Advanced Materials Ch2.ppt - Islamic University of...

POLYMERS

Chapter 2

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Dr. Anwar Abu-Zarifa . Islamic University Gaza . Department of Industrial Engineering

Ancient PolymersOriginally naturalpolymers were used:

Wood Rubber Cotton Wool Leather Silk

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Oldest known use:Rubber balls used by IncasNoah used pitch (a natural polymer) for the ark

gum based resins extracted from pine trees


Polymers

What is a polymer?Poly mer

many repeat unit

Adapted from Fig. 14.2, Callister 7e.

C C C C C CHHHHHH

HHHHHH

Polyethylene (PE)ClCl Cl

C C C C C CHHH

HHHHHH

Polyvinyl chloride (PVC)HH

HHH H

Polypropylene (PP)

C C C C C CCH3

HH

CH3CH3H

repeatunit

repeatunit

repeatunit


Polymer CompositionMost polymers are hydrocarbons

– i.e., made up of H and C Saturated hydrocarbons

Each carbon singly bonded to four other atoms Example:

Ethane, C2H6

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C C

H

H H H

HH


Unsaturated Hydrocarbons Double & triple bonds somewhat unstable Thus, can form new bonds

Double bond found in ethylene or ethene - C2H4

Triple bond found in acetylene or ethyne - C2H2

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C CH

H

H

H

C C HH


Structures of Polymers about hydrocarbons

Why? Most polymers are hydrocarbon (e.g. C, H) based

Bonding is highly covalent in hydrocarbons Carbon has four electrons that can participate in bonding, hydrogen

has only one Saturated versus unsaturated

C C

C C

H

H

H

H

H H

Ethylene

Acetylene

C C

H

H

H

H

H

HEthane

Unsaturated Saturated

• Unsaturated – species contain carbon-carbon double/triple bonds

• Possible to substitute another atom on the carbon

• Saturated – carbons have four atoms attached

• Cannot substitute another atom on the carbon

Dr. Anwar Abu-Zarifa . Islamic University Gaza . Department of Industrial Engineering 7

NOTE: See Table 15.3 for commercially important polymers – including trade names


Polymer Molecules

Molecules are giganticMacromoleculesRepeat unitsMonomer


Polymer chemistry Polymers are chain molecules. They are built up

from simple units called monomers. E.g. polyethylene is built from ethylene units:

which are assembled into long chains:

Polyethylene or polythene (IUPAC name poly(ethene)) is a thermoplastic commodity heavily used in consumer products (notably the plastic shopping bag). Over 60 million tons of the material are produced worldwide every year.


Molecular Structures for Polymers

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Adapted from Fig. 4.7, Callister & Rethwisch 3e.

Branched Cross-Linked NetworkLinear

secondarybonding

Direction of increasing strength

• Covalent chain configurations and strength:


– polymers in which the monomers units are connected end-to-end along the whole length of the chain

These types of polymers are often quite flexible• Van der waal’s forces and H-bonding are the two

main types of interactions between chains• Some examples – polyethylene, teflon, PVC,

polypropylene

Linear polymers


In cross-linked polymers, adjacent linear chains are joined to one another at various positions by covalent bonding of atoms. Examples are the rubber elastic materials.

Small molecules that form 3 or more active covalent bonds create structures called network polymers. Examples are the epoxies and polyurethanes.

Branched Cross-Linked NetworkLinear

secondarybonding

Polymer Crystallinity

Ex: polyethylene unit cell

Crystals must contain the polymer chains in some way Chain folded structure



amorphousregion

Polymer Crystallinity

• % Crystallinity: how much is crystalline.

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.)

crystalline region

Polymers rarely exhibit 100% crystalline Too difficult to get all those chains aligned

Mechanical Properties i.e. stress-strain behavior of polymers

brittle polymer

plasticelastomer

σFS of polymer ca. 10% that of metals

Strains – deformations > 1000% possible(for metals, maximum strain ca. 100% or less)

elastic modulus – less than metal


• Compare to responses of other polymers:-- brittle response (aligned, crosslinked & networked polymer)-- plastic response (semi-crystalline polymers)

Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along elastomer curve (green) adapted from Fig. 15.15, Callister 7e. (Fig. 15.15 is from Z.D. Jastrzebski, The Nature and Properties of Engineering Materials, 3rd ed., John Wiley and Sons, 1987.)

Tensile Response: Elastomer Caseσ(MPa)

ε

initial: amorphous chains are kinked, cross-linked.

x

final: chainsare straight,

stillcross-linked

elastomer

Deformation is reversible!

brittle failure

plastic failurex

x

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Polymer Structure

Processing of Plastics

Thermoplastic A thermoplastic is a polymer that turns to a liquid when heated and

freezes to a very glassy state when cooled sufficiently. can be reversibly cooled & reheated, i.e. recycled ex: polyethylene, polypropylene, polystyrene, etc.

Thermoset large crosslinking hard and brittle Once hardened a thermoset resin cannot be reheated and melted

to be shaped differently ex: urethane, epoxy

The response of a polymer to mechanical forces at elevated temperature is related to its dominant molecular structure. One classification of polymers is according to its behavior and rising temperature. Thermoplastics and Thermosets are the 2 categories.

Thermoset data


Polymer Processing

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1. Injection Molding2. Extrusion3. Thermo-Forming4. Blow Molding (injection blow molding with preforms

or extrusion blow molding with parison)5. Rotational Molding (combination of 3 and 4)


Processing Plastics - Molding Injection molding (most common 60-70%)

thermoplastic


Injection Molding Steps for Injection Molding

Clamping Injection Cooling Mold-opening Ejection

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Injection molding

Processing Plastics – Extrusion

Adapted from Fig. 15.25, Callister 7e. (Fig. 15.25 is from Encyclopædia Britannica, 1997.)

