Dmitry V. Pergushov and Felix A. Plamper
4.2 CORE-CORONA CO-ASSEMBLIES OF HOMOPOLYELECTROLYTE STARS COMPLEXED WITH LINEAR POLYIONS
Homopolyelectrolyte stars (Figure 4.1A) can host linear polyions bearing the oppo- site charge, thereby forming water-soluble IPECs, provided that their charged groups are present in a certain excess compared to the charged groups of the linear polymeric component [14–17]. Figure 4.2 shows a typical turbidimetric titration curve of an aqueous solution of a star-shaped homopolyelectrolyte with an aqueous solution of an oppositely charged linear homopolyelectrolyte. A window of Z-values Z<ZM(Z= cGPE/cHPE is the charge-to-charge stoichiometry of mixtures of oppositely charged polymeric components, withcGPE andcHPEbeing the molar concentrations of ionic groups of the linear guest polyelectrolyte (GPE) and the star-shaped host polyelec- trolyte (HPE), respectively) corresponds to a homogeneous system, strongly sug- gesting the formation of water-soluble IPECs. Direct confirmation that water-soluble IPECs are formed comes from analysis of such homogeneous mixtures (Z<ZM) by analytical ultracentrifugation [15].
Based on results of dynamic/static light scattering measurements, the structural picture of the formed macromolecular co-assemblies of star-shaped HPE with rather large number of arms hosting linear GPE has been proposed [14–17]. Indeed, a dominant fraction of the complex particles formed in homogeneous mixtures of oppositely charged polymeric components shows a hydrodynamic radius in the
Absorbance at λ = 500 nm 0.20
0.15
A B
0.10
0.05
0 0.2 0.4 0.6 0.8 1.0
Z Zmax
Figure 4.2 Turbidimetric titration curve of an aqueous solution of poly(acrylic acid) star having 21 arms (DPn,arm = 100) with an aqueous solution of exhaustively quaternized poly(2-vinylpyridine) (DPn=36): (A) homogeneous system; (B) heterogeneous system. Con- ditions: 0.1 mol/L NaCl, pH 7. Reprinted from [15] with permission from Springer.
k k nanometer range being very close to that of single (uncomplexed) star-shaped
macromolecules, and a weight-average molecular weight indicating only one star-shaped macromolecule incorporated into each of the complex species [15–17].
These findings suggest that chains of the linear GPE penetrate into the oppositely charged star-shaped HPE and are likely to settle in the vicinity of its branching point, thereby forming an inhomogeneous core-corona structure (Figure 4.3). Hence a sufficiently hydrophobic core is assembled from electrostatically coupled monomer units of the oppositely charged polymeric components in apparently about equivalent amounts, namely in the stoichiometric 1:1 ratio. A hydrophilic (ionic) corona is built up from the essentially free fragments of the star-shaped HPE, which are not complexed with the linear GPE that only partly compensates the charge of the homopolyelectrolyte star. Thus the corona grants solubility to the whole complex species and protects that species from aggregation.
+
+ – – +
– – –
CH3SO4 CH CH2
N CH3
(PANa)21 PMVPMS
Hydrophobic core
Hydrophilic (ionic) corona
I II
CH CH2
O Na C O
Figure 4.3 Core-corona (micelle-like) structure of macromolecular co-assemblies of homopolyelectrolyte stars hosting linear homopolyelectrolytes. Reprinted from [15] with permission from Springer.
k k Remarkably, the results from molecular dynamics simulations provide the evi-
dent support for the presence of a compartmentalized (micelle-like) structure of the complex species but also reveal some other important features of their internal struc- tural organization [18]. A snapshot of a typical conformation of a single complex species clearly shows its inhomogeneous structure of a core-corona type (Figure 4.4) that results from a pronounced non-uniform distribution of chains of the linear GPE within the volume occupied by the oppositely star-shaped HPE. Additionally, clearly seen in the figure is the distinct fractionation of the arms of the homopolyelectrolyte star between the two populations. Note that in one of the populations, the arms are completely embedded in a complex core of the star, whereas in the other population, the arms remain virtually free (uncomplexed) and so build up the corona.
Use of bis-hydrophilic (also often called as double hydrophilic) diblock copolymers, which comprise an ionic block and a hydrophilic non-ionic one, in
Figure 4.4 Snapshots of typical conformations of a homopolyelectrolyte star (top) and its complex with an oppositely charged linear homopolyelectrolyte (bottom). Reprinted from [18]
with permission from the American Chemical Society. (See color insert for color representa- tion of this figure).
k k interpolyelectrolyte complexation could have prevented the macroscopic phase
separation (observed at Z>ZMfor the described systems comprising a homopoly- electrolyte star complexed with the oppositely charged linear homopolyelectrolyte).
With the presence of the non-ionic block of the bis-hydrophilic diblock copoly- mer, the preparation would have boosted the water-soluble macromolecular co-assemblies, even at a 1:1 charge-to-charge ratio of the polymeric components in the aqueous mixture [2, 19–21]. Besides the enhanced solubility of the formed com- plex species in aqueous media, the desired properties and additional functionalities, such as biocompatibility and thermosensitivity, could then be imparted through the non-ionic block of the bis-hydrophilic diblock copolymer, as would be of definite interest for medical and biotechnological applications.
The macromolecular co-assemblies formed in such systems are supposed to have a compartmentalized (micelle-like) structure with a rather hydrophobic core. The core is assembled from electrostatically coupled monomer units of the oppositely charged polymeric components in apparently about equivalent amounts, namely in the stoi- chiometric 1:1 ratio. At Z=1, the complex species have a hydrophilic corona formed by the non-ionic blocks of the bis-hydrophilic diblock copolymer.
Indeed, the results of analytical ultracentrifugation, dynamic/static light scatter- ing, and cryogenic transmission electron microscopy indicate that such water-soluble core-corona macromolecular co-assemblies result from interpolyelectrolyte com- plexation of a homopolyelectrolyte star with the oppositely charged bis-hydrophilic diblock copolymer at Z = 1 [22]. In contrast with the above-considered systems, that is, complex species based on a homopolyelectrolyte star hosting chains of the oppositely charged linear homopolyelectrolyte, in most cases (but not always) they are of distinctly aggregated nature as follows from the results of dynamic/static light scattering and analytical ultracentrifugation measurements, incorporating several star-shaped macromolecules whose charges are fully compensated by the ionic blocks of the bis-hydrophilic diblock copolymer (Figure 4.5).