Silica Hybrid

Article pubs.acs.org/Langmuir

Superhydrophobic and Highly Luminescent Polyfluorene/Silica Hybrid Coatings Deposited onto Glass and Cellulose-Based Substrates Raquel de Francisco, Mario Hoyos,* Nuria García, and Pilar Tiemblo* Instituto de Ciencia y Tecnología de Polímeros, ICTP-CSIC, c/Juan de la Cierva, 3, 28006, Madrid, Spain ABSTRACT: Neat poly(9,9-dioctyl-9H-fluorene) (PFO) and composites of PFO and a modified organonanosilica P7 at weight ratios 90/10, 70/30, and 50/50 have been employed to prepare fluorescent and superhydrophobic coatings by spraying onto three different substrates: glass, Whatman paper, and a filtration membrane of mixed cellulose esters. The water repellency of the coatings and their photophysical properties are therein studied. It is found that, irrespective of the substrate and the composite composition, all coatings remain fluorescent. In some of the coatings prepared, confined morphologies are created, which fluoresce with a wavelength distribution resembling that of an ordered planar β-phase. Among the coatings prepared in this work, those with a ratio PFO/P7 of 50/50 are the ones with the strongest chain confinement and the highest surface roughness, being highly emissive at the β-phase wavelengths and also superhydrophobic. Depending on the substrate these materials are also tough and flexible (cellulose based substrates) or display a remarkable light transmittance (glass). A final merit of these multifunctional materials is the simplicity of the preparation procedure, adequate for large surfaces and industrial applications. an α-phase and a metastable α′-phase have been described, together with nematic and amorphous phases, a weakly ordered planar β-phase, and the recently described monoclinic γ phase, a transition state between the β and glassy phases of PFO.26,27 The β-phase of PFO has been related to an excellent performance in devices28−30 and improved optoelectronic properties.17,26 It involves a planar conformation of the main chain with an extended conjugation as proposed by Cadby et al.31 The β-phase is dependent upon the chains interactions,32 and either aggregation or chain folding can lead to its formation.33 Recently, the promotion of β-phase in confined environments, such as organic-inorganic hybrids, has been demonstrated.34,35 The absorption spectrum of PFO in solution is characterized at room temperature by a main band at 385 nm, caused by the S0 (ground state)/S1 (first excited state) transitions, and an additional band at 435 nm assigned to the S0/S1 0−0 transition of the β-phase.17,20,36 Though the β-phase is frequently a minor constituent in the photoabsorption, it tends to dominate the emission spectrum26,37,38 due to a combination of direct excitation of this phase and either Föster-type energy transfer or single exciton migration from the glassy PFO phase to the lower energy β-phase.26,39,40 The photoluminescence (PL) spectrum of PFO in solution is frequently composed of a set of S1/S0 bands with vibrational

1. INTRODUCTION Multifunctional superhydrophobic surfaces have been widely pursued over the past years for potential applications in interdisciplinary technological fields, including microelectronics, biosensors, smart structural coating materials, and microfluids.1−6 Superhydrophobic surfaces are those where the advancing water contact angle θadv > 150° and the difference between the advancing and receding contact angles (Δθ = θadv − θrec), hysteresis, is Δθ < 10°,7,8 and in practice these type of surfaces may behave as water-repellent. The development of water-repellent conjugated polymers (CPs), i.e., light emitting and/or conducting polymers for nonwettable electronics and waterproof electronics, is among the latest applications of superhydrophobicity.9,10 Despite the scarcity of references in this field, some preparative strategies have already been proposed,11−14 and many more will certainly appear in the near future given the obvious impact these materials will have. As a particularly important family of CPs, polyfluorenes were introduced in the 1990s by Fukuda and others15−17 and have attracted much attention over the past few years because of their excellent photophysical and optoelectronic characteristics.17−20 PFO is one of the best blue-light-emitting polymers due to its high photoluminescence (PL) quantum efficiencies and good thermal and chemical stability, as well as good chargetransporting characteristics.21 Molecular organization of PFO is critical to the control and tuning of its optoelectronic properties. PFO, well-studied16 in terms of color emission and lifetime,22 exists in different conformations in the solid state and in solution.23−25 Crystalline PFO is polymorphic, and © 2015 American Chemical Society

