Tensile modulus draw ratio effects

Figures 20.13 and 20.14 describe the effect of dibutyltin dilaurate (DBTDL) on the tensile strength and tensile modulus for the 25/75 LCP/PEN blend fibers at draw ratios of 10 and 20 [13]. As expected, the addition of DBTDL slightly enhances the mechanical properties of the blends up to ca. 500 ppm of DBTDL. The optimum quantity of DBTDL seems to be about 500 ppm at a draw ratio of 20. However, the mechanical properties deteriorate when the concentration of catalyst exceeds this optimum level. From the previous relationships between the rheological properties and the mechanical properties, it can be discerned that the interfacial adhesion and the compatibility between the two phases, PEN and LCP, were enhanced. Hence, DBTDL can be used as a catalyst to achieve reactive compatibility in this blend system. This suggests the possibility of improving the interfacial adhesion between the immiscible polymer blends containing the LCP by reactive extrusion processing with a very short residence time.

Figure 12 shows the influence of the nominal draw ratio on the tensile properties for the poly(ether ester) C. The initial tensile modulus was nearly independent of the draw ratio. A higher strains the modulus increased proportionally to the draw ratio. As can be seen from Fig. 13, the effect of the extrusion velodty on the tensile properties was rather small. 

The mechanical properties are usually determined from stress-strain tensile curves. Figure 3 shows the tensile strength and modulus in the most highly drawn direction as a function of the effective draw ratio in that direction. The data are taken from different papers involving different types of PP and different processing techniques. In spite of this wide range, the trend can be clearly seen. Undrawn PP has a modulus... 

An alternative approach was adopted by Kurelec et al. [84] who determined true stress-true strain curves at 80°C for a range of polyethylenes. It was shown that the slope of the tensile curve above the natural draw ratio (called the strain-hardening modulus) correlated well with the measured stress crack resistance (Figure 13.31). These results are entirely consistent with those obtained by Capaccio and co-workers and Ward and co-workers described above. Kurelec et al. found similar effects on the environmental stress cracking resistance (ESCR) performance with regard to short chain branches, and elaborated these in terms of the exact nature of the branches, particularly with regard to bimodal molecular weight distribution polymers. 

For higher extrusion temperatures, the annealing effect is favoured and molecular relaxation and recrystallisation are more likely to occur. Because of the variation in crystallinity with increasing draw ratio, tensile strength and modulus significantly increase as a fibrillar structure is formed starting from the spherulitic material. An increase of these properties with molecular weight also... 

Figure 10.5. The effect of extrudate draw ratio on (a) tensile toughness and (b) modulus for PMMA and PMMA-lwt%MWNT samples [31]...

Orientation of polyethylene introduces anisotropy with respect to virtually every physical property. At extreme levels of orientation, the degree of anisotropy developed surpasses that attainable by any other polyolefin and is unmatched by other organic polymers with the exception of carbon fibers. Most deliberate attempts to orient polyethylene to high degrees are made with the intent of improving mechanical properties, especially tensile modulus. It is therefore no surprise that the majority of literature references to highly drawn polyethylene detail the effects of orientation on such mechanical properties as elastic modulus, tensile strength, and draw ratio at break. 

---