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Intermolecular Interaction of Polymer Brushes Containing Phosphorylcholine and Inverse-Phosphorylcholine Saori Mihara, Kazuo Yamaguchi, and Motoyasu Kobayashi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01764 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 5, 2018
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Intermolecular Interaction of Polymer Brushes Containing Phosphorylcholine and Inverse-Phosphorylcholine Saori Mihara,a Kazuo Yamaguchi,b and Motoyasu Kobayashib*
a
Graduate School of Engineering, Kogakuin University, 2665-1 Nakano-cho, Hachioji,
Tokyo 192-0015, Japan b
Department of Applied Chemistry, School of Advanced Engineering, Kogakuin University,
2665-1 Nakano-cho, Hachioji, Tokyo 192-0015, Japan
*Corresponding author: Motoyasu Kobayashi, Department of Applied Chemistry, School of Advanced Engineering, Kogakuin University, 2665-1 Nakano-cho, Hachioji, Tokyo 192-0015, Japan. E-mail address: [email protected]
ABSTRACT Choline phosphate (CP) is a phosphobetaine-type zwitterionic functional group, referred to as inverse phosphorylcholine (PC) due to the reverse orientation of a positively charged quaternary amine and anionic phosphate in contrast to PC lipids in nature. The A unique dipole paring between CP and PC groups has attracted much attention in the bio-interface research field. Herein, to evaluate the molecular interaction between the CP and PC groups in water, force-distance curve measurements using scanning probe microscopy (SPM) with a PC-group-functionalized cantilever was carried out on the surface of polymer brushes bearing the CP groups. Three types of methacrylate monomers bearing CP with ethyl (Et), methoxyethyl (MOE), and isopropyl (iPr) phosphates were synthesized in 42-71% yields, and polymerized by surface-initiated atom transfer radical polymerization to form polymer brushes on silicon wafers. The surface free energy of CP-polymer brushes with Et, MOE, and iPr was estimated to be 64.0, 61.4, and 57.4 mN m-1, respectively, based on contact angle measurements. Force-distance curve measurements of polymer brushes having a CP group was conducted in water at 25 C by SPM using a spherical probe produced by attaching a
silica particle (SiP) (d = 25 μm) covered with PC or CP groups to a tipless cantilever. Adhesion force larger than 14 nN was observed between the CP-polymer brushes and PC-SiP, whereas PC-polymer brushes revealed extremely low adhesion force of less than 0.6 nN with PC-SiP and propylsilane-modified SiP. The specific attractive molecular interaction between CP and PC groups was quantitatively evaluated.
1. Introduction Zwitterionic phosphorylcholine (PC) is a phosphobetaine consisting of anionic phosphate and cationic quaternary amine groups that can be found in the polar head group at the outer surface of lipid bilayer cell membranes. Polyzwitterions with a PC pendant group have attracted much attention due to their useful application to various synthetic biomaterials. In particular, a methacrylate derivative bearing a PC group at the side chain MPC is a widely known zwitterionic monomers for the preparation of biocompatible polymers1. For example, poly(MPC) and its copolymers reveal excellent biocompatibility,2 blood compatibility,3 low friction,4-5 unique ionic strength dependency,6-9 and anti-fouling properties10-11 due to the extremely high hydrophilicity12 and low adhesive interaction with cells and proteins. Choline phosphate (CP) is another type of phosphobetaine, referred to as inverse-PC due to having a positively charged quaternary amine connected with anionic phosphate in the reverse orientation in contrast to PC. For instance, the cationic group of PC locates relatively outside the unit and the anionic phosphate group is connected near the backbone. In contrast, the phosphate and cationic group of inverse-PC are oriented opposite to their orientation in PC. Inverse-PC compounds were synthesized by Nakaya et al in 1996,13-14 and recently, unique molecular interaction behaviors of inverse-PC polymers were found15 as well as PC polymers. Brooks reported that the aggregation of red blood cells was induced by CP polymer with a methyl substituent as an alkyl phosphate group due to the specific attractive molecular interaction between CP and PC on the surface of cell membranes.16-19 Li et al. also reported
cell adhesion on a surface modified with CP molecules.20 Several investigations on effective drug delivery have also emerged through the use of micelles of a block copolymer of CP polymer,21-22 and liposomes composed of amphiphilic CP lipid molecules23. These results indicated that the inverse-PC group more likely interacted with the cell membrane surface rather than the PC group. On the other hand, Emrick et al. synthesized an inverse-PC polymer bearing n-butyl phosphate and reported that significant cytotoxicity, hemolytic activity, and aggregation of red blood cells were not observed in the presence of the inverse-PC polymer.24 Opposite cell adhesion behavior of CP polymers has been reported, however, a reasonable explanation for this behavior has not yet been obtained, even if the effect of alkyl substitutes of phosphate is considered. Nevertheless, the CP molecules are considered a promising candidate for useful functional group for novel biomaterial. To quantitatively evaluate the adhesive interaction between the PC group and inverse-PC molecules or polymers, we chose force curve (force-versus-distance) measurement using scanning probe microscopy (SPM). This is a useful methodology for directly measuring the interaction force between the probe surface and sample surface in the range of nanonewtons as a function of their mutual separation by using a cantilever featuring precise movements controlled by piezoelectric elements. The probe surface can be modified with various functional groups and molecules, therefore, the chemical interaction force between polymer brushes, proteins, and cells have been evaluated by force curve measurements.25-32 Ishihara et al. demonstrated force curve measurement to reveal extremely low adhesive interaction between a poly(MPC) brush surface and functionalized cantilever probe with carboxy, amino, and alkyl groups in a phosphate buffered saline aqueous solution, compared with that of a positively-charged polyelectrolyte brush and hydrophobic poly(butyl methacrylate) brush surfaces.33 In the present study, three types of methacrylate monomers bearing inverse-PC with ethyl (Et), isopropyl (iPr), and methoxyethyl (MOE) phosphate were synthesized to estimate the effect of hydrophilicity or hydrophobicity of alkyl groups on the molecular interaction. Surface-initiated atom transfer radical polymerization (SI-ATRP) of PC or CP monomers was carried out to prepare zwitterionic polymer brushes on flat silicon wafers to evaluate their interaction force with PC or CP functional groups chemically immobilized on a silica particle
(SiP) probe, using force curve measurement in water. The effect of the charge orientation of phosphobetaine on the adhesive interaction between CP and PC was also studied.
