Browsing by Author "Saed, Mohand O."
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Item Liquid-Crystal Order during Synthesis Affects Main-Chain Liquid-Crystal Elastomer Behavior(Royal Soc Chemistry, 2017-09-20) Traugutt, N. A.; Volpe, R. H.; Bollinger, M. S.; Saed, Mohand O.; Torbati, A. H.; Yu, K.; Dadivanyan, Natalia; Yakacki, C. M.; 0000-0001-5154-6378 (Saed, MO); Saed, Mohand O.This study presents the first direct comparison of the influence of liquid-crystal order during synthesis on the thermo-mechanical behaviors of main-chain liquid-crystal elastomers (LCEs) in thiol-acrylate networks. Six polydomain nematic elastomer (PNE) chemistries were compared directly by synthesizing with the mesogens in either an isotropic state (i-PNE) or a nematic state (n-PNE). The i-PNE networks were created in the presence of solvent, which disrupted any liquid-crystal order during network formation. Conversely, the n-PNE networks were created without the presence of solvent below the isotropic transition (T_{NI}). Differential scanning calorimetry (DSC) was first performed, and it showed that i-PNE networks experienced a clearly defined nematic-to-isotropic transition upon heating, whereas the transition in n-PNE networks was unable to be identified, which may be the result of a nematic-to-paranematic phase transition. Dynamic mechanical analysis (DMA) tests revealed that while both networks maintained elevated loss tangent in the nematic region, only i-PNE networks prominently displayed dynamic soft elasticity behavior. The two-way shape switching behaviors of LCE networks were examined using actuation tests under a 100 kPa bias stress. It showed that the strain amplitude strongly depends on synthesis history; it ranges from 66% to 126% in i-PNE samples and 3% to 61% in n-PNE samples. To help interpret the different actuation strain behaviors between i-PNEs and n-PNEs, wide-angle X-ray scattering (WAXS) was then performed where the LCE samples were strained to 40%. The results showed that order parameter (S) in n-PNE samples (ranging from 0.37 to 0.50) is lower than that in i-PNE samples (0.54 for all cases), and the parameter decreased as the cross-linking density increased. The stress-strain behaviors of the LCE networks measured from uniaxial tension tests revealed that all i-PNE samples had a lower soft-elasticity plateau during loading compared to the n-PNE samples. Finally, free-standing strain recovery of LCE samples after being strained to 100% was investigated. Immediately after removing stress on the samples, i-PNE and n-PNE samples recovered 14% to 38% and 27% to 73% of strain, respectively. We discuss the advantages and disadvantages of the different synthetic histories on LCE design.Item Molecularly-Engineered, 4D-Printed Liquid Crystal Elastomer Actuators(WILEY-VCH Verlag GmbH, 2018-11-27) Saed, Mohand O.; Ambulo, Cedric P.; Kim, Hyun; De, Rohit; Raval, Vyom; Searles, Kyle; Siddiqui, Danyal A.; Cue, John Michael O.; Stefan, Mihaela C.; Shankar, M. Ravi; Ware, Taylor H.; 0000-0001-5154-6378 (Saed MO); 0000-0001-7996-7393 (Ware, TH); Saed, Mohand O.; Ambulo, Cedric P.; Kim, Hyun; De, Rohit; Raval, Vyom; Searles, Kyle; Siddiqui, Danyal A.; Cue, John Michael O.; Stefan, Mihaela C.; Ware, Taylor H.Three-dimensional structures that undergo reversible shape changes in response to mild stimuli enable a wide range of smart devices, such as soft robots or implantable medical devices. Herein, a dual thiol-ene reaction scheme is used to synthesize a class of liquid crystal (LC) elastomers that can be 3D printed into complex shapes and subsequently undergo controlled shape change. Through controlling the phase transition temperature of polymerizable LC inks, morphing 3D structures with tunable actuation temperature (28 ± 2 to 105 ± 1 °C) are fabricated. Finally, multiple LC inks are 3D printed into single structures to allow for the production of untethered, thermo-responsive structures that sequentially and reversibly undergo multiple shape changes.Item Responsive, 3d Electronics Enabled by Liquid Crystal Elastomer Substrates(American Chemical Society, 2019-05-09) Kim, Hyun; Gibson, J.; Maeng, Jimin; Saed, Mohand O.; Pimentel, K.; Rihani, Rashed T.; Pancrazio, Joseph J.; Georgakopoulos, S. V.; Ware, Taylor H.; 0000-0001-7996-7393 (Ware, TH); Kim, Hyun; Maeng, Jimin; Saed, Mohand O.; Rihani, Rashed T.; Pancrazio, Joseph J.; Ware, Taylor H.Traditional electronic devices are rigid, planar, and mechanically static. The combination of traditional electronic materials and responsive polymer substrates is of significant interest to provide opportunities to replace conventional electronic devices with stretchable, 3D, and responsive electronics. Liquid crystal elastomers (LCEs) are well suited to function as such dynamic substrates because of their large strain, reversible stimulus response that can be controlled through directed self-assembly of molecular order. Here, we discuss using LCEs as substrates for electronic devices that are flat during processing but then morph into controlled 3D structures. We design and demonstrate processes for a variety of electronic devices on LCEs including deformation-tolerant conducting traces and capacitors and cold temperature-responsive antennas. For example, patterning twisted nematic orientation within the substrate can be used to create helical electronic devices that stretch up to 100% with less than 2% change in resistance or capacitance. Moreover, we discuss self-morphing LCE antennas which can dynamically change the operating frequency from 2.7 GHz (room temperature) to 3.3 GHz (-65 °C). We envision applications for these 3D, responsive devices in wearable or implantable electronics and in cold-chain monitoring radio frequency identification sensors. ©2019 American Chemical Society.