Multi-scale simulations of polymeric nanoparticle aggregation during rapid solvent exchange

Mixing versus Polymer Chemistry in the Synthesis of Loaded Polymer Nanoparticles through Nanoprecipitation

Polymer nanoparticles (NPs) loaded with drugs and contrast agents have become key tools in the advancement of nanomedicine, requiring robust technologies for their synthesis. Nanoprecipitation is a particularly interesting technique for the assembly of loaded polymer NPs, which is well-known to proceed under kinetic control, with a strong influence of the assembly conditions. On the other hand, the nature of the used polymer also influences the outcome of nanoprecipitation. Here, we investigated systematically the relative effects of mixing of the organic and aqueous phases and polymer chemistry on the formation of polymer nanocarriers. For this, two mixing schemes, manual mixing and microfluidic mixing using an impact-jet micromixer, were first evaluated, showing mixing times of several tens of milliseconds and a few milliseconds, respectively. Copolymers of ethyl methacrylate with charged and hydrophilic groups and different polyesters (poly(d-l-lactide-co-glycolide) and poly(lactic acid)) were combined with a fluorescent dye salt and tested for particle assembly using these "slow" and "fast" mixing methods. Our results showed that in the case of the most hydrophobic polymers, the speed of mixing had no significant influence on the size and loading of the formed NPs. In contrast, in the case of less hydrophobic polymers, faster mixing led to smaller NPs with better encapsulation. The switch between mixing and polymer-controlled assembly was directly correlated to the solubility limit of the polymers in acetonitrile-water mixtures, with a critical point for solubility limits between 15 and 20 vol % of water. Our results provide simple guidelines on how to evaluate the possible influence of polymer chemistry and mixing on the formation of loaded NPs, opening the way to fine-tune their properties and optimize their large-scale production.

Single core and multicore aggregates from a polymer mixture: A dissipative particle dynamics study

HYPOTHESIS

Multicore block copolymer aggregates correspond to self-assembly such that the polymer system spontaneously phase separates to multiple, droplet-like cores differing in the composition from the polymer surroundings. Such multiple core aggregates are highly useful capsules for different applications, e.g., drug transport, catalysis, controlled solvation, and chemical reactions platforms. We postulate that polymer system composition provides a direct means for designing polymer systems that self-assemble to such morphologies and controlling the assembly response.

SIMULATIONS

Using dissipative particle dynamics (DPD) simulations, we examine the self-assembly of a mixture of highly and weakly solvophobic homopolymers and an amphiphilic block copolymer in the presence of solvent. We map the multicore vs single core (core-shell particles) assembly response and aggregate structure in terms of block copolymer concentration, polymer component ratios, and chain length of the weakly solvophobic homopolymer.

FINDINGS

For fixed components and polymer chemistries, the amount of block copolymer is the key to controlling single core vs multicore aggregation. We find a polymer system dependent critical copolymer concentration for the multicore aggregation and that a minimum level of incompatibility between the solvent and the weakly solvophobic component is required for multicore assembly. We discuss the implications for polymer system design for multicore assemblies. In summary, the study presents guidelines to produce multicore aggregates and to tune the assembly from multicore aggregation to single core core-shell particles.

Rapid Precipitation of Ionomers for Stabilization of Polymeric Colloids

Polymeric colloids have shown potential as "building blocks" in applications ranging from formulations of Pickering emulsions and drug delivery systems to advanced materials, including colloidal crystals and composites. However, for applications requiring tunable properties of charged colloids, obstacles in fabrication can arise through limitations in process scalability and chemical versatility. In this work, the capabilities of flash nanoprecipitation (FNP), a scalable nanoparticle (NP) fabrication technology, are expanded to produce charged polystyrene colloids using sulfonated polystyrene ionomers as a new class of NP stabilizers. Through experimental exploration of formulation parameters, increases in the ionomer content are shown to reduce the particle size, mitigating a significant trade-off between the final particle size and inlet concentration; thus, expanding the processable material throughput of FNP. Further, the degree of sulfonation is found to impact stabilization with optimal performance achieved by selecting ionomers with intermediate (2.45-5.2 mol %) sulfonation. Simulations of single ionomer chains and their arrangement in multicomponent NPs provide molecular insights into the assembly and structure of NPs wherein the partitioning of ionomers to the particle surface depends on the polymer molecular weight and degree of sulfonation. By combining the insights from simulations with diffusion-limited growth kinetics and parametric fits to experimental data, a simple design formulation relation is proposed and validated. This work highlights the potential of ionomer-based stabilizers for controllably producing charged NP dispersions in a scalable manner.

