Facile Screening of Various Micellar Morphologies by Blending

Jun 21, 2017 - The self-assembly of macromolecules in selective solvents into defined nanostructures is one of the most studied fields in soft matter ...
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Facile Screening of Various Micellar Morphologies by Blending Miktoarm Stars and Diblock Copolymers Alexander A. Steinschulte,† Arjan P.H. Gelissen,† Andre Jung,†,‡ Monia Brugnoni,† Tobias Caumanns,§ Gudrun Lotze,∥ Joachim Mayer,§ Dmitry V. Pergushov,*,⊥ and Felix A. Plamper*,† †

Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, D-52056 Aachen, Germany GFE Central Facility for Electron Microscopy, RWTH Aachen University, Ahornstraße 55, D-52074 Aachen, Germany ∥ ESRF − The European Synchrotron Radiation Facility, ID02 - Time-Resolved Ultra Small-Angle X-Ray Scattering, 71, Avenue des Martyrs, CS40220, 38043 Grenoble Cedex 9, France ⊥ Department of Chemistry, M.V. Lomonosov Moscow State University, Leninskie Gory 1/3, 119991 Moscow, Russian Federation §

S Supporting Information *

ABSTRACT: A time-saving phase-diagram screening is introduced for the self-assembly of miktoarm star polymers with different arm numbers for the insoluble part. Agreeing with theory, all conventional micellar morphologies (spherical star-like micelles, cylindrical micelles and vesicles) can be accessed by adjusting the average arm number when blending miktoarm stars with diblock copolymers (at constant arm/ block lengths). Additionally, a rare clustered vesicle phase is detected. Hence, this approach permits an easy tuning of the equilibrium morphology and the size of the solvophobic domain. Such screening by scattering, ultracentrifugation, and electron microscopy techniques assists the targeted synthesis of miktoarm stars with a well-defined arm number, aimed at the morphology control of the nanostructures without blending. Specifically, we demonstrate a systematic variation of all classical micellar morphologies based on interpolyelectrolyte complexes (IPECs), consisting of a water-insoluble part formed by electrostatically coupled poly(styrenesulfonate) chains/quaternized poly(2-(dimethylamino)ethyl methacrylate) blocks, being stabilized by hydrophilic poly(ethylene oxide) blocks.

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way, stimuli-responsive behavior of self-assembled structures could be tuned.7 This enables a more facile way of fast screening for different properties and provides another tool for their flexible adjustment. On the other hand, blending can induce significant differences in the properties compared to the direct use of statistical or block copolymers with an intended solvophobic-solvophilic balance.8 In addition, this approach is exploited to access structures, which are not available if only one copolymer undergoes self-assembly.9−12 This could lead to a kinetic trapping, which preserves, for example, a mixture of structures. Hence, efforts need to be applied to obtain equilibrated structures such as solvent exchange from a common to a selective solvent1 or thin-film rehydration.13 In that context, complexation-induced self-assembly instead of segregationinduced self-assembly14 provides a facile way to circumvent these complications, as all polymeric components are wellsoluble under the conditions, at which eventually self-assembly occurs. This comprises electrostatically driven coassembly, also

he self-assembly of macromolecules in selective solvents into defined nanostructures is one of the most studied fields in soft matter research. This process is in most cases induced by an amphiphilicity of macromolecular building components.1 Hereby, the solvophobic segments undergo segregation and the subsequently formed aggregate is stabilized by the solvophilic segments.2 The equilibrium structure of such self-assemblies like spherical micelles, worm-like micelles and lamellae/vesicles is well understood from a theoretical point of view.3 In a simplified model, the actual self-assembled morphology depends on the ratio between lengths/volumes of the solvophilic and the solvophobic segments and can be comprehended by help of the packing parameter.1,4 Adjusting the morphology is important for the specific application (e.g., vesicles are suitable for transport and release, while cylindrical micelles have certain rheological properties). One promising method to control the self-assembled morphologies is blending chemical identical amphiphilic block copolymers with different ratios of lengths of solvophilic and solvophobic blocks. By such blending, one “average” block length ratio can be created. It was shown by Wright et al. that the self-assemblies obtained by this approach can lead to similar results as their directly synthesized counterparts.5,6 In a similar © XXXX American Chemical Society

