Heterogeneity and its Influence on the Properties of Difunctional Poly(ethylene glycol) Hydrogels: Structure and Mechanics

Difunctional polymer hydrogels, such as those prepared from poly­(ethylene glycol) diacrylate (PEGDA) macromers, are widely used for a number of potential applications in biotechnology and advanced materials due to their low cost, mild cross-linking conditions, and biocompatibility. The microstructure of such hydrogels is known to be heterogeneous, yet little is known about the specific structure itself, how it is impacted by the molecular parameters of the macromer, or its impact on macroscopic gel properties. Here, we determine the structure of PEGDA hydrogels using small-angle neutron scattering over a significant range of macromer molecular weights and volume fractions. From this, we propose a structural model for PEGDA hydrogels based on self-excluded, highly branched star polymers arranged into a fractal network. The primary implication of this structure is that heterogeneity arises not from defects in the cross-linking network, as is commonly assumed, but rather from a heterogeneous distribution of polymer concentration. This structural model provides a systematic explanation of the linear elasticity and swelling of PEGDA hydrogels

Decoupling Mechanical and Conductive Dynamics of Polymeric Ionic Liquids via a Trivalent Anion Additive

The mechanical and conductive properties of a polymeric ionic liquid (PIL) are decoupled through the addition of a fraction of trivalent anions to a chloride single-ion conductor. Trivalent phosphate ions strongly coordinate with polymer-bound imidazoliums, producing an increase in both the ionic conductivity and the polymer viscosity. Both the viscosity and the ionic conductivity increase with phosphate content, and the conductivity is superior to that of the neat PIL at larger trivalent anion concentrations. The interaggregate spacing (determined by X-ray scattering), glass transition temperature (measured by calorimetry), and free volume (estimated by rheology) are each sensitive to the presence of trivalent ions but not to changes in the phosphate concentration. Thus, the presence of a fraction of trivalent anions qualitatively changes the structure and interaction of ions, resulting in modified macroscopic properties of the PIL. We hypothesize that this step change in properties upon introducing phosphate ions is due to a densification of ion aggregates by the trivalent ion, which strongly binds to imidazolium ions. This provides a new mechanism for creating PILs with tailored conductive and rheological behavior

Elasticity of Nanoparticles Influences Their Blood Circulation, Phagocytosis, Endocytosis, and Targeting

The impact of physical and chemical modifications of nanoparticles on their biological function has been systemically investigated and exploited to improve their circulation and targeting. However, the impact of nanoparticles’ flexibility (<i>i.e.</i>, elastic modulus) on their function has been explored to a far lesser extent, and the potential benefits of tuning nanoparticle elasticity are not clear. Here, we describe a method to synthesize polyethylene glycol (PEG)-based hydrogel nanoparticles of uniform size (200 nm) with elastic moduli ranging from 0.255 to 3000 kPa. These particles are used to investigate the role of particle elasticity on key functions including blood circulation time, biodistribution, antibody-mediated targeting, endocytosis, and phagocytosis. Our results demonstrate that softer nanoparticles (10 kPa) offer enhanced circulation and subsequently enhanced targeting compared to harder nanoparticles (3000 kPa) <i>in vivo</i>. Furthermore, <i>in vitro</i> experiments show that softer nanoparticles exhibit significantly reduced cellular uptake in immune cells (J774 macrophages), endothelial cells (bEnd.3), and cancer cells (4T1). Tuning nanoparticle elasticity potentially offers a method to improve the biological fate of nanoparticles by offering enhanced circulation, reduced immune system uptake, and improved targeting

Synthesis of Oil-Laden Poly(ethylene glycol) Diacrylate Hydrogel Nanocapsules from Double Nanoemulsions

Multiple emulsions have received great interest due to their ability to be used as templates for the production of multicompartment particles for a variety of applications. However, scaling these complex droplets to nanoscale dimensions has been a challenge due to limitations on their fabrication methods. Here, we report the development of oil-in-water-in-oil (O₁/W/O₂) double nanoemulsions <i>via</i> a two-step high-energy method and their use as templates for complex nanogels comprised of inner oil droplets encapsulated within a hydrogel matrix. Using a combination of characterization methods, we determine how the properties of the nanogels are controlled by the size, stability, internal morphology, and chemical composition of the nanoemulsion templates from which they are formed. This allows for identification of compositional and emulsification parameters that can be used to optimize the size and oil encapsulation efficiency of the nanogels. Our templating method produces oil-laden nanogels with high oil encapsulation efficiencies and average diameters of 200–300 nm. In addition, we demonstrate the versatility of the system by varying the types of inner oil, the hydrogel chemistry, the amount of inner oil, and the hydrogel network cross-link density. These nontoxic oil-laden nanogels have potential applications in food, pharmaceutical, and cosmetic formulations

