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The effect of gamma irradiation and shelf aging in air on the oxidation of ultra-high molecular weight polyethylene
Applied Surface Science 252 (2006) 3316–3322 www.elsevier.com/locate/apsusc
The effect of gamma irradiation and shelf aging in air on the oxidation of ultra-high molecular weight polyethylene M.A. Al-Ma’adeed a, I.Y. Al-Qaradawi a, N. Madi a,*, N.J. Al-Thani b b
a Physics Department, College of Science, University of Qatar, Doha, Qatar Scientific and Applied Research Center (SARC), University of Qatar, Doha, Qatar
Available online 10 October 2005
Abstract This study has investigated the effect of shelf aging, for up to one year in air, on the properties of gamma-irradiated ultra-high molecular weight polyethylene (UHMWPE). A variety of techniques were used to characterize the properties of treated samples. Differential scanning calorimetery (DSC) was used to characterize the morphology. The extent of cross-linking in a polymer network was detected by swelling measurements. The durometer hardness test was used to measure the relative hardness of this material, and changes in density were also measured. Results from all these measurements were combined to explain the changes in the microstructure of the aged, irradiated UHMWPE. This study shows that crystallinity is increased with radiation dose and with aging due to chain scission, which leads to a reduction in the molecular weight of the material. This allows the chains to rearrange to form crystalline regions. Positron annihilation lifetime spectroscopy confirms these conclusions. Fractional free volumes have been deduced from lifetime parameters, which correlate with the data obtained by the other techniques. # 2005 Elsevier B.V. All rights reserved. Keywords: Free volume; Cross-linking; Crystallinity; UHMWPE; Oxidation; Gamma irradiation
1. Introduction Ultra-high molecular weight polyethylene (UHMWPE) is a high performance polymer having very high impact resistance, low wear rate, low friction, biocompatibility and it is sterilizable [1]. * Corresponding author. E-mail address: [email protected] (N. Madi).
Therefore, it is currently the standard articulating surface for use in total hip and knee replacement prostheses. These prostheses are commonly sterilized by gamma irradiation. Gamma irradiation and shelf aging create reactive and long lived free radicals in UHMWPE, greatly increasing its susceptibility to oxidative degradation. Oxidative degradation of polyethylene components promotes osteolysis and subsequent loosening of total joint replacements.
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.08.077
M.A. Al-Ma’adeed et al. / Applied Surface Science 252 (2006) 3316–3322
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Understanding the effects of sterilization method and shelf aging on the oxidation of UHMWPE is crucial in developing orthopedic total joint systems with longer outcomes. Gamma irradiation in air causes cross-linking but it will also lead to diffusion of oxygen. Increasing crosslinking in this material improves wear resistance and mechanical properties [2]. Free hydrogen atoms (free radicals) combine with oxygen and cut the long chains of UHMWPE, causing scission. Many models have been suggested to explain the oxidation of UHMWPE following irradiation [3], where oxygen enters the amorphous region in the material and reacts with the free radicals, increasing amorphous regions and creating cracks which will in the long term lead to a reduction in the lifetime of the polymer. The positron annihilation technique has been widely used for studying the micro- and nanostructure of polymers during the last two decades [4,5]. The method is based on the well-established fact that ortho-positronium (o-Ps), a bound triplet state of an electron and a positron, tends to be localized in regions of lower electron density which are interpreted as ‘‘free volume holes’’. Many studies of the effect of gamma irradiation on polymers have been carried out [6–9]. Understanding the effects of sterilization method and shelf aging on the oxidation of UHMWPE is crucial in developing orthopedic total joint systems with longer outcomes. The aim of this study is to investigate the influence of gamma irradiation in air and post-irradiation aging in air by assessment of morphology, gel content, hardness, change in size and amount of free volume as well as density change.
penetrate the 1 mm thick UHMWPE samples. After irradiation with various gamma doses, the samples were measured immediately after irradiation and then one year later to study the effect of gamma dose as well as the effect of shelf aging in air on the structure of UHMWPE.
