Method for monitoring secondary transformation of polymer in situ by utilizing cluster luminescence

Abstract

The invention discloses a method for monitoring secondary transformation of a polymer in situ by utilizing a fluorescence signal of cluster luminescence, which comprises the following steps of: testing the polymer with a cluster luminescence phenomenon by adopting a fluorescence spectrum, and determining the secondary transformation temperature of the polymer by monitoring the relationship of the autofluorescence intensity of the polymer along with the temperature change. By monitoring the relationship of the autofluorescence intensity of the polymer along with the temperature change, in-situ monitoring of the secondary transformation temperature of the polymer can be realized under the conditions of no damage and no need of contact with a force field. Meanwhile, the method is suitable for polymers in various forms, the demand quantity can be as low as milligram level, any processing is not needed, and the application range is wide.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer analysis, and particularly relates to a method for in-situ monitoring of polymer secondary transitions using the fluorescence signal of cluster luminescence. By monitoring the relationship between the spontaneous fluorescence intensity of the polymer and temperature changes, the polymer secondary transition temperature and its thermal motion characteristics are determined. Background Art

[0002] The thermal motion transitions of polymers at different temperatures are usually closely related to their thermal stability, mechanical properties, molecular structure, etc. The secondary transition temperature of polymer materials refers to the changes in thermodynamic or kinetic properties caused by the movement of polymer chain segments, usually including transitions at different levels such as α, β, γ, etc. These secondary transitions directly affect many key properties of the material, such as mechanical properties, heat resistance, barrier properties, wear resistance, etc. Therefore, the study of the secondary transition behavior of polymers is of great significance. In practical applications, the study of the secondary transitions of polymers is one of the key technologies for improving the use performance of materials and optimizing the design.

[0003] In polymer thermal transition testing, "in-situ" testing refers to directly monitoring the transition process of the material in its natural state without applying other influences (such as external forces or treatments) to the material, so as to directly observe the true behavior of the material. Existing polymer thermal transition monitoring methods mainly rely on thermal analysis techniques, such as differential scanning calorimetry (DSC), thermomechanical analysis (TMA), dynamic mechanical analysis (DMA), and dielectric spectroscopy (DS). Although these methods can accurately detect thermal transition parameters such as the glass transition temperature (T g ) and melting point of polymers, they cannot provide real-time and in-situ monitoring. Therefore, it is of great significance to develop a simple, real-time, and in-situ monitoring method to monitor the thermal transition process of polymers.

[0004] Fluorescence testing is non-destructive and does not require contact with a force field. Fluorescence monitoring reflects the molecular motion and thermal transition of a polymer by detecting changes in the intensity of the spontaneous fluorescence signal inside or on the surface of the polymer. The whole process does not require applying mechanical stress or physical contact to the material, and only relies on temperature control, so as to directly observe the spontaneous optical changes of the material during heating or cooling. This method will not have an additional impact on the physical state of the material itself. In addition, the response speed of fluorescence monitoring is very fast, and it can quickly record the minute changes in the activities of molecular chain segments during the glass transition of the material. It has high sensitivity to the molecular motion state and structural transition of the material, and can reflect information such as the glass transition temperature and other secondary transitions (such as β transition) of the material in real time.

[0005] In recent years, it has been found that for many polymers that do not contain traditional chromophores and do not emit light in the isolated state, they can emit bright and visible fluorescence in the aggregated state. This phenomenon is called clusteroluminescence (CL). Clusteroluminescence usually activates the originally forbidden transition through spatial interactions (such as spatial conjugation effect) and emits fluorescence. Therefore, the chemical structure and conformation of the polymer have a significant impact on the wavelength and intensity of clusteroluminescence. This enables the speculation of the thermal motion behavior transition of the polymer under different temperature environmental conditions by monitoring the change in the spontaneous fluorescence intensity of the polymer. Summary of the Invention

[0006] Aiming at the problems existing in the existing testing technologies for the secondary transition of polymer materials, the present invention discloses for the first time a method for in-situ monitoring of the secondary transition of polymers using the fluorescence signal of clusteroluminescence. By monitoring the relationship between the spontaneous fluorescence intensity of the polymer and the temperature change, the secondary transition temperature of the polymer can be in-situ monitored without damage and without the need for a contact force field. At the same time, the present invention is applicable to various forms of polymers, with the required amount as low as the milligram level, without any processing, and has a wide application range.

