Polymer Specimens Research Articles

In SituMeasurement of Elastic and Frictional Properties Using Atomic Force Microscopy

AbstractAtomic force microscopy (AFM) can determine mechanical properties, associated with surface topography and structure, of a material at the nanoscale. Force–indentation curves that depict the deformation of a target specimen as a function of an applied force are widely used to determine the elastic modulus of a material based on a contact model. However, a hysteresis may arise due to friction between the AFM tip and a specimen. Consequently, the normal force detected using a photodetector during extension and retraction could be underestimated and overestimated, respectively, and the extension/retraction data could result in a significant difference in the elastic modulus measurement result. In this study, elastic modulus and friction coefficient values were determined based on an in situ theoretical model that compensated for the effect of friction on force–indentation data. It validated the proposed model using three different polymer specimens and colloidal-tipped probes for the force–indentation curve and friction loop measurements. This research could contribute to the accurate measurement of mechanical properties using AFM by enhancing the interpretation of force–indentation curves with friction-induced hysteresis. Furthermore, the proposed approach may be useful for analyzing in situ relationships between mechanical and frictional properties from a fundamental tribological perspective.

Effect of Chlorine or Ester Incorporation on Diketopyrrolopyrrole -Based Polymeric Semiconductors

Organic photovoltaic materials incorporating either chlorine atoms or ester groups have been studied as replacement strategy for fluorinated compounds. However, the characteristics of such materials when used in organic field-effect transistors (OFETs) remain unclear. The present work synthesized three polymers via the copolymerization of diketopyrrolopyrrole (DPP) with monothiophene units having no substituents or incorporating either chlorine atoms or ester groups. All three materials were found to have similar photophysical and electrochemical properties even though their repeating units had different substituents. The maximum hole mobility values for the polymer specimens were determined to be 0.37 cm 2 V −1 s −1 . The introduction of chlorine into the DPP-based polymer generated an amorphous semiconductor material that makes it a very promising candidate for commercial OFETs since its properties are independent of fabrication conditions. The new polymeric semiconductors demonstrated herein suggest the viability of such materials with regard to incorporation in organic field-effect transistors. •Both ester groups and chlorine atoms were successfully introduced into DPP-based polymers. •Chlorination and esterification applied to polymeric OFET materials •The introduction of chlorine into the DPP-based polymer generated an amorphous semiconductor material.

Conjugated Polymer Mesocrystals with Structural and Optoelectronic Coherence and Anisotropy in Three Dimensions.

Semiconducting mesocrystalline bulk polymer specimens that exhibit near-intrinsic properties using channel-die pressing are demonstrated. A predominant edge-on orientation is obtained for poly(3-hexylthiophene-2,5-diyl) (P3HT) throughout 2 mm-thick/wide samples. This persistent mesocrystalline arrangement at macroscopic scales allows reliable evaluation of the electronic charge-transport anisotropy along all three crystallographic axes, with high mobilities found along the π-stacking. Indeed, charge-carrier mobilities of up to 2.3 cm2 V-1 s-1 are measured along the π-stack, which are some of the highest mobilities reported for polymers at low charge-carrier densities (drop-cast films display mobilities of maximum ≈10-3 cm2 V-1 s-1 ). The structural coherence also leads to an unusually well-defined photoluminescence line-shape characteristic of an H-aggregate (measured from the surface perpendicular to the materials flow), rather than the typical HJ-aggregate feature usually found for P3HT. The approach is widely applicable: to electrical conductors and materials used in n-type devices, such as poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (N2200) where the mesocrystalline structure leads to high electron transport along the polymer backbones (≈1.3 cm2 V-1 s-1 ). This versatility and the broad applicability of channel-die pressing signifies its promise as a straightforward, readily scalable method to fabricate bulk semiconducting polymer structures at macroscopic scales with properties typically accessible only by the tedious growth of single crystals.

