microstructured polyhedral oligomeric silsesquioxanes-based hybrid copolymers: Morphology evolution and surface characterization

Journal of Colloid and Interface Science 394 (2013) 386–393

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Nano/microstructured polyhedral oligomeric silsesquioxanes-based hybrid copolymers: Morphology evolution and surface characterization Jianzhao Liu a, Jizhou Fan a, Ze Zhang b, Qin Hu b, Tingying Zeng c, Bingbing Li a,⇑ a

Department of Chemistry, Science of Advanced Materials Doctoral Program, Central Michigan University, Mount Pleasant, MI 48859, United States Department of Electrical Engineering, Central Michigan University, Mount Pleasant, MI 48859, United States c Research Laboratory for Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, United States b

a r t i c l e

i n f o

Article history: Received 26 September 2012 Accepted 7 November 2012 Available online 5 December 2012 Keywords: Polyhedral oligomeric silsesquioxanes Electrohydrodynamic preparation Water droplet evaporation

a b s t r a c t Unique ‘‘micro-bean sprouts’’ with ‘‘nano-tails’’ were electrohydrodynamically prepared from poly[(propylmethacryl-heptaisobutyl-polyhedral oligomeric silsesquioxane)-co-(methylmethacrylate)] (POSSMMA). The nano/microstructured POSS-MMA substrates reveal superhydrophobic nature, with contact angles >160°. Contact angle versus time plots show that water droplet evaporation from the substrates with ‘‘micro-bean sprouts’’ falls into two stages: the initial stage, marked by steadily decreasing contact angles and the later stage, marked by rapidly decreasing contact angles. The turning point differentiating these two stages occurs at 120°–130°. The substrates consisting of ‘‘micro-bean sprouts’’ with larger ‘‘heads’’ experience pinned triple-line phase during the early stage of droplet evaporation. In contrast, the triple line for a droplet placed on ‘‘micro-bean sprouts’’ with smaller ‘‘heads’’ can move easily, leading to a rapid decrease in contact diameter. Meanwhile, the contact angle decreases only slightly during the rapidly moving triple-line phase. The contact angles of the fibrous substrates measured during the initial stage of water droplet evaporation are much lower than those of ‘‘micro-bean sprouts’’, even though the time period of the initial stage is extended. Both the architectures and the sizes of these POSS-MMA nano/ microstructures were shown to affect the energy barrier for the triple-line motion during water droplet evaporation and therefore the contact angle hysteresis. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Polyhedral oligomeric silsesquioxane (POSS), [(SiO1.5)nRn] is composed of an inorganic core [(SiO1.5)n] surrounded by an organic corona [Rn], where n = 6, 8, 10, or 12, and R can be various organic substitute groups or even long chain polymers [1]. The POSS family of molecules has been widely used to fabricate organic/inorganic hybrid materials [2]. The unique nano-cage structure of POSS molecules gives POSS-based materials distinct and desirable physicochemical properties such as reduced flammability, increased thermal stability, and enhanced hydrophobicity, as well as improved mechanical strength, modulus, and rigidity [1–5]. In particular, owing to their biocompatibility and low inflammatory response, POSS-based materials are very suitable for various biomedical applications [6–8]. For instance, Gu et al. demonstrated that incorporation of POSS into poly(e-caprolactone)-based multiblock thermoplastic-polyurethane (PCL-TPU) by either covalently bonding or physically blending can significantly suppress the ⇑ Corresponding author. Address: Department of Chemistry, Science of Advanced Materials Doctoral Program, Central Michigan University, 201 E Ottawa Ct. Dow 350, Mount Pleasant, MI 48859, United States. Fax: +1 989 774 3883. E-mail address: [email protected] (B. Li). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.11.015

enzymatic degradation of PCL-TPU [9]. Importantly, this strategy can be used to fabricate relatively stable PCL-based TPU surgical implants for use in long-term implantation [9]. Furthermore, POSS-containing PU physical networks evaluated in vivo over 24 weeks were determined to be biocompatible in terms of inflammatory and wound-healing responses, which are comparable to other clinically approved implants [10]. In another report, Seifalian and co-workers performed a series of intensive studies to examine the cytocompatibility, anti-thrombogenicity, and biostability of POSS-based poly(carbonate-urea)urethane (POSS-PCU) composites [6,7]. Their studies clearly demonstrated that POSS-PCU composites are suitable for use as biomaterial implants in contact with blood. POSS-PCU incorporated with growth factors and peptides can be further endothelialized by attracting endothelial progenitor cells from circulating blood [7]. The endothelialized POSS-PCU implants exhibit enhanced haemocompatibility [7]. The development of new POSS-based materials for biomedical applications requires an in-depth understanding of the materials’ surface properties for tuning cell adhesion, proliferation, migration, morphology, stem cell differentiation, and the deposition and remodeling of native extracellular matrix. Previous studies on the surface characterization of POSS-based composites focus primarily on measuring and interpreting apparent contact angles

