Semicrystalline Polyurethane Research Articles

Modular Design of a Polymer-Bilayer-Based Mechanically Compliant Worm-Like Robot.

Soft earthworm-like robots that exhibit mechanical compliance can, in principle, navigate through uneven terrains and constricted spaces that are inaccessible to traditional legged and wheeled robots. However, unlike the biological originals thatthey mimic, most of the worm-like robots reported to date contain rigid components that limit their compliance, such as electromotors or pressure-driven actuation systems. Here, a mechanically compliant worm-like robot with a fully modular body that is based on soft polymers is reported. The robot is composed of strategically assembled, electrothermally activated polymer bilayer actuators, which are based on a semicrystalline polyurethane with an exceptionally large nonlinear thermal expansion coefficient. The segments are designed on the basis of a modified Timoshenko model, and finite element analysis simulation is used to describe their performance. Upon electrical activation of the segments with basic waveform patterns, the robot can move through repeatable peristaltic locomotion on exceptionally slippery or sticky surfaces and it can be oriented in any direction. The soft body enables the robot to wriggle through openings and tunnels that are much smaller than its cross-section.

Bilayer Actuator with Overload Protection on the Basis of Semicrystalline Polyurethane.

Response to external stimuli plays a significant role in the environmental adaptation of living matters and intelligent devices. Most stimulus-response systems in nature can respond to appropriate stimuli, and inhibit the response under excessive stimuli, such as excessive heat or water, which could be called overload protection. However, even though various responsive materials have been developed for different stimuli, most of them are not protective against the overload stimuli. In this work, we have designed a bilayer actuator based on semicrystalline polyurethane, which can respond differently to proper stimuli and excessive stimuli, i.e., water. This actuator could bend gradually under the proper stimulation of water, but would straighten and even bend reversely with excessive stimulation. The mechanism behind the reversible and adjustable actuator with overload protection has been investigated both experimentally and theoretically, and the competition between dynamic factors and thermodynamic stability in the swelling process is considered the main cause. This article is protected by copyright. All rights reserved.

Bilayer Bending Actuators Based on Semicrystalline Polyurethanes with Large Thermal‐Expansion Coefficients

Thermoresponsive polymer‐based bilayer bending actuators rely on the dissimilar thermal expansion of the polymers that form the two layers. To maximize the heat‐induced change in curvature, the thermal expansion shall be large in one and small in the other material. Semicrystalline polymers display a large nonlinear thermal expansion across their melting transition, but as their mechanical integrity is lost upon melting, they cannot readily be used in bilayer actuators. To overcome this limitation, segmented polyurethanes (PUs) with crystallizable polyethylene glycol (PEG) soft segments and hard segments formed by the reaction of 1,6‐hexane diisocyanate and 1,4‐butanediol (BDO) is developed. The latter serve as physical cross‐links and inhibit flow at temperatures where the domains formed by the PEG segments have melted. The molecular weight of the PEG segments and the hard‐segment content in the polymer are systematically varied. As the nonlinear expansion of the PEG–PUs is associated with the melting of the crystalline PEG domains, the thermal expansion of these materials is correlated with their crystallinity and is highest for the polymers with the lowest BDO content and the highest PEG molecular weight. Electro‐thermally controlled bilayer bending actuators based on the new materials display high deflection and low switching temperatures.

Development of semi-crystalline polyurethane with self-healing and body temperature-responsive shape memory properties

Recently, polyurethane (PU) with multifunctionality has attracted significant attention in biomedical applications. This study reports semi-crystalline PUs with both shape memory and self-healing effects, which are composed of poly(ε-caprolactone) diol (PCL), isophorone diisocyanate (IPDI), and 1, 4-butanediol (BDO). The hard segment content greatly influences the structure, morphology and properties, and the increasing hard segment content promotes the phase separation and reduces the crystallizability of the soft segments. Herein, a new self-healing morphology of the semi-crystalline SMPUs is observed, and the self-healing mechanism is ascribed to the diffusion of soft segments and the stability of the hard domains. The relationship between the self-healing efficiency and the stability of hard domains is also explored. Furthermore, the shape memory mechanism of IPDI-based PU is clarified. Based on the partial melting state of the soft segment crystals, the PUs feature relatively good shape memory properties, and the response temperature can be designed as body temperature (37 °C). As the hard segment content increases, the value of shape fixity increases, while that of shape recovery decreases, which is related to hydrogen bonding between polymer chains. Considering the excellent biocompatibility, the as-designed semi-crystalline SMPUs are expected to be used as smart biomaterials in biomedical devices.

