Method for determining molecular weight of natural rubber

Abstract

The invention discloses a natural rubber molecular weight determination method, and belongs to the technical field of analysis and detection.The method comprises the steps that fresh natural rubber latex is subjected to deproteinization treatment through a urea aqueous solution and a lauryl sodium sulfate solution, then ester exchange reaction is conducted, and deproteinized and degreased natural rubber is obtained; and the molecular weight of the deproteinized and degreased natural rubber is determined by adopting a gel permeation chromatography, so that the content of insoluble gel is effectively reduced, the polymer is completely dissolved, and the detection result is more accurate and stable.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of analytical detection, and particularly relates to a natural rubber molecular weight determination method. BACKGROUND

[0002] Natural rubber (NR) is a natural macromolecule polymer with cis-1,4-polyisoprene as the main component, in which the polyisoprene content is as high as about 94%, and the non-rubber components such as protein, lipid, sugar and ash content is about 6%. The natural rubber molecules not only have a unique terminal structure, but also interact with protein, phospholipid and metal ions through ionic bonds and hydrogen bonds, cross-linking the linear natural rubber molecular chains to form a supramolecular network aggregate structure. This unique molecular network enables natural rubber to have excellent stretch-induced crystallization ability, making it not only have high strength, high elasticity and high tear resistance, but also exhibit comprehensive performance advantages such as fatigue resistance, low heat generation, modulus stability at high temperature, long service life and low creep. These characteristics make natural rubber irreplaceable in high-end manufacturing fields such as aviation tires, rail transportation, tank load wheels and air springs.

[0003] The molecular weight of natural rubber is closely related to the mechanical properties such as rubber elastic modulus and tensile strength, and is an important parameter for characterizing the processing performance of NR. Decades ago, thermal field flow fractionation technology has been used to analyze the molecular weight of polymers, especially for polymers with a molecular weight exceeding one million. Rubber processing analyzer (RPA) has become a common method for molecular weight analysis by analyzing the dynamic characteristics and rheological properties of NR. However, due to the time-consuming sample preparation process, RPA is not suitable for high-throughput applications. Although thermal pyrolysis gas chromatography mass spectrometry can accurately determine the molecular weight and content of NR, it has high detection cost and relies on professional spectral data analysis, and the requirements for instruments and operators are high, making it difficult to promote. Currently, gel permeation chromatography (GPC) technology has become a common method for NR molecular weight analysis due to its simple operation. However, when using GPC to determine the molecular weight of natural rubber, due to the lack of effective sample pretreatment, the protein and phospholipid in natural rubber are easily cross-linked with natural rubber to form a large amount of insoluble gel, affecting the solubility of the polymer, leading to a decrease in the accuracy and stability of the determination results, and also may cause the chromatographic column to be blocked, which restricts the wide application of GPC. Therefore, in the field of NR molecular weight research, it is urgent to establish a stable, efficient and accurate natural rubber sample preparation and molecular weight determination method. SUMMARY

[0004] To solve the above technical problems, the application provides a natural rubber molecular weight determination method, which can effectively reduce the protein and phospholipid in the natural rubber, remove the insoluble gel therein, increase the polymer solubility, accurately measure the NR molecular weight and distribution in the latex, reduce the measurement error, and provide reliable technical reference for NR biosynthesis and identification processing research.

[0005] To achieve the above object, the application provides the following technical scheme.

[0006] The application provides a natural rubber molecular weight determination method, which comprises the following steps.

[0007] Urea aqueous solution and sodium dodecyl sulfate (SDS) solution are added to fresh natural rubber latex, washed with water, and dried to obtain deproteinized natural rubber;

[0008] The deproteinized natural rubber is added to an organic solvent to obtain a natural rubber solution;

[0009] Sodium and methanol are added to the natural rubber solution to perform an ester exchange reaction in the dark;

[0010] After the ester exchange reaction is completed, the reaction solution is neutralized to neutral by using HCl-methanol solution, and is left to separate into layers; the upper layer solution is dropped into methanol to coagulate, and is washed and pressed into a tablet and then dried to obtain deproteinized and deesterified natural rubber (TE-DPNR); the deproteinized and deesterified natural rubber is dissolved in methanol, and the methanol is repeatedly used for the coagulation and drying steps to prepare a natural rubber sample;

[0011] The molecular weight of the natural rubber sample is determined by using a gel permeation chromatography (GPC) method;

[0012] In the gel permeation chromatography method, the chromatographic column is a tetrahydrofuran gel chromatographic column Styragel HR 5, and the mobile phase is tetrahydrofuran (THF).

