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Chemical changes in electron-beam-irradiated polymers
Ultramicroscopy 23 (1987) 329-338 North-Holland, Amsterdam
329
CHEMICAL CHANGES IN ELECTRON-BEAM-IRRADIATED POLYMERS D. VESELY and D.S. FINCH Department of Materials Technology, Brunel University, U.xbridge, Middlesex, UB8 3PH, UK Received 12 January 1987; received at editorial office 15 June 1987
Thin specimens of chlorinated polymers, mainly PVC, were exposed in the electron microscope to progressively increasing irradiation, and the irradiated areas were analysed by Fourier Transform Infrared (FTIR) spectroscopy. Changes in the individual absorption peaks with irradiation are discussed and correlated with X-ray analysis of chlorine loss. It is shown that there are different chemical processes occurring simultaneously. In order to understand the stability of some structures, different polymers with similar groups in a different environment arc compared, It is concluded thin! the electron beam sensitivity of polymeric compounds is not only dependent on the probability of formation of ions but also on their reactivity and on the ability of the structure to absorb and dissipate the excitation energy. It is shown that the unzipping mechanism could be particulariy important for the initial stages of dehydrochlorination of the irradiated polymer and thus for the explanation of the double e~ponential decay curves.
1. Introduction The decomposition of organic material in the electron beam has been studied by a number of different techniques, as reviewed for example by Isaacson [1] or Reimer [2]. These techniques can be divided into three different categories. The first is a relative evaluation of the rate of decomposition. A typical example is the "end dose" or the 1 / e measurement of crystalline polymers [3,4]. It is not quantitative and will not yield any information as to the chemical changes in the specimen. In the second category are measurements of the decay ct~rves and their analysis, such as mass loss or ele:~aental loss measurements. Accurate dosimetry is required for quantitative evaluation. Attempts for a mathematical description of the decay curves have been made (e.g. Deigado and Hutchinson [5], Egerton [6], Vesely [7], Vesely and Finch [8]). In the last two references the case for a double exponential curve is argued against more complicated equations. This analysis has led to the interpretation of the degradation by assuming two independent chemical processes: a fast degradation ,:,f t',;~ u,~g;aal ~tr,,cture and a slow degrada-
tion of the structure formed by irradiation. This provides some information on the rate of degradation and on the amount of residual mass, making the speculations on the chemical changes more quantitative. To the third catego~ belong the experiments where an analytical technique plays an important part. Mass spectroscopy, N MR. FSR, Raman and IR spectroscop3 were all utilized (for a review see Reimer [9]). The use of IR spectroscopy has a particular attraction, as small specimens, irradiated in the electron microscope, can be used. As an example of the application of this technique to polymers, works by Bahr et al. [10] or Burney [11] could be mentioned. Unfortunately, the analysis of IR spectra is not very easy and only recently has Fourier Transform Infrared (FTIR) spectroscopy with powerful computing facilities become available. The purpose of this work is to utilize the FTIR spectroscopy for the study of chemical changes in chlorinated polymers, mainly PVC and to correlate the results with chlorine loss curves, measured by X-ray energy dispersive analysis. Some question.', of interest are: what are the weak sites for deh3drochlorination, is the dehydrochlorination
0304-3991/87/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
330
D. Vesely, D.S. Finch / Chemical changes in electron-beam-irradiatedpolymers
sensitive to the molecular configuration, what are the resulting beam stable structures and finally, how to interpret the decay curves, which have two different dehydrochlorination rates.
2. Experiment Polymers used in this work were of commercial grades, but without additives. I'heir molecular weights and crystallinity were not measured as they are not important for this work, but it is believed that the molecular weight is slightly higher than 100,000 and that the crystallinity is very low for all the polymers used. The tacticity can be evaluated from the analysis of the IR data and will be discussed later. The abbreviations used throughout the text are as follows: P VC or commercial P VC for ICI suspension poly vinyl chloride; syndiotactic P VC for highly syndiotactic PVC (about 87%), prepared by the urea clathrate route; P VDC for polyvinilidene chloride SARAN, which is a co-polymer with about 20% of PVC; chlorinated P VC for LUCALOR with 67% of chlorination; chlorinated PE for HALOFLEX polyethylene: chlorinated Cu phth.cn, for fully chlorinated copper phthalocyanine, chlorosulphonated PE for HYP.,~LON polyethylene. Thin films of about 20-25 /~m in thickness were prepared from PVC and other polymers by solvent casting, by microtomy and by compression molding. The thickness has been selected for the best sensitivity of the IR technique, but it is too thick for a homogeneous irradiation through the specimen thickness. The beam spreading and the energy loss were therefore evaluated by MonteCarlo ,.,,,,..ul,~,ons ~.1. . . . ,: and found to .,m~,., ,h,~ sured exposure by only few percent, i.e. well withm the at curacy of practical dosimetry. This has been verified by experiments with specimens 200 nm thick. The solvents were evaporated over 24 h at 50°C under vacuum. In the case of commercial PVC. the residual THF was renloved by washing the specimens in di-ethyl ether using Soxhlet ap-
paratus. The specimens were placed on a thin copper substrate with a 1 mm hole and exposed to a progressively higher irradiation. The exposure has been established within the accuracy of a few percent by observing the following points: heating during carbon coating, accidental specimen preirradiation, specimen shrinkage, mechanical and electrical drift, accuracy of area measurement, efficiency of a Faraday cage, time and spatial fluctuation in current density of the elect~on beam, linearity and time delay of the signal recording system (e.g. dead time), contamination and the effect of the supporting carbon film. The irradiated specimens were quickly transferred from the electron microscope into a Nicolet 60 SX FTIR spectrometer. The microbeam attachment was used and the spectra averaged over 200 scans, with resolution 2 cm -1. The authors are aware of the problem with oxidation, occurring during the transfer of the specimens, and of the effect the interruption of the irradiation has on the chemical processes and therefore on the final IR spectra. For this reason spectra obtained by continuous irradiation were compared with spectra obtair~ed by cumulative exposures to the same irradiatio a, with several interruptions. No detectable differences were obse;ved.The carbonyl peak, which is a measure of oxidation, increased in height only after a prolonged exposure (days) of the irradiated specimen to air.
