Polymer Materials Flashcards
What is the basic polymer structure?
Consists of long chain molecules with a backbone of carbon atoms covalently bonded together
What is a Mer unit?
Basic repeat unit a chain is formed of
Types of polymerisation
Addition polymerisation
Condensation polymerisation
What by-products does condensation polymerisation produce?
Water or acetic acid
What are the three main types of polymer?
Thermoplastics
Thermosets
Elastomers
What are the properties of thermoplastics
Only secondary bonds Solids at room T Recyclable Injection moulded Amorphous or semi-crystalline
What are the properties of thermosets
Many covalent bonds (cross-links) High strength, stiffness and hardness Cannot be recycled Good chemical resistance Higher operating T than thermoplastics
What are the properties of elastomers
Few covalent cross-links
Chains uncoil under stress reducing entropy
What is the degree of polymerisation?
The average number of mer units in the chain
What are copolymers?
chains with two different mer units that can be regularly arranged, randomly arranged or formed into blocks. Chains of different mer units can also be attached – grafted on
How does cooling determine the level of crystallinity
Fast cooling - less crystalline
Slow cooling - more crystalline
What are spherulites?
The lamellae start from a central nucleation site and grow into spherical arrangements called spherulites
How do side groups affect crystallinity
Atactic molecules do not normally form crystals
Large side groups can also reduce chain mobility making crystal formation difficult.
What are Viscoelastic materials?
Exhibit elastic behaviour when loaded over a short time scale, but will flow, i.e. appear viscous when loaded over a longer time scale.
What is meant by Deflection temperature?
The temperature at which a given deformation of a beam occurs for a standard load -indicates the likelihood of creep.
Name some properties of Silicones
Made up of Si-O rather than C Generally thermosets Form at room T Flexible Resistant to weathering, oils
What are the properties of thermoplastic elastomers?
Rely on glassy regions where the chains pack together well to form lots of secondary bonding. Act like cross links to stop permanent displacement of chains relative to one another.
What are some examples of fillers?
can be sand, glass, clay.
What do fillers improve?
tensile and compressive strength, abrasion resistance, toughness and dimensional stability
What a plasticisers?
small molecules added to the polymer that obstruct secondary bonding. Reduce hardness and stiffness
Name four other polymer additives
Stabilisers
Flame retardants
Pigments
Anti-static agents
