Direct Ink Writing of Phenylethynyl End-Capped Oligoimide/SiO2 to Additively Manufacture High-Performance Thermosetting Polyimide Composites

The three-dimensional (3D) printing of a SiO2-filled thermosetting polyimide (SiO2@TSPI) composite with outstanding performance is realized via the direct ink writing (DIW) of polyamide acid (PAA) composite ink and thermal treatment conducted thereafter. The composite ink consists of phenylethynyl-terminated PAA and silica nanoparticles, where the SiO2 nanoparticles serve as the rheology modifier that is necessary for the DIW technique to obtain self-supporting feedstock during 3D printing and the reinforcement filler that is used to enhance the performance of the final composite. As a result, printed parts with complex geometry and robust thermal stability are obtained. Due to the extrusion-based DIW technique, the printed structures exhibit anisotropic mechanical strength that highly depends on printing roads. This simple and convenient means of realizing 3D structures of thermosetting polyimides is a promising strategy in aerospace and other fields.


Introduction
Polyimides (PIs) as a class of high-performance engineering materials have drawn considerable attention in tribology, aerospace, separating membranes, and micro-electronic applications owing to their diversity of synthetic routes and impressive properties, such as excellent mechanical strength, high long-term operating temperature, low dielectric constant, and chemical and radiation resistance [1][2][3][4]. Among the various polyimides, the thermosetting polyimide (TSPI) has attracted increasing attention due to its high glass transition temperature (Tg) and thermal stability, as it can be used above 300 • C for a long time. To date, plenty of thermosetting polyimides with diverse curing groups, including maleimide, norbornenimide, benzocyclobutene, cyano, and alkynyl groups, have been developed [5][6][7][8][9], all of which exhibit excellent thermo-oxidative stability, aging resistance, and outstanding toughness, and, therefore, extensive potential. However, the melting temperature of thermosetting polyimides cannot be observed, and their poor solubility makes the fabrication process difficult, which impedes their applications, especially when architectures with high complexity are necessary. In general, traditional compression molding and sintering with the assistance of high pressure or high temperature are used to manufacture architectures, but they need both expensive molds and complicated processes added to a 250 mL three-necked flask, which was equipped with a mechanical agitation system and an argon inlet/outlet. The total preparation procedure of the phenylethynylterminated polyamide acid (PAA) solution is shown in Scheme 1. First, after ODA was completely dissolved in NMP, the solution was moved into an ice bath for 20 min. Then, 6FDA was added gradually, and the reaction was stirred for 1 h in the ice bath. After that, the ice bath was removed, and the mixture was stirred for 5 h at room temperature. Then, the PEPA and the remainder of the NMP were added, the solution was stirred for another 24 h, and a phenylethynyl-terminated PAA solution with a solid content of 15% was obtained. The PAA solution was added to 300 mL of ethanol to obtain a PAA fiber. Then, the PAA fiber was filtered and dried at 70 • C for 12 h and smashed into powder. The cured thermosetting polyimide (C-TSPI) films were prepared by coating the PAA solution onto glass for thermal imidization and for the formation of a crosslink network in a muffle furnace in air environment, and the heat treatment route was as follows: 80 • C for 2 h, 120 • C for 1 h, 150 • C for 1 h, 180 • C for 1 h, 250 • C for 1 h, 280 • C for 2 h, 320 • C for 1 h, and 370 • C for 1 h. The uncured PI samples were prepared using the same procedure in the same conditions, but the heat treatment route was ended after running it at 280 • C for 2 h, both of them with a heating rate of 2 • C /min. M n = 18,700 g/mol, PD = 1.38.

Preparation of Thermosetting Polyimide and Polyamide Acid (PAA) Powders
The phenylethynyl-terminated PAA solution was synthesized according to the published literature [31]. ODA (24.57 mmol), 6FDA (23.57 mmol), and PEPA (2 mmol) were added to a 250 mL three-necked flask, which was equipped with a mechanical agitation system and an argon inlet/outlet. The total preparation procedure of the phenylethynylterminated polyamide acid (PAA) solution is shown in Scheme 1. First, after ODA was completely dissolved in NMP, the solution was moved into an ice bath for 20 min. Then, 6FDA was added gradually, and the reaction was stirred for 1 h in the ice bath. After that, the ice bath was removed, and the mixture was stirred for 5 h at room temperature. Then, the PEPA and the remainder of the NMP were added, the solution was stirred for another 24 h, and a phenylethynyl-terminated PAA solution with a solid content of 15% was obtained. The PAA solution was added to 300 mL of ethanol to obtain a PAA fiber. Then, the PAA fiber was filtered and dried at 70 °C for 12 h and smashed into powder. The cured thermosetting polyimide (C-TSPI) films were prepared by coating the PAA solution onto glass for thermal imidization and for the formation of a crosslink network in a muffle furnace in air environment, and the heat treatment route was as follows: 80 °C for 2 h, 120 °C for 1 h, 150 °C for 1 h, 180 °C for 1 h, 250 °C for 1 h, 280 °C for 2 h, 320 °C for 1 h, and 370 °C for 1 h. The uncured PI samples were prepared using the same procedure in the same conditions, but the heat treatment route was ended after running it at 280 °C for 2 h, both of them with a heating rate of 2 °C /min. Mn = 18,700 g/mol, PD = 1.38. Scheme 1. Preparation process of thermosetting polyimide film and polyimide acid (PAA powders).

