Session V25: Polymer-Mediated Structural Transitions in Soft Materials

Rouleaux formation: polymer induced Red Blood Cells aggregates

Plasma proteins such as fibrinogen induce the aggregation of red blood cells (RBC) into rouleaux, which are responsible for the pronounced shear thinning of blood, control the erythrocyte sedimentation rate (ESR) – a common hematological test – and are involved in many situations of physiological relevance such as structuration of blood in the microcirculation or clot formation in pathological situations. Confocal microscopy is used to characterize the shape of RBCs within rouleaux at equilibrium as a function of macromolecular concentration, revealing the diversity of contact zone morphology. Three different configurations that have only been partly predicted before are identified, namely parachute, male-female and sigmoid shapes, and quantitatively recovered by numerical simulations. Stable polymer induced clusters in capillary flow are also observed in-vivo in a mouse model by use of in intravital epiillumination microscopy and in-vitro in a microfluidic device, wherein the protein or the macromolecule concentration can be freely varied. The interaction energies between the cells are determined by use of single cell force spectroscopy and optical tweezers. Numerical simulations of flowing cells show again a good agreement with the experimental observations.   

Extracellular polymers control bacterial biofilm expansion and material properties

Biofilms, surface-attached communities of bacterial cells, are a concern in health and in industrial operations because of persistent infections, clogging of flows, and surface fouling. In this talk, I will explore the consequences to biofilm growth and robustness when the biofilm matrix functions as a material that is responsive to environmental perturbations such as changes in osmotic pressure. Using Vibrio cholerae as the model organism, we showed that matrix production enables biofilm-dwelling bacterial cells to establish an osmotic pressure differential between the biofilm and the external environment. The pressure difference promotes colony biofilm expansion on nutritious surfaces, controls growth of submerged biofilms, and enables matrix-producing cells in biofilms to exclude non-matrix-producing cheaters and to resist invasion by planktonic cells. Furthermore, we discovered how extracellular polysaccharides, proteins, and cells function together to define biofilm mechanical and interfacial properties.
  

Glass transition temperature of poly(lactic-co-glycolic acid) particles

Poly(lactic-co-glycolic acid) (PLGA), is a commonly used biodegradable biomaterial used to make many different biomedical products including drug delivery particles. The glass transition temperature (T_g) of a polymer particle can have a profound effect on its behavior in vivo such as aggregation behavior and drug release profile. Bulk PLGA has a T_g of approximately 40°C. Since normal body temperature is 37°C, it is assumed that PLGA particles will remain in their solid, glassy state during in vivo experiments. However, many factors can affect the T_g of a particle, such as thermal history. The method used to produce the PLGA particle will therefore affect its T_g. Most experiments are done with polymer samples in the bulk form that have been annealed to erase thermal history. In this work, PLGA particles are made by nanoemulsion, nanoprecipitation, and electrohydrodynamic jetting and the T_g of the particles is measured using the first heating scan of a modulated differential scanning calorimetry run to determine the role of processing conditions on the T_g of PLGA particles.   

Polyelectrolyte-Mediated Colloidal Interactions at the Interfaces of Liquid Crystals

Nematic liquid crystals (LCs) are complex fluids within which molecules exhibit long-range orientational order leading to an anisotropic elasticity. When introduced into a LC host, colloidal particles (microcargo) elastically strain the LC and generate topological defects. Consequently, the microcargo experience strong repulsive forces near interfaces of LCs (e.g., LC-aqueous, LC-air interfaces). In this presentation, we will show that formation of polyelectrolyte and polyelectrolyte-surfactant complexes at a LC interface modulates the interactions of colloidal microcargo with that interface. We will describe elastic and electrical double layer interactions occurring at these interfaces and how the interplay of the two interactions is modulated by the formation of complexes of polyelectrolytes and surfactants. We will also illustrate how this competition of interactions can be used to trigger the release of microcargo from the LC.   

Phase behavior and structure of polyelectrolyte-nanoparticle complex assemblies

Numerous examples of complex assemblies comprising oppositely charged polyelectrolytes and nanoparticles (or proteins) in aqueous environments exist in natural and synthetic systems. The structure and stability of these complex assemblies are dictated by numerous factors including solution conditions as well as polyelectrolyte and nanoparticle properties and are far from being fundamentally understood and controlled. In this work, we examine the effect of polyelectrolyte and nanoparticle concentrations, polyelectrolyte size, flexibility, charge density and degree of ionization, nanoparticle size and effective charge density and the pH and ionic strength of the solution on the structure and phase behavior of model nanoparticle-polyelectrolyte complex assemblies comprising negatively charged silica nanoparticles and diverse polycations. Conditions that lead to destabilization and phase separation of the complexes will be identified through turbidimetric and light scattering studies. Furthermore, X-ray and light scattering investigations on the structure and morphology of phase separated and the “soluble” stable complexes will be presented.   

Dielectric virial expansion of polarizable dipolar spheres

Predicting the static dielectric permittivity for composites with polarizable components remains a theoretical challenge. The typical Maxwell-Garnett mixing rule or its variant approximates the composite permittivity with a linear average of inclusion's polarizability, which neglects the inter-particle interactions. A systematic density expansion is developed to predict the composite permittivity that includes the contribution from interactions of multiple polarizable particles. The lowest order term in such expansion reduces to the Maxwell-Garnett rule. Including the second order term results in a parameter-free prediction, which matches the independently measured values for a set of seven polymer-nanoparticle composites.   

