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Synthesis and Properties of Plasma-Polymerized Methyl Methacrylate Via the Atmospheric Pressure Plasma Polymerization Technique

Synthesis and Properties of Plasma-Polymerized Methyl Methacrylate Via the Atmospheric Pressure Plasma Polymerization Technique

Article Synthesis and Properties of -Polymerized Methyl Methacrylate via the Atmospheric Plasma Technique

Choon-Sang Park 1, Eun Young Jung 1, Hyo Jun Jang 1, Gyu Tae Bae 1, Bhum Jae Shin 2 and Heung-Sik Tae 1,* 1 School of Electronics Engineering, College of IT Engineering, Kyungpook National University, Daegu 41566, Korea; [email protected] (C.-S.P.); [email protected] (E.Y.J.); [email protected] (H.J.J.); [email protected] (G.T.B.) 2 Department of Electronics Engineering, Sejong University, Seoul 05006, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-53-950-6563

 Received: 17 December 2018; Accepted: 22 February 2019; Published: 28 February 2019 

Abstract: Pinhole free layers are needed in order to prevent and water from damaging flexible electrical and bio-devices. Although polymerized methyl methacrylate (polymethyl methacrylate, PMMA) for the pinhole free layer has been studied extensively in the past, little work has been done on synthesizing films of this material using atmospheric pressure plasma-assisted electro-polymerization. Herein, we report the synthesis and properties of plasma-PMMA (pPMMA) synthesized using the atmospheric pressure plasma-assisted electro-polymerization technique at room temperature. According to the Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and time of flight-secondary mass spectrometry (ToF-SIMS) results, the characteristic peaks from the pPMMA chain were shown to have been detected. The results indicate that the percentage of hydrophobic groups (C–C and C–H) is greater than that of hydrophilic groups (C–O and O–C=O). The field emission-scanning microscope (FE-SEM) and thickness measurement results show that the surface morphology is quite homogenous and amorphous in nature, and the newly proposed pPMMA film at a thickness of 1.5 µm has high transmittance (about 93%) characteristics. In addition, the results of water contact angle tests show that pPMMA thin films can improve the hydrophobicity.

Keywords: methyl methacrylate; encapsulation; atmospheric pressure plasma; plasma polymerization; plasma-polymerized methyl methacrylate (pPMMA); time of flight-secondary ion mass spectrometry (ToF-SIMS); hydrophobicity

1. Introduction Plasma-assisted electro-polymerization is a new research field on the interaction of plasma with and electrolyte solution, and it uses plasma to drive the chemical reactions [1–6]. Electrochemically controlled formation of thin polymer films using plasma is one of the feasible routes to design new functional polymer materials. The creation of new polymer materials would require novel technological developments. Recently, there have been a number of interesting reports concerning an aerosol-through-plasma (ATP) technique for generating particulate materials [7–14]. In brief, this technology consists of passing aerosols through various plasma systems, particularly radio frequency (RF) and microwave, to create particulates, nanoparticles, and thin films. Among various plasma systems, by using a nonthermal atmospheric pressure plasma (APP) as a gaseous electrode, plasma-assisted electro-polymerization has attracted increasing attention in the green synthesis of polymer thin films and nanomaterials. Thanks to the low-temperature and cost-effective dry process,

Polymers 2019, 11, 396; doi:10.3390/polym11030396 www.mdpi.com/journal/polymers Polymers 2019, 11, 396 2 of 12 the APP polymerization process is a promising growth method for synthesizing polymer thin films with functional features, therefore being well-suited for versatile substrates and applications. The APP polymerization process can obtain high-quality polymer films and nanoparticles, with easily controllable deposition rate, size, and is well-suited for a wide range of substrates. Furthermore, APP reactors do not require a vacuum atmosphere and special equipment. In spite of intensive studies, only a few groups have reported successful plasma polymerization using APP [15–25]. Our previous works recently reported the stationary APP jets (APPJs) polymerization employing the new types of impinging technique and ATP system [26–30]. When discharge is initiated, the flowing inert argon can inject and into the vaporized monomer bubbles. This impinging technique and ATP system increase the charged particles and density by means of plasmas in the monomer fragmentation (or nucleation) and recombination regions. In this case, the polymer films can have different properties depending on the types of precursors and plasma conditions. Consequently, the advanced plasma-based process for synthesizing new materials including polymers requires a deeper understanding of plasma-assisted electro-polymerization. Recently, polymerized methyl methacrylate (polymethyl methacrylate, PMMA) having pinhole free characteristics has been widely used as organic thin films, encapsulations, biosensors, and electronics materials [21–25,31–36]. The plasma of porous conducting polymers, such as aniline, pyrrole, and , have been successfully implemented using our APP polymerization technique [26–30]. However, there is no report on the synthesis of plasma-PMMA (pPMMA) with pinhole free and nonporous characteristics as a dielectric polymer through plasma-assisted electro-polymerization with the novel APP polymerization technique. Accordingly, this study uses Fourier transform-infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), time of flight-secondary ion mass spectrometry (ToF-SIMS), field emission-scanning electron microscopy (FE-SEM), and transmittance measurement to analyze newly proposed pPMMA film synthesized by APPJs polymerization. The resultant surface wettability is also investigated.

