Structures and properties of polymers Flashcards
What is a polymer?
large molecule
(macromolecule) composed of
repeating structural units
typically connected by covalent
chemical bonds
How can polymers be classified ? and give an example of each.
Base on the source either
1.Natural Polymers: These polymers are found in plants and animals: proteins,
cellulose,
starch,
natural rubber
2. Synthetic Polymers: synthesised by chemical methods plastic (polyethylene),
synthetic fibres (nylon 6,6) and
synthetic rubbers (polybutadiene)
How can polymers be classified based on the structure of polymers? and give an example of each.
1.Linear Polymers: These polymers consist of long and straight
chains.
high density polyethene,
PVC,
2.Branched Polymers: These polymers contain linear chains having
some branches,
low density polyethene.
3.Cross-linked Polymers: These are usually formed from the
monomers with more than two functional groups and contain
strong covalent bonds between various linear polymer chains,
vulcanized rubber,
urea-formaldehyde resins,
Epoxy.
How can polymers be classified based on molecular forces?
Mechanical properties of polymers are governed by molecular weight and intermolecular forces. ie van der waals forces, hydrogen bonds etc.
Ie Elastomers
Fibres
Liquid resins
Plastics
Thermoplastic and
thermosetting plastic.
How are thermosetting polymers created?
mixing 2 components which react together and harden at either room temperature or heating.
They are heavily cross linked and so on re heating this prevents melting or viscous flow so it cannot be re-moulded. Further heating results in decomposition. much unlike thermoplastics.
How are thermoplastics structurally classified?
Linear or branched polymers
What is the typical properties and behaviour of elastomers?
They can be stretched several times their original length and return to their original dimension on unload. they have occasional crosslinks.
What are the 3 basic categories of molecular variables which cause processing and performance of polymeric materials?
Composition, molecular weight and intermolecular order.
What are processing and chemical conditions that affect the size of the polymer structure?
Processing : thermal treatment, flow induced orientation, cooling rates
chemical: constituents, topology, functionalities, compositions
What are main advantages/disadvantages of thermoplastic and thermosetting polymers?
Thermoplastics are more easily recyclable than thermosets. Thermosets can be very strong but can be quite brittle in behaviour.
What are the 5 main types of polymerisation and what is it?
a) Polymers are formed by polymerisation of monomers.
- Addition (or free radical) polymerisation
2.Condensation (step) polymerisation
3.Ring-open polymerisation
4.Living polymerisation
5.………(anionic, cationic polymerisation….)
What is free radical (addition) polymerisation?
1.Successive addition of monomer
molecules onto the reactive ends of a growing polymer called chain-growth polymerisation or addition polymerisation (vinyl type of monomers).
2. Free radical monomer attacks double bond of another monomer to form bond and propagate the free radical.
3. Most vinyl monomers are amenable to free-radical polymerisation.
Amorphous glass temp is more imporatn but semi crystalline melting temperature is more important.
What are the 3 main stages of addition polymerisation? (free radicals)
- Initiation
2 Propagation - free radicals can react with oxygen or another inert gas or it would take place in a vacuum. - Termination - either through disproportionation or combination
What is living polymerisation?
When a type of polymerisation that does not undergo termination reaction and so this ‘libing’ continues until the monomer supply is exhausted.
Why can living polymersisation be a positive/negative?
-ve When this happens, the free radicals become less active due to
interactions with solvent molecules. If more monomers are added
to the solution, the polymerisation will resume.
+ve Uniform molecular weights (low polydispersity) are characteristic
of living polymerisation. Because the supply of monomers is
controlled, the chain length can be manipulated to serve the
needs of a specific application. Assuming initiator is 100% efficient.
What is condensation polymerisation
Involves the build-up of molecular weight in a stepwise fashion, by the random combination of monomer molecules containing reactive
functional groups.
Each step of the process is accompanied by formation of a molecule of some simple compound, often water.
