Porous shape memory polymers: Design and applications

Porous shape memory polymers (SMPs) exhibit geometric and volumetric shape change when actuated by an external stimulus and can be fabricated as foams, scaffolds, meshes, and other polymeric substrates that possess porous three-dimensional macrostructures. These materials have applications in multiple industries such as textiles, biomedical devices, tissue engineering, and aerospace. This review article examines recent developments in porous SMPs, with a focus on fabrication methods, methods of characterization, modes of actuation, and applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1300–1318     

Recent Progress in Shape Memory Polymers for Biomedical Applications

Shape memory polymers (SMPs) as one type of the most important smart materials have attracted increasing attention due to their promising application in the field of biomedicine,textiles,aerospace et al.Following a brief intoduction of the conception and classification of SMPs,this review is focused on the progress of shape memory polymers for biomedical applications.The progress includes the early researches based on thermo-induced SMPs,the improvement of the stimulus,the development of shape recovery ways and the expansion of the applications in biomedical field.In addition,future perspectives of SMPs in the field of biomedicine are also discussed.     

A novel fractional viscoelastic constitutive model for shape memory polymers

Shape memory polymers (SMPs) are a class of smart materials which can recover from a deformed shape to their original shape by a certain external stimulus. To predict the deformation behaviors of SMPs, different constitutive models have been developed in the last few years. However, most of the constitutive models need many parameters to be determined by specific experiments and complex calibration processes. This drawback has limited their application in promoting the development of SMPs. Thus, it is imperative to develop a new constitutive model which is not only accurate, but also relatively simple. In our work, a novel fractional viscoelastic constitutive model coupling with time‐temperature superposition principle is first proposed for SMPs. Then, frequency sweep and temperature sweep experiments are conducted to determine the parameters of the model. Finally, the shape memory free recovery experiments are carried out to validate the predictive capability of the developed model. By comparing the predicted results with experimental data, we find that though our model has only eleven parameters in total, it could capture the thermomechanical behaviors of SMPs in very good agreement with experimental results. We hope the proposed new model provide researchers with guidelines in designing and optimizing of SMP applications. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1125–1134     

PCL-based Shape Memory Polymers with Variable PDMS Soft Segment Lengths

Thermoresponsive shape memory polymers (SMPs) are stimuli-responsive materials that return to their permanent shape from a temporary shape in response to heating. The design of new SMPs which obtain a broader range of properties including mechanical behavior is critical to realize their potential in biomedical as well as industrial and aerospace applications. To tailor the properties of SMPs, "AB networks" comprised of two distinct polymer components have been investigated but are overwhelmingly limited to those in which both components are organic. In this present work, we prepared inorganic-organic SMPs comprised of inorganic polydimethyl-siloxane (PDMS) segments of varying lengths and organic poly(ε-caprolactone) (PCL) segments. PDMS has a particularly low T(g) (-125 °C) which makes it a particularly effective soft segment to tailor the mechanical properties of PCL-based SMPs. The SMPs were prepared via the rapid photocure of solutions of diacrylated PCL(40)-block-PDMS(m)-block-PCL(40) macromers (m = 20, 37, 66 and 130). The resulting inorganic-organic SMP networks exhibited excellent shape fixity and recovery. By changing the PDMS segment length, the thermal, mechanical, and surface properties were systematically altered.     

4D Printing of Shape Memory‐Based Personalized Endoluminal Medical Devices

The convergence of additive manufacturing and shape‐morphing materials is promising for the advancement of personalized medical devices. The capability to transform 3D objects from one shape to another, right off the print bed, is known as 4D printing. Shape memory thermosets can be tailored to have a range of thermomechanical properties favorable to medical devices, but processing them is a challenge because they are insoluble and do not flow at any temperature. This study presents here a strategy to capitalize on a series of medical imaging modalities to construct a printable shape memory endoluminal device, exemplified by a tracheal stent. A methacrylated polycaprolactone precursor with a molecular weight of 10 000 g mol⁻¹ is printed with a UV‐LED stereolithography printer based on anatomical data. This approach converges with the zeitgeist of personalized medicine and it is anticipated that it will broadly expand the application of shape memory‐exhibiting biomedical devices to myriad clinical indications.