2. Experimental 2-1. Materials Commercially available 2-chloro-2-oxo-1,3,2-dioxaphospholane, (COP, Tokyo Chemical Industry Co. Ltd. (TCI), 95%), calcium hydride (CaH2, Nacalai Tesque Inc), sodium (Kanto Chemical Co.), lithium aluminum tetrahydride (LiAlH4, Kanto Chemical Co.), 2,2'-bipyridyl (bpy, Wako Pure Chemicals, 99.5%), anhydrous toluene (Kanto Chemical, 99.5%), hydrogen peroxide (H2O2, Kanto Chemical, 34.5%), sulfuric acid (H2SO4, Kanto Chemical, 95.0%), 5-hexen-1-ol
(TCI,
95%),
2-bromo-2-methylpropionyl
bromide
(Aldrich,
98%),
monomethylether hydroquinone (MEHQ, Aldrich, 98%), 3-aminopropyltrimethoxysilane (APS, TCI, 98.0%), and trimethoxypropylsilane (PrS, TCI, 98.0%) were used without additional purification. MPC monomer was kindly donated by NOF Corporation. Tetrahydrofuran (THF) was purified by refluxing with sodium for 6 h and distillation from LiAlH4 under nitrogen atmosphere. Triethylamine (Et3N, Kanto Chemical) was distilled from CaH2 under nitrogen atmosphere before use. Ethanol (EtOH), 2-methoxyethanol, and isopropanol (iPrOH) were purchased from Kanto Chemical and purified by refluxing over magnesium turnings for 3 h and successive distillation before use. Diethylether (Et2O, Kanto Chemical) was stored in a glass bottle in the presence of sodium chip. Copper (I) bromide (CuBr, Wako Pure Chemicals, 99.9%) was purified through successive washes with acetic acid and EtOH, and then dried under vacuum. Ethyl 2-bromoisobutylate (EB, TCI, 98%) and 2-dimethylaminoethyl methacrylate (DMAEMA, TCI, 98.5%) were distilled over CaH2 before use. 5'-Hexenyl 2-bromoisobutylate was synthesized from 5-hexen-1-ol and 2-bromo-2-methylpropionyl bromide, and purified by distillation with CaH2 under reduced pressure. The surface initiator, (2-bromo-2-methyl)propionyloxy hexyltrimethoxysilane (BHM) was synthesized by hydrosilylation of 5'-hexenyl 2-bromoisobutylate treated with trimethoxysilane in the presence of a Karstedt catalyst. The BHM monolayer was immobilized on silicon wafer (10×40×0.5 mm3) in dry toluene at 298 K for 4 h. Deionized water was purified using the Direct-Q 3UV system (Merck Millipore, Inc.). The silicon (111)
wafer (diameter = 100 ± 0.5 mm2, thickness = 500 ± 25 m, Matsuzaki Seisakusho Co., Ltd.) was cleaned by washing with piranha solution (H2SO4/H2O2 = 7/3, v/v) at 100 C for 1 h. Powder of silica particle (SiP) with 25-μm diameter (Kojundo Chemical Laboratory. Co. Ltd.) for force curve measurement, and silica sol (0.30 wt% SiP, d = 100 nm) dispersed in iPrOH solution (Nissan Chemical Industries Ltd.) for SI-ATRP were used as received.
