CN115782222A

CN115782222A - Method for preparing rigid multistage Kong Kuaiti polyimide by supercritical carbon dioxide foaming

Info

Publication number: CN115782222A
Application number: CN202211388291.7A
Authority: CN
Inventors: 王超; 孙慧婷; 王廷梅; 王齐华
Original assignee: Lanzhou Institute of Chemical Physics LICP of CAS
Current assignee: Lanzhou Institute of Chemical Physics LICP of CAS
Priority date: 2022-11-08
Filing date: 2022-11-08
Publication date: 2023-03-14

Abstract

The invention provides a method for preparing rigid block multi-stage pore polyimide by utilizing supercritical carbon dioxide foaming, and relates to the technical field of foaming. The N, N-dimethylformamide reduces the melt strength of polyimide, so that the micromolecular solvent permeates into the polymer, the interaction between chain segments is weakened, the free volume of the polymer is increased, carbon dioxide molecules can be smoothly diffused and adsorbed into the polymer, and meanwhile, the reduced melt strength is favorable for reducing the nucleation energy barrier of carbon dioxide gas, and the cells are easier to obtain; on the other hand, the N, N-dimethylformamide is a polar aprotic solvent, so that the polarity of a polymer system can be increased, and the dissolving capacity and selectivity of carbon dioxide to a polar polymer are obviously improved; porous polyimide is prepared firstly, and compared with solid nonporous polyimide, the introduction of pores is more beneficial to the diffusion of N, N-dimethylformamide and carbon dioxide molecules to a matrix.

Description

Method for preparing rigid multistage Kong Kuaiti polyimide by supercritical carbon dioxide foaming

Technical Field

The invention relates to the technical field of foaming, in particular to a method for preparing rigid multistage Kong Kuaiti polyimide by utilizing supercritical carbon dioxide foaming.

Background

In recent years, the non-toxic and non-flammable inert supercritical carbon dioxide gas gradually replaces the traditional physical foaming agents such as alkane, freon and the like, and has the advantages of rich sources, low price and simple operation. However, carbon dioxide has a low molecular cohesive energy density and a weak solvating power. The polymers which can be dissolved in carbon dioxide are mainly prepared from fluorine-containing polymers, polysiloxane and polycarbonate polymers. The adsorption of carbon dioxide in the polymer determines the plasticizing capacity of the polymer and the conditions for subsequent cell growth. The polyimide has compact chain packing property and weak chain fluidity, so that the adsorption amount of carbon dioxide in the polyimide is extremely low, and the plasticizing capacity of the polyimide is extremely weak; and a conjugated system formed by aromatic heterocyclic structures in polyimide molecules enables molecular chains to have strong rigidity and the energy barrier of free rotation of molecular chain segments to be higher, and the polyimide material has high glass transition temperature and higher melting point or softening point, shows the performance of no softening at the ordinary processing temperature in an experiment and cannot push the cells to grow, so that the research on preparing rigid porous polyimide by utilizing supercritical carbon dioxide foaming is less.

Disclosure of Invention

The invention aims to provide a method for preparing rigid multistage Kong Kuaiti polyimide by supercritical carbon dioxide foaming, which realizes the foaming of the polyimide by the supercritical carbon dioxide.

In order to achieve the above object, the present invention provides the following technical solutions:

the invention provides a method for preparing rigid multistage Kong Kuaiti polyimide by supercritical carbon dioxide foaming, which comprises the following steps:

cold-pressing and molding the polyimide powder, and then performing pressureless sintering to obtain porous polyimide;

soaking the porous polyimide into N, N-dimethylformamide to swell to obtain swelled polyimide;

and (3) placing the swelled polyimide in a high-pressure kettle to adsorb supercritical carbon dioxide until the saturated carbon dioxide is saturated, and then foaming to obtain the rigid hierarchical-pore block polyimide.

Preferably, the temperature of the impregnation is preferably 30 to 100 ℃.

