Liquid crystal elastomers (LCEs) films enable thermally responsive shape change. Mesogenic segments in the elastomeric network can be aligned into crystalline domains via mechanical deformation of the film. If subjected to a second-stage UV cure in the deformed shape, the crystalline domains are retained upon release of the external load. This induces a temporary shape in the LCE. Subsequent heating of the LCE above the nematic-to-isotropic temperature disorders the liquid crystals, and strain energy stored in the elastomer causes the LCE to return to the undeformed. In a reversible manner, the LCE returns to the temporary shape when cooled. In this work, we use thermally responsive LCE films applied to passive thin films, such as mylar and Kapton. The passive film enhances the mechanical strength and stiffness of the film but prevents alignment of the LCE crystalline domains through stretching. Instead, these bilayer films are restricted to folding deformations, wherein the LCE layer is used to induce thermally responsive shape change. We study the effects of layer thickness ratios on the reversibly self-folding bilayer films. The LCE films are synthesized directly on the passive film layer using a two-stage thiol-acrylate Michael addition and photopolymerization (TAMAP) reaction in which the first stage is a thermal cure, and the second stage is a UV cure. We demonstrate the reversible shape change between flat and folded states, and quantify the shape change in terms of the shape fixity and recovered flatness ratios. Potential applications for this system are actuators for deployable structures and soft robotics.
Flat-fabrication technology may enable the next generation of gigantic deployable architectures devoted to the detection of faint cosmological signals. We assess the applicability of a multifunctional roll-out structure based on shape memory polymer technology for the realization of a large space observatory to measure the cosmological Dark Ages radio signal. Roll-out solutions offer advantageous properties for probe class missions, such as the capability to morph the shape to achieve sufficient structural performance while ensuring high packaging efficiency. We characterize the feasibility of a roll-out observatory in the context of a 5 years-long heliocentric mission scenario. Our preliminary study demonstrates how a four-250 m-long arms architecture with 150 evenly spaced short dipole antennas potentially meets the basic mission requirements dictated by the Dark Ages science case. We conduct a quasi-static structural analysis considering axial and bending loads acting on the arms to assess the structural properties of the proposed architecture, identifying geometric ranges which enable the structure to withstand expected loads while satisfying mass and size constraints. Printable electronics are considered in the design due to the ease of integration with the polymer substrate. In this regard, we explore two distinct electronics configuration options—centralized and decentralized—discussing their benefits in terms of power demand and data management. If successful, such a design may set the stage for future technological development aiming to realize tomographic measurements of the cosmological Dark Ages.
In this work, we seek to fabricate self-folding liquid crystal elastomer (LCE) composite films. Liquid crystal elastomers (LCEs) are a class of smart, multifunctional materials that can be imparted with enhanced, anisotropic mechanical properties through the alignment of crystalline domains. Crystalline order decreases with increasing temperature, and long-range order is lost above the nematic to isotropic transition temperature, TNI. This enables programmable, reversible actuation in response to temperature changes. The envisioned composite films comprise domains of active, monodomain LCEs to drive reversible self-folding, which are adhered to passive, thin films that serve as a framework to guide the self-folding response. LCEs will be synthesized using a two-stage thiol-acrylate Michael addition and photopolymerization (TAMAP) reaction. The first-stage consists of a room temperature cure to form polydomain films, and a second-stage photopolymerization of the mechanically deformed LCE film forms aligned liquid crystal monodomains. Composite films will be molded to a folded state prior to the second stage cure such that heating above TNI produces a reversible unfolding response. We characterize the self-folding behavior of these materials using a series of single-fold and multiple, intersecting fold geometries. We envision application of these composite films as actuators in soft robotics and morphing surfaces.
Space represents a harsh environment for all materials. This is particularly challenging for shape memory polymers (SMPs), which show significant potential for lightweight actuators for deployable space structures. Relevant conditions include UV radiation, temperature variations, and vacuum. Polymers, when exposed to such environment for prolonged period (aging), begin to break down structurally and thermodynamic properties, such as enthalpy, entropy, and specific volume, change over time. This leads to permanent modification of mechanical properties such as decreased strength and increased brittleness of the polymer. The shape recovery performance of shape memory polymer is dependent on its thermodynamic properties and energy associated with UV aging can be evaluated through a differential scanning calorimetry (DSC) test. Previous studies focus on the effects of UV exposure on chemical degradation of polymers. However, limited research has been conducted towards studying shape recovery performance of UV aged polymer through thermomechanical prestrain followed by shape recovery process. In this study, we expose SMPs in a UV environment followed by shape recovery experiments where they are prestrained and recovered at various thermomechanical conditions such as recovery temperature, strain rate and aging time. Furthermore, we use characterization techniques such as FTIR and SEM to evaluate the amount of physical degradation of SMP as a result of UV aging process. The results obtained from this study will provide insight into recovery capabilities of a SMP for space exploration.
The Polymer Mechanics Research Laboratory at Auburn University contributes fundamental knowledge to the field of time dependent, thermomechanical behavior of polymers, which contributes to core functions of autonomy in engineered matter, such as sensory mechanisms, actuation capabilities, and adaptive mechanical-material frames. In this talk, we will provide an overview of ongoing research relevant to the multifunctional materials and adaptive structures community, with an emphasis on the deformation and actuation capabilities of polymeric materials and structures. Topics include: (1) mechanics of self-folding polymer origami: we utilize pre-strained polymer sheets that change shape in response to external stimuli. Compactly stored sheets can be transformed into three-dimensional structures on demand. These materials are proposed for use as actuators in deployable space structures. (2) Reconfigurable mechanical metamaterals: tessellated unit cells dictate the macroscopic behavior of the structure. Structures can be reconfigured through thermal and mechanical processes to tailor properties to specific applications. Of particular interest are bistable structures and auxetic metamaterials. (3) Electrospun smart sensors: the use of conducting polymers in the electrospinning processes provides additional functionality to non-woven mats. These sensors are envisioned for use at the human-machine interface. The processing conditions used to fabricate the non-woven mats influence sensor performance and human factors such as breathability. These research activities are broadly conducted by graduate and undergraduate researchers working in a collaborative environment.
