As An Environmentally Friendly Material, How Does PLA Degrade Effectively?
2022-04-27 14:28:12
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Polylactic acid (PLA) is a new type of bio-based and renewable biodegradable material, which is made from starch raw materials proposed by renewable plant resources (such as corn, cassava, etc.). The starch raw material is saccharified to obtain glucose, and then fermented from glucose and certain strains to produce high-purity lactic acid, and then chemical synthesis method is used to synthesize polylactic acid with a certain molecular weight. It has good biodegradability, and can be completely degraded by microorganisms in nature under specific conditions after use, and finally generates carbon dioxide and water, which does not pollute the environment, which is very beneficial to protect the environment and is recognized as an environmentally friendly material.
Polylactic acid (PLA) is a bio-based resin with a bio-based content of 100%. With the popularization and application of PLA in the fields of food, medicine, and daily necessities, it not only effectively alleviates the earth's energy crisis, but also significantly improves environmental pollution problems.
PLA degradation mechanism
The molecular structure of polylactic acid (PLA) is shown in the figure. The ester bond is easily hydrolyzed and can be degraded by microorganisms in the body or in the soil to generate lactic acid. The final products of metabolism are water and carbon dioxide, so it will not produce toxic and side effects on the human body. Safety. Therefore, polylactic acid has been used in many aspects of medicine, pharmacy, etc., such as surgical sutures, drug controlled release systems, and so on.
Due to the optical activity of lactic acid, there are three corresponding polylactic acids: PDLA, PLLA, and PDLLA (racemic).
PLLA and PDLA are partially crystalline polymers with good mechanical strength, and are often used as medical sutures and surgical orthopedic materials. Controlled drug release preparations often use PLLA and PDLLA, but more often use PDLLA. The degradation product of PLLA, L-lactic acid, can be completely metabolized by the human body, so it is more competitive than D-PLA.
Degradation in vivo
The hydrolysis of PLA is a complex process, mainly including four phenomena: water absorption, cleavage of ester bonds, diffusion of soluble oligomers and decomposition of fragments.
Main mode of degradation: bulk erosion.
After the PLA material is immersed in an aqueous medium or implanted in a human body, the material absorbs water first. The aqueous medium penetrates into the polymer matrix, resulting in the relaxation of the polymer molecular chain, the initial hydrolysis of the ester bond, the decrease in molecular weight, and the gradual degradation into oligomers.
The terminal carboxyl groups of polylactic acid (generated by polymerization and degradation) catalyze its hydrolysis. As the degradation progresses, the amount of terminal carboxyl groups increases and the degradation rate accelerates, resulting in autocatalysis.
Internal degradation is faster than surface degradation, which is attributed to the retention of carboxyl-terminated degradation products in the sample, resulting in a self-accelerating effect.
As the degradation progresses, there will be more and more carboxyl groups inside the material to accelerate the degradation of the internal material, further increasing the internal and external differences. When the inner material is completely converted into a soluble oligomer and dissolved in an aqueous medium, a hollow structure with a surface composed of a polymer that is not fully degraded is formed. Further degradation results in hydrolysis of the oligomers into small molecules, which are finally dissolved in aqueous media.
The entire dissolution process is from water-insoluble solids to water-soluble substances.
Macroscopically, the overall structure of the material is destroyed, the volume becomes smaller, gradually becomes fragments, and finally dissolves completely and is absorbed or excreted by the human body;
Microscopically, the polymer macromolecular chains undergo chemical decomposition, such as molecular weight reduction, molecular chain breakage and side chain breakage, etc., and become water-soluble small molecules that enter body fluids, are phagocytosed by cells, and are transformed and metabolized.
In vitro degradation
The decomposition of polylactic acid has two stages: after decomposition by hydrolysis reaction, it is decomposed by microorganisms.
In the natural environment, hydrolysis first occurs, and oligomers are formed by hydrolysis of unstable ester bonds on the main chain. Then, microorganisms enter the tissue and decompose it into carbon dioxide and water. Under the conditions of composting (high temperature and high humidity), the hydrolysis reaction can be completed easily and the decomposition rate is faster. In an environment where hydrolysis is not easy to occur, the decomposition process is gradual.
Microorganisms are ubiquitous in nature, and polylactic acid can be degraded by a variety of microorganisms. Such as Fusarium candida, Penicillium, humic bacteria and so on.
The degradation of polylactic acid with different configurations by different bacteria is different. The results showed that both Candida fusarium and Penicillium can completely absorb D,L lactic acid, and some can also absorb soluble polylactic acid oligomers.
Factors affecting degradation
1) pH value
| The picture comes from Baidu pictures Either acid or base can catalyze the hydrolysis of PLA. The degradation rate of polylactic acid under alkaline conditions > the degradation rate under acidic conditions > the degradation rate under neutral conditions.
2) Crystallinity
The degradation process always goes from the amorphous region to the crystalline region. This is because the molecular segments in the crystalline region are tightly packed, and water is not easy to penetrate. It penetrates into the amorphous region first, resulting in the breaking of ester bonds. When most of the amorphous region has been degraded, the degradation starts from the edge to the center of the crystalline region. During the hydrolysis of the amorphous region, low molecular weight substances with stereoregularity are formed, and the crystallinity increases, which delays the further hydrolysis.
3) Molecular weight and molecular weight distribution Molecular weight is inversely proportional to the degradation rate. The larger the molecular weight, the tighter the structure of the polymer, and the less likely the internal ester bond is to be broken; moreover, the larger the molecular weight, the longer the segment obtained by degradation, the less soluble in water, and the less water and hydrogen cations are generated, The pH value decreases slowly, which is one of the reasons why its degradation rate is lower than that of low molecular weight polylactic acid. For polymers with the same average molecular weight, the wider the molecular weight distribution, the faster the degradation rate. This is because the polymer with smaller molecular weight decomposes first, and the pH value of the environment changes from neutral to acidic, thereby accelerating the degradation rate.
4) The effect of stereoregularity Under alkaline conditions, the degradation rate is PDLA (PLLA)<P (LDL)A<PDLLA Since the methyl group is in a syndiotactic or atactic state, PDLLA absorbs water faster and degrades faster; for PLLA and PDLA, the hydrolysis is divided into two stages: the first stage, the diffusion of water molecules Enter the amorphous region, and then hydrolysis occurs; the second stage is the hydrolysis of the crystalline region, which is relatively slow.
5) Enzymes The main chain of polylactic acid contains ester bonds, which can be accelerated by esterases for degradation. Such as Rhizopus sp. lipase, porcine pancreatic lipase, porcine liver fusiform esterase.
