Study of microwave annealing effects on polymer crystallization

Rochester Institute of Technology

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11-1-1998

Study of microwave annealing effects on polymer

crystallization

Junmei Ji

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Recommended Citation

Study of Microwave Annealing Effects on

Polymer Crystallization

Junmei Ji

November, 1998

Thesis

Submitted in partial fulfillment of the requirements for the degree

of Master of Science in Chemistry

Approved:

Andreas Langner

Project Advisor

Gerald Takacs

Department Head

Study

of

Microwave

_Annealing

Effects

on

Polymer Crystallization

I,Junmei

_Ji,

_hereby

grant permissionto theWallace Memorial

_Library,

of

_R.I.T.,

to

reproduce_mythesisinwhole orinpart.

_Any

reproduction will notbe forcommercial use

orprofit.

Acknowledgements

Iwouldliketothank_myadvisor, Dr. Andreas

Langner,

forhisadvice and

support. His extensive knowledge and creative ideas made it possible to initiate this

thesis. His effective guidance and

_help

taughtmehowto becomeagoodresearcher and

experimentalist. Hishumorgaveme alotoffun

_during

_mystudies.

Ialsowouldliketo thank_mygraduate_committee,Dr. Gerald

Takacs,

Dr.

Joseph

_Homak,

andDr. Massoud

_MM,

forall their

_help

and support. I have benefited

from their suggestions and questions. I would like to extend _my thanks to Ms. Ruth

Willerthfor her

_help

inthe constructionoftheexperimental set-up.

Nowordsare enoughtoexpress_mythanks to_myparents.Withouttheir

understandingand _support, Iwould neverhave finishedthisthesis. I alsowould liketo

Abstract

In_{this project, the}effect of melt _annealing onthe crystallization ofpoly(ethylene

oxide),

(PEO),

was studied

_by

_comparingthermal _annealing to microwave annealing. After thesubsequent controlled_{crystallization, the}crystalline_morphologyoffilmswas

characterized

_{by Polarizing}

Light

_Microscopy

_(PLM);

the_melting curvesfor both films and bulk samples were measured

_by

Differential

_{Scanning Calorimetry}

(DSC). These

measurements revealed significant differences between microwave and

_thermally

annealed samples. We propose thatmicrowave _{annealing has} a "quantum on polymercrystallization. _{The difference in morphology}cannotbe explained

_by

a change inthetemperatureprofile ofthe two _annealingmethods.

By

investigating

thedifference betweenmicrowave_annealingandthermal

annealing, while _changing the crystallization_temperature, we found that microwave

absorption could change the polymer chain conformation more efficiently. The application of microwaves accelerates the dipolemoment oscillation thus _{speeding up}

rotation of polymer chain segments. In this way, microwave _annealing could

_destroy

the residual nuclei leftfroma prior crystalline_morphology,

_decreasing

the nucleation

site density.

_{Consequently,}

homogeneous nucleation becomes more important inthe case of microwave annealing, and the _morphology and _{melting behavior} of the

crystalline phase are both changed. As molecular weight ofPEO was

increased,

the

microwave effect became more evident. Some factors in the crystallization process

which_may mask the microwave effects are the surface area ofthe film samples, the

coolingrate andthe crystallizationtemperature.

Itisbelievedthatmicrowave _annealinghaspotentialtoproducenewcrystalline

Table

of

Contents

Chapter 1: Introduction 1

Chapter 2: Background 3

2.1

Morphology

ofCrystalline Polymers 3

2.2 NucleationandGrowth StepsofCrystallization 6

2.3 Homogeneous andHeterogeneous Nucleation 7

2.4

_Melting

BehaviorofCrystalline Polymers 8

2.5 Microwave

_Processing

ofPolymers 1 1

Chapter3: Experimental 16

3.1 Polyethylene oxide) Samples 16

3.2 Experimental

_Set-up

for Microwave

_Annealing

17

3.3

Morphology

Characterization

by

PLM 18

3.4Investigationof

Melting

BehaviorofCrystalline Polymers

by

DSC 18

Chapter 4: ResultsandDiscussions 21

4.1 Microwave EffectsonPEO Crystallization 21

4.2 The MechanismsoftheMicrowaveEffects 24

4.2.1 DependenceofMicrowaveeffects onCrystallization Temperature 25

4.2.2 Dependenceof

_Melting

BehavioronPEO Molecular Weight 42

Chapter 5: Conclusions 47

List

of

Figures

IV

Figure 1: Theoreticalplot ofTm*versusTcforindicatedchainlength

oflinearpolyethylene 10

Figure 2: Typicalvariationindielectric constantanddielectric losswith

frequency

forpolymers 13

Figure 3: _{Experimental set-up for}microwave_annealing 19

Figure4: The calibrationtemperatureofthepolymer meltsvs. _setting

temperature ofthereservoir 20

Figure 5:

_Morphology

of crystalline

_PEO,

crystallizedat25

C

inair after

a)thermalannealing, and_successively

_b)

microwave_annealing 22

Figure6:

_Morphology

of crystalline

_PEO,

crystallizedat25

C

inair after

a)microwave_annealing, and_successively

_b)

thermal _annealing 23

Figure 7:

_Morphology

of crystalline

_PEO,

crystallized at-44

C

on

dry

ice

after_a)microwaveannealing,

_b)

thermal_annealing 28

Figure 8:

_Morphology

of crystalline

_PEO,

crystallized 1 hrat55

_C,

after

_(a)

microwave annealing,

_(b)

thermal_annealing 29

Figure 9:

_Morphology

of crystalline

_PEO,

crystallized 15hr at55

_C,

after_a)microwave_annealing,

_b)

thermal_annealing 31

Figure 10: DSCthermogramsofthefilm samples afterdifferent_annealing

treatments 33

Figure 1 1: DSCthermogramsofthebulksamples afterdifferent_annealing

Figure 12:

_Morphology

ofcrystalline

_PEO,

crystallized at0

C,

after

a)microwave _annealing,

_b)

thermal_annealing 37

Figure 13:

_Morphology

of crystalline

_PEO,

crystallizedat40

C,

after

a)microwave_annealing,

_b)

thermal_annealing 39

Figure 14: DSCthermograms ofthebulkandfilmsamplescrystallized

at^4C 40

Figure 15: DSCthermograms ofthebulkandfilmsamples crystallized

atOC 41

Figure 16:

_Morphology

ofcrystallinePEO (Mn:2000g/mol), crystallized

at25

C

inair, after_a)microwave_annealing;

_b)

thermal_annealing 44

Figure 17:

_Morphology

of crystallinePEO (Mn:400,000 g/mol),crystallized at

VI

List

of

Tables

Table 1: Comparison of microwave andthermal

_heating

characteristics 12

Table 2: Samplecharacteristics of poly(ethylene_oxide) 16

Table3: Comparisonof_{melting behaviors for film}samples 26

Chapter

1 Introduction

Crystallinepolymersrepresenta class of materials_{that, in}solid_state, can

_develop

complex morphologies for which ordered structures occur over a wide range of

dimensions.

