Compositional Gradients in Siloxane Copolymers by

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Compositional Gradients in Siloxane Copolymers by Photocontrolled Surface Segregation Alessandra Vitale,*,†,‡ Sarah Touzeau,† Fang Sun,§ and Roberta Bongiovanni†,‡ †

Department of Applied Science and Technology, Politecnico di Torino, 10129 Torino, Italy INSTM−Politecnico di Torino Research Unit, 50121 Firenze, Italy § College of Science, Beijing University of Chemical Technology, 100029 Beijing, P. R. China ‡

S Supporting Information *

ABSTRACT: We demonstrate how to tune the surface chemistry and properties of copolymers obtained by the photopolymerization of polyoxyethylene (meth)acrylic monomers and a low polarity siloxane comonomer which spontaneously migrates to the free surface. By controlling the photopolymerization conditions, such as the light gradient through the thickness of the film and selecting the proper monomer functionality, it is possible to optimize and photoenforce the surface segregation of the siloxane comonomer. Photocured films containing the same amount of siloxane component (1 wt %) can exhibit a surface energy ranging from 42 to 22 mN m−1 depending on the process conditions. XPS and AFM analyses confirm that polymers with a compositional gradient are obtained and that the surface segregation can be finely photocontrolled. Photopolymerization is thus proven to be a facile, single step method for generating gradient films and for independently and simultaneously tune their surface and bulk properties.

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INTRODUCTION The study and control of the surface composition and structure of polymeric materials are essential for their application, as their surfaces are the primarily way of interaction with the environment. The chemical architecture of the first outermost molecular layers of a polymer governs numerous desirable surface characteristics, including wettability, adhesion, paintability, hardness, appearance (e.g., gloss), and haptic properties as well as biological response.1 The tailoring of surface chemistry could thus allow tuning of many surface properties, thereby facilitating advanced applications not accessible with homogeneous polymers. In order to impart specific properties to a polymer surface, chemical or physical functionalization processes,2,3 specifically designed to modify the material surface properties without substantially changing its bulk features, are generally used. Proposed techniques for surface modification include plasma treatment,4 surface grafting,5 and surface coating.2 Currently, polymers with different surface and bulk properties are typically obtained by multiple step processes,3 which require additional production time, are not well controlled, and/or deleteriously affect the final material properties, limiting the scope of application. Thus, alternative solutions to control and modulate both independently and simultaneously surface and bulk chemistry in polymer films, allowing the generation of materials that can be tailored for specific applications, are needed. In the case of copolymers containing structural units of different polarity and having a block or graft architecture, the surface composition can differ from the bulk:6 without any processing step, the material spontaneously arrange to have an © XXXX American Chemical Society

enrichment of the free surface by the less polar building block. The low surface energy component (generally Si-containing or F-containing polymeric segments), in fact, migrates to the free surface, driven by thermodynamic forces, in order to minimize the total energy of the system. It has been evidenced that siloxane chains are likely to form microphases, segregate, and thus be predominant on the surface of block copolymers,7 even at low bulk concentrations, due to their low surface energy relative to the other organic component. For instance, a surface mostly composed by siloxane is obtained when it is copolymerized with polymers such as polystyrene,8,9 bisphenol A polycarbonate,10 poly(methyl methacrylate),11,12 nylon-6,13 and polyurethane.6,14,15 In such systems, the bulk properties depend on both copolymer units, while the surface properties are mainly influenced only by the presence of the siloxane component. However, numerous variables, such as block length and sequence distribution, processing conditions (e.g., annealing and casting solvents), could affect the extent of the segregation and in turn of the properties of the surface.16 The surface segregation of siloxane chains has been studied not only for copolymers but also for melt and solution blends17−19 and in the case of the addition of surface active additives to polymer systems.20 In fact, when a siloxane additive is blended in small amounts (generally 1−2 wt %) with the host polymer melt or solution, a higher concentration of the additive is found at the free surface due to interfacial tension differences. Received: February 13, 2018 Revised: May 4, 2018

