Thermo-responsive Shape Memory Polymer Research Articles

One-way thermo-responsive shape memory polymer nanocomposite derived from polycaprolactone and polystyrene-block-polybutadiene-block-polystyrene packed with carbon nanofiber

Shape-memory polymer nanocomposite (SMPNC) blends with thermoresponsive shape memory capability have received increasing interests and have been a grooming research area due to its potential applications. In the present study, the polymer blend comprising of Polycaprolactone (PCL) and Polystyrene-block-Polybutadiene-block-Polystyrene (SBS) is prepared by melting and blending method, followed by the addition of carbon nanofibers (CNF), to provide the blend with excellent shape memory property stimulated by temperature. In this polymeric blend SBS, PCL, and CNF are acted as an elastomer, switch polymer and conductive filler, respectively. The two-phase morphology of components in the blend is confirmed using Scanning Electrode Microscopic (SEM) images, Polarised Optical Microscopic (POM) images, and stress-strain plots. The immiscibility between the microstructures is revealed from the Differential Scanning Calorimetric (DSC) results. Characterization techniques such as X-Ray Diffraction (XRD), Thermo Gravimetric Analysis (TGA) and shape memory test are also conducted. The crystallinity of the synthesized composite is influenced by the PCL content. As the PCL content increases, crystalline nature also increases. From TGA results, the thermal stability of the composites is found to be decreasing from PCL 5 to PCL 100. The tensile and shape memory results show an optimized one-way SMPNC system with good shape recovery and fixing performances.

Pre-Programmed Tri-Layer Electro-Thermal Actuators Composed of Shape Memory Polymer and Carbon Nanotubes.

Due to their high deformability, lightness, and safe interaction with the surrounding environment, flexible actuators are key ingredients in soft robotics technologies. Among these, electro-thermal actuators (ETAs), based on carbon nanotubes (CNTs), are used to generate agile movements when current is applied. The extent of movement is determined mostly by the coefficient of thermal expansion (CTE) of the materials arranged in a bi-/tri-layer structure. However, current CNT-based ETAs usually accomplish only simple actions with limited movements. In this work, we successfully developed novel ETAs that are capable of carrying out various controllable movements, such as extremely high bending curvature or unique actuations mimicking a wheel and a worm. These superior functionalities are achieved by adding a third layer or hinges composed of a thermo-responsive shape memory polymer (SMP) onto a bi-layer CNT-kapton ETA. To predict the unique movements of the "triangle" and "worm" actuators, finite element simulations were performed. The combination of SMP and electro-thermal behavior demonstrates its potential for applications in the field of soft actuators and robotics.

Shape memory polymer resonators as highly sensitive uncooled infrared detectors

Uncooled infrared detectors have enabled the rapid growth of thermal imaging applications. These detectors are predominantly bolometers, reading out a pixel’s temperature change due to infrared radiation as a resistance change. Another uncooled sensing method is to transduce the infrared radiation into the frequency shift of a mechanical resonator. We present here highly sensitive resonant infrared sensors, based on thermo-responsive shape memory polymers. By exploiting the phase-change polymer as transduction mechanism, our approach provides 2 orders of magnitude improvement of the temperature coefficient of frequency. Noise equivalent temperature difference of 22 mK in vacuum and 112 mK in air are obtained using f/2 optics. The noise equivalent temperature difference is further improved to 6 mK in vacuum by using high-Q silicon nitride membranes as substrates for the shape memory polymers. This high performance in air eliminates the need for vacuum packaging, paving a path towards flexible non-hermetically sealed infrared sensors.