Preferably, when the polyimide powder is YS20 or YHPI-P-100, the swelling degree of swelling is more than 30%

Preferably, the foaming mode is rapid temperature rise foaming; the temperature for adsorbing the supercritical carbon dioxide is 25-55 ℃, and the pressure is 8-30 MPa; the foaming temperature is 140-260 ℃ and the time is 1min.

Preferably, the foaming is carried out in an oil bath.

Preferably, the foaming mode is rapid pressure relief foaming; the temperature for adsorbing the supercritical carbon dioxide is 130-180 ℃, and the pressure is 8-30 MPa; the pressure relief time is 2-20 s.

Preferably, after the foaming, the method further comprises transferring the product obtained by foaming into an ice-water bath to shape the cells.

Preferably, the porous polyimide is washed prior to swelling.

The invention provides a method for preparing rigid multistage Kong Kuaiti polyimide by supercritical carbon dioxide foaming, which comprises the following steps: cold-pressing and molding polyimide powder, and then sintering under no pressure to obtain porous polyimide; soaking the porous polyimide into N, N-dimethylformamide to swell to obtain swelled polyimide; and (3) placing the swelled polyimide in a high-pressure kettle to adsorb supercritical carbon dioxide until the saturated carbon dioxide is saturated, and then foaming to obtain the rigid hierarchical-pore block polyimide.

The N, N-dimethylformamide reduces the melt strength of polyimide, so that the micromolecular solvent permeates into the polymer, the interaction between chain segments is weakened, the free volume of the polymer is increased, carbon dioxide molecules can be smoothly diffused and adsorbed into the polymer, and meanwhile, the reduced melt strength is favorable for reducing the nucleation energy barrier of carbon dioxide gas and is easier to obtain foam pores; on the other hand, the N, N-dimethylformamide is a polar aprotic solvent, so that the polarity of a polymer system can be increased, and the dissolving capacity and selectivity of carbon dioxide to a polar polymer are obviously improved; porous polyimide is prepared firstly, and compared with solid nonporous polyimide, the introduction of pores is more beneficial to the diffusion of N, N-dimethylformamide and carbon dioxide molecules to a matrix. According to the invention, N-dimethylformamide is adopted to cooperate with pores introduced into a polymer matrix in advance, and the hierarchical pore polyimide is successfully prepared through supercritical carbon dioxide foaming.

Furthermore, the invention can realize the regulation and control of the microstructure, the pore size distribution and the porosity of the foamed polyimide by controlling the swelling degree of the N, N-dimethylformamide to the polyimide. The method has the advantages of less time consumption, simple post-treatment, low production cost and contribution to industrial production.

Drawings

FIG. 1 is a graph of the glass transition temperature of polyimides at different swelling degrees;

FIG. 2 is a plot of the pore size distribution of the rigid hierarchical porous block polyimide prepared in example 1;

FIG. 3 is an SEM and close-up view of a rigid multi-stage porous block polyimide prepared in example 1;

FIG. 4 is a graph of the pore size distribution of the rigid hierarchical porous block polyimide prepared in example 2;

FIG. 5 is a graph of the pore size distribution of the rigid multilevel Kong Kuaiti polyimide prepared in example 3;

FIG. 6 is an SEM image of a rigid multilevel Kong Kuaiti polyimide prepared in example 3;

FIG. 7 is a pore size distribution plot of the porous YHPI-P-100 polyimide prepared in example 4 before foaming;

FIG. 8 is a graph of the pore size distribution of the rigid hierarchical porous block polyimide prepared in example 4;

FIG. 9 is a pore size distribution diagram of the porous YS20 polyimide in comparative example 1;

fig. 10 is a schematic diagram of mercury intrusion into and out of solid non-porous YS20 polyimide and porous YS20 polyimide of comparative example 1;

FIG. 11 is a graph showing the distribution of pore diameters of the rigid porous polyimide after foaming in comparative example 2.

Detailed Description

cold-pressing and molding polyimide powder, and then sintering under no pressure to obtain porous polyimide;

In the present invention, the starting materials used are all commercially available products well known in the art, unless otherwise specified.

