Synthesis and Multi-Stimuli-Responsive Behavior of Poly (N, N

Article pubs.acs.org/Langmuir

Synthesis and Multi-Stimuli-Responsive Behavior of Poly(N,N‑dimethylaminoethyl methacrylate) Spherical Brushes under Different Modes of Confinement in Solution Zhixin Dong, Jun Mao, Dapeng Wang, Muquan Yang, and Xiangling Ji* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China S Supporting Information *

ABSTRACT: We report the synthesis and solution behavior of photo-, temperature-, pH-, and ion-responsive weak polyelectrolyte spherical brushes under different modes of confinement. The spherical brushes were prepared by copolymerization of N,N-dimethylaminoethyl methacrylate (DMAEMA) and 7-(2-methacryloyloxyethoxy)-4-methylcoumarin anchored to silica nanoparticles via surface-initiated atom transfer radical polymerization. The photo-cross-linking and reversibility of the nanoparticle-attached coumarin entities are detected by UV−visible spectroscopy and dynamic light scattering (DLS). The cross-linking density of poly(DMAEMA) (i.e., PDMAEMA) brushes could be easily controlled by alternating irradiation at wavelengths of 365 and 254 nm. Moreover, solution behavior under different pH levels and ionic strengths is systematically investigated in the PDMAEMA brush−polyelectrolyte chains confined only by a hard core, the cross-linked PDMAEMA brush−polyelectrolyte chains confined by a hard core and cross-linking points, and the corresponding hollow nanocapsules after removal of silica by etchingpolyelectrolyte chains confined only by cross-linking points. These three models represent the different modes of confinement. DLS results indicate that the volume phase transition temperatures of the three models shift to lower temperatures with the increase in pH. The highest temperature is afforded to phase transition for hollow nanocapsules in solution, followed by the cross-linked PDMAEMA brushes. The hydrodynamic radius of the polyelectrolyte brush systems obviously decreases with the increase in ionic strength of the solution when adjusted by NaCl.

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INTRODUCTION When linear polyelectrolytes are densely grafted to a substrate and the grafting density (σ) is high enough, a polyelectrolyte brush system is formed.1−4 The system can be classified as a strong or a weak brush depending on the nature of the grafted linear polyelectrolyte chains, and the conformation responds to pH and/or ionic strengths. In a strong polyelectrolyte brush system, there is no variation of the conformation to pH but ionic strengths, which is attributed to the complete dissociation of ionic groups at all pH values. By contrast, weak polyelectrolyte brushes change their conformation to pH and ion strengths because the charge densities along the polymer chains can be altered at certain pH values.5−7 Poly(N,Ndimethylaminoethyl methacrylate) (PDMAEMA) is a typical weak polyelectrolyte and its apparent pKa value is 7.0−7.5.8,9 PDMAEMA is also a temperature-responsive polymer, which reveals a lower critical solution temperature (LCST) of 40−50 °C in aqueous solutions.10,11 Therefore, the abundant variations of conformation to environments enable PDMAEMA brushes not only of theoretical interest, but also relevant to numerous applications. The thickness of the PDMAEMA planar brush significantly increases with the change in pH from neutral to acidic and salt© 2015 American Chemical Society

induced contraction of quaternized PDMAEMA brushes at high ionic strength.12 The conformational change of the charged PDMAEMA planar brush is dominated by the counterion condensation, but this change is governed by the nonelectrostatic anion adsorption when uncharged, measured by quartz crystal microbalance with dissipation and surface plasmon resonance.13 Based on rheological measurements, Sui et al.14 found that the LCST of the PEO-g-PDMAEMA brushshaped copolymer strongly decreases with increasing pH. The unexpected shear thickening behavior is tuned by varying the pH, which results from the mobile nature and attractive force of the densely grafted hydrophobic chains of PDMAEMA at high pH. In situ spectroscopic ellipsometry and dynamic contact angle measurements revealed that increasing pH caused the largest decrease in hydration of PDMAEMA brushes, followed by the increasing temperature; by comparison, increasing the ionic strength gives the smallest change in hydration.15 The structure of a polymer brush depends on the distance between two anchoring polymer chains or, equivalently, on the σ of the Received: October 9, 2014 Revised: July 20, 2015 Published: July 26, 2015 8930

DOI: 10.1021/acs.langmuir.5b02159 Langmuir 2015, 31, 8930−8939

Article

Langmuir Scheme 1. Schematic of Three Models of PDMAEMA Brushes under Different Modes of Confinementa

a (a) weak polyelectrolyte brushespolyelectrolyte chains confined only by a hard core; (b) shell crosslinked weak polyelectrolyte brushes polyelectrolyte chains confined by both a hard core and crosslinking points; and (c) hollow nanocapsules after removal of the corepolyelectrolyte chains confined only by crosslinking points.

