Melting of PLA

The melting temperature (T ) of PLA occurs between 130 and 180°C according to the L-lactide content and the crystals formed during crystallization. The presence of meso-lactide in the PLA structure induces a decrease in the melting temperature according to the equation 8.3 where is the meso-lactide fraction in the matrix and 175 is the melting temperature of PLLA [5]. 

The melting peak is simple or double according to the crystalline forms and the lamellae thickness of the spherulites using the Gibbs-Thomson relation as shown in equation 8.4. 

The melting temperature for the thermodynamic equilibrium, T , representing the melting point of an infinite size and molecular mass crystal, equals 207°C for PLLA [10]. The crystal density, p, is reported to be 1.29 kg/L and the surface energy for the extremity of the lamellae, cr, is 53.6 x 10 J.m .  

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A DSC thermogram of a stereorandom copolymer is shown in Figure 5. Similar to the L copolymers discussed above, the R polymers exhibit an endotherm between 50 and 53 C corresponding to melting of the crystalline PEO mid-block. However, there are no further phase changes over the sampled temperature range. In this case, the copolymers do not show melting of PLA because the stereocenter dictates amorphous PLA end blocks. 

Epoxidized oils were also used to modify PLA Ali et ah (2009) reported that its use as a plasticizer to improve flexibility. Thermal and scanning electron microscope analysis revealed that epoxidized soybean oil is partially miscible with PLA. Rheological and mechanical properties of PLA/epoxidized soybean oil blends were studied by Xu and Qu (2009) Epoxidized soybean oil exhibited a positive effect on both the elongation at break and melt rheology. Al-Mulla et al. (2010b) also reported that plasticization of PLA (epoxidized palm oil) was carried out via solution casting process using chloroform as a solvent. The results indicated that improved flexibility could be achieved by incorporation of epoxidized palm oil. 

As in PP-based nanocomposite systems, the extended Trouton rule, 3r 0 (y t) = r E (so t), also does not hold for PLANC melts, in contrast to the melt of pure polymers. These results indicate that in the case of P LANC, the flow induced internal structural changes also occur in elongation flow [48], but the changes are quite different in shear flow. The strong rheopexy observed in the shear measurements for the PLA-based nanocomposite at very slow shear rate reflects the fact that the shear-induced structural change involved a process with an extremely long relaxation time. 

The physical properties and melt processing of PLA are similar to those of conventional packaging resins. It may thus be used as a commodity resin for general packaging application. In many aspects the basic properties of PLA lie between those of crystal PS and PET [ 14]. When plasticized by its own monomer lactic acid, PLA becomes increasingly flexible so that products that mimic PVC, LDPE, LLDPE, PP, and PS can be prepared [15]. Possible applications are espe-... 

The tacticity of PLA influences the physical properties of the polymer, including the degree of crystallinity which impacts both thermo-mechanical performance and degradation properties. Heterotactic PLA is amorphous, whereas isotactic PLA (poly(AA-lactide) or poly (55-lac tide)) is crystalline with a melting point of 170-180°C [26]. The co-crystallization of poly (RR-lactide) and poly(55-lactide) results in the formation of a stereocomplex of PLA, which actually shows an elevated, and highly desirable, melting point at 220-230°C. Another interesting possibility is the formation of stereoblock PLA, by polymerization of rac-lactide, which can show enhanced properties compared to isotactic PLA and is more easily prepared than stereocomplex PLA [21]. 

From voltammetric measurements with a Se(l) electrode in a melt of LiCI-KCl at around 700 K, Bodewig and Plambeck [70BOD/PLA] estimated the value of the Gibbs energy of reaction of ... 

Recently stereocomplexes of PLA appeared on the market and lead to promising applications in durable devices (see below PLA applications). The stereocomplexes are defined as the association of polymers with different tacticity or conformation. Three synthesis routes are used to produce a PLA stereocomplex, either in solution or in melt state during pol)nnerization or hydrolysis. The complexe formation is possible with (i) two monomers (L-lactide and D-lactide), (ii) polymer and monomer or (iii) two polymers (PLLA and PDLA). This synthesis is often performed with stannous tin and 1-dodecanol (lauryl alcohol) as initiator or co-initiator of the reaction [36-38]. According to Tsuji et al. [38], some other parameters affect the formation of stereocomplexes ... 

PLA crystallizes usually between 83 and 150°C but its fastest rate of crystallization occurs between 95 and 115°C [83]. The value of the crystallization half time (t, j) varies according to author. In the temperature range 95-115 °C the tj 2 of PLLA for crystallization from the melt varies between 1.5 min to 5 min [45, 79, 84]. Nevertheless the optimum, 1.5 min, is obtained at around 110°C for isothermal crystallization from melt (Figure 8.6) [45]. Not only does the tj of PLA depend widely on the crystallization temperature, but it is also linked to the crystallization type (isothermal or non-isothermal, from cold or melted state). So upon isothermal crystallization from the cold state, t is below 2 min [79, 85, 86]. Eventually, upon non-isothermal crystallization, t also lies around 2 min [85,87,88]. The further the isothermal crystallization is from this optimum, the more tj increases. For isothermal crystallization below 90°C or above 130°C, tj can be beyond 10 min [45, 69]. 

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