Combining Mass Spectrometry Diagnostic and Density Functional

Article pubs.acs.org/JPCB

Combining Mass Spectrometry Diagnostic and Density Functional Theory Calculations for a Better Understanding of the Plasma Polymerization of Ethyl Lactate S. Ligot,†,⊥ M. Guillaume,‡ P. Gerbaux,§ D. Thiry,† F. Renaux,∥ J. Cornil,‡,∥ P. Dubois,∥,⊥ and R. Snyders*,†,∥ †

Chimie des Interactions Plasma-Surface, Center of Innovation and Research in Materials & Polymers (CIRMAP), Université de Mons - UMONS, Place du Parc 23, B-7000 Mons, Belgium ‡ Service de Chimie des Matériaux Nouveaux, Center of Innovation and Research in Materials & Polymers (CIRMAP), Université de Mons - UMONS, place du Parc 23, B-7000 Mons, Belgium § Organic Synthesis and Mass Spectrometry Laboratory, Center of Innovation and Research in Materials & Polymers (CIRMAP), Université de Mons-UMONS, Place du Parc 23, B-7000 Mons, Belgium ∥ Materia Nova Research Center, Parc Initialis, Avenue N. Copernic 1, B-7000 Mons, Belgium ⊥ Service des Matériaux Polymères et Composites, Center of Innovation and Research in Materials & Polymers (CIRMAP), Université de Mons - UMONS, place du Parc 23, B-7000 Mons, Belgium S Supporting Information *

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.

1. INTRODUCTION The interest in poly(lactic acid) (PLA) has increased over the last two decades due to its biocompatibility, (bio)degradability and biobased features. Another advantage of the PLA is its adjustable architecture allowing its physical and mechanical properties to be modified. As a result, PLA is seen as a viable alternative to petrochemical-based polymers and offers important prospects in a wide range of commodity applications (biomedical, packaging, automotive and textile industry, electronic devices, ...).1−6 Nevertheless, in a certain case, there is a downside with the (bio)degradable properties of PLA. Indeed, when it is used in daily applications (i.e., food packaging), a high degradation rate of the material is required whereas it can have a detrimental effect on the long-term ones (i.e., mobile phone or computer housing). Because the (bio)degradation process starts with the hydrolysis of the ester bonds that are in the polymer, the ester function density © 2014 American Chemical Society

in the material is the key parameter that controls its degradation rate.1 Based on this and to widen the application field of PLA and tailor its degradation rate, it is necessary to control the diffusion rate of water molecules and/or the hydrophilicity of PLA. In this context, we have recently proposed to grow thin organic and cross-linked (bio)degradable barrier coatings on PLA substrates. These latter have been synthesized by plasma enhanced chemical vapor deposition (PECVD) using ethyl lactate as precursor, a biobased and (bio)-degradable derivative of lactic acid.7 By using this approach and modulating the process parameters, we expect to tune (i) the diffusion rate of water vapor and other gases within the ELPPF by controlling Received: November 15, 2013 Revised: March 18, 2014 Published: March 24, 2014 4201

dx.doi.org/10.1021/jp411244x | J. Phys. Chem. B 2014, 118, 4201−4211

The Journal of Physical Chemistry B

Article

Therefore, we believe that the knowledge generated by the present study is of importance for a broader audience than the thin films community. In the context of this work, both pulsed and CW plasma polymerization of ethyl lactate precursor have been studied. The chemical composition of the layers, especially the ester content, has been determined by XPS measurements combined with a derivatization method whereas the chemical composition of the plasma has been studied by mass spectrometry assisted by DFT calculations.

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. Indeed, it is well-known that by controlling the gas phase and the surface processes, the properties of the synthetized plasma polymer films can be tuned over a broad range by tuning the functional group density and on the cross-linking degree which enables applications such as drug delivery or permeability controls.8−10 In our previous work, using experimental design as working strategy, the influence of the applied power (PRF), the working pressure and the precursor flow rate have been investigated, revealing that PRF is the key parameter controlling both the ester density and the cross-linking degree of the ethyl lactate based plasma polymer films (ELPPF).7 In this study, the plasma discharge was generated through a continuous wave (CW) radio frequency signal in a power domain ranging from 60 to 280 W. In this experimental window, chemical derivatization combined to XPS measurements revealed that the ester function density can be modulated from 2 to 12% when decreasing the power. Because it is accepted that the hydrolysis of ester bonds is the key step of the (bio)-degradation of PLA-based materials, the higher the ester function density, the higher the (bio)degradation rate. Consequently, to widen the application fields of PLA, it is interesting to increase the density of ester function in the ELPPF. By using an ester as a precursor molecule, it is therefore necessary to promote the retention of monomer functionality in the film, which is often performed by pulsing the plasma.11−13 Indeed, with pulsed discharges, reactive species generated during the short plasma ON periods (ONtime) are consumed during the plasma OFF periods (OFFtime). Additionally, lower average power values than the minimum ones required to sustain the discharge in CW mode can be achieved. This might avoid excessive fragmentation of the precursor, and hence, the incorporation of the chemical group of interest is higher in the layers while the deposition rate is continually controlled.11−13 By tuning the mean power dissipated in the discharge, it should therefore be possible to tune the retention of ester functions in the resulting ELPPF. Nevertheless, a thorough control of the incorporation of ester in ELPPF can only be reached if the plasma polymerization process is deeply understood. The plasma polymerization process can only be obtained by a detailed characterization of the plasma phase, which allows providing a better understanding of the plasma−surface interactions during the growth of the plasma polymer films. These interactions are at the root of the growth film mechanisms which, in turn, define the PPF properties.13−15 As a consequence, residual gas analysis (RGA) mass spectrometry was performed to probe the chemistry of the ethyl lactate plasma phase with a special attention to the evolution of the ester function density. Theoretical calculations based on density functional theory (DFT) have been carried out to foster the interpretation of the mass spectrometry data and to identify potential reaction mechanisms in the discharge. The combination of these experimental and theoretical approaches enables a better understanding of the precursor fragmentation processes and, in the end, of the growth mechanism of the plasma polymer films. It has to be mentioned that due to the uniqueness of the chemical environment of a plasma, the understanding of the reactions of complex organic molecules in a plasma environment goes well beyond a “simple” technical applications.