2-2. Synthesis of monomers 2-Isopropoxy-2-oxo-1,3,2-dioxaphospholane (IOP) Et3N (71.9 mmol), iPrOH (131 mmol), and THF (100 mL) were charged into a three-neck round-bottom flask and stirred under nitrogen atmosphere. COP (66.8 mmol) diluted with THF (40 mL) was slowly added dropwise to the mixture at 78 °C for 1 h. The reaction mixture was stirred at -78 °C for 11 h, and further stirred at 25 °C for 9 h. The reaction mixture was filtrated with anhydrous THF and anhydrous Et2O to remove salts, and the resulting pale-yellow solution was concentrated by evaporator. Distillation of the solution from CaH2 was carried out under reduced pressure to give the IOP as a colorless oil in a 70% yield (b.p. 70.1-95.0 °C/ 3.3 mmHg) (Scheme 1). 1H NMR (δ: ppm, 400 MHz, methanol-d4): 1.34-1.35 (d, CH3, 6H, J = 6.2 Hz), 4.37-4.50 (m, CH2, 4H), 4.63-4.71 (m, CH, 1H). 13C NMR (δ: ppm, 100 MHz, methanol-d4): 23.72 (CH3), 67.85 (CH2), 75.56 and 75.62 (CH). 31P NMR (δ: ppm, 162 MHz, methanol-d4): 17.74. 2-[2-(Methacryloyl ethyl)dimethylammonio]ethyl isopropyl phosphate (MCP-iPr) Into a well-dried 100-mL flask, IOP 1 (19.9 mmol), DMAEMA (19.2 mmol), MEHQ (3.62 mmol) and acetonitrile (12 mL) were added under nitrogen atmosphere and stirred at 75 °C for 48 h. Dry Et2O was slowly added continuously until the precipitation of a white solid appeared and then was stirred at 25 °C for 2 h. The supernatant was removed by decantation, and the precipitate was purified by repeating the precipitation several times using dry THF to give MCP-iPr as a white powder in a 48% yield. 1H NMR (δ: ppm, 400 MHz, methanol-d4): 1.26-1.28 (d, CH(CH3)2, 6H, J = 6.2 Hz), 1.96 (s, αCH3, 3H), 3.27 (s, NCH3, 6H), 3.72-3.74 (m, CH2CH2OP, 2H), 3.86-3.88 (m, CO2CH2CH2, 2H), 4.26 (m, CH2OP, 2H), 4.40-4.48 (m,
methanol-d4): -0.29. 2-Ethoxy-2-oxo-1,3,2-dioxaphospholane (EOP) Et3N (52.4 mmol), EtOH (65.2 mmol), and THF (200 mL) were charged into a three-neck 500 mL round-bottom flask and stirred under nitrogen atmosphere. COP (43.5 mmol) diluted with THF (70 mL) was slowly added dropwise to the mixture at -78 °C for 5 h. The reaction mixture was stirred at -68 °C for 2 h, and further stirred at 25 °C for 13 h. The reaction mixture was filtrated with dry THF to remove salts, and the resulting pale-yellow solution was concentrated by evaporator. Distillation of the solution from CaH2 was carried out under reduced pressure to give the EOP as a colorless oil in an 83% yield (b.p. 84.9-89.8 °C/ 1.5 mmHg). 1H NMR (δ: ppm, 400 MHz, methanol-d4): 1.33-1.38 (t, CH3, 3H, J = 6.2 Hz), 4.13-4.23 (m, CH2, 2H), 4.39-4.51 (m, CH2, 4H), 13C NMR (δ: ppm, 100 MHz, methanol-d4): 16.39 and 16.45 (CH3), 66.28 and 66.34 (CH2), 68.07 (CH2)2.
31
P NMR (δ: ppm, 162 MHz,
methanol-d4): 18.40. 2-[2-(Methacryloyl ethyl)dimethylammonio]ethyl ethyl phosphate (MCP-Et) Into a well-dried two-neck 100 mL- flask, EOP (19.1 mmol), DMAEMA (19.2 mmol), MEHQ (4.59 mmol) and acetonitrile (18 mL) were added under nitrogen atmosphere and stirred at 70 °C for 96 h and at 25 °C for 38 h. Excess amount of dry Et2O was added to the reaction mixture and stirred vigorously to form white solid precipitates. The supernatant was removed by decantation, and the precipitate was further purified by repeating the precipitation four times using dry THF to give MCP-Et as a white powder in a 42% yield. The obtained MCP-Et was immediately diluted with TFE and stored in a freezer at 20 °C. 1H NMR (δ: ppm, 400 MHz, methanol-d4): 1.25-1.29 (t, CH2CH3, 3H, J = 7.1 Hz), 1.97 (s, αCH3, 3H), 3.27 (s, NCH3, 6H), 3.72-3.75 (m, CH2CH2OP, 2H), 3.86-3.88 (m, CO2CH2CH2, 2H), 3.90-3.97 (q, CH2CH3, 2H, J = 7.2 Hz), 4.27 (s, CH2CH2OP), 4.64 (s, CO2CH2, 2H), 5.71 and 6.15 (s, CH2, 2H). 13C NMR (δ: ppm, 100 MHz, methanol-d4): 16.81 and 16.89 (CH2CH3), 18.39 (αCH3), 52.92 (NCH3, 59.18 (CO2CH2), 60.03, 60.08 (CH2CH2OP), 62.67 and 62.73
(CH2CH3), 65.15 (CO2CH2CH2), 66.12 (NCH2), 127.29 (CH2), 137.08 (C=CH2), 167.60 (C=O). 31P NMR (δ: ppm, 162 MHz, methanol-d4): 0.03. 2-Methoxyethoxy-2-oxo-1,3,2-dioxaphospholane (MEOP) Et3N (43.3 mmol), 2-methoxyethanol (53.9 mmol), and THF (100 mL) were charged in a three-neck 300 mL round-bottom flask and stirred under nitrogen atmosphere. COP (35.9 mmol) diluted with THF (50 mL) was slowly added dropwise to the mixture at 78 °C for 2 h. The reaction mixture was stirred at -78 °C for 1 h, and further stirred at 25 °C for 20 h. The reaction mixture was filtrated with dry THF to remove salts. The resulting solution was concentrated by evaporator, and distilled from CaH2 under reduced pressure to give the MEOP as a colorless liquid in a 81% yield (b.p. 107-110 °C/ 1.5 mmHg). 1H NMR (δ: ppm, 400 MHz, methanol-d4): 3.38 (s, CH3, 3H), 3.59-3.62 (m, CH2CH2OP, 2H), 4.20-4.24 (m, CH2CH2OP, 2H), 4.40-4.52 (m, CH2, 4H). 13C NMR (δ: ppm, 100 MHz, methanol-d4): 59.09 (OCH3), 68.07 (O(CH2)2), 68.97 (CH3OCH2), 72.33 (CH2OP). 31P NMR (δ: ppm, 160 MHz, methanol-d4): 18.71. 2-[2-(Methacryloyl ethyl)dimethylammonio]ethyl methoxyethyl phosphate (MCP-MOE) Into a well-dried two-neck 100 mL-flask, MEOP (17.6 mmol), DMAEMA (17.8 mmol), MEHQ (4.03 mmol) and acetonitrile (20 mL) were added under nitrogen atmosphere and stirred at 70 °C for 85 h and at 25 °C for 38 h. Excess amount of dry THF was added to the reaction mixture and stirred to form a pale-yellow viscous liquid at the bottom of the flask. After the supernatant was carefully removed by decantation, the residue was stirred with dry THF, and isolated. The resulting viscous liquid was diluted with a small portion of 1,4-dioxane, and then freeze dried under reduced pressure for 2 h to give a MCP-MOE as a white powder in a 71% yield. The monomer was immediately diluted with TFA and stored in a freezer (20 °C). 1H NMR (δ: ppm, 400 MHz, methanol-d4): 1.96 (s, αCH3, 3H), 3.29 (s, NCH3, 6H), 3.36 (s, OCH3, 3H), 3.56-3.59 (m, CH2OCH3, 2H), 3.74-3.76 (m, NCH2CH2OP, 2H), 3.88-3.90 (m, CO2CH2CH2, 2H), 3.97-4.01 (m, CH2CH2OCH3, 2H), 4.32 (s, NCH2CH2OP), 4.67 (s, CO2CH2, 2H), 5.72 and 6.15 (s, βCH2, 2H).
