Influence of heat treatment on the properties of shape memory fibers. I. Crystallinity,hydrogen bonding,and shape memory effect

Shape memory fibers (SMFs) were prepared via melt spinning. The fibers underwent different heat treatments to eliminate internal stress and structure deficiency caused during melt spinning. The influences of heat treatments on the SMF crystallinity, molecular orientation, hydrogen bonding, and shape memory behavior were studied. It was found with increasing heat‐treatment temperature, the soft segment crystallinity, crystallite dimension, and microphase separation increased, and the hydrogen bonding in the hard segment phase increased. Low temperature heat treatments decreased the shape recovery ratios while increasing the shape fixity ratios as a result of internal stress releasing and molecules disorientation. High temperature heat treatments increased the hard segment stability. Increasing heat‐treatment temperature resulted in the improvement of both the shape recovery and fixity, because it promoted the phase separation. The results from DSC, DMA, XRD, and FTIR were used to illustrate the mechanism governing these properties difference. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Surface grafting of carbon fibers with artificial silver‐nanoparticle‐decorated graphene oxide for high‐speed electrical actuation of shape‐memory polymers

Poor heat conduction in the interface between the carbon fiber and polymer matrix is a problem in the actuation of shape‐memory polymer (SMP) composites by Joule heating. In this study, we investigated the effectiveness of grafting silver‐nanoparticle‐decorated graphene oxide (GO) onto carbon fibers to improve the electrothermal properties and Joule‐heating‐activated shape recovery of SMP composites. Self‐assembled GO was grafted onto carbon fibers to enhance the bonding of the carbon fibers with the polymeric matrix via van der Waal's forces and covalent crosslinking, respectively. Silver nanoparticles were further self‐assembled and deposited to decorate the GO assembly, which was used to decrease the thermal dissimilarity and facilitate heat transfer from the carbon fiber to the polymer matrix. The carbon fiber was incorporated with SMP to achieve the shape recovery induced by Joule heating. We found that the silver‐nanoparticle‐decorated GO helped us achieve a more uniform temperature distribution in the SMP composites compared to those without decoration. Furthermore, the shape‐recovery behavior and temperature profile during the Joule heating of the SMP composites were characterized and compared. A unique synergistic effect of the carbon fibers and silver‐nanoparticle‐decorated GO was achieved to enhance the heat transfer and a higher speed of actuation. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41673.     

Solid‐state polymerization of melt‐spun poly(ethylene terephthalate) fibers and their tensile properties

The production of high modulus and high strength poly(ethylene terephthalate) fibers was examined by using commercially available melt‐spun fibers with normal molecular weight (intrinsic viscosity = 0.6 dL/g). First, molecular weight of as‐spun fibers was increased up to 2.20 dL/g by a solid‐state polymerization, keeping the original shape of as‐spun fibers. Second, the polymerized as‐spun fibers were drawn by a conventional tensile drawing. The achieved tensile modulus and strength of as‐drawn fibers (without heat setting) were 20.0 and 1.1 GPa, respectively. A heat setting was carried out for the as‐drawn fibers. Tensile properties of the treated fibers were greatly affected by the condition of the heat setting. This was related to the increase of sample crystallinity and molecular degradation during the treatments. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1791–1797, 2007     

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     

Porous inorganic-organic shape memory polymers

Thermoresponsive shape memory polymers (SMPs) are a type of stimuli-sensitive materials that switch from a temporary shape back to their permanent shape upon exposure to heat. While the majority of SMPs have been fabricated in the solid form, porous SMP foams exhibit distinct properties and are better suited for certain applications, including some in the biomedical field. Like solid SMPs, SMP foams have been restricted to a limited group of organic polymer systems. In this study, we prepared inorganic-organic SMP foams based on the photochemical cure of a macromer comprised of inorganic polydimethylsiloxane (PDMS) segments and organic poly(ε-caprolactone) (PCL) segments, diacrylated PCL(40)-block-PDMS(37)-block-PCL(40). To achieve tunable pore size with high interconnectivity, the SMP foams were prepared via a refined solvent-casting/particulate-leaching (SCPL) method. By varying design parameters such as degree of salt fusion, macromer concentration in the solvent and salt particle size, the SMP foams with excellent shape memory behavior and tunable pore size, pore morphology, and modulus were obtained.     

