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Functional group analysis on oxidized surfaces of synthetic textile polymers
Talanta 76 (2008) 1136–1140
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Talanta journal homepage: www.elsevier.com/locate/talanta
Functional group analysis on oxidized surfaces of synthetic textile polymers Dierk Knittel ∗ , Eckhard Schollmeyer Deutsches Textilforschungszentrum Nord-West e.V. (DTNW), Adlerstrasse 1, D-47798 Krefeld, Germany
a r t i c l e
i n f o
Article history: Received 25 October 2007 Received in revised form 13 May 2008 Accepted 14 May 2008 Available online 7 July 2008 Keywords: Polymer surface analysis Functional groups Wet-chemical analysis Advanced oxidation process Synthetic textiles
a b s t r a c t A comprehensive collection of wet-chemical analyses of functional groups on oxidatively treated surfaces of hydrophobic polymers like poly(ethylene terephthalate) or polyolefine is presented. New methods are introduced. Textiles and foils have been subjected to advanced oxidation processes and the different oxygen functions have been quantified. Analysis of surface functional groups includes radical site determination with radical scavengers like diphenylpicrylhydrazyl, reduction of peroxides determined iodometrically, cationic dyestuff adsorption, carbonyl binding to Girard reagent P and surface hydroxyl group determination by surface nitrosation and subsequent azo-dye formation photometrically determinable. Use of potential surface swelling agents has been excluded except for addition of wetting agent. Wet-chemical analyses on textile surfaces bear the benefit of integrating over (relatively) large sample areas, a point which is interesting when regarding inhomogenities of textile or other surface constructions. In addition examples for visualisation for the existence of surface groups are described. © 2008 Elsevier B.V. All rights reserved.
1. Introduction During the course of a research project on the influence of ozone treatment (or ozone and UV-irradiation treatment, advanced oxidation process—AOP) on fibre surfaces of textile materials the importance of a quick analysis of surface oxidized functionalities posed an urgent demand [1]. In the literature AOP-treatment of plastics for surface functionalization has been reported frequently. Thereby mainly hydroxyl radicals and peroxy radicals are involved attacking the polymer. Some of the primarily formed species on the surface are rather long-lived other rearrange to stable functions. Quite often the oxygenated surface functions decline on storage (‘hydrophobic recovery’) due to polymer segment diffusion into the bulk polymer (especially with polymers of low glass transition temperature). In most of the papers XPS-spectroscopy was used in order to assess chemical groups on the surface [2–5,8,9]. Some chemical analysis methods for selected groups were mentioned too. These include the determination of peroxides, of carboxyl groups and of radicals [6–12]. An interesting chemical approach mainly for fluorescent ¨ derivatization on PE surfaces was given by Hollander in 2004 but requires rather uncomfortable calibration [13] and still introducing organic solvents.
∗ Corresponding author. Tel.: +49 2151 843 165; fax: +49 2151 843 143. E-mail addresses: [email protected], [email protected] (D. Knittel). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.05.036
Regarding drawbacks of XPS-analyses for quantitative determination on textile constructions (curvature of filaments, extremely small area accessible compared to practical production widths of >1.5 m, cost of equipment) it was felt necessary to establish wet-chemical analytics for the determination of oxygen containing functions on common, originally hydrophobic textile polymers exposed to advanced oxidation processes. For this aim some of the methods cited have been adopted and modified for easy analysis, also some new methods have been developed. The paper presented compiles the methods introduced. Only methods were chosen which can be applied from almost pure aqueous solutions. Thus it should be possible to minimise swelling effects of surface layers of synthetic polymers like poly(ethylene terephthalate) (PETP), polypropylene (PP) or similar polymers when working with organically soluble reagents thus keeping the (chemical) information depth as close to the surface as possible. Some of the methods established were tested too and described as qualitative spray-test method for visualisation of AOPtreatments. 2. Experimental 2.1. Materials used Poly(ethylene terephthalate) foil, thickness 190 m, 274 g/m2 (Cadillac Plastics), polypropylene foil, 75 m, 68 g/m2 (Leco); poly(ethylene terephthalate) (PETP) fabric, pleine weave,
D. Knittel, E. Schollmeyer / Talanta 76 (2008) 1136–1140
129 g/m2 , warp 24.8 cts/cm, weft 21.2 cts/cm (Testex); polyethylene fabric (PE), 104 g/m2 , warp 6.6 cts/cm, weft 6.2 cts/cm (Leco); polypropylene fabric (PP), pleine weave, 196 g/m2 , warp 22.5 cts/cm, weft 15.3 cts/cm (C.C. Cramer); poly(butylene terephthalate) (PBT) non-woven, 51.8 g/m2 ; thickness 390 m (Freudenberg). All materials have been extracted in a Soxhlet-apparatus prior to AOP with methanol/water (1:1). Advanced oxidation of textile samples (ozone, ozone + UV) has been done as described in [1], i.e. samples of textiles (30 cm × 15 cm) were treated with ozone (68–74 and 190–185 g/N m3 , respectively) in a cell equipped with a quartz window. Treatment time usually was 10–30 min. Some samples too were simultaneously irradiated with a mercury UV-lamp for AOP. 2.2. Apparatus used UV–VIS Diode Array Spectrophotometer HP 8452 A (HewlettPackard) with cuvettes of 1 cm optical path length and analyte volume of about 80–100 l (Brand) was used for measurements of dyes. A spectrophotometer Datamaster Type DC 3880 (Datacolor) was used for remission measurements. 