Craze Formation Research Articles

Opacification Kinetics of PLA during Liquid Water Sorption.

When in contact with water, poly(lactic acid), PLA, undergoes several physical changes. A very evident one is opacification, namely the change from the typical transparent appearance to a white opaque color. This phenomenon is particularly significant for many applications, including packaging, since opacity hinders the possibility of a clear look of the packed goods and also worsens the consumers' perceptions. In this work, we report an analysis of the time evolution of the phenomenon in different conditions of temperature and water concentration. The results allow us to define a time-scale of the phenomenon and to put it in relationship with the temperature and water content inside the material. In particular, opacification proceeds from the outer surface of the specimens toward the center. Both craze formation due to hydrolysis and crystallization contribute to the opacification phenomenon. Opacification becomes faster as temperature increases, whereas the increase in the solution density has the opposite effect. A model for describing the evolution of opacification was proposed and found to be consistent with the experimental data.

Long-term performance of HDPE extrusion welds aged at 85°C in synthetic leachate

The effect of ageing at 85°C of extrusion welds of 1.5 mm-thick high-density polyethylene (HDPE) geomembrane, immersed in synthetic municipal solid waste leachate, is investigated with respect to its standard oxidative induction time (Std-OIT) and stress crack resistance (SCR). Results show that the heat-affected zones (HAZ) adjacent to the squeeze-out bead (flashing) may exhibit faster STD-OIT depletion than the sheet. Generally, individual weld SCR failure times were observed to be between that of the notched and unnotched sheet. Welds with high welding-induced geometric irregularities (WIGI), and overheated fusion and extrusion welds are shown to result in SCR failure times close to that of a notched sheet. The average time to nominal failure (taken to be when tNF = 250 h) of Cool and Good welding parameter combinations ranges between 3% and 15% shorter than the unnotched sheet. Differences in tNF were attributed to accelerated craze formation in welds with high WIGI from stress concentration. No significant difference was observed between the SCR values of fusion and extrusion welds during 40 months of immersion in MSW-L3. This paper shows that overgrind negatively impacts the SCR of welds and accelerates the degradation of SCR.

Location-controlled crazing in polyethylene using focused electron beams and tensile strain

Abstract This study explores the use of focused electron beam (FEB) dot irradiation and tensile stress to control crazing formation in polymers at the microscale. Polyethylene (PE) containing carbon was subjected to FEB dot irradiation at 5–20 kV of accelerating voltage, followed by nominal strains of 0–70 %. The results revealed that FEB irradiation affects the structure and mechanical properties of PE. The Raman spectroscopy showed a glassy polymer of amorphous structure with the intensity changed after irradiation, which led to craze formation at the location of the electron beam dot irradiation after the tensile deformation. Rhombus-shaped crazes were observed at the locations of the electron beam irradiation in the dot area, which were evenly distributed and sized. Our findings provide insights into the control of crazing location in polymers and provide a promising approach to controlling the different shapes and sizes of crazing in polymers for future applications.

Regulating the formation and extent of crazing through the application of argon plasma surface functionalisation

Polymer crazing is a phenomenon observable as fine cracks on the surface of a material. For polymers crazing is often a precursor to mechanical failure with the capacity of a craze to propagate and develop into a larger structural crack. In this study polypropylene (PP) fibres, known to be susceptible to crazing, were subjected to argon (Ar) plasma treatment with the aim of creating a crosslinked surface that displays the ability to withstand the formation of crazes. To evaluate Ar plasma success at achieving this aim: the resulting Ar treated fibres were characterised to evaluate their capacity to avoid the formation of crazes and also identify any changes induced to the chemical and mechanical properties of PP as a consequence of Ar plasma exposure. This analysis was conducted by application of low voltage (LV)-scanning electron microscopy imaging, uniaxial tensile testing, Energy Dispersive X-ray Spectroscopy (EDS) and attenuated total reflectance - fourier-transform infrared spectroscopy (ATR-FTIR). The results of this study highlight the potential of the application of Ar plasma to influence the formation of crazes on PP fibres after exposure to repeated dynamic distention cycles. Ar plasma treated PP also showed a capacity to reduce bulk fibre oxidation when compared to that of non-treated PP when exposed to dynamic distention and subsequently immersed in an oxidative stress environment.