2. Experimental
2.3. Differential scanning calorimetry (DSC)
The samples used were from Goodfellow (UK) in the shape of sheets of about 1 mm and 1 cm thick, and molecular weight of 12,000 g/mol and density r = 0.93 g/cm3. The samples were irradiated in air at room temperature at the University of Qatar using a 60Co high dose research gamma irradiator from MDS Nordion International. The photons in this case have energies of 1.173 and 1.332 MeV, which easily
A small piece of UHMWPE (10–20 mg) was sealed in an aluminum pan and then heated in a temperature sweep from ambient to 150 8C at a rate of 10 8C/min in the DSC (Perkin-Elmer, model DSC7 Pyris). The total heat of melting DH (the area under the endotherm) was determined and, knowing the total heat of fusion of 100% crystalline UHMWPE (DHf = 293.6 J/g), the percentage crystallinity was calculated as 100 DH/DHf.
2.1. Positron annihilation lifetime spectroscopy (PALS) For the PALS study, two identical 1 mm thick samples were used to sandwich the 22Na source. Using the fast–fast coincidence technique, lifetime histograms were recorded. The system resolution was about 240 ps. 2.2. Measurements of cross-link density Swelling measurement is a good way to detect the extent of cross-linking in a polymer network. Polymers absorb some of the solvent and swell. The swollen gel can be characterized as an elastic, rather than a viscous, solution. Irradiated samples of a size of 1 cm 1 cm 1 cm were placed in boiling xylene for a time period of 12 h. Samples then were removed by 150 mm sieves. Duplicate tests were made for each dose. The swell ratio (SR) is then SR ¼
Wg Wd Wd
r ro
þ 1;
(1)
where Wg is the weight of the swollen gel after immersion, Wd the weight of dried gel, r the density of the solvent and ro is the density of polyethylene (here, r/ro = 1.08).
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2.4. Density measurements The determination of the density of polyethylene is an excellent measure of its crystallinity, which is related to many of its physical properties. The Automatic Densimeter was of the ‘‘displacement’’ type which determined the specimen density through measuring its mass (weight) first in air, and then again while immersed in water. Barometric pressure and changes in relative humidity are small affects and were thus ignored. 2.5. Durometer hardness The durometer hardness test was used to measure the relative hardness of this material. The test method is based on the penetration of a specified indenter forced into the material under specified conditions. The material was placed on a hard, flat surface and the pressure foot of the instrument was pressed onto the sample. The durometer hardness was read within one second after the pressure foot was in firm contact with the sample. A Shore hardness tester (model HHP2001) manufactured by Bareiss, was used.
3. Results and discussion 3.1. Free volume The PATFIT program [10] was used to extract the various lifetime components and their corresponding intensities from the PALS data. Four components were resolved corresponding to bulk annihilation, parapositronium ( p-Ps) formation and two types of orthopositronium (o-Ps). The important aspect of the results is the existence of o-Ps lifetime components, which implies that free volume is present in the samples [4,5,11]. The relation between the positron parameters and gamma radiation dose was compared with the parameters determined using other methods, such as crystallinity. The total intensity of o-Ps formation is obtained by adding the intensities of the two o-Ps components I3 and I4, while the mean o-Ps lifetime tm is calculated by determining the weighted mean of the two o-Ps lifetime components t3 and t4. By assuming a simple quantum mechanical model where an o-Ps atom resides in an infinite spherical
potential of radius Ro with an electron layer of thickness DR, the following analytical relation between the o-Ps lifetime t and the average radius R (Ro DR) of the free volume voids is obtained [12]: 1 R 1 R ¼2 1 þ sin 2p (2) t Ro 2p Ro The value of DR was found to be 0.166 nm [13]. Therefore, the average radius R of the free volume voids in polymers can be calculated from the measured o-Ps lifetime t and the average volume of the free volume voids: 4 v ¼ pR3 3
(3)
Fig. 1 shows the change in tm with gamma irradiation dose in the as-irradiated and aged samples. It can be seen that there is a large drop in the size of the free volume with increased dose. The trend is similar in both as-irradiated and aged samples but the values are generally higher in the aged samples. The reduction in the mean o-Ps lifetime, and therefore the size of the free volume, is a result of the production of free radicals by irradiation. This causes scission and hence an increase of electron density which decreases the lifetime. Information about the concentration of the free volume can be found from the intensity of the lifetime components. Fig. 2 shows the effect of gamma irradiation dose for as-irradiated and aged samples, on the o-Ps intensity. It can be noticed that there is also a
Fig. 1. The change in average o-Ps lifetime with gamma irradiation dose in as-irradiated and aged samples (lines drawn as eye-guide only).
