Broadband nanodielectric spectroscopy by means of amplitude modulation electrostatic force microscopy (AM-EFM)

In this work we present a new AFM based approach to measure the local dielectric response of polymer films at the nanoscale by means of Amplitude Modulation Electrostatic Force Microscopy (AM-EFM). The proposed experimental method is based on the measurement of the tip–sample force via the detection of the second harmonic component of the photosensor signal by means of a lock-in amplifier. This approach allows reaching unprecedented broad frequency range (2–3×104 Hz) without restrictions on the sample environment. The method was tested on different poly(vinyl acetate) (PVAc) films at several temperatures. Simple analytical models for describing the electric tip–sample interaction semi-quantitatively account for the dependence of the measured local dielectric response on samples with different thicknesses and at several tip–sample distances

Nanodielectric mapping of a model polystyrene-poly(vinyl acetate) blend by electrostatic force microscopy

We present a simple method to quantitatively image the dielectric permittivity of soft materials at nanoscale using electrostatic force microscopy (EFM) by means of the double pass method. The EFM experiments are based on the measurement of the frequency shifts of the oscillating tip biased at two different voltages. A numerical treatment based on the equivalent charge method allows extracting the values of the dielectric permittivity at each image point. This method can be applied with no restrictions of film thickness and tip radius. This method has been applied to image the morphology and the nanodielectric properties of a model polymer blend of polystyrene and poly(vinyl acetate)

Nanoscale dielectric properties of insulating thin films: From single point measurements to quantitative images

Dielectric relaxation (DR) has shown to be a very useful technique to study dielectric materials like polymers and other glass formers, giving valuable information about the molecular dynamics of the system at different length and time scales. However, the standard DR techniques have a fundamental limitation: they have no spatial resolution. This is of course not a problem when homogeneous and non-structured systems are analyzed but it becomes an important limitation for studying the local properties of heterogeneous and/or nano-structured materials. To overcome this constrain we have developed a novel approach that allows quantitatively measuring the local dielectric permittivity of thin films at the nanoscale by means of Electrostatic Force Microscopy. The proposed experimental method is based on the detection of the local electric force gradient at different values of the tip-sample distance. The value of the dielectric permittivity is then calculated by fitting the experimental points using the Equivalent Charge Method. Even more interesting, we show how this approach can be extended in order to obtain quantitative dielectric images of insulating thin films with an excellent lateral resolution

Literatura y ciencia. Hacia una integración del conocimiento

We examine in this article the relationships between literature and science through the last two centuries, paying special attention to the present situation and its future perspectives. We will see that there are deep connections between science and literature since both are fed by (and at the same time feed) common social imagery. We will also analyse different aspects of the many relations between science and literature, such as their mutual influences and the parallel development of new concepts and ideas. In this sense, we will show how the major paradigm shifts are usually expressed simultaneously by science, literature and other fields of knowledge. Finally, we will discuss some aspects related to exploration of the common spaces shared by literature and science, in order to strengthen the forms of knowledge by means of an epistemic and methodological hybridization.Examinaremos en este artículo las relaciones entre literatura y ciencia a lo largo de los últimos dos siglos, con especial énfasis en el estado actual y en las perspectivas futuras. Veremos aquí que existen profundos vínculos entre ellas como consecuencia de que ambas se nutren de un imaginario colectivo común a la vez que lo alimentan. Analizaremos también diversos aspectos tales como las influencias recíprocas y los desarrollos simultáneos de nuevas ideas y conceptos. En este sentido, comprobaremos cómo los grandes cambios de paradigma se manifiestan muchas veces a la vez en la ciencia, la literatura y otros ámbitos del saber. Finalmente, discutiremos algunos aspectos relativos a la exploración de los espacios comunes entre literatura y ciencia con el objetivo de potenciar las formas de conocimiento a partir de un mestizaje epistémico y metodológico

Imaging dielectric relaxation in nanostructured polymers by frequency modulation electrostatic force microscopy

We have developed a method for imaging the temperature-frequency dependence of the dynamics of nanostructured polymer films with spatial resolution. This method provides images with dielectric compositional contrast well decoupled from topography. Using frequency-modulation electrostatic-force-microscopy, we probe the local frequency-dependent (0.1–100 Hz) dielectric response through measurement of the amplitude and phase of the force gradient in response to an oscillating applied electric field. When the phase is imaged at fixed frequency, it reveals the spatial variation in dielectric losses, i.e., the spatial variation in molecular/dipolar dynamics, with 40 nm lateral resolution. This is demonstrated by using as a model system; a phase separated polystyrene/polyvinyl-acetate (PVAc) blend. We show that nanoscale dynamic domains of PVAc are clearly identifiable in phase images as those which light-up in a band of temperature, reflecting the variations in the molecular/dipolar dynamics approaching the glass transition temperature of PVAc

Measuring dielectric properties at the nanoscale using Electrostatic Force Microscopy

