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The application of Raman spectroscopy to the tribology of polymers
Tribology International Vol. 31, No. 11, pp. 687–693, 1998 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0301–679X/98/$19.00 ⫹ 0.00
PII: S0301–679X(98)00089–9
The application of Raman spectroscopy to the tribology of polymers B. H. Stuart
This paper reviews the use of Raman spectroscopy for investigations in polymer tribology. In particular, a number of studies dealing with the problem of surface plasticisation in engineering polymers have been reported and are reviewed in this paper. Solvent and thermally-induced plasticisation have been examined using Raman spectroscopy. The polymers poly(ether ether ketone), nylon and polycarbonate have been investigated and are discussed here. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Raman spectroscopy, polymers, tribology, surface plasticisation
Introduction Tribology has emerged in recent years as an interdisciplinary science. Chemical techniques can now be used in conjunction with the more conventional engineering testing methods in order to gain a better understanding of particular questions in polymer tribology. Raman spectroscopy has developed as a useful technique for the study of problems in polymer tribology and a number of investigations dealing with the phenomenon of surface plasticisation of polymers have been reported in the literature. Raman spectroscopy has been used to gain a better understanding of the chemical aspects of the phenomenon of surface plasticisation. The technique itself is based upon the examination of the manner in which electromagnetic radiation interacts with molecules. Raman spectroscopy is particularly suitable for studying the surface properties of polymers as it is a nondestructive technique based upon the inelastic scattering of radiation from the sample. Information regarding the experimental aspects of Raman spectroscopy have been detailed elsewhere1. This technique has previously been of limited use with polymers as fluorescence has previously often totally masked the Raman signal. However, with the advent of Fourier transform spectroscopy and improved instrumentation, Raman spectroscopy is now a viable method for use with polymer Department of Materials Science, University of Technology Sydney, PO Box 123, Broadway, NSW 2007, Australia Received 2 January 1998; accepted 15 June 1998
systems. A number of reviews of the application of Raman spectroscopy to polymers have appeared in the literature1–3. Polymers have been commonly used in unlubricated contacts because of what is termed their self-lubricating character, but significant improvement in performance may often be obtained by conventional liquid lubrication. However, in some polymer systems lubricants are believed to cause plasticisation (or softening) of the surface, which can be deleterious to good operation. When a lubricant is applied to a polymer it is possible for the lubricant molecules to penetrate the polymer and alter its mechanical properties. In certain cases surface softening may be useful as a means of improving efficiency, but bulk plasticisation obviously needs to be avoided. Thus, environmental plasticisation of polymers represents an important practical limitation in their effective utilisation. An understanding of how lubricants, or indeed how an active environment, can cause surface plasticisation is important in order to control and optimise this phenomenon. Raman spectroscopy can be used to gain a better understanding of the chemical aspects of the phenomenon of surface plasticisation. This paper provides an overview of the use of Raman spectroscopy in polymer tribology. A survey of recent literature demonstrates the emergence of Fourier transform Raman spectroscopy for use in polymer tribology and a review of these studies is provided here. The polymers poly(ether ether ketone), nylon and polycarbonate are discussed.
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Fig. 1 The chemical structure of PEEK
Poly(ether ether ketone) Poly(ether ether ketone) (PEEK) is a thermoplastic polymer which has been particularly useful in engineering applications because of its good mechanical properties and chemical resistance4. One of the reasons for the popularity of PEEK in this area is the excellent wear characteristics of the polymer. However, the sliding friction of PEEK is quite high and the associated generation of heat in a sliding contact can have a destructive effect on the polymer5. The phenomena has been described as scuffing, by analogy with failure modes commonly observed in lubricated metallic contacts. At some critical interface temperature or rate of energy dissipation an extremely rapid increase in frictional work is observed, coupled with severe chemical and mechanical damage to the contacting members. The chemical structure of PEEK is shown in Fig 1. The FT-Raman spectra of semi-crystalline and amorphous PEEK are shown in Fig 2. A number of Raman studies of PEEK have been reported and Raman spectroscopy has proved to be a convenient spectroscopic technique for the estimation of the nature of the thermal changes induced in PEEK. The relative intensity of the CJOJC stretching mode has been followed as a function of temperature in order to examine the crystallisation process of PEEK (Fig 3)6,7. The results obtained by Briscoe and co-workers support the theory developed by Nguyen and Ishida8 using infra-red spectroscopy that the polymer undergoes two processes
Fig. 2 The FT-Raman spectra of semi-crystalline and amorphous PEEK 688
Fig. 3 The relative intensity of the symmetric CJOJC stretching mode of PEEK as a function of temperature during crystallisation. The first process occurs in the region of 120–130°C and is associated with the partial bond rotation of the ether linkages prior to crystallisation. This is manifested by a decrease in the relative intensity of the symmetric CJOJC stretching mode. At temperatures greater than the glass transition temperature Tg (143°C) a second process involving the motion of the whole chain occurs and is characterised by the rotation of the benzophenone linkages near the melting temperature Tm (334°C). In the Raman spectrum, an increase in the intensity of the CJOJC stretching mode is observed at this temperature. Another particularly important property of PEEK has been its ability to resist chemical attack; there is a limited number of solvents for PEEK. Despite this, a series of recent studies has shown that certain solvents such as can be absorbed by PEEK and cause detectable plasticisation and induce crystallisation. The effect of toluene on the Raman spectrum of PEEK was examined by Briscoe et al.6 and the relative intensity of the symmetric CJOJC Raman stretching mode of the polymer was affected by the presence of toluene. Comparison with temperature experiments found that the presence of toluene in the polymer at room temperature mimics the spectral characteristics of the unsolvated polymer at a temperature of about 120°C. Thus, Briscoe and co-workers were able to assign a ‘fictive’ temperature of 120°C to the toluene solvated polymer. The presence of the solvent induces additional molecular mobility and the mechanical characteristics of the