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(advancing and receding angles) [11–14]. For instance, recent studies have demonstrated the feasibility of preparing POSS-based structured composites by electrospinning [15–19]. These studies focus mainly on using fluorodecyl POSS to fabric superhydrophobic and/or oleophobic substrates [15–17]. Advancing and receding angles were commonly measured to interpret the surface hydrophobicity of the as-spun fluorodecyl POSS-based composites. Until now, no study on the dynamics of water droplet evaporation on POSS-based materials has been reported. Water droplet evaporation is a common yet significant phenomenon that has direct relevance to potential biomedical applications of POSS-based materials. For instance, the dynamic behavior of contact angle and contact area versus time collected for POSS-based composites can be used to predict and control initial cell adhesion and its ramification that are crucial for the design of novel biomedical implants. Hence, the motivation of our study was to fabricate new POSS-based nano/microstructured composites using electrohydrodynamic preparation (i.e., electrospinning and electrospraying) and to investigate the dynamics of water droplet evaporation on these prepared nano/microstructured composites. The novelty of our study is demonstrated as follows: (1) Unique ‘‘micro-bean sprouts’’ with long ‘‘nano-tails’’ were prepared from POSS-MMA solutions with concentrations as low as 1 wt.% using flow rates ranging from 2.5 to 0.5 mL h1. This observation provides new evidence that the interplay between elastic response, solvent evaporation, and electrification can suppress or slow down the growth of Rayleigh instability. The reduction in the Rayleigh instability in turn leads to the formation of fibers or beaded fibers from polymer solutions with concentrations below critical concentration for chain entanglements (ccr). (2) A broad regime of morphological transitions, from smooth fibers to ‘‘micro-bean sprouts’’ with long ‘‘nano-tails’’, was captured and interpreted in terms of fluid relaxation time, the growth of Rayleigh instability, elastic response, and the known effects of chain entanglements on typical polymer systems. (3) Furthermore, the evaporation profiles of water droplets placed on micro/nanostructured POSS-MMA hybrid substrates (as-prepared and annealed at 85 °C) were obtained by analyzing the side images of water droplets captured during the evaporation process. The results were interpreted in terms of triple-line motion and its correlation with the sizes and architectures of POSS-MMA micro/nanostructures.

2. Materials and methods

Fig. 1. Chemical structure of POSS-MMA hybrid copolymer.

Table 1 Processing parameters for electrohydrodynamic preparation. #

Conc. (by wt.%)

Needle (gauge)

POO-MMA (25 wt.% of POSS) S1 17 18 S2 15 18 S3 10 18 S4 5 18 S5 2 18 S6 1 18 S7 1 18 S8 1 18 S9 1 18 S10 1 18 S11 1 18

Voltage (kV)

Flow rate (mL/h)

Distance (cm)

15 15 15 15 15 15 15 15 15 15 15

5 5 5 5 5 5 2.5 1.5 1 0.8 0.5

15 15 15 15 15 15 15 15 15 15 15

differential scanning calorimetry measurement using DSC Q2000 (TA Instruments, New Castle, DE) connected with a refrigerated cooling system (RCS 90). Accordingly, the thermal annealing of POSS-MMA substrates was performed in an 85 °C oven, which preheated for 30 min to reach thermal equilibrium. After 9 h of thermal annealing, the substrate samples were removed from the oven and cooled at RT; these substrates are referred to as ‘‘annealed’’ samples.

2.1. Electrohydrodynamic preparation

2.2. Scanning electron microscopy

POSS-MMA (25 wt.% POSS; Mn = 22  103 g mol1 and Mw = 57  103 g mol1; Sigma–Aldrich, St. Louis, MO), acetone, and chloroform were used as received. The chemical structure of the POSSMMA copolymer is shown in Fig. 1. POSS-MMA solution was prepared by dissolving POSS-MMA into a chloroform/acetone co-solvent (1:1, volume ratio), which was then stirred at room temperature overnight. The experimental setup used for electrohydrodynamic preparation consists of four major components: a high voltage power source (Gamma High Voltage, ES30P, Ormond Beach, FL), a syringe with a flat, blunt tipped needle, an aluminum collector, and a syringe pump (New Era, NE-300 series, Farmingdale, NY) [20]. Processing parameters for electrohydrodynamic preparation are summarized in Table 1. The nano/microstructured POSS-MMA substrates collected on the grounded aluminum collector were placed in an oven (Lindberg blue M, Thermo Scientific, Waltham, MA) to vacuum-dry at room temperature (RT) overnight or longer; the dried substrates are referred to as ‘‘as-prepared’’ samples. POSS-MMA exhibits the onset of glass transition temperatures at 78 °C, according to the

