Surfactant-Mediated Crystallization-Driven Self-Assembly of

Dec 7, 2016 - Zaizai Tong , Junyi Zhou , Rui-Yang Wang , Jun-Ting Xu. Polymer ... Runke Zhang , Runsheng Huang , Jie Zhou , Zaizai Tong , Guohua Jiang...
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Surfactant-Mediated Crystallization-Driven Self-Assembly of Crystalline/Ionic Complexed Block Copolymers in Aqueous Solution Zaizai Tong,†,‡,§ Runke Zhang,† Pianpian Ma,†,‡,§ Haian Xu,† Hua Chen,† Yanming Li,† Weijiang Yu,† Wangqian Zhuo,† and Guohua Jiang*,†,‡,§ †

College of Materials and Textiles, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China Key Laboratory of Advanced Textile Materials and Manufacturing Technology (ATMT), Ministry of Education, Hangzhou 310018, P. R. China § National Engineering Laboratory for Textile Fiber Materials and Processing Technology (Zhejiang), Hangzhou 310018, P. R. China ‡

S Supporting Information *

ABSTRACT: A series of crystalline/ionic complexed block copolymers (BCPs) with various compositions have been prepared by sequential reactions. The BCPs with different hydrophilic fractions can self-assemble into various morphologies, such as spindlelike, rodlike, and spherical micelles with different crystallinity of the core. Bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) is added as a surfactant to induce the morphological transition of BCPs in aqueous media. The introduced AOT can be tightly bound to the cationic units, and a water-insoluble unit in the corona forms, leading to a reduced tethering density. Consequently, morphological variety changing from rods to platelets to fibril to dendrite-like micelles can be observed.

1. INTRODUCTION Block copolymers (BCPs) can spontaneously form various shapes of nanoaggregates in selective solvents.1,2 The selfassembled morphologies can be manipulated by altering the block composition or tailoring the preparation process. Such a case is especially true for the crew-cut system, which exhibits various morphologies.3−5 Compared to amorphous BCP micelles, crystalline micelles have received considerable attention because of their controlled morphologies driven by crystallization.6,7 It has been reported that crystalline BCPs present morphological richness.8−14 Meanwhile, crystalline micelles have some unique characteristics, including living growth15−17 and the construction of block co-micelles,18,19 both of which are difficult to obtain in amorphous micelles. Although crystallization enriches the morphologies of BCPs, this process can be severely confined in a nanoscale environment, leading to a low or even no crystallinity of the core.20−22 Consequently, the final morphologies usually present spherical aggregates.23,24 A number of studies have been focused on the morphological transition from spherical to other geometric micelles by tailoring the temperature,25,26 pH,27−29 or additives.30−32 For instance, the effect of the aforementioned factors on the self-assembled morphology of poly(ε-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) has been systematically investigated.27,31,33 It is found that temperature regulates the crystallization driving force of the core-forming block33 and pH27 and salt30,31 alter the corona chain conformation swollen by the solvent, and thus a sphere-to© XXXX American Chemical Society

cylinder transformation of the micellar morphology occurs. In our previous work, it is also demonstrated that the presence of corona liquid-crystalline order alters the corona chain conformation to a certain extent, and well-developed single crystals are facilitated to form.34 However, the regulation of the crystalline micelles by the aforementioned factors is limited. A polyion that contains plentiful cations or anions can be easily modified by surfactants with hydrophobic spacers of variable length.35 Driven by a combination of electrostatic and hydrophobic interactions, polyions with oppositely charged surfactants form stoichiometric and water-insoluble complexes with higher stability.36 A series of studies have reported that the formation of core−shell aggregates originates from the oppositely charged surfactants and the charge-neutral block copolymers.37−41 It is said that the double hydrophilic BCPs containing polyionic blocks decorated by surfactants can assemble into vesicles42,43 or micelles.44,45 Meanwhile, the role of organic counterions in amorphous charged BCPs is well established when the micelles reach a new equilibrium.46−48 However, the effect of surfactants on the morphology of crystalline micelles remains unknown. The complexation between polyions and surfactants makes it possible for us to investigate the impact of corona conformation or corona hydrophilicity on the crystalline micelles. Received: August 6, 2016 Revised: December 5, 2016 Published: December 7, 2016 A

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corona in water decreases because of the lower hydration energy.49 It has been found that the stoichiometric ratio (Z) of AOT/qDM is an important factor in the variety of aggregation behavior. AOT reduces the solubility of the corona-forming block after partially replacing MO, and the tethering density becomes smaller. Then it will lead to crystal growth, and further morphological transformation of the crystalline micelles occurs. As a result, the morphological variety changing from rod to platelets to fibrils to dendrite-like micelles is observed, as shown in Scheme 1.