_Moreover,

these ordered crystallites are often

_intimately

mixed with

amorphousdomains_producinga composite structure. A_varietyof microstructures ofthis

kindprovide a good_possibilitytodesignthe crystalline polymeric materials withunique

properties.

_Therefore,

it is ofimportancetounderstandpolymercrystallizationto achieve

betterprocess control and obtaindesiredproperties.

Animportant issue inthe_studyof polymer crystallizationis the_{annealing behavior}

of polymeric materials below and above their melt temperatures. In this work,

"annealing"

refers to

_heating

thepolymer aboveitsmelttemperature. In a previous _study

about the crystallization kinetics of poly(ethylene oxide),

(PEO),

a model had been

proposedtosimulate the exothermicbehaviorofits crystallization(1). Themodelfitthe

experimental datawell for lowmolecularweight _polymers, butnot for high molecular

weight polymers. One explanation for this disagreement was the _memory effect of

previous _morphology on subsequent recrystallization. When crystalline polymers melt,

polymerchains can

_locally

break thebonds.

However,

itwilltakemore timeand_energy

for longer polymer chains to move around in a

_highly

viscous melt. As a _result, some

partiallyordered regions still exist after melting. The chains intheseregions can _rapidly

revertback toform bondswhenthe crystallization occurs uponcooling. Becauseof_this,

crystallization events. Conventional thermal_annealingofpolymer melts to eliminate this

local,

partial_{ordering is}tedious andtime-consuming.

_Therefore,

inthisstudy, weattempt

to introduce microwave radiation to speed_up annealing.

_By

_accelerating the rotation of

polymerchains, it is expected thatmicrowave radiation willbe able to heat thepolymer

melt _{volumetrically} and quickly,

_leading

to rapid

_disordering

ofits molten phase, and

achieving better subsequentcrystallizationprocess control.

Inthis_preliminarywork,wehavecarriedoutparallel experimentsto _studythe

effects of microwave _annealing

_(MA)

and thermal _annealing

_(TA)

on the PEO

crystallization. Sample morphology and the subsequent _{melting behavior} were

characterized

_by

_{polarizing light microscopy}

_(PLM)

anddifferential scanning calorimeter

(DSC),

respectively. Eachparallel experiment ofMA andTAhas beenconductedunder

thesame _coolingconditions. Theeffects ofsurface area, crystallization_temperature, and

molecularweight on the _morphology and _melting behaviors ofcrystalline

_PEO,

treated

Chapter

_{2 Background}

2.1.

_Morphology

Itiswellknownthat some polymermelts, ata certaindegreeof _supercoolingwill

rearrange themselves to form more ordered arrays characteristic of a crystalline state,

resulting in specific morphologies. A _{detailed study} of crystalline _morphology will not

only make it possible to better understand crystallization mechanism, but also

_help

to

achieve desiredpolymer properties.

_Here,

we will discuss fourtypicalmorphologies for

crystalline_polymers,

_including

theamorphous_structure,

_lamellae,

axialiteandspherulite,

aswell astheir_relationshipto crytallization conditions.

Amorphous Structure: One important feature of crystalline _polymers,

_{distinguishing}

them from small molecularcrystalline _solids, is that there is considerable fraction of a

disordered,

liquid-like,

amorphous component.

_Usually,

this component forms a

continuousmatrixinwhichthecrystalline regions are embedded. Therelativeamountof

the amorphous componentvaries _considerably

_depending

on the chemical nature ofthe

polymer chains and on the crystallization conditions. Each polymer chain _may be

incorporated

into,

or pass through several crystalline regions. As a _result, crystalline

regions are tied together_primarily

_by

covalent bonds fromsegments ofthe chainin the

amorphous region(2).

Lamellae:

_X-ray

diffraction experiments of semi-crystalline polymers often give a_very

chain is observed. On a larger scale, the

_{"chain-folding}

lamellae"

concept predicts that

the representative structural element ofthe crystallite is the plate-like lamellae(3). It is

typically

about 100

A

in _thickness,

_independent

ofmolecular weight. The molecular

chain axis or the C-axis of the unit cell lays in the direction of the thickness,

perpendicular to the plane surface ofthe lamellae. Each constituent molecule traverses

the lamellae_manytimes. Thethickness ofthelamellae grownfrom a melt is reasonably

uniform, and it is

_inversely

related to the degree of_supercooling at which the crystals

were grown.

Axialite and Spherulite: Single crystal lamellae can be formed under extreme

conditions.

_However,

upon_coolingof a polymer_{melt,many lamellae}aregenerated.

_They

may

break-up,

branching

throughscrew

_{dislocations,}

and

_finally

impingeorinterconnect

witheach otherthroughchainswhichare anchoredinmorethanone lamellae. Aggregate

lamellar layers _{may link} and align to take a characteristic _shape, either sheaf-like or

spherical. A sheaf-like _{morphology is} referred to as either axialite or

_hedrite,

while a

spherical oneis_usuallycalled a spherulite.

Relationship

betweenThese Structures: Themulti-layeredlamellaraggregates,formed

through the action of screw dislocations about one _{axis, may} appear sheaf-like

₍₃₎

if

viewed _along the edges ofthe lamellae. Axialites result when all the lamellae grow to

their final size without interconnection witheach other except _along the axis. Axialites

are formed at a temperature near the melt temperature.

_They

grow _slower, and the

axialite, all the layers tend to _splay apart from each other, with non-crystallizable

componentsinthegrowthfront

_being

rejectedinto theinterstitialsites(2-3).

The maltesecross-sectionimageof a _spherulite,viewedundera_{polarizing light}

microscope,

indicates

that all the radii are equivalent. In practice, spherulites are

composed of radial sequences oflamellaewhich are often linked

_by

screwdislocations.

Thegrowthproceeds

_by

theinitialformationofan_arrayofdominant

_lamellae,

composed

ofthe longest and mostregularmolecular chains present withinthe melt.

_Following

the

establishment ofthe dominant

_framework,

subsequent _subsidiary crystallization ofless

crystallizablematerial occurs attheinterstitial sites ofbranched lamellae. Componentsor

impuritiesunable to crystallizebecome trapped in amorphousdomains iftheir diffusion

is slow relativeto the_advancing growthfront (2).

Axialites are believed to represent the precursor to a full spherulite. Lamella

branching

or interconnection may take place anywhere in the growth

_front,

and screw

dislocations may occur around _{any axis,} when crystallization proceeds rapidly.

Eventually,

all solid angles will be filled and a near spherical envelope is generated.

Therefore,

the formation ofaxialites or _spherulites, under a particular set of growth

conditions, depends upon the

_frequency

with which the

_branching

occurs. These

structuresdiffer

_by

degreeratherthan

_by

_anymorefundamentalaspect(3).

Theprecisearrangementoflamellaand amorphousdomainswithinthebulk

polymerisof greatimportancein

_determining

microscopic properties.For_example,ithas

long

been accepted that as the crystallization _proceeds, the shorter, less crystallizable

spherulites. As such inter-spherulitic regions constitute the sites ofpotential weakness,

crack _propagation, or dielectric

break-down

_may start from these regions. In practice,

spherulite sizerepresentsanimportantparameterfor bulk propertycontrol(2).

