Polymers- Introduction

Question 1

Homopolymer, random copolymer, block copolymer

Answer 1

Homopolymer has only one type of monomer
Random copolymer has two types of monomer with no order to their arrangement in the polymer chain
Block copolymer has two types of monomer with regions along the chain with many of one type (-M-M-M-N-N-N-)

Question 2

How many CH2 units does typical molecule of polyethylene have?

Answer 2

Roughly 50,000

Question 3

Types of natural polymers

Answer 3

Polysaccharides (starch, cellulose)
Polypeptides (muscle, collagen, keratin)
Polynucleotides (DNA, RNA)
Natural rubber, gums, resins

Question 4

Types of synthetic polymers

Answer 4

Thermoplastics (solid when cooled, viscous liquid when heated)
Thermosets (precursors (viscous liquid) cross link to form infusible solid)

Question 5

Common types of polymers

Answer 5

See slide 5 to 9

Question 6

When can polymeric solids crystallise?

Answer 6

When their molecular structures are regular enough. Crystallisation often hampered by irregularities at molecular level

Question 7

Skeletal structure for crystal or amorphous

Answer 7

Linear polymer more likely to crystallise. Branched has side chains so less likely. Cross-linked likely to be amorphous

Question 8

Stereo-regularity for crystal or amorphous

Answer 8

Isotactic crystal
Syndiotactic (alternates sides) crystal
Atactic (random sides) amorphous

Question 9

Why can polyethylene form a random coil?

Answer 9

Rotation around the C-C bonds means planar zigzag conformation changed into random conformation typical of amorphous polymer

Question 10

Freely jointed chain model

Answer 10

Root mean square distance between the two chain ends is ^1/2. For freely jointed chain:
r sub f= ^1/2=n^1/2 l
n is number of bonds of length l
Blank space is r squared in arrows

Question 11

Restriction by the valence angle formula

Answer 11

=nl^2[(1-cosθ)/(1+cosθ)]

θ is valence angle between two adjacent bonds (v shape)

Question 12

Chain stiffness formula

Answer 12

=nl^2σ^2[(1-cosθ)/(1+cosθ)]

σ is steric parameter 1.5 to 2.5

Question 13

Characteristic ratio

Answer 13

C sub infinity=/rf^2=/nl^2

Measure of chain stiffness/flexibility

Question 14

Typical values of characteristic ratio for some polymers

Answer 14

Varies with temperature
Range from like 4 to 11
See slide 20

Question 15

Typical YM values for steel, glass, polymer glass, nylon, polyethylene, rubber

Answer 15

Steel 220 GPa
Glass 60 GPa
Polymer glass 3-30 GPa
Nylon 2 GPa
Polyethylene 0.2 GPa
Rubber 0.002 GPa

Question 16

Stress strain plots for various types of polymer

Answer 16

Very steep straight for high modulus polymer fibre.
Steep straight fails at slightly lower stress for glassy polymer.
Straightish fails at lower stress for semicrystalline polymer below Tg.
Less steep for semicrystalline polymer above Tg with peak at failure when below Tg then horizontal for ages until slight increase to failure.
Shallow curve never peak then curves up a bit near end for elastomer

Question 17

Temperature dependence graph for YM

Answer 17

Log(E) vs T (K). Starts high and near horizontal in glassy region. Curves to steep line down then levels off mostly in leathery region. Curves down bit more in rubbery region. Curves down bit more in rubbery flow region. Curves down bit more in viscous region. All regions similar with (T range) with later one’s getting bit smaller