Coordinated Responsive Arrays of Surface-Linked Polymer Islands

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Coordinated Responsive Arrays of Surface Linked Polymer Islands – CORALs. Oleg Davydovich, Elza Chu, Zachary Friar, Detlef-M. Smilgies, Preston B Moore, and Alexander Sidorenko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18305 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Materials & Interfaces

Coordinated Responsive Arrays of Surface Linked Polymer Islands – CORALs Oleg Davydovich, ‡1 Elza Chu,1† Zachary Friar,1 Detlef-M. Smilgies,2 Preston Moore,1‡* Alexander Sidorenko1‡* 1

Department of Chemistry & Biochemistry, University of the Sciences, Philadelphia, PA 19104

2

Cornell High Energy Synchrotron Source, Ithaca, NY 14853

KEYWORDS: Grafted polymers, interfaces, block copolymer assembly, molecular dynamics simulations, GISAXS ABSTRACT: The concept of CoOrdinated Responsive Arrays of surface Linked islands (polymer CORALs) is introduced. This study targets a responsive system capable of revealing or covering the substrate surface in response to environmental changes in a reversible way. A convenient method of fabrication of polymer CORALs is proposed. It is based on microphase separation that occurs in thin films of supramolecular assemblies of block copolymers with reactive blocks. Such blocks form nanometer-size domains that may serve as anchors for surfacelinked polymer islands. Two characteristics of the islands are critically important for the switching function: high grafting density within the islands and small lateral separation that allows interactions between polymer chains grafted to neighboring islands. This combination permits complete coverage of the substrate surface upon exposure to a good solvent (relaxed state). In a weak solvent, the chains collapse within the islands thus revealing the substrate (compact state). The morphology of the CORALs in both states and some details of switching process were studied with atomic force microscopy, grazing incidence small angle scattering, and coarse grained molecular dynamic simulations.

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INTRODUCTION The modification of solid surfaces with polymers via chemical grafting is a well-established method of surface modification. As a result, fundamental alterations of interfacial properties, such as adhesion, wetting, surface tension, electrical conductivity, etc. may occur. 1 Alexander and deGennes applied Flory’s classical macromolecular theory to surface-tethered polymer chains in the approximation of strong adsorption and no adsorption, respectively.2,3 The theory predicts three regimes depending on surface grafting density. At low grafting densities, the average coil-to-coil distance d is larger than the Gaussian coil diameter D (d>D), and the polymer chains adopt either mushroom-like (no adsorption) or pancake-like (strong adsorption) conformation.4,5 In the transition state the coils begin to interact when d~D. The polymer brush regime is characterized by strong overlap (D>d) due to high grafting density. The qualitative definition of polymer brushes has been given by Milner in his seminal work: "Polymer «brushes» are long-chain polymer molecules attached by one end to a surface or interface by some means, with a density of attachment points high enough so that the chains are obliged to stretch away from the interface, sometimes much farther than the typical unstretched size of a chain".6 In the overlap regime the height of the coils h scales with the grafting density δ as h ∝ Nδα ,

(1)

where N is polymerization degree and parameter α reflects the interactions between polymer and solvent: α=2/3 in a theta-solvent, α=1/3 in a good solvent, and approaches unity in a nonsolvent.7 The scaling equation expresses the fundamental property of a polymer brush: the sensitivity to the solvent quality at the given grafting density, or in a broader sense, response to the environment.


