"What is Tg, Tm, and Tc in Polymers: Explained with Definitions, Factors and Important aspects to remember"

Introduction: Why Tg, Tm, and Tc Matter in Polymers and Textile Science

Synthetic fibres reveal different forms of behavior, can be handled through several techniques, and have various uses due to their main thermal properties: Tg, Tm, and Tc from polymer science and fibre technology. These are some key and basic terms that everybody in the textile field should understand.

These thermal transitions control how a polymer behaves during:

• Fibre extrusion
• Drawing and crystallisation
• Heat setting and thermal finishing
• End-use performance (like dimensional stability and strength)

For students preparing for competitive exams such as GATE (Textile Engineering) or those entering textile industries, understanding these parameters is not just optional — it is foundational.

In this post, we will explore:

1. The definitions of Tg, Tm, and Tc
2. The physical changes occurring at each point
3. Molecular-level insights into what governs these transitions
4. Why these properties are crucial in fibre manufacturing and finishing
5. Common exam traps, values, and interpretation for GATE-level preparation

What is Tg (Glass Transition Temperature)?

As soon as the temperature passes the Tg, the polymer gets into a rubbery and flexible state below the Tg, it is stiff and non-flexible. The change happens only in polymers that are amorphous or partially amorphous polymers.

Unlike melting, the glass transition is not associated with latent heat. It is a second-order transition, meaning there is no abrupt change in enthalpy or volume — but there is a change in specific heat, thermal expansion, and mechanical properties.

At molecular level:

Below Tg, the polymer chains are “frozen” in position. They can vibrate, but there is no segmental motion.

Above Tg, chain segments gain enough energy to begin limited motion, making the material more flexible and less brittle.

Examples of Tg in textile polymers:

• Polyethylene terephthalate (PET): Tg ≈ 70–80°C
• Nylon 6: Tg ≈ 45–60°C
• Polyacrylonitrile: Tg ≈ 85–100°C

Tg determines the service temperature of a fibre. If a textile is exposed to temperatures above its Tg, its mechanical properties like strength, stiffness, and shape retention may deteriorate.

What is Tm (Melting Temperature)?

The temperature referred to as the melting temperature (Tm) is the point where the crystalline part of a polymer turns from solid to liquid. During Tm, latent heat is absorbed and there is a noticeable change in the material’s volume.

It occurs only in semi-crystalline polymers where orderly packed crystalline zones exist. At Tm, these zones lose their lattice arrangement and the material flows.

At molecular level:

Melting occurs when thermal energy disrupts intermolecular forces (such as hydrogen bonding or van der Waals forces) holding the crystalline structure.

The amorphous regions become mobile before Tm is reached, but crystallites resist flow until this critical point.

Typical Tm values:

• PET: Tm ≈ 255–265°C
• Nylon 6: Tm ≈ 220–225°C
• Polypropylene: Tm ≈ 160–170°C

Tm is the most important parameter in fibre spinning processes. The polymer must be heated above Tm to allow extrusion through spinnerets and to ensure proper fibre formation during melt spinning.

In GATE-level questions, the distinction between glass transition and melting is often tested with respect to:

Molecular weight

Crystalline vs amorphous behaviour

Heat flow in Differential Scanning Calorimetry (DSC)

📌 Notes:

• Acrylics and aramids do not have sharp melting points; they decompose before melting, making them unsuitable for melt spinning.

• Thermoplastic fibres like PET, PP, and Nylon have well-defined Tg and Tm and are processed using melt extrusion.

• These values can vary slightly depending on molecular weight, additives, or processing history.

What is Tc (Crystallisation Temperature)?

The crystallisation temperature (Tc) is the temperature at which a polymer transitions from the amorphous to crystalline phase during cooling. This transition typically occurs during fibre solidification after extrusion or melt spinning.

Tc is most relevant in semi-crystalline polymers, where ordered molecular arrangements begin to form when the polymer cools below Tm.

