Sierra-Bermúdez, Sergio L.

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    Probing nanoscale viscosity in polymer melts using magnetic nanoparticles.
    (2012) Sierra-Bermúdez, Sergio L.; Rinaldi, Carlos; College of Engineering; Acevedo, Aldo; Cordova, Ubaldo; Department of Chemical Engineering; Just, Frederick
    Oleic acid coated cobalt ferrite nanoparticles (CoFe2O4-OA), which relax through the Brownian mechanism, were dispersed in Polystyrene (PS) and Polybutadiene (PBD) of various molecular weights (MW). These suspensions were characterized by AC susceptibility measurements at various temperatures to determine the Brownian relaxation times of the nanoparticles in the polymer melts, with the objective of using the Debye model to determining the viscosity that the particles “feel” (i.e. “the nanoviscosity”) and compare these with macroscopic rheological measurements. Because accurate determination of MW is of essential importance to this study, all the samples were characterized using Gel Permeation Chromatography (GPC). The nanoparticles were synthesized through the high temperature thermal decomposition of organometallic precursors in organic solvents, which render particles with narrow size distribution and Brownian behavior up to at least 400 K. The colloidal stability of the nanoparticles in polymer melts was studied, as aggregates form anisotropic structures, precluding the use of the Debye model for the AC susceptibility in viscosity determination. Flory-Higgins interaction parameters between the nanoparticle surface and the polymers were calculated to predict miscibility, and good qualitative agreement was found with the experiments. The state of the nanoparticles, aggregated or dispersed, was studied using small angle xray scattering (SAXS) and transmission electron microscope (TEM), and was compared with susceptibility measurements. Good agreement was found between TEM images and susceptibility measurements. Nanoscale viscosities were higher than the macroscopic counterpart, which could be attributed to slight particle agglomeration in the polymer melt.