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Aligned Droplet Patterns by Dewetting of Polymer Bilayers Ming Chiu, Jared A. Wood, Asaph Widmer-Cooper, and Chiara Neto* School of Chemistry and The University of Sydney Nano Institute, The University of Sydney, Sydney, NSW 2006, Australia

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S Supporting Information *

ABSTRACT: Dewetting of polymer thin films has been extensively studied in the past two decades as an approach to produce patterned surfaces. Here we report a novel phenomenon where dewetted polymer droplets spontaneously align on a substrate, and the alignment extends over a large scale (millimeters). The patterns are formed by dewetting a bilayer system formed by poly(4-vinylpyridine) (P4VP) thin film (80 nm) on a polystyrene (PS) thin film (100 nm) prepared by spin-coating. We investigated the mechanism of the spontaneous droplet alignment and concluded that the final aligned pattern results from series of aligned defects on the P4VP, which are formed during spin-coating. We identified experimental parameters that control the appearance and the order of the resulting aligned droplets.

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INTRODUCTION Dewetting of polymer thin films has been extensively studied in the past two decades as an approach to produce patterned surfaces which may find various applications.1−3 For example, the Neto group has demonstrated the use of dewetting of polymer bilayers for patterning cells and proteins onto biological surfaces as well as producing patterned surfaces for atmospheric water capture.4−9 Polymer film dewetting occurs upon annealing of a metastable thin polymer film above its glass transition temperature on a nonwettable substrate.10 Dewetting takes place due to unfavorable intermolecular interactions at the film/substrate interface.11−13 This process entails a series of sequential stages: (i) random holes appear in the film by heterogeneous nucleation; (ii) the holes gradually grow with time, and the displaced polymer accumulates on rims around the holes; (iii) neighboring holes coalesce with each other, and their polymer rims are reduced to sections of a cylinder in cross section; (iv) the cylinder sections spontaneously break down to droplets driven by the Plateau−Rayleigh instability.10,12,14−16 This spontaneous and random process results in a characteristic Voronoi tessellation pattern with some degree of control possible on pattern feature size by controlling the thickness of polymer film, polymer molecular weight, and annealing method.15−17 However, a feature of this instability process is that the produced structures are inherently random; hence, the possible scope of application is limited. In order to overcome this constraint, attempts have been made to achieve spatial ordering by inducing the nucleation sites e.g. physically imprinting holes with the aid of lasers, stamps, electron beams, or prepatterned substrates.1,2 Methods used include tailoring the substrate with physical or chemical patterns,18−25 modifying the topography of the film surface,26,27 applying spatial confinement,28 and triggering instability of target thin film by controlled perturbation.29−31 The main principle © XXXX American Chemical Society

behind most of these approaches is to guide the nucleation of dewetting from controlled positions that have a spatial order. As the growth of dewetting holes on a polymer film is mostly isotropic, the final dewetted pattern will conserve the order of the initial nucleation spots.15 In this paper, we report a new phenomenon that leads to an aligned pattern of polymer droplets from a spin-coated poly(4vinylpyridine) (P4VP) film on top of a polystyrene (PS) film, a bilayer system that has been extensively studied in our group.7,9 The spreading coefficient S is negative for P4VP on PS, S = γPS − γPVP − γPS/PVP = γPVP(cos θ − 1) ≈ − 30.9 mN m−1, where γPS, γPVP, and γPS/PVP are the surface tension of PS, P4VP, and the interfacial tension of PS/P4VP, respectively. Therefore, P4VP films are expected to dewet from PS films. In this work, during spin-coating, if the P4VP−ethanol solution is dispensed on a stationary PS substrate and then the spinning motion is started, aligned pinholes spontaneously form in the P4VP film (Figure 1). The holes in the P4VP film are aligned radially, along the direction of centrifugal force experienced by the flowing polymer solution during spin-coating process. Upon either thermal or vapor annealing, these pinholes grow in size until the film completely dewets.16,27,32,33 When the dewetting process is complete, an ordered pattern of isolated polymer droplets, which extends over a large scale (millimeters), is obtained (Figure 2a). To our knowledge, this is the first time that such long-range two-dimensional alignment has been reported. When compared to other published approaches for ordered dewetting, the main feature of this spontaneous patterning method is that external intervention is not required to achieve the spatial order. Received: March 23, 2018 Revised: June 18, 2018