The morphology of nano/microstructured POSS-MMA hybrid substrates was examined by scanning electron microscopy (SEM, Hitachi VP-SEM S-3400N, Japan). Prior to SEM analysis, vacuumdried samples were sputter-coated with gold–palladium for 3 min under argon. All micrographs were collected using the secondary electron imaging function of the SEM set to 6 kV with a probe current of 6  1011 amps and a working distance of 5– 15 mm. ImageJ 1.46 software was used to measure the diameters of fibers and particles. The histograms for the size distributions of particles and fibers were generated using XLSTAT in Microsoft Excel 2010. 2.3. Droplet evaporation profiles Contact angles and contact diameters of evaporating water droplets were collected using an optical tensiometer (Theta-Lite, Biolin Scientific, Linthicum Heights, MD). A water droplet (5– 6 lL, ultrapure water) was dispensed on each of the selected POSS-MMA substrates by a syringe with a 30-gauge needle. The

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droplet was then left to evaporate at a constant temperature of 25 °C in a measurement chamber connected to a water-circulating bath. Side images (640  480 pixels) of droplets were captured every 15–20 s during the time course of evaporation with a firewire digital camera built into the optical tensiometer. Series of captured images were then analyzed with Attension Theta software (Version 4.1.9.8) to determine contact angles and water droplet evaporation profiles. All the experiments were repeated at least three times to validate the trends for contact angle and contact radius versus time during water droplet evaporation (see Figs. S1 and S2 in supporting information). Average contact angle versus time was plotted using the mean contact angle of three trials with standard deviation.

3. Results and discussion 3.1. Morphological evolution of POSS-MMA during electrohydrodynamic preparation Electrohydrodynamic preparation (i.e., electrospinning and electrospraying) is a convenient method used to produce polymer particles and fibers with diameters ranging from a few nanometers to several hundred micrometers [21–24]. The morphological and topographical features of prepared substrates strongly depend on the following conditions: (1) processing parameters including needle gauge, flow rate, applied voltage, distance between needle and collector (i.e., the working distance), and type of collectors; and (2)

physicochemical properties of polymer solutions, including molecular weight, solvent volatility, solution concentration, and polymer–solvent interactions. The versatility of polymers gives electrohydrodynamically prepared materials an array of potential applications, including wound dressing [25,26], drug delivery [27,28], and tissue scaffolds [29,30]. In general, the formation of uniform fibers requires a higher chain entanglement density with solution concentration approaching twice its critical concentration for chain entanglements (ccr) [31]. A lower chain entanglement density with solution concentration below ccr often results in the electrospraying of droplets and, subsequently, the formation of particulate morphologies [32,33(a)]. For instance, in a study on a series of linear and branched poly(ethylene terephthalateco-ethylene isophthalate) (PET-co-PEI) copolymers with molecular weights well above the entanglement molecular weights, the ccr was determined to be the minimum concentration required for the formation of beaded fibers and 2–2.5 times ccr to be the concentration for the formation of bead-free fibers [31]. Since the ccr for a given polymer depends on the solvent selected, electrospinning the same polymers with the same solution concentrations but in different solvents could lead to substantially different morphologies of electrospun membranes. Electrospinning of POSS-MMA in a co-solvent of dimethylformamide (DMF) and tetrahydrofuran (THF), for instance, provides only a narrow range of concentrations enabling the formation of fibers with rough surface features [18]. A slight increase or decrease in the solution concentration leads to the formation of interconnected, coarser fibers, or the formation of mushy webs, respectively. The selection of

S1

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Fig. 2. Scanning electron micrographs of POSS-MMA samples prepared from solutions of concentrations of (A-S1) 17 wt.%, (B-S2) 15 wt.%, (C-S3) 10 wt.%, (D-S4) 5 wt.%, (E-S5) 2 wt.%, and (F-S6) 1 wt.%, using a flow rate of 5 mL h1. A0 –D0 are histograms of fiber-size distributions corresponding to SEM images A–D, respectively. [Scale bars: 20 lm for SEM images A–F].

