Introduction to polymer chemistry Flashcards
The Glass Transition Temperature (T g )
the lowest temperature at which molecular motion of the polymer chain is possible
Elastomers
- crosslinked rubbery networks (above T g at room temp)
- can be stretched to high extension but recover when stress is released
- once crosslinked, it cannot be processed
- lightly crosslinked polymers which can be deformed
Thermosets
- rigid network polymers with a high crosslink density
- once formed, they cannot be processed or stretched
- rigid, inelastic structure so cannot be stretched/ deformed
Thermoplastics
- linear or branched polymers and solid at room temp
- when heated above a characteristic temperature, polymers can be processed as viscous liquids (they can be molded into shape)
- upon cooling they solidify but can be reheated and reprocessed, therefore recycled
Amorphous Thermoplastics
polymer chains are disordered even in solid state
- can be recycled like thermoplastics
Semi-crystalline thermoplastics
contains both amorphous and crystalline regions or domains
- they can only be reprocessed when heated above T m crystalline domains as above T g only the amorphous domains become mobile.
Amorphous domains
disordered chains with a glass transition temp (T g )
Crystalline domains
ordered chains with a melting point (T m )
Properties of linear/ branched polymers
soluble in some solvent
Properties of crosslinked/ network polymers
not soluble but can interact with solvent - they may swell which depends on crosslink density (thermosets with a high crosslink density may not swell at all)
Low density polyethylene (LDPE)
- comprised of chains with random branching therefore chain-packing/ crystallinity is inhibited by branching (e.g. plastic bags, food wrappers, etc)
- LPDE is less dense than HDPE so has a lower melting point and is soft and flexible
High density polyethylene (HDPE)
- comprised of chains with only a few branches therefore chains pack easily and the degree of crystallinity is high.
- HDPE has high density therefore a higher melting point and is hard, tough and rigid (e.g. plastic milk bottles)
Homopolymers
only one type of monomer
Copolymers
two or more types of monomer
- random
- alternating
- block (diblock, triblock, graft or comb block)
What are the conditions for Size Exclusion Chromatography (SEC)?
- in a good solvent for the polymer
- under ideal consitions where there are no enthalpic interaction between polymer and packing materials
What is K SEC a function of?
The loss of conformational entropy when a polymer chain enters a pore
How do SEC separate polymers?
In terms of the size of the polymer chain in solution relative to the size of the pores.
Examples of physical properties of polymers
they can be
- tough or brittle
- strong and durable
- rigid and inflexible
- flexible and rubbery
- hard or soft
(Properties vary with temperature thus allowing polymers to be processed)
Below T g
Polymer chains are effectively frozen and it is described as glassy.
Motion is restricted to short-range vibrations and rotations.
The polymer is hard, flexible and brittle.
Above T g
Chains have enough thermal energy to allow long range molecular motions so chains can move past each other.
Polymer chains are rubbery, can flow as a viscous liquid and be processed.
What determines the T g of a polymer? (stiff)
T g increases with increasing chain stiffness - stiffness is induced in polymer chains that have aromatic units in the polymer backbone or with bulky substituents.
What determines the T g of a polymer? (forces)
T g increases with increasing forces of intermolecular attraction.
T g of chain segments in a thermoset?
No T g with almost no mobility due to high crosslink density.
T g of chain segments in an elastomer?
Has a T g and retains mobility due to low crosslink density.
T g and T m in semi-crystalline polymers
T m is always higher than T g