PMMA films: Effects of surface energy and blend composition

Polymer 53 (2012) 4187e4194

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Morphological evolution of thin PS/PMMA films: Effects of surface energy and blend composition Dae Up Ahn a, **, Zhen Wang a, Ian P. Campbell b, Mark P. Stoykovich b, Yifu Ding a, * a b

Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, CO 80309-0427, USA Department of Chemical and Biological Engineering, University of Colorado at Boulder, Boulder, CO 80303-0596, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 May 2012 Received in revised form 5 July 2012 Accepted 14 July 2012 Available online 21 July 2012

We systematically compare the morphological evolution of thin PS/PMMA films, with varying compositions, during thermal annealing on preferential and non-preferential surfaces. On native silicon oxide surfaces, the phase evolution in the films was dictated by the preferential substrate-wetting of PMMA. However, the resulting PS relief structures on the PMMA wetting layer varied with the blend composition, and transitioned from capillary-fluctuation-mediated breakup to random nucleation with increased PS concentration. In contrast, the morphological evolution of the PS/PMMA films on nonpreferential surfaces was also dictated by the coarsening of PMMA domain, but proceeded without the formation of a PMMA wetting layer. Both the PS and PMMA domains maintain direct contact with both the substrate and free surfaces throughout the evolution of the morphology. The morphologies at the interfaces were highly correlated, but with distinctive length scales. A variety of dispersed and continuous phase-separated structures can thus be obtained. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Preferential wetting Neutral surface Polymer blend film

1. Introduction A polymer liquid in contact with other incompatible materials undergoes a variety of morphological changes as it proceeds towards its equilibrium state [1e4]. This evolution in morphology is manifested in both the dewetting of a thin polymer film from a non-wettable surface [1] and the de-mixing of a polymer blend [2]. The latter system has been investigated for decades both because the transient morphologies are important for practical applications and the slow kinetics are convenient for fundamental investigations of the phase separation process [2,5]. Phaseseparated polymer blends in thin films have been utilized to fabricate a variety of nano- to macro-scale structures that have a range of potential applications such as antireflective coatings and organic photovoltaic [6e10]. The ability to tune the morphologies and dimensions of phaseseparated blends is thus highly desirable [11e13], but it can be particularly challenging in thin films to achieve nanoscale structures. The presence of the substrate often dominates the morphological evolution of the blend because the blend components are * Corresponding author. Tel.: þ1 303 492 2036; fax: þ1 303 492 3498. ** Corresponding author. E-mail addresses: [email protected] (D.U. Ahn), [email protected] (Y. Ding). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.07.037

likely to have different wetting tendencies on the substrate [14e16]. For a classical blend of polystyrene (PS) and poly(methyl methacrylate) (PMMA) on a preferential surface such as Si wafer with a native oxide layer, the morphological evolution of the blend film is dictated by the substrate-wetting of PMMA. After long annealing, a so-called surface relief structure, featuring isolated PS droplets on a continuous PMMA wetting layer is observed for Si [17e21], glass substrate [22], and mica [23]. Further, the morphologies of PS/PMMA blend films after long annealing were also reported on metal substrates (Au and Co) [24], and even heterogeneous substrates [25]. The preferential wetting behavior can largely be suppressed by the addition of an organic or inorganic layer on the substrate to create a non-preferential surface [26e30]. This type of nonpreferential surface, often referred to as the “neutral” surface, has a surface energy lying between that of the blend components and is thus equally wet by each component. The neutrality of such surfaces has been demonstrated in their ability to control the orientation of block copolymer nanostructures in thin films, thereby enabling lamellar and cylindrical domains oriented perpendicular rather than parallel to the substrate [27,31e37]. For a PS-PMMA block copolymer, a neutral-like surface can be created with a layer of random copolymer of styrene and methyl methacrylate, self-assembled monolayers, an organosilane with controlled surface coverage, or a mixture of PS and PMMA brushes [38].