Revealing Deformation Mechanisms in Polymer-Grafted Thermoplastic Elastomers via In Situ Small-Angle X-ray Scattering.

The tunable properties of thermoplastic elastomers (TPEs), through polymer chemistry manipulations, enable these technologically critical materials to be employed in a broad range of applications. The need to "dial-in" the mechanical properties and responses of TPEs generally requires the design and synthesis of new macromolecules. In these designs, TPEs with nonlinear macromolecular architectures outperform the mechanical properties of their linear copolymer counterparts, but the differences in the deformation mechanism providing enhanced performance are unknown. Here, in situ small-angle X-ray scattering (SAXS) measurements during uniaxial extension reveal distinct deformation mechanisms between a commercially available linear poly(styrene)-poly(butadiene)-poly(styrene) (SBS) triblock copolymer and the grafted SBS version containing grafted poly(styrene) (PS) chains from the poly(butadiene) (PBD) midblock. The neat SBS (φSBS = 100%) sample deforms congruently with the macroscopic dimensions, with the domain spacing between spheres increasing and decreasing along and transverse to the stretch direction, respectively. At high extensions, end segment pullout from the PS-rich domains is detected, which is indicated by a disordering of SBS. Conversely, the PS-grafted SBS that is 30 vol % SBS and 70% styrene (φSBS = 30%) exhibits a lamellar morphology, and in situ SAXS measurements reveal an unexpected deformation mechanism. During deformation, there are two simultaneous processes: significant lamellar domain rearrangement to preferentially orient the lamellae planes parallel to the stretch direction and crazing. The samples whiten at high strains as expected for crazing, which corresponds with the emergence of features in the 2D SAXS pattern during stretching consistent with fibril-like structures that bridge the voids in crazes. The significant domain rearrangement in the grafted copolymers is attributed to the new junctions formed across multiple PS domains by the grafting of a single chain. The in situ SAXS measurements provide insights into the enhanced mechanical properties of grafted copolymers that arise through improved physical cross-linking that leads to nanostructure domain reorientation for self-reinforcement and craze formation where fibrils help to strengthen the polymer.

Long-term stress crack resistance of HDPE fusion seams aged at 85 °C in synthetic leachate

The stress crack resistance (SCR) and standard oxidative induction time (Std-OIT) reduction of high- density polyethylene (HDPE) geomembrane (GMB) fusion seams is examined for a 12-year old 1.5 mm HDPE GMB when welded using nine welding parameter combinations. The GMB experienced a decrease in Std-OIT from 135 to 87 min between manufacturing and welding. Welding did not significantly decrease the Std-OIT in the heat-affected zone (HAZs). In general, individual seam SCR failure times fell between the notched and unnotched sheet, with adhered squeeze-out bead seams exhibiting individual failure times closer to the notched sheet and an average time until nominal failure ( tNF) ∼21%–35% shorter than the unnotched sheet. Differences in tNF are attributed to residual stresses and (or) accelerated craze formation in seams with adhered squeeze-out beads. Non-adhered squeeze-out seams exhibited a slower tNF similar to that of the unnotched sheet, indicating a seam tNF less than that of the sheet may not be intrinsic to all HDPE fusion seams. High sheet temperatures at the time of welding and higher heat-applied welding parameters are thought to increase the likelihood of squeeze-out bead adherence. This paper shows that squeeze-out adherence can have a deleterious effect on seam SCR.

Stress crack resistance of unaged high-density polyethylene geomembrane fusion seams

The stress crack resistance (SCR) of high density polyethylene (HDPE) geomembrane (GMB) fusion seams is examined for two 1.5 mm HDPE GMBs and a range of welding parameters. Results are reported for both unnotched and notched seams as well as their corresponding sheet material. Unnotched seam SCR specimens are shown to preferentially initiate craze formation at the terminating edge of the squeeze-out bead, while incorporating potentially degraded areas, such as the seams heat-affected zone (HAZ), within the slow crack growth region of the specimen. In the short-term, little variation was observed between the majority of seams for the nine welding parameter combinations examined, with an average normalized seam SCR value (normalized with respect to the unnotched sheet SCR) of 0.3 ± 0.1, or about 30% of the SCR of the unnotched sheet. It is shown that squeeze-out geometry plays an important role in the SCR of fusion seams. Seams with weld track rippling, a known qualitative indication of overheating, were found to have average unnotched SCR values 45% lower than smooth weld track seams. Deleterious squeeze-out geometries are identified to provide a framework through which CQA engineers and researchers can more readily identify ‘higher risk’ seams with respect to stress cracking.

