19 nm‐Thick Grafted‐To Polymer Brushes onto Optimized Poly(Dopamine)‐Coated Surfaces

Grafting‐to polymer coatings are typically easy to apply, but the thickness of such coatings is typically limited to a few nanometers, which may hamper applications. This paper presents a grafting‐to coating approach that yields polymer brushes up to an unprecedented thickness of 19 nm. To this aim, an easy‐to‐apply poly(dopamine) (PDA) primer layer is optimized. PDA is an easy‐to‐apply, but highly complex and chemically not well understood primer layer. In this study, PDA is deposited on silicon substrates using several deposition protocols (pH 4–7 in presence of NaIO4, and from Tris solution at pH 8.5). The modified surfaces are characterized using X‐ray photoelectron spectroscopy, spectroscopic ellipsometry, static water contact angle measurements, and atomic force microscopy. Subsequently, block copolymers of poly(glycidyl methacrylate)20‐b‐poly(N‐isopropylacrylamide)n are attached onto the PDA films using a grafting‐to approach. The results indicate that the conditions of PDA deposition and the PDA film thickness strongly influence the stability and grafting efficiency of the block copolymers. PDA films deposited at pH 7 with NaIO4 are stable, and yield the most efficient grafting, with grafted polymer layers as thick as 19 nm. Polymer layers of such thickness are rarely achieved using grafting‐to procedures from solution.

hurdle to tackle. Grafting from melt, [27][28][29][30][31][32][33][34] or cloud-point grafting, [35][36][37][38][39][40] can partially overcome these limitations, but these procedures typically require highly specific reaction conditions. Aside from selecting the most optimal grafting conditions, the means of binding the polymer to the substrate requires careful consideration during the development of a grafting-to procedure. One of the prerequisites for achieving stable and dense polymer coatings is the ability of the polymer chains to efficiently and permanently bind to reactive sites on the surface. These reactive sites are often introduced by modification of the target substrate with a primer layer. [25] A modification agent with high potential is poly(dopamine) (PDA), mainly due to its ease of use, applicability on virtually any surface type and low cost. [41,42] Modification of a target substrate with poly(dopamine) introduces functional groups to the surface, such as amine and hydroxyl moieties, which can then be employed for further modification.
The chemical structure of poly(dopamine) has been the subject of many studies and is notoriously complex. [43][44][45][46][47] Despite extensive analysis using a range of different techniques, the exact structure of the highly cross-linked material as well as a complete understanding of the preceding polymerization mechanism remain elusive. [43,[45][46][47][48][49] To add to the complexity, it has been demonstrated that the chemical structure and morphological features of a poly(dopamine) film are influenced by both the type of substrate and the deposition conditions. [49][50][51][52][53] These properties can be expected to strongly influence the reactivity of the poly(dopamine) toward any subsequent modification steps. Evidently, insight in these factors is of great value for any procedure that involves modification of poly(dopamine) films.
One of the most commonly employed procedures for the synthesis of poly(dopamine) layers relies on polymerization induced by oxidation of dopamine in a basic Tris buffer. [41,42,54] For this procedure, control over thickness and roughness of the resulting film can be obtained by variation of deposition time and dopamine concentration. [55] Alternatively, the polymerization can be initiated by oxidizing agents, such as CuSO 4 , CuSO 4 /H 2 O 2 , [56] (NH 4 ) 2 S 2 O 8 , [57] or NaIO 4 . [50] Specifically, using NaIO 4 as oxidant can significantly speed up the deposition process, and yields low-roughness, highly hydrophilic surfaces. [50,58,59] Recently, our group introduced a highly efficient graftingto procedure to synthesize polymer brushes based on attachment of poly(glycidyl methacrylate)-b-poly(N-isopropylacrylamide) (poly(GMA)-b-poly(NIPAM)) block copolymers to poly(dopamine)-modified silicon surfaces. [60] The grafting densities that could be achieved using this procedure were relatively high for grafting-to procedures from solution (up to 0.12 chains nm −2 ). It is therefore of interest to investigate the potential of this procedure in more detail, explicitly scrutinizing the chemical and topological properties of the poly(dopamine) primer layer, and the effects those have on follow-up grafting-to polymer coatings.
