Photocatalyzed polymerization by semiconductors has recently contributed to the methods of polymer synthesis. In this work, we developed a new type of photocatalyzed polymerization with ring-opening polymerization of lactone. A complex catalyst was designed consisting of nitrogen-doped TiO2 (N–TiO2), iodonium salt (Ph2I+PF6−) and H2O. Poly(ε-caprolactone) (PCL) with a narrow molecular weight distribution was synthesized under visible light irradiation. By using electron spin resonance (ESR), in situ 1H-nuclear magnetic resonance (1H-NMR) and MALDI-TOF mass spectrometry, the mechanism studies clearly presented the reactions of the three components in the ternary catalyst. This polymerization was ruled by the route in which ε-caprolactone (ε-CL) was activated by H+. H+ was photogenerated from the oxidation reaction of H2O. Meanwhile, H+ was stabilized by the photogenerated PF6−, forming Brønsted acid H+PF6−. The relationship between the monomer conversions and PCL molecular weights was studied.
The design of three-dimensional ceramic nanofibrous materials with high-temperature insulation and flame-retardant characteristics is of significant interest due to the effectively improved mechanical properties. However, achieving a pure ceramic monolith with ultra-low density, high elasticity and toughness remains a great challenge. Herein, a low-cost, scalable strategy to fabricate ultralight and mechanically robust N-doped TiO2 ceramic nanofibrous sponges with a continuous stratified structure by conjugate electrospinning is reported. Remarkably, the introduction of dopamine into the precursor nanofibers is engineered, which realizes the nitrogen doping to inhibit the TiO2 grain growth, endowing single nanofibers with a smoother, less defective surface. Besides, the self-polymerization process of dopamine allows the construction of bonding points between nanofibers and optimizes the distribution of inorganic micelles on polymer templates. Moreover, a rotating disk receiving device under different rotating speeds is designed to obtain N-doped TiO2 sponges with various interlamellar spacings, further affecting the maximum compressive deformation capacity. The resulting ceramic sponges, consisting of fluffy crosslinked nanofiber layers, possess low densities of 12–45 mg cm−3, which can quickly recover under a large strain of 80% and have only 9.2% plastic deformation after 100 compression cycles. In addition, the sponge also exhibits a temperature-invariant superelasticity at 25–800 °C and a low heat conductivity of 0.0285 W m−1 K−1, with an outstanding thermal insulation property, making it an ideal insulation material for high-temperature or harsh conditions.
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