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TEM of spherulite structure in natural rubber(x30,000).Chain-folded lamellar crystallites(white lines) ~10nm thick extend radially.
Chapter 4- Polymer Structures
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ISSUES TO ADDRESS...
What are the basic
Classification?
Monomers and chemical groups?
Nomenclature? Polymerization methods?
Molecular Weight and Degree of Polymerization?
Molecular Structures?
Crystallinity? Microstructural features?
Chapter 4- Polymer Structures
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Polymer= many mers
Adapted from Fig. 14.2, Callister 6e.
Polymer Microstructure
Polyethylene perspective of molecule
A zig-zag backbone structure with covalent bonds
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Covalent chain configurations and strength:
Direction of increasing strengthAdapted from Fig. 14.7, Callister 6e.
Polymer Microstructure
Van der Waals, HMore rigid
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Common Examples
- Textile fibers: polyester, nylon
- IC packaging materials.
- Resists for photolithography/microfabrication.
- Plastic bottles (polyethylene plastics).
- Adhesives and epoxy.
- High-strength/light-weight fibers: polyamides,polyurethanes, Kevlar
- Biopolymers: DNA, proteins, cellulose
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Thermoplastics: polymers that flow more easily when
squeezed
, pushed, stretched, etc. by a load (usually atelevated T). Can be reheated to change shape.
Thermosets: polymers that flow and can be molded
initially but their shape becomes set upon curing. Reheating will result in irreversible change or decomposition.
Other ways to classify polymers. By chemical functionality (e.g. polyacrylates, polyamides,
polyethers, polyeurethanes).
Vinyl vs. non-vinyl polymers.
By polymerization methods (radical, anionic, cationic).
Etc
Common Classification
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Common Chemical Functional Groups
Saturated hydrocarbons(loose H to add atoms)
C C
H
H H
H
Ethylene(ethene)
C C
H
H C
H
H
HH
Propylene
(propene)=
1-butene
2-butenetrans cis
Acetylene(ethyne)
C CH H
Unsaturated hydrocarbons(double and triple bonds)
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Alcohols Methyl alcohols
Ethers Dimethyl Ether
Acids Acetic acid
Aldehydes Formaldehyde
Aromatic
hydrocarbonsPhenol
Common Hydrocarbon Monomers
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Some Common Polymers
C C
C
N
H
H H
Polyacrylonitrile (PAN)
C C
H
H H
X
C CH
H X
H
Vinyl polymers(one or more Hs of ethylene can be substituted)
Common backbone with substitutions
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Monomer-based naming:
poly________
e.g. ethylene -> polyethylene
if monomer name contains more than one word:
poly(_____ ____)
e.g. acrylic acid -> poly(acrylic acid)
Monomer name goes here
Monomer name in parentheses
Note: this may lead to polymers with different names but same structure.
C C C C
H
H
H
H
H
H
H
H
C C C C
H
H
H
H
H
H
H
H
polyethylene polymethylene
Nomenclature
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Polymerization Methods
H H
A. Free Radical Polymerization
1. Initiation
Free radical initiator
(unpaired electron)
C C
H
H H
H
monomer
C C
H
H H
R
H
RRadical
transferred
CC
HH
s bonds
p bond
R
H
H
C
HH
C
R
sp2 carbons
sp3 carbon
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Polymerization Methods
A. Free Radical Polymerization
2. Propagation
C C
H
H H
H
C C
H
H H
R
H
C C
H
H H
C
H
H
H
C
H
H
R
C C
H
H H
H
C C
H
H H
C
H
H
H
C
H
H
C
H
H
C
H
H
R
H
HC
HH
C
R
H H
CC
HH
Both carbon atoms will
change from sp2 to sp3.
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Polymerization Methods
A. Free Radical Polymerization
3. Termination
C C
H
H H
R
H
C C
H
H H
R
H
R+ C C
H
H H
R
H
R
C C
H
H H
R
H
+ C
H
H
C
H
H
C
H
H
C
H
H
R R
Intentional or unintentional molecules/impurities can also terminate.
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Polymerization Methods
B. Stepwise polymerization
RC
OH
O
NH2
+R
COH
O
NH2
RC
NH
O
NH2
RC
OH
O
C. Other methods
Anionic polymerization, cationic
polymerization, coordination
polymerization
RC
O
NH
n
HO
H+
HO
H+ (n-1)
Loses water
(condensation)
Proteins (polypeptides have similar composition)
CH C
O
NH
R n
Various R groups
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Molecular Weights
Not only are there different structures (molecular arrangements)
but there can also be a distribution of molecular weights(i.e. number of monomers per polymer molecule).
