UV-Induced Micropatterning of Complex Functional Surfaces by

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UV-induced micropatterning of complex functional surfaces by photopolymerization controlled by alkoxyamines Siham Telitel, Sofia Telitel, Julien Bosson, Jacques Lalevée, Jean-Louis Clement, Maxime Godfroy, Jean-Luc Fillaut, Huriye Akdas-Kilig, Yohann Guillaneuf, Didier Gigmes, and Olivier Soppera Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b01681 • Publication Date (Web): 24 Aug 2015 Downloaded from http://pubs.acs.org on September 2, 2015

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UV-induced micropatterning of complex functional surfaces by photopolymerization controlled by alkoxyamines. Siham Telitel, Sofia Telitel, Julien Bosson, Jacques Lalevée, Jean-Louis Clément, Maxime Godfroy, Jean-Luc Fillaut, Huriye Akdas-Kilig, Yohann Guillaneuf, Didier Gigmes, and Olivier Soppera*

Siham Telitel, Sofia Telitel, Jacques Lalevée, Olivier Soppera Institut de Science des Matériaux de Mulhouse, CNRS UMR 7361, Université de HauteAlsace 15 rue Jean Starcky, BP 2488, 68057 Mulhouse Cedex, FRANCE

Jean-Louis Clement, Julien Bosson, Yohann Guillaneuf, Didier Gigmes Aix-Marseille Université, CNRS, Institut de Chimie Radicalaire, UMR 7273 Av. Escadrille Normandie Niemen, 13397 Marseille Cedex 20, France

Maxime Godfroy, Jean-Luc Fillaud, Huriye Akdas-Kilig Institut des Sciences Chimiques de Rennes, CNRS UMR 6226, Campus de Beaulieu, 263 av. du Général Leclerc, 35042 Rennes, France

Corresponding Author: * [email protected] Keywords: controlled polymerization, photopolymerization, lithography, photopatterning, functional polymers

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Abstract We report on the use of an alkoxyamine (AA) for fabrication of functional micropatterns with complex structures by UV mask lithography. The living character of the polymer surface as well as the vertical spatial control of the repolymerization reaction from few tens of nm to few microns were demonstrated. The impact of the main parameters governing the controlled polymerization and the re-initiation process activated by light or heat was investigated. Micropatterning is shown to be a powerful method to investigate the physicochemical molecular phenomena. It is possible to control the polymer microstructure thickness from few tens of nm to few μm. In the last section, some applications are provided showing the potential of the AA for generating covalently bonded hydrophilic/hydrophobic micropatterns or luminescent surfaces. This demonstrates the high versatility and interest of this route.

1. Introduction Surface patterning of polymers, for controlling their topography and chemistry, at microand nanometer size1 has become increasingly important due to the wide range of potential applications, in biotechnology2 to study cell-surface interactions3, micro- and nanofluidic devices,4 stimulus-responsive polymer5,6, wetting/dewetting surfaces7,8, chemical sensing,9 or antifouling coating.10 Numerous techniques have been developed to fabricate patterned surfaces such as microcontact printing,11 capillary force lithography,12 chemical lithogra-


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phy,13 mask photolithography.14 All of these methods are well suited to form surface patterns on polymers but they present some limitations (size range, functionalities) and they usually require complex multistep processes. Alternatively, other processes such as laser ablation15 or photomodification by deep UV irradiation16-18 can be used but this is not applicable for the majority of surface chemistry processes. Another challenge is to obtain a covalent grafting of polymer micro-nanostructures on polymer surfaces, in order to ensure a durable anchoring of patterns. In this context, surface modifications by coupling (bio)polymer19,20 onto surfaces or by covalent binding to surfaceinitiators with grafted molecules such as iniferters21,22, atom transfer radical polymerization (ATRP)23,24, reversible addition fragmentation chain transfer polymerization25 and nitroxide mediated polymerization26,27 have been proposed. For these processes, lateral control of the polymer structures can be obtained by pre-patterning, by lithography techniques28 or contact molding.29, 30 It is also possible to generate the patterning via photoreactive surfaces. Bowman and coworkers31,32 fabricated photoreactive surfaces using photoiniferters based on dithiocarbamate (DTC). Surface properties such as surface roughness and/or hardness, biocompatibility were significantly improved.33 Upon UV irradiation, DTC groups incorporated in polymeric substrates were cleaved and generated radicals species that allowed the initiation of polymerization. The disadvantage of this method is that it produces dithiocarbamyl radical that can dimerize and produce secondary reactions leading to a loss of control during the polymerization.34, 35


