The Speakers

 Influence of charge fraction and sequence on polyelectrolyte solution and brush properties

Polyelectrolytes exhibit unique solution properties compared to neutral polymers due to charge repulsion along the backbone that increases chain size and results in viscoelastic behavior even at low polymer concentration. Consequently, polyelectrolytes are extensively used in industrial applications, including as thickeners and rheology modifiers for aqueous coatings and flocculation agents for colloids and wastewater treatment. They also play a fundamental role in biological processes, including intracellular phase separation and joint lubrication. Polyelectrolytes may also be anchored onto surfaces to create brush architectures that offer flexible design parameters for imparting tailored interfacial functionality at the nanoscale. The influence of charge sequence and fraction on polyelectrolyte solution and brush behavior, however, is lacking. Here, we use solid phase peptide synthesis (SPPS) and surface-initiated copper(0) controlled radical polymerization (SI-CuCRP) to produce polymers with controlled sequence and charge fractions. Systematic studies using small-angle X-ray scattering (SAXS) and 3D single molecule tracking reveal that charge fraction and sequence influence polyelectrolyte solution conformation and phase behavior, as well as brush height and transport properties. 

Is rigidity percolation the precursor of colloidal gelation?

One major goal in condensed matter is identifying the physical mechanisms that lead to arrested states of matter, especially gels and glasses. The complex nature and microscopic details of each particular system are relevant. However, from both scientific and technological viewpoints, a general, consistent, and unified definition is of paramount importance. Through Monte Carlo computer simulations of states identified in experiments, we demonstrate that dynamical arrest in adhesive hard-sphere dispersions is the result of rigidity percolation with coordination number, <nb>=2.4. This corresponds to an established mechanism leading to mechanical transitions in network-forming materials. The same physical scenario has been recently reported in patchy colloidal dispersions. Thus, our findings connect the concept of critical gel formation in colloidal suspensions with short-range attractive interactions to the universal concept of rigidity percolation. Finally, we also discuss how gravity might affect the process of gel formation.

Living Copolymerizations as a Tool to Tailor the Architecture and Fracture Properties of Soft Polymer Networks.

Soft materials find widespread use as elastomers and hydrogels because of their ability to sustain large reversible deformations. These materials are constituted of polymer networks and typically rely on chain friction to dissipate strain energy in the vicinity of cracks. Thus, soft materials have conventionally been viscoelastic and tough at room temperature or elastic and brittle at high temperatures or water concentrations. 

In 2003, Gong and co-workers introduced a network architecture that might help solve this problem, interpenetrating a stiff and brittle filler network within a soft and extensible matrix network. The resulting materials, referred to as multiple-networks, dissipate considerable strain energy by scission of filler network bonds and, as a result, are remarkably tough at high temperatures or water concentrations. However, how the architecture of the filler and matrix networks ultimately affects the fracture toughness still remains unknown.

In this talk, I will discuss the role of filler network architecture on the fracture toughness of multiple-networks. The key result is that more homogeneous filler networks, as enabled by controlled radical copolymerizations like RAFT or ATRP, afford tougher multiple-networks than analogues synthesized by free radical copolymerization because of stress delocalization in the vicinity of cracks. Such control over the architecture and fracture properties of multiple-networks is essential for designing the next generation of soft and tough materials.

Tuning reversible wet adhesion of catechol-inspired surface primer 

Nature-inspired wet adhesives based on marine organisms have emerged as a promising mechanochemical platform to develop robust wet adhesives. These adhesives contain catecholic 3,4-dihydroxyphenylalanine or DOPA. However, the oxidation of DOPA at basic pH limits its use in situations where adhesion is required over a wide range of pH values. To overcome this limitation, we are investigating hydroxypyridinone (HOPO) compounds, a catechol substitute derivative, as a candidate for multifunctional wet adhesives. Using a Surface Forces Apparatus, mica surfaces, and high ionic strength conditions, we report the pH dependence adhesion of HOPO compounds and compare their performance against catechol-containing compounds of similar molecular architecture. Our results reveal that HOPO’s peak adhesion force in acidic conditions (pH 3) is comparable to that of catechol-containing compounds (Ead = -8.5 mJ/m2). Furthermore, the peak adhesion force decreased 75% when switching from acidic (pH 3) to alkali (pH 10) conditions, like catechol-containing compounds. However, when changed back to acidic conditions (pH 3), HOPO compounds retained the initially measured peak adhesion force, which was not observed for catechol-containing compounds.  The observed robustness could be used to guide the design of a new generation of wet adhesives with improved performance.

