Session G33: Programmed Responsive Polymers and Soft Matter I

Modeling Liquid-Solid Phase Transitions in Suspensions of Compressible Microgels

Microgels are soft colloidal particles, made of crosslinked polymer gels, whose internal degrees of freedom allow them to respond to external stimuli by changing size. Their sensitive responses to changes in temperature and concentration inspire practical applications, e.g., to drug delivery and photonic crystals. To explore the influence of particle compressibility on thermodynamic phase behavior, we modeled suspensions of microgels that interact via the Hertz pair potential and swell/deswell according to the Flory-Rehner theory of polymer networks. From extensive Monte Carlo simulations that incorporate novel trial changes in particle size [1], we determined the liquid-solid phase boundary (microgel density vs. crosslink density) by computing free energies, osmotic pressures, and chemical potentials of both liquid and solid phases via thermodynamic integration methods [2]. Our results significantly extend previous studies of the phase behavior of (incompressible) hertzian spheres [3]. We further computed latent heats of melting, radial distribution functions, and static structure factors.

[1] M. Urich and A. R. Denton, Soft Matter 12, 9086 (2016).

[2] C. Vega et al., J. Phys.: Condens. Matter 20, 153101 (2008).

[3] J. C. Pàmies et al., J. Chem. Phys. 131, 044514 (2009).   

Swelling Behavior and Structure of Binary Mixtures of Ionic Microgels

Ionic microgels are soft, responsive colloids that exhibit unique swelling behavior due to their osmotic pressure-driven changes in volume. When dispersed in solution, microgels absorb solvent and can become charged by releasing counterions. Understanding the osmotic pressure and swelling behavior of ionic microgels is crucial for designing and optimizing applications, e.g., in drug delivery, tissue engineering, and biosensing. For binary mixtures of ionic microgels that carry fixed charges on their surfaces, we derive effective electrostatic interactions. We model steric interparticle interactions via Hertz elastic pair potentials and single-microgel swelling via Flory-Rehner theory of polymer networks. Within this coarse-grained modeling framework [1-3], we perform Monte Carlo simulations, with trial microgel size changes, to compute equilibrium swelling ratios and radial distribution functions. For size- and charge-asymmetric mixtures, we observe unusual self-healing behavior, in qualitative agreement with experiments [4]. Our results can illuminate fundamental principles governing swelling behavior and structural properties of ionic microgel mixtures, offering a foundation for designing novel responsive materials.

[1] T. J. Weyer and A. R. Denton, Soft Matter 14, 4530 (2018).

[2] A. R. Denton and M. O. Alziyadi, J. Chem. Phys. 151, 074903 (2019).

[3] M. E. Brito, A. R. Denton, and G. Nägele, J. Chem. Phys. 151, 224901 (2019).

[4] A. Scotti et al., Phys. Rev. E 96, 032609 (2017).   

A Granular Actuator Made of Electrically Conductive Grains

Phase-changing composites used for soft robotic actuation typically consist of an elastomer matrix containing pockets of low boiling point solvent. Under heat, the solvent undergoes a phase-change from liquid to gas, producing a rapid volumetric expansion of the bulk composite. Recently, such phase-changing composites have been discretized from a bulk composite into sub-millimeter-sized grains, called granular actuators. Granular actuators have multiple solvent cores and individually expand by up 700% when heated, opening possibilities for multi-scale actuation and tuneable actuator bulk properties. A single granular actuator can be used for microscale actuation, while aggregates for macroscale actuation. Granular actuators can also be jammed (solid-like) or flow (liquid-like) by modulating external pressure, and bulk actuator moduli can be tuned via the packing crystallinity of the actuating grains. However, granular actuators currently require a co-located or external heat source, which is difficult to integrate into a field-deployable soft robot and leads to relatively non-uniform heating that is difficult to control. To solve this, we fabricated granular actuators wherein each grain is coated in a conductive shell, imbuing electrical conductivity and Joule heating. We confirmed the conductivity of random packings made of conductive granular actuators, and further observed distributed actuation of individual grains throughout the packing. In this talk, I will discuss how the conductive coating impacts the mechanical and actuation metrics for individual grains, and how varying particle size can enhance or diminish the electrical conductivity of the particles. In the future, this system could be used for addressable granular actuators, with control over the actuation of an assembly on a localized scale. We also foresee this system being used as a tool to study and visualize the distribution of current through an assembly of deformable and active grains.   

