Conjugated Polymer Nanoparticles in Aqueous Media by Assembly

Article pubs.acs.org/Macromolecules

Conjugated Polymer Nanoparticles in Aqueous Media by Assembly with Phospholipids via Dense Alkyl Chain Packing Yeol Kyo Choi,† Dabin Lee,§ Sang Yup Lee,‡ Tae Joo Shin,∥ Juhyun Park,*,§ and Dong June Ahn*,†,‡ †

Department of Chemical and Biological Engineering and ‡KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 02841, Republic of Korea § School of Chemical Engineering and Materials Science, Institute of Energy Converting Soft Materials, Chung-Ang University, Seoul 06974, Republic of Korea ∥ UNIST Central Research Facilities & School of Natural Science, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea S Supporting Information *

ABSTRACT: Revealing the nature of chain packing in conjugated polymer nanoparticles (CPNs) is one of the important issues to polymer physics research. Surfactant-stabilized CPNs in water show significantly enhanced luminescence intensity in comparison to small molecular organic dyes and single polymer chains dissolved in solvents. The importance of the conjugated polymer structure in nanomaterials is undoubted. However, details of the relationship between alignment of conjugated polymer backbone in CPNs and its luminescent property have not been established. Furthermore, there are yet no methods that can predict the atom-resolved structure of conjugated polymer in the CPNs. Herein, we employ coarse-grained (CG) molecular dynamic simulations to investigate the structure of phase-separated film and the film shattering process for a mixture of poly[2,6-(4,4-bis(2-ethylhexyl)-4Hcyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) and 1,2-dioctanoyl-sn-glycero-3-phosphocholine (D8PC). The π−π stacked structure of PCPDTBT is significantly enhanced when the ratio of D8PC increases in both dried and water exposed film. We also show that the amount of D8PC is at least 2.5 times larger than that of PCPDTBT to wrap the conjugated polymer chain, and the direct retrieval of atomistic details is achieved through back-mapping from the morphology of CG. Finally, we confirmed that conjugated backbones inside the nanoparticles were completely shielded from the aqueous solution by the dense layers of alkyl chains, resulting in remarkably enhanced chain packing. These simulated results are correlated with experimentally observed structure through UV−vis−near-infrared (UV-vis-NIR) spectrometry, scanning electron microscopy (SEM), particle size analyzer (PSA), transmission electron microscopy (TEM), and grazing-incidence X-ray diffraction (GIXD).

1. INTRODUCTION Conjugated polymers as semiconductors, which have band gaps between their valence and conduction bands, can absorb energy upon light irradiation, exciting electrons from the valence to the conduction band. Consequently, the absorbed energy can be emitted as light, heat, or charge carriers (electrons and holes) and can be transferred to the surroundings to participate in photocatalytic reactions.1,2 When conjugated polymers are in the form of nanomaterials, such as nanodots, nanoparticles, and nanowires, one can further expect significantly enhanced luminescence intensity in comparison to small molecular organic dyes and single molecular polymer chains dissolved in solvents.3 Thus, conjugated polymer nanomaterials have attracted significant attention for applications in aqueous media, such as fluorescence imaging, photoacoustic imaging, photothermal therapy in biomedical fields,3−10 and photocatalytic reactions such as water splitting for hydrogen generation and organic pollutant degradation. Although there is a noticeable © XXXX American Chemical Society

improvement in the luminescence of conjugated polymer nanoparticles (CPNs) in comparison to single molecules, there remains room to improve the luminescence of these conjugated polymer nanomaterials. To improve their luminescence property, the importance of the conjugated polymer structure in nanomaterials is undoubted. Atomic force microscopy (AFM) has been employed to unravel the size and shape of phase-separated domains. However, this result does not provide any information on molecular structure inside CPNs. Furthermore, details of the relationship between the molecular structure of conjugated polymer in CPNs and their luminescent property have not yet been established. In this paper, we focus on the morphologies of CPNs prepared by shattering phase-separated films of a conjugated polymer and a phospholipid. As recently reported,3 in the Received: June 29, 2017

A

DOI: 10.1021/acs.macromol.7b01367 Macromolecules XXXX, XXX, XXX−XXX

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enhanced optical properties. MD is an attractive method to resolve this issue. For example, Saeki et al. examined how the polymer orientation impacts the charge separation and recombination at a bilayer heterojunction composed of PCPDTBT and fullerene using all-atom MD (AMD).14 However, many important CPNs assemblies are too large in length scale for AMD simulation. To overcome this limitation, CG treatment is significantly helpful. In our simulation, PCPDTBT and D8PC are coarse-grained using the conventional MARTINI scheme,15 which has been widely employed to study polymers, proteins, lipids, and DNA.16−19 The MARTINI force field (FF) parameters were fitted on the basis of molecular trajectories from AMD simulations of smaller systems of interest. With the significantly reduced overall system degrees of freedom from coarse-graining, the system length scale that CG-MD can handle is compatible with that of typical CPNs. The calculated assembled structures were compared with experimental data obtained by UV−vis−NIR spectrometer, particle size analyzer (PSA), scanning electron microscope (SEM), transmission electron microscope (TEM), and grazing-incidence X-ray diffraction (GIXD) for CPNs prepared by conventional lipid vesicle preparation, nanoprecipitation, and emulsion processes.