The invention is to cold press and mold polyimide powder, and then to sinter the polyimide powder without pressure to obtain the porous polyimide.

The particle size of the polyimide powder is not particularly limited in the present invention, and commercially available polyimide powders well known in the art may be used. The invention has no special requirement on the type of the polyimide powder, and polyimide varieties well known in the field can be used, and concretely can be YS30, YS20, YHPI-P-100 and the like. In an embodiment of the present invention, the polyimide powder is YS20 or YHPI-P-100 polyimide molding powder.

The cold press forming conditions of the present invention are not particularly required, and any cold press forming conditions known in the art may be used. In the embodiment of the invention, the pressure of the cold press molding is 5MPa, and the dwell time is 60min. The invention has no special requirements on the temperature and time of the pressureless sintering, and can realize that the mutually contacted parts among the powder particles are fused together in the pressureless sintering process, and the gaps among the powder particles are gradually connected to form mutually through pore passages. In the embodiment of the invention, the pressureless sintering temperature is 350 ℃ and the time is 2 hours. The porous polyimide with different apertures, porosities and appearances can be obtained by controlling the pressure and the pressure maintaining time of the cold press molding. According to the invention, porous polyimide is prepared, and compared with solid nonporous polyimide, the introduction of pores is more beneficial to the diffusion of N, N-dimethylformamide and carbon dioxide molecules to a matrix.

After the porous polyimide is obtained, the porous polyimide is soaked in N, N-dimethylformamide to swell, and the swelled polyimide is obtained.

Before the porous polyimide is soaked in the N, N-dimethylformamide, the porous polyimide is preferably washed by the method, and the washing preferably comprises washing with petroleum ether, acetone and ethanol in sequence. The size and shape of the porous polyimide are not particularly required, and in the embodiment of the invention, the porous polyimide is specifically cut into sheets of 18 × 18 × 2 mm.

The invention has no special requirement on the dosage of the N, N-dimethylformamide, and the porous polyimide can be completely immersed. In the present invention, the temperature of the impregnation is preferably 30 to 100 ℃, more preferably 40 to 90 ℃, and further preferably 50 to 80 ℃. The invention has no special requirement on the dipping time and can reach the proper swelling degree.

In the present invention, the swelling is intended to reduce the strength of the polyimide substrate and to allow carbon dioxide molecules to be smoothly adsorbed on the polyimide substrate. The requirement of polyimides with different molecular chain structures on the swelling degree of N, N-dimethylformamide is different, and the higher the strength of the general polyimides is, the more closely the molecular chains are arranged, the higher the swelling degree is required to achieve the plasticizing purpose.

Therefore, the present invention cannot be applied to all polyimides in general to limit the degree of swelling, but those skilled in the art can appropriately adjust the degree of swelling according to the principle of the present invention. In the present invention, when the polyimide powder is YS20 or YHPI-P-100, the swelling degree of swelling is preferably > 30%. The invention has no special requirement on the upper limit of the swelling degree and ensures that the polyimide is not dissolved.

The invention realizes the regulation and control of the microstructure, the pore size distribution and the porosity of the foamed polyimide by controlling the swelling degree of the polyimide.

The N, N-dimethylformamide reduces the melt strength of polyimide, so that the micromolecular solvent permeates into the polymer, the interaction between chain segments is weakened, the free volume of the polymer is increased, carbon dioxide molecules can be smoothly diffused and adsorbed into the polymer, and meanwhile, the reduced melt strength is favorable for reducing the nucleation energy barrier of carbon dioxide gas and is easier to obtain foam pores; on the other hand, the N, N-dimethylformamide is a polar aprotic solvent, so that the polarity of a polymer system can be increased, the dissolving capacity and selectivity of carbon dioxide to polar polymers are obviously improved, and foaming is facilitated.

After the swelling polyimide is obtained, the swelling polyimide is placed in an autoclave to absorb supercritical carbon dioxide until the swelling polyimide is saturated, and then foaming is carried out to obtain the rigid hierarchical-pore block polyimide.

