The temperature at which molecular or atomic motion in an amorphous substance or the amorphous regions of a polymer freezes, switching between the glassy and rubbery (supercooled liquid) states. Written Tg.
Japanese name: ガラス転移温度
Glass transition temperature appears in 7 practice questions across 4 topics, including Glasses and amorphous materials, Polymers and composites, Ceramics and glasses.
Field: Polymers and composites
As a liquid or polymer melt is cooled, molecular rearrangement gradually slows. When rearrangement can no longer keep up with cooling, the structure freezes in place and the material enters the glassy state, behaving like a solid. The temperature at which this switch occurs is the glass transition temperature Tg, where the temperature dependence of specific volume, heat capacity and thermal expansion coefficient shows a kink. In polymers, segmental motion, in which several units of the backbone move together, becomes active above Tg, giving a soft, rubbery state.
The melting point Tm is the temperature of a thermodynamic phase transition at which crystals melt, and it has a definite value. Tg, on the other hand, is a kinetic phenomenon determined by the balance between relaxation time and observation time, and its value changes with cooling rate, heating rate and measurement frequency. The faster the cooling, the higher the temperature at which the structure freezes, and the higher Tg. Crystalline polymers have both a melting point Tm for the crystalline regions and a Tg for the amorphous regions; Tg is always lower than Tm, often by a ratio of about 0.5–0.8 in absolute temperature. Amorphous polymers have no Tm, only a Tg.
The harder it is for chains to move, the higher Tg. Polymers with stiff structures such as aromatic rings in the backbone, bulky side groups, strong interchain interactions or many crosslinks have high Tg: about 150 °C for polycarbonate and about 100 °C for polystyrene. Conversely, polyethylene and silicone rubber, with flexible backbones, have Tg far below room temperature. Plasticizers and moisture enter between chains, increase free volume and lower Tg. In epoxy resins, the drop in Tg on moisture absorption is a reliability concern.
Tg is measured from the stepwise change in heat capacity in DSC, the kink in the expansion coefficient in thermomechanical analysis (TMA), and the sharp drop in storage modulus or the peak in the loss tangent tanδ in dynamic mechanical analysis (DMA). Even for the same material, the value can differ by more than 10 °C depending on the method, the heating rate and, for DMA, the frequency, so check the measurement conditions when comparing data. For semiconductor encapsulants and substrate resins, the expansion coefficient and modulus change greatly above Tg, so its relation to temperatures such as reflow matters.
Answer: C. The faster the cooling rate, the higher Tg
As a melt cools, its viscosity rises steeply, and at the temperature where atomic rearrangement can no longer keep up with cooling, the structure freezes in and the slope of specific volume versus temperature changes. This temperature is the glass transition temperature Tg, at a viscosity of roughly 10¹² Pa·s. The faster the cooling, the less time atoms have to rearrange, so the structure freezes in at a higher temperature and Tg is higher; the frozen-in structure is then a lower-density state. Tg is not the temperature of a thermodynamic phase transition like a melting point, so it is not fixed independent of cooling rate. This property also bears on removing residual stresses by annealing glass.
Answer: B. Tg is the temperature at which chain motion in the amorphous regions freezes or is released, and it is lower than Tm
A crystalline polymer consists of crystalline and amorphous regions. Tg is the temperature at which segmental motion of the chains in the amorphous regions freezes or is released; below Tg the polymer is hard and glassy, above it rubbery. Tm is the temperature at which the crystalline regions melt; Tg is always lower than Tm, and for many polymers Tg/Tm (as a ratio of absolute temperatures) is about 0.5–0.8. PET, for example, has a Tg of about 70–80 °C and a Tm of about 260 °C. It is Tm that represents melting of the crystalline regions; amorphous polymers such as polystyrene have no Tm, but they do have a Tg. Polymers such as polyethylene, whose Tg is below room temperature, are flexible because their amorphous regions are rubbery at room temperature.
An amorphous polymer first enters a rubbery region above Tg and begins to flow as the temperature rises further. Crosslinked thermosetting resins remain rubbery above Tg and do not flow.
Tg is a kinetic phenomenon, so its value changes with the time scale of the measurement. The DMA tanδ peak often comes out higher than the DSC midpoint value, so state which definition is used when comparing.
Both are glass transitions caused by the freezing of structure, and the mechanism is the same. The ranges of values differ widely, however: several hundred °C for silicate glasses versus roughly −100 °C to 200 °C for polymers.
This temperature is the glass transition temperature Tg, at a viscosity of roughly 10¹² Pa·s.
Which statement about the glass transition temperature Tg w…The weakened connectivity lowers the viscosity and the glass transition temperature, so shaping that requires high temperatures of about 2000 °C for silica glass becomes possible at much lower temperatures.
How do the structure and properties of SiO₂ glass change wh…Crystallinity Reduced glass transition temperature Non-bridging oxygen Coefficient of linear thermal expansion Rule of mixtures Thermosetting resin Viscoelasticity