- Email: [email protected]
Gamma-rays irradiation effects on polysulfone at high temperature
Available online at www.sciencedirect.com
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 265 (2007) 125–129 www.elsevier.com/locate/nimb
Gamma-rays irradiation effects on polysulfone at high temperature K. Murakami a, H. Kudo b
b,*
a Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565, Japan Nuclear Professional School, Graduate School of Engineering, University of Tokyo, 2-22 Shirakata-Shirane, Tokai, Naka, Ibaraki 319-1188, Japan
Available online 2 September 2007
Abstract The temperature dependence of the irradiation effects on polysulfone was studies by measuring the molecular weight, glass transition temperature, gel fraction and evolved gas. Polysulfone was irradiated with gamma-rays at room temperature, 100, 150, 180 and 210 C. The change of molecular weight distribution and glass transition temperature showed occurrences of a main chain scission at room temperature and cross-linking at high temperature. The decrease of gel dose, the increases of gel fraction and total gas evolution with increasing temperature was observed. The evolution of CO, CO2 and SO2 gases increased at high temperature, while yield of evolved H2 was independent of irradiation temperature. The probability of the cross-linking was clearly increased by irradiation at high temperature above 180 C, though the chain scission was not changed very much. 2007 Elsevier B.V. All rights reserved. PACS: 61.82.d Keywords: Polysulfone; Cross-link; Scission; High temperature irradiation
1. Introduction Polysulfone (PSF) (bisphenl A type Udel polyaryl sulfone), one of the engineering plastics having aromatic structure in the main chain, has excellent mechanical properties and are stable at high temperature. In addition, PSF shows high resistance to radiation. So we expect wide application of PSF for various severe environments, such as fusion reactors and space vehicles and satellites [1]. In space, for example, materials are exposed not only to high radiation field but also to high and cryogenic temperature environment. Furthermore PSF that have high glass transition temperature is used in higher temperature environments than many other thermoplastic polymers.
*
Corresponding author. Tel.: +81 29 287 8420. E-mail address: [email protected] (H. Kudo).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.08.037
Many studies have investigated irradiation effect on PSF at high temperature [1–5]. Nevertheless, these reports seem to contradict with each other, that is, some [1–3] have reported that PSF irradiated in vacuum environment are mainly degraded by main chain scission, while others [4,5] reported that cross-linking predominates over scission under vacuum in PSF. Thus, both cross-linking and main chain scission were reported for the irradiation effects on PSF. The irradiation effects on PSF have not been well understood even at room temperature, much less at high temperature. It was found that irradiation temperature greatly influences probabilities of cross-linking and main chain scission [6]. By considering these findings, past investigations on PSF would be carried out under the assumption that the polymer classification concerning to irradiation effects is not influenced by irradiation temperature. Irradiation effects on PSF at various temperatures must be studied on the probabilities of cross-linking and main chain scission by taking the significance of irradiation temperature into account.
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K. Murakami, H. Kudo / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 125–129
2. Experimental 2.1. Materials Polysulfone (PSF) (bisphenl A type Udel polyaryl sulfone) of thickness 50 lm supplied by TORAY Industries Inc. was used as received. The molecular weight of sample was 4.52 · 104 and glass transition temperature (Tg) of this polymer was 187 C. The chemical structure of this aromatic polymer is shown in Fig. 1.
gel fraction equals the ratio of insoluble polymer to initial polymer by weight. The amount of gaseous product accumulated in the glass ampoule was obtained by measurement of pressure and volume of the ampoule. Then the gas components were analyzed by two gas chromatographic apparatus (HITACHI 263-50, GC-1 and GC-2) with Helium carrier gas. Both GC’s have PID (photo-ionization detector). GC-1 contains a Molecular Sieve 5A column and detects light molecules such as H2 and CO. GC-2 has Porapak Q column and detects CO2 and SO2.
