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.     

Impact of poly(ɛ‐caprolactone) architecture on the thermomechanical and shape memory properties

We report the synthesis of linear‐ and brush‐type poly(?‐caprolactone) (PCL) networks and investigate their thermal, mechanical, and shape memory behavior. Brush‐PCLs are prepared by ring‐opening metathesis polymerization (ROMP) of a norbornenyl‐functionalized ?‐caprolactone macromonomer (MM‐PCL) of different molecular weights. The linear analog, diacrylate end‐functionalized PCL (linear‐PCL), having comparable molecular weight of side chain of brush‐PCL is also synthesized. These polymers are thermally cured by a radical initiator in the presence of poly(ethylene glycol) diacrylate crosslinker. Thermal and linear viscoelastic properties as well as shape memory performance of the resulting PCL networks are investigated, and are significantly impacted by the PCL architecture. Therefore, our work highlights that tailoring macromolecular architecture is useful strategy to manipulate thermal, mechanical, and resulting shape memory properties. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 3424–3433     

Two‐way multi‐shape memory properties of peroxide crosslinked ethylene vinyl‐acetate copolymer (EVA)/polycaprolactone (PCL) blends

Rare studies have investigated on the 2‐way shape memory crosslinked blends with multiple shape memory behavior up to date. To consider the merit of commercial cost‐competitive crystalline polymers, ethylene vinyl‐acetate copolymer (EVA) / polycaprolactone (PCL) blends (60/40 and 30/70) were peroxide‐cured to form the 2‐way multi‐shape memory crosslinked blends using a melt‐blending method. Both resins were selected to have a similar controlled crosslinking degree, which allowed us to distinctly evaluate their actuation contributions from the cooling‐induced elongation (crystallization) and from the entropy‐driven elongation during cooling process, respectively. In the 2‐way process for the 60/40 system, 2 respective peaks contributed from the cooling‐induced crystallization of EVA and PCL in the cooling curves based on the strain derivate rates at various temperatures were observed. After the cooling process under the loading stress of 150 kPa, the 2‐step heating‐induced contraction process with increasing temperature started at 54.1°C above the melting temperature of PCL at 52.3°C and EVA at 78.3°C, demonstrating 2‐way multi‐shape memory behavior. The multi‐step behavior was more prominent at higher PCL composition and higher load for the 30/70 system. It was found that the entropy‐driven contribution to the overall actuation magnitude increased with increasing nominal loads due to the increased orientation of molecular networks in the blends. The current approach offers numerous possibilities in preparing 2‐way multi‐shape memory crosslinked blends.     

Shape memory fiber spun with segmented polyurethane ionomer

The effect of cationic groups within hard segments on shape memory polyurethane (SMPU) fibers was studied and the cyclic tensile testing was conducted to assess the shape memory effect. Mechanical properties, hard segment crystallization, and dynamic mechanical properties of SMPU ionomer fibers composed of 1,4‐butanediol (BDO), N‐methyldiethanolamine (NMDA), 4,4′‐methylenebis(phenyl isocyanate) (MDI), and poly(butylene adipate)diol (PBA) were investigated using a universal tensile tester, differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). The results demonstrate that only 2 wt% NMDA can significantly change the glass transition temperature of the soft segment phase. DSC shows that the ionic group within hard segments can facilitate the crystallization of hard segments in unsteamed SMPU ionomer fibers. But for steamed fiber specimens, this effect is insignificant. Moreover, the ionic groups in hard segments with different hard segment contents (HSC) have different effects. In unsteamed fibers with 64 wt% HSC, 2 wt% NMDA increases the glass transition of soft segments from 63.5 to 70.6°C. However, in fibers with 55 wt% HSC, the glass transition temperature is lowered from 46.7 to 33.5°C. The post‐treatment, high‐pressure steaming is an effective way to remove the internal stress and subsequently improve the dimensional stability of SMPU ionomer fibers. Copyright © 2008 John Wiley & Sons, Ltd.     

Shape memory effect and reversible phase crystallization process in SMPU ionomer

It is the first attempt to reveal the effect of reversible phase crystallization process on shape memory effect in shape memory polyurethane (PU) ionomer. Thereof the cyclic tensile testing was conducted with various cooling time to fix the temporary deformation for assessing shape memory function. The crystallization process of the reversible phase, poly (ε‐caprolactone) (PCL) in shape memory PU ionomers composed of different ionic group contents, 1,4‐butanediol, 4,4′‐methylenebis(phenyl isocyanate) and PCL, was investigated by using isothermal crystallization kinetics under the thermal routine similar to that for the cyclic tensile testing. The results demonstrate that the ionic groups within hard segments significantly slow down the crystal growth of the reversible phase. When the physical crosslink is strong enough, the crystallization rate would be a predominant factor determining the shape fixity ratio after various cooling time. Instead, when physical crosslink is weakening, the influence of crystallization rate is much less on the cooling time dependence of fixity. Copyright © 2007 John Wiley & Sons, Ltd.     