A

DOI: 10.1021/acs.macromol.8b00339 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Chemical structures of PESiUA (a), PEGDA (b), PEGDMA (c), and photoinitiator (d). ≈ 33, R = short alkylic chain) and a Si content of 8.7 wt % was synthesized following the procedure described in a previous work28 from a hydroxyl-terminated polyether−siloxane copolymer (XIAMETER OFX-3667, Dow Corning Co. Ltd.). The chemical structure of PESiUA was confirmed by NMR and FT-IR analyses (see characterization details and spectra in the Supporting Information, Figures S1−S3). 1H NMR (δ, CDCl3, ppm): 0.07 (SiCH3), 0.83−1.26 (CHCH3 of PPG, CH3 of IPDI), 1.68−1.80 (CH2 of IPDI), 2.90−2.97 (CONHCH2), 3.36−3.81 (CONHCH, OCHCH3CH2O), 4.23−4.40 OCH2CH2O), 6.11−6.18 (COCHCH2), 5.85−5.87, 6.42−6.46 (COCHCH2). 29Si NMR (δ, CDCl3, ppm): −21.98 (−Si(CH3)2− O−). FT-IR (KBr, cm−1): 3330, 1532 (N−H); 2957, 2867 (CH3, CH2); 1724 (CO); 1640 (CC); 1374 (C−CH3); 1297, 802 (Si− CH3); 1242 (C−O−C); 1099, 1020 (Si−O−Si). The photopolymerizable polyoxyethylene (meth)acrylic monomers used are poly(ethylene) glycol diacrylate (PEGDA) with a molecular weight of 700 g mol−1 (Figure 1b) and poly(ethylene) glycol dimethacrylate (PEGDMA) with a molecular weight of 750 g mol−1 (Figure 1c). Both were purchased from Sigma-Aldrich and used as received. 2-Hydroxy-2-methyl-1-phenyl-propan-1-one (Sigma-Aldrich, Figure 1d) was selected as photoinitiator. All the other chemicals were obtained from Sigma-Aldrich. Photopolymerization Process. In order to obtain the final photocurable reactive formulations, 3 wt % of photoinitiator was added to monomers and comonomeric mixtures. The UV-sensitive formulations were spread on a glass substrate using a bar coater (film thickness h of 10−200 μm) and then irradiated until complete curing by means of a high-pressure mercury arc lamp Dymax ECE under a nitrogen atmosphere, using a light intensity I0 of 20−350 mW cm−2. The light intensity was measured by a UV Power Puck II detector by Electronic Instrumentation & Technology. Characterization of the Photopolymers. After UV irradiation for the time guaranteeing the maximum double-bond conversion for each system, films were characterized. Film thickness was measured with a QuaNix 7500 instrument (Automation Köln). Wettability was assessed by static contact angle (θ) measurements with water (γ = 72.1 mN m−1) and hexadecane (γ = 28.1 mN m−1) by means of a FTA 1000C instrument, equipped with a video camera and image analyzer, at room temperature with the sessile drop technique. On each surface at least five angles were measured immediately after the droplet deposition: the mean value and the standard deviation were determined. The surface energy was calculated by the Owens− Wendt geometric mean method.29 Contact angles were measured on both sides of the film: the surface exposed to air (i.e., air side) and the one in contact with the substrate (i.e., substrate side) during irradiation. Liquid surface tension of the monomeric mixtures, prior to photopolymerization, was measured by the pendant drop method using a DSA 100 (Krüss) surface tensiometer. Atomic force microscopy (AFM) experiments were performed using a Bruker Innova instrument. Film surface morphology and surface phase images were collected in tapping mode with RTESPA300 (Bruker) probes. Data treatment and presentation were realized with the help of Gwyddion Software. Root-mean-squared (RMS) roughness was evaluated from AFM surface morphology images with scan sizes of 20 × 20 μm2. For each sample at least three scans were made on different parts of the films. A PHI 5000 VersaProbe instrument (Physical Electronics) was utilized for X-ray photoelectron spectroscopy (XPS) analysis. A

