A. Maceiras

Synthesis and characterization of novel piezoelectric nitrile copolyimide films for high temperature sensor applications

A series of amorphous polyimides and copolyimides that contained nitrile were obtained by a two-step procedure. The first step consisted of a polycondensation reaction of 4,4’-oxydiphtalic anhydride (ODPA) with one or two aromatic diamines, namely 1,3-Bis-2-cyano-3-(3-aminophenoxy)phenoxybenzene (diamine 2CN) and 1,3-Bis(3-aminophenoxy)benzene (diamine 0CN). In the second step, a thermal cyclodehydration converted each poly(amic acid) or copoly(amic acid) into their corresponding polyimide films. The piezoelectric response was improved after corona poling of the films. A maximum d33 modulus value of 16 pC N−1 was obtained for the polymide with two cyano groups (poly 2CN). The polarization also showed time and thermal stability up to 160 °C. Additionally, the thermal stability of the amorphous polyimides, (β-CN)APB/ODPA, was studied by determining the glass transition temperature (T g ) and thermal decomposition through differential scanning calorimetry (DSC) and thermogravimetric analysis (TG), respectively. The high piezoelectric response (1–16 pC N−1), T g (160–180 °C) and degradation temperature (315–450 °C) make such polyamides excellent candidates for use as high temperature sensors.

Improving the Performance of High Temperature Piezopolymers for Magnetoelectric Applications

Piezoelectricity in amorphous polymers is mainly due to the orientation polarization of the molecular dipoles. Aromatic polyimides are high-performance polymeric materials possessing large molecular dipoles. We already reported good magnetoelectric performance of laminate composites with Vitrovac 4040® as magnetostrictive component and the 2,6(b-CN)APB/ODPA (poli 2,6) polyimide as the piezoelectric. Although the piezoelectric response of this polyimide is good, its mechanical properties can be improved. To combine the best mechanical and piezoelectric response in the same polymer, copolyimides have been synthesized by reaction of the 4,4-oxydiphtalic anhydride (ODPA) with a mixture of 1,3-Bis-2-cyano-3-(3-aminophenoxy) phenoxybenzene (diamine 2CN) and 1,3-Bis (3-aminophenoxy) benzene (diamine 0CN). We present the piezoelectric, mechanical and ME performance of laminate composites of these copolyimides.

Magnetic cellulose nanocrystal nanocomposites for the development of green functional materials

A magnetic cellulosic material composed of cellulose nanocrystals (CNC) and cobalt ferrite (CoFe2O4) nanoparticles was developed through evaporation-induced self-assembly (EISA). Nanoparticles demonstrated good dispersibility within the cellulose nanocrystal template. The addition of glucose to CNC network allows the development of homogeneous crack-free CNC-based films and does no modify neither the morphology nor the optical properties. In contrast, the introduction of CoFe2O4 nanoparticles produces a marked decrease in the amount of the transmitted light. 20wt.% of CoFe2O4 nanoparticles inside the CNC matrix induced a maximum magnetization value of 12.96emug-1, increased the real part of the dielectric constant (permittivity) from 10 (pure CNC film) to 12 and improved the thermostability of the nanocomposite as evidenced by the increase of the onset temperature from 165.1 to 220.4°C. Those features obtained in a non-petroleum-based composite provide insight into the development of the next generation of functional materials from natural origin.

Enhanced Charge-Transfer Emission in Polyimides by Cyano-Groups Doping

Conjugated polyimides (PI) are successfully synthesized by chemical imidization through in situ silylation of diamines by the alteration of different donor-acceptor monomers. A detailed photophysical characterization is performed in the monomers and different polymers in tetrahydrofuran solutions. The emission spectra of the related donor-acceptor polymers with electron withdrawing cyano groups showed broader and more intense fluorescence bands in comparison to the polymer without -CN groups. The new emission band results from the contribution of two different charge-transfer (CT) pathways: (i) An intramolecular CT (ICT) state in the donor monomers due to the presence of strong electron withdrawing CN groups, and (ii) a intramolecular CT complex (CTC) in the PI polymer between the donor-acceptor monomers, which is red-shifted and shows longer lifetime respect to the ICT of the diamine monomers. The very wide emission band is a very interesting feature for obtaining white light from UV light.

Radio Frequency Magnetoelectric Effect Measured at High Temperature

Magnetoelectric response at the radio frequency range has been studied as a function of temperature. For that purpose, we have fabricated sandwich-type laminated composites in which the Fe_61.6Co_16.4Si_10.8B_11.2 alloy was used as the magnetostrictive element and polyvinylidene fluoride (PVDF) as the piezoelectric one. The Fe_61.6Co_16.4Si_10.8B_11.2 amorphous ribbon shows good magnetic properties, with a magnetostriction close to 30 ppm and a piezomagnetic coefficient in as-quenched state and for a long ribbon of 21.4 × 10^-3 ppm/Am^-1. Even though PVDF shows a higher piezoelectric coefficient (d₃₃ = 15 pC/N) than the high temperature poly and copolymides, the advantage of using these poly- and copolymides is the great stability shown at temperatures close to 200 °C. Considering this, the influence of temperature in laminated composites and in their components (epoxy, piezoelectric, and magnetostrictive constituents) has been measured and discussed.