The brittle fracture of amorphous thermoplastic polymers

The brittle fracture properties of polyphenylene oxide, polysulfone, polycarbonate, and poly(methyl methacrylate) thermoplastic polymers were investigated over a wide range of temperatures. Fracture energy measurements were made using double edge-notched tensile samples. Tensile strength, tensile strain, and initial elastic modulus were measured for calculation of the fracture energy and further analysis of the polymer behavior. It was found that mechanical transitions in the tensile properties corresponded reasonably well with transitions in the fracture energy in the temperature range investigated. Fracture surface photographs permitted visual analysis of the fracture process. It was found that the roughest fracture surface corresponded to the maximum in the fracture energy for a given polymer. A theory for prediction of polymer tensile yield strain is presented, based on the volume dilation concept. The implications of this theory are discussed in terms of the crack tip flow process leading to brittle fracture.     

Effect of physical aging on the fracture behavior of crosslinked epoxies

The effect of physical aging on the fracture behavior, crack opening displacement, and plastic deformation zone of unmodified and rubber-modified epoxies was determined at two aging temperatures and different displacement rates. The strain energy release rate decreases to between 40 and 50% (for rubber modified and unmodified samples, respectively) of the unaged values after 35 days aging. Systematic dependence of the decrease in fracture toughness by aging on the rubber content is not apparent. The increased yield stress after physical aging is the main factor contributing to the reduction in fracture toughness, crack opening displacement, and plastic deformation zone. Physical aging suppresses the crack blunting mechanisms in epoxies.     

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     

The fracture behavior of surface embrittled polymers

Normally ductile polymers may exhibit unexpected brittle fractures because of a phenomenon termed surface embrittlement. The problem may arise whenever a thin brittle layer is present on the surface, which may result from the application of brittle paint or, more commonly, from surface degradation caused by exposure to elevated temperatures, ultra-violet radiation, and stress-cracking agents. In rubber-modified polymers such as acrylonitrile-butadiene-styrene and high-impact polystyrene, exposure to the outdoor environment inevitably results in the formation of a thin surface layer containing cross-linked rubber particles in a matrix of reduced molecular weight. Although the depth of material adversely affected is typically small compared to the bulk, a drastic reduction in ductility nonetheless has been observed. To better understand the mechanism of embrittlement, this paper examines the criterion for embrittlement by considering the mechanism for enhanced energy absorption of rubber-toughened plastics and elementary concepts of fracture mechanics. It has been shown that a critical coating thickness exists when multiple crazing is essentially inoperative and the deformation is localized, to the tip of a fast moving sharp crack restricting strain energy dissipation to a relatively small region.     

Mechanism of physical aging in crystalline polymers

The mechanism of physical aging in isotactic polypropylene was studied in the α-region (15–80°C) by thermally stimulated creep and by creep superposition. Aging for an elapsed time t_e was observed to have no effect on retardation elements at τ for τ ? t_e. Thus the pronounced effects observed in aging cannot be due to the movement of the retardation spectrum as a whole to longer times. The method of time-elapsed time (t?t_e) superposition fails because the retardation spectrum changes shape. This change in shape leads to systematic deficiencies in the superposed curves, an effect which shows up most clearly in superposition of the differentiated creep curves. The possible changes that take place during aging are a change in magnitude of the retardation elements and a shift to longer times. These changes will only occur in retardation elements at τ when τ ? t_e.     

Molecular modeling of physical aging in epoxy polymers

Epoxy resins are often exposed to prolonged periods of sub‐T_g temperatures which cause physical aging to occur. Because physical aging can compromise the performance of epoxies and their composites and because experimental techniques cannot provide all of the necessary physical insight that is needed to fully understand physical aging, efficient computational approaches to predict the effects of physical aging on thermomechanical properties are needed. In the current study, a new method is developed to efficiently establish molecular models of epoxy resins that represent the corresponding molecular structure at specific aging times. Although this approach does not simulate the physical aging process directly, it is useful in establishing molecular models that resemble physically aged states of epoxies. Such models are useful for predicting the thermomechanical properties of aged epoxy resins to facilitate the design of durable engineering structures. For demonstration purposes, the developed method is applied to an EPON 862/diethylene toluene diamine epoxy system for three different crosslink densities. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Ductile and brittle behavior of amorphous polymers. Relationship with activation energy for glass transition and mechanical fracture

An equation correlating the activation energy for the glass transition with T_R, a characteristic reference temperature, the fractional free volume, and the rate of change of the fractional free volume was developed. The resultant activation energies for about 30 polymers are given and favorably compare with the literature. The relationship between the activation energy and the bond-rupture energy indicates whether a polymer will fail in a ductile or brittle fashion. More accurate results are shown to be dependent on the stress, the stress concentration, molecular orientation, frequency of load application, and temperature. Equations correlating all these with the activation energies are given. These results are in agreement with the molecular domain model. Experimental observations from the literature seem to corroborate the suggestion that the molecular domain model holds in the amorphous solid, too.     

