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Synthesis and Characterization of Block Copolymers
For the sake of brevity, a shorthand notation is utilized throughout this article to describe the various block copolymers. Thus G, H, and B are used to represent glycerol monomethacrylate (GMA), 2-hydroxypropyl methacrylate (HPMA), and benzyl methacrylate (BzMA), respectively. Hence GxHyBz denotes poly(glycerol monomethacrylate-block-2-hydroxypropyl methacrylate-block-benzyl methacrylate), where x, y, and z indicate the mean degrees of polymerization (DP) of the three respective blocks.
The initial RAFT solution polymerization of GMA was conducted in ethanol at 70 °C to generate a near-monodisperse G37 macromolecular chain transfer agent (macro-CTA) (Mw/Mn = 1.19; see Figure S1 and Table 1). After purification, this water-soluble macro-CTA was utilized for the in situ RAFT aqueous solution polymerization of HPMA at 15% w/w solids, yielding a 100 g batch of G37H60 diblock copolymer precursor (see Figure 1a). 1H NMR studies indicated that more than 99% HPMA conversion was achieved within 2 h at 70 °C (see Figure S2), as expected from previous studies.34 Gel permeation chromatography (GPC) studies indicated that a near-monodisperse diblock copolymer was obtained with high blocking efficiencies and minimal macro-CTA contamination (Mw/Mn = 1.14; see Figure S1 and Table 1). The GPC trace was unimodal but a high molecular weight shoulder was discernible, which has been attributed to low levels of dimethacrylate impurity in the HPMA monomer (approximately 0.07 mol % as judged by HPLC analysis); this results in light branching of the PHPMA chains. Dynamic light scattering (DLS) studies of this G37H60 diblock copolymer reveal a relatively low count rate of 50 kcps, and 1H NMR studies confirm that the PHPMA block is fully soluble in water (see Figure S3), suggesting that self-assembly does not occur for this relatively short PHPMA block.
Synthesis of (a) G37H60B10–550 triblock copolymer via RAFT aqueous solution polymerization of HPMA followed by RAFT seeded emulsion polymerization of BzMA and (b) G92B28 diblock copolymer prepared via RAFT aqueous emulsion polymerization of BzMA....
Summary of 1H NMR-Derived Monomer Conversion, Apparent DLS Hydrodynamic Diameter (Dh) and Polydispersity, Number-Average Molecular Weight (Mn), and Polydispersity (Mw/Mn) Determined for a G37 Macro-CTA, a G37H60 Diblock Copolymer Precursor, a Series of...
Furthermore, no nanoparticles can be observed by TEM (see image in Figure 2), which again indicates that the PHPMA block is not sufficiently long to induce micellar nucleation. This is consistent with observations made by Blanazs and co-workers, who found that a minimum PHPMA DP of around 90 was required to induce nucleation when using a PGMA47 macro-CTA.34 However, it should be noted that this minimum critical DP is expected to be rather sensitive to the precise PISA formulation.37
TEM images obtained for dried dilute aqueous dispersions of the G37H60 diblock copolymer precursor, a series of seven G37H60Bz triblock copolymers (where z ranges from 10 to 550), the G37H90 diblock copolymer worms, and the G92B28 diblock copolymer spheres....
