As atividades piezo e piroelétricas do Poli(fluoreto de vinilideno) PVDF tem sido bastante estudadas e usadas em aplicações práticas nos últimos anos. Mais recentemente, além de ficar provada a ferroeletrecidade em uma das fases do PVDF, foi também detectada uma temperatura de Curie em um dos seus copolímeros, o Poli(flureto de vinilideno co triflúor etileno) P(VDF/TrFE). Com o objetivo de estudar algumas características peculiares das transições sofridas por este material, diversos experimentos estimulados termicamente foram efetuados em amostras de composição molar 60/40 e 70/30. Tem sido mostrado que o P(VDF/TrFE) 60/40 sofre uma transição ferro-para com forte indícios de primeira ordem, porém com queda da polarização remanente não tão abrupta como é previsto por teorias fenomenológicas. Um modelo composto de uma lenta relaxação tipo Debye e um parâmetro que controla a velocidade de transição foram propostos e ajustados aos dados experimentais com excelente precisão. É conhecido também, que o P(VDF/TrFE) 70/30 além de sofrer uma transição muito parecida com aquela há pouco discutida para o copolímero 60/40, sofre uma outra, a qual foi por nós assumida como sendo do tipo ferro-ferro. Usando uma dinâmica parecida com a que foi usada para o copolímero 60/40, acrescida da transição por último citada, um modelo foi usado para ajustar o que é previsto com os dados experimentais. Aqui, os resultados foram apenas razoáveis se comparados com aqueles obtidos para o copolímero 60/40. Analisando-se os dados experimentais, verificou-se que o campo elétrico de polarização exerce fortes influências, não somente nos fenômenos de transição, mas também sobre cargas de espaço detectadas no material, provavelmente oriundas do meio ambiente. Um modelo teórico simplificado foi proposto como ponto de partida para o estudo destes efeitos. / Piezoelectric and pyroelectric activities of the polyvinylidene fluoride have been intensively studied in the last years, and many practical applications have also been suggested. More recently, besides the detection of its ferroelectric activity, it was verified that the copolymer VDF/TrFE presents a Curie temperature. In the present work we have studied phase transitions on P(VDF/TrFE) with 60/40 and 70/30 molar composition, using thermally stimulated processes. It is already well-known that the copolymer 60/40 undergoes a ferro-to-paraelectric first-order phase transition, though the decay of the remanent polarization with the temperature is not abrupt as the predicted by the phenomenologic theories. So, we developed a model based on a very slow Debye relaxation combined with a parameter which controls the speed of the phase transition. The calculated values of the polarization fitted, with a high accuracy, the experimental values. The 70/30 copolymer undergoes, beside the F-P transition, another phase transition probably between two ferroelectric phases. A model, similar to that developed for the 60/40 copolymer, was also adjusted to the experimental results. From the experimental results we have observed that the electric field used to polarize the samples exerts some influence upon the phase transitions and also upon the space charges absorbed from the environment. For the latter case a simplified model was developed in attempting to explain isothermal current discharge measurements.

Usando-se a técnica do triodo de corona com corrente constante estudaram-se as propriedades ferroelétricas dos copolímeros P(VDF-TrFE) contendo diferentes teores de VDF. Descrevem-se as metodologias usadas na preparação de amostras por solução, a caracterização por raios X e DSC e a determinação do grau de cristalinidade. Apresentam-se o princípio de funcionamento da técnica do triodo de corona com corrente constante e a equivalência das curvas de potencial de superfície com as curvas de histerese. Mostra-se que a condução elétrica pode ser desprezada quando as medidas são realizadas em atmosfera com baixo valor de umidade. O valor do campo coercivo e da polarização remanente foram determinados a partir das curvas de subida do potencial de superfície, sendo o valor da polarização diretamente proporcional ao grau de cristalinidade das amostras. Esses resultados foram analisados supondo-se que os filmes de P(VDF-TrFE) podem ter duas fases cristalinas. Também são discutidos os modelos teóricos usados para interpretar as curvas de subida do potencial de superfície. / Ferroelectric properties of P(VDF-TrFE) copolymers having different contents of VDF were investigated using the corona triode with constant current. The casting procedure for the sample preparation, the characterization by X rays and DSC, and the determination of the degree of crystalinity were presented. The corona triode method allowing to charge samples with a Constant current and the equivalence between the potential buildup curves and hysteresis curves are presented. The coercive field and the remanent polarization were determined from the potential buildup curves and it was found that the polarization is linearly dependent on the degree of crystalinity. Results were discussed taking into account two different crystaline phases which could exists in P(VDF-TrFE). Also, it was presented theoretical models aiming to fit the potential buildup curves.

