A multi-stage cured shape memory polymer for 4D printing and a method of making the same

Abstract

The present application relates to the technical field of materials, in particular to a multi-stage solidification shape memory polymer for 4D printing and a preparation method thereof. In view of the problem that it is difficult to build gradient mechanical functions by precisely regulating the structure modulus in a single structure of 4D printing, the present application obtains a single-component multi-stage solidification shape memory polymer by regulating the ratio of toluene-2, 4-diisocyanate-hydroxypropyl acrylate, shape memory polymer, polyethylene glycol 400 and 1, 3, 5-tris (6-isocyanate hexyl) -1, 3, 5-triazine-2, 4, 6-trione under the test feedback of the printing structure modulus, and solves the problem that it is difficult to build gradient mechanical properties in a single structure of 4D printing.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of materials, in particular to a multi-stage solidification shape memory polymer for 4D printing and a preparation method thereof. BACKGROUND

[0002] The local gradient characteristics of material composition, structure and function are ubiquitous in nature. In many biological material systems (such as fish scales, silk threads and bones), the phenomenon of using non-uniform mechanical properties of the structure to achieve excellent performance can be found. Inspired by this, functionally graded materials (FGMs) have shown good engineering application potential in the fields of bioengineering optoelectronic devices and soft robots. The characteristic of FGMs is that the composition and structure gradually change with the change of volume, thereby causing the corresponding change of material properties. Different microstructure phases in FGMs have different functions, and their multi-functional characteristics come from the smooth transition of their constituent phases. They can be designed and manufactured for specific functions and applications, such as responding to specific stimuli (such as 4D printing structures), however, due to the need for FGMs to achieve spatial continuous change of material properties, the manufacturing process has always been the main bottleneck of its engineering application.

[0003] Digital Light Processing (DLP) 4D printing, which has both high precision and rapid forming characteristics, provides a new opportunity for this - through the digital forming of FGMs to create complex three-dimensional structures with gradient functional characteristics. However, the attention to FGMs in DLP 4D printing is still insufficient. Through composition gradient design, FGMs can endow the structure with differentiated functional characteristics. For example, by regulating the local glass transition temperature (T g ) change to produce non-uniform strain, or form a driving force field, thereby achieving precise regulation of the geometric deformation of 4D printing structures. It is worth noting that gradient design also endows the printing material with multi-stimulus response ability, which lays the foundation for the development of intelligent materials. Therefore, it becomes increasingly important to develop FGMs with mechanical adjustable characteristics suitable for DLP 4D printing. SUMMARY

[0004] In view of the problem that it is difficult to build gradient mechanical function by precisely regulating the structure modulus in 4D printing of a single structure, the application regulates the ratio of toluene-2,4-diisocyanate (TDI)-hydroxypropyl acrylate (HPA), shape memory polymer (SMP), polyethylene glycol 400 (PEG400) and 1,3,5-tris (6-isocyanate hexyl)-1,3,5-triazine-2,4,6-trione (HDI) under the test feedback of the printing structure modulus, that is, the single-component multi-level curing shape memory polymer (ATHP) is obtained by regulating the ratio of TDI-HPA, SMP, PEG400 and HDI, and specifically, the multi-level curing in the application refers to three-level curing, thereby solving the problem that it is difficult to build gradient mechanical properties in 4D printing of a single structure.

[0005] The application further provides a preparation method of the multi-level curing shape memory polymer for 4D printing.

[0006] (1) preparing a photocurable isocyanate monomer: using hydroxypropyl acrylate (HPA) and toluene-2,4-diisocyanate (TDI) as raw materials, preparing and modifying a double-curing monomer component under mild conditions, so as to obtain the photocurable isocyanate monomer, which is denoted as TDI-HPA;

[0007] The photocurable isocyanate monomer is a monomer material with a double bond that can be cured by ultraviolet light;

[0008] (2) first preparing a photocured resin, then preparing a photo-thermal double-curing resin by using the photocurable isocyanate monomer prepared in the step (1), and then mixing the photocured resin and the photo-thermal double-curing resin, so as to obtain the multi-level curing shape memory polymer for 4D printing.

[0009] Further, the mild conditions in the step (1) specifically refer to room temperature, N2 atmosphere and light shielding conditions.

[0010] Further, the molar ratio of the hydroxypropyl acrylate and the toluene-2,4-diisocyanate in the step (1) is 1:(0.8-1.5).

