Two Growth Modes of Semicrystalline Cylindrical Poly(ε-caprolactone

Article pubs.acs.org/Macromolecules

Two Growth Modes of Semicrystalline Cylindrical Poly(εcaprolactone)‑b‑poly(ethylene oxide) Micelles Wei-Na He, Bing Zhou, 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 *

ABSTRACT: The micelles of a poly(ε-caprolactone)-b-poly(ethylene oxide) block copolymer (PCL59-b-PEO113) in different mixed solvents were held at 53 °C for 5 min, and seed solutions with different micellar morphologies and amounts of micellar semicrystalline seeds were prepared. The crystallinity of these seed micelles was identified by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED). It is found that mostly amorphous spherical micelles are formed by heating micellar solutions in H2O/THF (5/1 v/v) and H2O/dioxane (5/1 v/v) mixed solvents, a mixture of amorphous spherical micelles and short semicrystalline cylindrical micelles is yielded in H2O/DMF (5/1 v/v), whereas mostly short semicrystalline cylindrical micelles are obtained in H2O/DMSO (5/1 v/v) mixed solvent. The seed solutions were placed at 4 °C for micellar growth. Transmission electron microscope (TEM) shows that micellar growth driven by epitaxial crystallization of core-forming PCL chains takes place and the length of grown cylindrical micelles increases with time. Two growth modes are observed. One is the growth of unimers (or amorphous spherical micelles) on the active ends of semicrystalline cylindrical micelles in micellar solution in H2O/DMF (5/1 v/v) at the initial growth period. The other is the growth by end-to-end coupling of cylindrical micelles in H2O/DMSO (5/1 v/v). The kinetics of micellar growth is strongly dependent on the growth mechanism. The growth of the cylindrical micelles in the H2O/DMF (5/1 v/v) solution is much faster than that in the H2O/DMSO (5/1 v/v) solution. On long time scale, micellar growth by end-to-end coupling of semicrystalline cylindrical micelles occurs with slow rate in both H2O/DMF (5/1 v/v) and H2O/DMSO (5/1 v/v) solutions, and the growth rate in H2O/DMF (5/1 v/v) solution is even slower than that in H2O/DMSO (5/1 v/v).

1. INTRODUCTION Amphiphilic block copolymers (BCPs) can self-assemble in selective solvent to form micelles with various morphologies.1−9 If the core-forming block is crystallizable, then the obtained micelles may be semicrystalline. We call the micelles with a semicrystalline core “semicrystalline micelles”. Compared with the amorphous micelles of BCPs, the semicrystalline micelles of BCPs have many excellent characteristics, which have attracted many researchers recently.10−53 First, the semicrystalline micellar morphology can be changed by more routes, comparing with amorphous micelles. Semicrystalline micelles with different morphologies can likely be formed not only by altering the solvent quality, the chain structure of BCP, or preparation method, which are usually used for amorphous BCPs, 1−5 but also by changing crystallization conditions while preparing the semicrystalline micelles.14,27,29,42 Moreover, the changed morphologies are more likely fixed as the result of the crystallization of the core.26 Second, compared with the amorphous BCP micelles, the unique characteristic of semicrystalline micelles of BCPs is the good regulation of the micellar size for one specific morphology, especially for cylinder. Winnik and Manners first found that by adding a homogeneous solution of the poly(ferrocenyldimethylsilane) (PFDMS)-containing BCP to the pre-existing semicrystalline seed micelles in a selective © 2012 American Chemical Society

solvent, the growth of the semicrystalline micelles took place and the length of the micelles was proportional to the amount of BCP added.54,55 Recently, living growth was also realized for the semicrystalline cylindrical micelles of poly(3-hexylthiophene)-b-poly(dimethyl siloxane) and poly(lactide)-b-poly(acrylic acid) BCPs,56,57 which indicates that the living growth may be a common property for semicrystalline cylindrical micelles. Furthermore, Winnik and Manners have successfully prepared triblock, pentablock, multiblock, scarf-shaped, and pointed-oval-shaped “co-micelles” from the PFDMS-containing BCPs.58−65 Triblock comicelles were prepared from polyethylene-containing BCPs as well.66 The regulation of the micellar size and shape is of great importance for the application of BCP micelles. For example, Discher found that long worm-like poly(ε-caprolactone)-bpoly(ethylene oxide) (PCL-b-PEO) micelles exhibited better performance in drug delivery than the spherical and short rodlike ones.67 So far, the semicrystalline cylindrical micelles with uniform length are usually prepared by self-seeding method, which is borrowed from cultivation of polymer single crystals.68,69 It is Received: June 20, 2012 Revised: November 19, 2012 Published: December 4, 2012 9768

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were not freeze-dried to avoid crystallization of the amorphous micelles resulting from a large supercooling.

