A method for characterizing a polyacrylonitrile fiber core-sheath structure

Abstract

The application relates to a polyacrylonitrile fiber skin-core structure characterization method and relates to the technical field of carbon fibers. The main technical scheme is as follows: a polyacrylonitrile fiber skin-core structure characterization method, which comprises the following steps: step 1): a plurality of polyacrylonitrile fiber tow samples with a set length are taken from polyacrylonitrile fiber tows to be characterized; the plurality of polyacrylonitrile fiber tow samples are subjected to dissolution treatment, wherein the dissolution treatment time t of different polyacrylonitrile fiber tow samples is different; the dissolution depth H of each polyacrylonitrile fiber tow sample after the dissolution treatment is calculated; and step 2): the skin thickness of the polyacrylonitrile fiber is determined according to the relationship between the dissolution depth H and the dissolution treatment time t. Compared with direct observation methods such as a scanning electron microscope and a transmission electron microscope, the method is simple in operation, low in cost, can be used to conveniently and quickly guide on-site adjustment of a wet spinning process.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon fibers, in particular to a method for characterizing the skin-core structure of polyacrylonitrile fibers. BACKGROUND

[0002] Carbon fibers have high specific strength, high specific modulus, heat resistance, corrosion resistance, fatigue resistance, creep resistance and other characteristics, and are a kind of high-performance fiber materials, which have been widely used in aerospace, national defense construction, sports and leisure products, medical devices and construction industry. Carbon fibers can be divided into polyacrylonitrile-based, pitch-based and viscose-based carbon fibers. Among them, polyacrylonitrile-based carbon fibers are the most widely used, the largest in quantity and the most rapidly developed, and they have an absolute advantage in carbon fiber production.

[0003] Polyacrylonitrile fibers (i.e., polyacrylonitrile filaments) prepared by wet spinning technology inevitably have a skin-core structure, that is, the surface structure of the fibers is dense and the core structure is relatively loose. If the skin layer of the fibers is thick and the structural difference between the skin layer and the core is large, the quality of the fibers is low and high-quality polyacrylonitrile-based carbon fibers cannot be produced. With the development of wet spinning technology, researchers try to modify the coagulation bath and adjust the coagulation forming process to control the skin-core structure of the fibers, so that the skin layer of the fibers is thin and the difference between the skin layer structure and the core structure is reduced. With further research, researchers urgently need a convenient, fast and accurate method for characterizing the skin-core structure of the fibers to timely and efficiently guide the process adjustment on site.

[0004] At present, the methods for characterizing the skin-core structure of the fibers include scanning electron microscope observation and transmission electron microscope observation. However, the equipment required for the scanning electron microscope observation and the transmission electron microscope observation is high-end and expensive, and it is rarely equipped in general enterprises. When the process is adjusted on site, samples need to be taken and sent out for testing, which results in a long testing period. Therefore, it is difficult for this method to timely and efficiently guide the process adjustment on site. In addition, the above-mentioned methods can only qualitatively characterize the existence of the skin-core structure, and the differentiation between the skin layer and the core layer and the determination of the thickness of the skin layer are greatly affected by human factors. SUMMARY

[0005] Therefore, the present application provides a method for characterizing the skin-core structure of polyacrylonitrile fibers, which aims to provide a timely and convenient method for characterizing the skin-core structure of the fibers.

[0006] To achieve the above-mentioned purpose, the present application mainly provides the following technical solutions:

[0007] In one aspect, the present application provides a method for characterizing the skin-core structure of polyacrylonitrile fibers, which includes the following steps:

[0008] Step 1): taking a plurality of polyacrylonitrile fiber tow samples of a set length from the polyacrylonitrile fiber tow to be characterized; dissolving the plurality of polyacrylonitrile fiber tow samples, wherein different polyacrylonitrile fiber tow samples correspond to different dissolving treatment times t; and calculating the dissolving depth H of each polyacrylonitrile fiber tow sample after the dissolving treatment.

[0009] Step 2): determining the skin layer thickness of the polyacrylonitrile fiber according to the relationship between the dissolving depth H and the dissolving treatment time t.

[0010] Preferably, in the step 1):

[0011] The calculation formula of the dissolving depth H is as follows:

[0012] H = r0-r;

[0013] Wherein, r0 is the fiber radius of the polyacrylonitrile fiber tow sample before the dissolving treatment;

[0014] r is the fiber radius of the polyacrylonitrile fiber tow sample after the dissolving treatment.

[0015] Preferably, the calculation formula of r0 is as follows:

[0016] r0 = [X m / (n x 1000 x p x p)] 1 / 2 x 1000;

[0017] Wherein, the unit of r0 is pm; X m is the weight of the polyacrylonitrile fiber tow sample of 1 m in length, in grams; the polyacrylonitrile fiber tow to be characterized is n k tows; n k represents that there are n x 1000 filaments in a bundle of fibers; p is the density of the polyacrylonitrile fiber tow sample, in g / cm 3 .

[0018] Preferably, the calculation formula of r is as follows:

[0019] r = [W / (n x 1000 x p x p)] 1 / 2 x 1000;

[0020] Wherein, the unit of r is pm; W is the fiber mass of the polyacrylonitrile fiber tow sample of 1 m in length after the dissolving treatment; the polyacrylonitrile fiber tow to be characterized is n k tows; n k represents that there are n x 1000 filaments in a bundle of fibers; p is the density of the polyacrylonitrile fiber tow sample, in g / cm 3 . Preferably, the step 1) comprises:

[0021] Step 11): taking a plurality of polyacrylonitrile fiber tow samples of a set length from the polyacrylonitrile fiber tow to be characterized;

[0022] Step 12) : drying treatment is performed on the plurality of polyacrylonitrile fiber tow samples; after cooling, the plurality of polyacrylonitrile fiber tow samples are respectively placed in different dissolving containers, a set amount of solvent is added in each dissolving container, and dissolving treatment is performed; wherein the different polyacrylonitrile fiber tow samples correspond to different dissolving treatment times; preferably, the mass ratio of the solvent to the polyacrylonitrile fiber tow sample is (400-600) : 1;

[0023] Step 13) : after the corresponding dissolving treatment time is reached, the fibers after dissolving treatment of each polyacrylonitrile fiber tow sample are washed, dried, cooled, and weighed to obtain the fiber mass W of each polyacrylonitrile fiber tow sample after dissolving treatment;

[0024] Step 14) : the fiber radius r of each polyacrylonitrile fiber tow sample after dissolving treatment is calculated respectively according to the fiber mass W of each polyacrylonitrile fiber tow sample after dissolving treatment, and the dissolving depth H of each polyacrylonitrile fiber tow sample after dissolving treatment is further calculated;

[0025] Preferably, in the step 12), the solvent is selected from one or more of dimethyl sulfoxide DMSO, dimethyl formamide DMF, and dimethyl acetamide DMAc;

[0026] Preferably, in the step 12), when the dissolving treatment is performed, the dissolving container needs to be placed in a constant-temperature shaking table at 25-30℃ for oscillation;

[0027] Preferably, in the step 12), the temperature of the drying treatment is 105-120℃, and the time of the drying treatment is 1.5-3h;

[0028] Preferably, in the step 13), when the washing is performed, the polyacrylonitrile fiber tow sample is first washed with an aqueous DMSO solution and then washed with flowing water;

[0029] Preferably, in the step 13), the temperature of the drying is 105-120℃, and the time of the drying is 2-4h.

