Qi Rongrong, Hu Keliang, Zhu Qingren, Pang Wenmin, Zhou Guien
(Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026,  China)

Received Nov. 2, 1999; Supported by the National Natural Science Foundation of China and the Youth Foundation of University of Science and Technology of China. .

Abstract  The luminescence properties of three sorts of polypropylene (PP) films, the unextracted PP film, the PP film extracted with hexane, and the extractant of PP were examined. The fluorescence intensity of the unextracted PP film is very strong. However, in the case of the film extracted with hexane, the fluorescence intensity became extremely weak. On the other hand, the extractant of PP showed almost similar fluorescence spectrum as that of the unextracted PP film. Identified by X-ray diffraction, FT IR and NMR, the extractant is mainly composed by low molecular weight atactic polypropylene (APP). The results indicated that atactic PP played an important part in the luminescence emission of commercial polypropylene.
Keywords luminescence, extract, extractant,  atactic polypropylene, ¹³C NMR

1. INTRODUCTION
In recent years, studies on electroluminescence (EL) of organic materials and polymers have caused  great attention with the development^[1-3]. The advantages of   luminescent organic materials are  low exciting voltage, easiness of color light emission and high EL efficiency. However, polymers used for EL are usually conjugated organic materials, which are very scarce and very difficult to be synthesized and  processed. Some conjugated organic EL materials/commercial polyolefins composites were prepared to improve the organic EL  processing of  the materials. Therefore, it is very important to reveal the luminescent mechanism of commercial polyolefins.
    The first observation of luminescence in commercial polyolefins was made by Charlesby and Partridge⁴. Since then a great deal of investigations^5-8 have devoted to the nature of the luminescent species, the characterization of polyolefins and the mechanism responsible for the photoluminescence from polyolefins. However, the luminescent mechanism of polyolefins still remains unclear. Some workers believed that the luminescent species were largely from polynuclear aromatic (PNA) impurities, which were released into the atmosphere in large quantities from the combustion and pyrolysis of fuels and lubricants^5-6. On the other hand, Allen et al.^{7, 8} reported that the fluorescence was primarily associated with the presence of enone and the phosphorescence of dienone chromophoric units. But all the studies were unable to give enough evidence.
    In this paper, the luminescent species in commercial polypropylene were studied and the results showed that the luminescence mainly originate from atactic polypropylene.
2. EXPERIMENTAL
2.1 Materials
Polypropylene powder (Sample 1) was supplied by Anqing Petroleum Company (melt flow index 1.4g/10min) and pellets of isotactic polypropylene (Sample 2) were from Mitsui Petrochemical Industries Co. Ltd. in Japan. Atactic polypropylene (Sample 4) was from Phillips Petroleum Co. in USA (Mn=30000).
2.2 Sample Preparation
The Unextracted Films: Powder and pellets placed between two aluminum foils were premelted at 200°C for 20min, then pressed in a laboratory press equipped with heating plates and quenched to room temperature.
    The Extracted Films: The films were extracted with 150ml of extra pure grade hexane for 48h.  The solvent in the samples was removed immediately under reduced pressure (1mmHg) for 20h.
    The Extractant: Extracted by acetone ( analytical reagent), some solid   materials were obtained from the above hexane solution and then were treated under reduced pressure for 10h to remove the solvents.
2.3 Measurements
Emission spectra: Fluorescent spectra were recorded at room temperature on a Hitachi 850 Fluorescence Spectrophotometer equipped with a xenon source and photomultiplier tubers. All spectra recorded were corrected for spectral distribution and photomulitiplier response. X-ray diffraction: Wide-angle X-ray diffraction (WAXD) patterns were obtained  on a x-ray rotating-anode diffractometer (D/max-rA, Rigaku) with graphite monochromatized Cu Ka radiation and height-pulse analyzer. FT IR spectra: The infrared examination was recorded on Nicolet Magna-IR^TM spectrometer. The film was coated on KBr.  NMR spectra: ¹³C NMR spectrum was taken on a Bruker DMX-500 nuclear resonance spectrometer. CDCl₃ was used as the solvent. Viscometric average molecular weight (M_h ) determination: M_h was determined by viscometry. Xylene was used as the solvent.
3. RESULTS AND DISCUSSION
Figure 1(a) shows the typical fluorescence excitation and emission spectra of commercial polypropylene (Sample 1). It shows that the excitation spectrum has two distinct maxima at 231 and 290nm, respectively. Exciting at 231nm wavelength, two peaks at 332 and 342nm in the fluorescent spectrum were observed. However, in the case of the PP film extracted with hexane, the fluorescence intensity becomes extremely weak (Fig. 1(c)). It seems that the luminescent species were extracted from the sample by hexane and transferred to the hexane. To further confirm it, the extractant was extracted from the mother liquor by acetone. Fig.1(b) is the luminescent spectra of the obtained extractant, we can see that the luminescent spectra of the extractant is almost similar to that of the unextracted PP films (Fig. 1(a)). The extractant was identified by X-ray diffraction, FT IR and  NMR etc.