conventional lipid vesicle preparation process, the conjugated polymer and phospholipid form a phase-separated film with nanometer-scale morphological features when they are dissolved in a cosolvent, followed by drying and solvent removal. The phase separation arises because of the distinct differences in the physicochemical properties between the polar lipid heads and the remaining hydrophobic moieties and the packing between the alkyl lipid tails and side chains of the conjugated polymer with comparable chain lengths. The film can then be shattered by the penetration of water molecules into the polar regions of the phase-separated film using ultrasonication, resulting in the dispersion of the CPNs in water. Our objective was to simulate the CPN preparation process, compare the results with experimental data, and gain an understanding of the assembly structures that influence the functions of the CPNs. We used poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3benzothiadiazole)] (PCPDTBT) and 1,2-dioctanoyl-sn-glycero3-phosphocholine (D8PC) (Scheme 1) and mimicked the Scheme 1. Chemical Structures of PCPDTBT and D8PC

2. THEORETICAL CALCULATIONS 2.1. Mapping. In the MARTINI approach, a MARTINI bead represents a group of four heavy atoms. More recently, smaller beads have been introduced to model the structure of cyclic compounds, such as cyclohexane, benzene, and DNA bases, which can be represented by three small interconnected beads. The mapping of the atomistic structure was done with the standard MARTINI FF scheme for rigid ring particles and a modified MARTINI FF scheme for side chains. Standard MARTINI beads use a Lennard-Jones (LJ) parameter, σ, of 0.47 nm, which is too large to model the interdigitation between the side chains of PCPDTBT and the alkyl tails of the D8PC lipid, and small beads also are not suitable for describing the density and diffusibility of alkyl chains because they are too small. Thus, to model the interdigitation of alkyl chains, we created smaller beads than that of standard MARTINI. We selected σ = 0.45 nm for this new bead type. These beads interact with each other using the reduced parameters (σ = 0.45 nm and epsilon reduced to 75%) but interact with standard and

CPN preparation process of the cosolvent drying, phase separation, and film shattering by coarse-grained molecular dynamics (CG-MD) simulations at different molar mixing ratios of PCPDTBT and D8PC. While many studies link the polymer morphology and processing conditions to the optoelectronic properties of the conjugated polymers,11−13 more detailed studies of the atomistic structure of conjugated polymers are necessary to determine the structural origin of the Table 1. Overview of the CG Simulation Setup system AA

CG

dilute solution back-mapped bilayerb solvent evaporationc

exposure to waterc

bilayer systemb

ratio of PCPDTBT/ D8PC

no. of PCPDTBT

no. of D8PC

no. of chloroform

no. of watera

1:2.5

1 12

308

1:1

400

2000

100000

∼1.25

0.2

1:3 1:5 1:1 1:3 1:5 1:1 1:3 1:5

200 100 400 200 100 12 12 12

3000 3500 2000 3000 3500 120 360 600

108000 106000

∼1.25 ∼1.25

0.2 0.2 1 1 1 1 1 1

evaporation rate (molecules/ (nm2 ns))

17000

0.1 0.1

1600

146756 146756 145939 4000 4000 4000

simulation time (μs)

a Number of CG water particles. The corresponding number of real water molecules is 4 times higher. bPCPDTBT consisted of 10-mers of CPDTBT. cPCPDTBT consisted of 5-mers of CPDTBT.