M.A. Al-Ma’adeed et al. / Applied Surface Science 252 (2006) 3316–3322
Fig. 2. The change in o-Ps intensity with gamma irradiation dose in as-irradiated and aged samples (lines drawn as eye-guide only).
sharp drop of the free volume concentration up to a radiation dose of about 50 kGy after which the amount of free volume is not greatly affected by irradiation. The amount of free volume is however, reduced by aging. The fractional free volume f can be expressed as a product of the free volume v and positronium intensity I [13]: f ¼ cvI
(4)
where c is a constant which was estimated empirically from the conventional free volume theory.
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Fig. 3 shows the effect of gamma irradiation dose, for as-irradiated and aged samples, on the fraction of the free volume in the sample. We notice that the change in this fraction with irradiation dose and aging follows a trend that is more or less similar to the intensity of o-Ps. This behavior may be attributed to reduction of free volume as a result of irradiation, which enhances cross-linking and/or crystallinity, depending on irradiation dose and polymer structure. In the aged samples, more oxidative degradation occurs, and hence a smaller positron lifetime is expected. However, the higher lifetime obtained after aging can be explained if we understand that one of the main effects of gamma irradiation is to induce polymer oxidation [14], which shortens the distance between crystalline regions in the degraded zone. This effect restricts the chain disentanglement of tie molecules and therefore decreases the voids size as irradiation proceeds. After aging in air oxygen enters, causing the lamella to link and the molecular ties to break, which in turn allows the molecules to rearrange themselves in a more ordered fashion. According to this model, small voids join together, reducing their number while increasing the void size, which appears as an increase in the o-Ps lifetime. This trend strongly correlates with crystallinity versus dose and density versus dose changes (see Fig. 5 and Table 1, respectively). 3.2. DSC measurements DSC thermograms from 100 kGy as-irradiated UHMWPE exhibited one distinct peak. In addition, a small overlapping shoulder region was present after the first heating of irradiated sample (Fig. 4a). The shoulder disappeared upon cooling and was no longer present during the second heating (Fig. 4b). The single Table 1 Density of as-irradiated samples and post-irradiation aged samples in air vs. irradiation dose Density (g/cm3)
Fig. 3. The change in fractional free volume with gamma irradiation dose in as-irradiated and aged samples (lines drawn as eye-guide only, error bars are within the size of the points).
Dose (kGy)
After one year
As-irradiated
0.89 0.9432 0.9456 0.9610 0.9568
0.8810 0.9025 0.9270 0.9446 0.9628
0 10 25 200 400
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Fig. 5. Crystallinity vs. irradiation dose. The solid and dotted lines are first-order regression of corresponding experimental data.
Fig. 4. (a) First heating of 100 kGy as-irradiated UHMWPE and (b) second heating of the same sample.
distinct peak of the DSC trace for the irradiated sample indicates that the material contains one population of crystalline lamellae. The small shoulder region present in the DSC trace is indicative of the presence of new crystalline forms within the amorphous region. It is suggested that the second population of crystallites could be a consequence of the radiation induced oxidation process. The finding that this shoulder region disappears upon second heating indicates that the small crystalline regions are exterminated upon reheating. The post-irradiated aged samples in air exhibit the same trend. The melting crystallization parameters for both samples are presented in Fig. 5. Evidently, the storage in air of irradiated UHMWPE led to much more increase in crystallinity compared to as-irradiated samples. Because of the higher oxygen permeability, oxygen can enter the polymer material more readily, thereby causing greater oxidation in the presence of
irradiation induced free radicals. Oxygen will react with free radicals in the polymer causing further chain scission. As a consequence, the crystallinity of UHMWPE increases as the scission of tie molecules led to reduced molecular weight, which in turn permits polymer chain in non-crystalline regions of the polymer to fold and crystallize, as well as to allow increasing perfection of existing folded-chain crystallites. Oxidation of polymers induced by gamma irradiation has been investigated by many authors [14]. Polymer oxidation can be characterized by oxygen uptake techniques. Turos et al. [15] developed a new Rutherford backscattering (RBS) set up to measure the depth distribution of oxygen uptake of ion-implanted polymers. For comparison, measurements were conducted both to as-irradiated samples and aged samples. Using this technique, it is observed an increase in oxygen uptake with post-irradiation aging in air as already confirmed by crystallinity, density and hardness measurements. A strong correlation between increase in polymer crystallinity and subsequent increase in melting point was also observed in DSC measurements. 3.3. Density measurements The average density of non-aged and shelf-aged UHMWPE samples following gamma irradiation is shown in Table 1. The density was found to increase significantly for both samples as the irradiation dose
