Irreversible Physisorption of PS-b-PMMA Copolymers on Substrates

Apr 16, 2019 - ABSTRACT: We present a direct approach to fabricating the perfect neutral layer for block copolymer (BCP) self-assembled thin films...
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Letter Cite This: ACS Macro Lett. 2019, 8, 519−524

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Irreversible Physisorption of PS‑b‑PMMA Copolymers on Substrates for Balanced Interfacial Interactions as a Versatile Surface Modification Wooseop Lee,† Yeongsik Kim,† Seungyun Jo,† Seongjin Park,† Hyungju Ahn,‡ and Du Yeol Ryu*,† †

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Korea Pohang Accelerator Laboratory, Pohang University of Science and Technology, 80 Jigok-ro, Nam-gu, Pohang 37673, Korea

‡

ACS Macro Lett. Downloaded from pubs.acs.org by ALBRIGHT COLG on 04/18/19. For personal use only.

S Supporting Information *

ABSTRACT: We present a direct approach to fabricating the perfect neutral layer for block copolymer (BCP) self-assembled thin films. An irreversible physisorption of polystyrene-bpoly(methyl methacrylate) (PS-b-PMMA) itself onto the bottom substrates, though extremely thin, offers such a compositional randomness for the substrates, directing the balanced interfacial interactions toward both blocks of the top-layer PS-b-PMMA. Owing to the neutral property from the skin layers, the chemically identical BCP self-assembles into perpendicular microdomains on the adsorbed layer composed of itself, as identified in symmetric PS-b-PMMA films. Intriguingly, the compositional randomness turns out to be valid when the correlation length (ξ) of the BCP layer adsorbed on the substrates is shorter than equilibrium lamellar spacing (L0) of the BCP. Our strategy provides its versatility applicable to various substrates without any necessity of specific random copolymer brushes or mats, enabling the design of a neutral platform for PS-b-PMMA films. olymer chain adsorption onto impenetrable flat substrates has been studied extensively in both theoretical and experimental points of view because the skin layer can tune over interfacial energy on the substrates. Such a phenomenon is practically important due to its applicability to the slip and wettability controls at the interfaces.1,2 Early studies put emphases on understanding the adsorbed chain conformation in equilibrium,3−8 although real systems show deviations from such an assumption.9−13 Granick and co-workers developed the adsorption kinetics for a dilute polymer solution, providing the idea of nonequilibrium, time-dependent chain adsorption structure to the solid substrate.14−20 Later, an irreversible physisorption of polymer melt chains to impenetrable substrates poses a great impact on the surface modification as a unique and feasible approach.21,22 Guiselin suggested a theoretical concept that tightly adsorbed polymer melt chains are unveiled by good solvents, and the loop-andtail conformations of chains are analogous to pseudobrush (or Guiselin brush).23 Such molecular interactions, including hydrogen bonding, van der Waals force, or dipolar interactions, facilitate the favorable conformations that maximize the contact area between the chain segments and substrates to overcome the loss in conformational entropy.6,21,24 A diffusioncontrolled adsorption process is immediate at the interfaces in contact with polymer melt chains due to the negligible energy barrier, and it can be further expedited by thermal annealing above glass transition temperature (Tg).22,24−26

P

© XXXX American Chemical Society

Recently, Koga and co-workers have reported the chain conformation of polystyrenes on a preferential substrate during irreversible adsorption.27 They also proposed the formation mechanism of adsorbed homopolymer layers to the substrates, where the thickness of the adsorbed layer approaches a plateau value of a quasiequilibrium state with increasing annealing time.28,29 A comprehensive description of the thermal adsorption process with polymer melts indicates that the early arriving chains form a tightly adsorbed (high-density) flattened layer, followed by an overlying loosely adsorbed layer from the late-arriving chains on the outermost skin.29 Interestingly, the studies demonstrate the flattened conformation of block copolymers (BCPs) on the substrates, where the constituent block chains are physically adsorbed side-by-side to generate a 2D percolating network structure no matter how preferential or nonpreferential it is to the substrates.30 It should be noted that this result is counterintuitive because it had been believed that the affinitive block chains strongly congregate near the substrate and the nonattracting block chains were relatively free from the substrate.8,31−35 On the other hand, a neutrality approach has an impact on directed self-assembly of BCPs to alleviate the energetic preference to the substrates, which has been developed using a Received: January 15, 2019 Accepted: April 16, 2019

