A Thermochromic Superhydrophobic Surface

Abstract

Highly enhanced solid-state thermochromism is observed in regioregular poly(3-hexylthiophene), P3HT, when deposited on a superhydrophobic polymer-SiO₂ nanocomposite coating. The conformal P3HT coating on the nanocomposite surface does not alter or reduce superhydrophicity while maintaining its reversible enhanced thermochromism. The polymeric matrix of the superhydrophobic surface is comprised of a blend of poly(vinylidene fluoride-co-hexafluoropropylene) copolymer and an acrylic adhesive. Based on detailed X-ray diffraction measurements, this long-lasting, repeatable and hysteresis-free thermochromic effect is attributed to the enhancement of the Bragg peak associated with the d-spacing of interchain directional packing (100) which remains unaltered during several heating-cooling cycles. We propose that the superhydrophobic surface confines π-π interchain stacking in P3HT with uniform d-spacing into its nanostructured texture resulting in better packing and reduction in face-on orientation. The rapid response of the system to sudden temperature changes is also demonstrated by water droplet impact and bounce back on heated surfaces. This effect can be exploited for embedded thin film temperature sensors for metal coatings.

A Thermochromic

Superhydrophobic Surface

Pietro Cataldi, Ilker S. Bayer, Roberto Cingolani,,, Sergio Marras, Ryad Chellali, & Athanassia Athanassiou

nanocomposite coating. The conformal

acrylic adhesive. Based on detailed X d interchain d- and reduction in face-on orientation. The rapid response of the system to sudden temperature changes

Most non-substituted conjugated polymers were not easily processed by solvents and thermoforming due to strong interchain interactions and chain stiness. In order to solubilize them in common solvents, side chains were introduced such as alkyl side chains for polythiophenes or similar silicon containing side chains for soluble pentacene polymers. Hence, substituted conjugated polymers have become commonplace for various applications such as organic solar cells, eld eect transistors, electrochromic windows and thin lm sensors15. Attachment of relatively long and exible side chains to conductive polymer backbones not only enables processing in solvents but also results in intriguing reversible color changes (chromism) under certain conditions, such as thermochromism, solvatochromism, piezochromism, and affinitychromism69. Among conductive conjugated polymers, poly(alkylthiophenes), have emerged as one of the most popular class of conducting polymers. In particular, a specic structure known as highly regio-regular 2 5 head-to-tail coupled poly(3-hexylthiophene) or P3HT, has advanced the technology of polymeric eld-eect transistors to the extent that eld-eect mobilities now reach 0.1 cm2V1s1 approximating that of -Si10. P3HT in solution is highly sensitive to various parameters, such as temperature variations, light, and solvent type. Discovery of thermochromism in poly(3-alkylthiophenes) due to conformational changes dates back to the late 1980s11. Such alkyl-substituted polythiophenes are in fact semicrystalline polymers and melting of the crystalline sections due to heating is associated with drastic changes in the chain conformation. Melting causes disorder within the alkyl side chains disrupting planarity and long cWonjugation lengths12. Heeger and co-workers also showed that P3HT can maintain a liquid crystalline state aer crystal melting13. Similarly, such order-to-disorder transformations are observed in solvent dispersions of P3HT due to temperature or solvent type accompanied by solution color changes (solvochromic eect)1416. For instance, P3HT can form nanorods in poor solvents or when cooled in good solvents accompanied by color changes15. In good solvents P3HT can also be found in the form of a mixture of random coils and nanorods17. Dissolving P3HT in chloroform (a good solvent) gives clear light red-orange solutions that slowly change color on standing for long times. Adding a poor solvent to this solution causes a drastic color change of the polymer to blue-violet in transmission. Upon further addition of the good solvent, the polymer recovers its initial color.

Earlier studies showed that thin lms of P3HT deposited on smooth glass or Si-wafer surfaces by spin coating from good solvents, are red-violet at room temperature, but when heated to temperature above 150 C

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change color to light red-orange18. On cooling, they slowly recover their initial color. The process was reported to be sensitive to oxidation by ambient oxygen levels. During transition, two phases can be discerned, a disordered (red-orange) phase with a high density of defects, and an ordered (red-violet) phase with a low density of defects18. The transition from the disordered to the ordered phase is characterized by very long relaxation times.

