Spontaneous Formation of Photochromic Coatings Made of

Oct 20, 2014 - DIBRIS Department, University of Genova, viale Causa 13, 16145, Genova, Italy. §. Nanochemistry, Istituto Italiano di Tecnologia, via ...
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Spontaneous Formation of Photochromic Coatings Made of Reversible Microfibrils and Nanofibrils on an Elastomer Substrate Reinier Oropesa-Nuñez,†,‡ Despina Fragouli,*,† Francesca Pignatelli,† Alice Scarpellini,§ Efisio Gigliotti,† Elena Samoylova,† and Athanassia Athanassiou*,† †

Smart Materials, Nanophysics, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy DIBRIS Department, University of Genova, viale Causa 13, 16145, Genova, Italy § Nanochemistry, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genova, Italy ‡

S Supporting Information *

ABSTRACT: We report the spontaneous formation of photochromic microcrystalline and nanocrystalline fibrils forming dense coatings of cactuslike supramolecular structures on the surface of a soft poly(dimethylsiloxane) (PDMS) elastomer. The initial deposition of the photochromic molecules of diarylethenes on the elastomer is done by dip adsorption, a process that permits the homogeneous distribution of the molecules not only on the surface but also in the inner part of the polymer. Detailed thermal and microscopy studies reveal that the growth process of the fibrils is initiated by the formation of crystal seeds of the diarylethene in the proximity of the elastomer’s surface empty voids and progresses toward the elastomer−air interface as a result of the high mobility of the molecules at room temperature. Fibril formation is possible only when the molecules are in the open form because the UV irradiation responsible for their transformation to the close isomeric form immediately after deposition totally prohibits the crystals’ formation. Furthermore, the UV irradiation of the grown supramolecular assemblies provokes their destruction, but when the irradiated samples are left to recover under ambient conditions, they form new assemblies of fibrils in a faster and more efficient way. The resulting systems exhibit superhydrophobic to slightly hydrophobic properties with differences of almost 80° in water contact angles upon dark storage−UV irradiation cycles. The proposed systems can be an alternative to the facile formation of reversible photochromic fibrils on soft polymer surfaces for utilization on diverse soft devices, where controlled surface morphology and wettability are desired.

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INTRODUCTION

the reversible or directional wettability if an external stimulus is applied.5−7 Alternatively, recent studies have demonstrated that organic synthetic molecules, such as the photochromic diarylethenes, can form crystalline nanofibrils directly on a surface, resulting in superhydrophobic systems with reversible wettability induced by an external stimulus.8−10 Generally, photochromic compounds using light excitation can be switched between two distinct states with different properties as a result of the photoisomerization process.11 Most photochromic compounds show light-induced reversible polarity12 but also significant volume changes if incorporated into polymers;13,14 therefore, wettability changes of surfaces composed of such materials can be easily controlled.15 Diarylethenes are extensively studied photochromic compounds that reversibly change their chemical and physical properties upon alternating irradiation with ultraviolet (UV) and visible light.16 Such systems are attractive for the development of optical memories and switches16 and photomechanical devices17,18 because of the thermal stability of

One of the most common mechanisms of crystals growth, observed in natural sciences but also in biology, proceeds by the initial formation of crystal nuclei upon aggregation of molecules or atoms. If the size of such nuclei is greater than a critical size, then they are stable and can act as centers for the formation of crystals. Under these conditions, the chemical species get attached to the nuclei in closely packed formations, and the rate of crystal growth depends on their mobility. Taking inspiration from such natural systems, a great number of studies have been focused on the fabrication of synthetic materials that present similar or, most importantly, improved or combined functional properties compared to natural systems. For example, scientists have developed in the last decades diverse nanocrystals in liquids, with impressive properties1,2 that can be easily combined with polymeric systems, thus producing novel composite materials that can once more mimic or go beyond natural systems. Specifically, such nanocomposites can have the desired surface micro/nanoroughness and the appropriate surface energy in order to form superhydrophobic or self-cleaning surfaces inspired by lotus or taro leaves.3,4 An improvement to these natural systems is © 2014 American Chemical Society

Received: September 2, 2014 Revised: October 3, 2014 Published: October 20, 2014 13058

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forming it into a superhydrophobic surface, and this is an exclusive property of the pristine open form of the diarylethene molecules. When, after deposition, UV irradiation switches the molecules from the open to the closed form, no fibrils are formed. Moreover, the UV irradiation of the formed fibrils results in their destruction, but when the samples are left to return to the most energetically stable open form under storage at ambient conditions, the crystalline fibrils are reformed. The kinetics, the mechanism of self-assembly, and the wetting properties of the system are presented in order to provide insight into the spontaneous formation of superhydrophobic surfaces on soft elastomers and their UV control.

