Note Cite This: Macromolecules XXXX, XXX, XXX−XXX
Specific Disassembly of Lamellar Crystalline Micelles of Block Copolymer into Cylinders Xiang-Yue Wang, Rui-Yang Wang, Bin Fan, Jun-Ting Xu,* Bin-Yang Du,* and Zhi-Qiang Fan MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *
■
interaction between corona and additive.43 Here we would like to develop a new way to induce disassembly of crystalline micelles via core-additive interaction. However, such an interaction should not be too strong so that the crystallinity of the micellar core would not be completely destroyed. Since n-hexanol is usually used to culture single crystals of PCL,47 it can dissolve PCL at high temperature, but PCL can also crystallize from n-hexanol solution at low temperature. As a result, we speculate that there exists a weak interaction between n-hexanol and PCL in the micellar core, and n-hexanol was chosen as the additive.
INTRODUCTION Recently crystallization-driven self-assembly (CDSA) of block copolymers (BCPs) has gained great attention because of its unique characteristics and advantages.1−3 First, the shape of crystalline micelles of BCPs can be regulated by changing crystallization conditions as well as chain structure.4−25 Second, since polymer crystals can grow, crystalline micelles with a uniform size may be prepared under suitable crystallization conditions.26−32 Moreover, complex and hierarchical structures in one, two, and three dimensions can be constructed via epitaxial crystallization.33−40 However, crystallization also has a strong vitrification effect on the formed micelles. This means that reversible switch of morphology is difficult for the crystalline micelles of BCPs upon environmental stimuli. Winnik and Manners reported that cylindrical micelles of crystalline poly(ferrocenyldimethylsilane) (PFDMS)-containing BCPs could be disassembled into short rods by sonication or spheres by heating to melt PFDMS.41,42 Our previous work showed that addition of some organic small molecules to the aqueous solution of poly(ε-caprolactone)-bpoly(ethylene oxide) (PCL-b-PEO) could lead to fragmentation of the long cylindrical micelles or growth of quasi-spherical micelles; thus, a reversible morphological transformation of crystalline micelles was achieved under mild conditions.43 The importance of study on the reversible morphology switch of crystalline micelles, especially disassembly, lies in following two aspects. First, the crystalline micelles of BCPs sometimes undergo environmental change in practical applications. The chemical substance in the new environment may induce the change of micelle size and shape; thus, the performance of micelles is affected accordingly.44−46 Second, additive-triggered morphological transformation may endow the crystalline micelles with the environment-responsive characteristic, which will be beneficial to their applications. Although transformation of long cylindrical crystalline micelles of BCPs into short rods or spheres has been realized, so far there is no report on specific disassembly of lamellar crystalline micelles into cylinders, to our best knowledge. The particularity of such a disassembly process results from the anisotropy of polymer crystals, which requires that the lamellar crystals decompose along a specific direction to form uniform cylinders. Herein we synthesized a poly(ε-caprolactone)-bpoly((dimethylamino)ethyl methacrylate) (PCL-b-PDM) BCP, and the disassembly behavior of its crystalline micelles in the aqueous solution was studied. On the other hand, in our previous work the disassembly of PCL-b-PEO crystalline micelles was triggered by the hydrogen (H)-bonding © XXXX American Chemical Society
■
EXPERIMENTAL SECTION
Materials. 1,1,4,7,10,10-Hexamethyltriethylenetetramine (HMTETA) and 2-bromoisobutyryl bromide (Aladdin, 98%) were used as received. Ethylene glycol (99%), n-hexanol (98%), and n-pentanol (98%) were purchased from Sinopharm Chemical Reagent Co., Ltd., and used as received, except for FT-IR measurement, in which nhexanol was dried by anhydrous magnesium sulfate. Et3N (Acros, 99%) was dried with NaOH. ε-Caprolactone (ε-CL, TCI, 99%) and 2(dimethylamino)ethyl methacrylate (DM, TCI, 98.5%) were distilled at reduced pressure just prior to use. Tin(II) 2-ethylhexanoate (Sn(Oct)2, Acros, 99%) was dried under reduced pressure and diluted with dried toluene. Copper bromide (CuBr, Aldrich, 98%) was washed with acetic acid and diethyl ether several times and dried under vacuum. Toluene and