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Page 1: Quantitative Chemical Determination of Organic Layers ... · Quantitative Chemical Determination of Organic Layers using ... information on specific functional groups is chemical

Quantitative Chemical Determination of Organic Layers using CDXPS: A Valid Experimental Procedure?

Pierre-Luc Girard-Lauriault1,2, Paul Dietrich1, Thomas Gross1, Wolfgang E.S. Unger1

1BAM Bundesanstalt für Materialforschung und –prüfung, D-12203 Berlin, Germany

2Department of Chemical Engineering, McGill University, Montréal, H3A 2B2, Canada

Fax : +48 30 8104 1827; E-mail : [email protected], [email protected]

Abstract: Functionalized organic layers prepared by plasma processes have complex surface chemistries. The problem of elucidating such chemistries is an extensively studied subject. One of the preponderant methods used to acquire information on specific functional groups is chemical derivatization XPS (CDXPS). In this contribution, we focus on the most widely used CDXPS procedures, namely the quantifications of primary amines and alcohols with 4-trifluoromethyl benzaldehyde (TFBA) and trifluoroacetic anhydride (TFAA), respectively.

While these procedures have been used for a long period by various laboratories, the use of slightly varying protocols is acknowledged, making for a difficult comparison of results. We discuss steps towards a valid experimental procedure; in particular, calculation of concentrations, preparation of test samples and crucial pitfalls and shortcomings are discussed in detail.

Keywords: Organic coatings, chemical derivatisation, XPS

1. Introduction

Plasma modification and plasma deposition of organic materials have found many applications, ranging from rapid in-line industrial processes for tailoring the wetability of polymeric films to high added-value applications, such as treatment of medical devices destined to be used in-vivo. The modification processes implicating highly energetic species and random reaction pathways as well as the intrinsic complexity of the chemistry of organic materials have permitted only a qualitative and often incomplete understanding of the nature of produced surfaces. However, as with most organic materials, the distribution of chemical functional groups is often the determining factor for the properties of these materials.

One of the most widely used techniques to obtain quantitative information on functional groups is chemical derivatization. It consists in using a marker molecule that reacts selectively with a specific functional group, which can in turn be easily

quantified. An example of this, which will also be one of the subjects of the present contribution, is the selective reaction of 4-trifluoromethylbenzaldehyde with primary amines as schematized below

F

FF

O

H+ NH2 R F

FF

NR

(1)

and the subsequent quantification of the chemically bound fluorine by X-ray photoelectron spectroscopy. According to this reaction, we expect that for each three detected fluorine atoms on a derivatized surface, a primary amino group was present on the underivatized surface. A similar widely used approach, based on the reaction of hydroxyl groups will triofluoroacetic anhydride, will also be described in details in this contribution. This second reaction is schematized hereunder.

F3C O CF3

O O

+ OH RF3C O

O

R + F3C OH

O

(2)

Derivatization reactions have been used for over two decades and have now become a standard method of characterization for plasma treated surfaces.

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However, experimental protocols and results are found to be inconsistent between different laboratories.[1-5] This has been highlighted in a recent study by Gross et al.[6], where hydroxyl groups where quantified on a plasma prepared test sample by 6 different laboratories. The reported results differed by up to 240%, underlining the need for validated experimental protocols.[6]

In this contribution, we will review the two reactions described above, present different approaches for calculation of functional group concentrations, conduct derivatization on commercial polymers, propose test samples, and finally, identify specific pitfalls and shortcomings that can occur during the derivatization procedure.

Calculating Concentrations

There are different approaches to derive concentration of functional groups, [G], from XPS atomic concentrations, and each laboratory has pursued its own preferences.[1-6] Typically, three different approaches are used to report concentrations. The ratio of a functional group to its constituting heteroatom, [G]H, is often used to describe the process selectivity. Otherwise, ratios of a functional group to the carbon content, [G]C, or to the total atomic content [G]T, are also used to represent surface concentrations.

