Anisotropic Creep in Amorphous Thermoplastics

The anisotropy of creep behaviour of oriented amorphous thermoplastics appears to have received even less attention than that of oriented semicrystalline thermoplastics. This may well be associated with the fact that early measurements based on standard tests demonstrated a marked 

Isochronous modulus-strain data for specimens ait at various angles, 0, to the fibre axis of an oriented PMMA sheet (drawn at 124°C, birefringence 0-001 

The variation of the 100 second tensile creep modulus with 100 second tensile strain is presented in Fig. 16. The behaviour of a specimen cut from an isotropic sheet (which had been subjected to the same thermal cycle as the drawn sheet) is included for comparison. It is apparent that all specimens exhibited non-linear viscoelastic behaviour, but there is little anisotropy of non-linearity. Furthermore the degree of non-linearity exhibited by the specimens from the drawn sheet is similar to that of a specimen from the isotropic sheet. At any chosen creep strain the anisotropy of modulus for the drawn sheet is relatively low. 

The results of creep rupture tests on a drawn PMMA sheet with a lovyer level of molecular orientation (birefringence of only (HXX)6) are presented in Fig. 18. This shows a high level of anisotropy of creep rupture which contrasts with the relatively low level of anisotropy of stiffness found in the more highly oriented sample. The level of molecular orientation, even in the sample with lower birefringence, must therefore be regarded as significant, and the low anisotropy of stiffness must be 

Wright et al. have also examined the stiffness behaviour of drawn PMMA using a uniaxial tensile test in which the load was alternately applied and removed at 30 second intervals. The modulus data from these tests showed reasonable agreement with the 100 second isochronous data presented above, when compared at equal birefringence. 

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