     

Fabrication of well‐defined shape memory graft polymers derived from biomass: An insight into the effect of side chain architecture on shape memory properties

Fully sustainable shape memory polymers (SMPs) derived from ethyl cellulose (EC, derived from cellulose), tetrahydrofurfuryl methacrylate (THFMA, derived from furfural), and lauryl methacrylate (LMA, derived from fatty acids) were prepared via “grafting from” atom transfer radical polymer (ATRP). The “grafting from” ATRP strategy allows to fabricate SMPs with EC as a backbone, and LMA and THFMA copolymer as a side chain. By utilizing the one‐pot and sequential monomer addition approach, two types of SMPs with random/semi‐block side chain architectures were obtained, respectively. Random/semi‐block side chain architecture of SMPs was confirmed by DSC, DMA, SAXS, and TEM. The presence of microphase separation in the SMPs with semi‐block side chain architecture provided two distinct thermal transitions, which was needed for triple‐shape memory behavior. Shape memory study showed that SMPs with semi‐block side chain architecture exhibited excellent triple‐shape memory property, and also had higher shape recovery speed and shape recovery ratio than those with random side chain architecture. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 1711–1720     

Reversible shape-shifting in polymeric materials

In recent years, significant progress has been made in polymeric materials, which alter shape upon external stimuli, suggesting potential applications in robotics, biomedical engineering, and optical devices. These stimuli-responsive materials may be categorized into two classes: (i) shape-changing materials in which a specific type of shape-shifting is encoded in the original material structure and (ii) shape-memory materials, which do not possess any predetermined shape-shifting as prepared, yet allow programming of complex shape transformations on demand. While shape alterations in shape-changing materials are intrinsically reversible, shape memory is usually a one-way transformation from a metastable (programmed) to an equilibrium (original) state. Recently, different principles for both one-way reversible and two-way reversible shape memory have been developed. These offer a powerful combination of reversibility and programmability, which significantly expands the range of potential applications. The goal of this review is to highlight recent developments in reversible shape-shifting by introducing novel mechanisms, materials, and applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1365–1380     

Hydrogel‐coated polyurethane/urea shape memory polymer foams

Shape memory polymers (SMPs) are a class of responsive polymers that have attracted attention in designing biomedical devices because of their potential to improve minimally invasive surgeries. Use of porous SMPs in vascular grafts has been proposed because porosity aids in transfer of fluids through the graft and growth of vascular tissue. However, porosity also allows blood to leak through grafts so preclotting the materials is necessary. Here hydrogels have been synthesized from acrylic acid and N‐hydroxyethyl acrylamide and coated around a porous SMP produced from lactose functionalized polyurea‐urethanes. The biocompatibility of the polymers used to prepare the cross‐linked shape memory material is demonstrated using an in vitro cell assay. As expected, the hydrogel coating enhanced fluid uptake abilities without hindering the shape memory properties. These results indicate that hydrogels can be used in porous SMP materials without inhibiting the shape recovery of the material. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1389–1395     

Body temperature triggered shape-memory polymers with high elastic energy storage capacity

Shape-memory polymers (SMPs) that respond near body temperature are attracting broad interest, especially in the biomedical fields. In this study, the triggering temperature of poly(caprolactone) SMP networks is precisely adjusted by inclusion of non-crystallizable molecular linkers and by variation of prepolymer molecular weight. Longer, non-crystalline linkers and lower molecular weight prepolymers interfere with crystallization, lowering the transition temperature. Networks are prepared with crystallization temperatures that are beneath the human body temperature and yet are above room temperature. Upon cooling such amorphous networks to room temperature, crystallization is sluggish. There, elastomers can be easily strained by several hundred-percent to induce crystallization, thereby fixing strained states. If subsequently heated, programmed SMPs can release significant amounts of stored strain energy (∼3 MJ/m³). SMPs that combine elastic energy storage and exhibit triggering temperatures near the human body temperature could benefit emerging applications in the biomedical space. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1397–1404     

Physical Microfabrication of Shape‐Memory Polymer Systems via Bicomponent Fiber Spinning

As emerging technologies continue to require diverse materials capable of exhibiting tunable stimuli‐responsiveness, shape‐memory materials are of considerable significance because they can change size and/or shape in controllable fashion upon environmental stimulation. Of particular interest, shape‐memory polymers (SMPs) have secured a central role in the ongoing development of relatively lightweight and remotely deployable devices that can be further designed with specific surface properties. In the case of thermally‐activated SMPs, two functional chemical species must be present to provide (i) an elastic network capable of restoring the SMP to a previous strain state and (ii) switching elements that either lock‐in or release a temporary strain at a well‐defined thermal transition. While these species are chemically combined into a single macromolecule in most commercially available SMPs, this work establishes that, even though they are physically separated across one or more polymer/polymer interfaces, their shape‐memory properties are retained in melt‐spun bicomponent fibers. In the present study, we investigate the effects of fiber composition and cross‐sectional geometry on both conventional and cold‐draw shape memory, and report surprisingly high levels of strain fixity and recovery that generally improve upon strain cycling.

     

A review of shape memory polymers bearing reversible binding groups

Shape memory polymers (SMP) can be deformed to a stable, temporary shape and recovered to their original shape by applying a stimulus. These networks rely on the presence of two types of net points to establish their permanent and temporary shapes. Classical strategies to stabilize temporary shapes rely on cooling below T_g/T_m where macromolecules become pinned in a stressed state. Recovery of the SMP usually involves heating to above the transition temperature where the permanent shape is remembered. Employing reversible binding groups (RBGs) in SMPs has emerged as an alternative strategy for stabilizing temporary shapes or imparting recyclability of the permanent shape. The use of dynamic chemistry often engenders additional functionality such as intrinsic self-healing characteristics or alternative shape recovery triggering strategies. SMPs bearing both supramolecular and covalent RBGs will be reviewed with an emphasis on hydrogen bonding, ionic interactions, metal–ligand coordination, and dynamic covalent exchange and addition reactions. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1340–1364     

Epoxy shape memory polymer (SMP): Material preparation,uniaxial tensile tests and dynamic mechanical analysis

The utilization of epoxy shape memory polymers (SMPs) as engineering materials for deployable structures has attracted considerable attention due to their excellent thermo-mechanical endurance and satisfactory processability. Knowledge of static and dynamic mechanical properties is essential for analyzing structural behavior and recovery, especially for new epoxy SMPs. In this paper, a new epoxy SMP was prepared with epoxy and aromatic amine curing agent. Uniaxial tensile tests and digital image correlation were used to obtain static mechanical properties. Dynamic mechanical analysis was carried out to evaluate glass transition temperatures that corresponded to the heat in the recovery process.It was found that elastic modulus, Poisson’s ratio and shear modulus are 1413 MPa, 0.35 and 591 MPa, respectively. The beginning of glass transition temperature of 37.4 °C could be effectively achieved by electrical heaters, validating the shape memory properties of epoxy SMPs. In general, this study could provide useful observations and basic mechanical properties of epoxy SMPs.     

Microstructural design for enhanced mechanical property and shape memory behavior of polyurethane nanocomposites: Role of carbon nanotube,montmorillonite, and their hybrid fillers

The recent rapid development of technology has demanded smart materials with tailoring a bridge between macro properties and sophisticated micro and nano characteristic. Principally, shape memory polymers (SMPs) will come to play as an indispensable part of numerous aspects of human activity. Nevertheless, the low mechanical strength and thermal conductivity of SMPs have primarily restricted their applications. To impart shape memory behaviour and mechanical properties, we fabricated a series of composites by a feasible and commercial melt-mixing method. Thus, a series of fast heat-actuated shape memory polymer composite with greatly enhanced stretch-ability, mechanical stiffness, dynamic-modulus, rheological qualities, recovery and fixity ratio was prepared by incorporating multi-walled carbon nanotubes (CNT), montmorillonite (MMT) and CNT:MMT hybrid into thermoplastic polyurethane (TPU). Noteworthy, CNT-based specimens exhibited superior mechanical properties than those of MMT-based samples, and interestingly, the hybrid composites featured a synergistic effect due to the sacrificial role of MMT nanoplatelets for adjusting the dispersion of CNT nanotubes. Microstructural observations indicated that the crystallization percentages of the composites were generally higher than that of pristine TPU; therefore, the shape-memory performance of the specimens improved notably in the case of the hybrid composites owing to creating more interfacial zone with CNT:MMT nanoparticles as compared to other simple composites. This study proved that the simultaneous incorporation of CNT and MMT nanoparticles not only granted outstanding mechanical properties, but also improved the overall shape memory behaviour of the composites by systematical localization of the nanoparticles without any functionalization or modification.     

Synthesis and characterization of hydrophilic polyester‐PEO networks with shape‐memory properties

Shape‐memory polyester‐polyethylene oxide networks are synthesized through photopolymerization of hydrophobic poly[(D,L‐lactide)‐co‐glycolide]tetraacrylate (PLGATA) and hydrophilic poly(ethylene glycol) dimethacrylate (PEGDMA). The materials can recover their original shape either quickly by heating stimulus or slowly upon immersion in water. The wettability, mechanical properties, and transition temperature of the polymer networks could be conveniently adjusted by variation of the compositions of PLGATA and PEGDMA. The hydrophilicity of PEGDMA could prospectively improve blood compatibility of polymer networks. They offer a high potential for biomedical applications such as smart implants, drug delivery systems, or medical devices. Copyright © 2010 John Wiley & Sons, Ltd.     

