Stimuli-responsive materials can enhance the field of three-dimensional (3D) printing by generating objects that change shape in response to external cues. While temperature and pH are common inputs for initiating a response in a 3D-printed object, there are few examples of using a mechanical input to afford a response. Herein, we report a suite of mechanochromic ionic liquid gel inks that can be used to fabricate 3D-printed objects that use a single mechanoactivation event to elicit both a mechanochromic response and an autonomous shape change. Direct-ink write 3D printing was used to deposit ionic liquid gel inks to create multimaterial objects that underwent a predetermined mechanoactivated shape change (mechanomorphic) when the sample was pulled and then released. When spiropyran was incorporated into the inks, the onset of spiropyran isomerization into its purple merocyanine form occurred at strains dependent upon the particular ion gel ink formulation. We suggest that the color onset could be used as a simple indicator for when the strain required to achieve a predetermined change in shape has been reached, potentially serving as real-time visual cue for the user. Such morphing 3D-printed structures with integrated instructions have potential application to areas including stowable structures as well as multiresponsive autonomous components and sensors.
Mechanochemistry
Mechanochemical Release of N‐heterocyclic Carbenes from Flex‐Activated Mechanophores.
We have discovered a new flex-activated mechanophore that releases an N-heterocyclic carbene (NHC) under mechanical load. The mechanophore design is based upon NHC-carbodiimide (NHC-CDI) adducts and demonstrates an important first step toward flex-activated designs capable of further downstream reactivities. Since the flex-activation is non-destructive to the main polymer chains, the material can be subjected to multiple compression cycles to achieve iterative increases in the activation percentage of mechanophores. Two different NHC structures were demonstrated, signifying the potential modularity of the mechanophore design.
The role of polymer mechanochemistry in responsive materials and additive manufacturing.
The use of mechanical forces to chemically transform polymers dates back decades. In recent years, the use of mechanochemistry to direct constructive transformations in polymers has resulted in a range of engineered molecular responses that span optical, mechanical, electronic and thermal properties. The chemistry that has been developed is now well positioned for use in materials science, polymer physics, mechanics and additive manufacturing. Here, we review the historical backdrop of polymer mechanochemistry, give an overview of the existing toolbox of mechanophores and associated theoretical methods, and speculate as to emerging opportunities in materials science for which current capabilities are seemingly well suited. Non-linear mechanical responses and internal, amplifying stimulus–response feedback loops, including those enabled by, or coupled to, microstructured metamaterial architectures, are seen as particularly promising.
Reduced Strain Mechanochemical Activation Onset in Microstructured Materials
In this study, we show that mechanochemical activation in responsive materials with designed, periodic microstructures can be achieved at lower applied strains than their bulk counterparts. Furthermore, by characterizing two responsive polymeric materials, we have developed a computational model capable of quantitatively predicting mechanochemical activation in geometrically complex structures.
The Intrinsic Mechanochemical Reactivity of Vinyl-Addition Polynorbornene
Herein we report the discovery of the intrinsic mechanochemical reactivity of vinyl-addition polynorbornene (VA-PNB), which has strained bicyclic ring repeat units along the polymer backbone. VA-PNBs with three different side chains were found to undergo ring-opening olefination upon sonication in dilute solutions. The sonicated polymers exhibited spectroscopic signatures consistent with conversion of the bicyclic norbornane repeat units to the ring-open isomer typical of polynorbornene made by ring-opening metathesis polymerization (ROMP-PNB). Thermal analysis and evaluation of chain scission kinetics suggest that sonication of VA-PNB results in chain segments containing a statistical mixture of vinyl-added and ROMP-type repeat units.
Additive Manufacturing with a Flex Activated Mechanophore for Nondestructive Assessment of Mechanochemical Reactivity in Complex Object Geometries
We used digital light processing additive manufacturing (DLP-AM) to produce mechanochemically responsive test specimens from custom photoresin formulations, wherein designer, flex activated mechanophores enable quantitative assessment of the total mechanophore activation in the specimen. The manufactured object geometries included an octet truss unit cell, a gyroid lattice, and an “8D cubic lattice”. The mechanophore activation in each test specimen was measured as a function of uniaxial compressive strain applied to the structure. Full shape recovery after compression was exhibited in all cases. These proof-of-concept results signify the potential to use flex activated mechanophore for nondestructive, quantitative volumetric assessment of mechanochemistry in test specimens with complex geometries. Additionally, the integration of DLP-AM with flex activated mechanophore build materials enabled the creation of customizable, three-dimensional mechanochemically responsive parts that exhibit small molecule release without undergoing irreversible deformation or fracture.
Investigations in Fundamental and Applied Polymer Mechanochemistry
The field of polymer mechanochemistry has experienced rapid growth over the past decade, propelled largely by the development of force-activated functional groups (mechanophores) and polymer structure-reactivity principles for mechanochemical transduction. In addition to fundamental guidelines for converting mechanical input into chemical output, there has also been increasing focus toward the application of polymer mechanochemistry for specific functions, materials, and devices. These endeavors are made possible by multidisciplinary approaches involving designer polymer synthesis, computational modeling and design, and different fields of engineering. Described herein are contributions from our group on the development of flex activated mechanophores for small molecule release and star polymer mechanochemistry, as well as collaborative efforts toward mechanochemically triggered depolymerizations and 3D printed mechanochromic materials.
Additive Manufacturing of Mechanochromic Polycaprolactone on Entry-Level Systems
This paper aims to explore and demonstrate the ability to integrate entry-level additive manufacturing (AM) techniques with responsive polymers capable of mechanical to chemical energy transduction. This integration signifies the merger of AM and smart materials.
Kinetic Analysis of Mechanochemical Chain Scission of Linear Poly(phthalaldehyde)
The kinetics of mechanochemical chain scission of poly(phthalaldehyde) (PPA) are investigated. Ultrasound-induced cavitation is capable of causing chain scission in the PPA backbone that ultimately leads to rapid depolymerization of each resulting polymer fragment when above the polymer’s ceiling temperature (Tc). An interesting feature of the mechanochemical breakdown of PPA is that “half-chain” daughter fragments are not observed, since the depolymerization is rapid following chain scission. These features facilitate the determination of rate constants of activation for multiple molecular weights from a single sonication experiment. Additionally, the degradation kinetics are modified with chain-end trapping agents through variation of the nature and amount of small molecule nucleophile or electrophile.
Modeling the Mechanochemical Degradation of Star Polymers
A model for predicting the molecular weight distributions of mechanochemically degraded star polymers has been developed. The model was shown to be in good agreement with experimental distributions and average total molecular weights obtained from ultrasonically degraded three-arm star poly(methyl acrylate)s. Generalization of the model to four- and n-arm star polymers was also achieved. The models are straightforward to use, and thus, all calculations were completed in Microsoft Excel.