[0007] The specific technical solution is as follows:

[0008] A method for in-situ monitoring of the secondary transition of polymers using the fluorescence signal of clusteroluminescence, comprising: testing a polymer with clusteroluminescence phenomenon by fluorescence spectroscopy, and determining the secondary transition temperature of the polymer by monitoring the relationship between the spontaneous fluorescence intensity of the polymer and the temperature change.

[0009] The method for in-situ monitoring of the secondary transition of polymers provided by the present invention is based on the discovery that: clusteroluminescence phenomenon can be observed in a variety of polymers. Even without including traditional polycyclic aromatic hydrocarbon chromophores, electron delocalization can be generated only by using spatial interactions, and a new and stronger fluorescence emission peak can be generated in the aggregated state. This fluorescence emission is closely related to the aggregation state and motion state of the polymer. Under non-disturbed conditions, temperature has a great influence on the motion of the polymer. From low temperature to high temperature, the motion units and motion forms of the polymer will change significantly, thus affecting the clusteroluminescence intensity and wavelength of the polymer. Therefore, the present invention can deduce the secondary transition temperature of the polymer by monitoring the fluorescence change of the polymer at different temperatures.

[0010] In the present invention, the polymer with clusteroluminescence phenomenon may contain electron-rich groups such as carbonyl, ether bond, benzene ring, etc. In some embodiments, the polymer with clusteroluminescence phenomenon may include polyester, etc.

[0011] The method for in-situ monitoring of polymer secondary transitions using the fluorescence signal of cluster luminescence, where the polymer secondary transition temperature may include the glass transition temperature and β-transition temperature of the polymer. The method provided by the present invention has good reliability and operability in realizing in-situ monitoring of various polymer secondary transitions, especially glass transition and β-transition, and has broad application prospects.

[0012] For the method for in-situ monitoring of polymer secondary transitions using the fluorescence signal of cluster luminescence, the excitation wavelength for fluorescence spectrum testing is preferably the excitation wavelength corresponding to the strongest emission peak of the polymer at room temperature (such as 20 - 25 °C, etc.).

[0013] In some embodiments, for the method for in-situ monitoring of polymer secondary transitions using the fluorescence signal of cluster luminescence, fluorescence spectrum testing of the polymer at different temperatures includes the steps of:

[0014] (1) Placing the polymer sample in a temperature-controlled fluorescence testing device;

[0015] (2) The temperature-controlled fluorescence testing device gradually increases the temperature within a set temperature range, records the corresponding fluorescence intensity of the sample at different temperature points, and plots a temperature-fluorescence intensity curve;

[0016] (3) Based on the temperature-fluorescence intensity curve, analyze the temperature dependence of the fluorescence intensity, and determine the polymer secondary transition temperature according to the temperature points where the change rate of the fluorescence intensity with temperature changes significantly.

[0017] In some preferred examples, in step (1), the polymer sample is pre-dried to remove the solvent.

[0018] In some preferred examples, in step (1), after the polymer sample is placed in the temperature-controlled fluorescence testing device, the sample area of the temperature-controlled fluorescence testing device is evacuated to reduce the interference of water vapor and oxidation in the environment.

[0019] In step (2), the set temperature range can be 80 - 400 K, and further can be 80 - 320 K, etc., to cover the temperature range of polymer secondary transitions.

[0020] In step (2), the temperature-controlled fluorescence testing device can use liquid nitrogen for cooling.

[0021] In some preferred examples, in step (2), at the temperature points where the fluorescence intensity of the sample is recorded, hold for a certain period of time (such as 3 minutes, etc.) to make the temperature of the polymer sample consistent with the ambient temperature.

[0022] In some embodiments, in step (3), determining the polymer secondary transition temperature according to the temperature points where the change rate of the fluorescence intensity with temperature changes significantly specifically includes:

[0023] The temperature-fluorescence intensity curve has temperature as the abscissa and fluorescence intensity as the ordinate. The absolute value of the second derivative of each test temperature point on the temperature-fluorescence intensity curve is calculated by the following formula One or more test temperature points with the largest absolute values of the second derivative are selected in descending order. The selected test temperature points can be divided into one or more groups of temperature points, and each group of temperature points consists of consecutive test temperature points. Different groups of temperature points are not continuous and belong to different temperature regions. The minimum temperature points in each group of temperature points are respectively determined as the secondary transition temperatures of the corresponding categories of polymers;

[0024]

[0025] In the formula: T represents the test temperature, I represents the fluorescence intensity, and i is a label.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] 1) Real-time in-situ non-contact monitoring: This method can monitor the change in fluorescence intensity of polymers during the thermal transition process in real time, avoiding the problem that traditional thermal analysis methods require extracting samples and performing off-line tests.

