Effect of heat pretreatment and strain rate on tensile properties of polycarbonate sheet

The effects of heat pretreatment and ambient gas (air and vacuum) on selected properties of the polycarbonate sheet have been studied. Changes in tensile properties as functions of heat pretreatment temperature (up to 160°C) and strain rate (wide range of 1.7 × 10^?4 ? 13.1 m/sec = 0.29 ? 2.3 × 10⁴ %/sec) were determined and these are discussed in relation to changes in differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) data. The performance characteristics of the present tensile testing are obtained over a wide range of extension rates without changing the mode of deformation and the shape of the test pieces. It was suggested from the experimental results that heat pretreatment below the glass transition temperature (T_g) causes ordered molecular domains to grow on the free surfaces of the sheet, consisting of thermally deteriorated macromolecules and possessing lower crazing stresses (exhibiting more brittle mechanical responses, leading to the decrease in breaking strain and energy). The effect of annealing above T_g on the tensile properties, and on the results of DSC and GPC, could not be precisely understood.     

Crazing,yielding, and fracture of polymers. I. Ductile brittle transition in polycarbonate

Most thermoplastics far below their glass transition give a brittle fracture when de-formed in uniaxial tension. Bisphenol-A polycarbonates are an exception and deform in a ductile manner. However, it has been observed in Izod impact studies of notched samples that the mode of failure changes from a ductile to a brittle fracture on annealing samples below T_g. It has been found that, when notched samples are stressed, a Griffith type flaw is formed under the notch. The criterion for the ductile brittle transition is evaluated in terms of σG (the stress required to propagate the Griffith flaw), and σ_y, the yield stress for the polymer. It has been found that the density and yield stress for the samples annealed at various temperatures are dependent upon previous thermal history and in particular on the molecular weíAght. On the basis of these measurements, it is concluded that many of the so-called anomalous effects observed with polycarbonate can be explained.     

Dielectric relaxations in poly [2,2-propane-bis-(4-phenyl thiocarbonate)]

The dielectric relaxation properties of poly[2,2-propane-bis-(4-phenyl thiocarbonate)] (PTC) have been studied. The existence of crystallinity, which can be eliminated by quenching, is detected. The degree of crystallinity of polymer samples was determined by differential scanning calorimetry in order to investigate the effect of this factor on the dielectric behaviour of this polymer. The thermal degradation of the samples was studied by thermogravimetry. The degradation of the polymer begins before the glass transition temperature T_g. The dielectric spectrum is complex showing several relaxation phenomena. With increasing temperature a γ relaxation can be observed at - 100°C (5 kHz). The activation energy obtained from an Arrhenius plot (lnfvs T^?1) is 6 kcal mol^?1. At 160°C the α relaxation which is associated with the glass transition temperature T_g is detected. The dielectric behaviour of this poly(thiocarbonate) is compared with the corresponding poly(carbonate).     

Isothermal rupture characteristics of a plasticized poly(vinyl chloride) in the glass–rubber transition zone

Constant strain-rate uniaxial extension tests to rupture were performed at 23°C on a plasticized poly(vinyl chloride) (PVC) in the glass-to-rubber transition zone, T_g = ?18°C, where experimental failure time t_f is equal to or greater than material relaxation time τ. Range of strain rate is from 1.8 × 10^?4 to 1.8 × 10^?1 sec^?1. The rupture characteristics in stress/strain time space are analyzed on the three coordinate planes. Time and deformation separability are examined in a nonlinear-constitutive relation. Rupture data on the coordinate planes are described by the Bueche-Halpin theory in which time effects are considered through a small-deformation viscoelastic property. Fracture surface morphology and separation processes in the crack tip are related to rupture characteristics.     