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Figure 1. Schematic representation of spin-coating a P4VP film onto a PS film and the resulting surface features, depending on the dispense method. The static dispense method leads to aligned pinholes that upon dewetting of the P4VP film result in an ordered pattern of isolated P4VP droplets. Prior to spin-coating the silicon wafers were thoroughly cleaned by sonication in ethanol, acetone, blown dried with high purity N2, and then exposed to a CO2 snow jet gun to remove particulate contaminants (Applied Surface Technologies, New Providence, NJ), followed by 1 min air plasma treatment (Harrick Plasma, Ithaca, NY, model PDC-002). PS solutions were filtered on PTFE filters with a pore size of 0.22 μm, and P4VP solutions were filtered on regenerated cellulose filters with a pore size of 0.2 μm. Instrumentation. Static contact angles were measured by a goniometer (KSV CAM 200). Spectroscopic ellipsometry (J.A. Woollam Co. Inc., M2000V) was used to establish film thickness, with measurements taken over three points across the surface on each sample. Dewetting and Imaging. Vapor annealing was conducted by placing the bilayer samples inside a custom-made Teflon cell with saturated vapor equilibrated from 30:70 % w/w ethanol and water mixtures (giving 1:1 vapor molar ratio) for 24 h unless otherwise specified, as this annealing method has been investigated by us previously.16 Thermal annealing was also conducted by heating bilayer samples directly on a temperature-controlled hot plate (ATV TR-124, Munich, Germany) at 180 °C, above Tg of both PS and P4VP. Dewetting was observed by optical microscopy (Nikon Eclipse LV150) and tapping mode atomic force microscopy (AFM, Bruker, Multimode 8). Image Analysis. Optical micrographs showing the dewetted patterns were analyzed using a set of custom scripts. The diameter and coordinates of each droplet were identified along with the direction of alignment for each image. The direction of alignment was determined by a linear least-squares fit to a single line of droplet centers, chosen manually from near the center of the image. Droplet size histograms were compiled. The order in the images were analyzed using a size dependent two-dimensional pair distribution function. A threshold for droplet size similarity and a minimum size cutoff were arbitrarily set in order to filter out random droplets from those which contributed to the aligned pattern Further details are provided in the Supporting Information.

The solvent in which the P4VP solution is dissolved dramatically changes the order in the final dewetted pattern. Spin-coating experimental conditions, including the position at which the P4VP solution is dispensed, and the surface area occupied on the substrate by the dispensed P4VP droplet were explored and also found to affect the resulting pattern significantly.

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METHODS

Bilayer Preparation. For most of the study the polymer bilayer system (P4VP/PS) was prepared by sequentially spin-coating polystyrene (PS350k, Mw = 350 kg mol−1, PDI 2.06, Sigma-Aldrich) followed by poly(4-vinylpyridine) (P4VP60k, Mw = 60 kg mol−1, Sigma-Aldrich) at room temperature in a high purity N2 environment. Other polymers used were PS93k (Mn = 93 kg mol−1, PDI 1.05, Polymer Source Inc.), P4VP19k (Mn = 19 kg mol−1, PDI 1.16, Polymer Source Inc.), and PMMA106k (Mw = 106 kg mol−1, PDI 1.05, Polymer Standard Service) which were also used for comparison studies. Samples were prepared in a laminar flow cabinet, and typical room conditions were 30% RH and 24 °C (see details in the Supporting Information). The PS films were spin-coated (Laurell Technologies, WS400B-6NPP-LITE, uses an internal N2 flow) at spin rate of 2000 rpm from 15.3 mg mL−1 solution in anhydrous toluene (>99.9%, Sigma-Aldrich) on square silicon wafers (approximately 12 × 12 mm2), with a 2 nm native oxide layer (MMRC Pty Ltd., Malvern VIC Australia). The P4VP films were then spin-coated from 16 to 23 mg mL−1 solution in ethanol at 2000−8000 rpm for 30 s to give a film thickness between 75 and 90 nm. The P4VP solution was dispensed on the PS substrate either before commencement of spinning motion (static dispense, Figure 1) or after the substrate fully accelerated to the target spin rate (dynamic dispense). Alternative solvents for P4VP (methanol, isopropanol, and n-butanol) were also used. “Ethanol treatment” consisted of spin-coating a droplet of pure ethanol on the PS film. Table 1 lists all the experimental parameters and conditions studied for spin-coating P4VP. The default parameters (bold text) were chosen as they provided the most pronounced patterns. B

