Electroactive phases of poly(vinylidene fluoride) : determination, processing and applications

A review on polymeric hydrogel membranes for wound dressing applications: PVA-based hydrogel dressings

A strain sensor has been fabricated from a polymer nanocomposite with multiwalled carbon nanotube (MWNT) fillers. The piezoresistivity
of this nanocomposite strain sensor has been investigated based on an improved three-dimensional (3D) statistical resistor network
model incorporating the tunneling effect between the neighboring carbon nanotubes (CNTs), and a fiber reorientation model. The
numerical results agree very well with the experimental measurements. As compared with traditional strain gauges, much higher sensitivity
can be obtained in the nanocomposite sensors when the volume fraction of CNT is close to the percolation threshold. For a small
CNT volume fraction, weak nonlinear piezoresistivity is observed both experimentally and from numerical simulation. The tunneling
effect is considered to be the principal mechanism of the sensor under small strains.

Tunneling effect in a polymer/carbon nanotube nanocompositestrain sensor

Despite its widespread use in nanocomposites, the effect of embedding graphene in highly viscoelastic polymer matrices is not well understood. We added graphene to a lightly cross-linked polysilicone, often encountered as Silly Putty, changing its electromechanical properties substantially. The resulting nanocomposites display unusual electromechanical behavior, such as postdeformation temporal relaxation of electrical resistance and nonmonotonic changes in resistivity with strain. These phenomena are associated with the mobility of the nanosheets in the low-viscosity polymer matrix. By considering both the connectivity and mobility of the nanosheets, we developed a quantitative model that completely describes the electromechanical properties. These nanocomposites are sensitive electromechanical sensors with gauge factors >500 that can measure pulse, blood pressure, and even the impact associated with the footsteps of a small spider.

/pdf/sensitive-electromechanical-sensors-using-viscoelastic-cyfjka25zb.pdf

Sensitive electromechanical sensors using viscoelastic graphene-polymer nanocomposites

This Review covers the recent developments (2005-2015) in the design, synthesis, characterization, and application of thermotropic ionic liquid crystals. It was designed to give a comprehensive overview of the "state-of-the-art" in the field. The discussion is focused on low molar mass and dendrimeric thermotropic ionic mesogens, as well as selected metal-containing compounds (metallomesogens), but some references to polymeric and/or lyotropic ionic liquid crystals and particularly to ionic liquids will also be provided. Although zwitterionic and mesoionic mesogens are also treated to some extent, emphasis will be directed toward liquid-crystalline materials consisting of organic cations and organic/inorganic anions that are not covalently bound but interact via electrostatic and other noncovalent interactions.

Ionic Liquid Crystals: Versatile Materials.

Thermoplastic elastomer/carbon nanotube composites are studied for sensor applications due to their excellent mechanical and electrical properties. Piezoresisitive properties of tri-block copolymer styrene–butadiene–styrene (SBS)/carbon nanotubes (CNT) prepared by solution casting have been investigated. The initial elastic modulus of the SBS/CNT composites increases with increasing the CNT filler content present in the samples, without losing the high deformation capability of the polymer matrix (∼1500%). Furthermore, above the percolation threshold these materials are unique for the development of large deformation sensors due to the strong piezoresistive response. Piezoresistive properties evaluated by uniaxial stretching in tensile mode and 4-point bending showed gauge factors up to 120. The linearity obtained between strain and electrical resistance makes these composites interesting for large strain piezoresistive sensors applications.

