Source: Danielle N. Beatty and Taylor D. Sparks, Department of Materials Science and Engineering, The University of Utah, Salt Lake City, UT
Differential scanning calorimetry (DSC) is an important measurement to characterize thermal properties of materials. DSC is used primarily to calculate the amount of heat stored in a material as it heats up (heat capacity) as well as the heat absorbed or released during chemical reactions or phase changes. However, measurement of this heat can also lead to the calculation of other important properties such as glassy transition temperature, polymer crystallinity, and more.
Due to the long, chain-like nature of polymers it is not uncommon for polymer strands to be entangled and disordered. As a result, most polymers are only partially crystalline with the remainder of the polymer being amorphous. In this experiment we will utilize DSC to determine polymer crystallinity.
As the name suggests, differential scanning calorimetry relies on a differential in heat flow between a sample of interest and a reference sample with known thermal properties. In fact, measuring heat accurately with a heat meter is very difficult. The measurement is further complicated by the fact that the sample is placed within a pan which also absorbs heat and the measurement typically occurs within a larger furnace. A more accurate measurement would involve monitoring the temperature of a sample and calculating what heat flow must have been present in order to produce the temperature change.
Therefore, DSC involves either the simultaneous or sequential measurement of temperatures of both a sample and a reference. To accurately measure heat in and out of the sample while accounting for thermal contributions and losses to the pan and surrounding environment, the measurement of both sample and reference should occur in the exact same environment and heat conditions. Preparations to the pan should also be consistent between reference and sample. These include crimping to seal the pan and poking a hole in the lid, to allow equilibration with the inert atmosphere in the furnace and avoid pressurization in the pan as phase changes occur in the sample.
A schematic of the DSC sample set-up and heat cell are shown in Figure 1. For each scan, the DSC contains an empty reference pan and a sample pan. The DSC reads the difference in power required to keep the reference pan and the sample pan at the set temperature (defined prior to measurement by the user). The sample pan will require more power to heat when the sample absorbs heat (in an endothermic reaction) and more power to cool when the sample gives off heat (in an exothermic reaction).
Figure 1: DSC sample set-up and heat cell schematic.
An empty pan is placed in the reference position for all DSC measurements. For all thermal characterization techniques, a baseline measurement is performed first with an empty pan inside the furnace in the sample position. This measurement accounts for atmospheric changes and is automatically subtracted from the following sample measurement. For a crystallinity measurement, a precisely measured amount of sample material is placed into a separate pan (which is placed in the sample position in the furnace) and run using the same measurement program as the baseline. Percent crystallinity is calculated using values obtained from the sample measurement. The equation used is:
% Crystallinity = (Equation 1)
A typical DSC results curve is shown in Figure 2. The heat of melting (ΔHm) is obtained by taking the area under the endothermic peak (present during the heating phase of the measurement) and the heat of cold crystallization (ΔHc) is obtained by taking the area under the exothermic peak (present during the cooling phase of the measurement); accompanying software is used to calculate these values from the sample measurement. The known heat of melting of a 100% crystalline form of the sample (ΔHm°) is a material property that must also be known to calculate polymer percent crystallinity.
Figure 2: Schematic of a DSC results curve. Exothermic and endothermic peaks are labeled.
When performing a heat capacity measurement, one additional step is added: prior to running the sample measurement, a measurement identical to the baseline is performed with a precisely measured amount of a standard material. The standard material should be a compound with well-characterized heat capacity, such as sapphire. The sample material is then run using the same measurement program as the baseline and standard. The heat capacity and heat flow in/out of the sample are also calculated by the user in accompanying software. The baseline measurement is subtracted and the heat capacity of the standard material is used to go from temperature to heat flow.
- Turn on the machine and allow it to warm up for about an hour.
- Check to ensure the compressed nitrogen tank and liquid nitrogen tank are both full and the valve connecting them is open. The compressed nitrogen pressure flow is set at 10 psi by the adjustment knobs on the regulator.
- Prepare two empty pans. Poke a small hole in the lid of each and seal by using the crimping press. Remove the three furnace lids and place the pans on the two circular sensors within the furnace. Replace all three lids.
- Click on the icon labeled DSC 3500 Sirius to launch the machine's software.
- Select File → New. The Measurement Definition window will open; four tabs are included that ask for information input. The first tab is the Setup tab. It contains information about the instrument and does not need to be altered for running a measurement using standard procedure.
- Click on the second tab, labeled Header. Select Correction under Measurement Type. This will save the baseline measurement as a correction file, which will later be subtracted from the sample measurement by the software.
- Input Baseline with the date as the sample Identity and Name under the Sample section.
