Thermally-induced two-way shape memory polymers: Mechanisms, structures, and applications

Thiol-ene/acrylate shape memory polymers (SMPs) have sufficient stiffness for facile insertion and precision placement and soften after exposure to physiological conditions to reduce the mechanical mismatch with body tissue. As a result, they have demonstrated excellent potential as substrates for various flexible bioelectronic devices, such as cochlear implants, nerve cuffs, cortical probes, plexus blankets, and spinal cord stimulators. To enhance the shape recovery properties and softening effect of SMPs under physiological conditions, we designed and implemented a new class of SMPs as bioelectronics substrates. In detail, we introduced dopamine acrylamide (DAc) as a hydrophilic monomer into a current thiol-ene polymer network. Dry and soaked dynamic mechanical analyses were performed to evaluate the thermomechanical properties, softening kinetics under wet conditions, and shape recovery properties. Modification of SMPs by DAc provided an improved softening effect and shape recovery speed under physiological conditions. Here, we report a new strategy for designing SMPs with enhanced shape recovery properties and lower moduli than previously reported SMPs under physiological conditions without sacrificing stiffness at room temperature by introducing a hydrophilic monomer.

In this dissertation, the application of smart polymers as actuators was investigated, with focuses on shape memory polymers and twisted-then-coiled artificial muscles. Thermomechanical models have been developed for various polymeric actuators, so as to facilitate interpretation of the underlying mechanisms and to provide guidance for future design. The classical one-way shape memory effect in amorphous shape memory polymers was first reproduced. The amorphous shape memory polymer was treated as a frozen-phase matrix with active-phase inclusions embedded in it. A phase evolution law was proposed from the physics perspective and the Mori-Tanaka approach was used to predict the effective mechanical properties. Then, a phenomenological constitutive model was developed based on the multiple natural configurations framework for the semi-crystalline two-way shape memory effect. The model elucidated how the programming procedure affect the crystallization behavior and eventually determine the two-way shape memory effect via storage of internal stress. Artificial muscles with hierarchical chiral structure that can offer a hundredfold increase in power over natural muscles of equivalent lengths have recently been demonstrated experimentally. To investigate the physical origin behind the remarkable tensile actuation behavior and, therefore, the correlation between the actuation performance and the intrinsic material parameters, a multi-scale modeling framework from macro-scale helical spring structure top-down to the molecular chain interaction has been developed Then, based on the prediction results of the multi-scale model, a new type of hierarchical chiral structured artificial muscle was fabricated using two-way shape memory polymer fiber. The usual improvement in the axial actuation of the twisted-then-coiled muscles were demonstrated both experimentally and theoretically.

Polymers can demonstrate shape memory (SM) effects by being temporarily fixed in a nonequilibrium shape and then recover their permanent shape when exposed to heat, light, or other external stimuli. Many previously developed shape memory polymers (SMPs) use the dramatic molecular chain mobility change around the glass transition temperature Tg to realize the SM effect. In these materials, the temporary shape cannot be repeated unless it is reprogramed, and therefore the SM effect is one way. Recently, a semicrystalline SMP, which can demonstrate both one- and two-way SM effects, was developed by one of our groups (Chung, T., Rorno-Uribe, A., and Mather, P. T., 2008, “Two-Way Reversible Shape Memory in a Semicrystalline Network,” Macromolecules, 41(1), pp. 184–192). The main mechanism of the observed SM effects is due to stretch induced crystallization. This paper develops a one-dimensional constitutive model to describe the SM effect due to stretch induced crystallization. The model accurately describes the complex thermomechanical SM effect and can be used for the future development of three-dimensional constitutive models.

This dissertation demonstrates the design of reversibly self-morphing novel liquid crystalline elastomer (LCE) antennas that can dynamically change electromagnetic performance in response to temperature. This change in performance can be achieved by programming the shape change of stimuli-responsive (i.e., temperature-responsive) LCEs, and using these materials as substrates for reconfigurable antennas. Existing reconfigurable antennas rely on external circuitry such as Micro-Electro-Mechanical-Systems (MEMS) switches, pin diodes, and shape memory alloys (SMAs) to reconfigure their performance. Antennas using MEMS or diodes exhibit low efficiency due to the losses from these components. Also, antennas based on SMAs can change their performance only once as SMAs response to the stimuli and is not reversible. Flexible electronics are capable of morphing from one shape to another using various techniques, such as liquid metals, hydrogels, and shape memory polymers. LCE antennas can reconfigure their electromagnetic performance, (e.g., frequency of operation, polarization, and radiation pattern) and enable passive (i.e., battery-less) temperature sensing and monitoring applications, such as passive radio frequency identification device (RFID) sensing tags. Limited previous work has been performed on shape-changing antenna structures based on LCEs. To date, self-morphing flexible electronics, including antennas, which rely on stimuli-responsive LCEs that reversibly change shape in response to temperature changes, have not been previously explored. Here, LCE antennas will be studied and developed. Also, the metallization of LCEs with different metal conductors and their fabrication process, by either electron beam (E-Beam) evaporation or optical gluing of the metal film will be observed. The LCE material can have a significant impact on sensing applications due to its reversible actuation that can enable a sensor to work repeatedly. This interdisciplinary research (material polymer science and electrical engineering) is expected to contribute to the development of morphing electronics, including sensors, passive antennas, arrays, and frequency selective surfaces (FSS).

