SCHM-503/703:
Structure & Properties of Polymeric Materials
603-1.1
Polymers - Macromolecules
Natural Polymers
Proteins – polyamides, enzymes, muscles, tissue,
Hair, wool, silk
Carbohydrates – polysaccharides, starch, cellulose,
rayon(reconstituted cellulose)
Nucleic Acids - DNA, RNA
Synthetic Polymers
Plastics - HDPE, LDPE, Lexan, plexiglass, teflon,
polystyrene
Fibers - nylon, orlon, kevlar, rayon
Elastomers - rubber, spandex, poly(urethane) foam,
silly putty
Adhesives - glues, cement, "scotch tape", epoxy,
Hair spray
Coatings - paints, varnishes, enamels, formica
Poly(urethane)
Specialty Polymers - graphite (lubricant), toners,
Diamond (insulator), polypyrrole
(conductor), poly(vinylidene fluoride)
(piezoelectric), photoresists (novolac
resins, PMMA)
603-1.2
Polymer Production
12000
80000
70000
10000
60000
8000
Millions of lb.
50000
6000
40000
30000
4000
20000
Thermoset Resins - Total
Synthetic Rubber - Total
2000
Synthetic Fiber - Total
10000
Thermoplastic - Total (right)
0
1988
1990
1992
1994
Year
1996
1998
0
2000
603-1.3
Key Concepts
Q:
What makes polymers unique?
Polymers are chain molecules that are characterized by a
molecular length that is much greater than the other dimensions of the
molecule. Since the bonding in the chain direction is covalent, and the
bonding between chains is weak, polymers can exhibit highly
anisotropic properties.
Polymeric material will also exhibit isotropic properties when the
chain segments are oriented randomly within the material. If the
random orientation occurs down to a molecular length scale, the
material is classified as being amorphous.
Most amorphous polymers have chains that are long enough for
them to entangle with one another. The molecular weight required for
chain entanglement to occur is referred to as the critical molecular
weight for the particular polymer.
Polymers with average molecular weights that are greater than
the critical molecular weight exhibit polymeric properties. An important
and unique polymeric property is viscoelasticity.
603-1.4
Plastics
Plastics are mainly structural materials that are capable of being
formed into different "plastic states" during processing. Plastics are
categorized as thermoplastic and thermosetting depending on whether the
plastic state is temporary or permanent.
Thermoplastics
The shape of these materials can be changed through the application of
heat and pressure. Although they are processed as viscous melts, these
materials are generally hard at their use temperature.
The thermo-mechanical behavior of thermoplastics is represented by
the viscoelastic curve.
Viscoelastic Curve
1.00E+10
B
A
1.00E+09
semi-crystalline
log(E) -{E in Pa)
1.00E+08
1.00E+07
amorphous
D
C
1.00E+06
low molecular
weight
1.00E+05
E
1.00E+04
0.5
0.7
0.9
1.1
1.3
1.5
Reduced Temperature - T/Tg
1.7
1.9
2.1
603-1.5
Thermo-Mechanical Transitions
The viscoelastic curve represents the relationship between a sample's
temperature and its modulus. The modulus is a measure of the sample's
resistance to being deformed by an imposed stress. The viscoelastic curve is
divided into five distinct regions, these have been labeled A - E on the
previous curve.
A - Glass. The material is rigid, yet brittle if not reinforced by
chemical cross-links or crystallites. Polystyrene and plexiglass are
amorphous glasses at room temperature.
B - Glass Transition Zone. The material starts to become compliant over
time (at constant temperature), or in a narrow temperature range. The
glass transition temperature, T g, identifies this zone.
C - Rubber. The material is very flexible, capable of being stretched to
several times its original dimensions without breaking. Commercial
rubbers are chemically cross linked to keep them from softening at
elevated temperatures.
D - Rubbery Flow Zone. The material becomes tacky, and will spread
like a liquid if pressure is applied. Pressure-sensitive adhesives are
designed to exhibit this behavior at room temperature.
E - Polymer Melt. As the polymeric material is heated beyond the
rubbery flow zone its viscosity steadily decreases. As the viscosity
(resistance to flow) decreases, so does the materials modulus. This is
the processing region of the viscoelastic curve.
The region between the two transition zones is affected by the
molecular weight of the polymer and by the degree of crystallinity of the
solid. Semi-crystalline polymers can absorbed a considerable amount of
energy in this region without fracturing. This is referred to as toughness.
603-1.6
Molecular Weight & Viscoelasticity
The effect of molecular weight on the thermo-mechanical properties of
polymeric materials is represented on the figure given below.