In this work we aim to investigate three facets: 1) to which degree do the properties of the underlying poly(dopamine) layer vary in response to the deposition conditions; 2) to which degree do the properties (thickness, density, stability) vary of a grafted-to polymer coatings that is deposited onto this poly(dopamine) layer, in dependence on such variations in the PDA layer; and 3) how thick can we make grafted-to polymer brushes in attempts to overcome the 10 nm-thickness limitation. To this aim, a simple two-step modification procedure (Scheme 1) was followed: silicon surfaces were coated with poly(dopamine) by deposition from Tris buffer (pH 8.5) or acetate buffer (pH 4 or 7), in the presence of NaIO 4 . Next, three poly(GMA) 20 -b-poly(NIPAM) n block copolymers were grafted to the thus obtained set of poly(dopamine)-coated substrates. The influence of the poly(dopamine) deposition conditions on the block copolymer grafting step was studied thoroughly, using X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry, static water contact angle measurements, and atomic force microscopy (AFM), and pointed to the exceptional efficacy of this grafting-to coating method.

Polydopamine Film Formation
As the first step (Scheme 1), various deposition methods of poly(dopamine) onto (native oxide-covered) silicon substrates were investigated. First, poly(dopamine) was deposited from a Tris-HCl buffer solution at pH 8.5. Alternatively, the deposition was carried out in the presence of oxidizing agent NaIO 4 at pH 4, 5, 6 or 7. For the experiments in the presence of NaIO 4 , the pH was regulated using a HOAc/NaOAc buffer solution. It has been demonstrated previously that the pH strongly influences both the oxidation of catechols and formation of poly(dopamine) films, specifically: a low pH reduces the rate of both. [55,61,62] Additionally, the deposition pH has been reported to affect the adhesive strength of the poly(dopamine) film. [63,64] The kinetics of the poly(dopamine) film deposition were investigated using spectroscopic ellipsometry ( Figure 1A). The Scheme 1. Schematic representation of the silicon surface modification procedure employed in this study.
www.advmatinterfaces.de evolution of layer thickness for poly(dopamine) from basic Tris solution onto (native) silicon oxide surfaces was relatively slow and reached 21 ± 2 nm after 4 h, which agrees well with previous reports for deposition on silicon surfaces. [41,65] The deposition reaction was found to be much faster for poly(dopamine) films deposited at pH 7 in the presence of NaIO 4 . Film formation was visibly observed within a few minutes, reaching thicknesses >40 nm within 1 h, and with kinetics that did not vary significantly upon changing from 10 to 20 mm NaIO 4 or from pH 7 to 6. These overlapping kinetic plots suggest that under these reaction conditions, film growth rate is not limited by NaIO 4 concentration or pH, but by the starting concentration of dopamine. [61] By contrast, when performing the reaction under more acidic conditions, pH 4 and 5, an induction period (up to 40 min at pH 4) was observed after which film formation accelerated. Under these conditions, poly(dopamine) film formation is apparently governed by two processes: first, adhesion of poly(dopamine) to the surface, and second, growth of the poly(dopamine) film. [54,66] The observed induction period for deposition at pH 4 is attributed to a lower oxidation rate of dopamine in acidic environment, as was reported in several previous studies. [59,61,63] While the ellipsometric thickness suggested little dopamine attachment during the induction period, the emergence of the N 1s signal in the XPS wide scan spectrum reveals that some dopamine attachment does take place (see Figure S1, Supporting Information). However, the low oxidation rate strongly hampered the cross-linking between dopamine species, and film formation was practically inhibited for the first 40 min. With time, dopamine is increasingly oxidized, allowing film thickness-increasing cross-linking reactions to become prevalent.
