Fullerene Nanoparticles as Molecular Surfactant for Dewetting of

Article pubs.acs.org/Macromolecules

Fullerene Nanoparticles as Molecular Surfactant for Dewetting of Phase-Separating Polymer Blend Films Diya Bandyopadhyay,† Jack F. Douglas,‡ and Alamgir Karim†,* †

Department of Polymer Engineering, The University of Akron, Akron, Ohio 44325, United States Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, United States

‡

ABSTRACT: We investigate the effects of fullerene nanoparticles (f-NP) on the dewetting morphology in the immiscible temperature regime of blend films of polystyrene (PS) and polybutadiene (PB) on silicon substrate. As in our former work in the miscible temperature regime of this blend film, competitive partitioning of the f-NPs to the polymer−polymer and the substrate interfaces in blend films requires a larger concentration of f-NPs (∼10 mass %) to suppress film dewetting than in homopolymer components (∼2 mass %). In contrast, however, phase-separated blend films rapidly dewet into hemispherical droplets due to finite interfacial tension of internal blend components unlike irregular shape droplets obtained in miscible blend films. The effect of the f-NPs (1 mass % to 5 mass % f-NP) is to simultaneously reduce the size and contact angle of the dewet droplets, but the hemispherical shape of droplets is maintained, suggesting the f-NPs act to (a) only reduce the phase-separated blend interfacial tension but not fully compatibilize it into single phase and (b) reduce the blend substrate interfacial tension progressively. Selective solvent etching of the PS blend component reveals a spherical PS core enclosed within a circular PB shell at all f-NP concentrations. Confocal fluorescence microscopy reveals that the f-NPs are distributed in both phases consistent with the hemispherical shape and calculations that predict only a weak reduction of interfacial tension. The dewet blend droplet contact angle (likewise polymer−substrate interfacial tension) measured by atomic force microscopy shows a bimodal behavior, reducing rapidly (by 40%) at low (0.1 mass %) f-NP levels and significantly slowing down at higher f-NP concentrations. Molecular surfactant like behavior of the f-NPs in the blend films then provides an effective means of tuning dewetting blend film morphology dimensions without compromising phase behavior for potential applications in nanotechnology and nanomedicine.

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such composite thin films to uniformly spread and stabilize on the underlying solid substrate.7 In most practical applications, a smooth, stable, and defect free coating is required, and it is imperative to maintain the stability of such thin films in order to retain their desired properties. Whereas homopolymer systems have received some attention in this regard,8−14 there have been very few studies on instabilities and dewetting (and their control) of multicomponent polymeric systems such as polymer blend thin films on solid substrates. This is in part due to the existence of a complex coupling between dewetting and phase separation/phase ordering that could complicate the interpretation of such measurements. However, given their vital significance in diverse and important applications, we believe it is necessary to understand how instabilities in such thin film systems related to phase separation and dewetting govern the ultimate morphology of these films and how the presence of the NPs influence the ultimate length scales of the surface patterns of fully annealed films of this type.

INTRODUCTION Nanoparticle-filled multicomponent polymer thin films have attracted significant interest in recent years because of their importance in a host of varied applications such as functional coatings, electronics, biosensors and drug-delivery systems, as a result of their potential for combining different homopolymer attributes to create superior molecular composite materials along with property enhancements provided by the nanofillers.1,2 When added to polymer matrices even in small amounts, nanoparticle additives are capable of generating considerable improvements in the mechanical, optical, conductive, thermal, and barrier properties of thin films. Fullerene nanoparticles (C60 “buckyballs”) for example, have generated wide interest due to their unique conductive and antioxidant properties, rendering them ideal for applications in organic photovoltaics in bulk heterojunction devices and potentially in flexible electronics as well as antioxidant agents for medical applications.3−5 In spite of the numerous attractive uses of nanofilled multicomponent polymer thin films, their successful implementation in practical applications is limited by the persistent difficulty of creating thermodynamically stable polymer− polymer and polymer−NP composites,6 as well as preparing © 2012 American Chemical Society

