Study on the thermal-induced shape memory effect of pyridine containing supramolecular polyurethane

In this paper, a series of pyridine containing supramolecular polyurethanes (PUPys) were synthesized from BINA, HDI and BDO. Then the structure, morphology and thermal-induced shape memory effect (SMEs) of PUPys were investigated systematically. Results show that strong hydrogen bonding is formed in the urethane group as well as in the pyridine ring; and phase separation consisting of soft phase and hard phase occurs in the PUPy. In addition, it is found that the lower limit of BINA content for PUPys exhibiting good SMEs is 30 wt%. PUPys with higher BINA content show higher shape fixity, higher shape recovery and better strain stability. Moreover, the shape recovery force increases with the decreasing of BINA content. Finally, the temperature-dependent FT-IR spectra support that the hydrogen bonding in the pyridine ring serves as the molecular switch; while the hydrogen bonding in the urethane groups acts as the physical netpoints for the utilization of PUPys as SMMs.     

Effect of diisocyanate on pyridine containing shape memory polyurethanes based on N,N‐bis(2‐hydroxylethyl)isonicotinamide

This article demonstrates a comparative investigation about the effect of diisocyanate on pyridine containing shape memory polyurethanes (Py‐SMPUs), which are synthesized with N,N‐bis(2‐hydroxylethyl)isonicotinamide (BINA) and four different diisocyanates: 1,6‐hexanediisocyante (HDI), isophorone diisocyanate (IPDI), methylene diphenyl diisocyanate (MDI), and tolylene diisocyanate (TDI). Results show that all BINA–SMPU systems have amorphous reversible phase. Comparatively, the MDI–BINA and TDI–BINA systems show higher T_g; and the HDI–BINA and IPDI–BINA systems show better thermal stability. In addition, the HDI–BINA and the IPDI–BINA systems exhibit good thermal‐induced shape memory effect and good moisture‐sensitive shape memory effect due to their better moisture absorption properties. Particularly, the HDI–BINA system has better response speed and better shape recovery. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40721.     

Novel moisture-sensitive shape memory polyurethanes containing pyridine moieties

In this communication, novel moisture-sensitive shape memory polyurethane (SMPU) was synthesized from 1,6-hexamethylene diisocyanate (HDI) and N,N-bis(2-hydroxylethyl) isonicotinamide (BINA) and 1,4-butanediol (BDO). Results show that the BINA based SMPUs have excellent moisture absorption properties which are not only influenced by the temperature, but also by the relative humidity (RH). As a result, high strain recovery with fast recovery speed is obtained after immersion in the moisture condition for a short time. FT-IR spectra provide a proof to the mechanism of moisture-sensitive SME which is based on the dissociation or disrupt of hydrogen bonding in the pyridine ring induced by moisture absorption. Thus, the strain recovery is achieved in the moisture-sensitive SMPUs by decreasing the glass transition temperature (T_g) below ambient temperature without external heating.     

Studies of the moisture‐sensitive shape memory effect of pyridine‐containing polyurethanes

Shape memory polymers have been much researched in recent years. In the work reported, moisture‐sensitive shape memory effects (SMEs) of novel pyridine‐containing shape memory polyurethanes (Py‐SMPUs) were investigated systematically. The results show that the strain recovery start immersion time (t_s), strain recovery immersion time (t_r) and final strain recovery immersion time (t_e) are prolonged with a decrease of relative humidity as well as a decrease of temperature. The final strain recovery decreases with a decrease of relative humidity as well as an increase of temperature. The key component affecting the moisture‐sensitive SME is the N,N‐bis(2‐hydroxyethyl)isonicotinamide (BINA) unit. The lower limit of BINA content for Py‐SMPUs to exhibit a good moisture‐sensitive SME is 30 wt%. The addition of diphenylmethane diisocyanate (MDI) and 1,4‐butanediol (BDO) enhances the moisture‐sensitive shape recovery. The final shape recovery decreases with a decrease of BINA content or an increase of MDI–BDO content. In addition, t_s, t_r and t_e become shorter in the Py‐SMPUs with higher BINA content or with lower MDI‐BDO content. Copyright © 2011 Society of Chemical Industry     

