The present study investigates the possibilities of manufacturing piezoelectric polymer fibres or yarns for integration into textile materials. The polymers known to give the strongest piezoelectric effects are poly(vinylidene fluoride) (PVDF) and some of its copolymers. PVDF is used in commercially available piezoelectric products, mainly in the form of films. The processing of PVDF towards these applications includes three necessary steps: the formation of polar β-phase crystallites, the application of electrodes and poling under high voltage. As the melt spinning process largely influences the molecular structure of a fibre, the processing (spinning) parameters can be expected to influence the formation of β-phase crystallinity in PVDF. In this study, results from both small/lab scale and large/industrial scale melt spinning are presented. By combining a range of processing parameters, it is shown that the degree of crystallinity increases with increasing melt draw ratio and is not affected by solid state drawing. Solid state drawing is necessary to produce fibres with β-phase crystallinity, the content of which increases with increasing draw ratio. Further, for maximum β-phase content drawing should take place at a temperature between 70-90°C and the draw rate should be as high as possible. Next, melt spinning of novel bicomponent fibres with high β-phase PVDF in the sheath and a carbon black/polypropylene (CB/PP) compound in the core has been successfully realized. The yarn’s core is electrically conductive, this paving the way to using CB/PP as inner electrode in melt spinning of piezoelectric bicomponent fibres. The combined analyses by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) showed that DSC-measurements are not sufficient to provide information about the crystalline structure of PVDF.
The manufacturing and characterisation of piezoelectric textile fibres are described in this thesis. A piezoelectric material is one that generates an electric voltage when deformed, a property which exists in a number of materials. The polymer with the strongest known piezoelectric effect today is poly(vinylidene fluoride) (PVDF), however it must be processed under certain conditions to become piezoelectric. This study shows that piezoelectric bicomponent PVDF-based fibres can be produced by melt spinning, which is a common and relatively simple fibre spinning method. The melt spinning process must include cold drawing, as this introduces a polar crystalline structure in the polymer. The fibres must also be electroded, which is done by producing bicomponent fibres with a core-and-sheath structure. The core is electrically conductive and constitutes an inner electrode consisting of a carbon black/polymer compound, whereas the sheath is PVDF and constitutes the piezoelectric component. Being sensitive to both deformation and temperature changes, these fibres are anticipated to be useful in a number of sensor applications. The flexibility and small size of the fibres makes it possible to include them as miniature-sensors in structures or garment without affecting the shape or comfort.
Melt Spun Piezoelectric Textile Fibres - an Experimental Study ANJA LUND Department of Materials and Manufacturing Technology Chalmers University of Technology ABSTRACT The manufacturing and characterisation of piezoelectric textile fibres are described in this thesis. A piezoelectric material is one that generates an electric voltage when deformed, a property which exists in a number of materials. The polymer with the strongest known piezoelectric effect today is poly(vinylidene fluoride) (PVDF), however it must be processed under certain conditions to become piezoelectric. This study shows that piezoelectric bicomponent PVDF-based fibres can be produced by melt spinning, which is a common and relatively simple fibre spinning method. The melt spinning process must include cold drawing, as this introduces a polar crystalline structure in the polymer. The fibres must also be electroded, which is done by producing bicomponent fibres with a core-and-sheath structure. The core is electrically conductive and constitutes an inner electrode consisting of a carbon black/polymer compound, whereas the sheath is PVDF and constitutes the piezoelectric component. Being sensitive to both deformation and temperature changes, these fibres are anticipated to be useful in a number of sensor applications. The flexibility and small size of the fibres makes it possible to include them as miniature-sensors in structures or garment without affecting the shape or comfort.
Nanoclay and carbon nanotubes (CNT) have been in focus recently as means of enhancing beta phase crystals formation in poly(vinylidene fluoride)(PVDF). Dominantly, the so-far work has been carried out on films/ thin sheets filled with nanoclay. It has been found, mainly from combined XRD and DSC data, that nanoclay influences the PVDF structure, and particularly the beta phase crystals formation is enhanced. Results published by various groups are in fairly good agreement. There are no results for nanoclay filled melt-spun PVDF fibres. The influence of CNT on PVDF structure has been less studied. XRD data indicating an enhancing role of multi-wall carbon nanotubes (MWNT) on beta phase crystals formation in solution compounded PVDF films are available. Published results for MWNT/PVDF films are not in good agreement. The only study into single-wall carbon nanotube (SWNT)/PVDF has been made on electrospun nanofibres. We explore above findings towards melt-spun nanofilled PVDF fibres. We present new results obtained by us for melt-spun PVDF fibres containing non-functionalized and amino-functionalized double-wall carbon nanotubes (DWNT). The key finding is that amino-DWNT can influence the beta to alpha polymorphic balance.
