Facile Preparation of Polymeric Dimers from Amphiphilic Patchy Particles

Communication
Macromolecular
Rapid Communications

933
DOI: 10.1002/marc.201100787 1.Introduction In the past decade, amphiphilic nanoparticles, which can be used as building blocks to fabricate complex super-structures, [ 1–4 ] have attracted much attention in academic and technological fi elds, due to their exceptional prop-erties. [ 3 , 5–7 ] These superstructures, such as tubular and sheet-like superstructures, [ 8 ]cylinders, [ 4 , 9–12 , 24 ]worm-like networks, [ 13 , 14 ]nanocavities [ 15 , 16 ] etc., which are dif-ferent to the structures of single nanoparticles, [ 17 ]have eximious properties in novel nanodevices and electronic/optical materials. [ 18–21 ] Recently, amphiphilic nanopar-ticles and their self-assembly into superstructures, such as the preparation of patchy nanoparticles from ABC tri-block copolymers and their self-assembly into undulated multicompartment cylinders, have been reported by many research groups. [ 22–29 ] It is well known that the preparation and the applications of these superstructures are very chal-lenging and signifi cant. However, superstructures with a confi ned number of particles, such as polymeric dimers and trimers, are rarely reported. Herein, we report an efficient and simple method for the fabrication of patchy particles (Janus particles) from mixed-shell micelles (MSMs) and the template-free self-assembly of these patchy particles into nano-scale polymeric dimers. In our previous work, semi-circular polymeric nanosheets were prepared from Janus nanoparticles due to intra-micellar complexa-tion between the PEO shell and the PAA core at pH 3.1 conditions. [ 8 ] Using a different method, amphiphilic micelles with mixed poly-(2-vinyl naphthalene)/poly-(ethylene oxide) shells were prepared in this work by non-covalent crosslinking of poly(acrylic acid) (PAA) blocks via the addition of 1,4-diaminobutane (BDA) to a mixed solution of poly-(2-vinylnaphthalene)- b-poly-(acrylic acid) (P2VN 38000-b-PAA24000 ) and poly(ethylene oxide)- b -poly(acrylic acid) (PEO 3500-b-PAA3800)(themass ratio of P2VN- b -PAA to PEO- b -PAA is 1:1); the sub-scripts denote the molecular weights of the respective blocks. The molar ratio of carboxyl/amidogen was 1:2 and the total polymer concentration was 1.0 mg ml − 1.After changing the common solvent (DMF) to a selective one (water), polymeric dimers were efficiently formed due to the cooperation of micro-phase separation and hydrophobic inter-particle interactions between MSMs. The structures of MSMs and dimers were fully char-acterized by dynamic light scattering (DLS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A straightforward strategy for assembling polymeric dimers from amphiphilic nanoparticles is reported. Amphiphilic polymeric nanoparticles with a mixed-shell of PEO/P2VN blocks and a fl exible core of PAA blocks are fabricated by a non-covalent crosslinking method. Uniform polymeric dimers are effi ciently and simply obtained via hydrophobic interactions under optimized conditions in selective solvent. The steric hindrance generated by reorganization of hydrophilic polymer brushes during the interparticle association is critical for morphological selectivity in the assembly. General applicability offers the possibility to organize functional NPs into superstructures with well-defi ned geometry and association numbers. Facile Preparation of Polymeric Dimers from Amphiphilic Patchy Particles Fengyang Wang, Lin Cheng,* Tianhong Chen, Dongsheng Zhu, Qiang Wen, Shaowu Wang *F.Wang,Dr.L.Cheng,T.Chen,D.Zhu,Q.Wen,Dr.S.Wang The Key Laboratory of Functional Molecular Solids of Ministry of Education of China; College of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, ChinaE-mail: chenglin@mail.ahnu.edu.cn; swwang@mail.ahnu.edu.cn Macromol. Rapid Commun. 2012, 33, 933−937© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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www.MaterialsViews.com934Blue opalescence appeared immediately, which indicated the formation of aggregates in the solution. The resultant solution was also stirred magnetically for 24 h before DLS, SEM and TEM measurements. 