A theory of healing at a polymer-polymer interface

Filled Elastomers: Mechanistic and Physics-Driven Modeling and Applications as Smart Materials

Elastomers are made of chain-like molecules to form networks that can sustain large deformation. Rubbers are thermosetting elastomers that are obtained from irreversible curing reactions. Curing reactions create permanent bonds between the molecular chains. On the other hand, thermoplastic elastomers do not need curing reactions. Incorporation of appropriated filler particles, as has been practiced for decades, can significantly enhance mechanical properties of elastomers. However, there are fundamental questions about polymer matrix composites (PMCs) that still elude complete understanding. This is because the macroscopic properties of PMCs depend not only on the overall volume fraction (ϕ) of the filler particles, but also on their spatial distribution (i.e., primary, secondary, and tertiary structure). This work aims at reviewing how the mechanical properties of PMCs are related to the microstructure of filler particles and to the interaction between filler particles and polymer matrices. Overall, soft rubbery matrices dictate the elasticity/hyperelasticity of the PMCs while the reinforcement involves polymer-particle interactions that can significantly influence the mechanical properties of the polymer matrix interface. For ϕ values higher than a threshold, percolation of the filler particles can lead to significant reinforcement. While viscoelastic behavior may be attributed to the soft rubbery component, inelastic behaviors like the Mullins and Payne effects are highly correlated to the microstructures of the polymer matrix and the filler particles, as well as that of the polymer-particle interface. Additionally, the incorporation of specific filler particles within intelligently designed polymer systems has been shown to yield a variety of functional and responsive materials, commonly termed smart materials. We review three types of smart PMCs, i.e., magnetoelastic (M-), shape-memory (SM-), and self-healing (SH-) PMCs, and discuss the constitutive models for these smart materials.

Dynamic Interfaces in Self-Healable Polymers

It is well-established that interfaces play critical roles in biological and synthetic processes. Aside from significant practical applications, the most accessible and measurable quantity is interfacial tension, which represents a measure of the energy required to create or rejoin two surfaces. Owing to the fact that interfacial processes are critical in polymeric materials, this review outlines recent advances in dynamic interfacial processes involving physics and chemistry targeting self-healing. Entropic interfacial energies stored during damage participate in the recovery, and self-healing depends upon copolymer composition and monomer sequence, monomer molar ratios, molecular weight, and polymer dispersity. These properties ultimately impact chain flexibility, shape-memory recovery, and interfacial interactions. Self-healing is a localized process with global implications on mechanical and other properties. Selected examples driven by interfacial flow and shape memory effects are discussed in the context of covalent and supramolecular rebonding targeting self-healable materials development.

Comparison of Mechanical Properties of Steam-Free and Steam-Based Specimens Made of Expanded Polypropylene Beads

Reducing the CO2 emissions of plastic parts is crucial in terms of sustainable product and process designs. Approaches include the use of recycled materials and reducing the energy demands of processes through more efficient technologies. In this context, this study shows the potential of the steam-free processing of particle foam beads into thin-walled moulded parts. Expanded polypropylene (EPP) particle foam beads have been processed in both a steam-free and steam-based process. For this purpose, specimens with different part densities and thicknesses were produced, the mechanical properties were investigated, and the surface quality was discussed. Specimens made of EPP with a part thickness of 5 to 20 mm and part densities of 60 to 185 g/L were produced steam-free. Lower part thicknesses and higher densities increase the mechanical properties. As the density increased, the homogeneity of the surfaces of the steam-free specimens also increased. In comparison, specimens with a thickness of 10 mm and part densities of 35 to 90 g/L were produced on a steam-based process. The results of the mechanical test were compared with those of the steam-free specimens. The steam-based specimens showed higher mechanical properties for the same density.

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.

