Relaxation suppression in a stretched copolymer matrix above Tg

Electrospun thermoplastic polyurethane (TPU) copolymer nanofiber mats are known to contract considerably upon heating up to ～90 °C, whereas cast TPU films expand as expected. This work examined contraction in single electrospun nanofibers. In contrast to nanofiber mats, where mat contraction appears like a critical phenomenon, no temperature threshold for contraction was measured for a single electrospun nanofiber. Unexpectedly, we demonstrate that cast TPU films can also massively contract upon heating, but only after thermomechanical programming which relies on film stretching (～100%) at a high temperature (～90 °C). This nonequilibrium stretched state is highly preserved, despite sample temperatures that significantly exceeded the glass transition temperature of the soft phase, and hard segments concentration in the TPU macromolecules is too low to form a percolated “solid” system. A possible physical explanation of the observed phenomenon is proposed. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1254–1259     

A predictive parameter for the shape memory behavior of thermoplastic polymers

We present an experimental and modeling study of the effect of programming conditions on the shape-memory behaviors of amorphous thermoplastic polymers. Experimentally we measure the influence of deformation temperature, strain rate and relaxation time on the thermomechanical properties and shape-memory response of poly(para-phenylene), which is a stiff and strong aromatic thermoplastic. To understand the underlying mechanism, we develop a viscoelastic model, which contains multiple discrete relaxation processes with broad distribution of relaxation time. The model parameters of the relaxation spectrum are obtained from the master curve of small strain–stress relaxation tests using time-temperature superposition. The model predictions show good agreement with experimental observations, including the stress response and shape-memory response under various conditions. We applied the model to study the effect of the programming conditions on the shape recovery performance. The results show that the relaxation modulus at the end of the programming process was a predictor of the recovery speed and recoverable strain ratio. This provides a design metric to optimize the shape programming process for shape recovery. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1405–1414     

Thermo-mechanical behavior of electrospun thermoplastic polyurethane nanofibers

Analysis of the thermo-mechanical behavior of electrospun thermoplastic polyurethane (TPU) block co-polymer nanofibers (glass transition temperature ∼−50 °C) is presented. Upon heating, nanofibers began to massively contract, at ∼70 °C, whereas TPU cast films started to expand. Radial wide-angle X-ray scattering (WAXS) profiles of the nanofibers and the films showed no diffraction peaks related to crystals, whereas their amorphous halo had an asymmetric shape, which can be approximated by two components, associated with hard and soft segments. During heating, noticeable changes in the contribution of these components were only observed in nanofibers. These changes, which were accompanied with an endothermic DSC peak, coinciding with the start of the nanofibers contraction, can be attributed to relaxation of an oriented stretched amorphous phase created during electrospinning. Such structure relaxation becomes possible when a portion of the hard segment clusters, forming an effective physical network, is destroyed upon heating.     

Shape‐memory behavior of segmented polyurethanes with an amorphous reversible phase: The effect of block length and content

Segmented thermoplastic polyurethanes (TPU)s with amorphous soft segments from the reaction of hexamethylene diisocyanate and 1,2‐butanediol and crystalline hard segments from 4,4′‐methylenediphenyl diisocyanate and 1,6‐hexanediol showed sharp glass‐transition temperatures that could be used as shape‐recovery temperatures. The thermal, mechanical, and shape‐memory effect of these TPUs of various block compositions and lengths were studied by differential scanning calorimetry, dynamic mechanical testing, and tensile testing. As the block lengths decreased, phase mixing increased and hysteresis in the shape‐memory behavior decreased. Too low a content of hard segments increased the hysteresis in the shape‐memory behavior. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2652–2657, 2000     

Shape‐memory effect in hyperbranched poly(ethyleneimine)‐modified epoxy thermosets

