CN116297299A - Polymer crystallinity measurement method based on terahertz time-domain spectroscopy - Google Patents
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- 229920000642 polymer Polymers 0.000 title claims abstract description 36
- 238000001328 terahertz time-domain spectroscopy Methods 0.000 title claims abstract description 9
- 238000000691 measurement method Methods 0.000 title claims abstract description 7
- 239000000523 sample Substances 0.000 claims abstract description 66
- 238000000034 method Methods 0.000 claims abstract description 34
- 238000001228 spectrum Methods 0.000 claims abstract description 31
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims abstract description 28
- 238000000862 absorption spectrum Methods 0.000 claims abstract description 26
- 238000005259 measurement Methods 0.000 claims abstract description 18
- 239000013074 reference sample Substances 0.000 claims abstract description 17
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 13
- 238000002425 crystallisation Methods 0.000 claims abstract description 8
- 230000008025 crystallization Effects 0.000 claims abstract description 7
- 238000001035 drying Methods 0.000 claims abstract description 5
- 239000013078 crystal Substances 0.000 claims description 21
- 238000012360 testing method Methods 0.000 claims description 11
- 238000010521 absorption reaction Methods 0.000 claims description 9
- 230000003595 spectral effect Effects 0.000 claims description 6
- 230000005684 electric field Effects 0.000 claims description 4
- 238000010926 purge Methods 0.000 claims description 4
- 230000008033 biological extinction Effects 0.000 claims description 3
- 238000012546 transfer Methods 0.000 claims description 3
- 229910001873 dinitrogen Inorganic materials 0.000 claims description 2
- 238000005516 engineering process Methods 0.000 abstract description 6
- 230000007547 defect Effects 0.000 abstract description 3
- 238000001514 detection method Methods 0.000 abstract description 3
- 238000002441 X-ray diffraction Methods 0.000 description 9
- 229920000747 poly(lactic acid) Polymers 0.000 description 9
- 239000004626 polylactic acid Substances 0.000 description 9
- 238000004364 calculation method Methods 0.000 description 7
- 238000000113 differential scanning calorimetry Methods 0.000 description 5
- 239000000463 material Substances 0.000 description 5
- 238000012512 characterization method Methods 0.000 description 2
- 238000000326 densiometry Methods 0.000 description 2
- 230000001066 destructive effect Effects 0.000 description 2
- 239000006185 dispersion Substances 0.000 description 2
- 238000002329 infrared spectrum Methods 0.000 description 2
- 230000003287 optical effect Effects 0.000 description 2
- 229920006381 polylactic acid film Polymers 0.000 description 2
- 238000004611 spectroscopical analysis Methods 0.000 description 2
- 239000000758 substrate Substances 0.000 description 2
- 230000009286 beneficial effect Effects 0.000 description 1
- 238000011161 development Methods 0.000 description 1
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- 230000000694 effects Effects 0.000 description 1
- 238000011156 evaluation Methods 0.000 description 1
- 230000010354 integration Effects 0.000 description 1
- 230000005865 ionizing radiation Effects 0.000 description 1
- 150000002605 large molecules Chemical class 0.000 description 1
- 238000007620 mathematical function Methods 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
- 230000008018 melting Effects 0.000 description 1
- 238000004476 mid-IR spectroscopy Methods 0.000 description 1
- 230000000149 penetrating effect Effects 0.000 description 1
- 239000002861 polymer material Substances 0.000 description 1
- 238000004451 qualitative analysis Methods 0.000 description 1
- 238000000926 separation method Methods 0.000 description 1
- 230000001052 transient effect Effects 0.000 description 1
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- G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
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- G01N21/3581—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation
- G01N21/3586—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light using far infrared light; using Terahertz radiation by Terahertz time domain spectroscopy [THz-TDS]
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Abstract
The invention relates to a polymer crystallinity measurement technology, and aims to provide a polymer crystallinity measurement method based on terahertz time-domain spectroscopy. Comprising the following steps: taking nitrogen as a reference sample, and taking a crystalline polymer sample after drying treatment as a sample to be detected; respectively collecting terahertz time-domain spectrums of a reference sample and a sample to be detected; performing fast Fourier transform on the obtained terahertz time-domain spectrum to obtain a terahertz absorption spectrum; determining a terahertz characteristic peak of a sample to be detected in a terahertz absorption spectrum; performing curve fitting on the terahertz absorption spectrum of the sample to be measured, and calculating the peak area according to the terahertz characteristic peak; the crystallinity of the polymer samples was calculated. The invention can effectively reflect the generation and the proportion of the polymer crystallization area, thereby rapidly, simply and nondestructively determining the relative crystallinity; the defect of the traditional detection technology can be made up to the greatest extent, and the method has practical application value.
