Transformations of Thermo-Sensitive Hyperbranched Poly(ionic liquid

Article Cite This: Langmuir XXXX, XXX, XXX−XXX

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Transformations of Thermosensitive Hyperbranched Poly(ionic liquid)s Monolayers Hansol Lee,† Alexandr V. Stryutsky,‡ Volodymyr F. Korolovych,† Emily Mikan,† Valery V. Shevchenko,‡ and Vladimir V. Tsukruk*,† †

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Institute of Macromolecular Chemistry of the National Academy of Sciences of Ukraine, Kyiv 02160, Ukraine

‡

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S Supporting Information *

ABSTRACT: We synthesized amphiphilic hyperbranched poly(ionic liquid)s (HBPILs) with asymmetrical peripheral composition consisting of hydrophobic n-octadecylurethane arms and hydrophilic, ionically linked poly(N-isopropylacrylamide) (PNIPAM) macrocations and studied low critical solution temperature (LCST)-induced reorganizations at the air−water interface. We observed that the morphology of HBPIL Langmuir monolayers is controlled by the surface pressure with uniform well-defined disk-like domains formed in a liquid phase. These domains are merged and transformed to uniform monolayers with elevated ridge-like network structures representing coalesced interdomain boundaries in a solid phase because the branched architecture and asymmetrical chemical composition stabilize the disk-like morphology under high compression. Above LCST, elevated individual islands are formed because of the aggregation of the collapsed hydrophobized PNIPAM terminal macrocations in a solid phase. The presence of thermoresponsive PNIPAM macrocations initiates monolayer reorganization at LCST with transformation of surface mechanical contrast distribution. The heterogeneity of elastic response and adhesion distributions for HBPIL monolayers in the wet state changed from highly contrasted two-phase distribution below LCST to near-uniform mechanical response above LCST because of the hydrophilic to hydrophobic transformation of the PNIPAM phase.

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stability and dimensional control.8−11 Overall, PILs are considered as building block materials for various applications, such as nonflammable electrolytes for batteries, solar/fuel cells or solid-state supercapacitors, ion-exchange resins for water purification, membranes for gas mixture separation, and sensors.12−15 Various star and branched polymer architectures of PILs have recently been introduced.10,16 The presence of multiple terminal groups can provide pathways to control the assembly of polymers in solutions, at surfaces and interfaces as well as their stimuli-responsive behavior.12,17,18 The assembly of hyperbranched PILs (HBPILs) can be controlled by changing the type of terminal ionic groups19 and the ratio of terminal

INTRODUCTION The assembly of functional polymeric materials provides exciting possibilities for the design and development of functional materials for a variety of applications, such as biomedical, sensing, energy storage and electrochemical applications.1−3 In this regard, poly(ionic liquid)s, (PILs) are promising as they can exhibit hierarchical assembly, forming various organized morphologies, such as micelles, vesicles, and cubosomes.4−6 PILs are a subclass of polyelectrolytes in which ionic liquid (IL) species are polymerized or incorporated as counterions.7,8 IL groups can diversify intra- and intermolecular interactions, providing multidimensional driving forces for the assembly of molecules. In addition, PILs possess intriguing physicochemical properties which combine the properties of ILs, such as high ionic conductivity, high chemical, thermal, and electrochemical stability, and low vapour pressure, with the intrinsic properties of polymers, such as improved mechanical © XXXX American Chemical Society

Received: June 27, 2019 Revised: August 8, 2019 Published: August 16, 2019 A

DOI: 10.1021/acs.langmuir.9b01905 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. Amphiphilic HBPIL with 24 hydrophobic n-octadecylurethane arms and hydrophilic 8 PNIPAM amine terminated macrocations (a). Red circle indicates the polyester core. Representative FTIR spectra of oligomeric hyperbranched polycarboxylic acid, HBP-32COOH and HBPIL, HBP-24Oct8[COO]−[PNIPAM]+ (b). 1H NMR spectrum of HBP-24Oct8[COO]−[PNIPAM]+ (c).

