On the phenomenology of yield in bisphenol-a polycarbonate

The phenomenology of yield in bisphenol-A polycarbonate is explored through tensile tests on thin rectangular specimens and through pressure-induced bulging of thin, clamped circular disks. In a tensile test, while the nominal critical stress at which yield initiates and the nominal draw stress at which a stable neck propagates along a specimen depend on the temperature and the strain rate, the ratio of the draw stress to the critical stress is shown to be approximately 0.75 over a temperature range of 22 to 65°C and strain-rates in the range of 10^?4 to 10⁰ s^?1. Specimens subjected to constant tensile loads between the draw and critical stresses are shown first to creep till stretches on the order of 1.06 are attained and then are shown to undergo stable necking. Tensile tests on thin, wide rectangular specimens show that yielding initiates through shear bands that broaden and intersect to generate necks, which subsequently propagate along the specimen. In pressure-induced bulging of clamped disks, biaxial stretching progresses monotonically under increasing pressures; strain localization does occur near the outer edges of the specimens, however. Heating of a specimen with a substantial stably necked region shows that the temperature-induced recovery of the specimen from its deformed state begins well below the transition temperature T_g of the material, although most of the recovery occurs at T_g.     

Effect of notches and glass fiber reinforcement on fatigue behavior of polycarbonate

In order to investigate the effect of a notch on the tensile properties of polycarbonate and 30% glass fiber-reinforced polycarbonate, two types of notched specimens were prepared. These notches were a sharp 60° notch and a dull notch with rounded tip 1.5 mm in radius at the base of the 60° notch. The notches decreased the tensile strength of polycarbonate. The sharp notch reduced tensile strength more effectively than the dull notch. In 30% glass fiber-reinforced polycarbonate, even the dull notch decreased the tensile strength considerably. Unnotched polycarbonate was subjected to cyclic tensile loading of 10⁴ cycles at 10 Hz, with varying cyclic stress. It was found that the elongation at break decreased rapidly with increase in cyclic stress. The notches considerably decreased the tensile fatigue strengths of polycarbonate and glass fiber-reinforced polycarbonate in 10⁴ cycles at 10 Hz.     

Effect of gas diffusion on creep behavior of polycarbonate

Biaxial tension-tension creep experiments were performed to study the effect of gas diffusion on creep behavior of polycarbonate. Experiments were conducted on a thin-walled tubular specimen by applying both gas pressure and axial tension at room temperature, and measuring axial strain and gas absorption. Experiments were performed with helium, nitrogen, air, carbon dioxide and Freon-22

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. It was found that the creep deformation was highly affected by the solubility controlled gases, carbon dioxide and Freon. The absorption measurements showed that the gas absorption behavior was clearly affected by the creep deformation of the material. Also an anomalous diffusion-type behavior appeared even in permanent gas-polymer systems under creep conditions. The modified superposition principle satisfactorily predicted the recovery following creep. The axial creep for the biaxial tension-tension experiments was also computed from data previously reported for creep under combined tension and torsion of a different sample of polycarbonate.     

Constitutive modeling of polycarbonate during high strain rate deformation

A constitutive model is presented for large strain deformation of polycarbonate (PC) at high strain rates (above 10² s^?1). The proposed model considers the primary process (α) and the two secondary rate‐activated processes (β and γ). It is shown that the secondary transitions in the material affect the yield and post yield behavior of the material at high strain rates. The constitutive model has been implemented numerically into a commercial finite element code through a user material subroutine. The experimental results, obtained using a split Hopkinson pressure bar, are supported by dynamic mechanical thermal analysis (DMTA) and DSR (Decompose/Shift/Reconstruct) method. These are employed to gain understanding of the material transitions, and to further the linkages between material viscoelastic, yield, and stress–strain behavior. Comparison of model predictions with experimental data demonstrates the ability of model to capture the characteristic features of stress–strain curve of the material such as initial linear elasticity, global yield, strain softening, and strain hardening at very high strain rates (up to 10,000 s^?1). POLYM. ENG. SCI. 2013. © 2012 Society of Plastics Engineers     

