Development of Shape-Tuned, Monodisperse Block Copolymer

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Development of Shape-Tuned, Monodisperse Block Copolymer Particles through Solvent-Mediated Particle Restructuring Jae Man Shin, Young Jun Lee, Mingoo Kim, Kang Hee Ku, Junhyuk Lee, YongJoo Kim, Hongseok Yun, Kin Liao, Craig J. Hawker, and Bumjoon J. Kim Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04777 • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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Chemistry of Materials

Development of Shape-Tuned, Monodisperse Block Copolymer Particles through Solvent-Mediated Particle Restructuring Jae Man Shin1, Young Jun Lee1, Mingoo Kim1, Kang Hee Ku1, Junhyuk Lee1, YongJoo Kim2, Hongseok Yun1, Kin Liao3, Craig J. Hawker*,4 and Bumjoon J. Kim*,1,2

1

Department of Chemical and Biomolecular Engineering, 2 KAIST Institute for NanoCentury,

Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea 3

Department of Mechanical Engineering, Khalifa University, Abu Dhabi, United Arab

Emirates 4

Materials Research Laboratory, University of California, Santa Barbara, California 93106,

United States

*E-mail: [email protected] (B. J. K.) *E-mail: [email protected] (C. J. H)

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ACS Paragon Plus Environment

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Abstract Control of the shape, size, internal structure, and uniformity of block copolymer (BCP) particles is crucial in determining their utility and functionality in practical applications. Here, we demonstrate a particle restructuring by solvent engineering (PRSE) strategy that combines membrane emulsification and solvent annealing processes to produce monodisperse BCP particles with controlled size, shape, and internal structure. A major advantage of the PRSE approach is the general applicability to different families of functional BCPs, including polystyrene-block-poly(1,4-butadiene) (PS-b-PB), polystyrene-block-polydimethylsiloxane (PS-b-PDMS), and polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP). PRSE starts with the production of monodisperse BCP spheres in a wide range of particle sizes (from hundreds of nanometers to several tens of microns) using membrane emulsification, followed by successful transformation to shape-anisotropic BCP particles by solvent annealing under neutral wetting conditions. Particle size monodispersity was maintained during the PRSE process with shape transformations from sphere to ellipsoids (i.e., oblate and prolate). The approach was effective in controlling the aspect ratio (AR) of both prolate and oblate ellipsoids over wide ranges. These ARs were well-supported by free energy calculations based on a theoretical model describing the particle elongation. Further investigation of the shape transformation kinetics during the PRSE process revealed that the morphology transformation was driven by reorientation of BCP microdomains with kinetics being strongly associated with the overall molecular weight of the BCP as well as the annealing time.

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Introduction

Colloidal polymeric particles with well-defined shape and internal morphology are of great interest due to their structure-dependent properties.1–6 Recently, self-assembly of block copolymers (BCPs) in evaporative emulsions has received significant attention as a simple and effective strategy to generate colloidal particles with precisely-controlled structures.7–9 For example, shape-anisotropic particles such as striped ellipsoids or convex lens-shaped particles could be obtained depending on the self-assembled BCP structures.10–15 While successfully demonstrating the control over particle structure, it has remained a significant challenge to implement these particles into a variety of potential applications, including optical lenses, sensors, catalysts, and dielectric resonators.16–20 In particular, two important features of BCP particle synthesis need to be addressed: 1) production of size- and shape-uniform particles in large-scale and 2) general applicability to a variety of BCPs having different backbones and side chains. Achieving both requirements has been a major challenge with partial solutions being developed. For example, various types of particles have been developed using functional BCPs that exhibit stimuli-responsiveness,14,21,22 redox-responsiveness,23,24 and specific interaction with functional molecules,25–27 but these examples were limited to polydisperse particles with very broad size distributions. On the other hand, the currently-available techniques of Shirasu porous glass (SPG) membrane emulsification are successful in producing monodisperse BCP particles at larger scales, but their utility is restricted to only a couple of BCP systems that have non-functional, hydrophobic blocks such as poly(styrene-block-1,4butadiene) (PS-b-PB)28–30 and poly(styrene-block-methyl methacrylate) (PS-b-PMMA)31. The major technological hurdle arises from the complexity of applying functional BCPs with two very distinct blocks to membrane emulsification, where the abundant silanol groups on the SPG 3