2.3. Chemicals used Methylene blue for microscopy (Merck). Ethyl red 97% (lithium-salt) (prepared from sodium salt, Aldrich). Intracronred® BF-3RM 150%, reactive dyestuff (Yorkshire). Nonionic wetting agent Marlipal® O13/80 (Sasol) and SDS (Aldrich). H-acid (4-amino-5-hydroxynaphthaline-2,7-disulfonic acid disodium salt hydrate 85%) (Aldrich or Hoechst). Sulfanilic acid (Fluka). Girard reagent P (N-acetylhydrazinopyridinium chloride) (Aldrich). 2,2-Diphenyl-1-picrylhydrazyl (DPPH) (Aldrich). Other reagents were of analytical grade. 2.4. Radical determination on surfaces The wet-chemical test is oriented on methods of MacManus et al. [11]. A DPPH-solution (2 mg/20 ml MeOH) is prepared and applied to (as-received and after surface oxidation) textile samples of about 120 mg in brown flasks. After shaking for several hours (up to 3 days) at room temperature the supernatant solution is quantified by photometry at 516 nm. From the decrease of DPPH-absorbance the amount of radicals reacted can be determined. This test can be modified for visualisation by simultaneous addition of an acid dye like the lithium salt of ethyl red. The dye anion binds to the C-cation of the surface. 2.5. Determination and visualisation of peroxidic groups on oxidized polymer surfaces Reaction vessels given in Fig. 3 are used for quantification (details in [1]). Briefly, they enable the treatment of small samples of fabric to be analysed with heating under inert atmosphere in presence of reaction partner. Additionally, titration of reagents formed may be done using microsyringes. These vessels may be used too for grafting reactions (occurrence of radicals or peroxides). For visualisation of the existence of peroxidic groups on oxidized polymer a graft polymerization of acrylic acid monomer may be induced by heating under inert atmosphere. After removal of
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homopolymers formed by alkaline washing staining is done with cationic dyestuff methylene blue. 2.6. Determination of hydroxyl containing surfaces: visualisation of surface hydroxyl groups A known amount of sample (0.1–0.2 g) is treated with 2.5 ml of 1 mol/l HCl and 1 ml of 15% Na-nitrite solution. After 10 min the textile sample is removed and rinsed intensively with 10 ml NaHCO3 -solution (1%, containing 0.5% Na-dodecylsulfate for wetting). After neutralisation (about 3 times with 20 ml dist. water) the sample is transferred to a 25 ml measuring flask and 1 ml of freshly prepared solution of sulfanilic acid (1%) is added. After 9 min of shaking 1 ml of H-acid (0.5% fresh solution) is added and shaken in the dark for 1.5 h. The resulting red solution can be analysed at 510 nm. Water introduced by the sample is corrected by weighing. Calibration is done simply by the use of a nitrite solution for the sulfanilic acid/H-acid procedure. This reaction sequence can be used for visualisation as spraying test sequence on the oxidized fabric: the sample is dried following the nitrite treatment and sprayed with sulfanilic acid followed by H-acid. Photographs can be taken after 1.5 h storage in the dark. Alternatively a surface oxidized sample may be stained by conventional reactive dyeing procedures and resulting colouration may be determined in reflectance measurements. 2.7. Determination of carbonyl groups on oxidized polymer surfaces 0.5 g of sample is shaken with 2.7 g of Girard P solution (1.11 g/l). After addition of 0.3 g acetic acid the vessel is stirred in a closed vessel (see Fig. 3) for 30 min at 90 ◦ C. After cooling the solution is transferred into a 100-ml flask. The decrease of reagent concentration due to binding of reagent to surface carbonyls is analysed photometrically at 260 nm. 2.8. Determination of carboxylic groups on surfaces of partially oxidized surfaces of hydrophobic polymers Samples (0.1–0.5 g) are treated at room temperature with a 0.05% solution of methylene blue in Britton–Robinson buffer (pH 7.1–7.4). The bluish samples are rinsed thoroughly and dried in air. For quantitative determination the sample is exposed to 20 ml of 84% aqueous acetic acid and shaken for 1 h. Photometric analysis is done at 650 nm. 3. Strategies for wet-chemical analysis The analytical methods elaborated enables the quantification of surface functional groups on synthetic polymers. The methods too can be used as a qualitative quick-test and for visualisation of surface effects obtained by AOP-treatment. In the following the scheme of analyses and chemical basics are described. 3.1. Radicals on surfaces of hydrophobic textile surfaces For the determination of surface radicals on PP or PETP the following reactions have been used (according to [12]) (Fig. 1) and widened for visualisation purposes by dye binding. First step after radical formation is an electron transfer to DPPH followed by irreversible protonation to diphenylpicryl hydrazine and a surface cation. The decay of the violet colour of DPPH can be followed pho-
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tometrically. The surface cation can be trapped by adding an acid dye.
3.2. Wet-chemical detection and determination of hydroxyl groups on oxidized polymers For quantification of hydroxyl group contents of oxidized surfaces the esterification by aqueous nitrous acid has been chosen. This reagent has been known for the use in homogeneous reactions for detection of alcohols [14]. It proved to be suitable even in heterogeneous application and one may expect no interference from possibly coexisting amino groups since –NH2 groups will be destroyed by HONO. Fig. 2 shows the reaction sequence used. In concentrated solutions of nitrous acid hydroxyl groups on the polymer surface are esterified. Those surface nitrous acid esters are stable against washing but can be cleaved afterwards by dilute acid. Cleavage is done on the washed surface in solutions capably of trapping HONO. The newly formed nitrous acid immediately is transformed to a diazonium salt, which can be used for formation of an azo-dye, i.e. with H-acid or similar. This reaction sequence can be used too for visualisation on the textile as a spray-test application.
