Method of achieving tunable gradient modulus and viscoelasticity and adhesive applications thereof

Abstract

The present application relates to a method for realizing adjustable gradient modulus and viscoelasticity and its adhesive application, and belongs to the field of bionic adhesion technology. The method utilizes the sensitivity difference of modulus and viscoelasticity of gradient functional layer material in different phases to temperature and the temperature gradient characteristics along the thickness direction, simultaneously realizes gradient modulus and gradient viscoelasticity along the thickness direction in a single gradient functional layer material, and the gradient modulus and gradient viscoelasticity are adjustable. Based on the method for realizing adjustable gradient modulus and viscoelasticity, three different adhesive states, i.e. strong adhesion, weak adhesion and no adhesion state, can be realized, and faster adhesion switch regulation can be realized, the adhesion switch cycle is shorter, and the adhesion switch regulation efficiency is higher. The method is simple and easy to realize, and can be widely applied in emerging technologies such as bionic wall-climbing robots, soft grippers, transfer printing, flexible electronic skin and wearable devices.

Description

Technical Field

[0001] This invention belongs to the field of biomimetic adhesion technology, specifically relating to a method for achieving adjustable gradient modulus and viscoelasticity and its adhesion application. Background Technology

[0002] Over millions of years of evolution, some organisms in nature have developed unique adhesive organs to cope with environmental challenges, including seticular adhesive pads and smooth adhesive pads. Geckos, as a typical example of seticular adhesive pads, possess a layered, directional micro-nano-scale seticular array structure on their adhesive toe pads. Furthermore, individual setae exhibit a gradient distribution of β-keratin in the longitudinal direction, causing the Young's modulus of a single seta to gradually decrease from 1725 MPa at the root to 95 MPa near the tip, demonstrating a significant gradient modulus characteristic. Similarly, the adhesive setae of ladybug tarsal pads show a gradual change in Young's modulus from 6.8 GPa at the base to 1.2 MPa at the tip. This gradient modulus characteristic along the longitudinal direction of the setae helps enhance adaptability to rough substrates and improve the stability of the seticular array. As a typical example of smooth adhesive pads, the tree frog's adhesive toe pads exhibit a contrasting gradient modulus characteristic. Atomic force microscopy nanoindentation tests show that the average effective elastic modulus of the keratinized epidermis of the tree frog's toe pads is approximately 14.4 MPa, gradually decreasing with increasing indentation depth. Furthermore, the tree frog's toe pad epidermal cells contain abundant glycoproteins, and the dermis contains a dense vascular network and large lymphatic spaces, all contributing to its excellent viscoelastic properties. Similarly, the grasshopper's smooth adhesive pads also exhibit a gradient modulus characteristic of being harder at the top and softer at the bottom in the thickness direction, with similar hemolymph and air sacs present within the adhesive pads. This gradient modulus and viscoelastic characteristic of being harder on the outside and softer on the inside makes the smooth adhesive pads flexible and easily deformable, thus also helping them adapt to different natural substrate surfaces, enhancing adhesion and friction properties, and improving the wear resistance of the adhesive pads.

[0003] Inspired by the microstructures of biological adhesion and the gradient of material properties, some biomimetic adhesion technologies with rigid-flexible composite structures have been proposed. Typically, Tian et al. prepared biomimetic dry adhesion materials with rigid core-flexible shell composites through an electrically responsive self-growing strategy. For comparison, the authors also prepared biomimetic dry adhesion materials with flexible layer-rigid layer composites and flexible micropillar-rigid layer composites, and conducted a detailed study on their adhesion properties on different rough substrate surfaces (Core–shell dryadhesives for rough surfaces via electrically responsive self-growing strategy, H.Tian, ​​D.Wang, Y.Zhang, et al. Nature Communications, 2022, 13(1):7659.DOI:10.1038 / s41467-022-35436-6.). Although these biomimetic dry adhesive materials with rigid-flexible composite structures have achieved success in adapting to rough substrate surfaces, improving stability, and enhancing adhesion, the abrupt change in elastic modulus at the interface between the rigid and flexible phases leads to stress concentration under long-term cyclic loading, resulting in interfacial delamination failure. This affects the strength and durability of the structure. Patent CN 110668398B discloses a method for preparing a biomimetic gecko-inspired extreme progressive rigid-flexible gradient micropillar structure and its application. The technical solution of this patent mainly involves adding magnetic nanoparticles of different sizes and a self-floating photoinitiating crosslinking agent to a polymer monomer to obtain a mixture. The prepared mixture is then transferred to a template with pores of different sizes and aspect ratios. After standing, the photoinitiating crosslinking agent is redistributed within the product. The magnetic nanoparticles are then redistributed within the product in a magnetic field. Finally, the polymer monomers are crosslinked and cured by ultraviolet irradiation. The template is then peeled off to obtain the rigid-flexible gradient micropillar array structure. However, this technical solution relies on the application of a gradient magnetic field and the pretreatment of the surface of Fe3O4 nanoparticles of different sizes, so the preparation process is relatively complex. In addition, the composite system prepared by mixing and curing has compatibility issues, and there is a risk of gradually losing the gradient material properties during use. Moreover, the gradient modulus properties of the cured material cannot be controlled.