This G37H60 diblock copolymer precursor was then utilized as a macro-CTA for the RAFT seeded emulsion polymerization of BzMA at 70 °C to produce a series of seven G37H60Bz triblock copolymers, where z ranged from 10 to 550 (see Figure 1a). 1H NMR studies confirmed that BzMA conversions greater than 92% were obtained in each case (see Figure S2 and Table 1). Dimethylformamide (DMF) GPC studies indicated that near-monodisperse triblock copolymers were obtained (Mw/Mn < 1.20, see Table 1) with high blocking efficiencies (see Figure S1). TEM images shown in Figure 2 and DLS studies (see Table 1) indicated that spheres with a hydrodynamic diameter (Dh) of 41 nm were formed when z = 10; thus chain extension with just 10 units of BzMA is sufficient to induce micellar nucleation. When targeting a PBzMA DP of 30 (and achieving a DP of 28), TEM studies indicated the formation of highly anisotropic worms (Figure 2), similar to those reported recently.36
These G37H60B28 worms were further characterized by SAXS. The worm model38−40 provided an excellent fit to the SAXS pattern over six orders of magnitude of X-ray scattering intensity (see Figure S4a). The mean worm contour length (Lw) was determined to be 653 nm, which is consistent with TEM observations. Assuming a circular worm cross-section, the mean worm width (Ww), was calculated to be 25.6 ± 1.7 nm, which is also consistent with that estimated from TEM images (for which Ww = 24.2 ± 3.2 nm), where Ww = 2Rsw + 4Rg, with Rsw representing the radius of the worm core cross section and Rg representing the radius of gyration of the corona chains. The Rg of the G37 corona block of these worms was determined to be 1.7 nm from the data fit to the SAXS pattern (see Figure 4a). This experimental value is comparable to a theoretical estimate: the projected contour length of a single GMA monomer is 0.255 nm (two carbon bonds in an all-trans conformation), the total contour length of a G37 block, LPGMA = 37 × 0.255 nm = 9.44 nm, and the literature value of the Kuhn length for poly(methyl methacrylate) is 1.53 nm,41 resulting in an Rg of (9.44 × 1.53/6)1/2, or 1.55 nm. A worm model fit to the SAXS data pattern of G37H60B28 (Figure S4a) indicated that the solvent volume fraction in the core (xsol) is 0.03, which suggests that the hydrophobic worm cores are essentially non-solvated. This is significantly different to xsol values reported recently by Warren et al.28 for G55Hy diblock copolymer vesicles, which ranged from 0.38 to 0.66 as y was increased from 200 to 1000. It is evident that extension with approximately 28 units of BzMA not only changes the nanoparticle morphology from spheres to worms but also drastically changes the extent of hydration of the nanoparticle cores.
SAXS data (open black circles) and fits (red lines) for (a) a G37 macro-CTA and dilute aqueous dispersions of (b) G37H60B28 triblock copolymer worms, (c) G37H60B186 triblock copolymer spheres, and (d) G92B28 diblock copolymer spheres. Insets: schematic...
Based on the PISA literature,37,42−45 it was anticipated that vesicular morphologies should be obtained for these G37H60Bz triblock copolymers as the target DP of the PBzMA block was gradually increased. However, only branched worms and spheres were obtained when z = 47 (see TEM images in Figure 2). Furthermore, both TEM and DLS studies indicated that only spheres were obtained when z ≥ 92 (see Figure 2 and Table 1, respectively). The spheres progressively increase in mean diameter from 45 to 120 nm as z was systematically varied from 92 to 550, but vesicular morphologies were never obtained. Hypothetically, the spherical morphology observed by TEM might actually correspond to small vesicles. However, the SAXS pattern recorded for the G37H60B186 triblock copolymer has a gradient that tends to zero at low q (see Figure S4b), indicating typical spherical particles46 rather than hollow spheres (or vesicles). Analysis of the G37H60B186 SAXS pattern using a spherical micelle model38−40,47 gave an excellent data fit over six orders of magnitude of X-ray scattering intensity (Figure S4b). The SAXS-derived mean sphere diameter (Ds) was calculated to be 56.2 ± 5.4 nm, which is similar to that reported by DLS (63 nm, see Table 1). The structure factor peak observed in the SAXS pattern at q ∼ 0.05 nm–1 (Figure S4b) suggests that the spheres are weakly aggregated. The Percus–Yevick correlation radius of packed spheres (RPY) was obtained to be 50.5 nm. The TEM images obtained for dispersions when z ≥ 92 also show that the spheres may be partially fused/weakly aggregated. However, the number-average diameter estimated from TEM images recorded for G37H60B92–550 triblock copolymer spheres corresponds quite closely to the hydrodynamic diameter obtained from DLS studies (see entries 6–9 in Table 1).