M.A. Al-Ma’adeed et al. / Applied Surface Science 252 (2006) 3316–3322
increased. In addition, for one-year shelf-aged samples density values were generally higher than for the un-aged gamma-irradiated samples; this is due to the increase in oxidation and the greater mass of oxygen compared to hydrogen and due to scission [16]. The increased density caused by gamma irradiation would seem to correlate with the percentage crystallinity. This indicates that the increased density caused by gamma irradiation and aging is predominantly due to increased crystallinity (crystalline PE being of higher density than the amorphous phase). The increased crystallinity is probably arises from chain scission caused by the irradiation and oxidation process lowering the molecular weight of the PE. Referring to Fig. 1, Fig. 5, and Table 1, it is clear that the changes in percentage crystallinity and density with irradiation dose strongly correlate with mean oPs lifetime versus dose. The explanation is that, as irradiation dose increases, crystallization is obtained by nucleation of new lamella [17] resulting in a decrease in the size of voids (positron trapping centers) and, consequently, a decrease in the mean lifetime of o-Ps as shown in Fig. 1. 3.4. Measurements of cross-linking density It has been reported that polyethylene cross-links when exposed to high-energy irradiation [18]. These cross-links occur preferentially in the amorphous region of the bulk material. Cross-linking density, as measured by the gel fraction measurements for as-
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Fig. 7. Change in Shore hardness with irradiation dose for asirradiated and aged UHMWPE samples (lines are drawn as eyeguide only).
irradiated and shelf-aged irradiated UHMWPE is shown in Fig. 6. The significant increase in gel content after irradiation indicates the cross-linking (which takes place mostly in the interfacial regions of lamellae at or near fold surfaces) is very sensitive to small doses of radiation; more than 80% of gel content is reached at low irradiation dose, and crosslinking does not change after that. Shelf aging causes a slight increase in cross-linking, suggesting that most of the cross-linking reactions occur during and/or after irradiation. 3.5. Measurements of bulk hardness Fig. 7 shows the relation between the Shore hardness and dose. Hardness – the resistance of a material to deformation – is a very important mechanical property. Hardness is purely a relative term and should not be confused with wear and resistance of plastic materials. An increase in hardness is due to the increase in crystallinity and the breakage of the long chains of the polymer, increasing brittleness and weakening the material.
4. Conclusions
Fig. 6. The change in gel percent with gamma dose for as-irradiated and aged UHMWPE (lines are drawn as eye-guide only).
Changes in a variety of experimentally determined physical parameters show that they are dependent upon the crystallinity, which itself is a function of radiation dose and time. Oxygen which enters the
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material and free radicals formed by irradiation cause changes in the structure of the polymer, which continues to oxidize with time. Reaction of oxygen with free radicals causes chain scission, and recrystallization occurs, increasing crystallinity, which is higher with aging in an air environment (rich in oxygen). An increase in crystallinity causes an increase in hardness and a consequent change in mechanical properties, which make the polymer inappropriate for use in implants. Free radicals produced by the gamma irradiation process can either react with oxygen, causing chain scission, or react internally, leading to cross-linking. Cross-linking is the predominant feature in low doses, where positronium formation and free volume are reduced. More advanced and controlled methods are needed to sterilize the polymer, depending on its final use – e.g., in acetabular cups or tibial plateaus – and more caution must be taken during storage of the polymer to reduce oxidation and subsequent degradation of the implant.
References [1] S.M. Kurtz, The UHMWPE Handbook, Elsevier, 2004, p. 37.
