SEC/GPC, DSC, and TGA Analysis of Low Molecular Weight Poly-L-lactic Acid

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James Browne, Jennifer Gough
TA Instruments, Waters
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Using a combination of analysis techniques for the analysis of PLLA, such as SEC, DSC, and TGA can reveal polymer characteristics that give insight into the manufacturing and processing of PLLA. The SEC analysis technique is very important in understanding the molecular weight distribution, and APC provides the molecular weight data for each injection in this experiment in 10 minutes, with a calibration curve provided in 30 minutes. Relative stability was determined by TGA and the results indicate the need to understand the effect of other structural characteristics on the stability of the polymer. DSC shows complex thermal transitions including crystallization from the melt and cold crystallization in 3 of the 4 samples. Each of the samples also shows enthalpic recovery due to enthalpic relaxation (aging) which potentially can be mitigated with process control. 

The analysis reveals an unusual distribution of the 40 KDa sample of 3 separate peaks. The 75 KDa and 6 KDa peaks can affect the melting and crystallization properties of the polymer, and these effects can potentially manifest in final product performance. The potential effect of the 75 KDa fraction on crystallization can be evaluated by either isothermal or non-isothermal kinetics methods. The effect of the 6 KDa fraction is apparent in plasticization of the sample resulting in the observed lower glass transition temperature and by implication lower modulus.

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