Photocleavage of the Corona Chains of Rigid-Rod Block Copolymer

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Photocleavage of the Corona Chains of Rigid-Rod Block Copolymer Micelles Hang Zhou,† Yijie Lu,† Huibin Qiu,‡ Gerald Guerin,*,† Ian Manners,*,‡ and Mitchell A. Winnik*,† †

Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom

‡

S Supporting Information *

ABSTRACT: A polyferrocenyldimethylsilane-block-poly(2-vinylpyridine) sample with a photocleavable o-nitrobenzyl ester (ONB) group at the junction (PFS35-hv-P2VP400) was synthesized by copper-catalyzed coupling of a P2VP-ONBalkyne with a PFS-azide. Rodlike core-crystalline micelles of uniform length and uniform width were prepared in 2propanol, a selective solvent for P2VP. Samples of these micelles were photoirradiated with UV-A light (peak emission 360 nm), which induced cleavage at the core−corona junction. Prolonged irradiation (24 h) led to aggregation and precipitation of the corona-cleaved micelles. One could see by TEM that the width of the micelles in the aggregates was significantly reduced (from 49 to 21 nm) because of the loss of the P2VP block, while the PFS core length (L) remained unchanged. For one micelle sample with Lw = 320 nm (650 polymer molecules per micelle), the time course of the irradiation was monitored by GPC, TEM, and multiangle light scattering. After 1 h irradiation, 60% of the corona chains were cleaved, but only small amounts of aggregates had formed. Most of the rodlike micelles maintained their colloidal stability even after 70% of the corona chains had been cleaved. By GPC, we detected formation of an unexpected PFS dimer that became more prominent as the irradiation continued. Dimer formation could be explained by a photoredox coupling of o-nitrosobenzaldehyde groups at the ends of adjacent PFS chains embedded in the micelle core.

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as precursors for magnetic ceramics37−39 and catalytically active materials.40,41 BCPs with asymmetric structures containing a short, crystallizable core-forming PFS block can form rigid-rod micelles in a selective solvent for the complementary block.42,43 Previous studies from our groups have established that these rodlike micelles can be utilized as building blocks to form hierarchical nano- and mesoscale structures.44−46 In a recent communication,47 we briefly described our preliminary results on the synthesis and self-assembly behavior of PFS−P2VP block copolymer with a photocleavable ONB group at the junction (P2VP = poly(2-vinylpyridine)). The synthesis employed reversible addition−fragmentation-transfer (RAFT) polymerization of 2VP with a chain transfer agent (CTA) containing both ONB and a terminal alkyne, combined with Cu-catalyzed “click” coupling to an azide-terminated PFS. Rigid-rod micelles formed by this block copolymer were irradiated in 2-propanol (2-PrOH) solution, resulting in cleavage of the P2VP corona chains. Deposition of PFS block copolymer unimers on the photochemically generated “naked” PFS cores led to the formation of higher order branched structures. A different reaction sequence, involving corona cross-linking and photocleavage, followed by solvent extraction of the PFS, led to the formation of uniform hollow nanotubes.

INTRODUCTION Stimuli-responsive polymers are sometimes referred to as “smart materials”.1−4 One important subset of these materials is photoresponsive polymers. There has been a recent and growing interest in photoresponsive block copolymers (BCPs). For BCPs that form micelles in selective solvents, the introduction of photoresponsive pendant groups such as azobenzene5−8 and photocleavable groups like pyrenemethanol9 or o-nitrobenzylesters (ONB)10−13 have been used to modify micelle properties. This approach is particularly attractive for micelle-based drug delivery applications,14−16 where a photoisomerization or a photocleavage reaction can lead to light-induced release of a cargo from the micelle. For polymers in the solid state, nanoporous materials prepared from BCPs with photocleavable junctions are of particular interest. Numerous applications have been found for nanoporous materials, including as gas storage materials,17 supports for catalysts,18 cell scaffolds,19 photonic band gap materials,20 filtration−separation membranes,21 and templates for structure replication.22 The most important applications of these photocleavable BCPs have been to generate periodic nanoporous thin films.23−28 We have a long-standing interest in polyferrocenyldimethylsilane (PFS) BCPs. They represent a class of main chain transition-metal-containing crystalline-coil BCPs.29 The iron atoms in the main chain of PFS introduce intrinsic functionality as a result of their redox activity30−36 and their ability to serve © 2015 American Chemical Society

Received: February 3, 2015 Revised: March 20, 2015 Published: April 6, 2015 2254

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mL, c = 0.500 mg/mL) in a 7 mL vial. The vial was placed in an oil bath at 80 °C for 30 min, followed by cooling slowly by allowing the oil bath to cool to room temperature. One day later, the vial was placed into a 70 W ultrasonic cleaning bath and sonicated for 30 min at 23 °C. An aliquot of the solution after sonication was taken for TEM analysis. Seed solutions were then diluted with 2-propanol to 10 μg/mL. Four portions of 2 mL 10 μg/mL seed solutions were transferred into four new vials for seeded growth experiments. Aliquots (4, 8, 20, and 40 μL) of polymer 3 in THF (5 mg/mL), referred to as “unimer” solutions, were added into each seed solution and swirled for 10 s. All the solutions were allowed to age in the dark for 1 day before TEM measurements. One can calculate a “theoretical” micelle length Ltheoretical based on the assumptions that (i) all added polymer deposits onto the ends of seed micelles present in the solution without forming new micelles and (ii) the number of polymer molecules per unit micelle length, the linear aggregation number Nagg,L, does not change during the growth.

Herein we report an in depth investigation of the photochemical cleavage reaction, examining rigid-rod micelles formed by crystallization-driven self-assembly in 2-propanol of PFS35-hv-P2VP400, where the subscripts refer to the numberaverage degrees of polymerization. We use the notation “-hv-” employed by Gohy and Fustin48 to emphasize the photocleavable nature of the coupling unit between the two blocks. We focus on the irradiation step. We examined its effect on the colloidal stability of the micelles by multiangle light scattering (static and dynamic) and transmission electron microscopy (TEM). In parallel, we studied its consequences on polymer composition by gel permeation chromatography (GPC). As expected, prolonged irradiation led to colloidal instability and flocculation. The micelles, however, largely retained their colloidal stability even when 70% of the corona chains had been cleaved. We also discovered an unexpected PFS dimerization reaction that occurred only for irradiation of PFS BCPs incorporated into micelles and presumably took place at the surface of the micelles. We believe that this photoinduced aggregation could be used for the construction of more complicated hierarchical superstructures.

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⎛m ⎞ Ltheoretical = ⎜ unimer + 1⎟Lseed ⎝ mseed ⎠

Here Lseed is the number-average length of the micelle seeds, munimer is the mass of polymer in THF that was added in the seeded growth, and mseed is the mass of polymer in the seeds to which the new polymer was added. The characteristics of these micelle samples, denoted A1A4, are summarized in Table 1.

EXPERIMENTAL SECTION

The synthesis and characterization of PFS35-hv-P2VP400 (3) are described in the Supporting Information. Instrumentation. UV irradiations were carried out with a model LZC-1 Luzchem photoreactor (8 UV-A lamps, Hitachi FL8BL-B, emission 320−400 nm, peak emission at 360 nm, power density = 2.5 mW/cm2). Analysis of PFS35-hv-P2VP400 contained in the micelles after their irradiation for different time periods was performed by GPC, using a Waters model 515 pump, a Waters Styragel HR 4E column equipped with a Viscotek VE 3580 RI detector, and a 2500 UV−vis detector. Poly(methyl methacrylate) standards were used for calibration, while the eluent was a THF solution containing 0.25 g/L tetra-nbutylammonium bromide (TBAB) (flow rate = 0.6 mL/min). TEM measurements were performed on a Hitachi D-7000 c-TEM (conventional-TEM) microscope operating at an accelerating voltage of 100 kV. All the samples for TEM were prepared in the same way. One drop of a sample solution was placed on a Formvar film coated 200-mesh copper grid. The excess solution was removed by touching the edge of the droplet with a filter paper. Images were analyzed using ImageJ, an image processing program developed at the National Institutes of Health. A minimum of 100, and more typically 300, individual cylinders were carefully traced by hand to determine the contour length. From this data the number-average length Ln and the weight-average length Lw of each sample of micelles was calculated with the expressions (L = length of object, N = number) n

Ln =

∑i = 1 NL i i n

∑i = 1 Ni

Table 1. Summary of the Characteristics Determined by TEM of the Micelles Prepared by Seeded Growth of PFS35hv-P2VP400

2 ∑i = 1 NL i i n

∑i = 1 NL i i

(1)

For a Gaussian distribution of lengths, the standard deviations (σ) of the measured lengths are related to length dispersity (Lw/Ln) through the following expression: ⎛ σ ⎞2 Lw −1=⎜ ⎟ Ln ⎝ Ln ⎠

polymer

sample ID

Ln (nm)

Lw (nm)

Lw/Ln

σ/Ln

PFS35-hv-P2VP400

seed A1 A2 A3 A4

85.9 160 235 476 797

101 169 247 484 823

1.17 1.06 1.05 1.02 1.03

0.42 0.24 0.23 0.13 0.18

A separate batch of micelles was prepared by this protocol for light scattering characterization and for a time evolution study of the micelles under photoirradiation. For this sample, 20 μL of polymer 3 in THF (10 mg/mL) was added into 2 mL seed solution (28 μg/mL), swirled for 10 s, and allowed to age. By TEM, these micelles are characterized by Ln = 313 nm, Lw = 320 nm, and σ/Ln = 0.15. Photoirradiation Experiments. UV Irradiation of PFS35-hvP2VP400 (3) in THF. To test the photocleavage behavior of the onitrobenzyl (ONB) ester moiety, PFS35-hv-P2VP400 was dissolved in THF at a concentration of 1 mg/mL. The solution was subjected to UV irradiation for 10 h followed by direct GPC analysis. UV Irradiation of Micelle Samples A1−A4 in 2-Propanol. Samples of micelles A1−A4 (Table 1) in 2-PrOH were subjected to UV irradiation for 24 h. A drop of each solution was placed on a copper grid to be examined by TEM. Time Evolution Study of UV Irradiation on a Well-Defined PFS35hv-P2VP400 Micelle Sample. A fresh batch of PFS35-hv-P2VP400 (3) micelle sample (c = 0.071 mg/mL in 2-PrOH) was prepared as described above. Aliquots of this solution were placed into six 2-dram vials, each containing about 2 mL of the micelle solution. The vials were placed in the Luzchem photoreactor under UV irradiation for 0, 1, 3, 10, 15, and 24 h. A drop of each solution was placed on a grid for TEM analysis. Then, the solvent for each sample was carefully removed by rotary evaporation followed by freeze-drying. To each vial 200 μL of THF was added to dissolve the polymers for GPC analysis. Micelle Characterization by Light Scattering. The micelle solutions after various irradiation times were diluted with 2-PrOH to a concentrations in the range 5.5−5.8 μg/mL for light scattering measurements. All measurements were carried out at 23.00 ± 0.05 °C for 90 s. Three angular ranges were investigated. The first range

n

Lw =

(3)

(2)

Corresponding equations can be written for the micelle width (d) to calculate the number-average (dn) and weight-average (dw) values. Laser light scattering measurements were carried out with a commercial light scattering spectrometer (ALV/SP-125) equipped with an ALV-5000 multitau digital time correlator and a He−Ne laser (output power = 35 mW at λ0 = 632.8 nm). Micelle Formation. A micelle seed solution of PFS35-hv-P2VP400 was prepared by adding block copolymer (2.20 mg) to 2-PrOH (4.40 2255

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Macromolecules Scheme 1. (A) Synthesis of Photoresponsive PFS-hv-P2VP; (B) Mechanism of Photocleavage of PFS-hv-P2VP and Dimerization Reaction of PFS

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consisted of scattering angles between 20° and 60° (at 2° intervals), second between 63° and 90° (at 3° intervals), and the last range consisted of angles between 95° and 145° (at 5° intervals). Static (SLS) and dynamic (DLS) light scattering measurements were carried out simultaneously. Filtered toluene was used as the standard solvent in the SLS experiments. For samples irradiated for 15 and 24 h, no meaningful light scattering data could be obtained. In SLS, we obtained structural and dimensional information by measuring the excess scattered intensity (known as the Rayleigh ratio, Rθ). Rθ is related to concentration c, the second viral coefficient A2, and the form factor P(q) as follows:

Kc 1 = + 2A 2 c Rθ M w P(q)

RESULTS AND DISCUSSION Polymer Synthesis. The azido-end-capped poly(ferrocenyldimethylsilane) was synthesized by anionic ringopening polymerization following a protocol described previously, 49 with details provided in the Supporting Information. End-group analysis by 1H NMR (see Figures S1 and S2, Supporting Information) gave a number-average degree of polymerization (DPn) of 35. This value is in reasonable agreement with the ([monomer]/[initiator] = 30) feed ratio. Alkyne-terminated P2VP (2) was synthesized by RAFT polymerization as shown in Scheme 1A. We take advantage of a commercially available chain transfer agent (CTA) with an alkyne and a dithiobenzoate ester linked by a photocleavable onitrobenzyl (ONB) ester group. When this reagent was first reported by Coughlin and Theato,26 they showed that the alkyne group can survive the RAFT polymerization process. Here, bulk RAFT polymerization of 2-vinylpyridine was carried out with a ratio of [monomer]:[CTA]:[AIBN] = 1200:1:0.2 at 80 °C. The reaction was quenched after 20 h, and monomer conversion was monitored by 1H NMR. The NMR spectrum of the isolated photocleavable P2VP sample is shown in Figure S3. By comparison of the integration of the methylene proton signal for CH2−CCH (peak B) and the CH2− of the onitrobenzyl group (peak C) with the pyridyl group signals at 6.15−6.50, we calculate DPn = 400 for the sample, with Đ ≈ 1.20. The block copolymer PFS35-hv-P2VP400 (3) was obtained by copper-catalyzed alkyne−azide coupling at 40 °C in the presence of CuBr/pentamethyldiethylenetriamine (PMDETA) as a catalyst using a small excess of the P2VP−CCH. After the “click” reaction with PFS35−N3, the alkyne peak at 2.64 ppm (Figure S3) disappeared in the 1H NMR (Figure S4). The GPC traces (see Figure S5), monitored by RI and UV−vis at

(4)

where K = 4π n (dn/dc) /(NAλ0 ), q = (4πn/λ0)/sin(θ/2), and NA, n, λ0, and θ are the Avogadro number, the solvent refractive index, the wavelength of the light in vacuum, and the scattering angle, respectively. In the Guinier regime of a very dilute solution qRg < 1; Rg is the root-mean-square z-averaged radius of gyration of the scatterers, and the 2A2c term can be neglected. The form factor P(q) is a linear function of q2: P(q) = 1 − (q2Rg2/3). Thus, for small values of q, eq 4 can be written as 2 2

4

2

Kc 1 (1 + q2R g 2/3) = Rθ Mw

(5)

Plots of Kc/Rθ vs q lead to Rg and to Mw by extrapolation to q → 0. For block copolymers, the specific refractive index increment (dn/ dc)polymer is related to the dn/dc values of the components via the weight fractions wj of the components of type j in the polymer. 2

(dn/dc)polymer =

2

∑ wj(dn/dc)j

(6)

provided that the dn/dc contributions of the multicomponent polymer in a single solvent are additive. Using values of (dn/dc)PFS = 0.240 mL/g and (dn/dc)P2VP = 0.220 mL/g in 2-propanol, we calculated (dn/dc)polymer = 0.223 mL/g. For analysis of the DLS data, refer to the Supporting Information for more details. 2256

