Wide Angle x-ray Diffraction (WAXD) analysis of fibers

Chapter 2. The synthesis of polyacrylonitrile (PAN)/cellulose nanocrystal

2.3 Results and Discussion

2.3.7 Wide Angle x-ray Diffraction (WAXD) analysis of fibers

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Azimuthal scans of PAN(100) plane are shown in Figure 2.21 and those of CNC(200) plane and its deconvoluted spectra is exhibited in Figure 2.22. The azimuthal scans of PAN(100) and CNC(200) were extracted at the 17° and 22.7°, respectively. The peaks at azimuthal scan of PAN(100) of post drawn fibers become sharp. This tendency can also be confirmed in the 2D WAXD pattern image (Figure 2.17) that PAN(100) peak becomes clearer with the increase of the draw ratio. In Figure 2.22, in the case of the azimuthal scan at 2θ ~22.7°, the peak of PAN around 30° and 150° which represents the four-point pattern on 2D WAXD images were overlapped. Therefore, only the peak of CNC(200) plane can be extracted through peak deconvolution. When comparing the CNC(200) peaks extracted in this way, in the case of post-drawn fiber especially TDR 18, the peak of in-situ PAN/CNC fiber is sharper and the intensity of the peak is slightly higher than ex-situ PAN/CNC, but there is no significant difference with the naked eye. For comparing the differences, Hermann’s orientation factor which means the degree of orientation of the crystalline phase formed in each plane direction was obtained by substituting the extrapolated azimuthal scan using Wilchinsky equation ^56,57. The calculated orientation factor of PAN(100) (fPAN(100)) and CNC(200) (fCNC(200)) are presented in Table 2.10 and the tendency of orientation factor according to the draw ratio is shown in Figure 2.23(a). fPAN(100) of in-situ PAN/CNC and ex-situ PAN/CNC fibers were larger than that of control PAN fibers. This means that the existence of CNC can influence on the PAN chain alignment. In addition, both orientation factor of PAN(100) and CNC(200) tends to increase with increasing draw ratio in in-situ PAN/CNC fiber and ex-situ PAN/CNC fiber, and the value of orientation factor of in-situ PAN/CNC fiber is slightly larger than that of ex-situ PAN/CNC fiber. As the fiber is drawn, the CNC inside the in-situ PAN/CNC fiber aligns better than ex-situ PAN/CNC fiber, and the CNC aligned this way also helps PAN chain alignment, therefore the orientation factor of PAN(100) peak of in-situ PAN/CNC fiber has a larger value than ex- situ PAN/CNC fiber.

In table 2.10, the crystal size of PAN (100) planes (Lc) of control PAN, in-situ PAN/CNC and ex-situ PAN/CNC calculated using the deconvoluted WAXD spectra of equatorial spectra was listed.

The Lc values of the as-spun fibers are 3.3137 nm, 3.3137 nm and 3.0302 nm, respectively, and the Lc

values of the drawn fibers have the maximum values of 14.3228 nm, 14.2575 nm and 14.2476 nm, respectively in control PAN, in-situ PAN/CNC and ex-situ PAN/CNC fibers. The maximum values of Lc of the three fibers are comparable, but at the low draw ratio, in-situ PAN/CNC fiber mostly has lower Lc values. This is predicted that Lc decreases due to the increase in chain edge portions due to the low molecular weight polymers inside the in-situ PAN/CNC ⁵⁸. In the case of in-situ PAN/CNC fibers, Lc at TDR 19.5 suddenly decreases to 13.8676 nm.

These tendencies can be compared with the previous DMA results in Table 2.5. It was confirmed that the interaction between the PAN chain and CNC in the in-situ PAN/CNC fiber was

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greater than ex-situ PAN/CNC and control PAN from DMA results. It was revealed numerically that the orientation of the PAN(100) plane of in-situ PAN/CNC fiber was also increased compared to ex-situ PAN/CNC fiber by the interaction between PAN and CNC. It can be suggested that this difference is due to the low molecular weight polymer of in-situ PAN/CNC because the only variable between the in-situ PAN/CNC and ex-situ PAN/CNC fibers were existence of low molecular weight polymers. In conclusion, compared with the tensile property results in Figure 2.15 and Table 2.7, it can be confirmed that the tensile strength and modulus of in-situ PAN/CNC fiber were larger than that of ex-situ PAN/CNC fiber due to the high degree of alignment of the polymer chains with nano fillers, however, decrease of Lc occurred the over-stretching of PAN chain and tensile strength reduction as it approaches the maximum draw ratio at TDR 19.5

