Nano-ordered thin films achieved by soft atmospheric plasma polymerization

Abstract

Plasma polymer thin films are of great interest in surface engineering in a wide range of applications. Herein, by using soft atmospheric plasma deposition parameters and by adapting these conditions to the used perfluorodecyl and dodecyl acrylates precursors, it is possible to get a high retention of monomer functionalities and a polymerization close to conventional methods. Molecular investigation revealed the presence of polymeric moieties and the mechanism of plasma polymerization has been mainly based on the polymerization by activation of the ethylenic groups. X-ray diffraction analyses have shown the presence of a smectic lamellar where the polyacrylate backbone was the amorphous phase and fluorinated and alkyl side chains were the hexagonal crystalline section. Wetting properties have been evaluated and finally showed hydrophobic surfaces

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Introduction

Polymers films with comb-like structures are of high interest in numerous applications and have been extensively studied both experimentally and theoretically due to specific structural, rheological or smart properties.^1,2 Significant developments regarding the radical polymerization control (RAFT, free radical polymerization, anionic-cationic polymerization,…) have extended the applications of such structures for encapsulation of actives molecules,³ optoelectronics,⁴ adhesion⁵ or anti-biofouling.⁶ The atom transfer radical polymerization (ATRP)^7,8 represents one of the most suitable synthesis methods to obtain comb-like polymers. This specific synthesis affords a well-controlled polymerization for a wide range of polymers or more extensive copolymers. This versatile method enables to obtain the desired polymeric architecture with well-defined molecular weights and low polydispersity. Comb-like polymer films synthesized by this polymerization method usually yielded a lamellar structure within a crystalline side chain region which is anchored on an amorphous region corresponding to the main backbone.^9,10

Recently, Coclite et al.¹¹ have demonstrated the efficiency of an Initiated Chemical Vapor Deposition (iCVD) technique to obtain comb-like polymers under vacuum, using a perfluorodecyl acrylate monomer to control the crystalline index and the crystallographic orientation of such a thin coating. A decrease in the crystalline index has been evidenced by increasing the (initiator)/(monomer) flow rate ratio, while the crystallographic orientation was influenced by precursors temperature. The crystallographic orientation is controlled with the temperature of the filament. In this paper, we describe an alternative method to design and functionalize surfaces consisting in different comb-like polymers from different monomers using a plasma process at atmospheric pressure. The promising results obtained by using glow¹² or homogeneous¹³ plasma discharge at atmospheric pressure led to a rising interest in such a deposition method.¹⁴ The syntheses of numerous chemically defined polymer thin films by using plasma processes are well-established.¹⁵ However, due to the excess of energy continuously delivered to the precursors during the plasma exposition, it appears difficult to induce selective chain scission or controlled plasma polymerization and to achieve complex structures mainly composed of short chains or branched structures with a high degree of cross linking.¹⁶ However in spite of this, plasma processes present many advantages such as: (i) a better adhesion of the coatings on various substrates due to the formation of activated species in the plasma or during the first steps of the deposition process (ii) an easy up-scaling to prepare thin coatings on large scale substrates without solvent¹⁷ (iii) the possibility to avoid the use of organic solvents and expensive pumping devices and finally, although Atmospheric Plasma Pressure (APP) exhibits few drawbacks, such as the gas and power consumptions involved in an open air configuration, these processes are attractive to cover a wide range of industrial topics. Moreover, it is well-known that working under atmospheric conditions^18,19 affords a soft plasma polymerization, by minimizing the energy transferred to the molecules. Then, it is reasonable to envisage to keep and maximize the chemical functionalities of the initial precursors.^20–22 This dominant trend represents the most promising way to access specific structural plasma polymer properties. More recently, interesting examples emphasized the potential of plasma polymers achieved at atmospheric pressure, adopting soft plasma polymerization conditions such as APPLD (atmospheric plasma pressure liquid deposition),²³ corona discharge³³ or helium glow discharge in DBD (dielectric barrier discharge) configuration.³¹