For the use of poly(vinylidene fluoride) (PVDF) as a piezoelectric material, the processing must include formation of polar β-phase crystallites, as well as the application of electrically conducting charge collectors, i.e. electrodes. In the present paper, results from melt spinning of PVDF yarns and a novel bicomponent PVDF-yarn with a conductive carbon black/polypropylene (CB/PP) core, are presented. Melt spinning has been done under conditions typical for industrial large-scale fiber production. The effects of varying spinning velocities, draw rates and draw temperatures on the resulting crystalline structure are discussed. The results show that for maximum α-to-β phase transformation, cold drawing should take place at a temperature between 70-90°C and draw ratio as well as draw rate should be as high as possible. It was observed that the cold drawing necessary to form β-phase crystallinity, simultaneously leads to a decrease in the core conductivity of the bicomponent yarns. In the present work, melt spinning of bicomponent fibers with high β-phase PVDF in the sheath and a CB/PP core was successfully realized. The core material remained electrically conductive, this paving the way to using a CB-polymer compound as inner electrode in melt spinning of piezoelectric bicomponent fibers.
When poly(vinylidene fluoride) (PVDF) is to be used as a piezoelectric material, the processing must include the formation of polar β-phase crystallites, as well as the application of electrically conducting charge collectors, that is, electrodes. In this article, results from the melt spinning of PVDF yarns and a novel bicomponent PVDF-yarn with a conductive carbon black/polypropylene (CB/PP) core are presented. Melt spinning has been done under conditions typical for industrial large-scale fiber production. The effects on the resulting crystalline structure of varying the spinning velocity, draw rate, and draw temperature are discussed. The results show that, for maximum α-to-β phase transformation, cold drawing should take place at a temperature between 70 and 90°C, and both the draw ratio and the draw rate should be as high as possible. It was observed that the cold drawing necessary to form β-phase crystallinity simultaneously leads to a decrease in the core conductivity of the bicomponent yarns. In this work, the melt spinning of bicomponent fibers with high-β-phase PVDF in the sheath and a CB/PP core was successfully accomplished. The core material remained electrically conductive, paving the way for the use of a CB-polymer compound as inner electrode in the melt spinning of piezoelectric bicomponent fibers.
The effect of melt spinning and cold drawing parameters on the formation of β-phase crystallinity in Poly(Vinylidene Fluoride) (PVDF) fibres, and ways of increasing such crystallinity, were studied. Fibres were melt-spun using four different melt draw ratios, and subsequently cold-drawn at different draw ratios. The maximum draw ratio in cold drawing was dependent on the draw ratio used in the melt spinning. The crystalline structure of the fibres was studied mainly by DSC and XRD. Results showed that the degree of crystallinity in the fibres was determined by the melt draw ratio, and that before cold drawing the crystalline structure of the fibres was predominantly in the α form. By cold drawing, α-phase crystallites could be transformed into the β-phase. It was established that, at certain conditions of melt spinning and cold drawing, PVDF fibres containing up to 80% of mainly β form crystallinity can be prepared. It is further proposed that fibres spun at a sufficiently high melt draw ratio consist to a large extent of extended-chain crystals, and this greatly affects the melting point of the PVDF. Thus DSC melting point data were shown to be insufficient to determine the crystalline phase of PVDF.
Poly(vinylidene fluoride) (PVDF) is a polymorphic polymer which, when made to crystallize in its polar form, yields piezo- and pyroelectric properties. In recent years PVDF has found its way towards intelligent textile applications, as researchers have attempted to create e.g. sensors for cardiorespiratory monitoring and energy harvesting by adding a commercially available PVDF film to a textile material. Present work is a first step towards constructing an electroactive fibre or yarn for true integration into textile materials. Important factors for achieving a fibre with useful electromechanical response, are high crystallinity and high content of polar β phase crystallites. The present study attempted to find how the spinning and drawing parameters should be adjusted to achieve such properties. Experimental results showed that crystallinity can be increased significantly (up to 90% crystalline content) by increasing melt draw ratio during spinning. The fibres drawn only in the melt showed mainly non-polar α phase crystallinity, and it was necessary to add a subsequent cold drawing step in which α- to β-phase conversion occurred. It was further seen that while a high crystallinity has a negative effect on the maximum draw ratio of fibres, an increase in the draw ratio during cold drawing had a positive effect on the conversion from α to the β form. Thus there seems to be an antagonistic effect between high degree of crystallinity and high degree of polar crystallites. An interesting finding was that an increased deformation speed during cold drawing appeared to promote conversion to the β form. The results indicate that with some fine-tuning of melt spinning and cold drawing parameters, it is possible to produce PVDF fibres with high crystallinity (above 80%) and almost completely in the β-form.
