CN110845870A - Surface covalent grafting modified hexagonal boron nitride nanosheet and preparation method thereof - Google Patents

Abstract

The invention discloses a surface covalent grafting modified hexagonal boron nitride nanosheet and a preparation method thereof, and the preparation method comprises the following steps: firstly, under the ultrasonic action assisted by alkali metal hydroxide, the preparation and surface hydroxylation of boron nitride nanosheets are realized in one step; and then, covalent grafting modification is carried out on the boron nitride nanosheets by using high-activity diisocyanate molecules, wherein one isocyanate group can perform hydrogen transfer reaction with hydroxyl groups on the surfaces of the boron nitride nanosheets, and the remaining other isocyanate group endows the boron nitride nanosheets with high dispersibility and chemical activity. The invention can not only improve the agglomeration problem of the boron nitride nanosheet in the organic solvent and the resin matrix, but also form a covalent interface between the boron nitride nanosheet and the resin matrix, improve the compatibility and be beneficial to large-scale production and practical application of the boron nitride nanosheet.

Description

Surface covalent grafting modified hexagonal boron nitride nanosheet and preparation method thereof

technical field

The invention belongs to the technical field of nanomaterials, in particular to a surface covalent graft modified hexagonal boron nitride nanosheet and a preparation method thereof.

Background technique

In recent years, hexagonal boron nitride (h-BN) has attracted much attention of researchers due to its excellent mechanical, thermal, and electrical properties. Similar to graphite, h-BN is formed by stacking multiple layers of 2D nanosheets, which can be further exfoliated into boron nitride nanosheets (BNNSs) with the help of external force. BNNSs not only have thermal conductivity and mechanical properties comparable to graphene, but also have excellent properties such as wide band gap, high thermal stability, strong corrosion resistance, small dielectric constant, low density, and low thermal expansion coefficient. These excellent properties make BNNSs stand out among many thermally conductive and insulating fillers, and become the most ideal lightweight nanofillers for thermally conductive polymer matrix composites with high insulation and low dielectric loss.

At present, the methods for preparing BNNSs mainly include "top-down" exfoliation methods such as ultrasonic exfoliation, mechanical exfoliation, and chemical exfoliation. In addition, "bottom-up" synthesis methods such as solid-phase reaction method and chemical vapor deposition method can also be used. obtained by law. A certain amount of BNNSs can be obtained through these two routes, but the surface of the prepared BNNSs lacks polar functional groups and is chemically inert. In addition to the extremely high specific surface area, BNNSs and other media (polymers, solvents, etc.) It is very easy to form a large number of agglomerates, which brings great difficulties to the further research and application of BNNSs. Also, because the surface of BNNSs does not have any functional groups that can be used for chemical bonding or physical interlocking, the interfacial force between BNNSs and the resin matrix is very weak, resulting in the performance of the obtained composite material far from the theoretical value, which limits its application in the field of composite materials application. Therefore, during the preparation of BNNSs, active groups were introduced on the surface or effective surface modification and functionalization of BNNSs were carried out through certain surface modification techniques to improve their dispersibility in the polymer matrix and the interfacial bonding force. It is necessary to obtain high-performance composite materials.

At present, most studies on the surface modification of BNNSs use coupling agents. However, BNNSs modified by coupling agents cannot exist in the polymer matrix for a long time and stably. Over time, BNNSs will gradually form aggregates or precipitates. The reason is that it is not easy to form a chemical bond between the coupling agent and the polymer matrix.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the deficiencies of the prior art and provide a simple and effective method for preparing BNNSs with highly active groups on the surface.