Usando-se a técnica do triodo de corona com corrente constante estudaram-se as propriedades ferroelétricas dos copolímeros P(VDF-TrFE) contendo diferentes teores de VDF. Descrevem-se as metodologias usadas na preparação de amostras por solução, a caracterização por raios X e DSC e a determinação do grau de cristalinidade. Apresentam-se o princípio de funcionamento da técnica do triodo de corona com corrente constante e a equivalência das curvas de potencial de superfície com as curvas de histerese. Mostra-se que a condução elétrica pode ser desprezada quando as medidas são realizadas em atmosfera com baixo valor de umidade. O valor do campo coercivo e da polarização remanente foram determinados a partir das curvas de subida do potencial de superfície, sendo o valor da polarização diretamente proporcional ao grau de cristalinidade das amostras. Esses resultados foram analisados supondo-se que os filmes de P(VDF-TrFE) podem ter duas fases cristalinas. Também são discutidos os modelos teóricos usados para interpretar as curvas de subida do potencial de superfície. / Ferroelectric properties of P(VDF-TrFE) copolymers having different contents of VDF were investigated using the corona triode with constant current. The casting procedure for the sample preparation, the characterization by X rays and DSC, and the determination of the degree of crystalinity were presented. The corona triode method allowing to charge samples with a Constant current and the equivalence between the potential buildup curves and hysteresis curves are presented. The coercive field and the remanent polarization were determined from the potential buildup curves and it was found that the polarization is linearly dependent on the degree of crystalinity. Results were discussed taking into account two different crystaline phases which could exists in P(VDF-TrFE). Also, it was presented theoretical models aiming to fit the potential buildup curves.

As atividades piezo e piroelétricas do Poli(fluoreto de vinilideno) PVDF tem sido bastante estudadas e usadas em aplicações práticas nos últimos anos. Mais recentemente, além de ficar provada a ferroeletrecidade em uma das fases do PVDF, foi também detectada uma temperatura de Curie em um dos seus copolímeros, o Poli(flureto de vinilideno co triflúor etileno) P(VDF/TrFE). Com o objetivo de estudar algumas características peculiares das transições sofridas por este material, diversos experimentos estimulados termicamente foram efetuados em amostras de composição molar 60/40 e 70/30. Tem sido mostrado que o P(VDF/TrFE) 60/40 sofre uma transição ferro-para com forte indícios de primeira ordem, porém com queda da polarização remanente não tão abrupta como é previsto por teorias fenomenológicas. Um modelo composto de uma lenta relaxação tipo Debye e um parâmetro que controla a velocidade de transição foram propostos e ajustados aos dados experimentais com excelente precisão. É conhecido também, que o P(VDF/TrFE) 70/30 além de sofrer uma transição muito parecida com aquela há pouco discutida para o copolímero 60/40, sofre uma outra, a qual foi por nós assumida como sendo do tipo ferro-ferro. Usando uma dinâmica parecida com a que foi usada para o copolímero 60/40, acrescida da transição por último citada, um modelo foi usado para ajustar o que é previsto com os dados experimentais. Aqui, os resultados foram apenas razoáveis se comparados com aqueles obtidos para o copolímero 60/40. Analisando-se os dados experimentais, verificou-se que o campo elétrico de polarização exerce fortes influências, não somente nos fenômenos de transição, mas também sobre cargas de espaço detectadas no material, provavelmente oriundas do meio ambiente. Um modelo teórico simplificado foi proposto como ponto de partida para o estudo destes efeitos. / Piezoelectric and pyroelectric activities of the polyvinylidene fluoride have been intensively studied in the last years, and many practical applications have also been suggested. More recently, besides the detection of its ferroelectric activity, it was verified that the copolymer VDF/TrFE presents a Curie temperature. In the present work we have studied phase transitions on P(VDF/TrFE) with 60/40 and 70/30 molar composition, using thermally stimulated processes. It is already well-known that the copolymer 60/40 undergoes a ferro-to-paraelectric first-order phase transition, though the decay of the remanent polarization with the temperature is not abrupt as the predicted by the phenomenologic theories. So, we developed a model based on a very slow Debye relaxation combined with a parameter which controls the speed of the phase transition. The calculated values of the polarization fitted, with a high accuracy, the experimental values. The 70/30 copolymer undergoes, beside the F-P transition, another phase transition probably between two ferroelectric phases. A model, similar to that developed for the 60/40 copolymer, was also adjusted to the experimental results. From the experimental results we have observed that the electric field used to polarize the samples exerts some influence upon the phase transitions and also upon the space charges absorbed from the environment. For the latter case a simplified model was developed in attempting to explain isothermal current discharge measurements.