According to the invention, N-dimethylformamide on the surface of the swelling polyimide is preferably wiped clean by using non-woven fabric, and then the swelling polyimide is placed in an autoclave for adsorbing supercritical carbon dioxide.

In the present invention, the foaming mode is preferably rapid temperature rise foaming or rapid pressure relief foaming.

When the foaming mode is rapid temperature rise foaming, the temperature for adsorbing the supercritical carbon dioxide is preferably 25-55 ℃, more preferably 30-50 ℃, and further preferably 35-45 ℃; the pressure is preferably 8 to 30MPa, more preferably 10 to 25MPa, and further preferably 15 to 20MPa; the foaming temperature is preferably 140-260 ℃, more preferably 160-220 ℃, and further preferably 180-200 ℃; the time for the foaming is preferably 1min. According to the invention, preferably, after the supercritical carbon dioxide is adsorbed to saturation, the sample after the carbon dioxide is adsorbed to saturation is quickly transferred to an oil bath, and the nucleation and growth of foam cells are induced. The temperature of the oil bath is the foaming temperature.

After the foaming is finished, the invention preferably transfers the foamed product into an ice-water bath to shape the foam holes so as to obtain the rigid hierarchical-hole block polyimide.

When the foaming mode is rapid pressure relief foaming, the temperature for adsorbing the supercritical carbon dioxide is preferably 130-180 ℃, more preferably 120-180 ℃, and further preferably 140-160 ℃; the pressure is preferably 8 to 30MPa, more preferably 8 to 25MPa, and further preferably 9 to 15MPa; the pressure-relief time is preferably 2 to 20s, more preferably 2 to 15s. The pressure is preferably relieved to atmospheric pressure in the present invention.

The invention induces the nucleation and growth of foam cells through pressure relief, and then transfers the foamed product into an ice-water bath to shape the foam cells to obtain the rigid hierarchical-pore block polyimide.

The following will describe the method for preparing rigid multi-stage porous block polyimide by foaming with supercritical carbon dioxide according to the present invention in detail with reference to the examples, but they should not be construed as limiting the scope of the present invention.

m _i Is the mass of the sample after swelling the solvent, m ₀ Is the dry weight of the sample.

Comparative example 1

Placing polyimide (YS 20) molding powder in a mold, cold-pressing and molding, wherein the molding pressure is 5MPa, the pressure maintaining time is 60min, and then carrying out pressureless sintering, wherein the pressureless sintering temperature is 350 ℃, and the pressureless sintering time is 2h to obtain porous polyimide; sequentially washing with petroleum ether, acetone and ethanol, and cutting into porous polyimide sheets of 18 × 18 × 2 mm;

and (3) soaking the porous polyimide sheet into N, N-Dimethylformamide (DMF) to swell to obtain swollen polyimide, and respectively obtaining swollen polyimide with swelling degrees of 20%, 30%, 50% and 80%.

The glass transition temperature of each swollen polyimide was measured, and the results are shown in FIG. 1. As can be seen from fig. 1, the 30% swelling degree and below the glass transition temperature of the YS20 polyimide was maintained at the pristine glass transition temperature of 265.4 ℃, at which melt strength cell growth by carbon dioxide plasticization and temperature induction was not possible. And the foaming result shows that after the porous polyimide with the swelling degree of 30% is foamed, the porosity and the appearance are not changed.

When the swelling degree is 50%, the glass transition temperature of the polyimide is reduced to 173.99 ℃, and a condition which is easy to realize is provided for subsequent foaming. As the swelling continued to increase to 80%, the glass transition temperature dropped to 165.71 ℃.

