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Crystal size shrinking in radiation-induced crosslinking of polytetrafluoroethylene: Synchrotron small angle X-ray scattering and scanning electron microscopy analysis
European Polymer Journal 59 (2014) 156–160
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Crystal size shrinking in radiation-induced crosslinking of polytetrafluoroethylene: Synchrotron small angle X-ray scattering and scanning electron microscopy analysis Zhongfeng Tang a,b, Mouhua Wang a, Feng Tian a, Lu Xu a, Guozhong Wu a,b,⇑ a b
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China Key Laboratory of Nuclear Radiation and Nuclear Energy Technology, Chinese Academy of Sciences, Shanghai 201800, China
a r t i c l e
i n f o
Article history: Received 24 March 2014 Received in revised form 5 July 2014 Accepted 10 July 2014 Available online 22 July 2014 Keywords: Polytetrafluoroethylene Small-angle X-ray scattering Crosslinking Radiation Crystal
a b s t r a c t The crystal structure of crosslinked polytetrafluoroethylene (XPTFE) and its morphological variations as compared with pure PTFE were investigated using differential scanning calorimetry, synchrotron small angle X-ray scattering (SAXS), and scanning electron microscopy (SEM). The XPTFE samples were obtained by irradiating PTFE at the melt state using different radiation doses. Results showed that the melting temperature of the XPTFE decreases with the increase in radiation dose. The crystal melting enthalpy increases under low-dose irradiation. The scattering intensity decreases with the increase in radiation dose, which indicates the decrease in crystal size. Based on the SAXS patterns, the XPTFE crystallite size is smaller than the PTFE crystallite size. The difference in crystal sizes is more pronounced for XPTFE obtained under high radiation doses. This finding is confirmed by the SEM images of the XPTFE fractured surfaces. These results provide a reasonable explanation for the improved transparency and the relatively low crystallinity and high wear resistance of XPTFE compared with PTFE. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Polytetrafluoroethylene (PTFE) exhibits remarkable chemical and physical properties, such as excellent chemical resistance, high thermal stability, and excellent dielectric properties. It is widely used in various applications under severe conditions. However, PTFE is sensitive to ionizing radiation, such that it degrades even under low irradiation, therefore, its application in radiation-related fields is limited [1–3]. In addition, PTFE shows significantly poor wear resistance, although it has a low friction coefficient. These drawbacks could be solved through the crosslinking of polymer chains with radiation processing at an ade⇑ Corresponding author. Address: P.O. Box 800-204, Shanghai 201800, China. Tel./fax: +86 21 39194526. E-mail address: [email protected] (G. Wu). http://dx.doi.org/10.1016/j.eurpolymj.2014.07.013 0014-3057/Ó 2014 Elsevier Ltd. All rights reserved.
quately high temperature just above the melting temperature of the polymer in an oxygen-free atmosphere [4–11]. The crosslinked PTFE (XPTFE) exhibits substantial improvement in its wear resistance, radiation resistance and other properties, such as excellent transparency, high yield strength and Young’s modulus. With these improved properties, XPTFE can be used in numerous applications [5–8,12–15]. XPTFE has been extensively studied using Fourier transform infrared spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), electron spin resonance (ESR), and 19F nuclear magnetic resonance to elucidate its distinct chemical structure [16–22]. However, limited studies have been conducted concerning the XPTFE crystal structure, which is important in understanding the relationship between its structure and properties. Irradiation breaks the long chains of PTFE, leading to the formation
Z. Tang et al. / European Polymer Journal 59 (2014) 156–160
of short chains that recombine and generate crosslinked structures. The XRD patterns of the polymer indicated that its amorphous part increases and its crystalline part decreases after irradiation [18,20], although the latter increases with low radiation doses. The crystal sizes of XPTFE are unknown, thus, correlating its polymer crystallinity with its properties is difficult. This study aim is to investigate the variations in the crystal sizes of XPTFE compared with that of PTFE, as well as its radiation dose dependence. The variations in the crystal sizes may confer specific XPTFE characteristics, such as improved transparency and excellent wear resistance. If the XPTFE crystal size can be maintained as small as possible during polymerization or crosslinking, then its specific properties can be improved. In this study, a bend source beam line of synchrotron small-angle X-ray scattering (SAXS) in the Shanghai Synchrotron Radiation Facility (SSRF) was used to investigate the crystal changes in PTFE and XPTFE. Moreover, the morphological changes in the XPTFE sheet were also examined through scanning electron microscopy (SEM). This study provides additional information on the micro-structural evolution of XPTFE. To the best of our knowledge, this study was the first to investigate the structural changes in XPTFE through SAXS and SEM experiments.
