DIFFERENTIAL SCANNING CALORIMETRY A brief Instruction manual with safety procedures Differential Scanning Calorimeter The Differential Scanning Calorimeter is a thermal analysis instrument that determines the temperature and heat flow associated with material transitions as a function of time and temperature. Applications The DSC can be used to determine 1. Glass transition 2. Melting temperature 3. Heat of fusion 4. Percent crystallinity 5. Crystallization kinetics and phase transitions 6. Oxidative stability 7. Curing kinetics Types of DSC Based on mechanism of operation DSC can be classified into two types. 1. Heat flux DSC 2. Power Compensated DSC Heat Flux DSC In a heat flux DSC, the sample material, enclosed in a pan and an empty reference pan are placed on a thermoelectric disk surrounded by a furnace. The furnace is heated at a linear heating rate and the heat is transferred to the sample and reference pan through thermoelectric disk. However owing to the heat capacity of the sample there exists a temperature difference between the sample and reference pans which is measured by area thermocouples and the consequent heat flow is determined by the thermal equivalent of Ohm’s law, q = T R q = sample heat flow T = temperature difference between sample and reference R = resistance of thermoelectric disk Power Compensated DSC In power compensated calorimeters, the sample and reference pan are in separate furnaces heated by separate heaters. Both the sample and reference are maintained at the same temperature and the difference in thermal power required to maintain them at the same temperature is measured and plotted as a function of temperature or time. We have a heat flux type DSC in our lab manufactured by TA instruments and Bingo! We have the model Q1000 DSC with the Tzero technology. So whats great. Lets go little deeper The simple relationship explained above for heat flux DSC considers that the differential heat flow is only due to the heat capacity associated with heating the sample. Hence the heat flow equation consists of only one term. It does not take into account the extraneous heat flow within the sensor or between the sensor and the sample pan. That is the heat flow due to cell imbalances between the sample and reference sides are not considered. The Tzero cell or Tzero technology are specifically designed to account for those heat flows. The cell sensor consists of a constantan body with separate raised platforms to hold the sample and reference pans. The platforms are connected to the heating blocks or the base by thin walled tubes that create thermal resistances between the platforms and the base. Thermocouples on the underside of each platform measure the temperature of the sample and the reference and a third thermocouple measures the temperature at the base. The heat flow expression for this cell arrangement is given by. T Rr Rs dTs dT q T 0( ) (Cr Cs ) Cr Rr RrRs dt dt Where T = measured sample temperature (Ts) minus measured reference temperature (Tf) T0 = measured base temperature of sensor minus measured sample temperature (T0-Ts) T0 = temperature for control Rr = reference sensor thermal resistance Rs = sample sensor thermal resistance Cr = reference sensor heat capacity Cs = sample sensor heat capacity. The first term in this expression is the equivalent of the conventional single-term DSC heat flow expression. The second and third terms account for difference between the sample and reference resistances and capacitances respectively. The fourth term accounts for the difference in heating rate between the sample and reference. The equation can be further modified to account for pan heat flow effects. Components of DSC The Q1000 DSC consists of 4 basic units: Furnace: This is the main assembly where the sample and reference are heated as per the set temperature program. Auto sampler: This unit works to load and unload the pans as per the commands of runs. Cooling system: This unit enables to cool the sample and assist in achieving the desired temperature program. Computer: This serves as an interface between the user and the instrument and enables automatic control of instrument as per the parameters set. Fig 1: Q1000 DSC with RCS Fig 2: Inner view of DSC cell The furnace or DSC cell dissipates heat to the specimens via a constantan disc. The disc has two raised platforms on which the sample and reference pans are placed. The platforms are connected to the heating block (base) by thin walled tubes that create thermal resistances between the platforms and the base. A chromel disc and connecting wire are attached to the underside of each platform, and the resulting chromel - constantan thermocouples are used to determine the differential temperatures of interest. Alumel wires attached to the chromel discs provide the chromel -alumel