Fig. 2. Scheme for quantification of hydroxyl groups on surfaces of oxidized polymers.
3.3. Wet-chemical detection and determination of peroxidic groups on oxidized polymers
Fig. 1. Detection of radical functions on oxidized polymers by reaction with diphenyl-picrylhydrazyl, followed by formation of dyestuff binding for visualisation.
The presence of peroxidic groups on oxidized polymer surfaces (ozone treatment) can be determined using potassium iodide (cf. [12]). There is no discrimination between hydroperoxides or dialkylperoxides. Using apparatuses as described in Fig. 3, a simple titrimetric analysis can be performed. This reaction vessel can be used too for other analyses on oxidized samples. In addition to titration peroxidic structures on surfaces may be used for inducing grafting reactions on the surface with acrylic monomers (cf. [4,15–18]). A successful grafting reaction, e.g. with acrylic or methacrylic acid can be visualised after-
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cationic dyestuffs on carboxylate functions, visualisation for documentation and then detachment of the cationic dyestuff by acid treatment and photometric analysis of the solutions obtained (cf. [6]). Preferable for this determination is the use of highly pure cationic dyestuff as used for microscopy (i.e. methylene blue). 4. Results on AOP-treated samples obtained by wet-chemical analyses 4.1. Results on AOP-treated polymers
Fig. 3. Testing equipment for radical-, peroxide-, carbonyl- and grafting-analysis.
wards by using a cationic dyestuff as described in Section 2.4 (cf. [16,17]). 3.4. Wet-chemical detection and determination of carbonyl groups on oxidized polymers
To illustrate the versatility of the analytical methods described some examples are given in Table 1 for surface group analysis on AOP-treated foils and Table 2 for textiles (details see [1]). Samples are analysed immediately after preparation. Analysis for radicals on treated PE-fabric showed 0.012 mol/g fabric just after AOP-treatment, decreasing to about 0.006 mol/g on storage for 6 days, traces are still detectable even after 22 days. As known from the literature, all the samples showed hydrophobic recovery on storage for several days. Thus the chemical surface analysis proves to be a rather quick and sensitive and inexpensive tool. 4.2. Visualisations of functional groups on surfaces of synthetic material
There are numerous reaction for the determination of carbonyl groups in solution. An important one for the purpose of surface analysis lies in the use of a water soluble Girard reagent (Girard P). Qualitative analysis using such a reagent can be done by reflectance measurements on oxidatively treated polymer surfaces after exposure to Girard P solutions. Performing this analysis on oxidized textiles with minimal volumes of reagent (i.e. almost not more than textile wet pick up) the decrease of Girard-concentration in the treating solution can be monitored and quantified by photometric analysis within cuvettes of about 100 l working volume.
Visualisation of hydroxyl groups can be done as test on the fabric or on the foil by spraying the reagents (nitrite/-HCl/sulfanilic acid/H-acid) onto the sample. Other examples of various staining reactions on oxidized polymer surfaces, differently treated with ozone and analysed for visualisation of carboxylic and peroxidic groups and radical centers, respectively are presented in Fig. 4. Radical detection on oxidatively treated polyesters and on polyolefins combines the reaction with DPPH resulting in a surface cation which is trapped by use of an anionic dyestuff like salts of ethyl red giving rise to visible staining. 5. Discussion
3.5. Wet-chemical detection and determination of carboxyl groups on oxidized polymers Detection and determination of carboxylic groups on oxidized polymer surfaces follows well known strategies, i.e. binding of
In this work it could be shown that almost all relevant oxygenated functions on polymer surfaces can be quantitatively assessed by wet-chemical analysis on textile constructions. Wetchemical analyses on textile surfaces bear the benefit of integrating
Table 1 Wet-chemical analysis of oxygen functionalities before and after ozone treatment of polymer foils (30 min O3 ) (sum of both sides) Sample PETP-original O3 -treated PP-original O3 -treated
Total –COOH (mol/cm2 ) (2.0 (4.2 (7.0 (1.7
± ± ± ±
−4
0.1) × 10 0.2) × 10−2 0.5) × 10−4 0.13) × 10−2
Total –C O (×10−2 mol/cm2 ) (2.7 (2.7 (1.22 (1.77
± ± ± ±
0.04) 0.04) 0.015) 0.02)
Total –OH (×10−2 mol/cm2 ) (1.4 (80 (5.0 (24
± ± ± ±
0.3) 17.6) 1.1) 5.3)
Table 2 Oxygen containing groups of AOP-treated fabrics Sample code
Total –COOH (mol/g)
Total –C O (mol/g)
Total –OH (mol/g)
Total peroxides (mol/g)
PETP-original PETP, 30 min O3 PBTP-original PBTP, 30 min O3 PBTP, 30 min O3 with UV-irradiation PP-original 30 min O3
0.11 ± 0.48 ± 0.37 ± 0.39 ± 1.62 ± 0.04 ± 0.07
0.17 ± 0.03 5.4 ± 0.03 n.d. n.d. n.d. 0.6 ± 0.02 2.4
3.4 ± 0.75 ∼29 ± 6.3 15 ± 3.3 8.4 ± 2.85 3.5 ± 0.8 0.4 ± 0.09 7.7 ± 1.7
0 6.6 ± 0.12 n.d. n.d. n.d. 0 2.04
n.d., not determined.