[0030] Preferably, in the step 11) :

[0031] The polyacrylonitrile fiber tow to be characterized is an n K tow, wherein n K represents that there are n x 1000 filaments in a bundle of fibers, and a length of 0.1-1m of the tow is taken from the polyacrylonitrile fiber tow to be characterized as the polyacrylonitrile fiber tow sample;

[0032] Preferably, the larger the value of n is, the smaller the length of the polyacrylonitrile fiber tow sample is;

[0033] Preferably, the polyacrylonitrile fiber tow sample is 0.5-1 m long;

[0034] Preferably, before the dissolving treatment, the polyacrylonitrile fiber tow sample is cut into 2-3 cm long sections and manually opened (manually separate the filaments as much as possible to facilitate the dissolving treatment).

[0035] Preferably, in the step 12), the dissolving treatment time ranges from 1-30 min.

[0036] Preferably, the plurality of polyacrylonitrile fiber tow samples of a set length include a first polyacrylonitrile fiber tow sample, a second polyacrylonitrile fiber tow sample, a third polyacrylonitrile fiber tow sample, a fourth polyacrylonitrile fiber tow sample, a fifth polyacrylonitrile fiber tow sample, a sixth polyacrylonitrile fiber tow sample, a seventh polyacrylonitrile fiber tow sample, an eighth polyacrylonitrile fiber tow sample, a ninth polyacrylonitrile fiber tow sample, and a tenth polyacrylonitrile fiber tow sample; wherein,

[0037] The first polyacrylonitrile fiber tow sample corresponds to a dissolving treatment time of 1-1.2 min, preferably 1 min; the second polyacrylonitrile fiber tow sample corresponds to a dissolving treatment time of 2-2.2 min, preferably 2 min; the third polyacrylonitrile fiber tow sample corresponds to a dissolving treatment time of 3-3.2 min, preferably 3 min; the fourth polyacrylonitrile fiber tow sample corresponds to a dissolving treatment time of 4-4.2 min, preferably 4 min; the fifth polyacrylonitrile fiber tow sample corresponds to a dissolving treatment time of 5-5.2 min, preferably 5 min; the sixth polyacrylonitrile fiber tow sample corresponds to a dissolving treatment time of 10-10.5 min, preferably 10 min; the seventh polyacrylonitrile fiber tow sample corresponds to a dissolving treatment time of 15-15.5 min, preferably 15 min; the eighth polyacrylonitrile fiber tow sample corresponds to a dissolving treatment time of 20-21 min, preferably 20 min; the ninth polyacrylonitrile fiber tow sample corresponds to a dissolving treatment time of 25-26 min, preferably 25 min; and the tenth polyacrylonitrile fiber tow sample corresponds to a dissolving treatment time of 29-30 min, preferably 30 min.

[0038] Preferably, the step 2) includes:

[0039] Step 21): In a rectangular coordinate system, plot the data points of the dissolving depth H versus the dissolving treatment time t;

[0040] Step 22): According to the distribution of the data points, fit the data points in the initial dissolving stage into a first straight line and the data points in the later dissolving stage into a second straight line.

[0041] Step 23): determining the intersection point of the first straight line and the second straight line, recording the coordinates of the intersection point; determining the value of the dissolution depth corresponding to the intersection point as the skin layer thickness of the polyacrylonitrile fiber tow to be characterized.

[0042] Preferably, in the step 22), the data points are fitted into the first straight line and the second straight line according to the distribution characteristics of the data points, and the dissolution processing time range corresponding to the initial dissolution stage and the dissolution processing time range corresponding to the later dissolution stage are determined.

[0043] Preferably, the data points of the initial dissolution stage refer to the data points corresponding to the dissolution processing time t in the range of ≤5 min, and the data points of the later dissolution stage refer to the data points corresponding to the dissolution processing time t in the range of >5 min, ≤30 min.

[0044] Preferably, the data points of the initial dissolution stage refer to the data points corresponding to the dissolution processing time t in the range of ≤5 min, and the data points of the later dissolution stage refer to the data points corresponding to the dissolution processing time t in the range of >5 min, ≤30 min.

[0045] Compared with the prior art, the characterization method of the polyacrylonitrile fiber skin-core structure has at least the following beneficial effects:

[0046] The characterization method of the polyacrylonitrile fiber skin-core structure according to the embodiment of the present application only needs to take a plurality of polyacrylonitrile fiber tow samples of a set length from the polyacrylonitrile fiber tow to be characterized, then perform dissolution treatment, and calculate the dissolution depth H of each polyacrylonitrile fiber tow sample after dissolution treatment. By making the dissolution processing time t of different polyacrylonitrile fiber tow samples different, and then according to the relationship between the dissolution depth H and the dissolution processing time t, the skin layer thickness of the polyacrylonitrile fiber can be determined. It should be noted that the characterization method of the present application evaluates the skin-core structure degree according to the difference in dissolution rate of the dense structure and the loose structure in the solvent. The dissolution rate of the surface dense part of the tow is relatively slow during the dissolution process, and the dissolution rate increases as the dissolution goes deeper into the fiber. The present application defines the transition point of the dissolution rate from slow to fast as the dissolution rate transition point, and defines the dissolution depth corresponding to the transition point as the fiber skin layer thickness (according to the coordinate graph of the relationship between the dissolution depth H and the dissolution processing time t, the dissolution depth corresponding to the transition point, i.e. the fiber skin layer thickness, can be obtained). With the skin layer thickness index, the skin-core structure degree of the fiber can be evaluated. The greater the skin layer thickness, the greater the skin-core structure degree. Compared with the direct observation method such as scanning electron microscopy and transmission electron microscopy, the method of the present application is simple to operate, low in cost, convenient, and can quickly guide the on-site adjustment of the wet spinning process.

