Structural evolution and acoustic phonon behavior in crystalline PTFE latex films
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Structural evolution and acoustic phonon behavior in crystalline PTFE latex films M. Pierno, C.S. Casari, A. Li Bassi, M.G. Beghi, R. Piazza, C.E. Bottani INFM and Dipartimento di Ingegneria Nucleare, Politecnico di Milano, Via Ponzio 34/3, 20133 Milano, Italy ABSTRACT The structural evolution of polytetrafluoroethylene (PTFE) crystalline polymer latex films is studied at hundreds nanometer length scale by atomic force microscopy and Brillouin light scattering. In a controlled sintering process the transition is observed from the original particle distribution towards a ‘fibrillar’ structure of crystalline regions embedded in a disordered matrix. This transition is accompanied by a cross-over from localized acoustic excitations to propagating acoustic phonons, related to mesoscopic elastic properties. After sintering, a ‘mark’ of the original particulate structure persists, suggesting that filming of crystalline polymers may be analogous to sintering of ceramic powders. Films of crystalline polymers can thus be exploited as model systems to study the elasto-optical properties of granular and disordered media. INTRODUCTION In many technologies, like paints, adhesives, printing inks, and friction reducers, the formation of polymer films is a crucial step. Film properties depend on both the nature of the polymer and the film preparation procedure. In many cases films are obtained from a latex: a dispersion of polymer particles of colloidal size, suspended in an immiscible fluid, and stabilized against coagulation by repulsive interparticle forces, obtained if necessary by specific surface-adsorbed additives. Film formation is a three steps process: the latex is cast on a substrate, dried by solvent evaporation, and finally sintered. Sintering occurs by polymer-chain diffusion, and dissolves the original particle morphology. Most polymers at normal temperatures are in an amorphous glassy phase. Film formation takes place beyond a temperature generally close to the glass-transition temperature, and has been thoroughly studied [1]. Less is known about filming of crystalline polymers, in which additional processes such as re-crystallization upon cooling have a significant role. Among these systems, polytetrafluoroethylene (PTFE) and its derivatives are of particular interest. PTFE has a crystalline phase, thermally stable up to the melting temperature, with a noticeable orientational disorder due the helical conformation of perfluorocarbon chains [2]. This distinctive structure contributes to yield the unique wetting, frictional and dielectric properties of perfluorinated polymers, that make them essential for hydrophobic or anti-sticking coatings, and in perspective may ensure them a role in high-frequency electronic hardware development [3]. Only the macroscopic thermal [4] and dielectric properties [3] of films produced from a perfluorinated polymer latex have been characterized. In the present work Brillouin light scattering (BS) and atomic force microscopy (AFM) are exploited to investigate the morpholo
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