Qi Ge

Ultralight, ultrastiff mechanical metamaterials

Microlattices make marvelous materials Framework or lattice structures can be remarkably strong despite their very low density. Using a very precise technique known as projection microstereolithography, Zheng et al. fabricated octet microlattices from polymers, metals, and ceramics. The design of the lattices meant that the individual struts making up the materials did not bend under pressure. The materials were therefore exceptionally stiff, strong, and lightweight. Science, this issue p. 1373 Ultralow-density materials that deform through tension or compression rather than bending show much higher stiffness. The mechanical properties of ordinary materials degrade substantially with reduced density because their structural elements bend under applied load. We report a class of microarchitected materials that maintain a nearly constant stiffness per unit mass density, even at ultralow density. This performance derives from a network of nearly isotropic microscale unit cells with high structural connectivity and nanoscale features, whose structural members are designed to carry loads in tension or compression. Production of these microlattices, with polymers, metals, or ceramics as constituent materials, is made possible by projection microstereolithography (an additive micromanufacturing technique) combined with nanoscale coating and postprocessing. We found that these materials exhibit ultrastiff properties across more than three orders of magnitude in density, regardless of the constituent material.

Active materials by four-dimension printing

We advance a paradigm of printed active composite materials realized by directly printing glassy shape memory polymer fibers in an elastomeric matrix. We imbue the active composites with intelligence via a programmed lamina and laminate architecture and a subsequent thermomechanical training process. The initial configuration is created by three-dimension (3D) printing, and then the programmed action of the shape memory fibers creates time dependence of the configuration—the four-dimension (4D) aspect. We design and print laminates in thin plate form that can be thermomechanically programmed to assume complex three-dimensional configurations including bent, coiled, and twisted strips, folded shapes, and complex contoured shapes with nonuniform, spatially varying curvature. The original flat plate shape can be recovered by heating the material again. We also show how the printed active composites can be directly integrated with other printed functionalities to create devices; here we demonstrate this by cr...

Active origami by 4D printing

Recent advances in three dimensional (3D) printing technology that allow multiple materials to be printed within each layer enable the creation of materials and components with precisely controlled heterogeneous microstructures. In addition, active materials, such as shape memory polymers, can be printed to create an active microstructure within a solid. These active materials can subsequently be activated in a controlled manner to change the shape or configuration of the solid in response to an environmental stimulus. This has been termed 4D printing, with the 4th dimension being the time-dependent shape change after the printing. In this paper, we advance the 4D printing concept to the design and fabrication of active origami, where a flat sheet automatically folds into a complicated 3D component. Here we print active composites with shape memory polymer fibers precisely printed in an elastomeric matrix and use them as intelligent active hinges to enable origami folding patterns. We develop a theoretical model to provide guidance in selecting design parameters such as fiber dimensions, hinge length, and programming strains and temperature. Using the model, we design and fabricate several active origami components that assemble from flat polymer sheets, including a box, a pyramid, and two origami airplanes. In addition, we directly print a 3D box with active composite hinges and program it to assume a temporary flat shape that subsequently recovers to the 3D box shape on demand.

Reduced time as a unified parameter determining fixity and free recovery of shape memory polymers

Shape memory polymers are at the forefront of recent materials research. Although the basic concept has been known for decades, recent advances in the research of shape memory polymers demand a unified approach to predict the shape memory performance under different thermo-temporal conditions. Here we report such an approach to predict the shape fixity and free recovery of thermo-rheologically simple shape memory polymers. The results show that the influence of programming conditions to free recovery can be unified by a reduced programming time that uniquely determines shape fixity, which consequently uniquely determines the shape recovery with a reduced recovery time. Furthermore, using the time-temperature superposition principle, shape recoveries under different thermo-temporal conditions can be extracted from the shape recovery under the reduced recovery time. Finally, a shape memory performance map is constructed based on a few simple standard polymer rheology tests to characterize the shape memory performance of the polymer.

Programmable, pattern-memorizing polymer surface.