[0004] Therefore, the above-mentioned prior art has at least the following defects: (1) It does not consider the gradient modulus and viscoelasticity of the material at the same time. Viscoelasticity is often ignored and the gradient modulus and viscoelasticity cannot be controlled; (2) After adhesion occurs, the deadhesion strategy based on peeling or directional adhesion is difficult to control; (3) The rigid-flexible composite structure is prone to interface failure and has poor durability; (4) The composite system based on the gradient distribution of reinforcing particles has compatibility problems, and the gradient material properties may be gradually lost; (5) The preparation process is relatively complicated. Summary of the Invention

[0005] In order to overcome the shortcomings of the prior art, the present invention aims to provide a method for achieving adjustable gradient modulus and viscoelasticity, and an adhesive application based thereon.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for achieving adjustable gradient modulus and viscoelasticity includes the following steps:

[0008] Step 1: Prepare a gradient functional layer with a thickness of t0;

[0009] The gradient functional layer material has different phase states depending on the temperature, including a rigid glassy state, a glass-rubber transition region, and a soft rubbery state, with a glass transition temperature T. g >40℃;

[0010] In the glass-rubber transition region, spanning a temperature range of 10℃ to 30℃, the modulus and viscoelasticity are most sensitive to temperature changes compared to the rigid glass and soft rubber states. Specifically, the modulus decreases sharply from the GPa level to the MPa level with increasing temperature, while the viscoelasticity first increases rapidly and then decreases rapidly with increasing temperature. g It reaches its strongest point nearby;

[0011] In the rigid glass state: the modulus decreases significantly with increasing temperature, and its sensitivity to temperature changes is moderate; the viscoelasticity is lower than that in the glass-rubber transition region and the soft rubber state, and its sensitivity to temperature changes is moderate.

[0012] In the soft rubber state: modulus and viscoelasticity are the least sensitive to temperature changes among the three phases, while viscoelasticity is in the middle.

[0013] Step 2: Fix the gradient functional layer onto the surface of the heating element;

[0014] The gradient functional layer is in direct contact with the heating element, and the heating element heats the gradient functional layer from the bottom surface.

[0015] Due to the difference between heating temperature T1 and ambient temperature T e There are differences between them, and the gradient functional layer has a certain thickness t0, which causes the gradient functional layer to generate a temperature gradient T(t) in its thickness direction, satisfying T(t=0)=T1, T(t=t0)=T2, T1>T2>T e , where t represents different thicknesses of the gradient functional layer, and T2 represents the temperature of the top surface of the gradient functional layer;

[0016] The temperature gradient is determined by the heating temperature T1 and the ambient temperature T. e The effect of gradient functional layer thickness t0: as T1 increases, T eDecreasing or increasing t0 will increase the temperature difference T1-T2, thus making the temperature gradient more pronounced; conversely, decreasing T1 and increasing T0 will increase the temperature difference T1-T2. e Increasing or decreasing t0 will reduce the temperature difference T1-T2, thus making the temperature gradient less obvious.

[0017] Step 3: By heating the gradient functional layer with a heating element to bring it to different phase states, different gradient moduli and gradient viscoelastic characteristics can be generated in the gradient functional layer along its thickness direction. Specifically:

[0018] By heating the gradient functional layer with a heating element to place it in the glass-rubber transition region, due to the existence of a temperature gradient and the fact that the modulus and viscoelasticity are very sensitive to temperature changes, the gradient functional layer produces obvious gradient modulus and gradient viscoelasticity characteristics with a hard top and soft bottom in its thickness direction.

[0019] By heating the gradient functional layer with a heating element to make it in a rigid glassy state, due to the existence of a temperature gradient and the fact that the modulus is more sensitive to temperature changes while the viscoelasticity is not sensitive to temperature changes, the gradient functional layer produces obvious gradient modulus characteristics of being hard at the top and soft at the bottom and weak gradient viscoelastic characteristics in its thickness direction.

[0020] By heating the gradient functional layer with a heating element to make it in a soft rubber state, although the temperature gradient is more obvious, the gradient modulus and gradient viscoelasticity characteristics of the gradient functional layer in its thickness direction are not obvious because the modulus and viscoelasticity are not sensitive to temperature changes.

[0021] Preferably, the method for achieving adjustable gradient modulus and viscoelasticity according to the present invention may also have the following characteristics: the gradient functional layer material in step 1 includes shape memory polymers, liquid crystal elastomers, liquid metals and their composites, and other temperature-sensitive materials with similar phase characteristics.

