Preparation and application of dual-responsive flexible composite membranes based on self-assembly behavior

Abstract

The present invention provides a preparation method of a dual-responsive flexible composite film based on self-assembly behavior, comprising the steps of: preparing a nanocellulose crystal suspension: mixing the nanocellulose crystals with deionized water to prepare a suspension; preparing a composite suspension: adding a small molecule sugar alcohol to the nanocellulose crystals in a mass ratio of (0.5-3):10, and adding an acid-base indicator in a mass ratio of (0.5-5):100 to the suspension; preparing a film: taking the prepared composite suspension into a container, sealing and standing for 24-72 hours, and then naturally drying to form a film. This preparation method compounds the nanocellulose crystals with common small molecule sugar alcohols, utilizing the plasticity of the sugar alcohols to solve the problem of the large brittleness of the nanocellulose crystal film to a certain extent; compounding with the sugar alcohols does not affect the unique optical properties of the nanocellulose crystal film, thereby expanding its practical value and application range.

Description

Technical Field

[0001] The invention belongs to the field of organic polymer materials, and particularly relates to a preparation method of a composite film with dual response functions, and application of the composite film. Background Art

[0002] Nanocellulose crystals are usually cellulose materials with a diameter of less than 100nm obtained by sulfuric acid hydrolysis. They are easy to obtain and degradable. Due to the sulfonate ions on their surface, nanocellulose crystals can self-assemble to form cholesteric liquid crystals when the concentration is above the critical concentration. This special structure gives nanocellulose unique optical properties, and the special structure can be retained in the formed film through a simple room temperature drying process, giving the film a unique structural color. Nanocellulose crystals are highly hydrophilic and have an adsorption effect on water molecules and other small molecules in the environment. They can change the pitch of the cholesteric liquid crystals and thus effectively adjust the color of the nanocellulose crystal film. Therefore, nanocellulose crystals are gradually being used in fields such as sensors.

[0003] Currently, nanocellulose crystal films are basically prepared by direct drying. Films that have not undergone any preparation process optimization will show a clear coffee ring phenomenon, and the color difference between the central ring and the edge of the film is large, which weakens its application value as a sensing material. In addition, due to the high brittleness of nanocellulose crystals, the films formed from them also have this defect. Some polymer plasticizers, such as polyethylene glycol, have been used in nanocellulose crystal films to improve mechanical properties, but the addition of polymer plasticizers can easily affect the special structure of nanocellulose crystal films, resulting in the loss of their unique optical properties and the disappearance of film color. Summary of the Invention

[0004] In view of the shortcomings of the existing technology, the first purpose of the present invention is to propose a method for preparing a dual-responsive flexible composite film based on self-assembly behavior to obtain a film that responds to both humidity and pH changes in color.

[0005] The second object of the present invention is to provide the prepared dual-responsive flexible composite film.

[0006] The third object of the present invention is to provide applications of the dual-responsive flexible composite film.

[0007] The technical solution for achieving the above-mentioned purpose of the present invention is:

[0008] A method for preparing a dual-responsive flexible composite film based on self-assembly behavior comprises the following steps:

[0009] (1) preparing a nanocellulose crystal suspension: mixing the nanocellulose crystals with deionized water to prepare a suspension, wherein the concentration of the nanocellulose crystals is 1.0-3.0 wt %;

[0010] (2) preparing a composite suspension: adding a small molecule sugar alcohol and nanocellulose crystals at a mass ratio of (0.5-3):10, and adding an acid-base indicator at a mass ratio of (0.5-5):100 to the suspension;

[0011] (3) Preparation of thin film: The composite suspension prepared in step (2) is placed in a container, sealed and allowed to stand for 24-72 hours, and then naturally dried to form a film.

[0012] The small molecule sugar alcohol is selected from one or more of sorbitol, xylitol, erythritol, mannitol and maltitol; the acid-base indicator is a plant acid-base indicator, selected from one of anthocyanin, shikonin and natural amaranth red pigment.

[0013] Preferably, the small molecule sugar alcohol is erythritol, and erythritol and nanocellulose crystals are added in a mass ratio of (1-1.2):10.

[0014] Wherein, the acid-base indicator is anthocyanin, and anthocyanin and nanocellulose crystals are added to the suspension at a mass ratio of (1-2):100.

[0015] Furthermore, in step (2), after adding the small molecule sugar alcohol and the acid-base indicator, stirring is continued for 1 hour, and then ultrasonic treatment is performed at 400-1000W for 2-5 minutes.

[0016] Wherein, in the step (3), the prepared composite suspension is put into a container so that the liquid surface thickness is 2 to 5 mm, sealed and allowed to stand for 24 to 72 hours, and then dried to form a film.

[0017] Further preferably, in the step (3), the composite suspension is placed in a container, sealed and allowed to stand for 45-50 hours, and then naturally dried at 20-28° C. and 30-60% RH to form a film.

[0018] The dual-responsive flexible composite film is prepared by the preparation method.

[0019] The dual-responsive flexible composite film is used for color response to humidity and pH.

