Questions of Assignment
Q.1 Write three monomers for each which can polymerize through:
(a) Addition polymerization
(b) Condensation polymerization
(c) Emulsion polymerization
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Q.2 Describe at least three methods or techniques for degradation of
polymers ?
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Q.3 Generate data points , calculate the number average (Mn) , weight
average (Mw) molecular weights and degree of dispersity(P).
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Q.4 Describe at least three methods for the preparation of colloidal
particles?
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Q.5 Generate data points and calculate potential of three colloidal
particles?
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Q.1 Write three monomers for each which can polymerize through:
(b) Addition polymerization
(c) Emulsion polymerization
The condensation polymers are formed by repeated condensation reaction
between two different bi-functional or tri-functional monomeric units. In these
polymerisation reactions, the elimination of small molecules such as water,
alcohol, hydrogen chloride, etc. take place.
Example.1
Bakelite is a condensation polymer of phenol and formaldehyde.
Example.2
A carboxylic acid monomer and an alcohol monomer can join in an ester
linkage.
Example.3
A carboxylic acid monomer and an amine monomer can join in an amide linkage.
The polymer made from these two six-carbon monomers is known as nylon-6,6.
b) Addition polymerization
Addition polymerization is a chemical reaction in which simple molecules
(monomers) are added to each other to form long-chain molecules (polymers)
without by-products.
Example.1
PVC (polyvinyl chloride) is formed from vinyl chloride (H2C=CHCl).
Example.2
Polypropylene made from the monomer propylene.
Example.3
Polystyrene is an aromatic polymer made from the monomer styrene.
c) Emulsion polymerization
A polymerization reaction that occurs in one phase of an emulsion. Emulsion polymerization involves the formation of synthetic latexes and resins by the
polymerization of amonomer-in-water emulsion.
Example.1
of tetrafluoroethylene monomer.
Example.2
Styrene-Butadiene-Rubber (SBR) is a synthetic rubber copolymer consisting of
styrene and butadiene.
Example.3
Acrylonitrile is used principally as a monomer to prepare the polyacrylonitrile.
Q.2 Describe at least three methods or techniques for degradation of polymers ?
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Polymer degradation
Polymer degradation is a change in the properties tensile strength, color, shape,
or molecular weight of a polymer or polymer based product under the influence
of one or more environmental factors, such as heat, light, chemicals and, in
some cases, galvanic action.
Following are the method or techniques for degradation of polymers:
1. Thermal degradation
2. UV degradation
3. Chemical degradation
1. Thermal degradation of polymers
Thermal degradation of polymers is molecular deterioration as a result of
overheating. At high temperatures the components of the long chain backbone
of the polymer can begin to separate (molecular scission) and react with one
another to change the properties of the polymer. Thermal degradation can
present an upper limit to the service temperature of plastics as much as the
possibility of mechanical property loss. Indeed unless correctly prevented,
significant thermal degradation can occur at temperatures much lower than
those at which mechanical failure is likely to occur. The chemical reactions
involved in thermal degradation lead to physical and optical property changes
relative to the initially specified properties. Thermal degradation generally
involves changes to the molecular weight (and molecular weight distribution) of
the polymer and typical property changes include reduced ductility and
embrittlement, chalking, color changes, cracking, general reduction in most
other desirable physical properties.
Ways of Polymer Thermal Degradation
Depolymerisation
Under thermal effect, the end of polymer chain departs, and forms low free
radical which has low activity. Then according to the chain reaction mechanism,
the polymer loses the monomer one by one. However, the molecular chain
doesn’t change a lot in a short time.This process is common for
polymethymethacrylate (perspex).
CH2-C(CH3)COOCH3-CH2-C*(CH3)COOCH3→CH2-C*(CH3)COOCH3+ H2=C(CH3)COOCH3
Random Chain Scission
The backbone will be break down randomly, could be occurred at any position
of the backbone. The molecular weight decreases rapidly, and cannot get
monomer in this reaction, this is because it forms new free radical which has
high activity can occurs intermolecular chain transfer and disproportion
termination with the CH2’group.
CH2-CH2-CH2-CH2-CH2-CH2-CH2’→ CH2-CH2-CH=CH2 + CH3-CH2-CH2’ or CH2’+CH2=CH-CH2-CH2-CH2-CH3
Side-Group Elimination
Groups that are attached to the side of the backbone are held by bonds which are
weaker than the bonds connecting the chain. When the polymer was being
heated, the side groups are stripped off from the chain before it is broken into
smaller pieces. For example the PVC eliminates HCL, under 100-120oC.
CH2(Cl)CHCH2CH(Cl)→CH=CH-CH=CH+2HCl
2. UV degradation of polymers
Many natural and synthetic polymers are attacked by ultra-violet radiation and
products made using these materials may crack or disintegrate. The problem is
known as UV degradation, and is a common problem in products exposed to
sunlight. Continuous exposure is a more serious problem than intermittent
exposure, since attack is dependent on the extent and degree of exposure.
