Shape Memory Cycle Research Articles

Three-Dimensional Printing of Shape Memory Composites with Epoxy-Acrylate Hybrid Photopolymer.

Four-dimensional printing, a new process to fabricate active materials through three-dimensional (3D) printing developed by MIT's Self-Assembly Lab in 2014, has attracted more and more research and development interests recently. In this paper, a type of epoxy-acrylate hybrid photopolymer was synthesized and applied to fabricate shape memory polymers through a stereolithography 3D printing technique. The glass-to-rubbery modulus ratio of the printed sample determined by dynamic mechanical analysis is as high as 600, indicating that it may possess good shape memory properties. Fold-deploy and shape memory cycle tests were applied to evaluate its shape memory performance. The shape fixity ratio and the shape recovery ratio in ten cycles of fold-deploy tests are about 99 and 100%, respectively. The shape recovery process takes less than 20 s, indicating its rapid shape recovery rate. The shape fixity ratio and shape recovery ratio during 18 consecutive shape memory cycles are 97.44 ± 0.08 and 100.02 ± 0.05%, respectively, showing that the printed sample has high shape fixity ratio, shape recovery ratio, and excellent cycling stability. A tensile test at 62 °C demonstrates that the printed samples combine a relatively large break strain of 38% with a large recovery stress of 4.7 MPa. Besides, mechanical and thermal stability tests prove that the printed sample has good thermal stability and mechanical properties, including high strength and good toughness.

Modeling the mechanical behavior of crystallizable shape memory polymers: incorporating temperature-dependent viscoelasticity

Shape memory polymers (SMPs) are soft active materials that have an ability to retain a temporary shape, and revert back to their original shape when triggered by a suitable stimulus, typically an increase in temperature. These materials are finding wide use in a variety of fields such as biomedical and aerospace engineering; hence it is important to model their mechanical behavior. Crystallizable shape memory polymers (CSMPs) is an important subclass of SMPs, and their temporary shape is fixed by a crystalline phase, while return to the original shape is due to the melting of this crystalline phase. In our earlier work, we have studied the mechanical behavior of CSMPs within a mechanical setting by considering the original amorphous network above the recovery temperature as a hyperelastic material. In this article, we extend our earlier work to incorporate the temperature-dependent viscoelasticity into the developed constitutive model to study the mechanical behavior of CSMPs. The viscoelastic behavior of the polymers at high temperature is simulated through a rate type model. Furthermore, the model of the semi-crystalline polymer after the onset of crystallization is developed based on the mixture theory and the theory of “multiple natural configurations”. In addition, we have applied the model to a specific boundary value problem, namely uniaxial extension. The shape memory cycles of the CSMPs under different stretch rates have been studied. The results are consistent with what has been observed in experiments.

In situ measurements of free volume during recovery process of a shape memory polymer

The physics and mechanics governing the shape memory effect in shape memory polymers is not fully understood at the nanoscale. A very recent publication highligths a possible link between the free volume of the polymer and its ability to recover the original shape in shape memory cycles. In this study, it is demonstrated that there is a reorganisation of the monomeric unit of polymer chains via molecular sized free volume that makes the shape recovery possible. For that purpose, the free volume properties are measured in situ during the shape recovery process of a crosslinked polycyclooctene shape memory polymer by employing positron annihilation lifetime spectroscopy. By this technique, the size and distribution of the dynamic subnanoscale free volume holes are obtained by measuring the lifetime of the positrons inside the polymer at different stages of the shape recovery.

Microscale Repeatability of the Shape-Memory Effect in Fine NiTi Wires

An experimental investigation into microscale transformation characteristics of polycrystalline NiTi wires of 500 µm diameter during shape memory cycling is discussed, with emphasis on the characterization of a pronounced heterogeneity in the strain distribution evident during detwinning of the martensite phase upon application of load and its persistence throughout the actuation cycle. Using scanning electron microscopy-digital image correlation, full-field strain maps at the microscale were obtained during shape memory cycling. It was found that the strains induced by detwinning were quite heterogeneous at the microscale, and could display a large degree of similarity with thermo-mechanical cycling that tended to increase as cycling progressed. Residual strain concentrated at locations where strain accumulation from detwinning and plasticity were significant, indicating that martensitic detwinning and the associated plasticity that occurs with it is spatially correlated to the subsequent accumulation of residual strain at the microscale.

