Crystallization Of Soft Segments Research Articles

4D-Printed Tool for Compressing a Shape Memory Polyurethane Foam during Programming.

Although several force application concepts are known that can be used to deform shape memory polymers (SMPs) within the scope of programming, controlled deformation is challenging in the case of samples with a cylinder-like shape, which need to be homogeneously compressed starting from the lateral surface. To solve this problem, this contribution follows a material approach that takes advantage of four-dimensional (4D) printing. Fused filament fabrication (FFF) was used as an additive manufacturing (AM) technique to produce a thermoresponsive tool in a cylindrical shape from a polyether urethane (PEU) having a glass transition temperature (Tg) close to 55 °C, as determined by differential scanning calorimetry (DSC). Once it was 4D-printed, a sample of laser cut polyester urethane urea (PEUU) foam with a cylindrical wall was placed inside of it. Subsequent heating to 75 °C and keeping that temperature constant for 15 min resulted in the compression of the foam, because the internal stresses of the PEU were transferred to the PEUU, whose soft segments were completely molten at 65 °C as verified by DSC. Upon cooling to -15 °C and thus below the offset temperature of the soft segment crystallization transition of the PEUU, the foam was fixed in its new shape. After 900 days of storage at temperatures close to 23 °C, the foam recovered its original shape upon reheating to 75 °C. In another experiment, a 4D-printed cylinder was put into hibernation for 900 days before its thermoresponsiveness was investigated. In the future, 4D-printed tools may be produced in many geometries, which fit well to the shapes of the SMPs to be programmed. Beyond programming SMP foams, transferring the forces released by 4D-printed tools to other programmable materials can further expand technical possibilities.

Single-Step Synthesis and Characterization of Non-Linear Tough and Strong Segmented Polyurethane Elastomer Consisting of Very Short Hard and Soft Segments and Hierarchical Side-Reacted Networks and Single-Step Synthesis of Hierarchical Hyper-Branched Polyurethane.

Polyurethane elastomers are among the most versatile classes of industrial polymers-typically achieved through a two-step synthesis of segmented block copolymers, comprising very long and soft segments that provide elasticity and significantly long and hard segments that provide strength. The present research focused on the design of a single-step synthesis of a new segmented polyurethane consisting of very short soft and hard segments, crosslinked by preferentially side-reacted hierarchical tertiary oligo-uret network structures, thus exhibiting significant strength, elasticity, and toughness. Despite the theoretically linear structure, both FTIR and solid-state 13C NMR spectroscopy analyses indicated the quasi-equal presence of urethane groups and tertiary oligo-uret structures in the resulting polymer, indicating a preferential consecutive side reaction mechanism. Thermal analysis indicated the significant crystallization of soft segments consisting of only four ethylene oxide units, which was, hereby, demonstrated to occur via an extended chain mechanism. Tensile mechanical properties included significant strength, elasticity, and toughness. Increasing the soft segment length led to a decreased tertiary oligo-uret secondary crosslinking efficacy. The preferential hierarchical side reaction mechanism was, hereby, further confirmed through the synthesis of a completely new type of hyper-branched polymer via diisocyanate and a mono-hydroxy-terminated reagent. The structure-property relations and reaction mechanisms demonstrated in the present research can facilitate the design of new polyurethanes of enhanced performance and processing efficacy for a variety of novel applications.

Nonisocyanate Polyurethane Segmented Copolymers from Bis-Carbonylimidazolides.

Bis-carbonylimidazolide (BCI) functionalization enables an efficient synthetic strategy to generate high molecular weight segmented nonisocyanate polyurethanes (NIPUs). Melt phase polymerization of ED-2003 Jeffamine, 4,4'-methylenebis(cyclohexylamine), and a BCI monomer that mimics a 1,4-butanediol chain extender enables polyether NIPUs that contain varying concentrations of hard segments ranging from 40 to 80wt. %. Dynamic mechanical analysis and differential scanning calorimetry reveal thermal transitions for soft, hard, and mixed phases. Hard segment incorporations between 40 and 60wt. % display up to three distinct phases pertaining to the poly(ethylene glycol) (PEG) soft segment Tg , melting transition, and hard segment Tg , while higher hard segment concentrations prohibit soft segment crystallization, presumably due to restricted molecular mobility from the hard segment. Atomic force microscopy allows for visualization and size determination of nanophase-separated regimes, revealing a nanoscale rod-like assembly of HS. Small-angle X-ray scattering confirms nanophase separation within the NIPU, characterizing both nanoscale amorphous domains and varying degrees of crystallinity. These NIPUs, which are synthesized with BCI monomers, display expected phase separation that is comparable to isocyanate-derived analogues. This work demonstrates nanophase separation in BCI-derived NIPUs and the feasibility of this nonisocyanate synthetic pathway for the preparation of segmented PU copolymers.

