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.     

Optimization of the shape memory effect in shape memory polymers

In this article, we show that given a thermoresponsive shape memory polymer, it is possible to alter a number of its properties, such as the recovery temperature, shape fixity ratio, maximum recovery stress, and final recovery stress (and even a right combination of some of them, e.g., the maximum recovery stress and final recovery stress), simply by means of selecting the programming temperature to achieve optimized performance. Some concerns for the implementation in real engineering practice are also discussed. Although the focus is on the case of a fixed maximum strain in programming, alternative programming approaches can be investigated in a similar way for optimized performance as well. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Investigation into stress recovery behavior of shape memory polyurethane fiber

In this work, the stress recovery behavior of shape memory polyurethane (SMPU) fiber was investigated. The as‐spun SMPU fibers were subjected to various programing‐recovery conditions. It was observed that recovering at 100 °C generated higher recovery stress than recovering at 150 °C. It was also found that, while hot‐drawn programed fiber has higher recovery stress than cold‐drawn programed fiber if recovered at 100 °C, cold‐drawn programed fiber has higher stabilized recovery stress than hot‐drawn programed counterpart when recovered at 150 °C. A morphological model was proposed based on the results from differential scanning calorimetry, Fourier transform infrared spectrometry, and X‐ray diffraction to understand the physics behind the different stress recovery behaviors. It is found that SMPU experiences different phase transitions and phase separations under different programing and stress recovery conditions. It is concluded that the two sequential phase separations taking place at 100 and 150 °C are primarily responsible for the differences in the stress recovery behavior. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1429–1440     

Dual-responsive shape memory and self-healing ability of a novel copolymer from epoxy/cashew nut shell liquid and polycaprolactone

In this work, the blends of epoxy (EP) and polycaprolactone (PCL) with a bio-based curing agent, viz. cashew nut shell liquid (CNSL) were studied for their dual-responsive shape memory and self-healing behaviors. The suitable EP/CNSL weight ratio was observed at 70/30. The increase of PCL content up to 20 wt% in EP-CNSL matrix significantly enhanced the shape memory response to both thermal and chemical stimuli. All specimens showed 100% thermo-responsive shape recovery and the recovery time decreased with increasing PCL content. In the case of chemo-responsive shape memory, the immersion times spent for 100% shape recovery in water and methanol substantially decreased when PCL was added. Moreover, after thermal treatment, the EP-CNSL matrix with 20 wt% PCL showed significant self-healing ability with high tensile strength recovery at 93.70%. The EP-CNSL/PCL copolymer could be a promising alternative bio-related smart material for various applications such as dual-activated sensors and coatings with self-healing ability.     

Recoverable strain storage capacity of shape memory polyethylene

Shape memory polymers (SMPs) are an important class of smart materials. So far the focus of such polymers was to find suited triggers for various application fields. Thus, the potential of most of these macromolecular networks regarding their maximally storable strain capability was not explored. In this study, the polyethylenes HDPE, LDPE, and ethylene‐1‐octene (EOC) were systematically investigated with respect to their strain storage potential. To achieve maximum strains, the polymers were chemically cross‐linked in such a way that they are at the borderline between thermoplastics and elastomers. All investigated polymers showed higher strain storage than literature reported systems and exhibited excellent shape memory parameters. The highest stored strain was found for networks of EOC with fully recoverable 1400%. Interestingly, this value could not be enlarged by using EOCs with higher molecular weight, which is probably due to increasing content of entanglements as confirmed by Mooney‐Rivlin. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1033–1040     

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     

Ultra low density and highly crosslinked biocompatible shape memory polyurethane foams

We report the development of highly chemically crosslinked, ultra low density (～0.015 g/cc) polyurethane shape memory foams synthesized from symmetrical, low molecular weight, and branched hydroxyl monomers. Sharp single glass transitions (T_g) customizable in the functional range of 45–70 °C were achieved. Thermomechanical testing confirmed shape memory behavior with 97–98% shape recovery over repeated cycles, a glassy storage modulus of 200–300 kPa, and recovery stresses of 5–15 kPa. Shape holding tests under constrained storage above the T_g showed stable shape memory. A high volume expansion of up to 70 times was seen on actuation of these foams from a fully compressed state. Low in vitro cell activation induced by the foam compared with controls demonstrates low acute bio‐reactivity. We believe these porous polymeric scaffolds constitute an important class of novel smart biomaterials with multiple potential applications. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Structure–property relationships of fatty acid swollen,crosslinked natural rubber shape memory polymers

This article investigates shape memory polymers (SMPs) fabricated by swelling sulfur crosslinked natural rubber with four different molten fatty acids: lauric, myristic, palmitic, and stearic acid. As inexpensive additives, they allow commodity natural rubber to be directly converted to SMPs. The shape memory properties are investigated as a function of wt% fatty acid, the choice of fatty acid, and the applied load during shape memory programming. It is found that increasing the wt% acid improves the shape fixity up to ca. 97% at ≥50 wt% fatty acid, at which point the recovery starts to decline with increasing wt% acid due to network failure during shape programming. The shape fixity is found to depend on the yield stress and modulus of the fatty acid network, which both increase with increasing wt% acid. The choice of fatty acid also varies the trigger temperature for shape memory, which scales with the melting point of the fatty acid. Serendipitously, it is found that alignment of the fatty acid crystals during programming produces stiffer networks whose modulus increase with applied load, which counterbalances the higher elastic energy stored in the rubber network to produce lower sensitivity of the shape fixity to the applied load. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 673–687     

