Effect of cyclic loading on tensile properties of PC and PC/ABS

The effects of cyclic loading on tensile fracture properties of polycarbonate (PC) and the alloy of polycarbonate and acrylonitrile-butadiene-styrene (PC/ABS) are experimentally investigated in the paper. Two digital cameras are used to record simultaneously the tensile deformation of specimens and the large deformation and the necking process of these polymers are discussed. Two lateral contractions are not identical at the later tensile stages and the contraction ratios in each lateral direction are related with the tensile strains in axial direction on width and thickness surface. The curvature radiuses at the minimum section during necking process are shown. The volume increases during necking process and then decreases gradually. The yield stress and fracture stress of PC/ABS are lower than that of PC. The degradation of the fracture stress and fracture strain due to the application of cyclic loading is larger for PC than that for PC/ABS, and these can be used to explain qualitatively why PC has higher fatigue crack growth rate than PC/ABS.     

Experimental observation on multiaxial ratchetting of polycarbonate polymer at room temperature

Multiaxial stress-controlled and mixed stress-strain-controlled cyclic tests were carried out to investigate the multiaxial ratchetting of polycarbonate (PC) polymer at room temperature. The effects of applied mean stress, stress amplitude, loading rate, loading path and loading history on the ratchetting are discussed. The results show that the multiaxial ratchetting mainly occurs in the direction of non-zero mean stress. In the multiaxial stress-controlled cases, the ratchetting strain increases with increasing mean stress and stress amplitude and decreasing stress rate. Different values of ratchetting strain were obtained in the multiaxial cyclic tests with seven different loading paths, and prior cyclic loading with higher stress level resulted in decreased ratchetting in the subsequent cyclic loading with lower stress level. In the multiaxial mixed stress-strain-controlled tests, the ratchetting increased with increasing axial (or equivalent shear) stress and torsional-angle (or axial-displacement) amplitude and decreasing applied deformation rate.     

Accelerated ratcheting testing of polycarbonate using the time-temperature-stress equivalence method

The long-term performance of polymers under cyclic loading is important for safety assessments in engineering applications. The deformation process under the cyclic loading can be accelerated through use of temperature and stress. As for asymmetric cyclic loading, so called ratcheting, a time-temperature-stress (TTS) equivalence method in which all the parameters have clear physical meanings and can be determined experimentally, was proposed to predict the long-term cyclic loading behavior for polycarbonate using short-term data. Taking into consideration the effects of both the mean stress and the stress amplitude, the ratcheting compliance was defined and its evolution function was also provided. Next, the TTS equivalence method was validated using the long-term ratcheting test results for the polycarbonate. Time, temperature, and stress do show equivalent effects on long-term ratcheting of polycarbonate. Using the proposed method, time and cost can be dramatically saved for the assessment of the long-term cyclic loading performance of polycarbonate.     

Experimental investigation and modeling of the mechanical behavior of PC/ABS during monotonic and cyclic loading

The purpose of this work is to characterize the mechanical behavior of blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene (ABS) during monotonic and cyclic loading. Compression experiments were performed using a SHIMADZU universal testing machine (10⁻⁴ to 10⁻² s⁻¹) and a split Hopkinson pressure bar (1600–5000 s⁻¹), with, the test temperatures ranging from 293 to 353 K. The influence of the rate and temperature on the deformation of PC/ABS is discussed in detail. Based on the investigation of numerous constitutive models, a phenomenological model called DSGZ was chosen to describe the compression behavior of PC/ABS. This model could not accurately reproduce the deformation of polymers at high strain rates when utilizing the same material coefficients for the low and high strain–rate deformations. In addition, this model was unable to capture the deformation features during unloading and subsequent reloading when adopting the original stress–strain updating algorithm. Hence, some improvements to the model have been implemented to better predict the deformation. Finally, the model predictions are shown to be consistent with the experimental results.     

