Hairy cylinders based on a coil-comb-coil copolymer

Abstract

Self-assembled complex structures with finely controlled arrangements are universal in biological systems but the corresponding formation mechanism usually remains unknown. Herein we report an example using fully synthetic copolymers with complex molecular structure and fuzzy hydrophilic/hydrophobic boundary to mimic multiple levels of self-assembly. We report the formation and possible mechanism insight of a complex hairy cylinder that is self-assembled from linear cylinders which are formed from an amphiphilic coil-comb-coil copolymer, PTMA₄₂-b-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-b-PTMA₄₂ (see Schemes 1 and 2). The copolymer was synthesized by two-step reversible addition–fragmentation chain transfer (RAFT) polymerization. The “comb” is the H-shaped [(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄ and the “coils” are linear PTMA₄₂. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) studies revealed the subtle nanostructure of hairy cylinders: a “head” made up of densely compacted linear cylinders and tens of surrounding “tentacles” (linear cylinders). The synergetic effect between the coil-comb-coil structure and the coils facilitates the formation of hairy cylinders. A control experiment where a coil-only copolymer without comb-like structure confirms that no hairy cylinders formed in this case. Compared with another coil (control), PTMA coils attached on two ends of the comb have stronger synergetic effect and then facilitate the formation of well-defined hairy cylinders. Finally, “embryo”, “baby”, “adult” and “ageing” hairy cylinders were observed by increasing the ratio of water to DMF during self-assembly. Overall, our study demonstrates a new insight for understanding the formation of hierarchical polymeric nanostructures and a new principle for designing complex polymeric nanoparticles.

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Introduction

Living systems are a result of a precise hierarchical self-assembly of molecules and macromolecules, and constructed with specific chemical signatures that direct supramolecular interaction between themselves and/or with water.¹ This self-assembly process is ubiquitous in nature and is the key of many biological transformations because it allows the formation of mesoscale structures with exquisite spatial and temporal control starting from the molecular level.

Amphiphilic block copolymers, which consist of hydrophilic and hydrophobic parts in one polymer, can self-assemble into various nanostructures in selective solvents.^2–4 For example, micelles,^5,6 cylinders,^7,8 vesicles,^9–12 toroids,¹³ nanosheets,¹⁴ and many other nanostructures^15–18 have been made from amphiphilic copolymers. Generally, the subtle nanostructure can be controlled by chemical structures of polymers,¹⁹ hydrophilic/hydrophobic ratios of polymers,^20,21 and self-assembly conditions such as solvent properties,^22–24 concentrations of polymers,²⁵ and crystallizations of polymers.^26–29 For example, spheres, rods, lamellae and vesicles were obtained by self-assembly of the same diblock copolymer at different water contents.³⁰

Hierarchical structures self-assembled from individual copolymer nanoparticles have attracted much interest because this bottom-up strategy is promising for constructing complex materials. Superstructures have been prepared by hierarchical assembly of spherical nanoparticles,³¹ multicompartment micelles,³² cylindrical co-micelles,^33,34 and nanofibers.³⁵ For example, Winnik et al. reported the use of colloidal polystyrene microbeads as a sacrificial template to create a hierarchical nanofibrous network coating consisting of linear PFS₃₀-b-P2VP₃₀₀ block copolymer micelles and the fibers of the network could be elongated by crystallization-driven self-assembly.³⁶ Recently, amphiphilic copolymers with complex structures such as graft,^37–39 star block^40,41 and dendritic^42–44 copolymers have been synthesized to afford various nanostructures by self-assembly. For example, Yan prepared unilamellar vesicles based on self-assembly of hyperbranched polymers.⁴⁵ Zhao synthesized a comb-like copolymer, which can self-assemble into toroidal micelles at 37 °C, and then necklaces upon external stimuli.⁴⁶ Müller and coworkers reported the hierarchical self-assembly of an ABC miktoarm star terpolymer comprising a polybutadiene (PB), a poly(tert-butyl methacrylate), and a poly(N-methyl-2-vinylpyridinium) segment.⁴⁷ The miktoarm architecture in conjunction with an increasing ratio of triiodide versus iodide counterions allows for a stepwise assembly of spherical micelles as initial building blocks into cylindrical structures and superstructures thereof. These examples indicate that the hierarchical nanostructures self-assembled from polymers with a complex structure are limitless and the self-assembly mechanism is very important for guiding the design of hierarchical nanostructures.

