Cooperation of Amphiphilicity and Smectic Order in Regulating the

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Cooperation of Amphiphilicity and Smectic Order in Regulating the SelfAssembly of Cholesterol-Functionalized Brush-Like Block Copolymers Lishan Li, Feng Zhou, Yiwen Li, Xiao-Fang Chen, Zhengbiao Zhang, Nianchen Zhou, and Xiulin Zhu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01946 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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Cooperation of Amphiphilicity and Smectic Order in Regulating the Self-Assembly of CholesterolFunctionalized Brush-Like Block Copolymers Lishan Li, 1,§ Feng Zhou, 1,§ Yiwen Li,2 Xiaofang Chen, 1 Zhengbiao Zhang*,1 Nianchen Zhou*,1 and Xiulin Zhu 1,3 1. State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China 2. College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, 610065, China 3. Global Institute of Software Technology, No 5. Qingshan Road, Suzhou National Hi-Tech District, Suzhou 215163, China ABSTRACT: Nanoparticle morphology significantly affects the application of nanometerscale materials. Understanding nanoparticle formation mechanisms and directing morphological control in nanoparticle self-assembly processes have received wide attention. Herein, a series of brush-like amphiphilic liquid crystalline block copolymers, PChEMAm-b-POEGMAn, containing cholesteryl mesogens with different hydrophobic/hydrophilic block ratios were designed and

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synthesized. The self-assembly behaviors of the resulting PChEMAm-b-POEGMAn block copolymers in different solvents (THF/H2O, 1,4-dioxane/H2O, and DMF) were investigated in detail. Desirable micellar aggregates with well-organized architectures, including short cylindrical micelles, nanofibers, fringed platelets, and ellipsoidal vesicles with smectic micellar cores, were observed in 1,4-dioxane/H2O with an increasing hydrophobic block ratio. Although both amphiphilicity and smectic order governed the self-assembly, these two factors were differently balanced in the different solvents. This unique supramolecular system provides a new strategy for the design of advanced functional nanomaterials with tunable morphologies.


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INTRODUCTION Continuing interest in energy and health-related topics has inspired the fabrication of advanced functional materials with sophisticated architectures and improved properties.1,2 In the past two decades, the self-assembly of amphiphilic polymers has attracted significant interest in the development of various nanostructures through a “bottom up” approach with promising properties.3–7 These amphiphilic polymers are able to self-assemble into various micellar structures, including spheres, cylinders, platelets, and vesicles, owing to their amphiphilic features. The morphology of self-assembled nanoparticles can play a crucial role in their applications. For example, spherical micelles have distinct advantages for reducing friction in lubricating base oils.8 In biomedicine, cylindrical micelles have long circulation times in vivo and alter cell internalization pathways compared with analogous spherical morphologies.9 Furthermore, platelets are used for pattern transformation in nanolithography applications.10 Vesicles are synthetic models for simple cells and coatings of virus particles, with applications in drug and protein delivery vessels or nanoreactors.11,12 Understanding the formation mechanisms and directing morphological control in self-assembly processes are important for realizing nanoparticles with diverse applications. The tailoring of macromolecular interactions and architectures has provided several robust strategies for precisely controlling and greatly enriching micellar morphologies.13 For

traditional

coil–coil

diblock

copolymers,

micelle

formation

is

driven

by

solvophilic/solvophobic forces, meaning that the distinctive morphology of self-assembled aggregates is governed by their amphiphilicity. Usually, cylindrical micelles are difficult to obtain from traditional coil–coil diblock copolymers14–16 because they have isotropic micelle