Example 1

Preparing rigid hierarchical pore block polyimide by adopting a rapid heating foaming mode:

placing polyimide (YS 20) molding powder in a mold, cold-pressing and molding, wherein the molding pressure is 5MPa, the pressure maintaining time is 60min, and then carrying out pressureless sintering, the pressureless sintering temperature is 350 ℃, and the pressureless sintering time is 2h, so as to obtain porous polyimide; sequentially washing with petroleum ether, acetone and ethanol, and cutting into porous polyimide sheets of 18 × 18 × 2 mm;

swelling a porous polyimide sheet in DMF (dimethyl formamide), with the swelling degree of 60%, wiping the DMF on the surface by using non-woven fabric, transferring the surface to a high-pressure kettle, introducing supercritical carbon dioxide, carrying out saturated adsorption for 3h at 35 ℃ and 15MPa, then quickly transferring to a high-temperature oil bath at 140 ℃ to induce cell nucleation and growth, transferring a sample to an ice-water bath after 1min, and shaping cells to obtain the rigid hierarchical-pore block polyimide.

The distribution of the pore size of the rigid hierarchical porous block polyimide prepared in example 1 is shown in FIG. 2. As can be seen from FIG. 2, the pore size is mainly 2.5 μm, 26nm and 11nm, and it has a hierarchical pore structure.

Fig. 3 is an SEM image and a partial magnified view of the rigid multi-stage porous block polyimide prepared in example 1. As can be seen from fig. 3, the rigid multilevel Kong Kuaiti polyimide has a distribution of many interconnected macropores, and in the lower enlarged view, it can be seen that the nanopores grown by carbon dioxide nucleation exhibit a distribution of micro-nano multilevel pore diameters.

Example 2

the porous polyimide sheet was prepared as in example 1.

Swelling a porous polyimide sheet in DMF (dimethyl formamide), with the swelling degree of 100%, wiping the DMF on the surface by using non-woven fabric, transferring the surface to a high-pressure kettle, introducing supercritical carbon dioxide, saturating at 35 ℃ and 9MPa for 3h, then quickly transferring to a high-temperature oil bath at 140 ℃ to induce cell nucleation and growth, transferring a sample to an ice-water bath after 5min, and shaping cells to obtain the rigid hierarchical-pore block polyimide.

The distribution of the pore diameter of the rigid hierarchical porous block polyimide prepared in example 2 is shown in FIG. 4. From FIG. 4, it can be seen that the peak of the pore diameter is mainly 1.632.5 μm and 12.24nm.

Example 3

Preparing rigid multi-level pore block polyimide by adopting a rapid pressure relief foaming mode:

the porous polyimide sheet was prepared as in example 1.

Swelling a porous polyimide sheet in DMF (dimethyl formamide), with the swelling degree of 60%, wiping the DMF on the surface by using non-woven fabric, transferring the polyimide sheet into a high-pressure kettle, introducing supercritical carbon dioxide, saturating the polyimide sheet at 180 ℃ under 15MPa for 3h, then quickly relieving pressure within 5s to induce nucleation and growth of foam cells, and then transferring a sample into an ice-water bath to shape the foam cells to obtain the rigid hierarchical-pore block polyimide.

The pore size distribution of the rigid multilevel Kong Kuaiti polyimide prepared in example 3 is shown in fig. 5, and it can be seen from fig. 5 that the rigid porous-level polyimide has mainly 9.06mm and 50.35nm pore sizes.

FIG. 6 is an SEM image of the rigid multilevel Kong Kuaiti polyimide prepared in example 3. As can be seen from FIG. 6, the pore diameter of the rigid multi-stage Kong Kuaiti polyimide prepared by rapid pressure relief is significantly larger than the cell structure obtained by rapid temperature rise, because the strength of the polyimide substrate is more reduced when the polyimide substrate is saturated at a temperature of 180 ℃, and the driving force for cell growth promoted by pressure is larger.

Example 4

Preparing porous YHPI-P-100 polyimide (pore diameter is 1.06 μm, see figure 7) by using the same preparation conditions as example 1, subsequently swelling YHPI-P-100 by using DMF until the swelling degree is 33%, wiping the surface by using non-woven fabric, adsorbing by supercritical carbon dioxide under the adsorption conditions of 35 ℃ and 9MPa, quickly transferring the adsorbed sample to a high-temperature oil bath at 260 ℃ for foaming after the adsorption is saturated, transferring the sample to an ice water bath after 1min, and shaping the cells to obtain the rigid multi-stage pore block polyimide.