Scheme 2a

a

(a) Synthesis of 7-(2-methacryloyloxyethoxy)-4-methylcoumarin and (b) silica-g-P(DMAEMA-co-CMA) nanoparticles via SI-ATRP.

brushes.16 Klitzing et al.17 investigated the effect of σ on the uptake of Au nanoparticles into PDMAEMA brushes. High particle uptake and better particle penetration are observed in PDMAEMA grafts in medium σ; the thickness of PDMAEMA brushes is increased by incorporation of water facilitated by cavity formation after immobilization of Au nanoparticles. The swelling behavior of PDMAEMA planar brushes in water vapor has been investigated by a combination of neutron and X-ray reflectivity and spectroscopic ellipsometry. Results show that PDMAEMA brushes are hydrated heterogeneously by the solvation of the polymer/air interface; quaternized brushes exhibit a more uniform swelling response to increasing humidity levels.18 Adsorption of high-density PDMAEMA spherical brushes (SiO2-g-PDMAEMA) onto the silica/aqueous interface was investigated by Tilton.19 Non-monotonic depend-

ences of the extent of adsorption on pH and ionic strength are observed. A strongly hysteretic adsorption response to altered pH and a greater tendency to adsorb under weak surface attraction conditions prevailed at high pH. However, reports about the solution behavior of PDMAEMA brushes under confinement are rare. This work aimed to study the solution behavior of weak polyelectrolyte spherical brushes under different modes of confinement. Three types of models with different degrees of confinement are designed and prepared based on PDMAEMA spherical brushes, namely, PDMAEMA brushes (polyelectrolyte chains confined only by a hard core), shell cross-linked PDMAEMA brushes (polyelectrolyte chains confined by a hard core and cross-linking points), and hollow PDMAEMA nanocapsules (polyelectrolyte chains confined only by cross8931


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using a JEOL JEM-1011 electron microscope at an acceleration voltage of 100 kV. A drop of sample suspension was deposited on the copper grid with a carbon film and dried for measurement. Atomic force microscopic (AFM) measurements were conducted on an Agilent 5500 SPM operated in the tapping mode. The THF suspension was spin-coated onto freshly cleaved mica surfaces, and then the phosphate buffer at pH of 7.0 was dropped, followed by drying using a stream of nitrogen. The molecular weights and molecular weight distributions of the P(DMAEMA-co-CMA) chains etched by HF solution were measured on GPC installed with a Waters 515 pump, Waters 717 plus autosampler, PL gel MIXED-BLS columns (300 × 7.5 mm, 10 μm) and Waters 2414 differential refractive index detector at 35 °C. Monodispersed polystyrene and dimethylformamide containing 0.1 wt % tetrabutylammonium bromide were used as the calibration standard and the eluent at a flow rate of 1.0 mL/min, respectively. Photo-cross-linking reaction was performed using a spectroline FC100/F UV lamp with λ = 365 nm (5 mW/cm2). For the photocleavage reaction, an EF-180C/FE UV lamp (1.29 mW/cm2) at λ = 254 nm was used at a distance of 5 cm to the sample dispersion. The UV− visible spectra were collected on a Shimadzu UV-2450 spectrophotometer. The hydrodynamic radius (Rh) was determined by a Malvern Zetasizer Nano ZS90 with a He−Ne laser at λ = 633 nm. The sample concentration at 0.2 g/L and phosphate buffer solutions at pH of 6.0, 7.0, and 8.0 were applied to investigate the volume phase transition behavior of the polyelectrolyte brush system. NaCl was added to the phosphate buffer solutions to keep the ionic strength constant, and final sample solutions with an ionic strength of 50 mM were prepared. The measurement was performed at least three times at every temperature and each temperature was equilibrated for 20 min before measurement. The heating step was set at 1 °C and the volume phase transition temperature (VPTT) in solution was defined as the temperature at which the maximum slope of the phase transition curve. In order to study the responsive behavior to different ionic strengths, the viscosities of the NaCl solutions (c ≥ 0.2 M) were measured using an Ubbelohde viscometer and by considering the increase in densities.

linking points) (Scheme 1). The photo-, temperature-, pH-, and ion-responsive behavior of PDMAEMA brushes have been extensively investigated by UV−visible spectroscopy and dynamic light scattering (DLS).