Received: September 8, 2014 Revised: March 6, 2015 Published: March 9, 2015 3718

DOI: 10.1021/acs.langmuir.5b00293 Langmuir 2015, 31, 3718−3726

Article

Langmuir structure, being the fundamental 0−0 transition under 420 nm. In the solid state (or in concentrated solutions),21 the PL emission bands also display vibrational structure located in the 420−550 nm range The S1/S0 0−0 vibrational transition of the phases N, α, α′, and β appears at progressively higher wavelengths, from about 423 nm (N) to close to 440 nm (β). Two other bands from excited vibrational states typically appear, where the same wavelength order stands as for the fundamental band, i.e., lowest wavelengths for N and highest for β, and so the spectrum of the β-phase is composed of three peaks at 440, 468, and 498 nm.40,41 We have described in previous work42,43 how adequate materials design can result in multifunctional polymer/silica hybrids with a superhydrophobic character. In this work, the intensity and spectral features of the PL emitted by PFO/silica hybrids deposited onto glass and cellulose-based substrates are studied in relation to the hybrids’ morphology and hydrophobicity. As a whole, these hybrids display very appealing properties, namely, the enhancement of the PFO luminescence, the promotion of the PFO β-phase emission and the superhydrophobicity of certain among them. The combination of these properties makes them true multifunctional superhydrophobic and light-emitting materials.

Table 1. Nomenclature, Composition, and Emission Maxima of PFO Solutions and PFO/P7 Suspensions in Toluene composition (wt %)a sample ‑5

S-10 S-0.1 S-0.7 S-1.4 S-2.8 S-2.8-10 S-2.8-30 S-2.8-50

PFO −5

10 0.1 0.7 1.4 2.8 2.8 2.8 2.8

P7

PL emission maxima (λem)

0 0 0 0 0 10 30 50

418/440/469 440/467/ 443/469/ 443/468/ 446/466/ 424/466/491 424/464/441/466/496

a PFO wt % with respect to toluene and P7 wt % with respect to the final solid content.

10 applications, the obtained thickness coating is around 1.5 μm. The list of coatings prepared, their composition and characterization appears in Table 2. They have been identified according to the type of

Table 2. Nomenclature, Composition, Static Water Contact Angle θw, and Emission Maxima of PFO and PFO/P7 Coatings Deposited on Different Supports sample

2. EXPERIMENTAL SECTION 2.1. Materials. Solvents for reaction, filtration, and chromatography were certified ACS grade and were purchased from SigmaAldrich. Hydrophilic fumed silica (Aerosil 200, nominal diameter 12 nm), hereafter called A200, was kindly provided by Degussa and polydimethylsiloxane (PDMS, silanol terminated; 4−6% −OH; viscosity 16−32 cSt) was purchased from ABCR. A detailed description of the materials for the polymerization of PFO is given elsewhere.42 Three different substrates were used for the preparation of the coatings: glass microscope slides (G), filtration membranes of mixed cellulose esters, cellulose acetate, and cellulose nitrate (HAWP02500 from Millipore with 0.45 μm of nominal pore) (C) and Whatman filter paper (pore size: 8 μm particle retention) (W). 2.2. Preparation of Organosilica (P7) and Synthesis of PFO. P7 organosilica is prepared by adsorption of PDMS onto hydrophilic fumed silica at room temperature in toluene solution. The particle aggregate size, Zave, and the polydispersity index, Pdi, of P7 were 187 nm ± 17 and 0.18 ± 0.05, respectively. A detailed description of the preparation and characterization of P7 and PFO is given elsewhere.42−44 2.3. Preparation of PFO Solutions and Coatings. Solutions with different percentages (w/w) of PFO in toluene were prepared to produce coatings on the different substrates. 10−5, 0.1, 0.7, and 2.8 wt % solutions were prepared and subsequently sprayed onto the three substrates: G, C, and W. The list of solutions, their composition, and characterization appear in Table 1. 2.4. Preparation of PFO Composite Coatings. The corresponding amount of PFO (2.8 wt %) was dissolved in toluene (3 mL) to form homogeneous solutions to which a certain amount of P7 was added to achieve coatings with a 10, 30, or 50 wt % of P7 with respect to the final solid content. Suspensions were stirred for 24 h, after which toluene was evaporated down to 75 wt % of the mixture. The purpose of the controlled evaporation was to gradually increase the viscosity of the mixture while stirring as this has been proven to enable a good dispersion of the organosilica in the final mixture and to prevent the deposition of the nanoparticles.42 The list of suspensions, their composition, and characterization appear in Table 1. Regardless of the substrate used, the coatings were prepared by spray-coating. This versatile methodology allowed the deposition of the different coatings in all cases. Coatings were deposited with a Sealy AB931 airbrush at 1.4 bar and at a distance of 35 cm to the support. The coating thickness prepared by this technique depends on the number of spray applications. For a conventional procedure with 7 to

