Polymer Glass Transition Temperature (Tg) Estimator

What Is Glass Transition Temperature (Tg)?

The glass transition temperature, almost always written as Tg, is the temperature at which an amorphous (non-crystalline) polymer switches between two very different physical states. Below Tg, the polymer chains are essentially frozen in place — the material is hard, stiff, and glass-like. Above Tg, the chains gain enough thermal energy to wiggle, slide past each other, and rotate around their backbone bonds, so the material becomes soft, flexible, and rubbery.

This isn't a sharp melting point like ice turning to water — there's no change in crystal structure and no defined heat of fusion. Instead, Tg is a gradual softening that shows up as a step-change in properties like stiffness, heat capacity, and volume, which is why it's usually measured with techniques like differential scanning calorimetry (DSC) or dynamic mechanical analysis (DMA) rather than simply watching a sample melt.

This calculator estimates Tg three different ways: the Fox equation for blends and copolymers with any number of components, the Gordon-Taylor equation for two-component blends where interaction effects matter, and the Flory-Fox equation for how a single polymer's own Tg changes with its molecular weight. It also includes a quick-reference library of Tg values for 15+ common polymers.

The Fox Equation for Blends and Copolymers

When two or more polymers are miscible — meaning they mix at the molecular level rather than separating into distinct phases — or when a copolymer is built from two or more different monomers distributed randomly along the chain, the resulting material shows a single, intermediate Tg somewhere between the Tg values of its individual components. The most widely used formula for predicting that blended Tg is the Fox equation:

• 1 / Tg(blend) = w1/Tg1 + w2/Tg2 + w3/Tg3 + ... (every Tg in Kelvin)
• wi is the weight fraction of component i, and all wi must add up to 1

How to Use the Fox Equation Mode

The 'Blend / Copolymer Tg (Fox Equation)' mode above lets you add as many components as your blend or copolymer needs. For each one, enter its individual Tg in Celsius and its weight — you can type in fractions, percentages, or arbitrary parts (like a recipe), and the calculator normalizes everything to add up to 100% automatically, so you don't need to do that math yourself first.

Quick-fill buttons for common polymers like polystyrene, polypropylene, and PMMA are built right into each component row, so you can build a realistic blend without having to look up Tg values elsewhere. The result shows the blended Tg in both Celsius and Kelvin (since the underlying formula only works correctly on an absolute temperature scale), along with a plain-language read on whether that blend will feel rubbery, glassy, or somewhere in between at everyday room temperature.

It's worth remembering that the Fox equation assumes the components are fully miscible and that there's no specific interaction (like hydrogen bonding) between them — it's a good first estimate, but real blends can deviate from it, especially when the two polymers interact strongly or only partially mix.

The Gordon-Taylor Equation

The Gordon-Taylor equation is a close cousin of the Fox equation, built specifically for two-component blends, and it adds one extra piece: a constant, K, that accounts for how strongly the two components interact and how their free volumes differ. When K happens to equal the ratio of the two components' Tg values in Kelvin, the Gordon-Taylor equation reduces to exactly the same answer as the Fox equation — so Fox is really just a special case of Gordon-Taylor.

• Tg(blend) = (w1·Tg1 + K·w2·Tg2) / (w1 + K·w2)
• K is usually found by fitting the equation to real experimental Tg data for a specific blend system

When to Use Gordon-Taylor Instead of Fox

Use the 'Two-Component Blend (Gordon-Taylor)' mode above when you already have (or want to test) a fitted K constant for a specific pair of polymers — for example, from a published paper or your own DSC data on that blend system. Because K can be tuned, this equation generally fits real experimental blend Tg data more closely than the simpler Fox equation, especially for blends where the components interact through hydrogen bonding or other specific interactions rather than mixing ideally.

If you don't have a fitted K value for your specific system, a reasonable starting estimate is K ≈ 1, which makes the equation behave close to a straight weighted average of the two Tg values — or you can enter the ratio Tg1(K)/Tg2(K) to approximate the Fox equation result instead.

The Flory-Fox Equation: Molecular Weight's Effect on Tg

Everything above assumes each individual polymer's Tg is a single fixed number — but that's only true for high-molecular-weight polymers. For lower-molecular-weight samples, Tg actually depends on chain length, because shorter chains have proportionally more free chain-end volume, which makes it easier for segments near the ends to move. As a result, Tg rises as molecular weight increases, then levels off at a plateau value once the chains get long enough. The Flory-Fox equation describes exactly this relationship:

• Tg(Mn) = Tg-infinity − K / Mn
• Tg-infinity is the plateau Tg reached at very high molecular weight
• K is a polymer-specific constant, usually determined experimentally

How to Use the Flory-Fox Mode

The 'Molecular Weight Effect (Flory-Fox)' mode above needs three inputs: the high-molecular-weight plateau Tg (Tg-infinity) for your polymer, the K constant for that specific polymer, and the number-average molecular weight (Mn) of your actual sample. It then reports the predicted Tg for that specific molecular weight, along with the 'Tg depression' — how many degrees lower that sample's Tg sits compared to the high-MW plateau value.