Sensitive polymers include thermoplastics, such as polypropylene, polyethylene,
and poly(methyl methacrylate) as well as speciality fibers like
aramids. UV absorption leads to chain degradation and loss of strength at
sensitive points in the chain structure. They include tertiary carbon atoms,
which in polypropylene occur in every repeat unit.
3. Chemically assisted degradation of polymers
Chemically assisted degradation of polymers is a type of polymer degradation
that involves a change of the polymer properties due to achemical reaction with
the polymer’s surroundings. There are many different types of possible
chemical reactions causing degradation however most of these reactions result
in the breaking of double bonds within the polymer structure.
Example of chemically assisted degradation
Degradation of rubber by ozone
One common example of chemically assisted degradation is the degradation of
rubber by ozone particles. Ozone is a naturally occurring atmospheric molecule
that is produced by electric discharge or through a reaction of Oxygen with solar
radiation. Ozone is also produced with atmospheric pollutants reacted with
ultraviolet radiation. For a reaction to occur, ozone concentrations only have to
be as low as 3-5 parts per hundred million (pphm) and when these
concentrations are reached, a reaction occurs with a thin surface layer (5x10-7 metres)
of the material. The ozone molecules react with the rubber which in
most cases is unsaturated (contains double bonds), however a reaction will still
occur in saturated polymers (those containing only single bonds).
When reaction occurs, scission of the polymer chain (breaking of double covalent bonds)
takes place forming decomposition products:
Chain scission increases with the presence of active Hydrogen molecules (for
example, in water) as well as acids and alcohols. Along with this type of
reaction, cross linking and side branch formations also occur by an activation of
the double bond and these make the rubber material more brittle. Due to the
increase in brittleness due to the chemical reactions, cracks form in areas of
high stress. As propagation of these cracks increases, new surfaces are opened
for degradation to occur.
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Q.3 From the following data points , calculate the number
average (Mn) , weight average (Mw) molecular weights and
degree of dispersity(P).
Number of Molecules
|
Mass of each molecule
|
7
|
1
|
5
|
3
|
4
|
6
|
3
|
9
|
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Mn = ΣNiMi
Σni
Step1: Multiply the weight of the molecule by the number of molecules of that weight
NiMi would be: 7(1) + 5(3)+ 4(6)+3(9) = 73
Therefore, Number Average Molecular Weight Mn = 73 / 19 = 3.842
Step 2: Formula for the calculation of weight average molecular weight
Mw = ΣNiMi ²/ΣNiMi = Σwi Mi /Σwi
The calculation of the weight average molecular weight requires the weight fraction, Wi, of each type of molecule. Weight fraction of individual molecules is the mass of each molecule (NiMi) divided by the total weight (ΣNiMi).
Ni
|
Mi
|
NiMi
|
Wi = NiMi/ΣNiMi
|
MiWi
|
7
|
1
|
7
|
0.095
|
0.095
|
5
|
3
|
15
|
0.205
|
0.615
|
4
|
6
|
24
|
0.328
|
1.968
|
3
|
9
|
27
|
0.369
|
3.321
|
ΣNiMi = 73
Weight Average Molecular Weight is ΣWiMi = 5.999
The ratio of Mw/Mn is a measure of polydispersity.
P = Mw/Mn
5.999/3.842 = 1.561
Q.4 Describe at least three methods for the preparation of colloidal particles?
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Preparation of Colloidal Solutions
(a) Preparation of Lyophilic Sols:
Hydrophilic colloids such as starch, gum, gelatin, etc. form colloidal solution
when warmed or left in contact with water for a long time. The particles are
already of colloidal size and they are readily dispersed in water forming
colloidal solution. Sols of water insoluble high molecular weight compounds
may similarly be obtained by their dispersal in suitable liquids.
(b) Preparation of Lyophobic Sols:
Lyophobic sols cannot be prepared by direct mixing of dispersed phase and
the dispersion medium. These are prepared by using special techniques.
The methods employed for the preparation of lyophobic colloids fall into
two categories:
(i) Dispersion methods in which larger macro-sized particles are broken down
to colloidal size.
(ii) Condensation methods in which particles of colloidal size are produced by
aggregation of single ions or molecules.
(i) Dispersion Methods
1. Bredig’s Electric Arc Method:
This method is employed to prepare sols of metals such as copper, silver, gold
or platinum. The two electrodes used in this method are made of the metal
whose sol is to be prepared. The electrodes are immersed in dispersion medium
such as water. The dispersion medium is cooled by immersing the container
in an ice bath and a trace of alkali is added. An arc is struck between the metal
electrodes held close together. The tremendous heat generated by the spark
across the electrodes vapourises some of the metal and the vapour condenses
immediately in the dispersion medium to give colloidal solution. The trace of
added alkali helps to stabilise the sol.
2. Mechanical Dispersion:
It is done with the help of a colloid mill. The mill consists of two steel discs
with a small gap between them. The discs rotate in opposite direction at high
speed. The substance whose sol is to be prepared is first ground as finely as
possible and then shaken with the dispersion medium to get a suspension. The
suspension is added to the colloid mill.The speed of the rotating discs is
adjusted so that the particles of the suspension are broken to produce the
particles of colloidal size.