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acetone/chloroform co-solvent in this study is motivated by the fact that the relatively weaker molecular interactions between POSS-MMA and acetone/chloroform, compared to those between POSS-MMA and DMF/THF, can accelerate the drying process and therefore prevent the formation of interconnected coarse fibers and webs. More importantly, the selection of acetone/chloroform co-solvent allows for a better understanding of the morphological evolution during electrohydrodynamic preparation of POSS-MMA at various solution concentrations with different elastic responses and fluid relaxation times. SEM images in Fig. 2, taken with the same magnification, show the morphological transition of POSS-MMA from fibers (A: 17 wt.%), to fibers with elongated beads (B: 15 wt.%), to beaded fibers (C: 10 wt.%), to fibers with large particles (D: 5 wt.%), to large crumpled particles (E: 2 wt.%), and to smaller bowl-shaped particles (F: 1 wt.%). The flow rate used to prepare these samples was 5 mL h1. The average fiber diameter of all above samples decreases from 4 lm to 200 nm with the reduction in solution concentrations from 17 to 5 wt.%, as shown in the histograms in Fig. 2A0 –D0 . This observation can be attributed to the zero-shear rate viscosity that increases when solution concentrations increases, giving rise to a higher density of entanglement couplings [31]. In addition, the initial elongational viscosity could also affect the fiber diameters. The initial elongational viscosity depends on parameters such as applied voltage, but it is weakly related to solution concentration. For POSS-MMA/acetone/chloroform system, it was found that the fiber diameters do not change with increases or decreases in applied voltage alone, even though the beads can be elongated or even disappear when the applied voltage is increased (results not shown here). Thus, zero-shear viscosity plays a dominant role in determining the fiber diameters for the POSSMMA/acetone/chloroform system. It is worth noting that the morphological features of the electrospun membranes reported here are substantially different from those prepared by electrospinning POSS-MMA/DMF/THF [15]. The interconnected coarse fibers and webs, which were shown in POSS-MMA/THF/DMF system, are absent in POSS-MMA/ acetone/chloroform system. More significantly, for 1 wt.% POSS-MMA solutions, further decreasing the flow rates from 5 to 2.5–0.5 mL h1 leads to a another morphological transition from particles to ‘‘micro-bean sprouts’’ with ‘‘nano-tails’’, respectively, as shown in Figs. 3 and 4. A closer view of the ‘‘nano-tails’’ shown in Fig. 3D0 and E0 reveal again the ‘‘nanobeads-on-string’’ morphology, similar to that of beaded fibers formed from electrospinning of polymer solutions with concentrations between the ccr and the 2  ccr. For a better comparison, the POSS-MMA particles formed at a flow rate of 5 mL h1 were also included in Fig. 3A (2 wt.% solution) and 3B (1 wt.% solution). The average diameters of the particles, the heads of ‘‘micro-bean sprouts’’, decrease from 5, 4, to 2 lm when decreasing the flow rate from 2.5 (Fig. 3C), 1.5 (Fig. 4A), to 0.5 mL h1 (Fig. 4B), respectively. The insets in Fig. 4 show the higher magnification micrographs and the histograms of particlesize distribution corresponding to the SEM images in Fig. 4A and B. Decreasing flow rate apparently affects the initial size of electrified droplets, as estimated by the Scaling law of d / Q1/3 [33]b. Thus, the diameter of the particles decreases with the decreasing flow rate, as seen in Fig. 4. The continuous transition from fibers to particles in POSS-MMA with solution concentrations ranging from 17 to 2 wt.% suggests that 2 wt.% of POSS-MMA could be well below the ccr. Therefore, the morphology transition from particles back to ‘‘micro-bean sprouts’’ with ‘‘nano-tails’’, as the result of the decreasing flow rate for POSS-MMA with sub-ccr concentrations, warrants further discussion as follows. According to the theory of electrohydrodynamic preparation, repulsive electrostatic forces between like charges accumulated on the surface of a pendant drop tend to pull the fluid jet toward

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the grounded collector [21–24,31,34–36]. Surface tension forces, on the other hand, tend to decrease the area of air/liquid interface. The balance between the electrostatic forces and the surface tension forces results in the formation of a Taylor cone. The applied electric field exerts tangential net forces on the Taylor cone, break up the force equilibrium at the apex of the cone, and draw a thin fluid jet from the needle tip toward the grounded collector. Electrospinning uniform fibers typically require polymer concentrations well above 2ccr [23,24,31,34–37]. Later, Rutledge and co-workers found that sufficient chain entanglements are not necessary for the formation of uniform and beaded fibers [38].

Fig. 3. Scanning electron micrographs of POSS-MMA hybrid substrates prepared from (A-S5) 2 wt.% solution using a flow rate of 5 mL h1 and (B–E) 1 wt.% solution using flow rates ranging from (B-S6) 5, (C-S7) 2.5, (D-S9) 1, to (E-S10) 0.8 mL h1, respectively. D0 and E0 highlight the ‘‘nano-tails’’ with ‘‘nanobeads-on-string’’ morphology triggered by the growth of Rayleigh instability along the secondary jet drawn from the ‘‘heads’’. [Scale bars: Images A, B, D, and E: 4 lm, and Image C: 50 lm].

