Method for analyzing the content of d-lactic acid repeating units in polylactic acid

Abstract

The method for analyzing the content of D-lactic acid repeating units in polylactic acid of the present disclosure is characterized in that without special chemical treatment of polylactic acid, the content of D in polylactic acid can be quickly and accurately analyzed by using NMR data of polylactic acid and hypothetical polylactic acid.

Description

[TECHNICAL FIELD]

[0001] Cross Reference to Related Applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2021-0050041, filed on April 16, 2021, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

[0003] The present disclosure relates to a method for analyzing the content of D-lactic acid repeating units in polylactic acid. [BACKGROUND]

[0004] Polylactic acid is a material having biodegradability and at the same time, excellent mechanical properties such as tensile strength and elastic modulus, and is widely used in various fields. Polylactic acid is a homopolymer, but due to its tacticity, it can have various structures. Polylactic acid is generally prepared by ring-opening polymerization of lactide. However, since optical isomers exist in lactide, the properties of polylactic acid can change depending on the arrangement of these optical isomers in the repeating units.

[0005] In particular, the D-lactic acid repeating units in polylactic acid greatly affect the properties of polylactic acid, and thus the analysis of the tacticity of polylactic acid is important. In the past, in order to analyze the content of D-lactic acid repeating units (hereinafter referred to as D content) in polylactic acid, polylactic acid was hydrolytically decomposed into individual monomers, which were analyzed by liquid chromatography (LC), or polylactic acid was hydrolytically decomposed into individual monomers and converted into lactic acid alkyl ester by esterification reaction, which was then analyzed by gas chromatography (GC). However, the above method is complex in process and time-consuming, and in particular, racemization can occur during the analysis, which can reduce the reliability of the actual analysis data.

[0006] As another method, unlike the above method, there is a method of analyzing optical rotation by dissolving polylactic acid in an organic solvent without decomposing the polylactic acid. However, this method has a problem in that the error increases as the concentration of polylactic acid in the organic solvent, and in the case of containing chiral impurities or additives, an error can occur.

[0007] On the other hand, even in the case of polylactic acid having the same D content, the ratio of L-lactide, meso-lactide, and D-lactide in the raw material can vary, which leads to a difference in the tacticity of polylactic acid and affects the physical properties. It is difficult to confirm the difference in the ratio of raw materials through conventional analysis methods such as the above method.

[0008] Therefore, there is a need to develop a method of rapidly and accurately analyzing the D content in polylactic acid and the relative ratio of L-lactide, meso-lactide, and D-lactide in the raw material, which is different from the conventional method. [SUMMARY]

[0009] [Technical Problem]

[0010] An object of the present disclosure is to provide a method for quickly and accurately analyzing the D content in polylactic acid and the relative proportions of L-lactide, meso-lactide, and D-lactide in a raw material.

[0011] [Technical Solution]

[0012] To achieve the above object, according to the present disclosure, there is provided a method for analyzing the D-lactic acid repeating unit content in polylactic acid, comprising the steps of:

[0013] 1) obtaining quantitative data of stereoregularity from the NMR spectrum of polylactic acid;

[0014] 2) obtaining the arrangement of a hypothetical polylactic acid using three parameters: L-lactide ratio parameter, meso-lactide ratio parameter, and racemization parameter;

[0015] 3) obtaining the stereoregularity data of the hypothetical polylactic acid of step 2;

[0016] 4) obtaining the standard deviation of the quantitative data of stereoregularity of step 1 and the quantitative data of stereoregularity of the hypothetical polylactic acid of step 3;

[0017] 5) repeating steps 2 to 4 to obtain L-lactide ratio parameter, meso-lactide ratio parameter, and racemization parameter that minimize the standard deviation of step 4; and

[0018] 6) obtaining the D content in polylactic acid from the L-lactide ratio parameter, meso-lactide ratio parameter, and racemization parameter obtained in step 5.

[0019] Polylactic acid is a polymer prepared by polymerization of lactide, which can have the following stereoisomer of L-lactide, meso-lactide, and D-lactide. Therefore, the repeating unit in polylactic acid can also have the respective stereoisomer resulting therefrom. Since such stereoisomer affects the physical properties of polylactic acid, it is necessary to analyze the stereoisomer in polylactic acid.

[0020]

[0021] On the other hand, a method for analyzing the stereoregularity of polylactic acid using NMR is known. Specifically, by analyzing the NMR spectrum of polylactic acid, information on the monomer arrangement in polylactic acid can be obtained. However, it is difficult to directly analyze from it how much D-lactic acid repeating unit is present in polylactic acid. The reason is that the arrangement of two kinds of polylactic acid corresponds to the stereoregularity of each analysis, and therefore the content of D-lactic acid repeating unit cannot be calculated.

[0022] In this regard, the method of the present disclosure is characterized in that, in addition to the stereoregularity information of polylactic acid using NMR, the content of D-lactic acid repeating units in polylactic acid and the contents of L-lactide, meso-lactide, and D-lactide in the raw material are analyzed by a method of obtaining stereoregularity information on the arrangement of hypothetical polylactic acid and then comparing the two stereoregularities with each other.

[0023] Now, the present disclosure will be described in detail with respect to each step.

[0024] (Step 1)

[0025] Step 1 of the present disclosure is a step of obtaining quantitative data of stereoregularity from the NMR spectrum of polylactic acid, which is a step of obtaining quantitative data of stereoregularity of the polylactic acid actually analyzed.

[0026] The lactic acid repeating units present in polylactic acid include L-lactic acid repeating units and D-lactic acid repeating units, and different NMR peaks are obtained depending on their arrangement. From this point of view, the quantitative data of stereoregularity of Step 1 refers to tetrad intensity data of polylactic acid. Specifically, the NMR peaks vary depending on the arrangement of four repeating units (tetrad) in polylactic acid, and in 13 C NMR and 1 H NMR, the following peaks can be obtained from the high chemical shift side to the low chemical shift side as shown in Table 1 below (Polymers 2019, 11, 725).

[0027] [Table 1]

[0028] 13 C NMR ​ mrr rrr mrm rrm mmm / mmr / rmm / rmr 1 H NMR ​ rmr rmm mmr mmm / rrr / rrm / mrr mrm

[0029] In Table 1, "m" indicates that two repeating units are the same as each other, and "r" indicates that two repeating units are different from each other. In one example, "rmr" means that the arrangement of four repeating units in polylactic acid is "LDDL" or "DLLD".

[0030] From these points of view, the NMR spectrum of Step 1 refers to the 13 C NMR spectrum and 1 H NMR spectrum of polylactic acid. In addition, the integral intensity of each peak can be obtained as quantitative data of stereoregularity, and a total of 8 kinds of quantitative data can be obtained depending on the arrangement of 4 repeating units (tetrad). In addition, for analysis, the ratio of each integral intensity to the sum of 8 integral intensities can be obtained as quantitative data.

[0031] (Step 2)

[0032] Step 2 of the present disclosure is a step of obtaining the arrangement of the hypothetical polylactic acid. For this purpose, three parameters are arbitrarily set, one is an L-lactide ratio parameter, another is a meso-lactide ratio parameter, and the other is a racemization parameter.

[0033] The "L-lactide ratio parameter" refers to the generation probability of the (L-lactic acid)-(L-lactic acid) repeat unit (LL) when generating the lactic acid repeat unit. That is, the "L-lactide ratio parameter" is "(generation probability of the LL repeat unit) / ((generation probability of the LL repeat unit) + (generation probability of the LD repeat unit) + (generation probability of the DL repeat unit) + (generation probability of the DD repeat unit))".

[0034] The "meso-lactide ratio parameter" refers to the generation probability of the (L-lactic acid)-(D-lactic acid) repeat unit (LD) and the generation probability of the (D-lactic acid)-(L-lactic acid) repeat unit (DL) when generating the lactic acid repeat unit. That is, the "meso-lactide ratio parameter" refers to "((generation probability of the LD repeat unit) + (generation probability of the DL repeat unit)) / ((generation probability of the LL repeat unit) + (generation probability of the LD repeat unit) + (generation probability of the DL repeat unit) + (generation probability of the DD repeat unit))".

[0035] For example, if the "L-lactide ratio parameter" is 0.90 and the "meso-lactide ratio parameter" is 0.05, the generation probability of the LL repeat unit is 0.90, the generation probability of the LD or DL repeat unit is 0.05, and the generation probability of the DD repeat unit is 0.05 when generating the lactic acid repeat unit.

[0036] The (generation probability of the LD repeat unit) and the (generation probability of the DL repeat unit) are considered not to affect the purpose of the present disclosure. However, if necessary, the ratio of the (generation probability of the LD repeat unit) to the (generation probability of the DL repeat unit) can also be introduced as another parameter.

[0037] The "racemization parameter" refers to the ratio of the L-lactic acid repeat unit (L) to the D-lactic acid repeat unit (D) and the D-lactic acid repeat unit (D) to the L-lactic acid repeat unit (L) with respect to any repeat unit among all the repeat units of the hypothetical generated polylactic acid. For example, if the "racemization parameter" is 0.10, it means that the stereomultiplication is reversed with respect to any 10% of all the repeat units of the hypothetical generated polylactic acid.

[0038] Specifically, an arbitrary L-lactide ratio parameter, meso-lactide ratio parameter, and racemization parameter are set. According to the L-lactide ratio parameter and the meso-lactide ratio parameter, the LL arrangement of L-lactide, the LD arrangement or the DL arrangement of meso-lactide, and the DD arrangement of D-lactide are probabilistically arranged according to the ratio of each parameter. Then, as many repeating units as the racemization parameter ratio corresponds to are randomly selected from the entire arrangement, and the arrangement of the hypothetical polylactic acid can be obtained by changing D-lactic acid to L-lactic acid in the case of L-lactic acid and changing L-lactic acid to D-lactic acid in the case of D-lactic acid.

[0039] On the other hand, the arrangement of the hypothetical polylactic acid can have a repeating unit length of 10,000 to 1,000,000.

[0040] (Step 3)

[0041] Step 3 of the present disclosure is a step of obtaining the stereoregularity data of the hypothetical polylactic acid of Step 2.

[0042] Since the information on the arrangement of the hypothetical polylactic acid obtained in Step 2 is known, stereoregularity data of the polylactic acid as in Step 1 can be obtained therefrom. For example, when the repeating unit arrangement of the hypothetical polylactic acid obtained in Step 2 is LLLDDLLLLL, quantitative data can be obtained by counting each of LLLD, LLDD, LDDL, DDLL, DLLL, LLLL, and LLLL once. Thereby, as in Step 1, a total of 8 kinds of quantitative data can be obtained from the arrangement of 4 repeating units (quadruplet).

[0043] (Steps 4 and 5)

[0044] Step 4 of the present disclosure is a step of obtaining the standard deviation of the quantitative data of the stereoregularity of the NMR spectrum from the polylactic acid obtained in Step 1 and the stereoregularity data of the hypothetical polylactic acid obtained in Step 3. In addition, Step 5 of the present disclosure is a step of repeating Steps 2 to 4 to obtain the L-lactide ratio parameter, the meso-lactide ratio parameter, and the racemization parameter that minimize the standard deviation of Step 4 by changing the L-lactide ratio parameter, the meso-lactide ratio parameter, and the racemization parameter.

[0045] Preferably, the standard deviation refers to the square root of the sum of the squares of the difference between the corresponding 8 kinds of quantitative data of the data of Step 1 and the data of Step 3. When the standard deviation is small, it can be considered that the arrangement of the actual polylactic acid and the hypothetical polylactic acid is similar.

[0046] Preferably, the standard deviation is less than 0.05. More preferably, the standard deviation is less than 0.01, or less than 0.005. On the other hand, it should be understood that the minimization of the standard deviation is not necessarily the smallest value theoretically possible, but includes the case of less than 0.05.

[0047] (Step 6)

[0048] Step 6 of the present disclosure is a step of obtaining the D content in the polylactic acid from the L-lactide ratio parameter, the meso-lactide ratio parameter, and the racemization parameter obtained in Step 5.

[0049] This is a step of obtaining the D content in the polylactic acid from the arrangement of the hypothetical polylactic acid having the minimum standard deviation obtained in Step 5. At this time, since it can be considered that the arrangement of the actual polylactic acid and the hypothetical polylactic acid is similar, the D content in the hypothetical polylactic acid can be predicted as the D content in the actual polylactic acid.

[0050] On the other hand, since the arrangement of the hypothetical polylactic acid is known by Step 5, the D content can be easily calculated therefrom.

[0051] [Advantages]

[0052] As described above, the method for analyzing the D-lactic acid repeating unit content in the polylactic acid is characterized in that the D content in the polylactic acid can be quickly and accurately analyzed using the NMR data of the polylactic acid and the hypothetical polylactic acid without special chemical treatment of the polylactic acid. [BRIEF DESCRIPTION OF DRAWINGS]

[0053] Figure 1 and 2 The NMR data of the polylactic acid measured in the examples of the present disclosure are shown. [DETAILED DESCRIPTION]

[0054] Hereinafter, the embodiments of the present disclosure will be described in more detail by examples. However, the following examples are provided for illustrative purposes only, and the content of the present disclosure is not limited by these examples.

[0055] Examples

[0056] The following five kinds of polylactic acid were used.

[0057] - L175 (Total Corbion)

[0058] - LX175 (Total Corbion)

[0059] - LX575 (Total Corbion)

[0060] - 2003D (NatureWorks)

[0061] - 4032D (NatureWorks)

[0062] 1) NMR spectrum of polylactic acid

[0063] Measurement of various polylactic acid 13 C NMR and 1 H NMR, results shown in Figure 1 and 2 .

[0064] In addition, for Figure 1 and 2 The integral intensity shown was analyzed, and quantitative data on stereoregularity were obtained for each type of tetraparticle in Table 2 below.

[0065] [Table 2]

[0066]

[0067] 2) Hypothetical arrangement of polylactic acid

[0068] Arbitrary L-lactide ratio, mesolactide, and racemization parameters were set for each of the three polylactic acids (PLAs), and the length of the repeating unit was set to 100,000. Specifically, LL, LD, DL, and DD arrangements were created based on the L-lactide ratio and mesolactide ratio parameters to obtain PLA arrangements with a repeating unit length of 100,000. Then, from the entire arrangement, an arbitrary number of repeating units, equal to the number corresponding to the racemization parameter ratio, were randomly selected, converting L-lactic acid to D-lactic acid and D-lactic acid to L-lactic acid.

[0069] Stereoregularity was analyzed from the obtained hypothetical polylactic acid (PLA) arrangements. This was based on the lactic acid arrangements in Table 2. By comparing the obtained quantitative data on various stereoregularities with the data in Table 2, the L-lactide ratio parameter, mesolactide ratio parameter, and racemization parameter were varied to minimize the standard deviation (the square root of the sum of the squares of the differences between the eight quantitative data points), and the hypothetical PLA arrangements were repeated.

[0070] The D-lactic acid unit content was calculated based on the hypothetical polylactic acid (PLA) arrangement that minimizes the standard deviation, and the results are shown in Table 3 below. In this case, the D-lactic acid unit content is calculated as the number of D-lactic acid repeating units compared to the total number of repeating units in the hypothetical PLA.

[0071] [Table 3]

[0072] L175 LX575 LX175 4032D 2003D D content (%) 0.87 2.23 3.72 2.10 4.23 L-lactide ratio parameter 0.987 0.977 0.961 0.981 0.974 meso-lactide ratio parameter 0.008 0.017 0.039 0.004 0.003 racemization parameter 0.00 0.008 0.018 0.004 0.018 standard deviation 0.00349 0.00917 0.01811 0.00359 0.01138

[0073] As described above, the analysis method of the embodiments of the present application is simple because chemical treatment of the sample is omitted, and is high in stability because no reagent or the like used for chemical treatment is used. In addition, because the arrangement of the lactic acid units can be determined by analyzing the tacticity, there is an advantage that it is possible to determine whether meso-lactide or D-lactide contributes more even at the same D content.

[0074] Comparative Example 1

[0075] Hydrolysis and esterification were performed using the method described in the literature (Lee Tin Sin, Polylactic Acid A Practical Guide for the Processing, Manufacturing, and Applications of PLA Second Edition) and converted into L-lactic acid methyl ester and D-lactic acid methyl ester. The D-lactic acid methyl ester content in the lactic acid methyl ester was analyzed to determine the D-lactic acid unit content in the polylactic acid.

[0076] Specifically, the polylactic acid sample was dissolved in a potassium hydroxide methanol solution under heating at 65°C. Sulfuric acid was added to the above solution and heated at the same temperature. Deionized water and dichloromethane were added to the above solution and mixed, and then allowed to stand to separate into two solution phases. The lower layer solution was collected and analyzed using chiral GC-FID.

[0077] The results are shown in Table 4 below.

[0078] [Table 4]

[0079] L175 LX575 LX175 4032D 2003D D content (%) of GC analysis 0.37 1.56 3.71 1.46 4.16

[0080] Comparing the analysis method of Comparative Example 1 with the analysis method of the embodiments of the present disclosure described above, the analysis method of Comparative Example 1 has a disadvantage in that the analysis method is complex because hydrolysis and esterification must be performed, and there is a problem in terms of the safety of the analyst because sulfuric acid must be used. In addition, the analysis method of the embodiments of the present disclosure can determine the arrangement of the lactic acid units by analyzing the tacticity, so it is possible to analyze whether meso-lactide or D-lactide contributes more even at the same D content. On the other hand, in the method of Comparative Example 1, this analysis is difficult.