O poli(fluoreto de vinilideno) (PVDF) é um polímero semicristalino que quando cristalizado a partir da fusão à temperaturas acima de 155°C apresenta uma variada morfologia cristalina, constituída por esferulitos anelados, não anelados e mistos. Abaixo dessa temperatura somente os esferulitos anelados são formados. Neste trabalho foi realizado um estudo morfológico do PVDF, procurando verificar a relação entre o tipo de esferulito formado e a fase cristalina predominante em suas lamelas. Foi verificado, por espectroscopia no infravermelho, que os esferulitos anelados apresentam exclusivamente a fase alfa, quando a cristalização ocorre a temperaturas inferiores a 155°C. Temperaturas superiores a essa induzem nessas estruturas uma transformação de fase alfa em gama, que aumenta a quantidade de fase gama com o tempo de cristalização. A taxa com que essa transformação ocorre aumenta com a temperatura de cristalização. Os esferulitos não anelados são constituídos predominantemente pela fase gama, cristalizada diretamente do fundido, com pequenas inclusões de fase alfa. O processo de fusão dos diferentes esferulitos, observado por microscopia ótica (MOLP) e medidas calorimétricas (DSC), mostraram que a temperatura de fusão da fase gama originada da transformação de fase é 8°C superior à daquela cristalizada diretamente do fundido. Micrografias de amostras aquecidas até 186°C e rapidamente resfriadas permitiram visualizar as regiões dos esferulitos anelados que sofreram a transformação de fase alfa em gama, para diferentes tempos e temperaturas de cristalização. Foi realizado, ainda, um estudo morfológico de blendas PVDF/P(VDF-TrFE), com diferentes composições e com o copolímero contendo 72% em mol de VDF. Análises térmicas (DSC) verificaram que os componentes da blenda são imiscíveis na fase cristalina, quando a cristalização ocorre a partir da fusão. Porém, micrografias obtidas por MOLP e MEV indicaram a presença de moléculas do copolímero nas regiões interlamelares dos esferulitos formados durante a cristalização do PVDF. Esse resultado sugere a miscibilidade entre os componentes no estado líquido, e que esta mistura íntima deve permanecer durante a cristalização, resultando na miscibilidade dos componentes na fase amorfa. / Poly(vinylidene fluoride) (PVDF) is a semi crystalline polymer that, when is crystallized from the melt at temperatures above 155°C, it presents a multiform morphology composed of ringed, non-ringed and mixed spherulites. Above this temperature only ringed spherulites are formed. In this work a morphological study of PVDF was performed, endeavoring to investigate the relation between the type of spherulite formed and the dominant crystalline phase in their lamellas. Infrared spectroscopy showed that the ringed spherulites are formed exclusively by the alpha phase when crystallization takes place at temperatures below 155°C. Higher temperatures induce a solid-state alpha in gamma phase transformation in these structures, increasing the amount of gamma phase with crystallization time. The rate at which this transformation takes place increases with crystallization temperature. The non-ringed spherulites consist predominantly of the gamma phase, crystallized from the melt, with small alpha phase inclusions. The melt process of the different spherulites, observed by optical microscopy (MOLP) and calorimetric measurements (DSC) showed that the melt temperature of the gamma phase originated from the phase transition is 8°C higher than that crystallized directly from the melt. Micrographs of samples heated up to 186°C and quickly cooled allowed visualization of the ringed spherulite regions, which underwent the alpha in gamma phase transformation at different crystallization times and temperatures. In addition, it was observed a morphological study of blends, with different compositions and the copolymer containing 72 mol% of VDF. Thermal analysis (DSC) showed that blend components are immiscible on crystalline phase, when the crystallization from the melt occurs. However, micrographs obtained from MOLP and MEV indicated the presence of copolymer on spherulites interlamellar regions, formed during PVDF crystallization. This result suggests miscibility between the components in the liquid state, and this intimate mixture must remain during crystallization, resulting in miscibility of the components in amorphous phase.

