High-performance Electrospun Polymer Nanofibres and Their Composite Materials with Hectorite

Abstract

Poly(bisbenzimidazobenzophenanthroline-dione) (BBB) is a heterocyclic rigid-rod polymer exhibiting excellent thermal and mechanical performances. It is made by the condensation of 1,4,5,8-naphthalene tetracarboxylic acid (NTCA) and 3,3’-diaminobenzidine (DAB) at a high temperature. The bottleneck in the use of BBB polymer is its insolubility in organic solvents and an extremely high softening point (higher than its decomposition temperature), that makes processing to films, fibres, sponges or, in fact, to any other form impossible. The aim of this work is to provide methods of processing BBB to fibres, sponges and membranes and study their thermal and mechanical properties. A new bottom-up approach was established for making BBB fibres by electrospinning to overcome the obstacle of insolubility and infusibility of BBB for processing fibre. The approach involved templet-assisted solid-state polymerization of the self-assembled starting monomers (NTCA and DAB) in fibre form. The self-assembled monomers were electrospun together with a templet polymer (any polymer that can be pyrolysed at a low temperature). Heating of the fibres at a temperature above 460 oC led to the polycondensation of self-assembled monomers to BBB polymer with simultaneous degradation and removal of the templet sacrificial polymer. A structural characterization, morphology study, thermal stability and mechanical testing were carried out for both a single and an aligned fibre belt. Electrospun BBB nanofibre showed an excellent thermal resistant property with the degradation onset temperature above 600 °C and a 5 % weight loss temperature of 640 °C in a N2 atmosphere. A single BBB and an aligned fibre belt showed a very high strength of 1.43 ± 0.26 GPa (226 nm diameter) and 364.75 ± 4.76 MPa, respectively. Furthermore, the BBB fibres were used for making three-dimensional open cell, low-density (≤ 13.9 mg cm-3), compressible (more than 90 % recovery after 50 % compression), intrinsic flame retardant and thermal insulating (thermal conductivity of 0.028-0.038 W m-1K-1) sponges. The sponges are made by the self-assembly of short BBB fibres (50–500 μm in length) in an aqueous dispersion during freeze-drying. The use of a sacrificial glue (poly(vinyl alcohol)) was systematically studied for improving the mechanical stability of the sponges. The short electrospun fibres’ dispersion can also be used for making a porous membrane using the wet-laid process. In this method, the vacuum-assisted filtration and drying of the short fibre dispersion provides a porous membrane by percolation of short fibres in the form of a randomly arranged fibre network. The BBB porous membranes made by this method were mechanically weak (0.04 MPa). Therefore, composite membranes of BBB with layered silicate (Na-hectorite) were studied. The composite membranes neither sustained flame, burned with smoke nor exhibited melt dripping. These porous, low-density and mechanical flexible Hec/BBB membranes showed a high char yield of 80–94 %, low thermal conductivity of 0.028–0.051…