Polymer-(Ramesh- GEC CLT)aroramesh.weebly.com/.../polymer-(ramesh-_gec_clt).pdf · 2018-09-10 ·...
Transcript of Polymer-(Ramesh- GEC CLT)aroramesh.weebly.com/.../polymer-(ramesh-_gec_clt).pdf · 2018-09-10 ·...
POLYMERS
Many Units
Dr. A. R. Ramesh
Assistant Professor1RAMESH-GEC Kozhikode- Polymer
The Structure and Properties of
Polymers
• Also known as
• Bonding +
• Properties
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A polymer is a large molecule (macromolecules) made by
linking together repeating units of small molecules called
monomers
Repeat unit in a polymer is called ‘mer’3RAMESH-GEC Kozhikode- Polymer
4.1 Ancient PolymersOriginally natural
polymers were used:
– Wood
– Rubber
– Cotton
– Wool
– Leather
– SilkOldest known use:
Rubber balls used by Incas
Noah used pitch (a natural polymer) for the ark
Noah's pitch
Genesis 6:14 "...and cover it inside and outside
with pitch."
gum based resins
extracted from pine
trees
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Examples of monomers and polymers
Monomer Polymer
HOCH 2CH 2OH
HO CO 2H
CH 2CH 2
CH 2CH 2O
CH 2CH 2O
O C
O
CH2
CH2
CH2
CHClCH
2CH
2
Cl
H2C CH
2
O
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Degree of polymerisation: number of repeating unit in the
chain of polymer
Highpolymers: DP > 100
Oligopolymers: DP < 100
Tacticity: Stereochemistry
Atactic: Random
Isotactic: Same Side
Syndiotactic: side groups in alternating fashion
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Nomenclature:
Homopolymer: A-A-A-A-A-A-A-A (identical units in linear fashion)
Copolymer: A-B-A-B-A-B-A-B (Different type of monomeric units)
Alternating Copolymer:A-B-A-B-A-B-A-B (alternating fashion)
Random Copolymer: A-B-A-A-A-B-A-B –B (Random fashion)
Block copolymer: A-A-A-A-A-A-B-B-B-B-B (Block of two polymers)
Graft copolymer: A-A-A-A-A-A-A-A-A- (homopolymergrafted in
another homopolymer)
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Four Types of Copolymers
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Block copolymer, example:
Poly(styrene)-block-poly(butadiene)
Random copolymer, example:
Poly(styrene-ran-butadiene)
Graft copolymer,
example:
Poly(styrene)-graft-poly(butadiene)
RAMESH-GEC Kozhikode- Polymer
Classification of polymers
1. Natural & Synthetic : Cotton , silk, wool Polyethylene (PE), PVC,
nylon
2. Organic & Inorganic: PE Glass, silicone
3. Thermoplastic & Themosetting: Soften on heating –linear/branched
(PE, PVC, nylon)
infusible & Insoluble mass on heating – network
(bakelite phenolformaldehyde resin)
4. Plastics, Elastomers, fibres & Liquid resins: Become hard & tough on
heating (PVC)
Vulcanized into rubbery products (natural rubber)
Long filament (nylon)
In liquid form as adhesives (Epoxy adhesivs)10RAMESH-GEC Kozhikode- Polymer
Types of polymerization
1. Addition or chain polymerization
(repeating units and monomers are same)
2. Condensation or step (repeating units and monomers
are not equal, combining two molecules by removing a
small molecule
3. Copolymerization
4. Coordination or Ziegler Natta polymerization
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Addition polymerization
• Exact multiple of original monomeric molecule
• No loss of material
• Formed by monomers with double bonds
• Initiated by heat, light, pressure, radiation or catalyst
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CH2=CH2 Polyethylene
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Mechanism
� Free radical
� Cationic
� Anionic
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Radical Polymerization
� Homolytic dissociation of initiator
� Initiator-Thermally unstable compound
� Easily decomposed to free radicals by the action of heat, light or
catalyst
� Eg: Acetyl or benzoyl peroxides 15RAMESH-GEC Kozhikode- Polymer
� Chain initiating species grows by successive addition of
monomer
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Chain termination step
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Cationic Polymerization
the alkene monomer
reacts with an electrophile
Formation of carbocation
AlCl3 + H2O [AlCl3(OH)]- + H+
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Monomers that are best able to undergo cationic polymerization are
those with electron-donating substituents
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Anionic Polymerization
Nonterminated chains are called living polymers
The chains remain active until they are killed
KNH2 K+ + NH2-
Bu-/NH2- +
CHAIN INITIATION
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+ NH3
+ NH2-
CHAIN TERMINATION
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Step-growth polymers OR condensation polymers
made by combining two molecules by removing a small molecule
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Nylon 6 is an example of a step-growth polymer formed
by a monomer with two different functional groups
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Addition Condensation
Example Polystyrene Nylon
Empirical formula No change from monomer.
