Plasma polymer thin films

The progress of the Diels-Alder reaction was assessed by contact angle measurements performed at room temperature (Fig. 19.7). Again, the reaction was studied systematically at six different temperatures. We observed that the Diels-Alder reaction could be described as a pseudo-second-order reaction (Fig. 19.8). Similarly to the Diels-Alder reaction on monolayers, the third-order rate constants koA. calculated from the least-squares fits shown in Fig. 19.8 for the Diels-Alder reaction in the polymer thin film, obey the Arrhenius equation (Fig. 19.9). The activation energy 3 = 48.1 3.7 kj moh and the activation entropy AS = -538.2 16.4 J mol at 298 K (Table 19.2) are determined at the polymer surface in the same way as for the monolayers. 

3 Comparison of Surface Reaction in Monoiayers and Piasma Poiymer Thin Fiims 

---

Incorporation of (meso-tetraphenylporphyrinato)cobalt(II), [Co(tpp)], into plasma polymer thin films for the possible use as dioxygen-selective permeable membranes has also been reported (91). Comparisons to... 

Choukorov, A. et al.. Scanning probe microscopy for the analysis of composite Ti/hydrocarbon plasma polymer thin films. Surf. Sci., 602, 1011, 2008. 

Fig. 19.1 (a) Tapping mode AFM topographic image of the optimized maleic anhydride pulsed plasma polymer thin film on a silicon wafer (b) the corresponding phase image. 

Fig. 19.2 Infrared spectra of (a) the maleic anhydride pulsed plasma polymer thin film (b) the allylamine in the gas phase (c) the plasma polymer after reaction with allylamine (d) the same surface as for (c), annealed at 120°C for 2 h. CH2 scis = scission.

A comparative study of the temperature dependence of the Diels-Alder reaction between [(trimethylsilyl)methyl]cyclopentadiene and dienophile groups confined in selfassembled monolayers or in pulsed plasma polymer layers has been done. The reactivity of dienophile groups confined in pulsed plasma polymer thin films is compared with the behavior of dienophile groups in monolayers, because of their well-known arrangement properties. 

Fig. 19.7 Advancing water contact angles, measured at 25 °C after Diels-Alder reaction at different temperatures with TMSM-Cp for pulsed plasma polymer thin films, versus reaction time.

Fig. 19.8 Linearization according to pseudo-second-order kinetics of the alkene group surface coverage on pulsed plasma polymer thin films. The solid lines correspond to least-squares fits of the data.

Both monolayers and plasma polymer thin films studied show pseudo-second-order surface Diels-Alder kinetics and obey the Arrhenius equation. The magnitudes of the rate constants calculated in the case of the Diels-Alder reaction on pulsed plasma polymer thin films are lower than the magnitudes of the rate constants on monolayers (Table 19.1). The rate constants seem to reflect the... 

Scheme 19.1 The confinement effect. The arrows illustrate the diffusion of the reactant (a) at the surface of the mono-layer and (b) into the alkene-functionalized plasma polymer thin film.

The activation energies and the parameters characterizing the transition state of the activated complex are calculated according Eqs. (5) and (6) and are summarized in Table 19.2 for the reactions on monolayers and on plasma polymer thin films as weU. 

In future studies, we propose to add to the contact angle measurements (which probe only 5-10 A of the layer) XPS and FTIR spectroscopy analysis, in order to understand the kinetics of the reaction in the interior of the pulsed plasma polymer thin film. Once quantitative elucidation of the reactivity of the pulsed plasma polymer thin film has been fuUy accomplished, adhesion strength measurements will be performed and correlations between adhesion parameters and thermodynamic parameters wiU be explored. This wiU be the subject of a further paper. 

The quantitative elucidation of the surface confinement effect of dienophile groups on the Diels-Alder reaction led to the conclusion that the reaction at the pulsed plasma polymer surface is significantly different from the reaction in the monolayer. The plasma polymer thin films are less reactive than the monolayers but the transition-state complex is more ordered. This means that this transition-state complex is more stable at the pulsed plasma polymer surface than on monolayers because of the chemical environment of the molecules. Since the reaction at the plasma polymer is significantly more confined than in mono-layers, the reaction is less affected by the temperature. These first results need to be completed by XPS and FTIR spectroscopic analysis in order to obtain quantitative elucidation of the reactivity in the entire pulsed plasma polymer thin film. 

This work also shed light on interesting properties of thin films due to the thermally reversible chemistry of the Diels-Alder reaction. Carefully designed, alkene- and diene-functionalized, pulsed plasma polymer thin films will make it possible to control adhesion between any kinds of solid surfaces. 

As such exemplary experimental material, plasma polymer thin films with embedded silver particles are selected [3]. These films were made by simultaneous or alternating plasma polymerization and metal evaporation. The films can be deposited as multilayers consisting of two polymer thin films and a nanoparticle-containing film between these films. Because of the two plasma polymer layers on either side, the particles are completely embedded in a homogeneous media. The multilayer systems are very appropriate for determining particle size and investigating the interface between metal particles and plasma polymer matrix, because here metal nanoparticles are embedded in one plane. This allows a simple determination of the particle size and shape in the TEM. 