Effect of thermal expansion on shape memory behavior of polyurethane and its nanocomposites

The effects of thermal expansion on shape memory performance of shape memory polyurethanes and their nanocomposites with organoclay, carbon nanofiber (CNF), silicon carbide (SiC), and carbon black (CB) were evaluated. The shape memory test cycle involved tensile deformation at above the trigger temperature to initiate shape memory function, cooling to room temperature to fix the shape, and shape recovery induced by heating to above the trigger temperature. Phenomenological models were used to interpret the experimental data on coefficient of thermal expansion (CTE). It was found that Kerner model showed good fit for composites of SiC and CB, and Halpin model gave better fit for composites of organoclay and CNF. It was observed that thermal expansion exerts negative effect on recovered strain, the extent of which depends on the magnitude of temperature gradient, CTE, and the level of tensile strain. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1437–1449, 2008     

Shape memory polybenzoxazines based on a siloxane‐containing diphenol

A siloxane‐containing diphenol is synthesized from 1,1,3,3‐tetramethyldisiloxane and o‐allylphenol, followed by the Mannich condensation with aniline, methylamine, and formaldehyde yielding two siloxane‐containing benzoxazines. The onset polymerization temperature of aniline‐based benzoxazine is higher than that of the methylamine counterpart. The dynamic mechanical properties of the polybenzoxazines depend on the structure of the starting primary amines. Both polybenzoxazines exhibit one‐way dual‐shape memory behavior in response to changes in temperature, and they show excellent shape fixity ratios in bending, tension, and tensile stress–strain tests, high shape recovery ratios in bending and tension tests, but relatively low shape recovery ratios in tensile stress–strain test. The network chain segments including the alkylsiloxane units serve as a thermal control switch based on the glass transition temperatures (39 and 53 °C) for the polybenzoxazines. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1255–1266     

Cold,warm, and hot programming of shape memory polymers

In this article, programming is classified as hot, warm, and cold, based on the temperature zone within which the programming is conducted. The strain and stress locking and releasing mechanisms are discussed within the thermodynamics framework. A new formula is developed for quantifying the strain recovery ratio of cold-programmed SMPs. Stress fixity ratio and stress recovery ratio are also defined based on the understanding of stress locking and recovery mechanisms. State-of-the-art literature on warm and cold programming is reviewed. Well-controlled programming as well as free strain recovery test and constrained stress recovery test are conducted, in order to validate the memory mechanisms discussed in this study. It is found that, while programming temperature has an insignificant effect on the final free shape recovery, it has a significant effect on the stress recovery. The recovery stress programmed by cold programming may be lower, equal to, or higher than that by hot programming, due to the different stress locking mechanisms and other factors such as damage during the thermomechanical cycle. Cold, Warm, and Hot Programming of Shape Memory Polymers © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1319–1339     

Effect of comonomer on thermal/mechanical and shape memory property of L‐lactide‐based shape‐memory copolymers

In this study, three kinds of L ‐lactide‐based copolymers, poly(lactide‐co‐glycolide) (PLGA), poly(lactide‐co‐p‐dioxanone) (PLDON) and poly(lactide‐co‐caprolactone) (PLC), were synthesized by the copolymerization of L ‐lactide (L) with glycolide (G), or p‐dioxanone (DON) or ε‐caprolactone (CL), respectively. The copolymers were easily soluble in common organic solvents. The compositions of the copolymers were determined by ¹H‐NMR. Thermal/mechanical and shape‐memory properties of the copolymers with different comonomers were compared. Moreover, the effect of the chain flexibility of the comonomers on thermal/mechanical and shape‐memory properties of the copolymers were investigated. The copolymers with appropriate lactyl content showed good shape‐memory properties where both the shape fixity rate (R_f)and the shape recovery rate (R_r) could exceed 95%. It was found that the comonomers with different flexible molecular chain have different effects on their thermal/mechanical and shape‐memory properties. Among them, PLGA has the highest mechanical strength and recovery rate while PLC copolymer has high recovery rate when the lactyl content exceeded 85% and the lowest transition temperature (T_trans). Copyright © 2007 John Wiley & Sons, Ltd.     

Comparative studies of polymer‐dispersed liquid crystal films via a thiol‐ene click reaction

In this work, the thiol‐ene click reaction is employed to fabricate polymer‐dispersed liquid crystal (PDLC) films by photoinitiated polymerization. The PDLC films are prepared by systematic variation of key conditions: variety and content of ‐ene monomer, liquid crystal (LC) content, curing time, and curing light intensity. We find that both the morphologies and electro‐optic properties of these films are adjustable. When increasing the length of alkyl main chain of ‐ene monomers, the driving voltages reduce, but in turn, the contrast ratio decreases. Increasing ‐ene monomer content raises the driving voltages as well as the response time, and the increase of LC content lowers the driving voltages but has a negative effect on the contrast ratio. The changes to the curing conditions (both curing time and UV light intensity) can be used to modify the driving voltages, response time, and contrast ratios of PDLC films. These comparative studies will elucidate new insights in commercial applications of intelligent PDLC films.     