Transition Temperatures for Thermoplastics
3
Reduced Transition Temperatures ----T/Tg
2.5
Gas
2
Liquid
Rubbery
1.5
Leathery
Waxy
1
Glass Transition, Tg
0.5
Glassy
Rubber-Liquid Transition, Tr
Melt Temperature, Tm
Thermal Decomposition, Td
0
0.01
0.1
1
10
100
Reduced Molecular Weight M/Mcr
Characteristic Thermo-Mechanical Behavior
Hard, Rigid - Polymer is below its Tg at its use temperature. For small
stresses, the behavior is independent of molecular weight.
Soft/Waxy - Polymer has a molecular weight below its critical molecular
weight. The material exhibits some crystallinity.
Tough/Leathery - The high degree of crystallinity leads to an increase in
the modulus. The crystallites are covalently linked by polymer chains.
Therefore, covalent bonds need to be broken to fracture the material.
Rubbery - The material can be stretched to large strains.
603-1.7
Production of Thermoplastics
Thermoplastics are by far the largest class of synthetic polymers
when measured based on annual production. Production of the 6 main
recyclable polymers amounted to 70 billion pounds in 1999, with
polyethylene leading the way.
Thermoplastic Resin Production
35000
30000
Millions of lb.
25000
Polyethylene (2,4)
Polypropylene (5)
PVC & Copolymers (3)
Styrene Polymers (6)
Thermoplastic polyester (1)
20000
15000
10000
5000
0
1988
1990
1992
1994
1996
1998
2000
Year
These materials are referred to as commodity plastics because of their
relatively low production cost. PET (recycle #1) is often considered to
be an engineering plastic because it combines good strength with a
reasonable cost. The combination of low cost/volume and high strength
is referred to as the materials performance. High-performance plastics
combine reasonable cost with high strength. Many of these latter
materials, such as kevlar, exhibit thermotropic liquid crystal behavior.
603-1.8
Thermoplastic Processing
The most common processing technique for thermoplastics is extrusion.
Solid polymer is added to a feed hopper (A). It then fills the spaces between
the vanes of the extrusion screw. As the material is forced down the barrel of
the extruder it is compressed and heated. Much of the heating results from the
friction between the particles. When the polymer reaches the metering and
mixing section of the extruder (B) it is a viscous melt. The melt is then
continuously forced through a narrow opening called a die. The shape of the
die orifice determines the cross-section of the final polymer product.
Thermoplastic sheet, film and fiber are produced in this way.
603-1.9
Fibers
Although fiber-forming polymers could certainly be classified as
thermoplastics, the fiber industry is separate from the plastics industry. Fibers
are defined as elongated structures that are considerably longer than they are
thick. Fibers exhibit highly anisotropic properties resulting from the
alignment of chains and crystalline regions along the fiber axis. The
amorphous regions between crystalline domains exhibit rubbery behavior.
As a consequence, fibers are flexible, yet tough. Since many of the chains are
aligned along the fiber axis, fibers exhibit high tensile strength. The trade-off
is poor shear strength, leading to fraying.
Annual fiber production is around 10 billion pounds. The biggest
growth in fiber production has been in polyolefins, particularly
polypropylene.
Synthetic Fiber Production
4500
4000
3500
Millions of lb.
3000
2500
2000
Polyester
1500
Nylon
Olefin
Acrylic
1000
Acetate & Rayon
500
0
1988
1990
1992
1994
Year
1996
1998
2000
603-1.10
Fiber Classification
The Federal Trade Commission lists the following classification scheme
for commercial fibers.
Acetate
Acrylic
Anidex
Aramid
> 92% acetate substituted cellulose - introduced by
Celanese in 1924.
> 85% polyacrylonitrile - introduced by DuPont in 1950.
> 50% polyacrylates - introduced by Rohn & Haas in 1970.
> 85% polyarylamide - introduced by DuPont in 1961.
Azlon
regenerated protein - marketed by Azlon.
Lyocell
solvent extruded cellulose - introduced by Acordis
Cellulosics in 1992.
> 50% crosslinked melamine resin.
Melamine
Modacrylic 35%-85% polyacrylonitrile - introduced by Union Carbide
in 1943.
Nylon
> 85% aliphatic polyamide - introduced by DuPont in 1939.
Olefin
> 85% polyolefin (PE and PP) - introduced by Hercules in
1961.
PBI
fiber containing polybenzimidazole - introduced by
Celanese in 1983.
