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Spontaneous, Phase-Separation Induced Surface Roughness: A New Method to Design Parahydrophobic Polymer Coatings with Rose Petal-like Morphology Caroline R. Szczepanski, Thierry Darmanin, and Frederic Guittard ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10222 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 27, 2016

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ACS Applied Materials & Interfaces

Spontaneous, Phase-Separation Induced Surface Roughness: A New Method to Design Parahydrophobic Polymer Coatings with Rose Petal-like Morphology Caroline R. Szczepanski, Thierry Darmanin, Frédéric Guittard* Univ. Nice Sophia Antipolis, CNRS, LPMC, UMR 7336, 06100 Nice, France [email protected] Tel: (+33)4 92 07 61 59

Keywords: Parahydrophobic, Superhydrophobic, Adhesion, Phase separation, Biomimetics

ABSTRACT While the development of polymer coatings with controlled surface topography is a growing research topic, a fabrication method that does not rely on lengthy processing times, bulk solvent solution, or secondary functionalization has yet to be identified. This study presents a facile, rapid, in situ method to develop parahydrophobic coatings based on phase separation during photopolymerization. A comonomer resin of ethylene glycol diacrylate (EGDA) and 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) is modified with a thermoplastic additive (PVDF) to induce phase separation during polymerization. If applied to a glass substrate and photopolymerized, the EGDA:PFDA copolymer forms a homogeneous network with a single glass transition temperature (Tg) and slight hydrophobicity (θw ~ 114°). When the resin is modified with PVDF, phase separation occurs during photopolymerization producing a heterogeneous network with two Tg’s. The phase separation causes differences in composition and cross-link density within the network, which leads to local variations in polymerization 1 ACS Paragon Plus Environment


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shrinkage across the non-constrained material interface. Domains with higher cross-link densities shrink and contract towards the bulk material more dramatically, permitting the formation of rough surfaces with sub-micron sized spheres enriched in PVDF dispersed in a continuous matrix of EGDA:PFDA copolymer. Both the surface roughness and hydrophobic components in the resin render these surfaces parahydrophobic with θw ~ 150°, high water adhesion, and a similar morphology to rose petals observed in nature.

1. INTRODUCTION Multi-scaled interfacial roughness and the self-assembly of surface patterns is a growing research topic, since the fabrication of coatings with unique and easily controlled surface morphology is very important for applications in membrane development,1 separations,2 catalysis,3 microfluidics,4,5 and textiles.6,7 When tailoring interfacial morphology for these purposes it is often done with the aim of controlling surface wettability. Specifically, both superhydrophobic surfaces defined by an apparent contact angle (θw) greater than 150° and low hysteresis (Hw, i.e. very low water adhesion)8-11 and parahydrophobic surfaces defined by high θw and high Hw (sticky behavior) are highly desirable.12-16 In nature, coatings with

these

different

wetting

properties

have

been

observed

on

lotus

leafs

(superhydrophobicity), gecko feet, and rose petals (parahydrophobicity).13-16 Studies of these natural coatings have determined that both multi-scaled rough surface structure as well as low-surface energy materials is necessary to obtain coatings with high contact angles.9,10,17,18 The use of low-energy materials without any interfacial roughness only yields coatings with

(Young’s Angle) on the scale of 125-130°19 and this angle can be predicted by the

Young-Dupre equation that relates the differences in surface tensions between the three phases present (SL: solid-liquid, SV: solid-vapor, LV: liquid-vapor):

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The wetting behavior on rough surfaces, including those with observed contact angles above the 125-130° threshold mentioned, is described with different theoretical relationships. The Cassie-Baxter20 equation can accurately the high contact angle and low adhesive behavior observed on superhydrophobic surfaces:

In this theory, a probe liquid is suspended on top of surface microstructures. Since the surface is rough and composite-like with both solid (φs) and air fractions (1- φs) a significant volume of air is trapped between the surface and liquid when wetted, leading to very low water adhesion. Additionally, with this wetting regime increases in the roughness ratio (rf) of the wetted surface amplifies the contact angle above the observed threshold on smooth surfaces predicted by the Young’s angle (θY). However, the Cassie-Baxter theory cannot accurately describe the formation of rough surfaces that have both high contact angles and adhesion. These properties were first classified by Marmur as parahydrophobic,21 and studies have tried to mimic this behavior that is already observed on gecko’s feet.15,22 For parahydrohobic behavior the Wenzel equation of wetting,23 which assumes homogeneous wetting of a surface across all surface microstructures can describe the high contact angle and adhesion with the following equation:

As in the Cassie-Baxter equation, the contact angle is amplified above the Young’s angle (θY) as a surface becomes rougher (r increases). Since the surface is wetted homogeneously, the presence of micro or nano-scale structures significantly increases the contact area between the solid surface and probe liquid, increasing adhesion. Investigations of naturally occurring parahydrophobic surfaces have shown various nano- and micro-scale surface structures that increase the solid-liquid contact area following this wetting regime. On gecko feet, high densities of nano- and micron-sized pillars with high aspect ratios have been found. In this case, capillary induced adhesion on the nano-structures is also cited as contributing to the 3 ACS Paragon Plus Environment


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strong adhesive behavior.15 On rose petals, which also have parahydrophobic properties, the surface morphology contains “nubs” or bumps that are 10-40 microns in size and also secondary submicron folds. Here both surface structures observed (micron sized bumps, and sub-micron sized folds) increase the solid-liquid contact area.24 Currently, the most common approaches to develop superhydrophobic and parahydrophobic surfaces include electrodeposition of conducting polymers,19,25 formation of polymer/nanoparticle composites,26-28 electrospinning,29,30 as well as solution immersion (dipcoating).31,32 While these approaches have been successful at generating surfaces with strong hydrophobic character, most of these methods are in fact multi-step fabrication processes that are often done in solvent solutions, and cannot be replicated in situ or on a larger scale. A method that permits rapid, in situ and facile formation of strongly hydrophobic surfaces is still needed, and could broaden the scope of applications of such interfaces. Towards this aim, studies have focused on utilizing phase separation to tailor surface topography and thus hydrophobicity. This has been done most commonly with vapor induced phase separation33-38 wherein a polymer solution undergoes phase separation during solvent evaporation. The rate of solvent evaporation and the speed of phase separation can be varied by exposure to vapor flow, which in turn varies the surface morphology. Phase separation during co-incident solvent evaporation and polymerization has also been studied, but to a limited extent.39-41 Here, since phase separation occurs simultaneously with polymerization and evaporation, many more factors can be used to tailor the surface morphology such as: rate of reaction, time-dependent concentration of solvent relative to polymer, as well as the change in compatibility with solvent as monomer converts to polymer. The methods listed thus far get closer at in situ surface fabrication since phase separation occurs directly on a substrate. However, the use of polymer/solvent incompatibility to generate surface roughness requires lengthy fabrication time that can be as great as 20 hours.39,41 Additionally, in some cases the surface roughness generated is not sufficient to obtain strongly hydrophobic properties so 4 ACS Paragon Plus Environment

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secondary functionalization is required.34,41 A truly rapid and facile method has yet to be identified. Instead of relying on a solvent to cause thermodynamic instability and phase separation, recent research has focused on exploiting the increases in overall free energy during polymerization in a solvent-less mixture of monomer and polymer, using an approach called polymerization-induced phase separation (PIPS). Here, the morphology is a result of the competition between thermodynamically driven phase separation and the physical changes associated with monomer conversion such as increases in viscosity, gelation and vitrification that can limit the diffusion of incompatible phases.42-45 The use of PIPS has already been exploited for a variety of applications including liquid crystal-based optical displays,46-50 composites,51 membrane fabrication,52 as well as for dental restorative materials.53 In this study we identify a new, rapid and in situ method that uses photo-initiated PIPS to generate surface roughness of a polymerizing resin, thus creating polymer coatings with unique wettability properties. The use of PIPS permits the rapid formation of multi-scale, rough surfaces in a solvent-less environment. Additionally, with the use of photo-initiation, the polymerization can be readily controlled both spatially and temporally making in situ network formation very facile. With this approach, parahydrophobic surfaces are formed with remarkable ease and efficiency that has yet to be demonstrated previously.

2. EXPERIMENTAL SECTION 2.1 Materials Two monomers were utilized in this study, as received (Sigma-Aldrich): ethylene glycol

di-acrylate

(EGDA)

and

1H,1H,2H,2H-perfluorodecyl

acrylate

(PFDA).