Herein, we present the preparation and possible mechanism insight of a novel hairy cylinders evolved from linear cylinders based on a coil-comb-coil copolymer, [poly((2-tetrahydrofuranyloxy)ethyl methacrylate)-block-[poly(α-methoxy-ω-vinylbenzyl poly(ethylene oxide))-alternating-poly(maleimidic poly(​ε-caprolactone))]-S-S-[poly(α-methoxy-ω-vinylbenzyl poly(ethylene oxide))-alternating-poly(maleimidic poly(​ε-caprolactone))]-block-poly((2-tetrahydrofuranyloxy)ethyl methacrylate)] ([PTMA₄₂-b-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-b-PTMA₄₂], see Schemes 1 and 2). The hairy cylinders have a “head” made up of densely compacted linear cylinders and many “tentacles” (linear cylinders) surrounding the “head”. As illustrated in Scheme 1, embryo linear cylinders are first formed at the beginning of self-assembly of the coil-comb-coil copolymer, and then the “embryo” cylinders are elongated, branched and aggregated to form “baby” hairy cylinders and finally well-defined hairy cylinders. We will discuss the importance of the synergetic effect between the coil-comb-coil molecular structure and its coil for the hairy cylinders formation because it can confine the arrangement of copolymers by the steric hindrance and promote the elongation of cylinders. The PTMA coil chains attached on two ends of the comb promote the formation of well-defined hairy cylinder as a result of stronger synergetic effect with the coil-comb-coil molecular structure.

Scheme 1 Illustration of the formation process of hairy cylinders with a head and surrounding tentacles from a coil-comb-coil copolymer PTMA₄₂-b-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-b-PTMA₄₂ (C1).

Scheme 2 Synthetic route to coil-comb-coil copolymer C1 by RAFT.

Experimental

Materials

All solvents, monomers, and other chemicals were purchased from Sigma-Aldrich unless otherwise stated. AIBN was recrystallized twice from ethanol. Dioxane was dried and distilled over CaH₂. P1 ([(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄)⁴⁸ and TMA⁴⁹ were synthesized and purified according to literature procedures.

Characterization

¹H NMR (400 MHz) spectra were recorded on a Varian spectrometer at 25 °C using CDCl₃ as a solvent. Apparent molecular weight (M_n, GPC) and polydispersity (M_w/M_n) of linear polymer (for L1) were measured on a Waters 150-C GPC using three Ultrastyragel columns (pore size 50, 100, and 1000 nm, with molecular weight ranges of 100–10 000, 500–30 000, and 5000–600000 g mol⁻¹, respectively) with 10 μm bead size at 35 °C. THF was used as an elution at a flow rate of 1.0 mL min⁻¹, and the samples were calibrated with PMMA standard samples. GPC with multiple angle laser scattering detection (GPC-MALLS) was used to determine the number-average molecular weight (M_n,_LS) of nonlinear polymers (for P1, C1, C2 and C3), in which GPC was conducted in DMF at 40 °C with a flow rate of 1.0 mL min⁻¹, and three MZ-Gel SDplus columns (pore size 103, 104 and 105 Å, with molecular weight ranges of 1000–40 000, 4000–500000, and 10 000–2 000 000, respectively) with 10 μm of bead size were used. Detection systems consisted of a RI detector (Optilab rEX) and a multiangle (14–145°) laser light scattering detector (DAWN HELEOS) with the He–Ne light wavelength at 658.0 nm. The refractive index increment dn/dc for samples were measured off-line by Optilab rEX refractive index detector (λ = 658 nm) at 25 °C using a series of different concentration solutions. Data were collected and processed by use of ASTRA software from Wyatt Technology, and molecular weights were determined by the triple detection method. FT-IR spectra were recorded on a Perkin-Elmer 2000 spectrometer using KBr discs. DLS measurements were carried out at 25 °C with a fixed scattering angle of 90° and Zetasizer Nano series instrument (Malvern Instruments ZS 90) which equipped with a multipurpose autotitrator (MPT-2). Each measurement was conducted for three runs. Transmission electron microscopy (TEM) images of C1 and C2 cylinders were obtained using a JEM-2100F electron microscope operating at an acceleration voltage of 200 kV equipped with a Gatan 894 Ultrascan 1k CCD camera. A carbon-coated copper grid was pre-disposed for 120 s with a plasma cleaner. The cylinder solution (8 μL) was dropped onto it and left dried at ambient temperature. The samples-loaded copper grid was then stained for 90 s with 1% aqueous phosphotungstic acid solution at pH 7.0. The atomic force microscope (AFM) experiments were carried out on Bruker Nanoscope VIII Multi-Mode operated in ScanAsyst in Air Mode at room temperature. Bruker silicon Tip on Nitride Lever cantilevers (T: 0.65 μm, L: 115 μm, W: 25 μm, fo: 70 μm, k: 0.4 N m⁻¹).

