Ductile Polymer Research Articles

Multiaxial characterisation and nonlinear thermoviscoelastic isotropic and orthotropic constitutive models of ETFE membranes

Ethylene-tetra-fluoroethylene (ETFE) is a stiff and ductile polymer widely employed in cladding elements for roofs and façades, formed in pneumatic cushions or tensioned foils. The material’s high thermoviscoelastic effects require a comprehensive time and temperature-dependent multiaxial characterisation and accurate constitutive model to foster the design of lightweight ETFE tensioned construction and their standards. A multiaxial experimental campaign is performed to assess the material response in a wide range of temperatures, from −20°C to 60°C, strain rates, from 0.01%/s to 1%/s, and mechanical loading conditions typical of ETFE buildings. Uniaxial and biaxial bulge tests on circular and elliptical samples are carried out to assess constant strain rates, pressure rates and temperatures, unloading, cyclic, creep and relaxation conditions until the material yield point. Two nonlinear thermoviscoelastic plane stress constitutive models, one isotropic and one orthotropic, are developed using the Eyring stress shift factor to capture material nonlinearities, in the framework of the Boltzmann and the time–temperature superposition principles. A freezing of the Eyring shift factor during unloading has been introduced to represent ETFE relaxation and cyclic conditions. The numerical implementation of the consecutive models within finite element software is made available open source and has enabled the their validation on independently acquired data. The results show high accuracy between measurements and predictions from both isotropic and orthotropic formulations, thus paving the way for their use in structural design to capture highly nonlinear time and thermal effects, such as multiaxial prestress loss after installation or creep caused by variable loads.

Thermomechanical coupling during large strain deformation of polycarbonate: Experimental study

Polycarbonate is a widely used ductile glassy polymer that can undergo large strain deformation before failure. During the plastic deformation process, some mechanical energy is converted to heat, which, if the specimen is loaded at rates sufficient that the heat cannot conduct out of the material, can result in significant temperature rises that affect the mechanical response. Typically, this is expected to result in a reduction in stress at large strain, compared to the behaviour under isothermal conditions; however, compared to other glassy polymers, it has been observed that less softening than expected is experienced in polycarbonate at high strain rates. The current paper describes a thorough investigation of temperature rises in polycarbonate. Compression experiments are performed using a screw-driven machine, a hydraulic machine, and a long split Hopkinson bar, all instrumented with a high-speed infrared camera to measure temperature rises at strain rates between 0.01 and 2600 s-1 at a starting temperature around 20°C. Further, temperature rises in compression experiments at 0.5 s-1 and starting temperatures between -80 to 150°C are measured using embedded thermocouples. These span the range of the secondary- to glass-transitions of polycarbonate, allowing investigation of the effect of these transitions. These experiments are supported by finite element simulations, which use a phenomenological viscoplastic model, to ensure the thermal boundary conditions are adiabatic. Temperature rises are observed in both temperature and rate-dependent tests: experiments at higher strain rates and lower temperatures experience a greater temperature rise because of the higher yield stress; however, there are differences in the conversion ratio between plastic work and heat (Taylor Quinney coefficient), which is both temperature and rate dependent, and strongly affected by both the secondary and glass transitions.

Analysis of mode I crack propagation in glassy polymers under cyclic loading using a molecular dynamics informed continuum model for crazing