Measurement of frequency-dependent shear properties of highly compliant materials using Kibble’s method

We present a method to determine the frequency-dependent properties of highly-compliant materials by measuring the response of rod-like specimens subjected to an applied torque. Kibble’s method, which utilizes a velocity measurement and two electrical measurements, is used to calculate the applied torque on the specimen as a function of frequency. Shear modulus and loss factor in shear are determined by fitting mechanical models to transfer functions between the applied torque and torsional response of the sample. We demonstrate this technique on a prototype apparatus using different polymer materials and specimen geometry. Estimates of the shear modulus and loss factor in shear are determined over a range of frequencies that include the first six torsional resonant modes. These material properties are compared with data obtained using a commercial Dynamic Material Analyzer wherein the samples are excited in a bending mode.

In-situ impact analysis during fatigue tests of open-hole carbon fibre reinforced polymer specimens

This paper presents the results for an experimental campaign of in-situ impact during tension-tension fatigue loading for open-hole carbon fibre reinforced polymer specimens. High-speed low energy impact was introduced to the specimen with the use of a canon, which was attached to testing bench enabling the impact without the need to remove the specimens from the test bench. Digital Image Correlation, C-scan and Acoustic Emission were utilized to record health monitoring data for damage diagnostics. A strain-based criterion was used to identify a common threshold for the timing of impact ensuring a fair comparison between the different tests. The results indicate that while an impact causes the total amount of damage to increase as one would expect, it does not necessarily increase the damage level in the critical area where final fracture occurs. A dependence on the moment of impact with the fatigue failure was found for specimens subjected to impact before the initiation of the fatigue loading. In contrast, impacting specimens in the presence of fatigue damage had no detrimental effect on the fatigue life, although it was observed that the damaged area was enlarged. Overall, the paper showcases the need to study systemically the effect of in-situ impact on the fatigue life in order to understand better the implications that may be introduced to the integrity of a composite structure.

Sliding friction and wear of human teeth against biocompatible polyether ether ketone (PEEK) under various wear conditions

For the application in dentistry, the friction and wear behaviour of the thermoplastic polymer polyether ether ketone (PEEK) was analysed. Therefore, the friction coefficient and specific wear rate of PEEK against human enamel/dentine was investigated under various wet conditions. The tooth specimens consisting of enamel and dentine were prepared from nine mandibular molars. The sliding friction as well as the material loss of the counterparts was determined by using a block-on-ring rotational test configuration with a boundary film lubrication (deionised water). The material loss was measured by profilometry. The results were statistically analysed by one-way analysis of variance (ANOVA). As a result, the counter surfaces of the tooth samples showed a higher hardness and were stiffer than the polymeric specimens made of PEEK. The initial sliding friction coefficient varied in the range of μ = 0.23-0.44 for the three investigated rotational velocities from 1 m/s to 0.1 m/s and the stationary values were measured in the range of μ = 0.54-0.58 for the total sliding path of 1500 m. The resulting specific wear rate of PEEK was significantly higher ws = 80 ± 65.5 10-6 mm³/Nm than the coefficient of enamel/dentine ws = 0.9 ± 0.7 10-6 mm³/Nm (p < 0.01). In most cases, a significant increase of averaged surface roughness of the tooth samples was determined after tribological tests. In general, the investigations obtained a low removal of dental hard tissue (enamel, dentine) compared to the material loss of PEEK.

Non-destructive evaluation of carbon fiber reinforced polymer using Golay coded thermal wave imaging

Infrared non-destructive evaluation is a valuable tool for characterization of wide variety of structural components/materials. It is based on capturing and processing of thermal response over the material to obtain information about its surface and sub-surface features or material inhomogeneities. This paper investigates the anomaly detecting abilities of Complementary Golay coded infrared thermographic technique to assess a carbon fiber reinforced polymer specimen containing artificial defects in the form of blind holes.

Constitutive Modelling of Polylactic Acid at Large Deformation Using Multiaxial Strains.