What are the 3 main differences between addition (free radical ) and condestaion polymerisation?
- The method of polymerisation influence both the average
chain length and the distribution of chain lengths. - The chain growth mechanism associated with vinyl
polymerisation creates high molar mass chains every time a
polymerisation is initiated. - The condensation polymerisation process involves the
successive coupling of small units, the chains grow slowlyand the final polymer material will retain significant traces of the original monomers.
What is ring- open polymerisation?
Modern synthetic methods have revolutionised polymer chemistry
through the development of new and powerful strategies for the
controlled synthesis of complex polymer architectures.
Why is hydrogen bonding between polymer chains positive?
enhances crystallinity
What are the 4 main methods of polymerisation?
Bulk, solution, suspension and emulsion
What is bulk polymerisation?
Reaction is carried out in
the absence of solvent,
diluent, or other materials.
Viscosity increases
dramatically during
conversion.
The method is used for
the polymerisation of liquid
state monomers including
PS, PVC, PMMA and
LDPE.
Heat removal is critical to
avoid formation of
explosive compounds. Takes place in a flask with a temperature probe and a mixer with both nitrogen in and out.
What is solution polymerisation?
Reaction in the presence of inert solvent & initiator (20%
monomer +80% solvent typical).
The viscosity of the solution does not increase and large reduction in heat generated
Polymer formed can be used for surface coating.
It is used for the production of Polyacrylonitrile, PVC,
Polyacrylamide, Polyvinyl alcohol, PMMA, Polybutadiene, etc
What are the advantages of solution polymerisation?
The solvent acts as a diluent & helps in facilitating continuous
transfer of heat of polymerisation and temperature control is easy.
The solvent allows easy stirring due to decrease of viscosity.
Solvent facilitates the ease of removal of polymer from the reactor.
Viscosity build up is negligible.
What are the disadvantages of solution polymerisation?
Potential toxicity, flammability and environmental pollution of solvents
Polymer product contains solvent impurities
Yield is significantly lower than in bulk polymerisation
Expensive due to additional solvent cost.
What is suspension polymerisation?
Liquid or dissolved monomer suspended in liquid phase like
water. The size of monomer droplets is 50-200 μm in diameter.
The dispersion is maintained by continuous agitation and the
droplets are prevented to coalesce (unite or merge) by adding small quantity of stabilizers.
The polymerisation takes place inside the droplet & product formed being insoluble in water.
The product separated out in the form of spherical pearls or beads of polymer.
What are advantages of suspension polymerisation?
Comparatively cheap: only water instead of solvents.
Viscosity increase is negligible.
Agitation & temperature control is easy.
Product isolation is easy since the product is insoluble in water.
What are disadvantages of suspension polymerisation?
Only for water insoluble monomers.
Difficult to control polymer size.
Polymer purity is low due to the presence of suspending &
stabilizing additives which are difficult to remove completely.
Highly agitation sensitive.
The method cannot be used for tacky polymers such as elastomers because of the tendency for agglomeration of
polymer particles.
What is emulsion polymerisation?
The system consists of:
water insoluble monomer,
dispersion medium (water)
emulsifying agents or surfactants (soaps and detergents)
and
a water soluble initiator (e.g. potassium persulphate).
The monomer is dispersed in the aqueous phase, not as a discrete droplets, but as a uniform emulsion.
The size of monomer droplet is around 0.5 to 10 μm in diameter.
The monomer in water is stabilized by a surfactant.
When it is put into a water, the surfactant molecules gather
together into aggregates called micelles.
The monomer molecules diffuse from monomer droplets to water & from water to the hydrocarbon centre of micelles.
The technique is used for the production of large number of
commercial plastics & elastomers.
What are advantages/ disadvantages of emulsion polymerisation?
More environmentally safe
Reaction occurs in the small droplets (or micelles), leading to
produce higher molecular weight of polymers
Role of water is that of a heat sink, viscosity remains close to that of water and is not dependent on molecular weight.
