The mechanical behavior of blends of polyethylene,polypropylene, and an ethylene–propylene block copolymer at −20°C

Compatibilization is necessary for most binary blends which display poor mechanical properties. The addition of an ethylene–propylene block copolymer to a blend of isotactic polypropylene and linear low-density polyethylene alleviates the problem of poor adhesion at the interface. This was observed through the improvement in overall performance of the blend. It was noted that it is not solely the “interfacial agent” which is responsible for the improvement in impact strength of this blend. © 1992 John Wiley & Sons, Inc.     

Microstructure of Horseshoe Nails Using Neutron Diffraction

Neutron diffraction allows nondestructive testing of the bulk microstructure of mechanical components. The microstructures of horseshoe nails made through three different processes have been explored as a function of position along the nail. Despite all nails being made of similar plain low-carbon steel and being process annealed after manufacture, the microstructures are far from the same. Nails made from strip, using a cold forging stamping process, show narrower diffraction peaks indicating a narrower distribution of lattice parameters and also show diffraction peak intensity ratios closer to those expected for unstrained steel. Thus, the distribution of the orientation of grains in these nails is closer to that of undistorted steel compared to nails made through the other two processes considered—one a drawing from wire and the other a combination of rolling and cold forging. The blades of the drawn nails showed little preferred orientation but the converse was true in the heads. Differing patterns of preferred orientation suggest that the various manufacturing approaches result in substantially different mechanical advantages for the three types of nails, a result in accord with mechanical testing.     

A Concordant Shift Model for Flow in Bulk Metallic Glasses

The homogeneous plastic flow in bulk metallic glasses(BMGs) must be elucidated by an appropriate atomistic mechanism. It is proposed that a so-called concordant shifting model, based on rearrangements of five-atom subclusters,can describe the plastic strain behaviour of BMGs in a temperature range from room temperature to the supercooled liquid region. To confirm the effectiveness of the atomic concordant shifting model, a comparative investigation between the vacancy/atom model and the concordant shifting model is carried out based on the estimation of the strain rate deduced from two models. Our findings suggest that the atomic concordant shifting model rather than the vacancy/atom exchange model can well predict the large strain rate in the superplasticity of BMGs.     

Micromechanics of stress relaxation in amorphous glassy PMMA. Part I. Molecular model for anelastic behaviour

This paper describes the development of a theoretical model for the strength of mechanical relaxation in terms of micromechanics of deformation. Polymethyl methacrylate (PMMA) is used as the model amorphous polymer. The internal molecular rearrangements during relaxation are identified and accounted for by the rotation of specific atomic groups. Voronoi tessellation is used as a method to characterise the nanostructure in amorphous glassy polymer below T_g. A theoretical model is postulated and shown to provide a limited quantitative prediction capacity of the anelastic deformation and the corresponding stress relaxation based on measurable molecular parameters without adjustable factors.     

A study of water absorption characteristics of DGEBA/DDS resin system using near infra-red spectroscopy

Summary Water absorption and desorption of the diglycidyl ether of bisphenol A (DGEBA)/4,4-diaminodiphenyl sulfone (DDS) resin systems, cured isothermally at three different temperatures (160, 180 and 220°C), were monitored at 80°C, as a function of soaking and drying times. New near infra-red spectroscopic (NIR) technique as well as the conventional gravimetric method were used.The data from two different techniques were reasonably identical showing that the new NIR technique is valid. The resin systems cured at higher temperature absorbed more water. The degree of structural packing of the network systems has been estimated from the observations in water absorption characteristics.     

Crystallization of poly(ε-caprolactone)

The isothermal crystallization behaviour of poly(ε-caprolactone), PCL, has been investigated by dilatometry and optical microscopy. Nucleation rates and spherulitic growth rates have been measured. At all temperatures tested a change in nucleation rate was observed early during the crystallization. Growth rates were linear over the whole of the crystallization range. The experimental results were analysed using the Avrami equation in which the experimentally observed time dependence of nucleation is used. The equation contains integer values of the Avrami exponent and describes adequately the crystallization behaviour of PCL. The difference between the apparent and true nucleation rates is emphasized, and difficulties in the calculation of rate constants are discussed.     

Microstructural effects and the toughening of thermoplastic modified epoxy resins

We have studied an epoxy resin formulation consisting of the diglycidyl ether of bisphenol-A (DGEBA), modified with phenolic hydroxyl-terminated polysulfone (PSF) and cured with an aromatic amine curing agent, diaminodiphenyl sulfone (DDS). A range of microstructures and fracture properties have been obtained by controlling the formulation cure conditions (cure temperature and cure cycle in an isothermal mode). The chemical conversion of the cured resins has been monitored by near-infrared spectroscopy (NIR). Although only a single material formulation was used, three distinct types of microstructure were identified by scanning electron microscope (SEM) observations on samples prepared at different cure temperatures. Surprisingly, the thermal and fracture properties of the cured samples did not vary noticeably, in spite of the significant microstructure variations. The consistency of these fracture toughness results with cure temperature changes was an unexpected result in the light of our earlier observations of a strong dependence of fracture toughness on cure temperature in neat resin systems. The difference in behavior between neat and modified resins reveals that the fracture toughness of the latter is dependent on a combination of the microstructure and the matrix resin properties. This hypothesis was also supported by an observation of high fracture thoughness in a sample cured in a two-step process, which we believe is due to the optimum microstructure and matrix resin properties, being achieved separately during precure and postcure, respectively. The increase in fracture toughness values caused by the modification (ΔG_IC) was calculated from the fracture toughness values of neat and modified resins, prepared under the same cure conditions, using a proposed theoretical equation. © 1993 John Wiley & Sons, Inc.