2. Experimental and Characterizations

2.1. Atmospheric Pressure Plasma Synthesis and Materials A stationary atmospheric pressure plasma (APP) polymerization technique by using liquid monomer was conducted by an APP jets (APPJs) and aerosol-through-plasma system mentioned in our previous work [26]. In the previous work, a newly proposed guide tube and a bluff body with impinging jet systems were introduced to suppress the quenching phenomenon from ambient air, thereby increasing the charged particles and plasma energies in the nucleation area [27]. In the case of conventional APPJs without the proposed guide tube and buff body, the plasma was only produced within the area of the three array jets because of the directional characteristics of the streamer-like discharges. In the case of the newly proposed guide tube and bluff body with impinging jet systems, the plasma produced in the impinging region was changed to a broadened glow-like discharge, thereby increasing the plasma region about 60-fold and the deposition area [27,29]. The proposed APPJs with impinging technique and ATP system can produce a broadened and intense plasma discharge with large area deposition and obtain high-quality polymers, with easily controllable morphology, deposition rate, deposition size, and is well-suited for a wide range of substrates. The detailed APP polymerization system, featuring the guide tube and bluff body, employed in this work, was described in the references of [26–30]. In the case of a low gas flow rate below 2500 standard cubic centimeters per minute (sccm) for the main discharge gas and below 300 sccm for vaporizing or carrier monomer gas flow rates, intense or stable or broadened plasma was not produced even though the proposed guide tube and bluff body with impinging jet systems was used [29]. In this experiment, therefore, the argon (Ar) gas was the main plasma discharge gas where its purity was high (99.999%) and its flow rate was fixed at 2500 sccm. Further Ar gas was supplied to the bubbler for vaporizing the liquid −1 methyl methacrylate (MMA) monomer (Sigma-Aldrich Co., St. Louis, MO, USA, Mw = 100 g·mol ) Polymers 2019, 11, 396 3 of 12 at a fixed flow rate of 300 sccm. For plasma generation, a sinusoidal voltage with a peak value of 12 kV and a frequency of 30 kHz were fixed and applied to the proposed APP polymerization device. The used substrates were and terephthalates (PETs). The pPMMA was directly deposited on substrates at room temperature by the novel APP polymerization technique. During the APP polymerization experiments, the plasma jet was not moved, and the as such experiments were conducted in stationary deposition.

2.2. Fourier Transform-Infrared Spectroscopy Analysis The Fourier transform infrared spectra (FT-IR) were used to determine the chemical changes of pPMMA introduced by the APP and measured on a Perkin-Elmer Frontier spectrometer (PerkinElmer, Waltham, MA, USA) between 650 and 4000 cm−1.

2.3. X-ray Photoelectron Spectroscopy Analysis An X-ray photoelectron spectroscopy (XPS) (ESCALAB 250XI surface analysis system, Thermo Fisher Scientific, Waltham, MA, USA) was used to investigate the surface chemical compositions and atomic concentration of the pPMMA films. In the XPS measurement, the voltage and current of the monochromatic Al Kα X-ray source (hv = 1486.7 eV) were 15 kV and 20 mA, respectively. The measurement angle was 60◦ and the measurement depth estimated to range from 8 to 10 nm. The measurement area was 500 µm × 500 µm and the pressure was about 10−8 Pa. The C 1s spectrum (285.0 eV) was used to calibrate the energy scale. Elements present on the deposited surfaces were identified from XPS survey scans and quantified with Thermo Avantage software (v.5.977, Waltham, MA, USA) using a Shirley background and applying the relative sensitivity factors provided by the manufacturer of the instrument. The relative sensitivity factors of C 1s and O 1s were 1.0 and 2.8, respectively. For high-resolution spectra, the constant analyzer energy modes were used at 200 eV for the survey scan and 50 eV pass energy for the element scan, respectively. Since the pPMMA samples and substrates were insulators, we used an additional electron gun to allow for surface neutralization to adjust the charge compensation during the measurements. To curve fit the high-resolution C 1s and O 1s peaks, the deconvolution of C 1s and O 1s peaks was analyzed by the Thermo Avantage software. The peaks were deconvoluted using Gaussian–Lorentzian peak shapes (constrained between 80% and 100% Gaussian) and the full-width at half maximum (FWHM) of each line shape was constrained between 2.0 and 3.0 eV.

