- Email: [email protected]
Diffuse reflection spectroscopy study of granular materials and their mixtures in the mid-infrared spectral range
Vibrational Spectroscopy 21 Ž1999. 17–26 www.elsevier.comrlocatervibspec
Diffuse reflection spectroscopy study of granular materials and their mixtures in the mid-infrared spectral range Celine Ventura, Marie Papini ´
)
IUSTI, UMR CNRS 6595, UniÕersite´ d’Aix-Marseille I, 5 rue Enrico Fermi, Marseille Cedex 13, 13453, France Received 3 February 1999; received in revised form 5 July 1999; accepted 9 July 1999
Abstract Diffuse reflection spectroscopy was used in the mid-infrared ŽMIR. wavelength range Ž2.5–22 mm. to characterize polymer particles and their behavior when mixed with inorganic materials such as calcium carbonate, glass spheres or silver-plated glass spheres. Different variables governing the reflection properties of these materials were examined. It was observed, on one hand, that MIR spectra are characteristic — they are related to the physical state and to the chemical composition of the samples — and, on the other hand, that the reflectances of mixtures are strongly affected by the optical characteristics of the added compound. Moreover, the reflectances of compacted samples are also significantly modified by the amount and the duration of the applied pressure. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Diffuse reflectance spectroscopy; FTIR; Mid-infrared; Powders; Polystyrene; Particle size
1. Introduction In general, spectral optical properties of materials Žreflection, transmission and absorption. are intrinsic characteristics of the sample related to their chemical and structural features as well as to their surface state. Moreover and from an energy point of view, radiative exchanges between various parts of a system Žfluidized or packed beds existing in industrial plants, for instance. depend upon the surface and volume properties of the corresponding constitutive materials. The existing theories consider the medium either as a single continuum or as a collection of individual particles w1–6x. Thus, any energy balance ) Corresponding author. Tel.: q33-491-106884; fax: q33-491106969; E-mail: [email protected]
requires the determination of the monochromatic optical properties, not only for the received radiation but also for any emitted one w7,8x. In the same way as a fingerprint, the spectra reveal the chemical and physical constitution of the material and the different fractions of each component for blends. A large number of widely used materials have a rough surface which causes the scattering of the incident light throughout the space. The potential of diffuse reflection spectroscopy is strong — it provides a destruction-free procedure to achieve the characterization of these materials and is particularly designed to study granular materials w6,9–18x. Absorption and scattering coefficients depend on orientation for non-spherical particles such as fibers, but are independent of direction for spherical particles w19x. For transmittance measurements,
0924-2031r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 9 . 0 0 0 4 0 - 5
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
18
solid materials may require grinding into powder and dispersing in a non-absorbing matrix. The potassium bromide pellet technique is a widespread technique of obtaining the spectra of solids rapidly when their identification is necessary w3,20–25x. The present study concerns, on one hand, the measurement of the diffuse reflectance of granular materials related to the influence of a KBr window Žrarely evoked in published papers. w6,8,26x and, on the other hand, how added compounds may affect the optical properties of a material.
2. Experimental Spectral reflectance measurements were performed using a Bio-Rad Fourier transform infrared ŽFTIR. spectrophotometer in the 2.5–22 mm wavelength range Ž4000–450 cmy1 .. The spectrophotometer, continuously purged with dry, carbon dioxide-free air, was fitted with an integrating sphere which collects the specularly and diffusely reflected radiation as displayed in Fig. 1. The inside wall of the integrating sphere consisted of a rough, highly reflecting and scattering gold surface w14,22,23,27– 34x. This experimental set up allowed a near-normal, directional, hemispherical, spectral reflectance measurement. This indicates that for measuring reflectance, the attachment used a directional incident radiation close to the normal incidence and a hemispherical collection of the reflected fluxes. When the
diffuse measurements were needed, a black light trap was placed in the direction of the specularly reflected radiation. For the studied granular materials, the specularly reflected component is small and thereby, the total near-normal reflection is widely dominated by the diffuse component. Polarization effects are negligible due to the multiple reflections from the sphere wall which distributes light uniformly inside the integrating sphere. The reference material was a diffuse gold-coated specimen with the same rough surface as the inside wall of the integrating sphere. The reflectance measurements used the substitution method which requires mounting the reference material Žor the sample. on the sample port alternately. A liquid-nitrogen-cooled mercury cadmium telluride ŽMCT. detector was used for the measurement of the reflected flux inside the sphere. Background reference spectra were acquired on a diffuse wall-mounted gold specimen, and the sample spectra were the ratio of 256 sample to 256 background scans with a resolution of 4 cmy1 w12x. The background represents the energy available by the MCT detector after reflection on the reference sample for each experimental setting. The differences in the background spectra obtained with the reference sample placed close to the sphere wall and behind a KBr window, 3 or 5 mm thick, are reported in Fig. 2. It must be noted that in this last case, the gold reference specimen is 3 or 5 mm behind the sample port and the position of the reference specimen causes a part of the reflected radiation
Fig. 1. Integrating sphere arrangement with wall mount for measuring the near-normal hemispherical spectral reflectance.
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
Fig. 2. Background spectra vs. wavenumbers Žcmy1 . given by a diffuse gold-coated specimen used as a reference sample: Ž1. without a KBr window; Ž2. with a KBr window 3 mm thick; Ž3. with a KBr window 5 mm thick.
to be lost: as expected, the energy collected by the detector is lower when a KBr window is used. Materials in a broad range of morphologies — powders, pellets and films — were used. In order to evaluate the influence of particle sizes on their spectral properties, powders were sieved through the following successively finer sieves: 1800, 1250,
19
1000, 800, 500, 250, 200, 160, 125, 100, 80, 53, 40, 20 and 10 mm to achieve grain size ranges. A weighed amount of powders was loaded at their natural packing density into a sample cup 9 mm high and 20 mm in diameter. Powders were also compacted into tablets by carefully controlling both the amount and duration of the applied pressure. The study of the spectral properties of polystyrene ŽPS., calcium carbonate, glass spheres and silver-coated glass ŽAg glass. spheres was carried out. It was observed that pellets prepared with PS powders with grain sizes larger than 100 mm were brittle. Loads used in the pellet preparation ranged from 2 to 15 tons. The reflectance properties of PS films with various thicknesses were also investigated. In our experimental setting, the sample was held in a vertical mount and this required that the powders were always covered with a potassium bromide window that modified the reflectivity of the sample. In other experimental devices, a transparent window may be necessary to protect the sample from its chemical environment. As shown in Fig. 3, the optical system of this sample cell consisted of air Žmedium 1., a non-absorbing, non-scattering KBr
Fig. 3. Multiple reflection in a sample cell consisting in the following media: Ž1. air; Ž2. KBr window; Ž3. spherical particles.
