Thermal-mechanical behavior of styrene-based shape memory polymer tubes

Styrene-based shape memory polymer (SMP) tubes were fabricated and their basic mechanical properties in different deformation states were investigated. The tensile, compression, bending and twisting shape memory properties of the tubes were analyzed and discussed, and the results indicated that SMP tubes exhibit good shape fixity ratio and shape recovery ratio. In addition, the shape recovery behavior was investigated at different heating rates. These experimental results will provide guidance for future applications of SMP tube structures.     

Optical and shape memory properties of semicrystalline poly(cyclooctene) upon cold‐drawing

Semicrystalline thermoplastic poly(cyclooctene) (PCO) shows significant improvement in transparency when cold‐drawn at room temperature, unlike other semicrystalline polymers whose fibrillated chains cause crazing upon cold‐drawing, making the polymers opaque to visible light. Upon heating, transparent cold‐drawn PCO recovers its original opacity as well as its undeformed shape. In situ wide‐ and small‐angle X‐ray diffraction and polarized Fourier transform infrared analyses show that molecular density differences between the PCO crystalline and amorphous phases were reduced due to strain‐induced crystallization and that fibrillated chains and voids, an indication of craze, were not observed due to chain entanglements concentrated in trans double‐bond regions. These two factors explain the unique optical properties of PCO. Finally, it is demonstrated that crosslinked PCO enhanced optical and shape memory recovery without deterioration of the transparency of the polymer. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1595–1607     

Thermo‐/moisture‐responsive shape‐memory effect of poly(2‐ethyl‐2‐oxazoline) networks

In this work, poly(2‐ethyl‐2‐oxazoline) (PEtOx) is crosslinked to realize a moisture‐ and thermo‐responsive shape‐memory polymer. The obtained PEtOx networks exhibit excellent shape‐memory properties with storable strains of up to 650% and recovery values of 100% over at least 10 shape‐memory cycles. The trigger temperature (T_trig) of 68 °C of a PEtOx network at a relative humidity (RH) of 0% decreases with increasing moisture and equals room temperature at an RH of 40%. Thus, programmed PEtOx networks trigger sensitively on a certain temperature/moisture combination and, further, can be programmed as well as triggered at room temperature exclusively by varying humidity. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1053–1061     

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     

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.     

Electrospinning‐induced shape memory effect in thermoplastic polyurethane characterization and thermoviscoelastic modeling

Electrospun thermoplastic polyurethane (TPU) nanofibers are known to contract considerably (～40%) on heating up to ～90 °C. This study investigates this thermomechanical behavior and the TPU shape memory capabilities. The shape memory effect was first studied in TPU films as a model system by applying classical thermomechanical cycles (programming and recovery). The films were able to fix the applied deformation during long‐term storage at room temperature, well above the material's calorimetric glass transition temperature and in the absence of a percolated structure of hard domains. Structural analysis (Fourier transform infrared, differential scanning calorimeter, and dynamic mechanical analysis) revealed broad thermal transitions indicating the presence of a mixed phase of hard segments dispersed in the soft segment matrix. Using a linear viscoelastic model together with time–temperature superposition, the shape memory effect was attributed to the thermoviscoelastic properties of TPU. In particular, the mixed phase was found to give rise to a very broad relaxation spectrum dominated by long relaxation times, which explains the suppression of strain recovery at room temperature. Finally, the electrospinning process was examined and was found to be similar to a programming cycle characterized by the strong elongation flow accompanied by massive solvent evaporation, whereas the contraction effect was interpreted as the recovery phase in a shape memory perspective. Thus, the contraction of electrospun TPU mats may be considered to be an electrospinning‐induced shape memory effect. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1590–1602     

Shape‐memory effect in hyperbranched poly(ethyleneimine)‐modified epoxy thermosets

A series of shape‐memory epoxy thermosets were synthesized by crosslinking diglycidyl ether of bisphenol A with mixtures of commercially available hyperbranched poly(ethyleneimine) and polyetheramine. Thermal, mechanical and shape‐memory properties were studied and the effect on them of the content and structure of the hyperbranched polymer was discussed. Measurements showed that the glass transition temperature can be tailored from 60 °C to 117 °C depending on the hyperbranched polymer content, and all formulations showed an appropriate glassy/rubbery storage modulus ratio. Shape‐memory programming was carried out at TgE′ given the excellent mechanical properties of the materials, with maximum stress and failure strain up to 15 MPa and 60%, respectively. The resulting shape‐memory behavior was excellent, with maximum shape recovery and shape fixity of 98% as well as a fast shape‐recovery rate of 22%/min. The results show that hyperbranched poly(ethyleneimine) as a crosslinking agent can be used to enhance mechanical and shape‐memory properties with different effects depending on the crosslinking density. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 924–933     

Biodegradable thermoset shape‐memory polymer developed from poly(β‐amino ester) networks