The WAXD images of carbonized fibers are shown in Figure 2.24 and calculated crystal size (Lc) and orientation factor of (002) plane of graphite (f(002)) are listed in Table 2.11. Unlike the WAXD images of the precursor fibers in which the PAN and CNC plane peaks were seen, the (002) graphite peak appeared prominently in the WAXD images of carbonized fibers. In each carbonized fiber, the Lc

and f(002) tend to increase as the carbonization temperature increases from 1300 ℃ to 1400 ℃, indicating the growth of graphitic structure. The two values are the largest in the in-situ PAN/CNC_iso fibers carbonized using the iso-strain method, and this result is comparable to the tensile results (Table 2.8, Table 2.9)

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Figure 2.17 2D WAXD pattern images of (a) control PAN, (b) in-situ PAN/CNC and (c) ex-situ PAN/CNC fibers of as-spun and various TDR.

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Figure 2.18 Peak assignment of 2D WAXD image patterns.

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Figure 2.19 The 2D WAXD integrated, equatorial and meridional scans of (a) control PAN, (b) in-situ PAN/CNC and (c) ex-situ PAN/CNC at various TDR.

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Figure 2.20 2D WAXD meridional scans in the range of 35-45° of (a) control PAN, (b) in-situ PAN/CNC and (c) ex-situ PAN/CNC.

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Figure 2.21 2D WAXD azimuthal scans of PAN(100) planes of (a) control PAN, (b) in-situ PAN/CNC and (c) ex-situ PAN/CNC at various TDR.

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Figure 2.22 2D WAXD azimuthal scans of CNC(200) planes of (a) in-situ PAN/CNC and (b) ex-situ PAN/CNC. The deconvoluted 2D WAXD azimuthal scans of CNC(200) planes of (a) in-situ PAN/CNC and (b) ex-situ PAN/CNC with TDR 18.

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Table 2.10 The crystal size (Lc) and Hermann’s orientation factor of PAN(100) plane and CNC(200) plane at various TDR.

TDR Lc (nm)¹ fPAN(100)2 fCNC(200)3

Control PAN

As-spun 3.3137 0.451 -

15 14.4004 0.871 -

16.5 14.0486 0.877 -

18 14.2608 0.897 -

19.5 14.3228 0.903 -

In-situ PAN/CNC

As-spun 3.3137 0.512 0.460

15 13.6915 0.918 0.899

16.5 13.9026 0.921 0.900

18 14.2575 0.923 0.911

19.5 13.8676 0.937 0.900

Ex-situ PAN/CNC

As-spun 3.0302 0.496 0.426

15 13.9262 0.915 0.864

16.5 14.1118 0.917 0.853

18 14.1059 0.922 0.854

19.5 14.2476 0.923 0.881

1 : Crystal size

2 : Orientation factor of PAN(100) planes

3 : Orientation factor of CNC(200) planes

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Figure 2.23 (a) The Hermann’s orientation factor of PAN(100) and CNC(200) planes of control PAN, in-situ PAN/CNC and ex-situ PAN/CNC fibers. (b) The crystal size of PAN(100) planes of control PAN, in-situ PAN/CNC and ex-situ PAN/CNC fibers.

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Figure 2.24 2D WAXD pattern images of carbonized fibers of (a) control PAN, (b) in-situ PAN/CNC, (c) ex-situ PAN/CNC and (d) in-situ_iso PAN/CNC fibers at carbonization temperature of 1300 ℃ and 1400 ℃.

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Table 2.11 The crystal size (Lc) and Hermann’s orientation factor of (002) plane (f(002)) of carbonized fibers at various carbonization temperatures.

Carbonization Temperature (℃)

Lc

(nm) f(002)

Control PAN 1300 1.73 0.810

1400 1.87 0.814

In-situ PAN/CNC 1300 1.77 0.812

1400 1.91 0.813

Ex-situ PAN/CNC 1300 1.74 0.811

1400 1.91 0.813

In-situ PAN/CNC_iso 1300 1.84 0.818

1400 1.95 0.819

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