Static water contact angles (CA) were measured for poly(dopamine) layers deposited from NaIO 4 -containing acetate buffer, at pH 4 and 7, and for poly(dopamine) layers deposited from Tris buffer, at pH 8.5 ( Figure 1B). The CA for poly(dopamine) deposited from the Tris buffer decreased slightly with increasing layer thickness, from 58° ± 7° at a layer thickness of 3 ± 1 nm to 40° ± 6° at the thickness of 21 ± 2 nm. Poly(dopamine) films deposited at pH 7 in the presence of NaIO 4 exhibited a contact angle of 51° ± 2° at a layer thickness of 3 ± 1 nm, which dropped substantially with increasing layer thickness to 24° ± 2° at 23 ± 1 nm, in accordance with previous findings. [50] The poly(dopamine) films deposited in the presence of NaIO 4 at pH 4 demonstrated roughly constant static water contact angles, ≈28°, irrespectively of layer thickness.
The surface roughness R q of the different substrates was analyzed using AFM (Figure 2 and Supporting Information).
In accordance with previous studies, poly(dopamine) films produced in the presence of NaIO 4 displayed lower R q values (in the range of 5-7 nm for layer thicknesses of 20 nm) than poly(dopamine) films produced through self-oxidative polymerization in basic environment, which reached values up to 34 ± 6 nm. [66] For the substrates that were submerged in the basic Tris solution, large aggregates of poly(dopamine) were visible on the surface. It appears that during deposition from basic Tris buffer, dopamine polymerization-which can to a small degree also still occur from a surface-initiated polymer growth process-occurs extensively in solution, yielding large, insoluble particles that end up on the surface. The results obtained from AFM and CA measurements demonstrate that the deposition procedure strongly influences both chemical features (wettability) and morphological features (surface roughness) of the poly(dopamine) film.
The more hydrophilic character of the poly(dopamine) films deposited in the presence of NaIO 4 , has been attributed to extensive oxidation of the poly(dopamine) film, which produces hydrophilic carboxylic acid groups and quinonoid structures. [50] Comparison of elemental ratios between C/N or C/O obtained from XPS wide scan did, however, not substantiate this hypothesis (see Figure S8A, Supporting Information). Remarkably, poly(dopamine) deposited at pH 7 in the presence of NaIO 4 , exhibited a higher contact angle at low thicknesses, than observed for poly(dopamine) deposited at pH 4 in the presence of NaIO 4 , despite their similar roughness values. By contrast, at increasing layer thickness, the contact angle for both films was ≈25°. A possible explanation for this is that dopamine at pH 7 is oxidized at higher rate than at pH 4, [61] yielding an increased, oxidation-induced fraction of cyclized compounds in the polymer film than at pH 4, which in turn would provide a more hydrophobic film. Then, as the amount of available NaIO 4 decreases with longer deposition times, the oxidation rate of dopamine is lowered and therefore less cyclization occurs.

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Incorporation of the uncyclized dopamine, in combination with increasing layer thickness and roughness, finally yields contact angles comparable to those of poly(dopamine) deposited at pH 4.
To test this hypothesis, an extensive XPS study was performed on poly(dopamine)-coated substrates produced from NaIO 4 -containing buffer, at pH 4 and 7, and for poly(dopamine) layers deposited from Tris buffer (see Figures S5-S8, Supporting Information). The XPS study involved thorough analysis of the N 1s and C 1s narrow scan spectra of these samples. The spectra were deconvoluted to represent the binding energies reported previously for the different functional groups present in the poly(dopamine) structure, [47] but no significant differences in chemical composition could be found between the films prepared using different procedures. This result implies that, to the degree that this is observable, the poly(dopamine) films exhibit roughly the same variety and ratios of functional groups. Yet, as the roughness values are highly similar, the differences observed in CA measurements are likely to originate from differences in chemical composition. Therefore, it is conceivable that due to the complex structure of poly(dopamine), which contains many different molecular structures each with their own binding energy, subtle differences in component ratios may go unnoticed using XPS analysis.
Overall, the poly(dopamine) deposition at pH 4 and pH 7 in the presence of NaIO 4 resulted in substantially more homogeneous and faster film formation than deposition from basic Tris solution. The remainder of this study will therefore focus on poly(dopamine)-modified substrates that were produced in the presence of NaIO 4 . The results for the grafting-to studies on poly(dopamine) films deposited from Tris buffer are included in the Supporting Information.

