Thermoresponsive Water-Soluble Polymer Layers and Water-Stable

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Applications of Polymer, Composite, and Coating Materials

Thermoresponsive water-soluble polymer layers and water-stable copolymer layers synthesized by atmospheric plasma initiated chemical vapor deposition François Loyer, Antoine Combrisson, Korantin Omer, Maryline Moreno-Couranjou, Patrick Choquet, and Nicolas D. Boscher ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14806 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 7, 2018

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ACS Applied Materials & Interfaces

Thermoresponsive Water-Soluble Polymer Layers and Water-Stable Copolymer Layers Synthesized by Atmospheric Plasma Initiated Chemical Vapor Deposition François Loyer, Antoine Combrisson, Korantin Omer, Maryline Moreno-Couranjou, Patrick Choquet, Nicolas D. Boscher*

Materials Research and Technology, Luxembourg Institute of Science and Technology, 41. rue du Brill, Belvaux, L-4422 Luxembourg

KEYWORDS: thermoresponsive thin film; copolymer; atmospheric‐plasma CVD; conventional polymerization; pulsed discharge

ABSTRACT

ACS Paragon Plus Environment

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The growth of thermoresponsive layers with the atmospheric pressure plasma-initiated chemical vapor deposition (AP-PiCVD) process is reported for the first time. N-vinyl caprolactam (NVCL) was successfully homopolymerized and copolymerized with ethylene glycol dimethacrylate (EGDMA), yielding water-soluble and water-stable thermoresponsive thin films, respectively. Strong chemical retention and high thermoresponsivity were achieved, highlighting the ability of AP-PiCVD to grow functional conventional homopolymers and copolymers.

INTRODUCTION Thermoresponsive polymers (TRP) are a class of smart materials characterized by a critical temperature threshold, inducing physico-chemical changes due to a reversible collapse or expansion of the polymer chain’s units defined through a physical shift from hydrophilic to hydrophobic. This transition temperature (i.e. lower critical solution temperature, or LCST, when the polymer transition from hydrophilic to hydrophobic as the temperature increase) is strongly dependent on the monomer chemistry, the polymeric chain length and the presence of comonomers, making TRP highly tunable compounds


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with a broad scope of application. Amongst the TRP, poly(n-isopropylacrylamide) (PNIPAM) is undeniably the most vastly documented due to its LCST close to 35°C and its viability for biomedical applications.1–4 Recently, there is a growing interest in Poly(nvinyl caprolactam) (PNVCL) based copolymers for their TRP behavior with a LCST close to the body temperature around 33°C,5,6 their complexation capability, stability against hydrolysis as well as biocompatibility,7–10 making them ideal candidates for biomedical applications.11 Among the many processes employed for the production of TRP layers, the most common methods involve the polymerization in solutions to form nanogels, since they are easily dispersed in aqueous solvent. In spite of the good control on the molecular mass afforded by such approach, it implies the need for chemical initiators and posttreatments.12 Thus, the deposition of the synthesized TRP in thin film form remains a challenge for many substrates and applications. Notably, the poor adhesion and high solubility of TRP in solvents complicate their practical use as thin films. Chemical vapor deposition (CVD) techniques provide an appealing route for the preparation of functional devices,13 enabling a solvent-free simultaneous synthesis and deposition of polymer layers. Moreover, CVD methods, such as initiated CVD (iCVD),


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allow the growth of conformal polymer layers with controlled thicknesses and are suitable for patterning.14–16 However, the low-pressure operating conditions of iCVD, dictated by the short lifetime of the initiated species at higher pressures, is an important drawback for upscaling the process to industrial levels. Up-to-recently, atmospheric-pressure CVD methods struggled to control the polymer thin films’ properties to the same degree as their vacuum counterparts. Plasma-enhanced CVD (PECVD) can efficiently initiate the freeradical polymerization reaction via the energetic species created by the discharge, even under atmospheric conditions, enabling the growth of organic thin films.17–20 Still, PECVD’s discharges yield a wide variety of species, with energies ranging from negligible (i.e. < 1 eV) to highly reactive (i.e. > 10 eV), that can induce non-specific reactions leading to the formation of “plasma-polymers”.21 Mostly based on fragmentation and recombination mechanisms, the plasma-polymerization of monomers induces strong disparities in the plasma-polymer chemistry and overall properties compared to their conventional polymer counterparts.17 The recently developed atmospheric pressure plasma-initiated chemical vapor deposition (AP-PiCVD) technique has been proven as an appealing answer to the current


