Method for analyzing natural rubber

Abstract

To provide a method for analyzing a natural rubber which can make an accurate separation, and a method for analyzing a natural rubber which can analyze the branch probability, the abundance ratio, and the molecular amounts of components with different branch degrees.SOLUTION: The present invention relates to a method for analyzing a natural rubber which can analyze a natural rubber by using a field flow fractionation device of a thermal flow type to which a multi-angle Light scattering detector and an evaporative light scattering detector are connected. The present invention also relates to a method for analyzing a natural rubber which performs a fitting analysis on the assumption that the branch structure of a natural rubber is a random 4-branch and calculates at least one selected from the group of the branch probability, the abundance ratio, and the molecular amounts of components with different branch degrees.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for analyzing natural rubber. [Background technology]

[0002] For example, natural rubber has traditionally been analyzed by dissolving a natural rubber sample in an organic solvent such as tetrahydrofuran (THF), filtering out impurities such as gel fraction that is insoluble in THF, and measuring the molecular weight distribution by gel permeation chromatography (GPC) (see Patent Documents 1-2, etc.). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2012-122796 A [Patent Document 2] JP 2013-221069 A Summary of the Invention [Problem to be solved by the invention]

[0004] Patent Documents 1 and 2 disclose a method for analyzing rubber using a general cross-flow type field flow fractionation (FFF) device. However, the cross-flow type requires a membrane to be laid inside the channel, and there is a concern that natural rubber may be adsorbed to the membrane and not be properly separated in the analysis. A differential refractometer (RID) detector is generally used for GPC, but it has the problem of low sensitivity.

[0005] In addition to the method of determining relative molecular weight by comparing retention time with standard polymers in GPC, etc., absolute molecular weight and radius of gyration can also be determined by using a concentration detector such as a RID in combination with a multi-angle light scattering detector (MALS).

[0006] In this case, a method such as a conformation plot has been proposed as a data analysis method, in which the molecular weight (log) obtained by measurement is plotted on the horizontal axis and the radius of gyration (log) on ​​the vertical axis, the slope and the molecular weight of the inflection point are evaluated, and it has been proposed that the greater the slope of the plot, the higher the linearity. However, evaluation using only a conformation plot has problems such as not being able to evaluate components with different branches and losing concentration information.

[0007] The present invention has an object to solve the above-mentioned problems and to provide an analytical method for natural rubber that enables accurate separation and enables analysis of the branching probability, abundance ratio, molecular weight, etc. of each component with a different degree of branching. [Means for solving the problem]

[0008] The first invention relates to a method for analyzing natural rubber, characterized in that natural rubber is analyzed using a thermal flow type field flow fractionation apparatus connected to a multi-angle light scattering detector and an evaporative light scattering detector.

[0009] The second invention relates to a method for analyzing natural rubber, characterized in that a fitting analysis is performed on the assumption that the branching structure of natural rubber is random four-branching, and at least one selected from the group consisting of the branching probability, abundance ratio, and molecular weight of each component having a different branching degree. Effect of the Invention

[0010] The present invention relates to a method for analyzing natural rubber (the first present invention), which comprises analyzing natural rubber using a thermal flow type field flow fractionation apparatus connected to a multi-angle light scattering detector and an evaporative light scattering detector, and a method for analyzing natural rubber (the second present invention), which comprises performing a fitting analysis assuming that the branching structure of natural rubber is a random four-branch structure, and calculating at least one selected from the group consisting of the branching probability, abundance ratio, and molecular weight of each component with a different degree of branching. Therefore, it is possible to provide a method for analyzing natural rubber which enables accurate separation, and a method for analyzing the branching probability, abundance ratio, molecular weight, etc. of each component with a different degree of branching. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 shows the separation temperature conditions of a Thermal Flow type FFF device. [Diagram 2] FIG. 1 shows ELSD chromatograms of Experimental Examples 1 and 2, respectively. [Diagram 3] FIG. 1 shows chromatograms of ELSD in Experimental Example 1 and RID in Experimental Example 3. [Figure 4] FIG. 2 shows conformation plots of natural rubber samples A and B. [Diagram 5] FIG. 1 shows fitting analyses of natural rubber samples A and B, respectively, assuming that the branching structure of the natural rubber is a random four-branch structure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The first invention is a method for analyzing natural rubber, characterized in that natural rubber (sample) is analyzed using a thermal flow type field flow fractionation (FFF) device connected to a multi-angle light scattering detector (MALS) and an evaporative light scattering detector (ELSD). Since the first method for analyzing natural rubber is of the thermal flow type, adsorption of polymers to the membrane, which occurs in the case of the cross flow type, is suppressed, improving separation and enabling accurate separation. In addition, since an ELSD is used, the sensitivity is high, and for example, the sensitivity is improved by about 10 times compared to the case of using a differential refractometer (RID).

