Experimental and theoretical studies of the growth mechanisms of ethyl lactate plasma polymer films

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 [1].
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 the mass spectrometry (MS) data supported by DFT calculations allow proposing a clear fragmentation pattern of the precursor as a function of the experimental conditions [2]. Nevertheless MS, as single diagnostic tool, does not allow evaluating the density of ester groups in the plasma phase. It is therefore necessary to combine MS with in situ infrared spectroscopy. The latter data are understood using synthetic IR spectra generated by DFT calculations.
In the defined experimental window, by increasing PRF, the ester content in the ELPPF decreases from 18 at.% to 1.4 at.% which is perfectly correlated to the decrease of the ester density in the plasma phase from 2.2x1015 to 9.1x1012 molecules/cm³. Altogether, this set of data paves the way for a reproducible and controllable synthesis of ELPPF with defined chemical composition.