Although these results are somewhat counterintuitive when compared to most of the recent PISA literature,37,42−45 it is perhaps not too surprising that only spheres are obtained when targeting higher DPs for the PBzMA block. For example, Cunningham et al.27 prepared a series of G51By diblock copolymer spheres via RAFT aqueous emulsion polymerization of BzMA, with y ranging from 50 to 1000. Only spherical nanoparticles were obtained in all cases, regardless of the total solids content. In the present study, a weakly hydrophobic PHPMA block lies between the hydrophilic PGMA and highly hydrophobic PBzMA blocks, which allows triblock copolymer worms to be prepared for compositions containing just 31 mol % PBzMA. However, targeting higher PBzMA contents only leads to the formation of triblock copolymer spheres. The most likely explanation for these unexpected observations is that the PBzMA block is enthalpically highly incompatible with the PHPMA block, whereas the PHPMA block is only rather weakly incompatible with the PGMA block. Thus, when the G37H60 diblock copolymer is chain-extended with BzMA, at least some fraction of the partially hydrated PHPMA block24 is gradually driven out of the increasingly hydrophobic core to become co-located with the PGMA stabilizer chains in the hydrophilic corona (see the schematic cartoon shown in Figure 3). If this is the case, it would lead to an effectively longer stabilizer block, with a theoretical maximum DP of 97 (i.e., the sum of G37 and H60).
Schematic cartoon to illustrate the conformational behavior of G37H60Bz triblock copolymer chains as z is systematically increased. Hydrophilic regions are represented by blue and hydrophobic regions are represented by orange. The packing parameter, ...
SAXS analysis allows this hypothesis to be examined.49 A SAXS pattern was collected for a 10% w/w aqueous solution of the G37 macro-CTA (i.e., for molecularly dissolved chains below their overlap concentration). A satisfactory data fit was obtained for this pattern using a Gaussian coil model,50 which indicated a Rg of 1.7 nm (see Figure 4a). This is very close to the Rg value for the stabilizer chains obtained from fitting the G37H60B28 SAXS pattern using the worm model (see Table S1). This suggests that all of the weakly hydrophobic PHPMA60 blocks are located within the core of the worms, while the hydrophilic PGMA37 blocks occupy the worm corona. To test this hypothesis, the worm model was slightly modified (see SAXS models given in the Supporting Information) by incorporating an additional fitting parameter (η) corresponding to the volume fraction of the PHPMA block within the core domain. This η parameter enables the volume of the core and corona to be determined, rather than fixing these values based on the known block compositions. By definition, if the whole PHPMA block is located within the core, η should be equal to unity. In contrast, η should be zero if the PHPMA block is solely located in the corona. A good data fit was obtained for the G37H60B28 SAXS pattern using the modified worm model (see Figure 4b). The fitting parameters were similar to those obtained when using the unmodified, original worm model (see Table S1). The Rg for the G37 corona block of this triblock copolymer was determined to be 1.7 nm, which is identical to that obtained for the G37 macro-CTA alone (see Figure 4a). Moreover, η tends toward unity, indicating that all of the PHPMA block is located in the worm core (see Figure 3).