[0013] The application establishes a new method for solving the technical limitations of the existing GPC method for determining the molecular weight of natural rubber. The core of the application is the pretreatment technology of the natural rubber latex. The application effectively removes the protein and phospholipid in the natural rubber sample through the deproteinization and deesterification reaction, solves the problem that the protein and phospholipid form insoluble gel by crosslinking with the natural rubber, and solves the problems of poor sample solubility and easy plugging of the GPC gel column, so that the molecular weight and distribution of the natural rubber can be stably, efficiently and accurately determined. THF has the advantages of good solubility for natural rubber, low viscosity, suitable boiling point (for controlling the operation temperature), low refractive index, low toxicity and high cost performance, and therefore, THF is selected as the mobile phase for determining the molecular weight of the natural rubber by using the GPC method. Since the mobile phase must be chemically compatible with the chromatographic column filler, the tetrahydrofuran gel is selected as the stationary phase.

[0014] Optionally, the organic solvent is toluene.

[0015] Further, the concentration of the urea aqueous solution is 0.1 wt%;

[0016] Further, the concentration of the sodium dodecyl sulfate solution is 1 wt%;

[0017] Further, after adding the urea aqueous solution and the sodium dodecyl sulfate solution, the concentration of the fresh natural rubber latex is 30 wt%.

[0018] Further, the step of obtaining the deproteinized natural rubber comprises: adding the urea aqueous solution and the sodium dodecyl sulfate solution into the fresh natural rubber latex, stirring and reacting, centrifuging for the first time, and taking the upper rubber phase; adding the sodium dodecyl sulfate solution into the upper rubber phase, stirring uniformly, centrifuging for the second time, taking the upper rubber phase again, washing with water, and drying to obtain the deproteinized natural rubber.

[0019] Further, the temperature of the stirring and reaction is 30℃, and the time of the stirring and reaction is 1 h.

[0020] Further, the relative centrifugal force of the first centrifugation is 22500 g, and the time of the first centrifugation is 30 min.

[0021] Further, the relative centrifugal force of the second centrifugation is 22500 g, and the time of the second centrifugation is 30 min.

[0022] Further, the concentration of the natural rubber solution is 2.5 wt%.

[0023] Further, the time of the transesterification reaction is 3 h.

[0024] Further, the ratio of the natural rubber, the sodium, and the methanol in the natural rubber solution is 1 g:0.3 g:7.5 mL.

[0025] Further, the concentration of the HCl-methanol solution is 4 wt%.

[0026] Further, in the gel permeation chromatography, the flow rate of the mobile phase is 0.8~1.2 mL / min, the column oven and the detector temperature are both 35~45℃, and the sample injection volume is 150~250 μL.

[0027] Further, the detector is a differential refractive index detector, and the sensitivity thereof is 3~5.

[0028] Further, when the gel permeation chromatograph is used for detection, the method further comprises the step of dissolving the natural rubber sample in the mobile phase and filtering with a filter membrane.

[0029] Compared with the prior art, the present application has the following advantages and technical effects:

[0030] The method of the present application can detect the molecular weight and its distribution of natural rubber with a weight average molecular weight ranging from 56,600 to 2330,000. After the natural rubber sample is treated by deproteinization and ester exchange reaction, the content of insoluble gel is effectively reduced, the polymer is completely dissolved, and the detection result is more accurate and stable. BRIEF DESCRIPTION OF DRAWINGS

[0031] The accompanying drawings, which form a part of the present application, are used to provide further understanding of the present application, and serve as an aid in explaining the exemplary embodiments of the present application and their descriptions, and do not constitute improper limitations on the present application. In the drawings:

[0032] Figure 1 is a gel chromatogram of polystyrene standard, in which the long dotted curve is, from left to right, standard peaks (740,000; 56,600) and solvent peaks; the short dotted curve is, from left to right, standard peaks (1700,000; 129,000) and solvent peaks; and the solid curve is, from left to right, standard peaks (2330,000; 321,000) and solvent peaks;

[0033] Figure 2 is a polystyrene standard calibration curve, in which Mw is the weight average molecular weight. DETAILED DESCRIPTION

[0034] Various exemplary embodiments of the present application will now be described in detail, which should not be considered as limiting the present application, but should be understood as a more detailed description of certain aspects, characteristics and embodiments of the present application.

