Session Y17: Responsive Polymers, Soft Materials, and Hybrids

Autonomous Heat Switch With Limit Cycle Behavior Based on a Shape Memory Elastomer

We explore the use of actuation in shape memory elastomers as a route to establish limit cycle temperature responses, and thereby, the realization of a heat switch. Temperature-dependent force generation on a small magnetic widget due to the thermomechanical response of liquid crystal elastomer is coupled with distance-dependent magnetic attraction of the widget for a fixed warm surface. The resulting motion is time-dependent, and exhibits limit cycle behavior, under appropriate conditions, with heat periodically transferred to a nearby cold surface. We examine the dynamics of the system and parameterize the stability of the oscillation in terms of substrate heat flow, thermal transport coefficients, and relevant thermophysical properties of the elastomer. A lumped heat transfer model is developed to investigate the optimal parametric conditions for maximum heat transfer efficiency by exploring the associated dimensionless variable space. We anticipate that self-sustained cyclical thermal transport of this variety will have application in waste heat harvesting. In particular, the thermal oscillator realized here is of interest for waste heat recovery using pyroelectric devices.   

Encryption and Decryption via Dual-Phase Nematic Elastomer Films

Liquid crystalline elastomers (LCE's) can exhibit large deformations in shape due to changes in temperature or ultraviolet light; this process occur when the material transitions from an ordered liquid crystalline state to an isotropic state. Patterning an elastomer with both isotropic and ordered liquid crystalline domains can further expand the palette of possible conformations by confining the expansion and contraction of the material to specific regions. In this study, we use a finite element simulation method to model such a dual-phase material that responds only in selected regions, forming a pattern of blisters buckling out of the plane when exposed to light. If this pattern of response is agreed upon beforehand by two users, the active regions of the LCE sheet can be used to decrypt messages hidden in a photomask that determines which portions of the elastomer will be exposed to light. The active bits comprising the message will be obscured amid bits matched to the inactive regions of the elastomer, effectively concealing the message in plain sight. Our numerical model demonstrates how this encryption protocol could be implemented, and how to optimize parameters such as the active liquid crystalline patch size and spacing.   

Programmable Shape Morphing of Liquid Crystal Elastomer

      Programming shape changes in soft materials require precisely controlling the directionality and magnitude of the mechanical response, which remains a challenge. As an ordered soft material, liquid crystal elastomer (LCE) provides a promising strategy to induce programmable shape-shifting by manipulating the molecular order. We propose a new technique to arrange molecular orientations in LCE without using any additive particles. Various complex reversible 3D shape transformations, including auxetic structures and undulating morphologies, are remotely programmed in a monolith via light irradiation. Besides, diverse functions such as bio-inspired shape changes of butterfly wings and flowers, dual-mode rotations, and dual-mode locomotion are also demonstrated. These programmable materials open opportunities in designing multifunctional devices in robotics, biomedicine, and flexible electronics.       

Constructive adaptation of synthetic polymers in response to typically destructive environmental stressors

Although engineering and biological systems are surrounded by similar destructive environmental stressors, such as loads, sunlight, water, and greenhouse gases, their responses are typically different. The biological systems exhibit extraordinary adaptive capacity by turning destructive environmental stressors into constructive factors. For example, bone and muscle turn the typically destructive mechanical loads into constructive factors to build mass and mechanical strength. Plants harness sunlight and carbon dioxide, which otherwise degrades substance, to constructively synthesize polysaccharides and grow stiffness and strength. The engineering systems, on the contrary, typically do not possess the intelligence of constructive adaptation but weaken in response to environmental stressors. Take synthetic polymers as an example, they typically fatigue, age, and degrade with weakening mechanical properties during exposure to environmental stressors, such as loads, sunlight, water, and greenhouse gases. Here, we will present our recent efforts in designing synthetic polymer networks capable of turning destructive environmental stressors into constructive factors for strengthening their bulk and interfacial mechanical properties. Our efforts may open the door for the design of synthetic engineering materials to imitate the constructive adaptation of the biological systems in response to environmental stressors, for applications such as artificial muscles, soft robotics, and flexible electronics.   