[0047] The above description is only a summary of the technical solutions of the present application. In order to more clearly understand the technical means of the present application, and to implement the content of the description, the following will describe the preferred embodiments of the present application in detail with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 is a schematic diagram of the change of the dissolution depth (H) of the fiber in the embodiment 2 of the present application with the dissolution time (t) ;

[0049] Figure 2 is a schematic diagram of the change of the dissolution depth (H) of the fiber in the embodiment 1 of the present application with the dissolution time (t) ;

[0050] Figure 3 is a schematic diagram of the change of the dissolution depth (H) of the fiber in the embodiment 3 of the present application with the dissolution time (t) ;

[0051] Figure 4 is a cross-sectional TEM photo of the carbon fiber filament prepared from the polyacrylonitrile fiber tow sample of the comparative example 1. DETAILED DESCRIPTION

[0052] To further explain the technical means and effects taken by the present application to achieve the predetermined object, the specific embodiments, structures, features and effects according to the present application are described in detail below in combination with the drawings and preferred embodiments. In the following description, different "an embodiment" or "embodiments" do not necessarily refer to the same embodiment. In addition, the specific features, structures or characteristics in one or more embodiments can be combined in any suitable form.

[0053] The inventive concept of the present application is as follows: the polyacrylonitrile fiber prepared by wet spinning has a skin-core structure, and the surface layer structure of the fiber is dense, and the core structure is relatively loose. The present application evaluates the degree of the skin-core structure according to the difference in the dissolution rate of the dense structure and the loose structure in the solvent. The dissolution rate of the surface dense part of the tow is slower in the dissolution process, and the dissolution rate increases as the dissolution proceeds to the inside of the fiber. The change of the dissolution depth with time in the process of the dissolution of the fiber from the surface to the inside can be obtained by means of the dissolution test, as shown in Figure 1 In the initial stage of dissolution, the change of the dissolution depth with time is relatively slow, and the test points in the initial stage of dissolution are fitted into a straight line, and the slope of the straight line reflects the dissolution depth per unit time in the initial stage of dissolution, i.e. the initial dissolution rate; in the later stage of dissolution, the change of the dissolution depth with time is relatively fast, and the test points in the later stage of dissolution are fitted into a straight line, and the slope of the straight line reflects the dissolution depth per unit time in the later stage of dissolution, i.e. the later dissolution rate; the present application defines the intersection point of the straight line fitted from the test points in the initial stage of dissolution and the straight line fitted from the test points in the later stage of dissolution as the transition point of the dissolution rate, and defines the dissolution depth corresponding to the transition point as the thickness of the fiber skin layer. The greater the thickness of the fiber skin layer, the greater the degree of the skin-core structure.

[0054] The scheme of the present application is as follows:

[0055] The embodiment of the present application provides a characterization method of the skin-core structure of the polyacrylonitrile fiber, which comprises the following steps:

[0056] Step 1): Take a plurality of polyacrylonitrile fiber tow samples of a set length from the polyacrylonitrile fiber tow to be characterized; dissolve the plurality of polyacrylonitrile fiber tow samples, wherein different polyacrylonitrile fiber tow samples correspond to different dissolution treatment times t; and calculate the dissolution depth H of each polyacrylonitrile fiber tow sample after dissolution treatment.

[0057] Preferably, the step 1) comprises:

[0058] Step 11): Take a plurality of polyacrylonitrile fiber tow samples (i.e., the lengths of the plurality of polyacrylonitrile fiber tow samples are the same) of a set length from the polyacrylonitrile-based fiber to be characterized.

[0059] If the polyacrylonitrile fiber to be characterized is a 1-6K tow, a 1m length of the tow is taken from the polyacrylonitrile fiber to be characterized as the polyacrylonitrile fiber tow sample; if the polyacrylonitrile fiber to be characterized is a 12-24K tow, a 0.5m length of the tow is taken from the polyacrylonitrile fiber to be characterized as the polyacrylonitrile fiber tow sample; and if the polyacrylonitrile fiber to be characterized is a 48-60K tow, a 0.1m length of the tow is taken from the polyacrylonitrile fiber to be characterized as the polyacrylonitrile fiber tow sample.

[0060] Step 12): Dry the plurality of polyacrylonitrile fiber tow samples; after cooling, place the plurality of polyacrylonitrile fiber tow samples in different dissolution containers respectively, add a certain amount of solvent (the mass ratio of the solvent to the polyacrylonitrile fiber tow sample is 400-600:1) in each dissolution container, and perform dissolution treatment; wherein different polyacrylonitrile fiber tow samples correspond to different dissolution treatment times.

[0061] Preferably, the polyacrylonitrile fiber tow sample needs to be cut into a length of 2-3cm before dissolution treatment. Preferably, the solvent is dimethyl sulfoxide (DMSO). Preferably, the dissolution container needs to be placed in a constant temperature (e.g., 25°C) shaking bed for oscillation during dissolution treatment. Preferably, the drying treatment temperature is 110°C, and the drying treatment time is 2h.

[0062] Step 13): After reaching the corresponding dissolution treatment time, wash, dry, cool, and weigh the fibers of each polyacrylonitrile fiber tow sample after dissolution treatment to obtain the fiber mass W of each polyacrylonitrile fiber tow sample after dissolution treatment.

[0063] Preferably, the washing method is as follows: first, wash with a DMSO aqueous solution, and then wash with flowing water. Further preferably, first wash with DMSO aqueous solutions with mass fractions of 88%, 55%, and 30% in sequence, and finally wash with flowing water at room temperature for more than 10min.

[0064] Preferably, the drying temperature is 110°C and the drying time is 2h.

[0065] Step 14) According to the fiber mass W of the polyacrylonitrile fiber towsample after the dissolution treatment (when it is not a 1m long sample, the fiber mass W after the dissolution treatment of a 1m long sample is calculated by multiplying the fiber mass W after the dissolution treatment of a 0.5m long sample by 2), the fiber radius r of each polyacrylonitrile fiber tow sample after the dissolution treatment is calculated, and the dissolution depth H of each polyacrylonitrile fiber tow sample after the dissolution treatment is further calculated;

[0066] The calculation formula of the fiber radius r of each polyacrylonitrile fiber tow sample after the dissolution treatment is as follows

[0067] r = [W / (n x 1000 x p x p)] 1 / 2 x 1000.