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membrane surface interact with the block having greater hydrophilicity, impeding the formation of uniform-sized emulsion droplets.32,33 As a result, while poly(2-vinylpyridine) (P2VP),10,14 poly(4-vinylpyridine) (P4VP),16,22 poly(dimethylsiloxane) (PDMS),34 and poly(Nisopropyl acrylamide)21 are interesting building blocks that can be metallized and/or respond to external stimuli such as pH and temperature, their use in membrane emulsification is limited. The extension of membrane emulsification to such BCP systems is therefore problematic. Solvent vapor annealing (SVA) is an effective approach for controlling and restructuring the BCP orientation and morphology by mediating the interactions between each block of the BCP, however development of a generalized process is a major challenge.35–38 Previous examples of BCP films investigated the effects of solvents that are good or selective for the BCP, either to screen or to induce favorable interactions with the film substrate, which allows control of the BCP nanostructure and orientation.39–43 Analogously, solvent annealing techniques can be applied to BCP particles dispersed in aqueous media.44,45 For example, Li et al. first introduced the SVA approach to restructuring the morphology of the BCP particles,46 and Deng et al. demonstrated the reversible shape change of BCP particles using the SVA method.22,47,48 More recently, our group applied a similar technique to demonstrate temperature-responsive, shape-changing BCP particles.21 Solvent engineering may therefore allow reconfiguring of the BCP structure within uniform-sized particles, leading to control over the shape and internal nanostructure for a wide variety of BCP systems. Herein, we report the development of a particle restructuring by solvent engineering (PRSE) strategy that combines membrane emulsification and SVA-driven reshaping processes to generate monodisperse BCP particles with controlled shape, size, and functionalities. The utility of the method is demonstrated by successful production of a variety of functional BCP particles including polystyrene-block-polydimethylsiloxane (PS-b-PDMS), polystyrene-block4


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poly(4-vinylpyridine) (PS-b-P4VP), and PS-b-PB polymers with different structures. First, monodisperse BCP spheres were produced using SPG membrane emulsification of an organic phase containing BCPs in a selective solvent (toluene). These spherical particles were then subjected to SVA using chloroform, a good solvent for both blocks, under neutral wetting of the BCPs to their surroundings. By utilizing dual surfactants, which successfully transformed the shape and internal structure of the particles into prolate or oblate particles, the monodispersity of the particle batch could be maintained. Importantly, these PRSE processes can be applicable to a broad range of particle sizes (from ~100 nm to ~10 microns), type of polymer, and molecular weights of BCPs. This allows large scale preparation of a series of anisotropically-shaped BCP particles with precisely-tuned aspect ratios (ARs) from 1.0 to 2.5 and 1.0 to 5.0 for prolate and oblate ellipsoids, respectively. In addition, the AR values were supported by calculations based on a theoretical model describing the BCP particle deformation. Furthermore, insight into the shape-transformation process could be obtained by monitoring the transformation kinetics of PS-b-P4VP particles as a function of the annealing time and BCP molecular weight.

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Experimental

Materials PS-b-PB, PS-b-PDMS, and PS-b-P4VP BCPs were purchased from Polymer Source, Inc., and the characteristics of the BCPs are summarized in Table 1. Sodium dodecyl sulfate (SDS), poly(vinyl alcohol) (PVA, weight-average molecular weight (Mw) = 13000-23000, 87%-89% hydrolyzed), and cetyltrimethylammonium bromide (CTAB) were purchased from Sigma-Aldrich. 16-Hydroxy-N,N,N-cetyl triethylammonium bromide (HO-CTAB) was synthesized according to the procedure described in previous literature.14 All organic solvents (toluene, chloroform, and chlorobenzene) were purchased from Samchun Chemical and used as received.