0.006 0.03 0.007 0.007 0.03 0.003
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Fig. 4. (A) Spray test on AOP-treated PP-fabric for visualisation of the existence of hydroxyl groups (nitrite/sulfanilic acid/H-acid). (B) Methylene blue staining of AOP-treated PET. (C) Same on PE. (D) Visualisation of peroxides on PET by KI/starch solutions. (E) Same for PP. (F) Visualisation of radical formation on AOP-treated synthetic polymers by simultaneous application of DPPH in the presence of an acid dye for marking the cationic centres formed. (G) Same for PE.
over (relatively) large sample areas, a point which is interesting when regarding inhomogenities of textile or other surface constructions. In addition, the wet-chemical analysis – being rather quick – avoids problems due to changes during sample storage. Especially important are the possibilities to characterize partially oxidized surfaces of rather large size. Obviously wet-chemical analysis includes surfaces of cracks or other morphological irregularities thus giving higher analytical values than an XPS-sampling. At present a preliminary comparison of values for carboxylic groups on surface indicates about 20-fold higher values for wet-chemical analysis on PETP-fabric than as detected by XPS. This higher value may be due sampling over large fabric areas including fibre irregularities like cracks. XPS-sampling is methodically restricted to specimen of some micrometer dimensions, often randomly taken for measurements. More comparative investigations on oxygenated films (wet-chemical vs. XPS-analysis) will be systematically done in future with synchronously performed analysis. This has to be done because it is known that surface functionalities disappear often rather quickly (hydrophobic recovery) and to correlate information depth obtained. Problems in detectability of functional groups on surfaces may arise when dealing with compact and smooth surfaces because of sensitivity of analytical methods introduced. In addition the methods presented in this work uses inexpensive equipment and can be done quickly in a normal lab even as process control. Acknowledgements We would like to thank the Forschungskuratorium Textil e.V. for funding this research project (AiF-No. 13924N). This project was funded with financial resources of the Bundesmin¨ Wirtschaft und Technologie (BMWi) with a grant isterium fur
from the Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto-von-Guericke” e.V., AiF). The final report is available from: Deutsches Textilforschungszentrum Nord-West e.V., Adlerstrasse 1, D-47798 Krefeld, FRG. References [1] Research project of DTNW, Federal Ministry for economics and technology, FRG, grant no. AiF 13924N, 2004–2006; D. Knittel, E. Schollmeyer, DTNW-Mitt, vol. 56, 2007, ISSN 1430-1954. [2] K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, VSP BV, Utrecht, NL, 1996, ISBN 90-6764-201-0. [3] M.J. Walzak, F. Flynn, R. Foerch, J.M. Hill, E. Karbashewski, A. Lin, M. Strobel, in: K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, VSP BV, Utrecht, NL, 1996, ISBN 90-6764-201-0, pp. 253–272. [4] O.D. Greenwood, B.J. Hopkins, J.P.S. Baydal, in: K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, VSP BV, Utrecht, NL, 1996, ISBN 90-6764201-0, pp. 17–32. [5] J.F. Friedrich, W. Unger, A. Lippitz, T. Gross, P. Rohrer, W. Saur, J. Erdmann, H.J. Gorsler, in: K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, VSP BV, Utrecht, NL, 1996, ISBN 90-6764-201-0, pp. 49–72. [6] D. Praschak, T. Textor, T. Bahners, E. Schollmeyer, Technol. Text. 41 (1998) 136–138. [7] P. Hedenberg, P. Gatenholm, J. Appl. Polym. Sci. 60 (1996) 2377–2385. [8] M. Strobel, M.J. Walzak, J.M. Hill, A. Lin, E. Karbashewski, C.S. Lyons, in: K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, VSP BV, Utrecht, NL, 1996, ISBN 90-6764-201-0, pp. 239–251. [9] A. Hartwig, K. Albinsky, FhG Fertigungstechnik und Angewandte Materialforschung, Bremen, AiF-Researchproj. 10900N/1, 1999. [10] S. Dasgupta, J. Appl. Polym. Sci. 41 (1990) 233–248. [11] L.F. MacManus, M.J. Walzak, N.S. McItyre, J. Polym. Sci. A: Polym. Chem. 37 (1999) 2489–2501. [12] P. DeAntonio, W.J. Bertrand, S.L. Coulter, K.G. Mayhan, J. Am. Chem. Soc., Polym. Preprints 31 (1990) 448–449. ¨ [13] A. Hollander, Surf. Interface Anal. 36 (2004) 1023–1028. ´ Z.J. Vejdelek, Handbuch der Photometrischen Analysen Organischer [14] B. Kakac, Verbindungen, VCH, Weinheim, 1974. [15] D.J. Carlsson, D.M. Wiles, Macromolecules 2 (1969) 597–606. [16] K. Fujimoto, Y. Takebayashi, H. Inoue, Y. Ikada, J. Appl. Poly. Sci. A: Polym. Chem. 31 (1993) 1035–1043. [17] M. Suzuki, A. Kishida, H. Iwata, Y. Ikada, Macromolecules 19 (1984) 1804–1808. [18] C.I. Simionescu, S. Oprea, J. Polym. Sci. C 37 (1972) 251–263.
