WAXS study of the structural reorganization of semi-crystalline polylactide under tensile drawing

This work deals with the in situ and ex situ WAXS study of the strain-induced structural changes in relation to the mechanical behavior of a semi-crystalline polylactide containing 4% of d-stereomer units. The crystals isothermally generated at 120 °C were in the α form. The draw temperature was in the range 65 °C < Td < 90 °C, i.e. just above Tg. In this Td range isothermal crystallization was so slow that only strain-induced crystals could be generated during the time scale of the experiments. This allowed studying samples with various initial levels of crystallinity, Xcr. For Xcr = 5%, the early occurrence of strain-hardening contrasted with the case of the amorphous material. This was due to the physical crosslinking of the macromolecular network by the crystallites that prevented chain relaxation since the onset of drawing. Strain-induced α′ crystals developed in parallel with the mesophase, the respective amount of the two phases being dependent on Td. For Xcr = 24%, the drawing behavior was that of a crosslinked and filled elastomer. Again, both strain-induced α′ crystals and mesophase occurred in various proportions in relation to Td. For Xcr = 40%, it was shown that the initial α crystals underwent partial destruction and subsequent reorganization into mesophase and/or α′ crystals. Whatever Td and Xcr, the total amount of ordered phases, i.e. crystal + mesophase, did not exceed the value of 40% that was the maximum accessible crystal content for the present material. A mechanism of plastic deformation involving twinning is proposed from the WAXS analysis of the crystalline texturing in the case Xcr = 40%. AFM observations as a function of strain corroborate the proposed deformation scheme.