157
5050 mm from the sample. The 2D SAXS scattering patterns were analyzed using Fit2D software. The synchrotron SAXS patterns were normalized using the primary beam intensity and corrected for background scattering. Changes in the scattering intensities caused by varying sample thicknesses were corrected by measuring the sample adsorption in ionization chambers. The 2D SAXS images were azimuthally averaged and then radially integrated to extract the 1D intensity profiles using the expression q = 4p(sin h)/k, where 2h is the scattering angle, and k is the wavelength. The long period (L), which represents the average distance between the regions with equivalent electronic densities, was calculated using the equation L = 2p/q, where q is the modulus of the scattering vector at the peak maximum. 2.4. SEM observation SEM images were obtained using a 1530 VP SEM (LEO Electron Microscopy Ltd., Cambridge, UK) with a voltage of 3.0 kV. To prepare the cross-sectioned samples, the PTFE and XPTFE sheets were immersed in liquid N2, fractured, and then coated with gold prior to analysis. 3. Results and discussion
2. Materials and methods
3.1. Crystallinity of PTFE and XPTE
2.1. XPTFE preparation and properties
Fig. 1 shows the PTFE, XPTFE500, and XPTFE3000 images. The white PTFE sheet was observed, but the sample became transparent as the polymer crosslinking progressed. The crosslinked PTFE was formed at the melt state, and the amount of crystallite that consist of lamellar stacks was enhanced by a small fraction of low-molecular weight species, which are induced by irradiation. The changes in optical properties could be due to the formation of the small crystallite. Moreover, the defect points caused by the crosslinking slightly disrupt the lamellar stacks because of a decrease in mobility when chains are incorporated into chain-folded lamella; this phenomenon could also lead to the formation of spherulites with poor qualities. In addition to the rough or imperfect lamellar surfaces with the increase in the number of crosslinks, the lamellar stack growth and quality decreases with a highly dispersed thickness. The crystallization of the crosslinked polymers was disrupted, resulting in a lower crystallinity degree and smaller crystals. The increase in transparency coincided with the decrease in the crystalline fraction of the polymer because of the crosslinking. Therefore, the transparency improves with the increase in crosslinking density, which is radiation dose-dependent. The changes in the transparency shown in Fig. 1 are in agreement with the data in [19]. The changes in the crystallinity with crosslinking were monitored using DSC. Fig. 2 shows the DSC curves for the PTFE with different crosslinking degrees. The spectral intensity was normalized with the specimen weight. With the increase in radiation, Tm shifted to a lower temperature and the peak intensity decreased and was accompanied with a broadened melting peak. The DHm of the samples
The PTFE sheet was crosslinked through irradiation under N2 atmosphere at the melt state (335 °C) [14]. The PTFE crosslinking degree was controlled by changing the total radiation dose from 150 kGy to 3000 kGy. After irradiation, all of the samples were cooled to room temperature at the same cooling rate. The PTFE sheets crosslinked using different doses were labeled XPTFE150, XPTFE500, XPTFE1000, and XPTFE3000. Compared with pristine PTFE, the wear and radiation resistances of XPTFE improved by three and two orders of magnitude, respectively, because of the formation of 3D crosslinked networks. Moreover, the other polymer properties, such as transparency, yield strength, and Young’s modulus were improved. 2.2. DSC characterization The melting temperature (Tm) and enthalpies (DHm) of the PTFE and XPTFE samples were measured by DSC (Mettler TA822e) under N2 atmosphere at 10 °C/min from 20 °C to 300 °C. DHm was determined from the crystallization peak area in the obtained DSC curves. 2.3. Synchrotron SAXS analysis Synchrotron SAXS measurements were performed with the use of the SSRF beam line BL16B1. The X-ray wavelength was 0.124 nm. The 2D SAXS patterns were obtained with a Mar165 detector (79 lm pixel size) at a distance of
158
Z. Tang et al. / European Polymer Journal 59 (2014) 156–160
Fig. 1. PTFE, XPTFE500, and XPTFE3000 images.
