Transcript polymeric materials

POLYMERIC MATERIALS
• Characteristics
• Applications
• Mechanisms of Polymerization
• Structure–Property Relationships
• Classification of Polymers
• Glasses
• Processing of Polymers
• Mechanical Properties
• Polymer Types
POLYMERIC MATERIALS
Important Characteristics of Polymers
The mechanical properties of polymers generally include low strength and high toughness. Their
strength is often improved using reinforced composite structures.
Size: Single polymer molecules typically have molcular weights between 10,000 and 1,000,000 g/mol-that can be more than 2000 repeating units depending on the polymer structure! The mechanical
properties of a polymer are significantly affected by the molecular weight, with better engineering
properties at higher molecular weights.
Thermal Transitions: The softening point (glass transition temperature) and the melting point of a
polymer will determine which applications it will be suitable for.
Crystallinity: Polymers can be crystalline or amorphous, but they usually have a combination of
crystalline and amorphous structures (semi-crystalline).
Interchain Interactions: The polymer chains can be free to slide past one another (thermoplastic) or
they can be connected to each other with crosslinks ( thermoset or elastomer). Thermoplastics can be
reformed and recycled, while thermosets and elastomers are not reworkable.
Interchain Structure: The chemical structure of the chains also has a tremendous effect on the
properties. Depending on the structure the polymer may be hydrophillic or hydrophobic (likes or
hates water), stiff or flexible, crystalline or amorphous, reactive or unreactive.
POLYMERIC MATERIALS
POLYMER composed primarily of C and H, they have low melting
temperature, poor thermal and electrical conductors. Some are crystals,
many are not, high plasticity, a few have good elasticity. Some are
transparent, some are opaque. Low density structures.
Main applications
• Films, foams, paints, fibers, and structural materials.
• Microelectronics industry, fabrication of semiconductor devices.
• Plastics, Liquid crystals, Adhesives and glues
• Containers and Water-resistant coatings (latex)
• Moldable products (computer casings, telephone handsets)
• Clothing (vinyl , polyesters, nylon), Biomaterials (organic/inorganic)
• Low-friction materials (teflon), Synthetic oils and greases
• Gaskets and O-rings (rubber), Soaps and surfactants
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Table 1 Composition and Molecular Structures for some of
the Paraffin compounds CnH2n+2
POLYMERIC MATERIALS
Industrially Important Polymers
Polymer
Example
Applications
Polyethylene (PE)
Electrical wire insulation,
flexible tubing, bottles
Polypropylene (PP)
carpet fibers, liquid
containers (cups, buckets,
tanks), pipes
Polystyrene (PS)
packaging foams, egg cartons,
lighting panels, electrical
components
Polyvinyl chloride (PVC)
bottles, pipes, valves,
electrical wire insulation, toys,
raincoats, automobile roofs
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Industrially Important Polymers
Table 2 Repeat units and applications for selected addition thermoplastics
POLYMERIC MATERIALS
Industrially Important Polymers
Table 2 Repeat units and applications for selected addition thermoplastics
(Continued)
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Industrially Important Polymers
Table 3 Repeat units and applications for complex thermoplastics
POLYMERIC MATERIALS
Industrially Important Polymers
Table 3 Repeat units and applications for complex thermoplastics (Continued)
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Table 4 Repeat units and applications for selected elastomers
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Table 4 Repeat units and applications for selected elastomers (continued)
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Table 5 Repeat units and applications for selected thermosets
POLYMERIC MATERIALS
Mechanisms of Polymerization
Polymerization is the formation of chemical linkages between relatively small molecules
or monomers to form very large molecules or polymers. These linkages are formed by
either one or two of the following two types: addition or condensation.
Addition polymerization process is characterized by the simple combination of
molecules without the generation of any by-products as a result of the combination. The
original molecules do not decompose to form reaction debris. When units of single
monomers are hooked together, the resulting product is a homopolymer, such as
polyethylene, that is made from the ethylene monomer. When two or more polymers are
used in the process, the product is a co-polymer.