2-3. Surface-initiated polymerization Commercially supplied 0.30 wt% SiP (d = 100 nm)/ iPrOH suspension solution (7.50 mL) was collected in a 300-mL round-bottomed flask and diluted with 25 mL of EtOH. A mixture of ammonia solution (28% NH3 aqueous solution, 6.0 mL) and EtOH (50 mL) was added dropwise to the SiP suspension with stirring, and the system was stirred for 2.5 h at 40 C. BHM (1.5 g, 4.8 mmol) diluted with dry EtOH (15 mL) was added dropwise to the suspension, and the reaction mixture was continuously stirred for another 13.5 h at 40 C. The reaction mixture was then condensed by the evaporation of EtOH and poured into an excess amount of n-hexane to remove unbound BHM. BHM-immobilized SiP was obtained by the centrifugation of n-hexane solution and was further purified by repeating the sonication and centrifugation in EtOH. A few sheets of BHM-immobilized silicon wafers, BHM-immobilized SiP (d = 100 um), and monomer/ TFE solution were charged in a well-dried glass tube with a stopcock, and degassed using a freeze-thaw process repeated three times. CuBr (0.030 mmol), bpy (0.060 mmol), and TFE (2.0 mL) were introduced in another glass tube with a stopcock, and degassed by a freeze-thaw process to give a red-colored solution. After a portion of copper catalyst solution was added to the monomer solution, the reaction mixture was degassed again by a freeze-thaw process three times, and then stirred in an oil bath at 60 C for 24 h under argon. The reaction was stopped by opening the glass vessel to air at 0 C. The silicon wafers were isolated and washed with methanol in a Soxhlet apparatus for 6 h. The reaction mixture was centrifuged to collect the SiP. SiP was treated with HF solution to decompose the SiO2. The reaction mixture was poured into THF to precipitate the cleaved polymer to measure the Mn by size exclusion chromatography (SEC).
The silica probe for force curve measurement was prepared as follows. Powder of SiP (d = 25 μm) was charged in a 100-mL round-bottomed flask and was diluted with 24 mL of EtOH. A mixture of 28% NH3 aqueous solution (7.41 mL) and EtOH (20 mL) was added dropwise to the SiP suspension with stirring, and the system was stirred for 2 h at 40 C. APS (0.98 mL 5.6 mmol) diluted with ethanol (10 mL) was added dropwise to the suspension, and the reaction mixture was continuously stirred for another 15 h at 40 C. The reaction mixture was then condensed by the evaporation of ethanol and poured into an excess amount of n-hexane to remove unbound APS. APS-immobilized SiNP (APS- SiNP) was purified by repeating the sonication and centrifugation in EiOH, and then dried under vacuum. PrS-immobilized SiNP (PrS-SiP) was prepared in a similar manner. Powder of APS-SiP (0.118 g) and methacrylate monomers (40 - 45 mg, 0.13 - 0.15 mmol) were charged into a 10-mL glass vial with a small magnetic stirring bar, and diluted with deionized water to obtain a 2.0wt% monomer solution. After the mixture was stirred for 6 h at 25 C to proceed with the Michael addition of an amino group to the methacrylate, methanol was added to the reaction mixture to form the precipitates. Resulting precipitates were purified by repeating decantation and stirring in methanol several times, and then were dried under vacuum to give a PC- or CP-immobilized SiP (PC-SiP, CPEt-SiP, CPiPr-SiP, CPMOE-SiP), as shown in Scheme 3. The Arrow TL1 tipless cantilever (NanoWorld, tip length = 500 μm, width = 100 μm, thickness = 1 μm, resonance frequency = 6 kHz, bending spring constant = 0.03 N·m–1) was used for force curve measurement. The torsion spring constant and sensitivity of the cantilever were 2.6 × 102 2.1 × 103 N·m–1 and 9.4 × 102 2.1 × 103 N·V–1, respectively, determined by thermal noise method34 in the atmosphere before use. A small amount of epoxy glue (EPOCLEAR, Konishi Co Ltd.) placed on the slide glass was picked up by the edge of the tipless cantilever head using piezo scanner manipulation. Then, the cantilever bearing adhesives approached the slide glass to pick up one SiP, which was air dried for 12 h at 25 C.