Received: January 4, 2012 Revised: March 6, 2012 Published: June 1, 2012 4716

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A decade ago, Barnes et al.8 demonstrated an unconventional route for controlling the dewetting of homopolymer films on nonwettable substrates that involved addition of a small quantity (on the order of 1% by mass) of fullerene NPs to homopolymer films. The C60 NPs were found to segregate to the film−substrate interface. This NP segregation modifies the polymer−substrate interaction9−14 and also enhances film stability by pinning the growing contact lines of any dewetting holes that might form in the film. A synergy between these thermodynamic and kinetic stabilization effects results in a strong and practically useful net film stabilization against dewetting. This phenomenon has been found to be very robust for other NP polymer thin film systems and suitable for the practical purpose of stabilizing films used in sensor applications.15 By comparison, the effect of nanoparticles on even the simplest multicomponent (binary) blend film structure and stability is substantially more complex than homopolymer films due to simultaneous nanoparticle partitioning to the polymer− polymer and blend−substrate interface.16 In the single phase region for the blend, surface segregation of the lower surface energy component occurs near the air surface for thin films and interfacial tension controls the segregation at the substrate boundary.16 In the two-phase region, surface segregation layers transform into wetting layers16 which serve to drive the polymer component segregation in their own fashion. There are distinct coexisting phases and associated interfaces at which the NPs can segregate depending on the interfacial tension of the phase-separated components and the relative interfacial energy of blend−air and blend−substrate interface. Previously, we illustrated several regimes of dewetting morphological behavior via the influence of C60 at varying concentrations on a polystyrene/polybutadiene (PS/PB) blend thin film system annealed in the one-phase temperature region at 130 °C.16 In contrast, this paper considers the dewetting− morphological behavior of the same ternary blend, but annealed in the two-phase regime at 95 °C, where we have the additional complication of polymer and NP phase separation and interaction interacting with the film dewetting process. We find several interesting differences and similarities between the one-phase versus two-phase regime of dewetting-phase controlled behavior in these thin film systems, and, in particular, we focus on surfactant like effects on the phase separation morphology that arise from the presence of NPs in the blend films. The present work indicates several novel effects related to the dewetting of the PS/PB/C60 ternary blend thin film behavior in the two-phase region for blend phase separation. First, the binary PS/PB blend film annealed at 95 °C dewets to form highly circular droplets that are quite unlike the highly irregular dewet droplets observed in one-phase blend films at 130 °C. The presence of these spherically symmetric droplets which are seen with or without NP indicate that the phase boundary does not shift dramatically in these thin dewet blend film morphology, nor are the blend films compatibilized into single phase by addition of the NPs. Second, competitive partitioning of C60 NPs to the polymer−polymer and the polymer−substrate interface progressively diminishes the size scale of the dewet droplet, while maintaining their spherical shape due to internal phase separation. This approach provides a practical means for tuning phase separation morphology in confined dimensions of high symmetry, an effect that is of great potential importance in the design of functional materials

where symmetry of pattern scale is important, e.g., shape- and size-based recognition by cells for tissue engineering or making surfaces of tunable wettability and interaction strength through the control of film nanoscale geometry.17,18