It has been recently shown that bioinspired nanostructured surfaces drastically enhance thermochromism in inorganic thermochromic materials such as vanadium dioxide19. To the best of our knowledge, thermochromic changes in polythiophenes on such bioinspired surfaces or on so polymeric nanostructured materials have not been studied so far. Many superhydrophobic surfaces have been developed following bioinspired strategies. Recent studies indicate that certain physicochemical phenomena can be enhanced on superhydrophobic surfaces such as electro-osmosis, diuso-osmosis, electrolyte diusion in fuel cells, surface enhanced Raman scattering and cell adhesion1923. For instance, superhydrophobic surfaces, due to their sub-micrometer surface texture and hydrophobic chemistry, can conne random analyte liquid spreading into a much smaller area compared to hydrophilic platforms, tremendously enhancing Raman scattering signals23. By the same token, in this work, we investigate and report on the thermochromic properties of thin regioregular P3HT lms deposited on hydrophobic polymer and superhydrophobic polymer/silica nanocomposite surfaces. We show that although solvent-cast regioregular P3HT lms demonstrate thermochromism on a so, smooth, and hydrophobic polymer surface, surprisingly, on a superhydrophobic polymer-silica nanocomposite surface, the thermochromism is remarkably enhanced with fast reversibility, excellent reproducibility and stability exceeding six months under ambient conditions.

The chosen hydrophobic polymer was poly(vinylidene uoride-co-hexauoropropylene), PVDF-HFP (average Mw ~400,000, average Mn ~130,000). It was purchased from Sigma-Aldrich. It is a crystalline copolymer with good thermal stability as well as good solvent resistance. PVDF-HFP copolymers are being extensively studied as new alternatives for traditional liquid electrolytes in application for lithium-ion batteries2429. P3HT having greater than 90% head-to-tail regiospecic conformation used in this study was produced by Rieke Metals,

Inc. Fumed hydrophobic silica nanoparticles (Aerosil R812S; 220 mg/m2, BET) treated with bis(trimethylsilyl) amine (HMDS) were obtained from Evonik Inc. Initially, PVDF-HFP pellets were dissolved in dimethylacetamide to form stock solutions containing 10% polymer by weight Spray solutions were prepared by diluting the stock solution with acetone to obtain solutions containing 3% polymer by weight. It was not possible to form pure PVDF-HFP coatings with rm substrate adhesion by spray. Spray-cast coatings of PVDF-HFP easily peeled o from aluminum substrates as free standing lms aer solvent evaporation and drying in an oven at 80C for one hour. In order to increase its metal substrate adhesion properties PVDF-HFP was blended in solution with an anaerobic cure acrylic adhesive (Loctite 270, Henkel) comprising polyethylene glycol dimethacrylate (65%), bisphenol A propylene oxide fumarate polymer (18%), poly (butyl methacrylate) (15%) and cumene hydroper-oxide (2%). The approach followed was identical to earlier reports on blending uorinated polymers with acrylic adhesives in solution to enhance nal coating substrate adhesion as well as its abrasion resistance30,31. The best substrate adhesion performance was displayed by the polymer matrix comprising a blend of 50% PVDF-HFP and 50% acrylic adhesive by weight.

Superhydrophobic nanocomposites were made by spraying acetone solutions containing PVDF-HFP, acrylic adhesive and Aerosil R812S SiO2 nanoparticles (NPs) onto aluminum substrates. Various amounts of fumed silica nanoparticles were added to the polymer solutions containing both PVDF-HFP and the adhesive in acetone. Mild sonication (Clion MU Series, Analogue, Unheated Ultrasonic Bath, 200 W, U.K.) was used for half an hour to ensure the nanoparticles were dispersed thoroughly in the polymer solutions. Aer spray coating, all the samples were le under ambient conditions overnight for solvent evaporation and subsequently thermally treated at 150 C in an oven for 1 minute to ensure complete solvent evaporation. The hydrophobicity of the coatings was changed by varying the concentration of the nanoparticles with respect to the polymer matrix. The nanoparticle concentration was ranged from 1% to 30% by weight with respect to the polymer on dry basis. The best performing coating contained 20% by wt. nanoparticles. The coating demonstrated static water contact angles exceeding 160 with negligible contact angle hysteresis; at the same time having good substrate adhesion measured by tape peel tests (see Supporting Information, Figure S1).