A very common and useful approach to studying the plasma polymerization process is the careful characterization of the polymer films produced. A specific property of the films is then measured as a function of one or more of the plasma parameters and mechanistic explanations are then derived from such a study. Some of the properties of plasma-polymerized thin films which have been measured include electrical conductivity, tunneling phenomena and photoconductivity, capacitance, optical constants, structure (IR absorption and ESCA), surface tension, free radical density (ESR), surface topography and reverse osmosis characteristics. So far relatively few of these measurements were made with the objective of determining mechanisms of plasma polymerization. The motivation in most instances was a specific application of the thin films. Considerable emphasis on correlations between mass spectroscopy in polymerizing plasmas and ESCA on polymer films with plasma polymerization mechanisms will be given later in this chapter based on recent work done in this laboratory. 

The hydrodynamic factors that influence the plasma polymerization process pose a complicated problem and are of importance in the application of plasma for thin film coatings. When two reaction chambers with different shapes or sizes are used and when plasma polymerization of the same monomer is operated under the same operational conditions of RF power, monomer flow rate, pressure in the reaction chamber etc., the two plasma polymers formed in the two reaction chambers are never identical because of the differences in the hydrodynamic factors. In this sense, plasma polymerization is a reactor-dependent process. Yasuda and Hirotsu [22] systematically investigated the effects of hydrodynamic factors on the plasma polymerization process. They studied the effect of the monomer flow pattern on the polymer deposition rate in a tubular reactor. The polymer deposition rate is a function of the location in the chamber. The distribution of the polymer deposition rate is mainly determined by the distance from the plasma zone and the... 

The rapid development of solid state physics and technology during the last fifteen years has resulted in intensive studies of the application of plasma to thin film preparation and crystal growth The subjects included the use of the well known sputtering technique, chemical vapour deposition ( CVD ) of the solid in the plasma, as well as the direct oxidation and nitridation of solid surfaces by the plasma. The latter process, called plasma anodization 10, has found application in the preparation of thin oxide films of metals and semiconductors. One interesting use of this technique is the fabrication of complementary MOS devices11. Thin films of oxides, nitrides and organic polymers can also be prepared by plasma CVD. 

The results from Miyachi et al. [64] showed that nonspecific adsorption of target DNA is decreased when SA embedded in a plasma-polymerized polymer thin film glass substrate is used (Fig. 3). The embedded SA on this substrate can selectively attach to biotinylated-probe ssDNA, which showed selective hybridization to complementary target DNA and a higher signal-to-noise ratio due to the low nonspecific DNA binding on substrate. However, we have to use a special polymerized system to fabricate thin polymer film by this method. 

A thorough understanding and careful control of plasma parameters can enable the process to be tailored to the desired application. It must be acknowledged however, that in spite of an impressive number of studies reported, an accurate prediction of the molecular structure of plasma polymers is not available. However, attempts have been made to examine the effects of various deposition parameters on the growth of plasma derived polymer thin films, and some of the results are summarized below. 

These plasma polymerized thin films have become increasing popular in the last few years, particularly, teflon like polymers, owing to their exceptional properties some of which are described below. 

N.K. Dutta, N.D. Tran, and N.R. Choudhury, Perfluoro(methylcyclohexane) plasma pol3fmer thin film Growth, surface morphology, and properties investigated hy scanning thermal microscopy, J. Polym. Sci. B., 43, 1392-1400 (2005). 

Ultrathin polymer films can be prepared using two kinds of technology. The first includes wet processes like LB, spreading, dipping or solvent casting methods. The other is dry processing, such as physical vapour deposition (PVD) and chemical vapour deposition (CVD). Of these methods, the CVD methods, such as plasma polymerisation, are frequently used to make polymer thin films [24-26]. 

Over the last decades, there have been a lot of efforts to fabricate polymer thin film. However, it is difficult to obtain a conducting polymer in thin film or monolayer owing to lack of processabihty and solubility. Among the conducting polymers, PANI thin film has a great potential to apply for chemical sensor due to its sensing ability and conductivity. Similar to PPy, there have been several methods to fabricate PANI thin film LB technique, self-assembly, electropolymerization, evaporation, and plasma-mediated polymerization. 

Ratner, B.D., Tyler, B.J., Chilkoti, A. (1993) Analysis of biomedical polymer surfaces polyurethanes and plasma-deposited thin films. Clin. Mater., 13, 71-84. 

Phillips HM, Li Y, Bi X, Zhang B (1996) Reactive pulsed laser deposition and laser induced crystallization of SnO transparent conducting thin films. Appl Phys A 63 347-351 Pique A, Auyeung RCY, Stepnowsk JL, Weir DW, Arnold CB, McGiU RA, Chrisey DB (2003) Laser processing of polymer thin films for chemical sensor applications. Surf Coat Technol 163-164 293-299 Randhaw H (1991) Review of plasma-assisted deposition processes. Thin Solid Films 196 329-349 Risti M, Ivanda M, Popovi S, Musi S (2002) Dependence of nanocrystaUine SnO particle size on synthesis route. J Non-Crystal Solids 303 270-280... 

Kwok CS, Horbett TA, Ratner BD. Design of infection-resistant antibiotic-releasing polymers II. Controlled release of antibiotics through a plasma-deposited thin film barrier. J Control Release 1999 62 301-11. http //dx.doi.org/10.1016/S0168-3659(99)00105. ... 

---