Cellulose sheathed nanotube fiber

Embodiments of the invention provide a cellulose-sheathed carbon nanotube fiber. One aspect of the invention provides a sheathed nanotube fiber comprising: a carbon nanotube fiber; and a cellulose sheath extending co-axially along at least a first portion of a length of the carbon nanotube fiber. Another aspect of the invention provides a method of forming a sheathed carbon nanotube fiber, the method comprising: co-electrospinning a carbon nanotube fiber gel core within a cellulose solution sheath.

TECHNICAL FIELD

Embodiments of the invention relate generally to conductive cable fibers and, more particularly, to conductive cable fibers with an insulating surface prepared by co-axial electrospinning of multi-walled nanotubes and cellulose.

BACKGROUND OF THE INVENTION

Carbon nanotubes (CNTs) have attracted significant interest for a wide variety of potential applications due to their strength, flexibility, and high electrical and thermal conductivities. The electrical properties of a polymer matrix have been enhanced by using shear mixing to introduce a very low loading of CNTs. However, the strong van der Waals interactions between CNTs make it difficult to achieve high CNT loadings, good dispersion, or alignment within polymer matrices. Therefore, the preparation of structured CNT nanocomposites with desirable electrical, thermal, and strength properties remains challenging.

Electrospinning is a method of extruding fibers at high speed from a solution using electrostatic charging and has been used to prepare various types of hybrid nanofibers by incorporating nanomaterials into polymer solutions. It has been demonstrated that fibers can still be extracted from materials that normally cannot be electrospun through co-electrospinning, using a double needle spinneret.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a cellulose-sheathed carbon nanotube fiber.

One aspect of the invention provides a sheathed nanotube fiber comprising: a carbon nanotube fiber; and a cellulose sheath extending co-axially along at least a first portion of a length of the carbon nanotube fiber.

Another aspect of the invention provides a method of forming a sheathed carbon nanotube fiber, the method comprising: co-electrospinning a carbon nanotube fiber gel core within a cellulose solution sheath.

The illustrative aspects of the present invention are designed to solve the problems herein described and other problems not discussed, which are discoverable by a skilled artisan.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide co-axial core-sheath nanofibers with multi-wall carbon nanotubes (MWNTs) cores and cellulose sheathes. In some embodiments, such nanofibers may be produced using dry jet wet electrospinning using room temperature ionic liquid solvents.

According to one illustrative embodiment of the invention, MWNTs are mixed with an ionic liquid, such as 1-ethyl-3-methylimidazolium acetate. Other ionic liquids include, but are not limited to, 1-butyl-3-methylimidazolium chloride and 1-allyl-3-methylimidazolium chloride. Such mixing may be by mortar and pestle, for example, to form a homogenous gel. This core gel typically comprises between about 1% and about 10% MWNTs by weight, although lesser or greater MWNT concentrations may be used.

A sheath solution is prepared by dissolving cellulose in an ionic liquid, such as 1-ethyl-3-methylimidazolium acetate to a cellulose concentration of about 1.5% by weight, although concentrations between about 1% and about 5% by weight may also be used. The sheath solution may be mixed using a magnetic stirrer or similar device to form a homogeneous solution. Solution formation may be aided by heating to about 80° C.

The core gel and sheath solution are then co-electrospun using a co-axial spinneret, such as those available from MECC (Fukuoka, Japan).FIG. 1shows such a co-axial spinneret100having an inner, core needle10and an outer, sheath needle20. The inner, core needle10has a diameter less than that of the outer, sheath needle20. In one embodiment, a diameter of the inner, core needle10is about 0.94 mm and a diameter of the outer, sheath needle20is about 2.50 mm. Needles having other absolute and relative diameters may be employed, however, the diameters given here being merely for the purpose of describing one embodiment of the invention.

Flow rates of the inner core gel and the outer sheath solution are, according to some embodiments of the invention, between about 280 μL/minute and about 320 μL/minute. At those flow rates, a voltage between about 18 kV and 22 kV applied to the spinneret typically achieves good electrospinnability of both the inner core gel and the outer sheath solution.

The co-axial electrospun fibers200are collected in a coagulation bath300, which typically includes a liquid mixture capable of removing the ionic liquid. Suitable mixtures include, for example, a water/ethanol mixture. Once the ionic liquid is removed in the coagulation bath, the co-axial electrospun fibers200solidify to form a fiber mat. Upon removal from the coagulation bath300, the fiber mat may then be washed with ethanol and dried under vacuum to remove residual water and ethanol.

FIG. 2shows a schematic cross-sectional view of a co-axial electrospun fiber200according to an embodiment of the invention. As can be seen, co-axial electrospun fiber200includes a MWNT core110and a cellulose sheath120. InFIG. 3, portions of the cellulose sheath120have been removed and portions112,114of the MWNT core110exposed. The exposed portions112,114of the MWNT core110may be used as electrical contacts. As shown inFIG. 3, portions112,114of the length of the carbon nanotube fiber200is adjacent a second portion120of the length of the carbon nanotube fiber200along which the cellulose sheath does not extend.

