Containing Palladium Nanoparticles for Active Packaging Applicati

19 Pages·2017·3.6 MB·English

nanomaterials
Article
Electrospun Oxygen Scavenging Films of
Poly(3-hydroxybutyrate) Containing Palladium
Nanoparticles for Active Packaging Applications
AdrianeCherpinski1,MelikeGozutok1,HilalTurkogluSasmazel2 ID,SergioTorres-Giner1 ID
andJoseM.Lagaron1,* ID
1 NovelMaterialsandNanotechnologyGroup,InstituteofAgrochemistryandFoodTechnology(IATA),
SpanishCouncilforScientificResearch(CSIC),CalleCatedráticoAgustínEscardinoBenlloch7,
46980Paterna,Spain;[email protected](A.C.);[email protected](M.G.);
[email protected](S.T.-G.)
2 DepartmentofMetallurgicalandMaterialsEngineering,AtilimUniversity,Incek,Golbasi,
06830Ankara,Turkey;[email protected]
* Correspondence:[email protected];Tel.:[phone];Fax:[phone]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:29May2018;Accepted:21June2018;Published:27June2018
Abstract: Thispaperreportsonthedevelopmentandcharacterizationofoxygenscavengingfilms
made of poly(3-hydroxybutyrate) (PHB) containing palladium nanoparticles (PdNPs) prepared
by electrospinning followed by annealing treatment at 160 ◦C. The PdNPs were modified with
the intention to optimize their dispersion and distribution in PHB by means of two different
surfactants permitted for food contact applications, i.e., hexadecyltrimethylammonium bromide
(CTAB)andtetraethylorthosilicate(TEOS).Analysisofthemorphologyandcharacterizationofthe
chemical,thermal,mechanical,andwaterandlimonenevaporbarrierpropertiesandtheoxygen
scavengingcapacityofthevariousPHBmaterialswerecarriedout. Fromtheresults,itwasseen
that a better dispersion and distribution was obtained using CTAB as the dispersing aid. As a
result,thePHB/PdNPnanocompositescontainingCTABprovidedalsothebestoxygenscavenging
performance. Thesefilmsofferasignificantpotentialasnewactivecoatingorinterlayersystemsfor
applicationinthedesignofnovelactivefoodpackagingstructures.
Keywords: polyhydroxyalkanoates;palladiumnanoparticles;packaging;electrospinning
1. Introduction
Animportantincreasingquantityofplasticwasteisbeinggeneratedyearlyforwhichtheprecise
neededtimeforitsbiodegradationiscertainlyunknown. Thisenvironmentalawarenesshasdriven
thedevelopmentandimprovementofnewbiodegradablepolymers,especiallyforsingle-useplastic
items [1]. In this sense, polyhydroxyalkanoates (PHAs) are well-known biopolymers that can be
producedmicrobiallybyavarietyofmicroorganismsasanenergystoragemechanism. Theyexhibit
similarperformanceintermsofmechanical,thermal,andbarrierpropertiesthanpetroleum-derived
polymersand,thus,theycanpotentiallyreplaceconventionalthermoplastics(e.g.,polyolefins)ina
widerangeofapplications[2]. Inparticular,barrierpropertiesareoffundamentalimportanceforfood
packagingapplications. Forinstance,therearemanyfoodproductsthatareverysensitivetooxidation
and,toovercomethisissue,packageswithreducedoxygenpermeabilityaredesirable. Additionally,
thewaterresistanceisalsoimportant,particularlyforplasticmaterialsintendedfordirectcontactwith
highmoisturefoodstuffaswellasmaterialstobeappliedinhighhumidityconditionsduringstorage
and/ortransport[3].
Nanomaterials2018,8,469;doi:10.3390/nano8070469 www.mdpi.com/journal/nanomaterials
Nanomaterials2018,8,469 2of19
Poly(3-hydroxybutyrate)(PHB)iscurrentlythemostcommonrepresentativeofPHAsandthis
biopolymerhasbeenproposedforshort-termfoodapplications[4].However,PHBisabrittlepolymer,
as its enzymatic polymerization leads to the formation of macromolecules with highly ordered
stereochemicalstructures,resultinginalargedegreeofcrystallinity[5]. Oneofthegreatadvantages
of PHB over many other biodegradable polymers is its biodegradability under aerobic as well as
anaerobicconditions[6]. Forthisreason,PHBandtheirblendswithotherbiopolymers,forinstance
polylactide(PLA),havebeenextensivelyinvestigatedforfoodpackagingapplications[7–9].Oneofthe
potentialapplicationfieldsofthesematerialsisthedevelopmentoffilmsforpackagingapplications.
Asanexample,Zhangetal.[10]studiedPHB/PLAblendsindifferentratiosandconcludedthatby
blending PLA with 25 wt. % of PHB some interactions between both biopolymer matrices can be
achieved. Furthermore,theirresultsalsoshowedimprovedmechanicalproperties.
Palladium nanoparticles (PdNPs) are well known by their ability to dissociate hydrogen
molecules to single atoms. This fact is further enhanced due to its nano-sized form and resultant
highsurface-to-volumeratio[11].Ithasbeendemonstratedthattheoxygenscavengingactivityof
palladium-basedoxygenscavengingfilmsisstronglydependentonthecoatingsubstrateaswellason
thepalladiumdepositionthickness.Optimizationoftheseparameterscanresultinactivescavenging
filmswheretheresidualheadspaceoxygenofpackagedfoodscanbescavengedveryquickly[10].
Thereisadrivetofindwaystoincorporateactivepackagingtechnologiesdirectlyintothepackage
walls. In spite of the advantages that they offer in maintaining quality and extending shelf life,
suchsystemsarestilllittleused. Thereasonstemsfromtheadditionalcostinvolved,thepotential
toxicity of the added scavenger in the food contact layer, and even more so because of the lack of
sufficienttechnicalinformationontheirperformanceandthelackofunderstandingofhowtoapply
themeffectively[12].
