Crystalline Structure of PLA

The crystalline phase of poly(lactic acid) has been mainly studied in the case of PLLA, while only limited data is available on PDLLA or PDLA. Nevertheless, the homopolymers probably present a similar pol)m vorphism, but their handedness of molecular chains in the crystal lattice is opposite [53]. PLLA crystallizes under three main forms dependent on the preparation conditions a (a, a ), P and y. 

The a-form is the most common form of PLLA. It expands under normal crystallization conditions, such as crystallization from molten, glassy state, or in solution [54]. The a-form is characterized by two antiparallel chains in a left-handed IO3 helix conformation [54, 55]. As shown in Table 8.3, the chains are stacked in an orthorhombic or pseudo-orthorhombic crystalline unit cell. A notable structural feature of the a-form is the distortion of the helix conformation and the crystal structure change. According to the authors named in Table 8.3 the interactions between closest neighbour chains, that is to say the electrostatic dipole-dipole interactions, are the cause of these changing data. 

The a -form was discovered by Marubayashi et al. [70]. The a -form is, like a -form, a disordered a-form due to the exposition to high-pressure CO below Tg. So below 40°C, for a pressure between 3 and 15 MPa, only the a -form is obtained. This a -form presents a poor chain packing and a lower crystal density, compared to a- and a -form [70]. 

The y-crystalline form, discovered by Lotz et al. has been the subject of only a few studies. It is obtained by epitaxial crystallization [53-55] on hexa-methylbenzene (HMB) substrate, but the formation mechanism is still to be determined. The y-form is characterized by two antiparallel helices with a 3j conformation packed in an orthorhombic cell unit whose dimensions are a = 0.995nm, b = 0.625nm, and c = 0.880nm [54]. 

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