Sheet specimens of a PLLA-based polymer have been extended at a temperature near to the glass transition in both uniaxial and planar tension, with stress relaxation observed for some time after reaching the final strain. Both axial and transverse stresses were recorded in the planar experiments. In all cases during loading, yielding at small strain was followed by a drop in true stress and then strain hardening. This was followed by stress relaxation at constant strain, during which stress dropped to reach an effectively constant level. Stresses were modelled as steady state and transient components. Steady-state components were identified with the long-term stress in stress relaxation and associated with an elastic component of the model. Transient stresses were modelled using Eyring mechanisms. The greater part of the stress during strain hardening was associated with dissipative Eyring processes. The model was successful in predicting stresses in both uniaxial and planar extension over a limited range of strain rate.

Influence of Infill Pattern on Mechanical Behavior of Polymeric and Composites Specimens Manufactured Using Fused Filament Fabrication Technology.

This paper presents the results of a comparative evaluation of the tensile strength behaviors of parts obtained by additive manufacturing using fused filament fabrication (FFF) technology. The study investigated the influences of the deposition printing parameters for both polymers and fiber-reinforced polymers. Polymeric materials that are widely used in FFF were selected, including acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), and nylon. Carbon and glass continuous fibers were used to reinforce the nylon matrix in composite materials. The study utilized two manufacturing methods. Polymers were manufactured using an Ultimaker 2 Extended+ device and the fiber-reinforced polymer specimens were obtained using a Markforged Mark Two printer. The entire set of specimens was eventually subjected to destructive monoaxial tensile tests to measure their responses. The main goal of this study was to estimate the effect of the different infill patterns applied (zig-zag, concentric, and four different orientations lines) on the mechanical properties of pure thermoplastic materials and reinforced polymers. Results show a spectacular increase in the tensile stress at break, which for polymers reaches an average value of 27.53 MPa compared to 94.51 MPa in the case of composites (increase of 70.87%). A similar increase occurs in the case of tensile stress at yield with values of 31.87 MPa and 105.98 MPa, respectively, which represents an increase of 69.93%. The influence of the infill of the fiber is decisive, reaching, in the 0-0 arrangement, mean values of 220.18 MPa for tensile stress at break and 198.26 MPa for tensile stress at yield.

Bending Properties of Lightweight Copper Specimens with Different Infill Patterns Produced by Material Extrusion Additive Manufacturing, Solvent Debinding and Sintering

Material extrusion additive manufacturing (MEX) is a versatile technology for producing complex specimens of polymers, ceramics and metals. Highly-filled filaments composed of a binder system and a high-volume content of sinterable powders are needed to produce ceramic or metal parts. After shaping the parts via MEX, the binder is removed and the specimens are sintered to obtain a dense part of the sintered filler particles. In this article, the applicability of this additive manufacturing process to produce copper specimens is demonstrated. The particular emphasis is on investigating the production of lightweight specimens that retain mechanical properties without increasing their weight. The effect of infill grades and the cover presence on the debinding process and the flexural properties of the sintered parts was studied. It was observed that covers could provide the same flexural strength with a maximum weight reduction of approximately 23%. However, a cover on specimens with less than 100% infill significantly slows down the debinding process. The results demonstrate the applicability of MEX to produce lightweight copper specimens.

Experimental investigation on tensile properties of the polymer and composite specimens printed in a Triangular pattern

