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

Abstract

We report the dielectric and viscoelastic relaxations in undiluted amorphous poly(d,l-lactic acid) (PLA). Three dielectric relaxations designated as Rn, Rs, and β are observed in order of decreasing temperature. The relaxation time for the Rn 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 R s 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 Rn, Rs, 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 M c () 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.

Dielectric Properties of 3D Printed Polylactic Acid

“…The VFT-formula smoothly fits both states. This implies only a single dipolar mechanism, which is contradictory to the two processes model proposed by Ren et al [33,35].…”

“…In particular, for frequencies above 117 kHz the main relaxation process is shifted into the temperature regime of the cold crystallization, which shows up in Figure 2(a) as kink in while the main relaxation develops. Ren et al [33,35] attribute the main relaxation to two processes: a sequential mode for < 350 K and a so-called "end-to-end" vector fluctuation for 350 K < < 390 K. The latter describes the orientation of huge fragments of the polymer. However, it is also possible to explain the dielectric feature only using the so-calledrelaxation, where the orientation of the molecules is partially hampered by the cold crystallization.…”

Structural Dependence of the Molecular Mobility in the Amorphous Fractions of Polylactide

“…In the low temperature range (-150° to 0°C), PLA exhibits a broad secondary relaxation process classically labeled  relaxation in the literature and assigned to twisting motions in the main chain. 64 Ren et al 65 estimated the amplitude of such motions by an average twisting angle of around 11°. For temperatures ranging from 50°C to 100°C, the main relaxation takes place, associated to the structural relaxation of PLA (dynamic glass transition).…”

Effects of crystallinity on dielectric properties of poly(L‐lactide)

“…It is assumed that the lower-temperature peak (Peak I) is due to segmental-mode relaxation, and that the higher-temperature peak (Peak II) is due to normalmode relaxation. The latter is attributed to the alignment of dipole moments in the direction parallel to the chain contour in the amorphous regions of the samples due to macroBrownian motion [11,12].…”