Thermomechanical Coupling in Monodomain and Polydomain Liquid Crystal Elastomers

Liquid crystal elastomers (LCEs) are rubbery polymer networks with liquid crystal mesogens linked to their polymer chains. These materials can generate large deformation and mechanical work upon temperature change or light illumination, making them ideal candidates for new applications including soft robots, metamaterials, and other stimuli-responsive devices. However, despite the rapid development of LCEs in new chemistry, manufacturing, and applications, the fundamental thermomechanical coupling between the microscopic temperature-controlled processes and the macroscopic mechanical behaviors of these materials remains poorly studied when combining theory and experiment. It remains further unclear how the modern fabrication methods such as the widely used "two-step polymerization" affect this thermomechanical coupling. This talk will present our recent progress in thermomechanical behaviors of nematic LCEs under both mechanical load and prescribed temperature. Through experiments with quantitatively controlled fabrication parameters, we show a nontrivial effect of the applied pre-stretch during polymerization on the thermomechanical behavior of nematic monodomain and polydomain LCEs. To understand the experimental observation, we establish continuum theories based on classical free energy models that incorporate the entropic elasticity of the polymer network and the long-range directional molecular interaction between mesogens. These efforts are hoped to help advance the fundamental knowledge of liquid crystal elastomers, bring together communities of relevant research fields, and further expand the large-scale applications of these materials.   

Interaction of amphiphilic block copolymers with abiotic lipid membranes and with cells

Certain commercial block copolymers in the poloxamer class (e.g., triblocks of poly(ethylene oxide) PEO and poly(propylene oxide) PPO) are known to interact with cell membranes, with either stabilizing or disruptive effects, depending on the situation. However, the mechanisms by which these effects arise are not yet established. In a broad effort to elucidate the key molecular variables, we have quantified the extent binding of a variety of PEO-PPO diblocks and triblocks with model lipid membranes, using pulsed-field-gradient NMR. In addition to these architectural variants, we have examined less hydrophilic blocks such as poly(butylene oxide) PBO, and also novel bottlebrush poloxamers. The ability of certain poloxamers to modulate lipid bilayer mechanical properties has been quantified using a micropipette aspiration assay. In order to relate the degree of binding to therapeutic efficacy, these molecules have also been compared in their ability to stabilize cell membranes against oxidative stress. The lessons learned from these various studies will be discussed, and remaining questions highlighted   

Self-growth of hydrogels under constraints

Using the 3D gel lattice spring model (gLSM), we explore the effects of boundary and structural constraints (stiff fibers) on polymer gel morphology during growth. During the initial stage of growth (Stage 0), a flat parent gel swells in a solution of monomers and cross-linkers until it reaches equilibrium. The presence of constraints during swelling causes a spatially non-uniform distribution of monomers in the gel sample that leads to buckling of the gel. The actual growth of the gel occurs during Stage 1, when the species, which diffused into the parent gel, undergo polymerization and cross-linking followed by inter-chain exchange to form a random copolymer network (RCN). The resulting RCN gel replicates the shape of the Stage 0 parent network and remains buckled even after the removal of the structural constraints. We explore scenarios of growth with various placements of the structural constraints (fibers).  The study could lead to potential applications in fabrication of shape morphing and controlled gradient materials.   

Adaptive networks: erosion of microfluidic channels

Microfluidics has enabled major advances in biology, where it has allowed to study and explore microscopic flows as they happen in biological systems. However, such microfluidic devices often fail to mimic a key aspect of living systems: their adaptability to incoming flows.

Here, we make a first step towards adaptive microfluidic networks by working with hydrogels (PEG-NB, polyethylene glycol norbornene) that erode in the presence of a flow of chemical. From the quantitative experimental data gathered, we show the erosion and absorption laws of the system, and develop consistent numerical simulation. Those allow us to easily explore the parameters space, and find adequate conditions to homogenize the flows in random networks. This homogenization is essential in biological systems to enhance the exchanges happening constantly at the interfaces.   

Influence of stimulus-responsive swelling on suspension properties of ionic microgels

Microgels can swell in response to environmental changes, enabling precise control over particle traits. Despite various models attempting to explain microgel swelling and its effects on bulk properties, a comprehensive theory linking microscopic behaviors to suspension properties remains a challenge.

In this work, we study theoretically the effects of swelling of weakly-crosslinked ionic microgels on bulk properties. By combining Poisson-Boltzmann theory for the electrostatic interactions with Flory theory for modeling polymer degrees of freedom, we present a theoretical model for swelling of stimulus-responsive microgels in dependence of concentrations, charge and pH. Our mean-field type calculations are corroborated with particle-based coarse-grained simulations in order to assess their accuracy and scope. Furthermore, we analyse the influence of different microgel architectures on swelling. With the help of density functional theory, we derive expressions for the microgel-microgel effective interactions for different architectures. We use the resulting mean-field effective pair potentials for analysing the impact of swelling on thermodynamic- and structural properties as well as on the phase behaviour. Our results are also compared with experimental results. This work provides a first-principle theoretical description that allows for an accurate characterisation of microgel suspensions, useful for an optimal synthesis and a trustworthy result interpretation.   

