Preparation method of shape memory polymer for improving bidirectional driving performance

Abstract

The invention discloses a preparation method of a shape memory polymer capable of improving bidirectional driving performance, and relates to the technical field of shape memory polymers, the preparation method comprises the following steps: taking two PEVA copolymers with different VA contents as basic polymers, the VA contents are respectively 12 wt% and 18 wt%, the mass ratio is 1: 1, and the shape memory polymer is prepared by mixing the basic polymers and the PEVA copolymers; adding 10wt% of benzoyl peroxide BPO as a first initiator, and mixing for 10 minutes in a double-screw extruder at the rotating speed of 10 rpm to form a main chain cross-linking system; according to the invention, through thermal crosslinking and gradient UV crosslinking, construction of primary and secondary bimolecular networks and gradient distribution, mechanical symmetry and driving efficiency are improved, easy fatigue of a single network and cycle attenuation caused by non-uniform stress distribution are overcome, a high recovery rate is maintained, and the service life is significantly prolonged.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of shape memory polymers, in particular to a shape memory polymer preparation method for improving bidirectional driving performance. BACKGROUND

[0002] Shape memory polymer materials are a kind of intelligent materials that can respond to changes in external conditions. Under the external stimuli of temperature, light, humidity, electric field, magnetic field and pH, the shape of the material can also change accordingly. According to the reversibility of the shape memory process, shape memory effect can be divided into unidirectional shape memory effect and bidirectional shape memory effect. Many scientists believe that unidirectional shape memory effect is not ideal and cannot be applied to many application fields, because unidirectional shape memory effect is the simplest and most common shape memory effect, which can only remember one shape memory cycle under external condition stimulation and cannot be applied to drivers and artificial intelligence fields. Therefore, many researchers have begun to explore and develop polymers with bidirectional shape memory effect.

[0003] Compared with irreversible unidirectional shape memory, bidirectional shape memory process is reversible, and reversible conversion between original shape and temporary shape can be achieved without external force for deformation again, which has attracted widespread attention from researchers around the world. It shows broad application prospects in the fields of artificial intelligence materials, drivers and sensors, self-repairing, aerospace, biomedical artificial muscles and the like. However, at present, flexible intelligent driving materials are faced with the problem of small driving. With the diversification of driving application scenarios, it is increasingly required to perform various soft deformation actions through high driving strain. Whether large driving deformation can be achieved has become the biggest challenge currently faced by shape memory polymer drivers.

[0004] Patent CN110746582B discloses a high-temperature-resistant high-performance shape memory polymer and a preparation method and application thereof. The above patent realizes the improvement of the temperature resistance, mechanical strength and chemical stability of the material.

[0005] The above patent improves the glass transition temperature of the epoxy resin shape memory polymer by introducing a modifier, but the shape memory polymer disclosed in the above patent adopts a single network, which is easy to fatigue and uneven stress, which can cause cycle attenuation.

[0006] Therefore, the present application provides a shape memory polymer preparation method based on molecular network for improving bidirectional driving performance, which can construct primary and secondary molecular networks and gradient distribution to improve mechanical symmetry and driving efficiency. SUMMARY

[0007] The present application aims to provide a shape memory polymer preparation method for improving bidirectional driving performance to solve the technical problems of single network fatigue and uneven stress distribution in the background art.

[0008] To achieve the above object, the present application provides the following technical scheme: a preparation method of shape memory polymer for improving bidirectional driving performance, the preparation method comprising the following steps:

[0009] Taking polyvinyl acetate (PEVA) copolymer with two different vinyl acetate (VA) contents as the base polymer, the VA contents are 12wt% and 18wt% respectively, the mass ratio is 1:1, 10wt% benzoyl peroxide (BPO) is added as the first initiator, and the main chain crosslinking system is formed by mixing in a twin-screw extruder at a speed of 10rpm for 10min;

[0010] After rapidly cooling the blend in the twin-screw extruder to room temperature, a first network structure with PEVA as the main chain is constructed by hot pressing at 130℃ and 10MPa for 20min;

[0011] The surface of the crosslinked film is treated by ultraviolet irradiation, using a UV light with a wavelength of 365nm and a power of 100mW / cm 2 for 5min to initiate the surface unsaturated bond photoreaction, and a second network structure with gradient crosslinking density in the thickness direction is formed;

[0012] The obtained double-network crosslinked film is subjected to 150% uniaxial stretching orientation treatment at 80℃, and cooled to room temperature at a rate of 5℃ / min, and the shape is fixed to realize stress memory;

[0013] The fixed polymer film is subjected to at least 50 times of heating and cooling reciprocating cycles in the range of 60-120℃ to activate and stabilize the bidirectional driving behavior.

