Interpreting Unexpected Events and Transitions in DSC ResultsTA039 Interpreting Unexpected Events and Transitions in DSC Results 1 TA039 Figure 1 Artificial DSC curve INTERPRETATION OF EVENTS AND TRANSITIONS
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TA039 and Transitions in DSC Results 1 TA039 Figure 1: Articial DSC curve Event 1: Large Endothermic Start-up Hook At the beginning of a programmed heating experiment, endothermic) which occurs primarily based on differences in the heat capacity of the sample and reference. Since heat capacity is directly related to weight, an endothermic shift indicates that the reference heightened by faster heating rates. When operating subambient, the thermocouple is transferred from the cell cooling head. This effect increases as the temperature is lowered and/or the time at lower temperatures is increased. Effects on Results A large start-up hook or a sloping baseline make during the rst 2-3 minutes of the experiment, transition temperatures and measured heat ow (DH) may not be reproducible. The purpose of this paper is to assist DSC users with results. The transitions discussed in the paper are several of the ones that most frequently create problems for the new user, and which can also fool even an experienced thermal analyst. By applying some of the recommended procedures and solutions, most laboratories will be able to improve the BACKGROUND analysis technique which measures the temperature and heat ow associated with transitions in materials as a function of temperature and time. Such measurements provide quantitative and qualitative information about physical and chemical changes that include endothermic/exothermic processes or changes in heat capacity. Specic information that can be obtained Glass transition temperatures Melting points & boiling points Crystallization time & temperature Percent crystallinity Heats of fusion and reaction Specic heat Oxidative stability Rate of cure Thermal stability Because of the wealth of information provided and because DSC is easy-to-use, DSC has become the most commonly used thermal analysis technique. Ease-of-use in this case refers to sample preparation transitions that can be the cause of less than optimum several of these events with causes and solutions. Figure 1 is an articial DSC curve which was generated to illustrate these events/transitions. The curve is articial in the sense that all of these events would not occur in the same real world DSC curve. 2 TA039 a series of reference pans of different weight (2 mg. increments). When running a sample use a reference pan that weighs 010% more than sample pan. Figure 2 shows results with an epoxy prepreg sample. Best results are achieved with 1.5 lids. 2 lids results in overcompensation and an exothermic start-up hook. Figures 3 and 4 show how the glass transition results are affected by correct compensation. Note: these results are obtained heating at 20°C/minute from a 100°C isothermal hold. The impact of the start-up hook can also be reduced by initiating the heating at a temperature that is at least 2-3 minutes below the range of interest at the heating rate chosen (ie. at 20°C/minute, start experiment at least 50°C below the rst thermal event of interest.) Figure 2: Effect of reference pan weight Figure 3: Start-up hook and T with no reference pan Figure 4: Start-up hook and T with correct reference pan If operating below 0°C, use 50cc/minute dry nitrogen purge gas through the cell base VACUUM PORT plus the normal purge gas. Figure 5 illustrates the typical improvement obtainable. Figure 5: Proper gas purging improves subambient baseline performance Event 2: Transition(s) at 0°C Weak transitions around 0°C indicate the presence of water in the sample or the purge gas. These transitions are usually endotherms, but may appear different than a melting peak. Since water can condense on both the sample and reference pans, the transition often appears as shown in Figure 6. Furthermore, the peaks may appear slightly lower than 0°C due to impurities dissolved by the moisture from the cell and pans. Effects on Results If water is in the sample, results may not be reproducible because it can act as a plasticizer and reduce transition temperatures. The water will also volatilize during the run, causing an endothermic peak and a shift in the baseline. 