Tribology International Volume 31 Number 11 1998
Raman spectroscopy application: B. H. Stuart Table 1 The relative intensities of the symmetric C–O–C stretching FT-Raman mode of PEEK: untreated and in various dodecane solutions Treatment Untreated Dodecane Dodecylamine in dodecane Decanoic acid in dodecane
Relative intensity 1.35 0.87 0.81 0.66
polymer appeared to be similar to those of the unsolvated polymer at some higher temperature. Similar findings were observed in a spectroscopic study of the effect of dodecane lubricant systems on the frictional properties of PEEK7. The Raman spectra of PEEK exposed to dodecane with the surfactant additives dodecylamine or decanoic acid show that these systems also produce a fictive temperature in the region of 120–130°C. Table 1 lists the relative intensities of the symmetric CJOJC stretching mode before and after treatment with dodecane solutions. These spectroscopic findings provide a useful comparison with sliding friction data5,7. Fig 4 shows the sliding frictional force of dodecane lubricated PEEK as a function of applied normal load7. The limit to the experiment is
brought about by the large frictional forces generated in the contact and the potential self destruction of the sample. It can be seen that while failure occurs at a load of 250 N where pure dodecane is used as lubricant, in the cases of dodecylamine and decanoic acid addition, the scuffing loads are reduced to around 225 and 200 N, respectively. Significantly, what is apparent from these studies is that the critical load for scuffing of PEEK depends very much on the type of lubricant applied. Dodecane with the additives decanoic acid or dodecylamine produce an initial reduction of the friction coefficient of PEEK, but promotes premature scuffing failure. It was surmised that the enhanced scuffing failure was due to the surface plasticisation occurring because of the presence of these chemical agents. Similar experiments to those reported by Briscoe and co-workers7 have been carried out on PEEK treated with chloroform9. In the high speed friction simulation, treatment of PEEK with chloroform was found to induce a similar response to that observed when dodecane lubricants were applied to the polymer. That is, a critical load is reached where the rate of increase of frictional work increases dramatically, and results in the destruction of the sample. Notably, the critical point in the case of chloroform treated PEEK occurs at a significantly lower load than that observed for any of the dodecane-additive systems reported by Briscoe et al.7. Fig 5 shows that the critical change occurs to the frictional response in the range 100–150 N, much lower than that found for PEEK lubricated by decanoic acid in dodecane (200 N)7. Spectroscopic data may be correlated with the friction experiments carried out on PEEK treated with chloroform. Exposure of amorphous PEEK to various chlori-
Fig. 4 The frictional force of PEEK lubricated with dodecane solutions as a function of normal load
Fig. 5 The frictional force of PEEK before and after treatment with chloroform as a function of normal load
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nated aliphatic hydrocarbons has been found to have a considerable effect on the properties of the polymer10,11. The solvents methylene chloride, chloroform and tetrachloroethane were found to cause swelling and induce crystallisation in PEEK. The Raman spectra of such solvated PEEK systems showed that the frequency of the CBO stretching mode is shifted to lower values compared to the untreated polymer, indicating an increase in crystallinity. In addition, the relative intensities of the symmetric CJOJC stretching modes were also affected by the presence of certain chlorinated solvents. The effect of chloroform on the crystallinity-sensitive Raman modes of PEEK is summarised in Table 2. Lewis acid-base interaction theory was used to rationalise why such solvents affect PEEK in this way. PEEK acts as a soft organic base owing to the presence of the CBO, CJOJC and aromatic groups in the structure, all of which act as electron donors. The chlorinated solvents act as electron donors owing to the electron-deficient hydrogen atoms which are activated by the strong electron-withdrawing influence of the chlorine groups. Fictive temperatures were also assigned to the aforementioned chlorinated solvents. These solvents have a greater plasticising effect than, say, toluene6, and it was shown that methylene chloride, chloroform and tetrachloroethane mimic the unsolvated polymer at temperatures near 250°C, close to the Tm.
requires about 10–15 w% PTFE in order to produce the majority of the friction reduction. Raman spectroscopy has also been used to study blends PEEK and PTFE of various compositions13. Stuart and Briscoe showed that the addition of PTFE to PEEK to form an immiscible blend causes changes to the Raman spectrum of PEEK. Fig 6 compares the FT-Raman spectra of PEEK, PTFE and a blend of PEEK/PTFE. The narrowing of the Raman modes of PEEK were taken as an indication of more order in the PEEK molecules. In addition, the reduction of the intensity of the CJOJC stretching mode with increasing PTFE concentration provided evidence of an increase in crystallinity in PEEK. Another important observation was that the most significant change to this crystallinitydependent mode occurs when about 10 w% PTFE is added to PEEK (Fig 7). Comparison with the frictional data produced by Briscoe and co-workers12 shows that this composition appears to produce the most generally effective bearing material where this blend is used in engineering applications.