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EXPERIMENTAL SECTION

Polystyrene (PS), average relative molecular mass Mw = 3000 g/mol and a polydispersity index (ratio of mass average to number-average relative molecular mass, Mw/Mn) of 1.09 and polybutadiene (PB), Mw = 2400 g/mol and a polydispersity index of 1.05 from Polymer Source Inc.19 were used for our experiments. The molecular masses were chosen so as to be similar to the ones used in a previous homopolymer−NP study.8 Laboratory grade toluene was used as the solvent for the polymers and was purchased from BDH chemicals.19 Homopolymer solutions of PS and PB in toluene were prepared separately by stirring the polymers in the solvent overnight. Blend solutions were made by dissolving the PS and PB in the common solvent toluene, such that the total polymer concentration in the solvent was 3% by mass, and the solutions were then stirred overnight. Both homopolymer and blend solutions were filtered thoroughly using a 0.2 μm PTFE filter. C60 NPs of purity >99%, from Sigma-Aldrich Chemical Co.,19 were dispersed in toluene, accompanied by ultra sonication, before addition to the blend solution. The nanoparticle solutions were handled inside a nitrogen purged glovebag. A series of NP-filled blend solutions consisting of C 60 concentrations varying from (0 to 5) % by mass, relative to total polymer blend weight were prepared by dissolving the C60−toluene mixture with the blend solutions. Thin homopolymer and blend films were prepared by spin coating at identical spin conditions of speed (2000 rpm), acceleration (1000 rpm), and time (60 s) on nitrogen blow dried and UVO cleaned p-type Si (100) wafers purchased from Silicon Quest International.19 This ensured that both homopolymer and blend films had the same thickness of 110 ± 2 nm, which was verified using a DI-Veeco Nanoscope V atomic force microscope (AFM) and a Filmetrics F20−UV thin film analyzer. Annealing temperatures were kept in the one phase region of bulk phase miscibility of this blend system as characterized by light microscopy in bulk films previously.20 Blend film compositions in this study were kept fixed at 50:50 PS:PB composition. The upper critical solution temperature (UCST) in neat bulk PS/PB blend films was determined to be at 110 °C at a critical composition of 0.5.20 The films were annealed at 95 °C, i.e., in the blend bulk two phase region, for 1, 2, 12, and 48 h under vacuum and then quenched to room temperature instantaneously. We observed that beyond 2 h, the morphology and film stability remained unchanged and hence in all our further studies we chose to analyze and present sample data for an annealing time of 2 h. In parallel, we examined the stabilization effect of the C60 under similar conditions on each of the blend component homopolymer films, as well as compared results with our previous homopolymer dewetting stabilization by NP studies. The annealed blend and homopolymer films filled with NP were characterized using an Olympus BX41 optical microscope, and a DI-Veeco Nanoscope V atomic force microscope (AFM).19 Further, a Zeiss confocal laser fluorescence microscope19 was used in order to map out the z-stacking distribution of C60 in the samples. Image J (NIH) data analysis software was used to quantify our results.

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RESULTS Thin film blend stability is a strong function of polymer mobility, the phase of the blend homopolymer components and surface energy of the films on which the films are cast. As these are temperature dependent phenomena, the film stability is consequently strongly dependent on the film annealing temperature. Prior to studying the effects of added fullerenes on the phase separation and dewetting of the polymer nanocomposite blend films, we carried out control measurements on as-cast nanoparticle (NP) filled and unfilled PS/PB 4717

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Figure 1. As-cast morphologies of 50/50 PS/PB blend thin films spin coated at room temperature (25 °C). (a and d) Optical micrographs of 0 and 5 mass % filled blend films respectively, where the inset in d is a confocal fluoresecnce optical image tracking the C60 distribution throughout the film. (b and e) AFM height figures of 0 and 5 mass % C60 filled blend films respectively indicating a bicontinuous morphology. (c and f) AFM phase images of 0 and 5 mass % filled blend films, respectively. AFM images for the 5 mass % C60 filled blend films are scanned on the nonclustered areas that are otherwise visible in the optical micrographs.

