Effect of Concentration on the Dissolution of One-Dimensional

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Cite This: Macromolecules XXXX, XXX, XXX−XXX

Effect of Concentration on the Dissolution of One-Dimensional Polymer Crystals: A TEM and NMR Study Gerald Guerin,*,† Gregory Molev,† Dmitry Pichugin,† Paul A. Rupar,‡ Fei Qi,† Menandro Cruz,† Ian Manners,‡ and Mitchell A. Winnik*,† †

Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON M5S 3H6, Canada School of Chemistry, University of Bristol, Bristol, U.K. BS8 1TS

‡

Macromolecules Downloaded from pubs.acs.org by YORK UNIV on 12/19/18. For personal use only.

S Supporting Information *

ABSTRACT: We report a study of the dissolution of corecrystalline polyferrocenyldimethylsilane-block-polyisoprene (PFS53-b-PI637, where the subscripts are the degrees of polymerization of the two blocks) micelle fragments in decane for different concentrations (ranging from 0.01 to 6 mg mL−1) by a combination of transmission electron microscopy (TEM) and high-temperature 1H NMR. We used self-seeding experiments at different temperatures as an efficient, although indirect, way to evaluate the dissolution of these micelles fragments. We annealed micelle fragment solutions at five different temperatures (50, 60, 65, 70, and 75 °C) for 30 min and cooled them to room temperature to regrow the micelles. The amount of micelle fragments that dissolved at the annealing temperature was then evaluated by comparing the length of the regrown micelles with that of the starting micelle fragments. We show that seed crystallites are less prone to dissolution as their concentration increases. In addition, by combining results of self-seeding experiments and 1H NMR measurements at 75 °C, we evaluated the percentage of unimer released upon the partial dissolution of seed fragments at 75 °C and established that the mechanism of seed fragment dissolution is also concentration dependent: at low concentrations, they dissolve in a cooperative process, whereas at high concentrations, they dissolve partially from both ends.

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INTRODUCTION Block copolymers (BCPs) self-assemble in selective solvents to form micelles consisting of a solvent-insoluble core surrounded by a solvent-swollen corona. Most BCP micelles are spherical, although other shapes can form.1−5 Rather different structures are obtained when the core-forming block can crystallize, particularly when crystallization of this block provides the driving force for micellization.6−8 This process is now termed crystallization-driven self-assembly (CDSA). CDSA of asymmetric BCPs consisting of a relatively short core-forming block and a much longer soluble block commonly leads to fiber-like one-dimensional (1D) micelles.9−23 These micelles have many of the fascinating properties of crystalline polymers. For example, addition of BCP unimer in a good solvent to a micelle suspension leads to seeded growth: the added unimer deposits epitaxially on the exposed crystal faces at the ends of the micelles, allowing the preparation of rod-like or fiber-like micelles of uniform length.24−27 In this way, one can prepare block comicelles by sequential addition of different BCPs with a common core forming block,28−31 and patchy micelles with different corona chains when the BCPs are added simultaneously,32−34 opening the door to the fabrication of sophisticated mesostructures with complex architectures.35−38 The core of these micelles consists of regions of different chain packing order and crystallinity, with a broad range of melting temperatures. When a micelle © XXXX American Chemical Society

suspension is heated within the range of dissolving temperatures of the crystalline core, the less crystalline regions dissolve to form a mixture of unimer and surviving micelle fragments. Upon cooling, the surviving crystals serve as seeds onto which the unimers regrow, leading to a suspension of micelle uniform in size, in a process called self-seeding.39−42 In this temperature range, a small increase in annealing temperature leads, after cooling, to a smaller number of larger single crystals, as observed for homopolymer single crystals in solution or in the solid state. While micelle formation and growth by CDSA has been extensively studied over the past decade, much less is known about how these micelles melt or dissolve upon heating. In this regard, self-seeding has become an essential tool for investigating the dissolution of core-crystalline micelles. We recently reported the first study of the dissolution step of a selfseeding experiment on a dilute solution (c = 0.02 mg mL−1) of micelle fragments of a polyferrocenyldimethylsilane-blockpolyisoprene BCP (PFS53-b-PI637) in decane.43 To follow the dissolution of these short micelles (mean length Ln = 63 nm), we heated the solution to various annealing temperatures, Ta, where increasing fractions of the micelle fragments dissolved. Received: October 3, 2018 Revised: November 22, 2018

