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Study on polytetrafluoroethylene aqueous dispersion irradiated by gamma ray
Journal of Fluorine Chemistry 127 (2006) 91–96 www.elsevier.com/locate/fluor
Study on polytetrafluoroethylene aqueous dispersion irradiated by gamma ray Jielong Su a,b, Guozhong Wu a,*, Yaodong Liu a, Hongyan Zeng b a
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Jialuo Road 2019, Jiading, Shanghai 201800, China b Department of Chemical Engineering, Xiangtan University, Xiangtan, Hunan, China
Received 1 September 2005; received in revised form 16 October 2005; accepted 18 October 2005 Available online 28 November 2005
Abstract Gamma-radiation induced degradation of polytetrafluoroethylene (PTFE) in 60 wt.% dispersion was studied in the dose range of 20–200 kGy and the change in property of PTFE was characterized by differential scanning calorimetry (DSC), photon cross correlation spectroscopy (PCCS), X-ray diffraction (XRD), scanning electron microscopy (SEM), FT-IR spectroscopy and X-ray photoelectron spectroscope (XPS). It was found that the mean particle size of PTFE reduces from 250 nm of the control to 170 nm at 100 kGy, as confirmed by dynamic laser scattering and SEM. The crystallinity degree of PTFE increased at 20 kGy but remained unvaried at higher dose level. G-value of scission, G(s), was determined to be 0.46 mmol/J. # 2005 Elsevier B.V. All rights reserved. Keywords: Radiation; PTFE; Dispersion; Degradation; G(s)
1. Introduction Polytetrafluoroethylene (PTFE) has attracted wide attention for its excellent properties such as low friction coefficient, high thermal and chemical resistance, non-wetting property. However, PTFE is highly sensitive to radiation and its molecular weight (Mn) decreases rapidly with radiation dose. Upon irradiation by electron beam or g-rays, the chemical structure, morphology and molecular weight of PTFE change considerably depending on the factors including dose rate, dose range, temperature, oxygen content, etc. [1,2]. The mechanism of radiation-induced degradation of PTFE has been extensively studied. PTFE predominantly undergoes chain scission when irradiated at room temperature in air and vacuum; even a very low dose can result in a significant decrease in molecular weight [3]. The high radiation sensitivity of PTFE is exploited converting PTFE into low molecular weight micropowders [4] that are widely used as additives for plastics, inks and coatings. To the contrary,
* Corresponding author. Tel./fax: +86 21 59558905. E-mail address: [email protected] (G. Wu). 0022-1139/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2005.10.005
radiation-induced cross-linking of PTFE was observed in a narrow temperature range above its melting point in the absence of oxygen [5–7]. The cross-linked PTFE shows remarkable improvement in radiation resistance, mechanical and frictional properties as compared to the original one. More recently, cross-linking of PTFE was also found to occur at the surface area under synchrotron radiation [8]. All these investigations have revealed that the properties of radiationmodified PTFE are greatly affected by the irradiation conditions. Owing to its low free surface energy and high specific density, it is very difficult to make PTFE fine powder stably dispersed in aqueous media, restricting the use in some applications. Since PTFE dispersion (60 wt.%) is commercially available, we suggest that PTFE inside the dispersion droplets will undergo degradation upon irradiation while the dispersion still remains stable. This type of low-molecularweight PTFE dispersion may find use in water-base adhesives, coatings, inks, etc. The motivation of this work is to investigate in detail the radiation-induced degradation of PTFE in dispersion. Radiation effects on molecular weight, crystallinity, medium pH, size and morphology of PTFE particles were studied over a wide dose range.