Poly(vinylidene fluoride) (PVDF, Sigma-Aldrich) was added to copolymer matrices to induce phase separation. The Mw of this polymer remained constant throughout the study at ~534,000. A Type I photo-initiator, 2-2-dimethoxy-2-phenylacetophenone (DMPA, Sigma-Aldrich) was 5 ACS Paragon Plus Environment


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added to all resins to induce polymerization upon UV exposure. The loading level of photoinitiator remained constant at 5 wt% in all resins. 2.2 Resin Formulation & Coating Fabrication To make monomer resin formulations, the desired mass of EGDA and PFDA were first placed in a vial and allowed to mix. If the formulations contained no PVDF modification, the desired mass of DMPA was added next and the sample was left to stir until all the photoinitiator had dissolved. For resins with PVDF modification, after mixing EGDA and PFDA monomers, the desired mass of DMPA was added to the solution, and the sample was stirred until complete dissolution of photo-initiator. Then the desired mass of PVDF was introduced into the vial. The EGDA/PFDA/PVDF/DMPA solution was then sonicated for 30 min at room temperature. This was done to uniformly disperse the PVDF powder into the liquid monomer solution. During this process the solution changed from a clear liquid to one that is slightly opaque with a milky appearance. Initial polymer coatings were fabricated on glass slides (20mm x 20mm). The liquid monomer formulation was applied to the slide using a pipette and allowed to spread across the entire surface. If needed, the sample was tilted to cover the entire surface with the liquid prior to curing. The glass slides were then place immediately under a lamp (SOL 500, Honle UV Technology) that emits a broad spectrum of UVA, UVB and visible wavelengths (280– 800nm). Since the photoinitiator used in this study (DMPA) absorbs in the UV region (~280380nm) the incident irradiation intensity (Io) was monitored at λ~360nm. All coatings were cured for 20 min (Io=25 mW cm-2) to ensure uniform initiation rates amongst different samples. After photocuring the surface was rinsed with ethanol to remove any un-reacted monomer and allowed to dry for 5 min before analysis. For coatings with controlled thickness, a spin coater (Laurell, WS-650HZ-8NPP/UD3) was used to evenly distribute the liquid monomer formulation across the glass slide prior to UV-curing. The desired volume of 6 ACS Paragon Plus Environment

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monomer was placed at the center of the glass slide (20mm x 20mm) and spun at 300 RPM for 1 min. This protocol was chosen as it was found that the different volumes of monomer would spread uniformly on a macroscopic scale without any loss of liquid for all volumes tested. 2.3 Thermal Property Analysis Glass transition (Tg) temperatures were measured utilizing differential scanning calorimetry (DSC, Perkin Elmer, Jade DSC). For each experiment, a polymer sample of mass 5-10 mg was placed in a 50 µL thin-walled aluminum pan. The sample was allowed to equilibrate at -25°C and then brought to a temperature of 200°C at a rate of 10°C min-1. After staying at 200°C for 1 min, the sample was then cooled back to -25°C at the same rate as the initial temperature ramp. 2.4 Surface Characterization Surface wettability was measured using a DSA30 goniometer (Krüss). The contact angles with distilled water were measured using the sessile drop method (drop volume: 2 µL, T~ 22 ± 1°C). Dynamic contact angles were measured using the tilted-drop method by surface inclination (drop volume: 6 µL, T~ 22 ± 1°C). With this method, the maximum surface inclination before the water droplet “rolls off” is the sliding angle (α). The contact angles in the back and the front of the water droplet during surface inclination are taken as the receding and advanced contact angles (θadv and θrec) and are used to measure the hysteresis (H = θadv − θrec). Surface morphology was characterized by both scanning electron microscopy (SEM) with a 6700 F microscope (JEOL) and atomic force microscopy (AFM) using a di Innova SPA microscope. For analysis via AFM, all images were collected in tapping mode using silicon probe tips (Veeco, MPP-11123-10). The cantilever had a spring constant of k = 40 N/m and resonance frequency (fo) of 300 kHz, and all images were collected with a scanning rate between 0.1 and 0.4 kHz. Mean arithmetic roughness (Ra) was determined by optical



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profilometry (Wyko NT 1100, Bruker). The measurements were completed with High Mag Phase Shift Interference (PSI), a 50X objective and 0.5X field view