The Impact of Soft Segment Crystallization and Cross‐Link Density on the Shape Memory Performance of the PCL‐PTMG/Graphene‐ Based Polyurethane Nanocomposites

AbstractIn this research paper, we investigated the impact of soft segment crystallization, the cross‐linking density (CLD) and elastic modulus of the synthesized thermoset polyurethanes on their shape memory performance (SMP). A group of tri‐block copolymers of poly(ϵ‐caprolactone) (PCL) and poly(tetramethylene glycol) (PTMG) with PCLx‐PTMG2000‐PCLx architecture were synthesized and used as polyols in a thermoset polyurethane (PU) structure. Crystallization of the soft segments was controlled through changing PCLx blocks’ length. Graphene nanoplatelets, with a fixed content of 0.50 wt.%, was used as the cross‐linker and the nano‐filler to tune elasticity of the PUs. Analysis of crystallization showed that by increasing PCLx’s length from 0 to 2000 D caused a drastic change in the crystallization behavior of the polyols. The polyols used in the thermoset PU nanocomposite led to a wide spectrum of elastic modulus at temperatures close to room temperature. The elastic modulus at room temperature ranged from 20 to 100 MPa. The changes in elastic modulus, CLD and soft segments’ crystallization resulted in a complex SMP behavior, which was studied in‐detail. The shape recovery rate was also studied, which showed the impact of crystallites’ melting, CLD and elastic modulus through a three‐stage shape recovery.

Study on polyurethane elastomer modification for improving low-temperature resistance of high-capacity polyurethane elastomeric bearing for bridges

• Side methyl groups in PTGL hindered the crystallization of soft segments in PUE. • Shear modulus of PUE decreased with incorporating PTGL at subzero temperatures. • HPEB prepared with new PUE still exhibited high enough vertical bearing capacity. • Adding PTGL could significantly improve the low-temperature resistance of HPEB. Seismic isolation laminated elastomeric bearings are applied worldwide in bridges to reduce vibration and prevent collapse. Currently, the high-capacity polyurethane elastomeric bearings (HPEB) are favored in long-span bridges due to their super-high bearing capacity, low cost, and simple manufacturing process. However, there are many countries and regions in the world with extremely cold climates and are covered by ice and snow. At subzero temperatures, the stiffness of polyurethane elastomers (PUEs), which are an important component of HPEB, increases significantly, rendering HPEB ineffective in protecting bridges in cold regions from earthquake hazards. In this work, an attempt was made to modify the PUE as well as to improve the low-temperature resistance of HPEB by introducing 3-methyl-tetrahydrofuran/tetrahydrofuran co-polyether glycol (PTGL). Specifically, a series of microscopic characterization experiments (FT-IR, AFM, DSC, and DMA) were performed to study microphase separation and crystallization phenomena of the newly developed PUE. Moreover, the mechanical verification experiments were performed for the PUE and HPEB with different amounts of PTGL, including tensile, compression, and shear tests. All tests are performed at 23, 0, −10, and −20 °C temperatures respectively. The mechanical experiments demonstrate that the introduction of PTGL significantly reduces the shear modulus ( G ) and horizontal stiffness ( K h ) of HPEB at low temperatures, that is, improves the low-temperature resistance of HPEB. Therefore, this research is of great significance for the application of isolation bearings in cold regions.

