CN111996583A - Self-assembly method of polystyrene colloidal particle crystals in aqueous medium - Google Patents

Abstract

The invention discloses a self-assembly method for polystyrene colloidal crystals in an aqueous medium, comprising the following steps: S1, pouring PS microspheres/water suspension synthesized by an emulsion polymerization method into a container, and placing it on a magnetic stirrer Stir; S2, alternately add anion exchange resin and cation exchange resin to the suspension, monitor the conductivity change of the suspension in real time with a conductivity tester, and stop when the conductivity no longer decreases; S3, filter out the anion exchange with a screen Resin and cation exchange resin, S4, in the state of constant stirring, heat and concentrate the suspension; S5, place the concentrated suspension in a constant temperature drying box, and start heating and assembling after standing for a period of time, until the dispersion medium water in the suspension Completely evaporated, the assembled PS colloidal crystals can be obtained; the present invention adopts the treatment process of deionization and concentrated suspension to improve the electrostatic repulsion of PS microspheres in the dispersion system, and then directly heats and assembles, which can efficiently realize PS Assembly of colloidal crystals.

Description

A kind of self-assembly method of polystyrene colloidal particle crystal in water medium

technical field

The invention relates to the technical field of PS colloidal crystals, in particular to a self-assembly method of polystyrene colloidal crystals in an aqueous medium.

Background technique

In the past ten years, two-dimensional or three-dimensional colloidal crystals assembled with micro- and nano-scale colloidal microspheres have attracted more and more interest, because they have a wide range of applications in many fields, such as: photonic crystals, optical filters , optical switches, high-density electromagnetic data storage, chemical and biochemical sensors, etc. In addition, three-dimensional ordered macroporous materials constructed with colloidal crystals as templates also have important applications in catalysis and adsorption materials.

There are many assembly methods for colloidal crystals, which are suitable for the self-assembly of colloidal microspheres of different materials. Polystyrene (PS) microspheres: micro-nano-scale spherical particles polymerized with styrene as a monomer. Methods suitable for PS microspheres include gravity deposition method, vertical deposition assembly method, room temperature rapid assembly method, suspension method. coating method, dip coating method, gas-liquid interface assembly method, etc. According to the different characteristics of the above-mentioned different methods, we can obtain colloidal crystals with different materials, different particle sizes and different surface characteristics. However, these methods have or have shortcomings such as long assembly time, large interference from external factors, or difficulty in achieving overall crystal ordering.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned deficiencies, the purpose of the present invention is to provide a method for self-assembly of polystyrene colloidal crystals in an aqueous medium. The present invention can realize the fast, efficient and ordered self-assembly of PS colloidal crystals, and solve the problem of self-assembly at the gas-liquid interface. The assembly method, the vertical deposition method and the gravity deposition method are difficult to achieve batch preparation, the assembly time is long, the interference from external factors is large, and it is difficult to achieve the overall orderly arrangement of the colloidal crystals.

To achieve the above object, the present invention is implemented according to the following technical solutions:

A self-assembly method of polystyrene colloidal particle crystals in an aqueous medium, comprising the following steps:

S1, pour the PS microspheres/water suspension synthesized by emulsion polymerization into a container, place the container on a magnetic stirrer, and add a magnetic stirrer in the container to stir;

S2, deionization: alternately add the regenerated anion exchange resin and cation exchange resin to the uniformly stirred suspension in S1, and at the same time use a conductivity tester to monitor the conductivity change of the suspension in real time, and stop when the conductivity no longer decreases;

S3, after the deionization of the suspension in step S2, the anion exchange resin and the cation exchange resin are filtered out with a screen, and the filtered suspension is poured into a container for preservation;

S4, heating and concentration: under the state of constant stirring, the suspension of step S3 is heated and concentrated to a mass percentage concentration of more than 30wt%;

S5, suspension self-assembly: pour the concentrated suspension in step S4 into a container and place it in a constant temperature drying box, and start heating and assembling after standing for a period of time. After the dispersion medium water in the suspension is completely evaporated, the assembly can be obtained PS colloidal crystals;

Further, in step S4, the temperature of the heating and concentration is maintained between 50-60 °C;

Further, in step S5, the temperature of the constant temperature drying box is set in the range of 60-70°C, and the time of constant temperature assembly is 5-6h;

Further, in step S3, the screen is a steel screen with a mesh of 70 and above.

Compared with the prior art, the self-assembly method of polystyrene colloidal particle crystals in an aqueous medium of the present invention has the following beneficial effects:

PS colloidal microspheres usually have a certain charge in the aqueous medium, which provides electrostatic repulsion between PS colloidal microspheres, which is the main reason for the stable existence of PS colloidal microspheres/water dispersion system for a certain period of time. The present invention is based on the fact that the PS colloidal microspheres are charged in an aqueous medium. In the present invention, by regulating the number of charges on the PS colloidal microspheres in the aqueous medium, the PS microspheres can survive in the aqueous medium under the action of strong electrostatic repulsion. complete the entire assembly process.

The invention adopts the treatment process of deionization and concentrated suspension to improve the electrostatic repulsion of PS microspheres in the dispersion system, and then directly heats and assembles. PS microspheres have strong electrostatic repulsion in the water medium, and with the continuous evaporation of the water medium to complete evaporation, the assembly of PS colloidal crystals can be realized quickly and efficiently.

Ion exchange resin deionization, sieving and filtering resin, and stirring and heating to concentrate the suspension are all conventional industrial operation methods, so they can be easily realized in production. Based on the mechanism of strong electrostatic repulsion driving the assembly process, the direct heating assembly process of the treated PS microspheres/water suspension is neither affected by the size of the container nor disturbed by external factors, so the suspension self-assembly can realize industrialized mass production a method. Not only that, the method of the present invention is also a method for preparing PS colloidal crystals with overall ordered PS microspheres.

Description of drawings

In order to explain the embodiments of the present invention or the technical solutions in the prior art more clearly, the following briefly introduces the accompanying drawings that need to be used in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only These are some embodiments of the present invention, and for those of ordinary skill in the art, other drawings can also be obtained from these drawings without any creative effort.

Fig. 1 is the process flow diagram of the present invention;

Fig. 2 is the Stern electric double layer model diagram of the implementation of the present invention;

Fig. 3 is the crystal structure diagram of PS colloid in aqueous medium;

4 is a schematic diagram of Bragg diffraction of PS colloidal crystals in an aqueous medium;

Fig. 5 is the PS colloidal particle crystal diagram that the present invention obtains;

Fig. 6 is the top surface SEM image of PS colloidal particle crystal of the present invention;

Fig. 7 is the lower surface SEM image of PS colloidal particle crystal of the present invention;

8 is a cross-sectional SEM image of the PS colloidal crystal of the present invention.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The exemplary embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention.

The PS colloidal microspheres used in this example are mainly synthesized by the emulsion polymerization method, and the PS microspheres after the synthesis are directly stored in the aqueous medium used in the synthesis process without any purification and separation treatment.

As shown in Figure 1, 80 mL of PS microspheres/water suspension sample synthesized by emulsion polymerization was poured into a beaker. The mass percentage concentration of PS microspheres in the suspension is about 10wt%, the beaker is placed on a magnetic stirrer, a magnetic stirrer is added to the beaker, and stirring is started; Anions..., re-cations...,..., are added to the beaker alternately in sequence, and at the same time, the conductivity changes of the suspension are monitored in real time with a conductivity tester to understand the deionization process of the ion exchange resin; the process of adding anion and cation exchange resin is suspended. The conductivity changes of the liquid are shown in Table 1:

Table 1. Data table of PS microspheres/water suspensions treated with ion exchange resins

According to the data in Table 1, the conductivity of PS microspheres/water suspension without ion exchange resin treatment is 617 μs/cm, which is relatively high and higher than the conductivity of laboratory tap water (about 386 μs/cm). cm) nearly 2 times. This is the dissociated ions (Na ⁺ , K ⁺ , HCO ₃ ^- , etc.) of various reagents and auxiliaries (emulsifiers, initiators and stabilizers, etc.) added during emulsion polymerization to improve the ionic strength of the aqueous medium, thereby increasing its electrical conductivity.