Received: April 29, 2017 Accepted: June 12, 2017

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DOI: 10.1021/acsmacrolett.7b00328 ACS Macro Lett. 2017, 6, 711−715

Letter

ACS Macro Letters Scheme 1. IPECs for Everyonea

a (Left box) Janus-like monostar IPECs formed at Z = 0.5;16 (right box) Variation of the average arm number A (the number of cationic arms per PEO arm) by blending a PEO-b-qPDMAEMA diblock copolymer and a PEO-(qPDMAEMA)4.3 miktoarm star, followed by a subsequent complexation with PSSNa, which leads to different self-assembled structures. The morphology changes with increasing the A from spherical star-like micelles to cylindrical micelles and further to vesicles and finally to aggregated (partly merged) vesicular structures.

[polycationic charges] approaches 1), the coassembled monostar IPECs undergo self-assembly. In a specific case, the hierarchical process of matched co- and self-assembly of oppositely charged miktoarm stars and linear polymers toward unilamellar vesicles (polymersomes) was observed.16,18 The prevalence of vesicles for this specific example can be explained by an entropically favorable decrease in the stretching of the complexed (qPDMAEMA/PSS) insoluble segments when allowing the lamellar (vesicular) morphology. Thermodynamic arguments indicated that with an increasing number of cationic arms (which become insoluble after they complex with PSSNa) the window corresponding to the polymersomes in the phase diagram becomes broader, under the prerequisite of a constant ionic arm length and Z ≈ 1.18 This theory covers the self-assembly into various morphologies, including spherical star-like micelles typically made of diblock copolymers and cylindrical (worm- or rod-like) micelles (see Supporting Information).18,28 The latter can be found only in a narrow intermediate window in the phase diagram. In our case, the theory predicts such rod-like objects only for the miktoarm stars with a rather small number of cationic arms. To screen for such a variety of different morphologies experimentally (see Scheme 1, right side), the precision synthesis of various miktoarm star-shaped polymers with the same arm length but varying cationic arm numbers is cumbersome. Hence, we exploit a different approach for screening, which could finally instruct the precision synthesis to obtain the desired morphology of IPECs. A miktoarm star and a diblock copolymer were synthesized with the same PEO arm (block) length and very similar qPDMAEMA arm (block) length. Specifically, we use PEO114-(qPDMAEMA39)4.3 and PEO114-bqPDMAEMA44 (the subscripts indicate the number-average degrees of polymerization per arm/block (DPn) and the number of ionic arms of the miktoarm star; the diblock copolymer is considered as a “star-shaped” copolymer with an ionic arm number of 1; experimental details on the system can be found in the Supporting Information). Simple mixing of different amounts of the miktoarm star and the diblock copolymer prior to the interpolyelectrolyte complexation (on addition of PSSNa) leads to different apparent average numbers of cationic segments (arms + blocks) per PEO moiety. Within one set of experiments, the base-molar concentration of ammonium groups [N+] is kept constant over the whole examined range of the average cationic arm numbers A,