Hydrophobic modification of polycarbonate for reproducible and stable formation of biocompatible microparticles

We propose a simple and effective scheme for the modification of the walls of microfluidic channels fabricated in polycarbonate (PC) after the device has been bonded. The method prevents both static and dynamic wetting of PC by aqueous solutions including viscous, non-Newtonian solutions of polymers as e.g. alginate. The procedure uses dodecylamine, which readily reacts with the carbonate groups of PC to produce a hydrophobic surface. We characterize the dependence of the contact angles and homogeneity of the modified surfaces on the time, temperature, and concentration-all important parameters-of the reaction and provide optimal conditions for the process.     

Analysis of the cyclic tensile behaviour of an elasto-viscoplastic polyamide

The present paper is concerned with the experimental and theoretical investigation of the progressive accumulation of inelastic deformation observed in cyclic tension tests performed on a particular polyamide. The elastic properties are not strongly affected by the strain rate, but the strain hardening induced by the plastic deformation is rate-dependent. Thus, the material behaviour is elasto-viscoplastic rather than viscoelastic or elasto-plastic. For the polymer studied in this paper, the kinematic hardening is much more significant than the isotropic hardening, and a negative plastic strain rate may occur even with a positive stress. The kinematic hardening is strongly dependent, not only on the accumulated plastic strain, but also on the loading rate. An elasto-viscoplastic mechanical model able to describe the cyclic inelastic behaviour for an arbitrary loading history is proposed. All parameters that arise in the theory are identified experimentally. The preliminary theoretical results concerning the modelling of cyclic load-unload tests are in good agreement with the experiments.     

Experimental investigation on electro-mechanically coupled cyclic deformation of laterally constrained dielectric elastomer

A series of electro-mechanically coupled cyclic tests at large deformation are carried out to characterize the cyclic deformation of a laterally constrained dielectric elastomer (DE) in this work. In the strain-controlled cyclic tests of the dielectric elastomer (e.g., VHB 4910 DE) with a constant or cyclic voltage, cyclic stress softening occurs and is influenced by the phase-angle difference between the applied cyclic strain and cyclic voltage. In the stress-controlled cyclic tests of VHB 4910 DE with a constant or cyclic voltage, ratchetting (a cyclic accumulation of inelastic strain) takes place; the ratchetting strain is considerably enhanced by applying higher voltage level, higher stress level and lower stress rate, and is also affected by the phase-angle difference between the applied cyclic stress and cyclic voltage. Moreover, the remarkable recovery of residual strain after the cyclic tests demonstrates that the cyclic stress softening and ratchetting of VHB 4910 DE mainly stem from the viscoelasticity. The comprehensive experimental observations are very useful to develop, calibrate and validate an electro-mechanically coupled constitutive model of dielectric elastomers.     

Self‐recovery and fatigue of double‐network gels with permanent and reversible bonds

Double‐network (DN) gels subjected to cyclic deformation (stretching up to a fixed strain followed by retraction down to the zero stress) demonstrate a monotonic decrease in strain with time (self‐recovery). Observations show that the duration of total recovery varies in a wide interval (from a few minutes to several days depending on composition of the gel), and this time is strongly affected by deformation history. A model is developed for the kinetics of self‐recovery. Its ability to describe stress–strain diagrams in cyclic tests with various periods of recovery is confirmed by comparison with observations on several DN gels. Numerical simulation reveals pronounced enhancement of fatigue resistance in multi‐cycle tests with stress‐ and strain‐controlled programs when subsequent cycles of deformation are interrupted by intervals of recovery. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 438–453     

Shear band formation and mode II fracture of polymeric glasses

Mode I and II fracture studies were performed from quasistatic to low velocity impact rates on polymethyl methacrylate (PMMA) and polycarbonate (PC). Mode II tests used an angled double‐edge notched specimen loaded in compression. The shear banding response of PMMA is shown to be highly sensitive to rate, with diffuse shear bands forming at low rates and sharp distinct shear bands forming at high rates. As the rate increases, shear deformation becomes more localized to the point where Mode II fracture occurs. PC is much less rate dependent and stable shear band propagation is observed over the range of rates studied with lesser amounts of localization. A new theory is formulated relating orientation in a shear band to intrinsic material properties obtained from true‐stress true‐strain tests. In a qualitative sense the theory predicts the high rate sensitivity of PMMA. A kinematic limit for orientation within a shear band is also derived based on entanglement network parameters. Mode II fracture in PMMA is shown to occur at this kinematic limit. For the case of PC, the maximum impact rates were not high enough to reach the kinematic limit. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Cyclic stress–strain behavior of the dry polycarbonate craze