Crystalline segments in a photo-detachable adhesive

Photo-induced detachable adhesive at room temperature is expected in biomedical and expensive precision instruments. However, its concomitant weak bonding strength has been the bottleneck restricting its wide application due to its lower glass transition temperature (Tg). Herein, a semi-crystalline polyurethane (PU) adhesive was prepared by the reaction of polycaprolactone diol (PCL-diol), 4,4′-methylenebis (cyclohexyl isocyanate) (HMDI) and chain extender mixture of 1,4-butanediol (BD) and a diol containing anthracene side chain (AERMT). The anthracene side group was used as the UV light-responsive reversible crosslinking site. The crystalline segments dominated by PCL-diol further enhanced the bonding strength, especially in photo-induced curing adhesive with Tg much lower than room temperature. The synthetizing and curing behavior of the adhesive was investigated by FTIR, NMR and UV–vis absorption spectroscopy. DSC, XRD and polarizing microscope were used to confirm the crystallization behavior of the adhesive before and after curing. The bonding strength was tested by using quartz glass sheet as the bonding substrate. The maximum strength was found to reach as high as 7.5 MPa. Then, under the irradiation of 254 nm UV light, the shear strength decreased significantly to achieve the purpose of disassembly process. This study can provide some guidance for photo-induced high strength detachable adhesives at room or moderate temperatures.

Recent Advances in the Synthesis and Application of Thermoplastic Semicrystalline Shape Memory Polyurethanes

The state of the art of materials science in the field of synthesis of promising functional thermoelastoplasts based on polyurethanes and poly(urethane ureas) is analyzed. Interest in these materials has remained high during the past two decades owing to the ease of production, biocompatibility, good thermal and mechanical characteristics, and the possibility to adjust properties to a specific application. The main focus is on describing semicrystalline polyurethanes, in which crystalline domains play the role of a temporary physical network, which provides an opportunity to design adaptive materials with the shape memory effect. The described materials are divided according to the types of external stimuli which are switches of mechanical properties. The effect of the nature and composition of thermoresponsive polyurethanes on the mechanical properties and main characteristics of the shape memory effect is considered. Recent achievements in the synthesis, modification, and characterization of polyurethane materials are discussed, and the most interesting examples of their application in medicine are presented.

Vibrational heat capacity of the linear 6,4-polyurethane

The calculated vibrational heat capacity of the linear 6,4-polyurethane (6,4-PU) in the solid state was estimated using its vibrational motions. The experimental low-temperature heat capacity of 6,4-PU collected from literature was related to approximate vibrational spectra based on the group and skeletal vibration contributions of 6,4-PU using the Advanced Thermal Analysis System (ATHAS) Data Bank. The group vibrational heat capacity was calculated based on ninety molecular vibrational motions derived from Raman and infrared spectroscopy. The skeletal vibrational heat capacity was described by a general Tarasov equation with Debye temperatures of Θ1 = 452 K, and Θ2 = Θ3 = 96 K and thirty skeletal vibrations. The liquid heat capacity of semicrystalline polyurethane (6,4-PU) was approximated from experimental data using a linear regression of the data above the melting region and the heat capacity of the fully amorphous material above the glass transition temperature. The equilibrium liquid heat capacity was expressed as Cp(liquid) = 1.0041T+233.24 in. J·K-1· mol-1. The solid and liquid heat capacities of linear polyurethane (6,4-PU) were applied as equilibrium baselines for advanced thermal analysis of the experimental, apparent heat capacity data.

Elevation of modulus and elongation of semicrystalline polyurethanes

ABSTRACTPolyurethanes were synthesized based on an amorphous polyester polyol, and different chain extenders, comparing the universally accepted symmetric chain extender (1,4‐butanediol) with an asymmetric (1,2‐propanediol) chain extender. The effect of incorporating a crystalline polyester polyol polyhexamethylene adipate (PHMA) was investigated. The resulting morphologies and tensile properties are dramatically different. Based on the thermal and infrared data obtained, the sample with the symmetric chain extender exhibits a phase‐separated morphology, which can change as a function of time and temperature. Conversely, the polyurethane with the asymmetric chain extender has a phase mixed morphology that remains stable under the same experimental conditions. Incorporating crystalline PHMA resulted in a substantial change in the mechanical properties of the materials, especially for the copolymer comprising asymmetric chain extender. The rate of crystallization and the degree of crystallization achieved depended on the type of chain extender used. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018

Preparation and characterization of titanium-segmented polyurethane composites for bone tissue engineering.