Static water contact angles and droplet roll-o angles of the samples were measured by a video based optical contact angle measuring instrument DataPhysics, Germany. Ten microliter of deionized water was gently placed on the surfaces. Measurements were conducted on four dierent locations and averaged for each sample. In order to measure the RAs, the substrates were tilted until the droplets started to roll o the surfaces. The substrate angle at the onset of droplet roll-o was recorded. All RA values were averaged over three dierent measurements on each sample. The standard deviation was 4 for the static contact angle measurements and 1 for the droplet roll-o angle measurements. All measurements were performed in ambient conditions. Changes in the static water contact angle as a function of tape peel events were recorded by the same optical contact angle measurement system. For this purpose, a plastic 3M tape with 820N/m adhesion strength on steel was used as reported by the manufacturer. Tapes were cut in proper sizes and adhered on the surface of the coatings by hand applying downward pressure at the same time. Aer the tapes were peeled o, water droplets were deposited on those spots where the tape was applied and contact angles were recorded. Four measurements were taken aer each peel event. Two dierent superhydrophobic surfaces were tested containing 20wt% and 30 wt% SiO2 nanoparticles with respect to the polymer binder. None of the coatings could be completely removed at the end of 15 peel events indicating that the coating adhesion to aluminum was larger than 820 N/m. However, the coating with 30 wt% SiO2 nanoparticles demonstrated lower water contact angles close to the generally accepted superhydrophobicity threshold (150).

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Figure 1. (a) AFM surface topography of the hydrophobic PVDF-HFP/adhesive coating. Randomly dispersed mostly spherical and smooth regions are attributed to the adhesive. (b) Fluorine EDX signal form the same surface (not exactly the same location). Dark regions correspond to cured adhesive domains. (c) AFM surface topography of the superhydrophobic polymer-silica nanocomposite. (d) Fluorine EDX signals form the same surface (not exactly the same location) showing much more uniform signal distribution. (e) An image sequence showing a water droplet (~10L) being dispensed on the superhydrophobic surface and its subsequent oscillation and rolling o the surface.

The enhanced thermochromic eect was observed when a thin layer (~0.15m) of P3HT was sprayed over the cured superhydrophobic coatings. For this, a solution containing 0.1% by wt. P3HT in chloroform was prepared. This additional P3HT layer did not reduce superhydrophobicity of the coatings as it conformed to the underlying texture. Aer spraying P3HT the solvent was completely evaporated overnight under standard hood ventilation. Further details regarding the thermochromic eect measurements are given under the results and discussion section of this letter. Atomic force microscopy (AFM) images were obtained by a Park System AFM instrument (XE-100) in true noncontact mode. The images were acquired in air using an anti-vibration table (Table Stable TS-150) and an acoustic enclosure. Single-beam silicon cantilevers tips (PPP-NCHR-10) were used for the data acquisition with less than 10nm nominal radius and 42N/m elastic force constant for high sensitivity. The resonance frequency was dened around 280kHz. The scan rate was between 0.2 and 1.0Hz.

Melting and crystallization temperatures were measured by heating and cooling the sample from 20 to 250C at a rate of 10C min1 using a dierential scanning calorimeter (TA Instruments, 2920 Modulated DSC).

The crystal structure was studied by X-ray diraction (XRD) using a Rigaku SmartLab X-Ray diractometer, equipped with a 9kW Cu K (= 1.542) rotating anode, operating at 40kV and 150mA. A Gbel mirror was used to convert the divergent X-ray beam into a parallel beam and to suppress the Cu K radiation (=1.392). The diraction patterns were collected at room and elevated temperatures, over an angular range of 4 to 35, with a step size of 0.05 and scan speed of 1.2/min. Grazing Incidence X-ray diraction (GI-XRD) measurements were conducted through University of Illinois beam time at the 12-ID-C beamline (energy = 11.26 keV, pixel size=79.6 m, wavelength=1.101, 2=0 20 at Argonne National Laboratory, USA.

The thermochromic response of the P3HT lms on the superhydrophobic nanocomposites was measured using diuse reection spectroscopy with an integrating sphere attachment to UV-Vis-NIR spectrophotometer (Varian Cary 6000i). The measurements were performed by placing the samples in front of the incident light window, and concentrating the light reected from the sample on the detector using a sphere having a barium sulfate-coated interior. The obtained value becomes the reectance (relative reectance) with respect to the reectance of the reference standard white board, which is taken to be 100%. A at resistive heater was attached on the back side of the coated aluminum substrates in order to collect reection spectra as a function of temperature (in steps of 1015C).

Results and Discussions

Figure1 shows atomic force microscope (AFM) topography and energy dispersive X-ray mapping (EDS) of the uorine atom distribution pertaining to PVDF-HFP/adhesive polymer surface (hydrophobic surface) and the superhydrophobic nanocomposite (with 20% wt. hydrophobic SiO2 NPs) prior to P3HT deposition.

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Figure 2. (a) Atomic force microscope (AFM) topography of the superhydrophobic polymer-silica nanocomposite coated with a P3HT which conforms to the texture. The diagonal line is used to obtain a random roughness prole on the surface shown in (b). (b) Roughness prole corresponding to the diagonalline in (a). The deepest valley is designated as zero. (c) 3D topological features of a 55m thermochromic superhydrophobic surface. (d) Normal (Gaussian) roughness distribution obtained from such as surface with an average roughness of 350nm.

