Dmitry V. Pergushov and Felix A. Plamper
4.4 VESICULAR CO-ASSEMBLIES OF BIS-HYDROPHILIC MIKTOARM STARS COMPLEXED WITH LINEAR POLYIONS
To prepare still more advanced macromolecular co-assemblies, bis-hydrophilic miktoarm stars can be involved in interpolyelectrolyte complexation instead of star-shaped homopolyelectrolytes. Such stars often have a non-centrosymmetric architecture, for instance, if their polyelectrolyte and non-ionic hydrophilic arms are segregated to different domains rather than homogeneously mixed with each other. Naturally, such miktoarm stars can host oppositely charged linear polyions.
k k
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0.0 0.1 0.2 0.3 0.4
Z = [–] / [+]
Optical Density at 500 nm
Figure 4.9 Turbidimetric titration curves of aqueous solutions of miktoarm stars poly(ethylene oxide)-quaternized poly(2-dimethylaminoethyl methacrylate) (1 PEO arm, DPn,PEO = 113, 2.5 PDMAEMAQ arms, DPn,PDMAEMAQ = 110) (○), poly(ethylene oxide)-quaternized poly(2-dimethylaminoethyl methacrylate) (1 PEO arm, DPn,PEO = 113, 2.5 PDMAEMAQ arms, DPn,PDMAEMAQ=40) (▴), and for a comparison an aqueous solution of homopolyelectrolyte star quaternized poly(2-dimethylaminoethyl methacrylate) (3.1 PDMAEMAQ arms, DPn,PDMAEMAQ=100) (◾) with an aqueous solution of poly(sodium styre- nesulfonate) (DPn=20). Conditions: 0.3 mol/L NaCl. Reprinted from [30] with permission from Elsevier.
Typically, water-soluble macromolecular co-assemblies are formed in a rather broad window of lower Z-values, that is, at Z < ZM, whereas stable colloidal systems are obtained at Z closer to 1 (Figure 4.9, triangles) [30]. Remarkably, the complex species observed at Z<ZMcomprise only one bis-hydrophilic miktoarm star (HPE) per particle as found by combining analytical ultracentrifugation and dynamic light scattering and apparently can keep the non-centrosymmetric feature of the original star-shaped polymeric component, thereby generating water-soluble macromolecular co-assemblies of Janus-like type [30]. Latter are rather rare for IPECs and mostly reported for micelle-like macromolecular co-assemblies formed by two oppositely charged bis-hydrophilic diblock copolymers with different non-ionic hydrophilic blocks undergoing phase separation in a corona, which results into double-faced micellar IPECs [31–33]. The formation of Janus-like structures from bis-hydrophilic miktoarm stars complexed with oppositely charged linear homopolyelectrolytes is corroborated by1H 2D NOESY nuclear magnetic resonance [30].
At Z = 1, electrostatically driven co-assembly between oppositely charged bis-hydrophilic miktoarm star and linear homopolyelectrolyte can lead to a rare vesicular structure [34] despite the intuitive expectation of core-corona structures.
k k Figure 4.10 Cryogenic transmission electron microscopy images of vesicular IPECs formed
by poly(ethylene oxide)-quaternized poly(2-dimethylaminoethyl methacrylate) (1 PEO arm, DPn,PEO= 113, 2.5 PDMAEMAQ arms, DPn,PDMAEMAQ=40) and poly(sodium styrenesul- fonate) (DPn =20) at Z =1. Conditions: 0.3 mol/L NaCl. Insets show particle diameter histogram and a gray-value histogram representing the transmitted electrons. Reprinted from [34] with permission from Wiley.
Such vesicular macromolecular co-assemblies (polymersomes) with the overall size in the nanometer range are clearly visualized by cryogenic transmission electron microscopy (Figure 4.10). The polymersomes comprise an insoluble IPEC domain (dark rings in Figure 4.10) containing essentially equivalent amounts of ionic groups of the oppositely charged polymeric components, which is sandwiched between two poly(ethylene oxide) brushes (not seen in Figure 4.10 because of their low contrast), which grant solubility (colloidal stability) to the whole macromolecular co-assembly in aqueous media. Schematic representation of vesicular IPECs based on bis-hydrophilic miktoarm stars complexed with oppositely charged chains of a linear homopolyelectrolyte is shown in Figure 4.11.