3. RESULTS AND DISCUSSION The basis for all formulations used in this study is a 50:50 (molar ratio) comonomer resin of ethylene glycol diacrylate (EGDA) and 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA). EGDA was chosen as the presence of a di-functional monomer leads to the formation of a cross-linked network with strong mechanical properties and high thermal resistivity. PFDA is chosen as a comonomer as it has a long fluoro-carbon chain present on the monomer backbone. As mentioned, the development of strongly hydrophobic properties depends on both the chemical and physical properties of the surfaces formed. The choice of a low surface energy, fluorinated comonomer can be considered a chemical modification of the surface to increase the hydrophobic behavior. The neat 50:50 EGDA:PFDA comonomer resin is used as a non-phase separating control for this study. Since both monomers are based in acrylate functionality a random copolymer is formed during photopolymerization of this resin. This is verified by DSC thermal analysis, in which a single Tg (128 ± 9°C) is observed, indicative of a uniform bulk network (Figure 1). When a bulk EGDA network is formed under the same irradiation conditions the Tg was found to be 159 ± 11°C (Figure 1). These findings are expected, as the introduction of PFDA as a comonomer will reduce the cross-link density and thus the Tg based on the molar concentration relative to the diacrylate monomer.


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Figure 1. Glass transition temperatures (Tg) of EGDA homopolymer, EGDA:PFDA copolymer, and PVDF-modified EGDA:PFDA copolymer networks after photocuring (Io=25mW cm-2). To induce phase separation a thermoplastic fluoropolymer, polyvinylidene fluoride, (PVDF) was introduced into the EGDA:PFDA matrix at a constant loading of 20 wt%. The use of a linear, thermoplastic additive has been utilized previously to induce phase separation in both linear and cross-linking resins.43,45,53-56 The addition of a linear polymer, such as PVDF, into the monomer formulation will decrease the overall entropy of mixing of the resin formulation (

). This occurs as a more constrained polymer chain with fewer degrees of

freedom (PVDF) decreases the overall concentration of small monomer molecules (PFDA, EGDA), thus decreasing

following the Flory-Huggins theory.57 The decrease in

with PVDF addition corresponds to an increase in the overall free energy of the resin formulation dictated by Gibbs Free Energy of Mixing:

Where:

= free energy,

= enthalpy,

Since phase separation occurs when

=entropy, and T=temperature. , the overall increase in

with PVDF

introduction promotes phase separation.



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PVDF was also chosen as the modifying polymer as it has moderate miscibility with both EGDA and PFDA, which makes uniform distribution of the initial monomer formulation onto a substrate facile. Additionally, the di-fluoride repeat unit on PVDF will decrease the surface energy of the resin formulation rendering the materials more hydrophobic. It should be noted that PVDF is not completely miscible with the comonomers, as indicated by the decrease in opacity of the monomer formulations observed with PVDF introduction. The limited miscibility indicates a higher enthalpy of mixing (

) of the PVDF-modified

resins. This contributes to the promotion of PIPS as it increases the overall free energy (

) further. To verify that phase separation occurs during polymerization, thermal analysis was

conducted on 50:50 EGDA:PFDA resins modified with 20wt% PVDF after photo-curing. The introduction of PVDF into the copolymer resin leads to the presence of two distinct Tg’s detected upon thermal analysis (Figure 1). Compositionally different domains that form as a result of phase separation will undergo thermal transitions at different temperatures based on the concentration of each component (EGDA, PFDA, PVDF) present, so the observation of multiple Tg’s indicates the successful formation of a heterogeneous, phase-separated network. The first Tg observed in the phase-separated resin, denoted as “Transition 1” occurs at 121°C, and the second (Transition 2) is observed at 81°C. It is difficult in a three-component system such as this to ascertain the exact compositions of the two phases formed upon phase separation. However, some broad conclusions can be made. Figure 1 illustrates that the Tg observed for bulk 50:50 EGDA:PFDA and Transition 1 for the PVDF-modified resins are nearly equivalent, and thus it is assumed that the two are very similar in composition. Transition 2 for the phase-separated resin is significantly lower, and is likely a phase that is enriched in PVDF and the 50:50 EGDA:PFDA copolymer. PVDF has a Tg of -38°C (supplied from Aldrich), which when combined with the copolymer resin would depress the overall Tg. If there were preferential segregation of one monomer into the PVDF-enriched domains, it 10 ACS Paragon Plus Environment

