What is Glass Transition Temperature?
The glass transition temperature of polymers (denoted Tg) is when a polymer loses its rigidity and adopts rubber-like characteristics. A substance below its Tg will have restricted movement resulting in a stiff and brittle substance. By contrast, a polymer above glass transition temperature adopts much greater mobility and becomes soft and rubbery.
How to Measure Glass Transition Temperature
One technique that can detect a glass transition is differential scanning calorimetry (DSC) which measures changes in the substance’s heat capacity. With this technique, the glass transition is where a sudden drop in heat flow (relative to a standard) indicates the sample has softened and is absorbing more heat. Another technique is thermo-mechanical analysis (TMA) which measures changes in a substance’s compressive resistance over a temperature range. Like DSC, TMA first exposes the sample to heating and cooling cycles then measures how far a probe kept at a constant force displaces from its starting position as the temperature increases. The glass transition is the change in slope on a displacement (measured in microns) versus temperature plot. The idea behind TMA is that above the glass transition, the slope will increase as the material softens and the probe more easily penetrates the sample. Figure 1 below compares a DSC with a TMA curve.
Glass Transition of Common Polymers
Glass transition temperature varies substantially between standard potting resins, impacting some key physical characteristics. A typical epoxy glass transition temperature is 40-60°C for Bis-A resins but can exceed 100°C with Novalac resins. The glass transition of silicone, by comparison, is around -60°C which is why silicones have such a low modulus at room temperature. The glass transition of polyurethane falls within a broad temperature spectrum depending on the prepolymers used, much like the cured hardness for these materials.
All figures were created using BioRender.com
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