hydrophilic−hydrophobic segments.20 In addition, various morphologies can be achieved by introducing terminal stimuli-responsive segments which respond to external stimuli, such as temperature, pH, and light.21−26 Poly(N-isopropylacrylamide) (PNIPAM) blocks with low critical solution temperature (LCST) behavior have been explored for this purpose as PNIPAM graft25−27 and macrocations.28 For example, bottlebrush copolymers composed of PIL with grafted PNIPAM formed disassociated nanoscale discoidal assemblies below LCST, while thicker circular discs above LCST.27 The HBPILs containing PNIPAM macrocations formed micellar assemblies with diverse morphologies depending on temperature.28 Temperature-induced transition in mechanical properties has been also observed for the polymer surface grafted with PNIPAM brushes. The collapse of PNIPAM chains above LCST causes increase in adhesion and elastic modulus as a result of the combination of dehydration and chain entanglement.29−32 Moreover, the incorporation of asymmetric composition of functional terminal groups can produce interesting morphologies by shifting the phase boundaries.33−37 For example, hyperbranched polyethers possessing homogenous peripheral chemical composition did not self-organize into ordered structures, while hyperbranched polyethers partially terminated with benzoyl groups formed macroscopic aggregates.38 However, to date, the majority of HBPIL studies focused on the synthesis and morphologies of HBPILs composed of symmetric chemical composition10,12,39−41 with less attention paid to the assembly of HBPILs at interfaces.19,20 It has been shown that amphiphilic branched polymers at the air−water interface exhibit interesting morphological transition upon lateral compression.17,42−46 For example, poly(styrene)-blockpoly(acrylic acid) dendrimer-like copolymers underwent “pancake-to-brush” transition under compression.43 Peculiar rod-to-globule and pancake-to-island transitions were also observed from brush block copolymers.45,46 Here, we report the synthesis and assembly of novel thermoresponsive amphiphilic HBPILs with asymmetric

peripheral chemical composition. These HBPILs are composed of hydrophobic polyester cores with 24 hydrophobic noctadecylurethane groups and eight hydrophilic PNIPAM macrocations as peripheral components with LCST transition (Figure 1a). Their assembly at the air−water interface is investigated at different temperatures and surface pressures in dry and wet conditions. These amphiphilic HBPILs showed domain and coalescent morphology due to the persistence of their shapes afforded by the asymmetric composition with the branched architecture. Thermally triggered phase reorganization resulted in alternating heterogeneous surface mechanical and adhesion distribution because of the transformation of the beneath PNIPAM phase.

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EXPERIMENTAL SECTION

Materials. PNIPAM amine terminated (PNIPAM, Mn = 2500 g/ mol) was obtained from Aldrich and used as received. Hyperbranched aliphatic oligoether polyol Boltorn H30 (Perstorp, Sweden) with a weight average molecular weight (Mw) of 3500 g/mol (comprising 32 terminal OH groups in outer shell) was purified by precipitation of dimethylformamide (DMF) solution in diethyl ether followed by vacuum drying at 25−30 °C for 6 h (an equivalent Mw measured by hydroxyl groups via an acetylation technique is equal to 117 g/ equiv).47 Phthalic anhydride was purified by sublimation. DMF, ethanol, diethyl ether, acetone, acetonitrile were dried and distilled before use. Ultrapure water used in all experiments was prepared in a three-stage Millipore Milli-Q Plus 185 purification system (resistivity ≥ 18.2 MΩ·cm) (see more detail in the Supporting Information). Langmuir Monolayers and Deposition. The pressure−area (Langmuir) isotherms at the air−water interface and Langmuir− Blodgett (LB) deposition onto a silicon substrate were conducted using a KSV 2000 minitrough assembled with water bath temperature controller below and above LCST (23 and 37 °C). Ultrapure water was used as the subphase for all depositions. The HBPIL solutions were prepared at a concentration of 0.2 mg/mL in chloroform and then spread uniformly onto the water surface in a dropwise fashion. The Langmuir monolayers were left undisturbed for 30 min to allow them equilibrate and evaporate the solvent. Afterward, the monolayers were compressed at a rate of 5 mm/min until the minimum barrier separation distance (for pressure−area isotherms) or to the target B


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Table 1. MW and DB Values for the Initial HBP-OH, Oligomeric Polycarboxylic Acid HBP-24Oct8COOH, and HBPIL HBP24Oct8[COO]−[PNIPAM]+ sample