O poli(fluoreto de vinilideno) (PVDF) é um polímero semicristalino que quando cristalizado a partir da fusão à temperaturas acima de 155°C apresenta uma variada morfologia cristalina, constituída por esferulitos anelados, não anelados e mistos. Abaixo dessa temperatura somente os esferulitos anelados são formados. Neste trabalho foi realizado um estudo morfológico do PVDF, procurando verificar a relação entre o tipo de esferulito formado e a fase cristalina predominante em suas lamelas. Foi verificado, por espectroscopia no infravermelho, que os esferulitos anelados apresentam exclusivamente a fase alfa, quando a cristalização ocorre a temperaturas inferiores a 155°C. Temperaturas superiores a essa induzem nessas estruturas uma transformação de fase alfa em gama, que aumenta a quantidade de fase gama com o tempo de cristalização. A taxa com que essa transformação ocorre aumenta com a temperatura de cristalização. Os esferulitos não anelados são constituídos predominantemente pela fase gama, cristalizada diretamente do fundido, com pequenas inclusões de fase alfa. O processo de fusão dos diferentes esferulitos, observado por microscopia ótica (MOLP) e medidas calorimétricas (DSC), mostraram que a temperatura de fusão da fase gama originada da transformação de fase é 8°C superior à daquela cristalizada diretamente do fundido. Micrografias de amostras aquecidas até 186°C e rapidamente resfriadas permitiram visualizar as regiões dos esferulitos anelados que sofreram a transformação de fase alfa em gama, para diferentes tempos e temperaturas de cristalização. Foi realizado, ainda, um estudo morfológico de blendas PVDF/P(VDF-TrFE), com diferentes composições e com o copolímero contendo 72% em mol de VDF. Análises térmicas (DSC) verificaram que os componentes da blenda são imiscíveis na fase cristalina, quando a cristalização ocorre a partir da fusão. Porém, micrografias obtidas por MOLP e MEV indicaram a presença de moléculas do copolímero nas regiões interlamelares dos esferulitos formados durante a cristalização do PVDF. Esse resultado sugere a miscibilidade entre os componentes no estado líquido, e que esta mistura íntima deve permanecer durante a cristalização, resultando na miscibilidade dos componentes na fase amorfa. / Poly(vinylidene fluoride) (PVDF) is a semi crystalline polymer that, when is crystallized from the melt at temperatures above 155°C, it presents a multiform morphology composed of ringed, non-ringed and mixed spherulites. Above this temperature only ringed spherulites are formed. In this work a morphological study of PVDF was performed, endeavoring to investigate the relation between the type of spherulite formed and the dominant crystalline phase in their lamellas. Infrared spectroscopy showed that the ringed spherulites are formed exclusively by the alpha phase when crystallization takes place at temperatures below 155°C. Higher temperatures induce a solid-state alpha in gamma phase transformation in these structures, increasing the amount of gamma phase with crystallization time. The rate at which this transformation takes place increases with crystallization temperature. The non-ringed spherulites consist predominantly of the gamma phase, crystallized from the melt, with small alpha phase inclusions. The melt process of the different spherulites, observed by optical microscopy (MOLP) and calorimetric measurements (DSC) showed that the melt temperature of the gamma phase originated from the phase transition is 8°C higher than that crystallized directly from the melt. Micrographs of samples heated up to 186°C and quickly cooled allowed visualization of the ringed spherulite regions, which underwent the alpha in gamma phase transformation at different crystallization times and temperatures. In addition, it was observed a morphological study of blends, with different compositions and the copolymer containing 72 mol% of VDF. Thermal analysis (DSC) showed that blend components are immiscible on crystalline phase, when the crystallization from the melt occurs. However, micrographs obtained from MOLP and MEV indicated the presence of copolymer on spherulites interlamellar regions, formed during PVDF crystallization. This result suggests miscibility between the components in the liquid state, and this intimate mixture must remain during crystallization, resulting in miscibility of the components in amorphous phase.

