There are different values for Tg of PE in the literature. The values are (-78, -110, -125 deg C). Are these correct. As I ran a DSC for my HDPE sample (1mm and 3mm) and got this curve. As per my understanding, there is no transition for Tg. Looking for your expert opinion.
Attached is the file.
Dear Hafiz Usman Khalid,
Tg of HDPE is lower than -100 ˚C. The minimum temperature for DSC analysis must be chosen at lower temperature (for example -120 ˚C) for HDPE to observe glass transition temperature.
Dear Hafiz Usman Khalid ,
For linear polymers, the glass transition (Tg) will depend on the polymer molecular weight Mw (amu).
Considering the structure of the homo or copolymer, the end of the polymer chain tends to be more flexible than the rest of the chain. By considering the chain increase, the molecular weight increases and, therefore, the mobility.
The generated plateau in the Tg x Mw graphics results from the relatively rigid resulting final polymer macrostructure configuration.
General speaking, all factors such as lower flexibility, bulkier groups, cross-linking, intense chain interactions that may affect the molecular mobility on a large scale have an increased effect over Tg.
However, as the polymer chain increases, the overall mobility will reduce up to a stable value as Mw goes to higher values; it is usually associated in the literature to the “infinite Tg temperature," reached at the effect saturation, where the Tg does not change in a significant way with a further increase of Mw, usually after Mw 5000 – 10000 amu.
So, in a mixture of different Mw components of one particular polymer or copolymer sample, it is usual to find more than one Tg, associated with their different Mw members contributions.
Best regards,
WNM
Hi,
I see that you are crystallizing the material on the cooling cycle. Although I am not sure how much, it seems to be a great part of it at least (if not all). If that is the case, you'll not be able to see any glass transition (wherever it is) of course. I'd suggest a faster cooling cycle to avoid (part of) the crystallization.
Best,
Joan
Hello Hafiz,
Firstly, there is no single Tg for any polymer. Tg depends on the method and conditions of measurement, which is why the method of measurement and conditions should always be stated when reporting Tg. Second, Tg will only be observed for the amorphous portions of the polymer as crystalline polymers exhibit melts (1st order thermodynamic transitions) and not glass transitions, which are 2nd order thermodynamic transitions. Since your polymer shows melting endotherms it obviously has crystalline components, however the percent of crystallinity is unknown based on your data and description. Regardless, this means that the amount of amorphous material will be a fractional percentage of the total polymer, and as stated above only the amorphous portions will exhibit Tg. If the amorphous fraction is small Tg will be difficult to observe. Third, DSC is not as sensitive to Tg as other methods like DMA. DMA is 10 - 100 times more sensitive to Tg as compared to DSC. As such, if possible I recommend that you try DMA. TMA is also another sensitive technique for Tg that you can try.
Joseph Marcinko Sir, what information is missing from my data which you are talking about ?
Wagner Da Nova Mussel Well HDPE has a linear structure, so there is just one polymer ethylene in it. So, it happens to have one Tg ?
Dear Hafiz Usman Khalid ,
It is linear; however, several molecular weights (Mw) are commercially available from the base (-CH2-CH2-)n repeating units; several polymers with different Mw are classified as HDPE.
High-density polyethylene, HDPE, is a polymer whose monomer is ethylene made up of a considerable number of repeating units (monomers), and its chemical formula can be generalized as (C2H4)n.
HDPE mostly features a low degree of branching where the linear molecules or the polymer chains are packed together tightly.
The presence of a strong intermolecular force results in a dense, highly crystalline material. For this reason, during the cooling cycle, you will experience some crystallization. Dr. Joan Ràfls-Ribé already pointed out the presence of crystallization in your experiment.
The long, chainlike molecules can be produced in linear or branched forms.
Branched versions are commercialized as low-density polyethylene (LDPE) or linear low-density polyethylene (LLDPE); linear versions are known as high-density polyethylene (HDPE) and ultrahigh-molecular-weight polyethylene (UHMWPE).
Linear polyethylene can even be produced in ultra-high-molecular-weight, with molecular weights of 3,000,000 to 6,000,000 atomic units. However, the HDPE is about 500,000 atomic units.
So, in your commercial or synthetic sample you end up with several HDPE different polymers with several different Mw.
Best regards,
WNM
Specifically, I am referring to the percent crystallinity for your samples, the molecular weight of your samples and any information about thermal history of your sample other than the thermal history that you imparted via DSC. HDPE can come in varying grades from different suppliers. For example where does one suppliers MDPE cut off before it becomes HDPE and likewise where does HDPE cut off before it becomes UHDPE. No polymer is perfect and the degree of crystallinity can vary with polymerization and with thermal history.
There are standard methods for measuring crystallinity via the DSC melting endotherms. Knowing the degree of crystallinity will be important to understanding how much amorphous material is in the HDPE. It is the amorphous portions of the molecule that will show Tg. Again, if knowing tg is important for you I would recommend trying DMA as it is much more sensitive to Tg.
Thanks for answers and guidance Joseph Marcinko Wagner Da Nova Mussel Mina Karimi-Avargani Joan Ràfols-Ribé
Are you aware that there are methods to alter the post polymerization second order glass transition temperatures of polyolefins!!
All true but when you do low temperature Gardner impact and physical property testing and the thermal analytical portion of the argument you will see that post polymerization modification of glass transition temperatures of both polypropylene and polyethylene and their blends can be altered to control whatever Tg properties you desire down to -35C. The practice is now used in both China, Russia, Canada and in the Artic. This is called taking the science into the real world
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