The aim of this work is to highlight that atmospheric plasma polymerization appears as an alternative way for the deposition of comb-like polymers. (1H,1H,2H,2H)-Perfluorodecyl acrylate and dodecyl acrylate have been chosen as monomer models to evidence the universal criterion of this concept. We expected to promote the polymerization reaction by the ethylenic groups from the monomers acrylate function by adapting the plasma parameters, leading finally to ordered structures. Fluorinated compounds are generally more stable than hydrocarbon species, which are more susceptible to dissociation or dehydrogenation processes. Indeed, the C–F bonds have a dissociation energy of about 500 kJ mol⁻¹ and represent one of the most stable bonds in plasma discharge and especially against the electronic impact compared to the C–H bond (410 kJ mol⁻¹) and C=C bond (370 kJ mol⁻¹).²⁴ Then, by using the appropriate plasma conditions, we showed that minimizing the fragmentation of monomers promotes polymerization only from the acrylate functions. Some groups already tried to use this family of monomers for the deposition of functional coatings and especially fluorinated compounds. The high retention of functional groups are mostly performed with the pulsed mode, however, these works have not evidenced the presence of comb-like or brush molecular structures^25,26 and not the formation of crystalline phases.

The main mechanism of the plasma polymerization, which occurred under the optimized soft plasma conditions, has been determined as based on the opening of the double bond from acrylate groups according to a multi techniques approach by using FTIR, MALDI-TOF/MS and solid state NMR. The presence of a smectic B crystalline phase in the side chains has been evidenced by X-Ray diffraction and Differential Scanning Calorimetry measurements, revealing the presence of isotropic transitions for both monomers. Finally, we highlighted that an atmospheric plasma process in specific adapted conditions represents a versatile and straightforward tool to obtain hydrophobic comb-like polymer thin films.

Materials and methods

(1H,1H,2H,2H)-Perfluorodecyl acrylate (PFDA) and dodecyl acrylate (DOCA) monomers, represented in Fig. 1, were deposited as received from Sigma Aldrich. The precursor used for the plasma polymerization consisted of monomer solution. A semi-dynamic Dielectric Barrier Discharge (DBD) plasma reactor at open atmospheric conditions described in a previous paper³⁴ has been used to deposit fluoro-acrylate plasma polymers. The DBD discharge was generated between an earthed bottom aluminium plate and two high-voltage aluminium top plates with a surface of 300 cm² separated by a boro-silicate glass dielectric. The gap between the electrodes was set to 2 mm. Silicon and glass substrates were positioned at the bottom electrode. The precursor was atomized in a TSI 3076 (from TSI Corp. Company) device at different pressures ranging from 0.5 to 4 bar. Helium gas has been introduced with a controlled flow meter with a gas flow of 10 slm. The gas mixture, containing the precursor aerosol and helium, was injected as nano-sized droplets into the plasma through slits between top electrodes.

Fig. 1 Schematic representation of a comb-like monomer (a) and chemical structure of (1H,1H,2H,2H)-perfluorodecyl acrylate (b) and dodecyl acrylate (c) monomers.

The deposition, in homogeneous plasma discharge conditions, was carried out at atmospheric pressure and at room temperature. Such coatings are adjustable on a wide range of substrates. During the deposition process for both monomers, plasma discharges were generated by an AC power supply. The frequency was set to 6 kHz and the voltage was a sinusoid of function of time. The thickness of the films was calculated from spectroscopic ellipsometry (AutoSE, Horiba scientific) measurements in the wavelength range between 450 and 1000 nm and at a constant angle of incidence of 70°. The ellipsometric angles ψ(λ) and Δ(λ) were used to calculate the film thickness and its refractive index using a double layer model in which the first layer was a semi infinite silicon substrate and the plasma film, modeled as an homogeneous and isotropic layer. The dispersion law of poly(methyl methacrylate) was used to fit n(λ) dispersion law of this last layer. The thickness and refractive index values given in the curves were the average (± one standard deviation) over 3 measurements performed along the major axis of the silicon wafer used to deposit the films. For convenience, the refractive index values will be given at λ = 632.8 nm, corresponding to the wavelength of the He–Ne laser.

X-ray photoelectron spectroscopy (XPS) analyses have been performed on a Hemispherical Energy Analyzer SPECS, PHOIBOS 150 with a monochromatic Al K radiation operating at 200 W with an anode voltage of 16 kV. The pressure in the analysis chamber was maintained at 10⁻⁹ mbar. XPS spectra were referenced with respect to the C1s peak at 284.6 eV originating from carbon contamination. Core peaks were analyzed using a nonlinear Shirley-type background and fitted using 70% Gaussian, 30% Lorentzian lineshapes.

Static contact angles (CA) were measured using Dataphysics OCA 15 + equipment, using a special borosilicate microtips with a exit diameter of 5 μm, purchased from WPI company, benefiting from a higher accuracy in the evaluation of the wetting properties. Wettability was evaluated by the sessile drop method at room temperature and constant relative humidity. Contact angles were determined with a mean of four measurements. Dynamic contact angles were determined according to the advancing/receding mode with high-purity deionized water.

