Lewis acid/base character and crystallisation properties ofpoly(butylene terephthalate)

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ContentslistsavailableatScienceDirect

Journal

of

Chromatography

A

j ou rn a l h om ep a ge :w w w . e l s e v i e r . c o m / l o c a t e / c h r o m a

Lewis

acid/base

character

and

crystallisation

properties

of

poly(butylene

terephthalate)

José

M.R.C.A.

Santos

∗

,

James

T. Guthrie

DepartmentofColourandPolymerScience,SchoolofChemistry,TheUniversityofLeeds,WoodhouseLane,Leeds,WestYorkshireLS29JT,UnitedKingdom

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received11September2014 Receivedinrevisedform 12December2014 Accepted15December2014 Availableonline29December2014 Keywords:

Inversegaschromatography Poly(butyleneterephthalate) Lewisacid/baseproperties Crystallisation

a

b

s

t

r

a

c

t

Twogradesofpoly(butyleneterephthalate)wereanalysedbymeansofinversegaschromatography

(IGC)andtheresultscorrelatedwiththerespectivecrystallisationproperties.Thefollowingparameters

weredeterminedbyIGC:thedispersivecomponentofthesurfacetension,theenthalpyandtheentropy

ofadsorptionofselectedpolarandapolarprobes,andtheLewisacidityandbasicityconstants,Kaand

Kbrespectively.TheinterpretationofthevaluesdeterminedforKaandKbisinagreementwiththeFTIR

spectrarelatingtothecarboxylend-groupandthehydroxylend-groupconcentrationsinthesepolymers.

Thedifferencesinthemolecularweightvaluesandintheend-grouptypeandconcentration,between

thetwogradesofPBT,donotcausedifferencesinthecrystallisationactivationenergy.Thisobservation

suggeststhatthereisaleadingcontributionoftheLewisbasicsitestothecrystallisationactivationenergy

ofthegradesofPBTthatwereanalysed.However,thelowervalueofKaandthegreatermolarmassof

oneofthePBTgradesleadtoacorrespondinglowercrystallisationdegree.

1. Introduction

Duetotheirsemi-crystallinecharacterandhighmelting tem-peratures,poly(butylene terephthalate), PBT, and poly(ethylene terephthalate), PET, are the more widely used thermoplastic polyesters.Common applicationsof PET include ﬁbrespinning, blowmouldingofbottles,injectionmouldingofengineeringparts andthermoformingoftrays.PETis,nevertheless,characterisedby lowcrystallisationrates.Thishaslimiteditsapplicationininjection mouldingappliances.Toovercomethisdrawback,fastcrystallising gradeshavebeendevelopedthatcontainnucleatingagents(e.g. BaSO4,ZnPO4andSb2O3)toincreasethenon-isothermal crystalli-sationtemperatureandreducethesizeofthespherulites,and/or containotherpolymers.PBT,ontheotherhand,exhibitsa signif-icantlyfastercrystallisationrate.The realisedshorter moulding cyclesandthelowerviscosityunderappropriateconditionshave resultedinawideruseofPBTthanPETformouldingapplications. PBTisactuallyoneofthefastercrystallisingpolymers[1–5].The chemicalnatureoftherepeatunitisshowninFig.1.Thehigh crys-tallisationrateisaconsequenceoftheconsiderablemobilitythat

∗ Correspondingauthor.Presentaddress:PolytechnicInstituteofBraganc¸a, Cam-pusSta.Apolónia,5300-253Braganc¸a,Portugal.Tel.:+351273330832; fax:+351273325405.

E-mailaddresses:josesantos@ipb.pt,josemrcasantos@gmail.com

(J.M.R.C.A.Santos).

isprovidedbythebutyleneunitinthechain.Usually,purePBThas acrystallinityextentthatisintherangeof30–40%.Theenthalpy offusion,Hf◦,ofthe100%crystallinePBTis142J/g[4,6–8].The meltingtemperatureTmisabout225◦Candthecrystallisation tem-perature,Tc,is approximately180◦C [9].The valueoftheglass transitiontemperature,Tg,ofthesemicrystallinestateoccursat about40◦C.ThecompletelyamorphousstateofpurePBTis difﬁ-culttoproduceduetothehighcrystallisationrateofthispolymer. Nevertheless,avalueof₋₂₅◦ChasbeendeterminedfortheTgof 100%amorphousPBT[7,10].

Becauseofitssemi-crystallinenature,PBToffersconsiderable chemical/solvent resistance but low dimensional stability, low ductility,low glasstransitiontemperatureand low Izodimpact strength.Consequently,core–shelltypeelastomersaremost fre-quentlyaddedtoPBTtoimproveitstoughness.Theseblendsshow lowmeltviscosityand,thus,areeasytoprocess.Moreover,the blendshavegoodmechanicalpropertiesandelectricalproperties, excellentsolventresistance,andgoodhydrolyticstability.Blends ofPBTwithelastomersandwithotherpolymershavebeenusedfor theinjectionmouldingofexteriorautomotivepartssuchasmirror housingsandbumpers,andareoftendesignedtoreplacemetalsin speciﬁcapplications.