Programmable shape morphing of composite hydrogels with tunable swelling and deswelling behaviors

Temperature-responsive hydrogels incorporated with photothermal materials can function in an aqueous medium is of great interest due to non-contact control by external heat and light stimuli. To date, however, the practical applications of this intelligent system have been largely impeded by the assembly of multiple material systems. Here, we take advantage of tunable swelling and deswelling behaviors of the composite hydrogels by the addition of the graphene oxide (GO) that induces changes in their internal structures. Next, we develop the single-sheet hydrogel actuators with GO-hydrogel patterns within the hydrogel matrix, to reach programmable shape deformation through temperature and light actuation. We anticipate this approach to program shape morphing without changing material composition will open a new path for promising applications of composite hydrogels in fields of targeted drug delivery and autonomous soft robotic.   

Spatiotemporally Controlled Shape Reconfiguration in Diels-Alder Polymer Networks through Selective Dissociation of Crosslinks

Dynamic covalent polymer networks crosslinked via the Diels-Alder (DA) reaction between furan and maleimide can undergo unique solid-state plastic deformation around 60 ~ 80 ^oC, on demand. Such on-demand network plasticity is enabled by the selective dissociation of the two isomers, endo and exo, of the DA adducts. The thermally less stable endo adducts can dissociate and be converted into exo at elevated temperatures between (60 ~80 ^oC). During this process, the Diels-Alder polymer (DAP) networks can undergo rapid stress relaxation and adapt to a new permanent shape. When the endo content is exhausted, the DAP networks can still participate in shape reconfiguration at a much slower rate through dynamic bond reshuffling of the thermally more stable exo adducts. Due to the higher thermal stability of exo adducts (dissociation is above 120 ^oC), the elastic modulus of the DAP network also increases with  the endo to exo conversion. The content of endo isomers in DAP networks can be easily controlled by varying the crosslinking density of the network. When 3D printed into relevant constructs for shape memory behavior, for example in bilayered strips or graded structures, the drastic difference in the thermal response between endo and exo adducts allows for spatiotemporal shape reconfiguration and actuation.   

Multiphase Direct Ink Writing (MDIW) of Layered Structures for Stimuli-Responsive Behaviors

Intelligent devices are extensively studied due to their versatile functionality and plethora of applications like targeted drug delivery for cancer cell treatment or deep ocean reach in soft robotics. Stimuli-responsive polymers, including smart materials or shape memory polymers (SMP), have attracted much attention in additive manufacturing due to their ability to produce complex structures in all shapes. This work demonstrated the one-step fabrication of composite multiphase smart materials using the newly developed multiphase direct ink writing (MDIW) 3D printing process. The SMPs were formulated using ether and ester-based thermoplastic polyurethane (TPU) mixed with polycaprolactone (PCL) and iron oxide nanoparticles (Fe₂O₃), respectively, providing the temperature and magnetic responsive properties. The MDIW mechanism produces alternative layers of the materials giving an isotropic responsive behavior of the as-fabricated structures to the external stimuli. The fabricated shape-memory structures also had a wide range of thermal actuation temperature (40 - 65°C), high strain, and shape recovery rate (up to 500%).   

Reversible and sequential self-folding of nanocomposite hydrogel/elastomer layers

Sequential self-folding ability is of great interest due to its potential to achieve task-specific shape transformation with complex final configuration. While different types of shape memory polymers have been integrated to achieve sequential folding, using a single material system can greatly simplify the fabrication process and improve structural integrity. Here, we develop reversible and sequential self-folding hydrogel/elastomer layers by controlling the microphase segregation of a temperature-responsive nanocomposite hydrogel. We show that the phase segregation of the nanocomposite hydrogels can be induced during crosslinking, leading to different swelling and deswelling kinetics. We anticipate this strategy for creating sequential self-folding without changing the material composition will suggest a rich area for fundamental studies and open a new avenue for developing autonomous soft actuators and robots with more complex geometries.   

Hydrogel swelling in confined geometries

Hydrogels are hydrophilic polymer networks capable of absorbing up to 10³ times their dry weight in water. Thus, they provide a nonliving material that can be used to study the interplay between volumetric growth and geometric constraints. Recent experiments in granular media reveal that hydrogels that must pass through narrow pores in order to expand show surprising fracturing behavior. In order to better understand what gives rise to these fractures, we simulate the swelling of a hydrogel through a pore using a thermodynamically consistent model that takes into account the transport of water and large deformations of the hydrogel. We explore the spatiotemporal distribution of stresses as a function of material properties and pore geometry, and comment on conditions that may lead to fracture.   

