A theoretical model is proposed in order to explain the mechanical behaviour of thin bioriented poly(ethylene terephthalate) (PET) films at low and medium stresses. This investigation requires one to introduce a theory of deformation of amorphous PET and coupling of the behaviour laws of the two phases (crystalline and amorphous). A newly developed molecular model of non-elastic deformation of glassy solids based on hierarchical constrained molecular movements and activation of defects is used and extended to the non-linear behaviour of the creep compliance function. Creep tests are carried out at low and medium stresses on amorphous and semicrystalline films in order to study (i) the linear domain, (ii) the transition to non-linear behaviour of the compliance (defined by some critical stress, crc) and (iii) the qualitative evolution of the creep compliance beyond this critical stress value. Semiquantitative agreement is found between theoretical predictions and experimental data in a large domain of stresses including the critical value a?. By assuming in a first step that partially crystalline films are two-phase systems, their mechanical behaviour can be explained by coupling an amorphous and a crystalline phase following Takayanagi's model. In this coupling, the crystalline phase can be considered as a perfect crystal with a Young's elastic modulus in the in-plane direction of about Ecl I ~ 110 GPa. In a second step, the mechanical response of the amorphous phase of biaxially oriented thin PET films is compared with that of bulk amorphous PET. We find that these two behaviours are very different, first because of an important reduction of'defect' concentration in the amorphous phase of thin biaxially oriented PET films and secondly because of an increase of relaxation characteristic time due to the proximity of crystallites.
Посилання на статтю:
Viscoelastic behaviour of thin bioriented poly (ethylene terephthalate) films under low and medium stresses / F. Bouquerel and P. Bourgin // Polymer. – 1992. – Vol 33. – P. 516-525.