Phase structure and transition behavior of zwitterionic polyurethane containing sulfobetaine

Zwitterionic polyurethane has been intensively studied due to their strong hydration ability and molecular interaction, with the most of reports focused on the post-polymerization functionalization. Herein, a series of block zwitterionic polyurethanes (SBZPUs) containing sulfobetaine-diol (SB-diol) are synthesized, and their crystallization, phase structure, and transition behavior are fully characterized with NMR, FT-IR, DSC, SAXS, WAXS, and AFM, to ascertain the effect of the ionic interactions. It is found that the introduction of zwitterionic SB-diol disrupts the hydrogen bonding of hard segments, while improves the crystallization of soft segment and micro-phase separation of SBZPUs due to the strong ionic interactions. The physical and mechanical properties of SBZPUs could be adjusted by varying the SB-diol amount, and the transition point could be tuned to around human body temperature. In combination with the excellent cytocompatibility, SBZPUs show a promising potential in applications of biomedical materials.

Effect of multi-armed chain extender on microphase morphology, stress-strain behavior and electromechanical properties of polyurethane elastomers

High dielectric constant and low modulus of thermoplastic polyurethane elastomers (PUEs) are required for various applications in actuators, sensors and energy harvestings. In this work, we controlled the microphase separated morphology of PUEs by regulation of the content of hydroxyl-terminated four-armed polycaprolactone (4amPCL) chain extender to achieve J-shaped stress-strain curve with low modulus and high electromechanical properties. Our results showed that the hydrogen bonding between hard segments (HSs) and crystallization of soft segment (SS) are significantly impeded by long-chain branched structure when added lower molar fraction (<20% of chain extenders) of 4amPCL as chain extender, whereas the branched long-chains are embedded in microphase separated hard and soft domains (HD and SD) to form strip shaped interconnected hard domain (HD) rich regions when added higher molar fraction (>20% of chain extenders) of 4amPCL as chain extender. The PUE with 29% of 4amPCL (PU-4amPCL-0.29) showed lower modulus (1.5 MPa), higher tensile strength (17.5 MPa) at 1048% of elongation, and 3.9% of actuation strain at 20 kV/mm, and higher contribution (48.1%) of Maxwell stress on actuation strain at 20 kV/mm. We found that the microphase separation of branched long-chains induced semi-interpenetration polymer network like structure of PUEs played an important role in exhibiting J-shaped stress-strain behavior. Our results can provide new insight on fabrication of high electromechanical performance of dielectric polyurethane elastomers.

Effect of crystalline structure on water resistance of waterborne polyurethane

A series of solvent-free waterborne polyurethanes (WPUs) with different soft segment structure were prepared by respectively using different number-average molecular weight (Mn) of polytetramethylene glycol (PTMG, Mn = 1000, 2000, 3000) and poly (1,4-butylene adipate) (PBA, Mn = 1000, 2000 3000) as main diol in this paper. Effect of soft segment crystallization on water resistance of WPUs was discussed. The crystallization properties of WPU films were investigated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The water resistance was determined by water absorption ratio. DSC and XRD results confirm that there are no crystalline structures in the WPU films which are synthesized with PTMG (Mn = 1000, 2000, 3000) and PBA (Mn = 1000), while the WPU films synthesized with PBA (Mn = 2000 and 3000) have obvious crystalline structures at room temperature. Meanwhile, the WPU films synthesized with PBA (Mn = 2000 and 3000) possess excellent water resistance because the existence of crystalline structure.

Elastocaloric properties of thermoplastic polyurethane

Very few studies have explored the elastocaloric effect of elastomers other than natural rubber (NR). The aim of the present article is thus to evaluate the elastocaloric properties of a thermoplastic polyurethane (TPU) in terms of microstructural characteristics and thermoelastic coupling. Calorimetric measurements showed two successive peaks at 240 K and 282 K, attributed to the crystallization and melting of soft segments, respectively. X-ray diffraction indicated that TPU exhibited a fully reversible strain-induced crystallization at room temperature. Thermomechanical experiments performed at different elongations revealed a minimum adiabatic temperature variation of about −8 K after retraction of a sample initially elongated at λ = 5. This is comparable to NR performances. However, for cycles carried out between λ = 1 and λ = 5, tensile stress/elongation curves showed a non-elastic behavior of TPU. A pseudo-elastic response was obtained for cyclic elongation when unloading was incomplete, in our case, when λ was between 3 and 5. The recorded peak-to-peak temperature variation decreased from 4.5 K to 3.3 K when the number of cycles was increased to 5000. Despite the fact that the issue of fatigue resistance for TPU needs to be addressed, this work opens new perspectives for studying the elastocaloric properties of various polyurethanes (whether crosslinked or thermoplastic) as well as other materials with a tendency for strain-induced crystallization, such as polychloroprene, hydrogenated acrylonitrile butadiene rubber, and others.

Polyurethane as smart biocoatings: Effects of hard segments on phase structures and properties

Development of smart polyurethane (PU) coatings for enhancing the biocompatibility of the metal scaffold has attracted great interest in the past few years, and the investigation of the relationship between phase structure and smart properties is necessary. Herein, two series of PU biocoatings with different hard segment structures and soft segment length were prepared, and the soft segments consist of the polycaprolactone diols (PCL) with different molecular weights, and the hard segments were made of 1, 4-butanediol (BDO) and hexamethylene diisocyanate (HDI) with linear structure, or BDO and isophorone diisocyanate (IPDI) with alicyclic structure, respectively. To understand the phase structures, the chemical structure, surface micromorphology, and segmental incompatibility of the PU films were characterized, which reveals that IPDI-based PU possesses a phase compatible structure with a single-phase transition above 0 °C. Besides, the IPDI-based PU shows the thermal-responsive deformation at the temperature of 45 °C, slightly higher than the human body temperature. In contrast, the HDI-based PU possesses the excellent shape memory property caused by the microphase separation structure, because of the crystallization of soft and hard segments from the melt. Further, based on the characteristics of phase structure, the potential application of the IPDI-based PU and HDI-based PU is explored, offering a useful approach to effectively design the structures and properties of PU coatings.

The Graft-Polymerization of Poly(Ethylene Glycol) Phenyl Ether Acrylate onto Polyurethane and Its Impact on the Mechanical Properties and Chain Packing

The poly(ethylene glycol) phenyl ether acrylate (PEPA) monomers are graft-polymerized onto polyurethane (PU), and its impacts on water compatibility, chain packing, tensile stress, and shape recovery capability are evaluated. The grafted poly(PEPA)s slightly increase the degree of cross-linking due to the linking between themselves. The water compatibility (water contact angle, water swelling, and water vapor permeation) are affected slightly by the grafted poly(PEPA). The grafted poly(PEPA) does not notably influence the melting of soft segments, but the crystallization of soft segments is reduced significantly. The grafted poly(PEPA) does not significantly affect the glass transition of soft segments. The grafted poly(PEPA) notably increases the breaking tensile stress with the increase in PEPA content, but the breaking tensile strain decreases slowly with increasing PEPA content. The grafted poly(PEPA) rapidly improves the shape recovery capability of PU and slowly decreases the shape retention capability with the increase in PEPA content. Therefore, the grafted poly(PEPA) slightly modifies the water compatibility but notably enhances the breaking tensile stress and shape recovery capability of PU.

The effect of hard segments structure on the functional properties of polyurethane elastomers based on oligoesterurethane epoxides

A series of polyurethane elastomers based on polyester urethane epoxy oligomers and cycloaliphatic diamines was synthesized. The relationship of the physico-mechanical properties of these elastomers with the structure of their polymer chain was studied. It was established that various options for increasing the strength of the final elastomers can be implemented, depending on the type of initial oligoesters and diamines. They may be associated with an increase in the degree of microphase separation between hard and soft polymer segments, and with partial crystallization of soft segments as well.

Protective role of vitamin E to reduce oxidative degradation of soft implantable polyurethanes: In vitro study

Abstract Vitamin E (VitE) additives are important in treating osteoarthritis inclusive cartilage regeneration due to their antioxidant and anti-inflammatory properties. The present research study focuses on the ability of biological antioxidant VitE (alpha-tocopherol isoform) to reduce or minimize oxidative degradation of soft implantable polyurethane (PU) elastomers after extended periods of time (5 months) in vitro. The effect of the oxidation storage media on the morphology of the segmented PUs was evaluated by mechanical softening, crystallization and melting behavior of both soft and hard segments (SS, HS) using dynamic mechanical analysis (DMA). Bulk mechanical properties of the potential implant materials during ageing were predicted from comprehensive mechanical testing of the biomaterials under tension and compression cyclic loads. 5-months in vitro data suggest that the prepared siloxane-poly(carbonate-urethane) formulations have sufficient resistance against degradation to be suitable materials for chondral long-term bio-stable implants. Most importantly, the positive effect of incorporating VitE (0.5 or 1.0% w/w) as bio-antioxidant and lubricant on the bio-stability was observed for all PU-types. VitE-additives protected the surface layer from erosion and cracking during chemical oxidation in vitro as well as from thermal oxidation during extrusion re-processing.