However, all current SMP applications focus on harvesting the macroscopic scale deformation, i.e. employing the SMP as structural materials. An intriguing capability of all SMPs, which remains largely unexplored, is their ability to memorize and recover nanoscale patterns or structures. Here we demonstrate that SMPs can memorize and faithfully recover their lithographically fabricated, permanent or even temporary surface patterns. More signifi cantly, tunable multi-pattern memory capability can be achieved in Nafi on fi lms. Considering the prevalence of nanostructured surfaces in emerging nanotechnologies, such pattern-memorizing surfaces could potentially transform these technologies. During a typical shape memory cycle, an SMPs permanent shape is fi rst “programmed” into a temporary shape under mechanical loading at a temperature higher than the transition temperature (either glass transition temperature, T g , or melting temperature, T m ) of the SMP. At the permanent shape, the polymer chains between crosslinking points can be considered at the equilibrium state, or the lowest energy state. The mechanical loading during the programming deforms the chains into a higher energy state (with lower entropic freedom), forming the temporary shape. Without the mechanical constraints, the SMP sample will return to their permanent shape to minimize the system energy. However, this temporary shape can be “fi xed” as the temperature decreases below the T g (or T m ) of the SMP before releasing the mechanical loading and remains stable indefi nitely. The SMP softens and recovers its permanent shape when exposed to an environmental stimuli such as heat, [ 4 , 5 ] light, [ 1 ] or even solvent vapors. [ 6 ] During the recovery, strain or stress can be harvested under free or constrained conditions, respectively. [ 7 , 8 ] However, beyond such structural applications, the potential applications of SMP surfaces have yet to be explored. King et al. reported that AFM-indented holes on a SMP surface can be recovered via heating which enables the AFM-based data storage. [ 9 , 10 ] Recently, Burke et al. showed that micron scale patterns embossed on liquid crystalline elastomer can be erased

Two-way reversible shape memory effects in a free-standing polymer composite

Shape memory polymers (SMPs) have attracted significant research efforts due to their ease in manufacturing and highly tailorable thermomechanical properties. SMPs can be temporarily programmed and fixed in a nonequilibrium shape and are capable of recovering the original undeformed shape upon exposure to a stimulus, the most common being temperature. Most SMPs exhibit a one-way shape memory (1W-SM) effect since one programming step can only yield one shape memory cycle; an additional shape memory cycle requires an extra programming step. Recently, a novel SMP that demonstrates both 1W-SM and two-way shape memory (2W-SM) effects was demonstrated by one of the authors (Mather). However, to achieve two-way actuation this SMP relies on a constant externally applied load. In this paper, an SMP composite where a pre-stretched 2W-SMP is embedded in an elastomeric matrix is developed. This composite demonstrates 2W-SM effects in response to changes in temperature without the requirement of a constant external load. A transversal actuation of ~ 10% of actuator length is achieved. Cyclic tests show that the transversal actuation stabilizes after an initial training cycle and shows no significant decreases after four cycles. A simple analytic model considering the programming stress and actuator dimensions is presented and shown to agree well with the transverse displacement of the actuator. The model also predicts that larger actuation can be achieved when larger pre-stretch of 2W-SMP is used. The scheme used for this polymer composite can promote the design of new shape memory composites at micro-xa0and nano-length scales to meet different application requirements.

Prediction of temperature-dependent free recovery behaviors of amorphous shape memory polymers

