Attenuated total reflection infrared spectroscopy for micro-domain analysis of polyethylene samples after accelerated ageing within weathering chambers

Vibrational Spectroscopy 34 (2004) 63–72

Attenuated total reflection infrared spectroscopy for micro-domain analysis of polyethylene samples after accelerated ageing within weathering chambers L. Ku¨ppera,*, J.V. Gulmineb, P.R. Janissekc, H.M. Heisea a

Institute of Spectrochemistry and Applied Spectroscopy at the University of Dortmund, Bunsen-Kirchhoff-Str. 11, D-44139 Dortmund, Germany b PIPE Centro Polite´cnico, Universidade Federal do Parana´, Caixa Postal 19011, CEP 81531-990 Curitiba-PR, Brazil c Centro Universitario Positivo UnicenP, Rua Professor Pedro Viriato Parigot de Souza, no. 5.300 Campo Comprido, CEP 81280-330, Curitiba-PR, Brazil Received 4 March 2003; received in revised form 5 May 2003; accepted 5 May 2003

Abstract Elucidation of the chemical changes that take place during the ageing of polyethylene by weathering can be accomplished by infrared (IR) spectroscopy. Accelerated ageing was applied to the most representative low density polyethylene (LDPE) found in markets today. The samples were exposed to UV- and xenon-arc radiation of varying duration and temperature cycles. The structural changes in the material surface were analysed by scanning electron microscopy (SEM) and chemical composition was studied by FTIR spectroscopy. By varying the crystal material in the conventional attenuated total reflection (ATR) experiments it was possible to analyse distinct layers from the surface. Micro-domain analysis was made possible by using a novel ATR microprobe based on a silver halide fibre of circular cross-section with an outer diameter of 700 mm. Layers at larger depth were reached by cross-sectioning the LDPE-samples and measuring the concentration profiles of oxidation products down to 500 mm with a spatial resolution lesser than 15 mm. The main chemical modifications were from carbonyl formation of various kinds identifiable in the ATR–FTIR spectra of degraded polyethylene samples. # 2003 Elsevier B.V. All rights reserved. Keywords: Infrared spectroscopy; Attenuated total reflection; Silver halide fibres; Microprobes; Polyethylene deterioration; Artificial weathering; Depth profiling of oxidation products

1. Introduction Infrared (IR) micro-spectroscopy is a powerful analytical method that can provide chemical information based on the recorded spectral data, in addition to the imaging capability as available in conventional microscopy [1]. Recently, special attention has been paid to the experimental design for mapping studies and recommendations have been published [2]. Besides spot analysis, which is most frequently carried out after visual inspection of the sample, the analysis along a line or by area mapping is often needed to provide enough information on the in-homogeneous composition changes within the material under investigation. For a conventional IR microscope equipped with a single detector, an X–Y mapping stage can be used for analysing the sample micro* Corresponding author. Present address: Infrared Fiber Sensors, Im Gillesbachtal 33, Aachen D-52066, Germany. Tel.: þ49-241-65609; fax: þ49-241-65617. E-mail address: [email protected] (L. Ku¨pper).

0924-2031/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2003.05.002

structure. For bulky samples, the attenuated total reflection (ATR)-technique is suited for sample surface analysis that is otherwise not accessible by transmission spectroscopy. Recently, we developed a microprobe based on a u-shaped silver halide fibre rendering the possibility of taking ATRspectra from micro-samples [3]. Applications using such a microprobe were already published by us: In particular, various sections of skin and hair samples from moor-mummified corpses or tissue biopsies have been studied [4,5]. The application we report in this paper is from the field of material analysis. Several polymers have been used in the manufacture of various mass products and for application in fields such as electro-techniques and electronics [6]. Polythene (Polyethylene, PE) is the polymer with the largest production and commercial application. They are classified into three types based on their density as high density (HDPE), medium density (MDPE) and low density (LDPE). The different densities occur due to the differences in the molecular structure with either linear or branched units. LDPE is produced without catalysts under high pressure,