the molecular chains. As a result, the crystallization proceeds. However, the crystal size of XPTFE is smaller than that of the unprocessed PTFE, resulting in a reduced Tm compared with the non-irradiated PTFE. The increase in crystallinity that coincided with a decrease in Tm can be observed in the DSC curves [23]. With the increase in radiation dose, XPTFE forms more networks at the melt state and the crosslinking point density significantly increases and dominates. Under the given conditions, the movements of the molecular chains were constrained by crosslinking points, and the crystallization was also suppressed. Only molecules with significantly long chains between crosslinking points formed crystals, which became increasingly smaller because of the molecular mobility restriction. This restriction caused a further decrease in Tm and DHm [23]. Fig. 2. DSC spectra for PTFE and XPTFE at different radiation doses.
3.2. XPTFE crystal size was calculated from the peak area in their corresponding DSC curves. The crystallinity degree, which is the ratio of the crystalline part (xc), was determined using the relationship xc = DHm/DHom, where DHom is the crystal melting enthalpy, which is assumed to be 60 J/g. Table 1 lists the parameters for PTFE with different crosslinking degrees. With the increasing in radiation dose, Tm continuously decreased from 327 °C for PTFE to 280 °C for XPTFE-3000. However, DHm increased under a small radiation dose of 150 kGy, and then decreased under higher radiation doses. Two competing process, namely, chain-scission and crosslinking, could simultaneously occur during irradiation. At low radiation doses, in which chain-scission dominates, short chains can be effectively generated in the molten PTFE matrices, thereby enhancing the mobility of
Table 1 Tm and DHm for PTFE and XPTFE samples at different radiation doses. Sample
Dose (kGy)
Tm (°C)
DHm (J/g)
PTFE XPTFE150 XPTFE500 XPTFE1000 XPTFE3000
0 150 500 1000 3000
327 324 307 300 280
26 40 35 31 23
The XPTFE crystal size was evaluated using the synchrotron SAXS data. Fig. 3 shows a diffused scattering ring in all of the 2D SAXS patterns, indicating the presence of crystals. With increasing dose, the scattering ring gradually moves further away from the beam center, indicating the decrease in the crystal size. The XPTFE meridional scattering in the patterns increased with the crosslinking degree, indicating that the crystals were preferentially oriented in the plane of the layer. Additional information on the evolution of the crystal size can be obtained from the integrated 1D SAXS curves (Fig. 4), which shows the shift of the scattered peak toward the lower q value and broadening of the peak width with increasing radiation doses. These phenomena indicate that crystal size was decreased and uncorrelated. With the increase in radiation dose, the scattering intensity decreases, whereas the lamellae content decreases. Thus, the amorphous phase changes because the intensities of the SAXS peaks are dependent on the crystallinity and on the difference between the electronic densities of the amorphous and crystalline phases. Upon cooling, the amorphous interlamellar component adopted a more compact molecular arrangement, probably because of the improved hydrogen bonding interactions. Meanwhile, the peak that represents lamellae interlayer distance shifted
Z. Tang et al. / European Polymer Journal 59 (2014) 156–160
159
Fig. 3. Synchrotron SAXS patterns of PTFE and XPTFE samples at different radiation doses. (a) PTFE, (b) XPTFE150, (c) XPTFE500, (d) XPTFE1000 and (e) XPTFE3000.
