Materials Engineering (Week 8) Flashcards
what are polymers, and how are they produced?
- Long chain molecules with repeats (mers) of a group of atoms
Polymer: A substance composed of macromolecules
* Macromolecule: A molecule of high relative molecular mass, the
structure of which essentially comprises the multiple repetition of
units derived, actually or conceptually, from molecules of
low relative molecular mass
- Produced by polymerising - joining the individual monomers together
e.g joining the group of atoms called ethylene tp create polyethylene
Natural Polymer Examples: (4)
wood
rubber
cotton
silk
Almost every polymer has the same…
carbon backbone
- Most polymers are…
(made of what 2 elements)
- Most polymers are hydrocarbons
– i.e., made up of H and C
Saturated Hydrocarbons:
- Each carbon singly bonded to four
other atoms - Example:
- Ethane, C2H6
Unsaturated Hydrocarbons:
- (Carbon) Double & triple bonds somewhat
unstable – can form new bonds - Double bond found in ethylene or
ethene - C2H4
Triple bond found in acetylene or
ethyne - C2H2
Polymerisation
* Three main steps in the chain
reaction:
- Initiation
-Production of free radical which
reacts with monomer - Propagation
-Joining of monomers together - Termination
-Put the end atom/group of atoms
on the chain
- Chains are entangled so
loading material moves…
Chains are entangled so loading material moves
chains NOT stretch the CC backbone
(Strong forces)
- Bonding between chains is by van der Waals forces (Weak forces)
Molecular Weight
- Factors which control chain length are: (4)
- Number of free radicals
- Number of end groups
- Time available for chain growth
- Catalysts
what are the 2 methods of defining molecular weight?
Number average (Mn, molecular weight) averaged over the number of molecules
equation: Mn = sum(NiMi) / sum(Ni)
- Weight average (Mw) averaged
over the weight of EACH chain - Where Ni is number of moles of i
and Mi is molecular weight of i
equation: Mw = sum(Ni x Mi^2) / sum(NiMi)
In a polymer a bundle of relatively short strings/ chains can be held together by …, and so increasing the strength of the polymer (harder to deform)
much longer string (this chain has a much higher molecular weight).
How do polymer chains behave?
Activity of movement is related
to: (3)
- Each chain moves
- Chain length
- Links between the chains
- Temperature
- Relative to Tg and Tm
Properties of Polyethylene
- Depend on molecular chain length, number of cross-links,
entanglements, crystallinity - Low density (LDPE), e.g shopping plastic bag
- High density (HDPE), type of engineering molecule
- Ultrahigh molecular weight (UHMWPE) (MW 2-8 million), used for drug replacements
- Cross linked (XLPE)
-Chemical resistant and electrically insulating
- Tough and relatively low coefficient of friction; low strength and poor resistance to weathering
Uses of polyethylene: (6)
-flexible bottles
-toys
-tumblers
-battery parts
-ice trays
-film wrapping materials
PE (polyethylene) types:
LDPE, HDPE and UHMWPE all linear
chains of increasing length so
increasing properties (except
elongation)
- Wear relatively easy as chains can be
pulled off surface - XLPE is irradiated (at high radiation
level) so develops cross links
between chains so more wear
resistant
Effect of Isotropy on PE:
- Isotropic PE is low modulus (c.1GPa) highly ductile (ef>300%) material
- Can be drawn into fibres, leading to orientation of PE chains, stiffer
and stronger than isotropic - Modulus can be upto 1TPa (from 1GPa)
- Used as reinforcing fibres
(Reinforce denture bases)
(Orthodontic braces)
Effect of crosslinks in polymers
- Increasing number of
crosslinks increases
the modulus - Diamond can be
considered to be
100% cross-linked
polymer
Polypropylene properties
-Resistant to heat distortion: excellent electrical properties and fatigue strength: chemically inert: relatively inexpensive: poor resistance to UV light
Polypropylene uses: (4)
-Sterilizable bottles
-Packaging film
-TV cabinets
-Luggage
As you increase the size of each molecule (no. of C and H atoms), (polymer) goes up, what else goes up?
boiling point
Fluorocarbons
Hydrogen atoms replaced with fluorine
- Very good blood compatibility
*Polytetrafluoroethylene (PTFE) or Teflon (used in non-stick pans)
- High crystallinity (<94%), high density 2.2Mg m-3
Expanded Polytetrafluoroethylene
(ePTFE)
-Expanded PTFE used as porous
material for tissue ingrowth
-Arterial grafts and ligament
replacements
-Gortex (waterproof yet still breathable)