It is assumed that such complex polymersomes result from a hierarchical assembly process. First, the bis-hydrophilic miktoarm stars get complexed with chains of the oppositely charged linear homopolyelectrolyte (electrostatically driven co-assembly), and then the formed primary complex species, each containing one macromolecule of the star-shaped polymeric component, self-assemble further, producing vesicular IPECs [30, 34].
Remarkably, such unilamellar IPEC vesicles were found before only for certain specific oppositely charged polypeptide-derived diblock copolymers undergoing electrostatically driven co-assembly in aqueous media [35–39]. In those publications, additional hydrogen bonding could be suggested, which helps stratifying the IPEC layers, thereby leading the self-assembly toward polymersomes.
At the same time, theoretical considerations (generalized theoretical mean-field approach [40]) on electrostatically driven co-assembly of bis-hydrophilic miktoarm
k k
PMOTAC SO3
PSS PEO O
O O
NB
NA
NB
R
S NC
NA
N+ –
Figure 4.11 Schematic representation of vesicular IPECs based on miktoarm star poly(ethylene oxide)-quaternized poly(2-dimethylaminoethyl methacrylate) complexed with poly(sodium styrenesulfonate) at Z=1. Reprinted from [34] with permission from Wiley.
stars with oppositely charged linear homopolyelectrolytes provide evidence that the star-shaped architecture of one of the polymeric components forces the morphology of the formed IPECs into a vesicular one [34]. Remarkably, the lamellar (vesicular) morphology is thermodynamically stable if
NB> N8∕5
A ⋅ ϕ3∕2 ⋅ 𝑣9∕10A ⋅ a9∕5 k2 ⋅ (VB+VC)3∕2
(kB ⋅ T γ ⋅ a2
)3∕5 ( 2 ⋅ 32∕5
π2 )1∕2
whereNBandkare the length and the number of the core-forming (polyelectrolyte) arms B, respectively,NAis the length of the corona-forming (non-ionic hydrophilic) arms A,ϕis the polymer volume fraction in the IPEC, υA is the excluded volume parameter of the monomer unit of the corona-forming (non-ionic hydrophilic) arms A, a is the length of the monomer unit (taken the same for all monomer units), VB andVCare volumes of the monomer units of the core-forming (polyelectrolyte) arms B and the linear homopolyelectrolyte C, respectively,γis surface tension at the core-corona interface,Tis the absolute temperature, andkB is Boltzmann constant, herein the core means a water-insoluble IPEC domain. This condition indicates that an increase in the numberk leads to a widening of the region of thermodynamic stability of vesicles. In particular, it implies that if diblock copolymers compris- ing a non-ionic hydrophilic block of length NA and one IPEC-forming polyelec- trolyte block of lengthk⋅NB assemble (at equilibrium) into spherical micelles, the miktoarm stars comprising the same non-ionic hydrophilic arm of lengthNAandk IPEC-forming polyelectrolyte arms each of lengthNB may form vesicles, provided thatkis sufficiently large to satisfy inequality in the equation above.
A simple physical explanation of this finding is the following [34]. The transition from spherical to cylindrical and further to lamellar morphology of the self-assembled aggregates is driven by a concomitant decrease in stretching and a corresponding gain in the conformational entropy of the core-forming blocks. In the specific case considered above, the polyelectrolyte arms’ charge is fully compensated by chains of
k k the oppositely charged linear homopolyelectrolyte, thereby rendering them insoluble.
At given volume (≈k⋅NB) of the IPEC domain, the conformational entropy penalty for stretching ofkarms, each of lengthNB, is larger than that for stretching of one arm of lengthk⋅NB. As a result, the equilibrium is shifted toward the planar-like (vesicular) morphology.
Remarkably, use of the miktoarm stars can allow a better fine-tuning of the thick- ness/permeability of the vesicle wall (e.g., by reducing the length of the single arms to degrees of polymerization, where the corresponding linear diblock copolymers would already form nonvesicular co-assemblies). In sum, the architecture of mik- toarm stars provides a facile transition from monostar co-assemblies to aggregates with a very high number of the miktoarm stars (vesicular co-assemblies) in a rather narrow window of Z-values.