Electrospinningisafiberproductionmethodthatemployshighelectricforcestodrawcharged
threadsofpolymersolutionsormeltsuptofiberdiametersbelow100nm. Itisalowstartupcost
processinwhichawidevarietyofbothpolymerandnon-polymermaterialshavebeenusedtoform
matscomposedofnanofiberswithahighsurfacearea-to-volumeratio[13].Theelectrospinningprocess
hasawidevarietyofapplicationssuchasmedical,filtration,tissueengineering,foodengineering,
packaging,etc.[14–16]. Untilnow,thisprocessingtechnologyremainedtoalaboratoryscale. However,
recentdevelopmentsininstrumentationandprocessaiddesignhaveallowedthisprocesstobescaled
to achieve the production volumes required in certain industrial commodity applications such as
fortifiedfoodsandactivepackaging[17].
Inactivepackaging,nanotechnologyhasasignificantpotentialbecausenanostructuresdisplay
ahighsurface-to-volumeratioandspecificsurfaceproperties. Consideringthehighsurfaceenergy
ofnanoparticles,whichtendtoagglomerateandtopreventthisaggregation,eitherpolarpolymers
orsurfactantscanbeusedasprotectiveagentsandstabilizersofthenanoparticles. Thisisextremely
necessarytoobtainmono-disperseduniformparticlesandtobe,thereafter,usedinvariousapplication
purposes[18–20]. Theobjectiveofthepresentstudywastoprepareandcharacterize,forthefirsttime,
PHBfilmsbytheelectrospinningprocessincorporatingPdNPs. Inordertoimprovethedispersionof
thePdNPsinthePHBmatrix,differentsurfactantsweretested.
2. MaterialsandMethods
2.1. Materials
BacterialaliphatichomopolyesterPHBwassuppliedbyBiomer(Krailling,Germany)asP226F.
According to the manufacturer, this is certified both as compostable and food contact grade,
presentingadensityof1.25g/cm3 andameltflowrate(MFR)of10g/10minat180 ◦Cand5kg.
The weight-average molecular weight (M ) estimated by the manufacturer was 500 kDa and the
w
polydispersityindex(PDI)was2.
Nanomaterials2018,8,469 3of19
2,2,2-trifluoroethanol(TFE)with99%purityand -limonenewith98%puritywerebothpurchased
D
fromSigma-AldrichS.A.(Madrid,Spain). Thetwotestedsurfactants,hexadecyltrimethylammonium
bromide(CTAB),with99%purity, andtetraethylorthosilicate(TEOS),with98%purity, aswellas
palladium(Pd)nano-powder,<25nmparticlesizemeasuredbytransmissionelectronicmicroscopy
(TEM)and≥99.5%tracemetalsbasis,werealsopurchasedfromSigma-AldrichS.A.Allproductswere
usedasreceivedwithoutfurtherpurification.
2.2. Electrospinning
A PHB solution for electrospinning was prepared by dissolving the biopolymer at 10 wt. %.
The PdNPs were added at 1 wt. % in relation to PHB to the solution. To improve the dispersion
ofthePdNPs,CTABandTEOSwerealsoaddedasdispersingaidsat0.5wt.%withthePHBand
PdNPsmixture.
Electrospinning was performed using a Fluidnatek® LE50 benchtop line from Bioinicia S.L.
(Valencia,Spain)withavariablehigh-voltage0–30kVpowersupply. Thisdevicewasequippedwitha
motorizedinjectorthatwasscanningtowardsametalliccollector,aimingtoobtainahomogeneous
electrospundeposition.Thedifferentbiopolymerssolutionsweretransferredtoa30-mLplasticsyringe
andthesyringewasconnectedthroughpolytetrafluoroethylene(PTFE)tubestoastainless-steelneedle
(Ø=0.9mm)whereastheneedletipwasconnectedtothepowersupply. Thesolutionwaselectrospun
for2hunderasteadyflow-rateof6mL/husingamotorizedinjector,scanninghorizontallytowardsa
metallicgrid. Thedistancebetweentheinjectorandcollectorwasoptimalat15cmandthevoltage
wassetat15kV.Thebiopolymersolutionswereelectrospuninacontrolledenvironmentalchamber
at 23 ◦C and 40% relative humidity (RH), for a given processing time and in optimal conditions
toachievesteadyfiberformation. Theprocessingandsolutioncharacterizationparametersforthe
optimalelectrospinningofthisPHAgradeweredeterminedandoptimizedinapreviouswork[21].
Finally,theobtainedelectrospunmatsweresubjectedtoanannealingstepbelowthebiopolymer’s
meltingpointwithoutapplyingpressureusingahydraulicpress4122-modelfromCarver,Inc.(Wabash,
IN,USA).Theannealingwasfoundoptimalat160◦C,withoutpressure,for5±1s,basedonour
previousstudy[21].Theresultantfilmswereaircooledatroomtemperature.Priortothermaltreatment,
theelectrospunmatswereequilibratedinadesiccatorat25◦Cand0%RHbyusingsilicagelforat
least1week.
2.3. Characterization
2.3.1. FilmThickness
FilmthicknesswasmeasuredwithadigitalmicrometerseriesS00014,having±0.001mmaccuracy,
fromMitutoyoCorporation(Kawasaki,Japan)atthreerandompositions. Thepost-processedsamples
hadathicknessoftypically55±4µm.
2.3.2. ScanningElectronMicroscopy
A S-4800 microscope from Hitachi (Tokyo, Japan) was used to observe by scanning electron
microscopy (SEM) the morphology of the electrospun PHB fibers and the film cross-sections and
surfaces. Cross-sectionsofthesampleswerepreparedbycryo-fractureoftheelectrospunPHBfilms
usingliquidnitrogen.Then,theywerefixedtobeveledholdersusingconductivedouble-sidedadhesive
tape,sputteredwithamixtureofgold-palladiumundervacuum,andobservedusinganaccelerating
voltageof5kV.ImageJLauncherv1.41softwarewasusedtodeterminetheaveragefiberdiameter
andstandarddeviationbymeasuringthediameterofatleast100fibers.