Polyester > 85% PET or a polyester of p-hydroxybenzoate introduced by DuPont in 1953.
Rayon
regenerated cellulose - introduced by American Viscose in
1910.
Saran
> 80% poly(vinylidene chloride) - introduced by Firestone
in 1941.
Spandex
> 85% segmented polyurethane - introduced by DuPont in
1959.
Sulfar
> 85% poly(phenylene sulfide) - introduced by Phillips in
1983.
Vinal
> 85% poly(vinyl alcohol)/poly(vinyl acetate).
Vinyon
> 85% poly(vinyl chloride) - introduced by FMC.
603-1.11
Elastomers
Elastomers and rubbers are flexible materials that can be stretched,
reversibly, to several times their original dimensions. Most commercial
elastomers are cross linked after polymerization. The degree of cross-linking
determines the classification of the rubber. The cross-linking in elastomers
prevents them from being stretched to more than double their original length.
The term rubber is reserved for more lightly cross-linked material. Most
elastomers are loaded with carbon black, or similar additives.
The annual production of elastomers is about half that of synthetic
fibers. The main elastomer product is styrene-butadiene rubber.
Synthetic Rubber Production
3000
2500
Styrene-Butadiene
Polybutadiene
Other
Ethylene-Propylene
Nitrile
Polychloroprene
Millions of lb.
2000
1500
1000
500
0
1988
1990
1992
1994
Year
1996
1998
2000
603-1.12
Thermosetting Resin
Thermosetting resins are polymeric materials that form a covalently
bonded network as the final product. The polymerization reactions involved in
network formation are generally of the step-growth type. The point at which a
continuous mesh of covalent bonds spans the entire part or film is called the
gel point. The conversion of monomer at the gel point is controlled by the
stoichiometry of bifunctional and multi-functional monomers.
Because the shape is unalterable once the polymerization is complete,
thermoset resins are generally produced in oligomeric, prepolymer form.
These prepolymers are then used as molding and casting compounds for
thermosetting plastics, or as part of an adhesive or coating formulation. The
Thermoset Resin Production
5000
4500
4000
Phenolic
Urea
Polyester
Epoxy
Melamine
Millions of lb.
3500
3000
2500
2000
1500
1000
500
0
1988
1990
1992
1994
1996
1998
Year
production of thermoset resins is about 10 billion pounds per year.
2000
603-1.13
Adhesives
Adhesives are polymer-based formulations used to bond two surfaces
together. Adhesives can be either reactive (form chemical bonds) or adhere
by physical (van der Waals, ionic, etc) bonds. In either case, it is important
that the adhesive can be reliably delivered to the joint that needs to be made,
and that the adhesion and cohesion of the final joint are optimized for the
particular use. Adhesion refers to the bonding between dissimilar surfaces.
Cohesion is the bonding within the polymer film itself.
Modern adhesives are classified either by the way that they are used or
by their chemical constituents.
Anaerobics - used to bond metal surfaces together when air is
excluded. They are generally based on acrylic resins.
Cyanoacrylates - reactive cyanoacrylic adhesives that cure in the
presence of moisture. They solidify in seconds.
Toughened Acrylics - general use, strong acrylic-based adhesive. It's
applied as a two-part, resin/catalyst system.
Epoxies - adhesives consisting of epoxy resin and hardener. These onepart, or two-part adhesives are extremely strong and versatile.
Polyurethanes - fast-cure, two-part adhesives used for bonding glassreinforced plastics.
Modified Phenolics - phenolic resins for bonding metal to metal, or
metal to wood. They require pressure and heat to cure.
Hot Melts - semi-crystalline polymers that bond physically. Joints form
quickly, but are not very strong.
Plastisols - poly(vinyl chloride) based dispersions that require heat to
harden. Once set, the joint is tough and resilient.
Rubber Adhesives - solutions or latexes of rubber that solidify by loss of
solvent. Do not give load-bearing joints.
Poly(vinyl acetates) - PVA emulsions for use in bonding porous
surfaces. Used in the packaging industry.
Pressure Sensitive Adhesives - used for labels and tapes. They don't
solidify, rather stay in the rubbery flow regime.
603-1.14
Polymer Design
Characterization
Testing
Microstructure ⇒ Morphology ⇒ Properties ⇒ Applications
Synthesis + Processing
H
H
H
H
H
H
H
H
H
H H
monomer
H
H H
H
H
H
H H
H
H
H
H H
H
H
H
H
polymer
microstructure
helix (ordered)
random coil
(disordered)
morphology
amorphous phase
crystalline phase