Product is marketed as polymer-water emulsions (e.g. water-borne paints, adhesives (white glue), etc.)
Difficult to remove the surfactant impurity in polymer
Can not be used for condensation, ionic or Ziegler- Natta polymerisation.
What is the molecular weight?
The mase of a mole of chains
What does molecular affect in polymer material properties?
Resistance to stress in tensile,
flexural and shear modes, toughness, creep resistance, and environmental stress-cracking.
How does the distribution of molecular weights influence polymers?
Can have either high or low molecular weight and distinguishes them from metals and ceramics. The length varies in terms of mean molecular weight.
What is the effect of molecular weight on tensile strength?
As the molecular weight increases (g/mol) then the tensile strength increases.
What are two main ways that the molecular weight distribution is defined?
The number average molecular weight (Mn) - total weight of polymer/ number of polymer chains and
The weight average molecular weight (Mw) - total number of polymer chainsmolecular weight^2/polymer chainsmolecular weight. Takes into account each molecules weight.
How is the breadth of the molecular weight distribution expressed?
Through the polydispersity index (PDI) which is the ratio of the weight average molecular weight to the number of average molecular weight. Mw/Mn. Must be greater than 1.
Can be bimodal, unimodal narrow or broad on a graph.
How does molecular weight effect polymerisation?
Chain polymerisation is much higher in molecular weight but narrower.
Condensation polymer is a broader distribution so much harder to control. It at a lower molecular weight.
Monomer is the lowest molecular weight and is 1 line.
What is a linear chain?
a single backbone with no branches.
What is a branched polymer?
a main chain with
one or more substituent side chains or branches.
The branching of polymer chains affects their ability to slide
past one another by altering intermolecular forces, in turn
affecting bulk physical polymer properties.
What are the main 4 molecular structures for polymers?
Linear, branched, cross linked, network
How does the length of the chain in branched polymers effect the material properties?
Long chain branches may increase polymer strength, toughness and the glass transition temperature due to an
increase in the number of entanglements per chain.
Random length and atactic short chains, on the other hand, may reduce polymer strength due to disruption of organisation and may likewise reduce the crystallinity of the polymer.
What are cross linked polymers?
form long chains, either branched or linear, that can form covalent bonds between the polymer molecules
Because cross-linked polymers form covalent bonds that are
much stronger than the intermolecular forces that attract other polymer chains, resulting in a stronger and more stable material.
Natural rubber is vulcanized by heating the sample for the
crosslink of the sulphur molecules in the rubber polymer chains formed covalent bonds with each other.
What is a polymer network?
A polymer molecule with a high degree of cross-linking
How does more cross linking make affect how the polymer will behave in processing/ how it should be treated?
More makes it more insoluble, infusible, thermosetting, thermosetting resin and thermosets.
Less is soluble, fusible, thermoplastic
What are the states of LDPE and HDPE in terms of branching?
LDPE has the most excessive branching and has a density of 0.910-0.925
g/cm3
. HDPE has minimal branching of its’ polymer chains and has a density of 0.941-0.965 g/cm3
.. Because it is denser it is more rigid and less permeable then the LDPE.
What are examples of copolymers arrangements?
Co polymers are two or more monomers polymerised together:
random – A and B randomly
positioned along chain
alternating – A and B alternate in polymer chain
block – large blocks of A units alternate with large blocks of B units
graft – chains of B units grafted onto A backbone
How does glass transition and melt transition determine the temperature material transition types?
Amorphous materials undergo a “glass transition”, Tg Crystalline materials have a “melt transition”, Tm
semi-crystalline can display both transition types
What factors affect Tm and Tg?
Both Tm and Tg
increase with
increasing chain stiffness
Chain stiffness increased by
presence of
Bulky side-groups
Polar groups or sidegroups
Chain double bonds and
aromatic chain groups
Regularity of repeat unit
arrangements – affects Tm
only
Both transition types originate from intermolecular forces.
