Plasma Processes and Polymers: 16th International Symposium on Plasma Chemistry Taormina, Italy June 22-27, 2003 - Hardcover

Hardcover

ISBN 10: 3527404872 ISBN 13: 9783527404872

Verlag: Blackwell Verlag GmbH, 2005

Inhaltsangabe

This volume compiles essential contributions to the most innovative fields of Plasma Processes and Polymers. High-quality contributions cover the fields of plasma deposition, plasma treatment of polymers and other organic compounds, plasma processes under partial vacuum and at atmospheric pressure, biomedical, textile, automotive, and optical applications as well as surface treatment of bulk materials, clusters, particles and powders.
This unique collection of refereed papers is based on the best contributions presented at the 16th International Symposium on Plasma Chemistry in Taormina, Italy (ISPC-16, June 2003). A high class reference of relevance to a large audience in plasma community as well as in the area of its industrial applications.

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The editors are leading scientists worldwide, well known from their publications, their work as editors as well as from conference organizations
* Dr. Riccardo d'Agostino, Professor, Director of the Department of Chemistry, University of Bari, Italy
* Dr. Michael R. Wertheimer, Professor, NSERC Industrial Research Chair on Low Pressure Plasma Processing of Materials, Ecole Polytechnique, Montreal Canada
* Dr. Christian Oehr, Head of Dept. Interfacial Engineering, Fraunhofer Institute for Interfacial Engineering and Biotechnology, Stuttgart, Germany
* Dr. Pietro Favia, Associate Professor, Department of Chemistry, University of Bari

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This unique collection of refereed papers is based on the best contributions presented at the 16th International Symposium on Plasma Chemistry in Taormina, Italy (ISPC-16, June 2003). A high class reference of relevance to a large audience in the plasma community as well as in the area of its industrial applications.

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Plasma Processes and Polymers

16th International Symposium on Plasma Chemistry Taormina, Italy June 22-27, 2003

John Wiley & Sons

Copyright © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
All right reserved.
ISBN: 978-3-527-40487-2

Chapter One

Polymer Surface Modification with Monofunctional Groups of Different Type and Density

J. Friedrich, G. Khn, R. Mix

Abstract

The retention of chemical structure and functional groups during pulsed-plasma polymerisation was used for producing adhesion-promoting plasma polymer layers with high concentrations of exclusively one sort of functional groups such as OH, N[H.sub.2], or COOH. The maximum content of functional groups was 31 OH using allyl alcohol, 18N[H.sub.2] using allylamine, or 24 COOH groups per 100 C atoms using acrylic acid. To vary the density of functional groups a chemical copolymerisation with ethylene as "chain-extending" comonomer or butadiene as "chemical crosslinker" was initiated in the pulsed plasma.

The composition of these copolymers was investigated by XPS and IR spectroscopy. The concentration of functionalities was measured by XPS after attaching fluorine-containing derivatives. These labelling reactions were audited with reference substances and different markers. A set of plasma parameters was found to be a compromise between a high number of functional groups and complete dissolubility in water, ethanol or THF as needed for further chemical processing. Here, these monotype functionalised polymers are used in metal-polymer composites as an adhesion-promoting interlayer to examine the influence of type and density of functional groups on the adhesion.

1.1 Introduction

The aim of this work was to produce plasmachemically monotype functionalized polymer surfaces as models for the investigation of the influence of each type of metal-functionality interactions to the adhesion (Fig. 1). Moreover, the density of monosort functionalization with different types of functional groups (OH, N[H.sub.2] and COOH) should be varied to study the influence of the concentration of metal-functional group anchoring points (Fig. 2). Three ways to produce monotype functionalized polymer surfaces were investigated, the [O.sub.2]-plasma treatment with subsequent wet-chemical reduction (vitride-Na-complex, [B.sub.2][H.sub.6], LiAl[H.sub.4]) of the majority of O functional groups to OH groups (process 1, Fig. 3), the pulsed-plasma polymerization of functional groups carrying monomers under retention of their functionalities (process 2, Fig. 3) and, in the same way as the pulsed-plasma polymerization, the plasma-initiated (chemical) copolymerization for varying the concentration of monotype functional groups (process 3, Fig. 3). In this work processes 2 and 3 were investigated, whereas process 1was studied previously.