The technical scheme that realizes the present invention is as follows:

A method for preparing hexagonal boron nitride nanosheets modified by surface covalent grafting. First, the peeling of BNNSs and the functionalization of surface hydroxyl groups are achieved by using alkali metal hydroxide to assist ultrasonic treatment. The molecules were modified by covalent grafting, and a large number of highly reactive isocyanate groups were introduced on the surface of BNNSs, which improved the dispersibility of BNNSs in organic solvents and resin matrix, and promoted the practical application of BNNSs. Specifically include the following steps:

(1) Disperse h-BN powder in deionized water and stir continuously, add a certain amount of alkali metal hydroxide, and continue stirring to obtain a mixed solution;

(2) placing the mixed solution prepared in step (1) in an ultrasonic disintegrator, and ultrasonically peeling it off for several hours to obtain a dispersion;

(3) Centrifuging the dispersion obtained in step (2), filtering the obtained supernatant, washing with deionized water, and drying the obtained product in a vacuum oven at 60-100 ^o C for 6-12 hours to obtain hydroxyl functionalization BNNSs (BNNSs-OH);

(4) Under the protection of inert gas and the action of a catalyst, the BNNSs-OH obtained in step (3) and the bifunctional isocyanate molecule are stirred and reacted at a temperature of 60-80 ^o C in a polar aprotic solvent for 6- 10h;

(5) Repeat the ultrasonic dispersion-centrifugation process for the mixture prepared in step (4) several times at room temperature to ensure that unreacted diisocyanate molecules and other impurities are completely removed, and the lower layer precipitate is dried to obtain surface covalent Graft-modified hexagonal boron nitride nanosheets are isocyanate covalently functionalized BNNSs.

Preferably, in step (1), the dosage ratio of h-BN powder and deionized water is 1g:100ml～1g:600ml.

Preferably, in step (1), the alkali metal hydroxide is sodium hydroxide or potassium hydroxide.

Preferably, in step (1), the mass ratio of h-BN powder to alkali metal hydroxide is 1:60-1:100.

Preferably, in step (2), ultrasonic peeling is performed at 60-80 kHz and 200-300 W power for 10-20 h.

Preferably, in step (3), centrifugal separation is carried out at a speed of 2000-4000 rpm for 20-50 min.

Preferably, in step (4), the catalyst is dibutyltin dilaurate organotin.

Preferably, in step (4), the bifunctional diisocyanate is any one of common diisocyanate molecules such as diphenylmethane diisocyanate, toluene diisocyanate, isophorone diisocyanate and hexamethylene diisocyanate. kind.

Preferably, in step (4), BNNSs-OH and polar aprotic solvent are mixed in a ratio of 1g:200ml~1g:600ml.

Preferably, in step (4), the dosage ratio of BNNSs-OH to difunctional diisocyanate is 1:5 to 1:20.

Preferably, in step (4), the polar aprotic solvent is N -methylpyridinone, dimethyl sulfoxide, N,N -dimethylformamide, and N,N -dimethylacetamide. any of the .

Preferably, in step (5), the ultrasonic dispersion-centrifugation separation process is repeated 3 to 5 times at room temperature.

Compared with the prior art, the advantages of the present invention are:

(1) The present invention can simultaneously realize the preparation of BNNSs and the functionalization of hydroxyl groups, and has the advantages of simple operation, low equipment requirements, mild conditions, low cost and high production efficiency, and the highest yield of BNNSs-OH can reach 33.8%.

(2) The dispersibility of the BNNSs modified by the covalent grafting of the diisocyanate molecules provided by the present invention in various polar solvents is effectively improved, and the subsequent processability is greatly improved.

(3) The BNNSs modified by covalent grafting of diisocyanate molecules provided by the present invention have a large number of highly active isocyanate groups on the surface, which solves the problem that the surface of BNNSs is chemically inert, and can be combined with common polar resins, such as polyurethane, Epoxy resin, silicone oil, silicone rubber, etc. form hydrogen bonds or covalent bonds to improve the compatibility and interfacial interaction of the two, thereby obtaining high-performance composite materials.

Description of drawings

1 is a transmission electron microscope image of the BNNSs-OH prepared in Example 1.

FIG. 2 is the infrared spectra of pristine h-BN and BNNSs-OH prepared in Example 1.

Figure 3 shows the high-resolution XPS results of B(a) and N(b) elements in the BNNSs-OH prepared in Example 2.

4 is the infrared spectrum of BNNSs-OH prepared in Example 1 and BNNSs-TDI prepared in Example 4.

FIG. 5 is the thermogravimetric curves of pristine h-BN and BNNSs-TDI prepared in Example 4.