Contents lists available at ScienceDirect
Talanta journal homepage: www.elsevier.com/locate/talanta
Functional group analysis on oxidized surfaces of synthetic textile polymers Dierk Knittel ∗ , Eckhard Schollmeyer Deutsches Textilforschungszentrum Nord-West e.V. (DTNW), Adlerstrasse 1, D-47798 Krefeld, Germany
a r t i c l e
i n f o
Article history: Received 25 October 2007 Received in revised form 13 May 2008 Accepted 14 May 2008 Available online 7 July 2008 Keywords: Polymer surface analysis Functional groups Wet-chemical analysis Advanced oxidation process Synthetic textiles
a b s t r a c t A comprehensive collection of wet-chemical analyses of functional groups on oxidatively treated surfaces of hydrophobic polymers like poly(ethylene terephthalate) or polyolefine is presented. New methods are introduced. Textiles and foils have been subjected to advanced oxidation processes and the different oxygen functions have been quantified. Analysis of surface functional groups includes radical site determination with radical scavengers like diphenylpicrylhydrazyl, reduction of peroxides determined iodometrically, cationic dyestuff adsorption, carbonyl binding to Girard reagent P and surface hydroxyl group determination by surface nitrosation and subsequent azo-dye formation photometrically determinable. Use of potential surface swelling agents has been excluded except for addition of wetting agent. Wet-chemical analyses on textile surfaces bear the benefit of integrating over (relatively) large sample areas, a point which is interesting when regarding inhomogenities of textile or other surface constructions. In addition examples for visualisation for the existence of surface groups are described. © 2008 Elsevier B.V. All rights reserved.
1. Introduction During the course of a research project on the influence of ozone treatment (or ozone and UV-irradiation treatment, advanced oxidation process—AOP) on fibre surfaces of textile materials the importance of a quick analysis of surface oxidized functionalities posed an urgent demand [1]. In the literature AOP-treatment of plastics for surface functionalization has been reported frequently. Thereby mainly hydroxyl radicals and peroxy radicals are involved attacking the polymer. Some of the primarily formed species on the surface are rather long-lived other rearrange to stable functions. Quite often the oxygenated surface functions decline on storage (‘hydrophobic recovery’) due to polymer segment diffusion into the bulk polymer (especially with polymers of low glass transition temperature). In most of the papers XPS-spectroscopy was used in order to assess chemical groups on the surface [2–5,8,9]. Some chemical analysis methods for selected groups were mentioned too. These include the determination of peroxides, of carboxyl groups and of radicals [6–12]. An interesting chemical approach mainly for fluorescent ¨ derivatization on PE surfaces was given by Hollander in 2004 but requires rather uncomfortable calibration [13] and still introducing organic solvents.
∗ Corresponding author. Tel.: +49 2151 843 165; fax: +49 2151 843 143. E-mail addresses: [email protected], [email protected] (D. Knittel). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.05.036
Regarding drawbacks of XPS-analyses for quantitative determination on textile constructions (curvature of filaments, extremely small area accessible compared to practical production widths of >1.5 m, cost of equipment) it was felt necessary to establish wet-chemical analytics for the determination of oxygen containing functions on common, originally hydrophobic textile polymers exposed to advanced oxidation processes. For this aim some of the methods cited have been adopted and modified for easy analysis, also some new methods have been developed. The paper presented compiles the methods introduced. Only methods were chosen which can be applied from almost pure aqueous solutions. Thus it should be possible to minimise swelling effects of surface layers of synthetic polymers like poly(ethylene terephthalate) (PETP), polypropylene (PP) or similar polymers when working with organically soluble reagents thus keeping the (chemical) information depth as close to the surface as possible. Some of the methods established were tested too and described as qualitative spray-test method for visualisation of AOPtreatments. 2. Experimental 2.1. Materials used Poly(ethylene terephthalate) foil, thickness 190 m, 274 g/m2 (Cadillac Plastics), polypropylene foil, 75 m, 68 g/m2 (Leco); poly(ethylene terephthalate) (PETP) fabric, pleine weave,
D. Knittel, E. Schollmeyer / Talanta 76 (2008) 1136–1140
129 g/m2 , warp 24.8 cts/cm, weft 21.2 cts/cm (Testex); polyethylene fabric (PE), 104 g/m2 , warp 6.6 cts/cm, weft 6.2 cts/cm (Leco); polypropylene fabric (PP), pleine weave, 196 g/m2 , warp 22.5 cts/cm, weft 15.3 cts/cm (C.C. Cramer); poly(butylene terephthalate) (PBT) non-woven, 51.8 g/m2 ; thickness 390 m (Freudenberg). All materials have been extracted in a Soxhlet-apparatus prior to AOP with methanol/water (1:1). Advanced oxidation of textile samples (ozone, ozone + UV) has been done as described in [1], i.e. samples of textiles (30 cm × 15 cm) were treated with ozone (68–74 and 190–185 g/N m3 , respectively) in a cell equipped with a quartz window. Treatment time usually was 10–30 min. Some samples too were simultaneously irradiated with a mercury UV-lamp for AOP. 2.2. Apparatus used UV–VIS Diode Array Spectrophotometer HP 8452 A (HewlettPackard) with cuvettes of 1 cm optical path length and analyte volume of about 80–100 l (Brand) was used for measurements of dyes. A spectrophotometer Datamaster Type DC 3880 (Datacolor) was used for remission measurements. 