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would reflect by a change in concentration of the resulting copolymer phase. However, since the bulk 50:50 EGDA:PFDA network Tg and Transition 1 in the phase-separated network are similar, this is not the case. The observed thermal behaviors of bulk 50:50 EGDA:PFDA copolymer and PVDFmodified resins indicate that PVDF modification is sufficient at 20 wt% loading to induce heterogeneous network formation via PIPS. The phase separation results in the formation of domains that differ in PVDF, EGDA, and PFDA overall concentrations. With one phase rich in 50:50 EGDA:PFDA copolymer and the other diluted with non-reactive PVDF, it is assumed that the reactive monomer concentration is higher in the copolymer-rich phase during polymerization. Since rate of propagation of polymerization has first order dependence with monomer concentration,58 the overall rate of polymerization and local conversion will vary between the two phases formed. To probe the influence of heterogeneous network formation via phase separation on surface topography and wettability, the static water contact angle (WCA) and surface roughness (Ra) were measured using goniometry and optical profilometry after photo-curing. All monomer resins were applied to glass substrates and cured (Io=25 mW cm-2, 20 min). The mean arithmetic surface roughness (Ra) and water contact angle (θW) vary significantly between the non-phase separated copolymer (50:50 EGDA:PFDA) and phase-separated (50:50 EGDA:PFDA, 20wt% PVDF) sample (Table 1). The surface roughness increases by three orders of magnitude with phase separation, such that the surface roughness changes from nano-scale to micron-scale in size. Table 1. Mean Arithmetic Surface Roughness (n=3) and Water Contact Angle (n=5) Sample

Ra (nm)

θW

50:50 EGDA:PFDA

3.1 x 101 (± 9 x 100)

114° (± 2.4)

50:50 EGDA:PFDA, 20wt% PVDF

3.1 x 103 (± 5.9 x 102)

150° (± 4.0)



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The significant increase in surface roughness in the heterogeneous polymer is attributed to non-uniform development of polymerization shrinkage during phase separation. Polymerization shrinkage and its corresponding stress are direct consequences of the reduction in free volume as monomer converts to polymer. Since shrinkage is driven by monomer conversion, materials with a higher density of double bond functionalities will experience a higher magnitude of polymerization shrinkage, given an equivalent fractional conversion of double bonds. This phenomena is well-characterized and studied especially in the formation of acrylate and methacrylate based networks.59-61 The development of polymerization shrinkage during PIPS has been studied with growing frequency and interfacial stress reduction is observed with phase-separating polymerizations.43,62 The mechanism for this behavior relies on internal volume rearrangement, especially along phase interfaces, as domains of different elasticity develop at non-equivalent rates. With the polymerization studied here, the non-uniform development of conversion and modulus in the two different phases formed, based on their differences in copolymer concentration, causes each domain to undergo polymerization shrinkage at different rates and overall magnitude. Specifically, the domain that is rich in EGDA:PFDA copolymer, and has a higher monomer concentration will experience a higher degree of polymerization shrinkage than the PVDFenriched phase which has a lower double bond concentration. In coating development, shrinkage occurs freely along the interface between the polymerizing resin and the atmosphere, causing the polymer to contract towards the substrate to which it is applied. It has been demonstrated that depending on the difference in shrinkage rate between phases and overall domain sizes formed, significant roughness and surface patterns can form during phase separation of thin films.63,64 Looking at how the phase-separation induced roughness influences wettability, it is first necessary to analyze the wetting behavior of the non-phase-separated copolymer. The contact angle of 50:50 EGDA:PFDA copolymer is found to be θW ~ 114° (Figure 2A), 12 ACS Paragon Plus Environment

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indicating slight hydrophobicity. This is expected based on the monomers chosen since PFDA has a long fluorocarbon chain that will decrease the surface energy of the copolymer formed. When phase-separation occurs in the 50:50 EGDA:PFDA, 20wt% PVDF resin, θW increases dramatically to 150°, indicative of strongly hydrophobic properties (Figure 2B). To ensure that this result is due to the phase-separation induced surface roughness, and not merely the addition of PVDF into the resin, a second control was tested in which the phase-separated resin is formed by photo-curing between two glass slides, forcing the formation of smooth surfaces. When this is done, θW of 50:50 EGDA:PFDA, 20wt% PVDF is only 99° ± 4 (Figure 2C), similar to that of the non-phase-separated control. This indicates that the surface roughening, due to phase separation is a significant driving force to making strongly hydrophobic interfaces.

