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Modifications of structural, optical and chemical properties of Li3+ irradiated polyurethane and polyetheretherketone
Radiation Physics and Chemistry 96 (2014) 181–185
Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Modifications of structural, optical and chemical properties of Li3 þ irradiated polyurethane and polyetheretherketone Paramjit Singh a, S. Asad Ali b, Rajesh Kumar a,n a b
University School of Basic & Applied Sciences, Guru Gobind Singh Indraprastha University, New Delhi-110078, India Department of Applied Physics, Aligarh Muslim University, Aligarh-202002, India
H I G H L I G H T S
We irradiated PU and PEEK with swift heavy ions at different fluences. Structural effects, optical and chemical changes after irradiation were examined by XRD, UV–visible and FTIR spectroscopy, respectively. Decrease in amorphous nature and band gap energy is observed with increase in fluence. Increase in average crystallite size and variation in Urback's energy is observed with increase of fluence. The modifications caused by SHI in various chemical bonds are also discussed.
art ic l e i nf o
a b s t r a c t
Article history: Received 11 August 2013 Accepted 26 September 2013 Available online 16 October 2013
Thin membranes of PU and PEEK polymers were irradiated with 50 MeV lithium ions for modifications in structural, optical and chemical properties. The XRD spectra indicated the alignment of polymeric chains in a regular pattern and hence there was decrease in the amorphous character of the irradiated polymers. The optical absorption of all irradiated samples shifted towards the visible region of the spectrum. It was attributed to the generation of a conjugated system of bonds which lowered the band gap energy of irradiated samples. The FTIR spectra obtained after irradiation exhibited decrease in absorbance for various bands in case of PU whereas PEEK showed minor changes in its chemical properties. The other results were discussed from the calculated parameters such as crystallite size, Urbach's energy and number of carbon hexagon rings per conjugation length from the analyses of the XRD and UV–visible data. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Polyurethane Polyether-etherketone Ion beam modification X-ray diffraction UV–visible FTIR
1. Introduction Ion beam modification has shown great potential for improving the structural, free volume, optical as well as surface and chemical properties of polymeric materials (Singh and Kumar, 2013; Singh et al., 2013; Kumar and Singh, 2013; Ali et al., 2013; Abdul-Kader et al., 2012). As a result, the past few years have seen many advances in the field of ion beam modification of polymeric materials in terms of applying conventional ion beam techniques to various types of polymers for industrial and scientific applications. The polymers under study are polyurethane (PU) and polyether-ether ketone (PEEK) used for biomedical and industrial applications. Polyurethanes are one of the most bio and blood-compatible materials. These materials play a major role in the development
n
Corresponding author. Tel.: þ 91 9718876101. E-mail address: [email protected] (R. Kumar).
0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.09.014
of many medical devices such as catheters and artificial hearts (Zdrahala, 1999). Properties such as durability, elasticity, fatigue resistance, compliance and acceptance or tolerance in the body during the healing became often associated with polyurethanes (Zdrahala, 1999). In addition to it, polyurethanes are used in industrial and scientific applications such as coatings, foam and nanocomposites (Chattopadhyay and Raju, 2007). The polyetherether ketone polymers are also used for biomedical research and fuel cell applications. The ion beam modification played a crucial role in the advancement of properties of both of these polymers. The surface of PEEK coated with titanium layer using electron beam deposition method was used in order to enhance its biocompatibility and adhesion to bone tissue by Han et al. (2010). The use of PEEK as a biomaterial for spinal applications has been reported in the literature (Toth et al., 2006). Radiationinduced crosslinking of PEEK film has been reported by Chen et al. (2010) for application in fuel cell. The crosslinking was carried out by 40 MGy dose of electron-beam irradiation on PEEK to prepare an aromatic hydrocarbon polymer electrolyte membrane (PEM).