Solid‐state microcellular polycarbonate foams. II. The effect of cell size on tensile properties

The effect of cell size on the tensile behavior of high‐relative‐density microcellular polycarbonate foams is investigated. Microcellular PC foams were produced in a way that allowed the average cell size to be varied, while the foam density was held constant. The polycarbonate‐CO₂ system offers an order of magnitude variation in the average cell size at a given density, allowing the tensile properties of microcellular polycarbonate to be investigated as a function of cell size. It was found that the tensile modulus, tensile strength, elongation to break, and toughness are not significantly affected when the average cell size is varied from 2.8 to 37.1 μm, and the nominal relative density is held constant at 0.5. This result is significant for solid‐state processing of microcellular polycarbonate foams of the type produced here, for it shows that regardless of the processing conditions and regardless of the average cell size, if two foams have the same density then they will also have the same tensile properties. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Prediction of long‐term service performance of polymeric materials from short‐term tests: Creep and prediction of the stress shift factor of a longitudinal polymer liquid crystal

The material studied is a longitudinal polymer liquid crystal (PLC). The creep behavior of the PLC is examined in the region of nonlinear viscoelasticity. The creep compliance D curves at nine different stress σ levels, from 10 to 50 J.cm^?3 at a constant temperature are determined and shifted along the log time axis for σ_ref = 10 J · cm^?3 to produce the D versus t/a_σ master curve. A fairly general formula for stress shift factor a_σ based on free volume v^f and the chain relaxation capability (CRC) derived by one of the authors is applied. The formula predicts values that agree with the experimental ones within the limits of the experimental accuracy. Thus, experiments at several stress levels can serve for prediction of long‐term behavior from short‐term tests. The same value of the Doolittle constant B is obtained separately from temperature shift and stress shift experiments for the PLC.     

Melt viscosity and flow birefringence of polycarbonate

Melt viscosity and flow birefringence of bisphenol A-type polycarbonate were measured and analyzed by the application of rubber-like photoelastic theory. The melt viscosity in the Newtonian flow region increased with the molecular weight to the power of 3.4. In polycarbonate, the shear stress of the Newtonian flow region was to 10⁶ dyn/cm², whereas in PMMA it was at most 3 = 10⁵ dyn/cm². The flow birefringence δn has a linear relation with shear stress S, that is δn = 5.7 × 10^?10 S. The principal polarization difference of flow unit α₁ – α₂ was 1.62 × 10^?23 cm³, which was obtained by the application of the rubber-like elastic theory. In PMMA, it was 3.9 = 10^?25 cm³; about 1/40 of that was polycarbonate. The anisotropy of polarizability of the flow unit of polycarbonate was also about 40 times larger than that of PMMA. So the anisotropy reflected the large flow birefringence of the polycarbonate.     

A comparison of constant and complex creep loading programs for several thermoplastics

Polymethyl methacrylate, polyacetal and polypropylene samples were subjected to constant-load uniaxial creep in tension and compression up to 3% strain in times up to 10⁵ sec. A higher creep resistance was obtained in compression compared with tension due to the influence of free volume on mobility. Square wave cyclic creep tests alternately in tension and equal compression for dwell times of 10, 100, 1000 sec were also conducted on each material up to a total time of 10⁵ sec. Under low cyclic creep stresses tension creep cancelled out compression creep in each successive half cycle so that there was no overall increase in creep strain as cycling proceeded. The unidirectional creep data was used successfully to predict this behavior. At higher cyclic stress levels creep strain range increased steadily throughout the test indicating a softening process. A few tests were conducted in unidirectional and cyclic stress relaxation on polymethyl methacrylate.     