Influence of programming strain rates on the shape‐memory performance of semicrystalline multiblock copolymers

Multiblock copolymers named PCL‐PIBMD consisting of crystallizable poly(ε‐caprolactone) segments and crystallizable poly[oligo(3S‐iso‐butylmorpholine‐2,5‐dione)] segments coupled by trimethyl hexamethylene diisocyanate provide a versatile molecular architecture for achieving shape‐memory effects (SMEs) in polymers. The mechanical properties as well as the SME performance of PCL‐PIBMD can be tailored by the variation of physical parameters during programming such as deformation strain or applied temperature protocols. In this study, we explored the influence of applying different strain rates during programming on the resulting nanostructure of PCL‐PIBMD. Programming was conducted at 50 °C by elongation to ε_m = 50% with strain rates of 1 or 10 or 50 mm min^?1. The nanostructural changes were visualized by atomic force microscopy (AFM) measurements and investigated by in situ wide and small angle X‐ray scattering experiments. With increasing the strain rate, a higher degree of orientation was observed in the amorphous domains. Simultaneously the strain‐induced formation of new PIBMD crystals as well as the fragmentation of existing large PIBMD crystals occurred. The observed differences in shape fixity ratio and recovery stress of samples deformed with various strain rates can be attributed to their different nanostructures. The achieved findings can be relevant parameters for programming the shape‐memory polymers with designed recovery forces. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1935–1943     

Parameters affecting transition temperatures of poly(lactic acid‐co‐polydiols) copolymer‐based polyester urethanes and their shape memory behavior

A series of polyester urethanes (PEUs) comprising poly(lactic acid‐co‐polydiol) copolymers as a soft segment, 4,4′‐diphenylmethane diisocyanate (MDI) and 1,4‐butanediol (BDO) as a hard segment were systematically synthesized. Soft segments, which were block copolymers of L ‐lactide (LA) and polydiols such as poly(ethylene glycol) and poly(trimethylene ether glycol), were prepared via ring opening polymerization. Glass transition temperatures (T_g) of the obtained PEUs were found strongly dependent on properties of copolymer soft segments. By simply changing composition ratio, type and molecular weight of polydiols in the soft segment preparation step, T_g of PEU can be varied in the broad range of 0–57°C. The synthesized PEUs exhibited shape memory behavior at their transition temperatures. PEUs with hard segment ratio higher than 65 mole percent showed good shape recovery. These findings suggested that it is important to manipulate molecular structure of the copolymer soft segment for a desirable transition temperature and design optimal soft to hard segment ratio in PEU for good shape recovery. Copyright © 2011 John Wiley & Sons, Ltd.     

A novel shape memory poly(ε‐caprolactone) network via UV‐triggered thiol‐ene reaction

A facile method to prepare shape memory polymers crosslinked by SiO₂ is described. A series of biodegradable shape memory networks were obtained through thiol‐ene reaction triggered by UV irradiation between surface‐thiol‐modified SiO₂ nanoparticles and end‐acrylate poly (ε‐caprolactone) (PCL). The highly selective thiol‐ene reaction ensured a uniform distribution of PCL chains between crosslinkers, contributing well‐defined network architecture with enhanced mechanical and shape‐memory properties. Thiol‐functionalized silica nanoparticle was characterized by using FTIR and XPS analysis, and ¹H NMR spectra was used to confirm the successful modification of terminal hydroxyl group of PCL diol. Surface‐modified silica particles were found well dispersible in acrylate‐capped PCL supported by SEM. Thermal and crystalline behaviors of the obtained polymers were analyzed by DSC and XRD, and DMA measurement proved good mechanical property. The shape memory behavior and tensile strength was somewhat tunable by the length of PCL. Acceptably, sample SiO₂‐SMP2k presented 99% recovery ratio and 97% shape fixity, and its relatively high tensile strength showed an attractive potential for biomedical application. Finally, a possible molecular mechanism accounting for the shape memory property was illustrated. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 692–701     