2.4. Time of Flight-Secondary Ion Mass Spectrometry Analysis The surface structure and composition of the pPMMA films were examined by the time of flight-secondary ion mass spectrometry (ToF-SIMS) V instrument (ION-TOF GmbH, Munster, + Germany) with a bismuth primary-ion (Bi3 ) gun source. The pressure in the ToF-SIMS chamber was −9 + maintained at less than 1 × 10 Torr. Bi3 (0.5 pA) accelerated at 30 keV was used as the analysis (primary) gun. The negative-ion and positive-ion mass spectra of a 500 µm × 500 µm area were + acquired at a Bi3 primary-ion beam of 30 keV. For a more accurate mass scale of pPMMA film, the − − spectra were calibrated based on the characteristic pPMMA ion, C4H5O2 including the known CH − and C4H ions.

2.5. Surface Morphology Study The surface morphology of the pPMMA films on the glass substrates was characterized using a field emission-scanning electron microscope (FE-SEM) (Hitachi SU8220, Hitachi, Tokyo, Japan) with accelerating voltage and current of 3 or 5 kV and 10 mA, respectively. The samples for FE-SEM were conductive platinum-coated before loading into the chamber. Polymers 2019, 11, 396 4 of 12

2.6. Thickness Measurement Polymers 2018, 10, x FOR PEER REVIEW 4 of 12 The thickness of pPMMA was determined using a KLA Tencor P-7 stylus profilometer (Milpitas, 2.7.CA, Optical USA) with Transmittance a diamond probe having a radius of 2 µm. 2.7. OpticalThe optical Transmittance transmittance (or transmission) of pPMMA film was performed on a glass substrate followed by measurement of its transmission by using of UV–Vis spectrophotometer (Cary 5G, The optical transmittance (or transmission) of pPMMA film was performed on a glass substrate Varian). The spectrum ranged from 300 to 800 nm with a scanning step of 0.5 nm. Glass and PET followed by measurement of its transmission by using of UV–Vis spectrophotometer (Cary 5G, Varian). plates were used as reference. The pPMMA samples were used for measurement of optical The spectrum ranged from 300 to 800 nm with a scanning step of 0.5 nm. Glass and PET plates were transmittance and the results were expressed as a mean value. used as reference. The pPMMA samples were used for measurement of optical transmittance and the results were expressed as a mean value. 2.8. Water Contact Angle Measurements 2.8. WaterThe sessile Contact drop Angle method Measurements using Drop Shape Analyzers (KRUSS, DSA100, Germany) was used to measureThe sessilethe surface drop wettability method using of the Drop pPMMA Shape Analyzersfilms grown (KRUSS, on the DSA100,glass and Germany) PET substrates. was used After to themeasure ultrapure thesurface water droplet wettability contacted of the the pPMMA sample films surfaces, grown the on contact the glass angles and were PET substrates.measured by After the built-inthe ultrapure charge-coupled water droplet device contacted (CCD) the sample video surfaces, camera the with contact the anglessoftware were provided measured by by the manufacturer.built-in charge-coupled The average device value (CCD) of the video contact camera angle with was the taken software after provided five measurements by the manufacturer. for each Thesample. average value of the contact angle was taken after five measurements for each sample.

3. Results and Discussion The FT-IR absorption spectrum of the newly proposed pPMMA film film grown on glass substrate when using the APP polymerization technique after 90 min deposition is shown in Figure1 1.. BandsBands from 3000 toto 36003600 cmcm−−11 ofof the the OH OH bonding bonding group, group, implying implying oxidation oxidation of of the particles induced by quenching from ambient air, were not observed thanks to using the newly proposed guide tube and bluff bodybody systems.systems. The The FT-IR FT-IR spectra spectra of theof the pPMMA pPMMA polymer polymer were were found found to exhibit to exhibit absorption absorption bands −1 −1 3 bandsat 2989 at and 2989 2951 and cm 2951. Thesecm . These bands bands were causedwere caused by –CH by3 asymmetric–CH asymmetric stretching. stretching. A sharp A sharp band bandlocated located at 1731 at 1731 cm− 1cmwas−1 was ascribed ascribed to theto the carbonyl carbonyl group. group. The The pPMMA pPMMA showed showed anan IRIR absorption −1−1 3 band at 1433 cm duedue to to the the asymmetric asymmetric bending bending vibration vibration (CH (CH) 3of) ofthe the methyl methyl group. group. The The peak peak at −1 −1 3 1381at 1381 cm cm waswas attributed attributed to OCH to OCH deformation3 deformation of pPMMA. of pPMMA. The The characteristic characteristic absorption absorption bands bands at 1265at 1265 and and 1145 1145 cm cm−1 − respectively1 respectively correspond correspond to to C–O–C C–O–C stretching stretching and and the the C–O C–O group group of of pristine −−11 3 PMMA polymer. The bandband atat 11931193 cmcm waswas due due to to –OCH 3 stretching.stretching. The The absorption absorption bands bands at 977 −−11 2 and 716 716 cm cm were were assigned assigned to to the the CH CH2 wagging wagging and androcking rocking modes modes of pPMMA, of pPMMA, respectively respectively [37–40]. [36–39].

