A polysiloxane having a shear thickening behavior and a method for preparing the same

Abstract

The application provides a polysiloxane with shear thickening behavior and a preparation method thereof. The preparation method comprises the following steps: a) reacting end double-hydroxyl terminated polydimethylsiloxane with boric acid to obtain a crude polysiloxane; the reaction temperature is 25-75 DEG C, and the reaction time is 1.5-10 h; b) removing unreacted boric acid from the crude polysiloxane to obtain the polysiloxane with shear thickening behavior. The polysiloxane prepared by the application not only has more significant'solid-liquid' transition characteristics, but also has shear thickening behavior in a wide temperature range.

Description

Technical Field

[0001] This invention relates to the field of organosilicon materials, and in particular to a polyborosiloxane with shear-thickening behavior and its preparation method. Background Technology

[0002] Polyborosiloxane (PBS), often referred to as a "shear-thickening / hardening gel," has wide applications in shock protection equipment and soft sensors. This "shear-thickening / hardening" behavior is mainly attributed to the dynamic bond exchanges present in the PBS system, such as B:O dynamic coordination bonds, hydrogen bonds, and transesterification.

[0003] The shearing behavior of PBS has been a focus of attention since the last century. However, the results of research on the shearing behavior of PBS over the years have been inconsistent. For example, AATrapeznikov was the first to discover that pure PBS has shear-thickening behavior in decahydronaphthalene solution, and believed that the shear-thickening behavior of PBS is related to the newly formed B:O dynamic coordination bonds in the system (AATrapeznikov et al. Some of The Rheological Characteristics of Boron-Siloxane Polymer Concentrates. Polymer Science USSR 1975, 17(9): 2244-2251); Although AATrapeznikov et al. found that PBS has shear-thickening behavior, their research was conducted in solution and could not be applied, and they did not conduct further research on the bulk of PBS. Commercially available PBS, also known as "DC3179 expanded compound," exhibits shear-thinning properties in rotational rheology tests (Plant, Daniel et al. Mechanical and rheological testing to develop thermoplastic elastomer-polyborodimethylsiloxane blends for personal impact protection. Polymer Testing 86(2020):106477). Currently, although PBS is referred to as a "shear-thickening / hardening gel," more attention is paid to the modulus changes of PBS at different frequencies or the changes in impact resistance at different impact rates. In fact, according to the fundamentals of rheology, the accurate definition of shear thickening should be the phenomenon that shear viscosity increases with increasing shear rate. The shear-thickening properties of PBS have guiding significance for its applications in sensors and impact-resistant materials. Summary of the Invention

[0004] This invention provides a polyborosiloxane exhibiting shear-thickening behavior and its preparation method. The polyborosiloxane prepared by this invention demonstrates that, under steady-state shear testing, its shear viscosity increases with increasing shear rate, thus achieving shear-thickening.

[0005] This invention provides a method for preparing a polyborosiloxane with shear-thickening behavior, comprising the following steps:

[0006] a) Reaction of terminally dihydroxyl-terminated polydimethylsiloxane with boric acid yields crude polyborosiloxane;

[0007] The reaction is carried out at a temperature of 25–75°C for a time of 1.5–10 hours.

[0008] b) Remove unreacted boric acid from the crude polyborosiloxane to obtain a polyborosiloxane exhibiting shear-thickening behavior.

[0009] Preferably, in step a), the number-average molecular weight of the terminally dihydroxyl-capped polydimethyl silicone oil is 700–20000 g / mol.

[0010] Preferably, the terminally dihydroxyl-terminated polydimethyl silicone oil is selected from one or more of dihydroxyl-terminated polydimethyl silicone oils with a number average molecular weight of 700 g / mol, 2000 g / mol, 3000 g / mol, 5000 g / mol and 20000 g / mol.

[0011] Preferably, in step a), the molar ratio of boric acid to hydroxyl groups in the terminally dihydroxyl-capped polydimethyl silicone oil is (0.5-1.5):1.

[0012] Preferably, step b) specifically includes:

[0013] The crude polyborosiloxane was dissolved in an organic solvent, followed by solid-liquid separation to remove unreacted boric acid. The organic solvent was then removed from the resulting separated liquid to obtain a polyborosiloxane exhibiting shear-thickening behavior.

[0014] Preferably, the organic solvent is an anhydrous solvent.

[0015] Preferably, the anhydrous solvent is selected from at least one of anhydrous tetrahydrofuran, anhydrous chloroform, anhydrous acetone, and anhydrous n-hexane.

