Polymer brushes grafted from nanostructured zinc oxide layers – Spatially controlled decoration of nanorods

European Polymer Journal 112 (2019) 186–194

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Polymer brushes grafted from nanostructured zinc oxide layers – Spatially controlled decoration of nanorods

T

Agata Pomorskaa,b, Karol Wolskia, Magdalena Wytrwal-Sarnac, Andrzej Bernasikc,d, ⁎ Szczepan Zapotocznya, a

Jagiellonian University, Faculty of Chemistry, Gronostajowa 2, 30-387 Krakow, Poland Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30- 239 Krakow, Poland c AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, Al. A. Mickiewicza 30, 30-059 Krakow, Poland d AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Al. A. Mickiewicz 30, 30-059 Krakow, Poland b

A R T I C LE I N FO

A B S T R A C T

Keywords: QCM PNIPAM ATRP ZnO nanocrystalline films

Poly(N-isopropylacrylamide) (PNIPAM) brushes were grafted from ZnO nanostructured layers with nanorod and hexagonal grains morphologies via Atom Transfer Radical Polymerization in various methanol/water mixtures. The brushes’ growth was followed using Quartz Crystal Microbalance and the obtained hybrid systems were characterized microscopically and using XPS profiling. The studied nanostructured ZnO has high potential for e.g., photovoltaic, sensor applications that may be enhanced in nanocomposites with spatial and compositional control on the organic/inorganic interface. Such systems were realized by grafting polymer brushes from both the convex tops of nanorods and concave areas between them. It was revealed that the growth of PNIPAM brushes on such nanostructured substrates may be controlled by varying the solvent composition. The polymerization performed at higher methanol content (xMe ≈ 0.31) led to decoration of the nanorods only with thin polymer layers around them while for the lower content the polymerization proceeded much faster leading to formation of the thick coating covering their tops. Importantly, such a layer did not block the brushes growth in the confined intercolumnar space. Thus, fine tuning of the solvent mixture should lead to desired relations between the mass/thickness of polymer brushes grafted from convex and concave areas that are crucial for applications of such nanocomposites.

1. Introduction Surface properties of nanostructured oxide materials with complex topography can be tuned by coating them with polymers in various forms for fabrication of e.g.: sensors [1–3], controlled delivery systems [4] or energy storage devices [5]. Only recently, biomimetic nanocomposites with columnar structure have been reported to exhibit previously inaccessible mechanical properties combining high stiffness, damping and light weight [6]. Such systems can be obtained through noncovalent interactions [7–12] but grafting of polymer layers in the form of brushes [13–15] from functionalized surfaces may offer additional advantages [16–18]. Application of surface-grafted polymer brushes enables precise control of the chain lengths [19], macromolecular conformations [20] together with the ability to tailor functionality at a given distance from the top surface [21]. The entropic repulsion between the neighboring polymer chains attached by one end

to a surface at sufficient density causes the chains in the brushes to extend in the direction perpendicular to the grafting surface, creating more ordered assembly in comparison to typical isotropic polymer films [22,23]. Therefore, it is possible to tune the desired brush thickness [24] matching surface morphology like e.g. pore size [25]. Moreover, conformation of the macromolecules in the brushes can be modified with external stimuli like temperature [26,27], pH [28], ionic strength [23], substrate topography [29], addition of nanoparticles [17] as well as environment confinement [30]. One of the most commonly applied method for fabrication of polymer brushes is the Surface-Initiated Atom Transfer Radical Polymerization (SI-ATRP) [31]. SI-ATRP enables grafting of low dispersity polymer chains from flat as well as rough supports with high surface area like nanocellulose [32], anodized aluminium oxide [4], mesoporous silica [29] or titanium dioxide nanoparticles [33]. The main problem with the polymer brushes formed within confined spaces is the fact that the crowding of chains increases

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Corresponding author. E-mail addresses: [email protected] (A. Pomorska), [email protected] (K. Wolski), [email protected] (M. Wytrwal-Sarna), [email protected] (A. Bernasik), [email protected] (S. Zapotoczny). https://doi.org/10.1016/j.eurpolymj.2019.01.012 Received 8 November 2018; Received in revised form 31 December 2018; Accepted 4 January 2019 Available online 05 January 2019 0014-3057/ © 2019 Elsevier Ltd. All rights reserved.

