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.     

Orthogonal Dual‐Triggered Shape‐Memory DNA‐Based Hydrogels

DNA‐based shape‐memory hydrogels revealing switchable shape recovery in the presence of two orthogonal triggers are described. In one system, a shaped DNA/acrylamide hydrogel is stabilized by duplex nucleic acids and pH‐responsive cytosine‐rich, i‐motif, bridges. Separation of the i‐motif bridges at pH 7.4 transforms the hydrogel into a quasi‐liquid, shapeless state, that includes the duplex bridges as permanent shape‐memory elements. Subjecting the quasi‐liquid state to pH 5.0 or Ag⁺ ions recovers the hydrogel shape, due to the stabilization of the hydrogel by i‐motif or C‐Ag⁺‐C bridged i‐motif. The cysteamine‐induced transformation of the duplex/C‐Ag⁺‐C bridged i‐motif hydrogel into a quasi‐liquid shapeless state results in the recovery of the shaped hydrogel in the presence of H⁺ or Ag⁺ ions as triggers. In a second system, a shaped DNA/acrylamide hydrogel is generated by DNA duplexes and bridging Pb²⁺ or Sr²⁺ ions‐stabilized G‐quadruplex subunits. Subjecting the shaped hydrogel to the DOTA or KP ligands eliminates the Pb²⁺ or Sr²⁺ ions from the respective hydrogels, leading to shapeless, memory‐containing, quasi‐liquid states that restore the original shapes with Pb²⁺ or Sr²⁺ ions.     

High‐Strain Shape Memory Behavior of PLA–PEG Multiblock Copolymers and Its Microstructural Origin

Shape memory properties of two thermoplastic multiblock copolymers composed of poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) having different PEG‐segment lengths of 6 and 11 kDa were studied. The performance as a shape memory polymer at high strain level (600%) and its interrelations with shape‐programming conditions, molecular orientation, and microstructural changes are elucidated. A significant contribution of strain‐induced crystallization of PLA segments to the improvement of temporary shape fixation was evidenced upon increasing draw ratio and/or shape‐holding duration as well as programming temperature (within certain range) without largely sacrificing the shape recoverability. Series of microstructural characterizations reveal the occurrence of fibrillar‐to‐lamellar transformation upon shape recovery (at 60 °C) of the samples programmed at 40 °C, generating shish–kebab crystalline morphology. Such phenomenon is responsible for the high‐strain shape memory effect of these materials. The unprecedented formation of shish–kebab structure at such relatively low temperature (instead of the melting temperature range) in solid state observed in these copolymers as well as their high‐strain shape memory functionality would bestow the promising future for their practicability in diverse areas. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 241–256     

pH‐ and Sugar‐Induced Shape Memory Hydrogel Based on Reversible Phenylboronic Acid–Diol Ester Bonds

A simple strategy is provided to construct a novel pH‐ and sugar‐induced shape memory hydrogel based on dynamic phenylboronic acid (PBA)–diol interactions formed by PBA‐modified sodium alginate (Alg‐PBA) and poly(vinyl alcohol) (PVA). The dynamic PBA–diol ester bonds serve as temporary cross‐links and stabilize the deformed shape of the hydrogel. The disassociation of the PBA–diol ester bonds is explored in acidic conditions and aqueous solutions of glucose and fructose, which endow the hydrogel with shape memory performances.

     

A Shape‐Memory DNA‐Based Hydrogel Exhibiting Two Internal Memories

The synthesis of a shape‐memory acrylamide–DNA hydrogel that includes two internal memories is introduced. The hydrogel is stabilized, at pH 7.0, by two different pH‐responsive oligonucleotide crosslinking units. At pH 10.0, one of the T‐A?T triplex DNA bridging units is dissociated, resulting in the dissociation of the hydrogel into a shapeless quasi‐liquid state that includes the other oligonucleotide bridges as internal memory. Similarly, at pH 5.0, the second type of bridges is separated, through the formation of C‐G?C⁺ triplex units to yield the shapeless quasi‐liquid state that includes the other oligonucleotide bridges as internal memory. By reversible pH triggering of the hydrogel between the values 10.0?7.0?5.0, the two internal memories cycle the material across shaped hydrogel and shapeless quasi‐liquid states. The two memories enable the pH‐dictated formation of two different hydrogel structures.     