2.3.3. TransmissionElectronicMicroscopy
ThemorphologyanddistributionofthePdNPswasstudiedusingaJEOL1010fromJEOLUSA,
Inc. (Peabody,MA,USA)byTEMusinganacceleratingvoltageof80kV.
Nanomaterials2018,8,469 4of19
2.3.4. DifferentialScanningCalorimetry
ThermalpropertiesoftheneatelectrospunPHBfibersandfilmsandofthemultilayersystems
wereevaluatedbydifferentialscanningcalorimetry(DSC)usingaPerkin-ElmerDSC8000(Waltham,
MA,USA)thermalanalysissystemundernitrogenatmosphere. Themeasurementwascarriedouton
~3mgofeachsampleusingatwo-stepprogramfrom0to200◦Cfollowedbyasubsequentcooling
downto−50◦C,bothataheatingrateof10◦C/min. TheDSCequipmentwaspreviouslycalibrated
withindiumasastandardandtheslopeofthethermogramswascorrectedbysubtractingsimilar
scansofanemptypan. Testsweredone,atleast,intriplicate.
2.3.5. ThermogravimetricAnalysis
Thermogravimetric analysis (TGA) was performed in a TG-STDA Mettler-Toledo model
TGA/STDA851e/LF/1600analyzerfromMettler-Toledo,LLC(Columbus,OH,USA).Thesamples,
with an initial weight typically about 15 mg, were heated from 50 to 500 ◦C at a heating rate of
10◦C/minundernitrogen/airflow.
2.3.6. InfraredSpectroscopy
Fouriertransforminfraredspectroscopy(FTIR)spectrawerecollectedcouplingtheattenuated
totalreflection(ATR)accessoryGoldenGateofSpecac,Ltd. (Orpington,UK)toBrukerTensor37FTIR
equipment(Rheinstetten,Germany). Singlespectrawerecollectedinthewavelengthrangefrom4000
to600cm−1byaveraging20scansataresolutionof4cm−1.
2.3.7. MechanicalTests
Dumbbell-shapedspecimensweredie-cutfromtheelectrospunfilmsandconditionedtoambient
conditions,i.e.,25◦Cand50%RH,for24hpriortotensiletesting. Testswerecarriedoutatroom
temperatureinauniversalmechanicaltestingmachineAGS-X500NfromShimadzuCorp. (Kyoto,
Japan)inaccordancewithASTMD638(TypeIV)standard. Thiswasequippedwitha1-kNloadcell
andthecross-headspeedwassetat10mm/min. Aminimumofsixspecimenswasmeasuredforeach
sampleandtheaverageresultswithstandarddeviationwerereported.
2.3.8. WaterVaporPermeability
The water vapor permeability (WVP) of the samples was determined, in triplicate, using the
gravimetricmethodASTME96-95. Tothisend,5mLofdistilledwaterwasplacedinsideaPayne
permeabilitycup(Ø=3.5cm)fromElcometerSprl(Hermalle-sous-Argenteau,Belgium)toexposethe
filmto100%RHononeside. Theliquidwasnotincontactwiththefilm. Oncethefilmsweresecured
withsiliconrings,theywereplacedwithinadesiccatorat0%RHcabinetat25◦C.Thedrynessof
thecabinetwasheldconstantusingdriedsilicagel. Cupswithaluminumfilmswereusedascontrol
samples to estimate solvent loss through the sealing. The cups were weighted periodically using
ananalyticalbalancewitha±0.0001gaccuracy. Watervaporpermeationwascalculatedfromthe
steady-state permeation slopes obtained from the regression analysis of weight loss data vs. time,
andtheweightlosswascalculatedasthetotallossminusthelossthroughthesealing.
2.3.9. D-LimonenePermeability
PermeabilitytolimonenevaporwasmeasuredasdescribedaboveforWVP.Forthis,5mLof
D-limonenewasplacedinsidethePaynepermeabilitycups. Thecupscontainingthefilmswereplaced
atcontrolledenvironmentalconditions,i.e.,23◦Cand40%RH.Limonenevaporpermeationrates
wereestimatedfromthesteady-statepermeationslopesandtheweightlosswascalculatedasthetotal
celllossminusthelossthroughthesealing. Thesamplesweremeasuredintriplicateandthelimonene
permeability(LP)valueswerecalculatedtakingintoaccounttheaveragefilmthicknessineachcase.
Nanomaterials2018,8,469 5of19
2.3.10. OxygenScavengingActivity
RNaonuomnadte-rbialos t2t0o18m, 8,S xc FhOlRen PkEEflR aRsEkVsIEfWro mVidraFocS.A.(Barcelona,Spain)withaPTFEstopc5 oocf k18a nda
headspacevolumeof50cm3wereusedfortheoxygenscavengingmeasurements.Theflaskscontained
2.3.10. Oxygen Scavenging Activity
avalveforgasflushinginandaO -sensitivesensorspot(PSt3,detectionlimit15ppb,0–100%oxygen)
2
Round-bottom Schlenk flasks from VidraFoc S.A. (Barcelona, Spain) with a PTFE stopcock and
fromPreSens(Regensburg,Germany)wasgluedontotheinnersideoftheflasksformeasuringoxygen
a headspace volume of 50 cm3 were used for the oxygen scavenging measurements. The flasks
depletion. Theelectrospunfibersandfilmswerecut(5×5cm2)andplacedintotheflasks. Theflask
contained a valve for gas flushing in and a O2-sensitive sensor spot (PSt3, detection limit 15 ppb,
wassubsequentlyflushedfor30sat1barwithagasmixturecontaining1vol. %oxygen,4vol. %
0–100% oxygen) from PreSens (Regensburg, Germany) was glued onto the inner side of the flasks for
hydrogen, and 95 vol. % nitrogen, which was provided by Abelló Linde, S.A. (Barcelona, Spain).