A series of shape‐memory epoxy thermosets were synthesized by crosslinking diglycidyl ether of bisphenol A with mixtures of commercially available hyperbranched poly(ethyleneimine) and polyetheramine. Thermal, mechanical and shape‐memory properties were studied and the effect on them of the content and structure of the hyperbranched polymer was discussed. Measurements showed that the glass transition temperature can be tailored from 60 °C to 117 °C depending on the hyperbranched polymer content, and all formulations showed an appropriate glassy/rubbery storage modulus ratio. Shape‐memory programming was carried out at TgE′ given the excellent mechanical properties of the materials, with maximum stress and failure strain up to 15 MPa and 60%, respectively. The resulting shape‐memory behavior was excellent, with maximum shape recovery and shape fixity of 98% as well as a fast shape‐recovery rate of 22%/min. The results show that hyperbranched poly(ethyleneimine) as a crosslinking agent can be used to enhance mechanical and shape‐memory properties with different effects depending on the crosslinking density. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 924–933     

Bio‐based epoxy vitrimers: Reprocessibility,controllable shape memory,and degradability

The research activities in the development of recyclable and reprocessable covalently crosslinked networks, and the construction of polymers from renewable resources are both stemmed from the economical and environmental problems associated with traditional thermosets. However, there is little effort in combination of these two attractive strategies in material designs. This article reported a bio‐based vitrimer constructed from isosorbide‐derived epoxy and aromatic diamines containing disulfide bonds. The resulted dynamic epoxy resins showed comparable thermomechanical properties as compared to similar epoxy networks cured by traditional curing agent. Rheological tests demonstrated the fast stress relaxation of the dynamic network due to the rapid metathesis of disulfide bonds at temperature higher than glass transition temperature. This feature permitted the recycling and reprocessing of the fragmented samples for several times by hot press. The dynamic epoxy resins also exhibited shape‐memory effect, and it is demonstrated that the shape recovery ratio could be readily adjusted by controlling the stress relaxation in the temporary state at programming temperature. Moreover, the degradability of the dynamic epoxy resins in alkaline aqueous solution was also demonstrated. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1790–1799     

Cold,warm, and hot programming of shape memory polymers

In this article, programming is classified as hot, warm, and cold, based on the temperature zone within which the programming is conducted. The strain and stress locking and releasing mechanisms are discussed within the thermodynamics framework. A new formula is developed for quantifying the strain recovery ratio of cold-programmed SMPs. Stress fixity ratio and stress recovery ratio are also defined based on the understanding of stress locking and recovery mechanisms. State-of-the-art literature on warm and cold programming is reviewed. Well-controlled programming as well as free strain recovery test and constrained stress recovery test are conducted, in order to validate the memory mechanisms discussed in this study. It is found that, while programming temperature has an insignificant effect on the final free shape recovery, it has a significant effect on the stress recovery. The recovery stress programmed by cold programming may be lower, equal to, or higher than that by hot programming, due to the different stress locking mechanisms and other factors such as damage during the thermomechanical cycle. Cold, Warm, and Hot Programming of Shape Memory Polymers © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1319–1339     

Structural relaxation behavior of strain hardened shape memory polymer fibers for self‐healing applications

In this study, shape‐memory polyurethane (SMPU) fibers were strain hardened by cold‐drawing programming (CDP) process. The programmed fibers are experimentally studied on the physical and thermomechanical properties. Structural relaxation, which determines shape memory capability of the SMP fibers, is quantified by conformational entropy change. Based on the entanglement tube theory and reptation theory, the entropic force is derived as a “bridge” to link the stress relaxation and structural relaxation, and thus structural relaxation can be evaluated by stress relaxation. It was found that the CDP SMPU fibers would still have good crackclosing capability after 13 years of hibernation in polymer matrix composite. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013 , 51, 966–977     

Effect of SSL and HSC on morphology and properties of PHA based SMPU synthesized by bulk polymerization method

Searching new shape memory polymer and the associating synthesis technology are critical on the development of smart materials. In this paper, a comprehensive study on Poly(hexylene adipate) PHA being the soft segment of shape memory polyurethane (SMPU) was presented. Bulk polymerization method was employed to synthesize the SMPU with different soft segment length (SSL) and hard segment content (HSC). The influences of SSL and HSC on its morphology and thermomechanical property using DSC, DMA, POM, and shape memory behavior were presented here. The results indicate that the thermal properties, dynamic mechanic properties, and crystal morphology of SMPU are influenced significantly by SSL and HSC. And it is found that the shape fixity increases with SSL but decreases with HSC. On the other hand, the shape recovery decreases with both SSL and HSC, and the associated recovery temperature increases either with the increasing SSL or with decreasing HSC. Lastly, it is concluded that in the PHA‐based‐SMPU, the lower limiting value of SSL for polyurethane having shape memory effect is 2000; their response temperature varied with SSL and HSC, changing from 41.0 to 51.9 °C. Stable hard segment crystal are formed at above 30% HSC sample in bulk polymerization, but shape memory behavior can also be observed when its physical crosslink point are formed in the lower HSC PHA‐based‐SMPU. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 444–454, 2007     

Connecting free volume with shape memory properties in noncytotoxic gamma‐irradiated polycyclooctene

The free volume holes of a shape memory polymer have been analyzed considering that the empty space between molecules is necessary for the molecular motion, and the shape memory response is based on polymer segments acting as molecular switches through variable flexibility with temperature or other stimuli. Therefore, thermomechanical analysis (TMA) and positron annihilation lifetime spectroscopy (PALS) have been applied to analyze shape recovery and free volume hole sizes in gamma‐irradiated polycyclooctene (PCO) samples, as a noncytotoxic alternative to more conventional PCO crosslinked via peroxide for future applications in medicine. Thus, a first approach relating structure, free volume holes and shape memory properties in gamma‐irradiated PCO is presented. The results suggest that free volume holes caused by gamma irradiation in PCO samples facilitate the recovery process by improving movement of polymer chains and open possibilities for the design and control of the macroscopic response. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1080–1088     

Influence of silica on shape memory effect and mechanical properties of polyurethane-silica hybrids

Polyurethane block copolymer (PU) was synthesized and was followed by a sol-gel reaction with tetraethoxysilane (TEOS) to prepare high performance polyurethane-silica hybrids with shape memory function. Their tensile and shape memory properties were compared as a function of TEOS content and PU hard segment content. A tensile test showed that the mechanical properties were largely influenced by TEOS content, and the maximum elongation-at-break as well as maximum breaking stress and modulus were obtained when TEOS at 10 wt% was used. Shape memory of hybrids was also obtained from a thermomechanical test, and showed good shape retention and shape recovery of more than 80% for all samples. Consequently, by silica hybridization, an improvement in the mechanical properties and shape recovery force of PU could be achieved without any decrease in their shape recovery effect.     

Room temperature deformable shape memory composite with fine‐tuned crystallization induced via nanoclay particles

It is known that particular types of semi‐crystalline/elastomer polymer blends exhibit shape memory effects (SME) due to the dispersion of two immiscible phases. In this study, the crystal structure of polylactic acid (PLA)/ thermoplastic polyurethane (TPU) based shape memory polymer (SMP) is altered by incorporating small amounts of montmorillonite (MMT) nanoclay. The results indicate the incorporation of MMT can improve the compatibility of the two different polymers. Moreover, the presence of MMT affects the total crystallinity of the SMP and improves mechanical properties. Lastly, uniaxial stretching deformation can be applied to the SMP at room temperature conditions while maintaining its shape memory properties. With 1 wt % MMT particles, the recovery ratio (Rr) was nearly 95%, which indicated a strong recovery effect. The shape‐fixing ratio (Rf) remained above 95% for all composites due to plastic deformation applied at room temperature. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1197–1206     

Preparation of Polylactide/Poly(ether)urethane Blends with Excellent Electro-actuated Shape Memory <Emphasis Type="Italic">via</Emphasis> Incorporating Carbon Black and Carbon Nanotubes Hybrids Fillers

In this work, hybrid conductive fillers of carbon black (CB) and carbon nanotubes (CNTs) were introduced into polylactide (PLA)/thermoplastic poly(ether)urethane (TPU) blend (70/30 by weight) to tune the phase morphology and realize rapid electrically actuated shape memory effect (SME). Particularly, the dispersion of conductive fillers, the phase morphology, the electrical conductivities and the shape memory properties of the composites containing CB or CB/CNTs were comparatively investigated. The results suggested that both CB and CNTs were selectively localized in TPU phase, and induced the morphological change from the sea-island structure to the co-continuous structure. The presence of CNTs resulted in a denser CB/CNTs network, which enhanced the continuity of TPU phase. Because the formed continuous TPU phase provided stronger recovery driving force, the PLA/TPU/CB/CNTs composites showed better shape recovery properties compared with the PLA/TPU/CB composites at the same CB content. Moreover, the CB and CNTs exerted a synergistic effect on enhancing the electrical conductivities of the composites. As a result, the prepared composites exhibited excellent electrically actuated SME and the shape recovery speed was also greatly enhanced. This work demonstrated a promising strategy to achieve rapid electrically actuated SME via the addition of hybrid nanoparticles with self-networking ability in binary PLA/TPU blends over a much larger composition range.     

Shape memory effects of polynorbornene modified by in-situ reactive thermoplastic polyurethane

Shape memory polymers (SMPs) based on polynorbornene (PNB) was prepared and modified by In-situ reactive thermoplastic polyurethane (TPU). Analysis shows that the TPU formed in PNB matrix slightly decreases T_g of PNB from 24.1 to ca. 23.4, which is beneficial to study the shape memory performance at room temperature. A small amount of TPU can be uniformly dispersed in PNB matrix to form interpenetrating network structure, which can significantly toughen and strengthen PNB. Simultaneously, the interpenetrating network can replace the physical entanglement of part of the PNB, increase the free volume among the molecular chains of PNB, make shape fixing easier, and reduce energy consumption in overcoming friction during the recovery process. Therefore, the PNB/TPU composites have higher shape fixing ratio and recovery ratio than PNB. When the content of TPU in PNB matrix is lower, the interpenetrating network of chain entanglements is formed with no phase separation; therefore, the improvement of shape memory performance is remarkable.     

Films from soft‐core/hard‐shell hydrophobic latexes: Structure and thermomechanical properties

Structured latexes provide a promising route to hard coatings without the use of coalescing aids. We studied the thermomechanical properties of films from structured soft‐core/hard‐shell hydrophobic latexes. We found that the mechanical properties of these films were closely related to their very particular organization. When the rigid phase was continuous, whatever its volume fraction, the films exhibited a high elastic modulus. An analysis of the viscoelastic properties of the films provided a good method for obtaining information about the interphase between the hard shell and soft core of the latex particles. By varying the film structure through annealing or the particle composition (core/shell ratio, core crosslinking, etc.), we were able to tune the mechanical properties of the films. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2989–3000, 2000     

Preparation of Polylactide/Poly(ether)urethane Blends with Excellent Electro-actuated Shape Memory via Incorporating Carbon Black and Carbon Nanotubes Hybrids Fillers

In this work, hybrid conductive fillers of carbon black(CB) and carbon nanotubes(CNTs) were introduced into polylactide(PLA)/thermoplastic poly(ether)urethane(TPU) blend(70/30 by weight) to tune the phase morphology and realize rapid electrically actuated shape memory effect(SME). Particularly, the dispersion of conductive fillers, the phase morphology, the electrical conductivities and the shape memory properties of the composites containing CB or CB/CNTs were comparatively investigated. The results suggested that both CB and CNTs were selectively localized in TPU phase, and induced the morphological change from the sea-island structure to the co-continuous structure. The presence of CNTs resulted in a denser CB/CNTs network, which enhanced the continuity of TPU phase.Because the formed continuous TPU phase provided stronger recovery driving force, the PLA/TPU/CB/CNTs composites showed better shape recovery properties compared with the PLA/TPU/CB composites at the same CB content. Moreover, the CB and CNTs exerted a synergistic effect on enhancing the electrical conductivities of the composites. As a result, the prepared composites exhibited excellent electrically actuated SME and the shape recovery speed was also greatly enhanced. This work demonstrated a promising strategy to achieve rapid electrically actuated SME via the addition of hybrid nanoparticles with self-networking ability in binary PLA/TPU blends over a much larger composition range.     

