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     

Effect of the crosslinking density and programming temperature on the shape fixity and shape recovery in epoxy–anhydride shape‐memory polymers

In addition to the fabrication of thermoset epoxy–anhydride shape‐memory polymers (SMPs), a systematic experimental investigation was conducted to characterize the crosslinking density, micromorphology, thermal properties, mechanical properties, and shape‐memory effects in the epoxy SMP system, with a focus on the influence of the crosslinking density and programming temperature on the shape‐fixity and shape‐recovery behaviors of the polymers. On the basis of the crosslinking density information determined by NMR technology, we concluded that the effect of the crosslinking density on the shape‐fixity behaviors was dependent on the programming temperature. The advantage of a nice combination of crosslinking density and programming temperature provided an effective approach to tailor the actual shape recovery within a wide range. The increasing crosslinking density significantly improved the shape‐recovery ratio, which could be further improved through a decrease in the programming, whereas the crosslinking density was more fundamental. This exploration should play an important role in the fabrication and applications of SMP materials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40559.     

Thermal,mechanical, and shape‐memory properties of nanorubber‐toughened,epoxy‐based shape‐memory nanocomposites

Epoxy‐based shape‐memory polymers (ESMPs) are a type of the most promising engineering smart polymers. However, their inherent brittleness limits their applications. Existing modification approaches are either based on complicated chemical reactions or done at the cost of the thermal properties of the ESMPs. In this study, a simple approach was used to fabricate ESMPs with the aim of improving their overall properties by introducing crosslinked carboxylic nitrile–butadiene nanorubber (CNBNR) into the ESMP network. The results show that the toughness of the CNBNR–ESMP nanocomposites greatly improved at both room temperature and the glass‐transition temperature (T_g) over that of the pure ESMP. Meanwhile, the increase in the toughness did not negatively affect other macroscopic properties. The CNBNR–ESMP nanocomposites presented improved thermal properties with a T_g in a stable range around 100 °C, enhanced thermal stabilities, and superior shape‐memory performance in terms of the shape‐fixing ratio, shape‐recovery ratio, shape‐recovery time, and repeatability of shape‐memory cycles. The combined property improvements and the simplicity of the manufacturing process demonstrated that the CNBNR–ESMP nanocomposites are desirable candidates for large‐scale applications in the engineering field as smart structural materials. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45780.     

Reinforcement in the mechanical properties of shape memory liquid crystalline epoxy composites

In this work, novel thermoresponsive shape memory composites based on glass fiber and nanosilica‐modified liquid crystalline epoxies (LCEs) with lateral substituent were prepared and characterized. According to the comprehensive analysis of polarized optical microscopy, wide‐angle X‐ray diffraction measurements, and tan δ data, the orientation of mesogen units were hindered by the introduction of nanosilica and lateral substituents of liquid crystalline epoxies, so that additional physical cross‐links except for similar chemical cross‐links emerged with the introduction of surface‐treated nanosilica. And the increased cross‐links could enhance the shape memory properties of the composites which could recover to their original state quickly in a time shorter than 30 s with high shape fixing ratios (>96%) and high shape recovery ratios (>98%), which indicated the composites could be applied into self‐deployable structural materials. Moreover, the reinforcement in the dynamic storage moduli, tensile modulus, and the tensile strength and shape memory properties indicated that glass fiber and nanosilica‐modified shape memory liquid crystalline epoxy composites could be high‐performance composites and could be used as new candidates for aerospace smart materials. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42616.     

Effect of phenol end functional switching segments on the shape memory properties of epoxy‐cyanate ester system

The effect of phenol end functional shape memory oligomers on the shape memory properties of an epoxy‐cyanate ester resin system was examined. The basic resin system consisted of diglycidyl ether of bisphenol A (DGEBA) cured with bisphenol A dicyanate (BADC). For conferring the shape memory properties, the switching segment (SS) components selected are α, ω‐phenol‐terminated poly(tetramethyleneoxide) (PPTMO), poly(ε‐caprolactone) (PPCL), and poly(propylene glycol) (PPPG). Epoxy‐cyanate ester blend of defined composition was analyzed for thermal, mechanical, thermo‐mechanical, and shape memory properties at two concentrations of the three SSs. The transition temperature of heavily SS loaded matrix increased in the order: PPTMO < PPCL < PPPG commensurate with crystallizability of SS segments at ambient. For same reason flexural property showed an increasing trend. This is in league with the increased crystallizability of the shape memory polymer components. The shape fixity, recovery extent, and recovery time followed a reverse order: PPPG < PPCL < PPTMO. In contrast to the alcohol terminated shape memory components, phenol terminal groups were helpful in integrating the shape memory segments into the matrix by way of reaction with both epoxy and cyanate groups. The coreaction was conducive for achieving better shape memory properties and decreasing the transition temperature. A direct relation existed between the modulus ratio and the shape recovery property. Higher concentration of the SSs caused a diminution in transition temperature but enhanced the shape memory properties, though the mechanical properties were adversely affected. The shape recovery increased with increase in temperature. All polymers possessed good mechanical properties and thermal stability. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41196.     

