A method for measuring the content of α′ and α crystal forms in polylactic acid based on terahertz time-domain spectroscopy

Abstract

The present application relates to polylactic acid crystal form measurement technology, and aims to provide a polylactic acid alpha' and alpha crystal form content measurement method based on terahertz time domain spectroscopy. Including: taking two polylactic acids containing only alpha' crystal form and only alpha crystal form as model samples, respectively collecting terahertz time domain spectroscopy, and converting into corresponding terahertz absorption spectrogram; extracting the terahertz characteristic peak frequency of the model sample, and establishing a calculation model between the terahertz characteristic peak frequency and the crystal form content; taking mixed crystal form polylactic acid as a sample to be measured, collecting the terahertz time domain spectroscopy of the sample to be measured, and converting into a terahertz absorption spectrogram; extracting the terahertz characteristic peak frequency of the sample to be measured in the terahertz absorption spectrogram, and calculating the content of the two crystal forms of the mixed crystal form according to the calculation model. The present application uses terahertz spectroscopy technology to establish a new method for measuring the crystal form content of polylactic acid, which can effectively reflect the crystal form change of polylactic acid, quickly, simply and non-destructively determine the relative content, and has practical application value.

Description

Technical Field

[0001] The present invention belongs to a polylactic acid crystal content measurement technology, and in particular relates to a method for measuring the α′ and α crystal content in polylactic acid based on terahertz time-domain spectroscopy. Background Art

[0002] Polylactic acid (PLA) is a linear aliphatic polyester acid synthesized from α-hydroxyl groups. As a green, bio-based polymer, PLA has broad research value and application prospects. PLA is also a semi-crystalline polymer with typical polymorphism, capable of forming different crystalline forms under different crystallization conditions, and even undergoing interconversion. Typically, PLA exists as an α′ crystalline form and / or an α crystalline form. At higher ambient temperatures, PLA forms the α crystalline form, while at lower temperatures, the α′ crystalline form is favored. Although these two crystalline forms have highly similar helical conformations, the α′ crystalline form of PLA exhibits thermodynamic instability and a looser molecular chain than the α crystalline form. Therefore, determining the content of the α′ and α crystalline forms in PLA is of great practical significance for better regulating the macroscopic properties of PLA.

[0003] Currently, commonly used methods for measuring the crystal structure of polylactic acid include differential scanning calorimetry, infrared spectroscopy, and X-ray diffraction. However, differential scanning calorimetry has the significant disadvantage of being destructive to samples; infrared spectroscopy cannot reflect the long-range structure of the polymer and can only characterize the crystal structure from a short-range perspective, lacking integrity, resulting in significant differences in the results calculated from different functional groups; and although X-ray diffraction is widely used for qualitative analysis, its data processing for quantitative analysis is cumbersome and easily affected by the test substrate.

[0004] Terahertz (THz) time-domain spectroscopy (THz) is an emerging and increasingly widely used low-frequency vibrational spectroscopy technique. It can obtain rich information about the overall skeletal motion of macromolecules based on their long-range structure, making it suitable for studying crystal structure, interchain interactions, and higher-order conformations. Furthermore, THz TDS offers the advantages of being non-destructive, transient, and highly penetrating. The lattice vibrations of different polylactic acid (PLA) crystalline forms can be expressed in THz spectra, with their content closely correlated with the characteristic peak frequency. Polylactic acid composed of the α′ and α forms generated by homogeneous crystallization exhibits distinct THz absorption features at 1.82 THz and 2.01 THz, respectively. However, while THz TDS can currently characterize the α′ and α forms of PLA and their transformation processes, it remains limited to qualitative analysis. To date, there have been no reports on the quantitative analysis of PLA crystalline forms, which is crucial for regulating the physical properties of PLA products.

[0005] Therefore, on the basis of the existing terahertz time-domain spectroscopy detection technology, based on the response characteristics of the α′ and α crystal forms of polylactic acid in the terahertz spectrum, there is a practical need to propose a faster and more convenient method for calculating the content of α′ and α crystal forms in polylactic acid. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and provide a method for measuring the content of α′ and α crystal forms in polylactic acid based on terahertz time-domain spectroscopy.