Thermally-Induced Triple-Shape Hydrogels: Soft Materials Enabling Complex Movements.

Shape-memory hydrogels enable directed movements of a specimen in response to temperature, whereby crystallizable switching segments incorporated as side chains resulted in constant degrees of swelling during the shape-memory cycle. Here we report about hydrogels exhibiting a thermally induced triple-shape effect that allows complex movements of soft materials with two almost independent shape changes. Potential applications for those soft triple-shape materials are two-step self-unfolding devices or temperature-sensitive hydrogel actuators, for example, smart valves for flow rate control in aqueous media. Series of hydrogels with two different hydrophobic crystallizable switching segments were prepared. The degrees of swelling of the triple-shape hydrogels were not affected for different shapes or temperatures, which avoided in this way interferences on the shape shifts. During the two-step programming procedure, two distinct shapes can be implemented as reflected by shape fixity ratios of generally >50%. Structural analysis of the switching domains during the triple-shape cycle by means of X-ray scattering indicates that longer side chains gain lower orientation after deformation and that shorter side chains orient perpendicular to the hydrophilic main chain. Furthermore, it is observed that increased orientation of the switching domains is not a key requirement for adequate shape fixity and recovery ratios of the triple-shape effect in hydrogels, thus longer side chains can be utilized as switching segments in other shape-memory hydrogels.

Shape memory induced structural evolution of high performance copolyimides

ABSTRACTIn 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 (Tg) 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 (Rf > 98%, Rr > 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

Dual-Triggered and Thermally Reconfigurable Shape Memory Graphene-Vitrimer Composites.

Conventional thermoset shape memory polymers can maintain a stable permanent shape, but the intrinsically chemical cross-linking leads to shape that cannot be altered. In this paper, we prepared shape memory graphene-vitrimer composites whose shape can be randomly changed via dynamic covalent transesterification reaction. Consecutive shape memory cycles indicate stable shape memory with undetected strain shift and constant shape fixity and recovery values (Rf > 99%, Rr > 98%). Quantitative characterization of shape reconfiguration by dynamic mechanical thermal analysis (DMA) shows prime reconfigurable behavior with shape retention ratio of 100%. Thus, the arbitrary 2D or 3D newly permanent shape can be easily obtained from a simple plain sample by facile thermal treatment at 200 °C above transesterification temperature (Tv). Besides, it is found that graphene-vitrimers show a ductile fracture in tensile test with a large breaking strain and classical yield phenomenon because of the well-dispersed graphene sheets in the vitrimer that endow effective stress transfer. As the graphene loading increases from 0% to 1%, the yield strength and breaking stain increase from 12.0 MPa and 6% to 22.9 MPa and 44%, respectively. In addition, graphene also serves as energy convertor to convert near-infrared (NIR) irradiation into thermal energy to induce a helix shape sample that is recovered totally within 80 s sequent NIR irradiation. These dual-triggered and reconfigurable shape memory graphene-vitrimers are expected to significantly simplify processing of complex shape and broaden the applications of shape memory polymers.

Network structure dependence on unconstrained isothermal-recovery processes for shape-memory thiol-epoxy “click” systems

