23 Nanomechanical Characterization of Polyolefin Materials with Multifrequency Atomic Force Microscopy

Tuesday, October 11, 2011: 2:30 PM
Meeting Room #16 (The I-X Center)
Dalia Yablon1, Roger Proksch2, Anil Gannepalli2, Jean Grabowski3 and Andy Tsou3, (1)Corporate Strategic Research, ExxonMobil Research and Engineering, Annandale, NJ, (2)Asylum Research, Santa Barbara, CA, (3)Corporate Strategic Research, ExxonMobil, Annandale, NJ
Atomic force microscopy (AFM) is a powerful technique with broad applications to characterization of surfaces, primarily used for nanoscale quantitative topographic measurements and qualitatively distinguishing between material properties on the surface.  Recent advances in multifrequency AFM enables imaging at 2 simultaneous frequencies, which has enabled the implementation of several new imaging modes (bimodal Dual AC, DART contact resonance, loss tangent imaging) with improved resolution and new contrast on surfaces over conventional AFM.  Bimodal Dual AC images a surface at two different cantilever eigenmodes with active feedback in only one mode; this provides new information in the amplitude and phase image on the surface as the higher order mode provides a different source of mechanical contrast and improved resolution on the surface.  A second new mode is Dual AC Resonance Tracking (DART) of contact resonance frequency, which can be used for measuring storage and loss moduli of viscoelastic materials by appropriate modeling of the tip-sample contact as a driven damped harmonic oscillator. 

In this study, bimodal Dual AC is applied to examine crystalline and amorphous morphology development on polyethylene (PE) blown films revealing new features and morphology previously not observed with conventional AFM.  DART contact resonance imaging is implemented on model polyolefin-polystyrene blends and more practical polypropylene-elastomer to quantify viscoelastic properties of these materials.  The nanoscale viscoelastic properties measured with DART contact resonance agree well with values calculated from DMA results.  Finally, we describe the ability to image polymer surfaces under tensile stress in a custom designed tensile stage for the AFM revealing information about the interface and material deformation as it responds to tensile stress.