believed that the growth proceeds through the epitaxial crystallization of the unimers in the solution on the semicrystalline seeds.59,61,70 However, this represents only one of the possible growth modes of semicrystalline micelles of BCPs. Sometimes, spheres-to-cylinders transformation of semicrystalline BCP micelles is observed as well.29,43,44,71 Recently, Winnik and Manners also reported that the amorphous spherical micelles of poly(ferrocenyldimethylsilane-b-2-vinylpyridine) (PFDMS-bP2VP) could slowly crystallize and further assemble into lenticular-shaped platelet micelles.72 To explore other growth modes of semicrystalline micelles of BCPs and understand the corresponding growth mechanism, different seed micelles of a PCL-b-PEO BCP (PCL59-bPEO113), such as total amorphous spherical micelles, mixture of amorphous spheres and short semicrystalline cylinders, or mostly uniform short semicrystalline cylinders, were prepared in different mixed solvents. Two different growth modes of semicrystalline cylindrical micelles of PCL-b-PEO were observed.

3. RESULTS AND DISCUSSION Preparation of the Seed Micelles. The crude PCL59-bPEO113 micelles in aqueous solution are a mixture of spheres and cylinders, as shown in Figure 1. The diameter of the

2. EXPERIMENTAL SECTION Materials. The BCP PCL59-b-PEO113 (the subscripts being the polymerization degrees of the blocks) with a narrow molecular weight distribution (Mw/Mn = 1.12) was synthesized according to our previous work.73 The number-average molecular weights of the PEO and PCL blocks were calculated from 1H NMR spectrum, and the molecular-weight distribution was determined by gel permeation chromatography (GPC). Analytical grade organic solvents, tetrahydrofuran (THF), dioxane, N,N-dimethyl formamide (DMF), and dimethyl sulfoxide (DMSO), were purchased from ACROS and used as received. Preparation of the Micellar Solutions. The PCL59-b-PEO113 BCP was dissolved in THF with a concentration of 1 mg/mL; then, 10 mL of the homogeneous solution was transferred to dialysis tubes (molecular-weight cutoff = 3500 g/mol) and dialyzed against twicedistilled water at room temperature to remove THF. After dialysis, the micellar solution was transferred to a volumetric flask. A certain volume of deionized water was used to wash the dialysis bags and added to the flask. The final solution volume was fixed at 100.0 mL, and the concentration of the micellar solution was 0.1 mg/mL. Five mL of micellar aqueous solution was taken out and 1 mL of organic solvent (THF, dioxane, DMF, or DMSO) was added. After being stirred for 1 h, the solution was heated to 53 °C and held for 5 min, then held at 4 °C for micellar growth. Characterizations. The micelles were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM). DLS measurements were performed on a Brookhaven Instrument BI200SM with a laser wavelength of 636 nm at 25 °C at a scattering angle of 90°. The data were analyzed with the software supplied by Brookhaven. TEM observations were carried out on a JEOL JEM-1230 electron microscope at an acceleration voltage of 80 kV. TEM samples were prepared by dropping 4 μL of the micellar solution onto carboncoated copper grids; then, the samples were negatively stained with phosphotungstic acid (PTA) aqueous solution. The stained TEM samples were frozen by liquid nitrogen and freeze-dried under the vacuum at −20 °C to avoid the reassembly and aggregation of the micelles during the drying process. The contour lengths of the cylindrical micelles were counted and analyzed with Image-Pro Plus software. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were performed on a JEOL JEM-2100 electron microscope at an operating voltage of 200 kV, equipped with a Gatan Orius CCD camera. The samples for HRTEM characterization were prepared by dropping 10 μL of the seed micellar solution onto ultrathin carbon-coated copper grids and were allowed to dry in air at room temperature. The samples

Figure 1. TEM micrograph of the freeze-dried PCL59-b-PEO113 micelles prepared from the crude aqueous solution.

spherical micelles is about 30−50 nm, and the length of the cylindrical micelles ranges from 80 to 200 nm. The coexistence of different micellar morphologies and wide dispersity in the cylindrical length limit the application of the micelles. A “self-seeding”-like method was used to prepare semicrystalline micellar seeds for further micellar growth.74 The crude micellar solution was first held at a temperature (Th) near the melting temperature of the micellar core for a certain time (holding time, th). After holding at Th for th, amorphous micelles and unimers coexist, with some surviving semicrystalline micellar seeds. These micellar seeds nucleated growth of semicrystalline cylindrical micelles when the seed solution was cooled by rapidly placing the seed solution at another temperature for crystallization (crystallization temperature, Tc) in the next step (Figure S1 in Supporting Information). It should be noted that the method used in the present work may be not real “self-seeding” because under some cases the semicrystalline micelles are just melted and only part of them form unimers. However, upon heating the aqueous solution of PCL59-bPEO113, we found that the micelles were not stable at the holding temperature and easily precipitated due to the decreased solubility of the PEO corona block. To solve this problem, we added different organic solvents including THF, dioxane, DMF, and DMSO to the micellar aqueous solution to stabilize the micelles. The volume ratio of H2O over the organic solvent is 5/1. To choose the proper holding temperature, differential scanning calorimeter (DSC) measurement was performed to follow the melting trace of the freeze-dried PCL59-b-PEO113 micelles, as shown in Figure 2. It is found that the melting peak temperature (Tm) of the freeze-dried PCL59-b-PEO113 micelles is 54.5 °C. It should be noted that the Tm of the micelles was measured in the solid state, although they were freeze-dried. It is highly probable that the Tm of the micelles in the solution is still a little lower than those in the dry state. Usually, the seeding temperature is slightly higher than the melting peak 9769

