Strain-Induced Molecular Ordering in Polylactide upon Uniaxial Stretching

This study deals with the tensile drawing behavior of a polylactide material containing 4% of D-stereoisomer units, in the amorphous state. The draw temperature domain spanned from the glass transition to the onset of thermal crystallization, namely 70-100 degrees C. The stress-strain curves exhibit a strain-hardening strongly sensitive to the draw temperature regarding both the onset and the slope of the phenomenon. A detailed structural investigation reveals that various strain-induced phase changes take place depending on the draw temperature. For T-d = 70 degrees C, a mesomorphic form develops from the strain- oriented amorphous chains, starting at a strain level epsilon approximate to 130%. In the case T-d = 90 degrees C, a well-defined crystalline phase grows beyond the strain epsilon approximate to 250%. In the midtemperature range, i.e. T-d = 80 degrees C, both the mesomorphic and the crystalline phases are generated in parallel. In all cases, the final weight content of ordered phases at rupture was roughly 30%, irrespective of their form. The observed evolution with increasing draw temperature of the strain-induced structure from mesomorphic to crystalline is quite surprising with regard to the concomitant drop of the strain-hardening. Indeed, if the latter finding is consistent with the thermal activation of plasticity, it also means that the mesomorphic form is almost as much cohesive as the crystalline form in spite of its imperfect ordering. The occurrence of the mesomorphic form is specifically discussed in terms of both chain mobility and thermodynamic metastability.