measuring oxygen depletion. The electrospun fibers and films were cut (5 × 5 cm2) and placed into
Theotxhye gfleanskcso. nTcheen ftlaraskti ownasi nsutbhseeqceulelnwtlya sflmusohnedit oforre d30b sy aat 1n boanr- wdeitsht rau gcatisv meimxteuares ucorenmtaeinnitngm 1e tvhool.d %u sing
theOoXxYyg-4enm, i4n vio(Pl.r e%S ehnysd)romguenlt,i -acnhda n9n5 evlofil.b %er noiptrtoicgeonx,y wgehnichm wetaesr pfororvsiidmedu ltbayn Aeobuelslór eLaidn-doeu, tSo.Af.u pto
4oxy(gBeanrceselonnsao, rsS,puaisne)d. Twhieth osxeyngseonr scobnacseendtroantioan2 imn mtheo pcteilcl awlfiabs emr.oOnxityorgeedn bcoy nac ennotnra-dtieosntrsuoctviveer time
weremmeeaasusurermedenbt ymleinthkoidn gustihneg ltihgeh Ot-eXmY-i4t tminigni( 6(P0r0e–S6e6n0s) nmmu)ltoi-pchtiacnanlefil bfiebresr toopttihce oflxyagsekns imnenteerr fsoern sing
spotss.imThueltasneenosuosr reemadit-souat coefr utapi ntoa 4m ooxuyngetno fselunsmorins,e ussceedn cweitdhe speennsodrisn bgaosendt hone oa x2y mgemn ocpotniccaeln ftirbaetri. onin
Oxygen concentrations over time were measured by linking the light-emitting (600–660 nm) optical
thecellthatwascalibratedtoyieldconcentrationbytheequipment. Allmeasurementswerecarried
outaftib2e3rs◦ Cto athned flaatsk5s0 %innaenr dse1n0s0in%g sRpHot.s. The sensor emits a certain amount of luminescence depending
on the oxygen concentration in the cell that was calibrated to yield concentration by the equipment.
All measurements were carried out at 23 °C and at 50% and 100% RH.
2.4. StatisticalAnalysis
T2.h4.e Stteatsitstdicaalt aAnwaelyrseis evaluated through analysis of variance (ANOVA) using STATGRAPHICS
CenturionThXe VteIstv d1a6ta.1 w.0e3ref reovmaluSatteadtP tohirnotugThe cahnnaloylsoigs ioefs ,vaIrnica.nce(W (AaNrrOenVtAon) ,usVinAg, SUTSAAT)G.RFAisPhHeIrC’sS least
signifiCceanntutrdioifnf eXreVnIc ev (1L6S.1D.0)3w farosmu sSetdataPtotihnte T95ec%hncoolnofigidees,n cInecl.e (vWela(rpre<nt0o.n0,5 )V.AM, eUaSnAv)a. lFuieshsearn’sd lsetaasnt dard
deviastiigonnifsicwanetr deiafflesroenccael c(uLSlaDte) dw.as used at the 95% confidence level (p < 0.05). Mean values and standard
deviations were also calculated.
3. ResultsandDiscussion
3. Results and Discussion
3.1. MorphologyandOpticalProperties
3.1. Morphology and Optical Properties
3.1.1. OpticalAppearance
3.1.1. Optical Appearance
Figure 1 shows the contact transparency pictures of the electrospun PHB fibers, Figure 1a–c,
Figure 1 shows the contact transparency pictures of the electrospun PHB fibers, Figure 1a–c,
as well as of their respective annealed films, Figure 1d–f. From these pictures, it can be observed
as well as of their respective annealed films, Figure 1d–f. From these pictures, it can be observed that
thatalltheelectrospunfibermatswerecompletelyopaqueduetotheultrathinsizeofthefibersthat
all the electrospun fiber mats were completely opaque due to the ultrathin size of the fibers that
generateasignificantlevelofporosityandhencerefractthelightverystrongly[21]. Ontheother
generate a significant level of porosity and hence refract the light very strongly [21]. On the other
hand,theannealedfilmspresentedanimprovedcontacttransparency,speciallythesamplewithCTAB.
hand, the annealed films presented an improved contact transparency, specially the sample with
DuetCoTtAhBe. pDruesee tno cteheo pfrtehseenPcdeN ofP tsh,et hPedNfiPlms, sthdee fvilemlosp deedvealnopeexdp aenc teexdpedcaterdk dcoarlko rc.olor.

FigurFeig1u.reC 1o. nCtaocnttatcrta ntrsapnsaprearnecnycyp ipcitcuturreess ooff tthhee eelleeccttrroossppuunn ppoloyl(y3(-h3-yhdyrodxryobxuytbyruattyer) a(tPeH)B(P) HfibBe)rsfi bers
containing palladium nanoparticles (PdNPs) and their respective annealed films: (a) PHB/PdNP
containingpalladiumnanoparticles(PdNPs)andtheirrespectiveannealedfilms:(a)PHB/PdNPfibers;
fibers; (b) PHB/PdNP/hexadecyltrimethylammonium bromide (CTAB) fibers; (c)
(b)PHB/PdNP/hexadecyltrimethylammoniumbromide(CTAB)fibers;(c)PHB/PdNP/tetraethyl
PHB/PdNP/tetraethyl orthosilicate (TEOS) fibers; (d) PHB/PdNP film, (e) PHB/PdNP/CTAB film, (f)
orthosilicate(TEOS)fibers;(d)PHB/PdNPfilm,(e)PHB/PdNP/CTABfilm,(f)PHB/PdNP/TEOSfilm.
PHB/PdNP/TEOS film.
Nanomaterials2018,8,469 6of19
3.1.2. MorphologyofElectrospunPHBMaterials
The morphology of the electrospun fibers and their annealed films were studied by SEM.
