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(6) Becker, 0. W.; TackM, P. M.; Bromer, W. W.; Lefeber. D. S.; Riggin, R. M. 810technol. Appl. 8bchem. 1988, IO,326-337. (7) Garnick, R. L.; Solli, N. J.; Papa, P. A. Anal. Chem. 1088, 60,
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2546-2558. ( 8 ) Hancock, W. S.; Canova-Davis, E.; Battersby. J.; Chloupek, R. I n 810technologicalv-med Medical Agents the Schnrific Besis of Their Reguletbn; Gueriguian, J. L., Fattorusso, V., Poggblini, D., Eds.; Christengraf Rome, 1987; pp 29-49. (9) Stachowlak, K.; Wilder, C.: Vestal, M. L.; Dyckes, D. F. J. Am. Chem. SOC. 1988, 110, 1758-1765. (IO) Fink. S. W.; Freas, R. B. Anal. Chem. 1989. 61,2050-2054. (11) Caprioli, R. M.: Fan, T.; Cottreil, J. D. Anal. Chem. 1988. 58, 2 949-2 954. (12) Henzel, W. J.; Boureil, J. H.; Stults, J. T. Anal. Biochem. 1990, 187, 228-233. (13) Hemling, M. E.; Roberts, G. D.; Johnson, W.; Carr, S. A,; Covey, T. R. Biomed. Environ. Mass Spectrom. 1990, 19,677-691. (14) Whitehouse, C. M.;Dreyer, R. N.; Yamashita, M.; Fenn, J. B. Anal. Chem. 1985, 57,675-679. (15) Covey, T. R.; Bonner, R. F.; Shushan, B. I.; Henion, J. Rapid Commun. &ass Spectrom. 1988, 2 ,249-256. (16) Smith, R. D.; Loo, J. A.; Edmonds, C. G.; Barinaga, C. J.; Udseth, H. R. Anal. Chem. 19S0, 62,882-899. (17) Fenn, J. 6.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Science 1989, 246, 64-71. (18) Bruins, A. P.; Covey, T. R.; Henion, J. D. Anal. Chem. 1987, 59, 2642-2646. -..- -. .. . (19) LOO, J. A.; Edmonds, C. G.; Udseth, H. R.; Smith, R. D. Anal. Chem. 1990, 62,693-698.
RECEIVED for review July 8, 1991. Accepted September 25, 1991.
Monitoring Polyvinyl Chloride Degradation Using Raman Microline Focus Spectrometry Michael Bowden, P a u l Donaldson, a n d Derek J. Gardiner* Department of Chemical and Life Sciences, Newcastle upon Tyne Polytechnic, Ellison Place, Newcastle upon Tyne,
U.K. NE1 8ST Joanne Birnie and Donald L. Gerrard B P Research Centre, Chertsey Road, Sunbury on Thames, Middlesex, U.K. TWIG 7LN
A line focus Raman microscope has been developed which utlllzes a CCD detector to obtain Raman spectra from a 100 pm length line on a sample. The technique has been applied to a study of chemical and thermal dehydrochlorination of PVC sheet. The results show the variation of degradation rates throughout the thickness of the PVC sheet and suggest that the modal polyene chain length giving rise to the resonantly enhanced spectra has 11-12 double bonds. I n a separate experiment the degradation characteristics of foambacked PVC sheet have been studled and the results suggest that competing thermal and chemical mechanisms are involved, producing modal polyene chain lengths of 13 and 19 double bonds.