[0013] The second invention is a method for analyzing natural rubber, characterized in that a fitting analysis is performed assuming that the branching structure of natural rubber (sample) is random 4-branching, and at least one selected from the group consisting of the branching probability, abundance ratio, and molecular weight of each component with a different branching degree is calculated. In the second method for analyzing natural rubber, for example, an accurate branching index can be calculated by assuming that polyisoprene is linear and that the branching structure of natural rubber (sample) is random 4-branching, and the branching probability, abundance ratio, and molecular weight can be calculated for each component with a different branching degree (each natural rubber component with a different branching degree) by separating each component. In this way, by separating each component and evaluating the branching probability, abundance ratio, and molecular weight, the branching structure of natural rubber can be more clearly clarified. Although it is a common method to obtain the branching probability (γM) of one component, in the present invention, it is possible to separate a sample containing a mixture of multiple components and evaluate the branching probability, abundance ratio, and branching degree of each component.

[0014] Hereinafter, embodiments of the first and second methods for analyzing natural rubber of the present invention will be described, however, the first method for analyzing natural rubber and the second method for analyzing natural rubber are not limited to the following embodiments.

[0015] In the above-mentioned first and second natural rubber analysis methods, for example, a dissolution step of dissolving natural rubber (sample) and a separation step of separating the soluble components from the natural rubber solution produced in the dissolution step are carried out, thereby separating the soluble components, and the soluble components are analyzed using a Thermal Flow type FFF apparatus connected to a MALS and an ELSD, thereby enabling the analysis of natural rubber (sample).

[0016] In the above first and second methods for analyzing natural rubber, examples of natural rubber (sample) include rubber having a polyisoprene skeleton obtained from sap produced from plant species such as Hevea brasiliensis.

[0017] The dissolving step can be carried out by any method capable of dissolving the sample (natural rubber) without any particular limitation, for example, by dissolving the natural rubber in an organic solvent. The organic solvent used to dissolve the sample (natural rubber) is not particularly limited, and any solvent capable of dissolving the sample can be used, for example, tetrahydrofuran (THF), chloroform, toluene, cyclohexane, etc. can be used alone or in combination. Among them, THF is preferable because of its good solubility. The sample concentration (concentration of natural rubber) in the organic solvent is preferably adjusted to 0.001 to 1% by mass. For example, a solution in which natural rubber is dissolved can be prepared by adding a sample to an organic solvent, adjusting the concentration, and leaving it for a predetermined time (for example, 12 hours or more) or by rotary shaking.

[0018] In the separation step, for example, the natural rubber solution prepared in the dissolving step is subjected to centrifugation to separate the soluble and insoluble components. The conditions for centrifugation are not particularly limited, and conditions that allow separation may be appropriately selected, and the centrifugation may be performed, for example, at 5,000 to 100,000 rpm for 1 to 300 minutes. In the separation step, filtration with a filter may be performed after centrifugation, if necessary. The filter is not particularly limited, and examples thereof include a PTFE filter.

[0019] The supernatant liquid of the natural rubber solution separated by the above dissolving step and separation step is collected to obtain a soluble component containing an ultra-high molecular weight component.

[0020] In the analytical method of the present invention, for example, natural rubber (sample) can be analyzed by analyzing the soluble components prepared above using a Thermal Flow type FFF device connected to MALS and ELSD.

