water evaporators were durable under varying salinity and pH conditions, and the ion removal rate for simulated seawater exceeded 99.9%.

Synthetic characterization of PAA-x and PI-x
The synthesis and characterization of CNDA are shown in supporting information according to our previous work (Figures S1 and S2). [52]To obtain a series of black PIs, the NDA molar fraction (x) was varied from 0 to 1.The corresponding polyamic acid and polyimides are designated PAA-x and PI-x, respectively.The chemical structures of PAA-x and PI-x were characterized by Fourier transform infrared spectroscopy (FTIR).For PAA-x, the distinct peaks at 1657, 1542, and 1498 cm −1 were assigned to the vibration of C=O, C-N(H)-C, and C=C, respectively (Figure S3).After thermal imidization, PI-x displayed asymmetric stretching and symmetric stretching of C=O (1779 cm −1 and 1720 cm −1 ), bending of C=O (721 cm −1 ), and stretching of C-N-C (1357 cm −1 ) (Figure 2A).The typical characteristic peak at about 2217 cm −1 is attributed to the vibration of -CN group in PI-x (x = 0, 0.33, 0.5, 0.66, 0.83).The NDA fraction in PI-x was quantitatively analyzed by X-ray photoelectron spectroscopy (XPS, Figure 2B).The N1s core-level spectra of PI-x (x = 0, 0.33, 0.5, 0.66, 0.83, and 1) could be deconvoluted into two peaks at 398.9 eV and 400.3 eV, which were attributed to C≡N and C-N bonds, and C≡N only derived from CNDA.The peaks at 398.9 eV gradually weakened with a decreasing CNDA content, indicating that the reactivities of the two diamines with the dianhydride were equivalent (Figure S4).The composition of PI-x could be adjusted by the feed ratio of the two diamines.Differential scanning calorimetry traces exhibited no glass transition temperatures (T g ) below 300 • C (Figure S5).Thermogravimetric analysis traces showed excellent thermal stability with decomposition temperatures (T d ) of ∼500 • C at 5% weight loss (Figure 2C).Powder X-ray diffraction showed all PI-x exhibited similar broad peaks, indicative of similar molecular stacking.The stacking order first increased and then decreased with an increasing NDA content, and PI-0.5 reached the maximum degree of order.Ordered molecular stacking could enhance molecular interactions and thus potentially provide strengthened photothermal conversion (Figure 2D).

2.2
Photothermal behavior of PI-x PI-x powders appeared increasingly dark with increasing content of NDA.When the NDA molar fraction reached 0.5, the PI powder appeared utterly black, indicating its strong ability to absorb light (Figure 3A).The photothermal properties of PI-x in the solid state were investigated using lasers of 365, 365-520, and 808 nm, as well as xenon lamps.The detailed thermal behavior of PI-0.5 has been summarized in Figure 3B.As shown in Figure 3C, PI-0 only presents a moderate temperature rise except for UV irradiation.In contrast, PI-0.5 showed optimal photothermal performance and a maximum temperature rise under various light sources.Specifically, when illuminated by lamps of 365, 365-520, and 808 nm, PI-0.5 reached the maximum equilibrium temperatures of 141, 39.7, and 238 • C, respectively.UV-Vis-NIR spectra showed that PI-x (x = 0, 0.5, 1) displayed extensive light absorption in the solar spectrum.Particularly, PI-0.5 showed the strongest light absorption due to the synergistic charge-transfer (CT) effect of the electrondonating monomer NDA and strong UV absorption of CNDA (Figure 4A).Additionally, photoluminescence spectra of PIx were obtained using laser irradiation with photoexcitation wavelengths (λ ex ) of 300 nm (Figure S6).An extremely weak fluorescence signal was observed in the range of 360-570 nm, indicating that most energy absorbed by electrons was dissipated non-radiatively, in consistence with the strong photothermal effect.Femtosecond transient absorption (fs-TA) spectroscopy was conducted with 360 nm femtosecond pulse excitation.The intensity of the 500-800 nm band increased from 0 to 0.28 ps (Figure 4C,D), indicating PI absorbed photons transited from the ground state to a singlet excited state.After 0.28 ps, the excited states returned to the ground state, and the intensity of the 500-800 nm band decayed with increasing quick amplitude.In contrast, PI-0 (Figure S7a,b) and PI-1(Figure S7c,d) showed much lower absorption intensities and narrower absorption wavelength ranges than PI-0.5. Figure 4B showed the schematic illustration of the photothermal process.The kinetics of PI-x showed that exciton decayed faster in the copolymers than in the homopolymers, indicating a faster hole transfer channel in the copolymers (Figure S8).The faster hole transfer process could result in higher CT yield in the active layer and therefore endow higher photothermal conversion efficiency.Density functional theory results show that the emission oscillator strength of PI-0.5 approximately equals zero due to the strong intramolecular and intermolecular CT interactions, indicating that electron relaxation tends to dissipate in a nonradiative form (Figure S9).The absorption oscillator strength could reach a high value of 6.4, suggesting that PI-0.5 possess strong light absorption capacity.

Preparation and performance of 3D water evaporator
As a proof of concept to illustrate the utilization of photothermal properties embraced by the newly designed PIs, PI-0.5 with the optimal photothermal effect was used to construct 3D water evaporators with vertical channels (V-3D-PI), wherein the cellulose nanofiber (CNF) and polyurethane (PU) were considered the waterway and heat insulation layer, respectively (Figure 5A).For comparison, PI-0.5 with non-aligned channels (3D-PI) was also prepared.FTIR confirmed the successful preparation of CNF-PI aerogels from the combination of CNF and PAA (Figure S10).UV-Vis-NIR spectra showed that CNF-PI aerogel presented extensive absorption from 250 to 2500 nm, covering most of the solar spectrum and indicating an excellent photothermal effect (Figure 5B).Morover, this CNF-PI aerogel in wet condition showed more strong absorption because the adsorbed water in the aerogel could reduce the refractive index and the reflectance of incident light.CNF-PI aerogel also showed excellent thermal stability, and the T d at 5% weight loss exceeded 308 • C (Figure S11).Scanning electron microscopy, Brunauer-Emmett-Teller analyses, and water contact angle measurements indicated this PI-containing aerogel should realize water transport (Figures S12-S14).Figure S15 presents that the napkin on the top surface of V-3D-PI was fully dyed into red after 35 s in a dish containing red ink, which demostrates V-3D-PI evaporator have outstanding water transport performance.Under one-sun irradiation, the surface temperature of CNF-PI and CNF aerogel rosed to 62 • C and 30 • C within 6 min, respectively, which confirmed the much stronger photothermal properties of PIs (Figure 5C).The evaporation rate (υ e ) of the 3D steam generators for pure water was low at 0.09 kg⋅m −2 ⋅h −1 in the dark and 0.47 kg⋅m −2 ⋅h −1 under one sun (Figure 5D), but υ e of V-3D-PI and 3D-PI water evaporators reached 2.31 kg⋅m −2 ⋅h −1 and 1.67 kg⋅m −2 ⋅h −1 , respectively.The comparatively high water evaporation rates were due to the effective photothermal conversion capabilities of PIs, vertically aligned channels and portion side evaporation of V-3D-PI.The water evaporation rate is not the highest among all reported materials, but it represents one of the best for engineering polymer materials.The solar-to-vapor conversion efficiency of V-3D-PI reached 94.8%, much higher than that of the 3D-PI (Figure S16).The infrared thermal image also suggested the superiority of the V-CNF-PI aerogel's directional structure for water evaporation (Figure S17).After continuous exposure to sunlight for 6 h, linear water loss was observed, and the evaporation rate was constant (Figure S18).Moreover, outdoor evaporation measurements were carried out in Xiangtan, Hunan Province, during a typical day (10:00-18:00) in June (Figure S19a).Impressively, the total yield of clean water during the 8 h of the day was ∼22 kg m −2 .Furthermore, υ e showed no significant changes with different pH values and salinity (Figure 4E).Moreover, the concentrations of the different ions (e.g., Na + , Mg 2+ , Ca 2+, and K + ) in seawater in the evaporated water dropped sharply after desalination (Figure S19b).These results demonstrate the successful application of intrinsically photothermal PI in solar water evaporation.

CONCLUSION
In summary, we have designed and synthesized a series of intrinsically photothermal PI-x via condensation copolymerization of PMDA, CNDA, and NDA.The molar fraction of NDA could adjust the photothermal performance of PI-x.When the NDA molar fraction reached 0.5, the PI-0.5 appeared black and possessed an optimal photothermal efficiency.The PI-0.5 as a photothermal absorber was applied to construct a 3D water evaporator through a unidirectional freeze-drying method.The proof-of-concept V-3D-PI evaporator exhibited effective water evaporation, persistent material stability, and a high ion removal rate.This work will inspire the design and synthesis of highly efficient photothermal engineering polymeric materials for solar energy utilization.