Plastics extrusion is a high-volume manufacturing process in which raw plastic is melted and formed into a continuous profile. Extrusion produces items such as pipe/tubing, weather stripping, fencing, deck railings, window frames, plastic films and sheeting, thermoplastic coatings, and wire insulation.

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Injection molding Software Simulation software for optimizing plastic injection

molding designs, reducing cycle time and improving product appearance.

Ex: CoreTech System Co., Ltd. (Moldex3D) was founded in 1995, it has provided the professional plastic injection molding simulation solution “Moldex” series for the plastic injection molding industry, and the current product “Moldex3D” is marketed worldwide.

--- Project

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Injection molding Simulation

Polymer Types: Elastomers

Elastomers – rubber Crosslinked materials

Natural rubber Synthetic rubber and thermoplastic elastomers

SBR- styrene-butadiene rubberstyrene

– Silicone rubber

butadiene

A number of new polymers having unique anddesirable combinations of properties have beendeveloped over the past several years.

This include: A niche in new technologies and/or

A satisfactorily replaced other materials

Some of these include:

Ultrahigh molecular weight polyethylene (UHMWPE) Liquid crystal polymers (LCPs) Shape memory polymers Nanotechnology

Advanced Polymers


Ultrahigh molecular weight polyethylene (UHMWPE)

Ultrahigh molecular weight polyethylene(UHMWPE) is a linear polyethylene that hasan extremely high molecular weight.

Also known as high-modulus polyethylene,(HMPE), or high-performance polyethylene(HPPE), it has extremely long chains.

Its typical Mw is approximately 4 x 106g/mol greater than that of high densitypolyethylene.

This results in a very tough material, withthe highest impact strength of anythermoplastic presently made.

UHMWPE in fibre form has trade name‘spectra’

UHMWPE


Some of the extraordinary characteristics of thismaterial are as follows: An extremely high impact resistance. Outstanding resistance to wear and abrasion. A very low coefficient of friction. A self-lubricating and nonstick surface. Very good chemical resistance. Excellent low-temperature properties. Outstanding sound damping and energy absorption

characteristics. Electrically insulating and excellent dielectric properties. Nontoxic

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This unusual combination of properties leads tonumerous and diverse applications for thismaterial, including:

bullet-proof vests, composite military helmets, fishing line, ski bottom surfaces, golf ball cores, bowling alley and ice skating rink surfaces, biomedicalblood filters, bulk material handling equipment (for coal, grain,

cement, gravel, etc.), bushings, pump impellers, and valve gaskets

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It is commonly used for the construction of articular portions of implants used for hip and knee replacements.

The AC socket are made from Cobalt-chrome metal alloy or ceramic, AC cup made from polyethylene


Liquid crystal polymers (LCPs) The liquid crystal polymers (LCPs) � are a group of chemically complex and

structurally distinct materials that have unique properties and are utilizedin diverse applications.

Liquid crystal materials generally have several common characteristics.Among these are a rod-like molecular structure, rigidness of the long axis,and strong dipoles and/or easily polarizable substituents.

In the solid state, molecules are highly ordered and have little translationalfreedom.

The characteristic orientational order of the liquid crystal state is betweenthe traditional solid and liquid phases and this is the origin of the termmesogenic state, used synonymously with liquid crystal state. Liquid-crystal polymers are present in melted/liquid or solid form.

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The following may be said about their processing andfabrication characteristics:

All conventional processing techniques available forthermoplastic materials may be used.

Low melt viscosity, which permits molding of thin sectionsand/or complex shapes.

Low heats of fusion; this results in rapid melting andsubsequent cooling, which shortens molding cycle times.

Anisotropic finished-part properties; molecular orientationeffects are produced from melt flow during molding.

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By incorporating an infrared-sensitive dye in the mixture, polymer films that undergo reversible shape deformations on heating are formed.

Illustration representation of preparation of a side chain LCP via hydrosilylation reaction between a linear siloxane polymer (green), mesogen (blue) and cross-linker (orange).


Applications The most common application of liquid crystal

technology is liquid crystal displays (LCDs.) This fieldhas grown into a multi-billion dollar industry, andmany significant scientific and engineering discoverieshave been made.

Liquid-crystal polymers: Exotic actuators Kevlar (solid)

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Introduced to United States in 1984 after its originalintroduction in Japan.

Shape-memory polymers (SMPs) are polymeric smartmaterials that have the ability to return from a deformedstate (temporary shape) to their original (permanent)shape induced by an external stimulus (trigger), such astemperature change.

SMPs can retain two or sometimes three shapes, and thetransition between those is induced by temperature.

Light weight Potentially biodegradable for surgical procedures. Cheaper to manufacture than Shape memory Alloys.

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Shape memory polymers


SMP: Linear block copolymers Representative shape-memory polymers in this

category are polyurethanes, polyurethanes with ionicor mesogenic components .

Other block copolymers also show the shape-memoryeffect, such as, block copolymer of polyethyleneterephthalate (PET) and polyethyleneoxide (PEO),block copolymers containing polystyrene and poly(1,4-butadiene), and an ABA triblock copolymer made frompoly(2-methyl-2-oxazoline) and polytetrahydrofuran.

The PEO-PET block copolymers can be crosslinked byusing maleic anhydride, glycerin or dimethyl 5-isopthalates as a crosslinking agent.

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Applications Self Deployment

Mechanisms Eyeglass Frames Automatic auto body

repairs. Biodegradable biomedical

stents. Artificial muscles Aeronautical Morphing

Wings


Shape memory Polymers

Advanced Materials Ch2.ppt - Islamic University of...

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