Table of Contents
Why Tg measurement under defined load is crucial
The glass transition temperature (Tg) marks the transition of a thermoplastic from a brittle, glass-like state to a rubber-like behavior. With classic measurement methods such as differential scanning calorimetry( Differential Scanning Calorimetry(DSC)), it is determined in a load-free state ; however, these often provide insufficient information for real applications.
Many thermoplastic molded parts – such as housings, seals or clamp connections – are subjected to mechanical stress in everyday life or work in ambient temperatures close to Tg. A material that has a Tg of 105 °C according to the data sheet can soften much earlier under heat plus force and lose its dimensional stability. For development, this means that a design based purely on DSC data can cause component failure – often without warning.
The Thermomechanical Analysis (TMA) offers a decisive advantage: it measures the change in length of a sample during the temperature rise under a defined mechanical force. With this method, the glass transition can be determined under a more realistic test condition, for example at a specific penetration or compression force. This methodical approach enables a more sensitive, practical Tg measurement that goes beyond the laboratory-based DSC observation and thus enables better material decisions to be made.
In the following article we show how TMA measurements can be carried out with defined force conditions and what empirically proven studies – for example on PMMA films and copper-PMMA composites – say about the difference to classic Tg measurements using DSC.
What thermomechanical analysis (TMA) can actually do
Thermomechanical analysis (TMA) is an established method for characterizing the thermal deformation behaviour of solid materials. In contrast to calorimetric methods such as DSC, TMA directly measures the change in length of a test specimen during controlled temperature control – under a defined mechanical force. This combination makes TMA particularly valuable for analyzing temperature-dependent structural changes, such as those that occur during the glass transition of amorphous or semi-crystalline thermoplastics .
Measuring principle
In a typical TMA measurement, a sample – e.g. a thin strip or a cylindrical test specimen – is placed on a firm base or, in the case of films or fibers, clamped between two clamps. A force is applied to the sample, whereby the size of the force can be variably adjusted (usually in the range of a few millinewtons to several newtons, depending on the material and test target).
While the temperature is increased at a constant heating rate (e.g. 2-5 K/min), the system records the change in length with high resolution.
In many amorphous thermoplastics, this change in length as a function of temperature shows a clear change in the gradient (a “kink point“), which corresponds to a change in the coefficient of expansion – precisely at the temperature at which molecular mobility increases: the glass transition. The Tg determined in this way is usually load-dependent and differs from the Tg determined under load-free conditions. In addition, the TMA method for determining Tg is significantly more sensitive than the DSC method. However, the glass transition temperatures determined depend not only on the selected method, but also on the respective heating rates and other test parameters. When specifying Tg, the measurement method used and the test conditions should therefore always be stated.
Relevant measurement modes
Different measurement modes can be used depending on the desired result:
- Expansion: The sample is exposed and expands due to heating under its own weight or minimal load. This mode is often used as a reference for undisturbed Tg measurement.
- Penetration: The measuring needle presses on the surface with a defined force – this mode is particularly suitable for simulating the behavior under punctual load.
- Measurement under oscillating force: During the measurement, an oscillating force with a frequency in the range of approx. 0.1 to 1 Hz is applied. The penetration muscle is usually used for this purpose.
All modes provide a characteristic length change curve over the course of the temperature. The glass transition is indicated by an abrupt change in the expansion behavior – usually a kink in the curve, which is determined using a tangent method or by comparing the thermal expansion coefficients before and after the transition point. When measuring with oscillating force, Tg is indicated by a strong increase in amplitude.
Case studies: Validated investigations for Tg determination with TMA
The following examples from the scientific literature show how thermomechanical analysis can be used to measure the glass transition temperature (Tg) of PMMA-based materials under defined mechanical conditions. The focus here is not on modeling extreme application loads, but on the possibility of drawing conclusions about sensitive structural changes in the material via the coupling of temperature and force – an advantage over purely calorimetric methods.
PMMA films: Tg deviation with TMA and DSC
In a study by Agarwal et al. (2010), films made of poly(methyl methacrylate) (PMMA) were analyzed using thermomechanical analysis (TMA) in tensile mode. At a test load of 10 N and a heating rate of 2 K/min, the elongation curve showed a striking transition at 82.1 °C. This value is well below the glass transition temperature of around 105 °C typically determined by DSC, as is often stated in data sheets.
The study shows that TMA can detect temperature-induced changes in mechanical behavior even below the Tg determined by DSC. This underlines the potential of TMA, especially for applications with tight tolerances or sensitive temperature ranges where mechanical reactions can start before the DSC Tg.
Copper-PMMA composites: Influence of fillers on the Tg behavior
Another study by Poblete and Álvarez (2023) was dedicated to the influence of nanoscale copper particles on the thermomechanical properties of PMMA composites. For this purpose, different volume fractions of copper were incorporated into a PMMA matrix and the resulting materials were analyzed using the thermomechanical analysis (TMA) method, among others.
The results show that the glass transition temperature decreases slightly at low filler contents (below 2 vol%), while it remains largely stable above about 10 vol%. The authors present both TMA and DSC measurement data, which differ for some compositions but show good agreement overall.
The TMA was able to depict the effects of the filler addition in a very differentiated manner – not only with regard to the Tg value, but also with regard to the temperature-dependent change in length. This provides valuable information for the development of PMMA-based composites, in particular for the targeted adjustment of material properties in the area of tension between thermal stability and mechanical performance.
PMMA-CCTO composites: No significant influence on Tg
In the study by Thomas et al. (2013), composites made of PMMA and ceramic CaCu₃Ti₄O₁₂ (CCTO) – a filler that is of interest for electronic applications due to its high dielectric constant – were investigated. The researchers analyzed the thermal properties of the materials using DSC. Devices from TA Instruments and Mettler Toledo were used for this.