Figure 2. Morphological and film stability evolution as a function of C60 nanoparticle concentration in 50/50 PS/PB blend thin films annealed at 95 °C for 24 h under vacuum: (a−d) optical micrographs of neat, 0.1, 1, and 5 mass % C60 filled blend films, respectively.

interconnected morphology of the cast blend film. This relative dispersion of the majority of the NPs away from the isolated NP cluster regions can be directly observed by confocal fluorescence microscopy (Figure 1d). There is insignificant difference between the surface morphology of the as-cast 0 mass % and 5 mass % C60 filled blend films. This could likely be attributed to the fact that before annealing, the NPs are dispersed over the entire film, both laterally and in the zdirection (depth) and in fact might have also begun to segregate to the substrate. Since the images in Figure 1, parts b and e, are only surface morphologies and do not indicate overall 3-dimensional structure, such differences in the structure between 0 and 5 mass % are therefore not directly manifested in these rapidly vitrified films. Upon annealing in the 2-phase region at 95 °C, unfilled and C60 NP filled 50/50 PS/PB blend films phase separate as well as dewet the substrate as seen in Figure 2, a−d. The neat blend film dewets into uniform circular droplets (Figure 2a) with diameters ranging from 20 to 25 μm. As shown in more detail later, etching the PS-rich phase only with a selective solvent confirms that a surface wetting layer of the lower surface energy component (PB) forms a hemispherical shell that encompasses the droplet due to surface-wetting effects observed in thin films.22 Competing unfavorable interactions with the underlying substrate and those arising due to blend incompatibility inside the drop result in minimization of

blend films. In these blend films, only the PB blend component was at a temperature above its Tg. As such these as-cast films are optically smooth and homogeneous, yet they exhibit a rough and interconnected morphology on a smaller scale, as revealed by AFM measurements. This fine structure, illustrated in AFM images shown in Figure 1, is attributed to the combined effect of the fast solvent evaporation21 induced quenching into the 2-phase region during spin coating and some small scale phase separation in the evaporating film before the film vitrified. No dewetting was observed at a temperature much below Tg because of the slowing down of the dynamics arising from vitrification, an effect primarily due to the PS component of the blend film. However, when the film is heated above the blend Tg, it dewets due to unfavorable interactions between both polymer blend components and the underlying silicon substrate. Dewetting occurs by the usual hole formation, growth and coalescence process, albeit modified by the simultaneously developing phase separation process. As baseline comparison, individual homopolymers (PS and PB) filled NP films behave in a qualitatively similar fashion, an effect studied previously.8,16 The C60 nanoparticles have a nonuniform distribution in the blend film that varies with NP concentration. For a 5 mass % C60, we see small aggregate regions forming, but majority of the C60 appears more or less uniformly dispersed throughout the film without disrupting the 4718

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Figure 3. (a−c) AFM (height) images tracking the evolution of phase separation and dewetting morphology as a function of C60 nanoparticle concentration annealed at 95 °C for 24 hr in 50/50 PS/PB blend thin films. Insets are corresponding AFM phase images. (d) Optical microscope image of 0 mass % C60 blend film. (e and f) Confocal fluorescence micrographs of 1 and 5 mass % C60 filled blend films indicating nonlocalized dispersion of C60 NPs. (g−i) Internal morphology analysis of neat and NP filled 50/50 PS/PB blend films annealed at 95 °C via selective solvent etching, AFM (height) images of PS-etched 0, 1, and 5 mass % C60 filled blend films respectively. (j) Optical micrograph of PS etched neat blend film. (e and f) confocal fluorescence images of 1 and 5 mass % C60 filled blend films respectively, etched with MEK (PS selective) showing remnant PB and C60 pinning sites around rims of dewet droplets.

polymer domain contact area with the substrate, leading to circularly shaped droplets with contact angles of approximately 20°. At NP concentrations of 5 mass % and above, the shape of the phase-separated dewet droplets tends to transition toward an interconnected elongated structure as compared to circular droplets at lower filler concentrations. This high concentration regime (5−20 mass %) and the related effects of NP segregation to the substrate on the structure of dewetting and film stability will be discussed in a later paper. When the surface curvature becomes sufficiently large, this surface layer enriched in PB ruptures and partially exposes the inner polymer core to reveal a core−shell structure. An example of this type of type of structure is illustrated in the AFM height and phase image in shown Figure. 3a.