[0011] Further, the double-curing monomer component preparation and modification under the mild conditions in the step (1) specifically refer to: under room temperature, N2 is introduced as a protective gas and the reaction process is ensured to be light shielding; under the condition of 30℃ and stirring, the toluene-2,4-diisocyanate is slowly added into the above-mentioned three-necked flask, and after the addition is completed, the stirring reaction is continued; then the heating is stopped and the stirring is continued, so as to obtain the photocurable isocyanate monomer.

[0012] Further, the stirring speed in step (1) is 300-600 rpm; after the end of dropwise addition, continue stirring for 0.5-2 h; then stop heating and continue stirring for 3-6 h.

[0013] Further, the preparation of the photocuring resin in step (2) is specifically as follows: taking aliphatic polyurethane diacrylate (AUD) as a crosslinking agent, isobornyl acrylate (IBOA) and benzyl acrylate (BA) as monomers, and taking diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide (TPO) as a photoinitiator, mixing the above AUD, IBOA, BA and TPO according to the mass ratio of (29-31):(19-21):(49-51):(1-2) to obtain the photocuring resin.

[0014] Further, the preparation of the photo-thermal dual-curing resin in step (2) is specifically as follows: mixing the TDI-HPA, 1,3,5-tris(6-isocyanatohexyl)-1,3,5-triazine-2,4,6-trione (HDI) and poly(ethylene glycol) 400 (PEG400) obtained in step (1) according to the mass ratio of (1.4-1.5):(0.7-0.73):(0.19-0.21) to obtain the photo-thermal dual-curing resin, which is denoted as SMP1.

[0015] Further, the photo-thermal dual-curing resin is mixed with the photocuring resin and stirred for 10 min in step (2) to obtain the three-stage cured shape memory polymer.

[0016] The application further provides a multi-stage cured shape memory polymer for 4D printing prepared by the above preparation method.

[0017] When the multi-stage cured shape memory polymer is used for 4D printing, the layer thickness and exposure time are set according to the composition of the photocuring resin, the structure is printed at room temperature, the printing method is photocuring printing, and after printing, the surface of the obtained three-dimensional structure is washed with anhydrous ethanol to remove the incompletely cured part.

[0018] Further, after printing is completed, the printed structure is first subjected to low-temperature ultraviolet curing (i.e., primary curing), then subjected to medium-temperature thermal curing (i.e., secondary curing or secondary curing), and finally subjected to high-temperature thermal curing, so that the finally obtained multi-stage cured printing structure has a mechanical gradient function characteristic.

[0019] Further, the low temperature refers to room temperature, the medium temperature refers to 85-95 DEG C, and the high temperature refers to 195-205 DEG C.

[0020] Further, the time for low-temperature ultraviolet curing is 30 min, the time for medium-temperature thermal curing is 180-200 min, and the time for high-temperature thermal curing is 0.5-1 h.

[0021] Compared with the prior art, the present application has the following beneficial effects:

[0022] The present application solves the problem that a single structure of 4D printing is difficult to construct a gradient mechanical property, and after the ATHP system is subjected to low-temperature ultraviolet curing, medium-temperature thermal curing and high-temperature thermal curing, the modulus is changed by more than three orders of magnitude, and after the structure is subjected to three-stage curing, the modulus reaches 1.2 GPa or more while the strain can be maintained at about 57%, which shows the characteristics of high strength and high toughness of the system. The material preparation process is simple, the reaction is controllable, and the material can be prepared in a large amount and industrialized, the cost is low, and after a long time of placement, the material still has a low viscosity and a stable state, which provides a high-performance shape memory polymer for developing a photocurable 4D printing material. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 is a reaction equation for preparing a photocurable isocyanate monomer;

[0024] Figure 2 is an NMR spectrum of the photocurable isocyanate monomer prepared in step (1) of Example 1;

[0025] Figure 3 is a Fourier transform infrared spectrum of a sample obtained by subjecting the three-stage cured shape memory polymer prepared in Example 1 to ultraviolet curing for different times;

[0026] Figure 4 is a Fourier transform infrared spectrum of a sample obtained by subjecting the three-stage cured shape memory polymer prepared in Example 1 to ultraviolet curing (300 s) and secondary curing (90 DEG C) for different times;

[0027] Figure 5 is a Fourier transform infrared spectrum of a sample obtained by subjecting the three-stage cured shape memory polymer prepared in Example 1 to ultraviolet curing (300 s), secondary curing (90 DEG C, 3 h) and three-stage curing (200 DEG C) for different times;