The general concept for the plasma polymerization was to approximate the structure of plasma polymers to that of classic polymers as much as possible. Therefore, low power input to the plasma, substrate and growing plasma polymer layer seems to be necessary because the average energy input per monomer molecule has to be <0.01eV for a classic (pure chemical) radical polymerization. Thus, the constant energy flux in continuous wave (cw) plasma is too high. Pulsed plasma with long off-time offers an alternative and possibly a tool to initiate pure chemical chain propagations (Fig. 4).

Using chemically reactive monomers, which are qualified to undergo a classic radical chain propagation only one activation incident with [approximately equal to] 1.5 eV is required to start a chain polymerization with a resulting molar mass of about 100 000 and the respective polymerization degree of X [approximately equal to] 1000. Therefore, 0.0015 eV per monomer are needed to initiate and propagate a chemical polymerization (Fig. 5). The advantage of a chemically produced polymer compared to a plasmachemically synthesized material is the defined structure, the exact stoichiometry, often the presence of a supermolecular structure (crystallinity), the defined properties and the significantly better ageing stability. Thus, we intend to exclusively initiate such a chemical polymerization through the plasma but the chain propagation should be performed on a pure chemical way. Monomers with active double bonds as vinyl or acrylic groups or moderately qualified allyl groups and dienes are a precondition of such a chemical chain propagation. Using nonclassic monomers and the cw mode only a nonchemical formation of low-quality polymer layers is possible. Examples for nonqualified monomers are alkanes (hexane), aromatic, cyclic, etc., monomers.

In practice, the low pressure and, therefore, the low sticking rate of monomers with a radical site of a growing macromolecular chain limit the chain propagation. Such termination reactions are radical recombination, chain transfer and disproportionation.. Therefore, the use of pulsed plasma is necessary to reinitiate the chain propagation. Short plasma pulses (0.01 to 1 ms) activate the monomer molecules and the surface of the growing polymer layer. During the plasma-off period reactive monomer molecules strike the radical sites at the polymer surface, graft and thus form the growing macromolecular chain. The chain propagation is a pure chemical process. Using typical pulse conditions as 0.1ms plasma-on and 1ms plasma-off the very reactive monomer styrene forms a polymer layer during every pulse that is thinner than <0.1-monolayer styrene. Comparing the deposition rates of cw r.f. plasma and pulsed r.f. plasma a difference of 1:2000 was found if the deposition rates were referenced to the same plasma-on durations. This difference in deposition rates demonstrates the dominant role of the chemical chain propagation using the pulsed plasma. Obviously the plasma polymerization in the cw plasma mode can be characterized as a process of only a slight preponderance of the deposition in comparison to the simultaneous plasma etching/sputtering of the growing polymer layer.

The structure of polymer sequences formed in the plasma-off period should correspond to those of classic polymers. However, the more degrees of freedom for adsorbing molecules in the low-pressure gas phase compared to polymerizations in the liquid phase should hinder all types of supermolecular structure or tacticity. Only amorphous and atactic polymers are anticipated. Nevertheless, such structures are also chemically defined. However, these defined structures and compositions are strongly disturbed by every new plasma pulse due to the UV irradiation and particle bombardment. In addition, no hints for a significant contribution of ion-molecule reactions to the polymer formation were identified on analyzing the polymer structure. The monomers used are all qualified for a radical and not for ionic polymerizations in the gas phase.

In 1971, Westwood found that the plasma polymers possess a chemically better defined structure and composition using low power input, however, unfortunately, thus-produced polymers are also characterized by inclusions of oligomers and monomers within the partially crosslinked polymer matrix. Tibbitt et al. proposed an actual model of such a plasma polymer. Pulsed low-power plasma helps to avoid the excessive monomer fragmentation in the plasma phase and reduces the number of plasma-induced damages in the polymers. This opens the way for enhancing the pure chemical radical polymerization in the gas phase or adsorption layer. Pulsed-plasma polymerization was introduced first by Tiller in 1972, later continued by Yasuda, Shen and Bell, and then further developed by Timmons and our group.