Condensation polymerization involves the chemical reaction of two or more chemicals
to form a new molecule. The chemical union of two molecules can be only achieved by
the formation of a by-product molecule with atoms from the two molecules to create the
link for the polymerization to continue. This chemical reaction produces a condensate or
non-polymerizable byproduct, usually water. A catalyst is often required to start and
maintain the reaction. It can also be used to control the reaction rate.
POLYMERIC MATERIALS
 Addition polymerization - Process by which polymer chains are built up
by adding monomers together without creating a byproduct.
 Unsaturated bond - The double- or even triple-covalent bond joining
two atoms together in an organic molecule.
 Functionality - The number of sites on a monomer at which
polymerization can occur.
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The addition reaction for producing polyethylene from ethylene molecules. The
unsaturated double bond in the monomer is broken to produce active sites, which then
attract additional repeat units to either end to produce a chain.
POLYMERIC MATERIALS
Addition (Chain) Polymerization
– Initiation
– Propagation
– Termination
POLYMERIC MATERIALS
 Condensation polymerization - A polymerization
mechanism in which a small molecule (e.g., water,
methanol, etc.) is condensed out as a byproduct.
Condensation (Step) Polymerization
POLYMERIC MATERIALS
Condensation (Step) Polymerization
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The condensation reaction for polyethylene terephthalate (PET), a common
polyester. The OCH3 group and a hydrogen atom are removed from the
monomers, permitting the two monomers to join and producing methyl alcohol
as a byproduct.
POLYMERIC MATERIALS
 Degree of polymerization - The average molecular weight of
the polymer divided by the molecular weight of the monomer.
Example: Calculate the degree of polymerization if 6,6-nylon has
a molecular weight of 120,000 g/mol.
SOLUTION: The molecular weights are 116 g/mol for
hexamethylene diamine, 146 g/mol for adipic acid, and 18
g/mol for water. The repeat unit for 6,6-nylon is
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POLYMERIC MATERIALS
Example SOLUTION (Continued)
The molecular weight of the repeat unit is the sum of the molecular
weights of the monomers, minus that of the two water molecules
that are evolved:
Mrepeat unit = 116 + 146 – 2(18) = 226 g/mol
Degree of polymerization = 120,000/226 = 531
The degree of polymerization refers to the total number of repeat
units in the chain. The chain contains 531 hexamethylene diamine
and 531 adipic acid molecules.
POLYMERIC MATERIALS
Structure–Property Relationships in Thermoplastics
 Branched polymer - Any polymer consisting of
chains that consist of a main chain and secondary
chains that branch off from the main chain.
 Crystallinity is important in polymers since it affects
mechanical and optical properties.
 Tacticity - Describes the location in the polymer
chain of atoms or atom groups in nonsymmetrical
monomers.
 Liquid-crystalline polymers - Exceptionally stiff
polymer chains that act as rigid rods, even above
their melting point.
POLYMERIC MATERIALS
Polymeric structure
Polymers are a group of materials characterized by chains of molecules made up of smaller units
called monomers, a majority of which is joined artificially.
Most polymers are organic (carbon-based) materials that contain molecules composed of various
combinations of hydrogen, oxygen, nitrogen and carbon. These four elements are among the most
common found in organic polymers. Carbon forms the spine in the polymer chain, and the other
constituents attach themselves to the carbon.
Crystalline polymers usually contain regions of well-packed chains separated by amorphous (liquidlike) regions. Pulling a fiber of a linear polymer causes the chains to line up, and can induce
crystallization. Besides viscosity, there are other factors that influence the ability of a polymer to
crystallize. One of them is the nature of the side groups on the polymer chains. With very bulky side
groups, or side groups that vary in an irregular way, the chains have a hard time organizing into an
ordered, crystalline solid. This effect is important, because crystalline polymers tend to be much
stiffer, harder, and denser than amorphous polymers. A good example of this phenomenon is
polypropylene, which can be made in either atactic, isotactic, or syndiotactic forms.
