We investigated how partial fluorination of a phenyl group affects the CH stretching vibrational absorptions of plastic optical fiber (POF) core materials based on poly-(phenyl methacrylate) and poly-styrene. We measured their attenuation spectra and evaluated the effects of fluorination on the CH vibrational potential curve. From the results, we confirmed that in partially fluorinated poly-styrene-based materials, the fluorination can decrease not only the number density of CH bonds but also the amount of CH absorption per bond. This suggests that the absorption reduction efficiency depends on the substituted position in the benzene ring and the chemical structure itself.
© 2013 OSA
In the recent years, Internet usage has grown worldwide, and the amount of information exchanged online increases day by day. In order to accommodate this situation, networks using single- mode glass optical fibers (SM GOFs) have been constructed in the trunk lines carrying Internet data to achieve ultrahigh-speed data transmission. However, the core diameter of SM GOFs is extremely small, resulting in the requirement of their high precision connection to transceivers. For short-reach networks, the graded-index plastic optical fiber (GI POF) has been proposed as the leading transmission medium because of its high flexibility and large core diameter, which allow for its easy handling and low-cost connection with other optical devices [1,2]. This GI POF is expected to be used for home, vehicle, and aircraft networks with different attenuation requirements. For example, the attenuation for home networks should be no more than 200 dB/km for a 670–680 nm wavelength. Therefore, the development of novel base materials with sufficiently low attenuation has become extremely important.
The main cause of attenuation in POFs is the presence of CH bonds which results in overtone absorptions due to their stretching vibrations at visible wavelengths. Therefore, one of the most effective ways to reduce attenuation is to substitute the hydrogen atoms in the fiber core base materials with heavier atoms such as fluorine [3–5]. Fluorination reduces the number density of CH bonds and thereby reduces the absorption loss due to the CH stretching vibrations. However, the partial fluorination of a phenyl group also results in peak wavelength shifts of overtone absorption due to the presence of other aromatic CH bonds [6–8]. This phenomenon can definitely affect the wavelength range of the low–loss optical windows. Nevertheless, the attenuation in POFs has been analyzed under the assumption that the CH bonds within the polymer structure are all independent and identical oscillators.
In this paper, we investigated the influences of fluorination on the CH vibration absorption spectra of fluorinated phenyl methacrylates and styrenes to understand the mechanisms of effects of fluorination on the attenuation spectra of POFs.
2. Experimental method
Rigorously distilled phenyl methacrylate (PhMA), styrene (St), and their derivatives shown in Fig. 1 were polymerized at optimum temperatures of 70–120 °C in a water/oil bath for 48 h with di-tert peroxide (DTBP) as a polymerization initiator and n-lauryl mercaptan (n-LM) as a chain transfer agent. The absorption spectra of the bulk polymers with optically polished surfaces were measured by a spectrophotometer (UV 3100, Shimazu). For spectral analysis, the densities were measured by the water displacement method. The refractive indices were also measured at wavelengths of 409.2, 650.3, 833.7, and 1546 nm by using a prism coupling method.
3. Results and discussion
3.1 Attenuation spectra of materials with a fluorinated phenyl group
Figures 2(a)–2(c) show the absorption spectra of PPhMA, PTFPhMA, and PPFPhMA in the indicated wavelength ranges, where the 3rd, 4th, and 5th overtone absorption bands of CH stretching vibrations were observed . PPhMA and PTFPhMA have both aromatic and the aliphatic CH overtone absorption peaks, whereas PPFPhMA has only the aliphatic CH overtone absorption peak. The aromatic CH absorption peak wavelengths of PTFPhMA are shorter than those of PPhMA, which are 1130 nm, 870 nm, and 710 nm for 3rd, 4th, and 5th overtone absorption bands, respectively. On the other hand, all the materials have comparable peak wavelengths for the 3rd, 4th, and 5th aliphatic CH overtone absorptions around 1175 nm, 900 nm, and 730 nm, respectively. These indicate that the fluorination resulted in an anharmonicity change or a potential curve change for only the aromatic CH bond vibration . Other absorption bands were also observed around 710 nm, 870 nm, and 1130 nm in the spectra of PTFPhMA and PPFPhMA. They are assumed be some combination absorption bands of aliphatic CH bonds, whose accurate assignments require more detailed investigations.