Nylon The strength and toughness of nylons has led to their acceptance as important engineering materials. However, nylons are known to be particularly affected by ambient water, with moisture causing plasticisation and
An alternative approach to the problem of attenuating the friction generated at PEEK contacts, while maintaining the attractive mechanical properties, is to use internal phase lubrication. This may be achieved by blending PEEK with a polymer of appropriate properties, such as polytetrafluoroethylene (PTFE). Briscoe et al.12 reported a study of the friction and wear of a number of PEEK/PTFE blends over a range of compositions. They observed that the overall unlubricated tribological performance of both PEEK and PTFE is improved by the inclusion of the other polymer into the matrix. For PEEK the friction is reduced and the load-bearing capacity is extended, at the expense of a modest loss in its wear resistance. The wear of PTFE, particularly the transfer wear, is greatly reduced by the inclusion of PEEK and the low friction is retained. The results of this study showed that the contact
Table 2 The effect of chloroform on FT-Raman modes of PEEK Treatment
Untreated amorphous Untreated crystalline Chloroform treated 690
Relative C⫽O stretching intensity C–O–C mode stretching frequency mode (cm−1) 1.35
1651
1.51
1644
1.64
1644
Fig. 6 The FT-Raman spectra of PEEK, PTFE and a PEEK/PTFE blend
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Fig. 7 The relative intensity of the CJOJC stretching mode of blended PEEK as a function of PTFE concentration having a profound effect on the mechanical properties of the polymer14. Clearly, this phenomenon raises problems in the manufacture and use of engineering components manufactured from nylon. Experimental studies of the effect of water on the mechanical properties of this polymer, particularly with respect to their surface mechanical and tribological properties, have been carried out15. An examination of the sliding frictional behaviour of nylon against a steel substrate showed that the friction decreases with increasing load, probably due to the formation of thermally softened interfaces in the contact. After exposure of this polymer to water, dramatic changes to the frictional behaviour of the nylon are observed; the friction increases with increasing load. Fig 8 illustrates the sliding frictional force of nylon before and after treatment with water as a function of normal load. From the application of the adhesion model of friction it was postulated that the observed changes are caused by extensive plasticisation of the nylon surface and, as a consequence, an increase in the contact area. This proposition is confirmed by the reported scratch hardness data. After treatment with water the scratch friction mechanism changes significantly and a notable reduction in the hardness of nylon is observed. Nylon 6 is an important engineering polyamide and its structure is shown in Fig 9. The effect of increasing temperature on the Raman spectra of nylon 6 has been investigated16 and the observed changes were
Fig. 8 The frictional force of nylon before and after treatment with water as a function of normal load
interpreted in terms of the thermal breakdown of the intermolecular hydrogen-bonding within nylon. Modes associated with the amide group show a reduction in intensity (Fig 10). The amide I band, due largely to CBO stretching, also shifts to greater frequencies with increasing temperature. Modes due to vibrations in the hydrocarbon chain of nylon 6 were also affected by an increase in thermal energy. In the same study the changes to the Fourier transform Raman spectrum of nylon 6 due to the presence of water was also examined (Fig 10). It was postulated that water molecules disrupt the inter-chain hydrogen-bonding which exist in the polyamide; water molecules have a greater propensity to interact at the basic carbonyl sites of nylon than adjacent nylon chains. Consequently, it was proposed that water produces augmented hydrogen-bonding in nylon 6.
Polycarbonate Polycarbonate (PC) is a high performance polymer with the useful properties of transparency and toughness. However, a number of organic solvents are known to be absorbed by PC and so alter the physical properties of this polymer. In particular, exposure of PC to the liquid xylene produces severe plasticisation and opaqueness, thus resulting in quite dramatic changes to the appearance and the mechanical properties of the
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Fig. 9 The chemical structure of nylon 6 polymer. These changes clearly have a serious effect on the applicability of PC in engineering applications. Briscoe and co-workers17 investigated the influence of xylene on the rheological and morphological properties of thin PC films. The solvent was observed to cause significant plasticisation of PC and the process altered the shear mechanism determined from friction experiments. The frictional properties of the unplasticised system are characteristic of a dry solid friction and the plasticised system was found to correlate with the deformation characteristics of a viscous fluid. It was concluded that it is a change in the molecular conformation and, hence, the morphological properties, caused by solvent induced plasticisation which determines the magnitude of frictional work.
solvent and this ordering is associated with the amorphous regions of the polymer. The introduction of the xylene molecules between the polymer chains increases the free volume in the polymer and allows for chain extension, which is in this case the cis–trans conformation.
