Temperature-modulated DSC techniques (TMDSC) are widely used in thermal analysis in industrial and university research laboratories to separate overlapping temperature-dependent and time-dependent thermal effects.
Methods used up until now have overlaid the isothermal temperature or heating ramp with a sinusoidal temperature modulation of just one frequency (single frequency method). In contrast, TOPEM®, the new advanced multi-frequency temperature-modulation technique, uses a large number of different frequencies (multi-frequency approach).
This webinar offers you the opportunity to learn more about the theory behind TOPEM® and to study some typical applications.
METTLER TOLEDO offers three different techniques for performing temperature-modulated DSC measurements. They are known as IsoStep, ADSC and TOPEM®.
The TOPEM webinar offered by METTLER TOLDO explains why temperature-modulated DSC (TMDSC) is so useful. An overview of various temperature-modulated DSC techniques is presented before going into more detail about the most advance technique – TOPEM. The webinar also discusses several examples that illustrate the kind of questions TOPEM can answer.
Theory of TMDSC
A temperature-modulated DSC experiment yields reversing and non-reversing heat flows. Under quasi-static conditions, such as exist in an ideal IsoStep experiment, the reversing heat flow corresponds to the sensible heat flow, and the non-reversing heat flow to the latent heat flow.
In an ADSC experiment, modulation occurs at a particular frequency. In this case, the statements “reversing heat flow equals sensible heat flow” and “non-reversing heat flow equals latent heat flow” are not generally valid.
TOPEM has a tremendous advantage: it allows you to distinguish between latent and sensible heat flows and at the same time investigate the frequency dependence and hence the dynamics of processes. To understand how this is possible, this webinar takes a closer look at the principles on which TOPEM is based, supported by real application examples.
Differential Scanning Calorimetry
Slide 0: TOPEM – A New Temperature-Modulated Technique
Ladies and Gentlemen,
Welcome to this seminar on temperature-modulated DSC techniques.
In particular, I would like to focus on an exciting new temperature-modulated DSC technique, which we call TOPEM. This technique has been developed and patented by METTLER TOLEDO.
Slide 1: Contents
First, I want to explain why temperature-modulated DSC is so useful. I will then present an overview of various temperature-modulated DSC techniques and describe TOPEM in more detail. After the theoretical background, I will discuss several examples that illustrate the kind of questions TOPEM can answer.
Slide 2: Why use Temperature Modulation?
Let me begin with a few remarks about possible reasons for using temperature-modulated DSC.
The slide shows the DSC curve of a sample of polyethylene terephthalate recorded at a heating rate of 10 degrees per minute. The curve displays the net heat flow in and out of the sample. In temperature-modulated DSC terminology, this heat flow is referred to as the total heat flow. In physical terms, the total heat flow consists of two parts, namely the sensible heat flow and the latent heat flow. The sensible heat flow is related to changes in sample temperature and hence to sample heat capacity. In contrast, the latent heat flow has to do with structural changes in the sample.
The heat flow curve shows three thermal events: a glass transition, cold crystallization, and melting. All three processes involve both sensible and latent heat.
For example, the step in the curve at the glass transition is caused by the change in heat capacity of the sample and corresponds to sensible heat flow, while the peak superimposed on the step is due to enthalpy relaxation and corresponds to latent heat flow.
The two heat flow components cannot be separated using conventional DSC. Temperature-modulated DSC methods, however, make such separations possible.
Slide 3 Temperature-Modulated DSC (TMDSC)
The temperature programs used in temperature-modulated DSC are more complex compared with those employed in conventional DSC experiments. Analysis of the resulting heat flow yields the so-called reversing heat flow and the non-reversing heat flow. The total heat flow is always the sum of the reversing and the non-reversing heat flows and corresponds to the heat flow measured in a conventional DSC experiment.
Separation of the total heat flow into reversing and non-reversing heat flow components allows you to understand and interpret the different thermal events that occur in the sample. Furthermore, overlapping effects can be separated. For example, in a partially cured resin system, the glass transition can be separated from the postcuring reaction.
METTLER TOLEDO offers three different techniques for performing temperature-modulated DSC measurements. They are known as IsoStep, ADSC and TOPEM.
The following slides summarize their most important features.
Slide 4 Different Approaches
The slide gives an overview of the temperature programs used in IsoStep, ADSC and TOPEM experiments.
The three techniques differ in the type of temperature program used for the measurement.
In IsoStep, the program consists of a sequence of alternate isothermal and heating segments. The data from the heating segments is used to determine the change in heat capacity and thus the reversing heat flow. The data from the isothermal segments yields the non-reversing heat flow.
In ADSC, a small sinusoidal temperature modulation is superimposed on the linear temperature program. The modulation is characterized by its amplitude and its period or frequency. The resulting temperature program causes the heat flow to vary in a sinusoidal manner. Continuous averaging of the heat flow over one cycle yields the total heat flow generated by the underlying heating rate. The reversing heat flow is determined from the amplitude of the modulated heat flow and the modulated heating rate. The difference between the total heat flow and the reversing heat flow gives the non-reversing heat flow.
In TOPEM, the linear heating profile is overlaid with a non-periodic temperature perturbation consisting of a series of temperature pulses of different length and defined intensity. Before we discuss TOPEM in more detail, let’s have a look at the basic theory of temperature-modulated DSC.
Slide 5 Theory of TMDSC
As I have already explained, from the physical point of view, the total heat flow consists of the sensible heat flow and the latent flow. A temperature-modulated DSC experiment, however, yields reversing and non-reversing heat flows. Under quasi-static conditions, such as exist in an ideal IsoStep experiment, the reversing heat flow corresponds to the sensible heat flow, and the non-reversing heat flow to the latent heat flow.
In an ADSC experiment, modulation occurs at a particular frequency. In this case, the statements “reversing heat flow equals sensible heat flow” and “non-reversing heat flow equals latent heat flow” are not generally valid. However, the frequency dependence of the measured heat flows allows us to draw conclusions about the dynamics of the processes that occur. To investigate frequency dependence using ADSC, we have to perform an individual experiment for each frequency of interest.
TOPEM has a tremendous advantage: it allows you to distinguish between latent and sensible heat flows and at the same time investigate the frequency dependence and hence the dynamics of processes. To understand how this is possible, we must now take a closer look at the principles on which TOPEM is based.
Slide 6 Principles of TOPEM
Let’s assume we want to measure the thermal properties of a sample. To do this, we need an instrument, in our case a DSC. The measurement result contains information about the sample and about the properties of the instrument. These two pieces of information are not so easy to separate. For a measurement, we need an input signal that allows us to obtain as much information as possible about the system. The stochastic temperature modulation used in TOPEM is an excellent way to do this: the modulation consists of short random pulses of defined pulse height superimposed on a relatively slow underlying heating program. This produces a heat flow output signal that appears to be very noisy. In fact, the signal contains all the information needed about the frequency behavior of the system.
The mathematical evaluation analyzes the correlation between the heating rate and the measured heat flow. This yields the reversing and the non-reversing heat flows, so that in contrast to ADSC, the non-reversing heat flow is measured directly.
Slide 7 Principles of TOPEM
The dynamic behavior of the entire system is characterized using mathematical functions. These functions describe the frequency response of the system at a particular temperature or time.
Slide 8 Principles of TOPEM
The frequency response of the system is determined in a window specified by the user. This allows the reversing, non-reversing and total heat flows, and the frequency-dependent and quasi-static heat capacity to be determined for that particular window at that particular temperature or time. The window is then moved across the entire set of measurement data. This yields heat flows and heat capacities as a function of temperature or time. The entire evaluation process is done automatically.
Slide 9 The TOPEM Measurement Process
This slide summarizes the TOPEM measurement and evaluation process.
In step 1, the stochastically modulated temperature curve and the resulting heat flow are measured. In step 2, the system is characterized with respect to its frequency behavior.
The quasi-static heat capacity, and the reversing, non-reversing, and total heat flows are then calculated in step 3. The reversing and non-reversing heat flows are determined under quasi-static conditions, which means that they correspond to the sensible and the latent heat flows.
If needed, the complex heat capacity can also be determined for any particular frequency in a fourth step.
Slide 10 Polyethylene Terephthalate by TOPEM
This slide shows the measurement of polyethylene terephthalate using TOPEM.
The uppermost curve is the measured heat flow. The curve below this is the quasi-static heat capacity. The curve exhibits a step-like increase at the glass transition at about 80 degrees. Cold crystallization begins at about 110 degrees. This is accompanied by a decrease in heat capacity because the heat capacity of the liquid phase is greater than that of the semicrystalline material.
The phase curve can be used to determine the two components of the complex heat capacity.
The bottom curves display the total, non-reversing and reversing heat flows.
The enthalpy relaxation would normally be observed in the non-reversing and total heat flow curves. In this example, the effect is very weak and is hardly visible.
The peak due to cold crystallization is only observed in the non-reversing and total heat flow curves.
Slide11 Polyethylene Terephthalate by TOPEM
This second slide of PET shows the heat capacity curve in more detail. The glass transition can be clearly seen with a midpoint at 77 degrees; cold crystallization occurs at about 115 degrees.
As expected, the glass transition is clearly frequency dependent, whereas the cold crystallization is independent of frequency.
Slide 12 Comparison of Methods
This slide gives an overview of the different temperature-modulated DSC techniques. All three separate the heat flow into reversing and non-reversing heat flow components.
