Letter pubs.acs.org/macroletters
Corona Liquid Crystalline Order Helps to Form Single Crystals When Self-Assembly Takes Place in the Crystalline/Liquid Crystalline Block Copolymers Zaizai Tong,†,‡,§ Yanming Li,† Haian Xu,† Hua Chen,† Weijiang Yu,† Wangqian Zhuo,† Runke Zhang,† and Guohua Jiang*,†,‡,§ †
Department of Materials Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China National Engineering Laboratory for Textile Fiber Materials and Processing Technology (Zhejiang), Hangzhou 310018, P. R. China § Key Laboratory of Advanced Textile Materials and Manufacturing Technology (ATMT), Ministry of Education, Hangzhou 310018, P. R. China ‡
S Supporting Information *
ABSTRACT: Crystalline/ionic liquid crystalline block copolymers (BCPs) with various compositions have been successfully prepared by sequential reactions. The effect of corona liquid crystalline order on self-assembly of BCPs in selective solvent is investigated in detail. It is found that twodimensional single crystals with well-developed shapes are formed when the liquid crystalline order is present. By contrast, ill-developed platelets with small size or one-dimensional worm-like micelles are assembled if the liquid crystalline order of the corona segments is lost. It is speculated that the preferred parallel arrangement of liquid crystalline block enables it to expose more growth front of crystals. Accordingly, epitaxial crystallization will proceed readily, leading to fabrication of the well-defined single crystals.
A
modified by adding additives or altering pH in aqueous solution, which allows to trigger morphological transformation of the micellar morphology by alteration of corona conformation.7,9 Related works are well-documented by Xu in a poly(ε-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) micellar solution.10 Inspired by the above results, a question is raised: what will be when liquid crystalline (LC) order is introduced to the micellar corona? It has been widely reported that morphological richness is encountered when the core-forming block possesses liquid crystallinity.11 Nevertheless, to our best of knowledge, the effect of corona LC order on the micellar morphology has not been yet explored. Generally, the LC block tends to parallel arrangement along the core surface with a low curvature to minimize the surface energy, thus it will provide an additional free energy to Fcorona. Despite that the concentration of micellar solution is low, once micellization or crystallization of the core-forming block takes place, the local concentration of the corona LC blocks that are tethered to both surfaces of the crystals is otherwise high.12 From this perspective, the freshly emerged LC order in the corona may in turn alter the self-
mphiphilic block copolymers (BCPs) can spontaneously form various morphologies, such as spheres, cylinders, and lamellae, which are driven by the balance of solvent-philic and solvent-phobic force.1 Recently, BCP micelles with a crystalline core-forming block have received rapidly increasing attention2 since crystallization can add some unique characters, such as “living growth”3 and construction of “block co-micelles”,4 to the crystalline micelles. Researchers have found that the crystallization-driven self-assembly (CDSA) is a powerful approach to prepare the aforementioned nano-objects, which is complicated to obtain in amorphous micelles.5 Despite the excellent works of crystalline micelles that have been carried out, most of them are focused on the crystalline blocks,6 while the effort devoted to the conformation of corona segments has been scarce. In crystalline micelles, the final morphology is determined by the competition of the stretching of soluble block and crystallization of the core-forming block.7 When the contribution of free energy of the corona (Fcorona) to the total free energy cannot be neglected, the micellar morphologies are not restricted to the lamellae, and spherical micelles or other rich morphologies can be observed. For example, scrolled single crystals were obtained by Cheng et al. in a copolymer of polystyrene-b-poly(ethylene oxide)-b-poly(1butene oxide) with a crystalline midblock.8 The unbalanced surface stress created by asymmetric soluble blocks could exert an additional energy to the Fcorona, which finally resulted in a nonflat single crystal. On the other hand, the Fcorona can also be © 2016 American Chemical Society
Received: June 5, 2016 Accepted: June 30, 2016 Published: July 5, 2016 867
DOI: 10.1021/acsmacrolett.6b00428 ACS Macro Lett. 2016, 5, 867−872
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ACS Macro Letters
AOT), as indicated in Figure 1. This analysis shows that there is close to equimolar AOT:PDM stoichiometry, indicative of essentially full complexation and no excess AOT, for all of the BCP samples. This is consistent with the energy-dispersive spectrometer (EDS) result, which shows no residual Na+ and I− for any of the complexed BCPs (Figure S2). Since the AOT surfactant tends to tightly bind to the cationic unit, the ionic complexed qDM/AOT unit can form an intimate ion-pair in selective solution.13 The self-assembly of crystalline/ liquid crystalline block copolymers is first investigated at various methanol/DMF ratios since methanol is a selective solvent for the qPDM/AOT block. When the methanol content reaches a critical value, PCL-b-qPDM/AOT can assemble together by the cooperation of micellization and crystallization of the hydrophobic PCL block. To optimize the self-assembled structures, methanol was progressively dropped into the DMF solution of PCL 170 -b-qPDM 22 /AOT. This process was monitored by UV−vis transmittance experiment as shown in Figure S3. It can be observed that the transmittance of the solution decreased until the methanol content reaches to 0.6− 0.7, and further addition of methanol results in a larger transmittance. It is known that the transmission signal is related to both the size and concentration of the aggregations. Larger size and denser concentration of the micelles will result in a more turbid solution, thus a lower transmittance. This phenomenon exhibits that the size of assemblies is similar when the methanol content is larger than 0.7 (Figure S4), and further addition of methanol could dilute the self-assembly solutions and thus a less turbid solution. A similar observation is also reported by Jiang.14 As a result, the methanol content is fixed to 67% (methanol/DMF = 2) in the following investigation unless with special instructions. Self-assembly of the PCL-b-qPDM/AOT with various PCL weight fractions (f PCL) was first explored. The TEM results shown in Figure 2 give morphologies of the self-assembled PCL-b-qPDM/AOT with various compositions. It is found that all the BCPs can form two-dimensional (2D) platelets with well-developed geometry, but different in size. To confirm the crystallinity of the PCL block, selected area electron diffraction (SAED) measurement was performed on a single platelet. The distinct two (200) and four (110) diffraction spots are clearly observed (as labeled in the inset of Figure 2a). Therefore, these 2D platelets can be considered as self-assembled analogues of
assembled behavior of block copolymer, leading to an unexpected morphology. In the present work, the crystalline/ionic liquid crystalline block copolymers with various compositions were prepared by sequential reactions, yielding poly(ε-caprolactone)-b-poly[quaternized 2-(dimethylamino)ethyl methacrylate]/bis(2-ethylhexyl) sulfosuccinate (PCL-b-qPDM/AOT for short), as described in Scheme 1. Samples with various compositions have Scheme 1. Synthetic Route of Complexed Diblock Copolymers, PCL-b-qPDM/AOT
been synthesized, and their molecular characteristics are given in Table S1. All the BCPs have a narrow polydispersity within PDI less than 1.16 (Figure S1). From the area ratio of the 1H NMR corresponding to the PCL (signal c) and PDM (signal j) in the PCL-b-PDM spectrum (Figure 1), the composition of BCPs can be calculated (Table S1). In the PCL-b-qPDM spectrum, the newly emerging signals of i, j, and k at 4.66, 4.26, and 3.60 ppm, respectively, imply the successful quaternized reaction between PDM blocks and CH3I. Furthermore, the absence of the unquaternized PDM signal, j at 2.58 ppm in the PCL-b-PDM spectrum, indicates the essentially complete quaternization, i.e., 100% quaternization. Finally, in the PCL-b-qPDM/AOT spectrum, a prominent upfield shift of the signals i, j, and k is detected after complexation with AOT, which implies AOT molecules are well electrostatically bounded to cationic units of qDM. On the other hand, the degree of complexation is determined by comparison of the signals e and 3 (belong to the PCL and AOT) with the b, d, and 4 signals (also belong to the PCL and
Figure 1. 1H NMR spectra of a representative block copolymer PCL70-b-PDM17 and its quaternized PCL70-b-qPDM17 and complexed derivatives, PCL70-b-qPDM17/AOT, with the peak assignments indicated. Solvent: CDCl3 for PCL70-b-PDM17 and PCL70-b-qPDM17/AOT and DMF-d7 for PCL70-b-qPDM17. * represents the residual solvent. 868
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platelet micelles is generally uncommon relative to other 1D morphologies that have been considerably controlled recently.16 For the PCL170-based family, with a decreasing f PCL, the axial and crosswise lengths of the single crystals are both decreased (Figure 2a−c). The size of the platelet in PCL170-bqPDM72/AOT is decreased to 3.0 μm in length and 1.2 μm in width (Figure 2c), which is obviously smaller than that in the other two samples. The detailed information along the height direction is further characterized with AFM measurement shown in Figure 2d (enlarged one is in Figure S5). One can see that the thickness of single crystals is about 13 nm, which covers one PCL lamellae and two soluble block layers tethered to two faces of the PCL lamellae. Based on the method reported by Cheng et al.,12 PCL and the qPDM/AOT layer are estimated to be 4.4 and 4.3 nm, respectively (see Supporting Information). The thickness of an individual layer of qPDM72/ AOT is much thicker than that of a longer amorphous block, such as P2VP170 (2.7 nm)17 or PEO114 (2.2 nm)18 block reported in the literature. This in turn confirms that the qPDM72/AOT block shows a rigid nature (possibly due to LC order) compared with an amorphous block. As for the PCL70 series, with a similar f PCL as PCL170-b-qPDM32/AOT, the single crystal of PCL70-b-qPDM17/AOT is quite similar to that of PCL170-b-qPDM32/AOT. This tendency is also true for PCL70b-qPDM32/AOT and PCL170-b-qPDM72/AOT, which has a similar axial ratio (the ratio of length and width) value of about 2.5. However, the crystal in PCL70-b-qPDM32/AOT is evidently larger than that in PCL170-b-qPDM72/AOT, which could possibly be attributed to the short PCL chains that have a better mobility and thus facilitate to crystallization. So far there are numerous reports on crystalline micelles, but single crystals prepared in a micellar solution are seldom reported.17,19 The solvent-soluble segments decorated by AOT molecules may have a crucial effect on the formation of single crystals since it presents a fully rigid ionic block that possesses liquid crystallinity. Once the concentration of rigid blocks is higher than the critical one, the rigid blocks may present LC habit.20 To study the effect of LC order on the self-assembly
Figure 2. Effect of block composition on the self-assembly morphologies: (a) PCL170-b-qPDM22/AOT, inset shows the SAED pattern of the single crystals; (b) PCL170-b-qPDM35/AOT; (c) PCL170-b-qPDM72/AOT; (d) AFM height image of PCL170-bqPDM72/AOT, the size is 10 × 10 μm; (e) PCL70-b-qPDM17/AOT; and (f) PCL70-b-qPDM32/AOT.
conventional polymeric single crystal, which can be laborious or challenging to prepare otherwise.15 The formation of 2D
Figure 3. TEM images of (a) PCL170-b-qPDM22/AOT, (b) PCL170-b-PDM85 ( f PCL = 0.59), and (c) PCL170-b-qPDM22/AOT0.75/MO0.25 ( f PCL = 0.61) with heterogeneous soluble block; (d) WAXS profiles of PCL170-b-qPDM22/AOT and PCL170-b-qPDM22/AOT0.75/MO0.25, the break of angles at 5−6° is due to the signal of polyimide that is difficult to subtract from the background; (e) illustration of the chain conformation of soluble block tethering to the surface of crystals with or without liquid crystalline order. 869
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Figure 4. TEM images of (a) PCL170-b-qPDM72/AOT; (b) PCL170-b-qPDM72/AOT0.5/MO0.5 with heterogeneous soluble block; and (c) WAXS profiles of PCL170-b-qPDM72/AOT and PCL170-b-qPDM72/AOT0.5/MO0.5, and samples for WAXS measurement were collected by centrifugation of micellar solution.
is detected at methanol/DMF = 1, although it is rather weak (Figure S7). Consideration of a much lower concentration of qPDM/AOT (1.7 wt %), in this case, the observation of LC order, should be contributed to the micellization or crystallization of PCL (a weak PCL crystalline peak is detected as indicated by an arrow), where the local concentration of the LC block is condensed. With further dropping of methanol, the LC peak progressively disappears because of a decreasing concentration of qPDM/AOT. Meanwhile, the crystalline peaks of PCL become stronger because of a poorer solvent for PCL after adding more methanol (Figure S7). From the above WAXS result, we can reasonably confirm LC order indeed exists when micellization or crystallization of PCL takes place. As it comes to the PCL170-b-qPDM72/AOT with a small f PCL, single crystals in smaller size are fabricated as shown in Figure 4a. The single crystals formed by PCL170-b-qPDM72/AOT also show a strong diffraction intensity at low angles (Figure 4c), indicating the corona segment possesses liquid crystallinity. Based on Bragg’s equation, the calculated distance of LC order is 2.80 nm, which is very close to that in PCL170-b-qPDM22/ AOT. Meanwhile, the (110), (111), and (200) crystalline planes of PCL are present at large angles, implying the aggregation in Figure 4a is highly crystalline. When 0.5 equiv of MO is mixed in the corona segment, the self-assembled aggregation is inclined to one-dimensional worm-like micelles, as indicated in Figure 4b, and the WAXS profile of PCL170-bqPDM22/AOT0.5/MO0.5 shows that the heterogeneous block loses its liquid crystallinity, as the diffraction peak at low angles disappears. Although the morphology of PCL170-b-qPDM22/ AOT0.5/MO0.5 changes significantly when its soluble segment loses liquid crystallinity, the crystalline diffractions of PCL at large angles show the core is crystalline, indicating the formation of worm-like micelles is also driven by crystallization. In the present case, the core of final nanoaggregates is crystalline regardless of the morphologies, implying the formation of nano-objects is driven by the crystallization. Generally, the micellar morphology is controlled by the competition between micellization and crystallization.23 When crystallization is dominated, the morphology tends to be large. If micellization prevails, then crystallization of the core is restricted within the micelles. The evolution of 2D single crystal to 1D rod-like micelle to 0D sphere is observed when the solvent quality toward the crystallization of PCL is altered (Figure S8), indicating crystallization is dominated to determine the morphology. Similarly, the crystallization ability of PCL should be changed when the conformation of the corona segment is altered, though it is not so profound as solvent. The LC order in the corona can create an additional
morphologies, controlled experiments were conducted. Block copolymers with the same crystalline block, yet with different soluble blocks, have been designed. First, we have investigated the PCL170-b-qPDM22/AOT sample that has a high PCL fraction. Uniform single crystals with an elongated truncated lozenge shape are observed in PCL170-b-qPDM22/AOT (Figure 3a). However, PCL170-b-PDM85 ( f PCL is similar to that of PCL170-b-qPDM22/AOT, Table S1) with a coil soluble block exhibits leaf-like platelets with size of about 1.5 × 0.8 μm as shown in Figure 3b. Such an observation is consistent with the result reported by Chen.19 Moreover, the shape of platelets is ill-developed, and the size is evidently smaller than that in PCL170-b-qPDM22/AOT. On the other hand, another sample that has a heterogeneous soluble block (PCL170-b-qPDM22/ AOT0.75/MO0.25) is designed, and the chemical structure is demonstrated in Figure 3e. PCL170-b-qPDM22/AOT0.75/MO0.25 was prepared from PCL170-b-qPDM22/MO21 via ion exchange because of the low hydration energy of AOT than that of MO.22 When 0.25 equiv of MO is mixed, the heterogeneous block possesses no liquid crystallinity due to the disturbance of original order arrangement of qPDM/AOT. This can be confirmed from the WAXS profiles of dried micellar aggregations at low diffraction angles (Figure 3d). One can see that the aggregation of PCL170-b-qPDM22/AOT0.75/MO0.25 lost its LC order since no diffraction peak is detected at small angles. By contrast, single crystals of PCL170-b-qPDM22/AOT display a strong diffraction peak ranging from 2 to 3°, indicative of a LC order in the corona segment. Based on Bragg’s equation, the long period of the liquid crystalline order is calculated to be 2.76 nm as schematically demonstrated in Figure 3e. In order to reveal the presence of corona LC order in the solution, different concentrations of qPDM42/AOT and qPDM42/AOT0.5/MO0.5 homopolymers (Table S1) in methanol/DMF solution were prepared. Then synchrotron WAXS experiment was conducted to examine the LC order (Figure S6). It is indicated that qPDM42/AOT exhibits LC order when the weight fraction reaches to 33 wt % since a discernible peak is observed, and a higher concentration results in an obvious peak. Such an observation is consistent with the result reported by Huang.20 By contrast, qPDM42/AOT0.5/MO0.5 shows no LC order at the same condition. To further reveal the presence of LC order when micellization or crystallization of PCL takes place in a BCP system, a higher concentration of PCL170-bqPDM22/AOT is prepared (50 mg/0.5 mL, BCP/DMF) since the LC order and crystallinity of PCL are difficult to detect at a micellar solution (0.2 mg/mL). With dropping methanol, micellization of PCL starts to occur. Then the LC of the corona 870
DOI: 10.1021/acsmacrolett.6b00428 ACS Macro Lett. 2016, 5, 867−872
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tion, Zhejiang Sci-Tech University (2015QN08), and the National Natural Science Foundation of China (51373155). The authors also thank beamline BL16B1 (Shanghai Synchrotron Radiation Facility) for providing the beam time.
interaction between soluble segments, which is favored in free energy, leading to a parallel arrangement (Scheme 2). This Scheme 2. Schematic Description of the Effect of Corona LC Order on the Self-Assembled Morphology
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allows the lateral surfaces at the ends of the micellar core to be highly exposed, and then epitaxial crystallization by the unimers will proceed readily, leading to the formation of 2D single crystals. By contrast, when the LC order is absent from the corona segment, the driving force of parallel arrangement of the corona blocks is lost. Accordingly, the soluble segments tend to be a random coil to minimize the conformational entropy. The overcrowded chains around the core surface may bend to cover the lateral surfaces of the crystals to minimize the surface energy. Thus, epitaxial crystallization is restricted, leading to formation of platelets in small size or even 1D worm-like micelles with a high curvature. Such a possible mechanism of the LC order on the self-assembled morphologies can be schematically depicted in Scheme 2. In summary, the LC order in the soluble segment allows the block to parallelly arrange along the surface core with a small curvature, favored by the free energy. As a result, more growth front of crystals is exposed, which is facilitated to epitaxial crystallization for the unimers. Consequently, well-developed 2D single crystals are fabricated. By contrast, ill-developed platelets with small size or even 1D worm-like micelles are obtained if the corona liquid crystalline order is absent, which is result from a retard crystallization of PCL.
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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/acsmacrolett.6b00428. Detailed experimental procedures and TEM or AFM morphologies at various conditions (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Tel.: +86 571 86843527. E-mail:
[email protected] (G.H. Jiang). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Science Foundation of Zhejiang Sci-Tech University (ZSTU) under Grant No. 15012082-Y, Zhejiang Top Priority Discipline of Textile Science and Engineering (2014YBZX02), the Young Researchers Foundation of Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Educa871
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