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Figure 2. (a) Aligned P4VP60k dewetted droplets over PS350k film on Si wafer substrate. (b) Photograph of overall dewetted P4VP/PS bilayer system after dispensing the P4VP on a stationary PS substrate and (c) after dispensing on an already spinning PS substrate. The orange line indicates the position of the ring pattern. The labels I−IV refer to the successive images. Substrate dimensions are roughly 12 mm × 12 mm. (d−g) Optical micrographs of P4VP/PS bilayer taken before and after annealing with 1:1 molar ethanol/water vapor: (d) area I, central area of the substrate, not ordered; (e) area II, aligned pinholes area; (f) area III, characteristic ring pattern, separating the aligned area from the random large droplets area; (g) area IV, P4VP surface after dynamic dispensing. Red arrows in (a), (e), and (f) indicate the outward radial direction of spinning. Scale bar applies to all optical micrographs. Insets: AFM micrographs of same sample area, scale bars = 20 μm, height scales = 40 nm (before) and 2000 nm (after).

Table 1. Experimental Parameters and Conditions Studied for Spin-Coating P4VPa solvent used for P4VP

method of dispensing solution

P4VP solution vol (μL)

P4VP concnb (mg mL−1)

spin rateb (rpm)

acceleration (rpm s−1)

ethanol treatment

ethanol methanol isopropanol n-butanol

static dynamic

15 30 50 100

16 20 23

2000 4000 8000

4000 6000 10000 25500

yes no

a

The values given in bold are the default parameters unless otherwise specified. The conditions listed in italics did not produce an ordered droplet pattern. bConcentration and spin rate were changed simultaneously in studies in order to achieve P4VP film thickness around 80 nm.

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RESULTS AND DISCUSSION Formation of Dewetted Patterns. Spontaneously aligned droplet patterns (Figure 2a) were produced after vapor annealing a P4VP/PS bilayer system prepared using default experimental settings (bold in Table 1). Dewetted P4VP droplets were aligned along the radial direction, starting from the center of the substrate and the alignment extended up to 2 mm in length (Figure 2b). Over the entire surface, the pattern showed a few key features, numbered I−III in Figure 2b, starting from the center toward the edge: area (I) sparsely scattered larger droplets in a Voronoi pattern, (II) aligned and densely packed smaller droplets, (III) again larger and sparser droplets in a Voronoi pattern; densely distributed random small droplets were found close to edges and corners. The transition between areas II and III is well-defined by a narrow continuous ring consisting of tiny droplets (highlighted by the orange line), whereas other transitions were often found to be quite diffuse. All of these patterned features were due to local P4VP60k film characteristics before dewetting. The central region was found to be featureless before dewetting (Figure 2d). Dewetting of P4VP film was initiated by random heterogeneous nucleation, hence resulting in a characteristic random Voronoi pattern.15 In area II, arrays of aligned preformed pinholes were found on P4VP film directly after spinning (Figure 2e). These pinholes were produced on the P4VP film during spin-coating. Their depth was 80 ± 5 nm, which matched the measured thickness of P4VP film, and the rims of holes were between 10 and 25 nm high (Figure 3a), depending on the hole diameter. When exposed to solvent vapor, the P4VP film dewetted from these pinholes as expected: the pinholes grew in size and the displaced polymer accumulated in rims (Figure 3b,g); as holes coalesced with their neighbors within the same line, their rims merged (indicated by yellow arrows) and then broke down into small droplets with diameters of a few micrometers or less (Figure 3c,h); pinholes continued to grow and eventually coalesced with holes in adjacent lines, and the rims merged into large ridges (Figure 3d,i, indicated by green arrows); the large ridges broke down into larger droplets (Figure 3e,j). As the pinholes were initially aligned and grew isotropically, the large ridges of polymer were also relatively linear, leading to aligned droplets. Random nucleation of holes also occurred occasionally between the lines of pinholes (Figure 3f,g), which contributed some irregularly to the pattern. Because of high density of pinholes, droplet size in the aligned region were significantly smaller on average than in the random region, and the time required for reaching final dewetting state was an order of magnitude shorter (2 h) compared to the rest of the surface (24 h). Moving further out from the center of the substrate, the aligned holes area was contained inside a ring of holes (Figure. 2f). After dewetting the ring region could still be seen as a circular band of tiny droplets, and this defined the transition between the aligned pattern and the Voronoi pattern. The outermost areas near the edges or the corners of the substrate are affected by irregular thickness34 and were excluded from analysis of surface patterns. Several experiments were conducted to demonstrate that the formation of aligned pinholes is intrinsic to spinning of a bilayer system. The observed aligned pattern did not depend on the particular spin-coater used (a different instrument was also tested, Specialty Coating System Inc., G3P-8) nor on the