both isomers, their fatigue-resistant property, and the capability of photochromism in the solid state.19,20 Although no remarkable polarity changes are observed during photochromic switching,21 the diarylethenes present an interesting property that deals with the formation of reversible photoresponsive crystalline microfibril and nanofibril structures when deposited on a solid substrate.8,9,22 In particular, upon deposition on metallic or glass substrates, some diarylethene molecules selfassemble into crystalline fibril photochromic structures after irradiation and/or heating of the system. In this way, the surfaces obtain a specific micro/nanoroughness that can transform them into superhydrophobic surfaces. The subsequent irradiation of the fibrils causes photoinduced shape changes in the crystals, resulting in a change in the topography of the crystalline surfaces and the modification of their wetting properties.8,9,22−24 This is a fascinating property that gives the possibility to form novel protoresponsive surfaces. The studies presented explain in detail the mechanism for specific types of diarylethenes deposited on hard substrates, which, however, limits the applicability in various soft devices. Therefore, recent studies are performed for the utilization of polymeric substrates (PMMA),25 though also in this case the extensive heating and the multiple steps followed for the fabrication is a limit for broad applications. Herein we report the spontaneous formation of supramolecular structures of photochromic microfibrils and nanofibrils on a soft PDMS elastomer, which is broadly used in fluidic devices and as a substrate for biological applications. We explore the capability of 2,3-bis(2,4,5-trimethyl-3-thienyl)maleimide diarylethene (inset of Figure 1) to self-assemble in

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MATERIALS AND METHODS

Materials. 2,3-Bis(2,4,5-trimethyl-3-thienyl)maleimide (DAE) was purchased from TCI, and chloroform was purchased from SigmaAldrich. PDMS was prepared by mixing a two-part heat-curable Sylgard 184 silicone elastomer kit (Dow Corning Corporation, Midland, MI). Preparation of the Film. The PDMS prepolymer was mixed with the curing agent in a weight ratio of 10:1 (prepolymer/curing agent) and subsequently deposited on a glass mold and degassed. The system was polymerized upon heating to 90 °C for 1 h. In this way, 350-μmthick PDMS films were formed and cut into pieces of 0.5 × 0.5 cm2. Next, the PDMS free-standing films were dipped in a chloroform solution containing DAE (10 mg/mL) for 10 min. During dipping, the PDMS swells and entraps DAE molecules in its volume. After removal from the solution and solvent evaporation, the polymer returns to its initial volume. The DAE-PDMS films were stored in the dark at room temperature. Surface Morphology Measurements. The surface morphology of the samples was investigated by Optical Microscopy, using a Nikon Eclipse 80i Digital Microscope and Scanning Electron Microscopy (SEM) using a JEOL JSM-6490LA (Jeol, Tokyo - Japan) equipped with a W thermionic source working at high vacuum with an acceleration voltage in the range of 5−15 kV. Imaging was obtained with secondary electrons. Samples have been previously sputter coated with a 10-nm-thick gold layer using a Cressington 208HR highresolution sputter coater (Cressington Scientific Instrument Ltd, U.K.). Absorption Measurements. UV−visible absorption spectra were obtained using a UV−visible−NIR spectrophotometer (Cary 6000iVarian) on DAE-PDMS samples on glass. Contact Angle Measurements. For the static wetting characterization, apparent water contact angle measurements (WCA) were performed using a OCAH 200 video-based optical contact angle measuring instrument (DataPhysics, Germany). The volume of the water droplets was 3 μL. UV−Visible Irradiation. UV−visible irradiation was performed with a Quanta-Ray (Spectra Physics) Nd:YAG laser operating at the second and fourth harmonics, λirr = 532 and 266 nm, respectively, τpulse ≈ 6 ns, with a pulse duration of 100 ps and a repetition rate of 20 Hz. The fluence (F) of UV and visible light used for the irradiation was F266 = 2 mJ cm−2 and F532 = 10 mJ cm−2, respectively. DSC Measurements. Thermal analysis was carried out using a PerkinElmer diamond differential scanning calorimeter (DSC). The instrument was previously calibrated with the indium standard. The baseline was recorded by a preliminary scan from −50 to 250 °C at a rate of 20 °C min−1. Thermal scans were performed in the temperature range of −50 to 250 °C at a rate of 20 °C min−1. For DSC measurements, high-volume sealed stainless steel pans were coated first with a thin layer of PDMS. Then a 40 μL DAE− chloroform solution of concentration 100 mg/mL was drop cast on PDMS, and the samples were placed in a desiccator chamber for 15 min in order to evaporate the solvent. The samples were tested at different time intervals ranging from 15 min to 3 days after preparation. DSC measurements were also conducted on UVirradiated samples. Such samples were formed following the same

