Chemistry: Polymers, Plastics, and Materials Science Worksheet

Question 1

Define the terms monomer and polymer. Use polyethylene as an example in your explanation.

Accepted Answer

A monomer is a small molecule that can join with other similar molecules to form a larger molecule. A polymer is a large molecule made of many repeating monomer units. In polyethylene, many ethene monomers join together to form long polyethylene chains.

Question 2

Ethene has the formula C2H4 and can form polyethylene through addition polymerization. Describe what happens to the carbon-carbon double bond during this process.

Accepted Answer

During addition polymerization, the carbon-carbon double bond in ethene opens up. Each carbon atom forms new single bonds with carbon atoms from neighboring ethene molecules, creating a long polymer chain.

Question 3

A polymer sample has an average molar mass of 84,000 g/mol. Its repeating unit has a molar mass of 28 g/mol. Calculate the average degree of polymerization.

Accepted Answer

The average degree of polymerization is 3,000 because 84,000 g/mol divided by 28 g/mol equals 3,000 repeating units per polymer chain on average.

Question 4

Compare thermoplastics and thermosetting plastics. Include one property difference and one example use for each type.

Accepted Answer

Thermoplastics soften when heated and can be remolded, so they are often used for items such as plastic bottles or food containers. Thermosetting plastics harden permanently after curing and do not remelt easily, so they are used for items such as electrical insulators, epoxy adhesives, or heat-resistant handles.

Question 5

Explain how cross-linking affects a polymer's flexibility, melting behavior, and strength.

Accepted Answer

Cross-linking connects polymer chains to each other with chemical bonds. More cross-linking usually makes a polymer stronger, harder, and less flexible. Highly cross-linked polymers do not melt easily because the chains cannot slide past one another freely.

Question 6

Nylon is formed when a diamine reacts with a dicarboxylic acid, releasing small molecules such as water. Identify this as addition polymerization or condensation polymerization and explain your choice.

Accepted Answer

This is condensation polymerization because monomers join together while a small molecule, such as water, is released. Nylon forms through this type of reaction when functional groups on different monomers react.

Question 7

A plastic labeled PET is commonly used in beverage bottles. Describe one useful property of PET for this application and one environmental concern related to PET bottles.

Accepted Answer

PET is useful for beverage bottles because it is lightweight, strong, transparent, and resistant to many liquids. One environmental concern is that PET bottles can persist for a long time if not recycled, contributing to plastic waste and pollution.

Question 8

Explain why low-density polyethylene, LDPE, is usually more flexible than high-density polyethylene, HDPE.

Accepted Answer

LDPE is usually more flexible because its polymer chains have more branching, which prevents the chains from packing closely together. HDPE has less branching, so its chains pack more tightly and form a stronger, more rigid material.

Question 9

A material scientist wants a plastic for a reusable food container that can handle repeated washing, moderate heat, and bending without cracking. List two polymer properties that should be tested and explain why each matters.

Accepted Answer

Thermal resistance should be tested because the container must not soften or deform during washing or exposure to warm food. Toughness or impact resistance should also be tested because the container needs to bend or be dropped without cracking.

Question 10

Explain the difference between biodegradable plastics and recyclable plastics.

Accepted Answer

Biodegradable plastics are designed to be broken down by microorganisms under certain conditions. Recyclable plastics are designed or collected so they can be processed into new materials. A plastic can be recyclable, biodegradable, both, or neither depending on its chemistry and the disposal system.

Question 11

A polymer has a glass transition temperature, Tg, of 80 degrees Celsius. Describe how the material is likely to behave at 25 degrees Celsius and at 100 degrees Celsius.

Accepted Answer

At 25 degrees Celsius, the polymer is below its glass transition temperature, so it is likely to be hard, rigid, or glassy. At 100 degrees Celsius, it is above its glass transition temperature, so it is likely to be softer, more flexible, or rubbery.

Question 12

A recycling facility receives a mixed batch of plastics containing PET, HDPE, PVC, and polystyrene. Explain why sorting plastics by type is important before recycling.

Accepted Answer

Sorting plastics by type is important because different polymers have different melting temperatures, chemical structures, additives, and processing requirements. Mixing incompatible plastics can lower the quality of the recycled product and may cause defects, contamination, or unsafe processing conditions.

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Chemistry: Polymers, Plastics, and Materials Science

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