Strain Recovery Rate Research Articles

Fluorinated waterborne shape memory polyurethane urea for potential medical implant application

AbstractA series of crosslinked fluorinated waterborne shape memory polyurethane urea (PUU) ionomers were synthesized from polycaprolactone diol, perfluoropolyether (PFPE) diol, dimethylolproionic acid, isophorone diisocyanate, ethylenediamine (EDA), and diethylenetriamine (DETA). The effect of PFPE content in the soft segment and the degree of crosslinking on the molecular structure and the properties of these PUU films was examined and studied. Differential scanning calorimetry showed that the transition temperature for these Tm type shape memory PUU could be fine tuned by PFPE weight percentage and EDA/DETA ratio in the range between 33 and 44°C, covering the range of body temperature. Although incorporating amorphous fluorinated units into semicrystalline soft segment compromised the shape memory performance of PUU with linear structure as expected, the introduction of crosslinking structure using DETA as a trifunctional chain extender could still retain quite high strain recovery rate (above 90%) at 100% stretching deformation. Furthermore, the relationship of these properties as well as thermal stability with hydrogen bonding was also discussed by evaluation of the carbonyl stretching region in Fourier transform infrared spectra. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Phase Transformation and Shape Memory Effect of Ti(Pt, Ir)

The martensitic transformation and shape memory effect of Ti50(Pt, Ir)50 with 5–37.5 at. pct Ir were investigated using differential thermal analysis (DTA), high-temperature X-ray diffraction (HT-XRD), and compression tests. The austenite finish temperature, A f, increased with increasing Ir content from 1331 K (1058 °C) in Ti-50 at. pct Pt to 1491 K (1218 °C) in Ti-12.5Pt-37.5Ir. The structure of the parent and martensite phases was identified as B2 and B19 in all tested alloys. A large strain recovery rate was obtained in Ti50(Pt, Ir)50 with 10 to 30 at. pct Ir. The highest shape recovery ratio was 57 pct in Ti-25Pt-25Ir after deformation at 1123 K (850 °C), followed by heating to above A f. Using HT-XRD, the dependence of lattice parameter change on Ir content and temperature was investigated. The volume change during phase transformation from B2 to B19 was estimated using the lattice parameter of the B2 and B19 phases. Strain recovery is discussed along with volume change and lattice parameter change.

Polyester‐Hydrophilic PEO Networks as Multifunctional Biomaterials

AbstractIn order to improve the hydrophilic and elastic properties of polyester networks, the amorphous polymer networks of hydrophobic polyester/hydrophilic poly(ethylene glycol) dimethacrylate (PEGDMA) were prepared by UV‐photopolymerization of PEGDMA and poly[(D,L‐lactide)‐co‐glycolide]tetraacrylate (PLGATA). The interpenetrating polymer networks (IPNs) based on polyesterurethane and PEGDMA were synthesized in situ by UV‐photopolymerization of PEGDMA and thermal polymerization of oligo[(D,L‐lactide)‐co‐ε‐caprolactone]teraol (PCLA) or poly[(D,L‐lactide)‐co‐glycolide]teraol (PLGA) with isophorone diisocyanate (IPDI). The polymer networks and IPNs are transparent soft materials, and show good shape‐memory properties. They recovered their permanent shape in 12 seconds when the environment temperature was above glass transition temperature (Tg). The strain recovery rate (Rr) and the strain fixity rate (Rf) of the polymer networks and IPNs were above 90%. The wettability, degradation rate, mechanical properties, and Tg of the polymer networks and IPNs could be conveniently adjusted by changing PEGDMA content. The elastic networks are suitable for potential soft substrates with tailored mechanical properties for clinical or medical use.

Strain‐Based Temperature Memory Effect for Nafion and Its Molecular Origins

AbstractA shape memory polymer traditionally refers to a polymer that can memorize one temporary shape and recover to its permanent shape upon exposure to an external stimulus. Although this basic concept has been known for at least half a century, recent advances have led to the discoveriy of previously uncovered memory properties that challenge the traditional concept of shape memory polymers. In particular, a temperature memory effect refers to the capability of a polymer to memorize temperatures instead of shapes. Thus far, the reported temperature memory effect has been established under iso‐strain stress recovery conditions, in which the maximum recovery stress appears at a temperature roughly identical to the deformation temperature. This effect can be called recovery stress based temperature memory effect. In this work, experiments were designed in an attempt to establish a temperature memory effect based on the stress free strain recovery behaviors of Nafion. The results show that, under carefully selected conditions, the temperature at which a maximum strain recovery rate is observed can indeed be quantitatively related to the deformation temperature. In addition, indications that the polymer is capable of memorizing more than one deformation temperature (i.e., multi‐temperature memory effect) are shown. The molecular origin of Nafion’s temperature memory effect is elucidated through small angle neutron scattering study.

Residual plastic strain recovery driven by grain boundary diffusion in nanocrystalline thin films

Residual strain in metals is typically considered to be irreversible. However, residual strain in nanocrystalline materials can be recovered over a period of time via diffusive mechanisms. In this study, free-standing copper films of submicron thickness with an average grain size of about 40 nm are mechanically loaded via a plane-strain bulge test, and residual strain recovery at room temperature is characterized after unloading. The specimens recover their residual strain in a period of time that can range from a few days to more than 1 month depending upon the surface conditions and heterogeneous residual strain distributions in multiple cycles of recoveries. A constant tensile stress of about 25 MPa is reached after each recovery finishes. Two characteristic strain rates occur during residual strain recovery, a transient strain recovery rate of the order of 10 −7 s −1 and a steady-state strain recovery rate of the order of 10 −9 s −1. A model of the plastic strain recovery is presented which demonstrates the plausibility that grain boundary diffusion driven by chemical potential gradients due to residual stresses and the presence of voids can rationalize the transient and steady-state plastic strain recovery rates, respectively.

Synthesis and characterization of biodegradable, amorphous, soft IPNs with shape‐memory effect

AbstractStar‐shaped oligo[(D,L‐lactide)‐co‐ε‐caprolactone]s (PCLA) with various number average molecular weights were synthesized via ring‐opening polymerization of D,L‐lactide (DLLA) and ε‐caprolactone (CL) with organic Sn as catalyst and pentaerythritol as an initiator. The elastic amorphous interpenetrating polymer networks (IPNs) of polyesterurethane/poly(ethylene glycol) dimethacrylate (PEGDMA) were synthesized in situ by UV‐photopolymerization of PEGDMA and thermal polymerization of PCLA with isophorone diisocyanate (IPDI). IPNs are transparent soft materials and the gel content of the IPNs is exceeding 87%. They are rubbery when PEGDMA content is above 10% at room temperature. IPNs show good shape‐memory properties. IPNs recover quickly its permanent form in 10 sec when the environment temperature is above its glass transition temperature (Tg). IPNs have only one single Tg between the Tg of PEGDMA and polyesterurethane. The strain recovery rate (Rr) and the strain fixity rate (Rf) are above 90%. No characteristic peaks of PEG crystallites in X‐ray diffraction pattern (XRD) demonstrate that they are amorphous polymer networks. The wettability, degradation rate, mechanical properties, and Tg of the IPNs could be conveniently adjusted by changing PEGDMA content in IPNs. The soft IPNs are promising suitable as potential soft substrates with tailored mechanical properties for potential clinical or medical use. Copyright © 2011 John Wiley & Sons, Ltd.

Shape memory effect and pseudoelasticity of TiPt

Martensitic transformation behavior and shape memory properties of Ti-50at%Pt were investigated using high-temperature X-ray diffractometry and loading–unloading compression tests. The structure of the parent and martensite phases was identified as B2 and B19, respectively. Changes in the lattice parameters of the martensite phase were investigated as a function of temperature. Applying the lattice parameter changes, the volume strain between B19 and B2 phases at 1473 K was estimated at 2.5%. The compression test was carried out at room temperature and at 1123 K. Strain was precisely measured using a strain gage for the test at room temperature and a CCD camera for the test at 1123 K. Strain recovery was observed during unloading at room temperature and at 1123 K, which was below the martensite temperature. Shape recovery was investigated for the samples tested at room temperature and at 1123 K by heating at 1523 K for 1 h. It was found that the strain recovery rate was 30–60% for the samples tested at room temperature and about 11% for the samples tested at 1123 K.

Biodegradable Shape-Memory Polymers Exhibiting Sharp Thermal Transitions and Controlled Drug Release

Biodegradable shape-memory polymer networks prepared by cross-linking star shape branched oligo(ε-caprolactone) (bOCL) with hexamethylene diisocyanate are introduced. The thermal and mechanical properties of these networks were investigated using differential scanning calorimetry and tensile testing, respectively, and the morphology of the phase structure was characterized by polarized optical microscopy. The shape-memory properties of the networks were quantified using thermomechanical tensile experiments and showed strain fixity rates R(f) higher than 97% and strain recovery rates R(r) as high as 100%. Of note, networks of OCL segments with a lower degree of polymerization (DP; 10) exhibited significantly improved temperature-sensitive shape recovery: 90% of the permanent shape was recovered upon heating to within a 2 °C range (37-39 °C). The networks exhibited complete shape recovery to the permanent shape within 10 s at 42 °C. Theophylline-loaded (10 and 20 wt %) shape-memory materials, prepared by cross-linking bOCL with hexamethylene diisocyanate in the presence of theophylline, are also described as a model for a controlled drug release device. The 10 wt % loaded material was sufficiently soft and flexible for complex shape transformation and also showed high R(f) (98%) and R(r) (99%). Sustained release of loaded theophylline was achieved over 1 month without initial burst-release in a phosphate buffer solution (PBS; pH 7.4) at 37 °C.