- Under Temperature Calibration, click Select. This will open a separate window; find the most recent temperature calibration file saved to the computer and select it.
- For a percent crystallinity measurement, click Select under Sensitivity Calibration and select the most recent sensitivity calibration file saved to the computer.
- Select the third tab, labeled Temperature Program.
- Check the Purge 2 and Protective boxes listed under Step Conditions. This turns the nitrogen purge gas on for all temperature steps.
- Select Initial under Step Category and input 20 °C as the Start Temperature.
- Select Dynamic under Step Category and input a temperature for the End Temperature. This end temperature should be approximately 30 °C higher than the reported melting temperature of the polymer sample. The maximum temperature allowed by the aluminum pans is 600 °C; as a precaution do not go higher than 550 °C. Enter 10 K/min as the Heating Rate.
- Select Dynamic under Step Category and input 20 °C for the End Temperature.
- At the top of the screen, click on the drop-down arrow under LN2 for the second cooling step, which brings the furnace back to room temperature. Select Auto. This tells the temperature program to automatically turn on the liquid nitrogen to cool the furnace after the heating step has finished.
- Select Final under Step Category. Input 20 °C as the End Temperature.
- The program will ask for an Emergency Reset temperature. Input a temperature 10 °C higher than the highest temperature set in the temperature program. This is a protective setting, which stops the machine from heating higher than a set temperature in case of machine malfunction. This protects the furnace from heating to a temperature that could vaporize the sample and damage the machine.
- The program will then ask for Final Standby information. This information will hold the furnace at the final temperature for up to 2 hours to keep it equilibrated but has no effect on the data collected. Input 20 °C for the Standby Temperature, 40 K/min as the Heating Rate, and a Max Standby Time of 2 hours.
- Select the Fourth Tab, labeled Last Items.
- To the right of File Name click Select. Choose a location on the computer to save the scan and name it Baseline with the date (same name as listed under the Header tab).
- Click Forward at the bottom right hand corner of the Measurement Definition Window. A new, smaller window will appear, listing the Initial temperature as defined in the temperature program and the current furnace temperature. To start the program, the current furnace temperature must be within 5 degrees of the initial temperature.
- If the furnace temperature is within 5 degrees of the initial program temperature, click Start and the measurement will begin. If the furnace temperature is too low, click start and the machine will do a heating and equilibration step prior to beginning the measurement. If the furnace is too hot, select Diagnosis → Gases and Switches. Check the box for LN2 and allow the liquid nitrogen to flow until the temperature reaches within 5 degrees of the initial. Then uncheck the LN2 box and press Start to start the measurement.
- After the Baseline scan has run, remove the empty baseline pan and replace it with the pan containing the sample. The pans are approximately 6 millimeters in diameter with a volume of 25 microliters, and so require a very small amount of sample. Cut the sample into small pieces that fit into the pan. To ensure even heat flow and accurate DSC readings, a thin layer of the sample pieces are placed so the entire bottom of the pan is covered.
- Select File → Open. Click Okay when the program asks to delete current configuration and find and open the Baseline scan.
- The Measurement Definition Window will open to a Fast Definition page. Select Correction plus Sample under Measurement Type.
- Under the Sample section, input the sample name under Identity and Name and input the sample mass in milligrams.
- At the bottom of the window, click Select. Choose a place to name and save the scan.
- Select Forward. Press Start when the smaller window appears.
- After the measurement has finished, close the program, turn off the compressed nitrogen tank and turn off the machine.
- Find the saved measurement for the sample scan and double click on it. This will open the scan in the Proteus Analysis software.
- Use the software to find the area under the melting and recrystallization curves. These values are the heat of melting and heat of cold crystallization of the polymer sample in Joules per gram.
- The percent crystallinity can be calculated using the equation listed above:
Differential scanning calorimetry or DSC is an important measurement technique used to characterize the thermal properties of materials especially polymers. The DSC measurement set-up consists of separate sample and reference pans, each with identical temperature sensors. The temperature of the sample pan containing the sample of interest and the reference pan which typically remains empty are controlled independently using separate but identical heaters. The temperature of both pans is increased linearly. The difference in the amount of energy or heat flow required to maintain both pans at the same temperature is recorded as a function of temperature. For example, if the sample pan contains a material that absorbs energy when it undergoes a phase change or reaction the heater under the sample pan must work harder to increase the pan temperature than the heater under the empty reference pan. This video will demonstrate how to use DSC to determine polymer phase transitions and calculate the percent crystallinity of a polymer.