[0068] The unit of r is pm; W is the fiber mass of a 1m long polyacrylonitrile fiber tow sample after the dissolution treatment; the polyacrylonitrile fiber tow to be characterized is n k tows; n k represents that there are n x 1000 filaments in a bundle of fibers; p is the density of the polyacrylonitrile fiber tow sample, and the unit is g / cm 3 .

[0069] The calculation formula of the fiber radius r0 of the polyacrylonitrile fiber tow sample before the dissolution treatment is as follows:

[0070] r0 = [X m / (n x 1000 x p x p)] 1 / 2 x 1000.

[0071] The unit of r0 is pm; Xm is the weight of a 1m long polyacrylonitrile fiber tow sample in grams; the polyacrylonitrile fiber tow to be characterized is n k tows; n k represents that there are n x 1000 filaments in a bundle of fibers; p is the density of the polyacrylonitrile fiber tow sample, and the unit is g / cm 3 .

[0072] The calculation formula of the dissolution depth H of each polyacrylonitrile fiber tow sample after the dissolution treatment is as follows:

[0073] H = r0 - r.

[0074] r0 is the fiber radius of the polyacrylonitrile fiber tow sample before the dissolution treatment;

[0075] r is the fiber radius of the polyacrylonitrile fiber tow sample after the dissolution treatment.

[0076] Step 2): According to the relationship between the dissolution depth H and the dissolution treatment time t, the skin layer thickness of the polyacrylonitrile fiber is determined.

[0077] In this step: a scatter plot of the dissolution depth (H) versus the dissolution time (t) is drawn (as shown in Figure 1 The initial dissolution data points are fitted with a straight line, and the late dissolution data points are fitted with a straight line according to the scatter point (data point) distribution of the scatter plot; the intersection of the two fitted straight lines is determined, and the coordinates of the intersection are recorded; the dissolution depth corresponding to the intersection is determined as the skin layer thickness.

[0078] In summary, compared with direct observation methods such as scanning electron microscopy and transmission electron microscopy, the characterization method of the skin-core structure of the polyacrylonitrile fiber proposed in the present application has the advantages of simple operation, convenient and fast acquisition of skin layer thickness data, and timely guidance of process adjustment; (2) The experimental equipment used in the method of the present application is simple, only a constant temperature water bath oscillator and a number of conical flasks are needed, and the equipment cost is low.

[0079] The present application is further described below through specific experimental examples:

[0080] The wet spinning coagulation forming process is adjusted to provide three different process state polyacrylonitrile fibers, and the three fibers are numbered A, B and C in turn. The specifications of the three fibers are all 3K. Examples 1-3 characterize the skin-core structure of the three specifications of polyacrylonitrile fibers.

[0081] Example 1

[0082] This example is used to characterize the skin-core structure of the 3K polyacrylonitrile fiber tow numbered A, which mainly includes the following steps:

[0083] 1) Ten samples (i.e., ten polyacrylonitrile fiber tow samples) are taken from the polyacrylonitrile fiber tow numbered A, each sample has a length of 1 m, and the mass of the ten samples is within the range of 0.370±0.0015g / m. The ten samples are labeled in turn as: 1#, 2#, 3#, 4#, 5#, 6#, 7#, 8#, 9# and 10#. Then, the ten samples are cut into lengths of 2-3 cm and manually fibrillated.

[0084] 2) The above ten polyacrylonitrile fiber tow samples are placed in a forced air drying oven and dried at a temperature of 110℃ for 2h and then cooled.

[0085] 3) Prepare ten 500ml triangular bottles, put 1#, 2#, 3#, 4#, 5#, 6#, 7#, 8#, 9#, 10# polyacrylonitrile fiber tow samples into the ten triangular bottles respectively; add 200mL of DMSO into the ten triangular bottles and oscillate in a constant temperature (25℃) shaker for dissolution treatment. Among them, different polyacrylonitrile fiber tow samples correspond to different dissolution treatment time t.

[0086] Specifically, the dissolution treatment time of 1# polyacrylonitrile fiber tow sample is 1min; the dissolution treatment time of 2# polyacrylonitrile fiber tow sample is 2min; the dissolution treatment time of 3# polyacrylonitrile fiber tow sample is 3min; the dissolution treatment time of 4# polyacrylonitrile fiber tow sample is 4min. The dissolution treatment time of 5# polyacrylonitrile fiber tow sample is 5min; the dissolution treatment time of 6# polyacrylonitrile fiber tow sample is 10min; the dissolution treatment time of 7# polyacrylonitrile fiber tow sample is 15min; the dissolution treatment time of 8# polyacrylonitrile fiber tow sample is 20min; the dissolution treatment time of 9# polyacrylonitrile fiber tow sample is 25min; the dissolution treatment time of 10# polyacrylonitrile fiber tow sample is 30min.

[0087] 4) After the dissolution treatment of the above ten polyacrylonitrile fiber tow samples, take out and wash them with 88%, 55%, 30% DMSO aqueous solution for 1min in turn, and finally wash them with running water at room temperature for more than 10min. Then, dry them in an oven at 110℃ for 2h, and after cooling, weigh the residual tow weight W (i.e. the weight W of the polyacrylonitrile fiber tow sample after dissolution treatment).

[0088] Among them, the weights of the ten polyacrylonitrile fiber tow samples after dissolution treatment are recorded as W1 (0.345g), W2 (0.342g), W3 (0.334g), W4 (0.331g), W5 (0.304g), W6 (0.246g), W7 (0.114g), W8 (0.071g), W9 (0.028g), W 10 (0.005g);

[0089] Then, the fiber radius r of the ten polyacrylonitrile fiber tow samples after dissolution treatment is calculated according to the following formula respectively; the fiber radius r of the ten polyacrylonitrile fiber tow samples after dissolution treatment is recorded as: r1 (5.55μm), r2 (5.52μm), r3 (5.46μm), r4 (5.43μm), r5 (5.21μm), r6 (4.68μm), r7 (3.19μm) 、 r8 (2.51μm), r9 (1.59μm), r 10 (0.7μm);

[0090] r = [W / (3 x 1000 x p x p)] 1 / 2 x 1000.

[0091] Wherein, the unit of r is pm; W is the fiber mass of 1 m long 3K polyacrylonitrile fiber tows after the dissolution treatment, 3K represents 3 x 1000 filaments in a bundle of fibers; p is the density of the polyacrylonitrile fiber tow sample, the unit is g / cm 3 , specifically 1.19 g / cm 3 .