Craze and crack propagation in glassy polymers under cyclic mode I loading are investigated by employing a recently developed continuum-micromechanical model for crazing. This model accounts for the local morphology change from microvoids to fibrils during craze initiation, viscoplastic drawing of bulk material into fibrils, and viscoelastic creep recovery of the fibrillated craze matter during unloading. To ensure consistency between the bulk and craze model parameters, the material parameters of the craze model are normalised and calibrated based on a hybrid approach integrating experimental findings from the literature and molecular dynamics results. This yields a generic, yet representative glassy polymer response.In the framework of 2D plane strain finite element simulations, we study brittle as well as ductile glassy polymers and assess the results by drawing comparisons to the experimental and numerical literature. For brittle materials, characterised by a purely elastic bulk behaviour, the model reproduces craze characteristics such as the craze opening contour, the craze length-to-width ratio, a double stress peak at the craze and crack tip, and a non-proportional stress redistribution during loading-unloading cycles. In ductile glassy polymers, the interaction of shear yielding in the bulk and crazing along the ligament is analysed. In particular, shear bands emanate from the crack tip in each loading cycle and arch forward towards the craze. This plastic zone shares resemblance to the so-called epsilon-shaped deformation zone. The current simulations capture normal fatigue crack propagation, where craze and crack growth occur near the peak load in every cycle and the craze length remains relatively constant across the loading cycles. Moreover, findings from this study suggest that plasticity-induced unloading of the craze adjacent to the crack tip impedes crack growth.

Suppression of cracking induced by epitaxial strain relaxation using a relaxation absorber during the transfer of epitaxial thin films of anatase-type Nb:TiO2

This work developed a technique for fabricating flexible epitaxial thin films of anatase-type Nb:TiO2 (epitaxial TNO) via a transfer process. In our prior work, epitaxial TNO deposited on LaAlO3 (001) single crystal substrates was lifted off and affixed to flexible polymeric sheets. However, many cracks occurred on the surfaces of the transferred thin films, such that the conductivity of the epitaxial TNO was drastically decreased. This loss of conductivity was attributed to the interruption of current paths by the cracks. These numerous cracks were linear and in most cases were oriented either parallel to the [100]anatase∥[100]LAO direction or parallel to the [010]anatase∥[010]LAO direction (where the subscripts anatase and LAO represent the materials for each Miller index). Therefore, cracking likely occurred as a consequence of the relaxation of epitaxial strain and a return to the original axial lengths of the TNO during the transfer process. In the present work, the surface of epitaxial TNO was coated with the ductile polymer SU-8 3050 as a relaxation absorber prior to the transfer. This modified transfer process greatly reduced crack formation in the TNO such that the conductivity of the thin film was improved by two orders of magnitude. Thus, pre-coating with a ductile relaxation absorber was shown to effectively suppress the cracking of flexible epitaxial TNO.

Injectable Dual Fenton/Enzymatically Cross-Linked Double-Network Hydrogels Based on Acrylic/Phenolic Polymers with Highly Reinforced and Tunable Mechanical Properties.

Injectable hydrogels have been extensively used as promising therapeutic scaffolds for a wide range of biomedical applications, such as tissue regeneration and drug delivery. However, their low fracture toughness and brittleness often limit their scope of application. Double-network (DN) hydrogel, which is composed of independently cross-linked rigid and ductile polymer networks, has been proposed as an alternative technique to compensate for the weak mechanical properties of hydrogels. Nevertheless, some challenges still remain, such as the complicated and time-consuming process for DN formation, and the difficulty in controlling the mechanical properties of DN hydrogels. In this study, we introduce a simple, rapid, and controllable method to prepare in situ cross-linkable injectable DN hydrogels composed of acrylamide (AAm) and 4-arm-PPO-PEO-tyramine (TTA) via dual Fenton- and enzyme-mediated reactions. By varying the concentration of Fenton's reagent, the DN hydrogels were rapidly formed with controllable gelation rate. Importantly, the DN hydrogels showed a 13-fold increase in compressive strength and a 14-fold increase in tensile strength, compared to the single network hydrogels. The mechanical properties, elasticity, and plasticity of DN hydrogels could also be modulated by simply varying the preparation conditions, including the cross-linking density and reagent concentrations. At low cross-linker concentration (<0.05 wt %), the plastic DN hydrogel stretched to over 6,500%, whereas high cross-linker concentration (≥0.05 wt %) induced fully elastic hydrogels, without hysteresis. Besides, DN hydrogels were endowed with rapid self-recovery and highly enhanced adhesion, which can be further applied to wearable devices. Moreover, human dermal fibroblasts treated with DN hydrogels retained viability, demonstrating the biocompatibility of the cross-linking system. Therefore, we expect that the dual Fenton-/enzyme-mediated cross-linkable DN hydrogels offer great potential as advanced biomaterials applied for hard tissue regeneration and replacement.