The shape-memory response (SMR) of “click” thiol-epoxy polymers produced using latent catalysts, with different network structure and thermo-mechanical properties, was tested on unconstrained shape-recovery processes under isothermal conditions. Experiments at several programming temperatures ( $T_{\mathrm{prog}}$ ) and isothermal-recovery temperatures ( $T_{\mathrm{iso}}$ ) were carried out, and the shape-memory stability was analyzed through various consecutive shape-memory cycles. The temperature profile during the isothermal-recovery experiments was monitored, and it showed that the shape-recovery process takes place while the sample is becoming thermally stable and before stable isothermal temperature conditions are eventually reached. The shape-recovery process takes place in two different stages regardless of $T_{\mathrm{iso}}$ : a slow initial stage until the process is triggered at a temperature strongly related with the beginning of network relaxation, followed by the typical exponential decay of the relaxation processes until completion at a temperature below or very close to $T_{\mathrm{g}}$ . The shape-recovery process is slower in materials with more densely crosslinked and hindered network structures. The shape-recovery time ( $t_{\mathrm{sr}}$ ) is significantly reduced when the isothermal-recovery temperature $T_{\mathrm{iso}}$ increases from below to above $T_{\mathrm{g}}$ because the network relaxation dynamics accelerates. However, the temperature range from the beginning to the end of the recovery process is hardly affected by $T_{\mathrm{iso}}$ ; at higher $T_{\mathrm{iso}}$ it is only slightly shifted to higher temperatures. These results suggest that the shape-recovery process can be controlled by changing the network structure and working at $T_{\mathrm{iso}} < T_{\mathrm{g}}$ to maximize the effect of the structure and/or by increasing $T_{\mathrm{iso}}$ to minimize the effect but increasing the shape-recovery rate.

Thermo-mechanical behavior and structure of melt blown shape-memory polyurethane nonwovens

New processing methods for shape-memory polymers allow for tailoring material properties for numerous applications. Shape-memory nonwovens have been previously electrospun, but melt blow processing has yet to be evaluated. In order to determine the process parameters affecting shape-memory behavior, this study examined the effect of air pressure and collector speed on the mechanical behavior and shape-recovery of shape-memory polyurethane nonwovens. Mechanical behavior was measured by dynamic mechanical analysis and tensile testing, and shape-recovery was measured by unconstrained and constrained recovery. Microstructure changes throughout the shape-memory cycle were also investigated by micro-computed tomography. It was found that increasing collector speed increases elastic modulus, ultimate strength and recovery stress of the nonwoven, but collector speed does not affect the failure strain or unconstrained recovery. Increasing air pressure decreases the failure strain and increases rubbery modulus and unconstrained recovery, but air pressure does not influence recovery stress. It was also found that during the shape-memory cycle, the connectivity density of the fibers upon recovery does not fully return to the initial values, accounting for the incomplete shape-recovery seen in shape-memory nonwovens. With these parameter to property relationships identified, shape-memory nonwovens can be more easily manufactured and tailored for specific applications.

Influences of Substrate Adhesion and Particle Size on the Shape Memory Effect of Polystyrene Particles.

Formulations and applications of micro- and nanoscale polymer particles have proliferated rapidly in recent years, yet knowledge of their mechanical behavior has not grown accordingly. In this study, we examine the ways that compressive strain, substrate surface energy, and particle size influence the shape memory cycle of polystyrene particles. Using nanoimprint lithography, differently sized particles are programmed into highly deformed, temporary shapes in contact with substrates of differing surface energies. Atomic force microscopy is used to obtain in situ measurements of particle shape recovery kinetics, and scanning electron microscopy is employed to assess differences in the profiles of particles at the conclusion of the shape memory cycle. Finally, finite element models are used to investigate the growing impact of surface energies at smaller length scales. Results reveal that the influence of substrate adhesion on particle recovery is size-dependent and can become dominating at submicron length scales.

Post-buckled equilibrium state of axially compressed polymeric rod in glass and rubbery transitions

Abstract Background Glass and rubbery transitions under cooling and heating of polymeric materials underlie a shape memory effect, that is a material ability to save temporarily the deformed shape and restore the original one under the external influence. The present work aims to model the shape memory effect for an axially compressed polymeric rod in its post-buckled equilibrium state, which is the generalization of Euler’s elastica for a glassy material case. Methods For modeling, we use a new type of constitutive relations describing the thermomechanical behavior of amorphous polymers over a wide temperature range. To define the model parameters for lightly-linked epoxy resin a series of experiments was conducted using the Dynamic Mechanical Analyzer. Results Post-buckled states of an epoxy rod equilibrium during the temperature change have been found from numerical simulation. The obtained results illustrate the shape memory effect in case of axially compressed rod buckling. Conclusion The thermomechanical shape-memory cycle includes the stages of deformation development and preservation and the subsequent recovery of the initial shape. According to the obtained results, maximum deflection corresponds to the first loading step at the rubbery material state, because the elastic modulus is very low. During cooling under a constant load the deformation remains constant. After unloading in glassy state the deflection decreases by a small value, because the glassy elastic modulus significantly exceeds the rubbery one. During subsequent heating the rod recovers its initial undeformed shape.