Ordered pattern formation in electro-responsive polymer ionic liquid blends

Formation of complex morphological patterns, through non-contact mechanisms, is one of the key techniques to design multifunctional systems from synthetic soft materials. These techniques have been harnessed to fabricate devices from soft matter for a variety of energy-efficient and biomimetic applications. Using modelling and simulation, here we develop an efficient mechanism, based on reaction-diffusion (RD) phenomena, that harnesses uniform electric field to create complex self-induced patterns in polymer ionic liquid (PIL) blends via phase separation. Our approach utilizes Poisson Boltzmann Nernst Planck (PBNP) equations to capture the long-range interactions of ionic liquid, both in the weak and strong segregation limits. We demonstrate that the ordered patterns in our PIL system are self-induced and robust and can be tuned by changing the direction of the electric field. Moreover, the calculations of the circularly averaged structure factor reveals that the electric field significantly increases the domain growth rate and their ordering remarkably. We believe this non-invasive technique is a significant step towards the development of ordered structures at microscopic length scales and can be utilized for in micro-scale fabrications from soft materials.   

Sequence and molecular weight controlled phase behavior of liquid crystalline oligomers

In polymeric liquid crystals (LCs), it is often difficult to deconvolute the effects of the mesogenic core from those of molecular weight (and its dispersity), non-mesogenic dilutions, and topology (such as linker length and branching). To better understand these relationships and target specific LC phase behavior, we employed an iterative exponential growth synthetic strategy to combine two mesogenic cores and prepare monodisperse triazole-linked LC oligomers of precise lengths and sequence. We analyzed the phase transitions of these oligomers using calorimetric techniques, contrasting them with those of comparable species prepared via conventional step-growth polymerization. The structures of these phases were elucidated through optical and X-ray studies, revealing phase behavior that is dependent on both molecular length and mesogenic sequence. We highlight the thermodynamic and kinetic factors determining phase transitions in these materials, thus developing design rules for tailored and precise LC behavior.   

Water vapor formation from hydrogels via photothermal heating of nanoparticles

Hydrogels are proposed to decrease the energy input needed to form water vapor in processes such as solar desalination. These hypotheses relate to the interactions between water and the gel, such as formation of surfaces or water clusters where evaporation is facilitated or changes in internal gel hydrophilicity. In addition, focusing energy at regions where vapor is most likely to form (rather than heating the bulk) also increases efficiency. Incorporation of larger (50-200 nm diameter) nanoparticles into/onto the hydrogel aids both processes - potentially creating additional internal disorder and, if the nanoparticle is photothermally active, providing a means to transition solar light into local heat. We present results on changes in vapor formation efficiency in sodium acrylate and N-isopropylamide hybrids, embedded with either melanin (MNP) or gold nanoparticles (GNP). Photothermal conversion is well established for GNP but impractical for applications. The photothermal properties of melanin are nascent, but MNPs ability to be mass produced and their innately biocompatible nature are significant advantages.   

Tuning non-ergodic aging in hydrogels using environment-dependent interfacial chemistry

Hydrogels are polymeric soft materials that can be engineered to suit a multitude of applications that exploit their tunable mechanochemical properties. Dynamic hydrogels formed of polymer-nanoparticle assemblies employing noncovalent, physically crosslinked networks dominated by either enthalpic or entropic effects enable unique rheological and stimuli-responsive properties. As opposed to enthalpy-driven dynamic networks that soften with temperature, entropic interactions result in largely temperature-independent mechanical properties. By engineering interfacial polymer-nanoparticle interactions, we induce a dynamic-to-covalent transition in entropic hydrogels that leads to non-ergodic aging in the microstructure. This transition is tuned by varying temperature and formulation environment such as pH, which allows for multivalent tunability in properties. These hydrogels can thus be designed to exhibit either temperature-independent metastable dynamic crosslinking or time-dependent stiffening based on formulation and storage conditions. Such robust materials with versatile and adaptable properties can be utilized in applications such as wildfire suppression, surgical adhesive films, and depot-forming injectable drug delivery systems.   

Modelling the effect of phase behaviour on the deformation in light-activated shape memory polymer blends

Light-activated shape memory polymers (LASMPs) are smart materials that transform back to their intended shape when stimulated by light. Although, unlike traditional SMPs, LASMPs have significantly faster response time and therefore, are enabling materials for a variety of remote actuation and non-invasive applications, their synthesis via chemical route is quite cumbersome. In contrast, blending constituent polymers to synthesize SMPs is straightforward, but their incompatibility leads to the formation of interfaces and thereby, results in inferior mechanical properties. Here, we develop a a non-linear viscoelastic constitutive model that captures the effect of interfacial properties and component miscibility on mechanical deformation in LASMP blends under isothermal conditions. Our investigations reveal that the interfacial stresses between phase-separated domains can be tweaked by manipulating the interfacial energy coefficient and thereby, increasing the mechanical strength of LASMP blends. The advantage of our non-empirical approach is that it provides an efficient pathway to design photo-chemical/thermal LASMPs with tailored properties. Our findings can also be extended to capture the structure-property relationships for other multi-component multi-phase polymer blends.