With the addition of the first batch of anion exchange resin, the conductivity of the suspension dropped rapidly from 617 μs/cm to 161.4 μs/cm, and after the addition of cation exchange resin, the conductivity dropped to 23.40 μs/cm; After the ion exchange resin, the conductivity of the suspension was further reduced to 0.065 μs/cm; then the third and fourth batches of ion exchange resin were added, and it was found that the conductivity of the system only decreased by a very small value or no longer decreased. The value of 0.065 μs/cm is already comparable to the conductivity of deionized or distilled water (0.055 μs/cm), indicating that all ions in the suspension have been basically removed. The efficiency of ion removal is related to various factors, such as the initial conductivity of the PS microsphere/water suspension, the amount of suspension processed, and the amount of ion exchange resin added per batch.

The purpose of deionization treatment is to increase the charge amount of PS microspheres in aqueous medium, that is, to increase its zeta potential. The effect of ion concentration in the suspension on the zeta potential of colloidal particles can be explained by the Stern electric double layer model. Figure 2 is a schematic diagram of the Stern electric double layer model. It can be seen from Figure 2 that when the ion concentration is increased, more counter ions will enter the solvation layer, and the thickness of the electric double layer will also become thinner, and the intersection of the potential curve and the tangential plane will decrease from ζ ₀ to ζ ₂ , the zeta potential will decrease; on the contrary, when deionized, the thickness of the electric double layer will increase, the intersection of the potential curve and the tangential plane will rise, from ζ ₀ to ζ ₁ , and the zeta potential increases.

After the deionization treatment of the suspension is completed, a steel 70 mesh screen is used to filter out the anion and cation exchange resin, and the filtered suspension is poured into a test tube with a plug for storage. Usually, the particle size of the anion and cation exchange resin particles ranges from 0.3 to 1.25mm. However, the aperture of the commonly used steel 70-mesh screen reaches 0.22mm, which is obviously smaller than the size of the ion exchange resin particles. Therefore, a steel screen of 70 mesh or more is used to filter out the anion and cation exchange resin.

After the suspension is filtered to remove the anion and cation exchange resin, the next step is to heat and concentrate the suspension, as shown in Figure 1. In this step, it is required to heat and concentrate in the temperature range of 50-60°C under the condition of constant stirring, and the mass percentage concentration of the suspension needs to be increased from the initial 10wt% to about 30wt%. After the concentration is completed, after a little standing (not more than 1min), the PS microspheres will self-assemble into PS colloidal crystals in the aqueous medium, and the crystals will show brilliant colors under visible light; The brilliant color is the result of Bragg diffraction of visible light. Figure 3 shows the structure of PS colloidal crystals in an aqueous medium, and Figure 4 is a schematic diagram of Bragg diffraction. Different from dry PS colloidal crystals, PS microspheres in PS colloidal crystals in aqueous media Each other is a state of orderly arrangement without contact. If the ordered PS microspheres in PS colloidal crystals are regarded as particles, the Bragg diffraction of visible light can be explained by Fig. 4.

When visible light is incident on PS colloidal crystals, since the particle size of PS colloidal microspheres is comparable to the wavelength of visible light, there must be a certain wavelength of visible light that satisfies the Bragg diffraction formula (see formula (1)), and the occurrence of Bragg diffraction will enhance this phenomenon. A specific wavelength of visible light, thus showing the color of the corresponding wavelength of visible light.

2 d sin θ = nλ (1)

The suspension after heating and concentration is not a single color, but a gradient color, this is because the inner wall of the test tube is an arc, which limits the surface of the PS colloidal crystal to be arc-shaped. Under the condition that the interplanar spacing d remains unchanged (the interplanar spacing does not change under a certain suspension concentration), the arc-shaped crystal surface will produce a continuous change of the incident angle θ , and a series of wavelengths λ will change continuously according to the Bragg diffraction formula. The visible light satisfies the diffraction condition and is strengthened, so that it is a gradient color observed by the naked eye.