Molecular characterization

The chemical and functional composition of the coatings was evaluated by FT-IR spectroscopy in transmission mode with a Bruker Optics Tensor 27 spectrometer. Substrates used for such analyses were silicon wafers. Matrix Assisted Laser Desorption-Ionisation coupled with a Time of Flight Mass Spectrometry (MALDI-ToF/MS) have been carried out on Autoflex III from Bruker company. Laser used is a Bruker Daltonics' smartbeam, working at a wavelength of 355 nm. After the deposition process, pp-PFDA and pp-DOCA films have been solubilised in tetrahydrofuran (THF) and 1,1,1,3,3,3-hexafluoro-2-propanol mixture (50 : 50) and THF respectively. Afterwards, a matrix has been used to protect plasma polymer moieties from laser energy and to preserve the original molecular structure. For pp-DOCA and pp-PFDA, the matrix used was the DCTB (trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile).

Thermal analyses have been performed on a Differential Scanning Calorimeter (DSC 204 F1) from Netzsch Company. Analyses have been performed under pure argon flow, used as protective gas. All the thermograms have been obtained under argon with a flow of 50 ml min⁻¹ after a heating and cooling step to avoid the materials’ thermal memory effect, then for thermal measurements, the heating rate was set to 10 K min⁻¹. Multiple cooling rates (1, 5, 10 and 20 K min⁻¹) have been applied to evaluate changes in the degree of cristallinity. XRD measurements have been achieved on a Panalytical instrument with a monochromatic Cu-Kα (λ = 1.54056 Å) radiation in reflection mode using an in situ heating stage from −60 °C to RT and from RT to 90 °C for the pp-DOCA and the pp-PFDA respectively.

Results and discussions

Plasma polymerization process at atmospheric pressure is mainly based on the free radical polymerization leading to tridimensional crosslinked structures. Under helium plasma conditions, all species could be affected by the Penning ionization phenomenon. Helium metastables possess high distribution energies centered at 19,82 eV and 20,61 eV for He (2³S) and He (2¹S) respectively,²⁷ which may ionized and activated the precursor’s molecules. The dissociation energy for common organic moieties is approximately 4–8 eV and the ionization process is evaluated generally higher than 9 eV. Several studies^20,28 highlighted the achievement of well-defined polyacrylic acid films by plasma polymerization at atmospheric pressure, when acrylic acid monomers were injected into the plasma as an aerosol. Authors demonstrated that monomer fragmentation is reduced, leading to higher deposition rate (few hundred nanometers per minute) and inducing at last stable functionalized plasma polymers. The induced reactions are controlled by the available energy transferred to the monomer molecules during the plasma exposure (W/F) and by the plasma discharge. The definition of the soft plasma polymerization regime generally concerns plasma carried out with low energy input but this definition can be extended and concerns up to now all plasma process conditions that affords a controlled polymerization with a selective activation of polymerization sites and then preserving other chemical functions of the monomers.^15,29 This particular regime can be monitored by the Yasuda factor (W/FM) and can be expressed more commonly in energy per monomer molecules or by plotting the deposition rate according to the W/F parameter for each precursor’s families (Fig. 2).³⁰ Two distinct regions, which represent different thin layer growth processes, could be observed (delimited by the dash line on Fig. 2). The domain between 0.025–0.05 kJ cm⁻³ g⁻¹ and 0.25–0.78 kJ cm⁻³ g⁻¹ for pp-DOCA and pp-PFDA respectively corresponded to the plasma soft plasma conditions. In these particular conditions the maximum deposition rate corresponded to 40 nm min⁻¹. In both cases, an important decrease in the deposition rate has been observed for W/F parameter higher than 0.05 kJ cm⁻³ g⁻¹ and 0.78 kJ cm⁻³ g⁻¹ for pp-DOCA and pp-PFDA respectively, due to competition reactions between the plasma polymerization and etching processes. This domain, associated to higher fragmentation conditions of the molecules, leads to an important loss of chemical groups from the initial monomers. This observation can be associated with the decrease of the soluble part (Fig. S1 in ESI†). In this energy range, the dissociation process of monomer molecules is predominant and the degree of crosslinking is obviously high.

Fig. 2 Deposition rate plots according to W/F parameter for (a) plasma polymer perfluorodecyl acrylate (pp-PFDA) and (b) plasma polymer dodecyl acrylate (pp-DOCA).