Changes as byproduct (often water) is given off.
How grows One monomer at a time
Monomer + dimer, hexamer +
octadecamer, etc.
Polydispersity Can be paucidisperse “Most probable”
Molecular weight Wide range: can be very high
Low (except biopolymers)
Synonym Chain growth polymerization
Step growth polymerization
Chain growth Step growth
RAMESH-GEC Kozhikode- Polymer
TABLE 1.1 Comparison of Step-Reaction and
Chain-Reaction Polymerization
Step Reaction Chain Reaction
Growth occurs throughout matrix by
reaction between monomers, oligomers,
and polymers
DPa low to moderate
Monomer consumed rapidly while
molecular weight increases slowly
No initiator needed; same reaction
mechanism throughout
No termination step; end groups still reactive
Polymerization rate decreases steadily as
functional groups consumed
Growth occurs by successive addition of
monomer units to limited number of
growing chains
DP can be very high
Monomer consumed relatively slowly, but
molecular weight increases rapidly
Initiation and propagation mechanisms different
Usually chain-terminating step involved
Polymerizaion rate increases initially as
initiator units generated; remains relatively
constant until monomer depleted
aDP, average degree of polymerization.. 29RAMESH-GEC Kozhikode- Polymer
Copolymerization
Two or more different types of monomers undergo polymerization
together
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Coordination or Ziegler Natta polymerization
� Produce stereospecific polymers
� Use Ziegler Natta catalyst
� Complex = {Alkyls + Metal ( group I-III) } + halides of transition
metals (IV-VIII)
� {(C2H5)3Al, (C2H5)2AlCl} + TiCl4
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Polymerization Processes
• TWO USEFUL DISTINCTIONS ;
– BETWEEN BATCH AND CONTINUOUS
– AND BETWEEN SINGLE - PHASE AND MULTI -
PHASE
• SINGLE - PHASE
– Bulk or Melt Polymerization
– Solution Polymerization
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Polymerization Techniques
These include:
• Bulk Polymerization
• Solution Polymerization
• Suspension Polymerization
• Emulsion Polymerization
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Bulk Polymerization
• Bulk polymerization is carried out in the absence of any solvent
or dispersantand is thus the simplest in terms of formulation.
• carried out by adding a soluble initiator to pure monomer in
liquid state.
• The reaction is initiated by heating or exposing to radiation.
• Gives the highest-purity polymer
• This process can be used for many free radical polymerizations
and some step-growth (condensation) polymerization.
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• Two stages
• First- Radical initiated thermal polymerization of monomer (heat)
• Cold water circulate
• Second- main reactor, 110-200oc
• 100% complete
• Unreacted monomer volatilize off
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Bulk Polymerization
Advantages:
• High yield per reactor volume
• Easy polymer recovery
• The option of casting the polymerisation
mixture into final product form
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Bulk Polymerization
Limitations:
• Difficulty in removing the last traces of
monomer
• The problem of dissipating heat produced
during the polymerization
– In practice, heat dissipated during bulk
polymerization can be improved by providing
special baffles
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Solution Polymerization
• Definition: A polymerization process in which
the monomers and the polymerization
initiators are dissolved in a inert liquid solvent
at the beginning of the polymerization
reaction. The liquid is usually also a solvent for
the resulting polymer or copolymer.
• Solvent control viscosity increase & heat
transfer
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Solution Polymerization
Disadvantages
• Chain transfer to the solvent- difficulty to get high
molecular weight polymers
• Removal of solvent
Advantages
• Polymers used in solution form (adhesives & coatings)
• Preparation of block copolymer
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Suspension Polymerization
• Definition: A polymerization process in which the monomer, or
mixture of monomers, is dispersed by mechanical agitation in a
liquid phase, usually water, in which the monomer droplets are
polymerized while they are dispersed by continuous agitation. Used
primarily for PVC polymerization
• If the monomer is insoluble in water, bulk polymerization can be
carried out in suspended droplets, i.e., monomer is mechanically
dispersed.
