Shape Memory Transition Temperature Research Articles

Tribological mechanism based on tribo-chemistry and molecular dynamics of polyurea/polyimide shape memory copolymers at high temperature

Exploring the tribological mechanism of the shape memory polymers plays an important role in designing intelligent lubricating materials. Here, a series of shape memory polyurea (PUA)/polyimide (PI) were prepared and their tribological performance were investigated. The result showed that the friction and wear of PUA/PI were higher at shape memory transition temperature (Ttrans) than at 200 ℃ and room temperature (RT). However, introducing PUA units into PI enhanced its shape memory stability and tribological performance. SEM and XPS analysis revealed that introducing PUA promoted the formation of polymer-based tribofilm. Molecular dynamics simulations further illustrated that the high relative atomic concentration of PUA/6PI at the sliding interface improves the bonding between the tribofilm and the counterpart, thereby improving overall tribological performance.

High-Tg shape memory polyimide composites with “spot-plane” directional thermally conduction structure based on AlN nanoparticles and acidified graphene

In order to enhance the thermal conduction property and shorten response time of shape memory polyimide (SMPI), “spot-plane” directional thermally conductive acidified graphene/AlN/SMPI composites were prepared, in which AlN nanoparticles were used as “thermal conduction spots” and acidified graphene was used as two-dimensional “thermal conduction planes”. The SMPI composite doped with 6 wt% AlN and 0.8 wt% acidified graphene had good heat conducting properties (the in-plane thermal conductivity of 5.90 W m−1 K−1), excellent mechanical propertied (the tensile strength of 134.5 MPa, and Young’s modulus of 3.8 GPa), high shape memory transition temperature (at 419 ℃), and great thermal stability and shape memory properties. Compared with SMPI resin matrix, the response time of the acidified graphene/AlN/SMPI composite was shortened by 140 s, only 20 s, and it can be applied in flexible electronics, intelligent response structures and active deformation structures in high-temperature environment.

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.

Hydroxyl terminated polybutadiene-based shape memory polyurethane nanocomposites containing nano metal-oxide fillers for microfluidic valve actuator applications

We report here a novel and unique thermo-responsive Shape Memory Polyurethane Nanocomposites (SMPUNCs) derived from Hydroxyl Terminated Polybutadiene (HTPB) and Poly Tetra Methylene Glycol (PTMG) blend encapsulated with nano metal oxides fillers such as CuO, NiO, Fe2O3, CuFe2O4, and Fe2NiO4 for possible use in polymer-based microvalve actuator systems. The synthesis methods adopted and the various characterization techniques employed are presented. The corresponding SMPUs are named PU−CuO, PU−Fe2O3, PU-NiO, PU−CuFe2O4, and PU-Fe2NiO4 respectively. HTPB and PTMG are used as the base polyols for reacting with isocyanates to form the polyurethane base matrix, named as PU-Neat for comparison. Different fillers were used to study the effect of these fillers on the enhancement of the shape memory properties of polyurethane. Morphological, thermal, mechanical, and shape-memory properties of the prepared SMPUs are fully characterized. Morphological studies confirm that all the nano-metal oxide fillers are uniformly dispersed within the SMPU matrix. The shape memory transition temperature (Ttrans), which plays an important role in the shape memory effect (SME), is obtained from DSC analysis. The results show that Ttrans is enhanced with the reinforcement of the nano-metal oxide fillers (resulting in about 10% enhancement in the shape recovery time). The metal oxide filler integrated samples show higher thermal stability as well as shape fixing and recovery performances than PU-Neat. The tensile test confirmed that the PU−Fe2O3 matrix possesses a high tensile modulus and is rigid when compared to other SMPUs. We also demonstrated a microvalve actuator system using PU−Fe2O3, as it possesses the optimum shape recovery and better shape fixing properties, compared to other prepared Shape Memory Polyurethane Nanocomposites. An improvement of 10% efficiency in the closing and opening of the valve for PU composite film was observed in the demonstrated actuator setup.