Spider silk inspired strong yet tough composite hydrogels

Nanofillers are frequently used to improve the mechanical properties of hydrogels, but it is still difficult to develop composite hydrogels with simultaneously enhanced strength, fracture strain, toughness and fatigue threshold at a high water content. Inspired from the unique structure of spider silk, we propose a “macromolecular chain engineered aramid nanofiber (ANF) reinforcement” strategy to prepare strong and fatigue resistance hydrogels with a relatively high water content. ANFs can decrease the crystallinity while increase the stretchability of PVA hydrogels. The stress can be effectively transferred from the soft and ductile polymer chains to the hard and strong ANFs through strong interfacial hydrogen bonding. The tensile strength, fracture strain, toughness and fracture energy of ANF reinforced hydrogels (ARHs) can reach as high as 2.07 ± 0.15 MPa, 1084 ± 116 %, 12.66 ± 1.1 MJ m−3, and 3196 ± 219 J m−2, respectively, at a water content of ∼80 %. Also, ARHs show great crack propagation resistance with the fatigue threshold up to ∼157 J m−2. The bioinspired ARHs with outstanding mechanical properties and anti-swelling performance have promising applications in the field of soft underwater robots, artificial muscles, and so on.

Morphological adaptation of expanded vermiculite in polylactic acid and polypropylene matrices for superior thermoplastic composites

AbstractThe morphological transformation of expanded vermiculite (VC) during injection molding with brittle PLA and ductile PP polymers along with its positive and negative contribution to mechanical response is investigated. Polymer/VC mixtures with 30 wt. % VC are prepared by thermokinetic high shear mixing at 4000 rpm without any ex‐situ exfoliation agents or compatibilizers. Obtained composite mixtures are then injection molded onto three‐point bending and tensile test specimens. Performed mechanical tests suggested a significant increase in tensile (110%) and flexural modulus (112%) of PP30VC samples. Presented fractographic and morphological investigations suggested that the root cause of measured improved tensile (36%) and bending strength (26%) of PP is the fibrillation of VC associated with PP/VC interactions under high shear. A similarly increasing trend for tensile (147%) and bending modulus (137%) was observed for PLA30VC samples. Contrary to PP30VC, a decreasing pattern was present in the case of tensile (−40%) and bending strength (−13%) of PLA30VC. The root cause for such reduction is determined to be (i) in situ exfoliation of VC inside PLA matrix and transformation of VC into micrometer‐sized platelets; (ii) evaporation of trapped water/crystalline water in the interlayer region of VC which caused in situ degradation of PLA during manufacturing.Highlights Vermiculite‐loaded PP and PLA composites are produced at high shear. Composites have superior modulus compared to neat polymers. Fibrillation of platelet vermiculite is observed in only the PP matrix.

Mechanical behaviour of ductile polymer cellular model structures manufactured by FDM

In this work, Acrylonitrile-Butadiene-Styrene model structures were manufactured by FDM, and their mechanical behaviour investigated under compression, both at small and at large strains. The structure design strategy adopted, based on the use of circular cross-section beam-like elements formed under controlled conditions, led to obtain open-cell structures (with a porosity degree of ≈ 65%) composed of unit cells with different shapes and dimensions assembled to form regularly repeating patterns. The stress-strain behaviour, from cube- and prism-shaped specimens with different sizes and loaded along different directions, was discussed in the light of the outcomes from (i) cyclic compression experiments and (ii) morphological analyses of cryogenic fracture surfaces of specimens compressed at high strain levels. The response along the 3D-stacking direction was traced back to the elastic-plastic case, with non-recoverable strain starting to accumulate between 3% and 5% strain and structure densification starting below 20%. The specimen size effects turned out to be little pronounced. Slightly higher levels of stiffness and strength were measured for the largest cube. This result was discussed on the basis of the peculiar morphology of the structure examined.

