The temperature and strain-rate dependence of mechanical properties in polyoxymethylene

The computer aided design approach used in current applications of semicrystalline polyoxymethylene (POM) requires high strain-rate mechanical data. The primary aim of this work has been to measure the room temperature modulus and tensile strength of injection molded samples of POM of different molecular weights at cross-head speeds of between 10^?5 ms^?1. We observe no major transition in bulk mechanical behavior in this range of test speeds, the Young's modulus E, in particular, showing little strain rate dependence. This is rationalized on the basis of tensile tests over a range of temperatures, these indicating room temperature to correspond to the plateau in the E(T) curves (T_g for these materials is taken to be ?70°C, and the DSC melting onset occurs at ～ 170°C). The tensile strength increases as ～log(d?/dt) and the behavior is found to be highly nonlinear, strains to fail of the order of 1 being observed even at the highest strain rates, depending on the molecular weight. It is believed that the yield stress of th crystalline regions determines the tensile strength above T_g, the higher degree of crystallinity associated with lower molecular weights resulting in a slightly higher tensile strength. Nevertheless, failure is qualitatively brittle, with no necking and relatively little permanent deformation. This behavior is discussed in terms of morphological investigations of the fractured samples by optical and scanning electron microscopy (SEM). In attempting to relate ultimate failure to the molecular/crystalline structure of the samples, measurements of the critical stress intensity for crack initiation in mode I opening, K_IC, as a function of crystallization temperature T_c have been carried out using compact tension specimens machined from injection molded and compression molded plaques. K_IC increases with molecular weight and decreases with T_c at low test speeds (in spite of an increase in crystallinity with T_c). This is accounted for in terms of a crack shielding model for crack initiation and of molecular rearrangements occurring during crystallization which lead to a decrease in the effective entanglement density with T_c. The implications of this model are then compared with K_IC results over a range of cross-head speeds and temperatures.     

EWF method to study long term fracture properties of cross‐linked polyethylene

The influence of cross‐linking on fracture properties of polyethylene has been studied by the method of essential work of fracture (EWF). Three distinct values of the cross‐link density were obtained by β irradiation with three distinct radiation doses, and characterized by sol‐gel and solvent swelling measurements. EWF tests were performed at speeds of 0.045 to 100 mm min^–1 at 80 and 110°C. The fracture toughness, as defined from the EWF method, is analyzed through essential (w_e) and nonessential (βw_p) components. Both values were found to be decreasing functions of the cross‐link density. But cross‐linking also suppresses the toughness decrease observed in linear PE at low speeds/high temperatures. It is hypothesized that cross‐linking prevents the chain disentanglement that occurs during crack growth. In conclusion, EWF tests performed at low speeds appear as an interesting method to characterize the influence of structural factors on the fracture properties of polyethylene. POLYM. ENG. SCI., 45:424–431, 2005. © 2005 Society of Plastics Engineers     

Gel permeation chromatography of polyoxymethylene

Gel permeation chromatography of polyoxymethylene has been studied using N,N-dimethylformamide as the solvent. Polyoxymethylene samples used here are a copolymer of tetraoxane with 1,3-dioxolane and a commercial polyoxymethylene whose molecular weight distributions are moderately broad. Their intrinsic viscosities [η] range from 1.4 to 2.8 dl/g. Factors affecting chromatograms are discussed, and the operating conditions were determined by using the analytical scale GPC. On the basis of these operating conditions, the molecular weight fractionation of polyoxymethylene was carried out by using the preparative scale GPC. It was found that polyoxymethylene can be effectively fractionated to give seven to ten fractions each of them containing the fractionated polymer ranging in weight from 0.2 to 8 mg when 40 mg polymer sample was used for a run of the measurement. The fractionated polymers were also found to have a narrow molecular weight distribution within a single peak, and their M_w/M_n values decrease with increasing molecular weight.     

Drop weight impact and work of fracture of short glass fiber reinforced polypropylene composites toughened with elastomer

Short glass fiber (SGF) reinforced polypropylene composites toughened with styrene‐ethylene butylene‐styrene (SEBS) or maleated SEBS (SEBS‐g‐MA) triblock copolymer were injection molded. Charpy drop‐weight impact properties and the impact essential work of fracture (EWF) of the SGF/SEBS/PP and SGF/SEBS‐g‐MA/PP hybrids were investigated. Drop‐weight impact results revealed that the SGF/SEBS/PP hybrid exhibits higher impact strength than the SGF/SEBS‐g‐MA/PP hybrid at low impact speeds. This was derived from the pull‐out of fibers from the SGF/SEBS/PP hybrid. At high impact speeds, the impact strength of the SGF/SEBS‐g‐MA/PP hybrid was slightly higher than that of the SGF/SEBS/PP hybrid. Impact EWF measurements showed that the hybrids only exhibit specific essential work (W_e) at a high impact speed of 3 ms⁻¹. The non‐essential work does not occur in the hybrids under high impact rate loading conditions. Moreover, SEBS or SEBS‐g‐MA addition was beneficial in enhancing the high‐rate specific essential work of the SGF/SEBS/PP and SGF/SEBS‐g‐MA/PP hybrid composites.     

An essential work of fracture study of the toughness of thermoset polyester coatings

The fracture toughness of a range of thermoset polyester paints with different cross-link densities has been studied, using the essential work of fracture (EWF) method. The glass transition temperature, T_g, of each of the materials was measured using differential scanning calorimetry, and found to lie between 8 and 46 °C. EWF tests were performed on the paint films at a range of temperatures around the measured glass transition temperature of each material. The essential work of fracture, w_e, at T_g was found to decrease with increasing cross-link density from around 20 kJ/m² at a cross-link density of 0.4 × 10⁻³ mol/cm³ to around 5 kJ/m² for cross-link densities of approximately 1 × 10⁻³ mol/cm³ or higher. A maximum in the essential work of fracture was observed at around T_g when w_e was plotted versus temperature, which could be attributed to the effect of an α-relaxation at a molecular level. The polyesters were found to be visco-elastic, and the applicability of the EWF test to the study of these visco-elastic thermoset materials is discussed.     

Studies of the fractionation of polyoxymethylene

The precipitation of polyoxymethylene in p-chlorophenol solution and the molecular weight fractionation of the polymer by mechanical agitation were investigated. The agitation of the solution was carried out in a glass vessel with a stirrer, usually at 60°C. After agitation for several minutes a fibrous polymer precipitated. High molecular weight polymer precipitated around the stirrer in an early stage, and therefore the method might be applied to the fractionation of polyoxymethylene. The method was applied to the fractionation of polyoxymethylene prepared in a solid-state and in a solution polymerization of trioxane, catalyzed by BF₃.OEt₂. It was found that the polymer from the solid state contained a small amount of extremely high molecular weight fraction, and that obtained from the solution had a relatively narrow distribution of molecular weight.     

The brittle‐ductile transition temperature of polycarbonate as a function of test speed

The fracture behavior of polycarbonate was studied as a function of temperature (?80°C to +80°C) and test speed (10^?5 to 10 m/s) using an instrumented, singleedged, notched tensile test (SENT). SENT tests give information on the fracture stress, fracture displacements, and fracture energies of polycarbonate, and from these data the average crack speeds were calculated and the brittle‐ductile transitions were determined. The fracture stress and the fracture energies of ductile fracturing samples increased with increasing test speed. The fracture surfaces were studied by scanning electron analysis, and sometimes a mixed mode fracture, part ductile and part brittle, could be seen. At high test speeds, a sharp brittle‐ductile transition was observed, while at low test speeds the transition was more gradual, via a mixed mode region. This mixed mode region decreased in size with increasing test speed and was absent at the higher test speeds. The average crack speeds in the ductile region were directly related to the test speeds. The brittle‐ductile transition temperature increased with the logarithmic of the test speed.     

Effects of molecular structure on the essential work of fracture of amorphous copolyesters at various deformation rates

The fracture behavior of amorphous copolyesters with different molecular structure was studied with double edge notched tensile loaded specimens (DENT) using the essential work of fracture (EWF) approach. Various deformation rates ranging from 1 to 1000 mm/min were employed. Amorphous poly(ethylene terephthalate) (aPET) exhibited considerably higher specific essential and non-essential work of fracture than the copolyesters containing either cyclohexylenedimethylene (aPET-C) or neopentyl glycols (aPET-N). At high deformation rates, ductile/brittle fracture transition was observed with aPET-C and aPET-N, while aPET always fractured in ductile mode within the entire deformation rate range. These phenomena were ascribed to the different molecular flexibility and entanglement density of the copolyesters. The specific EWF of the aPET as a function of deformation rate went through a minimum. The initial decrease in toughness was caused by the hampered segmental mobility due to the increased deformation rate. The subsequent increase in toughness was attributed to the adiabatic heating induced temperature rise in the process and plastic zones. Strain-induced crystallization of the aPET was observed at ν=500 and 1000 mm/min, which may also contribute to the increase of the specific EWF.     

Application of essential work of fracture concept to toughness characterization of high‐density polyethylene

Deformation and fracture toughness of high‐density polyethylene (HDPE) in plane‐stress tension was studied using the concept of essential work of fracture (EWF). Strain range for necking was determined from uniaxial tensile test, and was used to explain the deformation transition for 2‐staged crack growth in double‐edge‐notched tensile test. Through work‐partitioning, EWF values for HDPE were determined for each stage of the crack growth. Appropriateness of these EWF values to represent the material toughness is discussed. The study concludes that the EWF values for ductile polymers like HDPE may not be constant, but vary with the deformation behaviour involved in the crack growth process. POLYM. ENG. SCI., 47:1327–1337, 2007. © 2007 Society of Plastics Engineers     

Dielectric behaviour of a γ-irradiated polycarbonate

Summary Dielectric measurements on a -irradiated polycarbonate in a dose range 0–20 Mrad and at test frequencies of 10³ and 10⁴ Hz have been carried out. The glass transition temperature shows a maximum at 3.5 Mrad and then decreases at larger doses. This result is in line with other experiments (intrinsic viscosity, dynamic-mechanical measurements, etc) already reported: the Tg values are however larger in the present results. This is probably due to the plasticizing action of the absorbed moisture present in the previous tests. A Tg vs 1/[n]² plot (1/[n]² is proportional to 1/m ) allows to find the value of the glass transition point at an infinite molecular weight and the dependency of Tg on molecular weight.     

Essential work of fracture (EWF) analysis for compression molded alternating poly(propylene carbonate)

In this investigation, the main objective was to study the mechanical properties of alternating poly(propylene carbonate) copolymer (PPC). The PPC used in this study was derived from propylene oxide and carbon dioxide using zinc glutarate as catalyst. The molecular weight of the PPC copolymer used in this study has M?_n～33,000. The synthesized PPC was compression molded into sheets of thickness ～1mm. The fracture toughness of the PPC films was determined using the essential work of fracture (EWF) technique, at a laboratory temperature of 20°C, and a loading rate of 1 mm/min. During the EWF measurement, a significant amount of plastic deformation has taken place around the initial ligament region. The measured specific total fracture work (w_f) was observed to vary in a linear fashion with the specimen ligament (l), and hence satisfied the basic requirement for EWF analysis. The specific essential fracture work (w_e) for the PPC film was measured to be 11.0 kJ/m². The PPC showed a prominent recovery behavior. The severely deformed region surrounding the fracture ligament was observed to recover completely 8 days after fracture testing. Polym. Eng. Sci. 44:580–587, 2004. © 2004 Society of Plastics Engineers.     

Influence of molecular weight on fatigue behavior of polyethylene and polystyrene

Fatigue tests in reversed tension-compression have been carried out on samples of polyethylene and polystyrene of widely varying molecular weights, extending up to 2, 000, 000. All tests on polystyrene specimens were made at 1600 rpm. For polyethylene, tests speeds had to be reduced to 100 rpm in order to avoid serious temperature effects. For both materials, increasing molecular weight leads to improved resistance to alternating loading. For polystyrene, this improvement in ultimate properties even continued well beyond molecular weight values where T_g, becomes effectively independent of molecular weight. For polyethylene, samples of high molecular weight did not fail even after 10⁷ cycles of alternating loading at a stress level of 3000 psi.     

On the essential work of fracture method: Energy partitioning of the fracture process in iPP films

Summary The fracture properties of an iPP are investigated by the EWF method. A separation between crack initiation and propagation fracture parameters is done by splitting the total energy of the load-displacement curves in two. The influence of the DDENT specimen height and the test rate on these different parameters is studied, obtaining that varying the height has no influence in the range 40 to 80mm, but changing the crosshead speed (2 to 100mm/min) has an effect on the fracture parameters. It is interesting to note that the “Initiation Specific Essential Work” (w_e ^I) seems not to be sensible to the stress-state transition. Received: 20 October 1998/Revised version: 1 December 1998/Accepted: 7 December 1998     

Impact modification of engineering thermoplastics

Impact modification was studied for a variety of engineering thermoplastics to determine if notched Izod data obtained at various temperatures and modifier concentrations could be correlated with particle size or surface-to-surface interparticle distance of the modifier. Elastomers evaluated were characteristic of those used in commercial blend systems for those polymers, and both functionalized and nonfunctionalized materials were studied. For the single matrix polymer/elastomer-modified blend systems studied [poly(phenylene sulfide) (PPS), polyoxymethylene (POM), poly(butylene terephthalate) (PBT)], elastomer interparticle distance provides a better correlation to brittle–tough transition temperature than does particle size, as predicted by the Wu model. In POM, the dispersion morphology of the samples used was not adequate to achieve the critical interparticle distance required for supertoughening at room temperature. In this study, the critical interparticle distance has been shown to depend on the degree of crystallinity (PPS) and the modulus of the impact modifier relative to the matrix (PBT). Actual adhesion of the polymer to the matrix (variation of functionality levels) was not found to have a strong influence (PBT). In POM, the increase in impact at the brittle–tough transition was dependent on the molecular weight of the base resin. This is examined with respect to the ratio of the molecular weight (M_n) to the entanglement molecular weight (M_e), which determines the critical molecular weight necessary to achieve useful physical properties. In polyester (PET)/polycarbonate (PC)/elastomer blends, the molecular weight of the primary matrix resin (PET) determined impact properties within the molecular weight range of the resin studied. This was again related to the M_n/M_e ratio for PET and PC. © 1994 John Wiley & Sons, Inc.     

Strain rate dependence of the work of fracture response of an amorphous poly(ethylene‐naphthalate) (PEN) film

The essential work of fracture (EWF) response of an amorphous film of high molecular weight poly(ethylene‐2, 6‐naphthalate) (PEN) was determined on tensileloaded deeply double‐edge notched (DDEN‐T) specimens at various deformation rates (v=1, 10 and 100 mm/min) at ambient temperature. The DDEN‐T specimens showed full ligament yielding (marked by a load drop in the force‐displacement curves) based on which the yielding was separated from the subsequent necking. The yielding related specific essential work of fracture (W_e,y) changed parallel with the yield strength (σ_y), whereas the critical crack tip opening displacement (?_y, 0) remained practically constant in the v range studied. As a consequence, W_e,y(v) could well be estimated by the product of σ_y(v).?_y,0. Necking occurred by cold drawing superimposed by some strain‐induced crystallization at the highest deformation rate. The necking‐related EWF terms strongly depended on the deformation (strain) rate. Based on previous results on amorphous PENs of various molecular weight (MW), it was argued that increasing deformation rate corresponds to decreasing MW and vice versa. This was reasoned by assuming that only a part of the entangled molecular chain participates in the load distribution under high strain rate conditions.     

