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.

Table S3.The molecular weights in MALDI-TOF-MS spectra of the prepolymers and the depolymerized products (Figure 2h, Figure S4) of the hybrid Si-O-Si networks formed from the co-condensation of APTMS and TEOS with DEA or EDA as the catalyst, and their possible structural units.

Figure S2 .
Figure S2.Schematic chemical structures of the closed-loop recycled nanocomposite.The hybrid Si-O-Si networks with (a) DEA, (b) EDA, and (c) DEA and EDA.(f) The resulting silica-based nanocomposites containing (d) PTFPMS micelles with an APTES shell and (e) FAS.

Figure S3 .
Figure S3.The high resolution N1s peaks in XPS curves of solid prepolymer, the original and recycled solid prepolymers with DEA and EDA.

Figure S4 .
Figure S4.Schematic illustration of the partial depolymerization of the hybrid Si-O-Si network without a catalyst.Strong hydrogen bonds are formed between silanols and aminopropyl groups in the network without a catalyst.The hydrogen-bonded aminopropyl groups are less reactive, leading to the hybrid Si-O-Si network being only partially depolymerized.

Figure S5 .
Figure S5.The MALDI-TOF-MS spectra of the prepolymers and the depolymerized products of the hybrid Si-O-Si networks formed from the co-condensation of APTMS and TEOS with the catalyst: (a) DEA, (b) EDA, and (c) DEA and EDA.

Figure S6 .
Figure S6.The MALDI-TOF-MS spectra of the prepolymers and the depolymerized products of the poly(silsesquioxane) networks formed from the self-condensation of APTMS with the catalyst: (a) DEA, (b) EDA, and (c) DEA and EDA.

Figure S7 .
Figure S7.The depolymerization of the hybrid Si-O-Si networks formed from the co-condensation of APTMS and TEOS with DEA, EDA and the mixture of DEA and EDA: (a) 1 H-NMR spectra and (b) 13 C-NMR spectra of the corresponding prepolymers and the depolymerized products.

Figure S8 .
Figure S8.The depolymerization of the poly(silsesquioxane) networks formed from the self-condensation of APTMS with DEA, EDA and the mixture of DEA and EDA: (a) 1 H-NMR spectra and (b) 13 C-NMR spectra of the corresponding prepolymers and the depolymerized products.

Figure S9 .
Figure S9.Effect of the micelles on the formation of defect-free bulk materials after drying.Photographs showing that without micelles, the materials containing (a) DEA, (b) EDA, and (c) DEA and EDA, cracked after drying at 80°C for 3 days.(d) Photograph of the intact nanocomposite containing DEA and EDA with micelles after drying at 80°C for 3 days.

Figure S10 .
Figure S10.Elemental analysis of the nanocomposite: (a) XPS spectrum, (b) quantification table indicating the atomic species and their atomic percentages; (c) SEM image and corresponding element mapping of the fracture surface.

Figure S11 .
Figure S11.Mechanical properties of the nanocomposites containing DEA, EDA, and a mixture of DEA and EDA.(a-b) Nanoindentation tests: (a) load-displacement curves, and (b) hardness and modulus.(c-d) Threepoint bending tests: (c) stress-strain curves, and (d) flexural strength and flexural modulus.

Figure S12 .
Figure S12.The weight change in the nanocomposites with different catalysts in water with time.The weight ratio of the nanocomposite/water is 20/500.

Figure S13 .
Figure S13.Photos showing that (a) the surface of the glass can be easily scratched by a glass cutter, while no scratch is formed on (b) the surface of our nanocomposite containing DEA and EDA by the cutter under the same force.

Figure S14 .
Figure S14.TGA curves of the nanocomposite containing DEA and EDA in air and nitrogen.

Figure S15 .
Figure S15.(a) Contact angles and (b) mechanical properties of the nanocomposite containing DEA and EDA before and after immersion in different solvents for 24 h.

Figure S16 .
Figure S16.Photos of the polluted (a) glass and (b) nanocomposite containing DEA and EDA before and after wiping.(c) Photo of the water-based acrylic paint spray used as the pollutant.

Figure S17 .
Figure S17.Recycling ratio of the nanocomposites after different recycling cycles.

Figure S18 .
Figure S18.Photos of (a) the PTFPMS@APTES micelle dispersion and (b) the solution of depolymerized product in water.The size distributions of micelles in (c) the original prepolymer sol and (d) the depolymerized solution.TEM images of (e) the pristine PTFPMS@APTES micelles and (f) the micelles in the depolymerized solution.

Figure S19 .
Figure S19.The impact of the temperature of water on the depolymerization time of the nanocomposite.The weight ratio of nanocomposite/water was 20/500, and the depolymerization time was judged by the complete disappearance of the nanocomposite containing DEA and EDA and the formation of a clear solution.

Figure S20 .
Figure S20.TGA curves of the solid prepolymer powder.

Figure S21 .
Figure S21.(a) Photos of SiO 2 nanoparticles mixed with the liquid prepolymer.(b) Photos of the composite powders, photos of molded nanocomposites, and SEM images of the nanocomposites with different SiO 2 content.

Figure S22 .
Figure S22.Material properties of the nanocomposites with 0, 25 and 28 wt% SiO 2 nanoparticles.(a-d) Mechanical properties of the nanocomposites: (a) flexural stress-strain curves, (b) flexural strength and flexural modulus, (c) specific flexural strength, (d) impact toughness.(e-f) Thermal properties of the nanocomposites: (e) thermal conductivity and (f) TGA curves showing the thermal stability.

Figure S23 .
Figure S23.(a) Contact angles and (b) mechanical properties of the nanocomposites containing CuCl 2 and SiO 2 before and after immersion in water for 3 days.

Table S2 .
The chemical structural units of the prepolymers and the depolymerized products of the hybrid Si-O-Si networks formed from the co-condensation of APTMS and TEOS, as well as their molecular formulae, molecular weights, and symbols.

Table S4 .
Density and mechanical properties from nanoindentation tests of our nanocomposites and some common materials.