Continuous Binder-Free Fibers of Pure Imogolite Nanotubes

Abstract

Imogolite nanotubes (INTs) display a range of useful properties and provide an ideal material system to study the assembly of nanomaterials into macroscopic fibers. A method of wet spinning pure, binder-free imogolite fibers has been developed using double-walled germanium imogolite nanotubes. The nanotube aspect ratio can be controlled during the initial synthesis and is critical to the spinning process. Fibers made from short nanotubes (<100 nm) have very low gel strengths, while dopes with longer nanotubes (500–1000 nm) are readily spinnable. The tensile behavior of the resulting imogolite nanotube fibers is strongly influenced by relative humidity (RH), with a modulus of 30 GPa at 10% RH compared to 2.8 GPa at 85% RH, as well as a change in failure mode. This result highlights the importance of inter-nanotube interactions in such assemblies and provides a useful strategy for further exploration. Interestingly, in the absence of a matrix phase, a degree of misorientation appears to improve load transfer between the individual INTs within the porous fiber, likely due to an increase in the number of interparticle contacts. Imogolite nanotubes are an appealing analogue to other nanotube fiber systems, and it is hoped that learnings from this system can also be used to improve carbon nanotube fibers.

Wet spinning imogolite nanotube fibres: an in situ process study

“…As washing bath length increases, the INT orientation reduces (Fig. 7c), in agreement with previous data that less aligned INT bres are stiffer 13 but now identies that both the removal of CaCl 2 and structural rearrangements during washing are contributing process factors. As the bath length increased from 15 cm to 30 cm, packing density also increased from ∼20% to ∼40%.…”

“…Similarly to other rigid rod dispersions, aqueous solutions of imogolite nanotubes are highly shear thinning due to reorientation of the nanotubes in the flow direction 21,22 Capillary rheometry was used to study solutions with INT concentrations varying from 1 to 11 mg mL −1 (volume fraction, φ , 0.037% to 0.41%, based on a mean INT density 13 of 2.7 g cm −3 ) which represents the range of concentrations from isotropic to fully liquid crystalline solutions (Fig. S1†).…”

“…The alignment was characterised by fitting the angular dependence of the scattered intensity at 2.6 nm −1 to a Lorentzian function. As previously described, 13,32,33 the reciprocal space Lorentzian of the X-ray scattering pattern arises from a direct space orientational distribution function (ODF) of a Lorentzian to the power 1.5, with a half-width-at-half-maximum (HWHM), w d = 0.775 w r , where w r is the HWHM of the Lorentzian fit to the reciprocal space diffraction data. The Hermans' order parameter, S , is then calculated from the ODF aswhere f ( β ) is the Lorentzian function to the power 1.5 as a function of the polar angle β and the denominator is used here to normalise the ODF to 1.…”

“…19 The recent wet spinning process for pure INTs produced continuous bres with promising properties. 13 This work develops pure INT wet spinning, using in situ observation to understand the assembly mechanisms at each stage of the process. The aim was both to improve the properties of the INT bres and to develop a mechanistic understanding potentially relevant to a variety of rigid rod nanomaterials.…”

“…The use of aluminogermanate variants increases the scattering strength further, due to the substitution of Si atoms with highly-scattering Ge atoms. INTs can be easily synthesised at relatively low temperature (100–150 °C) and improvements in synthesis methods have enabled the growth of micron length INTs which may be spun into fibres using a wet spinning approach; 13 however, the spinning process is not yet optimised or fully understood.…”