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micelles after the addition of various organic solvents were also investigated. Figure 4 shows the TEM images of the freeze-

Figure 2. DSC melting curve of the freeze-dried PCL59-b-PEO113 micelles. The heating rate is 10.0 °C/min.

temperature but lower than the melting end temperature for cultivation of polymer single crystals in “self-seeding” method. After trials at different temperatures, finally the holding temperature Th is chosen as 53 °C, slightly lower than Tm. Upon holding at 53 °C, the apparent hydrodynamic diameter (Dhapp) of the micelles in different solutions was monitored by DLS to ascertain a proper holding time (th). Figure 3 shows the Figure 4. TEM micrographs of the freeze-dried seed micelles of PCL59-b-PEO113 in different solutions obtained by holding the solutions at 53 °C for 5 min. (a) H2O/THF (5/1 v/v), (b) H2O/ dioxane (5/1 v/v), (c) H2O/DMF (5/1 v/v), and (d) H2O/DMSO (5/1 v/v).

dried seed micelles of PCL59-b-PEO113 prepared from micellar solutions in different mixed solvents after holding at 53 °C for 5 min. It is observed that the seed micelles in H2O/THF (5/1 v/ v) and H2O/dioxane (5/1 v/v) are mostly spherical, whereas the seed micelles of mixed spheres (majority) and short cylinders (minority) are obtained for seed micelles in H2O/ DMF (5/1 v/v). For the seed micelles in H2O/DMSO (5/1 v/ v), most of the seed micelles are short cylinders. To explore the crystallinity of the seed micelles in different mixed solvents, we carried out HRTEM and SAED characterizations. In contrast with common TEM experiments, the samples for HRTEM observations were prepared by drying the solution at room temperature on the Cu grids to avoid the crystallization arising from a deep cooling. It should be pointed out that the morphologies of the seed micelles dried at room temperature in HRTEM and the freezedried micelles in common TEM experiments are a little different. Even for the seed micelles in H2O/THF (5/1 v/v), some cylindrical micelles are also observed by HRTEM. This may be due to coalescence of partial spherical micelles when the solvent is evaporated at room temperature. SAED shows that all cylindrical micelles are crystalline, irrespective of the solvent. For this reason, only spherical micelles are selected for SAED analysis, as shown in Figure 5. One can see from Figure 5a,d that the seed micelles in H2O/THF (5/1 v/v) are totally amorphous because neither lattice fringes nor diffraction patterns are observed. For the seed micelles in H2O/DMF (5/1 v/v), most of the seed micelles are also amorphous (Figure 5b,e). However, lattice fringes can be observed for some seed micelles and diffraction spots also appear in the SAED patterns. (The lattice

Figure 3. Changes of the apparent hydrodynamic diameter (Dhapp) of PCL59-b-PEO113 micelles in H2O/DMF (5/1 v/v) solution detected by DLS upon holding at 53 °C.

variation of Dhapp with holding time for the micelles in the H2O/DMF (5/1 v/v) solution. It is observed that Dhapp decreases with time and becomes stable after 5 min. Similar results were obtained for other solutions (Figure S2 in Supporting Information). The decrease in Dhapp shows that melting of the micelles with a large size leads to the formation of smaller ones. On the basis of the DLS results, the holding time for preparation of the seed micelles is set as 5 min. In addition, the crystallization temperature for growth of the cylindrical micelles Tc is chosen as 4 °C. Recently Qi et al. reports an example of a BCP in which nucleation to form a semicrystalline micelle is sensitive to solvent.75 They found that it took much longer time to form semicrystalline micelle nuclei for polyisoprene-b-poly(ferrocenyldiethylsilane) (PI-b-PFDES) in a better solvent (tert-butyl acetate) toward PFS, as compared in a poorer solvent decane.75 In our system, the morphologies of the seed 9770

