Surface Reaction in Monolayers and Plasma Polymer Thin Films

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 

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. 

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