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Another strategy recently proposed consisted in using living photoactivable systems to produce the first polymer layer. The major interest is that the dormant species can then be directly reactivated in a second step to generate the surface functionalization and/or patterning. Ir-catalyzed photomediated atom-transfer radical addition was used for direct photolithography process.36,37 Nitroxide mediated photopolymerization (NMP2)38,39 also proved to be a very effective system. Such systems exploit a photosensitive alkoxyamine that creates reactive radical species to initiate polymerization. With this process, radicals can be reactivated during exposure to UV irradiation. In a previous paper,41 we reported the proof of concept of using an alkoxyamine (AA) for the fabrication of functional micro- and nanopatterns by UV lamp and direct laser irradiation. The aim of the present paper is to provide an in-depth study of the main parameters governing the controlled polymerization with the alkoxyamine in the case of a UV lamp irradiation. The impact of photonic and chemical parameters on the repolymerization process was investigated and explained in the light of the expected molecular mechanism. We show that the thickness of polymerized layer can be finely controlled. The lateral size of patterns is ranging from 12 to 50 μm. Finally, applications to illustrate the potential of the AA for generating microstructured surface with control of both chemistry and topography are presented.

2. Experimental Chemicals: Structures of monomers are given in supporting information SI1. All monomers were used as received. Ebecryl 605 (E605) was purchased from Allnex. It is composed of 75


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wt % of bisphenol-A epoxy diacrylate and 25 wt % of tripropyleneglycol diacrylate. Trimethylolpropane triacrylate (TMPTA), pentaerythritol triacrylate (PETA) were purchased from Sartomer. Pentafluoropropylmethacrylate (PFPMA) was purchased from SigmaAldrich. The synthesis of the alkoxyamine used in this work was already described.41 The details of synthesis of the Ru complex monomer used in the last section are given in supporting information (SI2). RT-FTIR: The photopolymerization was monitored in situ by Real time Fourier Transform InfraRed spectroscopy (RT-FTIR) with a Thermo-Nicolet 6700 instrument IR-spectrometer. The sample is coated on a BaF2 pellet and irradiated at 365 nm using 100 W Mercury-Xenon Lamp (Hamamatsu, L2422-02) equipped with a band pass filter. Usually, a polypropylene thin film was used to cover the sample. The polypropylene thin film is used to avoid diffusion of oxygen from surrounding atmosphere. Glass coverslip cannot be used in FTIR experiments because of the strong absorption of glass in the IR region, covering some band of interest of the acrylate monomer. The conversion of the acrylate monomer at a given time is calculated using the area C=C stretching band at 1635cm-1. The conversion is given by : Conversion (%) = (At=0-At)/At=0 x 100 where At is the area of the C=C peak at a given time t. UV-visible absorption spectroscopy: UV-visible absorption spectra were recorded in the 250nm-500nm wavelength range using a Lambda 950 spectrophotometer from PerkinElmer. Quartz substrates were used for depositing the photopolymer film.