The plasticisation of PC by xylene has also been investigated using FT-Raman spectroscopy18. The FTRaman spectrum of PC before and after exposure to xylene was investigated and a stretching mode of the OJC(O)JO group and a phenyl ring vibration were found to change in intensity after PC was exposed to xylene (Table 3). The changes to the FT-Raman spectrum of PC in the presence of xylene are believed to be due to an increase in the amount of the cis–trans conformation of the polymer. Fig 11 illustrates the molecular conformations of PC. The plasticisation and subsequent swelling of PC occurs because the interaction of xylene with the polymer results in solution of some of the solvent in the polymer phase and solution of some of the polymer in the solvent phase. At the same time, ordering is induced in the polymer by the
Summary The application of Raman spectroscopy to problems in polymer tribology has been reviewed here. The technique has been shown to be particularly applicable to the investigation of the phenomenon of surface plasticisation for a number of polymeric systems. Both solvent-induced and thermally-induced plasticisation have been studied. Morphological changes to polymers on plasticisation can be characterised and correlated with changes in tribological properties, such as the frictional response. Raman spectroscopy offers a great deal of potential in the field of polymer tribology and allows an understanding of such systems at a molecular level.
692
Fig. 10 The FT-Raman spectra of nylon 6 at 25°C, 200°C and after exposure to water
Tribology International Volume 31 Number 11 1998
Raman spectroscopy application: B. H. Stuart Table 3 The effect of xylene on the relative intensities of FT-Raman modes of PC Frequency (cm−1) 890 641
Assignment
Untreated
O–C(O)–O stretching Phenyl ring vibration
Fig. 11 The molecular conformations of PC
References 1. Hendra, P. J., Jones, C. and Warnes, G., Fourier Transform Raman Spectroscopy: Instrumentation and Chemical Applications. Ellis Harwood, New York, 1991. 2. Agbenyega, J. K., Ellis, G., Hendra, P. J., Maddams, W. F., Passingham, C., Willis, H. A. and Chalmers, J., Applications of Fourier transform Raman spectroscopy in the synthetic polymer field. Spectrochimica Acta, 1990, 46A, 197–216. 3. Maddams, W. F., A review of Fourier transform Raman spectroscopic studies on polymers. Spectrochimica Acta, 1994, 50A, 1967–1986.
1.47 0.59
Xylene-treated 1.64 0.71
4. Jones, D. P., Leach, D. C. and Moore, D. R., Mechanical properties of poly(ether ether ketone) for engineering applications. Polymer, 1985, 26, 1385–1393. 5. Briscoe, B. J., Stolarski, T. A. and Davies, G. J., Boundary lubrication of polymers in model fluids. Tribology International, 1984, 17, 129–134. 6. Briscoe, B. J., Stuart, B. H., Thomas, P. S. and Williams, D. R., A comparison of thermal- and solvent-induced relaxation of poly(ether ether ketone) using Fourier transform Raman spectroscopy. Spectrochimica Acta, 1991, 47A, 1299–1303. 7. Briscoe, B. J., Stuart, B. H., Sebastian, S. and Tweedale, P. J., The failure of poly(ether ether ketone) in high speed contacts. Wear, 1993, 162, 407–417. 8. Nguyen, H. X. and Ishida, H., Molecular analysis of the crystallisation behaviour of poly(aryl ether ether ketone). Journal of Polymer Science, Polymer Physics Edition, 1986, 24, 1079–1091. 9. Briscoe, B. J. and Stuart, B. H., The surface plasticisation and lubrication of poly(ether ether ketone) by third body formation. Proceedings 22nd Leeds–Lyon Symposium on Tribology, Lyon 1995. Elsevier, Oxford, 1996. 10. Stuart, B. H. and Williams, D. R., The solvent-induced swelling of poly(ether ether ketone) by 1,1,2,2-tetrachloroethane. Polymer, 1994, 35, 1326–1328. 11. Stuart, B. H. and Williams, D. R., A study of the sorption of chlorinated organic solvents by poly(ether ether ketone) using vibrational spectroscopy. Polymer, 1995, 36, 4209–4213. 12. Briscoe, B. J., Yao, L. H. and Stolarski, T. A., The friction and wear of polytetrafluoroethylene-poly(ether ether ketone) composites: an initial appraisal of the optimum composition. Wear, 1986, 108, 357–374. 13. Stuart, B. H. and Briscoe, B. J., A Fourier transform Raman spectroscopy study of poly(ether ether ketone)/polytetrafluoroethylene blends. Spectrochimica Acta, 1994, 50A, 2005–2009. 14. Nelson, W. E., Nylon Plastics Technology. Newnes-Butterworths, London, 1976. 15. Stuart, B. H. and Briscoe, B. J., Surface plasticisation of Nylon 6,6 by water. Polymer International, 1995, 38, 95–99. 16. Stuart, B. H., A Fourier transform Raman study of water sorption by Nylon 6. Polymer Bulletin, 1994, 33, 681–686. 17. Briscoe, B. J., Stuart, B. H. and Thomas, P. S., Solvent induced morphological changes to polycarbonate. Materials Resources Society Symposium Proceedings, 1993, 304, 185–190. 18. Stuart, B. H. and Thomas, P. S., Xylene swelling of polycarbonate studied using Fourier transform Raman spectroscopy. Spectrochimica Acta, 1995, 51, 2133–2137.
