CN110746757B

CN110746757B - High-thermal-conductivity biodegradable polymer composite material and preparation method thereof

Info

Publication number: CN110746757B
Application number: CN201911052733.9A
Authority: CN
Inventors: 解孝林; 瞿昊; 叶昀昇; 尹亮亮; 周兴平
Original assignee: Huazhong University of Science and Technology
Current assignee: Huazhong University of Science and Technology
Priority date: 2019-10-31
Filing date: 2019-10-31
Publication date: 2021-03-16
Anticipated expiration: 2039-10-31
Also published as: CN110746757A

Abstract

The invention discloses a high-thermal-conductivity biodegradable polymer composite material, which comprises a thermal-conductivity nano filler and a biodegradable polymer with shape memory characteristics, wherein the thermal-conductivity nano filler is orderly arranged in the biodegradable polymer; the invention mixes the heat conduction nanometer filler which is not chemically modified with the biodegradable polymer, utilizes the characteristic of polymer shape memory to promote the ordered arrangement of the heat conduction nanometer filler in a mode of drawing induced self-assembly, simultaneously, the ordered arrangement of the heat conduction nanometer filler is taken as a physical cross-linking site to be beneficial to maintaining the orientation state of the material, after the obtained material is heated for the second time, the filler still keeps high orientation, and the constructed orientation structure is beneficial to lapping out an ordered filler network under a small amount of the heat conduction nanometer filler, thereby reducing the production cost, reducing the material density, enhancing the strength and the ductility of the material and improving the heat conductivity of the material.

Description

High-thermal-conductivity biodegradable polymer composite material and preparation method thereof

Technical Field

The invention belongs to the technical field of high-thermal-conductivity composite materials, and particularly relates to a high-thermal-conductivity biodegradable polymer composite material and a preparation method thereof.

Background

With the rapid development of electronic devices toward miniaturization and integration, efficient heat dissipation has become the foundation for ensuring their normal operation. The reliability of the operation of the electronic device is drastically deteriorated as the internal temperature thereof is increased. A major factor affecting the heat dissipation efficiency of electronic devices is the resistance to heat transfer between the heat source and the heat sink. The air has extremely low thermal conductivity (0.023 Wm) because of the gap between the two solids due to the non-close fit^-1k^-1) Resulting in preferential conduction of heat between two solids in localized contact. The heat accumulated locally in large quantity affects the rapid operation of equipment on one hand, and on the other hand, the equipment is easy to age, so that serious potential safety hazards appear.

The existing improvement is to fill a high thermal conductivity polymer between the heat source and the heat sink as a thermal interface material to improve heat transfer. Because the thermal conductivity of the polymer is low, a large amount of high-thermal-conductivity filler needs to be added, the preparation cost of the material is increased, and the mechanical property of the material is obviously reduced by the agglomeration formed by the non-uniform dispersion of the filler, so that the high-performance requirement of the thermal interface material cannot be met. In addition, the use of traditional petroleum-based polymers presents serious white pollution and is a great hazard to ecosystem. Moreover, due to the frequent replacement of electronic devices, the recyclability and degradability of the materials inside the devices becomes of particular importance. Therefore, the preparation of the high-thermal-conductivity biodegradable material is significant.

Patent CN106832877A discloses a method for preparing a vertically oriented boron nitride/high polymer insulating and heat conducting material, which comprises the steps of firstly modifying the surface of a boron nitride nanosheet by using dopamine or a silane coupling agent, then coating the modified boron nitride nanosheet between two layers of high polymers, then pressing the three layers of materials into a thin film with a certain thickness by using a hot pressing process, and finally laminating the thin film into a block or winding the thin film into a cylinder. According to the invention, a high-orientation heat conduction network is formed in the polymer by boron nitride, so that the rapid conduction of heat is easy. Although the surface modification can improve the dispersibility of the nano filler in the polymer, the modification process is complicated and takes longer time; meanwhile, the crystal structure of the nano filler is influenced in the modification process, so that the optimum intrinsic thermal conductivity cannot be achieved.