This is especially useful for oligomers, low-molecular-weight resins, and any polymer sample where Mn is in the low thousands or less, since that's exactly the range where the Flory-Fox correction matters most. Once Mn climbs into the tens of thousands, the K/Mn term becomes small and Tg approaches Tg-infinity closely enough that this correction usually isn't needed.

Typical Glass Transition Temperatures for Common Polymers

Tg varies enormously across different polymer families, from well below freezing for soft elastomers to hundreds of degrees Celsius for high-performance engineering plastics. The figures below are commonly published typical values for the unblended, high-molecular-weight homopolymer — actual Tg can shift with tacticity, plasticizer content, crosslink density, and measurement method, so always confirm with a manufacturer datasheet or your own DSC data when precision matters.

• Polydimethylsiloxane (PDMS / Silicone Rubber): approximately -125°C
• Polyethylene, low density (LDPE): approximately -125°C
• Natural Rubber (cis-Polyisoprene): approximately -70°C
• Polyethylene, high density (HDPE): approximately -100°C
• Polybutadiene Rubber: approximately -90°C
• Polypropylene (PP, atactic): approximately -10°C
• Poly(vinyl acetate) (PVAc): approximately 32°C
• Nylon 6 (Polyamide 6): approximately 47°C
• Poly(lactic acid) (PLA): approximately 60°C
• Poly(ethylene terephthalate) (PET): approximately 70°C
• Poly(vinyl chloride) (PVC, rigid): approximately 80°C
• Polystyrene (PS): approximately 100°C
• Poly(methyl methacrylate) (PMMA): approximately 105°C
• Polycarbonate (PC): approximately 147°C
• Polysulfone (PSU): approximately 185°C
• Polyetherimide (PEI / Ultem): approximately 217°C
• Polyimide (Kapton-type): approximately 360°C

Why Tg Matters in Polymer Selection and Processing

Glass transition temperature is one of the single most important numbers when choosing or designing a polymer for a real application, because it defines the usable temperature window where a material behaves the way you expect. A polymer used well below its Tg stays rigid and dimensionally stable, which is exactly what you want for structural parts, phone cases, or eyewear lenses — but push the same material above Tg and it can soften, warp, or creep under load.

The opposite is just as important: soft goods like rubber seals, gaskets, and flexible packaging films need a Tg well below their service temperature to stay pliable rather than turning brittle and cracking in cold conditions. Formulators use blending and plasticizers specifically to shift Tg into the target range — a plasticizer added to rigid PVC, for example, can drop its Tg from around 80°C down close to room temperature or below, turning a hard plastic into the flexible vinyl used in tubing, cables, and flooring. Process engineers also watch Tg closely during manufacturing, since operations like injection molding, film extrusion, and thermoforming all need the polymer to be processed well above Tg to flow properly, then cooled back below it to set the final shape.

Who Uses a Glass Transition Temperature Calculator

Polymer chemistry and materials science students use Tg blending equations constantly in coursework covering amorphous polymer behavior, since predicting blend and copolymer Tg is a standard exam and homework topic. Formulation chemists in plastics, adhesives, coatings, and rubber compounding use the Fox and Gordon-Taylor equations to plan new blends and plasticizer systems before running an actual DSC test, saving time and material in early-stage development.

Quality control labs cross-check measured DSC results against Fox-equation predictions to catch composition drift in a production blend. Packaging and product engineers reference typical Tg values, like the ones listed above, when choosing between polymer candidates for a part that needs to stay rigid — or stay flexible — across a specific temperature range.

A Quick Note on Accuracy

The Fox, Gordon-Taylor, and Flory-Fox equations used in this calculator are the standard, widely published relationships taught throughout polymer physics and materials science, and they give reliable first-pass estimates for coursework, formulation planning, and general reference use. Real measured Tg values can differ from these predictions because of factors the equations don't capture directly — partial miscibility, crosslinking, crystallinity, plasticizer content, measurement heating rate, and specific molecular interactions between components.

For a value that will be published, used in a regulatory filing, or relied on for a critical design decision, always confirm with actual DSC or DMA measurement on your specific formulation rather than relying on a predictive equation alone.