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whereas medium and high density PE are obtained under low pressure with the help of catalysts. The different grades can be discriminated by IR spectroscopy [7]. The LDPE plays an important role especially in the production of cables used for electricity-distribution. Due to increased demands for its thermo-mechanical properties, cross-linking of such a material is also affordable (XLPE cross-linked PE). Plastic products are exposed to external effects such as heat, weathering or electrical stress during their lifetime. Such effects often cause deterioration of the mechanical properties or, if used for insulation, their electrical characteristics. Chemical changes take place in the material during the deterioration which can be investigated by IR spectroscopy. Special decomposition effects, the so-called formation of water trees, have been observed in the PEinsulation material used for high voltage cables (see for example [8,9]). Surface degradation in particular takes place by weathering due to UV-radiation and oxygen exposure. Therefore, polymers are usually stabilised against thermoand photo-oxidative degradation by using radical scavengers [10] (for a recent IR spectroscopic study, see also [11]). Studies on LDPE when used for outdoor applications, particularly in the form of foils produced for greenhouse coverage, have recently been undertaken by Scoponi et al. [12]. In another study, the stabilisation of LDPE by additives, when cross-linked in the presence of peroxides, has been investigated by using FTIR spectroscopy [13]. Oxidation products are formed by the thermal oxidation of the polymer caused by the dissolving of the residual oxygen and atmospheric exposure. Weathering often leads to brittle-failure in otherwise ductile plastic products which results in a considerable reduction of their service life. In products with a large wall thickness, the degradation is usually limited to a surface layer, either due to limited oxygen diffusion or to a limited UV-radiation penetration [14]. But even with a small depth of such a layer (e.g. 0.5 mm)

compared to the whole wall thickness, e.g. of several millimetres, a brittle-fracture of the product can occur. In general, outdoor ageing is too slow to be used for quality control. Therefore, a number of accelerated weathering tests have been developed. Within the area of polyethylene analysis, investigations were carried out for assessing the degradation profiles in thick non-pigmented HDPE samples without UVstabilisation after intensive outdoor and artificial weathering [15]. Outdoor exposures were performed under climatic conditions in the state of Florida (USA) and the Netherlands. To study the depth profile, films of 10 mm thickness were microtomed from the sample and analysed by IR spectroscopy. LDPE samples were investigated by other authors [16]. The non-stabilised specimens were films of 150 mm thickness, exposed to the UV-radiation within a Sun tester for about 2000 h. A recent study on artificial ageing of HDPE samples by UV-radiation was reported by Carrasco et al. [17]. In this context, the performance of standard accelerated ageing methods for oxidation of PE samples within an inter-laboratory reproducibility study may be of interest to the reader [18]. Surface analysis and depth profiling of PE samples after accelerated ageing within two different weathering chambers under different conditions were studied by us using ATR-spectroscopy with crystals from ZnSe and Ge, respectively, under different reflection angles, allowing the estimation of depth profiles of the oxidation products [19]. The effects of accelerated ageing on such samples were also investigated using a fibre-optic microprobe as described above and exemplary results are reported in this paper. These results are compared to conventional measurements using large ATR crystals. Besides a surface analysis, the degradation effects along with the cross-section cuts of the polymer slabs were also determined by exploiting the special marker bands (for example, IR absorption due to the carbonyl stretching vibration).

Fig. 1. A silver halide fibre-optic probe for the analysis of solids: arrangement for micro-domain analysis.