Shape memory polymers (SMPs) are a new member of the smart materials family. SMPs have found wide applications or potential applications in almost all manmade structures and devices. In order to better design smart structures and devices using SMPs, thermomechanical constitutive modeling is essential. In this insight paper, we will focus on presenting several multi-length-scale and multi-physics modeling frameworks, including the thermodynamics consistent model, elasto-viscoplastic model, statistical mechanics model, and phase evaluation law model. The SMPs modeled will include amorphous one-way shape memory polymers, semicrystalline one-way shape memory polymers, semicrystalline two-way shape memory polymers, and functional and mechanical damage effects on SMPs. Finally, we will give some in-depth perspectives on future development in this area of study.

Shape memory polymers (SMPs) are an emerging class of active polymers that may be used for reconfigurable structures on air vehicles. In this study, epoxy-based SMPs were conditioned separately in simulated service environments relevant to Air Force missions, namely, (1) exposure to UV radiation, (2) immersion in jet-oil, and (3) immersion in water. The mechanical properties and shape recovery abilities of the unconditioned and conditioned SMPs were examined using large-scale compressive tests and small-scale nano-indentation tests. Results show that all the conditioned SMPs exhibit a decrease in Tg as compared to the unconditioned one. Under compressive loading, the SMPs undergo significant plastic deformation prior to fracture, except the UV conditioned sample. Environmentally conditionings generally result in higher moduli and yield strength of the SMPs. Environmental conditionings, in particular the UV exposure and water immersion, also affect the shape recovery abilities of the SMPs if the recovery temperatures are set low.

Applying thin electrically conductive coatings to Liquid Crystalline Elastomers (LCEs) is an effective way of functionalizing two-way shape memory polymers with the ability to respond to electrical currents. However, achieving robust adhesion between a given electrically conductive coating and the surface of LCEs can be challenging. This can limit the functionality, performance, and potential applications of these materials. This work describes a facile method to develop electrically responsive Liquid Crystalline Elastomer polymeric artificial muscles with strain-sensing, self-actuation-sensing, and joule-heating features. In this work, the effect of treating LCEs with polydopamine (PDA) prior to functionalizing the LCE with an electrically conductive gold-sputtered coating was explored. The findings confirmed that the PDA treatment considerably improved the adhesion of the gold sputter coating to the LCEs, thereby leading to the fabrication of multi-functional strain-sensing, electrically conductive, and electro-responsive LCEs.

Shape memory polymers (SMPs) usually have a one-way shape memory effect. In this paper, an easy-operating method to realize a two-way shape memory effect was demonstrated in a ring-shaped bilayer structure where the two layers are SMPs with different thermal transition temperatures. By designing specific thermomechanical processes, the mismatched deformation between the two layers leads to a morphology change of ring-shaped bilayer structures from a smooth ring to a gear-like buckling shape under cooling and a reversible recovery to the smooth shape under heating. Such a morphology change is ascribed to occurrence and recovery of thermoelastic buckling. This method was validated by finite element simulation. We experimentally investigated the influence of pre-strain on buckling, and it was found that both the buckling occurrence and recovery temperature vary with pre-strain. Furthermore, considering a ring-shaped SMP–SMP bilayer structure, finite element analysis was conducted to study the influence of film thickness and modulus ratio of two layers on buckling behavior. The results showed that the critical buckling wavelength was greatly influenced by film thickness and modulus ratio. We made a theoretical analysis that accorded well with the numerical results.

Novel two-way shape memory polymer composites (2WSMPC) consisting of interpenetrating polymer networks (IPNs) of poly(benzoxazine/urethane) reinforced with carbon fiber felt (CFF) were developed in this work. Sequential curing, i.e., moisture curing of urethane followed by thermal curing of benzoxazine (BA-a) was used to synthesize IPNs with molecular phase separation. The thermomechanical properties and two-way shape memory effect (2WSME) of the IPNs were systematically investigated by varying the urethane content. It was found that the glass transition temperature ( T g ) and 2WSME of the IPNs based on poly(benzoxazine/urethane) improved by optimizing the urethane content. Moreover, the electro-induced shape memory performance was found to be dependent on the CFF reinforcement. The results showed that the developed IPNs reinforced with CFF triggered by electric current exhibited high two-way shape memory performance, i.e., a fixity at room temperature (RT) of 97–98%, recovery to original shape at high temperature of 97–98%, and those to temporary shape at RT of 92–97%. The findings revealed that the electro-induced two-way shape memory composite from CFF-reinforced poly(benzoxazine/urethane) is a promising candidate for smart electric circuit breaker applications with highly stable reverse recovery between original and temporary shapes up to 20 cycles.

