Chapter Ⅳ Microstructure and Mechanical Properties of Poly-acrylonitrile-
4.2. Result and Discussion
4.2. Result and Discussion
58
Figure 4.2 (a) Density, (b) Tensile properties, (c) strain failure and effective diameter and (d)stress- strain curve of carbonized fiber.
Table 4.4 Mechanical properties of samples.
Sample
Density (g/cm3)
Linear Density (dtex)
Effective Diameter (𝜇𝑚)
Tensile strength (GPa)
Tensile Modulus (GPa)
Failure to strain (%)
SF 1.44 1.4 ± 0.1 11.0 ± 0.4 0.40 ± 0.03 9.9 ± 0.5 17.3 ± 2.9
400 oC 1.48 1.4 ± 0.1 10.8 ± 0.4 0.20 ± 0.20 8.7 ± 0.2 13.9 ± 2.4
500 oC 1.48 1.3 ± 0.1 10.5 ± 0.3 0.25 ± 0.25 10.0 ± 0.3 7.9 ± 0.6
600 oC 1.54 1.1 ± 0.1 9.7 ± 0.5 0.54 ± 0.54 18.8 ± 0.5 3.7 ± 0.8
700 oC 1.58 1.1 ± 0.1 9.3 ± 0.4 0.97 ± 0.11 35.0 ± 0.5 2.9 ± 0.3
800 oC 1.64 1.0 ± 0.1 8.7 ± 0.3 1.11 ± 0.24 61.2 ± 1.3 1.8 ± 0.4
1000 oC 1.81 0.7 ± 0.1 7.2 ± 0.3 2.36 ± 0.42 179.5 ± 8.7 1.4 ± 0.2
1050 oC 1.81 0.7 ± 0.1 7.2 ± 0.4 2.44 ± 0.46 182.7 ± 13.6 1.4 ± 0.2
1100 oC 1.81 0.7 ± 0.1 7.1 ± 0.4 2.46 ± 0.32 210.2 ± 10.4 1.2 ± 0.1
60
The mechanical properteis of carbonized fibers were shown in the Figure 4.2. and Table 4.4. The density trends of carbonized fibers were divide in 4 stage shown in Figrue 4.2 (a): Stage 1 (Stabilized fiber – 580 oC), Stage 2 (580 – 900 oC), Stage 3 (900 – 1200 oC) and Stage 4 (1200 – 1400 oC). During the carbonization process, the reactions during carbonization were known as combination of pyrolysis, condenstation and grapthitizaiton in the carbonized fiber [53]. With these reactions, the ladder strucutres in the stabilized fiber were crosslinked with releasing the heteroatomes, and then final carbonized fibers were formed the graphitic structure. As shown in Figrue 4.2 (a), the density of carbonized fiber increased up to stage 3 with maximum value and decreased at stage 4. With the pyrolysis during carbonization, the heteroatoms in the carbonized fiber were relaesd and these affected to remain the high component of carbon atoms in the final carbonized fiber. This indicated that the the condenstation reactions were occurred dominantly to form the grapitic structure under 1200 oC. In stage 4, the density of carbonized fiber decreased where the heteroatoms remained in the fiber relased and this lead to decrease of density of carbonized fiber.
The tenile properties, the strain to failure and effective diamter of carbonized fibers was shown in Figure 4.2 (b) and Figrue 4.2 (c). In stage 1, the tensile properties of carbonized fiber remained the same, while the strain to failure of carbonized fiber highly decrase. It might be the residence PAN structure in the stabilized fiber converted to the ladder structure by cyclization and dehydrogentation reaction [86].
After the stage 2, the tensile propreties of carbonized fiber increased upto maximum value at the staege 4. On the other hand, the effective diameter and strain to failure deacrease gradually at the stage 2 and reamin similar value above the stage 3. This results will be corrleated with structural evolation of carobnized fiber with following analysis.
4.2.2. Thermal analysis and elemental analysis
62
Figure 4.4 (a) Elemental percent and (b)Modified element percent during carbonization
The thermal properties of PAN stabilized fiber are appeared in Figure 4.3. The Figure 4.3 (a) represent the thermal degradation of PAN stabilized fiber during heat in nitrogen atmosphere. The fiber pyrolysis up to 1400. From the TGA curve, the weight loss step was represented by the peak of the Derivative TG (DTG) curve, and the slop the DTG curve indicated the rate of mass loss. From the results, there are four transition point in DTG curve at 400 oC, 580 oC, 700 oC and 1000 oC. The chemical reaction during carbonization process resulted the loss of weight. The fist transition point was related with cyclization in residence PAN structure and the second point indicated intramolecular crosslinking by dehydrogenation. The third point indicated the intermolecular crosslinking which had an activation energy transition point 580 oC shown in Figure 4.3 (b). This activation energy was calculated by the Arrhenius equation. This transition pointe indicated that the crosslinking structure developed the thermal stabilities. The fourth point started at 900oC which indicated the crosslinking by denitrogenation exhausted amount of HCN and N2.