Furthermore,inordertotakefulladvantageoftheuseful prop-erties ofPBT, and toovercomethe aforementionedlimitations, PBTisusually combinedwithpolymerssuchaspolycarbonates, PC,andPET(e.g.Xenoy®andValox®fromSABICInnovative Plas-tics,Makroblend®fromBayer,andPocan®fromLanxess,Sabre® http://dx.doi.org/10.1016/j.chroma.2014.12.042

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Fig.1. RepeatingunitinPBT.

Table1

Weightaveragemolecularweight(Mw),molecularweightdistribution(D)and

number-averagemolecularweight(Mn),carboxylend-groupconcentrationand

hydroxylend-groupconcentration,forPBTAandPBTB.

Mw(gmol−1) D Mn(gmol−1) −COOH(␮eq/g)

PBTA 108,500 3.18 34,100 48

PBTB 46,000 2.70 17,000 29

1600fromDowChemicalCompany, Stapron® EfromDSM and

Ultrablend®KRfromBASF).Inthepolymerblendsthusobtained,

controloverthecrystallinepropertiesofthePBTisnecessary.

Dur-ingprocessingatelevatedtemperatures,hydrolytic,thermal,and

oxidativedegradationofPBTcanoccurwiththeformationofnew

carboxylicacidend-groups,alongwiththereductioninthe

molec-ularweightand alterationof themolecularweightdistribution.

Theseeffects resultin higher crystallisationratesand

crystalli-sationextents[4,5,8,11].IntheparticularcaseofPBT/PCblends,

theformation of a PC-PBT copolyester results in a decrease in themolecularweightofthePCandofthePBT,inareductionin thedegreeofcrystallinityandinthecrystallisationtemperature [2,6,7,12–14].

The growingawareness of the importanceof solid surfaces, interfacesandinterphasesindeterminingtheusefulpropertiesof polymericcompositions,hasledtothedevelopmentofinversegas chromatography(IGC)asavaluabletechniqueforevaluatingthe potentialforinteractionofdifferentcomponentsofpolymerblends andcompositesandmulticomponentpolymericsystemsin gen-eral.Duetoitsapplicabilityindeterminingthesurfaceproperties ofsolidsinvariedformssuchasﬁlms,ﬁbresandpowdersofboth crystallineand amorphousstructures,IGC hasbeenextensively usedforsurfacecharacterisation.TheapplicabilityofIGCin mea-surementsofphysicochemicalpropertiesofvariousmaterialshas beenthoroughlydescribedintheliterature[15–18].Dataobtained fromIGCexperimentsmay,infavourablecases,correlatedirectly withobservedperformancecriteria,suchascolourdevelopment, gloss,rheologicalproperties,adhesionandmechanicalproperties [19–21].

Twopapershavebeenpublishedthatdealtwiththeanalysisof thethermodynamicpropertiesofPBT(surfacefreeenergy,surface LewisacidityandsurfaceLewisbasicity),onethatconcernsastudy carriedoutonaparticulargradeofPBTandasecondthatrelates tosurfacetensionstudiesonPBT,wherethedispersivecomponent ofthesurfacetensionwasquantiﬁedusingcontactangle measure-ments[22,23].Thespeciﬁccomponentofthesurfacefreeenergy (withoutquantifyingthecontributionoftheLewisacidicsitesand oftheLewisbasicsites)wasdeterminedthereof.

Differencesinend-group“concentrations”alongsidewith dif-ferencesintheaveragemolecularweightofPBTpermitthetailoring of thecontributionof thePBT phase towardstheproperties of thePBT-basedblends,namelytheviscosityandthecrystallinity. Thus,theyinﬂuencetheprocessabilityandmechanicalproperties ofthesepolymericsystems.Forthisstudy,IGCatinﬁnitedilution wasusedas atool by whichtoassess thedifferences between thesurfaceLewisacidic/basicpropertiesoftwogradesofPBT, dif-ferentiatedbytheirvaryingend-group“concentration”.Someof theeffectsthatthedifferencesinend-group“concentration”and molecularweighthaveonthecrystallisationpropertieswerealso examined.

Table2

RelevantcharacteristicsofcommonlyusedIGCprobes.

Probemolecule a(d

l)

0.5 (cm2_(mJ_cm−2₎0.5₎

DN(kJ/mol) AN*(kJ/mol)

n-Hexane 2.21E−16 n-Heptane 2.57E−16 – – n-Octane 2.91E−16 – – n-Nonane 3.29E−16 – – n-Decane 3.63E−16 – – Tetrahydrofuran(THF) 2.13E−16 84.42 2.10 Trichloromethane(TCM) 2.24E−16 0.00 22.68 Dichloromethane(DCM) 1.73E−16 0.00 16.38 Diethylether(DEE) 1.82E−16 80.64 5.88 Acetone(Acet) 1.65E−16 71.40 10.50 Ethylacetate(EtAcet) 1.95E−16 71.82 6.30

Another aspectof relevance to this study wasthe fact that

thespeciﬁccomponentoftheenthalpyofadsorptionofthepolar

probes onthesurface ofPBT A and onthe surfaceof PBT Bis

endothermic.