Influence of interchain forces and supermolecular structure on the drawing behavior of nylon 66 fibers in the presence of supercritical carbon dioxide

The drawing behavior and mechanical properties of as‐spun and highly oriented nylon 66 fibers drawn in supercritical carbon dioxide (SCCO₂) were investigated. Conditions including different temperatures, CO₂ pressures, and plasticizers with different polarity were systematically studied. Results indicate that CO₂ is an efficient plasticizer for as‐spun nylon 66 fibers as shown by decreases in the draw stress. In contrast, CO₂ shows only a slight influence on the drawability of highly oriented nylon 66 fiber. The effect of other plasticizers such as water, methanol, and ethanol on the drawability of nylon 66 fibers is very similar to that of CO₂. Tenacity and modulus of one‐stage drawn fibers were less than 0.8 and 5.0 GPa, respectively. Fibers with the highest tenacity and modulus, 0.96/5.04 and 1.06/5.04 GPa, were obtained by two‐stage drawing in SCCO₂ from as‐spun and drawn nylon 66 fibers, respectively. The main reason for the extremely low draw ratios (<6.0) of nylon 66 fibers was the presence of hydrogen bonds in the crystalline phase. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2282–2288, 2004     

Shape memory properties of olefin block copolymer (OBC)/poly(ɛ‐caprolactone) (PCL) blends

Conventionally, the chemically crosslinked shape memory polymer (SMP) blends are hard to recycle due to their network structure. Herein, the environmental SMP blends of olefin block copolymer (OBC), a unique thermoplastic elastomer, and poly(?‐caprolactone) (PCL) were physically crosslinked. Dicumyl peroxide was used as the compatibilizer to improve their miscibility, as evidenced by the reduced dispersed domain size of PCL in the OBC matrix and the increased complex viscosity. The peroxide modified OBC/PCL blend conferred enhanced tensile properties, increased dynamic storage modulus, increased crystallization temperature, and higher recovery stress. The shape memory behaviors of OBC/PCL blends predeformed under two different predeformation temperatures (30 and 65 °C) were investigated. The recovery stress showed respective maximum peak values corresponding to their predeformation temperatures. In addition, the modified blends gave the better shape memory performance at 65 °C. Besides the peroxide modification approach, a precycle training process via prestretching the samples and reducing the mechanical hysteresis was implemented to improve shape memory performance further. This is the first work on the OBC‐based SMP blends to enhance shape memory performance by combining the chemical modification using added peroxide compatibilizer and the process modification using a precycle training process. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45475.     

Effect of cationic group content on shape memory effect in segmented polyurethane cationomer

To illustrate the importance of cationic groups within hard segments on shape memory effect in segmented polyurethane (PU) cationomers, the shape memory polyurethane (SMPU) cationomers composed of poly(ε‐caprolactone) (PCL), 4,4′‐diphenylmethane diisocyanate (MDI), 1,4‐butanediol (BDO), and N‐methyldiethanolamine (NMDA) or N,N‐bis(2‐hydroxyethyl)isonicotinamide (BIN) were synthesized. The comparison of shape memory effect between NMDA series and BIN series was made. The relations between the structure and shape memory effect of the two series of cationomers with various ionic group contents were investigated. It is observed that the stress at 100% elongation is reduced for these two series of PU cationomers with increasing ionic group content. Especially for NMDA series, the stress reduction is more significant. The fixity ratio and recovery ratio of the NMDA series can be improved simultaneously by the insertion of cationic groups within hard segments, but not for the BIN series. Characterizations with DSC and DMA suggest that the crystallibility of soft segment in SMPU cationomers was enhanced by incorporation of ionic groups into hard segments, leading to a relative high degree of soft segment crystallization; compared with the corresponding nonionomers, incorporation of charged ionic groups within hard segments can enhance the cohesion force among hard segments particularly at high ionic group content. This methodology offers good control of the shape memory characteristic in thin films and is believed to be beneficial to the shape memory textile industries. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 545–556, 2007     