A Tailored Interface Design for Anode-Free Solid-State Batteries.

Anode-free solid-state batteries (AFSSBs) are considered to be one of the most promising high-safety and high-energy storage systems. However, low Coulombic efficiency stemming from severe deterioration on solid electrolyte/current collector (Cu foil) interface and undesirable Li dendrite growth impede their practical application, especially when rigid garnet electrolyte is used. Here, an interfacial engineering strategy between garnet electrolyte and Cu foil is introduced for stable and highly efficient AFSSBs. By utilizing the high Li ion conductivity of LiC6 layer, interfacial self-adaption ability arising from ductile lithiated polyacrylic acid polymer layer and regulated Li deposition via Li-Ag alloying reaction, the garnet-based AFSSB delivers a stable long-term operation. Additionally, when combined with a commercial LiCoO2 cathode (3.1mAhcm-2 ), the cell also exhibits an outstanding capacity retention due to the tailored interface design. The strategies for novel AFSSBs architecture thus offer an alternative route to design next-generation batteries with high safety and high density.

Experimental Validation of the Microplastic Index—Two Approaches to Understanding Microplastic Formation

The Microplastic Index (MPI) was presented in a previous paper as a method to assess the formation of microplastics during the application of impact and wear stresses, based on selected mechanical and physical properties of polymers. In this paper, the experimental validation of the MPI model is presented. A series of ten polymers was characterized to obtain the relevant parameters for the calculation of the MPI, i.e., the minimum particle size and volume of microplastics formed. The milling (addressing impact stress) and sanding experiments (addressing wear stress) resulted in particle sizes between 3 and 200 μm and 0.3 and 25 μm, respectively. These values were very well predicted by the MPI model, showing smaller particles for brittle polymers and larger ones for ductile polymers. In addition, the experimental-specific wear rates of impact and wear correlated well with the predicted ones, being 0.01–30 mm3/Nm for impact and 0.0002–0.012 mm3/Nm for wear. These results indicate that the MPI can be very well used to predict the tendency of a material to form microplastics. In the search for understanding and mitigating microplastic formation, the MPI can be used by both producers and end users to choose plastic grades that form fewer microplastics.

Determination of creep crack growth kinetics of ABS via the C* approach at different temperatures

Crack initiation and propagation under creep is one of the main failure modes of service equipment made of plastic. This paper explores the feasibility of using the creep C* integral approach to deduce creep crack growth data obtained for a polymeric material, specifically an acrylonitrile butadiene styrene polymer, which exhibits nonlinear mechanical behavior. Experimental creep crack growth data, measured at 60 and 80 °C, was used to obtain the relationship between the time rate of crack growth, da/dt, due to secondary creep and the applied value of C*. A master-curve could be determined, which describes experimental data at both temperatures. These promising results suggest that C* integral approach can be used as powerful and convenient tool to describe crack propagation of polymeric materials under secondary creep conditions. The approach may also be of interest for the prediction of creep lifetime for ductile polymers.

Plasticizer role of cellulose nanocrystals in the biodegradable polymer blend with ductile polymer as continuous phase

Plasticizer role of cellulose nanocrystals in the biodegradable polymer blend with ductile polymer as continuous phase

Investigating the heat sensitivity of frequently used excipients with varying particle sizes

During tablet manufacturing an increase in the production temperature can lead to an alteration of tablet characteristics. In the present study, the influence of the initial particle size on the tableting behavior of ductile polymers upon temperature rise was investigated. Different grades of the respective materials were tableted at temperatures ranging from 22 to 70 °C.Alterations in tableting behavior were affected by the initial particle size. Smaller particle sizes led to a more pronounced decrease in yield pressure and net work of compaction during compressibility analysis. The results were confirmed in the tabletability studies. Tablets from binary mixtures with lactose containing smaller polymer particles yielded a stronger increase in tensile strength. Differences in the tensile strength increase of two grades from the same material correlated with the ratio of their median particle sizes. The alteration of compactibility profiles was also particle size dependent. The increase in solid fraction was more prominent for binary mixtures containing polymers with smaller particle sizes. However, the ratio of the median particle sizes of the compared grades showed no systematic effect.The results underline the importance of controlling the structural properties of a material carefully during formulation development and production. If a formulation responds to temperature variations, an increase in particle size might be beneficial to decrease its heat sensitivity.