Fig. 4. SAXS curves of PTFE and XPTFE at different radiation doses. (a) PTFE, (b) XPTFE150, (c) XPTFE500, (d) XPTFE1000 and (e) XPTFE3000.
to larger h values as the radiation dose increases. Thus, the lamellae interlayer distance decreases with the increase in radiation dose. The evolution of long period (L) was also analyzed based on the 1D SAXS patterns (Fig. 5). Before irradiation, PTFE initially showed a maximum L value of approximately 82 nm. This value decreased to 21 nm when a radiation dose of 3000 kGy was applied. The apparently long period changes resulting in the two populations of lamellar stacks can be attributed to the variations in the amorphous layers within the lamellar stacks. The two lamellae populations were mainly formed through the combination of (1) lamellar deformation (the major mechanism) from the interlamellar compression of the amorphous layers in the lamellar stacks and (2) the crosslinking process that pro-
Fig. 5. Changes in long period (L) of PTFE and XPTFE at different radiation doses.
duces new crystalline lamellae (the minor mechanism). Since PTFE chains in melt state can be effectively crosslinked by radiation with high doses, the crystallization will be significantly suppressed, resulting the decrease of crystallite size. These results are similar to those in our previous observations. Using the long period values and DSC curves, the crystal size was determined to be from 82 nm to 21 nm. The current results are consistent with those presented in literature [4,6,14].
3.3. XPTFE micromorphological changes SEM images (Fig. 6) were obtained to confirm the SAXS patterns, which show the decrease in the XPTFE crystal
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Fig. 6. SEM images of PTFE and XPTFE samples at different radiation doses. (a) PTFE, (b) XPTFE500 and (c) XPTFE3000.
size. The fractured PTFE, XPTFE500, and XPTFE3000 sample surfaces were compared under the same magnification. Two kinds of fractured surface were observed, namely, dense (uniform) and loose (porous) regions. The dense regions have a fibrillar structure and are morphologically identical to XPTFE. At high radiation doses, the crystal sizes of the XPTFE were significantly smaller. These observations are in agreement with the results of the SAXS analysis. Although the SEM observations appear to be significantly useful in determining the morphology and qualitative evolutions of PTFE under irradiation, quantitative information cannot be obtained from these images. The results show that the size of XPTFE crystals is significantly small with high degree of crosslinking.
4. Conclusions We studied the crystal structures and morphological variations of PTFE and XPTFE through DSC, SAXS, and SEM techniques. The white PTFE sheet was observed, which became transparent upon crosslinking. The crystallization of the crosslinking part was disrupted, resulting in a low degree of crystallinity and small crystals. Based on the SAXS patterns and SEM images of the fractured surfaces, the XPTFE crystal size decreased compared with the original PTFE, and this effect was more pronounced for XPTFE obtained with high radiation doses. The variation in the crystal sizes is important for specific XPTFE characteristics. Our results provide sufficient explanation for the improved transparency, low degree of crystallinity, and high wear resistance of XPTFE, which were observed in a previous study.