The effect of glass transition on gas permeabilities

Gas permeabilities of a copolymer of acrylonitrile and methyl acrylate were examined at the temperature range of 25° to 80°C. The change of activation energy of gas permeability constant at the glass transition temperature (65°C) was observed with N₂, Ar, O₂, and CO₂, but no change was observed with He. The observation of the change of slope of the Arrhenius plot of gas permeability is discussed as a function of the diffusion constant (D) of a permeant gas at the glass transition temperature (T_g). The change occurs only if the value of D at T_g is smaller than a certain value (i.e., 5 × 10^?8), according to the analysis of data appearing in the literature.     

Synthesis and characterization of homo‐ and copolymers of 3‐(2‐cyano ethoxy)methyl‐ and 3‐[methoxy(triethylenoxy)]methyl‐3′‐methyl‐oxetane

Two oxetane‐derived monomers 3‐(2‐cyanoethoxy)methyl‐ and 3‐(methoxy(triethylenoxy)) methyl‐3′‐methyloxetane were prepared from the reaction of 3‐methyl‐3′‐hydroxymethyloxetane with acrylonitrile and triethylene glycol monomethyl ether, respectively. Their homo‐ and copolyethers were synthesized with BF₃· Et₂O/1,4‐butanediol and trifluoromethane sulfonic acid as initiator through cationic ring‐opening polymerization. The structure of the polymers was characterized by FTIR and¹H NMR. The ratio of two repeating units incorporated into the copolymers is well consistent with the feed ratio. Regarding glass transition temperature (T_g), the DSC data imply that the resulting copolymers have a lower T_g than pure poly(ethylene oxide). Moreover, the TGA measurements reveal that they possess in general a high heat decomposition temperature. The ion conductivity of a sample (P‐AN 20) is 1.07 × 10^?5 S cm^?1 at room temperature and 2.79 × 10^?4 S cm^?1 at 80 °C, thus presenting the potential to meet the practical requirement of lithium ion batteries for polymer electrolytes. Copyright © 2005 Society of Chemical Industry     

Tensile yield in phenolphthalein polyether ketone

Uniaxial tension tests to the yield point were performed on phenolphthalein polyether ketone (PEK-C) from room temperature to near the glass transition temperature (T_g) at a constant rate of 0.02 min^?1. At room temperature, some measurements were also made at strain rates from 0.002 min^?1 to 2 min^?1. Yield stress was a linear function of temperature and log strain rate. The temperature and the strain rate dependence of yield stress could be modeled using Eyring theory. Yield energy was found to be a linear function of temperature. Young's modulus, yield strain, elastic strain, and plastic strain all decreased with temperature. © 1994 John Wiley & Sons, Inc.     

Characterization of optical properties of acrylate based adhesives exposed to different temperature conditions

Optical adhesives combine the traditional function of structural attachment with a more advanced function of providing an optical path between optical interconnects. This article aims to characterize refractive index and birefringence of such adhesives under environmental exposure to different temperature conditions. Optical time domain reflectometery (OTDR) and prism coupling methods were employed to measure optical properties of an optical adhesive. Thermo‐optic coefficient (dn/dT) of the adhesive was observed to decrease noticeably from ?2 × 10^?4°C^?1 to ?4 × 10^?4°C^?1 around the glass transition temperature (T_g ～ 78°C). It is observed that refractive indices for both T_E and T_M modes increase with increasing annealing temperature, but the birefringence (T_E ? T_M) is decreasing. This suggests that the material has become more isotropic due to the annealing. The environmental changes in optical properties of the adhesive are discussed in the light of Lorentz–Lorenz equations. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 950–956, 2005     

Straining phenomena of POY revealed by thermal retraction and other techniques

Partially oriented polyesters yarns (POY) were strained at different strain rates (0.03–12.00 min^?1) and temperatures above and below T_g (3–92°C). Thermal retraction, density, DSC, and WAXS techniques show that strain-induced crystallization takes place by straining at temperatures above as well as below T_g. Above T_g, depending upon the strain rate, two regimes are observed: Below the strain rate of 1.5 min^?1, the flow regime; the degree of crystallinity is reduced as the strain rate increases. Above the strain rate of 1.5 min^?1, the strain-induced crystallization regime; the degree of crystallinity increases as the strain rate increases. Thermal retraction, stress–relaxation, and sonic modulus techniques indicate that, upon cold straining, instead of the original T_g at 65–69°C, two glass transitions occur: an upper T_g (u) and a lower T_g (l). For POY strained at 3°C and at a strain rate of 10 min^?1, the values are 78°C and 37°C, respectively. The higher the strain rate and the lower the straining temperature, the large the difference between T_g (u) and T_g (l).     