As a class of semi-crystallized polymers, shape memory polymers (SMPs) exhibit significant viscoelastic characteristics in the vicinity of the glass transition temperature Tg, which should affect their shape storage and recovery functionality. However, until now isothermal and elastic assumptions are commonly considered when studying the thermomechanical properties of this class of functionalized materials. This papers aims to present some experimental results about the viscoelastic characteristics of SMPs. Systematic thermomechanical experiments were performed on shape-memory polyurethane under uniaxial tensile loading, which includes the frozen/recovery tests under different constraint conditions, stress-strain cycles and stress relaxation at different temperatures. Based on the testing results, the viscoelastic characteristics effect on the shape frozen and recovery responses of SMPs are discussed, which is of importance in proposing suitable thermo-viscoelastic constitutions about this type of functional materials.

This paper presents a thermally actuated bilayer actuator fabricated from a synergistic combination of a liquid crystal elastomer (LCE) and a shape memory polymer (SMP). Capitalizing on their distinct thermomechanical properties, we innovatively fabricated the LCE-SMP bilayer using a spin-coating technique, which was validated as feasible through SEM analysis. The resulting actuator exhibits reversible and tunable deformation, coupled with significantly enhanced mechanical performance; its modulus is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2-3$</tex> times greater than that of single-layer counterparts. To demonstrate its practical utility, we explored diverse structural designs and proposed a specific application as a valve. This actuator design not only offers a cost-effective solution but also aligns with the engineering trend toward miniaturization and intelligent systems, thereby providing new impetus for innovation in related fields.

Additive manufacturing (AM), commonly referred to as 3D printing, has revolutionized the manufacturing landscape by enabling the intricate layer-by-layer construction of three-dimensional objects. In contrast to traditional methods relying on molds and tools, AM provides the flexibility to fabricate diverse components directly from digital models without the need for physical alterations to machinery. Four-dimensional printing is a revolutionary extension of 3D printing that introduces the dimension of time, enabling dynamic transformations in printed structures over predetermined periods. This comprehensive review focuses on polymeric materials in 3D printing, exploring their versatile processing capabilities, environmental adaptability, and applications across thermoplastics, thermosetting materials, elastomers, polymer composites, shape memory polymers (SMPs), including liquid crystal elastomer (LCE), and self-healing polymers for 4D printing. This review also examines recent advancements in microvascular and encapsulation self-healing mechanisms, explores the potential of supramolecular polymers, and highlights the latest progress in hybrid printing using polymer-metal and polymer-ceramic composites. Finally, this paper offers insights into potential challenges faced in the additive manufacturing of polymer composites and suggests avenues for future research in this dynamic and rapidly evolving field.

ABSTRACTA novel liquid crystal elastomer (LCE) synthesized by melt polymerization, which exhibits the capacity of shape memory, is reported here for the first time. The method of synthesize the shape memory LCE has been explored. A facile two‐step method to synthesize these anisotropic materials to realize reversible shape change behavior is reported. The first reaction is the addition of nematic liquid crystal molecules to form a kind of liquid crystal polymer. Subsequently, the polymer is crosslinked to trap the order of the liquid crystal into a crosslinked LCE. The LCE exhibits liquid crystalline behavior which has shape memory with excellent fixity and recovery. Its shape memory and actuating properties also have been studied. When reheating the LCE to 165 °C, the shape will recover. The main chains and crosslinked bonds of the LCE contain ester groups, which are sensitive to alkaline and acidic condition. It turns out that the LCE is intact under acidic condition, but it can be degraded under alkaline condition. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 389–394

Shape memory polymers (SMPs) are an emerging class of active polymers that have dual-shape capability, and are therefore candidate materials for multifunctional reconfigurable structures (i.e., morphing structures). However, the SMPs have not been fully tested to work in relevant environments (variable activation temperature, fuel and water swell, UV radiation, etc.) required for Air Force missions. In this study, epoxy-based SMPs were conditioned separately in simulated service environments designed to be reflective of anticipated performance requirements, namely, (1) exposure to UV radiation for 125 cycles, (2) immersion in jet-oil at ambient temperature, (3) immersion in jet-oil at 49°C, and (4) immersion in water at 49°C. The novel high-temperature indentation method was used to evaluate the mechanical properties and shape recovery ability of the conditioned SMPs. Results show that environmentally conditioned SMPs exhibit higher moduli in comparison to an unconditioned one. During free recovery, the indentation impressions of all SMPs disappeared as temperature reached above T_g, indicating that the material's ability to regain shape remains relatively unchanged with conditioning.

Shape memory polymer foams