2. EXPERIMENTAL SECTION The ELPPF were synthesized on 2 cm2 p-doped (100) silica wafers ultrasonically washed in hexane and rinsed with ethanol. The deposition chamber consists of a cylindrical stainless steel vacuum chamber (35.5 cm in diameter and 86.5 cm in length) equipped with a rotary pump (Varian Triscroll 600) and a turbomolecular pump (Oerlikon Leybold Vacuum MAG W 600 iP/L) connected in series, enabling a residual pressure of 5 × 10−6 Torr, as measured by a full-range gauge (Figure 1). The

Figure 1. 3D sketch of our experimental setup: (1) substrate holder, (2) load-lock system, (3) pumping system, (4) water-cooled copper coil, (5) precursor introduction pipe, and (6) mass spectrometer.

liquid precursor (ethyl lactate ≥99.0% purity supplied from Sigma Aldrich) is heated at 70 °C and introduced in the chamber using a mass flow controller. During the process, the working pressure is measured by using a baratron gauge and is accurately regulated by using a throttle valve. The water-cooled copper coil (4.5 cm internal diameter and 0.7 cm thickness) is located in front of the substrate and connected to a radio frequency (RF) power supply (13.56 MHz, Advanced Energy Cesar1310) via a matching network (Advanced Energy VM1000A). The distance between the coil and the substrate is fixed at 13 cm. The substrates are introduced under vacuum through a load-lock system, and then transferred to the deposition chamber using a transfer stick. During all deposition, the substrate is kept floating. The thickness and, hence, the deposition rates were determined by means of mechanical profilometry (Veeco Dektak150) using a diamond tip (2.5 μm curvature radius) with an applied force of 0.1 mN. ELPPF have been synthesized using both CW and pulsed RF power signal. In pulsed mode, it has been possible to grow 4202



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ELPPF for mean power as low as 5 W whereas in CW conditions, approximately 60 W was the lowest power value required to maintain a plasma. The power modulation in pulsed plasma deposition processes is defined by the duty cycle (Δ) given by the relationship between the plasma ON- and OFFtime. The input power during the plasma ON-time (Ppeak) multiplied by Δ defines the mean power dissipated in the discharge (Pmean); see eqs 1 and 2.11,12,15 Δ=

ONtime ONtime + OFFtime

As already mentioned in the Introduction, the chemical composition, particularly the ester concentration in the ELPPF, is very important in terms of the (bio)degradability of the materials. Due to the too low energetic resolution of our XPS machine (∼0.5 eV) and to the very close chemical environment of the carboxylic acid and ester functionalities, XPS does not allow us to resolve the contributions of carboxylic acid and ester functionalities. To evaluate the contribution of the ester bonds in the coatings, chemical derivatization reaction of carboxylic acid groups was performed using trifluoroethanol as probing agent.19 The ester contents (at. % COOC) have been calculated by using eqs 3 and 4 reported by Chilkoti et al. with a reaction yield of 0.87 ± 0.15:19

(1)

Pmean = Δ × Ppeak

(2)

In this work, Pmean ranged from 5 to 100 W while the Ppeak and the ON-time were fixed at 120 W and 0.25 ms, respectively. Table 1 reports the set of experimental values

at. % COOH =

(3)

Table 1. Range of Studied Pmean and the Correlated Variables Such as the Duty Cycle (Δ), the Pulse Frequency, and the OFF-Time (TOFF)a Pmean (W)

Δ (%)

frequency (s−1)

OFF-time (ms)

5 10 20 30 40 50 60 70 80 90 100

4 8 17 25 33 42 50 58 67 75 83

167 333 667 1000 1333 1667 2000 2333 2667 3000 3333

5.75 2.75 1.25 0.75 0.50 0.35 0.25 0.18 0.13 0.08 0.05

([F]/3) [COOH] 100 = × × 100 [C] [C] 87

at. % COOC = at. % COOX − at. % COOH

(4)