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Electron Spin Resonance (ESR) spectroscopy: Nitroxide radicals formation was followed using a Magnettech Miniscope MS400 spectrometer. Samples (10-3 M) were introduced in a 6mm internal diameter quartz tubes and the radicals were generated under light irradiation with a Xe–Hg lamp (Hamamatsu, L8252, 150 W; λ > 310 nm). Tert-butyl benzene was used as an inert solvent. Micropatterning procedure: The complete procedure is depicted in Figure 1. The first polymer layer was prepared from a drop of E605 containing 1wt % of AA, deposited on a Si wafer. UV-curing was led with a Hg-Xe UV lamp (Hamamatsu) equipped with a 365 nm pass-band filter, at room temperature. A glass coverslip was used to cover the sample in order to avoid unwanted inhibition of free-radical polymerization by oxygen from atmosphere. Typical intensity was 22 mW/cm2. E605 was selected for its good adhesion to the substrate.


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Figure 1: Schematic representation of the procedure to a) generate living AA doped polymer thin films by UV first polymerization and produce b) homogeneous thin film or c) micropatterns in a second UV polymerization. For second polymerization, a drop of monomer was deposited on the first polymer layer and the monomer was covered by a glass coverslip. Second irradiation could be carried out homogeneously (Figure 1-b) or locally for micropatterning (Figure 1-c). Micropatterning was obtained by UV-irradiation through a chromium mask. The chromium masks are Ronchi mask with 10 line/mm and 40 line/mm, on quartz substrates, provided by Edmund Scientific. A contact-lithography configuration was used to avoid the diffraction effects causing


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a loss of resolution. Intensity was set to 22 mW/cm2 and a band pass filter was used to select the 365 nm emission band of the UV lamp. After irradiation, the sample was rinsed with ethanol to remove the unreacted monomer, revealing micropatterns that are covalently bonded to the first polymer film. Micropatterns characterization: Atomic Force Microscopy (AFM) and interference microscopy were used to image the micropatterns. AFM images were recorded in air with a PicoPlus microscope operating in contact mode. The probes were commercially available silicon tips with a spring constant of 0.2 N/m (BS-CONT, from BudgetSensor). Interference microscopy analysis was performed with a Nikon microscope equipped with an interferential objective. This technique is interesting to monitor the uniformity of shape and height of the microstructures over a large area. A deviation less than 5% on the height was typically observed over few mm2. Typical images of patterned surfaces with both techniques can be seen in SI3.

3. Results and discussion 3.1. Homogeneous photopolymerization The molecular structure of the alkoxyamine that was used in this study is depicted in Figure 2-a. This alkoxyamine has been optimized from previous works in order to increase the inherent stability of the released nitroxide.39 Indeed, the nitroxide that was used to perform NMP2 experiments was similar to the 2,2-dimethyl-4-phenyl-3-aza- pentane-3-nitroxide prepared by Grubbs and coworkers.40 They showed that this nitroxide was hardly isolable


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and cannot be used to control polymerizations. The replacement of the methyl group by an isopropyl group on the beta position of the nitroxide moiety led to nitroxide similar to 2,2,5-trimethyl-4-phenyl-3-azahex- ane-3-nitroxide that is known to be stable and thus is widely used in classical NMP. The location of the chromophore group on the nitroxide moiety also ensures a reversible cleavage of the macroalkoxyamine during photopolymerization. The molecular pathway leading to free radical polymerization is depicted in Figure 2-a.

Figure 2: a) Molecular structure of the alkoxyamine (AA) and schematic view of the polymerization initiated by the AA. b) Absorption spectrum (Molar extinction coefficient) of the alkoxyamine synthesized for this study. c) Evolution of the ESR spectrum upon UV irradiation. Irradiation time between 2 spectra was 0.1 sec, from 0 sec to 1.5 sec.