Development of drug-releasing shape-memory polyurethane/hydroxyapatite composites - smart biomaterial for bone tissue implants

Event Abstract Back to Event Development of drug-releasing shape-memory polyurethane/hydroxyapatite composites - smart biomaterial for bone tissue implants Monika Bil1, Marcin Heljak1 and Wojciech Swieszkowski1 1 Warsaw University and Technology, Faculty of Materials Science and Engineering, Poland Introduction: Shape-memory polymers (SMPs) are mechanically active or smart materials, which can be deformed and fixed in a temporary shape and are able to return to their original, permanent shape when exposed to a suitable external stimulus. The unique attributes of shape memory effect in conjunction with biodegradability and drug delivery present enormous opportunities for the design of next generation, resorbable less invasive self-fitting medical implants, tissue scaffolds and medical devices[1]-[3]. The ultimate goal of the study is to develop self - deploying drug delivery system for bone tissue regeneration utilizing thermoplastic SMPs which could self-fitting to a target tissue defect and ensure a local drug release at the site of administration. Materials and Methods: Hydroxyapatite/polyurethane (HA/PU) composites were synthesized through in-situ polymerization with biodegradable hydroxyl terminated oligomers of polycaprolactone and poly(lactide-co-glycolide), 1,6-hexamethylene diisocyanate as a coupling agent and dibutyltin dilaurate as the catalyst. HA nanoparticles (Merck) and gentamicin sulfate (GS) ( 5% wt.) ultrasonically dispersed in dehydrated tetrahydrofuran (THF) were added to synthesis mixture. Chemical structure of the composites were analysed by Raman spectroscopy. Drug release study were performed in vitro in PBS and analyzed using UV spectrophotometer at a wavelength of 332 nm. Shape memory behavior was analyzed in thermomechanical test by dynamic mechanical analyzer (DMA) in water. The shape recovery ratio Rr and the fixity ratio Rf were calculated based on DMA measurement. The relationship between amounts of HA, drug content and shape memory properties were analyzed. Results and Discussion: A series of thermoplastic nano-HA/PU/GS composite with the content of HA in the range 3, 5 10 wt. % were synthesized. Quantitative assessment of the shape memory performance through thermomechanical shape memory cycles verified that all materials exhibited a shape-fixing ratio Rf from 80% to 85% and shape recovery ratio (Rr 80-100%). The best shape recovery ability Rf -100% in water at 37oC was observed for composite with 3% wt. of HA. The shape recovery ratio decreased slightly with an increment in HA nanoparticles weight fraction, due to formation of agglomerates that could be a hindrance in recovery of the macromolecule structure. In order to evaluate the effect of shape recovery on drug release profile the drug release study were performed for the samples subjected thermomechanical shape memory cycle. The results revealed sustained drug release over two weeks. However, initial burst release increased with increasing of HA weight fraction what could be connected with easier water penetration into polymer matrix through HA nanoparticles polymer matrix interface. Conclusion: We successfully prepared series of thermoplastic PU/HA composites based on biodegradable oligoesters combining controlled drug release and shape memory effect. All the drug-loaded samples have satisfactory shape memory properties with shape recovery within body relevant temperature, and drug release behavior, therefore appear to be potentially useful for biomedical applications. The authors would like to thank the National Centre for Research and Development (Grant no: LIDER/037/673/L-4/12/NCBR/2013) for providing financial support to this project

Thermoplastic biodegradable shape memory elastomeric composites by dual-electrospinning

