Regulated Fragmentation of Crystalline Micelles of Block Copolymer

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Regulated Fragmentation of Crystalline Micelles of Block Copolymer via Monoamine-Induced Corona Swelling Bin Fan, Jin-Qiao Xue, Xiao-Shuai Guo, Xiao-Han Cao, Rui-Yang Wang, 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

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S Supporting Information *

ABSTRACT: Because long cylindrical crystalline micelles of block copolymers (BCPs) are similar to the fibril structures related to some severe diseases to some extent, study of the disassembly process of crystalline micelles of BCPs may provide some conceptual inspiration to the therapy of these diseases. Herein the effect of amines on the fragmentation of cylindrical crystalline-polyelectrolyte polyethylene-block-poly(acrylic acid) (PE-b-PAA) micelles is investigated. It is found that long crystalline cylindrical micelles (150−400 nm) can be fractured into short stublike ones (20−50 nm) by adding monoamines like diethylamine, triethylamine, and lysine, while addition of diamines such as ethylenediamine and 2,2′-(ethylenedioxy)di(ethylamine) or inorganic base like ammonia and sodium hydroxide has little effect on the morphology of cylindrical PE-b-PAA micelles. Fourier transform infrared (FT-IR) characterization shows that the interaction between PAA and amines is electrostatic attraction. The fragmentation of PE-b-PAA cylindrical micelles can be ascribed to the stress release of PAA corona chains swollen by amines, which is related to the effective functionality and molecular size of amines. Calculation of free energy verifies the thermodynamic accessibility of the fragmentation of PE-b-PAA cylindrical crystalline micelles induced by monoamines.

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INTRODUCTION The self-assembly process based on soft matter has long been a fascinating topic in nanotechnology, bioscience, and material science.1 As a simple pathway to well-defined nanostructures, crystallization-driven self-assembly (CDSA) of block copolymers (BCPs) has received increasing attention in recent years.2−4 The CDSA of different BCPs with crystalline polyferrocenyldimethylsilane (PFDMS), 5−7 polyethylene (PE),8−10 poly(3-hexylthiophene) (P3HT),11−13 poly(L-lactide) (PLLA),14−16 poly(ethylene oxide) (PEO),17,18 poly(εcaprolactone) (PCL), 19−24 oligo(p-phenylenevinylene) (OPV),25,26 polyselenophene,27 etc., as the core-forming block has already been reported. Various methods like temperature,28−30 solvent quality,31−34 stereocomplexation,35 pH,36 adding extrinsic salts,37−40 and homopolymer41−48 can be adopted to regulate the morphology of crystalline BCPs micelles. The “living” epitaxial growth characteristic of crystalline BCPs also endows CDSA of block copolymers to © XXXX American Chemical Society

generate well-defined one-, two-, and three-dimensional nanostructures.47−52 Nevertheless, CDSA is a one-way process due to the strong solidification effect of crystallization and the fragmentation/ disassembly of crystalline BCP micelles can only be accessible through applying intensive external force such as thermal treatment and sonication to break the crystal structure of the core block.5,53 Therefore, crystalline micelles of BCPs are difficult to change their morphology reversibly upon mild external stimuli. As far as we know, there are only limited reports on the inverse process of CDSA or the disassembly of crystalline BCP micelles. Sonication was utilized to break long cylindrical micelles containing different crystalline coreforming blocks, such as PFDMS,54 P3HT,11 PCL,24 OPV,25 Received: May 29, 2018 Revised: September 5, 2018

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DOI: 10.1021/acs.macromol.8b01131 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules and polyselenophene,27 into shorter ones. There is no need to emphasize the importance of BCPs self-assembly. However, the disassembly of BCPs is equally important, which is usually ignored. For example, the irreversible fibrosis process of certain peptides or cells in human body is believed to be pathogeny of many severe diseases such as Alzheimer’s disease and organ fibrosis.55−58 Hopefully, research on the fragmentation/ disassembly of fibroid crystalline BCP micelles may provide important enlightenments for disassembly of amyloid fibrils59 and thus therapy of these deadly diseases. In our previous work, the reversible fragmentation and growth of cylindrical PCL-b-PEO crystalline micelles was realized in aqueous solution by utilizing the hydrogen-bonding interaction between the PEO coronal block and phenol or Lthreonine, respectively.21 Moreover, the disassembly of lamellar crystalline micelles into cylinders was also achieved for the BCPs with PCL as the core-forming block via addition of n-hexanol.60 These results provide new effective approaches to disassemble crystalline micelles of BCPs. However, the fragmentation mechanism of crystalline micelles and the influence of different small molecules have not been fully understood yet. In the present work, we prepared cylindrical polyethyleneblock-poly(acrylic acid) (PE-b-PAA) crystalline micelles in aqueous solution. Fragmentation of these long crystalline cylinders was achieved by addition of different monoamines like lysine, diethylamine (DEA), and ethylamine (TEA), which can interact with the PAA corona via electrostatic force. The effects of functionality and molecular size of amines were investigated. Furthermore, the free energy changes resulting from the corona swelling and fragmentation of the crystals in the micellar core were compared.