Additive manufacturing incorporates the utilization of polymers with additives to print 3D specimens in layers. FDM (fused deposition modelling) technique in AM (additive manufacturing) has gained much attention in recent years because of its ability to make complex shapes with great design flexibility. To explore the possibilities with new design pattern specimens with various percent infills is the need of the hour. This paper presents an investigation on the tensile properties of ABS, PLA, and CF (15% by weight fraction) reinforced with PLA specimens. Herein, to print the specimen, a newly triangular pattern is used in the gauge length with different infill percentages oriented at 0°, 45°, and 90°. The motivation is to understand the role of a pattern on mechanical properties. The analysis revealed that the reduction in percent strength corresponding to percent material removed from a particular infill is much less than expected. Thus fabricating a part in a specific pattern may lead to material saving and cost reduction. Besides, printing orientation also played a significant role in determining the ultimate strength. 0° orientation for maximum cases led to the highest strength, whereas 45° was the poorest printing orientation. As expected all the properties got enhanced with increasing infill percentages. Findings in line with the previous work revealed PLA + CF with the highest tensile strength while ABS has the least strength. PLA exhibited the largest Elastic modulus among the three materials. The SEM images in fracture morphology of the specimens indicated the bonding between the CF with PLA and failed layers. • The present work analyzes the tensile characteristic of composite and polymer. • Percent reduction in tensile strength decreased with a percent reduction of material infill. • Highest UTS (51.9 MPa) was observed for PLA + CF specimen oriented at 0°. • Designer can obtain required strength even at lower infills using specific pattern.

An innovative approach to partial discharge measurement and analysis in DC insulation systems during voltage transient and in steady state

Measuring partial discharges in DC insulation systems is an issue due to the lack of a reference relating the voltage waveform to the physics of discharge phenomena. Also, DC is not always steady state, due to voltage and load transients that generate electric field profile variations inside an insulation system, which can affect partial discharge inception likelihood and characteristics. Partial discharge measurement technology must be able to separate discharge pulses from noise and identify the type of sources generating partial discharge, which is related to condition assessment and maintenance. Eventually, measurement and analysis should be automatic and unsupervised, in order to get rid, partially or totally, of expert support. This study addresses a new approach to partial discharge measurements in DC insulation systems, presenting algorithms for separation, recognition and identification, which are effective both in DC steady state and during voltage (and load) transients. These algorithms are automatic and do not require expert support. Various cases of algorithm application on test objects consisting of multilayer polymeric specimens with an internal cavity and defective cable models are presented and discussed. Their effectiveness is proved, at least at a laboratory level, with effective noise separation and identification of discharge typology.

Experimental and numerical analysis of 3D printed suture joints under shearing load

The shear resistance of bio-inspired suture joints with different waviness is explored through an integrated experimental and numerical approach. Polymeric single lap shear specimens with different waviness are designed and fabricated via multi-material additive manufacturing. Quasi-static mechanical experiments are performed to quantify the effective shear stiffness and strength of the joints. SEM images show that the 3D printed suture joints fail under combined adhesive and cohesive failure mechanisms. To evaluate the effects of printing direction on the mechanical behaviors of suture joints, both tensile and single lap shear specimens are designed and printed in two orthogonal directions: parallel to the interface (H direction) and perpendicular to the interface (V direction). An inverse method was used to obtained the effective material properties of the interfacial layer printed in both directions. Nonlinear Finite Element models with cohesive interaction and mixed mode damage initiation and evolution are developed.

Hierarchical auxetic and isotropic porous medium with extremely negative Poisson’s ratio

We propose a novel two-dimensional hierarchical auxetic structure consisting of a porous medium in which a homogeneous matrix includes a rank-two set of cuts characterised by different scales. The six-fold symmetry of the perforations makes the medium isotropic in the plane. Remarkably, the mesoscale interaction between the first- and second-level cuts enables the attainment of a value of the Poisson’s ratio close to the minimum reachable limit of -1. The effective properties of the hierarchical auxetic structure are determined numerically, considering both a unit cell with periodic boundary conditions and a finite structure containing a large number of repeating cells. Further, results of the numerical study are validated experimentally on a polymeric specimen with appropriately arranged rank-two cuts, tested under uniaxial tension. We envisage that the proposed hierarchical design can be useful in numerous engineering applications exploiting an extreme auxetic effect.

Highly Dissipative Materials for Damage Protection against Earthquake-Induced Structural Pounding.