20
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
window Žmedium 2., and semi-transparent absorbing and scattering powders Žmedium 3.. A part of the incident flux will be reflected at the first interface, air–KBr, and transmitted through this interface. A part of the radiation arriving at the second interface, KBr–PS particles, will be converted into a reflected radiation at the second interface. Of the energy reflected from the medium 2–medium 3 interface, a part will be reflected at the medium 1–medium 2 interface and another part will be transmitted through this interface, back into the air. The process continues and the overall reflectance can be determined by adding the contributions of all the reflections and transmissions in the backward direction. The rest of the radiation arriving at the second interface, KBr–PS, will undergo transmission, refraction and absorption in varying degrees within the particles as displayed in Fig. 4 w5,35–37x. The specularly reflected rays Ž2. have never been inside the particles, while diffusely scattered rays Ž3. have been transmitted through the particles and contain spectral information about the powders. The direction of diffusely reflected light is random with respect to the incident beam. The background spectrum was always recorded, prior to the powder sample measurement, with the same KBr window placed against the rough
Fig. 5. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of a PS pellet in the MIR wavelength range: Ž1. both reference sample and PS pellet with a KBr window; Ž2. only the PS pellet with a KBr window; Ž3. both reference sample and PS pellet without a KBr window.
gold reference specimen. Fig. 5 illustrates how the measurement of the background modifies the obtained reflectances of a PS pellet when a KBr window 5 mm thick is placed in front of the sample. The published values show that the refractive index of KBr varies slightly with increasing wavelength: 1.537 to 1.493 for the mid-infrared ŽMIR. region w38x. The refractive index of a PS film has a mean value of 1.48 but it varies according to the wavelength where absorption bands occur: the value remains between 1.44 and 1.51 for the MIR region w39x.
3. Results and discussion
Fig. 4. Schematic of the incident Ž1. and specularly reflected Ž2. rays, represented by solid lines. Rays submitted to transmission through particles Ž3. are represented by dashed lines.
The total near-normal hemispherical spectral reflectance of chemically identical materials is related to their morphological nature as illustrated in Fig. 6 for PS with the following physical structures: a foam, a pellet and a film 32 mm thick. The pellet containing 1.25 g of PS was prepared using a load of 4 tons applied for 26 min. The physical nature of the sample greatly affects the shape of the spectra and the value of the reflectance, as shown for both the pellet and the foam spectra. The foam gives rise to intense absorption bands. On the contrary, for pellets, as the sample thickness increases, the absorption reaches higher values and the spectrum starts to flatten as shown in Fig. 6 Žcurve 2.. Nevertheless, a
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
Fig. 6. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of PS samples: Ž1. foam; Ž2. pellet containing 1.25 g of PS particles with a diameter d such as 40 - d-80 mm; Ž3. film 32-mm thick.
zero value of the reflectance is not obtained, due to direct surface reflection from the sample surface. In this spectrum, bands are distorted through saturation and stray light effects. For the thin PS film, interference fringes arise due to the different reflections of the incident beam within the film from the front and back of the film — the spectrum becomes sinusoidal. These multiple reflections are commonly used to determine the thickness of a thin film with accuracy. Bands due to the chemical composition of the sample are not clearly visible in the film spectrum as they are in the foam spectrum. The physical state of materials is one of the parameters controlling their radiative properties. The reflectance spectra of four chemically different samples consisting of glass spheres, silverplated glass spheres, calcium carbonate and PS powders are displayed in Fig. 7. The position of the absorption bands indicates the chemical composition of the material. Concerning PS, the observed and calculated frequencies of the in-plane or out-of-plane vibrations of the phenyl group, and the different interactions involving this group and the main chain are reported in several papers since films of PS are used to standardize spectrometers via their transmittance spectra w38–50x. All our results agree well with the group frequency assignments available in the published work. The phenyl CH vibration appears as a strong band around 3000 cmy1 . Assignments of combination and overtones of the fundamental frequencies can be made w40,41x. It is the case of the
21
Fig. 7. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of powders: Ž1. glass spheres; Ž2. silvercoated glass spheres; Ž3. calcium carbonate powders; Ž4. PS powders. A KBr window was placed in the experimental cell.
bands at 1942, 1870, 1802, 1744, 1664 and 1542 cmy1 , for instance, which are the combination between the following bands: 980, 964, 907, 842, 759 and 703 cmy1 . In the case of binary mixtures of powders, the spectrum displays the different absorption bands of each component as shown in Fig. 8 for a PS q CaCO 3 mixture Ž50% in wt.%. w51x. The optical properties of granular materials are sensitive to particle size. PS particles are large compared to the wavelength. The results described in this paper extend those obtained for PS powders in a previous work and in published papers w12,40, 45,48,50–58x. As illustrated in Fig. 9 for the MIR
Fig. 8. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of powders: Ž1. PS; Ž2. binary mixture of calcium carbonate and PS; Ž3. calcium carbonate. A KBr window was placed in the experimental cell.
22
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
Fig. 9. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of PS powders with increasing particle sizes: Ž1. ds 40 mm; Ž2. ds125 mm; Ž3. ds 250 mm. A KBr window was placed in the experimental cell.
region, the area of the absorption bands decreases when the particle size increases. The spectrum becomes flattened for the larger particles and particularly for bulk samples having a smooth surface state. A reverse effect is observed at higher frequencies in the near-infrared wavelength range for which the area of the absorption bands increases with increasing particle size w52,59x. As the particle size increases, the reflectance becomes smaller, the absorption becoming significant. This is also true for the PS foam walls, the thinness of which leads to an increase in the band area of the corresponding spectrum as shown in Fig. 6. The same particle size effect is also observed when powders with different diameters are pressed into a pellet as shown in Fig. 10 for powders 10 and 100 mm in diameter. Loads ranging from 3 to 15 tons were applied using a steel cylinder with a polished surface. Both the amount and the duration of the applied pressure are two other parameters strongly controlling the reflective properties of granular materials w56,60,61x. As a result, great care must be taken to control the pressure during the preparation of the packed samples. This is the case, for instance, in the well-known KBr procedure or when levelling the powder in a sample cup using a spatula; no compression may occur in either case as the volume fraction is an important parameter controlling the reflectance values of samples.
Fig. 10. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of PS pellets prepared with powders: Ž1. 10 mm; Ž2. 100 mm in diameter.
The effect of pressure on the reflectance values is presented in Fig. 11 where the increase of the pressure when preparing a pellet containing 1.25 g of PS powders induces a decrease of 0.1 in the reflectance values for loads varying from 3 to 15 tons. This effect can be explained by both the decrease in the scattering coefficient of the powder and in the distance between the grains: as a result, there are fewer reflections between the particles. It was observed that the applied pressure induces changes in the shape of the spherical particles and in the roughness of the sample: the pressure decreases the surface roughness, as expected.