Toughness optimization of elastomer‐modified glass–fiber reinforced PA6 materials

To investigate the influence of moisture and EPR‐g‐MA content on the fracture behavior of glass–fiber reinforced PA6 materials, brittle‐to‐tough transition temperatures (T_btt) were determined. Water absorption was taken into account by conditioning the analyzed materials. Tensile tests could reveal the temperature range of the largest moisture dependence of mechanical properties between 10 and 50°C. J‐integral values were used to describe the fracture behavior under conditions of impact load as a function of temperature. The brittle‐to‐tough transition of reinforced polyamides was found to be less approximate than in unreinforced materials. Two different characteristic temperature points T_s and T_e were identified, which were the intercept between elastic and elastic–plastic deformation on the one hand and the starting point of dominating stable crack propagation with strong plastic deformation on the other hand. Characteristic brittle‐to‐tough transition temperatures T_btt could be calculated as the arithmetic average of these two points. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Conducting blends obtained from maleic acid/dodecylhydrogensulfate‐doped polyaniline and polyvinyl chloride by solution processing

Solution processible polyaniline, obtained by our newly developed method via emulsion polymerization in the presence of maleic acid and sodium lauryl sulfate with benzoyl peroxide as the oxidizing/polymerizing agent, has been used for the fabrication of conductive blends with poly(vinyl chloride) (PVC), covering the conductivity range from 10^?5 to 5 × 10^?3 S/cm and showing the percolation threshold for the electrical conductivity f_p = 0.039. The fabricated blends combine good mechanical properties with enhanced electrical conductivity, since up to conducting phase content f = 0.231, their mechanical properties remain essentially the same and characteristic of pure PVC. Differential scanning calorimetry (DSC) measurements show a single glass‐transition temperature (T_g) increasing with growing content of the rigid conductive phase in the blend. This effect can be taken as an evidence of good miscibility of both components of the blend with phase separation at a submicrometric level. The latter has been confirmed by scanning electron microscopy studies showing a necklace‐like morphology which assures the continuity of the conductive phase. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1113–1119, 2007     

Effect of physical aging on fracture behavior of polyphenylquinoxaline films

The influence of physical aging on the tensile fracture behavior of notched Polyphenylquinoxaline (PPQ‐E) samples has been studied. The dependence of fracture stress and strain on physical aging has been explained. The glass transition temperature (T_g) and the endothermic peak at the end of T_g transition with different physical aging were characterized using differential scanning calorimetry (DSC) and the results have also been explained. The morphology of fracture surface was observed by scanning electron microscopy (SEM). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1275–1279, 2000     

Characterization of epoxies cured with bimodal blends of polyetheramines

DGEBA was cured with bimodal blends of polyetheramines as well as with single molecular weight amines while maintaining stoichiometry. Glass transition temperatures (T_gs) and moduli were measured using dynamic mechanical analysis (DMA). Fracture properties were measured using the compact tension geometry and testing was performed at both ambient and non‐ambient temperatures, investigating toughness changes as a function of temperature. For constant amine average molecular weights, the addition of high molecular weight amines caused increased glassy moduli at a constant T ? T_g and decreased densities while broadening the glass transition without changing the fracture toughness. The fracture behavior, specifically the slip‐stick to brittle transition, was affected by the broadened transitions. T_g, breadth of T_g, and total damping were found to be proportional to the volume fraction of amine in the system. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1621–1631, 2013     