3. Results and discussion IR spectroscopy is sensitive to chemical configurations and can thus help us to understand the changes with irradiation. The general changes in the IR spectra of irradiated PVC are shown in fig. 1 and of inadiated syndiotactic PVC in fig. 2. The most information we would like to obtain is which bonds or structures are preferentially degraded, what are the stable structures and what are the new strvctures formed? It is not very easy to interpret the individual peaks, and the information that is available in the literature is often confusing. Comparison with similar, or with known, structures could thus be valuable. In fig. 3 are the spectra of seven different chlorinated compounds.
D. Vesely, D.S. Finch / Chemical changes in electron-beam-irradiated polynwr.~
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Fig. 1. Changes of FTIR spectra of commercial PVC with irradiation.
The maip. interest is the loss of ciiiorine. Tile carbon-chlorine stretching region (500-900 cm -~) is therefore n-.ost important. Many problems are associated with the assignment of the peaks in this region, as the peaks, which number up to nine (Robinson et al. [12]) for PVC, show considerable overlap. For PVC we have three major peaks in this region: 612, 634 and 695 cm -~ (fig. 4). These peaks have been assigned [12-15] as being predominantly due to short planar zig-zag syndiotactic sequences, isotactic sequences and bent syndiotactic structures respectively. With irradiation, the peaks at 634 and 695 cm ~ disappear or shift more rapidly than the 612 cm-~ peak, indicating that the bent syndiotactic sequences are least stable, followed by isotactic sequences. There are, however, new peaks which are growing, in particular at 749 and 792 cm -~, which can be
assigned to involve chlorine (or GG isolated defects) and head-to-head -CI- CI-CHCI- configurations respectively. As these peaks are also present in unirradiated PVDC. chlorinated PE and also chlorinated PVC, this assignment is more probable than -CCI=CH- bonds. On the other hand, the spectrum of chlorinated c~'pper phthalocyanine shows most of the chlorine in these two peaks and thus the chlorine stretching peaks must be associated with double bonds. The possibility of peak overlap cannot be excluded, and both interpretations could be correct. For irradiated PVC. however, the formation of double bonds is more logical. The situation is further complicalod by tile presence of C - H out of plane bending and CH, rocking peaks on coi!iugated structures. Recent advancc~ in FTIR amd,, ~i.', using modern computing techniques have grcatk ira-
D, Vesely, D.S. Finch / Chemical changes in electron.beam-irradiated polymers
332
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Fig. 2. Change.,, of FTIR spectra of s2rndiotactic PVC with irradiation.
proved the analysis of peaks and their separation. it is likely that this will lead to resolving this uncertainty [16]. The second group of peaks between 861 and 970 cm -l is shown in fig. 5. There seem to be three overlapping peaks corresponding to C C stretching, C=C deformation and CH, rocking hnnd~ The decline and subs,.u,..cn, - . . . . . . , u peaks with exposure can be explained by the lo.,,s of C--C bonds and loss of hydrogen, ,aith the resulting double bond formation. qhe two major peaks between 1100 and 1400 c m 1 (fig. 6)car be assigned to C - H bending at 1258 and CH a twisting at 1333 cm--~. Their decrease witl~ irradiation is therefore expected.
The group of peaks between 1400 and 1480 cm-~ (fig. 7) is perhaps more interesting as the ratio of degradation is different for different peaks and some peaks are growing. Four peaks can be resolved: 1426 cm-1 (-CH2 scissor, or bending on planar syndiotactic sequences), 1434 cm -~ ( - C H 2 bending on syndiotactic sequences), 1443 and 1455 cm1 ~,--,,e tg"T_l" bending on very ~ ~1~. , 3"* . t ~cquc.t,c~j. , . c changes in these peaks with irradiation indicate shortening of syndiotactic sequences and also preferential loss of hydrogen from plan~r syndiotactic sequences. This confirms what is ogserved in the C-C1 stretching region. The increase of 1455 crn-t peak can only be partially explained by shortening of the syndiotactic sequences, as . . . . . . . . . . .
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D. Vesely, D.S. Finch / Chemical changes in electron-beam.irradiated polymers
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there must be a loss of CH 2 groups with irradia,.. tion. The 1455 cm-1 peak continues to grow e', c;:, auuv~ exposures u i I v u v , , . . / m , and it lIlllglit -' ' " po.,siblv overlap with a peak which can be assigned t C H 3 groups, as a result of chain scission. There are two growing peaks at 1623 cm-~ (fi~. 8) corresponding to C=C conjugated sequences and another at 1652 cm -~ corresponding to is(lated double bonds. These peaks are very irrportant but unfortunately small and masked by
the C=O peak (due to air oxidation) and tl:Js difficult to resolve and utilize for quantitative evaluation of poiyene sequences. Tiiis information can be obtained from spectroscopy in the UV-visible region or from Raman spectroscc, py. but these techniques are not currenl!v available to the authors.' The last group of peaks at 2830 and 3020 cm -1 (fig. 9) in the irradiated material corresponds to the C - H stretching region. The peaks at 2912
D. Vesely, D.S. Finch / Chemical changes in electron-beam-irradiated polymers
334
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Fig. 4. Changes of C-CI stretching peaks of commercial PVC with irradiation. The preferential loss of chlorine from isotactic and bent syndiotactic peaks is clearly demonstrated.
cm -~, corresponding to C H 2 stretching region and 2989 cm-~ to C-H stretching are not changing significantly. However. these peaks are quick;y
Fig. 6. Changes of C - H bending and C - H 2 twisting peaks of commercial PVC with irradiation. The loss of hydrogen is apparent.
hidden by the peaks at 2929, 2959 and 3013 cm --1 These peaks probably correspond to CH 2 symmetric and C H 2 asymmetric stretching peaks reC..n2-
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Fig. 5. Changes of C - f stretchivg. ( ' = ( deformation ano C - t t . rocking peaks of commercial P~'C ~vith irradiation. The peaks show the los, of hydrogen and C-(" bonds.
