In one line
In crystalline regions polymer chains are packed in an ordered, regular way, while in amorphous regions they are randomly coiled and tangled; most polymers are semi-crystalline, with both regions present.
Explained simply
Think of a drawer of socks. If the socks are neatly folded and stacked, that is crystalline: compact, dense and hard to disturb. If they are thrown in randomly, that is amorphous: loose and tangled. A polymer can have some neatly folded parts (crystallites) inside a tangled matrix, like a mostly messy drawer with a few tidy corners.
How it works
- In crystalline regions chains fold back and forth and line up in parallel (lamellae, often organised as spherulites). Strong, regular inter-chain forces act.
- In amorphous regions chains are randomly coiled with no long-range order.
- Degree of crystallinity = fraction (%) of the polymer that is crystalline. It is measured by X-ray diffraction, density or DSC.
Factors favouring crystallinity:
- Linear, regular chains (HDPE is more crystalline than branched LDPE).
- Stereo-regularity: isotactic and syndiotactic polymers crystallise; atactic usually do not (atactic polystyrene is amorphous).
- Strong inter-chain forces, especially hydrogen bonding (nylons).
- Small or no bulky side groups.
- Slow cooling and stretching (drawing of fibres) increase crystallinity; cross-linking and random copolymerisation reduce it.
Key formula
\( \%\,\text{crystallinity} = \dfrac{\rho_c(\rho_s - \rho_a)}{\rho_s(\rho_c - \rho_a)} \times 100 \)
\( \rho_s \) = density of sample, \( \rho_c \) = density of fully crystalline polymer, \( \rho_a \) = density of fully amorphous polymer (all in g/cm3). (Shown for understanding; first-year questions are usually descriptive.)
| Crystalline | Amorphous |
|---|---|
| Ordered chain packing | Random coiled chains |
| Sharp melting point Tm | No sharp melting point; softens over a range above Tg |
| Higher density, strength, stiffness, chemical resistance | Lower density; more flexible |
| Usually translucent or opaque (crystallites scatter light) | Usually transparent |
| e.g., HDPE, nylon 6,6, isotactic PP | e.g., atactic PS, PMMA, PC |
Quick example
Milk cans made of HDPE are stiff and opaque; transparent acrylic sheets (PMMA) are amorphous.
Common mistakes
- Saying a polymer is 100% crystalline; real polymers are semi-crystalline.
- Assigning Tg only to crystalline polymers; Tg belongs to amorphous regions, Tm to crystalline regions.
- Claiming crystalline polymers are transparent.
How MU asks it
"Distinguish between crystalline and amorphous polymers" (3 marks) or "What is crystallinity? Explain factors affecting it" (5 marks). For full marks: definition, a sketch of ordered and random chains, 4–5 comparison points, and factors with examples.