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2. Results and discussion 2.1. Radiation effect on size and morphology of PTFE particle As a technique allowing for measurement for particle size in the range of 1 nm to some mm in dispersions, photo cross correlation spectroscopy (PCCS) could investigate many samples even at high concentration, based on dynamic light scattering principle [9]. In this study, we take advantage of this and get detailed information about the change of PTFE particle size by radiation. Fig. 1(a) shows the particle size of the irradiated PTFE dispersion analyzed by PCCS. The mean particle size decreases from the original 250 to 190 nm at 20 kGy. With further increasing the dose, the size reduces gradually and reaches the minimum size of 170 nm at 100 kGy, but then increases slightly with dose. As also shown in Fig. 1(b), the particle size distribution becomes narrower with increasing dose. It is considered that in the course of irradiation, PTFE particles undergo direct radiation-induced decomposition, leading to a decrease in the apparent size. A narrowing of the size distribution under dosage of 100 kGy could be explained by that the larger particles absorb higher dose and split into smaller and comparatively uniform particles. At high dose level, however, owing to the reactions of the primary radicals (OH, HO2 and eaq) from water radiolysis with surfactant alkylphenol ethoxylates, which contains unsaturational structure, hydrophilic groups including –COOH and – OH will be formed on the surfactant molecules. This leads to formation of a thickener hydration shell around the solid particle and the ‘‘swollen’’ PTFE particles are observed. Consequently, at high dose, particle size is slightly increased and the size distribution becomes widening. Fig. 2 shows SEM observation of the unirradiated and irradiated PTFE particles; all of them have the shape of ellipse and rod, indicating no obvious topographic change by radiation. The particle size (long axis of each particle) was calculated from the SEM photographs using the Image-Pro Plus software. The particle size distribution obtained from the SEM analysis
generally agrees well with the PCCS test, although theoretically the PCCS test reflects the diameter of spherical particle according to the Stokes–Einstein equation. As confirmed by the direct and indirect methods mentioned above, PTFE particle size decreased remarkably at 20 kGy. 2.2. pH variation Fig. 3 shows the pH variation as a function of dose. The pH value of irradiated dispersion decreases monotonically from the original 10.0 to 7.9 at 200 kGy, indicating an increase in proton concentration with dose. We also found that the dispersion changed to be acidic (pH 4.7) at 350 kGy. Two sources are responsible for the increase of [H+]: water radiolysis and C–F scission of PTFE. H+ is produced as the counterpart of eaq in water radiolysis and its G-value should be the same as G(eaq). On the other hand, since F atom is formed as a result of radiation-induced C–F scission, it will react with H2O or surfactant molecule to form H+ and F while the G-value remains unknown. 2.3. Radiation effect on crystallinity and chemical structure Crystallinity of PTFE was previously found to increase by radiation [10], explained by the relief of residual stress in the amorphous region and the decrease in the probability of entanglement per molecule caused by radiation-induced chain scission. A model was proposed to describe the chain scission occurring near the disordered boundary regions of the crystallites. In our work, PTFE powder obtained from the irradiated PTFE dispersion was examined by XRD and the degree of crystallinity was calculated by the peak area method [11]. It is usually presumed that the crystallinity degree (F) is related to the integral intensity for the peaks. F can be calculated from the ratio of the integrated crystalline scattering to the total scattering according to: F ¼ Scr =ðScr þ KSa Þ
Fig. 1. (a) Dose dependence of the average particle size of PTFE in dispersion. (b) Intensity weighted size distribution of irradiated PTFE dispersion at: a, 0 kGy; b, 20 kGy; c, 50 kGy; d, 100 kGy; e, 150 kGy; f, 200 kGy.
J. Su et al. / Journal of Fluorine Chemistry 127 (2006) 91–96
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Fig. 2. SEM morphology and size distribution of PTFE particle irradiated at: a, 0 kGy; b, 20 kGy; c, 100 kGy.
where Scr is the sum of the areas of crystalline scattering curves and Sa is the area of amorphous scattering curve. The K value is probably affected by the processing condition, component of sample, etc. The default value is 1.00.
Fig. 3. Dose dependence of pH of PTFE dispersion.
The strongest peak is at 2u = 18.12–18.388, corresponding to the crystalline part in PTFE. In our studies, the crystallinity of PTFE increases from original 0.81 to 0.85 at 20 kGy, but keeps almost constant at higher dose level. The observed increasing of crystallinity with dose is generally consistent with the finding [12] in the electron beam irradiation of PTFE sheet, although in which the crystallinity is higher than 90% at 150 kGy. Numerous studies have been previously done to investigate the change of chemical structure of PTFE in various radiation conditions. It has been found that degradation of PTFE by irradiation in air leads mainly to the formation of acidic fluoride groups (COF), the COF groups hydrolyze to carboxylic acid groups (COOH) in the presence of atmospheric humidity [13]. When PTFE film immersed in water is irradiated by electron beam under atmospheric conditions [14], the end groups are the same as those irradiated at air. Fischer et al. [15] found that the partial degradation of PTFE by electron beam irradiation in air or oxygen leads to the formation of COF groups at 1884 cm1, which can be converted into COOH groups by hydrolysis. In this study, as
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Fig. 4. XRD, FT-IR spectra and the XPS wide scan spectra for PTFE samples irradiated at: a, 0 kGy; b, 20 kGy; c, 50 kGy; d, 100 kGy; e, 150 kGy; f, 200 kGy.