Nanoparticle-induced blend compatibilization toward stability against dewetting results in reduced contact angle of the dewet drop with respect to the substrate and thus increased wettability. Thus, the NPs act as a conventional surfactant and we next quantify this important effect, which is special to the immiscible blend films. A reduced contact angle corresponds to a lower surface curvature, which in conjunction with reducing interfacial tension between blend components has the auxiliary effect that the wetting PB outer layer becomes contiguous over the hemispherical droplet with increasing addition of nanofillers to the blend. AFM height and phase images in Figure 3, parts b and c, reveal the surface shell layer that had otherwise partially disintegrated in an unfilled system remains fully intact in an NP-filled blend film. This difference in morphology can be attributed to the enhancement of the NPs to the blend interface 4719

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forming a uniform surfactant like layer. The progressive increase in wettability with C60 concentration can be attributed to the simultaneous partitioning of the NPs to the blendsubstrate interface, along with intradrop PS/PB blend interface NP segregation. This is evident from confocal fluorescence imaging of the NP-filled dewet films that show a gradual transition from blend-localized C60 concentration for 1 mass % C60 concentration (Figure 3 e) to a more diffuse background fluorescence intensity from NPs at the substrate at 5 mass % C60 (Figure 3 f). The observed NP partitioning to the substrate is in good agreement with previous studies indicating a stabilization of homopolymer PS and PB, as well as PS/PB blend films16 on silicon substrates as a result of the formation of stabilizing C60 fractal structures14 and gel-like layers16 at the polymer−substrate interface. In order to better understand the internal morphology of phase separation and determine qualitatively the distribution of C60 NPs within the dewet polymer domains, selective solvent etching of PS was carried out using MEK. Figure 3 (g−l)reveal the etched core−shell morphology of phase separation and dewetting of unfilled and filled immiscible PS/PB blend films. In particular, solvent etching the spherical PS core leaves the remnant PB and C60 shell. Confocal fluorescence mapping of 1 mass % and 5 mass % C60 filled blend films with PS etched demonstrate the presence of the C60 NPs in the PB rim (shell) at both concentrations. However, at 5 mass % C60, a background NP concentration is also visible indicating the onset of significant nanofiller partitioning to the substrate. This, 5 mass % C60, and higher, is a different regime where the f-NPs have an altered effect on the film morphology, and we shall report this high NP concentration regime in a future study.

Figure 4. Plot of the average domain diameter of polymer blend dewet domains as a function of C60 nanoparticles concentration. Bars on data points indicate a standard uncertainty of ±2.5 of the average domain diameter.

interfacial energy reduction can be computed based on the contact angle measurements. Figure 7 shows the trend of the blend droplet−substrate interfacial energy reduction as a function of NP concentration. The interfacial energy reduction between the blend and the substrate follows the contact angle trend as expected, but in absolute magnitude, the decrease is not too significant. It reflects the effect of preferential NP segregation within the blend droplet to the blend-substrate interface. We infer from Figures 6 and 7 that the NPs rapidly form an interfacial layer at the substrate, but thereafter with increasing NP concentration, the tendency of the NPs is to slowly partition further to the substrate interface, as the NPs tend to saturate the blend droplets. The overall stabilizing effect of the NPs is further manifested in increased total polymer droplet coverage of the substrate with increasing NP concentration. Figure 8 shows that the integrated 2-D fractional areal coverage of all the blend droplets increases with NP concentration, despite the fact that the droplet size diminishes with increasing NP concentration (see Figure 4). This interesting feature is a consequence of a surfactant like effect of the NP on the film breakup process leading to a significantly larger number of droplets per unit area, that each have higher wettability (lower contact angle) on the substrate. From an observational standpoint, adding C60 nanofillers to the blend primarily alters the dewetting morphology of the film owing to partitioning of the NPs to the polymer−substrate interface. However, the C60 nanofillers should also weakly partition to the PS−PB phase-separated interface although this is difficult to detect in our experimental observations. The hemispherical phase-separated droplet geometry with a PB phase-separated outer layer does not lend itself to easy inference of enhancement of the NP to that interface. Our confocal fluorescence microscopy has typical 0.5 μm resolution, insufficient to differentiate NP distribution in a 0.23 μm high dewet blend droplet (see Figure 5). Preferential segregation of NPs to the blend−blend interface can however be estimated from basic interfacial energy calculations. A temperature dependence of surface tension γ for both PS and PB can be calculated as below,23,24 The surface tension of polystyrene at the blend annealing temperature of 95 °C is,

