Session K19: Self-Assembly and Phase Separation of Polymers and Charged Soft Matter II

Liquid-liquid phase separation between jammed soft particles

The recent discovery of phase separated liquid droplets within living cells has led to a burgeoning interest in liquid-liquid phase separation (LLPS). These liquid droplets act as membrane-less organelles, performing biochemical functions by condensing a variety of biomolecules as they form and participating in intracellular signaling processes. The complexity of the cell's interior creates the need to understand LLPS within more complex environments than in simple mixtures of ideal polymers. In a recent example of such work, LLPS was studied within a crosslinked polymer network, where the network's elasticity was shown to modulate the size and the ripening of phase separated droplets forming within it. We see a need to extend such ideas to systems of jammed soft particles, as the cell's interior can exhibit the rheological properties of jammed matter. In this presentation, we will discuss our investigations of LLPS within jammed emulsions. We will show how the presence of jammed emulsion droplets influences the phase diagram of silicone oil mixtures that undergo LLPS near room temperature. The phase behavior will be connected to measurements of the interfacial tension between the phase separated liquids and to the yield stress of the jammed emulsions.   

Obtaining stable bicontinuous cubic structures in lyotropic liquid crystals

Amphiphilic polymers can self-assemble into lyotropic liquid crystals of various stable phases such as micellar, hexagonal, lamellar, sponge, and bicontinuous cubic. Imidazolium-based amphiphilic monomers in polar solvents can self-assemble into bicontinuous cubic structures under the right composition and temperature. These lyotropic liquid crystal membranes, with their uniform and 3D continuous channels, are excellent candidates for several filtration applications. Molecular-level understanding is required to guide the design of these membranes with desired physical and chemical properties. To that end, we use molecular dynamics simulations to study the structure and transport through these membranes. We have developed procedures to construct all-atom and coarse-grain models of bicontinuous cubic structures that involve parameterization and assembly of the components. We then use these models to study the phase behaviors and their sensitivity to factors such as head group chemistry, monomer length, solvent dielectric properties, and so on. Moreover, we investigate the molecular transport through these membranes, including ways to optimize selectivity and permeability.   

Lyotropic Liquid Crystal Ionogels under Large Amplitude Oscillatory Shear (LAOS)

Amphiphilic block copolymers (BCPs) self-assemble in the presence of selective solvents and form different types of micellar structures depending on the composition and temperature. At relatively high concentrations of BCP, micelles form ordered lyotropic liquid crystals (LLCs) structures, also known as mesophases. LLCs usually have a polydomain structure with locally anisotropic ordered domains (grains) that are randomly orientated throughout the sample. In this study, ionogels are prepared from LLCs of poloxamer/ionic liquid/oil. We investigate the macroscopic alignment of the LLC ionogels under large amplitude oscillatory shear (LAOS). We show that the development of the overall orientation of the lamellar microstructure can be followed in situ, using the time progression of the dielectric loss modulus ε"(t) and conductivity of the samples. The evolution of Lissajous curves with alignment duration demonstrates mechanical signals of parallel and perpendicular alignments. The knowledge brought by this work can potentially be used in future research of templating technologies for microelectronics, biomaterials, and separations.   

Effect of Charged Block Sequence on Dilute Polyampholyte Solution Behavior

Polyampholytes (PAs) are polymers containing both positively and negatively charged groups along their backbone. Previous studies have found pH and salt greatly affect phase behavior and viscoelastic properties of PA solutions through electrostatic interactions. However, limitations in PA synthesis prevented further exploration of charge arrangement on these systems. Nonetheless, simulations have shown larger oppositely charged blocks lead to greater collapse due to attractive Coulombic interactions. Using solid phase peptide synthesis, we synthesized a set of poly-L-(lysine, glutamic acid) peptides with neutral net charge consisting of 32 residues arranged in increasing block sizes. Solubility of these PAs decrease as block size increases, and the addition of NaCl increases solubility. We observed charge block size may play a more complex role in the collapse of the PAs especially relative to ionization state and counterion interactions, from pH titration and scattering experiments, respectively. PAs at high salt concentrations behave more similarly to neutral polymers due to electrostatic shielding, and PAs at extreme pH's behave more consistent with polyelectrolytes. This study demonstrates charge sequence is as notable as charge density and symmetry in tuning PA materials.   

Effect of Co-ions on the Self-Assembly Behavior of Macroions

Simulating charged macromolecules (macroion) in aqueous solutions is challenging, as such systems usually include complex inter-/intra-molecular interactions. In addition, while the effects of smaller co-ions are known to be important, modeling the behavior of co-ions is difficult. In order to overcome these difficulties, a coarse-grained(CG) model was developed to capture the main features of macroions and compare it with an ideal model system of macroions to validate our method. {Mo₇₂Fe₃₀} polyoxometalates(POMs) is a rigid, hydrophilic, perfectly spherical macroion and known to self-assembly into vesicle-like “blackberry” structure in solutions. The smaller co-ion is K₇[α-PW₁₁O₃₉] (PW₁₁) POMs. The simulation results are found to be consistent with experimental observation—the {Mo₇₂Fe₃₀} can self-assemble into blackberry, while the Keggin co-ions stay well-dispersed in the solution, when they are the only species in the solution, respectively. When the macroion {Mo₇₂Fe₃₀} and the smaller co-ion PW₁₁ co-exist, they co-assemble into a thermodynamically favorable blackberry state that contains both {Mo₇₂Fe₃₀} and PW₁₁. Furthermore, the effect of hydrophobicity of the POMS compared to co-ions and the charge density of co-ions were also simulated and discussed in this presentation.   

Polarity Controlled Lithium Salt Partition in Diblock Copolymers and Polymer Blends

The role of polymer polarity in determining conductivity of polymer electrolytes is increasing in awareness. The inherent tradeoff between polymer polarities and segmental dynamics limits ionic conductivity for single-polymer electrolytes. We hypothesize that blending two polymers, one with fast dynamics (low viscosity) and the other with high polarities (strong ionic solvation), may be a viable strategy to mitigate the aforementioned limit. It is found that upon blending, lithium salt is partitioned in two phases with a prefence to the high-polarity polymer phase. This salt partition imposes a negative derivation from the average of ionic conductivity of these two polymers. To further understand the phase separation and polymer-polymer interactions with lithium salt addition, we investigated the self-assembly of a diblock copolymer consisting of high-polarity and low-polarity blocks. By simulating SAXS patterns of this copolymer with different loadings of lithium salts, we found that the polymer-polymer interactions decreased monotonically as the salt loading increased. We also quantifiy the salt partition between two blocks and our thermodynamic analysis revealed that this partition process is enthalpy controlled. Our results advance the fundamental understanding on polymer blend electrolytes and reveal that ionic conduction can be improved by promoting simultaneous miscibility and polarity contrast between the polymer hosts.   

Multiscale Modeling Approach of Multicompartment Micellization towards Nano-Reactor Application

Recently, multicompartment micelle (MCM) consisting of multiblock copolymers has attained lots of attention due to its application towards nanoscale molecular reactor. In order to successfully achieve such molecular reactor via multicompartment micelle, we need to have a theoretical/computational way to quantitatively predict the micelle structure from the chemical structures of monomeric units in multiblock copolymers. Although Flory-Huggins Chi parameter has been widely used in numerous experimental/theoretical studies, it should be noted that a method for chemical structure based assessment of Chi parameter has not been thoroughly completed yet. Therefore, the overarching goal in this study is to establish a computational method for quantitative assessment of Chi parameter for molecular pairs using DFT and molecular mechanics simulation. Through this study, interaction energy density and coordination number have been revisited and the molecular volume ratio is newly considered into our computational procedure. After calculating a set of chi parameters for a multiblock copolymer in the presence of solvent, we perform DPD simulations for actual multicompartment micelle formation.   

Understanding Coupled Transport in Semiconducting Polymers for Analysis of Energetic Trace Materials

Organic electrochemical transistors provide a means to examine the intimately coupled mass and charge transport that occurs in polymer mixed conductors. Here, we evaluate this coupled transport while applying polymer blends in the detection of improvised explosives, in particular ammonium nitrate. Specifically, our OECT devices utilized either a conductive poly(3,4-ethylene dioxythiophene) poly(styrene sulfonate) blend or a semiconducting poly[3,4-propylene dioxythiophene-(2,2,4,4-tetramethylpiperdin-1-yl)oxyl] homopolymer as their active layer materials. Exhibiting distinct conductivities and operation modes, their behavior is compared over a range of electrolyte compositions and concentrations while illustrating the device’s sensing capabilities. The polymers’ behavior is elucidated by modeling their transport exhibited when gated with dilute electrolytes reminiscent of anticipated sensor analytes. Additionally, OECTs that include a molecularly imprinted polymer (MIP) cast atop the device structure produce size-exclusive electrolyte gating. By properly preparing the OECT sensor, we explain the contributions of both the channel and electrolyte gate composition in ion and charge transport in these mixed conducting macromolecular systems.   

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The use of lithium metal anodes in all-solid-state batteries has emerged as the most promising technology for replacing conventional lithium-ion batteries. Here, we report a new class of elastomeric solid-state electrolytes having a three-dimensionally interconnected plastic crystal phase with an unprecedented combination of mechanical robustness, high ionic conductivity, low interfacial resistance, and high lithium-ion transference number. The in-situ-formed elastomer electrolyte on copper and lithium foils accommodates volume changes for prolonged lithium plating/stripping processes. Under the constrained conditions of a high-loading LiNi_0.83Mn_0.06Co_0.11O₂  cathode and limited lithium source, the full cells with the elastomer electrolytes deliver high specific energy exceeding 410 Wh kg_{anode+cathode+electrolyte}^-1 at ambient temperature. The elastomeric electrolyte system presents a powerful strategy for enabling stable operation of high-energy, all-solid-state lithium metal batteries.   

How does morphology impact the mechanical properties of ultrathin block copolymer films?

Thin films are critical to enhancing technologies such as packaging, membranes, and flexible electronics. However, below 100 nm in thickness, polymer chain behavior differs from the bulk. Here, we discuss a recent study that focuses on elucidating the role of entanglements and microstructural domains in determining mechanical strength and ductility for ultrathin films. We use poly(styrene-b-2-vinylpyridine) as a model system, where the block copolymer domains have similar properties. The morphology of the films is altered through solvent vapor annealing and measured using The Uniaxial Tensile Tester for UltraThin films (TUTTUT). These results show how relative positioning of phase-separated blocks within a thin film can significantly alter a film's mechanical response. For a freestanding thin film measurement, ordered lamellar morphology films withstand higher stresses than disordered films. Placing these films on a water surface during uniaxial extension caused local domain plasticization, increasing failure strains in comparison to freestanding films. Through this research, we are expanding the knowledge of how molecular entanglements and mobility influence the mechanical response of thin glassy polymer films and developing design principles for creating strong thin materials.   

Effect of Electrostatic and Hydrophobic Interactions on the Rheology of Aqueous Dispersions of OSA-Modified Phytoglycogen Nanoparticles

Phytoglycogen (PG) is a natural polysaccharide produced in the form of compact, 44 nm diameter, electrically neutral nanoparticles in the kernels of sweet corn. Its highly branched, dendritic structure and hydrophilic nature leads to interesting and useful properties that make the particles ideal as unique additives in personal care, nutrition and biomedical formulations. The applications of PG can be extended by increasing the hydrophobicity of the particles. We have covalently attached charged, hydrophobic octenyl succinic anhydride (OSA) chains to the surface of PG at low (OSA_LDS) and high (OSA_HDS) degrees of substitution, and we have measured the rheology of dispersions of the particles in water. The results for OSA_LDS-modified PG dispersions were dominated by electrostatics: the dispersions developed solid-like rheology at a much smaller concentration (20% w/w) than that for native PG dispersions (32% w/w) but quickly liquified with added NaCl. For OSA_HDS-modified PG dispersions, hydrophobic interactions were more significant and we observed evidence for clustering of the particles with added NaCl. These results highlight the unique interplay between electrostatic and hydrophobic interactions of the particles and suggest new applications for OSA-modified PG.   

Hydration Properties of Chemically Modified Phytoglycogen Nanoparticles

As a natural and sustainable biopolymer, phytoglycogen (PG) nanoparticles extracted from sweet corn are promising candidates for a wide range of applications.  We modified the particles by attaching charged chemical groups (both anionic and cationic), which produced distinctive changes to their physical properties.  Further changes were achieved through the addition of salts to tune the electrostatic interaction between particles. I will describe the hydration properties of PG nanoparticles that were modified using either anionic (carboxymethylation; CM-PG) or cationic (glycidyltrimethylammonium chloride; GTAC-PG) groups.  We used ellipsometry and attenuated total reflection infrared (ATR-IR) spectroscopy to probe changes in the swelling properties and network water structuring at relative humidity values ranging from 4 to 90%. We observed dramatic differences between the hydration properties of modified-PG and native PG particles, with significant changes produced by the addition of monovalent (NaCl) and divalent (CaCl₂) salts.  These studies demonstrate that chemical modification of PG nanoparticles can be used to create new nanomaterials with dramatically different physical properties for enhanced applications.