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P. Singh et al. / Radiation Physics and Chemistry 96 (2014) 181–185
Similar results for the preparation of fuel cells using radiation grafted PEEK had been reported by Manea and Mulder (2002). It is important to study the structural, optical, surface and chemical properties of these polymers after ion beam irradiation to understand the extent of modifications caused by the irradiated ion. The PU and PEEK polymers were irradiated previously with low energy (keV to few MeV) ions for the study of certain properties by various groups. Some important results of ion beam modification on these polymers are summarized below. PU was irradiated with electron beam (5–200 kGy dose) for chemical, thermal, structural and surface analysis by Murray et al., 2013. They demonstrated that cross- linking was emerged in the 200 kGy dose exposed polymer sample. Wong et al., 2006 studied the surface morphology and chemical structure of polyurethane fibers by implantation of 60 and 100 keV oxygen ions. They reported the formation of new chemical functional groups [C–(C ¼O)–C and C–N–C] due to near-surface chemical modifications by ion implantations. The secondary ion emission under MeV ion irradiation on PEEK as a function of the degree of crystallinity had been reported earlier (Nsouli et al., 1997). The structural and compositional changes of 40 keV Ni þ and Co þ implanted and subsequently annealed PEEK polymers were characterized by RBS, UV–vis and XPS by Mackova et al., 2013a, 2013b, 2009. They reported that the properties of implanted polymers strongly depend on the implantation ion fluence and on the properties of the implanted atoms. Their earlier studies of He þ (1.76 MeV) irradiation on PEEK showed similar results (Mackova et al., 2005). Depth profiling of implanted low energy ions in PEEK had also been reported in literature (Malinsky et al., 2012; Hnatowicz et al., 2008; Vacík et al., 2007). The early work in the discussed literature was mainly related to structural and compositional study of PU and PEEK by low energy ions and electron implantations. As far as we know, there have been very few examples of ion induced structural, optical and chemical modifications of these engineering plastic films. The present investigation is focused towards the modifications of structural, optical and chemical properties of the samples of PU and PEEK by lithium ions irradiation. The properties such as crystallite size, band gap energy, Urbach's energy and number of carbon hexagon rings per conjugation length are calculated from the obtained data of XRD and UV–vis.
2. Experimental methods
2.2. Irradiation The samples of size 1.5 cm 1.5 cm were mounted on a vacuum shielded vertical sliding ladder and exposed to 50 MeV Li3 þ beam from 15 UD pelletron accelerator at inter university accelerator centre (IUAC), New Delhi, India in the general purpose scattering chamber (GPSC) under high vacuum ( 7 10 6 Torr). Fluences were taken 5 1010, 2 1011, 5 1012 and 1.2 1013 ions/cm2. The beam current ( 0.5 pnA) was kept low to suppress thermal decomposition. The pristine and irradiated samples were characterized by X-ray diffraction (XRD), UV–visible (UV–vis) and Fourier transform Infrared (FTIR) spectroscopy techniques for the study of structural, optical and chemical modifications. The XRD studies were carried out using Bruker AXS system by Cu-Kα radiation (1.54 Å) for range of Bragg's angle 2θ (51 oθo501). UV–vis studies were carried out between 200 and 800 nm range using U-3300, Hitachi system. The FTIR studies were carried out using Thermo Nicolet Nexus 670 FTIR in the range 400–4000 cm 1. The projected range and electronic energy loss (Se) of 50 MeV lithium ion in both the polymers was calculated by SRIM code (Ziegler, 2010). The Se values are 5.14 and 6.83 eV/Å and the projected range values are 551 and 415.7 mm for PU and PEEK, respectively. The comparison of projected range and sample thicknesses of these polymers makes the ion implantation very less probable.