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PCL is limited. In H2O/DMF (5/1 v/v) solution, most of the semicrystalline micelles are melted, but a small number of semicrystalline cylindrical micelles are retained after heating, which can act as nuclei for growth in the next step. The solubility of DMSO toward PCL is the worst among these four organic solvents, and the remaining micelles are mostly short semicrystalline cylinders. Figure 5c,f shows that even for the spherical micelles in H2O/DMSO (5/1 v/v) most of them are crystalline. Growth of Semicrystalline Cylindrical Micelles in H2O/ DMF (5/1 v/v) Solution. After being held at 53 °C for 5 min, the micellar solutions were placed at 4 °C for epitaxial growth to create well-defined and monodisperse cylindrical micelles. As can be seen from Figure 4, the seed micelles in H2O/THF (5/1 v/v) and H2O/dioxane (5/1 v/v) solutions are amorphous spheres and indeed there are no semicrystalline seed micelles in these two solutions. At 4 °C, it will take a very long time for the amorphous micelles to crystallize to form crystalline nuclei, which must be required for growth of semicrystalline micelles. This leads to no growth of the micelles in a short time (Figure S4 in Supporting Information). Therefore, the growth of micelles in these two solutions will not be discussed anymore. Figure 6 shows the TEM images of the freeze-dried micelles after growth for different times in H2O/DMF (5/1 v/v) solution and the corresponding length distributions for the cylindrical micelles analyzed from the TEM images. For each image, more than 100 cylinders were carefully traced by hand to determine their length. One can see that as growth time increases the cylindrical micelles become longer. The diameter of the grown cylindrical micelles remains constant. The number-average length (Ln) and weight-average length (Lw) of the cylindrical micelles can be obtained by analyzing the TEM images of the micelles based on the following equations:

Figure 5. HRTEM (left) and SAED (right) images for the seed micelles prepared from different solutions. (a,d) H2O/THF (5/1 v/v), (b,e) H2O/DMF (5/1 v/v), and (c,f) H2O/DMSO (5/1 v/v). Insets in panels b and c are the Fourier transformation of the square-framed areas. The letters a and c in the HRTEM images indicate crystalline micelles and amorphous micelles, respectively.

n

fringes can be seen more clearly in the enlarged micrographs, as shown in Figure S3 in Supporting Information.) The Fourier transformation of the HRTEM image also verifies that a minor part of the seed micelles is crystalline. As for the seed micelles in H2O/DMSO (5/1 v/v), we can see that most of the micelles are crystalline, as revealed by the lattice fringes, the diffraction spots in the SAED pattern, and the Fourier transformation of the HRTEM image (Figure 5c,f). The effect of organic solvent on the morphology of the seed micelles may result from the solubility of the organic solvents toward the PCL block. The solubility parameters and their polar components for PCL and different solvents are listed in Table 1.76 As we can see from Table 1, THF and dioxane are

Ln =

δt ((MPa) ) δp ((MPa)1/2)

PCL

H2O

THF

1,4-dioxane

DMF

DMSO

18.9 3.3

48.0 22.8

18.5 11.0

20.7 10.1

24.8 13.7

26.7 16.4

n

∑i = 1 Ni

(1)

n

Lw =

2 ∑i = 1 NL i i n

∑i = 1 NL i i

(2)

where Li and Ni are the length of the counted cylindrical micelles and the number of the cylindrical micelles with a length of Li, respectively. The data were fitted to a Gaussian distribution using the following probability function: f (L ) =

Table 1. The Solubility Parameters (δt) and Their Polar Components (δp) for Solvents and PCL at 25 °C76 1/2

∑i = 1 NL i i

⎛ (L − μ)2 ⎞ 1 exp⎜ − ⎟ σ 2π 2σ 2 ⎠ ⎝

(3)

where L is the measured length of the micelles, σ is the standard deviation of the measured length, and μ is the mean length of the micelles. The standard deviation (σ) of the measured lengths is related to the length dispersity (Lw/Ln) by eq 4. ⎛ σ ⎞2 Lw −1=⎜ ⎟ Ln ⎝ Ln ⎠

good solvents for PCL because the solubility parameters of THF and dioxane are very close to that of PCL. Therefore, PCL is difficult to crystallize in the presence of THF and dioxane as cosolvent, and only amorphous spherical micelles are formed after heating. By contrast, there is a large difference between the solubility parameters of PCL and DMF, and the difference of the solubility parameter is even larger for PCL and DMSO. As a result, the solubility of DMF and DMSO toward

(4)

Figure 7 shows the variation of the length of the semicrystalline cylindrical micelles in H2O/DMF (5/1 v/v) solutions with growth time. It is found that the length of the cylindrical micelles measured by TEM increases with the growth time approximately in a linear way. The growth of the semicrystalline cylindrical micelles in H2O/DMF (5/1 v/v) is 9771

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Figure 7. Change of the number-average length of the cylindrical micelles measured by TEM with growth time for the PCL59-b-PEO113 micelles in H2O/DMF (5/1 v/v) solution. The numbers in the Figure indicate the length dispersities, Lw/Ln.