Figure 3. (a−e) Representative AFM micrographs of aligned defects on P4VP60k/PS350k bilayer annealed at room temperature with 30:70 % w/w ethanol/water mixture taken at different stages of dewetting in sequential order. Scale bars = 20 μm; height scales = 100, 300, 550, 1000, and 2000 nm, respectively. Timestamps indicate exposure time to vapor annealing. Inset: cross-sectional topography profile across defects indicated by blue line. (f−j) Representative optical micrographs for each dewetting stage shown in (a−e), respectively. Scale bars = 100 μm.

particular grade or polydispersity of the P4VP. Beautifully aligned dewetted patterns were reproduced with both of these changes (Figure S3a−c). Aligned pinholes formed on P4VP film with a different polymer substrate (PMMA), when P4VP was cast from methanol (but not from ethanol, Figure S3d,e). Thermal annealing of P4VP/PS bilayers led to similar droplet D

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Figure 4. (a, b) Particle size distribution of (a) aligned pattern and (b) random pattern; the size distribution of random pattern has a long tail extending up to 100 μm. Dash-dot line: the curve fit of the distribution by a sum of exponential decay function and a Gaussian function; Dashed line: the fitted Gaussian function. Dotted line: the fitted exponential decay function. Insets: binary images of corresponding optical micrographs and definition of parallel direction (d∥) and perpendicular direction (d⊥). (c, e) Size-dependent oriented pair distribution function (eq S1) plotted in the parallel direction and perpendicular direction, summed over all counted droplet pairs for (c) an aligned pattern, color scale 0−0.05%+, and (e) a random pattern, color scale 0−0.06%+. (d, f) contains the same data as (c, e), summed over the parallel direction (eq S3). (g) Size-dependent oriented pair distribution, summed over all d∥ < 200 μm to reduce the effect of row alignment deviations (eq S5). Color scale 0−0.54%+. (h) Sizedependent oriented pair distribution summed over all droplets in the same line (eq S4). Color scale 0−0.61%+. Insets: filtered PDF of droplets with a diameter between 11 and 12 μm (green solid line) and between 16 and 17 μm (purple dashed line)

patterns, confirming that the annealing method does not lead to the ordered patterns (Figure S3f), but rather the formation of the aligned pinholes does. The first element that is key in the formation of aligned pinholes is the dispensing method: when the P4VP solution

was dispensed on an already spinning substrate (dynamic dispense, as described in the Methods section) (Figure 2c), the resulting pattern was uniform across the entire substrate. The as-prepared P4VP film was mostly free of defects (Figure 2g); therefore, it formed a pattern very similar to Figure 2d. In the E

