Hello to all our future university students!
Welcome to our summary of "Polymers," a topic that appears frequently in the A-Level Applied Science exam. This topic isn't difficult at all because it’s all about the objects around us, from plastic bags and clothes to car tires, and even the starch we eat!
If you feel that chemistry or physical science is tough, don’t worry! I’ve broken down the content into easy-to-digest language with handy mnemonics that will help you score big on this chapter. Are you ready? Let’s go!
1. What is a polymer? (Understanding the basic structure)
Think of a "necklace of beads." Each individual bead is called a monomer. When we string many beads together into a long chain, we call it a polymer.
In short: Monomer (small unit) + Monomer + ... = Polymer (long chain) This process of connecting them is called "polymerization."
Did you know? The prefix Poly means "many" and Mer means "part." Put them together, and you get "a substance made of many parts joined together!"
2. Types of polymers (by origin)
We can categorize polymers into two main types based on where they come from:
1. Natural polymers: Occur naturally, such as starch, cellulose (plant cell walls), protein (meat), natural rubber, and DNA.
2. Synthetic polymers: Man-made in factories, such as all types of plastics, nylon, and synthetic rubber.
Key Point: Exam questions often ask which substances are natural polymers. Don't forget that "silk" and "cotton" are also polymers!
3. Polymer structures (The heart of this chapter!)
Different structures result in different properties (hardness, flexibility). They are divided into 3 types:
1) Linear Polymer
Like electrical wires laid side-by-side, packing tightly together. - Properties: High density, tough, high melting point, translucent. - Examples: PVC (water pipes), High-Density Polyethylene (HDPE - milk jugs, water buckets).
2) Branched Polymer
Like tree branches extending out, preventing the main chains from getting too close (creating empty spaces). - Properties: Low density, low melting point, flexible, clear. - Examples: Low-Density Polyethylene (LDPE - cold storage bags, food wrap).
3) Cross-linked Polymer
Like a net connected tightly in all directions. - Properties: Very hard, excellent heat resistance, "but if it gets too hot, it will break or burn (it will not melt)." - Examples: Melamine (tableware), Bakelite (pot handles, electrical switches).
Key Tip (How to remember): - Linear/Branched: When heated, they "melt" and can be reshaped. - Cross-linked: When heated, they "do not melt"—they just burn or shatter (hard as a rock!).
4. Thermoplastics vs. Thermosetting Plastics
Once you know the structure, we can classify plastics by how they react to heat:
1. Thermoplastic: - Derived from linear or branched structures. - Properties: Softens when heated, hardens when cooled. "Recyclable," can be melted and reshaped multiple times. - Examples: PET water bottles, plastic bags, PVC pipes.
2. Thermosetting plastic: - Derived from cross-linked structures. - Properties: Strong, durable, handles very high heat and pressure. "Cannot be melted." If exposed to extreme heat, it will char or turn to ash. - Examples: Melamine plates, electrical plugs, car tires (which have undergone vulcanization).
Common Pitfall: Many people assume all plastics can be recycled by melting. In reality, "thermosetting plastics cannot be melted!"
5. Rubber and Recycling Symbols
Natural Rubber
The monomer for natural rubber is "isoprene." - Raw rubber is soft and sticky when hot. - Rubber quality improvement: This is called vulcanization, which involves adding sulfur to create bonds between the polymer chains (making the rubber stable and more elastic).
Recycling symbols you should know (7 types)
1. PETE (1): Clear water bottles. 2. HDPE (2): Milk jugs, shampoo bottles (somewhat opaque). 3. PVC (3): Water pipes, hoses. 4. LDPE (4): Soft plastic bags, cling film. 5. PP (5): Food containers, hot-fill bags (good heat resistance). 6. PS (6): Foam, brittle plastic boxes. 7. OTHER (7): Others, such as polycarbonate.
6. Summary (Key Takeaways)
Memorize these before the exam: - Monomer is a small unit; Polymer is a long chain. - Linear/Branched = Thermoplastic = Meltable = Recyclable. - Cross-linked = Thermosetting = Very hard = Non-meltable (burns). - Vulcanization = Adding sulfur to rubber = Bouncier and stronger rubber. - Starch/Cellulose/Protein/Natural Rubber = Natural polymers.
If you understand the 3 structures (linear, branched, cross-linked), you’ll be able to answer almost every question regarding plastic properties!
Keep it up! Polymers aren't far from your daily life. Try looking around the room right now—how many types of polymers do you see? Connecting this to real life will help you remember it better!