Dear friend Je MIN Jung

Poly lactic acid (PLA) is a biodegradable and bioactive thermoplastic made from renewable resources, such as corn starch or sugarcane. The absence of a clear crystallization peak for PLA in Differential Scanning Calorimetry (DSC) is an interesting aspect tied to its polymer structure and behavior.

PLA has both amorphous and crystalline regions in its structure. The absence of a distinct crystallization peak in DSC can be attributed to a few factors:

1. **Amorphous Nature**: PLA tends to have a significant amorphous phase. Amorphous regions lack long-range order, and therefore, they do not exhibit a sharp melting or crystallization peak in DSC.

2. **Rapid Crystallization**: PLA has a relatively fast crystallization rate. In DSC, this can sometimes appear as a broad, less-defined crystallization region rather than a distinct peak.

3. **Multiple Polymorphs**: PLA can exist in different crystalline forms or polymorphs, each with its own distinct melting temperature. This diversity in crystalline structures can result in a broader or more diffuse thermal transition.

4. **Chain Stereoregularity**: The stereoregularity of the polymer chains in PLA affects its crystalline properties. PLA can have both L-lactide and D-lactide units, and the arrangement of these units can influence the crystallization behavior.

5. **Processing Conditions**: The conditions under which PLA is processed, such as the cooling rate during fabrication, can impact its crystallinity and the appearance of peaks in DSC.

It's worth noting that while PLA might not exhibit a sharp crystallization peak in DSC, it does have a distinct glass transition temperature (Tg) and melting temperature (Tm) associated with its amorphous and crystalline phases, respectively.

In summary, PLA's behavior in DSC is complex and influenced by its amorphous nature, rapid crystallization, polymorphism, and processing conditions. The absence of a clear crystallization peak in DSC does not mean PLA doesn't crystallize; rather, it crystallizes in a way that may not be as easily distinguishable in this particular analytical technique.