[0035] It should be understood that the terms described in the present application are only for describing the specific embodiments, and are not used to limit the present application. In addition, for the numerical range in the present application, it should be understood that each intermediate value between the upper limit and the lower limit of the range is also specifically disclosed. Each smaller range between any stated value or stated range and any other stated value or intermediate value within the stated range is also included in the present application. The upper limit and the lower limit of these smaller ranges can be independently included or excluded from the range.

[0036] 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 the present application pertains. Although preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In the case of conflict between any document incorporated by reference and the present specification, the present specification will control.

[0037] Many modifications and variations of the present application described in the specification are possible without departing from the scope or spirit of the application. Other implementations of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. The specification and examples are illustrative only.

[0038] As used herein, the terms "comprises", "comprising", "includes", "including", "has", "having" or variants thereof are open-ended, and specifically mean including, but not limited to.

[0039] The embodiment of the present application provides a natural rubber molecular weight determination method, comprising the following steps:

[0040] Urea aqueous solution and sodium dodecyl sulfate (SDS) solution are added to fresh natural rubber latex for deproteinization to obtain deproteinized natural rubber;

[0041] The deproteinized natural rubber is added into an organic solvent to obtain a natural rubber solution;

[0042] Sodium and methanol are added to the natural rubber solution, and an ester exchange reaction is carried out in the dark;

[0043] After the ester exchange reaction is completed, the reaction solution is neutralized to neutral by using an HCl-methanol solution, and is left to separate into layers; the upper layer solution is dropped into methanol for coagulation, and after washing and tabletting, the deproteinized and defatted natural rubber (TE-DPNR) is dried to obtain a natural rubber sample;

[0044] The molecular weight of the natural rubber sample is determined by using a gel permeation chromatography method;

[0045] In the gel permeation chromatography method, the chromatographic column is a tetrahydrofuran gel chromatographic column Styragel HR 5, and the mobile phase is tetrahydrofuran.

[0046] The GPC chromatographic column selected in the present application is a tetrahydrofuran column, and considering the compatibility of the chromatographic column and the mobile phase, tetrahydrofuran is selected as the mobile phase. If other incompatible mobile phases such as n-hexane or toluene (both of which can be used as solvents for natural rubber, but n-hexane has poor solubility and toluene has high toxicity) are used, the chromatographic column is easily damaged.

[0047] In the preferred embodiment of the present application, the organic solvent is selected from toluene.

[0048] In the preferred embodiment of the present application, the concentration of the urea aqueous solution is 0.1 wt%; the concentration of the sodium dodecyl sulfate solution is 1 wt%; and after the urea aqueous solution and the sodium dodecyl sulfate solution are added, the concentration of the fresh natural rubber latex is 30 wt%.

[0049] In the preferred embodiment of the present application, the concentration adjustment of fresh natural rubber latex is calculated according to the formula C1 x V1 = C2 x V2, wherein: C1 represents the starting latex concentration (DRC %); C2 represents the final latex concentration (30 wt %); V1 represents the starting latex volume (mL); and V2 represents the final latex volume (mL).

[0050] In the preferred embodiment of the present application, the step of obtaining deproteinized natural rubber is: adding a urea aqueous solution and a sodium dodecyl sulfate solution to the fresh natural rubber latex, stirring and reacting, first centrifuging, and taking the upper rubber phase; adding a sodium dodecyl sulfate solution to the upper rubber phase, stirring uniformly, second centrifuging, taking the upper rubber phase again, washing with water, and drying to obtain the deproteinized natural rubber.