Portions112,114may be exposed using cellulase to digest areas of the cellulose sheath120. In one embodiment, an aqueous cellulase solution may be applied to an absorbent material, such as hydrophilic poly(tetrafluoroethylene), which is then applied to an area of the cellulose sheath120. The co-axial electrospun fiber200may then be washed to remove any digested cellulose and expose portions112,114of the MWNT core110.

FIGS. 4A,4B,4C, and4D show scanning electron micrographs of MWNT-cellulose fiber mats at various MWNT loadings.FIG. 4Ashows a fiber mat at 45% MWNT, by weight.FIG. 4Bshows a fiber mat at 40% MWNT, by weight.FIG. 4Cshows a fiber mat at 30% MWNT, by weight.FIG. 4Dshows a fiber mat at 20% MWNT, by weight.

Tensile Strength

The tensile strength of co-axial electrospun fiber mats according to embodiments of the invention are proportional to the cellulose:MWNT ratio of the fiber mats. A fiber mat of pure cellulose having dimensions of 15 mm×15 mm×50-80 μm was determined to have a tensile strength of 6.22 MPa. A co-axial electrospun fiber mat with a MWNT loading of 45% by weight exhibited a tensile strength of 2.54 MPa.

Tensile strength measurements were performed using Instron Materials Testing Machine (Norwood, Mass.) model 5543 equipped with a 10 N static load cell and hydraulic grips (Instron 2712-001). Specimens were tested at 0.25 mm/min tension speed with a 5 mm gauge length. Both load and grip-to-grip distance were measured. Tensile strength was calculated by dividing the peak load by the initial cross-sectional area of the sample.

Thermal Characteristics

Surprisingly, it was found that the temperature of the onset of cellulose degradation increased with an increasing MWNT load. It is possible that either the cellulose crystallinity of the co-axial electrospun fiber is increased by MWNT loading or that MWNTs act as a heat sink or an antioxidant, protecting the cellulose from thermal degradation.

In evaluating the thermal characteristics of fiber mats of the invention, thermogravimetric analysis was performed using a computer-controlled TA Instruments (New Castle, Del.) TGA Q50. Temperature was ramped up at 20° C. per minute up to 700° C. with the furnace open to allow air flow along with nitrogen purge gas.

Conductivity

Pristine fiber mats were found to be non-conductive, due to the substantially insulating effects of the cellulose sheath120. Once portions112,114(FIG. 3) of MWNT core110were exposed, however, the fiber mats were found to have conductive properties following Ohm's law. Where portions112,114were separated by 1 cm, conductivity of the fiber mats was observed, confirming that MWNT core110was continuous for at least 1 cm.

In addition, it was found that the conductivities of the fibers increased with higher MWNT loading. In fact, a surprisingly large 2-order of magnitude increase in conductivity was obtained upon doubling the mass fraction of the MWNTs. This may suggest that the continuity of the MWNT core110improves at higher MWNT loading and/or that the MWNTs in the MWNT core110form denser wire- or cable-like core structures.FIG. 5shows a graph of conductivities obtained at MWNT loadings of 20%, 30%, 40%, and 45%. At 45% MWNT loading, conductivities as high as 10.7 S/m were obtained.

All of these conductivities are significantly higher than those reported with carbon nanotube-polymer fiber mats, which are typically around 3.7×10−2S/m for poly(styrene)/single-wall nanotube fiber materials and 5.05×10−6S/m for poly(ethyleneterephthalate)/MWNT fiber materials. The higher conductivities obtainable according to embodiments of the present invention may be due, at least in part, to the fact that the MWNT-cellulose fibers here consist of pure MWNTs free of a polymer matrix, which permits formation of a dense MWNT core that more effectively conducts an electrical current.

Conductivities of co-axial electrospun fiber mats according to the invention were measured by preparing two “sheath-off” areas, 1 cm apart, on a fiber mat between 50 μm and about 80 μm thick. The sheath-off areas were sandwiched by 20 μm thick aluminum foil and clamped at 0.7 MPa. Characteristic I-V curves were obtained at room temperature using a two-probe method using a Princeton Applied Research (Oak Ridge, Tenn.) model 273A electropotentiostat, with an applied voltage up to 1 V at a scan rate of 100 mV/second. Electrical conductivities were calculated according to the following equation:
σ=L/(R·A),
in which σ is electrical conductivity in Siemens per meter (S/m), L is the distance between sheath-off areas, R is resistance, and A is a cross-sectional area of the fiber mat.

The co-axial electrospun fibers according to some embodiments of the invention may be employed in any number of applications. For example, the cellulose sheath serves as an effective nanoscale insulating layer, permitting two parallel co-axial electrospun fibers to be used in electrical double-layer supercapacitor devices. In such a device, the distance between the two electrodes would be double the sheath thickness, often on the scale of hundreds of nanometers, permitting very high specific capacitances.

Other applications of the co-axial electrospun fibers according to embodiments of the invention include, for example, their use as the separator of a biomorph actuator, based on the formation of an electrical double layer. The thin, flexible separator achievable using the co-axial electrospun fibers of the invention may improve response properties of such an actuator.