Article pubs.acs.org/Macromolecules
Multiresponsive Square Hybrid Nanosheets of POSS-Ended Hyperbranched Poly(ether amine) (hPEA) Bing Yu, Xuesong Jiang,* and Jie Yin School of Chemistry & Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China S Supporting Information *
ABSTRACT: We demonstrated a novel square hybrid nanosheet with a ultrathin thickness for the first time, which was fabricated by self-assembly of hyperbranched poly(ether amine) (hPEA526) containing anthracene (AN) moieties and heptaisobutyl polyhedral oligomeric silsesquioxane (POSS). TEM and AFM images reveal that the average edge length and thickness of the hybrid nanosheets formed by HP1 is 1.2 ± 0.2 μm and 4.5 ± 0.5 nm, respectively. POSS prefers the ordered crystallized aggregation in the formation of the regular square nanosheets, which is confirmed by WAXD and DSC studies. Moreover, these nanosheets are cross-linked through dimerization of anthracene moieties which makes the nanosheets more stable. The other functional moieties such as naphthalene, pyrene, and dodecane can also be easily introduced into the hybrid nanosheets through the same way. The obtained hybrid nanosheets exhibit the multiresponses to temperature and pH, and their dispersion in water can be controlled by temperature. The fluorescence of the hybrid nanosheets decreases with the increasing temperature and pH. The discovery of the hybrid nanosheets is believed to provide a potential guiding significance on the preparation of the functional nanosheets by self-assembly of polymers. reprecipitation method.34 An et al. synthesized a compound cucurbit[8]uril, which can self-assemble with small organic molecules to form regular square nanosheets.29 The regular square shape of the nanosheet above was reported to be attributed to the existence of hydrogen bonds and π−π stacking in the nanosheets. However, all of these regular square nanosheets are fabricated by crystallization or self-assembly of small molecules. At the same time, hyperbranched polymers can be synthesized under a simple condition, and many delicate supramolecular structures from zero-dimension (0D) to threedimension (3D), such as micelles,37,38 fibers,39 tubes,40 and vesicles,41 have been prepared through the solution or interfacial self-assembly of amphiphilic hyperbranched poly-
1. INTRODUCTION Nanosheets are a subject of great interest because of their novel photochemical,1−4 electronics,5−8 mechanical,9 and optical10 properties and recently have attracted much attention for their potential applications in various areas of nanoscience, such as photocatalysts,11−14 supercapacitors,15 dye-sensitized solar cells,16 nanosensors,17,18 optical device,19 and template and platform for nanostructures.20,21 As a result, varieties of methods have been used to fabricate nanosheets, such as crystallization and exfoliation of some inorganic compounds including graphene, boron nitride, molybdenum sulfide, and tungsten sulfide.22−24 At the same time, self-assembly of some organic small molecules 25−32 and organometallic compounds33−36 is also an effective method. Nanosheets with regular shapes from 2D self-assembly have drawn much attention due to their easily tunable morphology and multifunctionality. Wang et al. reported a kind of porphyrin which can self-assemble into square nanosheets using a © 2012 American Chemical Society
Received: July 4, 2012 Revised: August 5, 2012 Published: August 17, 2012 7135
dx.doi.org/10.1021/ma301371h | Macromolecules 2012, 45, 7135−7142
Macromolecules
Article
Scheme 1. Structure of POSS/AN-Containing Hyperbranched Poly(ether amine) (HP1−HP5)
hPEA, POSS, and anthracene moieties, at a concentration of 0.5 wt %. The solution was left to equilibrate at 30 °C while ultrapure water was added very slowly to the solution (5 mL of water per hour to 1 mL of polymer solution). The polymer solution was gently stirred during the water addition until 1 h, and then the solution was exposed under an ultraviolet LED lamp (Uvata) at 365 nm with intensity about 8.4 mW/ cm2 to make part of the anthracene moieties in HP1 photodimerize. Except for the cases specifically mentioned, the irradiation time is 20 min. The solution with HP1 nanosheets was then dialyzed against ultrapure water for 24 h to remove 1,4-dioxane using cellulose membrane with a molecular weight cutoff of 3500, and cross-linked HP1 nanosheets aqueous solution with a concentration about 1 g/L was obtained. Assemblies by other molecules were prepared similarly, except that assemblies by HP6−HP8 were prepared without UV irradiation. Instruments and Measurements Nuclear Magnetic Resonance (NMR). 1H NMR and 13C NMR spectra was acquired with Mercury Plus spectrometer (Varian, Inc.) operating at 400 MHz by using CDCl3 as solvent and TMS as an internal standard at room temperature. 13C NMR quantitative spectra of hyperbranched poly(ether amine)s (hPEA) were carried out in CDCl3 on Advance400 spectrometer (Bruker, Switzerland) operating at 400 MHz. Fourier Transform Infrared Absorption Spectra (FTIR). FTIR measurements were carried out with Spectrum 100 Fourier transformation infrared absorption spectrometer (Perkin-Elmer, Inc.). The samples were prepared by dropping the polymer solution onto a KBr plate and dried below an infrared lamp. Gel Permeation Chromatography (GPC). The molecular weight of the products was measured by GPC on a LC-20AD (Shimadzu, Japan) system at 40 °C (50 μL injection column) with THF as eluent phase at a flow rate of 1 mL/min, and polystyrene was used for calibration. Dynamic Light Scattering (DLS). The DLS measurements were performed using a ZS90 Zetasizer Nano ZS instrument (Malvern Instruments Ltd., U.K.) equipped with a multi-τ digital time correlation and a 4 mW He−Ne laser (λ = 633 nm) at an angle of 90°. Regularized Laplace inversion (CONTIN algorithm) was applied to analyze the obtained autocorrelation functions. UV−vis Spectra. The UV−vis spectra of the HP1 nanosheets aqueous solution with different irradiation time were traced by a UV2550 spectrophotometer (Shimadzu, Japan). The HP1 nanosheets aqueous solution with different irradiation time was prepared according to the method mentioned in the synthesis section and diluted by 10 times before measurement. Fluorescence Spectra. The fluorescence emission spectra of the hyperbranched polymer nanosheets solutions were recorded under QM/TM/IM steady-state and time-resolved fluorescence spectrofluorometer (PTI Company) equipped with a thermo cell. The excitation wavelength was set at 381 nm, and the fluorescence emission spectra were recorded between 400 and 650 nm. The data were all normalized.