Autonomous self-healing optical sensors for damage intelligent soft-bodied systems

We introduce damage intelligent soft-bodied systems via a network of self-healing light guides for dynamic sensing (SHeaLDS). Exploiting the intrinsic damage resilience of light propagation in an optical waveguide, in combination with a tough, transparent, and autonomously self-healing polyurethane urea elastomer, SHeaLDS enables damage resilient and intelligent robots by self-healing cuts as well as detecting this damage and controlling the robot's actions accordingly. With optimized material and structural design for hyperelastic deformation of the robot and autonomous self-healing capacity, SHeaLDS provides reliable dynamic sensing at large strains (ε = 140%) with no drift or hysteresis, is resistant to punctures, and self-heals from cuts at room temperature with no external intervention. As a demonstration of utility, a soft quadruped protected by SHeaLDS detects and self-heals from extreme damage (e.g., six cuts on one leg) in 1 min and monitors and adapts its gait based on the damage condition autonomously through feedback control.

Dynamic Modeling of Intrinsic Self-Healing Polymers Using Deep Learning

The properties of self-healing polymers are traditionally identified through destructive testing. This means that the mechanics are explored in hindsight with either theoretical derivations and/or simulations. Here, a self-healing property evolution using energy functional dynamical (SPEED) model is proposed to predict and understand the mechanics of self-healing of polymers using images of cuts dynamically healing over time. Using machine learning, an energy functional minimization (EFM) model extracted an effective underlying dynamical system from a time series of two-dimensional cut images on a self-healing polymer of constant thickness. This model can be used to capture the physics behind the self-healing dynamics in terms of potential and interface energies. When combined with a static property prediction model, the SPEED model can predict the macroscopic evolution of material properties after training only on a small set of experimental measurements. Such temporal evolutions are usually inaccessible from pure experiments or computational modeling due to the need for destructive testing. As an example, we validate this approach on toughness measurements of an intrinsic self-healing conductive polymer by capturing over 100 000 image frames of cuts to build the machine learning (ML) model. The results show that the SPEED model can be applied to predict the temporal evolution of macroscopic properties using few measurements as training data.

Experimental Investigation on the Use of a PEI Foam as Core Material for the In-Situ Production of Thermoplastic Sandwich Structures Using Laser-Based Thermoplastic Automated Fiber Placement

Laser-based thermoplastic automated fiber placement (TAFP) is nowadays mainly used to produce pure carbon fiber-reinforced plastic (CFRP) structures. This paper investigates the feasibility of a novel application: The deposition of thermoplastic prepreg tapes onto a thermoplastic foam for the production of thermoplastic sandwich structures. Therefore, simple deposition experiments of thermoplastic PEEK/CF prepreg tapes on a PEI closed-cell foam were carried out. 3D surface profile measurements and peel tests according to DIN EN 28510-1 standard were used to investigate the joining area and bonding quality. The results show that a cohesive bond is formed between the deposited tapes and the foam core, however the foam structure in the area of the deposited tapes deforms in dependence of the process parameters, and increasingly with higher deposition temperatures. Due to the deformations that occur during tape deposition, the thermomechanical foam behavior under the TAFP process conditions was investigated in more detail in a subsequent study for an extensive parameter space using a simple experimental setup. Results show that for suitable process parameters, namely a short contact time and a high temperature, the foam deformation can be minimized with the simultaneous formation of a thin melting layer required for cohesive bonding. The inner foam core structure remains unaffected.

Healing Strategy Based on Space Adjustment for Cross-Linked Polymer Networks

Healable materials are notable for their ability to recover from mechanical damage. Most methods for preparing cross-linked healable materials require the introduction of healing agents or supramolecular interactions in solvent environments. Hence, a strategy without the addition of functional component remains a key challenge. Herein, a healing strategy based on space adjustment is proposed with cross-linked poly(octadecyl acrylate) as a model, and this strategy demonstrates that the predesigned holes in cross-linked networks can supply the possibility for polymer coils to move and decrease the space density of the networks during the annealing process. As a result, the motilities of coils are enhanced, which allows them to easily penetrate and entangle in fracture sites. In contrast with the untreated cross-linked poly(octadecyl acrylate), which cannot heal, the space-adjusted poly(octadecyl acrylate) readily heals, and the highest healing efficiency is 96%. The ways in which the extent of space adjustment and the content of the cross-linking agent affect the healing efficiency are discussed, and the mechanism of the space adjustment strategy is studied through rheology research. This strategy concentrates on adjusting the spatial density of the network without the need for any functional design, which may be applied in various polymer systems.