2. Materialandmethods

2.1. Materials

Twogradesofpoly(butyleneterephthalate),PBTAandPBTB,

werekindlysupplied bySABICInnovativePlastics(formerlyGE

PlasticsEurope),BergenopZoom,TheNetherlands.Theaverage

molecularweight,Mw,molecularweightdistribution,D,

number-averagemolecularweight,Mn,carboxylend-groupconcentration

and hydroxyl end-group concentration of these poly(butylene

terephthalate)sarepresentedinTable1.Thehydroxylend-group

and carboxyl end-group concentrations were determinedusing the –OH absorbance at 3550cm−1 and the –COOHabsorbance at3290cm−1,respectively.ThevaluesindicatedinTable1were obtainedfromthesupplier.Theglasstransitiontemperatureand themeltingtemperatureofbothPBTAandPBTB,are318Kand 503K,respectively(determinedbyDSC).

For the IGC analysis, analytical grade (Sigma–Aldrich Ltd.) probeswereusedwithoutfurtherpuriﬁcation.Theapolarprobes usedweren-heptane,n-octane,n-nonane,n-decane,andthepolar probestetrahydrofuran(THF,basicprobe),theamphotericprobes acetone (Acet) and diethyl ether (DEE), and the acidic probes chloroform (TCM) and dichloromethane (DCM). In Table 2 are summarisedrelevantpropertiesoftheprobemoleculesmentioned [17–21].Thechemicals usedasprobe moleculeswereobtained fromSigma–AldrichLtd.,Poole,UK.Methane(PhaseSeparations Ltd.,Deeside,UK)wasusedasanon-interactingreferenceprobe andthecarriergasusedwashelium(99.999+%purity,BOCGases Ltd.,Guildford,UK).

2.2. Inversegaschromatography 2.2.1. IGCdataprocessing

Themaindifferencebetweenconventionalgaschromatography (GC)andIGCliesinthefactthatthespeciesofprimaryinterestare notthevolatilecomponentsinjectedbutthematerialactingasthe stationaryphase,typicallyapowder,fibreorfilm.Thismaterialmay bepackeddirectlyintothecolumn,coatedontoasuitablesupport orcoatedontothewallsofthecolumn.Thisallowsthe investi-gationoftheinteractivenatureviathedegreeofinteractionwith well-characterisedvolatileliquids/vapours(“probes”). Quantifica-tionofthisinteractionmaybeachievedbythedeterminationof theretentiontime,tr,foragivenprobe.Inmostuses,thequantity ofprobevapourinjectedintothecarriergasisextremelysmall. Thus,theretentiondatarelatetothethermodynamicinteraction

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thatoccursbetweenpolymerandthevapourwhenthepolymeris highlyconcentrated,asinmostpracticalsituations.Furthermore, IGCexperimentsmaybecarriedoutoverappreciabletemperature ranges,sothatthetemperature dependence ofthermodynamic interactionsisnolongerindeterminate.

IGC data processing was carried out according to methods describedintheliterature[15,24,28,32].Theretentiontimesof apo-larprobemoleculesandofpolarprobemoleculesweredetermined atspeciﬁctemperatures,andthevaluesoftheretentionvolume,the energyofadsorption,theenthalpyandentropyofadsorption (dis-persiveandspeciﬁccomponents)oftheprobes,andofthesurface Lewisacidityandbasicityconstants,KaandKb,respectively,were computed.

InIGC,aninertcarriergaselutesaminutequantityofaprobe moleculethroughacolumnthatispackedwiththematerialunder study.Duetotheinteractionsbetweenthetwophases,theprobe moleculesareretainedforacertaintime,tr,whichisusedto cal-culatethenetretentionvolume,Vn,accordingtoEq.(1):

Vn=(tr−t0)F·C·J (1)

Here,Vnistheretentionvolume,t0isthedeadretentiontimeofthe markerprobe,Fisthecarriergasflowrate,Cisacorrectionfactor, allowingforthevapourpressureofthewateratthetemperatureof thebubbleflowmetreusedtodeterminetheflowrate,andJisthe correctionfactorforgascompressibility.Theretentiontimewas determinedusingthegeometrictechniqueoutlinedbyConderand Young[29,30].

Assumingthatexperimentstakeplaceatinﬁnitedilution,the freeenergyofadsorptionoftheprobeonthestationaryphase sur-facepermole,G,canbedeterminedfromtheretentionvolume, Vn,accordingto:

G=−RTln(Vn)+C1 (2)

Here,Ristheidealgasconstant,Tistheabsolutecolumn tempera-tureandC1isaconstant,whichdependsuponthechromatographic columnandthereferencestate[27].ThroughouttheIGCstudies, thestandarddeviationoftheenergyofadsorptionvaluesofthe probemoleculeswascalculatedasbeingtypicallybelow5%.

Considering that the dispersive and speciﬁc components, respectivelyGd_and_Gs_,_are_additive,_as_suggested_by_Fowkes [31],Eq.(2)canberewrittenas:

Gd+Gs=−RTln(Vn)+C1 (3)

Thefreeenergyofadsorptioncanberelatedtoadhesionwork, Wa,accordingto[27]:

−G=N·a·Wa (4)

Here,NisAvogadro’snumberand“a”thecross-sectionalareaof theprobetobetested(Table2).

Ifnonpolarcomponents(suchasn-alkanes)areused,only dis-persiveinteractionsoccurandtheadhesionworkisgivenby: Wa=2(sdld)

1/2

(5) Here,d

s and ld are,respectively,thedispersivecomponentsof surfacetensionofthesolid(stationaryphase)andofthe probe-molecule.

ReplacingEqs.(4)and(5)inEq.(2)leadsto: 2N(sd)

1/2

a(_ld)1/2+C1=RT ln(Vn) (6)

Theslopeofthestraightline,referredtoasthereferenceline, obtainedbyplottingRTln(Vn)versus2aN(_ld)1/2,forahomologous n-alkaneseries(Fig.4),leadstothedeterminationofd

sforagiven temperature.

Acid–base characteristics of surfaces were determined by analysingtheinteractionofthepolarprobeswiththesolid sur-faceandquantifyingthedeviationfromthereferenceline,leading totheestimationofthespeciﬁcfreeenergy,Gs_,_as:

−Gs₌_RT_ln(V

n)−RTln(Vnref) (7)

Here,Vn_ref istheretentionvolumeestablishedbythen-alkanes referenceline(Eq.(1)),Vnbeingnowtheretentionvolumeofthe polarprobes.ThiscalculationisalsoillustratedinFig.4.

Theadhesionworkbetweenthepolarprobestestedandthe solidsurface,Ws

a,canbeevaluatedfromthespeciﬁcfreeenergy, givenbyEq.(7),as Was= _NRT_·_aln

Vn Vnref

(8) Bycarryingoutexperimentsatdifferenttemperatures,itwas possibletodeterminetheenthalpyofadsorptionandtheentropy ofadsorption,respectivelyHandS,fromplotsofG/Tversus 1/T,(Fig.6),accordingtothefollowingequation:

G T =

H

T −S (9)

Theacidicandbasicconstants,respectivelyKaandKb,were cal-culatedfromtheplotof−Hs_/AN*_versus_DN/AN*,_according_to_Eq. (10)[26](Fig.8).

(−Hs₎ AN∗ =Ka

DN

AN∗+Kb (10)

Here, AN*and DN are,respectively, theGutmann’s modiﬁed acceptoranddonornumbersoftheprobestested[25,29](Table2). 2.2.2. IGCexperimentalset-up

InexperimentalworkinvolvingIGC,conventional GC equip-mentisgenerallyused,withsomeadaptations[15].

TheinstrumentusedwasaFisonsGC9100unit(FisonsScientific EquipmentLtd.,Loughborough,UK),equippedwithaFIDdetector. Themarkerprobeusedwasmethane.Typically,thesyringewas filledwith0.1␮lofeachprobe,flushed10times,inordertoensure thecreationofaHenry’sinfinitedilutionregion,andinjected man-ually.Theinjector washeatedto150◦Cand theFIDdetectorto 180◦C.Theattenuationwassetto1.Theflowratewascontrolled usinganeedlevalvepressureregulatoranddeterminedusinga bubbleflowmetrethatwasequippedwithaheliumtrap[33]and thermometer.Theinletpressure,Pi,wasmeasuredusingapressure gaugeandtheatmosphericpressure,Po,wasobtainedthroughthe BritishAtmosphericDataCentre(www.badc.rl.ac.uk).TheIGCunit waskeptoncontinuouslyduringtheentirecourseofthework.The temperaturewasincreasedincrementally.

Duetothelackofanappropriatesolvent,insteadofcoatinga supportmaterial,thepolymerwasusedasreceived,after grind-ingandsievingtoachieveanappropriateparticlesize.Tothisend, thepolymerparticleswereprocessedinacryogenicgrinderwhile beingcooledwithliquidnitrogen,followedbysievingthematerial through125␮mand250␮mfiltergauzesandthecolumnfilled. Thesievingoperationexcludedfineparticlesthatwouldincrease undesirablythepressuredropinthecolumn.

2.3. Differentialscanningcalorimetry

Theinstrumentusedwasa DSC2010DSCalorimeter. Allthe studieswerecarriedoutunderanitrogenﬂowrateof200cm3_/min. Thetypicalsamplemasswas3–9mg.Fortheheatingmode anal-ysis,thesampleswereheatedatarateof10◦C/min,from50◦C to440◦C.Thenon-isothermal crystallisationbehaviourof tape-extruded blends was studied using the following temperature programme, (i) heating thesample from roomtemperature to

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0 5 10 15 20 25 30 35 40 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

303 K

313 K

-Δ

G

a

(

kJ

/m

ol)

Carrier

ga

s flo

w ra

te (cm

3

/min)

Fig.2.Inﬂuenceofthecarriergasﬂowrateontheenergyofadsorptionofn-decane onthesurfaceofPBTB.

250◦Cat200◦C/min,(ii)keepingthesampleatthistemperature foroneminute,toreleaseallthestresseswithinthematerialand toerasethethermalhistoryand(iii)coolingthesamplefrom250◦C ataconstantrate(8◦C/min)to162◦C.Inpractice,thesamplewas heldat250◦Cfor2minand40s,duetothetemperature equilibra-tionstagethatwaspriortotheisothermalstep.Thenon-isothermal crystallisationpropertiesthatweredeterminedwerethe crystalli-sationtemperature,Tc,andtheenthalpyofcrystallisation,−Hc.

