Melting point, crystallization, and glass transition in polymers
Melting and crystallization
The change in the physical state of a solid from solid to liquid is known as melting. The heat supplied dissolves the crystal lattice with the temperature of the material remaining constant during the entire melting process. So, there is a defined melting temperature.
The reversal of this first order phase, the transition from the amorphous-liquid state of the melt to the crystalline state, is called crystallization. It is a kinetically controlled process and depends primarily on nucleation. Therefore, the crystallization temperature is always below the thermodynamically controlled melting temperature.
Non-crystalline materials such as polymers, on the other hand, have a glass transition. This is where completely or partially amorphous polymers change from a highly viscous or rubber-elastic, flexible state to a glass-like or hard-elastic, brittle state. To characterize the glass transition, the glass transition temperature Tg expresses the point at which half of the change in the specific heat capacity is reached. It is also called the glass transition temperature or softening temperature.
The thermal glass transition is observed when a melt which cannot crystallize is supercooled. The so-called cooperative molecular movements, such as rearrangements of main chain segments, side chain rotations, and end group rotations, then “freeze.” This results in sudden changes in mechanical and thermodynamic properties such as modulus of elasticity, specific heat capacity, and the coefficient of thermal expansion.
The cooling rate has a decisive influence on the glass transition temperature. If the melt is cooled down quickly, the glass transition temperature is higher. With an infinitely slow cooling, there is no glass transition, since no amorphous partial areas result.
Many common plastics are partially crystalline: they therefore have a glass transition temperature below which the amorphous phase freezes. At the same time, they also have a melting temperature at which the crystalline area dissolves.
Polymers can be classified based on their glass transition
Since every plastic has a specific glass transition, this is an important parameter for characterizing the material. Therefore, the determination of the glass transition temperature is often used in thermal analysis, in order to make such statements as the dimensional stability of a polymer under the action of heat.
For example, elastomers are only used in the rubber-elastic range, i.e. above the glass transition temperature. In contrast, amorphous thermoplastics are only used below Tg.
Since the glass transition depends on the type of plastic and its manufacture, information on changes in the material can be obtained by determining the Tg.
Among other things, the following relationships exist:
- Chemical structure: the more flexible the main chain, the lower the Tg
- Molar mass: the Tg increases as molar mass increases
- Molecular orientation, e.g. Tg increases in foils
- Crosslinking, the Tg increases as the degree of crosslinking increases
- Plasticizers: the Tg decreases as the plasticizer content increases.
The glass transition temperature also provides information on the physical aging of plastics, which occurs as an enthalpy relaxation peak in DSC measurements. Polymer mixtures can also be characterized using Tg. If the polymers are immiscible, the individual components appear phase-separated so that they exist side-by-side and several glass transitions can be measured. A comparison with the pure components can provide information about the proportions and quality of the mixing process.
Further information on determining the glass transition temperature.