Polyimide aerogels with novel bimodal micro and nano porous structure assembly for airborne nano filtering applications

Abstract

Aerogels have presented a very high potential to be utilized as airborne nanoparticles' filtration media due to their nanoscale pore size and extremely high porosity. The filtering performance of aerogels, such as air permeability and filtration efficiency, is highly related to the configuration of aerogels' nanostructure assembly. However, as aerogel morphology is formed with respect to the intermolecular forces during the gelation stage, tailoring the aerogel nanostructure assembly is still a challenge. In this work, a novel strategy for tailoring polyimide aerogel nanostructure assembly is proposed by controlled disturbing of the intermolecular forces. From the results, the nanostructure assembly of the 4,4'-oxydianiline (ODA)-biphenyl-tetracarboxylic acid dianhydride (BPDA) polyimide aerogel is tailored to a uniform bimodal micro and nano porous structure. This was achieved by introducing the proper fraction of thermoplastic polyurethane (TPU) chains to the polyimide chains in the solution state and through a controlled process. The fabricated polyimide/TPU aerogels with bimodal morphology presented enhanced filtration performance, with 30% improved air permeability and reduced cell size of 3.51 nm over the conventional ODA-BPDA polyimide aerogels. Moreover, the fabricated bimodal aerogels present the reduced shrinkage, density, and effective thermal conductivity of 6.3% and 0.063 g cm^-3, 28.7 mW m^-1 K^-1, respectively. Furthermore, the bimodal polyimide/TPU aerogels show the higher porosity of 96.5 vol% along with increased mechanical flexibility over the conventional polyimide aerogel with comparable backbone chemistry.

Fig. 1. The schematic of the ODA–BPDA polyimide aerogels cross-linked by the BTC.

Fig. 2. (A) picture of the fabricated aerogels, along with the SEM micrographs of (B) ODA–BPDA, (C) ODA–BPDA/10% TPU, & (D) ODA–BPDA/20% TPU samples.

Fig. 3. The nitrogen adsorption/desorption analysis for the ODA–BPDA (neat), ODA–BPDA–10% TPU, and ODA–BPDA–20% TPU samples.

Fig. 4. The DFT pore size distributions for the ODA–BPDA (neat), ODA–BPDA–10% TPU, and ODA–BPDA–20% TPU samples.

Fig. 5. Retained diameter of ODA–BPDA aerogels with varying TPU weight fraction.

Fig. 6. The Fourier transform infrared (FTIR) spectrum of fabricated aerogels along with the neat TPU.

Fig. 7. The TGA results fabricated aerogels and the neat TPU.

Fig. 8. The stress–strain graph of the fabricated aerogels with varying TPU fraction.

Fig. 9. Bimodal polyimide/TPU aerogels with increased filtration performance over the typical polyimide aerogels.