Shape-memory Polymer Networks Research Articles

Hydrophilic and hydrophobic shape memory polymer networks in high-intensity focused ultrasound fields

High-intensity focused ultrasound (HIFU) has been investigated as a remote and controlled activation method to noninvasively actuate shape memory polymers (SMPs), specifically in biomedical applications. However, the effects of aqueous environment on shape recoverability of in vivo HIFU-actuated SMPs have yet to be explored. HIFU directs sound waves into a millimeter-sized tightly focused region. In this study, the response of hydrophilic and hydrophobic photopolymerized thermoset SMP networks under HIFU activation in an aqueous environment was investigated. Acrylate-based SMP networks were copolymerized in specific ratios to produce networks with independently adjusted glass transition temperatures ranging from 40 to 80 °C and two distinct water uptake behaviors. The results link the polymer swelling behavior to shape recoverability in various acoustic fields. The presence of absorbed water molecules enhances the performance of SMPs in terms of their shape memory capabilities when activated by HIFU. Overall, understanding the interplay between water uptake and HIFU-actuated shape recovery is essential for optimizing the performance of SMPs in aqueous environments and advancing their use in various medical applications.

Reconfiguration, Welding, Reprogramming, and Complex Shape Transformation of An Optical Shape Memory Polymer Network Enabled by Patterned Secondary Crosslinking.

Stimuli-triggered generation of complicated 3D shapes from 2D strips or plates without using sophisticated molds is desirable and achieving such 2D-to-3D shape transformation in combination with shape reconfiguration, welding, and reprogramming on a single material is very challenging. Here, a convenient and facile strategy using the solution of a disulfide-containing diamine for patterned secondary crosslinking of an optical shape-memory polymer network is developed to integrate the above performances. The dangling thiolectones attached to the backbones react with the diamine in the solution-deposited region so that the secondary crosslinking may not only weld individual strips into assembled 3D shapes but also suppress the relaxation of the deformed polymer chains to different extents for shape reconfiguration or heating-induced complex 3D deformations. In addition, as the dynamic disulfide bonds can be thermally activated to erase the initial programming information and the excessive thiolectones are available for subsequent patterned crosslinking, the material also allows shape reprogramming. Combining welding with patterning treatment, it is further demonstrated that a gripper can be assembled and photothermally controlled to readily grasp an object.

Simulation study of shape memory polymer networks formed by free radical polymerization

Chemically crosslinked thermoset shape memory polymers (TSMPs) can combine the strength required for load-carrying structures and devices with stimuli-responsive properties that can allow them to perform crack-closing or other multifunctional properties. In particular, UV-curable shape memory polymers can be used for 3D printing, and to enhance the design process, a molecular dynamics (MD) modeling framework was developed to study them. The MD simulations were used to calculate thermophysical, mechanical, and shape memory properties for glycerolate dimethacrylate after UV-curing to form a polymer network. Semiquantitative agreement between simulation and experiment was achieved for the different properties compared. By examining various conditions for network formation, a range of distinct cross-linking topologies were generated and their effects on properties were evaluated. In particular, it was found that the percentage of monomers with the maximum number of possible links with other monomers had a greater correlation with recovery stress than the number of monomers reacted overall. A new topological fingerprint, the average minimum pathway between monomers, was proposed and found to have the strongest correlation with recovery stress than all properties examined.

Shape Memory Polymer Constructed by π–π Stacking with Ultrafast Photoresponse and Self-Healing Performance

Recently, polymer materials with light-responsive shape memory and self-healing ability have been extensively studied. Various strategies have been reported to prepare remotely controllable light-responsive shape memory materials by employing carbon-based nanophotothermal agents (e.g., carbon nanotubes (CNTs)) in a shape memory polymer network. However, enhancing the dispersion state and capacity of CNTs remains a challenge for improving the response capability and rate of light-responsive shape memory materials. In this work, poly(vinyl alcohol) (PVA) modified with a pyrene groups was introduced as the polymer network, and CNTs were introduced as the nanophotothermal agent to prepare a light-responsive shape memory material with fast photothermal responsiveness. Because of the stacking interaction between pyrene and CNTs, a large fraction of CNTs are dispersed in an orderly way in the polymer matrix. Moreover, stacking and hydrogen bond interactions build up stable cross-links and endow the material with shape memory and self-healing ability. Therefore, this work provides an innovative strategy for preparing a shape memory material with an ultrafast photoresponse and self-healing performance.

Influences of Crystallinity and Crosslinking Density on the Shape Recovery Force in Poly(ε-Caprolactone)-Based Shape-Memory Polymer Blends.

Shape-memory polymers (SMPs) show great potential in various emerging applications, such as artificial muscles, soft actuators, and biomedical devices, owing to their unique shape recovery-induced contraction force. However, the factors influencing this force remain unclear. Herein, we designed a simple polymer blending system using a series of tetra-branched poly(ε-caprolactone)-based SMPs with long and short branch-chain lengths that demonstrate decreased crystallinity and increased crosslinking density gradients. The resultant polymer blends possessed mechanical properties manipulable across a wide range in accordance with the crystallinity gradient, such as stretchability (50.5-1419.5%) and toughness (0.62-130.4 MJ m-3), while maintaining excellent shape-memory properties. The experimental results show that crosslinking density affected the shape recovery force, which correlates to the SMPs' energy storage capacity. Such a polymer blending system could provide new insights on how crystallinity and crosslinking density affect macroscopic thermal and mechanical properties as well as the shape recovery force of SMP networks, improving design capability for future applications.

Thermadapt shape memory polymers based on thermally induced dynamic covalent quinone methide–thiol click reaction

A novel thermadapt shape memory polymer networks were prepared based on the conjugate addition reaction of quinone methide (QM) end-group-functionalized polycaprolactone and a thiol cross-linker, which can undergo the dynamic exchange reaction under thermal stimulation conditions. The resultant cross-linked polymers possessed excellent elasticity-based shape memory performance, cycle stability and solid-state plastically characteristics under high temperature conditions. This thermal-induced QM-thiol dynamic covalent click chemistry provided an effective method for realizing the solid-state plasticity of the polymers.

3D fabrication of Shape-Memory polymer networks based on coumarin Photo-Dimerization

Photo-crosslinked shape-memory polymer (SMP) materials were obtained by the copolymerization of methacrylates bearing 2-ethylhexyl plasticizing group (EHMA) and photoactive coumarin unit (CoumMA). The photo-dimerization of coumarin units ensured the crosslinking of P(EHMA-co-CoumMA) and further allowed the fabrication by stereolithography of well-defined butterfly with ribbed wings to illustrate the potential of promising coumarin chemistry in additive fabrication. The influence of dye ratio on the optical and the mechanical properties of the transparent P(EHMA-co-CoumMA) films as well as the shape memory properties of the films in unfolding and twisting deformations were detailed and demonstrate the opportunity of coumarin-bearing photoprintable polymers for 4D printing.

NIR‐Encoded Shape Recovery and Regulated Reversible Stress of Shape Memory Polymer

AbstractNear‐infrared (NIR) light responsive shape memory polymers (SMPs) can realize remotely and regionally controlled shape changing that shows broad potential applications. Herein, an NIR‐responsive shape memory polymer (G/PNs) is prepared by introducing reduced graphene oxide to liquid crystal elastomers‐based polymer networks (PNs). The G/PNs present regulated reversible stress by tuning the NIR light irradiation. Furthermore, since the G/PNs are 3D‐printed with designed structure, the deformed 3D printing structure can be regionally patterned with varied shapes by controlling the moving path of NIR laser. In addition, the stress relaxation and cyclic stress–strain measurements are further studied to reveal the energy loss during the deformation and shape memory cycles. In compare with the previous work, the SMP networks provide a rather high reversible actuation stress and can realize remotely control by NIR. The NIR‐encoded shape memory effect can be used as actuators, artificial muscles.