[0007] To solve the technical problem, the solution of the present invention is:

[0008] A method for measuring the content of α′ and α crystal forms in polylactic acid based on terahertz time-domain spectroscopy is provided, comprising the following steps:

[0009] (1) Two types of polylactic acid containing only α′ crystal form and only α crystal form were used as model samples, and terahertz time-domain spectra were collected respectively and converted into corresponding terahertz absorption spectra;

[0010] (2) Extract the terahertz characteristic peak frequencies of the two model samples from the terahertz absorption spectrum and establish a calculation model between the terahertz characteristic peak frequencies and the α′ and α crystal content in polylactic acid;

[0011] (3) Using mixed crystalline polylactic acid as a test sample, collecting the terahertz time-domain spectrum of the test sample and converting it into a terahertz absorption spectrum;

[0012] (4) The terahertz characteristic peak frequency of the sample to be tested is extracted from the terahertz absorption spectrum, and the content of α′ and α crystal forms in the mixed crystal polylactic acid is calculated according to the calculation model.

[0013] As a preferred embodiment of the present invention, in step (1), the model sample is obtained in the following manner:

[0014] (1.1) preparing polylactic acid products of α′ crystal form and α crystal form respectively according to conventional process;

[0015] (1.2) The crystal content of the product was measured by X-ray diffraction, and two products with α′ crystal content and α crystal content of 100% were selected as model samples.

[0016] As a preferred embodiment of the present invention, in the steps (1) and (3), when collecting the terahertz time-domain spectra of the model sample and the sample to be tested, the scanning frequency range used is 0.25 to 2.5 THz.

[0017] As a preferred embodiment of the present invention, in steps (1) and (3), the terahertz time-domain spectrum is subjected to fast Fourier transform according to the following formula to obtain the terahertz absorption spectrum:

[0018]

[0019]

[0020] Where T(ω) is the transfer function; ω is the angular frequency; E sample (ω) and E reference (ω) are the sample signal and nitrogen reference signal respectively; n(ω) is the refractive index; c is the speed of light 3×10 8 m / s; i is the imaginary unit; a(ω) is the absorption coefficient; k(ω) is the extinction coefficient; d is the sample thickness.

[0021] As a preferred embodiment of the present invention, in step (3), the polylactic acid sample in the sample to be tested is any one of the following: L-polylactic acid, D-polylactic acid, stereocomplex polylactic acid or polylactic acid copolymer.

[0022] As a preferred embodiment of the present invention, in steps (2) and (4), when extracting the terahertz characteristic peak frequency of the sample from the terahertz absorption spectrum, the horizontal coordinate corresponding to the maximum vertical coordinate of the characteristic peak of the terahertz spectrum is used as the characteristic frequency.

[0023] As a preferred embodiment of the present invention, in step (2), the terahertz characteristic peak frequency of the model sample containing only α′ crystal (the α′ crystal and α crystal contained therein are 100% and 0%, respectively) is 1.82 THz; the terahertz characteristic peak frequency of the model sample containing only α crystal (the α′ crystal and α crystal contained therein are 0% and 100%, respectively) is 2.01 THz.

[0024] As a preferred embodiment of the present invention, in step (2), the calculation model is as follows:

[0025]

[0026] Where Xα′ and Xα are the contents of α′ and α crystal forms in PLA, respectively; fi is the characteristic frequency of mixed crystal form PLA in the sample.

[0027] As a preferred embodiment of the present invention, step (3) comprises:

[0028] (3.1) Preparing a mixed crystalline polylactic acid product according to conventional process;

[0029] (3.2) The crystal content in the product was measured by X-ray diffraction, and samples containing both α′ crystal and α crystal were selected as the test samples.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] Compared to traditional detection methods, terahertz time-domain spectroscopy (TDS) offers the advantages of being quick, simple, and non-destructive while sensitively responding to changes in crystal content. This method effectively addresses deficiencies in the rapid quantitative analysis of polylactic acid (PLA). This invention utilizes TDS to establish a new method for measuring the crystalline form content of PLA. This method effectively reflects changes in PLA crystalline form, allowing for a rapid, simple, and non-destructive determination of the relative content of the α′ and α crystal forms in PLA.

[0032] 2. The polylactic acid crystal content measurement method provided by the present invention achieves values ​​close to those obtained by X-ray diffraction. However, compared with X-ray diffraction, the present invention offers the advantages of being faster and more intuitive. Therefore, the present invention can largely overcome the shortcomings of existing traditional detection technologies and has practical application value.