Fig. 7. Changes of C - H . peaks of commercial PVC ~ith irradiau~m, shov, ing the prefen:ntial loss ot hydrogen from svndi~tactic ,,equences and shortenin~ of these sequences.
D. Vesely, D.S. Finch / Chemical change'~ in electron-beam-~ ,radiated polymers
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spectivel,, The C - H stretching peak at 3013 cm may be related to chains containing double bonds. It is possible that the absorption coefficients of these vibrations are greater than those of similarvibrations on PVC chains, due to the formation of an ordered structure. The absorption can also be increased by the presence of a double bond (polyene) or treble bond (terminal group) on the chain. This is not observed in more ordered highly syndiotactic PVC, where all of :he peaks are decreasing. and it is not at present clear why this is so.
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I: can be concluded that chlorine is lost preferentially from long isotactic and bent syndiotactic sequences and from defects. The amount of chlorine lost is well represented by the area under the C - C I stretching region and corresponds to X-ray data measured using an energy-dispersive spectrometer (fig. 10). It is interesting to note that syndiotactic PVC loses chlorine more quickly than PVC, which is consistent with the importance of long syndiotactic sequences. It is likely that long sequences are also present in chlorinated PE and chlorosulphonated PE. The IR spectroscopy therefore suggests a possibility of an unzipping mechanism. In order to understand the role of long sequences in the dehydrochlorination process, it is useful to appreciate how the energy from an electron is transferred to the molecute and what the possible mechanisms for dehydrochlorination are. An electron, passing sufficiently close to the molecule, will cause local polarization and therefore create a highly excited state. The excitation energy can be dissipated in a number of ways: for example, a phonon can be emitted, This represents the lowest cnerg~ level and is dependent on the vibrational properties of the molecule. For most polyrams this is between !0 -? and 1O- ~ eV, but for conjugated bonds it can be as high as 2 ~o 3 eV.
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Fig. 9. Changes of C - H stretching peaks of ccmmcic,al PV( with irradiation. The growth of the peaks indicates the formation of conjugated sequences and some structural ordering.
Fig. 10. Changes in relative values of chlorine content v.ith irradiation, measured by X-ra> analysis. Relative values of c14orinc ~' ".... ' on,,' measured from arem, under C-CI stre~chine peaks for t_omr~ercia| and for >vndiotactic PVC, z~re also indicated {© ).
336
D. Vesely, D.S. Finch / Chemical changes in electron-beam-irradiated polymers
For higher excitation energies an electron can be emitted. The energies involved must be higher than the ionization energies of the indbddua! atoms. These energies are [17]: 13.6 eV for H, 11.4 eV for C and 13 eV for C1. Some information on the probability of various energy transfers can be obtained from energy-loss measurements. For example, PE film shows maximum losses at 25 eV [18,19] with the first plasmon loss corresponding to the first ionization potential of carbon. This would suggest that ionization is the most probable energy transfer. Unfortunately it is difficult to obtain this information from undamaged specimens. The minimum exposure needed for the collection of the data is about 50 C / m 2, and therefore it would be very useful to utilize parallel recording a n d / o r large areas on the specimens. !saacson t,] tl and Ditchfield et al. [19], for example, have shown that substantial losses around 5 eV are present on undamaged specimens of adenine and polystyrene. These low energy transfers will be more dependent upon the conformation and configuration of the molecular chains and might explain why thc dehydrochlorination rate is critically dependent on the chemical structure. On the other hand, a large energy transfer can trigger off a series of chemical reactions. It might be useful at this stage to compare the energies transferred per monomer tlnit with ~he amount of chlorine evolved. The average energy lost by an electron in the specimen of unit thickness can be calculated from the Be:he equation (see e.g. Reimer [20]). The result indicates that one 100 keV will lose on average 400 eV per micron of travel in an organic (light element) specimen, This distance will, for PVC, contain about 250 monomer units. The energy lost per one monomer unit is therefore 1.6 eV. However, only at 100 C/re'- will this energy be transferred to all of the monomer units. At this exposure, 46% of the total chlorine is lost, which v.cans that 3.5 eV was needed per HCi molecule evolved. This is much ',u~,.:, ...... than 26.6 eV needed for H and CI ionizations. At the exposure of l0 C / m 2 where 10% of the chlorine for syndiotactic PVC (compared to 6°7c for PVC), even less energy is needed (1.6 eV per HCI molecule for syndio~actic PVC). This strong dependence of the average energy needed for HC1 formation is very
difficult to explain by a single-hit theory and it seems most logical to assume that the ionization c,f a carbon atom will initiate a chain reaction, forming initially 10 or more HCI molecules. A possible chain reaction due to charge transfer is shown in fig. 11. The values of chemical energies for different bonds can be found in the literature (e.g., ref. [12]) and a simple summation will show that 4 eV is needed for breaking C - H and C-C1 bonds and the formation of a double bond. On the other hand, 4.5 eV is obtained from HC1 formation. The reaction, therefore, is energetically favourable and a single ionization can dehydrochlorinate long chain sequences. The termination of the chain reaction can occur by the trapping of an electron or by the formation of a cross-link with the neighbouring chain. The availability of long uninterrupted sequences on the polymer chain is therefore needed for raoid dehydrochlorination. Since tile ionization energy of the atoms concerned (C, CI, H) is approximately ten times greater than the average energy dissipated by the ionizing electron, it might be assumed that the average length of the chain dehydrochlorinated by one ionization is about 10 monomer units for syndiotactic PVC and !ess for commercial PVC or for irradiated materials. As the sequences become shorter and shorter with progressive irradiation, the number of chlorine atoms lost eventually becomes equal to the number of atoms ionized. This model is consistent with decay curve measurements and also with the above IR observations. It can be used to explain a number of different observations, such as: (1) The electron beam sensitivity is high for polymers with long regular sequences, but low for is,~lated chlorine atoms on large molecules (i.e. low volume density of chlorine atoms).
H Cl
..•ct:Z•;Cl H H HH
H
CiH
CIH
H H HHH
Fig. 11. Schematic diagram of a chain reaction in a PVC molecule, initiated bv a loss of an electron.
D. Vesely, D.S. Finch / Chemical changes in electron-beam,irradiawd polymers
(2) The double exponential equation I/lo
--
(1 - A) e - k ' v + A e -~-'D
for the decay curves [7,8] can be interpreted in such a way that the first exponent k~ is related to the length of the reacting sequences, the second exponent k 2 to the density of chlorine atoms and A is the proportion of chlorine atoms to be removed by a single-hit mechanism. (3) The conjugated bonds, once formed, will transfer the charge without chemicql reactions. Chlorine and hydrogen atoms can therefore be attached to a relatively ,=..'.able structure. The formation of this stable structure is, however, dependent on the dehydrochlorination route and therefore on the chemical structure of the starting material. There are still many unanswered questions, For example, we do not know how long the polyene sequences are, if they are of cis or trans configuration and if chlorine atoms are present. We would like to know the detailed structure of the dehydrocl'~lorination sites, whether the probability of dehydrochlorination is dependent upon the proximity of chlorine atoms, and what the probability of C12, H2, cyclic structures, end groups and crosslink formation is. We believe that modern FTIR sp~'ctroscopy will in the future provide answers to many of these questions.
4. Conclusions It has been found from FTIR analysis that the electron beam sensitivity of chlorinated polymers is dependent on the vulnerability of the structure to unzipping chain reactions. This can explain why polymers with identical chemical composition but different configurational and conformational states exhibit such large differences in beam sensitivity'. T'~is can also explain the two different exponential terms in the equation for the dehydrochlorination curves, corresponding to two reaction rates. The evidence for the formation of long polyene sequences, which would confirm this mechanism, is however not yet available. The alternative mechanism previously proposed [7] assumed a decrease in the dehydrochlorinatic, n rate due to changes in the chemical environment. namely trapping of chlorine on co~ugated sequences. The infrared spec'{roscop5 gives evidence
33"7
for and against both mechanisms and only further more detailed and critical analv.,,is ,.,,~11provide the answer.
Acknowledgements The authors wish to grateful, ly acknowledge the cooperation with BP Research, Sunbury on Thames, MidO,esox. In particular the help and advice given t)y Mr. P.B. Tooke was invaluable. The authors wo..dd also like to thank to SERC for financial support.
References [1] M.S. !saacson, in: Principles and Techniques of E!ectr,-m Microscopy, Vol. ?, Ed. M. Hay~t (Van Nostrand-Reinhold, New York, 1977L [2] L. Reimer, Ultramicroscopy 14 (1984) 291. [3] D.T. Grubb and G.W, Groves, Phil. Mag. 24 t1971) 815. [4] E, Knapek, Ultramicroscopy 10 (1932) 71. [5] L.A. Delgado and T.E. Hutchin:.on, Ultramicroscopy 4 (1979) 163. [6] R.F. Egerton, Ultramicroscopy 5 (1980) 521. [71 D. Veselv, Ultramicroscopy 14 ti984) 27q. [8] D. Veselv and D.S. [inch, in: Electron Microscopy and Analysis 1985, Inst. Phys. Conf. S,:r. 78, Ed. G.J. Tatlock (Inst. Phys., London-Bristol, 19867 p. 7. [9] L. Reimer, Transmission Flectron Microscop> ~Springcr. Berlin, 1984). [10] G,F. Bahr, F.B. Johnson and E. Zeitler, Lab. Invest. 14 (1965) 377, [11] S.G, Burney, Radiation Phys. Chem, 13 (1979) 171. [12] E.R. Robinson. D.I. Bower and W.F. Maddams, Polymer 19 (1978) 773. [131 S. Krimm, V.L. Folt, J.J. Shipman and A.R, Berens, J. Polymer Sci B2 (1964) 1009:B3 (]965) 275. [14] H.V. ~,on Pohl and D.O. Hummel, Macron~ol, Chem, 113 (1968) 190 and 203. [15] P.P, Painter, M,M. Coleman and J.L. Koenig, The Theorx of Vibrational Spectroscopy and its Applications to Polxmerit Ma!erials (Wile>', New Y,~rk. !oS2! [le,] D.S, Finch. D. Veselv and P,B. I ooke, to be publi,,hed. [171 W.J. Moore. Physical ('hemistr~ (Longman. London. 1978), [18] D. Veseb,, in. Developments in Electron Micro.~cop.,, and Analysis 1977, Inst. Phvs Conf. Set. 36, Ed, D.L. Mi,,,ell (Inst. Phys., London-Bristol, 1977)p. 389. [19] R.W. Ditchfield, D.T. Grubb and M,3. \Vhelan, Phil. Mag. 27 (1~:73) 1267. [2~q k. Reimer. Scanning Electron Microscopy {Springer. Berlin. t9~;5). [21] J.D. Robert,,, and M.C. Caserio. Basic P,inciples of Organic Chemistry (BenJamin, New York, 1977).
329
CHEMICAL CHANGES IN ELECTRON-BEAM-IRRADIATED POLYMERS D. VESELY and D.S. FINCH Department of Materials Technology, Brunel University, U.xbridge, Middlesex, UB8 3PH, UK Received 12 January 1987; received at editorial office 15 June 1987
Thin specimens of chlorinated polymers, mainly PVC, were exposed in the electron microscope to progressively increasing irradiation, and the irradiated areas were analysed by Fourier Transform Infrared (FTIR) spectroscopy. Changes in the individual absorption peaks with irradiation are discussed and correlated with X-ray analysis of chlorine loss. It is shown that there are different chemical processes occurring simultaneously. In order to understand the stability of some structures, different polymers with similar groups in a different environment arc compared, It is concluded thin! the electron beam sensitivity of polymeric compounds is not only dependent on the probability of formation of ions but also on their reactivity and on the ability of the structure to absorb and dissipate the excitation energy. It is shown that the unzipping mechanism could be particulariy important for the initial stages of dehydrochlorination of the irradiated polymer and thus for the explanation of the double e~ponential decay curves.