Description
Technical Field
The invention belongs to a polymer crystallinity measurement technology, and particularly relates to a polymer crystallinity measurement method based on terahertz time-domain spectroscopy.
Background
Polymeric materials are defined as materials in which a plurality of identical, simple structural units are repeatedly linked by covalent bondsThe grafted high molecular weight compound (generally at 10 3 ～10 7 ). The regularly arranged regions of the polymer are referred to as crystalline regions, while the disordered regions are referred to as amorphous regions, the percentage of crystalline regions being referred to as crystallinity. In an actual crystalline polymer, the crystalline and amorphous regions are typically present at the same time. The degree of crystallization of a polymer significantly affects its various physical and mechanical properties. For example, the higher the crystallinity, the higher the melting point of the same polymeric material in general. Therefore, regardless of whether it is a theoretical study or practical application, crystallinity is an important index describing the properties of a polymer, and it is also a very important task to calculate its relative values quantitatively.
Currently, common methods for measuring the crystallinity of polymers are X-ray diffraction, mid-IR spectroscopy, differential scanning calorimetry, densitometry, and the like. The measurement and calculation of the X-ray diffraction method is limited by the purity of the sample and the test substrate, and different test substrates have obvious influence on the crystallinity calculation result and generate ionizing radiation on the sample. Mid-infrared spectra are generally applied to qualitative analysis of materials, and quantitative calculation of the crystallinity of polymers is limited by the remarkable degree and separation degree of absorption peaks of crystalline phases and amorphous phases, and in addition, most of mid-infrared spectra calculate the crystallinity based on the intensity difference of the absorption peaks, and the method lacks rigor. Differential scanning calorimetry can completely destroy the sample; the range of errors in the results of densitometry is large. Therefore, the existing conventional crystallinity detection technology is difficult to meet the rapid development of the polymer material characterization field to a certain extent.
Today, vibration spectroscopy techniques are being developed vigorously due to their advantages of non-destructive, penetrating, and transient properties. Terahertz spectroscopy is an emerging non-destructive technique for directly measuring dipole transitions, among other things. The 0.1-10 THz, 0.03-3 mm band is called the terahertz region. The terahertz time-domain spectroscopy system can obtain the optical physical information such as refractive index, absorption coefficient, dielectric constant and the like of the sample in the terahertz wave band.
Terahertz waves are very sensitive to the higher-order structure of the polymer, and the crystal structure of the polymer can be directly observed. Similar to the X-ray diffraction method, the vibration mode of a polymer in the terahertz wave band is derived from the crystalline region of the polymer, while the amorphous region of the polymer has no obvious spectral characteristics due to the lack of a long-range structure, so that the terahertz spectral shape changes along with the change of the crystallinity of the polymer. The terahertz spectrum obtained by the conventional terahertz measurement technology is still limited to qualitative characterization of materials at present, and effective measurement of the crystallinity of the polymer is not realized, and the biggest reason is that the vibration mode of the polymer is difficult to be well resolved due to the influence of a huge unit. The invention provides an effective method for calculating the crystallinity of a polymer through the integration area after terahertz absorption spectrum fitting.
Disclosure of Invention
The technical problems to be solved by the invention are as follows: the method for measuring the crystallinity of the polymer based on the terahertz time-domain spectroscopy is provided for overcoming the defects of the prior art.
In order to solve the technical problems, the invention adopts the following solutions:
the polymer crystallinity measurement method based on terahertz time-domain spectroscopy comprises the following steps:
(1) Taking nitrogen as a reference sample, and taking a crystalline polymer sample after drying treatment as a sample to be detected; respectively collecting terahertz time-domain spectrums of a reference sample and a sample to be detected;
(2) Performing fast Fourier transform on the obtained terahertz time-domain spectrum to obtain a terahertz absorption spectrum;
(3) Determining a terahertz characteristic peak of a sample to be detected in a terahertz absorption spectrum;
(4) Performing curve fitting on the terahertz absorption spectrum of the sample to be measured, and calculating the peak area according to the terahertz characteristic peak;
(5) The crystallinity of the polymer samples was calculated.
As a preferable mode of the present invention, the purity of the nitrogen gas used as the reference sample in the step (1) is 97% or more.
