Utilizing self-assembly and additive manufacturing to control mechanics and stimuli-response in structured polymer networks
Date
2020
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
The function and characteristics of polymer materials are highly dependent on the network structure, where changes in polymer chemistry or architecture drive shifts in mechanics, thermal behavior, and stimuli-responsive properties of the final material. Supramolecular chemistry, or the design and organization of building blocks through non-covalent interactions, has gained traction as a convenient avenue for accessing mechanically resilient and stimuli-responsive polymeric materials by relying on the association and dissociation of non-covalent chemical motifs. In this work, we examine how changes in the polymer network architecture can be used as a platform for tuning the behavior of supramolecular polymer networks. ☐ First, we examine how the overlaying of a covalently crosslinked polymer network on to a metal-coordinating supramolecular polymer, forming a bio-inspired supramolecular semi-interpenetrating network (SIPN), can be used as a platform for tuning the self-assembly behavior of the network. These SIPN materials exhibit improved mechanics with a lower supramolecular content (30 wt%), allowing for energy dissipation through cavitation to increase material toughness. The shift in mechanical behavior is further attributed to the morphology, where the size of the phase-separated droplets and nature of the continuous phase in these SIPNs contributes to the material mechanics. Furthermore, chemical gradients are applied to these systems through exposure to a competitive ligand, offering control over the localization of supramolecular interactions. These materials offer a framework to mediate mechanics while maintaining the ability to program gradient supramolecular interactions. ☐ Control over self-assembly within a supramolecular polymer elastomer is also afforded by the addition of silica nanoparticles (NPs) to form nanocomposite films. Not only do the NPs increase the stiffness of the supramolecular polymer nanocomposite, their surface chemistry can also be changed as an avenue for tuning morphology of the polymer matrix. When complementary supramolecular motifs are present on the NP surface, long fibers are formed after film casting; however, neat silica NPs bearing silanol groups only facilitate nanofiber growth after a thermal annealing step. These morphological shifts offer functional handles to control nanocomposite mechanics and stimuli-responsive behavior. ☐ Finally, digital light processing (DLP) 3D printing is utilized to print thermoresponsive actuating bilayer hydrogels. A simple resin formulation is devised by dissolving 50 wt% N-isopropylacrylamide (NIPAM) into 2-hydroxylethyl acrylate (HEA) and a small amount of photoinitiator to yield a printable ‘active’ layer, where the lower critical solution temperature (LCST) behavior of the NIPAM is used to control the swelling of the printed hydrogel in water. An unresponsive passive layer is printed from a neat HEA formulation, and when the active and passive layers are printed to form bilayer structures, they show actuation behavior at elevated temperatures driven by the collapse of NIPAM out of the aqueous media.
Description
Keywords
3D printing, Interpenetrating network, Nanocomposites, Self-assmembly, Supramolecular polymer