The atactic form doesn't pack well and therefore is normally amorphous, and is used in making
garbage bags and other applications where flexible plastics are needed. The isotactic form is
crystalline at ordinary temperatures (its melting point is 160oC), is translucent and much stiffer, and
is used to make jars, tupperware, etc.
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herein under license.
POLYMERIC MATERIALS
Three possible arrangements of nonsymmetrical monomers: (a) isotactic,
(b)syndiotactic, and (c)atactic.
POLYMERIC MATERIALS
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trademark used herein under license.
Melting vs. Glass Transition Temp.
The relationship between the density and
the temperature of the polymer shows the
melting and glass temperatures. Note that
Tg and Tm are not fixed; rather, they are
Adapted from Fig. 15.18, Callister 7e.
Both Tm and Tg increase with increasing
chain stiffness.
Regularity – effects Tm only
POLYMERIC MATERIALS
Table 4 Melting and glass temperature ranges (0C) for selected thermoplastics
and elastomers
POLYMERIC MATERIALS
Crystal structure of several polymers
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The folded chain ,model for crystallinity in
polymers, shown in (a) two dimensions and
(b) three dimensions.
Photograph of spherulitic crystals
in an amorphous matrix of nylon (
200). (From R. Brick, A. Pense and
R.
Gordon,
Structure
and
Properties
of
Engineering
Materials, 4th Ed., McGraw-Hill,
1977.)
POLYMERIC MATERIALS
Classification of Polymers
The terms Thermoplastic and thermoset refer to the properties of polymers. Polymers are separated
into these two general classifications. In general, the properties of polymers depend on the additives
used, materials added to increase polymer's strength; the amounts and properties of fillers used;
coloring agents used; and plasticizers, which are added as internal lubricants.
- Thermoplastic polymers
Thermoplastic polymers are generally available in films, sheets, rods, tubing, and several molded or
extruded shapes. Thermoplastic polymers often exhibit plastic, ductile properties. They can be
formed at elevated temperatures, cooled, remelted, and reformed into different shapes without
changing the properties of the polymer. The properties of thermoplastic polymers are determined by
the bonding method between polymer chains; in thermoplastic materials these bonds are weak.
Common thermoplastic polymers include acrylic, nylon, cellulose, polystyrene, fluorocarbons, and
vinyl.
- Thermosetting polymers
Thermosetting polymers have strong primary bonds, often formed by condensation polymerization.
Thermosetting polymers have strong primary bonds throughout, and their structure resembles one
large molecule. Their properties are the result of chemical changes undergone during processing,
under heat or through the application of a catalyst. Once hardened, thermosets can not be softened
or reshaped, due to the loss of a part of the molecule during the curing process. Once cured, if
further heat is applied to a thermosetting material, it will char, burn, or decompose.
Thermoplastics vs. Thermosets
• Thermoplastics:
-- little crosslinking
-- ductile
-- soften w/heating
-- polyethylene
polypropylene
polycarbonate
polystyrene
T
viscous
liquid
mobile
liquid
Callister,
rubber
Fig. 16.9
tough
plastic
crystalline
solid
Tm
Tg
partially
crystalline
solid
• Thermosets:
Molecular weight
-- large crosslinking
(10 to 50% of mers)
Adapted from Fig. 15.19, Callister 7e. (Fig. 15.19 is from F.W.
-- hard and brittle
Billmeyer, Jr., Textbook of Polymer Science, 3rd ed., John Wiley and
-- do NOT soften w/heating Sons, Inc., 1984.)
-- vulcanized rubber, epoxies,
polyester resin, phenolic resin
POLYMERIC MATERIALS
Polymer Additives
Improve mechanical properties, processability, durability, etc.

Fillers
– Added to improve tensile strength & abrasion resistance,
toughness & decrease cost
– ex: carbon black, silica gel, wood flour, glass, limestone, talc,
etc.