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limitations for the dry and atmospheric-pressure preparation of functional polymer layers.22,23 Indeed, in AP-PiCVD the polymer synthesis and deposition is operated in a single step that involves neither the use of vacuum, heating or solvent. The AP-PiCVD technique relies on temporally isolated discharges (ca. ton ≈ 100 ns) to initiate the freeradical polymerization of vinyl monomers while statistically lowering molecular fragmentation. Nanoseconds-short ton are combined with discharge off times adapted to the polymeric radical species lifetime (ca. toff = 1-100 ms) to promote the propagation of free-radical polymerization.24,25 AP-PiCVD demonstrated the ability to grow conventional and conformal polymer layers with an excellent retention of chemical functionalities, making it a strong candidate for the deposition of sensitive functional polymers such as TRP layers.24,25 In the present work, the growth of thermoresponsive polymer layers is demonstrated. Specifically, water-soluble PNVCL homopolymer layers and water-stable P(NVCL-co-EGDMA) copolymer layers, with both strong thermoresponsive properties, were successfully synthetized using AP-PiCVD. The chemical and functional characterizations of these NVCL-based thin films highlight the importance of the plasma


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pulses frequency, as well as the relevance of copolymerization on the resulting thin film properties.

EXPERIMENTAL METHODS

Materials and PiCVD

The thin films presented in this work were deposited using an atmospheric pressure dielectric barrier discharge (AP-DBD) setup as previously described (Figure S1).26 For every experiment, the discharge gap was maintained at 1 mm. The dielectric barrier discharges were ignited by square pulses of 6.5 kV, produced by an AHTPB10F generator from EFFITECH, allowing the generation of perfectly reproducible ultra-short discharges of several tens of nanoseconds (Figure S2a). For every experiment, all generator’s input parameters were kept identical except for the pulse repetition frequency which ranged from 10 Hz (i.e. ton ≈ 100 ns & toff ≈ 100 ms) up to 1,000 Hz (i.e. ton ≈ 100 ns & toff ≈ 1 ms). In order to generate a single discharge per cycle, the voltage’s pulse width was matched to the voltage’s off time for every discharge frequency presented in this work (Figure S2b). For all experiments reported in this work, no heating of the substrate


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was observed. Moreover, OES measurements revealed no sign of increasing the vibrational (Figure S2c) and rotational (Figure S2d) electronic temperatures, as the Ar and OH emissions remain almost perfectly identical for all the investigated frequencies. Since the discharges’ voltage and current traces does not change with the frequency either, the electronic density is expected to not increase significantly in the range of frequency studied. Indeed, the PiCVD approach implies ultrashort plasma pulses (ton = 100 ns) coupled to rather long plasma off-times (toff = 1 to 100 ms) that induce extremely low duty cycles (DC = ton/(ton + toff ) = 1 to 100 ms) that prevent heating of the substrates and excessive alteration of the monomers. The thermoresponsive polymers (TRPs) were either grown with n-vinyl caprolactam (NVCL, Sigma Aldrich 98 %) used alone or, when specified, copolymerized together with ethylene glycol dimethacrylate (EGDMA, Sigma Aldrich 98 %) used as a crosslinking agent. These two monomers were used as-supplied, i.e. without any purification step, and were directly carried to the deposition area using a bubbler setup with argon (Air Liquid 99.999 %) as a carrier gas. In order to give a more accurate description of the copolymerization mechanisms, the molar fluxes were calculated from the vapor pressure of each comonomer. An high-


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resolution stationary manometer 424-10 (Dwyer, IN) was used to determine the vapor pressure of EGDMA at 20°C and NVCL at 50°C, yielding Psat,EGDMA,20°C = 0.187 Torr and Psat,NVCL,50°C = 0.355 Torr. NVCL’s carrier gas flow was fixed to 4 L·min-1 for each experiment, which correspond to a molar flow of 0.582 mmol·min-1. On the other hand, the EGDMA’s carrier gas flow ranged from 0 L·min-1 to 4.1 L·min-1, corresponding to molar flows from 0.056 mmol·min-1, 0.174 mmol·min-1 and 0.285 mmol·min-1, respectively. The total gas flows from NVCL and EGDMA were completed with argon to a total of 20 L·min-1. To avoid oxygen and nitrogen contamination from atmosphere, “argon curtains” were added on both sides of the electrodes. Due to NVCL’s melting point being around 37°C, it was melted inside a bubbler using a water bath at 50°C that was maintained at this temperature during the whole experiment. All the gas lines were heated up to 60°C to avoid NVCL’s condensation inside the injection system. All depositions were carried on polished 4” silicon wafers (Siltronix), which were cleaned using a 95 %/5 % argon/oxygen plasma for 40 s prior to each experiment. All thermoresponsive coatings were stored in open air, remaining stable 3 weeks after their synthesis.


