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Differential Scanning Calorimeter DSC

Introduction DSC Differential: two calorimeters (for sample and reference) with the same heat transfer behavior (for compensation purpose) Scanning: the common operation mode is to run temperature or time scans Calorimeter: instrument to measure heat or heat flow What is DSC

1887: Le Chatelier measures the temperature of clays as a function of time. 1899: W. Roberts-Auston measures the temperature difference between the sample and an inert reference, thus establishing DTA. Heater Brief history of DTA/DSC

1955: Boersma invents the heat flow DSC- technique Ni Ceramic Plate Lit.: S. L. Boersma, J. Am. Ceramic. Soc., 38, , 1955 Brief history of DTA/DSC

Agenda Theory DSC Accessories Experimental Design Calibration Interpretation of Undesirable Events Software Applications

Differential Scanning Calorimetry A calorimeter measures the heat into or out of a sample A differential calorimeter measures the heat of a sample relative to a reference A differential scanning calorimeter does all of the above and heats/cools the sample with a linear temperature ramp

Differential Scanning Calorimetry l Differential Scanning Calorimetry (DSC) is most popular thermal analysis technique l DSC measures endothermic and exothermic transitions as a function of temperature –Endothermic heat flows into a sample –Exothermic heat flows out of the sample l Used to characterize polymers, pharmaceuticals, foods/biologicals, organic chemicals and inorganics l Transitions measured include Tg, melting, crystallization, curing and cure kinetics, onset of oxidation and heat capacity

DSC: Terminology Amorphous Phase - The portion of material whose molecules are randomly oriented in space. Liquids and glassy or rubbery solids. Thermosets and some thermoplastics. Crystalline Phase - The portion of material whose molecules are regularly arranged into well defined structures consisting of repeat units. Very few polymers are 100% crystalline. Semi-crystalline Polymers - Polymers whose solid phases are partially amorphous and partially crystalline. Most common thermoplastics are semi- crystalline. Melting - The endothermic transition upon heating from a crystalline solid to the liquid state. This process is also called fusion. The melt is another term for the polymer liquid phase.

DSC: Terminology Crystallization - The exothermic transition upon cooling from liquid to crystalline solid. Crystallization is a function of time and temperature. Cold Crystallization - The exothermic transition upon heating from the amorphous rubbery state to the crystalline state. This only occurs in semi-crystalline polymers that have been quenched (very rapidly cooled from the melt) into a highly amorphous state. Enthalpy of Melting/Crystallization - The heat energy required for melting or released upon crystallization. This is calculated by integrating the area of the DSC peak on a time basis.

DSC: Heat Flow Measurements Calorimeter Signals Time Temperature Heat Flow Signal ChangeProperties Measured Heat Flow, absoluteSpecific Heat Heat Flow, shiftGlass Transition Exothermic PeakCrystallization or Cure Endothermic PeakMelting Isothermal OnsetOxidative Stability

DSC: Typical DSC Transitions Temperature Heat Flow -> exothermic Glass Transition Crystallization Melting Cross-Linking (Cure) Oxidation or Decomposition Composite graph

LINSEIS DSC Instruments DSC PT10 DSC PT1600

DSC Cell Schematics DSC PT 10 PT 10 Sensor Sample pan Reference pan Gas purge Heater

Applications Thermoplastics Thermosets Pharmaceuticals Heat Capacity Glass Transition Melting and Crystallization Additional Applications Examples

Applications Pharma R&D Polymers Food

Thermoplastic Polymers Semi-Crystalline (or Amorphous) Crystalline Phase melting temperature Tm (endothermic peak) Amorphous Phase glass transition temperature (Tg) (causing  Cp) Tg < Tm Crystallisable polymer can crystallize on cooling from the melt at Tc (Tg < Tc < Tm)

DSC of Thermoplastic Polymers Tg Melting Crystallization Oxidative Induction Time (OIT) General Recommendations –10-15mg in crimped pan 10°C/min

DSC: Selecting Experimental Conditions Thermoplastic Polymers  Perform a Heat-Cool-Heat Experiment at 10°C/min.  First heat data is a function of the material and an unknown thermal history  Cooling segment data provides information on the crystallization properties of the polymer and gives the sample a known thermal history  Second heat data is a function of the material with a known thermal history

DSC: Thermoplastic: Heat/Cool/Heat Time (min) Heat Flow (W/g) [ ] Temperature (°C) First HeatCooling Second Heat

DSC: Thermoplastic: Heat Flow vs. Temperature for Heat/Cool/Heat Heat Flow (W/g) Temperature (°C) cooling first heating second heating

Selecting Experimental Conditions During first heat the maximum temperature must be higher than the melting peak end; eventually an isothermal period must be introduced –too high temperature/time: decomposition could occur –too low temperature/time: possibly subsequent memory effect because of the fact that crystalline order is not completely destroyed For non-crystallisable (amorphous) thermoplastics the maximum temperature should be slightly above Tg (removal of relaxation effects, avoid decomposition)

DSC: Selecting Experimental Conditions Thermoplastic Polymers (con't) Interpreting Heat-Cool-Heat Results: One of the primary benefits of doing Heat-Cool-Heat is for the comparison of two or more samples which can differ in material, thermal history or both  If the materials are different then there will be differences in the Cool and Second Heat results  If the materials are the same and they have had the same thermal history then all three (H-C-H) segments will be similar  If the materials are the same but they have had different thermal histories then the Cool and Second Heat segments are similar but the First Heats are different

Thermosetting Polymers Thermosetting polymers react (cross-link) irreversibly. A+B will give out heat (exothermic) when they cross-link (cure). After cooling and reheating C will have only a glass transition Tg. A + B C GLUE

DSC of Thermosetting Polymers Tg Curing Residual Cure General Recommendations –10-15 mg in crimped pan if solid; hermetic pan if liquid 10°C/min

DSC: Selecting Experimental Conditions Thermosetting Polymers Anneal the sample(if needed), then Heat-Cool-Heat at 10°C/min.  Anneal approximately 25°C above Tg onset for 1 minute to eliminate enthalpic relaxation from Tg (if needed)  First Heat is used to measure Tg and residual cure (un-reacted resin). Stop at a temperature below the onset of decomposition  Cooling segment gives the sample a known thermal history  Second Heat is used to measure the Tg of the fully cured sample. The greater the temperature difference between the Tg of the First and Second Heats the lower the degree of cure of the sample as received

DSC: Thermosets: Comparison of First and Second Heating Runs Temperature (°C) Heat Flow (W/g) Tg °C °C 20.41J/g Residual Cure First Second

Pharmaceuticals Tg Melting –Purity Polymorphs General Recommendations –Use TGA to determine pan type –Use 1-5 mg samples (use 1mg for purity) –Initial 10°C/min (1°C/min for purity) –If polymorphs present heat faster to inhibit polymorphic transformations

 What is it?  How is it observed and measured?  Methods for calculating specific heat capacity  What affects the specific heat capacity of a polymer? DSC: Specific Heat Capacity

WHAT IS HEAT CAPACITY?  Heat capacity is the amount of heat required to raise or lower the temperature of a material.  Specific heat capacity refers to a specific mass and temperature change for the material (J/g°C)

WHY IS HEAT CAPACITY IMPORTANT? Thermodynamic property of material(vs. heat flow) Measure of molecular mobility Provides useful information about physical properties of the material as a function of temperature

Heat Flow Due to Heat Capacity mW mW mW mW 5 °C/min 2.5 °C/min 10 °C/min 20 °C/min Temperature (°C) Heat Flow (mW) Sample: PMMA Size: mg

DSC: How are Heat Capacity and Specific Heat Measured? In a DSC experiment, heat capacity is measured as the absolute value of the heat flow, divided by the heating rate, and multiplied by a calibration constant. dH/dt = Cp (dT/dt) or Cp = [(dH/dt)/(dT/dt)] x K K = calibration constant

DSC Cp Measurement Where: K = Calibration constant HF S = Differential heat flow with sample HF MT = Differential heat flow with empty pans wt= weight of sample Heat Flow 0 HF MT HF S Temperature

Alternative DSC Cp Measurement Where: K = Calibration constant HF HR1 = Differential heat flow of sample at HR 1 HF HR2 = Differential heat flow of sample at HR 2 HR 2 = Heating rate 2 HR 1 = Heating rate 1 wt= weight of sample Heat Flow endo 0 HF HR1 HF HR2 Temperature

DSC: What Affects the Specific Heat Capacity? Amorphous Content Aging Side Chains Polymer Backbone Copolymer Composition Anything that effects the mobility of the molecules, affects the Heat Capacity

Effect of Amorphous Content on Cp Amorphous Cp is greater than Crystalline Cp Amorphous Content increases Specific Heat Capacity Crystalline polymers contain more order and thus fewer degrees of molecular motion. Less molecular motion results in lower specific heat capacity.

HEAT CAPACITY SUMMARY Anything that effects the mobility of the molecules, affects the Heat Capacity

What is it? How is it observed and measured? What affects the Glass Transition? DSC: The Glass Transition (Tg)

The Glass Transition The Glass Transition is the reversible change of the amorphous region of a polymer from, or to, a viscous or rubbery condition to, or from, a hard and relatively brittle one The Glass Transition Temperature is a temperature taken to represent the temperature range over which the glass transition takes place Detected by DSC as an increase in Cp

DSC: Measurements of the Tg TEMPERATURE (°C) endothermal HEAT FLOW exothermal T o T f T m T i T e T r T= Temperature of First Deviation (C) T=Extrapolated Onsettemperature (C) T=Midpoint temperature (C) T=Inflection Temperatere (C) T=Extrapolat ed Endset Temperature e (C) T=Temperaturee of Returnn -to - Baseline (C) o o f o m o i o e o r o 1/2h

DSC: Some Properties Affected at Tg Physical propertyResponse on heating through Tg Specific Volume Increases Modulus Decreases Coefficient of thermal expansion Increases Specific Heat Increases Enthalpy Increases Entropy Increases V, 1/E, CTE Cp H S Temperature Tg

DSC: What Affects the Glass Transition? Heating RateCrystalline Content Heating & CoolingCopolymers AgingSide Chains Molecular WeightPolymer Backbone PlasticizerHydrogen Bonding Filler Anything that effects the mobility of the molecules, affects the Heat Capacity

Effect of Heating Rate on the Tg Heating Rate (°C/min) Heat 80°C Tg Onset (°C) Tg Midpoint (°C) ½ Width of Tg (°C)

Effect of Cooling Rate on Tg increased amorphous fraction Quench 20 °C/min 10 °C/min 0.2 °C/min Cooling rates: 20, 10, 5, 2, 1 and 0.2°C/min Temperature (°C) Heat Capacity (J/g°C)

Glass Transition Summary The Tg is due to Amorphous material The Tg is the reversible change from a glassy to rubbery state & vice-versa DSC detects Tg’s by a step change in Cp Anything that effects the mobility of the molecules, affects the Heat Capacity

DSC: Melting and Crystallization Observations of Melting and Crystallization Crystallinity Calculations Applications

Observation of Melting °C °C 45.29J/g 12.71°C Peak Temperature Extrapolated Onset Temperature Area under the curve (Heat of Fusion) half height Heat Flow (W/g) Temperature (°C) Exo Up

DSC: Melting Points and Ranges  T o is the onset to melting  T p is the melting peak temperature  T e is the end of melting Pure, low molecular weight materials (mw<500 g/mol)  T o is used as the melting temperature (T m )  Between T o and T p the sample is melting  Between T p and T e the molten sample is returning to the DSC temperature Polymers  T p is used as the melting temperature (T m )  Between T o and T p crystal perfection is occurring (both melting and crystallization occurs simultaneously)  Between T p and T e the sample is finishing melting and returning to the DSC temperature

DSC: Polyethylene and Indium Melting Temperature (°C) Heat Flow (mW) °C J/g °C 28.42J/g °C °C

Baseline Types Sigmodial Baseline Linear Baseline

DSC: Crystallization Point Crystallization is a two step process:  Nucleation  Growth The onset temperature is the nucleation (T N ) The peak maximum is the crystallization temperature (T C )

DSC: Observation of Crystallization Temperature (°C) Heat Flow (mW) Tn Tc °C °C °C 36.61J/g Te

DSC: Super cooling of Water Temperature (°C) Heat Flow (mW) °C °C +

DSC: Melting and Crystallization Summary Melting and crystallization are phase changes from organized solid to amorphous phases and vice-versa. Melting is a one-step process while crystallization involves nucleation and crystal growth. The enthalpy of melting can be used to measure crystallinity. Any process that makes it easier for molecules to be organized will raise the melting temperature.

DSC: Effect of Polymer Type on Melting ClassStructureMelting Range Polyolefins-CH 2 –CH °C Polyamides-CH 2 -NH-C(O)-CH °C Polyesters-CH 2 -O-C(O)-CH °C Polyphenylene Sulfides-Ph-S °C

DSC: Effect of Molecular Weight on Melting Olefin FormulaMole. Wt.T m (g/mol) (°C) C 12 H C 24 H C 30 H C 35 H

DSC: Terminology Amorphous Phase - The portion of material whose molecules are randomly oriented in space. Liquids and glassy or rubbery solids. Thermosets and some thermoplastics. Crystalline Phase - The portion of material whose molecules are regularly arranged into well defined structures consisting of repeat units. Very few polymers are 100% crystalline. Semi-crystalline Polymers - Polymers whose solid phases are partially amorphous and partially crystalline. Most common thermoplastics are semi-crystalline. Melting - The endothermic transition upon heating from a crystalline solid to the liquid state. This process is also called fusion. The melt is another term for the polymer liquid phase.

DSC: Terminology Crystallization - The exothermic transition upon cooling from liquid to crystalline solid. Crystallization is a function of time and temperature. Cold Crystallization - The exothermic transition upon heating from the amorphous rubbery state to the crystalline state. This only occurs in semi-crystalline polymers that have been quenched (very rapidly cooled from the melt) into a highly amorphous state. Enthalpy of Melting/Crystallization - The heat energy required for melting or released upon crystallization. This is calculated by integrating the area of the DSC peak on a time basis.