Self-Assembly of Amphiphilic Cubes in Suspension

We study the self-assembly of amphiphilic cubic colloids using molecular dynamics as well as rejection-free kinetic Monte Carlo simulations. We vary both the number and location of the solvophobic faces (patches) on the cubes at several colloid volume fractions and determine the resulting size and shape distributions of the self-assembled aggregates. When the binding energy is comparable to the thermal energy of the system, aggregates typically consist of only few spontaneously associating/dissociating colloids. Increasing the binding energy (or lowering the temperature) leads to the emergence of highly stable aggregates, e.g., small dimers in pure suspensions of one-patch cubes or large (system-spanning) aggregates in suspensions of multipatch colloids. In mixtures of one- and multipatch cubes, the average aggregation number increases with increasing number of solvophobic faces on the multipatch cubes as well with increasing fraction of multipatch cubes. The resulting aggregate shapes range from elongated rods over fractal objects to compact spheres, depending on the number and arrangement of solvophobic patches on the cubic colloids. Our findings establish the complex self-assembly pathways for a class of building blocks that combine both interaction and shape anisotropy, with the potential of forming hierarchically ordered superstructures.

Eco-friendly fabrication of organic solar cells: electrostatic stabilization of surfactant-free organic nanoparticle dispersions by illumination

Earlier reports have discussed the manifold opportunities that arise from the use of eco-friendly organic semiconductor dispersions as inks for printed electronics and, in particular, organic photovoltaics. To date, poly(3-hexylthiophene) (P3HT) plays an outstanding role since it has been the only organic semiconductor that formed nanoparticle dispersions with sufficient stability and concentration without the use of surfactants. This work elucidates the underlying mechanisms that lead to the formation of intrinsically stable P3HT dispersions and reveals prevailing electrostatic effects to rule the nanoparticle growth. The electrostatic dispersion stability can be enhanced by photo-generation of additional charges, depending on the light intensity and its wavelength. This facile, additive-free process provides a universal handle to also stabilize surfactant-free dispersions of other semiconducting polymers, which are frequently used to fabricate organic solar cells or other optoelectronic thin-film devices. The more generalized process understanding paves the way towards a universal synthesis route for organic nanoparticle dispersions.

Self-Assembly of Polymer Blends and Nanoparticles through Rapid Solvent Exchange

Molecular dynamics simulations were performed to study the fabrication of polymeric colloids containing inorganic nanoparticles (NPs) via the flash nanoprecipitation (FNP) technique. During this process, a binary polymer blend, initially in a good solvent for the polymers, is rapidly mixed with NPs and a poor solvent for the polymers that is miscible with the good solvent. The simulations reveal that the polymers formed Janus particles with NPs distributed either on the surface of the aggregates, throughout their interior, or aligned at the interface between the two polymer domains, depending on the NP-polymer and NP-solvent interactions. The loading and surface density of NPs can be controlled by the polymer feed concentration, the NP feed concentration, and their ratio in the feed streams. Selective localization of NPs by incorporating electrostatic interactions between polymers and NPs has also been investigated, and was shown to be an effective way to enhance NP loading and surface density as compared to the case with only van der Waals attractions. This work demonstrates that the FNP process is promising for the production of structured and hybrid nanocolloids in a continuous and scalable way, with independent control over particle properties such as size, NP location, loading, and surface density. Our results provide useful guidelines for experimental fabrication of such hybrid nanoparticles.