A new x-shaped copolymer role in poly(L-lactide) shape memory

Thermo-responsive shape memory polymers which are a class of important smart materials can recover permanent shape from temporary shape by simple thermal stimulus, but most of the polymers lack of biodegradability and biocompatibility. Poly(L-lactide) (PLLA) is a green degradable thermoplastic, however, only limited studies involve controlling its shape change. In the work, a new x-shaped copolymer with defined architecture has been devised by using tetramethyltetravinylcyclotetrasiloxane (D4vi) as centronucleus and polytetrahydrofuran-co-poly(ε-caprolactone) (PTMG-co-PCL) as branch to achieve PLLA shape memory. When the weight ratio of DAPP and PLLA is 50/50, the composite exhibits good thermo-responsive shape memory capacity because a balance between Tm of the x-shaped polymer rand Tg of PLLA is kept. The shape fixation ratio is 84% at 15 °C, and the recovery ratio 94% at 55 °C. And the shape memory capability does not change evidently after the reversible spiral transition is repeated for 30 times. At the same time, the composite shows great flexibility. So the obtained composite is promising for medical and bionic devices, self-healing systems, self-assembly structures.

A lamination model for shape memory polymer/woven fabric composites

Based on continuum mechanics and finite element method, a lamination model was developed for shape memory polymer composites (SMPCs) reinforced by woven fabrics. SMPCs were modeled as a laminated structure with woven fabric reinforcements embedded in shape memory polymers (SMPs). Thermo-responsive SMPs were defined by a 3D phenomenological model based on the phase transition approach, while woven fabric reinforcements were characterized by an anisotropic hyperelastic model. The proposed model was validated by comparing numerical results of a SMPC in the shape memory cycle of bending deformation with experimental data. Applications of the lamination model were demonstrated on numerical simulations of a tube made of SMPC in three shape memory cycles with different deformation modes. The proposed model is simple and applicable in the simulations of various shape memory cycles related to SMPCs. It also provides a theoretical foundation for the analysis and design optimization of woven fabric reinforced SMPC structures.

Reversible Underwater Dry Adhesion of a Shape Memory Polymer

AbstractDry adhesives are believed to be inadequate to achieve high adhesion to a wet or submerged surface due to liquid layers intercalated at the contact interface. Here, the reversible dry adhesion of a thermoresponsive shape memory polymer (SMP) performing under water and other liquids such as oil is presented. It is revealed that applying pressure to the SMP in its compliant rubbery state enables to squeeze out the liquid from its contact interface, and that the interface is then maintained by fixing the SMPs shape in its glassy state. During this conformal and hermetic contact condition, the rigidity of the glassy state SMP provides remarkably high (≈18 atm, atmosphere) underwater yet dry adhesion. Lastly, the permanent shape recovery of the SMP allows a surrounding liquid to quickly squeeze in and separate the contact interface, which makes the SMP almost adhesiveless. These findings pave the way for inexpensive reversible dry adhesives working on submerged surfaces.

Pattern-driven 4D printing

Self-bending actuators have time and cost reduction benefits for applications like self-assembly and self-deployable structures. Three-dimensional (3D) printing is a promising rapid and accurate manufacturing method for controlling spatial self-bending actuation in custom-designed soft structures. This paper studies the features merely imparted by 3D printing fabrication in control of self-folding actuators. It is shown that 3D printing control parameters such as different spatial patterns of hinges affect the response time and bending angle of the actuator. A polystyrene (PS) pane as a representative of thermo-responsive shape memory polymers is used as the main material for being remotely stimulated via light emission while the actuation hinges are made of printed chitosan hydrogel ink. The hinges function as a heat source to absorb a wide range of light, particularly the infrared light, convert light energy to thermal energy and cause the underlying printed area to heat up faster than the unprinted area leading to actuation due to thermal stress gradients. A parametric study of physical properties of polymer pane incorporating 3D printed patterns is conducted. Also, experimental tests carried out to validate the proposed parametric model. The proposed model predicts the final shape of the actuator with excellent qualitative agreement with experimental studies. The validated results can provide guidelines for the design of functional, self-bending actuators using 3D printing.

