CN114479469A

CN114479469A - Preparation method of two-phase flexible PDMS composite material and wearable pressure sensor

Info

Publication number: CN114479469A
Application number: CN202210067002.7A
Authority: CN
Inventors: 林红; 任孟; 陈宇岳; 张德锁
Original assignee: Suzhou University
Current assignee: Suzhou University
Priority date: 2022-01-20
Filing date: 2022-01-20
Publication date: 2022-05-13

Abstract

The invention discloses a preparation method of a two-phase flexible PDMS composite material, which comprises the following steps: fully mixing graphene and sugar particles, adding pure PDMS sponge and a curing agent, tabletting by using a tabletting device after fully stirring, putting the uniform mixture into an oven for heating and curing, taking out and putting into water to melt sugar, thereby preparing the rGO/PDMS conductive sponge, namely the two-phase flexible PDMS composite material. By utilizing an in-situ sugar template method and a tabletting method, the problems that the conductive material is difficult to permeate into the porous sponge and is easy to fall off in the prior art are solved; the graphene is brought into the PDMS matrix through the sugar particles, so that the graphene is uniformly distributed in the porous PDMS sponge, the particles are tightly connected with each other, the particles are not easily separated from the inner surface of the PDMS sponge, the sensitivity and the durability are good, and the stable resistance response is ensured.

Description

Preparation method of two-phase flexible PDMS composite material and wearable pressure sensor

Technical Field

The invention relates to the technical field of flexible pressure sensors, in particular to a preparation method of a two-phase flexible PDMS composite material and a wearable pressure sensor.

Background

With the rapid development of the technology level, the wearable flexible pressure sensor has become a new research hotspot. When acted upon by an external stimulus, the wearable flexible sensor will convert and output the stimulus as a human-readable electrical signal. The flexible pressure sensor is one of the most commonly used wearable sensors, mainly comprises a flexible matrix and a conductive phase material, has the characteristics of high sensitivity, strong deformability and strong cycling stability, and can convert an externally input mechanical signal into an electric signal.

The graphene composite material is used as a new composite material, and due to the introduction of a synergistic effect and new performance between the materials, the application research of the graphene composite material is expanded. Due to the characteristics of zero band gap, extremely high electron mobility and the like, graphene can be used as a conductive filler in a polymer and used for a mechanical sensor for detecting pressure.

In the prior art, a flexible pressure sensor is generally prepared by firstly preparing porous PDMS (polydimethylsiloxane), and then adsorbing a conductive material by an evaporation coating, sputtering or dip coating method, but the sensor prepared by the method has the defects that the conductive material is difficult to permeate into the porous sponge and is easy to fall off from the surface, and particularly, the high sensitivity and stability of the sensor are difficult to obtain under frequent compression operation.

In order to solve the above problems, it is necessary to develop a flexible pressure sensor having high sensitivity and good stability.

Disclosure of Invention

The invention overcomes the defects of the prior art and provides a preparation method of a two-phase flexible PDMS composite material. In order to achieve the purpose, the invention adopts the technical scheme that: a preparation method of a two-phase flexible PDMS composite material is characterized by comprising the following steps:

s1, adding graphene into the dispersoid, and fully mixing;

s2, adding a curing agent into the PDMS sponge and mixing;

s3, mixing the mixture of S1 and S2, and shaping and curing;

and S4, adding water into the cured sample in the S3, dissolving dispersoids in the sample, and drying to obtain the rGO/PDMS conductive sponge, namely the two-phase flexible PDMS composite material.

In a preferred embodiment of the present invention, the doping concentration of the graphene is 2 to 8 wt%.

In a preferred embodiment of the invention, the dispersoid is a sugar.

In a preferred embodiment of the present invention, the dispersoid needs to be ground and screened before being mixed with the graphene.

In a preferred embodiment of the present invention, in S4, the dispersoid in the sample is dissolved by a water bath method.

In a preferred embodiment of the present invention, the mass ratio of the PDMS sponge to the curing agent is 8-12: 1.

In a preferred embodiment of the present invention, the doping concentration of the graphene is 3 to 5 wt%.