[0028] 2) High sensitivity and simplicity: Only milligram-level samples are required to sensitively detect the thermal transitions of polymers, such as the glass transition and β-transition processes, through the change in the intensity of cluster emission fluorescence.

[0029] 3) Wide application range: This method is applicable to a variety of polymer materials, especially polyester-based and polymers containing aromatic groups, and can provide information about molecular motion groups, chain motion sizes, etc. at different temperatures.

[0030] 4) No doping and no complex sample preparation: The method of the present invention is easy to operate, does not require additional synthesis of fluorescent temperature probes for doping, avoiding measurement errors that may be caused by uneven doping; it does not require complex sample preparation steps, is applicable to various forms of polymers, and can be directly used for fluorescence monitoring of polymer samples. Description of the Drawings

[0031] Figure 1 It is the fluorescence emission spectrum of the polymerization product prepared in Example 1 at room temperature.

[0032] Figure 2 It is the DSC test chart of the polymerization product prepared in Example 1.

[0033] Figure 3 It is the temperature-fluorescence intensity curve of the polymerization product prepared in Example 1 under vacuum conditions.

[0034] Figure 4Prepare the fluorescence emission spectrum of the polymerization product at room temperature for Example 2.

[0035] Figure 5 Prepare the DSC test chart of the polymerization product for Example 2.

[0036] Figure 6 Prepare the temperature-dependent dielectric spectrum of the polymerization product for Example 2.

[0037] Figure 7 Prepare the temperature-fluorescence intensity curve of the polymerization product for Example 2 under vacuum conditions.

[0038] Figure 8 Prepare the fluorescence emission spectrum of the polymerization product at room temperature for Example 3.

[0039] Figure 9 Prepare the DSC test chart of the polymerization product for Example 3.

[0040] Figure 10 Prepare the temperature-fluorescence intensity curve of the polymerization product for Example 3 under vacuum conditions.

[0041] Figure 11 Prepare the fluorescence emission spectrum of the polymerization product at room temperature for Example 4.

[0042] Figure 12 Prepare the DSC test chart of the polymerization product for Example 4.

[0043] Figure 13 Prepare the temperature-fluorescence intensity curve of the polymerization product for Example 4 under vacuum conditions.

[0044] Figure 14 Prepare the fluorescence emission spectrum of the polymerization product at room temperature for Example 5.

[0045] Figure 15 Prepare the DSC test chart of the polymerization product for Example 5.

[0046] Figure 16 Prepare the temperature-fluorescence intensity curve of the polymerization product for Example 5 under vacuum conditions. Detailed implementation manners

[0047] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. The operation methods without specific conditions noted in the following embodiments are usually in accordance with conventional conditions or in accordance with the conditions recommended by the manufacturer.

[0048] Example 1:

[0049] Paraldehyde (0.19 g, 1.46 mmol) and glutaric anhydride (0.5 g, 4.38 mmol) were added to a pressure-resistant tube in a molar ratio of 1:3, and then indium tribromide (15.5 mg, 4.38 μmol), a catalyst with a molar fraction of acid anhydride of 1%, was added. The reaction was carried out for 12 hours under anhydrous and anaerobic conditions to obtain a polyester with cluster luminescence phenomenon as shown in the following structure, where n represents the number of repeating units. The reaction product was dissolved in dichloromethane and then dropped into an ethanol solution for precipitation to remove unreacted monomers and catalysts. The polymer was placed in a vacuum oven at 45 °C (vacuum degree lower than 0.01 MPa) and dried for 24 hours to remove the solvent.

[0050]

[0051] Figure 1 The dried polymer was placed in a fluorescence test fixture. After the installation was completed, the fixture was placed in a temperature control element configured in a fluorescence tester, and the variable excitation spectrum of the polymer was measured at room temperature.

[0052] The sample placement part was evacuated to avoid the condensation of water vapor on the sample at low temperature, which would affect the test results.