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DOI: 10.1021/acsmacrolett.9b00036 ACS Macro Lett. 2019, 8, 519−524

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annealed at 190 °C under vacuum to generate the adsorbed layers onto the substrates, and the films were leached in toluene to remove the unadsorbed chains, as described by an irreversible adsorption in Figure 1a. Subsequently, the SM93 films applied to various adsorbed layers were annealed at 190 °C under vacuum for a day to ensure thermal equilibrium selfassembly. Figure 1b depicts 2D and 3D schemes of an adsorbed BCP layer to illustrate the correlation length (ξ) between two different phases composed of PS and PMMA and an adsorbed layer thickness (had), respectively. The compositional randomness was evaluated in terms of ξ as a function of Nad (degree of polymerization) of the adsorbed BCPs, where the sample codes are designated on the x-axis. The had of SM20, 42, 93, and 232 significantly increases at an early stage (∼7 h) during thermal annealing for 48 h, then approaches different plateau values (had*) on the native (2 nm) oxide and H-passivated Si substrates (Figure S1). Each quasi-equilibrium plateau (had*) is taken as the values of had measured at 24 h. A logarithmic plot of had*, as a function of Nad, is displayed in Figure 1c, of which the linear fit slope (x) indicates the scaling exponent in had* ∼ Nadx. The BCP chains adsorbed on H-passivated Si substrate reveals x = 0.37 akin to the homopolymer adsorption behavior having x = 0.38,43 because the substrate attraction is similar to both blocks, though the affinities are not strong. Meanwhile, the native oxide substrate more favorable with the PMMA block exhibits stronger Nad dependence, leading to a significant increase in x = 0.68.21,44,45 Here, we emphasize that the physically adsorbed layers on H-passivated Si are mainly

random copolymer brush or mat.36−38 Such morphologies find the lamellae and hexagonally packed cylinders oriented perpendicular to the substrate, although these methods demand specific functionalities to create a covalent tethering or cross-link.39−42 Inspired by an idea that the irreversibly adsorbed layers of BCPs to the substrates can serve as a random interlayer toward each block, we suggest a simple and feasible method for neutral substrates to achieve the perpendicular lamellae based on the compositional randomness from the underlying adsorbed BCP layer. Using a series of symmetric polystyrene-b-poly(methyl methacrylate)s (PS-b-PMMAs), as listed in Table 1, we Table 1. Sample Characteristics of PS-b-PMMAs sample

Mn (kg/mol)

Đ = Mw/Mn

ϕPS

L0,bulk (nm)

SM20 SM42 SM93 SM232

20.3 42.1 93.5 232

1.06 1.06 1.05 1.05

0.5 0.5 0.5 0.5

16.5 (disordered) 24.5 43.0 85.8

evaluated interfacial property of BCP sublayer in response to self-assembly of overlying BCP films having the same chemical identity, as depicted in Figure 1a. Sample codes were named after their constituent monomers (S and M denote styrene and methyl methacrylate, respectively) and the overall molecular weights (Mn), where the L0,bulk is an equilibrium lamellar spacing (or d-spacing) in the bulk state (measured in 4C beamline of Pohang Accelerator Laboratory (PAL), Korea). First, the thin (less than 30 nm) BCP films were thermally

Figure 1. (a) Schematic description of the irreversible adsorption of PS-b-PMMA and the self-assembly of the overlying PS-b-PMMA thin film. (b) 2D and 3D schemes of an adsorbed layer. (c) The plateau value (had*) of the adsorbed layer as a function of Nad, where the linear fit slope (x) indicates the scaling exponent in had* ∼ Nad x between the native (2 nm) oxide and H-passivated Si substrates. 520

DOI: 10.1021/acsmacrolett.9b00036 ACS Macro Lett. 2019, 8, 519−524

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Figure 2. (a−d) SFM phase images of the adsorbed layers from SM20, SM42, SM93, and SM232, where the color mapping and the phase scale range were customized for better display. (e) Intensity profiles line-cut along the horizon of αf = 0.134°, where an inset demonstrates a 2D GISAXS pattern of SM93 adsorbed layer and the linecut trace. The best-fit curves using Debye−Bueche model are superimposed on the experimental results of SM20, 42, and 93, while the SM232 exhibits a primary peak. (f) ξ or d-spacing of the adsorbed layers as a function of Nad.