[0022] Preferably, the method for achieving adjustable gradient modulus and viscoelasticity according to the present invention may also have the following characteristics: the thickness of the gradient functional layer in step 1 is on the order of millimeters, and the gradient functional layer is too thin, which is not conducive to the presentation of temperature gradient in the thickness direction.

[0023] More preferably, the method for achieving adjustable gradient modulus and viscoelasticity according to the present invention may also have the following characteristics: the top surface of the gradient functional layer in step 1 is a smooth flat surface or is covered by a biomimetic micro / nano structure array.

[0024] Preferably, the method for achieving adjustable gradient modulus and viscoelasticity according to the present invention may also have the following characteristics: the heating method of the heating element in step 2 includes resistance heating and other applicable heating methods, and the gradient functional layer is firmly bonded to the heating element.

[0025] This invention also provides an adhesion application based on a method for achieving adjustable gradient modulus and viscoelasticity, comprising the following steps:

[0026] Step 1: Heat the gradient functional layer using a heating element to bring it to T g In the nearby glass-rubber transition zone, a certain pre-pressure is applied to make the gradient functional layer contact the adherend and maintain it for a certain time. Then, the layer is unloaded. At this time, due to the obvious gradient modulus and gradient viscoelasticity in the thickness direction of the gradient functional layer, and the overall viscoelasticity is very strong, it is beneficial to increase the actual contact area and form a tight contact during contact formation. During unloading and pulling away, it is beneficial to enhance the fracture toughness of the contact interface and dissipate a large amount of elastic strain energy through viscoelasticity, contributing to the effective adhesion work. This makes the gradient functional layer and the adherend exhibit a strong adhesion state. Therefore, a large pulling force must be applied to pull the gradient functional layer away from the adherend.

[0027] Step 2: Heat the gradient functional layer to a soft rubber state using a heating element, apply a certain pre-pressure to make the gradient functional layer contact the adherend and maintain contact for a certain time, then unload. At this time, since the gradient modulus and gradient viscoelasticity are not obvious and the viscoelasticity is very weak, although a good contact state can be formed in the contact formation stage, the effective adhesion work is small due to the destructive effect of the release of stored elastic strain energy and the weak viscoelastic dissipation effect. Thus, the gradient functional layer and the adherend exhibit a weak adhesion state, so a small pulling force can be applied to pull the gradient functional layer away from the adherend.

[0028] Step 3: Heat the gradient functional layer to a rigid glassy state using a heating element. Apply a certain pre-pressure to make the gradient functional layer contact the substrate and maintain this contact for a certain period of time. Unload the pressure. At this point, because the gradient functional layer is in a rigid glassy state, its modulus is relatively high and its viscoelasticity is very weak. This hinders the formation of contact between the gradient functional layer and the substrate. A good contact state is a prerequisite for adhesion. This results in poor adhesion or almost no adhesion between the gradient functional layer and the substrate, presenting a non-adhesive state. Therefore, it is almost unnecessary to apply a pulling force to pull the gradient functional layer away from the substrate.

[0029] The strength of the adhesion state and the magnitude of the pulling force in the above steps are relative relationships between the three phases, and there is no specific numerical limitation. That is, the adhesion state in the glass-rubber transition zone is the strongest, the soft rubber state is in the middle, and the rigid glass state is the weakest.

[0030] Based on the above-mentioned adhesive application, this invention provides a method for controlling the on / off state of adhesion and for picking up and releasing the adhered object: firstly, the gradient functional layer is heated by a heating element to bring it to a T position. gIn the nearby glass-rubber transition zone, a certain pre-pressure is applied to bring the gradient functional layer into contact with the target adherend on the donor substrate surface and maintain this contact for a certain period of time. Then, the pressure is released, resulting in strong adhesion between the gradient functional layer and the target adherend, thus achieving adhesion activation. The gradient functional layer can then stably pick up the target adherend. Next, the target adherend is transferred to the top of the target substrate, and a certain pre-pressure is applied again to bring it into contact with the target substrate and maintain this contact for a certain period of time. Then, the gradient functional layer is heated by a heating element to a soft rubber state, at which point weak adhesion is achieved between the gradient functional layer and the target adherend, thus achieving adhesion deactivation. Finally, the gradient functional layer easily releases the target adherend onto the target substrate.

[0031] Preferably, the adhesive application involved in the present invention may also have the following characteristics: the adherend can be a one-dimensional, two-dimensional or three-dimensional solid with different shapes, sizes and materials, and its surface is smooth or rough, dry or slippery; the target substrate can be made of rigid or flexible material, with regular or irregular flat or curved surfaces.