[0014] Preferably, the mixing section temperature of the twin-screw extruder is set to 90℃, 95℃ and 100℃ to realize the micro-phase blending dispersion of PEVA with different VA contents under low shear, promote the synergistic regulation of crystalline and amorphous phases, and provide the basis for the molecular conformation of the subsequent double-crosslinked network;

[0015] The lateral ultrasonic vibration device is connected in parallel to the twin-screw extruder and vibrates synchronously at a power of 150W during the extrusion mixing process;

[0016] (1) The so-called "low shear" refers to the local shear rate of the material in the twin-screw extruder being less than 100s -1 (optimally 10-80s -1 ), corresponding to the extruder speed of 10-80rpm; in the embodiment, the low speed mixing at 10rpm corresponds to a shear rate of about 20-60s -1 ;

[0017] (2) Parallel ultrasonic vibration parameters: the power of the parallel lateral ultrasonic vibration device is 150 W, the frequency is about 20 kHz, and the amplitude is 1-10 μm (preferably 1-5 μm); the ultrasonic is turned on in parallel in the extrusion section to enhance the dispersion of the nanofiller and reduce agglomeration.

[0018] Preferably, the initiator includes, in addition to BPO, 0.5wt% of diisopropyl peroxide DCP, forming a double peroxide synergistic crosslinking system, and controlling the main chain crosslinking density at 2.0×10 -4 -3.5×10 -4 mol / cm 3 , effectively reducing the accumulation of mechanical fatigue caused by uneven crosslinking.

[0019] Preferably, the UV light crosslinking treatment is limited to the area of 20-50 μm in front of the film thickness, forming a gradient molecular crosslinking structure along the thickness direction;

[0020] UV surface layer depth control and determination method:

[0021] In order to accurately limit and realize the gradient crosslinking in the range of 20-50 μm of the surface layer, the following technical means are adopted:

[0022] (1) Based on Beer-Lambert law to estimate the penetration depth: I(z)=I0·exp(-α·z), where α is the absorption coefficient of the material at 365 nm, and I0 is the incident intensity. By measuring the light transmittance of the sample at 365 nm and the known incident intensity, the required irradiation dose (dose=intensity×time) can be calculated to achieve the expected crosslinking depth;

[0023] (2) Recommended process parameters:

[0024] UV wavelength: 365 nm;

[0025] Surface light intensity: 100 mW / cm 2 ;

[0026] Exposure time: 3-7 min (for a conventional PEVA film, 5 min corresponds to an active crosslinking depth of about 20-50 μm of the surface layer; the specific value depends on α, which is calibrated by the following determination);

[0027] When irradiating in air, first replace the surface with inert gas (nitrogen) to reduce oxygen inhibition; or cover a transparent nitrogen film;

[0028] (3) Depth determination method (verification):

[0029] Cut the film layer after UV treatment and use ATR-FTIR to do depth scanning to detect the decay curve of the absorption intensity of the crosslinking related functional groups with depth, so as to determine the actual crosslinking depth;

[0030] or cut into 10 μm slices, respectively, to determine the swelling degree and crosslinking density to verify the crosslinking gradient.

[0031] Preferably, 3 wt% of carboxyl-modified montmorillonite nanosheet MMT-COOH is added in the blending process as a nanometer inorganic crosslinking core to provide a molecular level inorganic physical confinement effect.

[0032] Preferably, the directional stretching step adopts a uniaxial tension mode, stretching at 80°C at a speed of 10 mm / min, keeping the stretched state for 2 min and then cooling to room temperature to obtain a pre-strain of 150%, inducing crystal phase rearrangement and establishing a "directional memory microstructure"; the measurable structural parameters of the "directional memory microstructure" and the measurement method are as follows:

[0033] (1) Crystal region, crystalline domain size: Scherrer formula calculation by XRD peak width, or direct measurement by TEM, typically controllable range of 5-200 nm (preferably 10-100 nm); the crystal region size of the application is preferably 20-80 nm, so as to balance the driving force and cycle stability;

[0034] (2) Lamella, lattice thickness: measured by TEM, preferably in the range of 5-50 nm;

[0035] (3) Orientation degree: Hermans orientation function f is calculated by 2D-XRD data, f value is defined as f = (3<cos2θ>-1) / 2, θ is the angle between the chain segment and the stretching direction. In the examples, f = 0.2-0.85 (preferably 0.4-0.8) can be obtained;

[0036] (4) Test conditions are exemplified: XRD (CuKα, 40 kV, 40 mA), 2θ scanning speed 2° / min; TEM sample preparation adopts freeze sectioning or sectioning method;

[0037] After isothermal pressure maintaining, the stretched and oriented film is annealed to 50°C at a rate of 5°C / min, and kept for 10 min to eliminate micro-arc distortion and optimize the crystal region size distribution.