shown in Fig. 4, there are three peaks at 505–640 cm1 that are assigned to CF2 rocking, CF2 bending and CF2 wagging, respectively. CF2 symmetrical and asymmetrical stretching bands appear at 1141 and 1200 cm1. Contrary to others’ work, the peaks due to carbonyl stretching vibrations and hydrogen-bonded carboxylic acid groups are not observed in the FT-IR spectra. It has been found that [16] PTFE has a large sensitivity to radiation and the degradation is significantly enhanced by oxygen. For irradiation in the powder state, oxygen plays an important role leading to the formation of peroxy radicals. However, for the irradiation of PTFE dispersion, since the oxygen concentration in water is limited and PTFE particles are wrapped by surfactants, radicals on the main chain of PTFE are difficult to react with oxygen to form peroxide. This leads to the observation of no significant C O groups in the FT-IR spectra. For further confirmation, the surface chemical composition was analyzed by XPS for the control and the sample at 100 kGy. Also as shown in Fig. 4, strong peaks around 699 eV and weak peaks around 302 eV due to F 1s and C 1s are observed, respectively. The absence of peak at 533 eV standing for O 1s indicates no detectable oxygen in the PTFE surface. This means no reaction of OH radicals from water radiolysis with
PTFE in droplets, in agreement with the FT-IR measurement. Owing to the scission of C–F bond by irradiation, ratio of F/C decreased at 100 kGy. 2.4. DSC measurement and G-value of scission Fig. 5 shows DSC curves for PTFE irradiated in dispersion at different doses. Melting (Tm) and crystallization (Tc) temperatures are determined from the appropriate peaks of DSC curves. The heats of melting (DHm) and crystallization (DHc) were calculated by the integration of relevant area. It can be seen that Tm decreases and Tc increases with increasing the irradiation dose. Based on the previous studies, the melting point is dependent on the molecular weight. The larger the molecular weight, the higher is the melting point, which could be explained by that the larger molecular weight polymer shows more pronounced behavior on superheating. The crystallization peak is markedly affected by radiation dose. As a linear polymer, PTFE predominantly undergoes chain scission under irradiation. This leads to shorter molecular chains. Shorter chain results in greater mobility of PTFE and less intra- and intermolecular entanglements make the crystallization more easily.
J. Su et al. / Journal of Fluorine Chemistry 127 (2006) 91–96
95 0
¯ n is the ¯ n is the molecular weight at dose D (kGy), M where M initial molecular weight of PTFE. G(s) is calculated to be 4.44 (n/100 eV), corresponding to 0.46 106 mol/J. This is consistent with the reported value for PTFE irradiated in ambient atmosphere [18]. 3. Conclusion The radiation effect on PTFE was investigated in the aqueous dispersion. PTFE particle undergoes predominantly degradation and G(s) is determined to be 0.46 106 mol/J, similar to that found in the irradiation of PTFE in the solid state. An increase of radiation dose leads to the decrease in medium pH but has no consequence in the topography of PTFE particles. This may be exploited making mono-disperse and low-molecular-weight PTFE dispersion. 4. Experimental section Fig. 5. DSC thermograms for the original and irradiated PTFE on heating and cooling, respectively, at various doses: a, 0 kGy; b, 20 kGy; c, 50 kGy; d, 100 kGy; e, 150 kGy; f, 200 kGy.