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DOI: 10.1021/acs.macromol.8b02126 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules We then trapped the surviving micelle fragments at Ta by adding a large excess of PFS60-b-PDMS600 (PDMS = polydimethylsiloxane) just before the solution was cooled to room temperature. Because the surviving fragments serve as seeds for micelle growth upon cooling, we referred to these experiments as “seed trapping” experiments. Upon cooling, both the PFS53 -b-PI637 and PFS60-b-PDMS600 unimers competed in adding on both ends of the surviving seeds and formed M(PFS−PI)-r-M(PFS−PDMS)-b-M(PFS−PI)-b-M(PFS−PDMS)-rM(PFS−PI) triblock comicelles, containing a large excess of PFS60-b-PDMS600 BCP on both sides of the seeds. By selectively staining the PI corona forming block and careful analysis of transmission electron microscopy (TEM) images, we could evaluate both the overall micelle length and the length of the seeds that were trapped at each annealing temperature. There were two important surprises from these experiments. The first was that the mean length and length distribution of the surviving seeds did not change substantially over the entire range of Ta examined, although the fraction of surviving seeds decreased from >50% to 1 μm) micelles were then sonicated at RT for three 10 min intervals. Self-Seeding Experiments. Separate sets of self-seeding experiments were performed on samples prepared from batch 1 and batch 2 of micelle fragment solutions. Batch 1:13 samples were prepared from the batch 1 mother solution to yield samples in decane at concentrations c = 0.01, 0.02, 0.05, 0.08, 0.12, 0.18, 0.3, 0.6, 1, 1.5, 2, 3, and 6 mg mL−1. Each sample was distributed into five different vials, which were all annealed in an oil bath at 50, 60, 65, 70, and 75 °C for 30 min, and then cooled in air to room temperature. These samples were then studied by TEM after being aged 1 week at room temperature to ensure complete regrowth of the micelles. A TEM study of the mother solution was presented in ref 45. For batch 2, five solutions were prepared from the mother solution (c = 0.2, 0.6, 1, 3, and 6 mg mL−1 in decane) and split in two series. One set of samples was annealed at 75 °C for 30 min, cooled to 23 °C, and studied by TEM. The second set of samples was examined by 1 H NMR after ca. 30 min annealing at 75 °C in the NMR probe. From the TEM image analysis (see below for a detailed description of the procedure), we could evaluate the number-average length B


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Macromolecules (Ln(c,Ta)1 for batch 1, Ln(c, 75)2 for batch 2) of the micelles prepared by self-seeding at the different annealing temperatures, Ta, and for each of the concentrations, c. NMR Spectroscopy Experiments. The NMR experiments were performed on an Agilent DD2 600 spectrometer equipped with a OneNMR(tm) probe. As a reference, a 1H NMR spectrum of a PFS53b-PI637 unimer solution in benzene-d6 (c = 6 mg mL−1) was taken at 23 °C. In addition, NMR spectra of micelle fragment solutions from batch 2 (c = 0.2, 0.6, 1, 3, and 6 mg mL−1) were recorded at 23, 75, and 100 °C. Because these PFS53-b-PI637 micelle fragments were dispersed in decane, a deuterium lock was provided by DMSO-d6 in a sealed glass capillary inserted into the NMR tube. This capillary tube also contained tetramethylsilane (TMS) and 1,4-dimethoxybenzene (DMB, 0.6 mg/mL) as external standards. The decane solvent signals were partially suppressed using a WET1D pulse sequence.50 Experiments were run with a 10 s relaxation delay, 4.5 s acquisition time, and 90° pulse angle. T1 measurements were performed using an inversion−recovery experiment with solvent suppression at 100 °C. T1 relaxation times of 2.5 s were measured for PFS signals at 3.7 and 4.1 ppm. Transmission Electron Microscopy. Bright-field TEM micrographs were obtained on a Hitachi H-7000 microscope operating at 100 kV. To prepare the TEM samples, all of the annealed solutions were further diluted at room temperature to c = 0.01 mg mL−1. A drop of each micelle solution was then placed onto a carbon-coated copper grid at room temperature, and the excess fluid was removed with a clean piece of filter paper. Before every electron microscopy session, the electron beam was aligned to minimize optical artifacts. Images were analyzed using the software package ImageJ, published by the National Institutes of Health. Statistical length analysis was performed on more than 200 micelles that were traced by hand to determine their contour length. The number- and weight-average micelle lengths (Ln and Lw, respectively) were calculated using eq 1, where N is the number of micelles examined in each sample and Li is the contour length of the ith micelle. N