DC conductivity and dielectric response in amorphous polycarbonate of Bisphenol-A

We report the dielectric response measurements of amorphous polycarbonate of Bisphenol-A at room temperature in the frequency range from 12 to 10⁵ Hz. The data is analyzed within the graphical representation of Wei and Sridhar. Such an approach has allowed us to estimate the DC conductivity value of the order of 5 × 10^?11 S/m in good agreement with the direct DC measurements.     

Thermal and strain rate sensitive compressive behavior of polycarbonate polymer - experimental and constitutive analysis

The mechanical behavior of polycarbonate (PC) polymer was investigated under the effect of various temperatures and strain rates. Characterization of polymer was carried out through uniaxial compression tests and split Hopkinson pressure bar (SHPB) dynamic tests for low and high strain rates respectively. The experiments were performed for strain rates varying from 10 ^?3 to 10³ and temperature range of 213 to 393 K. By conducting these experiments, the true stress–strain (SS) curves were obtained at different temperatures and strain rates. The results from experiments reveal that the stress–strain behavior of polycarbonates is different at lower and higher strain rates. At higher strain rate, the polymer yields at higher yield stress compared to that at low strain rate. At lower strain rate, the yield stress of the polymer increases with the increase in strain rate while it decreases significantly with the increase of temperature. Likewise, initial elastic modulus, yield and flow stress increase with the increase in strain rate while decreases with the increase in temperature. The yield stress increases significantly for low temperature and higher strain rates. On the basis of experimental findings, a phenomenological constitutive model was employed to capture the mechanical behavior of polymer under temperature and loading rate variations. The model predicted the yield stress of polymer at varying strain rate and temperature also it successfully predicted the compressive behavior of polymer under entire range of deformation.     

Damage and the nonlinear viscoelastic response of glassy polycarbonate and LaRC-TPI

In the bulk, the nonlinear viscoelastic response of glassy polymers is due to the irreversible work done on the body by the surroundings. The source of the irreversibility is plastic flow of material near distributed shear bands or microcracks in the polymer. Shear bands and microcracks also form new traction free boundaries in the body. The presence of these new boundaries diminish the load bearing capacity of the polymer. These changes in polymer lattice structure are a mechanism that promotes the release of stored strain energy. If the release of stored strain energy is stress controlled, then at sufficiently high levels of stress to cause a permanent structural arrangement of the polymer chains, polycarbonate and LaRC-TPI behave as nonlinear viscoelastic materials. If the current stress is less than the maximum stress the polymer has experienced, then the current energy release rate for the propagation of shear bands, crazes, etc., is less than the critical energy release rate. In this instance, damage production is a constant and the material can be modeled using linear viscoelasticity. It will be shown that the stress-induced nonlinear shift factors are a measure of the rate of damage production in glassy polycarbonate.     

Continuous extrusion of microcellular polycarbonate

Extruded microcellular foams have been obtained from mixtures of polycarbonate (PC) and n‐pentane. Cell diameters were in the range of 2 to 5 μm and the foam densities varied between 400 and 700 kg/m³. Although two types of PC have been investigated, one linear and one branched, the presence of side branchings did not modify the extruded foam characteristics. Use of carbon dioxide as the blowing agent was also attempted, and cell sizes below 10 μm have been successfully obtained. One prerequisite for microcellular foaming was believed to consist in a concentration of the blowing agent close to its limit of solubility as that defined under the actual processing conditions of pressure and temperature. This hypothesis was validated from the observation of extrusion of regular PC foams (intermediate to low densities and cell sizes ranging between 100 μm and 1 mm) using moderate concentrations of blowing agents, and from solubility and viscosity measurements on similar polymer/blowing agent systems.     

