It's known that PLA has a slower degradation rate in soil.Does it have also slower hydrolysis rate compared to other degradable polymers? If so why?
Dear Ozgun, soil biodegradation depends on a wide range of factors. It is practically difficult to find data where all biodegradable polymers have experienced on. Also the mechanisms of biodegradation are different. Hydrolysis is one mechanisms which depends essentially on the soil pH. Bacteria type and stress, enzyms,...all should be assembled to come to a structured comparison. My Regards
PLA hydrolysis proceeds at random. And during the hydrolysis from low-molecular fragments some unusual (non-linear) configurations may form. The latter slow down the whole process of hydrolysis.
Dear Ozgun,
The assertion that the PLA is the polymer with a slower (bio)degradation ability is not entirely true, and even it's not like that at all.
Owing to stereoisomeric nature of lactic acid, the family of PLAs members, poly(alfa-hydroxyacid)s, involves a number of the polyesters such as PLLA, PDLA, PLDLA, atactic PLA with different isomer ratio in the polymer matrix and hence with different crystallinity degree and crystalline morphology. Moreover, the situation is seriously complicated by the fact that certain ferments can interact with well-organized crystalline phase immediately (with elementary crystalline cell) due to the principle key-lock that is so well-famous in enzymology. But in a custom soil situation and especially at nonspecific hydrolysis in aqueous media, the crystallites, lamellae and spherulites should resist again an hydrolitic and enzymatic agent attack.
There is another family of biopolyesters , namely poly(beta-hydroxyacide)s or polyalkanoates, the members of which are characterized by the lower rate of biodegradation owing to their higher hydrophobicity and enhanced crystallinity degree (e.g. PHB, PHBV). This family with slower degradation rate competes with PLAs at polymer market, but simultaneously they complement each other via blending and designing PLA/PHB composites.So, blending begets the novel materials with controlled degradation for packaging, drug release etc.
Sorry for this long answer but the question is fairly sophisticated.
Best regards,
Alexey Iordanskii
I totally agree with Prof Alexey Iordanskii. I have been studing PLA-PHB blends during the last years and I can say that in composting PLA degrades much faster than PHB. PHB delays the PLA disintegration in compost of PLA-PHB blends due to the higher cristallinity of the blends. In fact, PHB promotes the PLA crystallization in PLA-PHB blends.
To express that the PLA has a low rate of degradation in soil as if it were a general behavior is not correct. The soil is a complex and variable matrix which does not allow to establish general degradation behaviors. In addition, the degradation of a polymer in soil depends on environmental and biotic factors, which can also be variable. It is important to remember that the rate of degradation of PLA depends primarily on the molecular size, crystallinity and molecular organization of the polymer; Secondly, it depends on the conditions of the medium, mainly temperature, humidity, pH and presence of chemical agents, this last of the medium has a great influence on the degradation.
The previous answers give a good overview of the different parameters you need to consider. I would like to add the glass transition temperature Tg : at temperatures above the Tg (50-60°C), the hydrolysis and biodegradation rates are significantly increased as amorphous phases become more accessible to degradation. In the contrary, it is quite slow below the Tg.
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