Shape memory polymers based on uniform aliphatic urethane networks

Aliphatic urethane polymers have been synthesized and characterized, using monomers with high molecular symmetry, to form amorphous networks with very uniform supermolecular structures, which can be used as photo‐thermally actuable shape memory polymers (SMPs). The monomers used include hexamethylene diisocyanate (HDI), trimethylhexamethylenediamine (TMHDI), N,N,N′,N′‐tetrakis(hydroxypropyl)ethylenediamine (HPED), triethanolamine (TEA), and 1,3‐butanediol (BD). The new polymers were characterized by solvent extraction, NMR, XPS, UV/VIS, DSC, DMTA, and tensile testing. The resulting polymers were found to be single phase amorphous networks with very high gel fraction, excellent optical clarity, and extremely sharp single glass transitions in the range of 34–153°C. Thermomechanical testing of these materials confirms their excellent shape memory behavior, high recovery force, and low mechanical hysteresis (especially on multiple cycles), effectively behaving as ideal elastomers above T_g. We believe these materials represent a new and potentially important class of SMPs, and should be especially useful in applications such as biomedical microdevices. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Significantly improving electro‐activated shape recovery performance of shape memory nanocomposite by self‐assembled carbon nanofiber and hexagonal boron nitride

This study presents two effective approaches to significantly improve the electro‐thermal properties and electro‐activated shape recovery performance of shape memory polymer (SMP) nanocomposites that are incorporated with carbon nanofibers (CNFs) and hexagonal boron nitrides (h‐BNs), and show Joule heating triggered shape recovery. CNFs were self‐assembled and deposited into buckypaper form to significantly improve the electrical properties of SMP and achieve the shape memory effect induced by electricity. The h‐BNs were either blended into or self‐assembled onto CNF buckypaper to significantly improve the thermally conductive properties and electro‐thermal performance of SMPs. Furthermore, the shape recovery behavior and temperature profile during the electrical actuation of the SMP nanocomposites were monitored and characterized. It was found that a unique synergistic effect of CNFs and h‐BNs was presented to facilitate the heat transfer and accelerate the electro‐activated shape recovery behavior of the SMP nanocomposites. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40506.     

A simulation method to analyze chemo‐mechanical behavior of swelling‐induced shape‐memory polymer in response to solvent

A network of thermally responsive shape‐memory polymers (SMPs) could imbibe a quantity of solvent molecules to swell, and subsequently induces a chemical potential change in polymer. When an equilibrium is reached between the mechanical load and the chemical potential of polymer network and solvent, the SMP polymer usually swells with a field of inhomogeneous and anisotropic deformation, which is considered to be equivalent to a hyperelastic field. We implement this theory in the free‐energy function equation, and analyze examples of swelling‐induced deformation and shape recovery behavior. This work may provide a powerful tool to study complex swelling‐induced shape‐memory behavior of SMPs in response to the immersing solvents. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

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

Thermoset 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.     

State diagram of phase transition temperatures and solvent‐induced recovery behavior of shape‐memory polymer

In this study we analyzed the phase and state transitions of shape‐memory polymers (SMPs)/solvent mixtures using the Flory–Huggins (FH) theory by extension of Vrentas and the Couchman–Karasz theory for glass transition, as well as Clausius–Clapeyron relation for melting transition. Using scaling relations of model parameters, we have obtained a theoretical prediction of state diagrams of the phase transition temperature and solvent‐induced recovery in SMPs. The inductive decrease in transition temperature is identified as the driving force for the solvent‐induced shape‐memory effect in SMPs Consequently, the thermodynamics of the polymer solution and the relaxation theory were employed to characterize the dependencies of shape recovery time on the FH parameter and the ratio of the molar volume of solute to solvent. With the estimated model parameters, we constructed the state diagram for SMP, which provides a powerful tool for design and analysis of phase transition temperatures and solvent‐induced recovery. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Demonstrating the mechanism and efficacy of water‐induced shape memory and the influence of water on the thermal properties of oriented poly(d,l‐lactide)