6 is a transmission electron microscope image of the BNNSs prepared in Comparative Example 1.

Figure 7 shows the high-resolution XPS results of B(a) and N(b) elements in the BNNSs prepared in Comparative Example 1.

8 is a scanning electron microscope image of the product obtained in Comparative Example 2.

FIG. 9 is an infrared spectrogram of the product obtained in Comparative Example 2. FIG.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and embodiments.

The principle of the present invention is: in the liquid phase ultrasonic process, on the one hand, ultrasonic energy is used to destroy the interlayer force of h-BN, thereby peeling off the nanosheet layer from the three-dimensional structure; In the alkaline solution, the boron atoms at the edges and defects of BNNSs in alkaline solution will be oxidized, thus realizing the preparation of BNNSs and the functionalization of hydroxyl groups at the same time. Moreover, the presence of hydroxyl groups can promote the formation of stable dispersions of BNNSs in water, thereby effectively improving the exfoliation efficiency. Furthermore, the hydrogen transfer reaction between hydroxyl groups and isocyanate groups was used to covalently graft bifunctional diisocyanate molecules on the surface of BNNSs-OH, thereby endowing BNNSs with high dispersibility and chemical activity.

The surface covalent graft-modified hexagonal boron nitride nanosheets of the present invention are mainly realized in two steps: firstly, the preparation and surface hydroxylation of boron nitride nanosheets are realized in one step under the assisted ultrasonic action of alkali metal hydroxide. . Subsequently, it was modified by covalent grafting with highly reactive diisocyanate molecules, in which one isocyanate group could undergo hydrogen transfer reaction with the hydroxyl groups on the surface of boron nitride nanosheets, while the remaining isocyanate group endowed the The high dispersibility and chemical activity of boron nitride nanosheets were obtained. It can not only improve the agglomeration problem of boron nitride nanosheets in organic solvents and resin matrix, but also form a covalent interface between boron nitride nanosheets and resin matrix, improve compatibility, and facilitate the growth of boron nitride nanosheets. Scale production and practical application.

Example 1

Disperse 1 g of h-BN powder in 300 ml of deionized water and stir continuously, then add 60 g of sodium hydroxide, and continue to stir to obtain a mixed solution. The mixed solution was placed in an ultrasonic breaker, and ultrasonically stripped for 15h under the conditions of 80kHz and 300W. After ultrasonic treatment, the dispersion liquid was centrifuged at 3000 rpm for 30 min to remove unstripped h-BN. The supernatant was filtered and washed with deionized water until the filtrate pH=7. The filter cake was placed in a vacuum oven at 80 ^° C and dried for 8 h to obtain 0.254 g of BNNSs-OH with a yield of 25.4% (yield=mass of BNNSs-OH/mass of original h-BN).

Figure 1 is the transmission electron microscope image of the prepared BNNSs-OH, it can be seen that it is almost transparent to the electron beam, and obvious wrinkles and ripples appear, confirming its soft and thin characteristics, indicating that the ultra-thin boron nitride nanometer successful preparation. Figure 2 shows the infrared spectra of the pristine h-BN and the as-prepared BNNSs-OH. It can be seen that the BNNSs-OH has a highly broadened stretching vibration peak of the B-OH bond near 3200 cm ^-1 , and at the same time at 1200 cm ^-1 The in-plane bending vibration absorption peak of BO bond appeared nearby (the absorption peak of h-BN near 3400cm ^-1 is generated by the surface adsorbed moisture). The results in Fig. 1 and Fig. 2 demonstrate that the alkali metal-assisted ultrasonic exfoliation can realize the exfoliation and hydroxyl functionalization of boron nitride nanosheets in one step.

Example 2

Disperse 1 g of h-BN powder in 300 ml of deionized water with constant stirring, then add 80 g of sodium hydroxide, and continue to stir to obtain a mixture. The mixture was placed in a sonicator, and ultrasonically stripped for 15h under the conditions of 80kHz and 300W. After ultrasonic treatment, the dispersion liquid was centrifuged at 3000 rpm for 30 min to remove unstripped h-BN. The supernatant was filtered and washed with deionized water until the filtrate pH=7. The filter cake was placed in a vacuum oven at 80 ^° C and dried for 8 h to obtain 0.338 g of BNNSs-OH with a yield of 33.8%.

