Improved Self-Healing of Polyethylene/Carbon Black Nanocomposites by Their Shape Memory Effect

Rapidly Self-Healable and Melt-Extrudable Polyethylene Reprocessable Network Enabled with Dialkylamino Disulfide Dynamic Chemistry

Catalyst-free, radical-based reactive processing is used to transform low-density polyethylene (LDPE) into polyethylene covalent adaptable networks (PE CANs) using a dialkylamino disulfide crosslinker, BiTEMPS methacrylate (BTMA). Two versions of BTMA are used, BTMA-S2, with nearly exclusively disulfide bridges, and BTMA-Sn, with a mixture of oligosulfide bridges, to produce S2 PE CAN and Sn PE CAN, respectively. The two PE CANs exhibit identical crosslink densities, but the S2 PE CAN manifests faster stress relaxation, with average relaxation times ∼4.5 times shorter than those of Sn PE CAN over a 130 to 160 °C temperature range. The more rapid dynamics of the S2 PE CAN translate into a shorter compression-molding reprocessing time at 160 °C of only 5 min (vs 30 min for the Sn PE CAN) to achieve full recovery of crosslink density. Both PE CANs are melt-extrudable and exhibit full recovery within experimental uncertainty of crosslink density after extrusion. Both PE CANs are self-healable, with a crack fully repaired and the original tensile properties restored after 30 min for the S2 PE CAN or 60 min for the Sn PE CAN at a temperature slightly above the LDPE melting point and without the assistance of external forces.

Self-Healing Polymeric Soft Actuators

Natural evolution has provided multicellular organisms with sophisticated functionalities and repair mechanisms for surviving and preserve their functions after an injury and/or infection. In this context, biological systems have inspired material scientists over decades to design and fabricate both self-healing polymeric materials and soft actuators with remarkable performance. The latter are capable of modifying their shape in response to environmental changes, such as temperature, pH, light, electrical/magnetic field, chemical additives, etc. In this review, we focus on the fusion of both types of materials, affording new systems with the potential to revolutionize almost every aspect of our modern life, from healthcare to environmental remediation and energy. The integration of stimuli-triggered self-healing properties into polymeric soft actuators endow environmental friendliness, cost-saving, enhanced safety, and lifespan of functional materials. We discuss the details of the most remarkable examples of self-healing soft actuators that display a macroscopic movement under specific stimuli. The discussion includes key experimental data, potential limitations, and mechanistic insights. Finally, we include a general table providing at first glance information about the nature of the external stimuli, conditions for self-healing and actuation, key information about the driving forces behind both phenomena, and the most important features of the achieved movement.

Review of Thermoresponsive Electroactive and Magnetoactive Shape Memory Polymer Nanocomposites

Electroactive and magnetoactive shape memory polymer nanocomposites (SMCs) are multistimuli-responsive smart materials that are of great interest in many research and industrial fields. In addition to thermoresponsive shape memory polymers, SMCs include nanofillers with suitable electric and/or magnetic properties that allow for alternative and remote methods of shape memory activation. This review discusses the state of the art on these electro- and magnetoactive SMCs and summarizes recently published investigations, together with relevant applications in several fields. Special attention is paid to the shape memory characteristics (shape fixity and shape recovery or recovery force) of these materials, as well as to the magnitude of the electric and magnetic fields required to trigger the shape memory characteristics.

Shape memory polymer composites (SMPCs) using interconnected nanowire network foams as reinforcements

Shape memory polymers (SMPs), although offer a suite of advantages such as ease of processability and lower density, lag behind their shape memory alloy counterparts, in terms of mechanical properties such as recovery stress and cyclability. Reinforcing SMPs with inorganic nanowires and carbon nanotubes (CNTs) is a sought-after pathway for tailoring their mechanical properties. Here, inorganic nanowires also offer the added advantage of covalently binding the fillers to the surrounding polymer matrices via organic molecules. The SMP composites (SMPCs) thus obtained have well-engineered nanowire-polymer interfaces, which could be used to tune their mechanical properties. A well-known method of fabricating SMPCs involving casting dispersions of nanowires (or CNTs) in mixtures of monomers and crosslinkers typically results in marginal improvements in the mechanical properties of the fabricated SMPCs. This is owed to the constraints imposed by the rule-of-mixture principles. To circumvent this limitation, a new method for SMPC fabrication is designed and presented. This involves infiltrating polymers into pre-fabricated nanowire foams. The pre-fabricated foams were fabricated by consolidating measured quantities of nanowires and a sacrificial material, such as (NH₄)₂CO₃, followed by heating the consolidated mixtures for subliming the sacrificial material. Similar to the case of traditional composites, use of silanes to functionalize the nanowire surfaces allowed for the formation of bonds between both the nanowire-nanowire and the nanowire-polymer interfaces. SMPCs fabricated using TiO₂nanowires and SMP composed of neopentyl glycol diglycidyl ether and poly(propylene glycol) bis(2-aminopropyl ether) (Jeffamine D230) in a 2:1 molar ratio exhibited a 300% improvement in the elastic modulus relative to that of the SMP. This increase was significantly higher than SMPC made using the traditional fabrication route. Well-known powder metallurgy techniques employed for the fabrication of these SMPCs make this strategy applicable for obtaining other SMPCs of any desired shape and chemical composition.

Self-Sensing Actuators Based on a Stiffness Variable Reversible Shape Memory Polymer Enabled by a Phase Change Material

Soft actuators with integrated mechanical and actuation properties and self-sensing ability are still a challenge. Herein, a stiffness variable polyolefin elastomer (POE) with a reversible shape memory effect is prepared by introducing a typical phase change material, i.e., paraffin wax (PW). It is found that the variable stiffness of POE induced by PW can balance the reversible strain and load-bearing capability of actuators. Especially, carbon nanotubes (CNTs) are concentrated in a thin surface layer by spraying and hot pressing in the soft state of POE/PW blends, providing signal transductions for the strain and temperature perception for actuators. Taking advantage of tunable reversible deformation and mechanical transformation of the POE/PW actuator, different biomimetic robotics, including grippers with high load-bearing capability (weight-lifting ratio > 146), walking robots that can sense angles of joints, and high-temperature warning robots are demonstrated. A scheme combining the variable stiffness and electrical properties provides a versatile strategy to integrate actuation performance and self-sensing ability, inspiring the development of multifunctional composite designs for soft robotics.

Modelling of environmental impacts of printed self-healing products

Products utilising self-healing materials have the potential to restore some of their function following damage, thereby extending the product lifespan and contributing to waste prevention and increased product safety. Despite the growing interest in these products, there a lack of comprehensive studies on the environmental implications of self-healing products and the parameters that influence impacts. The study presented in this paper combined life cycle assessment combined with a Taguchi experimental design and analysis of variance to investigate the effect of various parameters across the life stages of a self-healing composite product manufactured by 3D printing using poly-lactic acid (PLA) and self-healing polyurethane (PU). The results of this study suggests that impacts are primarily affected by avoided production due to the increased service of the product, followed by electricity requirements and material deposition rate (efficiency) of 3D printing. In the case of water consumption raw material manufacturing of PLA and PU are the highest and hence should be a target for research on reducing their water footprint. When comparing self-healing vs. regular products it is evident that most of the impacts are dominated by the electricity consumption of the manufacturing process. These results suggest that maximising avoided production can play a major role in reducing impacts of 3D printed products. The results are important for maximising the circularity of additive manufacturing products while minimising their life cycle impact.

Cellulose microfibers (CMFs) as a smart carrier for autonomous self-healing in epoxy coatings

.The cellulose microfibers (CMFs) were synthesized to assist the self-release of loaded product, that provides better corrosion inhibition and self-healing of epoxy coatings.

Large Improvement in the Mechanical Properties of Polyurethane Nanocomposites Based on a Highly Concentrated Graphite Nanoplate/Polyol Masterbatch

In this study, a highly concentrated graphite nanoplate (GNP)/polyol masterbatch was prepared by the exfoliation of natural graphite in an aqueous system using cetyltrimethylammonium bromide and the replacement of aqueous solution with a polyol, viz. poly(tetramethylene ether glycol), and it was subsequently used to prepare polyurethane (PU) nanocomposites by simple dilution. The polyol in the masterbatch efficiently prevented the aggregation of GNPs during the preparation of PU nanocomposite. In addition, the dispersed GNPs in the masterbatch exhibited rheological behavior of lyotropic liquid crystalline materials. In this study, the manufacture and application methods of the GNP/polyol masterbatch were discussed, enabling the facile manufacture of the PU/GNP nanocomposites with excellent mechanical properties. In addition, the manner in which the GNP alignment affected the microphase separation of PU in the nanocomposites was investigated, which determined the improvement in the mechanical properties of the nanocomposites. High-performance PU/GNP nanocomposites are thought to be manufactured from the GNP/polyol masterbatch by the simple dilution to 0.1 wt% GNP in the nanocomposite.

Synthesis of Self-Healing Polymers by Scandium-Catalyzed Copolymerization of Ethylene and Anisylpropylenes

Self-healing materials are of fundamental interest and practical importance. Herein we report the synthesis of a new class of self-healing materials, formed by the copolymerization of ethylene and anisyl-substituted propylenes using a sterically demanding half-sandwich scandium catalyst. The copolymerization proceeded in a controlled fashion, affording unique multi-block copolymers composed of relatively long alternating ethylene- alt-anisylpropylene sequences and short ethylene-ethylene units. By controlling the molecular weight and varying the anisyl substituents, a series of copolymers that show a wide range of glass-transition temperatures ( T_g) and mechanical properties have been obtained. The copolymers with T_g below room temperature showed high elastic modulus, high toughness, and remarkable self-healability, being able to autonomously self-heal upon mechanical damage not only in a dry environment but also in water and aqueous acid and alkaline solutions, while those with T_g around or above room temperature exhibited excellent shape-memory property. The unique mechanical properties may be ascribed to the phase separation of the crystalline ethylene-ethylene nanodomains from the ethylene- alt-anisylpropylene matrix.

Repeated Intrinsic Self-Healing of Wider Cracks in Polymer via Dynamic Reversible Covalent Bonding Molecularly Combined with a Two-Way Shape Memory Effect

To enable repeated intrinsic self-healing of wider cracks in polymers, a proof-of-concept approach is verified in the present work. It operates through two-way shape memory effect (SME)-aided intrinsic self-healing. Accordingly, a reversible C-ON bond is introduced into the main chain of crosslinked polyurethane (PU) containing an elastomeric dispersed phase (styrene-butadiene-styrene block copolymer, SBS). The PU/SBS blend was developed by the authors recently, and proved to possess an external stress-free two-way SME after programming. As a result, the thermal retractility offered by the SME coupled with the reversible C-ON bonds can be used for successive crack closure and remending based on synchronous fission/radical recombination of C-ON bonds. Moreover, multiwalled carbon nanotubes are incorporated to impart electrical conductivity to the insulating polymer. Repeated autonomic healing of wider cracks is thus achieved through narrowing of cracks followed by chemical rebonding under self-regulating Joule heating. No additional programming is needed after each healing event, which is superior to one-way SME-assisted self-healing. The outcomes set an example of integrating different stimuli-responsivities into single materials.

High Performance Shape Memory Epoxy/Carbon Nanotube Nanocomposites

A series of shape memory nanocomposites based on diglycidyl ether of bisphenol A (DGEBA) E51/methylhexahydrophthalic anhydride (MHHPA)/multiwalled carbon nanotube (MWCNT) with various stoichiometric ratios (rs) of DGEBA/MHHPA from 0.5 to 1.2 and filler contents of 0.25 and 0.75 wt % are fabricated. Their morphology, curing kinetics, phase transition, mechanical properties, thermal conduction, and shape memory behaviors are systematically investigated. The prepared materials show a wide range of glass transition temperatures (Tg) of ca. 65-140 °C, high flexural modulus (E) at room temperature up to ca. 3.0 GPa, high maximum stress (σm) up to ca. 30 MPa, high strain at break (εb) above 10%, and a fast recovery of 32 s. The results indicate that a small amount of MWCNT fillers (0.75 wt %) can significantly increase all three key mechanical properties (E, σm, and εb) at temperatures close to Tg, the recovery rate, and the repetition stability of the shape memory cycles. All of these remarkable advantages make the materials good candidates for the applications in aerospace and other important fields.

Healable shape memory (thio)urethane thermosets

(Thio)urethane networks combining SM properties and self-healability under mild temperature conditions via the DA chemistry were developed. The effect of various network architectures in SM-aided scratch healing was evaluated.

A tough, smart elastomeric bio-based hyperbranched polyurethane nanocomposite

A self-healable and shape-recoverable tough hyperbranched polyurethane and iron oxide–reduced graphene oxide nanocomposite is fabricated by an in situ polymerization technique.