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Chemical Analysis

Fourier transform infrared (FTIR) transmission measurements were performed on a Bruker Vertex 70 spectrometer (Ettlingen, Germany) equipped with a MCT detector. XPS measurements were performed on a Kratos Axis-Ultra DLD instrument, using an Al Kα source (1486.6 eV) with a pass energy of 20 eV and an energy resolution of 0.5 eV. A flooding gun was used to reduce charging effect on the samples surface. The polymers chain growth was determined by size exclusion chromatography using a Thermo Scientific (Sunnyvale, CA) Dionex UltiMate 3000 LC system. Each measurement corresponds to the soluble fraction of the polymer in THF and not necessarily the entire distribution. The elution times were translated into molecular weights using polystyrene standards ranging from 162 to 364,000 g mol−1 (Agilent EasiVial PS-M). Matrix-assisted laser

desorption/ionization

high-resolution

mass-spectrometry

(MALDI-HRMS)

measurements were performed on an AP MALDI PDF+ ion source from MassTech Inc. coupled to an LTQ/Orbitrap Elite from Thermo Scientific. Dithranol diluted in THF was selected as the matrix due to its known efficiency for the ionization of organic compounds


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and plasma polymers.24,27 0.1 μL of the matrix was then spotted on the thin films to form a co-crystal with the soluble part of the film when the THF evaporated.

Thin Film Characterizations

SEM imaging for topography inspection were carried out on a FEI Quanta 200F (Zürich, Switzerland). The growth rates were determined from the thicknesses measured by a KLA-Tencor P-17 Stylus profiler (Milpitas, CA) divided by the time spent under the electrodes. The 3D topographies were recorded in tapping mode at a scanning rate of 1 Hz with an atomic force microscope (AFM) MFP 3D Infinity (Asylum Research, Santa Clara, CA).

Thermoresponsive properties

The water-soluble PNVCL films’ lower critical solution temperature (LCST) was determined with an UV−vis spectrophotometer (Tecan Infinite M1000 Pro) in transmittance mode. The TRP films were dissolved in milliQ water, heated at the desired temperature and their absorption spectra obtained over the visible range (i.e. from 300 nm to 800 nm). To ensure an accurate determination of the LCST, each spectra was integrated before comparison. The water-stable PNVCL layers’ thermoresponsive


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properties were ascertained based on the water contact angle directly measured on the deposited films using a DSA100 (Krüss, Hamburg, Germany) equipped with a TC30 (Krüss, Hamburg, Germany) chamber, allowing measurements at temperatures ranging from 10°C to 60°C. The surface free energies (SFE) of the TRP films were determined at 22°C and 60°C by applying a linear fit to the extended Fowkes equations (eq. 1) using 3 liquids with distinct dispersions and polarizations energies (Table 1).28 𝛾𝑆𝐹𝐸 1 𝛾𝑑𝑖𝑠𝑝 1

∗

1 + cos 𝜃1 2

= 𝛾𝑝𝑜𝑙𝑎 2

𝛾𝑝𝑜𝑙𝑎 1 𝛾𝑑𝑖𝑠𝑝 1

(1)

+ 𝛾𝑑𝑖𝑠𝑝 2

Table 1. Total surface free energies γSFE together with their dispersive γdisp and polar γpolar components of water, ethylene glycol and diiodomethane.

γSFE

γdisp

γpola

(mJ·m-2)

(mJ·m-2)

(mJ·m-2)

Water

72.6

21.6

51.0

Ethylene glycol

48.8

32.8

16.0

Diiodomethane

50.8

49.5

1.3

Surfaces

RESULTS AND DISCUSSION


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The strategy toward the atmospheric-pressure chemical vapor deposition of thermoresponsive water-soluble polymer layers and water-stable copolymer layers relies on the use of ultra-short plasma discharges (ca. ton = 100 ns) and long plasma off-times (ca. toff = 1 to 100 ms), which have been previously proved highly advantageous for the growth of conventional and conformal polymer thin films.24,25 Ultra-short plasma discharges can efficiently initiate the free-radical polymerization reaction, while the plasma off-time selected in accordance with the lifetime of the formed radical species to promote the propagation of the free-radical polymerization reaction.

AP-PiCVD of water-soluble TRP layers

Previous studies have highlighted the possibility to either favor a free-radical polymerization pathway or plasma-polymerization route when tuning the ultra-short plasma pulses frequency.23–25,29,30 This first section aims at investigating the benefit, i.e. plasma-induced crosslinking, and drawbacks, i.e. loss of properties, afforded by the increase of the ultra-short plasma pulses frequency. Indeed, lower plasma pulses frequencies are known to better preserve the chemistry, most probably improving the coiling effect and thus the thermal response of these NVCL-based thin films, while higher


