Thermoplastic Shape Memory Polymer Research Articles

Design on thermal-mechanism coordination for binding reinforcing mesh based on shape memory polymer

The traditional rebar binding devices require complex drive and transmission mechanisms, which leads to large volume and complex structure. In this paper, a cylindrical thermoplastic shape memory polymer (SMP) fixture is proposed to verify the rebar binding method of thermal-mechanism coordination. The SMP fixture is manufactured by the injection molding technology through selecting suitable-ratio Polylactic acid and Polycaprolactone (PCL) blend materials. Besides, an additional auxiliary device is presented to overcome the incomplete recovery disadvantage existing in the thermoplastic SMP and completely achieve binding the rebar. On this base, two different binding methods are proposed to compare the mechanical performance after fixing the rebar, and the external force/thermal contributions are tested and discussed in detail. The tested results show that the binding contribution of heat could reach 70% while the binding contribution of external force could reach 30% above the transition temperature (Tg ). The maximum tensile force that the binding rebar can withstand under the thermal-mechanism coordination action could reach up to 657.7 N, which is higher than the maximum tensile force of the wire binding. In addition, the maximum friction force between rebar and notches of fixture could reach up to 94.1 N, which further verifies the feasibility of thermal-mechanism coordination for binding reinforcing mesh based on SMP fixture.

Variable Shape Tooling for Composite Manufacturing: A Systematic Review

The choice of material, manufacturing process, and molding tool significantly affects the quality, environmental impact, and cost efficiency of composite components. Producing one-piece hollow profiles with smooth inner surfaces and undercuts presents major challenges for conventional mold concepts. There is yet no thorough review of shape-variable mandrels in composite manufacturing to be found in the literature. This paper provides an overview of research on shape memory polymers and other shape-variable materials used in tooling applications for composite manufacturing. This work covers shape memory, heat shrink, and other deformable tooling concepts that enable the production of one-piece Type V pressure vessels, air intake ducts, or curved struts and tubes. A systematic literature review in combination with a state-of-the-art open-source active learning tool ASReview is conducted. Fifteen relevant studies were identified. Research on shape-variable tooling is mainly conducted by three research groups in the USA and the PRC. The tooling is mostly made of unreinforced thermosets, especially styrene-based ones. Thermoplastic resins are less common, and reinforcements limit the usable elongation in the temporary shape. The shape variability is either a shape memory and/or a softening process, which, in all studies, is activated by heating. Release agents are widely used to ease demolding. No ecological or economical assessment of the manufacturing methods was conducted in the reviewed studies. Three fields for further research that could be identified are as follows: (1) thorough ecological end economical assessment of shape-variable mandrels in comparison with conventional tooling; (2) thermoplastic shape memory polymer mandrels; and (3) further investigation of simulation capabilities for shape memory mandrels.

A phase-transition model of reprocessible thermadapt shape memory polymer

A new type of thermadapt shape memory polymer (SMP) has not only the processability of thermoplastic SMP, but also the excellent shape fixation of thermosetting SMP. To enhance the application of this thermadapt SMP within industrial sectors, a comprehensive constitutive model based on phase transition is being proposed as an indicative descriptor of the semi-crystalline thermadapt SMP’s salient features, predominantly related to its two-way shape memory effect (SME) and thermal reprocessability. The concept of cooling elongation is also introduced in this model for modeling the two-way SME during the crystallization process. The molecular mechanism of chain-packing has been studied and used to establish phenomenological formulas. In addition, to systematically assess the temperature-time dependence of the crystallization process, the Avrami equation is improved by incorporating the distribution of polymer chain segments. This strategy provides a detailed investigation into the evolving pattern of the crystallization process in response to various temperature and time conditions. Compared with the experimental results, it is found that our model can well capture mechanical behavior in multiple shape memory cycles, including the two-way SME and reshaping process caused by bond exchange reaction. Furthermore, the potential application of SMP in smart mandrels is explored because the cooling elongation feature is able to endow it with self-adaptive expansion ability.

Digital Laser Direct Writing of Internal Stress in Shape Memory Polymer for Anticounterfeiting and 4D Printing.

Shape memory polymers (SMPs) are a type of smart shape-shifting material that can respond to various stimuli. Their shape recovery pathway is determined by the internal stress stored in the temporary shapes. Thus, manipulating the internal stress is key to the potential applications of SMPs. This is commonly achieved by the types of deformation forces applied during the programming stage. In contrast, we present here a digital laser direct writing method to selectively induce thermal relaxation of internal stress stored in the two-dimensional (2D) shape of a thermoplastic SMP. The internal stress field, while invisible under natural light, can be visualized under polarized light. Consequently, the digital stress pattern can be used for anticounterfeiting. In addition, further uniform heating induces the release of the programmed internal stress within the 2D film. This triggers its transformation into a three-dimensional (3D) shape, enabling 4D printing. The simplicity and versatility of our approach in manipulating internal stress and shape-shifting make it attractive for potential applications.

Near-Infrared Remotely Controllable Shape Memory Biodegradable Occluder Based on Poly(l-lactide-co-ε-caprolactone)/Gold Nanorod Composite.

Biodegradable occluders, which can efficiently eliminate the complications caused by permanent foreign implants, are considered to be the next-generation devices for the interventional treatment of congenital heart disease. However, the controllability of the deployment process of degradable occluders remains a challenge. In this work, a near-infrared (NIR) remotely controllable biodegradable occluder is explored by integrating poly(l-lactide-co-ε-caprolactone) (PLCL) with poly(ethylene glycol)-modified gold nanorods (GNR/PEG). The caprolactone structural units can effectively increase the toughness of poly(l-lactide) and reduce the shape-memory transition temperature of the occluder to a more tissue-friendly temperature. Gold nanorods endow the PLCL-GNR/PEG composite with an excellent photothermal effect. The obtained occluder can be easily loaded into a catheter for transport and spatiotemporally expanded under irradiation with near-infrared light to block the defect site. Both in vitro and in vivo biological experiments showed that PLCL-GNR/PEG composites have good biocompatibility, and the PEGylated gold nanorods could improve the hemocompatibility of the composites to a certain extent by enhancing their hydrophilicity. As a thermoplastic shape-memory polymer, PLCL-GNR/PEG can be easily processed into various forms and structures for different patients and lesions. Therefore, PLCL-GNR/PEG has the potential to be considered as a competitive biodegradable material not only for occluders but also for other biodegradable implants.

Effects of recycling on polystyrene shape memory polymers for in-situ resource utilization

One-way, thermoplastic shape memory polymers (SMPs) used as actuators for self-folding origami are typically single-use materials that would be decommissioned upon completion of mission objectives. As a result, there exists an abundance of unutilized, single-use SMP waste. In-situ resource utilization (ISRU) and recycling offer solutions for the use and integration of sustainable SMP material infrastructures on Earth and for long-duration space missions. Unfortunately, mechanical recycling causes degradation of material properties. Therefore, it is imperative to quantify the effects of recycling on SMP properties. Herein, we utilize a thermo-mechanical recycling method applied to polystyrene SMPs. After recycling, we conduct Fourier transform infrared spectroscopy, differential scanning calorimetry, and dynamic mechanical analysis to investigate changes to the chemical structure, viscoelastic properties, and shape recovery response of the polymer. The results indicate negligible changes to the viscoelastic and shape recovery properties of the recycled material from one to six recycling sequences (extrusions) when compared to the non-recycled material. The most evident form of deterioration occurred in the physical appearance of the material. Otherwise, the shape recovery performance and thermo-mechanical properties remained consistent after recycling. Therefore, the recovery characteristics (recovery ratio, recovery time, and actuation stress) do not change significantly after six recycling sequences, making this material viable for ISRU applications in space environments.

Methodology for FDM 4D printing with thermo-responsive SMPs

Comparing and reproducing research results in 4D printing is challenging because researchers use different materials, printing parameters, and 3D printers. We propose a methodology aimed at helping to encounter this challenge. Thereby, we focus on fused deposition modelling (FDM) 4D printing with thermoplastic shape memory polymers (SMPs). SMPs are printed in a temporary shape and transform into a permanent shape under heat. Our methodology organises the influencing parameters in the 4D printing process. It provides a guide for four sections: Material Selection, Structure Design, Printing, and Activation. This allows to systematically identify further research topics.

4D Printing‐Encapsulated Polycaprolactone–Thermoplastic Polyurethane with High Shape Memory Performances

There are a few shape memory polymers (SMPs) like polylactic acid (PLA) and polyurethane (PU) that are 4D printable, and other SMPs must be synthesized with a complicated chemical lab effort. Herein, considering dual‐material extrusion printing and microscopic mechanism behind shape memory effect (SME), bilayer‐encapsulated polycaprolactone (PCL)–thermoplastic polyurethane (TPU) shape memory composite structures are 4D printed for the first time. The SME performance is investigated by assessing fixity, shape recovery, stress recovery, and stress relaxation under bending and compression loading modes. PCL, TPU, and melting temperature of PCL play the role of switching phase, net point, and transition temperature, respectively. Due to the destruction and dripping of molten PCL in contact with water, PCL is encapsulated by TPU. Encapsulation successfully solves the challenge of bonding/interface between printed layers, and the results show that the SME performance of the encapsulated structures is higher than bilayer PCL–TPU one's. Experiments reveal that maximum stress recovery in 4D‐printed composites remains constant over time. This is a great achievement compared to the previous extrusion‐based SMP structures that have great weakness in stress relaxation due to weak and low crystalline fractions and the unraveling of molecular entanglements in semicrystalline and amorphous thermoplastic SMPs, respectively.

Synthesis and characterization of poly(hexamethylene 2,6-naphthalate)-block-poly(tetrahydrofuran) copolymers with shape memory effect

Shape memory polymers (SMPs) are able to return to their previous shape after deformation under certain conditions. Several block copolymers can exhibit shape memory behavior. The blocks in copolymers are usually immiscible, which leads to phase separation into rigid and flexible segments. For that reason, these materials combine the mechanical properties of elastomers with the processing properties of thermoplastic. Thus, a poly(hexamethylene 2,6-naphthalate) (PHN) and a series of poly(hexamethylene 2,6-naphthalate)-block-poly(tetrahydrofuran) (PHN-b-N-pTHF) copolymers were synthesized via melt polycondensation. The content of the N-pTHF flexible segment varied from 15 to 50 wt.%. The materials were characterized with the use of Fourier transformed infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), X-ray diffraction (XRD) analysis, the tensile tests (static and cyclic), and the cyclic thermo-mechanical analysis. The PHN-b-N-pTHF 50/50 was proved to be a promising thermoplastic shape memory polymer (SMP).

Co-extrusion 4D printing of shape memory polymers with continuous metallic fibers for selective deformation

Shape memory polymers (SMPs), with the capability of transforming between temporary and permanent shapes, have fascinating applications in aerospace and mechanical engineering. However, for the transition of SMPs, the existing heating strategies, including water and oil bathing, usually involve global heating on structures with limited heating rates and untouchable surroundings. This limits the underlying applications of precise and selective deformation. Here, we report a co-extrusion 4D printing (CE-4DP) strategy that integrates continuous metallic fibers into thermoplastic SMP to build an electrical heating path in the polymer matrix. We systematically investigated the influence of crucial printing parameters on the fabrication and proposed an analytical model and a printable phase diagram to shed light on the mechanism of CE-4DP. The heating rate for the SMP composites was 70 times faster than that of the oil bath method. We also demonstrated that the SMP composites can be heated selectively and sequentially, thereby resulting in precise and programmable deformation. Besides, the capability of self-sensing is extended for monitoring the deformation.

Biobased Thermoplastic Elastomers: Structure-Property Relationship of Poly(hexamethylene 2,5-furanodicarboxylate)-Block-Poly(tetrahydrofuran) Copolymers Prepared by Melt Polycondensation.

A series of poly(hexamethylene 2,5-furanodicarboxylate)-block-poly(tetrahydrofuran) (PHF-b-F-pTHF) copolymers were synthesized using a two-stage procedure, employing transesterification and polycondensation. The content of pTHF flexible segments varied from 25 to 75 wt.%. 1H nuclear magnetic resonance (NMR) and Fourier transformed infrared spectroscopy (FTIR) analyses were applied to confirm the molecular structure of the materials. Differential scanning calorimetry (DSC), dynamic mechanical measurements (DMTA), and X-ray diffraction (XRD) allowed characterizing the supramolecular structure of the synthesized copolymers. SEM analysis was applied to show the differences in the block copolymers’ morphologies concerning their chemical structure. The influence of the number of flexible segments in the copolymers on the phase transition temperatures, thermal properties, as well as the thermo-oxidative and thermal stability was analyzed. TGA analysis, along with tensile tests (static and cyclic), confirmed the utilitarian performance of the synthesized bio-based materials. It was found that an increase in the amount of pTHF caused the increase of both number-average and weight-average molecular weights and intrinsic viscosities, and at the same time causing the shift of the values of phase transition temperatures toward lower ones. Besides, PHF-b-F-pTHF containing 75 wt.% of F-pTHF units was proved to be a promising thermoplastic shape memory polymer (SMP) with a switching temperature of 20 °C.

On the Size Effect of Additives in Amorphous Shape Memory Polymers.

Small additive molecules often enhance structural relaxation in polymers. We explore this effect in a thermoplastic shape memory polymer via molecular dynamics simulations. The additive-to-monomer size ratio is shown to play a key role here. While the effect of additive-concentration on the rate of shape recovery is found to be monotonic in the investigated range, a non-monotonic dependence on the size-ratio emerges at temperatures close to the glass transition. This work thus identifies the additives’ size to be a qualitatively novel parameter for controlling the recovery process in polymer-based shape memory materials.

Electrical and thermal stimuli responsive thermoplastic shape memory polymer composites containing rGO, Fe3O4 and rGO–Fe3O4 fillers

This work addresses a facile and broadly applicable method of fabricating a new thermoplastic shape memory polymer (SMP) composite by blending three biopolymers such as polyvinyl alcohol, polyvinyl pyrrolidone and polyethylene glycol. Reduced Graphene (rGO) is used as electrical conductive fillers. Iron Oxide (Fe3O4) and rGO-Fe3O4 combination provides thermo-electric stimulus. Under the thermal stimulus, shape recovery rate of the SMP with hybrid fillers was faster, having quick response time (28 s) compared to electrical stimulus response time (75 s). The conductivity of the SMP matrices increased by 7, 10 and 15 orders of magnitude by incorporating Fe3O4, rGO and rGO- Fe3O4 fillers, respectively. Moreover, the proposed shape memory polymer containing rGO-Fe3O4 filler exhibits a higher Young’s modulus (> 80%) compared to neat polymer (1.75 GPa at room temperature and 0.6 GPa at glass transition temperature, Tg). A maximum stress of 2.5 MPa and 4% recoverable strain was achieved in the SMP with 10 and 15 wt% of hybrid fillers and interestingly no stored strain evolves, upon cooling below Tg. The developed SMP may be applied in morphing wing and other smart actuator applications.

4D printed auxetic structures with tunable mechanical properties

Additive manufacturing has revolutionized the entire design cycle across a wide range of industries. The majority of materials utilized in 3D printing result in rather rigid structures with a fixed set of properties. Four-dimensional printing (4D printing) addresses this limitation by relaxing the geometric/properties rigidity over time. The careful selection of structural designs and the use of active and smart materials enables a unique set of shapes and properties to be programmed and achieved. This deviation from rigid structures with a fixed set of properties enables novel and unique applications in robotics, deployable structures, biomedical, and aerospace industries. The current work is focused on an interesting class of structures, 2D auxetic cellular solids, which exhibit a negative Poisson’s ratio. 4D printing of such structures allows for tunable structural stiffness, Poisson’s ratio, and rigidity. This work aims to experimentally investigate the tunability of stiffness and Poisson’s ratio in 4D printed auxetic structures fabricated using a thermoplastic shape-memory polymer. The attained structural stiffness was tuned in the range of 0.179−0.242 kN/mm while the in-plane Poisson’s ratio was controlled from -0.33 up to even positive magnitudes of +0.69. This wide range of elastic properties was obtained from a single structure programmed to different deformation levels. Experimental evaluation of 4D printed SMP structures under constant cyclic programming/recovery conditions induced residual strains which consequently affected the mechanical properties by degrading the Poisson’s ratio and structural stiffness. The magnitude of induced residual strains depends on the level of applied deformation during programing and the number of applied programing/recovery cycles. At the local level, full-field measurements revealed the localization of strains within the complex heterogeneous structure of the axially loaded auxetic honeycomb samples.

Light‐Coded Digital Crystallinity Patterns Toward Bioinspired 4D Transformation of Shape‐Memory Polymers

AbstractSpatially heterogeneous distribution of active components is key to the diverse shape‐morphing behaviors of biological species and their associated functions. Artificial morphing materials employing similar strategies have widened the design space for advanced functional devices. Typically, the spatial heterogeneity is introduced during the material synthesis/fabrication step and cannot be altered afterward. An approach that allows spatio‐selective programming of crystallinity in a shape‐memory polymer (SMP) by a digital photothermal effect is reported. The light‐patternable crystallinity affects greatly the shape morphing behavior. Consequently, a pre‐stretched 2D film with spatial heterogeneity in crystallinity can morph with time into designable 3D permanent shapes, achieving the 4D transformation. This approach utilizes a reprocessible thermoplastic SMP (polylactide) and the programming relies on a physical phase transformation (crystallization) instead of chemical heterogeneity. This allows repeated erasing and reprogramming using the same material, suggesting a versatile and sustainable means for manufacturing advanced morphing devices.

Stretchable Sensors for Nanomolar Glucose Detection

AbstractBiosensors that detect analytes in sweat face the challenge of maintaining sensitivity upon miniaturization. Various materials and processes have been developed to create nanostructured electrodes with high surface areas to mitigate this issue. The need remains, however, for biocompatible materials that can be scalably integrated into wearable devices. This paper details a gold thin‐film electrode fabricated using a thermoplastic shape memory polymer to create hierarchical wrinkled structures via the miniaturization process, followed by transfer onto a soft, stretchable substrate. The final stretchable electrode, which is ≈30 times smaller than that of the original, unshrunk electrode, retains its original surface area. Even more remarkably, further enhancement in current density is achieved upon stretching the electrode to 210% of its original length; improved sensitivity remains stable after relaxation of the polymer. Stretching aids diffusion limited reactions, such as the reduction and oxidation of [Fe(CN)6]3−/4−. The stretchable electrodes sensitively detect glucose without enzymes or additional labels at physiological pH in the range of 1 × 10−7–1 × 10−4 m with a calculated limit of detection of 2.22 × 10−8 m, among the lowest reported for a flexible, enzyme‐free sensor.

A novel thermoplastic shape memory polymer with solid-state plasticity derived from exchangeable hydrogen bonds.

A novel thermoplastic shape memory polymer (SMP) was synthesized by partly modifying the double bonds of trans-1,4-polybutadiene (TPB) and subsequently introducing mono-isocyanate as pendant groups. The comb-like chains consisted of abundant hydrogen bonds on their side groups, which could readily assemble into dynamic non-covalent polymer networks. The physical crosslink net-points formed by the hydrogen bonds were utilized to anchor the permanent shape of PBTP, while the soft trans-1,4-polybutadiene segments served as a switching phase to afford the temporary shape, and the glass transition temperature (Tg) was taken as the switching temperature. The optimal result was obtained with the sample with 30% modification, which showed an excellent shape fixity ratio and inferior shape recovery ratio of 100% and 89.57%, respectively. According to the results of the stress relaxation test, the relaxation time for this sample is 27 s when heated at 70 °C, implying that the dynamic network inside the sample can undergo topological rearrangement rapidly while maintaining the network integrity during the process. The high density of exchangeable hydrogen bonds constitutes a highly crosslinked non-covalent network, the dynamic nature of which is responsible for this outstanding network topological structure rearrangement behaviour, and endows this SMP with solid-state plasticity to reconfigure its permanent shape in the absence of catalysts or moulds. The highly complex permanent shape reconfiguration at a moderate temperature within a rational period will greatly promote the practical use of SMPs, and enable a wide variety of future engineering applications.