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Usually polymer brushes are considered by default as homogeneously distributed polymer chains. However, several examples of patterned polymer brushes have been reported using photolithography,8-13 microcontact printing,14-17 colloidal lithography,18-20 dip-pen nanolithography,21 electron beam lithography,22-24 and molecular transfer printing.25 The latter three methods are promising techniques that allow sub-100nm resolution, but low throughput limits their applicability. Recently, the phenomenon of molecular self-assembly was used to form island-like polymer brushes.26 Radical polymerization initiated from the reactive part of a binary self-assembled monolayer (SAM) results in islands of densely grafted polymer chains. This approach is limited by rather irregular sizes and shapes of the islands and their distribution. Here we introduce the concept of Co-Ordinated Responsive Arrays of surface-Linked polymer islands - polymer CORALs. The coordination, i.e. “the organization of the different elements of complex body or activity so as to enable them to work together effectively”,27 is achieved by an optimal neighbor-to-neighbor distance Di-i. The coordinated response of CORALs consists of a sharp switch in the morphology state. In the compact state (CS), polymers collapse upon themselves resulting in isolated islands of densely grafted polymer coils. Upon exposure to a good solvent, the polymer chains adopt a relaxed state (RS) by swelling and interacting with the chains from neighboring islands, thereby covering the entire surface (Figure 1). Therefore, the switch of state of each island is synchronous and coordinated with the neighbors. Upon drying, these CORALs retain their morphology. Both states are observed depending on the solvent and are dynamically captured, or “frozen”, in the glassy state of polymer chains. Apparently, the most favorable state – compact or relaxed - is a result of multiple interactions; the most important among them are polymer-polymer, polymer-solvent, polymer-substrate, and solventsubstrate.


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Microphase separation observed in block copolymers (BC) will produce patterns of domain sizes ranging from 5 to 50 nm and periodicities of 15 to 100 nm.28 This scale is suitable for the synthesis of CORALs, such that Di-i allows for polymer cross-island interactions. However, grafting of BCs typically results in polymer brushes that cover the entire surface.29-31 Although morphology of a typical BC brush may vary depending on the last solvent, it is not as dramatic as CORALs switching from RS to CS and vice versa. To obtain CORALs, we utilize a nongraftable additive that selectively associates with BCs creating supramolecular assemblies of block copolymers (ABCs). Similar to conventional BCs, the domains of ABCs may form spheres, cylinders, or lamellae, but are composed of the minor block of a parent BC and a low molecular weight additive.32-36 Molecules of the additive are associated with the monomers of the minor block via hydrogen bonds32,33 or other non-covalent interactions.34,37,38 As a result, a substantial part of the ABC can be removed by a selective solvent leaving organized arrays of nanoscopic pores. In order to apply this system for synthesis of polymer CORALs we deposited thin films of the ABC onto reactive SAMs.

Figure 1. The concept of polymer CORALs: coordinated switch of surface linked polymer islands from compact state (CS) to relaxed state (RS) occurs in response to a good solvent or vice versa in a weak solvent if optimal neighbor-to-neighbor distance Di-i and island radius Ri are provided.


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RESULTS AND DISCUSSIONS Synthetic approach to polymer CORALs. We used an ABC that consists of strongly asymmetric BC consisting of polystyrene-block-poly(4-vinyl pyridine) (PS: Mn=35,500, P4VP: Mn=4,400) and 2-(4’-hydroxybenzeneazo) benzoic acid (HABA). HABA is in stoichiometric proportion to monomeric units of P4VP of 1:1. HABA is associated with P4VP by hydrogen bonds resulting in cylindrical domains.32 Once deposited, thin films of ABC require annealing to adopt ordered morphologies. Annealing is a process that enables thermodynamic equilibrium for given conditions by exposing ABC samples to the temperature above glass transition. Conventional thermal annealing is not always applicable as it may result in separation of the additive from the BC matrix due to dissociation of hydrogen bonds. In contrast, solvent vapor annealing has been shown to be effective.33-42 Our experiments with the PS-P4VP/HABA ABC have shown that both 1,4-dioxane and chloroform are efficient solvents for vapor annealing. For example, thin (20-80 nm) films of the ABC adopt the conformation of hexagonally ordered cylinders oriented perpendicular to the substrate surface. In this case, one can speculate that P4VP blocks form patches at the interface with the substrate/SAM. Finally, the samples are exposed to vapors of a reagent that covalently links the monomers of the adjacent P4VP domains of the ABC with the reactive groups of the SAM. Pyridyls of the ABC can be effectively linked to pyridyls of the SAM, 4-pyridyltriethoxysilane, via a quaternization reaction using 1,4-diiodobutane (Figure 2). In respect to the SAM, the grafting occurs only in the spots where P4VP is present, i.e. occupied by the minor phase domains. Therefore, the domain distribution at the interphase of the ABC film and the SAM defines the distribution of the PS islands linked to the surface. Even a small fraction of reacted units of P4VP provides substantial grafting density of CORALs. Indeed, a single unit of P4VP block reacted via quaternization reaction secures the entire chain of the BC at the surface. It makes the chosen method of CORALs synthesis fairly simple and efficient.