Stress memory of a thermoset shape memory polymer

The 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 T_g) and 45°C (within the T_g 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.     

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.     

Highly filled blends of a vinylic copolymer with plasticized lignin: Thermal and mechanical properties

The objective of this study was the development of new vinyl flooring formulations with increased resistance to attack by fungi and microorganisms, formulated with plasticizers having chemical compositions different from that of common dioctyl phthalate (DOP). Alkyl phthalate plasticizers are considered to be toxicological and ecotoxicological hazards, although this is still under debate. It is suspected that during the service life of poly(vinyl chloride) (PVC) flooring, the attack of fungi and microorganisms leads to the degradation of DOP and the release of some volatile organic compounds. For this reason, in the new flooring formulations, the vinyl chloride/vinyl acetate copolymer (VC–VAc) was partially replaced with lignin, a natural polymer and a major component of wood and vascular plants. Besides its other functions in wood, lignin imparts resistance to microorganisms. An organosolv lignin from Alcell Technologies, Inc. (AL), was used as a partial replacement of PVC. The influence of the new plasticizers, as well as the influence of the partial replacement of VC–VAc with lignin, on the morphology and thermal and mechanical properties of the composites was investigated with scanning electron microscopy, differential scanning calorimetry, and tensile testing. Butyl benzyl phthalate and diethylene glycol dibenzoate were used as plasticizers; both were compatible with PVC and AL. The results indicated that diethylene glycol dibenzoate was the best plasticizer for these blend composites. In these formulations, AL could replace up to 20 parts of the copolymer. At this level of replacement, the key mechanical properties of the new composites compared very favorably with those of the DOP control formulations. The obtained formulation will be tested further for resistance to fungi and microorganisms. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2000–2010, 2003     

Microstructure and shape memory effect of acidic carbon nanotubes reinforced polyvinyl alcohol nanocomposites

Specimens of acidic multi‐walled carbon nanotubes (AMWNTs) reinforced polyvinyl alcohol (PVA) nanocomposites (AMWNTs‐PVA) were prepared using different amounts of AMWNTs by the traditional solution casting, involving ultrasonic wave agitation. The microstructures and tensile properties of AMWNTs‐PVA were investigated by scanning electron microscopy, dynamic mechanical analysis, and quasi‐static tensile testing. AMWNTs had good compatibility with PVA and dispersed evenly in the PVA matrix. The incorporation of AMWNTs improved the tensile modulus and strength of the PVA. The shape recovery testing revealed the shape recovery capacity of AMWNTs‐PVA. It was observed that the recovery ratio increased, and the shape recovery rate slightly decrease with the increase of AMWNTs content. The results showed that the AMWNTs had strong interaction with the segments of the PVA and hence affected the shape recovery behavior of PVA. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Multi‐stimuli responsive shape memory polymers synthesized by using reaction‐induced phase separation

Thermo‐induced multishape memory polymers are a growing focus of smart materials because of its promising applications. Multishape memory effect is generally attained by using polymers with broad phase transition and multiphase polymers. The latter is of particular interest for copolymerization and polymer compositing. One requirement has to be fulfilled to achieve multishape memory effect, which is to have two reversible phase transitions. In this study, we report synthesis of polymer composite composed of strong segregated polymers by using reaction‐induced phase separation. We demonstrate the method by using polyurethane (PU) and poly(methacrylic acid) (PMAA). With adjusting the weight ratio, the polymer composites exhibit a phase spectrum from phase separation to miscible composite. The composite with PU/PMAA =3:1 demonstrated triple‐shape memory effect. Based on the results, we argued the effect of segregation on the shape memory effect for polymer composites. With the addition of PMAA, the polymer composite also exhibits pH/water‐induced shape memory effect. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43534.     

Microwave responsive epoxy nanocomposites reinforced by carbon nanomaterials of different dimensions

This study fabricated nanocomposites consisting of epoxy‐based shape memory polymer (ESMP) matrix and carbon nanofillers. The nanofillers include zero‐dimensional carbon black, one‐dimensional multiwalled carbon nanotubes, two‐dimensional (2D) graphene nanoplatelets, and three‐dimensional (3D) functionalized graphene sheets, which are all efficient microwave‐absorbing materials that can transform microwaves into heat energy. As a result, the temperatures of the nanocomposites increased more rapidly than pristine ESMP in microwaves. The functionalized graphene sheets were found to transform the microwaves into heat more efficiently than the other nanofillers. Possible microwave propagation paths in the nanocomposites were proposed. Moreover, the nanocomposites displayed significantly higher mechanical strengths than pristine ESMP. The low cost and strong nanocomposites with fast microwave responses may be applied as actuators or deployable devices in medical treatments. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45676.     