20 mers 16 mers
10 mers
Average molecular weight =monomermonomer
MM 3.153
101620
This is what is called numberaverage molecular weight.
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Number average molecular weight:
Mn
NjMjj
Njj
mo Njj
j
Njj
NjMjj
Note: Total weight
Njj
Total # of polymer chains
Weight average molecular weight:
Mw
WjMjj
Wjj
NjMj2
j
NjMjj
Wj NjMj
In general: M
NjMj1
j
NjMj
j
MnIf = 0 then If = 1 then
Mw
Nj = # of polymer chains with lengthj
Mj= jmo mass of polymer chain with lengthj(mo
= monomer molecular weight).
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Molecular Weight: Different Notations
Mn
NjMjj
Njj
Mn xiMii
xi NiNj
j
Mw
NjMj2
j
NjMjj
Mw wiMii
wi NiMi
NjMjj
In Lecture Notes In Callister Textbook
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Examples
Light scattering: larger molecules scatter more light than smaller ones.
Infrared absorption properties: larger molecules have more sidegroups and light absorption (due to vibrational modes of side groups)
varies linearly with number of side groups.
Molecular WeightsWhy do we care about weight average MW?-some properties are dependent on MW (larger MW polymer chains can
contribute to overall properties more than smaller ones).
Distribution ofpolymer weights
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Polydispersity and Degree of Polymerization
Polydispersity:
MwMn 1
When polydispersity = 1, system is monodisperse.
Degree of Polymerization:
nnMnmo
Number avg degree of polymerization
nw Mwmo
Weight avg degree of polymerization
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Compute the number-average degree of polymerization forpolypropylene,given that the number-average molecular weight is 1,000,000 g/mol.
What is mer of PP?
Mer molecular weight of PP is
Example 1
C3H6
mo=3AC+6AH=3(12.01 g/mol)+6(1.008 g/mol)
= 42.08 g/mol
Number avg degree of polymerization
nnMnmo
106g/mol
42.08g/mol 23,700
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Example 2 (a, b, and c)A. Calculate the number and weight average degrees of polymerization
and polydispersity for a polymer sample with the following distribution.
Avg # of monomers/chain Relative abundance10 5
100 25
500 50
1000 30
5000 10
50,000 5
nn Mn
mom0
m0
jNjjNjj
jNjjNjj
5*10 25*100 50*500 30*100010 *5000 5 *50000
5 25 50 30 10 5 2860.4
nw Mw
mo
1
mo
(jmo )2Njj
Nj (jmo )j
j2NjjjNjj
5*102 25*1002 50*5002 30*10002 10 *50002 5 *500002
5*10 25*100 50*500 30*1000 10 *5000 5 *50000 35,800
Note: m0 cancels in all these!
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Example 2 (cont.)
B. If the polymer is PMMA, calculate number and weight averagemolecular weights.
Mw if monomer is methylmethacrylate (5C, 2O, and 8H)So m0= 5(12)+2(16)+8(1)= 100 g/mol
CH3
|-CH2-C-
|CO2CH3
Mnnnmo 2860.4(100g/mol) 286,040g/mol
Mwnwmo35,800(100g/mol) 3,580,000g/mol
Mw
Mn
3,580 ,000
286 ,040 ~12.52Polydispersity:
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Example 2 (cont.)
C. If we add polymer chains with avg # of monomers = 10 such that theirrelative abundance changes from 5 to 10, what are the new number
and weight average degrees of polymerization and polydispersity?
Add 5 more monomers of length 10 .nn =M
n
mo=
jNjj
Njj
=1 0 * 1 0 + 2 5 * 1 0 0 + 5 0 * 5 0 0 + 30 *1000 + 10 * 5000 + 5 * 50000
1 0 + 2 5 + 5 0 + 3 0 + 1 0 + 5
= 2750
Mw
Mn=
3,580,000
275000~ 13Polydispersity:
nw Mwmo
j2Njj
jNjj 35,800
Note: significant change in number average (3.8 %)
but no change in weight average!