Grafting-to Studies
Grafting-to reactions were performed using poly(GMA) 20 -bpoly(NIPAM) 210 that had been separately synthesized using RAFT polymerization in solution, following an earlier published procedure. [60] Briefly, GMA was polymerized in the presence of RAFT agent to synthesize poly(GMA) 20 , which then was used as macro-RAFT agent in a subsequent polymerization reaction of NIPAM, to yield poly(GMA) 20 -b-poly(NIPAM) 210 (see the Supporting Information for NMR, IR, and gel permeation chromatography (GPC) data). In a previous study, we demonstrated that for these polymers, the poly(GMA) block acts as an efficient anchoring segment, whereas the poly(NIPAM) block induces thermoresponsive and antifouling properties. [60] Here, Figure 2. A) Surface roughness R q of poly(dopamine) films deposited from NaOAc buffers of varying pH in the presence of NaIO 4 , or from Tris buffer, pH 8.5. Representative AFM images for silicon substrates modified with poly(dopamine) films at B) pH 4 with NaIO 4 (thickness = 19 ± 1 nm). C) pH 7 with NaIO 4 (thickness = 23 ± 1 nm) and D) pH 8.5 (thickness = 21 ± 2 nm).

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the influence of poly(dopamine) deposition conditions and deposition time on the efficiency and rate of the grafting-to reaction were investigated.
Poly(GMA) 20 -b-poly(NIPAM) 210 was attached to a range of poly(dopamine) substrates, obtained with various thicknesses and from several different preparation methods, via a graftingto procedure in DMSO. Spectroscopic ellipsometry measurements were performed following these reactions to determine the total layer thickness of the resulting coatings, so as to infer the thickness of the poly(GMA) 20 -b-poly(NIPAM) 210 layer, and these latter values for the top layer are given in Figure 3. From there we surmise that two factors determine the film thickness under these circumstances.
First, the stability of the poly(dopamine) films is a clear issue under these conditions, and unstable attachments evidently yielded overall thinner films. All grafting reactions performed on poly(dopamine) layers that were deposited in the presence of NaIO 4 at pH 7 demonstrated an increase in film thickness after the polymer grafting reaction. By contrast, for the reactions performed on thicker poly(dopamine) layers deposited in the presence of NaIO 4 at pH 4, a loss of poly(dopamine) film was observed following the reaction. Despite the various literature reports on the use of poly(dopamine) as anchoring layer, reports on the stability of poly(dopamine) layers are scarce. [56,67] It is, however, apparent that the adhesive strength of poly(dopamine) depends strongly on the type of substrate employed, making it hard to draw general conclusions on the stability of poly(dopamine) layers. [51,67,68] In this work, the stability against DMSO of the poly(dopamine) layer is highest when this was deposited at pH 7 in the presence of NaIO 4 , and-as shown by a control experiment with twice the amount of NaIO 4 (20 mm)-not affected by the concentration of NaIO 4 , similar to the observation made earlier on the deposition kinetics. Preliminary stability tests were performed on the produced surfaces, which turned out to be remarkably stable upon 24 h exposure to aqueous solutions in the pH range from 1 to 9 (see Figure S16, Supporting Information). By contrast, at pH 11 and 13, near-complete removal of the organic material was observed, and these latter results are in agreement with previous findings for poly(dopamine) films in strongly basic environments. [69] The second factor that plays an important role during polymer grafting is the thickness of the poly(dopamine) layer. Grafting reactions performed on thin poly(dopamine) films (<10 nm) gave rise to poly(GMA) 20 -b-poly(NIPAM) 210 layers of 6 ± 4 nm. Interestingly, for identical reactions performed on thicker poly(dopamine) films (>10 nm), significantly higher poly(GMA) 20 -b-poly(NIPAM) 210 layers could be achieved. In particular, grafted polymer layers reached thickness values up to 19 ± 3 nm, when deposited onto poly(dopamine) films thicker than 20 nm. A control experiment was performed to ensure that the observed increase in film thickness was not caused by swelling of the poly(dopamine) layer under the reaction conditions ( Figure S17, Supporting Information). Three poly(dopamine) substrates (average thickness 29.4 ± 0.7 nm) were exposed to grafting conditions without the presence of block copolymer. Following this experiment, the thickness of the poly(dopamine) layer remained virtually unchanged (average thickness 30.8 ± 2.4 nm). The increased thickness measured for the polymer grafting reactions must therefore indeed be caused by attachment of the block copolymer.