[2] O.K. Muratoglu, C.R. Bragdon, D.O. O’Connor, M. Jasty, W.H. Harris, R. Gul, F. McGarry, Biomaterials 20 (16) (1999) 1463–1470. [3] M. Goldman, R. Gronsky, G.G. Long, L. Pruitt, Polym. Degrad. Stab. 62 (1998) 104. [4] Y.C. Jean, Mater. Sci. Forum 59 (1995) 175. [5] Y.C. Jean, P.E. Mallon, D.M. Schrader (Eds.), Principles and Applications of Positron and Positronium Chemistry, World Scientific Publication, Singapore, 2003. [6] A. Badia, G. Duplaˆtre, Radiat. Phys. Chem. 54 (1999) 151. [7] T. Hirade, T. Kumada, Radiat. Phys. Chem. 60 (2001) 541. [8] R.A. Pethrick, Polym. Sci. 22 (1997) 1. [9] T. Suzuki, Y. Oki, M. Numajiri, T. Miura, K. Kondo, Y. Ito, J. Polym. Sci. B 30 (1997) 517. [10] P. Kirkegaard, M. Eldrup, O.E. Mogensen, N.J. Pedersen, Comput. Phys. Commun. 23 (1981) 307. [11] J. Liu, Q. Deng, H. Shi, Y.C. Jean, Multidimensional Spectroscopy of Polymers, ACS Symposium Series, vol. 598, 1995, p. 458. [12] S.J. Tao, J. Chem. Phys. 56 (1972) 5400. [13] Y.C. Jean, Microchem. J. 42 (1990) 72. [14] M. Zenkiewicz, M. Rauchfleisz, J. Czuprynska, Radiat. Phys. Chem. 68 (5) (2003) 799–809. [15] A. Turos, A.M. Abdul-Kader, D. Grambole, J. Jagielski, A. Pia˛tkowska, N.K. Madi, M. Al-Maadeed, Nucl. Instrum. Meth. B, submitted for publication. [16] M. Goldman, R. Grosky, R. Ranganthan, L. Pruitt, Polymer 37 (1996) 2909. [17] A. Bellare, M. Spector, R.E. Cohen, Symposium on Characterisation and Properties of Ultra High Molecular Weight Polyethylene, New Orleans, LA, USA, November 1996. [18] G. Lewis, Biomaterials 22 (4) (2001) 371–401.
The effect of gamma irradiation and shelf aging in air on the oxidation of ultra-high molecular weight polyethylene M.A. Al-Ma’adeed a, I.Y. Al-Qaradawi a, N. Madi a,*, N.J. Al-Thani b b
a Physics Department, College of Science, University of Qatar, Doha, Qatar Scientific and Applied Research Center (SARC), University of Qatar, Doha, Qatar
Available online 10 October 2005
Abstract This study has investigated the effect of shelf aging, for up to one year in air, on the properties of gamma-irradiated ultra-high molecular weight polyethylene (UHMWPE). A variety of techniques were used to characterize the properties of treated samples. Differential scanning calorimetery (DSC) was used to characterize the morphology. The extent of cross-linking in a polymer network was detected by swelling measurements. The durometer hardness test was used to measure the relative hardness of this material, and changes in density were also measured. Results from all these measurements were combined to explain the changes in the microstructure of the aged, irradiated UHMWPE. This study shows that crystallinity is increased with radiation dose and with aging due to chain scission, which leads to a reduction in the molecular weight of the material. This allows the chains to rearrange to form crystalline regions. Positron annihilation lifetime spectroscopy confirms these conclusions. Fractional free volumes have been deduced from lifetime parameters, which correlate with the data obtained by the other techniques. # 2005 Elsevier B.V. All rights reserved. Keywords: Free volume; Cross-linking; Crystallinity; UHMWPE; Oxidation; Gamma irradiation
1. Introduction Ultra-high molecular weight polyethylene (UHMWPE) is a high performance polymer having very high impact resistance, low wear rate, low friction, biocompatibility and it is sterilizable [1]. * Corresponding author. E-mail address: [email protected] (N. Madi).
Therefore, it is currently the standard articulating surface for use in total hip and knee replacement prostheses. These prostheses are commonly sterilized by gamma irradiation. Gamma irradiation and shelf aging create reactive and long lived free radicals in UHMWPE, greatly increasing its susceptibility to oxidative degradation. Oxidative degradation of polyethylene components promotes osteolysis and subsequent loosening of total joint replacements.
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.08.077
M.A. Al-Ma’adeed et al. / Applied Surface Science 252 (2006) 3316–3322
3317
Understanding the effects of sterilization method and shelf aging on the oxidation of UHMWPE is crucial in developing orthopedic total joint systems with longer outcomes. Gamma irradiation in air causes cross-linking but it will also lead to diffusion of oxygen. Increasing crosslinking in this material improves wear resistance and mechanical properties [2]. Free hydrogen atoms (free radicals) combine with oxygen and cut the long chains of UHMWPE, causing scission. Many models have been suggested to explain the oxidation of UHMWPE following irradiation [3], where oxygen enters the amorphous region in the material and reacts with the free radicals, increasing amorphous regions and creating cracks which will in the long term lead to a reduction in the lifetime of the polymer. The positron annihilation technique has been widely used for studying the micro- and nanostructure of polymers during the last two decades [4,5]. The method is based on the well-established fact that ortho-positronium (o-Ps), a bound triplet state of an electron and a positron, tends to be localized in regions of lower electron density which are interpreted as ‘‘free volume holes’’. Many studies of the effect of gamma irradiation on polymers have been carried out [6–9]. Understanding the effects of sterilization method and shelf aging on the oxidation of UHMWPE is crucial in developing orthopedic total joint systems with longer outcomes. The aim of this study is to investigate the influence of gamma irradiation in air and post-irradiation aging in air by assessment of morphology, gel content, hardness, change in size and amount of free volume as well as density change.