discussed in this study in order to ensure enough had for wide range of Nad. Figure 2a−d are scanning force microscopy (SFM) phase images of the adsorbed layers of which the color mapping and the phase scale range were customized for better display. The low value of phase (red to green color) visualizes PS domains, whereas the high value (pink to violet color) corresponds to PMMA domains. As the molecular weight (Mn,ad) of the adsorbed BCPs increases from 20 to 93 kg/mol, the microphase aggregates of the adsorbed layers gradually expand over the surfaces and their interfacial boundaries between two domains are still unsettled, being considered as a random segregation. However, when the Mn,ad increases to 232 kg/mol, the characteristic phases occur due to strong segregation power, displaying a sudden increase in domain size. This tendency was further analyzed by grazing incidence small-angle X-ray scattering (GISAXS) at an incidence angle (αi) of 0.120° above the critical angle (0.114°) of PS-b-PMMA adsorbed layers, which was were carried out at 9A beamline of Pohang Accelerator Laboratory (PAL), Korea. Figure 2e shows in-plane linecuts along the horizon of αf = 0.134° from 2D GISAXS patterns, in which the scattering intensity was calibrated by subtracting the background scattering. An inset displays a 2D GISAXS pattern of SM93 adsorbed layer and a linecut trace (as the red dashed line). Intriguingly, no discernible peaks along the in-plane direction are identified in the adsorbed layers of SM20, 42, and 93 so that they can be classified into the diffuse scattering regime, rather than the

Bragg scattering associated with strongly segregated periodic structures.46,47 Such systems are considered as nonparticulate two-phase systems of which the two inhomogeneous phases (i.e., PS and PMMA) are mixed irregularly but isotropically, which was well described by Roe.48 To explain this feature, we adopted the Debye−Bueche model,30,49,50 enabling us to calculate ξ by fitting the scattering intensity (I(q)) using the following equation: I(q) =

Cξ 3 (1 + ξ 2q2)2

(1) −1

where q and C are the scattering vector (in nm ) and a constant, respectively. Assuming a two-phase system consisting of PS and PMMA having a 3D unit region with volume (V) and cross-section (S), the average distance (V/S) between the two phases is expressed with ξ by

V ξ = S 4ϕ1ϕ2

(2)

where ϕi represents the volume fraction of the i-component (described in more detail in Figure S2). In a symmetric PS-bPMMA particularly with ϕPS = ϕPMMA = 0.5, the ξ (equal to V/ S) defines the compositional randomness of our adsorbed layers. As shown in Figure 2e, the best-fit curves using eq 1 are superimposed on the experimental results of SM20, 42, and 93, indicating a valid correlation with ξ in a q-range of 0.025− 0.300 nm−1. By contrast, the SM232 exhibits a primary peak (q* = 0.095 nm−1) arising from the Bragg rod, which 521

DOI: 10.1021/acsmacrolett.9b00036 ACS Macro Lett. 2019, 8, 519−524

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Figure 3. (a−d) 2D GISAXS patterns of SM93 films supported on various adsorbed layers of SM20, SM42, SM93, and SM232. The inset SFM phase images represent that the lamellar microdomains are oriented normal to the substrates. (e) Thickness window of perpendicular lamellae (of the overlying SM93) on various adsorbed layers and the normalized lamellar spacings (D/L0,SM93).

Figure 4. SFM phase images of various substrates (mica, glass, gold, and deposited aluminum) and SM93 thin film (∼40 nm) without (Bare) and with (Adsorbed) the irreversible adsorption of SM93 itself onto such substrates. The insets display the WCA images on each bare and adsorbed substrate.

corresponds to a distinct microphase separation having a specific d-spacing (d) of 66.1 nm by d = 2π/q*. Figure 2f shows ξ or d-spacing as a function of Nad (or Mn,ad). The values of ξ from the adsorbed layers for low-to-moderate Nad (SM20, 42, and 93) are smaller than L0,SM93 (43.0 nm) of SM93 (indicated by a red line), while an open symbol representing a d-spacing of SM232 is far above the L0,SM93. As a result, the adsorbed layers under the condition of ξ ≤ L0,SM93 would guarantee the compositional randomness toward SM93 to mediate the perpendicular lamellae of the overlying BCP films. To confirm our hypothesis on the compositional randomness, the SM93 films were applied onto the adsorbed layers

from SM20, 42, 93, and 232, followed by thermal annealing at 190 °C under vacuum. Figure 3a−d display 2D GISAXS patterns at αi = 0.140° and SFM phase images (insets) for ∼35 nm thick SM93 films supported on various adsorbed layers. As seen in Figure 3a−c, the consistent Bragg rods (q* = 0.146 nm−1) along an out-of-plane direction, corresponding to a film lamellar spacing (D) of 43.0 nm, represent that the lamellar microdomains are oriented normal to the adsorbed layers of SM20, 42, and 93, as also identified in SFM images (insets). The higher-order peak of q/q* = 1:3 coincides with a volumetric symmetry between the PS and PMMA lamellar microdomains, as marked with the black arrows. When 522