• The water phase becomes the heat transfer medium.41RAMESH-GEC Kozhikode- Polymer
Suspension Polymerization
• So the heat transfer is very good. In this system, the monomer
must be either
– 1) insoluble in water or
– 2) only slightly soluble in water, so that when it polymerizes it
becomes insoluble in water.
• The behavior inside the droplets is very much like the behavior of
bulk polymerization
• Since the droplets are only 10 to 1000 microns in diameter, more
rapid reaction rates can be tolerated (than would be the case for
bulk polymerization) without boilingthe monomer.42RAMESH-GEC Kozhikode- Polymer
Emulsion Polymerization
� Monomer dispersed as droplets in water containing soap or detergent
� Initiators-water/monomer soluble (persulphate)
� Suspension of high molecular weight polymers obtained
� Solid polymer obtained by coagulating the suspension by adding acid
� Starts with an emulsion incorporating water,
monomer, and surfactant.
� Common- oil-in-water emulsion
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Emulsion Polymerization – Schematic
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Emulsion Polymerization
Advantages of emulsion polymerization include:
• High molecular weight polymers can be made at fast
polymerization rates. By contrast, in bulk and solution
free radical polymerization, there is a tradeoff between
molecular weight and polymerization rate.
• The continuous water phase is an excellent conductor
of heat and allows the heat to be removed from the
system, allowing many reaction methods to increase
their rate.
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Emulsion Polymerization
Advantages Continued:
• Since polymer molecules are contained within
the particles, viscosity remains close to that of
water and is not dependent on molecular
weight.
• The final product can be used as is and does
not generally need to be altered or processed.
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Emulsion Polymerization
Disadvantages of emulsion polymerization include:
• For dry (isolated) polymers, water removal is an
energy-intensive process
• Emulsion polymerizations are usually designed to
operate at high conversion of monomer to polymer.
This can result in significant chain transfer to
polymer.
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Linear polymers can be represented by a
simple sequence such as: A-A-A-A-A .
Polystyrene
Styrene monomer
CH CH2
n
Nylon
Two monomers
make one
repeating unit.**There many different kinds of nylon.
H2N-(CH2)6-NH2
Nylon monomer
HOOCCOOH
Nylon 6,6
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Polydispersity is the term we use to describe the fact that
not all macromolecules in a given sample have the same
“repeat number” x.
size
#
size
#
size
Polydisperse Monodisperse Paucidisperse
Even in a pure sample, not all molecules will be the same.
Nature often does better than people do.
#
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The molecular weight of condensation (step growth)
polymers is limited to fairly low values.
Condensations: usually < 50,000 g/mol
Addition: can be quite high
(e.g., 46 x 106 for polystyrene)
Convert that to tons/mol
Nature makes huge polycondensates, but they are usually made in chain growth fashion!
Why?
RAMESH-GEC Kozhikode- Polymer
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Structure
Primary bonds (Chemical bond)
Secondary bonds (intermolecular force, weak Van der Waals force) –
Chains held together
� chain length
� high - Strength of van der Waals force increase (Molecular
weight) < 150 atoms
� Low molecular weight – Soft & gummy (sticky gum), brittle at
low temperature
� High Molecular weight – tough & heat resistant
� Polar groups (carboxyl, hydroxyl, Cl, F) increase interaction – nylon,
teflon, polyester52RAMESH-GEC Kozhikode- Polymer
� Slip movement –
� Simple & Uniform shape - easy movement (PE)
� PVC- lumps of Cl – restricted movement – tougher & Stronger
than PE
� Cross linked – covalent bonds in 3D – No movement – Most
strong & tough
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• The bonding process.
– When thermoplastic polymers are heated they become
flexible. There are no cross-links and the molecules can
slide over each other.
– Thermosettingpolymers do not soften when heated
because molecules are crosslinked together and remain
rigid.
PVCBakelite
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Thermoplastics
• No cross links between chains.
• Weak attractive forces between chains broken by
warming.
• Change shape - can be remoulded.
• Weak forces reform in new shape when cold.
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Thermoplastics
• Those which soften on heating and then harden again on cooling
These are called thermoplastic polymers because they keep their
plastic properties
• These polymer molecules consist of long chains which have only
weak bonds between the chains
• The bonds between the chains are so weak that they can be
broken when the plastic is heated
• The chains can then move around to form a different shape
• The weak bonds reform when it is cooled and the
• thermoplastic material keeps its new shape
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Thermosets
• Extensive cross-linking formed by covalent bonds.