B


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approach was employed by randomly removing 1% of the solvent molecules at predefined time intervals, a similar approach to that used for CG molecular simulations of the nanomorphology evolution of poly(3-hexylthiophene)/phenylC61-butyric acid methyl ester blends during solvent evaporation,31 because the phase-separated film of PCPDTBT/D8PC mixture was fabricated using solvent evaporation with N2 purging in experiment.3 In our approach, chloroform molecules were removed at 1.25 molecules/(nm2 ns), and we assumed that the system was quickly equilibrated itself upon solvent removal, leading to a negligible chloroform/PCPDTBT/D8PC concentration gradient in all directions because the evaporation rate of chloroform was too small compared to the ∼63 molecules/(nm2 ns) used in the experiment.32 To simulate the self-assembly behaviors of PCPDTBT/D8PC in the phaseseparated films while shattering the films, the modeled solventevaporated films were placed in the center of a 20 × 20 × 20 nm3 cubic cell and exposed to water. To avoid interference with the boundaries (for which periodic boundary conditions were used) an additional 5 nm layer of pure water was added. The final dimensions of the simulation box were 30 × 30 × 30 nm3. To investigate the optimized PCPDTBT to D8PC ratio, preassembled PCPDTBT was placed in the center of a periodic box of size 13 × 5 × 12 nm3, and D8PC was placed on both sides of the PCPDTBT layer. The production runs were carried out using two different ensemble schemes. In the case of solvent evaporation processes, the NPzAT ensemble was used, while the NPT ensemble was used for the self-assembly processes and during the shattering of the films with isotropic pressure coupling whereas, for a preassembled system, semiisotropic pressure coupling was used. 2.4. Atomistic Simulation. For the simulation of the PCPDTBT dilute solution, a single chain of PCPDTBT with CPDTBT units was inserted into the simulation box. The unit cell had average dimensions of 13 × 13 × 13 nm3, contained 17 000 CHCl3 molecules, and was equilibrated for 10 ns. The NPT production runs were carried out for 100 ns, and three independent production runs were carried out at 300 K, amounting to a total simulation time of 300 ns. For the simulation of the PCPDTBT/D8PC assembly system, the CG coordinates for the final configuration of the CG simulation were converted to the all-atom coordinates using the scripts “backward.py” and “initram.sh” developed by Wassenaar et al.33 Using this CG bead to atom mapping, 8880, 25 256, and 46 440 atoms were mapped from 180, 2464, and 4120 CG beads for PCPDTBT, D8PC, and water, respectively. The transferable intermolecular potential with 3-points (TIP3P) model was used for water because OPLS FF is parametrized based on TIP3P. Electrostatic interactions were calculated using particle mesh Ewald34 with a cutoff of 1.2 nm.

small beads using the Lorentz−Berthelot scheme. In our PCPDTBT model, the 2,6-(4,4-bis(2-ethylhexyl)-4Hcyclopenta[2,1-b;3,4-b′]-dithiophene) part was described by five beads and two virtual sites for the cyclopentadithiophene ring and four beads for the ethylhexyl side chain. The benzothiadiazole part was also described by three particles and one virtual site. The virtual site is massless and positioned at the center of mass of the three ring particles. Introducing a virtual site increases the numerical stability of the simulation. The triangle of constraint in the backbone ring would be difficult to solve and very sensitive to the forces of the connected beads, and this was alleviated by spreading the forces via the virtual site over the underlying atoms. The D8PC molecule was described by eight beads. The headgroup of D8PC was mapped using four beads as in the conventional MARTINI scheme. However, the parameters were slightly modified to introduce compatibility with the modified LJ parameters of the alkyl chain beads. The alkyl chain of D8PC was mapped using four beads, and the mapping scheme is shown in Figure S1. Detailed parameters, reference values, and force constants for bond lengths, angles, and dihedral potentials are also listed in the Supporting Information (Table S1). 2.2. Simulation Methods. All molecular dynamics runs were performed with Gromacs 4.6.720−22 using the MARTINI FF for CG-MD and the “Optimized Potentials for Liquid Simulations” (OPLS) FF23,24 with additional parameters for AMD because OPLS FF is widely used to model the molecular interaction in conjugated polymers.25 To control the temperature, a v-rescale thermostat26 was used. The pressure was maintained at 1 bar using the Berendsen27 and Parrinello− Rahman28 barostats for the equilibrium and production run, respectively. Neighbor lists were built using the Verlet cutoff scheme with a cutoff radius of 1.2 nm and updated at each step. The linear constraint solver (LINCS) algorithm was used to constrain the bond lengths.29,30 All simulations were conducted using a leapfrog integrator with time steps of 20 and 2 fs for CG-MD and AMD, respectively. All AMD and CG-MD configurations are reported in Table 1. The partitioning free energies of the CPDTBT building block are the main benchmark used to parametrize the nonbonded interactions in MARTINI.15,17 We determined the bead types for CPDTBT by calculating their partitioning free energies from water to hexadecane. We used the results from all-atom simulations to benchmark the CG partitioning behavior. All partitioning free energies were calculated by separately calculating the solvation free energies in water and hexadecane. The free energies of solvation were obtained by simulating the reverse process. We used Bennett’s acceptance ratio method (BAR), as implemented in the GROMACS tool g_bar, to convert these energy differences between the native and foreign λ (the degree of coupling parameter 0 ≤ λ ≤ 1) values into free energy differences. 2.3. Coarse-Grained Simulation. In this work, we focused on the CG simulation of the nanomorphology of the PCPDTBT/D8PC mixtures after solvent evaporation and self-assembly of the film after exposure to water. Therefore, to simulate the solvent evaporation processes, periodic boundary conditions were applied to x, y, and z directions of the simulation cells with an initial cell size of 20 × 20 × 40 nm3. The lateral dimensions (i.e., the x and y directions) of the simulation boxes were fixed, whereas the simulation cell in the z-direction was allowed to fluctuate to maintain constant pressure using the v-rescale scheme. A quasi-equilibrium