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core structures. Furthermore, because of the high surface free energy of the lateral surface, platelet micelles are also rarely observed and tend to close to form vesicles.17–19 Changing the hydrophobic coil block into a crystalline or liquid crystalline (LC) block allows additional parameters for the preparation of nanoaggregates with complicated morphologies and hierarchical architectures.20–22 Great attention has been paid to crystalline–coil block copolymers,23,24 in which various exotic micellar morphologies with a characteristic crystalline core have been reported. Besides, the liquid crystalline (LC) block is also a good candidate for directing micelle growth and dictating their final morphologies with ordered nanostructures within micellar cores, owing to strong interactions between neighboring macromolecules.22 Promising work has been reported in this area. For example, in 2007, Li et al. discovered that smectic order in the hydrophobic block was crucial for nanofiber formation in the LC block copolymer.25 Therefore, when vesicular structures were formed, they were usually faceted rather than round.26 Furthermore, physical cross-linking of the smectic structure in the hydrophobic core can strengthen the stability of the whole micellar aggregates.27 Recently, Li et al. obtained monodisperse cylindrical micelles with controlled lengths from a perfluorinated smectic LC block copolymer through a fragmentation–thermal annealing process.28 Furthermore, Jiang et al. investigated the effect of corona liquid crystalline order on the self-assembly of crystalline/ionic LC block copolymers in selective solvents, indicating that a parallel arrangement of the smectic LC block along the surface core facilitated epitaxial crystallization of the unimers, resulting in well-developed 2D single crystals.29 The morphology and structure of LC block copolymer micellar aggregates can be tuned by adjusting the copolymer composition, such as the topology30 and length of the hydrophilic/hydrophobic blocks,31 or peripheral parameters32. For example, our group recently investigated the effect of cyclic topological structures on the self-assembly of LC


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block copolymers, which indicated that the cyclic topology weakened the order of the amphiphilic LC polymer mesogens and determined the ultimate micellar morphologies in solution.30 Cholesterol-based polymers are exciting materials in biorelated topics, particularly bioimaging and drug delivery, owing to their biocompatibility and bioactivity. Furthermore, the unique ability of cholesterol to promote self-organization (smectic order) and hydrophobic interactions in aqueous media makes it a good candidate for the LC block in amphiphilic block copolymers.33–35 Engineering more diverse and robust micellar structures will undoubtedly expand the applications of cholesterol-based polymers. Brush-type block copolymers have received considerable attention owing to their worm-like cylindrical conformations and unique self-assembly behavior.36–40 Wan et al. found that amphiphilic mesogen-jacketed LC block copolymers POEGMA-b-PMBPS have self-assembly behaviors different to those of PEO-b-PMBPS because of the brush-type OEG hydrophilic block.41–45 Recently, Liu et al. presented an interesting thermoregulated dual transition from unimers to vesicles and, finally, micelles that was based on toothbrush-like double hydrophilic block copolymer PNIPAM-b-POEGMA. In contrast, PNIPAM-b-PEO showed only a single transition from unimers to either micelles or vesicles upon increasing the temperature.46 The self-assembly behaviors of amphiphilic brush-like side chain LC block copolymers with a smectic structure in solution have yet to be comprehensively addressed. Systematic research into some important issues, including the LC-driven process, hydrophilic effect, and solvent, is still lacking. Herein, we designed and synthesized cholesterol-functionalized amphiphilic LC diblock copolymers, PChEMAm-b-POEGMAn, with different hydrophobic/hydrophilic block ratios through reversible addition–fragmentation chain transfer (RAFT) polymerization. The effects of


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the rod-like block ratio and solvent on the self-assembly behavior of PChEMAm-b-POEGMAn were investigated using three different solvent systems, namely, THF/H2O, 1,4-dioxane/H2O, and DMF. We aimed to gain deeper insight into the unique self-assembly properties of cholesterol-based block copolymer systems and establish a general design principle to prepare micellar aggregates with desirable morphologies and functions by combining a brush-like hydrophilic block and smectic LC hydrophobic block. EXPERIMENTAL SECTION Materials. Amphiphilic liquid crystalline block copolymers PChEMAm-b-POEGMAn were synthesized as described in our previous report.30 Tetrahydrofuran (THF), 1,4-dioxane, and N,Ndimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Co., Ltd. THF was freshly distilled from sodium/benzophenone prior to use, 1,4-Dioxane was dried over CaH2 and distilled before use, and DMF was used without further purification. Self-assembly of PChEMAm-b-POEGMAn. In a typical procedure, the diblock copolymer was completely dissolved in THF or 1,4-dioxane at an initial concentration of 1.0 mg/mL. The polymer solution was then filtered through a PTFE filter (pore size, 0.22 µm) to remove any dust and obtain the stock solution. Milli-Q water was then added slowly (0.1 mL/h) to the stock solution (1.0 mL) with gentle shaking at room temperature until the water content reached a predetermined value. When DMF was used as the self-assembly solvent, the diblock copolymer (0.5 mg/mL) in DMF was stirred for 6 h at room temperature. The mixture was subsequently heated at 70 °C for 5 h. Finally, the dispersion was allowed to cool to room temperature with gentle shaking. Characterization. The number-average molecular weight (Mn) and polydispersity (Mw/Mn) of the polymers were determined by size exclusion chromatography using a TOSOH HLC-8320


