Tg and Melting Compared

Introduction

Glass transition and melting can both soften a polymer, but they involve different structures and different thermal signals. T g concerns mobility in amorphous regions; T m concerns loss of crystalline order. A semicrystalline sample may show both on one heating scan.

Core explanation

At T g, cooperative motions of amorphous chain segments become accessible on the measurement timescale. Calorimetry commonly shows a step in heat capacity rather than a sharp latent-heat peak. Because the material can relax and age, T g is often described as second-order-like, not a strict equilibrium second-order phase transition. At T m, polymer crystallites lose long-range order and absorb latent heat, producing an endothermic melting peak in differential scanning calorimetry. Melting is first-order in the ordinary thermodynamic description of crystalline order loss, though polymer crystals often contain imperfections and a range of lamellar thicknesses, so the peak may be broad. A fully amorphous polymer can show T g but no crystalline T m. An idealized fully crystalline portion has melting behaviour, but real polymer samples usually also contain amorphous material. In a semicrystalline sample, heating above T g makes amorphous regions mobile while crystallites still act as physical anchors. Only when those crystallites melt at higher temperature can the material flow more freely, provided it is not chemically cross-linked. Cooling history can shift crystallinity and melting peak shape, while heating rate can shift apparent T g. Reporting both temperatures without identifying sample morphology can therefore be misleading.

Step-by-step reasoning

Inspect whether the polymer is amorphous or semicrystalline. On a DSC heating trace, mark the baseline step as T g and the endothermic peak as melting. Describe what phase changes at each temperature. If a test temperature lies between them, expect mobile amorphous regions but persistent crystalline regions.

Visual explanation

Sketch a DSC curve with a small baseline offset at lower temperature and a larger endothermic peak at higher temperature. Draw disordered chain segments becoming mobile at the first event, while ordered lamellae disappear at the second.

Real-world analogy

Softening butter and melting an ice cube are not equivalent processes; one may involve gradual mobility changes while the other consumes heat to destroy crystalline order. In polymers, T g and T m likewise require separate descriptions.

Real-world example

A semicrystalline polyethylene sample can retain crystalline anchors above the mobility transition of its amorphous regions, then lose those anchors on melting. This distinction matters when choosing a processing temperature or service range.

Why?

Amorphous segments need thermal motion to relax but do not possess crystal order to melt. Crystallites have ordered packing whose loss requires fusion heat. Those different physical changes produce different calorimetric signatures.

Common misconception

A step at T g should not be labelled a melting peak. Likewise, a polymer above T g is not necessarily a freely flowing liquid: crystallites or chemical cross-links may still restrict motion.

Worked example

Question: A DSC heating curve shows a baseline step at 70 °C and an endothermic peak at 180 °C. Interpret them for a semicrystalline polymer. Reasoning: A step indicates increased heat capacity from amorphous segmental mobility; an endothermic peak indicates loss of crystal order. Answer: T g is near 70 °C and T m near 180 °C under the stated scan conditions.

Quick check

1. Which transition absorbs a latent heat of fusion, T g or T m? Answer: T m, the melting of crystalline regions.

Exam focus

Identify the thermal signal and the corresponding phase. If asked for exact values, mention that heating rate and sample history influence apparent transition positions and breadth.

Advanced insight

Lamellar thickness distribution can produce multiple or broad melting features, and cold crystallisation may occur during heating between T g and T m. A careful DSC interpretation must distinguish such exothermic ordering from the endothermic melting event.

Summary

T g is a time-dependent mobility transition of amorphous segments, usually seen as a heat-capacity step. T m is loss of crystalline order with latent heat, usually an endothermic peak. Semicrystalline polymers can show both, with crystallites persisting after the amorphous phase passes T g.

Practice questions

1. What DSC feature signals T g? Answer: A heat-capacity baseline step or shift.

2. What feature signals crystal melting? Answer: An endothermic peak associated with latent heat of fusion.

3. Can a completely amorphous polymer have T g but no T m? Answer: Yes, because it has amorphous segments but no crystallites to melt.

4. Does passing T g guarantee a semicrystalline polymer flows freely? Answer: No. Crystalline regions can remain as physical anchors until they melt.