G G-10‑5 G-0.1 G-0.7 G-2.8 G-2.8-10 G-2.8-30 G-2.8-50 G-P7 C C-10‑5 C-0.1 C-0.7 C-2.8 C-2.8-10 C-2.8-30 C-2.8-50 C-P7 W W-10‑5 W-0.1 W-0.7 W-2.8 W-2.8-10 W-2.8-30 W-2.8-50 W-P7

P7 (wt %)

PL emission maxima (λem)

θw (deg)

PFO Coatings − Glass Support (G) 54 ± 0 441/466/496 54 ± 0 444/466/494 107 ± 0 446/464/491 108 ± 0 423/460/491 107 ± 10 424/464/491 109 ± 30 424/439/465/491 157 ± 50 424/439/464/497 164 ± 100 165 ± PFO Coatings − Cellulose Membrane (C) 0 0 439/463 0 0 440/465/500 152 ± 0 442/464/496 150 ± 0 444/463/498 151 ± 10 424/455/489 107 ± 30 425/463/489 153 ± 50 440/466/495 165 ± 100 165 ± PFO Coatings − Whatman Paper (W) 0 0 439/466 0 0 441/464/497 0 0 445/466/498 138 ± 0 448/467/496 135 ± 10 424/439/464/495 123 ± 30 424/442/466/495 150 ± 50 440/467/497 167 ± 100 160 ±

2 2 1 1 1 1 1 2 2

Δθ (deg) >20 >20 >20 >20 18 ± 5± 3± 13 ±

2 2 1 2

3 1 4 2 1 3 2

14 ± 9± 9± >20 5± 0± 13 ±

4 2 5 1 0 2

5 1 1 1 1 2

13 ± 14 ± >20 9± 2± 15 ±

2 4 4 2 2

substrate and the amount of PFO, followed by the amount of P7 (e.g., C-2.8-50 corresponds to a coating prepared onto a filtration membranes of mixed cellulose esters from a suspension of 2.8 wt % of PFO in toluene and with a 50 wt % of P7). To allow comparison, coatings with only P7 have been prepared from a suspension of 1.7 × 10−3 wt % of P7 in toluene, and they are also included in Table 2. 2.5. Characterization. Scanning electron microscopy (SEM) images were recorded using a Philips XL30ESEM equipped with a 3719