3. Results and discussions 3.1. X-ray diffraction studies The diffraction patterns of pristine and irradiated PU and PEEK polymer samples are shown in Figs. 1 and 2, respectively. The broad peaks occuring at 2θ 19.71 and 20.131 (for PU and PEEK polymers, respectively) did not shift after irradiation, this indicated that lattice parameters did not change. The peak width decreased and peak intensity increased with increase of ion fluence signifying the decrease in the amorphous character after irradiation (Kumar et al., in press). The average crystallite size (L) was calculated using Scherrer formula (Singh and Kumar, 2013) using following equation. L ¼ kλ=ðb cos θÞ
ð1Þ
here b is the full width at half maximum (FWHM) of the peak (in radians). λ is the wavelength of the X-ray used (1.54 Å in our case for Cu-Kα radiation). θ is the angle which is calculated by taking 1/2
2.1. Membrane preparation The membranes of PU of thickness 70–80 mm were prepared by solution cast method. The details and experimental methods of solution cast method were used similar to the method already reported in the literature (Vijay, 2003; Vijay et al., 2002). Polyurethane was purchased from Indian Research Design, New Delhi in the form of small beads. The PU beads were dissolved in tetrahydrofurane (THF) with constant stirring over a magnetic stirrer. The solution was then put into flat-bottomed Petri-dish floating on mercury to obtain uniform thickness of the film. The solvent was allowed to evaporate for 24 h. The films so obtained were peeled off and dried in vacuum at 60 1C for next 24 h. The PEEK films of same thickness were purchased from Sigma Aldrich, India in the form of sheets and used as received. The films of both the polymers were cut into 1.5 cm 1.5 cm size for irradiation.
Fig. 1. X-ray diffraction spectra of pristine and Li3 þ ion irradiated PU polymer.
P. Singh et al. / Radiation Physics and Chemistry 96 (2014) 181–185
183
Fig. 3. UV–visible spectra of pristine and Li3 þ ion irradiated PU polymer.
Fig. 2. X-ray diffraction spectra of pristine and Li3 þ ion irradiated PEEK polymer.
Table 1 Calculated values of average crystallite size (L), Indirect band gap energy (Eg), number of carbon atoms per conjugation length (N) and Urback's energy (Eu) for pristine and irradiated samples of PU polymer. Fluence (ions/cm2)
Pristine 5 1010 2 1011 5 1012 1.2 1013
PU L (Å)
Eg (eV)
N
Eu (eV)
11.30 7 0.11 11.707 0.11 11.84 7 0.12 11.99 7 0.12 12.36 7 0.12
4.007 0.01 3.99 7 0.01 3.977 0.01 3.94 7 0.01 3.92 7 0.01
73 74 74 75 76
0.10 0.10 0.10 0.45 0.52 Fig. 4. UV–visible spectra of pristine and Li3 þ ion irradiated PEEK polymer.
Table 2 Calculated values of average crystallite size (L), Indirect band gap energy (Eg), number of carbon atoms per conjugation length (N) and Urback's energy (Eu) for pristine and irradiated samples of PEEK polymer. Fluence (ions/cm2)
Pristine 5 1010 2 1011 5 1012 1.2 1013
PEEK L (Å)
Eg (eV)
N
Eu (eV)
10.46 70.10 10.647 0.10 10.85 70.10 10.98 70.10 11.63 70.11
3.08 7 0.01 3.107 0.01 3.0770.01 3.067 0.01 3.05 7 0.01
124 122 125 125 126
0.15 0.12 0.11 0.10 0.11
of 2θ value of peak of diffraction as per given in Figs. 1 and 2. k is a constant of proportionality (Scherrer constant), its value is 0.9 (Sharma et al., 2007). The calculated values are given in Tables 1 and 2 for PU and PEEK, respectively. The values of L increase in irradiated samples of both the polymers. 3.2. UV–visible studies UV–visible spectra of pristine and irradiated samples are shown in Figs. 3 and 4 for PU and PEEK, respectively. The optical absorption edges of both the polymer samples shifted towards the higher wavelength regime after ion irradiation. This shift increased progressively with increase of ion fluence; it indicated a growing concentration of conjugated double bonds in the polymer and increase in conjugation length (Mackova et al., 2009; 2005). Similar results were observed by Mackova et al., 2009 in their study on PEEK by implantation of 40 keV Ni þ ions.