Fig. 1 Fluorescence excitation and emission spectra of commercial polypropylene (Sample 1) films a.unextracted film;  b.the extractant; c.extracted film

    Figure 2 is the WAXD patterns for the extractant. The peak is in a diffuse scattering shape. According to the region of the diffraction peak, the diffraction peak is attributed to the noncrystal polypropylene, whereas the peaks of d=5.27Å and 4.11Å are due to a -phase polypropylene. The above result shows that the extractant is main noncrystal polypropylene with a trace of a -phase.
    The infrared spectrum for the extractant is shown in Figure 3. The bands at 2870 and 2954cm^-1 are assigned to -CH₃, the bands at 2839 and 2918cm^-1 are attributed to -CH₂-, and the bands at 972 and 1376 cm^-1 are due to - CH₃. The band at 1459cm^-1 is assigned to CH₃ or CH₂, and the band at 1158 cm^-1 is assigned to -CHCH₃. From  which, we can see that the spectrum is a typical polypropylene IR spectrum. The intensity of the band at 998cm^-1 related to helix chains of polypropylene is very low, which shows that the molecular chains are very short. The above analysis indicates that the extractant is main low molecular weight polypropylene (The viscometric average molecular weight was determined by the viscometry to be about 6000). The band at 725cm^-1 for ethylene group is extremely weak, which shows that there is a very little amount of ethylene-propylene copolymer in the extractant.

	Fig. 2 X-ray diffraction patterns for the extractant
	Fig. 3 Infrared spectra for the extractant

    The extractant is further studied by   ¹³C NMR spectrum (Fig. 4). The peaks at 19.23 - 20.24 ppm are assigned to -CH₃. The peaks at 20.59-20.93ppm are attributed to heterotactic polypropylene (mr), the peaks at 21.24-21.76ppm are assigned to isotactic polypropylene (mm), and the peaks at 19.74-20.24ppm are attributed to syndiotactic polypropylene (rr). Their integral relative intensities (the intensities of mm, mr, rr are similar, and the intensity of mr is slightly stronger) show that the extractant is main atactic polypropylene. The peaks at 30.14 and 30.44ppm are assigned to (CH₂)_n (n³ 3)(EEE) and the intensity is very weak, which shows that the extractant contains little amount of ethylene-propylene copolymer. This result is consistent with that of the above IR spectrum. In general, the chemical shifts of PNA and enone are over 120ppm, but no peaks in the region of over 120ppm were observed, which indicates that PNA and enone are not detected.

Fig. 4 13C NMR spectrum of the extractant

     In order to further study at the effect of atactic PP on the luminescence properties of commercial PP, the luminescent spectra of the defferent tacticity PP, Sample 1 (from Anqing Petroleum Company) and Sample 2 (from Mitsui Petrochemical Co. Ltd. in Japan) were studied. The tacticity of Sample 1 and Sample2 are evaluated by ¹³C NMR spectra to be 91% and 98%, respectively. As shown in Fig. 5, the fluorescence spectrum of Sample 1 is apparently observed, whereas the fluorescence intensity in Sample2 is extremely weak. When the high isotactic PP (Sample 2) was mixed with the extractant (4%wt), a similar luminescent spectrum as that osf Sample 1 was obtained. The result indicates that the atactic PP has great effects on the luminescence of commercial polypropylene.   

Fig. 5 Fluorescence excitation and emission spectra for the three sorts of polypropylene samples [1]91% tacticity PP (by Anqing Petroleum Company) [2]98% tacticity PP (by Japan) [3]Sample 4 (the composites of the extraction (4% wt) with PP by Japan)		Fig. 6 Fluorescence excitation and emission spectra of atactic polypropylene (by USA) [1] before extracted [2] after extracted

   In general , just some low molecular weight materials in the commercial PP can be extracted by hexane, whereas the high molecular weight cannot be extracted by hexane. Although the extractant was identified to be low molecular weight atactic polypropylene and have great effects on the photoluminescent properties in commercial PP, it is not sure whether or not high molecular weight atactic PP have luminescent properties . The fluorescence spectra of atactic PP (Sample 3 with Mn=30000 from Phillips Petroleum Co. in USA.) (before and after extracted) were shown in Fig.6. The fluorescence spectra of Sample 3 were similar to that of commercial polypropylene. It is interesting to find that the fluorescent intensity of the extracted films (Fig. 6(2)) is almost equal to that of the unextracted films (Fig. 6(1)). The results indicate that the atactic polypropylene plays an important role in the fluorescent emission of commercial polypropylene. The fluorescent emission may be attributed to the defect energies which was originated from the crystal lattice defects in polypropylene with the presence of atactic PP. We have already found that the luminescent properties of polypropylene are sensitive to the crystal structure of PP. This relative work will be expounded in another paper.
4.CONCLUSION
The luminescent species in commercial polypropylene is proved to be mainly caused by atactic polypropylene. The crystal lattice defects from atactic polypropylene may lead to the luminescence of commercial polypropylene. Further study in this area is being carried our.

The authors would also like to acknowledge informative discussions with Professor Qu Baojun.

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