• Plasticizers
– Added to reduce the glass transition
temperature Tg
– commonly added to PVC - otherwise it is brittle
POLYMERIC MATERIALS
Polymer Additives

Stabilizers
– Antioxidants, Antistatic Agent
– UV protectants, Catalysts
• Lubricants
– Added to allow easier processing
– “slides” through dies easier – ex: Na stearate
• Colorants
– Dyes or pigments
• Flame Retardants
– Cl/F & B
• Reinforcements
POLYMERIC MATERIALS
Glasses
A glass is an inorganic nonmetallic material that does not have a crystalline
structure. Such materials are said to be amorphous.
Glasses have historically been used for low technology applications such as soda
bottles and window panes. However, glasses, like ceramics, have recently found
new application in high technology fields, particularly the semiconductor
microelectronics industry where silica is widely used as an insulator in
transistors and the fiber optic cable industry where high purity silica glass has
made advanced telecommunications possible.
Three of the most common uses for glasses: windows, liquid crystal displays, and
optical fibers.
Examples of glasses range from the soda-lime silicate glass in soda bottles to the
extremely high purity silica glass in optical fiber. As with ceramics, the list of
industrially important glasses also continues to grow.
POLYMERIC MATERIALS
Industrially Important Glasses
Glass
Ex.
Properties
SiO2
Used for optical fibers when it is very pure
Silica glass
Soda-lime glass SiO2-Na2OCaO
standard glass used for bottles and windows
due to its low cost and easy manufacturing
Borosilicate
glass
SiO2-B2O3
thermal shock resistance (glassware) and
low coefficient of thermal expansion
Lead glass
SiO2-PbO
high index of refraction
POLYMERIC MATERIALS
Polymer Crystallinity
crystalline
Polymers rarely 100% crystalline
region
 Too difficult to get all those chains
aligned
• % Crystallinity: % of material
that is crystalline.
-- TS and E often increase
with % crystallinity.
-- Annealing causes
crystalline regions
to grow. % crystallinity
amorphous
increases.
region
Adapted from Fig. 14.11, Callister 6e.
(Fig. 14.11 is from H.W. Hayden, W.G. Moffatt,
and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior,
John Wiley and Sons, Inc., 1965.)
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Polymer Processing and Recycling
Forming Processes for Thermoplastics:
• Extrusion
• Blow Molding
• Injection Molding
• Thermoforming
Forming Processes for Thermosetting polymers:
• Calendaring
• Spinning
• Compression Molding
• Transfer Molding
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Extrusion: The polymer is heated to the liquid state and forced through a die under
pressure resulting in an endless product of constant cross section. Examples include:
tubing, pipes, window frames, sheet, and insulated wire.
Film Blowing: Using the same method as extrusion the material coming out of the die is
blown into a film. An example is plastic wrap.
Injection molding: Similar to extrusion, the polymer is heated to the liquid state, but it is
prepared in metered amounts, and the melt is forced into a mold to create the part. It is
not a continuous process. Many toys are made by injection molding.
Blow molding: The melted polymer is put into a mold, and then compressed air is used to
spread the polymer into the mold. It is used to make many containers such as plastic soda
containers and milk jugs.
Compression molding: Solid polymer is placed in a mold; the mold is heated and puts
pressure on the polymer to form the part.
Reaction injection molding: Liquid monomers are placed in the mold avoiding the need to
use temperature to melt the polymer or pressure to inject it. The monomers polymerize in
POLYMERIC MATERIALS
Processing Plastics - Molding

Injection molding
– thermoplastic & some thermosets
Adapted from Fig. 15.24, Callister 7e. (Fig. 15.24 is from F.W. Billmeyer, Jr., Textbook of Polymer
Science, 2nd edition, John Wiley & Sons, 1971. )
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Processing Plastics – Extrusion
Adapted from Fig. 15.25, Callister 7e. (Fig. 15.25 is from Encyclopædia Britannica, 1997.)