Self-healable castor oil based tough smart hyperbranched polyurethane nanocomposite with antimicrobial attributes

Smart, tough hyperbranched polyurethane nanocomposite with excellent rapid and repeatable self-healing and antimicrobial attributes is fabricated.

Self-healing and shape memory capabilities of copper-coordination polymer network

A copper-coordination polymer network displays excellent shape memory and high self-healing functions under a mild condition.

Dually actuated triple shape memory polymers of cross-linked polycyclooctene-carbon nanotube/polyethylene nanocomposites

In this work, electrically and thermally actuated triple shape memory polymers (SMPs) of chemically cross-linked polycyclooctene (PCO)-multiwalled carbon nanotube (MWCNT)/polyethylene (PE) nanocomposites with co-continuous structure and selective distribution of fillers in PCO phase are prepared. We systematically studied not only the microstructure including morphology and fillers' selective distribution in one phase of the PCO/PE blends, but also the macroscopic properties including thermal, mechanical, and electrical properties. The co-continuous window of the immiscible PCO/PE blends is found to be the volume fraction of PCO (vPCO) of ca. 40-70 vol %. The selective distribution of fillers in one phase of co-continuous blends is obtained by a masterbatch technique. The prepared triple SMP materials show pronounced triple shape memory effects (SMEs) on the dynamic mechanical thermal analysis (DMTA) and the visual observation by both thermal and electric actuations. Such polyolefin samples with well-defined microstructure, electrical actuation, and triple SMEs might have potential applications as, for example, multiple autochoke elements for engines, self-adjusting orthodontic wires, and ophthalmic devices.

Nucleation and nonisothermal crystallization kinetics in cross-linked polyethylene/zinc oxide nanocomposites

In the present study organic–inorganic hybrid nanocomposites of cross-linked polyethylene (XLPE) with 0, 2, 5 and 10 wt% of trimethoxyoctyl-silane surface treated ZnO nanoparticles were prepared by melt mixing.

Polymers with dual light-triggered functions of shape memory and healing using gold nanoparticles

Shape-memory and stimuli-healable polymers (SMP and SHP) are two types of emerging smart materials. Among the many stimuli that can be used to control SMP and SHP, light is unique because of its unparalleled remote activation and spatial control. Generally, light-triggered shape memory and optically healable polymers are different polymers and it is challenging to endow the same polymer with the two light-triggered functions because of their structural incompatibility. In this paper, we describe a general polymer design that allows a single material to exhibit both light-controlled shape memory and optical healing capabilities. We show that by chemically cross-linking a crystalline polymer and loading it with a small amount of gold nanoparticles (AuNPs), the polymer displays optically controllable shape memory and fast optical healing based on the same localized heating effect arising from the surface plasmon resonance of AuNPs. The photothermal effect controls, on the one hand, the shape memory process by tuning the temperature with respect to Tm of the crystalline phase and, on the other hand, activates the damage healing through crystal melting and recrystallization. Moreover, we show that these two features can be triggered separately in a sequential manner.

Triple shape memory effects of cross-linked polyethylene/polypropylene blends with cocontinuous architecture

In this paper, the triple shape memory effects (SMEs) observed in chemically cross-linked polyethylene (PE)/polypropylene (PP) blends with cocontinuous architecture are systematically investigated. The cocontinuous window of typical immiscible PE/PP blends is the volume fraction of PE (v(PE)) of ca. 30-70 vol %. This architecture can be stabilized by chemical cross-linking. Different initiators, 2,5-dimethyl-2,5-di(tert-butylperoxy)-hexane (DHBP), dicumylperoxide (DCP) coupled with divinylbenzene (DVB) (DCP-DVB), and their mixture (DHBP/DCP-DVB), are used for the cross-linking. According to the differential scanning calorimetry (DSC) measurements and gel fraction calculations, DHBP produces the best cross-linking and DCP-DVB the worst, and the mixture, DHBP/DCP-DVB, is in between. The chemical cross-linking causes lower melting temperature (Tm) and smaller melting enthalpy (ΔHm). The prepared triple shape memory polymers (SMPs) by cocontinuous immiscible PE/PP blends with v(PE) of 50 vol % show pronounced triple SMEs in the dynamic mechanical thermal analysis (DMTA) and visual observation. This new strategy of chemically cross-linked immiscible blends with cocontinuous architecture can be used to design and prepare new SMPs with triple SMEs.