In general, the adhesive forms random micron-scale phase separated domains within PVDF-HFP as shown in Fig.1a,b. The superhydrophobic lm appears to have a dual scale hierarchical topology as can be seen in Fig.1c. The uorine atom distribution of this surface (Fig.1d) is much more uniform compared to the unlled polymer matrix (Fig.1b). The degree of superhydrophobicity of this surface is demonstrated by a sequence of images collected during the deposition of a 10L water droplet in Fig.1e. It is practically very difficult to maintain a water droplet on such surface as a slight tilt causes the droplets roll-o. Note that the superhydrophobicity is maintained when the surface was coated with P3HT. Further surface topography analysis of the superhydrophobic surfaces coated with P3HT was also made by AFM. Results indicate that surface roughness of the polymer-silica nanocomposite is preserved as seen in Fig.2a. Typical roughness prole on such a surface is shown in Fig.2b. The roughness prole is obtained from the diagonal line shown in Fig.2a where the deepest valley is set to zero in the plot (Fig.2b). Based on the measurements from dierent samples of identical nature, we found that the maximum surface roughness value does not exceed 450nm. A 3D AFM topology is exemplied in Fig.2c. The normal roughness distribution of the thermochromic superhydrophobic surface shown in Fig.2d was found to be narrow and remained below 1 m with an average roughness of ~350nm. Such a roughness prole is sufficient to create the self-cleaning lotus eect when combined with a uniformly formed hydrophobic surface chemistry as shown in Fig.1d in the form of EDS uorine signals.

Figure3 shows the thermochromic behavior of a P3HT layer deposited on both the hydrophobic surface polymer surface (Fig.3[1],[2]), and on the superhydrophobic nanocomposite (Fig.3[3],[4]), at temperatures between room temperature and 200 C (obtained on a hot plate under ambient conditions). Thermal conductivity of aluminum is very high (215 Wm1K1 at 25 C) enabling rapid heat transfer from the hot plate to the coating layers. As can be seen in Fig.3, for the P3HT applied on the hydrophobic polymer film, the color change perception is not signicant once the lm is heated to 200 C. P3HT on the superhydrophobic nanocomposite surface, however, displays a distinct purple-red color at room temperature that changes to light orange-yellow color at 200C. The degree of superhydrophobicity of the P3HT coated nanocomposite surface and its response to sudden temperature changes are demonstrated in Fig.3 ([5][8]). A sequence of high speed camera images is shown, in which a water droplet impacts on the surface maintained at 200C. As the room temperature droplet impacts and spreads, the color of the contact region immediately turns purple-red indicating the rapid response of the system to the changes in temperature. Once the droplet bounces away from the surface, the color of the

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Figure 3. Top row: [1] P3HT layer on hydrophobic PVDF-HFP/adhesive coating on aluminum at room temperature, [2] P3HT layer on hydrophobic PVDF-HFP/adhesive coating on aluminum at 200C, [3] P3HT layer on superhydrophobic polymer-silica nanocomposite on aluminum at room temperature, and [4] P3HT layer on superhydrophobic polymer-silica nanocomposite on aluminum at 200C. Panels [58] are sequence of a water droplet impacting on a hot surface ~200C. The droplet was released from plastic pipette (visible in the images). The lower section of the surface is cooler than the center as that edge of the coated aluminum was not in contact with the hot plate surface in order to enable handling during experiments. When the droplet makes impact, due to cooling, the color of the zone under the droplet immediately changes to purple-red. Video of this sequence is available in electronic supplementary information.

region underneath the droplet impact turns back to light orange-yellow. Upon impact, water droplet causes a decrease in the temperature of the surface and this is reected as a local color change due to re-crystallization and macromolecular rearrangement due to cooling. Note that during all this process the surface still maintains its superhydrophobicity. In fact, according to Ewbank et al.32, in all the solid phases of P3HT such as crystalline, meso, or kinetically trapped phase, the hydrophobic alkyl side chains always orient perpendicular to the plane of the substrate. Only the case of face-on morphology forces the waxy hydrophobic side chains to orient parallel to the substrate. It is therefore highly probable that during crystallization and degradation of crystal domains on the superhydrophobic surfaces the face-on orientation does not occur but rather the morphological changes can pertain to transformation from crystalline to meso or to kinetically trapped phase.

