title: High-performance Electrospun Polymer Nanofibres and Their Composite Materials with Hectorite
Creators: name: Zhu, Jian
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: Poly(bisbenzimidazobenzophenanthroline-dione) (BBB), Electrospinning, polymer/Hectorite nanocomposite
subject: 540
subject: 600
subject: 620

description: 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 W m-1K-1 and a great flame-shielding effect. Such membranes are highly promising for use as lightweight construction materials and protective clothing. Electrospun membranes with hierarchical pore structure and variable pore size are used extensively for air (high-efficiency particulate air [HEPA], ultra-low particulate air [ULPA]) and bacterial filtration. In a model study, as a last part of the work, a novel aspect of particulate filtration through electrospun thermoplastic porous membranes (TPU) is studied for the first time for making flexible, lightweight and strong polymer membranes with enhanced gas-barrier properties. The high-aspect ratio (≈20000) delaminated melt synthesized layered silicate (Na-hectorite) is self-assembled in a polymer matrix as a barrier-layer between two electrospun membranes by filtration and, subsequently, hot-pressed, which leads to enhanced gas-barrier properties. A highly filled (&gt; 25 wt.%) efficient gas-barrier polymer membrane with perfectly aligned synthetic high aspect ratio hectorite of a variable and considerable thickness (up to 5 μm) can be made. The tensile modulus increased by up to a factor of &gt; 60 and remarkable reduction of the oxygen permeability of 98.8 % for the composite membrane compared to the neat TPU was observed. The electrospun membrane polymer might be varied in the future, including the use of BBB polymer for combining material properties with layered silicates to generate high-performance membranes. The results are published in peer-reviewed journals: Nanoscale, 2017, 9(46), 18169-18174; Macromol. Mater. Eng., 2018, 303(4), 1700615; ACS Appl. Mater. Interfaces, 2019, 11 (12), 11876-11883; Macromol. Mater. Eng., 2019, 1800779.