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EXPERIMENTAL SECTION

Materials. N,N-(Dimethylamino)ethyl methacrylate (DMAEMA, 98%, Alfa) and anisole (98%, Alfa) were stirred overnight with CaH2 and distilled under reduced pressure. CuBr (98%, Shanghai Chemical Reagent Co.) was dispersed in glacial acetic acid overnight, washed with ethanol, and dried in a vacuum oven at 50 °C. Toluene, tetrahydrofuran (THF), and triethylamine (Et3N) were refluxed with sodium under nitrogen, and chloroform with CaH2, followed by distillation. 7-Hydroxy-4-methylcoumarin (97%, Aldrich), 2-bromoethanol (97%, Alfa), methylacryloyl chloride (97%, Alfa), tetraethoxysilane (TEOS, 98%, Alfa), 3-aminopropyltrimethoxysilane (APTMS, 97%, Aldrich), 2-bromoisobutyryl bromide (BIBB, 98%, Aldrich), N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA, 98%, Aldrich), and CuBr2 (99%, Shanghai Chemical Reagent Co.) were used as received. Regenerated cellulose membranes with molecular weight cutoff of 3400 and 14 000 g/mol were used to dialyze. The initiator-functionalized silica nanoparticles and 7-(2-methacryloyloxyethoxy)-4-methyl-coumarin (CMA) were synthesized according to the previous study20 and the literature,21 respectively (Supporting Information and Scheme 2a). Surface-Initiated Atom Transfer Radical Polymerization (SIATRP) of DMAEMA and CMA from Initiator-Functionalized Silica Nanoparticles. The procedures for preparation of silica-gP(DMAEMA-co-CMA) nanoparticles were as follows: the initiatorfunctionalized silica nanoparticles (50 mg, 0.011 mmol of the initiating sites), CuBr2 (1.2 mg, 0.0055 mmol), PMDETA (21.0 mg, 0.121 mmol), DMAEMA (1.38 g, 8.8 mmol), CMA (126.9 mg, 0.44 mmol), and anisole (1.0 mL) were added to a polymerization tube followed by dispersion using ultrasonication for 5 min. After two freeze−pump− thaw cycles to deoxidize, CuBr (7.9 mg, 0.055 mmol) was added under an argon flow. The mixture was degassed via two additional freeze− pump−thaw cycles. The tube was sealed and placed in an oil bath at 70 °C for polymerization. After 6 h, the mixture was cooled to room temperature, exposed to air, diluted with THF, and passed through a neutral Al2O3 column. The resulting solution was concentrated and the precipitates were formed via pouring into cold n-hexane. The products were separated, further purified via additional two redissolving/reprecipitating cycles with THF/cold n-hexane, and dried overnight at 50 °C in a vacuum oven. Cross-Linking and De-Cross-Linking of Silica-g-P(DMAEMAco-CMA) Nanoparticles. Silica-g-P(DMAEMA-co-CMA) nanoparticles (20 mg) was dissolved in deionized water (5 mL) overnight. In order to obtain the shell cross-linked hybrid nanoparticles, the hybrid nanoparticle dispersion was exposed to UV light at λ = 365 nm under stirring for 550 min. Then, the resulting dispersion was exposed to UV light at λ = 254 nm under stirring for 430 min to get the shell de-crosslinked hybrid nanoparticles. Preparation of Hollow Nanocapsules. The shell cross-linked silica nanoparticles (10 mg) were dispersed in deionized water (5 mL) under stirring followed by the addition of HF solution (47 wt %, 200 μL). After reaction for 12 h, the dispersion containing the hollow PDMAEMA nanocapsules was neutralized by introducing a Na2CO3 aqueous solution (5 wt %, 1 mL). The as-prepared mixture was purified via dialysis against deionized water for 72 h at room temperature, and the deionized water was refreshed every 6 h during this period. Finally, the PDMAEMA nanocapsules were obtained by a freeze−drying procedure. Characterizations. 1H NMR spectra were obtained by a Bruker400 MHz NMR instrument. Fourier transform infrared (FT-IR) spectra were collected by a Bruker VECTOR-22 IR spectrometer at 64 scans with a resolution of 2 cm−1. Thermogravimetric analysis (TGA) was conducted by a PerkinElmer Diamond TG/DTA from room temperature to 800 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. Transmission electron microscopy (TEM) was performed

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RESULTS AND DISCUSSION SI-ATRP of DMAEMA and CMA from the InitiatorFunctionalized Silica Nanoparticles. As shown in Scheme 2b, the ATRP reaction of DMAEMA and CMA on the surface of the initiator-functionalized silica nanoparticles was conducted in anisole, using silica-Br as the initiator and CuBr/ CuBr2/PMDETA as the catalyst. Synthesis of Initiator-Functionalized Silica Nanoparticles. Initiator-functionalized silica nanoparticles were synthesized according to a published procedure.20 Typically, silica nanoparticles were prepared using the Stöber method, followed by modification with APTMS to obtain amino-functionalized silica nanoparticles, and amidated with BIBB to afford initiatorfunctionalized silica nanoparticles. The as-prepared nanoparticles were characterized by FT-IR technique. The presence of the amide band (1540 cm−1, N−H stretching) confirmed the successful amidation (Figure S1b), compared to the curve of silica nanoparticles (1100 cm−1, Si−O stretching) (Figure S1a). The average diameter of the initiator-functionalized silica nanoparticles is approximately 50 nm, measured by TEM images (Figure S2), which shows a narrow distribution (Figure S 2d). The standard deviation (SD) is 3.7 nm. An approximately 2.9 wt % difference is counted from TGA curves between the weight retentions of the amino- and 2bromoisobutyrate-functionalized silica nanoparticles at 800 °C (Figure 1b,c). With the mass retention of the aminofunctionalized silica nanoparticles at 800 °C as a reference, and the density of the silica nanoparticles identical to the bulk 8932


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Figure 2. 1H NMR spectrum of the silica-g-P(DMAEMA-co-CMA) nanoparticles. Figure 1. TGA curves of (a) silica, (b) silica-NH2, (c) silica-Br, and (d) silica-g-P(DMAEMA-co-CMA) nanoparticles (6 h). TGA was conducted under nitrogen atmosphere at a heating rate of 10 °C/min.