[0021] The FFF device is a device that can separate molecular weights by passing a sample solution through a gap (channel) of, for example, 100 μm to 500 μm and applying a field when the sample passes through the channel. There are various methods of applying the field, such as the cross flow type, the thermal flow type, and the centrifugal force type. In the FFF device, the cross flow type is generally used for evaluating the molecular weight of polymers, but the thermal flow type is preferably used in the present invention. The thermal flow type applies a temperature gradient between the upper and lower plates, and the polymer molecules diffuse due to the force caused by the temperature gradient. Large polymers are separated by forming a layer at the bottom of the channel, and small polymers are separated at the center of the channel. The use of the thermal flow type improves separation.

[0022] In addition, when a differential refractometer (RID) is used as a detector, problems occur in which the peak area cannot be estimated correctly due to factors such as the peak being small and large noise in the negative direction occurring at the start of peak detection, whereas when an ELSD is used, the sensitivity is high and there is no noise, which improves the accuracy of concentration calculations.

[0023] The absolute molecular weight and radius of gyration can be calculated from the angle dependence of light scattering obtained by MALS and the concentration obtained by RID or ELSD. Examples of commercially available multi-angle light scattering detectors include the DAWN series manufactured by Shoko Science Co., Ltd.

[0024] In the analysis method of the present invention, the molecular weight Mw and the mean square radius of gyration of the soluble component are 2 For example, the above can be calculated using analytical software. As the analytical software, Postnova's analytical software (TF2000 control program ver.1.0.1.1) can be used.

[0025] Molecular weight Mw and mean square radius of gyration of the above soluble components 2 > can be analyzed using a conformation plot (horizontal axis: Log molecular weight, vertical axis: Log radius) that plots molecular weight (log) and radius of gyration (log). The greater the slope of the plot, the higher the linearity. In the case of linear polyisoprene, the slope of the conformation plot is about 0.6 to 0.7.

[0026] By analyzing using a conformation plot, the slope of the conformation plot can be evaluated as the linearity of the polymer, and the molecular weight of the inflection point can be evaluated as the molecular weight at which branching changes. For example, if the slope of the high molecular weight side is smaller than the slope of the low molecular weight side before and after the inflection point, the inflection point can be evaluated as the molecular weight at which branching begins to increase.

[0027] However, since information on concentration (amount) is lost in the conformation plot alone, it is not possible to evaluate, for example, the proportion of components with different branching degrees present.

[0028] On the other hand, in the analysis method of the present invention, by performing fitting analysis assuming that the branching structure of natural rubber is random 4-branching, it is possible to calculate the branching probability of each component with a different branching degree, the abundance ratio of each component with a different branching degree, the molecular weight of each component with a different branching degree (molecular weight between branch points), etc. In the present invention, the "branching degree" refers to the branching index g S (= 2 > b / 2 > l ​​​​This means a parameter indicating the degree of branching. The "branch probability" is γ M This refers to the parameter expressed as (=number of branch points / numerator). "Molecular weight between branch points" 1 / γ M The parameters shown in the table are:

[0029] The above fitting analysis is desirably carried out using, for example, the following theoretical formula.

number

[0030] In the above theoretical formula, the branch index g S is g S = 2 > b / 2 > l If the molecular structure of the sample is a linear polymer, the branching index is 1, and as the degree of branching increases, the branching index becomes smaller than 1. 2 > b is the mean square of the radius of gyration of the sample, 2 > l is the root mean square of the radius of gyration of the linear standard reference material. γ M (=number of branching points / molecule) is the branching probability. If we assume that the branching structure of natural rubber is random 4-branching, the branching index g S is g S =1 / [(1+γ M M / 6) 1 / 2 +(4γ M M / 3π)] 1 / 2 It becomes.

