If your material had irreversibly crystallized through the first heating cycle, you obviously cannot see any peaks of Tg and crystallization through the further heating cycles. I think, the absence of these peaks is an evidence for irreversibility of the processes occurred at the first heating.

Dear Mohamed,

Based on my practical experience regarding the characterizations by DSC of diverse grades of PLA, the different rates of cooling or heating could influence the thermal behavior of your polymer.

In your case, using a relatively slow cooling from the molten state, i.e. 5°C/min, the polymer could crystallize even that you didn’t see an evident crystallization exotherm on your DSC cooling curves. Thus, because your PLA is more or less crystallized, in the second heating it will be difficult to determine the glass transition temperature with a traditional DSC instrument, while it is possible to not have an additional cold crystallization process.

Regarding the second question, i.e., the annealing effect, for instance I will recommend two techniques: a) rapid cooling from the molten state to the temperature of interest and keeping in isothermal conditions the required time; b) Prepare firstly an amorphous PLA (e.g. by rapid quenching of melted polymer using nitrogen liquid or other cooling agent) and then heat the samples at the temperature of interest and keep them in isothermal conditions.

Finally, I will say that my comments are quite general. The nature of your PLA (molecular weights, isomer purity, presence or not of some additives, eventually a previous processing and degradation, etc.) could affect the thermal behavior of this polymer.

I will comeback with some selected references !

Good luck in your R&D study and best regards,

Marius

Marius Murariu

Dear Dr. Marius

Thank you for your great answer, and I'm looking forward to the references

Regards,

Mohamed

Dear Mohamed,

Regarding the annealing of PLA using DSC apparatus, the best method must be selected function of the main goal of your study and in some cases by considering the technical possibilities of your instrument (e.g., rate of cooling or heating). Below are only some papers that can be used for a first help in your study.

Good luck in your study and best regards,

Marius

Yongqi Feng, et al., The crystallization behavior of poly(lactic acid) with different types of nucleating agents, International Journal of Biological Macromolecules, Volume 106, January 2018, Pages 955-962 http://dx.doi.org/10.1016/j.ijbiomac.2017.08.095

Pluta M., Morphology and properties of polylactide modified by thermal treatment, filling with layered silicates and plasticization, November 2004, Polymer 45(24):8239-8251, DOI: 10.1016/j.polymer.2004.09.057

Liao, R. , Yang, B. , Yu, W. and Zhou, C. (2007), Isothermal cold crystallization kinetics of polylactide/nucleating agents. J. Appl. Polym. Sci., 104: 310-317. doi:10.1002/app.25733

Cui, L., Wang, Y., Guo, Y., Liu, Y., Zhao, J., Zhang, C., and Zhu, P. (2016) Cooperative effects of nucleation agent and plasticizer on crystallization properties of stereocomplex‐type poly(lactide acid). Polym. Adv. Technol., 27: 1301–1307. doi: 10.1002/pat.3795

Tábi, T., Sajó, I., Szabó, F., Luyt, A. S., & Kovács, J. G. (2010). Crystalline structure of annealed polylactic acid and its relation to processing. Express Polymer Letters, 4(10), 659-668. https://doi.org/10.3144/expresspolymlett.2010.80