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modification during their growth [60]. Selected samples were further subjected to X-ray Photoelectron Spectroscopy (XPS) profiling in order to get insight into the composition of the obtained nanocomposites in the direction perpendicular to the surface. The obtained samples were also visualized using atomic force microscopy (AFM) as well as scanning electron microscopy (SEM) and the observed differences were also analyzed with respect to confinement in which the polymerization proceeded. Our observations clearly indicate that by adjusting the solvent composition for SI-ATRP one can tailor the structure of the resulted polymer brushes grown on the nanostructured substrate.

the likelihood of recombination of the growing macroradicals leading ultimately to low molecular weights of the resulted chains [34]. Moreover, decreasing the pore size may affect the transport of monomers and catalysts to the growing macroradicals that leads to decrease of the overall polymer mass [13]. In the case of planar surfaces, the available volume for polymer chains increases approximately linearly with the distance from the interface, therefore providing a linear relationship between brush thickness and polymer chain length for a constant grafting density [13]. Polymer brushes at curved convex interfaces can be subjected to conformational transitions from concentrated to semi-diluted brush regimes with increasing distance from the surface. Therefore, with higher substrate curvature the chain growth should more closely resemble free solution-like behavior [35]. Hence, the resulted molecular weight of polymer brushes decreases as a function of substrate “arc” (with the highest weight in convex surface, lower for flat substrate and the smallest for concave interfaces) [36,37]. This effect is especially substantial for grafted macromolecules with a high degree of polymerization that may additionally block monomer diffusion as presented by Ye et al. [38]. Those findings were supported by molecular dynamics simulation [13]. The curvature of a substrate influences also the tendency of polymer brushes to swell [39]. Poly(N-isopropylacrylamide) (PNIPAM) is one of the most widely studied thermoresponsive polymer as it exhibits a lower critical solution temperature (LCST) in water at around 32 °C [40,41]. The chains are hydrated and adopt an extended coil conformation in aqueous solutions below LCST. Those chains undergo dehydration and collapse to a denser globular conformation above LCST [42]. Furthermore, PNIPAM exhibits an interesting co-nonsolvency effect [43,44] in methanol/water mixtures that was shown to affect also conformations of PNIPAM brushes [45–48]. Zinc oxide (ZnO) nanocrystalline films have gained a lot of attention as functional materials due to their desired electronic, optical and photonic properties [49]. They may be synthesized via the hydrothermal method [50], which offers straightforward adjustment of the crystal morphology [49,51]. Nanocrystalline ZnO in the form of columnar film (aligned nanorods) can be applied as e.g., gas sensors due to its large active surface and efficient carrier transport [52,53] offering a great promise for faster response and higher sensitivity than the planar sensor configuration. The surface of nanocrystalline ZnO can be also functionalized [54] for further tailoring of the sensor surface. Surface modification of the nanostructured ZnO with pyrene-1-carboxylic acid resulted in solar cells with higher short circuit current densities but lower open circuit voltage pointing to a better carrier collection and higher recombination [55]. Additionally, the improvement of efficiency of dye-sensitized solar cells was reported by Peng et al. by replacing ZnO nanoparticles with ZnO hierarchical nanorods as a photoanode. This was attributed to significantly enhanced light scattering of ZnO nanorods compared with ZnO nanoparticle, which led to efficient light harvesting [56,57]. Thus, a combination of responsive polymer brushes with arranged ZnO nanorods has a potential as an advanced material not only for sensing or photovoltaic applications but also as e.g. biomimetic material resembling tooth enamel with extraordinary mechanical properties [58]. Therefore, it is of high interest to explore the mechanism of polymer brush growth on such complex topography combining convex (tips of the rods) and concave (spaces between the rods) curvatures. Gaining control over the growth of functional brushes on such columnar ordered structures should open opportunities to fabricate also other hybrid materials grafted with polymer brushes with designed optical and electrical properties [59]. We report here on PNIPAM brushes that were grown from ZnO nanostructured layers with nanorods (NR) and hexagonal grains (HG) morphologies [54] via SI-ATRP at various solvent compositions (methanol/water ratio). The polymerization was followed in situ via Quartz Crystal Microbalance technique (QCM). This technique gives an insight into the reaction kinetics as well as polymer brush structure

