Session F43: Polymer Physics Prize

Prize Talk: Polymer Physics PrizeOrigin of the Entropic Driving Force in Polyelectrolyte Complex Coacervation

Mixing two solutions of oppositely charged polyelectrolytes under appropriate conditions results in a liquid–liquid phase separation into a polymer-rich coacervate phase and a coexisting polymer-poor supernatant phase. This polyelectrolyte complex coacervation (PCC) has received considerable attention in recent years due to its relevance to membraneless organelles in biology, and applications in biomedical and biomimetic systems. The complexation of oppositely charged polymers has been widely believed to be driven by the entropy gain due to counterion release. In this talk, we show that a large portion of the entropy change is due to solvent (water) reorganization, which we can extract by exploiting the temperature dependence of the dielectric constant. For weakly-to-moderately charged systems under common conditions (monovalent ions, room temperature in aqueous solvent), the solvent reorganization entropy, rather than the counterion release entropy, is the primary entropy contribution. We use this framework to examine the two elementary stages in the symmetric PCC—the complexation between a polycation and polyanion, and the subsequent condensation of the polycation–polyanion pairs by computing the potential of mean-force (PMF) using molecular dynamics simulation. From the calculated PMF, we find that the supernatant phase consists predominantly of polyion pairs with vanishingly small concentration of bare polyelectrolytes, and we provide an estimate of the spinodal of the supernatant phase. Finally, we show that prior to contact, two neutral polyion pairs can attract each other by mutually induced polarization, providing the initial driving force for the fusion of the pairs.   

Invited Talk: Frank Bates Structure and dynamics of tetrahedrally coordinated block copolymer particles

Since the discovery of the Frank-Kasper (FK) sigma phase in block polymers in 2010 there have been numerous experimental and theoretical reports of various tetrahedral close-packed particle phases in this class of soft matter. This presentation will summarize recent small-angle X-ray scattering (SAXS) and self-consistent mean-field theory (SCFT) results demonstrating the role of core block compatible homopolymer on the formation of the sigma, C14 and C15 phases, along with BCC and HCP structures. Mixtures of A-B and A¢-B¢ type diblocks will also be discussed, where super-cooling a disordered diblock copolymer melt led to the development of a dodecagonal quasicrystalline (DDQC) state, which transformed into the sigma phase after long time annealing. A judicious processing strategy afforded access to aperiodic (DDQC) and periodic (sigma) order in the same system at the same temperatures, providing unprecedented access to comparative X-ray photon correlation spectroscopy measurements, which revealed sizable differences in the associated particle dynamics. SCFT predictions motivated the investigation of compositionally asymmetric A-B and A-C diblock blends by SAXS, where the particle core-forming B and C blocks are thermodynamically incompatible. These most recent results also will be described.

 

Research conducted in collaboration with: Zachary Gdowski, Andreas Mueller, Aaron Lindsay, Ronald Lewis III, Quingteng Zhang, Suresh Narayanan, Timothy. P. Lodge, Kevin D. Dorfman, Mahesh. K. Mahanthappa

    

From Polymers to Bosons: Can AMO Physics Benefit from Polymer Field Theory?

Feynman’s work on path integral descriptions of helium-4 revealed a deep connection between the statistical mechanics of Bose superfluids and classical ensembles of reacting ring polymers. This analogy is the basis for the predominant finite-temperature quantum simulation technique, path integral Monte Carlo (PIMC). While PIMC is implemented using a basis of particle coordinates, equivalent field-theoretic representations of the quantum many-body problem utilize linear combinations of occupation number states known as coherent states (CS). We recently found that methods for simulating CS-inspired field theories of classical reacting polymers work equally well on quantum field theories of cold atoms. Moreover, quantum field-theoretic simulations offer similar advantages over coordinate-based methods as in the classical context, including linear scaling with system size and direct access to free energies.

We have applied this new field-theoretic simulation method to investigate the thermal phase behavior of a model of cold alkali atoms subject to Rashba spin-orbit coupling, a current topic in AMO physics and an exciting venue for creating mesostructured quantum states such as spin stripes. Our work has revealed that spin stripes in 2D melt into a new quantum “spin microemulsion” phase that resembles bicontinuous microemulsions observed in classical block copolymer and surfactant systems. The defect-mediated nature of the phase transition resembles that predicted by Toner and Nelson for the 2D nematic to isotropic phase transition of classical liquid crystals and verified in monolayers of cylinder-forming block copolymers by Kramer.   

Decoupling the Effects of Charge Density and Hydrophobicity on the Phase Behavior and Viscoelasticity of Complex Coacervates

Complex coacervation is an entropically driven, associative liquid-liquid phase separation that results in a polymer-rich coacervate and a polymer-poor supernatant. A number of studies have looked into the effect of salt concentration on the phase behavior and mechanical response of the resulting coacervate. However, the effect of copolymer chemistry on the phase behavior and viscoelasticity of complex coacervates is not well understood. To understand the influence of copolymer chemistry, we developed a library of oppositely-charged methacrylate copolymers of varying charge density and hydrophobicity, with which we studied the coacervate phase behavior and viscoelasticity as a function of salt concentration. Our results show that polymer charge density and hydrophobicity drastically affect the phase behavior, with charge density dictating the salt stability and hydrophobicity controlling the polymer concentration of the complexes. We used small amplitude oscillatory shear to study the viscoelastic response of complex coacervates, and time-salt superposition to examine the dependance of salt concentration. We take advantage of different copolymer chemistries to construct time-salt-copolymer master curves that have not been shown before in the literature. Our phase diagrams and rheological data show evidence of charge-dominated and hydrophobicity-dominated regimes. Finally, we highlight how copolymer chemistry can be used to tune the mechanical properties of complex coacervates.   

Solvation Time Scales in Polymer Electrolytes for Lithium Batteries

The term "solvation" generally applies to the immediate neighborhood of the working ion in dilute liquid electrolytes. In this limit, the neighborhood - approximately a sphere - is dominated by solvent molecules, whence the term solvation. The solvent that dissolves ions can either be a polymer or a low molecular weight solvent (liquid). In the case of polymers, the solvation shell comprises polymer segments that translate coherently with the working ion for a short while before they diffuse away to Brownian motion. The time scale for this coherent translation (solvation lifetime) was measured in a polymer electrolyte comprising poly(pentyl malonate) and a lithium salt by quasi elastic neutron scattering (QENS). We obtained a value of about 1 nanosecond. This measurement was enabled by a unique QENS signature of solvation lifetime, arising due to the presence of multiple chains in the solvation sphere. The experimental results are compared with molecular dynamics simulations without resorting to any adjustable parameters. Our measurements may be characterized as "ultraslow", compared to the solvation life time in aqueous electrolytes of 1 picosecond. We use our simulations to estimate the solvation lifetime of organic liquid electrolytes - whether this value is 1 picosecond or in excess of 100 picoseconds remains unresolved.