Equipment and methods have been developed which allow photomicrographic determination of the stress–strain properties of the individual craze. Serial cyclic tensile tests on polycarbonate crazes are described. Under stress the typical dry polycarbonate craze thickens solely by straining; no adjacent polymer of normal density is converted to craze material. The craze exhibits a yield stress followed by a recoverable flow to roughly 40–50% strain at 6000–8000 psi. On return to zero stress the craze exhibits creep recovery at a decelerating rate. The yield stress and loss factor of each cycle decrease with increasing initial strain and cycles initiating at 50% strain or more show completely Hookean behavior. Creep recovery results in recovery of yield stress and loss factor also. Craze tensile behavior is suggested to be essentially an extension of the craze formation process. Decrease in elastic modulus and yield stress with increasing strain are rationalized in terms of strain-produced decrease in density and resultant increase in stress concentration factor on the microscopic polymer elements of the craze. Polymer surface tension and the large internal specific surface area of the craze are suggested to be important factors in the large creep recovery rates of the craze.     

Temperature-dependent ratcheting of PTFE gaskets under cyclic compressive loads with small stress amplitude

Uniaxial stress-controlled ratcheting experiments on PTFE gaskets under cyclic compressive loads with small stress amplitude were performed. The effect of temperature on the deformation behavior was considered. Results showed that the compressive modulus decreases rapidly when the temperature increases from 100 °C to 200 °C. Compressive ratcheting deformation with cycles increase significantly with the increases of temperature. The ratcheting deformations at 100 °C, 150 °C and 200 °C are nearly two, three and five times that at room temperature, respectively. Most of ratcheting deformation mainly occurs during the first 20 cycles because the subsequent ratcheting rate and strain range are small and much less than those in the previous cycles. The accumulated deformation under cyclic loads with small stress amplitude is relatively approach to the static compressive creep with the same peak stress. Therefore, the accumulated deformation with time of PTFE gaskets obtained by cyclic compression with small stress amplitude can be estimated by the corresponding static creep deformation with good accuracy under the approximate stress rate and the same temperature, especially at room temperature.     

The influence of sub-Tg annealing on environmental stress cracking resistance of polycarbonate

To explore the effect of physical aging on environmental stress cracking (ESC) behavior of polycarbonate (PC), sub-T_g annealing was utilized as a method for accelerated aging. Injection molded samples were annealed at 130 °C for different time varying up to 96 h. A three point bending apparatus was used to evaluate critical stress for crazing and to record the variation of stress with immersion time at constant strain. The ESC results indicated that the critical stress for crazing initiation of PC in ethanol is increased by sub-T_g annealing. However, the resistance of annealed PC to ESC with immersion time during the stress relaxation test depends on the level of initial stress. When a relatively low initial stress was used, a short time (24 h) of sub-T_g annealing reduced the stress relaxation rate and decreased the number of cracks on the surface of PC. However, under higher initial stress, the stress relaxation rate of PC had a slight change only when the annealing time was prolonged about threefold (72 h). This can be explained by the formation of cohesional entanglement sites during the sub-T_g annealing process, which was demonstrated by the thermal and dynamic mechanical tests.     

Fracture toughness of gamma irradiated polycarbonate sheet using the essential work of fracture

Polycarbonate (PC), a ductile polymer, has been found by both linear elastic fracture mechanics and impact tests to present a ductile-brittle transition, which depends on notched specimen thickness, test speed and gamma irradiation. Owing to large amounts of plastic deformation, fracture toughness measurements by these test methods are not precise. In the present communication, a better method, the Essential Work of Fracture (EWF), to assess the fracture characteristics in plane state of stress was for the first time used to evaluate the fracture toughness of PC sheets subjected to gamma irradiation dose. Three-points bend tests of sharp pre-cracked specimens with different ligament lengths were 340 kGy gamma irradiated. EWF results showed that the total fracture work increased linearly with length for both non-irradiated and gamma irradiated conditions. A significant decrease in EWF fracture toughness was associated with brittleness promoted by gamma irradiation. This brittleness was also confirmed by macro and microscopy (SEM) evidence.     