Robust, Reprocessable, and Reconfigurable Cellulose-Based Multiple Shape Memory Polymer Enabled by Dynamic Metal-Ligand Bonds.

Smart materials with multiple shape memory capacities have gradually attracted the interest of a lot of researchers due to their potential application in textiles, smart actuators, and aerospace engineering. However, the design and sustainable synthesis of multiple shape memory polymers (SMPs) simultaneously possessing robust mechanical strength, reprocessability, and reconfigurability still remain full of challenges. Starting from a readily available biomass material cellulose, a well-defined SMP, cellulose-graft-poly(n-butyl acrylate-co-1-vinylimidazole) copolymer (Cell-g-(BA-co-VI)) was facilely synthesized by addition-fragmentation chain transfer polymerization (RAFT) and the subsequent metallosupramolecular cross-linking. Taking advantage of the dynamic bonding, i.e., the rapid reversible fragmentation and the formation of metal ion-imidazole coordination, polymer networks with highly tunable mechanical properties, excellent solid-state plasticity, and quadruple-shape memory capacity are handily attainable. Microscopically, the metal-ligand clusters have a strong tendency to phase segregate from the soft grafted copolymers indicated by atomic force microscopy (AFM), and these serve as netpoints to construct novel SMPs. This article represents our new exploration of the next-generation SMPs based on cellulose backbone where carrying with supramolecular cross-linked soft grafted copolymers. This architecture design allows achieving robust, reprocessable, and reconfigurable thermoplastic SMPs that are difficult to realize by many other methods. Integrating these properties into one system in a synergetic manner also provides a novel approach to the high value addition application of cellulose in the fabrication of advanced functional materials.

Reconfigurable Poly(urea‐urethane) Thermoset Based on Hindered Urea Bonds with Triple‐Shape‐Memory Performance

AbstractThermoset shape memory polymers (SMPs) have a series of advantages in practical applications when compared with thermoplastic SMPs. However, the reprocessing and reshaping of thermosets are difficult due to the covalent crosslinking which severely limits the reconfiguration of polymer networks even under high temperature. Here, hinered urea bond (HUB), a urea bearing a bulky substituent on its nitrogen atom that can reversibly dissociate to the corresponding bulky amine and isocyanate, is integrated into the poly(urea‐urethane) (PUU) crosslinked networks, enabling the networks to topologically reconfigure to other structures. Instead of having to cure the thermoset in a mold, the incorporation of HUBs can decouple the synthesis and shape‐forming steps, which is a huge advantage for the processing of thermoset materials compared to conventional thermosets. This new PUU thermoset shows a broad glass transition behavior and exhibits excellent triple‐shape‐memory performance. With the incorporation of this dynamic urea bonds, permanent shapes can be flexibly tuned via the network reconfiguration, which is often neglected but significant to the practical application of SMPs.

Synthesis and properties of shape memory poly(aryl ether ketone)s

Thermally responsive shape memory polymers have received great attention in the fields of biomedicine and aerospace. A new type of thermoplastic shape memory polymer with excellent mechanical and thermal properties was synthesized. A new reactable monomer with flexible alkyl chain was first designed and synthesized, then introducing it into the main chain structure of poly(aryl ether ketone)s. The alternating chain structure of the soft and hard segments and the H-Bonding between the carboxyl groups on the side chains both serve as physical cross-linking points in the polymer network, thereby imparting shape memory properties to the polymers. Polymers with more flexible segments exhibit better shape memory performance in terms of high shape fixing ratio and high shape recovery ratio.