Crystallization Kinetics of PLA

The PLA crystallization kinetics has been widely investigated, and most of the studies have been carried out on PLLA. The PLA crystallization, isothermal or non-isothermal, is related to the L-lactic acid content [39, 74, 75], the 

Upon isothermal crystallization, the spherulite growth rate of PLA (96% L) is between 0.2 and 3 pm.min depending upon the crystallization temperature according to the authors [76, 86, 89-91], with an optimum around 115°C [76]. Moreover, when the molecular weight is divided by a factor 5, the maximal growth rate increases from 3 to 7pm.min at 115°C [92]. The spherulite growth rate is increased by the stereoregularity of PLA. Di Lorenzo et al. have shown 

The isothermal crystallization kinetics is studied mainly with the Avrami equation (logarithmic form in equation 8.2), where )(f) is the relative crystallinity, n and are the Avrami exponent and crystallization rate constant respectively. 

The kinetic studies of the isothermal crystallization of neat PLA showed that is around 1.8-2.6 between 80 and 135°C [86, 93,94]. However, Lai et al. observed a of 3.98 at 124°C. [95]. In the same manner, the crystallization rate constant varies widely between 5.84x10 and 0.9 min [85,86,94]. 

Upon non-isothermal crystallization the Avrami exponent takes on values between 2.1 and 4.82, whereas the Avrami crystallization rate constant are found between 0.0104 and 0.685 [85,87]. 

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The classic Avrami Equation (1) is used to study the isothermal bulk crystallization kinetics of PLA. 

In this study, the effects of Talc and Montmorillonite clay on the crystallization kinetics of PLA were investigated under isothermal conditions using DSC technique. The... 

The effect of Talc and Montmorillonite (Closite Na+) on the crystallization kinetics of PLA was investigated. The lowest crystallization induction period and maximum crystallization speeds were observed at around 100°C. At this temperature the crystallization half-time of PLA was decreased from a few hours to 8 minutes by addition of 1% Talc. Further addition of Talc did not result in significant half-time reduction. The Montmorillonite Closite Na-i- was shown to be less effective than Talc as a nucleating agent and could not decrease the half-life below 30 min. Flow induced crystallization was observed... 

The detailed thermal behaviours of the starch/PLA blends have been studied by DSC [204]. The experimental data was evaluated using the well-known Avrami kinetic model. Starch effectively increased the crystallization rate of PLA, even at a 1 % content, but the effect was less than that of talc. The crystallization rate of PLA increased slightly as the starch content in the blend was increased from 1 to 40 %. An additional crystallization of PLA was observed, and it affected the melting point and degree of crystalhnity of PLA. 

As mentioned above, PLA should be addressed as a random copolymer rather than as a homopolymer the properties of the former depend on the ratio between L-lactic acid and D-lactic acid units. A few studies describe the influence of the concentration of D-lactic acid co-units in the PLLA macromolecule on the crystallization kinetics [15, 37, 77-79]. The incorporation of D-lactic acid co-units reduces the radial growth rate of spherulites and increases the induction period of spherulite formation, as is typical for random copolymers. In a recent work, the influence of the chain structure on the crystal polymorphism of PL A was detailed [15], with the results summarized in Figure 5.13. It shows the influence of D-lactic acid units on spherulite growth rates and crystal polymorphism of PLA for two selected molar mass ranges. 

When a plot of LnG + U lR Tc - Too) as a funetion of llT Alf is made, linear trends should be obtained with a slope that is indieative of the crystallization Regime according to eqns (6) and (7) and the partieular value of the j constant. The overall ciystallization kinetics of PLA has been investigated by the LH theory and Figure 3.9 shows the LH plots for four different samples. 

Kose and Kondo studied tbe size effects of cellulose nanofibres on the crystallization behaviour of PLA. They discovered that the smaller size of cellulose nanofibres on tbe nanoscale does not necessarily make a better nucleating agent for PLA. Table 9.1 summarizes the Avrami kinetic parameters for the isothermal crystallization of the PLA and PLA biocomposites with different types of nanocelluloses as compared to PLA composites with talc and nanoclay. With the addition of unmodified and silylated CNCs as nucleating agents, the t 2 value increases with increasing T similar to that of nanoclay and com starch, but opposite to that of talc. Comparing the... 