Charpy tests on brittle polymers

This paper describes a simple miss-spring model of Charpy's classical impact bending test. In this model, the mass represents the striker, and the spring the bent specimen. The stiffness of the latter is calculated from the formulas for static bending. The model has been tested by experiments on polymethyl methacrylate at room temperature. First, it is shown that the model correctly describes the effect of the dimensions of the specimen (span, width and thickness) on the fracture impact energy Secondly it is shown, that the fracture energy calculated from the measured fracture time agrees with the fracture energy determined experimentally. Third it has been found that the fracture energy in impact can be predicted by extrapolation of the results of slow bending tests at various deformation rates. Lastly, it has been proven experimentally that any stress waves generated by the impact of the striker have little effect on the measured fracture energy.     

Time-temperature superposition and physical aging in amorphous polymers

In the past, time-temperature superposition has been used to extend the time scale of creep tests in polymers from the short times easily obtained in the laboratory to long times seen in actual use. A fundamental assumption of time-temperature superposition is, however, that the polymer does not change in structure as a function of time. Because ductile amorphous thermoplastics physically age below T_g, the structure of the polymer changes on a time scale comparable to the time duration of the creep test. Thus, the time-temperature superposition prediction greatly exaggerates the amount of creep in amorphous thermoplastics. For samples aged at the test temperature for one hour before testing, the difference between the time-temperature superposition prediction and the actual creep data after 10 days is greater than a factor of ten in time.     

Epoxy network structure effect on physical aging behavior

Using dicyandiamide as curing agent, several epoxy networks are formed with different formulations and curing cycles. Both sub-T_g isothermal enthalpy relaxation and dynamic enthalpy relaxation in transition zone have been studied using differential scanning calorimetry (DSC). The isothermal enthalpy relaxation rates of all epoxy networks are quite similar and in good agreement with Arrhenius' law. Nevertheless, dynamic relaxation behaviors in the transition zone are very different. These observations are discussed in connection with relaxation mechanism and chemical structure of the networks. Evolutions of mechanical properties during sub-T_g annealing are monitored by means of three-points bending tests. The ductility of unprecured epoxy networks decreases with time; otherwise, the precured and/or filled networks present a stability with regards to mechanical properties. Explanations for these phenomena take into account a possible competition between the relaxation of residual stresses and the network structural relaxation.     

The change in mechanical behavior of linear polymers during photochemical aging

Films of low-density polyethylene, polypropylene, and rigid poly(vinyl chloride) were exposed for 1700 h to artificial aging in a Weather-O-Meter. Photochemical aging was characterized by tensile measurements. We noted that only ultimate properties are affected, whereas properties defined in the low strain range remain unchanged. The kinetics of strain and stress at break depend as much on the polymer's initial rheological characteristics as on the rate of the chemical degradation. The results show that aging results in localized chain breaking leading to defects at the supermolecular level. The average decrease of molecular weight does not seem to influence the mechanical behavior.     

Measurement of aging effects of ABS polymers

ABS (acrylonitrile-butadiene-styrene) plastics are one of the most common two-phase commercial polymer systems. They consist of a continuous rigid phase (styrene-acrylonitrile co-polymer) in which the elastomer phase (polybutadiene grafted with styrene and acrylonitrile) is finely dispersed in the form of spherical particles. Because of their properties and relatively low cost compared to other engineering thermoplastics, ABS resins are now being used increasingly in fields of application involving severe aging. The polybutadiene content, however, poses a problem in relation to ABS aging resistance, since it is a prime site for degradative attack at double bonds and tertiary carbon atoms. The present paper presents a concise account of the methods used in our labortory for measuring ABS aging. As an example, ABS degradation during processing and during natural and artificial light aging are discussed in more detail.     