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micrometers, but very uniform in width, characterized by dn = 49 nm and dw/dn = 1.01. In the second step, the micelles were subjected to mild sonication, yielding seed crystallite fragments (Figure S7B) characterized by Ln = 86 nm and Lw/Ln = 1.17. The third stage involved seeded growth. For these experiments, the seed crystallite solutions were first diluted to c = 0.010 mg/mL with 2-PrOH. Then different amounts of unimer solutions in THF (5 mg/mL) were added into 1 mL of the corresponding diluted seeds solution. The solutions were allowed to age in the dark for 1 day. Representative TEM images of four micelle samples are shown in Figure S7E−H. One can see that, after the addition of more polymer material, longer micelles were formed, characterized by a narrow length distribution, as indicated by their corresponding length distribution histograms shown underneath each image (Figure S7I−L). In Figure S8 we plot the mean length Ln of the micelles of PFS35-hv-P2VP400 obtained by seeded growth versus the ratio of the amount of polymer added to the amount of seeds. The dashed line is the theoretical predictions of length assuming that all added polymer adds to the seed crystallites and that the mass per unit length of the micelles does not change. In Figure S8, one sees that the micelles obtained match well with the theoretical prediction. Photoirradiation of Micelle Solutions. In initial studies of UV-irradiation of micelle solutions, we subjected samples of micelles A1−A4 (Table 1) to UV-A irradiation for 24 h. These samples formed aggregates that precipitated from solution. The most important conclusion to be drawn from the TEM images of these aggregates (Figure S9A−D) is that the photoreaction led to a significant decrease in the width of the micelles from dn = 49 nm (Figure S7D) in TEM images of the precursor micelles to dn = 21 nm (dw/dn = 1.01) (Figure S9E) for micelles in the aggregates. We infer that the photoreaction of the ONB group cleaved P2VP blocks from the micelle, destroying their colloidal stability. To obtain a deeper understanding of the photocleavage reaction of corona chains from PFS35-hv-P2VP400 micelles, we undertook a more quantitative study, using SLS and DLS, in parallel with TEM measurements to monitor changes as a function of irradiation time. For these experiments, we prepared a new batch of PFS35-hv-P2VP400 micelles by seeded growth (Lw = 320 nm, Lw/Ln = 1.02, determined by counting at least 300 micelles from multiple TEM images taken after 3 days of aging). A representative TEM image at low magnification is shown in Figure 2A. Figure 2B shows a CONTIN plot from DLS measurements taken at a scattering angle of 36°, showing a monomodal distribution centered at an apparent hydrodynamic radius Rh,app = 94 nm. Multiangle SLS results are presented in Figure 2C as a Holtzer−Casassa (HC) plot of qRθ/πM0Kc as a function of q, in which

450 nm, showed obvious shifts of retention time after the coupling reaction, leaving almost no PFS signal. There is a small high molecular weight shoulder in the GPC traces of both coupling products that increased the dispersity of the diblock copolymer product to Đ = 1.3. There is a report in the literature26 that amines like PMDETA may trigger the aminolysis of the dithiobenzoate end group of polymers, resulting in some disulfide dimerization. The polymer characteristics are summarized in Table S1. The UV spectra presented below in Figure 1A show that the block copolymer

Figure 1. (A) UV−vis spectra of PFS35-hv-P2VP400 (black line), PFS35 (red line), P4VP400 (green line), and the ONB containing RAFT chain transfer agent (blue line). The ONB concentrations were the same in each spectrum. The inset shows the spectra over a larger range of wavelengths. (B) GPC curves obtained from UV−vis detector for the PFS35-hv-P2VP400 unimer sample in THF after 10 h of UV irradiation.

itself does not have a clearly defined peak for the ONB group. Rather, this group contributes to the absorbance in the tail of the spectrum of PFS35-hv-P2VP400 between 320 and 380 nm. To test the photocleavage behavior of the ONB ester moiety, a sample of PFS35-hv-P2VP400 dissolved in THF (1 mg/mL) was irradiated for 10 h in the photoreactor box with eight overhead UV-A lamps (320−400 nm, with a peak output at 360 nm). GPC analysis of the irradiated solution monitored at 450 nm (see Figure 1B) showed complete disappearance of the block copolymer peak and a new symmetrical peak at a retention time consistent with regeneration of the PFS homopolymer. The PFS homopolymer peak was also observed in the RI signal (Figure S6A), along with the peak of the P2VP precursor that showed a slightly higher retention time than that of the BCP. Micelle Formation. Uniform rodlike BCP micelles of PFS35-hv-P2VP400 were prepared in three steps.4 First, a sample of the polymer was suspended in 2-PrOH, heated to 80 °C for 30 min, and allowed to cool to room temperature (23 °C, RT). This process yielded fiberlike micelles as shown in the TEM image in Figure S7A, with lengths on the scale of several

f (q) =

qR θ P(q)q = LNagg/ L πM 0Kc π

(7)

and for rigid rods of length L and cross-section radius RSLS P(q , L , R SLS) =

∫0

π /2

⎛⎡ ⎛ qL cos α ⎜⎢ ⎜ sin 2 ⎜⎢2⎜ qL cos α ⎜ ⎜⎢ 2 ⎝⎣ ⎝

(

⎞2

) ⎞⎟⎤⎥ J1(qR SLS sin α) ⎟ sin α dα ⎟⎟⎥ qR sin α ⎟ ⎟ SLS ⎠⎥⎦ ⎠

(8) 2257

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Figure 2. Characterization of PFS35-hv-P2VP400 micelles prior to irradiation. (A) TEM image of the micelles characterized as Ln = 313 nm, Lw = 320 nm, Lw/Ln = 1.02, and σ/Ln = 0.15 (scale bar is 500 nm). (B) CONTIN plot of the DLS data (c = 5.53 μg/mL) recorded at an angle of 36° as a function of the apparent hydrodynamic radius. (C) Plot of qRθ/πM0Kc as a function of q for the micelles. The red line is the best fit of the data to eq 7 for thick rigid rods. (D) Plot of Γ1/q2 as a function of qL for the micelles. The L value corresponds to Lw from SLS data. The red line corresponds to the best fit obtained from eq S2.