Preparation of Inks and 3D Printing of Thermoset Objects
The used viscous inks consisted of PAA powders with structures of phenylethynylterminated, N,N-dimethylformamide (DMF), and SiO2 nanoparticles, which serve as matrix, solvent, and reinforcement fillers and rheology modifiers, respectively. The phenylethynyl-terminated PAA powders were dissolved in DMF using the ball-milling method, and a PAA solution with a solid content of 70% was obtained. A series of PAA composite inks with SiO2 weight contents of 6.2%, 16.3%, and 26.3% were realized by gradually adding different dosages of SiO2 nanoparticles into 70% PAA solution and dispersed uniformly by repeatedly using the ball-milling method. After that, the rheological behaviors of the composite inks were evaluated using a rheometer (HAAKE, RS6000, Germany), which was equipped with a parallel plate model with a 35 mm diameter, and their heights were fixed at 1 mm. The shear viscosity of the inks was measured at 25 °C with Scheme 1. Preparation process of thermosetting polyimide film and polyimide acid (PAA powders).

Preparation of Inks and 3D Printing of Thermoset Objects
The used viscous inks consisted of PAA powders with structures of phenylethynylterminated, N,N-dimethylformamide (DMF), and SiO 2 nanoparticles, which serve as matrix, solvent, and reinforcement fillers and rheology modifiers, respectively. The phenylethynylterminated PAA powders were dissolved in DMF using the ball-milling method, and a PAA solution with a solid content of 70% was obtained. A series of PAA composite inks with SiO 2 weight contents of 6.2%, 16.3%, and 26.3% were realized by gradually adding different dosages of SiO 2 nanoparticles into 70% PAA solution and dispersed uniformly by repeatedly using the ball-milling method. After that, the rheological behaviors of the composite inks were evaluated using a rheometer (HAAKE, RS6000, Germany), which was equipped with a parallel plate model with a 35 mm diameter, and their heights were fixed at 1 mm. The shear viscosity of the inks was measured at 25 • C with shear rates from 10 −2 to 102 s −1 . The storage modulus (G') and loss modulus (G") were tested under room temperature at shear stress ranging from 0.1 to 6000 Pa with a frequency of 1 Hz. Viscoelastic inks were transferred into a syringe and degassed to remove air bubbles by centrifugation under the speed of 7000 r/min for 10 min before use. The inks with high viscosity were successfully extruded under the condition of additionally providing pneumatic pressure by shearing of the screw. The whole printing process is shown in Scheme 2. The syringe filled with inks was equipped with a gauge nozzle with a gauge of 18, 21, or 24 and an inner diameter of about 800 µm, 700 µm, or 300 µm, respectively. The process of thermal curing was conducted in a muffle furnace under an air atmosphere, and the whole curing procedure with a heating rate of 1 o C /min was carried out as follows: 80 • C for 4 h, 120 • C for 4 h, 150 • C for 4 h, 180 • C for 2 h, 250 • C for 2 h, 280 • C for 2 h, 320 • C for 2 h, and 370 • C for 1 h. Casting SiO 2 @TSPI (C-SiO 2 @TSPI) composite films were thermally cured the same way. tested under room temperature at shear stress ranging from 0.1 to 6000 Pa with a frequency of 1 Hz. Viscoelastic inks were transferred into a syringe and degassed to remove air bubbles by centrifugation under the speed of 7000 r/min for 10 min before use. The inks with high viscosity were successfully extruded under the condition of additionally providing pneumatic pressure by shearing of the screw. The whole printing process is shown in Scheme 2. The syringe filled with inks was equipped with a gauge nozzle with a gauge of 18, 21, or 24 and an inner diameter of about 800 μm, 700 μm, or 300 μm, respectively. The process of thermal curing was conducted in a muffle furnace under an air atmosphere, and the whole curing procedure with a heating rate of 1 o C /min was carried out as follows: 80 °C for 4 h, 120 °C for 4 h, 150 °C for 4 h, 180 °C for 2 h, 250 °C for 2 h, 280 °C for 2 h, 320 °C for 2 h, and 370 °C for 1 h. Casting SiO2@TSPI (C-SiO2@TSPI) composite films were thermally cured the same way. Scheme 2. The process of preparation of 3D printed parts of SiO2@TSPI composite.