Tribology International Volume 31 Number 11 1998
693
PII: S0301–679X(98)00089–9
The application of Raman spectroscopy to the tribology of polymers B. H. Stuart
This paper reviews the use of Raman spectroscopy for investigations in polymer tribology. In particular, a number of studies dealing with the problem of surface plasticisation in engineering polymers have been reported and are reviewed in this paper. Solvent and thermally-induced plasticisation have been examined using Raman spectroscopy. The polymers poly(ether ether ketone), nylon and polycarbonate have been investigated and are discussed here. 1999 Elsevier Science Ltd. All rights reserved. Keywords: Raman spectroscopy, polymers, tribology, surface plasticisation
Introduction Tribology has emerged in recent years as an interdisciplinary science. Chemical techniques can now be used in conjunction with the more conventional engineering testing methods in order to gain a better understanding of particular questions in polymer tribology. Raman spectroscopy has developed as a useful technique for the study of problems in polymer tribology and a number of investigations dealing with the phenomenon of surface plasticisation of polymers have been reported in the literature. Raman spectroscopy has been used to gain a better understanding of the chemical aspects of the phenomenon of surface plasticisation. The technique itself is based upon the examination of the manner in which electromagnetic radiation interacts with molecules. Raman spectroscopy is particularly suitable for studying the surface properties of polymers as it is a nondestructive technique based upon the inelastic scattering of radiation from the sample. Information regarding the experimental aspects of Raman spectroscopy have been detailed elsewhere1. This technique has previously been of limited use with polymers as fluorescence has previously often totally masked the Raman signal. However, with the advent of Fourier transform spectroscopy and improved instrumentation, Raman spectroscopy is now a viable method for use with polymer Department of Materials Science, University of Technology Sydney, PO Box 123, Broadway, NSW 2007, Australia Received 2 January 1998; accepted 15 June 1998
systems. A number of reviews of the application of Raman spectroscopy to polymers have appeared in the literature1–3. Polymers have been commonly used in unlubricated contacts because of what is termed their self-lubricating character, but significant improvement in performance may often be obtained by conventional liquid lubrication. However, in some polymer systems lubricants are believed to cause plasticisation (or softening) of the surface, which can be deleterious to good operation. When a lubricant is applied to a polymer it is possible for the lubricant molecules to penetrate the polymer and alter its mechanical properties. In certain cases surface softening may be useful as a means of improving efficiency, but bulk plasticisation obviously needs to be avoided. Thus, environmental plasticisation of polymers represents an important practical limitation in their effective utilisation. An understanding of how lubricants, or indeed how an active environment, can cause surface plasticisation is important in order to control and optimise this phenomenon. Raman spectroscopy can be used to gain a better understanding of the chemical aspects of the phenomenon of surface plasticisation. This paper provides an overview of the use of Raman spectroscopy in polymer tribology. A survey of recent literature demonstrates the emergence of Fourier transform Raman spectroscopy for use in polymer tribology and a review of these studies is provided here. The polymers poly(ether ether ketone), nylon and polycarbonate are discussed.
Tribology International Volume 31 Number 11 1998
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Raman spectroscopy application: B. H. Stuart
Fig. 1 The chemical structure of PEEK
Poly(ether ether ketone) Poly(ether ether ketone) (PEEK) is a thermoplastic polymer which has been particularly useful in engineering applications because of its good mechanical properties and chemical resistance4. One of the reasons for the popularity of PEEK in this area is the excellent wear characteristics of the polymer. However, the sliding friction of PEEK is quite high and the associated generation of heat in a sliding contact can have a destructive effect on the polymer5. The phenomena has been described as scuffing, by analogy with failure modes commonly observed in lubricated metallic contacts. At some critical interface temperature or rate of energy dissipation an extremely rapid increase in frictional work is observed, coupled with severe chemical and mechanical damage to the contacting members. The chemical structure of PEEK is shown in Fig 1. The FT-Raman spectra of semi-crystalline and amorphous PEEK are shown in Fig 2. A number of Raman studies of PEEK have been reported and Raman spectroscopy has proved to be a convenient spectroscopic technique for the estimation of the nature of the thermal changes induced in PEEK. The relative intensity of the CJOJC stretching mode has been followed as a function of temperature in order to examine the crystallisation process of PEEK (Fig 3)6,7. The results obtained by Briscoe and co-workers support the theory developed by Nguyen and Ishida8 using infra-red spectroscopy that the polymer undergoes two processes
Fig. 2 The FT-Raman spectra of semi-crystalline and amorphous PEEK 688
Fig. 3 The relative intensity of the symmetric CJOJC stretching mode of PEEK as a function of temperature during crystallisation. The first process occurs in the region of 120–130°C and is associated with the partial bond rotation of the ether linkages prior to crystallisation. This is manifested by a decrease in the relative intensity of the symmetric CJOJC stretching mode. At temperatures greater than the glass transition temperature Tg (143°C) a second process involving the motion of the whole chain occurs and is characterised by the rotation of the benzophenone linkages near the melting temperature Tm (334°C). In the Raman spectrum, an increase in the intensity of the CJOJC stretching mode is observed at this temperature. Another particularly important property of PEEK has been its ability to resist chemical attack; there is a limited number of solvents for PEEK. Despite this, a series of recent studies has shown that certain solvents such as can be absorbed by PEEK and cause detectable plasticisation and induce crystallisation. The effect of toluene on the Raman spectrum of PEEK was examined by Briscoe et al.6 and the relative intensity of the symmetric CJOJC Raman stretching mode of the polymer was affected by the presence of toluene. Comparison with temperature experiments found that the presence of toluene in the polymer at room temperature mimics the spectral characteristics of the unsolvated polymer at a temperature of about 120°C. Thus, Briscoe and co-workers were able to assign a ‘fictive’ temperature of 120°C to the toluene solvated polymer. The presence of the solvent induces additional molecular mobility and the mechanical characteristics of the
