Polymer isochrone

In order to know how is the variation of the mechanical properties of the polymers with temperature it is necessary to know the time of the measurements. In fact, E and D values obtained at different temperatures are comparable themselves if the time considered for the experiment is the same. Therefore the comparison of the experiments at different temperatures at the same time are isochrones [1-7,15-20], It is interesting to analyze the effect of the temperature on the elastic modulus. The classical schematic representation of this behaviour is shown on Fig. 2.4 ... 

Figures 2.37 and 2.38, show the isochronal curves of the permittivity and loss factor for P2NBM and P3M2NBM as a function of temperature at fixed frequencies. A prominent relaxation associated with the dynamic glass transition is observed in both polymers. Clearly the effect of the methyl substitution in position 3 of the norbornyl group is to decrease the temperature of this relaxational process.

Figure 7.8 gives some bundles of isochrones as supplied by a polymer manufacturer (GE). As we saw before, when comparing POM with PC, also here the higher rate of creep of a semi-crystalline polymer (PBTP), compared with the amorphous blend of PPE with PS, is obvious. 

Polymers can be divided into two broad classes, amorphous and semicrystalline. If observations are made at a fixed frequency, or isochronally, crystalline polymers often exhibit three major transitions as the temperature is varied, usually labelled a, p and y in decreasing order of temperature, whereas amorphous polymers generally exhibit two major transitions, labelled a and p in decreasing order of temperature. If other relaxations are seen at lower temperatures, they are labelled y or 8, respectively. 

In the case of (a), since there can be substantial variations in both compliance and strength (creep rupture) with angle, this may result in creep in some directions involving extremely low strains, and therefore presenting severe measurement problems, whilst in other directions very rapid large creep or rupture may occur thus limiting the information available. It has therefore been found preferable to employ (b) and to choose stress levels in different directions so as to produce equal strains after a specified creep time in all directions. Furthermore if correlation of creep behaviour with deformation mechanisms is sought it may well be desirable to compare the polymer response when the different mechanisms produce similar strains. Selection of appropriate stress-levels is achieved by use of the isochronous stress-strain curves. 

Oriented thermoplastics can show large anisotropy in creep behaviour, expecially in partially crystalline polymers. Significantly different patterns of behaviour occur in different materials. Not only is there anisotropy of isochronous stiffness, but also of creep rate and non-linearity. If stiffoess is regarded as a function of time, direction and stress or strain, the behaviour is such that the variables are not normally separable. 

Polymers exhibit this property of linear viscoelastic creep at low stresses (stresses sufficiently low that the strains are below 0.005). In a creep experiment, the plot of strain against stress at a specific time is known as an isochronal. 

For isochronal (constant frequency) experiments on Semicrystalline Polymers (qv) in the temperature range between the crystalline melting point and liquid nitrogen temperature (—196°C or 77 K), at least three relaxation processes are often foimd. The high temperature a process is often related to the crystalline... 

Fig. 24. Double logarithmic representation of the shear stress vs the shear strain for a polymer solution and for different isochrones, as indicated. After McKenna and Zapas (79).

Fig. 63. Half-step normal force response in a PMMA polymer glass, showing that the normal force response is independent of the duration ti of the first step. Points are data from the second step response for y = 0.05 after a step to y = 0.10. Lines are data from a single-step experiment at y = 0.05. (Solid lines mean dashed lines single standard deviation). Time values of 0.41 and 1677 s are isochronal values after the imposition of the step in the deformation. After McKenna and Zapas (113).

Figure 8.13 A 10 s isochronal creep modulus, measured at room temperature, as a function of draw ratio for a range of quenched (open symbols) and slowly cooled (closed symbols) samples of linear polyethylene drawn at 75 °C. ( ), Rigidex 140-60 (A, A), Rigidex 25 ( , ), Rigidex 50 (o, ), P40 (0, ), H020-54P. (Reproduced with permission from Capaccio, Crompton and Ward, J. Polym. Set, Polym. Phys. Ed., 14, 1641 (1976))...

Figure 10.1 Tensile creep of polypropylene at 60 °C. The stress and time dependence are approximately separable and therefore creep curves at intermediate stresses can be interpolated from a knowledge of two creep curves ( ) and the isochronous stress-strain relationship (X). (Reproduced with permission from Turner, Polym. Eng. Sci., 6, 306 (1966))...

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