Shape Memory Function Research Articles

Modified shape memory epoxy resin composites by blending activity polyurethane

AbstractA series of modified shape‐memory epoxy resin composites were prepared by blending activity polyurethane (APU). Fourier transform infrared spectroscopy (FTIR), tensile tests, scanning electron microscope (SEM), dynamic mechanical analysis (DMA), and fold‐deploy shape memory tests were used to characterize the structure, mechanical, morphology, thermodynamics, and shape memory performance of these materials. FTIR results suggest that APU has been introduced into the resin matrix resin. Tensile test results show that the addition of appropriate APU can increase the elongation at break significantly, compared with neat epoxy. SEM results indicate that the fracture mechanism has changed from brittle to ductile, suggesting that the brittleness of the material has been overcome. DMA results show that modified materials have lower glass transition temperature (Tg) and lower cross‐linking density for shape memory function. Furthermore, the fold‐deploy shape memory tests prove that the materials possess excellent shape memory properties. They can be deformed into different shapes and recover their original shapes fully within 2 min atTg,while they are hardly affected by ninefold‐deploy cycles. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Shape Memory RGD‐Containing Networks: Synthesis, Characterization, and Application in Cell Culture

AbstractHydrogels have found wide application in tissue engineering and cell mechanobiology research due to their tunable, and often biomimetic, biochemical and biomechanical properties. Although it has been known for more than a decade that hydrogels can be designed to exhibit shape memory functionality—the ability to change from one defined shape to another when triggered by a defined stimulus—shape memory hydrogels have not previously been exploited in tissue engineering or mechanobiology research. Here we report the development of a biodegradable and biocompatible hydrogel with tailored shape memory as well as desirable mechanical property for soft‐tissue applications. A shape memory hydrogel was synthesized by photopolymerization of PCL3k diacry‐lates together with acrylate‐PEG2k‐GRGDS in the presence of a crosslinker, tetrathiol, and photoinitiator, DMPA, through thiol‐ene chemistry. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were carried out to examine the thermal, mechanical and shape memory properties of the hydrogel in dry and wet states. Cell culture studies were performed to characterize material cytocompatibility. We found that the PCL phase was crystalline in the hydrogel, providing an excellent, reproducible shape memory effect. The transition temperature (shape memory “trigger”) was tuned to fall between room and body temperature. Both cell attachment and proliferation studies revealed that the presentation of GRGDS molecules in the hydrogel facilitated fibroblasts adhesion and spreading on the hydrogel surface. This hydrogel, tailored to exhibit shape memory behavior in the cell culture compatible temperature range, should provide new opportunities for “smart” shape‐changing scaffolds and substrates for application in tissue engineering and investigation of cell mechanobiology.

Synthesis and shape memory behavior study of hyperbranched poly(urethane-tetrazole)

A novel hyperbranched poly(urethane-tetrazole) (HPUTZ) was synthesized via the "A2+BB 2 ' " approach using hexadiisocya- nate (HDI) and 3-(bis-(2-hydroxyethyl)) aminopropyltetrazole (HAPTZ). The molecular structure was characterized by FTIR and 1 H NMR spectroscopy. The number average molecular weight was measured to be 1.05×10 4 g/mol with a polydispersity of 1.27 by GPC analysis. The HPUTZ was further cured by the semi-adduct (PEG-IPDI) from polyethylene glycol (PEG) reacting with isophorone diisocyanate (IPDI) to form the crosslinked HAPTZ-PU film in different ratio of HAPTZ to PEG-IPDI. The glass transition temperature of HAPTZ-PU increased from 44.9 to 56.4 °C as the HPUTZ content increased from 20% to 33% from the DSC analysis. The DMA results indicated that the HPUTZ-PU with 20% HPUTZ possessed the highest storage mod- ulus and loss tangent. However, the storage modulus increased with the increasing of HPUTZ segment at higher temperature. The shape memory study showed that all the films presented the excellent shape memory function. Over 98% shape recovery could be obtained for the HAPTZ-PU with 20%-33% HAPTZ segment content within 60 s in the tension deformation test and within 40 s at 80 °C in the bend deformation test. synthesis, hyperbranched poly(urethane-tetrazole), shape memory behavior, thermal property