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DISCUSSION The primary effect of the C60 nanofillers is to effectively minimize the overall energy of the system by reducing the unfavorable interactions between the silicon substrate and the polymer blend film. This is manifested in several ways, as modifications to the final dewet droplet size, the droplet contact angle with silicon substrate and the integrated droplet surface area. The effect of C60 NPs as compatibilizers on droplet size is quantitatively mapped out in Figure 4 illustrating a plot of dewet domain diameter as a function of increasing nanofiller, showing a trend of progressively decreasing domain size. Interestingly, with decreasing size of the polymer domains, the wettability of the drops increases as illustrated by its decreasing contact angle, so that the spreading of polymer increases with nanofillers concentration. Intuitively, one may have expected that as droplets get smaller, their contact angle should increase but this is not the case. The droplets get smaller and their contact angle decreases with increasing NP addition. This physical trend occurs because the droplet size is related to the initial breakup process and how it is modified in NP-filled films. Future studies could investigate this interesting scientific issue that requires a high speed camera to image the initial NP filled film breakup process. Figure 5 illustrates an example of estimating the contact angle of a polymer droplet that has dewet the substrate by constructing a section profile across the AFM topography image. These values are then plotted as a function of nanofillers concentration in Figure 6 that demonstrates the overall effect of C60 NPs on progressive wettability of the polymer on the underlying substrate. The blend dewet droplet-substrate 4720

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Figure 5. Estimation of polymer blend dewet droplet contact angle with respect to the underlying substrate. In the above example, an AFM height image of an unfilled 50/50 PS/PB blend film annealed at 95 °C is analyzed by drawing a section profile across a polymer dewet domain and the contact angle is measured from this profile. The scale on z-axis is in nm, i.e., the maximum of 300 nm on the z-axis is only 0.3 μm.

Figure 8. Plot of net polymer blend film coverage on the substrate (a measure of film wetting and thus stability against dewetting) as a function of C60 nanoparticles concentration, indicating progression toward increasing film stability at high concentrations. Bars on data points indicate a standard uncertainty of ±2.5 of the “% of net polymer area”.

Figure 6. Plot of the average contact angle of dewet domains with respect to the substrate as a function of C60 nanoparticles concentration. Different regimes of film wettability and morphology are observed as shown schematically.

γPS/C = ( 32.7 −

40 )2 = 0.15 mJ/m 2

γPB/C = ( 26.4 −

40 )2 = 1.4 mJ/m 2

60

60

The energy difference between NPs at the interface versus one embedded in a phase A can be expressed as,25 ΔE int =

πr 2 [γ − γB/NP + γA/B]2 γA/B A/NP

where A = PS, B = PB, and NP = C60 ΔE int = Figure 7. Blend droplet/substrate interfacial energy as a function of fNP concentration.