Mechanical Responses of 2-Hydroxyethyl Methacrylate-Methacrylonitrile and 2-Hydroxyethyl Methacrylate-Acrylonitrile Copolymer Networks

Abstract

The mechanical behaviour of 2-hydroxyethyl methacrylate-methacrylonitrile (HEMA-MAN)and 2-hydroxyethyl methacrylate-acrylonitrile (HEMA-AN) copolymer networks of various composition prepared in presence of 0.6 × 10^?4 mol cm^?3 ethylene dimethacrylate and 20 vol.-% dimethylformamide were investigated. The viscoelastic and stress-strain behaviour of samples swollen in water to equilibrium and the dynamic relaxation behaviour of dry polymers from liquid nitrogen temperature onwards was investigated.

It was found that with increasing concentration of the MAN or AN component the equilibrium degree of swelling in water decreases, which on the other hand is accompanied by a simultaneous decrease in the glass transition temperature, T _g of copolymers in the dry state. T_g passes through a minimum in this case. The HEMA-AN copolymers become swollen to a higher degree; their T_g temperatures in the dry state are lower compared to the HEMA-MAN copolymers. The effect of the decreasing degree of swelling with increasing content of the MAN or AN comonomers is predominantly reflected in shifting the viscoelastic data toward the main transition region, that is in the increasing distance from the mechanical equilibrium of networks measured at room temperature. In the molar concentration range of nitrile comonomers n ? 0.2 a minimum of the equilibrium modulus was observed, probably related to changes in the effectivity of crosslinking reaction with increasing content of the MAN or AN groups.

The tensile strength increased with increasing content of acrylonitrile and metha-crylonitrile, while strain-at-break passed through a maximum, the position of which depended on temperature and on the strain rate. The failure envelopes plotted as [sgrave]_bΛ_tT_oT^?1 vs. log ?b+ log 3(C₁ + C₂) shows that the ultimate behaviour of both types of copolymers is very similar, while the differences in behaviour can be predominantly attributed to different distances from the equilibrium.     

Effect of temperature on the compressive stress-strain properties of polystyrene

The compressive stress-strain behavior of a commercial polystyrene has been studied and the effect of deformation temperature on modulus, yield stress, percent yield strain and yield energy was determined. Yield energy is the only one of these parameters that is linear with temperature in the ductile region. A change in the mode of failure from ductile to brittle occurs between 5–30°C at a strain rate of O.1/in./in./min. At all temperatures studied, the yield or fracture stress varied linearly with the rate of deformation for strain rates ranging from 0.1 to 1.0 in./in./min. The yield data as a function of temperature were analyzed via a rate expression modified to incorporate the Coulomb-Navier yield criterion, Activation energy was found to be a function of deformation temperature with a change in slope occurring near the β transition. Activation volume increased linearly with deformation temperature, for the range studied. Agreement of dynamic mechanical and yield activation energies imply that the type of motion and the height of the energy barrier are similar for both. However, an increase in activation volume for stressed vs unstressed conditions suggests that a greater number of chain segments move as a result of stress biasing. Also the increase of both activation volume and activation energy with temperature implies that the correlated length of chain movement increases as temperature is increased. Similar to activation energy, yield stress exhibits a change in temperature dependence near the β transition. Data on other glassy polymers suggest that the highest temperature sub-T_g, transition is related to the change in the temperature dependence of yield stress.     

Influence of temperature on plane stress ductile fracture of poly(ethylene terephthalate) film

Abstract

Double edge notched poly(ethylene terephthalate) (PET) specimens of varying ligament lengths and 0·125 mm thickness have been pulled to complete fracture between 23 and 160°C. Within this temperature range, propagation of the crack was always stable, producing load–displacement curves at various ligament lengths that were geometrically similar to one another. Essential work of fracture (EWF) analysis was used to study the effect of temperature on fracture toughness. A linear relationship was obtained between specific total work of fracture W _f and ligament length over the entire temperature range under consideration. The slope of the line, which is termed specific non-essential work of fracture βw _p , showed a maximum near the glass transition temperature of the material (T _g ≈ 93°C). Beyond this point, βw _p decreased sharply with increasing temperature. The intercept of the line at zero ligament length, which is referred to as specific essential work of fracture w _e , showed three types of variation with respect to temperature. Below T _g , w _e was found to be more or less independent of temperature; above T _g it increased with temperature and reached a maximum value at the end of the leathery region (～120°C); beyond which it decreased steadily.     