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lies between 12 and 22 μm, indicating a high degree of order. This size range overlaps well with the middle of the Gaussian distribution that we earlier postulated as responsible for the aligned pattern. The inset in Figure 4g compares slices taken at 11 μm (transition between exponential and Gaussian distributions) and 16 μm (mean of Gaussian distribution). This comparison shows that the periodicity is considerably stronger for the droplets of diameter 16 μm than for those of 11 μm. These results support our earlier claim that the periodicity of the aligned pattern mainly arises from droplets with diameter around 16 μm. In contrast, along d∥ no obvious long-range periodicity can be observed even for droplets with diameter = 16 μm (Figure 4h and inset). Only one strong peak can be seen around d∥ = 20−60 μm, representing the distance between a droplet and its first neighbor. This peak shows a significant shift in separation distance with droplet size. This and the variation in size between neighboring droplets appears to be responsible for the lack of long-range order along d∥. For a perfect pattern created by Rayleigh instability, the spacing and droplet size should be regular (determined by the dominant wavelength of the instability11) and the order should be long-ranged. Instead, we find that there are sufficiently many randomly nucleated holes in our films to disrupt this wavelength. Experimental Variables and Their Effect on Patterns. The second element that is key to the formation of aligned pinholes is the solvent used to dissolve P4VP. Methanol, isopropanol, and n-butanol were selected as alternative solvents for P4VP, as they are good solvents for P4VP but nonsolvent for PS. Relevant information for each studied solvent is in Table S1. None of these solvents led to observable aligned defect formation like in the case of ethanol. Methanol solutions of P4VP resulted in a high number of defects resembling fragments of concentric ring features without aligned defects. Both isopropanol and n-butanol solutions produced mostly defect-free P4VP films (Figure S4a−c) and random dewetted patterns (Figure S4d−f). The third element that is key to the formation of aligned pinholes is the wettability of the PS substrate. Treating the PS film with ethanol prior to spin-coating the P4VP solution (i.e., spinning pure ethanol on the PS substrate or sonicating the PS substrate in pure ethanol followed by N2 drying) prevented any defect in the P4VP/PS bilayer (Figure 5a). Ethanol treatment temporarily increases the surface wettability of the PS substrate, facilitating the spreading of the P4VP solution. Interestingly, the effect of “ethanol treatment” was fully reversible: when the ethanol-treated PS substrate was dried

following sections, the Voronoi pattern resulting from random heterogeneous nucleation will be referred to as the “random pattern”. The ordered droplets resulting from aligned holes will be referred to as the “aligned pattern”. Quantitative Characterization of Patterns. Optical micrographs of the resulting patterns from both methods of dispensing were quantitatively analyzed for size and spatial distribution. The droplet size distribution in both the aligned pattern (Figure 4a) and the random pattern (Figure 4b) has a maximum around 2−3 μm, but the resolution of the micrographs (about 1 μm per pixel) limited the ability to resolve smaller droplets. The main difference between the two distributions is that the aligned pattern shows a significant second peak at around 16−17 μm, whereas the random pattern does not. This may indicate that the droplets with diameter around 16−17 μm are those which contribute to the aligned pattern, while the droplets below 11 μm, formed in earlier stages of dewetting (Figure 3c, marked by the yellow arrow) do not contribute to the alignment. This intuitive observation was confirmed by fitting the droplet size distribution to the sum of two independent functions: a Gaussian function fitted on the second peak and an exponential decay function approximately describing the rest of the distribution (as this is the simplest model giving a reasonable fit to the data). For the aligned patterns, the Gaussian portion of the fit had a mean diameter of 15.4 ± 0.3 μm and a standard deviation of 5.2 ± 0.5 μm for all studied micrographs. The transition between the exponential and Gaussian regions occurs at 11 μm (change of color in Figure 4a), which is about one standard deviation from the mean droplet diameter. This value was used in Figures 4c−f as the minimum droplet size, and only larger droplets were analyzed in these figures. We quantified the probability of finding pairs of droplets at different relative positions to one another using 2-D probability distribution functions (PDFs), with the axes oriented parallel (d∥) and perpendicular (d⊥) to the direction of droplet alignment, as illustrated in Figure 4a. Details of the specific functions used are provided in the Supporting Information. Figures 4c and 4e show the 2-D PDFs for the aligned pattern and the random pattern, respectively, quantifying the degree of spatial order in each pattern. The scale of each plot is normalized to sum to 1. Figure 4d shows the data from Figure 4c, with counts summed over the parallel direction. The aligned pattern shows a first peak around 75 ± 3 μm along d⊥, followed by regular peaks 75 ± 4 μm apart. At higher d⊥ the peaks are wider and less intense, indicating the gradual loss of ordering. The peak near 0 μm is due to droplets located at slightly different values of d⊥ along the same line. The corresponding plot for d∥ does not show obvious periodicity (Figure S1a). These results demonstrate that the aligned pattern has strong periodicity in the direction perpendicular to the alignment, whereas the random pattern does not show any regular features. On a scale of 0−1 where 0 denotes a completely random pattern and 1 a perfectly ordered pattern, the best aligned pattern scored 0.64 based on the distance with first neighboring peak, whereas the random pattern scored less than 0.05. Details on how this order parameter was calculated are provided in the Supporting Information. Figures 4g and 4h show the distributions plotted in Figure 4d and Figure S1a broken down by droplet size, where we have now included droplets smaller than 11 μm. Along d⊥ (Figure 4g) the peaks are very pronounced when the droplet diameter