The results of element analysis were shown in Figure 4.4. The Figure 4.4 (a) represented the element percent applied the retained mass. The retained mass of carbonized fiber was calculated with linear density of carbonized fiber and indicated the remained mass after the carbonization process. The modified element percent was calculated by multiplying retained mass and results of elemental analysis.
From the results, the hetero atoms, such as nitrogen, oxygen and hydrogen decreased and the carbon atom were mainly remained in the carbonized fiber in stage 3 and 4. The more detail of loss hetero atoms was described in Figure 4.4 (b). The oxygen and hydrogen decreased in similar way and they decreased highly in stage 2. On the other hand, the loss of nitrogen highly decreased in the stage 3. This result related with dehydrogenation and denitrogenation as mentioned in TGA analysis. Therefore, in the stage 2, the intermolecular crosslinking by dehydrogenation affect to release the hydrogen and oxygen atom in the carbonized fiber during heat treatment. In the stage 3, the intermolecular crosslinking by denitrogenation occurred dominantly an affect to loss of nitrogen contents.
64
4.2.3 Functional group and Crystal structure during LT carbonization
Figure 4.5 (a) X-ray diffraction and (b) FT-IR specturm during LT stabilization.
66
The Figure 4.5 indicated the X-ray diffraction and FT-IR results during the carbonization. The results of X-ray diffraction in Figure 4.5 (a), the unreacted PAN structure showed in SF fiber which meant PAN fiber not fully changed to the ladder structure during stabilization. The 400oC fiber showed the decrease intensity of 17o peak but remained still. The PAN structure fully disappeared in 600oC fiber which indicated the 25o peak only. This represented the remained PAN structure reacted in stage 1.
Therefore, in the stage 1, the cyclization of unreacted PAN structure might be dominated and that is less mass and element loss.
The Figure 4.5 (b) was represented the functional group of carbonized fiber. As mentioned in Chapter 3, the stabilized fiber showed the (C=N) and (C-H) resulted by cyclization and dehydrogenation reaction. In stage 1, the C=N and C-H peak increase by cyclization reaction detected from the X-ray result. In the stage 2, the broad peak appeared at ~1200cm-1 and developed up to 800oC. This broad peak indicated the aromatic structure of carbonized structure which hinder the vibration of bonding.
Form this result, in stage 2, the aromatic structure was formed, which the ladder structure crosslinked adjacent ladder structure.
The results of XPS Cls and N1s showed in Figure 4.6 (a) and (b), respectively. In the XPS C1s results, the carbon related bonds were appeared sp2, sp3, C-OH/C-N and COOH. In the XPS N1s, the pyridinic, pyridonic/pyrrolic, quaternary and oxidized nitrogen were deconvoluted with binding energies at 398.50, 401.05, 401.77, and 403.28 eV, respectively The amount of pyridinic N increased , but the amount of pyridonic/pyrrolic N decreased with an increase of temperature during stage 1 and 2.
During the stage 3 and stage4, the amount of pyridinic N decrease, but the amount of pyridonic/pyrrolic N increase. This mean the intermolecular crosslinking by dehydrogenation occurs the pyridinic structure and that decrease by evolution of HCN at 1000oC. On the other hand, quaternary and oxidized nitrogen gradually increase during carbonization process, that is related with graphenic structure growth.
Furthermore, peak was proposed from the deconvolution of C 1s. The amount of sp3 and oxygen bond decrease and the amount of sp2 structure increase during stage 1 and 2. The sp2 structure highly increase at start of stage 2 due to intermolecular crosslinking by dehydrogenation.
4.2.4 Crystal Structure Changes during Carbonization Process
68 Table 4.5 WAXD results for samples
Sample
PAN structure, (200),(110) plane Graphitic structure, (002) plane Graphitic structure, (10) plane
2𝜃 (o)
D-spacing (nm)
Crystal size (nm)
2𝜃 (o)
D-spacing (nm)
Crystal size (nm)
Orientation factor f
2𝜃 (o)
D-spacing (nm)
Crystal size (nm)
SF 16.65 0.532 1.25 25.7 0.347 1.31 0.51 43.8 0.207 2.56
400 oC 17.81 0.498 0.92 25.6 0.347 1.35 0.63 43.5 0.208 2.83
500 oC - - - 25.8 0.345 1.35 0.66 43.6 0.207 2.91
600 oC - - - 25.5 0.349 1.36 0.68 43.9 0.206 3.18
700 oC - - - 25.6 0.348 1.38 0.69 43.9 0.206 3.28
800 oC - - - 25.3 0.351 1.40 0.72 43.8 0.207 3.28
1000 oC - - - 25.7 0.346 1.45 0.79 43.7 0.207 3.62
1050 oC - - - 25.5 0.350 1.47 0.76 43.7 0.207 3.81
1100 oC - - - 25.3 0.351 1.47 0.80 43.6 0.207 3.96
1150 oC - - - 25.2 0.353 1.49 0.79 43.5 0.208 4.19
1200 oC - - - 25.5 0.350 1.59 0.80 43.3 0.209 4.55
1250 oC - - - 25.3 0.351 1.67 0.81 43.4 0.208 4.66
1300 oC - - - 25.2 0.353 1.70 0.80 43.5 0.208 4.66
1350 oC - - - 25.4 0.350 1.84 0.80 43.4 0.208 4.87
1400 oC - - - 25.3 0.351 1.87 0.81 43.4 0.209 5.38
In the Figure 4.7 shows the crystal size and Herman’s orientation factor from the WAXD analysis and the results were described in Table 4.5. The crystal size of (002) plane, Lc, calculated from the ~25o peak scanned from the equatorial scan and indicated the stacking size of graphitic structure. The crystal size of (10) plane, La, calculated from the ~43o peak scanned from the meridional scan and indicated the lateral size of crystal. The result of Figure 4.7 (a), the growth of crystal size shown the difference manner in each stage. In stage 1, La increase at early part while the Lc remain similar, then the similar manner shown in stage 2. During HT carbonization, the La highly increased while Lc is slightly increased, then both Lc and La increased in the stage 4. The lateral crystal size increased when the crosslinking occurred with adjacent molecules. For that reason, the crosslinking by hydrogenation increased the La initially, but similar value shown up to 800oC due to elongation process hinder the lateral growth. Therefore, with the shrinkage process, the lateral size of crystalline highly increase. On the other hand, Lc increase the stage 4 due to graphitization process which stack the turbostratic layer.