Ln =

∑i = 1 NL i i N ∑i = 1 Ni

Figure 1. Effect of the initial concentration (c) of PFS53-b-PI637 micelle fragments subjected to self-seeding at 75 °C on micelle length upon cooling and on the fraction of surviving seeds at 75 °C. (A) Plot of Ln(c,75°)1 as a function of c, with representative TEM images of PFS53-b-PI637 micelles obtained from solutions of c = 0.01, 0.18, and 2 mg mL−1. The corresponding histograms of length distribution are shown besides each TEM image. Ln(c,75°)1 was evaluated from TEM image analysis of more than 200 micelles for each sample. (B) Plot of the S(c,75°)1 as a function of concentration. An enlarged view of the plot for low concentrations is presented in the inset, with a dashed black line to emphasize the change in slope of the plot at low concentrations. For all TEM images, the scale bar corresponds to 500 nm.

N

Lw =

2 ∑i = 1 NL i i N

∑i = 1 NL i i

(1)

The average length of the initial seed fragments obtained upon sonication from batch 1 was Ln(seeds)1 = 53 nm and Lw(seeds)1 = 72 nm, while for batch 2, Ln(seeds)2 = 39 nm and Lw(seeds)2 = 54 nm.

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RESULTS AND DISCUSSION Self-Seeding Experiments on Samples Prepared from Batch 1. Because the self-seeding study was performed over a very wide range of concentrations, we first established that all of our experiments were done in the dilute regime by evaluating the overlap concentration of the micelle fragment solution, c*. For the micelle fragments in batch 1, we determined their hydrodynamic radius by multiangle dynamic light scattering and obtained a value of Rh = 46 nm at 23 °C for the most dilute sample. From this value, we calculated c* ≈ 58 mg mL−1 (calculation given in the Supporting Information), which is 10 times larger than the most concentrated solution used for this study. In Figure 1A, we show the evolution in length as a function of micelle fragment concentration for samples subjected to selfseeding with an annealing temperature of 75 °C for 30 min followed by cooling to RT. In our notation, we refer to the number-average lengths of these sample as Ln(c,75°)1. Representative TEM images and histograms of samples prepared at c = 0.01, 0.18, and 2 mg mL−1 are also shown. The effect of the initial concentration on the final micelle length is striking: the lengths of the regrown micelles decrease drastically as a function of concentration from Ln(0.01,75°)1 = 5500 nm to Ln(6,75°)1 = 60 nm.

From the values of Ln(c,Ta), we can evaluate the percentage of micelle fragment “surviving seeds” that persist upon annealing at Ta. S(c , Ta) = 100

Ln(seeds) (%) Ln(c , Ta)

(2)

These values of S(c,75°)1 for the sample annealed at 75 °C for 30 min are presented in Figure 1B. A low value of S(c,Ta) indicates that a large majority of the seeds initially present in the solution dissolved upon annealing, whereas a value approaching 100% indicates little dissolution. Values larger than 100% indicate an increase in the number of micelle fragments upon annealing. This situation can occur if some of the micelles in the sample undergo fragmentation upon heating.43,51 Binary fragmentation generates two seed fragments that can regrow into micelles as the solution is cooled. In Figure 1B, one observes that S(c,75°)1 increases monotonically with concentration, from S(0.01,75°)1 = 0.96% to S(6,75°)1 = 86%. This result is a clear indication that the seeds are more stable toward dissolution at higher C


