TX69434_02

2021-08-16 来源：乌哈旅游

CHAPTER2

InvestigationandSynthesisofLiquidCrystallinePolymersbyThinFilmPolymerization

SI-XUECHENGTAI-SHUNGCHUNG

1.INTRODUCTION

thediscoveryofliquidcrystallinephenomenonforlowmolecularweightliquidcrystals(LMWLCs)morethan100yearsago,anisotropicorderingbehaviorsofliquidcrystals(LCs)havebeenofconsiderableinteresttoacademe[1–8].Inthe1950s,FlorypostulatedthelatticemodelforvariousproblemsinLCsystemsandtheoreticallypredictedtheliquidcrystallinityforcertainpolymers[1–3].AspredictedbytheFlorytheory,DuPontscientistssynthesizedlyotropicLCPsmadeofrigidwhollyaromaticpolyamide.Later,Amoco,Eastman-Kodak,andCelanesecommercializedaseriesofthermotropicmain-chainLCPs[2].ThermotropicLCPshaveauniquecombinationofprop-ertiesfrombothliquidcrystallineandconventionalthermoplasticstates,suchasmeltprocessibility,highmechanicalproperties,lowmoisturetake-up,andexcellentthermalandchemicalresistance.Aromaticmain-chainLCPsarethemostimportantclassofthermotropicLCPsdevelopedforstructuralapplications[2,4–7].Becausetheyhavewideapplicationsinhighvalue-addedelectronicsandcomposites,bothacademiaandindustryhavecarriedoutcomprehensiveresearchanddevelopment.

Tounderstandtherelationshipsamongthemonomerstructures,synthesisconditionsandendusedpropertiesofLCPsareamongthemostimportantresearchareas.MostmonomersforLCPsynthesisdonothaveliquidcrys-tallinity[1–7].Duringthepolymerization,LCphaseforms,andtheLCtex-tureevolveswiththeprogressofthepolymerization,furtherannealingorcuringreaction.StudyingvariousdefectsinLCtexturesandthemorphol-ogyofLCPsbypolarizingmicroscopeisaconvenientwaytoinvestigatethe

INCE

structuralandphysicalpropertiesofthesematerials.Afewresearchgroupsareworkinginthisﬁeld.Geilandhisco-workerswerethepioneerstoem-ploythethinﬁlmpolymerizationtechniquetoinvestigatethemicrostruc-tureofLCPsduringthesynthesesofaseriesofaromaticpolymers,includ-ingpoly(p-oxybenzoate)(POBA)[9,10],poly(2,6-oxynaphthoate)(PONA)[11],poly(meta-oxybenzoate/2,6-oxynaphthoate)(P(mOBA/ONA))[12],andpoly(p-oxybenzoate/2,6-oxynaphthoate)P(OBA/ONA)[13].Intheirexperi-ments,athinlayerofmonomerswassandwichedbetweentwoglassslideswrappedinaluminumfoil,andthethinﬁlmpolymerizationwasconductedonahotstage.Themorphologyandcrystalstructureofthepolymerswerestudiedbyopticalmicroscope,TEM,andelectrondiffraction.Therewasamelting-polymerization-LCdomainformation-crystallizationprocessduringtheprepa-rationofpolymers[9–13].Afterpolymerization,thepolymerswithdifferentcrystalstructureswereobtained.POBAwascomposedoflamellarsinglecrys-˚thickness.SinglecrystalPONAwasobtainedandwasidentiﬁedtalswith100A

tohaveatleastthreedifferentmodesofchainpacking.P(mOBA/ONA)hadasimilarsinglecrystalstructuretothatofPOBA,butwithadoublingofbaxisdimensioninsomecrystals.ThinﬁlmpolymerizationofP(OBA/ONA)alsodevelopedlamellarsinglecrystals.However,contrarytohomopolymers,thelamellartextureofcopolymerswasnotsoobvious.Yee[14]andOber[15]studiedtheLCtextureevolutionduringcuringofliquidcrystallineepoxy.TheyfounddifferentcuringagentsledtovariousLCtextures.Chung’sgroup[16,17]usedthethinﬁlmpolymerizationtechniquetoinsituinvestigatethewholepro-cessofLCphaseformationandevolutionduringthepolymerizationofLCPs.Intheexperiments,athinlayerofmonomerswassandwichedbetweentwoglassslideswitharingspacertoreleasetheaceticacidgeneratedduringthereaction.Thereactionwascarriedoutonaheatingstageofapolarizingmi-croscope.Throughinsituobservationofthemicroscope,thewholeprocessofLCtexturegeneration,evolution,andannihilationofdifferentkindsofdefectsduringthepolymerizationreactionwerestudied.

AlthoughafairamountofknowledgeofLCPshasbeenobtained,manydebatableissuesstillremain,suchas

rWhataretheeffectsofreactionconditionsontheformationofliquid

crystallinityandtheend-usepropertiesofLCPs?andthepropertiesofcorrespondingpolymers?liquidcrystallinity?

rWhataretherelationshipsbetweenthestructuresofmonomericunitsrWhatarethecriticalcompositionsanddrivingmechanismstoyieldthe

ThethinﬁlmpolymerizationtechniqueprovidesapowerfultooltoanswerthesequestionsbyexperimentalapproachbecausethepolymerizationreactionsforLCPscanbeinsituinvestigatedontheheatingstageofthepolarizingmicroscope.Inaddition,thinﬁlmpolymerizationconsumesanextremelysmall

amountofmonomers,andthereactiontemperaturecanbeaccuratelycontrolled.The success of developing this technology yielded a quick and convenientmethod for scientists to solve the important issues in LCP area, such as tooptimize the synthesis conditions for LCPs, to investigate formation of liquidcrystallinity, to synthesize new materials, to evaluate new catalysts, and so on.

2. EXPERIMENTAL DETAIL OF THIN FILM POLYMERIZATIONMonomer mixture with a certain mole ratio was placed on a glass slide. Adrop of acetone was deposited on the glass slide to dissolve monomers. Afterevaporation of the solvent, a thin layer of reactant mixture was formed andattached to the glass slide and then sandwiched between two glass slides witha ring spacer. The monomers were attached on the top slide. The ring spacerwas made of stainless steel with a thickness of 0.5 mm. The whole package wasplaced on a heating stage (Linkam THMS-600) of a microscope and was heatedto a proposed temperature. The sample was held at that speciﬁc temperatureduring the whole reaction process. When the heating stage reached the proposedtemperature, the reaction time began to be recorded. The temperature of the topslide was calibrated by testing the melting points of the pure monomers as wellas by measuring with a thermocouple. The temperature difference between theheating stage set by the programmer and the top slide was 20 ± 2◦C in theexperimental temperature range. The polymerization reaction was carried outon the top slide, and all the temperatures mentioned refer to the temperaturesof the top slide. The reaction process was observed in situ by a polarizinglight microscope (Olympus BX50) with crossed polarizers between which ared plate having the retardation of 530 nm was inserted or not inserted. Theopticalimageswererecordedbyasoftwareprogram(Image-ProPlus3.0)aswellasadigitalvideocassetterecorder(SONYDHR-1000NP).Thedataofthemicrographswereanalyzedbythesoftwareprogram(Image-ProPlus3.0).Thesample package for thin ﬁlm polymerization is shown in Figure 2.1 [16,17].

3.MORPHOLOGICALCHANGESDURINGTHINFILM

POLYMERIZATIONOFLIQUIDCRYSTALLINEPOLYMERSMostmonomersforLCPsynthesisdonothaveliquidcrystallinity.Duringthepolymerization,LCphaseseparatesfromtheisotropicreactantmeltwhenthedegreeofpolymerizationreachesacertainvalue,andthemeltrapidlybe-comesturbidbecausetheorientationﬂuctuationsofthegrowingmesogenicdomainsareintherangeofthewavelengthofvisiblelight.Furtherremovalofvolatiles(forexample,aceticacid)fromthemeltisthekeytoincreasethemolecularweight.Theas-synthesizedLCtextureandmorphologynotonly

Polarizing microscopeGlass slidesReaction systemRing spacerHeating stageFigure 2.1 The sample package for thin ﬁlm polymerization.

determine processibility for subsequent process, but also affect the properties

of ﬁnal products. Because melt polymerization progresses from a homogeneousphase to a heterogeneous LC phase, different optical textures associated withstring-like and point-like defects can be observed. Some defects remain in theLCPs solidiﬁed from the melt polymerization, and the others disappear throughannihilation during the reaction. The density and types of defects will stronglyaffect the rheological behavior, mechanical properties, and optical propertiesof polymeric materials. So, it is essential to know exactly what occurs duringthe polymerization of LCPs.For different polymerization systems of LCPs with various monomer com-positions and at different temperatures, there are some differences in their mor-phological changes. However, the appearance of the LC phase and annihilationof disclinations can be observed for all systems. The reaction system with themonomer composition of 73/27 (mole ratio) ABA/ANA at a reaction tempera-ture of 250◦C was studied in detail as a typical example. The whole polymeriza-tion reaction system starts from mixed crystals, melts to a homogeneous phase,and then changes into a heterogeneous system with the following sequence ofmorphologicalchanges:generationofLCphase,annihilationofdisclinations,andformationofbandedtexture[17].

3.1.GENERATIONOFTHELIQUIDCRYSTALPHASE

Figure 2.2 illustrates a set of micrographs showing the typical LC phaseseparationfromahomogeneousphaseandtimeevolutionofLCtexturein

the early stage of thin ﬁlm polymerization. Figure 2.2(a) shows the crystals ofthe monomers. During heating, the monomers melt, and the whole view areabecomes an isotropic melt phase as shown in Figure 2.2(b). In the early stageof polycondensation reaction, oligomers form in the molten monomer phase.Their molecular weight and chain length increase with reaction time. When thechain length of the oligomers reaches a certain value, they form the anisotropicphase (LC phase) and separate from the isotropic melt. Figure 2.2(c) shows thereaction-inducedphaseseparationprocessduringthepolymerization.Thedark

Figure2.2Micrographsshowingmorphologicalchangesintheearlystageofpolymerizationreactionof73/27ABA/ANA.Allthemicrographsareobtainedfromthesameareaofthesamesample.Reactiontemperatureis250◦C.

area in the micrograph is the isotropic phase, while the bright area represents theanisotropic phase. The ﬁrst sign of forming the anisotropic phase is that manybright LC domains instantaneously appear in the view range. Because of thepolydispersity of the chain length, oligomers are partitioned within the isotropicand the anisotropic phases according to the chain length [18]. A fraction ofrelatively longer chain length forms anisotropic domains, while others remainin the isotropic phase.After the appearance of the anisotropic phase, the size of LC domains quicklyincreases, and, correspondingly, the number of domains decreases because ofdomain growth and coalescence between adjacent LC domains. Figure 2.2(g–l)illustrates a detailed coalescence process. According to the previous study,we know that black brushes originating from the points are regions wherethe director is either parallel or perpendicular to the plane of polarizationof incident light; therefore, the incident light is extinguished by the crossedpolarizers. When rotating the crossed polarizers, the position of the pointsremains unchanged but the brushes rotate continuously, showing that the ori-entation of the director changes continuously about the disclinations. If thesense of rotation is the same as that of polarizers, the disclination is a pos-itive one. On the contrary, if the sense of rotation is opposite to that of po-larizers, the disclination is a negative one [1]. The strength, S, of a disclina-tion is determined by the number, N, of the dark brushes around the singledisclination: |S |=N /4. In the thin ﬁlm polymerization experiments, all theLC domains formed in the isotropic melt have the disclination strength of+1. In Figure 2.2(g), the two domains with strength S =+1 are indicated bytwo bottom arrows. When coalescence happens, a negative disclination withstrength S =−1, as indicated by the top arrow, forms at the contact pointof these two domains as soon as they contact each other. The process fol-lowed is the annihilation of the two defects with opposite signs as shownin Figure 2.2(h–l). After the formation of the disclination of S =−1 dur-ingcoalescence,thisdisclination(indicatedbythetoparrow)andoneoftheadjacentdisclinationsofS=+1(indicatedbybottomarrow)immedi-atelymovetowardeachotherandthendisappeartogetherafter0.8softhecoalescence.Thus,alargedomainwithonlyonedisclinationofS=+1isformed.

ThegrowthoftheLCphaseoccurswhenreactiontimeisintherangefrom202sto392sforthissample.Asaresult,thetotalviewareabecomesananisotropicLCphase,andthesumofthestrengthofalldisclinationsinthesampletendstobezero.Thetypicalschlierentextureisformedatthismoment[17].3.2.ANNIHILATIONOFDISCLINATIONS

Asweknow,thedynamicpropertiesofLCParequitedifferentfromthatofLC,butthestaticpropertiesofLCParesimilartothatofLC[1].Inthe

continuum theory of LC, the distortion energy density is given by [19]

Fd = 1/2K1(div n)2 + 1/2K2(n · curl n)2 + 1/2K3(n × curl n)2 , (1)where n is the variable director and K1 , K2 , and K3 are elastic constants forsplay, twist, and bend, respectively. According to this formula, the larger theelastic constants are, the higher the distortion energy is. Normally, the elasticconstants decrease with increasing temperature and increase with increasinglength of molecules. According to Odijk’s deduction, the splay, twist, and bendelastic constants for the polymer chains that are not completely rigid can beexpressed as follows [3]:K1 ≈ 3K2(If the chain length, L, is less than the persistence length, q .) (2)K1 ∝ L (If the chain length, L, exceeds the persistence length, q .) (3)K2 ≈␾1/3(q /d)1/3(kT /d) (4)K3 ≈␾(q /d)(kT /d), (5)where the term (kT /d) redimensionalizes the constant to give the units of force,␾= 1 for thermotropic polymers, and q /d is the persistence ratio. Thus, theelastic constants increase with increasing persistence length. If the molecularlength exceeds the persistence length, the values of K2 and K3 will tend to be

increase monotonically with L.constant, while K1 will

Because the defects are caused by the deformation of molecular chains,they involve high distortion energy in the case of rigid or semi-rigid polymers.Disclinations with opposite signs tend to attract each other in order to releasethe energy [20–26]. This interaction leads to the annihilation and the decrease inthe number of defects. In the thin ﬁlm polymerization systems, the annihilationbetween two opposite sign disclinations and annihilation of the loops are thetwo methods to release the excess deformation energy.3.2.1. Annihilation Between Two Opposite Disclinations

During the thin ﬁlm polymerization reaction, the schlieren texture evolvesto inversion walls, and, at the same time, annihilation occurs between disclina-tions. The micrographs in Figure 2.3 are taken from the same area of the samesample when the range of reaction time is from 450 s to 479 s. They show theannihilation process of a pair of disclinations with opposite strength, S =+1and −1. According to Figure 2.3, we can see the distance between the pair ofdisclinationsgraduallydecreases.Atlast,twodisclinationsdisappeartogether.Duringthisprocess,themoleculeswithinthearearearrangetheirorientationandyieldalocallyperfectorientationaftertheannihilation.

Figure 2.3 Micrographs showing the annihilation process of a pair of disclinations with oppo-site signs. All the micrographs are obtained from the same area of the same sample. Monomer

◦composition of the reaction system is 73/27 ABA/ANA. Reaction temperature is 250C.

Figure 2.4 shows that the distance between the two disclinations decreases in

anapproximatelylinearmannerwiththereactiontimebecausetheslopeofthestraightlineinlogarithmicscaleis1.(Heretoisthetimethattwodisclinationsjoinedanddisappeared.)Asfrequentlyreported,forthenon-reactingsystems,theannihilationkineticsfollowapower-lawrelationship[20–23].BasedontheexperimentsofthinﬁlmpolymerizationofaromaticthermotropicLCPs,theannihilationkineticsforreactingsystemsalsofollowthepower-lawrelation-ship[17].Althoughtheviscosityincreaseswiththeprogressofpolymerizationreactionbecauseoftheincreaseinmolecularweight,itseffectonannihilation

1000D (µm)10010110100to-t (s)Figure2.4Timedependenceofthedistance,D,betweenapairofdisclinations.tisreactiontime,andtoisthetimethattwodisclinationsjoinedanddisappeared.Monomercompositionofthereactionsystemis73/27ABA/ANA.Reactiontemperatureis250◦C.

1000800600D (µm)

4002000400 450 500 550 600 650 700t (s)a bFigure 2.5 Annihilation of two pairs of disclinations during different reaction time intervals:

(a) 450 s–479 s and (b) 540 s–660 s. Monomer composition of the reaction system is 73/27

◦ABA/ANA. Reaction temperature is 250C.

cannot be observed. This is due to the fact that the change in viscosity is not

obvious because the annihilation happens within a relatively short time interval.At the beginning of the polycondensation reaction, the annihilation occursvery quickly, indicating the molecular weight at this stage is relatively low, and,thus, viscosity of the system is not very high. With an increase in the reactiontime, the annihilation rate slows down because the increasing viscosity restrictsthe motion of disclinations for further annihilation. When the energy neededfor the reorganization of molecular orientation is higher than that released bythe annihilation of defects, the annihilation process is completely retarded. Theannihilation between two pairs of defects during different reaction time intervalsin the same sample is compared in Figure 2.5. In this ﬁgure, the annihilationrate (v= dD/dt) for the same pair of defects remains approximately the sameduring the whole annihilation process. For the annihilation that occurred duringthetimeintervalof450sto479s,asalreadyshowninFigure2.4,theannihilationrateisabout31.2␮m/s.Whileforanotherannihilationthatoccurredduringthetimeintervalof540sto660s,theannihilationrateismuchslower,about2.8␮m/s.Inourreactionsystemat250◦C,nofurthermovementofdefectscouldbeobservedafterreactiontimereaches1000s.

However,annihilationbetweenthedefectsisacomplicatedprocessandhasnotbeenfullyunderstoodyet.Althoughannihilationoccursbetweentwopar-ticulardisclinations,infact,theannihilationprocessisaffectedbymanyfactors.Forexample,otherdefectssurroundingtheparticularpairofdefectsmoreorlessaffecttheannihilation.Inapolymerizationsystem,amorecomplicatedfactorisaddedtotheannihilationprocessduetotheincreaseinelasticconstantbecauseoftheincreaseinmolecularweight.However,itisauniversaltrendthattheanni-hilationratesfordifferentpairsofdisclinationsdecreasewithreactiontime[17].

Figure 2.6 Micrographs showing the shrinkage process of loops. All the micrographs are obtainedfrom the same area of the same sample. Monomer composition of the reaction system is 73/27ABA/ANA. Reaction temperature is 250◦C.

3.2.2. Shrinkage of Loop

Figure 2.6 shows the typical annihilation of loops for the polymerization at250◦C. The images were taken at reaction time ranging from 490 s to 518 s. Withan increase in the reaction time, the loops gradually shrink, change their shapesinto circular ones, and ﬁnally disappear. Time dependence of the particular loop(indicated by the arrow) area, A, and perimeter, P, is shown in Figure 2.7 (toisthetimethattheloopdisappeared).Itcanbefoundthatthedecreaseinareaandperimeteroftheloopfollowsapower-lawrelationshipwithinthemiddlestageoftheshrinkage.

1000000P10000010000A (µm2)

ASlope=1.4P(µm)

1000Slope=0.7100100.11101001000to-t (s)Figure2.7Timedependenceofarea,A,andperimeter,P,oftheloop.tisreactiontime,andtoisthetimethattheloopdisappeared.Monomercompositionofreactionsystemis73/27ABA/ANA.Reactiontemperatureis250◦C.

In the reaction systems, for the pairs of disclinations with opposite signs,in some cases, they annihilate and then completely disappear; while in othercases, the distance between two opposite disclinations decreases, but defectsstill exist in the system. For the loops, in some cases, they shrink, annihilate,and then disappear; while in other cases, they shrink into a small area or a spotand stay in the system. At the beginning stage of the polymerization, the densityof defects in the reaction system is considerably high. After annihilation, thedensity dramatically decreases [17].3.3. FORMATION OF BANDED TEXTURE

LCPs exhibit a variety of textures of different length scales originated fromﬂow and deformation. A frequently observed texture called “banded texture”or “stripe texture” often emerges in the sample where relaxation or shrinkageof molecular chains occurs. The conditions inducing this texture include beingsubjected to shear and/or elongation ﬂow [27], evaporating the solvent from alyotropic LC [28], and quenching a thermotropic LC from high temperatures[29]. As a consequence of stress relaxation after shear or volume deﬁciencyinduced by solvent evaporating or quenching, banded texture appears whilelocal order does not decrease signiﬁcantly.In the thin ﬁlm polymerization system, with the increase of molecular weight,the volume deﬁciency causes the appearance of banded texture because ad-hesion to the substrate prevents uniform shrinkage in three dimensions.Figure 2.8(a) is the typical schlieren texture with two disclinations of oppo-site signs. The dark line connecting the defects is quite diffuse, indicating thedefects are isolated. After the banded texture appears [Figure 2.8(b)], the situ-ation changes: the defects are connected by an inversion wall (the sharp darkline). From the stripes in Figure 2.8(c), one can deduce that the strength oftheleftdisclinationis−1andtherightoneis+1becausethemacromolecularchainorientationisperpendiculartothestripes.Thebandedtextureisfullydevelopedwithinmorethan10minutes.Thewidthofthestripesisabout5␮m.

Figure2.8Micrographsshowingtheformationofbandedtexture.Allthemicrographsareobtainedfromthesameareaofthesamesample.Monomercompositionofthereactionsystemis73/27ABA/ANA.Reactiontemperatureis250◦C.

After 120 min of reaction, the banded texture does not change much, but themorphology seems to become more coarse [17].

4. INVESTIGATION OF THE EFFECTS OF REACTIONCONDITIONS ON THE SYNTHESIS OF LIQUIDCRYSTALLINE POLYMERS4.1. EFFECTS OF REACTION TEMPERATURE ON THE THINFILM POLYMERIZATION4.1.1. Effect on the Kinetics of LC Texture Formation

At the beginning stage of polymerization reaction, LC phase is formed in theisotropic melt. The domain growth and coalescence of LC droplets lead to theformation of schlieren texture, which means the total view area becomes an LCphase. During the LC area increase process, the fraction of anisotropic phaseis deﬁned by X = Aanisotropic /Atotal, where Aanisotropic is the area of anisotropic

the total view area. Using the software to calculate X atphase and Atotal is

different reaction times, a series of curves for 73/27 ABA/ANA reaction systemcan be obtained and shown in Figure 2.9. It can be seen very clearly that thehigher the reaction temperature is, the earlier the LC forms, and the faster LCphase increases. The time interval from appearance of LC phase to formationof schlieren texture is strongly dependent on the reaction temperature. Forexample, it takes 332 s to form schlieren texture after generation of LC phase

◦◦

at 230C, whereas it only takes 10 s at 310C.4.1.2. Effect on the Morphology

Table 2.1 summarizes the observation results revealing the relationship be-tween reaction time and morphological transition at different temperatures.

10.8310 Co290 Co270 Co250 Co230 C02004006008001000oX0.60.40.20t (s)Figure2.9Thefractionofanisotropicphaseasafunctionofreactiontimeatdifferentreactiontemperatures.Monomercompositionis73/27ABA/ANA.

TABLE 2.1. The Effect of Reaction Temperature on the Transition Timeof Morphological Changes. Monomer Composition Is 73/27 ABA/ANA.

Temperature (◦C)Transition Time 230 250 270 290 310Generation of LC phase (s) 510 200 140 90 20Formation of schlieren texture (s) 842 392 247 138 30Appearance of banded texture (min) 180 40 18 10 2.5Full development of banded texture (min)300 50 21 11 3.3

From Table 2.1, we can clearly see that the time for morphological changedecreases with increasing reaction temperature. For example, when reaction

◦◦

temperatures are 230C and 310C, banded textures begin to form at 180 minand 2.5 min respectively.Figure 2.10 shows the morphologies of the late stage of thin ﬁlm poly-merization at relatively high reaction temperatures. After banded texture fullydevelops, the sample surface quickly turns into a non-smooth texture. Com-pared to the ﬁnal product obtained at a low temperature, the morphology ofhigh temperature product is coarser and fuzzier.4.2. EFFECTS OF CATALYST ON THE THIN

FILM POLYMERIZATION

For the polycondensation of wholly aromatic polyesters, several effectivecatalysts are known as having the capabilities of accelerating reaction as well

Figure2.10Themorphologiesofthelatestageofthinﬁlmpolymerizationatrelativelyhighreactiontemperatures:(a)290◦Cand(b)310◦C.Themicrographsareobtainedfromthesameareaforthesamesample.Monomercompositionis73/27ABA/ANA.

as resulting in high inherent viscosity. Representative examples of catalystssuitable for the reaction are derivatives of alkali metals, alkali-earth metals, andother metals, such as titanium, manganese, zinc, tin, and antimony for theiroxides, hydrides, hydroxides, halides, alcoholates, phenolates, organic and in-organic salts, complex salts, and so forth [30–35]. Of these compounds, theabove-mentioned alkali acetate compounds are particularly preferable. Gener-ally, the typical range of the catalytic amount is from 0.001 to 1 wt% basedon the total monomer reactant weight, with between 0.005 and 0.1 wt% beingpreferred [30–35].Because sodium acetate (CH3COONa) had been extensively employed inmany LCP patents and it obviously has an acceleration effect on the polymer-ization reaction, this catalyst was selected for extensive study. The catalyst isapplied in ABA/ANA polycondensation reactions with a monomer molar ratioof 73/27.As mentioned before, during the polycondensation reaction of an uncatalyzedsystem, the following morphological changes have been observed sequentially:generation of liquid crystal phase, coalescence of liquid crystal domains, for-mation of schlieren texture, annihilation of disclinations, and appearance ofbandedtexture.Foracatalyzedsystemwithalowpercentageofcatalyst,thesequence of morphological changes is the same. Figure 2.11 is a set of mi-crographstakenfromthesameareaofthesamesample,illustratingthewholeprocessofmorphologicalchangesduringthecatalyzedpolymerizationwith0.01wt%CH3COONaat250◦C.Itshowsthatthetransitiontimesofthemor-phologicalchangesforacatalyzedsystemareobviouslyearlierthanthatofanuncatalyzedsystem.Theappearanceoftheliquidcrystalphase,forexample,

Figure2.11Micrographsshowingthemorphologiesofthereactionsystematdifferentreac-tiontimesduringthinﬁlmpolymerization.Allthemicrographsareobtainedfromthesameareaofthesamesample.Monomercompositionis73/27ABA/ANA.Catalystcontentis0.01wt%CH3COONa.Reactiontemperatureis250◦C.

450400350300t (s)Formation of schlieren textureAppearance of LC phase2502001501005000 0.002 0.004 0.006 0.008 0.01c (wt%)Figure 2.12 The dependence of morphological transition time on the catalyst CH3COONa content.

◦Monomer composition is 73/27 ABA/ANA. Reaction temperature is 250C.

needs 202 s for the uncatalyzed system, but only 78 s for the catalyzed system.

Compared to the uncatalyzed system at same temperature, the banded texturefor catalyzed system becomes fuzzy or unclear [36].4.2.1. Effect of Catalyst Content on the Thin Film PolymerizationThe effect of catalyst content on polycondensation reactions has also beeninvestigated by varying the CH3COONa concentration over a range from 0 wt%to 0.01 wt%. Figure 2.12 summarizes the results from in situ observation of thecatalyst effect on the 73/27 ABA/ANA polycondensation reaction systems. Thesequences of morphological change for these reaction systems with differentcatalyst levels are similar. However, different catalyst concentrations result indifferent transition times. For example, it takes a much shorter time (168 s)for a catalyzed system with 0.002 wt% CH3COONa to show the appearanceof the LC phase than that for an uncatalyzed system (202 s). The transitiontime for each morphological change decreases with increasing catalyst content.Figure 2.12 also suggests that the transition times for LC phase appearanceand schlieren texture formation decrease almost linearly with an increase incatalyst content when the catalyst content varies from 0.002 wt% to 0.01 wt%.However, further increase in catalyst content does not shorten the transitiontime proportionally.The amount of a catalyst in polymerization not only determines the transitiontimes of morphological changes but also signiﬁcantly affects the morphology ofﬁnal products. Figure 2.13 shows various ﬁnal morphologies of LCPs synthe-sizedfromdifferentlevelsofcatalystconcentration.Forthecatalyzedsystemswithlowcatalystconcentrationsof0.002wt%to0.007wt%,theirmorpholo-giesarequitesimilartothatofanuncatalyzedsystematthesamereaction

Figure2.13ThemicrographsofﬁnalproductsofthereactionsystemswithdifferentcatalystCH3COONacontent(a)0.002wt%,(b)0.005wt%,(c)0.007wt%,(d)0.1wt%,(e)1wt%,and(f)5wt%.Monomercompositionis73/27ABA/ANA.Reactiontemperatureis250◦C.

temperature.Thesituationchangesifthecatalystpercentageisgreaterthan0.01wt%.TheLCPmorphologiesbecomecoarse,andthebandedtexturesseemtobefuzzyorunclearasdiscussedbefore.Thesefuzzymorphologiesarequitesimilartothatoftheuncatalyzedsystemreactingatahightemperature.Ifthesystemcontains5wt%catalyst,theresultantLCPhasatotallydifferentmorphology.Ithasanextremelyhighdensityofdefectsbecausethereactionproceedstoofastandthesystemviscosityincreasesveryrapidly;thus,thereisnotenoughtimefordefectstoannihilate,andmanydefectsstayintheﬁnalproduct.BecauseuncatalyzedandcatalyzedsystemspolymerizedatthesametemperatureproduceLCPswithdifferentmorphologies,thecatalysteffectonLCPprocesses,properties,andapplicationsmaynotbeignoredifthecatalystconcentrationishigh[36].

4.2.2.KineticsStudyofCatalyzedandUncatalyzed

PolycondensationReaction

PreviousworksconﬁrmthatthebulkcopolymerizationreactionofP(OBA/ONA)isabimolecularsecond-orderreactionforbothcatalyzedanduncatalyzedsystems[37].Thereactionrateequationcanbeexpressedasfollows:

−d[COOH]/dt=k[COOH]2

(6)

inwhich[COOH]istheconcentrationofacidgroupsandkisthereactionrate

121081/(1-P)64200 200 400 600 800 1000t (s)UncatalyzedCatalyzed: 0.005wt% CH3COONaFigure 2.14 The dependence of 1/(1 − P) on the reaction time for an uncatalyzed system and

Monomer composition is 73/27 ABA/ANA.a catalyzed system with 0.005 wt% CH3COONa.

Reaction temperature is 250◦C.

constant. The extent of polymerization reaction, P,isdeﬁned as the ratio ofthe reacted functional groups to the total functional groups, and the numberaverage degree of polymerization, DP,isdeﬁned as the ratio of initial acidgroup concentration to the current acid group concentration. So we have

DP = 1/(1 − P) = k[COOH]ot+1

(7)

where[COOH]oistheinitialconcentrationofacidgroupsandtisthereactiontime. Figure 2.14 shows the relationship between 1/(1 − P) and t for 73/27ABA/ANAthinﬁlmpolymerizationreactionsafterappearanceoftheLCphase.Forbothuncatalyzedandcatalyzedthinﬁlmpolymerizationsystems,theirreactionkineticsfollowthesecond-orderequation,whichisconsistentwiththeresultsobtainedfromthebulkpolymerization[37].

LCPpolymerizationisuniqueinthattherearetwokineticsregionsbecausethereactionsystemturnsfromahomogeneoustoheterogeneoussystemduetotheprecipitation(phaseseparation)ofoligomers.Thus,thereactionrateconstantsbeforeandafterthegenerationoftheLCphasemaybedifferent[37–39].Forthinﬁlmpolymerization,becausealldetectablemorphologicalchangesduringpolymerizationundermicroscopeobservationoccurafterthegenerationoftheLCphase,thestudyisfocusedonthesecondregion.

Becauseofthereleaseofaceticacidduringthepolycondensationreaction,thecontentofcarbon,hydrogen,andoxygenelementsinthesystemcontinuouslychangeswiththeprogressofpolymerization.Mathematically,onecaneasilydevelopthefollowingrelationshiptoexpresscarbonelementcontent,Ce,vs.theextentofreaction,P,as

Ce(wt%)=(120.96−24P)/(193.5−60P)

(8)

TABLE 2.2. The Effect of Reaction Temperature on Carbon Content, Ce,Reaction Extent, P, and Number Average Degree of Polymerization, DP,atCertain Morphological Transitions. Monomer Composition Is 73/27 ABA/ANA.

Temperature (◦C) 230 250 270 290

Ce Generation of liquid crystal phase 66.64 67.79 68.41 68.87(wt%) Formation of schlieren texture 68.10 69.28 69.56 70.10P Generation of liquid crystal phase 0.50 0.61 0.67 0.71

Formation of schlieren texture 0.64 0.74 0.77 0.81DP Generation of liquid crystal phase 2.0 2.6 3.0 3.5Formation of schlieren texture 2.8 3.8 4.3 5.3

In order to determine the exact extent of reaction at certain morphological

transitions, the reaction is stopped when the certain morphological transitionsoccur and then are characterized by the elemental analysis. The results areshown in Table 2.2. Figure 2.15 shows the transition times for the generationof the LC phase and the formation of schlieren textures in the catalyzed anduncatalyzed systems as a function of reaction temperature. It is very interestingto point out that the precipitation of oligomers is determined directly and muchprecisely from in situ observation for thin ﬁlm polymerization, while it canonly be calculated indirectly from the evolved acetic acid and the break in time-conversion curve for bulk polymerization. As shown in Table 2.2, the numberaverage degree of polymerization when the LC phase appears decreases with a

◦

decrease in the reaction temperature in the experimental range of 230–290C,

◦

and LC phase may form at the DP as low as 2 at 230 C. This low DP is becausethe onset of the LC phase should not be only dependent on the DP, but also on

9008007006005004003002001000220 240 260 280 300T ( C)UncatalyzedFormation of schlieren textureAppearance of LC phaseCatalyst: 0.005wt% CH3COONaFormation of schlieren textureAppearance of LC phaseFigure2.15ThedependenceofthemorphologicaltransitiontimeonthereactiontemperatureforthereactionsystemswithandwithoutcatalystCH3COONa.Monomercompositionis73/27ABA/ANA.

t (s)TABLE 2.3. The Reaction Rate Constants for Uncatalyzed and Catalyzed

Thin Film Polymerization Systems at Different Temperatures.

Monomer Composition Is 73/27 ABA/ANA.

Temperature (◦C)

k × 104 (L · mol−1 · s−1) (second region) 230 250 270 290Uncatalyzed 2.04 7.54 14.05 29.05

Catalyst: 0.005 wt% CH3COONa4.81 13.65 24.15 57.00

the temperature. At the constant DP, the rigidity of molecular chains increaseswith decreasing temperature; thus, the low DP may sufﬁciently yield mesogeniccharacteristics at a low reaction temperature.From the data of Table 2.2 and Figure 2.15, the reaction rate constant, k, canbe calculated by substituting the values of t and P into Equation (7), and theresults are tabulated in Table 2.3. Interestingly, the reaction rate constant, k, forthe thin ﬁlm polymerization reactions is much greater than the reported value, asshown in Table 2.4, for bulk polymerization reactions [37]. For example, the k in

−1 −1◦−4

the thin ﬁlm reaction at 270C without catalyst is 14.05 × 10L · mol· s,

◦

while the second region k for bulk polymerization without catalyst at 275Cis

−3 −1 −1−4 −1 −1

3.2 × 10L · mol· min(0.53 × 10L · mol· s) [37]. The great dif-ference is mainly due to the fact that the acetic acid in the thin ﬁlm polymeriza-tion is much easier and quicker to release than that in the bulk polymerization.Using the same method, k for the catalyzed system with different catalyst lev-els is calculated, and the results are shown in Figure 2.16. The reaction rateconstant increases with increasing catalyst content in an approximately linearmanner.To plot ln k vs. 1/T , the activation energy, Ea, can be obtained by the valuefrom the slope. As shown in Figure 2.17, for an uncatalyzed system, the Eais109.9kJ/mol,whichisquiteclosetothevalue(105.8kJ/mol)reportedforbulkpolymerization[37].Foracatalyzedsystemwith0.005wt%CH3COONa,Eadropsslightlytoabout96.9kJ/mol[36].

TABLE2.4.

ReferenceDataforReactionRateConstantsObtained

fromUncatalyzedBulkPolymerizationReactions.Monomer

CompositionIs73/27ABA/ANA.

Temperature(◦C)

2504.9

27513.03.20.53

30042.75.20.87

ReactionRateConstantk1×103(L·mol−1·min−1)(ﬁrstregion)k2×103(L·mol−1·min−1)(secondregion)or

k2×104(L·mol−1·s−1)(secondregion)

0.00250.0020.0015k (L•mol-1•s-1)

0.0010.0005000.0020.0040.006c (wt%)0.0080.01Figure2.16ThedependenceofreactionrateconstantonthecatalystCH3COONacontent.Monomercompositionis73/27ABA/ANA.Reactiontemperatureis250◦C.

5.INVESTIGATIONOFTHEFORMATIONOFLIQUIDCRYSTALLINITY

MostmonomersforLCPsynthesisdonothaveliquidcrystallinity.Duringthepolymerization,LCphaseformsandevolveswiththeprogressofthepolymer-ization,furtherannealingorcuringreaction.Thinﬁlmpolymerizationtechniqueisaveryconvenientwaytoinvestigatetheformationofliquidcrystallinity,be-causetheLCphasecanbeclearlyobservedduringthepolymerizationoncetheliquidcrystallinityforms,andthereactiontemperature,whichisacriticalparametertoformliquidcrystallinity,canbeaccuratelycontrolled.

-4-5-6-7-8-90.001750.00180.001850.00190.001951/T0.002UncatalyzedCatalyst: 0.005wt% CH3COONaFigure2.17ArrheniusplotsforthethinﬁlmpolymerizationofP(OBA/ONA).Monomercompo-sitionis73/27ABA/ANA.

lnK5.1. EFFECTS OF MONOMER STRUCTURES

ON LIQUID CRYSTALLINITY

Polymers that exhibit liquid crystallinity, either in the melt or in their solu-tions,usuallyconsistofcomparativelyrigidstructuresthatconferhighextensionon the molecular chains. Intermolecular attractive forces also may contributeto the stabilization of the LC state. Polarizability of the molecules and theirconstituent groups are the molecular feature that is required to render the in-termolecular force. Anisotropy of the polarizability tensors of the interactingmolecules confers a corresponding anisotropy on the dispersive attractions be-tween molecules. On this account, parallel molecular alignment may be ener-getically favored [1–3].To study the effects of monomer structures and reaction conditions on liq-uid crystallinity and crystallization, a series of homopolymers and copolymerswere synthesized. Their monomer compositions are ANA, ABA, AAA/IA,ANA/AAA/IA,andABA/AAA/IA.Thechemicalstructuresofthesemonomersare shown in Figure 2.18.

ANAandABAareamongthemostcommonlyusedmonomersformain-chainthermotropicLCPs.AtypicalexampleisthecommerciallyavailableLCPVectraAforwhichthemonomercompositionis73/27ABA/ANA[6,7].p-Acetoxybenzoicacid(ABA)providesthebenzeneringthatpotentiallycanformamesogenicunit.Phenyleneunitsinitshomopolymercanrotateand,thus,

ANA

CH3COOO

COH

ABA

CH3COHO

NO CAAA

CH3C CH3

OHO

CO C OHIA

Figure2.18Chemicalstructuresofthemonomers.

are not collinear. In the molecular chain, additional chain motion is possiblebecause the bridging group is an ester group. The ester group consists of angularbridging groups with parallel conﬁguration. Each ester linkage gives a sidewaysdisplacement, but keeps the 1,4-axes of the benzene rings lying parallel, so thatthe ester linkage provides little opportunity for a chain to kink and keeps thechain straight. Although the straightness satisﬁes the requirement of formingthe liquid crystallinity, the regularity and periodicity of the chain make thehomopolymer become a highly crystalline material and have a quite high Tm ifthe molecular weight is high enough [6,7]. 2,6-Acetoxynaphthoic acid (ANA)has a naphthoic ring to potentially form a mesogenic unit. It is usually used as amodifying unit for LCPs because of the side-step (crankshaft) effect associatedwith the naphthoic ring. Its homopolymer has a relative low Tm of 440◦C dueto the side-step effect. The homopolymer is also a crystalline material [11].Acetoxy acetanilide (AAA) is also a frequently utilized monomer for synthesisof main-chain thermotropic LCPs, such as in Vectra B [6,7], which has themonomer composition of 60/20/20 ANA/AAA/TA (terephthalic acid). AAAprovides a benzene ring potentially acting as a mesogenic unit, an amide group,and an ester group as bridging groups. Isophthalic acid (IA) is another monomerextensively employed to modify LCPs because its meta linkage can induce kinkinto the molecular chain. The resultant polymer may have a lower Tm. However,the meta linkage also has a detrimental effect to the stability of the LC phase,and it will disturb the liquid crystalline character if its percentage is too high[1–3]. Because of the structural character of the above monomers, the effects ofmonomer structures (length of mesogenic unit, kink) on the liquid crystallinitycan be easily determined.5.1.1. Morphological Changes during Homopolymerization

Reactions with Monomer Compositions of ANA,ABA, and AAA/IA

As known to all, at certain temperatures, polymers can be morphologicallydivided into amorphous, crystalline, and liquid crystalline states [40,41]. In thisstudy,thehomopolymersofANAandABAformliquidcrystallinephasesintheearlystageofthepolymerizationreactions,whilethepolymerofAAA/IAformscrystals.

5.1.1.1.FormationofLiquidCrystallinityduringPolycondensation

ReactionsofANAandABAAs shown in Figure 2.19, the homopolymerization system for pure ANAstartsfromahomogeneousphaseandthenchangesintoaheterogeneoussystem.Whenthesampleisheatedtotheproposedreactiontemperature,ANAcrystalsmelt,andthewholeviewareabecomesanisotropicmeltphase.Duringthepolycondensationreaction,oligomersforminthemoltenmonomerphase.As

50µmFigure 2.19 Micrographs showing morphologies of pure ANA polymerization reaction system atdifferent reaction times. All the micrographs are obtained from the same area of the same sample.

◦Reaction temperature is 320C.

shown in Figure 2.19, the dark area in the micrographs is the isotropic phase of

melt, while the bright area represents the LC phase. After the formation of LCdomains, their sizes increase, and coalescence of adjacent LC domains occurs.During the coalescence process, annihilation between disclinations occurs toreduce free energy. The LC phase increases quickly within a very short period.When the reaction time is 18 s, the total view area becomes the LC phase.After 136 s reaction, crystallization begins to occur. After 216 s, no obviousmorphological change can be observed, and the morphology is the same as theﬁnalmorphologyafterreactingfor60min.For polymerization of ABA, Figure 2.20 shows similar morphologicalchangestothatofANA.However,themajordifferencebetweenthemisthat

50µmFigure2.20MicrographsshowingmorphologiesofpureABApolymerizationreactionsystematdifferentreactiontimes.Allthemicrographsareobtainedfromthesameareaofthesamesample.Reactiontemperatureis320◦C.

polymerization of pure ABA has faster morphological transitions than that ofpureANA.Thismaybemainlyduetothefactthattheformerhasalowermeltingpoint than the latter. Homopolymer of ABA is known to be a highly crystallinematerial and has a very high melting point. In the reaction system consisting ofpure ABA, crystallization occurs much faster than that of pure ANA [41].5.1.1.2. Crystallization during Polycondensation Reaction of AAA/IACrystals of linear macromolecules can be grown not only from the melt orsolution, but also directly from the polymerization because of the increasingmolecular weight. Normally, melting point of the crystals increases with anincrease in molecular weight. At a certain reaction temperature, because of thecontinuous increase in molecular weight, crystals will appear once the melt-ing point of the crystals becomes higher than the reaction temperature. For thepolymeric crystals grown directly during polymerization, there are two impor-tant types of homogeneous nucleation: oligomer nuclei and folded chain nuclei.Oligomer nucleation is the intermolecular path on crystallization during poly-merization. It is the analogs to the fringed micelle nucleus of nucleation of analready polymerized molecule. Because the monomer is normally much shorterthan the critical nucleus dimension, nucleation occurs only when sufﬁcient reac-tion extent is reached. The folded chain nucleation means the molecules can stillpolymerizewhileundergoingnucleationandcrystalgrowth.Furthergrowthofthenucleusmayoccurthroughsimultaneousorsuccessivepolymerizationandcrystallization[42].Figure 2.21 shows the crystallization during the polycondensation reactionof50/50(moleratio)AAA/IA.Afternucleation,theradiusofthespherulitein-creaseswithtimealmostlinearlybeforethespherulitecontactsotherspherulites.

50µmFigure2.21Micrographsshowingmorphologiesof50/50AAA/IApolymerizationreactionsystematdifferentreactiontimes.Allthemicrographsareobtainedfromthesameareaofthesamesample.Reactiontemperatureis320◦C.

The crystal growth rate, which is deﬁned as the radius increase in unit time, isalmost constant, about 1.7 ␮m/s [41].5.1.2. Morphological Changes during Copolymerization Reactions

with Monomer Compositions of ANA/AAA/IA and ABA/AAA/IA5.1.2.1. Thin Film Polymerization Reactions of ANA/AAA/IA

IA is a kind of modifying monomer for LCPs that introduces the kink intomolecular chain to reduce the melting points of LCPs. However, the drawback ofmeta linkage is that it will disturb the liquid crystallinity if its content is too high.There is a critical ANA content for the reaction system ANA/AAA/IA to formliquid crystallinity or to crystallize. For example, when reaction temperature is320◦C, the critical ANA content is 20 ± 2%. If the ANA content is higher thanthis value, the LC phase is formed; if the ANA content is lower than this value,crystallization occurs.Figure 2.22 exhibits the generation of LC phase and time evolution of LCtexture for 60/20/20 (mole ratio) ANA/AAA/IA at the reaction temperature of

◦

320C.Forthissystem,theLCphaseappearssimultaneouslywithahighdensityof defects. And then, the annihilation between defects occurs rapidly to releasethe excess free energy. The circles in Figure 2.22 illustrate a typical exampleof the annihilation process when reaction time is between 48.0 s and 50.0 s, forwhich the annihilation rate is about 23 ␮m/s. In the 60/20/20 ANA/AAA/IA re-actionsystem,itonlytakesafewsecondstoaccomplishtheannihilationprocess.If ANA content is decreased to 40%, the defect density is increased in theﬁnal product, and, correspondingly, the annihilation rate decreases as shown inFigure 2.23.

50µmFigure2.22Micrographsshowingmorphologiesof60/20/20ANA/AAA/IApolymerizationre-actionsystematdifferentreactiontimes.Allthemicrographsareobtainedfromthesameareaofthesamesample.Reactiontemperatureis320◦C.

50µmFigure 2.23 Micrographs showing morphologies of 40/30/30 ANA/AAA/IA polymerization re-action system at different reaction times. All the micrographs are obtained from the same area ofthe same sample. Reaction temperature is 320◦C.

As we know, rigid rod-like molecular structures, such as ANA and ABA,can import the liquid crystalline character to the materials. Defect density andannihilation process in the LC phase depend not only on monomer structurebut also on monomer percentage. The higher the ANA content, the earlier theLC phase appears, and the lower the defect density after full annihilation. Thisinteresting phenomenon is caused by the higher elastic constant values for thesystem having a higher concentration of ANA. In other words, the persistencelength increases with an increase in corresponding ANA content.Further decreasing ANA content, crystallization can be observed during thereactioniftheANAcontentislowerthanthecriticalcontenttoformliquidcrystallinity. Figure 2.24 shows the crystallization during the polycondensa-tionreactionof10/45/45ANA/AAA/IA.Afternucleation,theradiusofthespheruliteincreaseswithtimealmostlinearly.Thecrystalgrowthrateislowerthanthevalueforthe50/50AAA/IAsystem.

Therearemanyfactorsaffectingtheabilityofapolymertocrystallize.Themostimportantfactormaybetheregularityofthemolecularstructure.With

50µmFigure2.24Micrographsshowingmorphologiesof10/45/45ANA/AAA/IApolymerizationre-actionsystematdifferentreactiontimes.Allthemicrographsareobtainedfromthesameareaofthesamesample.Reactiontemperatureis320◦C.

high regularity, molecular chains can approach sufﬁciently close so that theinterchain forces can maintain an ordered structure. The mobility of chains orchain segments is the other determining factor. Polymers with short repeatingunits and high symmetry can crystallize more readily than those with longrepeating units and low symmetry. Obviously, adding the ANA units disturbsthe regularity of the molecular structure, reduces the degree of periodicity alongthe chain, and frustrates crystal perfection. As a consequence, the crystal growthrate is lower for the system containing ANA [41].5.1.2.2. Thin Film Polymerization of ABA/AAA/IA

Similar to the ANA/AAA/IA system, LC phase appears in the ABA/AAA/IApolymerization system if the ABA content is higher than the critical content,and crystals form during the reaction if ABA content is lower than the criticalcontent.

ThegenerationofLCphaseandtimeevolutionofLCtexturefor60/20/20

◦

ABA/AAA/IA at the reaction temperature of 320 C is shown in Figure 2.25.Comparedtothereactionof60/20/20ANA/AAA/IA,theannihilationfor60/20/20ABA/AAA/IAisveryslow.AftertheLCtexturefullydevelops,thedensityofthedefectsisstillveryhigh.

Asmentionedbefore,theelasticconstantsincreasewithincreasingpersis-tencelength.Ifthemolecularlength,L,exceedsthepersistencelength,thevaluesofK2andK3willtendtobeconstant,whileK1willincreasemonotoni-callywithL.ThelengthsoftheANAandABAunitsinthepolymerchainsare

˚and6.35A,˚respectively[43].ComparedtotheANA/AAA/IAsystem,8.37A

themainreasontocauseahighdefectdensityandalowannihilationrateintheABA/AAA/IAsystemisduetomolecularstructuredifferencebetweenANAandABA.ANAhasarelativelylongercyclicunittoformalongermesogenicunit.BecausethemolarpercentagesofABAandANAinthesetworeactionsystemsareexactlythesame,thelongermesogenicunitofANAmayresultinagreaterpersistencelength.Becausetheelasticconstantsincreasewithin-creasingpersistencelength,theANA/AAA/IAsystemhasgreatervaluesof

20µmFigure2.25Micrographsshowingmorphologiesof60/20/20ABA/AAA/IApolymerizationre-actionsystematdifferentreactiontimes.Allthemicrographsareobtainedfromthesameareaofthesamesample.Reactiontemperatureis320◦C.

the elastic constants and a higher tendency than the ABA/AAA/IA system torelease energy and exhibit almost the same orientation. Thus, annihilation inthe former is faster, easier, and more complete than in the latter [41].5.2. EFFECTS OF REACTION TEMPERATURE

ON THE LIQUID CRYSTALLINITY

As predicted by Flory, the critical aspect ratio of rod molecules for ther-motropic liquid crystallinity is 6.4 [2,3]. However, in this prediction, the inﬂu-ence of temperature was not considered. The temperature effect was includedin the form of an orientationally dependent energy function in more recentdevelopment of the Flory lattice model [3]. For LMWLCs, the aspect ratio isdeﬁned as L /d, in which L is the length and d is the diameter of the molecule.For LCPs, the aspect ratio is replaced by persistence ratio through replacingmolecule length L by persistence length q [3].In ANA/AAA/IA and ABA/AAA/IA systems, the meta linkage in IA has an

◦

angular conformation (120). The kink created by the IA monomeric unit resultsin entanglement and shortens the persistence length of the polymer chain. Thus,the system containing higher content of IA (in other words, lower content ofANA or ABA) should be less likely to form liquid crystallinity because thepersistenceratioincreaseswithincreasingANAorABAcontent.WhenthecontentofANAorABAreachesacriticalvalue,theLCphasecanbeformed.Figure 2.26 shows the critical ANA and ABA content for the reaction systemsofANA/AAA/IAandABA/AAA/IAtoformliquidcrystallinityasafunctionofreactiontemperature.Forexample,whenreactiontemperatureis280◦C,thecriticalANAcontentis22±2%.IftheANAcontentishigherthanthisvalue,theLCphaseisformed;iftheANAcontentislowerthanthisvalue,crys-tallizationoccurs.Thisﬁgurealsosuggeststhat,forbothsystems,thecriticalcontentofANAorABAdecreaseswithincreasingreactiontemperature.Thisinterestingrelationshipmaybeduetothefollowingpossibility:reactionsat

6050

Critical30content (%)

100260

280

300

320T ( C)340

360

380

ABAANAFigure2.26ThedependenceofthecriticalANAandABAcontentonthereactiontemperature.

differenttemperaturesmayyielddifferentsequencesofmonomericunitsinthechains.Theotherreasonisduetothefactthattheoligomersmayformcrystalsatalowtemperature,butexhibittheLCstateatahightemperature.

Aninterestingphenomenonoccurswhenthereactiontemperatureis360◦C:thereisanamorphousregionbetweenthecriticalcontentofANAorABAtoformtheLCstateandthecrystallinestate.Forexample,ifANAcontentisintherangeof12%to21%,nothingcanbeopticallydiscerneduntiltheendofthepolymerization,whichsuggeststheproductsshouldbeintheamorphousstate.Thissurprisingphenomenonmightbeduetothereductionofpersistencelengthathightemperatures.Atlowtemperatures,chainrigidityishigh,andtheconformationofmoleculestendstobestraightandthusyieldstheLCstate.However,athightemperatures,chainrigidityreduces,andthechainshavetheabilitytoformentanglements.Asaresult,persistencelengthreduces,andtheamorphousstateisformed.However,byfurtherincreasingANAorABApercentage,theLCstatecanstillappearbecausetheresultantpolymerhasalongpersistencelength[40,41].

6.INVESTIGATIONOFTHEELECTRICRESPONSEOFLIQUIDCRYSTALLINEPOLYMERDURINGPOLYMERIZATION6.1.INTRODUCTIONOFTHEELECTRICFIELDEFFECTS

ONLIQUIDCRYSTALLINEMATERIALS

Sincetheelectro-opticaleffectofLCswasdiscoveredanditsextensiveapplicationswererecognized,theeffectsofelectricﬁeldontheliquidcrystalmaterialshavebeenofconsiderableinteresttobothacademyandindustry.ThedirectinﬂuencesoftheelectricﬁeldonLCincludeshiftofthephasetransitiontemperature,variationinorderparameter,andchangeinsymmetry[44,45].BecauseofthedielectricanisotropypropertyofLCs,theLCmoleculescanaligneitherparallelorperpendiculartotheelectricﬁeld,theoretically,accordingtotheirvaluesofdielectricanisotropy[44].However,undercertainconditions,theuniformdirectorreorientationinana-celectricﬁeldisunfavorable;thedo-mainstructurecorrespondingtoaminimumfreeenergyisformed.Thedomainpatternscanbeclassiﬁedintotwomaintypes:orientationaldomainswithpuredirectorrotationwithoutﬂuidmotionandtheelectrohydrodynamicdomainscausedbythecombinedeffectsoftheperiodicdirectorreorientationandregu-larvorticesofmaterialmoving[44].ThiskindofmovementofLCmaterialsiscalled“hydrodynamicﬂow,”mainlyresultingfromtheeffectsofconductivityanisotropyofLCmoleculesandionicelectriccurrent.

ThemovementofLCmaterialsinanelectricﬁeldisknownas“electro-hydrodynamicinstability”[44,45].Theoretically,itoccurswhenthedielectricanisotropyandconductivityanisotropyhavedifferentsigns,mostcommonly

withnegativedielectricanisotropyandpositiveconductivityanisotropy.Thedomainformationalsogreatlydependsontheparametersofexternalﬁeld.Inalow-frequencyregion,themostfrequentlyobservedoneiscalled“Williamsdomains”or“Kapustin-Williamsdomains,”causedbyaperiodicdistortionac-companyingacertainamountofcellularﬂow.Thespatialperiodisslightlyshorterthanthesamplethickness.Thisfrequencyregionisknownas“con-ductionregime.”Inmanycases,theWilliamsdomainpatternisasimpleone-dimensionalpattern,andthepatternstaysthesamewhentheelectricﬁeldreverses.However,thepatternmaybeinterrupted,andatwo-dimensionalpe-riodicitycanbefound.Thiskindoftwo-dimensionalpatternisidentiﬁedas“ﬂuctuatingWilliamsdomains”[45].Inahigh-frequencyregion,anarrowbandmodecalledthe“Chevronpattern”canbeobserved.Thisfrequencyregioniscalledthe“dielectricregime.”Inthisregime,thechargeoscillationscausetheoscillationsofLCmaterials,sothevorticesarenotstationarybutintheformof“toandfro”motion[44].

Toformthedomainstructure,thereusuallyexistsathresholdvoltage.ThethresholdvalueisafunctionofthephysicalpropertiesofanLC,suchasdielec-tricandconductivityanisotropies,viscoelasticproperties,andtheexperimentconditions,suchasamplitudeandfrequencyoftheexternalﬁeld,cellthickness,andinitialorientation.Previouswork[19,44]suggeststhatthethresholdvolt-agetoformtheWilliamsdomainsisnotstronglydependentonthefrequency,whilethethresholdvoltagefortheChevronpatternincreaseswithincreasingfrequency.Withfurtherincreaseinthevoltage,theﬂowvelocityincreases.Fi-nally,aboveacertainvoltage,anewregimeisreached:thedomainbecomesdisorderedandmobile,andtheﬂowbecomesturbulent.Thelatterphenomenonresultsinstronglightscattering,knownas“dynamicscattering”[19,44].

AlthoughthereisafairamountofknowledgeaboutLCPs,thestudyofelectri-calresponseofthermotropicmain-chainliquidcrystallinepolymers(MCLCPs)underelectricalﬁeldsisverylimited.Mostoftheresearchhasconcentratedonlowmolecularweightliquidcrystals(LMWLCs)[46–52].Onlyafewpapersre-portedtheeffectsofelectricalﬁeldonthermotropicMCLCPs[53,54].InstudiesbyKrigbaumetal.[53,54],theKapustin-Williamsdomainsandlightdynamicscatteringwereobservedforthepolymericnematics.TheelectrohydrodynamicinstabilityofLCPsissimilartothatofLMWLCs.Theprincipaldifferencesbe-tweenthemaretheslowdynamicsofthedomainformationandthedifﬁcultyofobtainingawell-alignedpattern[45,53,54].Thecriticalfrequencytoinduceinstabilityintheconductionregimewasfoundtobestronglydependentonthemolecularweight[53].Thedomainsandlightscatteringtexturescanbefrozenbycoolingthesamplesintheelectricﬁelds[44].

MostmonomersforLCPsynthesisdonotpossessliquidcrystallinity.Duringpolymerization,LCphaseforms,andtheLCtextureevolveswiththeprogressofthereaction.TheelectricresponseofthepolymerizationsystemofLCPalso

FunctiongeneratorReactionsystemConductiveglass slides Electric fieldHigh voltageamplifierPTFE filmas insulator

Steel clampsFigure 2.27 The thin ﬁlm polymerization under a-c electric ﬁeld.

evolves with reaction process. Through investigation of the electric response of

the reaction system, lots of important information about the properties of LCPcan be obtained.6.2. POLYMERIZATION OF LIQUID CRYSTALLINE POLYMER

UNDER ELECTRIC FIELDThe thin ﬁlm polymerization method is the same as the one without electricﬁeld.ThesamplepackageandconnectionofelectricdevicesareshowninFigure2.27. Monomer mixture 73/27 ABA/ANA was sandwiched between two con-ductive glass slides coated with indium tin oxide (ITO). A poly(tetraﬂuoroethy-lene) (PTFE) ﬁlm was used as a spacer as well as a insulator. The thicknessof the sample was 80 ␮m. The conductive glass slides were connected withthe outputs of a high-voltage ampliﬁer through the wires clamped by the steelclamps. Electric ﬁeld was generated by a function generator (HP 33120A) andwas ampliﬁed by the high-voltage ampliﬁer (Trek P0623A). The electric ﬁelds(square wave) with different frequencies were applied to the reaction systemafter the generation of LC phase [55].6.3. ELECTRIC RESPONSE OF THE POLYMERIZATION SYSTEM

OF LIQUID CRYSTAL POLYMER6.3.1. Morphology of the LCP Polymerization System BetweenConductive Glass SlidesInourexperiment,weappliedelectricﬁeldonthepolymerizationsystemafterthe whole view area became an LC phase. Figure 2.28 shows the appearance of

abcd

50µmgh

Figure2.28AppearanceofLCphasebetweentwoconductiveglasses(withoutelectricﬁeld).Monomercomposition:73/27ABA/ANA.Reactiontemperature:230◦C.

Figure2.29ThemorphologiesofLCphasewithoutelectricﬁeldandundera-celectricﬁeldsinlow-frequencyrange:(a)Withoutelectricﬁeld,(b)1Hz,and(c)10Hz.Voltageofthea-celectricﬁeld:25V.Rangeofreactiontime:20–30min.Monomercomposition:73/27ABA/ANA.Reactiontemperature:230◦C.

the LC phase before applying the electric ﬁeld. Unlike the reaction on a normalglass slide, the LC phase generated on the conductive coating layer tends toform large, separated droplets instead of a uniform LC thin layer because ofthe surface properties of the conductive coating layer. Figure 2.28(c–g), showsthat the bigger LC domains seem to attract the smaller LC domains to increasetheir areas. And, ﬁnally, a large LC domain forms and occupies the whole viewarea as shown in Figure 2.28(h).6.3.2. Response of the LC Phase under Electric Fields

with Different Frequencies

After the whole view area becomes an LC phase, we apply a-c electric ﬁeldswith different frequencies and a certain voltage to observe the effects of electricﬁelds on the LC phase. The reaction time ranges from 20 min to 30 min.Figure 2.29 shows the morphologies of the LC phase in low-frequency range.Under these low-frequency a-c ﬁelds, orientation of LC materials can be found.Figure 2.29(a) is the morphology of the LC phase without electric ﬁeld. Figure2.29(b) shows the morphologies of the LC phase after applying the externalﬁeld of 25 V (rms) and 1 Hz. The time interval to snap these two pictures is0.5 s. Under the electric ﬁeld, the LC phase becomes more transparent andtwinkles when the electric ﬁeld changes its direction. Figure 2.29(c) shows themorphologiesofreactionsystemundera10-Hzelectricﬁeld.Similarorientationphenomena can be seen. The major difference between the responses under1-Hz and 10-Hz ﬁelds is that the LC phase does not ﬂicker obviously under arelatively high-frequency ﬁeld.With an increase in frequency, electrohydrodynamic instability can be foundin a relatively wide voltage range, while the external ﬁeld effect on orientationbecomes less visible. When voltage is 25 V, the electrohydrodynamic ﬂowbegins to form once the frequency reaches 100 Hz. This kind of ﬂow occurs in awide frequency range, from 100 Hz to 60 kHz, as shown in Figure 2.30. The ﬂow

50µmcd

Figure2.30ThemorphologiesofLCphaseundera-celectricﬁeldsinmiddle-frequencyrange:(a)100Hz,(b)1kHz,(c)10kHz,and(d)50kHz.Voltageofthea-celectricﬁeld:25V.Rangeofreactiontime:20–30min.Monomercomposition:73/27ABA/ANA.Reactiontemperature:230◦C.

Figure 2.31 The morphologies of LC phase under a-c electric ﬁelds in high-frequency range:(a) 100 kHz, (b) 1 MHz, and (c) 10 MHz. Voltage of the a-c electric ﬁeld: 25 V. Range of reaction

◦time: 20–30 min. Monomer composition: 73/27 ABA/ANA. Reaction temperature: 230C.

is faster in the middle frequency region (400 Hz to 8 kHz) and becomes slower

if the frequency is outside this range. The ﬂow becomes faster if the voltageof electric ﬁeld increases. As shown in Figures 2.30(c,d) the ﬂow patternsin a relatively high-frequency range (10 kHz to 50 kHz) are quite similar tothe ﬂuctuating Williams domains that have been reported for other polymericnematic LCs [53,54]. However, our experiments indicate that there is no distinctmorphological change from the ﬂuctuating Williams domains to the dynamicscattering mode for the polymerized LCP system. The difference between themis the ﬂow rate, which is faster for the dynamic scattering mode.By further increasing the frequency, the effect of electric ﬁelds on the LCphase becomes undetectable because the director of LC cannot follow theﬁeld and exhibits an almost stationary distribution. When voltage is 25 V,the movements of LC material seem to become stationary when frequencyreaches 60 kHz. Figure 2.31 gives some examples of LC morphologies underhigh-frequency ﬁelds with the voltage of 25 V. Comparing these pictures withthe one without electric ﬁeld (Figure 2.29(a)), no distinct difference can befound [55].

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