Reversible Shape Memory of Nanoscale Deformations in Inherently Conducting Polymers without Reprogramming

By using inherently conducting polymers, we introduce new shape memory functionality for stimuli-responsive polymers. The shape memory process is unique in that it utilizes electrochemical control of the polymer redox state to conceal, and temporarily store, preformed nanoscale surface patterns, which can later be recalled. Unlike classical thermoset and thermoplastic shape memory polymers, the electrochemical control does not completely perturb the low entropy state of the deformed polymer chains, thus enabling the concept of reversible transition between the permanent and temporary shapes. This is demonstrated using electrochemical-atomic force microscopy/quartz crystal microbalance to characterize the modulation of nanoscale deformations in electroactive polybithiophene films. Experimental results reveal that cation/solvent exchange with the electrolyte and its effect on reconfiguration of the film structure is the mechanism behind the process. In addition to incorporating conductive properties into shape-memory polymers, the ability to reversibly modulate surface nanopatterns in a liquid environment is also of significant interest in tribology and biointerface applications.

Electro-active Shape Memory Properties of Poly(ε-caprolactone)/Functionalized Multiwalled Carbon Nanotube Nanocomposite

One type of electroactive shape memory nanocomposite was fabricated, including cross-linked poly(ε-caprolactone) (cPCL) and conductive multiwalled carbon nanotubes (MWNTs). The cross-linking reaction of the pristine poly(ε-caprolactone) (PCL) was realized by using benzoyl peroxide (BPO) as an initiator. The raw MWNTs (Raw-M) were prefunctionalized by acid-oxidation process and covalent grafting with poly (ethylene glycol) (PEG), respectively. Three kinds of nanocomposites containing cPCL/Raw-M, cPCL/acid-oxidation MWNTs (AO-M) and cPCL/PEG grafted MWNTs (PEG-M) were obtained, and the mechanical, electrical and shape memory properties were further investigated. The influence of in vitro degradation on their shape memory and mechanical properties was also evaluated. The methyl thiazolyl tetrazolium (MTT) assay was performed to estimate their biocompatibility. The results displayed that these nanocomposites could perform favorable shape memory recovery both in hot water at 55 °C and in electric field with 50 V applied voltage. In addition, compared with cPCL/Raw-M and cPCL/AO-M, cPCL/PEG-M composite possessed more favorable properties such as mechanical, biocompatible, and electroactive shape memory functions. Therefore, the nanocomposite may be potential for application as smart bioactuators in biomedical field.

In vitro investigation of Fe30Mn6Si shape memory alloy as potential biodegradable metallic material

Fe30Mn6Si alloy was investigated as a potential degradable biomaterial, with the recently well-developed biodegradable metals, pure iron and Fe30Mn alloy, as comparison. The microstructure, mechanical properties, shape memory effect, corrosion behavior and in vitro biocompatibilities were evaluated by X-ray diffraction, scanning electron microscopy, tensile tests, electrochemical tests, immersion tests in Hank's solution till 6 months, cytotoxicity and hemolysis tests. It's found that Fe30Mn6Si alloy consists of ε-martensite and γ-austenite at room temperature, the mechanical property of Fe30Mn6Si alloy is higher than that of the pure iron, and the corrosion rate of Fe30Mn6Si alloy is higher than that of Fe30Mn alloy. Additionally, Fe30Mn6Si alloy shows good performance for blood vessel related cellular application and the hemolysis percentage is less than 2%. In conclusion, Fe30Mn6Si alloy is a promising biodegradable metallic material with a shape memory function.