2.2. Irradiation 3. Results and discussion 1
About 5 g of material was evacuated to 10 Pa for more than 24 h and sealed in a glass ampoule with a breakable seal. The irradiation was carried out using Co-60 gamma rays at a dose rate of 5–7 kGy/h at various temperatures of 210, 180, 150, 100 C and room temperature. The ampoules were placed in a heating device, which maintained a constant temperature within 1 degree, during the irradiation. Samples were irradiated in the dose range from 0.1 to 4 MGy. 2.3. Measurements The number average and weight average molecular weight of polymer were measured by gel permeation chromatography (GPC). We used tetrahydrofurane (THF) as solvents and chromatographic apparatus (TOSOH HLC8120) to separate PSF. The column set was calibrated with mono-disperse polystyrene standards and the column oven was set at 40 C. The Tg was measured with a differential scanning calorimeter (DSC). The measurements were performed on 15–20 mg samples of polymer film cut to the size of the DSC pans using a punch. Aluminum sample pans were used for the measurements. The DSC measurements were made using a Perkin–Elmer DSC 7 which was programmed to acquire the DSC thermogram using a heating rate of 20 C/min. To avoid problems which might arise through the evolution of gas from the sample as the polymer passes through Tg, the measurement of Tg were made from the second scan of the thermogram. The value of Tg was taken to be the temperature maximum in the first differential of the thermogram. The gel fraction of PSF was obtained by placed about 100 mg samples in a stainless-steel cage holder of 100-mesh net and held this in chloroform. Materials remaining were determined by weighing after vacuum drying at 70 C. The
Fig. 1. The structure of the polysulfone (PSF, bisphenl A type Udel polyaryl sulfone).
3.1. The molecular weight distribution by GPC The molecular weight distributions of each sample are shown in Fig. 2. A solid curve displays the molecular weight distribution of unirradiated sample. The all chromatograms of irradiated PSF became broad compared to the original. The peak of PSF irradiated at room temperature (broken curve) to 0.3 MGy shifted to lower molecular weight direction. This shows that main chain scission was predominant and molecular weight decreased. However, a little increase in higher molecular weight fraction was also observed. On the other hand, the chromatogram irradiated at high temperature (dotted curve) to 0.2 MGy, became broad for higher molecular direction. Cross-linking predominates in PSF for irradiation at high temperature, while scission is not clearly observed from this experimental result. The chromatogram relates the size of molecules to the molecular weight for linear molecules. For crossliked polymer chains, the relation could be different from the linear ones. Though these fundamental aspects should be checked further, as another task in analytical technique, the chromatogram can be an indication of the molecular weight. The dose of this measurement is below gel
Fig. 2. The molecular weight distribution of polysulfone after gammairradiation. Solid curve: unirradiated, broken curve: irradiated at room temperature to 0.3 MGy, dotted curve: irradiated at 180 C to 0.2 MGy.
K. Murakami, H. Kudo / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 125–129
formation as shown in the later section; therefore the chromatogram covers whole range of molecular weight. The two chromatograms are single-peak, but the distribution is widened by irradiation and it is more obvious for 0.2 MGy at 180 C. As the two samples are 0.3 MGy at RT and 0.2 MGy at 180 C, it is clear that cross-linking is enhanced at high temperature than at RT. Although the limitation of the procedure may exist as above, the number average molecular weights (Mn) and weight average molecular weights (Mw) of PSF were computed by gel permeation chromatography (GPC). Fig. 3 shows the relation between Mn and Mw and dose for PSF irradiated at room temperature. Table 1 shows change in Mn and Mw on irradiation at various temperatures. Mn decreases with dose for most temperatures except for 180 C, which means main chain scission is predominant over scission. However, increase in Mw indicates that the polymer undergoes cross-linking at every temperature as well. At 180 C, it is presumed that cross-linking predominates over scission because both Mn and Mw increase with dose. G values of cross-linking, G(x) and main chain scission, G(s) were calculated from Mn and Mw by the following formula, where A = 1.04 · 1010 (kg), B = 5.18 · 1011 (kg) and D: dose (Gy):1/Mn(D) 1/Mn(0) = A[G(s) G(x)]D, 1/Mw(D) 1/Mw(0) = B[G(s) G(x)]D. Mw(0), Mn(0),
Fig. 3. The reciprocal of average molecular weight versus dose for polysulfone irradiated at room temperature; open circle: number average, closed circle: weight average.
