Introduction and Basic Concepts

Essentials of Polymer Science and

Engineering
ChE 297
Basic Concepts and Definitions
• Polymer - Greek poly (many) and mer (parts)
• Large molecule(macromolecule) built up by
the repetition of small chemical units
• Chemical units bound by covalent bonds
• Obtained through the chemical reaction of
small molecular compounds (monomers)
Basic Concepts and Definitions
Polyethylene from monomer ethylene

Polystyrene from monomer styrene

Repeating unit – chemical unit in square brackets

Structural unit – residue from the monomer used in the preparation of the polymer

n – chain length of the polymer; degree of polymerization (DP)
Basic Concepts and Definitions

Monomers – should have reactive functional groups or double/triple bonds to provide
the linkages

Polymer – could be derived from interaction of two or more monomers

Synthesis of Nylon 6,6 or poly(hexamethylene adipamide)

repeat unit consists of 2 structural units

n = 2 (dimer), 3 (trimer); 4 (tetramers) etc – oligomers (undesirable
thermal/mechanical properties)

polymer chain – resulting polymer molecule

n affects properties; e.g. n = 7 for polystyrene (viscous liquid)/ n > 1000 (solid;
commercial grade)
Degree of polymerization and
molecular weight
• DP
a way to quantify the molecular length or size of a
polymer
• We can also use MW(Polymer) = DP x MW (Repeat unit)

Polypropylene (PP)

MW (Repeat unit) = 8x12 + 1x8 = 104

MW of commercial PS with a DP of 1000 = 104,000

MW (Repeat unit) = 3x12 + 1x6 = 42

MW of commercial PP with a DP of 3 x 104 = 1.26 x 106
Molecular weight distribution
• Polymer sample is composed of millions of
polymer molecules (polymer chains)
• Length of a polymer chain is determined by
purely random events
• Mixture of molecules having different chain
lengths
distribution of molecular weight
Molecular weight distribution
curve

If you have a MW distribution, any
experimental measurement of molecular
weight in the given sample gives only an
average value

Number-average molecular weight -
derived from measurements that count
the number of molecules in the given
sample (Mn)

Weight-average molecular weight -
based on methods in which the
contribution of each molecule to the
observed effect depends on its size (Mw)

indicates how broad the differences in the chain lengths of the constituent
polymer molecules in a given sample are; polydispersity; heterogeneity index

If polymer molecules are of the same size (e.g. protein): ratio = 1

For synthetic polymers, ratio > 1
Molecular weight distribution
curve
Nylon 11 has the following structure

If the number-average degree of polymerization, Xn, for nylon is 100 and Mw =

120,000, what is its polydispersity?

Xn and n(DP) define the same quantity for two slightly different entities

DP for a single molecule is n; polymer mass is composed of millions of molecules,
each of which has a certain degree of polymerization (Xn)

N = total number of molecules in the polymer mass

Mr = molecular weight of repeating unit
ni = DP of molecule i
Classification of Polymers
• Natural vs Synthetic
• Biological – enzymes, nucleic acid, proteins
• Plant origin – starch, cellulose, natural rubber
• Synthetic – fibers, elastomers, plastics,
adhesives, etc
Classification of Polymers
• By polymer structure
• Linear, Branched, or Cross-Linked
• Functionality – interlinking capacity; number
of sites available for bonding w/ other
molecules (mono, bi, poly)
 Classification of Polymers by
Structure: Functionality

presence of two condensable groups (-NH2

and -COOH) makes each of these monomers

Styrene = extra pair of bifunctional
electrons in the double bond
endows it with the ability to
hexamethylenediamine has a functionality of 2 in
enter into the formation of two amide-forming reactions
bonds; bifunctional

in esterification reactions a diamine has a

functionality of zero
Classification of Polymers by
Structure: Functionality

Functionality depends on the reaction

butadiene
bifunctional
butadiene
tetrafunctional

at right conditions (high temperature; cross-linking) the remaining double bonds might
undergo addition reaction (w/c will result to tetrafunctionality)

Latent functionality – property of monomer where its functional groups react at
different conditions
Classification of Polymers by
Structure: Functionality

What if two monomers are both monofunctional?

reactive groups on the acid and alcohol are used up completely; product ester is
incapable of further esterification reaction

What if two monomers are both bifunctional?

ester is itself bifunctional, being terminated on either side by groups that are capable
of further reaction

same holds for polyfunctional molecules

To generate a polymer through the repetition of elementary units, monomer/s
must be at least bifunctional
Classification of Polymers by
Structure

Linear – when monomer/s is/are bifunctional (polystyrene, polyethylene)

Branched – linear backbone with branches emanating randomly from it
(functionality of monomer > 2)

Cross-linked – growing polymer chains become chemically linked to each other
 Classification of Polymers by
Structure

glycerol + phthalic anhydride

If reaction is allowed for higher

Reaction scheme at low conversion conversion, polymer becomes a 3-

Branched polymer is of low molecular dimensional network (cross-linked network,
weight gel)
Classification of Polymers by
Structure

Another example for cross-linked is cured epoxy
- Adhesives, coatings, aerospace composite matrix
- Epoxy exist as low molecular weight viscous fluid (prepolymer)
- Cures (converted to a cross-linked network) by amines/Lewis acids
 Classification of Polymers by
Structure

Epoxy curing in two steps:
1) Attack of epoxide group by the primary amine

2) Combination of the resulting secondary amine w/ a second epoxy group to form a

branch point

Presence of branch points leads

Functionality of hexamethylene diamine (= 2
to cross-linked, infusible,
for Nylon 6; = 4 for epoxy)
insoluble polymer
 Classification of Polymers by
Structure

Polystyrene Polyethylene Polyacrylonitrile Poly(vinyl chloride)

Poly(methyl methacrylate)

Examples of linear polymers

Pendant groups – substituent groups attached to the main chain of skeletal atoms
(-CH3, -Cl, -CN, etc); they could change properties of polymers (e.g. solubility)

Cross-linked polymers – generally insoluble; swelled by liquids; highly stable, rigid, high-melting
Classification of Polymers by
Structure
Ladder polymer - consists of two
parallel linear strands of molecules with
a regular sequence of crosslinks

have only condensed cyclic units in the
chain; commonly referred to as double-
chain or double-strand polymers

polymerization of aromatic
dianhydrides such as pyromellitic
dianhydride or aromatic tetracarboxylic
acids with orthoaromatic tetramines like
1,2,4,5-tetraaminobenzene

more rigid than that of conventional
linear polymers

exceptional thermal, mechanical, and
electrical behavior
poly(imidazopyrrolone)

thermal stability - requires that two bonds must
be broken at a cleavage site in order to disrupt the
overall integrity of the molecule
Show the polymer formed by the reaction of the
following monomers. Is the resulting polymer linear or
branched/cross-linked?
Show the polymer formed by the reaction of the
following monomers. Is the resulting polymer linear or
branched/cross-linked?
Show the polymer formed by the reaction of the
following monomers. Is the resulting polymer linear or
branched/cross-linked?
Show the polymer formed by the reaction of the
following monomers. Is the resulting polymer linear or
branched/cross-linked?

resulting secondary hydrogens in

the urea linkages are capable of
additional reaction depending on
the stoichiometric proportions of
reactants; urea molecule may be
polyfunctional (tetrafunctional)
Show the polymer formed by the reaction of the
following monomers. Is the resulting polymer linear or
branched/cross-linked?

Though the resulting polymer is linear, it

can be cross-linked in a subsequent reaction
due to the unsaturation on the main chain
using radical initiators.
 An example
When phthalic acid reacts with glycerol, the reaction leads first to the formation of fairly
soft soluble material, which on further heating yields a hard, insoluble, infusible material. If
the same reaction is carried out with ethylene glycol instead of glycerol, the product
remains soluble and fusible irrespective of the extent of reaction.

- either linear, branched, or both

- cross-linking

linear polymer
Classification of Polymers by
Structure: Crystalline polymers
• when polymers are cooled from the molten state or
concentrated in a solution, molecules are often attracted to
each other and tend to aggregate as closely as possible into a
solid with the least possible potential energy
• for some, individual chains are folded and packed regularly in
an ordered fashion
• impossible for chains to fit in a perfect arrangement (e.g. low
molecular-weight polymers)
degree of crystallinity
• PE and PET (crystalline)

Polyethylene

polytetrafluoroethylene Polyethylene terephthalate

Classification of Polymers by
Structure: Amorphous polymers
• polymer chains are incapable of ordered arrangement
• characterized by a short-range order of repeating units
• they vitrify, forming an amorphous glassy solid, polymer
molecules arranged at random and even entangled
• Example: polycarbonate and PMMA
Classification of Polymers by Structure:
Liquid crystalline polymers
• have phases characterized by structures intermediate between the
ordered crystalline structure and disordered fluid state
• their solids melt to form fluids in w/c much of the molecular order is
retained within a certain range of temperature
• ordering impart some solid-like properties on the fluid, but attraction
forces not strong enough to prevent flow
• Example: polybenzamide
• fabrication of lightweight, ultra-high-strength, and temperature-resistant
fibers and films such as Dupont’s Kevlar
 Classification of Polymers by Structure:
Homopolymer or Copolymer
• Homopolymer - only one repeating unit in the
polymer molecule
• Copolymer - composed of two different repeating
units in the polymer molecule

styrene and acrylonitrile polymerized in the same reactor

repeating unit and the structural unit of a polymer are not necessarily the same

Nylon 6,6 and PET - repeating units composed of more than one structural unit; still
homopolymers
Classification of Polymers by
Structure: Copolymer systems
• Random copolymer – repeating units arranged
randomly
• Alternating copolymer - ordered (alternating)
arrangement of 2 repeating units along the polymer
chain
• Block copolymer – consists of relatively long
sequences (blocks) of repeating unit
• Graft copolymer – sequences of one monomer
“grafted” onto the backbone of another monomer
type
Classification of Polymers by
Structure: Copolymer systems
random copolymer

alternating copolymer

block copolymer

graft copolymer
Classification of Polymers by Structure:
Fibers, Plastics, or Elastomers
• Fibers: linear polymers w/ high symmetry and high intermolecular
forces (secondary valence forces; van der Waals, hydrogen bond,
dipole bonds) that result usually from the presence of polar groups;
high modulus, high tensile strength; moderate extensibilities (less
20%)
• Elastomers: polymer molecules w/ irregular structure; weak
intermolecular attractive forces; very flexible polymer chains; in
absence of applied tensile stress, molecules assumed coiled shape;
high extensibility (up to 1000%) from w/c they recover rapidly upon
removal of imposed stress; low initial modulus in tension; stiffen
when stretched
• Plastics: fall between fibers and elastomers; polyamides and
polypropylene (usually classified as plastics) can also be made as
fibers; not clear boundary definition
Classification of Polymers by
Polymerization Mechanism
• old classification – addition, condensation
• Addition polymers – sequential addition of one
bifunctional or polyfunctional monomer to growing
polymer chains; usually very fast

poly(vinyl chloride)

ring-opening for poly(ethylene oxide)

if R in vinyl monomer is an aliphatic

hydrocarbon, then monomer is an olefin
(polyolefin)
Classification of Polymers by
Polymerization Mechanism
• Condensation polymers – formed from bifunctional or
polyfunctional monomers w/ the elimination of a small
molecular species

polyesterification (ethylene glycol +

terephthalic acid = PET)

polyamides (nylon)
Nylon 6,6 – poly(hexamethylene
adipamide)
Nylon 6 – poly(ω-aminocaproic
acid)
Classification of Polymers by Polymerization
Mechanism: Inconsistency with old classification
• Certain polymer molecules could be prepared
by one more than mechanism
For PE, 2nd reaction is neither
addition nor condensation

Typical addition polymerization

gives same polyamide (usual
condensation product)
Classification of Polymers by
Polymerization Mechanism: New Classes
• Chain-growth polymerization – consistent name for addition
polymerization
- individual molecules start growing, grow rapidly, and suddenly
stops
- reaction mass = monomer + non-growing polymer molecules
+ small number of rapidly growing polymer molecules
• Step-growth polymerization – monomers react w/ each other
at the beginning to form low-molecular weight polymer
- monomer is exhausted very quickly
- low-molecular weight polymer molecules continue to react w/
each other to form continually growing chains
Classification of Polymers by
Thermal Behavior
• thermo-mechanical response
• Thermoplastic – soften and flow under the action of heat and
pressure; upon cooling, assumes the shape of the mold or
container; can withstand heating and cooling cycles (like a
candle wax); e.g. PE, PP, nylon; linear/branched
• Thermoset – undergoes a chemical change when heated to
form cross-links and become a solid; initially exist as liquid
(prepolymer); can be shaped onto desired forms by heat and
pressure but incapable of undergoing repeated cycles of
softening and hardening; e.g. urea-formaldehyde, phenol-
formaldehyde, epoxy, polyester; cross-linked
Classification of Polymers by Thermal Behavior: Ideal thermo-
mechanical response of thermoset and thermoplastic
Classification of Polymers by
Preparation Technique
• Bulk polymerization: only monomer (sometimes w/ catalyst
and initiator) is fed into the reactor; polymer product is nearly
a solid mass; employed for manufacture of condensation
polymers; mildly exothermic and low viscosity for ready
mixing
• Solution polymerization: monomer polymerization in a
solvent; monomer and polymer product both soluble
• Suspension polymerization: polymerization in an aqueous
medium; monomer as dispersed phase
• Emulsion polymerization: similar to suspension
polymerization, but initiator is located in aqueous phase;
resulting polymer particles 10x smaller than that of
suspension polymerization
Classification of Polymers by End
Use
• diene polymers (rubber industry)
• olefin polymer (sheet, film, fiber industry)
• acrylics (coating/decorative materials)