Effect of Electron Beam Irradiation on the Properties of High Styrene Rubber/Styrene Butadiene Rubber Blends

High styrene rubber (HSR)/styrene butadiene rubber (SBR) blends at different ratios were exposed to various doses of electron beam irradiation. The effect of irradiation dose and blend ratios on the mechanical properties and shape memory characteristics in terms of strain fixation) rate (Rf) and strain recovery rate (Rr) was investigated. The results revealed that rich styrene blends displayed higher tensile strength and hardness than low styrene content blends at all irradiation doses. However, elongation at break, and toughness were lower for rich styrene content. Also, it was observed that for most specimens, the tensile strength, elongation at break and hardness increases up to100 kGy. Increasing irradiation doses resulted in slight deterioration in some mechanical properties only for low styrene content at150 kGy. According to the normalized tensile stress at 25% elongation, it was found that the contribution of irradiation in enhancing the mechanical properties is higher for rich butadiene blends. On the other hand, it was observed that rich styrene content blends possess higher Rf and Rr at all the irradiation doses and stretching temperatures. However, the increase of irradiation dose decreases Rf values; the extent of this decrease depends on the blend ratios. Conversely, for all blends, Rr were increased by increasing irradiation dose and styrene content ratios.

Mechanical behavior of shape memory fibers spun from nanoclay-tethered polyurethanes

This study examined the effect of nanoclays on the shape memory behavior of polyurethane (PU) in fibrous form. A cation was introduced into the PU molecules to disperse the organo-nanoclay (MMT) into poly(e- caprolactone) (PCL)-based PU (PCL-PU). The MMT/PCL-PU nanocomposites were then spun into fibers through melt-processing. The shape memory performance of the spun fibers was examined using a variety of thermomechanical tests including a new method to determine the transition temperature of shape memory polymers. The MMTs showed an improved the fixity strain rate of the MMT/PCL-PU fibers but a slight decrease in their recovery strain rate. This was explained by the limited movement of PU molecules due to the presence of nanoclays. The shape memory performance of the MMT/PCL-PU fibers was not enhanced significantly by the nanoclays. However, their recovery power was improved significantly up to a strain of approximately 50%.

Steady state creep and creep recovery behaviours of pre-aging Al–Si alloys

The creep and creep recovery of pre-aging Al–1 wt.%Si and Al–1 wt.%Si–0.1 wt.%Zr–0.1 wt.%Ti alloys have been investigated at room temperature under different constant stresses. The aging temperature dependence of steady creep rate, ɛ st, and the recovery strain rate, π, show that under the same test conditions first alloy yields creep or creep recovery rates much higher as compared with those of second alloy. The stress exponent n was found to change from 2.5 to 7.43 and 4.57 to 11.99 for two alloys, respectively, characterizing dislocation slipping mechanism. The activation energies of steady state creep of the two alloys were found to be 78.4 kJ/mol and 32.8 kJ/mol for Al–Si and Al–Si–Zr–Ti alloys, respectively. The microstructure of the samples studied was investigated by optical and transmission electron microscopy (TEM).

Amorphous, Elastic AB Copolymer Networks from Acrylates and Poly[(L‐lactide)‐ran‐glycolide]dimethacrylates

Three series of amorphous AB copolymer networks with shape-memory capabilities were obtained from poly[(L-lactide)-ran-glycolide]dimethacrylate (DM) as crosslinker and ethylacrylate-(AEt), butylacrylate-(ABu), or hexyl acrylate (AHe) as comonomers. The switching temperature Tsw of the shape-memory effect could be adjusted between 9 °C to 45 °C. Strain fixity rates and strain recovery rates above 96 % could be reached. Compared to the homonetwork from DM, the incorporation of polyacrylate segments into the network led to a significant improvement of mechanical properties. Reaching higher elasticity and the possibility to adjust Tsw to a temperature between room temperature and body temperature are considered to be substantial steps to improve the applicability of these polymer networks.

