Isothermal crystallization of PLA: Nucleation density and growth rates of α and α' phases

Abstract

From a technological point of view, poly(lactic acid) (PLA) is one of the most important polymers produced from renewable sources, due to its versatility, relatively acceptable processability, and low cost. However, a significant limitation exists in its slow crystallization kinetics, which results in amorphous products having low mechanical properties and thermal resistance. For this reason, quantitative knowledge of the phenomenon of crystallization kinetics is fundamental. In this work, the crystallization kinetics in quiescent conditions of a commercial grade of PLA was analyzed in terms of nucleation and growth rates by direct morphological observations at different crystallization temperatures (Tc) and by calorimetric analysis in isothermal and non‐isothermal conditions. The optical analysis showed a spherulitic morphology with radial growth of the lamellae. The analysis of the growth rate evidenced the α/α'‐crystals polymorphism with a transition temperature of ~120°C. Based on experimental observation, the crystallization kinetics of the two crystalline phases were assessed.

Effect of the application of low shear rates on the crystallization kinetics of PLA

“…In the case of iPP, it was found that a relationship holds between nucleation rate and excess growth rate (namely the difference between the growth rate observed in shear and the growth rate in quiescent conditions [ 34 ] at the same temperature). [ 6 ] This is justified by the fact that both phenomena are influenced by flow in a similar way.…”

“…The rheological characterization of PLA reported in a previous work [ 34 ] was used to determine the relaxation time and the molecular stretch. In particular, the dynamic data of the storage and loss moduli ( G ′ and G ″) were related to the relaxation time by the Cox‐Merz's and Laun's rules [ 34–37 ] as expressed in Equation ). …”

“…In order to find a correlation, the quiescent growth rate needs to be known. The characterization of quiescent crystallization kinetics of the material adopted in this work was presented in a previous work, [ 34 ] in which the relationship between growth rate and crystallization temperature was described by Hoffman‐Lauritzen model [ 40 ] : …”

“…At the temperatures analyzed in this work, only the α phase of PLA develops and thus the parameters of interest are those of this phase, which are reported in Table 2. The parameters were defined in a previous work [34] to describe the growth rate of α phase for the material used in this work. increase in Equation (18) of the melting temperature T m .…”

GRICU 2019 special issue section preface

“…Due to the nature of GRICU, and the broadness of the conference's scope, topics cover a number of fields, thus providing a convenient window on most prominent ongoing chemical engineering research in Italy. As a matter of fact, the paper fields encompass advanced modelling, [1,2] green fuel production (CO 2 hydrogenation [3] and methanation, [4] CH 4 reforming [5] ), advanced drug production, [6] biochemical food production (microalgae [7] and yeast [8] based), biomedical engineering, [9] process plant management, [10] effluent remediation (for waste water [11] and waste gas [12] streams), polymeric materials, [13] and chemical engineering fundamentals. [14] As it can be seen, all the papers are deeply rooted in chemical engineering science and technology.…”