Thermoset Shape Memory Polymers Research Articles

Multireusable Thermoset with Anomalous Flame-Triggered Shape Memory Effect.

While thermosets with high mechanical properties and shape memory capabilities have been developed in recent years, two main bottlenecks persist in their intrinsic nonreusability and flammability, especially for those shape memory thermosets used in recycling-required field with high glass transition temperatures ( Tg) and thus risky high temperatures to trigger shape recovery. Here, we report a new shape memory epoxy thermoset integrated with excellent fire retardancy, recyclability, high mechanical performance, and 100% shape recovery ratio. The shape memory effect of this new thermoset was directly triggered by high-temperature flame for the first time. Furthermore, the survival thermoset can be recycled by a simple solid-state recycling method and reused as reinforcing fillers for polyester. The highest recycling efficiency reached 85.4%, and the reinforced composite presented about four times higher storage modulus compared to that of neat sample. This work may open a door for application of thermoset shape memory polymers in many lightweight engineering structures and devices where fire hazard is a concern. The newly proposed concept of flame-triggered shape memory effect may also find applications in fire-protecting system.

Expandable proppants to moderate production drop in hydraulically fractured wells

Hydraulic fracturing is recognized as the primary technique to achieve economic oil and gas production from low permeability reservoirs like shale and tight-sand formations. One of the main challenges facing the oil and gas industry is maintaining the proppant functionality in the subsurface to guarantee a sustainable production rate and higher ultimate recovery. Proppant crushing and proppant embedment may diminish production from stimulated wells especially when bottomhole pressure is reaching low flowing pressures in soft and deep formations like Haynesville or Tuscaloosa Marine Shales. Experimental measurements and field observations have shown the strong impact of proppant stress and proppant embedment on reducing fracture conductivity. In this work, we introduce a novel material developed in order to achieve higher fracture conductivities at a minimum cost. The new type of proppants, so called "Expandable Proppants" (EP), is able to remotely control the expanding force and maintain the functionality of placed proppants. The presented proppants are made out of thermoset shape memory polymers which are activated by formation's in situ temperature to effectively maintain or even increase fracture's width. A fully coupled numerical model is developed to study the effectiveness of expandable proppants and evaluate fracture conductivity enhancement for different combinations and distributions of EP. In addition, a series of experiments were conducted in a modified API conductivity cell to verify the increase in fracture conductivity. Numerical and experimental results demonstrate that proppant expansion can increase the permeability up to 100%. Different conditions of confining stress and proppant sizes are studied to verify the optimum proppant design. This product can extend the lifetime of the fracture and ensure lasting production.

High enthalpy storage thermoset network with giant stress and energy output in rubbery state

Low output in stress and energy in rubbery state has been a bottleneck for wide-spread applications of thermoset shape memory polymers (SMPs). Traditionally, stress or energy storage in thermoset network is through entropy reduction by mechanical deformation or programming. We here report another mechanism for energy storage, which stores energy primarily through enthalpy increase by stretched bonds during programming. As compared to entropy-driven counterparts, which usually have a stable recovery stress from tenths to several MPa and energy output of several tenths MJ/m3, our rubbery network achieved a recovery stress of 17.0 MPa and energy output of 2.12 MJ/m3 in bulk form. The giant stress and energy release in the rubbery state will enhance applications of thermoset SMPs in engineering structures and devices.

Recyclable thermoset shape memory polymers with high stress and energy outputviafacile UV-curing

Engineering applications of current thermoset shape memory polymers are limited by three critical issues: demanding fabrication conditions (from 70 to 300 °C temperatures for hours or days), lack of reprocessability or recyclability, and low recovery stress and energy output.