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2. Experimental 2.1. Spectral measurements Standard ATR-measurements were carried using a BOMEM DA8 spectrometer (Bomem, Quebec, Canada) equipped with a DTGS detector. The ATR-accessory, model 300 (Spectra-Tech, Shelton, CT, USA) allowed multiplereflection spectra to be taken for different reflection angles between 308 and 608. The ATR crystals were either ZnSe or Ge and the contact area for both was 50  10 mm2. Spectra were recorded with a resolution of 2 cm1 using triangular apodisation; and 128 interferogram scans were averaged, providing spectra from 400 to 5000 cm1 (transmission) and from 650 to 5000 cm1 (ATR). Further IR spectra were recorded on a FTIR spectrometer (model Vector 22; Bruker Optik GmbH, Ettlingen, Germany), equipped with a silver halide optical fibre microprobe for remote ATR-spectroscopy. The IR beam exiting the FTIR spectrometer was coupled into the silver halide fibre, which was supplied by Infrared Fibre Sensors (Aachen, Germany), by an off-axis parabolic mirror. The silver halide fibre-optic microprobe consisted of a fibre used for transmitting the IR radiation and also passing it through the u-shaped ATR-element and finally guiding it to the detector. The transmitting fibre of a circular crosssection was about 20 cm in length with a diameter of 700 mm, providing a numerical aperture of 0.6 (for the experimental set-up, see Fig. 1). The sensitivity of the ATR-element depends on the bending radius of the silver halide fibre and the sample contact area. The maximum sensitivity of the ATR-element is reached at the lower end of the u-shaped fibre piece and can be increased by reducing the bending radius down to values close to the diameter of the fibre. For the experiments presented here, a u-shaped fibre ATR-element with an outer diameter of 4 mm was used. With the flat and hard samples studied, a contact area of about 15 mm  50 mm was realised with the smaller rectangle side length parallel to the gradient observed while oxidation with a maximum at the sample surface, so that maximum spatial resolution could be achieved. The fibre end was directly mounted to a liquid nitrogen-cooled semiconductive mercury–cadmium telluride detector (MCT) with a micro-lens formed at the fibre end which faced the 1 mm2 detector element from Infrared Associates (Stewart, FL, USA) with a few micrometre gap. The signal was amplified by a pre-amplifier which was matched to the detector to ensure low noise. Spectra were recorded with a spectral resolution of 4 cm1 using triangular apodisation; 256 interferogram scans were averaged for each individual spectrum when using the ATR microprobe.

Fig. 2. SEM micrograph of a non-degraded LDPE sample surface (A), and of a LDPE sample surface aged under WOM for 400 h (B).

as pellets. The samples were analysed in film, KBr disk and slab form. The film and disk measurements were carried out in transmission, while the slabs were measured by

2.2. Sample preparation Commercially available LDPE material (code PB 681/59) from OPP Polietilenos S.A. (Camac¸ari, Brazil) was received

Fig. 3. SEM micrograph of a LDPE sample surface aged under WOM for 1600 h (A), and of a LDPE sample surface aged under QUV for 800 h (B).

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exclusively using the ATR-technique. The films were prepared using a constant thickness film maker device from Graseby Specac (Orpington, UK). Using the assembly, a load of 2:77  107 Pa at 130 8C in a Graseby Specac press, model P/N 15620 was applied for 5 min to obtain films of 3 cm diameter and of varying thickness. The temperature was maintained by an automatic temperature controller and a water cooled and heatable platens system (model P/N 15515) from the same company. The KBr disks containing PE in dilution were prepared using PE-powder, supplied by Perkin-Elmer for preparing pellets for far-IR measurements, with a load of 5:68  108 Pa with no heating taking place in the stainless steel mould. The PE-slabs (10 cm  10 cm  0:1 cm) were prepared in a SCHULZ press, model PHS 15 (Joinville, Brazil), using aluminium moulds and polyester foils at a temperature of 130 8C for 5 min without any load, for 5 min with a load of 1:62  108 Pa, and for 2 min with a load of 3:25  108 Pa. The slabs were allowed to cool down to ambient temperature. The PE-samples were cut perpendicularly to the slab surface using a scalpel providing a fresh and plane surface,

which was investigated by the ATR microprobe. For this, the sample was clamped, so that it could be positioned by using an XYZ-micromapping stage. For example, spectra along a cross-section cut were taken for every 20 mm, starting from the edge of the degraded PE surface. 2.3. Artificial weathering Two types of artificial weathering equipment were used. The details of the apparatus and the weathering conditions are listed in Table 1. The exposure times, while using the Weather Ometer (WOM) equipment from ATLAS (Chicago, IL, USA), were 400, 800, and 1600 h, respectively. In the case of the QUV Weathering tester, which was from QPanel Lab Products (Cleveland, OH, USA), exposure to ageing conditions lasted for 200, 400, and 800 h, respectively. 2.4. Scanning electron microscopy (SEM) The SEM XL 30 equipment from Phillips (Hillsboro, OR, USA) was used to examine the polymer surfaces, which

Fig. 4. Transmission spectrum of a LDPE-film of 117 mm thickness (A), and absorbance spectra (B) obtained from a transmission measurement of a PEpowder diluted in a KBr-pellet (top) and from the surface measurement of a pure LDPE-slab (bottom) using the ATR-technique (ZnSe crystal with 458 reflection angle of incidence); the inset provides an enlarged section with the band doublet at 1470 cm1, illustrating different band intensities due to the measurement technique chosen.