[0051] In the preferred embodiment of the present application, the temperature of the stirring and reaction is 30℃, the time of the stirring and reaction is 1 h, the relative centrifugal force of the first centrifuging is 22500 g, and the time of the first centrifuging is 30 min; the relative centrifugal force of the second centrifuging is 22500 g, and the time of the second centrifuging is 30 min.

[0052] In the preferred embodiment of the present application, the concentration of the natural rubber solution is 2.5 wt %.

[0053] In the preferred embodiment of the present application, the time of the ester exchange reaction is 3 h.

[0054] In the preferred embodiment of the present application, the ratio of the natural rubber, sodium, and methanol in the natural rubber solution is 1 g: 0.3 g: 7.5 mL.

[0055] In the preferred embodiment of the present application, the concentration of the HCl-methanol solution is 4 wt %.

[0056] In the preferred embodiment of the present application, in the gel permeation chromatography, the flow rate of the mobile phase is 0.8-1.2 mL / min, the column oven and detector temperatures are both 35-45℃, and the sample injection volume is 150-250 μL.

[0057] In the preferred embodiment of the present application, the detector is a differential refractive index detector, the sensitivity of which is 3-5, and more preferably 5.

[0058] Temperature is a key factor affecting the stability and accuracy of GPC test results; the temperature of the column oven directly affects the solvent viscosity, flow rate stability, sample solubility, and column performance; the differential refractive index detector is extremely sensitive to temperature changes, and its temperature must be consistent with the mobile phase and column temperature to avoid baseline drift and signal distortion caused by thermal expansion and contraction, ensuring the stability and accuracy of the signal. The selection of the sensitivity range of the differential refractive index detector is based on the molecular weight distribution characteristics of natural rubber samples, and the operating conditions such as flow rate and temperature are optimized comprehensively, aiming to balance the detection limit, signal-to-noise ratio, and peak shape and resolution of the chromatographic peak. When applied to the molecular weight test analysis of different varieties of natural rubber samples, it has the characteristics of high accuracy (> 98%) and good reproducibility (RSD < 5%).

[0059] In the preferred embodiment of the present application, when detected by a gel permeation chromatograph, it further comprises the step of dissolving the natural rubber sample in the mobile phase and filtering with a filter membrane. For example, 10 mg of deproteinized and defatted sample after drying is dissolved in 10 mL of tetrahydrofuran, stirred until the sample is completely dissolved, filtered with a 0.45 μm polytetrafluoroethylene (PTFE) filter membrane, and 200 μL of the sample is detected by a gel permeation chromatograph. The retention time of the sample peak is substituted into the calibration curve equation to obtain the relative molecular mass of each peak, and the relative percentage of different molecular weight ranges can be calculated by area normalization method.

[0060] The term "fresh natural rubber latex" is a milky white viscous liquid flowing from the rubber tree tapping hole, which is the first raw material for manufacturing natural rubber and latex products.

[0061] Unless otherwise specified, the room temperature in the present application is 25±2℃.

[0062] The raw materials used in the embodiments of the present application are all obtained by commercial purchase.

[0063] It should be noted that the details not described in the present application are all conventional operating means in the art, and are not the focus of the present application, for example, the specific method of tabletting is completed by conventional method.

[0064] The technical solutions of the present application are further illustrated by the following examples.

[0065] Example 1 Determination of the molecular weight of defatted and deproteinized natural rubber (TE-DPNR)

[0066] 1. Chromatographic conditions

[0067] The chromatographic column was selected as Styragel HR 5, the front-end protection column was STYRAGEL INT’L HAZA; the mobile phase was tetrahydrofuran, the flow rate was 1.0 mL / min; the sample injection volume was 200 μL; the column oven and the differential refractive index detector temperature were both 40 °C, and the sensitivity of the differential refractive index detector was 4.

[0068] 2. Deproteinization treatment

[0069] 2.1 Determination of dry rubber content (DRC)

[0070] 1 g of fresh rubber latex of Hevea brasiliensis was weighed into a glass dish, lightly shaken to cover the bottom of the dish, 200 μL of 5% (v / v) glacial acetic acid solution was added, shaken well, and placed in a 50 °C ± 5 °C oven for drying after the latex coagulated. After cooling to room temperature in a desiccator, it was weighed. The drying, cooling and weighing operations were repeated until the difference between the two consecutive weighings was less than 0.5 mg. The obtained sample had a dry rubber content of 41%. The sample was stored in a desiccator for subsequent testing.