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Figure 2. Plots of (A) Ln(c,Ta)1 as a function of Ta for c = 0.01, 0.18, 2, and 6 mg mL−1. (B) is an enlarged view of (A) for Ln(c,Ta)1 < 800 nm. (C) A log−log plot of Ln(c,Ta)1 versus initial seed concentration, c, for all the self-seeding temperatures studied. The dashed lines are given as guides to the eye.

concentrations. A change in slope of S(c,75°) versus c at c ≈ 0.15 mg mL−1 can also be observed in the inset of Figure 1B, indicating that S(c,75°) reaches a plateau at low values of c, with a limiting value of ca. 0.9% as the concentration decreases toward 0. The presence of a limiting value suggests that even at extremely low concentrations some seed crystallites would survive annealing at 75 °C. Results like this emphasize that even at 75 °C these micelles do not have a characteristic critical micelle concentration (CMC). In a previous work,43 we have shown that when a solution of micelle fragments was annealed at a given temperature, the percentage of surviving seeds could be directly related to the distribution in crystallinity of the micelles: highly crystalline micelle fragments dissolve at high temperatures, while seeds containing a large fraction of amorphous PFS in the core dissolve at lower temperatures. The plateau observed at low concentrations might thus be explained by the presence of these highly crystalline seeds that would require a higher thermal energy to dissolve. Effect of Self-Seeding Temperature and Initial Concentration. In Figure 2A, we present Ln(c,Ta)1 as a function of Ta for four representative initial concentrations: c = 0.01, 0.18, 2, and 6 mg mL−1. The monotonic increase of the regrown micelle length with Ta observed for all the concentrations follows a typical self-seeding behavior.39−42 Figure 2A also shows that as the concentration was increased, a

higher temperature was required for Ln(c,Ta) to become noticeably larger than Ln(seeds), implying that the dissolution temperature of the micelle fragments was shifted toward higher values. This shift can also be seen for larger concentrations when one expands the y-axis scale for the portion of the plot at lower values of Ln(c,Ta) (Figure 2B). The effect of the initial concentration on the length of the regrown micelles is presented as a log−log plot for all the selfseeding temperatures investigated (Ta = 50, 60, 65, 70, and 75 °C, Figure 2C). Representative TEM micrographs and corresponding length distribution histograms of these samples are shown in the Supporting Information (Figures S1−S6). The most profound changes Ln(c,Ta) occur at concentrations between 0.02 and 0.5 mg/mL. In this plot, the error bars refer to the standard deviation of the length distribution, as determined by TEM image analysis. For all annealing temperatures, Ln(c,Ta) appears to reach a limiting value of ca. 50 nm at 6 mg/mL. For a self-seeding temperature of 50 °C the effect of concentration on the length of the micelle fragments after cooling is hardly noticeable and can only be seen in the slight shift of the length distributions (Figure S7) at very low concentrations (c < 0.1 mg mL−1). For self-seeding temperatures >50 °C, Ln(c,Ta) decreased monotonically as the concentration was increased, reaching a plateau value close to the number-average length of the initial seeds. This plateau D


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Macromolecules is, however, reached at higher concentrations when Ta is increased. On the basis of the data in Figure 2, we calculated the percent surviving seeds S(c,Ta)1 for each of the self-seeding temperatures. These values are plotted as a function of c in Figure 3A and on an expanded x-axis scale for low c in Figure