penetrate the 1 mm thick UHMWPE samples. After irradiation with various gamma doses, the samples were measured immediately after irradiation and then one year later to study the effect of gamma dose as well as the effect of shelf aging in air on the structure of UHMWPE.
2. Experimental
2.3. Differential scanning calorimetry (DSC)
The samples used were from Goodfellow (UK) in the shape of sheets of about 1 mm and 1 cm thick, and molecular weight of 12,000 g/mol and density r = 0.93 g/cm3. The samples were irradiated in air at room temperature at the University of Qatar using a 60Co high dose research gamma irradiator from MDS Nordion International. The photons in this case have energies of 1.173 and 1.332 MeV, which easily
A small piece of UHMWPE (10–20 mg) was sealed in an aluminum pan and then heated in a temperature sweep from ambient to 150 8C at a rate of 10 8C/min in the DSC (Perkin-Elmer, model DSC7 Pyris). The total heat of melting DH (the area under the endotherm) was determined and, knowing the total heat of fusion of 100% crystalline UHMWPE (DHf = 293.6 J/g), the percentage crystallinity was calculated as 100 DH/DHf.
2.1. Positron annihilation lifetime spectroscopy (PALS) For the PALS study, two identical 1 mm thick samples were used to sandwich the 22Na source. Using the fast–fast coincidence technique, lifetime histograms were recorded. The system resolution was about 240 ps. 2.2. Measurements of cross-link density Swelling measurement is a good way to detect the extent of cross-linking in a polymer network. Polymers absorb some of the solvent and swell. The swollen gel can be characterized as an elastic, rather than a viscous, solution. Irradiated samples of a size of 1 cm 1 cm 1 cm were placed in boiling xylene for a time period of 12 h. Samples then were removed by 150 mm sieves. Duplicate tests were made for each dose. The swell ratio (SR) is then SR ¼
Wg Wd Wd
r ro
þ 1;
(1)
where Wg is the weight of the swollen gel after immersion, Wd the weight of dried gel, r the density of the solvent and ro is the density of polyethylene (here, r/ro = 1.08).
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2.4. Density measurements The determination of the density of polyethylene is an excellent measure of its crystallinity, which is related to many of its physical properties. The Automatic Densimeter was of the ‘‘displacement’’ type which determined the specimen density through measuring its mass (weight) first in air, and then again while immersed in water. Barometric pressure and changes in relative humidity are small affects and were thus ignored. 2.5. Durometer hardness The durometer hardness test was used to measure the relative hardness of this material. The test method is based on the penetration of a specified indenter forced into the material under specified conditions. The material was placed on a hard, flat surface and the pressure foot of the instrument was pressed onto the sample. The durometer hardness was read within one second after the pressure foot was in firm contact with the sample. A Shore hardness tester (model HHP2001) manufactured by Bareiss, was used.
3. Results and discussion 3.1. Free volume The PATFIT program [10] was used to extract the various lifetime components and their corresponding intensities from the PALS data. Four components were resolved corresponding to bulk annihilation, parapositronium ( p-Ps) formation and two types of orthopositronium (o-Ps). The important aspect of the results is the existence of o-Ps lifetime components, which implies that free volume is present in the samples [4,5,11]. The relation between the positron parameters and gamma radiation dose was compared with the parameters determined using other methods, such as crystallinity. The total intensity of o-Ps formation is obtained by adding the intensities of the two o-Ps components I3 and I4, while the mean o-Ps lifetime tm is calculated by determining the weighted mean of the two o-Ps lifetime components t3 and t4. By assuming a simple quantum mechanical model where an o-Ps atom resides in an infinite spherical
potential of radius Ro with an electron layer of thickness DR, the following analytical relation between the o-Ps lifetime t and the average radius R (Ro DR) of the free volume voids is obtained [12]: 1 R 1 R ¼2 1 þ sin 2p (2) t Ro 2p Ro The value of DR was found to be 0.166 nm [13]. Therefore, the average radius R of the free volume voids in polymers can be calculated from the measured o-Ps lifetime t and the average volume of the free volume voids: 4 v ¼ pR3 3
(3)
Fig. 1 shows the change in tm with gamma irradiation dose in the as-irradiated and aged samples. It can be seen that there is a large drop in the size of the free volume with increased dose. The trend is similar in both as-irradiated and aged samples but the values are generally higher in the aged samples. The reduction in the mean o-Ps lifetime, and therefore the size of the free volume, is a result of the production of free radicals by irradiation. This causes scission and hence an increase of electron density which decreases the lifetime. Information about the concentration of the free volume can be found from the intensity of the lifetime components. Fig. 2 shows the effect of gamma irradiation dose for as-irradiated and aged samples, on the o-Ps intensity. It can be noticed that there is also a
Fig. 1. The change in average o-Ps lifetime with gamma irradiation dose in as-irradiated and aged samples (lines drawn as eye-guide only).
