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.     

The effects of physical aging on the brittle fracture behavior of polymers

The effects of physical aging on the failure behavior of a typical brittle polymer, polystyrene, have been studied. Properties examined were creep rupture lifetimes, fatigue lifetimes, and environmental stress cracking in ethanol. Fractured samples were examined both optically and by scanning electron microscopy to determine the degree of crazing. It was found that a longer physical aging time produced shorter lifetimes in all cases. The main reason for this is the reduction in craze strength caused by a reduced toughness due to physical aging. A long aging time was found to delay craze formation, but once formed, these crazes were much less stable than those formed with a short aging time. The effects of aging are important on failure prediction criteria and on testing methodologies, and the implications are discussed.     

Influence of crack speed on fracture energy in amorphous and rubber toughened amorphous polymers

Abstract

To study the influence of the crack speed at initiation on measurements of the toughness of amorphous polymers such as polycarbonate (PC) and rubber toughened poly(methyl methacrylate) (RTPMMA), ‘compact tension’ specimens were tested at a medium loading rate (0·6 m s^-1 ). The specimens were equipped to allow location of the position of the crack tip and to trigger a photograph on passage of the crack. The experimental procedure was validated using PC for which the crack speed was found to change suddenly over very short distances, owing to stick–slip propagation. Conversely, fracture tests on RTPMMA showed that the acceleration of the crack tip, due to the decrease in fracture energy with crack speed, is controlled by a process zone, the size of which did not allow the systematic use of linear elastic fracture mechanics. The rapid crack propagation regime in RTPMMA corresponds to a propagation stabilised at the macroscopic crack branching velocity. This constant crack speed corresponds to frustrated microbranching and leads to non-unique values of the fracture energy at the macroscopic branching velocity.     

The structure of amorphous polymers

Over the past ten years the structure of amorphous polymers has been extensively investigated. Publications on this topic were concerned with various aspects of the amorphous structure and were often based on specific molecular models. This review article attempts to discuss the interrelations between the different structural properties which were studied. The topics covered are the short range positional order, the orientational order, the chain conformation, the supermolecular structure and the relation between molecular and macroscopical properties both of the fluid and the glassy state. The basic theory of the fluid state is introduced at the beginning as well as the experimental techniques currently used to characterize the amorphous structure. Experimental results on different amorphous polymers and oligomers are then discussed and the information is pointed out which can be derived from these data. Finally models on the structure of amorphous polymers are considered briefly.     

The importance of char-forming reactions in thermoplastic polymers

The flammability and smoke generated from burning blends of acrylonitrile–butadiene–styrene (ABS) and polyvinylchloride (PVC) are discussed. Flammability was measured using standard oxygen index techniques and smoke production determined by the NBS method. The incorporation of some specific iron containing inorganic compounds into a range of blends of ABS and PVC considerably changes the burning characteristics of the polymer blend. Thermal stability at elevated temperatures and carbonaceous char formation are also discussed. The chemical role of iron compounds in reducing both the flammability and smoke production in ABS/PVC is considered.     

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.     

Shear test for thermoplastic polymers

A shear test is proposed as a complement to the tensile test in order to characterize the short time mechanical behavior of thermoplastic polymers. The proposed test geometry makes the test easy to perform and to evaluate and gives relevant information about anisotropy, plastic deformation, crack initiation, and propagation properties. Three different materials have been tested and the results are discussed.     

Effect of nanoparticle size and size‐distribution on mechanical behavior of filled amorphous thermoplastic polymers

Different types of polymer nanocomposites on the base of polystyrene, polymethylmethacrylate, and polycarbonate with alumina and SiO₂ nanoparticles and carbon nanotubes have been studied. Miniaturized, microdimensional samples were used, enabling a good control of morphology and distribution of particles by means of transmission and scanning electron microscopy. Special preparation techniques had been applied, which resulted in a very good dispersion of the nanoparticles. Using these materials with really nanosized particles of a few 10nm in size the effect on toughness enhancement could be studied without agglomerates as they often appear in the generally used large samples. Micromechanical mechanisms were studied in detail by TEM and SEM investigations of deformed samples. A “nanoparticle modulated crazing” could be detected as a toughness enhancing effect. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Thermal properties of strained thermoplastic polymers

Injection moulded tensile bars of several amorphous and semi-crystalline thermoplastic polymers have been deformed uniaxially to a macroscopic strain of 40% so that they showed necked and unnecked regions. From both the necked and unnecked regions of the tensile bars the specific heat and the thermal expansion coefficient were measured. The specific heat of the necked regions decreases about 20% with respect to unstrained samples for semi-crystalline polymers at temperatures between 40°C and 60°C and for amorphous polymers at temperatures between 50°C and 70°C. The thermal expansion coefficient becomes negative at these temperatures. A smaller negative expansion coefficient is observed even in the unnecked region although nearly no change in specific heat can be observed. There is also an increase in the heat of fusion of the semi-crystalline specimens of the necked region. Strain rate dependent effects on specific heat and thermal expansion were not observed within the accuracy of thermal analysis.     

External lubrication of high-temperature thermoplastic polymers

External lubrication of high-temperature thermoplastic polymers was studied experimentally. It was found that the nature of the lubrication fluid has a pronounced effect upon the behavior of the contact. It is postulated that premature failure of PEEK-lubricated contacts is caused by the migration of linear carboxylic acids into the polymer matrix at elevated temperatures, thereby producing significant plasticization. A sequence of events leading to failure is given and some rationale for it provided. © 1993 John Wiley & Sons, Inc.     