Table 1 Change in number average (Mn) and weight average (Mw) molecular weight on irradiation at various temperatures Irradiation temperature (C)
Dose (MGy)
Mn (·104)
Mw (·105)
– RT 100 100 150 180
Control 0.30 0.65 0.73 0.52 0.20
2.08 1.99 2.07 2.06 2.05 2.12
4.52 5.35 7.68 8.90 9.09 6.58
127
Mw(D) and Mn(D) are the initial weight (w)/number (n) average molecular weight of the initial (0)/after irradiation (D). The equations are based on the assumption that the initial molecular weight distribution is random, that is, Mw/Mn = 2. In our experiment, Mw/Mn of unirradiated PSF sample was 2.17, therefore the equations can be applied to calculating for G-value. Table 2 shows G(s) and G(x) for irradiation temperatures. The G(x) is clearly increased by irradiation at high temperature, though the G(s) scatters. We could not measure the sample irradiated at 210 C with GPC, because the polymer immediately formed gel. 3.2. The change of Tg by DSC Fig. 4 shows the change of Tg to absorbed dose. The Tg of samples that were irradiated at room temperature lowered at high dose. The Tg of sample irradiated at 180 C did not change, while the Tg of sample irradiated at 210 C increased with dose. DSC and GPC measurements show that PSF undergoes the main chain scission at room temperature and cross-linking predominates over scission at high temperature. 3.3. The gel fraction measurement Fig. 5 shows the relation between gel fraction and dose for PSF. The gel fraction increases with dose but levels off. It is clear that samples irradiated over 3 MGy at room temperature formed gel. The gel dose (Dg) decreases with increasing irradiation temperature, that is, Dg is 1.1 MGy at 100 C, 0.45 MGy at 180 C and 0.2 MGy at 210 C. The saturated gel fraction increases with irradiation temperature, namely from 30% at 40 C to 100% at 210 C. GPC and DSC measurements show that samples irradiated at room temperature undergo scission. Though it seems that gel formation contradicts these results, it corresponds with other measurement, because increase in higher molecular weight fraction was observed by GPC measurement. The GPC and DSC would give macroscopic average properties, whereas gel would be formed through local cross-linking. These facts show that both main chain scission and crosslinking occurred at room temperature irradiation. The G-value of scission and cross-linking were calculated with Charlesby–Pinner equation; s + s1/2 = G(s)/ [2G(x)] + C/[Mn(0)G(x)D], where s is sol fraction, C = 4.8 · 106 (kg1), D is dose in kGy, respectively. These G(s) and G(x) are shown in Table 2 with those computed by GPC measurement. The G(x) increases clearly with increasing irradiation temperature, though the G(s) scatters. The analysis of data for 210 C may be inappropriate because the polymer formed gel at very low dose, making the intersection of the Charlesby–Pinner plot negative, therefore we assume the probability of the scission is zero. The difference that G-values show between molecular weight and gel fraction would come from gel formation. However, we are considering that they show the similar
128
K. Murakami, H. Kudo / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 125–129
Table 2 G-values of scission and cross-linking for polysulfone Irradiation temperature (C)
By GPC measurements G(s)
G(x)
G(s)
G(x)
Room temperature 100 150 180 210
0.16 0.10 0.15 0.16 –
0.06 0.10 0.14 0.21 –
0.10 0.20 – 0.21 (0)
0.08 0.16 – 0.38 1.35
Fig. 4. The dependence of Tg of polysulfone on absorbed dose; (s) room temperature; (4) 100 C, (5) 180 C, (e) 210 C.
By gel fraction measurements
H2, CH4, CO, CO2 and SO2. The SO2 could not be detected from samples irradiated at room temperature. G-values of main gaseous products were computed in the low dose range and are shown in Table 3. Irradiation of PSF produced CO2, CO and SO2 as the major volatile product at higher temperature. The evolution of these three gases strongly depended on irradiation temperature and is presumably associated with the main chain scission. It is likely that the temperature dependence made the drastic increase of cross-linking. Whereas H2 and CH4 were not major gaseous products at higher temperature and yield of evolved H2 was independent of irradiation temperature. The production of H2 indicates C–H scission in the aromatic rings and in the isopropylidene units; the latter may be the major reaction [4]. These schemes are the primary process by radiation then it seems that these are not influenced by temperature very much. 3.5. Possible reaction mechanism It is reported that C–S main chain scission and decomposition of aliphatic group are primary step in the radiolysis [4,7].
