Local and global dynamics of polylactides.: A dielectric spectroscopy study

Abstract

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: 3.8×10 3< M n<4.7×10 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 ( τ∼ M n 3.1) reflects on the intermediate molecular weight regime ( M e∼6×10 3) from Rouse to entangled chains. These findings are compared with the viscoelastic results on the same system. Agreement in the c 2 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.