2.3. Chemicals used Methylene blue for microscopy (Merck). Ethyl red 97% (lithium-salt) (prepared from sodium salt, Aldrich). Intracronred® BF-3RM 150%, reactive dyestuff (Yorkshire). Nonionic wetting agent Marlipal® O13/80 (Sasol) and SDS (Aldrich). H-acid (4-amino-5-hydroxynaphthaline-2,7-disulfonic acid disodium salt hydrate 85%) (Aldrich or Hoechst). Sulfanilic acid (Fluka). Girard reagent P (N-acetylhydrazinopyridinium chloride) (Aldrich). 2,2-Diphenyl-1-picrylhydrazyl (DPPH) (Aldrich). Other reagents were of analytical grade. 2.4. Radical determination on surfaces The wet-chemical test is oriented on methods of MacManus et al. [11]. A DPPH-solution (2 mg/20 ml MeOH) is prepared and applied to (as-received and after surface oxidation) textile samples of about 120 mg in brown flasks. After shaking for several hours (up to 3 days) at room temperature the supernatant solution is quantified by photometry at 516 nm. From the decrease of DPPH-absorbance the amount of radicals reacted can be determined. This test can be modified for visualisation by simultaneous addition of an acid dye like the lithium salt of ethyl red. The dye anion binds to the C-cation of the surface. 2.5. Determination and visualisation of peroxidic groups on oxidized polymer surfaces Reaction vessels given in Fig. 3 are used for quantification (details in [1]). Briefly, they enable the treatment of small samples of fabric to be analysed with heating under inert atmosphere in presence of reaction partner. Additionally, titration of reagents formed may be done using microsyringes. These vessels may be used too for grafting reactions (occurrence of radicals or peroxides). For visualisation of the existence of peroxidic groups on oxidized polymer a graft polymerization of acrylic acid monomer may be induced by heating under inert atmosphere. After removal of
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homopolymers formed by alkaline washing staining is done with cationic dyestuff methylene blue. 2.6. Determination of hydroxyl containing surfaces: visualisation of surface hydroxyl groups A known amount of sample (0.1–0.2 g) is treated with 2.5 ml of 1 mol/l HCl and 1 ml of 15% Na-nitrite solution. After 10 min the textile sample is removed and rinsed intensively with 10 ml NaHCO3 -solution (1%, containing 0.5% Na-dodecylsulfate for wetting). After neutralisation (about 3 times with 20 ml dist. water) the sample is transferred to a 25 ml measuring flask and 1 ml of freshly prepared solution of sulfanilic acid (1%) is added. After 9 min of shaking 1 ml of H-acid (0.5% fresh solution) is added and shaken in the dark for 1.5 h. The resulting red solution can be analysed at 510 nm. Water introduced by the sample is corrected by weighing. Calibration is done simply by the use of a nitrite solution for the sulfanilic acid/H-acid procedure. This reaction sequence can be used for visualisation as spraying test sequence on the oxidized fabric: the sample is dried following the nitrite treatment and sprayed with sulfanilic acid followed by H-acid. Photographs can be taken after 1.5 h storage in the dark. Alternatively a surface oxidized sample may be stained by conventional reactive dyeing procedures and resulting colouration may be determined in reflectance measurements. 2.7. Determination of carbonyl groups on oxidized polymer surfaces 0.5 g of sample is shaken with 2.7 g of Girard P solution (1.11 g/l). After addition of 0.3 g acetic acid the vessel is stirred in a closed vessel (see Fig. 3) for 30 min at 90 ◦ C. After cooling the solution is transferred into a 100-ml flask. The decrease of reagent concentration due to binding of reagent to surface carbonyls is analysed photometrically at 260 nm. 2.8. Determination of carboxylic groups on surfaces of partially oxidized surfaces of hydrophobic polymers Samples (0.1–0.5 g) are treated at room temperature with a 0.05% solution of methylene blue in Britton–Robinson buffer (pH 7.1–7.4). The bluish samples are rinsed thoroughly and dried in air. For quantitative determination the sample is exposed to 20 ml of 84% aqueous acetic acid and shaken for 1 h. Photometric analysis is done at 650 nm. 3. Strategies for wet-chemical analysis The analytical methods elaborated enables the quantification of surface functional groups on synthetic polymers. The methods too can be used as a qualitative quick-test and for visualisation of surface effects obtained by AOP-treatment. In the following the scheme of analyses and chemical basics are described. 3.1. Radicals on surfaces of hydrophobic textile surfaces For the determination of surface radicals on PP or PETP the following reactions have been used (according to [12]) (Fig. 1) and widened for visualisation purposes by dye binding. First step after radical formation is an electron transfer to DPPH followed by irreversible protonation to diphenylpicryl hydrazine and a surface cation. The decay of the violet colour of DPPH can be followed pho-
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tometrically. The surface cation can be trapped by adding an acid dye.
3.2. Wet-chemical detection and determination of hydroxyl groups on oxidized polymers For quantification of hydroxyl group contents of oxidized surfaces the esterification by aqueous nitrous acid has been chosen. This reagent has been known for the use in homogeneous reactions for detection of alcohols [14]. It proved to be suitable even in heterogeneous application and one may expect no interference from possibly coexisting amino groups since –NH2 groups will be destroyed by HONO. Fig. 2 shows the reaction sequence used. In concentrated solutions of nitrous acid hydroxyl groups on the polymer surface are esterified. Those surface nitrous acid esters are stable against washing but can be cleaved afterwards by dilute acid. Cleavage is done on the washed surface in solutions capably of trapping HONO. The newly formed nitrous acid immediately is transformed to a diazonium salt, which can be used for formation of an azo-dye, i.e. with H-acid or similar. This reaction sequence can be used too for visualisation on the textile as a spray-test application.