Reconfiguration and shape memory triggered by heat and light of carbon nanotube–polyurethane vitrimer composites

ABSTRACTThermoset shape‐memory polymers (SMPs) are widely applied because of their superiority in maintaining permanent shapes. However, the inferiority of this material is also conspicuous, namely the loss of reprocessing ability owing to the chemically crosslinked structure. Fortunately, a new class of SMPs, known as “vitrimers,” was discovered, which can be reshaped or reprocessed via topological rearrangement due to the existence of dynamic covalent bonds. Thus, this new thermoset SMP could become a novel solution. In this paper, carbon nanotube–polyurethane vitrimer (CNT‐PUV) composites have been prepared, which possess the capability of thermally induced shape memory based on entropy changes and thermal reconfiguration based on transcarbamoylation reactions of carbamate bonds. In addition, the introduction of CNTs endows them with properties of near‐infrared (NIR) triggered shape memory and reconfiguration due to the photothermal conversion effect of CNTs. Besides, due to the character of the NIR laser, step‐by‐step shape recovery of CNT‐PUVs is realized from predefined temporary shapes to a permanent shape. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45784.

Shape memory polyurethanes with oxidation-induced degradation: In vivo and in vitro correlations for endovascular material applications.

This paper presents an in-depth analysis on the degradation of porous, shape memory polyurethanes (SMPs), including traditional surface characterization as well as model degradation compounds with absolute quantification. This combination of techniques allows for determination of rates of degradation as well as accumulation of individual degradation products. These behaviors are used for in vivo-in vitro comparisons for determination of real time degradation rates. Previous studies have primarily been limited to surface characterization without examination of degradation products and accumulation rates. To our knowledge, our work presents a unique example where a range of material scales (atomistic-scale model compounds along with macroscopic porous SMPs) are used in conjunction with ex planted samples for calculation of degradation rates and toxicological risk.

Catalyst-Free Thermoset Polyurethane with Permanent Shape Reconfigurability and Highly Tunable Triple-Shape Memory Performance.

Thermoset shape memory polymer (SMP) with dynamic covalent bonds in the network is a new class of SMPs for which the permanent shape can be reconfigured via topological rearrangement (plasticity). Catalyzed transcarbamoylation has recently been established as an effective exchange reaction for plasticity in cross-linked polyurethane networks. However, ensuring the plasticity severely constrains the network design which adversely affects the ability to tune other classical shape memory properties for practical applications. Facing this new challenge, we design an amorphous polyurethane system for which the cross-linking density can be adjusted in a wide range. We discovered that the use of an aromatic diisocyanate in the synthesis of the polyurethanes facilitates achieving plasticity without requiring any catalyst. The overall network design leads to tunable recovery stress and shape memory transition temperatures without sacrificing the plasticity. The versatility of our polyurethane SMP is further reflected in its triple-shape memory performance. We anticipate that our tunable polyurethanes will benefit a variety of potential SMP device applications.

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.

Thermoset shape memory polymers and their composites

This article reviews the main progress in the development of typical thermoset shape memory polymers. Thermoset polymers are usually stronger in their microstructure than thermoplastic polymers due to their covalently crosslinked three-dimensional networks. The virtues of thermoset polymers make them great source materials for shape memory polymers. However, their three-dimensional networks endow thermoset shape memory polymers with not only high shape fixity rates and shape recovery rates but also low deformation abilities and brittleness. In the past decade, many breakthroughs have been made regarding thermoset shape memory polymers. Typical thermoset materials, such as epoxy resin, cyanate resin, thermoset polyurethane, polyimide, and polystyrene, have been studied as raw materials for shape memory polymers. Studies on thermoset shape memory polymers have been carried out to not only improve their fundamental properties (i.e. broadening of their operating temperatures and service environments and enhancing their thermal stabilities and mechanical properties) but also introduce functional performance properties (e.g. responsiveness to light, electricity, magnetism, and chemical stimuli). This article aims to present a brief review of the development of thermoset shape memory polymers, including the selection and preparation of materials, the improvement of properties, the functionalization of their composites, and their potential applications.