monitor the dissolution of PFS53-b-PI637 micelle fragments in decane at 75 °C by 1H NMR. For these experiments, we used the second batch of seed fragments, b2, with the following characteristics: Ln(seeds)2 = 39 nm and Lw(seeds)2 = 54 nm (as evaluated by TEM; cf. Figure S8). Prior to examining the dissolution of PFS53-b-PI637 seeds at 75 °C, 1H NMR spectra of the micelle fragment solution (6 mg mL−1 in decane) were recorded at 25 °C (Figure 4A) and at 100 °C (Figure 4B). As a reference, we also ran a spectrum of the polymer in benzene-d6 (Figure S9), a good solvent for both blocks. At 25 °C, in decane, the signal due to the protons of the PFS cyclopentadienyl located at 3.89 and 4.09 ppm is barely visible (Figure 4A), while the peaks of the alkene units of the PI corona block (4.4−5.2 and 5.5−6 ppm) can clearly be observed. We have previously shown48 that the PI chains in the micelle corona are sufficiently mobile that there is no change in integrated intensity of these resonances as the micelles are heated from room temperature to the point where all of the polymer dissolves. The absence of signal from the cyclopentadienyl protons at 25 °C confirms that the PFS block is immobilized inside the micelle core and that no detectable PFS53-b-PI637 unimer is present in the solution at room temperature. At 100 °C, the peaks of the cyclopentadienyl protons are visible, and the block ratio calculated from the integrated intensities of the two PFS peaks at ca. 4 ppm to those of the PI peaks at 4.4−5.1 ppm (see the Supporting Information) is similar (within 5%) to the block ratio of PFS53b-PI637 in a good solvent, indicating that the solution only contains unimer at this temperature. We then examined the 1H NMR signal of the PFS53-b-PI637 micelle fragments at 75 °C. For this experiment, the NMR tube containing the seed solution was inserted in the probe preheated at 75 °C. The NMR tube also contained a capillary insert with DMSO-d6 as a deuterium lock and 1,4-dimethoxybenzene as an external proton standard. The sample was annealed for ca. 30 min, and the 1H NMR spectrum was recorded (Figure 4C). As expected, 1H NMR spectrum of the hot solution at 75 °C exhibited peaks from both the PI and the PFS protons. The ratio of the integrated PFS proton intensity compared to that of the PI protons was smaller than that for the reference sample at 100 °C, which indicates that some of the PFS units remain immobilized in the micelle core. This experiment was repeated for samples at five different concentrations, from c = 0.2 to 6 mg mL−1 (see Figures S10 and S11). From the ratios of the PFS proton peak intensities to those of the PI protons, we could evaluate the percentage of PFS53-b-PI637 polymer dissolved in solution as unimer for each concentration at 75 °C, UNMR(c,75°). These data are plotted in Figure 5A. In parallel, we used the batch 2 mother solution to prepare another set of solutions of micelle fragments with concentrations identical to those of the NMR experiments, i.e., c = 0.2, 0.6, 1, 3, and 6 mg mL−1, and performed self-seeding experiments at 75 °C. The micelles regrown at room temperature were imaged by TEM, and their average length, Ln(c,75°)2, was evaluated by TEM image analysis. Finally, we calculated the percentage of PFS53-b-PI637 unimer present in solution at 75 °C, UTEM(c,75°), according to eq 3:

Figure 3. (A) Plot of S(c,Ta)1 versus c at annealing temperatures Ta = 50, 60, 65, 70, and 75 °C. (B) is an enlarged view of A for S(c,Ta)1 < 50%. Values of S(c,Ta)1 larger than 100% are an indication that the micelles underwent some fragmentation, increasing their number. In (A) the dashed lines are given as guides to the eye.

3B. One observes that for Ta < 75 °C the percentage of surviving seeds increases with concentration to reach a plateau at 100%. This plateau is reached at higher concentrations as the temperature increased, indicating that an increase of Ta promotes micelle fragment dissolution of the more concentrated solutions. This plot also suggests that at Ta = 75 °C S(c,Ta) would eventually reach 100% for c > 6 mg mL−1. The data for the experiments at Ta = 75 °C presented in Figure 3B show a change of slope at low concentrations. A similar effect can also be seen for micelle fragments annealed at 70 °C, but the change in slope appears at lower concentrations for Ta = 70 °C (c ≈ 0.08 mg mL−1) than for Ta = 75 °C (0.15 mg mL−1). This change of slope is, however, not observable for Ta < 70 °C, suggesting that the concentrations investigated were already too high for these lower annealing temperatures. Study of the Seed Dissolution by 1H NMR. TEM is an excellent tool to study the effect of concentration on the selfseeding of polymer crystals, allowing one to monitor size changes that occur for experiments performed over an extremely wide range of concentrations. However, TEM remains an indirect method that needs to be confirmed by more direct techniques, particularly when the protocol of seed trapping is not used to isolate the surviving seeds at the annealing temperature. For this purpose, we decided to

ij L (seeds)2 yzz UTEM(c , 75°) = 100jjj1 − n z (%) j Ln(c , 75°)2 zz{ k

E

(3)