junctions for independently measuring the sample and reference temperature. A separate thermocouple embedded in the heating block measures the temperature of the base and serves a temperature controller for the programmed heating cycle. An inert gas is passed through the cell at a constant low rate of about 40 ml min. There are different types of cooling system that can be used in conjunction with DSC. The choice of cooling system depends on the temperature range that you wish to use for your experiments. Details of the types of cooling systems and their usage conditions can be found in the online DSC manual in the help section of the DSC software We have a refrigerated cooling system (RCS) in our lab which is used to cool DSC experiments. It consists of a two-stage, cascade, vapor compression refrigeration system and can be used for experiments requiring cooling within an operating range of -90°C to 550°C. The maximum rate of cooling depends on the temperature range of experiment. Using the instrument In order to perform DSC experiments you have to follow this general outline. In some cases, not all of these steps are required. The majority of these steps are performed using the instrument control software. The instructions needed to perform these actions can be found in detail in the online help in the instrument control program; therefore they will be discussed briefly here Calibrating the instrument Selecting the pan type and material Preparing the sample Creating or choosing the test procedure and entering sample and instrument information through the TA instrument control software Setting the purge gas flow rate Loading the sample and closing the cell lid Starting the experiment. To obtain accurate results, follow procedures carefully. Sample Preparation Do this before you set up the instrument; in fact make up all your samples so they are ready to run before you waste machine time (and that of your classmates) by trying to weigh and run the DSC at the same time. Selecting pan type Samples to be used in differential scanning calorimetry are small pieces that are enclosed in special aluminum pans for experiments upto 600ºC. Pans made of gold, copper, platinum or graphite are used for high temperature runs (upto 725ºC ) . Pans are crimped close using TA's Blue DSC sample Press. Crimped pans improve the thermal contact between the sample, pan and disc, reduce thermal gradients in the sample, minimize spillage, and enable retention of the sample for further study. For most experiments on solid polymeric materials, non-hermetic pans (ones that don't seal airtight) are appropriate. Hermetic pans are normally used only for special applications like studies of volatile liquids including specific heat, studies of materials that sublime, studies of aqueous solutions above 100°C, examinations of materials generating corrosive or condensable gases, and examinations of materials in self-generating atmospheres. Our sample press is normally set up for non-hermetic pans, but a few simple adjustments can be made for others. Generally the appropriate mass range for samples in DSC is 1 mg to 10 mg. If you have a bulk piece of material, it is necessary to convert this to a form that can be used in the DSC. Appropriately sized samples can usually be cut from a larger piece using a razor blade. Be careful while doing this that you don’t cut your fingers. It is advisable to wear nitrile gloves and/or wash your hands after cutting samples, especially when using samples containing lead or other harmful substances. No matter what, you should never touch a sample with your bare hands. If the bulk material is too hard to cut a big enough sample with a razor blade cut and collect small shavings so that they make a considerable mass to be used as a sample. It is important to accurately measure its mass because this is entered into the DSC so that it can give you data that is independent of sample size. Sample pans and lids are in the drawer below the computer. Again, do not touch the pan or lid with your bare hands at any point; oils from your skin can give their own results in the DSC. Use gloves and goggles while preparing samples. Start by placing a pan and a lid (they look very similar but the lid is in fact slightly smaller) open-side-up on the microbalance and note down its mass. Weigh your sample separately and note its mass Then, using tweezers place your sample in the pan and place the lid on top. Place them in the sample press as shown in Figure 3. Pull the lever down. If the press is set up correctly, it should press the lid down onto the pan and the side walls will wrap around the top Place the sample pan in the auto sampler tray and note down the position number You need a reference pan. For T4P heat flow we have a reference pan in the R2 position in the auto sampler