Figure4a demonstrates the evolution of the color change of the P3HT coated superhydrophobic nanocomposite containing 20% wt. SiO2 NPs. The event was recorded by a high speed camera at 600fps when the sample was suddenly placed on a hot plate maintained at 200C. The recorded frames were processed by a color tracking algorithm. For enhanced perception, the algorithm assigns blue to room temperature and green to 200C. The heating of the plate starts from two opposite corners and propagates to cover the whole plate within 1.3seconds. The initiation of the heating from the corners can be attributed to the plates curvature. The observed thermochromic response of the P3HT lms on the superhydrophobic nanocomposites was measured using diuse reection spectroscopy with an integrating sphere attachment to UV-Vis-NIR spectrophotometer (Varian Cary 6000i). The change in the reectance spectra between 450 nm to 750 nm is displayed in Fig.4b. As the temperature increases the spectra show a blue shi and at the same time the reectance intensity increases at all wavelengths. The shis in the spectra due to heating were followed by recording the changes in the wavelength corresponding to 45% reection that is approximately half of the reection range. The shis were denoted by R1/2 for every temperature step. Alternatively, the backward shi of the thermochromic coatings was also recorded during cooling back to room temperature to gauge the degree of repeatability as well as the presence of any hysteresis.

In the case of P3HT on the hydrophobic coating, the shi R1/2 is less than 10nm until 175C and becomes 30 nm at 200 C (see Fig.4). Upon cooling, however, a R1/2 of 10 nm is suddenly obtained and then no more change is detected until room temperature is reached. Upon heating again, a similar trend in R1/2 is observed but at the maximum temperature a lower R1/2 value is reached compared to the rst heating cycle. Aer the h cycle, R1/2 practically remains constant indicating termination of thermochromism. On the contrary, when P3HT was deposited on the superhydrophobic surface, the R1/2 changes were found to be highly repeatable with slight hysteresis for both heating and cooling cycles. During heating, the wavelength shi R1/2 is 20nm at 95C, and remains somewhat steady until 130C. Above this temperature an almost linear increase of the shi occurs until 200C and at 200C suddenly R1/2 jumps to 85nm (maximum shi). During cooling a similar trend was observed and the heating-cooling cycles were stopped aer twenty loops with no visible deterioration in thermochromism. Eect of hydrophobic SiO2 NPs concentration on the observed shis in the reective spectra was also studied. The maximum shi in R1/2 was found to correlate with the amount of SiO2 NPs as shown in the inset of Fig.5. The superhydrophobic surface with 20% wt. SiO2 NPs concentration demonstrates minimum contact angle hysteresis (see also Fig.1e) as well as a maximum shi in R1/2. This means that thermochromism of the

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Figure 4. (a) High speed camera image sequences (600fps) showing heating and resulting color change of the thermochromic superhydrophobic surface. The blue background corresponds to room temperature and green to 200C as a result of image processing for better perception. The red smudge on the lower right corner is the uncoated region for sample handling. (b) Temperature dependent shi in the surface reection measured by diuse reection spectroscopy between 450750nm.

Figure 5. Changes in the spectra as a function of temperature traced by recording the changes in the wavelength corresponding to 45% reection. The shis were denoted by R1/2 for every temperature step.

The hysteresis is also shown by cooling down the heated surfaces for P3HT on the superhydrophobic surface and on the polymer coating alone. The inset shows the value of R1/2 and water contact angle hysteresis as a function of SiO2 nanoparticle concentration within the polymer matrix.

P3HT on these non-wetting surfaces is optimum in terms of perceptional color changes. Although surfaces with more than 20wt% by wt. nanoparticle content were still superhydrophobic and with reasonable perceptional color changes, their tape-peel resistance (durability) decreased (see electronic Supplementary Information Figure S1).

Both PVDF-HFP and P3HT are semi-crystalline polymers and their crystallinity is well studied3236.

According to the DSC measurements, P3HT used in this study melts at around 213C upon heating and it crystalizes at about 173C. Hence, the highest temperature used in this study (200C) is very close but does not exceed the melting temperature of P3HT. The crystallinity of PVDF-HFP copolymer is lower than PVDF but it retains sufficient mechanical stability to allow it to act, for example, as a separator between the electrodes of a lithium-ion battery, while the amorphous phase can contain the liquid electrolyte37,38. The XRD spectrum of the hydrophobic coatings is shown in Fig.6a. The separate XRD measurements of the pure adhesive yielded no crystallinity. Within the coating, PVDF-HFP is mostly in crystalline form with non-polar transgauchetransgauche (TGTG) conformation. However, the band at 2 = 26.7 indicates some presence of the phase which has an intermediate polar TTTGTTTG conformation, occurring when the polymer is moderately stressed38. The polymer here was not mechanically stressed; however, the presence of random but mostly spherical adhesive domains shown in Fig.1 can induce various levels of internal stress to PVDF-HFP resulting in the partial appearance of the