Therefore, the method for constructing the ordered distribution of the filler can form a continuous heat-conducting network under a small filling amount, and is also an effective method for preparing the high-heat-conducting polymer composite material at present. CN108047569A discloses a functional composite material and a preparation method thereof, the method firstly uses silane coupling agent to modify the surface of filler, then mixes the modified filler, processing aid and polymer, prepares a blank by solution casting or extrusion casting, and then carries out unidirectional stretching or bidirectional stretching on the parison under the temperature condition between the glass transition temperature and the melting point of the polymer, so that the filler with asymmetric structure forms an oriented structure in a resin matrix, and the composite material with different oriented structures in different directions is obtained. In the method, the filler forms an oriented state structure in the polymer matrix, but permanent deformation is generated when the orientation and crystallization of polymer molecular chains are promoted in the stretching process, so that material defects generated in the stretching process of the polymer are fixed, such as the accumulation of the filler or the pores of the material, and the mechanical property of the material is reduced.

Disclosure of Invention

One of the objectives of the present invention is to provide a high thermal conductive biodegradable polymer composite material, which can be used to prepare a composite material with high orientation degree, order, excellent mechanical properties and thermal conductivity, and few defects without modifying the filler.

In order to achieve the purpose, the invention adopts the technical scheme that:

a high thermal conductive biodegradable polymer composite comprising a thermally conductive nanofiller and a biodegradable polymer having shape memory properties, the thermally conductive nanofiller being ordered within the biodegradable polymer.

When the composite material is prepared, the heat-conducting nano filler which is not chemically modified is mixed with a biodegradable polymer, the biodegradable polymer is specifically a polymer with a critical temperature and a specific transition temperature, the critical temperature is lower than the transition temperature, when the ambient temperature is raised to the transition temperature, the biodegradable polymer is in a stretchable state, the biodegradable polymer deforms under the action of an external force, then the deformation is fixed when the temperature is reduced to be lower than the transition temperature, when the temperature is secondarily heated to the critical temperature, the partial chains in the biodegradable polymer slowly move, the porosity in the composite material is reduced, the internal material defects generated in the stretching process of the polymer are reduced, and the mechanical property of the composite material is improved. The orientation structure with high orientation degree constructed by the method is beneficial to overlapping an ordered filler network under a small amount of heat-conducting nano fillers; the heat-conducting nano-filler can be orderly arranged in the biodegradable polymer without modifying the heat-conducting nano-filler or adding a processing aid into the polymer, so that the composite material with high orientation degree, orderliness, excellent mechanical property and heat-conducting property and less defects is prepared.

Further, the mass ratio of the biodegradable polymer to the heat-conducting nano filler is 100: 20-50.

Further, the mass ratio of the biodegradable polymer to the heat-conducting nano filler is 100: 30-40.

Further, the heat-conducting nano filler is at least one of a silicon carbide nano wire, a silver nano wire or a boron nitride nano sheet. The silicon carbide nanowires and the silver nanowires are one-dimensional materials, and the boron nitride nanosheets are two-dimensional materials.

Another object of the present invention is to provide a method for preparing a biodegradable polymer composite material with high thermal conductivity, which specifically comprises the following steps:

s1: adding the biodegradable polymer into an organic solvent, and ultrasonically stirring for 2-3h to obtain a dispersion liquid A;

s2: dispersing the heat-conducting nano filler in an organic solvent to obtain a heat-conducting nano filler dispersion liquid;

s3: dropwise adding the heat-conducting nano filler dispersion liquid obtained in the step S2 into the dispersion liquid A, stirring for 6-10 hours to obtain a heat-conducting nano filler-polymer mixed liquid, adding the mixed liquid into deionized water to separate out to obtain a precipitate, and washing, filtering, drying and thermoforming the precipitate by using the deionized water to obtain an initial composite material;

s4: heating and softening the initial composite material obtained in the step S3, then stretching to 1.5-3 times of the initial length of the initial composite material, cooling to room temperature, fixing and temporarily deforming to obtain a sample I with an oriented structure, and carrying out secondary heating on the sample I for 20-30min to obtain a sample II with an oriented structure, wherein the sample II is the biodegradable polymer composite material with high thermal conductivity.

In the preparation process of the composite material, the heat-conducting nano-filler which is not chemically modified is mixed with a biodegradable polymer, the biodegradable polymer has shape memory characteristic, when the environmental temperature is higher than the transition temperature of the biodegradable polymer, no processing aid is needed to be added, the processing performance and the extensibility of the polymer are greatly increased, under the induction of mechanical force, the heat-conducting nano-filler is highly orderly arranged in the biodegradable polymer, when the temperature is reduced to be lower than the transition temperature, the heat-conducting nano-filler which is orderly arranged is used as a physical cross-linking site for maintaining the orientation state of the material, when a sample is heated to the critical temperature for the second time, the molecular chain in the biodegradable polymer slowly moves again, the process reduces the porosity in the composite material, and reduces the material defects generated in the stretching process of the polymer, the mechanical property of the composite material is improved.