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plasma pulses frequencies may induce an innate crosslinking of the polymeric layers, affecting its molecular weight and consequently its LCST. Due to its Flory-Huggins thermoresponsive behavior, the adjustment of the PNVCL polymer chain length offers a unique way for the tuning of the LCST value without any comonomer.6,31,32 Hence, discharge frequencies from 10 Hz to 3,160 Hz were investigated to determine the kinetics of the free-radical polymerization of NVCL and the frequency threshold where the surface reactions, i.e. adsorption and free-radical polymerization, become preponderant over the gas phase mechanisms (Figure S3).24,25 From the growth rates, deduced from the thickness and the weight measurements, a maximum deposition rate is obtained for an ultra-short plasma pulses frequency of 316 Hz. Above this frequency, a decay of the growth rates is observed due to excessive gas phase reactions that yield to a pronounced fragmentation of the NVCL monomer. The importance of gas phase reactions above 316 Hz is also evidenced by the rough morphology of the NVCL-based thin films, such as observed by SEM (Figure S4). On the other hand, all the NVCL-based thin films elaborated at frequencies below 316 Hz exhibited a smooth morphology driven by the


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preponderant adsorption of plasma-formed radicals and monomers at the substrate’s surface and their subsequent polymerization (Figure S4). Previous studies linked the influence of the discharge frequency on the chemical structure to size exclusion chromatography (SEC) analyses, performed on thin films grown

by

AP-PiCVD.24,25

Notably,

the

increase

of

plasma-induced

fragmentation/recombination mechanisms – hence leading to crosslinking – are observable as a shift of the polymeric distribution toward longer elution time (Figure S5). In an identical manner, SEC analyses of the NVCL-based thin films display an important shift in average molecular mass in number (MW) (Figure 1a). Ranging from 4,100 g·mol-1 for the 10 Hz layer displaying a high molecular number plateau (Figure 1b) down to 950 g·mol-1 for the 1,000 Hz layer (Figure 1c), the strong effect of the discharge frequency on the size exclusion chromatogram suggests the formation of shorter and/or crosslinked structures at higher plasma pulses frequencies due to an excess of reactive species. While different frequencies appears to affect the chemistry of the coatings, their topography appear to


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Figure 1. Size exclusion chromatography (SEC) of NVCL-based thin films grown by APPiCVD at discharge frequencies of 10 Hz, 100 Hz and 1,000 Hz plotted (a) over the whole range of elution time and (b) zoomed on the short elution time of the Gaussian tails. (c) Average molar masses in weight (MW) of the NVCL-based thin films grown at different discharges frequencies.

To elucidate the impact of the plasma pulses frequency on the solubility/stability of NVCL-based thin films, immersion tests in milliQ water for 30 min were carried out. For each of the investigated plasma pulses frequency, the thin films were completely washedoff by water, such as evidenced by FTIR analyses, which only displayed the OH vibration


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from lingering water and Si-O vibration from the silicon wafer (Figure S6). This observation is not surprising since PNVCL is intrinsically entirely dissolved in water. However, by reference to other PECVD works, it can be reported that no sufficient crosslinking of the NVCL-based thin films can be achieved with these plasma process conditions. In order to check their thermoresponsivity, and despite their solubility in water, the films’ surface energies were investigated by instantaneous water contact angle (WCA) measurements at 25°C and 60°C. Interestingly, the NVCL-based thin films elaborated at the lowest plasma pulses frequencies exhibited a thermoresponsive property (Figure 2), with a maximum water contact angle variation for the thin film deposited using a plasma pulse frequency of 10 Hz (ΔWCA10 Hz = 20°). This range of variation is coherent with WCA values obtained for water-soluble PNVCL homopolymer layer obtained by iCVD.33 On the other hand, the NVCL-based thin films elaborated at the highest plasma pulses frequency,

i.e.

1000 Hz,

did

not

show

any

thermoresponsive

properties

(ΔWCA1,000 Hz = 0°).


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Figure 2. Water contact angle measurements performed at (a) 25°C and 60°C as well as (b) the corresponding angle variation of the NVCL-based thin films grown by AP-PiCVD at different discharge frequencies.

Such as observed from the SEC and WCA measurements, the plasma pulses frequency has a strong influence on the resulting chemical retention of the synthesized thin films, actively impacting their properties. Indeed, while significant alterations of the


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chemistry can be identified by Fourier transform infrared (FTIR) spectroscopy as the plasma pulses frequency increase, the free-radical polymerization pathway is evidenced for all the NVCL-based thin films by the disappearance of the vinyl vibrational bands (3,108 cm1 and 1,665 cm-1) (Figure 3). Moreover, the retention of the caprolactam function is ascertained through the carbonyl vibrational peak in the lactam (N-C=O)ring group at 1,620 cm-1 and, to a lesser extent, the peak ratios conservation between the CH2 vibrational bands (2,927, 2,859, 1,479, 1,445 and 1,424 cm-1) pointing to the preservation of the lactam ring structure.33,34 Nevertheless, as the frequency of plasma pulses is increased, a broad band centered at 3,300 cm-1 appears due to the formation of hydroxyl group. In addition, a broadening of the band located around 1,730 cm-1 corresponding to a series of different carbonyl C=O vibrational bands is observed, hinting on the lactam ring opening while increasing the plasma pulses frequency.34 The fragmentation of the NVCL-based thin films at higher ultrashort plasma pulse frequencies is also highlighted by XPS measurements (Figure S7). Such a deviation from the monomer chemistry is notably observed from the O 1s XPS spectra that reveal a broadening and shifting of the main peak as the ultrashort plasma pulse frequency increases. This is further


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corroborated by the increase of the single CH3 band at 1370 cm-1 for higher discharge frequencies, suggesting a transition from CH2,ring functions to terminal CH3, supporting the possibility to tune to some extent the film’s structure and thus its thermoresponsive properties using plasma-induced fragmentations.

