Lecture 3 Flashcards
Polymer materials consist of
long chain molecules usually with a back bone of carbon atoms
What bonding do we see in polymers?
covalent
Polymers can ……… that enables it to twist into a variety of positions
rotate around the axis of the C-C bond
What changes the polymers chain structure and properties?
Side groups
Mer unit is
the basic structure that the chain is formed from
Condensation polymerisation usually
combines two different monomers into the chain
Condensation polymerisation produces a small
molecule by-product
Which polymer; only secondary bonds between the polymer chains%
Which polymer; many cavalent bonds (cross-links) between the polymer chains%
Which polymer; rubbery materials can be thermoset or thermoplastic versions%
Thermoplastic polymers are
polymers that consist of many molecules that are linked to each other ONLY by secondary bonds and entanglement
A stronger thermoplastic polymer means what for the glass transition temp?
higher glass transition temp
The absence of primary bonds in thermoplastic polymers means that the chains can
slide past each other when loaded at a high temp
By heating a thermoplastic polymer what does it turn into? What can we do with this?
Viscous liquid - which can be formed and moulded easily
Differences in the depiction of an amorphous and semi-crystalline thermoplastic?
Amorphous - scribbled mess, intertwined and messy
Semi-crystalline - regular neat chains snaking around each other
Through a polymerisation process, what can we not determine without controlling the process parameters?
The average chain length
The degree of polymerisation DP represents the
average number of mer units in the chain
A high DP means
the chains are longer therefore a high molecular weight
A low DP means
the chains are shorter therefore a low molecular weight
If a polymer is linear, what does it mean?
Close packed and neat linear chain structures
Branched polymer, what does it mean?
the polymers during polymerisation become branched and cannot pack so closely together
Copolymers consist of
chains with two different mer units that can be regularly or randomly arranged or formed into blocks
Chains of different mer units can also be ——-
attached
In a thermoplastic, how do they usually form a region of regular crystalline structure?
When the long chains fold and align
The driving force for the thermoplastic forming a region of regular structure is%
Cooling rate in a thermoplastic determined the
level of crystallinity
Fast cooling =
Slow cooling =
low crystallinity
high crystallinity
Spherulites are formed when
lamellae start from from a central nucleation site and grow into spherical arrangements
What affects the way in which the chain molecules pack to for lamellae?
Arrangement of the side groups
Large side groups can make —– —— difficult and also reduces —– ——
crystal formation, chain mobility
What is the term used to refer to a polymer with side groups all attached on one side of the C-C bonds?
Isotactic
What is the term used to refer to a polymer with side groups attached on both sides, in alternating orders, of the C-C bonds?
Syndiotactic
What is the term used to refer to a polymer with side groups attached on both sides of the C-C bonds with no specific order?
Atactic
Higher crystallinity leads to:
- Higher melting point
- Increased solvent resistance
- Increased stiffness
- Reduced permeation of gases
- Light scattering - get opaque materials if crystals are large
Cooling an amorphous polymer, what are the characteristics observed?
- No distinct solidification or crystallisation
- Reformation of secondary bonds which limits motion of the motion of the particles — material turns into a rubbery material
- Once it has cooled right down below Tg, insufficient energy to break the secondary bonds — material turns brittle and glassy
Cooling a semi-crystalline polymer, what are the characteristics observed?
- Shows intermediate behaviour
- Change in volume at Tm as some of the chains for lamellae
- Amorphous regions between lamellae cease movement, around Tg it becomes glassy
Tg =
glass transition temperature
Tm =
melting temperature
Yield strength for a polymer is defined at what point?
When the neck begins to form
When tensile testing a thermoplastic, what sort of feature does it initially display?
Proportional stress/strain relationship
As a polymer strip is put under a tensile test and is stretched, what happens to the fibres?
Initially they are jumbled up but as the force is applied and the neck begins to form, the fibres all line up
Viscoelastic materials
exhibit elastic behaviour when loaded over a short time but will flow. appear viscous when loaded over a longer time
For many polymers, creep can occur at
room temperature or just above
Creep is
the deformation or stretching of polymers subjected to a constant stress, over a period of time
The deflection temp of a polymer indicates the ….
likelihood of creep
Mer unit of polyethylene
H H
| |
-C- C-
| |
H H
Polyethylene is typically used in
bags, electrical equipment, bottles
Few pros of polyethylene
- Low cost
- Tough
- Flexible
- Corrosion resistant
Mer unit of polypropylene
H H
| |
-C-C-
| |
H CH3
Few pros of polypropylene
- Low cost
- Tough
- flexible
- Stronger than PE and better at high temp
Polypropylene is typically used in
packaging, pipes and fittings, chairs
Mer unit of Polytetrafluoroethylene
F F
| |
-C-C-
| |
F F
Typical uses for Polytetrafluoroethylene
gaskets, inert lab equipment, electrical insulation
Pro of Polytetrafluoroethylene
low stick due to low surface energy
Nylon is known for the chemical name of
polyamide
Thermoset polymers ——– – —– once formed into a structure
cannot be reformed
Thermoset polymers are better than thermoplastics due to the reasons that they have a ….. and …..
good chemical resistance and higher operating temperature
3 examples of thermoset polymers
- Phenol-formaldehyde
- Urea-formaldehyde
- Melamine-formaldehyde
As thermoset polymers are linked together with —– —— ——, the material does not show many variations in properties …….
strong covalent bonds, with temperature as thermoplastics do
Two things oberved about thermoset polymers that are not oberved in thermoplastics
- Very little softening occurs when raised to a high temperature, too high and the material begins to degrade
- Tg is usually high, won’t “flow” but will become more compliant
What are the characteristics to an elastomer when put under a straining force?
- All deformation is elastic - no plastic deformation
- Bonds stretched as more force is applied to the material
- Sudden failure is seen
Thermoset elastometers have only a few….
cross links joining the polymer chains together
Silicones have —– —— but made up of Si-O
long chains
Polymer additives are
additional materials which are mixed into a polymer to change its mechanical properties, such as strength and wearability etc…
Filler additives are those added to
improve;
- tensile and compressive strength
- abrasion resisitance
- toughness
- dimensional stability
Plasticiser additives are
small molecules added to a polymer which obstruct the secondary bonding between molecules
Stabiliser additives are added to
prevent deterioration of the polymer in service due to exposure to light and atmosphere
Stabiliser additives can reduce the chance of
- colour change
- deterioration of mechanical properties
- cracking
- surface crazing
- UV bond breakage
——- ——- are added to polymers to reduce the flammability of the organic materials
Flame retardants