Representative images of all the electrospun samples are gathered in Figure 2. The images were
takenfromthesurfaceandcross-sectionsoftheobtainedfibersandfilms. AsshowninFigure2a,
thediameteroftheneatelectrospunPHBfiberswasdistributedprimarilyintherangeof200–600
nm,presentingasmoothandbead-freemorphology. Inparticular,themeandiameterwasfoundto
beat350±147nm. OnecanobservethatthepresenceofthePdNPsledtoafractionwithincreased
fiberdiameterandalsoresultedinaspindle-typebeadsformation. Thiseffectcanbeobservedin
Figure2d,g,correspondingtotheelectrospunPHB/PdNPandCTAB-containingPHB/PdNPfibers,
respectively. Thisobservationmaysuggestthat,insomefibrilarregions,partialagglomerationofthe
PdNPsmayoccur. Indeed,somedegreeofagglomerationisacommonphenomenonincomposites
duetothelargesurfaceareaandhightotalsurfaceenergyassociatedwithnanoparticlesincorporated
into polymer matrices that makes them amenable to clustering [22]. The nanofibers diameter was
previouslyreportedtodecreasewhenthesurfactantconcentrationincreasedintheelectrospinning
solution[23]. Interestingly,theTEOS-containingPHB/PdNPfibers,showninFigure2j,presenteda
significantfractionofthefiberswithreducedfiberdiameter. Thiseffecthasbeenpreviouslydescribed
forotherelectrospunmaterialsandithasbeenparticularlyattributedtoboththeexpecteddecreasein
surfacetensionandanincreaseinconductivity,whichinturnproduceanincreaseinthestretching
forcesinthejetandconsequentlydecreasesthefiberdiameter[24–26].
FromtheSEMimagesofthefiberscross-sections,onecanobservethatFigure2b,e,corresponding
to the neat PHB and PHB/PdNP fibers, respectively, showed similar morphologies. In particular,
both electrospun mats presented cross-sections with relatively high porosity and low compaction.
Alternatively,thecross-sectionofthesurfactant-containingPHB/PdNPfibers,includedinFigure2h,k,
wereseentobemorecompactedsincetheadhesionamongthefibersinthelayeredstructurewas
higher. For all layers, it was also possible to perceive some particles aggregation that may not be
necessarilyrelatedtothepresenceofthePdNPsbut,moreprobably,toadditivessuchasthenucleating
agentboronnitride,originallyincludedinthebiopolymerbythemanufacturer.
IntheSEMimagesofthefilmscross-sections,showninFigure2c,f,i,l,itcanbeobservedthatthe
electrospunfibersfusedandinterconnectedamongeachotheraftertheannealingtreatmentat160◦C,
successfullyleadingtothepackingofthefibermatintoacontinuousfilm. Amongthehere-prepared
films,theneatPHBfilmshowedamoreuniform,smooth,andhomogeneoussurface.Aftertheaddition
ofthePdNPsandsurfactants,thefilmsbecamemoreheterogeneous,rougher,andalsopresentedsome
cavities. ThismorphologychangecanbethenrelatedtothepresenceofthePdNPs,whichmorelikely
interferedinthefiberscoalescenceprocess.
3.1.3. DispersionofPdNPs
InordertoprovideamoreresolvedinformationaboutthedispersionofthePdNPsintothePHB
biopolymermatrix,TEMwasperformedonthenanocompositefibersandfilms. Thedistributionof
thePdNPsinsidetheelectrospunfibersareillustratedintheTEMimagesincludedinFigure3a,d,g.
From the TEM image included in Figure 3a, one can clearly discern that the PdNPs were mainly
agglomeratedincertainregionsofthesubmicronPHBfibers. Onecanalsoobservethattheaddition
ofbothsurfactants,i.e.,CTABandTEOS,asrespectivelyshowninFigure3d,g,successfullyimproved
thePdNPsdispersioninthePHBfibers. Dispersionanddistributionwas,however,seenhigherinthe
CTAB-containingsample.
Nanomaterials2018,8,469 7of19
Nanomaterials 2018, 8, x FOR PEER REVIEW 7 of 18

Figure 2. Scanning electron microscopy (SEM) images taken on the surface views and cross-sections
Figure2.Scanningelectronmicroscopy(SEM)imagestakenonthesurfaceviewsandcross-sectionsof
of the electrospun poly(3-hydroxybutyrate) (PHB) fibers containing palladium nanoparticles (PdNPs)
theelectrospunpoly(3-hydroxybutyrate)(PHB)fiberscontainingpalladiumnanoparticles(PdNPs)
and their respective annealed films: (a) Surface view of the neat PHB fibers; (b) Cross-section of the
andtnheeairt PreHsBp efcibtievrse; a(cn)n Ceraolsesd-sfiecltmiosn: o(af )thSeu rnfeaacte PvHieBw filomf;t h(de) nSeuarftaPceH vBiefiwb eorf st;h(eb P)HCBro/Psds-NsePc ftiiboenrso; f the
neatP(eH) CBrofisbs-esresc;ti(ocn) oCf rtohses P-sHeBct/PiodnNoPf fitbheersn; (efa) tCProHssB-sfiecltmio;n (odf) thSeu PrfHaBce/PvdiNewP fiolmft; h(ge) PSuHrfBa/ceP vdiNewP ofif bers;
(e)Crtohses -PsHecBt/iPodnNoPf/thheexaPdHecBy/ltPridmNetPhyfilabmerms;o(nfi)uCmr obsrso-mseicdteio (nCoTfAtBh)e fPibHerBs/; P(hd)N CProfislsm-s;ec(gti)onS uorff atchee view
ofthePHPHB/BPd/NPdP/NCPT/AhBe fxiabderesc; y(lit) rCimroestsh-syelcatmionm oofn ituhem PbHrBo/mPdidNeP(/CCTTAABB )fifilmb;e r(sj); (Shu)rfCacreo svsi-eswec toifo nthoe fthe
PHB/PPHdBN/PPd/NCPT/tAetBraefithbyelr so;rt(hi)osCilricoastse- s(eTcEtOioSn) foibfetrhs;e (Pk)H CBr/osPsd-sNecPti/oCn ToAf tBhefi PlmH;B/(Pj)dSNuPr/fTaEcOeSv fiiebwerso; f the
PHB/(lP) dCNroPss/-tseetcrtiaoent hoyf lthoer tPhHoBsi/PlidcaNtPe/(TTEEOOSS f)ilmfib. ers;(k)Cross-sectionofthePHB/PdNP/TEOSfibers;
(l)Cross-sectionofthePHB/PdNP/TEOSfilm.
