Synthesis of Hybrid Silica Nanoparticles Densely Grafted with Thermo

Article pubs.acs.org/Macromolecules

Synthesis of Hybrid Silica Nanoparticles Densely Grafted with Thermo and pH Dual-Responsive Brushes via Surface-Initiated ATRP Lei Wu,†,‡,§ Ulrich Glebe,† and Alexander Böker*,†,∥ †

Fraunhofer Institute for Applied Polymer Research IAP, Geiselbergstr. 69, 14476 Potsdam-Golm, Germany Lehrstuhl für Makromolekulare Materialien und Oberflächen, RWTH Aachen University, Forckenbeckstr. 50, 52056 Aachen, Germany § DWI−Leibniz Institut für Interaktive Materialien e.V., Forckenbeckstr. 50, 52056 Aachen, Germany ∥ Lehrstuhl für Polymermaterialien und Polymertechnologie, Universität Potsdam, 14476 Potsdam-Golm, Germany ‡

S Supporting Information *

ABSTRACT: This work introduces a synthesis of well-defined thermo and pH dual-responsive poly(N-isopropylacrylamide)-b-poly(4-vinylpyridine)grafted silica nanoparticles (SNPs-g-PNIPAM-b-P4VP) via surface-initiated atom transfer radical polymerization (ATRP). ATRP initiators were attached onto the surfaces of silica nanoparticles followed by ATRP of Nisopropylacrylamide (NIPAM). During the surface-initiated ATRP, free sacrificial initiator and halogen exchange were utilized to render the polymerization in a controlled manner. Because of the retention of the polymer chain’s end-group functionality, chain extension with 4-vinylpyridine (4VP) from the obtained PNIPAM-grafted SNPs (SNPs-gPNIPAM) was successfully conducted. Kinetics of the chain extension was studied in detail, showing the livingness of this reinitiation process. Subsequent quaternization of the outer P4VP block with methyl iodide led to the synthesized hybrid nanoparticles being well dispersed in aqueous solution. This well dispersibility affords the possibility to study the pH- and thermoresponsive behavior of the diblock copolymer chains on the nanoparticle surface.

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INTRODUCTION Silica nanoparticles (SNPs) have been extensively studied and applied in various areas such as colloid chemistry, catalysis, nanopatterning, photonics, drug delivery, and biosensing because of their high rigidity and thermal stability as well as physical and chemical resistance.1−10 For the vast majority of applications, SNPs are usually surface modified with organic materials, especially polymers, to form silica polymer core/shell nanohybrids, which are also called polymer-grafted SNPs.11,12 These hybrid nanomaterials combine the advantages of SNPs (rigidity, stability) and organic polymers (flexibility, processability, functionality) to enlarge the potential applications of SNPs.11 Within various methods to synthesize these silica polymer core/shell hybrid nanoparticles, surface-initiated controlled radical polymerization (CRP) is a powerful tool, especially for grafting a densely anchored polymer shell with a high degree of control on the size, structure, and unique uniformity of the polymer chains needed for versatile functionalization and application.13−18 Surface-initiated CRP includes surface-initiated atom transfer radical polymerization (ATRP), surfaceinitiated reversible addition−fragmentation chain transfer (RAFT) polymerization, and surface-initiated nitroxide-mediated polymerization (NMP).12,13 Because of the high compatibility of ATRP process with a wide range of monomers and its simple reaction system, many © XXXX American Chemical Society

polymers have been grown from the surfaces of SNPs not only in organic solvent16,19−21 but also in more environmentally friendly aqueous solution using the surface-initiated ATRP technique.22 Among the grafted polymers, poly(N-isopropylacrylamide) (PNIPAM) and poly(4-vinylpyridine) (P4VP) are the intensively studied and widely applied thermoresponsive and pH-responsive polymers, respectively.12,23,24 PNIPAM has a lower critical solution temperature (LCST) at around 32 °C. Below the LCST, the PNIPAM chains are hydrated (swollen state) by intermolecular hydrogen bonding and reversibly become hydrophobic (shrunken state) above the LCST by intramolecular hydrogen bonding.25−27 P4VP has a pKa value approximately in the range of 4−5.28,29 At pH > pKa, the P4VP chains are hydrophobic and shrink. When the pH decreases below the pKa, the polymer chains turn to hydrophilic and stretch due to the protonation of the 4VP units. Moreover, the pyridine nitrogen atoms on the P4VP chains have a strong ability to coordinate metal ions30−32 and can be quaternized by alkyl halides.33−36 Arising from the grafted PNIPAM or P4VP chains, the fabricated silica nanohybrids exhibit functional properties responsive to external stimulus, which bare SNPs do Received: August 17, 2016 Revised: December 2, 2016

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DOI: 10.1021/acs.macromol.6b01792 Macromolecules XXXX, XXX, XXX−XXX