Acknowledgement This work was supported by the National Natural Science Foundation of China (10775173) and the Program of the Shanghai Subject Chief Scientist (07XD14037). References [1] Golden JH. J Polym Sci 1960;45:534. [2] Katoh E, Kasai N, Sasuga T, Seguchi T. Radiat Phys Chem 1994;43:329. [3] Lunkwitz K, Burger W, Geissler U, Petr A, Jehnichen D. J Appl Polym Sci 1996;60:2017. [4] Sun JZ, Zhang YF, Zhong XG. Polymer 1994;35:2881. [5] Sun JZ, Zhang YF, Zhong XG, Zhu XL. Radiat Phys Chem 1994;44:655. [6] Oshima A, Tabata Y, Kudoh H, Seguchi T. Radiat Phys Chem 1995;45:269. [7] Tabata Y, Oshima A, Takashika K, Seguchi T. Radiat Phys Chem 1996;48:563. [8] Oshima A, Ikeda S, Seguchi T, Tabata Y. Radiat Phys Chem 1997;49:279. [9] Oshima A, Ikeda S, Seguchi T, Tabata Y. Radiat Phys Chem 1997;49:581. [10] Oshima A, Seguchi T, Tabata Y. Radiat Phys Chem 1997;50:601. [11] Oshima A, Ikeda S, Kudoh H, Seguchi T, Tabata Y. Radiat Phys Chem 1997;50:611. [12] Seguchi T. Radiat Phys Chem 2000;57:367. [13] Setogawa A, Nishi H, Yamamoto Y, Kusano H, Asai T. Hitachi Cable Rev 2002;21:83. [14] Tang ZF, Wang MH, Zhao YN, Wu GZ. Wear 2010;269:485. [15] Tang ZF, Wang MH, Zhao YN, Wu GZ. Radiat Phys Chem 2011;80:496. [16] Katoh E, Sugisawa H, Oshima A, Tabata Y, Seguchi T, Yamazaki T. Radiat Phys Chem 1999;54:165. [17] Lappan U, Geissler U, Lunkwitz K. J Appl Polym Sci 1999;74:1571. [18] Lappan U, Geissler U, Lunkwitz K. Radiat Phys Chem 2000;59:317. [19] Fuchs B, Scheler U. Macromolecules 2000;33:120. [20] Oshima A, Ikeda S, Katoh E, Tabata Y. Radiat Phys Chem 2001;62:39. [21] Ignat’eva LN, Buznik VM. Russ J Gen Chem 2001;79:677. [22] Bruk MA. High Energy Chem 2006;40:357. [23] Abdou SM, Mohamed RI. J Phys Chem Solid 2002;63:393.
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Crystal size shrinking in radiation-induced crosslinking of polytetrafluoroethylene: Synchrotron small angle X-ray scattering and scanning electron microscopy analysis Zhongfeng Tang a,b, Mouhua Wang a, Feng Tian a, Lu Xu a, Guozhong Wu a,b,⇑ a b
Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China Key Laboratory of Nuclear Radiation and Nuclear Energy Technology, Chinese Academy of Sciences, Shanghai 201800, China
a r t i c l e
i n f o
Article history: Received 24 March 2014 Received in revised form 5 July 2014 Accepted 10 July 2014 Available online 22 July 2014 Keywords: Polytetrafluoroethylene Small-angle X-ray scattering Crosslinking Radiation Crystal
a b s t r a c t The crystal structure of crosslinked polytetrafluoroethylene (XPTFE) and its morphological variations as compared with pure PTFE were investigated using differential scanning calorimetry, synchrotron small angle X-ray scattering (SAXS), and scanning electron microscopy (SEM). The XPTFE samples were obtained by irradiating PTFE at the melt state using different radiation doses. Results showed that the melting temperature of the XPTFE decreases with the increase in radiation dose. The crystal melting enthalpy increases under low-dose irradiation. The scattering intensity decreases with the increase in radiation dose, which indicates the decrease in crystal size. Based on the SAXS patterns, the XPTFE crystallite size is smaller than the PTFE crystallite size. The difference in crystal sizes is more pronounced for XPTFE obtained under high radiation doses. This finding is confirmed by the SEM images of the XPTFE fractured surfaces. These results provide a reasonable explanation for the improved transparency and the relatively low crystallinity and high wear resistance of XPTFE compared with PTFE. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Polytetrafluoroethylene (PTFE) exhibits remarkable chemical and physical properties, such as excellent chemical resistance, high thermal stability, and excellent dielectric properties. It is widely used in various applications under severe conditions. However, PTFE is sensitive to ionizing radiation, such that it degrades even under low irradiation, therefore, its application in radiation-related fields is limited [1–3]. In addition, PTFE shows significantly poor wear resistance, although it has a low friction coefficient. These drawbacks could be solved through the crosslinking of polymer chains with radiation processing at an ade⇑ Corresponding author. Address: P.O. Box 800-204, Shanghai 201800, China. Tel./fax: +86 21 39194526. E-mail address: [email protected] (G. Wu). http://dx.doi.org/10.1016/j.eurpolymj.2014.07.013 0014-3057/Ó 2014 Elsevier Ltd. All rights reserved.