3. Resultsanddiscussion

3.1. Inversegaschromatography

Carriergasflowrates,rangingfrom3cm3_/min_to₃₅_cm3_/min, wereusedtoassesstheinfluenceoftheflowrateonthe reten-tiontimesofn-decane,at303Kand313K,onthesurfaceofPBTB. Thispreliminarystudyisanessentialpre-requisiteinsurface ther-modynamiccharacterisationbyIGC,inordertoensurethatsurface adsorption,andnotbulkabsorption,isthedominantphenomenon. TheresultsarepresentedinFig.2.

Fig.2indicatesthatthecarriergasflowratedoesnotinfluence theadsorptionenergyofn-decanetoasignificantextent.Thus,it canbeconcludedthatsurfaceadsorptionisthedominant mecha-nism,andabsorptionoftheprobemoleculesintothebulkofthe polymercanbeneglected,iftemperatureslowerthantheTgare used,fortheflowraterangestudied.Thus,themeasurement tem-peratureusedinthestudyofthesurfacePBTwasvariedbetween 298Kand318K,inincrementsof5K,underacarriergasflowrate of10cm3_/min.

Inordertofurtherconﬁrmthatsurfaceadsorptionisthe gov-erningmechanismofinteractionsbetweenthepolymerandthe probes,andthattheexperimentalconditionschoseninthisstudy (ﬂowrateandtemperature)aresuitableforsurface characterisa-tion,aretentiondiagramwasdeveloped(Fig.3)byplottingLn(Vg) asafunctionof1/T[34].

InFig.3,nothermalchanges,duetothechangeinthe morphol-ogyofthesemi-crystallinepolymerastemperatureincreases,are observed.Thelinearrelationshipsobservedareanindicationofthe establishmentoftheequilibriumbetweentheprobesandthe sur-faceofPBTduetothehomogeneityofthesurface.Itcanthusbe concludedthatsurfaceadsorptionisthegoverningphenomenon.

Fig.3.RetentiondiagramfortheapolarprobesonthesurfaceofPBTB.

1.50E-016 2.00E-016 2.50E-016 3.00E-016 3.50E-016 4.00E-016

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 C7 C8 C9 C10 TCM DCM Acet THF EtAcet Y = -22.56 + 7.98E16X R = 1.00

-ΔG

_as

RT

Ln(V

g

) [kJ

/m

ol]

a

√(γ

l d

) [

cm

2

(mJ/cm

2

)

0.5

]

Fig.4.Surfacefreeenergyofadsorptionversusa

d

l,forthesurfaceadsorption

ofn-alkanes,andpolarprobes,onPBTA,at295K.

3.1.1. DispersivecomponentofthesurfacefreeenergyofPBTA andofPBTB

Fig.4illustrates thedeterminationof thedispersive compo-nentofthesurfacetensionofPBTA,at295K,accordingtoFowkes’ approach.

Table3summarisesthevaluesdeterminedforthedispersive component of thesurface tension of PBT A and of PBT B. The valuespresentedindicatethatthedispersivecomponentofthe sur-facetensionremainsconstant,withinexperimentalerror,forthe Table3

ValuesofthedispersivecomponentofthesurfacetensionofPBTAandofPBTB.

PBTA PBTB T(K) d s(mJ/m2) R2 sd(mJ/m2) R2 295 43.9±2.1 1.00 42.4±1.0 1.00 303 40.7±1.9 0.98 42.5±2.8 0.99 308 42.8±2.4 1.00 41.4±2.6 1.00 313 40.0±0.6 1.00 40.7±1.3 1.00 318 40.7±2.1 1.00 44.7±9.9 0.92

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TCM DCM Acet THF EtAcet 0 2 4 6 8 10 12

RT

L

n

(V

s

) (kJ/m

g

ol)

Probe

PBT

A

PBT B

Fig.5.Comparisonofthespeciﬁccomponentvaluesoftheenergyofadsorptionof thepolarprobesonthesurfaceofPBTAwiththoserelatingtothesurfaceofPBTB, at295K.

temperaturerangestudied,andequalisto41.6±1.7mJ/m2 _and

42.3±1.5mJ/m2_,_for_PBT_A_and_PBT_B,_{respectively.}_The_values_of

d

s forPBTAandforPBTBarepracticallyidentical.Thisleadsto

theconclusionthatthedifferentend-groupcomposition(Table1)

doesnotinﬂuencetoanoticeableextent,thedispersivecomponent ofthesurfacetension.Bearinginmindthatthemajordifference betweenPBTAandPBTBisintheOHend-groupconcentration, thevaluefoundford

s conﬁrmstheobservationsfoundin litera-ture[35]withregard tothefactthatthisfunctionalgroup,asa high-energysite,contributesmainlytotheformationofspeciﬁc intermolecularinteractions.Thevaluesfoundareinaccordwith surfacetensionstudiesonPBTbasedoncontactangle measure-ments[23].