Characterizations
Thermal stability and thermo-oxidative stability were measured using TA instruments (Netzsch STA 449C, Germany), and the tests were operated at a heating rate of 10 °C/min with the temperature from 25 °C to 800 °C under air and a nitrogen atmosphere, respectively. An electronic universal testing machine (SHIMADZU, EZ-Test, China) was used to evaluate the mechanical performances of cured PI and printed coupons at a load capacity of 500 N and at a speed of 5 mm/min. At least three independent test samples were used for each measurement. Pictures of the size, surface, and morphologies of objects and elemental analysis results were gained through optical microscopy (Olympus DP72, Japan) and scanning electron microscopy (SEM, JSM-6701, Japan) at 10 to15 KV. Gel permeation chromatography (GPC, Waters 1525) was utilized to calculate the molecular weight of the polyimide. The mobile phase was DMF, and styrene served as a standard sample. The vernier caliper was utilized to measure and calculate the volume of grids or cylinders before and after thermal treatment. Dimensional shrinkage was calculated as follows: In the formula, the parameters of L1 (V1) and L2 (V2) are the dimension (volume) of constructs before and after heat treatment, respectively. The shrinkages were measured three times for every sample.

Chemical Structures of Uncured Polyimide and Cured Thermosetting Polyimide
The process of synthesizing phenylethynyl-terminated polyimide by condensation of ODA, 6FDA, and PEPA is shown in Figure 1. Two kinds of polyimide with different molecular weights were designed, and detailed information is shown in Table 1. The PI-1 oligoimide with a designed molecular weight of 5000 g/mol was synthesized in order to Scheme 2. The process of preparation of 3D printed parts of SiO2@TSPI composite.

Characterizations
Thermal stability and thermo-oxidative stability were measured using TA instruments (Netzsch STA 449C, Germany), and the tests were operated at a heating rate of 10 • C/min with the temperature from 25 • C to 800 • C under air and a nitrogen atmosphere, respectively. An electronic universal testing machine (SHIMADZU, EZ-Test, China) was used to evaluate the mechanical performances of cured PI and printed coupons at a load capacity of 500 N and at a speed of 5 mm/min. At least three independent test samples were used for each measurement. Pictures of the size, surface, and morphologies of objects and elemental analysis results were gained through optical microscopy (Olympus DP72, Japan) and scanning electron microscopy (SEM, JSM-6701, Japan) at 10 KV to 15 KV. Gel permeation chromatography (GPC, Waters 1525) was utilized to calculate the molecular weight of the polyimide. The mobile phase was DMF, and styrene served as a standard sample. The vernier caliper was utilized to measure and calculate the volume of grids or cylinders before and after thermal treatment. Dimensional shrinkage was calculated as follows: In the formula, the parameters of L1 (V1) and L2 (V2) are the dimension (volume) of constructs before and after heat treatment, respectively. The shrinkages were measured three times for every sample.