Synthesis of PTMA₄₂-b-((St-PEG₁₆)-alt-PCL₂₁)₄-S-S-((St-PEG₁₆)-alt-PCL₂₁)₄-b-PTMA₄₂ coil-comb-coil copolymer (C1)

In a typical experiment, P1 (309 mg, 10.0 μmol), TMA (250 mg, 1.25 mmol), AIBN (1.00 mg, 6.10 μmol) were added to a Schlenk tube, and then dry dioxane was added until the total volume was 2.5 mL. The contents were degassed with bubbled nitrogen for 15 min and subjected to polymerization at 70 °C for 24 h. The polymerization solution was concentrated and precipitated into diethyl ether, and 472 mg (65.2% conversion) of C1 was obtained. PMEMA₄₄-b-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-b-PMEMA₄₄ (C2) was synthesized and purified according to a similar procedure.

Synthesis of C3 by cleavage of C1

C3 was obtained by treatment of C1 in the presence of 10 mM of DTT overnight and lyophilized after dialysis against deionized water.

Synthesis of PTMA homopolymer by ATRP

Ethyl α-bromoisobutyrate (20.0 mg, 0.100 mmol), TMA (900 mg, 4.52 mmol), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) (17.5 mg, 0.100 mmol) were added to methanol (1.5 mL), and the mixture was degassed for 30 min at room temperature. Cu(I)Br (14.5 mg, 0.100 mmol) was quickly added into the flask. Then the reaction was conducted at 50 °C for 8 h. The mixture was passed through a neutral Al₂O₃ column with dichloromethane and precipitated in n-hexane. PTMA was obtained after drying in a vacuum oven at 30 °C.

Synthesis of linear block copolymer L1: PEG₄₃-b-PCL₆₀-b-PTMA₃₀

PEG-b-PCL and PEG-b-PCL-Br were synthesized according to a previous method.⁵⁰ TMA was then polymerized by ATRP using PEG-b-PCL-Br as macro-initiator. The final polymer composition was PEG₄₃-b-PCL₆₀-b-PTMA₃₀ (determined by ¹H NMR).

Self-assembly of polymers

Typically, polymers were dissolved in DMF to form a homogenous solution with a concentration of 1.0 mg mL⁻¹ and distilled water was added dropwise under magnetic stirring. The solution was kept stirring for additional 30 minutes. Subsequently, DMF was removed by dialysis against deionized water in a dialysis tubing with a molecular weight cut off from 8000 to 14 000.

Turbidity test

The polymers were dissolved in DMF (4 mL), and the concentrations were 1.0, 0.473, 0.351 and 0.162 mg mL⁻¹ for C1, MI-PCL, PTMA and St-PEG, respectively. Then water (0.25 mL) was added gradually into the solution and the total volume of water added was 12 mL. After stirring for several minutes, the absorbance at 650 nm were collected for the turbidity analysis.

Results and discussion

Synthesis of PTMA₄₂-b-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄₁-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-b-PTMA₄₂ coil-comb-coil copolymer (C1)

A two-step RAFT polymerization was used to fabricate the coil-comb-coil copolymer (Scheme 2). First, an H-shaped polymer comb [(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄ (P1) was synthesized according to a previously reported protocol.⁴⁸ Second, PTMA₄₂-b-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-b-PTMA₄₂ coil-comb-coil copolymer (C1) was prepared by P1-mediated chain extension polymerization with TMA. Typical gel permeation chromatography, IR spectrum and ¹H NMR data were provided in Table 1, Fig. 1, S1 and S2.† As expected from controlled radical polymerization, their M_n and M_n,NMR values were consistent with each other. These results revealed the chain extension polymerization was efficiently performed to generate the target coil-comb-coil copolymers.

