Dielectric and Viscoelastic Studies of Segmental and Normal Mode Relaxations in Undiluted Poly(d,l-lactic acid)

We report the dielectric and viscoelastic relaxations in undiluted amorphous poly(d,l-lactic acid) (PLA). Three dielectric relaxations designated as αn, αs, and β are observed in order of decreasing temperature. The relaxation time for the αn relaxation increases with increasing molecular weight and is assigned to the normal mode relaxation due to the component of dipole vector aligned in the direction parallel to the chain contour. The αs relaxation is observed about 30 K above the glass transition temperature Tg (= 310 K) and is assigned to the local segmental mode due to the transverse component of the monomeric dipoles. The β relaxation is seen in the glassy state and is assigned to the secondary relaxation. From the relaxation strengths for the αn, αs, and β relaxations, the effective dipole moments for those relaxation processes are determined and compared with the parallel and transverse components of the dipole moment calculated theoretically with the semiempirical molecular orbital methods. The dielectric relaxation time for the normal mode increases with molecular weight M with the power of 3.5 in the range of molecular weight above the characteristic molecular weight Mc (= 13 000). The molecular weight between entanglements is calculated to be 7700 from the shear modulus in the rubbery plateau region. It is found that the dielectric normal mode relaxation time agrees approximately with the viscoelastic terminal relaxation time. The relaxation spectra for the viscoelastic relaxation are much broader than those for the dielectric relaxation as observed previously for polyisoprene.