Figure 2. Static water contact angles (θW) of copolymer coatings applied to glass substrates: (A) 50:50 EGDA:PFDA, (B) 50:50 EGDA:PFDA, 20wt% PVDF coating fabricated on a glass substrate, (C) 50:50 EGDA:PFDA, 20wt% PVDF cured between glass slides. Beyond the high static water contact angle (θW), the phase-separated coating exhibits sticky behavior and excellent water adhesion, indicating overall parahydrophobicity, which is characterized by high θW and high sliding contact angles ( ). A water droplet will remain on the surface when inclined to 90° (Figure 3A) or 180° (Figure 3B). Since the water adhesion is so high, there is no measurable sliding angle ( ). The advancing and receding contact angles were measured when =90° and as expected, there is a significant difference between the two resulting in a large hysteresis value (Table 2). Materials with very low water adhesion that are described as “self-cleaning” typically have hysteresis values (H) less than 5°. The materials 13 ACS Paragon Plus Environment


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presented here have hysteresis values approximately one order of magnitude greater than those with self-cleaning properties, which further validates the strong adhesive behavior observed.

Figure 3. Dynamic water contact angle behavior (6 µL) of 50:50 EGDA:PFDA, 20wt% PVDF copolymer coatings applied to a glass substrates and tilted to (A) 90° or (B) 180°.

Table 2. Dynamic Contact Angle Behavior of 50:50 EGDA:PFDA, 20 wt% PVDF phaseseparated networks (n=3). θadv (α = 90°) θrec (α = 90°) H 148° ± 3.2

101° ± 9.5

47 ± 7.2

The observed behavior can be explained with the Wenzel state of wetting, in which water is in full contact with the surface formed.23 The water contact angle of the smooth phase-separated surface, or in other words the Young’s Angle (θY) for this resin is greater than 90° (Figure 2C), indicating that the phase-separated polymer is intrinsically hydrophobic. As previously described, if the probe liquid on the polymer coating follows the Wenzel state of wetting, then increases in the roughness parameter (r) would correspond to amplification of the contact angle following the equation:

.65 When this resin is applied for

coating formation (Figure 2B) the contact angle is significantly increased (θW ~150°), and rough surface morphology is detected. This behavior is attributed to phase separation, as a single-phase homogeneous copolymer coating (50:50 EGDA:PFDA) yields an average roughness two orders of magnitude smaller (Table 1). The rough, phase-separated coatings also have strong “sticky”, adhesive behavior (Figure 3) and high hysteresis (Table 3). This further confirms that the wetting regime on the phase-separated coatings most closely 14 ACS Paragon Plus Environment

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resembles the Wenzel state, as increases in roughness correspond to both an increased contact angle and adhesive behavior. With both high adhesion and high contact angles, these coatings can be classified as parahydrophobic as defined by Marmur.12 The morphology of the phase-separated coatings was probed using scanning electron microscopy (Figures 4A & 4B) as well as atomic force microscopy (Figure 4C). The morphology is made of regular spherical domains that are sub-micron in size dispersed in a continuous matrix, which has slight variations in height and pitch, as observed in the SEM images. The height profile images obtained by AFM present the same morphology (Figure 4C) and a single line analysis (Supplementary Figure 1) shows that the spherical domains are roughly 0.2 – 0.5 µm in diameter, and 0.05 - 0.20 µm in height. The surface morphology observed by SEM and AFM is qualitatively similar to that observed on parahydrophobic rose petals (Figure 4D)13 since both surfaces have spherical, “nub”-like domains. On the naturally occurring surface (Figure 4D), the spherical domains are as large as 10 µm in diameter, and also have submicron secondary structures which are characterized as nanofolds.13 The globular surface structures on the coatings developed in this study are much smaller in diameter compared to the natural rose petals (0.5 µm vs. 10 µm, respectively). The surface morphology on the 50:50 EGDA:PFDA 20wt% PVDF coating is formed as a result of phase separation during a cross-linking polymerization, and therefore the domain size is dependent on the kinetics of reaction and the onset of network gelation which can limit phase development.42,44,45,54 On the fabricated coatings, the phase morphology resembles the structure associated with the Nucleation and Growth mechanism of phase separation.54 Following this mechanism, a dispersed phase will coalesce and grow in size until reaching thermodynamic equilibrium, assuming no external factors prohibit diffusion. Since the bulk EGDA:PFDA matrix has a significant degree of cross-linking, as indicated by the high Tg (Figure 1), network gelation limits the time available and the degree to which the 15 ACS Paragon Plus Environment