Figure 1. Absorption spectra of a DAE-PDMS film after irradiation with successive UV and visible laser pulses. (Inset) Scheme of the reversible transformation between the open and closed forms of DAE induced by UV−visible irradiation.

the form of crystalline fibrils, which, unlike other diarylethenes, offers great potential in biomedical and technological applications because of the presence of the maleimide group. In fact, maleimides linked to PEG chains are often used as flexible linking molecules to attach proteins to surfaces.26 Most importantly, we show that the crystalline structures are formed spontaneous on the surface of PDMS without any irradiation or heat pretreatment as done so far in other studies. The layer of fibrils, having a structure similar to that of cactus thorns, homogeneously covers the whole surface of PDMS, trans13059

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Figure 2. SEM images demonstrating changes in the surface morphology of the DAE-PDMS system: (a, d) 0, (b, e) 7, and (c, f) 14 days after sample preparation. procedure; 40 μL of UV-irradiated DAE chloroform solution of 100 mg/mL concentration was drop cast on PDMS, and the samples were placed in a desiccator chamber for 15 min in order to evaporate the solvent. The samples were tested at different time intervals ranging from 15 min to 3 days after preparation. The solution was irradiated using a Universal Arc Lamp (Oriel Instruments, Newport Corporation) operating at λirr = 266 nm for 40 min. Such UV-irradiation conditions cause the photochromic conversion of the open form to closed form as confirmed by absorption measurements (results not shown).

photochromic activity when they are incorporated into the polymer matrix. The morphology of the surfaces of the prepared samples is changing significantly with the time, as shown in Figure 2 (Supporting Information Figure S1). Specifically, Figure 2a−f shows low- and high-resolution SEM images of the same sample obtained 0, 7, and 14 days after preparation. A few hours after DAE-PDMS sample preparation (0 days), the surface of the sample is smooth, and only the presence of some small isolated fibrils or groups of fibrils can be detected (Figure 2a,d). After 7 days, the density of the grown fibrils is significantly increased (Figure 2b,e), whereas after 14 days the whole polymer surface is homogeneously covered with crystalline microfibrils and nanofibrils, as clearly demonstrated in Figure 2c,f. The results indicate that the first formed crystalline sites act as seeds that collect the surrounding molecules to become supramolecular cactuslike assemblies that continuously increase with time, both in density and in the size of the individual crystalline fibrils (Supporting Information Figure S2). Each thorn-shaped crystal has at the end of the procedure a length of tens of micrometers and a diameter of submicrometers to a few micrometers. The molecules or the small nuclei from which the formation of larger fibrils starts are expected to be located not only on the surface of the PDMS but also in the volume of the polymer because, as a result of the preparation process, the molecules are also entrapped in the whole free volume of the polymer. Indeed, SEM images reveal that the first self-assembled sites on which the subsequent crystalline structures are generated are mainly located in the empty voids of the PDMS (Supporting Information Figure S3). The kinetics of such process are significantly affected by the number of molecules entrapped in the PDMS film, which depends on the polymer’s free volume. The free volume can be tuned either by changing the type of PDMS and also by changing the time duration of the dipping process. In particular, preparing PDMS films with a lower density (higher free volume) or increasing the duration of dipping results in the

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RESULTS AND DISCUSSION The DAE-PDMS composite films are formed by dipping the PDMS films in a DAE chloroform solution of specific concentration. In this way, PDMS swells,27 facilitating the homogeneous entrapment of about 1.7 wt % photochromic molecules in the polymer’s free volume and assuring the maximum mobility of the molecules in the matrix. Such a process in fact minimizes the possibility of forming aggregates or photochromic complexes on the surface of the elastomers and is proven to be much more efficient for fibrils formation than the mixing process of the components and the subsequent polymer curing. In the latter case, no fibrils are formed in the time frame of the conducted experiments (results not shown). The absorption characteristics of the samples, as prepared and upon UV and visible laser irradiation, are shown in Figure 1. As demonstrated, upon UV irradiation (λirr = 266 nm, F266 = 2 mJ cm−2), two new absorption bands are formed at 375 and 524 nm, characteristic of the photoisomer of the closed form,28 with increasing intensity as a function of the number of irradiation pulses. After 50 pulses, the intensity of the new peaks stops increasing, demonstrating that the system has reached equilibrium. The subsequent irradiation with visible laser light (λirr = 532 nm, F532 = 10 mJ cm−2) causes the decrease in the intensity of these peaks, indicating that the molecules returned to the open form, a process that is almost complete after 80 pulses. Consequently, the DAE molecules conserve their 13060