Multiple shape-memory behavior and thermal-mechanical properties of peroxide cross-linked blends of linear and short-chain branched polyethylenes

Thermally induced shape-memory effect (SME) in tensile mode was investigated in binary and ternary blends of two ethylene-1-octene copolymers with a degree of branching of 30 and 60 CH3/1000C and/or nearly linear polyethylene cross-linked after melt mixing with 2 wt% of liquid peroxide 2,5-dimethyl-2,5-di-(tert.butylperoxy)-hexane at 190°C. The average cross-link density estimated by means of the Mooney-Rivlin equation on the basis of tensile test data was character- ized between 130 and 170 mol·m-3 depending on the blend composition. Thermal analysis points out multiple crystalliza- tion and melting behavior of blends caused by the existence of several polyethylene crystal populations with different perfection, size and correspondingly different melting temperature of crystallites. That agrees well with the diversity of blends phase morphology characterized by atomic force microscopy. However, triple- and quadruple-SME could be observed only after two- and accordingly three-step programming of binary and tertiary blends, respectively, at suitable temperatures and strains. Compared to performances obtained for the same blend after single-step programming above the maximal melting temperature the significantly poorer characteristics of SME like strain fixity and strain recovery ratio as well as recovery strain rate occurred after multi-step programming.

Shape-Memory Polymer Networks from Oligo[(ε-hydroxycaproate)-co-glycolate]dimethacrylates and Butyl Acrylate with Adjustable Hydrolytic Degradation Rate

Degradable shape-memory polymer networks intended for biomedical applications were synthesized from oligo[(epsilon-hydroxycaproate)-co-glycolate]dimethacrylates with glycolate contents between 0 and 30 mol % using a photopolymerization process. In addition AB copolymer networks were prepared by adding 60 wt % n-butyl acrylate as comonomer. All synthesized polymer networks are semicrystalline at room temperature. A melting transition T(m) between 18 and 53 degrees C which can be used as switching transition for the shape-memory effect can be attributed to the crystalline poly(epsilon-hydroxycaproate) phase. At temperatures below T(m) the elastic properties are dominated by these physical cross-links. At temperatures higher than T(m) the E modulus of the amorphous polymer networks is lowered by up to 2 orders of magnitude, depending on the chemical cross-link density. Copolymer networks based on macrodimethacrylates with a M(n) of up to 13,500 g x mol(-1) and a maximum glycolate content of 21 mol % show quantitative strain recovery rates in stress-controlled cyclic thermomechanical experiments. Hydrolytic degradation experiments of polymer networks performed in phosphate buffer solution at 37 degrees C show that the degradation rate can be accelerated by increasing the glycolate content and decelerated by the incorporation of n-butyl acrylate.

Novel interpenetrating networks with shape‐memory properties

AbstractNovel polyesterurethane/poly(ethylene glycol) dimethacrylate (PEGDMA) interpenetrating networks (IPNs) with good shape‐memory properties were synthesized using solvent casting method. The star‐shaped oligo[(rac‐lactide)‐co‐glycolide] was coupled with isophorone diisocyanate to form a polyesterurethane network (PULG), and PEGDMA was photopolymerized to form another polyetheracrylate network. IPNs were transparent and gel content exceeded 92%. The values of strain fixity rate and strain recovery rate were above 93%. PULG and PEGDMA networks in IPNs were amorphous and did not show any characteristic diffraction peaks in X‐ray diffraction spectra. Only one glass transition temperature (Tg) of the IPNs between Tg of PEGDMA and PULG was observed, which was proportional to PEGDMA content. PULG and PEGDMA networks were miscible when PEGDMA content was below 50 wt %. The hydrophilicity, transition temperatures, and mechanical properties of IPNs could be conveniently adjusted through variation of network compositions to match the promising potential clinical or medical applications. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 768–775, 2007

Degradable shape-memory polymer networks from oligo[(l-lactide)-ran-glycolide]dimethacrylates.

In this paper degradable shape-memory polymer networks synthesized from oligo[(-lactide)--glycolide]dimethaycrylates are introduced. The macrodimethacrylates are prepared a two-step synthesis: hydroxy telechelic oligo[(-lactide)--glycolide]s with number average molecular weights ranging from 1000 to 5700 g mol were synthesized by ring-opening polymerization from ,-dilactide, diglycolide and ethylene glycol as initiator using dibutyltin oxide as the catalyst. These oligodiols are reacted with methacryloyl chloride resulting in terminal methacrylate groups. Crosslinking of macrodimethacrylates is performed under exposure to UV light without applying a photo initiator. The polymer networks obtained are transparent and hydrolytically degradable. While the mechanical properties at temperatures higher than depend on crosslinking density, is almost constant at about 55 °C. The shape-memory functionality of the amorphous polymer network was investigated by cyclic, thermomechanical tests under the systematic variation of different programming parameters. Good shape-memory properties with strain recovery rates close to 100% were obtained under stress-controlled programming. Under strain-controlled conditions, it needs to be considered that relatively high stresses can be generated during programming. Potential biomedical applications are intelligent implants or smart drug release systems.