2. Experimental 2.1. Materials Perhydrol (H2O2, 30%, p.a.) was purchased from Stanlab (Lublin, Poland). Ammonia (30%, p.a.) and dichloromethane (p.a.) were obtained from Chempur (Piekary Slaskie, Poland). Methanol and ethanol (absolute for analysis) were purchased from Merck KGaA (Darmstadt, Germany). Zinc acetate dihydrate (98%), zinc nitrate hexahydrate (99%), hexamethylene-tetramine (99%), sodium citrate dihydrate (5 mM, 99%), 3-aminopropyltriethoxysilane (APTES, 99%), triethylamine (99.5%), 2-bromoisobutyryl bromide (BIB, 98%), copper(I) bromide (99.999%) and bis(2-dimethylaminethyl)methylamine (PMDETA, 99%) were obtained from Sigma-Aldrich (St. Louis, MO, USA). All the above mentioned reagents were used as received with no additional purification. N-isopropylacrylamide (NIPAM, 97%) was purchased from Sigma-Aldrich and purified by recrystallization (twice) from n-hexane prior to use. AT-cut quartz crystals with gold electrode (5 MHz, Cr/Au) were obtained from Fil-Tech Inc. (Boston, USA). 2.2. Methods Quartz Crystal Microbalance with an open cell (QCM200, Stanford Research System) was used to follow the kinetics of the polymer brush growth from modified quartz crystals. The heater (IKA® C-MAG HS7) with a temperature sensor was used to control the temperature during the QCM measurements. Atomic Force Microscopy (AFM) images were obtained for the samples after QCM experiments with a Dimension Icon AFM (Bruker, Santa Barbara, CA) working in the PeakForce Tapping® (PFT) and QNM® modes using standard silicon cantilevers for measurements in air (a nominal spring constant of 0.4 N m−1). Scanning Electron Microscopy (SEM) analyses were performed using HITACHI S4700 instrument for gold-coated samples obtained also after QCM experiments. The XPS spectra of the brushes were recorded using a PHI 5000 Versa Probe II (ULVAC-PHI, Chigasaki, Japan) spectrometer equipped with an Al Kα radiation source (1486.6 eV). XPS spectra were acquired from 400 × 300 μm2 areas. All XPS peaks were charge referenced to the neutral (C–C) carbon C 1s peak at 284.8 eV. The depth profiling of the sample was performed with an argon gas cluster ion beam (Ar-GCIB) and the average Ar cluster size was 2500 atoms. The beam energy was set to 10 keV, beam current was 5nA and the sputtered area was 3 × 3 mm2. The samples were rotated during sputtering (Zalar rotation) to minimize surface roughening due to cluster bombardment. Application of the Ar-GCIB allowed to minimalize side effects such as chemical modifications of organic materials typically induced by monoatomic ion beam. 2.3. Synthesis of nanocrystalline ZnO films ZnO nanocrystalline layers in NR and HR morphologies were synthesized directly on a gold substrate or quartz crystal gold electrode according to the previously described procedures [43]. The substrates were cleaned in the mixture of H2O: H2O2: NH4OH (1:1:1, for 1 h at 75 °C; handle with caution!) prior further treatments. Firstly, a freshly cleaned gold surface was covered with zinc seed layer by applying 187

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5 mM ethanolic solution of zinc acetate. A drop of the solution was left on the gold surface for 10 s while the sample was slowly moved from one side to the other for a uniform seed layer deposition. Subsequently, it was dried with a stream of argon. The procedure was performed five times with short ethanol rinsing step before drying. The sample was then annealed at 350 °C for 8 h. The whole Zn seed deposition process was performed twice for homogenous seed coverage. In the second step Zn seed covered samples were dipped into an aqueous solution of zinc nitrate and hexamethylene-tetramine (1:1 M ration; concentration of both reagents − 0.05 M) for 30 min at 90 °C to grow ZnO nanorods. Fabrication of a continuous ZnO film with hexagonal grains (HG) was achieved by the addition of a growth promoter, sodium citrate dihydrate (5 mM) to the aqueous solution. Such prepared samples were immediately used or stored in a desiccator prior experiments. 2.4. Synthesis of SI-ATRP initiator monolayer on nanocrystalline ZnO films ATRP initiator monolayer was prepared on the nanocrystalline ZnO surface according to the following procedure. Firstly, freshly synthesized samples were exposed to 3-aminopropyltriethoxysilane vapour under argon atmosphere for 3 h in a closed flask equipped with a small vessel containing 0.2 mL of this compound. The samples were then thoroughly rinsed with dichloromethane and put again into the flask with 60 mL of dichloromethane and 0.41 mL of triethylamine. After that, 0.37 mL of 2-bromoisobutyryl bromide was added under argon atmosphere and the samples were left to react for 1 h. Subsequently, the samples were rinsed with dichloromethane, methanol and dried in the stream of argon. 2.5. Monitoring of SI-ATRP of NIPAM A series of experiments were conducted via QCM by monitoring quartz crystal resonant frequency shift (ΔFq) and motional resistance shift (ΔR) [61,62]. The first parameter corresponds to the resonator mass change due to adsorption. Measuring electrical impedance of the quartz crystal provides motional resistance variations (ΔR). It serves as an indicator of energy dissipation through viscoelastic loss [63]. The modified quartz crystals (covered with the nanocrystalline ZnO film and functionalized with the initiator) were subjected to SI-ATRP of NIPAM. The process was performed under stable liquid flow (0.4 mL/ min) in Ar atmosphere. In the first step, NIPAM monomer solution (0.02 M) in water/methanol mixtures of a given methanol molar fraction (xMe) was pumped through QCM cell over the modified quartz crystal in a closed circle until stable ΔFq (drift < 2 Hz/h) and ΔR parameters were reached. Then, ATRP activation complex (CuBr (0.22 mM) and PMDETA (1.43 mM)) in NIPAM solution was injected into the system. The polymerization was conducted in the QCM cell or in a closed flask for 200 min. The detailed reaction set-up was presented previously [45]. The measurements were performed for NR films at xMe = 0.4, 0.31, 0.23, 0.16 and for HG layer at xMe = 0.31, 0.16.