πr 2 γPS/PB

[γPS/C − γPB/C + γPS/PB]2 60

60

with r = 1.1 nm for C60, we obtain the energy difference between f-NPs at the PS/PB interface versus embedded in PS phase as ΔEint = 1.45 × 10−21 J . Likewise, reversing so that A = PB and B = PS we obtain the energy difference between f-NPs at the PS/PB interface versus embedded in PB phase as ΔEint = 1.44 × 10−20 J. Thus, the f-NPs should have a weak preference to be segregated at the PS/PB interface and the PS phase rather than the PB phase. We estimate “weak” preference, since, furthermore, according to the Pieranski relationship,26 spherical NPs will be only be positioned at the interface if the polymer/ polymer interfacial energy exceeds the individual homopolymer/NP interfacial energy gradient, i.e., |γPS/C60 − γPB/C60| < γPS/PB. In the present case of our PS−PB−C60 system, |γPS/C60 − γPB/C60| = 1.25 mJ/m2 < γPS/PB = 2 mJ/m2. Thus, interfacial

11/9 ⎡ T⎤ γ95 = γ(0)⎢1 − ⎥ , Tcr ⎦ ⎣ 11/9 ⎡ 368 ⎤ γ95 = 63.3⎢1 − = 35.25 mJ/m 2 ⎥ ⎣ 967 ⎦

Similarly, the surface tension of polybutadiene at 95 °C is γ95 = 53.7[1 − (368/918)]11/9= 28.7 mJ/m2. In order to calculate the interfacial energy of polymer-NP systems, we apply the geometric mean equation, 4721

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BES Grant No. DE-FG02-10ER4779), and the Austen BioInnovation Institute in Akron (ABIA), supported Akron Functional Materials Center (AFMC). D.B. would also like to thank Gurpreet Singh and Prof. Matthew L. Becker for helpful discussions.

energy estimations suggest that C60 nanoparticles may only weakly segregate to the PS/PB interface as first approximation. Experimentally, with the aid of confocal fluorescence microscopy we do not find any well-defined NP layer at the PS−PB interface consistent, with the above theoretical estimate.

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CONCLUSIONS Our study reveals the influence of the technologically important fullerene nanofillers on the phase separation morphology, length scale and overall film stability of a model phaseseparated blend system as a function of competitive NP segregation to the interpolymer and polymer−substrate interface. We find that the competitive NP partitioning process greatly diminishes the NP stabilization of film effect that is observed in the individual PS and PB homopolymer films of which our blends are composed. Experimental fluorescent confocal microscopy observations and interfacial energy calculations suggest that the NPs are located substantially within the blend droplet with weak segregation to the polymer−polymer interface and increasingly at polymer substrate interface with increasing NP concentration. The segregation to blend-substrate interface results in a surfactantlike effect on dewetting leading to decreased phase-separated droplet size with increasing nanofiller concentrations. Further, we observe several novel effects related to the dewetting of the PS/PB/C60 ternary blend thin film behavior in its two-phase region. The two-phase blend dewet domains exhibit a perfectly spherical core−shell type of structure that provides additional interfaces and a potential driving energy for added C60 NPs to partition to the PS/PB interface. Spherical morphology is an interesting consequence of the finite interfacial tension of phase separation that imparts the spherical interfacial domain shape to the entire dewetting blend droplet, not just the PS/PB interface. While phase behavior (two-phase versus one-phase) controls the overall spherical morphology of the dewet droplet, the influence of competitive partitioning of C60 NPs from 0− 5% by mass is in fact to act as a molecular surfactant and tunanbly diminish the size scale of the dewet phase-separated droplet, while maintaining its spherical shape. The interfacial segregation of the C60 to the phase-separated PS/PB interface induces surfactant like effects leading to rupture of the dewetted phase-separated polymer domains into smaller droplets. Thus, the shape and size of the dewetting blend droplet can be controlled by whether the blend is in the one-phase or twophase region and the concentration of the nanofillers. This feature allow us to tune the morphology of dewet blend droplet by tuning its proximity to phase boundary through judicious choice of annealing temperature and tuning the nanofillers concentration, a subject of importance to morphology control in future potential applications such as bulk-heterojunctions for organic solar cells.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Telephone: (330) 972-8324. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the University of Akron Research Foundation (UARF), the U.S. Department of Energy (DOE4722

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