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Material Assembly From Collective Action of Shape Changing Polymers
Abstract
In this work, we develop synthetic aggregates through the collective action of programmable shape changing polymers. We explore the potential use of shape-morphing semicrystalline polymers to behave as building blocks for these aggregates. Initially, we developed a new method to program both the shape and surface topography of semi-crystalline polymeric networks at high resolutions using monomers that simultaneously undergo crystallization during polymerization. We find that the polymerization of high aspect ratio lines within ribbons leads to the alignment of crystals. An expansion along the direction perpendicular to the aligned crystals is observed upon heating. However, these actuators are weak and incapable of actuating in the presence of other ribbons due to melt-driven actuation.
Therefore, we focus on exploiting liquid crystal elastomers' (LCEs) reversible shape morphing capabilities. LCEs rely on the isotropic to nematic phase transition and can actuate around other ribbons. Therefore, mechanical entanglements between ribbons form, enabling a dispersion of LCE ribbons to reversibly switch into a macroscopic aggregate. Compression and rheological tests were performed to study the thermomechanical properties of these materials Moreover, coating ribbons with metals introduces multi-stimulus response and conductance to aggregates. These structures demonstrate the ability to heal damage and behave as electrical switches. Furthermore, we apply the exact mechanism to semicrystalline LCEs and liquid crystalline gels (LCGs) to develop aggregates that are stable at room temperature.
We further demonstrate that this mechanism can be expanded to biocompatible materials like hydrogels. The shape change of hydrogel ribbons is programmed using bilayers. Briefly, the active layer consists of a thermo-responsive hydrogel (~ 125 µm), and the inactive layer consists of a passive polymer film (~ 25 µm). Due to the preferential bending upon swelling and deswelling, ribbons will remain flat at room temperature and bend upon heating to physiologically relevant temperatures (37 °C). When multiple ribbons are allowed to bend in proximity, mechanical interlocking between ribbons is observed, giving rise to biocompatible aggregates. The degree of aggregation is evaluated as a function of temperature, ribbon length, and initial packing density through rheological and compression testing. The potential use of these materials as self-assembling scaffolds is explored.
Citation
Abdelrahman, Mustafa Kamal (2023). Material Assembly From Collective Action of Shape Changing Polymers. Doctoral dissertation, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /200000.