[0038] Preferably, the bidirectional driving performance test is carried out in a temperature-controlled cavity, the temperature is raised to 90°C, then shape switching is started, and then cooled to 25°C for rebound fixation, the whole process is carried out at a temperature control rate of 5°C / min, and the strain response rate and recovery ratio are monitored.

[0039] Preferably, after the formation of the double network crosslinked film, the melting crystallinity of the film is controlled in the range of 30-45%, forming a mechanical response gradient structure;

[0040] Through microstructure scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis, it is confirmed that the crosslinking network inside the film presents micron-sized ordered channels, providing a molecular-level guiding path for bidirectional driving performance.

[0041] The determination method of melt crystallinity is as follows:

[0042] The melt crystallinity Xc is determined by differential scanning calorimetry (DSC) and calculated according to the following steps:

[0043] (1) Instrument and conditions: DSC instrument, sample mass 5-10 mg; procedure: first heat from room temperature to 200 DEG C at 10 DEG C / min, cool to 0 DEG C, and then heat to 200 DEG C at 10 DEG C / min to record the second curve to obtain the melting enthalpy ΔHm;

[0044] (2) Calculation formula: Xc= (ΔHm / (ΔH0m* wPE)) * 100%, wherein ΔH0m is the melting enthalpy of 100% crystalline PE, and is taken as 293 J / g, and wPE is the mass fraction of PE component in the sample.

[0045] Preferably, after 50 driving cycle tests, the maximum strain recovery rate of the material is maintained at more than 80%, and the deformation response speed is within 0.5 s / cycle.

[0046] Preferably, the preparation method prepares a shape memory polymer film with double driving capabilities of thermal response and UV response, which can be integrated into flexible actuators, bionic artificial muscles and thermal response intelligent textiles, and combined with temperature control elements and ultraviolet control systems to realize programmed reversible driving.

[0047] The shape memory polymer film integrates a resistive heating network, and a micro-heating electrode of 100 mu m is formed by printing conductive silver paste, and cooperates with an embedded thermocouple temperature sensor.

[0048] Compared with the prior art, the beneficial effects of the present application are:

[0049] 1. The present application constructs primary and secondary double molecular networks through thermal crosslinking and gradient UV crosslinking, improves mechanical symmetry and driving efficiency through gradient distribution, overcomes the cycle attenuation caused by single network fatigue and uneven stress distribution, maintains high recovery rate, and significantly prolongs service life.

[0050] 2. The present application predefines oriented crystal phase in the crosslinking network through directional tensile stress memory microstructure, enhances the controllability of deformation energy storage and release, solves the problem that the deformation amplitude and recovery rate of traditional shape memory materials are difficult to balance, realizes fast rebound under 150% large pre-strain, and shortens the response time.

[0051] 3. The present application improves thermal conductivity and mechanical strength through molecular gradient and nanofiller synergistic reinforcement, solves the problems of slow material response, insufficient mechanical strength and uneven thermal conduction, shortens the response time and improves the recovery rate.

[0052] 4. The present application realizes precise balance of main chain and side chain crosslinking density through a dual-initiator synergistic crosslinking system, solves the problem of difficult to balance flexibility and strength of single initiation system crosslinking density, improves the rate of one network, and better network uniformity. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 is a schematic diagram of the bidirectional shape memory driving performance of the present application;

[0054] Figure 2 is a 2D-WAXS schematic diagram of the stretching of the polymer film sample of the present application when cooled under different stress conditions. DETAILED DESCRIPTION

[0055] The technical solutions in the embodiments of the present application will be described clearly and completely below in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0056] Example 1

[0057] Please refer to Figure 1 A shape memory polymer preparation method for improving bidirectional driving performance, using PEVA (20 g) with VA content of 12 wt% and BPO initiator (2 g, 10 wt%) as raw materials, placing the PEVA and BPO in a twin-screw extruder, setting the temperature section to 90℃, 95℃, 100℃, and the rotation speed to 10 rpm, mixing for 10 min, then extruding a long strip and rapidly air cooling to room temperature; slicing the cooled strip and laying it flat between hot pressing plates, hot pressing at 130℃, 10 MPa for 20 min to obtain a hot crosslinked film (sample P1) with a thickness of about 0.5 mm; placing P1 in a UV light crosslinking box, using a 365 nm wavelength, 100 mW / cm 3 UV lamp, irradiating from one side of the film for 5 min, only treating the 20 μm surface layer, forming a gradient crosslinked second network; heating the twice crosslinked film to 80℃, stretching to 150℃ pre-strain at a rate of 10 mm / min in a separate stretching clamp, maintaining the tension for 2 min, then cooling to room temperature at a rate of 5℃ / min to fix the temporary shape;

[0058] In the programmable temperature box, the temperature was increased to 90°C (shape switching) at a rate of 5°C / min, and then decreased to 25°C (shape fixing) at the same rate, for a total of 50 cycles, to obtain the cycle test results.

[0059] Example 2

[0060] See Figure 1 A method for preparing a shape memory polymer to improve bidirectional driving performance, using PEVA (20 g) with a VA content of 18 wt% and BPO initiator (2 g, 10 wt%) as raw materials, placing the PEVA and BPO in a twin-screw extruder, setting the temperature section to 90°C, 95°C, and 100°C, and rotating at 10 rpm, mixing for 10 min, then extruding a long strip and rapidly air cooling to room temperature; slicing the cooled strip and laying it between hot press plates, hot pressing at 130°C and 10 MPa for 20 min to obtain a hot crosslinked film (sample P2) with a thickness of about 0.5 mm; placing P2 in a UV light crosslinking box, using a 365 nm wavelength, 100 mW / cm 3 UV lamp, irradiating from one side of the film for 5 min, only treating the 20 μm surface layer, forming a gradient crosslinked second network; heating the twice crosslinked film to 80°C, stretching to 150°C pre-strain at a rate of 10 mm / min in a separate stretching clamp, maintaining the tension for 2 min, then cooling to room temperature at a rate of 5°C / min to fix the temporary shape;

[0061] In the programmable temperature box, the temperature was increased to 90°C (shape switching) at a rate of 5°C / min, and then decreased to 25°C (shape fixing) at the same rate, for a total of 50 cycles, to obtain the cycle test results.

[0062] Example 3

[0063] See Figure 1 A method for preparing a shape memory polymer to improve bidirectional driving performance, using PEVA (20 g) with a VA content of 18 wt% and BPO initiator (2 g, 10 wt%) as raw materials, placing the PEVA and BPO in a twin-screw extruder, setting the temperature section to 90°C, 95°C, and 100°C, and rotating at 10 rpm, mixing for 10 min, then extruding a long strip and rapidly air cooling to room temperature; slicing the cooled strip and laying it between hot press plates, hot pressing at 130°C and 10 MPa for 20 min to obtain a hot crosslinked film (sample P2) with a thickness of about 0.5 mm; placing P2 in a UV light crosslinking box, using a 365 nm wavelength, 100 mW / cm 3a UV lamp with a wavelength of 365 nm and a power of 100 mW / cm2, irradiating the film from one side for 7 min, only treating the top 20 pm layer, forming a second network of gradient crosslinking; heating the twice-crosslinked film to 80°C, stretching it to a pre-strain of 140°C at a rate of 10 mm / min in a separate stretching clamp, maintaining the tension for 3 min, then cooling it to room temperature at a rate of 5°C / min to fix the temporary shape;

[0064] In a programmable temperature box, the temperature was raised to 90°C at a rate of 6°C / min (shape switching), and then lowered to 25°C at the same rate (shape fixing), for a total of 60 cycles, to obtain the cycle test results.

[0065] Example 4

[0066] See Figure 1 A method for preparing a shape memory polymer with improved bidirectional driving performance, using PEVA with a VA content of 12 wt% (20 g), BPO initiator (1.8 g, 9 wt%), and DCP (0.1 g, 0.5 wt%) as raw materials, placing the PEVA, BPO, and DCP in a twin-screw extruder, setting the temperature zones to 90°C, 95°C, and 100°C, and rotating at 8 rpm, mixing for 15 min, then extruding a long strip and rapidly air cooling to room temperature; slicing the cooled strip and laying it flat between hot press plates, hot pressing at 130°C and 8 MPa for 18 min to obtain a hot crosslinked film with a thickness of about 0.5 mm (sample P4); placing P4 in a UV light crosslinking box, using a 365 nm wavelength, 100 mW / cm 3 a UV lamp with a wavelength of 365 nm and a power of 100 mW / cm2, irradiating the film from one side for 7 min, only treating the top 20 pm layer, forming a second network of gradient crosslinking; heating the twice-crosslinked film to 80°C, stretching it to a pre-strain of 140°C at a rate of 10 mm / min in a separate stretching clamp, maintaining the tension for 3 min, then cooling it to room temperature at a rate of 5°C / min to fix the temporary shape;

[0067] In a programmable temperature box, the temperature was raised to 90°C at a rate of 6°C / min (shape switching), and then lowered to 25°C at the same rate (shape fixing), for a total of 60 cycles, to obtain the cycle test results.