where [C] is the total amount of carbon and COOX is the global contribution of carboxylic acid (COOH) and ester (COOC) functions. According to the protocol developed by Chilkoti et al., the chemical derivatization reaction of carboxylic acid groups was performed in the vapor phase at room temperature with very few modifications. A 1.35 μL aliquot of 2,2,2-trifluoroethanol (TFE, Sigma Aldrich, ≥99.0% purity), 0.60 μL of pyridine (Sigma Aldrich, ≥ 99% purity), and 0.45 μL of 1,3-di-tertbutylcarbodiimide (Di-tBuC, Sigma Aldrich, 99% purity) were injected into a glass vial. The ELPPF sample was stuck on a glass slide in front of and above the liquid. The vial was then sealed with a parafilm, and the reaction was allowed to proceed for ∼24 h. After each chemical derivatization reaction, the derivatized samples were outgassed in vacuum (6 × 10−6 Torr) for 4 days, followed by XPS analysis. Plasma composition was investigated by mass spectrometry using a quadrupole Hal IV PSM002 supplied by Hidden Analytical. The source of the mass spectrometer was connected to the plasma chamber by a 100 μm extraction orifice located at around 30 cm of the coil. Neutral species entering the mass spectrometer are ionized by electron ionization (EI) using an electron kinetic energy fixed at 20 eV to avoid any further fragmentations from occurring in the ionization source.20−22 The multiplier, the first dynode, and the current of emission were fixed at 2500 V, −1200 V, and 350 μA, respectively. The detected ions are discussed in terms of relative intensities (Ire), calculated on the basis of eq 5.

The peak power (Ppeak) and the on-time (TON) were fixed at 120W and 0.25 ms, respectively. a

that have been used (Pmean, Δ, pulse frequency and OFF-time). For all deposition processes, the working pressure and the precursor flow rate were fixed at 10 mTorr and 5 sccm, respectively. The chemical composition of the ELPPF was investigated by XPS analysis using a PHI 5000 VersaProbe (ULVAC-PHI) hemispherical analyzer from Physical Electronics with a base pressure 225 W), the increase of the ion flow bombarding the growing film enhances the etching reactions, leading to a decrease of RD.29−31 Furthermore, it can be observed that within the confidence intervals, RD values are similar for a given Pmean, whichever plasma is used (continuous or pulsed). This could point toward the formation of identical reactive species produced by similar gas phase chemical reactions. Nevertheless, mass spectrometry measurements of plasma generated in CW and pulsed modes for identical mean powers reveal that the plasma chemistry is significantly different with the presence of a higher density of bigger fragments in pulsed mode. For example, for Pmean = 60 W, the intensity of the fragment at m/z 45 is 2.5 time higher in pulsed mode than in continuous mode (Figure S1, Supporting Information). Therefore, the close to similar RD values in CW and pulsed conditions for similar mean powers likely have another origin. One possible explanation would be to consider that, if in pulsed conditions the deposition process occurs mainly during ON-time, deposition can also occur during the OFF-time. This phenomenon has already been extensively reported and is dependent on the precursor chemistry being more important for unsaturated precursors.9 Nevertheless, it can also exist for saturated precursors, for which it is explained by the longer lifetime of radicals than reactive species (ions, photons, and electrons) when the plasma is turned off. Therefore, during the OFF-times, the surviving radicals can continue the gas phase radical polymerization reactions at the surface leading to the deposition of a surface layer with a structure close to the analogue conventional polymer.11,12,31−33 The other element that has to be considered is the peak power. Indeed, for Pmean = 100 W, the peak power used in pulsed mode (120 W) is higher to the power applied continuously in CW mode (100 W). This higher value of the peak power can induce higher erosion and ablation processes during the ON-time. Therefore, the similar RD values measured in pulsed and CW modes could be explained by the enhanced erosion and ablation phenomena occurring during ON-times, which counterbalance the possible deposition during the OFF-time. As already mentioned in the Introduction, the chemical composition, and particularly the ester concentration of the ELPPF, is very important considering the (bio)degradability of the materials. Therefore, the chemistry of the synthesized ELPPF has been studied by XPS. Figure 3 shows the evolution of the C/O atomic ratio and of the at. % COOC (calculated using eq 4, Experimental Section) as a function of Pmean. C/O increases linearly with Pmean from 1.8 to 5.7. This trend is widely reported in the literature for other organic precursors, and is explained by the decrease of the retention of functional groups coupled with the production, in the plasma phase, of oxygenated and stable molecules, such as CO and CO2, which do not contribute to the growth of the layer.7,12,18,32,34 As revealed by Figure 3, the linear increase of C/O with Pmean is coupled with a decrease of the ester density in the ELPPF. For the lowest power (5 W), at. % COOC (∼18%) is very close to the ester content of the precursor molecule (20%) giving a retention level of 90%. This means that, in this condition, the precursor molecules are barely fragmented and reach the

used to carry out the DFT calculations on radical systems instead of the restricted open-shell formalism; we have checked that spin contamination is low in all cases. For reactions involving biradical species, both spin states (singlet or triplet) were calculated and the more stable form was used to calculate the Gibbs free enthalpies. For each optimized geometry, we have performed a normal-mode analysis for the thermochemistry analysis (using a temperature of 298.15 K) and found a local energy minimum For radical cation and radical unimolecular decomposition, two different kinds of reaction may occur, namely single bond cleavages and rearrangement reactions. It is often observed that single bond cleavages are barrierless processes. As a consequence, the Gibbs free enthalpy difference between the reactants and products and the activation energies are identical or really similar. For rearrangement reactions, this is no longer the case due to the involvement of concomitant bond breakings and bond formations renders. Consequently, the rearrangement reactions are often characterized by significant activation energies. Nevertheless, it is often observed for small organic ions that the rearrangement reactions require less energy than the single bond cleavages. This prompted us to focus on thermodynamical data to avoid entering kinetic discussions.26 The possible thermodynamic pathways of the decomposition reactions are consequently discussed on the basis of the free enthalpies of reactions, estimated by simply calculating the energy difference between the reactants and products.20 The Gibbs free enthalpy variations for bond dissociation and rearrangement reactions (including the entropic effects) have been modeled through a statistical thermodynamic analysis on the rotational/translational/vibrational degrees of freedom, including zero point vibrational corrections. More details about the methodology can be found elsewhere.20 With the theoretical approach, the zero point correction requires a rescaling below 1%.27 All calculations have been performed using the Gaussian09 package.28

