Visualizing Thermal Degradation of Polystyrene by DSC with In-Situ Microscope Imaging

Thermal degradation can significantly complicate the interpretation of DSC thermograms, particularly when measurements are performed at elevated temperatures and in open sample pans. In this study, polystyrene (PS) was analyzed using a TA Instruments™ Discovery™ DSC equipped with the Discovery DSC Microscope Accessory, enabling simultaneous acquisition of calorimetric data and in-situ optical images. The sample was heated from 20 °C to 300 °C under a nitrogen atmosphere, and images were captured at selected temperatures during the first heating scan.

The combined thermal and visual analysis reveals the progressive onset of degradation, manifested by bubble nucleation, growth, and coalescence within the polymer melt. These bubbles originate from the release of volatile species and degradation products generated by thermal chain scission, even under inert conditions. As temperature increases, reduced melt viscosity and increased gas evolution lead to significant bubble expansion and coalescence at the highest temperatures.

A second heating scan shows a shift of the glass transition temperature to higher values, indicating irreversible changes in the material caused by thermal relaxation and degradation during the first scan. This work demonstrates how in-situ optical monitoring provides critical complementary information to DSC measurements, improving the understanding of degradationrelated phenomena and helping to distinguish true thermal transitions from irreversible chemical and physical processes.

Differential scanning calorimetry (DSC) is one of the most widely used techniques for characterizing the thermal behavior of polymers, providing key information on transitions such as the glass transition temperature (T_g), melting, and crystallization. However, at elevated temperatures, DSC thermograms may become increasingly difficult to interpret due to the onset of irreversible physical and chemical processes, including volatilization, relaxation, and thermal degradation. These effects are especially pronounced when measurements are performed in open pans, which are often required to avoid pressure build-up or to study real processing conditions.

In many cases, calorimetric features associated with degradation can overlap with or distort genuine thermal transitions, leading to ambiguous interpretation of DSC data. Conventional DSC alone cannot always distinguish whether a change in heat flow originates from a true thermodynamic transition or from morphological and chemical changes occurring within the sample.

The integration of in-situ optical microscopy with DSC provides a powerful solution to this limitation. By directly observing the sample during a DSC experiment, physical phenomena such as softening, shrinkage, bubbling, foaming, or mass loss can be correlated in real time with the recorded heat flow signal. This combined approach enables a more reliable interpretation of complex thermograms and improves the understanding of degradation mechanisms.

In this work, the thermal degradation of polystyrene is investigated using DSC combined with in-situ optical imaging. The study demonstrates how visual observation complements calorimetric data, allowing degradation-related effects to be clearly identified and separated from intrinsic thermal transitions such as the glass transition.

Approximately 20 mg of polystyrene (PS) was placed in an open aluminum Tzero™ DSC Pan (without lid). The measurements were performed using a Discovery DSC equipped with the Microscope Accessory, which is fully integrated with the DSC cell and allows simultaneous thermal analysis and in-situ optical observation of the sample during the experiment.

All measurements were conducted under a nitrogen atmosphere to minimize oxidative effects. A heat–cool–heat temperature program was applied, consisting of heating the sample from 20 °C to 300 °C, followed by cooling back to 20 °C, and a second heating to 300 °C. The heating and cooling rates were 20 °C/min.

During the first heating scan, optical images of the sample were captured at selected temperatures using the integrated microscope camera. These images document the physical evolution of the material, including softening, bubble formation, growth, and coalescence associated with thermal degradation. DSC thermograms were recorded simultaneously for both heating scans to correlate calorimetric features with the observed physical changes.

Thermogravimetric analysis (TGA) was performed using a TA Instruments Discovery SDT 650. Approximately 20 mg of polystyrene was analyzed under a nitrogen atmosphere. The sample was heated from room temperature to 320 °C at a constant heating rate of 20 °C/min. The resulting mass loss was recorded as a function of temperature and used to correlate gravimetric changes with the thermal and morphological phenomena observed during the DSC experiments.

At low temperatures (20–80 °C, Figures 1, 2, and 3), the DSC thermogram shows a stable baseline with no significant thermal events, and the optical images confirm that the PS sample remains homogeneous and rigid. The material is in the glassy state, and no visible physical changes or bubble formation are observed.

As the temperature exceeds the glass transition, observed at 96.3 °C, the DSC curve exhibits the characteristic step change associated with T_g. There is no visible change in the shape of the sample (Figure 4).

At intermediate temperatures (170–200 °C, Figures 5 and 6), small bubbles become visible on the sample surface for the first time. These bubbles are attributed to the release of volatile species such as residual monomer, oligomers, additives, or absorbed gases [1]. As viscosity decreases with temperature, these species can nucleate and expand locally. The DSC signal in this region begins to deviate slightly from ideal heat capacity behavior, reflecting the onset of irreversible processes.

The DSC thermogram obtained during the second heating scan (Figure 11) shows a noticeable shift of the glass transition temperature to higher values compared to the first scan. This change indicates that the material has undergone irreversible modification during the initial heating to 300 °C.

The observed T_g increase can be explained by the combined effects of thermal relaxation and degradation. During the first heating, the polymer experiences structural relaxation and erasure of its prior thermal history. In parallel, degradation leads to the preferential loss of low-molecular-weight species through volatilization, effectively increasing the average molecular weight of the remaining material. These effects outweigh the T_g-lowering influence typically associated with chain scission, resulting in an overall increase in T_g in the second scan.

At higher temperatures, the accelerated mass loss observed in the TGA curve is consistent with the rapid bubble growth and coalescence seen optically, reflecting increased gas evolution associated with advanced thermal degradation. The ability of the integrated camera to visually capture these subtle yet critical changes highlights its sensitivity to degradation phenomena that precede major mass loss events.

Overall, the correlation between TGA data and in-situ imaging emphasizes that visual observation can act as an early indicator of degradation, complementing gravimetric measurements and improving the interpretation of thermal analysis results, particularly in cases where degradation begins gradually rather than abruptly.