Figure 3. FTIR spectra of NVCL-based thin films grown by AP-PiCVD at different discharge frequencies. The monomer NVCL and conventionally polymerized PNVCL are showed as references.

To confirm the suggested chemistry, matrix-assisted laser desorption/ionization high resolution mass spectrometry measurements (MALDI-HRMS) were performed on our


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NVCL-based thin films. In accordance with FTIR analyses and for every discharge frequencies considered, a small amount of side-species and the strong retention of conventional polymer chains [(NVCL)n + H]+ are observed (Figure 4a). However, the detection of a non-negligible quantity of protons terminated polymer chains [H(NVCL)nH + H]+ (Figure 4b), points to a partial opening of the lactam ring. Consistently, the ring fragmentation increases as the duty cycle increases, together with side-species appearing and the overall distribution being shifted to lower masses. A recent study highlighted the competition between statistical fragmentations from high energy electrons and localized fragmentations at the molecule weakest points (i.e. low bond dissociation energies).29 This localized attack was notably shown to protect the rest of the molecule from fragmentation, acting as a chemical buffer. In the case of NVCL, aside from the vinyl promoting conventional polymerization, the lactam ring is undoubtedly the weakest function, offering 7 different opening pathways (Figure S8). Each of the ring’s possible fragmentation is likely to locally crosslink (i.e. via C● or N● anchor points), affecting the polymers average molecular weight – as evidenced by SEC analyses – and consequently the LCST.


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Figure 4. MALDI-HRMS spectra of films elaborated from NVCL at different discharge frequencies in the mass ranges (a) m/z = 50−750 and (b) m/z = 418.2−418.4 & 420.2−420.4.

Consequently, the optimal discharge frequency was defined as 10 Hz for the growth of thermoresponsive PNVCL water-soluble homopolymer layer below its LCST and its thermal response investigated by UV-vis spectrophotometry. The thermoresponsive PNVCL homopolymer dissolved in milliQ displayed a sharp transition around 40°C, from hydrophilic (i.e. dissolved) and completely transparent at room temperature to


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hydrophobic (i.e. precipitated) milky at 60°C (Figure 5), exhibiting an excellent repeatability between both heating and cooling cycles. The higher than usually reported LCST (ca. 33°C) is consistent with the Flory-Huggins behavior of the PNVCL TRP, since our PNVCL layer is composed of rather short homopolymer chains (Figure 1a), shifting the LCST to higher temperatures.5,6,31,32 While the use of higher frequencies undoubtedly displayed a strong influence on the molecular structure (Figure 1c), this effect was detrimental for the thermoresponsive properties – due to very short and crosslinked chains – and negligible on the water-stability (Figure 2 and 3).

Figure 5. Integrations of UV-vis absorbance spectra over the visible range (300 nm to 800 nm) performed at different temperatures for PNVCL homopolymer layer grown by AP-PiCVD at 10 Hz. The presented thermal transitions were determined by an integral


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Gaussian fitting and the pictures of the UV-cuvettes below and above the LCSTs are displayed.

AP-PiCVD of water-stable TRP layers

To compensate for the inherent solubility of PNVCL in water, we investigated the APPiCVD reaction of NVCL with a crosslinking monomer to yield water-stable thermoresponsive thin films. Ethylene glycol dimethacrylate (EGDMA) was selected for its bio-compatibility and its known efficiency as a plasma-activated crosslinker for copolymerization.35–38 In order to combine the functionalities of each comonomer into a water-stable thermoresponsive copolymer thin film, one must determine the optimal ratio of the two monomers. Indeed, the thermoresponsive and crosslinking monomers have opposite behavior, since NVCL coil and uncoil together with the temperature while EGDMA will promote the formation of a rigid structure. Consequently, keeping NVCL’s input constant; i.e. 0.582 mmol·min-1, different EGDMA molar flows are investigated –

i.e. 0.056 mmol·min-1, 0.174 mmol·min-1 and 0.285 mmol·min-1 – to identify the best compromise between stability and thermoresponsive properties.