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     

Blends composed of poly(N‐isopropylacrylamide) and an ethylene/vinyl alcohol copolymer: Thermal and morphological studies

Blends of poly(N‐isopropylacrylamide) (PNIPAM) and an ethylene/vinyl alcohol copolymer (EVAL) were obtained through casting from dimethyl sulfoxide (DMSO) solutions and phase inversion in 50/50 DMSO/H₂O solutions. The miscibility and morphology of the PNIPAM/EVAL blends were investigated with thermal and morphological analysis. Differential scanning calorimetry indicated that the crystallinity of EVAL decreased with increasing PNIPAM content and that the blends cast from DMSO/H₂O solutions were miscible in the melt state. Measurements of the melting point depression allowed the determination of the interaction energy density (B) and Flory–Huggins interaction parameter (χ₁₂) with the Nishi–Wang equation. The negative B and χ₁₂ values obtained were examined in terms of the specific intermolecular interactions between the polymers. Scanning electron micrographs revealed that blends obtained by the casting method led to dense membranes, whereas the phase‐inversion method rendered typical macroporous membranes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 501–505, 2004     

High temperature shape memory polyimide ionomer

In this work, a high temperature shape memory polymer based on polyimide (PI) ionomer is prepared by introducing ionic crosslinked interaction. The ionic crosslinked points are introduced to polymer networks through the reaction between polyamic acid and calcium hydroxide before thermal imidization. The crosslinked reaction, microtopography, mechanical, thermal, and shape memory properties of PI ionomers are systematically investigated. The results show the introduction of ionic crosslinked interaction could enhance the glass transition temperature, mechanical, and shape recovery performance of ODA‐ODPA, a PI. The prepared ionomers exhibit good high temperature shape memory properties around 270 °C. The shape fixation and shape recovery ratio are over 99% and 90%, respectively. This method provided a new sight of preparing high temperature shape memory polymer, which could be used in severe conditions, like aerospace industry field. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43630.     

Hyperbranched‐modified epoxy thermosets: Enhancement of thermomechanical and shape‐memory performances

In this study a complete characterization of the thermomechanical and shape‐memory properties of epoxy shape‐memory polymers modified with hyperbranched polymer and aliphatic diamine was performed. Focusing on the mechanical properties that are highly desirable for shape‐memory polymers, tensile behavior until break was analyzed at different temperatures and microhardness and impact strength were determined at room temperature. As regards shape memory performance, the materials were fully characterized at different programming temperatures to study how this influenced the recovery ratio, fixity ratio, shape‐recovery velocity, and switching temperature. Tensile testing revealed a peak in deformability and in the stored energy density at the onset of the glass transition temperature, demonstrating that this is the best programming temperature for obtaining the best shape‐memory performances. The Young's moduli revealed more rigid structures in formulations with higher hyperbranched polymer content, while microhardness showed higher values with increasing hyperbranched polymer content due to the increased crosslinking density. Impact strength was greatly improved as the aliphatic diamine content increases due to the energy dissipation capability of its flexible structure. As regards the shape‐memory properties, increasing the programming temperature has a minor effect on formulations with a lower hyperbranched polymer content and worsens these properties when the hyperbranched polymer content is increased. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44623.     

pH and thermo‐responsive poly(N‐isopropylacrylamide) copolymer grafted to poly(ethylene glycol)

pH and thermo‐responsive graft copolymers are reported where thermo‐responsive poly(N‐isopropylacrylamide) [poly(NIPAAm), poly A ], poly(N‐isopropylacrylamide‐co‐2‐(diethylamino) ethyl methacrylate) [poly(NIPAAm‐co‐DEA), poly B ], and poly(N‐isopropylacrylamide‐co‐methacrylic acid) [poly(NIPAAm‐co‐MAA), poly C ] have been installed to benzaldehyde grafted polyethylene glycol (PEG) back bone following introducing a pH responsive benzoic‐imine bond. All the prepared graft copolymers for PEG‐g‐poly(NIPAAm) [ P‐N1 ], PEG‐g‐poly(NIPAAm‐co‐DEA) [ P‐N2 ], and PEG‐g‐poly(NIPAAm‐co‐MAA) [ P‐N3 ] were characterized by ¹H‐NMR to assure the successful synthesis of the expected polymers. Molecular weight of all synthesized polymers was evaluated following gel permeation chromatography. The lower critical solution temperature of graft copolymers varied significantly when grafted to benzaldehyde containing PEG and after further functionalization of copolymer based poly(NIPAAm). The contact angle experiment showed the changes in hydrophilic/hydrophobic behavior when the polymers were exposed to different pH and temperature. Particle size measurement investigation by dynamic light scattering was performed to rectify thermo and pH responsiveness of all prepared polymers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Recent advance on constitutive models of thermal‐sensitive shape memory polymers