[0016] Preferably, the solid-liquid separation method is filtration;

[0017] The filtration membrane used in the filtration process has a pore size of less than 0.25 μm.

[0018] Preferably, the polyborosiloxane with shear thickening behavior obtained in step b) has the following specifications: boron content of 20 to 400 ppm, boron in the ternary borate ester unit accounts for 40 wt% to 65 wt% of the total boron content, and boron in the binary borate ester unit accounts for 20 wt% to 60 wt% of the total boron content.

[0019] The polyborosiloxane with shear-thickening behavior has a viscosity of 20–6000 Pa·s, a relaxation time of 0.4–5 s, and an elastic modulus of 100–5000 Pa.

[0020] The present invention also provides a polyborosiloxane with shear-thickening behavior prepared by the preparation method described in the above technical solution.

[0021] The preparation method provided by this invention controls the molecular weight and reactivity of the terminal hydroxyl silicone oil, regulates the boron content and other structural and relaxation behaviors of the polyborosiloxane, and obtains polyborosiloxanes with low boron content and high boron-to-total-boron ratios in the ternary and diborary esters. This results in polyborosiloxanes exhibiting a region where shear viscosity increases with increasing shear rate under steady-state shear experiments, i.e., shear thickening. The low boron content of the polyborosiloxane in this invention leads to a low content of B:O dynamic coordination bonds per unit volume, providing space for the formation of new B:O dynamic coordination bonds; the high boron-to-total-boron ratio in the ternary and diborary esters provides sufficient association sites for the formation of elastic active sites.

[0022] Experimental results show that the polyborosiloxane prepared by this invention not only has more significant solid-liquid transition characteristics, but also exhibits shear thickening behavior over a wide temperature range of 0–120 °C. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0024] Figure 1 The graph shows the frequency scan test results of the polyborosiloxanes obtained in each embodiment and comparative example.

[0025] Figure 2 The graph shows the steady-state shear test results of the polyborosiloxanes obtained in each embodiment and comparative example;

[0026] Figure 3 The graph shows the steady-state shear test results of the polyborosiloxane obtained in Example 1 at different temperatures. Detailed Implementation

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0028] In this article, the technical features described in an open-ended manner include both closed technical solutions composed of the listed features and open technical solutions that include the listed features.

[0029] As used herein, the term "and / or" includes any and all combinations of one or more of the related listed items.

[0030] In this document, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when a range refers to an integer, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be combined. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0031] In this article, when referring to units for data ranges, if the unit is only followed by the right endpoint, it indicates that the units for the left and right endpoints are the same. For example, 25~75℃ means that the units for the left endpoint "25" and the right endpoint "75" are both in degrees Celsius.

[0032] This document only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0033] This invention provides a method for preparing a polyborosiloxane with shear-thickening behavior, comprising the following steps:

[0034] a) Reaction of terminally dihydroxyl-terminated polydimethylsiloxane with boric acid yields crude polyborosiloxane;

[0035] The reaction is carried out at a temperature of 25–75°C for a time of 1.5–10 hours.

[0036] b) Remove unreacted boric acid from the crude polyborosiloxane to obtain a polyborosiloxane exhibiting shear-thickening behavior.

[0037] The preparation method provided by this invention controls the boron content and relaxation behavior of polyborosiloxane, enabling the polyborosiloxane to exhibit a region where the shear viscosity increases with increasing shear rate under steady-state shear experiments. In this invention, the polyborosiloxane has a low boron content, resulting in a low content of dynamic B:O coordination bonds per unit volume, providing space for the formation of new dynamic B:O coordination bonds. The high proportion of boron in the ternary and diboronate esters provides sufficient association sites for the formation of elastic active sites. Simultaneously, by controlling the molecular weight of the terminal hydroxyl silicone oil, the relaxation behavior of the polyborosiloxane is regulated; the system only exhibits shear thickening behavior when the physical entanglement relaxation mode of the polyborosiloxane is not dominant.

[0038] Regarding step a) :

[0039] a) Reaction of terminally dihydroxyl-terminated polydimethylsiloxane with boric acid yields crude polyborosiloxane.

