Surface Reconstruction in Hydrated Amphiphilic Block Copolymer Thin Films Probed by Fluid Cell Atomic Force Microscopy

Abstract

In many thin film materials, nuanced interplays of interfacial energies control the surface morphology and rearrangement. This work evaluates polymer–solvent interactions and solvent-driven surface reconstructions via ex situ and in situ fluid cell Atomic Force Microscopy (fc-AFM) analysis of amphiphilic block copolymer (BCP) thin films upon exposure to deionized (DI) water. We examine the differences in surface morphology, whole-film swelling, and force response in thin films of polystyrene-block-poly­(ethylene oxide) (PS-b-PEO) and polystyrene-block-poly­[(allyl glycidyl ether)-co-(ethylene oxide)] (PS-b-P­[AGE-co-EO]) processed into standing-up cylinder morphologies perpendicular to a silicon substrate (⊥C). Using Amplitude Modulation AFM (AM-AFM) and Amplitude-Phase Distance (APD) force spectroscopy, this work probes the mechanoresponsive nature of the dynamic surface layers of these films, unveiling surface layer stratification and surface chain rearrangement via minimal tip–sample stimulation. To help rationalize the observed reconfigurations, the energetic driving forces were estimated using the harmonic mean approximations of interfacial energies. Given the nonionizable nature of the minority P­(AGE-co-EO) block and the energetic driving forces for chain mobility, this work shows how the elimination of unfavorable PS–water interfaces drives chain rearrangement and coverage of the PS surface by chains of the hydrophilic block. This work highlights considerations for increasing the heterogeneity and complexity of BCP thin films via random blocks and how those changes to local interfacial energies may drive larger scale film morphology reconstructions, with broader implications for tuning interface hydrophilicity.

Introduction

Bulk block copolymers (BCPs) have a variety of mesoscale morphologies that arise from the interplay between enthalpic and entropic contributions of different blocks as well as their volume fractions. − The overall tunability of BCP composition and the control over properties that arises therefrom is of great use in a variety of fields. − This tunability is further expanded when BCPs are processed into thin films. − While the same bulk energetic considerations still hold in thin films, they may be balanced or even overridden by free energy contributions from interfaces with the substrate, air, or other blocks. Thin film behavior may also differ from the bulk because of altered chain dynamics at these interfaces. − The morphologies that arise from thin film processing are useful for applications ranging from coatings for biological applications, , soft electronics, organic photovoltaics, , and chemical templating. − Critically, many of these applications utilize BCP thin films in aqueous environments. While it has been shown that bulk BCPs can experience large swelling and property changes when exposed to aqueous environments, far less characterization work has been done on the surface layers of hydrated BCP thin films. This lack of characterization is due, in part, to a lack of in situ techniques that can probe how these film surfaces respond to aqueous environments with nanoscale resolution. − Further understanding of chain interactions and dynamics at hydrated BCP thin film surfaces is needed for controlling overall block interactions, surface morphology, and ultimately, properties. ,, In this work, we use in situ fluid cell AFM (fc-AFM) techniques to study surface reorganizations in evaporation induced self-assembly (EISA) directed BCP thin films that are exposed to DI water.

AFM is often used to characterize the morphology of BCP thin films. ,,, Fc-AFM delivers nanometer resolution topographic data that enables imaging of polymer surfaces in solvated environments, revealing surface rearrangements, defect interactions, and kinetics of swelling behaviors. In addition to topographic data, AFM can provide chemical and mechanical information via phase imaging (relative elastic modulus differences and adhesion, where phase lag is associated with tip–surface interactions) and tip modulation experiments to probe the force-response of solvated surfaces. − Further, amplitude-phase distance (APD) force spectroscopy can probe local intermolecular interactions through precise tip–sample forces, providing valuable mechanical insight into local chain properties and behaviors at the nanoscale. ,,− For example, using AFM, surface structure rearrangements in water were observed for amphiphilic diblock copolymers containing polystyrene (PS), such as PS-b-poly­(t-butyl acrylate) (PS-b-PtBA), , PS-b-poly­(2-vinylpyridine) (PS-b-P2VP), , and PS-b-poly­(acrylic acid) (PS-b-PAA). ,

Polystyrene-block-poly­(ethylene oxide) (PS-b-PEO) BCP thin films and their analogues have many potential applications as antifouling surface treatments, membranes for separations, and other areas, − however, their solvated structure remains poorly understood. In the handful of in situ AFM studies of BCPs mentioned above, the hydrophilic block is ionizable and as such (Coulombic) electrostatic interactions are attributed as the dominant driving force for the minority block swelling and surface reconfigurations. , In nonionic BCPs, however, other types of intermolecular interactions (e.g., hydrophobic interactions, hydrogen-bonding, etc.) can dominate the interfacial energies and result in surface rearrangements. For example, surface reconstructions could occur in which nonionic hydrophilic blocks rearrange to eliminate high-energy water-hydrophobic polymer interfaces to lower the overall thin film surface energy. More work is needed to understand thin film BCP systems in which intermolecular forces other than electrostatics are at play, and in which delicate interblock interactions might control surface morphology. Probing how nonionizable, polar polymer chains swell and interact in aqueous environments as described in the present study is important for applications where the surface structure and surface energy can affect macroscopic materials properties. ,,

Previously, we have studied both PS-b-PEO (Figure A, Table S1) and polystyrene-block-poly­[(allyl glycidyl ether)-co-(ethylene oxide)] (PS-b-P­(AGE-co-EO) thin films (Figure B, Table S1) as templates for confined surface reactions, as model surfaces in super-resolution microscopy probe development, as well as for crystal nucleation and growth, all in aqueous solutions. ,, In a previous study, we reported GISAXS measurements on similar block copolymer thin films that support the representation in Figure of hydrophilic-block cylinders extended from the film surface all the way to the substrate. Before immersion into aqueous solutions, both BCPs can self-assemble from organic solvents into thin films with cylindrical morphology, either standing-up perpendicular to the substrate cylinder (⊥C, Figure ) or lying-down parallel to the substrate cylinder (∥C) configurations based on controlled processing conditions. The PAGE block, via the pendant alkene, enables functionalization via “click chemistry” and, as a random coblock, mixes a relatively more hydrophobic component into the hydrophilic PEO block. Here, after their formation, we delve into the swelling and surface reorganization behavior of these amphiphilic, nonionizable BCP thin films in aqueous environments. The PS-b-PEO thin films, while important to study in their own right, serve as a reference system whose behavior has been previously studied in water. − The PS-b-P­(AGE-co-EO) thin films provide a more complex system through which to study how small changes in block chemistry and surface energy can affect surface chain reorganization and overall film swelling in aqueous environments. With in situ fc-AFM, we probe the surface layers of thin films before, during, and after exposure to water. This study shows how addition of the PAGE block affects both local surface layer film morphology and whole film swelling by changing surface energies and compatibility between the primarily hydrophobic and hydrophilic polymer blocks. This behavior could have large impacts on how we design templates, coatings, membranes, and more, for applications in aqueous environments.