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Figure 2. Synthetic approach towards CORALs: a film of ABC (a representative AFM topography image of 1x1 µm2 is shown on A, and schematic of microphase separation resulting in cylindrical domains of P4VP+additive is shown on B) is deposited on top of a reactive SAM with pyridyl functions available for linking with pyridyls of ABC via reaction with 1,4diiodobutane (C). Removal of ungrafted polymer results in coral-like surface (a representative AFM topography image of 1x1 µm2 is shown on D). The islands of PS (E, shown in green) are covalently linked to the surface via P4VP-SAM anchors in the spots previously occupied by P4VP+additive domains. Morphology of the polymer CORALs. Removal of all ungrafted components of ABC including HABA from the surface by a non-selective good solvent (e.g. chloroform) results in a relatively flat film of relaxed chains of grafted PS, i.e. RS. Both contact angle Θ measurements (Θ=90.5° vs ΘPS=93°) and AFM (Figure 3C) provide evidence that grafted PS chains completely cover the surface. The films demonstrate similar morphology upon rinse with toluene and 1,4-dioxane (Figure 3T and 3D): r.m.s. roughness of 0.8 nm and 0.5 nm respectively and Θ=90.3° in both cases. The latter two solvents are rather selective for the BC: both are good solvents for PS, and weak solvents for P4VP. (The case of 1,4-dioxane is controversial: it slowly dissolves oligomeric


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P4VP, such as short blocks of the BC.) The higher roughness of the CORALs observed after chloroform (1.6 nm, Figure 3C) can be explained by fast drying of solvent with the consequent water condensation. The integral grafting density (averaged over the entire surface) was measured in the RS using ellipsometry. In a typical experiment, the thickness of grafted PS is approximately 4 to 5 nm in RS. The polymer CORALs can be switched to a CS by transferring the sample from a good solvent to a weak one. In our system, a successful switch occurs from 1,4-dioxane or toluene to methanol. AFM reveals a drastic change in the morphology: polymer chains constrict towards the grafting patch and form isolated islands (Figure 2D, Figure 3M, Figure 4 CS and CSD). As a result, the substantial fraction of the substrate surface becomes uncovered. This lateral distribution of polymer coils opens a new dimensionality to grafted polymer chains of CORALs in comparison to homogeneous polymer brushes: the variables are island radius Ri, closest neighbor-to-neighbor distance Di-i, and fraction of surface covered by the polymer φPS. We observe the following parameters of CORALs in CS: average Ri of 15 nm, average height of 9.5 nm, surface coverage of about 36%, and periodicity Di-i of 35nm.

This set of parameters is optimal for the molecular weight of PS block of the chosen ABC, i.e. 35.5K. If the chains are shorter, they could not reach the neighboring island and form the RS. On the other hand, the CORALs composed of the polymer of a higher molecular weight will saturate the surface being unable to form the CS. These speculations are supported by the simulations as discussed below and illustrated in the Supporting Information Figure S9. In other words, the rational design of the CORALs requires that periodicity is to be commensurate with the polymerization degree of the major block.


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Figure 3. Examples of CORALs in a relaxed state (T, C, D) upon exposure to good solvents: toluene (T), chloroform (C), 1,4-dioxane (D), and in compact state (M, after switch with methanol): topography, tapping mode, 2×2 µm2, height scale 15 nm (left), height profiles (right), and r.m.s. roughness (inserts).