1. Introduction The decomposition of organic material in the electron beam has been studied by a number of different techniques, as reviewed for example by Isaacson [1] or Reimer [2]. These techniques can be divided into three different categories. The first is a relative evaluation of the rate of decomposition. A typical example is the "end dose" or the 1 / e measurement of crystalline polymers [3,4]. It is not quantitative and will not yield any information as to the chemical changes in the specimen. In the second category are measurements of the decay ct~rves and their analysis, such as mass loss or ele:~aental loss measurements. Accurate dosimetry is required for quantitative evaluation. Attempts for a mathematical description of the decay curves have been made (e.g. Deigado and Hutchinson [5], Egerton [6], Vesely [7], Vesely and Finch [8]). In the last two references the case for a double exponential curve is argued against more complicated equations. This analysis has led to the interpretation of the degradation by assuming two independent chemical processes: a fast degradation ,:,f t',;~ u,~g;aal ~tr,,cture and a slow degrada-
tion of the structure formed by irradiation. This provides some information on the rate of degradation and on the amount of residual mass, making the speculations on the chemical changes more quantitative. To the third catego~ belong the experiments where an analytical technique plays an important part. Mass spectroscopy, N MR. FSR, Raman and IR spectroscop3 were all utilized (for a review see Reimer [9]). The use of IR spectroscopy has a particular attraction, as small specimens, irradiated in the electron microscope, can be used. As an example of the application of this technique to polymers, works by Bahr et al. [10] or Burney [11] could be mentioned. Unfortunately, the analysis of IR spectra is not very easy and only recently has Fourier Transform Infrared (FTIR) spectroscopy with powerful computing facilities become available. The purpose of this work is to utilize the FTIR spectroscopy for the study of chemical changes in chlorinated polymers, mainly PVC and to correlate the results with chlorine loss curves, measured by X-ray energy dispersive analysis. Some question.', of interest are: what are the weak sites for deh3drochlorination, is the dehydrochlorination
0304-3991/87/$03.50 ~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
330
D. Vesely, D.S. Finch / Chemical changes in electron-beam-irradiatedpolymers
sensitive to the molecular configuration, what are the resulting beam stable structures and finally, how to interpret the decay curves, which have two different dehydrochlorination rates.
2. Experiment Polymers used in this work were of commercial grades, but without additives. I'heir molecular weights and crystallinity were not measured as they are not important for this work, but it is believed that the molecular weight is slightly higher than 100,000 and that the crystallinity is very low for all the polymers used. The tacticity can be evaluated from the analysis of the IR data and will be discussed later. The abbreviations used throughout the text are as follows: P VC or commercial P VC for ICI suspension poly vinyl chloride; syndiotactic P VC for highly syndiotactic PVC (about 87%), prepared by the urea clathrate route; P VDC for polyvinilidene chloride SARAN, which is a co-polymer with about 20% of PVC; chlorinated P VC for LUCALOR with 67% of chlorination; chlorinated PE for HALOFLEX polyethylene: chlorinated Cu phth.cn, for fully chlorinated copper phthalocyanine, chlorosulphonated PE for HYP.,~LON polyethylene. Thin films of about 20-25 /~m in thickness were prepared from PVC and other polymers by solvent casting, by microtomy and by compression molding. The thickness has been selected for the best sensitivity of the IR technique, but it is too thick for a homogeneous irradiation through the specimen thickness. The beam spreading and the energy loss were therefore evaluated by MonteCarlo ,.,,,,..ul,~,ons ~.1. . . . ,: and found to .,m~,., ,h,~ sured exposure by only few percent, i.e. well withm the at curacy of practical dosimetry. This has been verified by experiments with specimens 200 nm thick. The solvents were evaporated over 24 h at 50°C under vacuum. In the case of commercial PVC. the residual THF was renloved by washing the specimens in di-ethyl ether using Soxhlet ap-
paratus. The specimens were placed on a thin copper substrate with a 1 mm hole and exposed to a progressively higher irradiation. The exposure has been established within the accuracy of a few percent by observing the following points: heating during carbon coating, accidental specimen preirradiation, specimen shrinkage, mechanical and electrical drift, accuracy of area measurement, efficiency of a Faraday cage, time and spatial fluctuation in current density of the elect~on beam, linearity and time delay of the signal recording system (e.g. dead time), contamination and the effect of the supporting carbon film. The irradiated specimens were quickly transferred from the electron microscope into a Nicolet 60 SX FTIR spectrometer. The microbeam attachment was used and the spectra averaged over 200 scans, with resolution 2 cm -1. The authors are aware of the problem with oxidation, occurring during the transfer of the specimens, and of the effect the interruption of the irradiation has on the chemical processes and therefore on the final IR spectra. For this reason spectra obtained by continuous irradiation were compared with spectra obtair~ed by cumulative exposures to the same irradiatio a, with several interruptions. No detectable differences were obse;ved.The carbonyl peak, which is a measure of oxidation, increased in height only after a prolonged exposure (days) of the irradiated specimen to air.
3. Results and discussion IR spectroscopy is sensitive to chemical configurations and can thus help us to understand the changes with irradiation. The general changes in the IR spectra of irradiated PVC are shown in fig. 1 and of inadiated syndiotactic PVC in fig. 2. The most information we would like to obtain is which bonds or structures are preferentially degraded, what are the stable structures and what are the new strvctures formed? It is not very easy to interpret the individual peaks, and the information that is available in the literature is often confusing. Comparison with similar, or with known, structures could thus be valuable. In fig. 3 are the spectra of seven different chlorinated compounds.