Synergistic combination of 4D printing and electroless metallic plating for the fabrication of a highly conductive electrical device

Four-dimensional (4D) printing of stimuli-responsive materials with the ability to change their shapes or functions when subjected to external stimuli has garnered much interest due to their tremendous potential applications in various biomedical and engineering devices. Recently, 4D printing of electrically-conductive shape memory polymers (SMPs) have become increasingly attractive due to its prospects in the fields of biomedicine, soft robotics, flexible electronics etc. However, translation of these research into practical applications are often hindered by low electrical conductivity, as well as complicated programming and design. In this work, we report a facile approach to achieve highly conductive SMP-based 4D-printed devices. The SMP-based devices can be printed on a typical 3D digital light projection (DLP) or stereolithography (SLA) printer from photopolymer resins with the ability to tune the shape memory transition temperatures ranging from 20 °C to 50 °C. High electrical conductivity up to 2 × 106 S m−1 was achieved through electroless deposition of copper with high adhesion strength of up to 1.5 MPa. The incorporation of a metallic coating allowed for enhanced shape recovery due to enhanced surface thermal conductivity. An application of the technique to fabricate smart switches with the ability to monitor environmental temperature changes to be used in electrical safety devices was successfully demonstrated. The approach presented herein may be further explored and developed to capitalise on the stimuli-responsiveness and electrical conductivity of 4D-printed SMPs to find value in various engineering applications.

Effects of healing agent on shape memory, mechanical and self- healing properties of joint filler on cement concrete pavement

To improve the self-healing performance of joint filler on cement pavement and determine the healing agent content for preparing titanium dioxide/ shape memory polyurethane (TiO2/SMPU) composite as joint filler, the influences of PCL content on different properties of TiO2/SMPU joint filler were investigated before and after the two-stage uniaxial training, as well as after free recovery. Results indicate that the shape memory transition temperature of TiO2/SMPU joint filler and melting temperature of healing agent of thermoplasticpolycaprolactone (PCL) meet the requirements of two-stage uniaxial training and two-step self-healing of TiO2/ SMPU joint filler. Also, SMPU matrix has a good compatibility with PCL particles, and there are no chemical reactions between PCL and TiO2/SMPU. The physical cross -linking reactions between PCL and SMPU matrix occur during the self-healing at high temperature, providing a basis for PCL to repeat the self-healing. After the two -stage uniaxial training, TiO2/SMPU joint filler shows satisfactory shape memory performance, but when PCL content is more than 10%, TiO2/SMPU joint filler shows lower shape memory effects. Additionally, the two-stage uniaxial training method improves the mechanical properties and repeated self-healing ability of TiO2/SMPU joint filler after the two-step self-healing method. Finally, the PCL content of 10% is suggested to prepare TiO2/SMPU joint filler of expansion joint after the two-stage training to reduce the damages of cement concrete pavement.

Bio‐Based, Self‐CrosslinkableEucommia ulmoidesGum/Silica Hybrids with Body Temperature Triggering Shape Memory Capability

AbstractAt present, the synthesis of body temperature triggering shape memory polymers usually requires elaborate structural design, which limits their wide application. Herein, starting from bio‐basedEucommia ulmoidesgum (EUG), a series of EUG/silica hybrids (ESHs) are prepared through a facile one‐pot process, in which EUG is epoxied and then self‐crosslinked with SiO2by epoxy ring‐open reaction. Varying the amount of H2O2, the shape memory transition temperature (Ttrans) of ESHs is adjusted to 47.4–36.6 ℃, which is close to human body temperature (37 ℃). Among them, ESH‐17 exhibited the best body temperature triggering shape memory ability (Ttrans= 36.6 ℃), which can restore the permanent shape within 60 s at 37 ℃ with a shape fixity ratio of 99% and shape recovery ratio near 100%. In addition, the shape memory mechanism is discussed and shows some application scenarios of ESHs. The as‐produced materials can be used as smart biomaterials such as self‐tightening sutures, self‐sealing root canal filling materials, and so on.