[0092] The fiber radius r0 of the polyacrylonitrile fiber tow sample before the dissolution treatment is calculated by the following formula:

[0093] r0 = [X m / (3 x 1000 x p x p)] 1 / 2 x 1000.

[0094] Wherein, the unit of r0 is pm; X m is the weight of 1 m long 3K polyacrylonitrile fiber tows, 3K represents 3 x 1000 filaments in a bundle of fibers; p is the density of the polyacrylonitrile fiber tow sample, the unit is g / cm 3 , specifically 1.19 g / cm 3 .

[0095] The value of X m of 3K polyacrylonitrile fiber tows in this embodiment is 0.370 g / m.

[0096] The dissolution depth of each polyacrylonitrile fiber tow sample in the radial direction is calculated by the following formula:

[0097] H = r0 - r.

[0098] The dissolution depths of the above ten polyacrylonitrile fiber tow samples after dissolution are H1 (0.20 pm), H2 (0.23 pm), H3 (0.29 pm), H4 (0.32 pm), H5 (0.54 pm), H6 (1.07 pm), H7 (2.56 pm), H8 (3.24 pm), H9 (4.16 pm), and H 10 (5.05 pm).

[0099] 5) In the rectangular coordinate system, a scatter plot (data points) of the dissolution depth (H) versus the time (t) of the dissolution treatment is drawn; according to the distribution of the scatter points (data points) of the scatter plot, the data points in the early stage of dissolution (dissolution treatment time ≤ 5 min) are fitted to a straight line, and the data points in the later stage of dissolution (dissolution treatment time > 5 min) are fitted to a straight line; the intersection of the two fitted straight lines is determined, and the coordinates of the intersection point are recorded; the dissolution depth corresponding to the intersection point is determined as the skin thickness.

[0100] The dissolution depth (H) of the present embodiment as a function of the dissolution time (t) is shown in FIG. 1, wherein the determined skin thickness of the polyacrylonitrile fiber tow sample numbered A is shown in Table 1. Figure 2

[0101] Example 2

[0102] The present embodiment is used to characterize the skin-core structure of the 3K polyacrylonitrile fiber tow numbered B, and mainly comprises the following steps:

[0103] 1) Ten samples (i.e., ten polyacrylonitrile fiber tow samples) are taken from the polyacrylonitrile fiber tow numbered B, each sample having a length of 1 m, and the ten samples each having a mass in the range of 0.370 ± 0.0015 g / m. The ten samples are sequentially labeled as: 1#, 2#, 3#, 4#, 5#, 6#, 7#, 8#, 9#, and 10#. Then, the ten samples are each cut into a length of 2-3 cm and manually fibrillated.

[0104] 2) The above ten polyacrylonitrile fiber tow samples are placed in a forced air drying oven and dried at a temperature of 110°C for 2 h and then cooled.

[0105] 3) Ten 500 ml triangular flasks are prepared, and the 1#, 2#, 3#, 4#, 5#, 6#, 7#, 8#, 9#, and 10# polyacrylonitrile fiber tow samples are respectively placed in the ten triangular flasks; 200 ml of DMSO is added to each of the ten triangular flasks, and the ten triangular flasks are shaken in a constant temperature (25°C) shaking bed for dissolution treatment. Different polyacrylonitrile fiber tow samples correspond to different dissolution treatment times t.

[0106] Specifically, the dissolution treatment time of the 1# polyacrylonitrile fiber tow sample is 1 min; the dissolution treatment time of the 2# polyacrylonitrile fiber tow sample is 2 min; the dissolution treatment time of the 3# polyacrylonitrile fiber tow sample is 3 min; the dissolution treatment time of the 4# polyacrylonitrile fiber tow sample is 4 min. The dissolution treatment time of the 5# polyacrylonitrile fiber tow sample is 5 min; the dissolution treatment time of the 6# polyacrylonitrile fiber tow sample is 10 min; the dissolution treatment time of the 7# polyacrylonitrile fiber tow sample is 15 min; the dissolution treatment time of the 8# polyacrylonitrile fiber tow sample is 20 min; the dissolution treatment time of the 9# polyacrylonitrile fiber tow sample is 25 min; and the dissolution treatment time of the 10# polyacrylonitrile fiber tow sample is 30 min.

[0107] ​4) After the dissolution treatment of the above ten polyacrylonitrile fiber tow samples, they were taken out and washed with DMSO aqueous solution with mass fraction of 88%, 55%, and 30% for 1 min in turn, and finally washed with running water for more than 10 min at room temperature. Then, they were dried in an oven at 110°C for 2 h, and weighed after cooling to obtain the residual tow weight W (i.e., the weight W of the polyacrylonitrile fiber tow sample after the dissolution treatment).

[0108] wherein the weight of the ten polyacrylonitrile fiber tow samples after the dissolution treatment is sequentially recorded as W1 (0.340 g), W2 (0.329 g), W3 (0.322 g), W4 (0.305 g), W5 (0.274 g), W6 (0.144 g), W7 (0.062 g), W8 (0.02 g), W9 (0.004 g), and W10 (0 g). 10 (**g);

[0109] Then, the fiber radius r of the ten polyacrylonitrile fiber tow samples after the dissolution treatment was calculated according to the following formula, respectively; the fiber radius r of the ten polyacrylonitrile fiber tow samples after the dissolution treatment is sequentially recorded as r1 (5.51 μm), r2 (5.42 μm), r3 (5.36 μm), r4 (5.22 μm), r5 (4.94 μm), r6 (3.58 μm), r7 (2.35 μm) 、 r8 (1.35 μm), r9 (0.63 μm), and r10 (0 μm). 10 (**μm);

[0110] r = [W / (3 x 1000 x p x p)] 1 / 2 x 1000.

[0111] wherein the unit of r is μm; W is the fiber mass of the 3K polyacrylonitrile fiber tow sample with a length of 1 m after the dissolution treatment, and 3K represents that there are 3 x 1000 filaments in one fiber bundle; p is the density of the polyacrylonitrile fiber tow sample, and the unit is g / cm 3 , and specifically 1.19 g / cm 3 .

[0112] The fiber radius r0 of the polyacrylonitrile fiber tow sample before the dissolution treatment was calculated by the following formula:

[0113] r0 = [X m / (3 x 1000 x p x p)] 1 / 2 x 1000.

[0114] wherein the unit of r0 is μm; X m is the weight of the 3K polyacrylonitrile fiber tow sample with a length of 1 m, and 3K represents that there are 3 x 1000 filaments in one fiber bundle; p is the density of the polyacrylonitrile fiber tow sample, and the unit is g / cm3 , specifically 1.19 g / cm 3 .

[0115] The Xm value of the 3K fiber sample in this example is 0.370 g / m.

[0116] The dissolution depth of each polyacrylonitrile fiber tow sample in the radial direction is calculated by the following formula:

[0117] H = r0- r.

[0118] The dissolution depths of the above ten polyacrylonitrile fiber tow samples after dissolution are sequentially recorded as: H1 (0.24 pm), H2 (0.33 pm), H3 (0.39 pm), H4 (0.53 pm), H5 (0.81 pm), H6 (2.17 pm), H7 (3.4 pm), H8 (4.4 pm), H9 (5.12 pm), and H10 (6.4 pm). 10 (**pm).

[0119] 5) In the rectangular coordinate system, a scatter plot (data points) of the dissolution depth (H) versus the time (t) of dissolution treatment is drawn; according to the distribution of the scatter points (data points) of the scatter plot, the data points in the early stage of dissolution (dissolution treatment time ≤ 5 min) are fitted to a straight line, and the data points in the later stage of dissolution (dissolution treatment time > 5 min) are fitted to a straight line; the intersection of the two fitted straight lines is determined, and the coordinates of the intersection point are recorded; the dissolution depth corresponding to the intersection point is determined as the skin layer thickness.

[0120] The change of the dissolution depth (H) of this example with the dissolution time (t) is shown in Figure 1 , wherein the skin layer thickness of the polyacrylonitrile fiber tow sample numbered B determined is shown in Table 1.

[0121] Example 3

[0122] This example is used to characterize the skin-core structure of the 3K polyacrylonitrile fiber tow numbered C, which mainly includes the following steps:

[0123] 1) Ten samples (i.e., ten polyacrylonitrile fiber tow samples) are taken from the polyacrylonitrile fiber tow numbered C, each sample having a length of 1 m, and the mass of the ten samples is within the range of 0.370 ± 0.0015 g / m. The ten samples are sequentially labeled as: 1#, 2#, 3#, 4#, 5#, 6#, 7#, 8#, 9#, and 10#. Then, the ten samples are each cut into a length of 2-3 cm and manually fibrillated.

[0124] 2) The above ten polyacrylonitrile fiber tow samples are placed in a forced air drying oven and dried at a temperature of 110°C for 2 h and then cooled.

[0125] 3) Prepare ten 500ml triangular bottles, and put 1#, 2#, 3#, 4#, 5#, 6#, 7#, 8#, 9# and 10# polyacrylonitrile fiber tow samples into the ten triangular bottles respectively; add 200ml DMSO into each of the ten triangular bottles, and shake in a constant temperature (25℃) shaker for dissolution treatment. Different polyacrylonitrile fiber tow samples correspond to different dissolution treatment time t.

[0126] Specifically, the dissolution treatment time of 1# polyacrylonitrile fiber tow sample is 1min; the dissolution treatment time of 2# polyacrylonitrile fiber tow sample is 2min; the dissolution treatment time of 3# polyacrylonitrile fiber tow sample is 3min; the dissolution treatment time of 4# polyacrylonitrile fiber tow sample is 4min. The dissolution treatment time of 5# polyacrylonitrile fiber tow sample is 5min; the dissolution treatment time of 6# polyacrylonitrile fiber tow sample is 10min; the dissolution treatment time of 7# polyacrylonitrile fiber tow sample is 15min; the dissolution treatment time of 8# polyacrylonitrile fiber tow sample is 20min; the dissolution treatment time of 9# polyacrylonitrile fiber tow sample is 25min; the dissolution treatment time of 10# polyacrylonitrile fiber tow sample is 30min.

[0127] 4) After the dissolution treatment of the above ten polyacrylonitrile fiber tow samples, each of them is washed with 88%, 55% and 30% DMSO aqueous solution for 1min in turn, and finally washed with running water for more than 10min at room temperature. Then, dry them in an oven at 110℃ for 2h, and weigh the residual tow weight W (i.e. the weight W of the polyacrylonitrile fiber tow sample after dissolution treatment) after cooling.

[0128] Wherein, the weight of the ten polyacrylonitrile fiber tow samples after dissolution treatment is recorded as W1 (0.340g), W2 (0.328g), W3 (0.322g), W4 (0.300g), W5 (0.289g), W6 (0.161g), W7 (0.073g), W8 (0.038g), W9 (0.005g), and W10 (0.000g) in turn. 10 (**g);

[0129] Then, the fiber radius r of the ten polyacrylonitrile fiber tow samples after dissolution treatment is calculated according to the following formula respectively; the fiber radius r of the ten polyacrylonitrile fiber tow samples after dissolution treatment is recorded as r1 (5.51μm), r2 (5.41μm), r3 (5.36μm), r4 (5.17μm), r5 (5.08μm), r6 (3.79μm), r7 (2.55μm) 、 r8 (1.84μm), r9 (0.65μm), and r10 (0.00μm) in turn. 10 (**μm);

[0130] r = [W / (3x1000xp x p)] 1 / 2 x 1000.

[0131] Wherein, the unit of r is pm; W is the fiber mass of 1 m long 3K polyacrylonitrile fiber tows after the dissolution treatment, 3K represents 3x1000 monofilaments in a bundle of fibers; p is the density of the polyacrylonitrile fiber tow sample, the unit is g / cm 3 , specifically 1.19 g / cm 3 .

[0132] The fiber radius r0 of the polyacrylonitrile fiber tow sample before the dissolution treatment is calculated by the following formula:

[0133] r0 = [X m / (3x1000xp x p)] 1 / 2 x 1000.

[0134] Wherein, the unit of r0 is pm; X m is the weight of 1 m long 3K polyacrylonitrile fiber tows in grams, 3K represents 3x1000 monofilaments in a bundle of fibers; p is the density of the polyacrylonitrile fiber tow sample, the unit is g / cm 3 , specifically 1.19 g / cm 3 .

[0135] The Xm value of the 3K fiber sample in this example is 0.370 g / m.

[0136] The dissolution depth of each polyacrylonitrile fiber tow sample in the radial direction is calculated by the following formula:

[0137] H = r0-r.

[0138] The dissolution depths of the above ten polyacrylonitrile fiber tow samples after dissolution are recorded as H1(0.24 pm), H2(0.34 pm), H3(0.39 pm), H4(0.58 pm), H5(0.67 pm), H6(1.96 pm), H7(3.20 pm), H8(3.91 pm), H9(5.10 pm), and H 10 (**pm).