XPS analysis permit to obtain elemental compositions, [X], where X is C, N, O or F, which will be designated as [X]d or [X]nd depending on whether they are obtained from the spectrum of the derivatized or underivatized sample, respectively. From these compositions, the above explained concentrations can be calculated. For the case of the derivatization of primary amines with TFBA the three concentrations are calculated as follows, which correct for the added material:

d

dNH N

FNHG

][3

][][][ 2 == (3)

dd

d

nd

ndNCC

FC

F

C

NNHNHG

][8][3

][

][

][][][][ 22

=∗== (4)

)][][][38]([3

][

][][][][ 22

dddd

d

ndNTT

ONFC

F

NNHNHG

++−

=∗==

(5)

In the case of hydroxyl groups derivatization by TFAA, the calculations are made as follows:

dd

dOH FO

FOHG

][][3

][][][

−== (6)

dd

d

nd

ndOCC

FC

F

C

OOHOHG

][2][3

][

][

][][][][

=∗== (7)

)][][][]([3

][

][][][][

dddd

d

ndOTT

FONC

F

OOHOHG

−++

=∗== (8)

2. Experimental Methodology

Materials

Oxydianiline, 4,4 -methylenebis(2,6-diethylaniline), poly(allylamine), poly(allylamine) hydrochloride, poly(acrylic acid), poly(acrylonitrile), poly(vinyl alcohol-co-ethylene) 27%, 32%, 38% and 44 % ethylene, poly(ethyleneimine), and dioctadecylamine where acquired from Sigma-Aldrich. Poly(ethyleneimine) linear, 4-trifluoromethytl benzaldehyde and trifluoroacetic anhydride were acquired from Alfa Aesar. Thin coatings of the various poly(vinyl alcohol-co-ethylene) were obtained by melting a small amount of the polymer on a silicon wafer. Thin films of the other polymers were prepared by spin coating 3 % (w/w) aqueous solution on silicon wafers at 3500 RPM for 45 sec. Organic compounds were used as compacted powders in a specially designed sample holder.

Chemical Derivatization

Derivatization experiments were performed in gas phase at room temperature in a small vacuum chamber[4] (ca.250 ml), each derivatization reagent, either TFBA or TFAA having a dedicated chamber. Following the introduction of the sample, the

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chamber was pumped down by a membrane pump down to a pressure of ca. 10 mbar and filled with pure nitrogen; this pumping procedure was repeated three times. The chamber was afterwards connected to a reservoir containing the liquid reagent. The vapor pressure of TFBA or TFAA was established in the chamber during derivatization. After exposure for either 30 min (TFAA) or 180 min (TFBA), periods determined to ensure complete reaction by previous kinetic studies, the reservoir was isolated from the chamber and the pumping procedure was repeated three times before the sample was taken out. XPS were carried out with an AXIS Ultra DLD photoelectron spectrometer (Kratos Analytical, U.K.) using monochromatic Alka radiation following a procedure described elsewhere.[4]

3. Results and Discussions

Reaction of organic compounds with TFAA and TFBA

In order to validate the selectivity of the derivatization, reactions with TFBA and TFAA were conducted on all the commercially available organic compounds described in the experimental section and the products were further characterized by XPS. Atomic compositions were generally conforming to expectations, with a few notable exceptions.

Derivatization of the secondary amine containing linear poly(ethyleneimine) with TFBA yielded a [NH2]N close to 0.5 while a value of 0 was expected since reaction with TFBA is thought to be selective to primary amines. However, reaction of TFBA with dioctadecylamine resulted in a [NH2]N close to the expected value of 0. This difference is explained by the formation of aminals, which are favored when secondary amines are separated by two carbon atoms. While not a likely occurrence in plasma prepared materials, this possibility should be acknowledged.

Another interesting result was observed with the derivatization of poly(allylamine) hydrochloride, a polymer containing a positively charged primary amino group. Reaction with TFBA resulted in a [NH2]N of close to 0 while reaction with TFAA yielded a value close to 1. This indicates that

protonated amino groups in plasma polymers cannot be quantified by TFBA.

Finally, derivatization of poly(acrylic acid) and poly(acrylonitrile) with TFAA has shown that reactions indeed occur, albeit at a very slow rate.

Test samples

A previously conducted interlaboratory comparison has demonstrated that derivatization reactions conducted in different laboratories yielded inconsistent results, hereby highlighting the need for proper standard test sample. This sample should satisfy a few important constraints : i) chemical homogeneity (in depth) and stability, ii) stability to ultra high vacuum (UHV) and iii) having a concentration of functional group comparable to that of the typically analyzed materials. Requirement ii) mostly imposes that the test sample be polymeric while requirement iii) excludes most basic homopolymers.