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

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

Characterization and strain‐energy‐function‐based modeling of the thermomechanical response of shape‐memory polymers

Shape‐memory polymers (SMPs) are an emerging class of active polymers that can be used on a wide range of reconfigurable structures and actuation devices. In this study, an epoxy‐based SMP was synthesized, and its thermomechanical behaviors were comprehensively characterized. The stress–strain behavior of the SMP was determined to be nonlinear, finite deformation in all regions. Strain‐energy‐based models were used to capture the complicated stress–strain behavior and shape‐recovery response of the SMP. Among various strain energy functions, the stretch‐based Ogden model provided the best fit to the experimental observations. Compared to the sophisticated models developed for SMPs, the strain‐energy‐based model was found to be reliable and much easier to use for practical SMP designs. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41861.     

Structure development of poly(L‐lactic acid) fibers processed at various spinning conditions

Poly(L ‐lactic acid) (PLA) filaments were spun by melt‐spinning at 500 and 1850 mm^?1, and further drawn and heat‐set to modify the morphology of these PLA filaments. PLA yarns were characterized by wide‐angle X‐ray diffraction (WAXD) and sonic method. WAXD reveals that PLA yarns spun at 500 mm^?1 are almost amorphous while the PLA filaments spun at 1850 mm^?1 have about 6% crystallinity. This is different from PET filaments spun at the same speed that have almost no crystallinity. Both drawn‐ and heat‐set PLA filaments showed much higher crystallinity (60%) than do as‐spun fibers produced at 500 and 1850 mm^?1 speed, which is also higher than the usual heat‐set PET yarns. It appears that crystalline orientation rapidly reaches a value in the order of 0.95 at 1850 mm^?1 and that drawn‐ and heat‐set yarns have almost the same crystalline orientation values. Molecular orientation is relatively low for as‐spun PLA yarn, and molecular orientation increased to ～0.5 after drawing or heat–setting or both. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1210–1216, 2006     

Shape‐memory polyurethane/multiwalled carbon nanotube fibers

Shape‐memory polyurethane/multiwalled carbon nanotube (SMP–MWNT) composites with various multiwalled carbon nanotube (MWNT) contents were synthesized, and the corresponding SMP–MWNT fibers were prepared by melt spinning. The influence of the MWNT content on the spinnability, fracture morphology, thermal and mechanical properties, and shape‐memory behavior of the shape‐memory polymer was studied. The spinning ability of SMP–MWNTs decreased significantly with increasing MWNT content. When the MWNT content reached 8.0 wt %, the fibers could not be produced because of the poor rheological properties of the composites. The melt‐blending, extrusion, and melt‐spinning processes for the shape‐memory fiber (SMF), particularly at low MWNT contents, caused the nanotubes to distribute homogeneously and preferentially align along the drawing direction of the SMF. The crystallization in the SMF was promoted at low MWNT contents because it acted as a nucleation agent. At high MWNT contents, however, the crystallization was hindered because the movement of the polyurethane chains was restricted. The homogeneously distributed and aligned MWNTs preserved the SMF with high tenacity and initial modulus. The recovery ratio and recovery force were also improved because the MWNTs helped to store the internal elastic energy during stretching and fixing. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Thermomechanical and water‐responsive shape memory properties of carbon nanotubes‐reinforced hyperbranched polyurethane composites

Shape memory composites of hyperbranched polyurethane (HBPU) and acid‐treated multi‐walled carbon nanotubes (MWNTs) were prepared using an in situ polymerization method. HBPUs with different hard segments contents were synthesized via the A₂ + B₃ approach using poly(ethylene glycol) (PEG) as a soft segment, 4,4′‐methylene bis(phenylisocynate), castor oil, and 1,4‐butanediol as hard segment. Compared to HBPU, the HBPU/MWNT composites showed faster shape recovery and double the shape recovery stress in the thermomechanical shape memory test, which was dependent on the MWNTs content and HBPU hard segment content. The water‐responsive shape memory effect of HBPU/MWNT composites was considered to result from the combined contribution of hydrophilic PEG and well dispersed MWNTs in highly branched HBPU molecules. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Molecular structures and physical properties of heat‐drawn conjugate fibers