AB‐polymer Networks with Cooligoester and Poly(n‐butyl acrylate) Segments as a Multifunctional Matrix for Controlled Drug Release

Semi-crystalline AB-copolymer networks from oligo[(epsilon-caprolactone)-co-glycolide]dimethacrylates and n-butylacrylate have recently been shown to exhibit a shape-memory functionality, which may be used for self-deploying and anchoring of implants. In this study, a family of such materials differing in their molar glycolide contents chi(G) was investigated to determine structure-property functional relationships of unloaded and drug loaded specimens. Drug loading and release were evaluated, as well as their degradation behavior in vitro and in vivo. Higher chi(G) resulted in higher loading levels by swelling and a faster release of ethacridine lactate, lower melting temperature of polymer crystallites, and a decrease in shape fixity ratio of the programmed temporary shape. For unloaded networks, the material behavior in vivo was independent of the mechanical load associated with different implantation sites and agreed well with data from in vitro degradation studies. Thus, AB networks could be used as novel matrices for biofunctional implants, e.g., for urogenital applications, which can self-anchor in vivo and provide mechanical support, release drugs, and finally degrade in the body to excretable fragments.

Comparing techniques for drug loading of shape-memory polymer networks – effect on their functionalities

A family of oligo[(ɛ-caprolactone)-co-glycolide]dimethacrylate (oCG-DMA) derived networks of different glycolide contents as well as precursor molecular weights has been synthesized by crosslinking oCG-DMA, providing matrices of different hydrophilicity, network density, and morphology at body temperature. Such networks were loaded with a hydrophilic model drug, ethacridine lactate, either before crosslinking or afterwards by swelling in drug solution. Disadvantageous alterations of the shape-memory functionality and degradation characteristics were observed only in few loaded materials. Loading by swelling generally resulted in low payloads, which slightly increased for more hydrophilic polymer networks, and a substantial burst and fast subsequent release for all investigated materials. Loading before crosslinking gave almost no burst and higher subsequent release rates over longer periods of time. Overall, depending on the needs of a specific application, a material from this polymer family with the desired mechanical properties, shape-memory functionality, and degradation pattern can be selected and combined with drugs when considering that (i) loading by swelling is best suited for applications that require high initial doses and (ii) loading before crosslinking allows easy variation of payloads and low burst release for therapeutics that are non-sensitive to chemical alterations during crosslinking.

Hydrolytic aging of crystallizable shape memory poly(ester urethane): Effects on the thermo-mechanical properties and visco-elastic modeling

The shape memory functionality of a segmented poly(ester urethane) and its hydrolytically aged specimens has been studied by cyclic thermo-mechanical measurements with an imposed strain of 100%. The shape memory effect was triggered by a melting transition in the soft segment phase. Aging was enforced by immersion in hot de-ionized water. In the course of the immersion the tensile properties (secant moduli, stress and strain at yield and break) were impaired by hydrolysis. Advanced specimen embrittlement finally led to rupture during the first thermo-mechanical cycle. This happened after 68 days of aging at 55 °C and correspondingly after 8 days at 80 °C. The residual strain after the first cycle, which was about 25%, increased significantly with aging time. Therefore, the total strain recoverability became ever smaller: aged specimens needed conditioning by at least two cycles for a full development of shape recoverability. Likewise the recovery force decreased continuously. Despite these degradation effects, it was observed that the shape fixity and the cycle-related shape recovery of appropriately conditioned specimens (number of cycles N > 2) remained on a constant high level (at round 100% and between 90% and 100%, respectively) throughout the whole aging period. These observations are discussed within the framework of a simplified model of the behavior of crystallizable shape memory polymers. The amorphous state of the polymer is described by the equation of the linear visco-elastic solid. As for the semi-crystalline state the material is assumed to react elastically with respect to deviations from the configuration, which was frozen up under constraint conditions. The curves of the dependence of the material behavior on aging time at 55 °C match perfectly those at 80 °C when the time axis is adjusted by a factor of 8.5, from which the apparent activation energy for hydrolytic aging in the amorphous state of 82 kJ mol −1 could be deduced.

