Session E55: Smart Responsive Polymers I

Vapour-Induced Demixing in Solution-Processed Ferroelectric Films for Organic Memory Applications

Cost-effective production of organic memory devices requires ambient solution casting of ferroelectric polymers, such as poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)). However, in response to condensation of water vapor from the environment the drying polymer solution phase separates, which yields a fluid state morphology comprising polymer-rich micro-droplets in a solvent–rich medium. The resulting rough polymer films give poor device performance. Whereas this process of vapor-induced phase separation is useful in the production of microporous membranes, it must be avoided in the manufacture of thin-film memory devices. Through experiment and numerical calculation we identify a processing window for smooth films, determined by the evaporation rate, hygroscopicity and the ambient humidity. Modeling of the phase dynamics provides important insight on how microstructure and feature size emerge during simultaneous solvent evaporation and water condensation. Calculated morphologies are consistent with experiments and elucidate the coupling between domain size and ambient humidity. Our findings support a scenario wherein the dominant feature size in the dry polymer film is to a large extent already determined in the early stages of demixing.   

Concurrent reaction and diffusion in photo-responsive hydrogels

A hydrogel consists of a polymer network imbibing a solvent. It can undergo large deformation like a soft solid originating from the polymer network underpinned by the crosslinks, as well as exchange matter with the environment like a porous medium. Hydrogels with guest species undergoing a photoisomerization reaction can respond to light in the form of a volumetric change due to the different hydrophobicity of the photoisomers. In this work, we have combined hydrogel elasticity with chemical reaction under the framework of non-equilibrium thermodynamics to study the response kinetics of the photo-responsive hydrogels. As a result, the kinetic behavior of the photo-responsive hydrogels is governed by both the reaction of the photoisomerization and the diffusion of the solvent molecules. Our model allows us to investigate different kinetic processes in photo-responsive hydrogels, including reaction-limited, diffusion-limited, and coupled reaction-diffusion processes.   

Correlation between swelling/shrinking behaviour and nanorheology of microgel particles

During the last decades microgels made of N-isopropylacrylamide (NIPAM) have attracted much interest since they can be easily transferred into multiresponsive entities. That makes them interesting as drug delivery systems but also for the fabrication of multiresponsive coatings. Via specific comonomers the thermoresponsive PNIPAM gels get pH sensitive and the charge and hydrophobicity can be tailored. PNIPAM gels show co-nonsolvency in water/alcohol mixtures which induces also a swelling/shrinking process. In order to induce fast volume phase transitions external triggers like light or magnetic fields are useful. Therefore optical switchable azo-surfactants, gold nano particles or magnetic nanoparticles are embedded within the gel.
All these different triggers induce a volume phase transition of the microgels which has an impact for actuatorics. An application might be a touch screen with haptic functionality. In order to use these microgels as actuators and for haptics one has to understand the correlation between the volume phase transition and the rheological properties of the microgel particles. The talk focuses on the effect of different synthetic parameters (cross-linker, charge, additives) and outer parameters like solvent composition on the nanorheology. Therefore static and dynamic force experiments with high spatial resolution were carried out. It turned out that there is no simple correlation between the volume of a microgel and its rheological parameters G’ and G’’.   

Swelling, Structure, and Phase Behavior of Microgel Mixtures

Microgels are soft colloidal particles made up of cross-linked polymer networks that swell in the presence of a solvent. The compressibility of the particles allows them to swell and deswell in response to changes in surrounding conditions, such as concentration, temperature, and pH. Due to their responsiveness to the environment, microgels have applications as biosensors and drug delivery vehicles. Recent experiments indicate that mixtures of microgels exhibit unusual thermodynamic phase behavior, including self-assembly of equilibrium crystal structures under conditions where mixtures of hard particles would form amorphous solids. Motivated by such experimental results, we perform Monte Carlo simulations to model binary mixtures of microgels that interact according to Hertzian elastic forces and swell according to the Flory-Rehner free energy of cross-linked polymer gels. From our simulations, we obtain particle size distributions, osmotic pressure, radial distribution functions, and static structure factors. We also analyze the fluid-solid transition and stability of competing crystal structures. Our results help to interpret experimental observations and guide design of soft materials composed of tunable, responsive particles.
  

Swelling and Structural Properties of Polymer Microgels: Simulations of a Coarse-Grained Model

Microgels are microscopic particles composed of crosslinked polymer networks, which swell in a solvent. The equilibrium size and shape of a microgel respond to environmental conditions, such as pH, temperature, crowding, and ionic strength, facilitating applications to drug delivery and biochemical sensors. The swelling behavior of microgels depends directly on the distribution of tetravalent crosslinkers, which can vary from isotropic to a core-shell structure. Within a model in which polymers are represented as chains of coarse-grained beads, we perform molecular dynamics simulations to investigate the dependence of compressibility and swelling on network structure and the response of the equilibrium swollen radius to solvent quality and external stimuli. Our results provide a benchmark test of the Flory-Rehner theory of polymer networks, which relates the swelling of a polymer gel to the elasticity of the network and the entropy of mixing between the solvent and polymer. The model can be extended to explore swelling of ionic, polyelectrolyte microgels and diffusion of solutes throughout the gel network, which can help to guide the design of smart, responsive particles.   

Transport and Solvation of Penetrant Molecules in a Thermoresponsive Hydrogel

A wide range of modern soft functional materials, such as nanofilters, nanocarriers, and drug delivery systems, are based on the uptake, release, and diffusive transport of molecules through thermoresponsive polymer architectures. External stimuli, such as changes in temperature, trigger the transition from a swollen into a collapsed state of the polymer, which dramatically alters the thermodynamic and transport properties of penetrant molecules [1,2]. We employ molecular dynamics simulations of a Poly(N-isopropylacrylamide) (PNIPAM) hydrogel with explicit water in its collapsed as well as in the swollen state [3]. The simulations enable us to gain insights into the diffusion and solvation mechanisms of various penetrant molecules in the hydrogel. We find that formations of water pockets, the presence of crosslinkers in the hydrogel, and the polarity of the penetrants play a detrimental role for the diffusion mechanisms and solvation free energies. Finally, we discuss how can the observed phenomena be exploited in hydrogel-based applications of responsive nanoreactors for nanocatalysis [4].

[1] Wu et al. Angew. Chem. 51, 2229 (2012)
[2] Kim et al. Macromolecules 50, 6227 (2017)
[3] Kanduč et al. PCCP 19, 5906 (2017)
[4] Roa et al, ACS Catal. 7, 5604 (2017)   

Osmotic Swelling Behavior of Polyelectrolyte Microcapsules

Polyelectrolyte microcapsules are hydrogel shells composed of cross-linked polymer networks, 10-1000 nm in radius, that are swollen by a solvent. The ability of microcapsules to swell/de-swell in response to external stimuli (e.g., temperature, pH, ionic strength) makes them ideal for a wide range of applications, e.g., drug delivery, sensors, and catalysis. Equilibrium swelling behavior of ionic microcapsules is determined by a balance of electrostatic and elastic forces. The electrostatic contribution to the osmotic pressure – the difference in pressure between outside and inside a capsule – plays an important role in swelling. Within the spherical cell model, we derive an exact expression for the electrostatic contribution to the osmotic pressure, which we compute via molecular dynamics simulation and Poisson-Boltzmann theory. For the elastic contribution to the osmotic pressure, we use the Flory-Rehner theory of swollen polymer networks. By combining the electrostatic and elastic contributions, we predict the equilibrium size of a polyelectrolyte microcapsule as a function of salt concentration. Our results can help guide the design of smart, responsive particles.   

Modulating the Solution Phase Behavior of a Block Polymer with Light

Polymer solutions that are able to respond to external stimuli such as temperature or light have found applications ranging from sensors to optical devices. To enable the rational design of responsive materials, our work explores the solution phase behavior of a block polymer that is both thermo- and photoresponsive. The addition of a solvent that selectively swells one block provides a route to tuning the effective volume fraction of the blocks, and this selectivity can be controlled by both temperature and light exposure. The block polymer poly(methyl methacrylate)-block-poly(benzyl methacrylate-stat-(4-phenylazophenyl methacrylate)) (PMMA-b-P(BzMA-stat-AzoMA)) in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide displays these photoresponsive properties. The PMMA block is soluble in the ionic liquid, while the solubility of the P(BzMA-stat-AzoMA) block can be tuned by both temperature and light.   

Solvent Response of Ionic Co-Polymers at Interface

Ionizable co-polymers constitute effective media for controlled ion transport. Their response to the environment affects a large range of applications from biotechnology to clean energy. Though the interplay between highly segregating blocks is known to drive rich variety of phases, their response to solvents remains an open question. The higher the blocks segregation is, the more distinctive their response becomes. Here the response of a symmetric pentablock co-polymer (A-B-C-B-A) with a center polystyrene sulfonate (PSS) block tethered to polyethylene propylene (PEP), terminated with a t-butyl styrene(t-PS), to solvent triggers was studied for two PSS sulfonation 26% and 52% . Thin films of the order of 10-20 nm were spin coated on SiO₂ and their morphology was followed as a function of exposure time to toluene using atomic force microscopy. The pristine films of 26% consist of co-continuous domains. With exposure time to toluene the hydrophobic domains expend about 10% of their initial size within the first 20min. In contrast, the 52% sulfonation film forms well defined isolated assemblies that increase two folds within the first 20min. For both polymers however, the ionic networks remain unperturbed where the packing of the hydrophobic blocks is affected.