English
In the field of biodegradable materials, the issue of crystallinity of polylactic acid (PLA) has always been a focus of discussion among peers: why does the crystallinity of PLA seem to never break through the 50% -60% threshold no matter how the process is adjusted or nucleating agents are added? Nucleating agents can make crystallization faster, but they cannot increase this' upper limit '?
Today we will first clarify a core point: for crystallizable polymer materials such as PLA, there is an inherent upper limit to crystallinity determined by their own characteristics. Nucleating agents, annealing, and other methods can only shorten the time to reach the upper limit, and cannot break through the upper limit itself. Next, we will go from analyzing viewpoints to breaking down the reasons, and explain this matter thoroughly.
Initial viewpoint: The upper limit of material crystallization is a "fixed number", and nucleating agents only solve the "speed" problem
Firstly, it is important to clarify that the upper limit of crystallinity and the crystallization rate are two completely different concepts!
For PLA, its crystallization upper limit is "innate" - influenced by inherent properties such as molecular structure and chain segment mobility, even under the most ideal processing conditions (such as precise temperature control and sufficient insulation time), molecular chains cannot be arranged 100% orderly to form crystal regions. The upper limit of high crystalline PLA in the industry is generally stable at 50% -60%, which is the "ceiling" that the material itself cannot surpass.
On each repeating unit of PLA, there is a prominent "methyl group (- CH3)", which acts as a "small bump" stuck between molecular chains, greatly increasing the friction between chain segments and making the molecular chains stiff.
Compared to polyethylene (PE) - PE molecular chains are smooth "- CH ? - CH ? -" long chains, with flexible rotation of segments and easy crystallinity of 70% -90%; However, PLA molecular chains are "immobile" and almost completely stationary at low temperatures (below the glass transition temperature of 60 ℃). Even when heated above 60 ℃, the movement of chain segments is slow and difficult to quickly adjust to a regular arrangement.
More importantly, the melting temperature of PLA is about 170 ℃ - no matter how high the temperature is, the newly formed crystal zone will melt, falling into the dilemma of "low-temperature immobility, high-temperature melting zone". The molecular chains naturally cannot be fully arranged, and the upper limit of crystallization is stuck.
2. Polarized isomers "disrupt the rhythm": arranged neatly, but with "impurities"
PLA exists in three optical isomers: left-handed polylactic acid (PLLA), right-handed polylactic acid (DLLA), and a mixture of the two (PDLLA). Among them, only PLLA and DLLA are "ordered stereoisomers" that can form ordered crystal regions; But due to the random distribution of left-handed and right-handed segments, the molecular chains of PDLLA cannot align at all and are almost completely amorphous.
In actual production, even for "pure PLLA", it is difficult to achieve 100% optical purity, and a small amount of DLLA segments will always be mixed in. These 'impurity chains' will be embedded in the growing crystal region like' impurities', disrupting the orderly arrangement of molecular chains - just like suddenly inserting several people in the wrong direction while queuing, the queue can no longer extend neatly. As the crystal region grows, these "defects" will continue to accumulate, ultimately making it impossible for the crystal region to continue expanding, and the crystallinity can only stop at 50% -60%.
3. The contradiction between "speed and perfection" in crystallization: too fast to be neat, too slow to be full
The crystallization process of PLA also faces an irreconcilable contradiction: pursuing speed will sacrifice the perfection of the crystal region; If you want to pursue perfection, you will miss the opportunity to crystallize.
If pursuing fast crystallization: adding a large amount of nucleating agent and increasing annealing temperature, molecular chains will quickly arrange around the crystal nucleus, but because there is no time to adjust the posture, many small defects will appear in the crystal zone, which will limit the further growth of the crystal zone and the crystallinity cannot be improved;
If pursuing high perfection: reduce annealing temperature, extend insulation time, give molecular chains enough time to align, but at low temperatures, molecular chain movement is too slow, and some areas have just formed perfect crystalline regions. After the material has cooled and solidified, the remaining molecular chains can no longer move, and ultimately there are still a large number of amorphous regions, making it difficult to exceed the upper limit of crystallinity.
This contradiction between "fast and good" further compresses the room for improving the crystallinity of PLA, making the upper limit a "fixed number".
In fact, for practitioners of bio based materials, there is no need to overly focus on "breaking through the crystallization limit". Instead of pursuing numbers above 60%, it is better to precisely regulate according to the application scenario - making high-temperature resistant products and controlling the crystallinity at 45% -55% can meet the demand; Making transparent products, a crystallinity of 30% -40% can actually balance transparency and mechanical properties. Understanding the "temperament" of materials is more practical than forcefully breaking through the "ceiling".
Have you ever encountered the situation where the crystallinity cannot be improved even after countless process adjustments in PLA crystallization control? Welcome to share your experience in the comment section and explore more efficient solutions together!