[0028] Figure 6Stress-strain curves of samples prepared from the tertiary cured shape memory polymer prepared in Example 1 with different time of UV curing;

[0029] Figure 7 Stress-strain test plots of samples prepared from the tertiary cured shape memory polymer prepared in Example 1 with UV curing (300s) and secondary curing (90°C) for different time, where Fig. a is the stress-strain curves, and Fig. b is the modulus and elongation at break summary plot of the corresponding test samples;

[0030] Figure 8 Stress-strain test plots of samples prepared from the tertiary cured shape memory polymer prepared in Example 1 with UV curing (300s), secondary curing (90°C, 3h) and tertiary curing (200°C) for different time, where Fig. a is the stress-strain curves, and Fig. b is the modulus and elongation at break summary plot of the corresponding test samples;

[0031] Figure 9 Shape fixation rate calculation of samples prepared from the tertiary cured shape memory polymer prepared in Example 1 with tertiary curing, where Fig. a corresponds to the shape programming step, and Fig. b corresponds to the shape fixation step;

[0032] Figure 10 Shape recovery rate calculation of samples prepared from the tertiary cured shape memory polymer prepared in Example 1 with tertiary curing, where Fig. a corresponds to the shape programming step, and Fig. b corresponds to the shape recovery step;

[0033] Figure 11 Viscosity test curve of the tertiary cured shape memory polymer prepared in Example 1;

[0034] Figure 12 High-precision structure diagram of the tertiary cured shape memory polymer prepared in Example 1 formed by DLP 4D printing technology and with tertiary curing. DETAILED DESCRIPTION

[0035] In order to better understand the content of the present application, the present application will be further described below in combination with specific embodiments and drawings. The following embodiments are implemented based on the technology of the present application, and give detailed implementation modes and operation steps, but the protection scope of the present application is not limited to the following embodiments.

[0036] Please combine Figure 1 .

[0037] Example 1:

[0038] (1) Preparation of photocurable isocyanate monomer:

[0039] At room temperature, 1 mol of hydroxypropyl acrylate (HPA) was added to a three-necked flask, N2 was introduced as a protective gas and to ensure that the reaction process was strictly light-proof; 1 mol of toluene-2, 4-diisocyanate (TDI) was slowly added to the above three-necked flask under the condition of heating at 30°C and stirring at a speed of 300 rpm, and after the addition was completed, the stirring was continued for 1 h; then the heating was stopped, the stirring was continued at a speed of 300 rpm for 3 h, and the obtained uniform colorless viscous liquid was the photocurable isocyanate monomer, which was recorded as TDI-HPA;

[0040] (2) Preparation of a tertiary cured shape memory polymer:

[0041] ① Preparation of a photocured resin:

[0042] With aliphatic polyurethane diacrylate (AUD) as a crosslinking agent, acrylic acid (IBOA), benzyl acrylate (BA) and diphenyl (2, 4, 6-trimethyl phenol) phosphine oxide (TPO) as a photoinitiator, 1.73 g of a photocured resin was obtained after mixing AUD, IBOA, BA and TPO according to a mass ratio of 30:20:50:1;

[0043] ② Preparation of a photo-thermal dual-cured resin:

[0044] After mixing 1.44 g of TDI-HPA obtained in step (1), 0.72 g of 1, 3, 5-tris (6-isocyanate hexyl) -1, 3, 5-triazine-2, 4, 6-trione (HDI) and 0.2 g of polyethylene glycol 400 (PEG400), a photo-thermal dual-cured resin was obtained, which was recorded as SMP1; the molar ratio of isocyanate groups (-NCO) to hydroxyl groups (-OH) in SMP1 was 9:1;

[0045] ③ After mixing the photocured resin prepared in step ① with SMP1 obtained in step ② and stirring for 10 min, a tertiary cured shape memory polymer was obtained, which was recorded as ATHP.