Segmented polyurethanes were prepared with polycaprolactone diol as soft segment and 4,4-methylene-bis cyclohexyl diisocyanate and l-glutamine as the rigid segment. These polyurethanes were filled with 1 wt.% to 5 wt.% titanium particles (Ti), physicochemically characterized and their biocompatibility assessed using human dental pulp stem cells and mice osteoblasts. Physicochemical characterization showed that composites retained the properties of the semicrystalline polyurethane as they exhibited a glass transition temperature (Tg) between -35°C and -45°C, melting temperature (Tm) at 52°C and crystallinity close to 40% as determined by differential scanning calorimetry. In agreement with this, X-ray diffraction showed reflections at 21.3° and 23.6° for polycaprolactone diol and reflections at 35.1°, 38.4°, and 40.2° for Ti particles suggesting that these particles are not acting as nucleating sites. The addition of up to 5 wt.% of Ti reduced both, tensile strength and maximum strain from 1.9 MPa to 1.2 MPa, and from 670% to 172% for pristine and filled polyurethane, respectively. Although there were differences between composites at low strain rates, no significant differences in mechanical behavior were observed at higher strain rate where a tensile stress of 8.5 MPa and strain of 223% were observed for 5 wt.% composites. The addition to titanium particles had a beneficial effect on both human dental pulp stem cells and osteoblasts viability, as it increased with the amount of titanium in composites up to 10 days of incubation.

Flexoelectric response in soft polyurethane films and their use for large curvature sensing

The flexoelectric effect is simply defined as the coupling between the strain gradient and polarization in solid dielectrics. It may be seen as an alternative transduction mechanism to the piezoelectric effect to directly sense the curvature of bent flexible thin structures. In the case of large curvatures, flexible and compliant sensors are required and soft polar elastomers may be suitable for curvature sensing. In this study, we report the flexoelectric characterization of soft semi-crystalline polyurethane (PU) films with thicknesses ranging from 1.7 μm to 350 μm. Dynamic bending experiments have been performed on PU films deposited onto rigid steel substrates in the vicinity of the mechanical resonance frequency of the cantilever beams. Quasi-static flexoelectric coefficients of PU films could be obtained by using a classical oscillating model. A global large increase of μ12′ with the decreasing film thickness was found, especially for thicknesses lower than 25 μm. The variation of μ12′ is explained by the presence of a Young's Modulus gradient through the thickness of PU films. Besides, a concomitant uncommon dramatic decrease in the dielectric constant is observed. The combination of these two effects contributes to enhancing the flexocoupling “F” constant with the decreasing thickness. At last, the potential use of a 6.6 μm-thick soft PU film as a large curvature sensor has been experimentally demonstrated by subjecting a flexible Aluminum foil/Polyethylene terephthalate bilayered cantilever to large deflections. A curvature of about 80 m−1 (radius of curvature of ∼1.2 cm) could be sensed under low frequency (3 Hz) bending motion. These results may pave the way for the development of low cost and easy to implement soft flexoelectric elastomer-based large curvature sensors on highly flexible metallic structures.

Graft copolymerization of acrylic acid on to styrene butadiene rubber (SBR) to improve morphology and mechanical properties of SBR/polyurethane blend

ABSTRACTGraft copolymerization of acrylic acid (AA) on to styrene butadiene rubber (SBR) is carried out via free radical polymerization using benzoyl peroxide (BPO) as initiator. Graft yield (GY) and graft efficiency (GE) measurements reveal that the optimum grafting is achieved when 100 wt % of AA and 3 wt % of BPO are used for a reaction time of 6 h at 60 °C. The execution of the grafting process is confirmed through ATR‐IR spectroscopy and DMTA analysis. Tan δ thermograms indicate that the graft copolymerization occurs in the styrene segments of the SBR backbone. An in situ polymerized, semicrystalline polyurethane (PU) is then used to prepare a series of SBR‐g‐PAA/PU blends. It is found that the SBR‐g‐PAA with the highest GY exhibits the best compatibility with PU matrix. One‐phase morphology (SEM), as well as the appearance of only one glass transition (DMTA) verify the homogeneous miscibility of the modified blend compositions. Moreover, the integration of PUs crystalline structure into blends gives rise to elongation‐induced crystallinity as the prominent phenomenon in tensile testing, which proves to synchronously enhance tensile strength, modulus, elongation at break, and toughness. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43699.