Reversible plasticity shape memory polymers: Key factors and applications

Reversible plasticity shape memory (RPSM) polymers have been emerging as new smart materials with distinctions compared with conventional SMPs, such as easier shaping programming, stronger recovery stress, and higher recovery strain. For purposeful control of the structure, and therefore the physical and mechanical properties, a full understanding of the deformation habits of such materials under different conditions is essential. This perspective provides the context as to how the deformation temperature and fixing conditions influence the fixity and recovery behavior of RPSM polymers and what are the optimized conditions for RPSM. We hope that this will afford useful information for fabricating RPSM polymers with better memory properties and promote the technical development of new design methods of such materials for advanced applications © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1295–1299     

Memory chromic polyurethane with tetraphenylethylene

This article reports a chromic polymer, which is responsive to its shape memory properties and has both the behavior of shape memory polymers and chromic materials. We employed a strategy to fabricate such a smart material, which represents a new principle for making chromic materials. This material is made of shape memory polyurethane with tetraphenylethylene units (0.1 wt %) covalently connected to the soft‐segments (PCL, M_w = 4000). The material displays biocompatibility, shape fixity of 88–93%, and almost 100% shape recovery and has reversible mechanochromic, solvatochromic, and thermochromic shape memory effect. The memory chromism represented by the reversible change of emission intensity shows negative correlation with shape fixity, temperature, and existence of solvent. It may be explained that when the soft segments are molten or dissolved in solvent, the shape recovery switch is open, the AIE units are free from crystal binding and can migrate easily to larger areas, thus the AIE units/particles are far apart from each other and the barrier for rotation of phenyl groups is reduced, which lead to the reduction of emission intensity, appeared by no colors or pale colors, and vice versa. Since the switch is a fundamental structural character of SMPs, the shape memory properties have led to the chromism and we call this memory chromic. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014 , 52, 104–110     

Shape memory effect and recovery stress property of carbon nanotube/waterborne epoxy nanocomposites investigated via TMA

Shape memory polymers (SMPs) have received great attention and scientific interest in widespread technological development during last few decades. Besides the development of novel SMPs, various techniques have been practiced for characterization of shape memory effect (SME) of SMPs. In this study, the shape memory effect and recovery stress property of the carbon nanotube (CNT)/waterborne epoxy (WEP) nanocomposites below and above the glass transition temperature (T_g) of the nanocomposites and under isostrain and isostress were systematically investigated via thermal mechanical analysis (TMA), respectively. The experimental results showed that the nanocomposites exhibit excellent shape memory effect. The shape memory fixity and recovery ratios were approximately 100% even below glass transition temperature (T_g). A remarkable point is that the strain of the nanocomposites suddenly increased with the temperature decreasing in a certain period of the heating-cooling cycles under isostress condition and the strain increment increased with temperature in general. Especially at low temperature, the recovery stress was very sensitive to temperature under isostrain condition of ±0.25 °C temperature with differential of 25.5 °C developed pressure difference of 0.20 MPa. Moreover, TMA is a practical method for quantifying the SME and recovery stress properties of SMPs and their composites.     

A unified modeling approach for amorphous shape memory polymers and shape memory polymer based syntactic foam

Shape memory polymers (SMPs) and shape memory polymer composites have drawn considerable attention in recent years for their shape memory effects. A unified modeling approach is proposed to describe thermomechanical behaviors and shape memory effects of thermally activated amorphous SMPs and SMP‐based syntactic foam by using the generalized finite deformation multiple relaxation viscoelastic theory coupled with time–temperature superposition property. In this paper, the thermoviscoelastic parameters are determined from a single dynamic mechanical analysis temperature sweep at a constant frequency. The relaxation time strongly depends on the temperature and the variation follows the time–temperature superposition principle. The horizontal shift factor can be obtained by the Williams–Landel–Ferry equation at temperatures above or close to the reference temperature (T_r), and by the Arrhenius equation at temperatures below T_r. As the Arruda–Boyce eight‐chain model captures the hyperelastic behavior of the material up to large deformation, it is used here to describe partial material behaviors. The thermal expansion coefficient of the material is regarded as temperature dependent. Comparisons between the model results and the thermomechanical experiments presented in the literature show an acceptable agreement. Copyright © 2016 John Wiley & Sons, Ltd.     

形状记忆聚合物热力学本构方程

借鉴聚合物结晶学相关理论,建立了一个针对该类材料的新的热力学本构关系,通过与已有实验结果相比较,发现理论预测值与实验吻合较好.     

Stability and deterioration of a shape memory polymer fabric composite under thermomechanical stress

Fibres and fabrics are often used to reinforce shape memory polymers (SMPs) to improve their mechanical strength and properties, and such composites have been widely used in engineering. However incorporation of fibres and fabrics in SMPs is often accompanied with the deterioration of thermomechanical properties and shape memory effect. The thermomechanical properties and deterioration mechanisms of a shape memory polymer composite (SMPC) under repeated mechanical stress were investigated. Up to 100% extension, the SMPCs showed good shape memory effect with excellent shape recovery ratio, recovery stress and mechanical properties; while beyond that the recovery ratio and recovery stress of the composites deteriorated rapidly due to the significant delamination and debonding of fibres and fabrics from the SMP resin and accumulation of broken fibres.