The significance of melting point and glass transition temperature for polymers

The glass transition temperature (Tg) is the temperature at which a glassy material transitions between the glassy state and the elastomeric state. For amorphous polymers, by applying a constant force and observing the relationship between the resulting deformation and temperature, this is usually referred to as a temperature-deformation curve or thermomechanical curve. Amorphous polymers have three mechanical states, namely the glassy state, the elastomeric state, and the viscous flow state. At lower temperatures, the material is in a rigid solid state, similar to glass; under external forces, it only undergoes very slight deformation. This state is known as the glassy state. When the temperature rises to a certain level, the deformation of the material increases significantly, and within a certain temperature range thereafter, the deformation remains relatively stable. This state is known as the hyperelastic state. As the temperature continues to rise, the degree of deformation increases again, and the material gradually turns into a viscous fluid; at this point, the deformation cannot be reversed, and this state is known as the viscous flow state. We usually refer to the transition between the glassy state and the high-elasticity state as the glass transition, and the temperature at which this transition occurs is known as the glass transition temperature, or simply the glass temperature. Tg is the upper temperature limit for use in amorphous thermoplastics, and it is the lower temperature limit for use in rubber. Glass transition exhibits the behavior of a second-order phase transition, with the heat capacity of the material undergoing continuous changes. However, glass transition is actually a kinetic transformation. Therefore, the specific value of the glass transition temperature is related to the rate of temperature change. Common glassy materials include most polymer materials, glass, and so on. Other glassy materials with important industrial applications include glassy metals, etc.

Glass transition is an inherent property of amorphous polymer materials; it represents a macroscopic manifestation of the change in the mode of motion of polymers. It has a direct impact on the performance and processability of these materials, which is why it has long been a key focus of research in polymer physics. Since polymer structures are more complex than those of low-molecular-weight compounds, their molecular movements are also more complex and diverse. Depending on the form of molecular motion, the vast majority of polymer materials can typically exist in one of the following four physical states (or mechanical states): glassy state, viscoelastic state, elastomeric state (rubbery state), and viscous flow state. Glass transition, on the other hand, is the transition between the elastomeric state and the glassy state. From a molecular structure perspective, the glass transition temperature represents a relaxation process in which the amorphous portion of a polymer moves from a frozen state to a thawed state; it does not involve phase transition heat like phase transitions do. Therefore, it is neither a first-order phase transition nor a second-order phase transition (referred to as a primary transition in polymer dynamics). Below the glass transition temperature, polymers are in a glassy state, where neither the molecular chains nor the chain segments can move; only the atoms (or groups) that make up the molecules vibrate at their equilibrium positions. At the glass transition temperature, although the molecular chains cannot move, the chain segments begin to move, resulting in elastomeric properties. As the temperature rises further, the entire molecular chains start to move, leading to viscous flow properties. The glass transition temperature (Tg) is an important physical property of amorphous polymers, as well as a key issue and challenge in the fundamental theories of condensed matter physics. It is related to numerous cutting-edge problems in kinetics and thermodynamics. The theory of glass transition continues to evolve and be refined. From the free volume theory that emerged in the 1950s to the currently being developed modal vortex theory and many other theories, all of these can only address certain aspects of glass transition. A complete theory of glass transition still requires significant efforts from researchers. For amorphous polymers, applying a constant force to them and observing the relationship between the resulting deformation and temperature is typically referred to as a temperature-deformation curve or thermomechanical curve. Amorphous polymers have four mechanical states, namely the glassy state, the viscoelastic state, the elastomeric state, and the viscous flow state. At lower temperatures, the material is in a rigid solid state, similar to glass; under external forces, it only undergoes very slight deformation. This state is known as the glassy state. When the temperature rises to a certain level, the deformation of the material increases significantly, and within a certain temperature range thereafter, the deformation remains relatively stable. This state is known as the hyperelastic state. As the temperature continues to rise, the degree of deformation increases again, and the material gradually turns into a viscous fluid; at this point, the deformation cannot be reversed, and this state is known as the viscous flow state. We usually refer to the transition between the glassy state and the high-elasticity state as the glass transition, and the temperature at which this transition occurs is known as the glass transition temperature, or simply the glass temperature.

Thank you to the original poster for sharing the understanding of the basic concepts related to Tg, the glass transition temperature. I’m not from a materials science background, so I’m really a bit confused when trying to understand it.