[0071] 2.2 Deproteinization reaction

[0072] The fresh rubber latex (referred to as fresh latex) was adjusted to a concentration of 30wt% with 0.1wt% urea aqueous solution and 1wt% SDS solution. The specific steps were as follows: according to the formula C1×V1 = C2×V2, the required solution volume was calculated, wherein: C1 represents the starting latex concentration (DRC%); C2 represents the final latex concentration (30wt%); V1 represents the starting latex volume (mL); V2 represents the final latex volume (mL). In this example, C1 was 41%, and 100 mL of fresh latex (V2) was used for deproteinization reaction, and V1 was calculated to be 73 mL according to the formula. After adding 13.5 mL of 1wt% SDS solution to the fresh latex, it was mixed well with a magnetic stirrer, and then 13.5 mL of 0.1wt% urea aqueous solution was added. The reaction was stirred at 30 °C for 1 h with a magnetic stirrer. The reacted latex solution was poured into a centrifuge tube and centrifuged at 22500 g for 30 min. The upper rubber phase was taken and added with 17 mL of 0.5wt% SDS solution, which was stirred uniformly at room temperature. The upper rubber phase was taken and added with 100 mL of water, which was stirred uniformly and then spread on a glass dish. After drying in a 50 °C oven for 24 h, the latex became transparent, and deproteinized natural rubber was obtained.

[0073] 3. Transesterification treatment

[0074] 3.1 Preparation of rubber solution

[0075] Take 2.5 g of the above deproteinized natural rubber and dissolve it in 100 mL of toluene, then soak overnight, and then stir for 3 days until the sample is completely dissolved, to obtain a rubber solution with a concentration of 2.5wt%.

[0076] 3.2 Preparation of sodium methoxide

[0077] Cut the sodium metal soaked in kerosene into pieces and immerse it in excess toluene. Use tweezers to take out the sodium metal pieces from the toluene, quickly absorb the excess toluene on the surface with filter paper, then add 4 g of sodium metal to 100 mL of methanol one piece at a time to obtain a 4wt% sodium methoxide solution.

[0078] 3.3 Transesterification reaction

[0079] According to the ratio of rubber (g) : sodium metal (g) : methanol (mL) = 1 : 0.3 : 7.5, take 18.75 mL of 4wt% sodium methoxide solution into a separatory funnel, and pour 100 mL of 2.5wt% rubber solution into a beaker covered with a black bag; add the sodium methoxide solution to the rubber solution at a rate of 1 mL / min, stir in the dark for 3 h; after the reaction is completed, neutralize the rubber solution by adding 10% (v / v) HCl-methanol solution dropwise, immediately detect the pH value of the solution with pH paper after adding each time, until the pH value = 7, and let it stand to separate naturally; take 3 mL of the upper rubber solution each time and stir it in methanol, and collect the rubber as soon as it solidifies and place it in a petri dish. Then rinse the solidified rubber with methanol, press the rubber with a spatula until the rubber is completely solidified in methanol, and dry it in a 50°C oven. Dissolve the dried rubber in methanol at a ratio of 2.5% (w / v), solidify it in methanol again, and dry it to obtain a deproteinized and transesterified natural rubber sample (TE-DPNR, which can also be referred to as defatted and deproteinized natural rubber).

[0080] 3.4 Molecular weight determination

[0081] Take 10 mg of the deproteinized and transesterified natural rubber sample and dissolve it in 10 mL of tetrahydrofuran, stir until the sample is completely dissolved, filter with a 0.45 μm polytetrafluoroethylene (PTFE) filter membrane, and take 200 μL of the sample for detection with a gel permeation chromatograph. Record the peak area, calculate the regression equation and the molecular weight and molecular weight distribution of the detected sample with GPC software. The calculation formula is: regression equation LgMw = a + bx + cx 2 +dx 3 ; D = Mw / Mn; Lg is the common logarithm, a, b, c, and d are constants, which are 20.863, -4.5552, 0.4998, and -0.0203, respectively, x is the retention time tR (min), Mw is the weight average molecular weight, and Mn is the number average molecular weight.

