Polylactic acid (PLA) is a kind of aliphatic polymer compound, which is a new degradable polymer material. It has many advantages that other polymer materials cannot match. First of all, its synthetic raw materials are derived from renewable crops, and the resin can be degraded into H2O and CO2 after use, without causing environmental pollution; Secondly, PLA is non-toxic, non-irritating and has good biocompatibility, and is widely used in medical sutures and drug slow-release materials. Moreover, PLA has good mechanical properties and can be widely used in industrial packaging, textile, automotive industry and other fields. Due to these outstanding characteristics, PLA is increasingly becoming the focus of attention.

PLA polymeric monomer lactic acid has stereoisomerism, there are L-lactic acid and D-lactic acid two optical isomers, so polylactic acid also has L-L-lactic acid (PLLA), right-handed poly-D-lactic acid (PDLA) and racemic poly-DL-lactic acid (PDLLA) three isomers. Among them, PLLA and PDLA are crystalline polymers with crystallinity up to 40-60%. Racemic polylactic acid (PDLLA) is a kind of amorphous polymer, which is not crystalline and belongs to amorphous form. Compared with the non-crystalline polylactic acid (PDLLA), the glass transition temperature (Tg) and melting point (Tm) of crystalline polylactic acid (PLLA/PDLA) are slightly higher. At the same time, the crystalline polylactic acid (PLLA) has higher elastic modulus and smaller variability, and the mechanical properties show a large gap. In addition to different physical properties and mechanical properties, the crystallization of polylactic acid also has a certain impact on the degradation performance of polylactic acid, because the degradation of polylactic acid always starts from the non-crystalline zone, and the molecular chain arrangement in the crystal zone of crystalline polylactic acid is regular and tight, and it is not easy to be destroyed. At the same time, the crystallization of polylactic acid also helps to improve the gas permeability, heat resistance, thermal stability and so on.

The crystal type mainly depends on the heat treatment or processing technology, and under different conditions of the external environment, different types of crystal can be transformed into each other. Polylactic acid crystal type can be divided into α crystal type, β crystal type, γ crystal type and sc crystal type. This paper mainly introduces four different types of crystalline polylactic acid. The α crystal type was first discovered by De Sanctis and Kovacs in PLLA. Its structure is shown as follows: cell parameters a=1.07nm, b=0.645nm, c=2.87nm, and it is a quasi-orthorhombic system. Later, Hoogsteen et al. found that annealed PLLA fibers had the same crystalline structure, with slightly different cell structure parameters. Typically, alpha crystals can be obtained by crystallization from melt, solution, or solution spinning at low drawing temperatures and drawing rates.

D has the same crystalline structure, but the cell structure parameters are slightly different. Typically, alpha crystals can be obtained by crystallization from melt, solution, or solution spinning at low drawing temperatures and drawing rates.

gamma-form

γ-crystal form is obtained by the epitaxial growth of PLLA on hmethylol benzene, and its structure is very close to the β-crystal molecule model proposed by Brizzolara, except that there are two anti-parallel helical lines in the orthorhomorph cell. The structure is shown in the figure below, with parameters a=0.995nm, b=0.625nm, and c=0.88nm. Except for γ-crystals obtained by hexamethylbenzene epitaxy, no other method has been found to prepare γ-crystals.

sc crystal type

sc crystal is a special type of polylactic acid crystal, first proposed by Ikada in 1987, can be formed in the blending system of PLLA and PDLA, is a kind of stereo-composite crystal. Compared with the crystal form of pure PLLA and PDLA alone, sc crystalline material has good heat resistance (melting point increased by about 50℃) and chemical stability, so constructing sc crystal is an effective means to improve the comprehensive performance of PLA. The appearance of sc crystal is also different when PLLA and PDLA are mixed in different proportions.

Due to different crystallization conditions, pure PLLA/PDLA has α, β and γ crystal types. The α crystal is the most common and thermodynamically stable crystal. β crystal can be obtained at a higher tensile ratio (10~20) and tensile temperature (180~200 ℃). It was found that β crystal PLA has excellent impact resistance and heat resistance. Compared with α crystal, β crystal is unstable and its melting point is about 10 ℃ lower. The gamma-crystal form is obtained by the epitaxial growth of hexamethylbenzene by PLLA at around 140 ℃. sc crystal is a special crystal produced by mixing PLLA and PDLA, and the melting point is greatly increased, up to 225℃.