Table 1. Characteristics of BCPs used in this study. Block copolymera

Total Mn [kg/mol]

Mw/Mn (Ð)

Bulk Morphology

PS34k-b-PB25k

59

1.20

LAM

PS35k-b-PB11k

46

1.09

CYL

PS16k-b-PDMS17k

33

1.10

LAM

PS31k-b-PDMS17k

48

1.18

CYL

PS10k-b-P4VP10k

20

1.08

LAM

PS19k-b-P4VP22k

41

1.15

LAM

PS15k-b-P4VP7k

22

1.18

CYL

a

Subscripts indicate the number-average molecular weight (Mn) of each block

Preparation of BCP Spheres Using SPG Membrane Emulsification Solutions of BCPs (PS-b-PB, PS-b-PDMS, and PS-b-P4VP, Table 1) in toluene (3 mg/mL, 3 mL) were prepared as the disperse phase. Deionized (DI) water containing PVA (80 6


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mL, 20 mg/mL) was used as the continuous phase to form toluene-in-water emulsions. The polymer particles were obtained by passing the disperse phase through SPG membranes with different pore diameters (dpore), and the droplets formed on the membrane surface were detached to be dispersed in the continuous phase by shear force resulting from a stirring cell operated at 280 rpm. The temperature of the emulsification tank was kept constant at 30 ºC. To control the droplet size, membranes with various dpore values (5.1, 2.1, 1.1, and 0.5 μm) were employed. After emulsification, toluene was evaporated by stirring the emulsion in a container with an open top at room temperature for one day to generate monodisperse BCP spheres. Finally, the excess PVA surfactants were removed by repeated centrifugations at 10,000 rpm for 5 min and re-dispersion in pure DI-water.

Restructuring of BCP Spheres to Shape-Anisotropic Particles To provide neutral wetting conditions with the BCPs to the surrounding aqueous media, suspensions of BCP spheres were centrifuged and dispersed in DI-water (1.5 mL) containing dual surfactants (overall concentration of 3 mg/mL) by sonication for 30 min. A mixture of SDS and PVA, at a given weight ratio, was used for PS-b-PB and PS-b-PDMS, while a mixture of CTAB and HO-CTAB was used for PS-b-P4VP. The solution was transferred to a 20 mL vial, which was placed inside a larger vial (70 mL) containing 6 mL of chloroform, sealed tightly, and kept at 30 ºC. During annealing, the whole vial was shaken every hr in order to keep the particles dispersed. In the absence of agitation, the particles precipitate to the bottom of the vial, resulting in polydisperse particles. After annealing for a pre-scheduled time, the 20 mL vial containing the particles was removed from the larger vial and kept open to air, which allows slow evaporation of the chloroform.

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Characterization Field-emission scanning electron microscopy (SEM) (Hitachi S-4800) and transmission electron microscopy (TEM) (JEOL 2000 FX) were used to observe the surface and internal structures of the BCP particles. The samples were prepared by drop-casting BCP particle suspensions onto silicon wafers and TEM grids coated with a 20 nm thick carbon film. For TEM analysis, the prepared samples of PS-b-PB particles were exposed to OsO4 vapor to stain the PB domains, and PS-b-P4VP particles were subjected to iodine vapor to selectively stain the P4VP domains. The morphology of PS-b-PDMS was observed by TEM without staining due to large difference in the electron densities of PS and PDMS.

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Results and discussion

Scheme 1. Illustration showing the overall PRSE process. Monodisperse toluene-in-water emulsions containing BCP micelles were prepared using SPG membrane emulsification, and subsequent evaporation of toluene produced monodisperse BCP spheres. Reshaping by solvent vapor annealing transformed the monodisperse BCP spheres into anisotropic shapes (prolate and oblate particles).

Scheme 1 describes the overall PRSE process that combines membrane emulsification and the reconfiguration of particle shape by solvent annealing in order to fabricate monodisperse, shape-anisotropic BCP particles from a variety of BCPs. Using SPG membrane emulsification,28–30 BCPs dissolved in toluene (3 mg/mL) were emulsified in aqueous solution (20 mg/mL) containing PVA surfactant with subsequent evaporation of toluene giving monodisperse BCP spheres. An important step in the PRSE process is the choice of toluene, a PS selective solvent, as the disperse phase solvent since the formation of PS-corona micelles in toluene screens the favorable interactions of the functional block (here, PDMS or P4VP) with the SPG membrane surface, minimizing the coalescence of droplets near the membrane pore (Figure S1).49 After generation of the BCP spheres, the PVA surfactant was exchanged for novel, dual surfactant systems that favorably interact with each polymer block to generate 9