[0082] Method validation

[0083] 1. Preparation of calibration curve

[0084] Ten mg of each of six polystyrene standards with different relative molecular mass (2330,000, 1700,000, 740,000, 321,000, 129,000 and 56,600, respectively) were weighed and dissolved in 10 mL of chromatographic grade tetrahydrofuran, respectively. After complete dissolution, the solution was filtered with a 0.45 μm polytetrafluoroethylene (PTFE) filter membrane to obtain a standard solution for gel permeation chromatography analysis to obtain a polystyrene standard gel chromatogram (Fig. 1). Figure 1 The standard samples of different relative molecular mass eluted in order according to the size of the molecular weight, and were well separated in the chromatographic system. A third-order calibration curve was established using Breeze2 software, with the retention time t R (min) of the standard substance as the abscissa and the logarithmic value of the molecular weight (Mw) as the ordinate. Linear regression calculation was performed to obtain the calibration curve equation LgMw=20.863-4.5552x+ 0.4998x 2 -0.0203x 3 , where Mw is the weight average relative molecular mass, and R is 0.99995 (Fig. 2). Figure 2 The linear relationship between the logarithm of the weight average molecular weight of the control sample and the retention time was good. The calibration curve drawn was suitable for determining natural rubber with Mw in the range of 56,600-2330,000.

[0085] 2. Instrument precision test

[0086] The control sample solution with a weight average molecular weight of 740,000 was continuously injected for 6 times, and the weight average molecular weight of each injection was measured as 741,135, 742,485, 740,981, 736,520, 738,521 and 745,212, respectively, with RSD=0.10%, indicating that the instrument precision was good.

[0087] 3. Solution stability test

[0088] The rubber tree with an age of 8 years was used as the test material, and the defatted and deproteinized natural rubber solution was taken. The solution was injected into the needle at 0, 3, 6, 9 and 12 h according to the chromatographic conditions, and the weight average molecular weight of each time was measured as 7810,021, 7825,010, 7814,947, 7800,096, 7815,619 and 7805,028, respectively, with RSD=0.11%, indicating that the test rubber solution had good stability within 12 h.

[0089] 4. Reproducibility test

[0090] The defatted and deproteinized natural rubber solution was prepared by using 6 parts of the defatted and deproteinized natural rubber solution, and the molecular weight and dispersion coefficient were determined according to the chromatographic conditions. The results are shown in Table 1.

[0091] Table 1 Results of the repeatability test of the molecular weight of the rubber solution

[0092]

[0093] The relative standard deviations (RSD) of Mw, Mn and the dispersion coefficient in the results of Table 1 were 0.42%, 0.33% and 0.65% respectively, all less than 1%, indicating that the test repeatability was good.

[0094] Comparative Example 1 Determination of the molecular weight of the deproteinized natural rubber (DPNR)

[0095] 1. Chromatographic conditions

[0096] The same as in Example 1.

[0097] 2. Deproteinization treatment

[0098] The same as in Example 1.

[0099] 3. Determination of the molecular weight

[0100] 10 mg of the deproteinized sample (DPNR) obtained in step 3 after drying was dissolved in 10 mL of tetrahydrofuran, and stirred until the sample was completely dissolved. The sample was filtered with a 0.45 μm PTFE filter membrane, and 200 μL of the sample was detected with a gel permeation chromatograph. The peak area was recorded, and the regression equation and the molecular weight and molecular weight distribution of the sample were calculated by GPC software.

[0101] Comparative Example 2 Determination of the molecular weight of the deproteinized natural rubber (TENR)

[0102] 1. Chromatographic conditions

[0103] The same as in Example 1.

[0104] 2. Transesterification treatment

[0105] 2.1 Determination of the dry rubber content (DRC)

[0106] The same as in Example 1.

[0107] 2.2 Preparation of the rubber solution

[0108] 2.5 g of the dried natural rubber latex sample was weighed, dissolved in 100 mL of toluene, and soaked overnight, then stirred for 3 days until the sample was completely dissolved, to obtain a rubber solution with a concentration of 2.5 wt%.