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Figure 2-b shows the absorption spectrum of the alkoxyamine (AA) that was synthesized and used in the present study. The range of wavelengths suitable for photopolymerization is wide, up to 400 nm. The selected emission band of the Hg lamp (365 nm) is thus fitting well the absorption spectrum of the AA. The ESR spectra given in Figure 2-c demonstrate the formation of nitroxyl radical under UV irradiation. No other radical species were observed suggesting a selective O-C cleavage process, as expected. This alkoxyamine (AA) was used for repolymerization. The efficiency of the AA for initiating the first photopolymerization and repolymerization was first evaluated by RT-FTIR spectroscopy. The conversion of the monomer versus irradiation time was plotted for the first and second irradiations, as shown in Figure 3. The FTIR spectra before irradiation and after 50 sec, 100 sec and 200 sec are given in SI4 for the first irradiation. The first polymerization is followed as in classical RT-FTIR experiments. A polypropylene cover film (which does not absorb at 365 nm) is used to prevent the diffusion of oxygen from the surrounding atmosphere. The second polymerization is followed using the same set-up but in this case, the photopolymerization is not triggered homogeneously in the volume of the photopolymer as in usual RT-FTIR experiments but from the surface of the first film. The conversion that is deduced is thus an average value along the thickness of the film. In such conditions, the monitored conversion may result also from a further conversion of monomer 1. To avoid this, the conversion of the first polymer was chosen very close to the plateau of conversion. Then, further irradiation of the first polymer does not change significantly the conversion of the first polymer.


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The monitoring of the first polymerization (Figure 3-a) shows that the alkoxyamine is an efficient photoinitiating system for acrylates, in laminated conditions: after a short period of inhibition, corresponding to the time needed to consume the oxygen initially dissolved in the monomer, the conversion of the acrylate monomer is fast and reaches a plateau at almost 50 % after 200 sec. of irradiation. This maximum conversion is expected in the case of the polymerization of bulk multifunctional monomers since the gelation prevents further polymerization. Interestingly, the same experiment carried out without the polypropylene film did not give any significant polymerization. This suggests that the photoinitiating system is extremely sensitive to the oxygen inhibition.


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Figure 3: a) RT-FTIR following of the first polymerization, in laminated conditions (with a cover layer of polypropylene) and at air. E605 with AA (1 wt. %) was used and intensity of 22 mW/cm2 at 365 nm. b) Monitoring of the second photopolymerization. A film of TMPTA (without any photoinitiator) is casted on the first polymer film. Irradiation is carried out with the same intensity (22 mW/cm2). Two irradiation times for the first polymerization were compared: 50 and 200 sec. The re-initiation from the surface of the first film was also demonstrated by RT-FTIR. In this case, two irradiation times for the first polymerization were used. 50 sec. of irradiation corresponds to a conversion of 38 %, which confers to the polymer film good mechanical properties. A second irradiation time of 200 sec. was also investigated. In this case, this irradiation time corresponds to the maximum conversion of the first polymer film (46 % conversion). For the second layer, the reaction was followed over 200 s. FTIR spectra for different irradiation times are shown in SI5, in the case of a first irradiation of 200 sec. The kinetics of photopolymerization are plotted in Figure 3-b. It has to be emphasized that the results for the second layer can not be directly compared to first polymerization since, as illustrated in Figure 1, the conversion is not homogeneous in the thickness of the film. However the kinetics can be compared between the two experiments shown in Figure 3-b because the thickness is the same in both experiments. In both cases, the inhibition time is longer (50 sec.). This is probably due to relative lower concentration of radicals in this configuration because the radicals are formed at the surface and not in the bulk. The inhibition of oxygen has thus a more preeminent role. Interestingly, the polymerization was found to be more efficient in the case of a short first irradiation time (50 sec.). Both polymerization maximum rate (given by the slope of the conversion curve vs. time) and final conversion are indeed