INTRODUCTION The use of a cooled, slow-scanned, CCD detector in conjunction with a Raman microscope opens up the possibility of micro-Raman line scan or profiling studies on a realistic time scale. The fundamental requirement for such work is an instrument capable of focusing and collecting the Raman scatter from a microline illumination on the sample. This, coupled with a suitable filter/spectrograph and a two-dimensional detector, allows Raman spectra from the length of
* To whom correspondence should be addressed. 0003-2700/91/0363-2915$02.50/0
the focused microline to be recovered simultaneously. This approach is also applicable to fluorescence and luminescence spectrometry, and we have coined the acronym MiFS (microline focus spectrometry) for this type of instrumentation (1,2). Other workers have used line imaging to obtain spectral and spatial information using, for example, an imaging photomultiplier (3)or a CCD detector (4). The latter example utilized surface polariton field-enhanced Raman scattering to report one-dimensional line scans from Langmuir-Blodgett-Kuhn films with 50-pm resolution; however, no details of the illumination optics were reported. None of the previous Raman work has been carried out on an optical microscopy scale. The work described here involves the development and application of Raman MiFS instrumentation. Our fiit design of such an instrument (5) relied upon an electromechanical system to scan the laser beam across the back aperture of the focusing objective of a Raman microscope to generate a line. In addition a subtractive double monochromator was used as a laser line filter prior to a spectrograph stage to disperse the Raman spectra. This system, which demonstrating the potential of the method, suffered from severe alignment difficulties and system losses. The instrument described here relies on interference filters to remove the laser line and cylindrical optics coupled to the objective to generate the microline focus. The advantages of Raman MiFS over single-point Raman microscopy include: (i) improved sensitivity due to the electrooptical characteristics of the CCD detector (6); (ii) 0 1991 American Chemical Society
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Figure 1. Spectrometer schematic: (A) air cooled laser; (C) CCD camera; (E) entrance slit; (Fl) dichroic beam-splitter; (F2) edge filter; (G) grating; (Ll) cylindrical lens; (L2) matching lens; (L3) correcting lens; (Ml, M2, M3) spectrograph mirrors; (0)objective; (P) prism; (S) sample.
reduced risk of sample damage due to the distribution of the laser; (iii) simultaneous collection of spatially related Raman data obviating the need to monitor laser power and other experimental variations; (iv) on-chip data processing and data presentation; (v) speed of data collection; (vi) possibilities for Raman imaging. Dehydrochlorination of polyvinylchloride (PVC) results in the formation of all trans-conjugated polyene chains in the polymer backbone. The polyene chains, of varying chain length, have a broad electronic absorption in the visible region which supports a strong resonance enhancement of vibrations associated with the -C-C-Cbackbone. In particular the C=C stretch ( v 2 ) near 1500 cm-' and the C-C stretch (vl) near 1100 cm-' are enhanced sufficiently to allow detection of PVC degradation a t levels (7). The degree of resonance enhancement is determined by the proximity of the laser excitation wavelength to the electronic absorption band profile maximum. This in turn is determined by the polyene chain length distribution in the sample (8). Furthermore, the polyene chain length affects both the C=C and C-C vibration frequencies (9);thus for a given laser line the frequency of the resonantly enhanced C=C and C-C bands reflects changes in polyene chain length distibution. Intensity changes may result from overall polyene concentration variations or from changes in chain length distribution affecting the degree of resonance enhancement. Dehydrochlorination of PVC can be affected by heating, by UV and ionizing radiation, and by chemical attack. In polyurethane-foam-backed PVC it is possible for residual amines used as foam-initiating catalyst to migrate through to the foam and initiate degradation of the PVC by chemical dehydrochlorination (IO). An application of Raman MiFS to this process is outlined here along with some preliminary results on degradation rates.
INSTRUMENTATION Figure 1 shows the principal components of the MiFS spectrometer. The microline is formed by using two cylindrical lenses mounted in the input port of a BGSC RMIII Raman microscope (11). The cylindrical lenses are positioned so as to optimize the focused microline intensity profile. Figure 2 shows the 1332-cm-' Raman intensity profile of the microline obtained from a uniform diamond surface along with the middle 100-pm section which was used in these experiments. The intensity variation along this middle section of the line is less than &5%. The beam splitter in the microscope has been replaced by a dichroic filter with a cutoff at around a 400-cm-' shift from the Ar ion 514.5-nm laser line which was used to excite the spectra. This filter reflects 80% of the microscope input light onto the sample and transmits 90% of the scattered Raman
Position pm 1' 00
1330 1340 Raman shift cm-' Figure 2. MiFS line intensity profile of the 1332-cm-' Raman band obtained from a uniform diamond surface, along with the magnified middle 100-pm section, which was used in these experiments.
light whilst blocking any reflected and scattered laser light from the sample. A further broad-band filter before the entrance slits of the spectrograph combines with the beam splitter to provide a total blocking of 10" to the laser light. The Spex 1870,O.E~-mspectrograph is focused in the spectral dimension on the CCD chip. An aspheric lens is used to correct the astigmatism present in the system, by bringing the focus in the spatial dimension into coincidence on the CCD chip. In this way a spatial resolution of 3.5 pm/CCD pixel and a spectral resolution of 1.06 cm-l/CCD pixel was realized using a 1200 lines/" holographic grating and a X20 objective. The CCD detector was supplied by Wright Instruments Ltd. and incorporates an EEV 400 X 600 pixel chip which is operated in slow scan mode (3) a t 77 K.
EXPERIMENTAL SECTION PVC sheet was prepared containing PVC resin, plasticizer (a mixture of C9-Cll phthalates), tribasic lead sulfate, and calcium stearatein the proportions 100:50:3:0.5. Samples of the PVC sheet 10 X 30 X 1.5 mm were placed in sealed glass bottles along with a few crystals of triethylenediamine and kept at room temperature. Each bottle was opened after a specific period, and the PVC was removed and mechanically cleaned free of any adhering crystals. The PVC sheet was then cut across the narrow width and mounted in epoxy resin to expose the cross section. Finally, the samples were polished flat with 1pm grade paste. Raman MiFS data in the region 1400-1600 cm-' were obtained from across the 1.5-mm thickness of the samples by sequential collection of data from 100 pm length microlines. The 514.5-nm line of an Ar ion laser was used, providing a total power at the sample along the 100-pm microline of 5 mW. Exposure times of 5 s were used and each succeeding analysis was positioned such that it overlapped the previous position by 50 pm. In this way the whole 1.5-mm cross section was sampled. The spectral data were binned (summed CCD pixel data) 3-fold in both the spectral and spatial dimensions resulting in spatial data at 10-pm intervals and spectral data at 3-cm-' intervals. The sets of data were then collected together on the CCD chip to provide a complete Raman profile. Analogous sets of data were also collected from PVC sheets, of varying formulation, bonded to polyurethane foam, which had been held at 120 "C for 188 h. RESULTS PVC Sheet. Figure 3 shows a control MiFS profile across the thickness of a PVC sheet sample, which was not exposed to triethylenediamine. The bands a t 1436 and 1602 cm-l with a weak shoulder a t 1574 cm-' are due to the plasticizer and provide a useful internal intensity standard. Also in Figure 3 is a profile from a sample which had been exposed for 168
ANALYTICAL CHEMISTRY, VOL. 63,NO. 24, DECEMBER 15, 1991
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DAYS v2 band from the first 20 pm below the PVC surface (0)and from a 20-pm region in the center of the PVC Figure 5. Plots the intensity of the
sheet thickness ( 0 )as a function of time. Intensity
\
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1
1
1500 1600 Raman shift cm-'
Flgwe 3. MiFS profile spectra of the v2 band of an unexposed(control)
PVC sample and of a sample exposed to triethylenediamine for 21 days. Relative Intensity
21 days
7 days 4 days
I 1000
I
500
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Position um Flgure 4. Profiles showing v2 band integrated intensity variations as
a function of time and position through the PVC sheet.
h to triethylaminediamine crystals. The intense band a t 1521.3 f 1.5 cm-' is due to the resonantly enhanced C=C band of the polyene chains, and the intensity variation through the PVC sheet reflects the extent of dehydrochlorination. This can be seen to decrease toward the center of the sheet. The half-bandwidth remained constant a t 29.0 f 1.5 cm-'. The difference in intensity between the two ends of the profile we believe to be due to a small difference in bulk properties between the two surfaces. The uniformity of the plasticizer band across the sample provides a good intensity reference. Similar profiles were obtained for samples which had been exposed to triethylenediaminefor periods of 96 h and 21 days.
1600 Raman shift cm-' Figure 6. Profile of the v2 band obtained from a sample of PVC sheet which had been exposed to triethylenediamine on one side only, for a period of 63 days. 1400
The intensity of the plasticizer band at 1436 cm-' was used to normalize all three sets of data, and the intensity of the 1521 cm-' band was integrated to generate the intensity profiles shown in Figure 4. The profiles clearly show the pattern of dehydrochlorination in the PVC sheet both as a function of time at specific points in the sheet and at a specific time through the thickness of the sheet. Figure 5 plots the intensity of the 1521-cm-l band from the first 20 pm below the PVC surface and from a 20-pm region in the center of the PVC sheet thickness as a function of time. The plots represent dehydrochlorination rates at the two regions and show that the rate decreases more rapidly in the bulk center of the PVC than a t the surface. Figure 6 shows the profile of the 1521-cm-' band obtained from a sample of PVC sheet which had been exposed to triethylaminediamine on one side only, for a period of 63 days; once again the expected intensity distribution decreasing from the exposed side is observed. It has been shown previously (6)that the positions of the resonance-enhanced polyene bands in degraded PVC depend on the polyene sequence lengths producing the bands, and an equation has been derived (7) which relates the C=C Raman shift (v2) to the number of conjugated double bonds (n)in the polyene. Since there is a considerable overlap between the UV/visible absorption bands of the individual polyene sequences (12),the observed value of v2 will depend on the range of conjugated sequence lengths present and also on their relative concentrations. Hence the calculated value of n will be a weighted mean,
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h Foam
1400 1600 Raman shift cm-' Figure 7. MiFS profile of the degraded PVC sample.