1.

(A) PS-b-PEO and (B) PS-b-P­(AGE-co-EO): (i) BCP chemical structures and (ii) schematic of hexagonal cylinder packing perpendicular to a silicon wafer substrate.

Results and Discussion

Ex Situ AFM Imaging of PS-b-PEO and PS-b-P­(AGE-co-EO) Films Before and After Water Exposure

PS-b-PEO thin films were spin-coated onto a silicon substrate and annealed with benzene via previously established protocols, resulting in a hexagonally packed ⊥C morphology. , When these films were imaged in air (pre-DI) using amplitude modulation AFM (AM-AFM), we observed two distinct domains in the height (Figure A,i) and phase (Figure A,iii) channel images: the majority PS matrix (light) domain and minority PEO (dark) domains. The BCP films were then exposed to water for 2 h and allowed to equilibrate. Afterward, the films were removed, quickly dried (N₂, 5 min), and imaged. It is expected due to the hydrophilic nature of the minority block that there is some residual water in the polymer thin films after this quick drying. These films showed significant surface changes (Figure A,ii,iv). While qualitative hexagonal domains are maintained, they appear distorted, which is especially evident in the phase channel (Figure A,iv). If we make the AFM height images binary by setting thresholds of (−5 nm) and (−2 nm), respectively, for PEO and P­(AGE-co-EO) films via ImageJ (Figure S1), we can better visualize how the well distribution and sizing change before and after exposure to DI water. Not only do we “lose” many of the wells, but in many cases the qualitative hexagonal domains disappear and the characteristic well shape degrades. Comparing the height channels (Figure A,i,ii) and line-scans (Figure C) after equilibration in water, the matrix region has changed roughness significantly (R _q decreased from 1.4 to 0.3 nm), and the well depth decreased from ∼15 to ∼2 nm. Comparing the phase channels (in units of degrees of phase lag) we see a substantial decrease in the overall degree range (taken from Figure S2A, 34° to 4°), suggesting that the surface composition is more homogeneous after exposure to water. If we compare these conditions at the same degree range (−5° to 5°) we can see a clear qualitative difference that corresponds to the change in the surface morphology (Figure A,iii,iv).

2.

Ex situ AFM imaging of (A) PS-b-PEO and (B) PS-b-P­(AGE-co-EO) thin films. Height and phase channel images pre-DI (i, iii) and post-DI (ii, iv) water exposure, both imaged using similar parameters in air (free amplitude = 12 nm, amplitude set point = 0.86A₀). (C, D) Comparisons between the line scans taken at the relative positions shown by the colored lines in height channel images (i, ii) for cases (A, B) respectively. Corresponding phase line scans in Figure S2. (Dotted white lines and stars represent schematic snapshots used in Figure .) Scale bars = 100 nm. Probe spring constant k = 30.98 N/m used.

To examine the effects of adding a small amount of a relatively hydrophobic monomer to the hydrophilic PEO block of the parent system, PS-b-P­(AGE-co-EO) thin films were prepared via established methods also resulting in a hexagonally packed ⊥C morphology (Figure B,i). , When these films were imaged in air (pre-DI) using AM-AFM, like for the PS-b-PEO films, we observe two distinct domains in the height (Figure B,i) and phase (Figure B,iii) channel images: the majority PS matrix (light) domain and minority P­(AGE-co-EO) (dark) domains. Analogously to the PS-b-PEO films, PS-b-P­(AGE-co-EO) films were exposed to DI water for 2 h and then imaged after removal of water using the same drying process as described for the PS-b-PEO films (Figure B,ii,iv). It is assumed that due to the hydrophilic nature of the minority block (80% PEO) there is some residual water in the polymer thin films after this quick drying. For these films, unlike the parent films, after exposure to water, there is a qualitative loss of hexagonal order at the film surface, with less clear ⊥C hexagonal packing. Additionally, in contrast to the PS-b-PEO films, there appeared to be little quantitative change in the height channel and line-scans, with surface roughness (R _q = 0.6 to 0.55 nm) and well depths staying approximately the same despite the qualitative changes (Figure D). The phase channel image (Figure B,iii,iv) changes mirror that of PS-b-PEO: the total range has decreased significantly compared to the original samples (from Figure S2B, 18° to 3°), and when compared with the phase channel pre-DI at the same range (−5° to 5°) the decrease is qualitatively clear (Figure B,iii,iv). This lack of phase contrast suggests that there is possibly only one composition present at the surface after exposure to DI water.

To understand better if the observed changes induced by exposure to water were restricted to the film surface, macroscopic measurements of film thickness were conducted (Table S2, Figure S3). For PS-b-PEO, these measurements in air (∼47 nm), water (∼79 nm), and after removal from water (∼48 nm) reveal a substantial response of the nonionizable amphiphilic BCP thin film thickness to exposure to water (Figure S3C, blue line). In contrast, the PS-b-P­(AGE-co-EO) thin film shows minimal (<5%) changes in thickness upon hydration and subsequent removal from water (Figure S3C, green line).

In both films, the PS matrix is a hydrophobic, high glass transition temperature (T _g = 100 °C) block that would be in a glassy state at room temperature and would be unlikely to swell or change significantly upon introduction to DI water. The observed change in PS-b-PEO film thickness, therefore, must be attributed to the PEO domains, , which have energetically favorable interactions with water and could swell significantly. − While not typical, a 30–50% increase in film thickness has been seen in other systems with a hydrophilic minority block. Water is a good solvent for the PEO chains, so it would be reasonable for the chains at the surface to adopt a more extended conformation like at an air–water interface and in other PEO systems. , Then, when the water is abruptly removed, it would be reasonable that these extended, swollen chains would not fully dehydrate, and many would collapse onto the matrix, potentially explaining the change in qualitative appearance in Figure A,ii, the decrease in measured well depth, and the significantly decreased phase range seen in Figure A,iv.