3. EXPERIMENTAL SECTION 3.1. Materials. PCPDTBT (MW = 34 kDa, polydispersity index = 2.1, MW on a repeat unit basis = 534.845 g/mol) and D8PC (MW = 509.614 g/mol) were purchased from 1-Material, Inc. (Dorval, QC, Canada) and Avanti Polar Lipids, Inc. (Alabaster, AL), respectively, and used as received. Other chemicals were obtained from SigmaAldrich Co. and used without further purification. 3.2. Nanoparticle Preparation. As previously reported,3 CPNs were prepared as follows. PCPDTBT (1 mg) was dissolved in 10 mL of chloroform. After placing 1 mL of the resulting polymer solution (0.1 mg of PCPDTBT/mL, 18.7 μmol on a repeat unit basis) in a separate vial, 56.9 mg of D8PC (93.5 μmol) was added and dissolved by ultrasonication for 5 min. The molar ratio of the polymer to the C


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Macromolecules lipid was 1:5. Solutions with polymer-to-lipid molar ratios of 1:1 and 1:3 were also prepared by the same procedure. Then, the solvent of the final solution was completely removed by nitrogen flow in an oil bath at 32 °C to avoid the rapid precipitation induced by heat loss during evaporation. The samples were further dried under vacuum at room temperature for 12 h. Finally, the film that formed on the bottom and inner wall of the vial was shattered to nanoparticles by addition of 1 mL of deionized water and low power ultrasonication of the sample at a frequency of 40 kHz for 4 h, producing an aqueous solution in which the CPNs were dispersed. The size of thus-made particles ranges from 200 to 400 nm with molar ratio. The temperature of the sonication bath was controlled to remain below 40 °C to prevent possible damage to the polymers by a temperature increase during ultrasonication. Nanoprecipitation Process.35,36 PCPDTBT (1 mg) was dissolved in 10 mL of chloroform. Then, 50 μL of the resulting polymer solution (0.005 mg, ∼0.0025 wt %) was added dropwise into excess methanol (2 mL) with ultrasonication for 10 min. Emulsion Process.37 PCPDTBT (1 mg) and D8PC (4.76 mg) were dissolved in 10 mL of chloroform in two separate vials. Then, 0.5 mL of the D8PC solution (0.238 mg, 0.468 μmol) was added into 5 mL of DI water under stirring at 5000 rpm and further stirred for 10−15 min for pre-emulsification, followed by adding 0.5 mL of the PCPDTBT solution (0.05 mg, 0.0935 μmol) and stirring for 1 h. The molar mixing ratio of PCPDTBT to D8PC was 1:5. The final solution was treated with ultrasonication for 5 min to reach at a steady-state droplet size, and chloroform was removed by stirring the solution at 62 °C for 2 h until the solution became transparent. 3.3. Characterization. The morphology and size of the conjugated polymer nanoparticles were analyzed by using a fieldemission scanning electron microscope (FE-SEM, SIGMA, Carl Zeiss, USA), a high-resolution transmission electron microscope (HR-TEM, JEM 3010, JEOL Ltd., Japan), and a particle size analyzer (PSA) using dynamic light scattering (DLS,Nanoplus HD, Micromeritics, USA). The optical properties of the nanoparticles were examined with a UV− vis−near-infrared (NIR) spectrometer (V-670, JASCO, USA). Structural analysis using grazing-incidence X-ray diffraction (GIXD) was carried out at a synchrotron facility (6D UNIST-PAL and 9A USAXS beamlines of PLS-II at Pohang Accelerator Laboratory, South Korea). The X-rays coming from the bending magnet (6D) and invacuum undulator (9A) were monochromated (11.16 keV for 6D and 11.6 keV for 9A) using a double-crystal monochromator and focused using a toroidal (6D) and K-B type (9A) mirror system, respectively. A GIXD sample stage was equipped with a multiaxis motorized stage for the fine alignment of samples, and the incidence angle of X-ray was set to 0.13°, which was just above the critical angle of PCPDTBT:D8PC. Two-dimensional GIXD patterns were recorded with 2D CCD detectors (Rayonix MX225 for 6D and Rayonix SX165 for 9A). Diffraction angles were calibrated using a precalibrated sucrose sample (monoclinic, P21, a = 10.8631 Å, b = 8.7044 Å, c = 7.7624 Å, and β = 102.938°).38

simulation are shown in Table 2. The partition free energy of monomer unit of PCPDTBT was −61.7 kJ/mol in the AA Table 2. ⟨h2⟩1/2, Rg, and ΔΔGHW for Coarse-Grained and All-Atom Simulations of 10-Mer PCPDTBT ⟨h2⟩1/2 (nm) Rg (nm) ΔΔGHW (kJ/mol)