Fig. 5. Gel fraction of polysulfone by gamma-irradiation at various temperatures; (s) room temperature; (4) 100 C, (5) 180 C, (e) 210 C.
irradiation temperature dependence; G(x) increases drastically with increasing irradiation temperature, though G(s) remains almost constant. 3.4. Gas analysis Fig. 6 shows the yield of evolved total gas versus dose for PSF. Total yield of eliminated gases increases with increasing irradiation temperature. Components gases were
Fig. 6. Yield of total gas versus dose for polysulfone by gamma irradiation under vacuum; (s) room temperature; (4) 100 C, (5) 180 C, (e) 210 C.
K. Murakami, H. Kudo / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 125–129
129
Table 3 G-values (in 102) of main gaseous products for polysulfone under vacuum Irradiation temperature (C)
Total gas
H2
CH4
CO
CO2
SO2
Room temperature 100 180 210
2.1 12 34 61
0.77 1.3 1.3 1.8
0.26 1.0 1.4 1.8
0.22 1.7 3.9 10
0.36 1.5 2.1 12
0 5.0 23 14
Moreover, Brown et al. [4] mentioned that the scission of C–S bond may lead liberating SO2 and that the extraction of SO2 would be due to the phenylene sulfonyl radical either directly or via phenylene sulfonyl acid.
Since SO2 was detected at higher temperature than 100 C only, scission of C–S bonds is minor at room temperature and increased with temperature. Disintegration of isopropylidene unit would be major reaction at room temperature. Though the origins of CO and CO2 are unclear, it is possibly that CH3 radical attack to –SO2– group in polymer or SO2 radical attack to –C(CH3)– group in polymer. Brown and O’Donell [4] also considered that aliphatic group (isopropylidene unit) in PSF in involved in crosslinking. The cross-linking structure would be T-type, not H-type, because H-type cross-linking should evolve the H2 and CH4 upon cross-linking, but actually G(H2) and G(CH4) were about one order lower than G(s) and G(x) at all temperatures tested. This speculation is supported by Hill et al. [8] though C 13NMR (Nuclear Magnetic Resonance) measurement. Possible reaction resulting in cross-linking is combination between phenyl radical and methyl radical in the isopropylidene unit both by main chain scission. However, at room temperature, C–S bond scission may not arise, because SO2 was not detected in gas analysis. Therefore possible reaction mechanism at room temperature is, scission between isopropylidene unit and phenyl ring forms phenyl radical, and this radical combines with methyl radical in the aliphatic group.
On the other hand, at high temperature, C–S bond scission contributes to form phenyl radical accompanied with SO2 evolution as follows. Since the scission of C–S bond is enhance at high temperature as gas analysis implied, probability of cross-lining is increased with irradiation temperature.
4. Conclusion Polysulfone undergoes both cross-lining and main chain scission by gamma-rays irradiation. Main chain scission is regarded as predominant by irradiation at room temperature. At high temperature, on the other hand, cross-linking occurs effectively and G-value of cross-linking increases sharply in comparison with G-values of main chain scission. The cross-linking structure is presumed to be Ttype. References [1] E.A. Hegazy, T. Sasuga, M. Nishii, T. Seguchi, Polymer 33 (1992) 2897. [2] E.A. Hegazy, T. Sasuga, M. Nishii, T. Seguchi, Polymer 33 (1992) 2904. [3] T. Sasuga, N. Hayakawa, K. Yoshida, Polymer 28 (1987) 236. [4] J.R. Brown, J.H. O’Donnell, J. Appl. Polym. Sci. 19 (1975) 405. [5] R.W. Garrett, D.J.T. Hill, T.T. Le, K.A. Milne, J.H. O’Donnell, S.M.C. Perera, P.J. Pomery, American Chemical Society Book Series, Vol. 475, 1991, p. 146 (Chapter 10). [6] J. Sun, Y. Zhang, X. Zhong, X. Zhu, Radiat. Phys. Chem. 44 (1994) 655. [7] D.R. Coulter, M.V. Smith, F. Tsay, A. Gupta, J. Appl. Polym. Sci. 30 (1985) 1753. [8] D.J.T. Hill, D.A. Lewis, J.H. O’Donnell, J. Macromol. Sci. Part A 29 (1992) 11.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 265 (2007) 125–129 www.elsevier.com/locate/nimb
Gamma-rays irradiation effects on polysulfone at high temperature K. Murakami a, H. Kudo b
b,*
a Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565, Japan Nuclear Professional School, Graduate School of Engineering, University of Tokyo, 2-22 Shirakata-Shirane, Tokai, Naka, Ibaraki 319-1188, Japan
Available online 2 September 2007
Abstract The temperature dependence of the irradiation effects on polysulfone was studies by measuring the molecular weight, glass transition temperature, gel fraction and evolved gas. Polysulfone was irradiated with gamma-rays at room temperature, 100, 150, 180 and 210 C. The change of molecular weight distribution and glass transition temperature showed occurrences of a main chain scission at room temperature and cross-linking at high temperature. The decrease of gel dose, the increases of gel fraction and total gas evolution with increasing temperature was observed. The evolution of CO, CO2 and SO2 gases increased at high temperature, while yield of evolved H2 was independent of irradiation temperature. The probability of the cross-linking was clearly increased by irradiation at high temperature above 180 C, though the chain scission was not changed very much. 2007 Elsevier B.V. All rights reserved. PACS: 61.82.d Keywords: Polysulfone; Cross-link; Scission; High temperature irradiation
1. Introduction Polysulfone (PSF) (bisphenl A type Udel polyaryl sulfone), one of the engineering plastics having aromatic structure in the main chain, has excellent mechanical properties and are stable at high temperature. In addition, PSF shows high resistance to radiation. So we expect wide application of PSF for various severe environments, such as fusion reactors and space vehicles and satellites [1]. In space, for example, materials are exposed not only to high radiation field but also to high and cryogenic temperature environment. Furthermore PSF that have high glass transition temperature is used in higher temperature environments than many other thermoplastic polymers.
*
Corresponding author. Tel.: +81 29 287 8420. E-mail address: [email protected] (H. Kudo).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.08.037
Many studies have investigated irradiation effect on PSF at high temperature [1–5]. Nevertheless, these reports seem to contradict with each other, that is, some [1–3] have reported that PSF irradiated in vacuum environment are mainly degraded by main chain scission, while others [4,5] reported that cross-linking predominates over scission under vacuum in PSF. Thus, both cross-linking and main chain scission were reported for the irradiation effects on PSF. The irradiation effects on PSF have not been well understood even at room temperature, much less at high temperature. It was found that irradiation temperature greatly influences probabilities of cross-linking and main chain scission [6]. By considering these findings, past investigations on PSF would be carried out under the assumption that the polymer classification concerning to irradiation effects is not influenced by irradiation temperature. Irradiation effects on PSF at various temperatures must be studied on the probabilities of cross-linking and main chain scission by taking the significance of irradiation temperature into account.
126
K. Murakami, H. Kudo / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 125–129
2. Experimental 2.1. Materials Polysulfone (PSF) (bisphenl A type Udel polyaryl sulfone) of thickness 50 lm supplied by TORAY Industries Inc. was used as received. The molecular weight of sample was 4.52 · 104 and glass transition temperature (Tg) of this polymer was 187 C. The chemical structure of this aromatic polymer is shown in Fig. 1.
gel fraction equals the ratio of insoluble polymer to initial polymer by weight. The amount of gaseous product accumulated in the glass ampoule was obtained by measurement of pressure and volume of the ampoule. Then the gas components were analyzed by two gas chromatographic apparatus (HITACHI 263-50, GC-1 and GC-2) with Helium carrier gas. Both GC’s have PID (photo-ionization detector). GC-1 contains a Molecular Sieve 5A column and detects light molecules such as H2 and CO. GC-2 has Porapak Q column and detects CO2 and SO2.