calculated MW g/mol

found MW g/mol

mean DB %

HBP-OH HBP-24Oct8COOH HBP-24Oct8[COO]−[PNIPAM]+

3564 13 204 33 204

3744 12 857 32 857

38.7 38.7 38.7

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pressure (for LB monolayer deposition). For LB deposition, highly polished [100] silicon substrates (semiconductor processing) were cleaned with piranha solution (2:1 concentrated sulfuric acid to hydrogen peroxide mixture, caution: strong oxidizer!) according to the usual procedure.48 The substrates were then thoroughly rinsed with ultrapure water and dried with dry nitrogen before deposition. The LB monolayers were transferred via the vertical dipping method onto the precleaned silicon substrates at a rate of 1 mm/min at different pressures (0.3, 20, and 49 mN/m). Compression−decompression hysteresis isotherms were also recorded by compressing the monolayers to the maximum pressure before collapse and then expanding to the maximum trough area. Characterization. Fourier transform infrared (FTIR) spectra of synthesized HBPILs were collected using a Bruker Vertex 70 FTIR spectrophotometer operated in 200−4500 cm−1 range. Proton nuclear magnetic resonance (1H NMR) spectra were recorded with a Varian VXR-400 MHz spectrometer (Cambridge Isotope Laboratories, Inc.) using DMSO-d6 as a solvent. The surface morphology of HBPIL monolayers was investigated with an atomic force microscope (AFM) in the “light” tapping mode according to the usual procedure.49 High-resolution AFM probes (MikroMasch, Hi’Res-C15/Cr-Au) were used with a spring constant of 40 N/m. The scanning rate was kept in the range of 0.2−0.5 Hz, and the resolution was either 512 × 512 pixels or 1024 × 1024 pixels. The effective thickness of HBPIL monolayers was determined using an M-2000U spectroscopic ellipsometer with WVASE32 analysis software at three incident angles of 65°, 70°, and 75°. At least five separate locations on the sample were measured to determine the average thickness. The AFM scratch test was also conducted to measure the monolayer thickness. The monolayer samples were scratched with a sharp needle. After the scratched area was scanned over 10 × 10 μm2 area, the thickness of the HBPIL monolayers was obtained by subtracting the average height of the bare silicon region from the average height of the region where HBPILs were deposited using a height histogram distribution with NanoScope Analysis v1.4 software (Bruker, USA). The force-tapping mode [Bruker’s quantitative nanomechanics (QNM)] was conducted to map the nanomechanical properties of HBPIL monolayers. For the QNM mode, AFM probes from MikroMasch (HQ:XSC11/AL BS) were used with spring constants in the range of 1.5−2.2 N/m and a tip radius of ∼8 nm. Prior to each new sample measurement, tip characterization was performed. Deflection sensitivity was determined from force−distance curves (FDCs) on a sapphire crystal and the spring constant was calculated using the thermal calibration method. The HBPIL monolayers were scanned in the QNM mode at a scan rate of 0.5 Hz using the resolution of 512 × 512 pixels. These measurements provide simultaneous contrast variation for regions with difference in stiffness and adhesion. QNM measurements were conducted in air and in water below and above LCST. For the measurements above LCST, the samples were placed on a Peltier heating/cooling stage. Water was injected into the system by taking advantage of capillary forces between the AFM tip and sample. After adding water, the samples were allowed to equilibrate for 2 h. Water temperature was monitored to be maintained above LCST. For quantitative mechanical measurements, FDCs were collected from the selected regions with a ramp size of 200 nm and analyzed using a micromechanical analysis software (see the Supporting Information).50

RESULTS AND DISCUSSION

Synthesis of HBPILs. We synthesized amphiphilic HBPILs consisting of 24 hydrophobic n-octadecylurethane groups and eight hydrophilic carboxylate anions and PNIPAM macrocations as peripheral components (abbreviated as HBP24Oct8[COO]−[PNIPAM]+) (Figure 1).28 The synthesized polymers are considered as PILs because the components with ammonium cations and carboxylate anions are common for PILs.51,52 Boltorn H30 of third generation (HBP-OH) containing 32 terminal primary hydroxyl groups was used as an initial compound. The hydrophobicity of the hyperbranched oligoester core was enhanced by the incorporation of hydrophobic octadecyl urethane fragments (Figure 1). The change in the ratio of the hydrophobic octadecyl urethane fragments to hydrophilic PNIPAM macrocations allows control over the hydrophilic−hydrophobic balance. The synthesis of HBP-24Oct8[COO]−[PNIPAM]+ was based on the partial blocking of the terminal hydroxyl groups of the initial HBP-OH with n-octadecylisocyanate (OH/NCO = 4:3) followed by acylation of the residual hydroxyl groups of the reaction product with phthalic anhydride (OH/(CO)2O = 1:1) and neutralization of carboxyl groups by a primary amino groups of the PNIPAM (Figure S1). The chemical structure of the synthesized compound HBP24Oct8[COO]−[PNIPAM]+ was confirmed by FT-IR and 1H NMR spectroscopies (Figure 1b,c). The FT-IR spectrum of the synthesized HBPIL compound contains absorption bands of aliphatic fragments [ν C−H of CH2 (2874, 2922, 2972 cm−1), ν C−H of CH3 (1087−1312 cm−1), δ C−H of CH2 and δ as C−H of CH3 (1460 cm−1), δ sy C−H of CH3 (1367, 1387 cm−1)] of hyperbranched core, octadecyl urethane fragments and PNIPAM macrocations, bands of ν C−O−C bonds of ester fragments which overlap ν C−H of CH3 (1000−1312 cm−1), bands of ν CO bonds of ester and carboxylate groups (1703−1735 cm−1), bands of the characteristic amide groups of PNIPAM (ν CO amide I at 1649 cm−1 and δ N−H amide II at 1543 cm−1), and bands of aromatic rings (ν C−H at 3075 cm−1), ammonium cations, amide, and urethane groups (ν N−H at 3130−3700 cm−1) (Figure 1b). The characteristic peaks of HBP-24Oct8[COO]−[PNIPAM]+ and its distinctive feature from the initial oligomeric acid are signals from the carbonyl groups of PNIPAM (amide I and amide II) and a significant increase in the intensity of the alkyl signals (2800−3030 cm−1) (Figure 1b). 1 H NMR spectrum of the final compound contains signals of methyl (0.76−1.14 ppm) and methylene (1.24, 1.46, 2.25− 4.30 ppm) groups of the hyperbranched core, PNIPAM fragments, and octadecyl tails, signals of protons from tertiary carbon atoms of PNIPAM fragments (1.99, 3.85 ppm), urethane groups (4.80 ppm) of octadecyl tails, aromatic rings, amide groups (7.83−7.90 ppm), and ammonium cations (8.77 ppm) (Figure 1c). The chemical structure of HBPILs is confirmed by positions of characteristic peaks and ratios of integral areas of these signals (see Figure 1c and Supporting Information). C