3 TA039 If water is in the purge gas, it causes a perturbation in the near 0°C. Keep hygroscopic samples in a dessicator and load them into pans in a dry box. and after the run. A change in weight could explain an unexpected transition. Figure 6: DSC transition due to moisture in the purge gas Dry the purge gas by placing a drying tube in the line. Figure 7 shows an epoxy sample after loading at -100°C. The absence of any transitions at 0°C indicates that with proper precautions water condensation in the cell can be eliminated even under conditions which favor condensation. Note: Loading a sample at temperature below 0°C is only possible when using the liquid nitrogen cooling accessory (LNCA). Samples should always be loaded above 0°C with any other cooling accessory. Figure 7: Quench cooling samples at subambient temperatures Event 3: Apparent Melting at Glass Transition (T Stresses built into the material as a result of processing, handling or thermal history are released when the material is heated through its glass transition. The reason this occurs at T is that the molecule goes from a rigid to a exible structure and thus can move to relieve the stress. Effects on Results Molecular relaxation usually appears as a weak endothermic transition near the end of a glass transition. As shown in Figure 8, this behavior can be pronounced enough to either shift the measured glass transition of the T Figure 8: Molecular relaxation can cause T to appear as a melt Relieve the internal stresses in the material by heating it to at least 25°C above the T and then quench cooling it to a temperature below the T . Figure 9 shows the same material as in Figure 8 after curing at 200°C and then quench cooling to 25°C. 4 TA039 Figure 10: Effect of cooling rate on shape of T Event 4: Exothermic Peaks Below Decomposition Temperature While Heating Exothermic behavior results during curing of a polymer. The amount of heat associated with these and % crystallinity respectively provided scans of suitable standards are available. When an exotherm is obtained in a polymers DSC prole at a temperature which the operator suspects is too low to be a decomposition, running the material in the TGA aids evaluation. The absence of a TGA weight loss which coincides with the DSC exotherm indicates that the exotherm is crystallization or curing. Effects on Results The presence or absence of exothermic crystallization peaks in thermoplastic materials is very dependent on thermal history. Therefore, DSC results will not be reproducible if thermal history of the sample is not tightly controlled. Figures 11 and 12 illustrate the different results cooling at 10°C/minute respectively. The quenched material has a well-dened T amorphous structure which rearranges on heating to a crystalline structure before melting at about 235°C. The DH of crystallization is slightly less than the DH of melting which indicates that the initial structure is mostly amorphous. The slowly cooled material has a weak , indicating an initial structure that is almost entirely crystalline. Since it is crystalline at the start of the DSC experiment, no additional crystallization occurs prior to the melt at 235°C. When comparing thermoplastic materials, give the materials a common known thermal history by either quench cooling or program cooling from above the melting temperature. ASTM D3418-82 denes recommended procedures for giving polymers a known thermal history. Event 5: Baseline Shift After Endothermic or Exothermic Peaks Baseline shifts are caused by changes in sample weight, heating rate, or the specic heat of the sample. A change in specic heat often occurs after the sample has gone through a transition such as curing, crystallization, or melting. Sample weight often changes during volatilization or decomposition. Effects on Results Since H is calculated on the basis of sample weight change will be in error. 5 TA039 Integration of a peak which has a baseline shift is difcult and typically less accurate because of operator subjectivity weight loss has occurred. If crystallization or melting is the cause of the transition, compare the DH of the transitions, using different limits and types of baselines. Figure 13 illustrates an example where use of sigmoidal baseline is required. Event 6: Sharp Endothermic Peaks During Exothermic Sharp peaks similar to those in Figure 1 above 300°C, are usually the result of experimental phenomena rather than real material transitions. For example, rapid volatilization of gases trapped in the material can cause sharp peaks, as can rapid volatilization of gases trapped in a partially sealed Effects on Results melting peaks associated with minor components is possible. quantitative results since sample mass changes. If the DSC cell damage can occur with extended operation. weight loss has occurred. Reduce the temperature limit of further experiments if no useful information is obtained because of the volatilization. Use a Pressure DSC cell. By Leonard C. Thomas For more information or to place an order, go to http://www.tainstruments.com to locate your local sales ofce information.