A spherical micelle model38−40,47 was similarly modified by incorporating η as an additional fitting parameter (see SAXS models and Table S1). Analysis of the G37H60B186 spheres using this more sophisticated model gave a reasonably good data fit to the SAXS pattern over six orders of magnitude of X-ray scattering intensity (Figure 4c). Again, the fitting parameters were similar to those obtained when using the original unmodified sphere model (see Table S1). However, the Rg of the G37 corona block for this G37H60B186 triblock copolymer was determined to be 3.3 nm from this analysis, which is significantly larger than that obtained for the G37H60B28 worms. Notwithstanding the imperfect data fit at high q, this indicates that the stabilizer corona is somewhat thicker in the former case, even though the same G37H60 diblock precursor was used for the PISA synthesis of the G37H60B28 and G37H60B186 triblocks. Moreover, η was found to be 0.62, which suggests that a significant fraction of the PHPMA block is now located in the corona, rather than in the core (see Figure 3). This provides direct experimental evidence for a higher effective DP for the corona block when targeting a longer PBzMA core-forming block. For the G37H60B186 triblock copolymer spheres, SAXS analysis indicates that around 23 HPMA repeat units [(1 – 0.62) × 60 ≈ 23] in each PHPMA block are located within the corona, while the remaining 37 repeat units occupy the core along with the PBzMA chains. This increase in the effective stabilizer block DP leads to a reduction in the packing parameter, P, which in turn causes the observed worm-to-sphere transition (see Figure 3). The driving force for relocating approximately one-third of the PHPMA block within the corona is the greater incompatibility within the PHPMA and PBzMA blocks as the DP of the PBzMA block is increased. In this context, Mable et al.51 recently reported that systematically varying the PBzMA block DP (or z) from 25 to 400 led to an evolution in framboidal morphology for a series of G63H350Bz triblock copolymer vesicles. Thus it is not really surprising that enthalpic demixing between the PHPMA and PBzMA blocks leads to a dramatic change in morphology in the present work. In summary, SAXS provides useful insight into the unusual (and at first sight counterintuitive) evolution in copolymer morphology for this particular PISA formulation, which can be rationalized by considering subtle changes in the relative enthalpic incompatibilities between the three blocks during the growth of the PBzMA core-forming block.
In order to examine whether the intermediate PHPMA block is really essential for worm formation, a G92B28 diblock copolymer was synthesized via RAFT aqueous emulsion polymerization of BzMA using a G92 macro-CTA (see Figure 1b). The G92 block was designed to have a comparable DP to that of the combined DP of the G37 and H60 blocks, while a PBzMA DP of 30 was targeted because this produced worms for the ABC triblock formulation. 1H NMR studies indicated that 94% BzMA conversion was achieved after 4 h at 70 °C (see Figure S5). GPC studies indicated that a low-polydispersity diblock copolymer was obtained with a high blocking efficiency and minimal macro-CTA contamination (Mw/Mn = 1.14; see Figure S6 and Table 1). DLS studies indicate a mean Dh of 28 nm (see Table 1). TEM images confirmed the formation of very small spheres of around 11.3 ± 2.5 nm diameter (based on analyzing 100 nanoparticles) with no evidence for the presence of any worms (see Figure 2). SAXS analysis confirmed that spheres are indeed formed because the gradient of the SAXS pattern tends to zero in the low q region, which is characteristic of spheres.46 Analysis of this SAXS pattern using a star-like micelle model47,52 provided a satisfactory data fit over five orders of magnitude of X-ray scattering intensity (see Figure 4d). The mean sphere diameter, Ds, was calculated to be 21.0 ± 1.4 nm, which is comparable to that suggested by DLS, while the Rg of the G92 corona block for this G92B28 diblock copolymer was determined to be 3.0 nm. This experimental value is larger than the theoretical estimate (where Rg was calculated to be 2.45 nm) due to the star-like nature of the spheres. The spherical core diameter was determined to be 9.0 ± 1.4 nm, which is comparable to that estimated from TEM images. The correlation radius for densely packed spheres, RPY, was determined to be 19.3 nm. This is simply a consequence of the star-like nature of the former copolymer,47,53 which has a much higher effective volume fraction and hence a significantly lower critical overlap concentration. There is a pronounced upturn in the X-ray scattering intensity at low q (below 0.017 nm–1; see Figure 4d). This could indicate the formation of aggregates (or mass fractals) most likely due to the extensive overlap between stabilizer chains of the micelles. The formation of spherical star-like micelles by this G92B28 diblock copolymer suggests that an intermediate PHPMA block is an essential prerequisite for obtaining a worm morphology. A reasonable explanation for this unexpected observation is outlined in Figure 3.
