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 Tg. 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 Tg, 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.

Using the theoretical approach of Bersted and Slee, we developed a technique of determining the molecular weight distribution (MWD) from the viscosity vs. shear rate flow curve. Each molecular weight fraction was assumed to have a characteristic deformation rate. Below this rate, the viscosity was equal to the zero shear value. Above this rate, the viscosity was reduced to the zero shear viscosity of the molecular weight characteristic of the higher rate. The relationship between the weight fraction of each component and the viscosityjrate curve was derived from these assumptions. The flow curves of very well-characterized polystyrenes were determined. MWDs were then calculated using the Bersted-Slee approach and compared with results from size exclusion chromatography. Comparisons were also made with the results of Malkin and Teishev. They used the same assumptions as Bersted and Slee but chose a different methodology for determining the MWD.

SYNOPSISMeasurements using compact tension specimens machined from compression and injectionmolded plaques of polyoxymethylene show a systematic decrease in the plane-strain critical stress intensity K,, for crack initiation with crystallization temperature and with molecular weight, but little change in the yield stress in plane strain compression tests. The variation in with crystallization temperature has been accounted for in terms of the effective entanglement density in the damage zone ahead of the crack tip. The entanglement density is argued to decrease with increasing temperature owing to partial disentanglement of the polymer chains during lamellar folding.

The experimental arrangement for conducting fracture tests in the scanning electron microscope (SEM) are described together with the particular problems in applying this technique to polymers and composite materials. Several examples are given of mechanisms of fracture in epoxy resins both unfilled and reinforced with particulate and fibrous fillers. The paper is illustrated by still photos taken from the video recording of the fracture experiments and by photos taken from the SEM raster.

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