2.2 Nucleation

_Step

and

Growth

_Step

_during

Crystallization

Twosteps areinvolvedinthecrystallizationfroma polymermelt. Initially, a

nucleus ofcritical size mustbe formed. Ifthenucleus is too small, its high surface free

energy prevents it from

_being

stable

_long

enough to make crystal growth possible.

Nucleation cantakeplace _{spontaneously in 3}

_dimension,

or on an_existing surface, i.e. in

a growth front. Once the growth front is _established, crystal growth can take place.

During

crystal growth_stage, polymer chainsmust align themselves to the growthfront.

As chains _continually add to the growth

_front,

layers of polymers form the edge ofthe

lamellae. It is believed that the _morphology ofthe crystalline phase depends on the

relative rate ofnucleation to the rate at which polymer chains are "reeled in"

to the

growth front. The

_"reeling

in"

ofthe chains depends on the

_diffusion,

or the reptation

rate. At a small degree of _{supercooling,} or a high crystallization _temperature, the

reptation rateof polymerchainsis_relatively

high,

andthelateral growth rate of a nucleus

across the face ofthe crystal is much fasterthanthe nucleation rate. This leads to full

lateral growth oflamellaewith little interconnections. As a_result, axiallites are formed

undertheseconditions. In contrast,ifthedegreeof_supercoolingis sufficiently

_large,

the

nucleation rate will be much higher than the lateral growth rate. Multiple nucleation

events take place _{simultaneously,} and lateral growth ofnuclei is retarded. Now the

bridgeandimpingeto forma muchmore complexmorphology. The resulting structure is

thespherulite (4).

2.3 Heterogeneous

and

Homogeneous

Nucleation

Nucleationcanbeclassified ashomogeneous orheterogeneous. Homogeneous

nucleation involves a spontaneous, 3-dimentional aggregation of polymer chains at a

temperature _{below its melting}point, in a mannerwhich isreversible_up to a pointwhere

its critical size is reached. Beyond this point, the subsequent addition of chains is

irreversible,

andthepolymer aggregatewillgrow. Heterogeneousnucleation,ontheother

hand,

arises fromadventitious

_impurities,

either_{randomly distributed}throughout thebulk

or_{possibly localized}on a surface.Nucleation atthe surface ofthesample containeris an

example of the later case (2). These sites are one example of residual nuclei, as

mentioned earlier.

Insome crystallization studiesitwasfoundthatnucleation sites appearinsimilar

positions

_during

successive _melting and re-crystallization cycles. This suggests that

chemically different sites_may notbe _{necessary for} heterogeneous nucleation to occur.

Thoseregions ofphysical _{ordering may}persist abovethe _meltingpoint, and take a

_long

time todisperse. Someevidence

₍₅₎

suggested thatchainorientation inthepolymer melt

might indeed affect the rate of the subsequent crystallization process. It is

_likely

that

much ofthe data

_indicating

adependence onthemelt

_history

arise fromthis factor. These

residual_nuclei, defined

_by

Sharpies as

_"tiny

regions with ahigh degreeof order that_may

persistina meltfora

_long

timeand will act as predetermined nucleiforre-crystallization

ofresidual nuclei depends on the

_annealing

_conditions, such as _annealing temperature,

annealingtimeand_annealingmethod.

Thenumber of nuclei generatedinthecourse ofcrystallizationisof

importance,

because it

determines

thefinal size ofthegrowth-units. Thisparameterisnowrecognized

as

_being

correlated with the optical and mechanical properties of semi-crystalline

polymers.

_Therefore,

a_studyofthenucleation processisnecessary.

Considerableevidenceindicates thatheterogeneousnucleation predominatesin

practice. _{The majority}ofhomogeneousnucleation events require a muchhigher degree

of_{supercooling,} (e.g. 50

_C)

insteadof20

C,

whichis _usuallyneededforcrystallization

(2). The higher the nucleation _rate, the smaller the size ofthe growth-units (either

axiallite or spherulite). The nucleation rate will be the sum of the homogeneous

nucleation rate andtheheterogeneousnucleation rate.

2.4

_Melting

Behaviors

of

Crystalline Polymers

Althoughthe_meltingof polymersisafirst-order_transition,unlikethe _{sharp melting}

curves often observed forsmallmolecules, the_meltingcurves of crystalline polymers are

quitebroad. The

_broadening

ofthe_meltingrange canbe attributedto a largevariation of

surface _energyoriginatedfrom crystal size

_{distribution,}

andthecomplex _morphologyof

polymers. The polymer _morphology

_typically

depends on _processing conditions and

molecularproperties, such as_end-group effect, molecularweightand_{polydispersity,}and

the extent of chain regularity.

_Thus,

a detailed investigation ofthe polymer _melting

Moreover, the accurate measurement of the polymer _melting temperature is of

importance to

_calculating

the degreeof

_supercooling

forboth kinetic andthermodynamic

studies.

A broadrange of_{meltingtemperatures}suggeststhatalarge range of crystallite

stabilities, orqualities are involved. The high qualitycrystalsthatare formedwill melt at

a higher temperature than more disorderedcrystals. When a crystal melts, it consumes

energy.

_Therefore,

an endothermic peak is observed on a DSC curve. Tm* is _usually

definedas the _meltingtemperature at whichthe_{melting is} completed. Tm* _{may increase}

with

_{increasing heating}

_rate, and with the crystallization temperature for a specific

polymer sample. In this research, in order to investigate the effect of microwave

annealing and compareit withthermal annealing, the

_heating

rate and thecrystallization

temperature arekept the same in both cases.

_Therefore,

ifthere exist _{different melting}

curves between microwave _annealing andthermal annealing, it canbe deduced that the

differencesare caused

_by

the_annealingconditions.

Tm

isanothercharacteristic_meltingtemperature.Itis definedastheequilibrium

meltingtemperature of chains with finite molecular weight (4). Although

_Tm

cannot be

measured

_directly,

it is animportantparameter. Itcanbe approachedwhen a samplehas

been crystallized _very slowly. When this equilibrium _meltingtemperature is compared

withthose _{melting temperatures,}

_Tm*,

obtained fortypical polymer _systems, important

10

Tm

. themeltingtemperatureofthe perfectcrystal formed

by

infinite molecular

weightpolymers, is also a thermodynamic equilibrium_melting temperature(4). It canbe

seen that

Tm

isatheoretical parameterunderidealized conditions: no surface effects, no

end-group effects. Chains are

_fully

extended andequilibrium conditions are satisfied.

l-16r

Tm*

Tm

'3000

i?4 12?

Tc

Figure

1: Theoretical

plot ofTm* against

_Tc

for indicated

chain

length

of

linear

polyethylene

₍₆₎

The approachofHoffmanandWeeks

₍₆₎

canbeusedtoestimate

_Tm

and

Tm based

[image:19.561.71.486.198.512.2]

11

Hoffman-_{Weeks method is given in Figure 1. A plot}

of Tm* vs.

Tc

is made. The

intersection oftheextrapolatedcurveswiththe straightline

Tm*=Tc

givestheequilibrium

melting temperature Tm.