Advances and Challenges of Self-Healing Elastomers: A Mini Review

In the last few decades, self-healing polymeric materials have been widely investigated because they can heal the damages spontaneously and thereby prolong their service lifetime. Many ingenious synthetic procedures have been developed for fabricating self-healing polymers with high performance. This mini review provides an impressive summary of the self-healing polymers with fast self-healing speed, which exhibits an irreplaceable role in many intriguing applications, such as flexible electronics. After a brief introduction to the development of self-healing polymers, we divide the development of self-healing polymers into five stages through the perspective of their research priorities at different periods. Subsequently, we elaborated the underlying healing mechanism of polymers, including the self-healing origins, the influencing factors, and direct evidence of healing at nanoscopic level. Following this, recent advance in realizing the fast self-healing speed of polymers through physical and chemical approaches is extensively overviewed. In particular, the methodology for balancing the mechanical strength and healing ability in fast self-healing elastomers is summarized. We hope that it could afford useful information for research people in promoting the further technical development of new strategies and technologies to prepare the high performance self-healing elastomers for advanced applications.

Predicting mechanical properties of material extrusion additive manufacturing-fabricated structures with limited information

Mechanical properties of additively manufactured structures fabricated using material extrusion additive manufacturing are predicted through combining thermal modeling with entanglement theory and molecular dynamics approaches. A one-dimensional model of heat transfer in a single road width wall is created and validated against both thermography and mechanical testing results. Various model modifications are investigated to determine which heat transfer considerations are important to predicting properties. This approach was able to predict tear energies on reasonable scales with minimal information about the polymer. Such an approach is likely to be applicable to a wide range of amorphous and low crystallinity thermoplastics.

From passive to emerging smart silicones

Amassing remarkable properties, silicones are practically indispensable in our everyday life. In most classic applications, they play a passive role in that they cover, seal, insulate, lubricate, water-proof, weather-proof etc. However, silicone science and engineering are highly innovative, seeking to develop new compounds and materials that meet market demands. Thus, the unusual properties of silicones, coupled with chemical group functionalization, has allowed silicones to gradually evolve from passive materials to active ones, meeting the concept of “smart materials”, which are able to respond to external stimuli. In such cases, the intrinsic properties of polysiloxanes are augmented by various chemical modifications aiming to attach reactive or functional groups, and/or by engineering through proper cross-linking pattern or loading with suitable fillers (ceramic, magnetic, highly dielectric or electrically conductive materials, biologically active, etc.), to add new capabilities and develop high value materials. The literature and own data reflecting the state-of-the art in the field of smart silicones, such as thermoplasticity, self-healing ability, surface activity, electromechanical activity and magnetostriction, thermo-, photo-, and piezoresponsivity are reviewed.

Fusion Bonding Possibility for Incompatible Polymers by the Novel Ultrasonic Welding Technology: Effect of Interfacial Compatibilization

Fusion bonding for polymers has been successfully welded for the same and dissimilar materials. However, it is difficult to bond incompatible polymers due to poor interfacial adhesion. Usually, interfacial compatibilization can resolve this problem. According to the mechanism, an interlayer solder sheet (ISS) consisting of maleic anhydride-functionalized polypropylene (PP-g-MAH) and polyamide6 (PA6) was introduced into the ultrasonic welding (USW) device. In this way, it successfully realized the weldability between PP and PA6. The welding strength of PP-PA6 reached 22.3 MPa, about 84% welding strength for the PP body and 63% tensile strength for PP. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) showed the formation of PP-g-PA6 copolymer in blends. This copolymer played the role of an emulsifier, which enhanced the interfacial adhesion between PP and PA6 in two phases, leading to micron-scale homogeneity. In the USW process, the copolymer could act as a bridge between PP and PA6 molecular chains to realize the fusion bonding of incompatible polymers. Finally, we proposed the fusion bonding model for PP-PA6 interfaces.

Two-step heat fusion kinetics and mechanical performance of thermoplastic interfaces

Thermoplastic polymers and composites are ubiquitous in the industry for their reshaping and fusing capabilities at elevated temperatures. The quality of heat-fused thermoplastic interfaces is of great concern for adhesion, coating, and welding applications, especially those between dissimilar materials. Kinetic evolution of the microstructures defines the mechanical performance of heat-fusion thermoplastic interfaces, which is studied here using polyethylene and polypropylene as an example. Key factors such as the viscosity and compatibility of polymers and the time and temperature of fusion are discussed by combining molecular-level simulations and structural-level hot-compression experiments. Inter-diffusion and entanglement of polymer chains are identified as the two elementary kinetic steps of the fusion, which dominate the control on the stiffness and strength of the interfaces, respectively. Experimental data shows that the quality of fused interfaces can be improved by reducing the viscosity and the interaction parameter. Following the same set of time-scaling relations as identified in the simulations, the two-step characteristics and their effects on the stiffness and strength are experimentally validated. Both simulation and the experiment results show that Young’s modulus of fused interfaces recovers faster than the strength that is controlled by polymer entanglement to a large extent, rather than diffusion. These findings add insights into the design of fusion processes, laying the ground for the applications of thermoplastic polymers and composites.

Self-Healing Silicones for Outdoor High Voltage Insulation: Mechanism, Applications and Measurements

This paper discusses the state of the art in the application of self-healing silicone-based materials for outdoor high-voltage insulation. Both the dynamic behavior of the dimethyl side groups of silicone rubber and the diffusion of a bulk siloxane to maintain low surface energy are respectively reported as intrinsic mechanisms responsible for the self-healing of silicone rubber. Localization, temporality, mobility, and the type of synthesis are the aspects defining the efficiency of the self-healing ability of silicone rubber. In addition, the deterioration of the self-healing ability with filler loaded into silicone rubber insulation housing composites is discussed. Taking the self-healing property into consideration among the other properties of silicone rubber insulators, such as tracking and erosion resistance, can be a useful design practice at the material development stage. Hydrophobicity retention, recovery, and transfer measurements are discussed as useful indicators of the self-healing ability of silicone rubber. Nevertheless, there remains a need to standardize them as design tests at the material development stage. The paper is intended to shed the light on the hydrophobicity recovery, a key material design parameter in the development of silicone rubber outdoor insulating composites, similar to the tracking and erosion resistance.

Self-healing behavior of recycled asphalt prepared by residue oil of straw liquefaction based on molecular dynamics simulation

In order to accurately describe the self-healing behavior of recycled asphalt prepared by residue oil of straw liquefaction (ROSL), five ROSL contents of 2%, 4%, 6%, 10%, and 15% were added to the aged asphalt to represent the recycled asphalt and are denoted as ROSL-2, ROSL-4, ROSL-6, ROSL-10, and ROSL-15, respectively. Molecular simulation was used to simulate the healing behavior of nano cracks. A three-layer system was established in which the two sides are asphalt molecules and the middle is a 30 Å vacuum layer. The vacuum layer represented the internal nano cracks in the asphalt. The results show that the disappearance of cracks during the self-healing process was the result of the combined effect of model volume compression and asphalt molecule stretching. Self-healing is mainly affected by the van der Waals forces between the molecules. The self-healing rate of recycled asphalt is closely related to the content of ROSL, the higher the ROSL content, the greater the diffusion coefficient, which is more conducive to asphalt self-healing. However, as the time for ROSL-10 and ROSL-15 to reach the equilibrium distribution of relative concentration and density stability is basically the same, and the diffusion coefficient of ROSL-10 is basically the same as that of virgin asphalt, the optimal content of the ROSL is recommended to be 10%.

Transparent, self-recoverable, highly tough, puncture and tear resistant polyurethane supramolecular elastomer with fast self-healing capacity via "hard-soft" hard domain design

The integration of superior mechanical properties and fast healing efficiency for self-healing polyurethane supramolecular elastomers is challenging due to the confliction between high chain mobility for healing and high chain rigidity for mechanical properties. Herein, a strategy to design a "hard-soft" hard domain by the cooperation of quadruple hydrogen bonds (HBs) in the mainchain as restriction units and single HBs in the side chain as diffusion units is reported. The resulting transparent supramolecular elastomer exhibited fast self-recoverability, good puncture resistance and superior mechanical properties with a tensile strength of 20.5 MPa, an extensibility of 2043.7%, a toughness of 146.1 MJ m-3 and a tear resistance of 13.8 kJ m-2. Moreover, the fast self-healing capacity (healing efficiency > 82% within 3 h under moderate condition) was realized due to the soft effects of weak HBs in the side chain on the strong HBs in the mainchain. Taking advantage of the merits of the supramolecular elastomer, a flexible sensor was simply fabricated, which showed good self-repairable and stable sensing properties. Thus, the elastomer has great potential in the field of flexible electronics and wearable devices.

Citric acid induced room temperature self-healing polysiloxane elastomers with tunable mechanical property and untraditional AIE fluorescence

Polysiloxane plays an irreplaceable role in aviation, automobile, flexible electronic devices, electrical engineering, etc. However, the poor mechanical strength makes it prone to damage in the working process. Therefore, it...

Modulation of flexo-rigid balance in photoresponsive thymine grafted copolymers towards designing smart healable coating

Efficacy and durability of the photovoltaic device mandates its protection against hot, humid weather condition, high energy of UV light and unwanted scratches. Such challenges can be mitigated by smart polymeric coating with inherent properties e.g. hydrophobicity to prevent moisture, optimal viscocity for better processibility and crack-healing. The hydrophobic polymers TP1–TP4 containing pendant photo-crosslinkable thymine moieties are designed that undergo [2 + 2] photocycloaddition upon UV_B irradiation and can be dynamically reverted back upon irradiation with UV_C light. A judicious control of solvent environment, chain length, functionality% and concentration of the polymers regulate the aspects of photodimerization thereby, rendering intra or inter-chain collapse to form diverse nanostructures. Photodimerization of the thymine moieties renders coil to globule transformation in dilute condition whereas irradiation performed at high macromolecular concentration regime exhibits higher order nanostructures. The photoresponsive chain collapse leads to the formation of rigid crosslinked domains within flexible polymer chains akin to the hard–soft phases of thermoplastic elastomers. Such rigidification of the crosslinked segments endows a tool to photomodulate the glass transition temperature (T_g) that can dynamically revert back upon decrosslinking. Further, the structural modulation of the polymers is explored towards autonomic and nonautonomic self-healing behaviour at ambient conditions. Moreover, the self-healing efficacy can be tuned with the film thickness and it remains unaltered upon using solar simulator or direct sunlight. Overall, such hydrophobic low T_g polymers display photo-regulated self-healing mechanism consisting of both autonomic and non-autonomic self-healing and may find applications in designing smart protective coatings for photovoltaic devices.

Photo-crosslinking using [2 + 2] cycloaddition in thymine grafted low viscosity polymer generates flexorigid domain to result self-healing polymer with increased hydrophobicity for potential use as smart coating material.

The Factors That Affect the Expansion of the Tape for It to Avoid Side Effects in the Production of Composites in Online LATP Technology

During LATP (laser automated tape placement), the compaction roller contacts the prepreg and affects the pressure distribution directly. Moreover, the design parameters of the roller are optimized with the aim of improving pressure uniformity. This paper examines the impact of the contact line and surface that depends on the compaction force, the design of the roller, the angle of inclination and the angle of inclination of the strip. These factors significantly affect the expansion of the tape, and it is important to determine it to avoid side effects in the production of composites (formation of gaps or overlaps). Their presence increases the percentage of pores of the final material and thus reduces the mechanical properties. The results show that the pressure uniformity can be improved significantly by design optimization of the roller, which indicates that higher bond quality between layers is achieved. The lower the speed and higher the compact force in this technology give better intimate contact with a lower percentage of voids and good mechanical characteristics.