What happens at Tc?

When a molten polymer is cooled, it passes through Tc. At this point, nuclei form, and polymer chains start to align and fold into ordered, crystalline regions. The rate and degree of crystallisation affect the final fibre's:

• Strength
• Dimensional stability
• Dye uptake
• Moisture regain

Unlike Tm, which is measured during heating, Tc is observed during cooling using techniques like Differential Scanning Calorimetry (DSC).

Examples of Tc in Common Fibres:

• PET (Polyester): Tc ≈ 180–200°C
• Nylon 6: Tc ≈ 180–190°C
• Polypropylene: Tc ≈ 110–125°C

Note: Tc is not always fixed — it depends on:

• Cooling rate
• Molecular weight
• Presence of nucleating agents or plasticisers

In fibre production, controlled cooling through Tc is used to increase crystallinity and improve the mechanical performance of the final product.

Conceptual Differences Between Tg, Tm, and Tc

To avoid confusion in exams or practical use, here’s a clear conceptual understanding (without table formatting):

Tg (Glass Transition Temperature)

• Occurs in amorphous regions
• Transition from rigid to flexible
• Second-order transition (no heat absorbed)
• Important for service behaviour of the fibre

Tm (Melting Temperature)

• Occurs in crystalline regions
• Solid to liquid transition
• First-order transition (heat absorbed)
• Important for fibre formation and thermal processing

Tc (Crystallisation Temperature)

• Happens during cooling
• Polymer chains align into crystalline zones
• Determines crystallinity, shrinkage, and dyeability
• Important in drawing, heat setting, and annealing

Factors Affecting Tg, Tm, and Tc in Polymers

These values (Tg, Tm, and Tc) for polymers can vary from one to another. Both molecular properties as well as the structure of polymers and the temperature of the environment play a part in these thermal responses.

Knowing these factors matters in preparation for exams such as GATE and university viva but also for handling and making fibres in various industries.

Allow me to explain what each of them is.

1. Molecular Weight

• Tg goes up as the molecular weight increases but up to a certain level then no change occurs. When a polymer’s chain becomes longer, it needs to use extra energy to move each site along it.
• Tm tends to increase a small amount with molecular weight, not as much as Tg does.

So, the higher the molecular weight of a PET, the higher its Tg is expected to be.

2. Chain Flexibility

• Chains with solid backbones or heavy side groups blocks how much the material can stretch, so its Tg is greater.
• The low Tg of flexible polymers like polyethylene happens due to freer movement of their segments.

3. Intermolecular Forces

• An increase in intermolecular forces (for example, hydrogen bonding in nylons) causes both the glass transition temperature and melting point to rise.
• Nylon 6 is different from polyethylene since the hydrogen bonds between amide groups cause it to have both higher Tg and Tm.

4. Crystallinity

• Sharper and higher Tm is usually seen in highly crystalline polymers.
• Amorphous materials do not indicate a Tm temporally, only Tg can be found (for example in polystyrene).

What’s more, crystalline polymers usually become crystalline faster as they cool, so their Tc goes up.

5. Plasticizers

• Because of plasticizers, the molecules in the plastic matrix move more freely.
• Because of this, the Tg value is lowered. There could be a small drop in Tm because of compatibility.

The flexibility of PVC changes along with its Tg when using different types of plasticisers (like using rigid additives for piping, and flexible additives for cables).

6. Tacticity

• How the side groups are placed on the polymer chain affects the way the chains can pack and become crystallised.
• Since isotactic polymers like isotactic polypropylene are well crystallized, they have higher Tm.
• Atactic forms look like a random mass and have only the Tg phase.

7. Branching and Cross-Linking

• Branching leads to less crystallinity, which normally results in drops in Tm and Tg.
• Tg is increased a lot through cross-linking, as the movement of the chains slows down significantly.

Polyester nano composite fibers resist high temperatures due to their Tg values usually being higher.

