Note pubs.acs.org/Macromolecules
Bicontinuous Nanospheres from Simple Amorphous Amphiphilic Diblock Copolymers Beulah E. McKenzie,† Joel̈ F. de Visser,† Heiner Friedrich,† Maarten J. M. Wirix,† Paul H. H. Bomans,† Gijsbertus de With,† Simon J. Holder,‡ and Nico A. J. M. Sommerdijk*,† †
Laboratory of Materials and Interface Chemistry and Soft Matter Cryo-TEM Research Unit, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ‡ Functional Materials Group, School of Physical Sciences, University of Kent, Canterbury, Kent, CT2 7NH, U.K. S Supporting Information *
ABSTRACT: Bicontinuous nanospheres have been observed (although rarely) from a variety of block copolymers with architectural and compositional complexity, and often in the presence of additives. Unlocking key features involved in their formation presents possibilities for bicontinuous aggregates with varied functionality and application. An attractive prospect is the ability to form them from much simpler polymeric structures derived from facile syntheses. To that end, we herein report the formation of bicontinuous aggregates from simple amorphous amphiphilic diblock copolymers of poly(ethylene oxide)-b-poly(n-butyl methacrylate), analogous to our previous report of the same from a semicrystalline comb-like block copolymer. Moreover, we demonstrate that polymorphism can be achieved by altering the relative block proportions and the nonselective cosolvent. We find that the polymeric structure is not the dominating factor in the formation of bicontinuous nanospheres but that the choice of cosolvent for the hydrophilic block appears to have greater influence on determining the end morphology.
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INTRODUCTION In solution, amphiphilic block copolymers can form a variety of self-assembled morphologies of which the most common examples are spherical micelles, cylindrical micelles and vesicles (polymersomes),1−3 but more complex morphologies such as disk-like,4,5 toroidal,6−8 and helical structures9,10 have also been reported.11 In addition, there are a small number of accounts on the formation of discrete bicontinuous aggregates. These bicontinuous polymer nanospheres (BPNs) have been reported from block copolymers with little commonality in their molecular structure. They have been observed from triblock,12−14 ionic,14,15 single and double comb-like16,17 block copolymers and janus dendrimers18the majority in the presence of additives and solvent mixtures. We have previously reported the formation of BPNs from a semicrystalline comblike block copolymer of PEO−poly(octadecyl methacrylate) (PEO-b-PODMA)16 with side-chains consisting of 18 carbon atoms, in which the formation was induced by traversing the thermal transitions. Gaining insight into the parameters that determine the formation of BPNswhich (similar to their bulk counterparts)19−23 have potential for application in photovoltaics24 and as templating structures,25−28presents possibilities for BPNs with different chemistries and functionalities.29,30 Herein, we investigate the requirements for the formation of BPNs and present their formation from simple amorphous © 2013 American Chemical Society
diblock copolymers of PEO−poly(n-butyl methacrylate) (PEOb-PBMA) with four rather than 18 carbons in the side chain. The comparison with the BPNs previously reported from a semicrystalline comb-like block copolymer16 infers that neither the structural complexity nor crystallinity is a prerequisite for the evolution of BPNs. Furthermore, we show that the formation of the BPNs mainly depends on the relative PEO content of the block copolymers, as well as on the choice of the nonselective cosolvent.
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EXPERIMENTAL SECTION
Three PEO-b-PBMA block copolymers (PB1, PEO52-b-PBMA110, PEO = 14 wt %; PB2, PEO52-b-PBMA86, PEO = 17 wt %; PB3, PEO52-b-PBMA26, PEO = 40 wt %) were synthesized via copper(I) mediated atom transfer radical polymerization from a PEO macroinitiator (Mw = 2,500 kDa; Mw/Mn = 1.07). Aqueous dispersions were prepared in a procedure analogous to that of PEO39-b-PODMA17.16 Typically, 50 mg of polymer was dissolved in THF at 35 °C, followed by the slow addition of water to form a final concentration of 5g/L. The subsequent dispersion was dialyzed against water at 35 °C for 24 h to remove the THF (see Supporting Information for further details). The dispersion was cooled to 5 °C for comparison with the behavior of PEO39-b-PODMA17, which only produces BPNs below the Received: September 24, 2013 Revised: November 11, 2013 Published: December 3, 2013 9845
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PODMA Tc (20 °C). For cryoTEM analysis, 3 μL of the dispersion was deposited onto a holey carbon grid, blotted to create a thin film, and vitrified at 5 °C by plunging into liquid ethane.
smaller fraction of bilayered nanospheres of sizes between 50 and 150 nm. The bilayered nanospheres exhibit some internal phase separationthe morphology of which could not be fully resolved (Figure 1D). As judged from the 2D projection images, for PB1 (14 wt % PEO) the internally structured bicontinuous aggregates occasionally also showed the presence of some lamellar regions within the aggregates, similar to that reported for PEO39-b-PODMA1716 (Figure 1A). Nanospheres of PB2 (17 wt % PEO) were analyzed with cryo-electron tomography (cryoET), which revealed internal bicontinuity (Figure 2). Segmentation further showed that the BPNs also contain domains with the characteristics of an inverse micellar phase (Figure 2D). The pore diameter is 11 nm (± 2), as was measured from the 3D reconstructions smaller than that previously reported for BPNs of PEO-bPODMA (20 nm). This may be attributable to the difference in PEO content of PEO52-b-PBMA86 and PEO39-b-PODMA17 (17 and 25 wt % respectively). Aqueous dispersions of PB3 (40 wt %) yielded octopus-like morphologies: flattened bilayer sheets with cylindrical protrusions capped with spherical and toroidal structures (Figure 3A). Similar structures have previously been reported
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RESULTS AND DISCUSSION We observed that for PB1and PB2 (PEO contents ≤17 wt %) bicontinuous nanospheres with sizes of 100−750 nm are formed, which possess structure similar to the BPNs we have previously reported from 0.5 wt % dispersions of PEO39-bPODMA17 (Figure 1 A-C).16 BPNs of PB1 coexisted with a
Figure 1. CryoTEM micrographs of aggregates from PB1 (PEO52-bPBMA110) showing (A and B) bicontinuous nanospheres, (C) nanospheres with internal phase separation, and (D) bilayered nanospheres with internal phase separation. Scale bars represent 100 nm.
Figure 3. CryoTEM images of aggregates of PB3 (PEO52-b-PBMA26), showing (A) the octopus-like structures with cylindrical micellar protrusions and (B) flattened and ordered vesicular structures. Scale bars represent 100 nm.
Figure 2. Analysis of a PB2 (PEO52-b-PBMA86) aggregate: (A) cryoTEM image of a bicontinuous nanosphere, showing the internal phase separation; (B) gallery of z slices showing different cross sections of a 3D SIRT reconstruction of a tomographic series recorded from the vitrified film in part A; (C) the xy, xz, and yz slices from the center of the aggregate; (D) computer generated isosurface of a segmented volume made from the aggregate shown in part A (the yellow represents the surface between the two phases). Scale bars represent 100 nm. 9846
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for poly(ethylene oxide)−polybutylene block copolymers and described as an intermediate between the cylindrical micelle and lamellar morphologies.31−34 Surprisingly, groups of much smaller, organized flattened vesicular structures were also observed (Figure 3B). The apparent interaction between these previously unobserved morphologies currently cannot be explained when the chemistry of the block copolymer is considered, and thus requires further investigation. The relative block composition dependence of the aggregate morphology is well established, with the general observation that spherical micelles, cylindrical micelles and vesicles occur with decreasing hydrophilic content in the polymer (45% to 25%), and inverted phases form below a hydrophilic fraction of 25% in accordance with the classical packing parameter model.2,35 The BPNs presented here have hydrophilic contents of ≤17 wt %, and so this observation may be expected. Morphological complexity is more often achieved by significant changes to the block copolymer architecture (e.g., multiblock3,36,37 and miktoarm38,39 block copolymers). The majority of BPNs reported in the literature are formed from different types of ABC triblock copolymers,12−14,40 suggesting that structural complexity is a prerequisite for their formation. In line with this, we have previously postulated that the voluminous hydrophobic PODMA comb block was integral in the formation of the BPNs formed by the comb-like block copolymer PEO-b-PODMA (AB(C));16 however, the above results show that BPNs can be formed from simple diblock copolymers without additional architectural or chemical complexity, which possess PEO contents that, according to previous reports, promote the formation of inverted phases.41−43 Therefore, achieving the necessary relative block composition is of greater importance than the level of structural complexity. The fact that PB1 and PB2 (14 and 17 wt % PEO respectively) are amorphous, and have Tg’s of 9 and 14 °C respectively (see Supporting Information), also demonstrates that the semicrystalline nature of the PODMA is not a determining factor in the formation of PEO-b-PODMA BPNs. The loss of the bicontinuous structure at temperatures above the PODMA melting point may be due to the simultaneous increase in hydrophobic volume of the PODMA blocks, disturbing the hydrophilic−hydrophobic balance required for the BPN formation. A comparison with the morphologies of the nanospheres of PB1 and