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Fig. 4. Scanning electron micrographs of POSS-MMA substrates prepared from 1 wt.% solution using a flow rate of (A-S8) 1.5 and (B-S11) 0.5 mL h1. The histograms of particle (e.g. the ‘‘head of micro-bean sprout’’) size distributions were shown as insets of images A and B. [Scale bars (A and B): 50 lm, Insets of A and B: 5 lm].

Polymer solutions below ccr can exhibit elastic response when the fluid relaxation time is longer than the extensional deformation time. According to the study by Rutledge et al., uniform fibers can form by electrospinning un-entangled PEO/PEG solution with shear viscosities of less than 0.30 Pa s [38]. The ratio of fluid relaxation time to instability growth time, defined as a Deborah number (De), was evaluated in this study and found to be closely correlated with morphological evolution during electrospinning [38]. When De  1, the fluid relaxation time is longer than the instability growth time, and the capillary forces promote elastic response, suppress the Rayleigh instability, and delay the breakup of fluid jet [38]. The fluid relaxation time, which characterizes the elastic property of a polymer solution, depends on both molecular weight and solution concentration. Thus, the transition from particles to ‘‘micro-bean sprouts’’ prepared from 1 wt.% POSS-MMA, with presumably the same fluid relaxation time, is a direct consequence of the growth of Rayleigh instability concurrent with solvent evaporation. A steady jet drawn from a Taylor cone could be elongated further because of the repulsive forces between like charges. Rayleigh instability can be completely suppressed for polymer solutions (1)

at higher concentrations with sufficient chain entanglements [31] and (2) at lower concentrations with elastic stresses driven by extensional type of deformation [38]. In contrast, the growth of Rayleigh instability can give rise to (1) bead-on-string morphology, as the wave propagates along the surface of a liquid jet or to (2) particulate morphology, resulting from the breakup of liquid jet driven by the growth of the amplitude of propagating waves. The morphological features of POSS-MMA nano/microstructures discussed in this study are illustrated in Fig. 5. For 1 wt.% POSSMMA, the solution jet breaks into larger droplets at a higher flow rate of 5 ml h1. The larger droplets have smaller surface area compared to the smaller droplets produced at a lower flow rate. Thus, solvent evaporates slower in relatively larger droplets, allowing the elongated particles to retract back to their spherical shapes, which form crumpled particles upon drying, as shown in Fig. 5B. Decreasing flow rate could reduce the initial sizes of electrified droplets. The larger surface areas of smaller droplets produced at lower flow rates accelerate solvent evaporation, leading to further accumulation of like charges on the droplet surface [39]. The repulsive forces can draw a secondary nanoscale liquid jet from the droplet, resulting in the formation of a ‘‘nano-tail’’ upon rapid

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200

θ [degree]

(A)

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θ [degree]

(B)

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40 160

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B 80

0

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Fig. 6. Average contact angle versus time plots for a water droplet evaporating on samples (A) S7-2.5 mL h1 and S11-0.5 mL h1, ‘‘micro-bean sprouts’’, and (B) S1, uniform fibers.

Fig. 5. Illustration of morphological evolution during electrospraying of POSS-MMA hybrid co-polymer with sub-ccr concentrations: (A) growth of Rayleigh instability and liquid jet breakup, (B) droplet formation at higher flow rates, and (C) the formation of ‘‘micro-bean sprouts’’ with ‘‘nano-tails’’ at lower flow rates.

drying, as illustrated in Fig. 5B. Furthermore, the Rayleigh instability can grow along the secondary fluid jet drawn from the droplets, giving rise to beaded ‘‘nano-tails’’, as shown in Fig. 3D0 and E0 .

the fibrous substrate, is 140°, much lower than that for S7 and S11. The contact angle of a droplet evaporating on fibrous substrate steadily decreases for an extended period of time, as shown in Fig. 6B. When the droplet mass is further reduced after 35 min, the droplet distorts, and the images captured are no longer suitable for accurately measuring the contact angles. Thus, we focus on the evaporation dynamics of water droplets placed on S7 and S11, which exhibit well-defined baselines and thus enable accurate contact angle measurements. The profiles of evaporating water droplets were also collected on S7 and S11 after thermal annealing at 85 °C for 9 h. The static contact angles extracted from the droplet evaporation profiles at t = 0 are identical for as-prepared and annealed samples, as shown in Fig. 7A (S11) and B (S7). According to the theory of Cassieto-Wenzel transition [40,41], when a water droplet placed on a substrate is in an ideal Cassie state, it sits on the ‘‘air’’ trapped in the nanostructures and only slightly comes into contact with the edges of these solid nanostructures. The morphological features remain same after annealing (Fig. S1 in supporting information), and therefore, the liquid/solid contact area remains the same, leading to the almost identical static contact angle values during initial stages that are seen in both annealed and as-prepared samples. When the droplet progresses into a Wenzel state, the water droplet comes into contact with larger surface area of solid nanostructures. In this case, the surface chemistry and the roughness of these solid