The best fit of data gives three important parameters characterizing the micelles: (i) the weight-average length of the micelle Lw, (ii) the number of block copolymers per unit length of the micelles, a value we refer to as the linear aggregation number, Nagg/L, and (iii) the radius of the cylinder cross section, RSLS. Each parameter affects the fit in a different way, which simplifies the fitting procedure. The values of these parameters are Lw = 320 nm, Nagg/L = 2.0 chains/nm, and RSLS = 21 nm. Note the agreement with the values of Lw (320 nm) determined by TEM and by SLS as analyzed by a HC plot. We also calculated a value of ⟨Rg2⟩1/2 = 97 nm for the overall radius of gyration of the micelles. Further information about the micelles is available from the angular dependence of the DLS signal. In the SLS experiment one determines the average intensity of the scattered signal, whereas in DLS, one analyzes the fluctuations in the scattered intensity of the same signal due to the motion of the scatterers in solution. This signal is strongly related to the shape and dimensions of the scattering objects (see Supporting Information for details). To proceed, we examined ratio of the autocorrelation decay rate to the square of the scattering vector (Γ1/q2). We plot this ratio as a function of qL in Figure 2D and fit the data to the expression (eq S2) derived by Wilcoxon and Schurr51 for a dilute suspension of rigid rods. In the fitting we used L = 320 nm as the micelle length. One can see that this equation provides a good fit to the data, and in this way we calculated a value of RDLS = 36 nm for the hydrodynamic radius of the micelle cross section. It is not surprising that RDLS is significantly larger than RSLS.52 This is a consequence of the density distribution of the corona chains in the radial direction. Corona chains that extend to the fringe of the micelles affect the diffusive properties of the micelle, whereas segments of these solvent-swollen chains far from the

core are too dilute to contribute significantly to the scattering intensity monitored by SLS. Aliquots of this micelle solution were then subjected to irradiation with UV-A light for various periods of time (1, 3, 10, 15, and 24 h) and characterized as described above. We began by dissolving samples taken at each irradiation period in THF to examine their GPC traces. Curves from the RI detector are shown in Figure S6B, while those from the UV−vis detector at 450 nm are shown in Figure 3A. In the RI plot, with increasing irradiation time, the peak of the PFS35-hv-P2VP400 block copolymer decreased in intensity and shifted toward higher retention volume, while the intensity of the PFS35 homopolymer peak increased gradually. This is in accord with a photolysis mechanism in which the diblock copolymer is cleaved at the block junction. The UV−vis trace, however, shows that a third species was formed during the UV irradiation. Considering the retention volume of the new peak and that only polymers containing PFS are detectable at 450 nm, we believe that this peak is due to a PFS dimer formed during the UV irradiation. This type of dimerization has been found previously for the photoproducts of ONB irradiation: photocleavage leads to an o-nitrosobenzaldehyde, which can undergo a photoredox reaction to form an azobenzene bis(carboxylate).53,54 In Scheme 1B, we show the products of the photocleavage reaction and also how this reaction would lead to PFS dimer formation in our system. Because the test reaction of the polymer in dilute THF solution showed no evidence for PFS dimer formation, we conclude that the dimerization reaction occurred at the surface of the core of the micelles, most likely between adjacent o-nitrosobenzaldehydeterminated PFS chains. We attempt a more quantitative analysis of the GPC curves, focusing on the UV−vis (450 nm) traces, to which only the PFS-containing moieties contribute. We fitted these traces with 2258

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Figure 3. (A) GPC curves obtained from UV−vis detector for the PFS35-hv-P2VP400 micelle sample in 2-propanol after various UV irradiation times. The red box highlights new peaks formed, which we interpret as indicating the formation of PFS dimers in the solutions. (B). Time evolution of the amount of PFS-hv-P2VP remaining in the sample and of the amount of PFS homopolymer + dimer formed over time during the photolysis of PFS35-hv-P2VP400 micelles at 360 nm as evaluated from the GPC traces (see Supporting Information for details). The black line refers to PFS-hv-P2VP; the blue line refers to PFS dimer plus PFS homopolymer.