3.1.2. AdsorptionofpolarprobesonPBTAandonPBTB InFig.5arecomparedthevaluesof−Gs

aforthepolarprobes onthesurfaceofPBTAwiththoseconcerningthesurfaceofPBTB, at295K.TheretentiontimesforDEEonthesurfaceofPBTAwere verylowand,therefore,pronetolargeexperimentalerrors. Con-sequently,thedeterminationoftheenergyofadsorption,andof thespeciﬁccomponentoftheenthalpyofadsorptionofDEE,was notpossible.Thislowretentiontimeprobablyresultsfromsteric hindrance,thatis,structuralrestrictions,fromboththeadsorbate andtheadsorbent,hinderingthesemoleculesfromspatial con-formationsbecomingeffectiveintermsofspeciﬁcintermolecular interactions.Theoxygenatom(Lewisbasiccentre)ofdiethylether (DEE)ismoresusceptibletoshieldingbytheneighbouring hydro-genatoms[36]thanisthatof,e.g.acetone(Acet),whichisreadily accessibleforinteraction.Similarresultshavebeenobservedwhen CCl4wasusedasaprobe[37].

ForbothPBTAandPBT B,theadsorptionoftheLewisacidic probesisasstrongas,orstrongerthan,theadsorptionofLewis basic/Lewisamphotericprobes.Bearinginmindtherelativelylow acidityoftheacidicprobes,whencomparedtothebasicityofthe basicprobes(e.g.THF),itcanbeconcludedthatthesurfacesofboth PBTAandPBTBareamphoteric,beingstronglyLewisbasic.Fig.5 alsoshowsthatthesurfaceofPBTAiscomparabletothatofPBTBin termsofLewisacidity/basicity.Ananalysisofthespeciﬁc compo-nentoftheenthalpyofadsorptionwillprovideabetterinsightinto thedifferencesbetweentheLewisacidic/basicpropertiesofthe twoPBTs.Thevaluesoftheenthalpy,andentropy,ofadsorption ofthepolarprobes,alongwiththecorrespondingdispersiveand

Table4

Dispersivecomponentsoftheenthalpyofadsorptionandoftheentropyof adsorp-tion,Hd_a_,_and_Sd_a_,_{respectively,}_of_the_polar_probes,_on_the_surface_of_PBT_A.

Probemolecule −Hd a(kJ/mol) Sda(J/molK) R2 TCM 33.0 −127.0 0.91 DCM 26.2 −120.0 0.81 Acet 27.1 −121.0 0.83 THF 31.8 −126.0 0.89 EtAcet 29.7 −124.0 0.87 Table5

Speciﬁccomponentsoftheenthalpyofadsorptionandoftheentropyofadsorption, Hs

aandSda,Ssa,respectively,ofthepolarprobes,onthesurfaceofPBTA. Probemolecule −Hs_a_(kJ/mol) _Ss_a_(J/mol_K) _R2

TCM −38.4 147.0 0.94

DCM −23.1 138.0 0.82

Acet −20.9 101.0 0.89

THF −23.0 93.7 0.81

EtAcet −17.2 85.8 0.92

speciﬁccomponents,aresummarisedinTables4and5forPBTA,

andtheirdeterminationisillustratedinFig.6forthispolymer.The valuesrelatingtoPBTBhavealreadybeenreportedinaprevious publication[22].

The specific component of the enthalpy of adsorption of polarprobesonthesurfaceof PBT Ais endothermic. This con-firms the reported thermodynamic analysisof the surface and bulk PBT B [22]. Endothermic values of energy of adsorption have alsobeen reported in theliterature for theadsorption of polar probe molecules on titanium dioxide pigments [37,38], on a vinyl acetate-vinyl alcohol copolymer [39], and on 2-(N-morpholino)ethylmethacrylate [40]. A rearrangement of the surfaceuponthechemisorptionofpolarmoleculesisthoughtto occur,thusinitiatinganincreaseintheentropyofthesystem.This rationaleisconfirmedbythepositivevaluesofSs

a,andsupported bytheexperimentallyveriﬁedphenomenonoftherearrangement ofthesurfacelayersduetochemisorption[41].

The enthalpy of formation of individual hydrogen bonds is alwaysnegative.However,thetotalenthalpy(andentropy)related totheformation of hydrogenbondsis theresult of three con-tributions[42]:apositivecontributionthatresultsfrombreaking hydrogenbondsintheself-associatingpolymer,anegative con-tributionthatresultsfromforminghydrogenbondsbetweenthe self-associatingpolymerandtheadsorbentmolecule,and contrib-utionsfromvanderWaalsanddipoleforces.Whenpolarmolecules

0.0031 0.0032 0.0033 0.0034 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 DCM TCM Acet THF EtAcet -Δ Ga s /T [kJ /(m ol .K)] 1/T [K-1]

Fig.6. Determinationofthespeciﬁccomponentoftheenthalpyandoftheentropy ofadsorption,ofpolarprobesonthesurfaceofPBTA.

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TCM DCM Acet THF EtAcet 0 5 10 15 20 25 30 35 40 45 50 55 60

H

s

(kJ/mol)

a

Probe

PBT A

PBT B

Δ

Fig.7. Comparisonofthespeciﬁccomponentvaluesoftheenthalpyofadsorption ofthepolarprobesonthesurfaceofPBTAwiththoserelatingtothesurfaceofPBT B.