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Langmuir tungsten filament. The fractured hot pressed films were sputter-coated with Pd/Au (20/80) at an argon pressure of 0.1 Torr for 3 min at a current of 10 mA. UV−vis absorption spectra were recorded in toluene solutions in a Varian Cary 3-Bio UV−vis spectrophotometer and photoluminescence spectra were recorded in toluene solution or on supported coatings in a Varian Cary Eclipse fluorimeter. Excited and emission slits = 5 nm, emission filter = 1% attenuator, and spectral resolution = 0.5 nm. In order to determine the emission spectrum of a particular coating, the wavelength of maximum absorption is determined by UV−vis and the coating is excited at that wavelength. The excitation wavelength (λexc.) was 385 nm for all samples. Contact angles were measured at 25 °C with Milli-Q grade water by the sessile drop method using a conventional drop shape analysis technique (Attension Theta optical tensiometer). For dynamic measurements (advancing and receding contact angles required to obtain Δθ) the initial water drop volume was 5 μL and the volume was increased up to 10 μL by water disposal at 0.2 μL s−1 recording and analyzing 10 images per second during this process. A constant contact angle value is achieved, which is considered the advancing water contact angle. Then, 5 μL of water was removed at the same rate following identical analysis protocol which enables the determination of the receding contact angles. The determination of the contact angle from the captured images is done by the Young−Laplace method. In the experiments a ∼5 μL volume water drop was brought into contact with the surface in static conditions, and static water contact angles (θw) were determined. The contact angles values reported in Table 2 are the average of at least five measurements in different coating regions. The surface topography was examined using an AFM multimode controller (Veeco Nanoscope IVa, Santa Barbara, CA) in tapping mode to map the substrate morphologies at ambient conditions. The root-mean-square roughness, Rq, which is the standard deviation of feature height (Z) values within a given area, was calculated from 50 μm × 50 μm AFM images (0.3 kHz scan rate). The images were processed with Nanoscope software v 6.14R1 and WSxM software to perform the 3D roughness analysis.45

Figure 1. Filmstrip illustrates the water drop bouncing on the surface and finally being repelled away. Pictures in the lower row illustrate the transparency of G-2.8-50 (left) and the mechanical performance of W2.8-50 (right).

to the luminescence of the PFO solutions and PFO/P7 suspensions employed to prepare them, which appear collected in Table 1 and Figures 2 and 3. When considering the spectral characteristics of the solutions and suspensions in Table 1 and Figures 2 and 3 it is important to bear in mind that their concentrations are in the range of those required to coat a substrate, which are much higher than those employed in photophysical studies. Hence, phenomena such as quenching or reabsorption, as well as aggregation, are to be expected. In Figure 2a the absorption spectra of S-10−5 and S-0.1 are presented. The α-absorption at 385 nm is already saturated for S-0.1. The intensity of the α-absorption is overwhelmingly higher than that of the β-phase at 435 nm. An enlarged view of this region appears on the right inset of Figure 2a, showing that a shoulder is visible in the solution with larger concentration. The fluorescence spectra of the solutions appear in Figure 2b. That of S-10−5 exhibits a well-resolved vibronic structure with the 0−0 transition centered at ∼418 nm and other two peaks at 439 and 469 nm. More concentrated solutions produce less intense emissions, basically because the peaks at wavelengths under 418 nm disappear as the concentration increases. These spectra are composed of peaks between 439 and 469 nm and a long tail at around 500 nm, consistent with the formation of a β-phase.47 The disappearance of the low wavelength emissions in favor of the β-phase emission as the concentration increases is a frequently reported phenomenon attributed, as explained in the Introduction, to an efficient energy transfer to the lower energy aggregates or β-phase domains. This efficient energy transfer to the β-phase has been observed in films as well.39 Figure 3 collects the evolution of the PL spectrum as the P7 concentration content increases in the PFO/P7 suspensions: when 10 wt % of particles is added, the emission at 440 nm disappears and a clear shoulder at 418 nm, also present in S-2.830, is seen which strongly resembles the low wavelength emission seen in the less concentrated solution S-10−5 (Figure 2b). The particles seem to exert a dilution-like effect on the PFO suspensions.