[0032] Preferably, the adhesion application involved in the present invention may also have the following characteristics: the adhesion between the gradient functional layer and the adherend in step 1 is strongly dependent on the pre-pressure, holding time, or pull-off rate, and usually increases with the increase of the pre-pressure, holding time, or pull-off rate, while the adhesion between the gradient functional layer and the adherend in steps 2 and 3 is less dependent on the pre-pressure, holding time, or pull-off rate.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] This invention utilizes the temperature sensitivity differences in the modulus and viscoelasticity of graded functional layer materials in different phases, as well as the temperature gradient characteristics along the thickness direction, to simultaneously achieve gradient modulus and gradient viscoelasticity along the thickness direction in a single graded functional layer material. Furthermore, the gradient modulus and gradient viscoelasticity are adjustable, with the gradient modulus adjustable from the GPa level to the MPa level, and the gradient viscoelasticity adjustable from strong viscoelasticity to weak viscoelasticity. The fabrication process involved in this method is simple, easy to implement, and has lower costs.

[0035] Based on this adjustable gradient modulus and viscoelasticity method, three different adhesion states can be achieved: strong adhesion, weak adhesion, and no adhesion. It also enables rapid adhesion switching control; the adhesion stress reaches 600 kPa, the adhesion-to-onset ratio reaches 15, the adhesion-to-onset cycle is shorter, and the adhesion-to-onset control efficiency is higher. This method can be widely applied in emerging technologies such as biomimetic wall-climbing robots, soft grippers, transfer printing, flexible electronic skin, and wearable devices. Attached Figure Description

[0036] Figure 1It is a shape memory polymer film produced through a molding-transfer printing process;

[0037] Figure 2 This is a graph showing the relationship between the storage modulus and loss modulus of shape memory polymer films and temperature.

[0038] Figure 3 This is a schematic diagram of a shape memory polymer film fixed on the surface of a heating plate;

[0039] Figure 4 This is a force-displacement relationship curve of the shape memory polymer film under different phase states obtained from adhesion testing;

[0040] Figure 5 a and Figure 5 b are graphs showing the relationship between the effective modulus and internal friction of the shape memory polymer film under different phases and the downward displacement.

[0041] Figure 6 a and Figure 6 b are graphs showing the relationship between the adhesive force and adhesive stress of shape memory polymer films in different phases and the downward displacement.

[0042] Figure 7 This is a schematic diagram illustrating the pickup and release operation of the target object based on the adhesion switch control strategy of shape memory polymer film.

[0043] The labels in the diagram represent the following: 1-shape memory polymer film, 2-hot plate, 3-target substrate, 4-donor substrate, 5-target substrate, T1-temperature of the hot plate surface, T2-temperature of the top surface of the shape memory polymer film, T... e - Ambient temperature. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. Unless otherwise specified, the equipment and reagents used in the embodiments and experimental examples are commercially available. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0045] A method for achieving adjustable gradient modulus and viscoelasticity includes the following steps:

[0046] Step 1: Preparation of shape memory polymer film.

[0047] Shape memory polymer films are prepared by a copying-transfer process. First, PDMS prepolymer Sylgard 184 and curing agent are weighed at a weight ratio of 10:1 and mixed evenly by stirring. The mixture is then vacuumed in a vacuum drying oven for 30 minutes to remove air bubbles. The PDMS mixture is poured into a mold and then demolded after curing to obtain a PDMS elastomer negative mold.

[0048] In order to prepare a biomimetic micro-nano structure array, the biomimetic micro-nano structure array can be formed by etching on the surface of a silicon wafer first, and then the silicon wafer can be used as the bottom surface of the mold. If a smooth silicon wafer is used directly as the bottom surface of the mold, a smooth surface without the biomimetic micro-nano structure array will be obtained.

[0049] Then, resin monomer E44 and polyetheramine Jeffamine D230 were weighed at a weight ratio of 45:23. Resin monomer E44 was preheated to increase its fluidity. The mixture was stirred until fully homogeneous with the polyetheramine Jeffamine D230. The mixture was then placed in a vacuum drying oven and evacuated for approximately 1 hour to remove air bubbles. The resin mixture was poured into and filled into a PDMS negative mold. It was pre-cured at 100°C for 1 hour, followed by post-curing at 130°C for 1 hour. After cooling, the mold was demolded to obtain a shape memory polymer film with a thickness of approximately 2 mm. Figure 1 As shown.

[0050] The thermomechanical properties of the shape memory polymer film were characterized using a dynamic thermomechanical analyzer (DMA) with a temperature scan mode, an amplitude of 15 μm, a frequency of 1 Hz, and a heating rate of 3 °C / min.

[0051] Figure 2 This graph shows the relationship between the storage modulus and loss modulus of the shape memory polymer film and temperature. Figure 2 It is known that shape memory polymer films exhibit different phase states depending on the temperature, including a rigid glass state, a glass-rubber transition region, and a soft rubber state, with a glass transition temperature T. g It is approximately 57°C.

[0052] In the glass-rubber transition region, the temperature range is relatively narrow, approximately 20°C. The storage modulus and loss modulus are highly sensitive to temperature changes. Specifically, the storage modulus decreases sharply from the GPa level to the MPa level with increasing temperature; the loss modulus first increases rapidly and then decreases rapidly with increasing temperature. g It reaches its maximum near the vicinity. The loss modulus reflects the viscoelasticity of the material.