Table6

ValuesofKaandKbdeterminedforthesurfacesofPBTAandPBTB.

Ka Kb R2

PBTA −0.24 −0.97 0.97

PBTB −0.49 −0.96 0.96

adsorbonPBT,thedominantfactoristhepositivecontribution

aris-ingfromthebreakingofhydrogenbondsintheself-associating

polymer.Thisresultsinanendothermicenthalpyofadsorptionand

anincreaseintheentropyofthesystem[22].

Itshouldbenoticed(Tables4and5)thatthedispersive compo-nentoftheenthalpyofadsorption,Hd

a,isexothermic,thatthe correspondingchangeinentropy,Sd

a,negative,andthatthe val-uesaresimilarformostofthepolarprobes.Furthermore,thevalue of−Hd

aincreaseswithincreasinga×

d l.

InthecaseofPBTA,ananalysisofthespecificcomponentof theenthalpyof adsorption, leadstothefollowing ranking (the more negative, the stronger the interaction, as it is endother-mic):TCM>DCM=THF>Acet>EtAcet.ThisindicatesthatPBTAis amphotericandKboughttobehigh,inlinetheanalysisofthe spe-cificcomponentoftheenergyofadsorptionofthepolarprobes. Thechangeinentropyuponadsorptionisgreaterforthoseprobes whoseadsorptionischaracterisedbythegreaterspecific compo-nentoftheenthalpyofadsorption.Theanalysisof−Hs

aforPBT Aisalsoinlinewiththeanalysisoftheenergyofadsorption,and withtheresultsrelatingtothesurfaceandbulkPBTB[22].

Fig.7givesa comparisonof thevaluesofHs

a of thepolar probesonthesurfaceofPBTAwiththoseconcerningthesurface ofPBTB.

Fig.7suggeststhatthesurfaceofPBTAshouldbelessLewis acidicandlessLewisbasic,thanthatofPBTB.Thedetermination ofKaandofKbprovidesamorecompletedescriptionofthesurface Lewisacidity/basicity.

3.1.3. DeterminationofKaandKbforthesurfaceofPBTAandfor thesurfaceofPBTB

ThedeterminationofthesurfaceLewisacidityconstant,Ka,and ofthesurfaceLewisbasicityconstant,K_b,isillustratedinFig.8for PBTA.ThevaluesobtainedforbothPBTAandPBTBaresummarised inTable6. -30 -25 -20 -15 -10 -5 0 0 5 10 15 20 25 30 35 40 45 50 DCM THF Acet EtAcet TCM TCM

DN/AN*

Y = -0.97 - 0.24X R2 = 0.97

-Δ

H

a s

/AN

*

Fig.8.DeterminationoftheKaandoftheKbofthesurfaceofPBTA.

ItshouldbenotedthattheKaandKbvaluesarenegativedueto theendothermicadsorptionoftheprobemolecules.Thesurfacesof PBTAandPBTBareconcludedtobeLewisamphotericandstrongly Lewisbasic,inlinewiththeanalysisoftheenergyandenthalpyof adsorptionofpolarprobes.ThevaluesdeterminedforKaandKb areconsistentwiththeanalysisofthestructureofthismolecule (Fig.1).Thus,theLewisacidicsiteshavetheirorigininthehydrogen atomsofthe–O–CH2–segments,andinthehydrogenatomsofthe hydroxylend-groupand ofthecarboxylicend-group.TheLewis basicsiteshavetheiroriginsintheestermoiety,andintheoxygen atomsofthecarboxylicend-groupandofthehydroxylend-group [43].

The poly(butyleneterephthalate), PBT A, is significantly less LewisacidicthanPBTBandasLewisbasicasPBTB.Thiscanbe rationalisedonthebasisofananalysisofthecarboxylend-group andhydroxylend-groupconcentrationsinbothPBTAandPBTB, asdeterminedbyFTIR(Table1).Thehydroxylend-group concen-trationofPBTAislowerthanthatofPBTB,whichisreflectedin theloweracidiccharacterofPBTA.Thecarboxylend-group con-centrationdifferencesbetweenPBTAandPBTBdonotinfluence significantlytheLewisbasicity,asquantifiedbyKb.Thisobservation arisesfromthedominantcontributionoftheesterfunctionalityto thesurfacebasicity,alongsidewiththecontributionofthebasic oxygenatominthehydroxylgroups.ThefactthattheKbvaluefor thetwoPBTsdoesnotfullycorrelatewiththevalueofthespecific componentoftheenthalpyofadsorptionofDCMandofTCMmay berelatedtotherelativelylowvalueofAN*forDCMandTCM,when comparedtothevalueofDNfortheremainingpolarprobes.This resultstresses(i)theimportanceofusingasmanypolarprobesas possibleinthedeterminationofKaandKb,and(ii)theneedfora systematicandholisticanalysisofGs

a,−Hsa,KaandKb,in con-trasttoanindividualanalysisofthesethermodynamicparameters (asisusuallythecaseintheIGCliterature).