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Scheme 1. Molecular Structures of PE63-b-PAA74 Block Copolymer and Organic Amines Used in This Worka

a

DEA: diethylamine; TEA: triethylamine; EDA: ethylenediamine; EDDA: 2,2′-(ethylenedioxy)diethylamine.

with the software supplied by Brookhaven, and the apparent hydrodynamic diameters (Dh) were derived in terms of the Stokes− Einstein equation based on the assumption of spherical shape for the micelles.62 Wide-Angle X-ray Scattering (WAXS). WAXS patterns were performed on a beamline BL16B1 at Shanghai Synchrotron Radiation Facility. The wavelength of the X-ray is 0.123 nm. Aqueous solution of PE-b-PAA micelles was directly freeze-dried to remove water before measurement. Fourier Transform Infrared (FT-IR). FT-IR spectra of PAA/ organic amine mixtures were recorded on a Nicolet 6700 spectrometer at room temperature. The scan range is from 2000 to 400 cm−1 with a resolution of 2 cm−1. The data were processed using Thermo Nicolet OMNIC FTIR software. Heavy water (D2O) was used as solvent to reduce the overlap of the amide band with the strong water band. The sample solutions were prepared by dissolving PAA (Mw 45000) in D2O with a concentration of 2.0 wt %, followed by addition of amines (if needed). Atomic Force Microscopy (AFM). AFM observations were performed on Veeco multimode scanning probe microscope with Nano IVa controller in tapping mode. The scanning frequency was 1 Hz. AFM samples were prepared by dropping the micellar solution on freshly cleaved mica sheet. Preparation of Micellar Aqueous Solution. Generally, PE-bPAA BCP was first dissolved in DMF or DMF/o-xylene mixture with a concentration of 0.5 mg/mL; then the solution was held at 130 °C in an oil bath for 1 h, followed by quick transfer to another water bath with preset crystallization temperature (Tc = 75 or 30 °C) for 18 h to ensure the complete crystallization or micellization of BCP. Afterward, DMF/o-xylene solution of PE-b-PAA BCP micelles was dialyzed against DMF to remove o-xylene, followed by dialysis against deionized water (the molecular cutoff of dialysis tubes is 3500 g/mol). After dialysis, the micellar solution was transferred to a volumetric flask to give a final micellar aqueous solution with a concentration of 0.1 mg/mL. Addition of Amines into the Micelles Solution. Before using liquid amines like DEA, TEA, EDA, and EDDA were diluted for 100 times of volume with deionized water, and the concentrations of the solutions were 7.1, 7.3, 9.0, and 10.2 mg/mL, respectively. Solid lysine was dissolved in deionized water with a concentration of 15.3 mg/mL. A certain volume of amine solution was added into the original aqueous solution of PE-b-PAA micelles with pipet based on the designed molar ratio of amine over acrylic acid group ([amine]/ [AA]). For example, when 52.5 μL of DEA aqueous solution was added into 5 mL aqueous solution containing 0.5 mg of PE63-b-PAA74 cylindrical micelles, the molar ratio of [amine]/[AA] is 1.

EXPERIMENTAL SECTION

Materials. Polyethylene-block-poly(tert-butyl acrylate) (PE-bPtBA) BCPs were prepared through atom transfer radical polymerization (ATRP) of tert-butyl acrylate by using bromized polyethylene as macroinitiator, which was reported in our previous work.10 The PtBA block was fully hydrolyzed in toluene with p-toluenesulfonic acid (5 mol % relative to the tert-butyl ester content) at 110 °C to yield PE-b-PAA BCPs.61 Characterization data of polymers are given in the Supporting Information (Figures S1 and S2, Table S1). PAA homopolymer (Mw = 45000), p-toluenesulfonic acid monohydrate, diethylamine (DEA), triethylamine (TEA), and ethylenediamine (EDA) were purchased from Aladdin (China). 2,2′-(Ethylenedioxy)di(ethylamine) (EDDA) and L-lysine were purchased from J&K. The molecular structures of PE63-b-PAA74 block copolymer and different amines are illustrated in Scheme 1. The solvents N,N-dimethylformamide (DMF), o-xylene, and deionized water were filtered using polytetrafluoroethylene membrane with 0.45 μm pore size before use. If not specifically indicated, all the other chemicals were used as received. Transmission Electron Microscopy (TEM). TEM characterization was performed on a JEOL JEM-1230 electron microscope at an acceleration voltage of 80 kV. The TEM samples were prepared by dropping 4 μL solutions onto carbon-coated copper grids; afterward, the samples were stained by uranyl acetate aqueous solution to improve the contrast of PAA micellar corona. The contour sizes of micelles were analyzed with Image-Pro Plus software. To ensure accuracy and reliability, more than 200 micelles were counted for each sample. Dynamic Light Scattering (DLS). The apparent hydrodynamic diameters of the PE-b-PAA micelles in aqueous solution were measured by DLS on a Brookhaven Instrument BI-90Plus with a laser wavelength of 657 nm at 25 °C and a fixed scattering angle of 90°. The obtained electric field autocorrelation functions were analyzed B


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Figure 1. Representative TEM micrographs (a−f) and intensity weighted Dh distributions (g) of PE63-b-PAA74 micelles in aqueous solution (dialysis from DMF/o-xylene (1/1 v/v) mixed solvent) after adding different kinds of organic amines (molar ratio of amine molecule to acid groups [amine]/[AA] as 1). TEM samples were stained with uranyl acetate aqueous solution.