Shape memory ionomers

This review is focused on the use of ionomers in shape memory polymers. Ionomers are polymers that contain less than ∼15% ionic groups. The incompatibility between the ion-pairs and the polymer backbone drives microphase separation producing dispersed ionic aggregates, which can physically crosslink the polymer. Shape memory polymers are responsive materials that can be deformed to program a temporary shape and then recovered on application of an external stimulus. Through the review of the main types of ionomers used in shape memory polymers, polyurethanes and polyester ionomers, polyolefin and polyaromatic ionomers, and perfluorosulfonic acid ionomers (i.e., Nafion^®) it will be shown that ionomers can produce robust thermoplastic shape memory polymers and in many cases impart unique properties which allow advanced shape memory materials to be obtained including antibacterial, high temperature, and multishape memory polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1389–1396     

Temperature Responsive Shape‐Memory Scaffolds with Circumferentially Aligned Nanofibers for Guiding Smooth Muscle Cell Behavior

Structural simulation of the smooth muscle layer plays an important role in tissue engineering of blood vessels for the replacement of damaged arteries. However, it is difficult to construct small‐diameter tubular scaffolds to homogenously locate and align smooth muscle cells (SMCs). In this work, novel temperature responsive shape‐memory scaffolds are designed for SMC culturing. The scaffolds are composed of an outer layer of poly(lactide–glycolide–trimethylene carbonate) (PLGATMC) for programming the deformation from planar to small‐diameter tubular shape and an inner layer of aligned nanofibrous membrane of poly(lactide–glycolide)/chitosan (PLGA/CS) to regulate cell adhesion, proliferation, and morphology. The SMC behaviors and functions are dependent on the PLGA/CS ratios of membranes, and the scaffold with PLGA/CS 7:3 membrane exhibits the most suitable ability to regulate SMC behavior. The PLGA/CS@PLGATMC scaffold can be deformed into a temporary planar at 20 °C for convenient seeding and attachment of SMCs and then immediately self‐rolled into 3D tube at 37 °C. The proposed strategy offers a practical approach for the development of small‐diameter vascular scaffolds from 2D planar into 3D tubular shape by self‐rolling.     

Cover Picture: Angew. Chem. Int. Ed. 12/2002

The cover picture shows the thermally induced shape‐memory effect for a covalently cross‐linked polymer network. The polymer in its temporary shape (cube, picture on top) is heated from room temperature up to 70°C. Within 60 seconds the sample recovers its memorized, permanent shape of a nearly planar foil (picture on top left). The visual change of the material from opaque to transparent is caused by the melting of crystallites of the switching segments. The scheme in the center of the picture illustrates the molecular mechanism of the shape‐memory effect. The shown polymer network, which is synthesized from poly(ε‐caprolactone)dimethacrylate as macromonomer, is one of the first polymer systems that have specifically been developed for applications in the biomedical field. The net points (black) determine the permanent shape while the crystallites (blue) stabilize the temporary shape. More on the current state and the potential of this technology can be found in the review by A. Lendlein and S. Kelch on p. 2034 ff.     

A hydrogel‐forming liquid crystalline elastomer exhibiting soft shape memory

Research in the field of liquid crystalline polymers has recently witnessed the introduction of liquid crystalline hydrogels. Here, we report the synthesis and characterization of a new liquid crystalline network featuring elastomeric softness, water‐swelling and shape memory characteristics. By comparing with a nonmesogenic network prepared using the same procedure, we reveal structure–property relationships of the liquid crystalline and crystalline polymer networks. Wide angle and small angle X‐ray scattering studies were used to examine the liquid crystalline ordering in both dry and hydrated states. Such ordering was found to be related to the observed shape memory and actuation (two‐way shape memory) properties and these phenomena are highlighted with demonstrations of shape change in response to heat and water stimuli. This study provides insight into the mechanisms affecting the shape evolution of water activated anisotropic liquid crystalline hydrogels and enables the future design of materials or devices for a variety of applications such as biomaterials interacting with body fluids in a hydrated state. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 38–52     