2.2. Irradiation 3. Results and discussion 1
About 5 g of material was evacuated to 10 Pa for more than 24 h and sealed in a glass ampoule with a breakable seal. The irradiation was carried out using Co-60 gamma rays at a dose rate of 5–7 kGy/h at various temperatures of 210, 180, 150, 100 C and room temperature. The ampoules were placed in a heating device, which maintained a constant temperature within 1 degree, during the irradiation. Samples were irradiated in the dose range from 0.1 to 4 MGy. 2.3. Measurements The number average and weight average molecular weight of polymer were measured by gel permeation chromatography (GPC). We used tetrahydrofurane (THF) as solvents and chromatographic apparatus (TOSOH HLC8120) to separate PSF. The column set was calibrated with mono-disperse polystyrene standards and the column oven was set at 40 C. The Tg was measured with a differential scanning calorimeter (DSC). The measurements were performed on 15–20 mg samples of polymer film cut to the size of the DSC pans using a punch. Aluminum sample pans were used for the measurements. The DSC measurements were made using a Perkin–Elmer DSC 7 which was programmed to acquire the DSC thermogram using a heating rate of 20 C/min. To avoid problems which might arise through the evolution of gas from the sample as the polymer passes through Tg, the measurement of Tg were made from the second scan of the thermogram. The value of Tg was taken to be the temperature maximum in the first differential of the thermogram. The gel fraction of PSF was obtained by placed about 100 mg samples in a stainless-steel cage holder of 100-mesh net and held this in chloroform. Materials remaining were determined by weighing after vacuum drying at 70 C. The
Fig. 1. The structure of the polysulfone (PSF, bisphenl A type Udel polyaryl sulfone).
3.1. The molecular weight distribution by GPC The molecular weight distributions of each sample are shown in Fig. 2. A solid curve displays the molecular weight distribution of unirradiated sample. The all chromatograms of irradiated PSF became broad compared to the original. The peak of PSF irradiated at room temperature (broken curve) to 0.3 MGy shifted to lower molecular weight direction. This shows that main chain scission was predominant and molecular weight decreased. However, a little increase in higher molecular weight fraction was also observed. On the other hand, the chromatogram irradiated at high temperature (dotted curve) to 0.2 MGy, became broad for higher molecular direction. Cross-linking predominates in PSF for irradiation at high temperature, while scission is not clearly observed from this experimental result. The chromatogram relates the size of molecules to the molecular weight for linear molecules. For crossliked polymer chains, the relation could be different from the linear ones. Though these fundamental aspects should be checked further, as another task in analytical technique, the chromatogram can be an indication of the molecular weight. The dose of this measurement is below gel
Fig. 2. The molecular weight distribution of polysulfone after gammairradiation. Solid curve: unirradiated, broken curve: irradiated at room temperature to 0.3 MGy, dotted curve: irradiated at 180 C to 0.2 MGy.
K. Murakami, H. Kudo / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 125–129
formation as shown in the later section; therefore the chromatogram covers whole range of molecular weight. The two chromatograms are single-peak, but the distribution is widened by irradiation and it is more obvious for 0.2 MGy at 180 C. As the two samples are 0.3 MGy at RT and 0.2 MGy at 180 C, it is clear that cross-linking is enhanced at high temperature than at RT. Although the limitation of the procedure may exist as above, the number average molecular weights (Mn) and weight average molecular weights (Mw) of PSF were computed by gel permeation chromatography (GPC). Fig. 3 shows the relation between Mn and Mw and dose for PSF irradiated at room temperature. Table 1 shows change in Mn and Mw on irradiation at various temperatures. Mn decreases with dose for most temperatures except for 180 C, which means main chain scission is predominant over scission. However, increase in Mw indicates that the polymer undergoes cross-linking at every temperature as well. At 180 C, it is presumed that cross-linking predominates over scission because both Mn and Mw increase with dose. G values of cross-linking, G(x) and main chain scission, G(s) were calculated from Mn and Mw by the following formula, where A = 1.04 · 1010 (kg), B = 5.18 · 1011 (kg) and D: dose (Gy):1/Mn(D) 1/Mn(0) = A[G(s) G(x)]D, 1/Mw(D) 1/Mw(0) = B[G(s) G(x)]D. Mw(0), Mn(0),
Fig. 3. The reciprocal of average molecular weight versus dose for polysulfone irradiated at room temperature; open circle: number average, closed circle: weight average.
Table 1 Change in number average (Mn) and weight average (Mw) molecular weight on irradiation at various temperatures Irradiation temperature (C)
Dose (MGy)
Mn (·104)
Mw (·105)
– RT 100 100 150 180
Control 0.30 0.65 0.73 0.52 0.20
2.08 1.99 2.07 2.06 2.05 2.12
4.52 5.35 7.68 8.90 9.09 6.58
127
Mw(D) and Mn(D) are the initial weight (w)/number (n) average molecular weight of the initial (0)/after irradiation (D). The equations are based on the assumption that the initial molecular weight distribution is random, that is, Mw/Mn = 2. In our experiment, Mw/Mn of unirradiated PSF sample was 2.17, therefore the equations can be applied to calculating for G-value. Table 2 shows G(s) and G(x) for irradiation temperatures. The G(x) is clearly increased by irradiation at high temperature, though the G(s) scatters. We could not measure the sample irradiated at 210 C with GPC, because the polymer immediately formed gel. 3.2. The change of Tg by DSC Fig. 4 shows the change of Tg to absorbed dose. The Tg of samples that were irradiated at room temperature lowered at high dose. The Tg of sample irradiated at 180 C did not change, while the Tg of sample irradiated at 210 C increased with dose. DSC and GPC measurements show that PSF undergoes the main chain scission at room temperature and cross-linking predominates over scission at high temperature. 3.3. The gel fraction measurement Fig. 5 shows the relation between gel fraction and dose for PSF. The gel fraction increases with dose but levels off. It is clear that samples irradiated over 3 MGy at room temperature formed gel. The gel dose (Dg) decreases with increasing irradiation temperature, that is, Dg is 1.1 MGy at 100 C, 0.45 MGy at 180 C and 0.2 MGy at 210 C. The saturated gel fraction increases with irradiation temperature, namely from 30% at 40 C to 100% at 210 C. GPC and DSC measurements show that samples irradiated at room temperature undergo scission. Though it seems that gel formation contradicts these results, it corresponds with other measurement, because increase in higher molecular weight fraction was observed by GPC measurement. The GPC and DSC would give macroscopic average properties, whereas gel would be formed through local cross-linking. These facts show that both main chain scission and crosslinking occurred at room temperature irradiation. The G-value of scission and cross-linking were calculated with Charlesby–Pinner equation; s + s1/2 = G(s)/ [2G(x)] + C/[Mn(0)G(x)D], where s is sol fraction, C = 4.8 · 106 (kg1), D is dose in kGy, respectively. These G(s) and G(x) are shown in Table 2 with those computed by GPC measurement. The G(x) increases clearly with increasing irradiation temperature, though the G(s) scatters. The analysis of data for 210 C may be inappropriate because the polymer formed gel at very low dose, making the intersection of the Charlesby–Pinner plot negative, therefore we assume the probability of the scission is zero. The difference that G-values show between molecular weight and gel fraction would come from gel formation. However, we are considering that they show the similar
128
K. Murakami, H. Kudo / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 125–129
Table 2 G-values of scission and cross-linking for polysulfone Irradiation temperature (C)
By GPC measurements G(s)
G(x)
G(s)
G(x)
Room temperature 100 150 180 210
0.16 0.10 0.15 0.16 –
0.06 0.10 0.14 0.21 –
0.10 0.20 – 0.21 (0)
0.08 0.16 – 0.38 1.35
Fig. 4. The dependence of Tg of polysulfone on absorbed dose; (s) room temperature; (4) 100 C, (5) 180 C, (e) 210 C.
By gel fraction measurements
H2, CH4, CO, CO2 and SO2. The SO2 could not be detected from samples irradiated at room temperature. G-values of main gaseous products were computed in the low dose range and are shown in Table 3. Irradiation of PSF produced CO2, CO and SO2 as the major volatile product at higher temperature. The evolution of these three gases strongly depended on irradiation temperature and is presumably associated with the main chain scission. It is likely that the temperature dependence made the drastic increase of cross-linking. Whereas H2 and CH4 were not major gaseous products at higher temperature and yield of evolved H2 was independent of irradiation temperature. The production of H2 indicates C–H scission in the aromatic rings and in the isopropylidene units; the latter may be the major reaction [4]. These schemes are the primary process by radiation then it seems that these are not influenced by temperature very much. 3.5. Possible reaction mechanism It is reported that C–S main chain scission and decomposition of aliphatic group are primary step in the radiolysis [4,7].
