Jituo Hi-tech offers glass transition temperature testing service and analysis.

The basic principle of glass transition

• The physical state of polymer materials is not static, but closely related to temperature. With Tg as the boundary, the material exhibits two distinct mechanical behaviors: glassy state and elastomeric state (or rubbery state).

Glassy State

• When the ambient temperature is lower than the glass transition temperature (Tg) of a material, the segmental motion of the polymer chain is "frozen", and the molecular chain can only vibrate within a small range near its equilibrium position. At this time, the conformation of the molecular chain is fixed and cannot undergo large-scale cooperative motion. Macroscopically, the material appears hard, rigid, and brittle, similar to inorganic glass, hence the name "glassy state". The polystyrene (PS) plastic lunch boxes and polymethylmethacrylate (PMMA) advertising boards we use daily have Tgs much higher than room temperature, so they appear as hard solids at room temperature.

Rubbery State

• When the temperature rises above Tg, the molecular segments gain sufficient energy, breaking out of their "frozen" state and initiating vigorous, reversible cooperative movements. Although the centroid position of the entire polymer chain remains relatively fixed due to chain entanglement, the flexible movements of the segments enable the material to undergo significant deformation under external forces and recover its original shape upon removal of the external forces. On a macroscopic scale, the material appears soft and elastic. Elastomers such as natural rubber (Tg ≈ -70℃) and cis-polybutadiene rubber (Tg ≈ -100℃) possess the excellent elasticity we are familiar with precisely because their Tg is much lower than room temperature. Unlike the melting process, which involves lattice disruption and latent heat absorption, the glass transition is a second-order thermodynamic transition, primarily characterized by abrupt changes in physical properties such as specific heat capacity and thermal expansion coefficient of the material. One of the classical theories explaining this phenomenon is the free volume theory. This theory posits that there are voids within polymer materials that are not occupied by the molecules themselves, known as "free volume". As the temperature increases, the free volume expands; when it reaches a certain critical threshold, it becomes sufficient to accommodate cooperative motion of segments, and the material transitions from a glassy state to an elastomeric state. The temperature corresponding to this transition point is Tg.

Key factors affecting the glass transition temperature

• Chain structure and rigidity: The flexibility of the molecular chain is the core internal factor determining the glass transition temperature (Tg). Polymers with rigid groups such as benzene rings and heterocycles in the main chain exhibit high steric hindrance and difficulty in segmental motion, resulting in a higher Tg (e.g., polystyrene Tg ≈ 100°C). Conversely, polymers with flexible bonds such as C-C and C-O in the main chain allow for easier segmental motion, leading to a lower Tg (e.g., polyethylene Tg ≈ -120°C).
• Intermolecular forces: The presence of strong polar groups (such as amide groups and hydroxyl groups) between molecular chains can form strong secondary bonds such as hydrogen bonds, significantly constraining segmental motion and leading to an increase in Tg. For example, nylon (polyamide) has a much higher Tg than polyolefins, which have similar structures but lack hydrogen bonds, due to the presence of a large number of hydrogen bonds between molecules.
• Plasticizers: Adding low molecular weight substances (i.e., plasticizers) to polymers is the most commonly used method to adjust Tg in industry. Plasticizer molecules will intercalate between polymer chains, weakening the interchain interaction forces and increasing the free volume, thereby significantly lowering the Tg of the material. A typical example is polyvinyl chloride (PVC). Pure PVC (Tg ≈ 80°C) is a rigid plastic used to manufacture door and window profiles; however, after adding a large amount of plasticizers, its Tg can be lowered below room temperature, turning it into a soft film or artificial leather.