Plastic litter fate and contaminant transport within the urban environment, photodegradation, fragmentation, and heavy metal uptake from storm runoff

A significant portion of urban litter is plastic which contaminates the environment and threatens ecological safety. The conversion of plastic litter into small fragments called microplastics (MPs) intensifies their critical risks by facilitating their transport and altering their physicochemical features. This study focuses on low density polyethylene (LDPE) and polyethylene terephthalate (PET) as the main components of urban litter. The photodegradation of LDPE and PET MPs due to the accelerated weathering experiments is investigated through surface chemistry and morphology analysis. The influence of MPs’ photodegradation on their fragmentation behavior is evaluated through the innovative accelerated mechanical weathering experiments that simulated the abrasion of MPs with the road deposits. Furthermore, the role of MPs as the vehicles to transport the heavy metals from the urban environment to the water resources is evaluated by studying the kinetics of lead (Pb) uptake by new and weathered MPs in synthetic stormwater. The surface morphology investigation revealed the formation of crazes and the crack networks onto the MPs due to the weathering experiments. The surface chemistry analysis revealed the generation of several oxidized carbon surface functional groups onto the photodegraded MPs and their increased susceptibility to fragmentation due to the abrasion with the road deposits. The photodegradation increased the Pb accumulation onto the LDPE and PET MPs from 467 μg/m2 and 21 μg/m2 to 2290 μg/m2 and 725 μg/m2, after five days of metal exposure. The fundamental knowledge developed in this research provides a better conceptual understanding of the mechanisms controlling MPs persistence and contaminant transport within the urban environment, which is crucial to estimate their negative impacts on the ecosystem.

Fracture mechanism and scratch behaviour of MWNTs reinforced 70/30 (wt/wt) PC/ABS blends and their nanocomposites

In this work, multiwalled carbon nanotubes (MWNTs) are dispersed in 70/30 (wt/wt) polycarbonate (PC)/acrylonitrile butadiene styrene (ABS) (PC70ABS30) blends and their nanocomposites by the melt-processing method. The morphology observations suggest the development of matrix-droplet morphology. There exists an interaction between MWNTs and the PC phase, which is observed from Raman spectroscopic investigation. Dynamic mechanical analysis (DMA) studies show the increase in storage modulus values depicting the stiffening effect of MWNTs, tanδ value increase depicting the energy dissipation. The flexural and impact properties increase significantly. The mechanism of flexural fracture is craze formation in the skin layer with a shear band and facets formation in the bulk region as observed from the SEM investigation of a flexural fractured surface of MWNTs reinforced PC70ABS30 blends and their nanocomposites. In addition, the deformation at the edge, river-like pattern, striation associated shear bands, craze, cavitation, and facets formation are the impact fracture mechanism as observed from SEM investigation of impact fractured surface of MWNTs reinforced PC70ABS30 blends and its nanocomposites. Scratch studies reveal the improved dispersion of MNNTs at 0.5–1 wt% in PC70ABS30 blends and their nanocomposites with a slightly higher scratch coefficient of friction and scratching force.

Effects of Coarse-Graining on Molecular Simulation of Craze Formation in Polymer Glass

Effects of Coarse-Graining on Molecular Simulation of Craze Formation in Polymer Glass

Opto-Thermo-Mechanical Characterization of Crazing Phenomena Observed for Isotactic Polypropylene Fibers Using Digital Photoelasticity and the Computed Tomography Techniques

Digital photoelasticity was used to characterize the crazing phenomena of isotactic polypropylene (iPP) fibers as a precursor of fracture processes. In this study, the effect of stretching at different rates on the formation of crazes was investigated for the iPP fibers. Also, the influence of drawing the iPP fibers at different temperatures on the formation of the crazes was studied. In addition, the effect of heating the crazed iPP fibers on the formed crazes was characterized using digital photoelasticity. Computed tomography was used to obtain qualitative and quantitative measurements of the crazes densities along the cross-sections of the iPP fibers. Also, the birefringence (Δn) values of the crazes were measured for different conditions of stretching and temperature and compared to the Δn of the bulk material. The photoelastic patterns of the iPP fibers are given for illustrating the crazes behavior in the iPP fibers.

