Measurement of molecular orientation using longitudinal ultrasound and its first application in in-situ characterization

In-situ characterization of the molecular orientation during injection molding process remains a great challenge. Failing to penetrate the metallic mold, many conventional methods cannot realize the online measurements during the injection molding process. In comparison, longitudinal ultrasound provides a promising method due to its deep penetration depth and the ability to reveal the information of polymeric melts. In this study, a method for measuring the degree of molecular orientation with longitudinal ultrasonic waves is proposed. Based on the ultrasonic-wave equation and linear superposition principle, the relationship between the ultrasound velocity and degree of orientation was established. Online and offline experiments were performed to validate the proposed method by measuring the degrees of orientation with dichromic Fourier transform infrared spectroscopy and two-dimensional wide-angle X-ray diffraction. The experimental results show a negative correlation between the ultrasound velocity and degree of orientation, which agrees well with our mathematical model. Furthermore, the mechanism of the orientation measurement through the ultrasound velocity and the deviations of the proposed method were analyzed. The combined uncertainties of orientation in the offline measurement were found within the range of +/- 0.30-2.54%. In general, the proposed method is safe, non-destructive and valid for the insitu characterization of molecular orientation.