Well-Defined Polymeric Double Helices with Solvent-Triggered

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Well-Defined Polymeric Double Helices with Solvent-Triggered Destruction from Amphiphilic Hairy-Like Nanoparticles Lin Cheng,†,‡,* Xiang Lin,† Fengyang Wang,† Biao Liu,† Jincheng Zhou,† Jie Li,† and Wenlian Li† †

Key Laboratory of Functional Molecular Solids, Ministry of Education of China, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China ‡ State Key Laboratory of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: Fabrication of nanostructures with great complexity and regularity is a topic of intense current interest because of their importance both in fundamental theories and applications. Here we report an interesting finding of the formation of ordered polymeric double helices with precise structures and high stability. These uniform double helices are hierarchically self-assembling results of undulated multicompartment cylinders which formed from primary amphiphilic mixed-shell nanoparticles. Diblock copolymers of P2VN-b-PAA (poly(vinylnaphthalene)-block-poly(acrylic acid) and PEO-b-PAA (poly(ethylene oxide)-block-poly(acrylic acid)) were noncovalently/electrostatically cross-linked into amphiphilic PEO/P2VN-shelled nanoparticles by 1,6-hexyldiamine in a common solvent DMF. When the common solvent changed into selective ones (water), undulated cylinders were facilely obtained due to their hydrophobic interparticles interactions. After a certain time aging, these undulated cylinders self-organized into finally thermodynamic-stable double helices with complex but regular superstructures. The conditions and processes of assembling were carefully examined through extensive experiments.

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INTRODUCTION Polymer (block copolymer) is one of the most important building blocks in nanoscience. Amphiphilic block copolymers can agglomerate in a block-selective solvent to form various nanometer-sized micelle-like aggregates with different morphologies,1−5 which include spherical, vesicular,6,7 tubular,8−10 and discal and cylindrical11−14 nano-objects. The shape diversity of such aggregates which could be well adjusted by the compositions of original copolymers and proper interfacial intensions,15−19 facilitates their potential applications in novel nanodevices, electronic/optical materials, and drug delivery.20−22 More recently, the high complex polymeric structures such as helices have attracted tremendous researchers’ attention.23−25 As we know, helical structures are fascinating and general in biological systems such as α-helices in protein and double-strand helices of DNA. Motivated by nature, scientists have devoted great efforts to study and obtain helical structures from synthetic polymers. But it is uneasy to artificially fabricate these special complex structures from the self-assembly of block copolymers in solution process. UP to now, the helices formed by self-assembly of polymers were limited except for a few peptide hybrids.26−28 For example, the Pochan and Wooley group29 prepared both single and double helical superstructures with helix pitch adjustable from ABC triblock copolymers of poly(acrylic acid)-block-poly(methyl acylate)-block-polystyrene. Liu30,31 reported their interesting discovery of double and sometimes triple helices assembled © 2013 American Chemical Society

from a linear triblock terpolymer of poly(butyl methacrylate)block-poly(2-cinnamoyloxyethyl methacrylate)-block-poly(tertbutyl acrylate) in three binary solvent mixtures. Theoretical calculation32 also predicted the formation and stability of the complex single/double/triple helices from ABC triblock copolymers due to packing constraints. Herein, we reported a novel approach to produce double helical nanostructures effectively from undulated multicompartment cylinders and showed new insight into their assembling mechanism. Amphiphilic particles with mixed poly(2-vinyl naphthalene)/poly(ethylene oxide) shells were prepared first by noncovalently/electrostatically cross-linked poly(acrylic acid) (PAA) blocks as core in a common solvent DMF. After changing the common solvent (DMF) to a selective one (water), polymeric undulated multicompartment cylinders were efficiently formed due to the cooperation of microphase separation and hydrophobic interparticles interactions between amphiphilic nanoparticles. These metastable multicompartment cylinders spontaneously assembled into double helices by cooperations of thermodynamics and kinetics during a certain period of sample aging (illustrated in Figure 1). The structures of amphiphilic nanoparticles, undulated cylinders and double helices are fully characterized by Fourier transform infrared Received: August 19, 2013 Revised: October 9, 2013 Published: October 28, 2013 8644

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polymer concentration is 1.0 mg/mL (see Experimental Procedures). Amphiphilic nanoparticles were thus prepared, with PEO/P2VN as the mixed shell and a noncovalently/ electrostatically cross-linked PAA/HDA network as the core. Hydrodynamic radius of these amphiphilic nanoparticles in DMF was about 285 nm with a PDI (⟨μ2/Γ2⟩) 0.155, measured by dynamic light scattering (Figure S1 in Supporting Information), which agree with the SEM results (Figure 2a).