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Blown-Film Extrusion
Adapted from Fig. 15.26, Callister 7e. (Fig. 15.26 is from Encyclopædia Britannica, 1997.)
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Mechanical Properties
• Decreasing T...
-- increases E
-- increases TS
-- decreases %EL
brittle polymer
FS of polymer
10% that of metals
plastic
elastomer
• Increasing
strain rate...
-- same effects
as decreasing T.
elastic modulus
– less than metal
Adapted from Fig. 15.1, Callister 7e.
Strains – deformations > 1000% possible
(for metals, maximum strain ca. 10% or less)
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POLYMERIC MATERIALS
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Learning™ is a trademark used herein under license.
The stress-strain curve for 6,6-nylon, a
typical thermoplastic polymer.
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license.
Necks are not stable in amorphous
polymers, because local alignment
strengthens the necked region and
POLYMERIC MATERIALS
Mechanical Properties of Thermoplastics
Compare the mechanical properties of LD polyethylene, HD
polyethylene, polyvinyl chloride, polypropylene, and
polystyrene, and explain their differences in terms of their
structures.
Example 15.8 SOLUTION
Let us look at the maximum tensile strength and modulus
of elasticity for each polymer.
Example 15.8 SOLUTION (Continued)
We can conclude that:
1. Branching, which reduces the density and close packing of chains,
reduces the mechanical properties of polyethylene.
2. Adding atoms or atom groups other than hydrogen to the chain increases
strength and stiffness. The methyl group in polypropylene provides some
improvement, the benzene ring of styrene provides higher properties, and
the chlorine atom in polyvinyl chloride provides a large increase in
properties.
POLYMERIC MATERIALS
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Learning™ is a trademark used herein under license.
The effect of temperature on the stressrupture behavior of high-density
polyethylene.
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Learning™ is a trademark used herein under license.
Creep curves for acrylic (PMMA)
(colored lines) and polypropylene (black
lines) at 20°C and several applied
stresses.
POLYMERIC MATERIALS
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The effect of applied stress on the percent
creep strain for three polymers
Table 5 Deflection temperatures for
selected polymers for a 264-Psi load
POLYMERIC MATERIALS
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The stress-strain curve for an elastomer. Virtually all of the deformation is elastic;
therefore, the modulus of elasticity varies as the strain changes.
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Table 6 properties of selected elastomer
Table 7 properties of typical thermosetting
polymer.
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Polymer Types: Elastomers
Elastomers – rubber
 Crosslinked materials
– Natural rubber
– Synthetic rubber and thermoplastic elastomers
» SBR- styrene-butadiene rubber
butadiene
– Silicone rubber
styrene
POLYMERIC MATERIALS
Polymer Types: Fibers
Fibers - length/diameter >100
 Textiles are main use
– Must have high tensile strength
– Usually highly crystalline & highly polar
• Formed by spinning
– ex: extrude polymer through a spinnerette
• Pt plate with 1000’s of holes for nylon
• ex: rayon – dissolved in solvent then pumped through
die head to make fibers
– the fibers are drawn
– leads to highly aligned chains- fibrillar structure
POLYMERIC MATERIALS
Polymer Types

Coatings – thin film on surface – i.e. paint, varnish
–
–
–
To protect item
Improve appearance
Electrical insulation
• Adhesives –Usually bonded by Secondary and Mechanical bonding
1.Chemically Reactive Adhesives
2.Evaporation or Diffusion Adhesives
3.Hot-Melt Adhesives
4.Pressure-Sensitive Adhesives
5.Conductive Adhesives
• Films – blown film extrusion
• Foams – gas bubbles in plastic
POLYMERIC MATERIALS
Advanced Polymers

Ultrahigh molecular weight
polyethylene (UHMWPE)
– Molecular weight
ca. 4 x 106 g/mol
– Excellent properties for
variety of applications
» bullet-proof vest, golf ball
covers, hip joints, etc.
UHMWPE
Adapted from chapteropening photograph,
Chapter 22, Callister 7e.