In relation to the annealed films, the PHB/PdNP film without surfactant, shown in Figure 3b,c,
still presented a clear aggregation of the particles. As opposite, Figure 3e,f, for the CTAB-containing
Ifnilmre lsaatmiopnlet, oanthde Faignunreea 3lhed,i, fifolmr tsh,et hTeEOPSH-cBo/nPtadinNinPg fifillmm wsaimthpoleu, tcsleuarrflayc sthaonwt,esdh othwatn thine FPidgNuPres 3b,c,
stillpwreesree netveednlay calneda rrealgagtirveeglya twioenll odfistthriebuptaerdt iicnl eths.e APHsoBp fpilmossi twe,itFhioguut rfeor3me,ifn,gf oarggthloemCeTraAteBs-. cIonn tthaei ning
filmscaamsep olef ,tahne dTFEiOgSu-rceon3thai,ni,infogr ftihlme TsEamOpSl-ec, otnhtea innainnogpfiarlmticlsea mdisppleer,sciolena rwlyass hleosws eudnitfhoramt thwehPend NPs
wereceovmenplayreadn dto rtehlea tsiavmelpylew perlelpdairsetdri wbuitthe dCTinAtBh esuPrfHacBtafinltm dsuew tioth thoeu tabfosermncien ogf alagrggleo gmreeyra dtaersk. Ianretahse. case
In both PHB films, very small nanoparticles of approximately 5 ± 2 nm can be seen, being
oftheTEOS-containingfilmsample,thenanoparticledispersionwaslessuniformwhencompared
homogeneously dispersed along the biopolymer matrix. The present results are in agreement with
tothesamplepreparedwithCTABsurfactantduetotheabsenceoflargegreydarkareas. Inboth
the results showed by Shaukat et al. [27], where PdNPs were incorporated into polyamide 6
PHBfilms,verysmallnanoparticlesofapproximately5±2nmcanbeseen,beinghomogeneously
(PA6)/clay nanocomposites and the nanoparticles were largely separated from each other and
dispersedalongthebiopolymermatrix. Thepresentresultsareinagreementwiththeresultsshowed
bySh aukatetal.[27],wherePdNPswereincorporatedintopolyamide6(PA6)/claynanocomposites
andthenanoparticleswerelargelyseparatedfromeachotherandorientedinallpossibledirectionsin
Nanomaterials2018,8,469 8of19
Nanomaterials 2018, 8, x FOR PEER REVIEW 8 of 18
oriented in all possible directions in the polymer matrix. However, due to the absence of surfactant,
thepolymermatrix. However,duetotheabsenceofsurfactant,someparticlesstillagglomeratedinto
some particles still agglomerated into clusters of bigger sizes that increased at the PA6 interfaces with
clustersofbiggersizesthatincreasedatthePA6interfaceswiththenanoclays.
the nanoclays.

Figure 3. Transmission electron microscopy (TEM) images of the electrospun poly(3-hydroxybutyrate)
Figure3.Transmissionelectronmicroscopy(TEM)imagesoftheelectrospunpoly(3-hydroxybutyrate)
(PHB) fibers containing palladium nanoparticles (PdNPs) and their respective annealed films: (a)
(PHB) fibers containing palladium nanoparticles (PdNPs) and their respective annealed films:
PHB/PdNP fibers; (b,c) PHB/PdNP film; (d) PHB/PdNP/ hexadecyltrimethylammonium bromide
(a)P(HCBT/APBd) NfibPerfis;b (eer,sf); P(bH,cB)/PPdHNBP//CPdTANBP ffiillmm; ;(g(d) P)HPHB/BP/dPNdPN/tePt/rahetehxyald oercthyoltsriliimcaetteh (yTlEaOmSm) foibneirusm; (hb,ir)o mide
(CTAPBH)Bfi/PbdeNrsP;/T(eE,Of)SP fHilmB./ PdNP/CTABfilm;(g)PHB/PdNP/tetraethylorthosilicate(TEOS)fibers;
(h,i)PHB/PdNP/TEOSfilm.
3.2. Thermal Properties
3.2. ThermalProperties
3.2.1. Melting Profile
3.2.1. MeTlthine gthPerromfialel properties of the electrospun PHB fibers and films containing the PdNPs
nanoparticles were firstly investigated by DSC analysis. Table 1 gathers the melting temperatures
ThethermalpropertiesoftheelectrospunPHBfibersandfilmscontainingthePdNPsnanoparticles
(Tm1 and Tm2) and enthalpies (ΔHm) obtained from the first heating run. Likewise, the crystallization
werefirstlyinvestigatedbyDSCanalysis. Table1gathersthemeltingtemperatures(T andT )
temperature (Tc) was also obtained from the cooling run. One can observe that when tmhe1rmal m2
andenthalpies(∆H )obtainedfromthefirstheatingrun. Likewise,thecrystallizationtemperature
annealing was apmplied to the fibers, the melting profile of the PHB materials, both temperature and
(Tc)ewnathsaalplsyo, solibghtatliyn eddecfrreoasmedt.h Ine pcoarotliicnuglarr,u thne. TOmn vealcuaens oofb tsheer nveeatt hPaHtBw-bhaesendt fhibeerrms avlarainedn eina ltihneg was
appl1ie6d8–t1o70th °eCfi rbaenrgse,, tmheelmtineglt iinng a psrinogfillee poefatkh, ewPhHerBeams athteesreia vlsa,lubeost hwteerme pareoruantudr 3e a°Cn dloewnetrh ainlp tyh,esirli ghtly
decrceoausnedte.rpInarpt afirltmic usalamrp,tlehse. TIn avdadliutioens,o tfhteh iencnoeraptoPraHtioBn-b oafs ethde fiPbdeNrsPsv ainrtioe dPHinBt hined1u6c8ed–1 a7 0sl◦igChtr ange,
m
meltdinegcreiansea ins ibnogthle thpee make,ltiwngh teermeapsertahteusree avnadl uenetshawlpeyre asa wroeulln ads re3su◦Cltedlo iwn tehre ifnortmhaetiironc oouf mntuelrtpipaler t film
melting peaks. This observation may suggest that the nanoparticles interfered with the crystallization
samples. Inaddition,theincorporationofthePdNPsintoPHBinducedaslightdecreaseinboththe
process of the homopolyester, producing more imperfect crystals composed of thinner lamellae that
melting temperature and enthalpy as well as resulted in the formation of multiple melting peaks.
melted over a wider range at lower temperatures and with lower enthalpies [28]. This effect was more
Thisobservationmaysuggestthatthenanoparticlesinterferedwiththecrystallizationprocessofthe
intense in the case of the surfactant-containing film samples, which suggests that the PdNPs were
homopolyester,producingmoreimperfectcrystalscomposedofthinnerlamellaethatmeltedover
better dispersed then highly influencing the packing process of the PHB chains during cooling.
awiderrangeatlowertemperaturesandwithlowerenthalpies[28]. Thiseffectwasmoreintense
in th e case of the surfactant-containing film samples, which suggests that the PdNPs were better
dispersedthenhighlyinfluencingthepackingprocessofthePHBchainsduringcooling. Inrelationto
thecrystallizationfromthemelt,thePHBfilmsamplesincorporatingthePdNPsalsoshowedslightly
Nanomaterials2018,8,469 9of19
lowervaluesofT thanthefibers,whichconfirmsthatthenanoparticlesprovidedananti-nucleating
c
effect on PHB during the film formation. In any case, given that PHB is known to exhibit cold
crystallizationduringthedynamicDSCruns,itbecomesdifficulttodiscusswithcertaintypotential
changesincrystallinityandcrystallinemorphology[21,26].
Table1.Thermalpropertiesobtainedfromthedifferentialscanningcalorimetry(DSC)curvesinterms
ofmeltingtemperature(Tm),normalizedmeltingenthalpy(∆Hm),crystallizationtemperature(Tc),
andnormalizedcrystallizationenthalpy(∆Hc)forthevariouspoly(3-hydroxybutyrate)(PHB)and
palladiumnanoparticles(PdNPs)fibersandfilmswithandwithouthexadecyltrimethylammonium
bromide(CTAB)andtetraethylorthosilicate(TEOS).
Sample Tm1(◦C) Tm2(◦C) ∆Hm(J/g) Tc(J/g) ∆Hc(J/g)
PHBFibers - 169.1±0.9a 64.1±1.1a 110.2±0.9a 59.3±2.0b,c
PHBFilm - 168.4±1.3a 71.8±1.3e 110.5±1.2a 61.1±0.4a,b
PHB/PdNPFibers 156.4±2.1a,b 168.3±1.1a,b 59.4±0.9c,d 108.4±0.5a,b 63.1±1.5a
PHB/PdNPFilm 155.0±1.2a 168.4±1.2a,b 58.5±0.8d 109.5±2.1a 56.3±1.0c,d
PHB/PdNP/CTABFibers 156.7±0.5a,b 167.7±0.9a,b 62.2±1.4b 109.1±0.9a,b 60.3±0.8a,b
PHB/PdNP/CTABFilm 158.6±1.4c 165.1±1.4b 60.3±1.5c 106.8±0.3b 55.1±0.7d
PHB/PdNP/TEOSFibers 155.2±0.7a 169.9±0.5a 59.7±0.9d 110.8±1.0a 60.1±1.1a,b
PHB/PdNP/TEOSFilm 159.4±1.9c 165.5±1.2b 62.9±0.8a,b 110.0±2.3a 51.4±1.8e
a–e:Differentsuperscriptswithinthesamecolumnindicatesignificantdifferencesamongthesamples(p<0.05).
3.2.2. ThermalStability
Figure4showstheevolutionofthemasslossasafunctionoftemperatureforthePHBsamples,
includingthecurvesofthefirstderivativeanalysis(bluelines). AsshowninTable2,itcanbeseenthat
theneatPHBinitiateddegradationat207◦Cwhilethebiopolymerfullydecomposedintwosteps,
seenat262◦Cand348◦C,providingaresidualmassofapproximately3%. Thermaldegradationofthe
PHBfilmscontainingthePdNPsandthesurfactantsalsooccurredintwostageswithaslightincreasein
thermalstability. Inparticular,theonsetdegradationtookplaceinthe220–230◦Crangethatcontinued
tothefirstdegradationstageat270–275 ◦Cand, ataslowerrate, toover360–380 ◦C,remaininga
residueof3–4%oftheinitialmassofthesample. Inthissense,Díez-Pascualetal.[29]demonstrated
thattheincorporationofzincoxide(ZnO)improvedtheheatresistanceofPHB,whichwasascribedto
thebarriereffectofthenanoparticlesthateffectivelyhinderedthediffusionofdecompositionproducts
during thermal degradation. Therefore, the PdNPs probably also functioned as a thermal barrier,
absorbingheat,thusalsoresultinginanenhancedthermalstability.
ItisalsoworthytonotetheslightincreaseobservedinthethermalstabilityofthePHB/PdNP
filmswiththeincorporationofbothsurfactants,especiallyinthecaseofCTAB.Thissuggeststhat
the surfactant increased the matrix–nanoparticle interaction, inducing a positive delay in thermal
degradation. Taking into account that the ammonium salts degrade at the temperature range of
150–460◦C,thesurfactantscouldsuccessfullyprovideabondingeffectbetweenbothcomponentsof
thenanocompositeand,eventually,enhancethewholethermalstabilityofPHAs.
Nanomaterials2018,8,469 10of19
Nanomaterials 2018, 8, x FOR PEER REVIEW 10 of 18

FigurFei4g.urTeh e4r. mTohegrrmavoigmraevtirmiceatrnica lyansiasly(sTisG A(T)GcAu)r vceusrvoefst hoef ethleec terloescptruonsppuonl yp(o3l-yh(y3-dhryodxryobxuybtyurtaytrea)te()P HB)
(PHB) and palladium nanoparticles (PdNPs) films with and without hexadecyltrimethylammonium
andpalladiumnanoparticles(PdNPs)filmswithandwithouthexadecyltrimethylammoniumbromide
bromide (CTAB) and tetraethyl orthosilicate (TEOS) surfactants.
(CTAB)andtetraethylorthosilicate(TEOS)surfactants.
Table 2. Mean values of thermal stability obtained from the thermogravimetric analysis (TGA) curves
Table2.Meanvaluesofthermalstabilityobtainedfromthethermogravimetricanalysis(TGA)curves
of the electrospun poly(3-hydroxybutyrate) (PHB) and palladium nanoparticles (PdNPs) films with
oftheelectrospunpoly(3-hydroxybutyrate)(PHB)andpalladiumnanoparticles(PdNPs)filmswithand
and without hexadecyltrimethylammonium bromide (CTAB) and tetraethyl orthosilicate (TEOS) in
withotuertmhse xoafd deecgyrlatrdiamtieotnh ytelmampemraotunriue mat b5r%o moifd mea(sCsT lAosBs) (aTn5%d),t edtergareatdhaytlioonr ttheomspileircaattuere(T (ETOdegS)), ianndte rms
ofdegrersaiddautailo mnatsesm atp 5e0r0a °tuCr (eRa50t0)5. %ofmassloss(T5%),degradationtemperature(Tdeg),andresidual
massat500◦C(R ).
50F0ilm Sample T5% (°C) Tdeg1 (°C) Tdeg2 (°C) R500 (%)
PHB 207.0 ± 4.4 262.0 ± 3.8 348.0 ± 6.4 3.08 ± 0.04
FilmSample T (◦C) T (◦C) T (◦C) R (%)
PHB/PdNP 5% 224.1 ± 3.6 de2g711.3 ± 2.6 368d.2eg ±2 3.8 3.17 ± 0.05300
PHPBHB/PdNP/C2T0A7B.0 ±242.04.0 ± 8.12 622.078±.1 3±.8 3.4 39324.82. 0± ±5.46 .43.81 ±3 0.0.084± 0.04
PHB/PPHdNB/PPdNP/T2E2O4S.1 ±233.06.2 ± 2.52 712.375±.2 2±.6 4.1 38366.81. 2± ±2.33 .84.44 ±3 0.1.074± 0.03
PHB/PdNP/CTAB 220.0±8.1 278.1±3.4 392.2±5.4 3.81±0.04
PHB/PdNP/TEOS 230.2±2.5 275.2±4.1 386.1±2.3 4.44±0.04
3.3. FTIR Analysis
FTIR is a powerful tool, which is very sensitive to the molecular environment, to investigate the
3.3. FTIRAnalysis
structural changes that occur in the material during any chemical or physical process. In Figure 5, the
FFTTIIRR isspaecptroaw aerer fpurletsoeonlt,ewd hfoicr hthies vabeoryves-emnesnittiiovneetdo PthHeBm-boasleedcu fliabreresn avnirdo fnilmmesn. tF,rtoomi nthvee sgtiivgeante the
structsupreacltrcah oafn tghee sPtHhBa tmoactceuriralisn, tthhee bmanadt ecreinatledreudr iant g17a2n2y cmch−1e ims aicna linodricpahtivyes iocfa lthper Coc=eOs ss.trIentchFiinggu re5,
vibration for the biopolyester molecule. The absorption bands in the 1200–1300 cm−1 range were
theFTIRspectraarepresentedfortheabove-mentionedPHB-basedfibersandfilms. Fromthegiven
previously related to the presence of the C–O–C stretching vibrations whereas the band centered at
spectraofthePHBmaterials,thebandcenteredat1722cm−1isanindicativeoftheC=Ostretching
approximately 980 cm−1 has been ascribed to stretching bands of the carbon–carbon single bond
vibration for the biopolyester molecule. The absorption bands in the 1200–1300 cm−1 range were
(C–C) in PHAs [21,30]. In the spectra of the pure surfactants, it is interesting to be noted that the
previouslyrelatedtothepresenceoftheC–O–Cstretchingvibrationswhereasthebandcenteredat
bands at 2873 cm−1 and 2977 cm−1 correspond to the anti-symmetric C–H stretching of CTAB [19]
approwxihmilea ttehley b9a8n0dcs mlo−ca1tehda satb 1e1e6n8 acmsc−r1i, b1e1d03t ocmst−r1,e atcnhdi 1n0g80b acnmd−1s hoafvteh beeceanr absosnig–nceadr btoo nthsei nCgHle3 rboocknidng(C –C)
in PHaAnds C[2–1O,3 a0s]y.mInmethtreics sptreecttcrhainogf otfh TeEpOuSr, eressuprefcaticvtealny t[s3,1i]t. Fisori nthteer seusrtfiancgtatnot-cboentnaointeindg tPhHatB tfhilembs,a nds
at287t3hec amb−ov1ea snudrfa2c9t7an7tc bman−d1s coorr ortehsepros ncodutlod nthoet baen dtie-tseycmtedm metorsitc liCk–elHy dsutree ttoc hthine gloowf cCoTnAceBnt[r1a9ti]onw hile
thebainn dwshilcohc athteedse aatd1d1it6iv8ecsm ar−e 1p,r1e1se0n3t cinm t−he1 ,caonmdpo1s0i8te0. cm−1 havebeenassignedtotheCH rocking
3
andC–OasymmetricstretchingofTEOS,respectively[31]. Forthesurfactant-containingPHBfilms,

theabovesurfactantbandsorotherscouldnotbedetectedmostlikelyduetothelowconcentrationin
whichtheseadditivesarepresentinthecomposite.