Further, the organic solvent in step S1 and step S2 is at least one of N-N dimethylformamide, tetrahydrofuran and dimethyl carbonate.

Further, the concentration of the biodegradable polymer in the organic solvent in step S1 is 0.025-0.1 g/ml.

Further, the concentration of the heat-conducting nano filler dispersion liquid in the step S2 is 4-10 mg/ml.

Further, the biodegradable polymer is polypropylene carbonate (PPC).

Further, in the step S4, the heating temperature is 60-100 ℃, the heating time is 2-10min, and the secondary heating temperature is 30-40 ℃; the temperature of the secondary heating is 30-40 ℃, namely the critical temperature of the polypropylene carbonate.

Compared with the prior art, the invention has the beneficial effects that:

(1) the heat-conducting nano-fillers are highly orderly arranged in the biodegradable polymer, the orderly arranged heat-conducting nano-fillers are used as physical cross-linking sites to maintain the orientation state of the material (the orientation state can be judged according to the orientation degree), and the orientation process promotes the re-dispersion and orderly arrangement of the heat-conducting nano-fillers in an agglomeration state; and the highly orderly arranged heat-conducting nano-filler can provide a good transmission path for stress and heat, thereby obviously improving the thermal decomposition temperature and the heat-conducting property of the composite material.

(2) According to the invention, by improving the content and the type of the heat-conducting nano-filler, the whole process flow of the preparation method, the conditions of each step and the like, the problem of difficult dispersion caused by stacking and agglomeration of the heat-conducting nano-filler in a polymer can be effectively solved, and the glass transition temperature, the mechanical property and the heat-conducting property of the polypropylene carbonate can be improved. The preparation method is simple and feasible, is not limited by size, and is suitable for industrial large-scale production.

(3) The heat-conducting nano filler of the invention realizes uniform dispersion in biodegradable polymer without chemical modification or addition of surfactant, and can complete the orientation process of the material without addition of processing aid, thus greatly reducing the use of chemical reagents compared with the traditional interface modification and blending method, and having the advantages of energy saving and environmental protection.

(4) The orientation method provided by the invention has good universality on the one-dimensional silicon carbide nanowires, the one-dimensional silver nanowires and the two-dimensional boron nitride nanosheets, and the composite material can have good conductivity and gas barrier property by changing the types and dimensions of the filler, so that the composite material is favorable for storage of the material, has wide application prospect in the fields of degradable electronic packaging devices and degradable plastics for agriculture, and provides a new idea for improving the dispersibility of the inert nanofiller in polymers.

Drawings

FIG. 1 is a scanning electron microscope image of a brittle section of sample II described in example 1 in liquid nitrogen;

FIG. 2 is a polarization microscope photograph of the oriented structure of sample II described in example 1.

Detailed Description

The technical solutions of the present invention will be described clearly and completely with reference to the accompanying drawings, and it is obvious that the described embodiments are only some embodiments of the present invention, not all 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 1

S1: ultrasonically stirring 1g of polypropylene carbonate and 25ml of N-N dimethylformamide for 2 hours until the polypropylene carbonate and the N-N dimethylformamide are completely dissolved to obtain a dispersion liquid A;

s2: dispersing 0.4g of silicon carbide nanowires in 40ml of N-N dimethylformamide to obtain silicon carbide nanowire dispersion liquid with the concentration of 10 mg/ml; (the mass ratio of the polypropylene carbonate to the heat-conducting nano-filler is 100: 40)

S3: adding the silicon carbide nanowire dispersion liquid obtained in the step S2 into the dispersion liquid A at a speed of 2 drops/second, stirring for 6 hours to obtain a silicon carbide nanowire-polymer mixed liquid, adding the mixed liquid into deionized water to separate out to obtain a precipitate, and washing, filtering, drying and thermoforming the precipitate by the deionized water to obtain an initial composite material;

s4: heating the initial composite material in the step S3 at 100 ℃ for 2min for softening, then stretching to 3 times of the initial length of the initial composite material, and cooling to room temperature for fixing and temporarily deforming to obtain a sample I with an oriented structure; and (3) heating the sample I for the second time to 30 ℃, and keeping the temperature for 30min to obtain a silicon carbide nanowire/polypropylene carbonate sample II with an oriented structure.

FIG. 1 is the brittle fracture surface of the sample II in liquid nitrogen observed by a scanning electron microscope, and the orientation arrangement of the silicon carbide nanowires can be seen as shown in FIG. 1.

