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The mechanical properties of extruded films of isotactic polypropylene (iPP) and ethylene-propylene rubber toughened iPP (iPP/EPR) have been analyzed in terms of the specific essential and non-essential work of fracture using high speed tensile test apparatus modified for quasi-static testing in the range 0.0001 to 3 [ms.sup.-1]. Three iPP/EPR grades with modifier contents of 15, 21 and 30 vol% were investigated. As the deformation rate increased, relatively uniform necking of the whole ligament and extensive plastic deformation were progressively replaced by more localized plastic deformation in the blends, and by fully brittle fracture in the unmodified iPP. The non-essential work of fracture and the total fracture energy were highly sensitive to these changes in deformation mechanism. However, the essential work of fracture, although dependent on the test speed, was less correlated with the extent of global plasticity., INTRODUCTION Improvements in the impact properties of engineering polymers such as isotactic polypropylene (iPP) are usually achieved by incorporating dispersed second phase domains during either polymerization or melt processing (1, [...]

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.sup.-5] [ms.sup.-1] and 10 [ms.sup.-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.sub.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(dζ/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 the crystalline regions determines the tensile strength above [T.sub.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 1 opening, [K.sub.IC], as a function of crystallization temperature [T.sub.c] have been carried out using compact tension specimens machined from injection molded and compression molded plaques. [K.sub.IC] increases with molecular weight and decreases with T, at low test speeds (in spite of an increase in crystallinity with T,). 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.sub.c]. The implications of this model are then compared with [K.sub.IC] results over a range of cross-head speeds and temperatures., INTRODUCTION Polyoxymethylene (POM) is a highly crystalline polymer with a sub-ambient glass transition temperature [T.sub.g] and a melting point of approximately 170'C depending on the crystallization temperature. Its mechanical properties [...]

Transmission electron microscopy and optical studies of thin films of isotactic polystyrene (iPS) and polyoxymethylene (POM) provide evidence for a distinction between crazing mechanisms in semicrystalline polymers above and below T(sub g) In the latter temperature regime, deformation in iPS and POM crystallized at high supercooling has been discussed in terms of existing models for crazing in amorphous glassy polymers, based on entanglement ideas. Above T(sub g), where the difference in mechanical behavior of the amorphous and crystalline regions becomes marked, the fibrillar nature of local deformation appears to be a consequence of the inhomogeneity of the undeformed polymer.