Kinetics of re‐embrittlement of (anti)plasticized glassy polymers after mechanical rejuvenation

Mechanical rejuvenation is known to dramatically alter the deformation behavior of amorphous polymers. Polystyrene (PS)—for example, typically known as a brittle polymer—can be rendered ductile by this treatment, while a ductile polymer like polycarbonate (PC) shows no necking anymore and deforms homogeneously in tensile deformation. The effects are only of temporary nature, as because of physical aging the increasing yield stress, accompanied by intrinsic strain softening, renders PS brittle after a few hours, while for PC necking in tensile testing returns in a few months after the mechanical rejuvenation treatment. In this study, it is found that physical aging upon rejuvenation in both PS and PC can be delayed in two different ways: (1) by reducing the molecular mobility through antiplasticization and (2) by applying toughening agents (rubbery core–shell particles). For the first route, even though progressive aging is found to decrease with increasing amounts of antiplasticizer added, dilution of the entanglement network results in enhanced brittleness. Besides antiplasticization effects, also some typical plasticization effects are observed, like a reduction in matrix T_g. For the second route, traditional rubber toughening using acrylate core–shell modifiers also results in a reduced yield stress recovery, and ductile tensile deformation behavior is observed even 42 months after mechanical rejuvenation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 134–147, 2008     

Miscibility and cure kinetics studies on blends of bisphenol-A polycarbonate and tetraglycidyl-4,4′-diaminodiphenylmethane epoxy cured with an amine

Differential scanning calorimetry (DSC) has been applied to characterize the glass transition behavior of the blends formed by bisphenol-A polycarbonate (PC) with a tetrafunctional epoxy (tetraglycidyl-4,4′-diaminodiphenyl methane, TGDDM) cured with 4,4′-diaminodiphenylsulphone (DDS). A rare miscibility in the complete composition range has been demonstrated in these blends. Additionally, the blend morphology was examined using scanning electron microscopy (SEM) and a homogeneous single-phase PC/epoxy network has been observed in the blends of all compositions. Moreover, polycarbonate incorporation has been found to exert a distinct effect on the cure behavior of the epoxy blends. The cure reaction rates for the epoxy-PC blends were significantly higher due to the presence of PC. In addition, the cure mechanism of the epoxy blends was no longer autocatalytic. An n-th order reaction mechanism with n = 1.2 to 1.5 has been observed for the blends of DDS-cured epoxy with PC of various compositions studied using DSC. The proposed n-th order kinetic model has been found to describe well the cure behavior of the epoxy/PC blends up to the vitrification point. © 1995 John Wiley & Sons, Inc.     

Inverse relaxation-model and relation to recovery internal friction

Inverse relaxation is studied for hard elastic polypropylene (HEPP), rubber and non-elastic polypropylene. The results show that contractive stress, stress, and internal friction are three essential factors related to the phenomenon. A three-element model in which each element has a definite meaning is proposed to describe this phenomenon. The results also show that, in the first cyclic deformation, relaxation time increases with the increase of recovery for all the materials, which indicates that recovery viscosity increases with the increase of recovery, but the stress rising amplitude (SRA) of inverse relaxation has a maximum in the recovery range. Analysis indicates that SRA equals recovery internal friction (RIF) for ideal material in which stress is solely a function of strain, independent of paths, and approximately equals RIF for non-ideal material at a given strain. From this principle it is found that the order of the work counteracted by RIF for the four materials is the same as that of their second hysteresis loop, and the RIF of HEPP has a sudden increase at the later recovery range.     