tray. Take it out with tweezers weigh it and place it back in the tray. You need to do it only once. For subsequent runs you can use the same reading. If you don’t find a reference pan you can make one by taking an empty pan and lid, weighing it and then crimping it as explained above. You need to enter the sample and reference pan weights in the test procedure. Hence note it down in your notebook while sample preparation. Figure 3: DSC sample press Before You Begin Before you set up an experiment, ensure that the DSC and the controller have been installed properly. Make sure you Check all connections between the DSC and the controller (computer). Make sure each component is plugged into the correct connector. CAUTION: Before plugging the DSC power cable into the wall outlet, make sure the instrument is compatible with the line voltage. Check the label on the back of the unit to verify the voltage. Check if all gas lines are connected Set the instrument power switch to the ON (1) position. After the proper power up sequence, the TA Instruments logo will be displayed on the touch screen or on the TA Q series explorer window in the computer. Purge gas You can control the sample atmosphere during DSC experiments by connecting purge gases to the system. The DSC Q1000 model is equipped with a mass flow controller (MFC) to control the flow rate of the gas. For instruments with the mass flow controllers, the flow rate is controlled through settings chosen using the instrument control software. Turn on the nitrogen cylinder. Make sure that the pressure of your purge gas source is regulated between 100 and 140 kPa gauge (15 and 20 psig). Specify the connected gas on the Instrument Preferences/MFC Page using the instrument control software. It might already be specified. Just ensure Set the flow rate to the recommended value of 50 mL per minute for your experiments on the Notes Page of the Experiment View. Click Apply to save the changes. Turn on the power switch of the RCS NOTE: The RCS and DSC should be switched on atleast 30 min prior to beginning the experiments so that they maintain desired temperature level before beginning the experiment. Calibrate the DSC, if necessary. . Calibrating the instrument To obtain accurate experimental results you should calibrate the DSC cell when you first install it. For the best results, you should recalibrate periodically. The calibration procedures that you need to perform will vary depending on the instrument model and heat flow selection. These experiments will be performed using the instrument control software. See the chart below for a general guide to types of calibration. For details on how to perform that calibration, refer to the online documentation accessed through the instrument control software. NOTE: Please make sure that you run your experiments with the same gas that you used to calibrate the system. For example, if you calibrate using nitrogen, make your runs with nitrogen. Perform calibration runs that encompass the temperature range you plan to use in your experiments. For optimum experimental results you will need to perform all DSC calibrations whenever you change one of the following parameters: • First use of a new cell • Purge gas change • Cooling device or accessory (FACS, LNCS, RCS, or Quench Cooling Can) change. For optimum experimental results you will need to perform all DSC calibrations except Tzero whenever you change one of the following parameters: • Heat flow selection change (T4P, T4, etc.) • Pressure (in PDSC experiments) change. Calibration is performed in the instrument’s calibration mode, which is accessed through the controller. (This does not apply to heat capacity calibration, which is performed in both the standard and MDSC modes, where applicable). Our DSC is Q1000 which requires T-zero calibration only when you change the cell or purge gas or cooling accessory. The calibration which we mostly do is the cell constant and temperature calibration. This has to be done whenever we change the type of pan and heat flow (T4 or T4P etc). We generally use a standard crimped aluminium non-hermetic pan (T4P heat flow) for our polymer samples. People from other departments use high volume pans and T4 heat flow. Hence it is essential to check the current calibration status of the machine and do the calibration for your type of heat flow if necessary. There is a paper on the DSC machine with a note saying DSC is calibrated for high volume pans on one side and DSC is calibrated for standard pans on the other side. Whoever does the calibration has to flip the paper to indicate the current calibration status of the machine to the next user. If you need to do the cell constant and temperature calibration you can do it either by performing the actual run using the calibration wizard in the software or using