Tag Archives: TMA

Measuring Glass Transition Temperature (Tg)

Glass transition (Tg) describes a material moving from rubbery to glassy state.  Unlike phase transitions, which are first-order phenomena, glass transition is a second-order phenomenon.  Glass transition is a kinetic phenomenon where it occurs over a range of temperature and its activation energy can be evaluated and it has relevancy to time-temperature-superposition.

Thermal analysis is particularly useful for glass transition temperature (Tg) evaluations of polymers.  For example, Tg can be studied using DSC, DMA and TMA analyses.  We show cartoon drawings below to illustrate typical thermographs depicting glass transition using thermal analysis techniques.  Depending on material type and sample geometry limitation, certain techniques are better suited than others.

Because glass transition is a kinetic phenomenon, Tg varies with heat rate in the thermal analysis testing.  This is one reason that heat rate should always be specified when reporting Tg values.  In general, Tg measured with a slower heat rate (such as at 1C/min) is lower than Tg measured with a higher heat rate (such as at 20 C/min).  The difference in measured Tg values due to heat rate differences may be as little as a few degrees to as much as 10 deg-C.

And because different material properties are being measured depending on the techniques, the measured glass transition temperature from one technique to another should not be expected to be “identical” or “equivalent” even with the same heat rate.  This is the reason that the type of thermal analysis should always be specified when reporting Tg values.  Although the measured Tg values should not be expected to be identical, the variation in Tg values obtained from various techniques are typically to be less than 10 or 20 deg-C.

 

Typical Tg transition in DSC

Typical Tg transition in DSC

Typical Tg transition in TMA

Typical Tg transition in TMA

Typical glass transitions in DMTA

Typical glass transitions in DMTA

Acronym! Acronym! What are DMA, DMTA, and TMA?

For people who are new to thermal analysis, especially to “mechanical” analysis, the terms DMA, DMTA, TMA can be very confusing.  I mention this because I was confused when I was learning thermal analysis.  Now I pay special attention and ask for clarification when people ask me questions about DMA.  The reason these terms are confusing is because these abbreviations have overlapping and multiple meanings.

First, let’s tackle the most confusing and the most general term, DMA. DMA can mean “dynamic mechanical analysis”; or, DMA can also mean “dynamic mechanical analyzer”.

When it is “dynamic mechanical analyzer”, it refers to the actual machine, the analyzer, itself.  Most instrumentation vendors who sell DMA machines have their DMA model names starting with “DMA”, for example, TA Instruments’ “DMA Q800” and MT’s “DMA 1”.  These DMA machines are usually designed where the sample deformation occurs in the “flex” or “tension” mode, i.e., normal to the sample.

When the acronym DMA is used to refer to “dynamic mechanical analysis”, however, it implies to the analytical methodology where mechanical measurement of the sample is utilized to obtain information we are interested about the sample.  For example, ASTM E1640 “Standard Test Method for Assignment of the Glass Transition Temperatures by Dynamic Mechanical Analysis” describes various modes of DMA such as flexure, torsion, tension, compression, etc.

Similar to DMA, the acronym DMTA means “dynamic mechanical thermal analysis” or, “dynamic mechanical thermal analyzer”.

It is basically the same as DMA except the “T” referring to thermal is inserted into the acronym. Not too many people refer to DMTA nowadays.  It is used almost exclusively referring to the Polymer Laboratory and Rheometric Scientific’s DMA machines since that’s how they name their DMA models.  But since both companies no longer exist nor making DMA machines, this term is not used widely anymore.

Lastly, the acronym TMA refers to “thermo-mechanical analyzer” or “thermo-mechanical analysis”.  TMA refers to the mode of mechanical analysis where a constant load (in stress or strain) is applied to the sample instead of an oscillating load as in the case of the “dynamic” modes.