A polymer will normally have more than one characteristic temperature, including (in the normal ascending order):

• T_g: the glass transition temperature.
• T_m: the crystalline melting point
• T_p: the processing temperature (for thermoplastics).
• T_d: the degradation temperature.

As the temperature rises through the glass transition temperature, short segments of the polymer backbone which had insufficient energy for movement other than atomic vibration, start to move as a group of atoms.  On cooling through this temperature, it is normal to refer to segmental motion being frozen out.  The mechanical properties of the polymer are then:

• below T_g:  normally elastic and brittle (with good resistance to creep deformation)
• above T_g:  normally viscoelastic and tough (however creep deformation can be a problem)

For thermosetting resins, the glass transition temperature generally follows the maximum temperature experienced during the cure cycle (indicated by "*" in the Table below).

The crystalline melting point is not applicable to amorphous polymers and is usually only important in thermoplastics.  The crystalline melting point value is normally ~200 (±50) ºC above the glass transition temperature ( T_m ≈ T_g + 200 ºC ). T_m may be a narrow range of temperatures rather than a single point.

Additional data on T_g and T_m is available from

• DSC Transitions of Common Thermoplastics (PerkinElmer Instruments, 2000)
• Thermal Properties of Polymers (Netzsch-Gerätebau GmbH, 2011)
• Thermal Transitions of Homopolymers: Glass Transition and Melting Point (Aldrich, undated).

Thermal characterisation

There are a number of techniques which can be used to characterise the thermal behaviour of fibre-reinforced composites:

• TGA: ThermoGravimetric Analysis measures weight changes in a material as a function of temperature (or time) under a controlled atmosphere to determine the thermal stability and composition.
• DTA: (Differential Thermal Analysis) and DSC (Differential Scanning Calorimetry) measure the thermal transitions in materials by monitoring temperatures and heat flows.
  DTA is qualitative while DSC is quantitative.  Glass transition temperatures, phase changes, heat capacity, cure kinetics and thermal degradation can be monitored by these techniques.   Modulated DSC can be both faster and more accurate.
• DMTA: Dynamic Thermo-Mechanical Analysis measures the mechanical properties of materials under stress as a function of time, temperature, and frequency.
• DETA: Dynamic Electrical Thermal Analysis measures the electrical properties of materials as a function of time, temperature, and frequency.
• TMA: Thermo-Mechanical Analysis measures the change in dimensions or in a mechanical property of the sample while it is subjected to a temperature regime.
• Minimat is a miniature materials testing apparatus.
• STA: Simultaneous application of TGA and DSC to a single sample to yield more information than separate tests in different instruments.

Prajer (PhD thesis, University of Bath, 2011) characterised the thermal behaviour of polylactic acid (PLA) using a TA Instruments DSC2920 Differential Scanning Calorimeter. Figure 1 shows the response of material when melted at 190°C for 10 minutes then either air-quenched (left) or cooled at 5°C/min to 115°C then kept at temperature for 40 minutes (right).  The fast cooling rate suppresses crystallinity development in the quenched polymer so that the temperature ramp during DSC records a strong glass transition at ~60-70°C and a peak crystallisation temperature at about 120°C.  In the slowly cooled partially crystalline sample, the glass transition temperature is suppressed and there no crystallisation peak.  The crystalline melting point is in the range 160-180°C and moves to a slightly higher peak temperaure for the crystalline material.

         

Figure 1:  Differential scanning calorimeter thermal response plots for quenched/amorphous (left or above) and crystalline (right or below) polylactic acid (reproduced here with the permission of Dr Prajer).

Further reading

1. David Richard Mulligan, Samuel Gnanih and Graham Sims, Thermal Analysis Techniques for Composites and Adhesives (Second Edition), National Physical Laboratory Good Practice Guide (GPG No. 62), Teddington, 2000.
2. Wim M Groenewoud, Characterisation of polymers by thermal analysis, Elsevier, Amsterdam, 2001.  ISBN 0-444-50604-7.
3. Peter J Haines, Principles of thermal analysis and calorimetry, Royal Society of Chemistry, Cambridge, 2002.  ISBN 0-85404-610-0.
4. JD Menczel and RB Prime, Thermal Analysis of Polymers - fundamentals and applications, Wiley, 2009.  ISBN 978-0-471-76917-0.
5. Plastics - differential scanning calorimetry (DSC). Part 1: general principles, BS ISO 11357-1:1997.
6. Plastics - differential scanning calorimetry (DSC). Part 2: determination of glass transition temperature, BS ISO 11357-2:1999.
7. Plastics - differential scanning calorimetry (DSC). Part 3: determination of temperature and enthalpy of melting and crystallization, BS ISO 11357-3:1999.
8. Plastics - differential scanning calorimetry (DSC). Part 5: determination of characteristic reaction curve temperatures and times, enthalpy of reaction and degree of conversion, BS ISO 11357-5:1999.
9. Plastics - thermogravimetry (TG) of polymers. General principles. BS EN ISO 11358:1997.
10. Plastics - thermomechanical analysis (TMA). General principles. BS ISO 11359-1:1999.
11. Plastics - thermomechanical analysis (TMA). Determination of coefficient of linear thermal expansion and glass transition temperature. BS ISO 11359-2:1999.

UserCom (Mettler Toledo thermal analysis systems) TA Tips

1. Anonymous, Selection of the heating rate: influence on sensitivity, resolution and temperature accuracy, UserCom 2, December 1995, 1-2.
2. Anonymous, Investigation of unknown samples: sample preparation and experimental conditions, UserCom 3, July 1996, 1-2.
3. Anonymous, Optimum utilization of the internal database: extensive data management features (e.g. storage, search, filter), UserCom 4, December 1996, 1-3.
4. Anonymous, Crucibles in Thermal Analysis: application benefits offered by Al, Al₂O₃, Cu, Au, Pt and pressure pans, UserCom 5, June 1997, 1-4.
5. Anonymous, Calibration adjustment, calibration and reference substances: ADSC sample preparation and program parameters, UserCom 6, December 1997, 1-5.
6. Anonymous, Measuring specific heat capacity: comparison of different methods - direct Cp, sapphire method, steady state and sinusoidal ADSC, UserCom 7, June 1998, 1-5.
7. Anonymous, Tips on Model Free Kinetics: benefits, practical procedures and evaluations - crucibles for high temperature TGA, UserCom 8, 2/1998, 1-3.
8. Anonymous, Low temperature calibration: calibration substances for the range between –130 to 156°C, UserCom 9, 1/1999, 1-4.
9. Anonymous, DSC purity determination, UserCom 10, 2/1999, 1-5.
10. Anonymous, Interpreting DSC curves, Part 1: dynamic measurements - artifacts, conditions, physical transitions, lambda transitions, chemical reactions, UserCom 11, 1/2000, 1-7.
11. J Widmann, Interpreting DSC curves: isothermal measurements, crystallization, desorption, vaporization, drying, chemical reaction, UserCom 12, 2/2000, 1-4.
12. G Widmann, Interpreting TGA curves: gravimetric effects caused by mass losses and gains, melting, magnetic sample properties, buoyancy, experimental condition changes, UserCom 13, 1/2001, 1-4.
13. G Widmann, Interpreting TMA curves: dilatometry, TMA and dynamic load TMA, UserCom 14, 2/2001, 1-4.
14. G Widmann, J Schawe and R Riesen, Interpreting DMA curves, Part 1: advanced model free kinetics - IsoStep^TM, UserCom 15, 1/2002, 1-6.
15. J Schawe, Interpreting DMA curves, Part 2: complex modulus and compliance - the frequency dependence of modulus and compliance, UserCom 16, 2/2002, 1-5.
16. R Riesen and Jurgen Schawe, The glass transition temperature measured by different TA techniques, Part 1: overview, UserCom 17, 1/2003, 1-4.
17. R Riesen and J Schawe, The glass transition temperature measured by different TA techniques, Part 2: determination of glass transition temperatures, UserCom 18, 2/2003, 1-5.
18. M Wagner, R Bottom, P Larbanois and J Schawe, DSC measurements at high heating rates - advantages and limitations, UserCom 19, 1/2004, 1-5.
19. Ni Jing, The advantages of DSC cooling measurements for characterizing materials, UserCom 20, 2/2004, 1-4.
20. M Schnubell, Method development in thermal analysis, Part 1, UserCom 21, 1/2005, 1-4.
21. M Schnubell, Method development in thermal analysis , Part 2, UserCom 22, 2/2005, 1-4.
22. M Schnubell, How to determine optimum experimental parameters for DMA measurements, UserCom 23, 1/2006, 1-5.
23. M Zappa, Influence of absorbed water on the mechanical properties of polyamide 6, UserCom 24, 2/2006, 1-5.
24. R Riesen, Choosing the right baseline, UserCom 25, 1/2007, 1-6.
25. J Schawe, Optimum choice of method and evaluation in DMA measurements of composites, UserCom 26, 2/2007, 1-4.
26. R Riesen, Heat capacity determination at high temperatures by TGA/DSC, Part 1: DSC standard procedures, UserCom 27, 1/2008, 1-4.
27. R Riesen, Heat capacity determination at high temperatures by TGA/DSC, Part 1: applications, UserCom 28, 2/2008, 1-4.
28. M Zappa, Analytical measurement terminology in the laboratory. Part 1: trueness, precision and accuracy, UserCom 29, 1/2009, 1-7.
29. M Zappa, Analytical measurement terminology in the laboratory. Part 2: uncertainty of measurement, UserCom 30, 2/2009, 1-5.
30. A Hammer, Thermal analysis of polymers. Part 1: DSC of thermoplastics, UserCom 31, 1/2010, 1-6.
31. A Hammer, Thermal analysis of polymers. Part 2: TGA, TMA and DMA of thermoplastics, UserCom 32, 2/2010, 1-5.
32. A Hammer, Thermal analysis of polymers. Part 3: DSC of thermosets, UserCom 33, 1/2011, 1-5.
33. A Hammer, Thermal analysis of polymers. Part 4: TGA, TMA and DMA of thermosets, UserCom 34, 2.1011, 1-5.