Towards a better understanding of the plasma polymerization of ethyl lactate by combining mass spectrometry and IR absorption plasma diagnostics

Experimental and theoretical study of the plasma-surface interaction during the growth of ethyl-lactate plasma polymers
S. Ligot1, F. Renaux², P. Raynaud3, P. Gerbaux4, M. Guillaume5, R. Snyders1,2
1Chimie des Interactions Plasma-Surface, CIRMAP, Université de Mons, Place du Parc 23, B-7000 Mons, Belgium.
2Materia Nova Research Center, Parc Initialis, Avenue N. Copernic 1, B-7000 Mons, Belgium.
3Matériaux et Procédés Plasmas, Université Paul Sabatier, 118, route de Narbonne, F-31062 Toulouse, France
4Groupe de Recherche en Spectrométrie de Masse, Université de Mons, Place du Parc 23, B-7000 Mons, Belgium.
5Chimie des Matériaux Nouveaux, CIRMAP, Université de Mons, Place du Parc 23, B-7000 Mons, Belgium.

By being (bio-)degradable and derived from renewable resources, polylactide (PLA) has gained enormous attention as an alternative to conventional synthetic packaging materials. Nevertheless, PLA presents significant water and gas permittivity which lead to a high degradation rate of the material by hydrolysis of the ester bonds limiting its potential applications in the packaging industry. In order to improve the barrier properties of PLA substrate, we propose to cover it with a cross-linked ethyl lactate-based plasma polymer film (ELPPF). The control of both chemical composition (ester bonds density) and cross-linking degree would allow tuning the gas permittivity and, as a consequence, the degradation rate of the PLA substrate.[1]
In this work, in order to contribute to a deeper understanding of the growth mechanisms of the ELPPF, our objective is to correlate both plasma and film chemistries and, ultimately, to propose a clear picture of the plasma-surface interaction during the process.
ELPPF have been synthesized by PECVD using both continuous and pulsed RF power signal through an ICP copper coil. The applied power ranged between 5 and 400 W. The ELPPF chemistry, especially the ester function density, has been evaluated by the combination of chemical derivatization and XPS measurements and compared to the plasma phase composition. The latter has been measured by using RGA mass spectrometry and in situ infrared spectroscopy. These data, supported by DFT calculations of gas phase reaction allow understanding the plasma phase chemistry and the plasma-surface interaction during ELPPF growth.
Our data reveal that, in the defined experimental window, we are able, by increasing the injected RF power, to tailor the ester content in the ELPPF from 1.4 at.% to 18 at.%. This trend is perfectly correlated to the plasma diagnostic data revealing the loss of the ester content with quite the same rate. Indeed, the in situ FTIR spectroscopy supported by the DFT theoretical vibrations allows to exactly identifying the ester vibration peak. Consequently, the intensity of this peak can directly be correlated to the ester content in the plasma. Finally, the mass spectrometry and FTIR data support the plasma polymer film chemistry evolution as a function of power.

[1]S. Ligot, F. Renaux, L. Denis, D. Cossement, N. Nuns, P. Dubois, R. Snyders. Experimental study of the plasma polymerization of ethyl lactate. Accepted to Plasma Processes and Polymers, Augustus 2013