Figure 5. (a−c) Optical micrographs of P4VP/PS bilayer surface before and after solvent annealing. P4VP films were spin-coated on ethanol-treated PS substrate (a) immediately after treatment, (b) after 1 h of vacuum drying the PS, and (c) after overnight vacuum drying of the PS. Scale bars = 400 μm. F

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thickness striation, unaffected by the roughly 20 nm difference in thickness between the two regions. Based on the observed correlation between pinhole distribution and experimental conditions, the formation of the aligned pinholes appears to be instead related to the centrifugal force exerted on the P4VP solution as a result of spinning and the relative position to the center of rotation. To test this hypothesis, a trial experiment was designed. A 10 μL P4VP solution droplet was spread over a PS film substrate by blowing with pressurized air in either a single direction or a randomly changing one to prepare a P4VP film. After fully dried, the P4VP film was then dewetted via the same experimental procedure used previously. Although no defects can be directly seen prior to dewetting, the final pattern of the single direction sample showed significant ordering, whereas the randomly blown sample did not (Figure S6), indicating that the motion-driving force experienced by the polymer solution does play a role in formation of aligned pinholes. The diameter of the ring pattern, which was shown as transition from region II to region III in Figure 2b, was measured and compared to the diameter of the area occupied by a 50 μL deposited P4Vp solution droplet (Table 2). The

in vacuum for a long time (overnight, Figure 5c) and then P4VP was spun over this, dewetting of the P4VP film showed again aligned pattern, similarly to the case of nontreated substrates. When the substrate was dried for a shorter time (1 h), the P4VP film showed few defects as discrete ring-like patches (highlighted area, Figure 5b) and the aligned pattern did not appear. These observations provide strong evidence that the pinhole formation was related to the ability of the P4VP solution to spread over the PS substrate. The surface roughness of the PS substrate did not vary significantly with ethanol treatment (as shown by AFM imaging, data not shown). One parameter was found to affect the size of the area where the droplets are aligned. The larger the volume of the P4VP droplet deposited on the PS substrate, the larger the area covered by ordered dewetted droplets (Figure S5). The largest width containing aligned droplets was 3 mm, obtained with 100 μL droplets. When the P4VP droplet was deposited slightly off the center of rotation on the PS substrate, the share of the aligned area became skewed to one side but still retained the other main features (Figure S5). Neither changing acceleration rate on its own nor changing concentration of the P4VP solution and spin rate together had a dramatic effect on the size of the aligned ordered area or on the pattern. Mechanism of Pinhole Formation. Explaining the formation of ordered pinholes was not trivial. A common defect of spin-coated films is the presence of thickness fluctuations, and this effect was observed also in our P4VP, visible as radial color striations in the film.34,35 Preformed pinholes were predominantly found at the thinner (darker) area of the striations (Figure 6a). Since thinner portions of the