The effect of urea bond on structure and properties of toughened epoxy resins

In this article, modified poly(oxypropylene) diamines were synthesized and used as a new flexible curing agent for epoxy resins. The purpose of modification is to introduce urea group into epoxy resins. The reaction rate, mechanical properties, glass transition temperature (T_g), and fracture surface morphology of these toughened epoxy resins were investigated. Because of urea groups, the reactivity between poly(oxypropylene) diamines and epoxy resins was significantly enhanced. At the same time, the urea groups resulted in strong intersegmental hydrogen bonding between modified poly(oxypropylene) chain, which reduced the compatibility of poly(oxypropylene) with epoxy resins and resulted in higher T_g of toughened epoxy. The modified sample had tensile strength of 15.8 MPa and ultimate elongation of 118% at room temperature, whereas the unmodified sample only had 6.2 MPa and 70%. The scanning electron microscope analysis showed that the modified system displayed tough fracture feature, whereas the unmodified system showed typical brittle fracture. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effects of strain rate and comonomer on the brittle–ductile transition of polymers

The effect of rate on the brittle–ductile transition of polymers can be given by an Arrhenius-type equation with activation energy between those of α and β transitions and given by where E_b is the activation energy for brittle-ductile transition, E_α is that for α transition, E_β is that for β transition, T_g is the glass transition temperature, T_b is the brittle–ductile transition temperature at 0.1 min.^?1, T_α is the α transition temperature at 1 cps, and T_β is the β transition temperature at 1 cps. The plots of T_b versus the weight fraction (w) of comonomer are sigmoidal, with an inflection point at w = 0.5.     

Morphology and physical properties of poly(butylene terephthalate)

Liquid nitrogen-quenched PBT samples produce much larger spherulites of an optic axis orientation different from the of the air-cooled samples. Optical and scanning electron microscopy show that glass fibers in the glass-reinforced PBT sample nucleate the growth of well-defined spherulites along the glass fiber axis. Fracture studies at temperatures below and above the T_g indicate, respectively, brittle and ductile interspherulite boundary fracture. From dynamic mechanical studies, three transitions designated by α (flow transition), β (T_g), and γ (secondary relaxation) are observed. The magnitudes of the β and γ transitions are larger for the more amorphous quenched sample than the air-cooled sample, suggesting their amorphous phase origin. Addition of glass fibers raises the dynamic modulus and flow temperature, but suppresses the γ transition without significantly affecting the melting and glass transition temperatures.     

Fabrication and characterization of a solid polymeric electrolyte of PAN‐TiO2‐LiClO4

The ionic conductivity of PAN‐TiO₂‐LiClO₄ as a function of TiO₂ concentration and temperature has been reported. The electrolyte samples were prepared by solution casting technique. Their conductivity was measured using the impedance spectroscopy technique. The highest room temperature conductivity of 1.8 × 10^?4 S cm^?1 was obtained at 7.5 wt % of TiO₂ filler. It was observed that the relationship between temperature and conductivity were linear, fitting well in Arrhenius and not in Vogel‐Tamman‐Fulcher equation. The pre‐exponential factor, σ₀ and E_a are 1.8 × 10^?4 S cm^?1 and 0.15 eV, respectively. The conductivity data have been supported by differential scanning calorimeter (DSC) analysis. DSC analysis showed that there was a significant change in glass transition temperature (T_g) with the filler concentration. The SEM micrograph revealed that the TiO₂ particles are dispersed in the electrolyte, thus enhancing its conductivity. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010