Chemical Structures of Uncured Polyimide and Cured Thermosetting Polyimide
The process of synthesizing phenylethynyl-terminated polyimide by condensation of ODA, 6FDA, and PEPA is shown in Figure 1. Two kinds of polyimide with different molecular weights were designed, and detailed information is shown in Table 1. The PI-1 oligoimide with a designed molecular weight of 5000 g/mol was synthesized in order to clearly investigate the crosslinking of phenylethynyl in the polyimide structure. The structures of uncured PI-1 and cured PI-1 were characterized using Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) (Figure 2). Figure 2a shows the 1 H NMR spectrum of the uncured PI-1 oligomer, and all of the protons in the chemical structure of PI-1 are assigned as expected, implying that the obtained oligoimides have the ideal chemical structures. For the uncured PI-1 oligomer, the weak resonances at 88.54 ppm and 93.93 ppm could be assigned to the existence of the C≡C group (Figure 2b). Furthermore, Figure 2c compares the FTIR spectra of the PEPA oligoimides, uncured PI and cured PI. The absorption peak at 2216 cm −1 demonstrates the stretching vibrations of ethynyl groups (-C≡C-) in the oligoimides. The absorption peaks at 1779 and 1721 cm −1 are assigned to the vibration of the C=O asymmetric and symmetric stretching absorption bands, respectively. Meanwhile, the absorption peak at 1383 cm −1 belongs to the stretching vibration of C-N of the imine five-membered ring. These results suggest that phenylethynylterminated PI was successfully prepared via heat imidization. Compared to the C≡C peak of oligomer uncured PI-1, that at 2212 cm −1 disappeared after curing at 370 • C for 1 h, causing C≡C to react and thereby form a crosslink network in the heat treatment process [32]. clearly investigate the crosslinking of phenylethynyl in the polyimide structure. The structures of uncured PI-1 and cured PI-1 were characterized using Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance (NMR) ( Figure 2). Figure 2a shows the 1 H NMR spectrum of the uncured PI-1 oligomer, and all of the protons in the chemical structure of PI-1 are assigned as expected, implying that the obtained oligoimides have the ideal chemical structures. For the uncured PI-1 oligomer, the weak resonances at 88.54 ppm and 93.93 ppm could be assigned to the existence of the C≡C group ( Figure 2b). Furthermore, Figure 2c compares the FTIR spectra of the PEPA oligoimides, uncured PI and cured PI. The absorption peak at 2216 cm −1 demonstrates the stretching vibrations of ethynyl groups (-C≡C-) in the oligoimides. The absorption peaks at 1779 and 1721 cm −1 are assigned to the vibration of the C=O asymmetric and symmetric stretching absorption bands, respectively. Meanwhile, the absorption peak at 1383 cm −1 belongs to the stretching vibration of C-N of the imine five-membered ring. These results suggest that phenylethynylterminated PI was successfully prepared via heat imidization. Compared to the C≡C peak of oligomer uncured PI-1, that at 2212 cm −1 disappeared after curing at 370 °C for 1 h, causing C≡C to react and thereby form a crosslink network in the heat treatment process [32].

Three-Dimensional Printing of SiO2@TSPI Composites Using Direct Ink Writing (DI
Proper rheology is vital for DIW technology, and a trade-off between viscosi high elastic modulus is required to keep the shape during and after extrusion. The logical properties of the SiO2/PAA inks were analyzed using a rheometer. The PAA der was dissolved in DMF to obtain a solution with a concentration of 70 wt%. As in Figure 3a, when SiO2 nanoparticles were added into the PAA solution, the visco the mixture significantly increased compared with the PAA solution and exhibit characteristic of non-Newtonian fluid. Both the inks of PAA-70% and SiO2@PAA showed shear thinning behavior. Besides viscosity, the moduli test is another prere for DIW ink. Figure 3b shows the storage modulus (G′) and loss modulus (G″) und ferent shear stresses. We can see that the storage modulus is lower than the loss mo