PB2 at elevated temperatures will be part of future investigations. It is also notable that the majority of BPNs reported are obtained from organic solvent−water mixtures (namely, THF, DMF and DMSO).12,14,44,45 The degree of affinity of the cosolvent to the respective blocks and subsequent degree of swelling within them is therefore of significance for the formation of these bicontinuous nanospheres. To investigate the role of the cosolvent in selecting a specific morphology a dispersion of PB2 was prepared, replacing THF with dioxane. This yielded lamellar structures: multilamellar vesicles (Figure 4A), flattened bilayers (Figure 4B), and aggregates with internal twisted lamellar structure (Figure 4C), as well as the previously described internally filled vesicular structures. To understand the influence of the solvent on the morphologies formed, the relative affinities of the solvent for the two different polymer blocks should be considered. These affinities can be expressed by their solubility parameters (δ) which are 18.6, 20.5, 21.5, and 17.8 MPa1/2 for THF, dioxane, PEO and PBMA, respectively.46 Therefore, although the hydrophobic PBMA block is well
Figure 4. CryoTEM images of aggregates of PB2 (PEO52-b-PBMA86) formed from dioxane-water mixes. Key: (A) multilamellar vesicles, (B) a flattened bilayer surrounding a multilamellar vesicle, and (C) an aggregate with internal twisted lamellar morphology; (D) a representation of the difference in the unimer segment in THF and dioxane, signifying the corresponding change in morphology from bicontinuous nanospheres to lamellar structures. Scale bars represent 100 nm.
solubilized by both THF and dioxane, it has better solubility in THF as the δ values are closer. Concurrently, PEO is better solubilized in dioxane compared to THF and will therefore be more swollen in this solvent, effectively increasing the volume within the hydrophilic headgroup during the aggregate formation (Figure 4D). Lamellar structures then form as the increased volume in the PEO block suppresses the inverse curvature imposed by the high block asymmetry caused by the large PBMA content. Increasing the hydrophilic block volume increases the repulsion between the hydrophilic chains,47−49 thus curvature toward the PEO interface is restored. Therefore, for PB2 a cosolvent with higher affinity for the hydrophobic block seems key to the formation of bicontinuous nanospheres. More generally speaking, our results show that the use of solvents and additives is an important parameter to obtain the specific block volume required for BPNs to occur, which may not be easily attainable through polymer synthesis alone. Considering the small number of examples of BPN formation in the literature also suggests that the parameter space for these conditions is narrow, and that a delicate cooperation between structure and preparation conditions is at play.
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CONCLUSIONS In conclusion, we have shown that nanospheres with internal bicontinuous morphology can be obtained from simple amorphous diblock copolymers of PEO-b-PBMA. These BPNs occur when the PEO content within the block copolymer is ≤17 wt %. The simple diblock architecture shows that neither structural complexity, nor crystallinity within the block copolymer is a necessity for BPN formation. Moreover, the bicontinuous morphology is lost when the cosolvent is changed from THF to dioxane, which shows that besides a proper 9847
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hydrophilic−hydrophobic balance also solvent selectivity is an important parameter in attaining the BPN morphology. Furthermore, it suggests that bicontinuous nanospheres can be obtained from very structurally different block copolymers simply by taking the solvent parameters into consideration, and leads to exciting possibilities for their development from block copolymers with different chemistry to yield soft matter with specified functionality and ultimately application.
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ASSOCIATED CONTENT
S Supporting Information *
Materials and methods, synthesis of PEO macroinitiator and PEO-b-PBMA block copolymers, preparation of aqueous dispersions, molecular weight parameters by 1H NMR and SEC, DSC data for the block copolymers, comparison of particle size parameters from DLS and cryoTEM, 1H NMR spectra of the macroinitiator and block copolymers, DLS number-average curves of the PEO-b-PBMA dispersions and electron tomography movies of the reconstructed nanospheres. This material is available free of charge via the Internet at http://pubs.acs.org/.
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AUTHOR INFORMATION
Corresponding Author
*(N.A.J.M.S) E-mail:
[email protected]. Telephone: +31(0)40 247 5870. Notes
The authors declare no competing financial interest
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ACKNOWLEDGMENTS This work was funded by The Netherlands Organisation for Scientific Research (NWO).
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REFERENCES
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