Figure 4. Characteristics of PFS35-hv-P2VP400 micelles after 1 h of irradiation at 360 nm. (A, B) TEM images of individual micelles and aggregates. (scale bar is 500 nm). (C) CONTIN plot of the solution recorded at a scattering angle of 36° (concentration = 5.53 μg/mL).

a sum of three peaks corresponding to the remaining PFS35-hvP2VP400 block copolymer, the PFS homopolymer, and the smaller central peak which we have assigned to the PFS dimer. The fits are shown in Figure S10, and details are provided in the caption to this figure. We summarize the results in Figure 3B. According to this plot, 1 h irradiation leads to loss of 60% of the original PFS-hv-P2VP block copolymer, accompanied by a large increase in the amount of PFS homopolymer and dimer in solution. As we will see below, most of the PFS at early stages of the irradiation remains associated with the micelles. TEM and light scattering results for the sample irradiated for 1 h are presented in Figure 4. The TEM images in Figures 4A, B show aggregates in the presence of a sea of isolated micelles. The DLS CONTIN plot (Figure 4C) reveals a bimodal distribution, with peaks centered at Rh,app = 62 and 376 nm, consistent with observation of aggregates in the TEM images. Since scattering intensity increases with the sixth power of the radius of the objects, one can conclude from the CONTIN plot that the fraction of aggregates in the sample is small. We also carried out multiangle SLS measurements on this sample, but interpretation of the data is challenging. One problem involves accounting for the loss of 60% of the P2VP blocks from the micelles. We first calculate the expected molecular weight of the micelles Mw,th,1h after loss of this fraction of P2VP blocks, assuming that the number of PFS chains per micelle did not change. The cleaved P2VP chains in solution, because of their small size, do not contribute significantly to the scattering intensity, but their loss leads to a change in the dn/dc value characterizing the micelles. Fortunately, the differences in dn/dc values are not large

[calculated values for PFS, P2VP, and PFS−P2VP in 2propanol, using the Dale−Gladstone relationship,55 are (dn/ dc)PFS = 0.240 mL/g, (dn/dc)P2VP = 0.220 mL/g, and (dn/ dc)PFS−P2VP = 0.223 mL/g for PFS35-hv-P2VP400]. Using the information obtained from analysis of the GPC curves for the sample after 1 h irradiation (see Figure 3B and Figure S10) that 60% of the P2VP chains had been cleaved and lost to the solution, we obtained a revised value of dn/dc = 0.227 for the micelles and their aggregates. In Figure 5, we compare Zimm plots (Kc/Rθ vs q2, eq 4) of the data obtained for the initial micelle sample (Mw,micelle = 3.3 × 107 g/mol; ⟨Rg2⟩1/2 = 97 nm) with that of the data obtained after 1 h irradiation. For the irradiated micelles, the value of c was adjusted to account for the loss of 60% of the P2VP corona forming blocks. We also plotted the theoretical Zimm plot (as the straight line in Figure 5) for micelles that have lost 60% of their P2VP corona blocks, Mw,th,1h = 1.6 × 107 g/mol. In this calculation we assumed that the radius of gyration of the micelles was not affected by loss of the P2VP corona blocks. This value is primarily determined by the length of the rigid rod micelles, which remained unchanged. For the sample irradiated 1 h, we fitted the data at low scattering angle (q2 ≤ 200 μm−2) and calculated apparent values of Mw,1h = 3.9 × 107 g/mol and ⟨Rg,1h2⟩1/2 = 147 nm. The much larger apparent Mw,1h and the increase in the apparent Rg for the sample irradiated for 1 h are consistent with the influence of aggregates in the solution that contribute to the light scattering signal. 2259

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which are more prominent than in the sample irradiated for only 1 h. According to the DLS CONTIN plot, some aggregates are present in solution. Their size is so large (Rh,app larger than 1 μm) that some of them may settle out of the light beam, decreasing the intensity of the corresponding peak. After 10 h of UV irradiation, some isolated micelles could be seen in TEM images (Figure 6D), although other regions of the grid showed large aggregates. At 15 and 24 h irradiation, no meaningful light scattering signal could be obtained. As shown in Figure 6E,F, only aggregates could be seen in the TEM, with no well-resolved individual micelles.