[0033] 3. This invention can also provide a reference for qualitative and quantitative analysis of other polymers and even the entire field of materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a flow chart of the measurement method of the present invention;

[0035] Figure 2 This is the terahertz absorption spectrum of α′-crystalline polylactic acid used as a model sample;

[0036] Figure 3 This is the terahertz absorption spectrum of α-crystalline polylactic acid used as a model sample;

[0037] Figure 4 is a numerical diagram of characteristic frequencies of mixed crystalline polylactic acid as the sample to be tested;

[0038] Figure 5 It is a comparison chart of the α′ and α crystal content (calculated value) and the actual value in the tested sample. DETAILED DESCRIPTION

[0039] The present invention will be further described below with reference to the accompanying drawings and examples.

[0040] The present invention provides a method for measuring the content of α′ and α crystal forms in polylactic acid based on terahertz time-domain spectroscopy. This method uses the characteristic frequencies caused by the lattice vibrations of different polylactic acid crystal forms in the terahertz spectrum to identify and calculate the content of α′ and α crystal forms in mixed polylactic acid. The measurement method includes the following steps:

[0041] (1) Two types of polylactic acid containing only α′ crystal form and only α crystal form were used as model samples, and terahertz time-domain spectra were collected respectively and converted into corresponding terahertz absorption spectra;

[0042] (1.1) preparing polylactic acid products of α′ crystal form and α crystal form respectively according to conventional process;

[0043] (1.2) The crystal content of the product was measured by X-ray diffraction, and two products with α′ crystal content and α crystal content of 100% were selected as model samples.

[0044] (1.3) The terahertz time-domain spectra of the model samples were collected respectively, and the terahertz absorption spectra were obtained by fast Fourier transform according to the following formula:

[0045]

[0046]

[0047] Where T(ω) is the transfer function; ω is the angular frequency; E sample (ω) and E reference (ω) are the sample signal and nitrogen reference signal respectively; n(ω) is the refractive index; c is the speed of light 3×10 8 m / s; i is the imaginary unit; a(ω) is the absorption coefficient; k(ω) is the extinction coefficient; d is the sample thickness.

[0048] Terahertz time-domain spectroscopy acquires frequency-domain spectrum data. After obtaining parameters such as the sample's refractive index and extinction coefficient using other known testing methods, these values ​​are substituted into the above formula and transformed to obtain absorption spectrum data, which is then usable data for analysis.

[0049] (2) According to the horizontal coordinate corresponding to the maximum value of the vertical coordinate of the characteristic peak of the terahertz spectrum, the terahertz characteristic peak frequency of the model sample is extracted from the terahertz absorption spectrum.

[0050] A calculation model between the terahertz characteristic peak frequency and the α′ and α crystal content in polylactic acid was established, as shown in the following formula:

[0051]

[0052] Where Xα′ and Xa are the contents of α′ and α crystal forms in PLA, respectively; fi is the characteristic frequency of mixed crystal form PLA in the sample.

[0053] (3) collecting the terahertz time-domain spectrum of the mixed crystalline polylactic acid sample to be tested and converting it into a corresponding terahertz absorption spectrum;

[0054] (3.1) Preparing a mixed crystalline polylactic acid product according to conventional process;

[0055] (3.2) The crystal content in the product was measured by X-ray diffraction, and samples containing both α′ crystal and α crystal were selected as the test samples.

[0056] (3.3) Collect the terahertz time-domain spectrum of the sample to be tested and perform fast Fourier transform according to the following formula to obtain the terahertz absorption spectrum:

[0057]

[0058]

[0059] Where T(ω) is the transfer function; ω is the angular frequency; E sample (ω) and E reference (ω) are the sample signal and nitrogen reference signal respectively; n(ω) is the refractive index; c is the speed of light 3×10 8 m / s; i is the imaginary unit; a(ω) is the absorption coefficient; k(ω) is the extinction coefficient; d is the sample thickness.

[0060] (4) According to the horizontal coordinate corresponding to the maximum value of the vertical coordinate of the characteristic peak of the terahertz spectrum, the terahertz characteristic peak frequency of the sample to be tested is extracted from the terahertz absorption spectrum diagram.

[0061] Similarly, according to the calculation model in the aforementioned step (2), the contents of α′ and α crystal forms in the mixed crystalline polylactic acid are calculated. Specific embodiment 1

[0063] In this embodiment, homemade polylactic acid products are used as the model sample and the sample to be tested.

[0064] (1) Two types of polylactic acid, each with only α′ and α crystal forms, were used as model samples to collect terahertz time-domain spectra and convert them into corresponding terahertz absorption spectra; specifically, the following steps were performed:

[0065] (1.1) In the present invention, the polylactic acid sample can be any one of the following: L-polylactic acid, D-polylactic acid, stereocomplex polylactic acid or polylactic acid copolymer.