In the present work, crystalline/ionic complexed block copolymers poly(ε-caprolactone)-b-poly[quaternized 2(dimethylamino)ethyl methacrylate]/methyl orange (PCL-bqPDM/MO for short) were prepared as described in Scheme 1, Scheme 1. Polymer Structure of PCL-b-qPDM/MO and Schematic Illustration of the AOT-Mediated Morphological Transition

2. EXPERIMENTAL SECTION 2.1. Sample Synthesis. Initiator 2-hydroxyethyl 2-bromo-2methylpropanoate (2-HBMP) was described previously.50 The macroinitiators (PCL) and parent block copolymers (PCL-b-PDM) were synthesized according to our previous report,34 which is schematically depicted in Scheme S1. The detailed synthesis method of ionic complexed PCL-b-qPDM/MO is described in the Supporting Information. 2.2. Characterization. The molecular weight and the polydispersity index (PDI) were evaluated by gel permeation chromatography (GPC) using a Waters system calibrated with standard poly(methyl methacrylate). DMF with 1% lithium bromide (LiBr) was used as an eluent at a flow rate of 1.0 mL min−1. 1H NMR spectra were recorded on a Bruker DMX-400 MHz. The UV−vis spectra of the samples were recorded on a Hitachi U-3010 spectrophotometer. A FEI Quanta 200 FEG environmental scanning electron microscope equipped with an energy-dispersive spectrometer (EDS) was used to verify the presence or absence of Na+ and I− in the complexes. TEM observations were carried out using a JEOL JEM-1230 electron microscope at an

and the detailed synthesis routine is supplied in Scheme S1. In this article, the effect of AOT surfactant on the morphology of PCL-b-qPDM/MO in aqueous solution is mainly investigated because water is a selective solvent for block qPDM/MO. When MO is partially replaced by AOT, the solubility of the

Figure 1. Self-assembled morphologies of PCL-b-qPDM/MO. (A) PCL170-b-qPDM22/MO, (B) PCL170-b-qPDM35/MO, and (C) PCL170-bqPDM72/MO. Top, TEM images; middle, SAED patterns; bottom, HRTEM image, with the inset being the FFT image of the dashed box. B

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Langmuir acceleration voltage of 80 kV. High-resolution transmission electron microscopy (HRTEM) and selected-area electron diffraction (SAED) observation were conducted on a JSM-2100 electron microscope at an acceleration voltage of 200 kV. TEM or HRTEM samples were prepared by dropping 4 μL of the micellar solution onto carbon-coated copper grids, and then the samples were frozen by liquid nitrogen and freeze-dried under vacuum to avoid any possible reassembly and aggregation of the micelles during the drying process. Dynamic light scattering (DLS) measurements were performed in aqueous solution using a Horiba Zetasizer apparatus (LB-550 V) equipped with a 5.0 mW laser diode operating at 650 nm at room temperature. Tappingmode atomic force microscopy (AFM) was conducted on an XE-100E instrument (PSIA Cooperation, Korea). A drop of micellar solution was deposited on a piece of silicon wafer and spin-coated at 3000 rpm for 30 s. Differential scanning calorimetry (DSC) was conducted on a TA Q200 instrument under a N2 atmosphere. Wide-angle X-ray diffraction scattering (WAXS) experiments were performed at the BL16B1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF) in China. The X-ray wavelength was 1.24 Å, and the sampleto-detector distance was set as 358 mm. Two-dimensional (2D) WAXS patterns at room temperature were recorded. The 2D WAXS patterns were converted into one-dimensional (1D) WAXS profiles using Fit2D software. Samples for both DSC and WAXS experiments were freeze-dried from the micellar solution. 2.3. Preparation of the Micellar Solution. The complexed diblock copolymers were first dissolved in DMF to obtain a homogeneous solution at a concentration of 1 mg/mL. Then the sample solution was dialyzed against deionized water using a 3500 Mw dialysis bag at 20 °C with shaking. Finally, the micellar solution was diluted to 0.1 mg/mL. A surfactant (AOT)-mediated morphology transition was conducted by the following two methods. Method 1: A predetermined amount of AOT and the PCL-b-qPDM/MO sample were first mixed and dissolved in DMF, and then the mixture solution was dialyzed against deionized water to remove DMF and excess MO. Method 2: AOT aqueous solution was injected directly into the micellar solution in a predetermined ratio, and then the MO molecules were removed by dialysis. The molar ratios (Z) of AOT/qDM of different samples were fixed at 0.10, 0.25, and 0.50, respectively.

Figure 2. DLS size distribution of PCL170-based block copolymers in aqueous solution.

diffraction spots of the PCL crystal in the SAED pattern. Moreover, obvious lattice fringes with ca. 0.37 nm interplanar spacing assigned to (200) of PCL is found inside the PCL170-bqPDM22/MO aggregates (HRTEM, Figure 1A). The inset fast Fourier transformed (FFT) image shows the (200) spots of PCL. Here, the absence of other crystalline plane in HRTEM may be attributed to selective areas of the micellar core, which is frequently reported in other papers.29 For PCL170-bqPDM35/MO aggregation, the crystallization is also confirmed because of the existence of distinct (110), (200), (020), and (220) diffraction spots in the SAED pattern. In the HRTEM image, lattice fringes with ca. 0.42 nm interplanar spacing correspond to the (110) crystal plane of PCL. When the corona segment further increases, crystallization degenerates severely and no obvious diffraction spots except weak dots assigned to (020) of PCL exist in the SAED pattern. In the HRTEM image, no obvious lattice strips can be distinguished, indicating that the core of spheres formed by PCL170-b-qPDM72/MO is totally amorphous or possesses extremely low crystallinity. On the other hand, the self-assembly of PCL70-based BCPs with a similar f philic (Table S2) is also explored. The tendency of the morphological transition is similar to that of PCL170-based BCPs because a transition from rodlike micelles to typical spheres is observed when the f philic value increases (Figure S3). The above discussion shows the decreasing crystallinity of the aggregated cores when the hydrophilic segment increases. The surfactant is an interesting trigger when the corona solubility changes significantly. Herein, different AOT/qDM stoichiometric ratios, Zs, are adopted to change the solubility of the corona segment because AOT has hydrophobic alkyl tails.51 Because of the lower hydration energy of AOT compared to that of MO, AOT prefers to be attached to the qDM cationic unit, leading to a lower solubility of the qPDM/AOT block in water. The excess MO is removed by dialysis, whose content can be observed from the decreased intensity of the absorbance of azobenzene in UV−vis spectra (Figure S4). When MO is partially replaced by AOT, the compositions of PCL170-bqPDM72/MO for Z = 0.25 and 0.50 are further evaluated by 1H NMR (Figure S5) and EDS (Figure S6). The integration of all peaks is added to Figure S5. Isolated peak r and peak 2 represent MO and AOT, respectively. Thus, the AOT molar ratios from Figure S5b,c are calculated to be 0.30 and 0.48, respectively, and are very similar to Z = 0.25 and 0.50. Moreover, the EDS spectrum shows the absence of Na+ and I− (Figure S6), indicating that partial MO molecules are removed by dialysis. Two methods are applied to explore the effect of

3. RESULTS AND DISCUSSION The synthesis routine of PCL-b-qPDM/MO is described in Scheme S1, and the molecular characteristics obtained from GPC (Figure S1) and 1H NMR (Figure S2) are summarized in Table S1. Meanwhile, the molar ratio of monomer to initiator and the monomer conversion are also added to Table S1. The weight fractions of hydrophilic segments are concluded in Table S2. These molecular characteristics indicate the successful synthesis of PCL-b-qPDM/MO with different compositions. First, the self-assembled behavior of BCPs with different hydrophilic segments in aqueous solution is examined by TEM observation. As shown in Figure 1A, PCL170-bqPDM22/MO with a short hydrophilic block length (weight fraction of hydrophilic blocks, f philic = 0.35, Table S2) is inclined to form spindlelike micelles. However, when f philic rises to 0.46, the morphology tends to be rodlike micelles with a shorter length (Figure 1B). A further increase in f philic results in spherical micelles of PCL170-b-qPDM72/MO (Figure 1C). The hydrodynamic diameter evaluated by dynamic light scattering (DLS) decreases from 390 to 120 nm as f philic increases (Figure 2). This observation is reasonably consistent with the average size observed from TEM images. To reveal the crystallinity of micellar cores, selected-area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM) observation are further conducted. It can be observed that crystallization is dominant in PCL170-b-qPDM22/MO because of the presence of distinct (110), (200), (020), and (220) C

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Figure 3. Size distribution of PCL170-b-qPDM72/MO at various Z values prepared by (a) method 1 and (b) method 2.

Figure 4. TEM morphologies of PCL170-b-qPDM72/MO at various Z values prepared by method 2. The molar ratio (Z) of AOT/qDM is indicated.

Figure 5. Self-assembled morphologies of PCL170-b-qPDM72/MO at various Z values prepared by method 1: (a) Z = 0.10, (b) Z = 0.25, and (c) Z = 0.50. Top, TEM images; bottom, SAED patterns.

further evaluated by TEM observation. Mixed morphologies, including rodlike micelles, nanosheets, and lamellae, are observed by method 2 (Figure 4). This phenomenon is consistent with the result from DLS, which shows a wide size distribution. By contrast, the morphology and size distribution are relatively uniform by method 1 (Figure 5). The wide size distribution by method 2 is derived from the inhomogeneous complex between the added AOT solution and qDM cationic

AOT on the morphological transition as described in the Experimental Section. The size distributions of PCL170-bqPDM72/MO with different Z values prepared by two methods are first evaluated by DLS (Figure 3). One can see that the size distribution for each composition prepared by method 2 is relatively broader than that by method 1, especially for Z = 0.10. However, as Z increases, the size distributions become similar. Detailed morphologies prepared by two methods are D

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Figure 6. AOT-mediated morphological evolution of PCL170-b-qPDM35/MO. Images were recorded for Z = 0.10 (top) and 0.25 (bottom), respectively. (a, b, d, and e) TEM micrographs with different magnifications and (c and f) the corresponding SAED patterns of PCL170-b-qPDM35/ MO for Z = 0.10 and 0.25, respectively.

The effect of AOT on the morphological evolution of PCL170-b-qPDM35/MO is also probed. Because the soluble segment is relatively short in PCL170-b-qPDM35/MO, two Z values (i.e., 0.10 and 0.25) are selected to investigate the morphological transformation. Figure 6 shows the morphological structures of the self-assembled aggregates. For Z = 0.10 and 0.25, the corresponding DLS profiles and digital photographs of micellar solutions are supplied in Figure S8. It can be observed that fibril-like crystals instead of rodlike micelles exist when Z = 0.10 (Figure 6a,b). The SAED pattern shows discrete diffraction spots of (110), (200), (020) and (220), indicating that the core of the micelles is crystalline. The detailed microstructure of the core is evaluated by HRTEM (Figure S9), and lattice fringes with ca. 0.36 nm interplanar spacing assigned to (200) of PCL are clearly observed. For Z = 0.25, the selfassembled morphology transforms to unexpected aggregates, which are dendrite-like micelles with typical PCL diffraction spots of (110), (200), (020) and (220) (Figure 6d,e). HRTEM reveals the (200) lattice strips of PCL (Figure S10), indicating that the core is also crystalline. Because the dentritelike micelles are much larger than other aggregates (Figure S8a), the mechanism for the formation of dentritic micelles may be attributed to the aggregation of small particles. Moreover, some flocs are observed in the micellar solution (Figure S8b), which could be due to the smaller fraction of the hydrophilic segment for Z = 0.25. From the above discussion, we can conclude that the morphological transition is clearly caused by the change in hydrophilicity of the corona segment, which is triggered by AOT addition. In the crystalline micelles of BCPs, the soluble blocks are considered to graft to the top and bottom surfaces of the crystalline core. The micellar morphology of crystalline BCPs is related to the reduced tethering density (σ̃).6 AOT reduces the solubility of the corona-forming block after partial replacement by MO, thus the tethering density (σ̃) becomes smaller. This will lead to crystal growth, and further

units because the diffusion rate of AOT is much slower than the complexation rate of AOT bound to the cationic qDM unit in water. Such an effect becomes negligible as AOT increases, leading to uniform aggregation (Figures 4c and 5c). Therefore, the micellar solutions discussed below are all prepared by method 1. When the PCL170-b-qPDM72/MO sample is added with different amounts of AOT, the size of the aggregates also changes significantly (Figure 3). For Z = 0.10, the spheres transform to rodlike micelles (Figure 5a). The SAED pattern shows that the rodlike micelles are crystalline because the major diffraction spots of (110) that belong to PCL are observed. For Z = 0.25, nanosheets with irregular geometries are exclusively observed (Figure 5b). The thickness of the nanosheets characterized by the AFM height image is about 14 nm (Figure S7). Moreover, two symmetrical spots of (200)PCL and one asymmetrical spot of (020)PCL can be clearly observed from the SAED pattern, indicating that the platelets may be considered to be single crystals of PCL. Generally, the preparation of the single crystals is a complicated and time-consuming task (selfseeding method) according to previous reports.52,53 In fact, Eisenberg’s recent study has proposed a strategy in which homopolymers (PCLs) are used to manipulate the crystallization-driven self-assembly of block copolymer micelles. By incorporating the appropriate PCL chains within the micelle core, the spherical micelles in the mixture system of PCL-bPEO and PCL-b-PAA evolve passively to yield the unexpected single crystals, which is very similar to our observation. In our case, the addition of AOT may have a similar effect to homopolymer PCL in the mixture system of PCL-b-PEO and PCL-b-PAA, thus leading to the formation of single crystals. Finally, for Z = 0.50, exclusive fiberlike aggregates are obtained (Figure 5c). The corresponding SAED pattern shows that the core of fiberlike micelles is also crystalline, and typical diffraction spots or rings of (110), (200), (020), and (220) have been verified. E

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Figure 7. (a) DSC heating scan of the freeze-dried particles of PCL170-b-qPDM72/MO and corresponding samples for Z = 0.10 and 0.25. The heating rate is 10 °C/min, and the melting enthalpies are indicated. (b) Synchrotron WAXS patterns of the corresponding freeze-dried samples, noting that the wavelength of the X-ray is 0.124 nm. Samples for DSC and WAXS were freeze-dried from micellar solutions.

morphological transformation of the crystalline micelles occurs.54 For the case of PCL170-b-qPDM72/MO with Z = 0.10 and 0.25, the micelles show low curvature, and the crystallinity of the core-forming block is also enhanced. This conclusion can be further confirmed from the DSC curves of the freeze-dried particles as shown in Figure 7a. The spherical particles formed by PCL170-b-qPDM72/MO have a low melting temperature with a small melting enthalpy of about 2.3 J/g. This result is in accordance with the SAED pattern (Figure 1C), which shows an almost amorphous core of the spherical micelles. By contrast, when 0.10 equiv of AOT is introduced, a profound increase in the melting enthalpy (10.6 J/g) is obtained, indicating that the crystallinity of the rodlike micelles (a smaller σ̃) is enhanced. Platelet crystals with the smallest σ̃ are obtained for Z = 0.25, and a larger melting enthalpy (14.5 J/ g) is detected. The crystallinity of the micellar core is further evaluated by synchrotron WAXS, as shown in Figure 7b. It is found that the spherulitic micelle of PCL170-b-qPDM72/MO is almost amorphous, and no discernible crystalline peaks are observed. However, two obvious crystalline peaks of (110) and (200) are verified for Z = 0.10. And the intensity of the (110) crystalline peak becomes much stronger for Z = 0.25, indicating a higher crystallinity of the core. This result is in accordance with that from SAED and DSC. Therefore, it can be deduced that the various morphologies should be mainly attributed to variable σ̃, which is triggered by the addition of AOT.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 571 86843527. E-mail: [email protected]. ORCID

Zaizai Tong: 0000-0001-7115-0442 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of China (21604073), the Science Foundation of Zhejiang Sci-Tech University (ZSTU) under grant no. 15012082-Y, the Zhejiang Top Priority Discipline of Textile Science and Engineering (2014YBZX02), and the Young Researchers Foundation of the Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University (2016QN01). The authors also thank the Shanghai Synchrotron Radiation Facility for providing time on beamline BL16B1. R.K. Zhang also thanks the National Undergraduate Training Program for Innovation and Entrepreneurship (201610338007).

4. CONCLUSIONS PCL-b-qPDM/MO BCPs with different hydrophilic segments can self-assemble into various morphologies with different crystallinities of the core. AOT can reduce the solubility of the corona-forming block after partially replacing MO, thus the reduced tethering density (σ̃) becomes smaller. This will lead to crystal growth and further morphological transformation of the crystalline micelles, resulting in morphological richness from rods to platelets to fibrils to dendritelike micelles with increasing crystallinity of the micellar core.

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GPC curves, 1H NMR, UV−vis spectra, EDS, and AFM and HRTEM micrographs (PDF)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b02905. F

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DOI: 10.1021/acs.langmuir.6b02905 Langmuir XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.langmuir.6b02905 Langmuir XXXX, XXX, XXX−XXX