Fig. 11. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of compacted PS powders Žthe compaction load was applied for 26 min.: Ž1. 3 tons; Ž2. 7 tons; Ž3. 15 tons. Insert: variation of R, at 2500 cmy1 , vs. the applied load.
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
Reflectance values are linked to the composition of materials. Mixtures of sieved semi-transparent PS powders with glass spheres or with reflecting silvercoated glass spheres were prepared. Samples were prepared by mixing weighed amounts of each powdered component. Several small quantities of the mixture were carefully transferred in the experimental cell without shaking it. This experimental protocol allows the obtention of homogeneous samples and avoids segregation of one component with respect to the others caused by the differences in densities or in particles sizes. The diameter of the PS particles, glass spheres and that of the Ag glass spheres ranged from 40 to 80 mm, from 40 to 80 mm and from 5 to 30 mm, respectively. In Fig. 7, the comparison between the reflectance values of both PS and Ag glass spheres reveals that R Ag glass is higher than R PS in the whole MIR wavelength range. This is not the case when comparing the reflectance values of both PS and glass spheres, where R glass is higher than R PS for wavenumbers higher than 2200 cmy1 approximately. Nevertheless, the reflectance values of granular mixtures remain always comprised between that of each respective component of the PS q Ag glass mixture, as illustrated in Fig. 12 where they decrease with increasing PS content in the MIR region. The behavior varies according to the wavenumber Žor wavelength. for the PS q glass mixtures, as shown in Fig. 13. Our investigation, on granular PS q Ag glass mixtures in the near-infrared
Fig. 12. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of mixtures of PS powders with silvercoated glass spheres according to the following polymer contents, in vol.: 0, 40, 64, 75, 90 and 100 Žcurves 1 to 6, respectively.. The corresponding weights are: 0, 21, 41.5, 55, 78.5 and 100 wt.%.
23
Fig. 13. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of mixtures of PS powders with glass spheres according to the following polymer contents Žin vol..: 0, 10, 30, 61, 90, 100 Žcurves 1 to 6, respectively.. The corresponding weights are: 0, 4, 15, 40, 79 and 100 wt.%.
wavelength range, proved that this is not true for all the wavelengths: for instance, below 1.68 mm Žabove 5950 cmy1 ., the reflectance values are not always comprised between that of each component w59x. The variation of the areas of the absorption bands is also influenced by the nature of the added component as displayed in Fig. 14. Mixtures of PS powders and Ag glass spheres were used to investigate the influence of an applied pressure on reflectance values: they were shown to decrease with increasing applied load as shown in Fig. 15. In the as-prepared mixtures, the Ag glass spheres were randomly dispersed among the PS particles while compaction caused the Ag glass spheres
Fig. 14. Areas of the absorption band at 1940 cmy1 vs. the PS content Žin vol%. of mixtures: Ž1. PSqglass spheres; Ž2. PSq silver-coated glass spheres.
24
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
Fig. 15. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of compacted PSrsilver-coated glass spheres: Ž1. 2 tons; Ž2. 14 tons. The PS content was 49.7% in wt.% Ž29% in vol.%. and the compaction load was applied for 26 min. Insert: variation of R, at 2500 cmy1 , vs. the applied load.
Fig. 17. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of PS powders according to the thickness h of the experimental cell: Ž1. hs9 mm; Ž2. hs 0.1 mm; Ž3. hs 0.05 mm. A KBr window was placed in the experimental cell.
to be rearranged between PS particles, the number of neighbours depending on the relative diameters of particles w62x. The decreasing distance between PS particles with increasing pressure causes a decrease in the reflectance values. For a given pressure, the time at which this pressure is applied affects the reflectance values which decrease with increasing time as illustrated in Fig. 16, including a kinetic aspect in the deformation of the PS particles. The influence of the pressure appears in the first minutes of compaction. Fig. 16 shows the variation of the reflectance of two samples containing 1.25 g of PS powders and compacted with
a load of 4 tons applied for 5 and 26 min. A longer time of compaction Žup to an hour. induces practically no change in the reflectance values of the sample. As a result, great consideration must be given to the preparation of the samples, concerning the packing conditions, in particular. The reflectance of granular materials depends on the layer thickness h and increases with increasing depth, up to a given thickness generally considered infinitely thick, when it reaches 1 or 3 mm for small grain sizes. The reflectances of 40-mm diameter PS powders were shown to be strongly influenced by the thickness and particularly for low values and
Fig. 16. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of compacted PS powders with different durations of compaction: Ž1. 5 min; Ž2. 26 min.
Fig. 18. Areas of absorption bands vs. the thickness of the cell containing PS powders 40 mm in diameter: bands at Ž1. 1895 cmy1 ; Ž2. 1860 cmy1 ; Ž3. 1660 cmy1 .
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
small variations of h as illustrated in Fig. 17. In this case, the variations in reflectance values are practically the same, when h increases from 0.05 up to 0.1 mm, corresponding to approximately two or three layers of powders, as when it increases from 0.1 up to 9 mm. Concerning the areas of some absorption bands, they increase with increasing thickness up to 0.2 mm for h, while they remain unchanged for higher values of h, as shown in Fig. 18, for the following wavenumbers: 1895, 1860 and 1660 cmy1 .
4. Conclusions The results presented in this paper all refer to the reflection properties of granular materials which are shown to be strongly dependent on chemical and physical parameters. As expected, the chemical nature of the material assigns the position of the absorption bands and governs the shape of the spectra, thereby. The morphology of samples, as well as the particle sizes of granular materials are physical parameters influencing the radiative properties of materials. A kinetic aspect is observed during the preparation of pellets, as reflectance values vary with the pressure and the time during which the load is applied. The present paper is also concerned with reflection properties of mixtures of granular materials. The reflectance values of mixtures are influenced by the optical property of the added material and their behavior is wavelength-dependent.
References w1x w2x w3x w4x w5x w6x w7x w8x
P. Kubelka, F. Munk, Z. Tech. Phys. 12 Ž1931. 593. A. Mandelis, J.P. Grossman, Appl. Spectrosc. 46 Ž1992. 737. H.G. Hecht, J. Res. Natl. Bur. Stand. ŽU.S.. 80A Ž1976. 567. M. Kaviany, Principles of Heat Transfer in Porous Media, Springer, New York, 1991. B.P. Singh, M. Kaviany, Int. J. Heat Mass Transfer 35 Ž1992. 1397. G. Kortum, ¨ Reflectance Spectroscopy, Springer, New York, 1969. R. Siegel, J.R. Howell, Thermal Radiation Heat Transfer, Hemisphere Publishing, WA, 1992. M.Q. Brewster, Thermal Radiative Transfer and Properties, Wiley, New York, 1987.
