HW 3: Polymers

Question 1

1. How would you distinguish experimentally between an amorphous, crystalline, and a cross-linked polymer?

Answer 1

By studying thermal transitions by DSC, one could compare the behaviors of each polymer after heating. Amorphous polymers do not exhibit a sharp Tm, but do have a distinct Tg above which the polymer loses its strength (liquid). Crystalline polymers are rigid and have high Tm. Cross-linked polymers, such as thermosetting plastics, have extremely rigid structures which burn or decompose instead of melting.

Question 2

2. Based on your intuitive knowledge of chemistry or physics, suggest why rubber is an elastomer at room temperature but glass is not?

Answer 2

Rubber is an elastomer at room temperature because its Tg is below that, while glass has a Tg above room temperature and thus exists as an amorphous solid.

Question 3

4. What are linear and branched polymers? Can thermoplastics be branched?

Answer 3

A linear polymer consists of a long chain of skeletal atoms to which are attached the substituent groups. A branched polymer can be visualized as a linear polymer with branches of the same basic structure as the main chain. Thermoplastics can be branched polymers.

Question 4

5. Define:

a. Thermoplastic
b. Elastomers

Answer 4

A thermoplastic is any material that softens when it is heated.

An elastomer is a flexible polymer that is in the temperature range between its glass transition temperature and its liquification temperature.

Question 5

6. Define:

a. Addition polymerization
b. Initiator

Answer 5

Addition polymerization is a chemical reaction in which simple monomers are added together to create larger polymers (generally using the addition reaction of olefins, acetylenes, aldehydes, or other compounds with “unsaturated” bonds to form polymers).

Initiators are a species of compounds which are added in small quantities to reactions start the polymerization process.

Question 6

7. What is the structure of the repeating unit (mer) in:
   a. Polypropylene?
   b. Poly(vinylchloride)?

Answer 6

Question 7

Answer 7

LDPE

• Produced through free-radical polymerization,
• LDPE has the most long- and short-chain branching of any form of PE, resulting in its lower density.
• The branching keeps the molecular chains from packing tightly in its crystalline form, so LDPE has less tensile strength but greater ductility.
• That exceptional “formability” makes LDPE particularly useful for a range of applications, from rigid products like plastic bottles, buckets and bowls to filmy ones like plastic grocery bags and plastic cling-wrap.

HDPE

• On the opposite side of the polymer chain, we have HDPE, which is characterized by minimal branching of the polymer chain.
• Less branching means those nicely linear molecules pack together well during crystallization, making HDPE much denser and rigid.
• That added tensile strength means HDPE is the PE of choice for applications that require a bit more “backbone,” such as milk and detergent jugs, garbage cans, water pipes and children’s toys.
• It’s also one of the reasons why HDPE has largely replaced cardboard as the tube material of choice in making fireworks. An HDPE tube is less prone to shattering if a firework malfunctions, and once the boom inside the tube is exhausted, the HDPE tube is recyclable.

Question 8

Answer 8

• Ebonite is an interesting material. It is used as an electrical insulating medium and chemical-resistant material.
• Vulcanized ebonite is machinable and many engineering products can be made adopting regular engineering practices like turning, grinding, boring, drilling, etc.; however, it cannot be welded for making joints. It looks and feels like a hard plastic.
• It is called hard rubber because it is a modified version of soft rubber but not having the nature of soft rubber.
• Soft rubber has a low percentage of sulfur and is elastic, but hard rubber has lost most of its elastic or rubbery properties during conversion to the hard stage.

Question 9

Answer 9

Question 10

Answer 10

Question 11

12. Draw polymers with IMFs:

Dispersion

Dipole-dipole

H-bonding

Answer 11

Question 12

Answer 12

PMMA adds another methyl group above the acrylate.

Poly(methyl acrylate) is a white rubber at room temperature, but poly(methyl methacrylate) is a strong, hard, and clear plastic.

How soft or hard a polymer is at a given temperature is determined by what we call chain mobility, that is, how well the polymer chains wiggle past and around each other. The more they can move, the softer the polymer is.

Anything bigger than a methyl stops the polymerization in its tracks. Steric hindrance becomes too great for the incoming monomer to add on to the end of the reactive polymer chain.