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RESULTS AND DISCUSSION Effect of Amine on Fragmentation of Cylindrical Crystalline Micelles. Figure 1 shows the representative TEM micrographs of PE63-b-PAA74 crystalline micelles before and after adding different kinds of organic amine in aqueous solution. The TEM micrographs of low magnification and corresponding contour length distributions are shown in Figure S3. The initial cylindrical micelles were prepared via the CDSA of PE63-b-PAA74 BCP in DMF/o-xylene mixed solvent (VDMF/Vxylene = 1/1), followed by successive dialysis against DMF and water (details about the control of micellar morphology are provided in the Supporting Information). The crystallization of PE micellar core can be confirmed by the WAXS pattern (Figure S5). The average contour length (Ln) of the original cylindrical micelles is 209 ± 62 nm (Figure 1a and Figure S3a′). Addition of monoamine ([amine]/[AA] = 1) can induce the fragmentation of the long cylindrical micelles into uniform short stublike ones. The length of the micelles turns into 36 ± 8 nm after addition of DEA (Figure 1b and Figure S3b′) and 25 ± 5 nm after addition of TEA (Figure 1c

and Figure S3c′). By contrast, the addition of diamines like EDA and EDDA results in merely a small decrease (3−30 nm) in the micellar length (Figure 1d,e and Figure S3d′,e′). Lysine is also added into the aqueous solution containing cylindrical crystalline micelles of PE63-b-PAA74. It is found that addition of lysine leads to the fragmentation of long cylindrical micelles as well, and the resultant micelles have an average length of 49 ± 19 nm (Figure 1f and Figure S3f′). As shown in Scheme 1, there are two amine groups and one carboxyl group in one lysine molecule. We speculate that intramolecular electrostatic interaction may be formed between the carboxyl group and adjacent amine group; thus, lysine behaves like monoamines. This will be confirmed by FT-IR characterization. Because TEM micrographs only represent the morphology of local micelles in sight, we also used dynamic light scattering (DLS) to characterize the overall apparent hydrodynamic diameter (Dh) of cylindrical micelles before and after addition of different amines. The intensity weighted Dh distributions of the PE63-b-PAA74 micelles are shown in Figure 1g. It is observed that the average Dh of the original cylindrical micelles C


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Macromolecules dispersed in aqueous solution is about 294 nm. After addition of monoamines (DEA and TEA), the average Dh is immediately reduced to ∼80 nm, and it becomes ∼100 nm after addition of lysine. In contrast, addition of diamines such as EDDA or EDA only causes 20−70 nm decrease in Dh. The variation of number-weighted Dh distribution shown in Figure S3g is also in consistent with the result in Figure 1g. As a consequence, the DLS and TEM results are well consistent, which reveal that the ability of amines to fragmentize the cylindrical crystalline micelles of PE-b-PAA BCPs depends on their functionality. Monoamines can effectively induce the fragmentation of PE-b-PAA cylindrical crystalline micelles, while diamines have little influence on the micellar morphology. Such dependence is similar to that in fragmentation of PCL-b-PEO cylindrical crystalline micelles by hydroxyl-containing additives.21 We found that phenol, with single hydroxyl group in a molecule, could trigger the fragmentation of PCL-b-PEO cylindrical crystalline micelles, while L-threonine, which contains one hydroxyl, one carbonyl, and one amine, could not induce fragmentation and even resulted in the growth of micelles. Amines were also added into the aqueous solution containing the cylindrical crystalline micelles of another PEb-PAA BCP, i.e., PE53-b-PAA58. Both TEM and DLS results show that addition of lysine causes obvious fragmentation of the micelles and thus remarkable decrease in micellar length, whereas the change in micellar length is quite small after addition of EDDA (Figure S10). This further proves the feasibility of this method to fragmentize the cylindrical crystalline micelles of PE-b-PAA BCPs and the dependence of the fragmentation ability of amines on the functionality. Effect of [Amine]/[AA] Ratio. Different amounts of amine were added into the aqueous solution of PE63-b-PAA74 micelles to examine the effect of [amine]/[AA] ratio on the fragmentation of cylindrical crystalline micelles. With fixed solution volume (5 mL) and the concentration of PE63-bPAA74 micelles as 0.1 mg/mL, the [DEA]/[AA] ratio was changed from 0.25 to 1. TEM micrographs of the PE63-bPAA74 micelles at different [DEA]/[AA] ratios are shown in Figures 2a−c and 1b (TEM images of lower magnification are shown in Figures S11 and S12). In the enlarged TEM micrographs, one can discern the boundary and overlapped region of different micelles with darker contrast. Figure S11 gives a representative example for statistics of micellar length. It is found that the average Ln of the cylindrical micelles decreases gradually from ∼200 to ∼30 nm with increasing [DEA]/[AA] ratio. When the [DEA]/[AA] ratio exceeds 0.75, the average contour length of the micelles becomes constant and does not change with further increase of the amount of added DEA. The Dh measured by DLS changes with [DEA]/ [AA] ratio in a similar way (Figure 2d). However, the values of Dh are systematically larger than those of Ln. This difference originates from the different characterization principles of DLS and TEM and the different corona structures of micelles in dry state and in solution. Calculation of Dh is based on the Einstein−Stokes equation for diffusion of spherical particles in liquid, which is different from the real cylindrical morphology of micelles. Moreover, the PAA corona is swollen in solution, while it is collapsed when subject to TEM characterization. Comparing the TEM images and contour length distributions of the original micelles (Figure 1a and Figure S3a′) and the micelles after addition of DEA (Figures 2 and 1b, Figure S3b′), one can see that when only a very small amount of DEA

Figure 2. Representative TEM images (a−c) and histograms of the contour length distributions (a′−c′) of PE63-b-PAA74 micelles in aqueous solution after adding different amounts of DEA. The [DEA]/ [AA] ratio is 0.25 for (a, a′), 0.5 for (b, b′), and 0.75 for (c, c′). The samples for TEM observation were stained with uranyl acetate aqueous solution. (d) Variations of the apparent hydrodynamic diameter (Dh) and average contour length (Ln) of the micelles with [DEA]/[AA] ratio.

is added ([DEA]/[AA] = 0.25), long cylindrical micelles with contour length above 300 nm are fully fractured and micelles with contour length below 100 nm first appear (Figure 2a). This indicates that the fragmentation of cylindrical crystalline micelles takes place more easily in longer micelles. Nevertheless, increasing the amount of DEA will promote the D


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relatively weak peak at ca. 1560 cm−1 in D2O (black line in Figure 3) indicate the weak ionization characteristic of PAA and the formation of H-bonds among different PAA chains, which can account for the adhesion of different PE-b-PAA micelles in aqueous solution (Figure 1a). Nevertheless, the absorption band of uncharged PAA at ∼1700 cm−1 disappears in the presence of amines (DEA and EDDA at [amine]/[AA] = 1). Meanwhile, the absorbance intensity of the ionized carboxyl group at ca. 1560 cm−1 is enhanced dramatically. This shows that the addition of amines like DEA and EDDA will destroy the H-bonding among different PAA chains and promote the deprotonation of PAA because of the electrondonating ability of the amine group. Similarly, the addition of lysine can also lead to ionization of PAA in aqueous solution (Figure S14). In a word, FT-IR spectra prove the interaction between PAA and amines in aqueous solution is mainly an electrostatic interaction rather than H-bonding or other forces. Moreover, we notice that even in the FT-IR spectrum of neat lysine in D2O without the presence of PAA (Figure S14), the band of the uncharged COOH cannot be observed. This confirms the intramolecular electrostatic interaction in lysine; thus, lysine can be viewed as a monoamine, and the effective functionality of lysine is one. Proposed Mechanism for Fragmentation of Crystalline Micelles. As we mentioned above, long cylindrical PE-bPAA micelles can be fractured into short stublike ones after addition of monoamines like DEA, TEA, and lysine. Nevertheless, a question should be answered first: Where do the driving force and energy for fragmentation come from? One may speculate that the alteration of pH after addition of amines is the main source. The pH values of PE63-b-PAA74 micellar solution after adding lysine at [lysine]/[AA] = 1 and 100 are 8.5 and 10, respectively. In a control experiment, we adjust the pH of PE63-b-PAA74 micellar solution into 8.5 and 10 by adding 0.1 M sodium hydroxide solution. One can see that there is no obvious change of the micelle morphology in an alkaline environment (pH = 8.5, Figure 4a). Even in an extreme alkaline environment (pH = 10, Figure 4b), the PE-bPAA cylindrical micelles still cannot be fully fractured into short ones, which excludes the possibility that the fragmentation of the crystalline cylindrical micelles in solution is induced by the change of pH. The statistically average contour lengths (Ln) of the cylindrical micelles before and after adding different amines are summarized in Table 1. Monoamines like DEA, TEA, and lysine can induce the fragmentation of PE-b-PAA cylindrical micelles, while diamines like EDA and EDDA actually have little effect on the morphology of PE-b-PAA micelles. So the

fragmentation of other shorter cylindrical micelles so that micelles with a narrow length distribution can be prepared. When excessive amine is added, for example, at [DEA]/[AA] = 4 or [lysine]/[AA] = 100, the final length of the micelles will be close to that at [DEA]/[AA] = 1 (Figure S13). This means that the fragmentation of the PE-b-PAA cylindrical crystalline micelles will approach a limit when the micelle length is about 20−30 nm. Interaction between Amines and PAA. As a kind of weak polyelectrolyte, PAA will be partially deprotonated into anion in aqueous solution. As amines can bind H+ to form ammonium ion, the meeting of PAA and amines in aqueous solution may result in electrostatic attraction. Besides, it is reasonable to believe that amines and PAA can form hydrogen (H)-bonds in solution with PAA as the H-bonding donor and amines as the H-bonding receptor. To study the interaction between amines and PAA coronal block, infrared spectroscopy was employed to examine the charged state of PAA in the presence of organic amines in aqueous solution. Figure 3

