Lecture 10 POLYMER and Bio Materials

8/2/2019 Lecture 10 POLYMER and Bio Materials

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ISSUES TO ADDRESS...

Overview of bonding and materials

What are polymers?

How are polymers manufactured and processed?

POLYMERS


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Polymers

What is a polymer?

Poly mermany repeat unit

Adapted from Fig. 14.2, Callister 7e.

C C C C C C

HHHHHH

HHHHHH

Polyethylene (PE)

ClCl Cl

C C C C C C

HHH

HHHHHH

Polyvinyl chloride (PVC)

HH

HHH H

Polypropylene (PP)

C C C C C C

CH3

HH

CH3CH3H

repeat

unit

repeat

unit

repeat

unit


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Ancient Polymer History

Originally natural polymers were used

Wood Rubber

Cotton Wool

Leather Silk

Oldest known uses

Rubber balls used by Incas

Noah used pitch (a natural polymer)

for the ark


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Chemistry of Polymers

Free radical polymerization

Initiator: example - benzoyl peroxide

C

H

H

O O C

H

H

C

H

H

O2

C C

H H

HH

monomer

(ethylene)

R +

free radical

R C C

H

H

H

H

initiation

R C C

H

H

H

H

C C

H H

HH

+ R C C

H

H

H

H

C C

H H

H H

propagation

dimer

R= 2


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Chemistry of Polymers

Adapted from Fig.

14.1, Callister 7e.

Note: polyethylene is just a long HC

- paraffin is short polyethylene


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Bulk or Commodity Polymers


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MOLECULAR WEIGHT

moleculesof#total

polymerofwttotalnM

iiw

iin

MwM

MxM

Mwis more sensitive to

higher molecular

weights

Molecular weight, Mi: Mass of a mole of chains.

LowerM higherM

Adapted from Fig. 14.4, Callister 7e.


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Covalent chain configurations and strength:

Direction of increasing strengthAdapted from Fig. 14.7, Callister 7e.

Molecular Structures

Branched Cross-Linked NetworkLinear

secondarybonding


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Tacticity

Tacticity stereoregularity of chain

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

C C

H

H

H

R

C C

H

H

H

R

C C

H

H

H

R R

H

H

H

CC

C C

H

H

H

R R

H

H

H

CC

R

H

H

H

CC

R

H

H

H

CC

isotactic all R groups on

same side of chain

syndiotactic R groups

alternate sides

atactic R groups random


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Copolymers

two or more monomerspolymerized together

random A and B randomlyvary in chain

alternating A and Balternate in polymer chain

block large blocks of Aalternate with large blocks ofB

graft chains of B graftedon to A backbone

A B

random

block

graft

Adapted from Fig.

14.9, Callister 7e.

alternating


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ISSUES TO ADDRESS...

What are the tensile properties of polymers and howare they affected by basic microstructural features?

Hardening, anisotropy, and annealing in polymers.

How does the elevated temperature mechanical

response of polymers compare to ceramics and metals?

Characteristics, Applications &

Processing of Polymers

What are the primary polymer processing methods?


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Mechanical Properties

i.e. stress-strain behavior of polymersbrittle polymer

plastic

elastomer

FS of polymer ca. 10% that of metals

Strains deformations > 1000% possible

(for metals, maximum strain ca. 10% or less)

elastic modulus

less than metal

Adapted from Fig. 15.1,

Callister 7e.


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Tensile Response: Brittle & Plastic

brittle failure

plastic failure

(MPa)

x

x

crystalline

regionsslide

fibrillar

structure

nearfailure

crystallineregions align

onset of

necking

Initial

Near Failure

semi-crystalline

case

aligned,cross-

linkedcase

networkedcase

amorphousregions

elongate

unload/reload

Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from

Figs. 15.12 & 15.13, Callister 7e. (Figs. 15.12 & 15.13 are from J.M. Schultz, Polymer Materials Science, Prentice-

Hall, Inc., 1974, pp. 500-501.)


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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 ofEngineering Materials,

3rd ed., John Wiley and

Sons, 1987.)

Tensile Response: Elastomer Case

(MPa)

initial: amorphous chains arekinked, cross-linked.

x

final: chainsare straight,

stillcross-linked

elastomer

Deformationis reversible!

brittle failure

plastic failurex

x


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Thermoplastics:-- little crosslinking

-- ductile

-- soften w/heating

-- polyethylene

polypropylenepolycarbonate

polystyrene

Thermosets:-- large crosslinking

(10 to 50% of mers)

-- hard and brittle

-- do NOT soften w/heating

-- vulcanized rubber, epoxies,

polyester resin, phenolic resin

Adapted from Fig. 15.19, Callister 7e. (Fig. 15.19 is from F.W. Billmeyer,

Jr., Textbook of Polymer Science, 3rd ed., John Wiley and Sons, Inc.,

1984.)

Thermoplastics vs. Thermosets

Callister,Fig. 16.9

T

Molecular weight

Tg

Tmmobileliquid

viscousliquid

rubber

toughplastic

partiallycrystallinesolid

crystallinesolid


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Decreasing T...-- increases E

-- increases TS

-- decreases %EL

Increasing

strain rate...-- same effects

as decreasing T.

Adapted from Fig. 15.3, Callister 7e. (Fig. 15.3 is from T.S. Carswell and

J.K. Nason, 'Effect of Environmental Conditions on the Mechanical

Properties of Organic Plastics", Symposium on Plastics, American Society

for Testing and Materials, Philadelphia, PA, 1944.)