Organic ferroelectrics polymers have recently received much interest for use in nonvolatile memory devices. The ferroelectric copolymer poly(vinylidene fluoride- trifluoroethylene) , P(VDF-TrFE), is a promising candidate due to its relatively high remnant polarization, low coercive field, fast switching times, easy processability, and low Curie transition. However, no detailed study of charge injection and current transport properties in P(VDF-TrFE) have been reported in the literature yet. Charge injection and transport are believed to affect various properties of ferroelectric films such as remnant polarization values and polarization fatigue behavior.. Thus, this thesis aims to study charge injection in P(VDF-TrFE) and its transport properties as a function of electrode material. Injection was studied for Al, Ag, Au and Pt electrodes. Higher work function metals such as Pt have shown less leakage current compared to lower work function metals such as Al for more than an order of magnitude. That implied n-type conduction behavior for P(VDF-TrFE), as well as electrons being the dominant injected carrier type. Charge transport was also studied as a function of temperature, and two major transport regimes were identified: 1) Thermionic emission over a Schottky barrier for low fields (E < 25 MV/m). 2) Space-Charge-Limited regime at higher fields (25 < E <120 MV/m). We have also studied the optical imprint phenomenon, the polarization fatigue resulting from a combination of broad band optical illumination and DC bias near the switching field. A setup was designed for the experiment, and validated by reproducing the reported effect in polycrystalline Pb(Zr,Ti)O3 , PZT, film. On the other hand, P(VDF-TrFE) film showed no polarization fatigue as a result of optical imprint test, which could be attributed to the large band gap of the material, and the low intensity of the UV portion of the arc lamp white light used for the experiment. Results suggest using high work function metals for a memory application, as lower leakage would enhance fatigue endurance for P(VDF-TrFE) film.

Les objectifs de ce travail consistent à développer de nouveaux matériaux souples hybrides organiques/inorganiques en vue d'application pour des microsystèmes. Cette étude comprend l'élaboration et la caractérisation de nanocomposites à matrices polymères ferroélectriques P(VDF-TrFE), matériaux potentiels pour les applications à base de films minces, et de différents types de nanoparticules: ZnO, LiNbO3 (piézoélectriques) et Al2O3 (non-piézoélectriques). Les protocoles de dispersion ultrasoniques des nanoparticules dans des solvants et des solutions polymères ont été optimisés, afin de disperser de façon homogène des clusters de nanocristaux dans la matrice copolymère. Les films d'épaisseurs contrôlées de 0.1 µm à 100 µm ont été fabriqués par spin-coating et enduction, avec une qualité de surface adaptée aux micro-technologies. L'étude de la morphologie et de la cristallinité de P(VDF-trFE) a montré leur préservation en présence de nanoparticules jusqu'à 10 wt.%. Les nanocomposites gardent ainsi des propriétés piézoélectriques élevées tout en montrant un renforcement allant jusqu'à 30 % des propriétés mécaniques avec 10 wt.% de ZnO ou Al2O3. De plus, une augmentation des constantes élastiques avec la diminution de la taille des clusters de nanoparticules a été observée. Les films nanocomposites sur substrats ou autosupportés chargés jusqu'à 10 wt.% ont été polarisés avec succès par Corona sans contact. Des protocoles spécifiques pour réaliser des microdispositifs par photolithographie sur films nanocomposites tout en conservant les activités piézoélectriques des matériaux ont été développés. La caractérisation des dispositifs à ondes acoustiques réalisés est aussi présentée / The objective of this work is to develop flexible organic/inorganic hybrid materials for application in microsystems. This study included the preparation and characterization of nanocomposites based on ferroelectric polymer matrix P(VDF-TrFE), potential materials for applications based on thin films on substrates, and different types of nanoparticles: ZnO, LiNbO3 (piezoelectric) and Al2O3 (non-piezoelectric). The protocols of the ultrasonic dispersion of nanoparticles in solvents and polymeric solutions are optimized, allowing dispersing quite homogeneously clusters of nanocrystals in the matrix copolymer. Films of controlled thickness between 0.1 µm and 100 µm were fabricated by spin-coating and doctor blade coating with surface quality suitable for micro-technologies. Morphology and crystallinity of P(VDF-TrFE) are preserved in the presence of up to 10 wt.% of nanoparticles. Thus nanocomposites keep high piezoelectric properties and show an increased up to 30% of the mechanical properties for 10 wt.% ZnO or Al2O3. In addition, the increase in elastic constant with decreasing cluster size of nanoparticles was observed. Nanocomposite films on substrates or free-standing filled up to 10 wt.% were successfully polarized by corona without contact. Specific procedures for preparing microdevices by photolithography on nanocomposite polymer films, while keeping piezoelectric activities of materials, have been developed. The characterization of realized acoustic wave devices is also presented