This spontaneous enrichment of the low surface tension additive on the polymer surface allows to modify the material in a well-controlled manner, obtaining the desired surface characteristics but without substantially changing the overall bulk properties. This approach has thus been recognized as an attractive, low-cost, easy, reliable method for functionalizing polymer surfaces1,21 and obtaining materials that may be useful in different fields, including adhesives, hydrophobic, easy-toclean, antifouling, and self-replenishing coatings, and lubricants.22−25 The process of surface modification by spontaneous segregation is very versatile and has a wide field of application because of the large variety of possible functional groups that can be presented at the polymer surface. Moreover, interestingly, this functionalization method can be coupled with conventional polymer processing operations and can be purely controlled by physical mechanisms (e.g., diffusion, surface segregation, and shear).18 The simultaneous and autonomous tuning of the surface and bulk composition of a polymeric system can also be achieved in a single step by using photopolymerization, thanks to the temporal and spatial control inherent to this process. Cook and Guymon26,27 have reported that UV-curable monomers of similar polarity but different reactivity are capable of spontaneously producing copolymers with a gradient structure during photopolymerization. The monomer with the higher reactivity polymerizes faster at the surface of the film, where the light intensity is higher, and therefore more of that monomer diffuses from the bulk, as it is depleted at the surface. At the same time a counterdiffusion of the lower reactivity monomer from the surface into the bulk occurs, resulting in an enrichment of the polymer surface in the higher reactivity monomer. Building on the concepts of the spontaneous migration of low surface energy monomers to the free surface and of the photoenforced surface segregation, the goal of this work is to develop photocured gradient coatings yielding controllable surface chemistry and properties using comonomers with different polarity and reactivity. Specifically, a diacrylic siloxane oligomer is added in a low amount to poly(ethylene) glycol diacrylate and dimethacrylate, and photopolymerization is then conducted with defined conditions. The surface properties (e.g., wettability, surface energy, composition, morphology) of the photocured copolymers are studied, taking into account factors influencing the surface segregation, such as the surface tension of the monomers but also the light intensity, the thickness of the film, and the type of monomer functionality, and how the spontaneous rearrangements of copolymers interfere with the segregation induced by the light gradient.

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MATERIALS AND METHODS

Materials. A polyether siloxane urethane diacrylate (PESiUA) oligomer with Mn ≈ 5600 g mol−1 (Figure 1a, where n ≈ 16, m ≈ 11, p B

DOI: 10.1021/acs.macromol.8b00339 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules monochromatic Al Kα X-ray source (1486.6 eV, 15 kV voltage, and 1 mA anode current), a power of 25.2 W, and a pass energy of 187.85 eV were used. Analyses were carried out with a takeoff angle of 45° and with a 100 μm diameter X-ray spot size on a square area of 1400 × 1400 μm2, with the aim to have a good average and better statistics of film behavior. The investigation depth was around 10 nm. All binding energies were referenced to the C 1s line at 284.8 eV. An UV/vis spectrophotometer (ATI Unicam UV2) was used to determine the spectral absorbance of the reactive mixtures (monomers + photoinitiator).

linking reaction immobilizes the system forming a chemical three-dimensional network. This selective diffusion and segregation of the siloxane segments to the surface in contact with air was verified in PEGDA + PESiUA systems by contact angle measurements (Figure 2a): contact angle values on the

