--- title: "Glass Transition, Crystallinity and Entanglement in Polymers" description: "Polymers may contain ordered and amorphous regions. Segment mobility controls the glass transition, crystal order controls melting, and chain entanglements slow melt flow. Molecular structure and ther" canonical: https://lightmysky.com/learn/science/glass-transition-crystallinity-and-entanglement-in-polymers-mt_vGEYO5ILqt source: https://lightmysky.com/learn/science/glass-transition-crystallinity-and-entanglement-in-polymers-mt_vGEYO5ILqt.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Glass Transition, Crystallinity and Entanglement in Polymers Polymers may contain ordered and amorphous regions. Segment mobility controls the glass transition, crystal order controls melting, and chain entanglements slow melt flow. Molecular structure and thermal history affect these properties. Subject: Science · Area: Chemistry · Ages 23 to 24 Page: https://lightmysky.com/learn/science/glass-transition-crystallinity-and-entanglement-in-polymers-mt_vGEYO5ILqt ## Ready when they can - Explains what changes at the glass transition and why it is not a first-order transition - Explains how restricted segment motion can raise the glass transition temperature, with limits on predictions from side-group bulk or polarity alone - Distinguishes entanglement molar mass from the viscosity crossover and explains the steeper molar-mass dependence of zero-shear melt viscosity in the Newtonian regime - Identifies glass transition, crystallisation and melting from stated features of a differential scanning calorimetry trace ## Lesson: When plastics turn stiff or soft Polymers can be fully amorphous or partly crystalline. At the glass transition, Tg, motion of amorphous chain segments becomes fast enough on the observation timescale, and those regions soften. Crystals do not melt at Tg. This kinetic change has a heat-capacity change, not the latent heat of equilibrium melting. Measured Tg depends on timescale and thermal history. A broad temperature range alone does not distinguish Tg from polymer melting. Rigid backbones and interactions that restrict segment motion often raise Tg. Bulky side groups can restrict motion, but also change packing and free volume. Polarity alone cannot fix a Tg ranking. Compare measured values: amorphous regions below Tg are glassy; above Tg they are more mobile on the relevant timescale. Crystallinity and crosslinks also affect whole-sample stiffness. *(drawing: This change concerns amorphous regions. Crystalline regions melt in a separate process.)* Entanglements are temporary constraints between long chains that slow relaxation, not new bonds. Me describes molar mass between entanglements; the viscosity crossover Mc is related but need not equal Me. Compare similar linear-polymer melts at the same temperature using zero-shear viscosity, the low-rate Newtonian plateau where viscosity is independent of shear rate. Above Mc it rises more steeply with molar mass, without a jump. At higher shear rates, entangled melts can shear-thin. Differential scanning calorimetry, DSC, measures heat flow during a temperature scan. Tg usually appears as a baseline step because heat capacity changes. Crystallisation releases heat; melting absorbs heat. Check whether the plot draws heat absorption upward or downward before assigning peak direction. A heating scan need not show all three events. A sample that remains fully amorphous has no crystalline melting peak. **Example.** Suppose a heating scan has a baseline step near 50 C, heat release near 100 C and heat absorption near 150 C. These features are consistent with Tg, cold crystallisation and melting. Another sample may show a Tg step and a melting peak without a crystallisation peak: crystals present before heating can melt without a separate detectable event for new crystal formation. *(drawing: These are the stated observations in the example, not a required sequence for every polymer.)* **Recap.** Tg concerns amorphous segment motion; melting concerns crystals. Entanglements slow melt flow. Interpret DSC features using the plot convention and sample history. *Original lesson, questions, worked solutions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Controlled Radical Polymerisation and Block Architecture](https://lightmysky.com/learn/science/controlled-radical-polymerisation-and-block-architecture-mt_nQKYR0yEEs) ## Optional connections - [Phase Diagrams, the Clapeyron Equation and the Phase Rule](https://lightmysky.com/learn/science/phase-diagrams-the-clapeyron-equation-and-the-phase-rule-mt_U-zj6r4WFM)