Crystallinity degradation mechanism

The degradation time of PLLA is much slower than that of PDLLA, requiring more than 2 years to be completely absorbed. Copolymers of L-lactide and DL-lactide have been prepared to disrupt the crystallinity of L-lactide and accelerate the degradation process. The final crystallinity and mechanical properties of the polymer depend on the stereochemistry of the polymer backbone. PLA has a number of biomedical applications, such as sutures, stents, dialysis media, and drug delivery devices. " °... 

Sueoka, K., Nagata, M., Ohtani, H., Nagai, N., and Tsuge, S., Thermal Degradation Mechanisms of Liquid Crystalline Aromatic Polyester Studied by Pyrolysis-Gas Chromatography/Mass Spectrometry, /. Polym. Sci Part A, 29,1903,1991. 

The y-irradiation of cellulose in the atmosphere associates oxidation and degradation mechanisms, as one would expect from the particularly high energy involved. A recent investigation has also shown that this type of treatment favours some cellulose crosslinking [86, 87], as well as a decrease in crystallinity accompanied by thermal degradation, whose extent increases with increasing irradiation doses [88]. 

In parallel, the physical structure of PLA has been reported to affect its hydrolytic degradation mechanism. For example, it has been found that the hydrolytic chain cleavage proceeds preferentially in the amorphous regions, leading to the increase in polymer crystallinity. The rate of the degradation reaction seems also to be affected by the shape of the specimen and by the chemical structure of the polymer, as well as by the conditions under which the hydrolysis is performed, including pH and temperature. ... 

The chemical degradation of a polymer depends on a number of factors including composition and molecular structure, polydispersity index (PDl), crystallinity, surface area, and hydrophilic or hydrophobic characteristics (Tamariz and Rios-Ramirez, 2014). The chemical degradation of a polymer generally occurs via a random cleavage of covalent bonds. As such, the chain order, crystallinity, and mechanical properties of the material are disrupted. 

Metal ions need to be specified in low quantities, because Sn, Zn, Fe, and A1 cations accelerate polymerization, but may also affect hydrolysis, oxidation, or other degradation mechanisms. Sodium in particular causes racemization even in ppm amounts. Stereochemical purity expresses the sum of meso- a.c-tide and D-lactide in L-lactide and vice versa. The higher the stereochemical purity of the lactide monomer, the higher the stereochemical purity of the obtained PLA, which controls material properties such as melting point, crystallinity, and mechanical strength. 

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