Effect of a novel flame retardant containing silicon and nitrogen on the thermal stability and flame retardancy of polycarbonate

A novel flame retardant (PSiN), containing silicon and nitrogen, was synthesized using N-(β-aminoethyl)-γ-aminopropylmethyldimethoxysilane and diphenylsilanediol through solution polycondensation and it was added to polycarbonate (PC). The structure and thermal properties of PSiN were characterized by fourier transform infrared spectroscopy and thermogravimetric analysis (TG) tests. The effect of PSiN on the flame retardancy and thermal behaviors of PC was investigated by limited oxygen index (LOI), vertical burning test (UL-94), and TG tests. The results showed that the flame retardancy and the thermal stability of PC are improved with the addition of PSiN. When 1 mass% PSiN and 0.5 mass% diphenylsulfone sulfonate (KSS) are incorporated, the LOI value of PC is found to be 46, and class V-0 of UL-94 test is passed. The char structure observed by scanning electron microscopy indicated that the surface of the char for PC/KSS/PSiN system holds a firmer and denser char structure when compared with neat PC and PC/KSS system.     

Miscibility and deformation behavior in some thermoplastic polymer blends containing poly(styrene-co-acrylonitrile)

Miscibility in blends of poly(styrene-co-acrylonitrile) (PSAN) with several other polymeric components has been investigated over a range of compositions by means of thermal analysis and transmission electron microscopy. Systems in vestigated were (i) PSAN/polycarbonate (PC), (ii) PSAN/styrene-maleic anhydride-methyl methacrylate terpolymer (S/MA/MM), (iii) PSAN/polynorbornene nitrile (PNN), and (iv) PSAN//S/MA/MM//PC. PSAN/PC was demonstrated to be partially miscible in all proportions over the PSAN copolymer composition range 23–70 wt % AN, while the miscibility or lack thereof of PSAN//S/MA/MM depended on the relative AN and MA contents of the PSAN and S/MA/MM, respectively. In contrast, PSAN/PNN was found to be immiscible in all proporations, while the system PSAN//S/MA/MM//PC was shown to be partially miscible. Deformation studies performed on rubber-modified versions of these blends defined deformation mode and microstructural deformation behavior. Dual extensometer tensile testing yielded relative contributions of crazing and of plastic flow, which correlated both with blend composition and with toughness. TEM observations of deformed specimens indicated a deformation process in the multiphase matrix blends consisting of craze initiation and propagation in the rubber-containing phase, craze arresting in the ductile second matrix phase, and coordinated extensive deformation of the matrix phases and of the rubber particles, where the ability to support the latter coordinated forms of deformation were observed to increase with increasing proportion of plastically deforming phase.     

Flame retardancy and thermal properties of solid bisphenol A bis(diphenyl phosphate) combined with montmorillonite in polycarbonate

A series of flame retardant formulations of solid bisphenol A bis(diphenyl phosphate) (S-BDP) and organo-montmorillonite (OMMT) were prepared based on polycarbonate (PC) by a melt compounding procedure. OMMT was well dispersed into the matrix showing an intercalated-exfoliated morphology. S-BDP and OMMT exhibit a synergistic effect in the vertical burning test (UL-94) but an antagonistic effect in the limiting oxygen index (LOI) evaluation. Thermogravimetric analysis (TGA) of the flame retarded PC system both under nitrogen and air was performed. Migration of S-BDP and OMMT towards the surface occurs during combustion. The introduction of OMMT could especially enhance the thermal-oxidative stability of the material, which is further confirmed by the analysis of the char residues by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS).     

Elongational rheometry of polyethylene terephthalate/bisphenol-a polycarbonate blends

The elongational deformation properties of polyethylene terephthalate (PETP) and bisphenol-A polycarbonate (PC) blends were determined using a Rutherford elongational rheometer. The effects of temperature (in the thermoelastic region) and strain rate were studied on blends containing up to 50% PC. Addition of low levels of polycarbonate permits the thermoelastic processing of PETP over a wider temperature range. PETP is particularly sensitive to changes in temperature. Over the range studied, the effect of strain rate on elongational deformation is not very marked. However, the deformation temperature changes the strain levels at which strain stiffening occurs, an important observation in respect of processing and optimisation of physical properties of uniaxially oriented products. The effect of the addition of a phenoxy resin on the blend was studied, and although TEM analysis suggests that it did not compatibilise the blend, it did increase the available extension at higher temperatures.