In contrast, with the addition of the AGE monomers, there is now a hydrophobic component in both the majority and minority blocks. The AGE component could affect the diffusion of water through the whole film, suppressing whole-film swelling significantly except at the surface, where the chains have enhanced mobility. The lack of hexagonal ordering and decrease in phase contrast after equilibration in water suggests that, while whole-film swelling is limited in the PS-b-P­(AGE-co-EO) film, there are substantial changes occurring at the surface. Ex situ imaging of the BCP thin films showed several differences between the two films after incubation in water. Specifically, after hydration, the whole-film thickness of PS-b-PEO changes significantly more than PS-b-P­(AGE-co-EO), the well depths of the PS-b-PEO film decrease while they stay the same for the PS-b-P­(AGE-co-EO) films, and the PS-b-PEO film retains a clear hexagonal morphology while the PS-b-P­(AGE-co-EO) films lose that clear topographical definition. To better understand both films and how their surface compositions and structures evolve in water, it is desirable to probe the swollen films in situ.

In Situ Fluid Cell AFM Imaging of BCP Thin Films

To image these different behaviors of BCP thin films in situ, the thin films were transferred into an AFM fluid cell and equilibrated in DI water for 1 h. Imaging the films between 12 and 75 min after introduction of water showed no change in surface morphology (Figure S4). Time-resolved imaging of the initial swelling in DI water (<5 min) was attempted, but in both cases (PS-b-PEO and PS-b-P­(AGE-co-EO)) this process was too rapid and unstable to be properly imaged. After 1 h of incubation, the films were imaged using an optimized amplitude set point to ensure as gentle a force as reasonable to get good tip tracking (see Materials and Methods section for details).

In the PS-b-PEO films, we see minimal qualitative changes in the surface morphology when imaged in water relative to ex situ experiments. Looking at the height channel (Figure A,i) we see some of the cylinders appear to have occlusions in the middle of the wells (red circle) where there were none before (Figure A,i) and there are also small protrusions on the matrix portion of the BCP surface (green circle), also not seen in Figure A,i. Note that some of these features were partially resolved by phase imaging of the dry films (Figure A,iii) but were not correlated to any appreciable topographic features. Upon exposure to water, the well occlusions and matrix protrusions became more pronounced, measuring between 2 and 9 nm in height relative to the well depth or matrix level (Figure S5). The height channel of the film in DI water more resembles that seen in air (Figure A,i), with average well depths in-DI (10 nm) that are closer to those measured pre-DI (15 nm) and discernible hexagonal packing (see Figure S6). The phase channel, however, more resembles the film morphology seen post-DI exposure (Figure A,iii) with minimal phase range (<2°) suggesting the presence of one composition at the surface. All features seen in the height channel appear clearly in the phase channel as well, suggesting they are not artifacts (see Figures A,iii and S7). The relatively unchanged surface morphology supports the hypothesis that the whole film is swelling homogeneously and that the slight well-to-well variations and protrusions are the result of homogeneous PEO chain swelling for the different initial well depths and diameters already seen in air. This explains why the distribution of different well depths at the surface stays the same, even though they collectively swell.

3.

Amplitude modulation AFM imaging of PS-b-PEO (A) and PS-b-P­(AGE-co-EO) (B) thin films after 1 h incubation in DI water. Height (i, ii) and phase (iii, iv) channel images at original amplitude set point A₀ (i,iii) and reduced set points as shown (ii,iv). In A,i, the green circle indicates a matrix protrusion and the red circle a well inclusion. Schematics of proposed film swelling for (C) PS-b-PEO and (D) PS-b-P­(AGE-co-EO) stages of (i) Pre-DI, (ii) in DI, and (iii) Post-DI. Schematics in (C) and (D) correspond to locations in height images shown by starred dashed lines in Figures and (A,B). In part B, green dotted circles highlight same regions in height and phase channels. Scale bars: 200 nm. Probe spring constant k = 0.67–0.86 N/m was used.

When imaged in water, the PS-b-P­(AGE-co-EO) thin film surface morphology changes much more drastically than PS-b-PEO as compared to that of pre-DI (Figure B). When imaged using a similar amplitude set point ratio (r _sp) as the PS-b-PEO thin films (r _sp ∼ 0.95), there is no longer a clear hexagonally ordered surface (Figure B,i). Rather, large, half-ellipsoid globular structures (∼20 nm in height and ∼ 80–100 nm in diameter) appear on top of a bumpy, irregular background (see Figure S8A line scans for exact profile). The phase channel has also changed significantly, with the bumpy background exhibiting a very low phase range (<2°), and the globular protrusions (green dotted lines) providing the main phase contrast. The observed nonuniform swelling is mainly rationalized by the corresponding heterogeneity of the random AGE block of the P­(AGE-co-EO) minority block, where the local block variability might lead to local changes in hydration, swelling, and surface coverage that is subsequently observed in the AFM. Such large-scale surface morphology change, while unexpected, supports the hypothesis that almost all the hydration-induced changes in the PS-b-P­(AGE-co-EO) thin film are confined to the surface. Unlike the PS-b-PEO case, where the changes seen in situ fit nicely in the progression from pre-DI to post-DI states, here we capture a metastable surface state that seems unpredictable given the two ex situ end points.