Due to the long chain-like structure of polymers the strands can exhibit long-range order and are called crystalline or can be randomly organized, termed amorphous. Crystalline polymers undergo a phase transition from solid to liquid through melting. Whereas amorphous polymers transition from their rigid state, called a glass, to their rubbery state through a glass transition. These events can be measured using DSC. However most crystalline polymers are only partially crystalline with the remainder of chains being amorphous. These are called semi-crystalline polymers. In these materials the amorphous parts of the polymer undergo a glass transition during heating while the crystalline parts undergo melting.
These events are visualized in a DSC curve of heat flow versus temperature. Exothermic changes, meaning those that give off heat, are shown as peaks in the plot. While endothermic events, those that absorb heat, appear as valleys. Those peaks and valleys are specific to the certain phase changes in a polymer. The heat of melting, delta Hm, is the amount of energy required to induce melting in a crystalline polymer when the temperature increases. We can calculate the heat of melting by taking the area under the endothermic peak during the heating phase of the measurement.
The heat of cold crystallization, delta Hc, is the amount of energy released as the sample cools and re-crystallizes. Delta Hc is calculated using the area under the exothermic peak during the cooling phase of the measurement. So if we heat the sample past melting and then cool back to room temperature, we can determine both delta Hm and delta Hc. Then using this relationship, we can determine the percent crystallinity. Now that you've seen how to identify phase changes in a DSC plot, let's take a look at how to run the measurement and analyze the results.
To begin the DSC measurement switch the instrument on and allow it to warm up for about an hour. Check that the compressed nitrogen tank and liquid nitrogen tank are both full and that the valve connecting them are open. Now prepare the two pans. Choose a pan that is chemically inert and stable in the desired temperature range. Poke a small hole in the lids. Place the lids on each pan, and then seal them using a crimping press. Next remove the three furnace covers and place the two empty pans on the circular sensors within the furnace. Then replace the furnace covers. Launch the DSC software on the computer and create a new file.
The Measurement Definition window will open with tabs to define measurement parameters. Select Header and then Correction under Measurement Type. This will save the baseline measurement as a correction file which will be subtracted from the sample measurement by the software. Under the Sample section label the baseline measurement and date. Then under temperature calibration, select the most recent temperature calibration file. For a percent crystallinity measurement under Sensitivity Calibration, select the most recent sensitivity calibration file. Then under the temperature program tab, check the Purge2 and Protective boxes under the Step Conditions. This turns the nitrogen purge gas on for all temperature steps.
Select Initial under the Step Category and input 20 degrees Celsius as the start temperature. Then select Dynamic and input in an end temperature. It should be about 30 degrees higher than the reported melting temperature of the polymer sample. In this case we will use 260 degrees Celsius. Then click on the drop-down arrow under the liquid nitrogen icon to set up the cooling step. Select auto to automatically turn on the liquid nitrogen to cool the furnace after the heating step has finished. Select Final under the Step Category and input 20 degrees as the end temperature. Then set the Emergency Reset Temperature 10 degrees higher than the highest temperature in your program which shuts off the instrument in case of machine malfunction and overheating. Now with all of the parameters set, check the initial temperature that you defined and the current furnace temperature. To start the program, the furnace must be within five degrees of the initial temperature. Click Start to begin heating and the program will begin automatically.
After the baseline scan has run remove the empty baseline pan from the furnace. Obtain a new pan and lid and poke a hole in the lid. Weigh the empty sample pan and lid. Then cut the polymer sample into small pieces that will fit in the pan. To ensure even heat flow place a thin layer of the sample pieces in the pan so that the entire bottom of the pan is covered. Then place the lid on the pan and crimp it closed. Now weigh the full sample pan and subtract the weight of the empty pan to determine the weight of the sample.
Then place the pan in the furnace and close the cover. In the DSC software, select File, then Open. Click OK when the program asks to open the baseline scan. On the measurement definition window select Correction + sample under the Measurement Type. Then under the sample section input the sample name and mass. Select Forward and press Start when prompted to begin the scan. After the measurement has finished close the program, turn off the compressed nitrogen tank, and then turn off the instrument.
The DSC data of the polymer sample polybutylene terephthalate is presented as a plot of heat flow versus time. The red trace shows the increase of temperature to 260 degrees, and then the cooling back down to room temperature. Here the curve shows two distinct peaks. The first peak occurred during heating and is an endothermic peak corresponding to the heat of melting. The heat of melting is calculated using the area under the curve which equates to about minus 61 joules per gram. The second peak is an exothermic peak which occurred during the cooling step and corresponds to the heat of cold crystallization. The heat of crystallization is calculated by taking the area under the curve which is about 50 joules per gram. With these two values, along with the known heat of melting for a 100% crystalline sample of polybutylene terephthalate, we can calculate the percent crystallinity of the sample which is 78.6%.