As‐spun poly(trimethylene terephthalate) (PTT)/poly(ethylene terephthalate) (PET) side‐by‐side conjugate fibers were drawn to investigate the effects of drawing conditions on structure development and physical properties. Effects of draw ratio and heat‐set temperature were observed. In the state of an as‐spun fiber, the molecular orientation of PTT was higher than PET, whereas PET molecular orientation increased remarkably over PTT with increasing draw ratio. Crimp contraction increased sharply at a draw ratio over 2.0, where the crystalline structure of the PET developed sufficiently. A heat‐set temperature of at least 140°C was required to develop sufficient crimp contraction. The crystallinity and orientation of the PET were attributed mainly to the crimp contraction of the drawn fiber. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Processing,structure, and properties of gel spun PAN and PAN/CNT fibers and gel spun PAN based carbon fibers

Polyacrylonitrile (PAN) and PAN/carbon nanotube (PAN/CNT) fibers were manufactured through dry‐jet wet spinning and gel spinning. Fiber coagulation occurred in a solvent‐free or solvent/nonsolvent coagulation bath mixture with temperatures ranging from ?50 to 25°C. The effect of fiber processing conditions was studied to understand their effect on the as‐spun fiber cross‐sectional shape, as well as the as‐spun fiber morphology. Increased coagulation bath temperature and a higher concentration of solvent in the coagulation bath medium resulted in more circular fibers and smoother fiber surface. as‐spun fibers were then drawn to investigate the relationship between as‐spun fiber processing conditions and the drawn precursor fiber structure and mechanical properties. PAN precursor fiber tows were then stabilized and carbonized in a continuous process for the manufacture of PAN based carbon fibers. Carbon fibers with tensile strengths as high as 5.8 GPa and tensile modulus as high as 375 GPa were produced. The highest strength PAN based carbon fibers were manufactured from as‐spun fibers with an irregular cross‐sectional shape produced using a ?50°C methanol coagulation bath, and exhibited a 61% increase in carbon fiber tensile strength as compared to the carbon fibers manufactured with a circular cross‐section. POLYM. ENG. SCI., 55:2603–2614, 2015. © 2015 Society of Plastics Engineers     

Mechanical and shape‐memory behavior of shape‐memory polymer composites with hybrid fillers

Shape‐memory polymer (SMP) materials have several drawbacks such as low strength, low stiffness and natural insulating tendencies, which seriously limit their development and applications. Much effort has been made to improve their mechanical properties by adding particle or fiber fillers to reinforce the polymer matrix. However, this often leads to the mechanical properties being enhanced slightly, but the shape‐memory effect of reinforced SMP composites being drastically reduced. The experimental results reported here suggested that the mechanical resistive loading and thermal conductivity of a composite (with hybrid filler content of 7.0 wt%) were improved by 160 and 200%, respectively, in comparison with those of pure bulk SMP. Also, the glass transition temperature of the composite was enhanced to 57.28 °C from the 46.38 °C of a composite filled with 5.5 wt% hybrid filler, as determined from differential scanning calorimetry measurements. Finally, the temperature distribution and recovery behavior of specimens were recorded with infrared video in a recovery test, where a 28 V direct current circuit was applied. The effectiveness of carbon black and short carbon fibers being incorporated into a SMP with shape recovery activated by electricity has been demonstrated. These hybrid fillers were explored to improve the mechanical and conductive properties of bulk SMP. Copyright © 2010 Society of Chemical Industry     