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higher in the case of a shorter first irradiation. This may be due to two different reasons: first, a lower conversion of the first polymer layer is associated to a less crosslinked polymer network. This may favor the photolysis of the alkoxyamine embedded in the polymer matrix and radicals separation. Secondly, one can also argue that a shorter first irradiation will limit the side reaction of alkoxyamine degradation. In both cases, the results obtained are in agreement with a repolymerization that is more efficient with a shorter first irradiation. The second photopolymerization is thus very efficient, which is even surprising for a surface initiated polymerization that usually requires a relatively high concentration of photoinitiator at the surface. Here, there is no driving force to concentrate the alkoxyamine at the surface of the polymer film. This suggests that the second polymerization may partly result in an infiltrated network. The second monomer may indeed diffuse and swell the first polymer layer. The initiation would thus be within the first layer and not only on its surface. However, because the first monomer is chosen to produce a highly crosslinked first polymer network, the possibility of swelling is probably limited to a very thin layer but we can not exclude the contribution of this phenomenon. It could also be argued that the polymerization is initiated from unreacted alkoxyamine molecules and not from previously initiated polymers. Some unreacted alkoxyamine may indeed remain only embedded in the polymer matrix after the first irradiation, especially if low irradiation times are used. However, the results of second irradiation would then be new polymer chains weakly bonded to the first polymer. Such polymer chains would be eliminated when the sample is rinsed in ethanol. Having microstructures strongly bonded


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to the first polymer layer is a proof that second initiation by unreacted alkoxyamines is likely to be negligible.

3.2. Micropatterning In the configuration of micropatterning, the height of the micropattern corresponds to the thickness that is polymerized upon second irradiation. The height of the micropatterns can thus be used to monitor the efficiency and control of second polymerization under various conditions.


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Figure 4: Impact of second irradiation intensity on the height of microstructures. TMPTA was used as second monomer. First film was prepared with E605/AA (1%) and irradiated at 22 mW/cm2 for 50 sec. The lines are for eye guiding. The first irradiation time was fixed to 50 sec (22 mW/cm2). These moderate irradiation conditions correspond to optimal conditions to favor controlled second irradiation, as shown in previous work.38 The influence of the light intensity during the second polymerization is shown in Figure 4. Three different intensities were used and the results of change in thickness (Δh) during polymerization upon second irradiation were plotted versus dose to compare the effect of the intensity for the same quantity of energy deposited in the sample. The results obtained at 3.8 and 22 mW/cm2 are remarkably similar when the dose is below 2200 mJ/cm2. For these mild irradiation conditions, the height of the polymer layer is linear with the dose, confirming the control of polymerization. For high doses, two different behaviors were observed: i) at 3.8 mW/cm2, the linear behavior is maintained. Only a slight loss compared to a linear behavior was observed, which may be due to the loss of reactive species with time by oxygen quenching or radical recombination. ii) at 22 mW/cm2, a significant increase of the polymerization rate was observed for doses higher than 2200 mJ/cm2. The corresponding thicknesses (more than 2 microns) suggest a bulk polymerization of the polymer layer, and not only a surface initiated polymerization. The reasons accounting for such a behavior will be discussed below. Finally, at highest intensity (70 mW/cm2), an erratic growth of the polymer layer was observed. The thickness not only did not follow the linear behavior observed at lower intensi-


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ty but the growth was not even monotone and could not be reproduced from one experiment to another. This observation suggests that a special mechanism occurs when a high concentration of radicals is created leading to a bulk polymerization of the monomer film rather than a surface induced polymerization. It has to be noted that this regime is clearly to avoid if the objective is to build micro-nanostructures with controlled height. This will be discussed below after describing all different experimental conditions (Figure 6). Viscosity of the monomer is another parameter that significantly modifies the radicals recombination dynamic and may have consequences on the control of polymer growth. In Figure 5, we illustrate the role of the viscosity on the growth of the microstructures.