[0046] Application Example 1:

[0047] The tertiary cured shape memory polymer prepared in Example 1 was subjected to DLP printing performance test. DLP printing: using a self-assembled DLP 3D printer to print a structure at room temperature (the wavelength of the self-assembled DLP 3D printer was 405 nm, the light intensity was 19.85 mWcm -2), wherein each component of the self-assembled DLP 3D printer is purchased, and the self-assembled DLP 3D printer belongs to the prior art, which will not be described here. During printing, ultraviolet light curing printing is adopted, and according to the composition of the ultraviolet light curing resin, the thickness of each layer is set to 100 pm, and the exposure time is different from 0.8 s to 1.1 s. After printing, the surface of the obtained three-dimensional printed structure is cleaned with anhydrous ethanol to remove the incompletely cured part. Then, the three-dimensional printed structure is post-cured in the ultraviolet light curing box for 30 min to obtain an ultraviolet cured three-dimensional printed structure. Then, the ultraviolet cured three-dimensional printed structure is placed in an electric heating air blowing box, heated at 90 °C for 3 h to obtain a twice-cured three-dimensional printed structure, and finally heated and cured at 200 °C for 1 h to obtain the final three-dimensional printed structure.

[0048] Example 2:

[0049] (1) Preparation of a photocurable isocyanate monomer:

[0050] At room temperature, 1 mol of hydroxypropyl acrylate (HPA) was added to a three-necked flask, N2 was introduced as a protective gas and to ensure that the reaction process was strictly protected from light; under the condition of heating at 30 °C and stirring at a speed of 300 rpm, 0.8 mol of toluene-2,4-diisocyanate (TDI) was slowly added to the above three-necked flask, and after the addition was completed, the stirring was continued for 1 h; then the heating was stopped, the stirring speed was kept at 500 rpm, and the stirring was continued for 3 h. The obtained uniform colorless viscous liquid was the photocurable isocyanate monomer, denoted as TDI-HPA;

[0051] (2) Preparation of a three-stage cured shape memory polymer:

[0052] ① Preparation of a photocuring resin:

[0053] AUD, IBOA, BA and TPO were mixed in a mass ratio of 29:20:51:1 to obtain 1.73 g of a photocuring resin, wherein AUD is an aliphatic polyurethane diacrylate, IBOA is acrylic acid, BA is benzyl acrylate, and TPO is a diphenyl (2,4,6-trimethyl phenol) phosphine oxide;

[0054] ② Preparation of a photo-thermal dual-curing resin:

[0055] After 1.44 g of TDI-HPA obtained in step (1), 0.72 g of 1,3,5-tris (6-isocyanate hexyl)-1,3,5-triazine-2,4,6-trione (HDI) and 0.2 g of polyethylene glycol 400 (PEG400) were mixed, a photo-thermal dual-curing resin was obtained, denoted as SMP1; the molar ratio of isocyanate groups (-NCO) to hydroxyl groups (-OH) in SMP1 is 9:1;

[0056] Step 3: Preparation of the shape memory polymer with three curing modes (ATHP)

[0057] Example 3:

[0058] (1) Preparation of the photo-curable isocyanate monomer:

[0059] At room temperature, 1 mol of hydroxypropyl acrylate (HPA) was added to a three-necked flask, and N2 was introduced as a protective gas to ensure that the reaction was strictly protected from light. At a heating temperature of 30°C and a stirring speed of 300 rpm, 1.5 mol of toluene-2,4-diisocyanate (TDI) was slowly added to the above-mentioned three-necked flask. After the addition was completed, the reaction was continued for 1 h. Then the heating was stopped, and the stirring speed was maintained at 600 rpm for 3 h. The obtained uniform colorless viscous liquid was the photo-curable isocyanate monomer, which was denoted as TDI-HPA.

[0060] (2) Preparation of the shape memory polymer with three curing modes (ATHP):

[0061] Step 1: Preparation of the photo-curable resin:

[0062] The photo-curable resin was prepared by mixing 1.73 g of aliphatic polyurethane diacrylate (AUD) as a crosslinking agent, 0.72 g of acrylic acid (IBOA), 0.72 g of benzyl acrylate (BA), and 0.07 g of diphenyl (2,4,6-trimethylphenol) phosphine oxide (TPO) as a photoinitiator, according to a mass ratio of 30:21:49:1.

[0063] Step 2: Preparation of the photo-thermal dual-curing resin:

[0064] The photo-thermal dual-curing resin was prepared by mixing 1.44 g of TDI-HPA obtained in step (1), 0.72 g of 1,3,5-tris(6-isocyanatohexyl)-1,3,5-triazine-2,4,6-trione (HDI), and 0.2 g of polyethylene glycol 400 (PEG400), which was denoted as SMP1. The molar ratio of isocyanate groups (-NCO) to hydroxyl groups (-OH) in SMP1 was 9:1.

[0065] Step 3: Preparation of the shape memory polymer with three curing modes (ATHP)

[0066] Comparative Example 1:

[0067] (1) Preparation of the photo-curable isocyanate monomer:

[0068] This step was the same as step (1) of Example 1.