Failure of fluorocarbon copolymer pipes subjected to mechanical strain in an alkaline environment

A failure analysis was carried out on leaking fluorocarbon-based copolymer pipes used in a cleaning and disinfection plant. Pipes conveyed a solution of sodium hydroxide and showed leaking after two months of service. Leaks originated from reddish-colored cuts located on the bending curves of the external surface of the pipes. Experimental findings and literature review pointed out that a combined effect of chemical degradation and mechanical strain (similar to stress corrosion cracking) triggered the failure. This type of degradation developed in high pH hydroxide solution due to dehydrofluorination of the exposed surface, starting material embrittlement and formation of crazes and microcracks. An applied mechanical deformation accelerated the crack opening and the solvent diffusion, up to the consequent pipe leaking and failure.

Well-Adhered Copper Nanocubes on Electrospun Polymeric Fibers.

Electrospun polymer fibers can be used as templates for the stabilization of metallic nanostructures, but metallic species and polymer macromolecules generally exhibit weak interfacial adhesion. We have investigated the adhesion of model copper nanocubes on chemically treated aligned electrospun polyacrylonitrile (PAN) fibers based on the introduction of interfacial shear strains through mechanical deformation. The composite structures were subjected to distinct macroscopic tensile strain levels of 7%, 11%, and 14%. The fibers exhibited peculiar deformation behaviors that underscored their disparate strain transfer mechanisms depending on fiber size; nanofibers exhibited multiple necking phenomena, while microfiber deformation proceeded through localized dilatation that resulted in craze (and microcrack) formation. The copper nanocubes exhibited strong adhesion on both fibrous structures at all strain levels tested. Raman spectroscopy suggests chemisorption as the main adhesion mechanism. The interfacial adhesion energy of Cu on these treated PAN nanofibers was estimated using the Gibbs–Wulff–Kaischew shape theory giving a first order approximation of about 1 J/m2. A lower bound for the system’s adhesion strength, based on limited measurements of interfacial separation between PAN and Cu using mechanically applied strain, is 0.48 J/m2.

Physical and Morphological Properties of Tough and Transparent PMMA-Based Blends Modified with Polyrotaxane.

We prepared several novel, tough, and transparent poly(methyl methacrylate) (PMMA) blends modified with polyrotaxane (PR) and evaluated their physical properties and morphologies. A styrene/methyl methacrylate/maleic anhydride (SMM) copolymer that was miscible with PMMA was used as a reactive compatibilizer to enhance interfacial adhesion between the matrix resin and PR. A twin-screw melt-kneading extruder was used to prepare the polymer blends, and their thermal, morphological, optical, and mechanical properties were characterized. The effect of PR was evaluated by analyzing the deformation behavior of the blends in notched three-point bending tests. A PMMA/PR blend was immiscible and appeared to be a phase-separated system. However, when SMM was added as a compatibilizer, PR was partially miscible and did not form observable PR domains. Viscosity increased, and the glass transition temperature (Tg) of the matrix resin decreased. The surface hardness of a PMMA/SMM/PR blend was only 15% lower than that of PMMA. A 2.5-fold increase in elongation at breakage was observed, and the tensile strength and Young’s modulus decreased by 16%. The PMMA/SMM/PR blend had 60% higher impact strength than PMMA in notched Charpy impact test, which indicated that the balance between stiffness and ductility was excellent. PR served as a starting point for plastic deformation in the PMMA/SMM/PR blend. We found that PR could initiate void and craze formation, even when it was finely dispersed at the nanoscale. The stress-relieving effect of PR was effective when it was tightly bound at the interfaces. The materials obtained in this study are expected to make a significant contribution to reducing the weight of the products by applying them as a replacement for glass.

