Constrained Amorphous Interphase in Poly(L-lactic acid): Estimation of the Tensile Elastic Modulus

The mechanical properties of semicrystalline PLLA containing exclusively alpha'- or alpha-crystals have been investigated. The connection between experimental elastic moduli and phase composition has been analyzed as a function of the polymorphic crystalline form. For a complete interpretation of the mechanical properties, the contribution of the crystalline regions and the constrained amorphous interphase or rigid amorphous fraction (RAF) has been quantified by a three-phase mechanical model. The mathematical approach allowed the simultaneous quantification of the elastic moduli of (i) the alpha'- and alpha-phases (11.2 and 14.8 GPa, respectively, in excellent agreement with experimental and theoretical data reported in the literature) and (ii) the rigid amorphous fractions linked to the alpha'- and alpha-forms (5.4 and 6.1 GPa, respectively). In parallel, the densities of the RAF connected with alpha'- and alpha-crystals have been measured (1.17 and 1.11 g/cm(3), respectively). The slightly higher value of the elastic modulus of the RAF connected to the alpha-crystals and its lower density have been associated to a stronger chain coupling at the amorphous/crystal interface. Thus, the elastic moduli at T-room of the crystalline (E-C), mobile amorphous (E-MAF), and rigid amorphous (E-RAF) fractions of PLLA turned out to be quantitatively in the order of E-MAF < E-RAF <E-C, with the experimental E-MAF, value equal to 3.6 GPa. These findings can allow a better tailoring of the properties of PLLA materials in relation to specific applications.