Thermo‐responsive shape memory behavior of poly(styrene‐b‐isoprene‐b‐styrene)/ethylene‐1‐octene copolymer thermoplastic elastomer blends

Thermally‐triggered shape memory polymers (SMPs) are smart materials, which are capable of changing their shapes when they are exposed a heat stimulant. Blending semi‐crystalline and elastomeric polymers is an easy and low‐cost way to obtain thermo‐responsive SMPs. In this work, novel poly(ethylene‐co‐1‐octene) (PEO) and poly(styrene‐b‐isoprene‐b‐styrene) (SIS) thermoplastic elastomer blends were prepared via melt blending method. The morphological, mechanical, rheological properties and shape memory behaviours of the blends were investigated in detail. In morphological analysis, co‐continuous morphology was found for 50 wt% PEO/50 wt% SIS and 60 wt% PEO/40 wt% SIS (60PEO/40SIS) blends. The shape memory analysis performing by dynamic mechanical analyzer showed that the 60PEO/40SIS blend also exhibited the optimum shape memory performance with 95.74% shape fixing and 98.98% shape recovery. Qualitatively shape memory analysis in hot‐water pointed out that the amount of semi‐crystalline PEO promotes shape fixing ability of the blends whereas SIS content enhances shape recovery capability. Although the SIS and PEO are immiscible polymers, the blends of them were exhibited good elastomeric properties with regard to tensile strength, toughness, and elongation at break.     

Quadruple‐shape‐memory effect of TPI/LDPE/HDPE composites

Shape‐memory polymers are important smart materials with potential applications in smart textiles, medical devices, and sensors. We prepared trans‐1,4‐polyisoprene, low‐density polyethylene (LDPE), and high‐density polyethylene (HDPE) shape‐memory composites using a simple mechanical blend method. The mechanical, thermal, and shape‐memory properties of the composites were studied. Our results showed that the shape‐memory composites could memorize 3 temporary shapes, as revealed by the presence of broad melting transition peaks in the differential scanning calorimetry curves. In the trans‐1,4‐polyisoprene/LDPE/HDPE composites, the cross‐linked network and the crystallization of the LDPE and HDPE portions can serve as fixed domains, and all crystallizations can act as reversible domains. We proposed a schematic diagram to explain the vital role of the cross‐linked network and the crystallization in the shape‐memory process.     

Influence of programming strain rates on the shape‐memory performance of semicrystalline multiblock copolymers

Multiblock copolymers named PCL‐PIBMD consisting of crystallizable poly(ε‐caprolactone) segments and crystallizable poly[oligo(3S‐iso‐butylmorpholine‐2,5‐dione)] segments coupled by trimethyl hexamethylene diisocyanate provide a versatile molecular architecture for achieving shape‐memory effects (SMEs) in polymers. The mechanical properties as well as the SME performance of PCL‐PIBMD can be tailored by the variation of physical parameters during programming such as deformation strain or applied temperature protocols. In this study, we explored the influence of applying different strain rates during programming on the resulting nanostructure of PCL‐PIBMD. Programming was conducted at 50 °C by elongation to ε_m = 50% with strain rates of 1 or 10 or 50 mm min^?1. The nanostructural changes were visualized by atomic force microscopy (AFM) measurements and investigated by in situ wide and small angle X‐ray scattering experiments. With increasing the strain rate, a higher degree of orientation was observed in the amorphous domains. Simultaneously the strain‐induced formation of new PIBMD crystals as well as the fragmentation of existing large PIBMD crystals occurred. The observed differences in shape fixity ratio and recovery stress of samples deformed with various strain rates can be attributed to their different nanostructures. The achieved findings can be relevant parameters for programming the shape‐memory polymers with designed recovery forces. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1935–1943     

Shape Memory Properties and Enzymatic Degradability of Poly(ε‐caprolactone)‐Based Polyurethane Urea Containing Phenylalanine‐Derived Chain Extender