Figure 5: Impact of the viscosity of the second monomer on the height of the microstructures. First film was prepared with E605/AA (1%) and irradiated 50 sec. at 22 mW/cm2. Second polymerization was led with 22 mW/cm2. TMPTA was used in bulk and PETA was


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used as second monomer in solution (10 wt %). (Molecular structures of monomers can be found in SI1) TMPTA was used as a reference monomer for second polymerization. This monomer has the interesting property to be trifunctional (triacrylate) and with a low viscosity (70 cP at 25°C). A high degree of crosslinking is thus expected, which is favorable for microstructuring. In the case of TMPTA, for limited irradiation times, we confirmed the linear growth of the polymer layer versus irradiation time. Using more hydrophilic trifunctional monomers like PETA can be interesting for designing micropatterns with adjustable surface energy. However, pure PETA could not provide controlled polymerization, since, even in mild irradiation conditions (22 mW/cm2), no control over the polymerized thickness could be obtained.41 Interestingly, PETA exhibits a close molecular structure to TMPTA. The only difference is a hydroxyl group on the PETA monomer that induces a significant increase of its viscosity (7500 cP at 25°C). Interestingly, we show here that when PETA was used in diluted conditions in solvent (Acetonitrile) or in HEMA (to refrain from solvents and thus lead environmentally friendly systems), the control of the repolymerization was obtained, as seen in Figure 5. This confirms that the particular behavior of PETA versus TMPTA is due to its higher viscosity. A major difference of diluted monomers compared to bulk polymerization is the limitation of the maximum thickness to few hundreds nm in both cases. The dilution of the reactive acrylate functions probably explains these results. This approach is very useful is low thickness functionalization is wanted.


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To explain the effect of viscosity, one has to refer to the mechanism depicted in Figure 2. If the nitroxyl and carbon radicals cannot recombine fast enough in comparison to the propagation reaction with the monomer, then a loss of control of the polymerization is observed. In a highly viscous medium such as PETA, a high concentration of carbon radical is created at the surface. The probability for these radicals to diffuse in the bulk monomer becomes non negligible. The propagation can be faster than the diffusion of the nitroxide as controlled agent. The consequence is a random propagation of the crosslinking reaction. The result is a rough surface after washing the unreacted monomer with ethanol. These mechanisms are summarized in Figure 6. In this figure, mild conditions correspond to relatively low light intensity and low medium viscosity to favor the radical recombination. The conditions for the first polymerization have to be chosen to optimize the concentration of remaining alkoxyamine groups.

Figure 6: Schematic view of the mechanisms under mild and hard irradiation conditions.


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The repolymerization is also impacted by the nature of the first monomer. Figure 7 compares the results obtained using PETA and E605 as a first monomer. In both cases, 1 wt % of AA was used to induce the polymerization.

Figure 7: Impact of the chemical nature of the first monomer on the repolymerization kinetics. First film was prepared with PETA or E605 and AA (1 wt %). It was irradiated 50 sec. at 22 mW/cm2. TMPTA was used as second monomer. Second irradiation was led at 22 mW/cm2. (Molecular structures of monomers can be found in SI1) The repolymerization was found to be much more efficient when PETA was used as first monomer. Interestingly, in both cases, Δh followed a linear behavior with irradiation time. We explain this difference with different mechanical properties of the polymer films. Indeed, the films from E605 were found to be more rigid than those with PETA. E605 is a mix-


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ture of bisphenol A glycerolate diacrylate and tripropyleneglycol diacrylate optimized for hard coating formation. The softer film prepared from PETA probably favors the dissociation and diffusion of radicals issued from the photolysis of the alkoxyamine. Also, the diffusion of the second monomer (TMPTA) into the first polymer matrix may be favored in the case of PETA polymer, unlike for E605. In the case of E605, reinitiation may be only at the surface, accounting for a lower polymerization rate in step 2. Despite some difference in reactivity, one important conclusion from these results is that the first monomer can be adapted to tune the properties of the first film without compromising the possibility of repolymerization.

3.3. Applications: towards complex functional polymer surfaces

This last section is aimed at illustrating the potential of NMP2 for designing micropatterned functional polymers. In a first example, we present the possibility to prepare microstructures that can be post-functionalized in a second step by thermal polymerization. The microstructures are generated in a first step by classical lithography, with a polymer chosen for its optical (refractive index) or mechanical properties (rigidity, stiffness, flexibility...). These microstructures can then be functionalized to confer specific surface properties (hydrophilic, hydrophobic, anti-adhesive, etc...) or surface functionalities (biocompatibility, fluorescence, molecular recognition, etc....). As shown previously, the AA is to be able to be reactivated by photochemical reaction. We show here that the post-functionalization can


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also be achieved by thermal process, at moderate temperature. Results are shown in Figure 8.