Achieving water‐induced shape‐memory property in poly(D,L ‐lactide) (PDLLA), generated by means of advanced processing methods, opens possibilities to develop novel bioresorbable medical devices with shape‐memory properties activated by the human body without external heat. The main phenomena that affect the molecular movements that enable the water‐induced shape‐memory effect in an oriented PDLLA in an aqueous environment at physiological temperature are related to the water driven disruption of the intermolecular dipole‐dipole and/or hydrogen bonding of the oriented PDLLA chains and the subsequent decrease of the glass transition temperature (T_g) to the range of physiological temperature. The diffused water in the polymer matrix decreased the energy needed to finish the glass transition process explaining the higher shape‐recovery rate of the γ‐irradiated PDLLA with respect to the non‐γ‐irradiated PDLLA in an aqueous environment at physiological temperatures. The water‐induced decrease in the T_g was thermally reversible. The efficacy of the generated shape‐memory was tested with PDLLA shape‐memory nails in a pullout test, in which the pullout force of the PDLLA nails increased 360% during a seven day test period in vitro at 37°C. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4209–4218, 2013     

DNA/lipid complex organogel with shape‐memory behavior

DNA and amphiphilic N,N,N‐trimethyl‐N‐hexadecylammonium bromide (THAB) were stoichiometrically mixed together to produce a DNA–lipid complex, which was dissolved in dimethyl sulfoxide (DMSO) at 65°C and then crosslinked with isophorone diisocyanate (IPDI). The obtained organogel swelled reversibly in DMSO, responding to the variation of temperature. Interestingly, we observed that the gel exhibited a temperature‐dependent shape‐memory behavior. Above 65°C, whatever the shape the gel was deformed to, it could retain the new shape as the temperature was decreased to room temperature, while, when the gel was heated to 65°C again, it could recover its initial shape. The shape‐memory characteristic is supposedly originated from the transition between the close‐packed and the destroyed DNA/lipid conformation. The elastic urethane crosslinking bonds between base‐pair sites act as a fixing phase. The gel holds promise in its application as a gentle actuator. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 259–263, 2002     

Superhydrophobic films of UV‐curable fluorinated epoxy acrylate resins

Photopolymerization processes are often used in industrial applications because of their solvent‐free formulations and various advantages over conventional thermal processes. Fluorinated monomers and oligomers yield coatings of great interest because of the peculiar characteristics of fluorine atoms: these coatings show hydrophobicity, chemical stability, weathering resistance, etc. Novel UV‐curable fluorinated epoxy acrylate oligomers were synthesized from 1H,1H‐perfluorohexan‐1‐ol, 1,6‐hexamethylene diisocyanate (HDI) and epoxy acrylate (EA). The HDI plays the role of a spacer group in the side chain between the EA backbone chain and the fluorinated segment. This new spacer containing a urethane moiety with long alkyl groups can exhibit a self‐organization effect through the formation of strong hydrogen bonding. This resulted in a stiffening of the whole HDI urethane–perfluoalkyl chain to form nanostructure surface segregation. The designed fluorinated EA with fluoroalkyl (C₅F₁₁) units in the side chain exhibited a contact angle of about 151°, which is in the superhydrophobic range. Copyright © 2010 Society of Chemical Industry     

Numerical study of smart honeycomb core using shape memory polymers

Shape memory polymers (SMPs) attract widespread attention because they are able to maintain a temporary deformation after unloading and recover the initial shape under high temperature conditions. Based on a three‐dimensionally constitutive equation of SMPs, a finite element program is followed by compiling user‐defined material subroutine, which describes the shape memory behavior of thermo‐mechanical experiment. A honeycomb core using SMP is designed, which has the ability to recover the initial shape after deformation and be used as a smart core for sandwich structures. To prove their advantages in the engineering application, a series of thermodynamic behaviors of the SMP honeycomb core are simulated, including loading at high temperature, cooling, unloading at the low temperature, and recovering original shape on heating. Shape memory behaviors of tensile, compressive, bending, and locally sunken deformations are demonstrated and the effect of time and temperature on the recovery process is discussed. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45672.     