3. Ultrasonic Dispersion:
Sound waves having frequency more than that of the audible sound are called
ultrasonic waves. These waves can produce particles of colloidal size from
coarse suspension. This method is used to prepare a sol of mercury in water.
The ultrasonic waves produced from a quartz generator propagate through the
oil and strike the beaker containing mercury under water. The ultrasonic waves
transfer their energy to the atoms of mercury. Mercury gets vapourised and
the vapours disperse in water producing colloidal solution. This is the latest method
for the preparation of metal sols from their coarse suspension.
(ii) Condensation or Aggregation Methods
1. Lowering of solubility by exchange of solvent:
In this method, a substance is dissolved in a solvent and then the solution is
added to another solvent in which it is less soluble. For example if an alcoholic
solution (true solution) of sulphur is added in excess of water, a colloidal
solution of sulphur results. Sulphur is insoluble in water.
2. Passing vapours of an element into a liquid:
When the vapours of an element are passed through a liquid, condensation takes
place to give a colloidal solution. For example, colloidal solution of mercury
can be obtained by passing the vapours of mercury into cold water containing
suitable stabilising agents such as ammonium salts or citrates.
3. Excessive cooling:
The method can be used to get colloidal solution of ice in an organic solvent
like chloroform or ether. A solution of water in the required solvent is frozen.
The molecules of water, which can no longer be held in the solution, get
together to form particles of colloidal size.
Q.5 Generate data points and calculate potential of three colloidal particles?
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By directly measuring the electrophoretic mobility of a particle, the zeta
potential may then be determined using the Henry Equation:
UE = 2εzf(Ka)/3η
where UE is the electrophoretic mobility, ε is the dielectric constant, z is the
zeta potential, f(Ka) is Henry’s function, and η is the viscosity. Henry’s function
generally has value of either 1.5 or 1.0. For measuring zeta potential in aqueous
solutions of moderate electrolyte concentration, a value of 1.5 is used and this is
referred to as the Smoluchowski approximation.
1. Calculate the Zeta Potential of colloidal particle when the Particle size =235nm,Dielectric constant of water =78.5, Electric field = 8.13 V/cm,Viscosity of water = 0.890 cP and Mobility= -4.45 μ/s ?
UE = 2εzf(Ka)/3η
-4.45 = 2(78.5)z(1.5)/3(0.890)
-4.45 = z(235.5)/2.67
z(235.5) = (4.45)(2.67)
z(235.5) = 11.881
z = 11.881/235.5
z = +0.0504 mv
2. Calculate the Zeta Potential of colloidal particle when the Particle size = 150nm, Dielectric constant of water = 78.5, Electric field = 27.04 V/cm, Viscosity of water = 0.890 cP and Mobility = -1.07 μ/s?
UE = 2εzf(Ka)/3η
-1.07 = 2(78.5)z(1.5)/3(0.890)
-1.07 = z(235.5)/2.67
z(235.5) = (1.07)(2.67)
z(235.5) = 2.856
z = 2.856/235.5
z = +0.0121 mv
3. . Calculate the Zeta Potential of colloidal particle when the Particle size = 200nm, Dielectric constant of water = 78.5, Electric field = 25.78 V/cm, Viscosity of water = 0.890 cP and Mobility = -2.07 μ/s?
UE = 2εzf(Ka)/3η
-2.07 = 2(78.5)z(1.5)/3(0.890)
-2.07 = z(235.5)/2.67
z(235.5) = (2.07)(2.67)
z(235.5) = 5.526
z = 5.526/235.5
z = +0.0234 mv
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References:
(i) D. J. Shaw, Introduction to Colloid and Surface Chemistry, Butterworth (1980)
(ii) J. W. McBain, Colloid Science, D.C. Heath and Company, Boston (1950)
(iii) F.W. Billmeyer, Text Book of Polymer Science, Interscience Publishers Inc, New York (1962)
(iv) R.J. Hunter, ‘Zeta Potential in Colloids Science’, Academic Press, NY, 1981
(v) Principles of polymerization by George Odian New York fourth Edition
(vi) PhD Thesis ‘Mobility measurements By phase analysis’, Walther W.Tscharnuter.
(vii) Physical Chemistry ‘Surface Chemistry and Colloids’, By Dr A K Ghosh
(viii) Everett, D.H. (1994) Basic Principles Of Colloid Science, The Royal Society of Chemistry, UK.
(ix) Ross, S. and Morrison, I.D. (1988) Colloidal Systems and Interfaces,
John Wiley and Sons, USA.
(x) A.W. Admason and A.P. Gast, Physical Chemistry of Surfaces, Wiley, New York (1997).
Site Links:
i) en.wikipedia.org/wiki/Condensation_polymer
ii) dictionary.reference.com/browse/condensation+polymerization
iii) en.wikipedia.org/wiki/Addition_polymer
iv) en.wikipedia.org/wiki/Polymer_degradation
v) en.wikipedia.org/wiki/Colloid
vi) en.wikipedia.org/wiki/Colloidal_particle