[0053] Figure 2 The differential scanning calorimetry (DSC) spectrum of the polymer was given. It can be seen that the glass transition temperature of the polymer measured by this method is 261 K.

[0054] The best excitation wavelength of the polymer at room temperature, 340 nm, was selected as the excitation wavelength in the variable temperature test. In this experiment, the temperature was increased at intervals of 10 K until 320 K, and the polymer was kept at each set temperature (±0.5 K) for 3 minutes, and then the fluorescence data of the polymer at the excitation light wavelength of 340 nm were measured.

[0055] Figure 3 The temperature-fluorescence intensity curve was given, where the temperature was selected as the independent variable, and the highest point of the fluorescence emission peak at this temperature was used as the dependent variable, and the points were plotted on the coordinate.

[0056] Using the following formula:

[0057]

[0058] In the formula: T represents the test temperature, I represents the fluorescence intensity, and i is the label;

[0059] Calculate the absolute value of the second derivative of each test temperature point on the temperature-fluorescence intensity curve Select the top 5 test temperature points with the largest absolute value of the second derivative in descending order. The 5 selected test temperature points can be divided into two groups of temperature points, and each group of temperature points consists of consecutive test temperature points. The different temperature point groups are not continuous and belong to different temperature zones. The minimum temperature point in each group of temperature points is determined as the secondary transition temperature of the corresponding category of the polymer. Finally, the glass transition temperature of the polymer is obtained as 270K, and the β-transition temperature is 170K.

[0060] Example 2:

[0061] Add benzaldehyde (0.46 g, 4.38 mmol) and glutaric anhydride (0.5 g, 4.38 mmol) in a molar ratio of 1:1 to a pressure-resistant tube, and then add indium tribromide (15.5 mg, 4.38 μmol), which is 1% of the molar fraction of the acid anhydride, as a catalyst. React under anhydrous and anaerobic conditions for 12 hours to obtain a polyester with cluster luminescence phenomenon as shown in the following structure, where n represents the number of repeating units. After the reaction, the product is dissolved in dichloromethane and dropped into an ethanol solution for precipitation to remove unreacted monomers and catalysts. The polymer is placed in a vacuum oven at 45 °C (vacuum degree lower than 0.01 MPa) and dried for 24 hours to remove the solvent.

[0062]

[0063] Figure 4 The variable excitation spectrum of the polymer measured at room temperature is given after placing the dried polymer in a fluorescence test fixture and then placing the fixture in the temperature control element configured in the fluorescence tester after installation.

[0064] The sample placement part is evacuated to avoid the condensation of water vapor on the sample at low temperature, which may affect the test results.

[0065] Figure 5 The differential scanning calorimetry (DSC) spectrum of the polymer is given. It can be seen that the glass transition temperature of the polymer measured by this method is 268K.

[0066] Figure 6 The variable temperature dielectric spectrum of the polymer is given. It can be seen that the glass transition temperature of the polymer measured by this method is 280K, and the β-transition temperature is 180K.

[0067] Select the best excitation wavelength of 380 nm of the polymer at room temperature as the excitation wavelength in the variable temperature test. In this experiment, the temperature is increased at intervals of 10K until 320K, and the polymer is kept at each set temperature (±0.5K) for 3 minutes, and then the fluorescence data of the polymer at an excitation light wavelength of 380 nm is measured.

[0068] Figure 7A temperature-fluorescence intensity curve is given, where the temperature is selected as the independent variable and the highest point of the fluorescence emission peak at that temperature is selected as the dependent variable, and the points are plotted on the coordinate.

[0069] Using the same data analysis method as in Example 1, the glass transition temperature of the polymer was found to be 270 K and the β-transition temperature was 190 K.

[0070] Example 3:

[0071] Phenylacetaldehyde (0.53 g, 4.38 mmol) and glutaric anhydride (0.5 g, 4.38 mmol) were added to a pressure-resistant tube in a molar ratio of 1:1, and then indium tribromide (15.5 mg, 4.38 μmol), a catalyst with an acid anhydride molar fraction of 1%, was added. The reaction was carried out for 12 hours under anhydrous and anaerobic conditions to obtain a polyester with cluster luminescence phenomenon as shown in the following structure, where n represents the number of repeating units. The reaction product was dissolved in dichloromethane and then dropped into an ethanol solution for precipitation to remove unreacted monomers and catalysts. The polymer was placed in a vacuum oven at 45 °C (vacuum degree lower than 0.01 MPa) and dried for 24 hours to remove the solvent.