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Although the morphology of PS/PMMA films after annealing for long times (from several to tens of hours at w170  C) on neutral surfaces have been examined, the pathways by which the morphologies evolve remain unclear particularly at the early and intermediate stages of annealing [38]. In addition, the effect of the blend composition on the evolution of morphology on energetically different substrates is also unclear. Motivated by these questions, we characterized and compared the evolution of morphology of thin PS/PMMA films with varying compositions on both preferential and neutral-like surfaces. The morphological paths were found to be highly dependent on both the blend composition and the substrate surface energy, resulting in a rich set of non-equilibrium, micro and nanoscale morphologies. Understanding and controlling parameters such as blend composition and substrate surface energy may therefore provide the opportunity to spontaneously create a range of spatially correlated micro and nanostructures in thin films of polymer blends. 2. Experimental Monodisperse PS (weight average molecular weight, Mw,PS ¼ 48.1 kg/mol; polydispersity index, PDI ¼ 1.01; Scientific Polymer Products Inc.) and PMMA (Mw,PMMA ¼ 21.4 kg/mol; PDI ¼ 1.07; Polymer Source Inc.) were used as received. The glass transition temperature (Tg) of PS and PMMA was determined to be, Tg,PS ¼ 98  C, and Tg,PMMA ¼ 125  C, from the second scan of differential scanning calorimetry (NETZSCH DSC 204 F1) with a scan rate of 20  C/min. Two types of surfaces on Si substrates were prepared for the study: a native SiOx layer and a random copolymer (RCP) layer. For the native SiOx surface, the Si substrates were treated in piranha solution (a 70:30 solution of concentrated sulfuric acid:hydrogen peroxide by volume) at 80  C for 30 min, and rinsed with DI water. Further, some of these piranha-cleaned substrates were coated with a layer of RCP (poly(S-r-MMA-r-GMA)) with a composition of styrene (S, 61 wt.%), methyl methacrylate (MMA, 35 wt.%), and glycidyl methyl methacrylate (GMA, 4 wt.%) [32]. The RCP was synthesized by free-radical polymerization at 80  C for 72 h using an AIBN initiator and the desired ratio of monomers after purification on a basic alumina column and degassing in N2. The RCP product was recovered and purified by precipitation in methanol and subsequently dried under vacuum. The RCP was first spuncaste onto the silicon substrates from a dilute solution (0.03 wt.% in toluene), and then crosslinked into a dense layer after 5 h annealing at 170  C in a vacuum oven. Uncrosslinked chains of RCP were subsequently removed with toluene. PS/PMMA blend films with thickness of w50 nm were spun-cast from a toluene solution (1 wt.%) onto both surfaces described above. The as-cast films on both surfaces were then dried at 60  C for 2 h under vacuum to remove residual toluene. The SiOx surface is preferentially wet by PMMA but not PS [39], while the RCP surface is either neutral to both polymers or slightly favorable to one of them depending on the blend composition [31]. For each surface, PS/PMMA blends with three compositions, 60/40 (the weight fractions of PS and PMMA are 0.6 and 0.4, respectively), 67/ 33, and 80/20, were examined. These blends are referred to as S60M40, S67M33, and S80M20 in the remaining text for convenience. All the blend films were annealed at 160  C on a homebuilt hotstage (calibrated with 7-point standard materials from Aldrich) for different durations, and their surface morphologies were determined by atomic force microscopy (AFM, DI3100, Bruker) and optical microscopy (OM, Nikon LV150). AFM measurements were operated in the tapping mode under ambient conditions using silicon cantilever probe tips (Veeco, RTESP) with spring constants