v2 band from a foam-backed, thermally
dependant on these two factors. Since in most cases, degraded PVC will contain the whole range of possible n values from 1upward, the calculated value of n is largely governed by the relative concentrationsof those polyenes exhibiting resonance at the excitation wavelength being used. A value of 1521.0 f 1.5 cm-l for v2 corresponds (7) to n = 11.8, indicating a high concentrationof the 11-12 double-bond polyenes. Foam-Backed PVC Sheet. After heat treatment, the polyurethane-foam-backed samples showed visible signs of dehydrochlorination evidenced by a change in coloration from pale yellow to dark brown. The foam was removed from the PVC sheet, and the PVC was sectioned mounted and polished as before. A typical profile is shown in Figure 7 where dehydrochlorination is seen to be more extensive a t the foam interface than at the air interface. In addition there appears to be a change in the position and bandwidth of the C=C band. At the air interface the band maximum is at 1514.0 f 1.5 cm-l with a half-bandwidth of 30.0 f 1.5 cm-'. At the
foam interface the band maximum is at 1516.0 f 1.5 cm-l with a half-bandwidth of 41.5 f 1.5 cm-l. The changeover in band characteristics occurs about midway through the sheet and correspondswith the position of the color change front in the plastic sheet. At this front the band maximum is at 1496.0 f 1.5 cm-l with a half-bandwidth of 45.5 f 1.5 cm-l. This behavior was shown by several of the samples studied, and it is our intention to investigate this further, particularly with respect to excitation wavelength. It is clear however that more than one process is occurring, apparently generating different chain length distributions in the PVC. A crude application of the polyene chain length formula (7) to the frequencies of the band maxima leads to a chain length of around n = 13 at the air and foam interfaces and around n = 19 at the color change front. It is worth noting that these chain lengths are greater than those formed in the chemically degraded samples. Registry No. PVC,9002-86-2.
LITERATURE CITED (1) Bowden, M.; Birnie, J.; Donaldson, P.;Gardiner, D. J.; Southall. J. M. Proc. Int. Conf. Raman Spectrosc ., 12th 1990. 844-845. (2) Bowden, M.; Birnie, J.; Donaldson, P.; Gardiner, D. J.; Southall, J. M. Proc. Int. Conf. Raman Spectrosc.. 12th 1990, 842-843. (3) Ager. J. W., 111; Veirs, D. K.; Rosenblatt, G. M. Phys. Rev. B 1991, 43, 6491-6499. (4) Knobloch, H.; Knoll, W. J . Chem. Phys. 1991, 94, 835-842. (5) Bowden, M.; Gardiner, D. J.; Rice, G.; Gerrard, D. L. J . Raman Spectrosc. 1990, 21, 37-41. (6) Dierker, S. B.; Murray, C. A.; Legrange, J. D.; Schlotter, N. E. Chem. PhyS. Lett. 1987, 137,453-457. (7) Gerrard, D. L.; Maddams, W. F. Macromolecules 1981, 14, 1356- 1362. (8) Gerrard, D. L.; Maddams, W. F. Macromolecules 1975, 8 , 54-58. (9) Baruya, A.; Gerrard, D. L.; Maddams, W. F. Macromolecules 1983, 16, 578-580. (10) Bowley. H. J.; Gerrard, D. L.; Biggin, I. S. J . Vinyl Techno/. 1988, 10, 50-52. (11) Gardiner, D. J.; Bowden, M.; Graves, P. R. Phil. Trans. R . SOC.London, A 1986, 320, 295-306. (12) Sondheimer, F.; Ben-Efraim, D. A.; Wolovsky, R. J . Am. Chem. SOC. 1961, 83, 1675-1681.
RECEIVED for review May 10,1991. Accepted September 23, 1991. We thank BP Research for permission to publish and for financial support.