A two-phase flexible PDMS composite material is prepared by the method, wherein graphene in the two-phase flexible PDMS composite material is arranged on the inner hole wall of a PDMS sponge and has folds and rough edges, and the whole hole of the PDMS sponge is not filled with the graphene; the graphene surface on the PDMS sponge has a directional arrangement, is smooth and shows uniformly distributed graphene sheets.

A wearable pressure sensor is prepared by connecting the two-phase flexible PDMS composite material with a lead to assemble a flexible resistance type pressure sensor, and is combined with wearable equipment to test physiological signals of a human body.

In a preferred embodiment of the invention, the sensitivity of the wearable pressure sensor is increased and then decreased within a range of 2-8 wt% of doping concentration of graphene powder and under a certain pressure.

The invention solves the defects in the background technology, and has the following beneficial effects:

(1) the invention utilizes an in-situ sugar template method and a tabletting method, and solves the problems that the conductive material is difficult to permeate into the porous sponge and is easy to fall off in the prior art; the graphene is brought into the PDMS matrix through the sugar particles, so that the graphene is uniformly distributed in the porous PDMS sponge, the particles are tightly connected with each other, the particles are not easily separated from the inner surface of the PDMS sponge, the sensitivity and the durability are good, and the stable resistance response is ensured.

(2) According to the invention, pure PDMS sponge is adopted to be doped with graphene with different concentrations so as to obtain rGO/PDMS conductive sponge with different sensitivities and stabilities, the space arrangement structure of the conductive sponge meets the requirements of contact points and faces of the graphene, when the rGO/PDMS conductive sponge is compressed, the graphene attached to the interior of the PDMS sponge is extruded by the PDMS sponge as a conductive component to deform, and then the conductivity or resistivity changes, so that the requirements of a stress sensor are met; the two electrodes and the conductive sponge are assembled into a flexible resistance type pressure sensor in a sandwich mode, and the flexible pressure sensor is combined with the wearable textile and used for testing physiological signals of a human body.

(3) According to the invention, 4 wt% of graphene is doped in pure PDMS sponge, so that the winding and agglomeration phenomenon among particles in the conductor sponge is reduced, the conductivity of the graphene in the sponge is improved, and the resistivity change rate is ensured to be improved under the same pressure, thereby obtaining the flexible resistance type pressure sensor with the maximum sensitivity and stability.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below, it is obvious that the drawings in the following description are only some embodiments described in the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts;

FIG. 1 is a SEM image of a pure PDMS sponge and its cross section according to one embodiment of the present invention;

FIG. 2 is an SEM image of an embodiment of an rGO/PDMS conductive sponge with a graphene doping concentration of 4 wt% according to the first embodiment of the invention;

FIG. 3 is a sensitivity curve of rGO/PDMS flexible resistive pressure sensors at different graphene rGO concentrations in example II of the present invention;

FIG. 4 is a fitting curve of the sensitivity of rGO/PDMS flexible resistive pressure sensors of different graphene rGO concentration doping concentrations according to a second embodiment of the present invention;

FIG. 5 is a graph showing the responsiveness and recovery of the rGO/PDMS flexible resistive pressure sensor under three different pressures according to the third embodiment of the present invention;

FIG. 6 is a hysteresis curve of an rGO/PDMS resistive flexible pressure sensor according to a fourth embodiment of the present invention;

FIG. 7 is a graph of the cycling stability of an rGO/PDMS flexible resistive pressure sensor in accordance with a fifth embodiment of the present invention;

FIG. 8 is a stress-strain curve of rGO/PDMS conductive sponge in example six of the present invention;

FIG. 9 is a graph of the degree of finger bending measured by the flexible resistive pressure sensor according to the seventh embodiment of the present invention;

fig. 10 is a schematic view of a wearable human body pulse detection device according to a seventh embodiment of the invention;

fig. 11 is a pulse test chart of the flexible pressure sensor according to the seventh embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Example one

The flexible base material selected in the embodiment is polydimethylsiloxane PDMS, which is taken as a typical silicone rubber material and has a silicon-oxygen bond (Si-O) with a spiral structure, all side groups are methyl, and the bond angle of the PDMS silicon-oxygen bond is large, so that the PDMS silicon-oxygen bond is easy to rotate, and good flexibility is generated. PDMS is a thermosetting material and is divided into a main body and a curing agent, wherein the main body is a siloxane oligomer, the curing agent is a siloxane cross-linking agent, and the two components can generate cross-linking under the heating condition. Because a plurality of reaction sites exist between the main body of the PDMS and the curing agent, the PDMS can be cured and formed under the action of heat after being fully mixed.