Biodegradable shape memory polymers are promising biomaterials for minimally invasive surgical procedures. Herein, a series of linear biodegradable shape memory poly(ε‐caprolactone) (PCL)‐based polyurethane ureas (PUUs) containing a novel phenylalanine‐derived chain extender is synthesized. The phenylalanine‐derived chain extender, phenylalanine‐hexamethylenediamine‐phenylalanine (PHP), contains two chymotrypsin cleaving sites to enhance the enzymatic degradation of PUUs. The degradation rate, the crystallinity, and mechanical properties of PUUs are tailored by the content of PHP. Meanwhile, semicrystalline PCL is not only hydrolytically degradable but also vital for shape memory. Good shape memory ability under body temperature is achieved for PUUs due to the strong interactions in hard segments for permanent crosslinking and the crystallization‐melt transition of PCL to switch temporary shape. The PUUs would have a great potential in application as implanting stent.     

Soft bacterial polyester‐based shape memory nanocomposites featuring reconfigurable nanostructure

In this work, a novel soft shape memory polymer nanocomposite derived from a bacterial medium‐chain‐length polyhydroxyalkanoate, poly(3‐hydroxyoctanoate‐co‐3‐hydroxyundecenoate) (PHOU), used to form a covalent network grafted with polyhedral oligomeric silsesquioxane (POSS), a crystallizable inorganic–organic hybrid nanofiller, was prepared. The PHOU–POSS nanocomposite, PHOU–POSSw‐net [w (= POSS content, wt %) = 0, 20, 25, 30, and 38], is a completely amorphous elastomer (w ≤ 20) or contains POSS nanocrystals embedded in the amorphous PHOU matrix (w ≥ 25). The hybrid nanostructure of PHOU–POSSw‐net (w ≥ 25) is featured by its reconfigurability, based on aggregation and disaggregation of POSS covalently connected to the PHOU network, which enables excellent shape fixing and recovery. Furthermore, it exhibits soft and elastomeric mechanical properties even in the fixed state. Taking advantage of the shape memory ability as well as the softness in the fixed state, we demonstrate microscale dynamic surface topography of PHOU–POSSw‐net. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

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.

     

Shape memory induced structural evolution of high performance copolyimides

In this work, two kinds of high temperature shape memory copolyimides were prepared and the shape memory cycles induced structural evolution of macromolecular chains was investigated in detail. The glass transition temperature (T_g) of poly(benzoxazole‐co‐imide) (PI1) and poly(benzimidazole‐co‐imide) (PI2) are 280 °C and 355 °C, respectively. The results show that PI1 could keep stable macromolecular chain structure under shape memory cycles and exhibit outstanding shape memory performance (R_f > 98%, R_r > 97%) under different stretch condition. Whereas, shape memory cycles induced orientation with more ordered macromolecular chains packing is formed for PI2 after several thermal mechanical cycles, which strongly affect physical crosslinking points, thermal mechanical properties as well as shape memory behaviors. The study on macroscopic property and microscopic structure evolution will promote a better understanding of the shape memory effect of polyimides and accelerate development of high performance polyimides for shape memory applications. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3858–3867     

Synthesis and properties of shape memory polymers of LLA,TMC, and ε‐CL terpolymers

Biodegradable polylactide (PLA) and its copolymers with shape memory properties have attracted great interests because of their important application prospects in biomedical field. In this study, random poly(L‐lactide‐co‐trimethylene carbonate‐co‐ε‐caprolactone) (LTCL) terpolymers with different molar ratio were synthesized and characterized. Monomer ε‐caprolactone (ε‐CL) was used in this study instead of glycolide in preliminary study of LTG terpolymers to investigate the transition temperature and the shape memory performance. Characterization on crystallization, mechanical properties, shape fixing, and recovery ratios of the terpolymers was conducted to investigate the correlation between crystallization and shape memory performance of LTCL terpolymers. The results are consistent with the formation of crystallized LLA segments, which could act as crosslinks, strengthened the stationary phase within the polymer matrix, and significantly improved the shape memory performance of LTCL terpolymers. For example, LTCL801010 is a crystalline polymer with high shape fixity and shape recovery ratio; its shape recovery temperature is 39°C. LTCL terpolymers with high CL content do not show shape memory performance for the rubbery at room temperature. Based on this study, PLA materials with shape memory property can be designed through the selection of monomers or the adjustment of comonomer ratio. These polymers with recovery temperature close to 37°C are expected to be used in human body such as scaffolds in tissue engineering.     