Universal Self-Healing Poly(dimethylsiloxane) Polymer Crosslinked Predominantly by Physical Entanglements

Harsh conditions are inevitable for long-term use of self-healing polymers. However, the majority of reported self-healing materials cannot remain stable under harsh conditions due to the presence of vulnerable dynamic crosslinking sites. Herein, a universal self-healing poly(dimethylsiloxane) (PDMS) polymer is reported. In our design, the PDMS polymer chains are crosslinked predominantly through physical entanglements. Owing to the invulnerable nature of the entanglement junctions and high mobility of polymer chains, the as-synthesized polymer exhibits autonomous self-healing capabilities not only under ambient conditions but also in a variety of harsh environments, including aqueous solutions, organic solvents, and extreme conditions (strong acid/alkali, redox agents, freezing temperature). Moreover, this polymer can be easily integrated with a eutectic gallium-indium (EGaIn) alloy to achieve layer-by-layer self-healing electronic skin sensors, which realize the combination of excellent electrical conductivity, long-term sensing stability, and universal self-healing capability.

Review: Filament Winding and Automated Fiber Placement with In Situ Consolidation for Fiber Reinforced Thermoplastic Polymer Composites

Fiber reinforced thermoplastic composites are gaining popularity in many industries due to their short consolidation cycles, among other advantages over thermoset-based composites. Computer aided manufacturing processes, such as filament winding and automated fiber placement, have been used conventionally for thermoset-based composites. The automated processes can be adapted to include in situ consolidation for the fabrication of thermoplastic-based composites. In this paper, a detailed literature review on the factors affecting the in situ consolidation process is presented. The models used to study the various aspects of the in situ consolidation process are discussed. The processing parameters that gave good consolidation results in past studies are compiled and highlighted. The parameters can be used as reference points for future studies to further improve the automated manufacturing processes.

Dynamics and healing behavior of metallosupramolecular polymers

Self-healing or healable polymers can recuperate their function after physical damage. This process involves diffusion of macromolecules across severed interfaces until the structure of the interphase matches that of the pristine material. However, monitoring this nanoscale process and relating it to the mechanical recovery remain elusive. We report that studying diffusion across healed interfaces and a correlation of contact time, diffusion depth, and mechanical properties is possible when two metallosupramolecular polymers assembled with different lanthanoid salts are mended. The materials used display similar properties, while the metal ions can be tracked with high spatial resolution by energy-dispersive x-ray spectrum imaging. We find that healing actual defects requires an interphase thickness in excess of 100 nm, 10 times more than previously established for self-adhesion of smooth films of glassy polymers.

Laser Transmission Welding of Semi-Crystalline Polymers and Their Composites: A Critical Review

The present review provides an overview of the current status and future perspectives of one of the smart manufacturing techniques of Industry 4.0, laser transmission welding (LTW) of semi-crystalline (SC) polymers and their composites. It is one of the most versatile techniques used to join polymeric components with varying thickness and configuration using a laser source. This article focuses on various parameters and phenomena such as inter-diffusion and microstructural changes that occur due to the laser interaction with SC polymers (specifically polypropylene). The effect of carbon black (size, shape, structure, thermal conductivity, dispersion, distribution, etc.) in the laser absorptive part and nucleating agent in the laser transmissive part and its processing conditions impacting the weld strength is discussed in detail. Among the laser parameters, laser power, scanning speed and clamping pressure are considered to be the most critical. This review also highlights innovative ideas such as incorporating metal as an absorber in the laser absorptive part, hybrid carbon black, dual clamping device, and an increasing number of scans and patterns. Finally, there is presented an overview of the essential characterisation techniques that help to determine the weld quality. This review demonstrates that LTW has excellent potential in polymer joining applications and the challenges including the cost-effectiveness, innovative ideas to provide state-of-the-art design and fabrication of complex products in a wide range of applications. This work will be of keen interest to other researchers and practitioners who are involved in the welding of polymers.

