Molecular mobility draw ratio

We have reviewed the phase structure of the fiber samples as a function of the draw ratio in terms of molecular mobility by analyzing the spectrum at room temperature. In this section we consider the phase structure over a wide range of temperatures. 

Such increases of y- and -relaxation temperatures with increasing draw ratio are thought to be due to the limited mobility of molecular chains in the noncrystalline region. As pointed out in the foregoing section, the amorphous molecular chains align fairly well parallel to the drawing direction. Therefore, the conformational versatility and mobility of such molecular chains are considered to be much restricted. 

The variation of compliances with draw ratio for cold drawn polypropylene filaments examined at 20°C appeared very similar to that of high density polyethylene, with an increase in all compliances but Sii, which was insensitive to draw ratio. Ward aggregate theory was not applicable except for low draw ratios, implying that other processes intervened in addition to an orientation of pre-existing units. It was probable that even above the glass transition temperature increasing orientation led to a reduction in molecular mobility, as was known to occur in polyethylene terephthalate. ... 

At very high draw ratios and/or crystallinities the deformation deviates strongly from that of a network and resembles more closely that of a single crystal. In tension, for 20 < 0 < 60° where there is a substantial shear stress and a tensile component parallel to the extended molecules, the deformation of HDPE approximates to c-slip or intermolecular shear as described by many authors. " Even in these situations of potentially easy shear, if one lowers the temperature of HDPE, presumably until the high molecular mobility in the less crystalline regions is quenched out, then the network ideas become more relevant, especially in understanding the optical reorientation in deformation bands. ... 

In Fig. 14.8, the change of samples limiting draw ratio corresponding to their fracture in mechanical tests, as a function extrusion draw ratio X is shown. As one can seem the X,. increase at X growth is observed. Such behavior Xj is differed from traditionally observed one for oriented polymers, when X increasing decreases X,. owing to molecular chains mobility exhausting [19], Personally, it has been established, that for matrix UHMPE X enhancement from 3 up to 5 is accompanied by reduction from 1.18 uptol.l2[44]. 

The aggregate model would not appear to be generally applicable to high-density polyethylene and polypropylene. It appears that for polypropylene the aggregate model is applicable only at low draw ratios [78], As discussed above, there are simultaneous changes in morphology and molecular mobility at higher draw ratios. 

Fig. 16 Mean mobility amplitude at 25 C for anthracene labelled poly-isoprene networks versus the draw ratio. M denotes the average molecular weight of chain strands between adjacent crosslinks.

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