DOI: 10.1021/acsmacrolett.9b00036 ACS Macro Lett. 2019, 8, 519−524

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ACS Macro Letters supported on SM232 (Figure 3d), however, the two distinct Bragg rods are observed, as indicated by the blue and red arrows. Considering d = 66.1 nm of the SM232 adsorbed layer alone and L0 = 43.0 nm of bulk SM93, the deflation of SM232 to 62.2 nm (the blue arrow) and the inflation of SM93 to 50.7 nm (the red arrow) indicate the nontrivial association of lamellar spacings between the overlying SM93 and underlying SM232. Such an inflation of the overlying SM93 is identified by the SFM image (the inset of Figure 3d) displaying a larger domain size. Figure 3e shows the continuous thickness range of perpendicular lamellae as a function of Nad (or Mn,ad), in conjunction with the normalized lamellar spacing (D/L0,SM93), to verify the deviation from L0,SM93. It is inspiring that the perpendicular orientation is available in the broad range of film thickness, comparable to those made from the neutral random copolymers.40,51 A narrower thickness window observed in the films supported on SM232 and the inflation in D/L0,SM93 = 1.15 are attributed to the interplay between the overlying SM93 and underlying SM232. Such a tendency confirms our speculation over the compositional randomness of adsorbed layers only under the condition of ξ ≤ L0,SM93. Even cylinderforming PS-b-PMMA films supported on the adsorbed layers of themselves exhibit the continuous thickness range of perpendicular cylinders (Figure S3). Here, we highlight that a single BCP plays a double role of an irreversibly adsorbed layer and the self-assembly into perpendicular microdomains of its own. Figure 4 demonstrates the wide applicability to various substrates such as mica, glass, gold, and aluminum. An SM93 was used for both the adsorbed neutral layer as well as the lamellar pattern formation of the overlying BCP films. The insets show the water contact angle (WCA) before and after adsorption of an SM93, indicating that the surface modification by irreversibly adsorbed BCP layers could remarkably tune the surface property. Even if the WCA significantly varies from 0.0° (on bare mica) to 104.0° (on deposited aluminum), the irreversibly adsorbed layers (using an SM93) prepared on different substrates exhibit a narrow WCA range of 81.9 ± 3.0°, indicating a consistent surface modification, regardless of substrate types. With the overlying BCP films, a perpendicular orientation of lamellar microdomains is successfully achieved in the films supported on the adsorbed layers by the same SM93 itself, whereas on bare substrates, the morphology incurs hole/island and patchy lamellar structures. Along with the consistent results of SM42 films displaying smaller perpendicular lamellae (Figure S4), we put emphasis on the simplicity of our physisorption method because it is far more adaptable to various substrates than polymer brush grafting52,53 or crosslinked random copolymer mat38,54 techniques since it does not require any specific functionality or additives other than BCP itself. To summarize, we suggest an effective protocol to register the neutral interfaces to mediate the self-assembly of symmetric BCP by means of irreversible adsorption of BCP chains themselves onto the substrates. Such a compositional randomness of physically adsorbed BCP layers turns out to exhibit the well-balanced interfacial interactions toward both blocks of the overlying PS-b-PMMA, leading to directing perpendicular lamellae of the top-layer BCP films. Analogous to the random copolymer substrates, our results demonstrate the compositional randomness of adsorbed layers when the correlation length (ξ) of adsorbed layers is shorter than the L0

of the overlying BCP. Furthermore, our strategy proves its versatility on various substrates without any necessity of specific functionality, enabling the design of a neutral platform for other BCP films.

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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/acsmacrolett.9b00036.

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Experimental details and Figures S1−S4 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Du Yeol Ryu: 0000-0002-0929-7934 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the NRF Grants (2017R1A2A2A05001048, 2017R1A4A1014569) funded by the Ministry of Science, ICT & Future Planning (MSIP) and funding (20163030013960) from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE), Korea.

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