Differential Scanning Calorimetry A bulk analytical technique DSC: Differential Scanning Calorimetry A bulk analytical technique

What Does a DSC Measure? A DSC measures the difference in heat flow rate (mW = mJ/sec) between a sample and inert reference as a function of time and temperature

Endothermic Heat Flow Heat Flow Endothermic: heat flows into the sample as a result of either heat capacity (heating) or some endothermic process (glass transition, melting, evaporation, etc.)

Exothermic Heat Flow Heat Flow Exothermic: heat flows out of the sample as a result of either heat capacity (cooling) or some exothermic process (crystallization, cure, oxidation, etc.)

Temperature What temperature is being measured and displayed by the DSC? Sensor Temp: used by most DSCs. It is measured at the sample platform with a thermocouple (transducer), thermopile (series of thermocouples) or PRT (Platinum Resistance Thermometers) Used by most DSC’s but not the Q1000

Temperature What temperature is being measured and displayed by the DSC? Pan Temp: calculated by TA Q1000 based on pan material and shape Uses weight of pan, resistance of pan, & thermoconductivity of purge gas What about sample temperature? The actual temperature of the sample is never measured by DSC

Temperature What other temperatures are not typically being displayed? Program Temp: the set-point temperature is usually not recorded. It is used to control furnace temperature Furnace Temp: usually not recorded. It creates the temperature environment of the sample and reference

Understanding DSC Signals Heat Flow Relative Heat Flow: measured by many DSCs. The absolute value of the signal is not relevant, only absolute changes are used. Absolute Heat Flow: used by TA’s Q1000. Dividing the signal by the measured heating rate converts the heat flow signal into a heat capacity signal Absolute Heat Flow from Q1000 allows direct measurement of Cp

DSC Heat Flow

Tzero Heat Flow Equation Besides the three temperatures (Ts, Tr, T0); what other values do we need to calculate Heat Flow? Heat Flow Sensor Model How do we calculate these? Tzero heat flow equation applicable for Q100 & Q1000. Uses 4 part equation using the C’s & R’s

Measuring the C’s & R’s Tzero™ Calibration calculates the C’s & R’s Calibration is a misnomer, THIS IS NOT A CALIBRATION, but rather a measurement of the Capacitance (C) and Resistance (R) of each DSC cell After determination of these values, they can be used in the Four Term Heat Flow Equation showed previously

Measuring the C’s & R’s Preformed using Tzero™ Calibration Wizard Run Empty Cell Run Sapphire on both Sample & Reference side

Measuring the C’s & R’s Empty DSC constant heating rate Assume: Heat balance equations give sensor time constants

Measuring the C’s & R’s Repeat first experiment with sapphire disks on sample and reference (no pans) Assume: Use time constants to calculate heat capacities

Measuring the C’s & R’s Use time constants and heat capacities to calculate thermal resistances

A few words about the Cs and Rs The curves should be smooth and continuous, without evidence of noise or artifacts Capacitance values should increase with temperature (with a decreasing slope) Resistance values should decrease with temperature (also with a decreasing slope) It is not unusual for there to be a difference between the two sides, although often they are very close to identical

Good Tzero™ Calibration Run

Bad Tzero™ Calibration Run Can see that it is bad during Tzero™ cal run

Before Running Tzero™ Calibration System should be dry Dry the cell and the cooler heat exchanger using the cell/cooler conditioning template and the default conditions (2 hrs at 75°C) with the cooler off Preferably enable the secondary purge Do not exceed 75°C cell temperature with the cooler off, although the time can be extended indefinitely

Stabilization before Calibration System must be stable before Tzero™ Calibration Stabilization is achieved by cycling the baseline over the same temperature range and using the same heating rate as will be used for the subsequent calibration Typical systems will stabilize after 3-4 cycles, 8 cycles recommended to ensure that the system has stabilized

Example of Typical Results Characteristics of the thermal resistances and heat capacities: Both curves should be smooth, with no steps, spikes or inflection points. Thermal resistances should always have negative slope that gradually decreases. Heat capacities should always have positive slope that gradually decreases. This cell is very well balanced. It is acceptable and usual to have larger differences between sample and reference.

Tzero™ vs Conventional Baseline

Indium with Q Series Heat Flow Signals

Keeping the DSC Cell Clean One of the first steps to ensuring good data is to keep the DSC cell clean How do DSC cells get dirty? Decomposing samples during DSC runs Samples spilling out of the pan Transfer from bottom of pan to sensor

How do we keep DSC cells clean? DO NOT DECOMPOSE SAMPLES IN THE DSC CELL!!! Run TGA to determine the decomposition temperature Stay below that temperature! Make sure bottom of pans stay clean Use lids Use hermetic pans if necessary

TGA Gives Decomposition Temperature

Cleaning Cell If the cell gets dirty Clean w/ brush Brush gently both sensors and cell if necessary Be careful with the Tzero™ thermocouple Blow out any remaining particles

Brushing the Sample Sensor

It Does Matter What Pan you use Monohydrate Pharmaceutical sample Green curve is hermetic sealed pan, while blue curve is vented pan. Sealed pan shows crystalline melt, while vented shows loss of H2O. Sample is a channel hydrate and loss of water causes collapse of crystalline structure to amorphous.

Sample Shape Keep sample thin Cover as much as the bottom of pan as possible

Sample Shape Cut sample to make thin, don’t crush If pellet, cut cross section

Sample Shape Cut sample to make thin, don’t crush If pellet, cut cross section If powder, spread evenly over the bottom of the pan

Using Sample Press When using crimped pans, don’t over crimp Bottom of pan should remain flat after crimping When using Hermetic pans, a little more pressure is needed Hermetic pans are sealed by forming a cold wield on the Aluminum pans Crimped Pans Hermetic Pans Not Sealed Good Bad Sealed

Sample Size Larger samples will increase sensitivity but……………. Larger samples will decrease resolution Goal is to have heat flow of 0.1-10mW going through a transition

Sample Size Sample size depends on what you are measuring If running an extremely reactive sample (like an explosive) run very small samples (<1mg) Pure organic materials, pharmaceuticals (1-5mg) Polymers - ~10mg Composites – 15-20mg

Effect of Sample Size on Indium Melt

Purge Gas Purge gas should always be used during DSC experiments Provides dry, inert atmosphere Ensures even heating Helps sweep away any off gases that might be released Nitrogen Most common Increases Sensitivity Typical flow rate of 50ml/min

Purge Gas Helium Must be used with LNCS High Thermo-conductivity Increases Resolution Upper temp limited to 350°C Typical flow rate of 25ml/min Air or Oxygen Used to view oxidative effects Typical flow rate of 50ml/min

Sample Temperature Range Rule of Thumb Have 2-3 minutes of baseline before and after transitions of interest - if possible DO NOT DECOMPOSE SAMPLES IN DSC CELL Temperature range can affect choice of pans Just because the instrument has a temperature range of –90°C to 550°C (with RCS) doesn’t mean you need to heat every sample to 550°!

Start-up Hook

Heating Rate Good starting point is 10°C/min Faster heating rates increase sensitivity but……………. Faster heating rates decrease resolution Good starting point is 10°C/min

Effect of Heating Rate PMMA 10.04mg

Thermal History The thermal history of a sample can and will affect the results The cooling rate that the sample undergoes can affect : Crystallinity of semi-crystalline materials Enthalpic recovery at the glass transition Run Heat-Cool Heat experiments to see effect of & eliminate thermal history Heat at 10°C/min Cool at 10°C/min

Heat-Cool-Heat of PET