In Situ Imaging at Decreasing Amplitude Set Points

To further explore the dynamics of hydrated chains of the minority PEO and P­(AGE-co-EO) blocks, which seem to dominate the swelling behavior, these BCP thin films were examined by using in situ amplitude modulation AFM (AM-AFM). After the films had reached steady state (1 h in water) and while scanning the same region, the amplitude set point was decreased incrementally, giving different amplitude set point ratios (r _sp) (Figure S8). It is accepted that this ratio is indirectly related to tip–sample force, − thereby effectively increasing the force on the thin film surface with decreasing amplitude set point. It has also been shown that for decreased set point imaging, related to the increased forces, there is an increase in the indentation of the probe into materials of different moduli, particularly for compliant materials. , Such increased force and indentation can therefore be used to interrogate buried surface layers (or lack thereof). ,

For the PS-b-PEO films, as the amplitude set point is decreased to an r _sp of 0.75, the morphology reverts to that seen pre-DI (compare Figure A,ii,iv with Figure A,i,iii): the well depths increase to similar values (15 nm), and the well occlusions and matrix protrusions (highlighted in Figure A­(i) with red and green circles, respectively) disappear. While the phase range does increase once again to 17°, the prevalence of the phase change on only one side of the wells suggests that this is an artifact, rather than a real emergence of a separate phase (see Figure S7 for full data set). When masked for this artifact, the phase range once again drops to <5°, indicating only minimal differences in the mechanical properties across the film surface. This observation gives us a first clue that, while qualitatively the morphology is comparable to the pre-DI condition, the swollen film has been covered with extended PEO chains (Figure C). This proposed behavior is consistent with other published PS-b-PEO swelling studies and will be further explored in the following section using force measurements.

By comparison, the hydrated PS-b-P­(AGE-co-EO) films, which are dominated by large globular heterogeneities, display significant changes in morphology upon reducing the amplitude set point (Figure S8). The heterogeneities become less pronounced, giving way to a more regular hexagonal arrangement of the BCP domains (Figure B,ii). At an r _sp of 0.6 the large globular features are less obvious in the height channel, but still resolved in the phase channel until very low set point ratios of 0.5–0.4 (as seen in Figure S8). The height channel range decreases significantly (from a range of 31.3 to 13.5 nm), returning to values similar to those seen pre-DI in air (height channel range of 7.4 nm) (Figure S9). Like the PS-b-PEO case, the phase range increases significantly with reduced amplitude set point, and the pre-DI morphology is recovered. This behavior indicates that only a thin layer at the surface changes upon exposure to water, particularly forming globular heterogeneities, with most of the film experiencing little to no swelling (Figure D). Therefore, the addition of the AGE block in a random copolymer not only drastically changes the surface morphology as compared to PS-b-PEO, but also affects whole-film swelling. To visualize how these low set points (r _sp of ∼ 0.6) affect the film mechanically, we captured a combination image, where for a similar PS-b-P­(AGE-co-EO) thin film of mixed morphology we first imaged a smaller area at progressively lower set points (r _sp of 0.9–0.6, similar to Figure S8) and then increased the scan area substantially and scanned the area once again using similar parameters but returning to a high set point (an r _sp of ∼0.9). The results are shown in Figure S10 and show how the lower set point imaging deformed the smaller initial region of interest.

Force Spectroscopy Experiments on Hydrated BCP Thin Films

To gain further insights into the properties of the hydrated polymer films, we used Amplitude-Phase Distance (APD) force spectroscopy, an AFM technique for interrogating the mechanical behavior of surfaces on the nanoscale, particularly for compliant materials. − APD is based on oscillating the tip above a specific surface location and then approaching toward the surface while simultaneously monitoring the amplitude and phase of the tip oscillation. From the change in the amplitude and corresponding phase, information about the interactions between the tip, the surface, and the surrounding medium can be deduced as a function of the distance of the tip from the surface, which can be transformed to a conservative force approximation using the method by H. Söngen et al., as described in the Materials and Methods section and in Figure S11. Additionally, because we are working with compliant materials, by changing the set point of the approaching probe, we can potentially gain interaction information about layers and material below the surface. − We conducted APD experiments on both PS-b-PEO and PS-b-P­(AGE-co-EO) thin film surfaces in water (Figure ). One can visualize the films before and after taking the measurements in the corresponding height-phase channel imaging of the regions of force acquisition (Figure A,B,i–iv).

4.

In situ APD force curve measurements of (A) PS-b-PEO and (B) PS-b-P­(AGE-co-EO). (i, iii) Height channel and (ii, iv) phase channel AFM images taken in DI water. (v) Corresponding conservative force curves, at locations shown by color coded “x” in height images. (A,v) Force curves transformed from APD data on PS-b-PEO film at the lowest amplitude set point of 0.7A₀. (B,v) Force curves transformed from APD data on PS-b-P­(AGE-co-EO) film shown in B­(iii,iv). (v) Schemes representing probe indentations during experiments at different set points on surface layers of (A) PS-b-PEO and (B) PS-b-P­(AGE-co-EO) films (same as Figure C,D,ii). Scale bar (A) = 100 nm. Scale bar (B) = 50 nm. The stiffness of the cantilevers used for both experiments was 0.67 and 0.86 N/m, respectively.

For the case of PS-b-PEO (Figure A), we find almost no difference in the force curves collected from the well and matrix regions, again suggesting that the swollen surface is covered with a hydrated PEO layer. Both curves start at zero force far away from the surface, and upon interaction with the surface (or the environment close to the surface), there is a net attractive force between the probe and the sample. From the collected amplitude and phase data for PS-b-PEO (Figure S12A) we see that the phase values stay above 90° and the amplitude monotonically decreases; due to an interaction that extends to ∼15 nm. When the set point was pushed further to an r _sp of 0.6, the film began to noticeably tear during imaging, and no reproducible APD data could be collected.

We conducted similar APD experiments on PS-b-P­(AGE-co-EO) films (Figure B). Initially, at trigger set point ratios just below initial imaging in DI water (r _sp = 0.93, Figure B,v), the force curve mimics that seen in Figure A,v, giving an attractive force response with minimal differences between the well and matrix. The increasing trend in the phase-distance curves (Figure S12B,i,ii) for both the matrix and wells supports this force assignment, assuming similar probe properties as for the PS-b-PEO experiments. This similarly attractive (same as with r _sp of 0.7 in the PS-b-PEO case) force response supports the initial surface ordering, with the minority, hydrated P­(AGE-co-EO) blocks covering the whole surface. While the overall behavior of the interaction between the probe and the surface chains in this region is qualitatively similar, the attractive force magnitude is quite different. At the closest approach, the interaction force between the probe and surface is >5 times less than that of pure PEO chains (1 nN vs 0.2 nN). As we probe with a decreased r _sp, and therefore more force, however, we see the response changes from attractive to repulsive (this change can be verified in the sharper decreasing (amplitude) and flipping (phase) response of the amplitude-distance and phase-distance curves as well, Figure S12B,i,ii). At an r _sp of 0.7, for very small tip displacements in the APD curve, the force crosses from negative to positive. As we further decrease the set point ratio to just below one of the lowest r _sp of the in situ imaging (0.58), we see an even more drastic positive force (repulsion) as tip displacement decreases in the APD curves.