[0053] In the rigid glass state: the energy storage modulus decreases significantly with increasing temperature and is quite sensitive to temperature changes; the loss modulus is relatively small and increases with increasing temperature, and is also quite sensitive to temperature changes.

[0054] In the soft rubber state, the storage modulus and loss modulus are generally very low and are not sensitive to temperature changes.

[0055] Step 2: Fix the shape memory polymer film onto the surface of the heating plate.

[0056] like Figure 3 As shown, the shape memory polymer film 1 is directly fixed to the surface of the heating plate 2. In the curing step of the shape memory polymer film 1, the heating plate 2 is placed flat on the mold so that its heating surface is in full contact with the shape memory polymer mixture. After curing, demolding is performed to achieve the integral fabrication of the shape memory polymer film 1 and the heating plate 2. Alternatively, the shape memory polymer film 1 can be firmly fixed to the surface of the heating plate 2 using thermally conductive adhesives or mechanical clamps.

[0057] The heating plate 2 heats the shape memory polymer film 1 from its bottom surface using resistance heating. The surface temperature of the heating plate 2 is T1, the top surface temperature of the shape memory polymer film 1 is T2, and the ambient temperature is T. e .

[0058] Due to the difference between heating temperature T1 and ambient temperature T e There are differences between them, and the shape memory polymer film 1 has a certain thickness, which causes a temperature gradient T(t) to be generated in the shape memory polymer film 1 along its thickness direction, satisfying T(t=0)=T1, T(t=t0)=T2, T1>T2>T e , where t represents different thicknesses of shape memory polymer film 1, and t0 refers to the thickness of shape memory polymer film 1.

[0059] The temperature gradient is determined by the heating temperature T1 and the ambient temperature T. e The effect of shape memory polymer film thickness t0: as T1 increases, T e Decreasing or increasing t0 will increase the temperature difference T1-T2, thus making the temperature gradient more pronounced; conversely, decreasing T1 and increasing T0 will increase the temperature difference T1-T2. e Increasing or decreasing t0 will reduce the temperature difference T1-T2, thus making the temperature gradient less obvious.

[0060] The surface temperature T1 of the heating plate 2, the top surface temperature T2 of the shape memory polymer film 1 prepared in step 1, and the ambient temperature T were measured using a high-precision contact thermocouple thermometer. e The test should be repeated at least 10 times under the same test conditions, and the average result should be taken. The temperature of the test environment is T. eThe temperature is approximately 20.3 ± 0.4℃ (N = 14). Table 1 lists the measured values ​​of the surface temperature T1 of the heating plate 2 and the top surface temperature T2 of the shape memory polymer film 1 under different set temperature conditions:

[0061] Table 1

[0062]

[0063] As shown in Table 1, under different set temperature conditions, the temperature T2 of the top surface of the shape memory polymer film 1 is significantly lower than the temperature T1 of the surface of the heating plate 2, i.e., T2 < T1. Moreover, as the set temperature increases, the temperature difference (T1-T2) between the surface of the heating plate 2 and the top surface of the shape memory polymer film 1 gradually increases. This indicates that there is a temperature gradient in the thickness direction of the shape memory polymer film 1, and the higher the set temperature, the more obvious the temperature gradient.

[0064] The adhesive contact performance between shape memory polymer film 1 and a hemispherical glass indenter (9 mm in diameter) was studied using an adhesion testing device. The shape memory polymer film 1 was heated to different phase states (glass-rubber transition zone, rigid glass state, and soft rubber state) by a hot plate 2, and adhesion tests were conducted under different pressure displacement conditions. The pressure rate was 20 μm / s, the holding time was approximately 6 s, and the unloading and pull-out rates were 40 μm / s. Force-displacement-time data were acquired and recorded in real time throughout the entire test process. The test was repeated approximately 10 times under the same conditions, and the results were averaged.

[0065] Figure 4 The force-displacement relationship curves of shape memory polymer film 1 under different phases, obtained through adhesion testing, are shown. Based on Figure 4 The effective modulus under different downward displacements was extracted for the initial loading and unloading segments of the force-displacement relationship curve using Hertz elastic contact mechanics theory and the Oliver & Pharr method, respectively.

[0066] based on Figure 4 The force-displacement relationship curve in the figure is fitted and integrated using a law function to extract the elastic energy U1 stored in the loading segment and the elastic energy U2 dissipated due to viscoelasticity in the holding and unloading segments. The internal friction of the loading, holding, and unloading segments is then expressed as U2 / U1. Internal friction is used to evaluate viscoelastic hysteresis; therefore, the magnitude of internal friction reflects the strength of the material's viscoelasticity. For example... Figure 4 As shown, in T g The nearby glass-rubber transition zone (T) gThe force-displacement curves show significant viscoelastic hysteresis, corresponding to large internal friction and strong viscoelasticity. In the rigid glassy state and the soft rubbery state, the loading and unloading curves completely or almost completely overlap, and the viscoelastic hysteresis is not obvious, corresponding to low internal friction and weak viscoelasticity.