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Synthesis of BPME-Anchored SNPs. SNPs with an average diameter of 100 nm were synthesized by the Stöber method. Typically, ammonium hydroxide (18.0 mL, 29 wt %) was added to 300 mL of absolute ethanol under vigorous stirring which was continued for 30 min. Subsequently, TEOS (9.0 mL, 0.040 mol) was added in one patch. After 24 h, the nanoparticles were isolated from the opaque white suspension by centrifugation (5 × 8500 rpm) and dried under vacuum. For the functionalization of the SNPs’ surfaces with ATRP initiators, 2.50 g of SNPs was dispersed in 75 mL of toluene with the aid of ultrasonication for 1 h, followed by the addition of 850 μL of BPME (2.59 mmol). The mixture was stirred at room temperature for 2.5 days and subsequently heated at 100 °C for 8 h. The functionalized SNPs were washed with toluene (3 × 100 mL) and ethanol (3 × 100 mL), dried in a vacuum oven, and stored for further use. Surface-Initiated ATRP of NIPAM from BPME-Anchored SNPs. A 25 mL Schlenk tube was charged with 500.0 mg of BPMEfunctionalized SNPs and 10 mL of isopropanol, followed by 10 min of ultrasonication to disperse the nanoparticles. Then, NIPAM (1.80 g, 15.9 mmol), Me6TREN (106 μL, 0.397 mmol), and EBiB (3 μL, 0.02 mmol) were added carefully into the tube. The mixture was degassed by three freeze−pump−thaw cycles, and CuCl (19.8 mg, 0.200 mmol) was added at the frozen state under a strong flow of nitrogen gas. The system was subject to two more freeze−pump−thaw cycles, and the sealed Schlenk tube was put into an oil bath at 33 °C. After 14 h, the polymerization was quenched by exposure to air, and the suspension was diluted with 20 mL of isopropanol. The nanoparticles were centrifuged (3 × 10500 rpm) and dried under vacuum. The free PNIPAM, resulting from the sacrificial initiator EBiB, was collected from the supernatant by precipitation into 10-fold diethyl ether. To determine the molecular weight and molecular weight distribution of the grafted PNIPAM, 100.0 mg of the obtained nanoparticles was dispersed in THF with the aid of ultrasonication for 10 min, to which a drop of hydrofluoric acid (48 wt %) was added. The solution was gently stirred for 24 h in ventilation. Then, NaOH aqueous solution (35 wt %) was introduced to neutralize the excess hydrofluoric acid. The formed salt was removed by dialysis against a MWCO 1000 Da membrane for 1 week. Water was removed under vacuum, and the obtained polymer was subjected to further measurement. Caution: hydrof luoric acid is highly corrosive, and all operations with hydrof luoric acid should be handled with extreme care under proper safety protections in ventilation. Typical Chain Extension from PNIPAM-Grafted SNPs with 4VP. To a 25 mL Schlenk tube with a magnetic stirrer, 525.0 mg of PNIPAM-grafted SNPs (SNPs-g-PNIPAM) and 1.5 mL of isopropanol were introduced, followed by 30 min of ultrasonication to disperse the nanoparticles. Then, CuCl (10.0 mg, 0.100 mmol) was added quickly. The system was degassed by two freeze−pump−thaw cycles, and Me6TREN (107 μL, 0.400 mmol) was added at the frozen state under a strong flow of nitrogen gas. The mixture was subject to another freeze−pump−thaw cycle, and 4VP (2.5 mL, 23 mmol) was transferred into the tube at the frozen state. After two additional freeze−pump−thaw cycles, the sealed Schlenk tube was put into an oil bath at 55 °C. At different time intervals, e.g., 4 and 7 h, the polymerization was quenched by exposure to air. The reaction mixture was diluted with isopropanol and poured into n-hexane to collect the diblock copolymer-grafted SNPs. The synthesized hybrid nanoparticles were washed three times with isopropanol and dried in vacuum oven for 2 days. To determine the molecular weight and molecular weight distribution of the grafted diblock copolymers, 100.0 mg of the synthesized silica nanohybrids was directly added into 10 mL of hydrofluoric acid (48 wt %). The suspension was gently stirred for 2 days. NaOH aqueous solution (35 wt %) was introduced until pH > 9, and the polymers were collected by centrifugation at 35 °C. Caution: hydrof luoric acid is highly corrosive, and all operations with hydrof luoric acid should be handled with extreme care under proper safety protections in ventilation. Quaternization of P4VP Block in SNPs-g-PNIPAM-b-P4VP. SNPsg-PNIPAM-b-P4VP (50.0 mg, the chain extension time is 4 h) were

not have and are potential building blocks for smart nanodevices.12,23,24 However, many studies accomplished so far only addressed one kind of stimulus-responsive polymer.12 To the best of our knowledge, no synthesis of distinct dual stimuli-responsive diblock copolymer, e.g., poly(N-isopropylacrylamide)-b-poly(4vinylpyridine) (PNIPAM-b-P4VP), on SNPs using surfaceinitiated ATRP has been reported yet, and only one work is related to the synthesis of PNIPAM-b-P4VP from planar centimeter-sized surfaces.12,37,38 Herein, we describe a synthesis of well-defined PNIPAM-b-P4VP-grafted SNPs (SNPs-gPNIPAM-b-P4VP) via surface-initiated ATRP. First, PNIPAM was grown on the ATRP initiator-anchored silica nanoparticle surface. Thanks to the livingness of the ATRP process, the end group of the PNIPAM block can be reinitiated to continue the polymerization on the nanoparticle surface with a second monomer, 4VP, which leads to diblock copolymer-grafted SNPs.16,39 Afterward, partial quaternization of the outer P4VP block with methyl iodide not only renders the hybrid nanoparticles well-dispersed in aqueous solution but also maintains the pH-responsiveness of the P4VP block. Hence, the pH and thermo dual-responsiveness of the diblock copolymer chains on the nanoparticle surface can be studied by dynamic light scattering (DLS) together with in situ variabletemperature proton nuclear magnetic resonance (1H NMR) spectroscopy.

