Differential Scanning Calorimetry

Definitions • A calorimeter measures the heat into or out of a sample. differential calorimete r measures the heat of a sample relative to a reference. • A differential scanning calorimeter does all of the above and heats the sample with a linear temperature ramp. • Endothermic heat flows into the sample. • Exothermic heat flows out of the sample.

DSC: The Technique • Differential Scanning Calorimetry (DSC) measures temperatures and heat flows associated with transitions in materials as a function of time and temperature in a controlled atmosphere. • These measurements provide quantitative and qualitative information about physical and chemical changes that involve endothermic or exothermic processes , or changes in heat capacity .

What Can You Measure with DSC? Qualitative analysis Fingerprinting of minerals, clays, polymers Sample purity Melting points Heat capacity, cp Glass transition temperature, Tg Crystallization temperature, Tc Phase diagrams

Schematic of DSC Instrument Reference Sample T1 T2 Pt thermopile Pt thermopile Low mass 1 gram heater heater N2 flow DW

Glass Transition Step in thermogram Transition from disordered solid to liquid Observed in glassy solids, e.g., polymers Tg, glass transition temperature Glass transition dH/dt, mJ/s Tg Temperature, K

Crystallization Sharp positive peak Disordered to ordered transition Thermogram Sharp positive peak Disordered to ordered transition Material can crystallize! Observed in glassy solids, e.g., polymers Tc, crystallization temperature Crystallization dH/dt, mJ/s Tc Temperature, K

Melting Negative peak on thermogram Ordered to disordered transition Tm, melting temperature NB: melting happens to crystalline polymers; glassing happens to amorphous polymers Melting dH/dt, mJ/s Tm Temperature, K

Analysis Sharp positive peak Disordered to ordered transition Crystallization Sharp positive peak Disordered to ordered transition Observed in glassy solids, e.g., polymers Tc, crystallization temperature dH/dt, mJ/s Tc Temperature, K

DSC Thermogram > exothermic - Heat Flow Temperature 6 Oxidation Crystallisation Cross - Linking (Cure) > exothermic - Glass Transition Heat Flow Melting Temperature 6 Technical Group Talk

DSC: Main Sources of Errors Calibration Contamination Sample preparation – how sample is loaded into a pan Residual solvents and moisture. Thermal lag Heating/Cooling rates Sample mass Processing errors Technical Group Talk

Other DSC techniques These are three defferent types of DSC instruments. power-compensated DSC Heat-flux DSC Modulated DSC

Power-compensated DSC In this case, the sample and reference material are heated separately and continuously maintained at same temperature. While both temperatures are increased or decreased linearly. The differential power is recorded as a function of temperature and the peak area represents the energy of transition. Isothermal condition i.e Ts=Tr is obtained by placing a platinum resistant thermometer in the sample with an identical sensors in the reference holder. This DSC has lower sensitivity and capable of higher resolution than heat-flux DSC.

POWER-COMPENSATED DSC

Heat-flux DSC instrument In this case, the difference in heat flow into the sample and reference is measured while the sample temperature is changed at a constant rate. Both sample and reference are heated by a single heating unit. The differential heat flow to the sample and reference is monitored by chromel-constantan area thermocouples.

HEAT-FLUX DSC

Modulated DSC Benefits Increased Sensitivity for Detecting Weak (Glass) Transitions Eliminates baseline curvature and drift Increased Resolution Without Loss of Sensitivity Two heating rates (average and instantaneous) Ability to Separate Complex Thermal Events and Transitions Into Their Heat Capacity and Kinetic Components Ability to Measure Heat Capacity (Structure) Changes During Reactions and Under Isothermal Conditions Downside Slow data collection Modulated DSC

MODULATED DSC Reversible Transitions Glass Transition Melting Non-reversible Crystallisation Curing Oxidation/degradation Evaporation

Applications of DSC Qualitative analysis:-It is a finger-printing of minerals,clays,polymers,etc. Sample purity. Melting point. Heat capacity(Cp). Glass transition(Tg). Crystallization temperature(Tc). Phase diagrams.

In food industry for characterisation of edible fats and oils. The total time of analysis in DSC is 30minutes. It is used for identification of optical isomers. It is used for impurity studies in pharmaceutical industries.

It is used in polymer industry. Specific heat,heat of fusion,heat of dehydration,decomposition of samples can be determined. It is used to access the purity of any substances. Example:-carnauba wax.

REFERENCES INSTRUMENTAL ANALYSIS SKOOG INSTRUMENTAL METHODS OF CHEMICALANALYSIS GURUDEEP. R.CHATWAL,SHAM,K.ANAND. INSTRUMENTAL METHODS OF ANALYSIS,7TH EDITION WILLARD,MERRITT.

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