Download Modern Materials: Polymers and Biomaterials

Modern Materials:
Polymers and Biomaterials
Chapter 12
Supplementary Reading on Polymers
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http://courses.chem.psu.edu/chem112/materials/polymers.html
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Polymers: High molecular weight materials formed from many small molecules called monomers
Examples of polymers: plastics DNA proteins rubber
Examples of polymers: plastics, DNA, proteins, rubber
How can we understand this important group of molecules?
p
g p
Structure affects function MJ Bojan
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Most polymers are formed via one of these types of reaction
of reaction.
Polymer Synthesis
1. Addition polymers
Monomer containing a double bond polymerizes via addition reaction
Polyethylene, Polystyrene
Rubber cross‐linking
2. Ring opening
Cyclic monomer opens to form a linear chain
(e.g. polymerization of sulfur)
3. Condensation polymers
d
l
polymers form via condensation reaction, a small molecule
(H2O, HCl, CO2) is eliminated in the reaction
Silicone polymers, Polyurethanes, Nylon, Proteins
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Synthesis of ADDITION POLYMERS: Synthesis of ADDITION POLYMERS: Double bond opens up, two electrons are used to form two new C‐C single bonds connecting monomers.
ethylene
monomers
H
H
C
H
C
H
H
C
H
H
H
C
H
C
H
C
H
H
H
H
H
H
H
H
C
C
C
C
C
C
H
H
H
H
H
H
polyethylene
*
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*
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Synthesis of ADDITION POLYMERS: By modifying the monomer a variety of polymers can be made.
H
H
C
CH2
C
H
CH2
n
H
ethylene
polyethylene(gladwrap)
F
H
F
C
CF2
C
F
tetrafluoroethylene
H
C
CF2
n
F
C
stryrene
t
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polystyrene(Styrofoam)
H
CH3
CH3
C
n
H
Cl
polyvinyl chloride(PVC)
vinyl chloride
C
CH
CH2
C
CH
n
Cl
H
CH2
CH2
C
H
Teflon,TFal
H
C
H
C
O
C
CH3
O
methyl methacrylate MMA
CHEM 112 Polymers and BioMaterials
C
n
CH3
O
O
polyMMA
l MMA (Pl
(Plexiglass)
i l
)
5
Rubber is an example of a naturally occurring addition polymer
addition polymer.
Problem
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Ring Opening Polymerization: a cyclic monomer opens to form a linear chain
form a linear chain
Condensation Polymerization: Two molecules join to form a larger molecule by elimination of a small molecule such l
l l b l
f
ll
l l
h
as water
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Ethers, Esters, and Amides are formed via condensation reactions:
d
i
i
alcohol
R−O−H
+
alcohol → ether
+ H−O−R’ → R−O−R’
carboxylic acid + alcohol →
O
C
R
+ H−O−R’ →
R
OH
carboxylic acid + amine
O
R
C
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+
OH
ester
O
→
H N H
R
C
+
water
+ H2O
+
water
+
H2O
OR'
amide
O
→
R
C
+ water
+ H2O
NHR'
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Polyesters and polyamides are formed via condensation reactions.
condensation reactions.
Each monomer has two functional groups
two functional groups.
O
O
C
C
(CH2)4
H
N
(CH2)6
N
H
Nylon 6,6 (polyamide)
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There are several classifications or types of Polymers. Plastic: materials that can be formed into shapes.
p
Thermoplastic: materials that can be shaped more than once.
Thermosetting plastic: material that can only be shaped once.
Elastomer: material that is elastic in some way. If a moderate amount of deforming force is added, the elastomer will return to its original shape. Useful for fibers.
h
U f l f fib
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Properties of Polymers depend on their structure an bonding
bonding.
