Glass Transition & Melting 3 2004 Training Seminars DSC Interpreting DSC Data Glass Transition & Melting

Glass Transitions The glass transition is a step change in molecular mobility (in the amorphous phase of a sample) that results in a step change in heat capacity The material is rigid below the glass transition temperature and rubbery above it. Amorphous materials flow, they do not melt (no DSC melt peak)

Glass Transitions The change in heat capacity at the glass transition is a measure of the amount of amorphous phase in the sample Enthalpic recovery at the glass transition is a measure of order in the amorphous phase. Annealing or storage at temperatures just below Tg permit development of order as the sample moves towards equilibrium

Heat Flow & Heat Capacity at the Glass Transition Polystyrene

Measuring/Reporting Glass Transitions The glass transition is always a temperature range The molecular motion associated with the glass transition is time dependent. Therefore, Tg increases when heating rate increases or test frequency (MDSC®, DMA, DEA, etc.) increases. When reporting Tg, it is necessary to state the test method (DSC, DMA, etc.), experimental conditions (heating rate, sample size, etc.) and how Tg was determined Midpoint based on ½ Cp or inflection (peak in derivative)

Glass Transition Analysis Polystyrene 9.67mg 10°C/min

Glass Transition Analysis Polystyrene 9.67mg 10°C/min

Step Change in Cp at the Glass Transition PET 9.43mg % Amorphous = 0.145/0.353= 41%

Aged Epoxy Sample

Effect of Annealing Time on Shape of Tg

Importance of Enthalpic Relaxation Is enthalpic recovery at the glass transition important? …Sometimes Glass transition temperature, shape and size provide useful information about the structure of the amorphous component of the sample. This structure, and how it changes with time, is often important to the processing, storage and end-use of a material. Enthalpic recovery data can be used to measure and predict changes in structure and other physical properties with time.

Effect of Aging on Amorphous Materials Decreases Entropy Enthalpy Heat Capacity Coefficient of Expansion Increases Modulus Specific Volume Response on S Physical Property Storage Below Tg H Max Tg Storage V time H Equilibrium M Liquid S Equilibrium Glass Kauzmann Temp; Lowest Tg (Entropy of Crystal) Temperature Where H = High relative cooling rate M = Medium relative cooling rate S = Slow relative cooling rate

Suggestions for Finding Weak Glass Transitions Know your empty-pan baseline Get as much material in the amorphous state Cool slowly through the glass transition region Heat rapidly through glass transition region Use MDSC® Or use Quasi-Isothermal MDSC

Glass Transition Summary The glass transition is due to Amorphous material The glass transition is the reversible change from a glassy to rubbery state & vice-versa DSC detects glass transitions by a step change in Cp

Melting Definitions Melting – the process of converting crystalline structure to a liquid amorphous structure Thermodynamic Melting Temperature – the temperature where a crystal would melt if it had a perfect structure (large crystal with no defects) Metastable Crystals – Crystals that melt at lower temperature due to small size (high surface area) and poor quality (large number of defects)

Definitions (cont.) Crystal Perfection – the process of small, less perfect crystals (metastable) melting at a temperature below their thermodynamic melting point and then (re) crystallizing into larger, more perfect crystals that will melt again at a higher temperature True Heat Capacity Baseline – often called the thermodynamic baseline, it is the measured baseline (usually in heat flow rate units of mW) with all crystallization and melting removed…. assumes no interference from other latent heat such as polymerization, cure, evaporation etc. over the crystallization/melting range

Extrapolated Onset Temperature Melting of Indium Extrapolated Onset Temperature Heat of Fusion For pure, low molecular weight materials (mw<500 g/mol) use Extrapolated Onset as Melting Temperature Peak Temperature

Melting of PET For polymers, use Peak as Melting Temperature Extrapolated Onset Temperature Heat of Fusion Peak Temperature

Comparison of Melting

Analyzing/Interpreting Results It is often difficult to select limits for integrating melting peaks Integration should occur between two points on the heat capacity baseline Heat capacity baselines for difficult samples can usually be determined by MDSC® or by comparing experiments performed at different heating rates Sharp melting peaks that have a large shift in the heat capacity baseline can be integrated with a sigmoidal baseline

Baseline Due to Cp

Baseline Type

DSC of Polymer Blend More on this sample in the MDSC® section Where is the Cp baseline?

Is it a melt? YES! Onset shifts by only 0.3°C

Onset shifts by almost 30°C Is it a Melt? NO! Onset shifts by almost 30°C

Effect of Heating Rate on Melting

Effect of Heating Rate on Polymorph DSC at 1C/min DSC at 1C/min DSC at 10C/min Comparison of all 3 heating rates DSC at 50C/min

Effect of Impurities on Melting Effect of p-Aminobenzoic Acid Impurity Concentration on the Melting Shape/Temperature of Phenacetin 99.3% Pure 100% Pure Melting of Eutectic Mixture 96.0% Pure 95.0% Pure As sample becomes less pure the melting peak broadens and shifts to a lower melting temperature. Higher levels of impurities forms an Eutectic – The lowest melting point of an alloy or soln that is obtainable by varying the proportions of the components NBS 1514 Thermal Analysis Purity Set Approx. 1mg Crimped Al Pans 2°C/min

Van't Hoff Purity Calculation

Crystallization, Heat Capacity, 4 2004 Training Seminars DSC Interpreting DSC Data Crystallization, Heat Capacity, and Crosslinking

Crystallinity Definitions Crystallization – the process of converting either solid amorphous structure (cold crystallization on heating) or liquid amorphous structure (cooling) to a more organized solid crystalline structure Crystal Perfection – the process of small, less perfect crystals (metastable) melting at a temperature below their thermodynamic melting point and then (re) crystallizing into larger, more perfect crystals that will melt again at a higher temperature

Change in Crystallinity While Heating Quenched PET 9.56mg 10°C/min

Crystallization Crystallization is a kinetic process which can be studied either while cooling or isothermally Differences in crystallization temperature or time (at a specific temperature) between samples can affect end-use properties as well as processing conditions Isothermal crystallization is the most sensitive way to identify differences in crystallization rates

Crystallization Crystallization is a two step process: Nucleation Growth The onset temperature is the nucleation (Tn) The peak maximum is the crystallization temperature (Tc)

Effect of Nucleating Agents POLYPROPYLENE WITH NUCLEATING AGENTS WITHOUT NUCLEATING AGENTS crystallization melting Effect of Nucleating Agents

What is Isothermal Crystallization? A Time-To-Event Experiment Annealing Temperature Melt Temperature Isothermal Crystallization Temperature Temperature Zero Time Time

Isothermal Crystallization Polypropylene

Specific Heat Capacity (Cp) Heat capacity is the amount of heat required to raise the temperature of a material by 1°C from T1 to T2 True Heat Capacity (no transition) is completely reversible; the material releases the same amount of heat as temperature is lowered from T2 to T1 Specific Heat Capacity refers to a specific mass and temperature change for a material (J/g/°C) See slide

Why is Heat Capacity Important? Absolute thermodynamic property (vs. heat flow) used by engineers in the design of processing equipment Measure of molecular mobility Cp increases as molecular mobility increases. Amorphous structure is more mobile than crystalline structure Provides useful information about the physical properties of a material as a function of temperature Heat Capacity is an absolute thermodynamic property to measure and an important physical property to measure as a function of temperature.

Does DSC Measure Heat Capacity? DSC or MDSC® do not measure heat capacity directly. They measure heat flow rate which can be used to calculate heat capacity which is more appropriately called apparent heat capacity DSC calculated Cp signals include all transitions because the heat flow signal is simply divided by heating rate (an experimental constant) to convert it to heat capacity units A true value of Cp can only be obtained in temperature regions where there are no transitions

Calculating Heat Capacity (Cp) Depending on the DSC that you have there are three different ways to calculate Cp Three Run Method – ASTM E1269 Applicable to all DSC’s Direct Cp – Single Run Method Applicable to Q1000 only MDSC® - Single Run Method Any TA Instruments DSC w/ MDSC option Most accurate determination

Cp by Standard DSC Generally, three experiments are run in a DSC over a specific temperature range Empty pan run Sapphire run Sample run Traditionally, heat capacity is measured in a DSC by running three experiments. They are as shown in slide.

Calculating Cp by Standard DSC Three experiments are run over a specific temperature range Allow 5 minute isothermal at start Use 20°C/min heating rate Empty pan run Match pan/lid weights to ± 0.05 mg Used to establish a reference baseline

Calculating Cp by Standard DSC Sapphire run Used to determine calibration constant Use same weight of pan/lid as for baseline ± 0.05 mg Typical weight is 20 – 25 mg Sample run Typical weight is 10 – 15 mg Use same weight of pan/lid as before ± 0.05 mg

Cp by Traditional DSC – 3 Runs Heat Flow Baseline Run Sample Run Calibration Run

Cp by Traditional DSC – 3 Runs

Specific Heat Capacity MDSC® & Tzero™ DSC have the ability to calculate a heat capacity signal directly from a single run. Benefits of using a heat capacity (instead of heat flow) signal include: The ability to overlay signals from samples run at different heating rates The ability to overlay signals from heating and cooling experiments Advanced and patented techniques from TA Instruments offer the advantage of directly measuring heat capacity in a single run. The heat capacity signal offers additional benefits over using traditional heat flow signal.

Direct Cp from a Q1000 Absolute integral calculates total heat Latent Heat of Melting is Not Heat Capacity Absolute integral calculates total heat Latent Heat of Crystallization is Not Heat Capacity

Heat Flow w/ Different Heating Rates Heat Flow Signals Increase in Size with Increasing Heating Rate

Benefit of Plotting Heat Capacity Heat Capacity Signals Are Normalized for Heating Rate and Permit Comparison of Experiments Done at Different Heating Rates Remember, DSC and MDSC Cp signals are really Apparent Cp signals; crystallization and melting are latent heats, not Cp

Heat Flow & Cp Signals Polypropylene Size: 9.21 mg DSC Cycle @ 10degC/min Heat Flow on Cooling Heat Flow on Heating Heat Capacity on Cooling Heat Capacity on Heating

Weak Tg Visible in Cp Signal Heat Capacity on Cooling Heat Capacity on Heating Sample: Polypropylene Size: 9.21 mg DSC Cycle @ 10 C/min Slide shows advantage of using heat capacity signal for picking out weak Tg in polypropylene after heating and cooling scans. Also you can compare Tg on heating and cooling because Cp overlays but you would not see same comparison in Heat flow.

Thermoset Curing & Residual Cure A “thermoset” is a cross-linked polymer formed by an irreversible exothermic chemical reaction A common example would be a 2 part epoxy adhesive With a DSC we can look at the curing of these materials, and the Tg of full or partially cured samples

Curing of a Thermoset

Partially Cured System 2nd heat shows increased Tg, due to additional curing during 1st heat Note: Small exotherm due to residual cure

Photopolymer Cure by PCA The PCA is an accessory for the Q1000 / Q100 DSC modules that features a medium pressure mercury UV source and dual light guides. It is ideal for rapid initiation of unsaturated materials (e.g., acrylics)

Use PDSC to Study Phenolic Curing With ambient pressure, curing is not visible due to volatization of water. Water comes from the condensation reaction during the curing of the phenolic Decomposition After an "equilibrate at 200 oC" method segment, the isothermal temperature is stable to within 0.04 oC during the first 5 minutes and is then stable to within 0.01 oC for the remaining 95 minutes.