L. Ku¨ pper et al. / Vibrational Spectroscopy 34 (2004) 63–72 Table 1 Apparatus for artificial weathering and weathering conditions Weather Ometer Atlas model CI65 (norm ASTM G26-95—method A)a Xenon lamp 6500 W, irradiance: 0:35  0:03 W/m2 (340 nm), incidence: 908 Temperature: 63  5 8C (dry) and 50  5 8C (wet) Cycle: 102 h with Xe lamp only and 18 h with Xe lamp plus water spray Relative humidity: 60  5% (dry) and 80  5% (wet) QUV Weathering tester Q-Panel Lab Products, model QUV Spray—UV40 (norm ASTM G53-96)b Lamp UVB, irradiance: 0.60 W/m2 (313 nm), incidence: 908 Temperature: 60  5 8C (dry) and 50  5 8C (wet) Cycle: 8 h UV/4 h UV plus water spray a

Standard practice for operating light-exposure apparatus (xenon-arc type) with and without water for exposure of non-metallic materials, ASTM G26-95. Annual Book of ASTM Standards, American Society for Testing and Materials: Philadelphia, PA, 1995. b Standard practice for operating light- and water-exposure apparatus (fluorescent UV-condensation type) for exposure of non-metallic materials, ASTM G53-96. Annual Book of ASTM Standards, American Society for Testing and Materials: Philadelphia, PA, 1996.

were gold-coated by sputtering using a Baltec (Liechtenstein) SCD 005 equipment. Photomicrographs were taken with a 500-fold magnification. The surface of a freshly

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prepared PE slab is shown in Fig. 2A, while in Fig. 2B the surface after 400 h of WOM exposure is displayed. More drastic effects can be observed in Fig. 3A, which shows the surface of a PE-slab after 1600 h of WOM exposure; similar ridges and cracks were observed for PE-slabs after 800 h of QUV exposure (Fig. 3B).

3. Results and discussion For characterising the PE-samples by IR transmission spectroscopy, it is necessary to prepare a thin free-standing film. Such a spectrum of a film of about 120 mm thickness is presented in Fig. 4A. Interference fringes from multiple reflections within the film can be noticed in the baseline of the transmittance spectrum. To avoid the opaque spectral regions of the strongest absorption bands, we also recorded a PE-spectrum of a powdered PE-sample within a KBr-pellet, which is shown using absorbance units in Fig. 4B. Such a spectrum can be used for the calculation of absorptivities, which allows the determination of optical constants. An IR spectral characterisation of freshly prepared PE-slabs can only be made by ATR-spectroscopy because

Fig. 5. Surface measurements of artificially degraded LDPE-slabs, obtained under varying exposure duration within WOM- (A) and QUV-weathering equipment (B), using the ATR-technique (Ge crystal, reflection angle of incidence of 308).

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of their millimetre thickness. Such an ATR-spectrum is also presented in Fig. 4B (see different relative band intensities with the ATR-technique compared to those obtained by transmission spectroscopy). There are drastic differences in the band intensities of the doublet at 1470 cm1 (see inset in Fig. 4B), which are due to the band dispersion from the two neighbouring bands effecting the intensities when using the ATR-measurement technique. Within the ATR-spectrum of the non-degraded LDPE-sample, additional bands are observed in the interval between 3400 and 3200 cm1 and around 1600 cm1, which can be assigned to UVstabilisers of the hindered amine stabiliser (HAS)-type [20]. As these bands are not observed in the transmittance spectrum, their appearance can be explained by a possible migration and enrichment at the sample surface after extended exposure to elevated temperatures during slab formation. In the following, ATR-spectra from the surfaces of differently degraded PE-slabs are shown. Spectral measurements were done by using either a Ge crystal under 308 reflection or by using a ZnSe crystal under 458 reflection. Exemplary results from PE exposed to varying weathering

conditions are summarised in Figs. 5A and B using the first ATR-measurement conditions, whereas the spectra obtained by using the ZnSe crystal are illustrated in Figs. 6A and B. The model 300 ATR-accessory allowed a variable angle of incidence for the reflection measurements; the effective reflection angle for the measurements shown here using the Ge crystal was 33.58. With the refractive index of Ge nGe ¼ 4:0 at 1700 cm1 (useful range is between 900 and 5000 cm1 with a mean refractive index of 4.0) and an estimated refractive index of PE nLDPE ¼ 1:5 [21] (our own measurements taken recently using ellipsometry provided a lower value of 1.43 at 1750 cm1), the usual penetration depth for an ATR experiment [22] can be calculated to give a value of 0.57 mm for an exemplary wavenumber of 1700 cm1. For the measurement conditions (effective reflection angle 458 as given above) chosen with the ZnSe crystal (refractive index nZnSe ¼ 2:4 at 1700 cm1, useful range is between 700 and 20 000 cm1 with a mean refractive index of 2.42), a respective penetration depth of 1.17 mm can be calculated. The ATR-measurements on the drastically aged LDPE samples were complicated by the changes in hardness

Fig. 6. Surface measurements of artificially degraded LDPE-slabs, obtained under varying exposure duration within WOM- (A) and QUV-weathering equipment (B), using the ATR-technique (ZnSe crystal, reflection angle of incidence of 458).

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observed for the PE samples [19]. Further aggravation was due to the heterogeneity of the slab surfaces as can be seen from the SEM images shown in Fig. 3 which makes it difficult to have a good sample contact with the large ATR crystal. The smaller PE-band intensities are certainly a good indicator of these measurement complications. Some details on the oxidation of polyethylene under weathering conditions, which is well known from the works of Allen [23], will be given for the spectrum-interpretation of PE oxidation products. In the initial step of degradation, macro-radicals are formed which react with the oxygen of air. In the chain propagation sequence, the ROO radicals separate hydrogen from the polymer resulting in the formation of hydroperoxides. These hydroperoxides decompose fast, yielding highly reactive radicals, which accelerate the oxidation process where different types of oxidation products are formed, including ketones, esters and acids (see also our previous publication providing band assignments for the different C¼O stretching vibrations [19]). Enlarge-

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ments of the spectral region are also shown in Figs. 7 and 8, which were obtained with the silver halide ATR microprobe (see below). The bands due to the C–O stretching vibration of peroxides and hydroperoxides occur in the region between 1000 and 1300 cm1 (see Figs. 6B and 7C). The O–H stretching vibration of free hydroperoxides is located in the region between 3560 and 3530 cm1, whereas the hydrogenbonded OH vibration absorbs in the region between 3550 and 3230 cm1 [24]. The absorptions between 1100 and 1000 cm1 can be attributed to the C–O stretching vibration of hydroperoxides, confirmed by weak O–H bands above 3000 cm1. The intensity of the hydroperoxide C–O absorption decreases with ageing time due to the chemical reactions yielding ketones, esters and acids. The intensity of the band between 1250 and 1150 cm1 in Fig. 7A increases with ageing time (see also Fig. 7A with its top spectrum and Fig. 7C with its bottom spectrum). This band can possibly originate from long linear esters [25].

Fig. 7. Surface measurements of artificially degraded LDPE-slabs, obtained under varying exposure duration within WOM- (A), using the ATR-technique by means of an u-shaped silver halide fibre; sub-diagram (B) is provided for identifying the PE surface impurities after WOM-weathering. The surface spectra of LDPE samples after varying exposure within QUV weathering equipment are shown below (C).

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Surface measurements were also carried out using the silver halide microprobe, for which the problems of realising a satisfactory sample contact to the bent fibre are reduced compared to the situation using large ATR crystals for conventional measurements. Exemplary spectra obtained from micro-spots are displayed in Fig. 7A and C. For the samples obtained under WOM conditions, in particular, there are intensive bands in the region between 1300 and 800 cm1. By recording an XPS spectrum from such an unsputtered sample surface we identified spectral peaks that could easily be assigned to silicon [19]. A recording of a silicate spectrum (from silica gel, see also Fig. 7B) confirmed the identity of the impurities on the slab surfaces, which have their origin from the impurities of the water used during the spray periods of the wet weathering cycles. Also a treatment with distilled water using supersonic dispersion had the effect of reducing, i.e. washing out, the surface silicate contamination which could be confirmed after additional ATR-measurements. The wavenumbers of the most intensive silica absorption bands in Fig. 7B do not correspond perfectly to those of the artificially weathered PE in Fig. 7A, but for the latter only slight shifts to lower wavenumbers are noticed, whereas the