[0139] 5) In the rectangular coordinate system, a scatter plot (data points) of the dissolution depth (H) versus the time (t) of the dissolution treatment is drawn; according to the distribution of the scatter points (data points) of the scatter plot, the data points in the early stage of dissolution (dissolution treatment time ≤5 min) are fitted to a straight line, and the data points in the late stage of dissolution (dissolution treatment time >5 min) are fitted to a straight line; the intersection of the two fitted straight lines is determined, and the coordinates of the intersection point are recorded; the dissolution depth corresponding to the intersection point is determined as the skin thickness.

[0140] The dissolution depth (H) of the present embodiment as a function of the dissolution time (t) is shown in Figure 1. Figure 3 The skin layer thickness of the determined polyacrylonitrile fiber tow sample numbered C is shown in Table 1.

[0141] Table 1 shows the skin layer thickness of the polyacrylonitrile fibers numbered A, B and C characterized in Examples 1-3, as follows:

[0142] Table 1

[0143] Fiber No. Skin thickness (pm) A 0.32 B 0.42 C 0.67

[0144] Comparative Example 1

[0145] The polyacrylonitrile fiber tow sample used in Comparative Example 1 is the same as that of Example 3.

[0146] The polyacrylonitrile fiber tow sample of Comparative Example 1 is obtained by wet spinning. The wet fiber has a skin layer structure, which, after subsequent pre-oxidation and high-temperature carbonization treatment, forms a pore layer on the surface of the finally obtained polyacrylonitrile-based carbon fiber. By directly observing the pore layer on the surface of the cross-section of the carbon fiber with TEM, the thickness of the pore layer can be obtained. By converting the polyacrylonitrile fiber tow sample into carbon fiber, the shrinkage rate of the diameter during the conversion can be calculated to obtain the skin layer thickness of the polyacrylonitrile sample of the present comparative example.

[0147] The polyacrylonitrile fiber tow sample of Comparative Example 1 is subjected to conventional pre-oxidation and carbonization treatment to obtain the corresponding polyacrylonitrile-based carbon fiber. A single filament is randomly picked from the carbon fiber tow, and a cross-section sample (cross-section thickness required to be ≤100 nm) of the single filament is prepared by FIB-SEM and focused ion beam thinning technology, and then HRTEM (high resolution TEM) is used to observe the cross-section sample. Figure 4 The cross-section TEM photograph of the carbon fiber single filament prepared from the polyacrylonitrile fiber tow sample of Comparative Example 1 is shown in Figure 2. Figure 4 It can be clearly seen that there is a pore layer on the surface of the cross-section of the single filament, and the thickness of the pore layer is about 0.39 μm.

[0148] After conventional pre-oxidation and carbonization, the cross-section of the fiber will shrink. Generally speaking, the diameter ratio of the polyacrylonitrile fiber single filament to the corresponding carbon fiber single filament is 1.67:1 (statistical analysis of process practice). According to this direct shrinkage ratio, the skin layer thickness of the polyacrylonitrile sample can be calculated from the thickness of the pore layer obtained from the TEM photograph. The skin layer thickness of the sample of the present comparative example is 1.67 x 0.39 = 0.65 μm.

[0149] The polyacrylonitrile fiber tow sample of Comparative Example 1 is the same as the polyacrylonitrile fiber tow sample of Example 3, the skin layer thickness of the polyacrylonitrile fiber tow sample obtained by the method of Comparative Example 1 is basically the same as the skin layer thickness obtained by Example 3, which shows the accuracy of the characterization method of the present application.

[0150] Compared with Comparative Example 1, the beneficial effects of the present application are as follows:

[0151] (1) The method of Comparative Example 1 has high sample preparation and testing cost, long testing period (high FIB-SEM and ion thinning cost, high HRTEM observation cost, and at least more than one month of testing period), the total testing cost of the method of the present application is extremely low and can be carried out in a conventional laboratory, and the testing period is short, which facilitates timely feedback of process adjustment;

[0152] (2) Comparative Example 1 is a single fiber sample, and the final data obtained often only reflects the skin layer thickness of a single fiber, which is difficult to represent the average skin layer degree of 3k tow (3000 single fibers), and the method of the present application can obtain the average skin layer thickness of the tow, which can better represent the average skin layer level of the tow.

[0153] The above is only a preferred embodiment of the present application, and does not limit the present application in any form. Any simple modification, equivalent change and modification made according to the technical essence of the present application to the above embodiment still belongs to the scope of the technical solution of the present application.

Claims

1. A method of characterizing a polyacrylonitrile fiber core-sheath structure, characterized by, It comprises the following steps: Step 1): take a plurality of polyacrylonitrile fiber tows of a set length from the polyacrylonitrile fiber tows to be characterized; dissolve the plurality of polyacrylonitrile fiber tow samples, wherein different polyacrylonitrile fiber tow samples correspond to different dissolution treatment times t; calculate the dissolution depth H of each polyacrylonitrile fiber tow sample after dissolution treatment; Step 2): according to the relationship between the dissolution depth H and the dissolution treatment time t, the skin thickness of the polyacrylonitrile fiber is determined.

2. The method of claim 1, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 1): The calculation formula of the dissolution depth H is as follows: H=r0-r； Wherein, r0 is the fiber radius of the polyacrylonitrile fiber tow sample before dissolution treatment; r is the fiber radius of the polyacrylonitrile fiber tow sample after dissolution treatment.

3. The method of claim 2, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, The calculation formula of r0 is as follows: r0=[X m / (n x 1000 x p x pi)] 1 / 2 x 1000; wherein r0 is in units of pm; X m the weight in grams of a 1 m long polyacrylonitrile fiber tow sample; the polyacrylonitrile fiber tow to be characterized is the nk tow; nk indicates that there are n x 1000 filaments in a bundle of fibers; p is the density of the polyacrylonitrile fiber tow sample in units of g / cm 3 .

4. The method of claim 2, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, The calculation formula of r is as follows: r = [W / (n x 1000 x p x π)] 1 / 2 x 1000; Wherein, the unit of r is μm; W is the fiber mass of the polyacrylonitrile fiber tows sample after the dissolution treatment, the polyacrylonitrile fiber tows to be characterized is nk tows; nk indicates that there are n x 1000 filaments in a bundle of fibers; ρ is the density of the polyacrylonitrile fiber tows sample, the unit is g / cm 3 .