Event Abstract Back to Event Thermoplastic biodegradable shape memory elastomeric composites by dual-electrospinning Erin Mcmullin1, 2, Phillip A. Falcone1, 2, Jaimee M. Robertson1, 2 and Patrick T. Mather1, 2 1 Syracuse University, Syracuse Biomaterials Institute, United States 2 Syracuse University, Department of Biomedical and Chemical Engineering, United States Introduction: Electrospinning is a common processing technique employed to produce microscale or nanoscale polymeric fibers from a polymer solution using a high voltage differential. This technique has been used to fabricate shape memory materials – materials that have the ability to fix a temporary shape and return to the original shape upon application of an external stimulus, such as heat[1]. Dual-electrospinning[2], or the process of electrospinning two polymeric solutions simultaneously, is an easy fabrication method to make composites. In the present work, biodegradable shape memory elastomeric composites (SMECs) were fabricated by dual-electrospinning a synthesized thermoplastic, biodegradable, elastomeric polyurethane with linear polycaprolactone (PCL). Materials and Methods: Elastomeric polyurethanes were synthesized with short-chain PCL or caprolactone-containing soft segments and polyhedral oligomeric silsesquioxane (POSS), a nanoscale organic/inorganic hybrid material that has a well-defined, low melting point (~ 120ºC), as a hard segment. Figure 1: Dual-electrospinning a biodegradable elastomer and PCL. Once synthesized, this material was dual-electrospun with linear PCL to fabricate shape memory elastomeric composites. After electrospinning, films were made by compression molding the films above PCL’s melting point, but below the melting point of POSS, creating a condensed film of biodegradable elastomeric fibers bonded by PCL. The composition of the material was controlled by varying the flow rates of the two materials and was confirmed by differential scanning calorimetry. Results and Discussion: Figure 2: Scanning Electron Micrographs (SEMs) of electrospun fibers of the polyurethane, dual-electrospun fibrous web, and PCL. The elastomers were designed to have a biodegradable soft segment provide elasticity and control the degradation rate, while the biocompatible hard segment physically cross-linked the polymer via crystallization. These synthesized thermoplastic polyurethane elastomers have high molecular weights (> 100 kDa), low modulus (< 15 MPa), and are elastomeric with greater than 70% recovery upon stretching. The compacted dual-electrospun materials have high strain to failure (>1000% strain), composition dependent modulus, and shape memory properties with high fixing and recovery ratios with as little as 7% PCL. In this system, PCL fixes the temporary shape of the material while the elastomer drives recovery to the permanent shape. Further, the materials degrade via hydrolysis and are durable, having greater than 400% strain-to-failure after a month of degradation. Figure 3: Shape memory cycle of a shape memory elastomeric composite, with >90% shape fixing of the strained state and >90% recovery back to the original strain. Conclusion: In conclusion, biodegradable SMECs have been fabricated by dual-electrospinning. The properties of these composites – soft, elastomeric, biodegradable, and shape memory – allow them to be versatile biomaterials for soft applications, such as sutures, stents, or tissue engineering scaffolds for soft tissues. Gerry Fredrickson of Boston Scientific Corporation; Boston Scientific Corporation

Biodegradable anhydride-based reconfigurable shape memory elastomer

Event Abstract Back to Event Biodegradable anhydride-based reconfigurable shape memory elastomer Melodie I. Lawton1, 2, Halimatu S. Mohammed3, Wenbin Kuang1, 2, Devon A. Shipp3 and Patrick T. Mather1, 2 1 Syracuse University, Department of Biomedical and Chemical Engineering, United States 2 Syracuse University, Syracuse Biomaterials Institute, United States 3 Clarkson University, Department of Chemistry and Biomolecular Science, United States Introduction: Shape memory polymers are a class of smart materials capable of transitioning from a programmed temporary shape back to its permanent shape when stimulated[1]. While shape memory polymers have been developed to be degradable and biocompatible, none feature a reconfigurable permanent shape. We have developed a new, soft, biocompatible and biodegradable reconfigurable shape memory elastomeric composite[2] featuring a polyanhydride (PAH) matrix[3]-[6] and electrospun poly(ε-caprolactone) (PCL) fibers. This composite is capable of resetting its permanent shape as well as triggering shape recovery at body temperature. Materials: PCL, 4-pentenoic anhydride (PNA), pentaerythritol tetrakis(3-mercaptopropionate) (PETMP), azobisisobutyronitrile (AIBN), chloroform, N,N-dimethylformamide (DMF), trimethylolpropane diallyl ether (TMPDAE), and 1,6-hexanediol diacrylate (HDDA) were used as received. Thermoplastic polyurethane PCL was synthesized using 3,4-dihydroxy-1-butene, hexamethylene diisocyanate, toluene and poly(ε-caprolactone) diol. Methods: PCL was electrospun from solution prepared by dissolving PCL in chloroform/DMF. The PAH monomer was prepared by mixing PNA with PETMP (2:1 molar ratio) and AIBN. The PCL fiber mat was then imbibed with the PAH monomer, followed by UV and thermal curing of the PAH matrix. The shape memory properties were characterized quantitatively using a defined thermomechanical cycling method using a dynamic mechanical analyzer. Degradation of the composite was monitored by measuring the mass loss over time and evaluating the PCL content using differential scanning calorimetry. Scanning electron microscopy (SEM) was used to evaluate the morphological changes during degradation. Results and Discussion: The PAH elastomer was found to reconfigure its crosslinked structure toward a stress-free state when strained at temperatures greater than 40 °C, with stress relaxation completing over a time that decreased with increasing temperature. This reconfiguration in the solid state is due to dynamic covalent exchange reactions involving anhydride groups. Such reconfiguration did not occur when the elastomer was synthesized devoid of anhydride groups; i.e., using HDDA or TMPDAE instead of PNA. When combined with PCL fibers, PAH-PCL composite exhibited one way shape memory before and after shape resetting through melting and recrystallization of PCL. The thermal parameters of these shape memory cycles can be tuned by replacing PCL with a hard-block-free PCL-based polyurethane exhibiting a lower melting transition. Near-complete degradation of the PAH matrix occured after approximately four days, with about 92% of composite comprised of PCL compared to 18% prior to degradation. Microstructural analysis reveals a change in the erosion mechanics from surface erosion of the pure matrix to apparent bulk degradation for the composite. The composite maintained its fixed shape throughout degradation. Conclusions: Our polyanhydride elastomer is capable of resetting new permanent shapes, erasing any ‘memory’ of previous shapes. Incorporation of this elastomer as a matrix structured with PCL fibers provides a new reconfigurable elastomeric shape memory biomaterial capable of reprogramming its original shape as well as degrading in PBS at physiological conditions while maintaining a programmed shape.

High Performance Shape Memory Epoxy/Carbon Nanotube Nanocomposites.

A series of shape memory nanocomposites based on diglycidyl ether of bisphenol A (DGEBA) E51/methylhexahydrophthalic anhydride (MHHPA)/multiwalled carbon nanotube (MWCNT) with various stoichiometric ratios (rs) of DGEBA/MHHPA from 0.5 to 1.2 and filler contents of 0.25 and 0.75 wt % are fabricated. Their morphology, curing kinetics, phase transition, mechanical properties, thermal conduction, and shape memory behaviors are systematically investigated. The prepared materials show a wide range of glass transition temperatures (Tg) of ca. 65-140 °C, high flexural modulus (E) at room temperature up to ca. 3.0 GPa, high maximum stress (σm) up to ca. 30 MPa, high strain at break (εb) above 10%, and a fast recovery of 32 s. The results indicate that a small amount of MWCNT fillers (0.75 wt %) can significantly increase all three key mechanical properties (E, σm, and εb) at temperatures close to Tg, the recovery rate, and the repetition stability of the shape memory cycles. All of these remarkable advantages make the materials good candidates for the applications in aerospace and other important fields.

Fast switchable, epoxy based shape-memory polymers with high strength and toughness

A fast switchable, epoxy based shape-memory polymer is presented. For this, a partially crystalline epoxy resin was formed by cationic polymerization in the presence of a high meltable polyester polyol, namely poly(ω-pentadecalactone) (PPDL). Under certain polymerization conditions, this system generates a high degree of crystallinity compared to the respective poly(ɛ-caprolactone) (PCL) counterpart. Thus, such an epoxy/PPDL composite exhibits very fast shape-fixity and a good shape-memory cycle stability. Furthermore, the formation of a heterogeneous morphology by segregation and crystallization leads to enhanced strength and toughness compared to the respective epoxy/PCL composites.