Figure 1. Fourier transform infrared spectroscopy (FT-IR) spectra of plasma-polymerized methyl Figure 1. Fourier transform infrared spectroscopy (FT-IR) spectra of plasma-polymerized methyl methacrylate (pPMMA) thin films grown on a glass substrate when using the proposed atmospheric methacrylate (pPMMA) thin films grown on a glass substrate when using the proposed atmospheric pressure plasma (APP) polymerization technique after 90 min deposition. pressure plasma (APP) polymerization technique after 90 min deposition. The XPS survey spectra, with elemental composition percent (Figure2a, insets) of the atomic The XPS survey spectra, with elemental composition percent (Figure 2a, insets) of the atomic distribution and detailed high-resolutions of C 1s and O 1s spectra in the newly proposed pPMMA distribution and detailed high-resolutions of C 1s and O 1s spectra in the newly proposed pPMMA thin films grown on glass substrate when using the APP polymerization technique after 90 min deposition, are shown in Figure 2. The element content was calculated by the ratio of the corresponding integral area of the peaks. The XPS survey spectra in Figure 2a are typically observed in the signals corresponding to C 1s (285.0 eV) and O 1s (531.0 eV) electronic orbitals of pPMMA

Polymers 2019, 11, 396 5 of 12 thin films grown on glass substrate when using the APP polymerization technique after 90 min deposition, are shown in Figure2. The element content was calculated by the ratio of the corresponding integral area of the peaks. The XPS survey spectra in Figure2a are typically observed in the signals corresponding to C 1s (285.0 eV) and O 1s (531.0 eV) electronic orbitals of pPMMA [41,42]. The C and O atoms belong to the MMA monomer structure. To get further insight into the types and ratios of the surfacePolymers functional 2018, 10 groups, x FOR PEER ofthe REVIEW pPMMA, the curve fitting of the high-resolution C 1s and 5 of O 12 1s peaks was analyzed by the XPS peak. The assignments of the fitted components and the compositions of [40,41]. The C and O atoms belong to the MMA monomer structure. To get further insight into the the C 1stypes and O and 1s ratioslevels of are the summarized surface functional in Table groups1. The of C 1s the peaks pPMMA, of pPMMA the curve were fitting decomposed of the by curve fittinghigh-resolution into three C components;1s and O 1s peaks the componentwas analyzed at by 285.0 the XPS eV correspondingpeak. The assignments to C–C of andthe fitted C–H bonds, the componentcomponents at 286.8 and the eV compositions corresponding of the to C C–O 1s and bands, O 1s andlevels the are component summarized at in 289.1Table eV1. The corresponding C 1s to O–C=Opeaks bonds. of pPMMA The were O 1s decomposed peaks of pPMMAby curve fitting can into be dividedthree components; into two the component component at peaks, 285.0 which respectivelyeV corresponding indicate two to oxygen-containingC–C and C–H bonds, the groups; component C=O at (531.0 286.8 eV)eV corresponding and C–O (533.2 to C–O eV). bands, After fitting and the component at 289.1 eV corresponding to O–C=O bonds. The O 1s peaks of pPMMA can be the relative intensities of the two peaks, the peak corresponding to the C=O group was shown to be divided into two component peaks, which respectively indicate two oxygen-containing groups; C=O higher than(531.0 the eV) one and of C–O the C–O (533.2 group eV). After in Figure fitting2 c. the Comparing relative intensities the C 1s of spectra, the two the peaks, relative the peak intensities of the C–Ocorresponding and O–C=O to the groups C=O group were was lower, shown while to be thehigher relative than the intensities one of the C–O of the group C–C in and Figure C–H 2c. groups in the sameComparing spectra the wereC 1s spectra, higher the in relative Figure intensities2b. The of high the C–O peak and could O–C=O be groups attributed were lower, to cross-linking while reactionsthe and relative the formationintensities of of the new C–C C–C and bonds.C–H groups The in values the same in Tablespectra1 weredenote higher the in ratio Figure of carbon2b. The atoms high peak could be attributed to cross-linking reactions and the formation of new C–C bonds. The in the hydrophobic groups (C–C and C–H) to those in the hydrophilic groups (C–O and O–C=O) on values in Table 1 denote the ratio of carbon atoms in the hydrophobic groups (C–C and C–H) to the surface.those The in the results hydrophilic indicate groups that (C–O the andpercentage O–C=O) ofon hydrophobic the surface. The groups results is indicate greater that than the that of hydrophilicpercentage groups. of hydrophobic This means groups that ouris greater pPMMA than that surface of hydrophilic becomes groups. carbon-rich This means and eventuallythat our it is expectedpPMMA to increase surface the becomes water contactcarbon-rich angles and (WCAs)eventually after it is pPMMAexpected to increase [ 41the,42 water]. contact angles (WCAs) after pPMMA coating [40,41].