Biodegradable near-infrared-photoresponsive shape memory implants based on black phosphorus nanofillers

In this paper, we propose a new shape memory polymer (SMP) composite with excellent near-infrared (NIR)-photoresponsive shape memory performance and biodegradability. The composite is fabricated by using piperazine-based polyurethane (PU) as thermo-responsive SMP incorporated with black-phosphorus (BP) sheets as NIR photothermal nanofillers. Under 808 nm light irradiation, the incorporated BP sheets with concentration of only 0.08 wt% enable rapid temperature increase over the glass temperature of PU and trigger the shape change of the composite with shape recovery rate of ∼100%. The in vitro and in vivo toxicity examinations demonstrate the good biocompatibility of the PU/BP composite, and it degrades naturally into non-toxic carbon dioxide and water from PU and non-toxic phosphate from BP. By implanting PU/BP columns into back subcutis and vagina of mice, they exhibit excellent shape memory activity to change their shape quickly under moderate 808 nm light irradiaiton. Such SMP composite enable the development of intelligent implantable devices, which can be easily controlled by the remote NIR light and degrade gradually after performing the designed functions in the body.

Shape forming by thermal expansion mismatch and shape memory locking in polymer/elastomer laminates

This paper studies a novel method to fabricate three-dimensional (3D) structure from 2D thermo-responsive shape memory polymer (SMP)/elastomer bilayer laminate. In this method, the shape change is actuated by the thermal mismatch strain between the SMP and the elastomer layers upon heating. However, the glass transition behavior of the SMP locks the material into a new 3D shape that is stable even upon cooling. Therefore, the second shape becomes a new permanent shape of the laminate. A theoretical model that accounts for the temperature-dependent thermomechanical behavior of the SMP material and thermal mismatch strain between the two layers is developed to better understand the underlying physics. Model predictions and experiments show good agreement and indicate that the theoretical model can well predict the bending behavior of the bilayer laminate. The model is then used in the optimal design of geometrical configuration and material selection. The latter also illustrates the requirement of thermomechanical behaviors of the SMP to lock the shape. Based on the fundamental understandings, several self-folding structures are demonstrated by the bilayer laminate design.

PCL-PDMS-PCL Copolymer-Based Microspheres Mediate Cardiovascular Differentiation from Embryonic Stem Cells.

Poly-ɛ-caprolactone (PCL) based microspheres have received much attention as drug or growth factor delivery carriers and tissue engineering scaffolds due to their biocompatibility, biodegradability, and tunable biophysical properties. In addition, PCL and polydimethylsiloxane (PDMS) can be fabricated into thermoresponsive shape memory polymers for various biomedical applications (e.g., smart sutures and vascular stents). However, the influence of biophysical properties of PCL-PDMS based microspheres on stem cell lineage commitment has not been well understood. In this study, PDMS was used as soft segments of varying length to tailor the elastic modulus of PCL-based copolymers. It was found that lower elastic modulus (<10 kPa) of the tri-block copolymer PCL-PDMS-PCL promoted vascular differentiation of embryonic stem cells, but the range of 60-100 MPa PCL-PDMS-PCL had little influence on cardiovascular differentiation. Then different sizes (30-140 μm) of PCL-PDMS-PCL microspheres were fabricated and incorporated with embryoid bodies (EBs). Differential expression of KDR, CD31, and VE-cadherin was observed for the EBs containing microspheres of different sizes. Higher expression of KDR was observed for the condition with small size of microspheres (32 μm), while higher CD31 and VE-cadherin expression was observed for the group of medium size of microspheres (94 μm). Little difference in cardiac marker α-actinin was observed for different microspheres. This study indicates that the biophysical properties of PCL-PDMS-PCL microspheres impact vascular lineage commitment and have implications for drug delivery and tissue engineering.

PCL-PLLA Semi-IPN Shape Memory Polymers (SMPs): Degradation and Mechanical Properties.

Thermoresponsive shape memory polymers (SMPs) based on poly(ε-caprolactone) (PCL) whose shape may be actuated by a transition temperature (T trans ) have shown utility for a variety of biomedical applications. Important to their utility is the ability to modulate mechanical and degradation properties. Thus, in this work, SMPs are formed as semi-interpenetrating networks (semi-IPNs) comprised of a cross-linked PCL diacrylate (PCL-DA) network and thermoplastic poly(l-lactic acid) (PLLA). The semi-IPN uniquely allows for requisite crystallization of both PCL and PLLA. The influence of PLLA (PCL:PLLA wt% ratio) and PCL-DA molecular weight (n) on film properties are investigated. PCL-PLLA semi-IPNs are able to achieve enhanced mechanical properties and accelerated rates of degradation.

