Polymer–Nanoparticle Composite Films with Ultrahigh Nanoparticle Loadings Using Capillarity-Based Techniques

ConspectusPolymer–nanoparticle (NP) composites with ultrahigh loadings (more than 50 vol %) of NPs possess exceptional mechanical, transport, and physical properties, making them valuable for various applications. However, producing such polymer–NP composites poses significant challenges due to difficulties associated with mixing and dispersing high fractions of NPs in polymers. A promising approach to overcome these challenges involves infiltrating a polymer into the interstitial pores of a disordered NP packing, resulting in a polymer-infiltrated NP film (PINF). Recently, versatile capillarity-driven techniques have emerged, successfully enabling the production of PINFs. These capillarity-driven techniques allow for the fabrication of homogeneous (fully infiltrated), nanoporous (partially infiltrated), and heterostructured PINFs. Infiltrating polymers into stiff but brittle NP packings increases their toughness, attributed to the formation of polymer bridges between adjacent NPs or interchain entanglements. The physical confinement of polymer within the interstitial pore also enhances thermal stability and heat transfer of PINFs. Additionally, the tunable nanoporosity and heterostructures of PINFs lead to unique optical properties suitable for various practical applications.In this Account, we present recent advances and progress in capillarity-based techniques for the fabrication of PINFs and provide a summary of our latest finding on the infiltration process and the properties of PINFs which we have obtained after the publication of our 2021 review paper. We also discuss the stability of the resulting PINFs and demonstrate some practical applications. We conclude the Account by outlining the fundamental research and application directions for the future.In Section 2, we detail capillarity-driven techniques to infiltrate a polymer into a disordered packing of NPs, specifically capillary rise infiltration (CaRI), solvent-driven infiltration of polymer (SIP), and leaching-enabled CaRI (LeCaRI). The CaRI and SIP techniques involve thermal and solvent vapor annealing processes, respectively, while the LeCaRI technique is performed at room temperature without any solvent. For each technique, factors influencing the extent and dynamics of polymer infiltration, including nanoconfinement and polymer–NP surface interactions, are explained. In Section 3, we focus on the mechanical properties and thermal/photo degradation behaviors of the PINFs, which are closely linked to their stability, and explain how nanoconfinement and polymer–NP surface interactions affect these properties. We show that kinetics of infiltration and the properties of PINFs have nontrivial and at times counterintuitive dependence on the extent of nanoconfinement and the interaction strengths between polymers and NPs. In Section 4, we explore some practical applications of PINFs, demonstrating their multifunctionality in areas such as antireflection coatings and antifouling coatings. We highlight how PINFs with tunable refractive indices serve as effective antireflection coatings and how lubricant depletion-resistant slippery liquid-infused porous surfaces can be developed. In Section 5, we discuss the remaining challenges associated with capillarity-driven techniques and PINFs that need to be addressed and explore potential applications such as functional films, coatings, and membranes for sustainable development.