G-value of scission, G(s), is an important factor for the evaluation of radiation effect on PTFE. As an insoluble material, molecular weight of PTFE is difficult to be measured by conventional methods. Here we take the method by Suwa et al. [17] and the number-average molecular weight is calculated based on the DSC measurement: ¯ n ¼ 2:1 1010 DHc5:16 M
(1)
It should be pointed out that although the equation is valid in a certain range of molecular weight, this limit is not taken into account since no suitable formulae is available in the literature for PTFE with very low molecular weight. The calculated Mn values for different PTFE samples are listed on Table 1. It is obvious that molecular weight reduces with the increasing of irradiation dose. Since PTFE is recognized as a typical polymer of radiation degradation type, the cross-linking can be neglected and G(s) is calculated by: ¯ n Þ1 ¼ ðM ¯ 0n Þ1 þ 1:04 107 D GðsÞ ðM
(2)
Table 1 The summarized data from DSC measurement for original and irradiated PTFE samples Dose (kGy)
Tm (8C)
DHm (J/g)
Tc (8C)
DHc (J/g)
Mn (g/mol)
0 20 50 100 150 200
337.5 335.2 335.2 334.3 334.3 332.9
75.25 71.80 66.71 70.82 72.20 71.33
311.1 313.2 314.6 315.3 315.1 316.1
39.78 53.69 55.81 58.59 67.02 70.70
1.87 105 3.99 104 3.27 104 2.54 104 1.27 104 9.65 103
4.1. Materials and irradiation The aqueous dispersion of PTFE used in this work was purchased from Shanghai 3F New Materials Co., China. The composition is as follows: PFTE (59–61 wt.%), surfactants of alkylphenol ethoxylates (t–Oct–C6H4–(OCH2CH2)xOH, x = 7–8, 4–7 wt.%) and ammonium perfluorooctanoate (C7F15COONH4, 0.1 wt.%), and water (31.9–37.9 wt.%). The average particle size is about 0.3 mm and the pH of dispersion is 10. Samples (50 g) were sealed in glass tubes and exposed to gamma radiation from the 60Co source of Shanghai Institute of Applied Physics for a total dose of 20–200 kGy at a dose rate of 6.5 Gy/min. PTFE was precipitated by adding the irradiated sample into ethanol, followed by careful washing with distilled water and drying at 120 8C. The dried samples were used for FT-IR, DSC and XRD analysis. 4.2. Characterization of PTFE powder and dispersion The FT-IR spectra of PTFE were recorded on Nicolet infrared spectrophotometer (Avater-360) in KBr pellets. The surface chemical states of PTFE particles were measured by XPS. Photoelectrons were excited by Mg Ka irradiation. DSC measurement was performed with Mettler model TA822e. Samples (ca. 10 mg) were scanned from 200 to 380 8C in sealed aluminum pans under N2 atmosphere with a heating rate of 10 8C/min. When heated up to 380 8C, all samples were kept for 3 min and then recrystallized on cooling from the molten state to 50 8C at a rate of 10 8C/min. XRD patterns of PTFE were obtained by X-ray diffractometry (D/max 2550v) with Cu Ka radiation at 40 kV and 100 mA. For the evaluation of radiation effect on dispersion, the irradiated PTFE dispersion was directly used for pH measurement, particle size determination (PCCS instrument of Sympatec Company, samples were measured in the cuvette at 25 8C; scattering angle 908; light source: He–Ne Laser) and SEM observation (LEO1530VP).
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Acknowledgement This work was supported by the ‘‘Hundred talents’’ project of the Chinese Academy of Sciences. References [1] J.S. Forsythe, D.J.T. Hill, Prog. Polym. Sci. 25 (2000) 101–136. [2] U. Lappan, U. Geißler, K. Lunkwitz, Nucl. Instrum. Methods B 151 (1999) 222–226. [3] B. Fayolle, L. Audouin, J. Verdu, Polymer 44 (2003) 2773–2780. [4] K. Lunkwitz, H.-J. Brink, D. Handte, A. Ferse, Radiat. Phys. Chem. 33 (1989) 523–532. [5] M. Tutiya, Jpn. J. Appl. Phys. 11 (1972) 1542–1546. [6] A. Oshima, Y. Tabata, H. Kudoh, T. Seguchi, Radiat. Phys. Chem. 45 (1995) 269–273. [7] J. Sun, Y. Zhang, X. Zhong, Polymer 35 (1994) 2881–2883. [8] Y. Sato, D. Yamaguchi, T. Katoh, S. Ikeda, Y. Aoki, A. Oshima, Y. Tabata, M. Washio, Nucl. Instrum. Methods B 208 (2003) 231–235.
