UDK 678:542.428 Original scientific article/Izvirni znanstveni ~lanek

ISSN 1580-2949 MTAEC9, 45(3)191(2011)

J. KOVA^: SURFACE CHARACTERIZATION OF POLYMERS BY XPS AND SIMS TECHNIQUES

SURFACE CHARACTERIZATION OF POLYMERS BY XPS AND SIMS TECHNIQUES ANALIZA POVR[INE POLIMEROV Z METODAMA XPS IN SIMS Janez Kova~ Center of Excellence for Polymer Materials and Technologies, Tehnolo{ki park 24, 1000 Ljubljana, Slovenia. [email protected] Prejem rokopisa – received: 2011-02-14; sprejem za objavo – accepted for publication: 2011-03-10 Polymers are very often subjects of different surface treatments. To understand and control the basic processes during surfaces modifications, analyses with advanced analytical techniques are required. We present two of such state of the art techniques, X-ray photoelectron spectroscopy – XPS and secondary ion mass spectroscopy – SIMS, often applied for characterization of polymers surfaces. In this work we give an overview of these techniques and their advantages and limitations related to polymer characterization. The combined use of these techniques is demonstrated in the application example, in which the cotton fibers were characterized before and after deposition of fluorine-based thin coating. Keywords: X-ray photoelectron spectroscopy, XPS, TOF-SIMS, polymers, cotton Polimerni materiali so pogosto predmet povr{inskih obdelav. Za razumevanje in kontrolo osnovnih procesov, ki potekajo med povr{inskimi modifikacijami, je potrebna uporaba naprednih analitskih tehnik. Predstavljamo dve tak{ni moderni tehniki: rentgensko fotoelektronsko spektroskopijo – XPS in masno spektroskopijo sekundarnih ionov – SIMS, ki se pogosto uporabljata pri karakterizaciji povr{in polimernih materialov. V delu predstavljamo osnove teh tehnik ter njihove prednosti in omejitve, povezane s preiskavo polimerov. Kombinirana uporaba obeh analitskih tehnik je predstavljena na primeru preiskave bomba`nih vlaken pred nanosom tanke plasti na osnovi fluorovih spojin in po njem. Klju~ne besede: rentgenska fotoelektronska spektroskopija, XPS, TOF-SIMS, polimeri, bomba`

1 INTRODUCTION Polymers are widely used materials in everyday life. Their inertness, low specific weight, low production cost, variability of mechanical properties and other unique properties, make them even more attractive for future applications. Among modifications of polymers the polymer surface treatments play an important role since many surface related properties are crucial for technological applications of these materials. These applications are relevant for improvement of wettability, adhesion, surface barrier properties, biocompatibility, thin film deposition … The reason for surface modifications is often the intrinsic property of many polymers, i.e. low surface energy, which should be changed in order to improve polymers compatibility with other materials and processes. Another reason for modification can be to introduce new functional properties at the polymer surface. The technological processes for polymer surface modifications are physical (corona, plasma, UV, laser treatments …) and chemical (wet treatment, surface grafting …) modifications 1. The effect of such surface modification may be followed by different more or less sophisticated analytical tools. Surface analytical techniques with high surface sensitivity have been used to understand basic phenomena during surface modifications. Among them are X-ray photoelectron spectroscopy – XPS and secondary ion mass spectroscopy – SIMS, which were very often applied for advanced surface characterization of organic Materiali in tehnologije / Materials and technology 45 (2011) 3, 191–197

and inorganic solid surfaces 2–5. Both of them have very high surface sensitivity combined with high elemental and molecular sensitivity. There are many other methods to analyze polymer surfaces like optical, electron and scanning probe microscopy, vibrational spectroscopies (FTIR, Raman spectroscopy), methods for surface energy measurements like contact angle measurements and others 1. The aim of this work is to present the main features of XPS and SIMS techniques as two state of the art analytical techniques for surface polymer characterization. Both of them have very high surface, elemental and molecular sensitivity. The main features and limitations of these two techniques are described in relation to polymer surface characterization. An application of combined use of the XPS and SIMS technique is given for cotton fabric. 2 X-RAY PHOTOELECTRON SPECTROSCOPY – XPS XPS analyses give information on the chemical composition and chemical bonds of solid surface 2–4,6. During the XPS analysis, a sample is illuminated with the monochromatic X-ray light in an XPS spectrometer and the energy of emitted photoelectrons from the sample surface is analyzed. In the photoelectron spectrum, which represents the distribution of emitted photoelectrons as a function of their binding energy, 191

J. KOVA^: SURFACE CHARACTERIZATION OF POLYMERS BY XPS AND SIMS TECHNIQUES Table 1: Comparison of main features of two surface analytical techniques XPS and SIMS Tabela 1: Primerjava glavnih zna~ilnosti dveh povr{insko ob~utljivih tehnik: XPS in SIMS