Figure 3 shows the high-resolution XPS results of B(a) and N(b) elements in the prepared BNNSs-OH. It can be seen that there is a B-O peak, but no N-O peak, which means that the generated hydroxyl groups are related to the B atoms on the surface of BNNSs. bonding, not N atoms.

Example 3

Disperse 1 g of h-BN powder in 300 ml of deionized water with constant stirring, then add 100 g of potassium hydroxide, and continue to stir to obtain a mixture. The mixture was placed in a sonicator, and ultrasonically stripped for 15h under the conditions of 80kHz and 300W. After ultrasonic treatment, the dispersion liquid was centrifuged at 3000 rpm for 30 min to remove unstripped h-BN. The supernatant was filtered and washed with deionized water until the filtrate pH=7. The filter cake was placed in a vacuum oven at 80 ^° C and dried for 8 h to obtain 0.265 g of BNNSs-OH with a yield of 26.5%.

Example 4

Weigh 1 g of BNNSs-OH prepared according to the method of Example 1 of the present invention and disperse it in 300 ml of N,N -dimethylformamide, and place the dispersion in a water bath at 80 ^° C. Under nitrogen protection and a stirring speed of 600 rpm, 10 g of toluene diisocyanate and 4 drops of dibutyltin dilaurate organotin (catalyst) were added, and the reaction was stirred for 8 h. When the system was naturally cooled to room temperature, the mixed solution was ultrasonically dispersed at a power of 120W for 30min, and then centrifuged at a speed of 8000rpm for 15min. The centrifuged precipitate was dispersed in 300ml of N,N -dimethylformamide again, and after ultrasonic dispersion at 120W for 30min, centrifugation was performed again at 8000rpm for 15min. This ultrasonic dispersion-centrifugation process was repeated 4 times. Finally, the product was placed in a vacuum drying oven at 80 ^o C and dried for 10 h to obtain toluene diisocyanate covalently functionalized boron nitride nanosheets (BNNSs-TDI).

Figure 4 is the infrared spectrum of the prepared BNNSs-TDI and the BNNSs-OH prepared in Example 1. It can be seen from the comparison that the characteristic absorption peak of the hydroxyl group in BNNSs-TDI disappears, and at the same time appears near 2274 cm ^-1 Stretching vibration peaks of isocyanate groups. More importantly, the characteristic absorption peak of carbamate (-NHCOO-) group appeared near 1728cm ^-1 , which is a new functional group generated by the hydrogen transfer reaction between hydroxyl group and isocyanate group, It was shown that TDI molecules were successfully covalently grafted onto the BNNSs-OH surface through the hydrogen transfer reaction between hydroxyl groups and isocyanate groups. Figure 5 shows the thermogravimetric curves of the pristine h-BN and the as-prepared BNNSs-TDI. The results show that the graft ratio of toluene diisocyanate molecules on the surface of boron nitride nanosheets is as high as 12.1 wt%.

The prepared BNNSs-TDI was dispersed in N,N dimethylformamide, and ultrasonically dispersed for 15 min. The dispersion results are shown in Table 1. It can be seen that BNNSs-TDI can form a stable dispersion in N,N dimethylformamide. The dispersion concentration can be as high as 7mg/ml, and no obvious precipitation occurs within one month.

Example 5

Weigh 1 g of BNNSs-OH prepared according to the method of Example 2 of the present invention and disperse it in 300 ml of N -picoline, and place the dispersion in a water bath at 80 ^° C. Under nitrogen protection and a stirring speed of 600 rpm, 20 g of isophorone diisocyanate and 4 drops of dibutyltin dilaurate organotin (catalyst) were added, and the reaction was stirred for 8 h. When the system was naturally cooled to room temperature, the mixture was ultrasonically dispersed at a power of 120W for 30min, and then centrifuged at a speed of 8000rpm for 15min. The centrifuged precipitate was dispersed in 300 ml of N -picoline again, and after ultrasonic dispersion at 120W for 30 minutes, centrifugation was performed again at 8000 rpm for 15 minutes. This ultrasonic dispersion-centrifugation process was repeated 4 times. Finally, the product was placed in a vacuum drying oven at 80 ^° C for 10 h to obtain isophorone diisocyanate covalently functionalized boron nitride nanosheets (BNNSs-IPDI).

