Thermo-responsive Shape Memory Polymer Research Articles

Pattern transformation of thermo-responsive shape memory polymer periodic cellular structures

In this paper, pattern transformation behaviors of shape memory polymer (SMP) periodic cellular structures are investigated through numerical simulations. In order to describe SMP cellular structures behavior in the shape memory cycle, generalized thermo-mechanical viscoelasticity theory coupling time–temperature effect are utilized with the generalized Maxwell model and the Williams–Landel–Ferry (WLF) equation. Similar to other normal periodic cellular structures, SMP periodic cellular structures also display the interesting phenomenon of novel pattern transformation when the structures are loaded by compression force beyond a critical value. Different from other periodic cellular materials, novel transformed pattern for SMP material can be fixed via cooling to a temperature below the glass transition temperature Tg, and this fixed pattern can further be recovered to its original pattern by reheating to a temperature above Tg. Moreover, viscous property of SMP during shape memory cycle is taken into account by considering the effects of nominal strain rate and temperature on the pattern transformation. Time–temperature superposition principle is adopted to explain these effects. On the other hand, this transformation phenomenon for SMP can be triggered even by the stress relaxation process. It is also observed that the auxetic behavior (negative Poisson ratio) exists in the pattern transformation during both the compression process and the stress relaxation process for SMP periodic cellular structures. With present study, we are able to gain deeper insights and explain some of the new interesting physical phenomena observed in reported experiments for SMP periodic cellular structures. Besides, these new findings can be used to design appropriate SMP structures in special applications.

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.

Shape memory polymer/CNT composites and their microwave induced shape memory behaviors

In this paper, the practicability of actuating the shape recovery of thermo-responsive shape memory polymers (SMPs) filled with carbon nanotubes (CNTs) has been investigated. When exposed to microwave radiation, the embedded CNTs were expected to absorb the external electromagnetic energy and act as node heating sources in the SMP/CNT composites. In this way, the SMP could be volumetrically heated and subsequently this would lead to a fast actuation. Although the pure SMP was not responsive to microwave radiation, the microwave absorption ratio in the SMP/CNT composites significantly increased along with the increasing amount of added CNTs or the microwave frequency. The influence of the CNTs on the thermomechanical properties of the SMP composites has been investigated and analyzed using scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) methods. The shape recovery behaviors of the SMP composites with 1 wt%, 3 wt% and 5 wt% CNTs were successfully triggered by microwave radiation. The shape recovery process together with the temperature distribution were recorded with an infrared video camera simultaneously. Furthermore, the selective heating characteristic of the microwaves was also experimentally demonstrated on a SMP composite with a nonuniform CNT concentration. The microwave actuation method is potentially a better choice for the wireless remote shape control of SMP/CNT based functional structures.

Thermal management of a multiple mini-channel heat sink by the integration of a thermal responsive shape memory material

In this paper, a novel application of a thermo-responsive shape memory polymer (SMP) is proposed to smart-control the forced flow of water in a multi mini-channel heat sink. In particular, it is reported that millimeter-sized cylinders made of SMP could be used to smartly obstruct the fluid flow by adapting the flow cross section to the heat load to be removed. By integrating the sensing, the control and the actuation functions within a unique, millimeter-sized device, these micro-valves, unlike the traditional actuators normally used for flow control, could be easily embedded into small heat sinks, with significant space and energy saving, useful, in particular, in systems where several miniaturized components have to be cooled concurrently, such as the modern mainframes or the concentrated photovoltaic solar cells.Two possible configurations for the SMP were considered in this study: an “open” configuration, without any obstruction of the water flow free and an “obstructed” configuration, with the millimeter-sized cylinder partially occupying the mini-channel. A numerical, steady state analysis was carried out with water in single-phase forced convection, to determine the effect of these two states on the internal fluid flow characteristics under different conditions of heat flux and pressure drop and to evaluate the overall thermal behavior of the smart-controlled multiple mini-channel heat sink in terms of ability to control the temperature of the system and to reduce the energy consumption.

Inorganic–Organic Shape Memory Polymer (SMP) Foams with Highly Tunable Properties

Thermoresponsive shape memory polymers (SMPs) are a class of smart materials that can return from a temporary to a permanent shape with the application of heat. Porous SMP foams exhibit unique properties versus solid, nonporous SMPs, enabling their utility in different applications, including some in the biomedical field. Reports on SMP foams have focused on those based on organic polymer systems. In this study, we have prepared inorganic-organic SMP foams comprising inorganic polydimethylsiloxane (PDMS) segments and organic poly(ε-caprolactone) PCL segments. The PCL segments served as switching segments to induce shape changing behavior whereas the length of the PDMS soft segment was systematically tuned. SMP foams were formed via the photochemical cure of acrylated (AcO) macromers AcO-PCL(40)-block-PDMS(m)-block-PCL(40)-OAc (m = 0, 20, 37, 66 and 130) using a revised solvent casting/particulate leaching (SCPL) method. By varying the PDMS segment length, PDMS-PCL foams having excellent shape memory behavior were obtained that exhibited highly tunable properties, including pore size, % porosity, compressive modulus, and degradation rate.

Biodegradable solvent-induced shape-memory hyperbranched polyurethane

Solvent-induced shape-memory polymers are of immense interest in various fields, especially biomedicine, as the high transition temperatures required to induce shape recovery in thermoresponsive shape-memory polymers can have adverse effects. In this work, hyperbranched polyurethane was prepared from poly (e-caprolactone)diol, butanediol, monoglyceride of Mesua ferrea L. seed oil, triethanolamine, and toluene diisocyanate by a pre-polymerization technique. Biodegradation of the polyurethane was assessed by the broth culture technique, and adequate biodegradation was noted in Fourier transform infrared spectroscopy and scanning electron micrography studies, thermogravimetric analysis, and measurements of mechanical properties. The shape recovery of this hyperbranched polyurethane was tested by immersing it in various solvents with differing solubility parameters. The prepared polyurethane showed excellent shape recovery within a short time (10 min) in dimethylformamide. A differential scanning calorimetric study showed decreases in the glass transition temperature and the melting temperature of the sample when it had recovered its shape. The solubility parameter, polarity, and size of the solvent molecules were found to exert the strongest influences on the shape recovery. The studied hyperbranched polyurethane has the potential to be used as an advanced solvent-responsive shape-memory material.

A computational framework for the optimal design of morphing processes in locally activated smart material structures

A proof-of-concept study is presented for a strategy to obtain maximally efficient and accurate morphing structures composed of active materials such as shape memory polymers (SMP) through synchronization of adaptable and localized activation and actuation. The work focuses on structures or structural components entirely composed of thermo-responsive SMP, and particularly utilizes the ability of such materials to display controllable variable stiffness. The study presents and employs a computational inverse mechanics approach that combines a computational representation of the SMP thermo-mechanical behavior with a nonlinear optimization algorithm to determine location, magnitude and sequencing of the activation and actuation to obtain a desired shape change subject to design objectives such as prevention of damage. Two numerical examples are presented in which the synchronization of the activation and actuation and the location of activation excitation were optimized with respect to the combined thermal and mechanical energy for design concepts in morphing skeletal structural components. In all cases the concept of localized activation along with the optimal design strategy were able to produce far more energy efficient morphing structures and more accurately reach the desired shape change in comparison to traditional methods that require complete structural activation prior to actuation.

Porous inorganic–organic shape memory polymers

Thermoresponsive shape memory polymers (SMPs) are a type of stimuli-sensitive materials that switch from a temporary shape back to their permanent shape upon exposure to heat. While the majority of SMPs have been fabricated in the solid form, porous SMP foams exhibit distinct properties and are better suited for certain applications, including some in the biomedical field. Like solid SMPs, SMP foams have been restricted to a limited group of organic polymer systems. In this study, we prepared inorganic–organic SMP foams based on the photochemical cure of a macromer comprised of inorganic polydimethylsiloxane (PDMS) segments and organic poly(ε-caprolactone) (PCL) segments, diacrylated PCL40-block-PDMS37-block-PCL40. To achieve tunable pore size with high interconnectivity, the SMP foams were prepared via a refined solvent-casting/particulate-leaching (SCPL) method. By varying design parameters such as degree of salt fusion, macromer concentration in the solvent and salt particle size, the SMP foams with excellent shape memory behavior and tunable pore size, pore morphology, and modulus were obtained.