Fig. 1. AFM images of the ZnO NR sample before (A) and after SI-ATRP of NIPAM in the water/methanol solution with xMe = 0.16 (B) with respective RMS surface roughness parameter (Ra) and relative excess of actual surface area over the projected surface area (ΔS%).

projected surface area (ΔS% = 28%) than measured for HG sample (Fig. SI.1 in SI, Ra = 12.6 nm, ΔS% = 18%) as determined from the AFM images. It should be emphasized that the obtained AFM images are distorted due to the tip shape convolution especially for NR samples containing high aspect ratio objects. It limits the penetration of the confined space between the long neighboring nanorods (ca. 200–300 nm in length) [50] by the AFM tip that leads to reduction of the measured surface area and roughness. Thus, the actual ΔS% and Ra values for the NR sample should be even higher. Both the NR and HG samples were then used as substrates for growing PNIPAM brushes using SI-ATRP process that was carried out in water/methanol mixtures of various xMe. AFM images captured for PNIPAM brushes obtained in various xMe revealed serious differences of their morphologies (see Figs. 1B, 2B and C). Surface roughness of NR sample after PNIPAM grafting at xMe = 0.16 decreased substantially from 18.5 nm (for native NR sample) to 10.6 nm (for NR-PNIPAM-0.16) (compare Fig. 1A and B). Even more significant changes were detected for surface area – ΔS% decreased from 28% to only 1%, suggesting the formation of a thick

3. Results and discussion 3.1. Microscopic characterization of ZnO films before and after decoration with polymer brushes ZnO monocrystalline films with hexagonal grains (HG) and nanorod (NR) morphologies were synthesized using the methods previously described [50]. Freshly prepared ZnO films were analyzed using AFM and SEM. The microscopic measurements revealed homogenous coverage of nanocrystaline ZnO for both morphologies (see Figs. 1A, 2A, 3A and B) on quartz crystal gold electrode. The diameter of the nanorods at their tops was determined to be ca. 50 nm while the grains in the relatively flat HG films reached the lateral size of 100–200 nm. NR sample exhibited higher RMS roughness (Ra = 18.5 nm) and relative excess of actual surface area over the 188