Shape memory polymers (SMPs) are active materials that can fix a temporary shape and recover the permanent shape in response to environmental stimuli such as temperature, light, moisture or magnetic field. In order to provide insight into the mechanism for shape memory behavior and to predict the behaviors of targeted design, several constitutive models were developed in the past. Most of these models are complicated and require time-consuming experiments to obtain model parameters. However, for many engineers, an estimation of key features of shape memory behaviors, such as time for free recovery, is sufficient. Such estimation should be based on a simplified model involving only a few key parameters that can be quickly identified experimentally. In this paper, a simple theoretical solution was developed to predict the temperature dependent free recovery behaviors of amorphous SMPs. This solution is based on a modified standard linear solid (SLS) model with a Kohlrausch–Williams–Watts (KWW) stretched exponential function and requires only eight parameters that can be determined by stress relaxation tests. The theoretical predictions of free recovery behaviors show a good agreement with experimental results. Parametric studies using this solution reveal that the free recovery time can be reduced by increasing the equilibrium modulus (E0) or KWW stretching parameter (β), or by decreasing the nonequilibrium modulus (E1) or the relaxation time (τ0) and is most sensitive to the KWW stretching parameter β.

Highly Stretchable and UV Curable Elastomers for Digital Light Processing Based 3D Printing

Stretchable UV-curable (SUV) elastomers can be stretched by up to 1100% and are suitable for digital-light-processing (DLP)-based 3D-printing technology. DLP printing of these SUV elastomers enables the direct creation of highly deformable complex 3D hollow structures such as balloons, soft actuators, grippers, and buckyball electronical switches.

Thermomechanical behavior of a two-way shape memory composite actuator

Shape memory polymers (SMPs) are a class of smart materials that can fix a temporary shape and recover to their permanent (original) shape in response to an environmental stimulus such as heat, electricity, or irradiation, among others. Most SMPs developed in the past can only demonstrate the so-called one-way shape memory effect; i.e., one programming step can only yield one shape memory cycle. Recently, one of the authors (Mather) developed a SMP that exhibits both one-way shape memory (1W-SM) and two-way shape memory (2W-SM) effects (with the assistance of an external load). This SMP was further used to develop a free-standing composite actuator with a nonlinear reversible actuation under thermal cycling. In this paper, a theoretical model for the PCO SMP based composite actuator was developed to investigate its thermomechanical behavior and the mechanisms for the observed phenomena during the actuation cycles, and to provide insight into how to improve the design.

Mechanisms of triple-shape polymeric composites due to dual thermal transitions

Shape memory polymers (SMPs) are a class of smart materials capable of fixing a temporary shape and recovering the permanent shape in response to environmental stimuli such as heat, electricity, irradiation, moisture, or magnetic field, among others. Recently, multi-shape SMPs, which are capable of fixing more than one temporary shape and recovering sequentially from one temporary shape to another and eventually to the permanent shape, have attracted increasing attention. In general, there are two approaches to achieve a multi-shape memory effect (m-SME): the first one requires the SMP to have a broad temperature range of thermomechanical transition, such as a broad glass transition. The second approach uses multiple transitions to achieve m-SME, most notably, using two distinct transition temperatures to obtain a triple-shape memory effect (t-SME). The recently reported approach for designing and fabricating triple-shape polymeric composites (TSPCs) provides a much larger degree of design flexibility by separately tuning the two functional components (matrix and fiber network) to achieve optimum control of properties. The triple-shape memory behavior demonstrated by a TSPC is studied in this paper. This composite is composed of an epoxy matrix, providing a rubber–glass transition to fix one temporary shape, and an interpenetrating crystallizable PCL fiber network providing the system the melt–crystal transition to fix a second temporary shape. A one-dimension (1D) model that combines viscoelasticity for amorphous shape memory polymers (the matrix) with a constitutive model for crystallizable shape memory polymers (the fiber network) is developed to describe t-SME. The model includes the WLF and Arrhenius equations to describe the glass transition of the matrix, and the kinetics of crystallization and melting of the fiber network. The assumption that the newly formed crystalline phase of the fiber network is initially in a stress-free state is used to model the mechanics of evolving crystallizable phases. Experiments including uniaxial tension, stress relaxation, and triple-shape memory testing were carried out for parameter identification. The model accurately captures t-SME exhibited in experiments. The stress and stored energy analysis during the shape memory cycle provides insight into the mechanisms of shape fixing for the two different temporary shapes, the nature of both recovery events, as well as a guidance on how to design transitions to achieve the desired behavior.