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Figure 2. Langmuir isotherms of HBPILs (left) at 23 (a) and 37 (b). The solid and dashed lines are collected during compression and expansion, respectively. AFM height images (right) of HBPIL monolayers from HBP-ILs deposited at 0.3 (c,d), 20 (e,f), and 49 mN/m (g,h) at 23 (c,e,g) and 37 °C (d,f,h). Z scale is 11 nm for (c,d) and 7 nm for (e−h).

Table 2. Dimensions of the HBPIL Domains and the Surface Area per Molecule at Different Surface Pressures and Temperatures temperature (°C) surface area per molecule in the liquid phase, Al (nm2) surface area per molecule in the solid phase, As (nm2) surface pressure (mN/m) domain shape domain height (nm) domain diameter/width (nm) domain surface coverage (%)

23 56.4 10.3 20 disk 2.1 ± 0.5 86.1 ± 44.2 39.6 ± 5.2

56.4 10.3 49 ridge 1.1 ± 0.2 8.6 ± 1.0 11.3 ± 2.2

81.2 9.6 20 disk 2.1 ± 0.5 90.0 ± 19.0 40.6 ± 0.7

37 81.2 9.6 49 ridge 1.3 ± 0.3 10.5 ± 1.1 6.6 ± 1.1

81.2 9.6 49 island 4.3 ± 0.6 58.1 ± 31.3 4.9 ± 1.8

well-balanced hydrophilic−hydrophobic character. They have hydrophobic segments (core and octadecyl arms) sufficiently enough to overcome the hydrophilicity of PNIPAM segments and thus irreversible dissolution in the water subphase.55 At the same time, their PNIPAM macrocations are sufficiently hydrophilic to prevent the desorption and aggregation of HBPILs on top of the water surface. To investigate the thermally responsive behavior of Langmuir monolayers, the pressure−area isotherms were recorded below and above LCST (at 23 and 37 °C) (Figure 2a,b and Table 2). To determine the LCST of HBPILs, the transmittance of HBPIL aqueous solution (0.5 mg/mL) was measured at different temperatures in the range of 25−50 °C (Figure S3). The transmittance slightly and gradually decreased to 15% from initial transmittance at around 35 °C and dramatically dropped from 35 to 38 °C, which corresponds to the LCST range (see the Supporting Information).56,57 LCST can be defined as the temperature where the transmittance decreases to 10% from the initial transmittance when a sharp LCST transition was observed.58,59 According to this definition with slight modification, we defined LCST as the temperature where the transmittance decreases to 10% from the transmittance at the onset transition temperature (34.9 ± 0.1 °C) instead from the initial transmittance. LCST was defined at 35.8 ± 0.1 °C. Therefore, we chose 23 and 37 °C as below and

The degree of branching (DB) of the initial HBP-OH which is a ratio of sum of branched units and those containing terminal groups to sum of all units including linear ones was determined based on the 13C NMR spectra as described by our previous study53 (see the Supporting Information). The obtained DB values are 38−39% (mean value is 38.7%, Table 1), corresponding to the literature data for DB of the HBP-OH Boltorn H40 (36−43%)54 which has a similar chemical structure but with a different number of terminal hydroxyl groups (64 hydroxyl groups). The DB values are valid for both the oligomeric hyperbranched polycarboxylic acid, HBP-24Oct8COOH, (Supporting Information) and the thermally responsive HBPIL, HBP-24Oct8[COO]−[PNIPAM]+, because these compounds were obtained by successive polymer-analogous transformations of the initial cores without changing the total number of terminal functional groups (see the Supporting Information). The molecular weight (MW) for the HBPILs was determined as 32 857 g/mol based on the MW of oligomeric hyperbranched polycarboxylic acid (from acid−base titration technique) and terminal groups MW of PNIPAM and constitutes (Table 1). This value is close to the theoretical value (33 204 g/mol). Langmuir Monolayers. The HBP-24Oct8[COO]−[PNIPAM]+ compounds formed stable Langmuir monolayers transferable to solid surfaces because of their D