AA

CG

10.82 ± 0.51 3.43 ± 0.08 −61.7 ± 0.8

10.62 ± 1.29 3.44 ± 0.21 −65.5 ± 0.2

model, and a similar result was obtained in the CG model. We also confirmed that both models have similar Rg and ⟨h2⟩1/2 values. On the basis of these results, we confirmed that the developed CG model of PCPDTBT was valid, and we used this CG model to analyze the behaviors of mixtures containing different ratios of D8PC. 4.1. Effect of the Mixing Ratio on Structure of PhaseSeparated Films of PCPDTBT and D8PC. We calculated the radial distribution functions (RDFs) between the PCPDTBT dummy (DUM2) beads to investigate the phase separation between PCPDTBT and D8PC phases at different molar mixing ratios (1:1, 1:3, and 1:5). The formation of the phaseseparated film by solvent evaporation is shown in Figure 1a. The DUM2−DUM2 RDFs in the CG simulations show two peaks at approximately 0.5 and 1.2 nm (Figure 2). The first peak at 0.5 nm shows the packing by intermolecular interactions of PCPDTBT, and the second peak at 1.2 nm corresponds to the DUM2−DUM2 intramolecular distance. As the process of solvent evaporation continued, the peak at 0.5 nm increased because of the formation of intermolecular packing and the enhanced alignment of the polymer backbones. This tendency more clearly appeared at the 1:5 mixing ratio than the 1:1 ratio. In this simulation, the 1:1 ratio system has 2.85 times more PCPDTBT molecules than the 1:5 ratio system (400 PCPDTBT molecules in 1:1 ratio system, 140 molecules in 1:5 ratio system), but the RDF results show that the 1:5 ratio system has a much larger peak, about 4 times higher than that of the 1:1 ratio system at 0.5 nm. This result indicates that the portion of D8PC is the key factor affecting the formation of intermolecular packing between the PCPDTBT polymers and maintaining a well-ordered structure. In other words, as the amount of D8PC increases, PCPDTBT becomes more structurally stable by decreasing the portion of the polymer in direct contact with water. 4.2. Structure of the Phase-Separated Films in Water. The ⟨h2⟩1/2, RDF, and Rg analyses were performed to investigate the changes in the shapes of the phase-separated film formed via the solvent evaporation process when the film was exposed to water. The initial and final configurations according to each mixing ratio are shown in Figure 3. Comparing the film formed via solvent evaporation with that exposed to water for 1 μs, the PCPDTBT polymers are wrapped by D8PC in a molar mixing ratio of 1:1 and 1:3, as displayed in Figure 3a−d. When water and D8PC removed from the image to reveal the PCPDTBT alone, the condensation of PCPDTBT as it is exposed to water can be seen, as in the case of the 1:1 system. However, in the case of the 1:3 ratio, PCPDTBT forms well-ordered structures. This indicates that the self-assembly of PCPDTBT is possible when the D8PC content is greater than a certain ratio. In the case of the 1:5 ratio, some of the D8PC encapsulates PCPDTBT, inducing the self-assembly of PCPDTBT, similar to the

4. RESULTS AND DISCUSSION Our CG simulation results of the mixture consisting of PCPDTBT and D8PC were first validated by comparison with all-atom simulation results. The parametrization of our PCPDTBT CG model was based on three procedures. Initially, the nonbonded interactions were chosen based on the partitioning behavior of monomer of PCPDTBT. Then, the bonded interactions of PCPDTBT were selected by fitting based on the result of AMD. The bonded parameters are reported in detail in Table S1 of the Supporting Information. Finally, we confirmed the validity of the CG parameters by comparison with the radius of gyration (Rg) and end-to-end distance (⟨h2⟩1/2) values obtained in chloroform, which is a good solvent for PCPDTBT with a length of 10-mers, and compared with the results obtained by AMD. The partitioning free energies, Rg and ⟨h2⟩1/2 of the atomistic and coarse-grained D


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Figure 1. Snapshot of (a) PCPDTBT/D8PC mixture morphologies during solvent evaporation in the CG simulations. Top view of the final configuration in the solvent evaporated film. (b) 1:1, (c) 1:3, and (d) 1:5. PCPDTBT molecules are shown in gray. The choline, phosphate, and tail group of D8PC molecules are colored blue, red, and green, respectively.

