Crazing in polypropylene

The subject of crazing in crystalline polymers is reviewed and specific consideration given to crazing in polypropylene (PP). Tensile tests conducted over a wide spectrum of temperatures and strain rates indicate that, for a given test temperature, there exists a critical strain rate above which crazing is the dominant deformation mode of PP. Similarly, for a given strain rate, there exists a critical temperature which demarcates crazing from shear yielding as the characteristic process of deformation. High deformation rates and low temperatures favor crazing, while low rates and high temperatures favor shear yielding. Crazes in crystalline PP were found to be morphologically similar to those in glassy polymers: high reflectivity, large area-to-thickness ratio, and planarity. They have a higher tendency to bifurcate than those in glassy polymers. Two types of craze fibrils could be identified: those parallel to σ₁₁, and the randomly oriented interconnecting fibrils. It is demonstrated that microtome-trimming at low temperature followed by suitable chemical treatment is an effective technique of sample preparation for SEM examination of craze morphology in crystalline polymers. Further evidence has been provided that crazes in spherulitic polymers do not in general follow an interspherulitie path, but propagate through spherulites. The length of a craze in PP is not restricted to one spherulite diameter, nor does it grow radially.     

The fracture behaviour of a PXE/HIPS polyblend

As part of an overall examination of the fatigue crack propagation (FCP) behaviour of impact-modified polymers, a study of the fracture morphology of a PXE/HIPS polyblend polymer subjected to monotonic and cyclic loading conditions is reported. The HIPS rubbery-phase particles are found to fail by particle rupture in both fatigue and fast fracture. Another impact modifying addition, PE, is found to fail by a combination of interfacial rupture and tearing, the balance depending on the prevailing stress intensity value and the strain rate. Matrix failure is via multiple crazing at low fatigue crack growth rates, but shear yielding is believed to become a major fracture mechanism with increasing K. The degree of plastic deformation of the matrix increases with increasing strain rate. This fact is manifested by the increasing void size associated with the interfacial separation of the PE particles.     

Calculation and significance of the maximum polymer surface temperature T* in reciprocating cylinder‐on‐plate sliding

Heat generation and surface temperature rise are the main parameters controlling friction and wear of polymers, while certain transitions in sliding behavior often are difficultly related to intrinsic polymer transition temperatures. The sliding temperature cannot be accurately measured through physical limitations in the contact interface, and different calculation methods are available. Some temperature models are reviewed and applied to the reciprocating sliding of polyimide (PI) and polyethylene terephthalate against a steel counterface. The bulk temperature and asperity flash temperature models cannot explain transitions in friction and wear for polymers under reciprocating sliding. The bulk temperature model provides too low temperatures, representing a long‐range temperature, while the asperity flash temperature model provides too high temperatures, not considering visco‐elastic deformation of the polymer surface. An experimental model for the maximum polymer surface temperature T* under reciprocating sliding is developed, considering the environmental temperature, gradual heating of the steel counterface, and additional heating of the polymer surface. The proposed temperature model is analytically validated by differential thermal analysis and thermogravimetric analysis. Transitions in tribological behavior are controlled by the maximum polymer surface temperature T*, coinciding with an endotherm reaction (PIs) or a glass transition and melting (polyethylene terephthalate). POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

The role of crystalline phase on fracture and microstructure evolution of polytetrafluoroethylene (PTFE)

Polytetrafluoroethylene (PTFE) is a semi-crystalline polymer, which has been employed in a range of engineering applications due to its extremely low coefficient of friction, resistance to corrosion, and excellent electrical insulation properties. Despite failure-sensitive applications such as surgical implants, aerospace components, motor seals, and barriers for hazardous chemicals, the mechanisms of crack propagation in PTFE have received limited coverage in the literature. Moreover, PTFE exhibits complex crystalline phase behavior that includes four well-characterized phases with both local and long range order. Three crystalline structures (phases II, IV, and I) are observed at atmospheric pressure with transitions between them occurring at 19 and 30 °C. This observation provides a unique opportunity for investigation of the effects of a polymers crystalline phase on fracture and microstructure evolution. Moreover, due to the presence of three unique ambient pressure phases near room temperature, it is essential to develop an understanding of the effects of temperature-induced phase transitions on fracture mechanisms of PTFE to prevent failure over the normal range of operating temperatures. In this work, we present values for the J-integral fracture toughness of PTFE for a range of temperatures and loading rates employing the single specimen normalization technique. Crack propagation in PTFE is found to be strongly phase dependent with a brittle-to-ductile transition in the crack propagation behavior associated with the two room temperature phase transitions. Increases in fracture toughness are shown to result from the onset of stable fibril formation bridging the crack plane and increased plastic deformation. The stability of drawing fibrils is primarily determined by temperature and crystalline phase with additional dependence on loading rate and microstructure anisotropy. [LAUR-05-0004]     

Microdeformation mechanisms in thin films of amorphous semi-aromatic polyamides

Optical and transmission electron microscopy have been used to investigate the microdeformation mechanisms in thin films of amorphous semi-aromatic polyamides with different chemical structures and chain lengths. In the chosen range of test temperatures (between −120 °C and the principal mechanical relaxation temperature, T_α), three successive microdeformation mechanisms were identified: chain scission crazing (CSC) at the lowest temperatures, shear deformation zones (SDZs) at intermediate temperatures and chain disentanglement crazing (CDC) at the highest temperatures. The critical stress for SDZ formation was identified with the experimental yield stress, whereas the critical stresses for CSC and CDC were derived from model expressions, using experimental data for the molecular mass between entanglements, the monomeric friction coefficient and the plastic flow stress. Variations in the transition temperatures among the different polymers were attributed to differences in the temperature dependence of the yield stress, and hence to variations in chain mobility, which could be accounted for in terms of the chemical structure.     

Solvent crazing of polystyrene and polymethylmethacrylate

The minimum surface strain required to induce crazing in polymethylmethacrylate and polystyrene in the presence of alcohols and n-heptane has been determined at various temperatures by bending strips of the polymers around formers of varying curvature and immersing them in the liquid reagents. For each polymer/liquid system the long-term crazing strain was independent of test temperature except within a single 30°C interval in which it decreased as the test temperature was raised. It was found that this temperature region corresponded to the glass-rubber transition of the given polymer when, after extended periods of immersion it had achieved equilibrium liquid sorption. This suggested that, in the crazing tests, a condition approximating to equilibrium sorption was being established at the craze tip and that crazing occurred on applying small strains in the presence of the liquids because of their plasticising effect. It was found that not only the plasticizing effect of the liquid environment but also the liquid molecule size influenced the crazing strain; the larger-molecule liquids caused lower long-term crazing strains than did the smaller-molecule liquids.     

Effect of structure on the deformation and failure of oriented polymer films

This study compares the effect of anisotropic structure on the room temperature (23°C) deformation and failure behavior of two distinctly different polymers. One a single phase nematic polymer, PMDA-ODA polyimide [PI] with a glass transition, T_g, near 400°C; the other a previously studied anisotropic two phase molecular polymer composite, isotactic polypropylene [IPP] with a glass transition near 14°C. Although isotactic polypropylene has a two phase crystal-noncrystalline structure its deformation behavior is controlled by the noncrystalline phase. A series of oriented PMDA-ODA polyimide [PI] films are fabricated and their orientation characterized. The present study investigates whether the deformation and failure behavior of these anisotropic single phase nematic PMDA-ODA polyimide films (tested at a very low strain rate at room temperature) is similar to that previously observed for isotactic polypropylene at its low strain rate failure envelope limit.     

Filler/matrix-debonding and micro-mechanisms of deformation in particulate filled polypropylene composites under tension

Volume strain measurements of particulate filled polypropylene (PP) composites containing different glass beads and talc as filler were carried out in tension as a function of temperature and strain rate to determine the micro-mechanisms of deformation. While local cavitation mechanisms (micro-voiding, crazing, and micro-cracking) and subsequent debonding of the particles dominated as failure mechanisms at high strain rates and at room temperature, a more significant contribution of local shear yielding was observed with a reduced contribution of cavitational mechanisms at low strain rates or at 80 °C. This change in the dominating micro-mechanisms of deformation resulted in smaller volume strains during the tensile loading of the composites than for the respective neat matrix. Moreover, a novel approach is introduced for the detection of debonding using volume strain measurements, which takes into account the dilatational and deviatoric behavior of the neat matrix polymer and the composite. The results are supported by acoustic emission measurements carried out simultaneously on the same specimens.     

