Time and Temperature Evolution of the Rigid Amorphous Fraction and Differently Constrained Amorphous Fractions in PLLA

Abstract

In semi‐crystalline polymers, amorphous segments located in different regions and distance from the crystalline domains can exhibit different dynamics. In poly(l‐lactic acid) (PLLA), the existence of (i) two distinct mobile amorphous fractions (a completely mobile amorphous fraction and a slightly constrained mobile amorphous fraction), which vitrify/devitrify in the Tg region, and (ii) a rigid amorphous fraction, which vitrifies/devitrifies at temperatures higher than Tg, has been reported in the literature. The percentage of the three different amorphous fractions, characterized by different segmental mobility, is generally calculated at Tg. In the present study, for the first time, the time and temperature evolution of all the amorphous fractions of PLLA are measured in parallel with the crystalline phase (i) during crystallization at two different temperatures, and (ii) upon the successive cooling step down to room temperature. The kinetics of vitrification/devitrification of the different amorphous fractions in diverse conditions of crystallization and solidification has been determined during the entire thermal history of the material. Rigid amorphous fraction is found to develop not only during crystallization but also upon cooling, after completion of crystallization.

Crystallization of Polymers Investigated by Temperature-Modulated DSC

“…The crystallization process of polymers has been widely investigated by TMDSC [7,22,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38]. An accurate investigation of the crystallization process is a fundamental step for a full comprehension of the polymer structure, which in turn strongly influences the material properties.…”

Local and segmental motions of the mobile amorphous fraction in semi-crystalline polylactide crystallized under quiescent and flow-induced conditions

“…Apparently, the structural anisotropy induced in the liquid like-state plays no role on the apportionment of the different fractions. As observed by Righetti et al, [43,50] the establishment of the RAF depends on the crystallization temperature, i.e. on the molecular mobility of the polymer chains.…”

“…[47,48] RAF is commonly defined by opposition to the mobile amorphous fraction (MAF) in that its mobility is drastically restricted due to the geometrical constraints at the interface with the crystals. [49,50] Regarding the MAF, constrained and unconstrained MAF-depicted as intra-and interspherulitic amorphous phases respectively-have recently been reported for semicrystalline polyesters such as PET [51] and PLA. [43,52,53] Constrained and unconstrained MAF were revealed by differential scanning calorimetric measurements through the concept of cooperative rearranging regions (CRR) [54] as well as structural relaxation (i.e.…”

“Nothing Can Hide Itself from Thy Heat”: Understanding Polymers via Unconventional Applications of Thermal Analysis

“…This result is all the more surprising considering that the glass crystallization has yielded markedly less crystalline phase than crystallization from the melt. The complexity of the dynamics of the formation of the rigid amorphous phase has been revealed in studies by Righetti et al . The data on crystallization of poly[(R)‐3‐hydroxybutyrate] have demonstrated that under isothermal conditions at 30 °C, the rigid amorphous and crystalline phases develop in concert.…”

“…to hypothesize that the rigid amorphous phase may play an important role in crystallization. A study of poly(lactic acid) has again demonstrated that the rigid amorphous phase continues to develop after the formation of the crystalline phase …”