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neutral surrounding conditions, followed by solvent vapor annealing (chloroform).22,46 Chloroform, a good solvent for both blocks, swells the BCP spheres, leading to chain mobility increase and finally shape-anisotropic BCP particles, which are energetically-favorable. A key step in the PRSE process for the shape transformation of the particles is the exchange of the surfactants in order to provide neutral wetting conditions of the BCPs to the surrounding aqueous media. It is also important to maintain the size-uniformity of the particles during the PRSE process, which is crucial but often challenging due to the instability of the solventswollen BCP particles. In particular, Ostwald ripening can change the size distribution of the swollen BCP particles, which is driven by the difference in Laplace pressure, expressed as ∆𝑃 = ∆

2𝛾 𝑟

, of the droplets having an interfacial tension of γ and a radius of r. The chloroform

molecules carrying the polymer chains in the smaller droplets diffuse into larger droplets with smaller Laplace pressure through the aqueous phase.50–52 Therefore, the ripening of the swollen BCP particles can be significantly suppressed by having a monodisperse distribution of initial particle sizes (i.e., small difference in r) prior to the chloroform exposure. This illustrates the importance of achieving high monodispersity in the initial BCP particles through the use of membrane emulsification. As a result, the processing conditions for membrane emulsification were carefully optimized, including the choice of organic solvent, surfactant, and the emulsification process parameters (i.e., operation pressure and shearing speed), which are summarized in the Supporting Information for the PS-b-P4VP system as a representative example (Figures S1-S2).

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Figure 1. SEM and TEM images of PS31k-b-PDMS17k spheres (a, b, c) produced from membranes with dpore = (a) 0.5, (b) 1.1 and (c), 2.1 µm. The particles were transformed to oblate ellipsoids (d, e, f) by PRSE.

Figure 1(a-c) shows the SEM and TEM images of monodisperse spheres of PS31k-bPDMS17k, produced using membranes with dpore = (a) 0.5, (b) 1.1, and (c) 2.1 µm, respectively. The size distribution of each sample was evaluated by measuring the diameter of the spheres (dBCP) for more than 200 particles based on the microscopy images. The particles had dBCP values of (a) 0.27±0.04, (b) 0.71±0.07, and (c) 1.05±0.12 µm, which were produced from the membranes with dpore = 0.5, 1.1, and 2.1, µm, respectively. All of the particles had a narrow 11



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size distribution with a coefficient of variation (CV) value being 10-15% in each case. Subsequently, spherical BCP particles were subjected to PRSE and the BCP spheres were successfully transformed to oblate ellipsoids as shown in Figure 1(d, e, f). Neutral wetting conditions for both blocks is critical as it allows exposure of both blocks to the particle surface during spontaneous particle deformation in order to minimize the overall interfacial and chain stretching energies.10,14 The neutral surrounding conditions were achieved by exchanging the initial surfactant (PVA) with a mixture of surfactants that preferentially interact with each block of the BCP. For the PS31k-b-PDMS17k case shown in Figure 1(d, e, f), dual SDS and PVA surfactants were used, where each surfactant favorably interacts with the PS and PDMS blocks, respectively, and the weight ratio of SDS:PVA = 2:1 provided neutral wetting conditions. The effects of the concentration and relative ratio of the dual surfactants on the particle morphology have been investigated and optimized, which is described in detail in the Supporting Information (Figures S3-5). Importantly, the monodispersity of the particles was predominantly retained after the PRSE process, which was supported by the similar size distribution (i.e., CV value) of the particles before and after PRSE. Characterization of the spherical particles (dBCP) and the long axis length (L) and short axis length (S) of ellipsoids for PS-b-PB, PS-b-PDMS, and PS-b-P4VP BCPs is summarized in Table S1. For example, the PS31k-b-PDMS17k spheres produced from dpore = 1.1 µm before PRSE showed dBCP = 0.71±0.07 µm with CV of 9.9 % (Figure 1(b)). After the PRSE process, transformed oblate particles having an AR of 2.53 showed L = 1.04±0.10 µm with CV = 9.6 %, S = 0.41±0.04 with CV = 9.8 % (Figure 1(e)). This suggests that the overall CV values (monodispersity in size) of the BCP particles were almost unchanged after the PRSE process.

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Figure 2. SEM and TEM images of PS16k-b-PDMS17k spheres (a, b, c) produced from membranes with dpore = (a) 0.5, (b) 1.1, and (c) 2.1 µm. The particles were transformed to prolate ellipsoids (d, e, f) by PRSE. Scale bars in the insets (white) are 100 nm.