Electro-mechanical properties of triblock copolymer styrene-butadiene-styrene / carbon nanotube composites for large deformation sensor applications

Relationship between electromechanical response and percolation threshold in carbon nanotube/poly(vinylidene fluoride) composites

The piezoresistive response of epoxy/vapor-grown carbon nanofiber composites prepared by four different dispersion methods achieving different dispersion levels has been investigated. The composite response was measured as a function of carbon nanofiber loading for the different dispersion methods. Strain sensing by variation of the electrical resistance was tested through four-point bending experiments, and the dependence of the gauge factor as a function of the deformation and velocity of deformation was calculated as well as the stability of the electrical response. The composites demonstrated an appropriate response for being used as a piezoresistive sensor. Specific findings were that the intrinsic piezoresistive response was only effective around the percolation threshold and that good cluster dispersion was more appropriate for a good piezoresistive response than a uniform dispersion of individual nanofibers. The application limits of these materials for sensor applications are also addressed.

https://iopscience.iop.org/article/10.1088/0964-1726/21/7/075008/pdf

Effect of filler dispersion on the electromechanical response of epoxy/vapor-grown carbon nanofiber composites

The phase transformation from the α- to the electroactive β-phase of poly(vinylidene fluoride) (PVDF) extruded filaments submitted to different stretching conditions was investigated. Sample filaments of α-PVDF thermoplastic were extruded and stretched uniaxially at different temperatures (80 °C to 120 °C) and stretch ratios (1 to 6). The stretched samples were studied and characterised by x-ray diffraction and quasi-static mechanical experiments. High β-phase contents (~ 80%) are achieved using a stretch ratio of 5 independently of the stretching temperature, between 80 °C and 120 °C. Subsequently, in order to obtain filament geometries and material configurations suitable for application, a two layer filament with coaxial layers was produced by coextrusion. The inner layer consisted of a commercially available grade of a conductive thermoplastic with a polypropylene (PP) matrix. For the outer layer the same grade of PVDF was employed. The double-layer filament was also stretched under the same conditions of the PVDF filaments and the results obtained shows that the inner layer material, acting as an electrode, does not have any influence in the PVDF crystallization process: PVDF crystallizes in the α-phase for stretch ratios of 1 and the α-to β-phase transformation occurs for higher stretch ratios.

http://repositorium.sdum.uminho.pt/bitstream/1822/11965/1/pvdf_filaments_preprint.pdf

Extrusion of poly(vinylidene fluoride) filaments: effect of the processing conditions and conductive inner core on the electroactive phase content and mechanical properties

Cellulosic fibre-based smart materials exhibiting multiple capabilities are getting tremendous attention due to their wide application areas. In this work, multifunctional flax fabrics with piezoresistive response were developed through the combined functionalization with silver (Ag) and zinc oxide (ZnO) nanoparticles (NPs). Biodegradable polyethylene glycol (PEG) was used to produce AgNPs, whereas ZnONPs were synthetized via a simple and low-cost method. Flax fabrics with and without NPs were characterized by Ground State Diffuse Reflectance (GSDR), Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Spectroscopy (EDS), X-ray Diffraction (XRD), Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and Thermogravimetric analysis (TGA). After creating a conductive surface by flax functionalization with AgNPs, ZnONPs were synthetized onto these fabrics. The developed fibrous systems exhibited piezoresistive response and the sensor sensitivity increased with the use of higher ZnO precursor concentrations (0.4 M). Functionalized fabrics exhibited excellent antibacterial activity against Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria, higher hydrophobicity (WCA changed from 00 to >1000), UV radiation resistance, and wash durability. Overall, this work provides new insights regarding the bifunctionalization of flax fabrics with Ag/ZnO nanostructures and brings new findings about the combined effect of both NPs for the development of piezoresistive textile sensors with multifunctional properties.

/pdf/multifunctional-flax-fibres-based-on-the-combined-effect-of-2davoxxcr9.pdf

Armando Ferreira

Papers

Electro-mechanical properties of triblock copolymer styrene-butadiene-styrene / carbon nanotube composites for large deformation sensor applications

Relationship between electromechanical response and percolation threshold in carbon nanotube/poly(vinylidene fluoride) composites

Effect of filler dispersion on the electromechanical response of epoxy/vapor-grown carbon nanofiber composites

Extrusion of poly(vinylidene fluoride) filaments: effect of the processing conditions and conductive inner core on the electroactive phase content and mechanical properties

Multifunctional Flax Fibres Based on the Combined Effect of Silver and Zinc Oxide (Ag/ZnO) Nanostructures.