[0020] Among them, when placed in a humidity environment of 35%-85% RH, the dual-responsive flexible composite film can achieve reversible color response, and can also achieve color response when the environmental pH value ranges from 4 to 12.

[0021] The composite membrane was placed in an environment with 85% RH and 25°C for humidity response. After 3 minutes, the composite membrane changed from bluish-green (523 nm) to red (645 nm). The membrane quickly returned to blue when placed in an environment with 35% RH, and the response cycle was repeated multiple times. The composite membrane also exhibited color response when placed in solutions with pH values ​​ranging from 4 to 12.

[0022] The beneficial effects of the present invention are:

[0023] (1) The dual-responsive flexible composite film preparation method proposed in the present invention is simple and effectively improves the coffee ring effect, making the film color more uniform.

[0024] (2) This preparation method combines nanocellulose crystals with common small molecule sugar alcohols, and utilizes the plasticity of sugar alcohols to solve the problem of high brittleness of nanocellulose crystal films to a certain extent; the combination with sugar alcohols does not affect the unique optical properties of nanocellulose crystal films, and expands their practical value and application range.

[0025] (3) The dual-responsive flexible composite membrane prepared by this method has multiple responsiveness and reversibility. Based on the hygroscopicity of small molecule sugar alcohols and the pH sensitivity of anthocyanins, it can achieve rapid response to changes in environmental humidity and pH value. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Pictures of the flexible composite film prepared by sealing and drying a 1.5 wt% nanocellulose crystal solution after standing for different times.

[0027] Figure 2 AFM image of CNC suspension.

[0028] Figure 3 AFM images of dual-responsive flexible composite films with different amounts of erythritol added (addition amounts are 10%, 20%, and 30%, respectively).

[0029] Figure 4 AFM images of dual-responsive flexible composite films with different amounts of mannitol added (addition amounts are 10%, 20%, and 30%, respectively).

[0030] Figure 5 AFM images of dual-responsive flexible composite films with different amounts of maltitol added (addition amounts are 10%, 20%, and 30%)

[0031] Figure 6 The stress-strain curves of the dual-responsive flexible composite film with different amounts of erythritol added (the addition amount is 10%, 20%, and 30%)

[0032] Figure 7This is the cyclic response curve of the dual-responsive flexible composite film with an erythritol addition of 10%. DETAILED DESCRIPTION

[0033] The following examples are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0034] Unless otherwise specified, the test materials and testing instruments used in the instructions can be purchased commercially.

[0035] Unless otherwise specified, all parts in the examples are parts by mass and all percentages are by mass.

[0036] See also Figure 2 The nanocellulose crystals (CNC) used in the examples have a spindle-shaped rod morphology, a length of 150 to 330 nm, and a diameter of 10 to 20 nm.

[0037] Example 1:

[0038] The method for preparing a flexible composite film based on self-assembly behavior of this embodiment includes the following steps:

[0039] (1) Weigh 0.300 g of nanocellulose crystals in 20 mL of deionized water and stir magnetically for 30 min to mix the suspension evenly at a speed of about 1200 rpm to a nanocellulose crystal mass fraction of 1.5 wt %. Place in a refrigerator at 4 °C until ready for use.

[0040] (2) 3 mL of the suspension prepared in step (1) was placed in a 35 mm diameter disposable plastic Petri dish, sealed, and allowed to stand for 0, 24, 48, and 72 hours. After standing, the suspension was allowed to dry naturally at room temperature (25°C, 45% RH) to form a film, and the color uniformity of the film was observed.

[0041] See also Figure 1 As can be seen, the color range of the film after direct drying is relatively narrow, with most areas showing no color. However, as the sealed and standing time increases, the color range of the film is significantly expanded. At 72 hours of sealing and standing, the color range almost covers the entire film. However, a more obvious color range change can be seen after 48 hours of sealing and standing, so a 48-hour sealing and standing time was used in subsequent experiments.

[0042] By sealing and letting it stand, the nanocellulose crystals are given more time to self-assemble, reducing the problem of cholesteric liquid crystals being fixed in the film before complete self-assembly during the direct drying process, making the structure more orderly and consistent, and the color of the film more uniform from a macroscopic observation.

[0043] Example 2

[0044] The method for preparing a dual-responsive flexible composite film based on self-assembly behavior of this embodiment includes the following steps:

[0045] (1) Weigh nanocellulose crystals in 20 mL of deionized water and stir magnetically for 30 min to mix the suspension evenly at a speed of approximately 1200 rpm. The final nanocellulose crystal mass fraction is 1.5 wt %. Place in a refrigerator at 4 °C until ready for use.

[0046] (2) adding erythritol and nanocellulose crystals at a mass ratio of 10% and anthocyanin at a mass ratio of 2% to the solution of (1), continuing magnetic stirring for 1 hour and then ultrasonically treating at 400W for 2 minutes;

[0047] (3) Take 3 mL of the suspension in (2) and place it in a disposable plastic culture dish with a diameter of 35 mm. After sealing and standing for 48 hours, place it at room temperature (25°C, 45% RH) to dry naturally to form a film.

[0048] Figure 3 It is the microscopic morphology of the suspension obtained in step (2).