The simplest technique is IsoStep. Here, the two heat flow components are separated under quasi-static conditions. IsoStep uses sapphire as a standard to determine accurate heat capacity values.
In ADSC, separation of the total heat flow into reversing and non-reversing heat flows enables different overlapping effects to be distinguished from one another.
In TOPEM, the reversing and non-reversing heat flows correspond to the sensible and latent heat flows. This leads to a better understanding of the measurement results. Overlapping thermal effects can also be separated from one another. At the same time, TOPEM yields information about frequency behavior and hence about the dynamics of thermal processes. Furthermore, TOPEM provides the most accurate heat capacity values in comparison with all other techniques, including conventional DSC.
Slide 13 Industries and Applications
TOPEM has numerous potential applications in practically all industries. The table gives an overview of possible effects that can be investigated using TOPEM.
For example, in the automobile, aerospace and electronics industries, precise information about curing reactions is crucial. Here, TOPEM can be used to separate vitrification processes from reaction processes. In the pharmaceutical industry, melting and crystallization behavior, polymorphic transitions, and the influence of moisture are typical applications.
I would now like to present several different application examples that demonstrate the power and versatility of TOPEM.
Slide 14 TOPEM Application 1
This slide shows measurements performed on a powdered pharmaceutical substance. The particles sometimes clump together due to the effect of moisture. This of course interferes with the production step that follows. The black curve in the lower diagram is the total heat flow. It consists of a broad endothermic effect with two small peaks superimposed on it. The origin of these two peaks is not immediately clear.
TOPEM first yields the quasi-static heat capacity. This is the red curve in the upper diagram. At about 60 degrees, there is a step in the heat capacity. This characterizes the glass transition of the material. The peaks at this temperature in the total and non-reversing heat flow curves are due to enthalpy relaxation.
After the glass transition, the heat capacity decreases due to the increased evaporation of water. The heat capacity curve exhibits a small peak between about 120 and 130 degrees due to a melting process.
In this sample, the presence of moisture acts as a plasticizer and lowers the glass transition temperature causing the powder to become lumpy.
Slide 15 TOPEM Application 2
This slide shows the isothermal curing reaction of an epoxy resin performed at a temperature of 80 degrees. The lower diagram shows the total heat flow curve. This describes the course of the reaction. However, one does not know whether the reaction has gone to completion or not.
TOPEM provides you with additional information. The quasi-static heat capacity curve is shown in the upper diagram. At short reaction times, the heat capacity is relatively high. Between 70 and 120 minutes, it decreases in a step. But, what is the reason for this step? Let me explain:
At the beginning of the reaction, the resin is liquid. As crosslinking increases, the glass transition temperature shifts to higher temperatures. After about 70 minutes, the glass transition temperature is the same as the reaction temperature. This means that during the course of the isothermal reaction, the sample changes from the liquid to the glassy state, or as we say, the sample vitrifies. The time taken for this to occur is called the vitrification time. In this example, it is about 86 minutes.
During vitrification, the reaction rate slows significantly. The reason for this is that, in the liquid state, the reaction is under chemical control, whereas in the glassy state, it is diffusion controlled. Due to the limited molecular mobility in the glass, the reaction practically stops and no further crosslinking occurs. Under these conditions, the resin is not completely cured.
Slide 16 TOPEM Application 3
In practice, incomplete curing as a result of vitrification is one of the most frequent causes of failure in composite materials. If incompletely cured material is heated in a DSC, a so-called postcuring peak is observed immediately after the glass transition. As a result, it is often impossible to see the glass transition in a normal DSC measurement (here the green curve). This also makes it difficult to determine the postcuring enthalpy and hence the original degree of cure.
In the TOPEM measurement, the glass transition is clearly visible at about 210 degrees in the reversing heat flow curve. The small peak in the non-reversing and total heat flow curves is therefore due to the postcuring reaction. This peak can be evaluated and used for quality control.
Slide 17 TOPEM Application 4
The melting of polymers such as polypropylene can also be investigated using TOPEM. Such measurements must be performed using very small pulse heights and low heating rates.
Reversible melting can be seen in the reversing heat flow curve between 140 and 170 degrees. This corresponds to melting under equilibrium conditions.
The red curve is the non-reversing heat flow. It describes non-reversible melting, that is, the melting of locally superheated crystals. Such melting processes occur from about 160 degrees onward. Prior to this, exothermic crystallization is observed between 150 and 160 degrees. In this temperature range, reorganization processes take place in the material.
Slide 18 TOPEM Application 5
Not only polymers exhibit complex melting behavior. This slide shows the phase behavior of a sucrose-water solution containing 40% sucrose. The sample was cooled from room temperature to minus 100 degrees at 2 degrees per minute. During this process, ice crystals are formed and the solution of sucrose and water becomes a glass.
When the sample is heated, we first observe a broad glass transition in the reversing heat flow curve at about minus 45 degrees. A small endothermic peak due to enthalpy relaxation can also be seen in the non-reversing heat flow curve at about the same temperature.
After the glass transition, small amounts of water separate out from the sugar-water solution and crystallize. This process is observed as a small exothermic peak at about minus 37 degrees in the non-reversing heat flow curve. This is followed by an endothermic peak, which is attributed to the melting of very small unstable crystals.
In the reversing heat flow curve, the beginning of the main melting peak becomes apparent after the glass transition. First, small ice crystals melt in the sugar-water solution. The material is in a state of equilibrium so that the melting process is only visible in the reversing heat flow curve. This example illustrates how TOPEM can be used to investigate crystallization kinetics after the glass transition.
Slide 19 TOPEM Application 6
This slide shows an example in which TOPEM was used to separate overlapping processes in a sample of polystyrene containing a foaming agent. In a conventional DSC measurement, we see two endothermic peaks at about 60 and 110 degrees.
The TOPEM measurement again provides additional information. The bottom diagram shows the heat capacity curve with a glass transition at about 55 degrees. From 90 degrees onward, the heat capacity decreases slightly. This is due to a partial release of the foaming agent. The non-reversing heat flow curve exhibits a peak due to enthalpy relaxation followed by an endothermic peak due to loss of the foaming agent. This information allows us to interpret the conventional DSC curve. The first effect corresponds to a glass transition with enthalpy relaxation and the second peak to vaporization of the foaming agent. The differences in the peak temperatures between the conventional and the TOPEM measurements are due to the different heating rates.
Slide 20 TOPEM Application 7
Due to its measurement principle, TOPEM is more sensitive than conventional DSC. This is illustrated in the slide showing measurements of a polymer that should exhibit two glass transitions.
The conventional DSC curve exhibits a glass transition at about 5 degrees. One suspects that another thermal event could be present at about 70 degrees. This, however, is very uncertain.
The TOPEM measurement, however, clearly reveals a second glass transition at 70 degrees in addition to the glass transition at 5 degrees. This demonstrates that weak thermal events can be observed and identified using TOPEM.
Slide 21 TOPEM Application 8
This slide shows cooling measurements performed on a material that consisted of two different polymers. The conventional DSC curve shows a crystallization peak in the range plus 10 to minus 20 degrees. In the TOPEM measurement, the crystallization process is observed in the non-reversing heat flow curve and the glass transition in the reversing heat flow curve. The rather broad glass transition cannot be seen in the conventional DSC experiment because it is overlapped by the crystallization process.
Slide 22 TOPEM Application 9
Pure second order phase transitions only show a change in heat capacity. This means that they should only be observed in the reversing heat flow curve. Second order phase transition measurements are therefore an excellent way to check how well the heat flow is separated into sensible and latent heat flows.
The slide shows results obtained for the solid-solid transition of sodium nitrate at 275 degrees. The second order phase transition is only observed in the reversing heat flow curve. The reversing heat flow can therefore be interpreted as sensible heat flow.
Slide 23 Summary of TOPEM Features
What conclusions can we draw from all this?
In one single measurement, TOPEM can simultaneously yield information about the quasi-static heat capacity and about the frequency dependence of the heat capacity.
TOPEM is the DSC method that gives the most accurate values for the heat capacity of a material.
TOPEM measurements provide high sensitivity and resolution. Weak thermal events can be analyzed with excellent temperature accuracy.
The TOPEM method separates the total heat flow into reversing and non-reversing heat flow components. Physically these two heat flows correspond to the sensible and the latent heat flows. This allows overlapping processes to be separated.
Slide 24 Summary of TOPEM Features
TOPEM measurements can be used to investigate the frequency dependence of thermal effects. Frequency dependence is an important aid for interpreting measurement results.
The TOPEM evaluation technique is based on PEM, a parameter estimation method. This largely eliminates possible instrumental influences on the measurement result.
The quasi-static heat capacity is used as an internal reference curve to determine the frequency-dependent heat capacity. This enables the frequency-dependent complex heat capacity to be determined with great accuracy.
Slide 25 For More Information about TMDSC
The following slides list different articles that explain the basic principles and applications of temperature-modulated DSC. The articles have been published in our biannual UserCom technical customer magazine. You can download UserCom articles from the Internet.
Slide 26 For More Information about TMDSC
This slide presents a list of UserCom articles that describe the basic principles and applications of TOPEM as well as the TOPEM data sheet and the TOPEM literature list.
Slide 27 Thank You
This concludes my presentation on temperature-modulated DSC. Thank you very much for your interest and attention.