ranging between 20 and 80 N/m as specified by the manufacturer. To further distinguish the PS and PMMA phases, AFM measurements were carried out on the blend films after selectively removing the PS (or PMMA) by immersing the samples into cyclohexane (or glacial acetic acid) at ambient conditions for 2 h. The low solubility of PMMA (or PS) in cyclohexane (or acetic acid) limits its dissolution, which keeps the PMMA domains intact [40,41]. However, slight swelling of the remaining structures during the selective dissolution is possible [20]. 3. Results and discussion 3.1. Morphological evolution of PS/PMMA films on SiOx surfaces Fig. 1 shows the time-dependent morphological evolution of the S60M40 film on a SiOx surface. The as-cast film displayed dome-like domains with an average diameter of 200 nm and height of 10 nm across the film surface (Fig. 1a). After selective removal of PS (inset of Fig. 1a), PMMA cylinders with an average height of 36 nm were identified. In addition, AFM revealed nanoscale roughness (with an amplitude of a few nanometers and a lateral length scale of w100 nm) at regions between the PMMA cylinders, indicating that there was a PMMA wetting layer because the SiOx surface would be significantly smoother. After selective dissolution of the PMMA domains, holes were left in the films, indicating that a majority of the PMMA cylinders were exposed to air, consistent with previous reports [42,43]. Note that all the as-cast film morphologies are kinetically trapped, non-equilibrium states that will evolve toward more stable states upon annealing. In general, the rate of the evolution depends on the film thickness, composition, thermodynamic driving forces, and the viscosity of the constituents. Upon annealing, the elevated PMMA domains quickly sank into the film leaving depressions at the film surface, and larger-scale height fluctuations across the film surface took place, forming valleys up to 28 nm deep (as marked in Fig. 1b). This capillary fluctuation was highly correlated with the PMMA domain coalescence, with the coarsening occurring predominately at the valleys of the capillary wave (inset of Fig. 1b). Such a phenomenon may be attributed to the mass flow caused by surface fluctuations; the most significant lateral flow occurs at the valleys of the surface waves, which facilitates the coalescence of the PMMA domains. Such a phenomenon was also observed for the other films as discussed later. The amplitude of the capillary fluctuation increased over time, leading to a highly correlated ridge-like PS structure after 5 min (Fig. 1c) that broke up into isolated PS droplets after 20 min (Fig. 1d). Accordingly, the correlation length of the surface structures (lc) grew with the annealing time (ta, in seconds) as lc [mm] w 0:24 ta0:32 , as shown in Fig. 2. Such a growth rate is characteristic during the intermediate stage of phase separation and wetting of thin blend films [16]. During this period, the PMMA evolved into a continuous layer covering the entire SiOx surface (inset of Fig. 1e). The RMS roughness of the film surface as a function of annealing time for all the systems is shown in Fig. 2b. From 20 min up to 500 min, there was no evident change in the film morphology (and surface roughness) featuring highly correlated PS droplets on top of the PMMA layer (FFT image shown in inset of Fig. 1e). Such a phase inversion process shown in Fig. 1, i.e., PS evolving from forming a continuous matrix into isolated domains, is consistent with the report of the relief structure of a PS/PMMA blend with 70 wt% PS on a glass substrate [22]. This morphology is distinctive from the randomly-distributed polymer droplets that result from dewetting via random nucleation processes [44e46]. Compared to the S60M40 film, the as-cast S67M33 film contained a smaller number of PMMA cylinders sitting on a thin PMMA

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Fig. 1. Topographic AFM images (aed) and optical micrograph (OM) (e) of S60M40 on native SiOx, after annealing at 160  C for different durations as labeled. The lower right insets in (aee) are the topographic AFM images of the sample after selective removal of either PS (aed) or PMMA (e). The lower left insets in (cee) are the FFT images of the AFM image or OM (not to scale). (f) Schematic illustration of the early stages and evolution of the blend morphology.

wetting layer on the SiOx (Fig. 3a). When annealed at the same temperature, the morphological evolution of the S67M33 was similar to that of S60M40: coalescence and downward migration of the PMMA domains, accompanied by the surface capillary wave, drove the PS into highly correlated droplets on top of a continuous PMMA layer (Fig. 3b). Both the correlation length and average diameter of the PS droplets in the S67M33 film were larger than those in the S60M40 film (Fig. 1e versus Fig. 3b). Quantitatively, the number of the PS droplets over a 280 mm  200 mm surface area for S60M40 and S67M33 were 7200 and 3800, with average diameters of 1.4 mm and 2.3 mm, respectively. The differences can be rationalized by the observation that the coarsening of PMMA domains occurred at the valleys of the capillary fluctuation, i.e., fewer PMMA domains correspond to a longer capillary wavelength. Interestingly, morphological evolution in the S80M20 film was clearly different from that of the S60M40 and S67M33 blends. The as-cast S80M20 film showed a bi-layer-like configuration (PS on PMMA) with a rough interface (Fig. 4a). Upon annealing, the upper PS layer dewetted from the thin underlying PMMA layer via a conventional nucleation and growth (NG) mechanism (Fig. 4bed). After a relatively long incubation time (20 min), holes were randomly nucleated across the PS layer (Fig. 4b) [47], which was followed by the formation of PS stripes due to the impingement of holes (Fig. 4c) and the final breakup of these stripes caused by the capillary instability (Fig. 4d). All these observed morphologies are

1

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0

-1 2

10

The poly(S-r-MMA-r-GMA) random copolymer-treated substrate, as described in the experimental section, may be regarded as a “neutral” surface to PS-b-PMMA block copolymers or PS/ PMMA blends with a specific composition [32]. The RCP surface

b

S60M40 on SiOx S60/M40 on RCP S67M33 on RCP (surface) S67/M33 on RCP (interface) S80/M20 on RCP