In this work, efforts have been also focused on the investigation of the plasma polymerization mechanisms occurring with model monomers, which exhibit acrylate functions associated with alkyl and fluorinate chain. The power discharge and the monomer flow rate have been optimized to keep the structural composition of the monomer. Fig. 3 represents the FTIR spectra of monomers and plasma polymer coatings deposited in soft plasma conditions. In both cases, spectrum revealed a high retention of monomers’ structures. Indeed, the fluorinate (between 1240–1040 cm⁻¹) and alkyl chains (1458, 1378 and 1163 cm⁻¹) are preserved during the polymerization plasma. Moreover, an extinction of the band intensity between 1645 cm⁻¹ and 1620 cm⁻¹, has been observed for both plasma polymers, in favor of a complete disappearance of the double bond from acrylate groups. On the other hand, carbonyl and alkyl groups are not affected by the plasma process. In this frame, additional analytical characterizations (¹³C solid state NMR and mass spectrometry) have been performed to obtain molecular information regarding the plasma polymers (Fig. S2 in ESI†).

Fig. 3 FTIR spectrum of pp-PFDA and pp-DOCA in soft plasma conditions.

In this context, the MALDI-ToF/MS technique is an efficient method and is well adapted to investigate the relative molecular weight and the different species present in such plasma polymer films. Solubility tests have been performed according to the plasma parameters in HFIP/THF mixture and CHCl₃ for pp-PFDA and pp-DOCA respectively (Fig. S1 in ESI†). In both cases, a decrease of the soluble part has been observed by increasing the W/F parameter, revealing an increase of the degree of crosslinking in the film. Mass spectra of the pp-PFDA and pp-DOCA coatings, prepared under optimized soft plasma polymerization are represented on Fig. 4. In these specific plasma conditions, the soluble part has been estimated at 75 wt%. These analyses displayed series of peaks spaced by 518 Da and 240 Da, which corresponds to the PFDA and DOCA monomers mass respectively. Such results confirmed a predominant plasma polymerization by the C=C double bonds from the acrylate groups, disclosing a plasma polymer structure with a total of more than 12 and 20 monomers repetition units for pp-DOCA and pp-PFDA respectively, indicating a degree of polymerization which is of the same order for both monomers. These MS analyses provided a detection of distribution m/z values up to 12 000 Da centered at 4703 Da for the pp-PFDA and 1000 Da for the pp-DOCA. This is indicating a blend of oligomeric pp-PFDA and pp-DOCA. Moreover, MALDI-ToF/MS analyses distinguished different distributions with different chains and different ends groups on the polyacrylate backbone (Fig. S3 and S4 in ESI†). For pp-PFDA the wide distribution highlighted a low polydispersity index (PI) on detected species. This result was unpredictable because it is well known that plasma polymerization involves random growth processes. The mechanism of plasma polymerization was based by the activation of the ethylenic groups. These results suggested that soft plasma polymerization conditions might be in the suitable energy range to activate preferentially the C=C double bonds and to preserve the rest of the monomer structure, leading to an oligomeric with a polyacrylate backbone associated to selected side-chains according to the chosen monomers.

Fig. 4 MALDI ToF/MS spectrum of pp-PFDA (a) and pp-DOCA (b) obtain in soft condition.

Polymers synthesized from PFDA and DOCA monomers by conventional radical polymerization present semi-crystalline structures by their side chains despite of an amorphous backbone. In most cases, a bi-lamellar structure has been highlighted where the side chains might be interpenetrated or not. In this frame X-ray diffraction and DSC measurements have been performed on pp-DOCA and pp-PFDA coating respectively deposited at 0.02 kJ cm⁻³ g⁻¹ and 0.3 kJ cm⁻³ g⁻¹. Fig. 5 showed the DSC thermograms of pp-PFDA and pp-DOCA. Results reveal a reversible transition characteristic to the melting and crystallization phenomena. Melting temperature is estimated at 69 °C with an enthalpy of 8 J g⁻¹ for pp-PFDA and at 19 °C with an enthalpy of 23 J g⁻¹ for pp-DOCA. As mentioned by several authors, the thermal transition is due to the side chains. Temperatures values of thermal transition obtain on plasma polymer are similar to the polymer synthesized by conventional routes.^10,31,32 Calorimetric measurements on polymer plasma have been performed with different W/F ratio. Results evidenced a decrease of the enthalpy due to a decrease of the crystallite domains. Indeed the domain in heavy fragmentation induces a high dissociation of monomers molecules promote to a high cross-linking of the films.