2.6. Preparation of Polymeric Dimers Deionized water (2 mL) was added dropwise into a solution of MSMs in DMF (2 mL). Subsequently, the solution was dialyzed against deionized water for 1 d using a dialysis bag with a 14 kDa cut-off molecular weight. The total concentration of polymers was approximately 0.25 mg mL − 1 in the resultant aqueous solu-tion. After dialyzing against water, the MSMs changed into patchy particles for the confi ned micro-phase separation, and the patchy particles self-assembled into dimers simultaneously. These dimer superstructures were fully characterized by FTIR (see Supporting Information), SEM and TEM. 3. Results and Discussion In our pre-designed procedure, a specifi c mass ratio of P2VN- b-PAA/PEO- b -PAA of 1:1 was adopted. As shown previously, amphiphilic MSMs collapse into patchy parti-cles as a result of phase separation of chemically different blocks in the shells [ 8 , 20 ] when the solvent is converted from a common solvent to a selective one. Subsequently, the patchy particles formed in a selective solvent self-assemble into superstructures via hydrophobic inter-particle inter-actions. [ 26 ] Figure 1 a shows an SEM image of polymeric dimers (inset is the magnifi ed image). The SEM image shows a large number of uniform dimers, which are sim-ilar to dumbbell structures. It is apparent that these uni-form dimers are constructed from two patchy particles with different degrees of fusion and that their size ranges from 0.7 μ m to 1.1 μm. To gain further insight into the formation of the dimer superstructures, we used transmission electron microscopy (TEM) to probe the samples. The TEM image (Figure 1 b) also shows the formation of polymeric dimers 2. Experimental Section 2.1. Materials and Instruments Block copolymers of P2VN 38000-b-PAA24000 and PEO 3500-b-PAA3800 were purchased from Polymer Source Inc. The molecular weight polydispersity index (PDI, Mw/Mn)ofP2VN38000-b-PAA24000was1.09 and the PDI of PEO 3500-b-PAA3800 was 1.12. The subscripts indicate the molecular weights of the respective blocks. N,N -dimethylformamide (Shanghai Chemical Reagents Co.) of analyt-ical purity was purifi ed by reduced pressure distillation over CaH 2. 1,3-propanediamine and 1,4-butanediamine (Aladdin Chemistry Co.) were directly employed without further treatment. 2.2. Dynamic Light Scattering (DLS) A commercial spectrometer (ALV/DLS/SLS-5022F) equipped with a multi- τ digital time correlator (ALV5000) and a cylindrical 22 mW Uniphase He–Ne laser ( λ = 632 nm) as the light source was used. All DLS measurements were performed at 90 degree (angle). The 〈Rh〉 and polydispersity index (PDI, i.e., 〈 μ 2 / Γ 2 〉) were obtained by a CONTIN analysis mode. All solutions were fi ltered through a 0.45 μ m or 0.8 μ m PTFE Millipore filter. 2.3. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) SEM observations were conducted using a Hitachi S-4800 fi eld emission scanning electron microscope at room temperature. Samples for SEM imaging were prepared by natural evapora-tion of micelle and dimer aqueous solution at room temperature overnight, then coating with gold. TEM observations were con-ducted on a Jeol TEM 2010 electron microscope at an acceleration voltage of 200 kV. Samples for TEM observations were prepared by depositing a drop of the aqueous suspensions onto a carbon-coated copper grid. Excess solution was blotted away using a filterpaper. 2.4. Preparation of P2VN Symmetrical Shell Micelles In a typical procedure, P2VN- b -PAA (2 mg) was dissolved in dis-tilled DMF (2 mL) and the solution was stirred magnetically for 24 h. The molecular solubilization of the block copolymers in DMF was confi rmed by DLS measurements. Subsequently, 1,3-propan-ediamine (0.9 μ L) or 1,4-butanediamine (1.2 μ L) was added to the solution of diblock copolymer and blue opalescence appeared immediately, which suggested the formation of aggregates in the solution. The resultant solution was stirred magnetically for 24 h before SEM measurements (see Figure S4 in the Supporting Information). 2.5. Preparation of PEO/P2VN-Shelled Amphiphilic MSMs Similarly,P2VN-b -PAA (1 mg) and PEO- b -PAA (1 mg) were dis-solved in distilled DMF (2 mL) and the solution was stirred mag-netically for 24 h. Subsequently 1,3-propanediamine (1.0 μL)or1,4-butanediamine (1.3 μ L) was added into the mixture solution. Figure1 . a) SEM images and b) TEM images of polymeric dimers obtained after switching the solvent from DMF to water. Magni-fi ed image in a) is shown as inset and scale bar indicates 500 nm. The nano-objectives are obtained using 1, 4-diaminobutane as a crosslinking agent. Macromol. Rapid Commun. 2012, 33, 933−937© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim