Rule of Thumb in Polymer Engineering: Connection between Glass Transition and Melting Temperature of Amorphous Thermoplastics

Hello and welcome to this Rule of Thumb post. More Rule of Thumb posts in polymer engineering can be found here. 

Often there are situations where the information on the glass transition temperature is useful to have at hand e.g. during material selection process or processing. There are different ways to obtain such information. The most accurate way is by measuring it with analysis methods such as DSC, DMA, and TMA. 

However, there is an empirical way for a fast estimation of the Tg of amorphous polymers: starting by the melting temperature of the polymer and multiplying it by ⅔ and you obtain the glass transition temperature range (Figure 1). 

Figure 1: Rule of Thumb in polymer engineering - estimating the Tg from Tm.

Example PVC

Polyvinylchloride, PVC,  has a glass transition temperature of 338.15 K and melting point at 485.15 K. Applying the Tg=2/3Tm rule results in an estimated Tg of 320.19 K. The difference to the measured Tg with tools such as DSC is only 5%. As I mentioned before, it is a quick estimation tool and a certain deviation need to be kept in mind. 

If you have interest in the Tg of engineering and high performance polymers - I made an overview which can be found here. 

Conclusions

Altogether, this rule of thumb serves as a fast adjustment approach when you do not have the exact information at hand. However, it seems to have a good fit with most amorphous resins (Tg above the 2/3 and 1/3 line; Figure 2) and can only be restricted applied to certain amorphous (PVC, PS, PC as examples) and semi-crystalline polymers. Furthermore, this Tg/Tm Rule of Thumb was proposed 70 years ago by Raymond F. Boyer (thanks to Pawel for this input) and it is still considered valid for polymers having both symmetrical and unsymmetrical molecular structures. However, later studies have found the ratio to vary widely, though.

Figure 2: selected amorphous polymers with their Tg and Tm as well as 2/3 and 1/3 line.

Thanks for reading and #findoutaboutplastics

Greetings, 

Herwig Juster

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Literature:

[1] https://plasticranger.com/what-is-the-glass-transition-temperature-of-plastics/#What_is_a_rule_of_thumb_relating_Tg_to_TM

[2] https://www.researchgate.net/figure/DSC-melting-temperature-of-pure-PMMA-and-Ash-PMMA-composites_tbl1_262474618#:~:text=The%20coal%20ash%2FPMMA%20composite,the%20polymer%20matrix.%20...