Thermal tuning the reversible optical band gap of self-assembled polystyrene photonic crystals

Nano-sized polymeric colloidal particles could undergo self-organization into three-dimensional structures to produce desired optical properties. In this research, a facile emulsifier-free emulsion polymerization method was employed to synthesize highly mono-disperse sub-micron polystyrene colloids. A high quality photonic crystal (PhC) structure was prepared by colloidal polystyrene. The reversible thermal tuning effect on photonic band gap position as well as the attenuation of the band gap was investigated in detail. The position of PBG can be tuned from 420nm to 400nm by varying the temperature of the PhC structure, reversibly. This reversible effect provides a reconfigurable PhC structure which could be used as thermo-responsive shape memory polymers.

Enhancement of haze reflectance through thin-film buckling using a shape memory polymer foundation

The optical haze characteristics of buckled thin-films on a thermoresponsive shape-memory polymer(SMP) foundation have been investigated in the present study. The SMP was pre-stretched to exhibit negatively anisotropic thermal expansion, and coated with a layer of aluminum thin-film. Self-regularized grating patterns of which wavelengths range from 883nm to 5.0μm were obtained on the surface upon annealing due to the shape recovery of SMP. The optical haze spectra from the sample were measured using a spectrophotometer, and results show improvement of reflection haze up to 93%. The present study proposes a novel technique for facile and inexpensive fabrication of grating patterns for thin-film based organic devices.

Shape memory composite (SMC) self-healing coatings for corrosion protection

Abstract A shape memory composite (SMC) coating with a self-healing ability was prepared by a facile method based on a thermoresponsive shape memory polymer (SMP) that utilized carnauba wax microparticles as the healing agent. Damages to the SMC coating was healed via heating, which triggered a two-step healing mechanism consisting of defect closure through a shape memory effect at 65 °C and then defect sealing by molten wax at 90 °C. The surface morphologies of the scratched and healed coatings as well as a wax-free SMP coating were first studied by optical stereomicroscopy and scanning electron microscopy (SEM). To assess the recovery of the coating’s barrier properties, macroscopic and localized information was obtained by electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM), respectively. The healing performance was also evaluated by comparing the macroscopic morphologies of the intact, damaged and healed coatings after long-term immersion. The results from both tests were in agreement and confirmed the key roles of carnauba wax microparticles in the complete recovery of the barrier properties of initially damaged coatings upon thermally assisted self-healing.

A three-dimensional finite-strain phenomenological model for shape-memory polymers: Formulation, numerical simulations, and comparison with experimental data

Shape-memory polymers (SMPs) represent a class of smart materials able to store a temporary shape and to recover the original shape upon an external stimulus, such as temperature. The present paper proposes a three-dimensional finite-strain phenomenological model for thermo-responsive SMPs, which distinguishes between two material phases presenting different properties and is based on a rule of mixtures. The proposed model is motivated by the earlier work of Reese et al. (2010) and it considers several significant material features that had not been addressed in previous phenomenological approaches. Specifically, the model reproduces both heating-stretching-cooling and cold drawing shape-fixing procedures and it takes into account the non-ideal behavior of realistic SMPs (i.e. imperfect shape-fixing and incomplete shape-recovery). Several numerical tests are reported to assess model performances, from simple uniaxial and biaxial tests to complex simulations of biomedical devices. Comparisons with experimental data taken from the literature are also provided to validate the model. The proposed improvements increase the model applicability over a wide range of polymer types and operating conditions.

Thermoresponsive Shape-Memory Aerogels from Thiol–Ene Networks

Thermoresponsive shape-memory polymer aerogels have been produced from thiol–ene networks of 1,6-hexanedithiol, pentaerythritol tetrakis(3-mercaptopropionate), and triallyl-1,3,5-triazine-2,4,6-trione. The thiol–ene networks form organogels with either acetonitrile or acetone as the solvent, which can be subsequently removed using supercritical CO2 extraction. The resulting aerogels have nearly quantitative shape fixing and shape recovery with a glass transition temperature ranging from 42 to 64 °C, which serves as the thermal transition trigger for the shape-memory effect. The aerogels have a porosity of 72% to 81% but surface areas of only 5–10 m2/g.