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.     

Optimization of the surface modification process of cross‐linked polythiol‐coated chiral stationary phases synthesized by a two‐step thiol‐ene click reaction

A new platform technology for the preparation of stable chiral stationary phases was successfully optimized. The chiral selector tert‐butylcarbamoylquinine was firstly covalently connected to the polymer poly(3‐mercaptopropyl)methylsiloxane by thiol‐ene click reaction. Secondly, the quinine carbamate functionalized polysiloxane conjugate was coated onto the surface of vinyl modified silica particles and cross‐linked via thiol‐ene click reaction. The amount of polysiloxane, chiral selector, radical initiator, reaction solvent (chloroform and methanol), reaction time, and pore size of the supporting silica particles were varied and systematically optimized in terms of achievable plate numbers while maintaining simultaneously enantioselectivity. The optimization was based on elemental analysis data, chromatographic results, and H/u‐curves (Van Deemter) of the resultant chiral stationary phases. The results suggest that better chromatographic efficiency (higher plate numbers) at equal enantioselectivity can be achieved with methanol (a poor solvent for the polysiloxane that is dispersed rather than dissolved) and a lower film thickness of quinine carbamate functionalized polysiloxane. In this study, chiral stationary phases based on 100 Å silica slightly outperformed 200 Å silica particles (each 5 μm). The optimized two step material exhibited significantly reduced mass transfer resistance compared to the one step material and equal performance as a brush‐type chiral stationary phase.     

Three‐component negative‐type photosensitive polyimide precursor based on poly(amic acid), a crosslinker,and a photoacid generator

A new negative‐working and alkaline‐developable photosensitive polyimide precursor based on poly(amic acid) (PAA), 4,4′‐methylenebis[2,6‐bis(hydroxymethyl)]phenol (MBHP) as a crosslinker, and a photoacid generator (5‐propylsulfonyloxyimino‐5H‐thiophen‐2‐ylidene)‐2‐(methylphenyl)acetonitrile (PTMA) has been developed. PAA was prepared by ring‐opening polymerization of pyromellitic dianhydride with 4,4′‐oxydianiline. The photosensitive polyimide precursor containing PAA (65 wt %), MBHP (25 wt %), and PTMA (10 wt %) showed a clear negative image featuring 10 μm line and space patterns when it was exposed to 436 nm light at 100 mJ·cm^?2, post‐exposure baked at 130 °C for 3 min, followed by developing with a 2.38 wt % aqueous tetramethylammonium hydroxide solution at 25 °C. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 593–599, 2005     

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.     

Effects of graphene quantum dot (GQD) on photoluminescence,mechanical, thermal and shape memory properties of thermoplastic polyurethane nanocomposites

A group of shape memory polyurethane‐based nanocomposites containing graphene quantum dot nanoparticles (GQDs) were prepared via in‐situ polymerization method. GQD nanoparticles were synthesized by a facile and rapid microwave‐assisted method and characterized by Fourier‐transform infrared spectroscopy (FTIR), X‐ray diffraction pattern, field emission scanning microscopy, transmission electron microscopy, and fluorescence analysis. Chemical structure and hydrogen bonding index (HBI_[C=O]) of the nanocomposites were analyzed via FTIR spectra. The results show that the incorporation of GQDs in PU matrix reduces HBI_(C=O) of nanocomposites. Crystalline structure and thermal properties of the nanocomposites were investigated by differential scanning calorimetry. As results indicate, nucleation effect of GQDs raises crystallinity content of the samples. Mechanical examinations indicate that incorporation of GQDs improves Young's modulus of the nanocomposites, while their elongation at break values are reduced. In addition, shape memory analyses reveal that the presence of GQDs in PU matrix increases the shape fixity ratios in nanocomposites.     

Recovery performance and characteristics in shape memory effects of aliphatic polyether urethane

The analysis on the recovery performance and characteristics in shape memory effects is helpful for the optimal design and engineering applications of shape memory polymers and their composites. To investigate the relationships among recovery performance, material parameters, and loading conditions, by taking aliphatic polyether urethane as an example, the researchers simulate the shape memory behaviors numerically using a three‐dimensional viscoelastic model. The material parameters for this model are taken from stress relaxation tests, rather than dynamic mechanical analysis tests. Both the unconstrained and the constrained recovery behaviors during strain‐controlled shape memory processes are analyzed. The results reveal that the unconstrained recovery occurs at the same temperature regardless of the applied strain values. Another interesting result is that the shape recovery temperature in unconstrained recovery situations increases and the maximum recovery stress under constrained recovery conditions decreases with the increase of heating rates. Copyright © 2013 John Wiley & Sons, Ltd.