Self-assembly in biobased nanocomposites for multifunctionality and improved performance.

Concerns of petroleum dependence and environmental pollution prompt an urgent need for new sustainable approaches in developing polymeric products. Biobased polymers provide a potential solution, and biobased nanocomposites further enhance the performance and functionality of biobased polymers. Here we summarize the unique challenges and review recent progress in this field with an emphasis on self-assembly of inorganic nanoparticles. The conventional wisdom is to fully disperse nanoparticles in the polymer matrix to optimize the performance. However, self-assembly of the nanoparticles into clusters, networks, and layered structures provides an opportunity to address performance challenges and create new functionality in biobased polymers. We introduce basic assembly principles through both blending and in situ synthesis, and identify key technologies that benefit from the nanoparticle assembly in the polymer matrix. The fundamental forces and biobased polymer conformations are discussed in detail to correlate the nanoscale interactions and morphology with the macroscale properties. Different types of nanoparticles, their assembly structures and corresponding applications are surveyed. Through this review we hope to inspire the community to consider utilizing self-assembly to elevate functionality and performance of biobased materials. Development in this area sets the foundation for a new era of designing sustainable polymers in many applications including packaging, construction chemicals, adhesives, foams, coatings, personal care products, and advanced manufacturing.

Activation and Assembly of Plasmonic-Magnetic Nanosurfactants for Encapsulation and Triggered Release.

Multifunctional surfactants hold great potentials in catalysis, separation, and biomedicine. Highly active plasmonic-magnetic nanosurfactants are developed through a novel acid activation treatment of Au-Fe3O4 dumbbell nanocrystals. The activation step significantly boosts nanosurfactant surface energy and enables the strong adsorption at interfaces, which reduces the interfacial energy one order of magnitude. Mediated through the adsorption at the emulsion interfaces, the nanosurfactants are further constructed into free-standing hierarchical structures, including capsules, inverse capsules, and two-dimensional sheets. The nanosurfactant orientation and assembly structures follow the same packing parameter principles of surfactant molecules. Furthermore, nanosurfactants demonstrate the capability to disperse and encapsulate homogeneous nanoparticles and small molecules without adding any molecular surfactants. The assembled structures are responsive to external magnetic field, and triggered release is achieved using an infrared laser by taking advantage of the enhanced surface plasmon resonance of nanosurfactant assemblies. Solvent and pH changes are also utilized to achieve the cargo release.

Unique Orientation of the Solid-Solid Interface at the Janus Particle Boundary Induced by Ionic Liquids.

This study reveals the unique role on Janus particles of the solid-solid interface at the boundary in determining particle interactions and assembly. In an aqueous ionic liquid (IL) solution, Janus spheres adopt intriguing orientations with their boundaries pinned on the glass substrate. It was further discovered that the orientation was affected by the particle amphiphilicity as well as the chemical structure and concentration of the IL. Further characterization suggests that the adsorption on the hydrophilic side is due to both an electrostatic interaction and hydrogen bonding, while adsorption on the hydrophobic side is due to hydrophobic attraction. Through the concerted interplay of all these interactions, the amphiphilic boundary may attract an excessive amount of IL cations, which guide the unique orientations of the Janus spheres. The results highlight the importance of the Janus boundary that has not been recognized previously. Adsorption at the solid-solid interfaces may inspire new applications in areas such as separation and catalysis.

Thin Biobased Transparent UV-Blocking Coating Enabled by Nanoparticle Self-Assembly.

A waterborne, UV-blocking, and visually transparent nanocomposite coating was formulated with ZnO nanoparticles and 2-hydroxyethyl cellulose (HEC). The coating is highly effective (<5% UV and ∼65% visible transmittance), and the film thickness (0.2-2.5 µm) is ∼100 times thinner than the conventional coatings of similar UV-blocking performance. The superior properties are due to the fractal structures of ZnO nanoparticles assembled within the HEC matrix, revealed by scanning electron microscopy and small-angle X-ray scattering (SAXS). Changing the binder to 2-hydroxyethyl starch (HES) diminishes the UV-blocking performance, as ZnO nanoparticles form dense globular aggregates, with an aggregation number measured by SAXS 3 orders of magnitude larger than the HEC coating. Since HEC and HES share the same repeating glucose unit in the polymer backbone, it suggests that the conformational characteristics of the binder polymer have a strong influence on the nanoparticle aggregation, which plays a key role in determining the optical performance. Similar structures were achieved with TiO2 nanoparticles. This study not only offers a cost-effective and readily scalable method to fabricate transparent UV-blocking coating but also demonstrates that the unique fractal aggregation structures in a nanocomposite material can provide high performance and functionality without fully dispersing the nanoparticles.

Surfactant-Mediated Assembly of Amphiphilic Janus Spheres.

We investigate how amphiphilic Janus particle assembly structures, including clusters and striped two-dimensional (2D) crystals, are influenced by the addition of surfactant molecules. Janus particles are fabricated using silica particles coated with Au on one side, which is further modified with a hydrophobic self-assembled monolayer. Analysis on the cluster assembly structures suggests that in addition to hydrophobic attraction, van der Waals (VDW) attraction plays a significant role in the assembly process, which is modulated by the Au coating thickness. This is manifested by the cluster formation induced primarily by VDW forces when the hydrophobic attraction between particles is diminished by adding the surfactant. In the 2D crystal case, sodium dodecyl sulfate (SDS) and Tween 20 show opposite trends in how they affect assembly structures and particle dynamics. SDS shortens the stripes in 2D crystals and accelerates the rotation of particles, whereas Tween 20 extends the straight stripes and slows down the particle rotation. We interpret the results by considering SDS adsorption on the Au-coated hemisphere of the Janus particles and Tween 20 forming hydrogen bonds with the silica hemisphere of Janus particles. Our study offers a simple approach to change the assembly structures of Janus particles, and it provides principles and guidance for potential applications of Janus particles coupled with small amphiphilic molecules.

Drying mediated orientation and assembly structure of amphiphilic Janus particles.

Amphiphilic Janus particles demonstrate unique assembly structures when dried on a hydrophilic substrate. Particle orientations are influenced by amphiphilicity and Janus balance. A three-stage model is developed to describe the process. Simulation further indicates the dominant force is capillary attraction due to the interface pinning at rough Janus boundaries.