Origin of the anomalous decrease in the apparent density of polymer gels observed by multi-echo reflection ultrasound spectroscopy.

Multi-echo reflection ultrasound spectroscopy (MERUS), which enables one to simultaneously evaluate the attenuation coefficient α, the sound velocity v and the density ρ, has been developed for measurements of elastic moduli. In the present study, the technique was further developed to analyze systems undergoing gelation where an unphysical decrease in the apparent density was previously observed after polymerization. The main reason for this problem was that the shrinkage accompanying the gelation led to a small gap between the cell wall and the sample, resulting in the superposition of the reflected signals which were not separable into individual components. By taking into account the multiply reflecting echoes at the interface of the gap, the corrected densities were systematically obtained and compared with the results for the floating test. The present technique opens a new route to simultaneously evaluate the three parameters α, v and ρ and also the sample thickness for solid thin films.