Figure 3. FT-IR spectra of PAA in D2O in the presence of different amines. The [amine]/[AA] ratio is 1.

shows the FT-IR spectrum of 2.0 wt % PAA in pure D2O. According to literature, the absorption band at ca. 1710−1695 cm−1 is associated with the stretching vibration of the Hbonded carboxyl group of PAA (CO···H−O hydrogen bond), and the band located at ca. 1745−1731 cm−1 is ascribed to the stretching vibration of the non-H-bonded carboxyl group of PAA (“free” CO).63 Besides, the strong absorption band at ca. 1560 cm−1 is assigned to the stretching vibration of the ionized carboxyl group (−COO−). The wide absorption band of PAA from 1650 to 1750 cm−1 and the

Figure 4. TEM micrographs of PE63-b-PAA74 micelles at pH = 8.5 (a) and pH = 10 (b) and after addition of ammonia with [NH3]/[AA] = 1 (c). The samples for TEM observation were stained with uranyl acetate aqueous solution. E


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where Fcryst and Famorph are the free energies of the crystalline micelle core and the tethered amorphous corona chains, respectively, and Finterface represents the free energy of the interface between micellar core and outer solvent. Fragmentation of the micelles will cause the change of Fcryst, Famorph, and Finterface, which are designated as ΔFcryst, ΔFamorph, and ΔFinterface, respectively. After fragmentation, two new lateral surfaces are produced and the interaction of the crystalline polymer chains on them are weakened; thus, both ΔFcryst and ΔFinterface are positive. On the other hand, the repulsive interaction of the corona-forming chains nearby the newly formed surfaces is released, leading to decrease of Famorph, i.e., ΔFamorph < 0. The above analysis reveals that the conformational change of the corona-forming PAA chains is the main driving force for the fragmentation of crystalline micelles. The free energy of a single PE block with Xc crystallinity in the micellar core (Fcryst) can be expressed as

Table 1. Average Contour Length (Ln) and Apparently Hydrodynamic Diameter (Dh) of PE63-b-PAA74 Cylindrical Micelles in Aqueous Solution before and after Adding Different Amines at [Amine]/[AA] = 1 amines a

original effective functionality Mrb Lnc Dh d

209 292

DEA

TEA

lysine

EDA

1

1

1e

2

2

1

73 36 79

101 25 81

60 206 280

148 176 226

17 202 311

146 49 99

EDDA

NH3

a

Dialysis from DMF/o-xylene (VDMF/Vxylene = 1/1) solution of PE63b-PAA74 at Tc = 30 °C. bRelative molecular mass. cMeasured from TEM micrographs by counting at least 200 cylindrical micelles. d Measured by DLS. eLysine is viewed as a monoamine and its effective functionality is one.

fragmentation of PE-b-PAA micelles may be correlated to the functionality of amine. The entry of additives of monofunctionality into the micellar corona driven by a specific interaction (such as H-bonding or electrostatic interaction) will result in the swelling of the micellar corona. Accordingly, stress is produced inside the micellar corona. Such stress can be released through fragmentation to form extra lateral surfaces at the ends of cylindrical micelles, since the corona-forming chains are less crowded due to flipping to the lateral surfaces. On the other hand, for the additives with bifunctionality, physical cross-linking may be formed arising from simultaneous interactions of two functional groups with different corona-forming chains, leading to inability to release the stress. However, when ammonia, which is also a monoamine but has a lower molecular weight than other organic monoamines, is added into the aqueous solution containing PE63-b-PAA74 cylindrical micelles, no fragmentation of the micelles occurs, as illustrated in Figure 4c. This shows that besides functionality, the molecular size or molecular weight of amines is another key factor determining the fragmentation of PE-bPAA cylindrical micelles. If the molecular size of the additive is too small, the produced stress is not big enough to induce fragmentation of the micelles. This will be discussed more deeply in the next section. Calculation of Free Energy Change after Fragmentation. When small molecules enter the coils of polymer chains, it will cause swelling of polymer coils and produce stress inside the coils. It has been reported that swelling of hydrogel by solvent can result in the breakage of weak covalent bonds.64 This shows that the stress produced by swelling can be quite strong. Recently, Winnik and Manners also proposed that the highly stretched corona chains exert a pulling force to the crystalline micellar core in all directions, which may lead to dissolution of the seed micelles if the crystallinity of the micellar core is not high enough.65,66 To further understand the fragmentation of crystalline micelles of BCPs, herein the free energy change caused by swelling of PE-b-PAA micellar corona and the energy needed for fragmentation of crystalline micellar core are semiquantitatively calculated and compared. For an AB semicrystalline BCP with NA repeating units in the amorphous block and NB repeating units in the crystalline block, the total free energy for the crystalline micelles (Ftot) is given by67 Ftot = Fcryst + Famorph + Finterface

Fcryst = −2DPPE[Eint − (1 − Xc)ΔHf0]

(2)

where DPPE is the polymerization degree of PE block; Eint and ΔH0f are the lattice energy and heat of fusion per −CH2− unit in PE crystals, respectively. First, the thickness of PE crystals (lc) in the micellar core can be calculated in terms of Thomson−Gibbs equation:68 lc =

2σeTm0 ΔHf0(Tm0 − Tm)

(3)

where Tm is the measured melting temperature (91.6 °C for PE63-b-PAA74 cylindrical micelles, as determined by DSC, Figure S16), T0m is the equilibrium melting temperature of PE crystal with an infinite thickness (141 °C), ΔH0f is the heat of fusion in per unit volume (288 × 106 J/m3, or 6.69 × 10−21 J per −CH2− unit) with 100% crystallinity, and σe is the free energy of the folding surface of PE crystal (70 × 10−3 J/m2). The value of lc calculated from eq 3 is 4.1 nm. The folding number (nf) of PE block can be calculated as follows: n f = [(Lmax Xc)/lc] − 1

(4)

where Lmax is the extended chain length of PE block (15.8 nm for PE63) and Xc is the crystallinity of PE block, which is 74.7% determined by DSC (Figure S16). The calculated value of nf is ∼2, meaning that the PE blocks adopt twice-folding conformation in the crystalline micellar core. The tethering density (σ) of PAA chains on the folding surfaces of PE crystal, defined as the reciprocal of the area occupied by each PAA chain, can be calculated as n σ= 2(n f + 1)a0b0 where n is the number of PE stem in the unit cell of PE crystal (n = 2); a0 and b0 are the lattice parameters of PE orthorhombic unit cell along the a- and b-axis, which are 7.40 Å and 4.93 Å, respectively.69 The reduced tethering density (σ̃ ) of PAA chains is defined as σπRg2,70 where Rg is the radius of gyration of free PAA chain, which is 2.5 nm for PAA74.71 The parameter σ̃ can be used to evaluate the crowding degree of the tethered polymer chains. The calculated value of σ̃ is 17.8 for the PAA chains in the cylindrical crystalline micelles of PE63-b-PAA74, indicating that the PAA corona chains are highly stretched.72

(1) F


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Macromolecules Scheme 2. Structural Model for PE63-b-PAA74 Cylindrical Crystalline Micelles

crystals are in fact anisotropic, and the specific free energies for the folding and lateral surfaces of PE crystals are different.77 However, since the data about the water contact angle on these two different surfaces are not available, we just use the overall data of γPE and θ. The value of S is equal to Lwidth × lc = 22 nm × 4.1 nm = 90.2 nm2. Finally, the value of ΔFinterface is calculated as 0.92 × 10−17 J. As a result, the sum of ΔFcryst and ΔFinterface for a single fragmentation process of a PE63-b-PAA74 crystalline micelle is 3.02 × 10−17 J. For an amorphous chain with an end-to-end distance of R, Famorph can be estimated in terms of an ideal chain model:78

The structure model for the cylindrical crystalline micelles of PE63-b-PAA74 is shown in Scheme 2. AFM height image shows that the height of the cylindrical micelles (H) is 10.5−14 nm (Figure S15); thus, the sum thickness of amorphous PAA corona and amorphous PE on the top and bottom surfaces of PE crystals is calculated to be (14 − 4.1) nm/2 ≈ 5.0 nm. Note that in the dried micelles the thickness of amorphous layers on the top and bottom surfaces may be smaller than this value due to spreading and flipping of the corona chains upon evaporation of solvent. TEM images of PE63-b-PAA74 micelles stained by uranyl acetate demonstrate that the width of the cylindrical micelles (Lwidth) is 22 ± 3 nm. Moreover, since the parameter a0 is larger than b0 in orthorhombic PE crystal, the PAA corona chains are less crowded along a-axis and the cylindrical crystalline micelles tend to grow along this direction.73 The chain number on a cross section (Ncs) of PE63-b-PAA74 cylindrical micelles can be calculated as Ncs = Lwidth/[(nf + 1)b0] = 15. According to the literature, the heat of fusion (ΔH0f ) and lattice energy (Eint) per −CH2− group in PE crystals are 6.69 × 10−21 J and 1.28 × 10−20 J, respectively.68,74 The Fcryst of PE63-b-PAA74 chains on one cross section of cylindrical micelles (Fcscryst) is

Famorph(N , R ) =

(5)

The fragmentation of PE63-b-PAA74 cylindrical micelle means the disruption of one cross section of crystalline region, so ΔFcryst = −Fcscryst = 2.10 × 10−17 J. Fragmentation of the cylindrical micelles will lead to exposure of two extra lateral surfaces of micellar core to the solvent but the area of the folding surfaces almost remains the same, so the increment of the interfacial free energy is ΔFinterface = γPE/water × 2S

ΔR = (D h0 − D hamine)/2

(6)

D0h

(9)

Damine h

where and are the apparent hydrodynamic diameters of the quasi-spherical micelles in the aqueous solution before and after addition of amines, respectively. The intensity-weighted Dh distributions of spherical PE63-bPAA74 micelles after addition of DEA, EDDA, and NH3 are shown in Figure 5. It is found that the average Dh of the original PE63-b-PAA74 quasi-spherical micelles is 48 nm, while it becomes 56, 65, and 70 nm after addition of NH3, EDDA, and DEA, respectively. As the Dh characterized by DLS contains a part of hydrated coronal chains, generally Dh is slightly larger than the actual dimension of spherical micelles; thus, we only use the relative increment of Dh to calculate R.

where γPE/water is the specific free energy of PE/water interface and S is the area of the PE cross section in the micelle core. The value of γPE/water can be calculated using Young’s equation: γPE/water = γPE − γwater cos θ

(8)

where b and N are the length and number of Kuhn segment, respectively, k is Boltzmann constant, and F(N,0) is the free energy of the chain with both ends at the same point. To calculate the free energy change of coronal chains (ΔFcorona) before and after fragmentation, the end-to-end distance (R) of PAA coronal chains before and after addition of amines should be obtained first. DLS was chosen to measure the conformational change of PAA coronal chains in situ. However, for anisotropic long cylindrical micelles, the change of coronal size cannot be easily measured from Dh because it is an apparent value based on isotropic spherical model. Therefore, we measured the Dh of small quasi-spherical PEb-PAA micelles (Figure S4c), which can be prepared through the CDSA of PE63-b-PAA74 in DMF followed with dialysis against water. Subsequently, the increase of the end-to-end distance of PAA coronal chains (ΔR) after adding different kinds of amine can be yielded from following equation (as depicted in Scheme S1):

F cs cryst = −Ncs × 2DPPE × [Eint − (1 − Xc) × ΔHf ] = −2.10 × 10−17 J

3 R2 kT 2 + F(N , 0) 2 Nb

(7)

where γPE and γwater are the specific surface free energies of PE and water, respectively, and θ is the contact angle of water on PE surface. The values of γPE, γwater, and θ are 35.7 × 10−3 J/ m2,75, 72.8 × 10−3 J/m2,76 and 102°,75 respectively. Therefore, the calculated γPE/water is 50.8 × 10−3 J/m2. Note that the PE G


Article

Macromolecules ΔFPAA(DEA) =

2 2 3 RDEA − R water kT = 5.25 × 10−20 J 2 2 Nb

(10)

where Nb = 6Rg = 37.5 nm for PAA74 with Rg as 2.5 nm.71 The ΔFPAA after addition of NH3 is 2

2

ΔFPAA(NH3) =

2

2 2 3 R NH3 − R water kT = 1.45 × 10−20 J 2 2 Nb

(11)

According to the model of PE63-b-PAA74 cylindrical micelles (Scheme 2) and structure of PE orthorhombic crystal, there are 2NcsLx/a0 polymer chains along the length Lx of cylindrical micelles. After the addition of DEA, the free energy increment of PAA coronal chains within L x length range is 2ΔFPAA(amine)NcsLx/a0. For a single fragmentation process, the coronal chain swelling offers the free energy cost (ΔFcryst + ΔFinterface), so Lx can be determined by

Figure 5. Intensity-weighted Dh distributions of PE63-b-PAA74 quasispherical micelles in aqueous solution before and after adding DEA, EDDA, or ammonia ([amine]/[AA] = 1) measured by DLS. The original micellar solution was prepared by dialysis of DMF solution of PE63-b-PAA74 micelles against water at 30 °C.

Lx = (ΔFcryst + ΔFinterface)/[2ΔFPAA(amine) × Ncs/a0] (12)

For DEA, the calculated Lx is 14.2 nm, which means the release of the stress in DEA-swollen of PAA chains along 14.2 nm length range can overcome the free energy barrier for a single fragmentation process. The fragmentation of PE63-bPAA74 cylindrical micelles will generate two new lateral surfaces with an area of 22 nm × 4.1 nm, which greatly reduces the overcrowding of the coronal chains near the lateral surfaces. It is reasonable to believe that the two new lateral surfaces can influence PAA coronal chains along 14.2 nm length, so that the stress inside the PAA coronal chains with the presence of DEA can induce the fragmentation of cylindrical micelles. With consideration of two original lateral surfaces of cylindrical micelles, the actual length of cylindrical micelles should be longer than twice of Lx to offer fragmentation free energy costs. This means that the cylindrical micelles with length below 28 nm would not be fractured into

The values of ΔR resulting from addition of NH3, EDDA, and DEA are 4, 8.5, and 11 nm, respectively. For PE63-b-PAA74 aqueous solution (0.1 mg/mL), the degree of ionization is about 15−20%, as calculated from a pKa of 4.25 for propionic acid. The R of electrostatic screened PAA74 is about 6 Rg = 6 nm,71 so we estimate that the Rwater of PPA chains in the original PE63-b-PAA74 spherical micelles is about 9 nm. Therefore, the RNH3, REDDA, and RDEA of PAA chains in the micelles after addition of NH3, EDDA, and DEA are 13, 17.5, and 20 nm, respectively. The free energy increment per PAA74 chain (ΔFPAA) after addition of DEA is

Scheme 3. Schematic Diagram of PE63-b-PAA74 Cylindrical Crystalline Micelles after Addition of Monoamines (DEA, TEA, Lysine) (a), NH3 (b), and Diamines (EDDA, EDA) (c)

H


Article

Macromolecules

free energy resulting from release of the stress in PAA coronas swollen by larger monoamines like DEA is comparable to the free energy barrier for micellar fragmentation. Besides, the binding effect of diamines with carboxyl groups in different PAA chains will impede the release of stress, so that fragmentation of the cylindrical micelles is inaccessible through adding diamines like EDA and EDDA. As the chain swelling of PAA coronas can disrupt the crystal lattice of polymers with strong crystallizability like PE, this strategy can be applied to other crystalline-polyelectrolyte BCPs, which offers another simple and moderate pathway for the disassembly of crystalline micelles. Because essential amino acids like lysine can also induce the fragmentation of longer micelles into shorter ones, this approach may have potential applications in drug release and therapy of certain fibrotic lesion.

shorter ones, which is quite consistent with the limit length of the micelles after addition of sufficient monoamines (Figure 1 and Table 1). The stability of the stublike crystalline micelles can be understood with the aid of the concept of superblob developed by Bihrstein et al. for bottle-brush polymers.79,80 They proposed that if the backbone length of the bottle-brush polymers is comparable to the superblob diameter, which is equal to the diameter of bottle-brush cross section, the tension generated by side chain repulsion may induce fracture of backbone. Otherwise, the bottle-brush polymers are stable and would not be fractured. By contrast, with the addition of NH3, the calculated Lx is 51.4 nm. This means that the stress of PAA chains along length range of 51.4 nm is needed to counteract the free energy cost. Such a value of Lx is much larger than the length range that a new produced lateral surface can affect. As a result, the fragmentation of PE63-b-PAA74 cylindrical micelles cannot take place. The different abilities of DEA and NH3 to fracture PE-bPAA cylindrical crystalline micelles are related to their different molecular sizes, which further result in different swelling degrees of the PAA corona, as revealed by Figure 5. The addition of amines will promote ionization of PAA chains so that PAA chains tend to adopt more stretched conformation due to electrostatic repulsion, and this effect is quite similar for DEA and NH3. On the other hand, as depicted in Scheme 3 and Scheme S1, amine can also enter the corona of micelles to induce the chain swelling through steric repulsion effect, and this may largely influence or even dominate the fragmentation process of the cylindrical crystalline micelles. Because the molecular size of DEA is almost triple that of NH3 (see details in the Supporting Information), it can induce a larger swelling degree of PAA chains, so that fragmentation of cylindrical can take place. By comparison, a smaller swelling degree of the PAA chains is caused by NH3, leading to inability of fragmentation for the cylindrical crystalline micelles. We also notice that the chain swelling degree of the PAA corona induced by EDDA is quite close to that by DEA (Figure 5), but no obvious fragmentation of the cylindrical crystalline micelles is observed after addition of diamines like EDA and EDDA (Figure 1 and Table 1). This means that the effective functionality of amines also influences the fragmentation process of PE-b-PAA crystalline micelles. Wooley and Pochan et al. reported that diamines like EDDA could act as cross-linker and bind two carboxyl groups among different PSb-PMA-b-PAA micelles,81 leading to the aggregation of anisotropic shaped disklike micelles into well-defined onedimensional structures. As depicted in Scheme 3, there are two attraction modes between the PAA coronal chains and diamines: the attraction within single PAA chain and the attraction between two different PAA chains. The second attraction mode will impede the release of swelling stress through fragmentation.

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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.macromol.8b01131. 1 H NMR spectra and GPC traces of polymer used in the study, WAXS patterns, additional TEM images, contour length distribution and DLS data, DSC heating curve, AFM data, FT-IR results of lysine and PAA, calculation of molecular size of DEA and NH3 (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected]; Ph +86-571-87953164; Fax +86-57187952400 (J.-T.X.). *E-mail [email protected]; Ph+86-571-87953164; Fax +86571-87952400 (B.-Y.D.). ORCID

Rui-Yang Wang: 0000-0001-9561-3237 Jun-Ting Xu: 0000-0002-7788-9026 Bin-Yang Du: 0000-0002-5693-0325 Zhi-Qiang Fan: 0000-0001-8565-5919 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21774111 and 21674097) for financial support and beamline BL16B1 at SSRF for providing the beam time.

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REFERENCES

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CONCLUSION In summary, we have demonstrated that PE-b-PAA long cylindrical crystalline micelles can be fractured into short (∼30 nm) stublike ones by adding monoamines of large molecular size, such as DEA, TEA, and lysine. The length of the fractured micelles can be regulated via the [amine]/[AA] ratio. The electrostatic attraction between ionized PAA and amines and swelling of the PAA corona chains mainly accounts for the fragmentation of the cylindrical micelles. We proved that the I


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Macromolecules

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Regulated Fragmentation of Crystalline Micelles of Block Copolymer

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