Tuning Hydrophobicity To Program Block Copolymer Assemblies from

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Tuning Hydrophobicity To Program Block Copolymer Assemblies from the Inside Out C. Adrian Figg, R. Nicholas Carmean, Kyle C. Bentz, Soma Mukherjee, Daniel A. Savin, and Brent S. Sumerlin* George & Josephine Butler Polymer Research Laboratory, Center for Macromolecular Science & Engineering, Department of Chemistry, University of Florida, PO Box 117200, Gainesville, Florida 32611-7200, United States S Supporting Information *

ABSTRACT: Hydrophobicity inherently affects a solutes behavior in water, yet how polymer chain hydrophobicity impacts aggregate morphology during solution self-assembly and reorganization is largely overlooked. As polymer and nanoparticle syntheses are easily achieved, the resultant nanoparticle architectures are usually attributed to chain topology and overall degree of polymerization, bypassing how the chains may interact with water during/after self-assembly to elicit morphology changes. Herein, we demonstrate how block copolymer hydrophobicity allows control over aggregate morphology in water and leads to remarkable control over the length of polymeric nanoparticle worms. Polymerization-induced self-assembly facilitated nanoparticle synthesis through simultaneous polymerization, self-assembly, and chain reorganization during a block copolymer chain extension from a hydrophilic poly(N,N-dimethylacrylamide) macro-chain-transfer agent with diacetone acrylamide and N,N-dimethylacrylamide. Slight variations in the monomer feed ratio dictated the block copolymer chain composition and were proposed to alter aggregate thermodynamics. Micelles, worms, and vesicles were synthesized, and the highest level of control over worm elongation attained during a polymerization is reported, simply due to the polymer chain hydrophobicity.

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has recently been utilized by An and others for PISA.28−30 Additionally, the hydrophobicity of copolymers containing DMA and DAAm can be discretely tuned by varying the comonomer ratio during polymerization.31−33 We hypothesized that if these copolymers composed the core-forming block of polymer aggregates, the resulting nanoparticles would show composition-dependent thermoresponsive behaviors. Indeed, we observed that the thermal stability of the assemblies formed during PISA was directly dependent on the ratio of DMA to DAAm. More interestingly, minor changes in the monomer feed ratio and the resulting variation in hydrophobicity also led to dramatic differences in aggregate morphology. The effect of comonomer ratio within the core-forming block during PISA has only been considered relatively recently.34−37 The dispersion-based PISA mechanism in water (Figure 1) begins as chain extension from a water-soluble homopolymer with a water-soluble monomer that yields a hydrophobic block.38 When the second block reaches a critical degree of polymerization, the growing amphiphilic chains assemble in situ to form micelle-like morphologies, thus transitioning the homogeneous system into a dispersion polymerization. The

INTRODUCTION Block copolymer aggregation in solution can be exploited to synthesize complex nanomaterials and emulate natural structures. The in-situ synthesis of self-assembled block copolymer nanostructures by chain extending miscible polymers with an immiscible second block1−3 is redefining the field of block copolymer self-assembly. Dispersion-4−17 and emulsion-based18−21 polymerization-induced self-assembly (PISA)22−24 by reversible-deactivation radical polymerization has been used to achieve diverse polymer aggregate morphologies, with reversible addition−fragmentation chain transfer (RAFT) polymerization being the most utilized polymerization technique.25 Additionally, polymerization-induced thermal self-assembly26 was recently introduced, whereby thermoresponsive polymers27 were observed to yield different morphologies during block copolymerizations above a thermal transition temperature, thereby providing a route to a range of thermoresponsive polymeric nanoparticles. Initially, we sought to expand this route of synthesizing thermoresponsive systems to access block copolymers that selfassemble and dissociate at predetermined temperatures dictated by the composition of the responsive block. N,N-Dimethylacrylamide (DMA) is a hydrophilic monomer which yields a hydrophilic polymer, while diacetone acrylamide (DAAm) is a hydrophilic monomer that leads to a hydrophobic polymer and © XXXX American Chemical Society

Received: December 22, 2016 Revised: January 4, 2017

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DOI: 10.1021/acs.macromol.6b02754 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Polymerization-induced self-assembly process proceeding through a dispersion-based mechanism.