[0067] Figure 5 a and Figure 5 b shows the relationship between the effective modulus and internal friction (U2 / U1) of shape memory polymer film 1 in different phases and the displacement.

[0068] Step 3: Heat the shape memory polymer film 1 using the hot plate 2 to bring it to a T position. g Nearby glass-rubber transition zone, such as Figure 5 a and Figure 5 As shown in b, the effective modulus and internal friction both decrease significantly with the increase of downward displacement, indicating that the shape memory polymer film 1 has obvious gradient modulus characteristics and gradient viscoelastic characteristics with a hard upper part and a soft lower part in its thickness direction.

[0069] Step 4: Heat the shape memory polymer film 1 using the heating plate 2 to bring it to a rigid glassy state, such as... Figure 5 a and Figure 5 As shown in b, the effective modulus decreases significantly with the increase of the downward displacement, while the internal friction remains almost unchanged with the increase of the downward displacement. This indicates that the shape memory polymer film 1 has obvious gradient modulus characteristics of being hard at the top and soft at the bottom and very weak gradient viscoelastic characteristics in its thickness direction.

[0070] Step 5: Heat the shape memory polymer film 1 using the heating plate 2 to bring it to a soft rubber state, such as... Figure 5 a and Figure 5 As shown in b, the effective modulus and internal friction both show a certain decreasing trend with the increase of downward displacement, which indicates that the gradient modulus and gradient viscoelasticity characteristics of the shape memory polymer film 1 in its thickness direction are weak and not obvious.

[0071] from Figure 4 The maximum pull-off force (i.e. adhesive force) of the pull-off segment is extracted from the force-displacement relationship curve shown. Combined with the contact area parameter calculated by Oliver & Pharr method, the maximum pull-off force is divided by the contact area to further calculate the maximum pull-off stress (i.e. adhesive stress).

[0072] Figure 6 a and Figure 6b shows the relationship between the adhesive force and adhesive stress of shape memory polymer film 1 in different phases and the pressure displacement.

[0073] Accordingly, an adhesion application based on a method for achieving adjustable gradient modulus and viscoelasticity is provided, comprising the following steps:

[0074] Step 1: Heat the shape memory polymer film 1 using the hot plate 2 to bring it to a T position. g In the nearby glass-rubber transition zone, a certain pre-pressure is applied to make the shape memory polymer film 1 contact with the hemispherical glass indenter and maintain it for a certain period of time. Then, the pressure is unloaded. At this time, the shape memory polymer film 1 and the hemispherical glass indenter are in a strong adhesive state. Therefore, a large pulling force must be applied to pull the shape memory polymer film 1 away from the hemispherical glass indenter.

[0075] like Figure 6 a and Figure 6 As shown in b, in T g In the nearby glass-rubber transition zone, the adhesive force and adhesive stress of the shape memory polymer film 1 are significantly greater under different downward displacement conditions. In particular, the adhesive force reaches as high as 950 mN and the adhesive stress reaches as high as 600 kPa, and both adhesive force and adhesive stress show a strong dependence on the downward displacement. This is because there is a significant gradient modulus and gradient viscoelasticity in the thickness direction of the shape memory polymer film 1, and the viscoelasticity is very strong overall. This is beneficial for increasing the actual contact area and forming a tight contact during contact formation, and for enhancing the fracture toughness of the contact interface during unloading and pull-off. It also helps to dissipate a large amount of elastic strain energy through viscoelasticity, contributing to the effective adhesive work, thereby resulting in a strong adhesive state between the shape memory polymer film 1 and the hemispherical glass indenter.

[0076] Step 2: Heat the shape memory polymer film 1 with the heating plate 2 to make it soft rubber state, apply a certain pre-pressure to make the shape memory polymer film 1 contact with the hemispherical glass indenter and maintain it for a certain time, unload, at this time the shape memory polymer film 1 and the hemispherical glass indenter are in a weak adhesive state, so a small pulling force can be applied to pull the shape memory polymer film 1 away from the hemispherical glass indenter.

[0077] like Figure 6 a and Figure 6As shown in b, in the soft rubber state, the adhesive force and adhesive stress of the shape memory polymer film 1 are generally very low under different pressure displacement conditions. In particular, the adhesive force is as low as 60 mN and the adhesive stress is as low as 29 kPa, and the adhesive force and adhesive stress show only a very weak dependence on the pressure displacement. This is because the gradient modulus and gradient viscoelasticity are not obvious, and the viscoelasticity is very weak. Although a good contact state can be formed in the contact formation stage, the effective adhesive work is small due to the destructive effect of the release of stored elastic strain energy and the weak viscoelastic dissipation effect, thus resulting in a weak adhesive state between the shape memory polymer film 1 and the hemispherical glass indenter.