[0049] Example 3

[0050] The small molecule sugar alcohol added in Example 2 was adjusted to mannitol, and the other steps remained the same as in Example 2.

[0051] Example 4

[0052] The small molecule sugar alcohol added in Example 2 was adjusted to maltitol, and the other steps remained the same as in Example 2.

[0053] The suspensions obtained in Examples 2-4 were subjected to microscopic morphology observation and the films were subjected to mechanical property tests. The test results are shown in Figure 3-6 .

[0054] Depend on Figure 2-5 It can be seen that compared with mannitol and maltitol, erythritol can be better wrapped in the CNC crystal structure. This means that when CNC self-assembles, erythritol can be better embedded in the CNC cholesteric liquid crystal structure, regulating the color change of the film. Because erythritol is highly hygroscopic, when the humidity is high, erythritol absorbs moisture, causing the pitch of the CNC cholesteric phase structure to increase, and the color of the film to red shift.

[0055] Depend on Figure 6As shown in the left figure, when the erythritol addition level is 10%, the film tensile strength is the highest, the strain reaches 5.9%, and the film toughness is also the highest. As the erythritol addition level gradually increases, the tensile strength and toughness of the composite film show a gradual downward trend, and the strain drops to 1.9%. When the addition level is 10% and 20%, the film mechanical properties are better than those of pure CNC film. When the addition level is 30%, the film mechanical properties are lower than those of pure CNC film. This is because the appropriate amount of small molecule sugar alcohol can wrap around the nanocellulose crystals, improving their original brittleness and increasing the toughness of the film. However, the addition of too much erythritol will destroy its original self-assembled structure, resulting in an overall decrease in the film's mechanical properties.

[0056] The trend of the change in mechanical properties of the film with the addition of mannitol is the same as that with the addition of erythritol, see Figure 6 In the middle figure, the strain of the composite film with 10% mannitol addition reaches 4.4%, while the strain of the composite film with 30% mannitol addition drops to 2.8%. The mechanical properties of the film with maltitol addition increase with the increase of maltitol addition, see Figure 6 In the right figure, the strain of the composite film with 10% maltitol addition is 2.7%, while the strain of the composite film with 30% maltitol addition is 4.2%, which is better than the 2.3% strain of the pure CNC film. However, the excessive addition of small molecule sugar alcohol embeds into the CNC crystal structure, causing the original color of the film to redden, thus limiting its subsequent application.

[0057] Application Examples

[0058] The composite film with 10% erythritol added in Example 2 was placed in an environment of 85% RH and 25°C for humidity response. After 3 minutes, the composite film changed from blue-green (523nm) to red (645nm). It quickly returned to blue when placed in an environment of 35% RH, and multiple cycles of response were achieved. Figure 7 . When a long strip sample with a length of 20 mm and a width of 10 mm was placed in different pH buffer solutions (pH values ​​ranging from 4 to 12), the same color response phenomenon was observed. When placed in an acidic solution (pH 4-pH 6), the color of the film showed a slight red shift, which was mainly due to the hygroscopicity of CNC itself. When the solution was close to neutral (pH 7-pH 8), the film was almost colorless; as the alkalinity of the solution continued to increase, the color of the film changed from light gray (pH 9-pH 10) to light green and dark green (pH 11-pH 12), which was attributed to the pH sensitivity of anthocyanins.

[0059] The preparation method of the present invention uses nanocellulose crystals as raw materials, a small molecule sugar alcohol as a plasticizer, and anthocyanins as pH indicators. Based on the self-assembly behavior of the nanocellulose crystals, a film preparation process involving sealing, allowing the film to stand, and then drying is used to produce a dual-responsive flexible nanocellulose composite film. This method is simple, and all composite materials are safe, harmless, and readily available. The resulting film exhibits color responses to changes in both humidity and pH, and exhibits greater flexibility than films made of pure nanocellulose crystals.

[0060] Although the present invention has been described above through the embodiments, those skilled in the art should understand that any improvements and modifications made to the present invention without departing from the spirit and essence of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for preparing a dual-responsive flexible composite membrane based on self-assembly behavior, characterized in that: Including steps: (1) preparing a nanocellulose crystal suspension: mixing the nanocellulose crystals with deionized water to prepare a suspension, wherein the concentration of the nanocellulose crystals is 1.0-3.0 wt %; (2) preparing a composite suspension: adding erythritol and nanocellulose crystals at a mass ratio of (1-1.2):10, and adding an acid-base indicator at a mass ratio of (1-2):100 to the suspension, wherein the acid-base indicator is anthocyanin; (3) Preparation of thin film: The composite suspension prepared in step (2) is placed in a container so that the liquid surface thickness is 2-5 mm, sealed and allowed to stand for 45-50 hours, and then placed under conditions of 20-28°C and 30-60% RH to dry naturally to form a film.

2. The preparation method according to claim 1, characterized in that In step (2), after adding the small molecule sugar alcohol and the acid-base indicator, stirring is continued for 1 hour, and then ultrasonic treatment is performed at 400-1000W for 2-5 minutes.

3. A dual-responsive flexible composite membrane prepared by the preparation method according to any one of claims 1 to 2.

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