(CH3)2), 4.07 (CH2CH2N(CH3)2), 4.25−4.32 (COOCH2CH2O-aromatic), 6.15 (CH3CCHCOO), 6.83 (aromatic), and 7.53−7.55 (aromatic). By comparing the well-defined peak integrals of − OCH2CH2− (δ 4.07 ppm) of PDMAEMA, and − CH3CCHCOO− (δ 6.15 ppm) of CMA, the molar content of CMA monomer was calculated to be 5.3%, which matched the initial monomer feed ratio (DMAEMA/CMA = 20). This result agrees with the linear random copolymer of P(DMAEMA-co-CMA).23 The morphology of the silica-g-P(DMAEMA-co-CMA) nanoparticles was observed by TEM and AFM. In Figure 3a, the dark silica cores are coated by a light gray polymer shell. The average radius of the core nanoparticles is approximately 25 nm, and the overall radius of the hybrid nanoparticles is approximately 34 nm. The radius distribution is shown in Figure 3d and SD is counted to be 4.3 nm. Figure S5 displays the AFM topography image of silica-g-P(DMAEMA-co-CMA) nanoparticles on the freshly cleaved mica surface, treated by aqueous solution at pH of 7.0. The polymer layers can be observed easily, and the calculated nanoparticle radius is about 140 nm, which is much larger than the radius measured by TEM. The major reason is the shrinkage of polymer chains during drying procedure of TEM measurements, and greater extension by the electrostatic adsorption of PDMAEMA chains on the mica surface for AFM measurements. In order to calculate the σ of the tethered polymer chains on the surface of silica nanoparticles, TGA measurements are necessary. In Figure 1d, the weight retention of the silica-gP(DMAEMA-co-CMA) nanoparticles at 800 °C is approximately 18.9%. The weight content of grafted polymer relative to the silica cores is calculated to be 77.0%, by taking the weight retention of the silica-Br nanoparticles at 800 °C (82.1 wt %) as a reference. According to GPC results, the number-average molecular weight (Mn) of the grafted P(DMAEMA-co-CMA) chain was 110 000. Thus, the σ of the polymer chain is approximately 0.23 chain/nm2, and the grafted number of polymer chains on one silica nanoparticles is counted approximately to be 1806.

silica, i.e., 2.07 g/cm3,22 the grafting density of the ATRP initiators on the surface of the silica nanoparticles is determined to be 2.53 initiator/nm2. Synthesis of P(DMAEMA-co-CMA)-Grafted Silica Nanoparticles. P(DMAEMA-co-CMA) chains grafted on silica nanoparticles were prepared via SI-ATRP of DMAEMA and CMA from the initiator-functionalized silica nanoparticles in anisole. CuBr2 was added to ensure an efficient exchange between the dormant and active species to control the polymerization. The successful preparation of silica-g-P(DMAEMA-co-CMA) nanoparticles is confirmed by the characteristic peaks in the FT-IR spectrum assigned to DMAEMA and CMA, which are 2962 cm−1 due to C−H stretching of the methyl and methylene groups, 2822 and 2771 cm−1 due to C−H stretching of the − N(CH3)2 group, 1730 cm−1 due to carbonyl group, 1615 cm−1 due to ring CC stretching of coumarin, and 1460 cm−1 due to CH2 bending (Figure S1c). From Figure S3, with the polymerization time increased from 0 to 6 h, a linear and rapid growth on Rh of silica-g-P(DMAEMA-co-CMA) nanoparticles is observed from 25 to 151 nm. When the polymerization time is increased from 6 to 24 h, the Rh of silica-g-P(DMAEMA-co-CMA) nanoparticles shows a slow increase from 151 to 169 nm. The results indicate that the molecular weight of grafted polymer chains increases almost linearly at the initial stage and then grows slowly. P(DMAEMA-co-CMA) is cleaved from the silica nanoparticles (Supporting Information). From Table 1 and Figure S4, the molecular weight of P(DMAEMA-co-CMA) increases from 14.9 × 104 g/mol to 22.3 × 104 g/mol when the polymerization time is extended from 6 to 24 h, with PDIs of 1.35 and 1.44, respectively. A typical 1H NMR spectrum of silica-g-P(DMAEMA-coCMA) nanoparticles is shown in Figure 2 with the relevant signals labeled, and all peaks can be assigned with the chemical structures. 1H NMR (400 MHz, CDCl3), δ (ppm) = 2.30−2.32 (CH2N(CH3)2), 2.42 (CCH3CHCOO), 2.58 (CH2CH2N-

Table 1. Experimental Conditions and Results of silica-g-P(DMAEMA-co-CMA) Nanoparticles Prepared via SI-ATRP at 70 °Ca no.