D. Vesely, D.S. Finch / Chemical changes in electron-beam-irradiated polynwr.~
oo0
ag0o ........3z'oo
z~00
2~o0
edoo
~ d O ........i~00 .... ~qd/5-
IAAVENUMBERS
~oo--
i6oo
331
~6d -
g-0d......... O0
Fig. 1. Changes of FTIR spectra of commercial PVC with irradiation.
The maip. interest is the loss of ciiiorine. Tile carbon-chlorine stretching region (500-900 cm -~) is therefore n-.ost important. Many problems are associated with the assignment of the peaks in this region, as the peaks, which number up to nine (Robinson et al. [12]) for PVC, show considerable overlap. For PVC we have three major peaks in this region: 612, 634 and 695 cm -~ (fig. 4). These peaks have been assigned [12-15] as being predominantly due to short planar zig-zag syndiotactic sequences, isotactic sequences and bent syndiotactic structures respectively. With irradiation, the peaks at 634 and 695 cm ~ disappear or shift more rapidly than the 612 cm-~ peak, indicating that the bent syndiotactic sequences are least stable, followed by isotactic sequences. There are, however, new peaks which are growing, in particular at 749 and 792 cm -~, which can be
assigned to involve chlorine (or GG isolated defects) and head-to-head -CI- CI-CHCI- configurations respectively. As these peaks are also present in unirradiated PVDC. chlorinated PE and also chlorinated PVC, this assignment is more probable than -CCI=CH- bonds. On the other hand, the spectrum of chlorinated c~'pper phthalocyanine shows most of the chlorine in these two peaks and thus the chlorine stretching peaks must be associated with double bonds. The possibility of peak overlap cannot be excluded, and both interpretations could be correct. For irradiated PVC. however, the formation of double bonds is more logical. The situation is further complicalod by tile presence of C - H out of plane bending and CH, rocking peaks on coi!iugated structures. Recent advancc~ in FTIR amd,, ~i.', using modern computing techniques have grcatk ira-
D, Vesely, D.S. Finch / Chemical changes in electron.beam-irradiated polymers
332
I r ~ ~ ~ ~SYN" PVC0
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CIM2
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....
SYN.
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.
.
.
.
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zg0o ~ ,~od ........i~oo
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.......
goo
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~oo
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Fig. 2. Change.,, of FTIR spectra of s2rndiotactic PVC with irradiation.
proved the analysis of peaks and their separation. it is likely that this will lead to resolving this uncertainty [16]. The second group of peaks between 861 and 970 cm -l is shown in fig. 5. There seem to be three overlapping peaks corresponding to C C stretching, C=C deformation and CH, rocking hnnd~ The decline and subs,.u,..cn, - . . . . . . , u peaks with exposure can be explained by the lo.,,s of C--C bonds and loss of hydrogen, ,aith the resulting double bond formation. qhe two major peaks between 1100 and 1400 c m 1 (fig. 6)car be assigned to C - H bending at 1258 and CH a twisting at 1333 cm--~. Their decrease witl~ irradiation is therefore expected.
The group of peaks between 1400 and 1480 cm-~ (fig. 7) is perhaps more interesting as the ratio of degradation is different for different peaks and some peaks are growing. Four peaks can be resolved: 1426 cm-1 (-CH2 scissor, or bending on planar syndiotactic sequences), 1434 cm -~ ( - C H 2 bending on syndiotactic sequences), 1443 and 1455 cm1 ~,--,,e tg"T_l" bending on very ~ ~1~. , 3"* . t ~cquc.t,c~j. , . c changes in these peaks with irradiation indicate shortening of syndiotactic sequences and also preferential loss of hydrogen from plan~r syndiotactic sequences. This confirms what is ogserved in the C-C1 stretching region. The increase of 1455 crn-t peak can only be partially explained by shortening of the syndiotactic sequences, as . . . . . . . . . . .
~
"r'L~
D. Vesely, D.S. Finch / Chemical changes in electron-beam.irradiated polymers
IC
J
L
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PVC
CHLCRINATEDPE
CHLU~INATED
~...
J
/
I
CU.PHT.CN
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Fig. 3. FTIR spectra of some un-irradiated chlorinatedcompounds.
there must be a loss of CH 2 groups with irradia,.. tion. The 1455 cm-1 peak continues to grow e', c;:, auuv~ exposures u i I v u v , , . . / m , and it lIlllglit -' ' " po.,siblv overlap with a peak which can be assigned t C H 3 groups, as a result of chain scission. There are two growing peaks at 1623 cm-~ (fi~. 8) corresponding to C=C conjugated sequences and another at 1652 cm -~ corresponding to is(lated double bonds. These peaks are very irrportant but unfortunately small and masked by
the C=O peak (due to air oxidation) and tl:Js difficult to resolve and utilize for quantitative evaluation of poiyene sequences. Tiiis information can be obtained from spectroscopy in the UV-visible region or from Raman spectroscc, py. but these techniques are not currenl!v available to the authors.' The last group of peaks at 2830 and 3020 cm -1 (fig. 9) in the irradiated material corresponds to the C - H stretching region. The peaks at 2912
D. Vesely, D.S. Finch / Chemical changes in electron-beam-irradiated polymers
334
C lrn 2
elm2-0
~'~0
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590
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.
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.
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.
.
.
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Fig. 4. Changes of C-CI stretching peaks of commercial PVC with irradiation. The preferential loss of chlorine from isotactic and bent syndiotactic peaks is clearly demonstrated.
cm -~, corresponding to C H 2 stretching region and 2989 cm-~ to C-H stretching are not changing significantly. However. these peaks are quick;y
Fig. 6. Changes of C - H bending and C - H 2 twisting peaks of commercial PVC with irradiation. The loss of hydrogen is apparent.
hidden by the peaks at 2929, 2959 and 3013 cm --1 These peaks probably correspond to CH 2 symmetric and C H 2 asymmetric stretching peaks reC..n2-
A
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•
5
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Fig. 5. Changes of C - f stretchivg. ( ' = ( deformation ano C - t t . rocking peaks of commercial P~'C ~vith irradiation. The peaks show the los, of hydrogen and C-(" bonds.
Fig. 7. Changes of C - H . peaks of commercial PVC ~ith irradiau~m, shov, ing the prefen:ntial loss ot hydrogen from svndi~tactic ,,equences and shortenin~ of these sequences.
D. Vesely, D.S. Finch / Chemical change'~ in electron-beam-~ ,radiated polymers
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i J37
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-~'Y67 ....... I~Be IdtqVENUHBEI=I
.... l g 9 - 7
~gle
Fig. 8. Changes of C=C stretching peaks of eommercia! PVC with irradiation show tlte formation and destruction of isolated double bonds, formation of conjugated sequences and oxidation, -1
spectivel,, The C - H stretching peak at 3013 cm may be related to chains containing double bonds. It is possible that the absorption coefficients of these vibrations are greater than those of similarvibrations on PVC chains, due to the formation of an ordered structure. The absorption can also be increased by the presence of a double bond (polyene) or treble bond (terminal group) on the chain. This is not observed in more ordered highly syndiotactic PVC, where all of :he peaks are decreasing. and it is not at present clear why this is so.
C/m2- 6 0 ~
335
I: can be concluded that chlorine is lost preferentially from long isotactic and bent syndiotactic sequences and from defects. The amount of chlorine lost is well represented by the area under the C - C I stretching region and corresponds to X-ray data measured using an energy-dispersive spectrometer (fig. 10). It is interesting to note that syndiotactic PVC loses chlorine more quickly than PVC, which is consistent with the importance of long syndiotactic sequences. It is likely that long sequences are also present in chlorinated PE and chlorosulphonated PE. The IR spectroscopy therefore suggests a possibility of an unzipping mechanism. In order to understand the role of long sequences in the dehydrochlorination process, it is useful to appreciate how the energy from an electron is transferred to the molecute and what the possible mechanisms for dehydrochlorination are. An electron, passing sufficiently close to the molecule, will cause local polarization and therefore create a highly excited state. The excitation energy can be dissipated in a number of ways: for example, a phonon can be emitted, This represents the lowest cnerg~ level and is dependent on the vibrational properties of the molecule. For most polyrams this is between !0 -? and 1O- ~ eV, but for conjugated bonds it can be as high as 2 ~o 3 eV.
I00' BO-
!
60
E PVUC
\ % >
SYNOIOT~CTIC Pvc,
]
CL-----'~E
[:0
O -
200
400 -
-"
6C~C
0/1,12
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Fig. 9. Changes of C - H stretching peaks of ccmmcic,al PV( with irradiation. The growth of the peaks indicates the formation of conjugated sequences and some structural ordering.
Fig. 10. Changes in relative values of chlorine content v.ith irradiation, measured by X-ra> analysis. Relative values of c14orinc ~' ".... ' on,,' measured from arem, under C-CI stre~chine peaks for t_omr~ercia| and for >vndiotactic PVC, z~re also indicated {© ).
336
D. Vesely, D.S. Finch / Chemical changes in electron-beam-irradiated polymers
For higher excitation energies an electron can be emitted. The energies involved must be higher than the ionization energies of the indbddua! atoms. These energies are [17]: 13.6 eV for H, 11.4 eV for C and 13 eV for C1. Some information on the probability of various energy transfers can be obtained from energy-loss measurements. For example, PE film shows maximum losses at 25 eV [18,19] with the first plasmon loss corresponding to the first ionization potential of carbon. This would suggest that ionization is the most probable energy transfer. Unfortunately it is difficult to obtain this information from undamaged specimens. The minimum exposure needed for the collection of the data is about 50 C / m 2, and therefore it would be very useful to utilize parallel recording a n d / o r large areas on the specimens. !saacson t,] tl and Ditchfield et al. [19], for example, have shown that substantial losses around 5 eV are present on undamaged specimens of adenine and polystyrene. These low energy transfers will be more dependent upon the conformation and configuration of the molecular chains and might explain why thc dehydrochlorination rate is critically dependent on the chemical structure. On the other hand, a large energy transfer can trigger off a series of chemical reactions. It might be useful at this stage to compare the energies transferred per monomer tlnit with ~he amount of chlorine evolved. The average energy lost by an electron in the specimen of unit thickness can be calculated from the Be:he equation (see e.g. Reimer [20]). The result indicates that one 100 keV will lose on average 400 eV per micron of travel in an organic (light element) specimen, This distance will, for PVC, contain about 250 monomer units. The energy lost per one monomer unit is therefore 1.6 eV. However, only at 100 C/re'- will this energy be transferred to all of the monomer units. At this exposure, 46% of the total chlorine is lost, which v.cans that 3.5 eV was needed per HCi molecule evolved. This is much ',u~,.:, ...... than 26.6 eV needed for H and CI ionizations. At the exposure of l0 C / m 2 where 10% of the chlorine for syndiotactic PVC (compared to 6°7c for PVC), even less energy is needed (1.6 eV per HCI molecule for syndio~actic PVC). This strong dependence of the average energy needed for HC1 formation is very
difficult to explain by a single-hit theory and it seems most logical to assume that the ionization c,f a carbon atom will initiate a chain reaction, forming initially 10 or more HCI molecules. A possible chain reaction due to charge transfer is shown in fig. 11. The values of chemical energies for different bonds can be found in the literature (e.g., ref. [12]) and a simple summation will show that 4 eV is needed for breaking C - H and C-C1 bonds and the formation of a double bond. On the other hand, 4.5 eV is obtained from HC1 formation. The reaction, therefore, is energetically favourable and a single ionization can dehydrochlorinate long chain sequences. The termination of the chain reaction can occur by the trapping of an electron or by the formation of a cross-link with the neighbouring chain. The availability of long uninterrupted sequences on the polymer chain is therefore needed for raoid dehydrochlorination. Since tile ionization energy of the atoms concerned (C, CI, H) is approximately ten times greater than the average energy dissipated by the ionizing electron, it might be assumed that the average length of the chain dehydrochlorinated by one ionization is about 10 monomer units for syndiotactic PVC and !ess for commercial PVC or for irradiated materials. As the sequences become shorter and shorter with progressive irradiation, the number of chlorine atoms lost eventually becomes equal to the number of atoms ionized. This model is consistent with decay curve measurements and also with the above IR observations. It can be used to explain a number of different observations, such as: (1) The electron beam sensitivity is high for polymers with long regular sequences, but low for is,~lated chlorine atoms on large molecules (i.e. low volume density of chlorine atoms).
