Although PLA is one of the most commonly used materials in 3D printing and one of the most popular bioplastics, not many people know that there is more than one type of PLA out there. Variations of PLA are mostly based on the ways in which its chemical structure can be altered. The differences may be subtle, but they are significant enough for a few industries and are certainly worth discussing.
Lactic acid as a chiral molecule Before jumping into the different types of polymer chains that exist for PLA, we need to look at the molecular structure of its constituent monomer – lactic acid. Lactic acid is produced by fermentation of carbohydrate compounds which are typically derived from plant-based material, giving PLA its unique character as a more “sustainable” plastic. The chemical composition of lactic acid does not change – each unit of lactic acid contains exactly three molecules of carbon, three molecules of oxygen, and six molecules of hydrogen which are connected in a manner that creates specific functional groups. However, there can be variations in how these molecules are oriented in three-dimensional space. Lactic acid is what is known as a chiral molecule. This means its molecular structure has two mirror images that are distinct from each other. These mirror images are called enantiomers or optical isomers. To distinguish between these optical isomers, they are commonly referred to as either the L (laevorotatory) or the D (dextrorotatory) isomers based on the direction at which they polarize light. In the case of lactic acid, its optical isomers are called L-lactic acid and D-lactic acid. A mixture of the optical isomers can also be concocted, which is referred to as a racemic mixture of simply DL-lactic acid. The process by which lactic acid is created, including the bacteria used for fermentation, can have pronounced effects on which type of isomer lactic acid isomer is produced. Controlling these factors is important because these isomers can have distinct characteristics – for instance, both L-lactic acid and D-lactic acid have a freezing point that’s higher than DL-lactic acid.
Lactic acid as a chiral molecule Before jumping into the different types of polymer chains that exist for PLA, we need to look at the molecular structure of its constituent monomer – lactic acid. Lactic acid is produced by fermentation of carbohydrate compounds which are typically derived from plant-based material, giving PLA its unique character as a more “sustainable” plastic. The chemical composition of lactic acid does not change – each unit of lactic acid contains exactly three molecules of carbon, three molecules of oxygen, and six molecules of hydrogen which are connected in a manner that creates specific functional groups. However, there can be variations in how these molecules are oriented in three-dimensional space. Lactic acid is what is known as a chiral molecule. This means its molecular structure has two mirror images that are distinct from each other. These mirror images are called enantiomers or optical isomers. To distinguish between these optical isomers, they are commonly referred to as either the L (laevorotatory) or the D (dextrorotatory) isomers based on the direction at which they polarize light. In the case of lactic acid, its optical isomers are called L-lactic acid and D-lactic acid. A mixture of the optical isomers can also be concocted, which is referred to as a racemic mixture of simply DL-lactic acid. The process by which lactic acid is created, including the bacteria used for fermentation, can have pronounced effects on which type of isomer lactic acid isomer is produced. Controlling these factors is important because these isomers can have distinct characteristics – for instance, both L-lactic acid and D-lactic acid ha
ve a freezing point that’s higher than DL-lactic acid.
PLLA vs PDLA vs PDLLA
As we’ve mentioned, different isomers of lactic acid can have different chemical and physical properties. These differences also translate to their corresponding polymer products. Considering the concept of chirality, we now know there are also three possible PLA polymer chains:
PDLA (Poly D-lactic acid)
PDLLA (Poly DL-lactic acid)
Crystallization Both PLLA and PDLA can be manufactured by having pure mixtures of either L-lactic acid or D-lactic acid undergo a condensation reaction to produce long polymer chains. Both PLLA and PDLA polymers are naturally crystalline, which means that they take on ordered molecular structures. PDLLA is created by polymerizing a mixture of both L-lactic acid and D-lactic acid. A 1:1 ratio of both isomers is typically used, the degree of crystallinity of the final polymer can be altered by tweaking with this ratio. In general, however, PDLLA is less crystalline and more amorphous than its PLLA and PDLA counterparts.
Solubility The crystalline nature of both PLLA and PDLA gives them almost similar characteristics. They have melting temperatures somewhere within the 170 °C to 180 °C range and are selectively soluble. Crystalline PLLA does not dissolve in many common solvents like acetone, ethyl acetate, and tetrahydrofuran (THF). On the other hand, PDLA does not decompose when exposed to certain enzymes that can hydrolyze both PLLA and PDLLA.
Properties PDLLA is produced by the copolymerization of L-lactic acid and D-lactic acid or their lactide counterparts. When combined in a 1:1 ratio, the resulting PDLLA becomes an amorphous material with a glass transition temperature of 50 to 60 °C. The lack of a crystalline structure makes PDLLA more chemically reactive and more prone to biodegradation. Many solvents that do not react with the pure stereoisomers PLLA and PDLA can partially dissolve PDLLA. As with the pure stereoisomers, molecular weight plays a vital role in determining the physical and chemical characteristics of PDLLA. The 50 to 60 °C glass transition temperature applies for polymer chains with molecular weight of up to 30,000. Keeping the polymer chains short also results in gradual decrease of this glass transition temperature. As with other polymers, this also has effects on physical properties such as tensile strength and flexibility. Take note that these physical and chemical characteristics can still vary greatly depending on manufacturing methods. Factors such as the rate of crystallization, the molecular weight of the polymer chains, and the ratio of individual components play a significant role in determining the properties of the final polymer. Thus, these differences between PLLA, PDLA, and PDLLA are not absolute.
Applications Even without differentiating between the different types of PLA polymers, we already know some of its more common applications. PLA is a non-toxic plastic that is considered safe for food contact. The fact that it breaks down into non-toxic lactic acid makes it biocompatible and suitable for sutures and implants that are meant to be absorbed by the human body. High-molecular weight PLLA is the material of choice for stents and implants that need to maintain their mechanical properties over an extended period. PLLA takes several months to degrade, and this degradation time can be further extended by producing PLLA with higher molecular weight polymer chains. They have been used for implants meant to facilitate the reconstruction of tendons and ligaments, as well as embolic materials for arterial embolization. PLLA has better chemical stability, better withstands enzyme degradation, and has a much longer resorption time. PDLLA, on the other hand, breaks down inside the body relatively quickly. While this makes it unsuitable for long-term implants, PDLLA is actually one of the most well-researched bioplastics today. Its resorption behavior has been studied to a point where scientists can predict when it will degrade under normal physiological conditions. This unique characteristic has made PDLLA an ideal material for many drug-release mechanisms.
Conclusion Most of us who are into 3D printing have probably spent a lot of time working with PLA without really knowing how complex its chemistry could be. While we may not need to know the difference between PLLA, PDLA, and PDLLA, a little extra knowledge doesn’t hurt. If you ever find yourself in a situation where you need to 3D print medical implants or devices, it would be nice to know specifically which flavor of PLA you should use.
source: https://polylactide.com/plla-vs-pdla-vs-pdlla/