25
w9x H.W. Siesler, Makromol. Chem. Macromol. Symp. 52 Ž1991. 113. w10x W.WM. Wendlandt, H.G. Hecht, Reflectance Spectroscopy, Wiley, New York, 1966. w11x S.R. Culler, M.T. MacKenzie, L.J. Fina, H. Ishida, J.L. Koenig, Appl. Spectrosc. 38 Ž1984. 791. w12x M.P. Fuller, P.R. Griffiths, Anal. Chem. 50 Ž1978. 1906. w13x P.R. Griffiths, M.P. Fuller, in: R.J.H. Clark, R.E. Hester ŽEds.., Advances in Infrared and Raman Spectroscopy, Vol. 63, Chap. 2, Heyden, London, 1982. w14x F. Boroumand, J.E. Moser, H. VandenBergh, Appl. Spectrosc. 46 Ž1992. 1874. w15x D. Fassler, R. Gade, J. Mol. Struct. 246 Ž1991. 145. w16x J.M. Chalmers, M.W. MacKenzie ŽEds.., Advances in Applied Fourier Transform Infrared Spectroscopy, Wiley, New York, 1988. w17x M.K. Gunde, J.K. Logar, Z.C. Orel, B. Orel, Appl. Spectrosc. 44 Ž1990. 193. w18x K. Moradi, C. Depecker, J. Corset, Appl. Spectrosc. 48 Ž1994. 1491. w19x S.C. Lee, J. Quant. Spectrosc. Radiat. Transfer 36 Ž1986. 253. w20x D.J.J. Fraser, P.R. Griffiths, Appl. Spectrosc. 49 Ž1995. 623. w21x M.T. MacKenzie, J.L. Koenig, Appl. Spectrosc. 39 Ž1985. 408. w22x P.J. Brimmer, P.R. Griffiths, N.J. Harrick, Appl. Spectrosc. 40 Ž1986. 258. w23x T. Theophanides ŽEd.., Fourier Transform Infrared Spectroscopy, Reidel, Boston, 1984. w24x M. Falk, Can. J. Appl. Spectrosc. 36 Ž1991. 22. w25x H.G. Hecht, Appl. Spectrosc. 34 Ž1980. 161. w26x D.B. Judd, K.S. Gibson, Natl. Bur. Stand. ŽU.S.., RP 872, 16 Ž1936. 261. w27x W. Richter, W. Erb, Appl. Opt. 26 Ž1987. 4620. w28x W. Richter, Appl. Spectrosc. 37 Ž1983. 32. w29x B. Orel, Z.C. Orel, Sol. Energy Mater. 22 Ž1991. 259. w30x R.W. Frei, J.D. MacNeil, Diffuse Environmental Spectroscopy in Environmental Problem-Solving, CRC Press, Cleveland, 1973. w31x D.G. Goebel, Appl. Opt. 6 Ž1967. 125. w32x K. Gindele, M. Kohl, ¨ M. Mast, Appl. Opt. 24 Ž1985. 1757. w33x R.A. Shepherd, SPIE 1311 Ž1990. 55. w34x R.R. Willey, Appl. Spectrosc. 30 Ž1976. 593. w35x B.P. Singh, Int. J. Heat Mass Transfer 35 Ž1992. 1397. w36x M. Kerker, The Scattering of Light and Other Electromagnetic Radiation, Academic Press, New York, 1969. w37x D.E. Leyden, R.S.S. Murthy, Trends Anal. Chem. 7 Ž1988. 164. w38x R.E. Stephens, E.K. Plyler, W.S. Rodney, R.J. Spindler, J. Opt. Soc. Am. 43 Ž1953. 110. w39x R.T. Graf, J.L. Koenig, H. Ishida, Appl. Spectrosc. 39 Ž1985. 405. w40x P.C. Painter, M.M. Coleman, J.L. Koenig, The Theory of Vibrational Spectroscopy and its Application to Polymeric Materials, Wiley, New York, 1982. w41x R.A. Nyquist, Appl. Spectrosc. 38 Ž1984. 264. w42x B.H. Stuart, Vib. Spectrosc. 10 Ž1996. 79.
26
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
w43x B. Jasse, J.L. Koenig, J. Polym. Sci. Polym. Phys. 17 Ž1979. 799. w44x S. Krimm, Fortschr. Hochpolym.-Forsch. 2 Ž1960. 140. w45x A.A. Christy, Y.Z. Liang, C. Hui, O.M. Kvalheim, Vib. Spectrosc. 5 Ž1993. 233. w46x Polymer Handbook, J. Brandrup, E.H. Immergut ŽEds.., Wiley, New York, 1989. w47x H.W. Siesler, K. Holland-Moritz, Infrared and Raman Spectroscopy of Polymers, Marcel Dekker, New York, 1980. w48x N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1975. w49x D.I. Bower, W.F. Maddams, The Vibrational Spectroscopy of Polymers, Cambridge Univ. Press, Cambridge, 1989. w50x A. Lee Smith, in: P.J. Elving, J.D. Winefordner, I.M. Kolthoff ŽEds.., Applied Infrared Spectroscopy, Vol. 54, Wiley, New York, 1979. w51x T.M. Hancewicz, Appl. Spectrosc. 46 Ž1992. 1074.
w52x M. Papini, Infrared Phys. 34 Ž1993. 607. w53x G. Duyckaerts, Spectrochim. Acta 7 Ž1955. 25. w54x T. Burger, J. Kuhn, R. Caps, J. Fricke, Appl. Spectrosc. 51 Ž1997. 309. w55x P.W. Yang, H.H. Mantsch, Appl. Opt. 26 Ž1987. 326. w56x M.L.E. Te Vrucht, P.R. Griffiths, Appl. Spectrosc. 43 Ž1989. 1293. w57x P.W. Yang, H.H. Mantsch, F. Baudais, Appl. Spectrosc. 40 Ž1986. 974. w58x A.A. Christy, O.M. Kvalheim, R.A. Velapoldi, Vib. Spectrosc. 9 Ž1995. 19. w59x C. Ventura, M. Papini, Macromol. Symp. Polym. Spectrosc. 119 Ž1997. 137. w60x S.A. Yeboah, S.H. Wang, P.R. Griffiths, Appl. Spectrosc. 38 Ž1984. 259. w61x E.A. Schatz, J. Opt. Soc. Am. 56 Ž1966. 389. w62x C. Ventura, M. Papini, J. Near-Infrared Spectrosc. 5 Ž1997. 123.
Diffuse reflection spectroscopy study of granular materials and their mixtures in the mid-infrared spectral range Celine Ventura, Marie Papini ´
)
IUSTI, UMR CNRS 6595, UniÕersite´ d’Aix-Marseille I, 5 rue Enrico Fermi, Marseille Cedex 13, 13453, France Received 3 February 1999; received in revised form 5 July 1999; accepted 9 July 1999
Abstract Diffuse reflection spectroscopy was used in the mid-infrared ŽMIR. wavelength range Ž2.5–22 mm. to characterize polymer particles and their behavior when mixed with inorganic materials such as calcium carbonate, glass spheres or silver-plated glass spheres. Different variables governing the reflection properties of these materials were examined. It was observed, on one hand, that MIR spectra are characteristic — they are related to the physical state and to the chemical composition of the samples — and, on the other hand, that the reflectances of mixtures are strongly affected by the optical characteristics of the added compound. Moreover, the reflectances of compacted samples are also significantly modified by the amount and the duration of the applied pressure. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Diffuse reflectance spectroscopy; FTIR; Mid-infrared; Powders; Polystyrene; Particle size
1. Introduction In general, spectral optical properties of materials Žreflection, transmission and absorption. are intrinsic characteristics of the sample related to their chemical and structural features as well as to their surface state. Moreover and from an energy point of view, radiative exchanges between various parts of a system Žfluidized or packed beds existing in industrial plants, for instance. depend upon the surface and volume properties of the corresponding constitutive materials. The existing theories consider the medium either as a single continuum or as a collection of individual particles w1–6x. Thus, any energy balance ) Corresponding author. Tel.: q33-491-106884; fax: q33-491106969; E-mail: [email protected]
requires the determination of the monochromatic optical properties, not only for the received radiation but also for any emitted one w7,8x. In the same way as a fingerprint, the spectra reveal the chemical and physical constitution of the material and the different fractions of each component for blends. A large number of widely used materials have a rough surface which causes the scattering of the incident light throughout the space. The potential of diffuse reflection spectroscopy is strong — it provides a destruction-free procedure to achieve the characterization of these materials and is particularly designed to study granular materials w6,9–18x. Absorption and scattering coefficients depend on orientation for non-spherical particles such as fibers, but are independent of direction for spherical particles w19x. For transmittance measurements,
0924-2031r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 Ž 9 9 . 0 0 0 4 0 - 5
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
18
solid materials may require grinding into powder and dispersing in a non-absorbing matrix. The potassium bromide pellet technique is a widespread technique of obtaining the spectra of solids rapidly when their identification is necessary w3,20–25x. The present study concerns, on one hand, the measurement of the diffuse reflectance of granular materials related to the influence of a KBr window Žrarely evoked in published papers. w6,8,26x and, on the other hand, how added compounds may affect the optical properties of a material.
2. Experimental Spectral reflectance measurements were performed using a Bio-Rad Fourier transform infrared ŽFTIR. spectrophotometer in the 2.5–22 mm wavelength range Ž4000–450 cmy1 .. The spectrophotometer, continuously purged with dry, carbon dioxide-free air, was fitted with an integrating sphere which collects the specularly and diffusely reflected radiation as displayed in Fig. 1. The inside wall of the integrating sphere consisted of a rough, highly reflecting and scattering gold surface w14,22,23,27– 34x. This experimental set up allowed a near-normal, directional, hemispherical, spectral reflectance measurement. This indicates that for measuring reflectance, the attachment used a directional incident radiation close to the normal incidence and a hemispherical collection of the reflected fluxes. When the
diffuse measurements were needed, a black light trap was placed in the direction of the specularly reflected radiation. For the studied granular materials, the specularly reflected component is small and thereby, the total near-normal reflection is widely dominated by the diffuse component. Polarization effects are negligible due to the multiple reflections from the sphere wall which distributes light uniformly inside the integrating sphere. The reference material was a diffuse gold-coated specimen with the same rough surface as the inside wall of the integrating sphere. The reflectance measurements used the substitution method which requires mounting the reference material Žor the sample. on the sample port alternately. A liquid-nitrogen-cooled mercury cadmium telluride ŽMCT. detector was used for the measurement of the reflected flux inside the sphere. Background reference spectra were acquired on a diffuse wall-mounted gold specimen, and the sample spectra were the ratio of 256 sample to 256 background scans with a resolution of 4 cmy1 w12x. The background represents the energy available by the MCT detector after reflection on the reference sample for each experimental setting. The differences in the background spectra obtained with the reference sample placed close to the sphere wall and behind a KBr window, 3 or 5 mm thick, are reported in Fig. 2. It must be noted that in this last case, the gold reference specimen is 3 or 5 mm behind the sample port and the position of the reference specimen causes a part of the reflected radiation
Fig. 1. Integrating sphere arrangement with wall mount for measuring the near-normal hemispherical spectral reflectance.
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
Fig. 2. Background spectra vs. wavenumbers Žcmy1 . given by a diffuse gold-coated specimen used as a reference sample: Ž1. without a KBr window; Ž2. with a KBr window 3 mm thick; Ž3. with a KBr window 5 mm thick.
to be lost: as expected, the energy collected by the detector is lower when a KBr window is used. Materials in a broad range of morphologies — powders, pellets and films — were used. In order to evaluate the influence of particle sizes on their spectral properties, powders were sieved through the following successively finer sieves: 1800, 1250,
19
1000, 800, 500, 250, 200, 160, 125, 100, 80, 53, 40, 20 and 10 mm to achieve grain size ranges. A weighed amount of powders was loaded at their natural packing density into a sample cup 9 mm high and 20 mm in diameter. Powders were also compacted into tablets by carefully controlling both the amount and duration of the applied pressure. The study of the spectral properties of polystyrene ŽPS., calcium carbonate, glass spheres and silver-coated glass ŽAg glass. spheres was carried out. It was observed that pellets prepared with PS powders with grain sizes larger than 100 mm were brittle. Loads used in the pellet preparation ranged from 2 to 15 tons. The reflectance properties of PS films with various thicknesses were also investigated. In our experimental setting, the sample was held in a vertical mount and this required that the powders were always covered with a potassium bromide window that modified the reflectivity of the sample. In other experimental devices, a transparent window may be necessary to protect the sample from its chemical environment. As shown in Fig. 3, the optical system of this sample cell consisted of air Žmedium 1., a non-absorbing, non-scattering KBr
Fig. 3. Multiple reflection in a sample cell consisting in the following media: Ž1. air; Ž2. KBr window; Ž3. spherical particles.
20
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
window Žmedium 2., and semi-transparent absorbing and scattering powders Žmedium 3.. A part of the incident flux will be reflected at the first interface, air–KBr, and transmitted through this interface. A part of the radiation arriving at the second interface, KBr–PS particles, will be converted into a reflected radiation at the second interface. Of the energy reflected from the medium 2–medium 3 interface, a part will be reflected at the medium 1–medium 2 interface and another part will be transmitted through this interface, back into the air. The process continues and the overall reflectance can be determined by adding the contributions of all the reflections and transmissions in the backward direction. The rest of the radiation arriving at the second interface, KBr–PS, will undergo transmission, refraction and absorption in varying degrees within the particles as displayed in Fig. 4 w5,35–37x. The specularly reflected rays Ž2. have never been inside the particles, while diffusely scattered rays Ž3. have been transmitted through the particles and contain spectral information about the powders. The direction of diffusely reflected light is random with respect to the incident beam. The background spectrum was always recorded, prior to the powder sample measurement, with the same KBr window placed against the rough
Fig. 5. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of a PS pellet in the MIR wavelength range: Ž1. both reference sample and PS pellet with a KBr window; Ž2. only the PS pellet with a KBr window; Ž3. both reference sample and PS pellet without a KBr window.
gold reference specimen. Fig. 5 illustrates how the measurement of the background modifies the obtained reflectances of a PS pellet when a KBr window 5 mm thick is placed in front of the sample. The published values show that the refractive index of KBr varies slightly with increasing wavelength: 1.537 to 1.493 for the mid-infrared ŽMIR. region w38x. The refractive index of a PS film has a mean value of 1.48 but it varies according to the wavelength where absorption bands occur: the value remains between 1.44 and 1.51 for the MIR region w39x.
3. Results and discussion
Fig. 4. Schematic of the incident Ž1. and specularly reflected Ž2. rays, represented by solid lines. Rays submitted to transmission through particles Ž3. are represented by dashed lines.
The total near-normal hemispherical spectral reflectance of chemically identical materials is related to their morphological nature as illustrated in Fig. 6 for PS with the following physical structures: a foam, a pellet and a film 32 mm thick. The pellet containing 1.25 g of PS was prepared using a load of 4 tons applied for 26 min. The physical nature of the sample greatly affects the shape of the spectra and the value of the reflectance, as shown for both the pellet and the foam spectra. The foam gives rise to intense absorption bands. On the contrary, for pellets, as the sample thickness increases, the absorption reaches higher values and the spectrum starts to flatten as shown in Fig. 6 Žcurve 2.. Nevertheless, a
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
Fig. 6. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of PS samples: Ž1. foam; Ž2. pellet containing 1.25 g of PS particles with a diameter d such as 40 - d-80 mm; Ž3. film 32-mm thick.
zero value of the reflectance is not obtained, due to direct surface reflection from the sample surface. In this spectrum, bands are distorted through saturation and stray light effects. For the thin PS film, interference fringes arise due to the different reflections of the incident beam within the film from the front and back of the film — the spectrum becomes sinusoidal. These multiple reflections are commonly used to determine the thickness of a thin film with accuracy. Bands due to the chemical composition of the sample are not clearly visible in the film spectrum as they are in the foam spectrum. The physical state of materials is one of the parameters controlling their radiative properties. The reflectance spectra of four chemically different samples consisting of glass spheres, silverplated glass spheres, calcium carbonate and PS powders are displayed in Fig. 7. The position of the absorption bands indicates the chemical composition of the material. Concerning PS, the observed and calculated frequencies of the in-plane or out-of-plane vibrations of the phenyl group, and the different interactions involving this group and the main chain are reported in several papers since films of PS are used to standardize spectrometers via their transmittance spectra w38–50x. All our results agree well with the group frequency assignments available in the published work. The phenyl CH vibration appears as a strong band around 3000 cmy1 . Assignments of combination and overtones of the fundamental frequencies can be made w40,41x. It is the case of the
21
Fig. 7. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of powders: Ž1. glass spheres; Ž2. silvercoated glass spheres; Ž3. calcium carbonate powders; Ž4. PS powders. A KBr window was placed in the experimental cell.
bands at 1942, 1870, 1802, 1744, 1664 and 1542 cmy1 , for instance, which are the combination between the following bands: 980, 964, 907, 842, 759 and 703 cmy1 . In the case of binary mixtures of powders, the spectrum displays the different absorption bands of each component as shown in Fig. 8 for a PS q CaCO 3 mixture Ž50% in wt.%. w51x. The optical properties of granular materials are sensitive to particle size. PS particles are large compared to the wavelength. The results described in this paper extend those obtained for PS powders in a previous work and in published papers w12,40, 45,48,50–58x. As illustrated in Fig. 9 for the MIR
Fig. 8. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of powders: Ž1. PS; Ž2. binary mixture of calcium carbonate and PS; Ž3. calcium carbonate. A KBr window was placed in the experimental cell.
22
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
Fig. 9. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of PS powders with increasing particle sizes: Ž1. ds 40 mm; Ž2. ds125 mm; Ž3. ds 250 mm. A KBr window was placed in the experimental cell.
region, the area of the absorption bands decreases when the particle size increases. The spectrum becomes flattened for the larger particles and particularly for bulk samples having a smooth surface state. A reverse effect is observed at higher frequencies in the near-infrared wavelength range for which the area of the absorption bands increases with increasing particle size w52,59x. As the particle size increases, the reflectance becomes smaller, the absorption becoming significant. This is also true for the PS foam walls, the thinness of which leads to an increase in the band area of the corresponding spectrum as shown in Fig. 6. The same particle size effect is also observed when powders with different diameters are pressed into a pellet as shown in Fig. 10 for powders 10 and 100 mm in diameter. Loads ranging from 3 to 15 tons were applied using a steel cylinder with a polished surface. Both the amount and the duration of the applied pressure are two other parameters strongly controlling the reflective properties of granular materials w56,60,61x. As a result, great care must be taken to control the pressure during the preparation of the packed samples. This is the case, for instance, in the well-known KBr procedure or when levelling the powder in a sample cup using a spatula; no compression may occur in either case as the volume fraction is an important parameter controlling the reflectance values of samples.
Fig. 10. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of PS pellets prepared with powders: Ž1. 10 mm; Ž2. 100 mm in diameter.
The effect of pressure on the reflectance values is presented in Fig. 11 where the increase of the pressure when preparing a pellet containing 1.25 g of PS powders induces a decrease of 0.1 in the reflectance values for loads varying from 3 to 15 tons. This effect can be explained by both the decrease in the scattering coefficient of the powder and in the distance between the grains: as a result, there are fewer reflections between the particles. It was observed that the applied pressure induces changes in the shape of the spherical particles and in the roughness of the sample: the pressure decreases the surface roughness, as expected.
Fig. 11. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of compacted PS powders Žthe compaction load was applied for 26 min.: Ž1. 3 tons; Ž2. 7 tons; Ž3. 15 tons. Insert: variation of R, at 2500 cmy1 , vs. the applied load.
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
Reflectance values are linked to the composition of materials. Mixtures of sieved semi-transparent PS powders with glass spheres or with reflecting silvercoated glass spheres were prepared. Samples were prepared by mixing weighed amounts of each powdered component. Several small quantities of the mixture were carefully transferred in the experimental cell without shaking it. This experimental protocol allows the obtention of homogeneous samples and avoids segregation of one component with respect to the others caused by the differences in densities or in particles sizes. The diameter of the PS particles, glass spheres and that of the Ag glass spheres ranged from 40 to 80 mm, from 40 to 80 mm and from 5 to 30 mm, respectively. In Fig. 7, the comparison between the reflectance values of both PS and Ag glass spheres reveals that R Ag glass is higher than R PS in the whole MIR wavelength range. This is not the case when comparing the reflectance values of both PS and glass spheres, where R glass is higher than R PS for wavenumbers higher than 2200 cmy1 approximately. Nevertheless, the reflectance values of granular mixtures remain always comprised between that of each respective component of the PS q Ag glass mixture, as illustrated in Fig. 12 where they decrease with increasing PS content in the MIR region. The behavior varies according to the wavenumber Žor wavelength. for the PS q glass mixtures, as shown in Fig. 13. Our investigation, on granular PS q Ag glass mixtures in the near-infrared
Fig. 12. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of mixtures of PS powders with silvercoated glass spheres according to the following polymer contents, in vol.: 0, 40, 64, 75, 90 and 100 Žcurves 1 to 6, respectively.. The corresponding weights are: 0, 21, 41.5, 55, 78.5 and 100 wt.%.
23
Fig. 13. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of mixtures of PS powders with glass spheres according to the following polymer contents Žin vol..: 0, 10, 30, 61, 90, 100 Žcurves 1 to 6, respectively.. The corresponding weights are: 0, 4, 15, 40, 79 and 100 wt.%.
wavelength range, proved that this is not true for all the wavelengths: for instance, below 1.68 mm Žabove 5950 cmy1 ., the reflectance values are not always comprised between that of each component w59x. The variation of the areas of the absorption bands is also influenced by the nature of the added component as displayed in Fig. 14. Mixtures of PS powders and Ag glass spheres were used to investigate the influence of an applied pressure on reflectance values: they were shown to decrease with increasing applied load as shown in Fig. 15. In the as-prepared mixtures, the Ag glass spheres were randomly dispersed among the PS particles while compaction caused the Ag glass spheres
Fig. 14. Areas of the absorption band at 1940 cmy1 vs. the PS content Žin vol%. of mixtures: Ž1. PSqglass spheres; Ž2. PSq silver-coated glass spheres.
24
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
Fig. 15. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of compacted PSrsilver-coated glass spheres: Ž1. 2 tons; Ž2. 14 tons. The PS content was 49.7% in wt.% Ž29% in vol.%. and the compaction load was applied for 26 min. Insert: variation of R, at 2500 cmy1 , vs. the applied load.
Fig. 17. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of PS powders according to the thickness h of the experimental cell: Ž1. hs9 mm; Ž2. hs 0.1 mm; Ž3. hs 0.05 mm. A KBr window was placed in the experimental cell.
to be rearranged between PS particles, the number of neighbours depending on the relative diameters of particles w62x. The decreasing distance between PS particles with increasing pressure causes a decrease in the reflectance values. For a given pressure, the time at which this pressure is applied affects the reflectance values which decrease with increasing time as illustrated in Fig. 16, including a kinetic aspect in the deformation of the PS particles. The influence of the pressure appears in the first minutes of compaction. Fig. 16 shows the variation of the reflectance of two samples containing 1.25 g of PS powders and compacted with
a load of 4 tons applied for 5 and 26 min. A longer time of compaction Žup to an hour. induces practically no change in the reflectance values of the sample. As a result, great consideration must be given to the preparation of the samples, concerning the packing conditions, in particular. The reflectance of granular materials depends on the layer thickness h and increases with increasing depth, up to a given thickness generally considered infinitely thick, when it reaches 1 or 3 mm for small grain sizes. The reflectances of 40-mm diameter PS powders were shown to be strongly influenced by the thickness and particularly for low values and
Fig. 16. Total near-normal hemispherical spectral reflectance R vs. wavenumbers Žcmy1 . of compacted PS powders with different durations of compaction: Ž1. 5 min; Ž2. 26 min.
Fig. 18. Areas of absorption bands vs. the thickness of the cell containing PS powders 40 mm in diameter: bands at Ž1. 1895 cmy1 ; Ž2. 1860 cmy1 ; Ž3. 1660 cmy1 .
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
small variations of h as illustrated in Fig. 17. In this case, the variations in reflectance values are practically the same, when h increases from 0.05 up to 0.1 mm, corresponding to approximately two or three layers of powders, as when it increases from 0.1 up to 9 mm. Concerning the areas of some absorption bands, they increase with increasing thickness up to 0.2 mm for h, while they remain unchanged for higher values of h, as shown in Fig. 18, for the following wavenumbers: 1895, 1860 and 1660 cmy1 .
4. Conclusions The results presented in this paper all refer to the reflection properties of granular materials which are shown to be strongly dependent on chemical and physical parameters. As expected, the chemical nature of the material assigns the position of the absorption bands and governs the shape of the spectra, thereby. The morphology of samples, as well as the particle sizes of granular materials are physical parameters influencing the radiative properties of materials. A kinetic aspect is observed during the preparation of pellets, as reflectance values vary with the pressure and the time during which the load is applied. The present paper is also concerned with reflection properties of mixtures of granular materials. The reflectance values of mixtures are influenced by the optical property of the added material and their behavior is wavelength-dependent.
References w1x w2x w3x w4x w5x w6x w7x w8x
P. Kubelka, F. Munk, Z. Tech. Phys. 12 Ž1931. 593. A. Mandelis, J.P. Grossman, Appl. Spectrosc. 46 Ž1992. 737. H.G. Hecht, J. Res. Natl. Bur. Stand. ŽU.S.. 80A Ž1976. 567. M. Kaviany, Principles of Heat Transfer in Porous Media, Springer, New York, 1991. B.P. Singh, M. Kaviany, Int. J. Heat Mass Transfer 35 Ž1992. 1397. G. Kortum, ¨ Reflectance Spectroscopy, Springer, New York, 1969. R. Siegel, J.R. Howell, Thermal Radiation Heat Transfer, Hemisphere Publishing, WA, 1992. M.Q. Brewster, Thermal Radiative Transfer and Properties, Wiley, New York, 1987.