Table 2. Diameter of Ring Defect, Measured P4VP Solution Droplet Size on PS, and Theoretical Diameter of Droplet Based on Spherical Cap Assumption diameter (mm)

ring defect

measured

theoreticala

7.8 ± 0.5

8.2 ± 0.6

7.6

Calculation of diameter is done based on contact angle of θethanol−PS = 10° with eqs S7 and S8. a

diameter of the droplet was measured from the rim left by the naturally dried droplet on substrate. Because of minor liquid flow before evaporation, the diameter measured in this way should be slightly larger than the initial droplet diameter. The droplet diameter was also theoretically predicted based on contact angle of ethanol on PS. These numbers show that the size of the ring matches the original area of P4VP solution droplet, which is the initial three-phase contact line (i.e., P4VP solution/PS substrate/air) during spin-coating. The relation between P4VP solution volume and ring size also supports this claim (Figure S5d−f). The reason for the pinhole ring formation and for the region of aligned pinhole formation is not fully understood, but the following is our best explanation. First, to explain the formation of concentric rings of pinholes. In the first second or so before the spinning is started, when the P4VP solution droplet is deposited on the PS, the evaporation of ethanol starts producing a P4VP-rich region right at the contact line of the droplet. As P4VP has a higher surface tension (γ = 44.0 mN m−1) than ethanol (γ = 22.7 mN m−1) at room temperature, the surface tension gradient induces an outward Marangoni flow, and a larger P4VP-rich region forms. This leads to a highly viscous region near the edge of the solution droplet compared to the bulk, and a thin region of film with width on the micrometer scale. When spinning starts and the substrate accelerates, this highly viscous region likely stays immobile relative to the moving substrate, whereas the bulk solution experiences a radial force and spreads. Occasionally, the contact line of the solution becomes pinned due to substrate defects as it spreads, and whenever this happens, a coffee ring effect occurs and produces a ring of dry film at the

Figure 6. (a, b) Optical micrographs illustrating the preformed pinholes in P4VP film that are (a) directed in line with the direction of the thickness striation or (b) not in line with the striation. Scale bars = 200 μm. (c) AFM micrograph of the highlighted portion in part b. Scale bar = 10 μm; height scale = 40 nm.

metastable film are more susceptible to nucleate holes,36 it is very tempting to jump to the conclusion that the preformed pinholes were caused by P4VP film thickness fluctuations. However, further studies revealed that such matching was merely coincidence. In numerous experimental repeats, the center of the radial striations corresponded always to the position of the center of the spinning platform in the spincoater. On the other hand, the center of the distribution of pinholes was always determined by the position of the dispensed polymer droplet. Counterexamples produced by either placing the silicon wafer off-center on the spin-coater or dispensing the polymer solution in an off-center position on the substrate easily proved that there was no correlation between thickness fluctuation and pinhole formation, as shown in Figures 6b and 6c. The AFM micrograph in Figure 6c shows clearly that a line of pinholes crosses the ridge and trough of a G

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a pattern of aligned micrometric polymer droplets through dewetting of polymer bilayers. The alignment is highly reproducible and extends over the millimeter scale. We investigated the effect of various experimental conditions including the solvent used for solution preparation, method of dispensing solution, solution volume, solution concentration, spin rate, acceleration rate, and surface treatment. We identified the static dispensing method, the solvent used for the top polymer film, and wettability of the substrate as critical factors determining pinhole formation. The larger the volume of the P4VP droplet deposited on the PS substrate, the larger the area covered by ordered dewetted droplets. The mechanism was explained based on preliminary understanding of the phenomenon. Upon greater engineering control of film thickness and environmental conditions, it is likely that the patterns could be optimized to extend spatial ordering of the patterns over larger areas. If the density of particulate defects in the film could be reduced by operating in clean room conditions, more regular patterns could be obtained. As the approach itself is intrinsically upscalable, it offers the opportunity for overcoming the common limitations of most patterning technique, namely the cost and scale. The aligned droplet array may find application as functional engineered surfaces where ordering is beneficial, such as atmospheric water harvesting.