Melt spinning of a novel piezoelectric bicomponent fiber, with poly(vinylidene fluoride) as the electroactive sheath component, has been demonstrated. An electrically conductive compound of carbon black (CB) and high density polyethylene was used as core material, working as an inner electrode. A force sensor consisting of a number of fibers embedded in a soft CB/polyolefin elastomer matrix was manufactured for characterization. The fibers showed a clear piezoelectric effect, with a voltage output (peak-to-peak) of up to 40 mV under lateral compression. This continuous all-polymer piezoelectric fiber introduces new possibilities toward minimal single fiber sensors as well as large area sensors produced in standard industrial weaving machines.
This paper presents the melt spinning and characterisation of polymer actuator fibres; fibres that reversibly contract along the fibre axis in response to heat. Recently, Haines et al (1) showed that low-cost filaments, e.g. fishing lines, can be relevant precursors for artificial muscles. They demonstrated a reversible fibre-direction thermal contraction, which was significantly amplified when the fibres were twisted and coiled. The effect was explained to result from an increase in the conformational entropy of the amorphous phase. In earlier studies on negative thermal expansion in anisotropic polymer structures, it has been shown that the negative thermal expansion in oriented highly crystalline polymers approaches values typical of polymer crystals (2).
To further investigate the mechanisms behind these seemingly simple artificial muscles, we have melt spun fibres from poly(vinylidene fluoride) (PVDF) – Solef 1006 and 1008 kindly provided by Solvay (Milan, Italy) – and compared their properties to a commercially available PVDF-fishing line. The fibres were characterised with respect to their thermal actuation properties, crystal morphology and degree of orientation along the spin-line axis.
We have further done modelling on the molecular and macroscopic levels examining the possible mechanisms of negative thermal expansion in semi-crystalline PVDF. We believe that tie molecules (a polymer chain linking two crystalline regions) are the predominant factor influencing actuation. Two mechanisms are considered: an entropic effect and a conformational change effect. The entropic effect causes an increase in the elastic stiffness with an increase in temperature, effectively resulting in a contraction of a strained fibre. The conformational change effect is also expected to contribute to contraction as tie molecules, under strain, revert to their unloaded preferred conformation when heated.
This study reports on the poling and characteristics of a melt-spun piezoelectric bicomponent fiber with poly(vinylidene fluoride) (PVDF) as its sheath component and a conductive composite with carbon black (CB) and high density polyethylene (HDPE) as its core component. The influence of poling conditions on the piezoelectric properties of the fibers has been investigated. The poling parameters temperature, time and poling voltage have been varied and the piezoelectric effect of both contact- and corona-poled yarns have been evaluated. The results show that a high piezoelectric effect is achieved when the poling voltage is high as possible and the poling temperature is between 60°C and 120°C. It was also shown that permanent polarization is achieved in a time as short as 2 second in corona-poled fibers. A yarn exposed to a sinusoidal axial tension of 0.07% strain (the corresponding force amplitude was 0.05 N) shows an intrinsic voltage output of 4 V. The mean power from a 25 mm length of yarn is estimated to be 15nW. To demonstrate the fibers sensor properties, they are woven into a textile fabric from which a force sensor is manufactured and used to detect the heartbeat of a human.
Textile sensor for human motion detection in healthcare applications This project aims to develop a wearable and comfortable sensor system useful for continuous monitoring of symptoms of epilepsy and Parkinson´s disease, and progress during rehabilitation after a stroke. The system is to monitor both physiological electrical signals and movement, providing an objective assessment tool for hospital personnel monitoring the wearer’s progress. This gives a possibility to improve diagnosis, monitor disease progression or improvement and tailor treatments. By integrating the wearable sensors into a garment, preferably into the fabric itself, we aim to develop a functional demonstrator that is comfortable enough to be accepted by the patients for daily use. [1] The use of textile sensors in healthcare applications is one step closer to a more comfortable wearable sensor system for continuous measurements. For heart rate detection textile based electrodes in garments have been investigated. e.g. [2] Our current focus is on developing textile sensors for human motion detection, connected to the specific motions regarding the neurological disorders. One approach is to integrate electromechanical properties in the textile structure, creating strain sensitive structures which give an electrical output when stretched mechanically due to movement. When investigating a suitable textile construction developing textile sensors often takes a trial and error approach, which is time consuming. In a recent study[3] we showed that the textile construction influenced the performance of a textile sensor. The study pointed to a need of a more controlled developing method, such as computer simulation, to make more accurate predictions of the sensors performance. By investigating the possibility to combine existing computer simulation programmes, such as Comsol Multiphysics and TexGen, for an assessment of the behaviour and performance of the electromechanical properties of textile structures a new design method for smart textile sensors could be achieved. In an ongoing interdisciplinary research project, wearITmed, partners from healthcare, electronics and textiles development (Sahlgrenska Academy, Acreo ICT, The Swedish School of Textiles, Swerea IVF