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uptake of more DAE molecules by the polymer and faster fibril formation (Supporting Information Figures S4 and S5). To understand better the self-assembly mechanism that leads to the formation of the supramolecular structures, a DSC study was conducted. It has to be stressed that thermal tests were carried out on samples with a higher amount of DAE with respect to the samples used for optical and surface characterization for instrumental sensitivity reasons. The increase in the density of the molecules on the surface accelerates their nucleation, and this is the reason for the different time periods used in this study with respect to the ones used in the microscopy study. Indeed, as shown in the microscopy study of Supporting Information Figure S6, faster fibril formation is achieved when a larger number of DAE molecules is adsorbed in the film. Figure 3a shows the melting temperatures (Tm)

the sample, increasing thus the mobility of the DAE molecules on the surface (Supporting Information Figure S8). Indeed, as shown, initially most of the DAE molecules are in the unstable, amorphous state with low Tg and Tm but also with a small area of the crystallization peak, the latter indicating the number of crystalline molecules. However, as time passes the system is stabilized and a constant Tm is obtained whereas no Tg is observed, as ascribed to the change in the organization of the system during this time, from amorphous to ordered crystalline. This is in agreement with the microscopy study, which clearly shows a continuously growing number of needlelike and rodlike nanocrystals and microcrystals of DAE with time that collectively form the overall cactuslike structure. As discussed above, DAE spontaneously forms supramolecular structures of crystalline microfibrils and nanofibrils under room conditions as a result of the self-assembly of neighboring molecules. After fibril formation, subsequent UV irradiation of the DAE-PDMS samples causes the conversion of the open form of DAE to the closed photoisomer form as already shown in Figure 1, accompanied by an impressive modification of the overall surface morphology as shown in Figure 4. Specifically, after UV irradiation the crystalline nanofibrils that comprise the cactuslike structure are not evident anymore. This indicates that the photoisomerization process induced by UV irradiation causes the destruction of the crystals because, as assumed, in the closed form the molecules are no longer orderly packed. Indeed, as we show below, when the initial sample is irradiated immediately after fabrication, the nature of the surface is completely amorphous. However, after UV-irradiated crystalline structures are stored for 5 days in the dark at room temperature, new thin, long needle-shaped fibrils are formed. This, in combination with the evident color recovery of the irradiated area, indicates that the formation of crystals is exclusively attributed to the open form of the DAE molecules. During recovery, the molecules are assembled on these supramolecular structures that “survived” after UV irradiation. During the back-conversion from the closed to the open form, the needle-shaped crystals are growing faster than in the initial process after molecule deposition. In the latter case, the molecules need more time because the nuclei should form first, followed by the initial clusters and subsequently the crystals. In the case in which the DAE-PDMS system is irradiated with UV laser light before any formation of fibrils (day 0), we observe no crystal growth during this time, in contrast to the nonirradiated area. Indeed, Figure 5 shows the microscopy images of a DAE-PDMS film stored for 3 and 10 days after UV irradiation at day 0. As shown, in the irradiated area the crystal growth is inhibited, even after storage for 10 days, whereas outside the irradiated area, crystals are growing in accordance with the presented data. In fact, the DSC results obtained from the UV-irradiated DAE-PDMS systems (Supporting Information, Figures S9 and S10) show that the melting peak area, which indicates the quantity of the crystallized molecules, is half that of the nonirradiated sample. The reason that we continue to see the same behavior of Tm is most likely the noncomplete conversion of the molecules to the photoisomeric form and the achievement of dynamic equilibrium with about half of the molecules in the open form and half in the closed form. As a consequence, only the open form nucleates after deposition, giving rise to the melting peak in the thermogram, whereas the photoisomers of the closed form do not nucleate and do not show any phase transition in the investigated temperature

Figure 3. (a) Time evolution of the Tm of DAE molecules after deposition on PDMS films. (Inset) Corresponding DSC thermogram in each case. (b) Time evolution of Tg after the DAE-PDMS sample preparation.