_Moreover,

an extrapolation to infinitemolecular weight yields

the thermodynamic equilibrium temperature Tm.

_However,

the _{crystallinity level has}

beendemonstratedtobe_{important for obtaining}accurateresult(7).

2.6

Microwave

_Processing

of

Polymers

Theuse of microwaveradiationfor processingpolymers offers a number ofdistinct

benefits over conventional _processing approaches. These include: enhanced

polymerization rates

_(8),

increased glass transition temperatures

_(Tg)

ofthe cured _epoxy

(9),

improvedinterfaciai

_bonding

betweengraphite fibers andtheir matrix, andincreased

mechanical properties ofthe _resulting composites (10). The advantages ofmicrowave

heating

over conventional thermal

_heating

are summarized in Table 1 (11). The

characteristics of microwave

_heating

include: volumetric "inside to

_heating,

direct and quick

_heating,

high _selectivity, and high controllability. In additionto these

thermal _{characteristics, there} are so-called "microwave which include _anything

thateitherisdifferent fromthermal_processingorcan notbepredicted or_easilyexplained

by

the

_different,

temperatureprofile(12). Allofthesecharacteristicshave aroused alotof

research interests in exploiting microwave radiation to either produce new polymeric

materials with desired properties, oruse it as a new alternative technique forpolymer

12

willnot enumerate themhere.However, a more detailed discussionaboutthemechanism

[image:21.561.68.504.190.363.2]

ofmicrowave

_heating

willbehelpful for

_{understanding}

ourexperiment results.

Table 1:

Comparison

ofmicrowave and thermal

_heating

characteristics

(H)

Thermal

_Heating

Microwave

_Heating

Heating

Rate

Slow,

controlled

_by

heat transfer

Fast,

Direct coupling of energy into molecules

Selectivity

No,

Heating

is due to temperature gradient

Yes,

_Heating

is

proportional to material

loss factor and input

power

Heatmovement Outto

_in,

Surface driven

heating

from hottocold

In_{to out,}Volumetric

_heating

with

_boundary

heat loss

Controllability

No,

Heat source can't be readily controlled

Yes,

Microwave energy can be _readily removed

Themajormechanism

₍₁₃₎

of_couplingofmicrowave radiationtopolymersis

through dipole reorientation

_by

the electric field (Figure 2). The relaxation of this

reorientation has a maximum at a

_frequency

of about 10E+7

_Hz,

but there is a high

frequency

tail, which reaches the microwave region (10E+11 Hz).

_However,

the

maximum value ofthis relaxation varies

_during

a _processing cycle, as the temperature

increases. Put in other words, the dielectric loss

_factor,

which is related to microwave

absorption, isafunction ofboththe

_frequency

andthe temperature.

_{Consequently,}

as the

temperature changes, the microwave absorption changes. Anotherconsideration for the

use of microwave

_heating

is thatthe_processing temperature formostpolymers _{may be}

verycloseto the thermal degradationtemperatureofthepolymer.

_Therefore,

_{it is strictly}

tnrougnout the part

_being

processed, and it _{is necessary to}

precisely control the

temperatureofthe partinorder to avoidthermal_run-away.

C|

e.

So

a

ii

I

TTEHFACUL

PCUWZATON

s

ATOUC

BESONAWCE

afcmcMC

RESONANCE

-3 ₁₂ IS

[image:22.561.100.430.160.390.2]

LOG! (Hi)

Figure 2: Typical

variation

in dielectric

constant and

dielectric loss

with

_frequency

for

polymers

₍₁₃₎

Atthemolecular

level,

whenno electricfield ispresent, thedipolemoments are

randomly orientedthroughoutthe medium. Butwhen an electric field is_{applied, there is}

a

_tendency

fordipolemomentsto orientinthedirectionofthefieldto lowertheirenergy.

An _{imposed alternating}electric fieldwillcause oscillations ofdipole _moments, with the

adjacent groups _acting to inhibit the motion of the dipoles. The _{resulting friction}

14

The_efficiencyofthis_{coupling depends} on anumber of

_factors,

_including

thedipole strength, the_mobilityofthe

dipole

andthemass ofthedipole.

_Strong

dipole _couplingto the radiation leads to high _{efficiency for energy} transfer. In the condensed state, the mobility ofthemedium aroundthedipole isalso important. Smallpolarmoleculesobtain

trans_{lational energy in} additionto the_energy fromreorientation(rotationalenergy). This

will

_dramatically

increase the _coupling ofthese molecules to the radiation field as comparedwithasegment ofa largemacromolecule.

_Similarly,

adipole thatis apart of a

pendent_group will couplemore _stronglythana_{group in}themainchain, dueto itsgreater

mobility. To summarize the behaviors for polymers, the melt couples the strongest

followed

_by

the_rubberystate, glassypolymers and crystalline materials.

Amechanismfor explainingthe"microwave onpolymer_{processing has}

been proposed based on a non-equilibrium, non-uniform _{energy distribution between}

neighboring groups. For the solution imidization of a polymeric acid

_(12),

microwave

heating

increasedthe

"temperature"

ofthe_reacting groups

_by

_{approximately}

_50C,

over

the temperature range investigated. The energy couples

_directly

with a reactive polar

group in this system and dissipates through adjacent groups

_by

random collisions.

However,

ifthe _energy absorptionis fasterthan _{energytransport,} possibleat the initial

stages ofreaction, there will be a _{non-uniformity} in the temperature profile. This is consistent with some recent pulsedradiation studies

_(13),

inwhichthepulsed repetition rate was set equal torate of_energytransfer _alongthe chain. Theoretical calculations

_by

Brook et al

₍₁₄₎

predict that a small perturbation to the higher _energy region ofthe energy distributionwas sufficient to account for some "microwave which have

15

proposed forpolymers, butthe

details

are different. Polymer systems are not as

highly

ordered as ceramics systems. In _{this work,} for the _{first time,} we introduce microwave

heating

for annealing a polymer melt, and investigate the "microwave effect '

on the

16

Chapter 3 Experimental

3.1.

_Sampling

of

Poly(ethylene

_oxide)

Inthisstudy,threetypes of poly(ethyleneoxide),

(PEO),

were used. Thesematerials

were all_commerciallyavailablePEOwithnarrowmolecularweightdistributions. Major

informationaboutthePEOsamples is listed in Table 2.

Table

2: Sample

characteristics of

PEO

Molecular Weight Physical Description Manufacturer

2000 g/mol

_Waxy

flakes Fluka

20,000 g/mol

_Waxy

power Fluka

400,000 g/mol White fine power Aldrich

The PEOpowderswerespread on glass slides andwere moltenon ahot stageto

form liquid films. The films were annealed

_by

either TA or

_MA,

and then crystallized

into solid films for further characterization. The bulk samples were prepared in glass

tubes which were

_heated,

annealed and subsequently, cooled. At the same _{time, film}

samples could also be prepared in the tubes, because a liquid filmwas _{automatically}

formedontheupper wall ofthetubes whenthe powdersinthetube melted and shrank.

