Preparation method of elastic modulus programmable flexible substrate

Abstract

The invention discloses a preparation method of an elastic modulus programmable flexible substrate. Comprising the following steps: mixing an elastomer prepolymer with a corresponding curing agent, stirring, carrying out vacuum defoaming, spin-coating on the surface of a substrate coated with a polymer film, and heating and curing; carrying out plasma treatment and then spin-coating a photocuring adhesive on the surface of the substrate; irradiating the photo-curing adhesive region by using laser according to a pre-designed pattern, and adjusting the laser scanning times of different regions to form an elastomer substrate with a gradient elastic modulus; and removing the uncured light curing glue and stripping the obtained elastomer substrate from the substrate. The elastic substrate with the modulus gradient can be widely applied to flexible electronic devices, can effectively prevent devices from being damaged due to external force stretching especially in the aspects of protecting strain-sensitive electronic components, chip pins, strain-insensitive devices and the like, and has better adaptability and stability.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of flexible electronic device manufacturing, and particularly relates to a preparation method of an elastic modulus programmable flexible substrate, which is particularly suitable for the substrate construction of stretchable electronic devices that require local rigidity regulation. BACKGROUND

[0002] With the wide application of stretchable electronic devices in wearable devices, biomedical devices, intelligent robots and other fields, how to design a flexible substrate with excellent mechanical properties and reliability has become a key problem. An ideal flexible substrate should not only have excellent stretchability and flexibility, but also provide sufficient rigidity support in specific areas to effectively protect sensitive components and ensure the stability of the overall structure.

[0003] At present, some methods have been used to introduce materials with different hardness into the flexible substrate, embed rigid platforms (such as silicon chips or metal plates), or cover masks on the substrate and use laser irradiation with different light intensities to achieve local rigidity enhancement and modulus difference. Although these methods improve the mechanical properties of the flexible substrate to some extent, they generally have problems such as complex process, limited processing precision, poor repeatability, and difficulty in achieving continuous adjustable modulus gradient distribution, which limits their application in high-performance flexible electronic systems.

[0004] Therefore, there is an urgent need for a flexible substrate preparation method that is simple in process, flexible in design, and can achieve large-scale modulus regulation. The present application proposes a preparation method of an elastic modulus programmable flexible substrate, which irradiates an elastomer substrate covered with a light-cured glue by laser patterning, controls the curing degree of the light-cured glue by setting the number of scans, and thus prepares a flexible substrate with different modulus regions on the elastomer substrate. This method does not require complex mask preparation or multi-step material transfer process, the pattern design is completely digitally controlled, the modulus regulation range can reach two orders of magnitude, and the contrast is high. It can effectively isolate and protect the rigid device from strain while maintaining the overall stretchability. SUMMARY

[0005] The present application aims to address the deficiencies of the prior art, such as complex modulus regulation process and poor programmability of the flexible substrate, and proposes a preparation method of a modulus programmable flexible substrate. This method uses laser to pattern scan the pretreated elastomer substrate, controls the number of scans in different areas, and adjusts parameters such as the thickness ratio of the cured glue and the substrate, to achieve different elastic modulus in different areas, so as to obtain stretchable areas and high rigidity areas on the same flexible substrate, and meet the stress isolation and structural protection requirements of flexible electronic devices.

[0006] To achieve the above-mentioned purpose, the present application adopts the following technical solutions: A method for preparing a modulus programmable flexible substrate, comprising the following steps: 1) preparing an elastomer substrate: mixing and stirring elastomer prepolymer and corresponding curing agent in a certain mass ratio, and then spin-coating the obtained mixture on a substrate pre-coated with a polymer film after vacuum degassing, and obtaining a stretchable elastomer substrate after heating and curing.

[0007] 2) surface treatment: plasma treating the surface of the elastomer substrate to make it hydrophilic, so as to enhance the adhesion of the subsequent photocured adhesive layer.

[0008] 3) spin-coating photocured adhesive: spin-coating photocured adhesive on the treated elastomer surface.