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The latter parameter is directly related to the periodicity of minor block domains that contain reactive groups of P4VP. Depending on fine details of localization of these domains at the substrate surface, the periodicity may vary. Also, the Di-i values were measured using the FFT function of the AFM software. In the case of significant fraction of merged islands the reported Di-i will be overestimated. Fine details of switching and effect of surface-polymer interaction in CS are not in the scope of the present publication; it will get full consideration in the follow-up papers. Using the above data and assuming that the islands are round and flat in shape with a density of ρ=1.05×106 g·m-3 and neglecting any contribution from the P4VP block (which constitutes only ~10% of total volume of the parent BC), we calculated the number of PS chains in one island Ci:

= ℎ

(2)

where h is the islands height, Mn is the number-average molecular weight of PS block, NA is Avogadro’s number. The grafting density δ of the PS chains is the number of grafted chains per surface area: = ℎ

(3)

We found that an averaged δ within a collapsed island is 0.167 nm-2 and the number of chains in one island Ci is about 120. Using the grafting parameters established in the experiments with the ABC: Ci, Ri, and Mn we built a model for molecular dynamics (MD) simulations. We expected to obtain insights into the formation and morphology of polymer CORALs in both RS and CS. Simulations of polymers and polymer brushes have a long history43-45 and are able to test theory and interpret


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experiments.46 Simulations predict positions and velocities of individual particles providing details of the structure and dynamics at the molecular level of the systems. As a result, the relevant microscopic mechanism of particular phenomena can be clarified and/or predicted in a way inaccessible by both theory and experiment.47,48 In particular, previous coarse grained MD simulations have been applied to a polymer brush patterned as an isolated stripe of nanoscale width; the results were successfully confirmed in the experiments.49,50 Herein, we endeavored to use mesoscale simulations that have all the necessary interactions to model polymer CORALs at the molecular level with different polymer-polymer, polymersolvent, polymer-substrate, and solvent-substrate interactions in order to provide molecular insights into polymer CORALs. (See the simulation subsection in the experimental section for technical details.) We used a bead-spring model with harmonic bonds (springs) and LennardJones interaction potential for non-bonded beads. The adequate choice of the Lennard-Jones parameters ɛ and σ (interaction energy and bead radius, respectively) and harmonic parameters r0 and kb (equilibrium distance and bond spring constant, respectively) was confirmed by obtaining the geometric characteristics of the CORALs, such as h, Ci, Ri and Di-i that fit our experimental results. Moreover, we are able to construct a model of polymer CORALs that switch between the CS and RS morphologies. We have observed that there is only a limited interaction parameter space within which both the CS and RS can be observed. Furthermore, the adequate set of geometrical parameters and chain length is also crucial to obtain models of switchable CORALs. Our simulations match the AFM data obtained for PS islands linked to the surface, 120 chains per island. We simulated two islands in a hexagonal pattern spaced 80σ center-to-center apart, with 120 chains/island, 76 beads per chain, and an island radius of 30σ, within the extended chain distance. As noted above, the chain length is to be commensurate with the Di-i to be able to


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switch. Our simulations showed that chains either too short or too long gravitate to only one morphological state, CS or RS respectively (see Supporting Information, Figure S9). Switching of CORALS morphology – MD simulations. We simulated the tethered islands in both good and weak solvent. Once equilibrated we successfully switched the morphology from RS to CS and vice versa by changing the solvent-polymer interaction parameter ε. Once each system is equilibrated (evaluated by a static morphology), we simulated drying the samples by removing the solvent from the simulations instantaneously. Snapshots of our simulations are reported in Figure 4. The simulation results correlate quantitatively with the AFM and GISAXS data as shown below. The elementary input parameters of N, Ci, Ri and Di-i, give rise to morphologies observed in the experiments upon drying with striking similarity. We calculate the similar island heights (32σ, i.e. 18nm) and diameters (70σ, i.e. 38nm) for the CS and a flat surface and thickness (13.5σ, i.e. 7nm) for the RS as seen in the experiments.