• Bonds prevent chains moving relative to each other.
• What will the properties of this type of plastic be like?
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Thermosetting
• Those which never soften once they have been moulded
These are called thermosetting polymers because once set into a
shape, that shape cannot be altered
• These polymer molecules consist of long chains which have many
strong chemical bonds between the chains
• The bonds between the chains are so strong that they cannot be
broken when the plastic is heated
• This means that the thermosetting material always keeps its shape
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Physical state - Crystallinity
• The polymer chain layout determines a lot of material
properties:
• Amorphous:
• Crystalline:
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Crystallinity in Polymers• Ordered atomic
arrangements involving
molecular chains
• Crystal structures in terms of
unit cells
• Example shown
– polyethylene unit cell
Adapted from Fig. 4.10, Callister & Rethwisch 3e.
– Polymers can be crystalline (i.e. have
long range order)
– However, given these are large
molecules as compared to atoms/ions
(i.e. metals/ceramics) the crystal
structures/packing will be much more
complexRAMESH-GEC Kozhikode- Polymer
• Polymer crystallinity
– (One of the) differences between small molecules and
polymers
– Small molecules can either totally crystallize or become an
amorphous solid
– Polymers often are only partially crystalline
• Why? Molecules are very large
• Have crystalline regions dispersed within the remaining
amorphous materials
• Polymers are often referred to as semicrystalline
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Polymers rarely 100% crystalline
• Difficult for all regions of all chains to
become aligned
• Degree of crystallinity
expressed as % crystallinity.-- Some physical properties
depend on % crystallinity.
-- Heat treating causes
crystalline regions to grow
and % crystallinity to
increase.
Adapted from Fig. 14.11, Callister 6e.(Fig. 14.11 is from H.W. Hayden, W.G. Moffatt,and J. Wulff, The Structure and Properties of
Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)
crystalline region
amorphousregion
RAMESH-GEC Kozhikode- Polymer
• Polymer crystallinity
– Another way to think about it is that these are two phase
materials (crystalline, amorphous)
– Need to estimate degree of crystallinity – many ways
• One is from the density
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– What influences the degree of crystallinity
• Rate of cooling during solidification
• Molecular chemistry – structure matters
– Polyisoprene – hard to crystallize
– Polyethylene –not hard to crystallize
• Linear polymers are easier to crystallize
• Side chains interfere with crystallization
• Stereoisomers – atactic hard to crystallize (why?);
isotactic, syndiotactic – easier to crystallize
• Copolymers – more random; harder to crystallize
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Effect of crystallinity on properties
� Density of crystalline regions will be higher
� Less permeable
� Less ease of acid hydrolysis on cellulose
� Less attack of oxygen (PE, Polypropylene)
� High degree of crystallinity - Transparent
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4.12 Polymer crystals
– Chain folded-model
• Many polymers crystallize as very thin platelets (or lamellae)
• Idea – the chain folds back and forth within an individual plate
(chain folded model)
4.12 Polymer Crystallinity
• Crystalline regions
– thin platelets with chain folds at faces
– Chain folded structure
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Plastic Deformation
• Thermoplastics- structure deforms to plastic stage on the
application of heat & pressure
• Plastic stage – help to get moulded in the desired shape
• Linear polymers – maximum plastic deformation
• Plasticity decreases proportionally with fall in temperature
• Thermoset – cross linking – covalent bond – No chance for slip or
deformation
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Mechanical Properties
o Strength measured by stress-strain test
o Amount of stretch for a given stress (applied force) is a measure
of strain
o Stress strain graph – “necking and break point
o Necking – Stress becomes large- uncoiled and line up to a more
orderly arrangement
o Break point – stress becomes so large to break the sample
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Tensile Strength
� Defined as the tensile stress requires to stretch the test piece to
the break point
� Expressed in terms of breaking force per unit area of the original
cross sectional area
� Quantifies how much stress the material will endure (tolerate)
before failure
� Ability to withstand stress with getting pulled apart in linear
direction
� Measured by stretching a dumb bell test piece in universal testing
machine
� Increases with polymer chain length & crosslinking
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Testing gives information about
� How much force it takes to pull a polymer material apart (tensile
strength at break)
� How far a polymer material will stretch before break (elongation
at break)
� How it deforms as it is gets pulled apart (ratio of tensile stress to
tensile strain
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Mechanical Properties of Polymers
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Tensile strength