Insights into the Bead Fusion Mechanism of Expanded Polybutylene Terephthalate (E-PBT)

Expandable polystyrene (EPS) and expanded polypropylene (EPP) dominate the bead foam market. As the low thermal performance of EPS and EPP limits application at elevated temperatures novel solutions such as expanded polybutylene terephthalate (E-PBT) are gaining importance. To produce parts, individual beads are typically molded by hot steam. While molding of EPP is well-understood and related to two distinct melting temperatures, the mechanisms of E-PBT are different. E-PBT shows only one melting peak and can surprisingly only be molded when adding chain extender (CE). This publication therefore aims to understand the impact of thermal properties of E-PBT on its molding behavior. Detailed differential scanning calorimetry was performed on neat and chain extended E-PBT. The crystallinity of the outer layer and center of the bead was similar. Thus, a former hypothesis that a completely amorphous bead layer enables molding, was discarded. However, the incorporation of CE remarkably reduces the crystallization and re-crystallization rate. As a consequence, the time available for interdiffusion of chains across neighboring beads increases and facilitates crystallization across the bead interface. For E-PBT bead foams, it is concluded that sufficient time for polymer interdiffusion during molding is crucial and requires adjusted crystallization kinetics.

Polyhydroxyalkanoates: Next generation natural biomolecules and a solution for the world's future economy

Petrochemical plastics have become a cause of pollution for decades and finding alternative plastics that are environmental friendly. Polyhydroxyalkanoate (PHA), a biopolyester produced by microbial cells, has characteristics (biocompatible, biodegradable, non-toxic) that make it appropriate as a biodegradable plastic substance. The different forms of PHA make it suitable to a wide choice of products, from packaging materials to biomedical applications. The major challenge in commercialization of PHA is the cost of manufacturing. There are a lot of factors that could affect the efficiency of a development method. The development of new strategic parameters for better synthesis, including consumption of low cost carbon substrates, genetic modification of PHA-producing strains, and fermentational strategies are discussed. Recently, many efforts have been made to develop a method for the cost-effective production of PHAs. The isolation, analysis as well as characterization of PHAs are significant factors for any developmental process. Due to the biodegradable and biocompatible properties of PHAs, they are majorly used in biomedical applications such as vascular grafting, heart tissue engineering, skin tissue repairing, liver tissue engineering, nerve tissue engineering, bone tissue engineering, cartilage tissue engineering and therapeutic carrier. The emerging and interesting area of research is the development of self-healing biopolymer that could significantly broaden the operational life and protection of the polymeric materials for a broad range of uses. Biodegradable and biocompatible polymers are considered as the green materials in place of petroleum-based plastics in the future.

Linking interfacial work of deformation from deconvoluted macro-rheological spectrum to early stage healing in selected polyurethanes

The use of rheology and terminal flow relaxation times to predict healing behavior at long healing times is by now quite well accepted. In this work we go one step further and explore the use of macro-rheology (in particular the stored work of deformation) to predict the early stage interfacial healing properties (fracture resistance) of a set of self-healing polyurethanes. The interfacial healing is measured by single edge notch fracture experiments, using short healing times and a low healing temperature to exclude the effect of long range molecular motion on mechanical properties restoration. The systems based on aromatic diisocyanates show high fracture resistance after healing, while very limited restoration of the mechanical properties is observed for aliphatic and cycloaliphatic based polyurethanes. Linear sweep rheology and time-temperature-superposition allow obtaining the macro-rheological master curve and the mechanical relaxation spectra (H(t)). The application of a recently established deconvolution protocol to the H(t) gives the characteristic relaxation times and stored works of deformation associated to individual dynamic processes such as segmental motion, reversible bonds, and terminal flow. It is found that the calculated stored works of deformation related to the reversible bond relaxation reproduce the trend observed by fracture resistance at healed interfaces and reveal a qualitative correspondence between reversible bonds work of deformation and interfacial healing fracture resistance. Moreover, the method seems to point to the existence of a threshold interfacial work of deformation below which no efficient load transfer can be observed.

Modeling the Adhesion Bonding Strength in Injection Overmolding of Polypropylene Parts

In this work, the bonding strength of overmolded polypropylene is investigated and modeled. A T-joint specimen was designed to replicate the bonding between a base and an overmolded stem made of the same polymer: a previously molded plaque was used for the base, and the stem was directly overmolded. The effect of melt temperature, holding pressure, and localized heating was investigated following the design of experiments approach. Both the melt and base temperature positively affect the welding strength. On the contrary, the holding pressure negatively contributed, as the crystallization temperature significantly increases with pressure. Then, the bonding strength of the specimens was predicted using a non-isothermal healing model. Moreover, the quadratic distance of diffusion (based on the self-diffusion model) was calculated and correlated with the bonding strength prediction. The non-isothermal healing model well predicts the bonding strength when the reptation time is calculated within the first 0.09 s of the interface temperature evolution. The prediction error ranges from 1% to 35% for the specimens overmolded at high and low melt and base temperatures, respectively.