The optical band gap energy was calculated using Tauc's relation (Tauc et al., 1966) for indirect optical transitions. The details of the equation used has already been reported in our recently published article (Singh et al., in press). The calculated values of indirect band gap energy are tabulated in Tables 1 and 2 for PU and PEEK, respectively. The calculated values show that the band gap energy of polymers is decreasing with increase of ion fluence. The decrease is attributed to the conjugated bond formation (Sharma et al., 2007; Kumar et al., 2008; Dhillon et al., 2013). Increase in number of carbon atoms per conjugation length (N) are observed due to shift of the absorption edge towards longer wavelength (Kumar et al., 2011). The value of N was calculated by using following equation as reporetd by Fink et al., 1995. N ¼ ð34:3=Eg Þ2
ð2Þ
The values of N are calculated for pristine as well as irradiated samples and tabulated in Tables 1 and 2 for PU and PEEK, respectively. The value of N is increasing at higher fluences (5 1012 ions/cm2 and 1.2 1013 ions/cm2). The thermal fluctuations in the band gap energy were estimated by calculating Urbach's energy (Eu) (Skettrup, 1978; Urbach, 1953) using following equation. αðνÞ ¼ αo expðhν=Eu Þ
ð3Þ
The reciprocal of the slopes of the linear portion in the lower photon energy region of the plot of ln(α) as a function of photon energy (hν) gives the value of Urbach's energy (Eu) (Migahed and Zidan, 2006). The calculated values of Urback's energy for pristine
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and irradiated samples of PU and PEEK polymers are given in Tables 1 and 2, respectively. The value remained almost constant in case of PEEK for pristine and irradiated samples. However, it increased abruptly in case of PU at higher fluences. The main reason for the increase in value of Eu at higher fluences is the shift of the band gap energy due to temperature-dependent self energies of the electrons and holes interacting with the phonons (Skettrup, 1978; Dow and Redfield, 1978). It may be the reason for larger shift of absorption spectra of PU at fluences of 5 1012 and 1.2 1013 ions/cm2 towards visible region as compared to other fluences.
(Stuart, 2004) is described as: N–H wagging (649 and 911 cm 1), NH2 wagging and twisting (820 and 848 cm 1), N C stretching (2196 and 2292 cm 1), N–CH2 stretching (2760 cm 1), N–H stretching (3040 cm 1), amide symmetric stretching (3120 and 3193 cm 1) vibrations. The respective positions of functional groups are also indicated in Fig. 5. The infrared spectrum of PEEK conforms to the spectrum reported by Hnatowicz et al., 2008 in absorbance ATR mode. The FTIR spectra of PEEK polymer show minor changes for irradiated samples. There is small decrease in absorbance (increase in percentage transmittance) for bands at 1415 and 1500 cm 1. These wave numbers correspond to C–C stretching vibration.
3.3. FTIR studies 4. Conclusions The infrared spectra corresponding to pristine and irradiated samples of PU and PEEK are shown in Figs. 5 and 6, respectively. The FTIR spectrum of pristine PU conforms to the spectrum reported by Shibata and Ito (2003). The IR bands for irradiated samples show decrease in absorbance as compared to pristine sample. However, the NH (3330 cm 1) and C ¼ O (1725 cm 1) bonds do not show any change in peak intensity (Ravat et al., 2000). The decrease in absorbance for various functional groups
Polyurethane (PU) and polyether-ether ketone (PEEK) polymer films were investigated for structural, optical and chemical properties after irradiation with 50 MeV lithium ions. The amorphous nature of both the polymers decreased after irradiation. It was due to alignment of the polymeric chains in a regular pattern due to ion-solid interaction processes. The shift of the UV–vis spectra towards higher wavelength regime decreased the band gap energy of irradiated polymers. It was due to the formation of conjugated system of bonds. No new band formation was observed in the infrared study of irradiated samples.
Acknowledgments The authors are thankful to Dr. D. C. Aggarwal, Dr. F. Singh, Dr. P. Kulriya and staff of inter university accelerator centre, New Delhi, India for providing help during irradiation and characterizations. Financial assistance provided by Council of Scientific & Industrial Research (CSIR), Govt. of India to Mr. Paramjit Singh (to carry out this research work) as Senior Research Fellow (SRF) [Sanction No. 09/806(0026)/2012-EMR-I] is gratefully acknowledged. References
Fig. 5. FTIR spectra of pristine and Li3 þ ion irradiated PU polymer, here a,b,c and d stand for pristine, 5 1010, 2 1011 and 5 1012 ions/cm2, respectively.