[0040] In this invention, the terminally dihydroxyl-terminated polydimethyl silicone oil (i.e., terminally dihydroxyl-terminated polydimethylsiloxane) is a polydimethyl silicone oil with hydroxyl-terminated ends, preferably with a number-average molecular weight of 700–20000 g / mol. Specifically, it is preferred to use terminally dihydroxyl-terminated polydimethyl silicone oils within the above molecular weight range, and more preferably one or more of the following: 700 g / mol, 2000 g / mol, 3000 g / mol, 5000 g / mol, and 20000 g / mol. If the molecular weight is too high, the physical entanglement of the system contributes significantly, leading to the relaxation behavior of the synthesized polyborosiloxane being primarily based on physical entanglement. Furthermore, the boron content per unit volume is too low at this point, making it difficult to generate new elastic active sites. This invention controls the molecular weight within the above range and, combined with the control of the reaction degree, can control the boron content and relaxation behavior of the polyborosiloxane, thereby enabling it to possess shear-thickening properties. The present invention does not have any special restrictions on the source of the polydimethyl silicone oil with terminal dihydroxyl end capping, which can be a commercially available product or prepared according to conventional preparation methods in the art.

[0041] In this invention, the source of the boric acid is not particularly limited; it can be a commercially available product.

[0042] In this invention, the molar ratio of boric acid to hydroxyl groups in the polydimethyl silicone oil with terminal dihydroxyl end-capped hydroxyl groups is preferably (0.5-1.5):1, specifically 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, and more preferably 1:1.

[0043] In this invention, the reaction temperature is 25–75°C, specifically 25°C, 28°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, and 75°C, more preferably 25–40°C. If the reaction temperature is too low, the reaction efficiency is low; if the reaction temperature is too high, the diffusion rate of boric acid molecules is too fast, making it difficult to control and achieve the expected boron content and product structure of this invention. In this invention, the reaction time is preferably 1.5–10 h, specifically 1.5 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, and 10 h. If the reaction time is too short, the reaction degree is low, and the boron content is too low; if the reaction time is too long, the reaction degree is too high, and the boron content is too high, failing to achieve the expected product structure of this invention. After the above reaction, crude polyborosiloxane is generated in the system.

[0044] Regarding step b) :

[0045] b) Remove unreacted boric acid from the crude polyborosiloxane to obtain a polyborosiloxane exhibiting shear-thickening behavior.

[0046] In this invention, step b) preferably includes: dissolving the crude polyborosiloxane in an organic solvent, then performing solid-liquid separation to remove unreacted boric acid, and then removing the organic solvent from the resulting separated liquid to obtain polyborosiloxane with shear thickening behavior.

[0047] In this invention, the organic solvent is preferably an anhydrous solvent, more preferably at least one selected from anhydrous tetrahydrofuran, anhydrous chloroform, anhydrous acetone, and anhydrous n-hexane. The use of an anhydrous solvent in this invention serves two purposes: firstly, it prevents the hydrolysis of polyborosiloxane by water in the solvent; secondly, it prevents the dissolution of unreacted boric acid, thus reducing boric acid residue.

[0048] In this invention, the solid-liquid separation method is preferably filtration. In this invention, the pore size of the filter membrane used for filtration is preferably below 0.25 μm, thereby filtering out as much incompletely reacted boric acid as possible from the crude reaction product.

[0049] In this invention, after the above solid-liquid separation, the organic solvent is removed from the resulting separated liquid. The preferred method for removing the organic solvent is rotary evaporation and drying. The drying is preferably vacuum drying. The preferred temperature for vacuum drying is 35–65°C. After the above treatment, a polyborosiloxane exhibiting shear-thickening behavior is obtained.

[0050] Boric acid reacts with terminal hydroxyl silicone oil to produce ternary borate esters, diborary borate esters, and monoborary borate esters. In the ternary borate esters, boron is the central linker, serving as both a chemical and physical cross-linking point. In the diborary borate esters, boron is the physical cross-linking point, forming dynamic B:O coordination bonds and hydrogen bonds between boron hydroxyl groups. Hydrogen bonds can form at the chain ends in the monoborary borate esters. The degree of boric acid reaction, i.e., the boron content, needs to be controlled during the reaction. If the boron content is too high, the boron concentration per unit volume is too high, resulting in excessive chemical and physical cross-linking density, restricting molecular chain movement. If the boron content is too low, the boron concentration per unit volume is low, making association with other chains difficult. Therefore, the boron content needs to be controlled to ensure sufficient mobility for easy association with adjacent chains. Unreacted boric acid was present in the crude product; after purification, the boron content in the system was determined. The resulting polyborosiloxane was a blend of various borate ester structures, and the content of each borate ester structure was statistically analyzed. The total content of ternary borate esters and diborary borate esters should be high. If the total content of ternary borate esters and diborary borate esters is too low, there will not be enough association sites to form elastic active sites.