referred to as interpolyelectrolyte complexation, which typically takes place in aqueous media and yields so-called interpolyelectrolyte complexes (IPECs). IPECs are formed via complexing anionic and cationic chains, forming an insoluble complex domain because of mutual charge compensation. Hereby, the entropically favorable release of counterions is recognized as the main driving force of the process.15 Hence, the interaction strength between the polymeric components of an IPEC depends on the concentration of the background electrolyte (low molecular weight salt). We choose such a salt concentration (0.3 M NaCl), where rearrangements toward equilibrium can occur, but the extremes are prevented, for example, the formation of frozen coassemblies at low ionic strength16−18 or suppression of the electrostatic interaction between oppositely charged polymeric components at high ionic strength (which can be used as a release trigger).15,19−21 To now direct such a coassembly to IPECs within the colloidal size scale, it is essential to introduce a solvophilic (e.g., nonionic but hydrophilic) part, which is attached to at least one of the complexing polymeric components. The formed morphology of the IPECs depends then again on the ratio between volumes of the insoluble complex domain and the hydrophilic nonionic part. To date, a variety of different well-defined nanostructures like spherical micelles,17,22−24 cylindrical micelles,17,20,22−24 vesicles,18,20,22 and capsules25,26 are already accessible via interpolyelectrolyte complexation. This also includes multistimuli responsive systems.17,19,27 A conceptually new approach is to make use of bishydrophilic (ionic/nonionic) miktoarm stars, which can generate IPECs when interacting with oppositely charged polyions. We have investigated bis-hydrophilic cationic miktoarm stars, which comprise one poly(ethylene oxide) (PEO) arm and several quaternized poly(N,N-dimethylaminoethyl methacrylate) (qPDMAEMA) arms, interacting with short (DPn = 20) linear poly(sodium styrenesulfonate) (PSSNa).17−19 An excess of the miktoarm star complexing with such short polymeric counterions results in IPECs, which are coassemblies, containing only one star per particle (monostar complex). Hence, Janus-like monostar complexes can be observed due to the possible noncentrosymmetric nature of miktoarm stars (see Scheme 1, left side).16 When the charge of the star-shaped polymeric component becomes considerably compensated by chains of the oppositely charged linear polyelectrolyte (i.e., the charge-to-charge ratio of the polymeric components defined as Z = [polyanionic charges]/ 712

DOI: 10.1021/acsmacrolett.7b00328 ACS Macro Lett. 2017, 6, 711−715

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ACS Macro Letters

sediment too fast. This can be attributed to pronounced aggregation at these high A. In order to gain more structural insight and to unequivocally assign the regimes in Figure 1, we performed small angle X-ray scattering (SAXS) experiments (see Figure 2). Here, we see

changing only the PEO volume fraction (except stated otherwise). As demonstrated in the following, the screening reveals the formation of spherical micelles, cylindrical micelles, and vesicles upon increasing A (Scheme 1) for the complexation of PEO114(qPDMAEMA39)4.3/PEO114-b-qPDMAEMA44 mixtures with PSSNa (DPn = 20). The morphology obtained for a certain PEO114-(qPDMAEMA39)4.3/PEO114-b-qPDMAEMA44 mixture when complexing with the oppositely charged linear polymer is similar to the one obtained for a corresponding miktoarm star directly synthesized with the targeted number of cationic arms and their length. We emphasize that this blending approach has advantages compared to the common adjustment of the micellar morphology in block copolymer systems, which usually relies on variation of their block lengths, including blends of block copolymers with different block lengths. Specifically, the domain size of the insoluble complex part does not change considerably upon varying of A. We expect that the core size of the spherical/cylindrical micelles or the vesicular wall thickness will be unaffected by varying A within a certain morphology (provided that the amount of cationic charges incorporated into the insoluble core or wall is constant). This means that our approach could allow for a rather independent adjustment of the micellar morphology and the insoluble domain size, for example, thickness of the vesicular wall.18 For release applications, this envisions a fine-tuning of the permeability of the vesicular complex membrane (by varying the cationic arm length), while still preserving the vesicular morphology (at large cationic arm numbers). By help of analytical ultracentrifugation (AUC) and dynamic light scattering (DLS) (see Supporting Information), the number of PEO grafts N per co- and self-assembled complexes can be assessed in dependence of A (Figure 1). We can discern two plateau-like parts, one with A ∼ 1.2−1.7, one with A ∼ 2− 3, which might be a sign of coexistence of lighter and heavier (dominating) particles. Different micellar structures seem to prevail in these different regimes. At higher A, the sedimentation coefficient could not be detected as the particles

Figure 2. SAXS curves of IPECs (Z = 1; 0.3 M NaCl; [N+] = [SO3−] = 0.06 M) at different average cationic arm numbers A (the average number of qPDMAEMA arms per PEO arm; variation of A is achieved by blending PEO-b-qPDMAEMA with PEO-(qPDMAEMA)4.3 before the complexation with PSSNa); to obtain a better visibility of the curves, they are multiplied by 10 each; the curves with A from 1.0 to 1.5 are fitted with a combination of hard sphere and cylindrical fit model (1.0 A (purple), 1.2 A (dark blue), 1.3 A (light blue), 1.4 A (green), 1.5 A (light green)); the curves with A from 1.7 to 2.1 are fitted with a combination of cylindrical and vesicular fit models (1.7 A (dark yellow) and 2.1 A (orange)) and the curves with A from 2.5 to 3.8 are fitted with a vesicular fit model (2.5 A (red), 3.0 A (dark red), 3.8 A (brown)); all fits are presented by black lines.

indeed a continuous structural change when changing the average cationic arm number A. The IPEC with an A of 1.0 shows at low q a dependence with the slope close to q0 and the form factor can be fitted with a hard sphere form factor model. Upon increasing A, we observe an increase in the slope at low q to q−1 at A = 1.5. A dependence with the slope of q−1 at low q indicates cylindrical objects. The best fit for A = 1.5 is obtained with a cylindrical form factor model. The scattering of the IPECs with A between 1 and 1.5 can be fitted with a combination of fits for hard spheres and cylinders. Hereby, the contribution of the cylindrical fit increases with increasing A, to a maximum of pure cylinders at A = 1.5 (Table S2). At A = 1.7 and 2.1, the scattering patterns demonstrate a minimum, which can be attributed to the formation of hollow structures with a small radius. A mixture of cylindrical and vesicular (lamellar) structures might be responsible for the observed scattering (see Tables S3 and S5). The scattering curves for A = 2.5 and higher can be fitted best with a hollow sphere (vesicle) form factor. All fit results are summarized in the Supporting Information (see also Table S4), which are consistent with the theoretical prediction of the respective arm number for each morphological transition.

Figure 1. Estimation of the number of PEO grafts per self-assembled complex particle N (Z = 1; 0.3 M NaCl) with an increasing apparent arm number A of cationic arms per nonionic arms (variation of A is achieved by mixing PEO-b-qPDMAEMA with PEO-(qPDMAEMA)4.3 before complexation with PSSNa; results obtained by a combined DLS and AUC study; regimes are labeled with the individual structures (spherical star-like micelles, cylindrical micelles, and vesicles) obtained by SAXS and TEM/cryo-TEM; mixed phases comprising coexisting individual structures are observed between the indicated individual structures; line is a guide to the eye). 713

DOI: 10.1021/acsmacrolett.7b00328 ACS Macro Lett. 2017, 6, 711−715

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ACS Macro Letters

(∼0.06 nm−2) and for the vesicular species (∼0.02 nm−2), which differs by a factor of ∼3. Hence, these more and more diluted coronal PEO chains do not provide sufficient steric stabilization at A > 3. In contrast, the miktoarm approach gives access to a rare phase containing aggregated/partly merged vesicles, which is hard to obtain otherwise. These findings can be compared to vesicles obtained from the same system (a miktoarm star consisting of one PEO arm of the same length and qPDMAEMA arms of the similar length complexed with the same PSSNa) synthesized directly to have 2.5 cationic arms on average. This miktoarm star forms colloidally stable (over weeks) vesicles having nearly the same characteristics as the polymersomes described above.18 All these results indicate that a facile screening by help of blending can direct the synthesis to obtain IPECs with the desired morphology. In conclusion, we show that it is possible to achieve different equilibrium micellar/vesicular structures by just varying the relative amounts29 of the diblock copolymer and miktoarm star at the same conditions (e.g., without changing pH, salt concentration or temperature). In more detail, this flexibility provides the facile access to different micellar IPECs with a whole variety of morphologies, which are otherwise only accessible with tedious synthesis protocols. Hence, this modular approach enables the formation of Janus-like monostar IPECs at a charge to charge ratio Z apart from one (for complexes with pure miktoarm stars), while spherical micelles, cylindrical micelles, vesicular structures (polymersomes), or even supravesicular assemblies prevail at Z ≈ 1. Then, the actual morphology is just determined by the blending ratio of the miktoarm star and the corresponding diblock copolymer. Hence, we present a very versatile system, easily adaptable to each specific need. These results envision, for example, further applications in rationally engineered release systems, where permeability needs to be controlled independently.19

