Piezoelectric Response of Spun Polyvinylidene Fluoride and High Density Polyethylene Bicomponent Fibers with Carbon Black
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Date
2005-01-07
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Abstract
Sensors and actuators featuring biomimetic properties, with linear and angular resolution, good compliance and long term biostability are in growing demand for applications such as synthetic muscles, sensor equipped limps and other bio-engineering designs. Recent research papers have demonstrated that insulator materials coated with polypyrrole or polyaniline and combined with various dopants can achieve piezoresistive and dielectric properties, enabling the detection and displacement of local strains in polymer sheets, textile fibers and fabrics. It is known that composite films made from layers of carbon black (CB) filled polyvinylindene fluoride (PVDF) and high density polyethylene (HDPE) films provide stable piezoelectric behavior in the temperature range from 20 to 140 oC and low tensile loss on exposure to moisture and hydrolytic conditions. However, to date the literature contains no references to the use of this particular polymer system in fiber or textile form. Moreover, since the resistivity of such composites can be quantitatively specified by selectively localizing CB in one polymer phase or at the interface of an immiscible polymer blend, it was hypothesized that bicomponent fiber spinning might lead to similar piezoelectric properties within individual fibers. This research study was therefore aimed first at determining whether a blend of PVDF and HDPE polymers filled with CB could be melt spun and drawn into a series of composite or bicomponent fibers using a laboratory extruder and drawing machine. This was accomplished successfully with loadings of CB varying from zero to 27.7% by weight. The second goal was to determine the weight fraction of CB that should be added to PVDF / HDPE composite fibers in order to optimize their electrical functionality and piezoelectric performance. Analysis of the deformation of the as-spun and drawn fibers in their longitudinal direction during charging and discharging was conducted in a novel piezoresponse force microscope (PFM). It demonstrated that increasing the CB content also increased the ferroelectric hysteresis and piezoelectric constant of the composite fiber up to the percolation threshold of 20.7% of CB by weight. The CB was selectively located in the HDPE phase, resulting in a significant loss of crystallinity in the HDPE phase. At the same time, the PVDF phase was transformed from a non-polar to a polar form. The optimum spun and drawn composite piezoelectric fiber measuring 120 microns in diameter contained 56/32/12 PVDF/HDPE/CB by weight. Under the electric stimulation of a few volts it was predicted to be capable of producing a tensile force of about 2 x 10⁻² N for a 350 mm long fiber with 1 mm 2 cross-sectional area. It is anticipated that a bundle of such piezoelectric fibers measuring 26 mm² in cross-section could generate the force of 0.5 N required to complete flexion of a human distal interphalangeal (finger) joint. The incorporation of CB filled HDPE produces a conductive matrix phase within these bicomponent fibers, which acts as an electrode around the PVDF regions, facilitating a more uniform distribution of the piezoelectric charge within the PVDF phase. These encouraging results bode well for future piezoelectric fibers, which have both rapid electromechanical response and good biostability. Additional larger scale tests are recommended to evaluate the efficiency of these novel biomaterials for use in biomedical and electrotextile end-uses.
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carbon black, piezoresponse force microscopy, piezoelectric, thermal properties, composite fibers, high density polyethylene, polyvinylidene fluoride, percolation threshold., crystallinity, x-ray diffraction
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Degree
MS
Discipline
Textile and Apparel, Technology and Management