Shape Memory Properties and Enzymatic Degradability of Poly(ε‐caprolactone)‐Based Polyurethane Urea Containing Phenylalanine‐Derived Chain Extender

Biodegradable shape memory polymers are promising biomaterials for minimally invasive surgical procedures. Herein, a series of linear biodegradable shape memory poly(ε‐caprolactone) (PCL)‐based polyurethane ureas (PUUs) containing a novel phenylalanine‐derived chain extender is synthesized. The phenylalanine‐derived chain extender, phenylalanine‐hexamethylenediamine‐phenylalanine (PHP), contains two chymotrypsin cleaving sites to enhance the enzymatic degradation of PUUs. The degradation rate, the crystallinity, and mechanical properties of PUUs are tailored by the content of PHP. Meanwhile, semicrystalline PCL is not only hydrolytically degradable but also vital for shape memory. Good shape memory ability under body temperature is achieved for PUUs due to the strong interactions in hard segments for permanent crosslinking and the crystallization‐melt transition of PCL to switch temporary shape. The PUUs would have a great potential in application as implanting stent.     

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     

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     

具有热致形状记忆功能的热塑性多嵌段聚氨酯

以ＰＣＬ为软段、ＴＤＩ－ＢＤＯ为硬段，采用溶液聚合的方法合成了具有形状记忆功能的线性多嵌段聚氨酯．利用ＤＳＣ、ＤＭＴＡ、ＷＡＸＤ等测试手段对体系的结晶性、微相分离行为进行了研究．结果发现：聚氨酯中硬段的存在对软段的结晶有着很大的影响，当软段序列的平均分子量达到３０００以上时，软段才可以很好地结晶；并且，硬段含量也必须高于一定值才能形成较为完善的物理交联点．符合这些条件的试样能显示很好的形状记忆特征．此外还就拉伸比、多次形变以及组成等对材料的形状回复性能的影响进行了详细的研究．     

Memory chromic polyurethane with tetraphenylethylene

This article reports a chromic polymer, which is responsive to its shape memory properties and has both the behavior of shape memory polymers and chromic materials. We employed a strategy to fabricate such a smart material, which represents a new principle for making chromic materials. This material is made of shape memory polyurethane with tetraphenylethylene units (0.1 wt %) covalently connected to the soft‐segments (PCL, M_w = 4000). The material displays biocompatibility, shape fixity of 88–93%, and almost 100% shape recovery and has reversible mechanochromic, solvatochromic, and thermochromic shape memory effect. The memory chromism represented by the reversible change of emission intensity shows negative correlation with shape fixity, temperature, and existence of solvent. It may be explained that when the soft segments are molten or dissolved in solvent, the shape recovery switch is open, the AIE units are free from crystal binding and can migrate easily to larger areas, thus the AIE units/particles are far apart from each other and the barrier for rotation of phenyl groups is reduced, which lead to the reduction of emission intensity, appeared by no colors or pale colors, and vice versa. Since the switch is a fundamental structural character of SMPs, the shape memory properties have led to the chromism and we call this memory chromic. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014 , 52, 104–110     

A design of shape memory networks of poly(ε‐caprolactone)s via POSS‐POSS interactions

In this contribution, we reported a design of shape memory networks of poly(ε‐caprolactone)s (PCLs) via POSS‐POSS interactions. First, a series of novel organic‐inorganic PCL stars with polyhedral oligomeric silsesquioxane (POSS) termini were synthesized via the combination of ring‐opening polymerization of ε‐caprolactone and the copper (I)‐catalyzed cycloaddition of alkynyl with azido groups. It was found that the organic‐inorganic PCL stars significantly displayed shape memory properties with about 100% of recovery. The morphological observation showed that in the organic‐inorganic PCL stars, the POSS cages at the ends of PCL chains were self‐organized into the spherical POSS microdomains with the size of 10 to 20 nm in diameter. The POSS microdomains behaved as the netpoints, resulting in the formation of physically crosslinked networks. The novel physically crosslinked networks endowed the organic‐inorganic nanocomposites with shape memory properties.     

Basic structure–property behavior of UV curable polyurethane acrylates

Effects of hard segment type, soft segment type, soft segment length, and crystallization at various levels of reactive diluent have been studied in polyurethane acrylates. PTMG–IPDI-based PU acrylates gave better soft segment-hard segment phase separation as compared with PTMG–TDI-based ones. Among PTMG–, PCL–, and PTAd–IPDI-based materials, only PTMG-based one gave clean phase separation, and PTAd-based gave the poorest phase separation. With greater phase separation, higher modulus was obtained. Crystallization of soft segment in PTAd–IPDI-based material gave a significant increase in tensile modulus and elongation at break, with a notable difference in dynamic mechanical properties. © 1996 John Wiley & Sons, Inc.     

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     

PCL-based Shape Memory Polymers with Variable PDMS Soft Segment Lengths

Thermoresponsive shape memory polymers (SMPs) are stimuli-responsive materials that return to their permanent shape from a temporary shape in response to heating. The design of new SMPs which obtain a broader range of properties including mechanical behavior is critical to realize their potential in biomedical as well as industrial and aerospace applications. To tailor the properties of SMPs, "AB networks" comprised of two distinct polymer components have been investigated but are overwhelmingly limited to those in which both components are organic. In this present work, we prepared inorganic-organic SMPs comprised of inorganic polydimethyl-siloxane (PDMS) segments of varying lengths and organic poly(ε-caprolactone) (PCL) segments. PDMS has a particularly low T(g) (-125 °C) which makes it a particularly effective soft segment to tailor the mechanical properties of PCL-based SMPs. The SMPs were prepared via the rapid photocure of solutions of diacrylated PCL(40)-block-PDMS(m)-block-PCL(40) macromers (m = 20, 37, 66 and 130). The resulting inorganic-organic SMP networks exhibited excellent shape fixity and recovery. By changing the PDMS segment length, the thermal, mechanical, and surface properties were systematically altered.     

Lignin,a biomass crosslinker,in a shape memory polycaprolactone network

This work reports a new direction of natural lignin valorization, which utilizes lignin to produce crosslinked polycaprolactone (PCL) via a straightforward synthesis. Lignin's hydroxyl groups of its multibranched phenolic structure allow lignin to serve as crosslinkers, whereas the aromatic groups serve as hard segments. The modified natural lignin containing alkene terminals is crosslinked with a thiol‐terminal PCL via Ru‐catalyzed photoredox thiol‐ene reaction. The high rate of gel contents measured for all crosslinked polymers, with the least being 84% of gel content, indicates efficient crosslinking. The prepared flat rectangular shape lignin‐crosslinked PCL sample demonstrates rapid thermal responsive shape memory behavior at 10 °C and 80 °C showing interconversion between a permanent and temporary shape. The melting temperature of the lignin‐crosslinked PCL is tunable by varying the percent weight of lignin. The 11, 21, and 30 wt % lignin demonstrated T_m of 42 °C, 35 °C, and 26 °C, respectively. The role of lignin as a crosslinker presented in this work suggests that lignin can serve as an efficient biomass‐based functional additive to polymers. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2121–2130     

Water vapor permeability of shape memory polyurethane with amorphous reversible phase

Shape memory thermoplastic polyurethanes (TPUs), based on amorphous soft segment from the reaction of hexamethylene diisocyanate and 1,2‐butane diol, and the crystalline hard segment from 4,4′‐methylenediphenyl diisocyanate and 1,6‐hexanediol, were modified by hydrophilic segments, diol‐terminated poly(ethylene oxide) or dimethylol propionic acid (DMPA). Differential scanning calorimetry, dynamic mechanical testing, tensile testing, and the measurement of shape memory effect, water swell, and water vapor permeability were carried out to examine these TPUs. The hydrophilic segment increased the hysterisis in shape memory effect by reducing the crystallinity of the hard segment. The neutralized DMPA unit enhanced the sensitivity of the thermoresponsive water vapor permeability (WVP) by amplifying the increase of WVP at the temperature range above the glass transition temperature. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3009–3017, 2000     

Two-way shape memory behavior of styrene-based bilayer shape memory polymer plate

In this work, a bilayer shape memory polymer (SMP) composite plate with two-way shape memory behavior is simulated, in which two types of styrene-based SMPs with well-separated glass transition temperatures are assembled in parallel. The finite element (FE) software ABAQUS is selected to exhibit the two-way shape memory effect during the shape recovery step and the Generalized Maxwell Model with the WLF equation is applied to characterize the temperature-dependent properties of the SMP bilayer plates. The effect factors of axial predeformation, thermal expansion coefficient and plate thickness are all considered for the two-way shape memory behavior of the styrene-based bilayer SMP plate. After that, a smart gripper composed of four SMP composite plates is proposed to realize grabbing and releasing functions for one-step and staged heating recovery. The FE results provide some necessary theoretical guidelines for future soft smart structural designs and optimization.