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Langmuir above LCST, respectively. We also observed that the contact angle of water on the HBPIL monolayer films deposited in a liquid phase increased from 58° to 85° with increasing water subphase temperature from 23 to 37 °C (Figure S4). This result supports our choice of 37 °C as above LCST, confirming that HBPILs underwent temperature-induced phase transition with PNIPAMs being less hydrophilic at 37 °C. The pressure−area isotherms for both temperatures showed a steady increase in surface pressure upon compression, indicating the formation of stable Langmuir monolayers.53 The isotherms were shifted toward a larger surface area above LCST because of the increased hydrophobicity arising from LCST transition of PNIPAMs, which indicates the enhanced stability of monolayers.55 The surface pressure started to rise below 80 nm2 at 23 °C and below 100 nm2 at 37 °C (Figure 2). For the isotherm at 23 °C, a significant pseudoplateau region appeared for the surface area below 30 nm2, while the isotherm recorded at 37 °C showed a relatively small pseudoplateau region for the surface area below 20 nm2. When the surface area was below 10 nm2, surface pressure abruptly increased for both temperatures, indicating the formation of the solid monolayers. For the isotherm at 37 °C, a small shoulder appeared when the surface area reached 8 nm2. The surface area occupied per molecules in liquid and solid phases, Al and As, were calculated by extrapolating the steepest segment of the linear portion of the liquid and solid phases down to the zero surface pressure, respectively (Figure S5 and Table 2).60,61 The value of Al is much larger at 37 °C, indicating that the HBPIL molecules above LCST occupy a larger surface area because the hydrophobized PNIPAM segments are desorbed from the water subphase to the air− water interface and act as barriers which limit compact packaging between the molecules.17,62−65 The value of As was similar for both temperatures (Figure S4 and Table 2). The theoretical area occupied per molecule in a solid phase can be calculated by considering 0.2 nm2 as the known surface area occupied per alkyl tail in a densely packed phase.53 The total projected surface area per molecule occupied by 24 hydrophobic alkyl chains is theoretically estimated to be 4.8 nm2, suggesting that not only hydrophobic octadecyl tails but also the hyperbranched core govern the limiting surface molecular area of HBPIL monolayers in a solid phase. The sharp drop in surface pressure upon expansion indicates that the reconstruction of the condensed monolayer to the initial state is rather kinetically hindered at decompression (Figure 2).66−68 Then, the second compression results in the shift of Langmuir isotherms toward the lower surface due to the presence of the nuclei after first expansion and the formation of continuous domains during second compression (Figures S6 and S7).69,70 Monolayer Morphology. AFM images show a featureless, uniform morphology in the gas phase, a well-defined disk-like domains in the liquid phase, and eventually, a network of ridgelike morphology in the solid phase (Figures 2c−g and S8− S10). First of all, the disk-like domains observed at the surface pressure of 20 mN/m have similar average diameters (86 ± 44 and 90 ± 19 nm at 23 and 37 °C, respectively) and height of 2.1 ± 0.5 nm for both temperatures (Figures 3, 4, Table 2). The size distribution of the disk-like domains is broader below LCST than above LCST. Below LCST, about 16% of the disklike domains have a diameter larger than 150 nm, while very few (less than 1%) of the domains have a diameter larger than

Figure 3. High-resolution AFM images and height profiles of HBPIL monolayers at different surface pressures; 20 (a,b), 49 mN/m (c,d) and at different temperatures; 23 (a,c) and 37 °C (b,d). Z scale are 3 (c) and 6 nm (a,b,d).

150 nm above LCST (Figure 4). It should be noted that the disk-like domains exhibit a contrast between the outer region of about 10 nm and a center area of disk-like domains in AFM phase images (Figure 5a,b), having a concave shape with elevated rims, observed from height profiles of AFM images and three-dimensional images (Figures 3a,b and 6a,b). Further compression induces morphological transition where the disk-like structures are transformed into the network of ridges for both temperatures (Figure 3c). The average height of the elevated ridges is 1.1 ± 0.2 and 1.3 ± 0.3 nm at 23 and 37 °C, respectively (see profiles in Figure 3 and Table 2). Larger islands with a height of 4.3 ± 0.6 nm connected by the network of ridge-like domains are observed only at 37 °C (Figure 3d). The AFM phase and three-dimensional height images also show the morphological transition induced by temperature and surface pressure (Figures 5 and 6). At the first glance, the Langmuir isotherms and monolayer morphology resemble those commonly observed for linear amphiphilic block copolymers.71,72 As known, “pancake-tobrush” or “carpet-to-brush” transitions take place for neutral or ionic amphiphilic block copolymers, respectively, upon compression at the air−water interface as the hydrophilic blocks desorbed from the water surface and submerged into the subphase.43,72−75 In this case, the domain height should largely increase as surface pressure increases.64,76 However, the height of the ridge-like domains formed by HBPILs is close to 1 nm, which is much lower than that of the disk-like domains E