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Figure 6. (a) Room temperature XRD results of P3HT and PVDF-HFP/adhesive polymer spray coatedon aluminum and from P3HT coated on PVDF-HFP/adhesive polymer and on the superhydrophobic nanocomposite. (b) XRD results of P3HT coated on PVDF-HFP/adhesive polymer and the superhydrophobic nanocomposite at 200C.

phase39. When the PVDF-HFP/adhesive polymer matrix is kept at 200C, the well-dened (100) at 2=18.4 and (110) peaks at 2= 20 disappear indicating an amorphous state while some of the phase with +(021) still retains its form 2= 26.6 (results not shown)28.

As a result of various carefully conducted studies, there is now a consensus on a model for the ideal crystallinity of poly(alkylthiophenes) in which stacks of poly-conjugated main chains organize in layers, with the side chains extending in the regions between the stacked main chains3337. When P3HT is spray-coated from a chloroform solution on aluminum with subsequent solvent evaporation and short thermal curing, it gives rise to an X-ray diraction pattern as shown in Fig.6a. The band at 2=6.97.0 is normally indexed as (100) in the literature corresponding to the characteristic periodicity between adjacent layers of chains with non-interdigitating side chains. The present polymorph shows a broad diraction pattern in the region 2 = 2328 which can be indexed as (020). P3HT layers spray deposited on the hydrophobic coating and on the superhydrophobic nano-composites, however, displayed signicantly dierent changes in the X-ray diraction patterns at room temperature. Particularly, at room temperature on hydrophobic coatings, the (100) diraction signal was enhanced considerably and the broad (020) Bragg signal transformed into two bands (a narrow one at 2 = 22.4 and broader one at 2 = 23.5) that can be indexed by (010) as seen in Fig.6a. These out-of-plane (010) reections due to interchain stacking correspond to inter-chain distances of d[010] =3.81 andd[010] =3.96 , respec

tively. It is unusual to observe the splitting of the broad (020) peak which is characteristic of the spray cast lms on aluminum. However, this sort of changes in crystallinity was observed before in polyalkylthiopehenes. Prosa et al. attributes this splitting of the board halo of (020) to side chain disorder13. Similar crystallinity disorder observations were also reported by Wu et al. later on11. This unusual structural behavior can be ascribed to layers of interdigitated, tilted alkyl chains having intra-stack perpendicular chain-to-chain spacing and a dense side chain nearest neighbor spacing3740. As such, depending on the intra-stack chain repeat length and orientation, the broad (020) halo can be split into various reections. Herein we denote the split reections on the hydrophobic polymer with (010) as these could have mixed indices and in reality indexing them is quite hard as indicated by Yuan et al.37

The enhancement in intensity of the (100) diraction signal on the hydrophobic coatings indicates a higher degree of crystallinity in P3HT as well as the absence of smaller or scattered domains in which the polymer orientation is random or disordered4144. Foong et al. reported that nano-connement imposes changes in the crystallinity (reduction) and the orientation of the P3HT polymer45. For heterojunction solar cells, this is considered non-ideal for transport of charges from the exciton dissociation interface to the electrodes44,45. They observed that P3HT molecules within TiO2 nanotube arrays were less crystalline than unconned P3HT molecules with reduction in the intensity of the d[100] peak42. They also indicated that hydrophilicity of the underlying substrate imposes changes in P3HT conguration. On hydrophilic surfaces, P3HT would assemble over its backbone rather than the hydrophobic alkyl side-chains45,46.

The inter-chain distance d[100] of the P3HT packing on the hydrophobic surface is 16.6, which corresponds to the typical P3HT interchain packing distance for a lamella packing pattern reported in the literature4550. In

the case of the P3HT layer on aluminum the d[100] is 18.4 , higher than the generally accepted lamella packing value of 16.6 . The reduced d-spacing of the rst Bragg peak of P3HT on the hydrophobic surface indicates that P3HT is better packed along the interchain direction. When the P3HT is applied on the superhydrophobic nanocomposite containing 20% wt. SiO2 NPs, at room temperature, the (100) Bragg peak is preserved well with corresponding d[100] = 16.6 . This means that the superhydrophobic nanostructured texture does not disrupt the interchain packing distance. As mentioned earlier, the broad halo of P3HT indexed by (020) on the hydro-phobic polymer splits into two peaks, one narrow and one slightly broader resembling the original halo on the