Effect of physical aging on the shape-memory behavior of amorphous networks

This paper presents an experimental and modeling study of the effects of physical aging on the shape-memory performance of (meth)acrylate-based networks composed of tert-butyl acrylate (tBA) crosslinked by various concentrations of poly(ethylene glycol dimethacrylate) (PEGDMA). The experiments measured the unconstrained recovery response of samples stored at 20 °C (T_g ? 36 °C) for zero to 180 days and evaluated the effects of storage on the strain fixity, activation temperature, and initial recovery rate. A thermoviscoelastic model recently developed for amorphous networks near the T_g was applied to study the influence of structural and viscoelastic relaxation and the aging time and temperature on the recovery response. Results showed that the activation temperature and the initial recovery rate increased with the aging time, producing a sharper initial recovery response. The thermoviscoelastic model predicted that the magnitude of these effects depended on the aging temperature. There was an optimum aging temperature that maximized the initial recovery rate. These results suggest that physical aging can be manipulated to accelerate the recovery performance of shape-memory polymer devices.     

Modeling viscoelastic behavior of glasses during physical aging

The generalization of a multiordering parameter model to treat physical aging in polymeric glasses is more fully explored. In the past, this model has successfully rationalized most aspects of time dependent volume and enthalpy behavior; however, only qualitative agreement between predictions of the generalized model and experiment were previously achieved for physical aging. Now, by paying strict attention to the specific analytical form of the shift factor expression, quantitative predictions have been realized for physical aging behavior based on evaluations of the shift factor parameters using volumetric data. The model also reproduces the observed behavior of glasses subjected to more complex thermal treatment such as those which evoke “memory” in glasses, e.g., shift reversal and nonmonotonic variations of the steepness of relaxation curves in the primary transition region.     

The effects of thermooxidative aging on the fatigue behavior of a structural adhesive

The kinetics of autoxidation of FM-73U, a rubber-modified epoxy adhesive, were investigated by Fourier transform infrared analysis. Absorbance spectra for thin samples aged in hot, moist, oxygen-rich environments were used to assess a plausible reaction mechanism; rate constants, Arrhenius plots, and oxidation rates were determined. Concurrently, crack growth rates were measured on specimens which had been exposed to similar environments for periods of up to 8 months. Paris parameters measured in these tests were correlated with the results of the oxidation studies. These correlations were used to predict the crack growth rate of the adhesive after 10 years of aging in ambient conditions. Although the prediction indicates that the adhesive becomes more brittle with age, the changes are not severe.     

Surface tension measurements on film-forming polymers: Thermal aging effects

The surface property changes caused by exposure to specific time/temperature regimes have been followed by critical surface tension data (γ_c) in a series of styrene–acrylic polymers frequently used as film-formers. A simple, experimental method for γ_c evaluation, previously described proved useful for present purposes. Surface tensions change more rapidly when polymers age above their characteristic T_g. In the polymers studied, increases in γ_c indicate an apparent enrichment in the acrylic content of the surface, the rates apparently dominated by diffusion-controlled processes. The importance of the effect increases with overall acrylic content in the polymer and with molecular weight.     

The effects of physical aging on the thermal and mechanical properties of an epoxy polymer

The effects of free volume on the thermal and mechanical properties of epoxies from diglycidyl ether of bisphenol A (DGEBA) and ethylene diamine (EDA) were investigated. The degree of free volume was controlled by the different thermal history. For fully cured expoxy systems, the density and modulus of both aged and quenched specimens decreased with increasing EDA concentration. However the yield stress of quenched specimens showed a maximum at about equal stoichiometric formulation, while that of aged ones decreased with increasing EDA concentration.     

A viscosity model for technical polymers describing chemical and physical aging

Rheological techniques, like oscillating rheometry and high‐pressure capillary rheometry were used to measure and to describe chemical and physical aging influences on the rheology of polyamide and polybutylene terephthalate in an advanced viscosity model. Viscosity drop was found as cause of combined time and temperature effects and understood as chemical aging that was reduced to a thermally induced degradation. The influence of chemical aging on viscosity was described based on a novel structural change shift factor a_SC. In addition, viscosity drop was found as cause of residual moisture. Its influence was described by a novel physical change shift factor a_PC. The functional relationships for both factors were derived from measured data. Moreover, both factors contain additional parameters that were optimized with respect to specific processing and boundary conditions. The consideration of chemical and physical aging by means of a_SC and a_PC provides a better understanding with respect to rheological changes and thus a better control of process variations and an enhanced product quality. POLYM. ENG. SCI., 55:1628–1633, 2015. © 2014 Society of Plastics Engineers     

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.     

The viscoelastic and aging characteristics of polymers

The time dependent mechanical properties for incompressible polymer-like materials subjected to finite simple extension are studied. There are two parts: the viscoelastic effects and the aging characteristics. The constitutive equation developed by Christensen for finite deformation viscoelasticity is used. With two new material properties, the constitutive equation is extended to include the aging effects. An experiment is performed, and a method for determining these mechanical properties is presented in detail.