Integration of eq 5 yields: ln[M]0 − ln[M] = k1[S]t

(6)

where [S] is the concentration of the semicrystalline seed micelles, k is the growth rate constant, and [M]0 and [M] are the concentrations of the amorphous micelles and unimers at growth time t = 0 and t, respectively. After transformation of eq 6, we have: −ln[1 − X ] = k1[S]t

(7)

where X is equal to ([M]0 − [M])/[M]0 and is the fraction of the amorphous polymer chains already attached to the seed micelles (corresponding to the conversion in living anionic polymerization). The length of the semicrystalline cylindrical micelles, L, is approximately proportional to X, and thus we have: L = k 2[1 − exp( −k1[S]t )]

(8)

where k2 is a constant. At initial growth time, that is, small t, exp(−k1[S]t ) ≈ 1 − k1[S]t

(9)

Then, we have:

L = k 2k1[S]t

(10)

As a result, L is proportional to t at initial growth time. This is in accordance with what we observed in Figure 7. The length distributions and standard deviations of the cylindrical micelles at some growth time points are summarized in Table 2. One can see that polydispersity, Lw/Ln, is quite small at each growth time, indicating a uniform length distribution. Figure 6. TEM micrographs (left column) and length distributions of the cylindrical micelles analyzed from TEM images by software (right column) for the PCL59-b-PEO113 micelles in H2O/DMF (5/1 v/v) solution after holding at 53 °C for 5 min and then growth at 4 °C for different times. (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5 h. The samples for TEM observation were prepared by freeze-drying.

Table 2. Contour Lengths of PCL59-b-PEO113 Cylindrical Micelles Grown in H2O/DMF (5/1 v/v) Solution Analyzed from TEM Images

quite similar to polyaddition if the semicrystalline seed micelles and the amorphous micelles (and unimers) are viewed as the active species and monomer, respectively. Therefore, we have −d[M]/dt = k1[S][M]

(5) 9772

sample

time (h)

Ln (nm)

Lw (nm)

Lw/Ln

σ (nm)

σ/Ln

A B C D E F

0 1 2 3 4 5

15.5 31.1 42.3 58.9 110.6 118.9

16.2 32.7 43.8 62.0 113.7 128.8

1.04 1.05 1.04 1.06 1.04 1.08

3.2 7.2 8.0 14.5 19.9 34.0

0.21 0.23 0.19 0.25 0.18 0.29

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At the moment, we cannot unambiguously ascertain that the growth of the cylindrical micelles in the H2O/DMF (5/1 v/v) solution is due to the growth of amorphous spherical micelles or unimers in the solution on the active ends of semicrystalline cylindrical micelles. Winnik, Manners, and Schmalz proposed that the semicrystalline cylindrical micelles grew through the epitaxial crystallization of unimers in the solution on the ends of the cylindrical micelles.59,61,70 Considering the lower glasstransition temperature (Tg) of the PCL block in the amorphous spherical micelles and the slight solubility of DMF toward PCL, we are inclined to believe that semicrystalline cylindrical PCLb-PEO micelles in H2O/DMF (5/1 v/v) mixed solvent grow through a similar mechanism. When the unimers in the solution attach to and epitaxially grow on the active ends of semicrystalline cylindrical micelles, the PCL-b-PEO chains in the amorphous spherical micelles can transfer into the solution gradually. Growth of Semicrystalline Cylindrical Micelles in H2O/ DMSO (5/1 v/v) Solution. The TEM micrographs of the freeze-dried micelles and the corresponding length distributions of the cylindrical micelles analyzed from the TEM images after growth for different times in H2O/DMSO (5/1 v/v) solution at 4 °C are illustrated in Figure 8. The length distributions and standard deviations of the cylindrical micelles at corresponding growth times in H2 O/DMSO (5/1 v/v) solution are summarized in Table 3. It is found that the length of the micelles increases with time, indicating that the micelles in H2O/DMSO (5/1) solution can grow as well. Similarly, there is no change for the diameter of the grown micelles. The changes of the number-average lengths of the grown cylindrical micelles measured by TEM with growth time for the PCL59-b-PEO113 micelles in H2O/DMSO (5/1 v/v) are shown in Figure 9. It is observed that there is a linear relationship between the length of the micelles and growth time in the time range of several days. Because most of the seed micelles in the H2O/DMSO (5/1) solution after holding at 53 °C for 5 min are short semicrystalline cylinders (Figure 4d), the increase in cylindrical length with time means that different semicrystalline cylindrical micelles very likely attach each other by their ends. In such a case, the growth of the cylindrical micelles is quite similar to polycondensation. If all semicrystalline cylindrical micelles are viewed as both the active species and monomer, we have:

−d[M]/dt = k[M]2

Figure 8. TEM micrographs (left column) and length distributions of the cylindrical micelles analyzed from TEM images by software (right column) for the PCL59-b-PEO113 micelles in H2O/DMSO (5/1 v/v) solution after holding at 53 °C for 5 min and then growth at 4 °C for different times. (a) 2 h, (b) 1 day, (c) 2 days, and (d) 3.5 days. The samples for TEM observation were prepared by freeze-drying.