Three-Dimensional Printing of SiO 2 @TSPI Composites Using Direct Ink Writing (DIW)
Proper rheology is vital for DIW technology, and a trade-off between viscosity and high elastic modulus is required to keep the shape during and after extrusion. The rheological properties of the SiO 2 /PAA inks were analyzed using a rheometer. The PAA powder was dissolved in DMF to obtain a solution with a concentration of 70 wt%. As shown in Figure 3a, when SiO 2 nanoparticles were added into the PAA solution, the viscosity of the mixture significantly increased compared with the PAA solution and exhibited the characteristic of non-Newtonian fluid. Both the inks of PAA-70% and SiO 2 @PAA-6.2% showed shear thinning behavior. Besides viscosity, the moduli test is another prerequisite for DIW ink. Figure 3b shows the storage modulus (G ) and loss modulus (G") under different shear stresses. We can see that the storage modulus is lower than the loss modulus for PAA-70% and SiO 2 @PAA-6.2%. For SiO 2 @PAA-16.3%, the G is almost same as the G". Further increasing the addition of SiO 2 to 26.3% makes the G higher than the G", which means that only SiO 2 @PAA-26.3% exhibits the phenomenon of self-support (Figure 3b). The storage modulus overtakes the loss modulus at shear stress below 581 Pa. Therefore, SiO 2 @PAA-26.3% was chosen to print a variety of structures in order to investigate numerous properties. means that only SiO2@PAA-26.3% exhibits the phenomenon of self-support (Figu The storage modulus overtakes the loss modulus at shear stress below 581 Pa. The SiO2@PAA-26.3% was chosen to print a variety of structures in order to investig merous properties. We then studied the printability of the above inks. The inks of SiO2@PAA-6.2 SiO2@PAA-16.3% could extrude filaments, and they could deposit onto glass t grids. However, after a few minutes, the grids had collapsed (Figure 4). The grid nating from SiO2@PAA-26.3% had maintained their shape before and after heat tre and showed good dimensional stability. Then, the parameters for SiO2@PAA-26. were adopted to SiO2@PAA inks. As the resolution of 3D printed objects is primarily affected by the extrusion pr the z-spacing (the distance of the nozzle to the substrate), and the moving speed nozzles, we investigated the width of a single filament under different extrusion pr and moving speeds of the nozzles to achieve suitable conditions for DIW (Figure 5 We then studied the printability of the above inks. The inks of SiO 2 @PAA-6.2% and SiO 2 @PAA-16.3% could extrude filaments, and they could deposit onto glass to form grids. However, after a few minutes, the grids had collapsed (Figure 4). The grids originating from SiO 2 @PAA-26.3% had maintained their shape before and after heat treatment and showed good dimensional stability. Then, the parameters for SiO 2 @PAA-26.3% ink were adopted to SiO 2 @PAA inks.
for PAA-70% and SiO2@PAA-6.2%. For SiO2@PAA-16.3%, the G′ is almost same as the G″ Further increasing the addition of SiO2 to 26.3% makes the G′ higher than the G″, which means that only SiO2@PAA-26.3% exhibits the phenomenon of self-support (Figure 3b) The storage modulus overtakes the loss modulus at shear stress below 581 Pa. Therefore SiO2@PAA-26.3% was chosen to print a variety of structures in order to investigate numerous properties. We then studied the printability of the above inks. The inks of SiO2@PAA-6.2% and SiO2@PAA-16.3% could extrude filaments, and they could deposit onto glass to form grids. However, after a few minutes, the grids had collapsed (Figure 4). The grids originating from SiO2@PAA-26.3% had maintained their shape before and after heat treatment and showed good dimensional stability. Then, the parameters for SiO2@PAA-26.3% ink were adopted to SiO2@PAA inks. As the resolution of 3D printed objects is primarily affected by the extrusion pressure the z-spacing (the distance of the nozzle to the substrate), and the moving speed of the nozzles, we investigated the width of a single filament under different extrusion pressures and moving speeds of the nozzles to achieve suitable conditions for DIW (Figure 5a). The width of a single filament was measured by fixing the shear speed of the screw and by changing the extrusive pressure, z-spacing, and moving speed of the nozzles. The single filaments were deposited onto substrates, and their widths were measured. The filaments were obtained under the condition of extrusion pressures between 0.40 to 0.49 MPa with the moving speed of the nozzle varying from 1 to 4 mm/s. The higher extrusive pressures for the inks and the lower moving speeds of the nozzle corresponded to the more extrusive inks, which produced larger widths for every filament. An appropriate z-spacing is As the resolution of 3D printed objects is primarily affected by the extrusion pressure, the z-spacing (the distance of the nozzle to the substrate), and the moving speed of the nozzles, we investigated the width of a single filament under different extrusion pressures and moving speeds of the nozzles to achieve suitable conditions for DIW (Figure 5a). The width of a single filament was measured by fixing the shear speed of the screw and by changing the extrusive pressure, z-spacing, and moving speed of the nozzles. The single filaments were deposited onto substrates, and their widths were measured. The filaments were obtained under the condition of extrusion pressures between 0.40 MPa to 0.49 MPa with the moving speed of the nozzle varying from 1 to 4 mm/s. The higher extrusive pressures for the inks and the lower moving speeds of the nozzle corresponded to the more extrusive inks, which produced larger widths for every filament. An appropriate z-spacing is vital for printing geometry parts to avoid previously printed dragging filaments. As presented in one such instance (Figure 5b), the extrusion pressure, the moving speed of the nozzle, and the shear speed of the screw were kept constant, while the z-spacing was varied to fabricate neat grids. After completing the above investigation, the parameters to print random structures using SiO 2 @PAA-26.3% ink were determined, because they are composed of single filaments.
vital for printing geometry parts to avoid previously printed dragging filaments. As presented in one such instance (Figure 5b), the extrusion pressure, the moving speed of the nozzle, and the shear speed of the screw were kept constant, while the z-spacing was varied to fabricate neat grids. After completing the above investigation, the parameters to print random structures using SiO2@PAA-26.3% ink were determined, because they are composed of single filaments. In order to avoid the formation of cracks, guarantee shape stability, and fully release the internal stress during the heat treatment process for uncured parts, different curing procedures were utilized to prepare C-TSPI and C-SiO2@TSPI films and printed parts for SiO2@TSPI-26.3%. As the printed materials are thicker than films, resulting in different solvent volatilization rates between the surface, inner structures, and the larger internal stress, the printed materials need more time and a lower heating rate to decrease the influence by weakening the releasing rate of inner stress during the process of thermal treatment. The dimensional shrinkage of the structures was measured, and the average values were in the range of 13.65% to 16.27% ( Figure 6). The surface morphology of the printed SiO2@TSPI-26.3% material was analyzed using SEM. In Figure 7, from the EDS of Si, it can be seen that the distribution of nano-SiO2 In order to avoid the formation of cracks, guarantee shape stability, and fully release the internal stress during the heat treatment process for uncured parts, different curing procedures were utilized to prepare C-TSPI and C-SiO 2 @TSPI films and printed parts for SiO 2 @TSPI-26.3%. As the printed materials are thicker than films, resulting in different solvent volatilization rates between the surface, inner structures, and the larger internal stress, the printed materials need more time and a lower heating rate to decrease the influence by weakening the releasing rate of inner stress during the process of thermal treatment. The dimensional shrinkage of the structures was measured, and the average values were in the range of 13.65% to 16.27% ( Figure 6). nozzle, and the shear speed of the screw were kept constant, while the z-spacing was varied to fabricate neat grids. After completing the above investigation, the parameters to print random structures using SiO2@PAA-26.3% ink were determined, because they are composed of single filaments. In order to avoid the formation of cracks, guarantee shape stability, and fully release the internal stress during the heat treatment process for uncured parts, different curing procedures were utilized to prepare C-TSPI and C-SiO2@TSPI films and printed parts for SiO2@TSPI-26.3%. As the printed materials are thicker than films, resulting in different solvent volatilization rates between the surface, inner structures, and the larger internal stress, the printed materials need more time and a lower heating rate to decrease the influence by weakening the releasing rate of inner stress during the process of thermal treatment. The dimensional shrinkage of the structures was measured, and the average values were in the range of 13.65% to 16.27% ( Figure 6). The surface morphology of the printed SiO2@TSPI-26.3% material was analyzed using SEM. In Figure 7, from the EDS of Si, it can be seen that the distribution of nano-SiO2 was uniform. The results suggest that the nano-SiO2 particles were well-scattered in the oligomer ink. In Figure 8a,b, it can be observed that the printed grid maintained its shape after being cured. The microstructure of the printed grid was also observed. The results show that the filaments were separately positioned stack by stack and kept their shape, The surface morphology of the printed SiO 2 @TSPI-26.3% material was analyzed using SEM. In Figure 7, from the EDS of Si, it can be seen that the distribution of nano-SiO 2 was uniform. The results suggest that the nano-SiO 2 particles were well-scattered in the oligomer ink. In Figure 8a,b, it can be observed that the printed grid maintained its shape

