Angewandte
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PS but a poor solvent for PTFEP), the cryo-TEM image (after
24 h) showed the presence of poorly defined spherical
micelles with no evidence of a bicontinuous structure (see
Figure S9).
As the specific conditions to produce bicontinuous nano-
spheres are very difficult to achieve,[10] no other bicontinuous
nanospheres originating in the absence of additives, cosol-
vents, and/or structurally complex polymeric structures (i.e.
from a linear block copolymer) have been synthesized
previously, to the best of our knowledge. Instead, all reported
bicontinuous nanospheres have been obtained from mixtures
of an organic solvent and water.[11a] Moreover, the synthesis of
bicontinuous or lamellar structures is possible by the selective
variation of the affinity of the solvent for the constituent
blocks.[11a] Therefore, we studied the effects of the presence of
water on the structure of the bicontinuous aggregates formed
by BCP 6. When water (50 mL, 0.16 vol%) was added to the
solution of 6 at a concentration of 0.33 mgmLÀ1 in THF
(3 mL) at room temperature (this amount of water does not
change the block-polymer concentration significantly), DLS
after 24 h showed aggregates (Rh,app = 175 nm, PDI = 0.356;
see Figure S8) that were larger than those observed in pure
THF. These aggregates were studied by HRTEM, which
showed the presence of multilamellar vesicles (Dn = 250 nm,
Dw/Dn = 1.6, N = 70; see Figure 1e). According to the
Figure 2. a) Bright-field HRTEM images of toroidal micelles from
a 2 mgmLÀ1 solution of BCP 6 in THF. b) EDX line analysis (F in red,
N in green, and P in blue) along the yellow line (inset). Scale bar
corresponds to 200 nm. c) Cryo-TEM images of toroidal micelles from
a 2 mgmLÀ1 solution of BCP 6 in THF. Scale bar corresponds to
200 nm. d) AFM height image of toroidal micelles. Scale bar corre-
sponds to 100 nm. e) Depiction of the geometric parameters of the
toroid and AFM cross-sectional height profiles across two micelles.
d(MPa1/2
) values of both blocks (18.6 (PS) and 19.6
(PTFEP)) and those of the solvents (18.6 (THF) and 47.9
(H2O)[16]), THF is a better solvent for PS than for PTFEP, and
water is very poor solvent for both. Therefore, the addition of
water alters the balance between the excluded volumes of the
PS and PTFEP blocks, thus leading to a similar volume
occupation in the THF/water mixture, and a lamellar instead
of a bicontinuous structure. The addition of the same amount
(0.16 vol%) of cyclohexane (a good solvent for PS but poor
for PTFEP) or MeOH (a good solvent for PTFEP but poor
for PS) did not alter the balance between the excluded
volumes of the PS and PTFEP blocks with respect to those in
THF, thus resulting in bicontinuous nanospheres (see Fig-
ure S10).
confirming the perfect toroidal shape. The calculated Rg
value for our ring-shaped micelles is 140 nm (Rg = R[1 +
(3Z2/4)]1/2, Z = R/r, with R = 65 nm, 2r= 60 nm, both calcu-
lated by AFM; Figure 2b), and the value of Rh = 126 nm
(DLS), which gives the ratio Rg/Rh = 1.1. This value (1.1) is in
very good agreement with the expected value (1.06) for
a toroidal object.[11b] Again, when cyclohexane was used as
a solvent (2 mgmLÀ1), ill-defined spherical micelles were
observed (see Figure S9).
When the concentration of BCP 6 was raised from 0.33 to
2 mgmLÀ1 (1 mg of 6 in 3 mL of THF), DLS (after 24 h at
room temperature) showed a significant increase in Rh,app
from 74 to 125 nm (see Figure S8). The observation of these
aggregates by HRTEM and SEM revealed the exclusive
presence of pure and regular (in size and shape) toroidal
micelles (donut- or ring-shaped; Figure 2a). Line energy-
dispersive X-ray analysis (EDX) of the composition distribu-
tion of these micelles (Figure 2b, F in red, N in green, and P in
blue) demonstrated the presence of toroidal nanostructures,
in which the F, N, and P atoms were only detected in the dark
rings and not in the center of the nanostructures. Cryo-TEM
experiments (Figure 2c) confirmed the formation of the
toroidal micelles in solution before the evaporation of the
solvent on the solid substrate used for the TEM and SEM.
Micelles were also investigated by AFM (on a mica disk;
Figure 2d). The average diameter 2R of the toroid, as
determined from the peak-to-peak distance in Figure 2e,
was 130 nm, and the ring width (2r) 60 nm, which was very
similar to the height of the nanostructure, 65 nm, thus
The generation of bicontinuous nanospheres and toroidal
micelles from the same linear BCP 6 by simply adjusting its
concentration is of particular importance owing to the
simplicity of the experimental methodology. Furthermore,
we found that these two rare morphologies can be reversibly
transformed one into the other. Thus, THF (5.0 mL) was
added (over 15 min) to a solution of 6 in THF containing
toroidal micelles (2 mg of 6 in 1 mL of THF, C = 2 mgmLÀ1)
with slow magnetic stirring to reach a final concentration of
0.33 mgmLÀ1 (see the Supporting Information). The solution
was aged for 24 h (without stirring) and analyzed by DLS (see
Figure S8). A decrease in the value of the Rh,app from 126 (C =
2 mgmLÀ1) to 80 nm (C = 0.33 mgmLÀ1) was observed. Both
HRTEM and cryo-TEM (see Figure S2) confirmed the
presence of bicontinuous nanospheres with the same struc-
ture as that obtained when 1 mg of 6 was dissolved in 3 mL of
THF. When the solution containing the bicontinuous nano-
spheres was slowly evaporated (with slow magnetic stirring
under a flow of N2(g) in a bath at 208C to avoid the cooling
down of the solution) to reach a concentration of 2 mgmLÀ1
Angew. Chem. Int. Ed. 2016, 55, 1 – 7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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