How does glass transition temperatures arise?
It arises from the decoupling of the vibrational and translational motions.
What is the glass transition temperature?
The temperature at which
the polymer undergoes the
transformation from a
rubber to a glass
What is the motion that allows a polymer above its glass transition temperature?
long-range segmental motion.
How does volume of the polymer change with temperature? or does it even change?
The changes in conformation that occur above Tg require more volume, so plotting a graph of specific volume or thermal expansion coefficient against temperature will give a value for Tg. The actual volume of the molecules
stays the same through Tg, but the free volume (the volume through which they can move) increases.
What is the effect of temperature on the material modulus?
Below glass transition temperature: hard, brittle solid as a crystalline or amorphous.
Between Tg and Tm, the moduluse decreases bu the crystalline is a tough solid and the amorphous becomes rubbery.
Above Tm, they both viscously melt
What are the 5 viscoelastic behaviours for linear amorphous polymers? ie polystyrene
(1) glassy region; (2) the glass transition region; (3) the rubber plateau region; (4) the rubber flow region and (5) the liquid flow region.
What is the glassy region?
In glassy region, the polymer is glassy and frequently brittle (e.g. PS).
Young’s modulus for glassy polymer just below the glass transition temperature (Tg) is surprisingly
constant over a range of polymers, having the value of ~3Ga.
In the glassy state, molecular motions are largely restricted to vibrations and short-range rotational motions.
What is the glass transition region?
The polymers are leathery but T changes will affect the stiffness of the leather. Modulus drops by 1000…
For quasi-static measurements, the glass transition temperature, Tg, is often taken at the maximum rate of turndown of the modulus at the
elbow, where E ~ 0.1GPa.
A glass transition temperature is often defined as the temperature
where the thermal expansion coefficient undergoes a discontinuity
(Also have other definitions of Tg
: Enthalpic and dynamic etc) .
Qualitatively, the glass transition region can be interpreted as the onset of long-range, coordinated molecular motion, only 1-4 chain atoms are involved in motions below Tg
and 10-50 chain atoms attain sufficient thermal energy to move in coordinated manner in Tg region.
What is the rubbery plateau region?
After sharp drop of the
modulus taking place in the glass transition region, the modulus
becomes almost constant again in the rubbery plateau region with typical values of ~2 MPa.
In the rubbery plateau region, polymers exhibit long-range rubber
elasticity
What are the 3 cases in rubbery plateau region that need to be distinguished?
- The polymer is linear: the modulus will drop off slowly and
the width of the plateau is governed primarily by the molecular weight of the polymer; the higher the molecular weight, the longer is the plateau. - The polymer is crystalline (long dotted line): the modulus will drop to some extent but remain the same after Tg due to the crystals restricted the mobility of the chains. The modulus will drop greatly after melting.
- The polymer is cross-linked (short dotted line): improved rubber elasticity is observed with the creep portion
suppressed.
The dotted line follows the equation E = 3nRT, where n is the number of active chain segments in the network and RT represents the gas constant times
the temperature.
Tg≈ 1/2 - 2/3 of Tm
What is the rubbery flow region?
In this region, the polymer is marked by both rubber elasticity and flow properties, depending on the time scale
of the experiment.
For short time scale experiment, the physical entanglements are not able to relax and the material still behaves rubbery;
For longer times, the increased molecular motion imparted by the increased temperature permits
assemblies of chains to move in a coordinated matter (depending on the molecular weight) and hence to
flow.
This region does not occur for cross linked polymers.
What is the liquid flow region?
In this region, the polymer flows readily, often behaving like
molasses.
The increased energy allotted to the chains permits them to be unentangled rapidly and flow as individual
molecular chains.
For semicrystalline polymers, the modulus depends on the degree of crystallinity. The amorphous portions go through the glass transition, but the crystalline portion remains hard, resulting in a composite modulus.