Tribology International Volume 31 Number 11 1998
Raman spectroscopy application: B. H. Stuart Table 1 The relative intensities of the symmetric C–O–C stretching FT-Raman mode of PEEK: untreated and in various dodecane solutions Treatment Untreated Dodecane Dodecylamine in dodecane Decanoic acid in dodecane
Relative intensity 1.35 0.87 0.81 0.66
polymer appeared to be similar to those of the unsolvated polymer at some higher temperature. Similar findings were observed in a spectroscopic study of the effect of dodecane lubricant systems on the frictional properties of PEEK7. The Raman spectra of PEEK exposed to dodecane with the surfactant additives dodecylamine or decanoic acid show that these systems also produce a fictive temperature in the region of 120–130°C. Table 1 lists the relative intensities of the symmetric CJOJC stretching mode before and after treatment with dodecane solutions. These spectroscopic findings provide a useful comparison with sliding friction data5,7. Fig 4 shows the sliding frictional force of dodecane lubricated PEEK as a function of applied normal load7. The limit to the experiment is
brought about by the large frictional forces generated in the contact and the potential self destruction of the sample. It can be seen that while failure occurs at a load of 250 N where pure dodecane is used as lubricant, in the cases of dodecylamine and decanoic acid addition, the scuffing loads are reduced to around 225 and 200 N, respectively. Significantly, what is apparent from these studies is that the critical load for scuffing of PEEK depends very much on the type of lubricant applied. Dodecane with the additives decanoic acid or dodecylamine produce an initial reduction of the friction coefficient of PEEK, but promotes premature scuffing failure. It was surmised that the enhanced scuffing failure was due to the surface plasticisation occurring because of the presence of these chemical agents. Similar experiments to those reported by Briscoe and co-workers7 have been carried out on PEEK treated with chloroform9. In the high speed friction simulation, treatment of PEEK with chloroform was found to induce a similar response to that observed when dodecane lubricants were applied to the polymer. That is, a critical load is reached where the rate of increase of frictional work increases dramatically, and results in the destruction of the sample. Notably, the critical point in the case of chloroform treated PEEK occurs at a significantly lower load than that observed for any of the dodecane-additive systems reported by Briscoe et al.7. Fig 5 shows that the critical change occurs to the frictional response in the range 100–150 N, much lower than that found for PEEK lubricated by decanoic acid in dodecane (200 N)7. Spectroscopic data may be correlated with the friction experiments carried out on PEEK treated with chloroform. Exposure of amorphous PEEK to various chlori-
Fig. 4 The frictional force of PEEK lubricated with dodecane solutions as a function of normal load
Fig. 5 The frictional force of PEEK before and after treatment with chloroform as a function of normal load
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nated aliphatic hydrocarbons has been found to have a considerable effect on the properties of the polymer10,11. The solvents methylene chloride, chloroform and tetrachloroethane were found to cause swelling and induce crystallisation in PEEK. The Raman spectra of such solvated PEEK systems showed that the frequency of the CBO stretching mode is shifted to lower values compared to the untreated polymer, indicating an increase in crystallinity. In addition, the relative intensities of the symmetric CJOJC stretching modes were also affected by the presence of certain chlorinated solvents. The effect of chloroform on the crystallinity-sensitive Raman modes of PEEK is summarised in Table 2. Lewis acid-base interaction theory was used to rationalise why such solvents affect PEEK in this way. PEEK acts as a soft organic base owing to the presence of the CBO, CJOJC and aromatic groups in the structure, all of which act as electron donors. The chlorinated solvents act as electron donors owing to the electron-deficient hydrogen atoms which are activated by the strong electron-withdrawing influence of the chlorine groups. Fictive temperatures were also assigned to the aforementioned chlorinated solvents. These solvents have a greater plasticising effect than, say, toluene6, and it was shown that methylene chloride, chloroform and tetrachloroethane mimic the unsolvated polymer at temperatures near 250°C, close to the Tm.
requires about 10–15 w% PTFE in order to produce the majority of the friction reduction. Raman spectroscopy has also been used to study blends PEEK and PTFE of various compositions13. Stuart and Briscoe showed that the addition of PTFE to PEEK to form an immiscible blend causes changes to the Raman spectrum of PEEK. Fig 6 compares the FT-Raman spectra of PEEK, PTFE and a blend of PEEK/PTFE. The narrowing of the Raman modes of PEEK were taken as an indication of more order in the PEEK molecules. In addition, the reduction of the intensity of the CJOJC stretching mode with increasing PTFE concentration provided evidence of an increase in crystallinity in PEEK. Another important observation was that the most significant change to this crystallinitydependent mode occurs when about 10 w% PTFE is added to PEEK (Fig 7). Comparison with the frictional data produced by Briscoe and co-workers12 shows that this composition appears to produce the most generally effective bearing material where this blend is used in engineering applications.
