Papers by Author: Jivago Serrado-Nunes

Abstract: This paper describes a fully-integrated lab-on-a-chip device for testing and monitoring biochemical parameters in biological fluids. The major innovation of this microdevice is the application of an acoustic microagitation technique with automatic electronic control based on a β-PVDF piezoelectric polymer placed underneath the microfluidic structures. Experimental results regarding the influence of the thickness of the polymer on the reaction rate of biological fluids are presented. Moreover, the study of the transmittance curve of β-PVDF with transparent conductive electrodes is also presented. Transparent electrodes are a constraint once the polymer is incorporate underneath the reaction chamber due to the analytical measurement by spectrophotometry.

Abstract: In this work, commercial poly(vinylidene fluoride) PVDF, in its β phase was analysed before and after the samples were exposed to UV radiation using a xenon lamp, for ten weeks. Changes in chemical structures, crystallinity, dielectric and piezoelectric response were investigated. From the present study it can be concluded that PVDF shows high stability towards photodegradation and can be used for outdoor applications without interference in its performance.

Abstract: The phase diagram of the poly(vinylidene fluoride-trifluorethylene) (P(VDF-TrFE)) copolymer system shows for VDF contents of 50...85 mol% a ferroelectric (FE)-paraelectric (PE) phase transition below melting temperature. Investigations on P(VDF-TrFE) 75/25 samples revealed a slight anisotropic behaviour, which leads to a strongly anisotropic stretching effect both on the phase transition and on the amount and nature of the FE phase in samples subjected to mechanical stretching along the main directions of the film. In this work, both the refractive index n1,2 and the piezoelectric coefficient d33 of mechanically stretched P(VDF-TrFE) have been measured for samples with different levels of permanent deformation. These parameters are found to reflect the anisotropy of the permanently deformed samples. The stretching effect is most pronounced (n1,2) or limited (d33) to the vicinity of the yielding point of the material. Above the yielding point, almost the piezoelectric d33 coefficient of the non-deformed sample is observed for samples with large permanent deformation.

Abstract: Chain reorientation may be induced in polyvinylidene fluoride (PVDF) in its β-phase by applying a deformation perpendicular to the pre-oriented polymeric chains. This reorientation begins right after the yielding point and seems to be completed when the stress-strain curve stabilizes. As the deformation process plays an important role in the processing and optimisation properties of the material for practical applications, different deformation stress was applied to the PVDF lamellas and their topographic change and piezoelectric response were studied by means of scanning force microscopy in a piezo-response mode. The experimental results confirm the previously observed chain reorientation that occurs right after the yielding point and that is completed when the yielding region is passed. This reorientation is accompanied by a stretching of the granular structures observed in the topographical images and variations in the domain response. The observed results help to explain the variations in the macroscopic response of the material.