Millimeter-Sized Pickering Emulsion Droplets
Recently, Thompson et al. reported that G45H200 diblock copolymer vesicles were unstable with respect to dissociation when used as a Pickering emulsifier. However, chemical cross-linking of such vesicles using ethylene glycol dimethacrylate as a third block dramatically improved their stability toward high-shear homogenization: TEM studies of dried emulsion droplets confirmed that such covalently stabilized vesicles were adsorbed intact at the oil–water interface.54 More recently, Thompson et al. reported that G45H140 diblock copolymer worms similarly could not withstand high-shear homogenization, whereas G37H60B30 triblock copolymer worms proved to be highly efficient Pickering emulsifiers.36 Moreover, DLS studies showed that the former worms were thermoresponsive, as expected based on previous work by Verber et al.30 In contrast, the G37H60B30 triblock copolymer worms were not thermoresponsive; this indicates that introducing the more hydrophobic PBzMA block stabilizes the worm morphology. In the present study, we have used RAFT aqueous dispersion polymerization (see Figure 1a) to prepare G37H90 diblock copolymer worms, which were designed to be analogous to the G37H60B28 triblock copolymer worms. 1H NMR studies confirmed that more than 99% HPMA conversion was achieved after 2 h at 70 °C (see Figure S7). GPC studies indicated that a near-monodisperse diblock copolymer was obtained with a high blocking efficiency and minimal macro-CTA contamination (Mw/Mn = 1.11; see Figure S8 and Table 1). DLS studies (see Table 1) and TEM images (see Figure 2) were consistent with the targeted pure worm morphology. Rheology experiments for the G37H60B28 triblock copolymer worm gel were performed at 1.0% strain using an angular frequency of 1.0 rad s–1 (see Figure S9). Figure 5 shows the minimal change in gel moduli for this dispersion during a 25 °C to 2 °C to 25 °C thermal cycle. These G37H60B28 worms proved to be non-thermoresponsive, with a G′ of approximately 400 Pa being maintained over the entire temperature range.
Variation of storage moduli (G′, red) and loss moduli (G″, blue) for a G37H60B28 triblock copolymer worm gel at 13% w/w during temperature cycling at 1 °C min–1 with 5 min equilibration at each temperature: (i) cooling...
Incorporating the highly hydrophobic PBzMA block enables the G37H60B28 worms to act as an effective Pickering emulsifier. Previously, we reported that G37H60B28 worms can survive the high-shear homogenization conditions required for emulsification, whereas G45H140 worms undergo dissociation to form individual copolymer chains under these conditions.36 In the present study, we examined homogenization under much lower shear conditions, i.e., hand-shaking.
More specifically, both G37H60B28 and G37H90 worms were evaluated as putative Pickering emulsifiers for the stabilization of n-dodecane emulsion droplets in water. Aqueous worm dispersions (1.88 × 10–3 to 1.00% w/w) were hand-shaken with 20 vol % n-dodecane for 2 min at 20 °C to produce emulsions. In order to examine whether the worms were adsorbed intact at the oil–water interface, optical microscopy (OM) and laser diffraction were used to determine the mean oil droplet diameters (see Figure 6). According to OM studies, the oil droplets became larger as the G37H60B28 worm concentration was lowered, as shown in Figure 6a. These observations were corroborated by laser diffraction studies: the mean oil droplet diameter increased from 115 to 483 μm as the worm dispersion concentration was reduced from 1.00 to 0.0075% w/w (see Figure 6c). This concentration-dependent behavior is consistent with the formation of genuine Pickering emulsions (see Figure 7).5,55−57 This was expected because Thompson et al. recently reported that such triblock copolymer worms can withstand high-shear homogenization, so they should also survive low-shear homogenization.36 It is worth emphasizing that the mean oil droplet diameters are much larger when using hand-shaking for emulsification (approximately 115 μm at 1.00% w/w) compared to those obtained using high-shear homogenization (approximately 45 μm at 1.00% w/w).36 When the worm dispersion concentration was lowered to 1.88 × 10–3 % w/w, the oil droplets proved to be too unstable to be assessed by laser diffraction. However, the droplet diameter was estimated (from digital photographs recorded immediately after emulsification) to be 1.11 ± 0.42 mm (based on measuring 120 droplets). Droplet coalescence occurs within a few hours, but reconstitution of the original emulsions could be achieved via further hand-shaking. This differs from the highly stable millimeter-sized emulsions prepared using partially hydrophobized silica particles reported by Arditty et al.58 The instability observed in the present work suggests that worm desorption occurs; similar observations have been recently reported by Rizelli and co-workers for worm-stabilized Pickering non-aqueous emulsions.59 In contrast, it is emphasized that the finer o/w emulsions prepared at higher worm concentrations (≥0.03% w/w) remain stable indefinitely.
