Monthly Archives: April 2014

Directions of DSC Heat Flow Signals

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!

AFM and Roughness

AFMs can measure nanoscale roughness from sub micron to less than an angstrom.  However, there are two main factors which need to be considered when using these instruments for this type of measurement:

1) Vertical Resolution – How the system scans in Z
2) Spatial Resolution – How the system scans in XY

Vertical Resolution.  How well the system measures vertically is a function of the noise floor of the system.  If the sample feature size is 1nm, the system needs to have a vertical noise floor less than 1nm in order to measure the feature adequately.  The largest vertical feature that you can measure will be equal to the noise floor or the Z resolution limit.  This noise floor measures all of the mechanical and electrical movement or noise in the system.  It is easiest measured by setting the scan size to 0, using the parameters that you will use to measure the sample.  When the tip is not moving across the surface any measured variation over time is due to noise int he system and will mask any surface features once the tip starts to move.  For measuring the lowest noise floor achievable, many companies use a standard set of parameters so that they compare values.

Spatial Resolution.  Since the movement in XY is almost always larger than that in Z.  The lateral resolution of the an AFM isn’t limited by system noise, but by the radius of curvature (ROC) of the tip.  The sharper the tip, the better the resolution.  Super sharp commercial cantilevers have a ROC about 2-3nm.  Standard AFM probes have a nominal ROC of 6nm.  Any probe used for advanced modes usually has a coating not the to make them sensitive to a particular signal of interest.  These probes can have have ROCs 20nm or more.

In addition to the ROC, it is important to keep the tip sharp without blunting the tip to maintain a) the highest resolution possible and b) to maintain a stable and consistent image.  If the tip wears during the coarse of imaging.  The first image may look dramatically different from the last image on the same sample surface.  In cases where the feature size is much smaller than the tip size, a condition called tip imaging (tip self imaging, reverse imaging) will occur where the features surface features will be representative of the end of the tip rather than the surface itself.

What to do?  Understand the noise floor of your system.  This will determine what are the limits of the machine. Know what the specifications of your probe are and what condition it is in.  This will provide you with confidence that the images you are seeing are representative of the sample surface.

Happy Scanning….

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Thermoset Cure Characterization Using Dynamic Mechanical Analyzer (DMA)

One person’s question on the LinkedIn discussion forum “Dynamic Mechanical Analysis (DMA / DMTA)” prompted me to write this blog entry. Do you know that DMA is an effective tool to study cure of a thermoset material? Typically, cure characterization of a thermoset is studied by DSC for kinetics information and by rheometer for viscosity information.  However, in some
instances – such as instrumentation constraint or material constraint – you can study cure with DMA. The most effective way to study cure in DMA is by supporting your thermoset material with a mesh material, such as a plain weave glass fabric. You then run it in the cantilever mode or film mode with a constant heat rate. (My personal preference is to run it in single cantilever mode.) And you should get a DMA thermograph similar to the one illustrated below.

DMA cure

DMA cure

A good reference is the book “Thermal analysis of polymers” edited by J. Menszel & B. Prime. Look into chapter 5, section “DMA Techniques for Developing Cure Parameters”. Figure 5.44 shows a technique with wire mesh using a tensile fixture.