The prepared BNNSs-IPDI was dispersed in N,N dimethylformamide, and ultrasonically dispersed for 15 min. The dispersion results are shown in Table 1. It can be seen that the dispersion concentration of BNNSs-IPDI in N,N dimethylformamide can reach 5 mg. /ml, and no obvious precipitation occurred within one month.

Example 6

Weigh 1 g of the prepared BNNSs-OH and disperse it in 300 ml of N,N -dimethylacetamide, and place the dispersion in a water bath at 80 ^° C. Under nitrogen protection and a stirring speed of 600 rpm, 15 g of diphenylmethane diisocyanate and 4 drops of dibutyltin dilaurate organotin (catalyst) were added, and the reaction was stirred for 8 h. When the system was naturally cooled to room temperature, the mixture was ultrasonically dispersed at a power of 120W for 30min, and then centrifuged at a speed of 8000rpm for 15min. The centrifuged precipitate was dispersed in 300ml of N,N -dimethylacetamide again, and after ultrasonic dispersion at 120W for 30min, centrifugation was performed again at 8000rpm for 15min. This ultrasonic dispersion-centrifugation process was repeated 4 times. Finally, the product was placed in a vacuum drying oven at 80 ^o C and dried for 10 h to obtain diphenylmethane diisocyanate covalently functionalized boron nitride nanosheets (BNNSs-MDI).

The prepared BNNSs-MDI was dispersed in N,N dimethylformamide, and ultrasonically dispersed for 15 min. The dispersion results are shown in Table 1. It can be seen that the dispersion concentration of BNNSs-MDI in N,N dimethylformamide can reach 6 mg. /ml, and no obvious precipitation occurred within one month.

Example 7

Weigh 1 g of the prepared BNNSs-OH and disperse it in 300 ml of dimethyl sulfoxide, and place the dispersion in a water bath at 80 ^° C. Under nitrogen protection and a stirring speed of 600 rpm, 5 g of hexamethylene diisocyanate and 4 drops of dibutyltin dilaurate organotin (catalyst) were added, and the reaction was stirred for 8 h. When the system was naturally cooled to room temperature, the mixture was ultrasonically dispersed at a power of 120W for 30min, and then centrifuged at a speed of 8000rpm for 15min. The centrifuged precipitate was dispersed in 300 ml of dimethyl sulfoxide again, and after ultrasonic dispersion at 120W for 30 minutes, centrifugation was performed again at 8000 rpm for 15 minutes. This ultrasonic dispersion-centrifugation process was repeated 4 times. Finally, the product was placed in a vacuum drying oven at 80 ^° C for 10 h to obtain hexamethylene diisocyanate covalently functionalized boron nitride nanosheets (BNNSs-HDI).

The prepared BNNSs-HDI was dispersed in N,N dimethylformamide, and ultrasonically dispersed for 15 min. The dispersion results are shown in Table 1. It can be seen that BNNSs-HDI can form a stable dispersion in N,N dimethylformamide. The dispersion concentration can reach 5mg/ml, and there is no obvious precipitation within one month.

Comparative Example 1

The experimental procedure is the same as that of Example 1, but no alkali metal hydroxide is added in the experimental procedure.

FIG. 6 is a transmission electron microscope image of the boron nitride nanosheets prepared in Comparative Example 1, and the results show that ultrathin boron nitride nanosheets were successfully prepared by the comparative example. Figure 7 shows the high-resolution XPS results of boron (a) and nitrogen (b) elements in the boron nitride nanosheets prepared in Example 1. It can be seen that there are no hydroxyl groups on the surface of the boron nitride nanosheets prepared in Comparative Example 1.