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CONCLUSIONS By combining RAFT polymerization and copper-catalyzed azide−alkyne coupling, we synthesized a photocleavable PFS block copolymer denoted PFS35-hv-P2VP400 with an o-nitrobenzyl ester group at the junction. Irradiation of this group cleaves the block copolymer to its homopolymer components. Using a seeded growth protocol, rodlike micelles of uniform length and width were prepared. The PFS core of these micelles is semicrystalline. Irradiation of the micelles at 360 nm led to loss of the P2VP corona blocks accompanied by aggregation of the micelles and their eventual precipitation from solution. We also found a side product from irradiation of the micelle samples, a PFS dimer, presumably formed by redox reaction of neighboring o-nitrosobenzaldehyde groups that resulted from the photoreaction. One micelle sample was examined in much greater detail. These micelles had a narrow length distribution (Lw/Ln = 1.02) and a weight-average length of 320 nm as determined both by TEM and multiangle light scattering. From light scattering, we also determined a radius of gyration ⟨Rg2⟩1/2 = 97 nm and a molecular weight Mw = 3.3 × 107 g/mol, corresponding to an aggregation number of 650 block copolymer molecules per

Figure 5. Zimm plots of a micelle sample (c = 5.53 μg/mL) before (open squares) and after 1 h of UV-A irradiation (blue triangles). The intercept of the data for the sample before irradiation corresponds to Mw,micelle = 3.3 × 107 g/mol (650 polymer molecules per micelle). To plot the data for the irradiated sample, the mass concentration of polymers in the micelles was adjusted to 2.73 μg/mL to account for loss of 60% of the P2VP corona chains, and the refractive index increment was corrected to dn/dc = 0.227. The red line represents a theoretical calculation of the Zimm plot of isolated micelles corresponding the initial sample (open squares) after they lost 60% of their P2VP corona chains (Mw,th,1h = 1.6 × 107 g/mol, c = 2.73 μg/ mL, dn/dc = 0.227). In calculating the data for the theoretical plot, we also assumed that the radius of gyration of the micelles was determined by their length and remained unchanged after cleavage of the P2VP blocks from the original sample represented by the open squares.

Corresponding TEM images and a DLS CONTIN plot for the micelle sample irradiated for 3 h are presented in Figure 6. According to the GPC data summarized in Figure 3B, more than 70% of the PFS-hv-P2VP block copolymers lost their P2VP blocks. The TEM images in Figure 6A,B show a significant number of isolated micelles in addition to aggregates,

Figure 6. (A, B) TEM images of PFS35-hv-P2VP400 individual micelles and aggregates after 3 h of irradiation at 360 nm (scale bar is 500 nm). (C) CONTIN plot of the solution recorded at a scattering angle of 36° (concentration = 5.64 μg/mL). (D−F) TEM images of PFS35-hv-P2VP400 aggregates after 10, 15, and 24 h of UV irradiation, respectively (scale bar is 500 nm). 2260

DOI: 10.1021/acs.macromol.5b00238 Macromolecules 2015, 48, 2254−2262

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Macromolecules

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micelle. A Holtzer−Casassa analysis of the SLS data showed a linear aggregation number of 2.0 polymer molecules per nm and a cross section radius of gyration RSLS = 21 nm. After 1 h irradiation at 360 nm, ca. 60% of the P2VP block were cleaved from the block copolymer. Nevertheless, most of the micelles remained colloidally stable in solution. Some aggregates could be seen on TEM grids of the sample, and a second peak at larger radius was seen in a CONTIN plot of the DLS data. Even after 3 h irradiation, corresponding to loss of 70% of the P2VP blocks, TEM images showed the presence of many isolated micelles unchanged in length from the original sample. Aggregates were more prominent, but the most striking conclusion is that colloidally stable micelles were present when most of the corona chains were cleaved from the sample.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental details and characterization data (NMR, GPC, DLS), preliminary photoirradiation experiments, quantitative analysis of GPC traces from irradiated micelle solutions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Toronto authors thank NSERC Canada for their financial support. H.Q. is grateful to the EU for a Marie Curie Postdoctoral Fellowship. I.M. acknowledges the EU for an Advanced Investigator Grant.

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