Figure 8: a) Schematic view of the 2 routes for homogeneous grafting of polymer on microstructures achieved by photochemical or thermal process. b) Grafting by thermal repolymerization. First film was prepared with E605/AA (1%) and irradiated 50 sec. at 22 mW/cm2 and thermal grafting was led at 120°C. Second monomer is TMPTA. The AA dissociation was done at 120°C. An increase of the patterns height was obtained at the first stages of thermal heating showing that the formed radicals could initiate a polymerization of the TMPTA monomer. For further heating, a decreased of the height was observed, due to bridging between patterns. This example shows the possibility to generate


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covalently functionalized micropatterns either by thermal or photochemical route. The interest of thermal curing is to be easy to apply, versatile and global. Photochemical functionalization appears complementary, as it is faster, localized and it is an energy saving technique.

The high versatility of the grafting process is also related to the wide variety of acrylic monomers compatible. Fluorinated monomers are of high interest since they can be used to prepare hydrophobic surfaces. Grafting fluorinated polymers on polymer surfaces is not obvious because of the low chemical affinity. Moreover, achieving micropatterns with fluorinated monomers could open new possibilities in superhydrophobic surfaces or for the fabrication of complex microfluidic devices. Figure 9 shows the results of fluorinated monomer grafting using the NMP2 strategy. Pentafluoropropylmethacrylate (PFPMA) was used as a second monomer. The covalent grafting was demonstrated by AFM, water contact angle measurements and XPS. Water contact angle on the E605 polymer film was found to be 80°. After irradiation in contact with PFPMA, the contact angle increased to 120°. It has to be emphasized that a rinsing of the film after grafting was done with ethanol in order to remove the adsorbed PFPMA at the surface of the first polymer film. The change of surface energy is thus due the covalent grafting of the fluorinated monomer and not to adsorption of the PFPMA at the polymer surface. The grafting of the fluorinated monomer was also demonstrated by XPS analysis. The XPS spectra of the UV-irradiated zone and non UVirradiated zone were compared. The UV treated zone revealed specific peaks of F (F KLL at 834 eV and F1s at 688 eV). These peaks are absent in the non-treated part, demonstrating again that the F signal is not due to fluorinated monomer adsorbed at the surface. Interest-


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ingly, the AFM characterization brought additional information at local scale. Indeed, on the grafted area, the surface of the film was not smooth and regular but some holes could be observed. These defects are not linked to the first polymer film that exhibits a perfectly smooth surface as shown on the right side of Figure 9.

Figure 9: Grafting of fluorinated monomer (Pentafluoropropylmethacrylate) on the polymer surface by NMP2. Top images are AFM and water contact angle characterization of left) UV treated and right) non-UV treated surfaces. Bottom graph are the XPS spectra of bare polymer film with AA (grey) and UV treated polymer with fluorinated monomer. First film was prepared with E605/AA (1%) and irradiated 50 sec. at 22 mW/cm2. Second irradiation was led with 30 min at 56 mW/cm2.


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To explain these structures, one has to remind that the fluorinated monomer shows very low compatibility with the polar acrylate monomer. The consequence is the formation of air bubbles at the interface when the fluorinated monomer is casted on the first polymer film. These bubbles are imprinted in the second film. At this stage of the study, the holes could be hardly controlled in size and spatial distribution. So far, we could only achieve mm scale patterning of fluorinated monomer. Additional studies are underway to control this phenomenon that could allow the easy formation of superhydrophobic thin films on polymer by covalently grafting Fluor-containing microstructures on the polymer surface.

The NMP2 process was also used to graft luminescent monomers at the surface of the polymer film. Developing a method to covalently graft luminescent monomers is of high importance for fabrication of chemical sensors and for applications in photonic or biology. In this case, the NMP2 grafting has several advantages: i) a strong and durable anchoring of the luminescent probe is achieved. A covalent binding has advantages over doping a polymer layer with a fluorophore because it avoids the debinding of the fluorophore when the polymer layer is in contact with a liquid medium. ii) The thickness of the fluorescent film can be tuned down to several tens of nm, which is important to guarantee a maximum signal switching. iii) No photoinitiator is needed to achieve the photopolymerization of the fluorescent monomer, which limits the potential interaction with the chromophore. iv) Taking advantages of the possibility to obtain microstructures, complex optical devices including resonators can be produced.


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Figure 10 illustrates luminescent patterns that were produced using NMP2 grafting with masks (100 μm period). Patterns with 100-μm period were produced on a polymer film composed of E605 doped with AA (1 wt %). In Figure 10-b, the micropatterns were composed of TMPTA monomer doped with fluorescein. As seen in the fluorescent image, obtained with excitation at 405 nm, the spatially controlled emission at 488 nm of the fluorescein is recorded. The second example involved a Ru complex acrylate monomer (molecular structure shown in Figure 10-c). In this case, the luminescent complex is covalently bounded to the surface of the polymer. Contrary to the first method, the fluorophore is thus covalently bounded to the first polymer surface, which guarantee a durable anchoring.

Figure 10: Preparation of fluorescent micropatterns. a) is the optical microscopy image of the patterns prepared with E605/AA (1%). Irradiation was carried out during 50 sec. at 22 mW/cm2, through amplitude mask. b) and c) are confocal fluorescence microscopy images (in false colors) of b) TMPTA polymer doped with fluorescein (excitation at 405 nm, emis-


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sion recorded at 488 nm) and c) a Ru monomer diluted in acetonitrile (1/3) (excitation at 405 nm, emission recorded at 488 nm). Functionalization of patterns by fluorescent polymer was carried out in both cases by homogeneous UV-irradiation (200 sec. at 22 mW/cm2).

Ruthenium complexes have attracted high interest in the field of luminescent probes/chemosensors.42 In particular, their phosphorescence yield is known to be highly sensitive to the presence of oxygen in the surrounding medium. Such complexes have thus been proposed as probe material for oxygen sensor. As a preliminary work in this field, we show here the possibility to fabricate thin stable micropatterns, which will be very helpful to easily integrate the functional material onto the sensing optical device. Further works are underway to apply this to the fabrication of an integrated optical sensor.

4. Conclusion

NMP2 was successfully used to prepare living polymer surfaces able to reactivate freeradical polymerization under UV-light (365 nm). Under mild conditions (e.g. low light intensity), the re-initiation process is controlled and leads to a very good control of the thickness versus irradiation time. The conditions for providing an efficient recombination of the radicals were demonstrated to be necessary to achieve a good control of the height of polymerized layer, from few tens of nm to few microns. In particular, parameters such as monomer viscosity and the nature of the first monomer were also found to be important. Moreover, the grafting was proved to be possible also by thermal process. Based on these results,


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grafting of functional monomers with controlled dimension in the plan and out of the plan is possible. This opens very interesting possibilities in the frame of smart polymer surfaces and interfaces and sensors.

Acknowledgements The authors thank French Agence Nationale pour la Recherche (ANR) for funding (project IMPACT) under grant ANR2011-BS08-016. The authors also thank Philippe Fioux for running XPS analysis.

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Table of Contents artwork


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Supporting information

SI1 : Molecular structures of monomers SI2 : Synthesis of the Ru acrylate complex SI3 : Typical examples of microstructures visualized by microscopy SI4 : FTIR spectra during first polymerization SI5 : FTIR spectra during second polymerization


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UV-Induced Micropatterning of Complex Functional Surfaces by

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