To demonstrate the versatility of the PRSE approach for producing particles with various shapes, lamellae-forming PS16k-b-PDMS17k BCPs were used as the starting material for particle formation. Figure 2(a-c) shows the SEM and TEM images of spheres of PS16k-bPDMS17k, produced using membranes with dpore = (a) 0.5, (b) 1.1, and (c) 2.1 µm. In each example, the particles were monodisperse in terms of dBCP (diameter), showing (a) 0.30±0.03 (dpore = 0.5 µm), (b) 0.59±0.06 (dpore = 1.1 µm), and (c) 1.10±0.12 (dpore = 2.1 µm) with the CV value of approximately 10 % in each case. Subsequently, the spherical BCP particles were 13



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subjected to PRSE, and their shapes were successfully transformed to prolate ellipsoids as shown in the SEM and TEM images of Figure 2(d, e, f). It is noted that some clustering and deformation of the prolate ellipsoids is observed due to the rubbery PDMS blocks with low Tg= -125 ºC).53,54 Again, the surfactant was exchanged to SDS and PVA dual surfactants to achieve neutral wetting conditions, however a different weight ratio of SDS:PVA (1:1) was used considering the relative surface area of PS and PDMS domains in the prolate particles. It is noteworthy that with cylinder-forming BCPs, oblate ellipsoids are obtained (Figure 1), while the use of lamellae-forming BCPs results in the formation of oblate particles (Figure 2).10– 12,14,16

Importantly, the monodispersity of the prolate particles were nearly maintained during

the PRSE process, similar to the case of oblate ellipsoids. For example, the PS16k-b-PDMS17k spheres produced from dpore = 1.1 µm before PRSE showed dBCP = 0.59±0.06 µm with CV of 10.2 % (Figure 2(b)). After the PRSE process, transformed prolate particles having an AR of 1.55, having L = 1.36±0.15 µm with CV = 11.0 % and S = 0.88±0.09 µm with CV = 10.2 % (Figure 1(f)). Additional SEM and TEM images of PS31k-b-PDMS17k and PS16k-b-PDMS17k for dpore = 5.1 µm case, and the side-view SEM images of PS31k-b-PDMS17k oblate ellipsoids are shown in Figures S6 and S7.

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Figure 3. Plot of AR as a function of L for (a) prolate ellipsoids of PS16k-b-PDMS17k and (b) oblate particles of PS31k-b-PDMS17k, both of which were produced using PRSE. Each symbol denotes the particles produced from membranes with dpore = 0.5 (black), 1.1 (red), 2.1 (blue), and 5.1 µm (pink).

To investigate the tunability of the shape-anisotropy of the monodisperse particles achieved by the PRSE process, the ARs of the obtained prolate and oblate particles with different sizes were analyzed. Figure 3 shows scatter plots of AR as a function of L for monodisperse PS16k-b-PDMS17k prolate particles and PS31k-b-PDMS17k oblate particles after the PRSE process (dpore = 0.5, 1.1, 2.1, and 5.1 µm), constructed from 50 representative data points for each sample. Interestingly, the AR of both prolate and oblate particles was a strong function of particle size (L), showing an increase in AR for larger L values. For PS16k-bPDMS17k prolate particles, the AR of the particles generated from dpore = 0.5, 1.1, 2.1, and 5.1 µm showed a gradual increase from 1.44 to 1.55, 1.74, and 1.82. These results were in accordance with previous studies, where an increase of AR with larger particle size was observed.10,14,28 Such behavior was attributed to the combined effect of greater bulk elastic energy (increased number of lamellae stacks) that contributes to the particle elongation and 15



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lower surface energy (reduced surface area/volume ratio) that opposes the particle elongation. For PS31k-b-PDMS17k oblate particles, the AR of the particles generated from dpore = 0.5, 1.1, 2.1, and 5.1 µm showed an increasing trend from 1.66 to 2.53, 3.03, and 4.01, but the AR change is much more significant than in the case of the prolate particles. Therefore, the AR of the oblate particles was always greater than that of prolate particles at a comparable particle size. It is proposed that this difference is due to the geometry of the prolate and oblate particles. Assuming the spherical particles with perpendicular BCP domains undergo shape deformation to minimize the free energy of the system, the reduction of chain bending energy is more significant when the spheres deform into oblates than to prolates, considering that the chain bending penalty should be greater for a larger number of cylindrical domains along the edge of the oblate particles than that on the much smaller number of lamellae stacks at the two poles of the prolate ellipsoids. Therefore, the degree of deformation from spheres is larger for oblates than prolates, resulting in greater ARs.

