chapter 16: polymers. reading all of ch. 16 except sec. 16-10 and 16-11

Chapter 16: Polymers

Reading

All of Ch. 16 except

Sec. 16-10 and 16-11.

Homework No. 12

Problems 16-8, 16-10

Polymers• Natural polymers

• Synthetic polymers

Examples of natural polymers

• Cellulose (e.g., wood, wool, cotton)

• Starches

• Proteins

• Natural rubber

Examples of synthetic polymers

• Polyethylene

• Polyvinyl chloride

• Synthetic rubbers

Addition polymerization

.OH is the initiator.

Degree of polymerization

= No. of mers in a molecule

= Molecular mass / mer mass

Molecular mass is Mi for size interval i.

Population described asNumber fraction of molecules in size interval i = Xi

= No. of molecules in size interval i divided by No. of molecules in polymer

Number-average molecular mass = Σi(XiMi)

Population described asMass fraction of molecules in size interval i = Wi

= Mass of molecules in size interval i divided by mass of molecules in polymer

Mass-average molecular mass = Σi(WiMi)

Number-average molecular mass < Mass-average molecular mass

Example problem

• 18 g sugar (C6H12O6, M = 180 g/mole)

18 g water (H2O, M = 18 g/mole)• No. of molecules of sugar = 18/180 = 0.1 mole No. of molecules of water = 18/18 = 1.0 mole• Mass-average molecular mass = (0.50) (180 g/mole) + (0.50) (18 g/mole) = 99 g/mole• Number-average molecular mass = (0.1/1.1) (180 g/mole) + (1.0/1.1) (18 g/mole) = 32.7 g/mole

Polymer blend (a solid solution)

Homopolymer: 1 type of mer Copolymer: >1 type of mer

Copolymer

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Styrene-butadiene block copolymer

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Polyamide

Condensation polymerization

Polyester

Polyamide

FunctionalityNo. of reaction sites in a

monomer for polymerization

= 2 for linear polymers

>2 for network (3D) polymers

Due to stereohindrance, functionality = 3, i.e., each phenol ring is at most linked to 3 other phenol rings. One water molecule is formed per bridge.

Network polymer

Kinked conformation

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Isomers

Isotactic

Syndiotactic

Atactic

No side group

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Isotactic

Syndiotactic

Atactic

Tacticity

Fractions that are

atactic,

syndiotactic and

isotactic

Branching

Types of polymer

• Thermoplastic (softens upon heating)

• Thermoset (does not soften upon heating)

Compression molding

Necessary but not sufficient conditions for elastomers

• Noncrystalline at room temperature

• Glass transition temperature well below room temperature

• Macromolecular chains

- very long with many bends

- in constant motion at room temperature

- cross-linked every few hundred atoms

Degree of crystallinity

Degree of crystallinity = Fraction of the polymer that is crystalline

For the same cooling rate, different polymers have different tendencies toward crystallinity


Crystalline region


Orthorhomic unit cell of polyethylene


Network polymers

Glassy usually, because rearrangement is difficult.


Linear polymersFactors that affect the tendency toward crystallinity1. Character of the side groups (a) Bulkiness of the side groups (b) Arrangement of the side groups2. Amount of chain branching3. Macromolecular chain length (long molecules tend to

be kinked)4. Homopolymers tends to be more crystalline than

copolymers

Effect of the degree of crystallinity

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Description of a polymer

• Mer(s)• Type of copolymer• Molecular mass (degree of

polymerization)• Tacticity • Degree of crystallinity

chapter 16: polymers. reading all of ch. 16 except sec. 16-10 and 16-11

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