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From the observations made during the AP-PiCVD of NVCL-based thin films, an ultrashort plasma pulses frequency of 10 Hz was selected to prepare the copolymer layers. For all the NVCL:EGDMA molar ratios investigated, smooth and defect-free thin films were formed as confirmed by SEM (Figure S8) and AFM measurements (Figure 6). Notably, AFM highlighted the ultra-smooth surface of all the thin films prepared from a 10 Hz ultrashort plasma pulse frequency. This observation is consistent with the proposed deposition mechanisms for PiCVD of polymer layers, which relies on the successive adsorption and polymerization onto a surface.

Figure 6. AFM pictures of pure PNVCL and PEGDMA, along with the different P(NVCLco-EGDMA) copolymers grown by AP-PiCVD at 10 Hz. The root mean square height Sq is indicated.


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Due to the foreseen crosslinking of the P(NVCL-co-EGDMA) copolymer layers, no MALDI-HRMS and no SEC analyses were undertaken. Yet, the copolymerization between NVCL and EGDMA was chemically monitored by FTIR spectroscopy (Figure 7a) and XPS analyses (Figure S10). Notably, the presence of EGDMA within the thin films was evidenced by the ester function C(O)O (1,732 cm-1) and by the ether function COC contributions (1,169 cm-1 and 1,040 cm-1). By integrating the well-separated NVCL’s caprolactam ring (1,620 cm-1) and EGDMA’s ester (1,732 cm-1), the molar ratio of copolymerization within the thermoresponsive layers can be expressed from the BeerLambert equation (Figure S11). The determined NVCL:EGDMA molar ratios of the copolymer layers are equated to the monomers molar flows, yielding 99.4 %:0.6 % for 0.582 mmol·min-1/0.056 mmol·min-1, 1/0.174

98.5 %:1.5 %

for

0.582 mmol·min-

mmol·min-1, and 94.7 %:5.3 % for 0.582 mmol·min-1/0.285 mmol·min-1, in

accordance with the EGDMA chains acting as isolated bridges to retain the NVCL chains flexibility. The low input/output ratio of EGDMA was confirmed through the fitting of the XPS C 1s core level peak of the 98.5 %:1.5 % P(NVCL-co-EGDMA) layer, which yield a molar ratio (97.1 %:2.9 %) in accordance to the one estimated by FTIR (Figure S12).


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Since the FTIR quantifications were performed from the integration of the whole thin film thickness, in contrast to the very low depth analysis of XPS, the FTIR molar ratios will be used to further describe and identify the copolymer layers in the manuscript. Moreover, XPS fitting may artificially lead to an overestimation of the EGDMA content due to charge effects increasing its C=O contribution. Such a non-linear discrepancy between the monomers’ molar flows and the actual copolymer layers compositions is certainly related to the comonomers’ reactivity.39 Indeed, a poor reactivity of acetates with vinyl-lactams monomers has been reported on several occasions.40-42 Notably, Kahn et al. determined the copolymerization reactivity ratios between vinyl acetate (i.e. rVAc = 0.237) and n-vinyl pyrrolidone (i.e. rNVP = 2.28), which bear a chemistry close to a methacrylate and the nvinyl caprolactam, respectively.42 The high n-vinyl pyrrolidone ratio indicates that it is able to homopropagate easily (i.e. forming long chains of homopolymer), while the low vinyl acetate ratio shows that it would tend to crosspropagate (i.e. reacting mostly with the nvinyl pyrrolidone chains).43 This behavior matches with a reduced incorporation of EGDMA in the P(NVCL-co-EGDMA) copolymer as previously reported in Figure S11 and Figure S12.


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Interestingly, NVCL’s characteristic functions such as the lactam function (N-C=O)ring (1,620 cm-1) and the conservation of the CH2 vibrational bands (2,927, 2,859, 1,479, 1,445 and 1,424 cm-1), display a very high chemical retention, irrespective of the amount of crosslinker introduced in the structure. Finally, a higher integration of water is observed with higher EGDMA content into the NVCL-based thin films, such as expressed by the increasing intensity related to the hydroxyl OH band (3,250 cm-1).42 Such as for the NVCLbased thin films reported above, the water-stability of the P(NVCL-co-EGDMA) copolymer layers were determined by immersion in milliQ water for 30 minutes and subsequent FTIR analyses of the remaining layers (Figure 7b). Instantly, the 99.4 %:0.6 % copolymer layer appears completely rinsed indicating a lack of crosslinking between the polymer chains themselves and interactions with the substrate, while the chemical stability of the 98.5 %:1.5 % and 94.7 %:5.3 % copolymers was confirmed as their spectra are almost unaltered after the immersion. This is in accordance with previous literature works that report a crosslinker molar concentration around 1 mol% to stabilize the PNVCL chains.44 The layers’ stability was confirmed by SEM/EDX (Figure S9), also highlighting the conservation of the surface smoothness for the 98.5 %:1.5 % copolymer layer, the


27


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94.7 %:5.3 % copolymer displaying some scythe-like surface alterations from the water immersion.