Thermosetting epoxy resin/thermoplastic system with combined shape memory and self-healing properties

A novel and facile strategy was proposed to construct a thermosetting/thermoplastic system with both shape memory and self-healing properties based on commercial epoxy resin and poly(ϵ-caprolactone)-PCL. Thermoplastic material is capable of re-structuring and changing the stiffness/modulus when the temperature is above melting temperature. PCL microfiber was used as a plasticizer in epoxy resin–based blends, and served as a ‘hard segment’ to fix a temporary shape of the composites during shape memory cycles. In this study, the electrospun PCL membrane with a porous network structure enabled a homogenous PCL fibrous distribution and optimized interaction between fiber and epoxy resin. The self-healing capability is achieved by phase transition during curing of the composites. The mechanism of the shape memory effect of the thermosetting (rubber)/thermoplastic composite is attributed to the structural design of the thermoplastic network inside the thermosetting resin/rubber matrix.

Shape recovery studies for coupled deformations in an epoxy based amorphous shape memory polymers

In this work, the effect of sequence of shape setting on the shape recovery response for an epoxy based amorphous SMP was studied. The shape setting for coupled axial-twist deformations was done at two different temperatures within the glass transition band. A simple set-up with cameras was used to study the shape recovery behavior under free recovery experiments. Results show that the recovery behavior is independent of sequence of shape setting process but a shift in the shape recovery curve is noticed. The shape memory cycle for coupled deformations was also simulated in ABAQUS-VUMAT using the model proposed earlier by the authors based on multiple natural configurations. The simulated results show the capability of the model to analyze the memory effects of an amorphous polymer subjected to coupled deformations.

Constitutive modeling of the mechanics associated with triple shape memory polymers

Shape memory polymers (SMPs) are smart materials that alter their shape in response to external stimuli. Dual-shape SMPs are widely recognized and are characterized by two shapes involved in a typical shape memory cycle. Recently, triple-shape memory polymers (TSMPs) have been introduced that have the ability to remember three shapes. Thermo-sensitive TSMPs can perform two sequential shape changes in response to heat, which were programed previously in a triple-shape creation process (TSCP). TSMPs are technologically significant as their development of has led to emergence of many complex potential applications that cannot be achieved by dual-shape polymers (Zhao et al., 2013). Crystallizable TSMPs are a class of thermo-sensitive TSMPs, where the shape creation is due to formation of crystalline phases. Different TSCPs have been reported which enable to control the triple shape capability of these materials. In this work the mechanical behavior of TSMPs has been modeled using a framework that has been developed recently for studying crystallization in polymers (Rao, 2003; Rao and Rajagopal, 2001, 2002, 2004). The framework has been used successfully to model SMPs (Barot and Rao, 2006; Barot et al., 2008), light activated SMPs (Sodhi and Rao, 2010) and is based upon the theory of multiple natural configurations. The model has been used to simulate a uni-axial deformation cycle for different types of TSCPs and the results have been compared with experimental data.

Nanoscale Order and Crystallization in POSS–PCL Shape Memory Molecular Networks

The angstrom- and nanometer-scale crystallization and long-range ordering behavior of polyhedral oligosilsesquioxane–poly(e-caprolactone) (POSS–PCL) shape memory nanocomposites under thermomechanical shape memory cycles and uniaxial stretching were studied by simultaneous wide-angle and small-angle X-ray scattering (WAXS/SAXS). POSS/PCL cross-linked molecular networks featuring a single POSS moiety centered between two PCL network chains, previously reported [Alvarado-Tenorio Macromolecules 2011, 44, 5682−5692], with molecular weight of 2600 g/mol and exhibiting shape memory behavior, were synthesized with variation of cross-linking molar ratio (POSS–PCL diacrylate/tetrathiol cross-linker, 2:1, 2:1.5, and 2:2). The photocured networks exhibited a morphology consisting of POSS crystals embedded in an amorphous PCL matrix, and the POSS crystals were ordered in a cubic nanostructure. However, under tensile stress afforded by a shape memory cycle, the networks yielded a double-induced orientation (90° and 180...