Rubber-toughening in polypropylene

To deformation and fracture behavior of several polypropylene (PP) and rubber-modified PP materials have been investigated. Plastic deformation mechanisms of these systems depend upon the test rate and temperature with high rates and low temperatures being in favor of crazing. The ductility and toughness of these materials are explained in light of the competition between crack formation and the degree of plastic deformation through crazing and shear yielding. The second phase morphology with smaller average rubber particle diameter D appears to be more efficient than that with larger D in toughening PP. Theoretical calculations indicate that the stresses imposed upon the rubber particles due to volume shrinkage of PP during crystallization are sufficient to compensate for the stresses due to differential thermal contraction in cooling from solidification temperature to end-use temperature. The difference between these two is small, and therefore they provide very little contribution to interfacial adhesion between rubber particle and PP matrix, the adhesion being insufficient for the rubber particles to be effective in controlling craze propagation. The rubber particles, in addition to promoting crazing and shear yielding, can also improve the fracture resistance of PP by varying the crystalline structure of PP (e.g., reducing the spherulite dimensions).     

Limiting regimes of deformation of polymers

The relaxation transition of polymers under the influence of deformation from the fluid to the forced high-elastic (rubbery) and leathery states has been studied. It has been shown that the investigation of the polymers in question under the conditions of uniaxial extension allows one to estimate the properties of polymers at deformation rates exceeding by 4-5 decades the rates of deformation at which simple shear can be realized. A set of critical parameters has been found for the polymers investigated which determines the regimes of their transition from the fluid to the forced rubbery and leathery states and also their fracture properties. These parameters are subdivided into two groups. The parameters of the first group refer to critical values of stress and deformation. They are invariant to temperature and molecular mass. For different polymer-homologous series the critical stresses vary by more than 10-20 times; as regards the values of strains, they vary by more than several times. The parameters of the second group define the rates of strain which bear a simple relation to the initial viscosity and can be changed within many decades. This determines the success of the procedure of reduction of fracture properties to temperature and molecular mass.     

An atomistic simulation study of the mechanisms and kinetics of surface bond strengthening in thermally-treated cone-stacked carbon nanofibers

Bonding mechanisms and rates between the active edges of a cone-stacked CNF are examined by molecular dynamics simulations at temperatures up to 2273 K. Thermally treated nanofibers subjected to tensile deformation show a substantial increase in the elastic strain limit, albeit no change in elastic modulus, due to the resistance of surface bonds to crack propagation. Two bonding mechanisms; i.e., the formation of energetically stable loops from single dangling atoms and the folding of zigzag and armchair graphene bilayer edges, are shown to display predominant, yet distinct kinetics. This study reveals a critical transition temperature at 1000 K beyond which bilayer edge folding dominates over the formation of single atom loops in strengthening the surface of CNFs. This study also underscores the critical roles played by surface bond types, numbers, and distributions on the large failure strength dispersion observed experimentally in CNFs.     

Fatigue-fracture mechanism of slowly notched poly(ethylene terephthalate) polymers

Summary Fatigue fracture behavior of slowly notched polyethylene terephathalate (PET) polymers were investigated at temperatures close to their transition temperatures up to well above their glass transition temperatures. Detailed characterization on the morphology of the notched roots showed that the crack tip during crack propagation became more dull with increasing testing temperature. The failure cycle (N_f) of these samples increased with increasing temperatures until it reached the transition temperatures of PET polymers, and most of the increase in N_f is due to the increased time consumed in the initiation period. On the other hand, the initial crack growth rate increased significantly and N_f of these samples decreased dramatically as the temperature increased well above the glass transition temperature. This interesting temperature dependence of fatigue behavior is explained due to the change of molecular motion of PET polymers at this temperature range.     

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.     