[0009] 4) modulus regulation: using laser to irradiate the photocured adhesive area according to the preset pattern and power, controlling the scanning times of different areas by controlling the thickness ratio of photocured adhesive to elastomer substrate in different areas in 2), or setting different line segments in the pattern to control the scanning times of different areas, so as to control the elastic modulus of different areas. Subsequently, the unexposed and cured photocured adhesive is removed, and the obtained flexible substrate is peeled off from the polymer film to obtain an elastomer substrate with gradient modulus.

[0010] Further, the elastomer prepolymer is PDMS or Ecoflex. For PDMS, the corresponding curing agent used is a siloxane oligomer containing Si-H functional group and a platinum catalyst, and the mass ratio of PDMS to curing agent is usually 10:1. For Ecoflex, the corresponding curing agent used is a siloxane oligomer containing Si-H and a catalyst, and the mass ratio of Ecoflex to curing agent is usually 1:1.

[0011] Further, the polymer film is a PI film or a PET film.

[0012] Further, the photocured adhesive is an ultraviolet photocured adhesive containing acrylate oligomer, active diluent monomer and photoinitiator.

[0013] Further, the laser patterning irradiation specifically comprises: drawing a target scanning pattern by computer aided design (CAD) software, the pattern can be concentric square, grid shape, stripe shape, concentric circle or a combination thereof, and different line segment overlapping times are set according to the required modulus distribution in different areas. The drawn pattern data is imported into a laser scanning control system, and the ultraviolet femtosecond laser is controlled to scan the photocuring adhesive layer point by point or line by line according to the set path. During the scanning process, the energy accumulation in different areas is realized by controlling the line segment overlapping times in the pattern, for example, in one example of the present application, under the same conditions, the line segment overlapping times of the high modulus area are preferably 40-50 times, and the line segment overlapping times of the low modulus area are preferably 1-5 times. The laser irradiation initiates the free radical polymerization reaction of the acrylate monomers and oligomers in the photocuring adhesive, so that the high overlapping times area forms a cured layer with higher crosslinking density, thereby significantly improving the elastic modulus of the area; while the low overlapping times area has lower curing degree, and maintains lower elastic modulus and higher flexibility.

[0014] In addition, further, the pattern can be divided into multiple subareas, the same line segment overlapping times are controlled when forming the pattern in the same subarea; different subareas adopt different laser scanning overlapping times to form different modulus areas, thereby realizing clear and reliable gradient modulus transition, and effectively reducing stress concentration.

[0015] Further, under the condition that the laser irradiation conditions are the same, as the thickness ratio of the photocuring adhesive to the elastomer substrate increases, the elastic modulus after laser irradiation is higher, and the elastic modulus of different areas can also be regulated by changing the thickness ratio of the photocuring adhesive to the elastomer substrate in different areas.

[0016] Further, the gradient modulus design of the laser scanning pattern of the present application not only can form a strain isolation zone in the connecting area of the electronic device and the wire, and reduce the direct influence of local strain on the device performance, but also can effectively relieve stress concentration through the step-by-step transition of the modulus. In the traditional uniform modulus structure, the modulus mutation at the junction of soft and hard areas easily causes stress concentration, so that strain localization occurs at the place under tensile or bending load, resulting in crack generation or material failure. The present application can control the formation of continuous modulus distribution by regulating the laser scanning overlapping times, and can realize the smooth transition from the high modulus area to the low modulus area, so that the stress is dispersed and uniformly distributed in the interface area, thereby improving the overall tensile performance and cycle durability.

[0017] Compared with the existing technology for regulating the elastic modulus of flexible substrate, the present application does not need complex mask preparation or multi-step material transfer process, significantly simplifies the process flow; has strong design flexibility, can realize modulus pattern of any shape on the surface of elastomer through ultraviolet patterned irradiation, does not need to customize physical template, and the pattern design is completely digitalized control; and the modulus regulation range is wide and the contrast is high, the local high cross-linking density area formed through the photocuring process has an elastic modulus increased by more than 5 orders of magnitude compared with the untreated area, and has a significant modulus contrast effect. The modulus gradient can effectively isolate and protect the rigid device from strain, while maintaining the overall tensile performance of the system. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 The process flow diagram for the elastic modulus programmable flexible substrate of the present application.

[0019] Figure 2 The schematic diagram of laser pattern overlapping scanning principle of the present application.

[0020] Figure 3 The schematic diagram of the cross section of the sample in the process of preparing the elastic modulus programmable flexible substrate by using Ecoflex.

[0021] Figure 4 The schematic diagram of laser scanning pattern (left) and photocuring glue curing effect (right).

[0022] Figure 5 The relationship curve of the elastic modulus of photocuring glue with laser energy density.

[0023] Figure 6 The relationship of the elastic modulus of the elastic modulus programmable flexible substrate prepared by using Ecoflex with laser energy density and the thickness ratio of Ecoflex and photocuring glue.

[0024] Figure 7 The stress-strain curve of the elastic modulus programmable flexible substrate prepared by using Ecoflex.

[0025] Figure 8 The tensile test results of the LIG transferred on the Ecoflex elastomer substrate and the tensile test results of the LIG transferred on the Ecoflex elastomer substrate with strain isolation (a) and without strain isolation (b) respectively. Figure 8 Figure 8 The resistance change curve of the LIG in the tensile process under the conditions of no strain isolation and with strain isolation in the present application.

[0026] Figure 9 The process flow diagram of the flexible circuit device of Example 2 of the present application.

[0027] Figure 10 The process flow diagram of the flexible circuit device of Example 2 of the present application.​

[0028] Figure 11 To integrate liquid metal wires and chip resistors on an Ecoflex elastomer substrate and isolate them in a strain-free manner ( Figure 11 a) and strain isolation ( Figure 11 Schematic diagram of the tensile test results under condition b). Detailed Implementation

[0029] The technical solution of the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments: like Figure 1 As shown, this invention discloses a method for preparing a flexible substrate with programmable elastic modulus, comprising the following steps: S101: Preparation of Elastomer Substrate PDMS or Ecoflex is selected as the elastomer material. The elastomer prepolymer and the corresponding curing agent are mixed at a preset mass ratio (PDMS to curing agent mass ratio is 10:1, Ecoflex to curing agent mass ratio is 1:1). The mixture is stirred thoroughly for 1 minute with a stirring rod to ensure uniform mixing. The mixture is then placed in a vacuum degassing machine with a vacuum degree preferably -0.1 MPa for 30 minutes until all air bubbles are eliminated. The mixture is then spin-coated onto the surface of a glass substrate coated with a PI film, the PI film thickness preferably being 125 μm. The PI film is attached to the glass substrate surface using double-sided adhesive. The spin-coating speed and time are adjusted according to the target thickness. After spin-coating, the glass substrate is placed on a heating table for heat curing to obtain a uniform and stretchable solid elastomer substrate. For PDMS, the curing temperature is 120℃ and the curing time is 1 hour; for Ecoflex, the curing temperature is 65-120℃ and the curing time is 30 minutes. S102: Surface treatment, spin-coating UV-curable adhesive The cured elastomer substrate surface is subjected to plasma treatment. This plasma treatment disrupts the original carbon-hydrogen bond structure on the elastomer surface and introduces polar functional groups such as hydroxyl, carboxyl, and aldehyde groups, thereby significantly improving the surface's hydrophilicity and enhancing the wettability and adhesion of the UV-curable adhesive. After treatment, a layer of UV-curable adhesive is spin-coated onto the elastomer surface, preferably a formulation containing acrylate oligomers, reactive diluent monomers, and a photoinitiator. For PDMS substrates, the spin-coating speed is preferably 400 rpm and the spin-coating time is 10 s; for Ecoflex substrates, the spin-coating speed is preferably 300 rpm and the spin-coating time is 20 s, to obtain a uniform UV-curable adhesive layer thickness.

[0030] S103: Ultraviolet femtosecond laser patterned irradiation The target scanning pattern is drawn by computer-aided design (CAD) software, which can be concentric squares, grid shapes, stripe shapes, concentric circles, or combinations thereof. The scanning times and energy accumulation in different areas are controlled by setting different line segment overlap times in the pattern, as shown in Figure 2 Fig. 2. From top to bottom are the paths of each round of laser scanning. The central red area 21 has a high number of line segment overlaps and laser scanning times, and the scanning times of the black areas 22 gradually decrease from the outside, thereby achieving spatial distribution control of the modulus. During the scanning process, the laser moves point by point or line by line along the preset path on the surface of the photocuring adhesive layer. The scanning times of the high modulus area can be set to 40-50 times, and the scanning times of the low modulus area can be set to 3-5 times. The crosslinking density of the area with multiple scanning times is higher, thereby forming a high modulus cured layer; the area with fewer scanning times has a lower curing degree and maintains flexibility.