[0072]

[0073] Figure 8 The dried polymer was placed in a fluorescence test fixture. After installation, the fixture was placed in the temperature control element configured in the fluorescence tester, and the variable excitation spectrum of the polymer was measured at room temperature.

[0074] The sample placement part was evacuated to avoid water vapor condensation on the sample at low temperature, which would affect the test results.

[0075] Figure 9 The differential scanning calorimetry (DSC) spectrum of the polymer is given. It can be seen that the glass transition temperature of the polymer measured by this method is 265 K.

[0076] The best excitation wavelength of the polymer at room temperature, 340 nm, was selected as the excitation wavelength in the variable temperature test. In this experiment, the temperature was increased at intervals of 10 K until 320 K, and the sample was kept at each set temperature (±0.5 K) for 3 minutes, and then the fluorescence data of the polymer at the excitation light wavelength of 340 nm were measured.

[0077] Figure 10 A temperature-fluorescence intensity curve is given, where the temperature is selected as the independent variable and the highest point of the fluorescence emission peak at that temperature is selected as the dependent variable, and the points are plotted on the coordinate.

[0078] Using the same data analysis method as in Example 1, the glass transition temperature of the polymer was found to be 270 K and the β-transition temperature was 190 K.

[0079] Example 4:

[0080] Phenylpropionaldehyde (0.59 g, 4.38 mmol) and glutaric anhydride (0.5 g, 4.38 mmol) were added to a pressure-resistant tube in a molar ratio of 1:1, and then indium tribromide (15.5 mg, 4.38 μmol), a catalyst with an acid anhydride molar fraction of 1%, was added. The reaction was carried out for 12 hours under anhydrous and anaerobic conditions to obtain a polyester with cluster luminescence phenomenon as shown in the following structure, where n represents the number of repeating units. The reaction product was dissolved in dichloromethane and then dropped into an ethanol solution for precipitation to remove unreacted monomers and catalysts. The polymer was placed in a vacuum oven at 45 °C (vacuum degree lower than 0.01 MPa) and dried for 24 hours to remove the solvent.

[0081]

[0082] Figure 11 The dried polymer was placed in a fluorescence test fixture. After installation, the fixture was placed in a temperature control element configured in a fluorescence tester, and the variable excitation spectrum of the polymer was measured at room temperature.

[0083] The sample placement part was evacuated to avoid the condensation of water vapor on the sample at low temperature, which would affect the test results.

[0084] Figure 12 The differential scanning calorimetry (DSC) spectrum of the polymer was given. It can be seen that the glass transition temperature of the polymer obtained by this method is 269 K.

[0085] The best excitation wavelength of the polymer at room temperature, 340 nm, was selected as the excitation wavelength in the variable temperature test. In this experiment, the temperature was increased at intervals of 10 K until 320 K, and the sample was kept at each set temperature (±0.5 K) for 3 minutes, and then the fluorescence data of the polymer at the excitation light wavelength of 340 nm were measured.

[0086] Figure 13 The temperature-fluorescence intensity curve was given, where the selected temperature was used as the independent variable and the highest point of the fluorescence emission peak at that temperature was used as the dependent variable, and the points were plotted on the coordinate.

[0087] Using the same data analysis method as in Example 1, the glass transition temperature of the polymer was obtained as 260 K, and the β-transition temperature was 190 K.

[0088] Example 5:

[0089] α-Methylacetaldehyde (0.59 g, 4.38 mmol) and glutaric anhydride (0.5 g, 4.38 mmol) were added to a pressure-resistant tube in a molar ratio of 1:1, and then indium tribromide (15.5 mg, 4.38 μmol), a catalyst with a molar fraction of acid anhydride of 1%, was added. The reaction was carried out for 12 hours under anhydrous and anaerobic conditions to obtain a polyester with cluster luminescence phenomenon as shown in the following structure, where n represents the number of repeating units. The reaction product was dissolved in dichloromethane and then dropped into an ethanol solution for precipitation to remove unreacted monomers and catalysts. The polymer was placed in a vacuum oven at 45 °C (vacuum degree lower than 0.01 MPa) and dried for 24 hours to remove the solvent.

