Session P44: Focus Session: Block Copolymer Micelles and Polymersomes

Modeling of Interactions Between Spherical Micelles for Diblock Copolymers in Selective Solvents

The self-assembly of spherical micelles formed in systems with a diblock copolymer AB, consisting of a solvent-philic block (B) and a solvent-phobic block (A), in selective solvents (S) is studied here. Effective interactions between spherical micelles for a model system are quantified using self-consistent field modeling in real space, for the dilute regime $\phi_{\rm AB} < 0.2$, as well as using a pseudo-spectral implementation of SCFT, for the concentrated regime $\phi_{\rm AB} >0.2$. We show that the free energy of BCC, and FCC phases can be described in terms of a single effective pair potential that depends on micelle aggregation number, but the aggregation number changes significantly with concentration as well as temperature.   

Dissipative particle dynamics study of relationship between wall thickness and size in polymer vesicles

Vesicles and membrane properties have long been thought to be essential for reproducing the natural environment of living cells. By using dissipative particle dynamics method, we have studied the relationship between wall thickness and size of vesicles obtained from A1BnA1 block copolymers, where block A is hydrophilic and block B is hydrophobic. Our findings suggest that, the wall thickness is sensitive to the size of vesicles at a low block length ratio of B/A, but insensitive to the size at a large ratio. It shows both weak and strong effects with a crossover point in between. These behaviors are consistent with the experimental results of Eisenberg and co-workers. Besides, an additional crossover point also has been observed. With the B/A ratio increases, the relationship goes from strong to weak behavior, and this transformation first appears to affect the outer area for large sized vesicles, and then to the inner area for small sized vesicles. These results may also be useful in delivery applications through controlling the hydrophobic membrane and the hydrophilic coronas.   

Effect of Hydrophobic Block Length on Vesicle Formation of Block Copolymers

Self-assembly of amphiphilic block copolymers result in a wide range of aggregates in aqueous solution, including spheres, rods, lamellae, vesicles, and large compound vesicles with an inverted core-shell structure surrounded by hydrophilic surface. In the work, we carried out a systematic investigation of A1BnA1 triblock copolymers with different hydrophobic block length by using the dissipative particle dynamics (DPD). As the B/A ratio increase further, no well-defined disk-like micelles (bilayer structures) are seen. Spherical micelles rearrange internally and swell the hydrophilic group in the center until vesicles are formed. Because the longer chains appear to drift slowly, it is clear that the rearrangement process becomes slower in the system with longer hydrophobic block, which needs more time to form vesicles. The segregation of A segments (hydrophilic group) leaves in the internal layer when the two small spherical micelles merge into a larger one. We observe that the existence of the two pathways of vesicles formation based on the hydrophobic block length using DPD particle models.   

Nanostructured assemblies from amphiphilic ABC multiblock polymers

Amphiphilic AB diblock copolymers containing a water compatible segment can self-assemble in aqueous media to give supramolecular structures that include simple spherical micelles and macromolecular vesicles termed polymersomes. Amphiphilic ABA triblocks with hydrophobic end blocks can adopt analogous structures but can also form gels at high polymer concentrations. The structural and chemical diversity demonstrated in block copolymer micelles and gels makes them attractive for applications ranging from drug delivery to personal care products to nanoreactors. The inclusion of a third block in amphiphilic ABC triblock systems can lead to a much wider array of self-assembled structures that depend not only on composition but also on block sequence, architecture and incompatibility considerations. I will present our recent efforts on tuning micelle and gel structure and behavior using controlled architecture ABC triblocks. The combination of diverse polymer segments into a single macromolecule is a powerful method for development of self-assembled structures with both new form and new function.   

The Morphology of Lipid Aggregates based on the Interplay among Molecular Architectures, Hydrophobic-Hydrophilic and Coulombic Interactions and their Kinetics

Lipids resemble amphiphilic diblock copolymers in the many ways in terms of rich resultant structures, which are controlled by spontaneous curvature and molecular interaction, except that the molecular length scale of interest is smaller. One of the most commonly studied systems is so-called ``bicelle'' (``bilayered micelle''), which is composed of a long- and a short- chain lipid, spontaneously forming discoidal micelles in aqueous solutions under certain conditions. The lamellae of ``bicellar mixtures'' have been used as magnetically alignable templates for structural study of membrane-associated proteins since they provide a native bilayered environment for the proteins in study. In this talk, I will summarize how and why a variety of morphology can be obtained from the ``bicellar'' system based on the molecular architectures (spontaneous curvatures) of the lipids, inter-particle Coulombic interaction and hydrophobic-hydrophilic interaction. Most interestingly, many of these structures are kinetically controlled but have robust formation mechanisms and high stability. Currently, we are particularly interested in how the exchange rate of the lipid molecules affects the kinetics.   

Phase Behavior and Kinetics of Diblock Copolymer in Selective Solvent

Synchrotron based time-resolved small angle x-ray scattering (SAXS) was used to study the kinetics of the formation of a gyroid phase in solutions of a poly (styrene -isoprene) (SI) diblock copolymer in dimethyl phthalate, a selective solvent for the polystyrene block. Temperature ramp measurements over the range of 70-130C show the transition from hexagonally-packed cylinders (HEX) to Gyroid phase for 75{\%} and 80{\%} (w/v) samples to be 117C and 96C, respectively. Results of temperature jump experiments to different jump depths to examine the kinetics of this transition will be presented. In addition to the Bragg scattering from the ordered phases, we were able to observe the temperature dependence of the diffuse scattering near q=0. The temperature dependence of the correlation length shows a crossover from $\surd $T near the glass transition for polystyrene to linear in T near the HEX to Gyroid transition. The effect of adding low molecular weight linear homopolymer PS to the samples on the phase behavior will be discussed.   

Phase Behavior of Gradient Copolymer Solution

We study the behavior of amphiphilic linear gradient copolymer chains under poor solvent conditions. Using Bond Fluctuation model and parallel tempering algorithm, we explore qualitative behavior of this class of polymers with varying gradient strength; which is the largest difference in the instantaneous composition along the polymer chain. Under poor solvent conditions, the chains collapse to form micelles. We find a linear dependence of hydrophilic to hydrophobic transition temperature on gradient strength. Systematic analysis of these clusters reveals a strong dependence of micelle properties on gradient strength. Also, we discuss our results with reference to recent experiments on synthesis and cloud point depression in gradient copolymers confirming gradient strength as key parameter in tuning micelle properties.   

Self-Assembly of Novel Amphiphilic 21-Arm, Star-Like Coil-Rod Diblock Copolymers at Interfaces

A series of novel amphiphilic 21-arm, star-like diblock copolymers, poly(acrylic acid)-$b$-poly(3-hexylthiophene) (PAA-$b$-P3HT) based on $\beta $-cyclodextrin ($\beta $-CD) with well defined molecular architectures and ratio of two chemically distinct blocks were prepared, for the first time, via a combination of quasi-living Grignard metathesis method (GRIM), click reaction, and atom transfer radical polymerization (ATRP). The star-like PAA-$b$-P3HT diblock copolymers consist of hydrophilic coil-like PAA cores and hydrophobic rod-like P3HT shells with narrow molecular weight distribution and controllable molecular weight of each block. Owing to the compact structure, the amphiphilic star-like PAA-$b$-P3HT formed a unimolecular micelle. Vesicles based on these novel amphiphilic star-like, coil-rod diblock copolymers were readily produced at the oil/water interface by crosslinking hydrophilic coil-like PAA cores with a bifunctional crosslinker, ethylenediamine. They also self-assembled into a nanotubular structure at the air/water interface.   

Formation and Characterization of Anisotropic Block Copolymer Gels

Cylindrical micelles formed from block copolymer solutions closely mimic biological fibers that are presumed to guide mineral formation during biosynthesis of hard tissues like bone. The goal of our work is to use acrylic block copolymers as oriented templates for studying mineral formation reactions in model systems where the structure of the underlying template is well characterized and reproducible. Self-consistent mean field theory is first applied to investigate the thermodynamically stable micellar morphologies as a function of temperature and block copolymer composition. Small-angle x-ray scattering, optical birefringence and shear rheometry are used to study the morphology development during thermal processing. Initial experiments are based on a thermally-reversible alcohol-soluble system that can be converted to an aqueous gel by hydrolysis of a poly(t-butyl methacrylate) block to a poly(methacrylic acid) block. Aligned cylindrical domains are formed in the alcohol-based system when shear is applied in an appropriate temperature regime, which is below the critical micelle temperature but above the temperature at which the relaxation time of the gels becomes too large. Processing strategies for producing the desired cylindrical morphologies are being developed that account for both thermodynamic and kinetic effects.   

Stimuli-responsive block copolypeptides

Stimuli-responsive polypeptide block copolymers are appealing systems due to the morphological polymorphic states they can exhibit in selective solvents, including micelles, vesicles, fibrils, and more complex supramolecular aggregates. Their morphologies can be engineered a-priori by the relative block lengths, the solvent composition, and their concentration. However controlling the morphology upon external stimuli offers clear benefits, since changes in structure and morphology can be induced on demand. In this talk, we present two examples of stimuli-responsive block copolypeptides: in a first case, a photoresponsive PLGA-PEO diblock is discussed capable to reversibly undergo micellization-dissolution-micellization upon visible or Uv light exposure, due to spiropyrans units decorating the PLGA block; in the second case a PBLG-PDMS-PBLG triblock copolymer undergoing reversible thermal-induced organogelation is also presented. The changes in morphology are correlated, in both cases to the variations in molecular conformations of the polypeptide blocks.