Thermal Stability of TSPI Films and Printed SiO 2 @TSPI-26.3%
The TGA and DSC of the C-TSPI films and printed SiO 2 @TSPI-26.3% were measured in air and nitrogen as shown in Figure 9 and Table 2. The T g of both the C-TSPI film and printed SiO 2 @TSPI-26.3% was above 300 • C. The TGA results indicate that all the samples exhibited outstanding thermal properties and thermo-oxidative stability. There was no significant decline in the weight of the C-TSPI film or the printed SiO 2 @TSPI-26.3%, even under 500 • C in air and a nitrogen atmosphere. The degradative peak for C-TSPI was mainly caused by the degradation of the polymer from 552.0 • C to 612.0 • C, while the destructive peak for the printed SiO 2 @TSPI-26.3% occurred due to the degradation of the polymer from 554.0 • C to 714.0 • C. For SiO 2 @TSPI-26.3%, the destructive peak with a lower decomposition rate and a broader decomposition temperature suggests a composite with enhanced thermo-oxidative stability. At the same time, the char yield at 770 • C of C-TSPI in the nitrogen atmosphere was 54.71%, while the char yield for the printed SiO 2 @TSPI-26.3% was up to 65.96%. The above results indicate that the composite's higher thermal properties and thermo-oxidative stability are attributed to the nano-SiO 2 with outstanding thermal performance. The incorporation of nano-SiO 2 contributed to the T5%, and the char yields of the composites increased under air and the nitrogen atmosphere. enhanced thermo-oxidative stability. At the same time, the char yield at 770 °C of C-TSPI in the nitrogen atmosphere was 54.71%, while the char yield for the printed SiO2@TSPI-26.3% was up to 65.96%. The above results indicate that the composite's higher thermal properties and thermo-oxidative stability are attributed to the nano-SiO2 with outstanding thermal performance. The incorporation of nano-SiO2 contributed to the T5%, and the char yields of the composites increased under air and the nitrogen atmosphere.