band positions at 800 cm1 are identical. Depending on the structural units within silicate glasses and cation composition, shifting of the Si–O stretching frequencies has been observed, which can explain the differences in band position noticed here [26,27]. One of the referees mentioned that many of the commercially available PE products contain SiO2 in small quantities as anti-blocking agent [28], but the same PE raw material had been used for the production of slabs tested under both weathering conditions. However, our interpretation is supported by the fact that principally similar oxidation effects are expected, when UV-intensity and exposure time are both taken into account for quantifying the amount of oxidation products. Thus, the additional absorption bands observed under the Weather Ometer conditions are of different origin than due to PE oxidation. An estimate of the penetration depth while using the fibre ATR microprobe is difficult, since the average reflection angle is not known (the refractive index of the fibre core material, i.e. polycrystalline AgBr:AgCl:AgI, can be given with nfibre ¼ 2:2). In Figs. 8A and B the results from a mapping of a crosssection cut along the line from the surface to the inner central part of the slab are shown. Also shown is the absorbance

Fig. 8. ATR-absorbance spectra recorded along the perpendicular cross-section cut of a degraded polyethylene slab of 1 mm thickness using a silver halide fibre microprobe (surface measurement is shown that illustrates also the spectral contribution of silicate from spray water): LDPE-sample after weathering within the WOM-equipment (A) and within the QUV weathering tester (B).

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Fig. 9. Depth profiles of oxidation products in artificially weathered LDPE slabs with cross-section measurements using a silver halide ATR microprobe (LDPE-sample weathering was done using a QUV weathering tester).

noise spectrum from two consecutive single beam spectra recorded with the microprobe without a sample contact. The evaluation of the intensities of the absorption band at 1713 cm1 allowed us to plot the relative intensities normalised to the closest of the surface versus the distance from the sample surface, which is shown in Fig. 9 for two samples exposed to 200 h under WOM-conditions and after double the time. The depth for which oxidation within the PE-slab is still observed can be nicely correlated to the exposure time. From the IR studies a carbonyl and a vinyl index were defined by ratioing the band absorbances measured at 909 and 1714 cm1, respectively, to the peak absorbance at 1368 cm1 (see also our results published in [19]). It is interesting to note, that when spectra were taken from weathered PE samples that were measured below the slab surface, the oxidation effects as evident from the carbonyl stretching region were very similar. The microspectroscopic measurements were made possible by the available high spectral signal-to-noise ratio, which could be reached with the fibre configuration chosen and the coupling to the MCT-detector element. The set-up needs a certain rigidity to avoid changes of the fibre contact to the protecting Teflon1 tubing, which covers the major fibre length apart from the lower U-shaped section.

robes. In addition, the results from invasive cross-section cuts obtained from consecutive IR microscopic line mapping using a silver halide fibre microprobe showed the large potential of this uncomplicated measurement strategy. It must be also noted that such an equipment is much less expensive than an IR microscope equipped with an ATRmeasurement option. In general, the set-up presented is a convenient tool for analysing micro-domain surface degraded polymer, in particular with regard to test different compositions for material stabilisation against weathering effects.

Acknowledgements Two authors (Gulmine and Janissek) wish to thank Companhia Paranaense de Energia (COPEL) for financial support. Gulmine thanks LACTEC for a doctoral scholarship. Heise acknowledges the support given by a CNPq travel grant. Dr. H. Bubert and Mrs. Kittel (ISAS) are thanked for measuring the XPS-spectra of accelerated aged PE-samples. The authors from ISAS (Heise and Ku¨ pper) also acknowledge gratefully the financial support given by the Ministerium fu¨ r Wissenschaft und Forschung des Landes Nordrhein-Westfalen and by the Bundesministerium fu¨ r Bildung und Forschung, Germany.

4. Conclusions For the investigation of surface changes in bulky PE samples through different weathering conditions, ATR-spectroscopy is an extremely sensitive and versatile method, in particular when crystal material and reflection angles can be varied for the aim of non-invasive depth profiling. The depth profiles of oxidation products within the same material have been studied through the evaluation of such data that had been obtained from large sample areas, but published in another paper [19]. However, high-quality analysis of microdomain areas is possible by using the ATR-fibre microp-

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