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Figure 7. Room temperature grazing-incidence X-Ray diraction (GIXRD) measurements and schematics representing face-on and edge-on orientation of P3HT. 2D GIXRD pattern of (a) P3HT deposited on bare aluminum surface, (b) P3HT deposited on PVDF-HFP/adhesive coating, (c) P3HT deposited on the superhydrophobic coating and (c) room temperature 2D GIXRD pattern on P3HT deposited on the superhydrophobic surface aer cooling down from 10 seconds of 200 C exposure. The panel next to (b) represents a face-on orientation of P3HT which is generally associated with hydrophobic surface (see inset) and the panel next to (d) represents the edge-on orientation which is associated with superhydrophobicity due to the orientation of the hydrophobic alkyl chains (see inset).

hydrophobic polymer. On the superhydrophobic nanocomposite, this somewhat broader peak also vanishes. This might correspond to an edge-on type orientation when P3HT is deposited on the superhydrophobic surface textured mainly by the SiO2 nanoparticles. According to the review by Brinkmann50, the edge-on orientation reveals clear (010) reection whereas in the case of mostly face-on type structure, the (010) reection vanishes. Similarly, Yuan et al.37 indicated that edge-on or face-on orientation along with the side chain intra-alignments can produce sharp or diused and split (010) reections depending on the hybrid morphology, respectively. As seen, the broad peak at 2 = 23.5 disappears on the superhydrophobic surfaces, whereas the narrow peak at 2= 22.4 is strong. Hence, the interchain stacking conforms to a single distance at d[010] =3.96, indicating

that on the superhydrophobic surface at room temperature, P3HT chains are probably packed more uniformly along the interchain direction3941. When the P3HT layer on pure aluminum surface was heated to 200C, the (100) and (020) Bragg peaks signicantly lost their intensity (not shown for brevity) but aer cooling down to room temperature both crystalline indices reappeared. This is a well-known behavior related to the loss of -stacking order due to melting of the crystals4550. Upon heating P3HT coated hydrophobic polymer and superhydrophobic nanocomposite surfaces to 200C (Fig.6b), the Bragg peaks of PVDF-HFP associated with (100) and (110) are replaced by a much broader signal centered around 2 = 16, as a result the polymer loses its transgauchetransgauche (TGTG) conformation. P3HT on the polymer, on the other hand, maintains the Bragg peaks at (100) as well as the two peaks associated with the (010) plane observed at room temperature. The splitting of the (020) plane into two (010) planes at 200C is better dened, particularly, the original signal at 2= 23.5 is narrower and shows a slight shi to 23.1, indicating formation of more uniform interchain stacking distances. On the hot superhydrophobic nanocomposite surface, the rst Bragg peak of the P3HT maintains a strong d-spacing, d[100], still at 16.6. The second Bragg (010) diraction peak at 2= 23.1 is absent, like in the case of the room temperature analysis, and the (010) peak at 2= 22.4 is preserved. At the same time, the small Bragg diraction signal at 2= 26.6 pertaining to the phase of PVDF-HFP gets stronger. Summarizing, on the hydrophobic polymer surface or on the superhydrophobic nanocomposite surface, the inter-chain distance d[100] of the P3HT packing is strong and reduced compared to P3HT on aluminum and does not shi between heating and cooling, remaining very stable. On the other hand, the broad out-of-plane (020) reection due to interchain stacking of P3HT on aluminum transforms into two narrow out-of-plane interchain stacking that can be indexed by (010) on PVDF-HFP polymer. The suppression of one of these reection at 2=23.5 indicates reduction of polymorphism in regioregular P3HT and the fact that it also occurs at 200C indicates highly stable polymer chain stacking of P3HT on the nanocomposites.

In Fig.7, room temperature grazing-incidence X-Ray diffraction (GIXRD) patterns are shown. The 2D GIXRD images enable probing of the molecular-scale packing of the polythiophene on a particular surface, and are particularly sensitive to the efficient organization of P3HT in two orthogonal directions: the stacking

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of the aromatic thiophene rings, and the lamellar stacking resulting from the disordered alkyl side-chains. Note that the third orthogonal direction (along the polymer backbone) does not always yield well-dened GIXRD patterns. Moreover, existence of face-on crystals on the hydrophobic or superhydrophobic lms are not detected by 1D XRD measurements since their scattering vector is parallel to in-plane direction51. The intensity along the scattering rings can also be used to quantify the orientation distribution of the material, and thus the fractions of materials oriented in dierent directions. Figure7a shows the room temperature crystalline state of spray coated P3HT lm on a bare aluminum surface corresponding to the panel [1] of Fig.3. X-ray reections due to the (h00) crystal planes are not very clear except (100) plane. The (010) plane along the qz axis is rather broad and is located close to the qz axis indicating a face-on orientation. When P3HT is deposited on the polymer/adhesive coating (panel [2] in Fig.3) however, the (h00) crystal planes are better dened although higher order ones such as (300) and (400) are very diuse and of low intensity as seen in Fig.7b. The (010) crystal plane appears to be located at the qz axis but with less diuse more intense nature indicating that the hydrophobic polymer surfaces induces a better and more ordered face-on orientation of P3HT conrming the experimental observation of Kline et al.51

