Polymer 1

Question 1

Monomers

Answer 1

Small repeat units

Question 2

Polymerization

Answer 2

The process by which monomers are linked

Question 3

Cause of polymer special properties

Answer 3

Their constitution , the long molecular chains

Question 4

Three types of polymers

Answer 4

Question 5

T_g

Answer 5

The glass transition temperature (Tg), of a polymer is thelowest temperature at which molecular motion of the polymer chains is possible

Tg is the lowest temperature at which chains can flow or be processed

Question 6

Properties of Elastomers

Answer 6

Crosslinked rubbery networks (above Tg at RT) with low degree of crosslinking

Can be stretched to high extension but recover when stress is released

Once crosslinked cannot be processed

Question 7

Properties of Thermosets

Answer 7

Rigid network polymers with a high crosslink density

Once formed they cannot be processed or stretched (because of high crosslink density)

Question 8

Prperties of thermoplastics

Answer 8

Linear or branched polymers and solid at room temp

When heated above a characteristic temperature (Tg or Tm) polymers can be processed as viscous liquids

Upon cooling they solidify but can be reheated and reprocessed

Question 9

Amorphous Thermoplastics

Answer 9

Polymer chains are disordered even in the solid state

Glass transition temperature (Tg) is a property of amorphous polymer

When heated above Tg, amorphous polymers can

be processed as a viscous liquid

Upon cooling they solidify but can be reheated and reprocessed

Question 10

Semi-crystalline thermoplastics

Answer 10

Semi-crystalline polymers contains both amorphous and crystalline regions or domains

• Amorphous domains – disordered chains, with a glass transition temp (Tg)
• Crystalline domains – ordered chains with a melting point (Tm)

Semi-crystalline polymers therefore have BOTH a Tg andTm

Question 11

Semicrystaline thermoplastics

1 Heating above Tg

2 Heating above Tm

Answer 11

When heated above Tg, polymer chains in amorphous domains become mobile BUT polymer cannot be processed as viscous liquids – WHY?

Crystalline domains remain in tact, preventing macroscopic flow, until Tm

When heated above Tm crystalline domains melt and polymer can be processed as viscous liquid

Question 12

Cooling semi-crystalline polymers and examples

Answer 12

Upon cooling semi-crystalline polymers solidify (in two stages) but can be reheated and reprocessed

Examples of common SCT polymers include polyethylene, polypropylene, polyamide (Nylon) and poly(ethylene terephthalate) (PET)

Question 13

Solubility of linear or branched structure

Answer 13

Linear/branched polymers will be soluble in some solvent – not all solvents but at least some solvents.

Question 14

Solubility of network/ crosslink polymers

Answer 14

Crosslinked/network polymers are not soluble. They may swell in the presence of a suitable solvent. Degree of swelling will depend on crosslink density. Thermosets with a very high crosslink density may not swell at all.

Question 15

Low density polyethylene (LDPE)

Answer 15

Low density polyethylene (LDPE) comprises of chains with random branching.

Consequently, chain-packing/crystallinity is inhibited by branching. Therefore LPDE is less dense, has a lower melting temperature and is soft and flexible.

LDPE is used in food wrappers and shopping and rubbish bags.

Question 16

High density polyethylene (HDPE)

Answer 16

High density polyethylene (HDPE) comprises of chains with only a very few branches.

Consequently, chains pack easily and degree of crystallinity is high.

HDPE has higher density than LDPE, a higher melting temp and is hard, tough and rigid.

HDPE is commonly used in plastic milk bottles, and other containers.

Question 17

Classification of polymers according to composition

Answer 17

Question 18

Polymers size

Answer 18

Physical/mechanical properties of polymers shows a very strong dependence on size of polymer chains

Question 19

Characterisation of size

M, x, Mo

Answer 19

Molar Mass, M (molecular weight)

Mass of 1 mol of polymer (g mol-1 or Kg mol-1 )

Degree of Polymerisation, x (or n)

Number of monomer repeat units per chain

•M = xMo

Where Mo is molar mass of monomer unit

Question 20

Mi

Answer 20

Mi – molar mass of a given polymer chain comprising of ‘n’ monomer units

Question 21

Ni

Answer 21

Ni – total number of polymer molecules (moles) of molar mass Mi

Question 22

Wi

Answer 22

wi – weight fraction of polymer chains with molar mass Mi

wi = mass of polymer chains of molar mass Mi

total mass of polymer chains

Question 23

The graph

Answer 23

Question 24

Number-Average Molar Mass

Answer 24

Question 25

Weight-Average Molar Mass

Answer 25

Question 26

Dispersity

Answer 26

Dispersity (Đ) = Mw/Mn and since Mw is always greater than Mn, Đ is always greater than 1.0

Question 27

Measuring polymer molar mass – size exclusion chromatography

Answer 27

Separates and analyses on the basis of molecular size (hydrodynamic volume) – NOT molecular weight!!!!

Polymer molecules in solution distribute between

mobile phase (solvent) and stationary phase (column)

K - distribution constant

cm – conc of polymer in mobile phase

cs – conc of polymer in stationary phase

Larger (high molecular weight) chains elute in shorter times

Question 28

Thermodynamics of SEC!

Answer 28

SEC is usually carried out in a ‘good’ solvent for the polymer and under ideal conditions

there are no enthalpic interactions between polymer and packing materials c.f. HPLC

Therefore KSEC is a function of the loss of conformational entropy when a polymer chain enters a pore!

SEC columns separate polymers in terms of the size of the polymer chain in solution relative to the size of the pores.

For linear polymers, molecular size correlates extremely well with molecular weight! It is therefore an excellent method for measuring polymer molecular weight and molecular weight distributions.

Question 29

Polymers

Answer 29

Polymers are large molecules (macromolecules) made of many small repeat units – monomers