To understand these observations, we return to the proposed picture of film swelling in Figure C,D. In the case of PS-b-PEO films, there is an overlayer of swollen PEO that the probe would have to interact with and push through to directly “feel” the underlying PS matrix. As shown by the film thickness measurements (Table S2, Figure S3), however, there is barely any film thickness increase for the PS-b-P­(AGE-co-EO) films throughout exposure to water. At higher set point ratios, the tip does not have enough force to indent the thinner, swollen P­(AGE-co-EO) layer, and so we see very similar behavior to the PS-b-PEO films, with almost identical force response. In contrast, at lower set points, the probe is likely indenting through a thin layer of swollen P­(AGE-co-EO) chains at the surface and sensing the unswollen film below, so the force response switches to a repulsive regime. This interpretation could also explain why we see very little difference between the wells and the matrix (like the PS-b-PEO APD curves) until the lowest set point, as the probe is mainly feeling only one block and its mechanical properties. Once the probe has enough force, it feels the slight differences between the PS and P­(AGE-co-EO) regions. If the inflection point between the mainly negative force regime and mainly positive force regime represents the approximate depth of the swollen P­(AGE-co-EO) layer at the top of the film, we get values of around 5–6 nm, which agree well with the measured change in overall film thickness.

Overall, these APD experiments show that for both cases, most of the film thickness increase happens at the surface, as either the recorded lack of force behavior change indicates a thick surface layer of PEO, or the recorded change indicates the thickness of P­(AGE-co-EO) chains that agrees with the film thickness measurements. Also, via changes in the attractive force magnitude, we can gain some insight into the nanoscale hydrophilicity of the surface chains. These results, together with the in situ and ex situ height and phase channel imaging data, highlight the complex and heterogeneous nature of the film evolution when exposed to DI water. In addition, the APD experiments suggest that the hydrophilic minority block chains of the PS-b-PEO and PS-b-P­(AGE-co-EO) thin films in DI water cover the hydrophobic PS majority block surface, consistent with the interfacial energy calculations (see the next section), and for both cases provide a measure for the thickness and structuring of the hydrophilic surface layers of the BCP thin films.

Interfacial Energies Drive PS Surface Coverage via Minority Hydrophilic Blocks of BCP Thin Films Upon Hydration

When considering how a PS-b-PEO or PS-b-P­(AGE-co-EO) thin film with hexagonal cylinder morphology (⊥C) and a majority of PS (i.e., hydrophilic PEO or P­(AGE-co-EO) blocks constituting the cylinders) might reorganize upon exposure to water, we need to consider multiple different free and interfacial energies. Based on previous work with PS in water, PEO extension, , and polymer thin film expansion in water, ,,− we begin with two important assumptions. The first assumption is that the hydration free energy of the minority block chains (in our system) is highly favorable. The second assumption is that large areas of a PS–water interface are energetically unfavorable. In general overviews of literature, the PEO–water interfacial energy is thought to range from a large negative value to very small positive value depending on temperature, chain density, and a few other factors. − More specifically, based on previous work by Saito and Inutsuka, − the free energy of hydration for comparable PEO chains is negative on the order of 5–50 k _B T/nm² (although the magnitude varies based on chain density and extension). Similarly, work by Li and Walker , shows via force extension spectroscopy that the PS-water interface is highly energetically unfavorable.

Our experimental data suggest that the entire PS matrix surface is covered by PEO (or P­(AGE-co-EO)) minority block chains (Figures , S7, and S8). We hypothesize that the energy gained by eliminating the PS–water interface provides enough driving force in our system for complete PS surface coverage via the hydrophilic blocks. This hypothesis is further educated by the experimental observation that the observed thin-film morphologies deviate from previously reported “mushroom” morphologies, which are characterized by circular topographic features with sharp boundaries and arise due to the hydration and swelling of an electrostatically charged minority block (poly­(acrylic acid), PAA). ,,, The mushroom morphology results from hydration and swelling of the PAA block primarily driven by electrostatic chain repulsion of the minority block. , This mechanism is distinct from the process in the current study, which is driven by a structural rearrangement to minimize the interfacial surface energy by eliminating the unfavorable polystyrene-water interface. To evaluate the energetics for PS surface coverage via the hydrophilic blocks, we calculated the magnitude of the energy balance for complete coverage of the PS matrix, but with the simplification of only considering a single chain (eq and supplemental text). ,

Based on the assumptions described above, we consider five main contributions to the free energy calculation for chain extension in the case of PS-b-PEO (eq ).

1

The first is the enthalpic cost of creating a new PS/PEO interface (eq , red). The second is the energetic cost of dehydrating the PEO chain that will cover the PS chains as our energetic estimates are based on dry polymer–polymer surface energies (eq , dark blue). The third component is the energetic gain from the elimination of the unfavorable PS/water interface (eq , orange). The fourth component is the energetic gain of a new hydrated PEO chain at the top interface of the film with water (eq , light blue). The fifth component is the entropic penalty of stretching the minority block (PEO) chains anchored to the glassy PS matrix past their equilibrium swollen state in water (eq , purple). Surface and interfacial energy values (γ’s) were obtained from literature (sources shown in Supplemental Text 1) and from the harmonic mean surface energy approximation by Wu et al. , to yield eq . The Wu equation was chosen as it does not use the generalized bulk surface energies but instead uses the dispersive and polar surface energy components of the polymer and solvent interfacial energies, which, similar to Hansen Solubility Parameters (HSP) have been shown to be increasingly important in polar systems, often outperforming other common interaction parameters such as the Flory–Huggins chi parameter (χ) or the Hildebrand solubility parameter in complex polar systems due to inclusion of hydrogen bonding and polar interactions, not just dispersive ones. −

To calculate the interfacial areas (σ) of the hydrophilic block chains, we multiplied the polymer monomer cross-sectional area along the polymer chain (assuming the monomer has the shape of a cylinder, this value is given by r _monomer × (d _monomer + r _hyd) times the number of monomers per hydrophilic block, which we got from the BCP chain characteristics, i.e., the degree of polymerization of the hydrophilic blocks). Here, r _hyd refers to the size of the approximate hydration shell around the monomer. Information on how this value was calculated based on literature values can be found in the SI under Supplemental Text 1.