XPS analysis of the polymer-modified surfaces confirmed that the increase in layer thickness was indeed the result of addition of poly(GMA) 20 -b-poly(NIPAM) 210 (Figure 4A), as the silicon signal was no longer visible in the wide scan spectrum of the block copolymer-modified poly(dopamine) layer. Similarly, the iodine peak that was present after poly(dopamine) deposition had disappeared. It appears that any traces of iodate and iodic acid that were enclosed in the poly(dopamine) film leached out during the subsequent grafting step. Furthermore, the N 1s narrow scan spectrum showed a well-defined, narrow peak at 400.0 eV, arising from the amide functional groups present in the grafted block copolymer ( Figure 4B). In this N 1s XPS narrow scan, also no signals corresponding to CNC and CNH 3 + groups present in the poly(dopamine) structure were visible anymore, pointing to the presence of a significant overlayer on top of the poly(dopamine) layer.
The contact angles for the block copolymer-modified substrates reached 59° ± 1° for the samples with the thickest block copolymer layers (Figure 5A), which compare well with the value of 58° found for poly(NIPAM)-modified silicon surfaces in literature. [70] Quartz crystal microbalance with dissipation monitoring (QCM-D) was employed to investigate whether the thermoresponsive properties, characteristic for poly(NIPAM), could be observed in the produced surfaces. Generally, when thermoresponsive polymers are brought in an aqueous environment where the temperature is lower than the polymer's lower critical solution temperature (LCST), water molecules will coordinate to the polymers and the polymers swell. [71] Once the temperature is raised to exceed the LCST, water is entropically expelled and the polymers collapse. This reversible process can be monitored in polymer coatings using QCM-D, as the expulsion of water is accompanied by a measurable change in the stagnant mass present on the surface. [33,60,72] Poly(dopamine) deposition was carried out on SiO 2 -coated QCM-D sensors for 5 and 30 min, followed by grafting the poly(GMA) 20 -b-poly(NIPAM) 210 block copolymer. The sensors were then placed in an aqueous www.advmatinterfaces.de environment in the QCM-D and subjected to a temperature program that adjusted the temperature from 20 to 40 °C, and then back to 20 °C The produced surfaces indeed exhibited thermoresponsive behavior, with the LCST at ±32 °C, as indicated by the peaks in the plot ( Figure 5B; raw data in the Supporting Information), in line with previous studies. [60] Since Δ(Δf)/ΔT is directly related to the mass change on the sensors, the data display the amount of water that is expulsed and again bound when increasing and decreasing the temperature. [33,60] The SiO 2 sensors that were coated with poly(dopamine) for 30 min show the greatest amount of water expulsed and coordinated again over the applied temperature cycle. These surfaces thus have a larger capacity to bind water and thus likely have more polymer material grafted to the surface, suggesting agreement between these QCM-D data and the ellipsometry data discussed above.
The grafted layer thickness of 19 ± 3 nm obtained in this study is exceptionally high for grafting-to procedures performed from solution, particularly considering the relatively high molecular weight of poly(GMA) 20 -b-poly(NIPAM) 210 . [26,32,33,73,74] Alexander-de Gennes' theory enables approximation of the grafting density, σ, based on the measured dry thickness of the polymer layer, the size of the polymers, Avogadro's number and the polymer bulk density. [75] Based on a layer thickness of 19 ± 3 nm, the grafting density, or chains per nm 2 , can be approximated at 0.48 ± 0.07 chains nm −2 . Such high grafting densities for grafting-to reactions from solution have to our knowledge not yet been reported, [26] and are even more remarkable given the relatively long polymers (M n = 38 kDa) used here.
