DMA analysis is studied under cyclic loading of constant frequency at varying temperatures to display the visco-elastic behavior and to know the glass transition temperature (Tg)of PLA. Hence, DMA illustrates the damping factor and storage modulus ( which indicates the stiffness) of PLA as a function of temperature.

In my opinion, there are several reasons for using DMA to determine the dynamic mechanical properties of PLA:

1. because PLA behaves in different way when compared with many other polymers, requiring more study and analysis of the produced data. For e.g in some PLA one can observe a decrease of Tg with increasing crystallinity or dependence with heating rates, aging, shrinking during the glass transition (sharp peak in E''), etc.

2. because to understand PLA behaviour you need high sensitivity to the thermal variations and possibility of performing short-term experiments at higher temperatures. It allow also to extrapolate the results for time periods difficult to measure experimentally.

But, I am not a experient specialist in the field. Hope one specialist in the field can add here some comments.

What is the load and frequency we have to apply? Temperature ranges differs according to polymers,

It is known for each polymer the glass transition temperature... There are several ways to determine this temperature for PLA. The best method found through practical experiments is the one found by DMA.

Best regards

Dear Doctor

Go To

Dynamic Mechanical Analysis Investigations of PLA-Based Renewable Materials: How Are They Useful?

Mariana Cristea, Daniela Ionita, and Manuela Maria Iftime

Materials (Basel). 2020 Nov; 13(22): 5302.

Published online 2020 Nov 23. doi: 10.3390/ma13225302

"Conclusions

A survey of literature oriented to the use of the DMA technique in the study of polymers reveals that the tool is too often used only for the determination of the glass transition temperature. This is obtained mostly from tan δ peak of the plot tan δ vs. T, which resulted from a scanning temperature experiment, performed at one frequency. Nevertheless, an accurate understanding of the viscoelastic behavior of polymers requires the comparative examination of all the viscoelastic characteristics: elastic modulus E’, viscous modulus E’’ and loss factor tan δ. Such an approach is even mandatory in the case of PLA and PLA-based polymers that involve a whole hierarchy of phenomena which are triggered as the molecular mobility is changing from low temperature to high temperature. These aspects are summarized below.

• In the glassy region (T < Tg) the secondary relaxations are incidentally mentioned at −50 °C or lower. The β-relaxation was evidenced as a faint drop of E’ modulus or a shallow tan δ peak. Because of the brittleness of PLA, the DMA device is not able to perform reliable experiments at negative temperatures on samples that have a propensity toward cracking.
• When dealing with PLA, the glassy region means also room temperature condition. Therefore, the DMA investigations allow the determination of the elastic modulus E’ under usual working conditions.
• The effects that are noticed in the glassy region during composition-dependent studies are reported also for other classes of polymer. The particularity comes from the semicrystalline character of PLA. The processing conditions, the nature and the content of stereoisomers determine decisively the morphology of PLA in terms of crystallinity. Crystalline content can be tuned during the processing stage, inducing an envisaged change of the E’ modulus.
• By far the most challenging zone is the glass transition region. The chain mobility may be influenced, besides the temperature, by the history of the polymer (aging phenomena) and the applied stress/strain that can induce orientation effects.
• Typically, the glass transition temperature is considered to be the onset of E’ drop or the peaks of E’’ or tan δ. These indicators are often ambiguous in the situation of PLA because of overlapping phenomena that happen during the glass transition. The synergism of enthalpic relaxation, coordinated molecular movements and orientation/crystallization phenomena makes the determination of Tg by DMA fraught with difficulties.
• The E’ onset is often hidden by a hump that could appear just at the beginning of the glass transition on the E’ vs. T plot, because of enthalpic relaxation. As a result, contraction of the samples is obvious when the DMA experiment is performed under tension loading.
• The peaks of E’’ and tan δ for PLA are also deformed as compared to those of a well-behaved polymer that records during the glass transition only the coordinated movements of chain segments.
• The E’’ peak appears very sharp. This E’’ shape accounts for an instantaneous break of mobility growth due to refolding of polymer chains (shrinking).
• The tan δ peak is at least bimodal, its descending side is less abrupt, larger than the ascending side, and it may span partially the first rubbery plateau and the cold crystallization region. This is consistent with few underlying processes. Therefore, under the simultaneous action of temperature and force, orientation/crystallization phenomena are triggered even during the glass transition.
• The increase of crystalline content does not entail necessarily the increase of the glass transition temperature. From a certain level of the crystalline content upward, the values of the glass transition temperature decrease with the crystalline content. These results should be discussed in terms of cooperatively rearranging regions (rigid amorphous phase and mobile amorphous phase).
• The increased toughness that is obtained by adding a plasticizer is reflected in a lower Tg, but very often the height of tan δ peak monitored during the glass transition region decreases with the toughener quantity. Similarly, a reinforcement agent augments the E’ modulus. However, an opposite effect is reflected in the height of tan δ, i.e., it may increase as more reinforcement is included in the DMA. These patterns are consistent with the effects of already mentioned overlapping phenomena happening during the glass transition (enthalpic relaxation, shrinking, orientation/crystallization).
• The length of the first rubbery plateau depends on the PLA molecular weight. It can be considered as a gauge for the PLA level degradation in decomposition studies.
• In the presence of efficient plasticizers the first rubbery plateau is absent because the cold crystallization begins during or immediately the glass transition region.
• The cold crystallization is evidenced by a sudden increase of E’ modulus. When it follows the glass transition region (the first rubbery plateau is absent), the E’ vs. T plot has a V-shape.
• There are instances where the cold-crystallization is not encompassed by the extended tan δ descending side. It can appear as a separate, smaller, frequency-independent peak.
• The E’ value of the second rubbery plateau is lower than that of the glassy region; however it is stable until the abrupt decrease at melting.
• With regard to the heating rate, it is evident that its value is meaningful firstly for the point of view of DMA investigation accuracy. Then, the kinetic events that might take place as the temperature is raised require time for completion. Heating rates higher than 2–3 °C/min are not adequate for fulfillment of both conditions."