(a)

(b) (c)

Figure 2.Figure(a) X-ray2. (a) X-ray photoelectron photoelectron spectroscopy spectroscopy (XPS)(XPS) survey survey spectra, spectra, detailed detailed high-resolution high-resolution with with deconvolutionsdeconvolutions of (b) of C 1s,(b) andC 1s, (andc) O (c 1s) O spectra 1s spectra of pPMMA of pPMMA thin thin films films grown grown onon aa glassglass substrate substrate when when using the proposed APP polymerization technique after 90 min deposition. Insets in (a) using the proposed APP polymerization technique after 90 min deposition. Insets in (a) represent the represent the atom percent in the pPMMA film. atom percent in the pPMMA film.

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Table 1. The contents of functional groups of C 1s and O 1s core level spectra of plasma-polymerized Table 1. The contents of functional groups of C 1s and O 1s core level spectra of plasma-polymerized methyl methacrylate (pPMMA) film observed in the XPS spectra of Figure 2b,c. methyl methacrylate (pPMMA) film observed in the XPS spectra of Figure2b,c.

ConcentrationsConcentrations of of correlative Correlative Functional functional Group group (%) (%) Sample C 1sC 1s O 1s O 1s Sample C–C/C–H (285.0C–C/C–H eV) C–O (286.8C–O eV) O–C=OO–C=O (289.1 eV)C=O C=O (531.0C–O eV) C–O (533.2 eV) pPMMA 54.9(285.0 eV) (286.8 29.4 eV) (289.1 15.7eV) (531.0 eV) 68.9(533.2 eV) 31.1 pPMMA 54.9 29.4 15.7 68.9 31.1 The pPMMA films were characterized in both negative- and positive-ion modes using the ToF-SIMSThe pPMMA method in films order were to determine characterized the specific in both structures. negative- and The positive-ion negative-ion modes and positive-ion using the ToF-SIMSspectra (0–200 method amu) in of order ToF-SIMS to determine on the surface the specific of the newlystructures. proposed The negative-ion pPMMA thin and films positive-ion grown on spectraa glass substrate(0–200 amu) when of ToF-SIMS using the on proposed the surface APP of polymerization the newly proposed technique pPMMA after thin 90 minfilms deposition grown on aare glass shown substrate in Figure when3. Asusing shown the inproposed Figure3 aAPP using polymerization negative-ion mode,technique the ionsafter at90 m/zmin= deposition 31, 55, 71, − − − − − − are85, 141,shown and in 185 Figure were 3. assigned As shown to CHin 3FigureO ,C 33aH 3usingO ,C negative-ion3H3O2 ,C4H mode,5O2 ,C the8H ions13O 2at ,m/z and = C 31,9H 55,13O 71,4 , 85,respectively 141, and [ 43 185–46 ]. were These assigned fragment to ions CH arose3O−, from C3H3O the−, pPMMA C3H3O2−, chains. C4H5O The2−, C most8H13O abundant2−, and C fragment9H13O4−, − − respectivelyions were CH [42–45].3O and These C4H 5 fragmentO2 , which ions were arose the from monomer the pPMMA where the chains. methyl The group most (CH abundant3) was − − − fragmentremoved fromions were the ether CH3 linkageO and (HC4H3C–O–C)5O2 , which [44,45 were]. The the small monomer negative where hydroxyl the methyl ion OH group(m/z (CH= 17)3) wasimplying removed the oxidation from the ofether the linkage pPMMA (H thin3C–O–C) films induced [43,44]. byThe quenching small negative from ambienthydroxyl air ion was OH observed− (m/z = 17)thanks implying to using the the oxidation proposed of the guide pPMMA tube and thin bluff films body induced with impingingby quenching jet systems.from ambient As shown air was in observedFigure3b usingthanks the to positive-ionusing the proposed mode, some guide characteristic tube and bluff peaks body from with the impinging pPMMA polymerjet systems. chain As + + shownwere detected. in Figure The 3b ions using at m/z the= 15, positive-ion 27, 31, 39, 41, mode, 55, 59, some 69, 77, characteristic and 91 were peaks assigned from to CH the3 pPMMA,C2H3 , + + + + + + + + polymerCH3O ,C chain3H3 were,C3H detected.5 ,C4H7 The,C2 Hions3O 2at,C m/z4H =5 O15, ,C27,6 H31,5 39,, and 41, C 55,7H 59,7 , respectively.69, 77, and 91 These were fragmentassigned toions CH also3+, C originated2H3+, CH3O from+, C3 theH3+, pPMMA C3H5+, C chains.4H7+, C2H3O2+, C4H5O+, C6H5+, and C7H7+, respectively. These fragment ions also originated from the pPMMA chains.