[9] W. Witt, H. Geers, L. Aberle, in: Proceedings of the International Congress of Particle Technology, PARTEC 2004, Nuremberg, 16–18 March, 2004. [10] M. Takenaga, K. Yamagata, J. Polym. Sci. Polym. Phys. 18 (1980) 1643– 1650. [11] J.F. Rabek, Experimental Methods in Polymer Chemistry, Wiley-Interscience, New York, NY, 1980, Chapter 28. [12] S.M. Abdou, R.I. Mohamed, J. Phys. Chem. Solids 63 (2002) 393– 398. [13] K. Lunkwitz, U. Lappan, U. Scheler, J. Fluorine Chem. 125 (2004) 863– 873. [14] J.C. Caro, U. Lappan, K. Lunkwitz, Nucl. Instrum. Methods B 151 (1999) 181–185. [15] D. Fischer, U. Lappan, I. Hopfe, K.-J. Eichhorn, K. Lunkwitz, Polymer 39 (1998) 573–582. [16] M.D. Chipara˘, M.I. Chipara˘, Polym. Degrad. Stab. 37 (1992) 67–71. [17] T. Suwa, M. Takehisa, S. Machi, J. Appl. Polym. Sci. 17 (1973) 3253– 3257. [18] A. Oshima, S. Ikeda, H. Kudoh, T. Seguchi, Y. Tabata, Radiat. Phys. Chem. 50 (1997) 611–615.
Study on polytetrafluoroethylene aqueous dispersion irradiated by gamma ray Jielong Su a,b, Guozhong Wu a,*, Yaodong Liu a, Hongyan Zeng b a
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Jialuo Road 2019, Jiading, Shanghai 201800, China b Department of Chemical Engineering, Xiangtan University, Xiangtan, Hunan, China
Received 1 September 2005; received in revised form 16 October 2005; accepted 18 October 2005 Available online 28 November 2005
Abstract Gamma-radiation induced degradation of polytetrafluoroethylene (PTFE) in 60 wt.% dispersion was studied in the dose range of 20–200 kGy and the change in property of PTFE was characterized by differential scanning calorimetry (DSC), photon cross correlation spectroscopy (PCCS), X-ray diffraction (XRD), scanning electron microscopy (SEM), FT-IR spectroscopy and X-ray photoelectron spectroscope (XPS). It was found that the mean particle size of PTFE reduces from 250 nm of the control to 170 nm at 100 kGy, as confirmed by dynamic laser scattering and SEM. The crystallinity degree of PTFE increased at 20 kGy but remained unvaried at higher dose level. G-value of scission, G(s), was determined to be 0.46 mmol/J. # 2005 Elsevier B.V. All rights reserved. Keywords: Radiation; PTFE; Dispersion; Degradation; G(s)
1. Introduction Polytetrafluoroethylene (PTFE) has attracted wide attention for its excellent properties such as low friction coefficient, high thermal and chemical resistance, non-wetting property. However, PTFE is highly sensitive to radiation and its molecular weight (Mn) decreases rapidly with radiation dose. Upon irradiation by electron beam or g-rays, the chemical structure, morphology and molecular weight of PTFE change considerably depending on the factors including dose rate, dose range, temperature, oxygen content, etc. [1,2]. The mechanism of radiation-induced degradation of PTFE has been extensively studied. PTFE predominantly undergoes chain scission when irradiated at room temperature in air and vacuum; even a very low dose can result in a significant decrease in molecular weight [3]. The high radiation sensitivity of PTFE is exploited converting PTFE into low molecular weight micropowders [4] that are widely used as additives for plastics, inks and coatings. To the contrary,
* Corresponding author. Tel./fax: +86 21 59558905. E-mail address: [email protected] (G. Wu). 0022-1139/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2005.10.005
radiation-induced cross-linking of PTFE was observed in a narrow temperature range above its melting point in the absence of oxygen [5–7]. The cross-linked PTFE shows remarkable improvement in radiation resistance, mechanical and frictional properties as compared to the original one. More recently, cross-linking of PTFE was also found to occur at the surface area under synchrotron radiation [8]. All these investigations have revealed that the properties of radiationmodified PTFE are greatly affected by the irradiation conditions. Owing to its low free surface energy and high specific density, it is very difficult to make PTFE fine powder stably dispersed in aqueous media, restricting the use in some applications. Since PTFE dispersion (60 wt.%) is commercially available, we suggest that PTFE inside the dispersion droplets will undergo degradation upon irradiation while the dispersion still remains stable. This type of low-molecularweight PTFE dispersion may find use in water-base adhesives, coatings, inks, etc. The motivation of this work is to investigate in detail the radiation-induced degradation of PTFE in dispersion. Radiation effects on molecular weight, crystallinity, medium pH, size and morphology of PTFE particles were studied over a wide dose range.
