DSC is a convenient technique to determine the purity of an organic compound, such as monomers and low-molecular weight crystalline substances. The calculation utilizes the van’t Hoff equation, which describes the melting point depression from impurities. The DSC technique is relatively easy to perform and the purity value is quick to obtain since modern DSC equipment are equipped with sophisticated software where the software will calculate purity value once the DSC thermograph is obtained. It provides you with the information of total purity level in a few hours. However, keep in mind that this technique will not give any specifics to the nature of the impurities.
You can fine more information in the book “Thermal Analysis of Polymers – Fundamentals and Applications”. The ASTM E928 standard method is also a good reference where it describes the principle of the method and goes into details on how to perform the test method.
Scientists and engineers use the term “sub-ambient glass transition temperature” (aka “sub-Tg”) to denote the Tg of an uncured thermoset material such as resin or prepreg. This terminology comes about because a cured, engineered thermoset system has a Tg that is high (i.e., significantly above room temperature) whereas in its uncured state the Tg is below ambient temperature. The terminology is used intending to void confusion. When sub-Tg is used it is only referring to Tg of uncured thermoset materials. When the material is being cured, the glass transition increases with respect to its degree-of-cure and it is called the typical Tg instead.
Many thermoset materials are processed to thin sheets such as in resin film or prepreg. At temperature below the sub-Tg it remains a solid and the uncured resin film is brittle and easily flaked off if bent. At temperature above sub-Tg, the uncured resin film or prepreg becomes resinous and pliable. It is possible to relate sub-Tg to drapeability of prepreg or resin film for material handle ability purpose. Also the study of sub-Tg can be linked with the goal of determining the material usage time and allowable out time. In addition to drape, people have also been relating sub-Tg to prepreg tack, although tack is a material quality that has been elusive to quantify as it is found to also depend on environmental factors such as ambient humidity and processing factors such as peeling speed.
The sub-Tg can be measured by thermal analysis techniques such as DSC and DMA. Sample should be at ambient temperature during sample preparation and handling. Be sure to eliminate moisture condensation on the sample while it is equilibrating to ambient temperature. Use the same kind of care to perform sub-Tg measurement as you would with measuring Tg of a polymer.
There are powdery materials such as finely ground polymers, polymer microspheres and active pharmaceutical ingredients (APIs) that may require DSC analysis and characterization. Powder samples can be difficult to pack into DSC pans especially if they are static or low bulk density. It could be a messy business with jumpy powders where they cling to your spatula and both inside and outside of the DSC pan. Low bulk density prohibits the sample to have good contact with the bottom of the DSC pan. And if small transitions are of importance where a large sample amount is needed, low bulk density also makes it difficult to pack more powders in a pan.
One way to overcome these difficulties from a powdery material is to make it into a pellet. A pallet with its diameter slightly smaller than your DSC pan will enable you to pack a larger amount of sample into the pan with much improved surface contact and with no static issues. All you need is a pellet maker with the correct diameter, about 3/16” to fit into most DSC pans. If you work in an analytical lab and happen to have a KBr pellet maker (traditionally used for FTIR), I find it to be an excellent pellet maker for this purpose.
In part 1 blog, I mentioned that the “model-free” approach, which is based on the differential isoconversional method, is one of the recent approaches to determine chemical reaction kinetics. This approach is better suited for commercial formulations than prior approaches that rely on mathematical equations for the chemical conversion expression. This is because the “model-free” approach can be viewed as a curve fitting exercise and is more adaptable to commercial formulations that typically contain additives in addition to curing agents.
With this approach, one finds that the activation energy stays relatively a constant during conversion. However, one also notices that in most cases the activation energy varies significantly at the onset (conversion a < ~0.1) and at the end of reaction (conversion a > ~0.8). This is one main reason where specified mathematical models, which assume constant activation energy and pre-exponential factors do not predict reactivity well, especially at temperatures where reactivity is slow.
One potential downside to the “model-free” approach is the difficulties of applying the said kinetic parameters to further complete our quest in answering questions such as time-to-maximum-reactivity and safe process window of time and temperature. This is because instead of constant values, the parameters are now a row of values based on the degree of cure. Therefore, heat transfer models would require to be re-written to incorporate the row of values instead. Fortunately with abundant computing power available nowadays, this is usually not a major difficulty.
For people who work with thermoset polymers, cure information is important from manufacturing control to safe storage and transportation to cure process. In general, cure information of thermosets is characterized by DSC experiment and analysis. For most purposes a single DSC scan (i.e., with a 10C/min heat ramp) will suffice, which will give users its overall reactivity behavior.
However, if there is a need in numerical modeling for prediction purposes of thermal management, a lot more work is required so that cure kinetic parameters can be generated. It would involve several DSC scans of various heat rates and/or several isothermal DSC scans at various temperature values.
The chemical reaction rate depends on two factors, the reaction rate constant and the conversion. The reaction rate constant uses the Arrhenius expression where the pre-exponential factor and activation energy need to be determined from DSC experiment. The pre-exponential factor and activation energy are assumed constant over the entire range of conversion (from 0 to 1). The conversion expression can be expressed as a mathematical equation were one is assumed, for example, n-th order model or autocatalytic model. If so, additional kinetic parameters such as the reaction order and other reaction constants that are expressed in the mathematical equation would be determined from these DSC experiment as well. The beauty of using this approach is that everything is expressed in an elegant equation and parameters are constants. The downside however, is that the fit is at times not good enough.
Instead of using a specific equation to describe the conversion expression, one can apply the differential isoconversional method, which is now commonly referred to as the “model-free” approach. With model-free approach, only the pre-exponential factor and activation energy are the parameters to be determined. However, these parameters are no longer constant and they can vary over the range of conversion. It then becomes a surface-fitting exercise to determine these two parameters with respect to conversion. With powerful computing resources and commercially available software nowadays, it is becoming easier to analyze kinetic data using this approach.
Which way do you plot your DSC thermograph of the heat flow signal? Do you plot “exo up” or “exo down? Do you care? Why are there two conventions anyway? Have you been confused by it when you compared data from various sources or when you read a literature article? If you were used to DSC instrument of power compensation design (i.e., by PerkinElmer), you are most likely to see your DSC plotting as “exo down”. If you were used to DSC instrument designed with heat flux principal, however, you are more likely to see your DSC thermographs plotted as “exo up”.
In a power compensation designed DSC, during an exothermic event, more energy or heat is released from the sample, therefore less than expected energy or power is needed to maintain the temperature, which registers it as a negative event, or plotted as “exo down”. On the other hand, in a heat flux DSC, during an exothermic event, the temperature of the sample rises faster than it is specified by the heat rate program, therefore it is registered as a positive event, or plotted as “exo up”.
Another way to look at it is by defining your thermodynamic system. One definition of the system includes only the sample, which means in an exothermic event heat or energy is released to outside environment, making it “positive”. The other way to define the system is the entire DSC cell, which includes sample, reference, and oven. With this system definition, less energy is needed to maintain the system during an exothermic event. Therefore making it “negative”.
One advantage of plotting “exo down” is that your DSC thermograph (y-axis labeled “heat flow”) will trend the same way as the heat capacity of your sample. By this I mean that your heat flow trends upwards as temperature increases, which is the same way heat capacity behaves: heat capacity increases as temperature increases.
Regardless of how you plot your DSC thermograph, it certainly pays to pay attention on which way is up!