New Insights on the Strain-Induced Mesophase of Poly(D,L-lactide): In Situ WAXS and DSC Study of the Thermo-Mechanical Stability

This work deals with the study of the mesomorphic form or mesophase induced by tensile drawing from the amorphous state of a polylactide material containing 4 mol % of D-stereoisomer units. Investigations have been carried out over the draw temperature domain 45-90 degrees C, i.e. an interval spanning roughly +/- 20 degrees C about the glass transition temperature. In situ WAXS experiments during drawing, stress relaxation, and/or heating of stretched samples invariably showed the strain-induced occurrence of the mesophase as far as temperature did not exceed 70 degrees C. This seems to be the upper stability temperature of the mesophase identified in a previous study. DSC traces upon heating of drawn samples exhibit a post glass transition endothermic peak similar to the enthalpy relaxation phenomenon observed for aged polymers. The amplitude of this strain-induced endotherm proved to be strongly dependent on draw temperature and draw ratio. Draw ratio also appeared to strongly influence the temperature domain of cold crystallization. The quite different structural evolution of the drawn samples as a function of temperature, depending whether cold crystallization occurred close or far from the strain-induced endotherm, led us to the conclusion that this endotherm results from neither physical aging nor orientation relaxation but from "melting" of the mesophase. This proposal is thoroughly supported by the insensitivity of the endotherm enthalpy to the DSC scanning rate that gives evidence of a first order thermodynamic transition in contrast to the case of aging-induced endotherm. WAXS as a function of temperature on drawn samples annealed with free ends enabled to probe the persistence of chain orientation and the stability of the strain-induced structural changes in relation to drawing conditions.