[0066] In this example, a polylactic acid product containing only α′ and α crystal forms was prepared by melt crystallization at 120°C and cold crystallization at 80°C for a sufficient period of time. (Melt crystallization at 120°C produces only α crystal form, while cold crystallization at 80°C produces only α′ crystal form. After melt crystallization at 80°C, the ratio of α′ to α crystal forms in the polylactic acid changes with increasing crystallization time.)

[0067] (1.2) measuring the crystal form content of the polylactic acid product by X-ray diffraction, using polylactic acid products that meet the 100% content conditions of α′ and α crystal forms as model samples;

[0068] The model samples contained only a single crystalline form of both α′ and α-crystalline polylactic acid. The α′-crystalline polylactic acid contained 100% of the α′-crystalline form, while the α-crystalline polylactic acid contained 100% of the α-crystalline form. X-ray diffraction analysis revealed that the peak positions and peak quantities of the α′-crystalline and α-crystalline polylactic acid were different.

[0069] (1.3) Collect the terahertz time-domain spectrum of the model sample.

[0070] The terahertz time-domain spectrometer produced by Daheng Optoelectronics Co., Ltd. was used to collect spectra of the samples. All testing processes were carried out at room temperature (24°C), with a scanning frequency range of 0.25 to 2.5 THz. The spectral system was continuously purged with high-purity nitrogen to keep the relative humidity of the test environment always less than 3%. The acquired terahertz time-domain spectral data were saved in txt format. The measured samples were measured an average of three times, with each measurement including 1024 scans.

[0071] Perform fast Fourier transform according to the following formula to obtain the terahertz absorption spectrum:

[0072]

[0073]

[0074] Where T(ω) is the transfer function; ω is the angular frequency; E sample (ω) and E reference (ω) are the sample signal and nitrogen reference signal respectively; n(ω) is the refractive index; c is the speed of light 3×10 8 m / s; i is the imaginary unit; a(ω) is the absorption coefficient; k(ω) is the extinction coefficient; d is the sample thickness.

[0075] (2) According to the horizontal coordinate corresponding to the maximum value of the vertical coordinate of the characteristic peak of the terahertz spectrum, the terahertz characteristic peak frequency of the model sample is extracted from the terahertz absorption spectrum, and 1.82 THz and 2.01 THz are determined to be the characteristic peaks of standard α′ and α crystal polylactic acid, respectively.

[0076] A calculation model between the terahertz characteristic peak frequency and content of α′ and α crystal forms in polylactic acid was established, as shown in the following formula:

[0077]

[0078] Where Xα′ and Xα are the contents of α′ and α crystal forms in PLA, respectively; fi is the characteristic frequency of mixed crystal form PLA in the sample.

[0079] In this calculation model, the characteristic terahertz peak frequency of a model sample containing only the α′ crystal is 1.82 THz, with the α′ crystal and α crystal content being 100% and 0%, respectively. The characteristic terahertz peak frequency of a model sample containing only the α crystal is 2.01 THz, with the α′ crystal and α crystal content being 0% and 100%, respectively. 0.19 is a constant.

[0080] (3) Mixed crystalline polylactic acid was prepared by melt crystallization at 80°C for different times. Fifteen mixed crystalline polylactic acid samples were obtained according to the different crystallization times. Then, referring to the aforementioned operation for the model sample, the terahertz time-domain spectra of the mixed crystalline polylactic acid in the test samples were collected and converted into corresponding terahertz absorption spectra.

[0081] (4) According to the horizontal coordinate corresponding to the maximum value of the vertical coordinate of the characteristic peak of the terahertz spectrum, the terahertz characteristic peak frequency of the sample to be tested is extracted from the terahertz absorption spectrum, recorded as fi, and the characteristic peak exists in the range of 1.82~2.01THz;

[0082] Finally, the α′ and α crystal contents in the mixed crystal polylactic acid were calculated according to the above-mentioned calculation model, and the results were as follows: the terahertz frequencies of the 15 tested mixed crystal polylactic acids were 1.86, 1.93, 1.94, 1.94, 1.95, 1.95, 1.96, 1.97, 1.97, 1.97, 1.97, 1.97, 1.97, 1.97, and 1.97 THz, respectively.

[0083] Effect evaluation:

[0084] In order to evaluate the accuracy of the measurement method of the present invention, the concept of correlation coefficient R is introduced.

[0085] The correlation coefficient R specifically refers to a statistical indicator that reflects the closeness of the correlation between variables. The correlation coefficient is calculated using the product difference method, which is also based on the deviation of two variables from their respective means. The degree of correlation between the two variables is reflected by multiplying the two deviations. The expression is:

[0086]

[0087] Among them, x i and y i represent the actual value and calculated value of the crystal form content in the i-th polylactic acid sample to be tested, and represent the arithmetic mean of the actual value and the calculated value of the crystal form content in each polylactic acid sample to be tested, and n represents the number of polylactic acid samples to be tested.

[0088] Therefore, the accuracy of the crystal form content calculation can be evaluated based on the size of the R value. The closer the R value is to 1, the closer the calculated result is to the actual value.

[0089] like Figure 5 As shown, the α′ and α crystal content (calculated values) in 15 mixed crystalline polylactic acids to be tested were calculated based on the established calculation model and compared with the crystal content results obtained by X-ray diffraction measurement. The correlation coefficient of the results obtained in the present invention is greater than 0.99. Therefore, it can be proved that the measurement results of the α′ and α crystal content in mixed crystalline polylactic acid obtained by the present invention meet the accuracy requirements and have practical application significance.

Claims

1. A method for measuring the content of α' and α crystal forms in polylactic acid based on terahertz time-domain spectroscopy, characterized in that: The following steps are involved: (1) Two types of polylactic acid containing only α' crystal form and only α crystal form were used as model samples, and terahertz time-domain spectra were collected respectively and converted into corresponding terahertz absorption spectra; (2) Extract the terahertz characteristic peak frequencies of the two model samples from the terahertz absorption spectrum and establish a calculation model between the terahertz characteristic peak frequencies and the α' and α crystal content in polylactic acid; Among them, the terahertz characteristic peak frequency of the model sample containing only α' crystal is 1.82THz; the terahertz characteristic peak frequency of the model sample containing only α crystal is 2.01THz; The calculation model is shown below: Where Xα' and Xα are the contents of α' and α crystal forms in polylactic acid, respectively; fi is the characteristic frequency of mixed crystal form polylactic acid in the sample; (3) Using mixed crystalline polylactic acid as a test sample, collecting the terahertz time-domain spectrum of the test sample and converting it into a terahertz absorption spectrum; (4) The terahertz characteristic peak frequency of the sample to be tested is extracted from the terahertz absorption spectrum, and the content of α' and α crystal forms in the mixed crystal polylactic acid is calculated according to the calculation model.

2. The method according to claim 1, characterized in that In step (1), the model sample is obtained in the following manner: (1.1) preparing polylactic acid products of α' crystal form and α crystal form respectively according to conventional process methods; (1.2) The crystal form content in the product was measured by X-ray diffraction method, and two products with α' crystal form content and α crystal form content of 100% were selected as model samples.

3. The method according to claim 1, characterized in that In the steps (1) and (3), when collecting the terahertz time-domain spectra of the model sample and the sample to be tested, the scanning frequency range used is 0.25 to 2.5 THz.

4. The method according to claim 1, wherein In steps (1) and (3), the terahertz time-domain spectrum is subjected to fast Fourier transform according to the following formula to obtain the terahertz absorption spectrum: Where T(ω) is the transfer function; ω is the angular frequency; E sample (ω) and E reference (ω) are the sample signal and nitrogen reference signal respectively; n(ω) is the refractive index; c is the speed of light 3×10 8 m / s; i is the imaginary unit; a(ω) is the absorption coefficient; k(ω) is the extinction coefficient; d is the sample thickness.

5. The method according to claim 1, wherein In the step (3), the polylactic acid sample in the sample to be tested is any one of the following: left-handed polylactic acid, right-handed polylactic acid, stereocomplex polylactic acid or polylactic acid copolymer.

6. The method according to claim 1, characterized in that In the steps (2) and (4), when extracting the terahertz characteristic peak frequency of the sample from the terahertz absorption spectrum, the abscissa corresponding to the maximum value of the ordinate of the characteristic peak of the terahertz spectrum is taken as the characteristic frequency.

7. The method according to claim 1, characterized in that The step (3) comprises: (3.1) Preparing a mixed crystalline polylactic acid product according to conventional process; (3.2) The crystal content in the product was measured by X-ray diffraction, and samples containing both α' crystal and α crystal were selected as the test samples.

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