According to the embodiment, PDMS is used for replacing water, graphene rGO and sugar particles are fully mixed, PDMS sponge and a curing agent are added, a tabletting device is used for tabletting after full stirring, then a sample is placed into an oven for heating and curing for a certain time, the sample is taken out and placed into water to melt sugar, and therefore the porous rGO/PDMS two-phase flexible PDMS composite material is prepared, and the specific preparation method comprises the following steps:

s1, adding graphene into the dispersoid, and fully mixing;

s2, adding a curing agent into the PDMS sponge and mixing;

s3, mixing the mixture of S1 and S2, and shaping and curing;

According to the preparation method, the graphene rGO can be formed inside the PDMS sponge by utilizing the sugar blocks, the graphene rGO is uniformly distributed inside, the particles are tightly connected, the conductive particles are not easy to fall off, and the preparation method has good sensitivity and durability.

In this embodiment, the doping concentration of the graphene powder is preferably 4 wt%, and the two-phase flexible PDMS composite material, i.e., rGO/PDMS conductive sponge, is prepared and formed by using the above preparation method.

In this example, the prepared and formed rGO/PDMS conductive sponge was subjected to morphology characterization and analysis, and SEM images of pure PDMS sponge and rGO/PDMS conductive sponge were compared.

(1) SEM analysis of pure PDMS sponge

In order to explore the internal condition of pure PDMS sponge, the material was scanned by electron microscopy. The appearance of the pure PDMS sponge is shown in figure 1, the pure PDMS sponge has a white surface color, a plurality of pore structures are arranged inside the pure PDMS sponge, the surface of the pure PDMS sponge is smooth, and no impurity particles are attached to the pure PDMS sponge.

(2) SEM analysis of rGO/PDMS conductive sponge

The graphene is uniformly doped in the PDMS sponge, which is an important link for manufacturing the flexible conductive sponge. Fig. 2 is a physical diagram of rGO/PDMS conductive sponge, where it can be clearly seen that the surface color of PDMS sponge is black and the surface is rough, indicating that graphene is successfully combined with PDMS sponge. The scanning electron microscope observation of the material object shows that the graphene on the prepared rGO/PDMS conductive sponge is arranged in the PDMS sponge, the graphene is fully arranged on the wall of the sponge hole, and has folds and burrs, and the whole hole is not filled with the graphene. The high magnification of graphene on the sponge indicates that the graphene surface has a directional arrangement, is smooth and shows a uniform distribution of graphene sheets.

The spatial arrangement structure meets the requirement that the sponge can cause contact points and surfaces of graphene after being compressed again, so that the conductivity of the sponge can be correspondingly changed when the sponge is compressed again. When the rGO/PDMS conductive sponge is compressed, graphene attached to the interior of the PDMS sponge is extruded by the PDMS sponge as a conductive component to deform, and then the conductivity or resistivity changes, so that the requirements of the stress sensor are met.

Example two

In this embodiment, the two-phase flexible PDMS composite material as in the first embodiment is used to form a flexible resistive pressure sensor. The flexible resistance type pressure sensor is formed by connecting and assembling a two-phase flexible PDMS composite material and a lead, specifically, a prepared rGO/PDMS conductive sponge is placed on a workbench, conductive adhesive tapes are attached to the upper surface and the lower surface of the conductive sponge, and electrodes for testing are led out, so that the flexible resistance type pressure sensor is obtained.