liquid front edge. It seems that the coffee ring effect is determined by a short-lived pinning at the droplet contact line, which is stuck for a while, and then continues to slip outward. During annealing every one of the dried film rings starts to dewet, leaving nucleated pinholes seen later as ring features. All the above processes are completed within the very first second, during the acceleration stage where excess polymer solution is spun off from the substrate. And now for the explanation of the aligned pinholes. As the target spin rate is reached, the centrifugal force applies a shear to the P4VP solution that separated the homogeneous solution into alternating bands with slightly higher and slightly lower P4VP concentration. This shear-induced phase separation effect is well-known for polymer solutions under very high shear.37−39 Such concentration fluctuations are often associated with a characteristic wavelength and can become aligned on the substrate due to the centrifugal forces in spinning. Regions with lower P4VP content and higher ethanol content may remain plasticized (below Tg) for a slightly longer time compared to the ethanolpoorer regions. The differences in plasticity between the regions need not be large for a difference in dewetting ability to be seen. In the nanothin films under consideration here, the regions of mobile P4VP start to dewet earlier, leading to the formation of the aligned pinholes. Both the dewetting processes leading to ring and aligned pinholes stop when the P4VP reaches its Tg due to further loss of ethanol as spinning proceeds. The reason why the occurrence of the aligned pinholes depends on solvent used can be related to the Flory−Huggins parameter Χ12 between polymer and solvent for the system. Χ12 determines whether or not a polymer solution will remain homogeneous at a given volume fraction and environmental condition.40 For Χ12 values significantly smaller than the critical point ΧC ≈ 0.54, phase separation between the P4VP and the solvent under shear is likely to occur. The estimated Χ12 are provided (Table S2), calculated based on eq S9. Χ12 is used as a qualitative estimate because the solubility parameters reported for polymers are influenced by residue catalyst, contamination, and polymer diversity41 and vary significantly between different references.40−45 The estimated Χ12 for P4VP in isopropanol (0.41) and n-butanol (0.43) are smaller than for ethanol (0.51) and methanol (0.60), which means P4VP is less likely to undergo phase separation in isopropanol and in nbutanol. This is consistent with the observation that isopropanol and n-butanol solutions did not lead to any pinhole formation during spinning. On the other hand, the reason why aligned defects were not observed in methanol could be due to the fast evaporation rate. Because of its high vapor pressure (Table S1), methanol evaporated rapidly leaving insufficient time for defect development, and thus the ring defects observed were fragmented. In the P4VP/PMMA bilayer system, methanol, rather than ethanol, resulted in aligned patterns. The calculated Χethanol−PS is close to Χmethanol−PMMA. This could imply that the spreading of the P4VP solution also affects the formation of pinholes, especially the ring pattern. Therefore, the pinholes formation could be a complicated interplay between solvent affinity to both polymer and substrate, solvent evaporation rate, and substrate wettability by solvent.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00620.

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Details of image analysis method; calculation of theoretical polymer solution droplet on substrate; calculation of Flory−Huggins parameter between solvent and polymer based on respective solubility parameter; solvent and polymer information including solubility parameters, surface tensions, molar volume, and vapor pressure; optical micrographs of discussed surfaces (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.N.). ORCID

Asaph Widmer-Cooper: 0000-0001-5459-6960 Chiara Neto: 0000-0001-6058-0885 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the Australian Government Research Training Program (RTP) Scholarship. Part of the MATLAB program coding is supported by Mr. Felix Vüllers. Scripts used for image analysis are available upon request. Access to alternative spin-coater was offered by Dr Elena Kosobrodova from School of Physics, University of Sydney. A.W. was supported by the Australian Research Council via a Future Fellowship (FT140101061).

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CONCLUSION The usually overlooked formation of polymer film pinhole defects associated with spin-coating may be utilized to prepare H

DOI: 10.1021/acs.macromol.8b00620 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

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DOI: 10.1021/acs.macromol.8b00620 Macromolecules XXXX, XXX, XXX−XXX