(11)

Integration of above equation yields: 1/[M] = kt + 1/[M]0

(12)

Table 3. Contour Lengths of PCL59-b-PEO113 Cylindrical Micelles Grown in H2O/DMSO (5/1 v/v) Solution Analyzed from TEM Images

where k is the growth rate constant and [M]0 and [M] are the concentrations of cylindrical micelles at growth time t = 0 and t, respectively. When [M]0/[M] is set as Xn (corresponding to polymerization degree in condensation polymerization), we have: X n = k[M]0 t + 1

(13)

The length of the semicrystalline cylindrical micelles, L, is proportional to Xn, and thus based on eq 11, the value of L should linearly increase with growth time, t. This agrees with the result in Figure 9 very well. Recently, Winnik and Manners added a solution of semicrystalline homopolymer PFDMS to the cylindrical seed micelles of PI-b-PFDMS and found that PFDMS could first grow at both ends of semicrystalline cylindrical micelles of PI-b-

sample

time (day)

Ln (nm)

Lw (nm)

Lw/Ln

σ (nm)

σ/Ln

A B C D

0.08 1 2 3.5

49.6 64.8 83.2 102.2

52.9 70.1 89.1 113.7

1.07 1.08 1.07 1.11

12.8 18.4 22.2 34.5

0.26 0.28 0.27 0.34

PFDMS. The attached PFDMS could act as “glue” to couple another semicrystalline cylindrical micelle, leading to growth of the semicrystalline cylindrical micelles.64 However, in the present work, we tend to believe that the semicrystalline cylindrical micelles grow by end-to-end coupling without the 9773

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manipulation of the growth kinetics is allowed to some extent. Recently, Dove and O’Reilly reported that the addition of good solvent THF could facilitate the transformation from amorphous spherical micelles to semicrystalline cylindrical ones for poly(L-lactide)-b-poly(acrylic acid) micelles in aqueous solution.77 It was ascribed to higher unimer concentration in the presence of THF. In the present work, the faster growth rate of the cylindrical micelles in H2O/DMF (5/1 v/v) solution may also be due to the presence of more unimers and amorphous spherical micelles. However, because both growth modes of the semicrystalline micelles can take place simultaneously, the logarithmic length of the micelles should not linearly increase with growth time in the initial period; that is, the growth of the micelles should not be in a “living” manner in H2O/DMF (5/1 v/v) solution. Equation 7 is derived without consideration of the growth among different semicrystalline cylindrical micelles. Figure 9 shows that the growth by end-to-end coupling of semicrystalline cylindrical micelles is much slower than the growth of unimers (or spherical amorphous micelles) on semicrystalline cylindrical micelles, and thus the former can be neglected in H2O/DMF (5/1 v/v) solution and eq 7 is still applicable. We also use DLS to monitor the growth of PCL59-b-PEO113 micelles. It is found that the growth of the micellar size can still proceed but becomes very slow when the growth time exceeds 6 h for the H2O/DMF (5/1 v/v) solution or 4 days for the H2O/DMSO (5/1 v/v) solution (Figure S5 in Supporting Information). Moreover, one can see from the TEM images (Figures 6e and 8d) that both the spherical micelles in H2O/ DMF (5/1 v/v) solution after growth for 6 h and the short cylindrical micelles in H2O/DMSO (5/1 v/v) solution after growth for 4 days become fewer. As a result, we believe that the consumption of the most unimers and amorphous spherical micelles in the H2O/DMF (5/1 v/v) solution or the short semicrystalline cylindrical micelles in the H2O/DMSO (5/1 v/ v) solution (i.e., seed micelles prepared by holding at 53 °C) gives rise to the slowdown of micellar growth. In summary, not only unimers (or amorphous spherical micelles) but also semicrystalline cylindrical micelles can grow on the ends of semicrystalline cylindrical micelles. The growth rate strongly depends on the shape and size of the assembly units. Because the unimers are highly mobile in the solutions, they can easily attach to the ends of the semicrystalline cylindrical micelles and the semicrystalline cylindrical micelles grow fast through an epitaxial crystallization. By contrast, the growth by end-to-end coupling of short semicrystalline cylindrical micelles can take place only when the ends of two short cylindrical semicrystalline micelles collide. The probability of such an event is evidently smaller than that for attachment of unimers to the ends of the semicrystalline cylindrical micelles. Moreover, the growth of the crystalline micelles cannot proceed due to the mismatch of the crystal planes when the side part of a cylindrical micelle collides with the end of another cylindrical micelle. For the same reason, the growth between long cylindrical semicrystalline micelles is even slower. Therefore, there exists a stepwise decrease in rate for growths of unimers (or amorphous spherical micelles) and cylindrical micelles on semicrystalline cylindrical micelles, as shown in Scheme 1. The difference in rate between these two growth modes is so large that the growth of unimers (or amorphous spherical micelles) on semicrystalline cylindrical micelles can be viewed as “quasiliving”, although two growth modes may occur simultaneously.