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system equipped with refractive index and UV detectors and two TSKgel Super Multipore HZ-N (4.6×150 mm, 3 µm beads size) columns arranged in series, which can separate polymers of molecular weights ranging from 500 to 1.9×105 g/mol. THF was used as the eluent at a flow rate of 0.35 mL/min at 40 °C. Data acquisition was performed using EcoSEC software, and molecular weights were calculated using polymethyl methacrylate (PMMA) standards. 1H NMR spectra were collected using a Bruker NMR spectrometer (300 MHz) using tetramethylsilane (TMS) as the internal standard at room temperature. NMR samples were prepared at a concentration of ~20 mg/mL in CDCl3. Small-angle X-ray scattering (SAXS) experiments were used to identify the LC phase structures and performed using a high-flux X-ray instrument (SAXSess mc2, Anton Paar) equipped with a line collimation system and a 2200-W sealed-tube X-ray generator (CuKα, λ = 0.154 nm). Samples were wrapped in aluminum foil and sandwiched in a steel sample holder. The X-ray scattering patterns were recorded under vacuum on an imaging plate (IP). The scattering peak positions were calibrated with silver behenate. A temperature control unit (Anton Paar TC300) in conjunction with the SAXSess mc2 system was utilized to study structure evolution as a function of temperature. Samples for transmission electron microscopy (TEM) observation were prepared by placing a drop of self-assembled polymer solution onto a carbon-coated copper grid. After 30 s, excess fluid was removed using a piece of filter paper. A drop of staining agent phosphotungstic acid (1 wt%) was then placed on the grid, excess solution was removed with a piece of filter paper after 30 s, and the grid was left to dry under ambient conditions. TEM was performed on a HITACHI HT7700 instrument with an accelerating voltage of 120 kV. Hydrodynamic diameter (Dh) was measured by dynamic light scattering (DLS) using a Brookhaven NanoBrook 90Plus PALS instrument at 25 °C at a scattering angle of 90°. The surface morphology was measured by atomic force microscopy


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(AFM) in tapping mode (Veeco Instruments Inc., Nanoscope IV). The self-assembled polymer solution was dropped onto a precleaned silicon wafer and allowed to dry under ambient conditions. RESULTS AND DISCUSSION Synthesis and characterization of diblock copolymers PChEMA-b-POEGMA. A series of well-defined PChEMAm-b-POEGMAn amphiphilic LC diblock copolymers with different compositions were synthesized by sequential reversible addition–fragmentation chain transfer (RAFT) polymerization, as reported previously by our group.30 The chemical structure is described in Scheme 1. The chain length of the hydrophilic/hydrophobic block was varied by controlling the feeding molar ratio of monomer to RAFT agent and polymerization conversion. All diblock copolymers gave symmetrical distributions in SEC elution traces (Figure 1) and narrow polydispersities, with polydispersity indices (PDIs) below 1.15. This indicates that the RAFT polymerization were successfully conducted. However, there were weak shoulders in SEC curves of PChEMAm-b-POEGMAn, suggesting the existence of high molecular weight components, which may be formed by bi-radical coupling termination resulting from the fast polymerization rate.30 Comparing the self-assembly behavior of the purified PChEMA29-bPOEGMA14 after pre-GPC with the original sample, the result showed the effects of the high molecular weight components on the self-assembly of the diblock copolymers may be ignored in this work, due to its small fraction of proportion (less than 5% proportion). This detail can be seen from Supporting Information (Figure S2, Figure S3). The compositions of the diblock copolymers were calculated from their 1H NMR spectra (Figure S1). The characteristics of the diblock copolymers are summarized in Table 1.


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Scheme 1. Chemical structure of cholesterol-functionalized amphiphilic liquid crystalline diblock copolymers PChEMAm-b-POEGMAn.

Table 1. Characterization data of diblock copolymers PChEMAm-b-POEGMAn

a

Sample

Mn a(g/mol)

Mn b(g/mol)

Mw/Mnb

Hydrophobic characterc (wt.%)

PChEMA9-b-POEGMA14

9100

10800

1.13

53.7

PChEMA13-b-POEGMA14

11300

12000

1.11

61.1

PChEMA19-b-POEGMA14

14500

13200

1.10

70.0

PChEMA23-b-POEGMA14

16700

16700

1.11

74.8

PChEMA29-b-POEGMA14

19900

18700

1.11

78.9

PChEMA33-b-POEGMA14

22100

19600

1.10

81.0

Measured by 1H NMR spectra. b Determined by SEC measurements. c PChEMA weight fraction

in the PChEMAm-b-POEGMAn determined by 1H NMR spectra.


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Figure 1. SEC curves of block copolymers PChEMAm-b-POEGMAn. Self-assembly behaviors in three different solvents: THF/H2O, 1,4-dioxane/H2O, and DMF. The diblock copolymers listed in Table 1 could not be assembled in pure water owing to the strong hydrophobic nature of the PChEMA blocks. Therefore, we used the cosolvent method to produce assemblies. 1,4-Dioxane and THF are good candidates that are common solvents for cholesteryl-containing amphiphilic copolymers, as reported previously.30 The self-assembly of PChEMAm-b-POEGMAn was first conducted in THF/H2O to explore the effect of block ratio on aggregate morphology. To optimize the self-assembled structures, water was progressively dropped into the THF solution of PChEMAm-b-POEGMAn and the morphologies of the resulting self-assemblies were monitored by TEM. For PChEMA9-b-POEGMA14, spherical micelles with diameters of around 16.2 nm were obtained when the water content was 67% (Figure 2a), with no further morphological transition observed when more water was added. For PChEMA13-bPOEGMA14, when the water content was relatively low, such as 47.4%, spherical micelles with diameters of around 16.8 nm were obtained (Figure 2b). The micellar solution turned slightly


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turbid when the water content was further increased, indicating that large aggregates were formed. For instance, giant spherical vesicles with diameters of around 900 nm were obtained at a water content of 58.3% (Figure 2c). Similarly, spherical micelles with diameters of around 18.0 nm were observed for PChEMA19-b-POEGMA14 when the water content was 33.3% (Figure 2d), and further water addition induced the formation of giant vesicles, which had diameters of around 1.08 µm at a water content of 50.0% (Figure 2e). A similar phenomenon was also observed

in

PChEMA23-b-POEGMA14,


and

PChEMA33-b-

POEGMA14 (Figure S4). Apparently, a morphological transition from micelle to giant vesicle occurred as the hydrophobic PChEMA block length of the PChEMA-b-POEGMA copolymers increased.

Figure 2. TEM images (stained with phosphotungstic acid) of self-assembled morphologies of (a) PChEMA9-b-POEGMA14 at a water content of 67%, (b) PChEMA13-b-POEGMA14 at a water


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content of 47.4%, (c) PChEMA13-b-POEGMA14 at a water content of 58.3%, (d) PChEMA19-bPOEGMA14 at a water content of 33.3%, and (e) PChEMA19-b-POEGMA14 at a water content of 50.0% in THF/H2O. The initial concentration was 1.0 mg/mL in all cases. The cosolvent is an important factor affecting micellar morphology.4 When 1,4-dioxane was used as the cosolvent, water was added slowly to the polymer solution with gentle stirring until a water content of no less than 50% was achieved. Typically, PChEMA9-b-POEGMA14 formed uniform spherical micelles with diameters of around 12.6 nm (Figure 3a). PChEMA13-bPOEGMA14 formed short cylindrical micelles with diameters of around 13.2 nm (Figure 3b). When the hydrophobic block length was increased, as in PChEMA19-b-POEGMA14 and PChEMA23-b-POEGMA14, nanofibers were formed (Figures 3c and 3d, lengths of several micrometers with diameters of around 13.7 nm and 14.0 nm, respectively). Surprisingly, when the hydrophobic block length was further increased, as in PChEMA29-b-POEGMA14, fringed platelets with the thickness of around 14.2 nm were formed (Figure 3e). TEM samples were freeze-dried to prevent secondary aggregation, which was possible during drying at ambient temperature. The fringed platelet micelles were also observed using TEM (Figure S5). Finally, PChEMA33-b-POEGMA14 formed ellipsoidal vesicles with diameters of around 231 nm and 69 nm along the long and short axes, respectively, and a wall thickness of around 14.5 nm (Figure 3f). The diameters of micelles and wall thickness of vesicles changed with the increase of PChEMA blocks. The relationship between the observed sizes from TEM, calculated maximums by the Formulate S3 and degree of polymerization was discussed in Figure S6. The more versatile and intriguing self-assembly behavior of PChEMAm-b-POEGMAn in 1,4-dioxane/H2O motivated us to explore the formation mechanism and further direct the morphological control.


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Figure 3. TEM images (stained with phosphotungstic acid) of self-assembled morphologies of (a) PChEMA9-b-POEGMA14, (b) PChEMA13-b-POEGMA14, (c) PChEMA19-b-POEGMA14, (d) PChEMA23-b-POEGMA14, (e) PChEMA29-b-POEGMA14, and (f) PChEMA33-b-POEGMA14 in 1,4-dioxane/H2O. Initial concentration was 1.0 mg/mL in all cases. DMF, also selected as a self-assembly solvent, was only selective toward the POEGMA block. PChEMAm-b-POEGMAn was first dispersed in DMF at 70 °C and then the solutions were slowly cooled to room temperature. As shown by the TEM results in Figure 4, PChEMA9-bPOEGMA14,



and

PChEMA23-b-

POEGMA14 formed cylindrical micelles. The aggregates became less stable with increasing hydrophobic block length, such that the dispersions of PChEMA29-b-POEGMA14 and PChEMA33-b-POEGMA14 became turbid and some solids were generated. Nevertheless, cylindrical micelles mixed with other irregular structures were observed. Overall, the


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morphologies of the self-assembly aggregates of PChEMAm-b-POEGMAn in DMF were similar to those in 1,4-dioxane/H2O, which may comply with an in-plane smectic packing manner, for example, cylindrical micelle formation are inclined to be obtained.

Figure 4. TEM images (stained with phosphotungstic acid) of self-assembled morphologies of (a) PChEMA9-b-POEGMA14, (b) PChEMA13-b-POEGMA14, (c) PChEMA19-b-POEGMA14, (d) PChEMA23-b-POEGMA14, (e) PChEMA29-b-POEGMA14, and (f) PChEMA33-b-POEGMA14 in DMF. Concentrations were 0.5 mg/mL in all cases. Cooperation of amphiphilicity and smectic order in regulating the self-assembly process. To investigate the inner structures of the aggregates and further understand the assembly mechanism, we performed small-angle X-ray scattering (SAXS) experiments on the lyophilized samples of micellar aggregates formed in 1,4-dioxane/H2O (see Figure 5) and THF/H2O (see Figure S8). In Figure 5, the SAXS profiles of PChEMAm-b-POEGMAn samples, except


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PChEMA9-b-POEGMA14, show three scattering peaks in the low angle region with a q ratio of 1:2:3, indicating the formation of a long-range ordered lamellar structure within the micellar cores. Furthermore, the calculated d-spacing for the primary peak was around 4.42 nm, which was between l and 2 l, l = 2.50 nm being the length of the extended LC side chains of the PChEMA block. This might be attributed to the formation of an interdigitated smectic A (SmAd) structure, similar to that found in PEG5000-b-PAChol (14/86) copolymers reported by Li et al. 25 and PChEMA homopolymers reported by our group.47 The homopolymer PChEMA exhibited also an interdigitated SmAd phase as already discussed in the previous paper. The phase sequence of the homopolymer PChEMA24 with Mn = 13400 g/mol and Mw/Mn = 1.12 was g111.2 oC, -SmAd 206.4 oC-I, and the smectic layer spacing was 4.60 nm.47 Herein, the smectic arrangement of cholesteryl mesogens might exist within the cylindrical micelles, fringed platelets, and ellipsoidal vesicles, because of the strong in-plane interactions between cholesteryl mesogens.25,26,31


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Figure 5. SAXS profiles of freeze-dried micelle samples of PChEMAm-b-POEGMAn obtained in 1,4-dioxane/H2O. In general, when the length of the LC block is decreased, the liquid crystallinity is weakened.48,49 PChEMA9-b-POEGMA14 has the shortest liquid crystalline block. In the case, the self-assembly process was thought to mainly be determined by polymer amphiphilicity. Repulsion within the corona layer of the POEGMA block led to the formation of spherical micelles with high interfacial curvatures.4 At the meantime, the high curvature of spherical micelle tends to depress the stability of the smectic structure. So both the short liquid crystalline block and high curvature induce the PChEMA block could not keep the long range ordered smectic structure in solution assemblies. Compared with spherical micelles, the formation of cylindrical micelles is generally hard to achieve, partly because the narrow compositional range in the phase diagram offers a much smaller window of opportunity in which to observe them.14–16


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Therefore, facile and versatile preparative methodologies are essential to enable advances in the field. In this study, PChEMA-b-POEGMA copolymers formed cylindrical micelles with a broad range of compositions (hydrophobic weight fraction range of almost 15% (61.1%–74.8%)), mainly owing to the smectic order in the micellar cores arising from strong in-plane interactions between cholesteryl mesogens. The mesogens in the PChEMA block adopt parallel alignment to the interface and with the orientation along the long axis of cylinders, which are able to stabilize both the cylindrical assemblies and smectic layers. On the other hand, we note that, based on TEM study, the diameter of each cylinder is quite uniform and slightly increases from 13 to 14 nm with increasing the PChEMA block (Figure 3b-d). Assuming PChEMA block adopts extended conformation in terms of bottle-brush architecture, the diameter of cylinder is larger than the twice length of PChEMA block. We hypothesize that the PChEMA blocks are organized in a head-to-head manner in the micellar cores of the cylinders and the smectic layer normal is along the long axis of the cylindrical assemblies, as shown in Scheme 2. Experimentally, the correlation length of the smectic structure L was estimated by the Scherrer formula,50 which was found L > 21.7 nm for the copolymer PChEMA23-b-POEGMA14 (Figure S7). This means that the smectic layer normal should be along the long axis of the fibers, but not along their diameter, which was found to be only in the 14.0 nm range. This kind of packing model has been reported as the most probable one in cylindrical self-assembled aggregates in liquid crystalline block copolymer systems not only in solution assemblies but also in bulk.50, 51, 52 As the weight fraction of hydrophobic PChEMA increased, longer cylindrical micelles were observed. This might be due to the increase in liquid crystallinity enhancing the epitaxial growth of cylindrical micelles.31


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Figure 6. (a) AFM image and (b) height profile of fringed platelets formed by PChEMA29-bPOEGMA14 in 1,4-dioxane/H2O. Furthermore, a further increase in PChEMA block length caused the formation of fringed platelets because the POEGMA corona could not provide sufficient steric repulsion to balance the strong interfacial energy. To gain further insight into the fringed platelets formed by PChEMA29-b-POEGMA14, their height profiles were analyzed using AFM. As shown in Figure 6, the platelets had a thickness of around 13.0 nm, which was smaller than the result observed by TEM because of the collapse of soft POEGMA blocks. The thickness is about twice the length of fully extended PChEMA29, suggesting that the chains in the platelet region might be arranged in a sandwich structure consisting of a smectic core layer of PChEMA and two POEGMA layers, as shown in Scheme 2. To our knowledge, this is the first example of fringed platelets with an internal SmAd substructure obtained by self-assembly in solution. In general, the platelets are intermediate structures between cylindrical micelles and vesicles.19 The fringed platelets formed by PChEMA29-b-POEGMA14 were irregular and contained multiple cylinders, indicating simultaneous growth at multiple sites and the emergence of a significant kinetic factor.17 We assumed that the formation of the fringed platelets involved two steps: (i) Cylindrical micelles


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laterally aggregate into bundles, and (ii) fusion of the neighboring cylindrical micelles leads to the formation of the final fringed platelets.6,17 Furthermore, cylinders protruding from the fringed platelets might also be potential feeding materials grown into the lateral edges of the platelets.17 Generally, the excess lateral free energy would induce the closure of platelets to form vesicles.19 However, it was striking that ellipsoidal vesicles were formed instead of classical spherical vesicles, as reported previously in coil–coil block copolymers.4 Considering that bending the membrane parallel to the smectic layer did not change the layer spacing, while bending perpendicular to the smectic layer made the layer spacing in different sections of the membrane become unequal, as shown in Scheme S1, we speculated that the smectic layers were perpendicular to the major axis of the ellipsoidal vesicles.26 Furthermore, defects may exist in the membrane to relax topological constraints, which meant that smectic organizations were present in most parts of the membrane, except the two extremities of the ellipsoidal vesicles.26 Overall, both the amphiphilicity and smectic order govern the self-assembly process in 1,4-dioxane/H2O. Scheme 2 shows a schematic illustration of the smectic packing structure within a cross-section of the membrane formed by PChEMAm-b-POEGMAn with different PChEMA weight fractions in 1,4-dioxane/H2O. Scheme 2. Schematic illustrations of packing structures of self-assembled aggregates formed by PChEMAm-b-POEGMAn with different PChEMA weight fractions in 1,4dioxane/H2O.


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The liquid crystalline properties in self-assembled micelles of PChEMA19-b-POEGMA14 and PChEMA23-b-POEGMA14 in THF/H2O have also been studied via SAXS. As shown in Figure S8, the SAXS profiles only exhibit a weak and broad primary scattering peak, indicating the absence of long-range ordered smectic structure within the assemblies. In this case, the morphology transformation in THF/H2O was similar to that of traditional coil–coil diblock copolymers (Figure 2, Figure S4). The equilibrium of self-assembled morphology is mainly governed by amphiphilicity. Herein, we attempted to investigate the interactions between the PChEMA block and the solvents to explain why the self-assembly behaviors of PChEMAm-b-POEGMAn in THF and 1,4-dioxane were so different. The polymer–solvent interactions were estimated using χparameters, as shown by Flory–Huggins theory. The χ-parameters of PChEMA-THF and PChEMA-dioxane have been reported previously.31 The value of χPChEMA-dioxane (0.92) is much higher than that of χPChEMA-THF (0.63), indicating that THF was a much better solvent than 1,4-


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dioxane for the PChEMA block. In fact, the PChEMA homopolymer dissolved in THF more readily than in dioxane, which was consistent with the χ-parameter predictions. Therefore, as the interaction between 1,4-dioxane and the PChEMA block was less favorable, water addition induced strong in-plane interactions between cholesteryl mesogens, which resulted in extraordinary micellar aggregates forming in 1,4-dioxane/H2O compared with those in THF/H2O. CONCLUSIONS A series of brush-like liquid crystalline block copolymers with different hydrophobic/hydrophilic block ratios, PChEMAm-b-POEGMAn, were synthesized by RAFT polymerization. The block copolymers exhibited different self-assembly behaviors in THF/H2O, 1,4-dioxane/H2O, and DMF, respectively. Cylindrical micelles formed in DMF solvent. The morphology transition from micelles to vesicles occurred when THF was used as cosolvent. The transition point was mainly dependent on the hydrophobic/hydrophilic block ratio. However, using 1,4-dioxane as cosolvent resulted in spherical micelles, smectic cylindrical micelles, smectic fringed platelets, and smectic ellipsoidal vesicles, respectively, when the hydrophobic block ratio was increased. The smectic order arising from strong in-plane interactions between cholesteryl mesogens played an important role in micelle growth and their final morphological formation. Overall, this work established a general design principle to prepare micellar aggregates with desirable morphologies and functions. Ongoing work to develop micellar aggregates with targeting ligands on the surface, which have potential applications in targeting drug carriers and materials science, is underway in our laboratory. ASSOCIATED CONTENT


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The following files are available free of charge. Figure S1: 1H NMR spectrum of PChEMAm-b-POEGMAn, calculation formulas for DP and Mn; Figure S2: SEC curves of block copolymers before and after pre-GPC; Figure S3: TEM images of self-assembled morphologies of PChEMA29-b-POEGMA14 before and after pre-GPC; Figure S4 and Figure S5: TEM images

of self-assembled morphologies

in THF/H2O and 1,4-

dioxane/H2O; Figure S6: Average diameters versus degree of polymerization; Figure S7: SAXS profiles of freeze-dried micelle of PChEMA -b-POEGMA obtained in 1,4-dioxane/H2O; 23 14 Figure S8: SAXS profiles of freeze-dried micelle samples of PChEMA19-b-POEGMA14 and PChEMA23-b-POEGMA14 obtained in THF/H2O;

Scheme S1: Schematic illustration of a

smectic ellipsoidal vesicle of PChEMA33-b-POEGMA14 obtained in 1,4-dioxane/H2O . AUTHOR INFORMATION §

Authors contributed equally to this work.

Corresponding Authors *E-mail: [email protected] (N. C. Zhou). *E-mail: Zhengbiao Zhang @suda.edu.cn (Z. B. Zheng). Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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For Table of Contents use only

Cooperation of Amphiphilicity and Smectic Order in Regulating the SelfAssembly of Cholesterol-Functionalized Brush-Like Block Copolymers Lishan Li, 1,§ Feng Zhou, 1,§Yiwen Li,2 Xiaofang Chen, 1 Zhengbiao Zhang*,1 Nianchen Zhou*,1 and Xiulin Zhu 1,3 TOC Graphic


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Cooperation of Amphiphilicity and Smectic Order in Regulating the

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