The purpose of this study was to develop a degradable thermoset shape‐memory polymer from poly(β‐amino ester) (PBAE) networks. PBAE was chosen to be the crosslinker as it is biodegradable and has been projected as a potential material for biomedical applications. The low glass transition temperature of PBAE was increased to a biomedically relevant range using methyl methacrylate and methyl acrylate as the linear chain builders. The thermo‐mechanical properties of the networks were tailored such that they exhibited onset of glass transition temperature in between the room temperature (22 °C) and the body temperature (37 °C). Free‐strain recovery tests under heating and isothermal conditions were performed to quantify shape‐memory behavior. Testing showed that sampled programmed at 10 °C initiated deformation recovery at a lower temperature and a faster rate as compared to programming at 60 °C. Higher thermal conductivity of water enabled the samples to recover faster in water than in air. Samples with higher PBAE crosslinking densities exhibited higher normalized mass loss under regular and accelerated conditions. The amount of water absorption in the networks also increased with the crosslinker concentration independent of the testing conditions. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

A fully coupled thermo‐viscoelastic finite element model for self‐folding shape memory polymer sheets

The thermo‐mechanical response of heat activated shape memory polymers (SMPs) has been investigated using a thermo‐viscoelastic finite element analysis that accounts for external and internal heat sources. SMPs can be thermally stimulated by external heat sources, such as temperature and surface heat flux, or from internal viscous heating. Viscous heating can significantly affect the response of SMP sheets by increasing the temperature during pre‐strain, which accelerates stress relaxation. This stress relaxation results in a slower shrinking rate when the SMP is reheated. Viscous heating also causes an increase in temperatures during unconstrained recovery. The predicted results elucidate how the coupled thermo‐mechanical loading conditions affect folding and unfolding of SMP sheets in response to localized heating in a hinged region. A parametric study of sheet thickness, hinge width, degree of pre‐strain, and hinge surface temperature is also conducted. The validated results can provide guidelines for the design of functional, self‐folding structures. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1207–1219     

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.     

Experimental and numerical investigation of yielding phenomena in a shape memory polymer subjected to cyclic tension at various strain rates

This paper presents experimental and numerical results of a polyurethane shape memory polymer (SMP) subjected to cyclic tensile loading. The goal was to investigate the polymer yielding phenomena based on the effects of thermomechanical coupling. Mechanical characteristics were obtained with a testing machine, whereas the SMP temperature accompanying its deformation process was simultaneously measured in a contactless manner with an infrared camera. The SMP glass transition temperature was approximately 45 °C; therefore, when tested at room temperature, the polymer is rigid and behaves as solid material. The stress and related temperature changes at various strain rates showed how the SMP yield limit evolved in subsequent loading-unloading cycles under various strain rates. A two-phase model of the SMP was applied to describe its mechanical response in cyclic tension. The 3D Finite Element model of a tested specimen was used in simulations. Good agreement between the model predictions and experimental results was observed for the first tension cycle.     

Shape memory biomaterials prepared from polyurethane/ureas containing sulfated glucose

Covalently crosslinked polyurethane/urea polymers were synthesized using diamine monomers modified with pendant glucose groups and 2,4‐toluene diisocyanate, poly(ethylene glycol) (PEG), and 1,1,1‐tris(hydroxymethyl)ethane (triol) comonomers. The polymers showed shape memory behavior with a switching temperature dependent on the glass transition temperature. The glass transition temperature is tuned by varying the mole ratio between the glucose‐diamine and PEG used in the polymerization. Increasing PEG content resulted in decreasing glass transition temperature, and a glass transition temperature of 39 °C, close to physiological temperatures, was obtained. The fixed shape showed gradual shape recovery behavior, but a fixity of 70% was achieved when the material was stored at 25 °C. The polymer recovered to the permanent shape when heated to 50 °C. Finally, the surface of a film of the polymer can be sulfated to achieve increased blood‐compatibility without sacrificing the shape memory properties. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 2252–2257     

形状记忆高分子材料

作为一种新型的功能材料,形状记忆高分子不仅具有形变量大、赋形容易、形状恢复温度便于调整、加工方便的优点,而且种类丰富、质轻价廉.按形状记忆的方式,它可分为热致感应型、光致感应型和化学物质感应型等,能满足不同的应用需求.     

Recovery as a measure of oriented crystalline structure in poly(ether ester)s based on poly(ethylene oxide) and poly(ethylene terephthalate) used as shape memory polymers

Poly(ether ester)s consisting of poly(ethylene oxide) and poly(ethylene terephthalate) segments, EOET copolymers, could be used as shape memory polymers (SMP). Crystalline structural characters of the copolymers during the memory process were investigated by dynamic mechanical analysis, differential scanning calorimeter, wide-angle X-ray diffraction, polarizing microscopy, and recovery measurements. PEO crystals in stretched EOET copolymer preferentially oriented along fiber axis or stretch direction. During stretching, the structure of the copolymer undertake a transformation from spherulite to fiber, resulting in a crystalline morphology similar to shish-kebab, and recovery properties of stretched EOET samples were dependent on as-described crystalline structural characters that can be influenced by draw ratio. Driving forces for contraction come from the oriented chains, and only oriented or extended chains can be contributive to the recovery of deformation; these extended chains involve both crystalline and amorphous segments. The recovery process in shape memory behavior was noticed to be deorientation of oriented chains due to thermodynamic entropy effect, and was divided into three stages. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 101–112, 1999     