Co-assembly of anisotropic colloids and diblock copolymer mixtures

Block copolymers are excellent candidates to template the localisation of colloids, in order to create ordered arrays of nanoparticles, with uses in optical and electrical applications. The co-assembly of block copolymer nanocomposite materials can lead to highly ordered systems in the mesoscale. Computer simulations have been used to address the complex assembly of anisotropic rod-like colloids. The crossover between the block copolymer intrinsic length scales and the shape and orientation of anisotropic nanoparticles leads to a rich variety of phase behaviors, both in terms of polymeric morphologies and colloidal organisation.
Rod-like nanoparticles have been found to form side-to-side configuration in block copolymer ultrathin films. SEM images are compared with a hybrid Cell Dynamic/Brownian Dynamic computational method which is used to characterise the assembly of anisotropic nanoparticles in diblock copolymers.   

Predicting chi for polymers with different chain architectures using simulation

The Flory Huggins interaction parameter chi governs phase behavior in polymer blends and block copolymers. Chain architecture affects how chains pack in the melt, which can significantly influence chi. To explore this, we investigate chi for two different architectures of flexible bead-spring chains, using molecular dynamics simulations and our recently developed “morphing” method to compute the excess free energy of mixing. In the first case, we consider an idealized bead-spring model of polyethylene and polypropylene, in which all beads have the same interactions, but polypropylene chains have a side bead on every other backbone bead. In the second case, we examine blends in which both chain species have the “polypropylene” bead-spring structure, but one species has beads with a slightly weaker interaction – either the side beads (case 1), main chain beads (case 2) or branch point beads (case 3). We use our method to find c for all three cases, for which random mixing models would give identical results. Finally, we compare our values with predictions from PRISM, with correlation functions from simulations as input, as a purely simulation-based test of PRISM.   

Novel Dynamical behaviors of charged Macromolecules in a crowded environment

The phenomenon of “ordinary-extraordinary” dynamical transition in salt-free polyelectrolyte solutions is well known. We report the emergence of additional dynamical mode at higher molecular weights of the polyelectrolyte. We find that the key variable Cp/Cs, which is normally used in describing the onset of the ordinary-extraordinary transition (fast and slow modes), is inadequate in describing the dynamics of polyelectrolyte solutions. For concentrated solutions of sodium polystyrene sulfonate of higher molecular weights, we observe a third intermediate mode. The emergence of the new mode depends crucially on the polymer molecular weight and the concentration of the added salt. We find that Cp/C* (where C* is overlap concentration) is a key variable in addition to Cp/Cs.   

Similarity of Crambin Lattice Protein Homologues in the Semi-flexible H0P Model

The semi-flexible H0P lattice protein model is an extension of the HP model with an extra kind of “neutral” monomer and an energetic term for “bends”. Crambin (a 46 amino acids protein) has been mapped onto the H0P model and studied by replica-exchange Wang-Landau sampling. From the obtained density of states, thermal properties of the lattice protein are extracted at all temperature. With further study by multicanonical sampling, ground state degeneracy as well as the temperature-dependence of structural quantities are measured with high resolution. In this study, we examine five H0P lattice protein homologues of Crambin, i.e. proteins with similar sequences and structures in nature. Our results show that, at low temperature, thermal properties of these H0P-modeled homologues are close to those of Crambin itself, all showing two significant conformational changes. However, results also show that the previous value for the bending energy can lead to high degeneracies for some low excited states of the homologues. Therefore, a slight change has been made to this term, to keep the degeneracies of all tested homologues low and stable.   

Is there a universal equation of state for flexible polymers beyond the semi-dilute regime?

We reconsider the isothermal equation of state (EoS) for linear homopolymers in good solvents,
p = p(c,T), which relates the osmotic pressure, p, of polymers with the bulk concentration, c,
and the temperature, T. The classical scaling theory predicts the EoS in dilute and semi-dilute
regimes. We suggest a generalized EoS which extends the universal behavior of polymer solutions
up to the highly concentrated state and confirmed it by molecular dynamics simulations and using
available experimental data. Our conjecture implies that properties of polymer chains dominate
the EoS in the presence of many-body interactions. Our theoretical approach is based on a viral
expansion in terms of concentration blobs leading to a superposition of two power laws in the regime
of concentrated solutions.   

Gelation of hydrogel films and coatings induced by substrate swelling.

Hydrogel films used as membranes or coatings are essential components of devices interfaced with biological systems. Their design is greatly challenged by the need to find mild synthesis and processing conditions that preserve their biocompatibility and the integrity of encapsulated compounds. Here, we report an approach to produce hydrogel films spontaneously in aqueous polymer solutions [1]. This method uses the solvent depletion created at the surface of swelling polymer substrates to induce the gelation of a thin layer of polymer solution. Using a biocompatible polymer that self-assembles at high concentration (poly(vinyl alcohol)), hydrogel films are produced within minutes to hours with thicknesses ranging from tens to hundreds of micrometers. A simple model predicts how the solution composition, substrate geometry and swelling properties govern film growth. We also demonstrate the potential of this technique by incorporating other solutes to fabricate ceramic-hydrogel coatings for bone anchoring of osteo-articular implants and matrices for cell-encapsulation.
[1] Moreau et al. PNAS 113(47), 13295-13300 (2016).