properties of polymer
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Properties of polymers
Md. Hedayatullah.M.Tech I (G.E.T) 2ndSem.
Reg. No.12307003
GETY524 Waste to energy conversion
E-mail
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What is a polymer?
A long molecule
made up from lots
of small molecules
calledmonomers.
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Physical properties of polymers
Thermal Behavior: Melting point.
Glass Transition Temperature (Tg)
Crystalline melting temperature. (Tm) Co-efficient of thermal expansion
Solubility.
Surface and interface properties Surface dynamics
Surface energies
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Material properties.
1.Density:- The ratio of mass to volume of amaterial. Generally Polymers are Low density molecules. (in
compare to metals and ceramics)
It depends upon the Degree of Polymerisation., Crosslinkage between the chains of polymer and
Crytallinity in the Polymer.
Density Crystallinity , Cross linkage, DP
2. Electrical conductivity:-
Polymers are generly non-conducting electrically
except Polyacetylene.
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Mechanical Properties.
1.Elasticity:- Low modulus of elasticity (stiffness)
Eis two or three orders of magnitude
lower than metals and ceramics
2. Tensile strength:- Low tensile strengthTSis about 10% of the metal
3. Much lower hardness than metals or ceramics
4. Greater ductility on average
Tremendous range of values, from 1%
elongation for polystyrene to 500% or
more for polypropylene
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Figure 8.11 - Relationship of mechanicalproperties, portrayed as deformation resistance,as a function of temperature for an amorphousthermoplastic, a 100% crystalline (theoretical)thermoplastic, and a partially crystallized
thermoplastic
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Thermal behaviour.1. Free volume of polymers
varries with temperatue.
2. Much higher coefficient of
thermal expansion
Roughly five times the value
for metals and 10 times the
value for ceramics
3. Much lower melting
temperatures
Specific heats two to four times those of metals and
ceramicsThermal conductivities about three orders of
magnitude lower than those of metals
Insulating electrical properties
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Glass Transition Temperature (Tg)
The temperature at which amorphouspolymers undergo a transition from arubbery, viscous amorphous liquid, to abrittle, glassy amorphous solid is called
the Glass Transition temperature of thatpolymer.
It is denoted by Tg.
The glass transition temperature may beengineered by altering the degree ofbranching or crosslinking in the polymeror by the addition of plasticizer.
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Chemical properties of
polymers. 1. Molecular weight
2.Degree of polymerisation.
3.Polydispersity Index.
4.Conformation of chain in space.
5.Configuration of the chain
(Microstructure)
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MOLECULAR WEIGHT
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Molecular weight, M: Mass of a mole of chains.
high M
Polymers can have various lengths depending on the number of repeat
units. During the polymerization process not all chains in a polymer grow to the
same length, so there is a distribution of molecular weights. There are
several ways of defining an average molecular weight.
The molecular weight distribution in a polymer describes the relationship
between the number of molesof each polymer species and the molar
massof that species.
LOW M
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MOLECULAR WEIGHT DISTRIBUTION
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xi = number fraction of chains in size range i
iiw
iin
MwM
MxM
wi = weight fraction of chains in size range i
Mn= the number averagemolecular weight (mass)
Mi = mean (middle)
molecular weight of size
range i
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Degree of Polymerization, DP
DP= average number of repeat units per chain
m
MDP
n
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weightmolecularunitrepeatwhere m
Ex. problem 4.1b,
for PVC: m = 2(carbon) + 3(hydrogen) + 1(Clorine)
(from front of book) = 2(12.011) + 3(1.008) + 1(35.45)
= 62.496 g/mol
DP = 21,150 / 62.496 = 338.42
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Polymer Chain Lengths
Many polymer properties are affected by the length of thepolymer chains. For example, the melting temperature
increases with increasing molecular weight. At room temp, polymers with very short chains (roughly 100
g/mol) will exist as liquids.
Those with weights of 1000 g/molare typically waxy solidsand soft resins.
Solidpolymers range between 10,000and several milliong/mol.
The molecular weight affects the polymers properties(examples: elastic modulus & strength).
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Chain End-to-End Distance, r Representation of a
single polymer chain
molecule that has
numerous random kinks
and coils produced bychain bond rotations; it is
very similar to a heavily
tangled fishing line.
r is the end to enddistance of the polymer
chain which is much
smaller than the total
chain length. 19
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Molecular Structures-
Branched
Where side-branch chains have connected to main
chains, these are termed branched polymers. Linearstructures may have side-branching.
HDPEhigh density polyethylene is primarily a
linear polymer with minor branching, while LDPE
low density polyethylene contains numerous shortchain branches.
Greater chain linearity and chain length tend to
increase the melting point and improve the physical
and mechanical properties of the polymer due togreater crystallinity. 21
Branched Cross-Linked NetworkLinear
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Molecular StructuresCross-linked,
Network
In cross-linked polymers, adjacent linearchains are joined to one another at various
positions by covalent bonding of atoms.
Examples are the rubber elastic materials.
Small molecules that form 3 or more active
covalent bonds create structures called
network polymers. Examples are the epoxies
and polyurethanes. 22
Branched Cross-Linked NetworkLinear
secondarybonding
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Thermoplastics and
Thermosets The response of a polymer to mechanical forces at
elevated temperature is related to its dominant
molecular structure.
One classification of polymers is according to its
behavior and rising temperature. Thermoplastics andThermosets are the 2 categories.
A thermoplastic is a polymer that turns to a liquid when
heated and freezes to a very glassy state when cooled
sufficiently. Most thermoplastics are high-molecular-weight
polymers whose chains associate through weak Van
der Waals forces (polyethylene); stronger dipole-dipole
interactions and hydrogen bonding (nylon).23
Th l ti d
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Thermoplastics and
Thermosets Thermoplastic polymers differ from
thermosetting polymers (Bakelite,vulcanized rubber) since thermoplasticscan be remelted and remolded.
Thermosetting plastics when heated, will
chemically decompose, so they can not berecycled. Yet, once a thermoset is cured ittends to be stronger than a thermoplastic.
Typically, linear polymers with minor
branched structures (and flexible chains)are thermoplastics. The networkedstructures are thermosets.
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Examples of Thermoplastics
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More Examples of Thermoplastics
PTFE
Polymer
http://www2.dupont.com/Teflon/en_US/index.html -
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Specific Thermoplastic Properties
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Thermoset data
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Thermoset Properties
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Specific Elastomeric Properties
Elastomers, often referred to as rubber, can be a thermoplastic or a
thermoset depending on the structure. They are excellent for parts
requiring flexiblity, strength and durability: such as automotive andindustrial seals, gaskets and molded goods, roofing and belting, aircraft
and chemical processing seals, food, pharmaceutical and semiconductor
seals, and wire and cable coatings.
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3
Thermoplastics:
--little cross linking
--ductile
--soften with heating
--polyethylene
polypropylene
polycarbonatepolystyrene
Thermosets:
--large cross linking(10 to 50% of mers)
--hard and brittle
--do NOT soften with heating
--vulcanized rubber, epoxies,
polyester resin, phenolic resin
Callister,Fig. 16.9
T
Molecular weight
Tg
Tmmobileliquid
viscous
liquid
rubber
tough
plastic
partiallycrystalline
solidcrystalline
solid
Thermoplastic vs Thermoset
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Crystallinity in Polymers The crystalline state
may exist in polymericmaterials.
However, since itinvolves moleculesinstead of just atoms or
ions, as with metals orceramics, the atomicarrangement will bemore complex forpolymers.
There are orderedatomic arrangementsinvolving molecularchains.
Example shown is a
polyethylene unit cell 32
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Crystal Structures
Fe3Ciron carbide
orthorhombic crystal
structure
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The effect of temperature on the structure andbehavior of thermoplastics.
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Polymer CrystallinityPolymers are rarely 100%
crystalline
Difficult for all regions of all
chains to become aligned
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Degree of crystallinity
expressed as % crystallinity.-- Some physical properties
depend on % crystallinity.
-- Heat treating causes
crystalline regions to grow
and % crystallinity toincrease.
crystallineregion
amorphous
region
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Thank you..