2-4. Characterization 1
H NMR measurement was carried out with a Jeol ECX 400 spectrometer, using deuterated
chloroform or methanol-d4 as the solvent and tetramethylsilane as the references at 298 K. The number-average molecular weight (Mn) and the molecular weight distribution were
determined by size exclusion chromatography (SEC) using a LC-20Ai (Shimadzu Co., Ltd.) and CO-1024 (Jasco, Co. Ltd) equipped with three directly connected polystyrene gel columns of two TSKgel G3000PWXL and one G5000PWXL (Tosoh Co., Ltd.), and a refractive index detector RID-10A (Shimadzu Co., Ltd.), using 0.2 M of NaCl aqueous solution as the eluent at a flow rate of 0.5 mL min−1 at 313 K. Calibration curves were prepared using a series of polyethyleneoxide (PEO) standards (Mn = 3000, 50000, 107000, 250000, 520000 g mol−1). X-ray photoelectron spectroscopy (XPS) measurement was carried out with a Quantum 2000 system (Physical Electronics Inc.) at 1×10−9 Pa using a monochromatic Al K X-ray source operated at 100 W. XPS spectra were collected at a takeoff angle of 45°, and a low-energy (25 eV) electron flood gun was used to minimize sample charging. The survey spectra (0−1000 eV) and high-resolution spectra (narrow scan) of the C1s were acquired at an energy step of 1.0 and 0.1 eV, respectively. Surface topography images of the polymer brushes were acquired with a NanoWizard 3 system (JPK Instruments) mounted on a Nikon Ti-U-SK microscope, in dynamic force mode at optimal force under ambient atmosphere at room temperature. The rectangular shaped HyperDrive PPP-NCHAuD silicon cantilever (Nanosensors, Tip radius: 7 nm) with a spring constant of 42 N m−1 and resonance frequency of 330 kHz was used for imaging. Force curve measurement in water was also carried out by the NanoWizard 3 system using a silica probe made with the Arrow TL1 tipless cantilever and chemically modified SiP (PrS-SiP, PC-SiP, CPEt-SiP, CPiPr-SiP, and CPMOE-SiP), as shown in Scheme 3. Force-distance curves were recorded at various spots on the surfaces using a maximum applied force of 2 nN, Z length of 3.0 μm, a constant approach and retraction velocity of 2.0 μm s−1., and data duration of 1.5 s. The curves were acquired with the Z closed loop ON. Data was then automatically stored using JPK software to analyze the position on the retraction curve, force value, and detachment force (maximum negative force). The static contact angles against of liquid (2 L) were recorded with a Simage Entry system (Excimer Inc.) equipped with a video camera. The average of five measurements was used as the data. The thickness of the brush layers was determined with an alpha-SE, KKb spectroscopic ellipsometer (J.A. Woollam Co.) equipped with a Xenon arc lamp (wavelength 260–760 nm) at a fixed incident angle of 70°. Zeta potential of the brush surface was measured by Zeta-potential analyzer ELS-Z (Photal Otsuka
3. Results and Discussion 3-1. Synthesis of monomers and polymers The methacrylate monomers containing inverse PC were synthesized by a two-step process: (1) synthesis of alkoxy-2-oxo-1,3,2-dioxaphospholane using COP and alcohol, and (2) ring-opening reaction of alkoxy-2-oxo-1,3,2-dioxaphospholane by DMAEMA, as shown in Schemes 1 and 2. EOP, IOP, and MEOP were successfully obtained in relatively high yields, and purified by distillation under reduced pressure. Although the successive ring-opening reaction of each phospholane compound and DMAEMA proceeded smoothly in acetonitrile, it was hard to isolate the MCP-R (R = Et, iPr, MOE) monomers, because when they were highly purified and concentrated, their reactivity was so high that they easily polymerized to form gels. Therefore, the reaction carried out in the presence of MEHQ stabilizer, and then all the monomers were immediately diluted with TFA after purification by precipitation, and stored in the freezer at 20 C. 1H and 13P NMR spectra of MCP-iPr are shown in Figure 1. The phosphorous signal clearly shifted from 18 ppm to 0 ppm by the ring-opening reaction of IOP and the formation of MCP-iPr. 1H,
13
monomers
are
(MCP-Et,
MPC-MOE)
C, and 1H-1H COSY NMR spectra of the other shown
in
Figures
S1-1
to
S1-25.
Methoxy-2-oxo-1,3,2-dioxaphospholane (MOP) was also obtained in a 71% yield by the reaction of methanol and COP (Figures S1-26 to S1-27). Unfortunately, MCP bearing methyl phosphate was not obtained by the reaction of MOP and DMAEMA because only the by-product was formed instead of the desired compound. Although the successful synthesis of methyl-taaerminated MCP has been reported by Brooks et al., the same product could not be obtained by our synthetic protocol.
ATRP of MPC and MCP-R (R = Et, iPr, MOE) in TFE was carried out at 30 C for 24 h to
produce the corresponding polymers, as shown in Table 1. TFE is a useful solvent for controlled polymerization of ionic monomers.35 1H NMR spectra and SEC curves of the obtained polymers are shown in the Supporting Information (Figures S1-28 to S1-31 and Figure S2, respectively). All the poly(MCP-R) were soluble in water, methanol and TFE, but insoluble in THF, acetone, chloroform, and hexane. The solubility of poly(MCP-R) was similar to that of poly(MPC). The detailed solubility of monomers and polymers having PC and CP groups are listed in Table S3-1 (Supporting Information). Surface-initiated (SI) ATRP of MCP-iPr in TFE was carried out from BHM-immobilized Si wafers in the presence of unbound (free) initiator EB to give the free poly(MCP-iPr), but, a polymer brush with the appropriate thickness was not formed on the silicon wafers. Usually, a free initiator is used as the sacrificial initiator for SI-ATRP to control the polymerization rate and degree of polymerization, and to produce a free polymer with a similar molecular weight as that of a polymer brush generated from a surface initiator on the substrate. On the contrary, the ATRP of MCP initiated from EB in a solution proceeded more rapidly than that from a surface initiator, resulting in the production of quantitative free polymer and no polymer brush on the substrate surface. Therefore, BHM-immobilized SiP was used as the sacrificial initiator for SI-ATRP of MCP-R with a BHM-immobilized Si wafer. Both the sphere-shaped SiP and flat Si wafer simultaneously initiated SI-ATRP to generate poly(MPC-R) on their surfaces and give the polymer brushes (Scheme 2). For example, SI-ATRP of run 1 and run 3 in Table 1 was carried out with the free initiator EB and surface initiator BHM on the Si wafer, while that of runs 2, 4, 6, and 8 was carried out using only the surface initiator BHM-SiP and BHM-Si wafer in the absence of EB to produce a brush with a relatively large Mn and thickness on the Si wafer. Surface-grafted polymers were isolated from SiP through the decomposition of SiO2 using an HF aqueous solution. The Mn of surface-grafted polymers was determined by SEC. Formation of polymer brushes on silicon wafers was confirmed by ellipsometer and XPS (Figure S4).
3-2. Surface properties of polymer brushes containing inverse-PC groups
The static water contact angle of poly(MPC) and poly(MCP-R) brushes are listed in Table 2. Poly(MCP-R) brushes showed a relatively higher water contact angle compared with that of poly(MPC) brushes. The water contact angle increased in the order of poly(MCP-Et) < poly(MCP-MOE) < poly(MCP-iPr), corresponding to the alkyl groups of phosphate. The static contact angles of diiodemethane are also summarized in Table 2. The surface free energy (SV) of the brushes was estimated from the Owens-Wendt equation36 described by the dispersion force (SVd) and polar force (SVp) parameters, as follows: (1 + cos θ) LV = 2 (SVd LVd)1/2 + 2 (SVp LVp)1/2 SV = 2(SVd LVd)1/2 + 2(SVp LVp)1/2
(eq1) (eq2)
where SVd and LVd are dispersion force components, and SVp and LVp are the polar force components of the brush surface and probe liquid, respectively. Using these two equations and the contact angles of two types of liquids (water and diiodemethane) with different SVd and γSVp values, the surface free energy of poly(MPC), poly(MCP-Et), poly(MCP-MOE), and poly(MCP-iPr) brushes was estimated to be 73.7 mN m-1, 64.0 mN m-1, 61.4 mN m-1, and 57.4 mN m-1, respectively, indicating that poly(MCP-R) brushes are relatively more hydrophobic than poly(MPC) brushes. Zeta () potential of poly(MCP-iPr) was -9.5 mV, which was slightly more negative than poly(MPC) (-1.9 mV).
Force curve measurement of polymer brushes in deionized water at 25 C was conducted by using a cantilever having a silica particle (SiP) covered with monolayer of functional groups, such as PrS-SiP, PC-SiP, CPEt-Si, CPiPr-SiP, and CPMOE-SiP as shown in Scheme 3. Figure 3 shows some typical force curves of the polymer brush surface in water when the SiP probe approached the brush surface, contacted the surface, pressed with a load of 2 nN, and then retracted from the surface. The largest adhesion force observed in the detachment process was determined as the adhesion force Fad, as represented in Figure 2(d). In general, the interaction energy can be estimated by the area surrounded by the force curves of the approach and retraction process. However, the adhesive energy of the brushes in this study was not calculated because the actual contact area of the SiP probe on the brush surface was
not measured accurately. Therefore, we discuss the molecular interaction strength of PC and CP, in this study, by simply using the Fad observed in the force curve measurement, assuming that the contact area of the probe on the brush surface was constant in each experiment. As shown in Figure 2(a), (b), and (c), no specific adhesion was observed between the poly(MPC) brush and PrS-SiP or the PC-SiP probe in water, whereas a large adhesion force was detected between the poly(MPC) brush and the CPiPr-SiP probe. A relatively much larger Fad was also observed for the poly(MCP-iPr) brush and PC-SiP, as shown in Figure 2(d). Strong adhesion force by the combination of PC and inverse-PC groups indicated specific attractive molecular interaction between PC and CP in contrast to weak molecular interaction between identical PC and PC. Large adhesion force was also observed in the force curves of poly(MCP-Et), poly(MCP-MOE) brushes using PrS-, PC-, and CPiPr-SiP probes in water, as shown in Figures S5-1 to S5-4.
Figure 3 summarizes the adhesion forces determined by force curve measurement of poly(MPC) and poly(MCP-R) brushes using a chemically functionalized SiP probe. Every measurement was repeated over 100 times to determine the averaged value of adhesion force. The error bar in Figure 3 represents the standard deviation. Relatively larger adhesion forces were observed between the CP polymer and CP probe compared with the combination of PC groups. For example, the Fad between the poly(MPC) brush and PC-SiP was almost zero, whereas the Fad between the poly(MCP-iPr) brush and CPiPr-SiP or PrS-SiP was larger than 27 ~ 30 nN. Interestingly, the poly(MPC) brush showed markedly weak interaction (0.6 nN) with hydrophobic propyl-modified SiP, but had a large adhesive interaction (14 nN) with inverse-PC bearing the isopropyl phosphate group. In addition, PC-modified SiP probe showed low adhesion (0.4 nN) with poly(MPC) brush, but revealed adhesion force as large as 20 nN with poly(MCP-iPr) brush in water, suggesting that some specific attractive molecular interaction, such as dipole-dipole interaction, was produced between PC and CP. The average values of the adhesion force of PC-SiP vs poly(MCP-iPr) brush (30.7 ± 9.2 nN) was larger than that of CPiPr-SiP vs poly(MPC) brush (20.9 ± 6.0 nN), but the statistic
error bar of the adhesion force partially overlapped each other. If we consider these results strictly, it would be appropriate to regard the both adhesion forces were similar strength. However, we supposed that the difference might be produced by surface property of the functionalized SiP. The PC-SiP was merely covered with a very thin monolayer of PC group, as shown in Scheme 3. The surface of PC-SiP is expected to be more hydrophobic than that of poly(MPC) brush, because the surface free energy of the extremely thin film is often affected by the substrate. One possible mechanism is that the large adhesion force between the poly(MCP-iPr) brush with the PC-SiP might be produced not only by the interaction of the CP-dipole and PC-dipole, but also by hydrophobic affinity of CP-iPr. On the other hand, poly(MPC) brush has no interaction with the hydrophobic probe, such as PrSiP, resulting the slightly lower adhesion force of poly(MPC) vs CPiPr-SiP than that of poly(MCP-iPr) vs PC-SiP. Actually, the adhesion force of poly(MPC) brush vs hydrophobic PrSiP was almost zero, while that of poly(MCP-iPr) was 30 nN. This result indicates independency of hydrophobicity on adhesion force of poly(MPC) brush in water. The effect of hydrophobic interaction cannot be ignored for the adhesion force on the poly(MCP-R) brush surface. The adhesion forces of poly(MCP-Et), poly(MCP-MOE), and poly(MCP-iPr) brushes by using a SiP probe with identical CP groups were 15.9 ± 4.4, 23.8 ± 4.9, and 27.0 ± 10.8 nN, respectively. Larger adhesion force was observed on the hydrophobic poly(MCP-iPr) brush compared with the hydrophilic poly(MCP-Et) and poly(MCP-MOE) brushes, which agreed well with the order of hydrophilicity estimated by water contact angle measurement. The hydrophobicity of the isopropyl group in the MCP polymer brush might induce relatively large adhesion interaction in water with hydrophobic PrS-SiP or CPiPr-SiP. In general, adhesive interaction between hydrophilic surfaces is reduced in aqueous media, due to the reduction in the interfacial free energy between the hydration surface and water. Thus, the poly(MCP-Et) and poly(MCP-MOE) brushes might show relatively weaker interaction than the poly(MCP-iPr) brush. On the other hand, the standard deviation obtained by statistical analysis was so large that the error bar partially overlapped each other, indicating that strictly distinguishable differences among the adhesion force of poly(MCP-Et), poly(MCP-iPr), and poly(MCP-MOE) brushes were not observed in this experiment. Therefore, it is still difficult to describe clear correlation of adhesion force with
hydrophobicity derived from alkyl chain of CP groups, although apparent gap in the statistical average value of each adhesion force was found in Figure 3.
4. Conclusions In order to evaluate the interaction between PC and inverse-PC, three types of methacrylate monomers bearing inverse-PC groups, MCP-R (R = Et, iPr, MOE), were synthesized by the reaction of the corresponding alkoxy phospholane and DMAEMA to obtain 34-58% yields. Surface-initiated ATRP of MPC and MCP-R was carried out to give the corresponding polymer brush on silicon wafer and a silica particle, and the grafting polymers were isolated from the silica by acidic hydrolysis. Resulting isolated poly(MCP-R) was water-soluble; however, the surface free energy of the poly(MCP-R) brush was estimated to be 55.8-64.0 mN m1, which was lower than that of poly(MPC) (73.8 mN m1), indicating that the poly(MCP-R) brush surface was relatively hydrophobic compared with the poly(MPC) brush. Force curve measurement of the flat polymer brush surface in water by using a PC- or CP-modified silica particle probe was conducted to show the specific adhesive interaction of poly(MCP-R) and the PC group. For instance, the poly(MPC) brush showed extremely weak interaction with PC-modified silica probe, whereas adhesion force between poly(MPC) brush and the CPiPr-modified silica probe was clearly observed to be 14 nN. The poly(MPC-iPr) brush showed a larger adhesion force (21 nN) with the PC-modified probe. The scientific reason for the specific adhesion between PC and CP is still unclear, but we suppose that dipole-dipole interaction between the PC and CP groups contributed this specific adhesion. A force curve experiment in salt solution is in progress to understand the effect of ionic strength the on molecular interaction of the CP surface.
Supporting Information Supporting Information is available on the web site of Langmuir.
Acknowledgments SEC and Zeta potential measurement were performed under the Cooperative Research Program (No. 20161265, 20171318, 20181285) of the Network Joint Research Center for
Materials and Devices, with prof. Takahara in Kyushu University. This research was also supported by a Grant-in-Aid under the Japan Science Promotion Society (JSPS) KAKENHI Scientific Research C (No. 17K05887) from the Ministry of Education, Culture, Science, Sports and Technology of Japan (MEXT), and by Biomolecules System Research Center (BMSC) program for Strategic Research at Private Universities (Kogakuin University) from MEXT.