Micrometer-sized liquid-crystal (LC) droplets embedded in a polymer matrix may enable optical switching in the composite film through the alignment of the LC director along an external electric field. When a ferroelectric material is used as host polymer, the electric field generated by the piezoelectric effect can orient the director of the LC under an applied mechanical stress, making these materials interesting candidates for piezo-optical devices. In this work, polymer-dispersed liquid crystals (PDLCs) are prepared from poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and a nematic liquid crystal (LC). The anchoring effect is studied by means of dielectric relaxation spectroscopy. Two dispersion regions are observed in the dielectric spectra of the pure P(VDF-TrFE) film. They are related to the glass transition and to a charge-carrier relaxation, respectively. In PDLC films containing 10 and 60 wt% LC, an additional, bias-field-dependent relaxation peak is found that can be attributed to the motion of LC molecules. Due to the anchoring effect of the LC molecules, this relaxation process is slowed down considerably, when compared with the related process in the pure LC. The electro-optical and piezo-optical behavior of PDLC films containing 10 and 60 wt% LCs is investigated. In addition to the refractive-index mismatch between the polymer matrix and the LC molecules, the interaction between the polymer dipoles and the LC molecules at the droplet interface influences the light-scattering behavior of the PDLC films. For the first time, it was shown that the electric field generated by the application of a mechanical stress may lead to changes in the transmittance of a PDLC film. Such a piezo-optical PDLC material may be useful e.g. in sensing and visualization applications. Compared to a non-polar matrix polymer, the polar matrix polymer exhibits a strong interaction with the LC molecules at the polymer/LC interface which affects the electro-optical effect of the PDLC films and prevents a larger increase in optical transmission. / Mikrometer-große, in eine Polymermatrix eingebettete Flüssigkristall-Tröpfchen können als elektro-optische Lichtventile fungieren, da die Ausrichtung der Flüssigkristalle durch ein externes elektrisches Feld verändert werden kann. Wird nun ein ferroelektrisches Polymer als Matrix verwendet, so kann das durch den piezoelektrischen Effekt erzeugte und von der äußeren mechanischen Spannung abhängige elektrische Feld den Flüssigkristall ausrichten. Solche Materialien können daher als piezo-optische Lichtventile eingesetzt werden. Im Rahmen dieser Arbeit wurden PDLCs (polymer-dispersed liquid crystals) durch Einbettung von nematischen Flüssigkristallen in Poly(Vinylidenefluoride-Trifluorethylene) (P(VDF-TrFE)) erzeugt. Die Wechselwirkungen an der Grenzfläche zwischen Flüssigkristall und Polymer wurden mittels dielektrischer Spektroskopie untersucht. Im dielektrischen Spektrum des reinen P(VDF-TrFE) wurden zwei Dispersions-Regionen beobachtet, welche vom Glasübergang und einer Ladungsträgerrelaxation des Polymers herrühren. PDLC Folien mit unterschiedlichen Anteilen von Flüssigkristall-Tröpfchen (10 bzw. 60 Gewichtsprozente) zeigten beim Anlegen eines elektrischen Wechselfelds zusatzliche Relaxationseffekte, welche der Bewegung der eingebetteten Flüssigkristall-Moleküle zugeordnet werden konnten. Durch die Einlagerung der Flüssigkristall-Moleküle weist die Struktur eine Relaxation auf, die gegenüber vergleichbaren Prozessen im reinen Flüssigkristall deutlich verlangsamt ist. Des weiteren wurde das elektrooptische und piezo-optische Verhalten der mit 10 und 60 Gewichtsprozent Flüssigkristall geladenen Folien untersucht. Die Lichtstreuung hängt dabei ab von der Fehlanpassung der Brechungsindizes von Polymermatrix und Flüssigkristallen sowie von den Wechselwirkungen der Polymerdipole mit den Flüssigkristall-Molekülen an der Tröpfchenoberfläche. Es konnte erstmalig gezeigt werden, dass die Lichtdurchlässigkeit der PDLC-Folien durch eine externe mechanische Spannung gesteuert werden kann. Dieser Effekt macht das piezo-optische PDLC-Material für die Verwendung in Optik- und Sensoranwendungen interessant. Im Vergleich mit unpolaren Wirtspolymeren zeigen polare Wirtsmaterialien eine deutlich stärkere Wechselwirkung zwischen den Flüssigkristall-Molekülen an der Polymer/Flüssigkristall-Grenzfläche, welche den elektrooptischen Effekt beeinflusst und so die maximale transmissions änderung reduziert.

