What is Glass Transition Temperature (Tg)?

The glass-transition temperature Tg of a material characterizes the range of temperatures over which it undergoes glass transition. Glass transition is the process in which materials undergo a gradual and reversible transition from a hard and relatively brittle "glassy" state into a viscous or rubbery state as the temperature is increased. It is also called glass–liquid transition and is observed mainly in amorphous materials like amorphous polymers or amorphous regions in semi-crystalline materials/polymers. Tg indicates the region where the physical and mechanical properties of polymers change significantly.

Below Tg, the polymers look like glass; they are hard, brittle and have little flexibility. However, when the temperature exceeds Tg, it causes an increase in molecular mobility due to increased energy from the heat which makes them softer and more flexible like rubber.

The units used for measuring Tg values can be in °Celsius (°C), °Fahrenheit (°F) or Kelvin (K) depending on preference. These numbers are determined by how mobile chains are within a given polymer; the Tg of most man-made plastics is less than 500K (226.85°C).

PCBs have specific Tg values which can be selected while prototyping or quoting from a PCB supplier. Different Tg values affect the final PCB prototype manufacturing costs as prices increase with the Tg value of the PCB board. The higher the Tg value, the higher the reliability of the sheet; this is the most common grade index used to classify various types of substrates in the PCB manufacturing industry. The Tg of PCB substrate is divided into three grades: the Tg value of standard FR-4 substrate is 130°, 140°; the Tg value of High-Tg FR-4 substrate is greater than 170°, and the medium Tg value is 150°. The Tg value of PI material used in flexible circuit boards can reach 400°. PCB substrate sheets used for motherboards, consumer electronics, etc. generally have a Tg of 135°C, while the ones used for CPU boards, DDR3 memory substrates, IC packaging substrates, etc. have a Tg of 180°C.

Temperature vs. Stiffness graph of materials

Which type of polymers undergo glass transition? 

Polymers appear in different structural forms – amorphous, crystalline, or semi-crystalline. These three forms have widely varying ranges of Tg. This difference comes from the physical entanglement of polymer chains and the free-volume present in the structure. At lower temperatures, the more the free-volume, the easier it is for polymer chains to move easily.

An amorphous polymer has a chaotic molecular structure having the most free-volume with the weakest polymer chain links. When it reaches Tg it becomes like glass, so it is brittle, stiff and rigid when cooled down. Amorphous polymers do not have a specific melting point; instead, they gradually soften with increasing temperature. They are also more susceptible to stress failure because they contain hydrocarbons. Some examples include polycarbonate (PC), polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), acrylonitrile-butadiene-styrene (ABS) and general-purpose polystyrene (GPPS).  

Crystalline polymers have a highly ordered molecular arrangement. They don’t soften as the temperature rises as amorphous polymers do but possess a definite narrow melting point (Tm) instead. Usually, this point is higher than any seen in amorphous polymers or thermoplastics. The commonly found crystalline polymers are various polyolefins, polyether ether ketone (PEEK), polyethylene terephthalate (PET) and polyoxymethylene (POM). 

Semi-crystalline polymers have an intermediate state, featuring a blend of both random and ordered polymer chain structures. They have both amorphous and crystalline regions in their structures. The presence of crystalline regions restricts polymer chain movement, thereby elevating the Tg and hence they have higher Tg than amorphous polymers. It's important to note that Tg primarily characterizes the behavior of amorphous polymers, including the amorphous segments within semi-crystalline materials.

Effects of High Tg and Low Tg

PCBs with high Tg materials offer superior thermal and mechanical properties, making them suitable for high-temperature applications like lead-free soldering, automotive, and aerospace electronics. High Tg materials provide increased dimensional stability, reduced delamination risk, and improved reliability under harsh conditions. 

Some polymers are used below their Tg like Polystyrene, Poly (methyl methacrylate) They are hard and brittle because their Tg is higher than the room temperature on these polymers. 

Materials with low Tg values are more susceptible to softening and dimensional changes at elevated temperatures. PCBs with low Tg materials may experience solder joint failures, warping, and reduced mechanical strength, particularly in high thermal stress environments. 

Difference between Tg and Tm

In crystalline polymers or crystalline zones in semi-crystalline polymers, the polymer chains obtain enough thermal energy at a certain point temperature to move past each other at a significant speed at the molecular level. The temperature at which all chains start moving is called the melting point (Tm), which is higher than Tg. 

Melting is the property of the crystalline zones or polymers, while Tg is the property of the amorphous zones or polymers. Below Tg, the disordered amorphous solid has polymer chains with frozen movement but above Tg, the molecules begin to vibrate. The weaker and fewer the polymer chain links are, the higher the Tg will be. On the other hand, below Tm lies an ordered crystalline solid which becomes disordered melt above Tm.

Tm vs. Tg graphs

Various factors that influence the glass transition temperature (Tg) of polymers: 

• Chemical Structure: The chemical composition of polymers plays a crucial role in determining Tg. Different monomers and their arrangement within the polymer chain can affect the overall Tg.
• Molecular Weight: In straight-chain polymers, higher molecular weight leads to reduced chain end concentration, resulting in decreased free volume at chain ends. This reduction in free volume contributes to an increase in Tg.
• Molecular Structure: The insertion of bulky or inflexible side groups within the polymer chain restricts molecular mobility, thereby increasing Tg.
• Chemical Cross-linking: Introducing cross-links between polymer chains reduces their mobility, leading to a decrease in free volume and consequently an increase in Tg.
• Polar Groups: The presence of polar groups enhances intermolecular forces, interchain attraction, and cohesion within the polymer matrix. This reduction in free volume due to increased intermolecular interactions results in an elevation of Tg.

Glass transition temperatures of some common materials:

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