Figure 9. Change of the number-average length of the cylindrical micelles measured by TEM with growth time for the PCL59-b-PEO113 micelles in H2O/DMSO (5/1 v/v) solution. The numbers in the Figure indicate the length dispersities, Lw/Ln.

help of unimers or amorphous micelles, although the coupling is not directly observed. There are two reasons for such a speculation. First, the unimers or amorphous micelles become crystalline after attachment to the semicrystalline micelles through an epitaxial crystallization mechanism, and thus there is no difference for the ends of the semicrystalline cylindrical micelles before and after the attachment of the unimers or amorphous micelles. If a cylindrical micelle can couple another cylindrical micelle after attachment of the unimers or amorphous micelles, then they can directly couple end-to-end as well. Second, the number of unimers or amorphous micelles in H2O/DMSO (5/1 v/v) solution is limited and BCPs in the semicrystalline cylindrical micelles cannot go back to the solution due to the vitrification effect of crystallization after the unimers or amorphous micelles in the solution are consumed. As a result, after all unimers or amorphous micelles are attached to the semicrystalline cylindrical micelles, then the growth of the micelles should stop if the direct couple between two semicrystalline cylindrical micelles could not take place. Nevertheless, we observe that the semicrystalline cylindrical micelles can grow into long fiber-like micelles with a length of several micrometers. (See the next section.) Comparison of Two Growth Modes. The above results show that two different growth modes of the semicrystalline cylindrical micelles were obtained. One is the epitaxial crystallization growth of unimers or amorphous spherical micelles on the active ends of the semicrystalline cylindrical seed micelles, which takes place in H2O/DMF (5/1 v/v) solution. The other is the micellar growth by end-to-end coupling of different semicrystalline cylindrical micelles. Such a growth is observed in H2O/DMSO (5/1 v/v) solution. Comparing Figures 7 and 9, one can see that growth of the micelles in H2O/DMSO (5/1 v/v) solution is much slower than that in H2O/DMF (5/1 v/v) solution. It takes only 5 h for the micelles in the H2O/DMF (5/1 v/v) solution to grow into a length of ∼130 nm, whereas ∼4 days are needed for the micelles in H2O/DMSO (5/1 v/v) solution to grow into a similar length. In other words, the growth by end-to-end coupling of different semicrystalline cylindrical micelles is much slower than the growth of unimers or amorphous spherical micelles on the semicrystalline cylindrical seed micelles. By adding different organic solvents to the aqueous micellar solution, different seed micelles can be prepared, and thus 9774

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Scheme 1. Scheme for Variations of Logarithmic Length of the Cylindrical Micelles with Growth Time in Different Modes (A) and the Corresponding Growth Mechanisms (b)

