Dmitry V. Pergushov and Felix A. Plamper
4.3 CORE-SHELL-CORONA CO-ASSEMBLIES OF STAR-LIKE MICELLES OF IONIC AMPHIPHILIC DIBLOCK COPOLYMERS
The self-assemblies of a core-corona type (micelles) generated in aqueous solutions of ionic amphiphilic diblock copolymers very much resemble star-shaped polyelec- trolytes with a large number of arms (the latter is given by the aggregation number of micelles) provided that the hydrophobic core is considerably smaller than the ionic corona [13]. Such star-like micelles (Figure 4.1C) with polyelectrolyte coronae are naturally expected to host polyions bearing the opposite charge, thereby generating IPECs.
Figure 4.6 demonstrates that aqueous mixtures of the oppositely charged star-like micelles acting as HPE and the linear homopolyelectrolyte acting as GPE remain homogeneous at Z<ZMwhile they become heterogeneous at Z>ZM. Analysis of the homogeneous mixtures (Z<ZM) by analytical ultracentrifugation confirms the
k k ( CH2
CH3SO4⊖ CH3
CH ) ( CH2 CH )
( CH2 CH2 O ) PEO +
(PANa100)8
PMVPMS
Corona:
PEO blocks
Core:
IPEC Micellar structure
“core-corona”
COO⊖Na⊕ N⊕
Figure 4.5 Core-corona (micelle-like) structure of macromolecular co-assemblies of a homopolyelectrolyte star complexed with an oppositely charged bis-hydrophilic diblock copolymer. Reprinted from [22] with permission from Springer.
0.0 0.00 0.04 0.08 0.12 0.16
A B
0.20
0.2 0.4
ZM Z = [+] / [–] heterogeneous
system homogeneous
system
optical density at 500 nm
Z > ZM Z < ZM
0.6 0.8 1.0
Figure 4.6 Turbidimetric titration curve of an aqueous solution of star-like polyisobutylene-block-poly(methacrylic acid) micelles (DPn,PIB = 20, DPn,PMAA = 100) with an aqueous solution of exhaustively quaternized poly(4-vinylpyridine) (DPn = 285):
(A) homogeneous system; (B) heterogeneous system. Conditions: 0.1 mol/L NaCl, pH 9.
k k formation of water-soluble IPECs that are considerably enriched by ionic groups
of the micelle-forming polymeric component [23]. Thus this trend is very similar to that discussed in the previous section for the formation of macromolecular co-assemblies between the oppositely charged star-shaped HPE and linear GPE.
Further examination of water-soluble IPECs based on star-like micelles of ionic amphiphilic diblock copolymers by various techniques — including fluorescence spectroscopy (with the use of pyrene as a polarity probe), small-angle neutron and X-ray scattering, dynamic light scattering, and cryogenic transmission electron microscopy — suggests that such macromolecular co-assemblies have a peculiar core-shell-corona (or “onion-like”/multilayer) structure (Figure 4.7) [16, 23–28].
Specifically, each of the water-soluble complex species contains a hydrophobic core formed by nonpolar blocks of the ionic amphiphilic diblock copolymer (A in Figure 4.7). This core is surrounded by a compact complex inner shell assembled from oppositely charged fragments of the polymeric components (ionic blocks of the ionic amphiphilic diblock copolymer and linear homopolyelectrolyte) in apparently about 1:1 charge-to-charge ratio (B in Figure 4.7). The ionic corona is built up from excess fragments of the ionic blocks of the ionic amphiphilic diblock copolymer (Figure 4.7C), which are not complexed with the linear GPE (the linear GPE only partly compensates the charge of the polyelectrolyte corona of the star-like micelle).
Detailed analyses of the scattering curves obtained for such systems by small-angle neutron scattering (Figure 4.8) [24, 27] strongly suggest that the aggregation number of the original ionic amphiphilic diblock copolymer micelles hardly changes when they host the oppositely charged linear GPE, though for some systems the original self-assemblies can exhibit “dynamic” properties; that is, they can change their aggregation numbers upon variations in the conditions of the surrounding solution [29]. Further, interpolyelectrolyte complexation of such
“dynamic” micelles with the oppositely charged linear GPE does not make them
complex shell
B
C A
PMA
PVPEt
PMANa corona
corePIB
Figure 4.7 Core-shell-corona (“onion-like”) structure of macromolecular co-assemblies of a star-like micelle of ionic amphiphilic diblock copolymers hosting an oppositely charged linear homopolyelectrolyte. Reprinted from [24] with permission from Elsevier.
k k
q, Å–1
I(q), cm–1
0.002 0.01 0.1
1 2
0.1 1 10
0.5
Figure 4.8 Small-angle neutron scattering intensity as a function of the scattering vec- tor for aqueous solutions of star-like polyisobutylene-block-poly(methacrylic acid) micelles (DPn,PIB=20, DPn,PMAA=100) (1) and their water-soluble IPEC with exhaustively quater- nized poly(4-vinylpyridine) (DPn =285) at Z=0.4 (2). Conditions: 0.1 mol/L NaCl, pH 9.
Reprinted from [24] with permission from Elsevier.
“frozen” as their aggregation numbers remain still sensitive to variations in the pH of the surrounding solution [27].
As in the case of macromolecular co-assemblies of homopolyelectrolyte stars complexed with linear polyions (see the previous section), the hydrodynamic sizes of water-soluble IPECs based on star-like micelles are quite close to those of the corresponding original self-assemblies [24–27]. This observation again strongly suggests intra-coronal disproportionation of the polyelectrolyte blocks of the micelle-forming polymer component [16]. A certain fraction of such blocks is completely embedded in the complex shell, whereas the remaining polyelectrolyte blocks only contribute a small number of their monomer units in the shell domain and form ionic corona, which provides solubility of the resultant core-shell-corona macromolecular co-assemblies in aqueous media.