[DMAEMA] (mmol)

[CMA] (mmol)

reaction time (h)

Mwb (104 g/mol)

PDIb (Mw/Mn)

σc (chains/nm2)

Rgd/De

6h 24 h

8.8 8.8

0.44 0.44

6 24

14.9 22.3

1.35 1.44

0.23 0.22

6.3 7.4

a The ratio of initiator/CuBr/CuBr2/PMDETA was 1:5:0.5:10. bApparent values obtained by GPC with DMF and 0.1 wt % tetrabutylammonium bromide as the eluent and PS standards as calibration. P(DMAEMA-co-CMA) chains were cleaved from silica nanoparticles via etching with HF. cσ calculated by TGA. dRg: radius of gyration estimated by static light scattering. eDistance between neighboring grafted chains D = σ−1/2.

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Figure 3. TEM images of (a) silica-g-P(DMAEMA-co-CMA) nanoparticles, (b) shell cross-linked silica-g-P(DMAEMA-co-CMA) nanoparticles, (c) hollow P(DMAEMA-co-CMA) nanocapsules obtained by removing silica cores from cross-linked silica-g-P(DMAEMA-co-CMA) nanoparticles via etching with HF, and corresponding radius distributions of nanoparticles (d,e). Scale bar: 100 nm.

Scheme 3. Preparation of PDMAEMA Brushes, Cross-Linked PDMAEMA Brushes, and Corresponding Hollow Nanocapsules

light, these brushes can be cross-linked. Meanwhile, with exposure to λ < 260 nm light, these brushes become de-crosslinked (Scheme 3). As shown in Figure 3, compared with the silica-g-P(DMAEMA-co-CMA) nanoparticles (Figure 3a), the cross-linked gray shells are clearly visible, and the radius is approximately 44 nm (Figure 3b). The narrow radius distribution is observed in Figure 3e and the SD is 5.4 nm. Before cross-linking, the shell of the nanoparticles collapse on the silica surface. When the shell is cross-linked, the P(DMAEMA-co-CMA) chains are partially fixed, which only shows a slight collapse. The corresponding hollow nanocapsules are obtained when the silica core is removed by etching with HF. From Figure 3c, the individual hollow nanoparticles have an average radius of 23 nm and are smaller than the shell cross-linked silica nanoparticles. The corresponding radius distribution is shown in Figure 3f and the calculated SD is 6.0 nm. This property is attributed to the removal of silica sustainment and the shrinkage of polymer chains. Photoresponsive Behavior of PDMAEMA Brushes. In an aqueous solution of PDMAEMA brushes (0.2 mg/mL), we investigated the reversibility of the photo-cross-linking reaction

The average distance between neighboring chains (D) is denoted as σ−1/2. Table 1 shows that the Rg/D values of two samples are 6.3 and 7.4, which are much higher than 2. Thus, the P(DMAEMA-co-CMA) chains show the polymer brush conformation on the surface of the silica nanoparticles.16 Preparation of Photo-Cross-Linked, De-Cross-Linked PDMAEMA Brushes, and Hollow PDMAEMA Nanocapsules. Coumarin and its derivatives with reversible photodimerization and photocleavage under UV irradiation have been widely investigated for the application of photoresponsive materials.24−30 The reversible reaction is driven by the photodimerization of coumarin groups upon UV irradiation at λ > 300 nm and the photocleavage of dimers upon UV irradiation at λ < 260 nm.31,32 This may allow a polymer containing pendant coumarin groups to be reversibly crosslinked and de-cross-linked by two different wavelengths. In the present work, we applied this property of interest to construct a general strategy for the preparation of photoresponsive polyelectrolyte brushes via random copolymer grafting onto silica nanoparticles (Scheme 2). Upon exposure of the silica-gP(DMAEMA-co-CMA) nanoparticle solution to λ > 310 nm 8934


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Figure 4. UV−vis spectra and DLS results in an aqueous solution of PDMAEMA brushes showing the photocontrol of cross-linking: (a) decrease of the absorbance at 320 nm upon illumination at λ = 365 nm indicating the dimerization of coumarin groups in PDMAEMA brushes, with the inset being the plot of DD vs irradiation time; (b) dependence of corresponding Rh on DD; (c) increase of the absorbance at 320 nm when exposed to λ = 254 nm light indicating the reverse photoreaction of cyclobutane cleavage, with the inset being the plot of DD vs irradiation time; and (d) dependence of corresponding Rh on DD.