quately high temperature just above the melting temperature of the polymer in an oxygen-free atmosphere [4–11]. The crosslinked PTFE (XPTFE) exhibits substantial improvement in its wear resistance, radiation resistance and other properties, such as excellent transparency, high yield strength and Young’s modulus. With these improved properties, XPTFE can be used in numerous applications [5–8,12–15]. XPTFE has been extensively studied using Fourier transform infrared spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), electron spin resonance (ESR), and 19F nuclear magnetic resonance to elucidate its distinct chemical structure [16–22]. However, limited studies have been conducted concerning the XPTFE crystal structure, which is important in understanding the relationship between its structure and properties. Irradiation breaks the long chains of PTFE, leading to the formation
Z. Tang et al. / European Polymer Journal 59 (2014) 156–160
of short chains that recombine and generate crosslinked structures. The XRD patterns of the polymer indicated that its amorphous part increases and its crystalline part decreases after irradiation [18,20], although the latter increases with low radiation doses. The crystal sizes of XPTFE are unknown, thus, correlating its polymer crystallinity with its properties is difficult. This study aim is to investigate the variations in the crystal sizes of XPTFE compared with that of PTFE, as well as its radiation dose dependence. The variations in the crystal sizes may confer specific XPTFE characteristics, such as improved transparency and excellent wear resistance. If the XPTFE crystal size can be maintained as small as possible during polymerization or crosslinking, then its specific properties can be improved. In this study, a bend source beam line of synchrotron small-angle X-ray scattering (SAXS) in the Shanghai Synchrotron Radiation Facility (SSRF) was used to investigate the crystal changes in PTFE and XPTFE. Moreover, the morphological changes in the XPTFE sheet were also examined through scanning electron microscopy (SEM). This study provides additional information on the micro-structural evolution of XPTFE. To the best of our knowledge, this study was the first to investigate the structural changes in XPTFE through SAXS and SEM experiments.
157
5050 mm from the sample. The 2D SAXS scattering patterns were analyzed using Fit2D software. The synchrotron SAXS patterns were normalized using the primary beam intensity and corrected for background scattering. Changes in the scattering intensities caused by varying sample thicknesses were corrected by measuring the sample adsorption in ionization chambers. The 2D SAXS images were azimuthally averaged and then radially integrated to extract the 1D intensity profiles using the expression q = 4p(sin h)/k, where 2h is the scattering angle, and k is the wavelength. The long period (L), which represents the average distance between the regions with equivalent electronic densities, was calculated using the equation L = 2p/q, where q is the modulus of the scattering vector at the peak maximum. 2.4. SEM observation SEM images were obtained using a 1530 VP SEM (LEO Electron Microscopy Ltd., Cambridge, UK) with a voltage of 3.0 kV. To prepare the cross-sectioned samples, the PTFE and XPTFE sheets were immersed in liquid N2, fractured, and then coated with gold prior to analysis. 3. Results and discussion
2. Materials and methods
3.1. Crystallinity of PTFE and XPTE
2.1. XPTFE preparation and properties
Fig. 1 shows the PTFE, XPTFE500, and XPTFE3000 images. The white PTFE sheet was observed, but the sample became transparent as the polymer crosslinking progressed. The crosslinked PTFE was formed at the melt state, and the amount of crystallite that consist of lamellar stacks was enhanced by a small fraction of low-molecular weight species, which are induced by irradiation. The changes in optical properties could be due to the formation of the small crystallite. Moreover, the defect points caused by the crosslinking slightly disrupt the lamellar stacks because of a decrease in mobility when chains are incorporated into chain-folded lamella; this phenomenon could also lead to the formation of spherulites with poor qualities. In addition to the rough or imperfect lamellar surfaces with the increase in the number of crosslinks, the lamellar stack growth and quality decreases with a highly dispersed thickness. The crystallization of the crosslinked polymers was disrupted, resulting in a lower crystallinity degree and smaller crystals. The increase in transparency coincided with the decrease in the crystalline fraction of the polymer because of the crosslinking. Therefore, the transparency improves with the increase in crosslinking density, which is radiation dose-dependent. The changes in the transparency shown in Fig. 1 are in agreement with the data in [19]. The changes in the crystallinity with crosslinking were monitored using DSC. Fig. 2 shows the DSC curves for the PTFE with different crosslinking degrees. The spectral intensity was normalized with the specimen weight. With the increase in radiation, Tm shifted to a lower temperature and the peak intensity decreased and was accompanied with a broadened melting peak. The DHm of the samples
