Atomic Force Microscopy (AFM) and Ultrasonic Force Microscopy (UFM) have been applied to the characterization of composite samples formed by SrTiO3 (STO) nanoparticles (NPs) and polyvinyl alcohol (PVA). The morphological features of the STO NPs were much better resolved in UFM than in contact-mode AFM topography. For high STO concentrations the individual STO NPs formed nanoclusters, which gathered in microaggregates. The STO aggregates, covered by PVA, exhibited no AFM frictional contrast, but were clearly distinguished from the PVA matrix using UFM. Similar aggregation was observed for NPs in the composite samples than for NPs deposited on top of a flat silicon substrate from a milliQ water solution in the absence of polymer. In the hybrid films, most STO nanoparticles typically presented a lower UFM contrast than the PVA matrix, even though stiffer sample regions such as STO should give rise to a higher UFM contrast. STO NPs with intermediate contrast were characterized by an UFM halo of lower contrast at the PVA/STO interface. The results may be explained by considering that ultrasound is effectively damped on the nanometer scale at PVA/ STO interfaces. According to our data, the nanoscale ultrasonic response at the PVA/STO interface plays a fundamental role in the UFM image contrast.KEYWORDS. Atomic Force Microscopy. Ultrasonic Force Microscopy. Poly(vinyl alcohol). Stroncium Titanate. Nanoparticles. Nanocomposites. 1-IntroductionUltrasonic Force Microscopy (UFM) is a powerful technique to investigate the elastic and adhesive response of materials on the nanoscale [1,2]. The procedure is capable to provide material contrast in both soft and hard samples, bringing additional advantages when compared with other Scanning Probe Microscopy (SPM) approaches [3][4][5][6]. Here, UFM is applied to the characterization of composite samples formed by SrTiO3 (STO) nanoparticles (NP) and poly(vinyl alcohol) (PVA). We intend to further explore the capability of the technique to provide subsurface information, to characterize the nanostructures formed by STO NP in the PVA hybrid films, and to gain inside into the mechanisms of ultrasound propagation on the nanoscale and the origin of the UFM contrast.The insertion of ceramics nanoparticles into polymer matrix has led to the generation of novel hybrid materials with improved electrical and thermo-mechanical properties. In titanate-polymer composites, the titanates contribute with a high capacitance, and the polymers are typically easy to process. Hence, the composite films appear very attractive for the fabrication of integrated circuits [7,8]. Composites of STO with polymeric materials have been considered for microwave applications [9,10]. The dielectric properties of PVA mixed with PbTiO3 show promise for their application as supercapacitors and humidity sensors [11]. STO nanoparticles are being tested for the development of thin film transistors [8], batteries [12], photodiodes [13], and solar cells [14]. Also, they may exhibit photocatalytic activity [15][16...

Ureasil-Poly(ethylene oxide) (ureasil-PEO500) and ureasil-Poly(propylene oxide) (u-PPO400) films, unloaded and loaded with dexamethasone acetate (DMA), have been investigated by carrying out atomic force microscopy (AFM), ultrasonic force microscopy (UFM), contact-angle, and drug release experiments. In addition, X-ray diffraction, small angle X-ray scattering, and infrared spectroscopy have provided essential information to understand the films’ structural organization. Our results reveal that while in u-PEO500 DMA occupies sites near the ether oxygen and remains absent from the film surface, in u-PPO400 new crystalline phases are formed when DMA is loaded, which show up as ~30–100 nm in diameter rounded clusters aligned along a well-defined direction, presumably related to the one defined by the characteristic polymer ropes distinguished on the surface of the unloaded u-POP film; occasionally, larger needle-shaped DMA crystals are also observed. UFM reveals that in the unloaded u-PPO matrix the polymer ropes are made up of strands, which in turn consist of aligned ~180 nm in diameter stiffer rounded clusters possibly formed by siloxane-node aggregates; the new crystalline phases may grow in-between the strands when the drug is loaded. The results illustrate the potential of AFM-based procedures, in combination with additional physico-chemical techniques, to picture the nanostructural arrangements in polymer matrices intended for drug delivery.

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