[0068] Example 5

[0069] See Figure 1A method for preparing a shape memory polymer with improved bidirectional driving performance, using PEVA (20 g) with a VA content of 12 wt% and BPO initiator (2 g, 10 wt%) as raw materials, placing the PEVA and BPO in a twin-screw extruder, setting the temperature section to 90°C, 95°C, and 100°C, and rotating at 10 rpm, mixing for 10 min, then extruding a long strip and rapidly air cooling to room temperature; slicing the cooled strip and laying it between hot press plates, hot pressing at 130°C and 10 MPa for 20 min to obtain a hot crosslinked film (sample P5) with a thickness of about 0.5 mm; placing P5 in a UV light crosslinking box, first irradiating the front 20 μm surface layer at 100 mW / cm 3 for 5 min, then irradiating the middle 10 μm layer at 50 mW / cm 2 for 3 min; in a separate stretching clamp, first stretching at 10 mm / min to 100% at 80°C and holding for 2 min, then stretching at 5 mm / min to 150% and holding for 2 min, and then cooling at 5°C / min to room temperature;

[0070] In a programmable temperature box, heating at 5°C / min to 90°C (shape switching), then cooling at the same rate to 25°C (shape fixing), a total of 70 cycles, to obtain the cycle test results.

[0071] Example 6

[0072] See Figure 1 A method for preparing a shape memory polymer with improved bidirectional driving performance, using PEVA (20 g) with a VA content of 12 wt% and BPO initiator (2 g, 10 wt%) as raw materials, placing the PEVA and BPO in a twin-screw extruder, setting the temperature section to 90°C, 95°C, and 100°C, and rotating at 10 rpm, mixing for 10 min, then extruding a long strip and rapidly air cooling to room temperature; slicing the cooled strip and laying it between hot press plates, hot pressing at 130°C and 10 MPa for 20 min to obtain a hot crosslinked film (sample P5) with a thickness of about 0.5 mm; placing P5 in a UV light crosslinking box, first irradiating the front 20 μm surface layer at 100 mW / cm

[0073] In a programmable temperature box, heating at 5°C / min to 90°C (shape switching), then cooling at the same rate to 25°C (shape fixing), a total of 70 cycles, to obtain the cycle test results.

[0074] Example 7

[0075] See Figure 1A method for preparing a shape memory polymer with improved bidirectional driving performance, using PEVA (20 g) with a VA content of 12 wt% and BPO initiator (2 g, 10 wt%) as raw materials, placing the PEVA and BPO in a twin-screw extruder, setting the temperature section to 90°C, 95°C, and 100°C, and rotating at 10 rpm, mixing for 10 min, then extruding a long strip and rapidly air cooling to room temperature to obtain sample P7; placing P7 in a UV light crosslinking box, using a 365 nm wavelength, 100 mW / cm 3 of a UV lamp, irradiating from one side of the film for 5 min, only treating the front 20 μm surface layer; heating the crosslinked film to 80°C, stretching to 150°C pre-strain at a rate of 10 mm / min in a separate stretching clamp, maintaining tension for 2 min, then cooling to room temperature at a rate of 5°C / min to fix the temporary shape;

[0076] In a programmable temperature box, heating to 90°C at a rate of 5°C / min (shape switching), then cooling to 25°C at the same rate (shape fixing), a total of 10 cycles, i.e. fracture occurs.

[0077] Example 8

[0078] See Figure 1 A method for preparing a shape memory polymer with improved bidirectional driving performance, using PEVA (20 g) with a VA content of 12 wt% and BPO initiator (2 g, 10 wt%) as raw materials, placing the PEVA and BPO in a twin-screw extruder, setting the temperature section to 90°C, 95°C, and 100°C, and rotating at 10 rpm, mixing for 10 min, then extruding a long strip and rapidly air cooling to room temperature; slicing the cooled strip and laying it flat between hot press plates, hot pressing at 130°C and 10 MPa for 20 min to obtain a hot crosslinked film (sample P8) with a thickness of about 0.5 mm; placing P8 in a UV light crosslinking box, using a 365 nm wavelength, 100 mW / cm 3 of a UV lamp, irradiating from one side of the film for 5 min, only treating the front 20 μm surface layer, forming a second network of gradient crosslinking;

[0079] In a programmable temperature box, heating to 90°C at a rate of 5°C / min (shape switching), then cooling to 25°C at the same rate (shape fixing), a total of 50 cycles, to obtain the cycle test results.

[0080] Description:

[0081] Microstructure of shape memory polymer films was characterized by two-dimensional wide-angle X-ray scattering (2D-WAXS): The crystal structure changes of polymer films after cooling under different stress levels were studied by 2D wide-angle x-ray scattering (2D-waxs, SmartLab / ST / Hypix, Rigaku Corporation) with Cu Kα radiation (λ = 1.5418 Å) at room temperature, applying 40 kV and 30 mA to the copper anode. The exposure time for each sample was 30 min, and the collimator diameter was 100 µm. The distance between the sample and the detector was 27 mm, and the thickness of the sample was about 0.5 mm.

[0082] Conclusion: The micro-nano structure changes and mechanism of oriented crystals in shape memory polymer films after cooling under different stress conditions were studied by 2D-WAXS. The 2D-WAXS of stretched polymer film samples after cooling under different stress conditions are as follows: Figure 2 : The unstretched sample shows two continuous concentric Debye-Scherrer rings at 2θ = 21.4° and 23.7°, respectively, corresponding to the (110) and (200) faces of orthorhombic polyethylene, respectively, due to the randomness of crystal orientation. In the stretched polymer film sample, the continuous Debye-Scherrer ring shows a very obvious crystal orientation distribution, and with the increase of the stretching stress, the ring splits into four non-meridional patterns, and becomes more intense with the increase of stress, which indicates that the orientation of the crystal is parallel to the direction of the stretching stress, proving that the polymer film has memory for the size and direction of the initial external stress, which is due to the orientation of the internal crystal structure of the polymer.

[0083] UV irradiation "only processes the top 20-50 pm of the surface layer", which refers to a "gradient crosslinking" technique based on the depth limitation of UV light penetration. When UV light irradiates onto the material, the photoinitiator and polymer molecules in the material absorb photons. After the photoinitiator absorbs energy, it decomposes to generate free radicals, thereby initiating crosslinking reactions between polymer molecular chains. According to the Lambert-Beer law, the intensity of light decays exponentially with the depth of penetration when it propagates in a medium. In the surface region of the polymer film, the intensity of UV light is the strongest, the photoinitiator is activated in large quantities, and the crosslinking reaction is intense, forming a high crosslinking density region. In the middle region of the polymer film, as the depth of the polymer film increases, the intensity of UV light decays, the crosslinking reaction still occurs, but the intensity gradually weakens, forming a region with a decreasing crosslinking density gradient. In the internal region of the polymer film, the initiator cannot be effectively activated, and the crosslinking reaction basically stops. Therefore, the so-called "processing the top 20-50 pm", in essence, is the "effective reaction depth" determined by the light absorption characteristics of the material, rather than a clear boundary like mechanical processing. This boundary is a gradual process. We control the crosslinking reaction depth by adjusting the concentration of photoinitiator and the UV irradiation time. The higher the concentration of photoinitiator, the deeper the effective crosslinking depth under a certain irradiation dose. At the same time, increasing the UV irradiation time also increases the reaction depth.

[0084] The crystallinity, melting temperature (Tm) and crystallization temperature (Tc) were studied as the transition temperature trigger for shape change using differential scanning calorimetry (DSC). The results are as follows:

[0085] Thermal and mechanical properties of bidirectional shape memory films with different contents of thermal initiator (BPO) were characterized:

[0086]

[0087] Conclusion: From the table, it can be seen that the crystallinity of the crosslinked polymer PEVA is between 30%-45%.

[0088] Cycling performance test method:

[0089] Sample preparation: The film sample prepared in Example 1-8 with a thickness of 0.5 mm was cut to a size of 30 mm x 5 mm;

[0090] Test equipment: programmable temperature box, laser displacement sensor, strain measurement system and data acquisition instrument;

[0091] Test steps:

[0092] 1. Fix the sample horizontally on the clamp inside the temperature control box, and align the displacement sensor to the free middle section of the sample.

[0093] 2. Apply 150% of initial pre-strain:

[0094] Ramp to 80°C, hold for 1 min, then stretch to 150% pre-strain at 10 mm / min, hold tension for 2 min;

[0095] Cool to 25°C at 5°C / min, hold for 1 min, remove external force, measure initial set strain (ε0).

[0096] 3. Cycling procedure:

[0097] Ramp phase: from 25°C to 90°C at 5°C / min, record sample length recovery, when displacement recovers to 90% (0.9ε0), record the time required t up ;

[0098] Hold for 1 min;

[0099] Cooling phase: from 90°C to 25°C at 5°C / min, record sample length recovery, when displacement recovers to 90% (0.9ε0), record the time required t down ;

[0100] Hold for 1 min.

[0101] 4. Repeat the above heating and cooling procedures for a specified number of times, collect strain-time data throughout the process.

[0102] Performance indicators reference:

[0103] Maximum reversible strain recovery rate R (%): R = ε rec / ε0×100%;

[0104] Where ε rec is the highest recovery strain in the first cycle (take the highest value of the average cycle);

[0105] Response time t (s): upward driving time t up (reached 0.9ε0 required time); downward driving time t down (reached 0.1ε0 required time); take the average of the two as the single average response time t;

[0106] Cycling stability: record the retention rate of R after N cycles.

[0107] Cycling performance test results are as follows:

[0108]

[0109] Summary analysis:

[0110] 1. The significant advantages of the dual-network crosslinking system:

[0111] P1 still maintains 88% reversible strain recovery rate after 50 cycles, response time is about 0.55s, and cycle stability retention rate is 100%. Compared with P6, the double network cross-linked film effectively inhibits fatigue accumulation and cross-linking unevenness, significantly improves driving efficiency and cycle life.

[0112] 2. The balance of main chain composition and cross-linking density:

[0113] P2 shows greater deformation amplitude, but slightly lower recovery rate and slightly longer response time, indicating that increasing VA monomers improves network flexibility and phase change driving force, but may also lead to cross-linking density dispersion, chain segment relaxation, and slight reduction in cycle stability.

[0114] 3. Optimization of nano additives and synergistic initiators:

[0115] P3 and P4 both achieve more than 90% recovery rate and faster response under their respective optimal cycle conditions, with cycle retention rate of 98%. Nano-montmorillonite improves stress transfer and chain segment recovery through physical confinement and thermal conduction, and the dual-initiator system strengthens the primary and secondary network structures through more uniform and controllable cross-linking density distribution, both of which have performance improvements compared to Example 1.

[0116] 4. Synergistic effect of gradient cross-linking and gradient stretching:

[0117] P5, through layered UV cross-linking + double-rate stretching technology, maintains a recovery rate of 87% even after 70 cycles, with a response time of 0.5s, and significantly better symmetry and predictability than single-process samples, indicating that constructing a multi-scale stress-cross-linking gradient optimizes the stress release path and deformation recovery path within the network.

[0118] Gradient cross-linking means that the modulus (stiffness) of the material is continuously changing from the surface to the inside. When driving, this smooth transition of stiffness can effectively transfer stress from high cross-linking areas to low cross-linking areas, avoiding stress concentration at the interface, so that the overall deformation is more coordinated and uniform. For bidirectional shape memory effect (such as bending-straightening), the gradient structure ensures that the driving force and recovery force in both directions are matched during heating and cooling, just like a system composed of springs with different elasticities, whose movement is more controllable and more symmetrical than a single spring.

[0119] Quantitative determination and limitation of gradient cross-linking density; to meet the implementability, the following determination method and numerical limitation are used in this application:

[0120] (1) Swelling-Flory-Rehner method:

[0121] Reagents: non-polar reagent (toluene or dichloromethane), temperature 25℃, sample cutting different depth layers (surface layer 0-20 μm, middle layer 20-200 μm, bottom layer > 200 μm, or according to the actual film thickness segmentation), determination of equilibrium swelling degree Q;

[0122] According to the Flory-Rehner equation, the crosslinking density ve is calculated; the preferred surface layer ve of the present application is 2.2×10 -4 -3.5×10 -4 mol·cm -3 , the middle layer ve is 10-50% lower than the surface layer;

[0123] (2) Micro ATR-FTIR mapping:

[0124] Depth direction FTIR absorption intensity mapping is performed on the section sample to verify the gradient distribution by decreasing the crosslinking characteristic peak intensity;

[0125] (3) Nanoindentation modulus mapping:

[0126] The nanoindentation is used to measure the change of Young's modulus with depth in the thickness direction, which briefly reflects the crosslinking density gradient.

[0127] Working principle: Based on the "thermal induction main chain crosslinking" and "UV induced secondary chain crosslinking" double molecular network structure, the main network provides basic shape fixing support, and the secondary network enhances the stress distribution of the surface layer and the middle layer through gradient crosslinking, so that the material forms a reversible deformation mutual cooperation system in the process of thermal phase change and cooling solidification, realizing efficient thermal-optical double trigger driving response;

[0128] After the network crosslinking is completed, uniaxial stretching orientation is performed under heating state to induce crystal phase rearrangement and molecular chain orientation, and the pre-strain structure is fixed in the cooling process to form a "directional memory microstructure". When heated again, the microstructure releases the stored stress to drive shape recovery; when cooled, the network crosslinking and the residual stress of orientation jointly lock the recovered shape;

[0129] Gradient UV irradiation and the incorporation of nano fillers such as MMT-COOH are used to form crosslinking density and thermal conductivity gradients in the thickness direction and at the molecular scale, which cooperatively optimize the deformation driving path and heat transfer efficiency. The molecular gradient network ensures the macroscopic driving symmetry, and the nano filler enhances the mechanical strength and thermal response rate, realizing high cycle stability and fast response.