cannot completely undergo photocleavage into the free polymer chains. Moreover, the photoreaction in the suspension of PDMAEMA brushes was confirmed by four consecutive cycles of photodimerization and photocleavage. Figure 5 shows

and its effect on the size of the obtained brushes (Scheme 3). Figure 4a shows the UV−vis spectral change of the PDMAEMA brush dispersion upon irradiation of λ = 365 nm UV light. The decrease in the absorption of coumarin groups at ∼320 nm with irradiation time indicates the occurrence of dimerization and the formation of cross-linked polyelectrolyte brushes. From the spectra, the dimerization degree (DD) could be calculated using (1-At/A0), where A0 and At are the initial absorbance at 320 nm and the absorbance after an irradiation time t, respectively. The results in the inset of Figure 4a show the kinetic process, such that DD reaches a plateau value of 82.3% after 430 min of irradiation. DD determines the crosslinking density; thus, its effect on the size of polyelectrolyte brushes is investigated. A plot of the Rh versus DD for a PDMAEMA brush solution at 25 °C is shown in Figure 4b. In the figure, Rh decreases from 189 to 149 nm as DD increases from 0% to 82.3%, which indicates the shrinkage of PDMAEMA brushes after cross-linking. When the UV wavelength applied to the PDMAEMA brush solution is changed to 254 nm, the absorbance at 320 nm starts to recover because of the reverse photocleavage reaction bringing back coumarin side groups, as shown in Figure 4c. When the irradiation time increases to 400 min, DD decreases to 40% and has almost no change on DD with extending irradiation time, as shown in the inset of Figure 4c. The average photoinduced increase in Rh from de-cross-linking is from 149 to 173 nm as shown in Figure 4d. These results demonstrate that for PDMAEMA brushes, cross-linking density can be easily regulated by only adjusting the irradiation time, and the photocleavage of coumarin dimers appears incomplete. This result is attributed to the fact that the distance between neighboring coumarin units is close to each other due to high σ of PDMAEMA brushes. In addition, after long irradiation time at λ = 254 nm UV light, almost half of the total number of polymer brushes are obtained, which hinders the actual reversibility of the system. Thus, the cross-linked groups

Figure 5. Reversible changes in (a) DD and (b) Rh of PDMAEMA brushes upon alternating UV illumination at λ = 365 nm for dimerization (430 min exposure) and at λ = 254 nm for cleavage (430 min exposure).

that the DD could be switched between 44% and 85% and Rh between 170 and 148 nm by alternating the UV exposure between λ = 365 nm and λ = 254 nm. Therefore, the reversible photocontrol of the cross-linking density could be achieved to some extent. Solution Responsive Behavior of PDMAEMA Brushes under Different Modes of Confinement. Different modes of confinement in solutions are prepared (Scheme 1) and measured by DLS to investigate the temperature-, pH-, and ionresponsive behavior of the weak polyelectrolyte spherical brushes. These modes are PDMAEMA brushes (polyelectrolyte chains confined only by a hard core), shell cross-linked PDMAEMA brushes (polyelectrolyte chains confined by both hard core and cross-linking points), and hollow PDMAEMA nanocapsules after the removal of silica by etching (polyelectrolyte chains that are confined by cross-linking points alone). 8935


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Figure 6. Volume phase transition curves of PDMAEMA brushes (■), cross-linked PDMAEMA brushes (red ●) and hollow nanocapsules (blue ▲) in (a) pH = 6.0, (b) pH = 7.0, (c) pH = 8.0 aqueous solution during heating process and (d) schematic of morphological and VPTT change of three models.

49 °C at pH = 8.0, respectively (Table 2 and Figure 6). Figure 6d shows a schematic of the morphological and VPTT change of three models in solution during the heating process.

Temperature-Responsive Behavior of PDMAEMA Brushes under Different Confinement Modes. The DLS results showed the Rh changes of the nanoparticles during heating at a constant pH, and the phase transitions span a broad temperature range (Figure 6). As the temperature increases at constant pH, the Rh of three samples monotonically decreases, from 194 to 82 nm at pH = 6.0, from 169 to 49 nm at pH = 7.0, and from 149 to 60 nm at pH = 8.0 for PDMAEMA brushes; from 162 to 80 nm at pH = 6.0, from 134 to 53 nm at pH = 7.0, and from 124 to 52 nm at pH = 8.0 for cross-linked PDMAEMA brushes; from 141 to 83 nm at pH = 6.0, from 124 to 55 nm at pH = 7.0, and from 95 to 40 nm at pH = 8.0 for hollow nanocapsules. The corresponding intensity-weighted Rh distributions in the solution with pH = 7.0 are 0.192 for PDMAEMA brushes, 0.163 for cross-linked PDMAEMA brushes, and 0.150 for hollow nanocapsules at 25 °C, which decreased to 0.134, 0.125, and 0.104, respectively, after the phase transition as shown in Figure S6. The above results imply a narrow distribution for samples during the heating process. The onset Rh of the crosslinked brushes is considerably smaller as compared to that before cross-linking, which should be attributed to the shrinkage of polymer brushes after photo-cross-linking. The onset Rh for the hollow nanocapsules is smaller than that of cross-linked brushes before etching, which agrees well with the TEM results (Figure 3b and c). The phenomenon differs from the behavior of poly(N-isopropylacrylamide) (PNIPAM) hybrid silica nanoparticles in aqueous solutions. According to the study of Wu,33 the Rh of hollow PNIPAM nanocapsules is larger than that of cross-linked hybrid nanoparticles before etching when the temperature is 20 °C. Their group suggested that the PNIPAM shell of the cross-linked hybrid silica nanoparticles swelled upon removal of the silica core. However, the swelling degree of hollow nanocapsules of PDMAEMA is smaller than that of PDMAEMA grafted on a silica system in acidic or basic environments. The VPTTs of PDMAEMA brushes, cross-linked brushes, and hollow nanocapsules, shift toward higher temperature in sequence, which are 45, 53, and 60 °C at pH = 6.0; 41, 47, and 53 °C at pH = 7.0; 35, 42, and