(a)

Figure 3. Cont.

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(b) (b) Figure 3. (a) Negative-ion and (b) positive-ion spectra (0–200 amu) of time of flight-secondary ion Figure 3. (a) Negative-ion and (b) positive-ion spectra (0–200 amu) of time of flight-secondary ion massFigure spectrometry 3. (a) Negative-ion (ToF-SIMS) and on(b )the positive-ion surface of spectrapPMMA (0–200 thin films amu) grown of time on of a glassflight-secondary substrate when ion mass spectrometry (ToF-SIMS) on the surface of pPMMA thin films grown on a glass substrate when usingmass spectrometrythe proposed (ToF-SIMS)APP polymerization on the surface technique of pPMMA after 90 thin min films deposition. grown on a glass substrate when using the proposed APPAPP polymerizationpolymerization techniquetechnique afterafter 9090 minmin deposition.deposition. The characteristics of the conductor or semiconductor layer can be influencedinfluenced by the surfacesurface The characteristics of the conductor or semiconductor layer can be influenced by the surface morphology of the coated dielectric layer [[46,47].47,48]. In this sense, the plane and cross-sectional view morphology of the coated dielectric layer [46,47]. In this sense, the plane and cross-sectional view SEM images of the newly proposed pPMMA thin filmsfilms grown on glassglass substratessubstrates when using the SEM images of the newly proposed pPMMA thin films grown on glass substrates when using the proposed APP APP polymerization polymerization technique technique after after 90 min 90 min deposition deposition are displayed are displayed in Figure in Figure4. As shown 4. As proposed APP polymerization technique after 90 min deposition are displayed in Figure 4. As byshown the SEM by the results SEM of results Figure of4, theFigure pPMMA 4, the film pPMMA had a film deposition had a ratedeposition of about rate 0.023 of aboutµm·min 0.023−1, shown by−1 the SEM results of Figure 4, the pPMMA film had a deposition rate of about 0.023 andµm·min had no, and pits had and no pinholes. pits and The pinholes. surface of The the surface pPMMA of film the pPMMA was observed film wasto be observed homogeneous, to be µm·min−1, and had no pits and pinholes. The surface of the pPMMA film was observed to be amorphous,homogeneous, and amorphous, smooth, which and is consideredsmooth, which to be isone considered of the most to important be one featuresof the most of the important dielectric homogeneous, amorphous, and smooth, which is considered to be one of the most important layerfeatures required of the in dielectricorganic thin layer films, required encapsulations, in organic biosensors, thin films, and electronics encapsulations, applications. biosensors, The SEM and features of the dielectric layer required in organic thin films, encapsulations, biosensors, and imageselectronics of Figure applications.4 illustrating The the SEM pPMMA images films of with Figure the self-assembled 4 illustrating mesoscopic the pPMMA structure, films with confirm the electronics applications. The SEM images of Figure 4 illustrating the pPMMA films with the thatself-assembled the proposed plasma-assistedmesoscopic structure, electro-polymerization confirm that technique the enables proposed large-area plasma-assisted fabrication of self-assembled mesoscopic structure, confirm that the proposed plasma-assisted mesoscopicelectro-polymerization structured pPMMAtechnique films. enables large-area fabrication of mesoscopic structured pPMMA films.electro-polymerization technique enables large-area fabrication of mesoscopic structured pPMMA films.