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brush layer at least on top of NR structures (it is hard to reveal with AFM the morphology between the nanorods). SEM image (Fig. 3E) shows practically continuous polymer layer on top of the nanorods (small cracks are rather related to stiff sputtered gold covering soft polymer layer). Similar results were observed for PNIPAM brushes grafted from HG sample at xMe = 0.16 (HG-PNIPAM0.16, Figs. 2C and 3F). Typical hexagonal-shaped plates were still visible underneath the brush layer implying formation of uniform thick layer on top and in between the plates due to substantially smaller roughness on this type of substrate. For comparison, PNIPAM brushes grown previously at similar conditions (xMe = 0.16) but from flat gold surface resulted in the layer with the thickness of 22.4 nm [49]. AFM image (Fig. 2B) captured for the brushes obtained at higher methanol content (xMe = 0.31) for HG substrate (HG-PNIPAM-0.31) indicated formation of rather thin polymer layer that is consistent with the results obtained on flat gold substrate (although the decrease in the brush thickness was not such substantial on flat substrate) [45]. The actual surface area and roughness were practically the same as for the initial HR substrate. The same observations could be made for NR sample decorated at this conditions (NR-PNIPAM-0.31). AFM image (not shown) for this sample was indistinguishable from the one of the native substrate due to constraints of the measuring method – thin coatings around each nanorod were visible only in the SEM image (Fig. 3C). 3.2. In situ monitoring of the growth of PNIPAM brushes on nanostructured ZnO The influence of ZnO film morphology on PNIPAM brush growth was explored in situ via QCM technique. Time resolved QCM data together with respective relations between ΔR and ΔFq were presented in Fig. 4 for both morphologies of ZnO and two solvent compositions (xMe = 0.16 and 0.31). The changes of ΔR and ΔFq after 200 min of all surface-initiated polymerizations are collected in Table 1. It is clear that the largest absolute ΔFq values were found for the polymerizations in the solvent containing less methanol (xMe = 0.16) that indicates the largest overall masses of the formed brushes (compare Fig. 4A and B). This observation in not surprising as kinetics of SI-ATRP of NIPAM at this conditions is faster than for xMe = 0.31 and less controlled due to high water content [45,46]. The ΔFq signal reached plateau at the value ca. 3.5 times smaller than for NR, already after ca. 25 min of polymerization, while for NR morphology the polymerization proceeded until ca. 200 min. Larger final values of the frequency shifts (ΔFqF) for NR morphology might be related to larger surface area (see the AFM results) accessible for grafting of initiator and thus larger number of growing polymer chains both on tops and in the confined space between the nanorods. One has to take into account also the nonlinear response of QCM for high total masses of the grown polymers that may generate artifacts [64]. Such behavior might partially contribute to the observed plateau for the thickest brushes of NR-PNIPAM0.16 (Fig. 4A) but it does not affect the general conclusions. However, the initial rates of decrease of ΔFq were found to be very similar for both morphologies (see Fig. 4A) suggesting a comparable initial growth rate of the brushes (increase of the mass). The most exposed and easily accessible macroradicals could grow similarly and be terminated at similar times for both types of substrates. On the other hand, it seems that the more deeply buried macroradicals for NR morphology could have continued their growth for longer time due to increased local concentration of CuBr2 at the expense of CuBr implying better control of the SI-ATRP process as it was shown for PNIPAM brushes [24]. Nevertheless, such growth should be subsequently limited as crowding of longer chains increases the likelihood of recombination of the growing macroradicals due to space limitation in the concave geometry of ZnO NR [34]. The limited diffusion of the monomers and/or copper complexes into the confined space of ZnO columnar film, also due to formation of the barrier polymer layer at the top of the nanorods, may slow down the polymerization rate. However, polymer brushes growing

Fig. 2. AFM images of the ZnO HG sample before (A) and after SI-ATRP of NIPAM in the water/methanol solution with xMe = 0.31 (B) and xMe = 0.16 (C). Respective RMS surface roughness parameter (Ra) and relative excess of the actual surface area over the projected surface area (ΔS%) are also presented.

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Fig. 3. SEM images of ZnO films on quartz crystal gold electrode with NR (A) and HG (B) morphology, and the same films decorated with PNIPAM brushes obtained in the water/methanol solution with xMe = 0.31: NR-PNIPAM-0.31 (C), HG-PNIPAM-0.31 (D); xMe = 0.16: NR-PNIPAM-0.16 (E), HG-PNIPAM-0.16 (F); xMe = 0.23: NR-PNIPAM-0.23 (G); xMe = 0.4: NR-PNIPAM-0.4 (H).

polymer brushes exposed on top of the ZnO nanorods. The gradual slowdown in the polymerization on top of the nanorods may also contribute to this observation. Increasing the content of methanol to xMe = 0.31 in the polymerization mixture substantially slowed down the polymerization that was reflected in a smaller decrease of ΔFq during polymerization (compare Fig. 4A and B, Table 1). Moreover, the polymerization seemed to be practically terminated only after ca. 40 min as no further change of ΔFq was observed (Fig. 4B) that may be explained in terms of lower accessibility of the growing macroradicals due to the unfavored globular conformation of the chains at these conditions. An adsorbate composed of extended polymer brushes is more elastic, thus it is able to more readily dissipate energy through the

from the convex surfaces of the nanorods are less likely to recombine, as compared to the HG flat surface, due to increasing distance between the growing macroradicals with the polymerization time. All those features may explain the reported observations of much longer and effective growth of PNIPAM brushes on NR surface as compared to HG relatively flat morphology. The final value of ΔR is also larger for NR-PNIPAM0.16 than HG-PNIPAM-0.16 film (see Fig. 4A and Table 1). It indicates significant viscoelasticity of the polymer brushes decorating ZnO nanostructured NR film [45]. However, after the initial linear increase of ΔR with polymerization time (up to ca. 70 min) the growth of ΔR becomes less pronounced that might be related to the growth of polymer chains in the intercolumnar space that should have smaller impact on the viscoelastic properties of the whole layer as compared to the 190

