Liquid sensing properties of fibres prepared by melt spinning from poly(lactic acid) containing multi-walled carbon nanotubes

https://doi.org/10.1016/j.compscitech.2009.11.005Get rights and content

Abstract

Poly(lactic acid) (PLA)/multi-walled carbon nanotube (MWNT) composites were melt spun with different take-up velocities (max. 100 m/min) to obtain electrically conductive fibres. The incorporation of MWNT contents between 0.5 and 5.0 wt.% was realised in a previous melt mixing process using twin-screw extrusion. The relative resistance change of the fibres caused by contact with different solvents (water, n-hexane, ethanol, methanol) and solvent concentrations was used as liquid sensing response, whereas the time dependent resistance was recorded during immersion and drying cycles. Transmission electron microscopy and Raman spectroscopy indicated enhanced orientation of MWNT along the fibre axis with take-up velocity, resulting in decreased sensitivity during solvent contact. Additionally, sensitivity decreased as the weight content of MWNT increased and was furthermore dependent on the characteristics of used solvents. In context with the targeted application of leakage detection, fibres with low MWNT amount and low draw down ratio (as extruded fibres with 2 wt.% MWNT) are suitable, as they showed relative resistance changes of up to 87% after 10 min immersion in methanol even if the recovery upon drying was suppressed significantly.

Introduction

Besides the exceptional mechanical, thermal, and electrical properties [1], [2], [3], [4], the use of carbon nanotubes (CNT) as sensor materials represents an interesting field of research. One scientific field concerns the incorporation of CNT into a polymer matrix to create sensory composite materials. Up to now some work has been done to investigate the possibilities of detecting mechanical strain [5], [6], temperature changes [7], [8], and different chemicals in form of gases, liquids and vapours [9], [10], [11], [12], [13] by means of CNT polymer composites.

Basically, such sensory materials detect changes of environmental conditions, like mechanical strain, temperature, or presence of vapours or liquids, whereas the composites respond with changes of the conductive CNT network. This change of network structure normally goes along with an electrical response, which can be easily measured and analysed.

In case of liquid sensing, solvent sorption leads to swelling of the polymer matrix resulting in a partial disruption of contacts or increase in the distances between neighboured nanotubes above the tunnelling or hopping distances causing a resistance increase. Thus, a good dispersion of the nanotubes and an establishment of a percolated nanotube network within the polymer matrix are preconditions to use a composite for sensing trials. This aim provides a challenge, as nanotubes are produced in agglomerated structures [14], [15], which have to be dispersed. For MWNT within PLA we recently successfully optimised melt mixing conditions for preparation and dilution of a masterbatch using a twin-screw extruder in order to achieve a good filler dispersion [16]. For these composites the liquid sensing properties were investigated using compression-moulded plates [17] and aspects of the liquid sensing mechanism were examined [18]. It could be shown that the resistance reversibly changed upon the cycles with good reproducibility. Lower MWNT loadings resulted in larger resistance changes, indicating that the conductive MWNT network tends to readily disconnect due to the less dense structures as compared to higher loadings. Various solvents (n-hexane, toluene, chloroform, tetrahydrofuran, dichloromethane, ethanol, and water) were successfully detected, showing different degrees of resistance changes which could be related to the difference in the solubility parameters between PLA and the solvents used.

On the route towards applications in consumer goods reasonable devices have to be manufactured. Beside sensors made of bulk materials the use of fibres is very interesting because they can be used in textiles. Thus, flexible sensors or sensor arrays with large dimensions can be processed. However, it is well known that the morphology and thus electrical and sensing properties of CNT polymer composites is strongly related to the processing conditions. Consequently and based on previous work we further investigate the sensing behaviour of PLA/MWNT composites in the form of endless melt spun fibres within the framework of the European project INTELTEX. In a first step the processing conditions to manufacture fibres reasonable for liquid sensing were investigated. In the second step the triangle between processing, morphology, and sensing properties will be discussed.

One of the targeted applications is the use in textiles as applied for coatings in tanks, canisters, or containers in order to detect solvent leakage. Such a textile should contain sensing fibres announcing the occurrence of a leakage and possibly also the location of defects. For this application the main requirement is a fast and significant sensing response.

Section snippets

Materials and composite preparation

PLA L 9000 from Biomer (Germany) was chosen as matrix polymer. PLA usually hydrolyses at temperatures above glass transition temperature (Tg) of 60 °C and melts at 170 °C. Due to its susceptibility towards water absorption, the pellets were dried in vacuum for 18 h at 40 °C prior every processing step. Furthermore, PLA exhibits a good spinnability where high take-up velocities are achievable [19], [20], [21]. In addition, it can be produced from renewable resources and is a biodegradable and

Spinnability of the composites and as spun fibre resistance

A summary concerning the produced fibres, their diameters, and resistance ranges is given in Table 1. The melt spinning of PLA/MWNT composites with take-up velocities between 20 and 100 m/min was uncomplicated for filler amounts up to 3.0 wt.%, as their melting and flowing behaviour is still in the range of good processability. With a considerable exceedance of percolation threshold with filler contents above 3.0 wt.%, the increasing filler-filler interaction leads to a significant reduction of

Summary and conclusions

The liquid sensing properties of melt spun PLA/MWNT fibres have been investigated. Principally, such fibres are potential materials for liquid sensing and show relatively fast and reproducible responses on solvent immersion, especially in ethanol and methanol. During the immersion time of 10 min maximum relative resistance changes of 0.86 (extruded fibres with 2 wt.% MWNT in methanol) have been found. In contrast to compression-moulded samples the fibres did not return to their initial resistance

Acknowledgments

We gratefully acknowledge financial support from INTELTEX (Intelligent multi-reactive textiles integrating nano-filler based CPC-fibres), a European Integrated Project supported through the Sixth Framework Programme for Research and Technological Development of European Commission (NMP2-CT-2006-026626). We also thank Dr. Wolfgang Jenschke (Leibniz Institute of Polymer Research Dresden, Germany) for development of software to monitor the resistance changes.

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