Diblock copolymers, ionic amphiphilic

There is a vast body of diblock copolymer studies since block choice can be such that they resemble amphiphilic surfactants. For the sake of brevity, we will skip them. Instead, we present an interesting case of triblock copolymers of poly(ethylene oxide), PEO, and poly(propylene oxide), PPO, commonly known by one of its trade names, Pluronics [117]. They have been used as non-ionic surfactants for a variety of applications such as in emulsification and dispersion stabilization. In aqueous solutions, these copolymers form micelles, and there exists a well-defined critical micelle concentration that is experimentally accessible. Several groups have investigated colloidal suspensions of these polymers [118-122], The surface properties of the adsorbed monolayers of the copolymers have been reported with respect to their structures and static properties [123-126]. 

Abstract We present an overview of statistical thermodynamic theories that describe the self-assembly of amphiphilic ionic/hydrophobic diblock copolymers in dilute solution. Block copolymers with both strongly and weakly dissociating (pH-sensitive) ionic blocks are considered. We focus mostly on structural and morphological transitions that occur in self-assembled aggregates as a response to varied environmental conditions (ionic strength and pH in the solution). Analytical theory is complemented by a numerical self-consistent field approach. Theoretical predictions are compared to selected experimental data on micellization of ionic/hydrophobic diblock copolymers in aqueous solutions. 

A number of theoretical studies have been devoted to analysis of the self-assembly of amphiphilic ionic/hydrophobic diblock copolymers [13-24]. Most of these studies considered copolymers with strongly dissociating (also referred to as quenched ) PE blocks [13-18, 20] and extensively exploited the analogy between the conformation of PE blocks in a corona and that in a spherical PE brush [25-33] or PE stars (see [10] for a review). The micellization and the responsive behavior of nanostructures formed by copolymers with pH-sensitive PE blocks have also been systematically studied in recent years [19, 21-23]. 

We start with a brief reminder of the theory of self-assembly in a selective solvent of non-ionic amphiphilic diblock copolymers. Here, the focus is on polymorphism of the emerging copolymer nanoaggregates as a function of the intramolecular hy-drophilic/hydrophobic balance. We then proceed with a discussion of the structure of micelles formed by block copolymers with strongly dissociating PE blocks in salt-free and salt-added solutions. Subsequently, we analyze the responsive behavior of nanoaggregates formed by copolymers with pH-sensitive PE blocks. The predictions of the analytical models are systematically complemented by the results of a molecularly detailed self-consistent field (SCF) theory. Finally, the theoretical predictions are compared to the experimental data that exist to date. 

IPECs Based on Micelles of Ionic Amphiphilic Diblock Copolymers... 

Further investigations carried out by Chelushkin et al. [73] for various combinations of micelles formed in aqueous media by ionic amphiphilic diblock copolymers [PS-fc-P4VPQ, poly(styrene)-fetocA -poly(sodium acrylate) (PS- -PANa), and PS- -PMANa)] with oppositely charged linear PEs (polycarboxylates, polysulfonates, polyphosphates, and aromatic, aliphatic, and alicyclic quaternized polyamines) have demonstrated that the solubility of IPECs is decisively determined by (1) the aggregation state of the excess polymeric component (micelles versus individual polymeric coils) and (2) the procedure or method employed for the preparation of such macromolecular co-assemblies. 

The aforementioned group has also studied the interaction of amphiphilic diblock copolymers with surfactants carrying the same charge as the polyelectrolyte block [123]. The ionic block in this study was poly(A/-alkyl-4-vinylpyridinium bromide) and the cationic surfactants added were CTAB, DTAB, CPB, and DPB. It was shown that the surfactants form different structures with the charged amphiphilic diblock copolymers. The type of structure formed depends on the surfactant concentration. At low surfactant concentration (hydrophobic clusters are formed that consist of the surfactant alkyl chains and the hydrophobic block of the copolymer. At high surfactant concentration, a change in complexation character is reported [123]. 

Ionic amphiphilic diblock copolymers are well known to self-assemble into core-corona aggregates (micelles) in aqueous media. The micelle comprises a hydrophobic core formed by nonpolar blocks and a hydrophilic corona built up from polyelectrolyte blocks. The properties of such macromolecular self-assemblies are reviewed in detail elsewhere [57, 58]. In many cases, the micelles are characterized by a spherical morphology. When the radius of the hydrophobic core is considerably smaller than the thickness of the polyelectrolyte corona, such macromolecular self-assemblies are regarded as star-like micelles (Eig. 6b). 

They resemble star-shaped polyelectrolytes with a large number of arms, though the number of arms in such macromolecular self-assemblies might change if the micelles are of dynamic nature, that is, if they are able to change their aggregation numbers with variations in the environmental conditions. Historically, the micelles of ionic amphiphilic diblock copolymers were the first star-like polyionic species involved in interpolyelectrolyte complexation and their IPECs have attracted considerable attention during the recent years. 

A method for preparation of a composite based on the three-dimensional nanosized copolymer template has been discussed [118]. The IPEC of the metallo-containing PEI-Ag polycation with the ionic amphiphilic diblock copolymer PS-h-PAA was obtained for further synthesis of encapsulated metal NPs. The silver NPs with diameter 20-40 nm were successfully synthesized in coronas of micelles (Fig. 24). In this case, PEI was used as both reducing and stabdizing agent. The cryo-TEM images suggest that the Ag content determines the size and spatial distribution of silver NPs. 

Block copolymers are more complex. Only a few remarks arc made below, and immediately after we revat to the more common surfactants, a plan that is followed in the rest of the book. A hnear hydrophihe polymer such as polyethylene oxide (PEO) is attached at one of its terminals to a more hydrophobic polymer such as polypropylene oxide (PPO). The result is an amphiphile PEO-PPO, called a diblock copolyma-. Similarly, PEO-PPO-PEO is a triblock copolymer, another common block copolymer. The blocks range up to hundreds of repeat units. The insoluble block can crystallize or form glass. Ionic blocks are also available, although the nonionic block copolymers are more common. 

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