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Phase Identification and Elastic Property of Blend Copolymer Characterized by Force Modulation Microscopy and Force-Distance Curve

B. Kang, J.S. Lee, L. Pham and C. Sung
University of Massachusetts Lowell, US

Keywords: AFM, FMM, phase identification, force distance curve

Abstract:
The technique of polymer blending has been used to create new types of polymers with desirable properties in the past decade. Force Modulation Microscopy (FMM) provides a local contrast due to the local elasticity difference of a sample surface in addition to topography information [1]. The micro-structure depends on the composition of the blend, the compatibility of the two components, the fabrication of the blended materials, and the physical properties of two polymers. The mechanical properties are examined continuously over the extended area and force modulation mode is utilized to identify two phases to measure local elastic properties [2]. Poly(styrene-isobutylene-styrene) (SIBS) blended with poly(styrene-maleic anhydride) (SMA) was characterized by Atomic Force Microscopy (AFM) to understand phase separation. The blend ratios were 20%, 40%, and 80% of SMA with rest of percentage of SIBS. FIG 1. shows the FMM images and elastic moduli according to the SMA weight ratio. The scanning size of AFM was 30m x 30 m and the tip force constant was 40N/m at 325kHz resonance frequency. Youngs moduli were obtained by tensile testing machine from the slope of stress-strain curve [3]. Image analysis was employed to measure the volume fraction from FMM images at different blending ratios. The effective localized Youngs modulus can be calculated from the blended polymer utilizing Force versus Distance (F-D) curve. By using commercial AFM system, the F-D curve can be obtained and shown in FIG. 2. The cantilever was used the contact silicon tip and the spring constant was 0.9N/m with 105kHz resonant frequency. During the force measurement, the down speed of cantilever was 1.0 m/s and the up speed was 1.01.0 m/s. The localized elastic modulus of each phase is obtained by F-D curve through adopting the mathematical theory [4] and is compared with that of bulk material. Based on the FMM and modulus calculation by the F-D curves, the phase identification can be efficiently verified between the experimental results and the calculated values by the F-D data. The soft segment of SIBS showed much less elastic modulus than the hard segment of SMA, which was similar with the bulk elastic moduli of each phase.

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