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3.2. Morphological evolutions of PS/PMMA films on neutral-like surface

Surface Roughness (nm)

Correlation Length ( µ m)

a

similar to those characterized previously in planar PS films dewetting from a PMMA surface [44,48e50]. In contrast to the blends with lower concentrations of PS, the PS droplets in the annealed S80M20 film were not spatially correlated due to random nucleation dewetting (FFT image, inset of Fig. 4d). Note that the thickness of a PS layer, if completely segregated on top of the PMMA, would range from 30 to 40 nm for the three PS/ PMMA films. At this thickness range, NG is known to be the dominant mechanism for PS to dewet from the PMMA surface [51]. For the S60M40 and S67M33 films, however, surface capillary fluctuations associated with coarsening of the PMMA domain occurred quickly before the bi-layer configurations necessary for the standard NG mechanism could develop. However, regardless of the spatial correlations, the surface relief structure, with isolated PS droplets on top of PMMA wetting layer, can be regarded as the “final” or later stage morphology on SiOx surface, which is stable over more extended annealing.

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S60M40 on SiOx S60M40 on RCP S67M33 on RCP S80M20 on RCP

30 20 10 0

102

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Fig. 2. (a) Lateral domain correlation lengths, and (b) RMS surface roughness, of thin PS/PMMA films as a function of annealing duration, for a varying PS/PMMA composition.

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Fig. 3. (a) Topographic AFM images of as-cast S67M33 on a native SiOx layer. The inset is the topographic AFM image of the as-casted film after the selective removal of PS. (b) Optical micrograph of the S67M33 after annealing at 160  C for 100 min. The inset is the FFT image of the optical image.

treatment provides a surface energy that is intermediate to that of PS and PMMA, with the surface energy being tunable based on the ratio of styrene and methyl methacrylate units in the random copolymer. The degree of neutrality of the RCP-treated substrate also depends, however, on the composition of the overlying block copolymer or blend films [31,32]. Therefore the RCP-treated substrate used here, with 61 wt% PS and 39 wt% PMMA, might

not be truly “neutral” to all of the PS/PMMA blends that are examined. Nevertheless the relative extent of the preferential interactions between the blend domains and the RCP-treated substrate will be greatly modulated in comparison to those interactions on the SiOx surfaces. The as-cast morphology of the S60M40 and S67M33 films on the RCP surface was surprisingly similar to those on the SiOx surface

Fig. 4. (a) Topographic AFM image of as-cast S80M20 film on a SiOx surface. The inset in (a) is the topographic AFM image of the as-casted film after the selective removal of PS. (bed) Optical micrographs of S80M20 after annealing at 160  C for different tas, showing: (b) random nucleation of holes in the top PS layer, (c) growth and impingement of the holes, and (d) formation of isolated PS droplets from the breakup of the impinged PS lines. Inset in (d) is the FFT of the optical image. The sizes of these optical images are 400 mm  400 mm.

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Fig. 5. Topographic (a) and phase (bef) AFM images of S60M40 on the random copolymer-treated substrate after annealing at 160  C for different tas as labeled. The lower right insets are the corresponding topographic AFM images after the selective removal of either PS (aee) or PMMA (f). The lower left insets in (cee) are the FFTs of the corresponding lower right insets (not to the scale).

(compare Fig. 5a versus Fig. 1a and Fig. 6a versus Fig. 2a). All samples showed PMMA wetting layers and cylindrical domains with areal densities that decreased with a decrease in PMMA concentration. The as-cast morphology of the S80M20 blend on the RCP surface (Fig. 7a) largely resembled that on the SiOx surface (Fig. 4a) in that both showed a bi-layer-like morphology, but

a major difference was that the S80M20 blend on RCP surfaces exhibited a rougher interface between the PS and PMMA domains. We also examined PS/PMMA films 75 nm and 150 nm in thickness and with similar compositions but different molecular weights, which further confirmed that the morphology of as-cast PS/PMMA films was nearly identical on SiOx and RCP surfaces. Some studies in

Fig. 6. Topographic AFM images of S67M33 on the random copolymer-treated substrate after annealing at 160  C for different tas as labeled. The lower right insets are the corresponding topographic AFM images after the selective removal of either PS (aee) or PMMA (f). The lower left insets in (c and d) are the FFT images of the corresponding lower right insets (not to the scale).