[0069] (2) Preparation of the three-stage cured shape memory polymer:

[0070] ① Preparation of the photocuring resin:

[0071] This step is the same as step ① of step (2) in Example 1;

[0072] ② Preparation of the photo-thermal dual-curing resin:

[0073] The photo-thermal dual-curing resin was prepared by mixing 1.29 g of TDI-HPA obtained in step (1), 0.64 g of HDI and 0.4 g of PEG400, and was recorded as SMP2;

[0074] ③ This step is the same as step ③ of Example 1, except that SMP1 in step ③ of Example 1 was replaced by SMP2, and finally the three-stage cured shape memory polymer was obtained, which was recorded as ATHP.

[0075] Comparative Example 2:

[0076] (1) This step is the same as step (1) of Example 1;

[0077] (2) Preparation of the three-stage cured shape memory polymer:

[0078] ① Preparation of the photocuring resin:

[0079] This step is the same as step ① of step (2) in Example 1;

[0080] ② Preparation of the photo-thermal dual-curing resin:

[0081] The photo-thermal dual-curing resin was prepared by mixing 1.12 g of TDI-HPA obtained in step (1), 0.56 g of HDI and 0.6 g of PEG400, and was recorded as SMP3;

[0082] ③ This step is the same as step ③ of Example 1, except that SMP1 in step ③ of Example 1 was replaced by SMP3, and finally the three-stage cured shape memory polymer was obtained, which was recorded as ATHP.

[0083] The photo-thermal dual-curing resins (SMP1, SMP2, SMP3) prepared in step ② of step (2) in Example 1 and Comparative Examples 1-2 were tested for performance, and the results are shown in Table 1.

[0084] Table 1 Performance of photo-thermal dual-curing resins

[0085]

[0086] Based on the performance of the photothermal dual-curing resins in Table 1, SMP1, SMP2, and SMP3 exhibit similar elongation and modulus after UV curing. However, the SMP1 system has lower viscosity and a more stable state, which is more important for material preservation and use (UV curing in Table 1 refers to UV curing for 300 seconds). Therefore, SMP1 was selected as the key material in Example 1 of this invention, wherein the molar ratio of isocyanate groups (-NCO) to hydroxyl groups (-OH) is 9:1.

[0087] The photocurable isocyanate monomer prepared in step (1) of Example 1 was characterized by 1H NMR spectroscopy and 1H NMR spectroscopy. 1 The H NMR assay used deuterated chloroform (CDCl3) as the solvent. 500 µL of CDCl3 was transferred to an NMR tube, and 5 mg of TDI-HPA was mixed with CDCl3 until homogeneous before the assay. Results are as follows: Figure 2 As shown, characterization analysis using 1H NMR spectroscopy revealed the meaning of each peak, and the peaks corresponded to the TDI-HPA molecular structure. Specifically, 6.9-7.2 ppm represented hydrogen from the benzene ring of the TDI molecule, 5.8-6.5 ppm from the double bond position of the HPA molecule, 5.1-5.3 ppm from the methine group of the HPA molecule, 4.1-4.3 ppm from the methylene group, 2.1-2.3 ppm from the methyl group of the TDI molecule, and 1.3-1.4 ppm from the methyl group of the HPA molecule. The test results showed that the peak positions and integrals of the 1H NMR spectrum were correct, and the impurities were few, indicating that the product prepared by the reaction was a TDI-HPA monomer.

[0088] The three-stage curing shape memory polymer prepared in Example 1 was subjected to UV curing, secondary curing, and tertiary curing, respectively. Fourier transform infrared spectroscopy was performed on the resulting samples, and the results are as follows: Figure 3 , 4 As shown in Figure 5. Fourier transform infrared spectroscopy was used to further analyze the specific reaction sequence and products of each stage of the single-component tertiary curable resin solution. First, in the multi-stage polymerization reaction of the single-component tertiary curable resin, the photosensitive TDI-HPA monomer and the remaining acrylic monomers contained in SMP1 can ensure the rapid photocuring reaction in the first step to ensure the rapid shaping of the printed structure. Specifically, the tertiary curable shape memory polymer prepared in Example 1 was placed in an ultraviolet curing chamber, and samples were taken at the initial stage without radiation, 10s after radiation, 180s after radiation, 240s after radiation, and 300s after radiation, and Fourier transform infrared spectroscopy was performed. Figure 3 As shown, under ultraviolet light irradiation, at 1620cm -1 The carbon-carbon double bond reacts completely within 10 seconds. Furthermore, at 3300 cm⁻¹... -1The weak peak at 3335 cm"1is mainly attributed to the newly formed hydrogen bonding system of -N-H groups, indicating the formation of urethane molecular structure during the UV curing process, which is because the exothermic polymerization of acrylic monomers promotes the thermal polymerization reaction of hydroxyl groups with isocyanate groups under the condition of UV curing, but the degree of reaction is low, so it is acceptable. Therefore, in order to further ensure the complete reaction of the photocured monomers in the system, the UV curing time is extended to 5 min or more before the second curing (thermal curing) is carried out.

[0089] The tertiary cured shape memory polymer prepared in Example 1 was placed in a UV curing box for 300 s, and then the UV cured sample was placed in an electric heating air blowing box, and the sample was taken out and tested by Fourier transform infrared spectroscopy at the initial heating stage, 20 min, 40 min, 60 min, 80 min, 100 min, 120 min, 140 min, 160 min, 180 min, and 200 min at 90°C, respectively, as shown in Figure 4 The isocyanate groups in TDI-HPA and HDI can accelerate the polymerization reaction with the hydroxyl groups in PEG400 under the reaction condition of 90°C. As can be seen from the infrared test results, the hydroxyl groups in the system are gradually consumed as the reaction time is prolonged, and the characteristic peak signal of the urethane molecular structure at 2260 cm -1 The characteristic peak signal of the isocyanate group (-NCO) gradually weakens, and at the same time, the weak peak at 3335 cm -1 The infrared characteristic peak signal of the urethane molecular structure at 3335 cm -1 and 2860 cm -1 around the band is attributed to the C-H stretching vibration of -CH3 and -CH2-. The strong signal found at 1686 cm -1 in the spectrum is attributed to C=O in the triazine ring, which is a strong signal from HDI. The above results mean that the secondary curing of the system is completed, and the modulus of the corresponding system also changes twice. PEG400 in the tertiary cured shape memory polymer system is a monomer as well as an ideal crosslinking agent, which can be quickly crosslinked and cured by UV light under the action of a photoinitiator. Small molecule PEG400 not only provides the required hydroxyl groups for the reaction, but also plays a role in improving the tensile properties of the material and reducing the viscosity of the system. Lower viscosity is conducive to the use of the system for DLP 4D printing.

[0090] The tertiary cured shape memory polymer prepared in Example 1 was placed in a UV curing oven for 300 s, and then the UV cured sample was placed in an electric heating air oven for 3 h at 90 °C to obtain a secondary cured sample, which was then cured at 200 °C. The Fourier transform infrared spectroscopy test was performed on the sample at the initial stage of heating (here, it refers to heating at 200 °C), 30 min, 60 min, 90 min, 120 min and 150 min, respectively, and the results are shown in Figure 5 TDI-HPA and HDI provide isocyanate groups, which not only participate in the reaction with hydroxyl groups to form urethane, but also further react to self-polymerize at high temperatures. That is, under the reaction conditions of 200 °C, the isocyanate groups (-NCO) remaining in TDI-HPA and HDI after secondary curing self-polymerize to form isocyanurate six-membered rings (triazine rings) at high temperatures. The infrared test results show that under the reaction conditions of 200 °C, the infrared characteristic peak of isocyanate groups (-NCO) at 2260 cm -1 disappears completely after half an hour of reaction, and similarly, the strong signal (C=O in triazine ring) found at 1682 cm -1 is significantly enhanced after half an hour of reaction and may be degenerate with the polar functional group stretching vibration peak ester carbonyl (-CO) at 1725 cm -1 , which does not change significantly after half an hour, which means that the single-component multi-stage curing system has completed three times of curing. HDI in the tertiary curing system is a triisocyanate containing isocyanurate heterocyclic structure, which is a liquid and has lower viscosity compared to other diisocyanate trimers, and is usually used as a crosslinking agent for polyurethane materials. The stable six-membered ring, higher molecular weight and functionality make the prepared material have excellent mechanical properties. In addition, HDI provides more isocyanate groups, which can replace the role of a part of TDI-HPA monomers, to some extent, increasing the modulus of the single-component tertiary curing system.

[0091] The tertiary cured shape memory polymer prepared in Example 1 was placed in a UV curing oven for 300 s, and then the UV cured sample was placed in an electric heating air oven for 3 h at 90 °C to obtain a secondary cured sample, which was then cured at 200 °C. The Fourier transform infrared spectroscopy test was performed on the sample at the initial stage of heating (here, it refers to heating at 200 °C), 30 min, 60 min, 90 min, 120 min and 150 min, respectively, and the results are shown in Figure 6 To ensure the accuracy of the experimental results, the tensile samples prepared under each experimental condition were subjected to three tensile experiments, and the results of the three tensile experiments were averaged. As shown in

[0092] The tertiary cured shape memory polymer prepared in Example 1 was placed in a UV curing box for 300 s, and then the UV cured sample was subjected to secondary curing, specifically heating at 90℃ for 180 min, and finally the secondary cured sample was heated and cured at 200℃ for 60 min, and samples were taken for testing at 0.5h, 1h, 1.5h, 2h, 2.5h of heating (here, heating at 200℃), respectively, as shown in Table 2. Figure 7 In the secondary curing stage, as the heating time was prolonged, the strain of the sample gradually decreased from 178% to about 75%, and the corresponding modulus gradually increased from 2.36 MPa to about 158.26 MPa.

[0093] The tertiary cured shape memory polymer prepared in Example 1 was placed in a UV curing box for 300 s, and then the UV cured sample was subjected to secondary curing, specifically heating at 90℃ for 180 min, and finally the secondary cured sample was heated and cured at 200℃ for 60 min, and samples were taken for testing at 0.5h, 1h, 1.5h, 2h, 2.5h of heating (here, heating at 200℃), respectively, as shown in Table 2. Figure 8 As the heating time was prolonged, the strain of the sample gradually decreased from 75% to about 57%, but the corresponding modulus sharply increased from 158.26 MPa to about 1238.64 MPa. That is, after the modulus of the sample was enhanced to 1091.49 MPa by heating at 200℃ for half an hour, the strain could still be maintained above 56%, which is a very excellent performance, demonstrating the high strength and toughness characteristics of the ATHP polymer system.

[0094] The shape memory performance of SMP has a certain impact on its potential application prospects in the field of photocuring 4D printing, therefore, in order to further quantify the shape memory performance of the tertiary cured shape memory polymer system, the tertiary cured shape memory polymer prepared in Example 1 was poured into a Teflon mold, and then a layer of transparent glass sheet was covered, after which the mold was placed in a UV curing box (American Formlabs) for 5 min to obtain a UV cured sample, then the UV cured sample was heated at 90℃ for 180 min to obtain a secondary cured sample, and finally the secondary cured sample was heated at 200℃ for 60 min to obtain a sample with dimensions of 50mm x 10mm x 2mm (length x width x thickness), the above sample was first subjected to a shape programming step, specifically heated to 80℃ (T g 47.78℃ for the UV cured sample), and then an external force was applied to make the bending angle of the sample strip reach about 90° (the angle was recorded as α), and then a shape fixing step was performed, specifically the sample strip was cooled with an ice bath to fix the shape, and after shape fixing, the external force was removed and the final fixed angle was recorded as β.

[0095] Among them, the shape fixing rate calculation formula is as follows:

[0096]

[0097] In the formula, R f is the shape fixing rate, a is the bending angle, and b is the fixing angle.

[0098] The shape recovery rate calculation formula is as follows:

[0099]

[0100] In the formula, R γ is the shape recovery rate, b is the fixing angle, and g is the recovery angle.

[0101] The shape fixing rate R f and the shape recovery rate R γ of the three-stage cured shape memory polymer system are about 95.56% and 100% respectively. Figure 9 Figure 10 The results show that the three-stage cured shape memory polymer system has good shape memory performance.

[0102] The DLP printing performance test of the three-stage cured shape memory polymer (ATHP) prepared in Example 1 was carried out. As Figure 11 , the viscosity test results of the three-stage cured shape memory polymer system show that its maximum viscosity is only 740 mPa·s. According to the literature report, resin solution with viscosity higher than 3 Pa·s is not suitable for DLP 3D printing. Therefore, the three-stage cured shape memory polymer system proposed by the present application is suitable for DLP 4D printing.

[0103] As Figure 12 , using a self-assembled DLP printer, the three-stage cured shape memory polymer system can be printed to obtain a high-precision lattice structure through DLP 4D printing technology (using ultraviolet light curing printing) and three-stage curing (the printed structure is first subjected to 30 min of ultraviolet post-curing, then heated at 90°C for 3 h, and finally heated and cured at 200°C for 1 h), which shows that the system has excellent light-curing 3D printing capability.