3.2. Surface characterization of nano/microstructured POSS-MMA composites

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θ [degree]

The surface of ‘‘micro-bean sprout’’ structures displays an apparent contact angle greater than 160° (see Table S1 in supporting information), indicating its superhydrophobic nature. Dynamic contact angle plots were further analyzed to understand the equilibrium/meta-stable states of water droplets on representative nano/microstructured POSS-MMA hybrid substrates. Fig. 6 shows the average contact angle versus time plots for a water droplet evaporating on samples S1, S7, and S11, respectively. For the superhydrophobic substrates with ‘‘micro-bean sprouts’’ (S7 and S11), contact angle reduces slowly during the early stages of droplet evaporation, which is followed by its rapid decrease during the later stages of droplet evaporation, as shown in Fig. 6A. The initial evaporating stage lasts approximately 30 min for S7 (see morphology in Fig. 3C) and extends to 45 min for S11 (see morphology in Fig. 4B). The average turning points differentiating these two distinct stages for both S7 and S11 occur between 120° and 130°. The contact angles measured for S11 were constantly higher during the early stage of droplet evaporation than were those for S7 during the same stage. In contrast, the static contact angle for S1,

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t [minute] Fig. 7. Average contact angle versus time plots for a water droplet evaporating on as-prepared (RT) and annealed (85 °C) samples with ‘‘micro-bean sprout’’ morphology: (A) S11, 2 lm average diameter of the ‘‘heads of micro-bean sprouts’’ and (B) S7, 5 lm average diameter of the ‘‘heads of micro-bean sprouts’’.

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Fig. 8. Contact angle (s) and contact diameter (j) versus time plots for a water droplet evaporating on S7 (A) as-prepared substrate and (B) annealed substrate. [Morphological features were shown in Fig. 3C].

Fig. 9. Contact angle (D) and contact diameter (j) versus time plots for a water droplet evaporating on S11 (A) as-prepared substrate and (B) annealed substrate. [Morphological features were shown in Fig. 4B].

structures become dominant factors for determining contact angles. Annealing at 85°, above Tg of POSS-MMA, induces segmental mobility and structure relaxation with a concomitant decrease in free energy of system. It therefore favors the accumulation of lower free energy POSS moieties on the surface of nano/microstructures, which prevents water from entering the ‘‘air pockets’’ because of capillary adhesion force. Thus, the discrepancy in contact angles between as-prepared and annealed samples increases with time, leading to significantly higher contact angles measured for annealed substrates than for as-prepared ones, as shown in Fig. 7. Three models developed by Picknett and Bexon [42] have been used to interpret the mechanism of water droplet evaporation: the constant contact area model, the constant contact angle model, and the mixing model, the latter characterized by change in both contact angle and contact area [43–46]. For a spin-coated surface of POSS-MMA, the initial contact angle decreases rapidly; the contact diameter remains the same during the initial stage (see Fig. S1 in supporting information). This is followed by a constant contact angle phase along with a rapid decrease in contact diameter. Thus, the water droplet evaporating on spin-coated POSS-MMA is characterized by a constant contact radius phase (model) followed by a constant contact angle phase (model) that correspond to pinned and moving triple-line phases, respectively [47–49]. For ‘‘as-prepared’’ S7 with larger ‘‘heads’’ (5 lm, see morphology in Fig. 3C), a slight increase in contact diameter was observed over a period of time, as shown in Fig. 8A, suggesting that as-prepared substrates exhibit slight wettability, despite the apparent contact angles. In contrast, a droplet evaporates with a constant diameter on the annealed S7, as shown in Fig. 8B; this suggests a pinned triple-line phase during water droplet evaporation. The pinned tripleline phase observed here suggests that dynamic free energy cannot overcome the energy barriers caused by the nano/microstructures on the thermally annealed substrates. For the substrate S11 with smaller ‘‘heads’’ (2 lm, see morphology in Fig. 4B), the rapid decrease in contact diameters at the early stage of evaporation could suggest a smaller contact angle hysteresis and a lower energy barrier for triple-line motion, indicating that the droplet evaporation dynamics is strongly affected by the size of the nano/microstructures. For the annealed substrate S11, a typical constant contact angle phase accompanying a rapid decrease in contact diameter was observed during the initial 45 min of evaporation. The smaller the nanostructures, the lower the energy barrier for contact line motion. Thus, water droplets can easily slide off from the S11 substrates with a small sliding angle (8°, result not shown here) and small contact angle hysteresis (see Fig. 9).