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2. Results and discussion 2.1. Radiation effect on size and morphology of PTFE particle As a technique allowing for measurement for particle size in the range of 1 nm to some mm in dispersions, photo cross correlation spectroscopy (PCCS) could investigate many samples even at high concentration, based on dynamic light scattering principle [9]. In this study, we take advantage of this and get detailed information about the change of PTFE particle size by radiation. Fig. 1(a) shows the particle size of the irradiated PTFE dispersion analyzed by PCCS. The mean particle size decreases from the original 250 to 190 nm at 20 kGy. With further increasing the dose, the size reduces gradually and reaches the minimum size of 170 nm at 100 kGy, but then increases slightly with dose. As also shown in Fig. 1(b), the particle size distribution becomes narrower with increasing dose. It is considered that in the course of irradiation, PTFE particles undergo direct radiation-induced decomposition, leading to a decrease in the apparent size. A narrowing of the size distribution under dosage of 100 kGy could be explained by that the larger particles absorb higher dose and split into smaller and comparatively uniform particles. At high dose level, however, owing to the reactions of the primary radicals (OH, HO2 and eaq) from water radiolysis with surfactant alkylphenol ethoxylates, which contains unsaturational structure, hydrophilic groups including –COOH and – OH will be formed on the surfactant molecules. This leads to formation of a thickener hydration shell around the solid particle and the ‘‘swollen’’ PTFE particles are observed. Consequently, at high dose, particle size is slightly increased and the size distribution becomes widening. Fig. 2 shows SEM observation of the unirradiated and irradiated PTFE particles; all of them have the shape of ellipse and rod, indicating no obvious topographic change by radiation. The particle size (long axis of each particle) was calculated from the SEM photographs using the Image-Pro Plus software. The particle size distribution obtained from the SEM analysis
generally agrees well with the PCCS test, although theoretically the PCCS test reflects the diameter of spherical particle according to the Stokes–Einstein equation. As confirmed by the direct and indirect methods mentioned above, PTFE particle size decreased remarkably at 20 kGy. 2.2. pH variation Fig. 3 shows the pH variation as a function of dose. The pH value of irradiated dispersion decreases monotonically from the original 10.0 to 7.9 at 200 kGy, indicating an increase in proton concentration with dose. We also found that the dispersion changed to be acidic (pH 4.7) at 350 kGy. Two sources are responsible for the increase of [H+]: water radiolysis and C–F scission of PTFE. H+ is produced as the counterpart of eaq in water radiolysis and its G-value should be the same as G(eaq). On the other hand, since F atom is formed as a result of radiation-induced C–F scission, it will react with H2O or surfactant molecule to form H+ and F while the G-value remains unknown. 2.3. Radiation effect on crystallinity and chemical structure Crystallinity of PTFE was previously found to increase by radiation [10], explained by the relief of residual stress in the amorphous region and the decrease in the probability of entanglement per molecule caused by radiation-induced chain scission. A model was proposed to describe the chain scission occurring near the disordered boundary regions of the crystallites. In our work, PTFE powder obtained from the irradiated PTFE dispersion was examined by XRD and the degree of crystallinity was calculated by the peak area method [11]. It is usually presumed that the crystallinity degree (F) is related to the integral intensity for the peaks. F can be calculated from the ratio of the integrated crystalline scattering to the total scattering according to: F ¼ Scr =ðScr þ KSa Þ
Fig. 1. (a) Dose dependence of the average particle size of PTFE in dispersion. (b) Intensity weighted size distribution of irradiated PTFE dispersion at: a, 0 kGy; b, 20 kGy; c, 50 kGy; d, 100 kGy; e, 150 kGy; f, 200 kGy.