Supercritical CO2 and polycarbonate based nanocomposites: A critical issue for foaming

Supercritical carbon dioxide readily induced foaming of various polymers. In that context, supercritical CO₂ was applied to carbon nanotubes based polycarbonate nanocomposites to ensure their foaming. Surprisingly, efficient foaming only occurs when low pressure is applied while at high pressure, no expansion of the samples was observed. This is related to the ability of supercritical carbon dioxide to induce crystallization of amorphous polycarbonate. Moreover, this behaviour is amplified by the presence of carbon nanotubes that act as nucleating agents for crystals birth. The thermal behaviour of the composites was analysed by DSC and DMA and was related to the foaming observations. The uniformity of the cellular structure was analysed by scanning electron microscopy (SEM). By saturating the polycarbonate nanocomposites reinforced with 1 wt% of MWNTs at 100 bar and 100 °C during 16 h, microcellular foams were generated, with a density of 0.62, a cell size ranging from 0.6 to 4 μm, and a cellular density of 4.1 × 10¹¹ cells cm⁻³. The high ability of these polymeric foams to absorb electromagnetic radiation was demonstrated at low MWNT content as the result of the high affinity of the polycarbonate matrix for MWNTs, and therefore to the good MWNTs dispersion.     

A viscoelastic–plastic constitutive model for uniaxial ratcheting behaviors of polycarbonate

In this paper, a modified viscoelastic–plastic constitutive model has been proposed on the framework of Anand's work to describe the uniaxial ratcheting behavior of polycarbonate (PC) under tension–tension cyclic loading. The experimental observation illustrates that the previously accumulated deformation has an assignable influence on the subsequent material response during the ratcheting process of PC. Thus, the deformation resistance in the viscoelastic micromechanism is assumed to be evolving with the local accumulated inelastic strain rather than keeping unchanged in the original Anand's model. The proposed model is validated firstly by the monotonic tension and creep experiment results of PC. Then, its capability to describe the uniaxial ratcheting behaviors is compared with Anand model. Finally, the modified model is adopted to study the effect of mean stress, stress amplitude, loading rate, and peak holding time on the ratcheting behaviors of PC. It is shown that the proposed model can predict reasonably the uniaxial tension–tension ratcheting behavior of polymer. POLYM. ENG. SCI., 55:2559–2565, 2015. © 2015 Society of Plastics Engineers     

Open‐cell foams of polyethylene terephthalate/bisphenol a polycarbonate blend

Open microcellular foams of polyethylene terephthalate (PET)/polycarbonate (PC) blends were prepared by controlling their foaming behavior at the interface between these two polymers. Interface modification was a crucial factor in governing the foaming behavior and cell morphology of the blend foams: annealing at 280°C, i.e., conducting the transesterification reaction, generates a PET‐b‐PC copolymer, which lowers the interfacial tension, increases the affinity between PET and PC, and decreases the crystallinity of the PET domains. When CO₂ foaming was performed at the interface modified with the copolymer, an interesting fibril‐like structure was formed. The cell density of the PET/PC blend then increased, and its cell size reduced to the microscale while maintaining a high open‐cell ratio. The effect of heat annealing (transesterification reaction) on CO₂‐foaming was studied to reveal the relationship among the interface affinity, crystallinity, and degree of fibrillation. The optimal heat‐annealing procedure generated a fibril‐like structure in the PET/PC blend foams with a high cell density (7 × 10¹¹ cm^?3), small cell size (less than 2 μm), and 100% open‐cell ratio. POLYM. ENG. SCI., 55:375–385, 2015. © 2014 Society of Plastics Engineers     

Effect of CO2 sorption and desorption on the creep response of polycarbonate

In this article, experimental results on the effect of CO₂ sorption and desorption on the creep response of polycarbonate (PC) are presented. Tensile specimens machined from PC sheets were exposed to CO₂ and the absorbed gas mass fraction ranged from 0.045 to 0.12. The creep/creep recovery response of as‐received PC, saturated PC, and saturation‐cycled PC was characterized. It was found that the saturated PC showed a creep behavior similar to heating the PC to its glass transition temperature. The creep compliance of saturation‐cycled PC was found to change with the desorption or aging time. The tests on PC saturated and then desorbed for up to 60 days showed that the effects of exposure to CO₂ on PC creep properties persist long after the gas has left the polymer, and could be permanent. POLYM. ENG. SCI., 45:1639–1644, 2005. © 2005 Society of Plastics Engineers     

