Crystallinity and Mechanical Properties of Polypropylene-based Graphene Nanocomposites Studied with Atomic Force Microsc
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Crystallinity and Mechanical Properties of Polypropylene-based Graphene Nanocomposites Studied with Atomic Force Microscopy and Raman Spectroscopy Kjerstin Gronski, Robinson Flaig, Jorge Camacho, Yan Wu, and James P. Hamilton Nanotechnology Center for Collaborative Research and Development, Department of Chemistry and Engineering Physics,University of Wisconsin-Platteville, Platteville, WI 53818, USA. ABSTRACT Atomic force microscopy (AFM) and Raman spectroscopy were used to characterize the morphology and the local mechanical properties of polypropylene-based graphene nanocomposites. Amplitude Modulated AFM was used to perform phase angle measurements to estimate the loss tangent, along with the local elastic modulus of the nanocomposite’s surface as a function of graphene content. We have observed an increasing trend in phase angle as the graphene content increased. We also identified wrinkled graphene flakes embedded in the polymer matrix. The graphene corrugation and mismatched strain between polymer and graphene sheets show a variation in the phase angle that is corroborated with Raman measurements. Mechanically exfoliated graphene on SiO2 was characterized as a baseline to understand the effect of graphene wrinkles compared to graphene surfaces on phase angle. The Raman results revealed that there are changes in the crystalline morphology of the polymer with the addition of graphene. INTRODUCTION The mechanical properties of polymers can be enhanced by fillers. Polymer nanocomposites (NCs) represent an alternative to conventionally (macroscopically) filled polymers [1]. Graphene has more potential than other nanofillers, such as carbon nanotubes, due to its large interfacial area for mechanical and electrical enhancement along with low cost. There are many challenges that must be resolved for graphene-polymer NCs to be fully optimized for commercial use. There is evidence of poor interfacial adhesion in graphene-composites due to agglomeration of graphene sheets, lowering composite performance [2]. Investigating interaction of graphene and polymer will assist the optimization of polypropylene-based graphene (PPG) NCs. Amplitude modulation atomic force microscopy (AM-AFM) obtains topographic and compositional contrast images simultaneously in heterogeneous samples. Tip-sample interaction forces are time dependent and contain information on the viscoelastic, elastic, and adhesive properties of the sample, thus impacting the phase between the external excitation and the tip motion. The compositional contrast images are obtained by recording the phase angle difference, known as phase imaging [3]. Previous studies have shown that the cosine function of the phase is related to the conservative tip/sample interactions, thus the elastic properties of the sample, while sine function of the phase is related to the dissipative interaction, thus the viscoelastic behavior of the sample [4]. In previous research, it has been demonstrated that the crystalline morphology of polymer strongly influences the stiffness of NCs [5]
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