Method to Analyze Chemo-Mechanical Behavior of Shape Memory Polymer in Response to Solvent

Thermo-responsive shape-memory polymer (SMP) has been experimentally demonstrated that shape recovery can be induced by plasticizing solvent. Subject to being immersed into solvent, deformed SMP recover from the temporary shape to original shape, leading to shape recovery induced by the molecular interaction. The actuation of styrene-based SMP has been carried out by electrostatic dipole-dipole interaction and physical swelling effect, respectively. The model can be used to predict the effect of prestress, strain, volume change and chemical potential on SMP actuation in the solvent. Finally, it is found that the simulation agrees well with experimental results. The authors show that the chemo-mechanical instability occurs when the Hessian of the free-energy function ceases to the positive definite. Their calculations show that the shape recovery behavior of SMP is driven by the entropy change markedly, agreeing with existing experimental observations.

Synergistic effect of carbon nanofiber and sub-micro filamentary nickel nanostrand on theshape memory polymer nanocomposite

This work studies the synergistic effect of carbon nanofiber (CNF) and sub-microfilamentary nickel nanostrand on the thermal and electrical properties, as well as theelectro-active shape memory behavior, of a shape memory polymer (SMP) nanocomposite.The combination of electrical CNF and electromagnetic nickel nanostrand is used to renderinsulating thermo-responsive SMPs conductive. Subsequently, the shape memory behaviorof the SMP can be activated by the electrical resistive heating. It is shown thatsub-micro filamentary nickel-coated nanostrands significantly improved the electricalconductivity to facilitate the actuation of the SMP nanocomposite despite thelow nanostrand volume content and low electrical voltage. Also the CNFs areblended with the SMP resin to facilitate the dispersion of nanostrands and improvethe thermal conductivity to accelerate the electro- and thermo-active responses.

Development and characterization of semi-crystalline polyalkenamer based shape memorypolymers

A series of thermo-responsive shape memory polymers were developed from commerciallyavailable polyalkenamer by balancing competitive crystallinity and elasticity via amonofunctional peroxide based crosslinking process. The main thermomechanical andshape memory properties of covalently crosslinked samples were characterizedusing thermogravimetric analysis (TGA), differential scanning calorimetry (DSC),dynamical–mechanical–thermal analysis (DMTA), flexural mode stress–straintests and thermomechanical bending experiments. The obtained semi-crystallinepolymer networks were characterized by shape memory activation temperatures from − 10 to60 °C and shape recovery ratios from 95 to 100% by adjusting the melting temperatures andelasticity through a controlled density of covalent knots along the polymeric network froman accurate peroxide fraction.

PCL-based Shape Memory Polymers with Variable PDMS Soft Segment Lengths.

Thermoresponsive shape memory polymers (SMPs) are stimuli-responsive materials that return to their permanent shape from a temporary shape in response to heating. The design of new SMPs which obtain a broader range of properties including mechanical behavior is critical to realize their potential in biomedical as well as industrial and aerospace applications. To tailor the properties of SMPs, "AB networks" comprised of two distinct polymer components have been investigated but are overwhelmingly limited to those in which both components are organic. In this present work, we prepared inorganic-organic SMPs comprised of inorganic polydimethyl-siloxane (PDMS) segments of varying lengths and organic poly(ε-caprolactone) (PCL) segments. PDMS has a particularly low T(g) (-125 °C) which makes it a particularly effective soft segment to tailor the mechanical properties of PCL-based SMPs. The SMPs were prepared via the rapid photocure of solutions of diacrylated PCL(40)-block-PDMS(m)-block-PCL(40) macromers (m = 20, 37, 66 and 130). The resulting inorganic-organic SMP networks exhibited excellent shape fixity and recovery. By changing the PDMS segment length, the thermal, mechanical, and surface properties were systematically altered.

Magnetic Field Activation of Thermoresponsive Shape-Memory Polymer with Embedded Micron Sized Ni Powder

The shape memory polymer (SMP) materials are able to change these shape in response to external stimulus such as stress, temperature, solvent, PH, magnetic, electricity or light. The above-mentioned methods are only to recover initial shape of the deformed SMP, could not give the SMP predeformation. In this paper, magnetic field gives the shape memory polymer composite (SMPC) pre-deformation was studied, through on and off of magnetic field the two-way activation of SMPC was achieved. Shape memory effect of shape memory composite in magnetic field was studied. The SMPC was filled by nickel powder, and the nickel powder was treated by silane coupling agent. The Tg of shape memory composite was measured by dynamic mechanical analyzer. The surface element of treated nickel powder was analysized with XPS. The results indicated that the shape of composite filled by untreated nickel powder did not change in the magnetic field, while the composite filled by treated nickel powder was drawn in the magnetic field. The tensile stretch was decrease with the increase of nickel powder content in the shape memory composite. The addition of silane coupling agent onto nickel powder surface was helpful for the dispersion of nickel in polymer.

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

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

Shape memory polymers with silicon-containing segments.

Thermoresponsive shape memory polymers are stimuli-responsive materials whose shape is modulated by heat. They have been investigated as smart materials in a variety of biomedical, industrial and aerospace applications. The vast majority of shape memory polymers have been limited to those prepared from organic polymers. In this present work, shape memory polymers comprised of inorganic silicon-containing polymer segments (polydimethylsiloxane, PDMS) and organic poly(ε-caprolactone) (PCL) segments were developed. Because of its low T g, PDMS served as a highly effective soft segment. The photochemical cure of diacrylated PCL n -block-PDMS37-block-PCL n macromers with tailored PCL segment lengths produced networks with excellent mechanical properties, shape fixity, and shape recovery.

Mechanisms of the multi-shape memory effect and temperature memory effect in shape memory polymers

As recently demonstrated, after programming, thermo-responsive shape memorypolymers can exhibit the multi-shape memory effect (SME) upon heating. In addition, it is confirmed that the temperature corresponding to the maximum recovery stress in constrained recovery is roughly the temperature at which pre-deformation is conducted, a phenomenon known as the temperature memory effect (TME). In this paper, we propose a framework to investigate the underlying mechanisms behind both effects and provide the conditions for the TME. According to this framework, we can achieve fully controllable shape recovery following a very complicated sequence in a continuous manner.

The formation of micro-protrusions atop a thermo-responsive shape memory polymer

We present an experimental investigation on a novel approach to produce micro-sizedprotrusive features atop a thermo-responsive shape memory polymer (SMP). This approachincludes three steps, namely, indenting atop an SMP sample (using a Berkovich indenter inthis study), polishing and then heating it for full shape recovery. Apart from ordinarysamples, some SMP samples are pre-stretched in the in-plane direction or pre-compressedin the out-of-plane direction.The relationships between the height/shape of protrusion and the depth of indent areobtained for all indents in samples with/without pre-straining. Intrinsic relationshipsamong the indentation depth, polishing depth and height/shape of protrusion are furtherrevealed quantitatively in a dimensionless manner. The influence of pre-straining isdiscussed.

Electrical conductivity of thermoresponsive shape-memory polymer with embedded micron sized Ni powder chains

The electrical resistivity of a thermoresponsive polyurethane shape-memory polymer (SMP) filled with micron sized Ni powders is investigated in this letter. We show that, by forming conductive Ni chains under a weak static magnetic field (0.03T), the electrical conductivity of the SMP composite in the chain direction can be improved significantly, which makes it more suitable for Joule heat induced shape recovery. In addition, Ni chains reinforce the SMP significantly but their influence on the glass transition temperature is about the same as that of the randomly distributed Ni powders.

Polyether polyurethanes: Synthesis, characterization, and thermoresponsive shape memory properties

Thermoresponsive shape memory polymers based on polyurethane with soft segment consisting of poly (tetramethylene oxide) (PTMO) and hard segment arising from urethane reaction of tolylene diisocyanate with OH terminal of PTMO and 1,4 butane diol (BD) were synthesized by a two- step process. The molar ratios of the reactants were varied to get polymers of different soft – hard segment contents. These were characterized by DSC, IR, DMTA, XRD, SEM analyses, and mechanical properties. The shape memory behavior was evaluated by cyclic tensile tests. PTMO served as the switching segments whose crystalline melting was responsible for the switching behavior. As the hard segment-content increased, transition temperature (Ttrans) diminished and so did the elongation and tensile strength of the polyurethane. Higher glassy/rubbery modulus ratio observed for higher hard segment-content polymer was conducive to better shape recovery properties. The influence of hard/soft segments on the thermal, mechanical, damping and shape memory properties of the copolymers are presented and correlated to their phase morphology, as investigated by FTIR and SEM. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008