After annealing and crystallization, the tube was

broken,

and the polymer film on the

glasswallwas analyzed

_by

PLM

directly,

then the filmwas scratched offthe glass wall

[image:25.561.67.507.321.382.2]

17

3.2 Experimental

_Set-up

for

Microwave

_Annealing

Theexperimental_{set-up for annealing}ofthepolymer samplesis showninFigure 3.

The sampletubewas placedin a microwave oven

_{(Multiwave, MA-693M,}

Goldstar Co.

Themicrowave oven wasrun at 100%powerwitha

_frequency

of1.47

_GHz)

surrounded

by

a paraffin oil bath.

_During

microwave _{annealing, the} temperature of the polymer

sample could be controlled

_by

the paraffin oil

_bath,

which did not absorb microwave

radiation. A recirculating isothermal water systemwas used tojacket the oil

_bath,

and

take excess heat out ofthemicrowave oven in orderto

_keep

the sample at a constant

temperature. Afterthe _annealingwas complete, the temperature ofthe sample polymer

melt was

_immediately

measuredwitha digitalplatinum thermometer(Interstate Electric

DT6)

through a hole on

_top

ofthe oven.

_Therefore,

the calibration curve between the

setting temperature ofthe isothermalreservoirand the temperature ofthepolymer melt

couldbeobtained, as shownin Figure 4.

Itwasfoundthata_steadytemperatureofthepolymer melt couldbe achieved after

the microwave power was turned on for a period oftime. Ifthe set temperature was

above 60

C,

10 minuteswere_usuallyneededto reacha_steady temperature. At alower

set _temperature, alongertimewas required.As seenfromFigure

_4,

anincrease inthe set

temperatureledtoasmallertemperaturedifference betweenthepolymer melt andtheset

temperature. At _{higher temperatures,} the heat transfer was more _efficient, and the

18

Oncethemicrowave_annealingwas_{completed, the}polymer sampletubewas

quicklytakenout ofthemicrowave _oven, and placedinawater

_bath,

whichwas set ata

specifiedtemperature.

_By

_{this way, the} crystallization couldbecarried outisothermally.

Afterthe crystallization was

_finished,

the sampleswere characterized_{using differential}

scanning calorimetry

(DSC)

and_{polarizing light microscopy (PLM).}

Forthermal_annealing

_(TA),

polymer sampletubeswereplacedina controlledwater

bath. The temperature ofthe water bath was calibrated to maintain the same sample

temperatureas forMA. Crystallizationwasconducted asdescribedabove.

3.3

_Morphology

Characterization

_by

PLM

A polarizing lightmicroscope

_{(Reichert-micro)}

withaJVC TK-10video camera and

a Metier FP-52 hot stage was used to characterize the _morphology of film samples

(magnification: 200X). The morphology ofthebulk samples couldnotbe examined

_by

PLM because

_they

were too thick for the light to pass through. An attempt to cross

sectionbulksampleswitha microtomledtounclearimages.

3.4 Investigation

of the

_Melting

Behaviors

for

the

Crystalline

PEO

_by

DSC

The differential scanning calorimetry

(DSC)

isathermalanalysistechniquefor

accurate measurements oftransition temperatures and heat capacities. The DSC _curve,

refered to as a _thermogram, is proportional to the _{heat capacity} of a sample. In our

experiments, the_melting curves of allthe samples weremeasured at a

_heating

rate of5

19

different annealing methods, the

difficulty

in

transferring

ahot meltinto theDSC pan

preventedus from

_doing

so.

_Instead,

both filmandbulk samples werecharacterizedafter

theannealedmeltswere crystallized under controlledcoolingconditionsso thatwe could

obtainthe_melting curves for boththefilmand the bulksamples underthe same

heating

conditions.A SeikoInstrumentInc. DSC 220C Modulewas usedhere.

temperaturecontroller

thermometer

polymer

isothermal

reservoir

microwaveoven

[image:28.561.98.462.258.563.2]

ice bath

20

100

40

-? Tp=Ts s_{MA1 Omh.}

50 60 70 80

SetTemp,ofthe ReserviorTs_(oC)

[image:29.561.76.461.121.443.2]

100

Figure 4: The

temperature calibration curve

for

thepolymer melt and

21

Chapter

4

Results

and

Discussions

4.1 Microwave

Effects

on

PEO Crystallization

For_comparison,we wouldliketomaintainthesame_annealingtemperaturefor both

MA and TA samples.

_However,

the _energy can be coupled into the system

_directly,

quickly and_{volumetrically using}microwave annealing. In contrast, thermal annealing

requires alongertime toachieve_{thermal equilibrium,}becausepolymers are_usuallypoor

thermal conductors.

_Thus,

we

_{intentionally}

prolonged the _annealingtime forboth MA

and TA to eliminate potentialtemperature gradientsinside thepolymer_{melt, especially}

for TAsamples

Allthebulk PEOsamples

₍

Mn: 20,000 g/mol)wereannealed at100

C

forvarious

time periods,from 5minutesto2 hours. Afterannealing,theywereallquenched at25

C

inair untilthe crystallization was complete._{DSC melting behavior for} eachbulksample

wascharacterized_using=10_mgsemi-crystalline specimens removedfromthecenter of

thesampletube.

Itwasfoundthat_{DSC melting}curveswere superimposableforalltheMAbulk

samples when the _annealing time was larger than 10 _minutes, while the DSC _melting

curves were also superimposable forall the TA bulk samples when the _annealing time

was larger than 10minutes.

_However,

no DSC thermograms ofthe MA samples were

superimposable onthose oftheTA samples. The factthatan_annealing timelongerthan

had been reached

_by

10 minutes, no matter if MA or TA was used.

_Therefore,

the

difference

_between

_{MA and TA} cannot be explained

_by

the difference oftemperature

gradient

_inside

_{the polymer melt. There} must be a fundamental difference between

microwave

_annealing

and thermal annealing, as evidenced

_by

_{different DSC melting}

behaviors.

[image:31.562.67.500.205.523.2]

(a)

(b)

Figure 5:

_Morphology

of crystalline polymers-PEO crystallized at25

C

in air

after

_(a)

_{thermal annealing,} and subsequent

_(b)

microwave_annealing

(Annealing

temperature: 100

C;

_annealingtime: 10min.; Mn: 20,000 g/mol.)

Theeffect ofthe_annealingmethod onPEOcrystallizationbecamemore evident

when the crystalline_morphology offilm samples was examined _usingPLM. As seen in

subsequently

annealing the same sample with_{microwaves, the fine} texture inthe image

5bwas _{observed. Crystallization}conditionsforbothmicrographswere_exactlythesame.

(a)

(b)

Figure6:

_Morphology

of crystalline polymers-PEO crystallized at25

C

inair

after

_(a)

microwave_annealing,and subsequent

_(b)

thermal_annealing

(Annealing

temperature:

_100C;

_annealing time: 10 min.;Mn: 20,000 _g/mol.)

Whentheorder of_annealingconditions wasreversed, the_{resulting PLM} images

were also reversed. This is shown in Figure 6.

_Therefore,

it could be concluded that

different annealing methods gave rise to different crystalline morphologies. The result

was _{reproducible,} and was independent of the _annealing order. This was a true

[image:32.562.67.497.135.451.2]

24

As discussed inchapter

_2,

aMaltesecross patterninaPLMimageimpliedthatthe

lamellae of the crystalline polymer _aligned, branched and splayed in a spherical

symmetry,which wasobservedinthePLMimages oftheTAsamples.

_However,

thefine

texture in the PLM images ofthe MA samples implied that the structure might be

anisotropic.Anisotropicmechanical_property

₍₁₅₎

had beenreportedforpolyimide

films,

whenmicrowaves were employed to assistthe imidization. This could be explained

_by

the

_inability

for polymer chains to _regularly align to the film plane when _using

microwave radiation.

4.2 Mechanism

of

Microwave Effects

Beforewe couldcomparethe effects of microwave _annealingandthermal_annealing

by investigating

sample_morphologyandits melting

_behavior,

subsequent crystallization

had to be completed for all the samples. It is important to

_keep

inmindthat there are

someotherfactors affectingthePLM andDSCresultsfor PEOcrystalline_polymers, such

as _{annealing temperature, annealingtime,} crystallization_{conditions, sampling} type(bulk

or

_film),

and molecularweight.

_Here,

we have changed one

_factor,

andkeptalltheother

conditionsthe samewhile _studyingthe effects ofMAandTAonPEO crystallization. In

the next section, we will investigate themicrowave effects on PEO crystallization

_by

changing either the crystallization temperature or the molecular weight of PEO.

Moreover,

we will discuss the nature of microwave effects and _specify the conditions

25

4.2.1 Dependenceof microwave effects on crystallizationtemperature

Table3 andTable 4give a comparisonofthe_{melting behavior}ofMAandTA

samples. The precision of these measurements is within 2%. Similar trends were

observed for both film and bulk _samples, as exhibited in these tables. When the

crystallization was carried out at -44

C

(dry

ice

bath),

the extent of _{crystallinity for}

microwave _annealing was much lower than forthermal _annealing, as indicated

_by

the

enthalpychange

_during

the_meltingtransition._{The enthalpy}ofmelting, orAHoftheMA

sampleat-44

C

was about 10% lowerthan thatofthe TAsamplefor both filmandbulk

samples. Whenthe crystallization wasperformedina waterbathathigher_{temperatures,}

the difference ofthe _enthalpychangebetween MA andTAbecomesmuch smaller. This

phenomenonwasobservedin both filmandbulksamples. BasedontheseDSC_results,if

a difference in crystallinity existed for film samples between MA and

_TA,

a similar

differencewouldbe expectedto exist for bulksamples. Therole of substrate surface area

onthe crystallizationfor filmsampleswillbe discussed inmore detail later.

Althoughthe_meltingcurvesfortheMA andTAsamples crystallized at55

C

seemed to be similar, their crystallizationkinetics were different. After _{annealing, the}

polymer melts were

_immediately

transferred into a 55

C

water bath. Since the

temperaturewas_relatively

high,

the crystallizationwas slow, and couldbe observed

_by

eye. Forthe sample 7-18-1

_by

_MA,

countablecrystallites became visible afterthemelt

was cooledfor 10 minutes.

_However,

the sample 7-18-2

_by

TA exhibited atleast triple

the amount ofvisible crystalliteswith the samemass ofthemelt afteritwas placed into

26

thatmicrowave _annealing decreased the nucleation

_density

andretarded the rate ofthe

crystallization process. Although the final extent of_{crystallinity,} and the _meltingpoint

didnot change

_by

_{changing annealingmethod, the}kinetics ofthecrystallization process

of MA and TA were different. This might explain _why at the low crystallization

temperature-44

C,

the _{crystallinity}ofmicrowave annealed samples was lowerthanthat

ofthermal annealed samples. At such alow_temperature, the crystal growthis diffusion

(reptation)

limited.As a_{result, the} extentofcrystallization depended onthe number of

nuclei. Inthepolymer meltannealed

_by

_microwaves,the nuclei

_density

was

low,

because

mostoftheresidualnuclei weredestroyed

_by

microwave-assistedchain rotations.

Table

3: Comparison

of

_{melting behavior for}

film samples

Crystallization Conditions PEOSamples Number

Annealing

Method

Melting

Curves

AH,

mJ/mg Tmp,C

Dry

ice

_(-44C)

7-08-1 7-08-2

MA

TA

175 65.2

189 65.2

Icewater

_(0C)

7-01-1

7-01-3

MA

TA

183 65.1

178 65.1

Water

_(40C)

7-02-1

7-02-2

MA

TA

193 65.7

190 65.7

Water

_(55C)

lhr 7-08-3

7-08^1

MA

TA

203 68.3

200 67.7

(Annealing

temperature: 78

C: Annealing

time: 10min.;Mn: 20,000g/mol;

DSC

_heating

rate:

_5C/min.;

Tmp: meltingpoint atthepeak_position.)

PEOchains possess largedipolemoments atthehydroxylterminatedchain ends.An

applied_alternating electric fieldwill causethe oscillation ofthese dipole _moments, and

[image:35.561.72.503.383.527.2]

27

rotation_energy of_{the system, speeding up} the elimination ofresidual nucleiin themelt.

In contrast, _energy is transferred

_by

less efficient collision for thermal annealing, and

chain conformations do not change as easily. Some residual nuclei will persist after

annealing, and act asheterogeneousnucleiforsubsequent crystallization.

Table

4: Comparison

of

_{melting behavior for bulk}

samples

Crystallization conditions PEO Samples Number

Annealing

Method

Melting

Curves

AH,

mJ/mg Tmp,C

Dry

ice(-44

_C)

7-08-1

7-08-2

MA

TA

172 65.3

190 65.3

Icewater(0

C)

7-01-1

7-01-3

MA

TA

184 64.3

183 65.1

Water(40

_C)

7-15-1

7-15-2

MA

TA

189 65.8

194 66.4

Water(55

_C),

15hr 7-18-1 (a) 7-18-2w MA TA 217 68.3 216 68.5

(Annealing

temperature: 78

_C;

_Annealing

time:

_lOmin.;

Mn: 20,000g/mol;

DSC

_heating

rate:

_5C/min.;

Tmp: meltingpoint atthepeak_position.)

(a)

Countablecrystallitesbecamevisibleatthe 10thminutes afterthemelt was

placedin 55

C

waterbath.

(b)

Atleasttriple theamountofvisiblecrystallitespresentinthesame amount of

melt as comparedto(a) atthe4thminutesafterthemelt wasplacedin

55

C

waterbath.

As discussed

before,

homogeneousnucleationtakeslongertimeand requires lower

temperatures than heterogeneous nucleation. Homogeneous nucleation appears to

predominate in PEO crystallization

_following

microwave annealing. This gives an

[image:36.561.72.502.236.381.2]

7-08-2. For the

_latter,

_{heterogeneous} nucleation _{may be} the dominant mechanism. The

comparison isgivenin Table 4.

[image:37.562.68.501.148.424.2]

(a)

(b)

Figure 7:

_Morphology

of

PEO

crystallized at-44

C

on

dry

ice

after

_(a)

microwave _annealing,

_(b)

thermal

_annealing

(Annealing

temperature:

_78C;

_Annealing

time: 10rnin.;Mn: 20,000 g'mol)

Themorphologies ofthe filmsamples 7-08-1 and7-08-2wereexamined

_by

PLM.

As shown in Figure

_7a,

_only one crystallite was observed for the MA _sample, which

29

lamellae

werejust

locally

aligned, not_radially symmetric aroundthe center. In contrast,

as

indicated

in Figure

_7b,

more crystallites existed for the TA sample,

implying

more

nuclei. The Maltesecross patternindicatea_radiallysymmetric lamellaestructure.

(a)

(b)

Figure

8:

_Morphology

of

PEO

crystallized

for

1 hr

at

55C

after

_(a)

microwave annealing,

_(b)

thermal

_annealing

[image:38.562.70.499.196.495.2]

30

Figure 8representedthemorphologies forthefilmsamplesfor MA

_(7-08-3)

andTA

(7-08^1)

crystallized at 55

C

inthewaterbathforone

_hour,

thenkeptat25

C

inair.For

themicrowave-annealed _sample, as shownin Figure

_8a,

the lamellae arewell

defined,

smooth and of a large size. _{There may} be amorphous structures or less crystallized

domains in the dark regions. Since the crystallization had not been finished for this

samplewithin one

_hour,

itcontinuedtoformcrystalswith poor_quality

_during

the _cooling

at 25

C.

_However,

the image ofthe

_thermally

annealed sample in Figure 8b showed

more uniform crystallites with a rough contoured texture. This couldbe attributed to a

high crystallization rate due to a high residual nuclei density. The dark

"river"

in the

imageis a crackinthefilm.

Ifthe sampleswerekeptat55

C

fora

_long

enoughtime(e.g. 15

hours),

itwas

observedthatthemorphologies ofMAandTAsamplesbecamesimilar. Thisisrevealed

in Figure 9.

_During

the longer time period, polymer chains had an _opportunity to

reorganize into a crystalline state close to thermal equilibrium. Since at such a high

temperature, the chain reptation rate and

_impurity

diffusion rate were both

_high,

_any

smallunstable orpoor _qualitycrystalline entitiesmight

_dissipate,

andre-crystallize into

biggercrystal structures.

_{Consequently,}

_long

crystallizationtimes tendedto eliminatethe

morphology difference between TAandMA.

Next,

we_closelyexaminetheDSC thermogramsofMAandTAsamples aslisted in

Table3 and4. Thethermograms for filmsamples areshownin Figure

_lOa-d,

whilethose

comparison of thermograms ofMA and TA

_film

_samples reveals that the MA _melting

curves arenarrowerthanthe_{corresponding}TAcurve. _{The crystallinity}fortheMA film

(a)

(b)

Figure

9:

_Morphology

of

PEO

crystallized

for 15 hr

at

55

_C,

after

_(a)

microwave _annealing;

_(b)

thermal annealing.

(Annealing

temperature:

_78C;

_Annealing

time: 10_min.; Mn:20,000 _g/mol)

sample is lower than for the TA film one, and the morphologies for these two film

samples were different as indicated in Figure 7. For bulk _samples, Figure 11a shows

[image:40.562.71.501.148.425.2]

32

may be differentmorphologiesforthe twoMAandTAbulksamples,justaswasthecase

for the two film samples. We

hypothesize

that surface effects do not _play an important

role underthelowtemperaturecrystallization conditions.

Usually,

crystallization rate dependsonboth diffusionrate(therate ofpolymer chain

reptation) and nucleation rate. The diffusion rate increases with the crystallization

temperature, while the nucleation rate decreases with the temperature.

_Therefore,

the

maximum PEO crystallization rate is achieved at about 40C (degree ofsupercooling:

25C). At a low _temperature, such as _-44C, the diffusion rate is very slow, and the

nucleation rate ishigh. Asa_result,no extranucleation centerfromthesubstrateisneeded

forthecrystallization. Thatis_whysurface effects werenotpronouncedforthesesamples.

Atahighercrystallization_temperature, suchas 0

C

or40

_C,

the glass substrateof

the filmsamples couldbe used as heterogeneousnucleation sites for the _{crystallization,}

leading

toathe fastcrystallization rate. Thenucleationrate of polymer meltisslow. This

could explain_whytherewasno

_big

_{difference in morphology between MA}andTAfilm

samples at these temperatures (Figurel2 and 13).

_Similarly,

theirDSCthermograms also

tended tobe close to each_other, as depicted in FigurelOb and 10c.

_However,

Figurellb

and lieshoweddifferentthermogramsforthebulkcounterparts.

_Here,

a surface effectis

not important. The difference between TA and MAbulk samples couldbe attributedto

the fact thata heterogeneousnucleation mechanism dominates for the TA bulk _sample,

while a homogeneous nucleation mechanism took control for the MA one.

_Again,

O.OOE+00 -S00E+03

5

-100E+04 3 -1.50E+04 (/> O -200E+04 -250E+04 -aOOE+04

20 40 60 80

Temperature,

C

MA,Tc=-44C

TA,Tc=-44C

100 120

Figure

10a: DSC

thermogramsof

film

samples after

different annealing

treatments

(crystallized

at-44

C)

0.00E+00 -5.00E+03

^

-1.00E+04 3 O -1.50E+04 CO -2.00E+04 -2.50E+04 -3.00E+04 20

"~~^x

/

?k < '

MA Tr_{D C}1

\

!

!...

TA,Tc=0C

_J

\ ! i

\l

V 1

40 60

Temperafcjre,C

[image:42.561.92.419.106.279.2]

80 100

Figure 10b: DSC

thermograms of

film

samples after

different

_annealing

[image:42.561.99.413.377.555.2]

O.OOE+00 -5.00E-K33 5 -1.00E+04 3 O -1.50E-HM (0 Q -2.00E+04 -2.50E-K)4 -3.00E+04

20 30 40 50 60 70

Temperature,

C

[image:43.561.81.402.79.256.2]

80 90 100

Figure 10c. DSC thermograms

of

film

samples after

_{different annealing}

treatments

(crystallized

at

40

C)

O.OOE+OO

-5.00E+03

^

-1.00E+04 -1.50E+04 CO Q -2.00E+04 -2.50E+04 -3.00E+04

20 40 60 80

Temperature,

C

MA,Tc=_{55 C}

TA,Tc=_55C

100

Figure lOd: DSC

thermograms of

film

samples after

different

_annealing

treatments

(crystallized

at

55

C)

(Conditions for

_FigurelOa-d,

_Annealing

temperature:

_78C;

_Annealing

time: 10min.;

[image:43.561.67.417.356.523.2]

35 O.OOE+00 -5.00E+03

%

-1.00E+04 O -1.50E+04 Q -2.00E+04 -2.50E+04 -3.00E+04 / MA,Tc=-44C

TA. Tc=_-44C

0 20 40 60 80

Temperiurei

C

100 120

Figure

11a: DSC

thermograms of

bulk

samples

for different annealing

treatments

(crystallized

at-44

C)

0.00E+00

T

^

-5.00E+03 ^ -1.00E+04

O

W

-1.50E+04

Q

-2.00E+04 -2.50E+04

~"X

ff~

!

\

\i

1

fs

MA Tr

-0C

TA,Tc-0C

\s p,

V

!

i i i i i

20

40

60

80

Temperature,

C

100

120

Figure

lib:

DSC

thermograms of

bulk

samples

for different

_annealing

[image:44.561.74.432.88.281.2] [image:44.561.85.421.399.592.2]

3 O.OOE+00 -5.00E+03 -1.00E+04

O

-1.50E+04

Q

-2.00E+04 -2.50E+04 -3.00E+04 20

\

UL

M 40 60

Temperature,

C

80

MA

Tc=_40C

TATc=40C

[image:45.561.80.451.87.283.2]

100

Figure lie: DSC

thermograms of

bulk

samples

for different annealing

treatments

(crystallized

at

40

C)

0.00E+00 -5.00E+03

1

-1.00E+04

<i

-1.50E+04

Q

-2.00E+04 -2.50E+04 -3.00E+04

20 40 60 80

Temperature,

C

MA, Tc= 55

TA, Tc = 55

100 120

Figure lid:

DSC

thermograms of

bulk

samples

for different

_annealing

treatments

(crystallized

at

55

C

for

15

_hr)

(Conditions for Figurel la-d:

_Annealing

temperature:

_78C;

_Annealing

time: 10min.;

[image:45.561.86.437.385.577.2]

37

temperature)

and less time was left for crystal growth. The resulting crystals _{may have}

smaller size and larger surface area. Thiscould explain_whythe _{DSC melting} curves for

theMA samples showed_{lower melting}temperatures.

Atthecrystallization temperaturesof0 C and40

_C,

the microwave effectwas

observed forthe bulk samples evenin the DSC curves (Figure 1 lb and 1lc), which, as

mentioned

_before,

a less sensitive technique thanPLM to examine surfacemorphology.

Very

possibly, a

_big

difference in morphology also exists between MA and TA bulk

samples,althoughwecouldnot

_directly

characterizeitwith asuitable technique.

[image:46.562.71.508.289.621.2]

(a)

(b)

Figure 12:

_Morphology

of

PEO

crystallized at

0 C

(a)

microwave

_annealing;

(b)

thermal

_annealing

38

Atthehighestcrystallizationtemperatureof55

C,

thecrystallizationrateisdictated

by

nucleation. Inthis case,bothheterogeneousand

homogeneous

nucleation wouldtakea

long

time. Ithadbeen observedthatittook 10 minutes forcrystallitesto emergeforthe

MA _samples, while 4 minutes was needed for the TA samples.

_Possibly,

the surface

effect no longer played an important role in the crystallization at this _{extremely high}

temperature. Surface nuclei _may sustain even after formation.

_Therefore,

the different

morphology as indicated inFigure 8 fortheMA and TA sampleswas_primarilycaused

by

their _{different kinetics. When extending} the crystallization timefrom 1 hour to 15

hours at 55

_C,

the difference in DSC thermograms between MAand TAbulk samples

was _reduced, as shown in Figure lid. This couldbe attributed to

_long

time

melting-recrystallization or reorganizationofthe crystallites at thishightemperature. Thisresult

was consistentwiththe_morphologyexaminationforthefilmsamplesgivenin Figure 9.

ComparisonsofDSC thermogramsbetweenthebulkandfilmsamplesattwo

crystallizationtemperaturesare given in Figure 14a-band_{Figure 15a-b using TA}or

_MA,

respectively. Whenthecrystallizationtemperaturewas -44

C,

as shownin Figure

_14a-b,

thefilmthermogramwas closeto thebulkone, especiallyunderthe condition ofthermal

annealing (Figure 14a). As discussed earlier, the surface effect should not _play an

importantroleinthe crystallizationatthislowtemperature.

_Therefore,

it isreasonableto

assumethat thestructure ofthebulksamplewas similartothatofthe _{corresponding}film

sample. Ifthis is

_true,

there shouldbe differentmorphologiesbetween TAandMAbulk

samples, considering the fact that the morphologies between the TA and MA films

[image:48.562.73.502.65.364.2]

39

Figure 13:

_Morphology

of

PEO

crystallized at

_40C

(a)

microwave _annealing;

_(b)

thermal

_annealing

(annealing

temperature:

_78C;

_annealing time: 10min.; Mn:20,000g/mol)

Figure 15a-bshowsthatDSC thermogramsoffilmsamples are quitedifferentfrom

thoseofbulkones at a crystallizationtemperatureof0

C.

_Therefore,

the_morphologyof

film samples cannot be representative ofbulk ones. The surface effect for the film

samples must be taken into consideration at this _relatively high temperature. It was

interesting

to notice that _{different melting} behavior betweenthe film andbulk samples

occurred at the lowertemperaturepart ofthe_melting curves for

_MA,

and at the higher

40 O.OOE+00 -&OOE+03

^

-1.0CE+04 3 O -1.50E+O4 CO Q -20CE+04 -25CE+04 -aOCE+04 20 t

~x

r

\

!

TA(firn),Tc=-44C

---TA(rjulk)Tc=44C

^ i 1

\

i

J :

\i

40 60 80

Temperatire,

C

[image:49.561.93.432.104.292.2]

100 120

Figure 14a: DSC

thermogramsofthe

bulk

sample andthe

film

_sample,

crystallized at -44

C

(after

thermal

_annealing

treatments)

0.00E+00 -500E+03

^

-1.00E+04 3 6" -1.50E+04 CO Q -200E+04 -250E+04 -SOOE+04 20

u

C

\

: _t

i-TT

Y:

-MA(bulk),Tc=44C

-MA(iIm)Tc=_44C

40 60 80

Temperature,

C

100 120

Figure 14b:

DSC

thermograms ofthe

bulk

sample and the

film

_sample,

crystallized at -44

C

(after

microwave

_annealing

treatments)

(Conditions for_{Figure 12a-b: annealing}temperature:

_78C;

_annealingtime: 10min.;Mn:

[image:49.561.84.427.388.570.2]

41 O.OOE+00 -5.00E+03 = _-1.00E+04 CO Q -1.50E+04 -2.00E+04 -2.50E+04

~~"X

ff

\

1/

TA HVrt Tr- _Hf" TAbiik,Tc=OC

W

V

y

30 40 50 60 70 80

Temperature,

C

[image:50.561.81.434.90.280.2]

90 100

Figure 15a: DSC

thermograms ofthe

bulk

sample andthe

film

_sample,

crystallized at

0

C

(after

thermal

_annealing

_treatments)

0.00E+00 -5.00E+03

$

-1.00E+04 3 O -1.50E+04 CO Q -2.00E+04 -2.50E+04 -3.00E+04

-T

-C

XT

it

\i

MAfilm,Tc=_0C

MAbulKTc=0C

20 40 60 80

Temperature,

C

100 120

Figure 15b: DSC

thermograms ofthe

bulk

sample and the

_{film sample,}

crystallized at

0

C

(after

microwave

_annealing

_treatments)

(Conditions for Figurel5a-b: annealingtemperature:

_78C;

_annealingtime: 10_min.; Mn:

[image:50.561.76.427.374.560.2]