Fig. 5 DSC thermogramms of pp-PFDA (a) and pp-DOCA (b).

In situ temperature dependence diffraction measurements have been performed on polymer plasma coatings to evaluate the crystallization behavior. Diffraction pattern as shown on Fig. 6a of pp-PFDA coatings exhibits three peaks in the small angle region and assignable to the lamellar structure, precisely at 2θ = 2.6° and 2θ = 7.9° (Fig. S5 in ESI†), corresponding to a periodicity of 34.2 Å. The main peak at 2θ = 17.8° corresponds to the (100) plane hexagonal structure of fluorinated side chains packing. It is superimposed on an amorphous contribution, which are due to the cross linked part and the amorphous domain. The presence of such sharp peaks confirmed the existence of lamellar structures. The fluorinated side groups correspond to the hexagonal crystalline structure with an intermolecular distance of 5.7 Å. This type of organization, widely observed for film manufactured in conventional methods (considered as lamellar and organized along layers’ planes) was assimilated to a smectic B organization.^32,33 By applying the Debye-Scherrer equation, the crystalline domains sizes have been assessed at 130 Å. The crystalline index has been determined by the evaluating the ratio of the area of the crystalline peaks to the total area under the scattering curve. In a manner “soft” plasma polymerization the crystalline index is at 0.6, which is slight lower than value obtained by iCVD.¹¹Fig. 6. B summarizes the evolution of cell parameter according to the temperature. A sharp decrease of cell parameters has been observed at 75 °C for pp-PFDA films. This is in accordance with the thermal transition previously evidenced by DSC measurements, corresponding to the melting of the side chains crystallites. Moreover, a simultaneous decrease of the intensities of the lamellar and crystalline peak is observed at the same temperature. A schematic structure is given in the Fig. 7 where the smectic phases are embedded into the small cross linked parts of the film. The presence of such structures has a strong influence of the wetting performance and especially on the hysteresis comportment.³⁴

Fig. 6 In situ temperature dependence X-RD measurement (a) and evolution of structural properties according to the temperature (b) of pp-DOCA.

Fig. 7 Schematic representation of the smectic structure obtain in pp-PFDA coatings.

Crystallization behavior of pp-DOCA according to the temperature determined by X-RD is gathered in the Fig. 8. X-RD analyses at room temperature evidenced an amorphous structure. The formation of a crystalline structure is observed when the temperature decreases at 5°. This diffraction peak is related to the crystalline phase from the alkyl chains. As observed on pp-PFDA measurement, this peak corresponds to the alkyl side chains with a hexagonal structure. The distance between alkyl side chain is of 4.6 Å with a size domain at 80 Å. At −10 °C a second crystalline peak appears in the wide angle region at 2θ = 24°. The distance between the neighboring alkyl chain was calculated as 3.7 Å. The reduction of the distance between the alkyl chains leads to an orthorhombic packing of alkyl chains.^35–37 The thermal transition is in accordance with the value previously found by DSC measurements and corresponding to the melting of the side chains crystallites. However, thanks to X-RD measurements it was possible to distinguish two structural transitions according to the temperature. A schematic representation of crystalline pp-DOCA structure is given in Fig. 9. The crystallites parts, mainly resulted in the side chains, crystallized in hexagonal then in lower energy β-orthorhombic structures. The crystalline index calculated for plasma polymer with alkyl side chain is 0.2, which is low compared to the common alkyl side-chain polymers obtained by conventional synthesized process.³⁸ This value is mainly due to a shortage of the longer alkyl side chains. Several investigations have been devoted to the crystallization of the structure with alkyl side chains as a function of number of carbon atoms. Usually the alkyl side chain crystallizes with a minimum of 12–14 carbon atoms in the side chain depending to the backbone.^9,39 The thermal behavior of pp-DOCA is slightly different with the pp-PFDA films especially with the absence of lamellar structures which is due to a short alkyl side chain.⁴⁰ Moreover, the alkyl groups are more subject by the dissociation or dehydrogenation process leads to an increase of the cross linking.

Fig. 8 In situ temperature dependence X-RD measurement (a) and evolution of structural properties according to the temperature (b) of pp-DOCA

Fig. 9 Schematic representation of the crystalline phase obtained in pp-DOCA film.