Figure 2. Radial distribution function profiles of PCPDTBT/D8PC/chloroform mixture upon solvent evaporation: molar mixing ratios of (a) 1:1, (b) 1:3, and (c) 1:5 before evaporation (black) and after evaporation (red). (d) Comparison of RDFs at each molar mixing ratio when the solvent had completely evaporated.

continuously maintained in the case of the 1:1 ratio, and half of the reduced distance is recovered in the case of the 1:3 ratio. In the case of the 1:5 ratio, ⟨h2⟩1/2 is fully recovered at a simulation time of 0.4 μs. We calculated the RDFs between the DUM2 beads of PCPDTBT to investigate the formation of the ordered structure of PCPDTBT at different mixing ratios (1:1, 1:3, and 1:5). When the phase-separated films were exposed to water, the formation of intermolecular packing and the enhanced alignment of the polymer backbones were observed in all cases (Figure S3). In particular, in the case of the 1:5 ratio, the intermolecular packing is significantly enhanced, and these

tendency seen in the 1:3 system. Then, the remaining D8PC forms cylindrical micelles by self-assembly, as shown in the red circle in Figure 3f. To quantitatively analyze the self-assembly of PCPDTBT, we carried out ⟨h2⟩1/2 and RDF analyses. When the ratios of PCPDTBT/D8PC are 1:1, 1:3, and 1:5 in the phaseseparated film, the ⟨h2⟩1/2 values are 5.13, 5.3, and 5.35 nm, respectively. This indicates that as the content of D8PC increased, the PCPDTBT molecules become more linear. When exposing these films to water, ⟨h2⟩1/2 sharply decreased in all cases because PCPDTBT minimizes its exposure to water, as shown in Figure 4. In addition, when observing the change of ⟨h2⟩1/2 with time, we found that the shrinking state is E


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Figure 3. Initial (a, c, e) and final (b, d, e) configurations of the CPNs prepared by shattering the phase-separated dry films of PCPDTBT and D8PC. The molar mixing ratios of 1:1 (top), 1:3 (middle), and 1:5 (bottom) of the PCPDTBT/D8PC mixture. PCPDTBT molecules are shown in gray, D8PC molecules in green, and water in cyan. Water and D8PC are removed in the images in the second row to show the PCPDTBT clearly (scale bar is 5 nm).

indicates that the side chain of PCPDTBT and the tail part of D8PC are interdigitated. The structure of PCPDTBT chains in the back-mapped atomistic bilayer excluding side chains for clarity is shown in Figure 5e. The PCPDTBT chains form wellordered structure, and the interchain distance is about 5 Å. To experimentally confirm the calculated results, we prepared CPNs via our film shattering process at three different molar mixing ratios of PCPDTBT to D8PC at 1:1, 1:3, and 1:5. In addition, CPNs were prepared by nanoprecipitation and an emulsion process to investigate the dependency of the CPN properties on the preparation method. The CPNs were first characterized by their absorption spectra in Figure 6. The absorption maxima of the CPNs from the five different preparation conditions were 799, 797, 797, 779, and 782 nm. The absorption maximum of PCPDTBT dissolved in a nonpolar organic solvent, such as chloroform, appeared at 718 nm, and the strongly red-shifted absorption maxima of CPNs measured in this study are attributed to the enhanced intermolecular π−π electron delocalization and subsequent energy level stabilization.39 Such bathochromic shifts were also found in active layers for photovoltaic cells based on conjugated polymers.40−45 Noticeably, the absorption maxima of the CPNs prepared from the phase-separated film shattering method are about 20 nm more red-shifted than those of the CPNs prepared by the nanoprecipitation and emulsion processes. This result indicates that the close intermolecular packing of conjugated backbones and resulting stabilization of energy levels are more prominent in the CPNs prepared by film shattering methods than those from the latter methods, indicating that these properties are dependent on the nanoparticle preparation process. In contrast, the CPNs prepared from the film shattering method at a 1:1 mixing ratio of PCPDTBT to D8PC and by the emulsion process absorb light in the NIR region up to 1200 nm. Because the average size and distribution of the CPNs under these two conditions are large and irregular, as described in the following paragraph, we believe that light

Figure 4. End-to-end distance of phase-separated films of the PCPDTBT/D8PC mixture upon exposure to water: molar mixing ratios of 1:1 (black), 1:3 (red), and (c) 1:5 (blue).

results agree well with the results visualized in snapshots of the simulation and recovery of ⟨h2⟩1/2. To investigate the optimal ratio of PCPDTBT and D8PC, we observed the self-assembly phenomenon of excess D8PC in the presence of a preassembled PCPDTBT layer (Figure 5). We observed that some of the D8PC enveloped the PCPDTBT, while the remaining part forms a micelle. The number of initial D8PC molecules was 360, and excluding D8PC molecules in micelles, the number of D8PC forming the bilayer with PCPDTBT was 308. This indicates that the quantity of D8PC required to form a stable bilayer assembly is 2.5 times greater than the number of PCPDTBT monomers. Consequently, we back-mapped the system consisting of D8PC and PCPDTBT in the bilayer and analyzed the bilayer thickness and density distribution to investigate the structural properties. The thickness of the CG bilayer, measured from the peaks of the phosphate distribution, was 3.4 nm. The density distributions of the PCPDTBT side chain and the D8PC tail overlap. This F