The copolymerisation in pulsed plasmas was designed as a plasma-initiated (chemical) radical chain propagation reaction preferentially in the plasma-off period. This kind of copolymerisation is strictly different from those introduced by Schler or Yasuda. They simply mixed two gases or monomers without any consideration of their chemical reactivities. Using the cw plasma, which fragments all monomer molecules and allows their random recombination as nondefined plasma polymers, the reactivity of comonomers does not play any role. However, the more the chemical reactions dominate the more important the chemical copolymerisation ability of comonomers becomes. In polymer chemistry this behavior is expressed in terms of copolymerisation parameters (coefficients) [r.sub.a] and [r.sub.b] for free-radical copolymerizations to linear, branched or crosslinked copolymers. For [r.sub.i] = [r.sub.a] and [r.sub.i] = [r.sub.b] two of five cases are important: [r.sub.i] = 1 both comonomers add to any active center with equal probability if [A] = [B] and [r.sub.i] = ( only homopolymerization occurs, no copolymerization. Hence, the pairs of comonomers must be accurately compiled to avoid homopolymerization. An example for a genuine classic copolymerisation is that of styrene and methylmethacrylate with [r.sub.s] = 0.52 and [r.sub.m] = 0.46. Vinyl, acrylic, allyl and diene comonomers are generally suited for plasma copolymerization. In Fig. 6 examples of co- and homopolymerization and also chemical crosslinking in the pulsed plasma are shown.

1.2 Experimental

Plasma polymerisations were carried out in a vacuum system with a base pressure of [10.sup.-3] Pa or lower. The principal design of the plasma reactor was described earlier. The r.f. power was varied from 50 to 300W, the duty cycle from 0.05 to 1 and the pressure was 25 Pa. The samples are kept under floating potential. The XPS data acquisition was performed with a SAGE 150 Spectrometer (Specs, Berlin, Germany) using nonmonochromatized Mg[K.sub.[alpha]] or Al[K.sub.[alpha]] radiation with 12.5 kV and 250 W settings at a pressure [approximately equals to] [10.sup.-7] Pa in the analysis chamber. This instrument is equipped with a plasma reactor separated by a gate valve from the UHV system, where surface treatments can be carried out at a pressure of [10.sup.1]-[10.sup.-7] Pa. XPS spectra were acquired in the constant analyser energy (CAE) mode at 90 take-off angle. Peak analysis was performed using the peak fit routine from Specs. The FTIR spectra were recorded with a NEXUS instrument (Nicolet, USA) using the ATR technique (attenuated total reflectance) with a diamond cell ("Golden Gate", Specac, Kent, UK). Contact-angle measurements were performed in the sessile drop mode using water, formamide, ethylene glycol, benzyl alcohol and diiodomethane as test liquids. The equipment consists of a G2 goniometer and the appropriate software (Kruess, Hamburg, Germany). The derivatization of OH groups was performed using trifluoroacetic anhydride (TFAA) or m-trifluoromethylphenylisocyanate (TMPI), that of N[H.sub.2] by applying pentafluorobenzaldehyde (PFBA) or 4-trifluoromethylbenzaldehyde (TFMBA) and that of COOH by exposure to trifluoroethanol (TFE). The number of functional groups was calculated by considering the percentage of the introduced fluorine (F1s peak) and the theoretical stoichiometry of the derivatized polymer. It was supposed that the XPS analysed outermost layer ([approximately equal to] 3 nm) was homogeneously derivatized. The completeness of the derivatization and the absence of nonconsumed functional groups in the deposited polymer layer were checked using ATR-FTIR spectroscopy. Further methods were the C1s peak fitting (C[F.sub.3], COOR) or the measuring of the concentrations of introduced oxygen (OH, COOH-O1s peak) or introduced nitrogen (N[H.sub.2] - N1s peak). The metal evaporation onto plasma-polymer-modified PP foils was performed in situ by using a plasma reactor equipped with two sources for thermal evaporation (Ilmplasma 1200, Saskia, Ilmenau, Germany). This technique was applied for analytical purposes. For measuring the peel strength the samples are transferred to a metallizer equipped with the electron beam technique (Edwards, UK). The 90 peeling technique of metal-PP composites follows DuPont's preparation and peeling procedure. It was described in detail elsewhere.