5. The method of claim 1-4, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, The step 1) comprises: Step 11): take a plurality of polyacrylonitrile fiber tows of a set length from the polyacrylonitrile fiber tows to be characterized; Step 12): dry the plurality of polyacrylonitrile fiber tow samples; after cooling, place the plurality of polyacrylonitrile fiber tow samples in different dissolution containers respectively, add a set amount of solvent to each dissolution container, and perform dissolution treatment; wherein different polyacrylonitrile fiber tow samples correspond to different dissolution treatment times; wherein the mass ratio of the solvent to the polyacrylonitrile fiber tow sample is (400-600):1; Step 13) after reaching the corresponding dissolution treatment time, wash, dry, cool and weigh the fiber of each polyacrylonitrile fiber tow sample after dissolution treatment to obtain the fiber mass W of each polyacrylonitrile fiber tow sample after dissolution treatment; Step 14) according to the fiber mass W of the polyacrylonitrile fiber tow sample after dissolution treatment, calculate the fiber radius r of each polyacrylonitrile fiber tow sample after dissolution treatment, and further calculate the dissolution depth H of each polyacrylonitrile fiber tow sample after dissolution treatment.

6. The method of claim 5, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 12), the solvent is selected from one or more of dimethyl sulfoxide DMSO, dimethyl formamide DMF, and dimethyl acetamide DMAc.

7. The method of claim 5, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 12), when performing dissolution treatment, the dissolution container needs to be placed in a constant temperature shaking bed at 25-30°C for oscillation.

8. The method of claim 5, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 12), the drying temperature is 105-120°C, and the drying time is 1.5-3h.

9. The method of claim 5, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 13), when washing, first wash with a DMSO aqueous solution, and then wash with flowing water.

10. The method of claim 5, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 13), the drying temperature is 105-120°C, and the drying time is 2-4h.

11. The method of claim 5, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 11): The polyacrylonitrile fiber tows to be characterized are n K tows, where n K represents that there are n x 1000 filaments in a bundle of fibers, and 0.1-1m long tows are taken from the polyacrylonitrile fiber tows to be characterized as polyacrylonitrile fiber tow samples.

12. The method of claim 11, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 11): the larger the value of n, the smaller the length of the polyacrylonitrile fiber tow sample.

13. The method of claim 11, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 11): take 0.5-1m long tows as polyacrylonitrile fiber tow samples.

14. The method of claim 11, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, Before the dissolving treatment, the polyacrylonitrile fiber tow sample needs to be cut into 2-3 cm long sections and manually opened.

15. The method of claim 5, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 12), the dissolving treatment time ranges from 1 to 30 minutes.

16. The method of claim 1, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, The plurality of polyacrylonitrile fiber tow samples of different lengths includes a first polyacrylonitrile fiber tow sample, a second polyacrylonitrile fiber tow sample, a third polyacrylonitrile fiber tow sample, a fourth polyacrylonitrile fiber tow sample, a fifth polyacrylonitrile fiber tow sample, a sixth polyacrylonitrile fiber tow sample, a seventh polyacrylonitrile fiber tow sample, an eighth polyacrylonitrile fiber tow sample, a ninth polyacrylonitrile fiber tow sample, and a tenth polyacrylonitrile fiber tow sample. The dissolving treatment time corresponding to the first polyacrylonitrile fiber tow sample is 1-1.2 minutes, the dissolving treatment time corresponding to the second polyacrylonitrile fiber tow sample is 2-2.2 minutes, the dissolving treatment time corresponding to the third polyacrylonitrile fiber tow sample is 3-3.2 minutes, the dissolving treatment time corresponding to the fourth polyacrylonitrile fiber tow sample is 4-4.2 minutes, the dissolving treatment time corresponding to the fifth polyacrylonitrile fiber tow sample is 5-5.2 minutes, the dissolving treatment time corresponding to the sixth polyacrylonitrile fiber tow sample is 10-10.5 minutes, the dissolving treatment time corresponding to the seventh polyacrylonitrile fiber tow sample is 15-15.5 minutes, the dissolving treatment time corresponding to the eighth polyacrylonitrile fiber tow sample is 20-21 minutes, the dissolving treatment time corresponding to the ninth polyacrylonitrile fiber tow sample is 25-26 minutes, and the dissolving treatment time corresponding to the tenth polyacrylonitrile fiber tow sample is 29-30 minutes.

17. The method of claim 16, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, The dissolving treatment time corresponding to the first polyacrylonitrile fiber tow sample is 1 minute, the dissolving treatment time corresponding to the second polyacrylonitrile fiber tow sample is 2 minutes, the dissolving treatment time corresponding to the third polyacrylonitrile fiber tow sample is 3 minutes, the dissolving treatment time corresponding to the fourth polyacrylonitrile fiber tow sample is 4 minutes, the dissolving treatment time corresponding to the fifth polyacrylonitrile fiber tow sample is 5 minutes, the dissolving treatment time corresponding to the sixth polyacrylonitrile fiber tow sample is 10 minutes, the dissolving treatment time corresponding to the seventh polyacrylonitrile fiber tow sample is 15 minutes, the dissolving treatment time corresponding to the eighth polyacrylonitrile fiber tow sample is 20 minutes, and the dissolving treatment time corresponding to the ninth polyacrylonitrile fiber tow sample is 25 minutes. The dissolving treatment time corresponding to the tenth polyacrylonitrile fiber tow sample is 30 minutes.

18. The method of claim 1, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, The step 2) includes: Step 21): In a rectangular coordinate system, a data point graph of the dissolution depth H changing with the dissolving treatment time t is drawn; Step 22): According to the distribution of the data points, the data points in the initial stage of dissolution are fitted into a first straight line, and the data points in the later stage of dissolution are fitted into a second straight line. Step 23): determining the intersection point of the first straight line and the second straight line, recording the coordinates of the intersection point; determining the value of the dissolution depth corresponding to the intersection point as the skin layer thickness of the polyacrylonitrile fiber tow to be characterized.

19. The method of claim 18, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 22): When fitting the data points into the first straight line and the second straight line, according to the distribution characteristics of the data points, the dissolution processing time range corresponding to the initial dissolution stage and the dissolution processing time range corresponding to the late dissolution stage are determined.

20. The method of claim 19, wherein the polyacrylonitrile fiber core-sheath structure is characterized by, In the step 22): The data points of the initial dissolution stage refer to the data points corresponding to the dissolution processing time t in the range of ≤5 min; the data points of the late dissolution stage refer to the data points corresponding to the dissolution processing time t in the range of >5 min, ≤30 min.

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