Facile control of high temperature shape memory polymers

Glass transition temperature (T_g) is crucial in determining application areas of high temperature shape memory polymers (SMPs), but some T_gs are difficult or uneconomic to be obtained. Here we introduce a facile way to prepare high temperature SMPs with controllable T_gs from 183 to 230 °C by copolymerization of polyimides, and relationships between T_gs and diamine components of the shape memory copolyimides agree with Fox Equation. These copolyimides can fix temporary shape and return to original shape nicely, and the possible mechanisms of their high shape fixity and shape recovery are analyzed on the basis of thermomechanical properties and molecular structures. The copolymerization of shape memory polyimides has offered an effective way to obtain high temperature SMPs with desired properties. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44902.     

Thermomechanical and shape memory properties of thermosetting shape memory polymer under compressive loadings

Shape memory polymers (SMPs) are an emerging class of active polymers that may be used for a range of reconfigurable structures. In this study, the thermomechanical and shape memory behavior of a thermosetting SMP was investigated using large‐scale compressive tests and small‐scale indentation tests. Results show that the SMP exhibits different deformation modes and mechanical properties in compression than in tension. In glassy state, the SMP displays significant plastic deformation and has a much higher modulus and yield strength in comparison to those obtained in tension. In rubbery state, the SMP behaves like a hyperelastic material and again has a much higher modulus than that obtained in tension. The SMPs were further conditioned separately in simulated service environments relevant to Air Force missions, namely, (1) exposure to UV radiation, (2) immersion in jet‐oil, and (3) immersion in water. The thermomechanical and shape recovery properties of the original and conditioned SMPs were examined under compression. Results show that all the conditioned SMPs exhibit a decrease in T_g as compared to the original SMP. Environmental conditionings generally result in higher moduli and yield strength of the SMPs in the glassy state but lower modulus in the rubbery state. In particular, the UV exposure and water immersion, also weaken the shape recovery abilities of the SMPs. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Soybean oil‐based shape‐memory polyurethanes: Synthesis and characterization

Shape‐memory polymers (SMPs) have wide range of applications due to their ability to sense environmental stimuli and reshape from a temporary shape to a permanent shape. Plant oil‐based polymeric materials are highly concerned in recent years in consideration of petroleum depletion and environmental pollution. However, plant oil‐based polymers are rarely investigated regarding their shape‐memory characteristics though bio‐based SMPs are highly desired nowadays. In this study, a series of soybean oil‐based shape‐memory polyurethanes (SSMPUs) are prepared through a mild chemo‐enzymatic synthetic route, and their properties are fully characterized with tensile testing, DSC, dynamic mechanical analysis (DMA), and shape‐memory testing. Results show that SSMPUs are soft rubbers with tensile strength in the range of 1.9–2.2 MPa and glass transition temperature in the range of 2–5°C, and possess good shape recoveries at RT when stretching ratio is 10, 20, and 30%, respectively. This work would promote the development of high‐value‐added plant oil‐based shape‐memory polyurethanes. Practical applications: Using annual renewable plant oil as feedstock, the synthesized SSMPUs show good shape recovery properties, which will make them applicable as potential alternatives to petroleum‐based shape‐memory materials. The simple and mild preparation process also contributes to the further exploration of plant oil to value‐added functional materials.     

