Dynamic properties of polyurea-milled glass composites Part I: Experimental characterization

Polyurea (PU) is an elastomer, which exhibits unique thermo-mechanical properties. It is synthesized from a di-functional amine, e.g. Versalink P-1000 and a diisocyanate, e.g. Isonate143L. In this study, various volume fractions milled glass (MG) was added to create polyuria-milled glass composites (PU-MG). Milled glass is a micro fiber with cylindrical shape. The distribution of the milled glass in the polyurea matrix was observed under the scanning electron microscope. The dynamic properties of pure polyurea and the PU-MG composites were measured by dynamic mechanical analysis (DMA) in the low frequency range (1-20 Hz) and by ultrasonic wave measurement in the high frequency range (0.5-1.5 MHz). Both experiments show that increasing the milled glass volume fraction drastically increases both the storage and loss moduli of the composites. DMA results show that dynamic Young's modulus increases with increasing frequency. However, longitudinal and shear moduli from ultrasonic wave measurement appears to be insensitive to frequency within the range of 0.5-1.5 MHz. The experimental dynamic moduli master curves of PU and PU-MG composites were constructed and compared. The relaxation function or creep compliance are generally useful than dynamic moduli for modeling of material response under complex histories. It is of practical use to convert dynamic mechanical data from the frequency domain into the time domain. The discrete relaxation spectra of the composites were calculated by fitting Prony series to the master curves, using least square nonlinear regression. Retardation spectra were then calculated using the interrelation between relaxation modulus and creep compliance in Laplace domain. Finally, the time domain relaxation modulus and creep compliance for each composite were obtained from the two spectra. In order to extend our understanding of the dynamic behavior of the PU-MG composite, micromechanical models have been created and are discussed in the accompanying paper Nantasetphong (2016a).