At the melting temperature the modulus drops rapidly to that of the corresponding amorphous materials in the liquid flow region.
What are the 2 morphologies of polymers and how are they arranged?
Crystalline materials have their molecules arranged in repeating patterns. As such, they all tend to have highly ordered and regular structures.
Amorphous materials, by contrast, have their molecules arranged randomly and in long chains which twist and curve around one-another, making large regions of highly structured morphology unlikely.
The morphology of most polymers is semi-crystalline: a combination with the tangled and disordered regions surrounding the crystalline areas.
What is the equation for crystallisation and crystal growth?
Crystallisation = nucleation + crystal growth
Nucleating agents can be added to semi-crystalline polymers for faster processing
What is homogenous nucleation?
In pure materials this can take a long time and it is possible to take a material well below its Tm before crystallisation occurs
what is heterogenous nucleation?
If impurities/inclusions are present the energy barrier to nucleation is much lower and crystallisation occurs much
faster
What are crystalline regions and single crystals structures for polymers?
Electron micrograph – multilayered single crystals (chain-folded
layers) of polyethylene
Single crystals – only for slow and carefully controlled growth rates
Crystalline regions: thin platelets with chain folds at faces to form Chain folded structure
What is the crystal structure on semi crystalline polymers?
Some semi-crystalline polymers form spherulite structures
Alternating chain-folded
crystallites and amorphous regions
Spherulite structure for
relatively rapid growth rates
What is used and what is a maltese cross in each spherulite?
Cross polarized light is used
The bands or rings in the
spherulite image result from twisting of the lamellar crystals as they extend like ribbons from the centre.
What are common defects in polymers?
Screw dislocation, branch, vacancy, chain ends, edge dislocation, dangling or loose chain etc
Is polymer 100% crystallinity common?
Polymers rarely 100% crystalline- Difficult for all regions of all chains to
become aligned
Degree of crystallinity expressed as % crystallinity. Heat treating causes crystalline regions to grow and increase.
What is a positive of being a more crystalline polymer?
Areas in polymer where chains
packed in regular way.
Both amorphous and crystalline
areas in same polymer.
Crystalline - regular chain
structure - no bulky side groups.
More crystalline polymer -
stronger and less flexible.
What are the ways to crystallise polymers?
Solutions, solidification from the melt, stretching
How can polymers be crystallised from solutions?
This process depends on the degree of dilution:
in dilute solutions, the molecular chains have no connection with each other and exist as a separate polymer coils in the solution.
Increase in concentration which can occur via solvent evaporation, induces interaction between molecular chains. Crystallisation from solution may result in the highest degree of
polymer crystallinity.
how does crystallinity increase from melt solidification?
Nucleation starts with small, nanometer-sized areas where as a result of heat motion some chains or their segments occur parallel.
Those seeds can either dissociate, if thermal motion destroys the molecular order, or grow further, if the grain size exceeds a certain critical value.
Apart from the thermal mechanism, nucleation is strongly affected by
impurities, dyes, plasticizers, fillers and other additives in the polymer. This is also referred to as heterogeneous nucleation.
How can crystallisation by stretching increase?
The mechanism considers crystallization from the melt, which is important for injection moulding of plastic components.
Another type of crystallization occurs upon extrusion used in making fibres and films.
In this process, the polymer is forced through, e.g., a nozzle that
creates tensile stress which partially aligns its molecules. Such
alignment can be considered as crystallization and it affects the
material properties.
The strength of the fibre increases so the anisotropy is enhanced with fillers added. This increases polymer strength through extrusion and blow moulding.
Very good for amorphous elastomers that have rapid crystallisation on stretching.
What are the main structural characteristic that affect the crystallinity of polymers?
Structural regularity, bond flexibility, close packing ability and inter-chain attraction, will affect the crystallinity and Tm of the polymers.
Close packing capability, rigid molecules, strong inter chain attraction, cross linking.
How does structural regularity affect the polymer ability to crystallise?
If the structure of the polymer is regular and orderly, it will pack into
crystals easily. If it’s not, it won’t.
Atactic polystyrene
– amorphous
Syndiotactic polystyrene
– crystalline.
What is the effect of intermolecular forces on polyamides (nylon)?
the polar amide groups in the backbone chain of nylon 6,6 are strongly attracted to each other. They form strong
hydrogen bonds. This strong binding holds crystals together. This
raises the melting point compared to polymers without strong
intermolecular interactions, like polyethylene.
What are the main factors that affect crystallinity?
Degree of polymerisation: polymer containing long chains are more
difficult to crystallise.
Complexity of the polymer: crystallisation is easiest for the polymers with simple structures such as PE (& PS.
Cooling rate: slow cooling, which permits more time for the chains to become aligned, encourage crystallisation.
Annealing: heating an amorphous structure just below the melting
temperature provides the thermal activation that permits crystals to
nucleate and grow.
Deformation: slow deformation of the polymer between the melting and glass transition temperatures may promote crystallisation by straightening the chains, thus permits them to align and close
together.
What is a method to determine crystal structure?
X- rays
How does crystallinity affect the density of the polymer?
Density of a crystalline polymer will be greater than an amorphous one of the same material and molecular weigh, as the chains are more closed packed together for crystalline structure.
It is directly proportional.
What is the equation for degree of crystallinity by weight? EQ PROVIDED
% crystallinity = [ρc(ρs – ρa) / ρs (ρc – ρa
)] x 100
Where ρc is the density of totally amorphous polymer and ρs is the density for which precentage crystallinity is determined and ρa is the density of the totally amorphous polymer.
How does the differing intermolecular order affect crystalline and amorphous polymers?
Processing requires heating above the melting point. Processing at high
temperature possesses the danger of degradation, especially if traces of water are present.
For an amorphous polymer, the polymer is in the rubbery or “soft” state above Tg. This limits its service temperature range.
For a crystalline polymer the crystalline form retains dimensional stability almost up to the melting point. Thus, crystalline polymers can be used at higher service temperatures.
How does crosslinking benefit the polymers glass transition temperature over intermolecular order?
The polymer chains are chemically bonded to each other via
crosslinks.
Crosslinking raises the Tg and increase melt viscosity above Tg. The higher of the degree of crosslinks, the higher of the modulus (the more
brittle) of the polymers
What order is amorphous and crystalline polymers in?
Amorphous – no short range order Crystalline – short range order
Rubber is lightly crosslinked
What is the right way to process thermoplastics vs Thermoset polymers?
Thermoplastic
Use melt in liquid shaping stage
Harden by freezing melt
Liquid-solid reversible
Scrap recovery possible
Ceiling service temperature
Processing usually orient chains
Thermoset*
Use lower Mw or rubbery polymer at shaping stage
Harden by cross-linking using
temperature or catalyst
Liquid goes irreversibly to solid
Scrap cannot be recovered directly
Can withstand high T
Process with low orientation
At what processing conditions can random coils be oriented more?
High speed and High shear rate
How does rapid cooling and molecular weight affect relaxation for orientation?
Rapid cooling during processing does not allow enough time for complete relaxation, leaving a substantial amount of orientation and resultant frozen-in stress in the part.
The higher the molecular weight, the longer it takes for orientated molecules to relax at any given temperature.
What factors affect frozen in stress and how can it be removed?
The frozen-in stress can be removed by heating above the
material’s softening temperature.
The greater the shrinkage and distortion on heating, the greater the frozen-in stress. Generally, the smaller the change on heating, the less likely is the part to fail.
What is a polymer blend? And is it likely for them to blend well?
physical mixing of two or more different polymers. Tends to be quite unlikely polymers will mix well as they are thermodynamically incompatible.
What is a compatibiliser?
Compatibilisers can be used to improve mixing
A compatibiliser can be for example a block copolymer of thetwo polymer we want to blend