Preparation and characterization of hydrophilic temperature‐dependent polyurethane containing the grafted poly(N‐isopropylacrylamide)

The polyurethane (PU) structure is modified with dimethylolpropionic acid (DMPA) and poly(N‐isopropylacrylamide; poly[IPA]) to prepare hydrophilic temperature‐dependent PU that is investigated with reference to the degree of crosslinking, thermal properties of soft segment, tensile and shape memory performance, hydrophilic conversion of surface, and temperature‐dependent water swelling and water vapor permeability (WVP). The thermal properties of soft segment (melting, crystallization, and glass transition) are significantly affected by poly(IPA). Breaking tensile stress also increases with increasing IPA monomer content due to crosslinking effect but breaking tensile strain does not significantly decrease with increasing IPA monomer content. Shape recovery capability at 10°C steeply inclines to over 90% from 46.9% for unmodified PU by the grafting of poly(IPA), whereas shape retention at −25°C does not decrease below 90% with the increase in IPA content. Poly(IPA)‐grafted PU can display temperature‐dependent control of water swelling and WVP due to transformation of the grafted poly(IPA) depending on the surrounding temperature. POLYM. ENG. SCI., 59:1719–1728 2019. © 2019 Society of Plastics Engineers

The temperature-sensitive water vapor permeation control of polyurethane membrane using the graft-polymerized poly(N-isopropylacrylamide) and the impact on the tensile strength and shape recovery effect

ABSTRACTPoly(N-isopropylacrylamide) (poly(NIPAM)) was grafted onto polyurethane (PU) using a graft-polymerization method to develop a thermo-responsive PU and to investigate the impact on cross-link density, solution viscosity, soft segment thermal transitions, tensile properties, shape memory effect, and water vapor permeation through PU membrane. The soft segment crystallization peak sharply decreased with the increase in NIPAM content, whereas the glass transition temperature (Tg) slightly increased with the increase of NIPAM content. The breaking tensile stress rapidly increased with the increase in NIPAM content due to the cross-linking effect between the grafted poly(NIPAM) chains, whereas the strain at break did not significantly decreased as the NIPAM content increased. The shape recovery at 10°C rapidly increased from 46.9% for plain PU to above 90% after the grafting of poly(NIPAM) onto PU, and the shape retention at −25°C slightly decreased with the increase in NIPAM content. Finally, the grafting of poly(NIPAM) onto PU demonstrated the temperature-responsive control of water vapor permeation through PU film due to the conformational change of the grafted poly(NIPAM) with the increase of temperature and the potential applications of the resulting PUs are discussed.

Grafting of Triphenylmethyl Group onto Polyurethane and the Impact on the Shape Recovery and Flexibility at Extremely Low Temperature

The bulky and rigid triphenylmethyl group was grafted onto polyurethane (PU) to reduce the molecular attractions between hard segments and to improve the mobility of the PU chain under freezing conditions. The triphenylmethyl-grafted PU exhibited improvement in the cross-link density, solution viscosity, maximum tensile stress, shape recovery at 10 °C, and low temperature flexibility compared with the plain PU. The soft segment melting was not affected by the grafted triphenylmethyl group, whereas the soft segment crystallization disappeared with the increase of the triphenylmethyl group content. The glass transition temperature (Tg) increased with the increase of the triphenylmethyl group content. The rapid increase of the tensile strength and shape recovery at 10 °C resulted from the cross-linking effect, whereas the strain at break and shape retention at -25 °C slightly decreased with the increase of the triphenylmethyl group content. The triphenylmethylgrafted PU displayed an excellent low temperature flexibility even at -50 °C due to the improved mobility of the PU chain compared to ordinary PU.

