Combining Mass Spectrometry Diagnostic and Density Functional Theory Calculations for a Better Understanding of the Plasma Polymerization of Ethyl Lactate

Abstract

The focus of this work is on the growth mechanism of ethyl lactate-based plasma polymer film (ELPPF) that could be used as barrier coatings. In such an application, the ester density of the plasma polymer has to be controlled to tune the degradation rate of the material. Our strategy consists of correlating the plasma chemistry evaluated by RGA mass spectrometry and understanding, via DFT calculations, the chemistry of the synthesized thin films. The theoretical calculations helped us to understand the plasma chemistry in plasma ON and OFF conditions. From these data it is unambiguously shown that the signal m/z 75 can directly be correlated with the precursor density in the plasma phase. The combination of XPS and chemical derivatization experiments reveal that the ester content in the ELPFF can be tailored from 2 to 18 at. % by decreasing the RF power, which is perfectly correlated with the evolution of the plasma chemistry. Our results also highlight that the ELPPF chemistry, especially the ester content, is affected by the plasma mode of operation (continuous or pulsed discharge, at similar injected mean power) for similar ester content in the plasma. This could be related to different energy conditions at the interface of the growing films that could affect the sticking coefficient of the ester-bearing fragments.

Mechanical, Optical and Barrier Properties of PLA-layered Silicate Nanocomposites Coated with Organic Plasma Polymer Thin Films

“…These results support the hypothesis of internal stress in ELPPF synthesized at higher power and the low resistance to crack propagations. This behavior has been indeed predicted by the high H/E r ratio -where H is the hardness and E r is the reduced Young's modulus-determined by nanoindentation tests in our previous paper [29]. Such a kind of phenomenon has already been reported, for example, for metal oxide coatings deposited on polymeric substrates.…”

“…As a consequence, there is a good affinity between the polar groups at the surface of the coating and the oxygen molecules, which could increase the sorption coefficient and limit the barrier efficiency of the ELPPF. On the opposite, the ELPPF prepared at high power are strongly cross-linked and contain much less polar groups [28,29]. This should have the consequence to decrease the OTR and the diffusion pathway of oxygen molecules through.…”

“…It has been shown that for low power, the ELPPF are slightly cross-linked and contain much more ester functions, which result in polar and hydrophilic surfaces [28,29]. As a consequence, there is a good affinity between the polar groups at the surface of the coating and the oxygen molecules, which could increase the sorption coefficient and limit the barrier efficiency of the ELPPF.…”

“…In previous works, we reported the possibility to finely control the physicochemical properties of the ELPPF by modulating the experimental parameters. For example, by increasing the injected radiofrequency (RF) power in the plasma discharge, the ester function density in the ELPPF is reduced from 18 to 2 % and the cross-linking degree of the ELPPF increases by a factor 2 [28,29]. By using this deep control of the ELPPF synthesis, we expect to tune: (i) the diffusion rate of water vapor and other gases within the ELPPF by controlling the degree of cross-linking of the network; and (ii) the degradation rate of the ELPPF through the control of the ester bond density in the material.…”

Challenges in the characterization of plasma polymers using XPS

“…As examples, Ligot et al [150], and Nisol et al [29] combined XPS and mass spectrometry to investigate the polymerization mechanisms of ethyl lactate, allyl methacrylate (AMA) and propyl isobutyrate (PiB). Ligot could link, although she needed to go through derivatization for the XPS part, the proportion of ester in the gas phase (low pressure plasma) to the coating content, as a function of the plasma parameters, as shown in figure 15 [150].…”