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For an asymmetric monomer
T H T H+
T H T H
T H H T
H T T H
C
H
F
C
H
H
C
F
H
C
H
H
C
H
F
C
H
H
C
H
H
C
F
H
e.g. poly(vinyl fluoride):
H to T T to T
H to H
Random arrangement
e.g. PMMA
C
C
CH3
C
H
H
C
C
CH3
C
H
H
O OCH
3 OCH
3
O
C
C
CH3
C
H
H
C
C
CH3
C
H
H
OO
CH3
OCH
3
O
H to T H to TH to T
Exclusive H to T arrangement (Why?)
Sequence isomerism
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Regularity and symmetry of side groups affect properties
Stereoisomerism: (can add geometric isomerism too)
Polymer Molecular Configurations
IsotacticOn one side
SyndiotacticAlternating sides
AtacticRandomly placed
- Conversion from one stereoisomerism to another is notpossible by simplerotation about single chain bond; bonds must be severed first, then reformed!
PolymerizeCan it crystallize?
Melting T?
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Regularity and symmetry of side groups affect properties
Polymer Geometrical Isomerism
cis-structure trans-structure
with R= CH3 to form rubber
Cis-polyisoprene trans-polyisoprene
HH
-Conversion from one isomerismto another is notpossible by simplerotation about chain bond because double-bond is too rigid!
-See Figure 4.8 for taxonomy of polymer structures
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Polymer Structural Isomerism
Some polymers contain monomers with more than 1 reactive site
e.g. isoprene
CH2
CCH
CH2
CH3
trans-isoprene
trans-1,4-polyisoprene
CH
2
CCH
C
H2
CH3
1
42
trans-1,2-polyisoprene
n
C
H2
C
CH
CH2
CH3
n
3
3,4-polyisoprene
C
H2
CH
C
CH2
CH3
n
Note: there are also cis-1,4- and cis-1,2-polyisoprene
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Covalent chain configurations and strength:
Direction of increasing strengthAdapted from Fig. 14.7, Callister 6e.
Polymer Microstructure
Van der Waals, H More rigid
Short branching
Long branchingStar branching Dendrimers
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Random, Alternating, Blocked, and Grafted
CoPolymers
Synthetic rubbers are often copolymers.
e.g., automobile tires (SBR)
Styrene-Butadiene Rubber random polymer
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Molecular Structure
How do crosslinking and branching occur in polymerization?
1. Start with or add in monomers that have more than 2 sites that bond
with other monomers, e.g. crosslinking polystyrene with divinyl benzene
stryene polystyrene
Control degree of
crosslinking by
styrene-divinyl
benzene ratio
+
styrene
divinyl benzene crosslinked polystyrene
Monomers with trifunctional groups lead to network polymers.
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Molecular Structure
Branching in polyethylene (back-biting)
CH2
CH2 R
CH2
CH
2
CH2
CH
2
CH2
C
H
H
RC
CH
2
CH2
CH2
C
H
HH
H
Same as
Radical moves to a different carbon
(H transfer) RC
CH
2
CH2
CH2
C
H
H
H
H
Polymerization continues from this carbon
Process is difficult to avoid and leads to (highly branched) low-density PE .When there is small degree of branching you get high-density PE.
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Example 3Nitrile rubber copolymer, co-poly(acrylonitrile-butadiene), has
Calculate the ratio of(# of acrylonitrile) to (# of butadiene).
Mn106,740 g/mol
nn 2000
3 C = 3 x 12.01 g/mol
3 H = 3 x 1.008 g/mol
1 N = 1 x 14.007 g/mol
m0= 53.06 g/mol
4 C = 4 x 12.01 g/mol
6 H = 6 x 1.008 g/mol
m0= 54.09 g/mol
1,4-addition product
moMnnn
106,740
2000 53.57g/molWe need to use an
avg. monomer MW:
mo f1m1 f2m2 f1(m1m2)m2
f1
m0 m2m1m2
53.37 54.09
53.06 54.09 0.7 f2 1 f1 0.3
f2
f1
0.7
0.37 : 3
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Crosslinking in elastomers is called vulcanization, and is achieved byirreversible chemical reaction, usually requiring high temperatures.
Vulcanization
Sulfur compounds are added to form chains that bond adjacent
polymer backbone chains and crosslinksthem. Unvulcnaized rubber is soft and tacky an poorly resistant to wear.
e.g., cis-isoprene Stress-strain curves
+ (m+n) S (S)n
(S)m
Single bonds
Double bonds
See also sect. in Chpt. 8
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Molecular weight, Mw: Mass of a mole of chains.
Tensile strength (TS):--often increases with Mw.