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Figure Captions Scheme 1. Synthesis of methacrylate monomers bearing inverse PC, and chemical structure of MCP-R and MPC Scheme 2. Surface-initiated ATRP of MCP-R monomers from flat Si wafer and silica particle (SiP), and isolation of the surface-grafted polymers from SiP by HF treatment. Polymer-grafted SiP was only used for determination of molecular weight of the isolated polymer brush. Scheme 3. Surface modification of silica particle (SiP) by propyltrimethoxysilane (PrS) and 3-aminopropyltrimethoxysilane (APS), and successive Michael addition of MPC and MCP-R (R= Et, iPr, MOE). The resulting PrS-SiP, PC-SiP, and CP(R)-SiP were used for force curve measurement. Figure 1. (A) 1H NMR of MCP-iPr, (B) 31P NMR of MCP-iPr, and (C) 31P NMR of IOP Figure 2. Typical force-distance curves of poly(MPC) brush at approach and retraction of (a) PrS-SiP, (b) PC-SiP, and (c) CPiPr-SiP probes, and (d) poly(MCP-iPr) brush by using PC-SiP probe in water at 25 C. Adhesion force (Fad) was estimated by gap between the bottom of retraction trace and retrace line, as shown in (d). Figure 3. Adhesion force (Fad) of poly(MPC), poly(MCP-iPr), poly(MCP-Et), and poly(MCP-MOE) brushes measured by surface modified SiP probes with PrS, PC, CP-iPr, CP-Et, and CP-MOE in water at 25 C.
Graphical abstract.
Attractive molecular interaction between PC and inverse-PC polymers
Table 1. Surface-initiated ATRP of MPC and MCP-R in TFE at 30 C for 24 h a amount of reagents, mmol run
monomer
BHM-SiP
conc. b
yield c
thickness f
Mn
CuBr
bpy
EB
g
mo L-1
%
theo. d
SEC e
Mw/Mne
nm
1
MPC, 2.55
0.0325
0.0681
0.0060
0
1.0
96
132000
143000
1.08
24.0±0.1
2
MPC, 3.79
0.0381
0.0750
0
0.2085
0.80
100
-
381000
1.34
46.3±0.3
3
MCP-iPr, 3.09
0.0325
0.0681
0.0060
-
1.2
89
148000
209000
2.05
39.9±4.0
4
MCP-iPr, 3.20
0.0310
0.0670
0
0.1521
0.58
100
-
427000
1.73
73.7±0.3
5
MCP-Et, 3.90
0.0272
0.0536
0.0120
-
0.89
-
96800
72600
1.70
-
6
MCP-Et, 3.75
0.0310
0.0670
0
0.1536
0.58
100
-
420000
2.52
35.9±0.2
7
MCP-MOE, 3.90
0.0272
0.0536
0.0133
-
0.89
-
99700
68300
1.46
-
8
MCP-MOE, 4.50
0.0366
0.0776
0
0.2183
0.80
100
-
-
-
11.2±0.4
a Surface initiated ATRP was carried out in 2,2,2-trifluoroethanol (TFE) using CuBr and 2,2'-bipyridyl (bpy) initiated with ethyl 2-bromoisobutyrate (EB) or BHM-immobilized silicon wafer or BHM-immobilized silica particle (SiP) at 30 C for 24 h in an argon atmosphere. BHM-immobilized silicon wafer was not used in run 5 and 7. Mn and Mw represent the number-averaged and weight-averaged molecular weights, respectively. b Concentration of monomer in reaction mixture. c Gravity. d Mn(theo.) = [monomer]/ [EB] × yield/ 100 × [MW of monomer] + [MW of EB]. e Determined by SEC using 0.2 M-NaCl aqueous solution as an eluent and calibration of polyethyleneoxide standards. f The thickness of polymer brush on silicon wafer was determined by spectroscopic ellipsometry.
Table 2. Contact angle and surface free energy of polymer brushes a H2O in air
CH2I2 in air
SV
SVd
SVp
degree
degree
mN m-1
mN m-1
mN m-1
poly(MPC)
3.8
24.8
73.7
30.6
43.2
poly(MCP-Et)
30.3
39.9
64.0
26.0
38.0
poly(MCP-MOE)
35.9
35.9
61.4
28.7
32.7
poly(MCP-iPr)
43.3
44.6
57.4
23.1
34.3
polymer brushes
Contact angles of water and diiodomethane were measured with a 2 μL of droplet under atmosphere. Surface free energy (SV) was estimated by Owens-Wendt equation using a dispersion force parameter (SVd) and polar force parameter (SVp). Parameter value of probe liquid were as follows; SVd(H2O) = 21.8 mN m-1, SVp(H2O) = 51.0 mN m-1, SVd(CH2I2) = 49.5 mN m-1, SVp(H2O) = 1.3 mN m-1.
Cantilever Surface-modified SiP probe Polymer y brushes
SiP Probe vs Polymer Brushes
Ad dh hessio on nF Forrce e (F ( aad)), nN nN
Page 29 of 30
40 30
20.9 nN 14 1 nN 14.1
20 10
0 6 nN 0.6 N
30 7 nN 30.7
23 8 nN 23.8
27.0 nN 15 9 nN 15.9
0 4 nN 0.4 N
0 PrS PC vs vs poly(MPC) poly(MPC)
CP-iPr CP iPr PC PrS CP iPr CP-iPr CP Et CP-Et CP MOE CP-MOE vs vs vs vs vs vs poly(MPC) poly(MCP-iPr) poly(MCP-iPr) poly(MCP-MOE) poly(MCP-iPr) l (MCP iP ) poly(MCP-Et) l (MCP Et)
