Polymer 1 Flashcards
Monomers
Small repeat units
Polymerization
The process by which monomers are linked
Cause of polymer special properties
Their constitution , the long molecular chains
Three types of polymers
Tg
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
Properties of Elastomers
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
Properties of Thermosets
Rigid network polymers with a high crosslink density
Once formed they cannot be processed or stretched (because of high crosslink density)
Prperties of thermoplastics
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
Amorphous Thermoplastics
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
Semi-crystalline thermoplastics
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
Semicrystaline thermoplastics
1 Heating above Tg
2 Heating above Tm
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
Cooling semi-crystalline polymers and examples
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)
Solubility of linear or branched structure
Linear/branched polymers will be soluble in some solvent – not all solvents but at least some solvents.
Solubility of network/ crosslink polymers
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.
Low density polyethylene (LDPE)
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.
High density polyethylene (HDPE)
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.
Classification of polymers according to composition
Polymers size
Physical/mechanical properties of polymers shows a very strong dependence on size of polymer chains
Characterisation of size
M, x, Mo
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
Mi
Mi – molar mass of a given polymer chain comprising of ‘n’ monomer units
Ni
Ni – total number of polymer molecules (moles) of molar mass Mi
Wi
wi – weight fraction of polymer chains with molar mass Mi
wi = mass of polymer chains of molar mass Mi
total mass of polymer chains
The graph
Number-Average Molar Mass
Weight-Average Molar Mass
Dispersity
Dispersity (Đ) = Mw/Mn and since Mw is always greater than Mn, Đ is always greater than 1.0
Measuring polymer molar mass – size exclusion chromatography
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
Thermodynamics of SEC!
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.
Polymers
Polymers are large molecules (macromolecules) made of many small repeat units – monomers