4. Conclusions POSS-MMA hybrid fibers and unique ‘‘micro-bean sprouts’’ with long ‘‘nano-tails’’ were produced using electrohydrodynamic preparation. Evaporation dynamics of a water droplet placed on these nano/microstructured substrates was also discussed in this study. Thermally annealed hybrid substrates with larger ‘‘heads’’ of ‘‘bean sprouts’’ exhibit a pinned triple-line phase during the early stage of droplet evaporation. In contrast, the triple line for a droplet placed on an annealed substrate with smaller ‘‘heads’’ could move easily, leading to a rapid decrease in contact diameter along with contact angles remained almost constant during the moving triple-line phase. Both the architectures and the sizes of nano/microstructures can significantly affect the energy barrier for contact line motion and therefore the contact angle hysteresis, as well as the droplet evaporation dynamics. The present work sheds light on the electrohydrodynamic preparation and surface characterization of POSS-based nano/microstructured composites that are directly relevant to their biomedical applications as implants and tissue scaffolds. Acknowledgments J. Liu would like to thank Philip Oshel for SEM training. Authors would like to thank financial support by CMU start-up grant and Early Career Investigator Award (613771) to B. Li. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2012.11.015. References [1] (a) S.W. Kuo, F.C. Chang, Prog. Polym. Sci. 36 (2011) 1649–1696; (b) J. Wu, P.J. Mather, Polym. Rev. 49 (2009) 25–63; (c) W. Lee, S. Ni, J. Deng, B.S. Kim, S.K. Satija, P.T. Mather, A.R. Esker, Macromolecules 40 (2007) 682–688; (d) H.J. Kim, J. Deng, J.H. Lalli, J.S. Riffle, B.D. Viers, A.R. Esker, Langmuir 21 (2005) 1908–1916. [2] (a) Y. Liu, Z. Shi, H. Xu, J. Fang, X. Ma, J. Yin, Macromolecules 43 (2010) 6731– 6738; (b) J. Wu, Q. Ge, P.T. Mather, Macromolecules 43 (2010) 7637–7649; (c) P.T. Knight, K. Lee, H. Qin, P.T. Mather, Biomacromolecules 9 (2008) 2458– 2467. [3] J. Deng, B.D. Viers, A.R. Esker, J.W. Anseth, G.G. Fuller, Langmuir 21 (2005) 2375–2385. [4] B. Tan, H. Hussain, C. He, Macromolecules 44 (2011) 622–631.

J. Liu et al. / Journal of Colloid and Interface Science 394 (2013) 386–393 [5] Q. Guo, P.T. Knight, J. Wu, P.T. Mather, Macromolecules 42 (2010) 4991– 4999. [6] A. de Mel, F. Murad, A.M. Seifalian, Chem. Rev. 111 (2011) 5742–5767. [7] H. Ghanbari, B.G. Cousins, A.M. Seifalian, Macromol. Rapid. Commun. 32 (2011) 1032–1046. [8] J. Xu, J. Song, Proc. Natl. Acad. Sci. USA 107 (2010) 7652–7657. [9] X. Gu, J. Wu, P.T. Mather, Biomacromolecules 12 (2011) 3066–3077. [10] P.T. Knight, J.T. Kirk, J.M. Anderson, P.T. Mather, J. Biomed. Mater. Res., Part A 94 (2010) 333–343. [11] R. Chen, W. Feng, S. Zhu, G. Botton, B. Ong, Y. Wu, Polymer 47 (2006) 1119– 1123. [12] O. Monticelli, A. Fina, A. Ullah, P. Waghmare, Macromolecules 42 (2009) 6614– 6623. [13] A.K. Nanda, D.A. Wicks, S.A. Madbouly, J.U. Otaigbe, Macromolecules 39 (2006) 7037–7043. [14] R. Misra, B.X. Fu, S.E. Morgan, J. Polym. Sci., Part B: Polym. Phys. 45 (2007) 2441–2455. [15] A. Tuteja, W. Choi, M. Ma, J.M. Mabry, S.A. Mazzella, G.C. Rutledge, G.H. McKinley, R.E. Cohen, Science 318 (2007) 1618–1622. [16] W. Choi, A. Tuteja, S. Chhatre, J.M. Mabry, R.E. Cohen, G.H. McKinley, Adv. Mater. 21 (2009) 2190–2195. [17] A. Tuteja, W. Choi, J.M. Mabry, G.H. McKinley, R.E. Cohen, Proc. Natl. Acad. Sci. USA 105 (2008) 18200–18205. [18] Y. Xue, H. Wang, D. Yu, L. Feng, L. Dai, X. Wang, T. Lin, Chem. Commun. (2009) 6418–6420. [19] V.A. Ganesh, A.S. Nair, H.K. Raut, T. Tan, C. He, S. Ramakrishna, J. Xu, Mater. Chem. 22 (2012) 18479–18485. [20] J. Doshi, D.H. Reneker, J. Electrostat. 35 (1995) 151–160. [21] D.H. Reneker, A.L. Yarin, H. Fong, S. Koombhongse, J. Appl. Phys. 87 (2000) 4531–4547. [22] S.V. Fridrikh, J.H. Yu, M.P. Brenner, G.C. Rutledge, Phys. Rev. Lett. 90 (2003) 144502. 1–4. [23] M.G. McKee, T. Park, S. Unal, I. Yilgor, T.E. Long, Polymer 46 (2005) 2011–2015. [24] D. Li, Y. Xia, Adv. Mater. 16 (2004) 1151–1170. [25] J.-P. Chen, G.-Y. Chang, J.-K. Chen, Colloids Surf., A 313–314 (2008) 183–188.