Several electrostatic force microscopy (EFM) - based methods have been recently developed to study the nanoscale dielectric properties of thin insulating layers. Some methods allow measuring quantitatively the static dielectric permittivity whereas some others provide qualitative information about the temperature-frequency dependence of dielectric properties. In this chapter, all these methods are described and illustrated by experiments on pure and nanostructured polymer films. A section is dedicated to EFM probe - sample models and especially to the Equivalent Charge Method (ECM)

Dielectric properties of thin insulating layers measured by Electrostatic Force Microscopy

In order to measure the dielectric permittivity of thin insulting layers, we developed a method based on electrostatic force microscopy (EFM) experiments coupled with numerical simulations. This method allows to characterize the dielectric properties of materials without any restrictions of film thickness, tip radius and tip-sample distance. The EFM experiments consist in the detection of the electric force gradient by means of a double pass method. The numerical simulations, based on the equivalent charge method (ECM), model the electric force gradient between an EFM tip and a sample, and thus, determine from the EFM experiments the relative dielectric permittivity by an inverse approach. This method was validated on a thin SiO2 sample and was used to characterize the dielectric permittivity of ultrathin poly(vinyl acetate) and polystyrene films at two temperatures

Tuning molecular dynamics by hydration and confinement: Antiplasticizing effect of water in hydrated prilocaine nanoclusters

In glass-forming substances, the addition of water tends to produce the effect of lowering the glass transition temperature, Tg. In a previous work by some of us (Ruiz et al., Sci. Rep., 2017, 7, 7470) we reported on a rare anti-plasticizing effect of water on the molecular dynamics of a simple molecular system, the pharmaceutically active prilocaine molecule, for which the addition of water leads to an increase of Tg. In the present work, we study pure and hydrated prilocaine confined in 0.5 nm and 1 nm pore size molecular sieves, and carry out a comparison with the bulk compounds in order to gain a better understanding of the microscopic mechanisms that result in this rare effect. We find that the Tg of the drug under nanometric confinement can be lower than the bulk value by as much as 17 K. Through the concurrent use of differential scanning calorimetry and broadband dielectric spectroscopy we are able to observe the antiplasticizing effect of water in prilocaine also under nanometric confinement, finding an increase of Tg of up to almost 6 K upon hydration. The extension of our analysis to nanoconfined systems provides a plausible explanation for the very uncommon antiplasticizing effect, based on the formation of water-prilocaine molecular complexes. Moreover, this study deepens the understanding of the behavior of drugs under confinement, which is of relevance not only from a fundamental point of view, but also for practical applications such as drug delivery.Peer ReviewedPostprint (author's final draft