The orientation factor of samples shown in Figure 4.5 were obtained from azimuthal scan along (002) plane. The orientation factor increased in gradually in LT carbonization and sharply increased in start point of HT carbonization. The orientation is related with the growth of La, because large sized of graphite layer is helping to form the high order structure. With the cyclization in stage 2, the PAN structure changed to ladder structure and increase the La initially [87]. In stage 2, the crosslinking of ladder structure increased the orientation in fiber, but the later sized maintain the same size. This described the misoriented ladder structure crosslinked, but the growth of lateral size limited due to elongation process. On the other hand, the orientation increased with highly on the stage 3, even in the shrinkage reaction. The crosslinking by denitrogenation resulted the shrinkage which is the considered for manufacturing the high strength carbon fiber. The previous research was correlated with this result need for shrinkage and hard effect to carbon fiber.
70 4.2.5 Microstructure Changes during Carbonization Process
Figure 4.8 2D gradient maps in (a) Surface, (b) Skin and (c) Core of stabilized and carobnized fiber.
72
The Raman spectra Along with the spectral changes at around 1350 and 1580 cm-1 for D- and G- bands, respectively, noticeable changes of contour lines in the vicinity of 1200 and 1500 cm-1 are observed during carbonization The bands at around 1200 and 1500 cm-1 are reported to originate from the transpolyacetylene(TPA) and amorphous (A) structures, respectively
The Fig 4.8 showed the 2D gradient maps from the Raman spectra. The 2D gradient map was the first derivatives of the intensity as a function of the carbonization temperature(T). In 2D gradient map, red and blue lines indicate the positive and negative values of the d(I)/d(T), respectively. In the stage 1, the D-band increased due to increase of ladder structure by cyclization reaction. In stage 2, D-band decrease with intermolecular crosslinking by dehydrogenation. In the stage 3, the amorphous region increased highly in the core due to the shrinkage process. In the stage 4, the D- and G- band increased because the graphitization occurred in the carbonized fiber.
The structure parameters of carbonized fiber were shown in Figure 4.9. There were three points were detected. First, the ID/IG of core region had the sudden decrease than the ID/IG of surface in the stage 2. The gas could easily escape in the surface region and this affected the bigger size of graphitic structure. On the other hand, the gas could not escape in the core region and this affected smaller size of graphitic structure. Above the stage 3, the graphitic structure became the similar as shown in the ID/IG
of core and surface region. Secondly, the ITPA/IG and IA/IG sharply increased in the stage 3. In the stage 3, the shrinkage process applied in the HT carbonization process. Moreover, the heat diffusion occurred slowly in the core than the surface region due to low heat conductivity. Therefore, the amorphous structure formed in the core region in stage 3. The last point was the decrease of ITPA/IG and IA/IG
intensity in stage 4. As the carbonization temperature increased, the amorphous carbon structure decreased due to the diffusion of the heat.
4.3. Conclusion
The correlation between structure and mechanical properties was studied in this experiment. The trend of mechanical properties changed in four stage during carbonization process. In stage 1, the residual PAN structure cyclized to ladder structure with decrease of strain to failure. In stage 2, the aromatic structure was developed by intermolecular crosslinking by dehydrogenation and this affected increased tensile properties and density. In stage 3, the graphitic structure was developed by intermolecular crosslinking by denitrogenation with significant increase of tensile properties and density. Also, shrinkage process affected formed the amorphous structure in the core where the lateral graphitic structure size developed smaller than the surface at stage 2. In stage 4, as the carbonization temperature increased, the tensile properties increased up to maximum value with developed graphitic crystallites and loss of amorphous structure.
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