The transition from the glassy state to the highly elastic state of the polymer material is usually called the glass transition, and the corresponding temperature is called the glass transition temperature(Tg). Glass transition temperature only occurs in polymers, which is one of their unique characteristics. When the polymer is cooled below this temperature, it becomes hard and brittle, just like glass. Some polymers need to be used above their glass transition temperature, while others need to be used below their glass transition temperature.
Polymers have the following five physical states: glass state, glass transition, high elastic state (rubber state), viscoelastic transition, and viscous flow state. From the perspective of molecular structure, the glass transition temperature is a relaxation phenomenon of the polymer amorphous part from the frozen state to the thawed state, and there is no phase transition heat, so it is not a primary phase transition nor a secondary phase transition.
When the temperature is below the glass transition temperature, the molecular chains and segments inside the material cannot move, but the atoms or groups that make up the molecules can vibrate at their equilibrium positions. This is called the glass state of the material, the amorphous parts inside the material are in a frozen state. The material in the glass state is non-viscous and inelastic and is a rigid solid, similar to glass.
When the temperature rises and reaches the glass transition temperature, the chain segments inside the material begin to move, the molecular chains still cannot move. This state is called the highly elastic state of the material, and the material is in a thawed state. The material in the highly elastic state exhibits high elasticity, with a significant increase in deformation, and reaches relative stability within a certain temperature range. The highly elastic state is also called the rubber state.
When the temperature continues to rise and exceeds the glass transition temperature, the molecular chains inside the material begin to move. At this time, the molecular chains and chain segments move together. This state is called the viscous flow state of the material. The material is in an amorphous state and the material is like the fluid. The material in the viscous flow state exhibits viscous flow, and the deformation gradually increases and cannot be restored.
The test methods for the glass transition temperature
The transition of polymers is not a thermodynamic equilibrium process, and the vitrification state is also a non-equilibrium state. The test methods for Tg are affected by the heating and cooling rate.
The faster (slower) the heating rate, the higher (lower) the measured Tg of the polymer moves, then the higher (lower) the measured Tg is. This is a hysteresis phenomenon: the glass transition is a transition from freezing to the movement of polymer chain segments, which is a relaxation process and takes a certain amount of time. If the heating rate is accelerated, the movement of the chain segments will lag behind the heating process. When the chain segments undergo glass transition, the external display temperature will be higher than the actual transition temperature inside the system, and we cannot know the transition temperature, but use the external display temperature to approximate it, resulting in a higher measured polymer Tg.
The faster (slower) the temperature decrease rate, the higher (lower) the measured Tg will be. The faster the cooling rate, the greater the cooling rate of the system, and this rate will be much greater than the rate at which the system deviates from the structural adjustment. In this way, since the system does not have time to make structural adjustments to form an irregular solid, that is, the tendency of glassy state formation is enhanced, which helps to create a glassy state, so that the glass transition occurs at a higher temperature, and the glass transition temperature is naturally higher. The faster the cooling rate, the greater the tendency of glass transition, so that Tg moves toward the high-temperature direction. It is a hysteresis phenomenon: the result of the system structural adjustment lagging behind the system cooling rate.
The applications of glass transition temperature product
Taking the glass transition temperature (Tg) of acrylic resin as an example, according to the comprehensive requirements of coating types, performance, and special properties, the specific situation is as follows:
The higher the Tg, the harder the coating is and the stronger the scratch resistance is, but the coating cannot be brittle; at the same time, after painting, the better the drier the coating film, and also the faster the solvent release is. At the same time, the higher the Tg, the greater the final viscosity of the resin reaction, and the better the solvent resistance and corrosion resistance are after painting.
The Tg of the resin used in thermoplastic paint for topcoat is generally higher than 70°C; the Tg of the resin used in plastic coating for primer can be controlled at 45-60°C;
Thermoplastic acrylic metal coating resin Tg for topcoats of televisions, mobile phones, computers, etc. Tg is preferably 90-110℃.
ABS plastic coatings have high requirements for comprehensive performance. The Tg of acrylic resin must be as high as possible, generally 100-110 ℃; the Tg of thermoplastic acrylic resin modified by PP plastic primer is preferably 50-65 ℃.
The glass transition temperature is an important physical property of the amorphous polymers. It affects the performance of polymer materials and is an important part of long-time research on polymer materials.
At the glass transition temperature, the deformation and modulus of polymers change, and many physical properties, such as volume, expansion coefficient, specific heat, thermal conductivity, dielectric constant, etc., will also change greatly. Therefore, studying the glass transition temperature is also studying these physical changes, which is of great significance to the research of polymer materials.