FIG. 2 is a polarization microscope image of the alignment structure of a brittle cross-section of the sample II in liquid nitrogen, and it can be seen that the material as a whole exhibits a good dispersion state and a highly aligned structure.

The sample I had a porosity of 52% and a tensile strength of 36MPa as measured by ASTM C20-2000 standard test.

Cutting the sample II into a standard sample strip by using a cutter, performing a tensile test to obtain a maximum tensile strength of 46MPa (the maximum tensile strength of pure polypropylene carbonate is 23MPa), obtaining the glass transition temperature of 34 ℃ by using Differential Scanning Calorimetry (DSC) (the glass transition temperature of pure polypropylene carbonate is 25 ℃), and measuring the temperature of 5% of thermal decomposition of the sample II by using a thermogravimetric analyzer (TGA) (the temperature of 5% of thermal decomposition of pure polypropylene carbonate is 218 ℃); testing the orientation degree of the sample II by using an x-ray diffractometer to obtain the orientation degree of 0.98; the sample II was measured to have a thermal conductivity of 1.22Wm by the hot wire method^-1k^-1. Sample II had a porosity of 22%.

The degree of orientation of 1 indicates that the filler is completely arranged in one direction, and the degree of orientation of 0 indicates that the filler is completely arranged in disorder.

Example 2

Example 2 differs from example 1 in that:

s1: 1g of polypropylene carbonate and 25ml of tetrahydrofuran are ultrasonically stirred for 2 hours until the propylene carbonate and the tetrahydrofuran are completely dissolved to obtain a dispersion A.

S2: 0.3g of silver nanowires was dispersed in 42ml of tetrahydrofuran to obtain a silver nanowire dispersion having a concentration of 7 mg/ml. (the mass ratio of the polypropylene carbonate to the heat-conducting nano-filler is 100:30)

S3: adding the silver nanowire dispersion liquid obtained in the step S2 into the dispersion liquid A at a speed of 2 drops/second, stirring for 8 hours to obtain a silver nanowire-polymer mixed liquid, adding the mixed liquid into deionized water to separate out to obtain a precipitate, and washing, filtering, drying and thermoforming the precipitate by the deionized water to obtain a composite material;

s4: heating the composite material in the step S3 at 60 ℃ for 10min to soften, then stretching to 2 times of the initial length of the composite material, and cooling to room temperature to fix the deformation to obtain a sample I with an oriented structure; and heating the sample I for the second time to 40 ℃, and keeping the temperature constant for 20min to obtain a silicon carbide nanowire/polypropylene carbonate sample II with an oriented structure.

The rest of the procedure was the same as in example 1.

The sample I had a porosity of 58% and a tensile strength of 33MPa as measured by ASTM C20-2000 standard test.

Cutting the sample II with the recovered shape into a standard sample strip by using a cutter, performing a tensile test to obtain a maximum tensile strength of 45MPa, obtaining a glass transition temperature of 33.5 ℃ by using Differential Scanning Calorimetry (DSC), obtaining a temperature of 249 ℃ for 5% of thermal decomposition of the sample II (the temperature of 5% of thermal decomposition of pure polypropylene carbonate is 218 ℃) by using a thermogravimetric analyzer (TGA), and performing an orientation degree test on the sample II by using an x-ray diffractometer to obtain an orientation degree of 0.99; the sample II was measured to have a thermal conductivity of 1.05Wm by the hot wire method^-1k^-1. Sample II had a porosity of 24%.

Example 3

Example 3 differs from example 1 in that:

s1: and (3) ultrasonically stirring 3g of polypropylene carbonate and 30ml of dimethyl carbonate for 2 hours until the polypropylene carbonate and the dimethyl carbonate are completely dissolved to obtain a dispersion liquid A.

S2: boron nitride nanosheets were dispersed in 0.15g of dimethyl carbonate in 37.5ml to give boron nitride nanosheets having a concentration of 4 mg/ml. (the mass ratio of the polypropylene carbonate to the boron nitride nanosheet is 100:20)

S3: dropwise adding the boron nitride nanosheet dispersed liquid obtained in the step S2 into the dispersed liquid A, stirring for 10 hours to obtain a boron nitride nanosheet-polymer mixed liquid, adding the mixed liquid into deionized water to separate out to obtain a precipitate, and washing, filtering, drying and thermoforming the precipitate by using the deionized water to obtain a composite material;

s4: heating the composite material obtained in the step S3 at 60 ℃ for 5min to soften, then stretching to 1.5 times of the initial length of the composite material, and cooling to room temperature to fix and deform, so as to obtain a sample I with an oriented structure; and (3) heating the sample I for the second time to 35 ℃, and keeping the temperature for 30min to obtain a silicon carbide nanowire/polypropylene carbonate sample II with an oriented structure.