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to larger active surface area (as revealed from AFM data) exploited for initiator grafting and polymerization of NIPAM, hence PNIPAM chains may grow both on the tops of the nanorods and between them like in concave and convex structures [29]. However, smaller |ΔRF|/|ΔFqF| in comparison to the value reached for respective sample at xMe = 0.16 indicates that PNIPAM chains adopts globular rigid conformation during SI-ATRP at xMe = 0.31 on ZnO NR [45]. We have recently presented that the rate of SI-ATRP of NIPAM in methanol/water mixtures strongly depends on methanol content. For high methanol content (xMe ≈ 0.31) propagation of the growing macroradicals is rather slow leading to the formation of thin PNIPAM layers (up to 10 nm for the brushes grafted on gold). Therefore the monomer molecules are able to penetrate between nanorods as it is not blocked by elongated chains in the external layer of nanocrystalline film. Thus, these polymerization conditions imply formation of the brush layer both on top of the nanorods and in the intercolumnar confined space. Furthermore, the phenomenon leads to weaker energy dissipation in comparison to extended chains. Thin polymer brushes obtained at xMe = 0.31 on the nanorods can be observed by comparison of the SEM images of the native NR sample and NR-PNIPAM-0.31 (Fig. 3A and C). In order to get more insight into the polymerization process in the confined space of NR films, additional QCM experiments were performed for xMe = 0.4 and 0.23. Final shifts values of frequency (ΔFqF) and motional resistance (ΔRF) after 200 min of the polymerization at xMe = 0.4, 0.31, 0.23, 0.16 on NR films are presented in Table 1 and the phase diagram (Fig. 5; for initial stage of the polymerizations) as well as diagram of final parameter shift as a function of xMe (Fig. SI.3). The increased amount of water up to xMe = 0.23 led to the enhancement of the QCM response in comparison to xMe = 0.31 (Figs. 3G, SI.3, Table 1). Such large increase of the parameters shifts (ΔFqF = −5080 Hz and ΔRF = +1908 Ω) and relative contribution of ΔR (|ΔRF|/ |ΔFqF| = 0.375 [Ω/Hz]) indicate significant mass uptake combined with energy dissipation due to viscoelastic effects at the interface [67]. Newly grown brushes at xMe = 0.23 adopted a different conformation than the ones obtained at xMe = 0.31. It is hard to distinguish mass uptake from the lower and upper part of the nanocrystalline film since QCM data is a sum of all interfacial phenomena at the crystal/liquid interface [54]. However, final shifts of the parameters were smaller than for xMe = 0.16 indicating thinner polymer coverage on NR tips. SEM images of both surfaces show thin layer coatings on the nanorods (Fig. 3E and G), while the layers seem to be more uniform for lower methanol content (xMe = 0.16). It may be explained by faster polymerization for the system with lower methanol content [45]. For a more detailed evaluation of QCM output, time resolved data were converted to ΔR-ΔFq phase diagrams [68] that help to compare viscoelastic

Fig. 4. The results of QCM measurements for the growth of PNIPAM on ZnO films with NR and HG morphologies. Time resolved data of ΔFq and ΔR for polymerization in water/methanol mixture with xMe = 0.16 (A) and xMe = 0.31 (B).

Table 1 The final (after 200 min of polymerization) shifts of the parameters in QCM experiments, ΔFqF, ΔRF and their ratio for various xMe in the polymerization mixture. xMe

ΔFqF [Hz]

ΔRF [Ω]

|ΔRF|/|ΔFqF| [Ω/Hz]

NR-PNIPAM 0.16 0.23 0.31 0.40

−6112 −5080 −928 −4038

4558 1908 30 478

0.745 0.375 0.032 0.118

HG-PNIPAM 0.16 0.31

−1744 −480

1250 −2

0.716 0.004

motion of the solvated macromolecules. When they shrink the adsorbate becomes more rigid and interacts less with the liquid environment [65]. Those differences are additionally highlighted in the phase diagrams based on QCM results (Fig. SI.2). The ratio of the final motional resistance shift modulus to the final frequency shift modulus, |ΔRF|/|ΔFqF|, for xMe = 0.31 (Table 1) showed that the relative contribution of motional resistance was larger for NR morphology in comparison to the respective shift value obtained for HG film. Such a tendency indicates that the formed organic adsorbate is stiffer on HG surfaces. QCM response for HG-PNIPAM-0.31 showed typical linear mass uptake (> 2% of crystal mass uptake, no significant ΔR) [62,66], leading to creation of thin and uniform organic coating that followed HG shape as it was visualized by AFM and SEM (Figs. 2B and 3D). Larger parameters shifts for NR morphology can be additionally related

Fig. 5. Phase diagram converted from time resolved QCM data (for initial 25 min) for series of experiments on ZnO NR films as a function of xMe. 191

