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Polymer chemistry

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Polymer Chemistry
Chap 1.The Scope of Polymer Chemistry
Terminology
• Monomer: A substrate that can be converted into a polymer.
monomer
(ethylene)
(Amino acid)
polymer
(Polyethylene)
(Polypeptides)
• Dimer: Two monomer molecules react together.
• Trimer: Dimer react with a third monomer.
• Oligomer: A molecular complex of chemicals that consists of a few repeating
units.
Glycolic acid
(monomer)
Dimer
Trimer
Example of Oligomers
Benzene
(cyclic trimer)
Acetylene
(monomer)
Polyacetylene
Trioxane
(cyclic trimer)
Formaldehyde
(monomer)
Polyformaldehyde
Polymer
-The term polymer is used to describe high-molecular-weight substances.
A polymer (Greek poly-, "many" + -mer, "part") is a large molecule,
or macromolecule, composed of many repeated subunits. (from Wikipedia)
- However, this is a very broad definition, and in practice it is convenient to
divide polymers into subcategories according to their molecular weight and
structure.
- Generally, low polymers = MW 10,000 ~ 20,000
High polymers = MW 20,000 ~ several million
- The numbers of repeating units in a chain of polymer is known as degree of
polymerization (DP).
Types of Polymers
Nature polymers
Synthetic polymers
• Collagen
• Polyethylene terephthalate (PET)
• Gelatin
• High Density Polyethylene (HDPE)
• Silk
• Polyvinyl Chloride (PVC)
• Wool
• Low Density Polyethylene (LPDE)
• Natural rubber
• Polypropylene (PP)
• DNA
• Polystyrene (PS)
Examples of Polymers
• Polypropylene (PP) - Carpet, upholstery
• Polyethylene low density (LDPE) - Grocery bags
• Polyethylene high density (HDPE) - Detergent bottles, toys
• Poly(vinyl chloride) (PVC) - Piping, decking
• Polystyrene (PS) - Toys, foam
• Polytetrafluoroethylene (PTFE, Teflon) - non-stick pans, electrical insulation
• Poly(methyl methacrylate) (PMMA, Lucite, Plexiglas) - Face shields, skylights
• Poly(vinyl acetate) (PVAc) - Paints, adhesives
• Polychloroprene (cis + trans) (Neoprene) - Wetsuits
Characteristics of Polymers
•
Low Density.
•
Low coefficient of friction.
•
Good corrosion resistance.
•
Good mold ability.
•
Excellent surface finish can be obtained.
•
Can be produced with close dimensional tolerances.
•
Economical.
•
Poor tensile strength.
•
Low mechanical properties.
•
Poor temperature resistance.
•
Can be produced transparent or in different colors.
Physical Properties of Polymers
The physical properties of a polymer, such as its strength
and flexibility depend on:
•
•
•
•
Chain length - in general, the longer the chains the stronger the polymer;
Side groups - polar side groups give stronger attraction between polymer
chains, making the polymer stronger;
Branching - straight, un branched chains can pack together more closely
than highly branched chains, giving polymers that are more crystalline and
therefore stronger;
Cross-linking - if polymer chains are linked together extensively by covalent
bonds, the polymer is harder and more difficult to melt.
Properties of Polymers
Polymer material properties depend on
1.
Degree of Polymerization
2.
Molecular Weight of the Polymer
3.
Molecular Weight Distribution
4.
Glass Transition Temperature
5.
Percentage of Crystallinity
6.
Structure and Distribution of Chain Branching
Linear polymer
- A long chain of skeletal atoms to which are attached the substituent groups.
- Linear polymers are usually soluble in some solvent, and in the solid state at normal
temperatures they exist as flexible materials, or glasslike thermoplastics.
EX) Polyethylene, Poly(vinyl chloride, PVC), Poly(methyl methacrylate, PMMA),
Polyacrylonitrile, Nylon
Branched polymer
- A linear polymer with branches of the same basic structure as main chain.
- They are often soluble in the same solvents as the corresponding linear polymer.
- However, they can sometimes be distinguished from linear polymers by their
lower tendency to crystallize or by their different solution viscosity or light
scattering.
Crosslinked polymer
- Polymer in which chemical linkages exit between the chains.
- Actually, the amount by which the polymer is swelled by a liquid depends on
the density of crosslinking.
- Degree of crosslinking is high enough, the material may be a rigid, high-melting, and
unswellable solid.
Star polymer
-Polymer which has arms radiating from a common core.
- The number of arms may vary from three to six or more.
- Such polymers are prepared either by growing the arms by polymerization
from a multifunctional core, or by linking preformed polymer molecules to a
core through reactive end groups on the polymer.
Tri-star polymer
Dendrimeric polymer
- One which has three-dimensional and repetitively branched molecules.
- Preparation method
1.
Core-first: They are accessible through the reactions of a multifunctional core
with a tri- (or higher) functional monomer. The growth of the molecule is usually
carried out in successive layers or “generations” moving further and further
from the core.
2. Arms-first: Each highly branched arm is synthesized first, and several of these are
then linked in a final step to the core.
1st generation
2nd generation
3rd generation
Cyclolinear polymer
- A special type of linear polymer formed by the linking together of ring systems.
-The properties resemble those of conventional linear polymers, except that
the solubility of the cyclolinear species is often low.
-The tendency for crystallization may be very high.
Ladder polymer
- Polymer consists of linear molecules in which two skeletal strands are linked
together in regular sequence by crosslinking units.
- Ladder polymers have a more rigid molecular structure than do conventional
linear polymers and they are often much less soluble.
Cyclomatrix polymer
- One in which ring systems are linked together to form a three-dimensional
matrix of connecting units.
- Since 3D network of bonds is formed in these systems, the polymers are highly
insoluble, rigid, very high melting, and usually stable at elevated temperatures.
Silicon resins
Copolymer
-
Polymer made from two or more different monomers
- Different types of sequencing arrangements
a.
Radom Copolymer
b.
Alternative copolymer
c.
ABA Block copolymer
d.
Graft copolymer
Terpolymer
- A terpolymer contains three different monomer units. These can be
sequenced randomly or in blocks.
https://doi.org/10.1155/2014/350832
Telechelic Polymers
- A telechelic polymers is one that bears reactive functional groups at one or
both of its chain ends.
- These are frequently produced by living polymerization processes.
Average Molecular Weights and Distributions
Number-average molecular weight
σπ’Š π‘΅π’Š π‘΄π’Š
𝑴𝒏 = ෍ π‘Ώπ’Š π‘΄π’Š =
σπ’Š π‘΅π’Š
π’Š
Ni = the number of molecules of molecular weight Mi
Xi = the number fraction or mole fraction of molecules of molecular weight Mi
Weight-average molecular weight
π‘΄π’˜ = σπ’Š π‘Ύπ’Š π‘΄π’Š =
σπ’Š π’˜π’Š π‘΄π’Š
σπ’Š π’˜π’Š
=
σπ’Š π‘΅π’Š π‘΄πŸπ’Š
σπ’Š π‘΅π’Š 𝑴 π’Š
wi = the weight of molecules of molecular weight Mi
Wi = the weight fraction of molecules of molecular weight Mi
Polydispersity index (PDI, fraction MW/Mn)
The fraction is a measure of the molecular weight distribution.
For most polymers Mw/Mn: 1.5 – 2.5
For monodisperse polymer: Mw/Mn = Mz/Mw = 1
Mw
> 1 Polydisperse
Mn
Mw
= 1 Monodisperse
Mn
Influence of Molecular weight of Polymers
The influence of molecular weight on the bulk properties of polyolefin's,
an increase in the molecular weight leads to
Increase in:
• Melt viscosity
• Impact strength
Lowers in:
• Hardness
• Stiffness
• Softening point
• Brittle point
High molecular weight polymer does not crystallize so easily as lower
molecular weight material crystallizes due to chain entanglement and
that reflect in bulk properties of the high molecular weight polymer.
Average Molecular Weights and Distributions
- A sample of a synthetic polymer has no single, fixed molecular weight.
- Instead, there is a distribution of different molecular weights in the same sample
of material.
Classification of Polymers
Thermoplastics
- Polymer material that becomes pliable or moldable at a certain elevated temperature and
solidifies upon cooling. (from Wikipedia)
Elastomers
- A flexible polymer that is in the temperature range between its glass transition
temperature and its liquefaction temperature.
- Crosslinked rubbery polymers that can be stretched easily to high extensions and
which rapidly recover their original dimensions when the applied stress is released.
https://www.sciencedirect.com/topics/chemistry/elastomer
Plasticizers
- Additives that decrease the plasticity or decrease the viscosity of a material. These
are the substances which are added in order to alter their physical properties. (from
Wikipedia)
Thermosetting Resin
- A polymer that is irreversibly hardened by curing from a soft solid or viscous
liquid prepolymer or resin. (from Wikipedia)
Polymer blends
- Two or more polymers are mixed together mechanically.
- Many polymer blends display properties that are different from those of the
individual polymers.
- Polymer blends can be of two main types, miscible or immiscible.
Glass Transition Temperature (Tg)
- The temperature below, which a polymer is hard and above which it is soft, is
called the glass transition temperature (Tg).
- The molecular mobility is just starts above that temperature or below which
mobility arrested called Tg .
- Tg is unique to amorphous thermoplastics. It occurs at a specific temperature
that depends on pressure and specific volume and is lower than melting point.
Comparison of the thermal behavior of amorphous,
crystalline, and liquid crystalline polymers
Tacticity
Isotactic : All the substituents are located on the same side of the macromolecular
backbone.
Syndiotactic : The substituents have alternate positions along the chain.
Atactic : The substituents are placed randomly along the chain.
Condensation Polymers (Step Reactions)
- Step-growth polymers, also called condensation polymers. This type of polymers is
generated by the condensation of two monomer units with the loss of small molecules
such as H2O, HCl, and NH3 etc. Here the monomer units must have two functional
groups in order to do condensation reaction.
Nylon 6,6 — 2 monomers: 6-carbon diamine + 6-carbon diacid
hexamethylene diamine + adipic acid
Polyester from a di-alcohol and a di-acid
A polyester is a synthetic condensation polymer formed by a series
of esterification reactions between monomers.
Monomers with two functional groups, –OH (hydroxyl) group of
alcohols and –COOH (carboxyl) group of carboxylic acids, form chains by
undergoing condensation reactions at both ends.
As each monomer molecule is added to the chain, the condensation reaction
produces a small by-product molecule, such as water.
http://biology.reachingfordreams.com/chemistry-cheat-sheet/organic-chemistry/35-polymers
Phenol-formaldehyde resins
These thermosetting resins are obtained by reacting phenol C6H5OH and
phormaldehyde CH2O. Formaldehyde forms −CH2− bridges between two phenol
molecules, producing chains. Linear chains are obtained when the reaction ratio
is 1:1. Phenol, however, may also react with a third formaldehyde molecule.
Whenever this happens, a branch is formed in the chain.
http://www.whatischemistry.unina.it/en/bakelite.html
Urea-formaldehyde resins
Urea formaldehyde (UF) resins are primarily made up of urea and formaldehyde
with formaldehyde acting as the crosslinker. The UF resins are formed in water at
a pH above 7 at the start of the reaction, because the methylol derivatives that
form in the first steps condense rapidly at acidic conditions.
http://polymerdatabase.com/polymer%20classes/UreaFormaldehyde%20type.html
Inorganic condensation polymers : polyphosphate
Polyphosphate, which is known as inorganic condensation polymer, is formed by removing
water from the appropriate di-, tri-, or tetrahydroxy monomer.
https://www.odinity.com/investigating-polyphosphates-spectroscopy/
Biological condensation polymerizations
When two amino acid monomers are close together, they may be joined to each
other by peptide bonds to form a polypeptide chain. The reaction is a
condensation reaction. Polypeptides can vary in length from a few amino acids to
a thousand or more. The polypeptide chains are then joined to each other in
different ways to form a protein. It is the sequence of the amino acids in the
polymer that gives a protein its particular properties.
https://cnx.org/contents/[email protected]/Organic-Macromolecules-Biological-macromolecules
Addition Polymers
Addition polymers are formed by the sequential addition of the monomer units with
the help of a reactive intermediate such as free radicals, cation or anions without loss
of small molecules. The addition polymerization generally involves three steps called
initiation, propagation and termination. These steps apply to all types of addition
polymerization such as free-radical, cation and anion. In this process alkenes are
typically used as monomers and polymerization results by successive additions across
the double bonds.
Examples of Addition Polymer
Ring-Opening Polymerizations
Ring opening polymerization is a versatile process to polymerize a wide range of
cyclic monomers.
Examples of Ring-Opening Polymer
Rhombic sulfur
Plastic sulfur
trace of
n
acid or base
Octamethylcyclotetrasiloxane
Poly(dimethylsiloxane)
Preparation of polyphosphazene
Initiators
A few polymerization reactions take place spontaneously, but most require the addition of
small quantities of compounds called initiators.
Benzoyl Peroxide
C O O C
O
O
CN
H3C
CN
C N N C CH3
CH3
CH3
Azobisisobutyronitrile (AIBN)
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