As a preferred embodiment of the present invention, the operation content of the measurement in the step (1) specifically includes:
under the room temperature condition, carrying out spectrum acquisition on a sample to be detected by using a terahertz time-domain spectrometer; continuously purging with nitrogen in the acquisition process, and keeping the relative humidity of the test environment to be less than 3% all the time; and storing the acquired terahertz time-domain spectrum data in txt format.
As a preferred embodiment of the present invention, the step (2) specifically includes:
recording frequency domain spectrums of terahertz electric fields of a reference sample and a test sample respectively, and performing fast Fourier transform on the obtained terahertz time domain spectrums according to the following formula to obtain terahertz absorption spectrums:
wherein T (ω) is a transfer function; omega is the angular frequency; e (E) sample (omega) and E reference (ω) sample signal and reference signal, respectively; n (ω) is the refractive index; c is the light speed of 3X 10 8 m/s; i is an imaginary unit; a (ω) is the absorption coefficient; k (ω) is an extinction coefficient; d is the sample thickness.
As a preferred embodiment of the present invention, the step (3) specifically includes:
and analyzing the terahertz absorption spectrum, and determining a terahertz characteristic peak according to the spectral characteristics of the sample to be detected.
As a preferred embodiment of the present invention, the step (4) specifically includes:
(4.1) performing curve fitting on a terahertz absorption spectrum of a sample to be tested by utilizing a Lorentz function based on a Lorentz oscillator model;
(4.2) according to the terahertz characteristic peak condition of the sample to be measured in the fitting curve, taking the integral intensity of the crystallization area and the amorphous area as peak areas, respectively marking as I crystal And I amorphous 。
As a preferred embodiment of the present invention, the step (5) specifically includes:
calculating the crystallinity X of the sample to be measured according to the following C ：
X C ＝I crystal /(I crystal +I amorphous )
Wherein X is C For the crystallinity of the sample to be tested, I crystal To fit the peak area of the crystalline region in the curve, I amorphous To fit the peak areas of the amorphous regions in the curve.
Compared with the prior art, the invention has the beneficial technical effects that:
1. the invention is based on terahertz absorption spectrum, utilizes Lorentz function fitting curve, can effectively reflect the generation and occupied proportion of polymer crystallization area, thereby rapidly, simply and nondestructively determining the relative crystallinity.
2. The method for measuring the crystallinity of the polymer provided by the invention has the advantages that the crystallinity measured value is close to the data obtained by a differential scanning calorimeter and an X-ray diffraction method, and the measurement accuracy meets the actual application requirements. Under the premise, compared with a differential scanning calorimeter and an X-ray diffraction method, the invention has the additional advantages of rapidness and no damage. Therefore, the invention can make up the defects of the traditional detection technology to the greatest extent and has practical application value.
Drawings
FIG. 1 is a flow chart of the method of the present invention;
FIG. 2 is a schematic diagram of a terahertz absorption spectrum curve fitting process of a polymer sample;
FIG. 3 is a graph showing the comparison of the crystallinity results obtained by the measurement of the present invention with the crystallinity results obtained by the measurement of the X-ray diffraction method;
FIG. 4 is a graph comparing the results of crystallinity obtained by the inventive measurement with the results of crystallinity obtained by differential scanning calorimetry.
Detailed Description
The invention is further described below with reference to the drawings and examples.
The invention provides a polymer crystallinity measurement method based on terahertz time-domain spectroscopy, which comprises the following steps:
(1) Taking nitrogen as a reference sample, and taking a crystalline polymer sample after drying treatment as a sample to be detected; respectively collecting terahertz time-domain spectrums of a reference sample and a sample to be detected;
the operation content of the measurement specifically comprises: under the room temperature condition (such as 24 ℃), respectively carrying out spectrum acquisition on nitrogen and a sample to be tested by using a terahertz time-domain spectrometer, and continuously purging with high-purity nitrogen in the acquisition process so as to keep the relative humidity of a test environment to be less than 3% all the time; the sample to be measured is collected at least three times, 1024 scans are carried out for each collection, and the obtained terahertz time-domain spectrum data are stored in txt format.
The spectrum acquisition of the sample to be detected is carried out in a nitrogen environment, so that nitrogen (test background) is used as a reference sample. Therefore, after the terahertz spectrum of the sample to be detected is obtained, the influence of nitrogen is eliminated.