Nylon The strength and toughness of nylons has led to their acceptance as important engineering materials. However, nylons are known to be particularly affected by ambient water, with moisture causing plasticisation and
An alternative approach to the problem of attenuating the friction generated at PEEK contacts, while maintaining the attractive mechanical properties, is to use internal phase lubrication. This may be achieved by blending PEEK with a polymer of appropriate properties, such as polytetrafluoroethylene (PTFE). Briscoe et al.12 reported a study of the friction and wear of a number of PEEK/PTFE blends over a range of compositions. They observed that the overall unlubricated tribological performance of both PEEK and PTFE is improved by the inclusion of the other polymer into the matrix. For PEEK the friction is reduced and the load-bearing capacity is extended, at the expense of a modest loss in its wear resistance. The wear of PTFE, particularly the transfer wear, is greatly reduced by the inclusion of PEEK and the low friction is retained. The results of this study showed that the contact
Table 2 The effect of chloroform on FT-Raman modes of PEEK Treatment
Untreated amorphous Untreated crystalline Chloroform treated 690
Relative C⫽O stretching intensity C–O–C mode stretching frequency mode (cm−1) 1.35
1651
1.51
1644
1.64
1644
Fig. 6 The FT-Raman spectra of PEEK, PTFE and a PEEK/PTFE blend
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Fig. 7 The relative intensity of the CJOJC stretching mode of blended PEEK as a function of PTFE concentration having a profound effect on the mechanical properties of the polymer14. Clearly, this phenomenon raises problems in the manufacture and use of engineering components manufactured from nylon. Experimental studies of the effect of water on the mechanical properties of this polymer, particularly with respect to their surface mechanical and tribological properties, have been carried out15. An examination of the sliding frictional behaviour of nylon against a steel substrate showed that the friction decreases with increasing load, probably due to the formation of thermally softened interfaces in the contact. After exposure of this polymer to water, dramatic changes to the frictional behaviour of the nylon are observed; the friction increases with increasing load. Fig 8 illustrates the sliding frictional force of nylon before and after treatment with water as a function of normal load. From the application of the adhesion model of friction it was postulated that the observed changes are caused by extensive plasticisation of the nylon surface and, as a consequence, an increase in the contact area. This proposition is confirmed by the reported scratch hardness data. After treatment with water the scratch friction mechanism changes significantly and a notable reduction in the hardness of nylon is observed. Nylon 6 is an important engineering polyamide and its structure is shown in Fig 9. The effect of increasing temperature on the Raman spectra of nylon 6 has been investigated16 and the observed changes were
Fig. 8 The frictional force of nylon before and after treatment with water as a function of normal load
interpreted in terms of the thermal breakdown of the intermolecular hydrogen-bonding within nylon. Modes associated with the amide group show a reduction in intensity (Fig 10). The amide I band, due largely to CBO stretching, also shifts to greater frequencies with increasing temperature. Modes due to vibrations in the hydrocarbon chain of nylon 6 were also affected by an increase in thermal energy. In the same study the changes to the Fourier transform Raman spectrum of nylon 6 due to the presence of water was also examined (Fig 10). It was postulated that water molecules disrupt the inter-chain hydrogen-bonding which exist in the polyamide; water molecules have a greater propensity to interact at the basic carbonyl sites of nylon than adjacent nylon chains. Consequently, it was proposed that water produces augmented hydrogen-bonding in nylon 6.
Polycarbonate Polycarbonate (PC) is a high performance polymer with the useful properties of transparency and toughness. However, a number of organic solvents are known to be absorbed by PC and so alter the physical properties of this polymer. In particular, exposure of PC to the liquid xylene produces severe plasticisation and opaqueness, thus resulting in quite dramatic changes to the appearance and the mechanical properties of the
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Fig. 9 The chemical structure of nylon 6 polymer. These changes clearly have a serious effect on the applicability of PC in engineering applications. Briscoe and co-workers17 investigated the influence of xylene on the rheological and morphological properties of thin PC films. The solvent was observed to cause significant plasticisation of PC and the process altered the shear mechanism determined from friction experiments. The frictional properties of the unplasticised system are characteristic of a dry solid friction and the plasticised system was found to correlate with the deformation characteristics of a viscous fluid. It was concluded that it is a change in the molecular conformation and, hence, the morphological properties, caused by solvent induced plasticisation which determines the magnitude of frictional work.
solvent and this ordering is associated with the amorphous regions of the polymer. The introduction of the xylene molecules between the polymer chains increases the free volume in the polymer and allows for chain extension, which is in this case the cis–trans conformation.