Feature Main information Surface sensitivity Sensitivity for elements Need for ultra-high vacuum ambient Spatial resolution In-depth information for thin films faautoImaging 3D analysis Quantification Typical acquisition time for high resolution spectrum Required expertise for data interpretation

XPS Elemental composition and type of chemical bonds of surface atoms From 3–6 nm for polymers All, except H and He Yes 1–100 μm Yes Limited No Yes 1–3 min

SIMS Type of atoms, molecules and pendant groups at surface 1–2 uppermost monolayers (1 nm) All Yes Less than 1 μm Yes Yes with high resolution and sensitivity Yes with high resolution and sensitivity Limited 5–15 min

High

High

peaks can be observed which are typical of elements present on the sample surface. The analysis area can be from some microns to mm in diameter and the signal during the XPS analysis originates from the surface layer up to 6 nm in thickness. During the analysis, two types of XPS spectra are usually recorded. Firstly, a spectrum through a wide energy range is acquired, in which the peaks of present elements are identified. Their concentration is calculated by dividing the peak intensities with the relative sensitivity factors provided by the XPS spectrometer manufacturer 6. The attained results are normalized to 100 %. The relative error at the calculation of surface composition is approximately 20 % of reported value, while the elemental sensitivity is about 0.5 %. The XPS method does not enable the analysis of hydrogen and helium. In addition to wide energy range spectra, high-energy resolution spectra of characteristic peaks of the elements like C 1s, O 1s and others (N 1s, S 2p, Cl 2p, F 1s, Si 2p …) are recorded through a narrow energy range. From the shape and binding energy of the peaks within these XPS spectra, the chemical bonding of surface elements can be identified with the help of data from the literature. For example in the case of organic materials one can identified from the carbon C 1s spectrum (energy range 282–297 eV) the bonding of carbon atoms like C-C/C-H, C-O, C=O, C-N, O-C-O, O=C-O, C-F and others. In addition to this other spectral features like shake-up peak in C 1s spectrum at 292 eV helps to identify aromatic pendant groups. During the analysis, the polymer samples are usually charging electrically, thus, a low-energy electron gun-neutralizer should be used. Prior to the spectra processing, spectra should be shifted, so that within the spectrum of carbon C 1s, the peak typical of the chemical bonds C-C/C-H is at the binding energy of 285.0 eV. The analysis took place in an ultra-high vacuum, which was during the analysis approximately 10–7 Pa or less. For precise XPS analysis of chemical bonding of surface atoms a special X-ray source should be used. In such a source X-rays are 192

emitted from the Al-anode with energy of 1486.6 eV and they are additionally monochromatized by special monochromator in order to reduce their energy spread to about 0.25 eV. The total recommended energy resolution during polymer analysis is 0.6 eV. In order to get depth distribution of elements beneath the surface, ion bombardment with argon ions with energy of few keV was traditionally applied what results in a control removal of surface material. By subsequent XPS analyses one can in this way obtain XPS depth profiles presenting the concentration of elements as a function of sputtering time what can be converted into depth. In this way also polymers and thin organic films can be analyzed. Unfortunately due to local damage of chemical bonds in soft material like polymers and preferential sputtering effects very limited information on subsurface chemical bonds can be obtained by depth profiling. In last years large progress was achieved towards reducing surface damage introducing ion bombardment by cluster ion beams like C60+. Table 1 gives some characteristic features of the XPS technique. XPS technique was often applied for characterization of surface modifications of polymeric materials by other and also by our research group 7–24. 3 SECONDARY ION MASS SPECTROSCOPY – SIMS SIMS is an analytical technique for compositional analysis of the solid surfaces and thin films 2,5. Due to its ability to identify the molecular structure i.e. type of molecules at the surface, SIMS technique was extensively applied in last decades just for polymer analyses 12,15,24. During the SIMS analysis the surface is bombarded with focused high energy (1–30 keV) ion beam what results in the ejection or sputtering of the species from the solid surface. Most of the emitted particles are neutral but small fraction of them is charged ions which are called secondary ions. They are measured Materiali in tehnologije / Materials and technology 45 (2011) 3, 191–197

J. KOVA^: SURFACE CHARACTERIZATION OF POLYMERS BY XPS AND SIMS TECHNIQUES

with a mass analyzer to determine the elemental composition and molecular structure of the surface. SIMS is a very surface sensitive technique because the emitted particles originate from the 1–2 top-most monolayers (1 nm). During the SIMS analysis the surface atoms are removed from the surface therefore the SIMS is locally destructive. In order to obtain chemical information of the original surface, the primary ion dose must be low enough (