Glass transition temperature of polymers

The glass transition temperature is the temperature at which an amorphous or semicrystalline polymer transitions from a glass-like, brittle state into a rubber-elastic state. In this range, the molecular mobility of the polymer chains changes drastically, which leads to a change in the mechanical properties. In contrast to crystalline substances, amorphous materials do not have an orderly crystal lattice, but rather unordered molecular chains.

Polymers

Polymers are large, chain-like macromolecules that are composed of many similar monomers. Polymers, also called plastics, can be artificially produced but are also found in nature, e.g. in polysaccharides or polypeptides. Amorphous and semicrystalline polymers have a so-called Glass transition temperature (T_g).When these amorphous or semicrystalline polymers are heated, the bonds of the disorderly polymer chains present in the amorphous phase along with the bonds among these are reduced. The polymer becomes softer and deformable until it becomes soft, rubber-like and deformable above the T_g. Artificial polymers can be divided into the following categories based on their mechanical properties:

• Thermoplastics: In thermoplastics, the polymers are arranged in chains that are not linked to each other. They melt or deform as heat is applied. They are further subdivided into amorphous (without crystal structure) and semi-crystalline thermoplastics. Semi-crystalline means that they have both amorphous (unordered) and crystalline (ordered) regions in their molecular structure. The operating temperature of thermoplastics normally ranges between -40°C and 150°C.
• Duroplasts: In duroplasts, the polymers have a very strongly bond to each other and each monomer has more than two bonds to other monomers. This creates grid-like, tightly-meshed 3D bonds. They are hard, brittle and temperature-resistant. The operating temperature range of duroplasts can vary greatly depending on the type. Certain duroplasts can withstand temperatures of up to 300°C or higher, while others can already fail at lower temperatures.
• Elastomers: Elastomers are a mixed form of thermoplastic and duroplast with regard to the bonding structure of the individual molecular chains. They consist of longer chain sections as well as wide-meshed 3D bonds. They are elastic, i.e. they reassume their original state after deformation. The operating temperature range of elastomers varies greatly depending on the type of elastomer. Typical operating temperatures can be between -50°C and 150°C.

Production: Polymerization, polycondensation, polyaddition

There are various manufacturing processes to convert monomers into polymers. Monomers are small, chemically reactive molecules with the ability to combine with each other to form polymers by bonding (polymerization). The choice of method depends on the monomers, the desired molecular structure and the product requirements. However, the basic requirement is always that a monomer with at least one double bond is present in order to be able to trigger a chain reaction.

Polymerization distinguishes between radical and ionic (cationic or anionic) polymerization. The process of polymerization itself is divided into the start of the chain, chain growth and chain termination. A cation is added to a monomer, e.g. ethylene, to start the chain during cationic polymerization. The positively charged cation reacts with the monomer and forms a bond with it. The originally existing double bond between the carbon atoms of the monomer is lost as a result and is occupied by the binding of the cation. The positive charge resulting from this turns it into a cation itself. This allows another monomer to be integrated, which continues in endless steps.

The chain growth is only interrupted by adding an anion, thus forming the end product, e.g. polyethylene. However, only long chains are created during polymerization, which is why only thermoplastics can be produced wwith this method. For polycondensation and polyaddition, monomers are used that have more than two functional groups with which 3D bonds can be created at the end. Depending on the size of the monomers, this either results in a duroplast (small monomers, because the mesh is tight) or elastomers (large monomers, because the mesh is wide). During polycondensation, one molecule is also split off as a by-product.

What materials have a glass transition temperature

Not only glass, but also other amorphous or semi-crystalline materials such as polymers have a glass transition temperature, also abbreviated as T_g. The glass transition temperature T_g is an important thermodynamic property of a polymer that is closely linked to its structure and properties. It is not to be confused with the melting temperature at which a material transitions from a solid state to a liquid state. These are two different processes, since energy supplied during melting - in contrast to the glass transition - is needed to dissolve the crystalline grating. However, it is possible that a material has both a glass transition temperature and a melting temperature.

Measuring the glass transition temperature

There are various ways to determine the glass transition temperature of different materials:

• FTIR spectroscopy: It measures the changes in molecular vibrations that occur near the T_g.
• Thermo-mechanical analysis (TMA): The occurrence of a characteristic change in the deflection of the sample is identified. As it approaches T_g, the sample starts to soften and deform, which leads to a visible increase in deflection.
• Dynamic differential calorimetry (DSC): The energy absorbed or released during the transition is measured.
• Dynamic vapor sorption (DVS): This method measures a change in sorption behavior (the ability of the polymer to absorb water vapor).
• Dynamic mechanical analysis: The polymer is deformed by means of periodic deformation or oscillation. T_g is identified in the DMA diagram as the point at which the phase shift of the sample increases significantly or its elasticity properties change drastically.
• Dielectric analysis (DEA): T_g is often identified as the point at which dielectric properties, especially the loss factor, shows a sharp rise or change.

Influence factors on the glass transition temperature

Knowledge of the glass transition temperature plays a key role when selecting the right polymer material for certain applications. The glass transition temperature is influenced by various factors:

Molecular weight

The glass transition temperature depends on the molecular weight of the respective polymer. The molecular weight determines the length of the long chains generated during the formation of polymers. Higher molecular weights generally lead to higher glass transition temperatures, since longer polymer chains require more energy to move.

Chemical structure

The type and strength of the chemical bonds and functional groups in a polymer influence its glass transition temperature. Polymers with stronger bonds often have higher T_g values.

Crystallinity

Amorphous plastics that do not have an ordered crystal structure tend to have lower glass transition temperatures compared to semicrystalline polymers. The crystalline areas are strongly ordered and remain so even after the T_g has been exceeded. They form the material structure and ensure that semi-crystalline materials can still be used above their T_g.

Chain stiffness

Polymers whose chains are flexible and have a high freedom of movement tend to have lower T_g values. Rigid polymer chains require more energy to move, which leads to higher T_g values.

Fillers and additives

The addition of fillers, plasticizers or other additives can affect the glass transition temperature by modifying the polymer structure with these substances. Many fillers, in particular inorganic fillers such as glass fibers, carbon fibers or minerals, can significantly improve the mechanical properties of the polymer. They act as reinforcing elements and increase the tensile strength, flexural strength and hardness of the polymer. Fillers can also increase the stiffness of the polymer by limiting the flexibility of the polymer chains. By increasing the thermal conductivity, they can also make a polymer more temperature-stable.

Additives are often used to improve the processability of the polymer. Plasticizers are an example of this. They influence the polymer structure by interacting between the polymer chains and loosening their bonds. This leads to reduced T_g and increased polymer flexibility. Antioxidants and UV stabilizers, for example, can also be used to protect the polymer structure from aging and degradation by exposure to light, heat or oxygen.

Effect on processing

The glass transition temperature also influences the processing of polymers. At temperatures above T_g, polymers can be formed more easily, while processing can become more difficult below T_g, since the polymer is brittle and breaks easily. T_g influences, for example:

• the choice of processing technology,
• the processing temperature, and
• processing parameters such as speed, pressure and cooling.

Thermoplastic polymers, such as polystyrene, can be readily processed above T_g. Polystyrene is then in a flowable state and is easily moldable, which is why injection molding, extrusion or thermoforming can be used as the processing method. Hard polyethylene is also suitable for blow molds, for example, because it can melt and flow well at higher temperatures, which makes it suitable for the production of bottles, canisters and containers for food packaging.

MISUMI supplies a portfolio of plastics with various properties.