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Figure 4. 1H NMR spectra of PFS53−b-PI637 micelle fragments in decane (6 mg mL−1) at (A) 25, (B) 100, and (C) 75 °C. The peaks of the protons of the cyclopentadienyl units of the PFS block are located at 3.89 and 4.09 ppm, while the peaks due to the protons of the alkene units of the PI block are located at 4.4−5.2 and 5.5−6 ppm. For all the spectra, a capillary containing DMSO-d6 was inserted inside the NMR tube to lock and shim the magnetic field. For polyisoprene, integration of the peaks a, b, c, and d was used to evaluate the ratio of 3,4 (x), 1,2 (y), and 1,4 (z) isomers. For PFS53−PI637, x = 0.48, y = 0.37, and z = 0.15. The fraction of polymer dissolved as unimer was evaluated by comparing the integration of the PFS peaks at 3.89 and 4.09 ppm to those of the PI peaks. Note: the spectra were obtained using a WET1D pulse sequence to suppress the decane peak.

[UNMR(c,75°) − UTEM(c,75°)] as a function of the starting concentration of the micelle fragments (Figure 5B). One should expect UNMR(c,75°) to be equal to or larger than UTEM(c,75°) for all the concentrations studies. However, this is not the case when c < 3 mg mL−1, where UTEM(c,75°) > UNMR(c,75°). The underestimated values of UNMR(c,75°) most likely find their origin in the error associated with peak integration due to poor signal-to-noise ratio at these concentrations (see, for example, the 1H NMR spectrum of the most dilute sample, c = 0.2 mg mL−1, before apodization; Figure S12). We thus assume that at these low concentrations the differences between UNMR(c,75°) and UTEM (c,75°) are negligible. The uncertainty of the evaluation of UNMR(c,75°) at low concentration does not affect the general trend observed in Figure 5B, i.e., the increase of [UNMR(c,75°) − UTEM(c,75°)] as a function of c. The fact that UNMR(c,75°) is very close to UTEM(c,75°) at low concentrations suggests that the unimer present in solution comes essentially from micelle fragments that dissolved entirely, while at higher concentrations, since UNMR(c,75°) > UTEM(c,75°) (reaching ca. 16% at c = 6 mg mL−1), the micelle

In Figure 5A, we present the plot of both UNMR(c,75°) and UTEM(c,75°) as a function of the initial micelle fragment concentration. In both types of experiments, the solutions were annealed at 75 °C for 30 min. Both sets of data show that U strongly depends on the initial micelle fragment concentration and decreases monotonically with increasing concentration. At low concentrations (c < 1 mg mL−1) UTEM appears to be larger than UNMR [UNMR(0.6,75°) = 62% and UTEM(0.6,75°) = 70%]. At higher concentrations, however, UNMR becomes larger than UTEM [UNMR(6,75°) = 30% and UTEM(6,75°) = 11%]. Figure 5A shows a few intriguing features that need to be addressed. First, we recall that the unimer present in a micelle fragment solution annealed at high temperature comes not only from fragments that dissolve entirely but also from micelle fragments that partially dissolve. While the 1H NMR spectra account for all the unimer present in solution,52 TEM imaging of the micelles regrown at room temperature only provides information about the amount of unimer originating from seeds that dissolved entirely. To emphasize the difference between the values of U(c,75°) obtained from NMR with those obtained by TEM, we plotted the difference F


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block copolymer, play an important role in the striking behavior reported here.

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SUMMARY We used a combination of TEM and 1H NMR measurements to study the effect of polymer concentration, c, on self-seeding behavior of PFS53-b-PI637 micelle fragments at 50, 60, 65, 70, and 75 °C. From TEM image analysis we could evaluate the fraction of micelle fragments that resist dissolution in decane at these temperatures and obtained the surprising result that this fraction increased substantially as c increased from 0.02 to 6 mg mL−1. 1H NMR measurements allowed us to determine the fraction of polymer that dissolved as unimer at 75 °C. This fraction decreased with increasing c. By comparing these values, we could show that two processes contribute to the unimer concentration at 75 °C: micelle fragments that dissolve completely (responsible for the unimer contribution to the self-seeding effect) and micelle fragments that only partially dissolve. The contribution of partial dissolution of the micelle fragments becomes increasingly important at the higher polymer concentration. This result points to a change in dissolution mechanism for the micelle fragments. At low concentration, the major dissolution mechanism involves a cooperative process (a “conformational explosion”) in which the entire fragment disintegrates into unimer. The dissolution temperature reflects the degree of crystal perfection in the individual fragments. At higher micelle fragment concentrations, this mechanism is suppressed. Dissolution occurs from the ends of the fragments, which persist to high annealing temperature. The origin of this change in mechanism is not yet understood but may be related to interactions involving the micelle corona. Studies are in progress to explore this effect.

Figure 5. (A) Plot of UNMR(c,75°) and UTEM(c,75°) versus concentration for PFS53-b-PI637 seed solutions annealed for ca. 30 min at 75 °C obtained from 1H NMR (blue squares) and TEM (red diamonds). (B) Plot of the percentage of partially dissolved unimer, UNMR(c,75°) − UTEM(c,75°), as a function of c. UNMR(c,75°) was obtained from the integration of the PFS and PI 1H NMR signals, while UTEM(c,75°) was evaluated by TEM eq 1. For TEM experiments, the samples were annealed for 30 min, cooled in air to 23 °C, and aged at room temperature for several days before imaging. The dashed lines are given as guides to the eye.

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ASSOCIATED CONTENT

S Supporting Information *

fragments dissolve only partially. This result suggests that at low concentrations explosive (cooperative) dissolution of the micelle fragments is the dominant process, while for concentrated solutions the majority of the seeds are more resilient and dissolve only partially via unimer dissociation from the ends of the micelles. One should note that the percentage of unimer evaluated from TEM images is somewhat underestimated since it involves assumptions about the system that may not be entirely correct. For example, we showed that seed crystallites annealed at high concentration (6 mg mL−1) and high temperature (75 °C) become wider by ca. 30%.45 Accounting for this effect, however, would lead to only a slight increase in UTEM(6,75°) from 11.4 to 14.8%, a value that is still much lower than UNMR(6,75°) = 27.9% . The mechanistic origin of the concentration effect on the dissolution of core-crystalline micelles is not yet clearly understood. There are, however, some common features among the small number of previous studies that varied concentration in self-seeding experiments and did not observe a concentration effect. Two sets of experiments were performed on polyethylene homopolymer,46,53 and one set of experiments was performed on a diblock copolymer with a liquid crystalline core-forming block and a block ratio close to 1.0.54 The one other study that did show a concentration effect on self-seeding involved a block copolymer with a block ratio close to 10.47 These experiments provide a hint that the length of the corona forming block, and possibly the block ratio of the

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b02126.

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Additional TEM images and corresponding histograms of self-seeded samples of batch 1 (Figures S1−S7) and batch 2 (Figure S8); additional NMR spectra (Figures S9−S12) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (G.G.). *E-mail [email protected] (M.A.W.). ORCID

Gerald Guerin: 0000-0003-4997-0561 Paul A. Rupar: 0000-0002-9532-116X Ian Manners: 0000-0002-3794-967X Mitchell A. Winnik: 0000-0002-2673-2141 Present Address

P.A.R.: Department of Chemistry, University of Alabama, Tuscaloosa, AL 35487. Author Contributions

G.G. and G.M. contributed equally to this work. Notes

The authors declare no competing financial interest. G


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ACKNOWLEDGMENTS The Toronto authors thank NSERC Canada for their support of this research.

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