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spherical domains can coalesce and grow in size during polymerization. This is why the 50:50 EGDA:PFDA, 20wt% PVDF resins yield significantly smaller surface morphologies after polymerization when compared to the natural rose petals. Despite the difference in the size of surface structures between the rose petal and 50:50 EGDA:PFDA, 20wt% PVDF coatings, both still achieve similar wetting properties. Strong water adhesion afforded by high solid/liquid contact area has been demonstrated on various types of surface structures studied previously, such as micron sized bumps, nanopillars, etc.15,24 Since the structures on 50:50 EGDA:PFDA, 20wt% PVDF coatings are submicron in size the water/polymer contact area is significantly large even without any secondary surface structuration, such as that observed on rose petals, and is sufficient for high water adhesion and sticky, parahydrophobic behavior. As mentioned, the surface structure on 50:50 EGDA:PFDA, 20wt% PVDF coatings is reminiscent of the typical phase structure from systems that undergo phase separation via the Nucleation and Growth mechanism.66 Based on the high concentration of reactive functionalities in the comonomer-enriched phase, as well as it’s propensity to shrink more dramatically, it is assumed that the continuous matrix is rich in EGDA:PFDA copolymer. Therefore, the spherical domains that form the majority of the surface structure and roughness are composed of the PVDF-enriched phase, which does not contract and shrink as dramatically during polymerization due to a lower concentration of monomer units. Since the surface wetting follows the Wenzel regime, where all microstructures are uniformly wetted, both the copolymer- and PVDF-enriched phases are in full contact with water during wetting. Therefore, the parahydrophobic behavior does not depend on which phase is manifested as the bulk continuous network and which is the dispersed domain in the final phase-separated network. The parahydrophobic behavior is most significantly influenced by the increase in solid/liquid contact area permitted by the formation of rough surface structure.


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Figure 4. Microscopic images of phase-separated surface (50:50 EGDA:PFDA, 20 wt% PVDF). SEM images at 10,000X (A) and 25,000X (B) magnification, AFM height profile image (C) (2.41 µm x 2.41 µm). SEM Image of a natural rose petal surface (D) for comparison taken in ref. 13.

To probe the versatility of this material system, the coating thickness was varied to see if the surface morphology, wettability and adhesive properties changed with this control parameter. Coatings of varying thickness were made on glass slides by depositing increasing volumes of the initial monomer formulation and spin coating to spread the liquid resin, followed by UV-curing. The resulting thicknesses, static contact angles with water (θW), and dynamic contact angle behavior (θadv , θrec , H) for these coatings are summarized in Table 3. It should be noted that the curing conditions and cleaning procedure of the coatings after fabrication is done in the same manner for these experiments as described in the experimental



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section, and that the measurements presented for Sample #2 are the results from the simple pipette cast-coating that have been reported thus far. Table 3. Wetting behavior of 50:50 EGDA:PFDA, 20% PVDF coatings as a function of bulk thickness (n=3 for all measurements). *Sample 2 summarizes the results presented thus far for the rough, pipette casting method. θrec (α = 90°)

H

147° (± 6.7)

104° (± 5.0)

43 (± 12)

150° (± 4.0)

148° (± 3.2)

101° (± 9.5)

47 (± 7.2)

78 (± 3)

150° (± 7.0)

150° (± 1.2)

96° (± 1.5)

53 (± 0.6)

4

100 (± 0.5)

147° (± 3.4)

151° (± 5.0)

94° (± 2.5)

57 (± 4.0)

5

133 (± 15)

143° (± 0.4)

145° (± 3.5)

100° (± 7.2)

45 (± 11)

Sample

Thickness (µm)

θW

1

28 (± 3)

145° (± 1.0)

2*

47 (± 5)

3

θadv (α = 90°)

From these results, it can be seen that the coating thickness does not greatly impact the wetting behavior. All materials displayed strong adhesion with water, as no drop roll-off was observed up to inclination angles of 90°. As the coatings become sufficiently thin or thick (Samples 1 and 5, respectively) the static contact angle and hysteresis behaviors begin to decrease slightly, but the difference is not significant as calculated with a paired t-test with a 95% confidence interval. The morphologies of the thinnest and thickest coatings were investigated using AFM, and found to be similar to the initial results (Supplemental Figure 2). Here, the spherical domains have diameters on the scale of 0.2 - 0.4 µm, and the local variation in height between spherical domains is 0.1-0.2 µm. Some larger height variations are observed (Supplementary Figure 2B & 2D), but this is attributed to macroscopic differences in height and pitch, as observed in Figure 4A, and does not actually represent the height of the spherical surface structures. It should be noted that the low viscosity of the monomer solution makes it much more difficult to form macroscopically uniform coatings of a constant thickness that is greater than what is reported here, as the resin flows easily off the substrate. For formation of thicker coatings, a resin with higher viscosity must be developed. To probe the durability of the surfaces fabricated in this study, a thermal stability test was conducted. Coatings 1-5 of varying thicknesses (Table 3) were placed in an oven at 18 ACS Paragon Plus Environment