[0031] Specifically, the fitting analysis can be carried out, for example, by the following method. Measure standard polyisoprene (molecular weight 85,000) and calculate the coefficients from the measured values ​​(e.g., a = 0.6, b = 1.30 × 10 -2 ). The branching structure of natural rubber (sample) is assumed to be random 4-branched.​​​​ Regarding the actual measured values ​​of the sample (natural rubber), Mw (the molecular weight of the soluble component) and 2 > z 1 / 2 Plot the relationship (actual measurements) (vertical axis: 2 > z 1 / 2 , horizontal axis: Mw). Using the above theoretical formula, a solid line showing the theoretical value of standard polyisoprene (linear polyisoprene, molecular weight 85,000) is obtained. For the line showing low branching components (solid line) and the line showing high branching components (solid line), the branching index g S , the root mean square of the radius of gyration of the sample 2 > b Calculate. When the actual measured value lies on the line indicating low branching components, it is judged to be a low branching component, when it lies on the line indicating high branching components, it is judged to be a high branching component, and when the actual measured value lies on or above the solid line of standard polyisoprene (linear), it is judged to be a linear component. In addition to the molecular weight and radius, the actual measured values ​​are also linked to concentration information (calculated from ELSD), so the concentrations of each point determined to be linear, low-branched, and highly branched components can also be determined. Branch component gamma M The value (branch probability) is calculated based on the actual measured value of the sample. Here, γ M If you change the input value, the slope of the solid line will change, so adjust γ so that the actual measured value fits the solid line. M Enter the value of γ for each component with different branching degree. M A value is required. Furthermore, the abundance ratio of each component and the average molecular weight of each component are calculated. Furthermore, the abundance ratio and molecular weight (molecular weight between branching points) of each component with a different degree of branching are also calculated. EXAMPLES

[0032] In the following, examples (embodiments) that are considered to be preferable for carrying out the present invention will be shown, but the scope of the present invention is not limited to the examples. ​​​

[0033] <Common matters> (Sample Preparation) A natural rubber sample (TSR20) is added to THF to a concentration of 1-2 mg / mL, and extracted by slowly rotating and shaking at room temperature for one week. The natural rubber sample extract is centrifuged (15,000 rpm, 15 minutes) and then filtered using a PTFE filter (pore size 5 μm). The filtrate (soluble components) is analyzed according to each of the following experimental examples.

[0034] As the standard polyisoprene, polyisoprene (linear) having a weight average molecular weight of 85,000 is used. Calibration of ELSD and MALS is performed using a 5 mg / mL THF solution of the above polyisoprene. The analysis software TF2000 that accompanies the FFF-MALS is used to collect data. In Experimental Example 2, software dedicated to AF4 is used. Data analysis results show the average value of N=3 for each sample.

[0035] <Experimental Example 1> Using a FFF device (FFF-MALS) with a thermal flow type separation section and a MALS and ELSD connected as detectors, fitting analysis was performed assuming that the branching structure of natural rubber is random 4-branching, the branching probability of each component with a different branching degree was obtained, and the abundance ratio and molecular weight (molecular weight between branching points) of each component with a different branching degree was calculated. Details of the device used are as follows. Separation unit: Postnova TF2000 (Thermal FFF System) Detector: Postnova PN3510 (ELSD), Postnova PN3621 (MALS) Spacer: 350μm thickness Mobile phase: THF (contains stabilizer) Flow rate: 0.3mL / min Injection volume: 50μL Separation conditions: Initial temperature gradient: 100°C, Initial temperature holding time: 5 minutes Analysis method: Random 4-branch fitting

[0036] FIG. 1 shows the separation temperature conditions for the TF2000 (Thermal FFF System).

[0037] <Experimental Example 2> The separation section is a cross flow type, and the detector is an FFF device (FFF-MALS) connected to MALS and ELSD. Details of the device used are as follows. Separation part: Postnova AF4 (FFF System) Detector: Postnova PN3510 (ELSD), Postnova PN3621 (MALS) Membrane: NavaRC 10kDa Mobile phase: THF (contains stabilizer) Flow rate: 0.5mL / min Injection volume: 50μL