To broaden the scope of this grafting procedure and further investigate the cause for the observed grafting densities, block copolymers of increased sizes were synthesized, specifically poly(GMA) 20 -b-poly(NIPAM) 404 and poly(GMA) 20 -b-poly(NIPAM) 566 . These polymers were grafted to poly(dopamine) substrates under identical conditions as described for poly(GMA) 20 -b-poly(NIPAM) 210 . The thickness measured for the resulting grafted layers was highly similar to the results obtained earlier, showing an increase in block copolymer layer on thicker poly(dopamine) films ( Figure 6A). Evidently, a change in poly(dopamine) features with increasing thickness is responsible for the higher grafting efficiency. We therefore hypothesize that at lower poly(dopamine) thickness, the grafting reaction is limited by the number of suitable reactive groups present near the surface of the poly(dopamine) layer. With increasing poly(dopamine) layer thickness, a larger total number of reactive groups will be available throughout the increasingly porous, granular structure of the poly(dopamine) film, [76] leading to a wide variety of starting heights of the grafted polymers and thus a rapidly increasing roughness.
AFM measurements were performed to determine the surface roughness of the substrates after polymer grafting  www.advmatinterfaces.de ( Figure 6B). The roughness increased markedly with increasing block copolymer layer thickness and followed the trend that was previously observed for roughness of poly(dopamine) (Figure 2A). To rule out a roughness increase caused by the poly(dopamine) layer during the reaction, the roughness of three poly(dopamine) substrates was measured before and after exposure to grafting conditions in the absence of the block copolymer ( Figure S17, Supporting Information). The average roughness of these substrates did not increase, indicating that the increased roughness was a result of block copolymer attachment. The roughness values obtained for poly(GMA) 20b-poly(NIPAM) 566 grafted on poly(dopamine) layers that had a PDA film thickness of 18.7 ± 0.7 nm or 28.9 ± 1.9 nm, with characteristic roughnesses of 4 and 8 nm, respectively) are of particular interest. The attached block copolymer layers for these substrates have similar thicknesses of 12.6 ± 1.7 and 12.4 ± 4.3 nm, respectively (as determined by spectroscopic ellipsometry). The AFM-derived R q values differ significantly and are 11.8 ± 0.6 nm for grafting on the thinner-and relatively smoother-poly(dopamine) film and 24.4 ± 5.4 nm for grafting on the thicker-and relatively rougher-poly(dopamine) film. This implies that grafting of the block copolymer amplifies the topological features of the underlying poly(dopamine) film and thereby raises the surface roughness.
Interestingly, the AFM images obtained from the samples with thickest block copolymer layers showed thread-like regions that were not observed in the other samples ( Figure 6C and Supporting Information) and could not be removed by extensive washing or sonication. The structures have a length and width of roughly 1-2 and 0.3 µm, respectively, and are therefore too large to correspond to single polymer chains. Possible explanations for these features may be clustering of the surface-attached polymers, cross-linking between poly(glycidyl methacrylate) segments of the block copolymers or partial rupture of the poly(dopamine) layer. Cross-linking of the polymers would result in large, bottlebrush-like structures that correspond to the size range observed in the AFM image. A control experiment was performed to determine whether cross-linking of the block copolymers occurs in solution. Grafting conditions were simulated by heating a solution of poly(GMA) 20 -bpoly(NIPAM) 210 in DMF with 5% triethylamine (TEA) to 80 °C overnight without the presence of a poly(dopamine) substrate. GPC measurements were then performed and compared to the measurements taken before control experiment. The traces were near-identical, indicating that no reaction took place between the polymer chains in the reaction solution (see Figure S24, Supporting Information). Moreover, the previously described experiment in which thick poly(dopamine) films were exposed to grafting conditions without the presence of polymer did not result in thread-like features on the surface (see Figure S17, Supporting Information). Furthermore, sonication of a substrate with this thread motif in acetone for 10 min did not cause any further rupture of the film (see Figure S25, Supporting Information). As a result, the thread-like motifs are believed to be the result of clustering of the polymers on the topological features of the poly(dopamine) films.