3.2. StudyofthecrystallisationpropertiesofPBTAandPBTB ThevaluesofKaandKbforbothPBTAandPBTBindicatethat theintramolecularinteractionsandtheintermolecularinteractions betweenPBTmoleculesarestrong.Thismoleculeischaracterised bythenon-existenceofbulkyside-groups,bythechemically regu-larstructure,andbyhighmobility(duetothebutyleneunitinthe chain).Thus,boththethermodynamicandthestructural require-mentsforcrystallinityexistinthecaseofPBTmolecules.

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0 20 40 60 80 100 450 455 460 465 470 475 480

T

c

(K)

Weight fraction of PBT A (%)

Fig.9.Non-isothermalcrystallisationtemperatureversusPBTAloadinginsamples ofPBTgranulate.

Toassesstheinﬂuenceoftheend-grouptypeand concentra-tion,and of molecularweightdifferences onthecrystallisation propertiesof PBT, a seriesof PBT A plusPBT Bgranulate sam-pleswaspreparedwithincreasingamountsofPBTA.Thevalues determinedforthecoldcrystallisationtemperature,Tc,andforthe enthalpyofcrystallisation,−Hc,ofthesesamplesarepresented inFigs.9and10,respectively.

ThecrystallinitydegreeofPBTAiscalculatedas56%andthat ofPBTBas65%(consideringthevalueof142J/gforthe−Hcof 100%crystallinePBT).Theextentofcrystallisationdecreases lin-earlywithincreasingamountofPBTAinthePBTgranulatesample. ThegreaterextentofcrystallinityofPBTBisinterpreted,inview ofthevalues determinedforKaand Kb,tobea consequenceof greaterstrengthofspeciﬁcLewisacid/baseintermolecular inter-actionsthan in PBT A. Thisis in accordance withthe fact that highenergy sitesin polymericmaterials(asassessedby IGCat inﬁnitedilution)arewellknowntoactasnucleating spotsthat initiatethecrystallisationprocess[44,45].Thedifferencesinthe averagemolecularweightofPBTAandPBTBalsoplayaroleinthe crystallisationpropertiesofthispolymer.Lowermolecularweight valueswouldbeexpectedtoleadtogreatervaluesofcrystallinity degree.However,inthisstudyitwasnotpossibletodistinguishthe

0 20 40 60 80 100 76 78 80 82 84 86 88 90 92 94

-H

c

(J/g)

Weight fraction of PBT A (%)

Δ

Fig.10.EnthalpyofcrystallisationversusPBTAloadinginsamplesofPBTgranulate.

relativecontributionofmolecularweightdifferencesandof end-grouptypeandconcentrationdifferences.Thisisduetothefact thatthegreater–OHend-groupconcentrationisdirectlyrelated withtheloweraveragemolecularweightofthispolymer.

Fig.9alsoshows,veryinterestingly,thatthenon-isothermal crystallisationtemperatureisnotsignificantlyinfluencedbythe weightfractionofPBT Ainthesample.Thus,differencesinthe end-grouptypeandconcentrationandintheaveragemolecular weightofthesePBTsdonotcausechangesinthecrystallisation activationenergy.InviewofthevaluesfoundforKaandKb,one canarguethatthecrystallisationactivationenergyis,inthiscase, dominantlydefined bytheenergyof theLewisbasicsites.This observationderivesfromthesignificantlygreatervalueofKbwhen comparedtothevalueofKa,forbothPBTAandPBTB,andfromthe similarvalueofKbforthesegradesofPBT.

4. Conclusions

TheabilityofPBTtointeractthroughdispersiveintermolecular forcesisnotinﬂuencedbytheend-grouptypeandconcentration orbythemolecularweight.Theenthalpyofadsorptionofpolar moleculesonPBTwasconﬁrmedtobeendothermic. Thisresult wasinterpretedasthedominantcontributiontotheenthalpyand entropyof adsorptionof breakinghydrogenbondsin the poly-mer,comparedwiththecontributionofforminghydrogenbonds betweenthepolarprobemoleculesandthePBTmolecules.

IGC characterisation of the surface Lewis acidic/basic prop-erties ofPBT A and of PBT Bcorrelateswell withthecarboxyl end-groupconcentrationandhydroxylend-groupconcentration, asdeterminedbyFTIR.Furthermore,theanalysisofthesurface LewisacidityandsurfaceLewisbasicity,asquantiﬁedbyKaand KbanddeterminedbyIGC,alongsidewiththeanalysisofthe phys-icalstructureofthesepolymers,isusefulintheinterpretationof thecrystallinityand,thus,oftheexcellentsolventresistanceand thermalstabilityofPBT.

Thedifferencesintheend-grouptypeandconcentration,and intheaveragemolecularweight,betweenPBTAandPBTB,donot causedifferencesinthecrystallisationactivationenergy,andthus, inthecrystallisationtemperature.Thisevidenceisinterpretedin termsofdominantcontributionoftheLewisbasicsitestothe crys-tallisationactivationenergy.Theextentofcrystallisationislower forPBTAthanitisforPBTB.ThisisaconsequenceofweakerLewis acid/baseintramolecularandintermolecular interactionsand of greatermolecularweightinthecaseofPBTA.

Acknowledgements

Theauthorswishtoacknowledgethesupportandthe collabora-tionofSABICInnovativePlastics,BergenopZoom,TheNetherlands, inthisproject.

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