H Cl
..•ct:Z•;Cl H H HH
H
CiH
CIH
H H HHH
Fig. 11. Schematic diagram of a chain reaction in a PVC molecule, initiated bv a loss of an electron.
D. Vesely, D.S. Finch / Chemical changes in electron-beam,irradiawd polymers
(2) The double exponential equation I/lo
--
(1 - A) e - k ' v + A e -~-'D
for the decay curves [7,8] can be interpreted in such a way that the first exponent k~ is related to the length of the reacting sequences, the second exponent k 2 to the density of chlorine atoms and A is the proportion of chlorine atoms to be removed by a single-hit mechanism. (3) The conjugated bonds, once formed, will transfer the charge without chemicql reactions. Chlorine and hydrogen atoms can therefore be attached to a relatively ,=..'.able structure. The formation of this stable structure is, however, dependent on the dehydrochlorination route and therefore on the chemical structure of the starting material. There are still many unanswered questions, For example, we do not know how long the polyene sequences are, if they are of cis or trans configuration and if chlorine atoms are present. We would like to know the detailed structure of the dehydrocl'~lorination sites, whether the probability of dehydrochlorination is dependent upon the proximity of chlorine atoms, and what the probability of C12, H2, cyclic structures, end groups and crosslink formation is. We believe that modern FTIR sp~'ctroscopy will in the future provide answers to many of these questions.
4. Conclusions It has been found from FTIR analysis that the electron beam sensitivity of chlorinated polymers is dependent on the vulnerability of the structure to unzipping chain reactions. This can explain why polymers with identical chemical composition but different configurational and conformational states exhibit such large differences in beam sensitivity'. T'~is can also explain the two different exponential terms in the equation for the dehydrochlorination curves, corresponding to two reaction rates. The evidence for the formation of long polyene sequences, which would confirm this mechanism, is however not yet available. The alternative mechanism previously proposed [7] assumed a decrease in the dehydrochlorinatic, n rate due to changes in the chemical environment. namely trapping of chlorine on co~ugated sequences. The infrared spec'{roscop5 gives evidence
33"7
for and against both mechanisms and only further more detailed and critical analv.,,is ,.,,~11provide the answer.
Acknowledgements The authors wish to grateful, ly acknowledge the cooperation with BP Research, Sunbury on Thames, MidO,esox. In particular the help and advice given t)y Mr. P.B. Tooke was invaluable. The authors wo..dd also like to thank to SERC for financial support.
References [1] M.S. !saacson, in: Principles and Techniques of E!ectr,-m Microscopy, Vol. ?, Ed. M. Hay~t (Van Nostrand-Reinhold, New York, 1977L [2] L. Reimer, Ultramicroscopy 14 (1984) 291. [3] D.T. Grubb and G.W, Groves, Phil. Mag. 24 t1971) 815. [4] E, Knapek, Ultramicroscopy 10 (1932) 71. [5] L.A. Delgado and T.E. Hutchin:.on, Ultramicroscopy 4 (1979) 163. [6] R.F. Egerton, Ultramicroscopy 5 (1980) 521. [71 D. Veselv, Ultramicroscopy 14 ti984) 27q. [8] D. Veselv and D.S. [inch, in: Electron Microscopy and Analysis 1985, Inst. Phys. Conf. S,:r. 78, Ed. G.J. Tatlock (Inst. Phys., London-Bristol, 19867 p. 7. [9] L. Reimer, Transmission Flectron Microscop> ~Springcr. Berlin, 1984). [10] G,F. Bahr, F.B. Johnson and E. Zeitler, Lab. Invest. 14 (1965) 377, [11] S.G, Burney, Radiation Phys. Chem, 13 (1979) 171. [12] E.R. Robinson. D.I. Bower and W.F. Maddams, Polymer 19 (1978) 773. [131 S. Krimm, V.L. Folt, J.J. Shipman and A.R, Berens, J. Polymer Sci B2 (1964) 1009:B3 (]965) 275. [14] H.V. ~,on Pohl and D.O. Hummel, Macron~ol, Chem, 113 (1968) 190 and 203. [15] P.P, Painter, M,M. Coleman and J.L. Koenig, The Theorx of Vibrational Spectroscopy and its Applications to Polxmerit Ma!erials (Wile>', New Y,~rk. !oS2! [le,] D.S, Finch. D. Veselv and P,B. I ooke, to be publi,,hed. [171 W.J. Moore. Physical ('hemistr~ (Longman. London. 1978), [18] D. Veseb,, in. Developments in Electron Micro.~cop.,, and Analysis 1977, Inst. Phvs Conf. Set. 36, Ed, D.L. Mi,,,ell (Inst. Phys., London-Bristol, 1977)p. 389. [19] R.W. Ditchfield, D.T. Grubb and M,3. \Vhelan, Phil. Mag. 27 (1~:73) 1267. [2~q k. Reimer. Scanning Electron Microscopy {Springer. Berlin. t9~;5). [21] J.D. Robert,,, and M.C. Caserio. Basic P,inciples of Organic Chemistry (BenJamin, New York, 1977).