Mechanical Properties of Printed Specimens
Mechanical properties, which are among the most vital properties of a polymer, are assessed by the uniaxial tensile test. In order to evaluate the strength of printed filaments and the adhesive force among layers, the printed routes of the specimens were divided into transverse (X, SiO 2 @TSPI-26.3%-X) and axial (Y, SiO 2 @TSPI-26.3%-Y) orientations, respectively. Therefore, the impacts of orientation on the printed specimens were analyzed. Meanwhile, the mechanical properties of the plain C-TSPI and C-SiO 2 @TSPI-26.3% films with a lack of orientation were also measured. The results are shown in Figure 10 and Table 3. Compared with the plain C-TSPI films, the C-SiO 2 @TSPI-26.3% films have a decreased tensile strength and increased elastic modulus, which may be due to the aggregation of nano-SiO 2 . In addition, when comparing the C-SiO 2 @TSPI-26.3% films with the SiO 2 @TSPI-26.3%-X composite obtained by printing with the transverse orientation method, there was no significant difference in the average tensile strength. For the C-SiO 2 @TSPI-26.3% films and the print roads of the transverse and axial SiO 2 @TSPI-26.3% composites, the average tensile strengths are 96.46 MPa, 94.97 MPa and 34.89 MPa, respectively, and their average elastic modulus values are 2.96 GPa, 6.51 GPa, and 1.8 GPa, respectively. The decrease in modulus for the printed SiO 2 @TSPI-26.3% composite was caused by a defective stack among the filaments. Furthermore, these results indicate that both the elastic modulus and tensile strength for the Y orientation were related to poor performance. Compared to samples with a printing road in the Y orientation, samples with a printing road in the X orientation showed higher tensile strength and exhibited excellent mechanical properties. Therefore, the printed samples with the X and Y orientations exhibited highly anisotropic behavior [33]. The major reasons for this phenomenon are the limitations of a direct ink 3D printing machine and the poor adhesive force of each layer caused by the poor diffusion and entanglement between the filaments. As a result, a poorer adhesive force between the filaments than between other regions brought about a decrease in the mechanical performance of the whole part.
tively, and their average elastic modulus values are 2.96 GPa, 6.51 GPa, and 1.8 GPa, respectively. The decrease in modulus for the printed SiO2@TSPI-26.3% composite was caused by a defective stack among the filaments. Furthermore, these results indicate that both the elastic modulus and tensile strength for the Y orientation were related to poor performance. Compared to samples with a printing road in the Y orientation, samples with a printing road in the X orientation showed higher tensile strength and exhibited excellent mechanical properties. Therefore, the printed samples with the X and Y orientations exhibited highly anisotropic behavior [33]. The major reasons for this phenomenon are the limitations of a direct ink 3D printing machine and the poor adhesive force of each layer caused by the poor diffusion and entanglement between the filaments. As a result, a poorer adhesive force between the filaments than between other regions brought about a decrease in the mechanical performance of the whole part.    The 5% weight loss temperature, dimensional shrinkage, and mechanical strength in this work and some published results are listed in Table 4. The SiO 2 @TSPI composites obtained in this work exhibit the lowest shrinkage while maintaining competitive thermal and mechanical properties in comparison with the reported 3D printed composites. The critical factor of MPLA, SLA, UV-assisted DIW, and DLP is using UV-induced free radical polymerization of photosensitive groups in molecular chains to form a crosslinked network. The heat treatment was carried out for thermal imidization or further crosslinking, and, then, high-precision structures were obtained. The introduction of photosensitive agents and other photocurable agents decreased the thermo-mechanical properties compared with the imidization products, and a large dimensional shrinkage occurred. For FDM, the oligomers with phenylethynyl-terminated PI were first prepared. During the process of extrusion at a high temperature, the crosslinked network was formed. Therefore, the manufacturing process is not easy due to the block of inks in the nozzle. MPSL and mechanical properties in comparison with the reported 3D printed composites. The critical factor of MPLA, SLA, UV-assisted DIW, and DLP is using UV-induced free radical polymerization of photosensitive groups in molecular chains to form a crosslinked network. The heat treatment was carried out for thermal imidization or further crosslinking, and, then, high-precision structures were obtained. The introduction of photosensitive agents and other photocurable agents decreased the thermo-mechanical properties compared with the imidization products, and a large dimensional shrinkage occurred. For FDM, the oligomers with phenylethynyl-terminated PI were first prepared. During the process of extrusion at a high temperature, the crosslinked network was formed. Therefore, the manufacturing process is not easy due to the block of inks in the nozzle.

Conclusions
In summary, we established a simple strategy to 3D print thermal setting polyimides using DIW technology. Based on phenylethynyl end-capped SiO2@oligoimide self-supporting ink, the printing parameters of extruding pressure, the moving speed of the nozzle, and z-spacing were studied, and structures with tailored geometry were successfully printed. After the curing procedure was operated, the maximum dimensional shrinkage SLA critical factor of MPLA, SLA, UV-assisted DIW, and DLP is using UV-induced free radical polymerization of photosensitive groups in molecular chains to form a crosslinked network. The heat treatment was carried out for thermal imidization or further crosslinking, and, then, high-precision structures were obtained. The introduction of photosensitive agents and other photocurable agents decreased the thermo-mechanical properties compared with the imidization products, and a large dimensional shrinkage occurred. For FDM, the oligomers with phenylethynyl-terminated PI were first prepared. During the process of extrusion at a high temperature, the crosslinked network was formed. Therefore, the manufacturing process is not easy due to the block of inks in the nozzle.

