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 °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 °C, a mesomorphic form develops from the strain-oriented amorphous chains, starting at a strain level ε ≈ 130%. In the case T d = 90 °C, a well-defined crystalline phase grows beyond the strain ε ≈ 250%. In the midtemperature range, i.e. T d = 80 °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.

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 °C, i.e. an interval spanning roughly ±20 °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 °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.

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The crystallization kinetics and the resulting structure and morphology of polylactide (PLA) were investigated in the presence of carbon nanotubes (CNTs). Nanocomposite samples prepared by solution and melt mixing present homogeneous filler dispersion, as observed by scanning electron microscopy. Calorimetric characterization of the nonisothermal and isothermal crystallization behavior analyzed according to Avrami’s theory provides evidence of the significant impact of CNTs on the crystallization kinetics of the PLA matrix. The nucleating effect of the nanofillers is confirmed by Raman spectroscopy experiments. Indeed, during isothermal crystallization, the nanotube characteristic vibrations are strongly affected by the development of polymer crystalline phase. Additionally, CNTs increase the number of nucleation sites and thereby decrease the average spherolite size as observed by optical microscopy. The PLA crystal structure is not modified by the presence of CNTs, as probed by X-ray diffraction.

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Crystallization is among the easiest ways to improve polymer barrier properties because of the tortuosity increase within the material and the strong coupling between amorphous and crystalline phases. In this work, poly(lactic acid) (PLA) films have undergone α' thermal crystallization or different drawing processes. Although no effect of α' thermal crystallization on water permeability is observed, the drawing processes lead to an enhancement of the PLA barrier properties. This work clearly shows that, in the case of PLA, the crystallinity degree is not the main parameter governing the barrier properties contrary to the crystalline and amorphous phase organizations which play a key role. X-ray analyses confirm that the macromolecular chain orientation in the amorphous phase is the main cause of the improvement of the drawn PLA water barrier property. This improvement is due to the orthotropic structure formation for sufficient draw ratios, particularly when using the Simultaneous Biaxial drawing mode. Moreover, independently of the draw conditions, the drawing process tends to reduce the plasticization coefficient. Consequently, the drawn material barrier properties are not much affected by the water passage.

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