3. RESULTS AND DISCUSSION All PFO/P7 coatings described in this work are luminescent and hydrophobic. However, there is a marked effect of the substrate nature, the concentration of the solution, and the P7 particle content on the wetting behavior and on the emission wavelength distribution and its intensity. Being PFO a hydrophobic polymer by itself, the hydrophobicity of PFO/P7 composites depends on the coating’s final surface roughness, which in turn depends on the ratio of P7 and on the characteristics of the substrate chosen (G, C, or W). Thus, depending on the combination of substrate and content of P7 particles, the PFO/P7 coatings show wetting behaviors ranging from ultrahydrophobic to superhydrophobic or even pearl bouncing droplet (θadv > 150° and Δθ = 0°) where a water droplet carefully deposited on a plane surface will bounce away.46 Together with the luminescence and strong hydrophobicity, these materials add the properties of the substrate on which they are deposited. Several illustrations of this multifunctionality appear in Figure 1. The top four pictures show the highly efficient pure blue luminescence of C-2.8-50 in Table 2, together with the pearl bouncing behavior of a water droplet deposited on it. On the lower row, the relative transparency of G-2.8-50 and the paperlike properties of W-2.8-50 are shown, both being also fluorescent and superhydrophobic as C-2.8-50. 3.1. Photoluminescence of the PFO Solutions and PFO/P7 Suspensions. Before studying the characteristics of the coatings’ luminescence, some consideration has been given 3720


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Figure 2. (a) Absorption spectra of S-10−5 (black) and S-0.1 (blue) in toluene and (b) PL spectra of all the PFO solutions: S-10−5, S-0.1, S-0.7, and S-2.8 in toluene.

Figure 3. PL spectra of the PFO solution S-2.8 and the PFO/P7 suspensions: S-2.8-10, S-2.8-30, and S-2.8-50 in toluene.

3.2. Emission of the PFO and PFO/P7 Coatings on Different Substrates. 3.2.1. Coating Morphology and Surface Roughness. The PFO coatings sprayed onto the three different substrates, G, C, and W, retain their PL activity and show varying degrees of water repellency, as collected in Table 2. The varying degrees of hydrophobicity originate on the different surface roughness of the coatings, qualitatively visible in the SEM images in Figure 4. G coatings are continuous and homogeneous in all cases, and in the absence of P7 (G-2.8) look quite smooth. Aversely, C coatings are not continuous (C-2.8) and PFO organizes itself into microspheres on the surface of the cellulose membrane because of the porous structure of the substrate (clearly visualized in the SEM image) and the very different polarity of substrate (hydrophilic) and polymer (hydrophobic). On W, which is chemically similar to C but less porous, the coating morphology is in between G and C. Addition of P7 makes the coatings become continuous or almost on the three substrates; see W-2.8-10 and C-2.8-10. For G coatings this is not a strong morphological and topographical change, as the coating is already continuous in G-2.8, but for W and mostly for C, the coating morphology suffers a dramatic change on addition of 10 wt % P7. As more particles are added to the blend, the surface becomes rougher and more alike on

Figure 4. SEM images of the coatings onto the different substrates: G (left), C (middle), and W (right). All dimension bars correspond to 2 μm scale except for those of the first row which corresponds to 5 μm scale.

the three substrates, and coatings with 50 wt % P7 have very similar surface characteristics irrespective of the substrate. This surface evolution has its manifestation in the coatings’ hydrophobicity. θadv is ∼108° in G-2.8, which is the value for PFO coatings on a smooth surface. As the P7 content increases, surface roughness does as well (Figure 4) and a concomitant increase in the hydrophobicity of the coatings deposited on G occurs, so that G-2.8-50 shows θadv ∼ 164° and Δθ = 3 ± 1°, i.e., superhydrophobic. In W, and especially in C, the incorporation of a 10 of wt % P7 makes W-2.8-10 and C-2.810 not rougher but smoother (Figure 4) in comparison with W2.8 and C-2.8, and consequently, hydrophobicity decreases instead of increasing (Table 2). The effect is obviously more 3721


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Figure 5. 2D and 3D AFM images at 50 μm × 50 μm of (a) C-2.8-50, (b) G-2.8-30 coatings, and (c) corresponding topographical profiles.