Stress memory of a thermoset shape memory polymer

ABSTRACTThe performance of stress recovery and shape recovery are equally important for high performance shape memory polymers (SMPs) in emerging applications. However, unlike shape recovery, stress recovery does not always follow a monotonic behavior, i.e., “stress plateau,” “stress overshoot,” and “stress undershoot” can be observed. In order to reveal the complicated stress memorization and recovery behavior, this study employs a phenomenological model which considers the recovery stress as the sum of residual programming stress, memorized stress, thermal stress, and relaxed stress for amorphous crosslinked SMPs. This model is demonstrated by a stress recovery experiment in which a polystyrene based SMP was programmed at two prestrain levels above the glass transition temperature, i.e., 20% (neo‐Hookean hyperelastic region) and 50% (strain‐hardening region), and two fixation temperatures, i.e., 20°C (below Tg) and 45°C (within the Tg region), respectively. In addition, a clear distinction between the memorized stress and recovery stress is presented. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42112.

High strain epoxy shape memory polymer

Epoxy polymers represent a recently emerged class of thermoset shape memory polymers with superior thermo-mechanical endurance and excellent processability.

Silicone Membranes to Inhibit Water Uptake into Thermoset Polyurethane Shape-Memory Polymer Conductive Composites.

Electroactive shape memory polymer (SMP) composites capable of shape actuation via resistive heating are of interest for various biomedical applications. However, water uptake into SMPs will produce a depression of the glass transition temperature (Tg ) resulting in shape recovery in vivo. While water actuated shape recovery may be useful, it is foreseen to be undesirable during early periods of surgical placement into the body. Silicone membranes have been previously reported to prevent release of conductive filler from an electroactive polymer composite in vivo. In this study, a silicone membrane was used to inhibit water uptake into a thermoset SMP composite containing conductive filler. Thermoset polyurethane (PU) SMPs were loaded with either 5 wt% carbon black (CB) or 5 wt% carbon nanotubes (CNT) and subsequently coated with either an Al2O3- or silica-filled silicone membrane. It was observed that the silicone membranes, particularly the silica-filled membrane, reduced the rate of water absorption (37 °C) and subsequent Tg depression versus uncoated composites. In turn, this led to a reduction in the rate of recovery of the permanent shape when exposed to water at 37 °C.

Effects of programming and healing temperatures on the healing efficiency of a confined healable polymer composite

In previous work, a biomimetic close-then-heal (CTH) healing mechanism was proposed and validated to repeatedly heal wide-open cracks in load carrying engineering structures by using constrained expansion of compression programmed thermoset shape memory polymers (SMPs). In this study, the effects on healing efficiencies of variation of temperature during both thermomechanical programming and shape recovery (healing) under three-dimensional (3D) confinement are evaluated. The polymer considered is a polystyrene shape memory polymer with 6% by volume of thermoplastic particle additives (copolyester) dispersed in the matrix. In addition to the programming and healing temperatures, some of the parameters investigated include the flexural strength, crack width and elemental composition at the crack interface. It is observed that while increase of the programming temperature is slightly beneficial to strength recovery, most of the strength recovered and damage repair are strongly dependent on the healing temperature. The best healing efficiency (63%) is achieved by a combination of a programming temperature above the glass transition temperature of the polymer and a healing temperature above the bonding point of the copolyester.

Strain rate effect on the thermomechanical behavior of a thermoset shape memory polymer

Driven by conformational entropy, a shape memory polymer (SMP) is capable of memorizing typical programming information, such as level of induced strain, deformation sequence and temperature profile. Because strain rate is one of the variables during programming, it is desirable to know whether the strain rate effect can be memorized or not. In this study, the effects of programming strain rate on the thermomechanical behavior of a thermoset SMP were investigated. A polystyrene based thermoset SMP was programmed under various strain rates (1500, 0.1 and 0.0001 s−1) in the glassy state, with the same prestrain level and temperature profiles. The thermomechanical response and some crucial physical properties were investigated via multiple characterization techniques. The test results indicate that programming strain rate has an influence on the glass transition temperature. A higher strain rate consistently leads to higher relaxation of prestrain and thus lower strain fixity. Moreover, the recovery strain and recovery strain rate rely on the pseudo-plastic strain during programming, which is associated with the programming strain rate.

