Session K28: Focus Session: Microphysical Properties of Block Copolymer Aggregates II

Polymer Vesicles in Biomimetic Applications

The performance of phospholipid vesicles in targeted pharmaceutical delivery is greatly improved by the addition of water-soluble polymeric tethers. The resulting solvated brush resists protein adsorption and access of biological entities to the hydrophobic membrane core, evading immune response. Polymeric vesicles have, due to their increased stability and greater variability in chemical and mechanical properties, huge potential relative to lipsomes. They may ultimately form the basis for artificial cells and scavengers, or in non-biomedical applications, distributed microreactors. To this end, we explore the incorporation of biomimetic features in polymer lamellae: membrane phase separation and ``rafts'', bending fluctuations and budding, triggered release, and dynamic engulfment. Vesicle phase separation, bending, lamellar disruption, and lamellar wetting all are rooted in block copolymer and polymer brush physics. For instance, vesicles blended from two block copolymers, polystyrene-co-poly(ethylene oxide) (PS-PEO) and poly(butadiene)-PEO (PBD-PEO) exhibit no macroscopic phase separation, though blends of PS and PBD at the same molecular weight do. This represents either an upward shifting of the UCST or a suppression of large-scale morphology. Also interesting is the ability of PS-PEO to form robust, vesicle-sized, capsules carrying aqueous phase cargo. While we doubt the true vesicular nature of these structures, they are often robust to passing of a liquid-air contact line. Their morphology is highly dependent on copolymer composition. The capsules are subject to buckling instabilities and in some instances, glassy fracture with sudden content release. Lamellar physics also plays an important role in the scavenging and adhesion capabilities of polymer vesicles: While the density of adhesive groups on the chain ends of incorporated tethers acts in ways easy to anticipate, engulfment dynamics depends dramatically on dynamic membrane bending.   

Transition from Unilamellar to Bilamellar Vesicles Induced by an Amphiphilic Biopolymer

We report some unusual structural transitions upon the addition of an amphiphilic biopolymer to unilamellar surfactant vesicles. The polymer is a hydrophobically modified chitosan and it embeds its hydrophobes in vesicle bilayers. We study vesicle-polymer mixtures using small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM). When low amounts of the polymer are added to unilamellar vesicles of \textit{ca}. 120 nm diameter, the vesicle size decreases by about 50{\%}. Upon further addition of polymer, lamellar peaks are observed in the SANS spectra at high scattering vectors. Using a model developed by Nallet \textit{et al}., we show that the SANS data corresponds to a co-existence of unilamellar vesicles and bilamellar vesicles (i.e., vesicles with two bilayers). The transition to bilamellar vesicles as well as the changes in unilamellar vesicle size are further confirmed by cryo-TEM. A mechanism for the polymer-induced transition from uni- to bilamellar vesicles is proposed.   

Temperature and pH Response of PB-P(Lys) Block Copolymer Assemblies

Amphiphilic block copolymers consisting of poly(butadiene) and poly(L-lysine) (PB-P(Lys)) were synthesized and their solution properties studied using dynamic light scattering and transmission electron microscopy. We exploit secondary structure changes that occur in the P(Lys) chain to observe changes in solution morphology as a function of solution conditions. At high pH, the P(Lys) chain assumes either an $\alpha$-helical or a $\beta$-sheet conformation depending on temperature, while at lower pH the side chains become protonated, resulting in an expanded coil configuration. In these studies, four molecular weights and compositions of PB-P (Lys) were studied. For short P(Lys) blocks, these block polymers assemble into vesicles that swell with decreasing pH. For longer P(Lys) blocks, cylindrical micelles are formed that undergo a morphological shift to spherical micelles with decreasing pH.   

Understanding the Self-Assembly of Amphiphilic Diblock Copolypeptides for Controlled Biomaterial Design

Copolypeptides with a hydrophilic lysine (K) block and a hydrophobic leucine (L) block were designed to self-assemble due to their amphiphilic nature and the defined secondary structure of the hydrophobic block. In aqueous solution, these copolypeptides assemble into stiff, porous hydrogels at low volume fractions of polymer (vol. fraction polypeptide (0.5 wt\%). Assembly is dictated by the secondary structure of the hydrophobic block and the polyelectrolytic character of the hydrophilic block, as revealed through microscopy and neutron scattering experiments. Understanding the self-assembly mechanism of amphiphilic diblock copolypeptides has led to the formation of a disparate range of materials. Vesicles can be produced by inducing curvature at the interface between the hydrophilic to hydrophobic blocks by altering either the charge density or the molecular weight of the polypeptide. Manipulation of the assembly kinetics, by dissolution of the polypeptide into an organic solvent with subsequent addition of water followed by evaporation of the organic component, produces twisted fibrils and hexagonal platelets. Characterization of these materials demonstrates that assembly is intrinsically controlled on the nanoscale by molecular design, most importantly by the presence of the alpha–helical hydrophobic block, and can also be influenced by assembly kinetics and manipulation of electrostatic interactions of the charged blocks.   

Laminated, Nontwisting Beta-Sheet Fibrils Constructed via Peptide Self-Assembly

A de novo designed peptide has been characterized that self-assembles into beta-sheet fibrils exhibiting a nontwisted, laminated morphology. The laminated morphology is constituted by 2.5nm wide filaments that laterally associate to form flat fibril laminates exceeding 100nm in width and microns in length. The height of each fibril is determined by the length of exactly one peptide momomer in an extended beta-strand conformation, approximately 7nm. Once formed, these fibrils are highly stable over a range of temperatures and pH and exhibit characteristics similar to those of amyloid fibrils. Kinetic parameters of pH and temperature can be used to affect the rate of beta-sheet formation and, consequently, the degree of lamination. Finally, the importance of peptide sequence on the resultant fibril morphology is demonstrated via rational peptide design and discussed in the context of current theories of fibril twisting.   

Physical Properties of Anionic Peptide Amphiphile Fibers Grown in the Presence of Cationic Proteins

We analyze the structure and mechanical properties of fibers formed by anionic peptide amphiphiles (PA) in the presence of cationic proteins with varying charge. The PA molecules, which are composed of a hydrophobic alkyl tail, a beta-sheet forming region, and a hydrophilic epitope region, self-assemble into cylindrical micelles in water at 1{\%} weight concentration in the presence of multivalent salts. The fibers grow in one dimension by forming an internal beta sheet along the middle segment; the hydrophobic tail hides inside the micelle and the epitope region is exposed to the water. Rheology and electron microscopy are used to investigate the physical properties of the resulting PA gels. The correlation between the charge of the cationic proteins used for gelation and the resulting PA gel's structure and mechanical properties are discussed.   

Dewetting instability during formation of polymerosomes from block-copolymer-stabilized double emulsions

We study the formation of polymer vesicles, or polymerosomes, from double emulsion droplets of controlled architecture produced via a microfluidic device. A volatile organic solvent containing an amphiphilic diblock copolymer is employed as the middle phase of a water-in-oil-in-water emulsion. The block copolymer assembles at the oil-water interfaces, stabilizing the interior water droplet against coalescence with the exterior aqueous phase. Upon evaporation of the organic solvent, a thin vesicle of the block copolymer is formed. We find that the presence of excess diblock copolymer in the oil phase gives rise to a dewetting phenomenon, in which the shrinking oil droplet partially wets a thin film of solvated block copolymer. This yields acorn-like morphologies similar to those commonly encountered in systems of three immiscible fluids, ultimately resulting in an inhomogeneous polymerosome structure. We propose that the dewetting may be driven by a depletion effect due to the excess diblock copolymer.   

Condensed States of a Semiflexible Copolymer in a Poor Solvent: Figures of Eight and Discrete Toroids

We examine the problem of a semiflexible (stiff) copolymer chain in a selective solvent. In the homopolymer case toroids often result. In the copolymer case the phase diagram is much richer. One can obtain striped toroids, figure eights, cages and many other stuctures. We will present simple analytical results for some of these along with computer simulations showing a variety of morphologies. One major result, is that for copolymers the structures must be discrete so the size of the toroids is effectively quantised.