25
w9x H.W. Siesler, Makromol. Chem. Macromol. Symp. 52 Ž1991. 113. w10x W.WM. Wendlandt, H.G. Hecht, Reflectance Spectroscopy, Wiley, New York, 1966. w11x S.R. Culler, M.T. MacKenzie, L.J. Fina, H. Ishida, J.L. Koenig, Appl. Spectrosc. 38 Ž1984. 791. w12x M.P. Fuller, P.R. Griffiths, Anal. Chem. 50 Ž1978. 1906. w13x P.R. Griffiths, M.P. Fuller, in: R.J.H. Clark, R.E. Hester ŽEds.., Advances in Infrared and Raman Spectroscopy, Vol. 63, Chap. 2, Heyden, London, 1982. w14x F. Boroumand, J.E. Moser, H. VandenBergh, Appl. Spectrosc. 46 Ž1992. 1874. w15x D. Fassler, R. Gade, J. Mol. Struct. 246 Ž1991. 145. w16x J.M. Chalmers, M.W. MacKenzie ŽEds.., Advances in Applied Fourier Transform Infrared Spectroscopy, Wiley, New York, 1988. w17x M.K. Gunde, J.K. Logar, Z.C. Orel, B. Orel, Appl. Spectrosc. 44 Ž1990. 193. w18x K. Moradi, C. Depecker, J. Corset, Appl. Spectrosc. 48 Ž1994. 1491. w19x S.C. Lee, J. Quant. Spectrosc. Radiat. Transfer 36 Ž1986. 253. w20x D.J.J. Fraser, P.R. Griffiths, Appl. Spectrosc. 49 Ž1995. 623. w21x M.T. MacKenzie, J.L. Koenig, Appl. Spectrosc. 39 Ž1985. 408. w22x P.J. Brimmer, P.R. Griffiths, N.J. Harrick, Appl. Spectrosc. 40 Ž1986. 258. w23x T. Theophanides ŽEd.., Fourier Transform Infrared Spectroscopy, Reidel, Boston, 1984. w24x M. Falk, Can. J. Appl. Spectrosc. 36 Ž1991. 22. w25x H.G. Hecht, Appl. Spectrosc. 34 Ž1980. 161. w26x D.B. Judd, K.S. Gibson, Natl. Bur. Stand. ŽU.S.., RP 872, 16 Ž1936. 261. w27x W. Richter, W. Erb, Appl. Opt. 26 Ž1987. 4620. w28x W. Richter, Appl. Spectrosc. 37 Ž1983. 32. w29x B. Orel, Z.C. Orel, Sol. Energy Mater. 22 Ž1991. 259. w30x R.W. Frei, J.D. MacNeil, Diffuse Environmental Spectroscopy in Environmental Problem-Solving, CRC Press, Cleveland, 1973. w31x D.G. Goebel, Appl. Opt. 6 Ž1967. 125. w32x K. Gindele, M. Kohl, ¨ M. Mast, Appl. Opt. 24 Ž1985. 1757. w33x R.A. Shepherd, SPIE 1311 Ž1990. 55. w34x R.R. Willey, Appl. Spectrosc. 30 Ž1976. 593. w35x B.P. Singh, Int. J. Heat Mass Transfer 35 Ž1992. 1397. w36x M. Kerker, The Scattering of Light and Other Electromagnetic Radiation, Academic Press, New York, 1969. w37x D.E. Leyden, R.S.S. Murthy, Trends Anal. Chem. 7 Ž1988. 164. w38x R.E. Stephens, E.K. Plyler, W.S. Rodney, R.J. Spindler, J. Opt. Soc. Am. 43 Ž1953. 110. w39x R.T. Graf, J.L. Koenig, H. Ishida, Appl. Spectrosc. 39 Ž1985. 405. w40x P.C. Painter, M.M. Coleman, J.L. Koenig, The Theory of Vibrational Spectroscopy and its Application to Polymeric Materials, Wiley, New York, 1982. w41x R.A. Nyquist, Appl. Spectrosc. 38 Ž1984. 264. w42x B.H. Stuart, Vib. Spectrosc. 10 Ž1996. 79.
26
C. Ventura, M. Papinir Vibrational Spectroscopy 21 (1999) 17–26
w43x B. Jasse, J.L. Koenig, J. Polym. Sci. Polym. Phys. 17 Ž1979. 799. w44x S. Krimm, Fortschr. Hochpolym.-Forsch. 2 Ž1960. 140. w45x A.A. Christy, Y.Z. Liang, C. Hui, O.M. Kvalheim, Vib. Spectrosc. 5 Ž1993. 233. w46x Polymer Handbook, J. Brandrup, E.H. Immergut ŽEds.., Wiley, New York, 1989. w47x H.W. Siesler, K. Holland-Moritz, Infrared and Raman Spectroscopy of Polymers, Marcel Dekker, New York, 1980. w48x N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press, New York, 1975. w49x D.I. Bower, W.F. Maddams, The Vibrational Spectroscopy of Polymers, Cambridge Univ. Press, Cambridge, 1989. w50x A. Lee Smith, in: P.J. Elving, J.D. Winefordner, I.M. Kolthoff ŽEds.., Applied Infrared Spectroscopy, Vol. 54, Wiley, New York, 1979. w51x T.M. Hancewicz, Appl. Spectrosc. 46 Ž1992. 1074.
w52x M. Papini, Infrared Phys. 34 Ž1993. 607. w53x G. Duyckaerts, Spectrochim. Acta 7 Ž1955. 25. w54x T. Burger, J. Kuhn, R. Caps, J. Fricke, Appl. Spectrosc. 51 Ž1997. 309. w55x P.W. Yang, H.H. Mantsch, Appl. Opt. 26 Ž1987. 326. w56x M.L.E. Te Vrucht, P.R. Griffiths, Appl. Spectrosc. 43 Ž1989. 1293. w57x P.W. Yang, H.H. Mantsch, F. Baudais, Appl. Spectrosc. 40 Ž1986. 974. w58x A.A. Christy, O.M. Kvalheim, R.A. Velapoldi, Vib. Spectrosc. 9 Ž1995. 19. w59x C. Ventura, M. Papini, Macromol. Symp. Polym. Spectrosc. 119 Ž1997. 137. w60x S.A. Yeboah, S.H. Wang, P.R. Griffiths, Appl. Spectrosc. 38 Ž1984. 259. w61x E.A. Schatz, J. Opt. Soc. Am. 56 Ž1966. 389. w62x C. Ventura, M. Papini, J. Near-Infrared Spectrosc. 5 Ž1997. 123.