It is a common situation that seismic excitations may lead to collisions between adjacent civil engineering structures. This phenomenon, called earthquake-induced structural pounding, may result in serious damage or even the total collapse of the colliding structures. Filling the gap between two buildings erected close to one another by using visco-elastic materials can be considered to be one of the most effective methods to avoid seismic pounding. In this paper, a new polymer–metal composite material made of polyurethane and closed-cell aluminum foam is proposed as a pounding energy absorber for protection against earthquake hazards. The composite was created in two versions, with and without an adhesive interface. A series of experiments which reflect the conditions of seismic collision were performed: quasi-static compression, dynamic uniaxial compression and low-cycle dynamic compression with 10 loops of unloading at 10% strain. The composite material’s behavior was observed and compared with respect to uniform material specimens: polymer and metal foam. The experimental results showed that the maximum energy absorption efficiency in the case of the new material with the bonding layer was improved by 34% and 49% in quasi-static and dynamic conditions, respectively, in comparison to a sole polymer bumper. Furthermore, the newly proposed composites dissipated from 35% to 44% of the energy absorbed in the cyclic procedure, whereas the polymer specimen dissipated 25%. The capacity of the maintenance of the dissipative properties throughout the complete low-cycle loading was also satisfactory: it achieved an additional 100% to 300% of the energy dissipated in the first loading–unloading loop.

Experimental research of formability limits in different thicknesses of polycarbonate sheets

This experimental investigation evaluates the methodologies for determining the forming limits by necking and fracture in polycarbonate (PC) sheets. The proposed approaches are based on two methodologies that are commonly utilized in sheet metal forming. To that end, Nakajima tests for polymer specimens with different strain loading paths were carried out for 1 mm, 2 mm and 3 mm thickness. The evaluation of the principal strains was accomplished by means of Digital Image Correlation (DIC) and thickness measurements to obtain failure strains were performed. The experimental results highlighted the different behaviour attained in polymers and metals, and the need of establishing a suitable adaptation of the methodologies used in sheet metal forming, especially due to the elastic recovery of polymers that cannot be neglected. This analysis allowed obtaining accurate formability limits by necking and by fracture for the PC sheets of 1, 2 and 3 mm of thickness, additionally these results were compared with the formability limits for PVC sheet of 3 mm of thickness. The representation of these formability limits within the principal strain space allowed establishing a general framework for analysing the fracture limit, which is the relevant forming limit in incremental sheet forming processes.

An ultrasound method for characterization of viscoelastic properties in frequency domain at small deformations

Background The ultrasound technique, usually based on the transmission mode, is capable of providing the viscoelastic properties of polymers. Further techniques involving pulse-echo methods were also described in literature, but they still exhibit inaccuracies in the evaluation of the acoustic properties. Objective The manuscript focuses on an innovative approach for the characterization of the viscoelastic behavior of polymers employing the ultrasound methodology. The proposed procedure is based on the pulse-echo method in order to overcome possible inaccuracies in acoustic properties evaluation and in issues related to transmitter mode applications. Methods Starting from the pulse-echo method adopted for the acquisition, a novel formulation for data processing has been developed and described, allowing to determine the wave attenuation coefficient, in comparison to the commonly employed procedures involving ultrasound in polymers characterization, based on transmitter mode inspections. To carry out the study, a specifically designed ultrasound bench has been set up and three different polymers have been tested in the temperature range of interest. Results According to the proposed methodology, the loss factor towards the temperature is determined starting from the data acquired considering the identified attenuation coefficient and the measured sound velocities. The trustworthiness of the novel procedure has been proved comparing the obtained viscoelastic loss factor quantities to the reference master curves obtained by the standard Dynamic Mechanical Analysis characterizations carried out on the same polymer specimens. Conclusions A novel methodology involving ultrasound technology aiming to evaluate the viscoelasticity of the polymers using non-destructive approach has been developed. The results obtained are agreement with the standard viscoelastic master curves determined through the DMA.