Figure 4. Plane and cross-sectional views of scanning electron microscopy (SEM) images of pPMMA Figure 4. Plane and cross-sectional views of scanning electron microscopy (SEM) images of pPMMA thin films grown on a glass substrate when using the proposed APP polymerization technique after 90 thinFigure films 4. Plane grown and on cross-sectionala glass substrate views when of using scanning the proposedelectron microscopy APP polymerization (SEM) images technique of pPMMA after min deposition. 90thin min films deposition. grown on a glass substrate when using the proposed APP polymerization technique after The90 min variation deposition. in the film thickness of the newly proposed pPMMA thin films grown on glass The variation in the film thickness of the newly proposed pPMMA thin films grown on glass substrates when using the proposed APP polymerization technique at different deposition times is substratesThe variation when using in the the film proposed thickness APP of polymerization the newly proposed technique pPMMA at different thin films deposition grown ontimes glass is substrates when using the proposed APP polymerization technique at different deposition times is

Polymers 2019, 11, 396 8 of 12

Polymers 2018, 10, x FOR PEER REVIEW 8 of 12 shown in Figure5. Under various deposition times, the deposition rates were about 0.025 µm·min−1 for 60 minshown deposition in Figure time5. Under and various about 0.023depositionµm· mintimes,− 1thefor deposition 90 min deposition rates were a time,bout 0.025 which μm·min was estimated−1 for 60 min deposition time and about 0.023 μm·min−1 for 90 min deposition time, which was to be low.for However, 60 min deposition the deposition time and ratesabout were0.023 increased μm·min for significantly 90 min deposition and maintained time, which after was 120 min estimated to be low. However, the deposition rates were increased significantly and maintained deposition time at about 0.036 µm·min−1 in the proposed APP polymerization technique. This means after 120 min deposition time at about 0.036 μm·min−1 in the proposed APP polymerization that thetechnique deposition. This rates means in that our the APP deposition polymerization rates in our system APP polymerization remain almost system constant remain almost after 120 min depositionconstant time. after 120 min deposition time.

Figure 5. Variation in the film thickness of pPMMA thin films grown on glass substrates when using Figure 5. Variation in the film thickness of pPMMA thin films grown on glass substrates when using the proposed APP polymerization technique at different deposition times. the proposed APP polymerization technique at different deposition times.

The opticalThe optical transmittance transmittance of of pPMMA pPMMA playsplays an an important important role role when when used usedas optical as opticalmaterial. material. Therefore,Therefore, the variation the variation in the in optical the optical transmittance transmittance (or transmission) (or transmission) of theof the newly newly proposed proposed pPMMA thin filmspPMMA grown thin on films glass grown substrates on glass when substrates using when the proposed using the proposed APP polymerization APP polymerization technique at differenttechnique deposition at different times deposition is shown times in Figure is shown6. Thein Figure pPMMA 6. The thin pPMMA film thinhad film a high had transparencya high transparency within the range of visible wavelength (400–700 nm), which could reach up to 93% at within the range of visible wavelength (400–700 nm), which could reach up to 93% at 600 nm. As the 600 nm. As the deposition times were increased, the corresponding transmittances decreased depositionslightly. times were increased, the corresponding transmittances decreased slightly.

Figure 6. Variation in the optical transmittance (or transmission) of pPMMA thin films grown on glass Figure 6. Variation in the optical transmittance (or transmission) of pPMMA thin films grown on substrates when using the proposed APP polymerization technique at different deposition times. glass substrates when using the proposed APP polymerization technique at different deposition times. To calculate the surface wettability or hydrophobicity of the newly proposed polymer, the WCAs of pPMMA were measured [43,47–49]. The WCAs analysis is a simple, rapid, and direct method to evaluate the hydrophobic or hydrophilic feature of a surface. The variation in the WCAs on the pristine (bare) substrates and pPMMA thin films grown on glass and polyethylene terephthalate (PET) substrates when using the proposed APP polymerization technique after 90 min deposition is shown Polymers 2018, 10, x FOR PEER REVIEW 9 of 12