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EXPERIMENTAL SECTION

Materials. N-Isopropylacrylamide (NIPAM) and 4-vinylpyridine (4VP) were purchased from Sigma-Aldrich. NIPAM was purified twice by recrystallization from toluene/n-hexane (v/v, 1/1). The inhibitors in 4VP were removed by a basic aluminum oxide column and vacuum distillation twice. Allyl alcohol (>99%), 2-bromoisobutyryl bromide (BiBB, 98%), triethylamine (TEA, >99%), hydrochloric acid (37 wt %), hydrofluoric acid (48 wt %), Karstedt’s catalyst (platinum(0)−1,3divinyl-1,1,3,3-tetramethyldisiloxane complex solution in xylene, Pt ∼ 2%), trimethoxysilane (95%), formic acid (98%), formaldehyde (37 wt % in H2O), tris(2-aminoethyl)amine (TREN, 96%), tetraethoxysilane (TEOS, 99%), ammonium hydroxide (29 wt %), ethyl 2-bromoisobutyrate (EBiB, >99%), methyl iodide (>99%), NaHCO3 (>98%), MgSO4 (>98%), NaOH (>98%), CuCl (>98%), sinapinic acid (>98%, SA), and trifluoroacetic acid (99%) were received from Sigma-Aldrich and used without further purification. ATRP ligand tris(2-(dimethylamino)ethyl)amine (Me6TREN) was synthesized according to a literature procedure.40,41 All solvents used were purchased from VWR with high purity (>99%). Millipore pure water with an electrical resistance of 18.2 MΩ·cm was used. Synthesis. Synthesis of ATRP Initiator-Functionalized Silane, [3(2-Bromo-2-methyl)propionyloxylpropyl]trimethoxysilane (BPME). BPME was synthesized by a two-step process.42,43 In a 250 mL round-bottom flask, allyl alcohol (6.8 mL, 100 mmol), TEA (15.5 mL, 110 mmol), and 100 mL of DCM were mixed and cooled in an ice bath. Then, BiBB (18.5 mL, 149 mmol) was slowly added within 15 min. The mixture was stirred at 0 °C for 1 h and gradually warmed up to room temperature. After 1 day, the resultant suspension was washed with 1 M hydrochloric acid (2 × 200 mL), saturated NaHCO3 aqueous solution (2 × 200 mL), and Millipore water (2 × 200 mL). The organic phase was isolated and dried over MgSO4. After filtration and removal of solvent under reduced pressure, a light yellow liquid (19.6 g, 95 mmol, 95% yield) was obtained. The yellowish liquid from the first step was dissolved in 200 mL of toluene. Trimethoxysilane (25.5 mL, 200 mmol) and Karstedt’s catalyst (550 μL) were sequentially added within 30 min. Then, the mixture was heated to 100 °C under a nitrogen atmosphere. After 12 h, the solution was cooled to room temperature. After removal of volatile compounds under vacuum, a light yellow oil (28.6 g, 85.5 mmol, 90% yield) was obtained. B


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Macromolecules dispersed in 35 mL of DMF in a 50 mL flask under gentle ultrasonication. The mixture was then stirred for 15 min, and methyl iodide (100 μL, 1.61 mmol, 35-fold to the number of 4VP units) was added slowly with great care. The flask was tightly sealed and gently heated at 40 °C overnight. The nanoparticles were isolated by centrifugation (2 × 13000 rpm) for 1 h. Caution: methyl iodide is volatile and toxic and should be handled with great care in ventilation. It is important that the f lask is tightly sealed. Characterization and Measurements. Transmission electron microscopy (TEM) measurements were performed on a ZEISSLIBRA120 PLUS electron microscope operated at 180 kV. Sample preparation was performed by dipping a drop of the solution on a carbon-coated copper grid and subsequently placing the copper grid onto a piece of filter paper to remove excess solvent. Fourier transform infrared (FT-IR) spectroscopy was recorded on a Nexus 470 (Thermo Nicolet) (spectral resolution 8 cm−1) with samples pressed in KBr pellets. The peak fitting was conducted in Origin 8 software. Polymer number-average molecular weight (Mn,GPC), weight-average molecular weight (Mw,GPC), and molecular weight distribution (also polydispersity index, PDI) were determined by gel permeation chromatography (GPC) using DMF with LiBr (1 mg/mL) as eluent, a flow rate of 1.0 mL/min, a high pressure liquid chromatography pump (Bischoff HPLC Compact pump), and a RI detector (Jasco RI-2031 plus). Calibration was achieved using poly(methyl methacrylate) (PMMA) standards. Results were evaluated using WinGPC Unity software. Matrix-assisted laser desorption/ionization time-of-flight (MALDIToF) mass spectra were collected on a 1 kHz 337 nm laser Bruker ultrafleXtreme spectrometer with pulsed ion extraction. 50.0 μL of polymer solution (10.0 mg/mL in methanol), 50.0 μL of the matrix SA solution (20.0 mg/mL in methanol), and 30.0 μL of trifluoroacetic acid were mixed together. 0.5 μL of the mixture was dipped on a ground steel target and dried at room temperature. The MALDI-ToF measurement was externally calibrated using a series of standard polystyrene (PSt). Positive ion reflective mode was used. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker DPX-400 spectrometer at 400 MHz. Deuterated chloroform (CDCl3, 99.8%) or deuterated water (D2O, 99.8%) was used as solvent. The signal of nondeuterated solvent was used as internal standard. Variable-temperature 1H NMR spectra were recorded in D2O on a temperature-controlled Bruker 600 MHz spectrometer. Dynamic light scattering (DLS) was conducted on a Malvern Zetasizer Nano ZS (Malvern Instruments GmbH) in a glass cuvette, using a He−Ne (633 nm) laser and a thermoelectric temperature controller. X-ray photoelectron spectroscopy (XPS) was performed on an Ultra AxisTM spectrometer from Kratos Analytical (Manchester, UK). The samples on the substrate were excited with an Al Kα irradiation with energy of 1486.6 eV and power of 150 W. All spectra were adjusted relative to the aliphatic/aromatic carbon (C−C, C−H, or C−CC) of C 1s photoline at 285.0 eV. Thermogravimetric analyses (TGA) were conducted on the thermo-micro-weight TG-209C from Netzsch. All samples were measured in a temperature range of 25−700 °C under a nitrogen atmosphere with a heating rate of 20 °C/min.

