Tag Archives: glass transition

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

Glass Transition, Degree-of-Cure, and Cure Kinetics Relationship of a Thermoset

There are many reasons to study a thermoset system from its uncured state to the fully cured state.  The ultimate material properties and attributes in a product is intimately tied with a well cured polymer.  For example, people want to know the rate of advancement at elevated temperature for curing an epoxy formulation.  We may want to know:  Is the resin fully cured?  What is the glass transition temperature when the resin is fully cured?  Can we speed up cure?  Can we optimize cure in terms of time, temperature and pressure?  Would the resin be cured properly without safety hazard?

Resin advancement is expressed mathematically by the degree-of-cure, α.  The degree-of-cure may be evaluated experimentally by DSC or various chemical analyses such as HPLC, NMR, or FTIR, depending on the thermoset chemistry. Frequently people are interested in the relationship between degree-of-cure and glass transition temperature, which helps us relate material properties to process conditions.  As the materials cure, the Tg values increase with respect to its degree-of-cure.  Therefore, we can track the curing process by measuring the Tg values of a thermoset material.  For example, we may be interested in knowing the ultimate Tg when the thermoset is completely cured.

For many thermoset systems it is found that there is a unique relationship between Tg and α.  In other word, a material cured at different temperatures with the same a has the same Tg.  See graphs below.  This unique relationship implies that the molecular structure of materials cured at different temperature is the same or is not significantly affecting Tg.

cure_temp1

Time and degree-of-cure relationship from isothermal experimental data

cure_tg

Glass transition temperature and degree-of-cure relationship

The one-to-one relationship between Tg and α is convenient because Tg in most cases is easier to measure than α.  As I mentioned above, α may be evaluated experimentally by DSC or various chemical analyses such as HPLC, NMR, or FTIR.  Tg can be analyzed by thermal analysis techniques such as DSC, TMA or DMA.  Therefore, Tg measurement can be easily applied to characterize thermoset materials and to quality control testing.

This one-to-one relationship between Tg and α also suggests that a kinetic model (i.e., nth order, autocatalytic, or model-free) can be applied to describe the polymerization reaction of the resin in the temperature region.  We can use the kinetic model to predict material behaviors under all kinds of temperature conditions.  For example, we can use the kinetic model to predict thermal safety such as the time-to-maximum reaction rate under adiabatic condition, “TMRad”, or to develop a cure cycle for a large and bulky part.

Best practices in presenting Tg data

Due to the nature of glass transition, which may occur over a range of temperature 5 – 10 deg-C and which is a second-order transition, it can be difficult and confusing to identify.  In addition, because thermal analysis by nature is dynamic (i.e., temperature increase or decrease), it is necessary to accompany Tg data with experimental details, preferably the thermograph with sample preparation information and test parameters.  Without knowing the testing details, it is impossible to compare Tg.  Ideally, it is best to compare Tg when the samples are tested side-by-side with the same method.  This means the same sample preparation, same test procedures, and the same equipment.

Tg data should always be presented with the way it is measured and how it is identified.  If a standard method such as ASTM is followed, the method should be specified along with its required report information.  If it is not a standard method, sample preparation, test parameters and how the transition measurement is selected should be available, at the very least.  For example, DSC, heating rate 10C/min, mid point temperature.

For comparison between samples, the temperature value should be rounded and not contain any decimal points.

Why am I interested in glass transition temperature, Tg?

You are going to hear about glass transition temperature when you work with polymers, be it a thermoplastic or a thermoset or a composite.  Even with non-polymer materials, glass transition can be of importance.  For example, an uncured epoxy mixture (mind you – it is still in the monomer stage, not yet a polymer) typically shows a glass transition below room temperature, aka “sub-Tg”.  Another example, an active pharmaceutical ingredient (API) that exhibits polymorphism, may also show glass transition.

The reason I care about glass transition and glass transition temperature is because Tg can dictate material selection for a specific application.  Above Tg, the material can be described as rubbery or soft or molecularly flexible.  Below Tg, the material is glassy or rigid or molecularly inflexible. For example, elastomers are commonly used above its Tg.  And carbon fiber reinforced plastics (CFRP) in structural applications are commonly used below its Tg.

Another reason I care about Tg is because it gives me insight into molecular structure of the molecule, which also affects material properties.  For example, the glass transition of a phenolic material (a thermoset) can vary due to the extent of cure and crosslinking density.  Another example is the glass transition of a semi-crystalline thermoplastic.  The glass transition can vary due to the degree of crystallinity of the polymer.

These are the main reasons I am interested in Tg.  How about you?  What makes you study and measure Tg of your materials?