Fused deposition modeling permits rapid, low-cost fabrication of polymeric lattices. Frequently, these lattices are evaluated under isothermal conditions at ambient temperatures. However, the mechanical properties of polymeric lattices are sensitive to variations in temperature. We evaluate the mechanical behavior of polymeric lattices across a range of temperatures utilizing a coupled computational and experimental approach. 3D printed unit cells are subjected experimentally to sequential tension and compression and results are compared to a multiphysics finite element framework. Unit cells are then tessellated to form larger lattices that are analyzed computationally, wherein the effects of uniform and gradient temperatures are considered.
Traditional origami patterns can be applied to pre-strained polystyrene (PSPS) sheets to create precisely fabricated three-dimensional shapes through self-folding. The basis of these folded shapes are the tessellated repeat units, comprising only a few faces and folds. Subtle modifications of the geometry of the patterns allows generation of many final shapes based on the same fundamental repeat unit. These subtle changes allow for variations in attributes like packing density or curvature. Further, self-folded PSPS sheets represent a novel kind of engineering material in which the mechanical properties depend on the fold pattern and the extent of folding . After folding, the adjacent hinges and faces of these structures differ from each other in thickness, temperature history, and orientation relative to loading directions. We seek to characterize the mechanical properties of self-folded, periodic structures to gain a better understanding of how to design and utilize them in engineering applications. Miura-ori patterns will be applied to PSPS sheets, which self-fold in response to infrared light absorption. Folded samples with a range of face sizes and pattern angles will be subjected to compressive testing. Modification of geometric parameters, along with exposure time to the infrared light, has a significant effect on orientation of faces relative to the direction of loading, which will allow control over the final shape and mechanical properties.
This Conference Presentation, “Large deformation of 3D printed reconfigurable cylindrical shells with multiple stable states,” was recorded for the Smart Structures + Nondestructive Evaluation 2021 Digital Forum.
This Conference Presentation, “Computational modeling of hot rolling process for biaxial prestraining of shape memory polymer sheets,” was recorded for the Smart Structures + Nondestructive Evaluation 2021 Digital Forum.
Polymeric lattices offer lightweight structures in which subtle changes in temperature can be used to manipulate lattice shape and mechanical properties. We seek to elicit the interrelations between inhomogeneous deformations, spatially and temporally non-linear mechanical properties, and the resulting mechanical properties of polymeric lattices. Computational modeling allows the measurement of local parameters, e.g. stress gradients and viscous strains, that are difficult or impossible to evaluate experimentally. We implement a coupled thermo-mechanical finite element analysis framework to evaluate lattices subjected to the shape memory cycle. Insight gained from this work advances the understanding of lattice structures with adaptive mechanical properties.
In mechanical metamaterials, geometry and material properties dictate the structural response to mechanical loads. These materials are comprised of a tessellated array of repeat unit cells, which form a lattice that populates the domain of the structure. While the mechanical behavior of 2D lattices is well understood, recent advances in polymer based Additive Manufacturing (AM) usher in a new era of metamaterials research. However, previous work in this area has failed to address the effects of time and temperature on the transient response of polymerbased mechanical metamaterials. We seek to investigate the effects of thermal loads on the mechanical properties of mechanical metamaterials, in particular, the stiffness and damping properties of lattice structures comprised of bowtie and honeycomb representative unit cells. Towards this goal, in the present paper, an experimental approach is used to investigate the mechanical behaviour of 2D lattice structures. Experimental samples are prepared using Fused Deposition Modeling (FDM) AM. These samples are subject to quasi-static mechanical tests at room temperature. Additionally, we investigate the effects of mismatched unit cells (defects) on the mechanical behavior of the lattice. Mechanical metamaterials with adjustable stiffness and damping properties have applications in the aerospace and automotive industries, including sandwich composites, damping and impact protection.
Shape memory polymers (SMPs) are studied extensively for self-folding origami due to their low cost, large strain recovery and low activation energy. SMPs utilize viscoelastic strain recovery to induce shape change, wherein an external stimulus, e.g. light or electricity, heats the material above the glass transition temperature to accelerate the recovery. Application of electric current to a conductive SMP composite produces Joule-heating, which provides higher energy density and a shorter self-folding time compared to other stimuli. Previous research has focused on Joule-heat induced shape recovery using SMP samples containing uniformly dispersed conductive fillers. Application of an electric field to these samples causes them to heat and change shape uniformly, thus limiting the ability to fold locally. In contrast, the present study focuses on shape recovery of a prestrained SMP sheets using localized resistive Joule-heating via a nichrome wire. The localized heat input applied to the SMP enables self-folding in specific regions of the sample. A previously prestrained polymer sample, experiences a differential shrinking between its top and bottom surface when subjected to the local Joule-heat. The differential shrinking causes the polymer to have a strain gradient along the thickness, which results in self-folding of the sample. This paper studies the thermal and mechanical response of Joule-heat induced self-folding of polymer sheets subjected to varying applied current and electrical resistance. Furthermore, an in-house polymer prestraining sequence is also reported.
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