tetrahydrofuran (THF) for polymerization were refluxed with sodium prior to use, and dimethylformamide (DMF) was used as-received. Synthesis and Characterization of PCL-b-PDM. Poly(εcaprolactone) with an end group Br (PCL-Br) was first synthesized via ring-opening polymerization (ROP) of ε-CL using 2-hydroxyethyl 2-bromoisobutyrate (HEBiB) as the initiator,48 and then atom transfer radical polymerization (ATRP) of DM was carried out using PCL-Br as macroinitiator.49 The details for synthesis and characterization of PCL-b-PDM are presented in the Supporting Information. The obtained PCL-b-PDM with a polydispersity (Mw/Mn = 1.13) was denoted as PCL67-b-PDM41, where the subscripts are the polymerization degrees of the PCL and PDM blocks, respectively. Preparation of the Micellar Solution. The block copolymer (BCP) was first dissolved in dimethylformamide (DMF), a good solvent to all blocks. The homogeneous solution was injected with redistilled water (10% v/v) at a slow rate (1.0 mL/h) under mild stirring. Then the solution was dialyzed against deionized water to remove the organic phase at room temperature. The micellar solution was diluted with redistilled water to a certain concentration. For TEM, AFM, and DLS tests, if not specified, micellar concentration was 0.1 Received: November 12, 2017 Revised: February 3, 2018
A
DOI: 10.1021/acs.macromol.7b02406 Macromolecules XXXX, XXX, XXX−XXX
Note
Macromolecules mg/mL. For WAXS and DSC measurements, concentrated micellar solutions (1 mg/mL) were prepared in order to get a mass of freezedried micelles. Characterizations. Gel permeation chromatography (GPC) was performed on a Waters system calibrated with standard poly(methyl methacrylate) using DMF with 0.05 M LiBr and 2‰ Et3N as the eluent at a flow rate of 1.0 mL min−1. 1H nuclear magnetic resonance (NMR) spectra (400 MHz) were recorded at room temperature on a Bruker Avance III 400 instrument using CDCl3 as solvent. The micellar morphology was observed by transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM was performed on a JEOL JEM-1230 electron microscope at an acceleration voltage of 80 kV. TEM samples were prepared by dropping 3 μL micellar solution onto the carbon-coated copper grids frozen with liquid nitrogen and freeze-drying under vacuum. AFM was carried on a Veeco multimode scanning probe microscope in the tapping mode. The measurements were performed using a Si3N4 tip with a scanning rate of 1 Hz. AFM samples were prepared by dropping 10 μL micellar solution onto freshly cleaved mica sheet and dried in a vacuum oven at room temperature overnight. The apparent hydrodynamic diameter of micelles in aqueous solution was measured by dynamic light scattering (DLS). DLS measurement was carried out on Brookhaven instrument BI-90Plus with a laser wavelength of 657 nm at room temperature. The data were processed with a software supplied by Brookhaven to yield the intensity distributions of the micelles. Fourier-transform infrared (FT-IR) spectra were performed on a Thermo Fisher Scientific LLC Nicolet 6700 spectrometer. The neat PCL homopolymer, neat PDM homopolymer, and PCL/n-hexanol mixture were dissolved in CH2Cl2, while PDM was simply dissolved in equivalent n-hexanol at room temperature to obtain a PDM/n-hexanol mixture. The sample solution was dropped onto KBr disk at room temperature and scanned among the range of 400−4000 cm−1 with a spectral resolution of 2 cm−1. The absorbance peaks were deconvolved using Origin Software with multiple Lorentz functions.50,51 The crystallinity of PCL in PCL-b-PDM bulk or freeze-dried micelles was measured by wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). WAXS patterns were recorded on a BL16B1 beamline in Shanghai Synchrotron Radiation Facility (SSRF). The wavelength of the X-ray was 1.24 Å. The range of scanning angle was 2θ = 1°−30°, and the scanning step was 0.02°. The data were analyzed with JADE software. DSC measurements were performed on a TA Q200 DSC calorimeter. Specimens were weighed 3−5 mg in an aluminum pan and heated from 20 to 100 °C at a rate of 10 °C/min.
Figure 1. TEM images of PCL67-b-PDM41 original micelles in aqueous solution (a) and the micelles after 30 days of addition of n-hexanol (0.74% v/v) (b).
Figure 2. WAXS patterns (a) and DSC heating scans (b) of bulk PCL67-b-PDM41 (1), freeze-dried lamellar micelles (2), and freezedried cylindrical micelles formed by disassembly of the lamellar micelles in the present of n-hexanol (3). The wavelength of X-ray (λ) is 1.24 Å.
disassembled into cylinders after 30 days, as revealed by the TEM image (Figure 1b). The length and width of the cylindrical micelles measured from the TEM image are 1250 ± 430 nm and 48 ± 3 nm, respectively. The corresponding length and width distributions are 1.12 and 1.01, respectively. This shows that the formed cylinders are uniform in width but not so uniform in length. We notice that the length of some longer cylinders is nearly 2 μm (Figure 1b), which is similar to the maximal length of the lamellar micelles along the a-axis. However, the possibility of growth on the cylinders cannot be completely excluded. Figure 3 shows the AFM height image and height profile of the PCL67-b-PDM41 micelles after addition of n-hexanol. It is found that the cylindrical and lamellar micelles have a similar height (∼15−17 nm). Combining the TEM and AFM results, one can see that the height of the disassembled micelles is evidently smaller than the width in the dry state. This makes the disassembled micelles more like ribbons. However, in the solution the soluble PDM block tends to adopt a much more extended conformation due to solvation and overcrowding in the micellar corona. It means that the height of the disassembled micelles in solution may become comparable with the width. As a result, we still would like to call the disassembled micelles as “cylinders”. WAXS and DSC characterizations reveal that the crystallinity of PCL in the micellar core is retained after the lamellar micelles are disassembled into cylinders (Figure 2). However, when comparing the WAXS pattern of the cylindrical and lamellar micelles (Figure 2a), one can see that the peak area ratio of the (110) reflection over the (200) reflection (A(110)/ A(200)) is 5.64 for the cylindrical micelles and 2.83 for the original lamellar micelles (Table S2), showing the significant
■
RESULTS AND DISCUSSION Disassembly of PCL67-b-PDM41 Micelles Induced by nHexanol. The PCL67-b-PDM41 micelles with crystalline PCL as core were prepared by dialysis of the DMF (common solvent for PCL and PDM) solution of PCL67-b-PDM41 against deionized water. TEM image shows that PCL67-b-PDM41 forms uniform spindle-like lamellar micelles in the aqueous solution (Figure 1a), which is similar to that reported in the literature.52 The length of lamellar micelles is 1.9 ± 0.1 μm along the a-axis and 700 ± 50 nm along the b-axis, and the size distributions (Lw/Ln) along the a- and b-axes are both 1.01 (see Supporting Information for detailed calculation). The WAXS pattern and differential scanning calorimetry (DSC) heating scan of the dried lamellar micelles are presented in Figure 2. The (110) and (200) reflections of PCL orthorhombic crystals can be clearly observed by WAXS (Figure 2a), and the lamellar micelles exhibit a melting peak around 58.6 °C (Figure 2b).48 These results confirm the crystallinity of the PCL blocks in the lamellar micelles. When a small amount of n-hexanol (0.74% v/ v) is added to the aqueous solution of PCL67-b-PDM41, it is observed that most of the lamellar micelles are specifically B
DOI: 10.1021/acs.macromol.7b02406 Macromolecules XXXX, XXX, XXX−XXX
Note
Macromolecules
Figure 3. AFM height image (a) and height profile (b) of PCL67-b-PDM41 micelles in aqueous solution after 8 days of addition of n-hexanol (0.74% v/v).
decrease in the relative intensity of the (200) reflection after disassembly. This implies that disassembly of the lamellar micelles proceeds in the manner of cleavage along the (200) crystal plane. In PCL crystals, the d-spacing of the (200) crystal plane is smaller than that of the (110) crystal plane. This indicates that the PCL chains are closer and thus the chemically linked PDM blocks are more crowded along the direction of the (200) crystal plane, leading to stronger stress in this direction. Consequently, the stress can be released more effectively by cleavage along the (200) crystal plane. Moreover, because of such a cleavage direction and the spindle-like shape of the original lamellar micelles, the formed cylindrical micelles after disassembly have a broader length distribution. The TEM and AFM results show that addition of n-hexanol can induce specific disassembly of PCL67-b-PDM41 lamellar crystalline micelles into cylinders. Eisenberg et al. reported that cylindrical crystalline micelles of PCL-b-PEO could be rafted into lamellar ones with tassels upon addition of a small amount of PCL homopolymer.53,54 Herein the n-hexanol-induced disassembly can be viewed as the inverse process of such a morphological transformation. Effect of n-Hexanol Concentration and Disassembly Kinetics. Different amounts of n-hexanol were added into the aqueous solution containing PCL67-b-PDM41 lamellar crystalline micelles and the variations of micellar size and shape with time were monitored by dynamic light scattering (DLS) and TEM, respectively. Figure 4 shows the average apparent
hydrodynamic diameters of the micelles at different times after addition of n-hexanol, which were measured by DLS and derived from the intensity distribution (the intensity distribution and CONTIN plots are shown in Figure S7). One should bear in mind that considering the micelles are lamellar and cylindrical other than spherical, only the relative values of apparent diameter are meaningful. It is observed that addition of n-hexanol causes a dramatic decrease of the apparent diameter in the initial time (