[0031] S104: cleaning and peeling of the finished product After laser patterning and curing, the sample surface is rinsed with a cleaning solvent (preferably isopropanol) to remove uncured photocuring adhesive residues. Subsequently, the flexible substrate is integrally peeled from the PI carrier film and the glass substrate at room temperature, thereby obtaining a flexible substrate with a preset modulus gradient pattern. The high modulus area deforms less under tension, and the low modulus area has high extensibility, thereby achieving the characteristics of overall stretchability and local non-stretchability.

[0032] The elastic modulus programmable flexible substrate prepared by the above method has a difference in elastic modulus between the high and low modulus areas of up to 5 orders of magnitude, achieving a tunable range from KPa to hundreds of MPa. The modulus gradient distribution can provide effective strain isolation and protection for rigid components in flexible electronic systems, while maintaining the overall structure's stretchability and stability.

[0033] Example 1

[0034] In this embodiment, the above method is used, and Ecoflex is selected as the elastomer material to prepare an elastic modulus programmable flexible substrate.

[0035] As shown in Figure 3 Fig. 1(a), a PI film with a thickness of 125 μm is attached to the surface of a glass substrate as a carrier layer. According to the above steps S101 and S102, the Ecoflex elastomer prepolymer and the curing agent are mixed and stirred at a mass ratio of 1:1, and then degassed under vacuum for 30 minutes. The mixture is then spin-coated on the surface of the PI film at a spin-coating speed of 300 rpm and a spin-coating time of 10 s. Subsequently, the mixture is heated and cured (temperature 80°C, time 30 minutes) to form a solid elastomer substrate.

[0036] As shown in Figure 3A layer of UV-curable glue was spin-coated on the surface of the cured Ecoflex, at a spin-coating speed of 400 rpm and a spin-coating time of 20 s, to obtain a uniform layer of the UV-curable glue.

[0037] As shown in Figure 3 As shown in (c), the layer of the UV-curable glue spin-coated on the surface of the elastomer substrate was subjected to patterned scanning irradiation by using a femtosecond laser to achieve local curing and modulus regulation. Figure 4 The laser scanning pattern (left) and the curing effect of the UV-curable glue after scanning (right) are shown. The laser scanning pattern is a concentric square array with a spacing of 60 μm, and a zoned overlap number distribution is adopted: the four squares in the innermost circle are all 40 times of overlap, and the squares outward are 20 times, 16 times, 10 times, 5 times and 3 times, respectively, and the same overlap number is maintained every four circles. This zoned overlap number distribution pattern design can achieve a clear and reliable gradient modulus transition, effectively reduce stress concentration, and thus better protect the stability and integrity of the device in the stretching process.

[0038] After curing, isopropanol was used to clean and remove the uncured UV-curable glue, and the elastomer substrate was integrally peeled off from the PI film to obtain a flexible substrate with a gradient elastic modulus as shown in Figure 3

[0039] As shown in Figure 5 The relationship curve of the elastic modulus of the layer of the UV-curable glue in the above flexible substrate with a gradient elastic modulus with respect to the laser energy density is shown. In this experiment, the laser energy density was increased by increasing the overlap number of the line segments in the scanning pattern. With the increase of the laser energy density, the elastic modulus of the material was significantly improved: at 4 J / cm 2 , the modulus was about 1 KPa; at 20 J / cm 2 , the modulus was increased to about 100 MPa; when the energy density reached 400 J / cm 2 , the modulus was more than 150 MPa, and the modulus contrast reached more than 5 orders of magnitude. This indicates that continuous programmable regulation from extremely soft to high stiffness can be achieved by adjusting the laser energy density.

[0040] As shown in Figure 6 The effects of the laser energy density and the thickness ratio of the layer of the UV-curable glue to the layer of Ecoflex on the elastic modulus of the prepared elastic modulus programmable flexible substrate are shown. In Figure 6 (a), when the thickness ratio of the layer of the UV-curable glue to the layer of Ecoflex was 1:1, the simulated value, the calculated value and the measured value of the elastic modulus were highly consistent by changing the laser energy density, indicating that the regulation model in this energy density range was accurate and reliable. In Figure 6 (b), when the laser energy density was fixed at 20 J / cm 2 ​As the thickness ratio of the UV-curable adhesive to the Ecoflex layer increases, the elastic modulus of the substrate shows a significant upward trend, and the simulated, calculated, and measured values ​​remain in good agreement, further validating the effectiveness and predictability of the thickness ratio in modulus control. Furthermore, Figure 6 Figure (c) presents a heatmap of elastic modulus changes under different combinations of laser energy density and thickness ratio, which can intuitively reflect the synergistic control effect of the two: simultaneously increasing the thickness ratio and laser energy density can significantly improve the modulus of the substrate, by more than five orders of magnitude. This result verifies the effectiveness and predictability of the method of the present invention in precisely adjusting the modulus of flexible substrates over a wide range.

[0041] like Figure 7 The figure shows the stress-strain curves of the aforementioned programmable elastic modulus flexible substrate under tension. The untreated Ecoflex (blue curve) exhibits extremely low stress levels throughout the tensile process, demonstrating excellent stretchability; with an energy density of 4 J / cm²... 2 The mechanical intensity of the region treated with ultraviolet laser (red curve) did not change significantly; while the region treated with an energy density of 476 J / cm² showed little change. 2 The region treated with ultraviolet laser (yellow curve) showed a significant increase in stress at low strain, with a maximum stress of about 6 MPa and a strain capacity close to 100%, indicating that the stiffness of the region was significantly enhanced while still having a certain degree of ductility.

[0042] like Figure 8 As shown, this embodiment further verifies the protective effect of modulus-controlled strain isolation on laser-induced graphene (LIG). Specifically, the prepared LIG film was transferred onto the surface of an Ecoflex elastomer substrate, and strain isolation was achieved under conditions of no modulus control (…). Figure 8 (a) and modulus control ( Figure 8 Tensile tests were conducted under the conditions described in (b). Without strain isolation, when the substrate was stretched to 50% strain, numerous obvious cracks appeared on the LIG surface, severely damaging the conductive network. However, with modulus control, the LIG maintained a relatively complete continuous morphology under the same strain conditions, and crack formation was significantly reduced, indicating that this method can effectively alleviate the damage to the brittle conductive layer caused by stress concentration during tensile testing.

[0043] like Figure 9The resistance change of LIG in the stretching process was measured in both cases. The results show that, under the condition of no strain isolation, the resistance increases significantly with strain, and reaches several times or even orders of magnitude of the initial value at about 50% strain, showing a large-scale failure of the conductive path; while under the condition of strain isolation, the resistance change remains at a very low level within the full range of strain, verifying the effectiveness and superiority of the modulus regulation method of the application in maintaining the electrical stability of the flexible conductive layer.

[0044] Example 2

[0045] The present embodiment provides a flexible circuit device based on an Ecoflex elastomer substrate and a preparation method thereof, aiming to verify that by introducing modulus regulation in the key areas of the device, strain isolation is achieved, thereby effectively protecting the structural integrity of the liquid metal wire and the electrical performance stability of the resistor device.

[0046] As shown in Figure 10 , the method comprises the following steps: 1. Prepare the Ecoflex elastomer substrate according to steps S101 and S102, and spin-coat a layer of light-cured glue on its surface.

[0047] 2. Draw the target scanning pattern using CAD software, and use ultraviolet femtosecond laser to selectively irradiate and cure the light-cured glue layer. The cured area includes the connection between the electronic components and the wires, the predetermined resistance placement position, and the peripheral circuit lead connection position. By controlling the scanning path and the number of overlaps, a local modulus enhancement structure is formed in the above-mentioned key areas, achieving the function of strain isolation.

[0048] 3. Perform plasma treatment on the other side of the cured area to significantly improve the wettability and adhesion of the liquid metal on the surface of the Ecoflex substrate, ensuring that the wire does not easily peel off or break during stretching deformation.