Pre‐necking and post‐necking relaxation and creep behavior of polycarbonate: A phenomenological study

Experiments have been performed to investigate the mechanical response of unfilled polycarbonate vis‐à‐vis the influence of prior deformation on stress relaxation and creep. Piecewise linear deformation histories, which involve strain‐controlled tensile loading of a specimen to a maximum load and partial unloading to a target strain/stress point as prologue to a relaxation test, have been shown to qualitatively influence the recorded stress‐time behavior. In particular, the stress magnitude during relaxation first increases and is then followed by a decrease. Analogously, in creep tests during unloading, the strain might decrease and then increase. Time characteristics for this U‐turn in the deformation response are influenced by the placement of the test. The influence of prior specimen conditioning on this phenomenon is investigated by comparing test data from virgin samples to that of specimens having high (～85%) inelastic strain from prior tensile elongation. Findings suggest that the observed persistence in the occurrence of this reversal effect for both types of specimens is evidence of the need to incorporate this behavior into the fold of material modeling. Additionally, this novel relaxation and creep behavior has been observed in other amorphous (poly(phenylene oxide)) and crystalline (high‐density polyethylene) polymers. Polym. Eng. Sci. 44:1783–1791, 2004. © 2004 Society of Plastics Engineers.     

Combined tension-torsion creep experiments on polycarbonate in the nonlinear range

Constant stress creep and recovery experiments were performed in the nonlinear range under combined tension and torsion on a tubular specimen of polycarbonate. Nonlinear viscoelasticity theory was employed to describe the test results. It was found that the first three orders of stress terms and a power function of time adequately described the experimental results. The modified superposition principle satisfactorily predicted the recovery following creep. Recovery was almost complete in several days following two hours of creep.     

聚碳酸酯液液分离器的流体力学模拟和中试试验

对聚碳酸酯混合液,通过液液间歇沉降实验得到分散相液滴的Sauter平均粒径为330 μm。采用Mixture多相流模型对聚碳酸酯液液静态分离器进行了模拟和分析。对平行板、波纹板和斜板进行了比较,得出斜板更适合聚碳酸酯液液分离。考察了斜板长、板间距、倾斜角度、入口流量对液液分离效果的影响,得出对于水油相体积比为0.6的物料,最优的聚结板参数为斜板倾角20°、板长0.8 m、板间距40 mm、筒内最大流速5.3×10^-3 m·s^-1。设计了聚碳酸酯液液分离器,进行了中试试验,水油两相出口浓度接近各自的溶解度值,筒内最大流速为3.9×10^-3 m·s^-1,结果表明该分离器起到了很好的液液分离效果,可以取代原有的碟片离心机设备,降低了设备和操作费用。     

Modes of deformation and failure of polycarbonate

Electron and optical microscopy studies of the modes of deformation and failure of polycarbonate are reported. The high toughness of glassy polycarbonate is controlled by the ease of shear-band deformation and the surface craze characteristics. Such crazes form in tension prior to macroscopic necking and cold-drawing and serve as sites for ultimate fracture. The surface craze characteristics and the role they play in the fracture processes are reported as a function of strain-rate (10^?2?10⁺² min^?1) from scanning electron microscopy studies of the fracture topographies and edges of polycarbonate specimens fractured in tension at room temperature. The mechanism by which surface crazing in polycarbonate is enhanced by handling is also reported. The surface regions that come into contact with islands of finger-grease are plasticized, and fabrication stresses within these regions relax near T_g at a faster rate than in the unplasticized surroundings. Microcracks which are produced at the boundary between the plasticized and unplasticized regions serve as sites for craze initiation and growth. The craze processes in thin polycarbonate films strained directly in the electron microscope are also reported. Undeformable ～10 nm sized nodular regions were observed during the craze flow processes in these thin films.