Figure 6. Photoluminescence spectra of coatings prepared on G (a), C (b), and W (c) substrates with solutions of increasing PFO concentration (upper row), and with suspensions with increasing ratio of P7 particles (lower row).

of G-2.8-30 and C-2.8-50. The Rq50 values extracted from these images are 306 ± 4 nm and 485 ± 78 nm for G-2.8-30 and C2.8-50, respectively. In accordance with their different surface roughness, the former has a θadv = 157 ± 1° and a Δθ = 5 ± 1° while the latter displays pearl bouncing droplet behavior, with a θadv = 165 ± 3° and a Δθ = 0°.

conspicuous in the C series than in the W one due to the porosity. Irrespective of the substrate, addition of 30 and 50 wt % P7 makes all coatings superhydrophobic, with θadv > 150° and hysteresis Δθ < 10°. Not all coatings with 30 and 50 wt % P7 are, however, equally superhydrophobic, and clear differences are seen among them. Figure 5 collects the surface topography 3722


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ratios of P7 exert a dilution-like effect on the fluorescence of PFO. To summarize, basically two types of spectra are seen in the different coatings and solutions, one with components at 423, 455, and 483 nm and the other red-shifted with its components at about 444, 469, and 497 nm. The first type of spectrum corresponds to N and/or α-phases and the second to the βphase. Thus, the β-phase is clearly promoted under some of the experimental conditions tested in this work. As illustrated in Figure 7, β is promoted in (i) coatings on very porous

A more detailed study on the hydrophobicity of the coatings can be found elsewhere.42 3.2.2. Fluorescence Spectrum of the Coatings Deposited on the G, C, and W Substrates. Effect of PFO Confinement. Figure 6 shows the emission of the coatings deposited on G, C, and W prepared with solutions of increasing PFO concentration (upper row) and of the coatings prepared with suspensions with increasing ratio of P7 particles (lower row). On G substrates (Figure 6a upper row), as the concentration of the parent solution progressively increases the spectral distribution of the derived coating red-shifts and its intensity increases. G-0.1 has its maximum at 440 nm, while raising the concentration makes the maximum wavelength shift to about 460 nm. Recall that something similar occurs with the solutions, with the maximum wavelength shifting from 418 nm for S-10−5 to 440 nm for the more concentrated solutions (Figure 2b). This is attributed to the occurrence of a β-phase, both in the solid state and in solution.21−27 The effect of incorporating particles in the final coating, illustrated in Figure 6a (lower row), is very interesting: G-2.8-10 changes little either the intensity or the emission position with respect to G2.8; in G-2.8-30 a maximum in the 440 nm range appears, emissions at 468 nm are enhanced in detriment of 455 nm, and a clear increase in the intensity of the overall spectrum occurs. In G-2.8-50 these effects are even more pronounced. Overall it looks as if the addition of >30 wt % P7 is promoting the emission from a β-phase. The emission of PFO coatings on C prepared with solutions of increasing concentration appears in Figure 6b (upper row). General features resemble those of G coatings; however, some key differences are noted. The most conspicuous is that in C the ratio of emission I440/I468 is for all coatings higher than in G, the peak at 440 nm being present in all the C coatings regardless of their concentrations. As happened on G, the emission intensity increases strongly on adding particles (Figure 6b, lower row), though on C the effect is much stronger: C-2.8-50 is over 10 times more intense than C-2.8, but note that the final intensity values of C-2.8-50 and G-2.8-50 are very similar, as far as the unavoidable saturation of these spectra allows comparison. The emission from coatings on W, which appears in Figure 6c (upper row), is similar to those on G and C. With regard to wavelength distribution and intensity, coatings on W display features in between those on G and those on C, though more similar to the latter. See, for example, the existence of a component at 440 nm in W-2.8 as in C-2.8 which does not exist in G-2.8. This strongly suggests that the porosity together with the chemical nature of the substrates are key issues on the emission wavelength distribution of solid PFO coatings. On adding P7 (Figure 6c, lower row), the intensity increase in W2.8-50 with respect to W-2.8 is 3-fold, and when increasing the amount of particles the wavelength shift is identical to that for the coatings in G and C. Note that there is a remarkable resemblance in the effect of P7 addition in the coatings on C and W (Figure 6) and in suspension (Figure 3). Figure 6 shows that when incorporating 10−30 wt % of P7 to the PFO coatings the spectra change notably, becoming similar to that of an N or α-phase; when the P7 ratio increases more and reaches 50 wt %, the β-phase emission reappears, with 440 nm becoming stronger and 455 shifting to 468 nm. This is exactly what happens in the concentrated solutions and suspension shown in Figure 3. As occurred in suspension (Figure 3), also in the solid state low