Fabrication of a functionally graded and magnetically responsive shape memory polymer using a 3D printing technique and its characterization

In this study, a three‐dimensional printing technique is applied for the fabrication of novel functionally graded magnetic shape memory polymers (SMPs) to create high‐resolution multimaterial shape memory architectures. This approach is applied to a copolymer network of photocurable methacrylate using high projection stereolithography. Carbonyl iron particles (CIPs) were physically embedded in a polymer matrix to add magnetic functions to the SMPs. The glass transition characteristics and shape memory effect were also investigated by varying the composition of the SMP. The microstructured, lightweight SMPs showed interesting shape memory behaviors, as observed in hot environment. The almost perfect strain recovery rate of poly(ethylene glycol) dimethacrylate was measured (99.95% using a tension set bar). The results of dynamic mechanical analysis and thermogravimetric analysis reveal an increment in the thermal conductivity after embedding the CIPs. Further, the results of dynamic mechanical analysis, differential scanning calorimetry, and scanning electron microscopy reveal close interaction between the particles and matrix. X‐ray diffraction was used to characterize the iron particles and polymer structure. These results, along with the electrical and magnetic tests, strongly support the remote controllability of the material properties of the present functionally graded magnetic SMPs for a broad range of temperature and/or magnetically responsive material applications by using eddy current heating and/or magnetorheological polymeric effects. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45997.     

Sustainable shape memory polymers based on epoxidized natural rubber cured by zinc ferulate via oxa-Michael reaction

Although various shape memory polymers (SMPs) or diverse applications have been widely reported, the SMPs based on rubbers have been rarely realized due to the low triggering temperature of rubbers. In another aspect, the SMPs based on sustainable substances are highly desired for the growing shortage in fossil resources. In the present study, we accordingly developed the sustainable SMPs with tunable triggering temperature, based on natural rubber (NR) and ferulic acid (FA) as the raw materials. Specifically, the SMPs are based on a crosslinked network of epoxidized natural rubber (ENR) crosslinked by in situ formed zinc ferulate (ZDF) via oxa-Michael reaction. The excellent shape memory effect (SME) is found in these SMPs, as evidenced by the high fixity/recovery ratio and the tunable triggering temperature. With the incorporation of natural halloysite nanotubes (HNTs), the stress and recovery rate of the SMPs are found to be tunable, which widens the application of this kind of SMPs. The combination of adoption of sustainable raw materials, and the excellent and tunable SME makes these SMPs potentially useful in many applications, such as various actuators and heat-shrinkable package materials.     

Thermomechanical properties and shape memory effect of epoxidized natural rubber crosslinked by 3‐amino‐1,2,4‐triazole

A thermally induced shape memory polymer based on epoxidized natural rubber (ENR) was produced by curing the ENR with 3‐amino‐1,2,4‐triazole as a crosslinker in the presence of bisphenol‐A as a catalyst. Dynamic mechanical and tensile analysis was conducted to examine the variation of glass transition temperature, stiffness, and extensibility of the vulcanizates with the amount of curatives. Shape memory properties of the ENR vulcanizates were characterized by shape retention and shape recovery. It was revealed that the glass transition temperature of the ENR vulcanizates could be tuned well above room temperature by increasing the amount of curing agents. Also, ENR vulcanizates with T_g higher than ambient temperature showed good shape memory effects under 100% elongation, and the response temperatures of the recovery were well matched with T_g of the samples. Copyright © 2006 Society of Chemical Industry     

Fabrication of Shape‐Memory Aerogel Based on Chitosan/Poly(ethylene glycol) Diacrylate Semi‐Interpenetrating Networks via a Facile and Eco‐Friendly Strategy

While the field of shape memory polymers (SMPs) has developed rapidly, it is still highly challenging to obtain SMPs in the form of aerogels (SMPAs) due to the unique technique used for the fabrication of the aerogels and their high porosity. Herein, a thermally induced SMPA based on chitosan/poly(ethylene glycol) diacrylate (CS/PEGDA) semi‐interpenetrating networks is reported that are produced using an eco‐friendly strategy. The main network is responsible for the shape memory effect (SME) and can be easily tuned by varying the feed ratio of the two PEGDA precursors, which have different molecular weights. The crystalline segment in poly(ethylene glycol) diacrylate (PEGDA) with higher molecular weight acts as the molecular switch, and the PEGDA with lower molecular weight endows the network with an efficient degree of crosslinking. Meanwhile, the chitosan (CS) is interpenetrated into the main network to enhance the aerogel. The SME is realized both at the macroscale and the microscale, as is further demonstrated for three different models with various shapes.