Determining concentration depth profiles in fluorinated networks by means of electric force microscopy

5 páginas, 6 figuras.-- et al.By means of electric force microscopy, composition depth profiles were measured with nanometric resolution for a series of fluorinated networks. By mapping the dielectric permittivity along a line going from the surface to the bulk, we were able to experimentally access to the fluorine concentration profile. Obtained data show composition gradient lengths ranging from 30 nm to 80 nm in the near surface area for samples containing from 0.5 to 5 wt. % F, respectively. In contrast, no gradients of concentration were detected in bulk. This method has several advantages over other techniques because it allows profiling directly on a sectional cut of the sample. By combining the obtained results with x-ray photoelectron spectroscopy measurements, we were also able to quantify F/C ratio as a function of depth with nanoscale resolution.L.A.M., M.M.K., G.A.S., A.A., and J.C. acknowledge the financial support provided by the Basque Country Government (Grant No. IT-436-07), the Spanish Ministry of Science and Innovation (Grant Nos. MAT 2007-63681 and CSD 2006- 00053). The Donostia International Physics Center (DIPC) financial support is also acknowledged.Peer reviewe

Quantitative mapping of mechanical properties in polylactic acid/natural rubber/organoclay bionanocomposites as revealed by nanoindentation with atomic force microscopy

Quantitative mapping of the mechanical properties of a series of polylactic acid/natural rubber/organoclay bionanocomposites has been accomplished by using nanoindentation with atomic force microscopy. Topography, elastic modulus and adhesion maps were obtained simultaneously revealing nanoscopic mechanical features in the samples associated to the different phases. For polylactic acid and polylactic acid/natural rubber a single distribution of Young's moduli was obtained whose maximum correlates well with the macroscopic measurements. Bionanocomposites with high organoclay loads exhibit a bimodal distribution of elastic moduli whose maxima can be associated to the polylactic acid matrix and to the reinforcing levels provided by the organoclay component. Adhesion maps allow one to obtain mechanical contrast between polylactic acid and organoclay, at high loadings, revealing the good compatibility of the organoclay with the polymer.Peer Reviewe

On the dielectric properties of polymers at nanometric scale by afm microscopy Autores: Mohammed Musthafa Kummali

[ES]: El objetivo de este trabajo de tesis consiste en desarrollar y analizar técnicas basadas en microscopía de fuerza atómica (AFM) para la caracterización mecánica y dieléctrica de materiales poliméricos. Para ello, se utilizó nanoespectroscopía dieléctrica (nDS) y nanoimaging dieléctrico (nDI) para estudiar la dinámica de polímeros en diferentes condiciones. Estas técnicas, recientemente desarrolladas, se emplearon en el estudio de sistemas poliméricos con propiedades dieléctricas complejas, en conjunto con otras técnicas de caracterización macroscópicas tales como calorimetría diferencial de barrido (DSC) y la espectroscopia dieléctrica de banda ancha (BDS). En la mayoría de los casos, además de la caracterización dieléctrica a nanoescala, también se realizó una caracterización mecánica utilizando técnicas de AFM. La aplicación de técnicas dieléctricas a nanoescala incluye el estudio del nivel de compatibilidad en films delgados de mezclas de poliestireno-polivinilacetato (PS/PVAc) con diferentes tratamientos térmicos. A este respecto, se encontró que dichos polímeros son incompatibles, y que su incompatibilidad se incrementa conforme la temperatura del tratamiento térmico aumenta. Estas técnicas fueron también aplicadas al estudio de la estructura y dinámica de cauchos sintéticos estireno-butadieno-caucho (SBR) reforzados con nanopartículas de silica. La caracterización mecánica a nanoescala reveló la presencia de una capa fina de polímero sobre las partículas, que, sin embargo, presenta la misma dinámica que el resto del material polimérico debido a una unión flexible con las partículas. Finalmente, se estudió el origen estructural de la relajación dieléctrica en copolímeros polietileno-polivinilacetato (EVA) semicristalinos. En este caso, el comportamiento dieléctrico de los materiales en bulk fue modelado exitosamente combinado BDS y nDI. Estos estudios muestran que las técnicas nanodieléctricas son una poderosa herramienta en el diseño y caracterización de nanocompuestos poliméricos.I thank University of the Basque Country for financially supporting me to pursue my PhD.Peer reviewe

Effect of Pore Connectivity on the Behavior of Fluids Confined in Sub-Nanometer Pores: Ethane and CO₂ Confined in ZSM-22

The behavior of fluids under nano-confinement varies from that in bulk due to an interplay of several factors including pore connectivity. In this work, we use molecular dynamics simulations to study the behavior of two fluids—ethane and CO2 confined in ZSM-22, a zeolite with channel-like pores of diameter 0.55 nm isolated from each other. By comparing the behavior of the two fluids in ZSM-22 with that reported earlier in ZSM-5, a zeolite with pores of similar shape and size connected to each other via sinusoidal pores running perpendicular to them, we reveal the important role of pore connectivity. Further, by artificially imposing pore connectivity in ZSM-22 via inserting a 2-dimensional slab-like inter-crystalline space of thickness 0.5 nm, we also studied the effect of the dimensionality and geometry of pore connectivity. While the translational motion of both ethane and CO2 in ZSM-22 is suppressed as a result of connecting the pores by perpendicular quasi-one-dimensional pores of similar dimensions, the effect of connecting the pores by inserting the inter-crystalline space is different on the translational motion of the two fluids. For ethane, pores connected via inter-crystalline space facilitate translational motion but suppress rotational motion, whereas in the case of CO2, both types of motion are suppressed by pore connection due to the strong interaction of CO2 with the surface of the substrate

