Characteristic properties of different material types Flashcards
(36 cards)
What are polymers?
Large hydrocarbon molecules composed of many monomers
Describe the bonding within the chains of a polymer
The atoms are covalently bonded and van der Waals forces exist between the chains
Identify the different types of polymers
- linear
- branched
- crosslinked
- network
Describe linear polymers
- repeat units joined end to end in single chains extensive van der Waals and hydrogen bonding between chains example: polyethylene, PVC
- greater packing efficiency than branched
- high density
Describe branched polymers
- side branches result from reactions during polymer synthesis
- packing efficiency reduced
- lower density
- LDPE-branched, weaker and cheaper than HDPE
Describe crosslinked polymers
- adjacent chains joined by covalent bonds at various points
- crosslinking occurs during synthesis or chemical reactions by additive atoms or molecules during covalent bonds with the polymer chains
Describe network polymers
- 3D networks formed when molecules form 3 or more covalent bonds
- can be brittle
- very rigid and hard
- retain properties at high temperatures
Which properties of polymers are affected by chain length?
melting or softening temperature viscosity tensile strength young’s modulus
Calculating the number average molecular weight
Formula:
Calculating the weight average molecular weight
Formula:
Identify the different types of plastics
thermoplastics thermosetting elastomers/rubbers
Characteristics of thermoplastics
composed of long polymer chains with/without branches limited cross-linking chains tangled but easily detangled when tensile stress is applied emorphous or crystalline soften and melt on heating easily recycled
Characteristics of thermosetting
long polymer chains substantial amount of cross-linking between chains amorphous structure usually stronger than thermoplastics more brittle decompose on heating difficult to reprocess after formation difficult to recycle
Characteristics of elastomers/rubber
capable of sustaining very large plastic deformations thermoplastics or lightly cross-linked thermosets
What happens in a polymer when the temperature is greater than the melting temperature?
the polymer is a liquid and the chains have enough energy to move easily
Define crystallisation
an ordered or partially ordered crystalline solid phase forms
Define a nuclei
regions of the polymer that become ordered and aligned thickness remains constant increased spherulitic radius
Describe what happens at the glass transition temperature
Some polymers cool below the melting temperature to an amorphous solid state which is stable, rigid but not ordered
Draw a graph showing polymer transitions
Graph
Describe the transition of glass or amorphous polymer
it becomes more viscous and rubbery during cooling elastic modulus, specific volume and thermal expansion change significantly around the glass transition temperature
Describe the transition of semi-crystalline polymers
change is limited to the amorphous regions the glass transition temperature corresponds to the temperature at which rotation about single bonds is possible due to the increased temperature
Describe the transition of crystalline solids
at the melting temperature, there is a molecular rearrangement from ordered to a disordered state
Vulcanisation of rubbers
crosslinking in rubbers using sulphur compounds stiffness is determined by the degree of cross-linking
Why is cured rubber hard to recycle
sulphur compounds link the long-chain molecules irreversibly
