Polyoxymethylene, oriented

From Fig. 2a it is evident that as the test temperature raises from 0 up to 70 °C the resulting load-displacement curves display a decrease of the maximum load and an increase of the elongation at failure. On the other hand, as reported in Fig. 2b, when the displacement rate increases the load-displacement curves evidence an increase of the maximum load and, quite surprisingly, also the elongation at break increases. It is worth noting, that this rather unexpected rate effect on the load-displacement curves of notched samples has been already reported by Karger-Kocsis and co-workers for both biaxially oriented filled PET [7] and amorphous copolyester [9], and by Plummer et al. for polyoxymethylene tested at high temperature [14]. 

The influence of external variables, i.e. testing and environmental conditions such as rate of loading, temperature and moisture, on fracture mechanisms and hence on fracture toughness in short fibre polymer composites is investigated as a function of fibre orientation with respect to the fracture plane. Unidirectionally oriented materials with different polymer matrix (polyamide 6.6 and polyoxymethylene) and different glass fibre content (15wt% and 30wt%) are examined. 

Fig. 5.22 Experimental and simulated contour plots of two-dimensional spectra of highly oriented polyoxymethylene. The bottom part of the diagram shows the sub-spectra corresponding to jumps through various angles. The main diagonal runs from bottom left to top right In each figure. (Reproduced by permission of Academic Press.)...

In injection molding of semicrystalline thermoplastics, for example polyamide, polyoxymethylene, ultrahigh molecular polyethylene (very high melt viscosity), linear polyesters (PET, PBT), local variations in molecular orientation result in dimensional variations in the part (warpage, memory effect), especially if the toothed gear is exposed to a thermal stress load. 

The pioneering work of Slichter, and Hyndman and Origlio was followed by a systematic investigation of oriented linear polyethylene and oriented polyoxymethylene over a wide range of temperatures by Olf and... 

Figure 1.10. Surface replica of molded polyoxymethylene fractured at liquid nitrogen temperatures. While the lamellae to the lower left are oriented at an angle to the fracture surface, the lamellae elsewhere are nearly parallel to the fracture surface. The important point is that lamellae (resembling those in single crystals) are stacked up like cards or dishes in the bulk state. (Geil, 1963.)...

Other Thermoplastics. Polyoxymethylene (POM) was imaged by afm, revealing oriented polymer chains parallel with the machine axis of sample extrusion (Fig. 15). Atomic scale resolution of the chains demonstrated the helical nature of the polymer chains. Long-range correlation between poljuner chains was observed as well (97). Imaging of extended chain crystals of POM closely matched molecular models for this material, allowing for the molecular packing and order in the extended chain crystal to be well understood. The authors were able to describe the polymer chain orientations with respect to the crystal (98). 

The SSE process based on changes in polymer-billet shape is, first of all, characterized by the EDR value. The limited EDR value is determined by polymer type, molecular weight and morphology. For example, for HDPE it may reach more than 40 (1). For polyoxymethylene (POM), polypropylene (PP), PTFE the maximum values of draw ratio equal 10,6, and 4 (20-22), respectively. For polymethylmethacrylate (PMMA), polystyrene (PS), polycarbonate (PC), and other amorphous polymers they are even lower (21,23). The extruded specimens are highly oriented and possess improved mechanical properties. In the case of semicrystalline polymers, the extrusion results in a considerable increase of tensile modulus and strength (1,2). With amorphous polymers, a considerable increase of plasticity is observed alongside with increase of tensile moduli (21,23). 

For oriented samples of polyoxymethylene, neutron up-scattering spectra were measured by Trevino and Boutin (1966,1967). The observed peak positions (568, 430 and 248cm" ) correspond fairly well with the positions of amplitude-weighted distribution peaks. However, the peak at 430cm was observed to be appreciably longitudinal, in contrast to the low anisotropy theoretically expected from vibrational displacements of hydrogen nuclei... 

Snetivy D, Vancso GJ. Selective visualization of atoms in extended-chain crystals of oriented polyoxymethylene by atomic force microscopy. Macromolecules 1992 25 3320. 

Figure 8.12 Polymer decoration epitaxial crystallization on the fold surface. Polyethylene vapors were deposited on the surface of single crystals of polyoxymethylene (POM) at room temperature. The resulting PE decoration is made of edge-on lamellae oriented normal to the POM growth faces, with the deposited PE chains parallel to the growth front. The distance between POM fold planes (0.386 nm) imposes the monoclinic (010) PE contact plane, in which chains are separated by =0.4 nm [68].

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