Melt Spinning of Thermoplastic Polyurethane-Based Bulk Ionofibers Filled with Carbon Nanotubes

Abstract

Ionotronic textiles or i-textiles offer in-air electrochemical applications and sensing due to their ionic character, mimicking phenomena of organisms. To manufacture different i-textiles with unique functions and characteristics, it is necessary to have a range of ionically conductive textile fibers or ionofibers to choose from. However, their means of production are not sufficiently explored to provide knowledge that meets the fabric manufacturing needs. For a textile application, surface functionalization is usually explored as a convenient way to build upon an already known textile material. In contrast, bulk functionalization allows for superior production rate, versatility, and durability. Additionally, the use of the synergy between ionic liquids and carbon nanotubes is seldom explored. Therefore, in this study, melt spinning is investigated regarding the use of an ionic liquid (IL) initially without and ultimately with multiwalled carbon nanotubes (CNTs) for the tailoring of the electrical and mechanical properties of ionofibers. Based on thermoplastic polyurethane (TPU) elastomers, IL-containing pellets are prepared using 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMIm OTf) at different weight ratios. About the melt-spun monofilaments, their extrusion temperatures, their morphology through scanning electron microscopy with energy-dispersive X-ray, their fiber conductivity through electrochemical impedance spectroscopy and cyclic voltammetry, and their tensile properties are investigated. An optimum of the ratios of IL and CNTs is observed for the melt-spinning process, which results in fiber conductivities within the range of 10^-2 μS cm dtex^-1. Compared to a monofilament melt-spun with no IL and a CNT weight ratio above percolation threshold, the fiber conductivity is twice higher due to its intricate segregated network. Thus, this industrial textile-compatible process offers an alternative within the development of ionotronic fabrics.

Keywords: carbon nanotubes; conductive polymer composites; i-textiles; ionic liquids; melt-spinning; monofilaments.

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(a) TGA curves under a nitrogen purge (60 mL min^–1). (b) Degradation temperature at 5 wt % loss. The confidence interval at 95% for TPU-IL65­(MB) was 2.04 °C.

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(a) DSC curves of the prepared pellets. Additional plots with the enlarged view of the endothermic peaks, the crystallization peaks, and with the full range of TPU-IL65­(MB) are available in the Supporting Information. (b) Extracted crystallization temperatures from peaks. *: No noticeable peak.

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(a) Backscattered electron images of the cross sections of TPU-IL20 with (b) its pseudocolored EDX map sum of fluor and sulfur and (c) TPU-IL50.

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(a) Backscattered electron image of the longitudinal section of bulk ionofiber TPU-IL65­(MB) with (b) its pseudocolored EDX map sum of fluor and sulfur.

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Close-up of backscattered electron images of the cross sections of the CNT-containing samples (a) TPU-CNT1.5-IL5, (b) TPU-CNT1.5-IL15, (c) TPU-CNT1.5-IL20, and (d–f) their respective pseudocolored EDX map sum of fluor and sulfur (same scale). Carbon EDX maps highlighting the segregated networks are available in the Supporting Information.

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(a) Tensile moduli (columns) and fiber conductivities (points) of the samples. (b) Specific stress of the melt-spun samples up to 180% strain. 1 mN tex^–1 is equivalent to 1.12 MPa for virgin TPU. A close-up of the first 30% strain is available in the Supporting Information. (c) Nyquist plot of the most conductive CNT-containing samples along with a typical equivalent circuit that was used for the fitting. The extracted R _ct was used for calculating the fiber conductivity.

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Fitted n-values for allometric scaling of the cyclic voltammetries. Values further away from 1 (ohmic conduction) represent bigger tunneling contributions.

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Melt-spun monofilaments (a) TPU-IL20, vTPU, and (b) TPU-CNT1.5-IL5, and the knitted fabrics made using (c) TPU-IL20 and (d) TPU-CNT1.5-IL5.