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Figure 4. Size distribution of disk-like domains in HBPIL monolayers at 20 mN/m and different temperatures; 23 (a) and 37 °C (b).

at lower pressure (Table 2). Therefore, the interfacial behavior of HBPILs upon compression cannot be explained by the common pancake−brush transition. Monolayer Formation. Considering that AFM monitors only the surface morphology, we also analyzed the data on heights and effective thickness of the monolayers under different conditions as measured by ellipsometry and the AFM scratch test (Figure 7). As we observe, the effective thickness of the monolayers in the liquid phase is 1.5−1.6 nm below and above LCST. In the solid phase, the average effective thickness increases to 2.4−3.3 nm at 23 °C and 4.1− 4.4 nm at 37 °C. The larger effective thickness above LCST is associated with the presence of the islands with height over 4 nm. The effective thickness of the monolayers, teffective is calculated as Adomain(tdomain + tunder) + Aunder(tunder), where Adomain and Aunder are the area coverage of the domains and underlying layer, respectively, in percentage (Adomain + Aunder = 100%), tdomain is the height of the domains, and tunder is the thickness of the underlying sub-layer.42 From this equation, the thickness of the underlying layer was calculated to be 0.6−0.8 nm for the monolayers in the liquid phase both below and above LCST. Therefore, a model for the monolayer organization in liquid and solid states that fulfills these morphological observations is suggested in Figure 8. In this model, below LCST, the disk-like

Figure 5. High-resolution AFM phase images of HBPIL monolayers at different surface pressures; 20 (a,b), 49 mN/m (c,d) and at different temperatures; 23 (a,c) and 37 °C (b,d). Z scale is 8° for all images.

Figure 6. Three-dimensional AFM images of HBPIL monolayers at different surface pressures; 20 (a,b) and 49 mN/m (c,d) and at different temperatures: 23 (a,c) and 37 °C (b,d). F


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Figure 7. Effective thickness (a,c) and underlying layer thickness (b,d) of HBPIL monolayer films measured by ellipsometry (a,b) and the AFM scratch test (c,d).

pancake shape (1.5−1.9 nm)77 and smaller than the estimated length of fully extended octadecyl chain (2.4 nm, see Figure S11). Therefore, we suggest that the disk-like domains are formed by the molecules with a flattened hyperbranched core and randomly oriented alkyl tails. Regarding thin underlying sublayer, we suggest that it is formed by hydrophilic PNIPAM macrocations, which spread between hydrophobic segments and hydrophilic silicon oxide surface (Figure 8). The formation of the ridge-like network morphology in the solid state can be attributed by the high stability of disk-like domains under compression caused by branched architecture and asymmetric chemical composition of HBPILs (Figures 3 and 6). We suggest that the branched architecture and asymmetry in chemical composition of HBPILs favor the formation of the disk-like domains that have a concave shape with elevated rims, making the pancake-to-brush or carpet-tobrush transition less likely to occur and allowing the disk-like domains to preserve their shape under high lateral compression. Meanwhile, the steric and electrostatic repulsions induced by the PNIPAM macrocations may not be strong enough to convert their structure from flat carpet-like to brushlike structure even at high surface pressure; thus, initial disklike domains are preserved in the merged structures with interdomain boundaries formed by peripheral rims at the original contact lines (Figure 8). Moreover, the thickness of the underlying layer in a solid phase is close to the sum of the initial height of disk-like domains (2.1 nm) and the thickness of the underlying PNIPAM layer in a liquid phase (0.6−0.8 nm) (Figure 7). This result suggests that the underlying layer of the monolayers in a solid phase is formed by the integration of the disk-like domains, and the ridge-like network is produced as the alkyl tails of HBPIL molecules are vertically oriented when trapped and compressed at the interdomain boundaries. This is further supported by the large contact angle of water over 90° (Figure S4), indicating that hydrophobic components constitute the surface of the monolayers. Indeed, our previous study found that alkyl tails of hyperbranched polyesters can be vertically

Figure 8. Models of HBPIL monolayer at the air−water interface at 23 °C (top) and at 37 °C (bottom) at different surface pressures (20 and 49 mN/m) (left) and molecular packing of monolayers at the solid substrate (right). The dashed line shows the ridge-like domains formed by the molecules with alkyl tails in an up-right orientation.

domains are formed in the liquid phase because hydrophobic segments of HBPILs tend to combine across the air−water interface to avoid unfavorable interaction with water, while the hydrophilic carboxylate terminal groups are anchored at the interface, ionically linked with PNIPAM macrocations that submerge in the water subphase. The hydrophobic−hydrophobic interactions of terminal alkyl chains of HBPILs facilitate large disk-like domain formation. It is also worthy to mention that the height of disk-like domains is around 2 nm, and this value is slightly larger than the height of the hyperbranched core being flattened to a G