Triple and Two-Way Reversible Shape Memory Polymer Networks with Body Temperature and Water Responsiveness

Shape memory polymers (SMPs), as a class of intelligent materials, have shown great potential in biomedical and robotic fields. Although efforts have been made to design and fabricate SMPs in the p...

Prolonged recovery of 3D printed, photo-cured polylactide shape memory polymer networks.

Shape memory polymers are materials that are able to retain a deformed state until an external stimulus, most typically heat, triggers recovery to the original geometry. Whereas typically, shape memory polymers are required to recover fast (seconds to minutes), many applications, particularly in the medical field, would benefit from a slow recovery (days to weeks). In this work, we exploit the broad glass transition range of photo-cured poly(D,L-lactide) dimethacrylate networks to obtain recovery times of up to 2 weeks, at 11 °C below the peak glass transition temperature of 58 °C. Recovery times decreased considerably for higher recovery temperatures, down to ∼10 min at 55 °C. A large spread in glass transition values (53.3–61.0 °C) was observed between samples, indicating poor reproducibility in sample preparation and, hence, in predicting shape recovery kinetics for individual samples. Furthermore, a staged recovery was observed with different parts of the samples recovering at different times. The ability to prepare complex structures using digital light processing stereolithography 3D printing from these polymers was confirmed. To the best of our knowledge, this work provides the first experimental evidence of prolonged recovery of shape memory polymers.

Effect of the addition of diurethane dimethacrylate on the chemical and mechanical properties of tBA-PEGDMA acrylate based shape memory polymer network

There is a great demand for the synthesis of acrylate based thermoset shape memory polymer (SMP) associated with one monomer and one crosslinker such as tert-butyl acrylate (t-BA) with poly (ethylene glycol) dimethacrylate (PEGDMA). The present work describes the synthesis of a new thermoset SMP wherein a second monomer such as diurethane dimethacrylate (DUDMA) has been added to the existing tBA + PEGDMA SMP matrix. The synthesized thermoset shape memory polymer exhibited a glass transition temperature (Tg) of 55 °C, higher Young's Modulus of 3.23 GPa, transmittance of 95% and 100% shape recovery. The SMP exhibited response to both thermal and chemical stimuli. The shape recovery rate of the SMP network is 20 s compared to 24 s observed for SMP based on tBA + PEGDMA. The obtained SMP is very transparent and possesses higher stiffness (8 MPa) and hence may be suitable for biomedical shape memory lens and orthopedic application.

Sequence-Rearranged Cocrystalline Polymer Network with Shape Reconfigurability and Tunable Switching Temperature.

Switching temperature (Tsw) is a key parameter governing the service condition of shape memory polymers (SMPs). However, tuning Tsw of SMPs often requires sophisticated synthesis or intricate processing. Herein, we report a simple yet effective strategy to prepare the SMPs with tunable Tsw and good reconfigurability by using the cocrystalline polyesters as the reversible phase. The cocrystallizable copolyesters with rearranged sequences were prepared by the transesterification of mixed polyester diols and then photo-cross-linked to achieve the SMP networks. Cocrystallization of copolymer blocks endows the SMP networks tunable melting point and relatively high crystallinity, affording the network good shape fixing and recovery ability at body temperature. Besides, the dynamic nature of transesterification, that enables the network to have good shape reconfigurability, allows for the easy processing of SMPs with complicated shapes. The reconfigurable SMPs capable of actuating at the body temperature show great potential for use as biomedical devices.

Millisecond Response of Shape Memory Polymer Nanocomposite Aerogel Powered by Stretchable Graphene Framework.

