Fuente: Journal of applied polymer Lugar: RESEARCH ARTICLE Differences observed when fixed or swipped ultrasound frecuencies are used.

ABSTRACT
The effect of ultrasonic frequency sweeping on the preparation of polypropylene (PP) composites reinforced with multi-walled carbon nanotubes (MWCNTs) was investigated to improve the dispersion of the MWCNTs within the PP matrix. The MWCNTs loading in the composites was maintained fixed at 5 wt%. Ultrasonic treatment was carried out using a static ultrasonic mixer positioned at the outlet of a single-screw extruder operating without an internal mixing configuration. Ultrasonic radiation was applied at a constant frequency of 20 kHz, using frequency sweeping modes within three ranges: 15–20 kHz, 20–35 kHz, and 35–50 kHz. The resulting composites were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scattering electron microscopy (SEM) and measurements of thermal and electrical conductivity. Tensile behavior was also evaluated as a mechanical property. Additionally, the molecular weight of the PP before and after sonication was assessed via gel permeation chromatography (GPC). A 27% reduction in the molecular weight of PP was observed as a function of increasing ultrasonic frequency. This demonstrates the significant effect of ultrasonic radiation on the polymer structure. Composites prepared under ultrasonic frequency sweeping treatment exhibited superior MWCNTs dispersion compared to both non-sonicated and fixed-frequency sonicated materials. These composites also demonstrated enhancements in elastic modulus (increased 31%), thermal stability (onset increased 15°C and offset increased 4°C) and electrical conductivity five orders of magnitude and thermal conductivity (10%) compared to the composite without ultrasound. These composites also showed improvements in elastic modulus (increased 31%), thermal stability (onset increased 15°C and offset increased 4°C) and electrical conductivity increased five orders of magnitude and thermal conductivity (increased 10%) compared to the composite without ultrasonic energy.