Cicala et al.⁴¹ have also observed a hexagonal crystalline structure in fluorinate-based plasma polymer manufactured by pulsed plasma process at low pressure with a low duty cycle. Zang et al.⁴² have deposited successfully a crystalline structure deposited at atmospheric pressure plasma process. In both cases, the perfluorohexane (C₆F₁₄) precursor has been used to obtain single-crystalline PTFE films due to high preservation of –CF₂ groups during the plasma phase. In our work, the crystalline structures are obtained thanks to the polymerization of acrylate groups that lead to the growth of oligomeric species. The microstructure of the polymer plasma is composed of a crystalline part linked to a cross linked amorphous network.

These results on structural properties in the plasma polymer films highlighted similar structural behavior between polymer elaborated by conventional polymerization process.^43,44 However, the crystalline index and the crystalline domains’ size in both cases are smaller than those obtained on conventional polymers. Plasma polymers are composed of a blend of amorphous structures and small linear chains. The oligomers chain growth is based on polyaddition mechanisms. We hypothesize that the initiation of the doubles is mainly induced by electron impacts of and metastable species with monomers molecules. These reactions induce radical and ionic species, leading to the propagation of linear chains. At atmospheric pressure, the ionic species’ mean free path is so small that the energy impact is reduced, rendering their effects neglected. In future works, the analysis of the gas phase will be an asset to determine the reactions between excited species in the plasma and monomers acrylate groups.

Finally, surface properties have been evaluated by XPS and contact angle measurements. Fig. 10 shows the high resolution C_1s peak. The chemical composition of the different coatings is given in Table 1. In the case of pp-PFDA, peaks have been assigned to seven carbon moieties corresponding to, C_xH_y at 284.9 eV, CC(O)=O at 285.7 eV, CO/CCF_n at 286.7 eV, CF at 287.7 eV, OC=O/CFCF_n at 289.1 eV, CF₂ at 291.3 eV and CF₃ at 293.2 eV.^45,46 The presence of –CF components indicates a partial fragmentation of fluorinate chains. However, spectra revealed a quite good conservation of CF₂ and CF₃ moieties, and also the acrylate groups. The F/C ratio of pp-PFDA was very close to the theoretical value of conventional polymer. For pp-DOCA coatings the spectra revealed the different components of the poly(dodecyl acrylate). Quantitative analyses showed carbon and oxygen contents close to those from the conventional polymer too. The surface of pp-DOCA evidenced more oxidized species, which were due to reaction with oxygen during and after the plasma deposition. These results highlighted an important conservation of the fluorinate and alkyl side chains, which are conform to a preferential activation of ethylene groups by species from the plasma discharge. To consolidate theses results, water contact angle have been done at room temperature. Results evidenced an advancing/receding contact angle measurements of 124°/120° and 88°/85° for pp-PFDA and pp-DOCA respectively. Wettability properties are very close to those obtained on polymers deposited by conventional methods.

Fig. 10 C 1s XPS spectrum of pp-PFDA and pp-DOCA deposited under soft-condition after 20 passes.

Table 1 Chemical composition determined by XPS measurements

	C	O	F	F/C	O/C
PFDA (theoretical)	40.6	6.3	53.1	1.3	0.15
pp-PFDA	36	5.6	58.4	1.2	0.15
DOCA (theoretical)	88.2	11.8			0.13
pp-DOCA	85.3	14.7			0.17

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Conclusion

In summary, our studies have demonstrated that plasma polymerization at atmospheric pressure is a versatile and straightforward method to hydrophobic syntheses with a low hysteresis thin films with a comb-like structure. Molecular studies highlighted that the plasma polymerization can be controlled. In soft deposition conditions, the growth of coatings is mainly based on the opening double bond from the acrylate groups. DSC and X-RD results revealed the presence of a bi-lamellar smectic B phase with an isotropic transition. This new generation of plasma polymer coatings presents a modular crystalline rate, inducing a thermal transition at low temperature and exhibiting structural properties that can finally lead to new applications for plasma polymers such as nonlinear optical devices, biomedical, energy application, wettability properties or smart surfaces. In further work, our efforts will be focused on the influence of such switchable or stimuli-responsive attributes on the wettability and rheological properties. Moreover, co-plasma polymerization of comb-like monomers will be an attractive asset to extend the properties of plasma polymer.

Acknowledgements

The authors would like to thank the Luxembourg funding organization ‘‘Fonds National de la Recherche’’ (FNR) for financial support through the FlexProtect project and the grant AFR PHD-08-047 (Julien Petersen).

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Footnote

† Electronic Supplementary Information (ESI) available. See DOI: 10.1039/c2ra21833j

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