Ion Transport in Dynamic Polymer Networks Based on Metal–Ligand Coordination: Effect of Cross-Linker Concentration

The development of high-performance ion conducting polymers requires a comprehensive multiscale understanding of the connection between ion–polymer associations, ionic conductivity, and polymer mechanics. We present polymer networks based on dynamic metal–ligand coordination as model systems to illustrate this relationship. The molecular design of these materials allows for precise and independent control over the nature and concentration of ligand and metal, which are molecular properties critical for bulk ion conduction and polymer mechanics. The model system investigated, inspired by polymerized ionic liquids, is composed of poly­(ethylene oxide) with tethered imidazole moieties that facilitate dissociation upon incorporation of nickel­(II) bis­(trifluoro­methylsulfonyl)­imide. Nickel–imidazole interactions physically cross-link the polymer, increase the number of elastically active strands, and dramatically enhance the modulus. In addition, a maximum in ionic conductivity is observed due to the competing effects of increasing ion concentration and decreasing ion mobility upon network formation. The simultaneous enhancement of conducting and mechanical properties within a specific concentration regime demonstrates a promising pathway for the development of mechanically robust ion conducting polymers

Controlling Complex Nanoemulsion Morphology Using Asymmetric Cosurfactants for the Preparation of Polymer Nanocapsules

Complex nanoemulsions, comprising multiphase nanoscale droplets, hold considerable potential advantages as vehicles for encapsulation and delivery as well as templates for nanoparticle synthesis. Although methods exist to controllably produce complex emulsions on the microscale, very few methods exist to produce them on the nanoscale. Here, we examine a recently developed method involving a combination of high-energy emulsification with conventional cosurfactants to produce oil–water–oil (O/W/O) complex nanoemulsions. Specifically, we study in detail how the composition of conventional ethoxylated cosurfactants Span80 and Tween20 influences the morphology and structure of the resulting complex nanoemulsions in the water–cyclohexane system. Using a combination of small-angle neutron scattering and cryo-electron microscopy, we find that the cosurfactant composition controls the generation of complex droplet morphologies including core–shell and multicore–shell O/W/O nanodroplets, resulting in an effective state diagram for the selection of nanoemulsion morphology. Additionally, the cosurfactant composition can be used to control the thickness of the water shell contained within the complex nanodroplets. We hypothesize that this degree of control, despite the highly nonequilibrium nature of the nanoemulsions, is ultimately determined by a competition between the opposing spontaneous curvature of the two cosurfactants, which strongly influences the interfacial curvature of the nanodroplets as a result of their ultralow interfacial tension. This is supported by a correlation between cosurfactant compositions that produces complex nanoemulsions and those that produce homogeneous mixed micelles in equilibrium surfactant–cyclohexane solutions. Ultimately, we show that the formation of complex O/W/O nanoemulsions is weakly perturbed upon the addition of hydrophilic polymer precursors, facilitating their use as templates for the formation of polymer nanocapsules

Nucleation under Soft Confinement: Role of Polymer–Solute Interactions

Nucleation of a crystalline phase almost always occurs at interfaces. However, the lack of fundamental understanding of the impact of interfacial properties on nucleation hinders the design of nucleation active materials for regulating crystallization in practice. In particular, the role of intermolecular interactions is often neglected in nucleation under confinement such as those provided by nano- and microporous materials. Herein, we report the use of a novel material, polymer microgels with tunable microstructure and chemistry, for understanding the role of intermolecular interactions in nucleation under confinement and for controlling crystallization from solution in general. We demonstrate that by tuning the polymer–solute interactions, solute nucleation kinetics were promoted by up to 4 orders of magnitude. Moreover, the effect of polymer–solute interactions was manifested by the split of nucleation time scales due to the presence of nucleation sites of distinct chemical compositions in the microgels, characterized by small angle neutron scattering. Our mechanistic investigations suggest that the polymer matrix facilitates nucleation by enhancing effective solute–solute interactions due to solute adsorptive partitioning and by promoting molecular alignment inferred from preferred crystal orientations on polymer surfaces. Our results provide new insights into nucleation at interfaces and help enable a rational material design approach for directing nucleation of molecular crystals from solution