Figure 1. Schematic illustration of the formation of double helix from primary amphiphilic mixed-shell nanoparticles.

spectroscopy (FTIR), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

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EXPERIMENTAL PROCEDURES

Materials. Block copolymers of poly(2-vinylnaphthalene)-b-poly(acrylic acid) (P2VN3,0800-b-PAA2,4000) and poly(ethylene oxide)-bpoly(acrylic acid) (PEO3500-b-PAA3800) were purchased from Polymer Source Inc. The molecular weight polydispersity index (PDI, Mw/Mn) of P2VN30800-b-PAA24000 is 1.09 and the PDI of PEO3500-b-PAA3800 is 1.12. The subscripts indicate the number-average molecular weights (Mn) of the respective blocks. 1,6-Hexyldiamine (HDA, Aldrich) of analytical purity was used without further treatment. Preparation of PEO/P2VN-Shelled Amphiphilic Mixed-Shell Micelles (MSMs). In a typical procedure, P2VN-b-PAA (1 mg) and PEO-b-PAA (1 mg) were dissolved in distilled DMF (2 mL) and the solution was stirred magnetically for 24h. Subsequently, proper amount of 1,6-hexyldiamine (3.6 mg, 0.031 mmol) was added into the mixture solution. The molar ratio of carboxyl/amidogen is about 1:5 and the total polymer concentration is 1.0 mg/mL. Blue 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. Preparation of Polymeric Cylinders and Double Helical Superstructures. Deionized water (2 mL) was added dropwise into a solution of MSMs (above-mentioned) in DMF (2 mL). Subsequently, the solution was dialyzed against deionized water for 1 day using a dialysis bag with a 14 kDa cutoff molecular weight. The total concentration of polymers was approximately 0.5 mg/mL in the resultant aqueous solution. After dialyzing against water, the MSMs changed into patchy particles for the confined microphase separation, and the patchy particles spontaneously self-assembled into undulated cylinders immediately. After a certain period (15 d) of sample aging, the double helix formed due to cooperations of thermodynamics and kinetics (During this process, there is no external stimulus (e.g., stirring) exists). To trigger the dissociation of the superstructures, the helices were cured in a common solvent THF for 12 h before TEM characterization.

Figure 2. (a) SEM images of the mixed-shell nanoparticles; (b) TEM images and (c) the diagram of the formation of undulated multicompartment cylinders.

It worth noted that these amphiphilic particles are not typical or simple micelles but with more complexed and jel-like flexible cores. Here, a proper mass ratio of P2VN-b-PAA/PEO-b-PAA (1/1) is essential to prepare targeted nanoparticles which could further regularly assemble in selective solvent as discussed in the Supporting Information (Figures S6 and S7). In our previous work, semicircular polymeric nanosheets were prepared from Janus nanoparticles due to intramicellar complexation between the PEO shell and the PAA core at pH 3.1 condition33 and polymeric dimers were produced from patched nanoparticles by using 1,4-butanediamine as crosslinker.34 We found interestingly that the bigger nanoparticles with a relative smaller curvature were obtained along with longer chain diamine used as cross-linker. The amphiphilic nanoparticles with relative bigger diameter and lower curvature in this case supplies more precisely microphase separation and adjustability in nanoparticles’ self-assembling to produce more complex superstructures.15,16,35 Owing to the association of flexible core and proper curvature, a dipole-like nanoparticle with two P2VN domains at opposite sides of the micellar sphere formation is reasonable (Figure 1), and the hydrophobic interactions of these P2VN domains prompt their further selforganization. Undulated Multicompartment Cylinders. Undulated multicompartment cylinders were facilely and efficiently obtained from above-mentioned primary amphiphilic nanoparticles after changing the solvent from DMF to water. Figure 2b is the TEM images of the samples, from which we can see a lot of cylindrical superstructrues with the width about 250 nm, which is well agree with the results obtained by SEM (see Figure S2 in Supporting Information). The typical undulated multicompartment structures showing clearly different contrast are distinguishable both in TEM and SEM images. The darker domains are attributed to P2VN segments, and the lighter ones