• Mechanical behavior of amorphous and semi-
crystalline polymers is strongly affected by Tg
• In general
• Polymers whose Tg is above the service
temperature are strong, stiff and sometimes
brittle• e.g. Polystyrene (cheap, clear plastic drink cups)
– Polymers whose Tg is below the service temperature
are weaker, less rigid, and more ductile
• Polyethylene (milk jugs)
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Tear Resistance (strength)
• Measure of how well a material can withstand the effects of
tearing
• How well a material resists the growth of any cuts when under
tension
• Measured in kN/m
• Sample is held between two holders and a uniform pulling force
applied
• Tear resistance is calculated by dividing the force applied by
thickness of material
75RAMESH-GEC Kozhikode- Polymer
Abrasion Resistance
� Defined as the resistance to wear or reciprocal of the abrasion
loss
� Scratch test – material subjects to many scratches (abrasive
wheel) and abrasion determined by loss of weight
� Footwear industry
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Resilience
� The physical property of a material that can return to its original
shape or position after deformation that does not exceed its
elastic limit
� Ability of the material to absorb energy when it is deformed
elastically and release that energy upon unloading
� Measure how far the test specimen of the polymer will recover
the original dimension on the release of stress
� Pendulum rebound testing machine
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Density
� Density = mass per unit volume (volumetric mass density)
� Relative density or specific gravity – ratio of the density to
the material to the standard material (water)
� Measured to
� Identify a material
� Follow physical changes
� Understand the uniformity of a material among different
sampling units
� Understand the porosity of the material
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Glass transition temperature (Tg)
The glass transition temperature (Tg) describes
the temperature at which amorphous polymers
undergo (a second order) phase transition from
a hard brittle, glassy amorphous solid (glassy
state) to a soft, rubbery, viscous amorphous
solid (rubbery/viscoelastic state)
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Melting temperature (Tm)
• The (Tm) when applied to polymers
suggests not a solid-liquid phase transition,
but a transition from a crystalline phase to
a solid amorphous phase. Crystalline
melting is only discussed with
thermoplastics, as thermosets will
decompose at high temperatures rather
than melt.
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Glass-rubber-liquid
• Amorphous plastics have a complex thermal profile with 3
typical states:
Glassy (Elastic-high modulus)
Leathery
(Elastic-low modulus)
Thermoplastic (uncrosslinked)
Tg Tm
Modulu
s o
f ela
sticity
Temp.
Rubbery Plateau
Elastic at high strain rate
Viscous at low strain rate
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How polymers are effected by temperature changes
• Heats solids made of small molecules – melt to form liquid an eventually boil
• Polymers not so simple• E.g. rubber cooled in liquid nitrogen becomes brittle
and can be smashed• It becomes GLASSY• poly(propene) becomes brittle at about -10 C• Structure of many polymers mixture of ordered
areas (crystalline) and random (amorphous)• In glassy state the amorphous regions become
‘frozen’ so cant can’t change shape if it has to move it does so breaking
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How polymers are effected by temperature changes
• If you heat the glassy material, polymer chains reach a temp at which they move relative to each other. This is the glass transistion temperature (Tg)
• When polymer is warmer than this, we see the typical plastic properties we expect-
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How polymers are effected by temperature changes
• On further heating we reach the melting temperature (Tm)
• The crystalline regions break down and polymer becomes a viscous fluid
• These processes are reversible for thermoplastics
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Matching polymer properties to needs
• Pure poly(chloroethene)- PVC has a Tg of about 80 C – rigid and quite brittle at room temp
• Used to make drain pipes
• Sometimes called unplasticised PVC or uPVC
• To make it more flexible the Tg needs to be lowered.
• One way of doing this is to copolymerise the chloroethene with a small amount of ethenyl ethanoate
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Matching polymer properties to needs
• Introduces different side groups into the polymer chain
• Chains pack together less well – attractive forces are weaker
• Polymer is more flexible because the chains can move over one another more easily
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Plasticizers
Are small molecules which occupy position
between polymer chains (like adding
water to mud to make it easy in molding)
1. To increase flexibility, elongation and to
reduce hardness and stiffness.
2. To lower the processing temperature
(energy saving, decomposition
preventing)
87RAMESH-GEC Kozhikode- Polymer
Matching polymer properties to needs
• Another way is to use a ‘molecular lubricant’ – a plasticiser
• Allows the PVC chains to slide over each other more easily
88RAMESH-GEC Kozhikode- Polymer