The results show that the glass transition remained largely constant even with high filler contents of up to 38% by volume. The measured Tg was consistently around 107 °C. This indicates that CCTO as a ceramic filler only slightly influences the molecular mobility of the polymer matrix.
This means for materials engineering practice: Functionalized materials with ceramic additives can be developed without significant changes to their thermomechanical properties being expected. TMA can be used here – in addition to DSC – to check at an early stage whether new fillers influence the mechanical behavior over the temperature curve.
Significance of the measurement results for material evaluation
The case studies presented show that thermomechanical analysis (TMA) is a particularly sensitive method for determining the glass transition temperature (Tg) – especially when the determination is carried out under defined mechanical stress. Compared to conventional methods such as differential scanning calorimetry (DSC), several studies have shown that the Tg value depends on the measurement methods and conditions. This deviation is not a measurement inaccuracy, but an expression of the different physical questions posed by the methods and the respective test conditions.
While DSC measures the energy input during the transition to a new thermodynamic equilibrium, TMA records the start of the macroscopic change in shape – i.e. the point at which the material yields its structure under a small defined force. The TMA therefore provides a value that is directly relevant in practice: it is not the complete glass transition that is decisive, but the temperature at which a component begins to show signs of deformation or settling.
This difference is particularly relevant for applications with high demands on dimensional accuracy, fit or clamping behavior – for example in the field of..:
- of connectors and housings,
- of optical components,
- or medical plastic parts that are exposed to temperature fluctuations inside the body.
The investigation of the copper-PMMA composites also proves that low filler contents can also influence the Tg behavior. This is an important criterion in the development of functionalized polymer systems, for example for electrical engineering or sensor technology. At the same time, the CCTO composites show that not all additives lead to relevant Tg shifts. This is also an important finding, as it helps to focus on materials that retain their thermomechanical properties during filler integration.
Application notes for materials engineering practice
Thermomechanical analysis (TMA) offers a reliable way of detecting thermally induced changes in the length of plastics under a defined force. This makes it particularly suitable for determining the glass transition temperature (Tg) under application-related conditions – provided that the method is used in a targeted and reproducible manner.
To ensure that the measurement results are technically usable, some key aspects should already be taken into account when planning the test:
Setting the test parameters
The significance of a TMA measurement depends heavily on the parameters selected:
- Heating rate: Moderate heating rates of 2 to 5 K/min are recommended. Higher rates can lead to a distortion of the transition point, as the material is not heated through evenly.
- Force: The applied force should remain within the range that does not cause plastic deformation, but only serves to detect thermally induced deformation. Typical forces are in the range of 50 to 500 mN, depending on the material and specimen geometry.
- Sample thickness: A homogeneous sample geometry is crucial. Particular attention should be paid to uniform distribution and orientation of the particles, especially with materials containing fillers.
By combining these parameters, the TMA measurement can be adjusted in such a way that it not only provides comparable results, but also makes specific effects visible.
Interpretation of the measurement results
A central aim of the TMA measurement is to determine the temperature range in which the material reacts with increasing flexibility. This is typically indicated by a change in the slope angle of the strain curve, i.e. a buckling point. The glass transition temperature is usually interpreted as the beginning of this transition range.
Other typical evaluation parameters are
- The coefficient of thermal expansion (CTE) before and after the glass transition
- The deformation rate under constant force in the Tg range
- The comparison of different sample conditions (filled, unfilled, processed, conditioned)
It is advisable to always carry out several measurements under slightly different conditions in order to check the robustness of the results.
Limits and combinations
TMA is ideal for investigating amorphous and semi-crystalline thermoplastics that are used in the temperature range between room temperature and approx. 300 °C. For very thin layers, highly viscoelastic materials or components with a multilayer structure, supplementary methods such as DMA or microscale methods (e.g. nano-thermomechanics) may be useful. Nevertheless, TMA in its simple form offers an easily accessible and practical method to reliably quantify the deformation behavior in the relevant temperature window.
The role of TMA in material characterization
The results from research and application clearly show that thermomechanical analysis (TMA) offers decisive methodological added value when it comes to determining the glass transition of thermoplastic materials. In contrast to calorimetric methods such as DSC, which record the Tg under idealized, load-free conditions, TMA allows the material to be evaluated under defined mechanical stress.
The combination of temperature and force influence enables a differentiated statement to be made about the point at which a material begins to change its shape – i.e. precisely the limit that is of decisive importance for the development of dimensionally stable, mechanically stressed components.
TMA is not a competitor to other methods, but a useful addition to the method network. Especially for:
- the validation of plastics for precise fits,
- the development of filled or reinforced polymer compounds,
- as well as in the analysis of processing or aging influences
it provides insights that remain hidden from other methods. Its high sensitivity to small changes in length makes it the ideal method for detecting the start and progression of the glass transition.
References
Agarwal, A. et al. (2010): Investigation of Thermomechanical Properties of PMMA
https://www.researchgate.net/publication/252928444_Investigation_of_Thermomechanical_Properties_of_PMMAPoblete, V. H. & Álvarez, M. P. (2023): Mechanical, Electrical, and Glass Transition Behavior of Copper-PMMA Composite sheets fabricated via melt mixing.
https://www.mdpi.com/2073-4352/13/3/368Thomas et al. (2013): Thomas, S., Stephen, R., Grohens, Y., & Pothan, L. A. (2013). Thermal and dielectric behavior of PMMA/CaCu₃Ti₄O₁₂ nanocomposites. Journal of Thermal Analysis and Calorimetry, 112, 1175-1182.
https://arxiv.org/abs/1301.4218