The maximum distance chains must stretch over the PS surface (l) was derived from the experimental data, i.e., from the distance required for full coverage of the PS matrix. We made our calculations based on two assumptions: (1) We consider a chain that originates from the PS/PEO or PS/P­(AGE-co-EO) interface that is located at the top of the original hydrophilic cylinder (i.e., we assume the chain does not have to travel upward along the cylinder wall before it bends over to the PS surface); (2) We consider a chain that stretches to the farthest distance from the cylindrical hydrophilic domain to cover the PS matrix (we call this distance the Maximum Chain Extension Distance (MCED), see the red line and red star in Scheme S1). We thereby chose the scenario of maximum entropic penalty per chain (as for the regions between the cylinders, rather than the middle of the interstitial space, the degree of stretching is smaller).

We also acknowledge that chains may be extending volumetrically into the solvent but do not treat this explicitly in our system, focusing solely on the energetics of full matrix coverage by the minority block chains. F _RC is the free entropic energy penalty (assuming an ideal chain; random coil approximation) of stretching (in Joules [J]) of the minority blocks beyond the initial chain conformation, where the chains are anchored at the cylinder-matrix interface. Values, as well as expansion and definition of variables, are given in the SI Supplemental Text, pp 16 and 17. For the extension component free energy F _RC,PEO we approximated the minority block chains as hydrated PEO chains in a good solvent, as water is generally a good solvent for PEO. ,−

The result of this analysis, ΔG _{PEO,extension} = −8664.8 kT, suggests that the energy gained by the system via PS-water interface elimination is more than sufficient to drive the covering of the PS matrix with PEO chains. In short, while the creation of the new PS/PEO interface and the stretching of the chains are energetically costly, it is far outweighed by the energy of the system gained by elimination of the PS-water interface.

Using a similar approach, we formulated the free energy associated with the extension of the P­(AGE-co-EO) minority block across the majority PS block surface (eq ):

2

In this case, similarly based on the experimental evidence in Figures and S8 we posited that the hydrated minority block chains (P­(AGE-co-EO)) extend across the entirety of the PS block surface area, creating a new PS/P­(AGE-co-EO) interface and eliminating the highly unfavorable PS/water interface. We used the same interfacial energy values for the PS and PEO blocks as in eq . Finding values for P­(AGE-co-EO) and the associated P­(AGE-co-EO)/X interfaces proved more challenging due to the lack of previous work in the literature using this monomer, especially in an amphiphilic random block. To address this challenge, the dispersive Hansen Solubility Parameter (HSP) of allyl glycidyl ether (AGE) from literature was used, with a root-mean-square of the polar and hydrogen bonding components employed to approximate the typical “polar” interfacial energy component. This is a good approximation, as the dispersive HSP is generally equal to the interfacial energy dispersive component, whereas the typical interfacial energy polar component lies between the polar and hydrogen bonding HSPs. In eq we use a weighted mixture of the surface energies to calculate the interaction energies. Here, X and Y denote the volume ratios of the EO and AGE blocks, respectively, which allow us to approximate a mixture of surface energies between those of PEO and PAGE. We believe this approximation to be appropriate for several reasons. First, the minority block in this case is overwhelmingly the PEO (80%). Second, while generally described as hydrophobic in literature, ,, AGE as a molecule still has decent solubility in water (14% w/v). In addition, it was shown for slightly hydrophobic segments (and in one case for PAGE specifically) that it loses some of its hydrophobic character when the segment ratio is smaller than half that of the hydrophilic block. , Third, based on their HSP distance (“Ra”, an approximation that estimates polymer–solvent compatibility) PAGE and water (36.7) are much more similar to that of PEO and water (37.9) than PS and water (41.5).

In eq the area (σ) was calculated using the same method as above for PS-b-PEO. In this case, the size of the monomer was kept at an approximation of ∼1 nm as it seems reasonable that hydrated PAGE would be similar in size to a doubly hydrated PEO monomer. The same cross sectional area approximation was used and then multiplied by the degree of polymerization (see Supporting Information Scheme S1). For the extension component free energy, F _{RC,P(AGE‑co‑EO)}, we used the values for PEO, relying here on the fact that the P­(AGE-co-EO) chains are 80% PEO. For this calculation we approximated the MCED distance in a similar manner to that of the PS-b-PEO case (see Scheme S1). We acknowledge that this assumption does not consider how covalently bound chains might shield each other from solvent interactions and change the extension width or length, but we believe that this would be an interesting subject of future work.

In summary, eq provides a free energy balance for the coverage of the PS matrix by minority P­(AGE-co-EO) chains, simplified to the case of a single chain. The result of this analysis, ΔG _{P(AGE‑co‑EO),extension} = −3124.1 kT, suggests that the energy gained by the system via the PS/water interface elimination is also sufficient to drive the coverage of the PS matrix with P­(AGE-co-EO) chains.

Keeping in mind that our calculations are at the level of a single chain only, it is still interesting to compare the different contributions to the free energy balance in both cases and examine their impacts. An important point is that the energetics, as approximated here, suggest that in both BCP cases, entire surface coverage by minority block chains is highly energetically favorable, although the driving force for the case of PS-b-P­(AGE-co-EO) is less than half of that for PS-b-PEO. Depending on how one chooses to define or include terms, however, this conclusion might not always be the case. Here we have included terms for both the dehydration and hydration of the interfaces that are in contact with the PS matrix and water respectively, and these terms hold great weight. For example, if one includes only the energy penalty for the dehydration of the minority block chains and not the energy gain from the new hydrated interface, the energetics change significantly. In this example, assuming a moderate free energy of hydration for PEO of about 10k _B T/nm² from literature, ,, the chain extension now becomes energetically unfavorable for the PS-b-PEO case (ΔG _{PEO,extension} = +1152.7 kT), but remains energetically favorable for the PS-b-P­(AGE-co-EO) case (ΔG _{P(AGE‑c‑EO),ext.} = −900.1 kT), a different outcome from our current results. While the AGE block does act as a compatibilizer between the PEO and PS chains, lowering the energetic cost of the new polymer–polymer interface only from 0.887k _B T/nm² to 0.804k _B T/nm² (less than 10% difference), its addition alters the energy of (de)­hydration much more, i.e., from 10k _B T/nm² to 6.35k _B T/nm². Another interesting point is that the penalty from chain extension is negligible (almost 2 orders of magnitude less than the interfacial energy contributions) based on our approximations. Considering that the chain does not have to stretch very far compared to its fully extended conformation and in both cases we are extending the chain into a good solvent, this observation makes sense. Depending on the system scale or solvent, however, this conclusion may no longer be valid. In our system, this analysis also highlights the dominance of the interfacial energy terms.