Extremely oriented tunicin whiskers in poly(vinyl alcohol) nanocomposites

In order to utilize the excellent mechanical properties of cellulose whiskers (CWs) along their length, the present work was undertaken to embed CWs with highly oriented forms in a polymer matrix. Nanocomposite fibers were prepared using poly(vinyl alcohol) (PVA; degree of polymerization of 1500) as the matrix and a stable aqueous suspension of CWs extracted from tunicates as the reinforcing phase. Macroscopically homogeneous suspensions of PVA–CW were gel‐spun in a methanol coagulating bath. The as‐spun fibers included CWs oriented along the fiber axis and showed a significant increase in dynamic storage modulus. Hot drawing of the PVA–CW as‐spun fibers to their maximal draw ratio led to extremely high orientation of the CWs together with a drastic reduction in voids in the fiber matrix. Outstanding mechanical properties of the drawn composites were obtained by the incorporation of only a small amount (1 wt% of solid PVA content) of CWs. The stress transfer mechanism in the fibers was studied using an X‐ray diffraction technique by applying stress to the whole composite with in situ monitoring of stress on the incorporated CWs. The applied external stress was found to be translated efficiently to the incorporated CWs through the PVA matrix, suggesting strong interfacial bonding between filler and matrix. The strong interaction and efficient stress transfer between matrix and filler are suggested as the cause for the observed improvements in mechanical properties of the composites. Copyright © 2011 Society of Chemical Industry     

Investigations of the preparation technology for polyglycolic acid fiber with perfect mechanical performance

Polyglycolic acid (PGA) fibers were prepared by melt‐spinning process in this report. The effects of spinning parameters, such as windup rates and drawn ratio, on the mechanical properties of the fibers were discussed by analyzing the internal stress of as‐spun fibers, axial sound velocity, fiber tenacity, etc. The results showed that windup rate had a slight effect on the macromolecular orientation degree of the as‐spun fibers, which was quite unusual for melt spinning, whereas, the subsequent drawing process effectively increased the macromolecular orientation degree of the PGA fibers and consequently increased the tensile strength of the fibers. Low internal stress of as‐spun fibers obtained at lower windup rate led to higher drawing ratio, and the drawn fibers possessed relatively excellent mechanical properties. As a contrast, higher windup rate resulted in the strong internal stress of the as‐spun fibers, which had a negative influence on the drawing process, and so the tensile strength of the drawn fibers was relatively poor. Therefore, PGA fiber with perfect mechanical performance could be prepared at the technical parameters of lower windup rate and higher drawing multiples as well as slow drawing rate. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Structural changes in the melt spinning,cold drawing,and annealing of poly(4‐methylpentene‐1) fibers

Isotactic poly(4‐methylpentene‐1) melt‐spun fibers were investigated. Prior investigators of melt‐spun fibers found that these fibers have a tetragonal unit cell (Form I). We obtained the same unit cell structure in melt‐spun fibers. We found that higher draw‐down‐ratio fibers had d‐spacings closer to the previously cited values of Form I. We also found that cold‐drawn fibers had similar values to those of melt‐spun fibers. However, after these were annealed at 200°C, the unit cell was changed. It is possible that this new unit cell was the orthorhombic form of He and Porter. We also observed the birefringence of these fibers. The values changed after the melt‐spun fibers were cold drawn and annealed. The melt‐spun fiber values reached 0.006. The values for the drawn fibers were as high as 0.007. We suggest that the intrinsic birefringence is about 0.0075. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 130–137, 2005     

Study on the liquefied‐MDI‐based shape memory polyurethanes

A series of liquefied‐MDI‐based polyurethanes having shape memory behavior, with various soft segments, chain extenders, and micro‐phase separation promoters were synthesized. Their morphology and properties were investigated in terms of thermal properties, dynamic mechanical properties, and shape recovery behavior. The results indicate that the soft segment formed with longer chain segment incline to crystallize during the cooling scans and the resulting SMPU have the higher crystallinity. Meanwhile, the chain extenders, which can enhance the polarity of hard segment, incline to have excellent shape memory properties with bigger storage modulus in rubbery state too. It was also found that the micro‐phase separation promoters have great influence on the shape memory behavior due to the enhancement of micro‐phase separation of SMPU. Furthermore, it was proved again that SMPU with longer soft segment and lower hard segment contents usually showed good shape memory behavior. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007