Hydrophilic/Hydrophobic Balance

According to the mechanism we presented for a µ-BVT miktoarm star terpolymer³¹ (in-cluded as µ-BV2T in Table 5-1), we hypothesized that such hierarchical processes should generally be applicable to all miktoarm star systems containing P2VPq segments. It is well-established that the molecular composition drastically affects the solution behavior of amphiphilic block copolymers, i.e. that spherical micelles, cylindrical micelles and ves-icles are formed with decreasing fraction of the corona block.^7,11,36 Here, three polybutadiene-b-poly(2-vinylpyridine) diblock copolymers having an alkyne-function between the blocks and with comparable molecular weights but decreasing P2VP frac-tion (BV1, BV2 and BV3) were conjugated with the same azido-funcfrac-tionalized polysty-rene homopolymer PS-N3. In that way, µ-BVS miktoarm star terpolymers with different weight fractions of the hydrophilic block (whydophilic) were synthesized, the details of which are given in Table 5-1.

The P2VP segments of all three µ-BVS miktoarm star terpolymers were first quaternized with methyl iodide resulting in µ-BVqS with a permanently charged and hy-drophilic P2VPq arm. After dialysis from dioxane into water (without added iodine), dif-ferent micellar structures were visualized by TEM (Figure 5-1). The contrast primarily derives from the P2VPq phase, which is inherently stained with iodide. The PB and PS phases exhibit only low contrast and cannot be distinguished. Spherical micelles were observed for µ-BVq1S (dmicelle ~20.5 ± 2.0 nm, Figure 5-1A) and µ-BVq2S (dmicelle ~27.0 ± 1.5 nm, Figure 5-1B), whereas in the latter case also some linear assemblies could be seen (inset in Figure 5-1B). Due to the shorter P2VPq block these micelles are slightly larger than those of µ-BVq1S. In contrast, µ-BVq3S with only 23 wt-% hydrophilic seg-ments mainly yielded vesicular aggregates with some cylindrical micelles (Figure 5-1C) being present as well. This was supported by cryo-TEM investigations (inset in Figure 5-1C and Figure 5-S5A). As expected for vesicles; the size-distribution was rather broad (~50 to 200 nm in diameter from cryo-TEM). The dimensions of the vesicles were further confirmed by DLS (R_h,app = 117 nm, D.I. = 0.14 Figure 5-S6). Both in TEM and cryo-TEM

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145 some spherical structures were also detected, mostly at the end or kinks of cylindrical micelles, indicating fusion/fission processes during vesicle formation.³⁷ Thus, reducing the fraction of the solubilizing block expectedly induces a transition from spherical mi-celles (µ-BVq1S, µ-BVq2S) to vesicles coexisting with cylinders (µ-BVq3S).

Figure 5-1. TEM micrographs of micellar aggregates of µ-BVq1S (A), µ-BVq2S (B), and µ-BVq3S (C) in water.

No staining was performed. The inset in (C) displays a cryo-TEM micrograph of the same sample. The con-centration was 0.2 g/L for TEM and 0.41 g/L for the cryo-TEM in (C). The schematic illustrations represent the main aggregation form.

When iodine (0.25 equiv with respect to P2VPq) was added to the µ-BVqS dioxane solu-tions prior to dialysis into water, drastic changes regarding micellar morphology are ob-served (Figure 5-2). Visual inspection showed an increase of the turbidity, clearly indicat-ing the formation of larger structures. For µ-BVq1S, mainly spherical micelles and a small fraction of cylinders were present, but also large superstructures with a periodic lamellar fine structure (Figure 5-2A). These structures are analogous to our previous findings for µ-BVqT and the observations are confirmed by cryo-TEM (Figures 5-2D, 5-S7A). Here, superstructures of up to several µm in length were found and the P2VPq segments are only visible at the sheet edges, probably due to an increased segmental density directly at the surface (see gray scale analysis in the inset in Figure 5-2D). In contrast to the

“woodlouse” structure reported earlier,³¹ the superstructures consist of rather stacked lamellae instead of back-folded and bent lamellae. This is most probably caused by the higher hydrophilic weight fraction whydophilic (0.60 for BVq1S as compared to 0.38 for µ-BVqT) which enables sufficient shielding of the hydrophobic edges. Comparably stacked/segmented structures were also reported for triblock terpolymer systems

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where attractive interactions of the terminal stabilizing block were induced by interac-tion with divalent counterions²⁹ or decreased solvent quality.³⁸ In case of µ-BVq2S, again a mixture of spherical micelles (smaller fraction) and cylinders or superstructures of disk-like aggregates (majority fraction) was found. In some cases, also multilamellar vesicles were observed and cryo-TEM confirms the coexistence of all these morphologies (Figure 5-2B, 5-2E and Figure 5-S7B). In contrast to µ-BVq1S, the periodicity of the inner seg-mented fine structure is not as pronounced.

Figure 5-2. TEM (A, B, C) and cryo-TEM micrographs (D, E, F) of micellar aggregates obtained from dialysis of µ-BVq1S (A, D), µ-BVq2S (B, E) and µ-BVq3S (C, F) to water in the presence of 0.25 equiv I₂ (with respect to P2VPq units). Except for the inset in (C), where OsO₄ staining (selective for PB) was performed, the contrast results from the quaternized P2VP corona. The concentration was 0.2 g/L for TEM and 0.35 g/L for cryo-TEM. With decreasing length of the hydrophilic block a transition from lamellar structures to multilamellar vesicles is observed, as illustrated in the scheme at the bottom.

However, for µ-BVq3S, the addition of iodine induced the formation of multilamellar vesicles with sizes up to 500 nm (Figure 5-2C). As a consequence of the low Tg of the PB block, which represents the majority phase of the wall, the structures are rather flat-tened on the substrate in the dried state. In contrast, cryo-TEM (Figure 2F, Figure 5-S5B to D) reveals intact multilamellar vesicles with up to 5 lamellae and, additionally, the presence of cylindrical micelles. In some cases also fusion/fission processes during

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147 cle formation can be visualized (Figure 5-S5B). The fusion-induced increased size and high dispersity of the vesicles with diameters of 200 to over 500 nm (as observed from cryo-TEM) is corroborated by DLS studies (Rh,app = 236 nm, D.I. = 0.60, DLS in Figure 5-S6).

When higher amounts of iodine (0.40 equiv) were added before dialysis, precipitation occurred for µ-BVq1S and µ-BVq2S, most probably due to insufficient coronal stabiliza-tion of the disk-like building units, whereas µ-BVq3S yielded colloidally stable solustabiliza-tions.

TEM revealed huge, disperse aggregates of vesicles larger than 500 nm in diameter, which sometimes appear in a stacked manner (Figure 5-3A). For some features a lamel-lar inner structure, similamel-lar to the “woodlice” was visible as indicated by the white arrow in Figure 5-3A. Cryo-TEM confirms the coexistence of multilamellar vesicles and elongat-ed structures with an internal lamellar arrangement perpendicular to the longitudinal axis (Figure 5-3B). DLS measurements support the increased dimensions (R_h,app = 391 nm, D.I. = 0.12, DLS in Figure 5-S6). Since we did not observe any huge aggregates of several micrometers in length in cryo-TEM, we attribute their presence in the dried state to drying effects. Overall, the multilamellar constitution of the vesicles is much more pronounced than for the sample with 0.25 equiv of I₂ (Figure 5-2F). The increased amount of triiodide accordingly favors fusion into vesicles filled with deformed vesicles due to decreased coronal solubility (lower inset in Figure 5-3B). At a certain threshold value we assume the internal crowding of the vesicles to force rearrangement into elon-gated structures with a parallel aligned interior to decrease surface curvature (upper inset in Figure 5-3B). These structures are quite similar to the “woodlouse” structures reported for µ-BVq2T.³¹

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Figure 5-3. TEM (A) and cryo-TEM (B) micrographs of vesicular structures obtained from the dialysis of µ-BVq3S with 0.40 equiv I₂ to water. For the micrograph at the right side of (A) the polybutadiene phase of the core was selectively stained with OsO4. In case of TEM the concentration was 0.2 g/L and for cryo-TEM 0.50 g/L, respectively.

The composition of the core segment for superstructures formed by µ-BVq3S will be shortly discussed according to TEM data. Although the phase segregation parameter, χN

= 15.8, would suggest the PS and PB arms to be within the strong segregation re-gime,^39,40 no compartmentalization of the vesicle walls was observed after selective staining of the PB phase with OsO₄ (inset in Figure 5-2C and 5-3A). Additionally, even despite a lower χN of 8.8, differential scanning calorimetry (DSC) of µ-BV2S implies phase segregation between PS and PB as the glass transition of PB at -2 °C is detected (Figure 5-S8). We hypothesize that compartmentalization is not visible in the insets in Figure 5-2C and 5-3A, because the shorter PS block (DP = 36) is shielding the PB phase (DP = 223) from the P2VPq by the formation of a thin continuous PS lamella. This situa-tion is hard to visualize in TEM as staining with RuO4 simultaneously enhances the con-trast in the P2VPq phase. Similar findings were made by Hückstädt et al. for bulk mor-phologies of SBV miktoarm star terpolymers with larger volume fractions of PB than PS.⁴¹

Nevertheless, for all three miktoarm stars different hierarchical superstructures were observed when iodine was added prior to dialysis to water. Whereas for BVq1S and µ-BVq1S similar processes seem to operate as identified for µ-BVqT earlier (aggregation of spherical micelles into cylinders or disks and superstructures thereof),³¹ in case of µ-BVq3S (displaying the lowest fraction of P2Vq) the fusion of vesicles into multilamellar structures with up to 500 nm in diameter took place, followed by partial rearrangement

5 – Triiodide-Directed Self-Assembly of Different Miktoarm Star Polymers

149 into “woodlouse”-like structures upon increasing the triiodide content. Even though in all cases discussed rather non-ergodic structures or, more precisely, mixtures of differ-ent morphologies were found, our methodology of counterion-mediated hierarchical self-assembly is clearly applicable to µ-BVqS miktoarm star terpolymers of different composition.

The P2VP/P2VPq segments have the added advantage that they are capable to coor-dinate metal ions.^42,43 We exploited this to directly visualize the P2VPq domains within segmented disk-like structures formed by µ-BVq1S. Therefore, HAuCl4 was added to a micellar solution obtained by dialysis with 0.25 equiv I2 and subsequently reduced with NaBH4. The resulting hybrid superstructures are seen in the cryo-TEM micrograph in Figure 5-4A, with the majority of the Au nanoparticles being located within alternating compartments. Further, a preferential location of the Au nanoparticles at the edges of the sheets is seen. The size distribution of the nanoparticles is quite broad, with diame-ters between 2-14 nm (see inset in Figure 5-4A). Besides the visualization in TEM the successful nanoparticle generation was also proven by UV-vis measurements (Figure 5-4B). The observed surface plasmon resonance band at 546 nm is blue-shifted as com-pared to the ~525 nm expected from the size range of the nanoparticles.⁴⁴ This might be explained by the dense NP population at the surface of the individual sheets and the decreased interparticle distance, which influences the position of the surface plasmon resonance band.⁴⁵

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Figure 5-4. (A) Cryo-TEM micrograph of micellar aggregates from µ-BVq1S (c ~0.35 g/L) with in-situ formed Au nanoparticles and (B) UV-vis spectrum of the aqueous micellar solution.