Crystallization kinetics of a commercial poly(lactic acid) based on characteristic crystallization time and optimal crystallization temperature

Abstract

A model is proposed to fit differential scanning calorimetry (DSC) isothermal crystallization curves obtained from the molten state at different temperatures. A commercial 3D printing polylactic acid (PLA) sample is used to test the method. All DSC curves are fitted by a mixture of two simultaneous functions, one of them being a time derivative generalized logistic accounting for the exothermic effect and the other, a generalized logistic, accounting for the baseline. There is a rate parameter, which is allowed to vary across different temperatures. The rate parameter values obtained at different temperatures were jointly explained as a result of three crystallization processes, each one defined by a characteristic crystallization time, a characteristic temperature, and a dispersion or width factor. Apart from the very good fittings obtained at all temperatures, the results agree with the existence of a few crystal forms of PLA, which were demonstrated by other authors. Thus, the main significance of this work consists in providing a new approach in order to mathematically describe the isothermal crystallization kinetics of a polymer from the melt. Such a kinetic description is needed in order to predict the extent of a crystallization process as a function of time at any isothermal temperature. The approach used here allows to understand the overall crystallization of the PLA used in this work as the sum of three crystallization processes, each of them corresponding to a different crystal form. Each experimental crystallization exotherm, which may include more than one crystal form, can be reproduced by a generalized logistic function. The overall rate factor at a given temperature is the weighted sum of the rate factors of the different crystal structures at that temperature. The rate factor of each of these three processes is described by a Gaussian function whose parameters are a crystallization time, a characteristic temperature and a temperature dispersion factor. Therefore, the crystallization rate for each crystal form can be interpreted as a relative likelihood to crystallize at a given temperature. On the other hand, the characteristic crystallization time parameter refers to the time needed for a given crystal structure to be formed at the temperature at which the relative likelihood to crystallize of that form is highest.

Properties of Mosquito Repellent-Plasticized Poly(lactic acid) Strands

“…Table 1 reports the glass transition ( T g ) values together with the cold crystallization temperatures ( T cc ), the melting temperatures ( T m ), cold crystallization enthalpy (Δ H cc ) and melting enthalpy (Δ H m ). Two melting peaks are observed for the neat PLA as found previously by [ 40 , 41 , 42 , 43 ]. They correspond to the presence of the stable orthorhombic α crystalline form and the metastable α′-modification [ 44 ].…”

Indomethacin: The Interplay between Structural Relaxation, Viscous Flow and Crystal Growth

“…Correspondingly, the lower ΔH in the case of the 50–125 μm powder fraction needs to be explained either by the different polymorph (the larger portion of α-IMC) or different crystallization mode (the dominant crystallization caused by mechanically induced defects), associated with a lower release of heat. Note that the simultaneous formation of the two polymorphic forms may be the main reason for the divergence of experimental data, even in the case of the isothermal measurements [ 47 ]. It is thus fully possible that this phenomenon is responsible for the changes in the crystallization kinetics in the case of the present non-isothermal measurements, where the impacts of the potential differences in the activation energy are augmented by the continuous increase in temperature.…”

A Logistic Approach for Kinetics of Isothermal Pyrolysis of Cellulose

“…It can be observed in Figure 4 how the b parameter values calculated from different tests clearly follow an exponential trend. While in crystallization processes, a Gaussian dependence of the rate factor, b, on temperature was found [24]; now it follows an exponential trend. That is not surprising since it is well known that crystallization and thermal degradation are intrinsically different and while the crystallization rate has a maximum somewhere in the middle of the glass transition temperature and the melting temperature, the degradation rate always increases with temperature.…”

“…The aim of the work is to obtain an accurate mathematical description of the degradation rate with respect to the temperature. For that, using a similar approach to that described in a recent crystallization kinetics study [24], optimal fittings of individual isothermal TG curves are performed. The method for the fittings was previously described [11,25,26].…”