FIG. 8 is a plot of the pore size distribution of the rigid multi-stage pore block polyimide prepared in example 4. As can be seen from fig. 8, the pore size was changed, and compared with fig. 7, which is a schematic diagram of the pore size before foaming, since partial pore fusion combined causes the main pore size to decrease to 484.69nm, and in addition, a new pore size peak appeared at 9.06nm, and a multi-stage porous polyimide was also obtained.

From examples 1 to 4, it can be seen that, by adopting different foaming methods and control of foaming conditions, not only can the pore size be controlled, but also the porosity can be controlled.

Comparative example 1

And (3) filling YS20 polyimide powder into a die, and applying a pressure of 20MPa in a high-temperature environment at 350 ℃ to completely melt and bond the powder particles together to obtain the solid non-porous YS20 polyimide.

YS20 polyimide powder was charged into a mold under the conditions of example 1, cold press molded, and then pressureless sintered at 350 ℃ for 2 hours to obtain porous YS20 polyimide having a pore size distribution as shown in FIG. 9. As is clear from FIG. 9, the pore diameter of the porous YS20 polyimide was 3.18. Mu.m.

The change of the amount of carbon dioxide adsorbed with time under the same saturated adsorption conditions was investigated, and the results are shown in fig. 10. Fig. 10 is a schematic diagram of mercury intrusion and desorption of solid nonporous YS20 polyimide and porous YS20 polyimide in comparative example 1, and it can be seen from fig. 10 that the adsorption amount of the polyimide introduced into the pores is much higher than that of the solid nonporous polyimide, and the higher carbon dioxide adsorption amount has a stronger plasticizing effect on the polyimide, which is favorable for foaming.

Comparative example 2

The distribution of the pore diameter of the rigid porous polyimide obtained by subjecting the porous YS20 polyimide of comparative example 1 to saturation adsorption at 35 ℃ and 9MPa and foaming at 265 ℃ in a high-temperature oil bath for 1min without swelling with DMF is shown in FIG. 11, and it is understood from FIG. 11 that the pore diameter is 3.1. Mu.m. The pore diameter is substantially the same as that before the supercritical carbon dioxide treatment.

This is because supercritical carbon dioxide molecules are directly used for adsorption, and although supercritical carbon dioxide has high diffusivity and high solubility, after saturation at 35 ℃ and 9MPa, the weak interaction and solvation capability of carbon dioxide on polyimide are limited, and the low carbon dioxide adsorption amount and the high glass transition temperature of polyimide are insufficient for nucleating and growing cells through rapid temperature rise foaming. In fig. 11, the pore size of the porous polyimide is substantially unchanged from that of fig. 9, and is represented by a single pore size.

The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing rigid multistage Kong Kuaiti polyimide by supercritical carbon dioxide foaming comprises the following steps:

and (3) placing the swollen polyimide in an autoclave to absorb supercritical carbon dioxide until the swollen polyimide is saturated, and then foaming to obtain the rigid hierarchical-pore block polyimide.

2. The method according to claim 1, characterized in that the temperature of the impregnation is preferably 30 to 100 ℃.

3. The method as claimed in claim 1 or 2, wherein the swelling degree of swelling is > 30% when the polyimide powder is YS20 or YHPI-P-100.

4. The method of claim 1, wherein the foaming is by rapid temperature rise foaming; the temperature for adsorbing the supercritical carbon dioxide is 25-55 ℃, and the pressure is 8-30 MPa; the foaming temperature is 140-260 ℃ and the time is 1min.

5. The method according to claim 4, characterized in that the foaming is carried out in an oil bath.

6. The method according to claim 1, wherein the foaming is by rapid pressure relief foaming; the temperature for adsorbing the supercritical carbon dioxide is 130-180 ℃, and the pressure is 8-30 MPa; the pressure relief time is 2-20 s.

7. The method of claim 1, 4, 5 or 6, wherein after the foaming, the method further comprises transferring the product obtained by foaming into an ice-water bath to shape the cells.

8. The method of claim 1, wherein the porous polyimide is washed prior to swelling.