Dielectric investigation of molecular dynamics of blends: III. Effect of molecular weight in TMPC/PS blends

Dielectric and calorimetric measurements have been carried out for tetramethyl polycarbonate/polystyrene (TMPC/PS) blends with different compositions. The effect of varying the molecular weight of the weakly polar component (PS) on the molecular dynamics of the polar segments of TMPC has been thoroughly studied over wide ranges of frequency (10⁻²−10⁵ Hz), temperature (50–220°C) and number average molecular weight, M̄_n, (6500–560 000 g mol⁻¹). All blends were found to be compatible regardless of the composition ratio and the molecular weight of PS. Some new and interesting experimental findings have been observed concerning the effect of molecular weight on the glass temperature and on the broadness of the glass transition and relaxation. Neither the kinetics nor the distribution of relaxation times of the local process observed in pure TMPC was affected by blending with PS, regardless of the composition ratio or the molecular weight of PS. It has been concluded that the mixing of the polymeric components to form a homogeneous single phase (compatible blend) does not take place on a segmental level but on a structural one. The size of this structural level has been suggested to have the same volume as the cooperative dipoles, which is assumed to be the minimum volume responsible for a uniform glass transition (10–15 nm). The molecular weight dependence of the relaxation characteristics of the glass process and temperature could be attributed to the variation in the size and packing of the structural units.     

Analysis of the effect of the test rate on polymer essential work of fracture parameters for the small punch test

In recent decades, one of the non‐standard tests that has been consolidated as a viable alternative in those cases where there is not sufficient material to carry out standard tests is the small punch test. This test basically consists of deforming a miniature specimen using a high strength punch. It is possible for this miniature specimen to have an initial pre‐notch with the aim of improving the fracture behavior estimation of the material analyzed. Recently, to characterize the fracture properties of polymer sheets under plane stress conditions, there has been an attempt to establish the feasibility of applying the essential work of fracture (EWF) method in polymer pre‐notched miniature specimens. This article intends to go one step beyond and focuses on the test rate, which is an important aspect in the EWF application. Its effect on the EWF parameters in polymer pre‐notched miniature specimens has been analyzed and its correlation has been established with the results obtained from standard specimens. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43314.     

Influence of molecular weight on the fracture properties of aliphatic polyketone terpolymers

The influence of polymer molecular weight on the mechanical properties of aliphatic polyketones was investigated. The molecular weight varied from 100,000 to 300,000 g mol⁻¹. The crystallinity was found to be independent of polymer molecular weight, as was the glass transition temperature. The yield strength and stiffness of the aliphatic polyketone terpolymers were also found to be independent of molecular weight. The post yield behaviour showed strong dependency on polymer chain length. The draw stress was increased significantly with higher molecular weight material. The impact resistance was increased with molecular weight, resulting in ductile fractures with large energy consumption upon fracture. The brittle-to-ductile transition temperature was lowered with increasing chain length. The difference in material deformation was linked to the higher mechanical connectivity and more stable post yield behaviour of the polymers with an increased molecular weight.     

On the measurement of fracture toughness of soft biogel

In this study, the rate dependent energy dissipation process and the fracture toughness of physical gels were investigated using agarose as a sample material. Both the J‐integral and Essential work of Fracture (EWF) methods were examined. To assess the quasi‐static fracture toughness of gels, linear regression was performed on critical J (J_c) values at different loading rates resulting in a quasi‐static J_c value of 6.5 J/m². This is close to the quasi‐static EWF value of 5.3 J/m² obtained by performing EWF tests at a quasi‐static loading rate (crosshead speed of less than 2 mm/min). Nearly constant crack propagation rates at low loading rates, regardless of crack length, suggest viscoplastic chain pull‐out is the fracture mechanism. At high loading rates failure was highly brittle, which is attributed to sufficient elastic energy accumulation to precipitate failure by chain scission. We conclude that in physical gels quasi‐static fracture toughness can be evaluated by both the J‐integral and EWF methods provided the effects of loading rate are investigated and accounted for. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers