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RichardE.Eitel
AssociateProfessor
Email:[email protected]
Office:Burchard307B
DepartmentofChemical
EngineeringandMaterials
Science
StevensInstitute
of
Technology
E344:MaterialsProcessing
05 StructureandProperties
ofPolymers
2
Thegoalofthisdiscussionistobeabletodescribethe
macromolecularstructure
of
polymers
and
relate
this
to
theirengineeringproperties..Bytheendofthisdiscussionyoushouldbeableto:
Describetheroleofpolymerchain
lengthonkeyengineering
properties.
Classifythebasicstructuresof
polymerchains.
Predicttrendsinthecrystallinity
andpropertiesofpolymerbasedon
theirmolecularweightandchain
structure.
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Polymersaregiantmacromoleculesconsistingof10sto
many1,000sofcovalentlybondedrepeat(mer)units.
mer
GuidedInquiryI:AverageMolecularWeight
(10
minutes)
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A. 0.0012
B. 831.75
C. 832
D. 1,472,800
E. I don't know.
ConceptCheck:What is the DP for polypropylene with amolecular
weight of 35,000 g/mol?
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A. B. g/C. D. It is unit less
E. I don't know.
ConceptCheck:
What are the units for DP?
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Thechainlengthofpolymersischaracterizedbythedegree
ofpolymerization(DP)andthemolecularweight.
PolymersareALWAYSmixtureofmacromoleculesof
varyingmolecularweights
NumberAverageMolecularWeight
:
Where:
isthemedianmolecularweightofachainintheithsizerange.
isnumberfractionofchainsintheithsizerange
isweightfractionofchainsintheithsizerangeismolecularmassoftherepeat(mer)unit
=
WeightAverageMolecularWeight:
Degreeofpolymerization(DP):
Polydispersity Index(PDI):
0
0.05
0.1
0.15
0.2
0.25
FractionalContent
MolecularWeightRange(g/mol)
FractionalDistrubtionofMolecularWeights
WeightFraction(wi)
NumberFraction(ni)
GuidedInquiryII:MolecularWeightCalculations
(8
minutes)Considerasampleofplasticwhichyouknowis15mol%polymer
moleculeswithmolecularweight500,000g/mol,and85mol%
polymermoleculeswithmolecularweight150,000g/mol.
NumberAverageMolecularWeight:
WeightAverageMolecularWeight:
Degreeofpolymerization (DP):
Polydispersity Index(PDI):
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A. 119,600g/mol
B. 202,500g/mol
C. 280,000g/mol
D. Idontknow
ConceptCheck:What is the number average molecular weight forthe
above mixture?
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A. 119,600g/mol
B. 202,500g/mol
C. 280,000g/mol
D. Idontknow
ConceptCheck:
What is the weight average molecular weight forthe above
mixture?
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A.Yes
B.No
C.isalwayslessthan.D.Idontknow
ConceptCheck#3:Can belessthan?Why?
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Formanysimplelinearpolymers,materialpropertiesare
dictatedby
chain
length
and
weak
secondary
bonding
betweenchains.
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GuidedInquiryIII:MWandProperties.
(5minutes)
Considertwodifferentplatesofspaghetti.Oneinwhichallthe
strandsarethefulllengthandoneinwhichthestrandhavebeencutintoshortsegments.
A. 100,000 g/mol
B. 200,000 g/mol
C. Theyhavethesamestrength.
D. Idontknow
ConceptCheck#4:
Which has a higher strength; polyethylene of100,000 g/mol or
200,000 g/mol?
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ConceptCheck#5:Which flows more easily; polyethylene of
100,000g/mol or 200,000 g/mol?
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A. 100,000 g/mol
B. 200,000 g/mol
C. Theyflowequally.
D. Idontknow
Polymersmayalsobecharacterizedbychainlengthand
meanend
to
end
Distance.
Total(stretchedout)chainlength(L):
2Meanendtoenddistance(r):
Where:
d bondlength
N numberofbondsinchain
anglebetweenadjacentatoms
=109.5o forCC
L
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Thepropertiesoflinearpolymerscanbemodifiedby
introducingbranchingorcrosslinking.
Linear
secondary
bonding
Branched
Cross-Linked Network
Likemetalsandceramicspolymersmayexhibithighly
orderedcrystalline
regions.
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1
GuidedInquiryIV:PolymerCrystallinity.
(10minutes)
a) Polyethyleneorpolystyrene.
b) Polyethylenewithmanylongbranchesorpolyethylene
withafewshortbranches.
c) Atactic polypropyleneorisotacticpolypropylene.
d) Polypropyleneorpoly(ethyleneterephthalate)(PET).
e) Polyethyleneorarandomcopolymerof50%ethylene
repeatunitsand50%propylenerepeatunits.
f) PETmeltedandrapidlycooledtoroomtemperatureor
PETmelted,cooledto150 C,heldtherefor1hour,and
thencooledtoroomtemperature.
Reportout:
Which of the following pairs is more crystalline?
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1
Theproductionofmostpolymermacromoleculesfrom
theirmonomeroccursbyoneoftwopolymerization
routes.
Addition(Chaingrowth)
polymerization:
Condensation(stepreaction)
Polymerization
(byproductproducedoftenwater)
Initiatormonomer
Polyethylene:
Polyethyleneterephalate
Thermoplasticpolymersaretypicallylinearorslightly
branchedand
soften
on
heating
allowing
them
to
be
easily
(re)formedandreadilyrecycled.
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1
Thermosetpolymersreactonmixingorunderactionof
heatformingextensivecrosslinks.
Example: Bakelitefirstcompletely
syntheticplastic,
1907.