In conclusion, the amount of polymer that can be grafted on poly(dopamine) and the resulting surface features are highly dependent on the poly(dopamine) layer thickness. It is to be expected that such a dependence translates to any grafting-to reaction performed on poly(dopamine) films. Additionally, in a much broader sense, these findings plainly illustrate the strong influence that poly(dopamine) properties have on the outcome of any subsequent modification step. With poly(dopamine) on the forefront in the development of universally applicable coating procedures, it is of the essence for potential future application that studies of poly(dopamine) concerning modification efficiency and film stability are thoroughly performed. [77] In that regard, this study contributes to development of a straightforward, substrate-independent grafting-to procedure for the synthesis of stable polymer brush coatings.

Conclusions
Poly(dopamine) deposition conditions greatly affect the properties of the poly(dopamine) film and, as a result, the efficiency of a subsequent grafting-to reaction using three block copolymers, specifically poly(GMA) 20 -b-poly(NIPAM) 210 , poly(GMA) 20b-poly(NIPAM) 404 , and poly(GMA) 20 -b-poly(NIPAM) 566 . Using optimized deposition conditions for both the poly(dopamine) layer and for the grafted-to polymer layer on top of this, we www.advmatinterfaces.de were able to graft polymer layers up to 19 nm thick. To get to this result, first poly(dopamine) was deposited at pH 4-7 in the presence of NaIO 4 , and at pH 8.5 from Tris buffer, and distinct differences in deposition rate, hydrophilicity, and surface roughness were observed. In addition, after grafting of the block copolymer poly(GMA) 20 -b-poly(NIPAM) 210 , variations in stability and grafting efficiency were apparent. Concerning the stability, poly(dopamine) deposited at pH 7 with NaIO 4 was the only film to survive the grafting conditions. Furthermore, the amount of block copolymer grafted to poly(dopamine) was shown to increase with poly(dopamine) layer thickness. The ultimate 19 ± 3 nm thick poly(GMA) 20 -b-poly(NIPAM) 210 layers had an approximated grafting density of 0.48 ± 0.07 chains nm −2 . We expect such thicker and still easy to apply grafted-to surface coatings to have improved properties for a wide range of applications, including antifouling and self-healing coatings.
Prior to use, GMA was purified by washing with 0.1% m/v KOH solution and NIPAM was recrystallized from hexane.
Modification of Silicon Substrates with Poly(dopamine): 1 × 1 cm Si substrates were rinsed using acetone, ethanol, and MilliQ and then dried under a gentle stream of argon. The Si substrates were placed in a petri dish containing freshly prepared solution of dopamine HCl. For deposition in the presence of NaIO 4 , this solution consisted of dopamine HCl (40 mg, 0.21 mmol, 10.5 mm) and NaIO 4 (10 or 20 mm) in 50 mm HOAc/NaOAc buffer (20 mL) of the desired pH. For deposition from basic Tris buffer, the solution consisted of dopamine HCl (40 mg, 0.21 mol, 10.5 mm) in 10 mm Tris HCl (Tris) buffer (pH 8.5). The petri dish was closed, sealed using parafilm, and placed on an automated shaker at RT, 60 RPM. The surfaces were removed from the solution after the appropriate time, cleaned using MilliQ and subsequently dried under nitrogen flow.
AFM: Atomic force microscopy was performed using an Asylum MFP-3D Origin AFM (Oxford Instruments, UK). The instrument was operated in tapping mode and equipped with a silicon cantilever (AC240TS-R3, k = 1.3 N m −1 ) with a nominal tip radius of ≈7 nm. The acquired images were processed and analyzed using Gwyddion opensource software for SPM data analysis.
Ellipsometry: Ellipsometric angles Δ and Ψ of the synthesized polymer brushes were measured using an EP4 imaging ellipsometer (Accurion, Germany). The measurements were performed in air at room temperature in the wavelength range of λ = 491-761.3 nm at an angle of incidence of 50°. The acquired Δ and Ψ were fitted in the EP4 modeling software using a multilayer model to obtain dry polymer brush thickness and refraction index values. The poly(dopamine) layer was described using a Cauchy-Urbach model to account for the light absorption of poly(dopamine) The following parameters were used: A = 1.54, B = 3000 nm 2 , α = 0.161 ± 0.58, β = 0.242 ± 0.121, and E b = 3.0 eV. The poly(GMA) 20 -bpoly(NIPAM) 210 layer for polymer-modified substrates was described using a Cauchy model with parameters A = 1.50 and B = 3000 nm 2 .