Magnetic and Viscoelastic Properties of Iron Oxide–Poly(ethylene oxide) Nanocomposites

Current research aims to characterize the magnetic and viscoelastic properties of iron oxide (Fe₃O₄) nanoparticle (NP) filled poly(ethylene oxide) (PEO). Iron oxide NPs with varying surface chemistries are used to prepare nanocomposites with concentrations of less than 1% iron oxide by weight. Magnetic heating, magnetic property characterization and nanoindenation experiments were carried out on hot pressed cylindrical samples. Samples were subjected to stress relaxation experiments where loading rates were varied while hold times and unloading rates were held constant. Previously, Huang-Lu showed that loading curves can be used to extract viscoelastic properties of materials. Using this model, loading curves were fit and the influence of loading rate on loading response as a function of NP surface chemistry and NP loading was determined. Additionally, the dependence of heat generation mechanisms and interfacial heat transfer on the matrix molecular weight and nanoparticle concentration was explored. Understanding the rotational dynamics of magnetic NPs is important in the application areas of these nanocomposites. The relative importance of Brownian and Neel relaxation processes can be determined by altering the polymer matrix viscosity.   

Shapes of Deformable Charge-patterned Nanoparticles

Deformable nanoparticles have received attention as candidates for designing stimuli-responsive nanocontainers for therapeutic applications. These systems are also investigated as dynamic building blocks for designing reconfigurable materials. Using molecular dynamics based simulated annealing, we investigate the shape of deformable nanoparticles for a variety of surface charge patterns including octahedral charged patches, single-charged spherical caps, and oppositely-charged spherical caps. Our simulations reveal that deformable nanoparticles of diameters within 20 - 100 nm adapt their shape differently in response to changes in pattern types and salt concentration over 0.01 - 10 mM. Shape transitions to faceted and rounded octahedra, spherocylinders, squared and rounded disks, capsules, variously-deformed bowls, and prolate spheroids are observed. Effects of counterion condensation on the stability of shape deformations are discussed. The observed shape transitions, controlled by pattern type and screening length, can change the directional specificity of interactions between nanoparticles, which may in turn hinder or promote reconfiguration in assembled materials.   

Mechanical behavior of magneto-active elastic shells under an arbitrary magnetic field

We develop a theoretical model of magneto active hemispherical shells and verify it by comparing with experimental results. Magneto-active elastomers which contain the magnetic filler within the elastomeric matrix have been studied due to their features that are soft, wireless and responsive. However, they have been studied only in the uniform magnetic field, although they are magnetized in different directions at each point. To generalize and understand the mechanical behavior induced by magnetic fields, we establish the theoretical model using Hooke's law and Biot-Savart law. Also, we conduct experiments where thin hemispherical shells are fabricated by injection molding with magneto-active elastomer, and evaluated by measuring the displacement of the pole under magnetic fields. Moreover, we vary the weight percent of the NdFeB powder and thickness of the shells to change the magnetic and mechanical properties of the shells. We find that our theoretical model better agrees with the experimental results than the FEM. Beyond the simple hemispherical shells, our theoretical model could be directly used in various applications: capsule drug delivery pumps, soft grippers, and soft robots.   

Characterization of a self-standing and stress-free two way shape memory effect in semicrystalline networks based on poly(ethylene glycol)/poly(ε–caprolactone)

The two-way shape memory effect (SME) is a peculiar feature of some polymeric materials, among which semicrystalline networks, that allows to obtain reversible shape changes upon cycling the material across its crystallization and melting regions under the application of an external load. Recently, the possibility of achieving this effect also under stress-free conditions was proved by subjecting semicrystalline networks to a suitable thermomechanical protocol. This early treatment leads to the formation of an internal stress at the macromolecular level, usually obtained by only partially melting the crystalline structure. Here, we synthesized networks based on poly(ethylene glycol)/poly(ε-caprolactone) by photocrosslinking, starting from methacrylated precursors of different molecular weights and in different ratios. The stress-free two-way SME was obtained and the role of some thermomechanical parameters, such as the applied pre-strain and the actuation temperature of the reversible cycle, was investigated. The composition of the material was fundamental for the achievement of a significant effect and it is related to the presence of a low-melting phase which elongates during crystallization and of a crystalline skeleton which provides the driving force.