Physical Evidence of Stress-Induced Conformational Changes in Polymers
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RESEARCH PAPER
Physical Evidence of Stress-Induced Conformational Changes in Polymers N. Huynh 1 & G. Youssef 1 Received: 6 May 2020 / Accepted: 7 October 2020 # Society for Experimental Mechanics 2020
Abstract Background Polymer mechanics and characterization is an active area of research where a keen effort is directed towards gaining a predictive and correlative relationship between the applied loads and the specific conformational motions of the macromolecule chains. Objective Therefore, the objective of this research is to introduce the preliminary results based on a novel technique to in situ probe the mechanical properties of polymers using non-invasive, non-destructive, and non-contact terahertz spectroscopy. Methods A dielectric elastomer actuator (DEA) structure is used as the loading mechanism to avoid obscuring the beam path of transmission terahertz time-domain spectroscopy. In DEAs, the applied voltage results in mechanical stresses under the active electrode area with far-reaching stretching in the passive area. Finite element analysis is used to model and simulate the DEA to quantify the induced stresses at the observation site over a voltage range spanning from 0 V to 3000 V. Additionally, a novel analysis technique is introduced based on the Hilbert-Huang transform to exploit the time-domain signals of the ultrathin elastomeric film and to defy the limits set forth by the current state-of-the-art analysis techniques. Results The computational result shows a nonlinear relationship between the effective stresses and the applied voltage. Analysis of the terahertz time-domain signals shows a shift in the delay times and a decrease in signal peak amplitudes, whereas these characteristics are implicitly related to the change in the index of refraction. Conclusions In all, the results evidentially signify the interrelationship between the conformational changes and applied mechanical stress. Keywords In-situ characterization . Dielectric elastic actuators . Terahertz spectroscopy . Polymer response
Introduction Polymers continue to be an important class of materials for a plethora of load and non-load bearing applications, where polymers constitute a large portion of raw materials used in engineered products such as cellular phones, automobiles, and household goods, to name a few examples. The diversity of the sub-classification of polymers mirrors the variety of applications they are used in, including thermosets, thermoplastic, elastomers, biodegradable, and biopolymers. While this classification is not meant to be comprehensive nor exclusive, it is instead a shortlist to elude to the differences in the underlying molecular structure. Typical to any of these subclasses is the * G. Youssef [email protected] 1
Experimental Mechanics Laboratory, Mechanical Engineering Department, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182-1323, USA
sensitivity to applied loads, regardless of the amplitude and directionality, which gives rise to molecular rearrangements that may be reversibl
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