Fig. 2. Scheme for quantification of hydroxyl groups on surfaces of oxidized polymers.
3.3. Wet-chemical detection and determination of peroxidic groups on oxidized polymers
Fig. 1. Detection of radical functions on oxidized polymers by reaction with diphenyl-picrylhydrazyl, followed by formation of dyestuff binding for visualisation.
The presence of peroxidic groups on oxidized polymer surfaces (ozone treatment) can be determined using potassium iodide (cf. [12]). There is no discrimination between hydroperoxides or dialkylperoxides. Using apparatuses as described in Fig. 3, a simple titrimetric analysis can be performed. This reaction vessel can be used too for other analyses on oxidized samples. In addition to titration peroxidic structures on surfaces may be used for inducing grafting reactions on the surface with acrylic monomers (cf. [4,15–18]). A successful grafting reaction, e.g. with acrylic or methacrylic acid can be visualised after-
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cationic dyestuffs on carboxylate functions, visualisation for documentation and then detachment of the cationic dyestuff by acid treatment and photometric analysis of the solutions obtained (cf. [6]). Preferable for this determination is the use of highly pure cationic dyestuff as used for microscopy (i.e. methylene blue). 4. Results on AOP-treated samples obtained by wet-chemical analyses 4.1. Results on AOP-treated polymers
Fig. 3. Testing equipment for radical-, peroxide-, carbonyl- and grafting-analysis.
wards by using a cationic dyestuff as described in Section 2.4 (cf. [16,17]). 3.4. Wet-chemical detection and determination of carbonyl groups on oxidized polymers
To illustrate the versatility of the analytical methods described some examples are given in Table 1 for surface group analysis on AOP-treated foils and Table 2 for textiles (details see [1]). Samples are analysed immediately after preparation. Analysis for radicals on treated PE-fabric showed 0.012 mol/g fabric just after AOP-treatment, decreasing to about 0.006 mol/g on storage for 6 days, traces are still detectable even after 22 days. As known from the literature, all the samples showed hydrophobic recovery on storage for several days. Thus the chemical surface analysis proves to be a rather quick and sensitive and inexpensive tool. 4.2. Visualisations of functional groups on surfaces of synthetic material
There are numerous reaction for the determination of carbonyl groups in solution. An important one for the purpose of surface analysis lies in the use of a water soluble Girard reagent (Girard P). Qualitative analysis using such a reagent can be done by reflectance measurements on oxidatively treated polymer surfaces after exposure to Girard P solutions. Performing this analysis on oxidized textiles with minimal volumes of reagent (i.e. almost not more than textile wet pick up) the decrease of Girard-concentration in the treating solution can be monitored and quantified by photometric analysis within cuvettes of about 100 l working volume.
Visualisation of hydroxyl groups can be done as test on the fabric or on the foil by spraying the reagents (nitrite/-HCl/sulfanilic acid/H-acid) onto the sample. Other examples of various staining reactions on oxidized polymer surfaces, differently treated with ozone and analysed for visualisation of carboxylic and peroxidic groups and radical centers, respectively are presented in Fig. 4. Radical detection on oxidatively treated polyesters and on polyolefins combines the reaction with DPPH resulting in a surface cation which is trapped by use of an anionic dyestuff like salts of ethyl red giving rise to visible staining. 5. Discussion
3.5. Wet-chemical detection and determination of carboxyl groups on oxidized polymers Detection and determination of carboxylic groups on oxidized polymer surfaces follows well known strategies, i.e. binding of
In this work it could be shown that almost all relevant oxygenated functions on polymer surfaces can be quantitatively assessed by wet-chemical analysis on textile constructions. Wetchemical analyses on textile surfaces bear the benefit of integrating
Table 1 Wet-chemical analysis of oxygen functionalities before and after ozone treatment of polymer foils (30 min O3 ) (sum of both sides) Sample PETP-original O3 -treated PP-original O3 -treated
Total –COOH (mol/cm2 ) (2.0 (4.2 (7.0 (1.7
± ± ± ±
−4
0.1) × 10 0.2) × 10−2 0.5) × 10−4 0.13) × 10−2
Total –C O (×10−2 mol/cm2 ) (2.7 (2.7 (1.22 (1.77
± ± ± ±
0.04) 0.04) 0.015) 0.02)
Total –OH (×10−2 mol/cm2 ) (1.4 (80 (5.0 (24
± ± ± ±
0.3) 17.6) 1.1) 5.3)
Table 2 Oxygen containing groups of AOP-treated fabrics Sample code
Total –COOH (mol/g)
Total –C O (mol/g)
Total –OH (mol/g)
Total peroxides (mol/g)
PETP-original PETP, 30 min O3 PBTP-original PBTP, 30 min O3 PBTP, 30 min O3 with UV-irradiation PP-original 30 min O3
0.11 ± 0.48 ± 0.37 ± 0.39 ± 1.62 ± 0.04 ± 0.07
0.17 ± 0.03 5.4 ± 0.03 n.d. n.d. n.d. 0.6 ± 0.02 2.4
3.4 ± 0.75 ∼29 ± 6.3 15 ± 3.3 8.4 ± 2.85 3.5 ± 0.8 0.4 ± 0.09 7.7 ± 1.7
0 6.6 ± 0.12 n.d. n.d. n.d. 0 2.04
n.d., not determined.