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Langmuir oriented at high surface pressure, forming aggregate domains with a height of 0.6−0.8 nm above the beneath sublayer.53 Increasing temperature above LCST does not change the overall shape and dimension of the domains but affects their size distribution in a liquid phase (Figures 3, 4). Above LCST, the disk-like domains have diameters mostly within 120 nm, showing more uniform size distribution, compared to below LCST. This can be explained by the effect of the collapsed PNIPAM macrocations on limiting the domain combination. Above LCST, PNIPAM macrocations collapse and become partially hydrophobic, thus promoting PNIPAM desorption from the water subphase and anchoring at the air−water interface. This anchoring results in the formation of a thicker outer region of disk-like domains and causes increased steric repulsion between HBPIL molecules, preventing full merge of the initial domains (Figure 5). Nanomechanical Mapping. QNM surface mapping was further performed to reveal the surface properties (apparent elastic modulus and adhesion distribution) below and above LCST (Figures 9 and S12). For both temperatures, under dry conditions, the disk-like domains possess slightly higher elastic modulus and adhesion compared to the interdomain region (Figure 9a). Further analysis of FDCs was performed to obtain values of elastic modulus and adhesion (Figures S14 and S15). The FDC analysis shows the similarity of the average elastic

modulus of 2.5−3.5 GPa within experimental deviation (Figure 10a). These values are characteristics of ordered alkyl chain surfaces, in the range of 1−5 GPa.78,79 This observation suggests that the topmost surface of the HBPIL monolayer might be covered by terminal octadecyl tails, which supports the proposed model (Figure 8). In addition, the adhesive forces for the domains and interdomain region are similar within experimental deviation with modest temperature dependence (Figure 10b). The uniformity of surface adhesion distribution also supports our model that the topmost surface does not consist of dissimilar components, and hydrophobic components are located at the surface with the beneath hydrophilic sublayer. QNM mapping and FDC analysis were also conducted for HBPIL monolayers deposited in a solid phase (Figures S13 and S16−S18). QNM mapping for those deposited at 23 °C shows that the ridge-like domains have a higher elastic modulus and lower adhesion than the interdomain region, supporting the suggestion that the ridge-like network is formed by vertically oriented alkyl chains (Figure S13a,c). The vertically oriented alkyl chains are more likely crystallized and thus expected to have higher elastic modulus and lower adhesion compared to randomly oriented alkyl chains.80,81 On the other hand, the FDC analysis shows that the ridges and interdomain region have similar average values of elastic modulus (around 2.5 GPa) and adhesion (around 4 nN) (Figure S18). This similarity further supports that the monolayers have the topmost layer composed of hydrophobic and stiff octadecyl tails with the underlying sub-layer formed by hydrophilic terminal arms. For the monolayers deposited at 37 °C, the elastic modulus distribution does not show a significant contrast between domains and the interdomain region, but adhesion mapping shows a remarkable difference between domains and the interdomain region with much lower adhesion within the island domains (Figure S13b,d). This observation suggests that the surface components for the islands are dissimilar from those for ridges and the interdomain region, supporting the proposed model that the islands are formed by the integration of hydrophobized PNIPAM arms. The FDC analysis also shows the similarity in elastic modulus and adhesion between the ridges and interdomain region. Notably, island domains possess a lower elastic modulus of 1.4 ± 0.4 GPa, suggesting that the island domains are formed by PNIPAM macrocations, which are much softer than the hyperbranched core and octadecyl tails.78,79,82−84 It should be noted that all measurements discussed above were conducted in ambient air, which is a bad solvent for all components of HBPILs and for all temperatures. Therefore, with all molecular components collapsing in air, the topmost surface of the monolayers can show a relative uniformity of elastic properties irrespective of constituents. In contrast to dry conditions, a clear heterogeneity of mechanical properties was observed by QNM measurements in water below and above LCST (Figure 11). First, the individual domains show a lower elastic modulus than the interdomain region below LCST (Figure 11a), which is also confirmed by the FDC analysis (Figure 12a). The surface adhesion distribution displays that the individual domains possess higher adhesion than the interdomain region (Figure 11c), and the average adhesive force for the domains is four-fold higher than that for the interdomain region (1.1 and 0.3 nN, respectively) according to the FDC analysis (Figure 12a).

Figure 9. Elastic modulus (a,b) and adhesion (c,d) distribution of HBPIL monolayers at surface pressure 20 mN/m and at 23 (a,c) and 37 °C (b,d). Z scale is 250 MPa and 5 nN for the modulus and adhesion images, respectively. H


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Figure 10. Elastic modulus (a) and adhesion (b) of the domain and interdomain region of HBPIL monolayers at surface pressure 20 mN/m. All measurements were performed in a dry state.