Scheme 1. General Synthetic Procedure To Fabricate SNPsg-PNIPAM-b-P4VP: (a) Immobilization of ATRP Initiator, BPME, onto SNPs; (b) Synthesis of SNPs-g-PNIPAM via Surface-Initiated ATRP; and (c) Synthesis of SNPs-gPNIPAM-b-P4VP by Chain Extension from SNPs-gPNIPAM with 4VP

ATRP initiator-functionalized silane, BPME, in a two-step process (Scheme S1 and Figure S1 in Supporting Information). Then, SNPs were surface modified with BPME to introduce ATRP initiator moieties onto the nanoparticle surface (Scheme 1a). The obtained BPME-functionalized SNPs distributed uniformly on TEM grids with an average diameter of 100 nm without aggregation or impurities (Figure 1a,b). No visible morphology change appeared compared with bare SNPs. However, the size of the nanoparticles from DLS measurement showed an increase from 123.5 to 126.3 nm through the surface functionalization (Figure S2), demonstrating that a monolayer of BPME is anchored on the nanoparticle surface. Further evidence from FT-IR and XPS confirms the success of the surface functionalization. In FT-IR spectra, besides the three absorption bands at 1100 cm−1 (Si−O stretching), 953 cm−1 (Si−OH bending), and 800 cm−1 (Si−O−Si bending) from bare SNPs (Figure 2a), the characteristic absorption of the carbonyl groups from the attached initiators on the nanoparticle surface was observed at 1720 cm−1 (Figure 2b).44 Meanwhile, as shown in XPS spectra (Figure S3a), the characteristic binding energy peaks of Si 2p, Si 2s, and O 1s, which arise from bare SNPs, were discerned at 103.5, 154.0, and 532.8 eV, respectively.45 The C 1s peak at 285.6 eV is ascribed to the incomplete hydrolysis and condensation of the ethoxysilyl residues from TEOS during the SNPs preparation.46 After BPME attachment, the characteristic peak of Br 3d at 68.0 eV appeared (Figure S3a).45,47 Moreover, the grafting density of the attached BPME on the nanoparticle surface can be quantitatively determined by TGA analysis (Figure S4 and eq S1). The weight retention of bare SNPs at 700 °C is 94.83% (Figure S4), which originates from the continued condensation and associated water loss.48 With this mass retention as the reference, the grafting density of BPME was calculated as 3.09 initiators/nm2.

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RESULTS AND DISCUSSION Synthesis of BPME-Functionalized SNPs. Scheme 1 shows the general synthetic routine to fabricate diblock copolymer-grafted SNPs, SNPs-g-PNIPAM-b-P4VP. To obtain BPME-anchored SNPs, bare SNPs with an average diameter of 100 nm were first synthesized by the Stöber method. Because of the hydroxyl groups on the nanoparticle surface, various functional ATRP initiators can be conveniently anchored onto SNPs for surface-initiated ATRP. For example, Patten and coworkers prepared several kinds of ATRP initiator-functionalized silanes.42 The functional silane contains a methoxy, ethoxy, or chloro moiety at one end for direct attachment onto the nanoparticle surface via condensation of the O−Si−O groups.42 The alkyl halide group is introduced on the other side for ATRP initiation.42 Inspired by this work, we synthesized an C


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solvent, catalyst complex, and free sacrificial initiator, respectively (Scheme 1b). The addition of sacrificial initiator quickly accumulates an appropriate amount of CuII via the termination of polymer radicals at early stage of the polymerization. The CuII species ensures a fast deactivation process and suppresses further termination reactions from radical coupling, thus controlling the polymerization by the persistent radical effect.12,42 Also, free polymers produced by the sacrificial free initiator form an entangled network structure which prevents nanoparticle diffusion, diminishes interparticle coupling, and avoids gelation or aggregation.12,42 Meanwhile, the addition of adequate CuCl-based catalyst was used for halogen exchange to produce large quantities of the less active chloride-terminated dormant species, which lowers the propagation rate.50 The lower propagation rate results in a high initiation efficiency and keeps the polymerization more controlled.50−52 After 14 h of polymerization, the reaction system was exposed to air and the nanoparticles were isolated by centrifugation. The morphology of SNPs-g-PNIPAM was characterized by TEM. From Figures 1c and 1d, it is obvious that a polymer shell, with an average thickness of 7.3 nm, appears surrounding each nanoparticle. Meanwhile, the hybrid nanoparticles were dispersed in isopropanol for DLS measurements. The DLS analysis of SNPs-g-PNIPAM revealed a narrow size distribution (Figure S2) with an average diameter of 163.5 nm, about 40 nm larger than that of the BPME-anchored SNPs. Besides isopropanol, the SNPs-g-PNIPAM can be dispersed very well in DMF, ethanol, water, and THF because of the unique solubility of the PNIPAM shell. FT-IR and XPS characterizations further verify the grafted PNIPAM chains. As shown in Figure 2c, the amide I band (1649 cm−1, CO stretching) and amide II band (1550 cm−1, N−H stretching) appear.53 The asymmetric deformation vibration of two methyl groups on the isopropyl moiety appears at 1469 and 1459 cm−1.53 In the meantime, the two bands at 1390 and 1369 cm−1 with nearly equal intensity are assigned to the symmetric deformation of the two methyl groups.53 In XPS analysis, a new peak appeared which can be attributed to the binding energy of N 1s from the grafted PNIPAM chains (Figure S3a).45 Meanwhile, because the thick PNIPAM layer on the nanoparticle surface shields the silica cores beyond the depth of XPS probing, the two binding energy peaks of Si 2s and 2p almost disappeared (Figure S3a). GPC and MALDI-ToF mass spectrometry are powerful tools to analyze the molecular weights, molecular weight distributions, and chain structures of the grafted and free PNIPAM. The grafted PNIPAM was cleaved from the nanoparticle surface by etching the silica core with hydrofluoric acid. The GPC curve of the grafted PNIPAM is almost identical with that of the free PNIPAM (Figure 3a). Using a linear PMMA standard, the Mn,GPC and PDI of the grafted PNIPAM were determined as 25 500 g/mol and 1.25, respectively, very close to those of the free PNIPAM, 25 900 g/mol and 1.28. Thus, the free polymer synthesized from the sacrificial initiator not only favors the surface-initiated polymerization in a controlled way but also can be used to monitor the grafted polymer. From MALDI-ToF mass spectrum, the absolute molecular weight (Mn,MALDI) and PDI of the free PNIPAM were calculated as 8293 Da and 1.20, respectively (Figure 3b). Mn,MALDI differs from Mn,GPC, and the deviation has a ratio of Mn,MALDI/Mn,GPC ≈ 1/3. A similar phenomenon was observed by Schilli.54 This pronounced difference arises from the GPC