The plasticisation of PC by xylene has also been investigated using FT-Raman spectroscopy18. The FTRaman spectrum of PC before and after exposure to xylene was investigated and a stretching mode of the OJC(O)JO group and a phenyl ring vibration were found to change in intensity after PC was exposed to xylene (Table 3). The changes to the FT-Raman spectrum of PC in the presence of xylene are believed to be due to an increase in the amount of the cis–trans conformation of the polymer. Fig 11 illustrates the molecular conformations of PC. The plasticisation and subsequent swelling of PC occurs because the interaction of xylene with the polymer results in solution of some of the solvent in the polymer phase and solution of some of the polymer in the solvent phase. At the same time, ordering is induced in the polymer by the
Summary The application of Raman spectroscopy to problems in polymer tribology has been reviewed here. The technique has been shown to be particularly applicable to the investigation of the phenomenon of surface plasticisation for a number of polymeric systems. Both solvent-induced and thermally-induced plasticisation have been studied. Morphological changes to polymers on plasticisation can be characterised and correlated with changes in tribological properties, such as the frictional response. Raman spectroscopy offers a great deal of potential in the field of polymer tribology and allows an understanding of such systems at a molecular level.
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Fig. 10 The FT-Raman spectra of nylon 6 at 25°C, 200°C and after exposure to water
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Raman spectroscopy application: B. H. Stuart Table 3 The effect of xylene on the relative intensities of FT-Raman modes of PC Frequency (cm−1) 890 641
Assignment
Untreated
O–C(O)–O stretching Phenyl ring vibration
Fig. 11 The molecular conformations of PC
References 1. Hendra, P. J., Jones, C. and Warnes, G., Fourier Transform Raman Spectroscopy: Instrumentation and Chemical Applications. Ellis Harwood, New York, 1991. 2. Agbenyega, J. K., Ellis, G., Hendra, P. J., Maddams, W. F., Passingham, C., Willis, H. A. and Chalmers, J., Applications of Fourier transform Raman spectroscopy in the synthetic polymer field. Spectrochimica Acta, 1990, 46A, 197–216. 3. Maddams, W. F., A review of Fourier transform Raman spectroscopic studies on polymers. Spectrochimica Acta, 1994, 50A, 1967–1986.
1.47 0.59
Xylene-treated 1.64 0.71
4. Jones, D. P., Leach, D. C. and Moore, D. R., Mechanical properties of poly(ether ether ketone) for engineering applications. Polymer, 1985, 26, 1385–1393. 5. Briscoe, B. J., Stolarski, T. A. and Davies, G. J., Boundary lubrication of polymers in model fluids. Tribology International, 1984, 17, 129–134. 6. Briscoe, B. J., Stuart, B. H., Thomas, P. S. and Williams, D. R., A comparison of thermal- and solvent-induced relaxation of poly(ether ether ketone) using Fourier transform Raman spectroscopy. Spectrochimica Acta, 1991, 47A, 1299–1303. 7. Briscoe, B. J., Stuart, B. H., Sebastian, S. and Tweedale, P. J., The failure of poly(ether ether ketone) in high speed contacts. Wear, 1993, 162, 407–417. 8. Nguyen, H. X. and Ishida, H., Molecular analysis of the crystallisation behaviour of poly(aryl ether ether ketone). Journal of Polymer Science, Polymer Physics Edition, 1986, 24, 1079–1091. 9. Briscoe, B. J. and Stuart, B. H., The surface plasticisation and lubrication of poly(ether ether ketone) by third body formation. Proceedings 22nd Leeds–Lyon Symposium on Tribology, Lyon 1995. Elsevier, Oxford, 1996. 10. Stuart, B. H. and Williams, D. R., The solvent-induced swelling of poly(ether ether ketone) by 1,1,2,2-tetrachloroethane. Polymer, 1994, 35, 1326–1328. 11. Stuart, B. H. and Williams, D. R., A study of the sorption of chlorinated organic solvents by poly(ether ether ketone) using vibrational spectroscopy. Polymer, 1995, 36, 4209–4213. 12. Briscoe, B. J., Yao, L. H. and Stolarski, T. A., The friction and wear of polytetrafluoroethylene-poly(ether ether ketone) composites: an initial appraisal of the optimum composition. Wear, 1986, 108, 357–374. 13. Stuart, B. H. and Briscoe, B. J., A Fourier transform Raman spectroscopy study of poly(ether ether ketone)/polytetrafluoroethylene blends. Spectrochimica Acta, 1994, 50A, 2005–2009. 14. Nelson, W. E., Nylon Plastics Technology. Newnes-Butterworths, London, 1976. 15. Stuart, B. H. and Briscoe, B. J., Surface plasticisation of Nylon 6,6 by water. Polymer International, 1995, 38, 95–99. 16. Stuart, B. H., A Fourier transform Raman study of water sorption by Nylon 6. Polymer Bulletin, 1994, 33, 681–686. 17. Briscoe, B. J., Stuart, B. H. and Thomas, P. S., Solvent induced morphological changes to polycarbonate. Materials Resources Society Symposium Proceedings, 1993, 304, 185–190. 18. Stuart, B. H. and Thomas, P. S., Xylene swelling of polycarbonate studied using Fourier transform Raman spectroscopy. Spectrochimica Acta, 1995, 51, 2133–2137.
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