Table 2. Volume Phase Transition Temperatures of PDMAEMA Brushes, Cross-Linked PDMAEMA Brushes and Hollow Nanocapsules in Aqueous Solutions (pH = 6.0, 7.0, and 8.0) during Heating Process pH

PDMAEMA brushes/° C

cross-linked PDMAEMA brushes/° C

hollow nanocapsules/° C

6.0 7.0 8.0

45 41 35

53 47 42

60 53 49

According to a study by Müller’s group,11 PDMAEMA is a type I temperature-sensitive polymer. Its phase transition temperature shifts toward lower temperatures with increasing polymer molar mass. The grafted PDMAEMA chains are divided into several short parts by cross-linking points from CMA groups for cross-linked brushes, which is equal to a large quantity of low-molecular-weight PDMAEMA chains grafted onto silica nanoparticles. Therefore, the VPTT is clearly higher in the cross-linked brushes than in the un-cross-linked brushes in solution. That is, the VPTT increases with the degree of confinement. On the other hand, more water is incorporated into the brush matrix with the increasing cross-linked density, which causes the phase transition of cross-linked PDMAEMA brushes at higher temperatures. Among the three types of confinement, the highest thermal energy for phase transition is necessary for hollow nanocapsules. In this hollow nanocapsule system, the silica cores are removed and more cavities are formed. Moreover, the PDMAEMA chains are confined, which results in the entry of more water molecules. Finally, more thermal energy is needed to weaken the hydrogen-bonding interactions and release the water molecules. Consequently, the hollow nanocapsules display the highest VPTT values among the three systems. 8936


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Langmuir Phase Transition Behaviors of Three Models in Buffer Solutions at Different pH Levels. The effects of pH on the VPTT of the three systems are shown in Table 2 and Figure 7;

Figure 8. Effect of ionic strength on Rh change of PDMAEMA brushes (■), cross-linked PDMAEMA brushes (red ●) and hollow nanocapsules (blue ▲) in NaCl solutions at pH = 6.0.

Figure 7. Effect of pH on VPTTs of PDMAEMA brushes (■), crosslinked PDMAEMA brushes (red ●) and hollow nanocapsules (blue ▲).

to above two systems in NaCl solutions. Actually, the PDMAEMA chains are more charged when the solutions of three models are at pH = 6.0, and the electrostatic repulsion is governed. Because of the electrostatic and osmotic screening, the polyelectrolyte chains collapse with the addition of NaCl. Thus, the Rh of three PDMAEMA brushes systems under different confinement decreased with increase of ionic strength.