Propiedades eléctricas de compuestos de caucho cargados con negro de humo

Los compuestos de caucho con negro de humo son ampliamente utilizados en la industria, principalmente en la del neumático, por la posibilidad de sintonizar las propiedades físicas a los requerimientos del producto. Sin embargo, a pesar de que estos materiales se utilizan con éxito desde hace más de un siglo, y a la gran cantidad de esfuerzo y dinero dedicados a la investigación de los mismos, aun es poco lo que se conoce acerca de los mecanismos físicos que determinan sus propiedades físicas. Muchos de los procedimientos utilizados, por ejemplo en su formulación, obedecen más a un arte, o a un conocimiento adquirido,que a un método científico. La estabilidad térmica de estos materiales compuestos presenta una relevancia tecnológica significativa y un interesante desafío para la investigación científica. Se ha observado desde hace tiempo, que variaciones importantes se producen en las propiedades físicas de los compuestos de caucho cargados con negro de humo durante el envejecimiento a temperatura constante, aun en ausencia de esfuerzos mecánicos. Estos cambios pueden ser observados a través de la medición de las propiedades eléctricas o mecánicas de estos compuestos. Numerosos estudios han sido publicados en relación con este comportamiento. En ellos, los cambios en las propiedades físicas han sido relacionados con la floculación o aglomeración de las partículas de negro de humo en la matriz polimérica. Esto significa la difusión de los agregados de negro de humo, debido al movimiento Browniano, para formar aglomerados termodinámicamente más estables. Generalmente la ecuación de Stokes es utilizada para estimar la constante de difusión (Δ=kT/6πηa). En esta ecuación, k es la constante de Boltzmann, T la temperatura absoluta, η la viscosidad y a el radio de los agregados. Sin embargo, algunos resultados parecen contradecir las predicciones de la teoría de la floculación. La floculación no ha sido observada por TEM y la modificación física o química de la superficie del negro de humo produce resultados anómalo que no pueden ser explicados sólo en términos de la teoría de la floculación. El propósito de este trabajo es determinar si los cambios observados en las propiedades eléctricas y mecánicas, durante el envejecimiento de las muestras a temperatura constante, se deben exclusivamente al movimiento de los agregados de negro de humo, si es que este movimiento existe, o si otros mecanismos físicos son los responsables de estos cambios. Para ello, numerosas muestras han sido preparadas utilizando diferentes matrices poliméricas, diferentes tipos y cantidades de negro de humo con distintos tratamientos térmicos y diferentes procesos de mezclado entre otras variables. Las muestras fueron envejecidas a varias temperaturas (entre 60 y 150°C) durante diferentes tiempos (entre 10 minutos y varios días). La resistividad eléctrica y el módulo dinámico fueron medidos antes, durante y después del envejecimiento. Además, la evolución de la cantidad de caucho ligado (bound rubber)con el tiempo de envejecimiento fue medida para tener en cuenta el efecto de la interacción entre el negro de humo y el polímero. Se diseñó y construyó un dispositivo experimental para medir la evolución de la resistividad eléctrica en función del tiempo, de una muestra de caucho con negro de humo, durante su envejecimiento a temperatura constante. El dispositivo permitió realizar mediciones en aire y en vacío, controlar la rampa de temperatura y medir en un amplio rango de condiciones experimentales. Se observó que junto con el aumento de la conductividad eléctrica (o del módulo dinámico) se producía un incremento de la cantidad de bound rubber. Más aun, se observó que la dependencia con la temperatura y el tipo de polímero era la misma para ambas cantidades. Cuanto mayor era el incremento de la cantidad de bound rubber, mayor era el incremento de la conductividad eléctrica. De acuerdo con trabajos previos de Medalia y jachym, y con los resultados obtenidos en este trabajo, es posible que las cadenas de polímero, unidas químicamente a los agregados de negro de humo, actúen como hilos conductores entre agregados próximos. De este modo, un incremento de la cantidad de bound rubber implicaría un incremento de la conductividad eléctrica. Mediciones en vacío de la variación de las propiedades eléctricas con el tiempo, más nuevos ensayos mecánicos y la irradiación gamma de algunas muestras, han permitido verificar la hipótesis de conducción eléctrica a través de las cadenas de polímero y explicar la reversión observada para tiempos largos, tanto en las propiedades eléctricas como mecánicas. Finalmente se analizan los mecanismos de conducción eléctrica y el incremento del cluster percolante a partir del incremento de bound rubber. Un nuevo modelo de percolación por enlaces dependientes del tiempo es propuesto para describir el fenómeno en términos de la nueva interpretación propuesta.Carbon black filled compounds are fully used in the industry, mainly in tire industry, due to their ability to tune the physical properties to product requirements. However, in spite of these materials have been used successfully since a century ago, and too much effort and money have been put in research, still is not much what we know about the physical mechanisms that fix their physical properties. Most of used procedures, for instance in their formulation, seem more like an art, or acquired knowledge, than a scientific method. Thermal stability of these compounds present an important technological relevance and an interesting challenge for scientific research. It has been observed that appreciable changes occur in carbon black filled rubber compounds during aging at constant temperature, even in the absence of shear. These changes can be monitored through the electrical and mechanical properties of these compounds. Several studies have been published concerning this behavior and are related to flocculation of the filler entides. That means by diffusion of aggregates of carbon black, due to Brownian motion, to form thermodynamically stable agglomerates. Usually the Stokes equation is used to estimate the diffusion constant (Δ=kT/6πηa). In the previous equation k is the Boltzmann constant, T the absolute temperature, η the viscosity and a the aggregate radius. However, some results seem to be contradictory with the flocculation theory. The flocculation has not been observed by TEM and the physical and chemical modification of carbon black surface produce an anomalous behavior, which cannot be explained within the flocculation frame only. The purpose of this work is to elucidate if the observed changes on electrical and mechanical properties, during thermal aging, are due exclusively to the movement of the carbon black aggregates, if exist, or if other mechanisms responsible for those changes may exist. In order to do that, several samples were prepared using different polymer matrixes, different carbon black grades at various loading levels and also using physically and chemically modified carbon blacks. The samples were aged at several temperatures (between 60 and 150°C) during different times (between 10 minutes and several days). The electrical resistivity and mechanical modulus were measured before, during and after aging. Additionally, the evolution of the bound rubber with aging time was measured to take into account the effect of the polymer-filler interaction. An experimental device was designed and built to measure the electrical resistivity as a function of the time, for a carbon black filled rubber sample, during aging at constant temperature. This device allowed measuring sample resistivity in air and vacuum, controlling temperature ramp and measuring in a broad range of experimental setups. A simultaneous increment of the amount of bound rubber and electrical conductivity was observed. Furthermore, the temperature and polymer type dependence of both quantities is similar noticing that the higher the increment of bound rubber the higher the increment in the electrical conductivity. According to Medalia and jachym, and the results obtained in the present work, it is possible that polymer chains, grafted to carbon black aggregates, act like conductive hairs between close aggregates. In this way, an increment in the amount of bound rubber implies and increment in the electrical conductivity. Vacuum measurements of the electrical properties, new mechanical measurements and gamma irradiation have allowed verifying the hypothesis of electrical conduction through polymer chains and explaining the observed reversion for long times in both, electrical and mechanical properties. Finally, electrical conduction mechanisms and the increment of the percolating cluster from the increment of bound rubber have been analyzed. A new model of dynamic bond percolation is proposed in order to describe the behavior in terms of the new proposed interpretation.Fil:Schwartz, Gustavo Ariel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina