Elastomeric Block Copolymer Research Articles

Reprocessable and Mechanically Tailored Soft Architectures Through 3D Printing of Elastomeric Block Copolymers

AbstractThermoplastic elastomers (TPEs) are nanostructured, melt‐processable, elastomeric block copolymers. When TPEs that form cylindrical or lamellar nanostructures are macroscopically oriented, their material properties can exhibit several orders of magnitude of anisotropy. Here it is demonstrated that the flows applied during the 3D printing of a cylinder‐forming TPE enable hierarchical control over material nanostructure and function. It is demonstrated that 3D printing allows for control over the extent of nanostructural and mechanical anisotropy and that thermal annealing of 3D printed structures leads to highly anisotropic properties (up to 85 × anisotropic tensile modulus). This approach is leveraged to print functional soft 3D architectures with tunable local and macroscopic mechanical responses. Further, these printed TPEs intrinsically achieve melt‐reprocessability over multiple cycles, reprogrammability, and robust self‐healing via a brief period of thermal annealing, enabling facile fabrication of highly tunable, robust, and recyclable soft architectures.

Janus nanoparticles as efficient interface compatibilizer in blends of polylactide and elastomers: Importance of interfacial relaxation on toughening

For blending immiscible polymers, such as in the toughening modification of polylactide (PLA) via blending with rubbery materials, interfacial compatibilization is of great significance while the mechanism, especially the role of interfacial rheology, remains elusive. In this study, styrene-butadiene block copolymer elastomer (SBC) was employed to toughen PLA and a dumbbell-shaped Janus nanoparticle (JNP) consisting of polymethyl methacrylate and polystyrene spheres with equal size (∼80 nm) was used as the compatibilizer. Located at the interface, JNPs exhibited a great compatibilization efficiency in PLA/SBC blends, as demonstrated by the good morphology stabilization against droplet coalescence under static annealing and low shear flow conditions, as well as by the resistance against droplet breakup under high shear flow conditions. Moreover, as revealed from the linear viscoelasticity of JNP compatibilized blends, when JNP loading is more than 2 phr, aside from shape relaxation, an interfacial relaxation dominated by Marangoni stress was observed, indicating the possibility of particle redistribution on droplet surfaces. However, when loading is more than 4 phr, relaxations in the terminal zone no longer exist, implying the possible formation of a particle network on the droplet surface. This is consistent with the mechanical properties. The blend shows the greatest toughness at JNP loading around 3 phr, while the toughness is very poor when JNP loading is either too low or too high. This suggests interfacial relaxation to be crucial to guarantee a good toughening effect of SBC in PLA.

Conductive block copolymer elastomers and psychophysical thresholding for accurate haptic effects.

Electrotactile stimulus is a form of sensory substitution in which an electrical signal is perceived as a mechanical sensation. The electrotactile effect could, in principle, recapitulate a range of tactile experience by selective activation of nerve endings. However, the method has been plagued by inconsistency, galvanic reactions, pain and desensitization, and unwanted stimulation of nontactile nerves. Here, we describe how a soft conductive block copolymer, a stretchable layout, and concentric electrodes, along with psychophysical thresholding, can circumvent these shortcomings. These purpose-designed materials, device layouts, and calibration techniques make it possible to generate accurate and reproducible sensations across a cohort of 10 human participants and to do so at ultralow currents (≥6 microamperes) without pain or desensitization. This material, form factor, and psychophysical approach could be useful for haptic devices and as a tool for activation of the peripheral nervous system.

Morphology of poly-3-hexyl-thiophene blends with styrene-isoprene-styrene block-copolymer elastomers from X-ray and neutron scattering.

The nano- and micron scale morphology of poly(3-hexylthiophene) (P3HT) and polystyrene-block-polyisoprene-block-polystyrene (PS-PI-PS) elastomeric blends is investigated through the use of ultra-small and small angle X-ray and neutron scattering (USAXS, SAXS, SANS). It is demonstrated that loading P3HT into elastomer matrices is possible with little distortion of the elastomeric structure up to a loading of ∼5 wt%. Increased loadings of conjugated polymer is found to significantly distort the matrix structure. Changes in processing conditions are also found to affect the blend morphology with especially strong dependence on processing temperature. Processing temperatures above the glass transition temperature (Tg) of polystyrene and the melting temperature (Tm) of the conjugated polymer additive (P3HT) creates significantly more organized mesophase domains. P3HT blends with PS-PI-PS can also be flow-aligned through processing, which results in an anisotropic structure that could be useful for the generation of anisotropic properties (e.g. conductivity). Moreover, the extent of flow alignment is significantly affected by the P3HT loading in the PS-PI-PS matrix. The work adds insight to the morphological understanding of a complex P3HT and PS-PI-PS polymer blend as conjugated polymer is added to the system. We also provide studies isolating the effect of processing changes aiding in the understanding of the structural changes in this elastomeric conjugated polymer blend.

Resilient and reprocessable physical gels based on crystalline olefin block copolymer elastomer and mineral oil toward optically transparent electronic packaging

Due to the urgent demand of high-performance electronic packaging materials, thermoplastic elastomer physical gels (TPEG) have emerged as a promising alternative to traditional hydrogels and crosslinked rubbers due to their good thermal stability and reprocessability. In this work, olefin block copolymer (OBC) elastomer was blended with soft-block selective mineral oil (MO) to tune the mechanical property and transparency. It was found that the MO molecules can disperse well within amorphous phase with effective filling ratio ranging from 9.85 wt% to 44.82 wt%. The hard segments pack into spherulites and gradually develop into segregated crystals of ∼800 nm with the increase of MO concentration. Interestingly, fast quenching induces breakout crystallization morphology while thermal annealing at 130 °C can result in confined crystallization morphology with increased domain spacing due to the selective distributed MO since the corrected crystallinity remains constant and the crystal structure maintains as orthogonal crystals. Due to the combined effect of weak mesophase separation and decreased crystal size, OBC gels has an increased transparency up to 80% in visible region. The extensibility and cyclic property of OBC-based TPEG were finely tuned along with high transparency and excellent dielectric insulation property. Furthermore, the OBC gels are highly stable over a wide temperature and can be recycled. Overall, our work highlights the potential of thermoplastic elastomer physical gels as a sustainable and high-performance alternative for transparent electronics packaging materials.

Progress in the catalyst for ethylene/α‐olefin copolymerization at high temperature

AbstractEthylene/α‐olefin copolymers are one of the most widely‐used polyolefin materials. With the continuous improvement of polyolefin catalysts, high‐performance polyolefin materials were synthesized by adjusting the chain microstructure, changing the comonomer type and comonomer insertion amount, among which the ethylene/α‐olefin random copolymer elastomer (POE) and olefin block copolymer elastomer (OBC) are the most famous and well accepted by the market. The excellent properties of POE and OBC first depend on their polymer chain microstructure. The chain microstructure of polyolefins is fundamentally determined by the catalysts, polymerization conditions, comonomer feed policies, and reaction engineering. High‐performance ethylene/α‐olefin copolymer elastomers are currently prepared by high‐temperature solution polymerization process, which needs to be carried out at a temperature above the melting point of the polymer and is beneficial to speed up the polymerization reaction rate and control the polyolefin chain microstructure. However, the high‐temperature solution polymerization process launched more stringent requirements for the olefin coordination polymerization catalyst. Systematic reports on catalysts for high‐temperature solution copolymerization of ethylene and α‐olefins are lacking. In this review, we screened some catalysts suitable for the controllable copolymerization and high‐temperature solution copolymerization of ethylene/α‐olefin based on the catalyst's heat resistance, copolymerization activity, comonomer insertion ability, molecular weight, and distribution of the copolymer, including traditional Z–N catalysts, metallocene catalysts, and post‐metallocene catalysts. And the future development of catalysts for high‐temperature solution copolymerization of olefins, catalysts for precise control of polyolefin chain microstructures, and catalysts for olefin copolymerization with polar monomers at high temperature are envisaged.

Atomic insights into shock-induced spalling of polyurea by molecular dynamics simulation

Some block-copolymer elastomers are regarded as competitors to metal for lightweight impact protection design. However, the microscopic spalling mechanism of polymer is still unclear owing to the complex macromolecule structure. Molecular dynamics (MD) can overcome the drawback in the experiment and reveal the nature of dynamic damage at the microscale. In this work, the spalling of polyurea under different impact velocity is analyzed using MD method and coarse-grained molecular dynamics model. The simulation results show that spalling failure originates from the nucleation and development of void-damage. The flow stress has a plateau in the early and middle stages of void flow. The Hugoniot relation is obtained. Furthermore, the free surface velocity is calculated, which shows the typical characteristics observed in the experiment. The spalling strength is calculated by the direct and indirect methods, respectively. The non-monotonic variation of spalling strength with impact velocity is the result of the competition between the strain-rate strengthening and temperature weakening. The molecular mechanism is further analyzed, which indicates that the molecules are oriented during void flow, and the hard segments are more rigid and hysteretic. Besides, microphase-separated block copolymers have a special fibril toughening mechanism. The relationship between the flow stress and deformation of a fibril is quantitatively analyzed. Finally, the unit cell simulation, which represents one point in macroscopic, is compared with the spalling simulation.

A novel elastomeric copolymer-based phase change material with thermally induced flexible and shape-stable performance for prismatic battery module

In this study, a novel thermally induced flexible phase change material (PCM), which consists of an elastomeric block copolymer called styrene-b-ethylene-co-butylene-b-styrene triblock copolymer (SEBS), paraffin (PA) and expanded graphite (EG), is successfully prepared. Here, SEBS and EG serve as a supporting material and a thermal enhanced component. The effects related to latent heat, thermal stability, thermal conductivity and shape stability of different SEBS mass fractions are investigated and the test results reveal the good thermal performance of the PA/SEBS/EG blends. Besides, the further rheological analysis points out that the property of shape stability and thermal induced flexibility are based on the physical crosslinking mechanism between SEBS and PA. As a result, PA/SEBS/EG composite considerably reduces thermal contact resistance due to narrower gap, endowing the battery module with much better heat dissipation efficiency and temperature uniformity. The 3 C discharge of the battery module test shows that the maximum temperature of battery module using PA/SEBS/EG blend is effectively controlled within 42.2 °C, while the maximum temperature difference is below the safety threshold of 5 °C during the 5-cycling discharge and charge tests.

Extraction of intrinsic effects of glassy domain cross-linking on the tensile properties of ABA block copolymer elastomers via photo cross-linking approach

Chemical or physical cross-links into the A glassy domains have been reported to enhance mechanical properties for glassy-b-soft-b-glassy ABA triblock copolymer-based elastomers. However, no reports dealt with the pure effects of the domain cross-linking, since the cross-linking processes inevitably altered the morphological characteristics. This study extracts the intrinsic effects of domain cross-linking on the tensile properties based on the ABA triblock copolymer with the A blocks cross-linked via photo cross-linking without heat and cross-linker molecules. Differential scanning calorimetry, viscoelasticity measurements, and small-angle X-ray scattering (SAXS) with a model function analyses were used to confirm preservation of the segmental dynamics of the B block and the morphology of the self-assembled structure after cross-linking, which are all important for a fair discussion. Tensile properties were then investigated by simple elongation and cyclic tests, revealing that the domain cross-linking enhanced the stress in the small deformation region and the recovery property primarily. These changes of the tensile properties were discussed in terms of the deformation resistance of the micro-phase separated structure based on the SAXS data under elongation. The superiority or inferiority of cross-linking the end block domains or middle block strands for mechanical property enhancement was interpreted based on the observed difference between the neat and cross-linked samples. In the last section, effects of domain photo cross-linking for the different morphologies were also examined. Throughout the article, this study can provide new and essential knowledge of the mechanical properties of block copolymer-based materials, and some valuable hints for further modification of the physical properties of these materials.

Bent‐But‐Not‐Broken: Reactive Metal‐Organic Framework Composites from Elastomeric Phase‐Inverted Polymers

AbstractA phase inversion process is used to fabricate composite beads from the metal–organic framework (MOF) UiO‐66‐NH2 and the elastomeric block copolymer poly(styrene‐block‐ethylene‐ran‐butylene‐block‐styrene) (SEBS) for improved performance over activated carbon‐based protective textiles. Textiles incorporating the best MOF‐composite materials reduced steady‐state permeation of the chemical warfare agent (CWAs) simulant, 2‐chloroethyl ethyl sulfide (CEES), by ≈60% versus activated carbon cloth. Additionally, the protection capabilities of the composite beads could be further tuned by adjusting bead size—textiles fabricated from smaller composite beads show enhanced diffusional properties whereas textiles generated from larger beads maximize protective capacity. The elastomeric beads also do not exhibit any attrition when subjected to a hardness test, whereas traditional pressing techniques result in materials with only a 20% retention of particle size. Furthermore, composites with increasing MOF content result in enhanced chemical capacity and reactivity, with the best composite outperforming activated carbon cloth against the CWAs soman (91% versus 17% reacted, respectively) and mustard (97% versus 0% reacted, respectively) over 24 h. The 80 wt% samples also have ≈1.4 and 2.2× higher adsorption capacity for soman and CEES, respectively, versus carbon cloth. Finally, mixing multiple MOFs results in optimal composites for the simultaneous filtration of chlorine and ammonia.

Thermoplastic Polyolefin Elastomer Blends for Multiple and Reversible Shape Memory Polymers

This work reports a novel type of reversible shape memory polymers (RSMPs) from commercially available thermoplastic polyolefin elastomers (POEs). Two RSMPs based on Dow Chemical’s POE Engage 8003, Engage 8137, Engage 8180, and olefin block copolymer elastomer Infuse 9007 were prepared by simple blending of the commercial products having different melting temperatures. The multiple and reversible shape memory performances of the as-prepared blends were clearly demonstrated. This work provides a facile and low-cost method for the large-scale preparation of the reversible shape memory materials. It broadens the source of the materials for the design and development of RSMPs.

Ag nanowire-based transparent stretchable tactile sensor recognizing strain directions and pressure

In the last decade, extensive studies have been conducted to realize the functions of human skin based on stretchable electronics. An artificial skin, recognizing complex mechanical stimulation including pressure, strain and shear, and composed of transparent material, is an essential goal but has hardly been achieved. We fabricated a transparent integrated sensor system that can sense the strain direction and normal pressure of applied mechanical stimulation. Each sensor is composed of micropatterned Ag nanowire, forming a composite stretchable conductor with a block copolymer elastomer. The micropatterning and transfer process using thermoplastic elastomer facilitates the transparent conductor to show high transmittance with low sheet resistance at the same time. The designed transparent strain sensor responds linearly to strain, but does not respond to the orthogonal direction, which enables it to have strain-directionality. The applied mechanical signal, comprising normal force and directional strain, can be interpreted through the electrical signal observed from integrated sensors.

Block Copolymer Elastomers for Stretchable Electronics.

As industrial needs for healthcare sensors, electronic skin, and flexible/stretchable displays increase, interest in stretchable materials is increasing as well. In recent years, the studies on stretchable materials have spread to various pivot components, such as electrodes, circuits, substrates, semiconductors, dielectric layers, membranes, and active nanocomposite films. The block copolymer (BC) elastomers have been playing considerable role in the development of stretchable materials. Since BCs are soft elastomersbased on physical cross-links, they show differences in physical properties from normal elastomers formed with chemical cross-linking. BC elastomers does not require additional chemical cross-linking procedure, so they can be easily processed after dissolved in various solvents. Their viscoelasticity and thermoplasticity enable the BCs to become moldable and sticky. Although their unique physical properties may serve as disadvantages in some cases, they have been actively applied to create various stretchable electronic materials and their uses are expected to be enlarged more than ever. In this Account, we summarize recent successful applications of BCs for the stretchable electronic devices and discuss the possibility of further uses and the challenges to be addressed for practical uses. Studies on BC-based stretchable materials have focused initially on the fabrication process of stretchable conductors; mixing conductive fillers physically with BCs, infiltrating BCsin a conductive filler layer, and converting metal precursors into metal nanoparticles inside BCs. When conductive fillers with high aspect ratios, such as nanowires or nanosheets are used, the fillers can be infiltrated by the BCs after deposited. Since the contacts between the fillers are maintained during the infiltration process, even thin composite films possess high conductivity and stretchability. The metal precursor solution printing is suggested as a promising approach because it is compatible with traditional printing techniques without clogging the nozzles and allows high filler loading efficiency. When using a BC as a substrate, it is advisable to use a BC/PDMS double layer because of viscoelastic and thermoplastic properties of BCs. If BC/PDMS double layer is used with much thicker PDMS layer instead of viscoelastic BC alone, the double layer substrate can show a perfect elastomeric behavior, and the advantages of the BC substrate are preserved. Additionally, the use of conventional manufacturing techniques is important for commercialization of the stretchable devices. BC substrates having preformed microfibril network on their surfaces facilitate the fabrication of high-resolution circuitry by directly depositing metals through a mask on the substrate. Recent successes of fabricating stretchable organic transistors were obtained based on in situ phase separation of polymer semiconductors to form nanofibril bundles on the surface of a BC substrate. They have led to the achievement of high resolution transistor array printed in large area. BCs are expected to expand their applicability, including stretchable batteries, since they make it feasible to fabricate various hybrid nanocomposites, pore size-controlled membranes, and microstructured surfaces. However, it is necessary to secure long-term stability under heat, solvent, and UV; in addition, there is a need for the synthesis of functional BCs for use in stretchable implanted biomedical devices.

