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chemistry

Glass transition

Glass transition is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Glass transition rather than just read about it. In short: The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or "rubbery" state as the temperature is increased. An amorphous solid that exhibits a glass transition is called a glass.

Glass transition — main illustration
Glass transition — illustration

Key takeaways

  • Glass transition belongs to chemistry; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Glass transition to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Glass transition from memory before moving on to harder problems.

Reference excerpt

The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or "rubbery" state as the temperature is increased. An amorphous solid that exhibits a glass transition is called a glass. The reverse transition, achieved by supercooling a viscous liquid into the glass state, is called vitrification. The glass-transition temperature Tg of a material characterizes the range of temperatures over which this glass transition occurs (as an experimental definition, typically marked as 100 s of relaxation time). It is always lower than the melting temperature, Tm, of the crystalline state of the material, if one exists, because the glass is a higher energy state (or enthalpy at constant pressure) than the corresponding crystal. Hard plastics like polystyrene and poly(methyl methacrylate) are used well below their glass transition temperatures, i.e., when they are in their glassy state. Their Tg values are both at around 100 °C (212 °F). Rubber elastomers like polyisoprene and polyisobutylene are used above their Tg, that is, in the rubbery state, where they are soft and flexible; crosslinking prevents free flow of their molecules, thus endowing rubber with a set shape at room temperature (as opposed to a viscous liquid). Despite the change in the physical properties of a material through its glass transition, the transition is not considered a phase transition; rather it is a phenomenon extending over a range of temperature and defined by one of several conventions. Such conventions include a constant cooling rate (20 kelvins per minute (36 °F/min)) and a viscosity threshold of 1012 Pa·s, among others. Upon cooling or heating through this glass-transition range, the material also exhibits a smooth step in the thermal-expansion coefficient and in the specific heat, with the location of these effects again being dependent on the history of the material. The question of whether some phase transition underlies the glass transition is a matter of ongoing research.

… excerpt ends here. Continue reading the full article.

Illustrations

Glass transition: Two-dimensional, schematic, representation of the lattices of quartz (a), silica (b), and of silica based glasses (c).[1]
Two-dimensional, schematic, representation of the lattices of quartz (a), silica (b), and of silica based glasses (c).[1]
Glass transition: Determination of Tg by dilatometry.
Determination of Tg by dilatometry.
Glass transition: Measurement of Tg (the temperature at the point A) by differential scanning calorimetry
Measurement of Tg (the temperature at the point A) by differential scanning calorimetry
Glass transition: Specific heat of several noncrystalline solids, plotted as ( T 2 , c / T ) {\displaystyle (T^{2},c/T)} graph, showing linear dependence component in the low-temperature regime.[47]
Specific heat of several noncrystalline solids, plotted as ( T 2 , c / T ) {\displaystyle (T^{2},c/T)} graph, showing linear dependence component in the low-temperature regime.[47]
Glass transition: Entropy difference between crystal and undercooled melt
Entropy difference between crystal and undercooled melt

Worked examples

Example 1 — a first encounter with Glass transition

Start with the simplest possible case. Write down what Glass transition claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Glass transition before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Glass transition ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Glass transition

In research
Glass transition appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Glass transition in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Glass transition is common in secondary-school and first-year university syllabi. It links to neighbouring topics Cryobiology, Glass engineering and science, Glass physics, so understanding it makes those chapters shorter.
In everyday life
Look for Glass transition outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Glass transition in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Glass transition means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Glass transition out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Glass transition in simple terms?

The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or "rubbery" state as the temperature is increased. An amorph…

Why does Glass transition matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Glass transition?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Glass transition.

Tags

  • Cryobiology
  • Glass engineering and science
  • Glass physics
  • Phase transitions
  • Polymer chemistry
  • Rubber properties
  • Threshold temperatures

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