Polyelectrolyte Complex Scaffoldings for Wet Adhesives and 3D Bioprinting Inks

Photocrosslinkable precursors (small molecules or polymers) undergo rapid crosslinking upon photoirradiation, forming covalently crosslinked hydrogels. The spatiotemporally controlled crosslinking, which can be achieved in situ, encourages the utility of photocrosslinked hydrogels in biomedicine as bioadhesives, bioprinting inks, and extracellular matrix mimics. However, the low viscosity of the precursor solutions results in handling difficulties owing to unwanted flows and dilution and compromises the strength of the photocrosslinked hydrogels. In this talk, I will introduce oppositely charged triblock polyelectrolytes as additives for precursor solutions that transform the precursor solution into a self-assembled polyelectrolyte complex (PEC) hydrogel with enhanced shear strength and viscosity, providing interim protection against precursor dilution and mitigating secondary flows. The PEC network also augments the properties of the photocrosslinked hydrogels. Crosslinking of the precursors upon photoirradiation results in the formation of interpenetrating polymer network hydrogels with PEC and covalently-linked networks that exhibit shear moduli exceeding the linear combination of the moduli of the constituent networks and overcome the tensile strength–extensibility tradeoff that restricts the performance of covalently-linked hydrogels. The reinforcement approach will be shown to be compatible with four types of photocrosslinkable precursors, not require any modification of the precursors, and introduce minimal processing steps, paving the way for broader translation of photocrosslinkable materials for biomedical applications.

Self-assembly of magnetic Janus particles via Brownian Dynamics simulations 

The potential anisotropy in Janus colloidal particles offers additional control over their aggregation into various cluster sizes and morphologies. This attribute is equally applicable to magnetic Janus particles, where the magnetic dipole is located towards the particle's surface, away from its center of mass. To understand the cluster structures that evolve over time and gain valuable microstructural insights, we employ Brownian dynamics (BD) simulations of a dilute suspension of these particles. The simulations generate a phase diagram that illustrates different dynamic formations of clusters, based on the dipolar shift (the ratio between the displacement of the dipole and the particle radius) and the dipolar coupling constant (the ratio between the magnetic dipole-dipole and Brownian forces). Each region in the phase diagram corresponds to unique nucleation and growth behaviors, as well as the orientational ordering of dipoles inside clusters. To extend this work further, we conducted simulations on self-propelled particles, a typical outcome of catalytic Janus particles. The final structure formation is dictated by two competing mechanisms: the process that restores the cluster structure (magnetic interactions) and another process that is enabled by self-propulsion, which breaks down the clusters at fast propulsion speeds. This deviation from the results obtained in the absence of self-propulsion underscores the influence of propulsion in driving the formation of structures in these systems.

Lipid sorting and the signature of protein-lipid interactions at the membrane interface

Cell membranes provide a barrier from the surroundings and between compartments in the cell. Lipids are major constituents of cell membranes and self-assemble into a bilayer due to their amphiphilic nature. Lipid diversity and composition varies greatly across organisms and even among organelles inside the cell. Lipids are actively involved in cellular processes and signaling cascades; yet, our understanding of their implication at the molecular level is just emerging. This study focuses on modeling protein-lipid interactions in the context of mechanisms of cell death and membrane permeabilization. MLKL is a pseudo-kinase that translocates to and permeabilizes the plasma membrane during the last stage of necroptosis. This form of programmed cell death results in leakage of the cell contents, producing an inflammatory response. It is of great interest to understand the molecular mechanisms of this cellular pathway as it can be leveraged for treatment of cancer and inflammatory diseases. Using all-atom molecular dynamics, we identified specific protein-lipid interactions between an MLKL monomer and a realistic model for the plasma membrane using six lipid species to mimic the charge and structural properties of this bilayer. Our results indicate initial binding is driven by electrostatic interactions of Lys and Arg residues. Inositol and cholesterol lipids are repeatedly recruited to the protein binding site, though different protein bound conformations result in a specific molecular signature after lipid lateral resorting. We show the four-helical bundle binds and inserts deeper into the binding leaflet, in agreement with predictions from experimental observations. In general, our simulations provide insights into molecular interactions and remodeling at the lipid membrane interface.

A first principles theory of non-equilibrium amorphous states of matter.

Glassy and gelled states are observed in both manufactured and natural materials, and are influenced by dynamic arrest mechanisms, phase separation, and aging phenomena. These materials' properties are time-dependent and influenced by their preparation method, unlike ordinary equilibrium phases (gases, liquids, and crystalline solids) that maximize entropy. The complexity in their physical description, thus, poses one of the greatest unsolved problems for theoretical physics and condensed matter. In this presentation, however, I will review the development of a theoretical description of irreversible processes, fully conceived by the Mexican statistical physics community, and which can reveal the mystery of amorphous solids, their non-equilibrium phases, and their time-evolving phase diagrams, given the underlying molecular interactions and the preparation protocol. In particular, I will focus on the non-equilibrium scaling laws that the theory predicts for the aging and equilibration dynamics during glass formation. Finally, I will touch on some exciting applications, such as characterizing soft materials containing composite mixtures of different sizes, systems with competing interactions, and ferrofluids.