[0090]

[0091] Figure 14 The dried polymer was placed in a fluorescence test fixture. After the installation was completed, the fixture was placed in the temperature control element configured in the fluorescence tester, and the variable excitation spectrum of the polymer was measured at room temperature.

[0092] The sample placement part was evacuated to avoid the condensation of water vapor on the sample at low temperature, which would affect the test results.

[0093] Figure 15 The differential scanning calorimetry (DSC) spectrum of the polymer was given. It can be seen that the glass transition temperature of the polymer measured by this method is 251 K.

[0094] The best excitation wavelength of the polymer at room temperature, 340 nm, was selected as the excitation wavelength in the variable temperature test. In this experiment, the temperature was increased at intervals of 10 K until 320 K, and the sample was kept at each set temperature (±0.5 K) for 3 minutes, and then the fluorescence data of the polymer at the excitation light wavelength of 340 nm were measured.

[0095] Figure 16 The temperature-fluorescence intensity curve was given, where the temperature was selected as the independent variable and the highest point of the fluorescence emission peak at that temperature was selected as the dependent variable, and the points were plotted on the coordinate.

[0096] Using the same data analysis method as in Example 1, the glass transition temperature of the polymer was obtained as 240 K, and the β-transition temperature was 180 K.

[0097] In addition, it should be understood that after reading the above description of the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

Claims

1. A method for in situ monitoring of polymer secondary transformation using cluster luminescence fluorescence signals, characterized in that: include: Fluorescence spectroscopy is used to test polymers with cluster luminescence phenomenon. The secondary transition temperature of the polymer is determined by monitoring the relationship between the spontaneous fluorescence intensity of the polymer and the temperature.

2. The method according to claim 1, characterized in that: The polymer secondary transition temperature includes the glass transition temperature and the beta transition temperature of the polymer.

3. The method according to claim 1, characterized in that: The excitation light wavelength for fluorescence spectrum testing is selected to be the excitation light wavelength corresponding to the strongest emission peak of the polymer at room temperature.

4. The method according to any one of claims 1 to 3, characterized in that: Fluorescence spectroscopy testing of polymers at different temperatures includes the following steps: (1) placing a polymer sample in a temperature-controlled fluorescence testing device; (2) The temperature-controlled fluorescence testing equipment gradually increases the temperature within a set temperature range, records the corresponding sample fluorescence intensity at different temperature points, and draws a temperature-fluorescence intensity curve; (3) Based on the temperature-fluorescence intensity curve, the temperature dependence of the fluorescence intensity is analyzed, and the secondary transition temperature of the polymer is determined according to the temperature point at which the rate of change of the fluorescence intensity with temperature changes significantly.

5. The method according to claim 4, characterized in that In step (1), the solvent is removed from the polymer sample in advance.

6. The method according to claim 4, characterized in that In step (1), after the polymer sample is placed in the temperature-controlled fluorescence testing device, the sample area of ​​the temperature-controlled fluorescence testing device is evacuated.

7. The method according to claim 4, characterized in that In step (2), the set temperature range is 80 to 400K.

8. The method according to claim 4, characterized in that In step (2), the temperature-controlled fluorescence testing equipment uses liquid nitrogen to cool down.

9. The method according to claim 4, characterized in that In step (2), the temperature point where the fluorescence intensity of the sample is recorded is kept warm for a period of time to allow the polymer sample temperature to be consistent with the ambient temperature.

10. The method according to claim 4, characterized in that In step (3), determining the secondary transition temperature of the polymer according to the temperature point at which the rate of change of the fluorescence intensity with temperature significantly changes specifically includes: The temperature-fluorescence intensity curve uses temperature as the horizontal axis and fluorescence intensity as the vertical axis. The absolute value of the second-order derivative of each test temperature point on the temperature-fluorescence intensity curve is calculated by the following formula: One or more test temperature points with the highest absolute value of the second-order derivative are selected from the largest to the smallest. The selected test temperature points can be divided into one or more temperature point groups, and each temperature point group is composed of continuous test temperature points. Different temperature point groups are discontinuous and belong to different temperature zones. The minimum temperature point in each temperature point group is determined as the secondary transition temperature of the corresponding polymer category. Where: T represents the test temperature, I represents the fluorescence intensity, and i represents the label.

Images