Thermomechanical behavior of thermoset shape memory polymer programmed by cold-compression: Testing and constitutive modeling

Programming is a key process for thermally activated stress or strain recovery of shape memory polymers (SMPs). Typically, programming requires an initial heating above the glass transition temperature ( T g ), subsequent cooling below T g and removal of the applied load, in order to fix a temporary shape. This work adopted a new approach to program thermoset SMPs directly at temperatures well below T g , which effectively simplified the shape fixing process. 1-D compression programming below T g and free shape recovery of a thermoset SMP were experimentally investigated. Functional stability of the shape fixity under various environmental attacks was also experimentally evaluated. A mechanism-based thermoviscoelastic–thermoviscoplastic constitutive model incorporating structural and stress relaxation was then developed to predict the nonlinear shape memory behavior of the SMP trained below T g . Comparison between the prediction and the experiment showed good agreement. The structure dependence of the thermomechanical behavior of the SMP was further discussed through a parametric study per the validated constitutive model. This study validates that programming by cold-compression is a viable alternative for thermally responsive thermoset SMPs.

Shape recovery in a thermoset shape memory polymer and its fabric-reinforced composites

A shape memory polymer (SMP) can be deformed from a permanent to a temporary shape above their transfor- mation temperature. Upon reheating, the SMP spontaneously returns to the permanent shape. SMP's show high deformabil- ity, but the recovery stresses are very low, thus limiting the size of the components. This paper presents the first results of an ongoing research to develop large sized components based on SMP. To achieve higher recovery stresses, asymmetric fibre reinforced shape memory composites were produced (SMPC) using resin transfer moulding. The results show a 30- fold increase in recovery stress, compared to the neat SMP resin. The recovery stress is independent of the deformation tem- perature, but is strongly affected by the degree of deformation. At higher deformation levels, crazing occurs. Even though the visible effects of the crazing disappear during reheating, it does influence the recovery stress. This indicates that the ability to recover the permanent shape might change in cyclic loading. All composites tested show complete recovery upon reheating. The rate of shape recovery is higher when the fibre reinforcement is loaded in compression.

Enhancing the Deformation of Shape Memory Sandwich Panels

Abstract: Composite sandwich panels fabricated using a thermosetting shape memory polymer matrix material and a corresponding thermoset shape memory polymer foam core offer the potential to demonstrate large, recoverable, deformations in otherwise stiff structures, under flexural loading. However, as with flexure of thin, fibre-reinforced shape memory matrix laminates, deflection is limited by fibre compression buckling because of the reduced shape memory matrix stiffness at elevated temperature. A hybrid matrix concept has been developed for sandwich panels loaded in flexure in a single direction. This concept uses a non-shape memory resin as the matrix for a fraction of the plies on the surface of the facesheet loaded in compression. It is predicted that, at the elevated temperatures required for the generation of deformation in the shape memory structural sandwich panel, the shape memory matrix and foam moduli will be substantially reduced, while the modulus of the non-shape memory resin will not. Thus, at elevated temperature this effectively leads to a shift of the neutral axis towards the non-shape memory surface, keeping the low stiffness shape memory matrix material in tension and extending the range of deformation prior to onset of fibre buckling. The experiments performed demonstrate that this hybrid matrix approach enables a three-fold increase in mid-span deformation prior to buckling of fibres in the compression surface plies. Furthermore, the force measured to attain the deformed geometry, at elevated temperature, only increases approximately 10–15%, while the magnitude of the force required remains very low. Thus, the hybrid matrix approach functions as predicted and enables the development of sandwich panel structural elements which can undergo large, recoverable deformations.

Radiation crosslinked shape-memory polymers

Shape-memory polymers (SMPs) are active smart materials with tunable stiffness changes at specific, tailored temperatures. The use of thermoset SMPs has been limited in commodity applications because a variety of common low-cost plastics processing techniques are not possible with network polymers. In this study of thermoset SMPs, beyond adjusting the glass transition temperature (Tg) between 25 and 75 °C and tuning the recoverable force between 0.5 and 13 MPa, a novel manufacturing process, Mnemosynation, is described. The customizable mechanical properties of traditional SMPs are coupled with traditional plastic processing techniques to enable a new generation of mass producible plastic products with thermosetting shape-memory properties: low residual strains, tunable recoverable force and adjustable Tg. The results of this study are intended to enable future advanced applications where mass manufacturing, the ability to accurately and independently position Tg and the ability to tune recoverable force in SMPs are required.

Thermomechanical experiment and analysis on shape recovery properties of shape memory polymer influenced by fiber reinforcement

Shape memory polymers (SMPs) are currently investigated as potential materials for large deployable space structures [1–3]. The thermomechanical properties of these polymers significantly change on reaching their glass transition temperature, which yields the excellent feature of shape fixity and shape recovery [4]. As another aspect, the modulus of these materials is not sufficient since they are polymeric materials. In actual applications, the fiber reinforcement is effective for ensuring the sustainability of the deployed structures. However, while the fiber reinforcement has advantages for increasing the stiffness, it has a negative influence on the shape recovery behavior of SMPs. Experimental studies for shape memory polymer composite were conducted by Gall et al. [5, 6] for SiC powder-reinforced nanocomposite, Ohki et al. [7] for short-glass-fiber reinforcement, and Lan et al. [8] for SMP reinforced with plain-weave fabrics, and the increase of residual strain after shape recovery process was confirmed. In order to maximize both the stiffness and shape-recovery behavior of fiber-reinforced SMPs, it is essential to know how the fibers block the shape recovery behavior of polymers. However, the mechanism was not modeled. Therefore, we focus on the mechanism underlying the degradation of shape recovery behavior due to fiber reinforcement. To investigate this mechanism, we first conducted thermomechanical cycle tests for pure SMP and SMP reinforced with short-carbon fibers. We used polyurethane series of thermoset SMP, Diary MP-5510 (curing temperature 100 C), provided by SMP Technologies Inc. The cured polymer has a glass transition temperature of 55 C, and the temperature range of glass transition between glassy state and rubbery state is 30 C. Short carbon fibers T700S (Toray Industries Inc.) were embedded in the polymer. The fibers with approximately 5 mm length were randomly embedded in the prepolymer before curing, and the weight fraction of fibers was set to 0, 2, and 4 wt%. It should be noted that the bundle of carbon fibers (12,000 fibers) was embedded and not dispersed in these model experiments, as also modeled later. After curing, strip specimens were cut out (approximately 40 mm gauge length, 20 mm width, and 2 mm thickness in average). The following thermomechanical cycle was applied to these specimens using a tensile test machine (INSTRON 5566), as shown in Fig. 1:

Electroactive thermoset shape memory polymer nanocomposite filled with nanocarbonpowders

This paper concerns an electroactive thermoset styrene-based shape memory polymer (SMP)nanocomposite filled with nanosized (30 nm) carbon powders. With an increase of theincorporated nanocarbon powders of the SMP composite, its glass transition temperature (Tg) decreases and storage modulus increases. Due to the high micro-porosity andhomogeneous distributions of nanocarbon powders in the SMP matrix, the SMP compositeshows good electrical conductivity with a percolation of about 3.8%. This percolationthreshold is slightly lower than that of many other carbon-based conductive polymercomposites. Consequently, due to the relatively high electrical conductivity, asample filled with 10 vol% nanocarbon powders shows a good electroactive shaperecovery performance heating by a voltage of 30 V above a transition temperature of56–69 °C.