[0104] The above is only an embodiment of the present application, and does not limit the present application in any form. The present application can also have other forms of embodiments according to the above structure and function, which are not listed one by one. Therefore, any skilled person in the art, without departing from the scope of the technical solutions of the present application, according to the technical essence of the present application, any simple modification, equivalent change and modification of the above embodiments, still belongs to the scope of the technical solutions of the present application.​

Claims

1. A method for the preparation of a multi-stage cured shape memory polymer for 4D printing, characterized by, Specifically comprising the following steps: (1) Preparation of photocurable isocyanate monomer: using hydroxypropyl acrylate and toluene-2,4-diisocyanate as raw materials, the preparation and modification of the double-cured monomer component are carried out under mild conditions, thereby obtaining a photocurable isocyanate monomer, denoted as TDI-HPA, wherein the molar ratio of hydroxypropyl acrylate to toluene-2,4-diisocyanate is 1:(0.8-1.5); (2) First, prepare a photocuring resin, then use the photocurable isocyanate monomer prepared in step (1) to prepare a photo-thermal dual-curing resin, and then mix the above photocuring resin and photo-thermal dual-curing resin to obtain a multi-stage curing shape memory polymer for 4D printing; Wherein, the preparation of the photocuring resin specifically adopts the following steps: taking aliphatic polyurethane diacrylate as the crosslinking agent, taking acrylic acid and benzyl acrylate as the monomers, and taking diphenyl (2,4,6-trimethyl phenol) phosphine oxide as the photoinitiator, mixing the above aliphatic polyurethane diacrylate, acrylic acid, benzyl acrylate and diphenyl (2,4,6-trimethyl phenol) phosphine oxide in a mass ratio of (29-31):(19-21):(49-51):(1-2) to obtain the photocuring resin; The preparation of the photo-thermal dual-curing resin specifically adopts the following steps: mixing TDI-HPA obtained in step (1), 1,3,5-tris(6-isocyanate hexyl)-1,3,5-triazine-2,4,6-trione and polyethylene glycol 400 in a mass ratio of (1.4-1.5):(0.7-0.73):(0.19-0.21) to obtain the photo-thermal dual-curing resin.

2. The method of claim 1, wherein the multi-stage cured shape memory polymer for 4D printing is characterized by, The mild conditions in step (1) specifically refer to room temperature, N2 atmosphere and light shielding conditions.

3. The method of claim 1, wherein the multi-stage cured shape memory polymer for 4D printing is characterized by, The preparation and modification of the double-cured monomer component under mild conditions in step (1) specifically refers to: at room temperature, hydroxypropyl acrylate is added to a three-necked flask, N2 is introduced as a protective gas to ensure that the reaction process is light shielding; under the condition of stirring at 30°C, toluene-2,4-diisocyanate is slowly added to the above three-necked flask, and after the addition is completed, the stirring reaction is continued; then the heating is stopped and the stirring is continued to obtain the photocurable isocyanate monomer.

4. The method of claim 3, wherein the multi-stage cured shape memory polymer for 4D printing is characterized by, In step (1), the stirring speed of the stirring is 300-600 rpm; after the addition is completed, the stirring reaction is continued for 0.5-2 h; then the heating is stopped and the stirring is continued for 3-6 h; In step (2), the photocuring resin and the photo-thermal dual-curing resin are mixed and stirred for 10 min to obtain the multi-stage curing shape memory polymer.

5. The multi-stage curing shape memory polymer for 4D printing obtained by the preparation method of any one of claims 1-4.

6. The multi-stage cured shape memory polymer of claim 5, wherein, When used for 4D printing, the layer thickness and exposure time are set, the structure is printed at room temperature, and the printing method is photocuring printing.

7. The multi-stage cured shape memory polymer of claim 6, wherein, After printing, the surface of the obtained three-dimensional structure is washed with anhydrous ethanol to remove the incompletely cured part, then the printed structure is first subjected to low-temperature ultraviolet curing, then subjected to medium-temperature thermal curing, and finally subjected to high-temperature thermal curing, thereby obtaining a multi-stage cured printed structure.

8. The multi-stage cured shape memory polymer of claim 7, wherein, Low temperature refers to room temperature, medium temperature refers to 85-95°C, and high temperature refers to 195-205°C; The time for low temperature UV curing is 30 min, the time for medium temperature thermal curing is 180-200 min, and the time for high temperature thermal curing is 0.5-1 h.

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