Highly conductive, stretchable and biocompatible Ag-Au core-sheath nanowire composite for wearable and implantable bioelectronics.

Wearable and implantable devices require conductive, stretchable and biocompatible materials. However, obtaining composites that simultaneously fulfil these requirements is challenging due to a trade-off between conductivity and stretchability. Here, we report on Ag-Au nanocomposites composed of ultralong gold-coated silver nanowires in an elastomeric block-copolymer matrix. Owing to the high aspect ratio and percolation network of the Ag-Au nanowires, the nanocomposites exhibit an optimized conductivity of 41,850 S cm-1 (maximum of 72,600 S cm-1). Phase separation in the Ag-Au nanocomposite during the solvent-drying process generates a microstructure that yields an optimized stretchability of 266% (maximum of 840%). The thick gold sheath deposited on the silver nanowire surface prevents oxidation and silver ion leaching, making the composite biocompatible and highly conductive. Using the nanocomposite, we successfully fabricate wearable and implantable soft bioelectronic devices that can be conformally integrated with human skin and swine heart for continuous electrophysiological recording, and electrical and thermal stimulation.

Effect of Block Immiscibility on Strain-Induced Microphase Segregation and Crystallization of Model Block Copolymer Elastomers

Main-chain liquid crystalline elastomers (LCE) of ABA block copolymer chains are simulated using a polymer bead–spring model to understand how their sawtooth tensile behavior is affected by changes...

Block Copolymer Elastomer with Graphite Filler: Effect of Processing Conditions and Silane Coupling Agent on the Composite Properties.

The control of morphology and interface in poly(styrene-ethylene/butylene-styrene) (SEBS) composites with graphitic fillers is extremely important for the design of piezoresistive sensors for body motion or flexible temperature sensors. The effects of a high amount of graphite (G) and silane coupling agent on the morphology and properties of SEBS composites with anisotropic mechanical properties are reported. The physical and chemical bonding of silane to both G and SEBS surface was proved by EDX and TGA results; this improved interface influenced both the thermal and mechanical properties of the composite. The vinyltriethoxysilane (VS) promoted the formation of char residue and, being tightly bound to both SEBS and G, did not show separate decomposition peak in the TGA curve of composites. The mechanical properties were measured on two perpendicular directions and were improved by both the addition of VS and the increased amount of G; however, the increase of storage modulus due to orientation (from 5 to 15 times depending on the composition and direction of the test) was more important than that provided by the increase of G concentration, which was a maximum of four times that obtained for 15 wt % graphite. A mechanism to explain the influence of G content and treatment on the variation of storage modulus and tan δ depending on the direction of the test was also proposed.

Dynamic mechanical properties of elastomeric nanoparticle composites

The blends of two crosslinked polystyrene-b-polyisoprene (PS-b-PI) copolymers with homopolystyrene were investigated for their dynamic mechanical properties. Elastomeric block copolymer nanofibers utilized as filler were prepared by employing the phase separation property of a polystyrene-b-polyisoprene copolymer, which was then exposed to a crosslinking agent (S2Cl2) for cold vulcanization. The crosslinking density and morphology of nanofillers resulted in three elastomeric block copolymer nanofillers: fully cross-linked nanofiber (FCF), fully crosslinked multi-junctioned nanofiber (FCM), and partially crosslinked multi-junctioned sample (PCM). Uncrosslinked PS-PI block copolymer (UBC) was also studied as a comparison. Crosslinking density was calculated by measuring the change in intensity of the double bond peaks using FTIR spectroscopy. Thermo-mechanical properties and morphology of the blends were characterized by dynamic mechanical analysis (DMA) and scanning electron microscope (SEM). DMA results show that modulus increases with increasing filler loading in the terminal region for both PS/FCM and PS/FCF systems and the increasing rate is related to crosslinking density. Open image in new window