--Why? Longer chains are entangled (anchored) better.
% Crystallinity: % of material that is crystalline.--TS and E often increase
with % crystallinity.
--Annealing causes
crystalline regions to grow.
% crystallinity increases.
crystalline
region
amorphousregion
Adapted from Fig. 14.11, Callister 6e.
Molecular Weight and Crystallinity
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Polymer Crystallinity
polyethylene Some are amorphous.
Some are partially crystalline (semi-crystalline). Why is it difficult to have a 100% crystalline polymer?
%crystallinity c (s a )
s (ca )
100%
s = density of specimen in question
a
= density of totally amorphous polymer
c = density of totally crystalline polymer
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%crystallinityMcrystalline
Mtotal100%
cVc
sVs100%
c
s
fc100%
Volume fraction of crystalline component.
MtotalMcrystallineMamophous
MsM
cM
asVs cVc aVa
s cVcVs
aVaVs
cfc afa cfc a(1 fc) fc(c a) a
Using definition of volume fractions:
fc VcVs
faVaVs
fc
s a
c a
Substituting in fc into the original definition: %crystallinity c(s a )
s (c a )100%
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Polymer Crystallinity
Degree of crystallinity depends on processing conditions (e.g.
cooling rate) and chain configuration.
Cooling rate: during crystallization upon cooling through MP,polymers become highly viscous. Requires sufficient time for
random & entangled chains to become ordered in viscous liquid.
Chemical groups and chain configuration:
More Crystalline
Smaller/simper side groups
Linear
Isotactic or syndiotactic
Less Crystalline
Larger/complex side groups
Highly branched
Crosslinked, network
Random
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Semi-Crystalline Polymers
Fringed micelle model: crystalline region embedded in amorphous region.
A single chain of polymer may pass through several crystalline regions aswell as intervening amorphous regions.
fc s a
c a
Crystalline volume fractions Important
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Semi-Crystalline Polymers
Chain-folded model: regularly shaped platelets (~10 20 nm thick)
sometimes forming multilayers.Average chain length >> platelet thickness.
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Semi-Crystalline Polymers
Spherulites: Spherical shape composed of aggregates of chain-folded crystallites.
Natural rubber
Cross-polarized light through
spherulite structure of PE.
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Diblock copolymers
Representative polymer-polymer
phase behavior with different
architectures:A) Phase separation with mixed
LINEAR homopolymers.
B) Mixed LINEAR homopolymers and
DIBLOCK copolymergives
surfactant-like stabilized state.
C) Covalent bond between blocks in
DIBLOCK copolymergive
microphase segregation.F. Bates, Science1991.
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Thermoplastics:--little cross linking
--ductile
--soften w/heating
--polyethylene (#2)
polypropylene (#5)
polycarbonatepolystyrene (#6)
Thermosets:--large cross linking (10 to 50% of mers)
--hard and brittle--do NOT soften w/heating
--vulcanized rubber, epoxies,
polyester resin, phenolic resin
Callister,Fig. 16.9
T
Molecular weight
Tg
Tmmobileliquid
viscousliquid
rubber
toughplastic
partiallycrystallinesolid
crystallinesolid
Adapted from Fig. 15.18, Callister 6e.
Thermoplastics vs Thermosets
Tm: melting over wide range of T
depends upon history of sample
consequence of lamellar structure
thicker lamellae, higher Tm.
Tg: from rubbery to rigid as T lowers
P ki f P l
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Packing of spherical atoms as in ionic and metallic crystals led to
crystalline structures.
How polymers pack depend on many factors:
long or short, e.g. long (-CH2-)n.
stiff or flexible, e.g. bendy C-C sp3.
smooth or lumpy, e.g., HDPE.
regular or random single or branched
slippery or sticky, e.g. C-H covalent (nonpolar) joined via vdW.
Analogy:Consider dried (uncooked) spaghetti (crystalline) vs.cooked and buttered spaghetti (amorphous).
pile of long stiff spaghetti forms a random arrangement.
cut into short pieces and they align easily.
Candle wax more crystalline than PE, even though same chemicalnature.
Packing of Polymers
Wh t A E t d P ti ?
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Would you expect melting ofnylon 6,6to be lower than PE?
What Are Expected Properties?
N
|
H
H
|
C
|
H
6
N
|
H
O
||C
H
|
C
|
H
4
N
|
H
O
||C
N
|
H
H
|
C
|
H
6
N
|
H
O
||
C
H
|
C
|
H
4
N
|
H
O
||
C
+
++
+
++
H
C
H
H
C
H
H
C
H
H
C
H
+
+
+
+
+
+
nylon 6,6 polyethylene
a) What is the source of intermolecular cohesion in Nylon vs PE?
b) How does the source of linking affect temperature?
Hydrogen bonds
Van der Waals bonds
With H-bonds vs vdW bonds, nylon is expected to have (and does) higher melting T.
Wh t A E t d P ti ?
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Which polymer more likely to crystallize? Can it be decided?
What Are Expected Properties?
Linear syndiotactic polyvinyl chloride Linear isotactic polystyrene
Linear and syndiotactic polyvinyl chloride is more likely to crystallize.
The phenyl side-group for PS is bulkier than the Cl side-group for PVC.
Generally, syndiotactic and isotactic isomers are equally likely to crystallize.
For linear polymers, crystallization is more easily accomplished as chain
alignment is not prevented.
Crystallization is not favored for polymers that are composed of
chemically complex mer structures, e.g. polyisoprene.
Wh t A E t d P ti ?
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Which polymer more likely to crystallize? Can it be decided?
What Are Expected Properties?
Linear and highly crosslink
cis-isoprene
Not possible to decide which might crystallize. Both not likely to do so.
Networked and highly crosslinked structures are near impossible to
reorient to favorable alignment.
H+
+ H20
Networked
Phenol-Formaldehyde
(Bakelite)
Wh t A E t d P ti ?
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Which polymer more likely to crystallize? Can it be decided?
What Are Expected Properties?
alternatingPoly(Polystyrene-Ethylene)
Copolymer
random
poly(vinyl chloride-tetra-fluoroethylne)
copolymer
Alternating co-polymer more likely to crystallize than random ones, as they are
always more easily crystallized as the chains can align more easily.
D t t
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Soap is a detergent based on animal or vegetable product, some
contain petrochemicals
Detergents
grease
water
detergent
What properties of soap molecules do you need to remove grease?green end must be hydrophilic. Why?
Opposite end must be hydrocarbon. Why?
Water must be like oxygen (hoard
electrons and promote H-bonding)
greasee.g., oxy-clean
Si l l El l + B SLIME!
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Simple polymer: Elmers glue + BoraxSLIME!
Chemistry Elmers glueis similar to poly (vinyl alcohol) with formula:
Borax is sodium tetraborate decahydrate (B4Na2O7 10 H2O).
The borax actually dissolves to form boric acid, B(OH)3.
This boric acid-borate solution is a buffer with a pH of about 9 (basic).Boric acid will accept a hydroxide OH- from water.
B(OH)3 + 2H2O B(OH)4- + H3O
+ pH=9.2
OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH
this is a SHORT, n=15 chain of poly(vinyl alcohol)
Hydrolyzed molecule acts in a condensation reaction
with PVA, crosslinking it.
Simple polymer: Elmers glue + Borax SLIME!
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MatSE 280: Introduction to Engineering Materials D.D. Johnson 2004, 2006, 2007-08
Simple polymer: Elmers glue + Borax SLIME!
Hydrolyzed molecule acts in a condensation reaction with PVA,
crosslinking it.
B(OH)3 + 2H2O B(OH)4- + H3O
+ pH=9.2
Crosslinkingties chains via weak non-covalent(hydrogen) bonds, so it flows slowly.
Crosslinked
Range of Bonding and Elastic Properties
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MatSE 280: Introduction to Engineering Materials D.D. Johnson 2004, 2006, 2007-08
Range of Bonding and Elastic Properties
Is slime a thermoset or thermoplastic, or neither?
Thermoset
bondingThermoplastic
bonding
Induced dipolar bondsform crosslinks
Slime?
Stiffness increases
Where is nylon?
Covalent bonds form
crosslinks H-bonds form
crosslinks
Summary
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Polymers are part crystalline and part amorphous.
The more lumpy and branched the polymer, the less denseand less crystalline.
The more crosslinking the stifferthe polymer. And, networked
polymers are like heavily crosslinked ones.
Many long-chained polymers crystallize with a Spherulite
microstructure - radial crystallites separated by amorphous
regions.
Optical properties: crystalline -> scatter light (Bragg)amorphous -> transparent.
Most covalent molecules absorb light outside visible spectrum, e.g.PMMA (lucite) is a high clarity tranparent materials.
Summary