Shape memory polymers (SMPs) change shapes as-designed through altering the chain segment movement by external stimuli, promising wide uses in actuators, sensors, drug delivery, and deployable devices. However, the recovery speed of SMPs is still far slower than the benchmark shape memory alloys (SMAs), originating from their intrinsic poor heat transport and retarded viscoelasticity of polymer chains. In this work, monolithic nanocomposite aerogels composed of bicontinuous graphene and SMP networks are designed to promote the recovery time of SMP composites to a record value of 50 ms, comparable to the SMA case. The integration of a stretchable graphene framework as a fast energy transformation grid with ultrathin polycaprolactone nanofilms (tunable at 2.5-60 nm) enables the rapid phase transition of SMPs under electrical stimulation. The graphene-SMP nanocomposite aerogels, with a density of ∼10 mg cm-3, exhibit a fast response (175 ± 40 mm s-1), large deformation (∼100%), and a wide response bandwidth (0.1-20 Hz). The ultrafast response of SMP nanocomposite aerogels confers extensive uses in sensitive fuses, micro-oscillators, artificial muscles, actuators, and soft robotics. The design of bicontinuous ultralight aerogels can be extended to fabricate multifunctional and multiresponsive hybrid materials and devices.

Shape memory polymer networks based on methacrylated fatty acids

In this study, bio-based materials were prepared from fatty acid precursors and variable amounts of styrene or divinylbenzene by free-radical polymerization. The obtained materials were highly translucent and resulted insoluble in common organic and aqueous solvents. They exhibit a complex microstructure due to the formation of highly cross-linked arrangements (micro/nanogels) closed packed into clusters. The bio-polymers prepared with divinylbenzene (a tetra-functional monomer), due to their excessive cross-linkage, resulted in brittle samples whose mechanical response could not be obtained. On the other hand, bio-based polymers obtained from styrene (a bi-functional monomer) resulted in materials with a moderate cross-linking density, a sharp glass-rubber transition and acceptable mechanical properties, leading to polymers with interesting shape memory response (switching temperature, fixity and recovery ratios) that could be tailored by changing the composition. In these materials it was corroborated that higher (but not excessive) cross-linking density led to a greater shape recovery, since cross-links are the responsible for memorizing the original shape of the material.

Photo-Cross-Linkable Coumarin-Based Poly(ε-caprolactone) for Light-Controlled Design and Reconfiguration of Shape-Memory Polymer Networks

Photochemically cross-linked shape-memory polymer (SMP) materials have been achieved by functionalizing chain-ends of star-shaped poly(e-caprolactone) (PCL) with 7-hydroxypropoxy-4-methylcoumarin f...

Digital coding of mechanical stress in a dynamic covalent shape memory polymer network

Controlling stresses in materials presents many unusual opportunities for their engineering applications. The potential for current approaches is severely limited by the intrinsic tie between the stress and the geometric shape. Here, we report a material concept that allows stress management in a highly efficient digital manner while decoupling the stress and the geometric shape. This is realized in a dynamic covalent shape memory polymer network, for which the elastic shape memory sets the baseline stress level and maintains the geometric shape while the plasticity enabled by the dynamic bond exchange allows stress tuning. With a digital gray scale photothermal mechanism, any arbitrarily defined stress distribution can be created in a free-standing polymer film. The naturally invisible stresses can be further visualized as mechanical colors under polarized light, revealing its potential for encoding hidden information. Our approach expands the technological potential in many areas for which stresses are relevant.

Programming a crystalline shape memory polymer network with thermo- and photo-reversible bonds toward a single-component soft robot.

The need to support the two most basic functions [three-dimensional (3D)-shaped support and actuation] independently for a typical robot demands that at least two components should be used in its construction. Therefore, component assembly is unavoidable despite the ultimate dream of creating assembly-free robots. We devise a strategy that uses a programmable crystalline shape memory polymer with thermo- and photo-reversible bonds to create a single-component robot. The global 3D-shaped structural support is fabricated via a plasticity-based origami technique enabled by the thermo-reversible bonds. More critically, precisely controlled localized actuation can be programmed into the 3D origami via spatially defined reversible shape memory using the photo-reversible bonds. The overall result is that a polymer thin film can be programmed into various soft robots including a 3D crane and an elephant. Besides reversible shape memory, other types of actuation mechanisms can be potentially introduced via a similar principle. Thus, our strategy represents a general method to create single-component soft robots.

High-strain slide-ring shape-memory polycaprolactone-based polyurethane.

To enable shape-memory polymer networks to achieve recoverable high deformability with a simultaneous high shape-fixity ratio and shape-recovery ratio, novel semi-crystalline slide-ring shape-memory polycaprolactone-based polyurethane (SR-SMPCLU) with movable net-points constructed by a topologically interlocked slide-ring structure was designed and fabricated. The SR-SMPCLU not only exhibited good shape fixity, almost complete shape recovery, and a fast shape-recovery speed, it also showed an outstanding recoverable high-strain capacity with 95.83% Rr under a deformation strain of 1410% due to the pulley effect of the topological slide-ring structure. Furthermore, the SR-SMPCLU system maintained excellent shape-memory performance with increasing the training cycle numbers at 45% and even 280% deformation strain. The effects of the slide-ring cross-linker content, deformation strain, and successive shape-memory cycles on the shape-memory performance were investigated. A possible mechanism for the shape-memory effect of the SR-SMPCLU system is proposed.

A Metallosupramolecular Shape‐Memory Polymer with Gradient Thermal Plasticity

Solid-state plasticity by dynamic covalent bond exchange in a shape-memory polymer network bestows a permanent shape reconfiguration ability. Spatio-selective control of thermally induced plasticity may further extend the capabilities of materials into unexplored domains. However, this is difficult to achieve because of the lack of spatio-control in typical polymer network synthesis. Metal-ligand interactions possess the high strength of covalent bonds while maintaining the dynamic reversibility of supramolecular bonds. Metallosupramolecular shape-memory polymer networks were designed and prepared, which demonstrated solid-state plasticity. The metallo-coordination bonds within these networks permit facile tuning of the plasticity behavior across a wide temperature range, simply by changing the metal ion. By controlling the diffusion of two different metal ions during preparation of a polymer film, a plasticity behavior with a spatial gradient was achieved, providing a unique shape-morphing versatility with potential in shape-memory devices.

A New Type of Photo-Thermo Staged-Responsive Shape-Memory Polyurethanes Network.

In this paper, we developed a photo-thermo staged-responsive shape-memory polymer network which has a unique ability of being spontaneously photo-responsive deformable and thermo-responsive shape recovery. This new type of shape-memory polyurethane network (A-SMPUs) was successfully synthesized with 4,4-azodibenzoic acid (Azoa), hexamethylenediisocyanate (HDI) and polycaprolactone (PCL), followed by chemical cross-linking with glycerol (Gl). The structures, morphology, and shape-memory properties of A-SMPUs have been carefully investigated. The results demonstrate that the A-SMPUs form micro-phase separation structures consisting of a semi-crystallized PCL soft phase and an Azoa amorphous hard phase that could influence the crystallinity of PCL soft phases. The chemical cross-linking provided a stable network and good thermal stability to the A-SMPUs. All A-SMPUs exhibited good triple-shape-memory properties with higher than 97% shape fixity ratio and 95% shape recovery ratio. Additionally, the A-SMPUs with higher Azoa content exhibited interesting photo-thermo two-staged responsiveness. A pre-processed film with orientated Azoa structure exhibited spontaneous curling deformation upon exposing to ultraviolet (UV) light, and curling deformation is constant even under Vis light. Finally, the curling deformation can spontaneously recover to the original shape by applying a thermal stimulus. This work demonstrates new synergistically multi-responsive SMPUs that will have many applications in smart science and technology.