Deformation zones and entanglements in glassy polymers

Deformation zones are regions of drawn but unfibrillated material which grow from crack tips in thin films of glassy polymers which have a low value of I_e′, the chain contour length between entanglements. The growth of these zones is observed optically and their final structure characterized by transmission electron microscopy. By microdensitometry of the electron image plate the average value of the extension ratio within the deformation zone, λ_DZ, is measured. Such deformation zones have been grown in thin films of four homopolymers and a series of polymer blends. It is found that λ_DZ is approximately 0.6 λ_max, where λ_max is a predicted maximum extension ratio derived from a simple model in which the entanglement points are assumed to act as permanent crosslinks with no chain slippage or scission occurring. This value of λ_DZ is lower than the extension ratios previously measured for crazes grown in the same polymers; typical λ_craze values lie much closer to λ_max. This result can be rationalized by realizing that at least a limited degree of chain scission/slippage must occur during crazing to permit the generation of the void-fibril network. For those polymers where both crazing and deformation zones may form, the latter grow rapidly whereas the formation of crazes requires longer times. This observation also indicates the importance of the kinetic process of chain scission/chain slippage for crazing. Annealing of the polymer films below the glass transition temperature leads to an increased tendency for crazing relative to the growth of deformation zones.     

Properties and strain hardening character of polyethylene terephthalate containing Isosorbide

Polyethylene terephthalate containing Isosorbide (PEIT) polymers made from renewable corn‐derived Isosorbide monomer exhibit a wide range of glass transition temperatures (80–180°C) and are therefore able to be used in many applications. Stress–strain curves for high Isosorbide content copolymers show strain softening, which impairs the molecular orientation during orientation of films and bottles. It is therefore necessary to find ways to modify deformation behavior of PEIT copolymers. Deformation characteristics of PEIT and other polyesters have been evaluated to define stretching parameters and necessary composition for making oriented bottles for hot fill applications. In the presence of polymeric nucleating agents, (polymeric ionomers or polyesters containing sodium ions), strain‐hardening parameters become almost temperature‐ independent below solid state deformation temperature of 125°C. We developed a methodology to achieve molecular orientation comparable with films and articles made by conventional processing of poly(ethylene terephthalate), PET. Polyesters containing sodium ions are efficient nucleating agents for PEIT, and their required concentration is dependent on deformation temperature. Both strain hardening and stress at 250% strain depend on the concentration of nucleating agents and deformation temperatures. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Unsolved problems in the fracture of homogeneous thermoplastic

The steps which occur when a homogeneous polymer breaks are considered with reference to plane strain and plane stress conditions. Under plane strain fracture generally seems to take place by cavitation and crazing, though the reason for the different measured energies of crack propagation are not well understood. Other obscure steps concern the basic reason for crazing and the mechanism of craze separation. The mechanism by which polymer separates after yielding by a simultaneous process of crack propagation and plastic straining is also largely unknown as is the reason why non strain softening uniformly extending polymers appear to generate dispersed voids. Only the thermal fracture process seems to raise no fundamental difficulties.     

Rate dependent finite deformation stress-strain behavior of an ethylene methacrylic acid copolymer and an ethylene methacrylic acid butyl acrylate copolymer

The large strain deformation behaviors of an ethylene methacrylic acid (EMAA) copolymer and an ethylene methacrylic acid butyl acrylate (EMAABA) copolymer are evaluated and compared in compression over nearly eight orders of magnitude in strain rate, from 10⁻⁴ to almost 10⁴/s. Transition regimes are quantified using dynamic mechanical analysis. The stress-strain behavior of these copolymers exhibits a relatively stiff initial behavior followed by a rollover to a more compliant response. The low strain modulus, the rollover stress and the large deformation stress-strain behavior are strongly dependent on strain rate. The proximity of the material glass transition to the room temperature test conditions results in a substantial change in the nature of the rate sensitivity of the stress-strain behavior as one moves over the range of strain rates. The mechanical behavior of the EMAA is contrasted to that of a corresponding EMAABA terpolymer and to its sodium-neutralized counterpart (EMAABA_Na). The nature of the rate sensitivity of the room temperature stress-strain behavior of EMAA transitions from a behavior near the glassy end of the leathery regime at low rates to a near glassy behavior at high rates. The butyl acrylate content in the EMAABA lowers the glass transition temperature and leads to a more compliant mechanical behavior (reduced initial stiffness, reduced rollover stress, reduced post-rollover stress level) at room temperature. The EMAABA behavior transitions from a rubbery-like behavior at the lowest rates to a leathery-like behavior at the highest rates. Upon sodium neutralization, the overall stiffness and flow stress levels are enhanced likely due to the presence of the ionic aggregates; the glass transition of EMAABA_Na is broadened in comparison to the EMAABA, giving a rate dependent room temperature behavior that transitions through the leathery regime with increasing strain rate. A constitutive model that separately accounts for the distinct deformation resistances of the crystalline domains and the amorphous domains is able to capture the changes in rate dependent deformation behavior of the EMAA copolymers studied herein. The crystalline domains provide resistance to flow across a wide window in rate and temperature whereas the amorphous domains provide increasing resistance as the strain rate is increased and the material effectively transitions through the glass transition regime, providing a mechanism for changing rate sensitivity.     