To calculate the surface wettability or hydrophobicity of the newly proposed polymer, the Polymers 2019, 11, 396 9 of 12 WCAs of pPMMA were measured [43,47–49]. The WCAs analysis is a simple, rapid, and direct method to evaluate the hydrophobic or hydrophilic feature of a surface. The variation in the WCAs in Figureon 7 the. From pristine Figure (bare)7, after substrates deposition, and pPMMA we can thin see filmsthat the grown WCAs on ofglass the and pPMMA polyethylene thin films terephthalate (PET) substrates when using the proposed APP polymerization technique after 90 min gradually increased for both glass and PET substrates. The results of the WCA tests showed that the deposition is shown in Figure 7. From Figure 7, after deposition, we can see that the WCAs of the pPMMApPMMA thin filmsthin films could gradually increase increased the WCAs for both after glass deposition. and PET Thesubstrates. increased The results WCAs of of the the WCA pPMMA thin filmstests wereshowed presumably that the pPMMA due tothin the films cleavage could increase of the hydrophilicthe WCAs after groups deposition and. theThe newlyincreased formed hydrophobicWCAs of groups the pPMMA (C–C thin and films C–H). were In presumably addition, we due observed to the cleavage a change of the in hydrophilic the WCAs groups depending and on the substrates.the newly The formed surface hydrophobic roughness groups (root (C mean–C and square C–H). roughness,In addition, Rweq) observed of the pPMMA a change thin in the films on the glassWCAs substrate depending was on 25.9 the nm,substrates. whereas The the surface Rq of roughness the pPMMA (root mean thin filmssquare on roughness, the PET R substrateq) of the was pPMMA thin films on the glass substrate was 25.9 nm, whereas the Rq of the pPMMA thin films on 0.6 nm in Figure S1 and Table S1. The WCA of the glass sample decreased as the Rq was increased. the PET substrate was 0.6 nm in Figure S1 and Table S1. The WCA of the glass sample decreased as The changed WCAs of the pPMMA thin films on both glass and PET substrates were due to the the Rq was increased. The changed WCAs of the pPMMA thin films on both glass and PET substrates differenceswere due in the to the surface differences roughness in the surface [50,51 ].roughness It is a typical [50,51]. phenomenon It is a typical phenomenon that appears that on appears hydrophilic surfaceson dependinghydrophilic surface on thes Wenzeldepending theory on the [52 Wenz]. Theel theory detailed [52]. atomic The detailed force microscopeatomic force microscope images and the Rq of theimages pPMMA and the thin Rq of films the pPMMA surface thin grown films on surface glass grown and PET on glass substrates and PET are substrates shown are in Figureshown in S1 and Table S1.Figure S1 and Table S1.

Figure 7. Variation in the water contact angles (WCAs) on the pristine (bare) substrates and pPMMA thin filmsFigure grown 7. Variation on glass in the and water polyethylene contact angles terephthalate (WCAs) on the (PET) pristine substrates (bare) substrates when using and pPMMA the proposed APP polymerizationthin films grown techniqueon glass and after polyethylene 90 min deposition.terephthalate ( InsetsPET) substrates represent when photographs using the proposed of the water APP polymerization technique after 90 min deposition. Insets represent photographs of the water droplets on the pristine and pPMMA film. droplets on the pristine and pPMMA film. 4. Conclusions 4. Conclusions In thisIn work, this work, we usedwe used for for the the first-time first-time APPAPP polymerization techniques techniques to provide to provide further further insightinsight into the into process the process of electro-polymerization of electro-polymerization of MMA monomer. monomer. The The pPMMA pPMMA thin thinfilms filmswere were successfullysuccessfully obtained obtained by the by atmospheric the atmosphe pressureric pressure plasma-assisted plasma-as electro-polymerizationsisted electro-polymerization technique. The experimentaltechnique. The results experimental show that results the show newly that proposed the newly pPMMA proposed has pPMMA quite has homogenous, quite homogenous, amorphous, pinholeamorphous, free, and pinhole high transmittance free, and high transmittance characteristics. characteristics. In addition, In addition, the results the results of the of the water water contact angle testscontact show angle that tests the show pPMMA that thinthe pPMMA films can thin improve films canthe hydrophobicity. improve the hydrophobicity. The plasma-assisted The plasma-assisted electro-polymerization technique presented in this study is very simple, highly electro-polymerization technique presented in this study is very simple, highly reproducible, and reproducible, and permits fabrication of large areas of mesoscopic structured films having potential permitsas fabricationmembranes, organic, of large inorganic areas of materials, mesoscopic and photonic structured . films having potential as membranes, organic, inorganic materials, and photonic molecules.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4360/11/3/396/s1, Figure S1: Two- (2D) and three-dimensional (3D) AFM images of pPMMA film surfaces grown on glass and PET substrates when using the proposed APP polymerization technique after 90 min deposition, Table S1: Root mean square roughness (Rq) and average roughness (Ra) obtained from AFM images of pPMMA film surfaces grown on glass and PET substrates when using the proposed APP polymerization technique after 90 min deposition. Author Contributions: C.-S.P., E.Y.J., and H.-S.T. conceived and designed the study; C.-S.P., E.Y.J., H.J.J., and G.T.B. performed the experiments; C.-S.P. and E.Y.J. contributed analysis tools; C.-S.P., E.Y.J., and H.-S.T. analyzed the data; C.-S.P. and H.-S.T. wrote the majority of the paper and all authors reviewed and approved the final version. Polymers 2019, 11, 396 10 of 12

Funding: This research was funded by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MOE) (No. 2018R1D1A1B07046640) and Korea government (MOE) (No. 2016R1D1A1B03933162). Acknowledgments: Authors would like to thank Sang-Geul Lee and Weon-Sik Chae at the Korea Basic Science Institute (Daegu) for useful discussion and providing the FTIR data. Conflicts of Interest: The authors declare no conflict of interest.

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