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the literature have reported that the as-cast morphology of PS/ PMMA films on SiOx and gold surfaces was identical [24], whereas other studies have shown different as-cast morphologies [39,52]. It is therefore suggested that the morphology of the as-cast blend film is relatively independent of the substrate surface energy and highly sensitive to the spin-coating and thin film processing conditions [40,53]. Upon annealing at 160  C, PMMA domains in the S60M40 blend film started to coalesce, while PS penetrated the PMMA wetting layer, as verified by AFM characterization after removing the PS domain (Fig. 5b). From then on, both the PS and PMMA domains were continuous throughout the thickness of the film and contacted the RCP-treated substrate and the free surface (Fig. 5bee). The morphology showed high spatial correlation (FFT image, inset of Fig. 5c), and the correlation lengths between PMMA phases (lc,PMMA) increased from w400 nm at 2 min to w1.4 mm at 100 min (Fig. 5bef), following lc,PMMA [mm] w 0:087 ta0:32 , consistent with the characteristic growth rate of intermediate stage of the morphological evolution in thin blend films (Fig. 2) [16]. Clearly, the characteristic power-law growth behavior (with an exponent

w1/3) remains independent of substrate surface energy, despite the strongly different morphology involved. During annealing from 2 to 200 min, the areal coverage of the PS phase on the RCP-treated substrate remained around 50e55%, as estimated from the AFM images (over a 10 mm  10 mm area) after selective removal of PS. Since the coverage was lower than its volume fraction (w60% as PS and PMMA have similar densities), the vertical thickness of PS domains was thicker than that of the PMMA domains. The fact that both PS and PMMA domains in the S60M40 film form stable contacts with the RCP surface during annealing confirms that the RCP is indeed effectively non-preferential to both polymers. This observation is in stark contrast to the morphological evolution of the S60M40 blend on a SiOx surface (Fig. 1), where the preferential wetting of PMMA led to a completely different morphological path. The PS phase in the S67M33 blend film also quickly penetrated the PMMA wetting layer to contact the RCP surface (inset of Fig. 6b). At the film-air surface, large-scale fluctuations in thickness started to arise (Fig. 6c and d). Both the PS/PMMA morphology at the RCP surface (insets of Fig. 6c and d) and at the free surface (Fig. 6c and d)

Fig. 7. Topographic (a, e) and phase (bed and f) AFM images of S80M20 on the random copolymer-treated substrate after annealing at 160  C for different tas as labeled. The lower right insets are the topographic AFM images after the selective removal of either PS (aec, e, and f) or PMMA (d). (g) and (h) are the optical images of the film after 200 min and 1000 min, correspondingly.