Table 1 Results for the synthesis of disulfide-linked coil-comb-coil copolymers by RAFT and linear copolymer L1 by ATRP

Polymer	C^a%	Mn^b	Mw/Mn^b	Mn,NMR^c
a Monomer conversion determined by gravimetry.b Number-average molecular weight and polydispersity determined by GPC-MALLS (for P1, C1 and C2) or GPC (for L1).c Number-average molecular weight determined by ^1H NMR analysis.
P1	85.6	30 500	1.14	30 900
C1	65.2	48 800	1.14	47 600
C2	67.5	44 000	1.13	43 600
L1	64.4	9680	1.27	14 300

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Fig. 1 GPC traces of polymers P1, C1, C2 and L1.

Self-assembly of C1 into hairy cylinders

The coil-comb-coil copolymer (C1) can self-assemble into hairy cylinders in H₂O/DMF (2 : 1; v/v) mixture solvent followed by dialysis to remove DMF. TEM analysis was conducted to reveal the morphology of hairy cylinders (Fig. 2a and 3e) and all samples were stained by phosphotungstic acid (PTA). The hairy cylinders have a compact head (ca. 200 nm) with many surrounding tentacles. The hydrodynamic diameter (D_h) of the hairy cylinders by DLS is 304 nm (PDI = 0.176, see Fig. 4d), which is consistent with the TEM results. A similar hairy cylinder morphology was also confirmed by AFM with a width of about 260 nm and height of 30 nm, clearly showing the head of hairy cylinders (Fig. 2c–e). To clearly reveal the tentacles, the colour temperature of the image in Fig. 2d is decreased and the tentacles (blue) can be clearly seen surrounded from the head of hairy cylinders and even attached onto the head (Fig. 2e). More TEM and AFM images of hairy cylinders are provided in Fig. S3.†

Fig. 2 TEM and AFM images of an “adult” hairy cylinder self-assembled from coil-comb-coil copolymer C1: (a) typical TEM image of hairy cylinders, showing a densely packed head and surrounding tentacles; (b) inverse fast Fourier transform (IFFT) image of (a); (c) corresponding AFM image of a hairy cylinder and (d) 3D view of AFM image; (e) a lower color temperature treatment of (d) to highlight the surrounding tentacles (blue). TEM images at low magnification were shown in Fig. 3e.

Fig. 3 TEM images of C1 cylinders at different stages developed at different volume ratios of water to DMF (r): (a) “embryo hairy cylinders”: only linear cylinders (r = 0.5 : 1); (b) an enlarged image from (a), clearly showing the physical bridges of adjacent cylinders; (c) “baby hairy cylinders”: the heads of hairy cylinders appeared with some coexisting linear cylinders (r = 0.7 : 1); (d) “baby hairy cylinders”: hairy cylinders with more surrounding tentacles appeared (r = 1 : 1); (e) “adult hairy cylinders”: only hairy cylinders (r = 2 : 1); (f) “ageing hairy cylinders”: dissociation of tentacles appeared (r = 3 : 1). DMF was removed by dialysis after self-assembly.

Fig. 4 DLS data of C1 cylinders at different volume ratios of water to DMF (r).

The ratio of water to DMF during self-assembly determines the developing stages of C1 hairy cylinders, as confirmed by TEM studies in Fig. 4. With a low water/DMF ratio (r = 0.5 : 1), long linear cylinders are formed (Fig. 3a) and the physical bridges of adjacent cylinders can be seen clearly (Fig. 3b). Increasing r to 0.7 : 1, some linear cylinders begin to branch, aggregate and form the head of hairy cylinders (Fig. 3c and Scheme 1). Also, long cylinders and the head without surrounding cylinders appeared. Increasing r to 1 : 1, tentacles are grown from the head to form ill-defined baby hairy cylinders, with few coexisting linear cylinders (Fig. 3d and Scheme 1). When r is further increased to 2 : 1, well-defined hairy cylinders are formed with a densely-compacted head and many surrounding tentacles (Fig. 3e and Scheme 1). When more water is added (r = 3 : 1), the hairy cylinders tend to be dissociated to form “ageing” hairy cylinders (Fig. 3f and Scheme 1). The turbidity studies of the cylinder formation process were also discussed in the ESI (see Fig. S4†). It is noteworthy that the absorbance decreased quickly after r is up to 0.5, corresponding to the morphology transition detected by TEM when the linear cylinders turned into hairy cylinders (r increases from 0.5 to 0.7).