A modified neo-Hookean model for semi-crystalline thermoplastics assessed by relaxation and zero-stress creep tests of recycled polypropylene and polyoxymethylene

The mechanical behavior of thermoplastics is strongly rate-dependent, and oftentimes it is difficult to find constitutive models that can accurately describe their behavior in the small to moderate strain regime. In this paper, a hyperelastic network model (modified neo-Hookean) and a set of experiments are presented. The testing consists of monotonic tensile loading as well as stress relaxation and zero stress creep. Two materials were tested, polyoxymethylene (POM) and recycled polypropylene (rPP), representing one more rigid and brittle and one softer and more ductile semi-crystalline polymer. The model contains two main novelties. The first novelty is that the stiffness is allowed to vary with the elastic deformation (in contrast to a standard neo-Hookean model). The second novelty is that the exponent governing viscous relaxation is allowed to vary with the viscous deformation. The basic features of the new model are illustrated, and the model was fitted to the experimental data. The model proved to be able to describe the experimental results well.

Effects of UV Irradation on Electrospun PLLA and PAN in the Production of Short Electropun Fibres Using Ultrasonication Method

This work showed that exposure of ductile electrospun polymers, namely poly-L-Lactide acid (PLLA) and polyacrylonitrile (PAN) to UV-Ozone, leads to the embrittlement of fibres. Young’s modulus for PLLA and PAN increased by 39% and 78%, respectively. Meanwhile, the ductility was reduced by 23% for PLLA and 40% for PAN. The SEM images show that the UV irradiation resulted in a surface pitted of PLLA and no changes in PAN surface morphology. The ATR-FTIR results indicate that this treatment did not change the chemical structure of the electrospun PLLA and PAN fibres. The as-spun polymers that failed to be scission directly using ultrasonication can now be fragmented into micron-length short fibres after the UV irradiation treatment. The minimum time to produce the short fibres is 18 mins for PAN and 29 mins for PLLA. It indicates ultrasonication is suitable for producing short electrospun fibres, even for ductile materials.

Influence of Coffee Variety and Processing on the Properties of Parchments as Functional Bioadditives for Biobased Poly(butylene succinate) Composites.

Fermented polymers like biobased poly(butylene succinate) (BioPBS) have become more relevant as technical substitutes for ductile petrochemical-based polymers but require biogenic functional additives to deaccelerate undesired thermo-oxidative degradation and keep a fully biobased character. In this paper, the influence of coffee parchment (PMT) from two different varieties and processings on the thermo-oxidative stabilization and mechanical properties of poly(butylene succinate) composites up to 20 wt.-% PMT were investigated. Micronized with a TurboRotor mill, both PMT powders differ in particle size and shape, moisture ab- and adsorption behavior and antioxidative properties. It could be shown that pulped-natural PMT consists partially of coffee cherry residues, which leads to a higher total polyphenol content and water activity. The homogeneous PMT from fully washed processing has a higher thermal degradation resistance but consists of fibers with larger diameters. Compounded with the BioPBS and subsequent injection molded, the fully washed PMT leads to higher stiffness and equal tensile strength but lower toughness compared to the pulped-natural PMT, especially at lower deformation speed. Surprisingly, the fully washed PMT showed a higher stability against thermo-oxidative decomposition despite the lower values in the total phenol content and antioxidative activity. The required antioxidative stabilizers might be extracted at higher temperatures from the PMT fibers, making it a suitable biogenic stabilizer for extrusion processes.