4. RESULTS AND DISCUSSION Figure 2 shows the evolution of the deposition rate (RD) as a function of Pmean in pulsed and continuous mode. In our experimental windows, RD ranges from 0.4 to 42.9 nm/min

Figure 2. ELPPF deposition rate measured in pulsed and CW mode as function of Pmean. 4204



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The EI mass spectrum of ethyl lactate in the gas phase (plasma OFF) has been recorded and is presented in Figure 4a.

Figure 3. Evolution of the ELPPF relative ester content and C/O atomic ratio as a function of Pmean. Figure 4. Mass spectrum of ethyl lactate gas phase (plasma OFF): (a) m/z = 1−125; (b) m/z = 1−80.

substrate while keeping their initial structures. For higher power (280 W), the retention level drops to 10% corresponding to at. % COOC of 2%. In this condition, the fragmentation degree is important, leading to the production of more radicals, ions, and photons that will affect the film chemistry and limit the incorporation of ester functions in the growing film.11,12,14,15 Comparing the set of data obtained for CW and pulsed mode, we can observe that for equivalent Pmean, the retention of the ester group is higher, and thus, the C/O ratio is lower when the pulsed conditions are used. Similar evolution has already been observed by Voronin et al. reporting the increase of the retention for long OFF-times in a pulsed process.12 The authors explain this evolution by the fact that when the OFFtime increases, the deposition process approaches the energy deficient regime representing a lower energy input per molecule and consequently a reduced fragmentation of the precursor. Additionally, the incident energy flux on the substrate surface is different as a function of the plasma mode production inducing different surfaces processes affecting the plasma polymer film chemistry.8 These results show that, by tuning the power and the mode of the plasma production, it is possible to finely control the ester content in the ELPPF. To correlate the evolution of the chemical properties of the coatings with the chemical composition of the plasma phase and to get a better understanding of the growth mechanism at a molecular level, the chemical composition of the plasma was measured by mass spectrometry in RGA mode. Because only charged species can be detected by mass spectrometry, radical species have to be ionized in the source of the spectrometer by means of fast moving electrons (20 eV). However, the electron ionization, which is a mandatory step, is known, even at 20 eV kinetic energy, for affording extensive fragmentation, following typical dissociative ionization schemes inducing the observation of numerous peaks in the mass spectra that are not representative of the plasma composition. Among all ions detected by mass spectrometry, the discrimination between the species arising from the plasma chemistry (radical and neutral chemistry) and the species arising from dissociative ionization in the ion source (ion chemistry) is definitely not straightforward. However, such a distinction is a prerequisite for better describing the plasma-generated reactive species that are involved in the film chemistry.

As expected for such a brittle molecule,35,36 no molecular ions are detected at m/z 118 among the ionic species and the heaviest detected ions appear at m/z 75. This already confirms that ethyl lactate molecules suffer from decomposition, on a large extent, upon electron ionization in the mass spectrometer source. At first sight, this is a major issue for our study, because it will be hard to estimate the degree of survival of ethyl lactate molecules when the plasma is switched ON by online mass spectrometry. Figure 4b shows the (m/z 1−80) part of the plasma-OFF ethyl lactate mass spectrum and the most significant ions at m/z 75, 46, 45 (base peak), 44, 43, 31, 28, and 18. Among all those ions, we can already identify signals that originate from well-identified contaminations of the experimental setup. The intense signal detected at m/z 18 is readily allocated to ionized water H2O•+. The presence of water molecules cannot be avoided even under vacuum due to their adsorption in the cold stainless-steel walls of the instrument. A second intense signal is detected at m/z 28 and is related to a mixture of isobaric CO•+/C2H4•+ radical cations, respectively. The formation of those small radical cations is readily explained by precursor ion dissociations following numerous dissociation pathways in the ionization source.37 The most intense peak, i.e., the base peak, observed in this mass spectrum corresponds to the m/z 45 cations. On the basis of the molecular structure of the precursor molecule, these C2H5O+ cations are CH3CHOH+, namely (hydroxymethyl)carbenium ions that are stabilized by resonance. We will demonstrate in a subsequent part of this report that single bond cleavage reactions, typical for ionized carbonyl compounds, are responsible for producing those ions.35,36 The signal at m/z 44 is observed both in the mass spectrum of the residual vacuum (not shown) and in the mass spectrum of ethyl lactate. Therefore, it is more than likely that the m/z 44 ions are CO2•+ ions coming from residual atmosphere. However, on the basis of the elemental composition of the ethyl lactate molecule and the important intensity of the m/z 44 peak, a part of the CO2•+ ions also arises from the ion chemistry of ethyl lactate. The detection of m/z 43 signal can be associated with C3H7+ or C2H3O+ ions. In the case of the hydrocarbon cations, the m/z 43 signal is usually accompanied 4205



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by signals at m/z 41 and m/z 39 for C3H5+ and C3H3+ ions, respectively. In our study, these subproducts are not detected and we correlate the detected m/z 43 ions with CH3CO+ acylium cations. Finally, m/z 29 ions could be assigned to the fragment CHO+, whereas the m/z 31 ions are likely to have a CH3O composition.37−39 Table 2 reports all ionic species detected in the ethyl lactate electron ionization mass spectrum, together with structure assignments.

Figure 5 shows the energy diagram depicting the various intermediates and the dissociation states involved in the decomposition reactions of ionized ethyl lactate. Starting from neutral ethyl lactate, the calculation ended up on an adiabatic electron ionization (EIa) of 9.5 eV to reach the stable radical cations. For the sake of clarity, the energy level corresponding to those radical cations is defined as the zero on the relative energy scale. Starting then from ionized ethyl lactate, four dissociation pathways are likely to occur to account for the production of the detected ionic species, see Figure 5.35,36,40−42 Single bond cleavage processes, namely the α-cleavages, characteristic of carbonyl compounds were calculated to require 0.64 and 1.37 eV, respectively, for the isomeric CH 3 CH(OH) • and CH3CH2O• losses (Figure 5, red and blue, respectively). These two reactions yield isomeric CH3CH2OCO+ and CH3CH(OH)CO+ acylium ions both detected at m/z 73. Those ions are then prone to decarbonylation reaction. Starting from CH3CH(OH)CO+ (cleavage α2), the CO loss clearly appears as a spontaneous process (−0.25 eV) generating the stable hydroxyl methyl carbenium cations, i.e., CH3CHOH+, representing the base peak of the ethyl lactate mass spectrum (Figure 4). On the other hand, the CO loss from the second acylium ions, CH3CH2OCO+, would generate the unstable ethoxyl cations (CH3CH2O+) with a prohibitively large energy demand (+ 0.98 eV) that renders this decomposition unlikely to occur.43,44 It is then more likely that the CH3CH2OCO+ cations suffer from decarbonylation via another mechanism, immediately generating the stable CH3CHOH+ cations (m/z 45). The presence of an Hγ in ethyl lactate immediately suggests the presence of a McLafferty rearrangement.35,36,40−42 Such a reaction is initiated by the migration of the Hγ to the oxygen atom of the carbonyl moiety affording a distonic radical cation.

Table 2. Masses Detected in the Ethyl Lactate Gas Phase Mass Spectrum and the Corresponding Proposal Fragmentsa mass

fragment

mass

fragment

mass

fragment

1 2 15 18 27

H H2 CH3 H2O C2H3

28 29 30 31 43

CO/C2H4 CHO CHOH/C2H6 CH2OH C2H3O

44 45 46 47 75

CO2/C2H3OH/C3H8 C2H5O C2H5OH/13C2H5O CH3O2 C3H7O2

a

All molecules listed are monocharged positive ions.

To further investigate the mass spectrometry behavior of ethyl lactate, we decided to undertake a thorough mechanistic study based on computational chemistry using DFT calculations. Takahashi et al. reported a mechanistic study dealing with the unimolecular decompositions of ionized ethyl lactate using mass-analyzed ion kinetic energy spectrometry (MIKES).40,41 In this paper, they mainly focused on the m/z 75 ions, arising from a C3H7• loss, and they tentatively proposed, without any theoretical support, that those C3H7O2+ fragment ions are produced from the ethyl lactate molecular ions following a double hydrogen migration (DHM) rearrangement.40,41

Figure 5. Proposed fragmentation pathways of ionic ethyl lactate dissociations occurring in the ionization source of the mass spectrometer. The dash lines shows the vertical ionization. 4206



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Figure 6. Proposed fragmentations pathways of ethyl lactate dissociations occurring in the plasma discharge and leading to radical fragments.

provided in these reports to confirm these conclusions. From Figure 5 (black), it is observed that a 1,4-hydrogen migration process is likely to generate a new distonic radical cation and interestingly that the corresponding energy level (0.63 eV) is located below the internal energy excess from the vertical ionization process (0.8 eV). Of course, such a process is expected to be accompanied by a kinetic barrier but the energy threshold should be reasonably low because (i) the reaction involves a five-membered ring and (ii) the hydrogen atom moves from one oxygen atom to another oxygen atom. Recent literature related to the study of the chemistry of gas phase ions reveals that hydrogen-bridged radical cations (HBRC) are ubiquitous in the unimolecular decompositions of radical cations. Numerous examples report that those HBRC as really stable ion-neutral complexes that associate a neutral molecule with a radical species by the intermediary of a proton.40,45 For the present ionic system, the distonic intermediate reaches, thermodynamically and in a spontaneous way, an HBRC intermediate that strongly associates a molecule of acetaldehyde with a C2H5OC•O radical. It is interesting to note, from Figure 5, that this specific complex lies at −0.35 eV below ionized ethyl lactate. A further CH3C•O loss also remains overall exoenergetic (−0.2 eV) and produces the C3H7O2+ detected at m/z 75 in Figure 4. From these stable ions, an ethene molecule can be released to create the ionic fragment CH(OH)2+ observed at m/z 47. The corresponding level remains below the internal energy excess level. Clearly, the overall reaction pathway generating the m/z 75 ions corresponds to a DHM mechanism. However, on the basis of recent literature and with the support of DFT calculations, we propose here a slightly different mechanism. The total absence of the molecular ions at m/z 118 is in full agreement with the vertical nature of the ionization process. We propose that, immediately after their preparation, the ethyl lactate radical cations dissociate, affording m/z 75 ions following the DHM pathway and m/z 45 more likely after the single bond cleavage α1 (Figure 5). On the basis of the mass spectrum of the ethyl lactate measured for plasma OFF and on the theoretical results, the

It is really interesting to note that the corresponding level is located only 0.35 eV above ionized ethyl lactate (Figure 5, green). Moreover, such a process involves a six-membered ring transition state and is consequently really favorable. Starting from these distonic radical cations, a consecutive ethene loss is then expected and only requires 0.21 eV to occur, making the overall process 0.56 eV endoenergetic from ionized ethyl lactate. The ionized carboxylic acid is not observed in the EI mass spectrum (at m/z 90), and we thus suspect consecutive fragmentation reactions. For instance, it is tempting to consider consecutive hydroxyl and carbon monoxide losses as a third putative pathway to account for the production of the abundant m/z 45 ions. Nevertheless, the hydroxyl radical loss induced by a single bond cleavage process can be proposed but seems unlikely on the basis of the DFT calculations, revealing an energy difference of about 1.16 eV (1.72 eV above ionized ethyl lactate). At this stage, the (hydroxymethyl)carbenium cations are likely to be produced, following single bond cleavage processes and further decarbonylation, instead of a McLafferty rearrangement. This conclusion is even strengthened by remembering that electron ionization with a fast moving electron is a vertical process generating excited molecular ions. The corresponding internal energy excess was also calculated and is shown in Figure 5 (dashed brown line). It appears that, upon vertical ionization,35,36,40−42 the molecular ions of ethyl lactate are generated with an 0.8 eV internal energy excess. In first approximation and ruling out other excitation processes, such an energy content exceeds the energy requirements for cleavage α1 and for the McLafferty rearrangement to occur. However, this internal energy is not sufficient to drive cleavage α2 nor the hydroxyl loss from m/z 90 ions. Finally, the ions detected at m/z 75 deserve peculiar attention. Indeed, those ions are not produced following the common dissociation reactions of ionized carbonyl compounds, namely, single bond cleavages and McLafferty rearrangement. However, as previously stated, the literature hints that C2H5 OC(H)O+H cations are produced from the ethyl lactate molecular ions following a double hydrogen migration (DHM) rearrangement.40,41 A theoretical support was not 4207



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moment, we did not identify any fragment that could be considered as diagnostic of the plasma chemistry. It is also important to recall that the situation is even more complicated because ethyl lactate cannot be detected in EI mass spectrometry. Nevertheless, hydrogen bridged radical complexes are purely ionic species and are not expected to play a role in the radical plasma chemistry. In other words, if a signal is detected in the mass spectrum at m/z 75, this would highlight that intact ethyl lactate molecules enter the ionization chamber of the mass spectrometer. In Figure 6 (black), we observe that the 1,4hydrogen migration only requires 1.46 eV and yields a biradical species. Further evolution of the system will generate a complex, similar to the previous hydrogen bridged radical cation, associating a molecule of acetaldehyde with the (C2H5O−C−OH)•• carbene. Our calculations strikingly prove that such a complex is a local minimum on the potential energy surface. Moreover, the production of this complex starting from the biradical species is only 0.39 eV endoenergetic and remains far less energy demanding than both the Norrish Ia and Ib processes. The DFT calculations further demonstrate that this complex is marginally stable and will probably collapse immediately by dissociation into acetaldehyde and (C2H5O− C−OH)•• carbene. Indeed, the corresponding level is located 0.18 eV below the acetaldehyde/carbene complex. In other words, this complex, similar to the HBRC identified in the ion chemistry, is unlikely to further evolve to generate C3H7O2• radicals that would be detected at m/z 75 with mass spectrometry. In other words, the m/z 75 ions clearly find their origin in the gas phase ion chemistry of ethyl lactate and no radical reactions have been identified for producing the corresponding neutrals. To experimentally support the hypothesis that C3H7O2• is not or barely created in the plasma, the apparition energy of the m/z 75 ions has been determined. Such energy corresponds to the minimum kinetic energy of the ionizing electrons that is required for the m/z 75 signal to appear. By linearly increasing the electron energy from 5 to 100 eV in the ionization source of the mass spectrometer, we can observe the energy requested to form the corresponding fragment through the evolution of the signal m/z 75 intensity. Figure 7 shows an experimental apparition energy around 10 eV. According to the fragmentation expected in the spectrometer source, the DFT calculations indicate that the ion C3H7O2+ appears at an energy level of −0.2 eV, implying that 9.3 eV must be consumed to create it. On the other hand, 3.1 eV would be required to create the corresponding radical fragment (C3H7O2•), following the homolytic fragmentations in the plasma phase. The experimental apparition energy therefore better matches the energy to create the ions in the spectrometer source than with the energy to create the radicals in the plasma. As a consequence, the nonpreferential hydrogen migration from the HBR intermediate to C3H7O2• fragment and the good agreement between DFT and the experimental apparition energy allows us to conclude that any precursors for the m/z 75 ions are not produced in the plasma discharge. Its detection in the mass spectra should thus be the consequence of the fragmentation of precursor molecules according to the ionic DHM reaction in the spectrometer source. This implies that the intensity of m/z 75 signal is proportional to the density of undamaged precursor molecules in the plasma. On the basis of these conclusions and to simplify the interpretation of the mass spectra, the m/z 75 signal has

ionic fragmentation pathways of the precursor occurring in the spectrometer source have been determined. Nevertheless, the aim of this work is to determine as meticulously as possible the chemical composition of the plasma to suggest some fragmentation pathways occurring in plasma and to have a better understanding of the plasma surface interactions. Due to the electron energy distribution function reported for our kind of plasma, these fragmentations are mainly the result of collisions with electrons having an energy ranging between 1 and 4 eV.46 Because the ionization threshold of organic molecules (9−13 eV) is much higher than the energy from their chemical bonds (3−4 eV), free radicals are generally considered to be the dominant species controlling the PPF growth.47,48 Nevertheless, both may contribute significantly to the total mass deposition and should be considered when plasma polymerization is described. Indeed, positive ions are accelerated to surfaces in contact with the plasma phase by the potential drop between the plasma and the substrate. As examples of ions bombardment impact, Choukourov et al. as well as Brookes et al. reported for high ion energy, the substitution of oxygen for nitrogen atoms in allylamine-based plasma polymer films and a higher cross-linking degree of Sibased plasma polymer films, respectively.49,50 However, it has been reported for ICP discharges that the ion/neutral flux ratio, which is the important parameter for depositing plasmas, decreases when the working pressure increases and rapidly increases with the injected power but tends to stabilize for power higher than 12 W.51 And yet, in our experimental conditions, the working pressure is relatively high (10 mTorr) and the injected power is always higher than 30 W. These conditions would therefore limit the value of the ion/neutral flux ratio. Therefore, even if it is obvious that ions play an important role in defining the PPF properties, we believe that focusing in a first approach on the neutral species makes sense. The fragmentation pattern shown in Figure 5 related to the ionic chemistry has been consequently recalculated to account for the presence of radical and neutrals species. In this case, the neutral ethyl lactate molecule is fragmented according to the same reactions than in Figure 5, not leading to ionic fragments any longer but only to radical fragments. Figure 6 depicts the radical unimolecular chemistry of neutral ethyl lactate. Well-known radical processes, i.e., reactions Norrish Ia and Ib, require 2.82 and 3.29 eV to occur, respectively. Again, decarbonylation reactions may spontaneously occur to finally form the radical fragment CH3CHOH•. This implies that Norrish Ia and Ib are feasible processes in the plasma phase and that the stable CH3CHOH• radicals are likely to be the final products (Figure 6, red and blue). The third dissociation process studied is Norrish II. This process is related to the McLafferty rearrangement in the ionic chemistry (green mechanism in Figure 6). Calculations reveal that the formation of neutral carboxylic acid is nearly thermoneutral. Indeed, 0.2 eV is necessary to reach the biradical intermediate but 0.17 eV is then released when the hydroxyl propionic acid is created accompanied by an ethene loss. Similarly to the ion chemistry, a consecutive loss of the hydroxyl radical is by far too energy demanding and, consequently, Norrish II seems not to be the most favorable way to produce the radical fragment CH3CHOH•. It is quite embarrassing to observe that, whatever the nature of the decomposing species, ions or radicals, the dominant expected species is CH 3 CHOH • in the radical chemistry and CH3CHOH+ in the ionic chemistry. This implies that, for the 4208



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Figure 7. Evolution of the raw intensity of m/z 75 detected in mass spectrometry as a function of the electron energy applied in the ionization source of the mass spectrometer.

consequently been assimilated as the precursor peak. The mass spectra of ethyl lactate obtained at 5, 60, and 280 W have therefore been corrected accordingly. The correction consisted of recalculating the mass spectra on the basis of the relation proposed by Voronin et al. (eq 6).12 Ir(m) = Ion(m) − Ioff (m)

Ion(75) Ioff (75)

(6)

where Ir(m) is the recalculated peak intensity corresponding to the mass m, Ion(m) is the experimental peak intensity obtained from the mass spectra, and Ioff(m) is the intensity when the plasma is off. The corrected RGA mass spectra of ethyl lactate plasma measured for Pmean = 5, 60, and 280 W are shown in Figure 8a− c, respectively. It has to be noted that the data were recorded from m/z 3 to 80 due to the saturation of the intensity at m/z 1 and 2 for high power conditions, showing the important concentrations of hydrogen radicals and H2 molecules. At 5 W (Figure 8a), masses 75, 45, 29, and 28 are mainly detected. We can infer that plasma chemistry is there mainly made of large and functional fragments such as C3H7O2• and CH3CHOH•. Furthermore, as previously discussed, the detection of the m/z 75 ions implies that undamaged precursor molecules are still present in the reactor. This low fragmentation degree at Pmean = 5 W is also supported by the XPS study, revealing a low C/O atomic ratio and a high ester content in ELPPF in these conditions. When Pmean increases to 60 W (Figure 8b), the chemistry of the plasma is strongly affected. The signals of the organic fragments at m/z 75 and 45 have disappeared and strongly decreased, respectively, while the signal at m/z 28 for ionized ethene and carbon monoxide has strongly increased. At Pmean = 280 W (Figure 8c), the peak at m/z 45 has almost disappeared whereas the peaks at m/z 44, 28, 26, and 18 corresponding to CO2•+, CO•+ and C2H4•+, C2H2•+, and H2O•+, respectively, have strongly increased. This indicates that the degree of precursor fragmentation increases with Pmean, inducing the decrease of oxygenated functional fragments for the benefit of byproduct molecules. This behavior has been widely reported in literature for other organic precursors.12,18,21,22 Additionally, the plasma chemistry is well correlated with the chemical composition of the ELPPF. Indeed, the substantial increase of the signals at m/z 28 (CO

Figure 8. Corrected mass spectra of ethyl lactate plasma phase acquired at Pmean = (a) 5 W, (b) 60 W, and (c) 280 W.

contributions) and m/z 44 (CO2) with Pmean supports the decrease in oxygen content in the ELPPF, revealed by the XPS results. As stable and nonreactive byproducts of the process, CO and CO2 are pumped out the system and hence do not take part in the ELPPF growth. This trend has been already observed for methyl isobutyrate plasma polymerization process.18 On the basis of the key hypothesis that m/z 75 ions are mainly created in the ionization source of the mass spectrometer and consequently that its signal is proportional to the amount of precursor in the plasma, the raw intensity of the m/z 75 signal in the mass spectra gives us an estimation of the ester content in the plasma phase. Figure 9 correlates, for all power conditions and for the type of discharge used, the ester content in the plasma phase and in the plasma polymer films. Figure 9 shows that there is a clear correlation between the plasma chemistry and the ester content in the synthesized PPF. This correlation can be described by two linear evolutions: one having a strong slope for low content of ester function in the plasma (high Pmean conditions) and the other having a lower slope for higher content of ester in the plasma (low Pmean conditions). This could explain the fact that the sticking coefficients of ester-bearing fragments depend on the chemistry of the receiving surface or on the energetic conditions during the growth. Additional work that is out of the scope of the present paper is definitively required to answer this question. On the other hand, comparing data obtained for continuous and pulsed discharges for similar ester content in the plasma, we observe a higher ester density in the films prepared in pulsed conditions. This could explain that during the OFF4209



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be used to explain the difference observed between the set of data measured in continuous and pulsed mode. We believe that the strategy proposed in this work, based on the combination of plasma diagnostic with gas phase DFT calculations opens up the way to a better understanding of the overall plasma polymerization mechanism. As regards the ELPPF, our data clearly provide a thorough control of the ELPPF chemistry that can be obtained by a picture of the plasma chemistry. Future work will include additional measurements to understand, from a fundamental point of view, the observed correlation as well as the evaluation of functional properties of these coatings such as the gas permeation and the degradation rate.

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ASSOCIATED CONTENT

S Supporting Information *

Comparison of mass spectra of ethyl lactate plasma phases. This information is available free of charge via the Internet at http://pubs.acs.org

Figure 9. Correlation of the ester contents in ELPPF and plasma determined by XPS and mass spectrometry measurements, respectively.

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AUTHOR INFORMATION

Corresponding Author

*R. Snyders: tel, +32 (0) 65 55 49 45; fax, +32 (0) 65 55 49 41; e-mail: [email protected].

time, condensation of the ester-bearing fragments which have been activated during the ON-time can occur without any disturbance within the plasma, which is not the case in continuous mode. As a consequence, for a given amount of ester in the plasma, the retention is higher in the pulsed mode. These data clearly open up the way to future works for understanding the observed dependencies.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S. Ligot is grateful to the “Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture” (F.R.I.A.) for its financial support. The Belgian Government through the “Pôle d’Attraction Interuniversitaire” (PAI, P7/05, “PlasmaSurface Interaction”, Ψ and PAI P7/05 “Functional Supramolecular Systems”) and the DG06-Wallonia Region through the program of excellence “OPTI2MAT” are also thanked. P. Gerbaux is senior Research associate of the “Fonds de la Recherche Scientifique” (FRS-FNRS) and is grateful to the FRS-FNRS for its continuing support. J. Cornil is an FNRS Research Director. DFT calculations have been performed on the computational facilities of the Laboratory for Chemistry of Novel Materials, as supported by BELSPO (PAI 7/05), Région Wallonne (OPTI2MAT Excellence program), and FNRSFRFC.

5. CONCLUSIONS In this work, the synthesis of ethyl lactate based plasma polymer films (ELPPF) using both continuous and pulsed RF discharges was investigated. Such films could be used as barrier coatings with tunable (bio)degradation rate mainly controlled by their ester function density. The aim of this work was therefore to control the latter by understanding the growth mechanisms of the ELPPF. The chemical analysis of the deposited ELPPF assessed by combining XPS measurements and chemical derivatization reveals an increase of the ester density from 2% to 18% when the injected RF power is decreased from 280 to 5 W. This evolution correlates with the evolution of the plasma chemistry, evaluated through RGA mass spectrometry and understood thanks to DFT calculations. The latter applied to plasma “ON” and plasma “OFF” situations allow fragmentation pathways for the ethyl lactate molecule in the ionization source and in the plasma, respectively. Among the information available, it is demonstrated that the signal at m/z 75 observed in the plasma ON spectra can unambiguously be directly correlated with the unfragmented precursor density in the plasma phase, enabling the ester density to be evaluated in the plasma phase. Comparing the ester density in the plasma with the one measured in the ELPFF highlights a strong correlation that is valid for both continuous and pulsed mode. We can indeed observe a two-step linear evolution with, first, a strong slope evolution in which the ester density in the plasma is low (high mean injected power) and, second, a lower slope corresponding to higher values of ester density (low mean injected power). This could be explained by different energetic conditions at the surface of the growing film, which could affect the sticking coefficient of ester-bearing fragments. A similar argument can

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Combining Mass Spectrometry Diagnostic and Density Functional

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