Page 133 - Biodegradable Polyesters

5.2 Crystal Polymorphism in Poly(L-lactic acid) 111

5.2
Crystal Polymorphism in Poly(L-lactic acid)

Similar to other polyesters, PLA displays crystal polymorphism and four different
crystal modifications have been identified so far, named α-, β-, γ-, and ε-forms.
The α-form of PLA grows upon melt- or cold-crystallization, as well as from
solution [18–25]. Hot-drawn, melt-spun, or solution-spun PLA fibers of a
high-draw ratio show the β-form [21–23]. The γ-form is obtained via epitaxial
crystallization on hexamethylbenzene substrate [24] and the ε-modification is a
crystalline complex formed below room temperature in the presence of specific
organic solvents such as tetrahydrofuran and N,N-dimethylformamide [25].
Besides these four main crystal polymorphs, two disordered modifications of the
′ ′′ ′
α-form, named α and α , were recently proposed for PLA. The α -crystals grow
∘
′′
via melt- or cold-crystallization below 110 C [21, 26–29], whereas the α -form
develops upon crystallization under special processing conditions, below the
glass transition temperature and in the presence of carbon dioxide [30].
The equilibrium melting point of the α-crystals of PLA was determined by
a number of authors using various approaches, such as the Hoffman–Weeks
approach [31], Gibbs–Thomson approach [32], or the method developed by
Marand et al. [33]. The reported values vary in a wide temperature range, from
∘
199 to 227 C [34–40]. The large scattering of data is caused not only by the
different methods of calculation, as the Hoffman–Weeks linear extrapolation
usually leads to an underestimation of the equilibrium melting temperature [33],
but also by the different molecular characteristics of the used PLA grades, which
in some cases, were not homopolymers but contained D-lactic acid comonomer.
Moreover, many investigations included analysis of melting temperatures of
′
PLA crystals formed at low temperatures, where α -crystals grow, ultimately
resulting in an incorrect determination of the equilibrium melting temperature
of the α-crystals. In addition, the literature data of the bulk enthalpy of fusion of
PLA α-crystals are largely scattering, as they vary between 81 and 135 J g −1 [37,
38, 40–42]. Again, the determination of the enthalpy of melting was conducted
′
without taking into account the α/α crystal polymorphism of PLA. In a recent
′
paper, the equilibrium melting enthalpies of both α-and α -forms were reported:
′
96 ± 3Jg −1 for α-crystals and 53 ± 3Jg −1 for the α -modification [42].
′
Upon usual processing conditions, only α-and α -crystals grow, which makes
these two polymorphs of foremost interest for both academic and industrial
research. The two crystal modifications have a similar chain packing, with a
10 helix conformation and orthorhombic (or pseudo-orthorhombic) unit cell
3
′
[20, 23]. The lattice spacings for the (110)/(200) and (203) planes of α -form
′
crystals are larger than those of their α-counterparts, indicating that the α -form
has slightly larger lattice dimensions [20, 23]. The main difference between α-
′
′
and α -crystals is their chain conformation, as α -crystals have conformational
disorder, which classifies this crystal modification as a condis crystal [26, 43].
′
Compared to the α-form, the α -crystal has weakened specific carbonyl and
′
methyl interactions. Vibrational spectra of α -and α-rich PLA samples suggested