Fig. 6. FTIR spectra of pristine and Li3 þ ion irradiated PEEK polymer, here (a), (b), (c), (d) and (e) stand for pristine, 5 1010, 2 1011, 5 1012 and 1.2 1013 ions/cm2, respectively.
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Contents lists available at ScienceDirect
Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Modifications of structural, optical and chemical properties of Li3 þ irradiated polyurethane and polyetheretherketone Paramjit Singh a, S. Asad Ali b, Rajesh Kumar a,n a b
University School of Basic & Applied Sciences, Guru Gobind Singh Indraprastha University, New Delhi-110078, India Department of Applied Physics, Aligarh Muslim University, Aligarh-202002, India
H I G H L I G H T S
We irradiated PU and PEEK with swift heavy ions at different fluences. Structural effects, optical and chemical changes after irradiation were examined by XRD, UV–visible and FTIR spectroscopy, respectively. Decrease in amorphous nature and band gap energy is observed with increase in fluence. Increase in average crystallite size and variation in Urback's energy is observed with increase of fluence. The modifications caused by SHI in various chemical bonds are also discussed.
art ic l e i nf o
a b s t r a c t
Article history: Received 11 August 2013 Accepted 26 September 2013 Available online 16 October 2013
Thin membranes of PU and PEEK polymers were irradiated with 50 MeV lithium ions for modifications in structural, optical and chemical properties. The XRD spectra indicated the alignment of polymeric chains in a regular pattern and hence there was decrease in the amorphous character of the irradiated polymers. The optical absorption of all irradiated samples shifted towards the visible region of the spectrum. It was attributed to the generation of a conjugated system of bonds which lowered the band gap energy of irradiated samples. The FTIR spectra obtained after irradiation exhibited decrease in absorbance for various bands in case of PU whereas PEEK showed minor changes in its chemical properties. The other results were discussed from the calculated parameters such as crystallite size, Urbach's energy and number of carbon hexagon rings per conjugation length from the analyses of the XRD and UV–visible data. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Polyurethane Polyether-etherketone Ion beam modification X-ray diffraction UV–visible FTIR
1. Introduction Ion beam modification has shown great potential for improving the structural, free volume, optical as well as surface and chemical properties of polymeric materials (Singh and Kumar, 2013; Singh et al., 2013; Kumar and Singh, 2013; Ali et al., 2013; Abdul-Kader et al., 2012). As a result, the past few years have seen many advances in the field of ion beam modification of polymeric materials in terms of applying conventional ion beam techniques to various types of polymers for industrial and scientific applications. The polymers under study are polyurethane (PU) and polyether-ether ketone (PEEK) used for biomedical and industrial applications. Polyurethanes are one of the most bio and blood-compatible materials. These materials play a major role in the development
n
Corresponding author. Tel.: þ 91 9718876101. E-mail address: [email protected] (R. Kumar).
0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.09.014
of many medical devices such as catheters and artificial hearts (Zdrahala, 1999). Properties such as durability, elasticity, fatigue resistance, compliance and acceptance or tolerance in the body during the healing became often associated with polyurethanes (Zdrahala, 1999). In addition to it, polyurethanes are used in industrial and scientific applications such as coatings, foam and nanocomposites (Chattopadhyay and Raju, 2007). The polyetherether ketone polymers are also used for biomedical research and fuel cell applications. The ion beam modification played a crucial role in the advancement of properties of both of these polymers. The surface of PEEK coated with titanium layer using electron beam deposition method was used in order to enhance its biocompatibility and adhesion to bone tissue by Han et al. (2010). The use of PEEK as a biomaterial for spinal applications has been reported in the literature (Toth et al., 2006). Radiationinduced crosslinking of PEEK film has been reported by Chen et al. (2010) for application in fuel cell. The crosslinking was carried out by 40 MGy dose of electron-beam irradiation on PEEK to prepare an aromatic hydrocarbon polymer electrolyte membrane (PEM).
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Similar results for the preparation of fuel cells using radiation grafted PEEK had been reported by Manea and Mulder (2002). It is important to study the structural, optical, surface and chemical properties of these polymers after ion beam irradiation to understand the extent of modifications caused by the irradiated ion. The PU and PEEK polymers were irradiated previously with low energy (keV to few MeV) ions for the study of certain properties by various groups. Some important results of ion beam modification on these polymers are summarized below. PU was irradiated with electron beam (5–200 kGy dose) for chemical, thermal, structural and surface analysis by Murray et al., 2013. They demonstrated that cross- linking was emerged in the 200 kGy dose exposed polymer sample. Wong et al., 2006 studied the surface morphology and chemical structure of polyurethane fibers by implantation of 60 and 100 keV oxygen ions. They reported the formation of new chemical functional groups [C–(C ¼O)–C and C–N–C] due to near-surface chemical modifications by ion implantations. The secondary ion emission under MeV ion irradiation on PEEK as a function of the degree of crystallinity had been reported earlier (Nsouli et al., 1997). The structural and compositional changes of 40 keV Ni þ and Co þ implanted and subsequently annealed PEEK polymers were characterized by RBS, UV–vis and XPS by Mackova et al., 2013a, 2013b, 2009. They reported that the properties of implanted polymers strongly depend on the implantation ion fluence and on the properties of the implanted atoms. Their earlier studies of He þ (1.76 MeV) irradiation on PEEK showed similar results (Mackova et al., 2005). Depth profiling of implanted low energy ions in PEEK had also been reported in literature (Malinsky et al., 2012; Hnatowicz et al., 2008; Vacík et al., 2007). The early work in the discussed literature was mainly related to structural and compositional study of PU and PEEK by low energy ions and electron implantations. As far as we know, there have been very few examples of ion induced structural, optical and chemical modifications of these engineering plastic films. The present investigation is focused towards the modifications of structural, optical and chemical properties of the samples of PU and PEEK by lithium ions irradiation. The properties such as crystallite size, band gap energy, Urbach's energy and number of carbon hexagon rings per conjugation length are calculated from the obtained data of XRD and UV–vis.
2. Experimental methods
2.2. Irradiation The samples of size 1.5 cm 1.5 cm were mounted on a vacuum shielded vertical sliding ladder and exposed to 50 MeV Li3 þ beam from 15 UD pelletron accelerator at inter university accelerator centre (IUAC), New Delhi, India in the general purpose scattering chamber (GPSC) under high vacuum ( 7 10 6 Torr). Fluences were taken 5 1010, 2 1011, 5 1012 and 1.2 1013 ions/cm2. The beam current ( 0.5 pnA) was kept low to suppress thermal decomposition. The pristine and irradiated samples were characterized by X-ray diffraction (XRD), UV–visible (UV–vis) and Fourier transform Infrared (FTIR) spectroscopy techniques for the study of structural, optical and chemical modifications. The XRD studies were carried out using Bruker AXS system by Cu-Kα radiation (1.54 Å) for range of Bragg's angle 2θ (51 oθo501). UV–vis studies were carried out between 200 and 800 nm range using U-3300, Hitachi system. The FTIR studies were carried out using Thermo Nicolet Nexus 670 FTIR in the range 400–4000 cm 1. The projected range and electronic energy loss (Se) of 50 MeV lithium ion in both the polymers was calculated by SRIM code (Ziegler, 2010). The Se values are 5.14 and 6.83 eV/Å and the projected range values are 551 and 415.7 mm for PU and PEEK, respectively. The comparison of projected range and sample thicknesses of these polymers makes the ion implantation very less probable.
3. Results and discussions 3.1. X-ray diffraction studies The diffraction patterns of pristine and irradiated PU and PEEK polymer samples are shown in Figs. 1 and 2, respectively. The broad peaks occuring at 2θ 19.71 and 20.131 (for PU and PEEK polymers, respectively) did not shift after irradiation, this indicated that lattice parameters did not change. The peak width decreased and peak intensity increased with increase of ion fluence signifying the decrease in the amorphous character after irradiation (Kumar et al., in press). The average crystallite size (L) was calculated using Scherrer formula (Singh and Kumar, 2013) using following equation. L ¼ kλ=ðb cos θÞ
ð1Þ
here b is the full width at half maximum (FWHM) of the peak (in radians). λ is the wavelength of the X-ray used (1.54 Å in our case for Cu-Kα radiation). θ is the angle which is calculated by taking 1/2
2.1. Membrane preparation The membranes of PU of thickness 70–80 mm were prepared by solution cast method. The details and experimental methods of solution cast method were used similar to the method already reported in the literature (Vijay, 2003; Vijay et al., 2002). Polyurethane was purchased from Indian Research Design, New Delhi in the form of small beads. The PU beads were dissolved in tetrahydrofurane (THF) with constant stirring over a magnetic stirrer. The solution was then put into flat-bottomed Petri-dish floating on mercury to obtain uniform thickness of the film. The solvent was allowed to evaporate for 24 h. The films so obtained were peeled off and dried in vacuum at 60 1C for next 24 h. The PEEK films of same thickness were purchased from Sigma Aldrich, India in the form of sheets and used as received. The films of both the polymers were cut into 1.5 cm 1.5 cm size for irradiation.
Fig. 1. X-ray diffraction spectra of pristine and Li3 þ ion irradiated PU polymer.
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Fig. 3. UV–visible spectra of pristine and Li3 þ ion irradiated PU polymer.
Fig. 2. X-ray diffraction spectra of pristine and Li3 þ ion irradiated PEEK polymer.
Table 1 Calculated values of average crystallite size (L), Indirect band gap energy (Eg), number of carbon atoms per conjugation length (N) and Urback's energy (Eu) for pristine and irradiated samples of PU polymer. Fluence (ions/cm2)
Pristine 5 1010 2 1011 5 1012 1.2 1013
PU L (Å)
Eg (eV)
N
Eu (eV)
11.30 7 0.11 11.707 0.11 11.84 7 0.12 11.99 7 0.12 12.36 7 0.12
4.007 0.01 3.99 7 0.01 3.977 0.01 3.94 7 0.01 3.92 7 0.01
73 74 74 75 76
0.10 0.10 0.10 0.45 0.52 Fig. 4. UV–visible spectra of pristine and Li3 þ ion irradiated PEEK polymer.
Table 2 Calculated values of average crystallite size (L), Indirect band gap energy (Eg), number of carbon atoms per conjugation length (N) and Urback's energy (Eu) for pristine and irradiated samples of PEEK polymer. Fluence (ions/cm2)
Pristine 5 1010 2 1011 5 1012 1.2 1013
PEEK L (Å)
Eg (eV)
N
Eu (eV)
10.46 70.10 10.647 0.10 10.85 70.10 10.98 70.10 11.63 70.11
3.08 7 0.01 3.107 0.01 3.0770.01 3.067 0.01 3.05 7 0.01
124 122 125 125 126
0.15 0.12 0.11 0.10 0.11
of 2θ value of peak of diffraction as per given in Figs. 1 and 2. k is a constant of proportionality (Scherrer constant), its value is 0.9 (Sharma et al., 2007). The calculated values are given in Tables 1 and 2 for PU and PEEK, respectively. The values of L increase in irradiated samples of both the polymers. 3.2. UV–visible studies UV–visible spectra of pristine and irradiated samples are shown in Figs. 3 and 4 for PU and PEEK, respectively. The optical absorption edges of both the polymer samples shifted towards the higher wavelength regime after ion irradiation. This shift increased progressively with increase of ion fluence; it indicated a growing concentration of conjugated double bonds in the polymer and increase in conjugation length (Mackova et al., 2009; 2005). Similar results were observed by Mackova et al., 2009 in their study on PEEK by implantation of 40 keV Ni þ ions.
The optical band gap energy was calculated using Tauc's relation (Tauc et al., 1966) for indirect optical transitions. The details of the equation used has already been reported in our recently published article (Singh et al., in press). The calculated values of indirect band gap energy are tabulated in Tables 1 and 2 for PU and PEEK, respectively. The calculated values show that the band gap energy of polymers is decreasing with increase of ion fluence. The decrease is attributed to the conjugated bond formation (Sharma et al., 2007; Kumar et al., 2008; Dhillon et al., 2013). Increase in number of carbon atoms per conjugation length (N) are observed due to shift of the absorption edge towards longer wavelength (Kumar et al., 2011). The value of N was calculated by using following equation as reporetd by Fink et al., 1995. N ¼ ð34:3=Eg Þ2
ð2Þ
The values of N are calculated for pristine as well as irradiated samples and tabulated in Tables 1 and 2 for PU and PEEK, respectively. The value of N is increasing at higher fluences (5 1012 ions/cm2 and 1.2 1013 ions/cm2). The thermal fluctuations in the band gap energy were estimated by calculating Urbach's energy (Eu) (Skettrup, 1978; Urbach, 1953) using following equation. αðνÞ ¼ αo expðhν=Eu Þ
ð3Þ
The reciprocal of the slopes of the linear portion in the lower photon energy region of the plot of ln(α) as a function of photon energy (hν) gives the value of Urbach's energy (Eu) (Migahed and Zidan, 2006). The calculated values of Urback's energy for pristine
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and irradiated samples of PU and PEEK polymers are given in Tables 1 and 2, respectively. The value remained almost constant in case of PEEK for pristine and irradiated samples. However, it increased abruptly in case of PU at higher fluences. The main reason for the increase in value of Eu at higher fluences is the shift of the band gap energy due to temperature-dependent self energies of the electrons and holes interacting with the phonons (Skettrup, 1978; Dow and Redfield, 1978). It may be the reason for larger shift of absorption spectra of PU at fluences of 5 1012 and 1.2 1013 ions/cm2 towards visible region as compared to other fluences.
(Stuart, 2004) is described as: N–H wagging (649 and 911 cm 1), NH2 wagging and twisting (820 and 848 cm 1), N C stretching (2196 and 2292 cm 1), N–CH2 stretching (2760 cm 1), N–H stretching (3040 cm 1), amide symmetric stretching (3120 and 3193 cm 1) vibrations. The respective positions of functional groups are also indicated in Fig. 5. The infrared spectrum of PEEK conforms to the spectrum reported by Hnatowicz et al., 2008 in absorbance ATR mode. The FTIR spectra of PEEK polymer show minor changes for irradiated samples. There is small decrease in absorbance (increase in percentage transmittance) for bands at 1415 and 1500 cm 1. These wave numbers correspond to C–C stretching vibration.
3.3. FTIR studies 4. Conclusions The infrared spectra corresponding to pristine and irradiated samples of PU and PEEK are shown in Figs. 5 and 6, respectively. The FTIR spectrum of pristine PU conforms to the spectrum reported by Shibata and Ito (2003). The IR bands for irradiated samples show decrease in absorbance as compared to pristine sample. However, the NH (3330 cm 1) and C ¼ O (1725 cm 1) bonds do not show any change in peak intensity (Ravat et al., 2000). The decrease in absorbance for various functional groups
Polyurethane (PU) and polyether-ether ketone (PEEK) polymer films were investigated for structural, optical and chemical properties after irradiation with 50 MeV lithium ions. The amorphous nature of both the polymers decreased after irradiation. It was due to alignment of the polymeric chains in a regular pattern due to ion-solid interaction processes. The shift of the UV–vis spectra towards higher wavelength regime decreased the band gap energy of irradiated polymers. It was due to the formation of conjugated system of bonds. No new band formation was observed in the infrared study of irradiated samples.
Acknowledgments The authors are thankful to Dr. D. C. Aggarwal, Dr. F. Singh, Dr. P. Kulriya and staff of inter university accelerator centre, New Delhi, India for providing help during irradiation and characterizations. Financial assistance provided by Council of Scientific & Industrial Research (CSIR), Govt. of India to Mr. Paramjit Singh (to carry out this research work) as Senior Research Fellow (SRF) [Sanction No. 09/806(0026)/2012-EMR-I] is gratefully acknowledged. References
Fig. 5. FTIR spectra of pristine and Li3 þ ion irradiated PU polymer, here a,b,c and d stand for pristine, 5 1010, 2 1011 and 5 1012 ions/cm2, respectively.
Fig. 6. FTIR spectra of pristine and Li3 þ ion irradiated PEEK polymer, here (a), (b), (c), (d) and (e) stand for pristine, 5 1010, 2 1011, 5 1012 and 1.2 1013 ions/cm2, respectively.
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