Evaluation of candidate polymers to maximize geotechnical performance of oil sands tailings

Oil sands fluid fine tailings deposits are challenging to reclaim due to their inherently high natural water content, low permeability, and low strength. Combinations of polymers and (or) coagulants are used by operators to improve the dewatering and strength properties of the tailings. However, considerably more work has been done to evaluate polymer performance with short-term dewatering metrics rather than with long-term metrics such as consolidation properties. This paper evaluates the potential of four novel polymers for use in fluid fine tailings treatment compared to a commercially available polymer and untreated tailings specimen. The performance of the polymers was assessed through initial screening with respect to short-term dewatering, evaluation of the consolidation and strength properties using large-strain consolidation tests, shear sensitivity in pipeline transport, and finally, large-strain consolidation modelling to appraise the relative potential performance under different strategies, such as terrestrial or aquatic reclamation options. One polymer exhibits remarkably fast dewatering at high void ratios, while another demonstrates dense and shear-resistant flocs. The paper discusses each polymer’s distinctive tailings fabrics and how their unique merits and limitations would benefit different reclamation eventualities. Finally, potential improvements of the polymers are suggested for future assessment.

Investigation of the dynamic behavior of CFRP with an embedded smart layer based on fiber-optic sensors

Currently, special interest arises in the use of various systems for monitoring the state of structures during operation. Structures during operation, as a rule, experience dynamic loads. As a result, the monitoring system must correctly and accurately record this response. Most modern monitoring systems use fiber-optic sensors. Embedding fiber optic sensors directly into the structure of composite structures is a difficult task due to the fragility of the fiber. The team of authors has developed a prototype of the Smart-layer, which allows for safe implementation into structures made of carbon fibre reinforced polymer. However, the issue of decrypting the data obtained from the Smart layer is an urgent and laborious task. The paper presents the results of studies on the registration and interpretation of the dynamic response obtained from the Smart-layer embedded in carbon fibre reinforced polymer specimens under dynamic loading of the specimens. The results of field experiments on dynamic loading of samples were compared with model values. In the course of the study, good agreement was found between the model and experimental results.

Flexural Properties and Polished Surface Characteristics of a Structural Colored Resin Composite.

The aim of this study was to determine the flexural properties and surface characteristics of a structural colored resin composite after different finishing and polishing methods, in comparison to those of conventional resin composites. A structural color resin composite, Omnichroma (OM, Tokuyama Corp, Chiyoda City, Tokyo, Japan), and two comparison resin composites, Filtek Supreme Ultra (FS, 3M, St Paul, MN, USA) and Tetric EvoCeram (TE, Ivoclar Vivadent, Schaan, Liechtenstein), were used. The flexural properties of the resin composites were determined in accordance with the ISO 4049 specifications. For surface properties, 70 polymerized specimens of each resin composite were prepared and divided into seven groups of 10. Surface roughness (Sa), gloss (GU), and surface free energy (SFE) were investigated after the following finishing and polishing methods. Three groups of specimens were finished with a superfine-grit diamond bur (SFD), and three with a tungsten carbide bur (TCB). After finishing, one of the two remaining groups was polished with a one-step silicone point (CMP), and the other with an aluminum oxide flexible disk (SSD). A group ground with SiC 320-grit was set as a baseline. The average flexural strength ranged from 116.6 to 142.3 MPa in the following order with significant differences between each value: FS > TE > OM. The average E ranged from 6.8 to 13.2 GPa in the following order with significant differences between each value: FS > TE > OM. The average R ranged from 0.77 to 1.01 MJ/mm3 in the following order: OM > FS > TE. The Sa values of the OM groups polished with CMP and SSD were found to be significantly lower than those of the other resin composites, regardless of the finishing method. The GU values appeared to be dependent on the material and the finishing method used. The OM specimens polished with SSD showed significantly higher GU values than those polished with CMP. Most of the resin composites polished with SSD demonstrated significantly higher γS values compared to the other groups. Extremely strong negative correlations between Sa and GU in the combined data from the three resin composites and each resin composite and between Sa and γS in the OM specimens were observed; GU showed a strong positive correlation with γS in the same material. These findings indicate that both flexural and surface properties are material dependent. Furthermore, the different finishing and polishing methods used in this study were observed to affect the Sa, GU, and SFE of the resin composites.