Le travail présenté ici porte sur la réalisation d’un matériau polymère piézoélectrique destiné à être l’élément sensible d’un capteur de déformation de tissus biologiques. Cela comprend notamment l’étude de l’assouplissement du copolymère P(VDF-TrFE) nécessaire pour se rapprocher des propriétés mécaniques d’une artère, sans dégrader son coefficient piézoélectrique. Des films de P(VDF-TrFE) plastifiés avec du phtalate de diéthyle (DEP) ont été réalisés selon différents protocoles incluant enduction ou spin-coating et polarisation sous haute tension pour activer les propriétés ferroélectriques. Selon les conditions d’élaboration, deux structures distinctes de films ont été obtenues avec des propriétés physiques propres à chacune. Dans le premier type de film, l’étude de la morphologie et des courbes d’hystérésis polarisation-champ électrique a permis de mettre en évidence une nouvelle structuration du matériau, avec la démixtion du plastifiant dans la matrice. Le champ coercitif est dans ce cas fortement abaissé ce qui permet une réduction de la haute tension de polarisation nécessaire allant jusqu’à 40%, même lorsque que le film ne contient plus que 50wt% de P(VDF-TrFE). Le second type de film, obtenu après recuit à plus basse température, présente au contraire une structure quasi homogène et des propriétés proches d’une loi de mélange. Le champ coercitif reste comparable à celui du P(VDF-TrFE) pur mais la flexibilité du matériau est fortement accrue. L’étude des propriétés mécaniques a montré que le plastifiant peut réduire le module de Young du copolymère à 40MPa avec 30wt% de DEP dans le film. De surcroit la polarisation rémanente et le coefficient piézoélectrique sont également renforcés. Des tests in vitro et in vivo, réalisés sur des artères, de capteurs basés sur ces derniers films ont démontré le haut potentiel du matériau à détecter des déformations de tissus mous et à fonctionner aux fréquences biologiques humaines / The work presented here focuses on the preparation of a piezoelectric polymer material aimed to be the sensitive element of a strain sensor of biological tissues. This includes the study of the softening of the copolymer P(VDF-TrFE) necessary to be close of the mechanical properties of an artery, without reducing the piezoelectric coefficient. Plasticized P(VDF-TrFE) films with diethyl phthalate (DEP) were made according to different protocols including doctor blade technique or spin-coating and polarization under high voltage to activate the ferroelectric properties. Depending on the preparation conditions, two distinct structures were obtained with physical properties specific to each of them. For the first type of film, the study of the morphology and the hysteresis loops polarization-electric field showed a new structure of the material, with a demixing of the plasticizer in the matrix. In this case, the coercive field is strongly reduced which allows a decrease of the required high polarization voltage up to 40%, even if the film only contains 50wt% of P(VDF-TrFE). The second type of film, obtained after an annealing at lower temperature, has an almost homogeneous structure and properties close to a mixing law. The coercive field remains comparable to that of the pure P(VDF-TrFE) but the flexibility of the material is greatly increased. The study of the mechanical properties showed that the plasticizer can reduce the Young modulus to 40MPa for 30wt% of DEP in the film. In addition, the remanent polarization and the piezoelectric coefficient are also reinforced. In vitro and in vivo experiments, performed on arteries, of sensors based on these films demonstrated the high potential of the material to detect the strain of soft tissues and to function at biologic human frequencies