Segmental mobility in linear polylactides of various molecular weights

In this work we synthesized and studied a series of linear polylactides (PLA) with a fixed L/d -lactide stereoisomer ratio, covering together the wide range of molecular weights, M-n, from similar to 6 to similar to 80 kg/mol. The basic aims are the segmental mobility mapping, in terms of time scale and fragility, the recording of the M-n dependences of the glass transition temperature and fragility, and, finally, the comparison with corresponding findings from the literature. Obviously, the direct M-n effects on T-g are assessed by preserving PLA at the amorphous state, i.e., via melting and relatively fast cooling. The segmental mobility is studied by conventional and temperature modulated calorimetry, whereas the molecular dynamics is assessed employing the technique of broadband dielectric spectroscopy in combination with proper critical analysis and evaluation routes. The calorimetric and dielectric T-g(M-n) trends were found consistent between each other as well as with previous findings in similar systems, both with respect to experimental findings and theoretic predictions for PLAs of similar L/D ratio. The M-n threshold of entanglements seems to lay between 13 and 28 kg/mol. The fragility index of the alpha relaxation, i.e., the dielectric analogue of the glass transition, exhibits a similar dependence to that of T-g(M-n). When decreasing M-n to very low values, the high fragility (similar to 150) drops down to 0 (M(& ntilde;)6 kg/mol), denoting the vanishing of the polymer chains' cooperativity. Qualitatively interesting alternations are recorded in the dielectric strength of the alpha relaxation and the relaxation times width. The effects are discussed in terms of the various parameters co-affecting the polymer dynamics, namely, chain lengths, free volume and dynamical heterogeneities.