The PTFE sheet was crosslinked through irradiation under N2 atmosphere at the melt state (335 °C) [14]. The PTFE crosslinking degree was controlled by changing the total radiation dose from 150 kGy to 3000 kGy. After irradiation, all of the samples were cooled to room temperature at the same cooling rate. The PTFE sheets crosslinked using different doses were labeled XPTFE150, XPTFE500, XPTFE1000, and XPTFE3000. Compared with pristine PTFE, the wear and radiation resistances of XPTFE improved by three and two orders of magnitude, respectively, because of the formation of 3D crosslinked networks. Moreover, the other polymer properties, such as transparency, yield strength, and Young’s modulus were improved. 2.2. DSC characterization The melting temperature (Tm) and enthalpies (DHm) of the PTFE and XPTFE samples were measured by DSC (Mettler TA822e) under N2 atmosphere at 10 °C/min from 20 °C to 300 °C. DHm was determined from the crystallization peak area in the obtained DSC curves. 2.3. Synchrotron SAXS analysis Synchrotron SAXS measurements were performed with the use of the SSRF beam line BL16B1. The X-ray wavelength was 0.124 nm. The 2D SAXS patterns were obtained with a Mar165 detector (79 lm pixel size) at a distance of
158
Z. Tang et al. / European Polymer Journal 59 (2014) 156–160
Fig. 1. PTFE, XPTFE500, and XPTFE3000 images.
the molecular chains. As a result, the crystallization proceeds. However, the crystal size of XPTFE is smaller than that of the unprocessed PTFE, resulting in a reduced Tm compared with the non-irradiated PTFE. The increase in crystallinity that coincided with a decrease in Tm can be observed in the DSC curves [23]. With the increase in radiation dose, XPTFE forms more networks at the melt state and the crosslinking point density significantly increases and dominates. Under the given conditions, the movements of the molecular chains were constrained by crosslinking points, and the crystallization was also suppressed. Only molecules with significantly long chains between crosslinking points formed crystals, which became increasingly smaller because of the molecular mobility restriction. This restriction caused a further decrease in Tm and DHm [23]. Fig. 2. DSC spectra for PTFE and XPTFE at different radiation doses.
3.2. XPTFE crystal size was calculated from the peak area in their corresponding DSC curves. The crystallinity degree, which is the ratio of the crystalline part (xc), was determined using the relationship xc = DHm/DHom, where DHom is the crystal melting enthalpy, which is assumed to be 60 J/g. Table 1 lists the parameters for PTFE with different crosslinking degrees. With the increasing in radiation dose, Tm continuously decreased from 327 °C for PTFE to 280 °C for XPTFE-3000. However, DHm increased under a small radiation dose of 150 kGy, and then decreased under higher radiation doses. Two competing process, namely, chain-scission and crosslinking, could simultaneously occur during irradiation. At low radiation doses, in which chain-scission dominates, short chains can be effectively generated in the molten PTFE matrices, thereby enhancing the mobility of
Table 1 Tm and DHm for PTFE and XPTFE samples at different radiation doses. Sample
Dose (kGy)
Tm (°C)
DHm (J/g)
PTFE XPTFE150 XPTFE500 XPTFE1000 XPTFE3000
0 150 500 1000 3000
327 324 307 300 280
26 40 35 31 23
The XPTFE crystal size was evaluated using the synchrotron SAXS data. Fig. 3 shows a diffused scattering ring in all of the 2D SAXS patterns, indicating the presence of crystals. With increasing dose, the scattering ring gradually moves further away from the beam center, indicating the decrease in the crystal size. The XPTFE meridional scattering in the patterns increased with the crosslinking degree, indicating that the crystals were preferentially oriented in the plane of the layer. Additional information on the evolution of the crystal size can be obtained from the integrated 1D SAXS curves (Fig. 4), which shows the shift of the scattered peak toward the lower q value and broadening of the peak width with increasing radiation doses. These phenomena indicate that crystal size was decreased and uncorrelated. With the increase in radiation dose, the scattering intensity decreases, whereas the lamellae content decreases. Thus, the amorphous phase changes because the intensities of the SAXS peaks are dependent on the crystallinity and on the difference between the electronic densities of the amorphous and crystalline phases. Upon cooling, the amorphous interlamellar component adopted a more compact molecular arrangement, probably because of the improved hydrogen bonding interactions. Meanwhile, the peak that represents lamellae interlayer distance shifted
Z. Tang et al. / European Polymer Journal 59 (2014) 156–160
159
Fig. 3. Synchrotron SAXS patterns of PTFE and XPTFE samples at different radiation doses. (a) PTFE, (b) XPTFE150, (c) XPTFE500, (d) XPTFE1000 and (e) XPTFE3000.