Fracture properties of thin brittle MTM clay coating on ductile HEC polymer substrate

Thin clay coatings can be deposited from water dispersions for the purpose of improved gas barrier properties and fire retardancy of polymeric materials. Mechanical properties of the coatings are difficult to assess, since they are very thin (≈1µm). In-situ tests using a micro tensile stage in a scanning electron microscope reveal a thickness-dependent microcracking mechanism, and Weibull parameters for coating fracture are extracted. Complex fracture events are identified, related to a weak clay coating-polymer substrate interface. A micromechanical finite element formulation provides values of 5 MPa for interfacial shear strength and 1 J/m2 for interfacial fracture toughness. From the multiple cracking behavior of the clay coating, a clay strength ≈ 225 MPa is estimated by the Weibull strength parameter from fragmentation diagrams. The method may be extended to other combinations of brittle coating-ductile substrates.

Fracture toughness of semi-regular lattices

Previous studies have shown that the kagome lattice has a remarkably high fracture toughness. This architecture is one of eight semi-regular tessellations, and this work aims to quantify the toughness of three other unexplored semi-regular lattices: the snub-trihexagonal, snub-square and elongated-triangular lattices. Their mode I fracture toughness was obtained with finite element simulations, using the boundary layer technique. These simulations showed that the fracture toughness KIc of a snub-trihexagonal lattice scales linearly with relative density ρ̄. In contrast, the fracture toughness of snub-square and elongated-triangular lattices scale as ρ̄1.5, an exponent different from other prismatic lattices reported in the literature. These numerical results were then compared with fracture toughness tests performed on Compact Tension specimens made from a ductile polymer and produced by additive manufacturing. The numerical and experimental results were in excellent agreement, indicating that our samples had a sufficiently large number of unit cells to accurately measure the fracture toughness. This result may be useful to guide the design of future experiments.

Evaluation of In-Die Compression Data for a Deeper Understanding of Altered Excipient Properties upon Temperature Rise

The thermodynamic analysis of tablet formation includes the thermal and mechanical analysis during compression. The aim of this study was to evaluate alterations of force–displacement data upon temperature rise as an indicator for changed excipient properties. The tablet press was equipped with a thermally controlled die to imitate the heat evolution from tableting on an industrial scale. Six predominantly ductile polymers with a comparably low glass transition temperature were tableted at temperatures ranging from 22-70°C. Lactose served as a brittle reference with a high melting point. The energy analysis included the net and recovery work during compression, from which the plasticity factor was calculated. The respective results were compared to the changes in compressibility obtained via Heckel analysis. Elevated temperatures reduced the necessary work for plastic deformation for the ductile polymers, which was reflected in decreasing values for the net work of compaction and the plasticity factor. The recovery work slightly increased for the maximum tableting temperature. Lactose showed no response to temperature variations. Changes in the net work of compaction showed a linear correlation to the changes in yield pressure, which could be correlated to the glass transition temperature of a material. It is therefore possible to detect material alterations directly from the compression data, if the glass transition temperature of a material is sufficiently low.Graphical

Role of conformation transition of high acyl gellan in the design of double network hydrogels

Double network hydrogels (DNs) with excellent strength and toughness have been preliminarily applied in the preparation of artificial foods. To evaluate the effect of conformation transition of ductile polymers on the physicochemical properties of DNs, we firstly prepared agarose (AR)/high acyl gellan (HAG) DNs and investigated their mechanical properties, and then calcium ion (Ca2+) was introduced into optimized AR/HAG DNs to regulate the conformation of ductile chains (HAG) for further increasing their mechanical properties. The mechanical strength of the optimized AR/HAG gel is 5 times and 2 times that of AR and HAG gel, respectively. Compared with adding Ca2+ method, immersing Ca2+ solution endowed optimized DNs with 5-fold increase in mechanical strength, outstanding textural properties and lower swelling ratio, which was attributed to the extended conformation of ductile chains. Furthermore, the obtained DNs were reminiscent of beef omasum based on their physicochemical properties. Optimized AR/HAG DNs after immersing in 2 wt% CaCl2 solution exhibited comparable texture properties with beef omasum by three correlation analysis methods and sensory evaluation, providing a new strategy to fabricate biomimetic food with high chewiness by regulating the conformation of ductile polymers in DNs.