the data file previously used to calibrate and stored in computer. It is recommended to do the actual run and calibrate only once in a while (i.e once in 6 months) otherwise you can do the cell constant and temperature calibration from the previous data without conducting the run using the following steps. Quick recalibration from file 1. Set the pan type and operator in the notes tab 2. Open calibration wizard (calibrate>calibration wizard) 3. Select heat flow type (T4P for hermetic, T4 for high volume) 4. Select the center radial button (cell constant & temperature calibration) 5. Click next on page asking for calibration material, mass, etc 6. Uncheck "conduct experiment" in the window showing the procedure 7. Open file name for the appropriate pan type (indium) (The file can be found at DSC/ ME / JIM / calibrate / standard pan). 8. Send changes to the machine NEXT FINISH 9. If absolute heat capacity is desired, change capacity constants in the calibration table (calibrate>calibration table), if using MDSC change the reversible and nonreversible constants. Running a DSC Experiment CAUTION: Drying out the DSC cell may be needed, if it has been exposed to humid conditions. It is important to be certain that the instrument ground is adequately connected to the facilities ground for safe operation. Run the "Cell/Cooler Conditioning" test template to dry out the cell: 1 External Event Off 2 Equilibrate at 75°C 3 Isothermal for 120.00 min. Once the nitrogen is set and the RCS is also on after about 30 minutes you can begin setting your experimental run. Now let’s start talking about the DSC software. The standard view is "experimental", which will look like this. This has the sequence frame on the left showing each run. The right frame shows the current machine conditions. The temperature and flange temperatures are important to note at this point. The temperature field is the temperature of the sample cell. Usually in standby this is 40°C. If it is not, put it in standby (File>go to standby). The flange temperature is the low temperature produced by the RCS. It takes some time to cool down once the RCS is turned on. You always want to make sure the flange temp is steady at -80°C before starting runs. That is why we need to turn the RCS on up to a half hour before your first run. Figure 4: Experimental screen view. The middle frame has three tabs. The left tab (Summary) is where you choose the run mode (standard, calibrate, or MDSC). Then below it are more "test" options. For the standard mode there is ramp, heat cool heat, isothermal similar options for the MDSC mode. Below these options is where you input the pan mass, sample mass, reference mass, auto-sampler locations, and run comment. Below this is the filename. It is good to use a common naming scheme and good folder organization. My folder is in TA>Data>DSC>Kadhiravan. Make your own folder in TA>Data>DSC>your name and store your file. Figure 5: Summary screen Figure 6: Procedure screen. The middle tab (procedure) is where you input the experimental procedure. There are standard templates already there (isothermal, ramp, heat cool heat etc ) which is shown in figure 6) where you need to enter the required parameters. If none of those suit you then you can make your custom procedure. You can edit the procedures from the edit box down at the bottom of the procedure tab. Most of the procedures are self-explanatory. The common ones are ramp, isothermal, etc. You make a procedure in the editor window by dragging the action into the window. So, if you drag "ramp" into the window a line will appear saying ramp at -°C/min to _°C. You put in the ramp rate and what temp to stop ramping at here. Say, ramp at 10°C/min to 190°C. This will ramp from whatever the current temp of the cell is (when that ramp command is executed) to 190°C at 10°C/min. If you then follow that command with isothermal you will only input a time in the isothermal command. Say 10 min. Now the sample will stay isothermal at190°C, since that was the last specified temp. Now say you want to cool the sample and heat it again over the same range. You would simply insert another ramp line after the isothermal, enter your ramp rate again and enter the end temperature. The ramp rate is absolute value, it knows whether to heat or cool based on the current temp and the end point temp. So you put in 10°C/min again and 50°C for the end temp. Now say you hold it isothermal there again for 10 min. Then you heat it again up to 190°C at the same rate. This is a standard heatlcool/heat run. But when you get into the analysis software the two heating runs will be on top of each other and very difficult to see which was the first and which was the second heating run. To overcome this, simply insert "mark end of cycle" between each command in the procedure editor. So