Closed-Loop Recyclable Silica-Based Nanocomposites with Multifunctional Properties and Versatile Processability

Abstract

Most plastics originate from limited petroleum reserves and cannot be effectively recycled at the end of their life cycle, making them a significant threat to the environment and human health. Closed-loop chemical recycling, by depolymerizing plastics into monomers that can be repolymerized, offers a promising solution for recycling otherwise wasted plastics. However, most current chemically recyclable polymers may only be prepared at the gram scale, and their depolymerization typically requires harsh conditions and high energy consumption. Herein, it reports less petroleum-dependent closed-loop recyclable silica-based nanocomposites that can be prepared on a large scale and have a fully reversible polymerization/depolymerization capability at room temperature, based on catalysis of free aminopropyl groups with the assistance of diethylamine or ethylenediamine. The nanocomposites show glass-like hardness yet plastic-like light weight and toughness, exhibiting the highest specific mechanical strength superior even to common materials such as poly(methyl methacrylate), glass, and ZrO₂ ceramic, as well as demonstrating multifunctionality such as anti-fouling, low thermal conductivity, and flame retardancy. Meanwhile, these nanocomposites can be easily processed by various plastic-like scalable manufacturing methods, such as compression molding and 3D printing. These nanocomposites are expected to provide an alternative to petroleum-based plastics and contribute to a closed-loop materials economy.

Keywords: chemical recycling; dynamic bonding; mechanical properties; organic-inorganic hybrid material.

Figure 1

a) Classification of Si─O‐based materials, including poly(siloxane), poly(silsesquioxane), and silicate. b) Recycling mechanisms of Si─O‐based materials, including soft poly(siloxane), and hard silicate. c) Schematic diagram of the aminopropyl‐functionalized hybrid Si─O─Si network in this work. d) Comprehensive performance of our material in this work compared with common Si─O‐based materials.

Figure 2

Dynamic Si─O─Si bonds and the reversible depolymerization and repolymerization of hybrid Si‐O‐Si networks. a) The previously reported reaction of the aminosilane with Si─OH group to form Si─O─Si bond at room temperature. b) Illustration of reversible Si─O─Si and Si─OH bonds using the catalyst of DEA (NHEt₂) in this work. c) Illustration of the preparation process of the hybrid Si─O─Si networks. d) FTIR spectra of the solid prepolymers. e) The high‐resolution N1s peaks in XPS curves of the solid prepolymers. f) ²⁹Si MAS NMR spectra and condensation degrees of solid prepolymers and solid hybrid networks. g) Photos of the resultant hybrid material after sintering at 300 °C for 3 h and the depolymerized solutions with/without DEA or EDA.

Figure 3

Material design and properties. a) Preparation process of the nanocomposite. b) Photos of the resultant nanocomposite. c) Load‒displacement curves from the indentation of our nanocomposite and common materials. d) Comparison of the hardness and density of the nanocomposites with common materials. e) Comparison of the calculated hardness‐to‐Young's modulus (H/E) ratio and elastic recovery rate (W_e ) of the nanocomposites with common materials. f) Flexural stress‒strain curves and g) specific strength. h) Charpy impact toughness of the nanocomposite and common materials. i) Optical transmittance of glass, PMMA, and the nanocomposite. j) Thermal conductivity of the nanocomposite and common materials. k) Photographs of the nanocomposite and PMMA on the alcohol lamp.

Figure 4

Closed‐loop recycling of the nanocomposite. Schematic illustration of the nanocomposite with a) the original hydrophobic surface and the fractured surface switching from (b) hydrophilic to (c) hydrophobic. d) The change in the contact angle of the freshly fractured surface with time. e) Photographs of the nanocomposites in various solvents after 24 h. f) The retention rate of the flexural strength of the nanocomposites after immersion in various solvents for 24 h. g) The closed‐loop recycling process: the testing bars of the original nanocomposites, the freshly broken bars exposing hydrophilic surfaces in water, the depolymerized products in water, the regenerated products after drying, the ground powder, and the recycled testing bars. h) Mechanical properties of the original and recycled nanocomposites. i) The selective recycling of our nanocomposite from a plastic waste stream (acrylonitrile butadiene styrene plastic (ABS), polyethylene (PE), polypropylene (PP) and polycarbonate (PC)).

Figure 5

Processing methods of the nanocomposite. a–f) Compression molding: a) Photographs of the compression molding process and the resultant samples. b–e) Photographs and cross‐sectional SEM images of the samples prepared under 5 MPa (b1‐b2), 10 MPa (c1‐c2), 15 MPa (d1‐d2), and sintered nanocomposites (e1‐e2). f) Photos of the colored nanocomposites by adding different amounts of CuCl₂. g,h) Sol–gel–solid processing: g) the schematic illustration of sol–gel–solid processing, and h) photo of the obtained nanocomposite. i,j) Water‐vapor processing: i) schematic illustration of water‐vapor processing, and j) photos of the obtained cups for liquids. k–m) 3D printing: k) schematic illustration of 3D printing, photos of (l) 3D printing process and (m) the printed structure.