increasing chain length of the hydrophobic block causes the copolymers to rearrange into thermodynamically favored morphologies dictated by the packing parameter, the degree of polymerization of the hydrophobic block, and the interfacial curvature and surface tension between the hydrophilic and hydrophobic blocks.39 As we changed the composition of the core-forming block by varying the monomer feed ratio, we observed differences in the morphological progression during the course of the polymerization. These results suggested aggregate morphology was being dictated by the inherent hydrophobicity of the block copolymers, a phenomenon that affects chain mobility and unimer exchange between micelles.40−43 Therefore, we realized that our system provided an opportunity to explore another fundamental aspect of block copolymer self-assembly and reorganization: how hydrophobicity affects nanoparticle morphology during PISA. In the work presented here, a hydrophilic poly(N,Ndimethylacrylamide) (PDMA) macro-chain-transfer agent (macro-CTA) was first synthesized by RAFT polymerization to high monomer conversion (Scheme 1a). In a one-pot approach,44,45 this macro-CTA was chain extended with varying molar ratios of DAAm to DMA to result in statistical copolymer segments with compositions dependent on the monomer feed ratio (Scheme 1b). Various nanoparticle morphologies were observed (e.g., micelles, worms, branched worms, or vesicles) with a remarkable dependence on polymer composition. In addition to control over nanoparticle morphology, the proposed changes in aggregate thermodynamics through comonomer modulation provided unprecedented evidence of controlled worm growth during PISAa phenomenon unrealized in most self-assembly techniques in selective solvents,46 except seeded crystallization-driven self-assembly.47 Worm−worm fusions of polymeric nanoparticles have been reported following self-assembly,48−57 but as far as we are aware, there have been no reports of controlling worm length to the extent described here for amphiphilic block copolymer in solution. Here, we present our results on the dramatic morphology changes that result from small variations in block

Scheme 1. (a) Reversible Addition−Fragmentation Chain Transfer Polymerization of N,N-Dimethylacrylamide (DMA) To Yield the Hydrophilic Macro-Chain-Transfer Agent (Macro-CTA); (b) Polymerization-Induced Self-Assembly Aqueous Chain Extension of the Macro-CTA with DMA and Diacetone Acrylamide (DAAm) and Subsequent CrossLinking of the DAAm Groups To Provide “Locked” Nanostructures

copolymer composition and, more broadly, provide insight into how tuning block copolymer solvophilicity can lead to extraordinary control over polymer self-assembly and reorganization.

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RESULTS AND DISCUSSION A PDMA macro-CTA (Mw,MALS = 8040 g/mol, ĐM = 1.17) was synthesized in water, reaching full monomer conversion (>95% by 1H NMR spectroscopy) after 3 h. Through a one-pot polymerization approach, the macro-CTA solution was diluted with an aqueous monomer solution to 15 w/w% concentration (see Supporting Information for experimental details). B


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Figure 2. Size exclusion chromatography characterization of self-assembled nanoparticles formed during polymerization-induced self-assembly (after cross-linking): (a) diacetone acrylamide (DAAm) to N,N-dimethylacrylamide (DMA) monomer feed ratio of 90:10 (DAAm90); (b) DAAm to DMA monomer feed ratio of 85:15 (DAAm85); (c) DAAm to DMA monomer feed ratio of 80:20 (DAAm85); (d) DAAm to DMA monomer feed ratio of 75:25 (DAAm75).

linking of the hydrophobic block was achieved by reacting a difunctional alkoxyamine with the ketone functional groups of the DAAm units to result in hydrolytically stable oxime linkages.58−60 These “locked” nanostructures could be readily characterized by dynamic light scattering (DLS), SEC-MALS, and transmission electron microscopy (TEM). DAAm75 was the lowest incorporation of DAAm considered, as polymerizations with lower ratios of the hydrophobic monomer gelled during cross-linking, presumably from blocks not being sufficiently hydrophobic to induce phase separation. The unimer weight-average molecular weights (Mw,unimer) were determined by SEC-MALS. Purified aliquots of the polymerization solution prior to cross-linking offered evidence of excellent blocking efficiency and low polymer molar mass dispersities by SEC-MALS for all compositions at DP2 = 50, 80, 130, and 200 (Figure 2a−d, solid lines). Importantly, the unimers targeting the same DP 2 , but with different compositions, possessed similar molecular weights regardless of the resultant morphology (Table 1). For example, unimers with DP2 of 141 (entries 9−12) all had absolute molecular weights of approximately 22 000−24 000 g/mol, facilitating evaluation of aggregate morphology based solely on inherent chain hydrophobicity. Following cross-linking, the reactions were cooled to room temperature before DLS size measurements were performed (Figures S8−S11). Variation in observed nanoparticle sizes and size distributions (i.e., unimodal versus multimodal) across

Polymerization kinetics using molar monomer feed ratios of 90% DAAm to 10% DMA (DAAm90, Figure S2), 85% DAAm to 15% DMA (DAAm85, Figure S3), 80% DAAm to 20% DMA (DAAm80, Figure S4), and 75% DAAm to 25% DMA (DAAm75, Figure S5) were monitored at 70 °C to determine the copolymerization behavior of the comonomers. Similar reactivity ratios (rDAAm = 0.86 and rDMA = 0.80) of the monomers were found that suggested relatively statistical monomer incorporation during PISA (Figure S7). The compositional effects of the block copolymers on the resultant morphologies were studied by varying both comonomer feed ratio and the degree of polymerization of the second block (DP2). Copolymerizations targeting a specific DP2 and containing monomer feed ratios of DAAm90, DAAm85, DAAm80, or DAAm75 were left for 16 h so that each polymerization achieved >95% monomer conversion. Aliquots from the polymerizations were taken periodically, diluted with DMAc (i.e., a nonselective solvent), and characterized by NMR spectroscopy for monomer conversion and size-exclusion chromatography with multiangle light scattering detection (SEC-MALS) for molecular weights of the unimers. Because the block copolymer aggregates that formed during chain extension of the macro-CTA with DMA and DAAm were thermoresponsive (i.e., susceptible to dissociation on cooling), the aggregates that resulted during PISA were cross-linked immediately after polymerization at 70 °C to facilitate characterization of morphology and size. CrossC


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dotted lines) were consistent with the size distributions observed during DLS characterization and further suggested that monomer feed composition had a large effect on the resultant nanoparticles. Inevitably, some SEC results may be affected by the variety of anisotropic structures that could be present, but qualitative comparisons can still be made. When an intermediate value of DP2 for DAAm80 and DAAm75 was characterized, the nanoparticles eluted at shorter retention times with broad and multimodal distributions. Subsequently, for polymerizations with higher DP2, the nanoparticles eluted at longer retention times, and symmetric traces were observed. This change in elution time and distribution shape could be indicative of an anisotropic−isotropic morphology change that would accompany a worm-to-sphere transition. Nanoparticle traces for DAAm90 and DAAm85 contrasted these results where a steady decrease in retention time was observed according to increasing DP2. Additional discussion on the effect of unimer composition on nanoparticle weight-average molecular weight (Mw,NP), aggregation number (Nagg), and radius of gyration (Rg) attained during nanoparticle MALS is in the Supporting Information. One conclusion drawn from the constant increases in Mw,NP, Nagg, and Rg during MALS analysis is that regardless of composition or DP2, the nanostructures may not be kinetically trapped and instead favor reorganization. This reorganization would cause the chains to adopt more thermodynamically favorable conformations, agreeing with recent SAXS studies by Armes.61 However, only correlational conclusions can be drawn using MALS because the apparent

Table 1. Characterization Data of Nanoparticle Constituent Unimers Obtained Using Size Exclusion Chromatography Equipped with a Multiangle Laser Light Scattering Detector entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

second block degree of polymerization, DP2a

mol % DAAm in monomer feed

unimer weight-average molecular weight, Mw,unimer (×103 g/mol)

54

90 85 80 75 90 85 80 75 90 85 80 75 90 85 80 75

11.6 11.7 11.3 14.3 15.6 15.6 18.0 18.7 21.7 22.2 23.8 22.2 35.2 33.1 30.6 39.5

87

141

217

a

According to initial monomer feed ratio and >95% monomer conversion by 1H NMR spectroscopy.

similar unimer molecular weights suggested that increasing the amount of DMA in the copolymer significantly affected morphology. The nanoparticle SEC traces (Figure 2a−d,

Figure 3. Transmission electron microscopy images of polymerization-induced self-assembly synthesized nanoparticles with varying monomer feed ratios of diacetone acrylamide (DAAm) to N,N-dimethylacrylamide (DMA) and second block degrees of polymerization (DP2) of 54, 87, 141, and 217; (a−d) 90% DAAm:10% DMA (DAAm90); (e−h) 85% DAAm:15% DMA (DAAm85); (i−l) 80% DAAm:20% DMA (DAAm80); (m−p) 75% DAAm:25% DMA (DAAm75). PTA = sodium phosphotungstate negative stain; UA = uranyl acetate negative stain. D


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to the micelles achieved with the other compositions (Figure 3m). At a DP2 = 87, worms were observed (Figure 3n); however, these worms appeared shorter in length than those of the other monomer compositions previously discussed, and many micelle-like structures were also present. Upon further increasing the second block degree of polymerization to 141, highly extended worm structures were achieved (Figure 3o). Worm entanglements proved to encumber dispersion of the worms in solution needed to image isolated worms, but the lengths were observed to be up to micrometers in length. Additionally, few branching points can be seen over the length of the worms. At the highest DP2 of 217, predominately vesicles were observed (Figure 3p). Importantly, all the spherical morphologies at a DP2 value of 217 displayed a bilayer-like structure, and their propensity to hexagonally pack on the TEM grids provides evidence of their narrow size dispersity. The morphological transformations of DAAm75 proved to be particularly interesting. Micelle-to-worm transitions appeared to occur at a higher DP2 than in the other monomer feed ratios, which could be attributed to the DMA imparting more hydrophilicity to the chains. The enhanced hydrophilicity may have led to a later onset of aggregation and subsequent reorganization of unimers. Worm phases were also observed over a large range of DP2, and the worms appeared to predominantly favor elongation over branching at DP2 values of 111 and 127 (Figure 4a,b). Elongation continued to result in worms up to micrometers in length with minimal branching points and a few lamellar structures at a DP2 of 141 (Figure 4c). These results are in stark contrast to previous PISA reports where the linear worm phase was abrupt and branched worms were commonly observed.23 The worm-to-vesicle transition observed at DP2 = 176 resulted in morphologies previously observed during PISA morphological transitions, including lamellar bilayers, “octopi”, and branched structures (Figure 4d).38 Additionally, the majority of the vesicles appeared to be adhered to worms, which may result from a morphological transition or due to interparticle attachment (black arrows, Figure 4d). A similar transformation was reported by Zhu and Hayward for blends of poly(ethylene oxide)-b-polystyrene copolymers during postpolymerization self-assembly.63 The bilayers were attributed to local phase separation due to the introduction of copolymers containing different block lengths. The nanoparticles reported here may be undergoing similar transitions, where chains possessing lower molecular weight PDMA blocks are partitioning into the core of the worms to form a bilayer. However, further mechanistic insight is required to fully understand this newly observed PISA transformation. The morphological phase diagram (Figure 5a) and transitions observed (Figure 5b) emphasize the significant impact of the hydrophobicity of the self-assembling polymer block on aggregate morphology. Vesicles were observed only when higher amounts of hydrophilic DMA were incorporated into the feed ratio, while worms were observed over a larger compositional range. By introducing more hydrophobicity (through a higher DP2 or increasing the amount of hydrophobic monomer in the responsive-block composition), the solvent would become increasingly poor for the hydrophobes. This change in solvent quality would lead to continued collapse of the hydrophobic regions, which would yield a low hydrophobic-to-hydrophilic volume fraction and, eventually, change the morphology. Alternatively, by incorporating additional hydrophilic monomer into the responsive-block composition, the relative volume of the core block can increase

variety of nanoparticle shapes resulted in difficult characterization and data analysis. Fortunately, this characterization limitation between nanoparticles possessing a similar unimer DP2 provides compelling evidence that chain composition drastically affected the morphology. TEM imaging is perhaps the most useful characterization technique to understand the morphological transitions of polymer chains during aggregation and reassembly (although the technique is limited by sample size). As with the DLS and SEC-MALS results, we observed dramatic effects of copolymer composition on nanoparticle morphology. DAAm90 exhibited only spherical micelle structures at all DP2 values analyzed (Figure 3a−d). Though vesicles have been reported for copolymers containing DAAm self-assembled blocks with a high DP2,28 we did not observe any bilayer structures over the ranges considered. With increasing amounts of DMA within the hydrophobic block (DAAm85), worms were observed at a DP2 of 87 (Figure 3f), while a DP2 of 141 (Figure 3g) appeared to result in a worm-to-sphere transition and a smaller population of vesicular structures which remained constant at DP2 = 217 (Figure 3h). The transitions exhibited similarity to other worm−micelle transitions observed in postpolymerization self-assembly.48,54,62 Presumably, the core-forming block is becoming increasingly hydrophobic as the polymerization ensues; therefore, the interfacial energy would be expected to increase between the poly(DAAm-co-DMA) block (DP2) and water. To quell the unfavorable interaction, the core-forming chains could be stretching to lower the core−solvent surface interactions. Chain stretching would lower the interfacial area between the hydrophilic and hydrophobic blocks; however, bringing the corona chains closer together increases steric repulsion. To relieve the coronal steric congestion, the length of each worm could shorten to provide a higher number of spherical end-caps, which provide thermodynamic relief through a small hydrophilic−hydrophobic interfacial area and more volume for the corona chains.49 The constant decrease in solvent quality for the core chains most likely explains why only micelle-like morphologies were observed for DAAm90. The high interfacial tension between the hydrophobic block and water may inhibit any morphological transition, as the chains could favor sacrificing the entropic penalty of elongation over the enthalpic surface tension penalty. Further increasing the DMA content to 20% in the monomer feed ratio (DAAm80) yielded morphological transitions more customary of PISA. Micelles were first observed with a DP2 = 54 (Figure 3i), worms at DP2 = 87 (Figure 3j), branched worms and other intermediate structures such as “jellyfish” and “octopi” at a DP2 = 141 (Figure 3k), and vesicles at a DP2 = 217 (Figure 3l). However, a worm-to-sphere transition was still observed at this composition. Following the worms at DP2 = 87 and 141, a minor population appeared to undergo reversion to micelles as the degree of polymerization was increased. Therefore, in the final morphological composition (DP2 = 217, Figure 3l), mostly vesicles with a small population of micelles persisted. Though a micelle-to-worm-to-fused wormto-vesicle transition for DAAm80 seemed to have occurred, the presence of a minor worm-to-sphere transition inhibits the block copolymers formed at that monomer feed ratio from truly encompassing a hierarchical morphological transformation, showing multiple possible morphologies at high DP2. DAAm75 yielded the most diverse and distinct morphological transitions. A DP2 of 54 resulted in micelles comparable E


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Figure 5. (a) Phase diagram depicting the effect of varying the ratio of diacetone acrylamide (DAAm) to N,N-dimethylacrylamide in the monomer feed, resulting in a variety of morphologies. (b) Observed transition of morphology according to monomer feed ratio and the degree of polymerization of the block copolymer.

Figure 4. Transmission electron microscopy images of cross-linked nanoparticles using a monomer feed ratio of 75:25 diacetone acrylamide to N,N-dimethylacrylamide (DAAm75) for the second block and varying degrees of polymerization (DP2): (a) DP2 of 111 showing worm and micelle morphologies; (b) DP2 of 127 showing predominately worm morphologies; (c) DP2 of 141 showing elongated and branched worm morphologies (black arrows denote branching or points of lamellar formation); (d) DP2 of 176 showing a worm-tovesicle transition through branch points and budding from worms (black arrows denote points of vesicles budding from worms).

nm across a range of DP2 values (Figure 6a). Upon increasing the DMA content (DAAm80), the worm length peaked around a DP2 of 87, after which a broadening of the length distribution was observed as branching and multiple junction points developed (Figure 6b). The polymerizations with the highest DMA content (DAAm75) yielded worms that grew at least an order of magnitude longer (from 76 to 770 nm) over the range of DP2 values considered, until the worm-to-vesicle transition was approached and worm length plummeted (Figure 6c, only isolated worms were measuredthe actual average worm length is expected to be higher). We reasoned that worm elongation proceeded through a stepwise process where the growing polymer chains induced an increase in packing parameter and necessitated a lower interfacial curvature. As the volume of the hydrophobic blocks increases, the worm end-caps become the most thermodynamically unfavorable section of the nanoparticles.39 The difference in energy between the cylindrical worm and spherical end-cap could cause the ends to undergo inelastic collisions with each other, while most collisions that do not include two end-caps would be elastic (Figure 6d). Consequently, worm length would grow exponentially as spherical ends fused (since the

with degree of polymerization to provide higher-order morphologies. Therefore, nanoparticle morphology was heavily dependent upon the hydrophobicity of the core-forming polymer block, where the hydrophobicity dictated progression of a polymer aggregate via a hierarchical morphological transition or an alternative reversion to micelles. Importantly, the composition-dependent morphology trends presented in this article have been reproduced once with the same macroCTA and also observed using two other macro-CTAs of differing lengths at 70, 45, and 25 °C (data not shown). The average worm length for each of the compositions was determined by TEM to observe the differences in worm−worm fusions during the polymerization. With low concentrations of DMA (DAAm85), the worm length stayed between 76 and 94 F


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Figure 6. Measured worm lengths and standard deviations from transmission electron microscopy (TEM) images with representative images of each worm at the measured second block degree of polymerization (DP2): (a) diacetone acrylamide (DAAm):N,N-dimethylacrylamide (DMA) monomer feed ratio of 85:15 (DAAm85); (b) DAAm:DMA monomer feed ratio of 80:20 (DAAm80); (c) DAAm:DMA monomer feed ratio of 75:25 (DAAm75); (d) proposed stepwise worm elongation controlled through the hydrophobicity of the constituent unimers.

vesicles budding off from worms). Regulation of block copolymer amphiphilicity also yielded the highest level of control over worm elongation reported during or after polymerization without a seeded initiator. Therefore, this elucidation of the contributions of block copolymer hydrophobicity during self-assembly may provide powerful new insight and strategies for programming the morphology and dimensions of block copolymer solution aggregates.

length of the would have no effect on the reactivity of the end groups), while branching along the cylindrical portions of the assemblies via fusion of worms would be much slower. However, if the growing polymer chains become too hydrophobic, a high interfacial energy favors spheres over cylinders.49,64 The balance between the increasing volume of the hydrophobic block and the inherent hydrophobicity of the block (i.e., the number of worm end-caps present) would then dictate worm length. Therefore, the worm elongation (and reversion back to micelles) observed here is controlled by the hydrophilic−hydrophobic balance of the core chains, where the interfacial energy between the core and corona acts to inhibit or induce worm elongation during polymerization.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02754. Additional figures, experimental details, worm characterization data, and transmission electron microscopy images (PDF)

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CONCLUSIONS New examples of aqueous PISA systems have been difficult to identify and effectively modulate due to the stringent monomer/polymer requirements. We have demonstrated that nanoparticle morphology is defined by the hydrophobic nature of the growing amphiphilic chains. Moreover, predetermined morphologies can be targeted by carefully controlling incorporation of hydrophilic comonomers during PISA. Insight into the contribution of chain solvophilicity during polymer self-assembly and reorganization provided an understanding of the delicate balance between entropic and enthalpic penalties during polymer self-assembly and reorganization. This concept led to the strategic synthesis of a variety of nanoparticles, including micelles, worms, micrometer-length worms, branched worms, and vesicles, and the elucidation of intermediate morphologies not observed previously during PISA (e.g.,

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]fl.edu (B.S.S.). ORCID

Brent S. Sumerlin: 0000-0001-5749-5444 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation (DMR-0846792 and DMR-1410223). G


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K.C.B. was supported by The Gulf of Mexico Research Initiative (GoMRI 2012-II-798). D.A.S. was supported by NSF CHE 1539347.

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