Crystallization of PLLA

Digital images of SALS scattering patterns at selected tunes during the isothermal crystallization of PLLA at 90°C. 

Very similar results have been recently obtained by Shin el al. [36] in PLLA-6-PEO-6-PLLA samples Ain values were in the range of 30-50 kg/moi). They found that PLLA and PEO blocks were miscible in the melt upon cooling from the melt at 2°C/min, sequential crystallization of PLLA and PEO occurred. The PLLA formed negative spherulites at higher temperatures then, at lower temperatures, the PEO crystallized within the PLLA spherulites, and the sign of the birefringence was maintained, only the retardation increased in an additive fashion. 

PLA stereo-complexes are obtained by co-crystallization of PLLA and PDLA in bulk or in organic solvents such as chloroform or acetonitrile. It has been shown that the minimal degree of polymerization (DP) of PLLA or PDLA oligomers for crystallization is 11, with corresponding melting temperature (Tm) at 59 °C and melting enthalpy (AH i) of 39 J g. In contrast, in blends of PLLA and PDLA oligomers, crystallization or stereo-complexation is... 

Figure 3.2 TEM micrographs of single crystals of PLLA grown from a 0.08 wt% solution in a) toluene at 55 °C and b) p->ylene at 90 °C. The insert shows an electron diffraction pattern of one of the crystals. ...

Polarized light optical micrographs (PLOM) during isothermal crystallization of PLLA a) after 8 min. at 140 °C (modified from Ref. 37), b) after armealingat 160 °C for 2 h followed by slow cooling to 120 °C (modified from Ref. 21). 

Thus, an increase in polymer crystallinity should generally decrease polymer degradation rates, as extensively reported for PLA matrices under alkaline hydrolytic degradation conditions.Other researchers reported accelerated hydrolysis in neutral media with increasing polymer crystallinity for PLLA and PLLA/PDLA blends. A possible explanation to this behaviour has been related to the consideration that, upon crystallization of PLLA, hydrophilic terminal groups (-OH and -COOH) can be condensed in the amorphous area between the crystalline regions (see... 

J.F. Mano, Y Wang, J.C. Viana, Z. Denchev and M J. Oliveira, Cold crystallization of PLLA studied by simultaueous SAXS and WAXS, Macromol. Mater. Eng., 289 (2004) 910-915. 

As mentioned above, the of sc-PLA is much higher than that of he crystals of PLLA or PDLA. The highest experimentally determined for sc-PLA is 240°C [2, 4, 9], and the equilibrium is reported to be 279° C [10], much higher than that of a crystals (190°C) [11, 12], The crystallization temperature (Tc) of sc crystals is uncertain, although their crystallization rate is apparently faster than that of he crystals. In many cases, sc crystallization is accompanied by he crystallization. The glass transition temperature (Tg) of sc-PLA is reported to be 65-72°C [13], which seems to be slightly higher than that of the ordinary PLLA samples [11], probably because the chain mobility in the amorphous domain is affected by the crystalline states. 

TABLE 5.2 Various Crystal Parameters Reported for the he Crystals of PLLA... 

Depending on the preparation conditions, three different crystalline modifications (a, (3, y) can be attained for PLLA. Recently, the fine details of dynamic processes during the crystallization of PLLA have become a matter of keen interest. The orthorhombic structure of the 3-form, previously suggested from X-ray studies, was confirmed by trichroic vibrational analysis [43]. 

As for isothermal crystallization of PLLA, the spherulite growth rate is determined by crystallization temperature and molecular weight. As the crystallization temperature rises, the spherulite growdi rate first increases and then decreases. As the molecular weight of PLLA increases, the spherulite growth rate decreases. The maximum of spherulite growth rate occurs at 120°C and isothermal overall crystallization rate occurs at 105°C [221]. 

Maillard D, Prud homme RE (2008) Crystallization of ultrathin films of polylactides from chain chirality to lamella curvature and twisting. Macromolecnles 41 1705-1712 Mano JF, Wang Y, Viana JC, Denchev Z, Oliveira MJ (2004) Cold crystallization of PLLA studied by simultaneous SAXS and WAXS. Macromol Mater Eng 289 910-915... 

In the PLLA/PMMA blend, the crystallization of PLLA was restricted by PMMA. Once the thermal history of the blends was destroyed. PLLA and PMMA were miscible. 

The aim of this paragraph is to explain when the alignment of PLLA chains in the perpendicular direction of the PE chain starts. In other words, to find out whether there are already oriented chains of PLLA in the amorphous phase or not. This will provide different crystallization scenarios about the epitaxial cold crystallization of PLLA on the PE substrate. If some PLLA chains oriented perpendicular to the chain direction of PE in the as-prepared sample due to intermolecular interaction, these oriented PLLA chains will act as prenuclei. The transformation of these prenuclei into stable crystal nuclei in the subsequent cold crystallization process initiates the growth of fold chain lamellae with high orientation. Otherwise, the alignment of the PLLA chains in the random coil amorphous state should start in the nucleation process during cold crystallization. For this purpose, the orientation status of PLLA in the as-prepared sample, that is, in amorphous phase, has been followed by polarized FTIR. 

Fig. 5.2. Growing spherulites observed during the crystallization of PLLA in an optical microscope (polarized light, crossed nicols) (by Cho, FMF, Universitat Freiburg, 2006)...

As stated by Lotz and co-workers [134,135], crystal morphology and structure can be regulated by epitaxial crystallization on the substrates of low-MW substances or polymers. The epitaxial crystallization of PLLA on the hexamethylbenzene surface induces the formation of a new crystal modification, i.e., y-form [135]. The epitaxial crystallization of PBA on the highly oriented PE or i-PP film favors the formation of the p-form crystals, rather than the thermally stable a-form [136,137]. 

---