Performance of 3D-Printed Continuous-Carbon-Fiber-Reinforced Plastics with Pressure

Fused Deposition Modeling (FDM) has been investigated as a low-cost manufacturing method for fiber-reinforced composites. The traditional and mature technology for manufacturing continuous-carbon-fiber-reinforced plastics is Automated Fiber Placement (AFP), which uses a consolidation roller and an autoclave process to improve the quality of parts. Compared to AFP, FDM is simple in design and operation but lacks the ability to pressurize and heat the model. In this work, a novel method for printing continuous carbon-fiber-reinforced plastics with a pressure roller was investigated. First, the path processing of the pressure roller was researched, which will reduce the number of rotations of the pressure roller and increase the service life of the equipment and the efficiency of printing. Thereafter, three specimens were printed under different pressures and the tensile and bending strength of specimens were tested. The tensile strength and bending strength of specimens were enhanced to 644.8 MPa and 401.24 MPa by increasing the pressure, compared to the tensile strength and bending strength of specimens without pressure of 109.9 MPa and 163.13 MPa. However, excessive pressure will destroy the path of the continuous carbon fiber (CCF) and the surface quality of the model, and may even lead to printing failure.

Post-Process Effects of Isothermal Annealing and Initially Applied Static Uniaxial Loading on the Ultimate Tensile Strength of Fused Filament Fabrication Parts

Fused filament fabrication (FFF) is one of the most popular additive manufacturing (AM) techniques used to fabricate polymeric structures. However, these polymeric structures suffer from an inherent deficiency of weak inter-laminar tensile strength. Because of this weak inter-laminar strength, these parts fail prematurely and exhibit only a fraction of the mechanical properties of those fabricated using conventional means. In this paper, we study the effect of thermal annealing in the presence of an initially applied static uniaxial load on the ultimate tensile strength of parts fabricated using FFF. Tensile specimens or dogbones were fabricated from an acrylonitrile butadiene styrene (ABS) filament with a glass transition temperature (T_g) of 105 °C; these specimens were then isothermally annealed, post manufacture, in a fixture across a given range of temperatures and static loads. Tensile testing was then performed on these specimens to gauge the effect of the thermal annealing and static loading on inter-laminar tensile strength by measuring the ultimate tensile strength of the specimens. A design of experiments (DOE) approach was followed to calculate the main and interaction effects of the two factors (temperature and static loading) on the ultimate tensile strength, and an analysis of variance was conducted. Cross-sectional images of the specimens were studied to observe the changes in the mesostructure of the specimens that led to the increase in inter-laminar strength of the parts. The results show that temperature plays a dominant role in increasing the ultimate tensile strength and an 89% increase in the average ultimate tensile strength was seen corresponding to an annealing temperature of 160 °C. A change in the mesostructure of the parts is seen, which is characterized by an increase in bond length and void coalescence. These results can be helpful in studying the structural strength of 3D printed parts, and thus could eventually guide the fabrication of components with strength comparable to those of conventional manufacturing techniques.

Detailed Analysis of the Influencing Parameters on the Self-Healing Behavior of Dynamic Urea-Crosslinked Poly(methacrylate)s

For this paper, the self-healing ability of poly(methacrylate)s crosslinked via reversible urea bonds was studied in detail. In this context, the effects of healing time and temperature on the healing process were investigated. Furthermore, the impact of the size of the damage (i.e., area of the scratch) was monitored. Aging processes, counteracting the self-healing process, result in a decrease in the mechanical performance. This effect diminishes the healing ability. Consequently, the current study is a first approach towards a detailed analysis of self-healing polymers regarding the influencing parameters of the healing process, considering also possible aging processes for thermo-reversible polymer networks.