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RESULTS AND DISCUSSION Photoenforced Surface Modification. Small quantities of siloxane PESiUA oligomer were added to the poly(ethylene glycol)-based monomer α,ω-terminated with acrylic groups (PEGDA). The reactive mixtures were casted on a glass substrate and irradiated with UV light in order to obtain photocross-linked copolymeric networks in the form of films. Specifically, copolymers containing 0.5, 1, and 5 wt % of the siloxane comonomer were synthesized. The photopolymerization reaction of PEGDA + PESiUA systems is very fast and leads to a nearly complete monomer-to-polymer conversion in less than 30 s; a high insoluble fraction is obtained and found to be independent of the film thickness (for the photopolymerization kinetics and gel content details see the Supporting Information, Table S1 and Figure S4). On the macroscopic level, the copolymeric films appear to be transparent, smooth, and uniform, without visual defects. The bulk properties of the photocured copolymers (i.e., the glass transition temperature Tg and the degradation temperatures of the networks) are not influenced by the inclusion of the siloxane comonomer and are unaltered compared to those of the corresponding PEGDA homopolymer (Table S2). The effect of the inclusion of PESiUA on the surface properties of the copolymers was also investigated. According to the literature and to previous data collected by the authors,30 a photo-cross-linked film containing comonomers of different polarity and prepared by casting on a glass substrate shows a chemical composition changing along its thickness, which is remarkably influenced by the surfaces in contact with the film. In fact, in any binary mixture surface segregation can occur owing to the difference in surface tension between the components: in order to minimize the total free energy of the system, the component of lower surface tension is enriched on the free surface of the sample, and its concentration gradually decreases from the surface to the bulk. Interfacial tension is hence the thermodynamic driving force for surface segregation.31−34 In photocurable systems, the compositional gradient generally depends on diffusion happening before irradiation, when the polymer chains are still mobile. Once the cross-linking reaction is completed, the film shows the surface properties deriving by the chemistry of the segregated monomer, while the bulk properties are only dependent on the matrix polymer, if the concentration of the apolar additive is very low (e.g., below 1 wt %). Evidently, as previously reported,35 the migration process can be modified and tuned by changing the substrate and/or using two substrates on both sides of the film. In our case the photopolymerizable formulations are made of two reactive monomers, and the apolar one is in low amount: once they are cast on a glass substrate in air, the siloxane component is expected to segregate at the free surface minimizing interfacial tension with air and forming a nonuniform gradient structure prior irradiation. Then, the photo-cross-

Figure 2. Effect of PESiUA content and monomer functionality on surface properties: static water contact angles θ of PEGDA + PESiUA (a) and PEGDMA + PESiUA (b) systems, measured on the air side (θa) and on the substrate side (θs). Photopolymerization conditions: I0 = 50 mW cm−2, h = 200 μm. θPESiUA is the water contact angle of PESiUA homopolymer and is equal to 104°. When the error bar is not visible on data points, it means that it is within the symbol size.

substrate side (θs) are comparable to those of the polyoxyethylene acrylate homopolymer; whereas contact angle values on the air side (θa) increase with the concentration of PESiUA. By adding as low as 1 wt % of siloxane oligomer, it is possible to make the films hydrophobic: PEGDA is hydrophilic (contact angle with water of 54°), 0.5 wt % of PESiUA increases the contact angle of the materials, and from 1 wt % of siloxane additive the copolymers present a strong hydrophobic character, as θa overcomes 90°, and a plateau is reached. However, this value (θa = 93°) is lower than the water contact angle of PESiUA homopolymer (θPESiUA = 104°). This means that with the system under investigation, even increasing the amount of the low surface energy PESiUA, the surface properties of the cross-linked copolymers are still partially determined by the presence of the polar comonomer PEGDA. In order to further decrease the copolymer wettability, we applied the approach of photoenforcing the surface segregation by changing the monomer functionality.26,27 A poly(ethylene glycol)-based dimethacrylate monomer (PEGDMA), equivalent to the PEGDA used in this study was selected as main monomer, and small quantities of siloxane PESiUA oligomer were added. Photo-cross-linked copolymeric films containing 0.5, 1, and 5 wt % of the siloxane comonomer were produced, as for PEGDA-based systems. Also in this case the photopolymerization conversion is nearly complete, the insoluble fraction is high and independent of the film thickness (Table C

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Figure 3. Effect of incident light intensity I0 on surface properties. (a) Static water contact angles θ of PEGDA + 1% PESiUA and PEGDMA + 1% PESiUA systems, measured on films with h = 200 μm and photocured by using different light intensities. (b) Surface energy (divided in the dispersive γd and the polar γp component) of PEGDA and PEGDMA and their copolymers containing 1 wt % of PESiUA, measured on films with h = 200 μm and photocured by using different light intensities.