Shape memory and mechanical properties of cross-linked polyethylene/clay nanocomposites

Shape memory polymer (SMP) such as cross-linked low-density polyethylene (XLDPE), can return from its temporary shape to the original (permanent) shape upon heating. SMP in comparison with shape memory alloy (SMA) and shape memory ceramic (SMC) has lower stiffness, so generates lower recovery force when it is being used as an actuator. Also, when SMP is reinforced with traditional micro-fillers, it often loses its shape memory effect due to the high weight fraction of filler (20-30%). To overcome these disadvantages, nanoclays can be used. The smart resultant nanocomposite, even in small clay loading level (0-10 wt.%), shows higher modulus, strength, and the other physical properties such as higher recovery force, required to act as an actuator.In this work, the effect of modified montmorillonite on mechanical and shape memory properties as well as the force generation of a shape memory cross-linked low density polyethylene were investigated.The results show that the modulus of elasticity, the recovery temperature, the recovery force and force recovery rate increase with increasing organoclay in nanocomposites, but final recovery strain decreases slightly.     

Connecting free volume with shape memory properties in noncytotoxic gamma‐irradiated polycyclooctene

The free volume holes of a shape memory polymer have been analyzed considering that the empty space between molecules is necessary for the molecular motion, and the shape memory response is based on polymer segments acting as molecular switches through variable flexibility with temperature or other stimuli. Therefore, thermomechanical analysis (TMA) and positron annihilation lifetime spectroscopy (PALS) have been applied to analyze shape recovery and free volume hole sizes in gamma‐irradiated polycyclooctene (PCO) samples, as a noncytotoxic alternative to more conventional PCO crosslinked via peroxide for future applications in medicine. Thus, a first approach relating structure, free volume holes and shape memory properties in gamma‐irradiated PCO is presented. The results suggest that free volume holes caused by gamma irradiation in PCO samples facilitate the recovery process by improving movement of polymer chains and open possibilities for the design and control of the macroscopic response. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1080–1088     

Qualitative separation of the physical swelling effect on the recovery behavior of shape memory polymer

The physical swelling effect-induced shape recovery is studied in a thermo-responsive styrene-based shape memory polymer (SMP). Fourier transform-infrared (FTIR) test reveals no apparent change in the characteristic polar bonds of CO and O-H after immersing the SMP into toluene solvent. Based on the rubber elastic and relaxation theory, the decrease in internal energy is identified as the driving force for the shape recovery. Subsequently, the rubber elastic theory is further applied to investigate the swelling-induced free/constraint shape recovery in this SMP, and the free-energy function is utilized to analyze the swelling-induced homogenous/inhomogeneous deformation. This study provides a framework to study both the swelling effect-induced shape recovery and complex shape memory behavior in solvent-responsive SMPs.     

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     

Two-way shape memory behavior of styrene-based bilayer shape memory polymer plate

In this work, a bilayer shape memory polymer (SMP) composite plate with two-way shape memory behavior is simulated, in which two types of styrene-based SMPs with well-separated glass transition temperatures are assembled in parallel. The finite element (FE) software ABAQUS is selected to exhibit the two-way shape memory effect during the shape recovery step and the Generalized Maxwell Model with the WLF equation is applied to characterize the temperature-dependent properties of the SMP bilayer plates. The effect factors of axial predeformation, thermal expansion coefficient and plate thickness are all considered for the two-way shape memory behavior of the styrene-based bilayer SMP plate. After that, a smart gripper composed of four SMP composite plates is proposed to realize grabbing and releasing functions for one-step and staged heating recovery. The FE results provide some necessary theoretical guidelines for future soft smart structural designs and optimization.     

Predicting shape memory characteristics of polyurethane in three‐point bending deformation

As a kind of an exceptional material, the temperature‐responding shape memory polymer can fix a temporary shape when cooled down and recover to its original shape when reheated up. Several models have been developed to describe the process. In the present work, we use the generalized Maxwell model to predict the shape memory characteristics of polyurethane in three‐point bending deformation by means of three‐dimensional finite element simulation. The deformation–temperature–stress behavior is obtained numerically. The results reveal that under the condition of accelerating reheating rates at the stage of stress‐free recovery, the maximum recovery rate occurs at higher temperature whereas the recovery ratio decreases. When the deformation degree enlarges, the maximum recovery stress increases in the situation of constrained recovery. The micro‐motion theory of polymer segments is used to explain the characteristics in the shape memory cycle. The simulated results agree well with previous researches, which demonstrate that the simplified model and the numerical simulation method are helpful for both scientific research and engineering development about shape memory polymers. Copyright © 2014 John Wiley & Sons, Ltd.