[0078] Step 3: Heat the shape memory polymer film 1 with the heating plate 2 to make it in a rigid glass state. Apply a certain pre-pressure to make the shape memory polymer film 1 contact the hemispherical glass indenter and maintain it for a certain time. Unload. At this time, the adhesion between the shape memory polymer film 1 and the hemispherical glass indenter is very poor or almost no adhesion, and it is in a non-adhesive state. Therefore, it is almost unnecessary to apply a pulling force to pull the shape memory polymer film 1 away from the hemispherical glass indenter.

[0079] like Figure 6 As shown in a and 6b, in the rigid glass state, the adhesive force and adhesive stress of the shape memory polymer film 1 under different downward displacement conditions are almost zero, showing no dependence on the downward displacement. This is because the shape memory polymer film 1 is in a rigid glass state, with a relatively high modulus and very weak viscoelasticity. This hinders the formation of contact between the shape memory polymer film 1 and the hemispherical glass indenter. A good contact state is a prerequisite for adhesion. Therefore, the adhesion between the shape memory polymer film 1 and the hemispherical glass indenter is poor or almost non-existent.

[0080] Step 4: Combining steps 1 and 2, implement the on / off control of the adhesion and the picking up and releasing of the adhered object. For example... Figure 7 As shown, the shape memory polymer film 1 is first heated by the heating plate 2 to a temperature of T. g Near the glass-rubber transition zone, a certain pre-pressure is applied to make the shape memory polymer film 1 contact the target adherend 3 on the surface of the donor substrate 4 and maintain it for a certain period of time. Figure 7 (a) Unloading: At this point, the shape memory polymer film 1 and the target object 3 are in a strong adhesive state, i.e., the adhesion is activated, and the shape memory polymer film 1 can stably pick up the target object 3. Figure 7 (b)); Next, the target adherend 3 is transferred above the target substrate 5, and a certain pre-pressure is applied again to make the target adherend 3 contact the target substrate 5 and maintain it for a certain period of time. Figure 7(c)); then the shape memory polymer film 1 is heated by the heating plate 2 to make it into a soft rubber state. At this time, the shape memory polymer film 1 and the target adherend 3 exhibit a weak adhesion state, that is, the adhesion is closed; finally, the shape memory polymer film 1 easily releases the target adherend 3 onto the target substrate 5. Figure 7 (d)).

[0081] like Figure 6 As shown in a and 6b, the adhesion on / off ratio (i.e., the ratio of strong adhesion to weak adhesion performance) of the shape memory polymer film 1 is as high as 15, which facilitates stable pickup and easy release of the target adhered object 3. On the other hand, the adhesion on / off control method based on shape memory polymers usually requires heating to a temperature much higher than T. g The soft rubber forms a contact, and then it is cooled to a temperature far below T. g The rigid glassy state is fixed and deformed to achieve the adhesive open state, while to achieve the adhesive closed state, it is necessary to reheat to a temperature much higher than T. g The deadhesion of the soft rubber state is problematic because the temperature range between the rigid glass state and the soft rubber state is approximately 65°C. The slow heating and cooling process undoubtedly results in a long adhesion-on-off cycle, which is detrimental to rapid on / off adhesion control. However, in this invention, it is only necessary to set the temperature so that the shape memory polymer film 1 is at T... g The nearby glass-rubber transition zone enables the adhesion to be turned on, while heating the glass-rubber transition zone to a soft rubber state enables the adhesion to be turned off. The heating range is about 20°C, which significantly shortens the adhesion switching cycle and thus can significantly improve the picking up and releasing efficiency of the target object 3.

[0082] As a preferred embodiment, the target adherend 3 can be a one-dimensional, two-dimensional, or three-dimensional solid with different shapes, sizes, and materials, and its surface can be smooth or rough, dry or slippery; the target substrate 5 can be made of rigid or flexible material, and has regular or irregular flat or curved surfaces.

[0083] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for achieving adjustable gradient modulus and viscoelasticity, characterized in that, Includes the following steps: Step 1: Prepare a material with a thickness of t Gradient function layer with a value of 0; The gradient functional layer has different phase states depending on the temperature, including a rigid glass state, a glass-rubber transition region, and a soft rubber state, with its glass transition temperature... T g >40 o C; In the glass-rubber transition region: the temperature range spanned is 10°C. o C~30 o C. Modulus and viscoelasticity are most sensitive to temperature changes compared to the rigid glassy and soft rubber states. Specifically: the modulus decreases sharply from the GPa level to the MPa level with increasing temperature, while viscoelasticity first increases rapidly and then decreases rapidly with increasing temperature. T g It reaches its strongest point nearby; In the rigid glass state: the modulus decreases significantly with increasing temperature, and its sensitivity to temperature changes is moderate; the viscoelasticity is lower than that in the glass-rubber transition region and the soft rubber state, and its sensitivity to temperature changes is moderate. In the soft rubber state: modulus and viscoelasticity are the least sensitive to temperature changes among the three phase states, while viscoelasticity is in the middle. Step 2: Fix the gradient functional layer onto the surface of the heating element; The gradient functional layer is in direct contact with the heating element, and the heating element heats the gradient functional layer from the bottom surface of the gradient functional layer, thereby generating a temperature gradient in the thickness direction of the gradient functional layer. Step 3: By heating the gradient functional layer with a heating element to make it in different phase states, the gradient functional layer can produce different gradient moduli and gradient viscoelastic characteristics in its thickness direction.

2. The method for achieving adjustable gradient modulus and viscoelasticity as described in claim 1, characterized in that, In step 2: due to the heating temperature T 1. Ambient temperature T e There are differences between them, and the gradient functional layer has a certain thickness. t 0, which causes the gradient functional layer to generate a temperature gradient along its thickness direction. T ( t ),satisfy T ( t= 0) = T 1, T ( t=t 0) = T 2, T 1> T 2> T e ,in, t Representing different thicknesses of the gradient functional layer T 2 represents the temperature of the top surface of the gradient functional layer; Temperature gradient is affected by heating temperature T 1. Ambient temperature T e and gradient functional layer thickness t The effect of 0: T 1. Increase height T e Reduce or t Increasing the temperature difference will make the temperature difference greater. T 1- T 2 increases; T 1. Reduce T e Increase or t A decrease of 0 will cause a temperature difference T 1- T 2 becomes smaller.

3. The method for achieving adjustable gradient modulus and viscoelasticity as described in claim 1, characterized in that, In step 3: By heating the gradient functional layer with a heating element to place it in the glass-rubber transition region, the gradient functional layer will produce obvious gradient modulus and gradient viscoelasticity characteristics in its thickness direction, which are hard at the top and soft at the bottom. By heating the gradient functional layer with a heating element to make it in a rigid glassy state, the gradient functional layer will produce obvious gradient modulus characteristics of being hard at the top and soft at the bottom and weak gradient viscoelastic characteristics in its thickness direction. The gradient functional layer is heated by a heating element to make it into a soft rubber state, and the gradient modulus and gradient viscoelasticity characteristics of the gradient functional layer in the thickness direction are not obvious.

4. The method for achieving adjustable gradient modulus and viscoelasticity as described in claim 1, characterized in that: In step 1, the gradient functional layer material is a thermosensitive material with the desired phase characteristics.

5. The method for achieving adjustable gradient modulus and viscoelasticity as described in claim 1, characterized in that: In step 1, the thickness of the gradient functional layer is on the order of millimeters.

6. The method for achieving adjustable gradient modulus and viscoelasticity as described in claim 5, characterized in that: The top surface of the gradient functional layer is a smooth flat surface or is covered by a biomimetic micro / nano structure array.

7. The method for achieving adjustable gradient modulus and viscoelasticity as described in claim 1, characterized in that: In step 2, the heating method of the heating element includes resistance heating, and the gradient functional layer is firmly bonded to the heating element.

8. An adhesive application of a method for achieving adjustable gradient modulus and viscoelasticity based on any one of claims 1-7, characterized in that, Includes the following steps: Step 1: Heat the gradient functional layer with a heating element to place it in the glass-rubber transition zone, apply pre-pressure to make the gradient functional layer contact the substrate and maintain it for a certain time, unload, at this time the gradient functional layer and the substrate exhibit a strong adhesive state. Step 2: Heat the gradient functional layer with a heating element to make it soft rubber state, apply pre-pressure to make the gradient functional layer contact the substrate and maintain it for a certain time, unload, at this time the gradient functional layer and the substrate exhibit a weak adhesion state. Step 3: Heat the gradient functional layer with a heating element to make it in a rigid glassy state. Apply pre-pressure to make the gradient functional layer contact the substrate and maintain it for a certain period of time. Unload the pressure. At this time, the gradient functional layer and the substrate are in a non-adhesive state.

9. A method for controlling the on / off state of adhesion and picking up and releasing the adhered object based on the adhesive application of claim 8, characterized in that: First, the gradient functional layer is heated to the glass-rubber transition zone using a heating element. Pre-pressure is applied to bring the gradient functional layer into contact with the target substrate on the donor substrate surface and maintain this contact for a certain period of time. Then, the pressure is released, at which point the gradient functional layer and the target substrate exhibit strong adhesion, i.e., adhesion is enabled, and the gradient functional layer can stably pick up the target substrate. Next, the target substrate is transferred to the target substrate, and pre-pressure is applied again to bring the target substrate into contact with the target substrate and maintain this contact for a certain period of time. Then, the gradient functional layer is heated to a soft rubber state using a heating element, at which point the gradient functional layer and the target substrate exhibit weak adhesion, i.e., adhesion is disabled. Finally, the gradient functional layer easily releases the target substrate.

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