[0109] 2.3 Preparation of sodium methoxide

[0110] The same as Example 1.

[0111] 2.4 Transesterification reaction

[0112] The same as Example 1, except that the rubber solution is a rubber solution without deproteinization treatment.

[0113] 2.5 Molecular weight determination

[0114] 10 mg of the transesterification reaction sample (TENR) after drying was weighed and dissolved in 10 mL of tetrahydrofuran, stirred until the sample was completely dissolved, filtered with a 0.45 μm polytetrafluoroethylene (PTFE) filter membrane, and 200 μL of the sample was taken for detection by gel permeation chromatography. The peak area was recorded, and the regression equation and the molecular weight and molecular weight distribution of the sample were calculated by GPC software.

[0115] The molecular weight and dispersity coefficient of the control group (NR) without previous treatment, the deproteinized defatted natural rubber (TE-DPNR) in Example 1, the deproteinized sample (DPNR) in Comparative Example 1, and the transesterification reaction sample (TENR) in Comparative Example 2 were measured by gel permeation chromatography, and the results are shown in Table 2.

[0116] Table 2 Molecular weight and dispersity coefficient determined by different methods

[0117]

[0118] As can be seen from Table 2, the Mw, Mn and standard deviation of the dispersity coefficient of the deproteinized defatted natural rubber (TE-DPNR) are smaller than those of the control group (NR) without previous treatment, the deproteinized sample (DPNR) and the transesterification reaction sample (TENR) in Comparative Example 2, indicating that the TE-DPNR method has the best repeatability in determining the molecular weight of rubber, and the solubility of rubber has the least effect on the molecular weight of rubber.

[0119] Example 2

[0120] The same as Example 1, except that the flow rate of the mobile phase is 0.8 mL / min, the injection volume is 150 μL, the column oven and the differential refractive index detector are both at a temperature of 35°C, and the sensitivity of the differential refractive index detector is 3.

[0121] The GPC results show that the Mn is 120487±2342, the Mw is 762135±3892, and the dispersity coefficient is 6.33±0.13.

[0122] Example 3

[0123] The same as Example 1, except that the flow rate of the mobile phase was 1.2 mL / min, the injection volume was 250 μL, the column oven and refractive index detector temperatures were both 45°C, and the sensitivity of the refractive index detector was 5.

[0124] The GPC results showed Mn to be 122850 ± 2615, Mw to be 770284 ± 3415, and the dispersity to be 6.27 ± 0.14.

[0125] Example 4

[0126] The same as Example 1, except that the chromatographic column was a tetrahydrofuran gel chromatographic column Styragel HR 5, the pre-column guard was STYRAGEL INT'L HAZA; the mobile phase was tetrahydrofuran, the flow rate was 1.2 mL / min; the column oven and detector temperatures were both 35°C, the sensitivity of the refractive index detector was 4; and the injection volume was 250 μL.

[0127] The GPC results showed Mn to be 121247 ± 2728, Mw to be 763892 ± 3827, and the dispersity to be 6.30 ± 0.15.

[0128] Example 5

[0129] The same as Example 1, except that the chromatographic column was a tetrahydrofuran gel chromatographic column Styragel HR 5, the pre-column guard was STYRAGEL INT'L HAZA; the mobile phase was tetrahydrofuran, the flow rate was 1.2 mL / min; the column oven and detector temperatures were both 42°C, the sensitivity of the refractive index detector was 3; and the injection volume was 150 μL.

[0130] The GPC results showed Mn to be 123518 ± 2485, Mw to be 768145 ± 3624, and the dispersity to be 6.22 ± 0.13.

[0131] Comparative Example 3

[0132] The same as Example 1, except that the chromatographic column was a tetrahydrofuran gel chromatographic column Styragel HR 4.

[0133] Comparative Example 4

[0134] The same as Example 1, except that the chromatographic column was a tetrahydrofuran gel chromatographic column Styragel HR 6.