Dielectric spectroscopy at the nanoscale by atomic force microscopy: A simple model linking materials properties and experimental response

The use of an atomic force microscope for studying molecular dynamics through dielectric spectroscopy with spatial resolution in the nanometer scale is a recently developed approach. However, difficulties in the quantitative connection of the obtained data and the material dielectric properties, namely, frequency dependent dielectric permittivity, have limited its application. In this work, we develop a simple electrical model based on physically meaningful parameters to connect the atomic force microscopy (AFM) based dielectric spectroscopy experimental results with the material dielectric properties. We have tested the accuracy of the model and analyzed the relevance of the forces arising from the electrical interaction with the AFM probe cantilever. In this way, by using this model, it is now possible to obtain quantitative information of the local dielectric material properties in a broad frequency range. Furthermore, it is also possible to determine the experimental setup providing the best sensitivity in the detected signal. © 2014 AIP Publishing LLC.We acknowledge the financial support provided by the Basque Country Government (IT-654-13) and the Spanish Ministry of Science and Innovation (MAT2012-31088). Financial support from EU-funded “European soft matter infrastructure” (reference 262348 ESMI) is also acknowledged.Peer Reviewe

AFM based dielectric spectroscopy: Extended frequency range through excitation of cantilever higher eigenmodes

In the last years, a new AFM based dielectric spectroscopy approach has been developed for measuring the dielectric relaxation of materials at the nanoscale, the so called nanoDielectric Spectroscopy (nDS). In spite of the effort done so far, some experimental aspects of this technique remain still unclear. In particular, one of these aspects is the possibility of extending the experimental frequency range, to date limited at high frequencies by the resonance frequency of the AFM cantilever as a main factor. In order to overcome this limitation, the electrical excitation of cantilever higher eigenmodes for measuring the dielectric relaxation is here explored. Thus, in this work we present a detailed experimental analysis of the electrical excitation of the cantilever second eigenmode. Based on this analysis we show that the experimental frequency range of the AFM based dielectric spectroscopy can be extended by nearly two decades with a good signal-to-noise ratio. By using the combination of first and second cantilever eigenmodes we study dissipation processes on well known PVAc based polymeric samples. Both, relaxation spectra and images with molecular dynamics contrast were thus obtained over this broader frequency range.We acknowledge the financial support provided by the Basque Country Government (IT-654-13) and the Spanish Ministry of Science and Innovation (MAT2012-31088). Financial support from EU-funded ‘European Soft Matter Infrastructure’ (Reference 262348 ESMI) is also acknowledged.Peer Reviewe

Study of the dynamic heterogeneity in poly(ethylene-ran-vinyl acetate) copolymer by using broadband dielectric spectroscopy and electrostatic force microscopy

Dynamic heterogeneities in ethylene-vinyl acetate (EVA) random copolymers were studied using broadband dielectric spectroscopy (BDS) over a broad frequency and temperature range. BDS data for EVA copolymers show relatively broad spectra extending over several frequency decades, which make interpretation of the data complicated. Thus, microscopic dielectric characterizations of the samples were done using single pass electrostatic force microscopy (SP-EFM) images. From these experiments two distinct dielectric contributions were found in the semicrystalline samples. Dielectric spectra of EVA copolymers were fitted using a model based on the SP-EFM images. According to this analysis, the broad dielectric responses of semicrystalline EVA copolymers were constituted from two dielectrically active phases, referred to as the constrained and nonconstrained phases, exhibiting different segmental relaxations. In the constrained phase, vinyl acetate (VA) motions were greatly restricted by crystalline microstructures, whereas in the nonconstrained phase, VA monomers were assumed to be completely free from crystalline effect. The glass transition temperature derived from the dynamics of the VA segments in the constrained phase was found to correspond well with that determined from DSC representing the overall amorphous phase in the more crystallized state. Moreover, crystallinity of the sample obtained using DSC was found to follow a similar variation with temperature to that of the dielectric relaxation strength of the constrained phase of the copolymers. However, at the earlier stage of crystallization, where crystallinity increases rapidly, the dielectric relaxation remains insensitive to it. © 2013 American Chemical Society.We acknowledge the financial support from Projects No. MAT2012-31088 (Spanish Government), No. IT654-13 (Basque Government), and the European Soft Matter Infrastructure (ESMI) Program (Grant Agreement number: 262348).Peer Reviewe

Compatibility studies of polystyrene and poly(vinyl acetate) blends using electrostatic force microscopy

The effect of thermal treatment on the phase separation process of the components of a polymer blend was investigated using electrostatic force microscopy (EFM). EFM technique is an advance on conventional atomic force microscopy, which enables us to measure locally the dielectric properties of the samples under investigation providing compositional information. In this work, we studied the phase separation process of the polymer blend thin films made of polystyrene and poly(vinyl acetate) (PS/PVAc) (75/25 weight fraction). The samples were subjected to different thermal treatments. It was found that at low annealing temperature, PVAc forms many small islands within PS matrix. As the annealing temperature increases, the number of PVAc islands decreases with an increase in the size of the islands. These islands take spherical-like shape when annealed at a temperature well above the glass transition temperatures of both the component polymers. Despite these morphological/topographical changes, EFM images evidence that there is no interdiffusion which was further confirmed by quantitatively measuring the value of the dielectric permittivity across the interphase. © 2011 Wiley Periodicals, Inc.Peer Reviewe