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Figure 4. SEM and TEM images of (a, b) PS34k-b-PB25k, (c, d) PS16k-b-PDMS17k, (e, f) PS10kb-P4VP10k particles before (a, c, e) and after (b, d, f) PRSE by chloroform for 2 days, produced from SPG membrane with dpore = 1.1 µm.

Next, in order to illustrate the generality of PRSE approach for producing monodisperse, non-spherical particles, extension to different BCP systems was studied. Figure 4(a, c, e) shows the SEM and TEM images of monodisperse spheres of PS34k-b-PB25k, PS16kb-PDMS17k, and PS10k-b-P4VP10k, produced using membranes with dpore = 1.1 µm. These particles had similar dBCP values regardless of the BCP type, having 0.62±0.06 µm, 0.59±0.06 and 0.61± 0.09 µm for PS34k-b-PB25k, PS16k-b-PDMS17k, and PS10k-b-P4VP10k, respectively 17



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with all examples showing CV values within 10-15 %. Significantly, after the spherical BCP particles were subjected to PRSE, all were transformed to prolate ellipsoids while maintaining the monodispersity of particle size, as shown in Figure 4(b, d, f). In these cases, the PS34k-bPB25k spheres showed dBCP = 0.62±0.06 with CV of 9.7 % (Figure 4(a)), while prolate particles generated by PRSE (AR = 1.28) showed L = 1.00± 0.11 µm with CV = 11.0 %, and S = 0.78±0.08 µm with CV = 10.2 % (Figure 4(b)). Similarly, PS16k-b-PDMS17k spheres showed dBCP = 0.59±0.06 with CV of 10.2 % (Figure 4(c)), while prolate particles generated by PRSE (AR = 1.55) showed L = 1.36±0.15 with CV = 11.0 %, and S = 0.88±0.09 with CV = 10.2 % (Figure 4(d)). PS10k-b-P4VP10k spheres before PRSE showed dBCP = 0.61±0.08 with CV of 13.1 % (Figure 4(e)), while prolate particles generated by PRSE (AR = 2.07) showed L = 1.19±0.17 with CV = 14.2 %, and S = 0.58±0.07 with CV = 12.0 % (Figure 4(f)). From these disparate diblock copolymers, size-uniformity was retained during the PRSE process. Additionally, applying the PRSE process to three different BCPs having cylinder-forming fractions (PS35k-b-PB11k, PS31k-b-PDMS17k and PS15k-b-P4VP7k spheres) allowed the shape transformation to oblate particles regardless of particle size, which are summarized in Figures S8-11. These results clearly illustrate the PRSE approach as a general strategy for other BCP particle systems leading to accurate control over size and shape.

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Figure 5. Plot of AR as a function of L in monodisperse prolate particles of (a) PS34k-b-PB25k, (b) PS16k-b-PDMS17k, and (c) PS10k-b-P4VP10k after PRSE. Scatter data points are fitted with theoretical calculations shown as green line.

Figure 5 shows scatter plots of AR as a function of L for monodisperse prolate ellipsoid particles based on PS34k-b-PB25k, PS16k-b-PDMS17k, and PS10k-b-P4VP10k. The particles were initially produced from different-sized membranes of dpore = 0.5, 1.1, 2.1, and 5.1 µm, and shape-transformed by the PRSE process. As discussed earlier in Figure 3, the AR of ellipsoids increased with larger L for all the three BCPs (PS34k-b-PB25k, PS16k-b-PDMS17k, and PS10k-b-P4VP10k). Interestingly, at a given dpore, ARs increased in the sequence of PS34k-bPB25k < PS16k-b-PDMS17k < PS10k-b-P4VP10k. For example, AR values were AR = 1.28, 1.74 and 2.41 for PS34k-b-PB25k, PS16k-b-PDMS17k, and PS10k-b-P4VP10k, respectively, for the particles produced using a membrane with dpore = 2.1 µm. To quantitatively describe such dependence of AR as a function of L and the type of BCP, ARs of the particles were theoretically calculated for each of the BCPs and plotted as green lines along with the experimental values, as shown in Figure 5. Calculation details are summarized in the Supporting Information (equation S1) and also described in the previous literature.10,14,29 The 19



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calculated AR values based on this model correspond well with the experimental values. Increasing particle size results in a greater contribution of bulk elastic energy relative to the surface energy contribution to the system, which allows particle elongation to larger ARs for all three BCPs. Furthermore, increasing ARs in the sequence of PS34k-b-PB25k < PS16k-bPDMS17k < PS10k-b-P4VP10k is attributed to the increase in the Flory-Huggins interaction parameter (χ) (i.e., χPS-b-PB = 0.04, χPS-b-PDMS = 0.21 and χPS-b-P4VP = 0.53) and the corresponding increase in the interfacial energy between the two blocks.55–57 Additionally, the AR values of the oblate particles from cylinder-forming BCPs of PS35k-b-PB11k, PS31k-b-PDMS17k, and PS15kb-P4VP7k were analyzed (Figure S12). The AR values show an increasing trend in the sequence of PS35k-b-PB11k< PS31k-b-PDMS17k< PS15k-b-P4VP7k, similar to the case of the prolate ellipsoids. As a result, for the different BCP systems, the PRSE strategy provides a unique methodology for universal shape-transformation of spherical particles to monodisperse, sizecontrolled prolates and oblates, while also allowing for the accurate control of AR with values ranging from 1.0 to 2.5 and 1.0 to 5.0 for prolate and oblate ellipsoids, respectively.

Figure 6. TEM images showing morphology transformation kinetics from spheres to ellipsoids of (a–e) PS10k-b-P4VP10k particles and (f–j) PS19k-b-P4VP22k particles produced from a dpore = 20


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1.1 µm membrane. Each image shows a representative example of the most dominantly observed particles that were obtained after exposure to chloroform for a given time between 0 and 24 h, followed by freeze-drying in vacuum (i.e., 0.07 mbar) for 1 day. P4VP domains appear dark due to iodine staining.

To gain deeper insight into the shape transformation process, the structural evolution of particle shape and the internal morphology at different time durations during the PRSE process was monitored, using PS-b-P4VP as a model BCP system. Since the morphology transformation process of the BCP particles is strongly associated with the swollen state in which the reorganization of the BCPs occurs, the particle structure was captured at different times (tswell) of exposure to chloroform during the PRSE process. Figure 6 shows TEM images of particle morphologies for PS10k-b-P4VP10k and PS19k-b-P4VP22k that were obtained by freezing with liquid nitrogen after a given exposure time to chloroform vapor, followed by drying in vacuum (i.e., 0.07 mbar) for 1 day to capture the particle morphology in the swollen state. For PS10k-b-P4VP10k, onion-like particles (Figure 6(a)) were first observed upon swelling with chloroform vapor. After tswell = 1 h and 3 h, particles with an intermediate morphology between onion and ellipsoid were observed, indicating a switch in the orientation of lamellae morphology from parallel to perpendicular relative to the particle surface (Figure 6(b, c)). After tswell = 6 h, the orientation of lamellae was mostly perpendicular relative to the particle surface (Figure 6(d)). Further annealing allows reorganization of the polymer chains and transformation to ellipsoids (Figure 6(e)). These observations suggest that the dominant process during shape transformation of spherical BCP particles involves the reorientation of PS-b-P4VP domains from a parallel to perpendicular orientation with regard to the particle surface. Similar reorientation of BCP domains has been extensively demonstrated in thin films, 21