The effect of chitosan form on the shape memory properties of polyurethane based composites

The continuous progress in the medicine and biotechnology demands development of novel biomaterials with multiple functions. This multifunctionality may be accomplished through combining synthetic polymers with shape memory properties and natural polymers. Shape memory composites consisting of poly(caprolactone-urethane) and chitosan (PU-2/3 PCL CH) were synthesized and evaluated in terms of structure–function correlation. Chitosan was incorporated in a form of microspheres (CH-Mic) or as the macromolecule unit (CH-m) of PCL-based polyurethane matrix (PU-2/3 PCL). The effect of chitosan on shape memory properties and transition temperature was evaluated. The PU-2/3 PCL CH-m composite demonstrated a lower transition temperature (Ttrans) of the shape recovery in comparison to neat PU-2/3 PCL matrix and PU-2/3 PCL CH-Mic composites. The composites with CH-m exhibited shape recovery immediately after reaching Ttrans whereas neat polymers and composites consisting of CH-Mic required five minutes pre-heating time before initialization of the shape recovery process.

Three-Dimensional-Printed Shape Memory Biomass Composites for Thermal-Responsive Devices.

In this study, unique three-dimensional (3D)-printed shape memory biomass composites were prepared by the melt blending and extrusion of polyurethane, polycaprolactone (PCL), and wood flour (WF) with adjustable contents. The addition content of PCL was used to adjust the shape memory transition temperature and improve the shape fixing rate of composites. The crystallization, thermal, mechanical, and shape memory properties of different composites were investigated. The results of X-ray diffraction and differential scanning calorimetry tests showed that the crystallization peak and melting temperature of different composites were not obviously changed. As the PCL content increased, the tensile strength of the composites decreased first and then increased, and the elongation at break gradually decreased. Thermal response shape memory test results showed that, when the PCL content was 30 wt.%, the composites had high shape recovery rate and fixed rate (both ∼100%). In addition, carbon black (CB) was added as a photothermal conversion material to the composite with a preferred ratio to achieve the photothermal response shape memory performance. With the addition of CB, the thermal conductivity of composites was improved. Under the same conditions, the thicker the 3D-printed specimens, the longer the specimen shape recovery time; the greater the light intensity, the shorter the specimen shape recovery time. Compared with the composite without CB, the flower model printed with the composites containing CB had a better photothermal response shape memory performance.

Tailoring new bisphenol a ethoxylated shape memory polyurethanes

AbstractShape memory polyurethanes (SMPUs) have generated great attention because of their unique properties. These properties are result of a particular molecular structure consisting of flexible molecular chains with low glass transition temperatures alternating with hard urethane segments. In this field, bisphenol A (BA) has been used for a long time as chain extender due to the good properties of the obtained SPMU materials. Nevertheless, the high toxicity of this compound has caused a high decrease on its use. For this reason, it has been selected a lower toxicity compound, bisphenol A ethoxylate (BAE). In this work, it is described a new SMPUs based on BAE and the influence of the hard segment on the thermo‐mechanical properties and shape memory capacity. For that, both the proportion of the components and the diisocyanate employed (2,4‐toluene diisocyanate (TDI), 4,4′‐methylene bis(phenylisocyanate) (MDI) or a TDI/MDI mixture) have been modified. Then, depending on the molecular architecture achieved, the polyurethanes present different properties, which were studied by different techniques, such as thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic‐mechanical thermal analysis (DMTA). It has been observed that glass transition temperature (Tg) increases as the hard phase content in the PU samples increases. In addition, Tg‐MDI > Tg‐MDI‐TDI > Tg‐TDI, so it is possible to control the Tg of the material, that is, shape memory transition temperature varying the diisocyanate. Finally, the shape memory capacity of the PUs was evaluated by thermo‐mechanical analysis (TMA). All the synthesized PUs have shown good shape memory effect with fixation ratios up to 80% and recovery ratios close to 100%.

Degradation properties of a biodegradable shape memory elastomer, poly(glycerol dodecanoate), for soft tissue repair.

Development of biodegradable shape memory elastomers (SMEs) is driven by the growing need for materials to address soft tissue pathology using a minimally invasive surgical approach. Composition, chain length and crosslinking of biocompatible polymers like PCL and PLA have been investigated to control mechanical properties, shape recovery and degradation rates. Depending on the primary mechanism of degradation, many of these polymers become considerably stiffer or softer resulting in mechanical properties that are inappropriate to support the regeneration of surrounding soft tissues. Additionally, concerns regarding degradation byproducts or residual organic solvents during synthesis accelerated interest in development of materials from bioavailable monomers. We previously developed a biodegradable SME, poly(glycerol dodecanoate) (PGD), using biologically relevant metabolites and controlled synthesis conditions to tune mechanical properties for soft tissue repair. In this study, we investigate the influence of crosslinking density on the mechanical and thermal properties of PGD during in vitro and in vivo degradation. Results suggest polymer degradation in vivo is predominantly driven by surface erosion, with no significant effects of initial crosslinking density on degradation time under the conditions investigated. Importantly, mechanical integrity is maintained during degradation. Additionally, shifts in melt transitions on thermograms indicate a potential shift in shape memory transition temperatures as the polymers degrade. These findings support the use of PGD for soft tissue repair and warrant further investigation towards tuning the molecular and macromolecular properties of the polymer to tailor degradation rates for specific clinical applications.

Electrospun PCL-based polyurethane/HA microfibers as drug carrier of dexamethasone with enhanced biodegradability and shape memory performances

Shape memory polymers (SMP) with better biodegradability and better stability have great potential applications in biomedical fields, such as the drug carriers or the tissue engineering scaffolds. In this study, poly(e-caprolactone)(PCL)-based polyurethane(PU) microfibers were fabricated with the containing of hydroxyapatite(HA) to enhance the biodegradability and to exhibit excellent shape memory performance. The composition, the morphology, the thermal stability, and the mechanical properties of the microfibers were characterized and detected using Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (1HNMR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC). and dynamic mechanical analysis (DMA), etc. And dexamethasone was selected as drug model to investigate the delivery and release behaviors of the carrier of the microfibers. It was revealed that HA enhanced the degradation rate of the shape memory polyurethane (SMPU) fibers, and the fibers could guarantee a sustained long time drug release. The detection on the shape memory performance found that, with the different addition amounts of HA, the composite microfibers of (SMPU) and HA exhibited the different shape memory transition temperature (Ttrans) values. And with the addition of 3 wt% of HA, the excellent shape recovery ratios of Rr (> 97%) and the shortest recovery time of ~ 6 s could be obtained. With further increase of the amounts of HA, the recovery force and the recovery time were reduced and prolonged, respectively. The obtained results proved that the biodegradable SMPU/HA composite microfibers have more valuable application prospects in biomedical fields.

Green and biomass-derived materials with controllable shape memory transition temperatures based on cross-linked Poly(l-malic acid)

An interesting biomass-derived shape memory polymer (SMP) based on cross-linked poly(l-malic acid) (PMA), with reconfigurable permanent shape and tunable shape memory transition temperature (Ttrans), was developed in this work. The amorphous cross-linked network was constructed in PMA using eco-friendly 1,8-octanediol as the cross-linker through a catalyst-free two-step way. The relations between crosslinking density (d) and glass transition temperature (Tg), as well as the mechanical strengths of as-obtained cross-linked PMA (CPMA) were then evaluated. Both the strengths and the Ttrans are easily tunable by controlling curing times. Moreover, the permanent shape of CPMA is reconfigurable due to additional crosslinking in the heat treatment process. It also shows good diversity of processing approaches. This work is the first report to achieve good strengths on the biodegradable PMA and to activate its shape memory effect, opening up a window of application of PMA as the smart material or even as the common plastics.

4D Printing of a Digital Shape Memory Polymer with Tunable High Performance.

Shape memory polymers (SMP) with 3D geometries and tunable shape-shifting behavior can open up new opportunities in intelligent devices. Achieving both simultaneously is difficult for conventional approaches. 4D printing allows fabrication of complex 3D SMP geometries that can change shapes (i.e., the fourth dimension is time), but tuning the shape memory response is challenging because of the printing constraints. Here, we report a material and process concept that allows digital light fabrication of SMP with fine control of not only the geometries but also the shape memory characteristics, within a printing time of 30 s. Digital light modulation allows spatio-temporal tuning of the material properties including shape memory transition temperature, rubbery modulus, and maximum elongation (up to 250%). Consequently, the process allows producing multiple-SMP within a single material construct using the same printing precursor. We demonstrate that this unique attribute is beneficial in constructing unusual shape-shifting 3D nano-photonic and electronic devices. The simplicity and versatility of our approach facilitates its future expansion into a wide range of geometrically complex devices with advanced functions.

A novel shape memory poly(ɛ-caprolactone)/hydroxyapatite nanoparticle networks for potential biomedical applications

A series of smart shape memory poly(ɛ-caprolactones) (PCL)/hydroxyapatite (HA) networks with different PCL arm lengths are designed and fabricated by the thiol-ene click reaction of thiol-modified HA particles with functional acrylate-terminated PCL. Compared with traditional physical blending of nanoparticles into polymer, this paper constructs a well-defined network architecture on the basis of the molecular level. Thermal and crystalline results indicate that the shape memory transition temperature (Ttrans) of the composites is correlated to the initial PCL diol molecular weight, the melting and crystallization temperature of the networks gradually increase with increasing PCL diol molecular weight. Meanwhile, the HA-PCL networks all exhibit good shape memory capability under thermal stimulus with good shape fixing ratio and recovery ratio irrespective of the molecular weight of PCL diol. An advantage of these network is that the intrinsic biocompatibility of HA nanoparticles as a netpoint for biodegradable PCL renders it a good prospects in the bone biomedical applications.

Thermal, optical, interfacial and mechanical properties of titanium dioxide/shape memory polyurethane nanocomposites

To further understand effects of titanium dioxide (TiO2) nanoparticles on thermal, optical, microstructural, interfacial and mechanical properties of shape memory polyurethane (SMPU), TiO2/SMPU nanocomposites with different TiO2 contents were synthesized. Then various properties of TiO2/SMPU nanocomposites were characterized. Results indicate that the melting temperature of soft segments in SMPU can be used as the shape memory transition temperature of TiO2/SMPU nanocomposites. TiO2 nanoparticles are almost filled in SMPU pores to form compact skeleton structures in TiO2/SMPU when the TiO2 content is 3% by weight. Further, the used TiO2 is rutile phase, and lowers the SMPU crystallinity. The suitable TiO2 content can increase the absorptivity to UV light and enhance the reflectivity to visible light of TiO2/SMPU nanocomposites, lowering its photo-aging properties and prolonging its service life. Also, TiO2/SMPU shows a higher scattering intensity and a faster decreasing trend than SMPU due to the larger electron density difference between TiO2 and SMPU. The microphase separation and ordered structures in SMPU are decreased due to added TiO2 nanoparticles. There are electron density fluctuations at the interfaces between hard and soft phases in SMPU, and between SMPU and TiO2 nanoparticles. Finally, the prepared TiO2/SMPU nanocomposites have better shape memory effects and tensile properties when TiO2 content of 3% is proposed to synthesize TiO2/SMPU nanocomposites for practical engineering applications.

Mechanical properties and shape memory effect of thermal-responsive polymer based on PVA

In this study, the effect of content of glutaraldehyde (GA) on the shape memory behavior of a shape memory polymer based on polyvinyl alcohol chemically cross-linked with GA was investigated. Thermal-responsive shape memory composites with three different GA levels, GA-PVA (3 wt%, 5 wt%, 7 wt%), were prepared by particle melting, mold forming and freeze–drying technique. The mechanical properties, thermal properties and shape memory behavior were measured by differential scanning calorimeter, physical bending test and cyclic thermo-mechanical test. The addition of GA to PVA led to a steady shape memory transition temperature and an improved mechanical compressive strength. The composite with 5 wt% of GA exhibited the best shape recoverability. Further increase in the crosslinking agent content of GA would reduce the recovery force and prolong the recovery time due to restriction in the movement of the soft PVA chain segments. These results provide important information for the study on materials in 4D printing.

Post-Crosslinked Polyurethanes with Excellent Shape Memory Property.

Shape-memory polymers are highly desirable in implant biomaterials for minimally invasive surgical procedures. However, most of them lack suitable transition temperature, mechanical properties, and biodegradability. Here, a series of shape-memory polyurethanes are synthesized by postcrosslinking in hard-segment domains using a flexible crosslinker. The materials used are all nontoxic and biodegradable. Through postcrosslinking of unsaturated linear polyurethanes with flexible and biodegradable crosslinker, the crosslinked polyurethanes (CPUs) show good mechanical properties, excellent shape-memory property, and repeatability. The post-crosslinking structure and shape-memory mechanism of CPUs are investigated by Fourier transform infrared spectroscopy, differential scanning calorimetry, and dynamic mechanical analysis tests. The crosslinker endows the fixed phase enough crosslinking and inserts in the hard segments to give the fixed phase certain elasticity. The elastic hard segments make them form more hydrogen bonds with soft segments during shape deformation. The low-molecular-weight poly (ε-caprolactone) offers the samples a shape-memory transition temperature at around 37 °C, which is suitable for implant devices in vivo. This work expands CPUs with an elastic crosslinking structure as potential candidates for implant biomaterials. Since the post-crosslinking polymerization is facile, it can be convenient for industrial production.

Catalyst-Free Thermoset Polyurethane with Permanent Shape Reconfigurability and Highly Tunable Triple-Shape Memory Performance.

Thermoset shape memory polymer (SMP) with dynamic covalent bonds in the network is a new class of SMPs for which the permanent shape can be reconfigured via topological rearrangement (plasticity). Catalyzed transcarbamoylation has recently been established as an effective exchange reaction for plasticity in cross-linked polyurethane networks. However, ensuring the plasticity severely constrains the network design which adversely affects the ability to tune other classical shape memory properties for practical applications. Facing this new challenge, we design an amorphous polyurethane system for which the cross-linking density can be adjusted in a wide range. We discovered that the use of an aromatic diisocyanate in the synthesis of the polyurethanes facilitates achieving plasticity without requiring any catalyst. The overall network design leads to tunable recovery stress and shape memory transition temperatures without sacrificing the plasticity. The versatility of our polyurethane SMP is further reflected in its triple-shape memory performance. We anticipate that our tunable polyurethanes will benefit a variety of potential SMP device applications.

Shape memory effect of thermal-responsive nano-hydroxyapatite reinforced poly-d-l-lactide composites with porous structure

In this study, the effect of the inclusion of nano-hydroxyapatite (nano-HAp) and the porous structure on the shape memory behavior of poly-d-l-lactide (PDLLA) composites was investigated. Porous, thermal-responsive shape memory composites, PDLLA/nano-HAp, were prepared by a newly developed polymer coagulation, particulate leaching, and cold compression moulding technique. The inclusion of nano-HAp to PDLLA led to a higher shape memory transition temperature, improved mechanical strength and larger recovery force as compared with the neat PDLLA. The nanocomposite with 10 wt% of nano-HAp exhibited the best shape fixity and recoverability with the shortest recovery time. Further increase in the filler content of nano-HAp would reduce the recovery force and prolong the recovery time due to restriction in the movement of the amorphous PDLLA chain segments. The morphologies, thermal properties and shape memory behavior were measured by scanning electron microscopy, thermal gravimetric analyzer, differential scanning calorimeter, dynamic mechanical analysis, physical bending test and cyclic thermo-mechanical test. These results provide important information for designing biomedical devices using this novel type of bioabsorbable shape memory polymer composite.