Poly(vinyl alcohol-co-ethylene) was found to be a good test sample candidate for the derivatization of hydroxyl groups with TFAA as it conforms to all three criteria stated above. Figure 1 presents this evaluation for our derivatization scheme. The kinetic study demonstrates that saturation is obtained after a reaction time of 30 min. The [OH]C values at saturation indicate that the procedure systematically overestimates functional group concentrations by a factor of a few percents. A correction factor could be derived in order to improve the precision of the quantification.

An important consideration for reaction kinetics is that they not only depend on the derivatization system, but also on the nature of the test sample. Therefore, kinetic studies need to be conducted on the materials to be analyzed to determine the proper derivatization time.

Pitfalls and Shortcommings

In the course of our thorough investigation of the derivatization reaction, we identified a series of pitfalls and shortcoming that need to be acknowledged for successful experimentation.

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0 20 40 60 80 100 120 140 160 180 2000

10

20

30

40

50

[OH

] c (%

)

Time (min)

Poly(vinyl alcohol-co-ethylene) 27% ethylene

Poly(vinyl alcohol-co-ethylene) 32% ethylene

Poly(vinyl alcohol-co-ethylene) 38% ethylene

Poly(vinyl alcohol-co-ethylene) 44% ethylene

Figure 1. Kinetics of the chemical derivatization of poly(vinyl alcohol-co-ethylene) with TFAA. Expected values are indicated as dashed lines.

A first important point is the purity of the reagents obtained from commercial sources and used without further purification. It has been once reported before, but discussed many times in private conversations, that some reagent batches would introduce unwanted contamination, typically silicon and potentially inorganic fluorine (identified by a second peak in the F 1s spectra at 685 ev), to derivatized surfaces. Such contamination influences quantification and should therefore be identified and avoided by purifying reagents or by adequate sourcing.

Another critical shortcoming of derivatization is the contamination of laboratory air and characterization equipments by derivatization reagents. It has been often observed in our many experiments that samples that had not been subjected to derivatization would contain small amounts of derivatization reagents. This has been found to occur both due to the contamination of the laboratory air and by the release of derivatization reagents in our XPS apparatus. Highly volatile and reactive reagents such as TFAA were more susceptible to cause problems. For laboratories working with biomedical applications the functionalization of the outermost layer of the material can have severe effects on properties and should be avoided at all costs. Our recommendation is to house all derivatization equipment and reagents in fume hoods and to ensure proper degassing of derivatized surfaces before introduction in characterization equipment.

4. Conclusions

The intrinsic complexity of derivatization reactions has been described by many previous authors, however, the unique and necessary nature of the information it provides justifies significant efforts towards its validation. Methods to calculate the concentration, a basic parameter for which there is no actual consensus, have been described in detail. Previous interlaboratory comparisions have highlighted to need for proper standard test samples in order to compare results obtained from different derivatization setups. Poly(vinyl alcohol-co-ehtylene) was found to be adequate to validate TFAA derivatization procedures. The importance of avoiding contamination of laboratory air and equipments with derivatization reagents was also identified as a critical point for obtaining robust and reliable procedures.

Acknowledgements

P.-L. Girard-Lauriault is grateful to the Adolf Martens Fellowship Program of BAM and to the Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT) for post-doctoral fellowship support.

References

[1] P. Favia, M. V. Stendardo, R. d’Agostino, Plasmas Polym. 1996, 1, 91.

[2] F. Truica-Marasescu, M. R. Wertheimer, Plasma Processes Polym. 2008, 5, 44.

[3] A. A. Meyer-Plath, K. Schröder, B. Finke, A. Ohl, Vacuum 2003, 71, 391.

[4] E. Yegen, U. Zimmermann, W.E.S. Unger, T. Braun, Plasma Processes Polym. 2009, 6, 11.

[5] C.P. Klages A. Grishin, Plasma Processes Polym. 2008, 5, 359.

[6] T. Gross, F. Pippig, B. Merz, R. Merz, U. Vohrer, R. Mix, H. Steffen, W. Bremser, W.E.S. Unger WES, Plasma Processes Polym. 2010, 7, 294.