The sensitivity refers to the sensitivity of the sensor to external pressure when the sensor is stimulated by the external pressure, and is mainly determined by the slope of the curve of the delta R/R0. This embodiment test sensor's sensitivity compares the influence of different graphite alkene rGO concentrations to flexible resistance type pressure sensor sensitivity. In the method for testing the sensitivity of the sensor in the embodiment, the rGO/PDMS flexible resistance type pressure sensor is tested by adopting a push-pull meter and an LCR digital bridge. The flexible pressure sensor was placed on the platform of the test rig and connected to an LCR digital bridge, then differential pressure was applied to the sensor and data collected and recorded.

As shown in fig. 3, the sensitivity curves of rGO/PDMS flexible resistive pressure sensors at different graphene rGO concentrations in this example are shown. In order to test the response of the sensor to pressure, the sensitivity of the sensor doped with graphene powder with different concentrations is compared.

As can be seen from fig. 3, when the doping concentration of the graphene is in the range of 2 wt% to 8 wt%, the resistance change rate of the sensor under the same pressure decreases as the doping concentration of the graphene increases.

When the doping concentration of the graphene is increased to 4 wt%, the resistance change rate of the sensor is increased to a maximum value, and then the resistance change rate of the sensor is reduced if the doping concentration of the graphene is increased. The reason is that when the doping concentration range of the graphene is 2 wt% -4 wt%, the conductive path inside the conductive sponge is not saturated yet, and at this time, if the doping concentration of the graphene is increased, the conductive path in the conductive sponge gradually changes more, so that the resistance change rate under the same pressure is increased along with the increase of the doping concentration of the graphene. When the doping concentration of the graphene rGO exceeds 4 wt%, the graphene in the conductive sponge is saturated, and the further filling of the graphene can cause the phenomenon of winding and agglomeration among particles, so that the conductivity of the graphene in the sponge can be reduced. If the doping concentration of the graphene is continuously increased at this time, the resistance change rate of the sensor is gradually reduced under the same pressure.

In FIG. 4, (a) to (g) are fitting curves of sensitivity of rGO/PDMS flexible resistive pressure sensor with doping concentration of 2 wt% to 8 wt%, and (h) is a sensitivity curve of the sensor. In this embodiment, sensitivity fitting is performed on 7 rGO/PDMS flexible resistive pressure sensors with different doping concentrations, which are 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt% and 8 wt%, within pressure ranges of 0 to 0.5kPa, 0.5 to 1.5kPa and 2.5 to 5.0kPa, respectively, so as to obtain corresponding sensitivity values. The sensitivity values in the range of 0 to 0.5kPa in the present example are shown in Table 1.

Table 10-0.5 kPa sensitivity values at different graphene doping concentrations

As can be seen from fig. 4 and table 3, when the doping concentration of the graphene is in the range of 2 wt% to 4 wt%, the sensitivity gradually increases with the increase of the doping concentration, and when the doping concentration is 4 wt%, the sensitivity reaches a peak of 76.321kPa^-1However, as the doping concentration of graphene further increases, the sensitivity begins to gradually decrease.

Meanwhile, compared with the sensitivity values in the pressure ranges of 0-0.5 kPa, 0.5-1.5 kPa and 2.5-5.0 kPa, the sensitivity of the sensor is found to be reduced along with the increase of the pressure. The reason is that when no pressure is applied to the sponge, the pore structures in the conductive sponge are almost not contacted, the internal structures are looser, and the Poisson ratio is higher, so that the conductive sponge can be greatly deformed when a smaller pressure is applied, graphene particles attached to the interior of the conductive sponge are connected with each other, conductive paths are rapidly increased, the conductivity of the conductive sponge is rapidly increased, and the sensitivity is higher; however, as the pressure increases, the poisson ratio of the conductive sponge decreases, and even if a large pressure is applied, only a small deformation occurs, and the conductive path inside the sponge is about to reach saturation, so that the sensitivity gradually decreases.

EXAMPLE III

The first embodiment and the second embodiment show that the sensitivity of the flexible resistance type pressure sensor is the maximum when the doping concentration of the graphene is 4 wt%. In the embodiment, the response time of the flexible pressure sensor is tested by adopting a weight and an electrochemical workstation, the sensor and the electrochemical workstation form a series circuit, then an i-t test program is selected from the programs of the electrochemical workstation, the weight is placed on the flexible pressure sensor, and the response time of the flexible pressure sensor to the weight is recorded.