Clearly the addition of the PAGE monomer, even in very small proportions, has big implications for the microscopic behavior of the films. The ratio of PS to PEO chains in both polymers is almost identical (0.18 by volume fraction). If one were to approximate the behavior of the minority chains in the second case to only PEO, one would expect the swollen state height to be well over 90 nm (a change of at least 30 nm in water). Instead, the swollen state height change is less than 5 nm (except for rare globules of 15–20 nm, which might be more purely PEO behavior). We can learn from our simple energetic model for coverage of the PS matrix by minority chains that the AGE term affects block compatibilization and chain hydration, but extending these approximations to the whole system will require future computational and theoretical backing.

Conclusions

Using AM-AFM imaging in air and in DI water as well as APD experiments, we have captured the behavior of PS-b-PEO and PS-b-P­(AGE-co-EO) thin films upon exposure to aqueous solutions. Together, these data are consistent with surface energy calculations using the harmonic mean surface energy approximation method and suggest that the nonionizable, hydrophilic minority blocks PEO or P­(AGE-co-EO) are driven by interfacial energy to eliminate unfavorable PS/water interfaces by covering the PS matrix block. This rearrangement creates a hydrated, highly mobile polymer chain layer on top of the PS matrix, thus drastically changing the surface morphology and chemical nature of the film/water interface.

Surface rearrangements in a range of different types of materials are often governed by interfacial energies. Previously studied BCP systems have mainly considered polymer compositions in which electrostatic interactions between polymer chains are the main driving force for interfacial rearrangements. In the current work, we consider systems with nonionizable hydrophilic blocks in which there is a more delicate balance of interfacial energies. Based upon our calculations as well as ex situ and in situ AFM experiments, we show that in these nonionizable cases, one must consider all the different components of interfacial energy (including polymer–polymer and polymer–solvent interactions) to derive a driving force that appropriately describes the experimental system. Furthermore, this work highlights how additional complexity in block chemistry (in this case a relatively hydrophobic AGE monomer randomly copolymerized with the hydrophilic EO monomer), even at small amounts (<5% by volume) can drastically affect both the large scale and surface level morphology and film behavior under water. Looking at the corresponding interfacial energies, we see that this impact may be because of changes to block hydration energies and block compatibilization effects with the majority PS block. Not only are the dynamics driven by the energetics of the PS, PEO, P­(AGE-co-EO), and water interfaces, but these same forces also play a role in creating a viscoelastic, solvent-responsive surface layer. Our results are relevant both for fundamental polymer thin film physics and also for application driven areas such as polymer electrolytes, biomedical coatings, and crystal growth templating where the precise hydrated structure and surface dynamics of thin films in aqueous solutions are critical to as they dictate the material’s function. More research is needed to characterize how these hydrated polymer surfaces change and reconstruct using in situ and in operando techniques. Such an understanding of surface reconstructions in BCP thin films will impact future research in these areas and open potential avenues for more dynamic responsive surfaces in BCP thin films and beyond.

Materials and Methods

Polymers and General Materials

Polystyrene-block-poly­(ethylene oxide) with a polydispersity of 1.09 and a M _n of 229 kDa was synthesized via anionic polymerization. The volume fractions of the two blocks were 0.82 PS and 0.18 PEO. Polystyrene-block-poly­[(allyl glycidyl ether)-co-(ethylene oxide)] (PS-b-P­(AGE-co-EO)) with a polydispersity of 1.11 and M _n of 75.6 kDa was synthesized via anionic polymerization. The volume fractions of the two blocks were 0.76 PS and 0.24 P­(AGE-co-EO). For full polymer information, see Table S1 and refs and . Deionized (DI) water provided by Barnstead Thermo Fisher water systems, 18.2 MΩ·cm resistance. Silicon wafers from Electron Microscopy Sciences were cut to 1 cm × 1 cm pieces and cleaned by sonication in acetone and 2-propanol for an hour, dried by N₂, then annealed at 100 °C for 1 h.

BCP Thin Films Processing

Films of PS-b-PEO were prepared by spin coating a 1 wt % benzene solution onto a freshly cleaned silicon wafer piece. Samples were annealed for 21.5 h in a saturated benzene atmosphere in a closed evaporation dish. Samples were left to dry in air for 1–3 h. Films of PS-b-P­(AGE-co-EO) were prepared by spin-coating a 1 wt % benzene solution onto cleaned silicon wafer pieces. Samples were then solvent annealed in a saturated benzene atmosphere at room temperature for 21.5 h. The samples were then left to dry (covered) in air at room temperature for 1–3 h before being prepared for AFM imaging.

Atomic Force Microscopy in Air

All AFM measurements and images were taken on a Bruker MultiMode 8 SPM system. All samples were reinforced on a magnetic stub before imaging and blown with N₂ just before imaging. For air (Pre-DI, Post-DI) imaging, Bruker RTESPA-150 tips with a resonance frequency of 150 kHz were used in Soft Tapping Mode with a MMTRCH Probe Holder. The spring constant was measured to be 30.98 N/m based on thermal tune and a Lorentzian fit. Tips were plasma cleaned for 30 s on medium power in a Harrick Plasma cleaner before use, with some being used as-is for added comparison (data not shown). Samples were scanned at 0.5–1.0 Hz. Exact parameters used for images are supplied in the main text or SI figure captions.