Synthesis of precursors PAA-x and PI-x
The monomer (CNDA) was synthesized following our previous work, and the detailed process was provided in the supplementary information (Figures S1, S2).The precursors PAA-x (x is the molar fraction of NDA among two diamines and x = 0, 0.33, 0.5, 0.66, 0.83, and 1) were synthesized by reacting diamines CNDA and NDA with dianhydride PMDA in anhydrous DMF (Scheme 1), yielding a viscous solution with a solid content of approximately 15 wt%.For example, to synthesize PAA-0.5, PMDA (0.218 g, 1 mmol) was added to anhydrous DMF (3 mL) in a 25 mL Schlenk tube equipped with a magnetic stir bar.The dianhydride PMDA was completely dissolved by ultrasound.Then, NDA (0.1 g, 0.5 mmol) and CNDA (0.118 g, 0.5 mmol) were slowly added to the transparent solution and stirred at 0 • C for ∼24 h under nitrogen, generating a dark-green viscous PAA solution.The solution was precipitated in distilled water.The precipitates of PAA-0.5 were filtered and washed several times.The purified product (yield: 90%) was obtained after drying under vacuum.PAA-x powders were placed in bottles at room temperature and then imidized to PI-x on a hotplate stepwise at 80 • C for 1 h, 150 • C for 1 h, 200 • C for 1 h, and 250 • C for 1 h.The resulting PI-x powders were naturally cooled down to room temperature.

Preparation of 3D steam generators
Uniform suspensions of PAA precursors and CNF were poured into an epoxy resin mold with an iron sheet at the bottom and then frozen in liquid nitrogen.CNF-PAA aerogels with vertically oriented channels were generated after freezedrying due to the bottom-up temperature gradient caused by the presence of the iron sheet.Immediate thermal imidization of CNF-PAA yielded CNF-PI aerogels.As a control, pure CNF aerogel was prepared in a similar manner.PU was filled into CNF-PI and pure CNF aerogels to form 3D steam generators with vertical channels (V-3D-PI), as shown in Scheme 2. For comparison, 3D steam generators without vertical channels (3D-PI) were prepared by ordinary freezing.

F
I G U R E 1 (A) Conceptual illustration of photothermal polyimide (PI).(B) Synthesis scheme of PI-x.F I G U R E 2 (A) Fourier transform infrared spectroscopy (FTIR) spectra, (B) X-ray photoelectron spectroscopy (XPS) spectra, (C) thermogravimetric analysis (TGA) traces, (D) X-ray diffraction (XRD) diffractograms of Polyimide (PI)-x.F I G U R E 3 (A) Photograph of polyimide (PI)-x.(B) IR images of PI-0.5 illuminated by wavelength lasers at 365, 365-520, and 808 nm and xenon lamps.(C) Surface of PI-0.5 illuminated by wavelength lasers at 365, 365-520, and 808 nm and xenon lamps (AM 1.5G).F I G U R E 4 (A) UV-Vis-NIR spectra of polyimide (PI)-x.(B) Schematic illustration of the photothermal process.(C and D) The ultrafast time evolving transient absorption difference spectra of PI-0.5 from 500 to 800 nm at the indicated time delays following a 360-nm laser excitation.

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I G U R E 5 (A) Preparation of a directional 3D steam generator.(B) UV-Vis-NIR absorption spectra of CNF-PI aerogel in dry and wet conditions.(C) Surface temperature of cellulose nanofiber (CNF) and CNF-polyimide (PI) aerogels as a function of illumination time under one Sun.(D) 3D-PI and V-3D-PI aerogel induced water mass change as a function of illumination time, compared with bare water in the dark and under one Sun.(E) Water evaporation rates of V-3D-PI over wide pH and salinity ranges.
This work was financially supported by National Natural Science Foundation of China (NNSFC 21975215 and 22275158), Funding project of Furong Scholars Award Program and XiangTan University-Zhuzhou Feilu High-tech Material Technology Co., LTD., and Joint Training Base of Industry-Education Integration of Graduate Students.C O N F L I C T O F I N T E R E S T S TAT E M E N TThe authors declare no conflict of interest.O R C I DHe-Lou Xie https://orcid.org/0000-0003-4103-2634RE F E R E N C E S