mers. There are also some reports on 2D self-assembly of polymers.42−49 However, as the complexity of the polymer chains, ultrathin nanosheets with regular square shape formed by self-assembly of macromolecules, especially hyperbranched polymers are really less reported. As a result, it is really significant if nanosheets with regular square shape can be obtained by assembly of hyperbranched polymers. Herein, we reported the square hybrid nanosheets of amphiphilic hyperbranched polymer, which are fabricated through self-assembly of hyperbranched poly(ether amine) (hPEA526) ended with anthracene (AN) moieties and heptaisobutyl polyhedral oligomeric silsesquioxane (POSS) (Scheme 1). The obtained hybrid square nanosheets possess the ultrathin sandwich-like structure with a thickness of 4−5 nm, which is comprised of a hydrophilic outer layer of hPEA526 and a hydrophobic inner layer of POSS and AN moieties. As the smallest precisely defined cubic silica nanoparticle, POSS is of great interest because it can be used as an important nanobuilding block to fabricate variety of micro- and nanostructures including vesicles, cylinders, and spheres.50−57 Because of the photodimerization under UV light, the introduction of AN moieties can lead to photo-cross-linking to make the assemblies much more stable. The obtained hybrid square nanosheets of POSS-ended hPEA exhibit high fluorescence which is responsive to external stimulus such as temperature and pH. To the best of our knowledge, this is the first example of the hybrid nanosheets with regular square shape formed by self-assembly of hyperbranched polymers, which might open up new perspectives for the preparation of the functional nanosheets by self-assembly of polymers.
2. EXPERIMENTAL SECTION Materials. Poly(ethylene glycol) diglycidyl ether (PEG526-DE, Mn = 526 g/mol, Sigma-Aldrich), ethylene glycol diglycidyl ether (EG-DE, M = 174 g/mol, TCI), N-ethylethylenediamine (NEED, Alfa Aesar), (3-glycidyl)propoxyheptaisobutyl polyhedral oligomeric silsesquioxane (E-POSS, Sigma-Aldrich), 9-anthracenemethanol (H-AN, Alfa Aesar), 1-naphthalenemethanol (H-NA, J&K Chemical), 1-pyrenemethanol (H-PY, TCI), 1,2-epoxydodecane (E-DA, TCI), and 3-chloro-1,2epoxypropane (ECH, Sinopharm Chemical Reagent) were all used without further purification. Other chemicals are of analytical grade except as noted. Synthesis of POSS-Ended Hyperbranched Poly(ether amine)s (HP1−HP9). Hyperbranched polymers HP1−HP9 were synthesized according to our previous reports,38,52 and their synthesis process is shown in the Supporting Information. Preparation of Hyperbranched Polymer Nanosheets. HP1 was first dissolved into 1,4-dioxane, which is a good solvent for both 7136
dx.doi.org/10.1021/ma301371h | Macromolecules 2012, 45, 7135−7142
Macromolecules
Article
Figure 1. (a) TEM image of the nanosheets formed by HP1. (b) AFM image of the nanosheet formed by HP1 and its height profile along the two lines of the AFM image. (c) Length distribution histogram of the square nanosheets formed by HP1 determined by TEM image. (d) Thickness distribution histogram of the nanosheets formed by HP1 determined by AFM image; 30 nanosheets were counted for the distribution. Wide-Angle X-ray Diffraction (WAXD). WAXD experiments were performed at room temperature on a glass slide using Rigaku Xray diffractometer D/MAX-2200/PC with rotating anode source operated at 40 kV and 20 mA, and the spectra were recorded in the scattering angle (2θ) range of 5°−40° (step size 0.02°). Cu Kα radiation with wavelength λ = 0.154 18 nm was used for the measurements. Differential Scanning Calorimetry (DSC). The crystallization behaviors of the samples were determined by a modulated differential scanning calorimeter (TA Instruments Q2000) under continuous nitrogen purge (50 mL/min). The nanosheets were concentrated into 50 g/L and freeze-dried at −60 °C before measurement. The heating and cooling rate for each sample was 20 °C/min, and the heating and cooling runs between −60 and 180 °C were recorded. Transmission Electron Microscopy (TEM). The TEM images were obtained using a JEM-2100 (JEOL Ltd. Japan) transmission electron microscope operated at an acceleration voltage of 200 kV. The sample was prepared by dropping the hyperbranched polymer nanosheets solution onto copper grids coated with a thin polymer film and then dried at 30 °C for 24 h. No staining treatment was performed for the measurement. Scanning Electron Microscopy (SEM). The SEM images were obtained using a JSM-7401F (JEOL Ltd., Japan) field emission scanning electron microscope operated at an acceleration voltage of 5 kV. The samples were prepared by dropping the hyperbranched polymer assemblies solution onto silica wafers and dried at 30 °C for 24 h. Then the samples were sputter-coated with gold to minimize charging. Atom Force Microscopy (AFM). The AFM images were obtained by using a scanning probe microscope (Nanoscope III, Digital instrument Co., Ltd.) equipped with a MikroMasch silicon cantilever (NSCII, radius