Optical microscopy images obtained for n-dodecane-in-water emulsion droplets prepared using either (a) G37H60B28 or (b) G37H90 worms under low-shear conditions (i.e. hand-shaking). (c) Plots of mean droplet diameter (obtained by laser diffraction) vs...
Schematic illustration of the attempted formation of Pickering emulsions using either G37H90 or G37H60B28 worms under low-shear conditions (i.e., hand-shaking).
Remarkably, both OM and laser diffraction studies indicated that the mean oil droplet diameter remained relatively constant on lowering the concentration of the G37H90 worms (Figures 6b and 6c). This indicates that these linear worms are so delicate that they cannot survive even low-shear hand-shaking. Instead, dissociation to form individual G37H90 copolymer chains occurs, which then adsorb at the oil–water interface to stabilize the oil droplets (see Figure 7). Again, mean oil droplet diameters were significantly larger (∼136 μm) than those reported previously when using high-shear homogenization (∼45 μm).
For emulsions stabilized using either G37H90 or G37H60B28 worms, creaming of the low-density oil droplet phase occurred on standing for 24 h at 20 °C. The lower aqueous phase, which contained excess non-adsorbed copolymer, was carefully removed and analyzed by DLS to examine whether the worms remained intact after hand-shaking. DLS studies of the G37H90 aqueous phase indicated a hydrodynamic diameter of 41 nm (polydispersity = 0.18) and a much lower count rate (2500 kcps) than that observed for the original worms (37 000 kcps). This 93% reduction in count rate is fully consistent with substantial worm dissociation occurring during hand-shaking. In contrast, DLS studies of the aqueous phase removed from the G37H60B28-stabilized emulsion indicated an apparent hydrodynamic diameter of 153 nm, a polydispersity of 0.23, and count rate of 21 000 kcps, which are comparable to the DLS data obtained before emulsification. These observations confirm that these G37H60B28 worms remain intact after emulsification via hand-shaking.
Finally, closely related emulsions were prepared using n-hexane instead of n-dodecane to enable more convenient removal of the oil phase via evaporation at ambient temperature. Figure 8 shows TEM images obtained from emulsions prepared using the G37H60B28 and G37H90 worms. In the latter case, the surface of the dried emulsion droplet is smooth and featureless, with no evidence for any adsorbed nanoparticles (see Figure 8a). Similar TEM observations were reported for both G45H150 diblock copolymer worms and G45H200 diblock copolymer vesicles in previous studies of shear-induced dissociation of diblock copolymer nano-objects.36,54 In contrast, the dried emulsions prepared using the G37H60B28 worms clearly comprise intact worms adsorbed at the oil–water interface (see Figure 8b). Thus all the experimental evidence suggests that, regardless of their morphology, GxHy nanoparticles are not sufficiently robust to survive emulsification under any conditions, even low-shear hand-shaking. However, incorporating highly hydrophobic PBzMA as a third block produces much more robust linear worms that can withstand high-shear homogenization and allow the formation of millimeter-sized emulsion droplets.
TEM images obtained for n-hexane-in-water emulsion droplets dried at 20 °C using (a) 0.25% w/w G37H90 diblock copolymer worms and (b) 0.25% w/w G37H60B28 triblock copolymer worms. The edge (blue) and top surface (red) of the dried emulsion droplets...