[0135] The weight average molecular weight detection range of the chromatographic column Styragel HR 4 used in the present application Comparative Example 3 is 5,000-600,000, while natural rubber is a high molecular compound, and the molecular weight is mostly greater than 600,000, which exceeds the upper limit of the detection of the column, so the Styragel HR 4 can only detect the rubber molecular weight of a small number of experimental materials. The weight average molecular weight detection range of the chromatographic column Styragel HR 6 used in Comparative Example 4 is 2,000,000-10,000,000, which covers the molecular weight range of some natural rubber samples, but the chromatographic separation resolution is insufficient, which easily leads to peak overlapping. The weight average molecular weight detection range of the chromatographic column Styragel HR 5 used in Example 1 is 50,000-4,000,000, and the molecular weight detection range is suitable for most natural rubber samples. Compared with the Styragel HR 6 chromatographic column, the column aperture distribution of the column Styragel HR 5 chromatographic column is narrower, which effectively improves the separation degree of the high molecular weight component and the low molecular weight component, significantly improves the chromatographic peak separation effect, and avoids the problem of chromatographic peak overlapping.

[0136] The above merely describes the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method for determining the molecular weight of natural rubber, characterized by, The method comprises the following steps: adding urea aqueous solution and sodium dodecyl sulfate solution into fresh natural rubber latex to remove protein and obtain deproteinized natural rubber; adding the deproteinized natural rubber into an organic solvent to obtain a natural rubber solution; adding sodium and methanol into the natural rubber solution to perform an ester exchange reaction in the dark; after the ester exchange reaction is completed, neutralizing to neutral with HCl-methanol solution, standing and separating, taking the upper solution to drop into methanol to coagulate, and after washing and tabletting, drying to obtain defatted and deproteinized natural rubber; dissolving the defatted and deproteinized natural rubber in methanol, and repeating the coagulation and drying steps to prepare a natural rubber sample; determining the molecular weight of the natural rubber sample by gel permeation chromatography; in the gel permeation chromatography, the chromatographic column is a tetrahydrofuran gel chromatographic column Styragel HR 5, the mobile phase is tetrahydrofuran, the flow rate of the mobile phase is 0.8-1.2 mL / min, the column oven and detector temperatures are both 35-45℃, and the sample injection volume is 150-250 μL.

2. The method of claim 1, wherein the natural rubber is a cis-1,4-polyisoprene. The concentration of the urea aqueous solution is 0.1wt%. And / or, the concentration of the sodium dodecyl sulfate solution is 1wt%. And / or, after adding the urea aqueous solution and the sodium dodecyl sulfate solution, the concentration of the fresh natural rubber latex is 30wt%.

3. The method of claim 1, wherein the natural rubber is a natural rubber latex. The step of obtaining the deproteinized natural rubber is: adding urea aqueous solution and sodium dodecyl sulfate solution into fresh natural rubber latex, stirring and reacting, centrifuging for the first time to take the upper rubber phase; adding sodium dodecyl sulfate solution into the upper rubber phase, stirring uniformly, centrifuging for the second time, taking the upper rubber phase again, washing with water, and drying to obtain the deproteinized natural rubber.

4. The method of claim 3, wherein the natural rubber is a natural rubber latex. The stirring and reacting temperature is 30℃, and the stirring and reacting time is 1 h. And / or, the relative centrifugal force of the first centrifugation is 22500 g, and the time of the first centrifugation is 30 min. And / or, the relative centrifugal force of the second centrifugation is 22500 g, and the time of the second centrifugation is 30 min.

5. The method of claim 1, wherein the natural rubber is a natural rubber latex. The concentration of the natural rubber solution is 2.5wt%.

6. The method of claim 1, wherein the natural rubber is a natural rubber latex. The ester exchange reaction time is 3 h.

7. The method of claim 1, wherein the natural rubber is a natural rubber latex. The ratio of natural rubber, sodium and methanol in the natural rubber solution is 1 g:0.3 g:7.5 mL.

8. The method of claim 1, wherein the natural rubber is a natural rubber latex. The concentration of the HCl-methanol solution is 4wt%.

9. The method of claim 1, wherein the natural rubber is a natural rubber latex. When detected by a gel permeation chromatograph, the step of dissolving the natural rubber sample in a mobile phase and filtering with a filter membrane is further included.

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