Shape memory polymer based self-healing syntactic foam: 3-D confined thermomechanical characterization

In this study, the thermomechanical behavior of a shape memory polymer (SMP) based syntactic foam under three-dimensional (3-D) confinement was investigated through strain-controlled programming and fully confined shape recovery tests. The 3-D confinement was created by encasing the foam in circular confining tubes and subjecting the foam cylinder to uniaxial compression. The parameters investigated included two programming temperatures, three types of confining tubes with varying lateral confinements, three prestrain levels, and one fully-confined recovery condition. A three-layer plane-stress analytical model was also developed to estimate the volume change of the specimen by prestressing. It is found that the stress recovery ratio is the highest with rubber liner and the recovered stress is the highest with nylon liner. The stress recovered in the foam specimen which is confined by the nylon liner is as high as 26 MPa, making it possible as actuators. While volume reduction during programming is the key for the foam to self-close cracks, the volume reduction must be within a certain limit; otherwise, the foam loses its shape memory functionality.

Constitutive modeling of shape memory polymer based self-healing syntactic foam

In a previous study, it was found that the shape memory functionality of a shape memory polymer based syntactic foam can be utilized to self-seal impact damage repeatedly, efficiently, and almost autonomously [Li G., John M., 2008. A self-healing smart syntactic foam under multiple impacts. Comp. Sci. Technol. 68(15–16), 3337–3343]. The purpose of this study is to develop a thermodynamics based constitutive model to predict the thermomechanical behavior of the smart foam. First, based on DMA tests and FTIR tests, the foam is perceived as a three-phase composite with interfacial transition zone (interphase) coated microballoons dispersed in the shape memory polymer (SMP) matrix; for simplicity, it is assumed to be an equivalent two-phase composite by dispersing elastic microballoons into an equivalent SMP matrix. Second, the equivalent SMP matrix is phenomenologically assumed to consist of an active (rubbery) phase and a frozen (glassy) phase following Liu et al. [Liu, Y., Gall, K., Dunn, M.L., Greenberg, A.R., Diani J., 2006. Thermomechanics of shape memory polymers: uniaxial experiments and constitutive modeling. Int. J. Plasticity 22, 279–313]. The phase transition between these two phases is through the change of the volume fraction of each phase and it captures the thermomechanical behavior of the foam. The time rate effect is also considered by using rheological models. With some parameters determined by additional experimental testing, the prediction by this model is in good agreement with the 1D test result found in the literature. Parametric studies are also conducted using the constitutive model, which provide guidance for future design of this novel self-healing syntactic foam and a class of light-weight composite sandwich structures.

Polymer Networks Combining Controlled Drug Release, Biodegradation, and Shape Memory Capability

A triple functional polymer network system that combines shape-memory capability, biodegradability, and drug release is developed. The choice of network structure and switching segment prevent that drug incorporation substantially changes the thermal and mechanical properties as well as the shape-memory functionality (see recovery curves). A diffusion-controlled release that is independent from biodegradation is enabled. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Effect of soft segment crystallization and hard segment physical crosslink on shape memory function in antibacterial segmented polyurethane ionomers

Shape memory polyurethane (SMPU) ionomers containing constant 75 wt.% soft segment content were synthesized using poly(ε-caprolactone)diol, 4,4′-diphenylmethane diisocyanate, 1,4-butanediol and/or N, N-bis(2-hydroxyethyl)-isonicotinamide. To introduce substrate bonding antibacterial activity, pyridinium was prepared through a neutralization reaction using 1-iodooctane as neutralization agent. For the SMPU ionomer film obtained, tensile testing at 70 °C and dynamic mechanical analysis suggests that, at temperatures > T ms (the melting point of soft segments), 6.72 and 29.55 mol.% pyridinium within hard segments significantly decreased the mechanical properties such as the stress at 100% elongation (70 °C), the initial modulus (70 °C) and the elastic modulus (75–110 °C). Cyclic tensile investigation demonstrated that the two factors, soft segment crystallization and hard segment physical crosslink, play a very important role in shape memory function in SMPU ionomers. For the each individual specimen, the fixity ratio increased, and the recovery ratio decreased with the extension of cooling time. After sufficient cooling time, the fixity ratio of all specimens can reach a high value (∼95%). Owing to the disrupted physical crosslink in the sample containing 29.55 mol.% pyridinium, the crystallization rate of soft segments has less effect on shape fixity. Therefore, a high fixity ratio (93.8%) can be achieved in a short cooling time (30 s). In the control sample, the fixity ratio is only 73.7% after 30 s cooling. In addition, the admirable substrate bonding antibacterial activity of prepared SMPU ionomers was verified using standards AACTT 147 and ASTM E2149 in comparison with the control sample. The antibacterial activity of SMPU ionomers on Gram-positive bacteria ( Staphylococcus aureus) is significant, and the rate of reduction of bacteria is 100%; the antibacterial activity on Gram-negative bacteria ( Klebsiella pneumoniae) increases from 83.6% to 90.7% with increase in pyridinium content from 6.72 to 29.55 mol.%.

Nanoscale Shape-Memory Function in Highly Cross-Linked Polymers

Topographic engraving of structures in polymer surfaces attracts widespread interest for application in imprint lithography and data storage. We study the nonlinear interaction of nanoindents written in close proximity, 20-100 nm, to one another in a highly cross-linked polystyrene matrix. The indents are created thermomechanically by applying heat and force stimuli of 10 micros duration to a tip, thereby raising the polymer temperature to 250 degrees C and exerting contact pressures of up to 1 GPa. We show that on the nanoscale plastic deformation is highly reversible providing outstanding shape-memory functionality of the material.

Hydrolytic degradation and functional stability of a segmented shape memory poly(ester urethane)

In order to understand the effects of water and hydrolytic ageing on semi-crystalline poly(ester urethane) and its shape memory functionality, water immersion experiments at elevated temperature have been performed on a model substance and various parameters were monitored: change of the melting/crystallisation temperatures, substantial increase in crystallinity, temperature dependence of the water diffusion coefficient and solubility, hydrogen-bonding index and phase mixing by peak deconvolution of the FT-IR carbonyl region and day-to-day tensile and thermo-mechanical cyclic tensile tests. A rising fraction of freezable water agglomerates in the polymer was found for specimens cooled from the immersion temperature. The degradation process could be divided into three phases: an induction phase, a phase of continuous degradation and a phase of accelerated degradation. Shape recovery remains fairly constant during phase one and decreases slowly during phase two. The increase in crystallinity in phase two is accompanied by an increase in shape fixing ability.

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

AbstractThe 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 Actuation by Resistive Heating in Polyurethane Composites of Carbonaceous Conductive Fillers

AbstractThe dependence of electrical resistivity on specimen temperature and imposed tensile strains was determined for shape memory polyurethane (SMPU) composites of carbon nanofiber (CNF), oxidized carbon nanofiber (ox-CNF), and carbon black (CB). The SMPU composites with crystalline soft segments were synthesized from diphenylmethane diisocyanate, 1,4-butanediol, and poly(caprolactone)diol in a low-shear chaotic mixer and in an internal mixer. The materials synthesized in the chaotic mixer showed higher soft segment crystallinity and lower electrical percolation thresholds. The soft segment crystallinity reduced in the presence of CNF and ox-CNF; although the reduction was lower in the case of ox-CNF. The composites of CB showed pronounced positive temperature coefficient (PTC) effects which in turn showed a close relationship with non-linear thermal expansion behavior. The composites of CNF and ox-CNF did not exhibit PTC effects due to low levels of soft segment crystallinity. The resistivity of composites of CNF and ox-CNF showed weak dependence on strain, while that of composites of CB increased by several orders of magnitude with imposed tensile strain. A corollary of this study was that a high level of crystallinity may cause a PTC effect and prevent any actuation through resistive heating. However, a carefully tailored compound which has reduced crystallinity and which requires minimum amount of filler may prevent PTC phenomenon and could supply necessary electrical conductivity over the operating temperature range, while offering enough soft segment crystallinity and rubberlike properties for excellent shape memory function.