Figure 1. TEM images of (a, b) BPME-functionalized SNPs, (c, d) SNPs-g-PNIPAM, and (e, f) SNPs-g-PNIPAM-b-P4VP. The chain extension time from SNPs-g-PNIPAM with 4VP is 10 h. All scale bars represent 250 nm.

Figure 2. FT-IR spectra of (a) bare SNPs, (b) BMPE-anchored SNPs, (c) SNPs-g-PNIPAM, and (d) SNPs-g-PNIPAM-b-P4VP. The chain extension time from SNPs-g-PNIPAM with 4VP is 10 h.

Synthesis of SNPs-g-PNIPAM via Surface-Initiated ATRP. SNPs-g-PNIPAM has been successfully fabricated via surface-initiated ATRP, in either aqueous or organic media.23,49 Herein, the surface-initiated ATRP of NIPAM was carried out at 33 °C using isopropanol, CuCl/Me6TREN, and EBiB as D


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polymer chain cannot fly and be distinguished from the noise.55 This results in an underestimate of the molecular weight of polymers using MALDI-ToF mass spectrometry. However, due to the reliable and absolute information obtained, we still chose MALDI-ToF mass measurement rather than GPC to characterize the synthesized polymer especially for molecular weight, chain structure, and end-group functionality. Moreover, the end-group functionality of the free PNIPAM chains was analyzed from the MALDI-ToF mass spectrum (Figure 3c). The expanded mass spectrum in the range of 7500−7900 Da demonstrates a repeating set of two peaks separated from corresponding neighboring peaks by about 113 Da, matching well with the mass of the repeating unit NIPAM (Figure 3c). The two peaks in one set can be ascribed to two chains having different alkyl halide end groups. For example, the peaks at 7844.1 and 7884.9 Da were determined as (NIPAM)68−Cl/H+ (7840.8 calcd) and (NIPAM)68−Br/H+ (7884.7 calcd). The large portion of polymer chains with terminal chloride originates from the halogen exchange during polymerization. Because of the simultaneous initiation and propagation process on the nanoparticle surface and in the solution dominated via the ATRP mechanism, it is widely accepted that the grafted and free PNIPAM are consistent with each other in molecular weight, polymer chain structure, and endgroup functionality.12 Therefore, it is feasible to analyze the grafted PNIPAM based on the free PNIPAM characterization. Hence, the molecular weight of the grafted PNIPAM can be assumed as 8293 g/mol and the polymer grafting density was calculated as 0.81 chains/nm2 using the weight retention of BPME-anchored SNPs as the reference (Figure S4 and eq S1). The grafting density is higher than most other results previously reported.56 This is probably due to the underestimate of the polymer molecular weight using MALDI-ToF mass spectrometry discussed above. Furthermore, from the MALDI-ToF mass analysis of the free PNIPAM, the grafted PNIPAM chains have either a bromide- or chloride-terminal group which can be reinitiated for chain extension with a second monomer, e.g., 4VP, to form diblock copolymers. Chain Extension from SNPs-g-PNIPAM with 4VP. Compared to BPME-anchored SNPs, SNPs-g-PNIPAM has less number of initiating sites in the same mass amount due to its higher molar mass. Thus, a high solid content of the nanohybrids during the chain extension is needed to afford enough initiators.16 However, the high solid content together with the free polymers propagated from sacrificial initiators makes the reaction solution too viscous and prevents the diffusion of monomers, catalysts, and nanoparticles. Indeed, we observed that gelation happened quickly within 1 h when free sacrificial initiators were introduced. To avoid this, free sacrificial initiators were not used during the chain extension from SNPs-g-PNIPAM (Scheme 1c). Meanwhile, halogen exchange was employed by introducing adequate CuCl catalyst. The halogen exchange produces large quantities of less active chloride-terminated rather than bromide-terminated dormant species, which lowers the propagation rate.50 This lower propagation rate ensures high reinitiation efficiency and a better controlled polymerization.50−52 Figures 1e,f show typical TEM images of the synthesized SNPs-g-PNIPAM-b-P4VP, in which the chain extension with 4VP was performed for 10 h. Compared with SNPs-gPNIPAM, the polymer shell became thicker, growing from 7.3 to 13.4 nm. Correspondingly, the size of the nanohybrids

Figure 3. (a) GPC curves of the free PNIPAM (solid line) from the sacrificial initiator EBiB and the grafted PNIPAM (dashed line) cleaved from SNPs-g-PNIPAM. (b) MALDI-ToF mass spectrum of the free PNIPAM. (c) Detailed analysis of the MALDI-ToF mass spectrum from (b) in the range of 7500−7900 Da together with the corresponding structures.

calibration using PMMA. In the same elution volume, PMMA and PNIPAM show different molecular weights because of their different hydrodynamic sizes in the same eluent.54 This means that the Mn,GPC of PNIPAM calculated using PMMA standard calibration is less accurate compared to the Mn obtained from MALDI-ToF measurement.54 Meanwhile, the detection limit of the MALDI-ToF mass measurement should be noticed and discussed. The charge distribution of polymers with high PDI (>1.2) usually leads to an upper limit above which the larger E


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chain extension process (Figure S5). Furthermore, at different polymerization times, the compositions of 4VP and NIPAM units in the polymer shell were compared using the FT-IR bands at 1620−1580 cm−1 (vibration of pyridine ring in 4VP) and 1690−1580 cm−1 (stretching vibration of carbonyl group in NIPAM), as shown in Figure S6.59 Through simulation of multipeak fitting, the integrated intensity ratio of the two characteristic bands was calculated which is proportional to the content ratio of 4VP to NIPAM unit in the polymer shell (Figure S6b−e). The content ratio of the two monomers in the polymer gradually increased with the chain extension time (Figure S6f), matching well with the consumption of 4VP monomer. In addition, the grafted diblock copolymers were detached from the nanoparticle surface by etching with hydrofluoric acid, and the molecular evolution was monitored by GPC. Figure 5a shows GPC elution curves of the grafted PNIPAM-b-P4VP diblock copolymers at different chain extension times in comparison with that of the grafted PNIPAM at the chain extension onset. The GPC curve of the diblock copolymer shifts to a lower elution time (higher molecular weight) with polymerization proceeding. This indicates that the alkyl halogen groups on the grafted PNIPAM chain ends were reinitiated for the chain extension with 4VP. Using a PMMA calibration, Mn,GPC and PDI values of the grafted PNIPAM-bP4VP diblock copolymers were calculated, as summarized in Table 1. Mn,GPC of the PNIPAM-b-P4VP increases with the monomer conversion. The slightly higher PDI is probably caused by chain terminations through radical coupling during polymerization because of the high local concentration of propagating free radicals on the nanoparticle surface. MALDI-ToF mass spectra of the grafted diblock copolymers were not achieved probably owing to the relatively high PDI and molecular weight as well as the difficulty to choose a proper matrix/ionizing agent system compatible for both polymer blocks. However, using Mn,MALDI of the grafted PNIPAM, in combination with NMR analysis (Figure 5b), we can speculatively derive Mn,MALDI of the grafted PNIPAM-b-P4VP using eq 1. M(NIPAM) and M(4VP) represent the molecular weights of the monomers. R denotes the molar ratio of 4VP to NIPAM in the diblock polymer. In 1H NMR spectra, the signal at 3.98 ppm associates with the methyne proton on the isopropyl group of NIPAM and the signal centered at 8.3 ppm

from DLS measurements increased from 163.5 nm for SNPs-gPNIPAM to 273.6 nm for SNPs-g-PNIPAM-b-P4VP (Figure S2). In the FT-IR spectrum of the diblock copolymer-grafted SNPs, the appearance of three strong absorption bands at 1599, 1416, and 820 cm−1 is associated with the characteristic vibrations of pyridine rings (Figure 2d).57,58 The characterization with TEM, DLS, and FT-IR confirms that a P4VP block has been grown by chain extension from the grafted PNIPAM. However, XPS was unable to detect a difference before and after the chain extension since the binding energy of N 1s from the NIPAM and 4VP units is identical (Figure S3b). The chain extension kinetics was investigated to illustrate that the chain extension with 4VP was conducted in a controlled living manner. The linear increase of ln([M]0/ [M]) versus chain extension time demonstrates a pseudo-firstorder kinetic behavior (dashed line in Figure 4). This indicates

Figure 4. Pseudo-first-order kinetic behavior of the chain extension with 4VP from SNPs-g-PNIPAM: plots of 4VP conversion and ln([M]0/[M]t) versus chain extension time.

an almost constant number of propagating radical species throughout the polymerization course, which results in the consumption of monomer at a linear rate (solid line in Figure 4). It is assumed that the linear consumption of monomer leads to a gradual increase in the polymer content. Accordingly, TEM statistics illustrates that the thickness of the polymer shell surrounding the silica core increased continuously during the

Figure 5. (a) GPC curves of the grafted PNIPAM-b-P4VP diblock copolymers at different P4VP chain extension times. Eluent phase is DMF, and chain extension time at 0 h refers to GPC of the grafted PNIPAM. (b) 1H NMR spectra of the grafted diblock copolymers in CDCl3. All block copolymers were cleaved from the silica polymer hybrid nanoparticles by hydrofluoric acid. F


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Macromolecules Table 1. Overview of SNPs-g-PNIPAM and SNPs-gPNIPAM-b-P4VP entry

sample

1 2

SNPs-g-PNIPAM SNPs-g-PNIPAM-bP4VP (4 h)a SNPs-g-PNIPAM-bP4VP (7 h)a SNPs-g-PNIPAM-bP4VP (10 h)a SNPs-g-PNIPAM-bP4VP (13 h)a

3 4 5

Scheme 2. Fabrication of SNPs-g-PNIPAM-b-QP4VP by Quaternizing the 4VP Repeating Units

shell thicknessb (nm)

Mn,GPCc (g mol−1)

PDIc

Mn,MALDId (g mol−1)

7.3 10.3

25500 53500

1.25 1.43

8293 12839

11.1

59300

1.48

15921

13.4

64300

1.45

20000

15.9

69500

1.46

23318

a Chain extension time with 4VP from SNPs-g-PNIPAM. bThe thickness of the polymer shell in dry state was obtained from TEM statistics as shown in Figure S5. cMn,GPC and PDI of the grafted PNIPAM and PNIPAM-b-P4VP were determined by GPC using PMMA as calibration. dMn,MALDI of the grafted PNIPAM was obtained from MALDI-ToF mass spectrometry of free PNIPAM and Mn,MALDI of the grafted PNIPAM-b-P4VP at different chain extension times was calculated using eq 1.

further confirms the highly positive charges of the QP4VP block. Meanwhile, the 1H NMR spectrum of the synthesized SNPsg-PNIPAM-b-QP4VP was collected to estimate the quaternization degree of the 4VP units on the polymer chains (Figure S8). Calculated from the spectrum, approximately 66% of 4VP units in the polymer chain were quaternized, probably due to the steric hindrance and crowded environment in the polymer shell. Similar partial quaternization of vinylpyridine groups on polymer chains was observed by Schmalz and co-workers.60 The partial quaternization of the outer P4VP block with methyl iodide not only renders the hybrid nanoparticles well-dispersed in aqueous solution but also maintains the pH-responsiveness of the P4VP block. This allows us to study the pH and thermo dual-responsiveness of the diblock copolymer chains on the nanoparticle surface. Dual Stimuli-Responsiveness of Diblock Copolymer Chains on the Nanoparticle Surface. DLS and corresponding zeta-potential measurements of SNPs-g-PNIPAM-b-QP4VP in aqueous dispersion as a function of pH show the pHresponsive behavior of the diblock copolymer chains on the nanoparticle surface. From Figure 6, the nanohybrids remain highly dispersible and positively charged over a wide range of pH from 3.3 to 12.0. The positive charge arises from the cationic quaternary ammonium groups on the 4VP aromatic rings while the high dispersibility originates from the watersoluble PNIPAM and QP4VP blocks. A significant swelling occurs at pH < 8.5 and is very sensitive to pH variation (Figure 6a). This swelling can be ascribed to the protonation of unquaternized 4VP repeating units.61 Correspondingly, the zeta-potential shows an increase during the chain swelling (Figure 6b). The phase transition has a pronounced shift from pH 4.0 for pure P4VP to pH 8.5 for QP4VP.28,29,36 This is probably due to the interactions of the quaternized pyridine groups.36 Moreover, the swelling is slightly decreased for pH < 4 because the inorganic ions introduced during adjusting the pH of the solution screen the positively charged quaternized or protonated pyridine groups and reduce the charge repulsion.62 To investigate the thermoresponsive behavior of the diblock copolymer chains on the nanoparticle surface, the hydrodynamic diameter of the nanohybrids was studied in a temperature range from 23 to 57 °C. PNIPAM is a thermoresponsive polymer with a LCST at around 32 °C. The conformation of PNIPAM chains reversibly changes from

is assigned to the aromatic protons of 4VP (Figure 5b). Based on the ratio of the integrated intensities of the two signals at different chain extension times, R can be estimated as 0.59 for 4 h, 0.99 for 7 h, 1.52 for 10 h, and 1.95 for 13 h. Thus, Mn,MALDI of the grafted PNIPAM-b-P4VP at different chain extension times was calculated and summarized in Table 1. Consequently, the polymer grafting density was determined by eq S1 together with Figure S4. The graft density is approximately equal to 0.79 chain/nm2 during the chain extension. M(PNIPAM‐b‐P4VP) = M n,MALDI(PNIPAM) ⎞ ⎛ M n,MALDI(PNIPAM) +⎜ M(4VP)R ⎟ M(NIPAM) ⎠ ⎝

(1)

Quaternization of P4VP Block with Methyl Iodide. Owing to the excellent solubility of PNIPAM and P4VP in common organic solvents, the synthesized SNPs-g-PNIPAM-bP4VP disperses well in isopropanol, ethanol, methanol, THF, DMF, chloroform, etc. However, the prepared SNPs-gPNIPAM-b-P4VP cannot disperse in water because of the shielding effect of the outer hydrophobic P4VP block. This disadvantage prevents us from studying the dual stimuliresponsive behavior of PNIPAM-b-P4VP chains on the nanoparticle surface and further restricts the potential application of these nanoparticles. It is well-known that methyl iodide reacts with and quaternizes the pyridine units in P4VP chains, forming quaternized P4VP (QP4VP). QP4VP is a polyelectrolyte and soluble in water in a broad pH range.33−36 The quaternization of P4VP with methyl iodide was easily achieved in DMF at mild conditions (Scheme 2). The synthesized SNPs-g-PNIPAM-bQP4VP dispersed easily in water by gentle shaking without any further aid. The size of the monodispersed nanohybrids remained constant for a long time demonstrating their extreme chemical and dispersion stability in aqueous solution. The average diameter is significantly larger than that before the P4VP quaternization (Figures S2 and S7a). The increase in size indicates that the QP4VP chains are more stretched, which stems from the electrostatic repulsion between nearby cationic quaternary ammonium groups on the aromatic rings (Scheme 2). The zeta-potential measurement, as shown in Figure S7b, G


Article

Macromolecules

Figure 6. (a) Hydrodynamic diameter and (b) zeta-potential of SNPs-g-PNIPAM-b-QP4VP as a function of pH in aqueous solution. The inset scheme shows the pH-responsiveness of the block copolymer chains on the nanoparticle surface.

Figure 7. (a) Hydrodynamic diameter of SNPs-g-PNIPAM-b-QP4VP as a function of temperature. The inset scheme demonstrates the swollen and shrunken state of the inner PNIPAM block below and above the LCST. (b) In situ variable-temperature 1H NMR spectra of the nanohybrids at 25 and 55 °C in D2O.

pulls the QP4VP block closer to the nanoparticle and thereby also closer to the neighboring QP4VP chains, leading to more steric constraint and strengthened electrostatic repulsion between the charges on the QP4VP block. To counter this, the flexible QP4VP chains stretch to release the constraint and keep the distance between the charges on the polymer chain, which compensates for the shrinking of the inner PNIPAM chains (scheme in Figure 7a). This compensation maintains the hydrodynamic diameter of the nanohybrids nearly constant (Figure 7a). Furthermore, heating−cooling cycles can be repeated several times and the nanohybrid’s size remains constant with minor changes.

hydrated swollen state below the LCST (intermolecular hydrogen bonding) to hydrophobic shrunken state above the LCST (intramolecular hydrogen bonding).25−27 However, the expected size variation as a function of temperature was not observed for SNPs-g-PNIPAM-b-QP4VP. Instead, the size of the nanohybrids kept constant around 480 nm with little fluctuations (Figure 7a). The nanohybrids were dispersed in D2O, and variabletemperature 1H NMR spectra were collected using HDO as reference (Figure 7b).63 The measurement affords direct information on the conformational change of the PNIPAM chains with temperature in situ on the nanoparticle surface. In Figure 7b, at 25 °C, the signals at approximately 1.2 and 3.9 ppm represent the methyl and methyne protons on the isopropyl groups of NIPAM units. The two signals centered around 8 ppm belong to 4VP aromatic ring protons, and the signal at 4.2 ppm is associated with the protons of the methyl groups of quaternary pyridine moieties.64 In contrast, when temperature is higher than the LCST of PNIPAM, e.g., 55 °C, the signals assigned to NIPAM repeating units disappear. This demonstrates that the hydrophilic−hydrophobic transition of PNIPAM occurs and the polymer chains collapse. Meanwhile, the signal intensities from pyridine rings remain consistent. Thus, we propose a new explanation for the nanohybrid’s size dependence on temperature. When the solution temperature is higher than the LCST, the inner PNIPAM block transforms from swollen to shrunken state and collapses. This

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CONCLUSIONS In summary, we reported a facile synthesis of well-defined dual stimuli-responsive diblock copolymer-grafted silica nanoparticles, i.e., SNPs-g-PNIPAM-b-P4VP, via surface-initiated ATRP. BPME, an ATRP initiator-functionalized silane, was first attached onto the surfaces of SNPs to initiate ATRP of NIPAM from the nanoparticle surface. Halogen exchange and sacrificial initiator were introduced to render the polymerization in a controlled manner. Characterization with TEM, DLS, FT-IR, XPS, and TGA verified the success of the surfaceinitiated ATRP of NIPAM. Furthermore, the grafted PNIPAM was cleaved from the nanoparticle surface to study its molecular weight and molecular weight distribution in comparison with those of free PNIPAM from the added sacrificial initiator. The H


Macromolecules nearly identical overlap of the GPC curves shows the simultaneous polymerization process, which allows to estimate chain structure, especially end-group functionality, of the grafted PNIPAM from the MALDI-ToF mass spectrum of free PNIPAM. Because of the retention of the end-group functionality, chain extension from the obtained SNPs-gPNIPAM with 4VP was successfully conducted. The kinetics was analyzed by calculating ln([M]0/[M]t) and monomer conversion versus polymerization time. The pseudo-first-order kinetic behavior shows the livingness of the chain extension process. After quaternization of the P4VP block with methyl iodide, the hybrid nanoparticles dispersed well in aqueous solution which allowed us to study the pH- and thermoresponsiveness of the diblock copolymer chains on the nanoparticle surface. DLS measurement detected the swelling of the diblock copolymer chains at pH < 8.5, which arises from the protonation of pyridine groups in the pH-responsive P4VP chains. The phase transition showed a noticeable shift compared to pH 4.0 for pure P4VP due to the interactions of the quaternized 4VP groups. Interestingly, the expected size decrease of the nanohybrids did not occur when temperature increased from below to above the LCST of PNIPAM. Instead, the size remained constant with minor variations. However, in situ variable-temperature 1H NMR spectroscopy proved the hydrophilic−hydrophobic phase transition of the inner PNIPAM block. Accordingly, a new explanation was proposed that the shrink of the inner thermoresponsive PNIPAM chains was compensated by the stretch of the flexible outer QP4VP chains, thus leading to the constant size of the nanohybrids observed during DLS analysis. Moreover, heating and cooling cycles were repeated, and the size kept constant. The surfaceinitiated ATRP technique provides a new tool to synthesize well-defined diblock copolymer chains on the nanoparticle surface, which supplies a platform to study the polymer chain behavior on a nanoscale surface. In perspective, the fabricated dual stimuli-responsive diblock copolymer-grafted SNPs are potential building blocks for smart nanodevices which is promising in nanoelectronics.

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ACKNOWLEDGMENTS

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REFERENCES

L.W. thanks CSC (China Scholarship Council) for a PhD scholarship. The authors thank Joachim Roes for XPS measurements.

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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.macromol.6b01792. Synthetic scheme and 1NMR spectrum of BPME, particle size distributions, XPS spectra, TGA data, equation for calculating the grafting density of anchored BPME or polymer, TEM images with statistics of the polymer shell thickness, and FT-IR spectra with peak fitting and splitting (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.B.). ORCID

Alexander Böker: 0000-0002-5760-6631 Notes

The authors declare no competing financial interest. I


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Synthesis of Hybrid Silica Nanoparticles Densely Grafted with Thermo

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