Miscibility of polyethyloxazoline with thermoplastic polymers

Polyethyloxazoline (PEOx) blends with several other thermoplastic polymers were examined by differential scanning calorimetry (DSC) for miscibility. Styrene/acrylonitrile (SAN) copolymers having compositions in the range of about 20–40% acrylonitrile (AN) by weight were found to be miscible with PEOx whereas SANs outside this range were not. The polyhydroxyl ether of bisphenol A (Phenoxy) was also found to be miscible with PEOx. A vinylidene chloride copolymer (Saran) was found to be partially miscible with PEOx, whereas poly(methyl methacrylate) and polycarbonate were not miscible at all.     

Dispersivity of rubbers in thermoplastic polymers

The main factors influencing the dispersivity of rubbers (nitrile and ethylene–propylene) in nylon 6 (PA-6) and polypropylene (PP) are investigated. On the basis of an equation, describing the influence of interfacial tension and viscosity ratio of disperse and matrix phases on the average size of dispersed phase particles, analysis of dispergation process in researched blended systems has been conducted. The limits of applicability of a given equation were established. It was shown that the best dispersivity of rubbers in the PA matrix is observed in that case when the viscosity values of initial components of a blend are close to each other. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1563–1567, 1999     

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 equilibrium water content of some thermoplastic hydroxyalkyl methacrylate polymers

The equilibrium water content (EWC) of thermoplastic 2-hydroxypropyl methacrylate and 2-hydroxyethyl methacrylate homopolymers from technical grade monomer and of copolymers of the latter with methyl methacrylate has been measured in distilled water and in phosphate-buffered saline (PBS). In distilled water, as expected, EWCs increased monotonically with the proportion of hydrophilic monomer in the polymer and showed only a small dependence on molecular weight. In contrast, the degree of swelling (and in some cases dissolution) and “freezable” water content of polymers in PBS were shown to be anomalously high and to be a consequence of ionization at pH 7.4 of methacrylic acid incorporated into the polymer in small quantity (< 1 mol %) as an impurity. Initial studies indicated a strong relationship between copolymer molecular weight and small molecule permeability in distilled water.     

The essential work of fracture of a thermoplastic elastomer

This paper presents the fracture behaviour of a thermoplastic elastomer, HYTREL 5556. Since with this material it is not possible to successfully apply the LEFM nor the EPFM, it has been studied by following the ESIS protocol for determining the essential work of fracture in plane stress and extended for mixed-mode conditions, which should give a material constant, independent of the sample geometry. DDENT specimens were used in two different thickness, and results showed that the essential works of fracture in plane stress and mixed-mode were the same for both thickness for this material. Received: 24 March 1997/Revised: 30 May 1997/Accepted: 30 May 1997     

Stability of blends of thermotropic liquid crystalline polymers with thermoplastic polymers

Breakup of fibers of a thermotropic liquid crystalline polymer (TLCP) above the melting temperature in various ordinary polymers has been studied by capillary instability experiments on single TLCP fibers and by annealing experiments on extruded TLCP/thermoplast blends. The TLCP was an aromatic copolyester, Vectra A900, the matrix polymers were PP, PS, PC, PEL PES, and PEBT. Both types of experiments show that the fiber/matrix morphology is, in general, highly unstable in the molten state. The TLCP fibers break up into droplets by a combination of Rayleigh distortions, end-pinching and retraction, depending on the system and shape of the fiber. Fibers of a thickness of ～1 μm can break up in a few seconds. Breakup times of fibrous blends and individual fibers are in agreement provided size effects are accounted for. Rayleigh distortions develop exponentially in time up to relative distortions of 0.5 to 0.6. Breakup occurs within a range of wave numbers rather than at one distinct dominant wave number, which is shown to be the consequence of relatively large initial distortions. Apparent values for the interfacial tensions calculated with Tomotika's theory turned out to be of the correct order of magnitude, ranging from 7 mN/m for Vectra/PES to 24 mN/m for Vectra/PP and to yield correct values of the interfacial tensions of PP/PS, PP/PC, and PS/PC using Antonow's rule.     

Comparative swelling behavior of various thermoplastic polymers

The elasticity of polymer melts is of major concern in the processing of plastics. It is usually reflected by dimensional changes. Since the swelling of polymer extrudates depends on the capillary dimensions and the volumetric flow rate, the blow-up must be examined over a range of conditions. Of course, the swelling is also dependent on polymer structure. Consequently, variations in materials and operating conditions necessiate changes in tooling. This paper describes the swelling behavior of several different polymer types and illustrates that viscosity measurements can not be used to predict elasticity.     

Sintering rheology of amorphous polymers

The activation energy for sintering of poly(methyl methacrylate) particle pairs is shown to be similar to their activation energy for Newtonian flow. Sintering progress with time is in good agreement with the Frenkel's coalescence theory. Typical sintering shear rates are shown to be very low and potential energy change (two particles) is small in comparison with the surface energy change. These results lead to the conclusion that the coalescence sintering mechanism of amorphous polymers above their glass transition temperature is essentially a Newtonian viscous flow mechanism where surface tension is the major driving force. A periodical phenomenon associated with sintering progress with time is reported and a supporting mechanism is proposed.