[0051] The final polyborosiloxane obtained by this invention has the following specifications: boron content of 20–400 ppm, boron in the ternary boronate ester accounting for 40 wt%–65 wt% of the total boron content, and boron in the diborate ester accounting for 20 wt%–60 wt% of the total boron content. It has a low boron content and high ternary and diborate ester contents. The low boron content results in a low content of dynamic B:O coordination bonds per unit volume in the system, providing space for the formation of new B:O dynamic coordination bonds. The high proportion of boron in the ternary and diborate esters provides sufficient association sites for the formation of elastic active sites, enabling the polyborosiloxane to exhibit a region where shear viscosity increases with increasing shear rate under steady-state shear experiments, i.e., shear thickening. This allows it to be used in fields such as impact-resistant protective materials.

[0052] The polyborosiloxane obtained by this invention has a viscosity of 20–6000 Pa·s, a relaxation time of 0.4–5 s, and an elastic modulus of 100–5000 Pa.

[0053] Experimental results show that the polyborosiloxane prepared by this invention not only has more significant solid-liquid transition characteristics, but also exhibits shear thickening behavior over a wide temperature range of 0–120 °C.

[0054] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims of the present invention.

[0055] The raw materials used in the following examples were both self-made and commercially available. The dihydroxyl-terminated polydimethylsiloxanes were purchased from Sigma Aldrich, USA, with number average molecular weights of 700 g / mol, 2000 g / mol, 3000 g / mol, 5000 g / mol, and 20000 g / mol. Boric acid was also purchased from Sigma Aldrich, USA.

[0056] Example 1

[0057] Dihydroxyl-terminated polydimethylsiloxane (molecular weight 2000 g / mol) and boric acid were blended at a hydroxyl molar ratio (boric acid:silicone oil) of 1:1 and reacted at 50 °C for 3 h. The resulting reaction product was then rapidly dissolved in anhydrous n-hexane, unreacted boric acid was removed by filtration, the solvent was removed by rotary evaporation, and the product was then vacuum dried at 80 °C for 12 h to obtain the polyborosiloxane product.

[0058] Example 2

[0059] Dihydroxyl-terminated polydimethylsiloxane (molecular weight 2000 g / mol) and boric acid were blended at a hydroxyl molar ratio (boric acid:silicone oil) of 1:1 and reacted at 28 °C for 4 h. The resulting reaction product was then rapidly dissolved in anhydrous n-hexane, unreacted boric acid was removed by filtration, the solvent was removed by rotary evaporation, and the product was then vacuum dried at 80 °C for 12 h to obtain the polyborosiloxane product.

[0060] Example 3

[0061] Dihydroxyl-terminated polydimethylsiloxane (molecular weight 700 g / mol) and boric acid were blended at a hydroxyl molar ratio (boric acid:silicone oil) of 1:1 and reacted at 28 °C for 2 h. The resulting reaction product was then rapidly dissolved in anhydrous n-hexane, unreacted boric acid was removed by filtration, the solvent was removed by rotary evaporation, and the product was then vacuum dried at 80 °C for 12 h to obtain the polyborosiloxane product.

[0062] Comparative Example 1

[0063] The experiment was carried out according to Example 3, except that the reaction temperature was increased to 120°C.

[0064] Comparative Example 2

[0065] The reaction was carried out according to Example 3, except that the reaction conditions were adjusted to react at 20°C for 1 hour.

[0066] Comparative Example 3

[0067] The experiment was carried out according to Example 1, except that the silicone oil raw material was replaced with dihydroxy-terminated polydimethylsiloxane with a molecular weight of 75000 g / mol, and the reaction conditions were adjusted to react at 60°C for 5 h.

[0068] Product Testing:

[0069] The structure and rheological properties of the polyborosiloxane products obtained in each example and comparative example were characterized. The results are shown in Table 1.

[0070] The test process for steady-state shear behavior is as follows: On the Anton Paar MCR702 rotational rheometer, a Peltier plate temperature control system was used to conduct the test with a parallel plate with a diameter of 25 mm. The temperature was set to 25 °C and the gap was 1 mm.

