On the Origins of Morphological Complexity in Block Copolymer Surfactants

ABSTRACT:We report the formation of a highly entangled and interconnected, selfassembled, wormlike-cylinder network of polystyrene-block-poly(ethylene oxide) in N, N-dimethylformamide/water. In this system, N,N-dimethylformamide was a common solvent and water was a selective solvent for the poly(ethylene oxide) blocks. The degrees of polymerization of the polystyrene and poly(ethylene oxide) blocks were 962 and 227, respectively. The network was formed at copolymer concentrations higher than 0.4 wt % and consisted of self-assembled, wormlike cylinders that were interconnected by Y-shaped, T-shaped, and multiple junctions. The network morphology was visualized with transmission electron microscopy. Capillary viscometry measurements revealed an order-of-magnitude increase in the inherent viscosity of the colloidal system upon the formation of the network. A similar effort to obtain a wormlike-cylinder network in an N,N-dimethylformamide/acetonitrile system, in which acetonitrile was a selective solvent for the poly(ethylene oxide) blocks, was unsuccessful even at high copolymer concentrations; instead, the wormlike cylinders showed a tendency to align. The viscosity measurements also did not show a substantial increase in the inherent viscosity. Thus, the solvent played a critical role in determining the formation of the self-assembled, wormlike-cylinder network. This formation of the network resulted from an interplay between the end-capping energy, bending energy (curvature), and configurational entropy of the self-assembled, wormlike-cylinder micelles that minimized the free energy.

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Self‐assembled, wormlike‐cylinder network of polystyrene‐block‐poly(ethylene oxide) micelles in solution

Bhargava¹,

Zheng²,

Quirk³

et al. 2006

“…As the hydrophilic fraction is increased in the copolymer, vesicles, cylindrical micelles, and spherical micelles (in the order of increasing interfacial curvature,) are the most common morphologies (Figure 1). 5,9,10 Physically, this is explained as a balance between entropic freedom of the hydrophilic coronal chains and shielding of the hydrophobic blocks from the aqueous solution; as the hydrophilic fraction increases, the chains are more able to effectively stabilize these assemblies without close-packing, and the free energy of the system is lowered when the coronal chains are provided more entropic freedom/mobility through increased curvature. …”

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confidence: 99%

Self‐assembly and responsiveness of polypeptide‐based block copolymers: How “Smart” behavior and topological complexity yield unique assembly in aqueous media

Ray¹,

Johnson²,

Savin³

2013

Polypeptide-based amphiphilic block copolymers are an attractive class of materials given their ability to form welldefined aqueous nanoassemblies that respond to external stimulus through secondary structure transitions. This report will highlight recent literature in the area of polypeptide-based block copolymer self-assembly, with the major focus being on how the responsive nature and structural complexity of the polypeptide blocks can be incorporated into systems with complex topologies such as ABA/BAB/ABC triblock copolymers, AB 2 and A 2 B star copolymers, and miktoarm l-ABC star terpolymers. In particular, the role of interfacial curvature changes and how they result in morphology transitions will be discussed. The 'smart' assembly properties of peptides in complex block copolymer topologies can lead to enhanced responsiveness, morphological complexity, and unique morphological transitions with varying solution conditions. V C 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 508-523 KEYWORDS: amphiphiles; biomimetic; block copolymers; selfassembly INTRODUCTION Amphiphilic block copolymers are able to self-assemble into well-defined nanostructures in aqueous solution. [1][2][3][4][5] The equilibrium morphology and aggregation number of diblock copolymer assemblies is primarily determined by the balance of three energetic factors: (1) interfacial tension, (2) corona chain crowding, and (3) core chain stretching. The balance of these three factors dictates an equilibrium curvature for the aggregate. 6-8 Typically, solution morphologies formed by amphiphilic block copolymers follow a trend of increasing interfacial curvature. As the hydrophilic fraction is increased in the copolymer, vesicles, cylindrical micelles, and spherical micelles (in the order of increasing interfacial curvature,) are the most common morphologies (Figure 1). 5,9,10 Physically, this is explained as a balance between entropic freedom of the hydrophilic coronal chains and shielding of the hydrophobic blocks from the aqueous solution; as the hydrophilic fraction increases, the chains are more able to effectively stabilize these assemblies without close-packing, and the free energy of the system is lowered when the coronal chains are provided more entropic freedom/mobility through increased curvature.

“…[1][2][3] For each of these morphologies, the idealized packing parameter p 4 is readily determined by a comparison of the core volume per molecule to its interfacial area: p < 1/3 for spherical micelles, p ¼ 1/3 À 1/2 for cylindrical worm micelles, and p ¼ 1/2 À 1 for membrane-bound vesicles. This simple and widely used shape parameterization serves to show that worm micelles not only are an intermediate microphase but also occupy the narrowest sliver of the phase diagram: Dp sph ¼ 0.333 versus Dp wm ¼ 0.167 versus Dp ves ¼ 0.50.…”

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confidence: 99%

Block copolymer worm micelles in dilution: Mechanochemical metrics of robustness as a basis for novel linear assemblies

Vijayan

Discher

2006

Over the last decade, it has become increasingly clear that amphiphilic block copolymers composed of strongly segregating blocks will self-direct their assembly in water into the same basic aggregate structures found for small surfactants: spheres, cylinders, and vesicular membranes. [1][2][3] For each of these morphologies, the idealized packing parameter p 4 is readily determined by a comparison of the core volume per molecule to its interfacial area: p < 1/3 for spherical micelles, p ¼ 1/3 À 1/2 for cylindrical worm micelles, and p ¼ 1/2 À 1 for membrane-bound vesicles. This simple and widely used shape parameterization serves to show that worm micelles not only are an intermediate microphase but also occupy the narrowest sliver of the phase diagram: Dp sph ¼ 0.333 versus Dp wm ¼ 0.167 versus Dp ves ¼ 0.50. In other words, worm micelles not only require more carefully proportioned hydrophilic-to-hydrophobic block ratios to achieve stability but also are likely to be less stable than other morphologies. Indeed, although spherical micelles and vesicles of lipids and surfactants have been extensively characterized, there are relatively few studies and applications of lipid-or surfactantbased worm micelles, in part because stresses and thermal defects accumulate over the length of any embedded linear object and tend to disrupt it. Block copolymers lead to more robust assemblies and thus open up new opportunities.What block copolymers provide, of course, is a broadened choice of both the composition and molecular weight versus small amphiphiles. 2,5 The simplest strong segregation theory balances the interfacial tension against the chain elasticity and predicts that core diameter d of worm micelles will scale with molecular weight N of the hydrophobic block. Within experimental error, 6 the scaling iswhere a is the monomer size. To understand the implications of such scaling for chemical stability as well as mechanical stability, particularly in stirring, extrusion, and sonication, old and new theories of surfactancy and soft-matter stability can be invoked. First, recent studies of polymer vesicles and polymersomes have taught us that interfacial tension c, established at the semisolvated interface between the core and corona, is independent of N. 7 For a series of diblock copolymers composed of poly(ethylene oxide)