Transient dynamics of cold‐rolled and subsequently thermally rejuvenated atactic‐polystyrene using broadband dielectric spectroscopy

AbstractThe effect of plastic deformation on the molecular dynamics of atactic polystyrene (a‐PS) was studied by broadband dielectric relaxation spectroscopy (BDRS), Fourier‐transform infrared spectroscopy (FTIR) and polarized‐light microscopy. Sheets of a‐PS have been subjected to cold rolling, that is, mechanical rejuvenation, followed by a quenching step and fast heating above its glass‐transition temperature, resulting in thermal rejuvenation. Cold rolling revealed, in addition to the known α‐ and γ(I)‐relaxations, four hitherto unknown relaxation processes (II, III, IV and V). Using the framework of craze formation and multiplicity of the glass transition (E. Donth, G. H. Michler, Colloid Polym. Sci. 1989, 267, 557–567), supported by an activation‐enthalpy/entropy analysis (Starkweather, W. Howard, Macromolecules 1981, 14, 1277–1281), the following physical picture emerges: (a) processes I and II represent local conformation transitions γ referring to chains of two different degrees of stretching (T/G‐ratio); and (ii) processes III and IV were identified as helix‐inversion processes of T2G2 helices as reported earlier for syndiotactic‐rich PS—an assignment supported by FTIR results. Finally, the relaxation V could be attributed to the onset of the fibrillar glass transition (within crazes), leading to stress release by collapse of the fibrils and hence dying out of process V. Polarized‐light microscopy confirmed the creation of oriented structures and internal stresses upon cold rolling, and their removal upon thermal rejuvenation.

Annealing effect on tensile property and hydrolytic degradation of biodegradable poly(lactic acid) reactive blend with poly(trimethylene terephthalate) by two-step blending procedure

A blending procedure utilizing a crystallized, tough, biodegradable poly(lactic acid) (PLA) blend with poly(trimethylene terephthalate) (PTT) and reactive compatibilizer, poly(ethylene-co-glycidyl methacrylate) (PEGMA), produced by two-step blending, was investigated in this research. The effects of the reactive compatibilizer and annealing temperature were investigated using an annealing PLA/PTT/PEGMA blend. The tensile properties of the annealed PLA/PTT/PEGMA blend were drastically improved compared with the annealed neat PLA and PLA/PTT blends. Craze formation was observed in the elongated area of the annealed PLA/PTT/PEGMA blend, and it was suggested to contribute to the toughening of the blend. The crystal's lack of spherulitic morphologies also contributed to the improved tensile properties, as confirmed by polarized optical microscopy (POM) and small-angle light scattering (SALS). The crystallization kinetics of the blends during annealing were also studied. Moreover, hydrolytic degradation of the blend could be controlled, after blending with PTT and PEGMA.

Slow crack growth resistance of modern PA-U12 grades measured by cyclic cracked round bar tests and strain hardening tests

A critical failure mechanism in long-term applications of plastic structures, such as piping systems, is considered to be crack initiation and subsequent Slow Crack Growth (SCG). Thus, safe installation and operation during service lifetime of such structures are not conceivable without the knowledge of the resistance against SCG for any new material, like Unplasticized PolyAmide 12 (PA-U12). Unfortunately, long-term static tests at elevated temperatures lead to unreasonably long test times and measurements may also be affected by thermal aging and hydrolysis. Against this backdrop, the current study examines two accelerated test methods, namely the cyclic Cracked Round Bar (CRB) test as well as the Strain Hardening (SH) test, in order to characterize the SCG behavior. Both were originally developed for PolyEthylene (PE) and successfully implemented in recent years. While the cyclic CRB test measures SCG directly, accelerated through cyclic loading, the SH test quantifies the molecular disentanglement resistance, which determines craze formation and breakdown. The focus of this work was initially put on the extension of the CRB test towards PA-U12 grades, checking the occurrence of actual crack initiation and propagation. Afterwards a correlation to SH test results was done in terms of SCG resistance. Thereby, adequate test conditions were developed and the influence of the Molecular Weight (MW), expressed by the Viscosity Number (VN) of the PA-U12 grades, was considered. Results confirm the suitability of each method to rank SCG resistance and show a good correlation of the abovementioned SMall scale Accelerated Reliable Test (SMART) methods, highlighting their sensitivity to long-term relevant molecular parameters.