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115°C for 1.5 hours. This temperature was chosen as it is just slightly below that of the Tg of the bulk EGDA:PFDA phase (Transition 1, Figure 1 - 121°C), but above that of the PVDFenriched phase (Transition 2, Figure 1 - 81°C). After the thermal annealing the dynamic and static contact angles of all materials were probed again, and the results are summarized in Table 4. In general, the materials were found to have a high level of thermal stability. After thermal annealing at 115°C, all samples still display very strong adhesion, as no droplet rolloff was observed up to an inclination of 90°, indicating strong thermal stability of the materials. There are, however, slight decreases in the static contact angles with water (θW), which are all just slightly below 150°. For Samples 3-5 the hysteresis values decrease slightly. These decreases are most likely due to structural changes in the PVDF-enriched domains, as the bulk material was held above the Tg associated with this phase. However, since the PVDFrich domains are dispersed within a EGDA:PFDA network, the decrease in contact angle is not very dramatic, as the densely cross-linked EGDA:PFDA copolymer phase that surrounds the spherical structures maintains its rigidity at elevated temperatures, so no significant morphological changes can occur. If the surfaces were held above both Tg’s observed for the phase separated network (i.e. above 121°C), there would be a much more significant change in wettability as the surface morphology would change significantly. Table 4. Wetting behavior of 50:50 EGDA:PFDA, 20% PVDF coatings as a function of bulk thickness (n=3 for all measurements) after thermal annealing (T=115°C) for 1.5 hours. Sample

Thickness (µm)

θW

1

28 (± 3)

142° (± 5.0)

2

47 (± 5)

3

θadv (α = 90°)

θrec (α = 90°)

H

146° (± 7.7)

104° (± 15)

42 (± 7.1)

142° (± 2.6)

146° (± 1.7)

96° (± 3.4)

50 (± 3.0)

78 (± 3)

142° (± 4.4)

147° (± 2.1)

104° (± 0.5)

43 (± 2.6)

4

100 (± 0.5)

144° (± 6.0)

146° (± 5.6)

100° (± 1.2)

46 (± 6.6)

5

133 (± 15)

142° (± 1.0)

142° (± 1.0)

104° (± 4.6)

38 (± 4.0)



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4. CONCLUSION In summary, this work has presented an in situ, low temperature, rapid approach to develop polymer coatings with unique parahydrophobic properties using photo-initiated polymerization induced phase separation (PIPS). A hydrophobic comonomer resin composed of EGDA and PFDA was modified with a thermoplastic additive (PVDF) to increase the formulation entropy and induce phase separation during polymerization. The EGDA:PFDA pure copolymer has only one glass transition temperature (Tg) indicating homogeneous network structure. With PVDF addition, a second Tg is detected, indicating the formation of a heterogeneous, phase-separated network with one phase enriched in PVDF and another enriched in EGDA:PFDA copolymer. In PVDF-modified resins, PIPS leads to significant surface roughness due to non-uniform shrinkage development in the two phases formed during polymerization, resulting in a surface that has rough spherical domains dispersed in a continuous matrix. As observed on rose petals, this surface topography contributes to both the high water contact angle (θW) and sticky behavior, making it parahydrophobic. Phaseseparated coatings of varying thicknesses (28-133 µm) were fabricated and all displayed strong water adhesion and high contact angles, and the surface morphology did not vary with this control parameter. To probe the thermal stability of the surfaces, annealing at 115°C was conducted. The strong adhesive behavior remained after annealing; indicating that the majority of the sub-micron surface morphology is thermally stable. The method presented here is a significant improvement over previous approaches to design rough surfaces with unique wettability, as it can be used in a variety of environments and applied under very mild processing conditions, which will broaden the scope of applications for strongly hydrophobic polymer coatings but with sticky behavior, such as in the field of biosensors or antibioadhesion or in applications where loss-less liquid transport is needed.


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ASSOCIATED CONTENT Supporting Information AFM height profile images and single line height analysis of 50:50 EGDA:PFDA, 20wt% PVDF surfaces of 27, 48 and 133 µm thicknesses

ACKNOWLEDGEMENTS We thank J.P. Laugier and S. Pagnotta from the CCMA (Centre Commun de Microscopie Appliquée) of the Université de Nice Sophia Antipolis for the SEM imaging.



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