Fig. 4. SAXS curves of PTFE and XPTFE at different radiation doses. (a) PTFE, (b) XPTFE150, (c) XPTFE500, (d) XPTFE1000 and (e) XPTFE3000.
to larger h values as the radiation dose increases. Thus, the lamellae interlayer distance decreases with the increase in radiation dose. The evolution of long period (L) was also analyzed based on the 1D SAXS patterns (Fig. 5). Before irradiation, PTFE initially showed a maximum L value of approximately 82 nm. This value decreased to 21 nm when a radiation dose of 3000 kGy was applied. The apparently long period changes resulting in the two populations of lamellar stacks can be attributed to the variations in the amorphous layers within the lamellar stacks. The two lamellae populations were mainly formed through the combination of (1) lamellar deformation (the major mechanism) from the interlamellar compression of the amorphous layers in the lamellar stacks and (2) the crosslinking process that pro-
Fig. 5. Changes in long period (L) of PTFE and XPTFE at different radiation doses.
duces new crystalline lamellae (the minor mechanism). Since PTFE chains in melt state can be effectively crosslinked by radiation with high doses, the crystallization will be significantly suppressed, resulting the decrease of crystallite size. These results are similar to those in our previous observations. Using the long period values and DSC curves, the crystal size was determined to be from 82 nm to 21 nm. The current results are consistent with those presented in literature [4,6,14].
3.3. XPTFE micromorphological changes SEM images (Fig. 6) were obtained to confirm the SAXS patterns, which show the decrease in the XPTFE crystal
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Z. Tang et al. / European Polymer Journal 59 (2014) 156–160
Fig. 6. SEM images of PTFE and XPTFE samples at different radiation doses. (a) PTFE, (b) XPTFE500 and (c) XPTFE3000.
size. The fractured PTFE, XPTFE500, and XPTFE3000 sample surfaces were compared under the same magnification. Two kinds of fractured surface were observed, namely, dense (uniform) and loose (porous) regions. The dense regions have a fibrillar structure and are morphologically identical to XPTFE. At high radiation doses, the crystal sizes of the XPTFE were significantly smaller. These observations are in agreement with the results of the SAXS analysis. Although the SEM observations appear to be significantly useful in determining the morphology and qualitative evolutions of PTFE under irradiation, quantitative information cannot be obtained from these images. The results show that the size of XPTFE crystals is significantly small with high degree of crosslinking.
4. Conclusions We studied the crystal structures and morphological variations of PTFE and XPTFE through DSC, SAXS, and SEM techniques. The white PTFE sheet was observed, which became transparent upon crosslinking. The crystallization of the crosslinking part was disrupted, resulting in a low degree of crystallinity and small crystals. Based on the SAXS patterns and SEM images of the fractured surfaces, the XPTFE crystal size decreased compared with the original PTFE, and this effect was more pronounced for XPTFE obtained with high radiation doses. The variation in the crystal sizes is important for specific XPTFE characteristics. Our results provide sufficient explanation for the improved transparency, low degree of crystallinity, and high wear resistance of XPTFE, which were observed in a previous study.
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