Correlating Nanoparticle Dispersion to Surface Mechanical Properties of TiO 2 /Polymer Composites
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Correlating Nanoparticle Dispersion to Surface Mechanical Properties of TiO2/Polymer Composites Yongyan Pang, Stephanie S. Watson, Aaron M. Foster, and Li-Piin Sung Polymeric Materials Group, Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899 ABSTRACT The objective of this study is to characterize the nanoparticle dispersion and to investigate its effect on the surface mechanical properties of nanoparticle-polymer systems. Two types of TiO2 nanoparticles were chosen to mix in two polymeric matrices: solvent-borne acrylic urethane (AU) and water-borne butyl-acrylic styrene latex (latex) coatings. Nanoparticle dispersion was characterized using laser scanning confocal microscopy. Overall, Particle A (PA, without surface treatment) dispersed better than Particle B (PB, organic treatment) in both systems. The AU-PA system exhibited the best dispersion of the four systems, however PB forms big clusters in both of the matrices. Surface mechanical properties, such as surface modulus at micron and submicron length scales were determined from depth sensing indentation equipped with a pyramidal tip or a conical tip. The surface mechanical properties were strongly affected by the dispersion of nanoparticle clusters, and a good correlation was found between dispersion of nanoparticle clusters near surface and the modulus-depth mapping using a pyramid tip. INTRODUCTION The addition of nanoparticles into polymeric coatings and composites has potential to improve performance and increase the number of applications compared to their micron-sized pigmentary counterparts [1-3]. However, poor dispersion and agglomeration of nanoparticles in polymeric matrices still remain a challenge in research and industrial applications. Agglomeration and poor dispersion adversely affect appearance, service life, and mechanical properties of polymer nanocomposites. Traditional mechanical testing methods, such as dynamic mechanical thermal analysis and tensile tests, detect changes in bulk mechanical properties upon the addition of nanoparticles; however, they are not always sensitive to the changes in local structural features. In this study, laser scanning confocal microscopy (LSCM) was used to map TiO2 nanoparticle dispersion in polymeric coatings and depth sensing indentation (DSI) techniques were used to measure the film mechanical properties within the first few micrometers from the surface. Nanoparticle-polymer coating systems having different dispersion states were prepared with two types of TiO2 nanoparticles incorporated into two different polymeric matrices. The effect of indenter acuity on the material indentation response was investigated using pyramidal and conical indenters. The impact of particle dispersion on local modulus was investigated by imaging the area after indentation. Good correlation was found between nanoparticle dispersion and surface modulus mapping using a pyramidal tip. EXPERIMENTAL# Materials #
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