(2) Performing fast Fourier transform on the terahertz time-domain spectrum to obtain a terahertz absorption spectrum; the method specifically comprises the following steps:
recording frequency domain spectrums of terahertz electric fields of a reference sample and a test sample respectively, and performing fast Fourier transform on the obtained terahertz time domain spectrums according to the following formula to obtain terahertz absorption spectrums:
wherein T (ω) is a transfer function; omega is the angular frequency; e (E) sample (omega) and E reference (ω) sample signal and reference signal, respectively; n (ω) is the refractive index; c is the light speed of 3X 10 8 m/s; i is an imaginary unit; a (ω) is the absorption coefficient; k (ω) is an extinction coefficient; d is the sample thickness.
(3) And analyzing the terahertz absorption spectrum, and determining a terahertz characteristic peak according to the spectral characteristics of the sample to be detected.
(4) Based on the Lorentz vibrator model, performing curve fitting on the terahertz absorption spectrum of the sample to be tested by utilizing a Lorentz function;
according to the terahertz characteristic peak condition of the sample to be measured in the fitting curve, taking the integral intensity of the crystallization area and the amorphous area as peak areas, respectively marking as I crystal And I amorphous 。
Because the experimental data points are discontinuous, the invention uses a mathematical function model to fit the experimental data to obtain better experimental results, and uses the fitted curve as the calculation basis of the crystallinity.
(5) Calculating the crystallinity X of the sample to be measured according to the following C ：
X C ＝I crystal /(I crystal +I amorphous )
Wherein X is C For the crystallinity of the sample to be tested, I crystal To fit the peak area of the crystalline region in the curve, I amorphous To fit the peak areas of the amorphous regions in the curve.
Example 1
In the example, polylactic acid is used as a sample to be detected, and nitrogen with the purity of more than 97% is used as a reference sample.
(1) Respectively preparing 7 polylactic acid films with different crystallinity by a solution crystallization method, and taking the polylactic acid films as samples to be tested after drying treatment, so as to prevent moisture from affecting terahertz time-domain spectrum measurement results;
and respectively measuring the reference sample and the sample to be measured by using a terahertz time-domain spectrometer produced by a large constant-voltage optical company so as to acquire terahertz time-domain spectrums. All measurement operations are carried out at room temperature (24 ℃), the sample to be measured is measured for three times on average, and each measurement comprises 1024 scans; continuously purging with high-purity nitrogen in the measurement process to keep the relative humidity of the test environment to be less than 3% all the time; and storing the acquired terahertz time-domain spectrum data in txt format.
(2) Performing fast Fourier transform on the obtained terahertz time-domain spectrum to obtain a corresponding terahertz absorption spectrum: recording time domain spectrums of terahertz electric fields of the reference sample and the test sample, and performing fast Fourier transform on the obtained time domain spectrums and terahertz spectrum absorption according to the formula to obtain terahertz absorption spectrums.
(3) And analyzing the terahertz absorption spectrum, and determining a terahertz characteristic peak according to the spectral characteristics of the polylactic acid.
Taking fig. 2 as an example, in the terahertz spectrum of polylactic acid, the relative absorption intensity at 2.01THz is significantly higher than the absorption intensity at the nearby frequency, and the peak state is presented, and the characteristic peak frequency of polylactic acid is determined.
(4) Based on the Lorentz vibrator model, performing curve fitting on the terahertz absorption spectrum of the sample to be tested by utilizing a Lorentz function; according to the terahertz characteristic peak condition of the sample to be measured in the fitting curve, taking the integral intensity of the crystallization area and the amorphous area as peak areas, respectively marking as I crystal And I amorphous 。
Taking fig. 2 as an example, the crystallinity is calculated as in the X-ray diffraction method, and in the figure, by lorentz function fitting, the base line divides two regions of the fitting curve line: a crystal phase (black region) and an amorphous phase region (diagonal line region), the upper half of the stippling line is a characteristic vibration caused by the crystal phase, a significant characteristic peak appears, the lower half is a vibration caused by the amorphous phase, and no characteristic peak appears.
(5) Calculating the crystallinity X of the sample to be measured according to the following C ：
X C ＝I crystal /(I crystal +I amorphous )
Wherein X is C For the crystallinity of the sample to be tested, I crystal To fit the peak area of the crystalline region in the curve, I amorphous To fit the peak areas of the amorphous regions in the curve.
In this example, the crystallinity results of 7 polylactic acid samples to be measured were obtained by calculation and were 18.1%, 21.4%, 25.1%, 25.7%, 26.4%, 28.5% and 31%, respectively.
Evaluation of results:
in order to evaluate the accuracy of the measuring method of the invention, the concept of a correlation coefficient R is introduced.
The correlation coefficient R specifically means: a statistical indicator reflecting how closely the variables are related. The correlation coefficient is calculated according to a product difference method, and the correlation degree between two variables is reflected by multiplying the two dispersions on the basis of the dispersion of the two variables and the average value of the two variables, wherein the expression is as follows:
wherein x is i And y i Respectively representing the actual value and the calculated value of the crystal form content in the ith polylactic acid sample to be detected,
and->
Respectively representing the actual value of the crystal form content in each polylactic acid sample to be detected and the arithmetic average value of the calculated value, and n represents the number of the polylactic acid samples to be detected.
Therefore, the accuracy of the calculation of the crystallinity can be evaluated according to the magnitude of the R value. The closer the R value is to 1, the closer the calculation result is to the actual value.
As shown in fig. 3 and 4, the crystallinity results obtained by the measurement of the present invention are compared with the crystallinity results obtained by the measurement of the present invention by using an X-ray diffraction method and a differential scanning calorimetry method, and the correlation coefficients of the crystallinity (calculated value) of the obtained 7 polylactic acid samples and the data measured by the X-ray diffraction method and the differential scanning calorimetry method are all greater than 0.98. Therefore, the crystallinity result obtained by the measuring method provided by the invention can meet the accuracy requirement, and has practical application significance.
Claims (7)
1. The polymer crystallinity measurement method based on terahertz time-domain spectroscopy is characterized by comprising the following steps of:
(1) Taking nitrogen as a reference sample, and taking a crystalline polymer sample after drying treatment as a sample to be detected; respectively collecting terahertz time-domain spectrums of a reference sample and a sample to be detected;
(2) Performing fast Fourier transform on the obtained terahertz time-domain spectrum to obtain a terahertz absorption spectrum;
(3) Determining a terahertz characteristic peak of a sample to be detected in a terahertz absorption spectrum;
(4) Performing curve fitting on the terahertz absorption spectrum of the sample to be measured, and calculating the peak area according to the terahertz characteristic peak;
(5) The crystallinity of the polymer samples was calculated.
2. The method according to claim 1, wherein the purity of nitrogen gas used as the reference sample in the step (1) is 97% or more.
3. The method according to claim 1, wherein the operation content of the measurement in step (1) specifically includes:
under the room temperature condition, carrying out spectrum acquisition on a sample to be detected by using a terahertz time-domain spectrometer; continuously purging with nitrogen in the acquisition process, and keeping the relative humidity of the test environment to be less than 3% all the time; and storing the acquired terahertz time-domain spectrum data in txt format.
4. The method according to claim 1, characterized in that: the step (2) specifically comprises:
recording frequency domain spectrums of terahertz electric fields of a reference sample and a test sample respectively, and performing fast Fourier transform on the obtained terahertz time domain spectrums according to the following formula to obtain terahertz absorption spectrums:
wherein T (ω) is a transfer function; omega is the angular frequency; e (E) sample (omega) and E reference (ω) sample signal and reference signal, respectively; n (ω) is the refractive index; cIs 3X 10 of the light speed 8 m/s; i is an imaginary unit; a (ω) is the absorption coefficient; k (ω) is an extinction coefficient; d is the sample thickness.
5. The method according to claim 1, characterized in that: the step (3) specifically comprises:
and analyzing the terahertz absorption spectrum, and determining a terahertz characteristic peak according to the spectral characteristics of the sample to be detected.
6. The method according to claim 1, characterized in that: the step (4) specifically comprises:
(4.1) performing curve fitting on a terahertz absorption spectrum of a sample to be tested by utilizing a Lorentz function based on a Lorentz oscillator model;
(4.2) according to the terahertz characteristic peak condition of the sample to be measured in the fitting curve, taking the integral intensity of the crystallization area and the amorphous area as peak areas, respectively marking as I crystal And I amorphous 。
7. The method according to claim 1, characterized in that: the step (5) specifically comprises:
calculating the crystallinity X of the sample to be measured according to the following C ：
X C ＝I crystal /(I crystal +I amorphous )
Wherein X is C For the crystallinity of the sample to be tested, I crystal To fit the peak area of the crystalline region in the curve, I amorphous To fit the peak areas of the amorphous regions in the curve.
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| CN117332258A (en) * | 2023-12-01 | 2024-01-02 | 奥谱天成(成都)信息科技有限公司 | Near infrared absorption peak identification method, system and medium based on multi-scale Lorentz |
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| CN117332258B (en) * | 2023-12-01 | 2024-01-30 | 奥谱天成(成都)信息科技有限公司 | Near infrared absorption peak identification method, system and medium based on multi-scale Lorentz |
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