0.006 0.03 0.007 0.007 0.03 0.003
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Fig. 4. (A) Spray test on AOP-treated PP-fabric for visualisation of the existence of hydroxyl groups (nitrite/sulfanilic acid/H-acid). (B) Methylene blue staining of AOP-treated PET. (C) Same on PE. (D) Visualisation of peroxides on PET by KI/starch solutions. (E) Same for PP. (F) Visualisation of radical formation on AOP-treated synthetic polymers by simultaneous application of DPPH in the presence of an acid dye for marking the cationic centres formed. (G) Same for PE.
over (relatively) large sample areas, a point which is interesting when regarding inhomogenities of textile or other surface constructions. In addition, the wet-chemical analysis – being rather quick – avoids problems due to changes during sample storage. Especially important are the possibilities to characterize partially oxidized surfaces of rather large size. Obviously wet-chemical analysis includes surfaces of cracks or other morphological irregularities thus giving higher analytical values than an XPS-sampling. At present a preliminary comparison of values for carboxylic groups on surface indicates about 20-fold higher values for wet-chemical analysis on PETP-fabric than as detected by XPS. This higher value may be due sampling over large fabric areas including fibre irregularities like cracks. XPS-sampling is methodically restricted to specimen of some micrometer dimensions, often randomly taken for measurements. More comparative investigations on oxygenated films (wet-chemical vs. XPS-analysis) will be systematically done in future with synchronously performed analysis. This has to be done because it is known that surface functionalities disappear often rather quickly (hydrophobic recovery) and to correlate information depth obtained. Problems in detectability of functional groups on surfaces may arise when dealing with compact and smooth surfaces because of sensitivity of analytical methods introduced. In addition the methods presented in this work uses inexpensive equipment and can be done quickly in a normal lab even as process control. Acknowledgements We would like to thank the Forschungskuratorium Textil e.V. for funding this research project (AiF-No. 13924N). This project was funded with financial resources of the Bundesmin¨ Wirtschaft und Technologie (BMWi) with a grant isterium fur
from the Arbeitsgemeinschaft industrieller Forschungsvereinigungen “Otto-von-Guericke” e.V., AiF). The final report is available from: Deutsches Textilforschungszentrum Nord-West e.V., Adlerstrasse 1, D-47798 Krefeld, FRG. References [1] Research project of DTNW, Federal Ministry for economics and technology, FRG, grant no. AiF 13924N, 2004–2006; D. Knittel, E. Schollmeyer, DTNW-Mitt, vol. 56, 2007, ISSN 1430-1954. [2] K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, VSP BV, Utrecht, NL, 1996, ISBN 90-6764-201-0. [3] M.J. Walzak, F. Flynn, R. Foerch, J.M. Hill, E. Karbashewski, A. Lin, M. Strobel, in: K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, VSP BV, Utrecht, NL, 1996, ISBN 90-6764-201-0, pp. 253–272. [4] O.D. Greenwood, B.J. Hopkins, J.P.S. Baydal, in: K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, VSP BV, Utrecht, NL, 1996, ISBN 90-6764201-0, pp. 17–32. [5] J.F. Friedrich, W. Unger, A. Lippitz, T. Gross, P. Rohrer, W. Saur, J. Erdmann, H.J. Gorsler, in: K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, VSP BV, Utrecht, NL, 1996, ISBN 90-6764-201-0, pp. 49–72. [6] D. Praschak, T. Textor, T. Bahners, E. Schollmeyer, Technol. Text. 41 (1998) 136–138. [7] P. Hedenberg, P. Gatenholm, J. Appl. Polym. Sci. 60 (1996) 2377–2385. [8] M. Strobel, M.J. Walzak, J.M. Hill, A. Lin, E. Karbashewski, C.S. Lyons, in: K.L. Mittal (Ed.), Polymer Surface Modification: Relevance to Adhesion, VSP BV, Utrecht, NL, 1996, ISBN 90-6764-201-0, pp. 239–251. [9] A. Hartwig, K. Albinsky, FhG Fertigungstechnik und Angewandte Materialforschung, Bremen, AiF-Researchproj. 10900N/1, 1999. [10] S. Dasgupta, J. Appl. Polym. Sci. 41 (1990) 233–248. [11] L.F. MacManus, M.J. Walzak, N.S. McItyre, J. Polym. Sci. A: Polym. Chem. 37 (1999) 2489–2501. [12] P. DeAntonio, W.J. Bertrand, S.L. Coulter, K.G. Mayhan, J. Am. Chem. Soc., Polym. Preprints 31 (1990) 448–449. ¨ [13] A. Hollander, Surf. Interface Anal. 36 (2004) 1023–1028. ´ Z.J. Vejdelek, Handbuch der Photometrischen Analysen Organischer [14] B. Kakac, Verbindungen, VCH, Weinheim, 1974. [15] D.J. Carlsson, D.M. Wiles, Macromolecules 2 (1969) 597–606. [16] K. Fujimoto, Y. Takebayashi, H. Inoue, Y. Ikada, J. Appl. Poly. Sci. A: Polym. Chem. 31 (1993) 1035–1043. [17] M. Suzuki, A. Kishida, H. Iwata, Y. Ikada, Macromolecules 19 (1984) 1804–1808. [18] C.I. Simionescu, S. Oprea, J. Polym. Sci. C 37 (1972) 251–263.