Experimental and theoretical studies of the plasma surface interactions during the growth of ethyl lactate plasma polymer films

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.

Polylactic acid (PLA), derived from renewable resources, is a (bio)degradable polymer accepted as a good alternative to conventional polymers for packaging applications. Nevertheless, as for other polymers, it presents too high water and gas permittivity. In order to improve its barrier properties and, as a consequence, its degradation rate, we are developing a cross-linked ethyl lactate-based plasma polymer film (ELPPF). The control of both chemical composition (hydrolysable ester bonds density) and cross-linking degree would allow tuning the degradation rate of the polymer by ester bond hydrolysis.
In this work, our objective is to correlate the 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 (PRF) through an ICP copper coil (5 W < PRF < 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 measured by using RGA mass spectrometry and in situ infrared spectroscopy. These plasma diagnostic data are discussed in view of DFT calculations.
Our data reveal that, in the defined experimental window, by increasing PRF, the ester content in the ELPPF increases from 1.4 at.% to 18 at.% which is perfectly correlated to the in situ FTIR spectroscopy data revealing the loss of the degradation of the ester function in the gas phase with a similar rate. On the other hand, the mass spectrometry data supported by DFT calculations allow proposing a clear fragmentation pattern of the precursor as a function of the experimental conditions. Altogether, this set of data paves the way for a reproducible and controllable synthesis of ELPPF with defined chemical composition.