Reprint of: Pendant allyl crosslinking as a tunable shape memory actuator for vascular applications

Thermo-responsive shape memory polymers (SMPs) can be programmed to fit into small-bore incisions and recover their functional shape upon deployment in the body. This property is of significant interest for developing the next generation of minimally-invasive medical devices. To be used in such applications, SMPs should exhibit adequate mechanical strengths that minimize adverse compliance mismatch-induced host responses (e.g. thrombosis, hyperplasia), be biodegradable, and demonstrate switch-like shape recovery near body temperature with favorable biocompatibility. Combinatorial approaches are essential in optimizing SMP material properties for a particular application. In this study, a new class of thermo-responsive SMPs with pendant, photocrosslinkable allyl groups, x%poly(ε-caprolactone)-co-y%(α-allyl carboxylate ε-caprolactone) (x%PCL-y%ACPCL), are created in a robust, facile manner with readily tunable material properties. Thermomechanical and shape memory properties can be drastically altered through subtle changes in allyl composition. Molecular weight and gel content can also be altered in this combinatorial format to fine-tune material properties. Materials exhibit highly elastic, switch-like shape recovery near 37°C. Endothelial compatibility is comparable to tissue culture polystyrene (TCPS) and 100%PCL in vitro and vascular compatibility is demonstrated in vivo in a murine model of hindlimb ischemia, indicating promising suitability for vascular applications. Statement of SignificanceWith the ongoing thrust to make surgeries minimally-invasive, it is prudent to develop new biomaterials that are highly compatible and effective in this workflow. Thermo-responsive shape memory polymers (SMPs) have great potential for minimally-invasive applications because SMP medical devices (e.g. stents, grafts) can fit into small-bore minimally-invasive surgical devices and recover their functional shape when deployed in the body. To realize their potential, it is imperative to devise combinatorial approaches that enable optimization of mechanical, SM, and cellular responses for a particular application. In this study, a new class of thermo-responsive SMPs is created in a robust, facile manner with readily tunable material properties. Materials exhibit excellent, switch-like shape recovery near body temperature and promising biocompatibility for minimally-invasive vascular applications.

Study on a Thermoresponsive Shape Memory Polymer Used in Down-Hole Packer

Shape memory polymer (SMP) is being studied and anticipated to be used in a novel down-hole packer. SMP is capable of recovering a predetermined shape following environmental changes, allowing the packer to be altered to a smaller size during run in and to retain its original configuration after activated by the heat in the wellbore. This paper describes the work involved in studying the shape recovery performance of a polyethylene based thermoresponsive SMP under different conditions. The influence of temperature, pressure and degree of deformation on the SMP’s shape recovery performance has been characterized.

Photonic Crystals: Optically Bistable Macroporous Photonic Crystals Enabled by Thermoresponsive Shape Memory Polymers (Advanced Optical Materials 11/2015)

Photonic Crystals: Optically Bistable Macroporous Photonic Crystals Enabled by Thermoresponsive Shape Memory Polymers (Advanced Optical Materials 11/2015)

Thermo-responsive shape memory polymer blends based on alpha olefin and ethylene propylene diene rubber

Thermally stimulated novel shape memory polymer blend of ethylene octene copolymer (EOC) and ethylene propylene diene terpolymer (EPDM) has been developed. This novel polyolefinic blends possess superior shape memory behaviour in presence of heat. Shape memory behaviour study at 60 °C of the blends shows that the blends containing higher fraction of EPDM exhibits superior shape memory behaviour in terms of shape fixity (f) and shape recovery ratio (RR). Structural similarities of the pristine components lead to higher degree of compatibility which also results better physicomechanical behaviour. Lower modulus value followed by low relaxation ratio of the blend containing higher proportion of EPDM also supports the superior shape memory behaviour of the EPDM rich blend. Thus EPDM rich blend shows better shape memory effect among the various blends.