Soluble salt-driven matrix swelling of a block copolymer for rapid fabrication of a conductive elastomer toward highly stretchable electronics

A simple soluble salt-driven matrix swelling method was developed for fast fabrication of a silver nanoparticle-based conductive elastomer. Taking advantage of the synergistic effects of silver trifluoroacetate and methanol, the swelling capability of an elastomeric block copolymer polystyrene-block-polyisoprene-block-polystyrene (SIS) was greatly enhanced. The interaction of silver ions and double bonds of the copolymer enables fast infiltration of soluble salt into the polymer network, resulting in a high-content loading of silver ions, which is a prerequisite for fabrication of a highly conductive polymer. In-situ reduction was performed under a swollen state to allow the reducer to effectively penetrate into the polymer matrix, thoroughly reducing the silver salt into silver nanoparticles. A three-layered conductive elastomer was obtained, with a high concentration of silver nanoparticles formed on both surfaces of SIS copolymer. Typically, the conductivity of the reduced SIS elastomer was measured to be >2×105S/m when ε=0, and can maintain to be over one seventh of its initial value even at a 200% elongation. The conductive elastomer can withstand over 1000 stretch-release cycles under a strain of 100%. A radio-frequency antenna was prepared with the conductive elastomer, which gives the ability of dimension tuning, to realize high-quality frequency-selective radiation at a wide bandwidth of larger than 2.6GHz.

Hemocompatibility of styrenic block copolymers for use in prosthetic heart valves.

Certain styrenic thermoplastic block copolymer elastomers can be processed to exhibit anisotropic mechanical properties which may be desirable for imitating biological tissues. The ex-vivo hemocompatibility of four triblock (hard–soft–hard) copolymers with polystyrene hard blocks and polyethylene, polypropylene, polyisoprene, polybutadiene or polyisobutylene soft blocks are tested using the modified Chandler loop method using fresh human blood and direct contact cell proliferation of fibroblasts upon the materials. The hemocompatibility and durability performance of a heparin coating is also evaluated. Measures of platelet and coagulation cascade activation indicate that the test materials are superior to polyester but inferior to expanded polytetrafluoroethylene and bovine pericardium reference materials. Against inflammatory measures the test materials are superior to polyester and bovine pericardium. The addition of a heparin coating results in reduced protein adsorption and ex-vivo hemocompatibility performance superior to all reference materials, in all measures. The tested styrenic thermoplastic block copolymers demonstrate adequate performance for blood contacting applications.

2次元小角X線散乱と応力-ひずみ同時測定によるエラストマー特性を有するブロックコポリマーフィルムの一軸延伸にともなう構造と物性の変化の研究

Simultaneous measurements of small-angle X-ray scattering with stress-strain tests were conducted to reveal changes in microphase-separated structures being correlated with changes in the mechanical properties upon the uniaxial stretching of an elastomeric block copolymer film containing glassy spherical microdomains embedded in the rubbery matrix. For this purpose, high brilliant synchrotron X-ray source was utilized at SPring-8, Japan, and cycled stretching-and-reversing process was conducted three times. While the tensile modulus decreased to be less than the half for the second cycle of the stretching, affine deformation for the changes in the microphase-separated structures was confirmed not only for the second but also for the third cycles. The Mullins effect (stress-induced softening) may be attributed to stress-induced disentangling of the soft-segment chains between two adjacent glassy spherical microdomains (as physical crosslinking points). It was found that the number density of the active network chain (νe ) is more than 22-times higher than that of the soft-segment chains between two adjacent glassy spherical microdomains (νe°) in the virgin sample. Such a big number of the active network chain suggests that the soft-segment chains between two adjacent glassy spherical microdomains are heavily entangled. Furthermore, it was found that the ratio νe /νe° is still around 10 for the second and third stretching cycles, suggesting that appreciable number of entanglements can be relaxed while comparable number of entanglements cannot be. The latter is so referred to as “trapped entanglement due to crosslinking”.