[0038] <Experimental Example 3> The separation section is a Thermal Flow type, and the detector is an FFF device (FFF-MALS) connected to MALS and RI. Details of the equipment used are as follows. Separation unit: Postnova TF2000 (Thermal FFF System) Detector: Postnova PN3150 (RID), Postnova PN3621 (MALS) Spacer: 350μm thickness Mobile phase: THF (contains stabilizer) Flow rate: 0.3mL / min Injection volume: 50μL Separation conditions: Initial temperature gradient: 100°C, Initial temperature holding time: 5 minutes

[0039] <Experimental Example 4> The separation unit and detector are the same as in Experimental Example 1, and the conformation plot is used as the analysis method. Details of the equipment used are as follows. Separation unit: Postnova TF2000 (Thermal FFF System) Detector: Postnova PN3510 (ELSD), Postnova PN3621 (MALS) Spacer: 350μm thickness Mobile phase: THF (contains stabilizer) Flow rate: 0.3mL / min Injection volume: 50μL Separation conditions: Initial temperature gradient: 100°C, Initial temperature holding time: 5 minutes Analysis method: Conformation plot

[0040] (Comparison of Experimental Examples 1 and 2) FIG. 2 shows the chromatograms of the ELSD in Experimental Example 1 and the chromatograms of the ELSD in Experimental Example 2, respectively. In Experimental Example 1 (Thermal flow type), the peak shape is close to normal distribution, there is little peak noise, and it returns to the baseline. On the other hand, in Experimental Example 2 (Cross Flow type), the peak tails and does not return to the baseline at the end of the elution time, which indicates that the peaks are not completely separated. These results show that experimental example 1, which used a Thermal Flow type FFF apparatus connected to a MALS, allows for more accurate separation than experimental example 2, which used a Cross Flow type FFF apparatus connected to a MALS.

[0041] (Comparison of Experimental Examples 1 and 3) FIG. 3 shows the chromatograms of the ELSD of Experimental Example 1 and the RID of Experimental Example 3. Compared to the ELSD chromatogram, the RID chromatogram has smaller peaks and a large amount of noise in the negative direction at the start of peak detection, making it difficult to accurately estimate the peak area. These results show that the accuracy of concentration calculation is higher in Experimental Example 1, which used a Thermal Flow type FFF device connected to a MALS, than in Experimental Example 3, which used a Thermal Flow type FFF device connected to a RID.

[0042] (Comparison of Experimental Examples 1 and 4) FIG. 4 shows the conformation plots for natural rubber samples A and B, respectively. Molecular weight Mw, mean square radius of gyration 2 > is calculated using Postnova's analysis software (TF2000 control program ver.1.0.1.1). When analyzing the conformation plot (horizontal axis: Log molecular weight, vertical axis: Log radius), the slope of the conformation plot becomes smaller as the molecular weight increases in all samples, and there is a tendency for branching to increase. It is also clear that the molecular weight at which branching begins to increase (the molecular weight at the inflection point of the conformation plot) differs between samples (Figure 4, Table 1). Here, since information on concentration (amount) is lost in the conformation plot alone, it is not possible to evaluate the proportion of components with different branching degrees, etc.

[0043] [Table 1]

[0044] Therefore, assuming that the branching structure of natural rubber is random 4-branching, a fitting analysis is performed using the theoretical formula described in the above formula 1.

[0045] FIG. 5 shows fitting analysis of natural rubber samples A and B, respectively, assuming that the branching structure of the natural rubber is random four-branched.