The sample I had a porosity of 50% and a tensile strength of 27MPa as measured by ASTM C20-2000.

Cutting the sample II with the recovered shape into a standard sample strip by using a cutter, performing a tensile test to obtain a maximum tensile strength of 32MPa, obtaining a glass transition temperature of 28 ℃ by using Differential Scanning Calorimetry (DSC), obtaining a temperature of 223 ℃ for 5% of thermal decomposition of the sample II (the temperature of 5% of thermal decomposition of pure polypropylene carbonate is 218 ℃) by using a thermogravimetric analyzer (TGA), and testing an orientation degree of the sample II by using an x-ray diffractometer to obtain an orientation degree of 0.78; the sample II was measured to have a thermal conductivity of 0.82Wm by the hot wire method^-1k^-1. The porosity of sample II was 26%.

Example 4

Example 4 differs from example 1 in that:

s1: 1g of polypropylene carbonate was ultrasonically stirred with 25ml of tetrahydrofuran for 2h until completely dissolved.

S2: 0.5g of silicon carbide nanowires was dispersed in tetrahydrofuran to obtain 100ml of a silicon carbide nanowire dispersion having a concentration of 5 mg/ml. (the mass ratio of the polypropylene carbonate to the heat-conducting nano-filler is 100:50)

The rest of the procedure was the same as in example 1.

The sample I had a porosity of 53% and a tensile strength of 38MPa as measured by ASTM C20-2000.

Cutting the sample II with the recovered shape into a standard sample strip by using a cutter, performing a tensile test to obtain the maximum tensile strength of 42MPa, obtaining the glass transition temperature of 34.2 ℃ by using Differential Scanning Calorimetry (DSC), obtaining the temperature of 253% of the sample II after thermal decomposition by using a thermogravimetric analyzer (TGA), and performing an orientation degree test on the sample II by using an x-ray diffractometer to obtain the orientation degree of 0.98; the sample II was measured to have a thermal conductivity of 1.18Wm by the hot wire method^-1k^-1. The porosity of sample II was 23%.

Comparative example 1

Comparative example 1 differs from example 1 in that:

s1: 3g of polypropylene carbonate and 30ml of N-N dimethylformamide are stirred ultrasonically for 2h until the polypropylene carbonate is completely dissolved.

S2: 0.15g of silicon carbide nanowires was dispersed in 21ml of N-N dimethylformamide to obtain a silicon carbide nanowire dispersion having a concentration of 7 mg/ml. (the mass ratio of the polypropylene carbonate to the heat-conducting nano-filler is 100:0.5)

The rest of the procedure was the same as in example 1.

The sample I had a porosity of 53% and a tensile strength of 22MPa as measured by ASTM C20-2000.

Cutting the sample II with the recovered shape into a standard sample strip by using a cutter to perform a tensile test to achieve that the maximum tensile strength is 29MPa, obtaining that the glass transition temperature of the sample II is 26 ℃ by using a Differential Scanning Calorimetry (DSC), obtaining that the temperature of 5% of thermal decomposition of the sample II is 221 ℃ (the temperature of 5% of thermal decomposition of pure polypropylene carbonate is 218 ℃) by using a thermogravimetric analyzer (TGA), and testing the orientation degree of the sample II by using an x-ray diffractometer to obtain that the orientation degree is 0.98 (the orientation degree is 1 to indicate that the filler is completely arranged along one direction, and the orientation degree is 0 to indicate that the filler is completely arranged in disorder), and obtaining that the thermal conductivity of the material tested by using a hot wire method is 0.22Wm^-1k^-1. The porosity of sample II was 26%.

When the content of the heat-conducting nano filler is too small, a continuous heat-conducting structure cannot be formed, so that the sample II has poor heat-conducting performance.

Comparative example 2

Comparative example 2 differs from example 1 in that:

S2: 1.8g of silicon carbide nanowires were dispersed in 21ml of N-N dimethylformamide to obtain a silicon carbide nanowire dispersion having a concentration of 7 mg/ml. (the mass ratio of the polypropylene carbonate to the heat-conducting nano-filler is 100:60)

The rest of the procedure was the same as in example 1.

The sample I had a porosity of 55% and a tensile strength of 34MPa as measured by ASTM C20-2000.

Cutting the polypropylene carbonate composite material film with the recovered shape into a standard sample strip by using a cutter to perform a tensile test to achieve that the maximum tensile strength is 45MPa, obtaining a material with a Differential Scanning Calorimetry (DSC) glass transition temperature of 35 ℃, obtaining a temperature of 255 ℃ for thermal decomposition of the material to 5% by using a thermogravimetric analyzer (TGA) (the temperature of 5% for thermal decomposition of pure polypropylene carbonate is 218 ℃), obtaining a degree of orientation of 0.94 by using fiber orientation analysis (the degree of orientation is 1, the filler is completely arranged along one direction, the degree of orientation is 0, the filler is completely disordered and the thermal conductivity of the material is 1.19Wm by using a hot wire method^-1k^-1. Sample II had a porosity of 24%.

When the content of the heat-conducting nano filler is excessive, the heat-conducting nano filler forms a complete heat-conducting orientation structure in the polypropylene carbonate, so that the heat conductivity cannot be increased continuously by adding the heat-conducting nano filler, the production cost is increased, and the material density is also improved.

Comparative example 3

2g of polypropylene carbonate and 30ml of N-N dimethylformamide are stirred ultrasonically for 2h until the polypropylene carbonate is completely dissolved. And dripping the N-N dimethylformamide solution of the polypropylene carbonate into deionized water for precipitation. Washing the obtained precipitate with deionized water, filtering, drying, and thermoforming. The above materials were heated to 90 ℃ for 5 minutes. Then stretched to 2 times its original length and cooled to room temperature to fix the deformation, resulting in sample I with an oriented structure. And (3) heating the sample I for the second time to 30 ℃, and keeping the temperature for 25min to obtain a silicon carbide nanowire/polypropylene carbonate sample II with an oriented structure.

The sample I had a porosity of 50% and a tensile strength of 19MPa as measured by ASTM C20-2000.

The sample II was cut into a standard sample with a cutter and subjected to tensile test to a maximum tensile strength of 25MPa, a glass transition temperature of 25.2 ℃ was obtained by Differential Scanning Calorimetry (DSC), a temperature at which 5% thermal decomposition of the finished product was achieved was 218.1 ℃ by thermogravimetric analysis (TGA), and a thermal conductivity of 0.22W m by hot wire method^-1k^-1。

Since the polypropylene carbonate is not filled with the heat conductive nano filler, the tensile strength and the heat conductivity thereof are not changed even if the sample is heated, stretched and secondarily heated for recovery treatment.

Comparative example 4

Comparative example 4 differs from example 1 in that:

s4: heating the composite material obtained in the step S3 at 80 ℃ for 3min for softening, and then cooling to room temperature to obtain a sample III;

cutting the above sample III into standard sample strips by a cutter, performing tensile test to obtain a maximum tensile strength of 41MPa, performing Differential Scanning Calorimetry (DSC) to obtain a glass transition temperature of 33.8 ℃, performing thermogravimetric analysis (TGA) to obtain a temperature of 247% of thermal decomposition of the initial sample of 5%, performing orientation degree test on the initial sample by an x-ray diffractometer to obtain an orientation degree of 0.02, and performing hot-wire method to obtain a thermal conductivity of 0.76W m^-1k^-1. The rest of the procedure was the same as in example 1.

The results show that, although the thermal conductivity is improved after the silicon carbide nanowires are doped in the polypropylene carbonate, the silicon carbide nanowires are only simply doped with the polypropylene carbonate because the initial sample is not stretched, the silicon carbide nanowires are not oriented in the polypropylene carbonate, and the silicon carbide nanowires are in an agglomerated state in the polypropylene carbonate, so that the thermal conductivity of the initial sample is low.

Comparative example 6

Comparative example 5 differs from example 1 in that:

s4: heating the composite material obtained in the step S3 at 70 ℃ for 5min to soften the composite material, then stretching the composite material to 1.3 times of the initial length of the composite material, and cooling the composite material to room temperature to fix the composite material for deformation to obtain a sample I with an oriented structure; and heating the sample I for the second time to 38 ℃, and keeping the temperature for 28min to obtain a silicon carbide nanowire/polypropylene carbonate sample II with an oriented structure.

The rest of the procedure was the same as in example 1.

The sample I had a porosity of 42% and a tensile strength of 36MPa as measured by ASTM C20-2000.

Cutting the sample II into a standard sample strip by a cutter to perform tensile test, wherein the maximum tensile strength is 41MPa, the glass transition temperature of the sample II is 33.8 ℃ by DSC, the temperature of 5% thermal decomposition of the sample II is 247 ℃ by TGA, the sample II is subjected to orientation degree test by an X-ray diffractometer to obtain the orientation degree of 0.45, and the thermal conductivity of the sample II is 0.91Wm by a hot wire method^-1k^-1. The porosity of sample II was 26%.

The results show that when the composite material is stretched to 1.3 times the original length, the degree of orientation and the thermal conductivity of the sample II are not high because the silicon carbide nanowires are partially oriented and a part of the silicon carbide nanowires are still in an agglomerated state.

Comparative example 6

Comparative example 6 differs from example 1 in that:

s4: heating the composite material obtained in the step S3 at 80 ℃ for 3min for softening, then stretching to 3.2 times of the initial length of the composite material, and cooling to room temperature for fixing deformation to obtain a sample I with an oriented structure; and (3) heating the sample I for the second time to 30 ℃, keeping the temperature for 25min, and recovering the initial shape of the sample I to obtain a silicon carbide nanowire/polypropylene carbonate sample II. The rest of the procedure was the same as in example 1.

The sample I had a porosity of 56% and a tensile strength of 40MPa as measured by ASTM C20-2000.

Cutting the sample II into a standard sample strip by using a cutter to perform tensile test, wherein the maximum tensile strength is 46MPa, the glass transition temperature of the material is 34.1 ℃ by using Differential Scanning Calorimetry (DSC), the temperature of 5% thermal decomposition of the material is 252 ℃ by using a thermogravimetric analyzer (TGA), and the orientation degree of the sample II is tested by using an x-ray diffractometer to obtain the orientation degree of 0.98, and the thermal conductivity of the material is 1.22W m by using a hot wire method^-1k^-1. Sample II had a porosity of 25%.

The result shows that after the polypropylene carbonate is doped with the silicon carbide nanowires with the same content, the orientation degree of the composite material reaches 0.98 when the polypropylene carbonate is stretched to 3 times of the initial length, the stretching proportion is further increased, the orientation degree and the dispersion degree of the composite material are not obviously changed, and the performance of the composite material is kept unchanged.

Comparative example 7

Comparative example 7 differs from example 1 in that:

s2: dispersing 0.3g of boron nitride nanosheets in 30ml of 5mg/ml polydopamine water solution, ultrasonically dispersing for 5h, then washing with deionized water, and drying to obtain polydopamine-modified boron nitride nanosheets, and dispersing 0.3g of the polydopamine-modified boron nitride nanosheets in 30ml of N-N dimethylformamide to obtain a dispersion liquid with the concentration of 10 mg/ml. The rest of the procedure was the same as in example 1.

Cutting the material film into standard sample strips by a cutter, performing tensile test to obtain a maximum tensile strength of 48MPa, performing DSC to obtain a glass transition temperature of 34.7 ℃, performing TGA to obtain a temperature of 250 ℃ at which the thermal decomposition of the material reaches 5%, and performing an orientation degree test on the sample II by using an X-ray diffractometer to obtain an orientation degree of 0.98, and performing a heat ray method to test the thermal conductivity of the material to be 0.89W m^-1k^-1。

The result shows that after the modified boron nitride nanosheets are doped in the polypropylene carbonate, the interface action between the boron nitride nanosheets and the polypropylene carbonate is increased due to the fact that the boron nitride nanosheets are modified by polydopamine, so that the mechanical property and the glass transition temperature of the composite material are slightly improved, but the heat conducting property of the composite material is reduced due to the fact that a polymer layer modified on the surfaces of the boron nitride nanosheets is a poor heat conductor.

Comparative example 8

Comparative example 8 differs from example 1 in that:

s4: heating the composite material obtained in the step S3 at 60 ℃ for 10min to soften, then stretching to 2 times of the initial length of the composite material, and cooling to room temperature to fix the deformation to obtain a sample I with an oriented structure; and heating the sample I for the second time to 60 ℃, and keeping the temperature constant for 20min to obtain a silicon carbide nanowire/polypropylene carbonate sample II with an oriented structure.

The sample I had a porosity of 49% and a tensile strength of 37MPa as measured by ASTM C20-2000.

Subjecting the sample toCutting the product II into standard sample strips by using a cutter, performing tensile test to obtain the maximum tensile strength of 40MPa, obtaining the glass transition temperature of the sample II by using Differential Scanning Calorimetry (DSC) of 33 ℃, and obtaining the temperature of 251 ℃ when the thermal decomposition of the sample II reaches 5% by using a thermogravimetric analyzer (TGA) (the thermal decomposition 5% temperature of pure polypropylene carbonate is 216 ℃); testing the orientation degree of the sample II by using an x-ray diffractometer to obtain the orientation degree of 0.94; the sample II was measured to have a thermal conductivity of 1.09Wm by the hot wire method^-1k^-1. Sample II had a porosity of 22%.

The result shows that when the secondary heating temperature of the ordered material is higher, the surface of the sample II generates wrinkles due to shrinkage, so that the orientation degree, the heat conduction performance and the mechanical property of the material are reduced.

In summary, it is found by comparing examples 1 to 4 with comparative examples 1 to 2 that when the content of the thermally conductive nano filler is too small, a continuous thermally conductive structure cannot be formed, so that the sample II has poor thermally conductive properties; when the content of the heat-conducting nano filler is too high, the heat conductivity is not increased, and the waste of the filler and the cost are increased due to the fact that the heat-conducting nano filler is continuously increased; when the mass ratio of the polypropylene carbonate to the heat-conducting nano filler is 100: 20-50, the composite material with excellent oriented structure, high tensile strength, high thermal decomposition temperature and good heat-conducting property can be prepared. It was found through a comparative study of example 1 and comparative example 3 that the tensile strength and thermal conductivity were not changed when the thermally conductive nano filler was not filled even though the sample was heated, stretched, oriented and heat-treated. Through a comparative study of example 1 and comparative example 4, it was found that, even though the initial sample was doped with the thermally conductive nano-filler, the thermally conductive nano-filler was not oriented within the polypropylene carbonate when the initial sample was not stretched, and the thermally conductive nano-filler was in an agglomerated state in the polypropylene carbonate, resulting in a low thermal conductivity of the initial sample. Through the comparison study of example 1 and comparative examples 5 and 6, when the stretching length is lower, because the silicon carbide nanowires are partially oriented, and a part of the silicon carbide nanowires are still in an agglomerated state, the orientation degree and the thermal conductivity of the sample II are not high; when the draw length is higher, the composite orientation is not significantly changed and the dispersion degree is not significantly changed, and the properties of the composite are maintained. Through comparison and research of the embodiment 1 and the comparative example 7, the composite material prepared by the poly-dopamine-modified boron nitride nanosheet and the polypropylene carbonate is found that the polymer layer modified on the surface of the boron nitride nanosheet is a poor conductor of heat, so that the heat conducting performance of the composite material is reduced.

Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims

1. A biodegradable polymer composite material with high thermal conductivity is characterized by comprising thermal-conductive nano-fillers and a biodegradable polymer with shape memory characteristics, wherein the thermal-conductive nano-fillers are orderly arranged in the biodegradable polymer; the biodegradable polymer is polypropylene carbonate; the mass ratio of the biodegradable polymer to the heat-conducting nano filler is 100: 20-50;

the preparation method of the high-thermal-conductivity biodegradable polymer composite material comprises the following steps:

s4: heating and softening the initial composite material in the step S3, wherein the heating temperature is 60-100 ℃, and the heating time is 2-10 min; and then stretching to 1.5-3 times of the initial length of the sample, cooling to room temperature, fixing and temporarily deforming to obtain a sample I with an oriented structure, heating the sample I for 20-30min for the second time, wherein the second heating temperature is 30-40 ℃, and obtaining a sample II with an oriented structure, wherein the sample II is the high-thermal-conductivity biodegradable polymer composite material.

2. The biodegradable polymer composite material with high thermal conductivity as claimed in claim 1, wherein the mass ratio of the biodegradable polymer to the thermal conductive nano filler is 100: 30-40.

3. The biodegradable polymer composite material with high thermal conductivity according to claim 1, wherein the thermally conductive nano-filler is at least one of silicon carbide nano-wires, silver nano-wires or boron nitride nano-sheets.

4. The biodegradable polymer composite material with high thermal conductivity according to claim 1, wherein the organic solvent in step S1 and step S2 is at least one of N-N dimethylformamide, tetrahydrofuran and dimethyl carbonate.

5. The biodegradable polymer composite material with high thermal conductivity according to claim 1, wherein the concentration of the biodegradable polymer in the organic solvent in step S1 is 0.025-0.1 g/ml.

6. The biodegradable polymer composite material with high thermal conductivity of claim 1, wherein the concentration of the thermally conductive nanofiller dispersion in step S2 is 4-10 mg/ml.