Shape‐memory behavior of segmented polyurethanes with an amorphous reversible phase: The effect of block length and content

Segmented thermoplastic polyurethanes (TPU)s with amorphous soft segments from the reaction of hexamethylene diisocyanate and 1,2‐butanediol and crystalline hard segments from 4,4′‐methylenediphenyl diisocyanate and 1,6‐hexanediol showed sharp glass‐transition temperatures that could be used as shape‐recovery temperatures. The thermal, mechanical, and shape‐memory effect of these TPUs of various block compositions and lengths were studied by differential scanning calorimetry, dynamic mechanical testing, and tensile testing. As the block lengths decreased, phase mixing increased and hysteresis in the shape‐memory behavior decreased. Too low a content of hard segments increased the hysteresis in the shape‐memory behavior. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2652–2657, 2000     

Two‐way multi‐shape memory properties of peroxide crosslinked ethylene vinyl‐acetate copolymer (EVA)/polycaprolactone (PCL) blends

Rare studies have investigated on the 2‐way shape memory crosslinked blends with multiple shape memory behavior up to date. To consider the merit of commercial cost‐competitive crystalline polymers, ethylene vinyl‐acetate copolymer (EVA) / polycaprolactone (PCL) blends (60/40 and 30/70) were peroxide‐cured to form the 2‐way multi‐shape memory crosslinked blends using a melt‐blending method. Both resins were selected to have a similar controlled crosslinking degree, which allowed us to distinctly evaluate their actuation contributions from the cooling‐induced elongation (crystallization) and from the entropy‐driven elongation during cooling process, respectively. In the 2‐way process for the 60/40 system, 2 respective peaks contributed from the cooling‐induced crystallization of EVA and PCL in the cooling curves based on the strain derivate rates at various temperatures were observed. After the cooling process under the loading stress of 150 kPa, the 2‐step heating‐induced contraction process with increasing temperature started at 54.1°C above the melting temperature of PCL at 52.3°C and EVA at 78.3°C, demonstrating 2‐way multi‐shape memory behavior. The multi‐step behavior was more prominent at higher PCL composition and higher load for the 30/70 system. It was found that the entropy‐driven contribution to the overall actuation magnitude increased with increasing nominal loads due to the increased orientation of molecular networks in the blends. The current approach offers numerous possibilities in preparing 2‐way multi‐shape memory crosslinked blends.     

A novel shape memory poly(ε‐caprolactone) network via UV‐triggered thiol‐ene reaction

A facile method to prepare shape memory polymers crosslinked by SiO₂ is described. A series of biodegradable shape memory networks were obtained through thiol‐ene reaction triggered by UV irradiation between surface‐thiol‐modified SiO₂ nanoparticles and end‐acrylate poly (ε‐caprolactone) (PCL). The highly selective thiol‐ene reaction ensured a uniform distribution of PCL chains between crosslinkers, contributing well‐defined network architecture with enhanced mechanical and shape‐memory properties. Thiol‐functionalized silica nanoparticle was characterized by using FTIR and XPS analysis, and ¹H NMR spectra was used to confirm the successful modification of terminal hydroxyl group of PCL diol. Surface‐modified silica particles were found well dispersible in acrylate‐capped PCL supported by SEM. Thermal and crystalline behaviors of the obtained polymers were analyzed by DSC and XRD, and DMA measurement proved good mechanical property. The shape memory behavior and tensile strength was somewhat tunable by the length of PCL. Acceptably, sample SiO₂‐SMP2k presented 99% recovery ratio and 97% shape fixity, and its relatively high tensile strength showed an attractive potential for biomedical application. Finally, a possible molecular mechanism accounting for the shape memory property was illustrated. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 692–701