Impact of an external acoustic field on diffusion of nanoparticles in polymer nanocomposites

Prior work has shown that we can use isothermal crystallization to organize nanoparticles into the amorphous regions of a semicrystalline polymer. A specific challenge of the processing is the slow crystallization rate required to achieve organization. By adding an acoustic field to the processing, the nanoparticle diffusivity increases, increasing the critical crystallization rate to achieve organization. The acoustic radiation force, dependent on particle volume, matrix compressibility, and acoustic contrast factor, acts in conjunction with Stokes drag to drive the elevated diffusion of NP in response to an external acoustic field. To test this effect, nanocomposites of polyethylene oxide (PEO) and silica NP (20% mass loading) were studied during isothermal crystallization under controlled acoustic conditions using a probe sonicator. Small-angle X-ray scattering (SAXS) was employed to characterize the resulting anisotropic structures and NP ordering. The crystalline lamellar long period increased as the acoustic field directed particle mobility, forming ordered domains not seen in the absence of external fields. Specifically, results show that PEO at a crystallization temperature of 54ºC, the presence of an acoustic field provokes organization within a brief period as compared while without an acoustic field, nanoparticle organization was not observed. Work continues to study the kinetics of this behavior as a function of matrix molecular weight, acoustic parameters and particle size.