The prepared BNNSs were dispersed in N,N dimethylformamide, and ultrasonically dispersed for 15 min. The dispersion results are shown in Table 1. It can be seen that due to the lack of active groups, the uncovalently modified BNNSs are in N,N dimethylformamide. Formamide cannot form a stable dispersion and is prone to agglomeration.

Comparative Example 2

The experimental procedure involved replacing ultrasonic treatment with stirring treatment, similar to Example 1, as follows:

Disperse 1 g of h-BN powder in 300 ml of deionized water and stir continuously, then add 60 g of sodium hydroxide, and continue to stir to obtain a mixed solution. At room temperature, the mixture was stirred at 500 rpm for 15 h. After the stirring treatment, the dispersion liquid was centrifuged at a centrifugal speed of 3000 rpm for 30 min to remove the unstripped h-BN. The supernatant was filtered and washed with deionized water until the filtrate pH=7. The filter cake was placed in a vacuum oven at 80 ^° C and dried for 8 h.

FIG. 8 is a scanning electron microscope image of the product obtained in Comparative Example 2, and it can be seen that the obtained product is still a relatively thick block. FIG. 9 is the infrared spectrum of the product obtained in Comparative Example 2, and it can be seen that the characteristic absorption peak of the hydroxyl group does not appear in the sample. The results in Figures 8 and 9 show that without the effect of ultrasound, h-BN could neither be effectively exfoliated nor oxidized efficiently.

Table 1 Dispersion effect of BNNSs in N,N dimethylformamide before and after covalent graft modification

In summary, the development of a simple and large-scale production of BNNSs with highly active groups on the surface not only plays a key role in promoting the basic research of BNNSs, but also can endow BNNSs with new properties and promote their application in composite materials, biomedicine. and practical applications in the field of optoelectronic devices.

Claims (10)

1. a preparation method of surface covalent grafting modified hexagonal boron nitride nanosheet, is characterized in that, specifically comprises the steps: (1) Disperse h-BN powder in water and stir continuously, add alkali metal hydroxide, and continue stirring to obtain a mixed solution; (2) ultrasonically peeling off the mixed solution prepared in step (1) to obtain a dispersion; (3) centrifuging the dispersion obtained in step (2), filtering and washing the supernatant, and vacuum drying the obtained product to obtain hydroxyl-functionalized BNNSs; (4) Under the protection of an inert gas and the action of a catalyst, the hydroxyl-functionalized BNNSs prepared in step (3) and the bifunctional diisocyanate are stirred and reacted at a temperature of 60-80 ^o C in a polar aprotic solvent. 6～10h; (5) Repeating the ultrasonic dispersion-centrifugation process of the mixture prepared in step (4) several times, and drying the lower layer precipitate to obtain the surface covalently grafted modified hexagonal boron nitride nanosheets. 2. The method according to claim 1, wherein the dosage ratio of h-BN powder to water is 1g:100ml～1g:600ml. 3. The method of claim 1, wherein the alkali metal hydroxide is sodium hydroxide or potassium hydroxide. 4 . The method of claim 1 , wherein the mass ratio of h-BN powder to alkali metal hydroxide is 1:60˜1:100. 5 . 5 . The method of claim 1 , wherein, in step (2), ultrasonic peeling is performed at 60-80 kHz and 200-300 W power for 10-20 h to obtain a dispersion. 6 . 6 . The method of claim 1 , wherein, in step (3), centrifugal separation is performed for 20 to 50 min at a rotation speed of 2000 to 4000 rpm. 7 . 7. The method of claim 1, wherein in step (3), the obtained product is vacuum-dried at 60-100 ^° C for 6-12 h to obtain hydroxyl-functionalized BNNSs. 8 . The method of claim 1 , wherein, in step (4), the catalyst is dibutyltin dilaurate organotin. 9 . 9 . The method of claim 1 , wherein, in step (4), the diisocyanate of the bifunctional group is diphenylmethane diisocyanate, toluene diisocyanate, isophorone diisocyanate and hexamethylene diisocyanate. 10 . Any of the isocyanates. 10 . The method of claim 1 , wherein, in step (4), the hydroxyl-functionalized BNNSs and the polar aprotic solvent are mixed in a ratio of 1g:200ml-1g:600ml. 11 .

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