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were spatially correlated with distinctive length scales. From the FFT of the AFM images after removing the PS domain (insets of Fig. 6d), two correlation lengths were observed: lc1 w 330 nm corresponding to the morphology at the substrate surface and lc2 w 1.8 mm for the morphology at the free surface. At the early stages of the evolution of the phase-separated morphology, both structures have similar power-law dependences of lc1 [mm] w 0:056 ta0:31 and lc2 [mm] w 0:27 ta0:33 (Fig. 2). As shown in Fig. 2, the correlation length of the S67M33 morphology at the RCP interface was similar to that of the S60M40 morphology at the RCP interface, while the correlation length at the free surface was comparable to that observed in the S60M40 blend on the SiOx substrates. From the inset of Fig. 6d, it is clear that the PMMA domain coarsening occurred at the valleys of the capillary fluctuations. After 20 min, ridge-like PS domains developed while the PMMA evolved into a continuous domain (Fig. 6e). Small but appreciable PMMA domains remained embedded in the PS phase under the ridges of the capillary fluctuations. Finally, the PS ridges broke up into isolated droplets after 100 min due to the capillary instability (Fig. 6f). In addition, holes were nucleated in the PMMArich phase in the S67M33 film, as highlighted in Fig. 6f, suggesting that the RCP surface is not preferentially wet by PMMA at least for ultrathin PMMA layers (10e20 nm in Fig. 6f). Fig. 7 shows the morphological evolution of S80M20 on the RCP surface. The as-cast film displayed a corrugated PS/PMMA interface with a correlation length of lc ¼ 210 nm (Fig. 7a). According to AFM, the amplitude of the interfacial corrugation was as high as 20 nm, while the roughness at the free surface was less than 2 nm. After annealing for 1 min, isolated PMMA domains were observed with diameters ranging from several nm to w100 nm (Fig. 7b). The larger PMMA domains (w100 nm) were likely to evolve from the breakup of the PMMA-rich domain in the as-cast film, since their diameter was comparable with the width of the continuous PMMA phases in Fig. 7a. In contrast, the smaller ones (10 nm or less) likely resulted from a secondary phase separation process via a nucleation and growth mechanism from the PS-rich phase, within which the effective PMMA concentration was significantly lower than 20% and thus lies in the metastable region of the PS/PMMA phase diagram. These PMMA domains became larger through coalescence, which led to depth-through PMMA cylinders with diameters of 100  50 nm that were well-dispersed within the PS matrix film (Fig. 7c). This morphology enabled the formation of free-standing PMMA pillars (or depth-through holes) with a height (or depth) of 50 nm, as demonstrated in the inset of Fig. 7c (or d). Both the height of the pillars and the depth of the holes were identical to the film thickness. Note that the topographic AFM images of the films after removing PS (insets of Fig. 7b and c) showed larger PMMA domains compared to those in the phase image (Fig. 7b and c), which is most likely due to convolution of the probe tip with the PMMA features. In comparison, the size of the holes left after PMMA removal in the inset of Fig. 7d appeared similar to that of the phase image (Fig. 7d). The depth-through PMMA cylinder morphology was established within 2 min and remained stable up to 10 min. Capillary fluctuations started to evolve on the S80M20 surface after 20 min and led to a characteristic ridge and valley structure by 100 min (Fig. 7e). Similar to that observed in Figs. 1 and 6, the PMMA domain coarsened mostly at the valleys of the capillary fluctuation (Fig. 7e), while the rest of the PMMA cylinders remained within the PS-rich domain. Prior to the complete breakup of the PS droplets, the correlation length of the surface structure was lc [mm] w 0:38 ta0:29 (Fig. 2). In addition, holes within the PMMA-rich phase were observed and the bare RCP surface was exposed as marked in Fig. 7e, similar to that observed in Fig. 6f. This resulted in the formation of intriguing partial coreeshell droplets, composed

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Fig. 8. Schematics of the later stage morphology after annealing on RCP surfaces for all three blend compositions.

of a PS-rich inner core and PMMA-rich outer shell (Fig. 7f), that were highly correlated in space (Fig. 7h). Fig. 8 schematically summarizes the later stage morphology of all three blend films after annealing on RCP surfaces. 4. Conclusions To summarize, we systematically investigated the evolution of micro and nanostructured morphologies in thin films of PS/PMMA blends as a function of the blend composition and the surface energy of the substrate. For thin films of the three PS/PMMA blends on SiOx surfaces, PMMA evolved into a continuous layer that preferentially wet the SiOx surface, while PS droplets formed on top of the PMMA layer either driven by capillary fluctuations (S60M40 and S67M33) or a random nucleation process (S80M20). Nonpreferential wetting was observed for the PS/PMMA blends on substrates treated with a random copolymer with a surface energy intermediate to that of PS and PMMA. Both PS and PMMA formed contacts with the substrate and highly correlated phase structures were obtained. Capillary fluctuations, correlated with the PMMA domain coalescence, also dominated the intermediate stage of the morphological evolution by forming PS-rich ridges and PMMA-rich valleys and thus the resulting isolated PS-rich structures were spatially correlated. It was observed that both the PS and PMMA domains formed direct contact with the RCP surface during the evolution of the morphology suggesting that the surface was indeed neutral-like to both polymers. For both the S67M33 and S80M20 polymer blends, holes in the PMMA phase were observed which further confirms that the RCP surface, unlike the SiOx surface, was not a uniformly wettable or preferential surface for the PMMA component. Because the thickness of the PMMA phase on the RCP surface decreased with its concentration in the blend, more dramatic dewetting of PMMA was observed for the S80M20 film than the S67M33 film. Similarly, no holes were observed in the PMMA layer for the S60M40 blend. In addition to the 50 nm films presented here, we have also examined PS/PMMA blend films with larger molecular weight (slower kinetics) and in thicker films (75 nm and 150 nm). The morphological behavior of these other systems are consistent with the observations for the 50 nm films, in the sense that both the PS and PMMA domains contact the RCP surfaces in the annealed films. For the thicker films that were studied, no dewetting in the PMMA phases were observed. To the best of our knowledge, this is the first systematic experimental comparison on the phase evolutions of thin blend films on non-preferential and preferential surfaces. The unique non-equilibrium morphologies that are

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