8. Copolymerization

• Adding another monomer in the copolymer blends can affect the regularity of the chains.
• Because of this, the polymer can become less crystalline, turning more amorphous and so altering Tm as well as raising or lowering Tg based on the comonomer used.

PBT (polybutylene terephthalate) behaves differently from the standard PET used in fibres since it is a copolyester.

9. Cooling Rate (for Tc only)

• A rapid temperature drop during cooling stops the polymer from forming some ordered crystals, which means Tc is lower when it cools rapidly.
• If moisture is slowly removed from the liquid, it makes the sugar molecules organize better for stronger crystals.

This becomes very important in the melt spinning and annealing stages since cooling controls the characteristics of the fibres.

Summary Insight:

Tg, Tm, and Tc are not only basic properties given by each substance. Their behaviors are affected by the structure, how they process information, and their surroundings. Being able to manage these factors is very important in making fibres and polymers.

Why Tg, Tm, and Tc Matter in Textile Processing

Thermal properties are not just academic—they directly impact how textiles are spun, dyed, finished, and even worn.

• During melt spinning, the polymer must be heated above Tm to flow and extrude through spinnerets.
• During drawing and orientation, cooling through Tc ensures proper crystallinity and dimensional stability.
• During heat setting, temperature must be above Tg but below Tm to relax stresses without melting the fibre.
• In garment use, the material should maintain its structure below Tg, or else it will lose shape and strength.

For example, if polyester is not properly heat set above Tg, the final fabric may shrink or wrinkle during ironing or washing.

Even fabric finishing techniques like calendaring, embossing, and crimping are heavily dependent on fibre Tg and Tm. A minor error in setting temperature may lead to fibre distortion, yellowing, or even surface melting.

🎓GATE & Competitive Exam Insights

If you're preparing for GATE (TF) or university-level technical exams, here’s what you should keep in mind:

1. Know the typical Tg and Tm values of common fibres — these are often directly asked.
2. Understand the sequence: Tg < Tc < Tm for semi-crystalline polymers.
3. Pay attention to definitions and differences — examiners often test clarity between amorphous and crystalline behaviour.
4. Learn how processing conditions affect Tc — such as cooling rate or drawing ratio.
5. Avoid the myth that all polymers have a clear melting point. PAN and aramids, for example, do not melt—they degrade.
6. DSC (Differential Scanning Calorimetry) curves are sometimes included in questions — know how to interpret peaks (Tm) and shifts (Tg).

A solid grasp of these thermal transitions gives you an edge not only in exams but also in interviews, internships, and technical writing.

📌 Final Takeaway

Tg, Tm, and Tc are far more than theoretical values. They define the behaviour, stability, and processability of polymers in every stage of textile manufacturing.

Whether you're spinning yarns, engineering smart fabrics, or preparing for GATE, your understanding of these thermal transitions determines how confidently you work with materials. As the textile industry moves towards smarter and more sustainable fibres, having a command over these fundamentals will place you far ahead in both academic and industrial careers.

🔗 Internal Links

• Polymerisation in Nylon 6 and Nylon 66: Explained Simply
• Lyocell Fibre: The Future of Sustainable Textiles
• Yarn Count Explained: Linear Density Systems in Textiles

FAQ Section

Q1: Why do some fibres like acrylic not have a sharp melting point?

Because they degrade thermally before reaching a melting point. Such fibres require solvent or gel spinning techniques instead of melt spinning.

Q2: What happens if a fibre is heated above Tg but below Tm?

The fibre becomes soft and flexible but does not melt. This temperature range is often used for heat setting or thermal relaxation.

Q3: Which thermal transition affects dye uptake the most?

Crystallinity, which is related to Tc, significantly affects dye diffusion. Higher crystallinity often reduces dyeability in synthetic fibres.

Q4: Is Tc observed in amorphous fibres?

No. Only semi-crystalline polymers exhibit Tc during controlled cooling. Fully amorphous fibres do not crystallise.