The above water content-dependent evolution process was also monitored by DLS (Fig. 4). The D_h of linear cylinders (r = 0.5 : 1) is 907 nm, which is consistent with TEM studies. Increasing r to 0.7 : 1, the D_h decreased sharply to 204 nm, as a result of the branching, aggregation and formation of the head of hairy cylinders, as revealed by the TEM studies. This morphology transition is also reflected by the increase in the diameters of cylinders at different r (Table 2), which will be discussed later.

Table 2 Evolution stages of hairy cylinders at various volume ratio of water to DMF

H2O : DMF^a	Evolution stage^b	Dh^c	Dcylinder^d
a The volume ratio of water to DMF during self-assembly.b The developing stages of hairy cylinders.c The hydrophobic diameter (nm) monitored by DLS.d The number-averaged mean diameter (nm) of cylinders.
0.5 : 1	“Embryo”	907	10.2 ± 0.9
0.7 : 1	“Baby”	204	14.2 ± 2.4
1 : 1	“Baby”	231	16.5 ± 1.4
2 : 1	“Adult”	304	18.9 ± 2.0
3 : 1	“Ageing”	390	20.6 ± 2.7

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Possible mechanism insight of hairy cylinders formation

We suppose that the coil-comb-coil molecular structure of copolymer is essential for the formation of hairy cylinders and there is a synergetic effect between the molecular structure and its coil on determining the formation of well-defined hairy cylinders.

First, the coil-comb-coil molecular structure of copolymer is essential for the formation of hairy cylinders. For example, a coil-only copolymer [PEG₄₃-b-PCL₆₀-b-PTMA₃₀] (L1) without the comb but with similar block compositions to that of C1, was synthesized (see Fig. 1, Table 1 and Fig. S5†) and self-assembled at the same condition (Fig. 5a and b). Besides other ill-defined irregular aggregates, no hairy structures were formed.

Fig. 5 Chemical structures and corresponding TEM images of self-assembled nanostructures: (a) a coil copolymer L1 with similar block composition to that of coil-comb-coil copolymer C1 and (b) corresponding TEM images of L1 self-assemblies; (c) coil-comb copolymer C3 by cleavage of C1 and (d) corresponding TEM images of C3 self-assemblies. The self-assembly of L1 and C3 was conducted in H₂O/DMF (2 : 1; v/v).

Furthermore, another coil-comb-coil copolymer with the same “comb body” (P1) but different “coil” of poly(methoxyethyl methacrylate) (PMEMA₄₄) was synthesized as the control. This PMEMA₄₄-b-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-b-PMEMA₄₄ coil-comb-coil copolymer (C2) was prepared by a similar protocol to C1 (see Fig. 1, Table 1, and Fig. S1 and S6†). The self-assembly of C2 was conducted at the same conditions as C1. TEM studies in Fig. 6b revealed that some hairy structures similar to C1 hairy cylinders in Fig. 2e were self-assembled from C2. However, the hairy cylinders prepared from C2 (with the comb structure but without PTMA) are not as good as that from C1 (with both comb structure and PTMA), indicating the synergistic effect between the comb-like molecular structure and PTMA coil for the formation of well-defined hairy cylinders.

Fig. 6 (a) Chemical structure of a control coil-comb-coil copolymer (C2): PMEMA₄₄-b-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-b-PMEMA₄₄; (b–d) TEM images of cylinders and ill-defined hairy cylinders formed at different water contents (r: H₂O/DMF); (b) r = 1 : 1; (c) r = 2 : 1; (d) r = 3 : 1; (e) corresponding DLS data.

To further verify this synergic effect, we cleaved the disulfide bond in C1 by DTT-triggered reduction to afford a coil-comb copolymer, C3 (see Table 1, Fig. 5c, and S7†). Compared with C1, the coil-comb C3 has half amount of PTMA chains and comb, suggesting a compromised property for generating hairy cylinders. TEM studies in Fig. 5d confirmed that hairy cylinders from C3 are better than C2 but worse than C1, as expected.