S1), and the bulk properties of the copolymers are not affected by the inclusion of the siloxane comonomer (Table S2). As the two matrix monomers PEGDA and PEGDMA have analogous structure, similar viscosity and surface tension (i.e., 39.4 and 40.4 mN m−1 for PEGDA and PEGDMA, respectively), and build up networks with comparable mobility (i.e., the Tg of PEGDA and PEGDMA is very close), the diffusion process of PESiUA leading to selective surface segregation should be the same. In fact, liquid surface tension data show that comonomeric mixtures containing 1 wt % of PESiUA have a similar surface tension when PEGDA and PEGMA are used (27.3 and 28.8 mN m−1, respectively). Wettability data for PEGDMA + PESiUA copolymers are represented in Figure 2b and clearly show the selective segregation of the siloxane segments to the free surface. Interestingly, in this case, the air side of the copolymers presents a water contact angle as high as the water contact angle of PESiUA homopolymer (θPESiUA = 104°). Thus, the results demonstrate that an enhanced segregation occurs when PEGDMA is used as matrix monomer and indicate that the wettability changes cannot only be explained by the surface segregation induced by the interfacial tension: as the two monomers differ only in functionality type, it is evident that also the kinetics of photopolymerization reaction controls the PESiUA segregation and consequently the copolymer surface properties. In radical reactions methacrylic groups are less reactive than acrylic groups due to the sterical hindrance of the methyl group and the induced stabilization of the radical.36,37 This is evident also in our experimental conditions (see the photopolymerization conversion curves of PEGDA and PEGDMA in Figure S4). In the investigated copolymeric systems, due to thermodynamic reasons, there is first a surface segregation of the siloxane component in the coating step. Then, when the main monomer is PEGDMA, PESiUA has additional time during irradiation to further diffuse and migrate toward the air surface before the reaction is completed. Instead, the photo-cross-linking reaction of PEGDA + PESiUA systems has a faster kinetics, and the time for the migration of the siloxane oligomer to the free surface is hence reduced: in this case surface segregation is mostly happening before irradiation. Accordingly, a more efficient surface photoenforced modification by PESiUA takes place with methacrylic networks, as shown by the experimental data.

As the contact angle results suggest that the surface wettability of the studied copolymers containing siloxane additives is influenced by the polymerization reactivity speed and in turn by the functionality of the comonomers, the incident light intensity I0 of the UV radiation is expected to have an impact on the surface segregation. Photopolymerization efficiency is in fact influenced by absorption coefficient values and by UV light intensity.38 By increasing I0, the reaction rate is incremented, the time for the migration of siloxane chains to the surface of the film before reacting should be reduced, and thus the resulting contact angle should decrease. Figure 3a shows the effect of incident light intensity I0 on the surface properties of PEGDA and PEGDMA systems containing 1 wt % of PESiUA: the contact angle θa of the copolymers decreases when the reaction rate is faster, and this feature is especially evident for the systems having PEGDMA as matrix. At the highest reaction rate (i.e., highest light intensity) there is nearly no difference between PEGDA and PEGDMA copolymers: the wettability relies on the PESiUA surface concentration obtained by the enrichment happening before irradiation and depending on the diffusion at the liquid state (as said before, diffusion is expected to be very similar in PEGDA and PEGDMA monomers). The compositional surface modification means also the change of surface energy γ of the photocured films, which was estimated with the Owens−Wendt model using the values of contact angle with water and hexadecane as test liquid. The γ values for the copolymers (h = 200 μm) photocured at different light intensity are reported in Figure 3b. With the addition of 1 wt % of PESiUA, the surface energy of the systems significantly decreases, and due to the apolar character of the siloxane oligomer, the polar component of the surface energy (γp) is drastically reduced. While for PEGDA there is only a limited effect of intensity on the value of γ, in the case of PEGDMA there is a stronger reduction of surface energy when the copolymer is obtained by irradiation at low light intensity. The same trend is even more evident when the intensity effect is evaluated for films as thin as 10 μm: in this case, for PEGDMA + 1% PESiUA, γ = 42 and 22 mN m−1 for a light intensity of 50 and 20 mW cm−2, respectively. This indicates that even in thin films, where the surface-to-volume ratio is higher, the amount of the siloxane oligomer is enough to impart a highly hydrophobic character, which is obtained if the curing rate is adequately low for complete segregation to happen. D