Properties depend on:
1) Structure,
Structure orientation of chains
2) Identity of side groups
3) Chain length
4) Degree of cross‐linking
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Degree of crystallinity is the amount of ordering in a polymer
Polymer chains tend to be flexible and easily entangled or folded; tend to be disordered
Crystallinity affects − hard to crystallize; often amorphous
hard to crystallize; often amorphous
physical properties:
crystalline polymers are stiffer, harder, more
stiffer, harder, more dense
Crystallinity affects optical properties:
1) Amorphous polymers are transparent (glasslike)
2)) Partly crystalline and partly glassy polymers are
y y
p yg yp y
translucent
Side chains
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There are ways to Increase order or crystallinity in a polymer
Interactions between chains of a polymer lend elements of order to the structure of polymers.
Stretching (or extruding) the polymer chains as they form can increase the amount of order, leading to a greater degree of crystallinity
y
y in the polymer.
p y
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Average molecular mass affects degree of crystallinity.
crystallinity
low and high density polyethylene
LDPE: average molecular mass of 104 amu
Sodium polyacrylate:
O-
HDPE: average molecular mass of 106 amu
-O
O
CH2
C
C
CH
CH
CH2
O
CH2
better known as…
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Polymer properties can be modified by crosslinking polymer chains together
crosslinking polymer chains together.
• Crosslinking: covalent bonds between chains: increases stiffness, strength
Examples: rubber, Silicone
• Metal
Metal ions can serve as cross
ions can serve as cross‐linking
linking agents
agents
Example: Sodium alginate
• Intermolecular
Intermolecular forces can hold polymer chains together:
forces can hold polymer chains together:
a) LDF’s hold chains in nonpolar polymers together: Polyethylene, polypropylene
b) Dipole dipole forces: PVC
b) Dipole‐dipole forces: PVC
c) H‐bonding PVA
plays an important role in biological polymers
DNA Proteins,
DNA, Proteins
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Cross linking with covalent bonds formed between chains make the polymer stiffer; more crystalline
Example: Vulcanization of Rubber
•
•
Natural rubber is too soft and chemically reactive to make a useful material.
B
By vulcanizing
l i i the rubber (crosslinking
th
bb (
li ki the chains with sulfur) useful th h i
ith lf )
f l
materials are made.
S8
heat
Isoprene (monomer)
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Natural rubber
(gummy)
CHEM 112 Polymers and BioMaterials
Cross‐linked rubber
(tough elastomer)
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Silicones represent a different kind of polymer: based on silicon rather than carbon
based on silicon rather than carbon.
Silicones are chains of alternating silicon and oxygen atoms with organic substituents on the silicon.
organic substituents on the silicon
Monomer: R2SiCl2
silicon compound with organic groups (R) attached. Formed via Condensation reaction
E
Example:
l
dichlorodimethylsilane [(CH3)2SiCl2] with water (H2O). The silicone formed is dimethylsiloxane
The silicone formed is dimethylsiloxane.
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By changing the R group, the properties of the polymer are changed. changed
Properties depend on:
Properties of Silicones
1)
2)
3)
Nontoxic, stable (unreactive)
Nontoxic,
stable (unreactive)
flame resistant
hydrocarbon substituents act to repel water.
repel water.
Silicones are either rubbers or oils
depending on the chain length and degree of cross‐linking.
g
g
R group
R‐group
Chain length
Degree of cross‐linking
–
No crosslinks: liquid silicone oils
No crosslinks: liquid silicone oils
high T lubricants, hot oil baths
–
Few cross links; silicone rubber
Few
cross links; silicone rubber
caulking material
–
More cross links: silicone resins
More
cross links: silicone resins
coatings and adhesives
–
All
All cross links: SiO
li k SiO2 = quartz
t
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Uses:
Lubricants
Car polish
Gaskets, sealants
Waterproofing (organic groups form barrier)
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Metal ions can serve as cross‐linking agents
Gummy worms: polymer of Sodium alginate
Alginate is a polysaccharide found in seaweed: (It’s edible!)
Used to thicken food (like milkshakes)
To make gummy worms: add crosslinker: CaCl2
add crosslinker: CaCl
Ca+2 ions hold chains together. MJ Bojan
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Intermolecular forces can hold polymer chains together:
Polyvinylalcohol (PVA) is an Addition Polymer
catalyst
CH2
CH2
CH
heat
vinyl alcohol
vinyl alcohol
OH
CH
OH
x
linear chain polymer
x = 103 ‐ 104
poly(vinyl alcohol)
Add a cross‐linker: borax B4O72−(forms H‐bonding crosslink)
+ B 4O72−
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viscoelastic20
Biological Polymers: Proteins are polyamides.
The monomers are amino acids:
Th
i
id
React via condensation polymerization
H
H
N
C
H
O
+
C
OH
R
*
H
H
N
C
R
H
H
N
C
H
O
R
H
N
C
C
H
R
O
H
N
C
H
OH
O
C
+
C
H
H
H
N
C
R
O
C
OH
O
C
*
R
Protein
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amide linkage or peptide bond
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The sequence of amino acids is the primary structure of the protein. Secondary structure of proteins:
Alpha helices and beta sheets
Tertiary structure (protein folding) is even more complex. This structure is p
p
y
important in protein activity.
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Biological Polymers DNA and RNA are polyesters
((of H3PO4)).
The monomers are nucleotides
React via condensation polymerization
p y
Monomer; nucleotide
Polymer; polynucleotide
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The DNA and RNA strands are held together by
H‐ bonds. Bases are paired
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Biomaterials: are any materials that have biomedical applications.
For example, the materials that are used to fill teeth are biomaterials.
Characteristics of Biomaterials
The biomaterials must be biocompatible:
The body’s immune system must not attack th bi
the biomaterial.
t i l
Physical requirements:
Biomaterials must be created for a specific environment.
Artificial heart valves must open and close 70 to 80 times per minute
Artificial heart valves must open and close 70 to 80 times per minute.
Chemical requirements:
g
Biomaterials must be of medical grade.
Polymers are very important biomaterials: beware of fillers, stabilizers, etc.
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The different types of The different types of Polymers have bio applications. Polymers have bio applications. Plastic: materials that can be formed into shapes.
Thermoplastic: materials that can be shaped more than once. (Used as replacements for blood vessels.)
Thermosetting: materials that can only be shaped once (Used in dental devices, and orthopedics such as hip replacements.)
Elastomer: material that is elastic in some way. If a moderate amount of deforming force is added, the elastomer will return to its original shape. Used as catheters, and for covering leads on implanted electronics, like pacemaker.
,
p
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Polymers are be used in many biological applications.
Examples of Biomaterial Applications
Heart Replacement and Repairs:
Polyethylene terephthalate, called Dacron™, Polyethylene
terephthalate called Dacron™
is often used in the manufacture of artificial heart valves.
Dacron™ is used because tissue will grow through a polyester mesh.
Vascular grafts:
g
A vascular graft is the replacement for A
vascular graft is the replacement for
a piece of blood vessel.
Dacron™ is used for large arteries.
*
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F
F
C
C
F
F
*
Polytetrafluoroethylene, is used for smaller vascular grafts.
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Polymers are be used in many biological applications.
Artificial Tissue:
•
•
•
•
Artificial skin, which is grown in the laboratory, is used to treat patients with extensive skin loss.
The challenge with growing artificial skin is
The challenge with growing artificial skin is getting the cells to align properly.
Therefore a scaffold must be used for the cells.
The most successful scaffold is lactic acid‐
glycolic acid copolymer.
Hip Replacements:
•
About 200 000 total hip replacements are performed each year
About 200,000 total hip replacements are performed each year.
•
A metal ball, a cobalt chromium alloy, is often used in a hip replacement.
•
This alloy is attached to a titanium alloy and cemented using a tough thermoset
polymer.
• The acetabulum, which accommodates the femur, is lined with a polyethylene layer
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