Poly stress—strain state

Notwithstanding this great variety of mechanical properties the deformation curves of fibres of linear polymers in the glassy state show a great similarity. Typical stress-strain curves of poly(ethylene terephthalate) (PET), cellulose II and poly(p-phenylene terephtha-lamide (PpPTA) are shown in Fig. 13.89. All curves consist of a nearly straight section up to the yield strain between 0.5 and 2.5%, a short yield range characterised by a decrease of the slope, followed by a more or less concave section almost up to fracture. Also the sonic modulus versus strain curves of these fibres are very similar (see Fig. 13.90). Apart from a small shoulder below the yield point for the medium- or low-oriented fibres, the sonic modulus is an increasing, almost linear function of the strain. 

Ethylenediamine carbamate of (EDA-C) is particularly advantageous. Results of stress-strain tests and compression set of a poly(VDF-co-HFP) copolymer cured by EDA-C show that 0.85 part of EDA-C produces a state of cure equal to that obtained with one part of HMDA-C (Table 9 [ 111 ]). 

Since PHB is a polyester it might have been expected that it could be drawn and oriented into fibres quite easily using similar techniques to those developed for poly (ethylene terephthalate). This is not the case. PET is normally melt-spun into a fibre which is then quenched and wound up as an amorphous, glassy material.In a separate stage it is then drawn over a hot pin at a temperature above its 7 and crystallized in this oriented state to produce the familiar strong polyester fibres. This type of stress-induced crystallization from the quenched amorphous state does not seem to be possible with PHB. Indeed, the application of strain to a supercooled PHB melt can inhibit recrystallization rather than promote it. 

Figures 11.5 and 11.6 illustrate the effects of time and temperature (6). The poly(vinyl chloride) in Figure 11.6 retains significant ductility even in the glassy state. Polystyrene, by contrast, breaks before the yield point at about 2.5% extension see Tables 11.1 and 11.2. Both sets of data show yield stresses, followed by strains nearly independent of the stress. Note the well-defined yield points. Many people use the yield strength for design rather than the...

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