In this embodiment, the highest sensitivity rGO/PDMS conductive sponge is selected for the response and recovery tests, and the response time and recovery time when the pressure is 1kPa, 2.5kPa, and 5kPa are tested, and the test result is shown in fig. 5.

When the applied pressure is 1kPa, the response time of the sensor is 0.3s, and the recovery time is 0.4 s; when the applied pressure is 2.5kPa, the response time of the sensor is 0.6s, and the recovery time is 0.8 s; when the applied pressure was 5kPa, the response time of the sensor was 0.9s and the recovery time was 1.0 s.

The results show that the response time and recovery time of the flexible resistive pressure sensor are longer the greater the applied pressure, because as the pressure continues to increase, the greater the degree of deformation of the conductive sponge when compressed, the longer the time corresponding to the interconnection of the graphene, which is conductive, within the sponge, resulting in longer response time and recovery time.

Example four

In this embodiment, the hysteresis stability of the flexible resistive pressure sensor prepared in the above embodiment is tested, and the test result is shown in fig. 6. The hysteresis in this embodiment refers to the degree of coincidence of the two sensitivity curves after the flexible pressure sensor is subjected to pressure and pressure is removed, that is, whether the flexible pressure sensor can return to the original state after pressure is removed, and the stability of the sensor is shown. In the embodiment, a push-pull meter and an LCR digital bridge are adopted to test the rGO/PDMS flexible pressure sensor, and the change of the resistance value after pressure is applied is compared with the change of the resistance value after pressure is removed, so that the hysteresis of the sensor is verified.

As can be seen from fig. 6, the rGO/PDMS sensor has very small hysteresis, and the two sensitivity curves substantially coincide, which indicates that the flexible resistive pressure sensor is not easily damaged and can be well restored to the original state after pressure is removed.

EXAMPLE five

In this embodiment, in order to further verify the cycling stability performance of the rGO/PDMS flexible resistive pressure sensor under different compressive deformation amounts, the conductive sponge with the highest sensitivity in the above embodiments is selected for the stability test. In the embodiment, the electrochemical workstation and the universal material testing machine are adopted to test the circulation stability performance of the flexible pressure sensor. Specifically, a conductive sponge sample is placed on a universal material testing machine, the sample is connected and assembled into a flexible pressure sensor through a conductive adhesive tape and a lead and is connected with an LCR digital bridge, and cycle data of 200 times of compression of the rGO/PDMS flexible pressure sensor is tested.

A cyclic compression stability experiment in which the deformation amount was 30% and the compression number was 200 was performed using a conductive sponge having a graphene doping concentration of 4 wt%, and resistance change rates in which the compression deformation amounts were 10%, 20%, and 30% were compared, and the test results are shown in fig. 7.

From fig. 7, it can be seen that the valleys and peaks of the cyclic curve almost form two parallel straight lines during 200 times of repeated compression, which shows that the flexible sensor has better repeatability, and the resistance change rate gradually increases with the increase of the deformation amount.

EXAMPLE six

In this embodiment, the conductive sponge with the highest sensitivity in the above embodiments is selected to perform a cyclic compression type stress-strain experiment, a universal material testing machine is used, stress-strain curves after 300 times and 500 times of compression are selected to be compared, and a test result is shown in fig. 8.

As can be seen from the figure, the stress-strain curves after 300 times and 500 times of compression are basically overlapped, which shows that the flexible conductive sponge can be restored to the original size after multiple times of compression and has better resilience performance.

EXAMPLE seven

The first embodiment and the sixth embodiment show that when the doping concentration of the graphene is 4 wt%, the flexible resistance type pressure sensor has the maximum sensitivity, has fast responsiveness, good hysteresis, good repetition stability and good resilience.

In order to show the application potential of the prepared flexible conductive sponge, a wearable pressure sensor is prepared by the preparation method of the two-phase flexible PDMS composite material, and the wearable pressure sensor is assembled by connecting the two-phase flexible PDMS composite material and a lead to form a flexible resistance type pressure sensor, and is combined with wearable equipment to test physiological signals of a human body.

Specifically, the degree of bending of the joints of the human fingers is detected by preparing a flexible resistance type pressure sensor by connecting a flexible conductive sponge with a lead. The flexible resistance type pressure sensor is attached to the joint position of a human finger, then a test lead of the sensor is connected to an electrochemical workstation, then an i-t test program is selected from the programs of the electrochemical workstation, the bending degree of the finger joint is continuously changed, data are collected and recorded, and the test result is shown in fig. 9.

Fig. 9 shows a change curve of the current value of the flexible resistive pressure sensor when the finger is bent to different degrees, fig. 9(a) is a change curve of the current when the finger is bent and stays for a period of time and returns to a straight state, and fig. 9(b) is a change curve of the current when the finger is bent and returns to the straight state immediately. It can be seen from the curve analysis that the value of the current flowing through the sensor increases with the increase of the bending angle of the finger, and the value of the current flowing through the sensor returns to the initial state again after the finger is straightened again. The results show that the flexible resistive pressure sensor can detect the degree of bending of the finger and has low hysteresis.

In this embodiment, a wearable human body pulse detection device is prepared, and the wearable human body pulse detection device combines a textile and the flexible resistance type pressure sensor prepared as described above.

The prepared conductive sponge is connected with the conducting wire and fixed on the wrist guard, the wrist guard is worn on the wrist, and the position of the wrist guard is adjusted to determine the pulse position of the conductive sponge on the human body, so that the pulse condition of the wrist can be monitored in real time, as shown in fig. 10. It can be seen from fig. 11 that the sensing device can detect pulse signals of the wrist, each peak in the graph represents a pulse beat, and the frequency of the pulse beat is 65 times/min and the pulse beat rate of a normal person is 60-100 times/min by calculating the detected peaks. The result shows that the flexible resistance type pressure sensor has wide application potential in the aspect of real-time monitoring of human body pulse.

In light of the foregoing description of the preferred embodiment of the present invention, it is to be understood that various changes and modifications may be made by one skilled in the art without departing from the spirit and scope of the invention. The technical scope of the present invention is not limited to the content of the specification, and must be determined according to the scope of the claims.

Claims

1. A preparation method of a two-phase flexible PDMS composite material is characterized by comprising the following steps:

s1, adding graphene into the dispersoid, and fully mixing;

s2, adding a curing agent into the PDMS sponge and mixing;

s3, mixing the mixture of S1 and S2, and shaping and curing;

2. The method of claim 1, wherein the method comprises the following steps: the doping concentration of the graphene is 2-8 wt%.

3. The method of claim 1, wherein the method comprises the following steps: the dispersoid is sugar.

4. The method of claim 1, wherein the method comprises the following steps: the dispersoids need to be ground and screened before being mixed with the graphene.

5. The method of claim 1, wherein the method comprises the following steps: in S4, the dispersoid in the sample is dissolved by a water bath method.

6. The method of claim 1, wherein the method comprises the following steps: the mass ratio of the PDMS sponge to the curing agent is 8-12: 1.

7. The method of claim 1, wherein the method comprises the following steps: the doping concentration of the graphene is 3-5 wt%.

8. A two-phase flexible PDMS composite prepared using a method of preparing a two-phase flexible PDMS composite of any one of claims 1-7, comprising:

in the two-phase flexible PDMS composite material, graphene is arranged on the inner hole wall of the PDMS sponge and has folds and rough edges, and the whole hole of the PDMS sponge is not filled with the graphene; the graphene surface on the PDMS sponge has a directional arrangement, is smooth and shows uniformly distributed graphene sheets.

9. A wearable pressure sensor, which is prepared by the method of preparing a two-phase flexible PDMS composite material according to any one of claims 1-7, wherein: the wearable pressure sensor is formed by connecting and assembling two-phase flexible PDMS composite materials and a lead to form a flexible resistance type pressure sensor, and is combined with wearable equipment to test physiological signals of a human body.

10. A wearable pressure sensor according to claim 9, characterized in that: under the condition that the doping concentration of graphene powder is 2-8 wt% and a certain pressure, the sensitivity of the wearable pressure sensor is increased firstly and then decreased.