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RESULTS AND DISCUSSION Amphiphilic Nanoparticles. In this experiment, 1,6hexyldiamine (HDA) used as cross-linkers was added into a mixed solution of poly(2-vinylnaphthalene)-b-poly(acrylic acid) (P2VN-b-PAA) and poly(ethylene oxide)-b-poly(acrylic acid) (PEO-b-PAA) to produce polymeric nanoparticles with a proper mass ratio of P2VN-b-PAA to PEO-b-PAA being 1:1. The molar ratio of carboxyl/amidogen is 1:5 and the total 8645

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that the driving forces underlie the helix formation are uniform across the whole assembling procedure. Fluorescence microscope of the superstructure of the polymeric double helices was also observed for their aggregation of naphthyl groups (Figure S5). FTIR Characterization and Mechanism of the Formation of Helical Superstructures. For further understanding the mechanism of the formation of these uniform double helices, Fourier transform infrared spectroscopy (FTIR) was conducted to track the changing of polymers groups before and after assembling. The peak at 1561 cm−1 indicated that the carboxylic acid groups of PAA blocks are predominantly deprotonated with oppositely charged amines (Figure 4). Thus,

are assigned to PAA/HDA complexations. The morphology is well consistent with what was expected for the mixed-shellmicells assembling.36−38 These subtle structures further confirmed that the cylinders were assembled from the original amphiphilic nanoparticles by hydrophobic interparticles interactions as shown in schematic illustration (Figure 1 and Figure 2c). We can safely deduce that the amphiphilic nanoparticles reconstructed into dipole-like (divalent) nanoparticles due to insoluble P2VN blocks accumulation in water, and subsequently hydrophobic interparticles interactions made the cylinders forming. The noncovalently cross-linked core of flexible amphiphilic nanoparticles supplied the possibility of precisely microphase separation, and the interfacial tension between P2VN domain and water was driving force to construct these undulated cylinders, and solvated PEO chains protected their stability in aqueous (Figure 2c). Polymeric Double Helices. After the former multicompartment cylindrical micelles suspension was sealed tightly and aged for 15d at room temperature without stirring, the helical superstructures were thus forming. Figures 3,S3, and S4

Figure 4. FTIR spectra of pure block copolymers of P2VN-b-PAA (1), PEO-b-PAA (2), and the double helical superstructures (3). The signals at 1101, 956, and 848 cm−1 in the spectrum of the helices are assigned to the stretching vibration of the C−O bond in the PEO blocks. A signal at 1561 cm−1 could be seen clearly, which confirms the existence of COO− and protonated N−H. The stretching vibration of N−H is overlapped with that of the associated O−H groups at higher wavenumber ∼3400 cm−1.

the amphiphilic nanoparticles with P2VN/PEO blocks as shell and noncovalently/electrostatically cross-linked PAA/HDA network as core were formed. After dialyzing against water by using 14 KD cutoff membranes, the excess amine was thoroughly moved away, which is supported by FTIR data and our previous reports. 33,34 Owing to the confined microphase separation and hydrophobic interparticles interactions, undulated multicompartment cylinders finally fabricated as discussed above. Pochan et al. reported their findings that the single strand cylinders bended and coiled into helices with PAA chains on their surface, and they proposed both the uniaxial compression and electrostatic repulsion along the cylinder surface were possible driving forces.29 Extremely different to that, in this study, the uniform double helices formed from mixture of two simple diblock copolymers (P2VN-b-PAA and PEO-b-PAA) stably existed in pure water, which was also different to Liu’s binary solvent mixtures.30,31 We believe that this is the first report on the fabrication of uniform double helices from diblock copolymers solution hierarchical self-assembly. A reasonable speculation is that, when the undulated multicompartment cylinders formed from spontaneous selfassembly of original amphiphilic nanoparticles, there are still some P2VN domains not protected enough by solvated PEO chains and exposed to water as shown in Figure 1 (green segment represents possible P2VN domain). These metastable undulated cylinders have a strong inclination to associate with each other as well as uniaxial compression along the long axis of

Figure 3. SEM images (a) and TEM images (b, c) of double helices formed from undulated cylinders; (d) TEM images of double helices after ultrasonic treatment for 30 min (the gaps between two cylinders are distinguishable as red circles).

are the SEM and TEM images of the formed polymeric double helices. These double helices all showed their uniformity and regularity with two closely packed cylinders. The breadth of these double helices is 495 nm, which is about 2-fold to their single constituted cylinder’s width 254 nm as shown in SEM images (Figure 3a). The width of single cylinder in double helices both in TEM and SEM images is very near to that of the undulated cylinders in Figure 2b. All these further confirmed that the double helices were formed from those original multicompartment cylinders. These uniform double helices show regular helix pitch about 620 nm in TEM images in Figure 3, parts b and c and Figure S4, which is slightly smaller than that calculated from SEM images in Figure 3a. The regular micrometer-long helical superstructures in Figure 3b indicated 8646

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the cylinders. Thus, thermostable and regular double helices well formed from two closely bound cylinders at last with a relative long time aging. To confirm this mechanism of the formation of double helices from primary amphiphilic particles, an experiment of the double helices disassembling was conducted. Figure 5 shows the results of the destroyed double

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ASSOCIATED CONTENT

S Supporting Information *

Materials, synthesis of MSMs, undulated multicompartment cylinders, and helical superstructures, characterization methods, DLS profiles, and more SEM and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (L.C.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (20904001); NSF of Anhui province (090414198) and Foundation of State Key Laboratory of Molecular Engineering of Polymers (Fudan University) (K2012-02).

Figure 5. TEM images (a, b) of dissociated double helices after curing in common solvent THF for 12 h.

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helices after curing them in a common solvent THF for 12 h. From which there are still a lot of deformed nanoparticles existing after the helices dissociated. These result further demonstrated the formation pathways of undulated cylinders and double helices from primary particles. It is worth noting that the molecular chirality and specific intermolecular interactions are not necessities for the constructing of helical superstructures in this experiment. There are right-handed and left-handed double helices coexisting as shown in Figure 3 and Figure S4. It is necessary to study in next intensive investigation that whether an external stimulus (e.g., shear force) could control the chirality of these double helices. The stability of the uniform double helices was further confirmed. After 30 min ultrasonic treatment, the contours of these double helices are almost intact as shown in Figure 3d.

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CONCLUSION In summary, we have successfully prepared highly swollen amphiphilic nanoparticles based on a HDA cross-linked 1:1 (w/w) mixture of P2VN-b-PAA and PEO-b-PAA diblock copolymers in DMF. Changing the common solvent DMF to selective one (water) led to the subtle microphase separation of the naoparticles shells and resulted in symmetric nanoparticles with two small P2VN domains at both opposite sides. Owing to the subsequent hydrophobic intermicelles interactions of P2VN domains, undulated multicompartment cylinders were facilely formed. After a certain period aging, those metastable cylinders wind with each other into uniform double helices. The reasonable driving forces are association of uniaxial compression along the long axis of the cylinders and hydrophobic interaction (even π−π bonding interaction) of expressible P2VN domains, and the solvated PEO chains around the helix ensured their stability in aqueous. In brief, an efficient approach was realized to construct complexed superstructures (such as double helix) from simple building blocks (amphiphilic nanoparticles) by hierarchical and programmable self-assembling method. This assembling strategy may throw light on the mechanism of forming double and triple helices in nature, and open up a promising way to fabricate varies functional superstructures with increased topological complexity and regiospecificity. 8647

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