During the entire heating and concentration process, the test tube showed a changing trend of only milky white (no color), to the appearance of color, and then to a brilliant gradient color. According to the above analysis, the first appearance of color is the result of Bragg diffraction of visible light, and Bragg diffraction reflects that PS microspheres in the suspension have self-assembled into PS colloidal crystals. As the concentration of the suspension increases gradually from 10%, 20% to 30%, it can be seen that the distance between the PS microspheres in the suspension will gradually decrease; as the distance between the PS microspheres decreases, the electrostatic repulsion will increase rapidly.

In the deionization process, the zeta potential of the particles is increased by deionization, thereby increasing the electrostatic repulsion between the PS microspheres; in the step of concentrating the suspension, the PS microspheres are significantly improved by shortening the distance between the PS microspheres electrostatic repulsion. The functions of the two process steps are the same, both are to improve the electrostatic repulsion between PS microspheres. After two processing steps, strong electrostatic repulsion was produced between PS microspheres. It is well known that in addition to electrostatic repulsion, colloidal particles in aqueous media also have van der Waals attraction and Brownian motion. However, compared with the electrostatic repulsion, especially after the two processing steps of deionization and concentration, the van der Waals attraction is negligible, that is, the electrostatic repulsion is the main force between the PS microspheres. Brownian motion is the basic characteristic of particles in colloidal system that always exists, and it is an important factor to maintain the stability of colloidal system. But for our suspension dispersion system, after deionization and concentration, the electrostatic repulsion between PS microspheres in the system forms a competitive relationship with the Brownian motion of PS microspheres: Brownian motion makes PS microspheres tend to disordered arrangement, while electrostatic repulsion tends to orderly arrangement. After heating and concentration, the suspension samples showed strong and brilliant colors. The electrostatic repulsion became dominant, and the Brownian motion became negligible, and this dominant effect became more and more obvious with the increase of the concentration of the suspension.

After concentrating the suspension, the next step is the suspension self-assembly process of PS colloidal crystals, the process of which is shown in Figure 1. Pour a certain volume of prepared (deionized and concentrated) PS microsphere suspension into the beaker, place the beaker in a constant temperature drying oven, set the temperature in the range of 60-70 °C, assemble for 5-6 hours, and assemble the beaker for 5-6 hours. When the water in the dispersion medium in the suspension is completely evaporated, the assembled PS colloidal crystals can be obtained in the beaker.

Figure 5 shows the block PS colloidal crystals taken at different angles after the suspended self-assembly. It can be seen from the figure that the PS colloidal crystals are broken into some small crystals of different sizes, similar to the size of "soybean grains". piece. The massive PS colloidal crystals accumulated at the bottom of the beaker showed two colors of emerald green and pale pink. Under different display angles, the surface of the massive PS colloidal crystals presents a variety of brilliant colors such as pink, yellow, light green and emerald green. The difference in color is still the result of Bragg diffraction. The specific analysis has been analyzed in the heating and concentration step, and will not be repeated here.

Figure 6 shows the SEM image of the upper surface of the PS colloidal crystals obtained after suspension self-assembly. It can be seen that the PS microspheres are in contact with each other and exhibit a highly ordered arrangement, which is obviously different from the structure shown in Figure 3 in which the PS microspheres do not contact each other in the PS colloidal crystal. Because PS colloidal crystals in aqueous media are fluid, there is currently no suitable characterization method to explore their internal structure. The structure of Figure 3. Figure 7 is the lower surface of the PS colloidal particle crystal, from which it can be seen that the PS microspheres are also highly ordered, and have fewer point defects and line defects than the upper surface. Figure 8 is a cross-section of the PS colloidal crystal, and the cross-section reflects the internal structure of the PS colloidal crystal. It can be seen from Figure 8 that the PS microspheres are ordered in the form of face-centered cubic, and there are basically no point and line defects. This indicates that the arrangement of the inner PS microspheres is more ordered during suspension self-assembly.

In Fig. 6/7/8, the arrangement of the upper and lower surfaces of the PS colloidal crystals to the inner PS microspheres is in an ordered state, indicating that the suspension self-assembly method is an effective method to realize the overall orderly assembly of PS colloidal crystals. . This is a significant advantage over other assembly methods. In other assembly methods, the PS colloidal crystals at the end of the assembly or in contact with the wall of the assembled vessel (especially the bottom of the vessel) will produce great defects, and the overall orderly assembly cannot be achieved.

The ability of suspension self-assembly to obtain overall ordered PS colloidal crystals is closely related to the assembly mechanism of this method. As mentioned above, after the two key steps of deionization and concentration, the PS microspheres in the system generated strong electrostatic repulsion between each other. Although there are also van der Waals attraction and Brownian motion between PS microspheres, both are negligible compared with the strong electrostatic repulsion. Driven by the strong electrostatic repulsion, the PS microspheres in the aqueous medium cannot move at will, and will spontaneously form the ordered structure shown in Figure 3, which we call PS colloidal crystals. During the assembly process in the constant temperature drying box, with the evaporation of the water medium, the spacing of the PS microspheres is continuously reduced, and the electrostatic repulsion becomes stronger. This further drives and adjusts the position of the PS microspheres, making them more orderly. At the same time, the strong electrostatic repulsion maintains the PS colloidal crystal structure like the "spring force" and cannot be easily changed, which makes it undisturbed by external factors during the process of moving the sample or suspending the self-assembly.

According to the DLVO theory, with the continuous evaporation of the water medium, the distance between the PS microspheres is continuously reduced. Although the electrostatic repulsion will increase significantly, the van der Waals attraction will also increase. When the distance between the PS microspheres is reduced to a certain extent, that is, the potential barrier formed by the repulsive potential energy and the attraction effect is overcome, the PS microspheres will come into contact with each other under the condition of maintaining the previous ordered structure. Until the water in the dispersion medium is completely evaporated, PS colloidal crystals become PS colloidal crystals.

The technical solutions of the present invention are not limited to the limitations of the above-mentioned specific embodiments, and all technical deformations made according to the technical solutions of the present invention fall within the protection scope of the present invention.

Claims (4)

1. A self-assembly method of polystyrene colloidal particle crystals in an aqueous medium is characterized by comprising the following steps:

s1, pouring the PS microsphere/water suspension synthesized by the emulsion polymerization method into a container, placing the container on a magnetic stirrer, and adding a magnetic stirrer into the container for stirring;

s2, deionization: alternately adding the regenerated anion exchange resin and the regenerated cation exchange resin into the uniformly stirred suspension in the S1, and simultaneously monitoring the conductivity change of the suspension in real time by using a conductivity tester until the conductivity is not reduced any more;

s3, after the suspension is deionized in the step S2, filtering out the anion exchange resin and the cation exchange resin by using a screen, and pouring the filtered suspension into a container for storage;

s4, heating and concentrating: heating and concentrating the suspension obtained in the step S3 under the condition of continuous stirring until the mass percent concentration is more than 30 wt%;

s5, suspension self-assembly: and (4) pouring the concentrated suspension liquid obtained in the step S4 into a container, placing the container in a constant-temperature drying oven, standing for a period of time, starting heating and assembling, and obtaining the assembled PS colloidal particle crystal after the dispersion medium water in the suspension liquid is completely evaporated.

2. The self-assembly method of polystyrene micelle crystals within an aqueous medium according to claim 1, wherein the temperature of the heating concentration is maintained between 50-60 ℃ in step S4.

3. The self-assembly method of polystyrene micelle crystals within an aqueous medium according to claim 1, wherein the temperature of the constant temperature drying oven is set within the range of 60-70 ℃ and the constant temperature assembly time is 5-6 hours at step S5.

4. The method for self-assembling polystyrene micelle crystals within an aqueous medium according to claim 1, wherein the mesh is a steel mesh with 70 mesh or more in step S3.

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