Shape memory effect and reversible phase crystallization process in SMPU ionomer

AbstractIt is the first attempt to reveal the effect of reversible phase crystallization process on shape memory effect in shape memory polyurethane (PU) ionomer. Thereof the cyclic tensile testing was conducted with various cooling time to fix the temporary deformation for assessing shape memory function. The crystallization process of the reversible phase, poly (ε‐caprolactone) (PCL) in shape memory PU ionomers composed of different ionic group contents, 1,4‐butanediol, 4,4′‐methylenebis(phenyl isocyanate) and PCL, was investigated by using isothermal crystallization kinetics under the thermal routine similar to that for the cyclic tensile testing. The results demonstrate that the ionic groups within hard segments significantly slow down the crystal growth of the reversible phase. When the physical crosslink is strong enough, the crystallization rate would be a predominant factor determining the shape fixity ratio after various cooling time. Instead, when physical crosslink is weakening, the influence of crystallization rate is much less on the cooling time dependence of fixity. Copyright © 2007 John Wiley & Sons, Ltd.

Polyurethane smart fiber with shape memory function: Experimental characterization and constitutive modelling

Segmented polyurethane (PU) polymers are known to have shape memory function, i.e., when they reach certain temperatures, they deform into the memorized shape from any temporary one. In the present study, PU polymers were spun into fibers using the conventional extrusion process to investigate the feasibility of producing smart fibers with shape memory function. The shape memory polymers (SMPs) and their spun fibers were characterized using DSC, DMTA, and tensile test. In particular, the thermo-mechanical deformation behavior, which enables to evaluate the shape memory performance of the SMPs, was characterized using DMTA. Then, the linear viscoelastic theory was utilized for mathematical modelling of the thermo-mechanical behavior of the SMPs. For the shape memory fibers, the large deformation characteristics were also investigated using the thermo-mechanical test, necessitating the development of nonlinear viscoelastic theory to formulate a constitutive equation and to provide an effective tool for designing smart textile structure.

Degradable shape-memory polymer networks from oligo[(l-lactide)-ran-glycolide]dimethacrylates.

In this paper degradable shape-memory polymer networks synthesized from oligo[(-lactide)--glycolide]dimethaycrylates are introduced. The macrodimethacrylates are prepared a two-step synthesis: hydroxy telechelic oligo[(-lactide)--glycolide]s with number average molecular weights ranging from 1000 to 5700 g mol were synthesized by ring-opening polymerization from ,-dilactide, diglycolide and ethylene glycol as initiator using dibutyltin oxide as the catalyst. These oligodiols are reacted with methacryloyl chloride resulting in terminal methacrylate groups. Crosslinking of macrodimethacrylates is performed under exposure to UV light without applying a photo initiator. The polymer networks obtained are transparent and hydrolytically degradable. While the mechanical properties at temperatures higher than depend on crosslinking density, is almost constant at about 55 °C. The shape-memory functionality of the amorphous polymer network was investigated by cyclic, thermomechanical tests under the systematic variation of different programming parameters. Good shape-memory properties with strain recovery rates close to 100% were obtained under stress-controlled programming. Under strain-controlled conditions, it needs to be considered that relatively high stresses can be generated during programming. Potential biomedical applications are intelligent implants or smart drug release systems.