[0049] 4. Cover the side of the Ecoflex elastomer substrate that has completed plasma treatment with a release film mask to define the liquid metal printing area.

[0050] 5. In the area defined by the mask, form a liquid metal wire by direct printing, which has both flexibility and high conductivity, and can maintain electrical connection under a large tensile strain.

[0051] 6. Place and solder the fixed-value resistor at the predetermined position to form reliable electrical contact with the liquid metal wire, forming a complete flexible circuit structure.

[0052] 7. Solder external wires at the edge positions of the Ecoflex substrate to realize connection with the test circuit or functional module (i.e. the flexible circuit structure described in 6), facilitating subsequent performance testing and application verification.

[0053] like Figure 11 As shown, the flexible circuit device prepared in this embodiment and the control sample without modulus control were subjected to tensile tests at 0% strain and 250% strain, respectively. Figure 11 As shown in (a), in the control sample without modulus control, the liquid metal acting as a conductor showed significant fracture upon stretching to 250% strain, resulting in loss of conductivity. However, in the sample with modulus control (e.g., ...), ... Figure 11 As shown in (b), the liquid metal wires maintain continuity under the same strain conditions, demonstrating a significant strain isolation effect and verifying the effectiveness of the method of the present invention in protecting flexible electronic devices.

[0054] The embodiments described above are merely some preferred embodiments of the present invention, and are not intended to limit the invention. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the invention. Therefore, all technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A method of making a modulus programmable flexible substrate, characterized by, The method comprises the following steps: 1) preparing an elastomer substrate: mixing and stirring elastomer prepolymer and corresponding curing agent in a preset mass ratio, and degassing under vacuum conditions; spinning the degassed mixture on the surface of a substrate pre-coated with a polymer film, and curing under heating conditions to obtain an elastomer substrate; 2) surface treatment and spin-coating of light-curing adhesive: plasma treating the surface of the elastomer substrate to improve the surface hydrophilicity, and spin-coating light-curing adhesive on the surface thereof; 3) patterned laser irradiation: scanning and irradiating the light-curing adhesive according to a preset pattern using a laser, so that the irradiated area is cross-linked and cured, and by controlling the thickness ratio of the light-curing adhesive to the elastomer substrate in different areas in step 2) or controlling the number of scanning times during laser irradiation, cured areas with different elastic moduli are formed; 4) removing uncured adhesive: removing the uncured light-curing adhesive, and peeling the obtained elastomer substrate from the polymer film to obtain an elastomer substrate with gradient elastic modulus.

2. The method of claim 1, wherein, The elastomer prepolymer is polydimethylsiloxane (PDMS) or Ecoflex.

3. The method of claim 1, wherein, The light-curing adhesive comprises acrylate oligomer, active diluent monomer and photoinitiator.

4. The method of claim 1, wherein, The laser is an ultraviolet femtosecond laser.

5. The method of claim 1, wherein, The preset pattern is generated by computer-aided design (CAD) software, and the pattern is concentric square, grid, stripe, concentric circle or a combination thereof. Under the same conditions, the more times the laser scans overlap at the same position on the elastomer substrate, i.e. the more laser energy is absorbed, the higher the elastic modulus at that position.

6. The method of claim 5, wherein, The pattern is divided into multiple sub-zones, and the number of overlapping scanning lines is controlled to be the same when forming the pattern in the same sub-zone to avoid interference and differences caused by laser scattering between adjacent patterns; different sub-zones use different numbers of overlapping laser scans to form different modulus areas.

7. The method of claim 1, wherein, Under the same laser irradiation conditions, as the thickness ratio of the light-curing adhesive to the elastomer substrate increases, the elastic modulus after laser irradiation is higher.

8. The method of claim 1, wherein, The obtained elastomer substrate with gradient elastic modulus has an elastic modulus ranging from 1 kPa to 150 MPa.

9. An elastomeric substrate having a gradient modulus, characterized by, The method is prepared according to any one of claims 1-8.

10. A flexible electronic device, characterized by The elastomer substrate with gradient elastic modulus is prepared according to claim 9. The elastomer substrate with gradient elastic modulus is prepared according to claim 9.

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