III. Characterization of Polypropylene (PP) by Raman Spectroscopy

As an emerging material characterization technique, Raman spectroscopy can reveal the relevant information of molecules inside the sample by inelastic scattering of photons through the interaction between a beam of monochromatic light and sample. It has typical advantages of short detection time, non-destructiveness, and no need for sample preparation.

Figure 1. Complete Raman spectrum of polypropylene (PP).png

Figure 3. Complete Raman spectrum of polypropylene (PP)

Figure 3 shows the overall Raman spectrum of the polypropylene sample used in the experiment in the Raman shift range between 0 ~ 3750 cm^–1. Through non-destructive Raman analysis, the characteristic vibration peaks of the sample are mainly concentrated in the Raman shift ranges of 50 ~ 700 cm^–1, 700 ~ 1500 cm^–1, and 2500 ~ 3100 cm^–1. The following three subsections will discuss the motion states of chemical bonds within the polypropylene molecules corresponding to the three Raman shift ranges in detail.

(a) 50 ~ 700 cm^–1

Figure 2. Partial Raman spectrum of polypropylene between 50 ~ 700 cm–1.png

Figure 4. Partial Raman spectrum of polypropylene between 50 ~ 700 cm^–1

As shown in Figure 4, in the Raman shift range of 50 ~ 700 cm^–1, there are 5 Raman bands mainly observed at 95 cm^–1, 177 cm^–1, 399 cm^–1, 461 cm^–1, and 528 cm^–1. It is hard to tell the vibration modes at 95 cm^–1 and 177 cm^–1. However, they may be caused by the translation or translational vibration of crystal lattices of polypropylene.

Table 2. Vibrational motion states of each Raman bands of polypropylene between 50 ~ 700 cm^–1

(δ = bending, r = rocking, v = stretching)

Raman shift of peaks (cm^-1)	Vibration modes
95	Unknown
177	Unknown
342	v(C–C)
399	v(C–C), δ(CH)
461	v(C–C)
528	v(C–C), r(CH₂), δ(C–C)

Table 2 shows the assignment of Raman bands in this range. From the table, it can be suggested that the skeletal stretching of C–C is the most common vibration modes found at 342 ~ 528 cm^–1. Besides, there also exists bending of CH at 399 cm^–1 and rocking of CH₂ and bending of C–C at 528 cm^–1.

(b) 700 ~ 1500 cm^–1

Figure 3. Partial Raman spectrum of polypropylene between 700 ~ 1500 cm–1.png

Figure 5. Partial Raman spectrum of polypropylene between 700 ~ 1500 cm^–1

As shown in Figure 5, in the Raman shift range of 700 ~ 1500 cm^–1, there are 8 Raman bands mainly observed at 810 cm^–1, 843 cm^–1, 976 cm^–1, 1153 cm^–1, 1171 cm^–1, 1332 cm^–1, 1360 cm^–1, and 1460 cm^–1.

Table 3. Vibrational motion states of each Raman bands of polypropylene between 700 ~ 1500 cm^–1

(δ = bending, r = rocking, v = stretching, t = twisting, w = wagging, d = deformation)

Raman shift of peaks (cm^-1)		Vibration modes
810		r(CH₂), v(C–C)
843	821	r(CH₂), v(C–C)
	855	r(CH₂), r(CH₃), v(C–C)
976		r(CH₃), v(C–C)
1153		v(C–C), δ(CH)
1171		v(C–C), r(CH₃), w(C–C)
1332		t(CH₂), w(CH₂), v(C–C)
1360		d(CH₃)
1460	1464	δ(CH₃)
	1465	δ(CH₃)

Table 3 shows the assignment of Raman bands in this range. Snyder et al. (1964) suggested that there are two decomposable bands in this range. The one is at 843 cm^–1, which can be further decomposed into the rocking of C–H in CH2 and skeletal stretching of C–C at 821 cm^–1. The other is at 1460 cm^–1 and can be further decomposed into the bending of C–H in CH₃ at 1464 cm^–1 and 1465 cm^–1. From the table, it was found that the skeletal stretching of C–C is the most common vibration mode at 810 ~ 1332 cm^–1, while in the remaining range there are deformation and bending of C–H in CH₃. In general, the vibration modes are the most in this range, in which the main ones are skeletal stretching of C–C and rocking of CH₂ and CH₃. Besides, there are other vibration modes of CH, CH₂ and CH₃, including bending, wagging, twisting, or deformation.

(c) 2500 ~ 3100 cm^–1

Figure 4. Partial Raman spectrum of polypropylene between 2700 ~ 3100 cm–1.png

Figure 6. Partial Raman spectrum of polypropylene between 2700 ~ 3100 cm^–1

As shown in Figure 6, in the Raman shift range of 2700 ~ 3100 cm^–1, there are 5 Raman bands mainly observed at 2841 cm^–1, 2883 cm^–1, 2906 cm^–1, 2924 cm^–1, and 2961 cm^–1.

Table 4. Vibrational motion states of each Raman bands of polypropylene between 2700 ~ 3100 cm^–1

(v = symmetric stretching, v' = antisymmetric stretching)

Raman shift of peaks (cm^-1)		Vibration modes
2841		v(CH₂)
2883		v(CH₃)
2906		v(CH)
2924	2918	v'(CH₃)
	2938	v'(CH₃)
2961		v'(CH₃)

Table 4 shows the assignment of Raman bands in this range. Snyder et al. (1964) suggested that the Raman band at 2924 cm^–1 can be decomposed into antisymmetric stretching at 2918 cm^–1 and 2938 cm^–1. From the table, it was intuitively concluded that symmetric or antisymmetric stretching of CH, CH₂ and CH₃ dominate the vibration modes in this range.

References

[1] Snyder, R. G., Schachtschneider, J. H. (1964). Valence force calculation of the vibrational spectra of crystalline isotactic polypropylene and some deuterated polypropylenes. Spectrochimica Acta, 20(5), 853–869. https://doi.org/10.1016/0371-1951(64)80084-9

Characterization Analysis of Polypropylene (PP)

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