Growth of Semicrystalline Cylindrical Micelles on Long Time Scale. To verify the growth by end-to-end coupling of different semicrystalline cylindrical micelles, we performed experiments on long time scale. The TEM micrographs of the freeze-dried micelles in H2O/DMF (5/1 v/v) and H2O/DMSO (5/1 v/v) solutions after long time growth at 4 °C and the corresponding length distributions of the cylindrical micelles analyzed from the TEM images are shown in Figures 10 and 11, respectively. It is observed that in both solutions the growth of the semicrystalline cylindrical micelles indeed takes place on long time scale. This is also clearly revealed by the plots of the length of the cylindrical micelles versus growth time (Figure 12). The data in Figure 12 show that the length of the cylindrical micelles approximately increases with growth time in a linear way in both solutions. This indicates that the mechanism for growth by end-to-end coupling of different cylindrical micelles on long time scale is still like condensation polymerization. The data in Figures 7 and 9 show that the growth by end-toend coupling of different cylindrical micelles in the H2O/ DMSO (5/1 v/v) solution is slower than the growth of unimers or amorphous spherical micelles on cylindrical micelles in the H2O/DMF (5/1 v/v) solution in the initial period. However, one can see from Figure 12 that on long time scale the growth rate in the H2O/DMSO (5/1 v/v) solution is larger than that in the H2O/DMF (5/1 v/v) solution. The slower growth by end-to-end coupling of different cylindrical micelles in the H2O/DMF (5/1 v/v) solution is probably due to the better solubility of DMF toward to PEO, leading to more expanded conformation and thus larger reduced tethering density of PEO. As a result, the PEO chains at the ends of the semicrystalline cylindrical micelles are more crowded in the H2O/DMF (5/1 v/v) solution and tend to cover parts of the lateral surface of PCL crystals, which hinders the growth between different cylindrical micelles, resulting in a slower growth rate. This shows that the growth by end-to-end coupling of different semicrystalline cylindrical micelles also depends on the organic solvent added. Comparing Figures 9 and 12, one can see that the growth by end-to-end coupling of different cylindrical micelles in H2O/ DMSO (5/1 v/v) solution on short time scale is faster than that on long time scale. This implies that the growth between different cylindrical micelles becomes slower as the length of the cylindrical micelles increases, probably due to the strong interference, such as entanglement, among the long micelles. We also notice in Figures 10 and 11 that after growth for about 1 month there seems to be a sudden change in micellar morphology. Lots of long and short cylindrical micelles are observed. The length of the short cylindrical micelles is even smaller than the length of the micelles grown at shorter time,

Figure 10. TEM micrographs (left column) of the freeze-dried PCL59b-PEO113 micelles in H2O/DMF (5/1 v/v) solution after long time growth at 4 °C and the corresponding length distributions (right column) of the cylindrical micelles analyzed from the TEM images. (a) 0.5, (b) 8, (c) 14, (d) 17, (e) 32, and (f) 37 days. 9775

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Figure 12. Variations of the number-average length of the cylindrical micelles measured by TEM with growth time at long time scale for the PCL59-b-PEO113 micelles in H2O/DMF (5/1 v/v) and H2O/DMSO (5/1 v/v) solutions. The numbers in the Figure indicate the length dispersities, Lw/Ln.

that there may exist an unknown mechanism for the growth on very long time scale, and further work is still in progress.

4. CONCLUSIONS The TEM results reveal that the seed micellar solution of PCL59-b-PEO113 with different micellar morphologies and amount of micellar semicrystalline seeds can be papered by a self-seeding-like method after the addition of different organic solvents. Under the same seeding conditions, mostly amorphous spherical seed micelles are formed in the H2O/ THF (5/1 v/v) and H2O/dioxane (5/1 v/v) solutions; a mixture of amorphous spherical micelles and short cylindrical semicrystalline micelles is obtained for the H2O/DMF (5/1 v/ v) solution, but most of the seed micelles are short semicrystalline cylinders micelles in the H2O/DMSO (5/1 v/ v) solution. Two different growth modes are observed: growth of unimers (or amorphous spherical micelles) on the active ends of cylindrical semicrystalline micelles and growth by endto-end coupling of different semicrystalline cylindrical micelles. The growth rate of the latter is much smaller than that of the former. On long time scale, growth by end-to-end coupling of different cylindrical micelles is observed in both solutions, but the growth rate in the H2O/DMF (5/1 v/v) solution is slower than that in the H2O/DMSO (5/1 v/v) solution. This shows that the growth kinetics of semicrystalline cylindrical micelles of PCL-b-PEO BCP can be regulated to some extent by the addition of different organic solvents. It is also found that the growth by end-to-end coupling of different cylindrical micelles becomes slower with increase in the length of the cylindrical micelles.

Figure 11. TEM micrographs (left column) of the freeze-dried PCL59b-PEO113 micelles in H2O/DMSO (5/1 v/v) solution after long time growth at 4 °C and the corresponding length distributions (right column) of the cylindrical micelles analyzed from the TEM images. (a) 5, (b) 14, (c) 18, (d) 27, and (e) 37 days.

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ASSOCIATED CONTENT

S Supporting Information *

Schematic temperature profile for preparation of the seed micelles and micellar growth, changes of Dhapp with holding time for different micellar solutions at 53 °C, enlarged HRTEM images, TEM images of micelles in H2O/THF (5/1 v/v) and H2O/dioxane (5/1 v/v) solutions after growth for a long time, and changes of Dhapp with time ojn a long time scale for H2O/ DMF (5/1 v/v)and H2O/DMSO (5/1 v/v) solutions. This

which is out of our expectation. It is possible that the short cylinders are produced by fragmentation of the long ones upon sample preparation. The length of the long cylindrical micelles can reach several micrometers, which is much longer than the length expected from the normal growth kinetics. This indicates 9776

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material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86-571-87952400. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the financial support from the National Natural Science Foundation of China (20974099 and 21274130) and Specialized Research Fund for the Doctoral Program of Higher Education (20090101110054).

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