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Figure 5. (a) Initial configuration of the PCPDTBT/D8PC mixture composed of 360 D8PC and 12 PCPDTBT molecules with a chain length of 10mer. (b) Final configuration of the PCPDTBT/D8PC mixture at a simulation time of 4 μs and (c) the back-mapped atomistic bilayer from (b) excluding the D8PC molecules forming micelles. (d) Comparison of the density distributions obtained from atomistic (solid lines) and coarsegrained (dashed) simulations. The densities of water, phosphate, terminal tails in D8PC, and the side chains of PCPDTBT are shown in blue, red, green, and gray, respectively. The bilayer center is located at 0 nm. (e) Snapshot of PCPDTBT chains in the back-mapped atomistic bilayer (left) and enlarged image of PCPDTBT chains (right). Side chains of PCPDTBT are removed for clarity.

and bright areas. In a strong similarity to the CG-MD results at a 1:5 mixing ratio, the dark region is attributed to close-packed assemblies of PCPDTBT chains, whereas the bright area is attributed to areas of assembled D8PC. The morphology inside the particle bears a resemblance to a skeleton network of closely assembled PCPDTBT chains and D8PC molecules. These are surrounded by D8PC-only assemblies because of the excess of D8PC. This skeleton network structure of PCPDTBT chains strongly supports the assembly structure calculated by CG-MD and shown in Figure 3f. In comparison, we did not observe such a distinct skeleton network structure of PCPDTBT chains for CPNs assembled at the 1:1 and 1:3 mixing ratios, as shown in Figures 7a and 7b. Interestingly, the CPNs prepared via a nanoprecipitation process have a skeleton network structure consisting of only PCPDTBT chains, as shown in Figure 7d, probably because of their sudden exposure to a nonpolar environment and strong tendency for aggregation arising from the π−π interactions between the conjugated backbones. No such structures were found in the TEM images of the CPNs prepared by an emulsion process because the CPNs are enveloped by the polar heads of the amphiphilic molecules (D8PC in this study) during the emulsification process. Structural information concerning the CPNs was obtained from the GIXD patterns of the CPNs prepared from the filmshattering process at 1:1, 1:3, and 1:5 molar mixing ratios of PCPDTBT to D8PC, which are presented in Figure 8a−e. These were compared with the AMD results shown in Figure 8f. The 2D GIXD patterns shown in Figure 8a−c clearly show the improvement of the molecular assemblies as the portion of D8PC increase from 1:1 to 1:5. We also prepared CPNs at 1:2 and 1:4 mixing ratios and measured their GIXD patterns (Figure S6). The GIXD results show that higher order peaks appear for CPNs prepared at 1:3, 1:4, and 1:5 mixing ratios,

Figure 6. UV−vis absorption spectra PCPDTBT nanoparticles assembled with D8PC at molar mixing ratios of 1:1, 1:3, and 1:5 prepared via a nanoprecipitation process and an emulsion process.

scattering has an influence on the increase in the absorption intensity in the NIR region. The size and morphology of the CPNs were examined by SEM image (Figure S4) analysis, DLS (Figure S5), and TEM (Figure 7). The average particle sizes of CPNs prepared via our film shattering process at three different molar mixing ratios of PCPDTBT to D8PC at 1:1, 1:3, and 1:5 were 286.5 ± 143.0, 362.2 ± 132.7, and 302.5 ± 62.7, respectively. We found that the size distributions of the CPNs prepared at a 1:1 mixing ratio of PCPDTBT to D8PC was irregular and bimodal, including a portion of large particles with sizes larger than 500 nm (Figure S5), and the CPNs assembled at the 1:5 mixing ratio had the most uniform size distribution. TEM images measured for the five CPNs are shown in Figure 7. Noticeably, the high-resolution TEM image of CPNs prepared at a 1:5 mixing ratio shows a morphology with dark G


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Macromolecules

Figure 7. TEM images of the PCPDTBT nanoparticles assembled with D8PC at molar mixing ratios of (a) 1:1, (b) 1:3, and (c) 1:5 and prepared via (d) a nanoprecipitation process and (e) an emulsion process.

Figure 8. 2D GIXD patterns for PCPDTBT nanoparticles assembled with D8PC at molar mixing ratios of (a) 1:1, (b) 1:3, and (c) 1:5. The corresponding GIXD line cuts along (d) in-plane and (e) out-of-plane directions. (f) Results of calculation.

shaped diffractions along the Debye rings are shown in 1:1 CPNs, indicative of poorly ordered and widely distributed assembly structure. The 1:3 CPNs sample shows intermediate structural characteristics between that of 1:5 and 1:1 CPNs, i.e., moderate molecular ordering and layer distribution. These results strongly support the CG-MD result, indicating that the

while no such higher order peaks exist in the GIXD patterns for CPNs prepared at 1:1 and 1:2 ratios. The spotty diffractions extended to much higher orders are clearly seen in 1:5 CPNs sample, suggesting the strong ordering and sharp boundaries between layers, as well as well-ordered 3D structure in both inplane and out-of-plane direction. On the contrary, the circularH


Macromolecules

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molar mixing ratio should be greater than 1:2.5 to obtain ordering of the PCPDTBT chains with the D8PC molecules. The assembly layer structure of PCPDTBT chains and D8PC, illustrated in Figure 5c, appears in the qz (out-of-plane direction) GIXD profiles (Figure 8e). The layer spacing of 1:1, 1:3, and 1:5 CPNs sample, calculated from the qz profiles, are 33.1 (qz = 0.190 Å−1), 32.0 (qz = 0.196 Å−1), and 34.1 Å (qz = 0.184 Å−1), respectively. The layer spacing of 1:5 CPNs is almost identical to the calculated thickness of the CG bilayer, 3.4 nm (Figures 8f and 5d), and similar to the d-spacing (3.3 nm) in the assembly of PCPDTBT and n-octylbenzoic acid in which PCPDTBT chains are embedded in the bilayer assembly of n-octylbenzoic acid.46 It should be mentioned that the double-split diffractions in qz profiles, shown in Figure 8e, are attributed to the diffractions from the transmitted and reflected beam, where lower-q peaks are attributed to the transmitted beam and higher-q peaks to the reflected beam.47 The ordering tendency of CPNs at different mixing ratios in the in-plane direction is also seen at qxy profiles (Figure 8d). There exists no distinct diffraction peak in the qxy > 1 Å−1 range for 1:1 CPNs sample, except for amorphous halo. On the contrary, distinct diffraction peaks are observed for 1:3 and 1:5 CPNs samples in that qxy range, such as peak at qxy = 1.31 Å−1 (d-spacing = 4.8 Å) for 1:3 CPNs and peaks at qxy = 1.25 and 1.43 Å−1 (dspacing = 5.0 and 4.4 Å) for 1:5 CPNs sample, respectively, suggesting the improved in-plane ordering with increasing the D8PC content. In particular, the peak at qxy = 1.25 Å−1 (dspacing = 5.0 Å) seemingly originates from the diffraction by (20l) planes of an assembly structure with an orthorhombic unit cell, supporting highly ordered lateral packing of conjugated planes (Figure S7).

Article

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01367. Figures S1−S7 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.J.A.). *E-mail: [email protected] (J.P.). ORCID

Yeol Kyo Choi: 0000-0002-4218-7139 Juhyun Park: 0000-0003-1300-5743 Dong June Ahn: 0000-0001-5205-9168 Author Contributions

Y.K.C. and D.L. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Korea Research Foundation (Grants NRF-2017R1A2B3006770, 2016M3D1A1952972, and 2016R1D1A1A02937538) and Korea University Grant, Republic of Korea. Experiments at PLS-II 6D and 9A beamlines were supported in part by UCRF, MSIP, and POSTECH. The authors especially thank Dr. Byeongdu Lee at Argonne National Laboratory, who has developed a peak indexing program in the Matlab environment for analyzing GIXD patterns.

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5. CONCLUSION In this study, we confirmed that the conjugated backbone alignment in the nanoparticles based on assemblies of a conjugated polymer, PCPDTBT, and a choline-based phospholipid, D8PC, can be increased by preparing the nanoparticles by shattering a phase-separated film after solvent drying. We simulated the nanoparticle preparation process by a CG-MD model in which a new force field was applied to the PCPDTBT part. Calculations on the phase separation between the PCPDTBT and D8PC phases, after solvent drying followed by film shattering upon water penetration to the hydrophilic regions of the phase-separated film, showed that the molecular ordering is significantly enhanced upon increasing the D8PC mixing ratio. In addition, the simulation results confirmed that the PCPDTBT monomers and D8PC molecules were selfassembled into the bilayer structure at the latter exceeding 2.5 times larger in population than the former. Higher-order peaks, which originate from the repetition of the bilayer structure, only appeared at a mixing ratio greater than 1:3, and a skeleton network structure of PCPDTBT chains was found inside the nanoparticles assembled at a 1:5 mixing ratio. Such calculated results agreed remarkably well with GIXD and TEM experimental observations on the structural information. These morphological characteristics are responsible for the enhanced optical properties of the CPNs, in which the conjugated backbones become stabilized and aligned by isolation from the aqueous environment. With the combined insights provided from simulation and experiment, the CPNs assembled with phospholipids would be valuable for various applications in the biomedical fields.

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Conjugated Polymer Nanoparticles in Aqueous Media by Assembly

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