Fig. 5. Gel fraction of polysulfone by gamma-irradiation at various temperatures; (s) room temperature; (4) 100 C, (5) 180 C, (e) 210 C.
irradiation temperature dependence; G(x) increases drastically with increasing irradiation temperature, though G(s) remains almost constant. 3.4. Gas analysis Fig. 6 shows the yield of evolved total gas versus dose for PSF. Total yield of eliminated gases increases with increasing irradiation temperature. Components gases were
Fig. 6. Yield of total gas versus dose for polysulfone by gamma irradiation under vacuum; (s) room temperature; (4) 100 C, (5) 180 C, (e) 210 C.
K. Murakami, H. Kudo / Nucl. Instr. and Meth. in Phys. Res. B 265 (2007) 125–129
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Table 3 G-values (in 102) of main gaseous products for polysulfone under vacuum Irradiation temperature (C)
Total gas
H2
CH4
CO
CO2
SO2
Room temperature 100 180 210
2.1 12 34 61
0.77 1.3 1.3 1.8
0.26 1.0 1.4 1.8
0.22 1.7 3.9 10
0.36 1.5 2.1 12
0 5.0 23 14
Moreover, Brown et al. [4] mentioned that the scission of C–S bond may lead liberating SO2 and that the extraction of SO2 would be due to the phenylene sulfonyl radical either directly or via phenylene sulfonyl acid.
Since SO2 was detected at higher temperature than 100 C only, scission of C–S bonds is minor at room temperature and increased with temperature. Disintegration of isopropylidene unit would be major reaction at room temperature. Though the origins of CO and CO2 are unclear, it is possibly that CH3 radical attack to –SO2– group in polymer or SO2 radical attack to –C(CH3)– group in polymer. Brown and O’Donell [4] also considered that aliphatic group (isopropylidene unit) in PSF in involved in crosslinking. The cross-linking structure would be T-type, not H-type, because H-type cross-linking should evolve the H2 and CH4 upon cross-linking, but actually G(H2) and G(CH4) were about one order lower than G(s) and G(x) at all temperatures tested. This speculation is supported by Hill et al. [8] though C 13NMR (Nuclear Magnetic Resonance) measurement. Possible reaction resulting in cross-linking is combination between phenyl radical and methyl radical in the isopropylidene unit both by main chain scission. However, at room temperature, C–S bond scission may not arise, because SO2 was not detected in gas analysis. Therefore possible reaction mechanism at room temperature is, scission between isopropylidene unit and phenyl ring forms phenyl radical, and this radical combines with methyl radical in the aliphatic group.
On the other hand, at high temperature, C–S bond scission contributes to form phenyl radical accompanied with SO2 evolution as follows. Since the scission of C–S bond is enhance at high temperature as gas analysis implied, probability of cross-lining is increased with irradiation temperature.
4. Conclusion Polysulfone undergoes both cross-lining and main chain scission by gamma-rays irradiation. Main chain scission is regarded as predominant by irradiation at room temperature. At high temperature, on the other hand, cross-linking occurs effectively and G-value of cross-linking increases sharply in comparison with G-values of main chain scission. The cross-linking structure is presumed to be Ttype. References [1] E.A. Hegazy, T. Sasuga, M. Nishii, T. Seguchi, Polymer 33 (1992) 2897. [2] E.A. Hegazy, T. Sasuga, M. Nishii, T. Seguchi, Polymer 33 (1992) 2904. [3] T. Sasuga, N. Hayakawa, K. Yoshida, Polymer 28 (1987) 236. [4] J.R. Brown, J.H. O’Donnell, J. Appl. Polym. Sci. 19 (1975) 405. [5] R.W. Garrett, D.J.T. Hill, T.T. Le, K.A. Milne, J.H. O’Donnell, S.M.C. Perera, P.J. Pomery, American Chemical Society Book Series, Vol. 475, 1991, p. 146 (Chapter 10). [6] J. Sun, Y. Zhang, X. Zhong, X. Zhu, Radiat. Phys. Chem. 44 (1994) 655. [7] D.R. Coulter, M.V. Smith, F. Tsay, A. Gupta, J. Appl. Polym. Sci. 30 (1985) 1753. [8] D.J.T. Hill, D.A. Lewis, J.H. O’Donnell, J. Macromol. Sci. Part A 29 (1992) 11.