We report the synthesis and solution characterization of poly(L-lysine)-b-poly(propylene oxide)-b-poly(L-lysine) (KPK) triblock copolymers with high lysine weight fractions (>75 wt%). In contrast to PK diblock copolymers in this composition range, KPK triblock copolymers exhibit morphology transitions as a function of pH. Using a combination of light-scattering and microscopy techniques, we demonstrate spherical micelle-vesicle and spherical micelle-disk micelle transitions for different K fractions. We interpret these morphology changes in terms of the energy penalty associated with folding the core P block to form a spherical micelle in relation to the interfacial curvature associated with different charged states of the K block.

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.

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