Chain Exchange Kinetics of Bottlebrush Block Copolymer Micelles

Scaling Relationship of Complex Coacervate Core Micelles: Role of Core Block Stretching

The scaling relationship of complex coacervate core micelles (C3Ms) has been investigated experimentally and theoretically. The C3Ms are formed by mixing two oppositely charged block copolyelectrolyte solutions (i.e., AB + AC system) and are characterized by small-angle neutron (SANS) and X-ray scattering (SAXS). Scaling relationships for micellar structure parameters, including core radius, total radius, corona thickness, and aggregation number, all with respect to the core block length, are determined. A scaling theory is also proposed by minimizing the free energy per chain, leading to four regimes depending on the core and corona chain conformations. Although the corona block is significantly longer than the core block, the structure of our C3Ms is consistent with that of the crew-cut I regime. A highly swollen core by water enables the core blocks to be stretched significantly and corona chains to be minimally overlapped.

Effect of Bottlebrush Poloxamer Architecture on Binding to Liposomes

Poloxamers─triblock copolymers consisting of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO)─have demonstrated cell membrane stabilization efficacy against numerous types of stress. However, the mechanism responsible for this stabilizing effect remains elusive, hindering engineering of more effective therapeutics. Bottlebrush polymers have a wide parameter space and known relationships between architectural parameters and polymer properties, enabling their use as a tool for mechanistic investigations of polymer-lipid bilayer interactions. In this work, we utilized a versatile synthetic platform to create novel bottlebrush analogues to poloxamers and then employed pulsed-field-gradient NMR and an in vitro osmotic stress assay to explore the effect of bottlebrush architectural parameters on binding to, and protection of, model phospholipid bilayers. We found that the binding affinity of a bottlebrush poloxamer (BBP) (B-E₁₀⁴³P₅¹⁵, M_n ≈ 26 kDa) is about 3 times higher than a linear poloxamer with a similar composition and number of PPO units (L-E₉₃P₅₄E₉₃, M_n ≈ 11 kDa). Furthermore, BBP binding is sensitive to overall molecular weight, side-chain length, and architecture (statistical versus block). Finally, all tested BBPs exhibit a protective effect on cell membranes under stress at sub-μM concentrations. As the factors controlling membrane affinity and protection efficacy of bottlebrush poloxamers are not understood, these results provide important insight into how they adhere to and stabilize a lipid bilayer surface.

Dynamics and Equilibration Mechanisms in Block Copolymer Particles

Self-assembly of block copolymers into interesting and useful nanostructures, in both solution and bulk, is a vibrant research arena. While much attention has been paid to characterization and prediction of equilibrium phases, the associated dynamic processes are far from fully understood. Here, we explore what is known and not known about the equilibration of particle phases in the bulk, and spherical micelles in solution. The presumed primary equilibration mechanisms are chain exchange, fusion, and fragmentation. These processes have been extensively studied in surfactants and lipids, where they occur on subsecond time scales. In contrast, increased chain lengths in block copolymers create much larger barriers, and time scales can become prohibitively slow. In practice, equilibration of block copolymers is achievable only in proximity to the critical micelle temperature (in solution) or the order-disorder transition (in the bulk). Detailed theories for these processes in block copolymers are few. In the bulk, the rate of chain exchange can be quantified by tracer diffusion measurements. Often the rate of equilibration, in terms of number density and aggregation number of particles, is much slower than chain exchange, and consequently observed particle phases are often metastable. This is particularly true in regions of the phase diagram where Frank-Kasper phases occur. Chain exchange in solution has been explored quantitatively by time-resolved SANS, but the results are not well captured by theory. Computer simulations, particularly via dissipative particle dynamics, are beginning to shed light on the chain escape mechanism at the molecular level. The rate of fragmentation has been quantified in a few experimental systems, and TEM images support a mechanism akin to the anaphase stage of mitosis in cells, via a thin neck that pinches off to produce two smaller micelles. Direct measurements of micelle fusion are quite rare. Suggestions for future theoretical, computational, and experimental efforts are offered.

High-χ, low-N micelles from partially perfluorinated block polymers

Kinetically trapped ("persistent") micelles enable emerging applications requiring a constant core diameter. Preserving a χN barrier to chain exchange with low-N requires a commensurately higher χ_core-solvent for micelle persistence. Low-N, high-χ micelles containing fluorophobic interactions were studied using poly(ethylene oxide-b-perfluorooctyl acrylate)s (O₄₅F_X, x = 8, 11) in methanolic solutions. DLS analysis of micelles revealed chain exchange only for O₄₅F₈ while SAXS analysis suggested elongated core block conformations commensurate with the contour lengths. Micelle chain exchange from solution perturbations were examined by characterizing their behavior as templates for inorganic materials via SAXS and SEM. In contrast to the F₈ analog, the larger χN barrier for the O₄₅F₁₁ enabled persistent micelle behavior in both thin films and bulk samples despite the low T_g micelle core. Careful measures of micelle core diameters and pore sizes revealed that the nanoparticle distribution extended through the corona and 0.52 ± 0.15 nm into the core-corona interface, highlighting thermodynamics favoring both locations simultaneously.

Unravelling the Mechanism of Viscoelasticity in Polymers with Phase-Separated Dynamic Bonds

Incorporation of dynamic (reversible) bonds within polymer structure enables properties such as self-healing, shape transformation, and recyclability. These dynamic bonds, sometimes refer as stickers, can form clusters by phase-segregation from the polymer matrix. These systems can exhibit interesting viscoelastic properties with an unusually high and extremely long rubbery plateau. Understanding how viscoelastic properties of these materials are controlled by the hierarchical structure is crucial for engineering of recyclable materials for various future applications. Here we studied such systems made from short telechelic polydimethylsiloxane chains by employing a broad range of experimental techniques. We demonstrate that formation of a percolated network of interfacial layers surrounding clusters enhances mechanical modulus in these phase-separated systems, whereas single chain hopping between the clusters results in macroscopic flow. On the basis of the results, we formulated a general scenario describing viscoelastic properties of phase-separated dynamic polymers, which will foster development of recyclable materials with tunable rheological properties.

Free Energy Trajectory for Escape of a Single Chain from a Diblock Copolymer Micelle

We use umbrella sampling to compute the free energy trajectory of a single chain undergoing expulsion from an isolated diblock copolymer micelle. This approach elucidates the experimentally unobservable transition state, identifies the spatial position of the maximum free energy, and reveals the chain conformation of a single chain as it undergoes expulsion. Combining umbrella sampling with dissipative particle dynamics simulations of A₄B₈ micelles reveals that the core block (A) of the expelled chain remains partially stretched at the transition state, in contrast with the collapsed state assumed in some previous models. The free energy barrier increases linearly with the Flory-Huggins interaction parameter χ up to large interaction energies, where the structure of the otherwise spherical core apparently deforms near the transition state.