the VPTTs exhibit a strong dependence on the solution pH. The VPTTs of PDMAEMA brushes, cross-linked brushes, and hollow nanocapsules decreased with the increase in pH. For PDMAEMA brushes, the VPTT is 45 °C at pH = 6.0, 41 °C at pH = 7.0, and 35 °C at pH = 8.0; for cross-linked brushes, 53 °C at pH = 6.0, 47 °C at pH = 7.0, and 42 °C at pH = 8.0; for hollow nanocapsules, 60 °C at pH = 6.0, 53 °C at pH = 7.0, and 49 °C at pH = 8.0, respectively. The results also demonstrate that the VPTTs of PDMAEMA brushes, cross-linked brushes, and hollow nanocapsules shift toward higher temperatures in sequence at constant pH. In addition, the onset sample Rh decreases with increasing pH: approximately 194 nm at pH = 6.0, 169 nm at pH = 7.0, and 149 nm at pH = 8.0 for PDMAEMA brushes; 162 nm at pH = 6.0, 134 nm at pH = 7.0, and 124 nm at pH = 8.0 for cross-linked brushes; 141 nm at pH = 6.0, 124 nm at pH = 7.0, and 95 nm at pH = 8.0 for hollow nanocapsules (Figure 6). The conformational change in three types of model systems is influenced by electrostatic repulsion and hydrogen-bonding interaction at pH of 6.0 and 7.0. However, the variation of the conformation is controlled by hydrophobic interaction at pH of 8.0. Thus, the VPTTs of the model systems move to a lower temperature with increasing of pH values. Response Behaviors of Three Confinement Models to Different Ionic Strengths. It is well-known that PDMAEMA is a weak polyelectrolyte, whose charge density can be adjusted with the pH. Herein, the pH is fixed at 6.0 and NaCl aqueous solutions with different concentrations are prepared. The ionresponsive behavior of PDMAEMA spherical brushes under different modes of confinement is investigated. As shown in Figure 8, when the concentration of NaCl (CNaCl) increases from 0 to 1.0 M, an evident decrease in Rh is found for PDMAEMA brushes from 194 to 83 nm, for cross-linked PDMAEMA brushes from 162 to 72.6 nm, and for hollow nanocapsules from 141 to 60.5 nm. These results indicate that in NaCl aqueous solutions the cross-linked PDMAEMA brushes and hollow nanocapsules exhibit the obvious ionresponsive behavior. The hollow nanocapsules have the smallest Rh among the three model systems, followed by cross-linked brushes with CNaCl from 0 to 1.0 M. The PDMAEMA brushes give the largest values in Rh compared

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CONCLUSIONS In summary, we designed and synthesized three PDMAEMA copolymer spherical brush models under different modes of confinement. The photo-, temperature-, pH-, and ionresponsive behaviors of the prepared weak polyelectrolyte brushes in aqueous media were elucidated. The PDMAEMA brushes with coumarin can be reversibly cross-linked and decross-linked by using light at two different wavelengths in a reaction that can be repeated at least four times. However, the photocleavage process itself is incomplete because of the high σ of the PDMAEMA brushes. This can be achieved by optically adjusting the cross-linking degree in preparing the PDMAEMA brushes. Furthermore, hollow PDMAEMA nanocapsules were obtained by cross-linking the polyelectrolyte brushes after removing silica cores. The VPTTs of the model system displayed significant dependence on the degree of confinement and pH. The VPTTs of PDMAEMA brushes, cross-linked brushes, and the hollow nanocapsules shifted toward higher temperatures at a constant pH. On one hand, the grafted PDMAEMA chains are divided into several short parts by crosslinking points from CMA groups, which is equal to several PDMAEMA chains with low molecular weights grafted onto silica nanoparticles. On the other hand, more water is incorporated into the brush matrix with increased cross-linked density. Therefore, more thermal energy is required for the phase transition of the cross-linked PDMAEMA brushes. The hollow nanocapsule system exhibits the highest phase transition temperature to weaken the hydrogen-bonding interactions and release more water molecules. When the solution pH changes from 6.0 to 8.0 in the model system, the conformational change is predominated by the hydrophobic interaction rather than the electrostatic repulsion and hydrogen-bonding interaction. As a result, the VPTTs move to a lower temperature with the increase in pH. When the ionic strength of the aqueous solution increases, an evident decrease in the Rh of three model systems is observed, which indicates an obvious ion-responsive 8937


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Langmuir

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behavior in NaCl aqueous solutions at pH = 6.0. These photo-, temperature-, pH-, and ion-responsive polyelectrolyte brushes and hollow nanocapsules could have potential applications in controlled-release, nanocarriers, antimicrobial surfaces, and smart optical devices.

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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.5b02159. Procedure for synthesis of the initiator-functionalized silica nanoparticles, 7-(2-hydroxyethoxy)-4-methylcoumarin, and 7-(2-methacryloyloxyethoxy)-4-methylcoumarin, cleavage of the grafted P(DMAEMA-co-CMA) from the silica nanoparticles, FT-IR curves of (a) silica, (b) silica-Br, and (c) silica-g-P(DMAEMA-co-CMA) nanoparticles, TEM images of (a) silica, (b) silica-Br nanoparticles and corresponding diameter distributions (c,d), effect of polymerization time on Rh of silica-gP(DMAEMA-co-CMA) nanoparticles, GPC curves, samples with different polymerization time and P(DMAEMA-co-CMA) chains were cleaved from silica nanoparticles, AFM topography image of silica-g-P(DMAEMA-co-CMA) nanoparticles treated by a dilute phosphate buffer with pH of 7.0 onto freshly cleaved mica, and Rh distributions of (a) PDMAEMA brushes, (b) cross-linked PDMAEMA brushes, and (c) hollow nanocapsules at pH = 7.0 solution during heating process (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-431-85262876. Fax: 86-431-85262075. E-mail: xlji@ ciac.ac.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by National Natural Science Foundation of China (Key Project: 50633030, Innovation Group: 50921062).

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REFERENCES

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Synthesis and Multi-Stimuli-Responsive Behavior of Poly (N, N

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