Claims

1. A method for preparing a shape memory polymer for improved bidirectional actuation, characterized by: The preparation method comprises the following steps: The polyvinyl acetate ethylene copolymer (PEVA) with two different vinyl acetate (VA) contents of 12wt% and 18wt% is used as the base polymer, the mass ratio is 1:1, 10wt% benzoyl peroxide (BPO) is added as the first initiator, and the main chain crosslinking system is formed by mixing in the twin-screw extruder at a speed of 10rpm for 10min; After the blend in the twin-screw extruder is rapidly cooled to room temperature, the first network structure with PEVA as the main chain is constructed by hot pressing at 130℃ and 10MPa for 20min. The surface of the crosslinked membrane was treated by ultraviolet irradiation using UV light with a wavelength of 365 nm and an intensity of 100 mW / cm 2 for 5 min to initiate a surface unsaturated bond photoreaction to form a second network structure with a gradient crosslinking density in the thickness direction. The obtained double-network crosslinked film is subjected to 150% uniaxial stretching orientation treatment at 80℃, and is cooled to room temperature at a rate of 5℃ / min, and the shape is fixed to realize stress memory; The fixed polymer film is subjected to at least 50 heating and cooling reciprocating cycles in the range of 60-120℃ to activate and stabilize the bidirectional driving behavior.

2. The method for preparing a shape memory polymer with improved bidirectional driving performance according to claim 1, characterized in that: The mixing section temperature of the twin-screw extruder is set to 90℃, 95℃ and 100℃ to realize the micro-phase blending dispersion of PEVA with different VA contents under low shear, promote the synergistic regulation of the crystal phase and the amorphous phase, and provide the basis of molecular conformation for the subsequent double-crosslinked network.

3. The method of claim 1, wherein the shape memory polymer has improved bidirectional actuation performance. The bidirectional driving performance test is carried out in a temperature-controlled cavity, the shape switching is started after heating to 90℃, and then the shape is fixed by rebounding after cooling to 25℃, the whole process is carried out at a temperature control rate of 5℃ / min, and the strain response rate and recovery ratio are monitored.

4. The method for preparing a shape memory polymer with improved bidirectional driving performance according to claim 1, characterized in that: After the formation of the double-network crosslinked film, the melting crystallinity of the film is controlled in the range of 30-45% to form a mechanical response gradient structure. The difference in crosslinking density between the skin and the mid layer was calculated by swelling-Flory-Rehner, the skin crosslinking density was 2.2 x 10 -4 -3.5 x 10 -4 mol cm -3 and the skin crosslinking density was 10-50% higher than the mid layer.

5. The method for preparing a shape memory polymer with improved bidirectional driving performance according to claim 1, characterized in that: The shape memory polymer film is integrated with a resistive heating network, and a micro-heating electrode of 100μm is formed by printing conductive silver paste, which cooperates with an embedded thermocouple temperature sensor.

6. A method of making a shape memory polymer for improved bidirectional actuation, characterized by: The initiator includes, in addition to BPO, 0.5wt% of diisopropyl peroxide DCP, forming a double peroxide synergistic crosslinking system, controlling the main chain crosslinking density at 2.0×10 -4 -3.5×10 -4 mol / cm 3 , effectively reducing the mechanical fatigue accumulation caused by uneven crosslinking.

7. A method of making a shape memory polymer for improved bidirectional actuation, the method comprising: providing a shape memory polymer; and applying a magnetic field to the shape memory polymer to induce a magnetic field within the shape memory polymer. The UV light crosslinking treatment is limited to the area of 20-50μm in the thickness of the film, and a gradient molecular crosslinking structure along the thickness direction is formed.

8. A method of making a shape memory polymer for improved bidirectional actuation, the method comprising: providing a shape memory polymer; and applying a magnetic field to the shape memory polymer to induce a magnetic field within the shape memory polymer. 3wt% of carboxyl-modified montmorillonite nanosheet (MMT-COOH) is added as a nano-inorganic crosslinking core during the blending process to provide a molecular-level inorganic physical confinement effect.

9. A method of making a shape memory polymer for improved bidirectional actuation, the method comprising: providing a shape memory polymer; and applying a magnetic field to the shape memory polymer to induce a magnetic field within the shape memory polymer. The uniaxial tension mode is used in the orientation stretching step, the stretching is carried out at a speed of 10mm / min at 80℃, the stretched state is maintained for 2min, and then the temperature is cooled to room temperature to obtain a pre-strain of 150%, induce crystal phase rearrangement and establish a "directional memory microstructure".

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