Shape memory polymers (SMPs) are a novel class of shape memory materials which can store a deformed (temporary) shape and recover an original (permanent) shape under a shape memory thermomechanical loading–unloading cycle. The deformation mechanisms of SMPs are very complicated, but the SMPs also have a lot of advantages and the widespread application value and prospect. So developing proper constitutive models that describe thermomechanical properties of SMPs and the shape memory effect is very challenging and of great theoretical and application value. Based on the deformation mechanisms and considerable experimental investigations of SMPs, researchers have developed many constitutive models. This article investigates the deformation mechanism and introduces the recent research advance of the constitutive models of thermal‐sensitive SMPs. Special emphases are given on the micromechanical constitutive relations in which the deformation is considered being based on the microstructure of the SMPs. Finally, the lack of research and prospects for further research are discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Dynamic mechanical and shape memory properties of polybenzoxazines based on aminopropyl‐terminated siloxanes

Four siloxane‐containing benzoxazine monomers and telechelic benzoxazine oligomers were synthesized from 1,3‐bis(3‐aminopropyl)‐1,1,3,3‐tetramethyldisiloxane, α,ω‐bis(3‐aminopropyl)polydimethylsiloxane, phenol, o‐allylphenol, and formaldehyde. The length of the siloxane segment affects the polymerization reaction of the benzoxazine monomers and telechelic benzoxazine oligomers. The dynamic mechanical properties of the corresponding polybenzoxazines depend primarily on the structure of phenol and the length of the siloxane segment. The polybenzoxazines exhibit one‐way dual‐shape memory behavior in response to changes in temperature. The thermally induced shape memory effects of the polybenzoxazines were characterized by bending and tensile stress–strain tests with a temperature program based on their glass transition temperatures. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44121.     

Partially constrained recovery of (meth)acrylate shape‐memory polymer networks

The purpose of this study was to characterize the partial strain recovery of a thermoset shape‐memory polymer under a constraining stress. Three polymer networks were synthesized from tert‐butyl acrylate and poly(ethylene glycol) dimethacrylate (PEGDMA) solutions. The molecular weight and the weight fraction of the PEGDMA crosslinking monomer was altered systematically to maintain a constant glass transition temperature (T_g = 54°C) but tailorable rubbery moduli, which varied by almost an order of magnitude for the three polymer networks (E = 1.8–11.3°MPa). The shape‐recovery behavior of the polymers under a constraining stress was characterized for programming temperature below (20°C) and above (70°C) the T_g. The experiments revealed a peak in the recovered strain for samples programmed at 20°C. Recovered strain scaled linearly with the constraining stress by the rubbery modulus. The work performed by the shape‐memory polymer networks was observed to be primarily a function of constraining stress and crosslinking density, while programming temperature had a relatively mild influence; however, the efficiency of the shape‐memory effect was shown to be a function of constraining stress and programming temperature, but was independent of crosslinking density. Maximum work efficiencies (up to 45%) were observed for programming temperature of 70°C. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

The influence of heat treatment on the properties of shape memory fibers. II. Tensile properties,dimensional stability,recovery force relaxation,and thermomechanical cyclic properties

Shape memory fibers (SMFs) were prepared via a melt spinning process. The fibers were subject to different heat treatments to eliminate internal stress and structure deficiency caused during the melt spinning process. The influences of heat treatments on the SMF thermal properties, molecular orientation, tensile properties, dimensional stability, recovery force relaxation, and thermomechanical cyclic properties were studied. It was found that the heat treatments increased soft segment crystallinity and phase separation while decreased molecular orientation. The low‐temperature heat treatment increased the breaking elongation, shape fixity ratios, and decreased boiling water shrinkage while shape recovery ratios were decreased. High‐temperature treatment increased both the shape recovery ratios, fixity ratios, recovery stress stability and at the same time decreasing the fiber mechanical strength. The results from differential scanning calorimetry, molecular orientation apparatus, and cyclic tensile testing were used to illustrate the mechanism governing the mechanical properties and shape memory effect. To obtain comprehensive outstanding properties, the SMF is expected to be treated at a high temperature because of the hard segment high glass transition temperature. Unfortunately, the heat treatment could not be conducted at a too high temperature because the SMF became too tacky and soft due to the melting of the soft segment phase. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009