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Figure 4. Effect of film thickness on surface properties, I0 = 50 mW cm−2. (a) Static water contact angles θa of PEGDA + 1% PESiUA and PEGDMA + 1% PESiUA systems, measured on films with thickness h = 10 μm and h = 200 μm. (b) Surface energy (divided in the dispersive γd and the polar γp component) of PEGDA and PEGDMA and their copolymers containing 1 wt % of PESiUA measured on films with thickness h = 10 μm and h = 200 μm. (c) Calculated decrease of light intensity I across the sample thickness, following eq 1.

As before, this phenomenon is enhanced in the systems having PEGDMA as matrix due to the slower polymerization rate of the methacrylate. For thinner films there is no difference within the experimental error between acrylate and methacrylate copolymers. Clearly the same effect could be obtained keeping the thickness constant and changing the photoinitiator and/or its concentration. Other options to tune the light gradient, and in turn the photoenforcing process as well as the surface properties of the films, are the use of dyes, photostabilizers, and any other additive containing a chromophore. We have also explored how the surface segregation is affected by imposing a waiting time between the film formation and the UV irradiation (Table S3), as reported for fluorinated systems.34 It is interesting to notice that the surface segregation and consequently the contact angle values can be improved by the waiting time prior to irradiation, but this effect takes place over long times. In fact, in order to optimize the surface hydrophobicity, i.e., obtaining a contact angle higher than 90° and 100° for the PEGDA and PEGDMA copolymers, respectively, a waiting time of around 24 h is needed. Thus, the modification process applied to reach the minimum value of surface energy can be preferably conducted by tuning the photopolymerization conditions (irradiation intensity and light gradient) instead of imposing a waiting time prior to irradiation. The siloxane segregation in different photocuring conditions can be estimated by calculating the surface composition using the Cassie model,39 which describes the effective contact angle of a liquid θ on a composite surface as a function of the area fractions of the components (eq 2):

The surface segregation could also be tuned by changing the film thickness h (Figure 4a,b): this effect is due to the decrease of the light intensity I across the sample depth. In other words, the higher the thickness, the lower the reaction rate in the bulk of the film and the lower the resulting wettability (i.e., higher contact angle and lower surface energy). The evolution of spectral radiation intensity within the sample thickness (Figure 4c) can be described knowing the absorption coefficients of the system and applying the Lambert−Beer law,38 as in eq 1: I(h) =

∫λ

λ2

1

I0e−αλh dλ

(1)

where I is the light intensity at a certain depth h in the sample, I0 is the incident light intensity (i.e., light intensity at the sample surface h = 0 μm), αλ is the sample absorption coefficient (αλ = ελ c), ελ is the molar extinction coefficient of the photoinitiator, which strongly depends on the wavelength λ considered,38 c is the photoinitiator concentration, and λ1 and λ2 are the wavelengths delimiting the photoinitiator efficiency range, namely 300 and 380 nm, respectively. In our case αλ was experimentally measured by UV/vis spectroscopy and is the same for all systems, as the same photoinitiator at the same concentration was used. Thicker films show a remarkable light intensity gradient along their thickness and a significant difference ΔI between the two faces, which slows down the reaction in the bulk and better allows the diffusion of the siloxane chains to the free surface of the film. In a 200 μm thick film the decrease in intensity between the top surface (h = 0 μm) and the bulk (midthickness, h = 100 μm) is 73%, while for a 10 μm thick film the decrease is 8% (Figure 4c). E

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Table 1. Area Fractions of the Siloxane and Polyoxyethylene Components on the Surfaces of Copolymers Containing 1 wt % of PESiUA, Calculated by Eq 2 PEGDA + 1% PESiUA

PEGDMA + 1% PESiUA

air side air side air side substrate side air side air side air side substrate side

h= h= h= any h= h= h= any

−2

10 μm, I0 = 50 mW cm 200 μm, I0 = 50 mW cm−2 200 μm, I0 = 200 mW cm−2 photocuring conditions 10 μm, I0 = 50 mW cm−2 200 μm, I0 = 50 mW cm−2 200 μm, I0 = 200 mW cm−2 photocuring conditions

θ (deg)

f PESiUA

f PEGD(M)A

72 91 88 57 63 103 87 45

0.32 0.74 0.67 0.04 0.25 0.98 0.67 0

0.68 0.26 0.33 0.96 0.75 0.02 0.33 1

Figure 5. Surface morphology of photocured PEGDA and PEGDMA copolymers containing 1 wt % of PESiUA. (a) Photograph of transparent PEGDA + 1% PESiUA and PEGDMA + 1% PESiUA films on a glass substrate. (b−f) Tapping mode AFM 5 × 5 μm2 phase images of 200 μm thick films, photo-cross-linked using different light intensities: (b) substrate side of PEGDMA + 1% PESiUA, I0 = 50 mW cm−2, (c) air side of PEGDA + 1% PESiUA, I0 = 50 mW cm−2, (d) air side of PEGDA + 1% PESiUA, I0 = 200 mW cm−2, (e) air side of PEGDMA + 1% PESiUA, I0 = 50 mW cm−2, (f) air side of PEGDMA + 1% PESiUA, I0 = 200 mW cm−2.

cos θ = fPESiUA cos θPESiUA + fPEGD(M)A cos θPEGD(M)A

Summarizing the previous results, PESiUA in low amount modifies the surface of photo-cross-linked polymers. The hydrophobicity is selectively obtained on the free surface and can be tuned changing different process parameters. Segregation toward the air surface of the siloxane segments is governed by both the interfacial tension driving force, the inherent monomer reactivity differences, and the light gradient. For the best effectiveness of the siloxane oligomer, it should be copolymerized with methacrylic monomers, at low light intensity in the presence of a substantial light gradient. Surface Morphology and Composition. PESiUA oligomer was selected in this work as its polyoxyethylene groups in the main chain ensure solubility in the main matrix monomer, thus avoiding bulk phase separation. In fact, as said before, the copolymeric films obtained have a single Tg and are smooth and transparent at a visual inspection (Figure 5a). At a submicrometric scale, when the materials are examined by AFM, a RMS roughness lower than 15 nm (independently on

(2)

being f PESiUA and f PEGD(M)A the area fractions of the comonomers and θPESiUA and θPEGD(M)A the contact angles of pure, homogeneous photocured homopolymers. Results are reported in Table 1 and show that PESiUA chains occupy up to 98% of the air surface of the copolymeric film when a methacrylic monomer is used, the film thickness is high, and the material is cured with a low light intensity. On the substrate side, the siloxane area fraction is instead close to or even equal to zero. Therefore, there is a photocontrolled molecular migration of the low surface tension PESiUA chains to the film/air boundary. In the case of thin films, high light intensity, or fast reacting monomers, the photo-cross-linking kinetics could be too quick to allow a complete surface segregation of the siloxane oligomer (see also additional data in Table S4). F

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Table 2. Atomic Concentration Measured by XPS Analysis of Polar Homopolymers PEGDA and PEGDMA, Their Copolymers Containing 1 wt % of Siloxane PESiUA, and Homopolymer PESiUA atomic concentration (%) PEGDA PEGDA + 1% PESiUA

PEGDMA PEGDMA + 1% PESiUA

PESiUA a

experimentala calculated h = 10 μm, I0 = 50 mW cm−2 h = 200 μm, I0 = 50 mW cm−2 h = 200 μm, I0 = 200 mW cm−2 calculated experimentala calculated h = 10 μm, I0 = 50 mW cm−2 h = 200 μm, I0 = 50 mW cm−2 h = 200 μm, I0 = 200 mW cm−2 calculated calculated

Any photocuring conditions.