1.3 Results

1.3.1 Kinetics of the Deposition of Copolymers

In Fig. 7 the deposition rates of different copolymerized mixtures of allyl alcohol and allylamine with ethylene, styrene and butadiene are plotted as a function of the composition of the comonomer mixture. The deposition rates of these mixtures are shown to be not quite a linear combination of those of the comonomers as expected for comonomers with different copolymerisation coefficients.

The curve regressions (nonlinearity) are characteristic of chemical copolymerizations. However, they are not yet completely understood. Moreover, the type of substrate and therefore the possibility of dissipation of charges have an influence on the deposition characteristics. Attempts were made to interpret the deposition characteristics in terms of classic copolymer kinetics. Here, the Fineman and Ross approaches were applied. For example, it could be shown that the copolymer formation of the allyl alcohol-butadiene copolymerisation can be linearized by applying these methods. Two linear dependencies are found for different compositions of the comonomer mixture corresponding to preferred homopolymerization (of butadiene) and preferred copolymerisation. The structure of copolymers is alternating if [r.sub.a] = [r.sub.b] = 0, which was assumed for the investigated copolymer systems.

1.3.2 Variation of the Density of Functional Groups

Ethylene and styrene as linear "chain extenders" and 1,3-butadiene as "chemical crosslinker" were copolymerized with allyl alcohol, allylamine or acrylic acid as carrier for OH, N[H.sub.2] or COOH groups. Thereby the composition of comonomer mixtures was systematically varied. Then the resulting number of OH, N[H.sub.2] or COOH groups in the copolymers was measured by applying the derivatization methods in connection with the XPS and IR spectroscopy as described before. The structure of the copolymers is reflected in the respective C1s-XP spectra as shown for an ethylene-allyl alcohol copolymer (Fig. 8). The spectrum of the copolymer seems to be a linear superposition of the spectra of homopolymers.

To determine the extent of retained functional groups the copolymers were derivatized in the same manner as the homopolymers. Figure 9 shows a nearly linear correlation between the concentration of functional groups in the resulting copolymer and the composition of the comonomer mixture for the copolymerisation of acrylic acid-butadiene in the subsequent measurement of COOH groups as TFE derivatives. In Fig. 10 the number of OH groups of an ethylene-allyl alcohol copolymer, determined by derivatization with TFAA, is plotted. In the range of 60 to 100% allyl alcohol in the gas-vapor mixture of ethylene only homopolymerization of allyl alcohol can be observed. From 0 to 60% copolymerization occurs. This behavior can be observed for both the copolymerization at 100Wor 300W. Using 300W the maximum yield in OH groups is 23 per 100 C atoms whereas the maximum concentration is near 31OH groups per 100 C atoms using 100W. This can be interpreted as leaving the region of soft plasma power responsible for the preferred chemical chain propagation at polymer deposition and passing to the region of plasma parameters characteristic for the preferred fragmentation of monomers and the random recombination of fragments and atoms to irregular polymer structures. Therefore, the chemical structure of copolymers should change from a more chemically defined (100 W) to a more irregular one (300 W).

In contrast to the ethylene-allyl alcohol system the copolymerization of butadieneallyl alcohol is dominant in the full range of allyl alcohol in the gas/vapor comonomer mixture as shown by the (nearly) linear increase in the number of OH groups with growing percentage of allyl alcohol (Fig. 10). Homopolymerization seems to be negligible. The maximum concentrations of OH groups were 29 OH groups per 100 C atoms using 100W and 20OH per 100 C atoms using 300W. Copolymerization with styrene is dominated by its homopolymerization (0 to 50 mole% allyl alcohol, see Fig. 10).

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Details zur Ausgabe

• Verlag: Blackwell Verlag GmbH
• Erscheinungsdatum: 2005
• Sprache: Englisch
• ISBN 10: 3527404872
• ISBN 13: 9783527404872
• Einband: Tapa dura
• Anzahl der Seiten: 545
• Herausgeber: D'Agostino Riccardo, Favia Pietro, Oehr Christian, Wertheimer Michael R.
• Kontakt zum Hersteller: Nicht verfügbar
• Verantwortliche Person: Nicht verfügbar