J. Su et al. / Journal of Fluorine Chemistry 127 (2006) 91–96
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Fig. 2. SEM morphology and size distribution of PTFE particle irradiated at: a, 0 kGy; b, 20 kGy; c, 100 kGy.
where Scr is the sum of the areas of crystalline scattering curves and Sa is the area of amorphous scattering curve. The K value is probably affected by the processing condition, component of sample, etc. The default value is 1.00.
Fig. 3. Dose dependence of pH of PTFE dispersion.
The strongest peak is at 2u = 18.12–18.388, corresponding to the crystalline part in PTFE. In our studies, the crystallinity of PTFE increases from original 0.81 to 0.85 at 20 kGy, but keeps almost constant at higher dose level. The observed increasing of crystallinity with dose is generally consistent with the finding [12] in the electron beam irradiation of PTFE sheet, although in which the crystallinity is higher than 90% at 150 kGy. Numerous studies have been previously done to investigate the change of chemical structure of PTFE in various radiation conditions. It has been found that degradation of PTFE by irradiation in air leads mainly to the formation of acidic fluoride groups (COF), the COF groups hydrolyze to carboxylic acid groups (COOH) in the presence of atmospheric humidity [13]. When PTFE film immersed in water is irradiated by electron beam under atmospheric conditions [14], the end groups are the same as those irradiated at air. Fischer et al. [15] found that the partial degradation of PTFE by electron beam irradiation in air or oxygen leads to the formation of COF groups at 1884 cm1, which can be converted into COOH groups by hydrolysis. In this study, as
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Fig. 4. XRD, FT-IR spectra and the XPS wide scan spectra for PTFE samples irradiated at: a, 0 kGy; b, 20 kGy; c, 50 kGy; d, 100 kGy; e, 150 kGy; f, 200 kGy.
shown in Fig. 4, there are three peaks at 505–640 cm1 that are assigned to CF2 rocking, CF2 bending and CF2 wagging, respectively. CF2 symmetrical and asymmetrical stretching bands appear at 1141 and 1200 cm1. Contrary to others’ work, the peaks due to carbonyl stretching vibrations and hydrogen-bonded carboxylic acid groups are not observed in the FT-IR spectra. It has been found that [16] PTFE has a large sensitivity to radiation and the degradation is significantly enhanced by oxygen. For irradiation in the powder state, oxygen plays an important role leading to the formation of peroxy radicals. However, for the irradiation of PTFE dispersion, since the oxygen concentration in water is limited and PTFE particles are wrapped by surfactants, radicals on the main chain of PTFE are difficult to react with oxygen to form peroxide. This leads to the observation of no significant C O groups in the FT-IR spectra. For further confirmation, the surface chemical composition was analyzed by XPS for the control and the sample at 100 kGy. Also as shown in Fig. 4, strong peaks around 699 eV and weak peaks around 302 eV due to F 1s and C 1s are observed, respectively. The absence of peak at 533 eV standing for O 1s indicates no detectable oxygen in the PTFE surface. This means no reaction of OH radicals from water radiolysis with
PTFE in droplets, in agreement with the FT-IR measurement. Owing to the scission of C–F bond by irradiation, ratio of F/C decreased at 100 kGy. 2.4. DSC measurement and G-value of scission Fig. 5 shows DSC curves for PTFE irradiated in dispersion at different doses. Melting (Tm) and crystallization (Tc) temperatures are determined from the appropriate peaks of DSC curves. The heats of melting (DHm) and crystallization (DHc) were calculated by the integration of relevant area. It can be seen that Tm decreases and Tc increases with increasing the irradiation dose. Based on the previous studies, the melting point is dependent on the molecular weight. The larger the molecular weight, the higher is the melting point, which could be explained by that the larger molecular weight polymer shows more pronounced behavior on superheating. The crystallization peak is markedly affected by radiation dose. As a linear polymer, PTFE predominantly undergoes chain scission under irradiation. This leads to shorter molecular chains. Shorter chain results in greater mobility of PTFE and less intra- and intermolecular entanglements make the crystallization more easily.