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Figure 4. Simulations of CORALs. IS: initial setup of two islands of 120 chains, Ri is the island radius, and Di-i is islands center-center distance. RS: snapshot of equilibrated CORALs in a good solvent, relaxed state; RSD - dried RS. CS: snapshot of equilibrated CORALs in a weak solvent, compact state; CSD - dried CS.


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Beyond reproducing the different morphologies, the model provides insight into the molecular structure and dynamics which is difficult to obtain with experimental methods. The thermodynamic local minima corresponding to the CS and RS can be realized by different processing of the systems. To elaborate, in a weak solvent the islands “ball” up similar to the CS with no island to island contacts. Upon drying (e.g. removing solvent), the islands shrink resulting in even more compact isolated polymer islands. However, in a good solvent the chains of the neighboring islands may overlap and possibly entangle. Drying results in a uniform decrease in thickness maximizing bead-to-bead interactions including chains from neighboring islands. It results in a flat RS layer instead of isolated islands of CS. Both morphologies are local minima, and maximize chain interactions. The nature of polymer CORALs – surface tethered polymer chains – is similar to polymer brushes in synthetic and some other characteristics. However, the characteristics of islands and the exposed substrate between the islands make CORALs unique and distinct from polymer brushes. Specifically, the three-component interface of the polymer at the edge of the island, the substrate surface, and solvent, allow for unique material properties and morphologies. Importantly, large morphology changes in response to stimuli are not possible with polymer brushes. To illustrate some of the differences between brushes and CORALs we have also simulated homogenously grafted polymer brushes. The density profiles φ(z) (i.e. changes of polymer density as a function of distance z from the surface) of conventional polymer brushes is a parabolic function in a theta and a good solvent.6 The height of the brush in different solvents follows the obvious trend: weak solvent < theta solvent < good solvent, due to different degree of polymer swelling (Eq. 1). Our simulations of a conventional polymer brush reproduce this trend as well as the parabolic density profile in a theta and a good solvent. However, the CORALs density profiles are not parabolic nor is the height trend in different solvents (Figure 5). The good solvent gives the lowest height and densest polymer packing profile among the three


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solvents. This is due to the island-like morphology and the substrate/solvent interactions outside the island. Indeed, the polymers tethered near the boundary of the island will lay flat on the surface (instead of “standing up” as in a brush) with a solvent monolayer between the substrate and the polymer. These interactions result in a decrease in height of the islands. In a theta solvent, CORALs swell similarly to a brush but with no sharp polymer/solvent interface. In a weak solvent, the islands collapse on themselves similar to a brush. These are some of the most important distinctions and just one example of numerous significant differences between CORALs and polymer brushes.

Figure 5. Polymer density profiles (from simulations) as a function of distance from the “wall” for weak, theta, and good solvents: polymer brush (left) and isolated islands within CORALs (right). Both have the same local grafting density (δ=0.167nm-2). Monomer packing can be seen against the wall at a distance of σ = 4 as expected. Reversible switching of CORALS – experimental study. The switching capability is an outstanding feature of polymer CORALs. The morphological changes from RS to CS and vice versa are reversible. The result of switching between these extremes can be easily seen with AFM (Figure 6) of dry samples. RS is achieved by immersion in a good solvent, such as toluene or 1,4-dioxane. The CORALs adopt CS conformation in methanol or ethanol. We found that the


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switching can be repeated many times (at least 30 cycles with no degradation) as evidenced in Figure 7. Equally, the switch from dry CS to RS occurs by exposure to a good solvent or even vapors. However, exposure of dry CORALs in RS to a weak solvent has no effect on the conformation. We hypothesize that this conformational stability is due to a web-like film of PS chains linked to neighbor islands. Due to preferred polymer-polymer interactions of chains across the entire surface the CORALs maintain RS conformation even in a weak solvent. Our simulations are consistent with this hypothesis and show that PS chains forming inter-island bridges make about 6200 contacts in a good solvent and 28200 upon drying.

Figure 6. Switch from RS to CS. Upper row: AFM images (tapping mode, topography, 2×2µm2, z-scale 15 nm) of a sample exposed to toluene (A), toluene/methanol 1:1 (B), toluene/methanol 1:3 (C), and methanol (D). Bottom row, E-H: GISAXS images of the same sample exposed to the same solvent sequence. Periodicity (d-spacing) in G and H is 45nm.


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Figure 7. Multiple switching of a CORALs sample from CS to RS up to 30 cycles (60 times) shows no degradation of the CORALs. Switching from 1,4-dioxane (RS) to ethanol (CS).

Some details of the process of RS-CS switching can be obtained by exposure to a sequence of binary solvents with gradual decrease of solvent quality. We obtained a series of AFM images that reveal the evolution of the morphological changes upon immersing the samples in a sequence of toluene → toluene/methanol 1:1 → toluene/methanol 1:3 → methanol. A sample of CORALs was exposed to the solvent of a given composition for 15 minutes, dried by Argon flow, and scanned with an AFM. As illustrated by Figure 6A-D, flat PS film in RS evolve to isolated clusters of CS. First, the depressions in the film are formed upon exposure to toluene/methanol 1:1 binary solvent (Figure 6B). We speculate that it takes place in the locations where the surface grafting is rare or absent, e.g. between the grafting patches. The surface is still occupied by PS as evidenced by contact angle measurements (Θ=91±1°). The same is observed in 1:2 mixture. Abruptly, upon exposure to 1:3 toluene/methanol, a new phase with the characteristic morphology of isolated clusters of CS is formed by a nucleation and growth mechanism (see Supporting Information Figure S2). Yet, some clusters are not separated: the PS


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chains of neighboring islands retain occasional contacts. At this stage, the contact angle Θ drops to 73° that indicates about 0.45 surface coverage (calculated according to the Cassie equation, ΘSAM of the substrate covered by the SAM is 57°). Finally in neat methanol, the clusters of PS chains linked to islands are separated forming the CS conformation (Figure 3M). The surface coverage is about 0.34 as derived from contact angle measurements (ΘCS=70°); it is in good agreement with the value of 0.36 obtained from AFM. The CORALs is a product of self-organization of ABCs in thin films. As such, the characteristic feature of both ABCs and CORALs is periodicity. An advanced method of analysis of such systems is GISAXS. It was introduced in 1989 by Levine at al. 51 Recently, the application of GISAXS to characterize nanostructured polymer films has been reviewed by Mueller-Buschbaum.52 More specifically, a method and a program for GISAXS analysis of surfaces decorated with nanoscale-size islands of different geometry was proposed in Ref. 53 and 54. A particular strength of GISAXS is the opportunity to perform the experiments in-situ and in real time.41 We performed a series of GISAXS measurements of CORALs samples that match the binary solvent compositions used to observe the RS-CS switch with the AFM. It allowed characterization of the surface morphologies on macroscopic surface area, as opposed to the local surface characterization by AFM. Figure 6 (bottom row, E-H) shows the set of GISAXS images of the same sample of CORALs. It can be clearly seen that the signs of lateral separation can be observed upon exposure to toluene/methanol 1:3 and further develop after neat methanol. Exposure to methanol reveals the features of CORALs in CS: a lateral separation of 45 nm and a uniform height of the clusters. Moreover, the extended form factor scattering is indicative of a relatively uniform shape of the clusters. More GISAXS results on similar CORALs samples are provided in the Supporting Information, Figures S4 and S5, as well as results of modeling of GISAXS images using IsGISAXS (S8).