The overall nucleation and crystalhzation rates of PLA tmder heterogeneous conditions are relatively higher than in homogenous conditions. The nucleation and crystallization rates of propylene-ethylene copolymer are increased tmder isothermal conditions. Addition of nucleating agent accelerates crystallization. Avrami equation is in popular use in the analysis of isothermal crystallization kinetics of polymers ... 

Typically, polymer-grade l-LA with high chemical purity and optical purity (i.e., over 98-99% l-LA and less than 1-2% d-LA) is used for commercial PLA production. When l-LA is dehydrated at high temperature into L-lactide, some l-LA may be converted into d-LA. d-LA mixed with l-LA contributes to meso-lactide (the cyclic dimer of one d-LA and one l-LA) and heteropolymer PLA (with both d-LA and l-LA units). Heteropolymer PLA exhibits slower crystallization kinetics and lower melting points than homopolymer PLA (of pure l-LA units or pure d-LA units). 

The kinetics of melt-crystallization of PLA has been analyzed by a number of research groups [14, 35, 39, 71-75]. Isothermal bulk crystallization rates were determined in a wide temperature range from 70 to 165 C [71,72]. The maximum crystaUization rate is observed around 100 C, and the most peculiar behavior is a discontinuity in the phase change kinetics around 110-120°C, an example of which is shown in Figure 5.10. Figure 5.10a reports the half-time of crystallization of PLA as function of the isothermal crystallization temperature. The data set shows a broad minimum around 108 C and a step/discontinuity around 116-118°C, indicated by the arrow. The sudden variation in crystallization rate... 

Tale and triphenyl phosphate (TPP) have been reported as nueleation agents for PLLA. The isothermal kinetics of neat PLA and its blends with TPP and tale were reported by Xiao et al7 However, the crystallization rate decreased with the ineorporation of TPP. 

Semicrystalline PLA has a tensile strength of approximately 50-70 MPa, tensile modulus of 3000-4000 MPa, elongation at break of 2-10%, flexural strength of 100 MPa, and flexural modulus of 4000-5000 MPa [ 1-6], PLA specimens obtained by a typical injection molding process are generally almost amorphous, because of the slow crystallization kinetics characterizing this material. In Tables 11.1 and 11.2, the results of physicochemical and mechanical characterization of, respectively, PLLA and PDLLA injection molded specimens with different molecular weights are presented [7]. 

With talc reinforcements, both crystallization kinetics and crystal microstructure of the polymer (PLA) are significantly altered, for example, the overall crystallinity is reached more quickly due to increase in crystallization kinetics. The degree of crystallinity obtained from DSC measurements increased from 3.6% in neat PLA to 15.4% in PLA-talc composites with a filler content of 7.0 wt% [68]. This in turn can lead to reduced cycle time in injection molding and stiffer materials than neat PLA, when molded under the same conditions. At similar loading level of talc (7.0 wt%), the tensile modulus increased by 15%, while flexural modulus increased by 22% [68]. 

We will consider these groups one after the other and systematically try to understand the processes of crystallization as it concerns each group. To simplify these processes, we will consider first, the system of neat blends (i.e., without the deliberate addition of nanoparticles). The nanoparticles will be considered as one of the heterogeneities inside the blend. However, we should remember that the effect of the nanoparticles will depend greatly on the localization of the nanoparticles. In such ternary blends, preferential localization of the third component could be in either of the two polymers [21,22], which eventually affects the crystallization behavior. The preferential localization is driven by factors such as (i) thermodynamic (enthalpic interaction between each polymer and the third material) and/or (ii) kinetic factors (e.g., viscosity ratios of the two polymers) [36]. Wu et al. [22] reported the effect of localization of nanofillers, clay, and CNTs, on the morphology and crystallization of PLA/polycapro-lactone (PCL) blends. 

Whatever the crystallization type, the kinetic study of the PLA crystallization, according Avrami, showed a two-dimensional or three-dimensional growth process, depending on the temperature, from a predetermined nucle-ation with a combination of sporadic nucleation [96,97]. 

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