Martensitic transformations and functional properties of thermally and thermomechanically treated Ti–Ni–Nb-based alloys

The aim of the present work is the detailed investigation of the structure, martensitic transformations and functional properties of the wide-hysteresis Ti–Ni–Nb-based shape memory alloys intended for the thermomechanical coupling (TMC) development. The 45Ti–45Ni–10Nb and 42.5Ti–46.5Ni–8Nb–3Zr (at.%) alloys have been studied. Zr addition results in martensitic hysteresis widening that promotes an improvement of the material functional properties. High-temperature thermomechanical treatment (HTMT) is an effective method for the optimization of Ti–Ni–Nb-based shape memory alloys properties. HTMT increases recovery strain and shape recovery rate as well as provides a favorable effect on the recovery stress generation and relaxation. The alloys compositions and treatments which allow designing the room-temperature-stored TMCs for tubes or pipes joints, reliable within the temperature range above −60 °C and easily dismantled when cooling in liquid nitrogen, have been proposed.

Synthesis and characterization of two shape-memory polymers containing short aramid hard segments and poly(ε-caprolactone) soft segments

A number of segmented copolymers were synthesized by reacting 4-aminobenzoyl end-functionalized poly(ε-caprolactone)s of M n 2000, 3000 and 4000 g mol −1 with three aromatic diacid dichlorides in the presence of chlorotrimethylsilane. Polymer purity, molar mass, thermal and mechanical properties were characterized by NMR, MALDI-TOF mass spectrometry, GPC, DSC, and DMTA. Promising shape-memory properties were observed for two polymers that contained comparatively short, semicrystalline poly(ε-caprolactone) soft segments of molecular weight 3000 g mol −1 and either terephthalamide or 2,6-naphthalenedicarboxyamide hard segments. Cast films of these polymers were soft and elastomer-like at room temperature. Loading could be conveniently achieved by cold-drawing at room temperature and strain recovery took place upon heating above the melting temperature of the soft segment (35 °C). Cast films reached uniform deformation properties with strain recovery rates as high as 99% and strain fixity values of 78% after passing through only one or two training cycles.

Synthesis, Shape‐Memory Functionality and Hydrolytical Degradation Studies on Polymer Networks from Poly(rac‐lactide)‐b‐poly(propylene oxide)‐b‐poly(rac‐lactide) dimethacrylates

Amorphous, hydrolytically degradable, multi‐phase block copolymer networks were obtained by photocrosslinking. The macrodimethacrylate precursors were prepared by ring‐opening polymerisation (ROP). The aim of this investigation to tailor the materials' elasticity in the temporary shape could be achieved. Stress‐controlled, cyclic thermomechanical tensile experiments showed the materials' excellent shape‐memory properties with strain recovery rates up to 99 %. The presented amorphous, biodegradable polymer network system with shape‐memory presents a new generation of multi‐functional biomaterials enabling applications in keyhole surgery or controlled drug release.

Effect of Nb Addition on Shape Memory Behavior of Ti–Mo–Ga Alloys

Effect of Nb addition on mechanical properties and shape memory behavior of Ti–Mo–Ga alloys was investigated by cyclic loadingunloading tensile tests, tensile tests at various temperatures and Vickers hardness tests. The martensitic transformation start temperature (Ms) decreased by 20 K with 1 at% increase of Nb content in the Ti–6Mo–3Ga(at%) alloy. The shape memory effect was observed in the Ti–6Mo– 3Ga(at%) alloy. The superelastic strain increased with increasing Nb content. The increase of the yield stress during the cyclic deformation decreased the strain recovery rate in the Ti–6Mo–3Ga(at%) alloy. Aging at intermediate temperatures resulted in increase in hardness of the Ti– 6Mo–3Ga–(0–4)Nb(at%) alloys. The increase in hardness decreased significantly with increasing Nb content because the addition of Nb was effective to suppress the ! phase hardening. The yield stress decreased and the strain recovery rate increased with increasing number of cycle in the Ti–6Mo–3Ga–4Nb(at%) alloy.