[0071] Table 1: Structure and performance of products obtained in each embodiment and comparative example

[0072]

[0073] During shearing, the molecular chains of polyborosiloxanes orient themselves. When the degree of reaction in the system is low, new B:O dynamic coordination bonds are formed as the shear rate increases, increasing the number of elastic active sites and exhibiting shear thickening behavior, as shown in Examples 1-3. Among the examples, Example 1 has the highest boron content, the longest relaxation time, the largest elastic modulus, and the highest viscosity, while Example 3 has the lowest. This is because the terminal hydroxyl silicone oils used in the examples have the smallest molecular weight and the highest hydroxyl equivalent per unit volume, resulting in a high boron content per unit volume in the prepared polyborosiloxane. Comparative Example 1 has a high reaction temperature and a high degree of reaction, resulting in a high boron content. At this time, the density of elastic active sites in the system is high, and under shearing, new B:O dynamic coordination bonds cannot be formed. Instead, the existing B:O dynamic coordination bonds are broken, resulting in shear thinning. Comparative Example 2 had a low reaction temperature and short reaction time, resulting in a very low degree of reaction. Therefore, the boron content in the system was low, and the sample did not form a gel, making it impossible to characterize its relaxation time and elastic modulus. The system contained a large amount of unreacted silicone oil, with large distances between molecular chains, making it difficult to form dynamic B:O coordination bonds, thus exhibiting Newtonian fluid behavior. Comparative Example 3 used a hydroxyl-terminated silicone oil with a molecular weight much higher than the entanglement molecular weight, resulting in entanglement in the system. Under shearing, the B:O dynamic coordination bonds in the system were broken before the molecular chains were oriented. Furthermore, due to the large molecular weight, there were few hydroxyl reaction sites per unit volume, and the boron content was also very low. Under shearing, new elastic active sites formed in the system, thus exhibiting shear-thinning.

[0074] The frequency scanning results of each embodiment and comparative example are as follows: Figure 1 As shown, the polyborosiloxanes of Examples 1-3 exhibiting shear-thickening behavior also demonstrate more significant solid-liquid transition characteristics. The steady-state shear results for each example and comparative example are as follows: Figure 2 As shown, Examples 1-3 exhibited significant shear thickening, Comparative Examples 1 and 3 showed shear thinning, and Comparative Example 2 exhibited Newtonian fluid behavior. The polyborosiloxane of Example 1 showed shear thickening throughout the temperature range of 0℃-120℃, as... Figure 3 As shown.

[0075] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of these embodiments are merely to aid in understanding the method and core ideas of the present invention, including the best mode, and to enable any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that those skilled in the art can make various improvements and modifications to the present invention without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims. The scope of protection of this patent is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements similar to those expressed in the claims, or if they include equivalent structural elements that are not substantially different from those expressed in the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. A method for preparing a polyborosiloxane with shear-thickening behavior, characterized in that, Includes the following steps: a) Reaction of terminally dihydroxyl-terminated polydimethylsiloxane with boric acid yields crude polyborosiloxane; The reaction is carried out at a temperature of 25~60℃ for a time of 1.5~10h. The number-average molecular weight of the terminally dihydroxyl-capped polydimethyl silicone oil is 2000~5000 g / mol; b) Dissolve the crude polyborosiloxane in an organic solvent, then perform solid-liquid separation to remove unreacted boric acid, and then remove the organic solvent from the resulting separated liquid to obtain a polyborosiloxane with shear thickening behavior. The solid-liquid separation method is filtration; the filtration uses a filter membrane with a pore size of less than 0.25 μm; The polyborosiloxanes with shear-thickening behavior obtained in step b) have the following specifications: boron content 20~400ppm, boron in the ternary borate ester unit accounts for 40wt%~65wt% of the total boron content, and boron in the binary borate ester unit accounts for 20wt%~60wt% of the total boron content; The shear thickening refers to the region in a steady-state shear experiment where the shear viscosity increases with increasing shear rate.

2. The preparation method according to claim 1, characterized in that, In step a), the molar ratio of boric acid to hydroxyl groups in the terminally dihydroxyl-capped polydimethyl silicone oil is (0.5~1.5):

1.

3. The preparation method according to claim 1, characterized in that, The organic solvent is an anhydrous solvent.

4. The preparation method according to claim 3, characterized in that, The anhydrous solvent is selected from at least one of anhydrous tetrahydrofuran, anhydrous chloroform, anhydrous acetone, and anhydrous n-hexane.

5. The preparation method according to claim 1, characterized in that, The polyborosiloxane with shear-thickening behavior has a viscosity of 20~6000 Pa·s, a relaxation time of 0.4~5s, and an elastic modulus of 100~5000 Pa.

6. A polyborosiloxane with shear-thickening behavior prepared by any one of claims 1 to 5.

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