Theoretical predictions on microphase separation in polyurethane: Combinatorial design, synthesis and demonstration of shape memory property

Hydrogen bonding interactions among hard segments and crystallization of soft segments being critical factors for microphase-segregated morphology in shape memory polyurethane, tuning these parameters would lead to enhancement of the shape memory behavior. We investigated this morphological importance and contribution of each segment by combinatorial design of polyurethane using polyethylene glycol, toluene diisocyanate, 4,4′-methylenebis(phenyl isocyanate), butane-1,4-diol and ethylene diamine as building blocks. In a dual approach, a combined theoretical and experimental approach was adopted for optimization of shape memory property. The hard segments were modeled using density functional theory (DFT) using B3LYP method and 6–31G** basis set. Extent of phase separation and mixing were predicted from bond distances of H-bonded carbonyls (NH⋯OC) and that between NH group and ether oxygen (NH⋯O). The frequency shifts were computed and stability was determined from binding energies using localized pseudospectral methods (PS-LMP2). MDI_EDA system with shorter predicted bond length imparted better phase separation. PU samples were synthesized and characterized by spectroscopy (IR, NMR), polarized optical microscopy, XRD, DSC, DMA and AFM analyses. Tuning of phase segregation was achieved by varying both soft segment length and hard segment content. PEG6000_MDI_EDA showed improved properties with 340% elongation at break and tensile strength of 11 MPa. Thermomechanical cyclic tensile testing revealed shape recovery ratio of the sample to be 96.4% while fixity ratio was 51.4%. The shape memory behavior was successfully demonstrated in an electric circuit where polyurethane actuator acted as circuit breaker at time of overload thus ensuring circuit safety. Furthermore, an intrinsic fluorescence from samples in film and fiber form was observed which widens the scope of research in area of light emitting shape memory polyurethanes.

Degradation of polymeric bellows effecting life cycle of constant-velocity joint

ABSTRACTOne broken and three new polymeric constant-velocity boots (bellows) being a spare part of a constant-velocity joint in two military vehicles were analyzed to identify basic composition and to determine if composition or structure of broken bellows differ. Infrared spectroscopy, differential scanning calorimetry, and thermogravimetry were used for analyses. Bellows for both vehicles are produced from thermoplastic elastomer. Broken bellows is chemically degraded and material just from the fracture is deformed plastically. The life cycle of bellows depends especially on applied stress and temperature (that about −20°C seems to be critical due to crystallization of soft segments).

Graft polymerization of norbornene onto polyurethane and the characterization of its effect on the shape recovery and low-temperature flexibility of polyurethane

Graft polymerization of norbornene monomers onto polyurethane (PU) was tested to reduce the intermolecular attractions between PUs using the rigid bicyclic ring structure of norbornene, to control the soft segment thermal properties, and to enhance the tensile, shape memory, and low-temperature flexibility properties compared with those of unmodified PU. The soft segment crystallization temperature (T c) sharply decreased with the increase in norbornene, whereas the soft segment melting temperature (T m) was not significantly changed by the norbornene content. However, the glass transition temperature (T g) increased with the incorporation of norbornene. The tensile strength sharply increased with the increase in norbornene due to the chemical cross-linking between the grafted poly(norbornene), whereas a significant decrease in the maximum strain was not observed. The shape recovery at 10 °C sharply increased from 47% to approximately 90% after the graft polymerization of norbornene, and the shape retention at −25 °C decreased with the increase in norbornene. Finally, the graft polymerization of norbornene onto PU demonstrated enhanced low-temperature flexibility due to the reduced restrictions of rotational and translational mobility.

Preparation of hybrid polyurethane–silica composites by a lateral sol-gel process using tetraethyl orthosilicate

Hybrid polyurethane–silica composites were prepared using a sol-gel reaction between tetraethyl orthosilicate and triethoxysilyl groups grafted onto polyurethane. The TS series contained the same polyurethane composition but the tetraethyl orthosilicate content varied. Soft segment Tm and Tg values were not affected by the tetraethyl orthosilicate content, while ΔH for soft segment melting and crystallization increased with the tetraethyl orthosilicate content. The tensile strength sharply increased from the cross-linking of the grafted silica at low tetraethyl orthosilicate concentrations but decreased with the increase of tetraethyl orthosilicate content because excess silica inhibited cross-linking. The shape recovery of the TS series improved during cyclic tests, and shape retention slightly decreased as the test cycles repeated. Low temperature flexibility moderately improved with the formation of grafted silica. Therefore, the grafted sol-gel silica formation demonstrated a tensile strength improvement and reproducible shape recovery.