DSC can be used to study thermodynamic events in other samples and materials as well. For example, DSC can be used to analyze phase transitions in biological samples. In this experiment the phase transition of a cell suspension was analyzed in order to understand its freeze-drying properties. Freeze-drying or lyophilization is commonly used for long-term storage of biologicals. Here cell suspensions were prepared and frozen under different conditions in the DSC instrument. The frozen suspensions were then heated and the glass transition measured. Later, the cells were analyzed with electron microscopy to determine which freezing condition promoted cell survival. An understanding of the freeze-drying process via phase transition temperatures helps tailor the process in order to improve cell storage.
The enthalpy change occurring during a chemical reaction is a measure of the amount of heat absorbed in the case of an endothermic reaction or released in the case of an exothermic reaction. The enthalpy change during a reaction can be measured using DSC by performing the chemical reaction inside of the sample pan and measuring the heat flow. In this example, the enthalpy of decomposition of calcium carbonate to form calcium oxide or quicklime was measured by DSC. The decomposition of calcium carbonate occurs endothermically as evidenced by the positive peak at 853 degrees Celsius. The enthalpy of decomposition of calcium carbonate is calculated from the area under the peak and is approximately 160 kilojoules per mole.
You have just watched JoVE's introduction to studying polymer phase transitions using differential scanning calorimetry. You should now understand the different phase transitions for a crystalline and amorphous polymer, and how to identify the events, and calculate crystallinity using DSC. Thanks for watching.
Figure 3 shows the result of a DSC percent crystallinity sample scan on a polybutylene terephthalate (PBT) polymer sample. The result is displayed as a DSC power reading (in milliwatts per milligram of sample) verses time. The power reading, the blue trace in Figure 3, indicates how much additional power was required to change the temperature of the sample pan in comparison to the empty reference pan. The temperature program is also displayed as the dashed red line in Figure 3. The first peak in the blue trace is an endothermic peak; its area gives a value for the heat of melting of the polymer sample. The second peak is an exothermic peak whose area gives a value for the heat of crystallization of the polymer sample.
Figure 4 shows zoomed views of the endothermic and exothermic peaks from the PBT scan (Figure 3). The area of each peak is shown (calculated using the Proteus Analysis software). From these calculated values, the percent crystallinity of this PBT polymer sample is calculated using Equation 1 and a reported value of 142 J/g for ΔHm°:
% Crystallinity = = 78.6% crystalline
Figure 3: DSC reading vs time for a polybutylene terephthalate polymer sample, run using the DSC 3500. The temperature program used is also shown as the red dashed curve.
Figure 4: Zoomed view of the endothermic peak (A) and the exothermic peak (B) of PBT polymer DSC scan. Areas under each curve are calculated; these correspond to the heat of melting and heat of cold crystallization of the PBT polymer sample, respectively.
Applications and Summary
Differential scanning calorimetry is a technique used to determine many thermal properties of materials, such as heat of melting, heat of crystallization, heat capacity, and phase changes. DSC measurements can also be used to calculate additional material properties including glassy transition temperature and polymer percent crystallinity. The DSC requires very small samples that must conform to the size and shape of the pans used in the machine and is based on a differential heat comparison between an empty reference and a sample. Polymer percent crystallinity calculations are relatively simple if the heat of melting of a 100% crystalline form of the polymer being tested is known. Other characterization methods that can determine percent crystallinity include density measurements, which also require a 100% crystalline and a 100% amorphous version of the polymer, and X-ray diffraction, which requires a sample that can be thoroughly mixed with a standard material such as silicon.
Percent crystallinity is an important parameter that significantly contributes to many of the properties of polymer materials used every day. Percent crystallinity plays a role in how brittle (high crystallinity) or how soft and ductile (low crystallinity) a polymer is. Polyethylene is one of the most widely used polymer materials and is a good example of the importance of crystallinity to material properties. HDPE (high density polyethylene) is a more crystalline form and thus is a harder, more brittle plastic used in garbage bins and cutting boards, whereas LDPE (low density polyethylene) has a lower crystallinity and is thus a ductile plastic used in disposable plastic shopping bags. Polymer crystallinity can also affect transparency and color; polymers with higher crystallinity are more difficult to color and are often more opaque. Percent crystallinity plays a large role in how we create and use different plastics and different forms of the same plastic every day, from polymers used in fabrics, to those used in bullet proof vests. Other polymeric characteristics that can affect these properties, and can contribute to percent crystallinity values, include previous heat treatments and degree of crosslinking.