Based on the above results, we proposed the possible formation mechanism of hairy cylinders, as shown in Schemes 1 and 3. For C1, linear cylinders are formed with the hydrophobic PCL side chains and PTMA coil as the hydrophobic inner core whereas the hydrophilic PEG as the outer coronas at the beginning of self-assembly. Then cylinders elongate to form long linear cylinders and some physical bridges exist in adjacent linear cylinders. With more and more water adding, the linear cylinders are not stable in DMF/H₂O system owing to the high interfacial potential. So these linear cylinders tend to fuse together to decrease the interfacial potential.¹⁶ As the diameter of cylinders increases (Table 2), more bridges exist and certain branched cylinders are formed. These linear and branched cylinders further self-aggregated into baby hairy cylinders and finally well-defined hairy cylinders. The driving force is the equilibrium of interfacial potential contributed by the crystallinity of the PCL comb and PTMA coil at the DMF/water mixture, leading to the morphology transition.

Scheme 3 Possible formation mechanism of hairy cylinders.

The synergetic effect between the coil-comb-coil structure and the coil significantly influences the formation of hairy cylinders. The coil-come-coil structure has an important impact on the cylinders elongation because of the steric hindrance and promotes cylinders branching. As calculated in Scheme 2, the contour length of the main chain of C1 is ca. 18 nm, which is the same as that of PCL (ca. 18.4 nm). The more flexibility of coil-comb structure (C3) is not favoured for cylinders elongation because it can easily adjust its conformation to end a cylinder. So there are more hairy cylinders found in C1 and C2 than C3 (see Scheme S1†). More importantly, according to our previous report, the comb-like copolymer (without a coil) P1 can only form cylinder rafts and cocoons (enclosure of cylinder rafts).⁴⁸ After a coil was introduced to P1, the copolymer become more hydrophobic with a higher interfacial potential and the synergetic effect with a coil promotes the formation of hairy cylinders. The stronger synergetic effect with a PTMA coil is liable to fabricate well-defined hairy cylinders and the reason may be more hydrophobic effect and different steric hindrance compared to PMEMA coil.

Conclusions

In summary, we prepare novel hairy cylinders based on a coil-comb-coil copolymer, PTMA₄₂-b-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-S-S-[(St-PEG₁₆)-alt-(MI-PCL₂₁)]₄-b-PTMA₄₂ and present the possible formation mechanism. The stronger synergetic effect between coil-comb-coil structure and PTMA coil block facilitate the 3D hierarchical self-assembly for the formation of uniform hairy cylinders from linear cylinders at a controllable water content. The coil-comb-coil molecular structure is essential for the hairy cylinder formation because it confines the arrangement of copolymers chain due to the steric hindrance and promotes the elongation and branching of linear cylinders. Furthermore, the control experiments based on a coil copolymer with similar composition but without a comb structure doesn't afford hairy cylinders. The stronger synergetic effect with PTMA coil leads to uniform hairy cylinders and the reason may be the slight crystallization of PTMA. The driving force of 3D hierarchical self-assembly is the equilibrium of interfacial potential aroused by the crystallinity of the PCL comb and PTMA coil at the DMF/water mixture, which determines the transition from linear cylinders to hairy cylinders. Such strategy may provide a new approach to design and synthesize hierarchical nanostructures from amphiphilic copolymers with complex hierarchical structure.

Acknowledgements

JD is supported by national natural science foundation of China (21374080, 21674081 and 21611130175), Shanghai 1000 Talents Plan, Shanghai International Scientific Collaboration Fund (15230724500) and the Fundamental Research Funds for the Central Universities (0500219211 and 1500219107). YZ appreciates the financial support from NSFC (21274096 and 21474070). Prof. Afang Zhang and Mr Xin Zhao at Shanghai University were appreciated for their assistance in the AFM study.

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Footnote

† Electronic supplementary information (ESI) available: IR spectra, ¹H NMR spectra data, GPC data, turbidity test as well as additional TEM and AFM images, Scheme S1 and calculation. See DOI: 10.1039/c6ra20862b

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