Tand Strain Rate: Thermoplastics

20

40

60

80

00 0.1 0.2 0.3

4C

20C

40C

60Cto 1.3

(MPa)Data for the

semicrystalline

polymer: PMMA

(Plexiglas)


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Melting vs. Glass Transition Temp.

What factors affect Tm and Tg?

Both Tm and Tg increase with

increasing chain stiffness

Chain stiffness increased by

1. Bulky sidegroups2. Polar groups or sidegroups

3. Double bonds or aromatic

chain groups

Regularity effects Tm only

Adapted from Fig. 15.18,

Callister 7e.


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Polymer Additives

Improve mechanical properties, processability,durability, etc.

Fillers

Added to improve tensile strength & abrasionresistance, 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


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Polymer Additives

Stabilizers

Antioxidants

UV protectants

Lubricants

Added to allow easier processing slides through dies easier ex: Na stearate

Colorants

Dyes or pigments

Flame Retardants Cl/F & B


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Back to the Beginning

Early in the course, we tried several ways to categorize polymers, such as

condensation vs. addition, etc. From a processing point of view, the main

classes are:

Thermoplastic: the resin is heated to make a viscous liquid and thenprocessed into a usable object without much additional chemistry. Example:

polyethylene, polystyrene.

Thermoset: upon heating, further reaction occurs to make molecules set up

into a useful product. Chemistry occurs, so these are sometimes called

reactive polymers. The resin may be provided as either small molecules or

prepregspartially polymerized stuff. Example: polyurethanes, phenol-

formaldehyde, melamine-formaldehyde, epoxy glue.


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Polymer Types

Coatings thin film on surface i.e. paint, varnish

To protect item

Improve appearance

Electrical insulation

Adhesives produce bond between two adherands Usually bonded by:

1. Secondary bonds

2. Mechanical bonding

Films blown film extrusion Foams gas bubbles in plastic


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Advanced Polymers

Ultrahigh molecular weightpolyethylene (UHMWPE)

Molecular weight

ca. 4x106 g/mol

Excellent properties forvariety of applications

bullet-proof vest, golf ball

covers, hip joints, etc.

UHMWPE

Adapted from chapter-

opening photograph,

Chapter 22, Callister 7e.


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Biomaterials


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A biomaterial is "any substance (other than

drugs) or combination of substances synthetic

or natural in origin, which can be used for any

period of time, as a whole or as a part of a

system which treats, augments, or replaces

any tissue, organ, or function of the body".

Biocompatibility The ability of a material to

perform with an appropriate host response in a

specific application

Host Response The response of the host

organism (local and systemic) to the implanted

material or device.


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Some Commonly Used

Biomaterials

Material ApplicationsSilicone rubber Catheters, tubingDacron Vascular graftsCellulose Dialysis membranesPoly(methyl methacrylate) Intraocular lenses, bone cementPolyurethanes Catheters, pacemaker leadsHydogels Opthalmological devices, Drug DeliveryStainless steel Orthopedic devices, stentsTitanium Orthopedic and dental devices

Alumina Orthopedic and dental devicesHydroxyapatite Orthopedic and dental devicesCollagen (reprocessed) Opthalmologic applications, wound

dressings


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An Interdisciplinary Field

Bioengineers

Material Scientists

Immunologists

Chemists

Biologists

Surgeons


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A Little History on Biomaterials

Romans, Chinese, and Aztecs used gold indentistry over 2000 years ago, Cu not good.

Ivory & wood teeth

Aseptic surgery 1860 (Lister)

Bone plates 1900, joints 1930

Turn of the century, synthetic plastics cameinto use

WWII, shards of PMMA unintentionally gotlodged into eyes of aviators

Parachute cloth used for vascularprosthesis

1960- Polyethylene and stainless steel beingused for hip implants


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Uses of Biomaterials

Replace diseased part dialysis Assist in healing sutures

Improve function contacts

Correct function spinal rods

Correct cosmetic nose, ear

Aid dx probe

Aid tx catheter

Replace rotten amalgam Replace dead - skin


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Intraocular Lens

3 basic materials - PMMA, acrylic, silicone


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Vascular Grafts


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Second generation implants

engineered implants using common and borrowed

materials

developed through collaborations of physicians and

engineers

built on first generation experiences

used advances in materials science (from other fields)

titanium alloy dental and orthopaedic implants

cobalt-chromium-molybdinum orthopaedic implants

UHMW polyethylene bearing surfaces for total joint replacements

heart valves and pacemakers

Examples Second generation implants


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Artificial Hip Joints

http://www.totaljoints.info/Hip.jpg


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Third generation implants bioengineered implants using bioengineered materials

few examples on the market

some modified and new polymeric devices

many under development

Example - Third generation implants

tissue engineered implants designed to regrow rather than replace tissues

Integra LifeSciences artificial skin

Genzyme cartilage cell procedure

some resorbable bone repair cementsgenetically engineered biological components (Genetics Institute and

Creative Biomolecules BMPs)


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Substitute Heart Valves


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Metals

Semiconductor

Materials

Ceramics

Polymers

Synthetic

BIOMATERIALS

Orthopedic

screws/fixation

Dental Implants Dental Implants

Heart

valves

Bone

replacements

Biosensors

Implantable

Microelectrodes

Skin/cartilageDrug Delivery

DevicesOcular

implants

Lecture 10 POLYMER and Bio Materials

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