Figure 7. FTIR spectra of the P(NVCL-co-EGDMA) copolymer layers grown by AP-PiCVD (10 Hz) for different NVCL:EGDMA molar ratios. (a) as-deposited and (b) after being immersed for 30 min in milliQ water.

The thermoresponsive properties of the different P(NVCL-co-EGDMA) as-deposited copolymer layers were assessed from the WCA measurement at 22°C and 60°C (Figure 8). Interestingly, the thermoresponsive behavior of the P(NVCL-co-EGDMA) copolymer layers increases with the amount of EGDMA crosslinker up to a maximum of


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52° (from 19° to 71°) for the 98.5 %:1.5 % molar ratio P(NVCL-co-EGDMA) layer. Such an increase of the ΔWCA from 20° for the PNVCL layer to 52° for the best P(NVCL-coEGDMA) copolymer layer correlates with the increased water uptake observed by FTIR for higher EGDMA content (Figure 7). Indeed, copolymerization of NCVL with a crosslinker affects the stability of the formed hydrogel and those with a higher water uptake consistently display a better swelling capacity, and therefore a better thermoresponsive performance.44 Unsurprisingly, lower content of EGDMA crosslinker (i.e. 99.4 %:0.6 % molar ratio) is not sufficient to confer the EGDMA’s contribution to the P(NVCL-co-EGDMA) copolymer layers, and both the thermal response and the water-solubility are very close to PNVCL homopolymer layers due to a lack of polymer/polymer and polymer/substrate interactions. By the same token, a high amount of crosslinker (i.e. 94.7 %:5.3 % molar ratio) yields properties comparable to pure EGDMA, with strong water stability and no thermoresponsive behavior resulting that the PNVCL films are completely bound by 3D network and unable to coil/uncoil with the temperature. As such, the NVCL:EGDMA molar ratio of 98.5 %:1.5 % appears as the best compromise between film stability and polymer


29


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chains flexibility, allowing the growth of adherent and water-stable thermoresponsive P(NVCL-co-EGDMA) copolymer layers. Finally, the total immersion of the P(NVCL-coEGDMA) copolymer layers in milliQ during 30 minutes induced a partial washing of the surface, together with a drop in the water contact angle variation – from ΔWCA ≈ 52° down to ΔWCA ≈ 10° – indicating a retention of the thermoresponsive properties. This decrease of the thermoresponsivity might arise from the dissolution of the most flexible thermoresponsive chain (i.e. less crosslinked), leaving the ones with the strongest polymer-substrate and polymer-polymer interactions. Moreover, as evidenced by the increase of hydroxyl vibrations (3,250 cm-1) measured by FTIR, the introduction of water generating hydrogen bonding within the thin film might stiffen the TRP chains, hindering the chains uncoiling at low temperature.


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Figure 8. Water contact angle measurements performed at (a) 22°C and 60°C as well as (b) the corresponding angle variation of different molar ratios of P(NVCL-co-EGDMA) asdeposited copolymer layers grown by AP-PiCVD at 10 Hz.

To ascertain the degree of liberty of the PNVCL chains crosslinked by EGDMA units (Figure 9a), surface free energy (SFE) measurements were performed on the 98.5 %:1.5 % molar ratio P(NVCL-co-EGDMA) layer at 22°C and 60°C using the


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extended Fowkes equations. Both analyses were conducted using milliQ water, ethylene glycol and diiodomethane, allowing to extract the copolymer thin film surface free energies as well as their respective dispersive (i.e. non-polarized environment) and polar components (i.e. polarized environment) (Figure 9b). In a very intuitive manner, the polar component (33.4 mJ m-2) of the SFE is slightly higher than the dispersive one (26.9 mJ m-2) at low temperature (i.e. 22°C), indicating that the most polarized C=O bonds are evenly distributed on the surface of the P(NVCL-co-EGDMA) copolymer layer. When the temperature is raised and cross over the LCST the dispersive component increases to 40.9 mJ m-2, while the polar component drops considerably to 8.1 mJ m-2, implying that most of the C=O bonds previously available at the surface of the P(NVCL-co-EGDMA) copolymer layer at 22°C are involved in the coiling the PNVCL chains through hydrogen bonding at 60°C. Consequently, the SFE measurements point to an optimal flexibility from the PNVCL chains adequately coupled to the rigidity brought by the EGDMA units.


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Figure 9. (a) Scheme of the P(NVCL-co-EGDMA) copolymer conformation together with uncoiled and coiled form of the chains depending on the temperature. (b) Fit of the extended Fowkes equations at 22°C and 60°C with the corresponding surface free energies and their dispersive and polar components for the as-deposited P(NVCL-coEGDMA) layer with a 98.5 %:1.5 % NVCL:EGDMA molar ratio.

Finally, the lower critical solution temperature (LCST) of the P(NVCL-co-EGDMA) copolymer layer grown from a 98.5 %:1.5 % NVCL:EGDMA molar ratio was determined by WCA measurements (Figure 10). The thermoresponsive P(NVCL-co-EGDMA) copolymer layer displayed a very long thermal transition between 15°C and 45°C with its


33


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LCST at 27.6°C, several degrees lower than conventional PNVCL homopolymer layers, possibly due to changes in the polymeric molar mass distribution.6,31,32 This highlights the influence of a comonomer (crosslinker in the present case) on the thermal response of NVCL-based thin films. Considering the narrow window of processing for the copolymer ratio, the tuning of LCST of thermoresponsive P(NVCL-co-EGDMA) copolymer layer will require the use of different comonomers with different surface free energy. As an example, one may copolymerize NVCL with N-vinylacetamides or vinylesters to tune up or down the LCST of the NVCL-based thin films.44 Aside from the chemical composition affecting the PNVCL-based copolymer layers’ thermoresponsivity, one may also take advantage of the reported conformality24,25 of the thin films deposited by AP-PiCVD. Indeed, the conformal deposition of P(NVCL-co-EGDMA) copolymer layers on patterned or rough substrates can allow a tuning of the thermal transition from hydrophilic to hydrophobic.


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Figure 10. Water contact angle measurements from 7°C to 65°C for the as-deposited P(NVCL-co-EGDMA) layer with a 98.5 %:1.5 % NVCL:EGDMA molar ratio. The presented thermal transitions were determined by an integral Gaussian fitting.

CONCLUSION Using ultra-short plasma pulses (i.e. ton ≈ 100 ns) and long plasma off-times (i.e. 100 ms), thermoresponsive water-soluble PNVCL layers and water-stable P(NVCL-coEGDMA) layers were simultaneously synthesized and deposited from a substrateindependent, dry and up-scalable process operating at atmospheric-pressure. The PNVCL homopolymer synthesized at a plasma pulses frequency of 10 Hz displayed an average molecular mass in weight of 4,100 g·mol-1 and a strong thermoresponsive


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behavior, such as evidenced by WCA measurements. Above the LCST (ca. 40°C), the PNVCL layer was insoluble in water, whereas lowering of the temperature below its LCST made it water soluble. Due to its reversible and fast transition, the developed PNVCL homopolymer in thin film form may meet biomedical and histological applications. Furthermore, we demonstrated for the first time the potential of AP-PiCVD to form copolymer layers and combine the properties of interest of two different monomers. Notably,

copolymerizing

NVCL

with

EGDMA

successfully

combined

the

thermoresponsiveness of PNVCL and the crosslinking properties of EGDMA in a waterstable thermoresponsive P(NVCL-co-EGDMA) copolymer layer. For a NVCL:EGDMA molar ratio of 98.5 %:1.5 %, the water contact angle variation was significantly increased to ΔWCA = 52° and the LCST lowered to ca. 27.6°C. This observation opens possibilities for a better control of the thermoresponsive properties of NVCL-based thin films by using different comonomers.

ASSOCIATED CONTENT


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Supporting Information: Schematic representation and picture of the AP-DBD reactor setup, traces of AP-PiCVD’s voltage pulse and current discharges, deposition rate per second in thickness and in weight of n-vinyl caprolactam (NVCL) according to the discharge frequency, SEM top-view imaging of NVCL-based homopolymer grown by APPiCVD at different discharge frequencies, average molar masses in weight of PMMA and PBMA grown by AP-PiCVD at different discharge frequencies, FTIR spectra of PNVCL homopolymer layers grown by AP-PiCVD at different discharge frequencies measured after being immersed for 30 min in milliQ water, potential 7 fragments resulting from a single caprolactam ring-opening through a plasma-induced homolytic cleavage, SEM images and EDX spectra of NVCL:EGDMA copolymers before and after being immersed in milliQ water for half an hour, full FTIR quantification of NVCL:EGDMA copolymers. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author * E-mail [email protected]; Tel +352 275 888 578


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ORCID Nicolas Boscher: 0000-0003-3693-6866 François Loyer: 0000-0002-5636-3885 Maryline Moreno-Couranjou: 0000-0003-0041-5532 Patrick Choquet: 0000-0001-8696-5812 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT

The Luxembourg National Research Fund (fnr.lu) is thanked for financial support through the NANOPOLYPULSE project (C14/MS/8345246). We thank Dr. R. Quintana for insightful discussions, Dr. S. Bulou for OES measurements, J. L. Biagi for the SEM pictures and EDX measurements, P. Grysan for AFM measurements, Dr. J. Guillot for XPS measurements, Dr. G. Frache and D. El Assad for acquisition of the MALDI-HRMS spectra and Prof. A. Shaplov for the solution-based synthesis of conventional poly(n-vinyl caprolactam).

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For Table of Contents Only


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Thermoresponsive Water-Soluble Polymer Layers and Water-Stable

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