In Situ Fluid Cell Experiments

To interrogate film structure in aqueous environments, the PS-b-PEO and PS-b-P­(AGE-co-EO) thin films were immersed in DI water in a Bruker Scientific Fluid Cell MTFML-V2 and imaged with plasma cleaned (30 s on medium power) Bruker SNL-10 tips with a resonance frequency of about 60 kHz. The tips were measured to have a spring constant between 0.67 and 0.86 N/m based on thermal turning and a simple harmonic oscillator fit. Tapping mode was used in a solution of DI water passed through a filtered syringe at a rate of 0.1–0.3 mL/min to eliminate any debris on the surface. PS-b-PEO samples were also imaged statically. Samples were imaged at around 0.6–0.9 Hz for an average of 1–1.5 h. For force experiments, the amplitude set point (drive amplitude) was recorded, then decreased (increased) incrementally, then increased (decreased) again for extended comparison imaging. The initial set-point was at a value carefully chosen to be as delicate as possible while still faithfully tracking the surface topography, achieved by initially contacting the surface, then fully retracting the tip to be oscillating in air, and then slowly bringing the tip back into contact with the surface until proper tracking was recovered. From there, the amplitude set point was decreased by steps of ∼10% (20% for PS-b-P­(AGE-co-EO)). Interestingly, the set point could not be lowered past 68% for PS-b-PEO and 40% for PS-b-P­(AGE-co-EO) reliably, as the films began to tear, and no proper images or APD curves could be collected. After fluid imaging, samples were dabbed with a Kimwipe and/or very gently blown with N₂ to remove excess water, then imaged at the time points referenced in air. Films were very stable in DI water with no noticeable delamination even after submersion overnight. Films also appeared stable over long periods of storage (covered in the dark) in air, with little degradation seen after imaging many months later.

Film Thickness Measurements

Film heights were measured by scraping the films with a new surgical knife and then measuring the samples in air and fluid, from the top of the film to the bare silicon surface. For each measurement result, at least 3 separate samples were probed, and within each sample, at least 3 measurements were taken for each type (pre-DI, DI, post-DI). Analysis was initially performed on proprietary Bruker NanoScope Analysis 2.0 software using step and section functions but then verified in Gwyddion 2.4 software using masking and plane leveling techniques for the silicon background, which often gave a flatter silicon substrate for comparison. Values were averaged, and standard error analysis procedures were followed to get the values seen in Table ST2.

Amplitude-Phase Distance (APD) Force Spectroscopy Experiments

New, plasma cleaned probes were used for air (RTESPA-150, k ∼ 30 N/m) and fluid (SNL-10, k ∼ 0.7 N/m) force spectroscopy measurements. For most samples, the force curves were collected after general imaging so as not to disturb the surface for image collection and not damage the tip. Curves were collected in both deflection mode (not shown) and amplitude tapping mode. Most curves were collected at areas far enough away from each other (larger than tip radius and probable area of effect ∼50–100 nm) and then processed and analyzed using Bruker Nanoscope 2.0 software, Gwyddion 2.4 software, and custom MATLAB code. Curves were acquired at different approach and retraction rates, and multiple curves were taken to ensure reproducibility. A period of relaxation (1, 10, 30 min) was allowed for different samples in between curve collection to ensure accurate data collection. Amplitude, phase, and correction deflection curves were analyzed via a custom MATLAB code. The deflection sensitivity was measured via the standard “infinite” hard surface (silicon wafer piece) and verified by thermal tuning and a Lorentzian fit in DI water found to be 33.10 nm/V. The deflection sensitivity correction was put at a standard 1.08, and the spring constant was measured for each different tip used. The quality factor (Q) was found to be 2.7. The nominal free amplitude for both force curve measurements in DI water was ∼510 mV, with the set points indicated in the images. Calibration was done after imaging to ensure that calibration did not damage the tip. These parameters from tuning and thermal fitting were then used for transforming the force curves from that session. New calibrations were performed for each new tip used and each new session, but during the same collection of force curves, only measurement parameters were changed; the whole setup was not recalibrated. Tip parameters were measured in situ, or approximations were taken from Bruker data sheets where needed. We converted the key instrument observables, namely the amplitude and phase shift, to force–distance curves using the method of H. Söngen et al., an abbreviated version of which is shown in Figure S11. In short, using the harmonic approximation, we made the simplifying approximation in our system that from the tip sample force we only dealt with the even force component, mainly targeting the conservative force (acknowledging that the nonconservative force also affects the even force component). Using these approximations and the even component of the force, we could obtain the force gradient, which could then be integrated over the closest tip–sample distance to infinity (signifying no interaction, with a cutoff defined primarily by the phase channel). This was then input into the overall force equation to receive the total force given in the curves in Figure (and Figure S12). The odd component of the force was not treated in our system.

Glossary

Abbreviations

AFM

Atomic Force Microscope

APD

Amplitude-Phase Distance Curve

BCP

Block Copolymer

∥C

Lying-Down Parallel Cylinder

⊥C

Standing-Up Perpendicular Cylinder

fc-AFM

Fluid Cell Atomic Force Microscopy

T _g

Glass Transition Temperature

PS

Polystyrene

PEO

Poly­(ethylene oxide)

PAGE

Poly­(allyl-glycidyl ether)

PS-b-PEO

Polystyrene-block-poly­(ethylene oxide)

PS-b-P­(AGE-co-EO)

Polystyrene-block-poly­[(allyl glycidyl ether)-co-(ethylene oxide)]

PS-b-PAA

Polystyrene-block-poly­(acrylic acid)

R _sp

Amplitude set point ratio

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.macromol.5c03044.

• Block copolymer parameters, additional AFM analysis, film thickness measurements, harmonic mean energetic calculations and variables, and force curve analysis and reconstruction details (PDF)

K.R.H. prepared most samples and conducted all AFM experiments. D.V.A prepared key thin film samples and provided polymer science insights. W.R.T.T. prepared some PS-b-PEO block copolymer samples and provided polymer science insights. K.W.O. synthesized PS-b-P­(AGE-co-EO) block copolymer and provided initial AFM and polymer sample support. Y.M.L. and Q.Z. synthesized PS-b-PEO block copolymer. J.T. assisted with key AFM experiments, theoretical work and manuscript feedback. E.N. provided valuable theoretical, technical, and manuscript input. L.A.E. and U.B.W. conceived the initial experiments, oversaw all work, and provided key insights into data analysis and interpretation. K.R.H. prepared first manuscript draft, with feedback for subsequent iterations from L.A.E., U.B.W., and E.N.

DOE Grant DE-SC0010560 and FWP 81642. DOE SCGSR Solicitation 2020 2 – Conducted at Pacific Northwest National Laboratory. CCMR NSF MRSEC Grant DMR-1719875.

The authors declare no competing financial interest.