Synthesis and micromechanical properties of graphene oxide-based polymer nanocomposites

The authors have studied the effect of small (≤ 1 wt%) additions of thermally reduced graphene oxide on the microhardness and microindentation kinetics in two types of polymers: polystyrene (i.e. thermoplastic with a glass transition temperature of Tg ≈ 373 K) and polyester resin (i.e. thermosetting plastic, Tg ≈ 300 K). The room temperature creep of nanocomposites under an indenter is described using a three-element rheological Kelvin–Voigt model. The study determines the parameters of this model and how graphene oxide (GO) affects them. In a polystyrene nanocomposite with 0.3 wt % graphene oxide, unrelaxed and relaxed elastic moduli, and the modulus characterizing high-elastic deformation, increase by 11%, 40% and 87%, respectively, as compared to the initial polystyrene; at the same time, microhardness increases by 38% and 45% for the different series of samples. The results obtained indicate that the presence of graphene oxide in the nanocomposite severely restricts the mobility of molecular segments. The addition of 0.3 wt% graphene oxide to polyester resin is accompanied by an increase in the mechanical glass transition temperature of the resin by at least 5 K. This leads to a change in the relaxation state of this polymer: while at room temperature the polyester resin behaves like an elastomer, a polyester resin nanocomposite with 0.3 wt% graphene oxide exhibits glassy properties. At room temperature, the microhardness of polyester resin-glass fabric-graphene oxide nanocomposites with a GO content of 0.5 and 1 wt% increases by 20% and 80% respectively, as compared to that of a polyester resin-glass fabric composite. The authors have obtained the temperature dependences of the microhardness of nanocomposites with a polyester matrix in the range 77–298 K, and have also identified temperature regions where the microdeformation of composites is reversible, which is associated with the formation of crazes with a lower glass transition temperature.

Energy dissipation characteristics of additively manufactured CNT/ABS nanocomposites

PurposeThis study aims to discuss the effect of carbon nanotubes (CNTs) on the mechanical properties of acrylonitrile–butadiene–styrene (ABS) composites fabricated by additive manufacturing (AM). Insight into the energy-dissipation mechanisms introduced and/or enhanced by the addition of CNTs is presented in this study.Design/methodology/approachABS/CNT filaments were fabricated with different concentrations of CNTs. Using a fused deposition modeling approach, unidirectional specimens were printed using a MakerBot Replicator 2X (MakerBot Industries, Brooklyn, NY, USA). Specimens were tested under static and dynamic conditions, with the loading coinciding with the printing direction, to determine elastic modulus, strength and viscoelastic properties.FindingsA CNT reinforcing effect is evident in a 37 per cent increase in elastic modulus. Likewise, the strength of the composite increases by up to 30 per cent with an increase in weight fraction of CNTs. At low dynamic strain amplitudes (0.05 per cent), a correlation between dissipated strain energy of the butadiene phase and strength of the composite is found such that less dissipation, from constraint of the butadiene particles by the CNTs, leads to higher strength of the composite. At higher dynamic strains, the presence of a high concentration of CNT leads to increased energy dissipation, with a maximum measured value of 24 per cent higher loss factor compared to baseline specimens. Because the trend of the composite behavior is similar (with a higher absolute value) to that of neat ABS, this study’s results indicate that well-established polymer/CNT dissipation mechanisms (such as stick-slip) are not significant, but that the CNTs amplify the dissipation of the ABS matrix by formation of crazes through stress concentrations.Originality/valueThis study provides knowledge of the dissipation behavior in additively manufactured ABS/CNT composites and provides insight into the expansion to new printable materials for dynamics applications.

Low-Temperature Features of the Micromechanical Properties of Polystyrene

The microindentation of thin polystyrene plates, which is an amorphous linear thermoplastic polymer with a glass transition temperature Tg ≈ 373 K, was carried out in the 77–295 K temperature range. A decrease in the temperature from 295 K to 140 K generated a two-fold linear increase in microhardness, which is in good agreement with the temperature dependence of the forced elasticity limit of polystyrene measured by other authors during uniaxial compression deformation. At temperatures of T = 77–140 K the local deformation of polystyrene was reversible: during heating to room temperature the impressions on the sample surface disappeared completely. Partial deformation reversibility was observed upon indentation in the temperature range of 140–190 K, but at indentation temperatures T > 190 K the impressions remained. The microstrain reversibility of polystyrene in nitrogen at, and close to, 77 K is due to the effect of the adsorption-active medium on the formation of crazes (fibrillar-porous nanostructures), the glass transition temperature of which is tens of degrees below that of bulk polymer.