Constituent analysis of stress memory in semicrystalline polyurethane

ABSTRACTSimilar to shape memory, the stress in a stimulus responsive polymer can also be programmed, stored, and retrieved reversibly upon an external stimulus, and known as stress memory. Herein, the stress analysis in a semicrystalline polyurethane is investigated to unveil the total stress–strain components of the memory polymer. The evolution of stress under different temperature and strain levels is determined experimentally. A constitutive model based on phase transition was further used to predict and characterize the individual stress components during the thermomechanical process. In contrast to earlier models, a new approach of using relaxed modulus (RM) has been proposed to predict the stress components in tensile programming condition. The predicted results are having significant agreement with the experimental data. The quantitative stress analysis can help in engineering the products more precisely, where the controllable stimulus responsive stress is needed in multidisciplinary arenas such as pressure garments, massage devices, and artificial muscles etc. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 941–947

Effect of Cobalt Ferrite Loading on Differential Scanning Calorimetry of Magnetic Polymer Composites

Effects of magnetic cobalt ferrite (CoFe2O4) on thermal properties of polypropylene (PP), polyurethane (PU) and natural rubber (NR) matrices were studied by differential scanning calorimetry. The endothermic peak area of PP was reduced with increasing loading of CoFe2O4 from 0 to 45 % and such peak in semicrystalline PU was entirely absent when the CoFe2O4 loading was over 25 part per hundred rubber. The results indicated that magnetic clusters embedded at the interchain spacing inhibited the nucleation of order segments in polymer matrix. The presence of CoFe2O4 particles also had influence on the mobile amorphous portions of PU chains with polar functional group. No endothermic evidence on the thermograms of non-polar amorphous NR was observed.

Designing temperature-memory effects in semicrystalline polyurethane

Facile ways to design temperature-memory effects in semicrystalline polyurethane by thermomechanical programming are reported.

Programming of Temperature-Memory Onsets in a Semicrystalline Polyurethane Elastomer

We demonstrate that phase-segregated poly(ester urethane) (PEU) with crystallizable switching segments of poly(1,4-butylene adipate) (PBA) excels as high-performance temperature-memory polymer. Temperature-memory effects (TMEs) with regard to strain and stress recovering could be programmed by polymer elongation at temperatures below or within the PBA melting transition, followed by cooling under constant stress below the PBA crystallization transition and unloading. Beyond that conventional approach, a novel TME programming route was designed, mostly consisting in specimen elongation and unloading at the same temperature. As a result, an enhanced control over the onsets of strain and stress recovering could be achieved. With these findings, the TME could be exploited to switch quick response (QR) codes in recently developed information carriers from unreadable to readable. We conjecture that such behavior can be programmed into virtually all semicrystalline elastomers and anticipate applicability as labe...

AB type polyaddition route to thermoplastic polyurethanes from fatty acid derivatives

Naturally occurring oleic acid and undecylenic acid derivatives were successfully utilized to synthesize a series of new semi-crystalline polyurethanes via the AB-type polycondensation approach. A set of four novel AB-type monomers was prepared via the use of environmentally benign thiol–ene chemistry. These AB-type monomers were self-polycondensed (with or without catalyst) in two ways: (a) the hydroxyl–acyl azide approach and (b) the melt transurethane method in order to obtain reasonably high molar mass polyurethanes. The resulting polymers were thoroughly characterized by NMR, SEC, DSC, TGA and MALDI-TOF mass spectroscopy. The MALDI-TOF analysis confirmed the formation of linear polyurethanes with a trace amount of macrocycles. All the polyurethanes exhibited fair thermal stability with no significant weight loss below 200 °C. The polyurethanes based on undecylenic acid as a monomer displayed a thermoplastic behavior with well-identified melting transitions. Further, DSC analysis revealed that polyurethanes prepared from undecylenic acid by different methods showed different glass and melting transitions. Two glass transition temperatures due to soft and hard segments were observed for polyurethanes synthesized from oleic acid derivatives, indicating a phase-separated morphology.

Electrical conductivity of PUR/MWCNT nanocomposites in the molten state, during crystallization and in the solid state

The electrical conductivity of a nanocomposite constituted of multiwalled carbon nanotubes (MWCNT) dispersed in a semicrystalline polyurethane matrix, is investigated during cooling from the melt to the solid state. The same percolation threshold, ϕc=0.85wt.%, is obtained in the molten and in the solid state, although the exponent t of the percolation equation is significantly higher in the solid state. A remarkable increase of the conductivity during crystallization is observed for nanocomposites of MWCNT content above ϕc, but for contents below ϕc the conductivity decreases. Combined conductivity and PVT results, lead us to discard the hypothesis of an increase of the density of the conductive network (associated with volume shrinkage) as being the cause of the conductivity enhancement during crystallization. Instead, the analysis of the parameters of the percolation equation for the molten and the solid state, suggests a transition from a less effective conductive network to a more performing three dimensional network.

Poly‐2‐oxazoline‐derived polyurethanes: A versatile synthetic approach to renewable polyurethane thermosets

Abstract2‐Nonyl‐2‐oxazoline and 2‐(9‐decenyl)‐2‐oxazoline have been copolymerized in different proportions by cationic ring‐opening polymerization to obtain a series of random linear copolymers with tailored molecular weight and double bond functionality in the side chains. Thiol‐ene addition of 2‐mercaptoethanol has been used to produce a set of polyoxazoline–polyols under mild conditions and with quantitative double bond transformation. The polyols obtained in this way were reacted with methylene‐bis(phenylisocyanate) to yield a series of amorphous and semicrystalline polyurethane networks. The thermal stability and the thermomechanical properties of these thermosets have been studied and related with the structure of the parent polyols. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Sugar‐based hydrophilic polyurethanes and polyureas

AbstractNovel linear homogeneous polyurethanes and polyureas with enhanced hydrophilic character have been successfully prepared from sugar‐based monomers having their hydroxyl groups free or partially protected. By the reaction of primary hydroxyl groups of xylitol with dimethyl hexamethylene dicarbamate (HMDC) or di‐tert‐butyl‐4,4′‐diphenyl methyl dicarbamate (MDC), two new linear semicrystalline polyurethanes [PU(X‐HMDC) and PU(X‐MDC)] have been prepared. Likewise, by the reaction of xylitol with the analogous diisocyanates hexamethylene diisocyanate (HMDI) or 4,4′‐methylenebis(phenyl isocyanate) (MDI), similar polyurethanes [PU(X‐HMDI) and PU(X‐MDI)] were obtained. However, these latter polyurethanes present some degree of crosslinking because of the higher reactivity of the diisocyanate comonomers. Linear hydrophilic polyureas having free hydroxyl groups joined to the main chain have also been prepared by the reaction of the same diisocyanates (HMDI and MDI) with 1,6‐diamino‐1,6‐dideoxy‐D‐mannitol and 1,6‐diamino‐1,6‐dideoxy‐3:4‐O‐isopropylidene‐D‐mannitol. As far as we are aware, this kind of polyhydroxylated polyurea has not been previously described in the literature. The new polymers were characterized by standard methods (elemental analyses, gel permeation chromatography, IR, and NMR). The polyurethanes were hydrolytically degradable under physiological conditions, in contrast with less‐hydrophilic linear polyurethanes previously described. The thermal properties of the novel polymers were investigated by thermogravimetric analysis and differential scanning calorimetry. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

Polyurethanes from polyols obtained by ADMET polymerization of a castor oil-based diene: Characterization and shape memory properties

The acyclic diene metathesis polymerization (ADMET) of 1,3-di-10-undecenoxy-2-propanol, a castor oil based diene, is reported. 10-Undecenol was used as renewable comonomer to end-cap polymer chains and limit the molecular weight. The poly ols obtained in this way were reacted with 4,4′-methylenebis(phenylisocyanate) (MDI) to yield a series of amorphous and semicrystalline polyurethane networks. The thermal stability and the thermomechanical and mechanical properties of these thermosets have been studied and showed good shape memory properties for the semicrystalline polymer. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011