Similarity of dynamic mechanical transitions of thermotropic polyesters in extension and torsion

The frequency dependence of the dynamic mechanical transitions of several thermotropic polyesters were measured in tension on highly oriented fibres and in torsion on moulded bars. The transition temperatures were not affected by the differences in the mode of deformation and in the degree of orientation of the test specimens. It is concluded that the molecular motions responsible for the transitions, and the environment in which they occur, are independent of the macroscopic orientation. They are determined, rather, by the local state of orientation, which is highly ordered in both test specimens. The results are also consistent with other data, which suggest that the macroscopic tensile modulus of liquid-crystal polymers, over a range of time and of temperature, is strongly influenced by variation of the shear modulus.     

The detergent stress-cracking of polythene

Whenever the fracture of a polymer is preceded by crazing or other types of cavity formation, energy must be supplied both for local yielding and for the generation of a new surface. However previous calculations for rigid glassy polymers have shown that the surface energy contribution required for the formation of a craze, compared with that associated with plastic and elastic strains is small. In order to study this problem further, we have now made a finite element analysis of the cavitation of an elastic-plastic solid introducing a surface energy term. This shows that the ratio of the plastic energy requirement to surface energy depends, not unexpectedly, on (I) the ratio of yield stress/ surface tension and (II) cavity size. Thus for polymers with a low yield stress such as polythene, the surface tension becomes significant if the hole diameter is below 100 nm. In these circumstances crazing and cracking is predictably accelerated by dilute aqueous detergents which do not need to penetrate the polymer, provided of course that the detergent has access to the cavities. Hole growth also depends on the extent of orientation hardening during plastic strain and this contributes to the greater stress crack resistance of high molecular weight polythene. The occurrence of a larger amount of plastic deformation on the fracture surfaces of high molecular weight polymer has been demonstrated by Harman for high density polythene and is confirmed here in scanning electron micrographs of low density material. Other electron micrographs show that very small cacities are indeed formed when high density polythene is broken in the coventional detergent stress cracking test, thus accounting for the accelerating effect of the detergent.     

Viscoelastic properties of linear polymers in the fluid state and their transition to the high-elastic state

The experimental results of the Viscoelastic properties of linear polymers of narrow molecular weight distribution (MWD) and of their mixtures have been analyzed and generalized. Based on the study of the properties of polymers of narrow MWD, we propose a classification of high molecular weight compounds. It specifies a distinct boundary between oligomers and polymers, assuming that the most important feature of polymers is the manifestation of large high-elastic recoverable deformations of entropy character. For polymers to be characterized, not the absolute molecular weight is essential, but the molecular weight referred to the boundary values. The corresponding state for polymers is attained at temperatures 100°C away from the glass temperature. The transition from the fluid to the high-elastic state with increasing deformation rate (or frequency for cyclic deformation) has been studied. Transition to the high-elastic state takes place over a narrow stress range (0.1-1.0 dynes/cm²), independent of molecular weight, whereas the critical deformation rates (frequencies), like viscosity, depend greatly on molecular weight. An increase in the amount of deformation shifts, to u certain extent, this transition to lower Kites of deformation (frequencies). In the region of deformation rates (frequencies) corresponding to the high-elastic state, the effect of large deformations during shear manifests itself largely in the tear-off of polymers Iron, the confining surfaces and in specimen rupture. Polydispersity has a strong effect on the properties of polymeric systems. As the rate of deformation is increased, the transition proceeds successively from the higher molecular weight components. This relaxational transition is tantamount to a change of the structure for polymeric systems. It is responsible for non-linear, particularly, non-Newtonian behavior of such systems. The transition to the high-elastic state and all the related phenomena are observed also in concentrated solutions of high molecular weight polymers. The long-term durability of un-cured rubbers in the high-elastic state is described by the same relationships.