J. Su et al. / Journal of Fluorine Chemistry 127 (2006) 91–96
95 0
¯ n is the ¯ n is the molecular weight at dose D (kGy), M where M initial molecular weight of PTFE. G(s) is calculated to be 4.44 (n/100 eV), corresponding to 0.46 106 mol/J. This is consistent with the reported value for PTFE irradiated in ambient atmosphere [18]. 3. Conclusion The radiation effect on PTFE was investigated in the aqueous dispersion. PTFE particle undergoes predominantly degradation and G(s) is determined to be 0.46 106 mol/J, similar to that found in the irradiation of PTFE in the solid state. An increase of radiation dose leads to the decrease in medium pH but has no consequence in the topography of PTFE particles. This may be exploited making mono-disperse and low-molecular-weight PTFE dispersion. 4. Experimental section Fig. 5. DSC thermograms for the original and irradiated PTFE on heating and cooling, respectively, at various doses: a, 0 kGy; b, 20 kGy; c, 50 kGy; d, 100 kGy; e, 150 kGy; f, 200 kGy.
G-value of scission, G(s), is an important factor for the evaluation of radiation effect on PTFE. As an insoluble material, molecular weight of PTFE is difficult to be measured by conventional methods. Here we take the method by Suwa et al. [17] and the number-average molecular weight is calculated based on the DSC measurement: ¯ n ¼ 2:1 1010 DHc5:16 M
(1)
It should be pointed out that although the equation is valid in a certain range of molecular weight, this limit is not taken into account since no suitable formulae is available in the literature for PTFE with very low molecular weight. The calculated Mn values for different PTFE samples are listed on Table 1. It is obvious that molecular weight reduces with the increasing of irradiation dose. Since PTFE is recognized as a typical polymer of radiation degradation type, the cross-linking can be neglected and G(s) is calculated by: ¯ n Þ1 ¼ ðM ¯ 0n Þ1 þ 1:04 107 D GðsÞ ðM
(2)
Table 1 The summarized data from DSC measurement for original and irradiated PTFE samples Dose (kGy)
Tm (8C)
DHm (J/g)
Tc (8C)
DHc (J/g)
Mn (g/mol)
0 20 50 100 150 200
337.5 335.2 335.2 334.3 334.3 332.9
75.25 71.80 66.71 70.82 72.20 71.33
311.1 313.2 314.6 315.3 315.1 316.1
39.78 53.69 55.81 58.59 67.02 70.70
1.87 105 3.99 104 3.27 104 2.54 104 1.27 104 9.65 103
4.1. Materials and irradiation The aqueous dispersion of PTFE used in this work was purchased from Shanghai 3F New Materials Co., China. The composition is as follows: PFTE (59–61 wt.%), surfactants of alkylphenol ethoxylates (t–Oct–C6H4–(OCH2CH2)xOH, x = 7–8, 4–7 wt.%) and ammonium perfluorooctanoate (C7F15COONH4, 0.1 wt.%), and water (31.9–37.9 wt.%). The average particle size is about 0.3 mm and the pH of dispersion is 10. Samples (50 g) were sealed in glass tubes and exposed to gamma radiation from the 60Co source of Shanghai Institute of Applied Physics for a total dose of 20–200 kGy at a dose rate of 6.5 Gy/min. PTFE was precipitated by adding the irradiated sample into ethanol, followed by careful washing with distilled water and drying at 120 8C. The dried samples were used for FT-IR, DSC and XRD analysis. 4.2. Characterization of PTFE powder and dispersion The FT-IR spectra of PTFE were recorded on Nicolet infrared spectrophotometer (Avater-360) in KBr pellets. The surface chemical states of PTFE particles were measured by XPS. Photoelectrons were excited by Mg Ka irradiation. DSC measurement was performed with Mettler model TA822e. Samples (ca. 10 mg) were scanned from 200 to 380 8C in sealed aluminum pans under N2 atmosphere with a heating rate of 10 8C/min. When heated up to 380 8C, all samples were kept for 3 min and then recrystallized on cooling from the molten state to 50 8C at a rate of 10 8C/min. XRD patterns of PTFE were obtained by X-ray diffractometry (D/max 2550v) with Cu Ka radiation at 40 kV and 100 mA. For the evaluation of radiation effect on dispersion, the irradiated PTFE dispersion was directly used for pH measurement, particle size determination (PCCS instrument of Sympatec Company, samples were measured in the cuvette at 25 8C; scattering angle 908; light source: He–Ne Laser) and SEM observation (LEO1530VP).
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