Conclusions
In summary, we established a simple strategy to 3D print thermal setting polyimides using DIW technology. Based on phenylethynyl end-capped SiO2@oligoimide self-supporting ink, the printing parameters of extruding pressure, the moving speed of the nozzle, and z-spacing were studied, and structures with tailored geometry were successfully printed. After the curing procedure was operated, the maximum dimensional shrinkage UV-DIW polymerization of photosensitive groups in molecular chains to form a crosslinked network. The heat treatment was carried out for thermal imidization or further crosslinking, and, then, high-precision structures were obtained. The introduction of photosensitive agents and other photocurable agents decreased the thermo-mechanical properties compared with the imidization products, and a large dimensional shrinkage occurred. For FDM, the oligomers with phenylethynyl-terminated PI were first prepared. During the process of extrusion at a high temperature, the crosslinked network was formed. Therefore, the manufacturing process is not easy due to the block of inks in the nozzle.

Conclusions
In summary, we established a simple strategy to 3D print thermal setting polyimides using DIW technology. Based on phenylethynyl end-capped SiO2@oligoimide self-supporting ink, the printing parameters of extruding pressure, the moving speed of the nozzle, and z-spacing were studied, and structures with tailored geometry were successfully printed. After the curing procedure was operated, the maximum dimensional shrinkage

534
-45 [28] DLP agents and other photocurable agents decreased the thermo-mechanical properties compared with the imidization products, and a large dimensional shrinkage occurred. For FDM, the oligomers with phenylethynyl-terminated PI were first prepared. During the process of extrusion at a high temperature, the crosslinked network was formed. Therefore, the manufacturing process is not easy due to the block of inks in the nozzle.

Conclusions
In summary, we established a simple strategy to 3D print thermal setting polyimides using DIW technology. Based on phenylethynyl end-capped SiO2@oligoimide self-supporting ink, the printing parameters of extruding pressure, the moving speed of the nozzle, and z-spacing were studied, and structures with tailored geometry were successfully printed. After the curing procedure was operated, the maximum dimensional shrinkage FDM agents and other photocurable agents decreased the thermo-mechanical properties compared with the imidization products, and a large dimensional shrinkage occurred. For FDM, the oligomers with phenylethynyl-terminated PI were first prepared. During the process of extrusion at a high temperature, the crosslinked network was formed. Therefore, the manufacturing process is not easy due to the block of inks in the nozzle.

Conclusions
In summary, we established a simple strategy to 3D print thermal setting polyimides using DIW technology. Based on phenylethynyl end-capped SiO2@oligoimide self-supporting ink, the printing parameters of extruding pressure, the moving speed of the nozzle, and z-spacing were studied, and structures with tailored geometry were successfully printed. After the curing procedure was operated, the maximum dimensional shrinkage DIW (this work) pared with the imidization products, and a large dimensional shrinkage occurred. For FDM, the oligomers with phenylethynyl-terminated PI were first prepared. During the process of extrusion at a high temperature, the crosslinked network was formed. Therefore, the manufacturing process is not easy due to the block of inks in the nozzle.

Conclusions
In summary, we established a simple strategy to 3D print thermal setting polyimides using DIW technology. Based on phenylethynyl end-capped SiO2@oligoimide self-supporting ink, the printing parameters of extruding pressure, the moving speed of the nozzle, and z-spacing were studied, and structures with tailored geometry were successfully printed. After the curing procedure was operated, the maximum dimensional shrinkage

Conclusions
In summary, we established a simple strategy to 3D print thermal setting polyimides using DIW technology. Based on phenylethynyl end-capped SiO 2 @oligoimide self-supporting ink, the printing parameters of extruding pressure, the moving speed of the nozzle, and z-spacing were studied, and structures with tailored geometry were successfully printed. After the curing procedure was operated, the maximum dimensional shrinkage was only 16%. Additionally, the printed samples exhibited highly anisotropic behavior, which could be described as the average tensile strengths for when the X and Y orientations were 95.0 MPa and 34.9 MPa, respectively. Both the printed SiO 2 @TSPI composite and TSPI showed considerable thermal properties and thermo-stability, and their T5% values were higher than 515 • C. The thermal properties and thermo-stability of the printed SiO 2 @TSPI composite were superior to those of the TSPI composite. Overall, this method may extend the applications of thermosetting polymers by providing a simple and convenient strategy to manufacture 3D parts with outstanding performance.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.