Figure 7. Effect of (a) the substrate, (b) the solution concentration, and (c) the particle ratio on the PFO coating spectra.

substrates (all C coatings, W coatings to a lesser extent) (Figure 7a), (ii) coatings at the lowest concentrations G-0.1, C-0.1, and W-0.1 (Figure 7b), and (iii) coatings with 50 wt % of P7 (G-2.850, W-2.8-50, and C-2.8-50) (Figure 7c). In the opposite, the more concentrated solutions, the smoother substrate (G) and small percentages of P7 particles (C-2.8-10, W-2.8-10, and G2.8-10) promote phases which exhibit the 423/455/483 nm component, i.e., N and/or α-phases. 3723


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Figure 8. Deconvoluted spectra of C-2.8 and G-0.1 making use of only the β-phase components at 444/465/496 nm. In black, the experimental curve; in red, the fitted curve; and in blue, the deconvolution components.

and W-2.8-50 show a component distribution characteristic of abundant β-phase. 3.2.3. Fluorescence and Superhydrophobicity. Effects of Surface Roughness. It is proven, then, that morphologies which tend to confine PFO (e.g., microspheres, thin films, and highly loaded hybrids), will promote the β-phase emission, and the spectra will show components at 440, 468, and ∼500 nm, while those not promoting this phase will produce spectra with components at 423, 455−460, and 483 nm. Interestingly, the microspherical PFO morphologies on porous C (C-2.8) and the highly loaded hybrids (C-2.8-50, W-2.8-50, and G-2.8-50) not only promote the β-phase emission, but also give rise to very rough surface topographies and thus to superhydrophobic coatings. In fact, C-2.8 is the only coating which in the absence of nanoparticles displays both the promotion of the β-phase and a superhydrophobic character, and it is a surprising material which combines the fluorescence and water repellency of the coating with the high flexibility, toughness, and robustness of the substrate. Surface roughness seems to have more than one effect on the overall properties of these materials. Recall that on adding particles, fluorescence intensity increases in all coatings, and C2.8-50, W-2.8-50, and G-2.8-50 are more intense than C-2.8, W-2.8, and G-2.8. Note that the fluorescence intensity of C-2.850, W-2.8-50, and G-2.8-50 is quite similar, and the same occurs to their hydrophobicity. This suggests that the surface topography is at the origin of both the intensity increase and the water repellency. As a matter of fact, the surface topographies of C-2.8-50 and G-2.8-50 are extremely similar, dominated as they are by the structure of the hybrid coating rather than by that of the substrate, as illustrated and commented in Figure 4. Fluorescence arises basically from the film surface and not from the bulk,50 and surface roughness increases the real surface area (as opposed to the geometrical), more so the rougher the topography. It seems very reasonable that the higher intensity in the composites is caused by the increase in real surface area. Hence, by carefully designing the surface roughness and the confinement level of PFO in PFO/P7 composites, it is possible to tune the emission wavelength and intensity of PFO, and the water repellency of the coating. It is possible to prepare, for instance, coatings with enhanced emission from the PFO βphase, being simultaneously very strongly water repellent. The final material can add properties provided by the substrate, and for example, coatings deposited on glass or glass-like supports will be quite transparent, while those deposited on paper-like

Those spectra in Figure 7, claimed to be originated basically from a β-phase, have been deconvoluted making use of only three components at 444, 465, and 496 nm, and the result appears in Figure 8. The spectra are well fitted with the β-phase components only and the same occurs with W-2.5-50 (Figure 7c), not included for the sake of space. What do coatings (i) with high ratio of particles, (ii) deposited on porous substrates, and (iii) prepared with low concentration solutions have in common that enhances the βphase at the expense of the other phases? According to other authors,34 environments that result in PFO chain confinement promote the development of the PFO β-phase. Thin films,48 microdoplets,49 and of course organo−inorganic hybrids with a large inorganic ratio34,35 have been proved to produce confinement and subsequent modification of the semicrystalline morphology in PFO and other polymers. It is very plausible that PFO coatings prepared from low concentration solutions or with high ratio of particles, or on porous substrates, result in PFO morphologies where chain confinement occurs, as illustrated in the cartoons in Figure 7. In the case of a highly porous substrate like C, PFO is at least partly in the form of microspheres of roughly 2−3 μm (Figure 4), a classical morphology resulting in chain confinement. The connection between the microspherical morphology and the βphase emission is nicely illustrated by the simultaneous disappearing of the microspheres (SEM images of Figure 4) and of the β-phase emission (Figure 6b) which takes place in C-2.8-10 as compared to C-2.8. With regard to the effect of the solution concentration, this experimental condition modifies the thickness of the resulting film. In fact, it is our experience that until a given solution concentration is achieved, the films formed are discontinuous. Over the threshold concentration at which film continuity is achieved, further increases in concentration result in larger coating thickness, as illustrated in Figure 7b. Coatings prepared from dilute solutions will result in discontinuous films (morphologically similar to deposited microspheres) or continuous thin films, situations classically related to chain confinement. In thicker films, chain confinement will be progressively reduced until it disappears. In accordance, the emission spectra of coatings prepared with increasingly more concentrated solutions evolve toward the reduction of the components assigned to a β-phase. Because of the high P7 ratio in C-2.8-50, G-2.8-50, and W-2.8-50, confinement of the PFO chains is highly expectable, for polymer domains in these hybrids will obviously be very small (Figure 7c). It is thus not surprising that C-2.8-50, G-2.8-50, 3724


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Langmuir supports will be flexible and degradable. The multifunctionality and versatility of these easily processed and scalable hybrid coatings is thus demonstrated.

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4. CONCLUSIONS Fluorescent and superhydrophobic coatings have been achieved by a composite material made of varying amounts of an organosilica and PFO, sprayed onto smooth microscope slides and cellulose based porous substrates. These multifunctional materials are prepared in a very simple processing way by spraying of these coatings onto different substrates, enabling their use in a wide range of areas with easy adaptation to industrial applications. It is found that though all of the coatings are fluorescent, the emission wavelength distribution and the intensity depend on the coatings’ characteristics. All coating morphologies that result in PFO confinement, such as thin coatings, PFO microspheres, or PFO composites highly loaded with organosilica, emit with a wavelength distribution where the bands of the β-crystalline phase of PFO predominate. In the opposite, in morphologies where PFO is less constrained, such as thicker coatings or little loaded composites, the fluorescence components arising from other PFO phases, crystalline or amorphous, predominate. Of special interest are the hybrid composites, where the organosilica and PFO are at a wt % ratio of 50/50. These composites, irrespective of the substrate on which they are deposited, display an intensity-enhanced fluorescent emission originating at the β-phase and a superhydrophobic character, with very strong water repellent behavior. Both higher emission intensity and superhydrophobicity are thought to arise from the surface roughness induced by the addition of organosilica. This versatile method of forcing PFO self-organization during spraycasting of organosilica/PFO suspension could serve as the basis for novel superhydrophobic electronics.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.H.). *E-mail: [email protected] (P.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no PIIF-GA-2012-327563. Raquel de Francisco acknowledges a predoctoral fellowship (JAE-Pre) administered by CSIC. We acknowledge the Spanish Science and Innovation Ministry (MAT2011-29174-C02-02) and CSIC PIE 201160E109 for the financial support.

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REFERENCES

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