[0081] Comparative Example 2

[0082] The content of D-lactic acid in the polylactic acid was calculated by a statistical model method using the paper (Polymers 2019, 11, 725).

[0083] Specifically, for the quantitative data of the stereoregularity obtained as shown in Table 2, the values of each variable (p1, p2, f2) described in Table 3 of this paper were obtained using the search function of Excel. At this time, p1 represents L / (L+D), p2 represents LL / (LL+DD), and f2 represents the variable in the "Two-State" model of this paper.

[0084] The results are shown in the following Table 5, and the results of Tables 3 and 4 are also described for comparison.

[0085] [Table 5]

[0086] L175 LX575 LX175 4032D 2003D D content (%) of GC method 0.37 1.56 3.71 1.46 4.16 (*D content in Table 3) 0.87 2.23 3.72 2.10 4.22 D content (%) 0.88 2.76 4.78 2.26 4.27 p2 0.996 1 1 0.988 0.977 p1 0.919 0.782 0.885 0.778 0.899 f2 0.939 0.873 0.586 0.948 0.751 standard deviation 0.00982 0.0230 0.0449 0.0094 0.0320

[0087] Comparing the analysis method of Comparative Example 2 with the analysis method of the above-described embodiments of the present disclosure, it can be indirectly determined whether the D content is from meso-lactide or from D-lactide depending on whether the Single Addition Factor is high or the Pair Addition Factor is high, but it has the disadvantage that it is difficult to express numerically. In addition, since the analysis method of Comparative Example 2 calculates the ratios of various stereoregularities based on probability theory, the standard deviation is large. Therefore, there is a disadvantage of reduced precision in quantifying the D content.

Claims

1. A method for analyzing the content of D-lactic acid repeating units in polylactic acid, comprising the following steps: 1) Quantitative data on stereoregularity were obtained from the NMR spectra of polylactic acid; 2) The hypothetical polylactic acid arrangement is obtained using the following three parameters: L-lactide ratio parameter, mesolactide ratio parameter, and racemization parameter; 3) Obtain the stereoregularity data of the hypothetical polylactic acid obtained in step 2; 4) Obtain the standard deviation of the quantitative data on stereoregularity in step 1 and the quantitative data on stereoregularity of hypothetical polylactic acid in step 3. 5) Repeat steps 2 to 4 to obtain the L-lactide ratio parameter, mesolactide ratio parameter, and racemicization parameter that minimize the standard deviation of step 4; and 6) Obtain the content of D-lactic acid repeating units in polylactic acid from the L-lactide ratio parameter, mesolactide ratio parameter and racemization parameter obtained in step 5; in: The quantitative data of stereoregularity in step 1 is the integral intensity data of the polylactic acid tetrad. The L-lactide ratio parameter in step 2 is the probability of generating (L-lactic acid)-(L-lactic acid) repeating units (LL) when lactic acid repeating units are generated. The meso lactide ratio parameter in step 2 refers to the generation probability of (L-lactic acid)-(D-lactic acid) repeating units (LD) and (D-lactic acid)-(L-lactic acid) repeating units (DL) when lactic acid repeating units are generated. The racemic parameter in step 2 is the ratio of L-lactic acid repeating unit (L) to D-lactic acid repeating unit (D) and D-lactic acid repeating unit (D) to L-lactic acid repeating unit (L) relative to any repeating unit among all the repeating units of the hypothetical polylactic acid generated.

2. The method of claim 1, wherein: The polylactic acid tetrads are mmm, mrm, mmr, rmm, rmr, mrr, rrm, and rrr.

3. The method of claim 1, wherein: The NMR spectrum in step 1 is for polylactic acid. 13 C NMR spectra and 1 H NMR spectrum.

4. The method of claim 1, wherein: The stereoregularity data in step 3 is the quantitative data of the hypothetical polylactic acid tetrad in step 2.

5. The method of claim 1, wherein: The standard deviation mentioned in step 4 is the square root of the sum of the squares of the differences between the corresponding eight quantitative data points of the data in step 1 and the data in step 3.

6. The method of claim 1, wherein: The standard deviation in step 4 is less than 0.05.

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