M.A. Al-Ma’adeed et al. / Applied Surface Science 252 (2006) 3316–3322
Fig. 2. The change in o-Ps intensity with gamma irradiation dose in as-irradiated and aged samples (lines drawn as eye-guide only).
sharp drop of the free volume concentration up to a radiation dose of about 50 kGy after which the amount of free volume is not greatly affected by irradiation. The amount of free volume is however, reduced by aging. The fractional free volume f can be expressed as a product of the free volume v and positronium intensity I [13]: f ¼ cvI
(4)
where c is a constant which was estimated empirically from the conventional free volume theory.
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Fig. 3 shows the effect of gamma irradiation dose, for as-irradiated and aged samples, on the fraction of the free volume in the sample. We notice that the change in this fraction with irradiation dose and aging follows a trend that is more or less similar to the intensity of o-Ps. This behavior may be attributed to reduction of free volume as a result of irradiation, which enhances cross-linking and/or crystallinity, depending on irradiation dose and polymer structure. In the aged samples, more oxidative degradation occurs, and hence a smaller positron lifetime is expected. However, the higher lifetime obtained after aging can be explained if we understand that one of the main effects of gamma irradiation is to induce polymer oxidation [14], which shortens the distance between crystalline regions in the degraded zone. This effect restricts the chain disentanglement of tie molecules and therefore decreases the voids size as irradiation proceeds. After aging in air oxygen enters, causing the lamella to link and the molecular ties to break, which in turn allows the molecules to rearrange themselves in a more ordered fashion. According to this model, small voids join together, reducing their number while increasing the void size, which appears as an increase in the o-Ps lifetime. This trend strongly correlates with crystallinity versus dose and density versus dose changes (see Fig. 5 and Table 1, respectively). 3.2. DSC measurements DSC thermograms from 100 kGy as-irradiated UHMWPE exhibited one distinct peak. In addition, a small overlapping shoulder region was present after the first heating of irradiated sample (Fig. 4a). The shoulder disappeared upon cooling and was no longer present during the second heating (Fig. 4b). The single Table 1 Density of as-irradiated samples and post-irradiation aged samples in air vs. irradiation dose Density (g/cm3)
Fig. 3. The change in fractional free volume with gamma irradiation dose in as-irradiated and aged samples (lines drawn as eye-guide only, error bars are within the size of the points).
Dose (kGy)
After one year
As-irradiated
0.89 0.9432 0.9456 0.9610 0.9568
0.8810 0.9025 0.9270 0.9446 0.9628
0 10 25 200 400
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Fig. 5. Crystallinity vs. irradiation dose. The solid and dotted lines are first-order regression of corresponding experimental data.
Fig. 4. (a) First heating of 100 kGy as-irradiated UHMWPE and (b) second heating of the same sample.