[0046] First, measure the standard polyisoprene (molecular weight 85,000) and calculate the coefficients from the measured values ​​(a = 0.6, b = 1.30 × 10 -2 ). The branching structures of natural rubber samples A and B are assumed to be random 4-branched. In Figure 5, ○ indicates the actual measured value (data calculated using TF2000 dedicated software) and Mw and 2 >​​z 1 / 2 The plot is thinned out to make it easier to see (5 to 15 points. The number of points can be changed depending on the data). The solid line labeled "Linear (theoretical value)" shows the theoretical value of standard polyisoprene (linear polyisoprene, molecular weight 85,000) (vertical axis is the left axis). 2 > z 1 / 2 =bMw a In the equation shown above, a = 0.6, b = 1.30 × 10 -2 is obtained by substituting The solid line labeled Line (low branching component B) is the γ M The solid line labeled B (B component, low-density branching) and line (high-branching component C) are γ M C (C component, high density branching) line, and the above g S =1 / [(1+γ M M / 6) 1 / 2 +(4γ M M / 3π)] 1 / 2 In the formula shown below, each γ M (branch probability), g S is calculated. Furthermore, the above g S = 2 > b / 2 > l In the formula: 2 > l (the radius of rotation of linear standard polyisoprene) is obtained by the solid line marked as Linear (theoretical value) above, so g S and 2 > l from, 2 > b is calculated. When the measured value (○) is on the solid line labeled Line (Low Branching Component B), it is determined to be component B (low branching). When the measured value (○) is on the solid line labeled Line (Highly Branched Component C), it is determined to be component C (highly branched). ​​​​​​When the measured value (○) is on the solid line labeled "Linear (theoretical value)" or is above the solid line labeled "Linear (theoretical value)", it is determined to be component A (straight-chain). The actual measured values ​​(○) are linked to the molecular weight and radius as well as concentration information (calculated from ELSD), so the concentrations of each of the points determined to be components A, B, and C can be determined. The values ​​are converted to relative values ​​(normalized) so that the highest concentration is 1, and the molecular weight and concentration (relative value: vertical axis is on the right axis) for each component are plotted in a dot plot for wA (component A, linear), wB (component B, low branching), and wC (component C, highly branched). The dot plot of w (normalized) is the sum of components A, B, and C. In Fig. 5, the branch component γ M The value (branch probability) is calculated based on the actual measured value (○) of the sample. Here, γ M If you change the input value, the slope of the solid line will change, so adjust γ so that the actual measured value (○) fits on the solid line. M Enter the value of γ for each component with different branching degree. M values ​​are obtained. Furthermore, the ratio of each component present and the average molecular weight of each component can be calculated. As shown in Figure 5, it is suggested that natural rubber samples A and B contain component A (straight-chain component) which is above the solid line labeled Linear (theoretical value), component B (low-density branched component) which is on the solid line labeled Line (low-branched component B), and component C (high-density branched component) which is on the solid line labeled Line (high-branched component C).

[0047] As shown in Figure 5 and Table 2, both natural rubber samples A and B contain linear and branched components, and it is suggested that the branched components consist of two components with different branching densities. The branching probability of component B, which is the main component and has a relatively low density of branches, does not differ between samples, but the abundance ratio is thought to differ. The figures also show that sample B has a higher branching density from a relatively low molecular weight. These results show that by assuming the branching structure of natural rubber to be randomly four-branched and performing fitting analysis using the above theoretical formula, it is possible to calculate the branching probability, abundance ratio, and molecular weight of each component with a different degree of branching.

[0048] [Table 2]

[0049] The present invention (1) is a method for analyzing natural rubber, characterized in that natural rubber is analyzed using a thermal flow type field flow fractionation apparatus connected to a multi-angle light scattering detector and an evaporative light scattering detector.

[0050] The present invention (2) is a method for analyzing natural rubber, characterized in that a fitting analysis is performed on the assumption that the branching structure of natural rubber is random four-branching, and at least one selected from the group consisting of the branching probability, abundance ratio, and molecular weight of each component having a different degree of branching.

[0051] The present invention (3) is the method for analyzing natural rubber according to the present invention (2), in which fitting analysis is performed using the following theoretical formula:

number

Claims

1. A method for analyzing natural rubber, comprising the steps of: analyzing natural rubber using a thermal flow type field flow fractionation device connected to a multi-angle light scattering detector and an evaporative light scattering detector.

2. A method for analyzing natural rubber, comprising: performing a fitting analysis on the assumption that the branching structure of natural rubber is a random four-branch structure; and calculating at least one selected from the group consisting of the branching probability, abundance ratio, and molecular weight of each component having a different degree of branching.

3. The method for analyzing natural rubber according to claim 2, wherein the fitting analysis is carried out using the following theoretical formula: [0010]

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