Crystallization of Poly(l-lactic acid) Probed with Dielectric Relaxation Spectroscopy

Abstract

The properties of the amorphous component in poly(l-lactic acid), PLLA, depend on its crystalline structure, where the nanoconfinement effects may be monitored using dielectric relaxation spectroscopy. In this work this technique is used to monitor isothermal crystallization in PLLA by probing the evolution of the loss peak with crystallization time. It was found, for two different molecular weight materials and by crystallization from either the glassy or melt states, that the data could be given by a linear combination of three loss processes, where just their intensities vary: the α-process of the amorphous material, the constrained α-process present in the fully crystallized material, corresponding to the segmental motions of the amorphous phase confined by the crystalline lamellae, and the sub-glass β-relaxation. The appearance of the confined process was detected in the earlier stages of the crystallization, suggesting that the confinement effects are effective during primary crystallization. In this case, the cooperative segmental motions are restricted by the primary lamellae, leading to a dynamics characterized by a broader relaxation time distribution shifted to higher values. It was observed that the crystallization, as investigated by monitoring the evolution of the amorphous phase, is quicker for lower molecular weight PLLA and is slower for the material crystallized from the melt with respect to the cold crystallization. It was found that the confined dynamics is not very dependent on the thermal history prior to the crystallization step. The features of the β-relaxation were found to be similar for amorphous and semicrystalline systems, indicating that the sub-glass process in PLLA is not influenced by the crystalline confinement.

Structural evolution of the amorphous phase during crystallization of poly(l-lactic acid): A synchrotron wide-angle X-ray scattering study

“…The results present a clear trend, and the obtained A 1 and A 2 were found to be little dependent on the initial guesses used in the fitting procedure. The profiles for the progress of A 1 and A 2 are very similar to the evolution found in the dielectric strengths of the bulk-like 10 and confined dielectric loss peaks, respectively [24,25]. Moreover, as observed for the dielectric case, such evolution follows the progression of the crystallinity development.…”

“…The position and broadness of such processes are essentially independent on the degree of crystallinity, and just their relative weight varies with the development of crystallinity. The same argument was transposed to the dielectric behavior of PLLA during crystallization [24,25]: the loss peaks of the a and b relaxations of the two extreme materials, AM and SC, were fitted with single Havriliak-Negami functions, and the loss peaks of the polymer for intermediate crystallinity developments could be fitted by assuming a simple linear combination of the intensities of the pure loss peaks. This means that the evolution of the loss peak does not correspond to a continuous transformation from the AM-loss peak to the SC-one, but rather by a progressive decline and concomitant rising of the two respective processes upon crystallization.…”

“…The change in the amorphous phase of PLLA during crystallization at 80°C was probed using dielectric relaxation spectroscopy [24,25]. From such studies it was possible to monitor the change in the dynamics of the amorphous fraction of this polymer by analyzing the loss spectra assigned to the a-relaxation, that gradually shifts to lower frequencies upon crystallization.…”

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

“…The results obtained for the TCR material are related to the specific nature of the intra-spherulitic MAP. This phase is separated from the crystals by the RAF acting as a buffer zone 64 . The remarkable increase in the degrees of freedom of the relaxing part of the macromolecule is a secondary effect of the local restriction of the mobility occurring in the RAF.…”

Tie molecules, morphology and confinement effects in semi – crystalline poly(ethylene naphthalene-2,6-dicarboxylate) (PEN)

“…The molecular mobility in semi-crystalline polymers has been studied by several authors using a large number of systems and spectroscopic methods [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] . However, there are still few works analyzing simultaneously in details their molecular and nano structure in relation with their dynamics [9][10][11][12] .…”