A polycrystalline solid is made of many small crystals or grains with different orientations. The textbook also discusses polymers here: large chain molecules formed by repetition of small molecular units.
Learning Outcomes
Polycrystalline solids contain many grains.
Grain orientations are random and separated by boundaries.
Bulk polycrystalline materials are usually isotropic.
Polymers contain repeating molecular units joined into long chains.
Polymers may be natural or synthetic.
Intermediate Level
Polycrystalline Structure
Each crystallite or grain is internally ordered, but neighbouring grains have different orientations and distinct grain boundaries. The textbook gives a typical grain-size range of 10² nm to 10³ nm.
Why Bulk Behaviour Is Isotropic
A single grain may be anisotropic, but randomly oriented grains average their directional properties, so the overall material often behaves similarly in all directions.
Polymers
Polymerization joins many small molecules into large chains containing mainly carbon, hydrogen, oxygen and nitrogen.
Type
Examples
Natural polymers
rubber, resin, wood
Synthetic polymers
polythene, polystyrene, polyvinyl chloride
Typical Polymer Properties
Low specific gravity.
Good strength-to-weight ratio.
Toughness and corrosion resistance.
Poor conduction of heat and electricity.
Low cost.
Applications
Most inorganic metals, many ceramics, rocks and ice are polycrystalline. Polymers are used in packaging, construction, medicine, electrical insulation and lightweight products.
Common Mistakes
Classify a solid from particle arrangement, not appearance alone.
Use crystalline, polycrystalline, polymeric and amorphous precisely.
Support comparisons with structure and physical properties.
Short-Question Preparation
Q. Define a polycrystalline solid.
A. A solid made of many small crystals with random orientations.
Q. What are grains and grain boundaries?
A. Grains are small crystals; grain boundaries separate crystals of different orientation.
Q. What is polymerization?
A. The process of joining repeating small molecules into long chains.
Q. Differentiate polycrystalline and polymeric.
A. Polycrystalline refers to many crystal grains; polymeric refers to chain molecules.
Sticky Notes
Polycrystalline solids contain many grains.
Grain orientations are random and separated by boundaries.
Bulk polycrystalline materials are usually isotropic.
Polymers contain repeating molecular units joined into long chains.
Polymers may be natural or synthetic.
60-Second Revision
A polycrystalline solid is made of many small crystals or grains with different orientations. The textbook also discusses polymers here: large chain molecules formed by repetition of small molecular units.
Summary
A polycrystalline solid is made of many small crystals or grains with different orientations. The textbook also discusses polymers here: large chain molecules formed by repetition of small molecular units.
Ask-the-Tutor Modes
Explain it simply.
Compare the structures.
Give material examples.
Prepare a board-style answer.
Test me with an MCQ.
Did you understand this topic?
Your answer is saved with your learning record. It does not mark the topic as mastered.
Exam preparation
Exam Focus
Important points
Polycrystalline solids contain many grains.
Grain orientations are random and separated by boundaries.
Bulk polycrystalline materials are usually isotropic.
Polymers contain repeating molecular units joined into long chains.
Polymers may be natural or synthetic.
Common mistakes
Base classification on atomic arrangement.
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.
Polycrystalline and Polymeric Solids
Sticky Notes · Quick Revision
1.
Key Point 1
Polycrystalline solids contain many grains.
2.
Key Point 2
Grain orientations are random and separated by boundaries.
3.
Key Point 3
Bulk polycrystalline materials are usually isotropic.
4.
Key Point 4
Polymers contain repeating molecular units joined into long chains.