derived from the thermograms (inset) recorded at different times after the deposition of DAE on a PDMS surface. As demonstrated, the Tm and the melting peak area (Supporting Information Figure S7) are sharply increased in the first 5 h after deposition, revealing the dynamic nucleation of the molecules into crystalline structures. Furthermore, 1 h after deposition, the glass-transition temperature (Tg) of the film (Figure 3b) is measured at −10 °C and increases with time, reaching 6 °C after 4 h (Figure 3b). After that time, no Tg is evident, demonstrating that the system is completely crystalline, as already proven by the Tm study. In summary, in the first few hours after deposition, the molecules are highly mobile at room temperature, so they spontaneously interact with their neighbors through intermolecular noncovalent interactions, such as π−π interactions and/ or hydrogen bonds, in order to form the initial assemblies that can be considered to be the seeds for further crystal growth. This process can be further accelerated by slightly heating of 13061

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Figure 4. Morphological difference on the surface of a DAE-PDMS sample between a nonirradiated and a UV-irradiated area measured by SEM microscopy. (a) Image of the crystalline film, where the left part is irradiated by UV. Higher-magnification image of the (b) nonirradiated and (c) irradiated areas. (d) Irradiated area 5 days after UV irradiation, demonstrating the recrystallization process.

range. In fact, the macroscopic appearance of these samples, after prolonged irradiation, suggests that the photoisomers are in a jelly or liquid state. Therefore, the closed form of DAE is not forming any crystalline fibrils under ambient conditions, in contrast to the open form. On the top, the possibility to modify the surface morphology of the soft DAE-PDMS film using UV light irradiation and by specifically selecting the time after the formation of the samples that this irradiation can be performed offers the possibility to form various types of patterns with the desired characteristics. To assess possible applications in this regard, measurements of wettability were made to evaluate the wetting properties of the fabricated materials. Figure 6 shows the WCA evolution of DAE-PDMS films during storage at room temperature in the dark. As demonstrated, there is an increase in the WCA with time, consistent with the cactuslike crystal growth and the subsequent increase in the surface roughness. Indeed, the superhydrophobicity is reached after 7 days of deposition where the WCA exceeds 160°, whereas at day 14 the WCA is almost 180°. It is noteworthy that after gentle deposition the water drops are able to slide from the surface even at a very low inclination of the surface (1−5°). The subsequent irradiation with UV light results in the destruction of the cactuslike morphology, leading to the sudden decrease in WCA by almost 80 to 100.6°, a value even lower than that for pure PDMS (110°). Upon storage in the dark, the WCA of the irradiated sample increases because of the re-formation of the crystalline fibrils as discussed above,

Figure 5. Microscope images of the DAE-PDMS film. (a) Nonirradiated area 3 days after film preparation. (b) UV-irradiated area 3 days after film preparation and irradiation. (c) Nonirradiated area 10 days after film preparation. (d) UV-irradiated area 10 days after film preparation and irradiation.

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Present Addresses

(E.G.) Oxford Instruments Omicron NanoScience. The Felbridge Centre Willard Way, East Grinstead RH19 1XP, UK. (E.S.) Ludwig-Maximilians University, Department of Physics, Oettingenstrasse 67, Munich, Germany. Author Contributions

The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

■ Figure 6. Surface wettability modification of the surface of a DAEPDMS film before and after UV irradiation as a function of time.

reaching the superhydrophobic region 4 days after UV irradiation and the maximum WCA value after 6 days. This behavior is in agreement with results shown in the previous experiment of UV irradiation and confirms that the molecules are recrystallizing, changing the surface morphology.

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CONCLUSIONS We have investigated the spontaneous growth of well-defined microfibrils and nanofibrils of DAE photochromic molecules on soft PDMS films. The growth process is attributed to the ability of the open form of diarylethene to form crystal seeds as a result of the high mobility of the molecules at room temperature. The use of the PDMS polymer and the fabrication process of the composite contributes to the large-scale homogeneity of the formed crystalline area and to the continuous supply of molecules, entrapped in the free volume of the PDMS, to the fibril surface. This is a property exclusively attributed to the open form because after UV irradiation crystals are destroyed or not formed. This process can be an alternative to the facile and effective formation of photochromic fibrils on soft polymer surfaces for utilization on diverse soft devices.

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ASSOCIATED CONTENT

S Supporting Information *

Optical images of changes in the morphology of the DAEPDMS system. SEM images of the DAE molecules on different substrates. Optical images of the DAE-PDMS system after different dipping times and of DAE-elastomer systems using elastomers with different free volumes. Optical images of the DAE-PDMS system dipped in DAE solutions with different concentrations. Time evolution of the melting peak area of DAE molecules after deposition on PDMS films. Optical images of the DAE-PDMS system after heating. Thermal analysis of DAE molecules after deposition on PDMS films before and after irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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