Figures 3(a)–3(c) show the absorption spectra of PSt, PpFSt, PPFSt, and P2TFMSt. All the poly-styrene materials have both the aromatic and aliphatic CH overtone absorptions except PPFPhMA without aromatic CH bonds. Poly-styrene has absorption bands around 1140 nm, 870 nm, and 700 nm corresponding to the 3rd, 4th, and 5th aromatic CH overtones, respectively. The 3rd, 4th, and 5th aliphatic CH overtone absorption bands of PSt appeared around 1200 nm, 930 nm, and 750 nm, respectively. In the spectrum of PpFSt, we observed a fluorination-induced peak wavelength shift of the aromatic CH overtone absorption similar to PTFPhMA. However, we also observed a peak shift to shorter wavelengths for the aliphatic CH overtone absorptions in the spectra of PPFSt and P2TFMSt. This indicates that aromatic CH bond fluorination can also change the vibrational potential curve of the aliphatic CH bond in poly-styrene materials. The different effects of fluorination on PPhMA and poly-styrene could be due to their different chemical structures.
To evaluate the effects of fluorination quantitatively on CH bond potential curves, we estimated the anharmonicity constant χ of partially fluorinated materials by fitting the peak absorption wavelengths to those given by the Schrödinger equation for the Morse potential curve . Table 1 lists the χ values of the evaluated polymer materials. The anharmonicity constants of aliphatic CH bonds in PPhMA materials could not be measured because the CH absorptions in the visible wavelengths were extremely weak to enable the measurement of the absorption peak wavelengths, as shown in Fig. 2(a).
The anharmonicity constants of CH bond potentials are reduced by fluorination for both PPhMA and poly-styrene materials. The anharmonicity constants for fluorinated materials are lower for greater fluorine content in the benzene ring. This is likely due to the electron withdrawing effect of fluorination , which leads to an increase in absorption resonance frequencies through the potential curve changes. PTFPhMA has the lowest χ value of all the values of aromatic CH bonds according to Table 1. This suggests that the electron withdrawing effects could be stronger because of more fluorine in the benzene ring than those of the other partially fluorinated materials.
We also note that the χ values for the aliphatic CH bonds in PPFSt and P2TFMSt are quite low compared to PSt, whereas PTFPhMA and PPFPhMA have comparable peak wavelengths to that of PPhMA, as shown in Fig. 2(a). The different effects of fluorination between PPhMA and poly-styrene materials are attributed to the carbonyl group between the aromatic and the aliphatic CH bonds. For PPhMA materials, the carbonyl group can act as a barrier to disturb the mutual interaction of the aromatic and the aliphatic CH bonds, whereas fluorination of the aromatic CH bonds can affect the aliphatic CH vibrational potentials for poly-styrene materials. Furthermore, we note that P2TFMS has a lower aliphatic anharmonicity constant value than PPFS, suggesting that the substituent group of CF3 has a stronger effect of fluorination [11,12].
3.2 CH overtone absorptions per bond
Fluorination can also affect the absorption band strengths of CH bonds through a potential curve change or an anharmonicity constant change [13,14]. Therefore, we also evaluated the CH absorption per bond by fitting the molar absorption coefficient spectra to the sum of the pseudo Voigt profiles [15,16] with the integral band strengths, the bandwidths, and the resonance wavenumbers. Based on the fitted results, we estimated the average integral band strengths per bond for aromatic and aliphatic CH bonds, whereas CH absorption actually depends on the bond positions. For the spectra in the visible wavelengths, we also accounted for the contributions of light scattering and electron transition absorption .
Figures 4(a) and 4(b) show the average integral band strengths of the aromatic and aliphatic CH absorptions of partially fluorinated PPhMA materials as a function of the substituted position numbers with F. It was confirmed that the integral band strengths or CH absorptions per bond are hardly affected by fluorination. This suggests that the influences of the potential curve change on absorption transition rates can be negligibly small.
Figures 5(a) and 5(b) show the average integral band strengths of the aromatic and aliphatic CH absorptions of partially fluorinated styrene materials as a function of the substituted position numbers with F or CF3. As for partially fluorinated poly-styrenes, the average integral band strengths of both the aromatic and the aliphatic CH bonds dramatically decreased with an increase in the fluorine content in the benzene rings. This indicates that fluorination can decrease not only the number density of CH bonds but also the CH absorption per bond. These results suggest that the absorption reduction efficiency depends on the substituted position in the benzene ring and the chemical structure itself.
We studied the effects of fluorination on partially fluorinated phenyl methacrylate and styrene materials in the overtone absorption bands of CH stretching vibrations. For partially fluorinated phenyl methacrylate materials, the fluorination resulted in the peak wavelength shifts of the aromatic CH absorptions with small changes in the integral band strengths per bond. On the other hand, in partially fluorinated styrene materials, fluorination significantly shifted the peak wavelengths of both the aliphatic and the aromatic CH absorptions with significant reductions in the integral band strengths per bond. This indicates that the effect of fluorination depends on the chemical structure of the materials. We are presently analyzing the effects of fluorination to elucidate their origins by the absorption spectral analysis of various fluorinated materials with the quantum chemical calculations of their molecular orbitals.
This research is supported by the Japan Society for the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovative R&D on Science and Technology (FIRST program),” initiated by the Council for Science and Technology Policy (CSTP).
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