the editor will look something like this 1. ramp at 10°C/min to 190°C 2. mark end of cycle 0 3. isothermal for 10 min 4. ramp at 10°C/min to 50°C 5. mark end of cycle 0 6. isothermal for 10 min 7. mark end of cycle 0 8. ramp at 10°C/min to 190°C. Now you will be able to separate each of those commands in the analysis software by their cycle number. Even though in the procedure listed above they all appear to be cycle 0 they actually start count starting at 1. So the first heating scan is cycle 1 and the second heating scan is cycle 5. You can now see what ever cycle you want to using the cycle limit function in the analysis software (data>data limits>cycle limits). Scan rates The whole concept of DSC is that the sample remains in thermal equilibrium, which is clearly not possible while changing temperature. The only way to achieve this is to have an infinitely slow heating rate. So the slower the heating rate the better the results. However the runs need to completed in practical time also. For standard DSC I have found that 10°C/min seems to be a reasonable value which is also mostly used. If you want really good results for a single scan you can go down to 5°C/min, or less. But then the run takes longer, so usually you want to find the medium that gives you run times that are reasonable but still gives you good results. Scan rate is something you can play with when you first start running experiments. Also keep in mind what runs you want to get data from. For the example above if you don't care about the cooling scan from 190 to 50 you could set that to 20°C/min or higher just to get it done. Be aware though that sometimes cooling rates can have an effect on Tg of a heating run. So just try to be consistent with the cooling. In short scanning rate is one which you have to decide based on literature and your observations. The last tab on the window is notes where you enter the operator name and the pan type. The standard pan type is just aluminum. Once you enter all parameters verify once and click apply wherein the instructions will be stored for that run. If you want to add one more run you can click append and enter the pan details and sample details and so on for the next run. Thus you can build a sequence of runs and then click start or the green arrow on the top left corner of the screen. Figure 7: View of notes screen The runs will be done automatically and sequentially but it is recommended that you keep a watch in the beginning for some time and then check back periodically to see if anything malfunctions and error message appears. This is what you have to do in short. 1. Enter your name in the logbook. 2. Wear your goggles and gloves. 3. Prepare the samples and place in autosampler. 4. Turn on the nitrogen and set it at 20 psig. 5. Turn on the DSC. 6. Turn on the RCS. 7. Take a break for half an hour ( Go drink water or whatever you feel like doing). 8. Come back and check the temperature of cell (40°C )and flange temperature (- 80°C). 9. Calibrate for pan type if necessary. 10. Once it is ready, enter all the experimental parameters for the runs and start the runs. 11. Once all the runs are completed leave the set up on for half an hour.(Important). 12. Then turn off the RCS and DSC and close the nitrogen. 13. Fill out the logbook. Clean the place and leave. If you are leaving late in the day (after 5.00pm) or the last person to leave the department don’t forget to close the lab door.(Very Very important). Precautions: think about these things before you run the instrument! o Make sure the purge nitrogen is on at the right rate (outlet pressure is 20psi). (The new tank comes with about 2500 psi and uses about 100 psi for a day. If the tank pressure is below 100 psi be careful not to set up a long experiment. It is better to leave some nitrogen in the tank than to deal with what happens if it runs out during a run). o Never run halogenated materials: they eat the cell! o Never degrade samples in the cell: but you have to know the degradation temperature first; how? By running a TGA or capillary mp first! o Make sure sample is dry or leave the lid off to let volatiles leave. o Use clean sample pans so nothing contacts the cell surface; be sure not to let the sample you are analyzing melt and flow onto the cell! o Use gloves and tweezers to handle sample and pans o Never heat above 500C with aluminum pans: they'll melt! o Make sure calibration has been done at your heating rate and for your pan type. o Cooling to subambient requires very special precautions: don't do it alone! o Never try to open the cell in between runs. If something is wrong wait till the cell temperature reaches standby temp (40°C) and then try to open lid after canceling autosampler. o Don’t forget to wear goggles and gloves.
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