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where the interaction between swollen BCPs and the film surface is important for determining orientation.40,41,58–60 For example, Cavicchi et al. observed the orientation shift of polyisopreneb-polylactide (PI-b-PLA) BCP microdomains as a function of the swollen film thickness, where perpendicular orientation to the film surface is obtained as the interfacial interaction between swollen BCPs and the substrate became less selective. In this case, the morphology transformation of PS10k-b-P4VP10k particles from onion to ellipsoid is driven by neutral interaction of swollen PS-b-P4VP with CTAB/HO-CTAB surfactants coated at the particle surface. As PS-b-P4VP chains are swollen with chloroform, CTAB preferentially adsorbs to the swollen PS block while HO-CTAB favorably interacts with the swollen P4VP block to reorient the lamellae from parallel to perpendicular by exposing both PS and P4VP domains to the surrounding aqueous phase. Next, the effect of overall molecular weight for symmetric PS-b-P4VP on the kinetics of particle morphology transformation was examined. Interestingly, it was observed that the transformation kinetics of a higher molecular weight diblock, PS19k-b-P4VP22k (Figure 6(f-j)) is different compared to those of PS10k-b-P4VP10k. For example, after membrane emulsification using toluene as the disperse phase solvent, PS10k-b-P4VP10k spheres had an onion-like initial morphology (Figure 6(a)) whereas PS19k-b-P4VP22k spheres consist of BCP micelles with a PS corona (Figure 6(f)). The difference is attributed to the greater selectivity of toluene towards the PS block for higher molecular weight PS-b-P4VP.17,61,62 The morphology transformation sequence of PS19k-b-P4VP22k particles was also different from that of PS10k-b-P4VP10k particles, following the sequence: micelle → onion → intermediate → ellipsoid (Figure 6(f-j)). Due to the different initial morphology (micelles, Figure 6(f)), two processes occurred together during swelling with chloroform: interfusion of P4VP micelles into a continuous lamellae domain, and 22


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reorientation of the microdomains from parallel to perpendicular. When the P4VP core of the micelles are plasticized, the swollen P4VP chains fuse together to form a continuous domain.63,64 Therefore, an onion-like morphology appeared at tswell = 3 h (Figure 6(g)). Once the P4VP domains are interconnected, the orientation of the BCP domains starts to shift, and an intermediate morphology (half-ellipsoid and half-onion) was predominantly observed at tswell = 6 h (Figure 6(h)). It is worth noting that the reorientation starts from the tip of a particle, due to a local decrease in Tg induced by confinement of chain ends at the surface which increases free volume.65,66 Further annealing (tswell = 12 h) results in particles with BCP domains mostly perpendicular to the particle surface (Figure 6(i)) and well-ordered ellipsoids were obtained after tswell = 24 h (Figure 6(j)). As a result, the overall time taken for full transition to ellipsoid was 12 h for PS10k-b-P4VP10k, while nearly 24 h is required for PS19k-b-P4VP22k particles to undergo full shape transition. The additional time in the swollen state for PS19k-bP4VP22k is likely due to the micelle-like initial morphology, where BCP chains have to overcome the kinetic constraint of breaking the P4VP micelles and undergoing interfusion into connected P4VP domains. Additionally, higher molecular weight BCPs are more strongly segregated and will generally take longer to reach equilibrium due to chain diffusion and interfacial width considerations.67

23



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Conclusions A novel approach to controlling the shape and internal morphology (PRSE) of polymer particles is reported. This approach combines membrane emulsification and solvent annealing to scalably produce monodisperse, non-spherical BCP particles from a wide variety of starting BCPs. This PRSE strategy involves first producing monodisperse BCP spheres using SPG membrane emulsification in a selective solvent, and subsequent shape transformation to prolate and oblate particles by chloroform annealing under neutral wetting conditions. Importantly, this PRSE strategy was widely applicable to various functional BCPs, such as PS-b-PB, PS-bPDMS, and PS-b-P4VP, allowing non-spherical and monodisperse BCP particles to be prepared for a wide range of particle sizes and shape-anisotropy. The AR values from theoretical calculations matched well with the observed ARs. Investigation of the shape transformation process during PRSE, achieved by observing the time-dependent morphology, revealed that the shape transitions were induced by orientation shifts of BCP microdomains with the overall molecular weight of the BCP and the annealing time providing further control over the full transformation to anisotropic shapes.

24


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ASSOCIATED CONTENT Supporting Information. Additional SEM and TEM images of the particles, information about the particle size, and plots analyzing the AR of different particles are provided in the supporting information. This material is available free of charge at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected]

ACKNOWLEDGMENT This work was supported by the Agency for Defense Development of the Republic of Korea, under the contract number UD160085BD. This research was supported by the Korea Research Foundation

Grant,

funded

by

the

Korean

Government

(2012M3A6A7055540,

2017M3D1A1039553). C.J.H acknowledges support from the National Science Foundation, Division of Materials Research under the Materials Research Science & Engineering Centers Program (UCSB MRSEC, NSF DMR 1720256). We also acknowledge additional support for this work from the Research Project of the KAIST-KUSTAR.

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Development of Shape-Tuned, Monodisperse Block Copolymer

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