393

[26] M.S. Khil, D.I. Cha, H.Y. Kim, I.S. Kim, N. Bhattarai, J. Biomed. Mater. Res., Part B 67 (2003) 675–679. [27] H.S. Yoo, T.G. Kim, T.G. Park, Adv. Drug Delivery Rev. 61 (2009) 1033–1042. [28] T.J. Sill, H.A. Von Recum, Biomaterials 29 (2008) 1989–2006. [29] W.J. Li, C.T. Laurencin, E.J. Caterson, R.S. Tuan, F.K. Ko, J. Biomed. Mater. Res. 60 (2002) 613–621. [30] H. Yoshimoto, Y. Shin, H. Terai, J. Vacanti, Biomaterials 24 (2003) 2077–2082. [31] M.G. McKee, G.L. Wilkes, R.H. Colby, Macromolecules 37 (2004) 1760–1767; (b) G. Eda, S. Shivkumar, J. Appl. Polym. Sci. 106 (2007) 475–487. [32] S.L. Shenoy, W.D. Bates, H.L. Frisch, G.E. Wnek, Polymer 46 (2005) 3372–3384. [33] (a) Y. Wu, J.A. MacKay, J.R. McDaniel, A. Chilkoti, R.L. Clark, Biomacromolecules 10 (2008) 19–24; (b) J. He, Y. Wan, J. Yu, Polymer 46 (2005) 2799–2801. [34] J. Deitzel, J. Kleinmeyer, D. Harris, N. Beck Tan, Polymer 42 (2001) 261–272. [35] A.L. Yarin, S. Koombhongse, D.H. Reneker, J. Appl. Phys. 90 (2001) 4836–4846. [36] M.M. Hohman, M. Shin, G. Rutledge, M.P. Brenner, Phys. Fluids 13 (2001) 2201–2220. [37] R.R. Klossner, H.A. Queen, A.J. Coughlin, W.E. Krause, Biomacromolecules 9 (2008) 2947–2953. [38] J.H. Yu, S.V. Fridrikh, G.C. Rutledge, Polymer 47 (2006) 4789–4797. b. [39] E. Scholten, H. Dhamankar, L. Bromberg, G.C. Rutledge, T.A. Hatton, Langmuir 27 (2011) 6683–6688. [40] A.B.D. Cassie, S. Baxter, Trans. Faraday Soc. 40 (1944) 546–551. [41] R.N. Wenzel, Ind. Eng. Chem. 28 (1936) 988–994. [42] R.G. Picknett, Interface Sci. 61 (1977) 336–350. [43] J.-H. Kim, S.I. Ahn, J.H. Kim, W.-C. Zin, Langmuir 23 (2007) 6163–6169. [44] X. Fang, B. Li, E. Petersen, Y. Ji, J.C. Sokolov, M.H. Rafailovich, J. Phys. Chem. B 109 (2005) 20554–20557. [45] X. Zhang, S. Tan, N. Zhao, X. Guo, X. Zhang, Y. Zhang, J. Xu, Chem. Phys. Chem. 7 (2006) 2067–2070. [46] S. Kulinich, M. Farzaneh, Appl. Surf. Sci. 255 (2009) 4056–4060. [47] E. Bormashenko, A. Musin, M. Zinigrad, Colloids Surf., A 385 (2011) 235–240. [48] H.Y. Erbil, Adv. Colloid Interface Sci. 170 (2012) 67–86. [49] N. Anantharaju, M. Panchagnula, S. Neti, J. Colloid Interface Sci. 337 (2009) 176–182.