Comparing the wall thickness of the insoluble complex walls (see the Supporting Information), we find similar thicknesses for all hollow structures, irrespective of A (∼10 nm). We also see a similar diameter for the insoluble complex cores of the spherical/cylindrical micelles (∼18 nm). Hence, these findings point to the beneficial possibility to independently adjust the micellar morphology (by fine-tuning the average cationic arm number) and the size of the insoluble complex domain (by finetuning the cationic arm length). The spherical and cylindrical micelles as well as the vesicles are also visualized by TEM, showing that we can obtain diverse morphologies (Figure 3). The spherical micelles observed for A

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Figure 3. TEM images of IPECs (Z = 1; 0.3 M NaCl; [N+] = [SO3−] = 0.002 M; scale bars 200 nm): (a) TEM of spherical micelles, A = 1.0; (b) TEM of cylindrical micelles, A = 1.5; (c) cryo-TEM of vesicles, A = 2.5.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00328. Preparation details; characterization details; details on the combined DLS and AUC study to determine the aggregation number; and detailed SAXS evaluation (PDF).

= 1.0 have a radius of about 10 nm, which corresponds to the size determined by SAXS. The observed cylinders (Figure 3b, A = 1.5) have a length of about 100 nm and a radius of about 10 nm, which is also in good agreement with the data obtained for such systems by SAXS. Please note that TEM in the dry state is recorded after removal of the background electrolyte by dialysis for spherical and cylindrical micelles (which will freeze the micellar structures and eliminate the salt crystals occurring during drying of the sample on a solid support). In contrast, all AUC, DLS, and SAXS data are obtained under equilibrium conditions (0.3 M NaCl), as it was mimicked for the cryo-TEM images obtained for vesicular samples. The vesicles (Figure 3c, A = 2.5) show high polydispersity in size (from 20 to 70 nm radius), which corresponds well to the SAXS results. The wall thickness of the vesicles is on the other hand quite uniform at around 10 nm. One effect, which cannot be seen from the SAXS curves due to the limitation in the q range, is the aggregation of vesicles as soon as we come to large cationic arm numbers (A > 3). The number of PEO segments is not enough to avoid aggregation or even merging of vesicles, which is visible in microscopy images (Figure S3, A = 4.3). Such a clustering of vesicles can be also understood when comparing the PEO grafting density for the spherical star-like micelles

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +49-241-8094750. *E-mail: [email protected]. Tel.: +7-495-9393124 ORCID

Felix A. Plamper: 0000-0002-0762-6095 Present Address ‡

Institut für Bauforschung der RWTH Aachen University, Schinkelstr.3, 52062 Aachen, Germany. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. 714

DOI: 10.1021/acsmacrolett.7b00328 ACS Macro Lett. 2017, 6, 711−715

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ACS Macro Letters Funding

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Deutsche Forschungsgemeinschaft (DFG) Project Grant PL 571/3−2. Russian Foundation for Basic Reseach (Project Number 15−03−06974). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Deutsche Forschungsgemeinschaft (DFG; Project Grant PL 571/3-2 and SFB 985) and by the Russian Foundation for Basic Reseach (Project Number 1503-06974). We thank Walter Richtering for continuous support and Oleg Borisov for fruitful discussions.

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ABBREVIATIONS A, average polycationic arm number; IPEC, interpolyelectrolyte complex; PSSNa, poly(sodium styrenesulfonate); PEO, poly(ethylene oxide); qPDMAEMA, quaternized poly(N,N-dimethylaminoethyl methacrylate); SAXS, small angle X-ray scattering; TEM, transmission electron microscopy

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DOI: 10.1021/acsmacrolett.7b00328 ACS Macro Lett. 2017, 6, 711−715