distinct peak of the DSC trace for the irradiated sample indicates that the material contains one population of crystalline lamellae. The small shoulder region present in the DSC trace is indicative of the presence of new crystalline forms within the amorphous region. It is suggested that the second population of crystallites could be a consequence of the radiation induced oxidation process. The finding that this shoulder region disappears upon second heating indicates that the small crystalline regions are exterminated upon reheating. The post-irradiated aged samples in air exhibit the same trend. The melting crystallization parameters for both samples are presented in Fig. 5. Evidently, the storage in air of irradiated UHMWPE led to much more increase in crystallinity compared to as-irradiated samples. Because of the higher oxygen permeability, oxygen can enter the polymer material more readily, thereby causing greater oxidation in the presence of
irradiation induced free radicals. Oxygen will react with free radicals in the polymer causing further chain scission. As a consequence, the crystallinity of UHMWPE increases as the scission of tie molecules led to reduced molecular weight, which in turn permits polymer chain in non-crystalline regions of the polymer to fold and crystallize, as well as to allow increasing perfection of existing folded-chain crystallites. Oxidation of polymers induced by gamma irradiation has been investigated by many authors [14]. Polymer oxidation can be characterized by oxygen uptake techniques. Turos et al. [15] developed a new Rutherford backscattering (RBS) set up to measure the depth distribution of oxygen uptake of ion-implanted polymers. For comparison, measurements were conducted both to as-irradiated samples and aged samples. Using this technique, it is observed an increase in oxygen uptake with post-irradiation aging in air as already confirmed by crystallinity, density and hardness measurements. A strong correlation between increase in polymer crystallinity and subsequent increase in melting point was also observed in DSC measurements. 3.3. Density measurements The average density of non-aged and shelf-aged UHMWPE samples following gamma irradiation is shown in Table 1. The density was found to increase significantly for both samples as the irradiation dose
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increased. In addition, for one-year shelf-aged samples density values were generally higher than for the un-aged gamma-irradiated samples; this is due to the increase in oxidation and the greater mass of oxygen compared to hydrogen and due to scission [16]. The increased density caused by gamma irradiation would seem to correlate with the percentage crystallinity. This indicates that the increased density caused by gamma irradiation and aging is predominantly due to increased crystallinity (crystalline PE being of higher density than the amorphous phase). The increased crystallinity is probably arises from chain scission caused by the irradiation and oxidation process lowering the molecular weight of the PE. Referring to Fig. 1, Fig. 5, and Table 1, it is clear that the changes in percentage crystallinity and density with irradiation dose strongly correlate with mean oPs lifetime versus dose. The explanation is that, as irradiation dose increases, crystallization is obtained by nucleation of new lamella [17] resulting in a decrease in the size of voids (positron trapping centers) and, consequently, a decrease in the mean lifetime of o-Ps as shown in Fig. 1. 3.4. Measurements of cross-linking density It has been reported that polyethylene cross-links when exposed to high-energy irradiation [18]. These cross-links occur preferentially in the amorphous region of the bulk material. Cross-linking density, as measured by the gel fraction measurements for as-
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Fig. 7. Change in Shore hardness with irradiation dose for asirradiated and aged UHMWPE samples (lines are drawn as eyeguide only).
irradiated and shelf-aged irradiated UHMWPE is shown in Fig. 6. The significant increase in gel content after irradiation indicates the cross-linking (which takes place mostly in the interfacial regions of lamellae at or near fold surfaces) is very sensitive to small doses of radiation; more than 80% of gel content is reached at low irradiation dose, and crosslinking does not change after that. Shelf aging causes a slight increase in cross-linking, suggesting that most of the cross-linking reactions occur during and/or after irradiation. 3.5. Measurements of bulk hardness Fig. 7 shows the relation between the Shore hardness and dose. Hardness – the resistance of a material to deformation – is a very important mechanical property. Hardness is purely a relative term and should not be confused with wear and resistance of plastic materials. An increase in hardness is due to the increase in crystallinity and the breakage of the long chains of the polymer, increasing brittleness and weakening the material.
4. Conclusions
Fig. 6. The change in gel percent with gamma dose for as-irradiated and aged UHMWPE (lines are drawn as eye-guide only).
Changes in a variety of experimentally determined physical parameters show that they are dependent upon the crystallinity, which itself is a function of radiation dose and time. Oxygen which enters the
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material and free radicals formed by irradiation cause changes in the structure of the polymer, which continues to oxidize with time. Reaction of oxygen with free radicals causes chain scission, and recrystallization occurs, increasing crystallinity, which is higher with aging in an air environment (rich in oxygen). An increase in crystallinity causes an increase in hardness and a consequent change in mechanical properties, which make the polymer inappropriate for use in implants. Free radicals produced by the gamma irradiation process can either react with oxygen, causing chain scission, or react internally, leading to cross-linking. Cross-linking is the predominant feature in low doses, where positronium formation and free volume are reduced. More advanced and controlled methods are needed to sterilize the polymer, depending on its final use – e.g., in acetabular cups or tibial plateaus – and more caution must be taken during storage of the polymer to reduce oxidation and subsequent degradation of the implant.
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