Chapter Ⅳ Microstructure and Mechanical Properties of Poly-acrylonitrile-
4.2 Experimental
4.2.1 Materials
A commercial PAN precursor fiber used for this experiment was obtained from BlueStar Co Ltd, China. The precursor fiber contains 93 % acrylonitrile, 6 % methyl methacrylate and 1 % itaconic.[52]
The PAN precursor fiber was stabilized in ST135 condition, which was described in Chapter 2.
4.2.2 LT and HT carbonization process
The stabilized fiber was heat treated through the LT carbonization oven under a nitrogen gas flow of 3 l/min. During the LT carbonization, the stabilized fiber was heat treated 400 oC first and re-heat treated up to 800 oC at 100 oC intervals. The elongation ratio was controlled by different drawing speed between two driving roller and the elongation ratios of each temperature process was 2, 2, 3, 5 and 3%.
The residence time of each temperature process was 25 seconds and the total residence time was 125 seconds. During the HT carbonization process, the LT carbonized fiber was heat treated through the HT carbonization oven under a nitrogen gas flow of 3 l/min. The LT carbonized fiber was heat treated in various temperature from 1000 to 1400 oC at 50 oC interval for 2 min with 3% of shrinkage ratio for each temperature condition. The lists of overall carbonized fibers in this chapter were shown in the Table 4.2. and Table 4.3.
Table 4.2 Carbonization condition during LT carbonization.
Sample
Final temperature in LT carbonization
(oC)
Total Elongation ratio (%)
Total residence time (sec)
400 oC 400 2 25
500 oC 500 4 50
600 oC 600 7 75
700 oC 700 12 100
800 oC 800 15 125
Table 4.3 Carbonization conditino during HT carbonization.
Sample Final temperature in HT carbonization (oC)
Shrinkage (%)
Residence time (min)
1000 oC 1000 3 2
1050 oC 1050 3 2
1100 oC 1100 3 2
1150 oC 1150 3 2
1200 oC 1200 3 2
1250 oC 1250 3 2
1300 oC 1300 3 2
54 4.2.3 Characterization
4.2.3.1. Density
The densities of carbonized fiber were measured by He pycnometer (Accupyc II 1340, Micromeritics, Inc) in room temperature (23 oC). A carbonized fiber weighed in 700 ± 50 mg and loaded in a 1 cc cup. The densities of carbonized fibers were reported in average value obtained through 10-time trials, which was satisfied conditions of 0.005 standard deviation or less [41].
4.2.3.2. Single fiber tensile test
The mechanical properties of carbonized fibers were performed with a single filament tensile testing machine (FAVIMAT+, Measured Solutions Inc.). The linear densities of the carbonized fibers were measured by a vibroscope prior to tensile testing for a fiber gauge length of 25.4 mm. The linear densities were converted to an effective diameter and the retain mass. The effective diameters of carbonized fibers were calculated correlation between linear density and density of carbonized fibers.
The retained mass of carbonized fibers represented the remained mass after the carbonization process compared to stabilized fiber and was calculated by following equation [76, 77],
Retained mass (%) = 100 − (LDs− LDc
LDs ) × 100 (4-1) Where LDs is linear density of stabilized fiber, LDc is linear density of carbonized fiber and TE is total elongation ratio of carbonized fiber during carbonization process. For tensile tests, 30 filaments were tested. The fracture morphologies of carbonized fiber converted tensile test were examined by the scanning electron microscope (SEM, NOVA NanoSEM 230, FEI, USA) at an accelerating voltage of 10 kV.
4.2.3.3. Thermogravimetric analysis
Thermogravimetric analysis (TGA) was performed using a Q50 (TA instruments, USA). The stabilized fiber was heated under nitrogen atmosphere from 40 oC to 1400 oC at heating rate of 5, 10, 20 and 30 ℃/min. The activation energy was calculated by application of the Arrhenius equation [78, 79]:
k = Ae−ERTA (4-2)
or, alternatively,
ln k = −EA
RT+ ln A (4-3)
Where k is the preexponential Arrhenius constant, EA is activation energy during carbonization, R is gas constant, which equals to 8.314 J/Kmol, and T represents temperature.
4.2.3.4. Elemental analysis
Elemental analysis (Flash 2000, Thermo Scientific, USA) was performed to measure amounts of oxygen in the bulk of stabilized fibers by analyzing CO gas after thermal decomposition process at 1300 °C.
4.2.3.5. X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo Scientific, USA) was performed using monochromated Al Kα (1486.6 eV) X-rays to examine chemical bonds of each element of the carbonized fibers. The survey spectrum was collected from 0 to 1350 eV. Binding energies were referenced to the C1s line at 284.4 eV.
4.2.3.6. FT-IR
Fourier Transform Infrared (FT-IR) spectroscopy was conducted on the carbonized fibers with an infrared microscope (Cary 670, Varian, Co.) using Attenuated Total Reflectance (ATR) mode The absorbance spectra was obtained in the range of 600 cm-1 to 4000 cm-1 at spectral resolution of 2 cm-1 by accumulating 256 scans.
4.2.3.7. Wide-angle X-ray diffraction
Wide-angle X-ray diffraction (WAXD) patterns of carbonized fibers were obtained by PSL-II 6D UNIST-PAL beamline of Pohang Accelerator Laboratory (PAL) in Korea. The WAXD patterns were
56
and Bragg angle of the corresponding peak, respectively; Kc and Ka is constants equal to 0.9 and 1.84, respectively.
The orientation factor, f(002), is a parameter to obtain the orientation of crystal (002) plane in carbonized fiber. The crystal orientation was calculated by the Herman’s orientation function as following equation [83-85]
f(002)=3⟨cos2∅⟩ − 1
2 (4-6)
Where, ⟨cos2∅⟩ is defined as
⟨cos2∅⟩ =∫oπ 2⁄ I(∅) cos2∅ sin ∅ d∅
∫0π 2⁄ I(∅) sin ∅ d∅ (4-7) and ∅ is the azimuthal angle, I(∅) is the diffraction intensity of (002) plane using the azimuthal scan at 2θ = 25.5 o.
4.2.3.8. Raman Spectroscopy
Raman spectra of samples were obtained using a using an Alpha 300s micro Ramana spectrometer (WITec) equipped with a 532nm laser and a 0.5 mW power. The 50 × objective lens was used resulting in a spot size of approximately 1 μm in diameter. The 10 sites of spectrum were obtained by 1 second exposure time with 400-time accumulations. To obtain local scattering information, the laser beam was focused at accurately designated positions such as surface, skin and core. To obtain the Raman spectrum on surface of carbonized fiber, the carbonized fiber mounted on microscope slides. Raman spectra from the fiber cross-section were obtained by first embedded in epoxy resin and cured at room temperature for 1 day. These specimens were then polished using Multiprep (Allied High tech, USA). The point of skin was measured 0.7 μm away from the surface toward the core. Cross-sectional surfaces of fibers were obtained using an optical microscope (Leica, DM2500M) at 100 × magnification.
Figure 4.1 The surface, skin and core region of the carbonized fiber in Raman analaysis
4.2. Result and Discussion