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properties of the obtained polymer brushes in the series of experiments as a function of solvent composition on ZnO NR film (Fig. 5) in the initial stage of reaction. It shows that in the initial phase of the polymerization all the polymer brushes tend to grow as uniform thin films without major viscoelastic effects (very small ΔR values) except for the sample synthesized at xMe = 0.16 that revealed significant viscoelasticity from the very beginning of the polymerization. Polymer brushes synthesized at xMe = 0.31 developed no substantial viscoelastic effects along the whole in situ experiment indicating the tight globular structure of the formed brushes as it was also confirmed by the SEM imaging (Fig. 3C). According to our previous results on PNIPAM brushes synthesized by SI-ATRP on flat gold surface [45] the phase diagram for various solvent compositions resembles the one for free PNIPAM chains in solution [44]. The PNIPAM chains exhibit the drastic conformational change from a fully swollen coil at low xMe (xMe < 0.17) to a globular state at intermediated xMe (xMe = 0.17–0.4) to again a fully swollen coil at high xMe (xMe > 0.4) – the co-nonsolvency effect [68]. Similar behaviour was also observed for ZnO NR (Fig. SI.3, Table 1). QCM parameters presented the highest values for xMe = 0.16. They decrease till xMe = 0.31 (with the most significant variation between xMe = 0.23 and 0.31, Table 1) and rise again at xMe = 0.4. Such high parameter shifts obtained for xMe = 0.4 clearly prove substantial mass uptake, although upper polymer coverage on top of NR tips is much thinner in comparison to xMe = 0.23 and 0.16 (Fig. 3E, G and H). It leads to the conclusion that also the brushes grown within the concave space of the lower parts of ZnO NR should substantially contribute to the measured mass uptake. The samples after QCM experiments were further characterized in terms of swellability in the solvents exchange experiments (Fig. SI.4, Table SI.1) and at various temperatures (Fig. SI.5–7 and detailed discussion in SI). 3.3. XPS studies of PNIPAM ZnO nanostructured films synthesized at xMe = 0.16 Previous in situ QCM and microscopic studies were mainly exploring the upper part of the brushes on ZnO nanostructured films. Thus, XPS depth profiling (Fig. 6) was used to investigate the lower layer of polymer brush content synthesized at xMe = 0.16 on both types of ZnO topographies. Unfortunately, PNIPAM brushes obtained at xMe = 0.31 did not reveal sufficient signal from the organic coating on top of ZnO nanocrystals for meaningful profiling (data not shown). XPS spectra confirmed the presence of Zn, O, C and N elements in ZnO samples [69] decorated with PNIPAM brushes (see Fig. SI.8 for NRPNIPAM-0.16). The detailed analyzes of C 1s and O 1s bands are shown in SI for NR-PNIPAM-0.16 (Fig. SI.9). Fig. 6 reveal changes of contribution of each element (and bond type) with varying sputtering time for both samples (NR-PNIPAM-0.16, HG-PNIPAM-0.16). Decrease of the signals attributed to PNIPAM (C, N, and O in C]O), together with strong increase of Zn and O (in Zn-O) signals only after ca. 30 min of sputtering time on NR-PNIPAM-0.16 sample (Fig. 6A) indicates formation of a relatively thick brush layer on top of the nanorods and relatively slow (based on the C 1s signal) decrease of the brush content below the tops of the nanorods. The corresponding exchange of signal intensity for organic brushes and inorganic substrate was observed also for HG-PNIPAM-0.16 (Fig. 6B) but just after 5–10 min of the sputtering indicated much thinner top PNIPAM layer. Moreover, the organic/inorganic interface was found much narrower for HG substrate - C content levels off after ca. 30 min of sputtering. The XPS profiling data indicated that polymer brush coating in NR-PNIPAM-0.16 sample grew significantly below the top of the nanorods confirming previous conclusions from QCM measurements. Thus, it seems that even at the condition enabling fast growth of PNIPAM brushes, the chains grafted from the tops of the nanorods do not completely block the growth of the brushes in the intercolumnar space although the contribution of the brushes gradually decreases while moving to the bottom of the nanorods. Such behavior may be expected due to limited diffusion of the

Fig. 6. Atomic concentration with the sputtering time for elements at different chemical states: NR-PNIPAM-0.16 (A); HG-PNIPAM-0.16 (B).

monomer molecules and less space at the bottom of the nanorods for the growing macroradicals for which the likelihood of recombination increases due to concave geometry [34]. 4. Conclusions The growth of PNIPAM brushes via SI-ATRP on nanostructured ZnO substrates was followed in situ using QCM measurements and the structure of the obtained hybrid systems was characterized microscopically (AFM, SEM) as well as using XPS profiling. The studies on the polymerization performed both in the convex and concave confined environment offered in the ZnO nanorods layer revealed that the growth of the PNIPAM brushes may be controlled by varying the solvent (water/methanol) composition. The polymerization performed at high methanol content (e.g., xMe = 0.31) led to decoration of the nanorods only with thin PNIPAM layer around them. At lower methanol content (xMe = 0.16) the polymerization proceeded much faster, especially on the convex surface of the tops of nanorods forming an almost continuous coating on them. Nevertheless, such a top layer did not seem to block totally the diffusion of monomers and growth of the brushes in the intercolumnar space as indicated by QCM measurements and XPS profiling. The polymerization performed at the same conditions but using ZnO substrate with relatively flat morphology was selfterminated much faster than for the nanorods. It may be explained by kinetically advantageous grafting from the convex tops of ZnO rods (dilution of macroradicals with increasing distance from the grafting surface) and observed growth of the brushes in the concave surface 192

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formed between the nanorods. Nevertheless, it is difficult in the studied systems to fully discriminate the polymerization processes running in parallel in convex and concave locations so the detailed description of the kinetics remains speculative and can only be revealed after significant modifications of the system (e.g. selective blocking of the initiation sites on the tops of the nanorods). However, it is important to emphasize that fine tuning of the polymerization mixture should lead to the desired relation between the mass/length of polymer brushes grown in the intercolumnar space to the top brush layer. It is important for tailoring swelling and mechanical properties of the formed nanocomposites that should depend on intercolumnar soft bridging realized by the growing chains at various distances from the top. This way one may also spatially control the growth of the brushes as well as closing or trapping different substances in the pores between the neighboring nanorods. The obtained results are of high importance for potential electronic or photovoltaic applications of nanostructured ZnO that require spatial and compositional control on organic/inorganic interface.

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Acknowledgements A. P. would like to acknowledge National Science Centre for financial support [grant number 2013/08/S/ST5/00565]. S. Z. would like to acknowledge financial support of TEAM Programme [grant number: TEAM/2016-1/9] of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund. K. W. would like to thank the Foundation for Polish Science for the financial support [START 96.2018]. Data availability The processed data required to reproduce these findings can be found in the Supplementary Materials file. The raw data required to reproduce these findings cannot be shared at this time due to technical or time limitations. Appendix A. Supplementary material AFM images of ZnO substrates; phase diagrams based on QCM monitoring of the growth of PNIPAM brushes; investigation of PNIPAM brushes in water and methanol and their thermoresponsive behaviour by QCM. XPS depth profiles of PNIPAM brushes on ZnO. Supplementary data to this article can be found online at https://doi.org/10.1016/j. eurpolymj.2019.01.012. References [1] X. Zhang, W. Wang, Z. Hu, G. Wang, K. Uvdal, Coordination polymers for energy transfer: preparations, properties, sensing applications, and perspectives, Coord. Chem. Rev. 284 (2015) 206–235, https://doi.org/10.1016/j.ccr.2014.10.006. [2] X. Zhang, K. Wang, M. Liu, X. Zhang, L. Tao, Y. Chen, Y. Wei, Polymeric AIE-based nanoprobes for biomedical applications: recent advances and perspectives, Nanoscale 7 (2015) 11486–11508, https://doi.org/10.1039/C5NR01444A. [3] T.M. Reineke, Stimuli-responsive polymers for biological detection and delivery, ACS Macro Lett. 5 (2016) 14–18, https://doi.org/10.1021/acsmacrolett.5b00862. [4] M. Szuwarzyński, L. Zaraska, G.D. Sulka, S. Zapotoczny, Pulsatile releasing platform of nanocontainers equipped with thermally responsive polymeric nanovalves, Chem. Mater. 25 (2013) 514–520, https://doi.org/10.1021/cm303930y. [5] Y. Shi, L. Peng, Y. Ding, Y. Zhao, G. Yu, Nanostructured conductive polymers for advanced energy storage, Chem. Soc. Rev. 44 (2015) 6684–6696, https://doi.org/ 10.1039/c5cs00362h. [6] B. Yeom, T. Sain, N. Lacevic, D. Bukharina, S.H. Cha, A.M. Waas, E.M. Arruda, N.A. Kotov, Abiotic tooth enamel, Nature 543 (2017) 95–98, https://doi.org/10. 1038/nature21410. [7] I. Banerjee, R.C. Pangule, R.S. Kane, Antifouling coatings: recent developments in the design of surfaces that prevent fouling by proteins, bacteria, and marine organisms, Adv. Mater. 23 (2011) 690–718, https://doi.org/10.1002/adma. 201001215. [8] V. Georgakilas, J.N. Tiwari, K.C. Kemp, J.A. Perman, A.B. Bourlinos, K.S. Kim, R. Zboril, Noncovalent functionalization of graphene and graphene oxide for energy materials, biosensing, catalytic, and biomedical applications, Chem. Rev. 116 (2016) 5464–5519, https://doi.org/10.1021/acs.chemrev.5b00620.

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