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Addition of an outermost layer to account for the roughness of the measured substrates using Bruggeman's effective medium approximation did not improve the fit of the model. [83] Control experiments were carried out to verify the model by scratching the surface of a modified silicon oxide substrate and measuring the thickness using AFM (see the Supporting Information).
Following Alexander-de Gennes' theory, the ellipsometric thickness, h dry , was used to approximate the grafting density of the poly(GMA) 20b-poly(NIPAM) 210 block copolymer chains on the poly(dopamine) films [75] Here, σ is the grafting density in chains nm −2 , ρ b is the density of the grafted polymer layer, N A is Avogadro's constant, and M n is the polymer molecular weight. Bulk density of poly(NIPAM), ρ b , was 1.07 g cm −3 used. [84] ATR FT-IR Spectroscopy: FT-IR spectra were obtained on a Bruker Tensor 27 spectrometer with platinum attenuated total reflection accessory. The samples were applied as powder or oil on top of the crystal. 64 scans were performed with a resolution of 4 cm −1 .
X-Ray Photoelectron Spectroscopy: XPS measurements were performed using a JPS-9200 photoelectron spectrometer (JEOL Ltd., Japan). All samples were analyzed using a focused monochromated Al Kα X-ray source (spot size of 300 µm) at a constant dwelling time for wide-scan 50 ms and narrow-scan of 100 ms and pass energy: wide-scan 50 eV, narrow-scan: 10 eV, under UHV conditions (base pressure: 3 × 10 −7 Pa). The power of the X-ray source was 240 W (15 mA and 9 kV). Charge compensation was applied during the XPS scans with an accelerating voltage of 2.8 eV and a filament current of 4.8 A. All narrow-range spectra were corrected with a linear background before fitting. The spectra were fitted with symmetrical Gaussian/Lorentzian (GL(30)) line shapes using CasaXPS. The wide scan spectra and C 1s narrow scan spectra were referenced to the C 1s peak attributed to CC and CH atoms at 285.0 eV. The fits employed for deconvolution were attributed to CC and CH (285.0 eV), CN (286.1 eV), CO (286.8 eV), CO (288.6 eV), and π-π* shake-up (291.1 eV).
The N 1s narrow scan spectrum was referenced to the N 1s peak attributed to C-NH-R at 400.0 eV. The fits employed for deconvolution were attributed to CNC (389.9 eV), CNHR (R = C,H; 400.0 eV) and CNH 3 + (401.5 eV). Static Water Contact Angle Measurements: The wettability of the modified surfaces was determined by automated static water contact angle measurements using a DSA 100 goniometer (Krüss, Germany). The volume of a drop of demineralized water employed was 3 µL. Contact angles from sessile drops measured by the tangent method were estimated using a standard error propagation technique involving partial derivatives.
QCM-D Measurements: Measurements were performed by using Q-Sense E4 QCM-D (Biolin Scientific, Sweden). Prior to the temperaturedependent measurements, MilliQ was pumped via a peristaltic pump (Ismatec high precision multichannel dispenser) with a flow rate of 400 µL min −1 for at least 15 min. The flow rate was then reduced to 25 µL min −1 . The temperature was set at 20 °C and the system was left to stabilize for 1 h. The temperature was then increased with steps of 2 °C with a rate of 1 °C min −1 , where each step was followed by a stabilization period of 1 h. After reaching 40 °C and stabilization of 1 h, the temperature was decreased with steps of 2 °C with a rate of 1 °C min −1 , where again each step was followed by a stabilization period of 1 h. The measurement was stopped following 1 h of stabilization after returning to 20 °C.
To account for any possible influence of the poly(dopamine) during these measurements, the experimental data were corrected by subtracting the frequency changes reference experiments were performed using poly(dopamine)-coated SiO 2 sensors.
Statistical Analysis: All statistical results were calculated using Origin 2019 software. Data were displayed as average values ± standard deviation.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.