(see panel next to Fig.7b). On the superhydrophobic surface, the higher order (h00) crystal planes which correspond to the intermolecular backbone layer and stacking order are somewhat dened close to the qz axis, however an abrupt shi of the (010) plane to the qxy axis (although with lesser intensity, Fig.7c) indicates some P3HT molecules assumed an edge-on chain conformation with side chains oriented in a standing-up conguration on the superhydrophobic surface. Upon heating the surface to force a color change and later cooling down to room temperature (Fig.7d), the resultant crystalline order practically appears similar to the state shown in Fig.7c, with less diused higher order (h00) crystal planes. The (010) maintains its position near the qxy axis indicating the presence of edge-on orientation of the P3HT (see panel next to Fig.7d). Although a more in-depth crystal-lographic analysis of these surfaces is beyond the scope of this work both 1D and 2D XRD measurements show that on the superhydrophobic surface at least some portion of the P3HT polymer assume an edge on orientation evidenced by the disappearance of the (010) plane near the qz axis. However, P3HT coating also conforms to the submicron roughness associated with the superhydrophobic surface, rendering data collection and interpretation more challenging compared to for instance, a spin coated poly (3-alkylthiophene) crystals on a smooth surface.

As such, long-lasting thermochromic superhydrophobic surfaces which can function reversibly many times can nd a wide range of applications particularly for metallic surfaces. Some potential and immediate applications can be in the form of smart non-wetting coatings for heat exchanger surfaces, waterproof temperature indicators or sensors for reactors, or outdoor storage enclosures to name a few. Future work can focus on various aspects related to use of other thermochromic organic molecules or polymers and also on fundamental aspects related to the assembly of the thermochromic polymers on superhydrophobic composite surfaces with submicron roughness scales.

Conclusions

In conclusion, regioregular P3HT displays enhanced, long-lasting, and repeatable thermochromic eect when deposited on a superhydrophobic polymer nanocomposite comprising PVDF-HFP and hydrophobic SiO2 NPs.

The rst P3HT Bragg peak associated with the d-spacing of interchain directional packing is highly enhanced both on the hydrophobic and the superhydrophobic nanocomposites, indicating better crystalline state compared to untreated aluminum surfaces. This does not change when the system is heated to 200 C. Moreover, the nanocomposite surface imposes interconversions within the out-of-plane interchain stacking of P3HT resulting in better packing and reduction in polymorphism. We propose that the perceptional enhancement in P3HT thermochromism and most importantly its stability and non-hysteretic repeatability is due to the existence of interchain stacking with uniform d-spacing conned into a nanostructured surface texture. It has been reported that P3HT does not lose its functionality when it assembles into superhydrophobic structures on textured surfaces or in composites as it is also the case in the present study. Previously superhydrophobic P3HT surfaces demonstrated eective electrical properties as well as photo-response while preserving self-cleaning properties52,53. In this work, we also demonstrated that it can maintain its solid-state thermo-responsive characteristics on superhydrophobic surfaces. This eect could be useful for designing thin lm temperature detectors particularly for metal surfaces simply by spray coating. Future work will be conducted on better understanding of connement of such thermochromic molecules bioinspired surface textures.

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We would like to acknowledge Alice Scarpellini for her help with the EDS measurements. We also acknowledge experimental support and assistance from University of Illinois.

Author Contributions

I.S.B. conceived the study and conducted preliminary experiments and wrote the manuscript P.C. produced samples and conducted the experiments, analyzed data and wrote the manuscript R.C. designed the experiments and performed data analysis and supervised the project S M. conducted the XRD measurements R.C. performed the image processing analysis to track the color changes during heating and cooling A.A. was involved in the data analysis and interpretation of the experimental results and wrote the manuscript

Additional Information

Supplementary information accompanies this paper at http://www.nature.com/srep

Competing nancial interests: The authors declare no competing nancial interests.

How to cite this article: Cataldi, P. et al. A Thermochromic Superhydrophobic Surface. Sci. Rep. 6, 27984; doi: 10.1038/srep27984 (2016).

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