Above LCST, the modulus distribution changed to rather uniform across all domains and interdomain region (Figure 11b). The FDC analysis also shows the reduced difference in the average elastic modulus between the domains and the interdomain region. The average elastic modulus for the domains increases from 250 MPa below LCST to 470 MPa above LCST, while for the interdomain region, the average values of elastic modulus are about 700 MPa for both temperatures. In addition, the adhesion distribution shows higher adhesion for the interdomain region, which is in contrast to that observed below LCST (Figure 11d). However, the FDC analysis shows that the adhesive forces for the domains and interdomain region are similar above LCST (0.9 and 1.1 nN, respectively) (Figure 12b). These changes in mechanical contrast distribution suggest additional reorganization of HBPIL molecules because of the hydrophilic− hydrophobic transformation of PNIPAM chains. PNIPAM fragments are transited from the swollen to collapsed state preferably in the domains with their subsequent aggregation accompanied by orientation of hydrophilic ionic ammonium and carboxylate groups on the surface of the aggregates formed. This reorganization might enable the hyperbranched core to be exposed to the surface, leading to the increased elastic modulus above LCST, and the ionic groups oriented onto the surface could predominate the adhesive forces across the entire area of the monolayers. It should be also added that the increase in elastic modulus of PNIPAM fragments above LCST can contribute to the increased elastic modulus because of water repulsion from collapsed shells. It has been reported that the elastic modulus of PNIPAM films increases by 2 orders of magnitude from around 10−100 kPa in the highly swollen state below LCST to several MPa in the collapsed state above LCST.82,88,89 In addition, the temperature-induced change in mechanical response confirms that the PNIPAM fragments provide thermally responsive surface behavior, supporting the proposed model that they have a critical role in the formation of HBPIL monolayers with temperature-dependent morphology.

Figure 11. Elastic modulus (a,b) and adhesion (c,d) distribution of HBPIL monolayers deposited at surface pressure 20 mN/m at different temperatures: 23 (a,c) and 37 °C (b,d). The images were collected in water at temperature below (a,c) and above LCST (b,d). Z scale is 250 MPa and 5 nN for the modulus and adhesion images, respectively.

These results suggest that the topmost surface of the domains is composed of softer and more hydrophilic components than the interdomain region.85 In addition, such heterogeneity of mechanical response is in stark contrast to that under dry conditions, indicating that the molecular reorganization occurs in water below LCST with PNIPAM chains saturating the water−domain interface. When exposed to water, hydrophilic and soft segments, such as terminal ionic groups and PNIPAM macrocations, beneath hydrophobic and stiff components of the dried films swell and emerge to the surface, while the hydrophobic segments collapse in water, which is a bad solvent for these terminal groups.86,87

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CONCLUSIONS In this paper, we report the synthesis and assembly behavior of novel amphiphilic HBPILs at the air−water interface. The HBPILs studied here consist of asymmetrical peripheral composition of hydrophobic octadecyl tails and hydrophilic PNIPAM macrocations, which provides HBPILs with amphiphilicity as well as thermo-responsive behavior. Stable HBPIL monolayers were successfully formed with diverse morphologies at the interface. The disk-like domains were I


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Langmuir

Figure 12. Elastic modulus (a) and adhesion (b) of the domain and spreading areas of HBPIL monolayers at surface pressure 20 mN/m. All measurements were performed in water below and above PNIPAM LCST temperature.

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formed in a liquid phase and transformed into almost uniform with the network of ridge-like domains representing interdomain boundary in a solid phase because of the high stability of disk-like morphology. In the solid phase, islands connected by the network of ridge-like domains were only observed above LCST because of the integration of hydrophobized PNIPAM macrocations into monolayers. Moreover, we observed the transition in mechanical response of HBPIL monolayers from homogenous distribution in the dry state to heterogeneous distribution in the wet state, suggesting the surface reconstruction under wet conditions, which further supports that the as-assembled HBPIL monolayers are composed of the hydrophobic surface and hydrophilic underlying sublayer. Further change in the mechanical response was observed when water temperature increased above LCST, which confirms the important role of thermoresponsive PNIPAM macrocations in the monolayer formation. The distribution of mechanical properties changed from highly contrasted two-phase distribution below LCST to near-uniform distribution above LCST because of molecular reorganization associated with the hydrophilic−hydrophobic transformation of PNIPAM macrocations above LCST.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.9b01905. Synthesis of HBPILs, repeated compression−extension hysteresis experiment, AFM images, molecular models, and FDC analysis (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 404-894-6081. Fax: 404-385-3112. ORCID

Vladimir V. Tsukruk: 0000-0001-5489-0967 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the National Science Foundation DMR 1505234. We would like to thank Luke Pittner for technical assistance. J


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