Stability and deterioration of a shape memory polymer fabric composite under thermomechanical stress

Fibres and fabrics are often used to reinforce shape memory polymers (SMPs) to improve their mechanical strength and properties, and such composites have been widely used in engineering. However incorporation of fibres and fabrics in SMPs is often accompanied with the deterioration of thermomechanical properties and shape memory effect. The thermomechanical properties and deterioration mechanisms of a shape memory polymer composite (SMPC) under repeated mechanical stress were investigated. Up to 100% extension, the SMPCs showed good shape memory effect with excellent shape recovery ratio, recovery stress and mechanical properties; while beyond that the recovery ratio and recovery stress of the composites deteriorated rapidly due to the significant delamination and debonding of fibres and fabrics from the SMP resin and accumulation of broken fibres.     

可生物降解多嵌段聚(酯-氨酯)的合成及其热致形状记忆性质

以端羟基L-丙交酯/乙交酯共聚物(PLLG-diol)和端羟基ε-己内酯/乙交酯共聚物(PCG-diol)为硬段和软段,通过与二异氰酸酯反应制得了软、硬分子量和组成均可调的多嵌段聚(酯-氨酯),表征了它们的形状记忆行为.多嵌段聚(酯-氨酯)具有良好的形状记忆性质,应变固定率达98%~99.5%,应变恢复率达93%~98.5%;通过转变温度的调节,可使多嵌段聚(酯-氨酯)在37℃体温下不发生形状变化,而在稍高于体温的温度(40~50℃)下恢复原始形状,其形状恢复速率可通过温度和升温速率来调节.     

Synthesis and characterization of photocrosslinked poly(ε‐caprolactone)s showing shape‐memory properties

Poly(ε‐caprolactone) (PCL) with a pendent coumarin group was prepared by solution polycondensation from 7‐(3,5‐dicarboxyphenyl) carbonylmethoxycoumarin dichloride and α, ω‐dihydroxy terminated poly(ε‐caprolactone) with molecular weights of 1250, 3000, and 10,000 g/mol. These photosensitive polymers underwent a rapid reversible photocrosslinking upon exposure to irradiation with alternating wavelengths (>280/254 nm) without a photoinitiator. The thermal and mechanical properties of the photocrosslinked films were examined by means of differential scanning calorimetry and stress–strain measurements. The crosslinked films exhibited elastic properties above the melting temperature of the PCL segment along with significant decrease in the ultimate tensile strength and Young's modulus. Shape‐memory properties such as strain fixity ratio (R_f) and strain recovery ratio (R_r) were determined by means of a cyclic thermomechanical tensile experiments under varying maximum strains (ε_m = 100, 300, and 500%). The crosslinked ICM/PCL‐3000 and ‐10,000 films exhibited the excellent shape‐memory properties in which both R_f and R_r values were 88–100% for tensile strain of 100–500%; after the deformation, the films recovered their permanent shapes instantaneously. In vitro degradation was performed in a phosphate buffer saline (pH 7.2) at 37 °C with or without the presence of Pseudomonas cepacia lipase. The presence of the pendent coumarin group and the crosslinking of the polymers pronouncedly decreased the degradation rate. The crosslinked biodegradable PCL showing a good shape‐memory property is promising as a new material for biomedical applications. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2422–2433, 2009     

Study on poly(ε‐caprolactone)‐based shape memory copolymer fiber prepared by bulk polymerization and melt spinning

In this paper, a poly(ε‐caprolactone) (PCL)‐based shape memory polyurethane fiber was prepared by melt spinning. The shape memory switching temperature was the melting transition temperature of the soft segment phase mainly composed of PCL at 47°C. The mechanical properties especially shape memory effect were explicitly characterized by thermomechanical cyclic tensile testing. The results suggest that the prepared fiber has shape memory effects. The prepared 40 denier shape memory fiber had a tenacity of about 1.0 cN/dtex, and strain at break 562–660%. The shape fixity ratio reached 84% and the recovery ratio reached 95% under drawing at high temperature and thermal recovery testing.¹ Finally, the fiber thermal/mechanical properties were measured using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Copyright © 2007 John Wiley & Sons, Ltd.