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The surface coverage in RS and CS was further studied by Cyclic Voltammetry (CVA). We used ITO glass as the electroconductive substrate. We successfully deposited pyridyl SAM and fabricated CORALS on ITO samples according to the procedure identical to those described for Si wafers. Potassium hexacyanoferrate(III) (1×10-2 mol/L solution in 0.1 M KCl electrolyte) was used as electrochemical label. Notably, the pyridyl SAM on ITO creates no barrier to RedOx transformations of [Fe(CN)6]-3 species as CVA traces are identical in both cases. The CORALs in CS decrease the peak current ip from 7.99×10-4A·cm-2 to 5.24×10-4A·cm-2, which corresponds the surface coverage fraction of 0.345, in good agreement with CA and AFM results. The switching from CS to RS drastically decrease the available surface as observed from the ip drop of almost two orders of magnitude, down to 1.04×10-5A·cm-2. The surface coverage can be successfully restored by switching the CORALs back to CS, ip = 4.62×10-4A·cm-2. Figure 8 illustrates the observed behavior.

Figure 8. CVA of CORALs in RS (dashes), CS (dots), and recovered CS (dash-dots) deposited on ITO glass. Surface availability was probed by K3[Fe(CN)6] (0.01M) in 0.1 M KCl electrolyte. ITO modified with 4-pyridyltriethoxysilane SAM was used as reference (solid line CVA). Insert: Magnified CVA of CORALs in RS.


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CONCLUSIONS We targeted a method of in-situ control of surface properties of solid substrates. We designed and fabricated a series of nanometer scale constructs of polymer islands linked to the surface by covalent bonds and laterally separated within the distance of the extended polymer chain. Such polymers may cover the entire surface in RS when preferably interacting with the chains linked to the neighboring islands. In a weak solvent, polymers collapse to CS and reveal the surface. In principle, either a top-down or bottom-up approach for polymer patterns at the nanometer scale can be utilized. We chose a bottom-up synthetic pathway as a more feasible, scalable and technologically applicable for objects with complex shapes as well as particulate and fibrous materials. Considering the requirements for lateral scales of 10-50 nm, the phenomenon of microphase separation seems to be the most viable, although other approaches may be considered. We report for the first time a facile synthetic route to polymer CORALs based on ABC. We have characterized the surface characteristics of the CORALs using AFM, GISAXS and mesoscale MD simulations. All data are consistent. The polymer islands are separated by length scales that allow polymer-polymer interactions between neighboring islands. The fabricated CORALs are responsive to different stimuli (e.g. solvent) via reversible morphological changes and surface coverage. The uniqueness of CORALs as a novel class of reversible responsive systems consists of revealing the substrate surface to the environment. In addition, the approach may be used to establish the distribution of domains in BCs at the interface with the substrate –important parameter of self-assembly that is a challenge to measure directly. CORALs may serve as a


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convenient model for studies of chain distribution and reconstruction for other switchable polymeric systems. The applications of CORALs may span virtually all heterogeneous processes: electrochemistry including battery manufacturing, heterogeneous catalysis, adhesion, adsorption including cell-substrate interactions and protein adsorption. EXPERIMENTAL SECTION Materials. 2-(4-Pyridylethyl) Triethoxysilane (PETOS) (Gelest, Inc.) and poly(styrene-b-4vinylpyridine) (Mn: PS (35500 g mol−1 ), P4VP (4400 g mol−1 ), Mw/Mn = 1.09 for both blocks, PS−P4VP, (purchased from Polymer Source, Inc.) were used as received. 2-(4hydroxyphenylazo)benzoic acid (HABA), 1,4-diiodobutane, and solvents were purchased from Sigma-Aldrich. All solvents were HPLC grade and used with no further purification. Deionized (DI) water was prepared using a Milli-Q50 UltraPure water system. Silicon (Si) wafers (Addison Engineering, {100} orientation,) were cut to size and successively washed in ultrasonic baths of dichloromethane (Pharmco-AAPER), methanol, and DI water for 15 min each, then exposed to „piranha” solution at 80 °C for 45 min. They were then thoroughly rinsed with DI water and blown dry with argon gas prior to use. Synthesis. Si wafers were silanized in a 1% solution of PETOS in toluene overnight to form a uniform self-assembled monolayer (SAM). The samples were abundantly rinsed in neat toluene and dried under argon gas. The layers of the SAM were flat and featureless as examined by AFM: r.m.s.

Coordinated Responsive Arrays of Surface-Linked Polymer Islands

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