C

O

N

Si

Si/C

65.2 66.67 71.3 63.8 64.4 66.68 68.1 68.00 64.2 63.2 62.0 67.99 67.46

34.4 33.33 21.5 26.5 27.2 33.26 30.6 32.00 29.2 26.1 27.0 31.94 26.04

0 0 1.4 2.9 1.8 0.01 0 0 1.2 1.8 1.8 0.02 1.77

0 0 5.0 6.8 6.5 0.05 0 0 5.3 9.0 7.7 0.05 4.73

0 0 0.07 0.11 0.10 0 0 0 0.08 0.14 0.12 0 0.07

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CONCLUSIONS We have demonstrated that photopolymerization allows for a facile, single step method of generating films with a chemical composition gradient when two monomers having different surface tension are reacted. The surface modification (i.e., the surface enriching of Si-containing segments) is driven by thermodynamic forces making the apolar monomer to migrate to the free surface; this feature can be better controlled taking advantage of the different reactivity of the comonomers and photoenforcing the initial surface migration of the siloxane chains. Using a methacrylic monomer for the matrix and exploiting its slower reactivity kinetics, the enrichment was hence optimized obtaining films with a surface tension as low as γ = 22 mN m−1, in the presence of 1 wt % of siloxane comonomer. Moreover, the surface properties of the material could also be tuned by changing the light intensity used for the photocuring process and the light gradient in the film (e.g., by modifying the film thickness): depending on the local polymerization kinetics, different surface compositions and surface wettabilities were obtained. We demonstrated that for the best effectiveness of the low surface tension siloxane acrylic oligomer, it should be copolymerized with methacrylic monomers, at low light intensity and in the presence of a substantial light gradient. The ability to independently and simultaneously control and modulate the surface and bulk properties of polymers was proven, enabling the production of gradient materials that can be tailored for specific applications.

the composition and photocuring conditions) is measured. At this scale the substrate side is still homogeneous (Figure 5b) as no siloxane is present according to the wettability results; on the contrary, a clear phase separation can be detected on the film/air surface (Figure 5c−f). The domains appearing in the AFM images are likely to be formed by the siloxane blocks of PESiUA. Upon comparison of Figures 5c with 5d and Figures 5e with 5f, it is evident that the surface morphology depends on the photocuring conditions (e.g., light intensity). Also, the functionality of the main matrix monomer is relevant: the increased size of the siloxane domains on the surface of PEGDMA-based films is due to the slower reaction kinetics, which enables a higher migration of the low polarity chains to the free surface and an enhanced phase separation process prior to complete cross-linking. We conducted XPS analysis to examine the composition of the outermost surface of the photocured polymers and determine the siloxane localization level. The XPS survey spectra of the copolymers containing 1 wt % of PESiUA possess four main characteristic peaks, associated with C, O, N, and Si. Table 2 lists the experimental values measured by XPS for atomic concentration of both homopolymers and copolymers as well as the calculated theoretical values. Regarding homopolymeric systems, the experimental data correspond accurately to the theoretical ones, whereas the results for copolymers PEGDA + 1% PESiUA and PEGDMA + 1% PESiUA indicate that most of the siloxane moiety introduced in the copolymer structure is located in the outermost layer of the cross-linked network. In fact, the calculated Si concentration is 0.05 at. %, while in all the studied copolymers a concentration higher than 5 at. % is detected. The Si content reaches 9 at. % when the best segregation conditions are applied (i.e., slow kinetics: low reactivity comonomer, high film thickness, low light intensity). This value overcomes the calculated Si concentration of PESiUA and approaches that of the siloxane precursor used to synthesize it (12 at. %). These data thus confirm that the low wettability of the copolymers on the air side is linked to variations in the nature of the chemical groups on the solid state (i.e., migration of siloxane chains to the air surface) and that the film surface is enriched with siloxane chains.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00339. PESiUA oligomer characterization, photopolymerization reaction kinetics, bulk properties of photocured copolymers, effect of time prior to irradiation on the surface properties of photocured copolymers, relationship film thickness−irradiation intensity (PDF)

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Corresponding Author

*E-mail [email protected] (A.V.). G

DOI: 10.1021/acs.macromol.8b00339 Macromolecules XXXX, XXX, XXX−XXX

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Alessandra Vitale: 0000-0002-8682-3125 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. Mauro Tortello for assistance with AFM analyses. REFERENCES

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