Local and global dynamics of polylactides. A dielectric spectroscopy study

Polylactides (PLAs) are a family of degradable plastics having a component of the dipole moment both perpendicular and parallel to the polymer backbone (i.e. is a type-A polymer). We have studied the sub-glass. segmental and global chain dynamics in a series of fully amorphous samples having an L:D ratio of 80:20, with molecular weights in the range: 3.8 x 10(3) < M-n < 4.7 x 10(4). Finite size effects have been observed for both the segmental and the sub-glass process. The segmental mode was found to have a steeper temperature and pressure dependence compared to the longest normal mode in accord with findings from other type-A polymers. The molecular weight dependence of the longest normal mode relaxation times (tau similar to M-n(3.1)) reflects on the intermediate molecular weight regime (M-c similar to 6 x 10(3)) from Rouse to entangled chains. These findings are compared with the viscoelastic results on the same system. Agreement in the c, value of the WLF equation is found but the scaling of the normalized longest relaxation time derived from the rheological measurements shows a stronger than expected dependence on the molecular weight. (C) 2002 Elsevier Science B.V. All rights reserved.