The Preparation, Characterization, Mechanical and Antibacterial Properties of GO-ZnO

17 Pages·2017·15.78 MB·English

materials
Article
The Preparation, Characterization, Mechanical and
Antibacterial Properties of GO-ZnO Nanocomposites
with a Poly(L-lactide)-Modified Surface
MingweiYuan1,2,ChengdongXiong2,LinJiang1,HongliLi1andMinglongYuan1,* ID
1 EngineeringResearchCenterofBiopolymerFunctionalMaterialsofYunnan,YunnanMinzuUniversity,
Kunming650500,China;[email protected](M.Y.);[email protected](L.J.);
[email protected](H.L.)
2 ChengduInstituteofOrganicChemistry,ChineseAcademyofSciences,Chengdu610041,China;
[email protected]
* Correspondence:[email protected]@188.com;Tel.:[phone]
Received:27December2017;Accepted:13February2018;Published:23February2018
Abstract: Graphene oxide (GO) was employed for the preparation of GO-zinc oxide (ZnO).
ThehydroxylgrouponthesurfacewasexploitedtotriggertheL-lactidering-openingpolymerization.
A composite material with poly(L-lactide) (PLLA) chains grafted to the GO-ZnO surface,
GO-ZnO-PLLA,wasprepared.Theresultsdemonstratedthattheemployedmethodallowedone-step,
rapidgraftingofPLLAtotheGO-ZnOsurface. ThechemicalstructureoftheGOsurfacewasaltered
byimproveddispersionofGO-ZnOinorganicsolvents,thusenhancingtheGO-ZnOdispersioninthe
PLLAmatrixandtheinterfacebondingwithPLLA.Subsequently,compositefilms,GO-ZnO-PLLA
and GO-ZnO-PLLA/PLLA, were prepared. The changes in interface properties and mechanical
propertieswerestudied. Furthermore,theantibacterialperformanceofnano-ZnOwasinvestigated.
Keywords: GO-ZnO-PLLA;compositematerials;nano-ZnO;antibacterialperformance;mechanical
properties
1. Introduction
Theincreasingseverityofglobalwhitepollutioncausedbyfoodpackingmaterials,inaddition
tothepetroleumcrisis, haspromotedstudiesongreenandeco-friendlyfoodpackagingmaterials
thatarerenewable,degradable,andnon-toxic. Todate,therehavebeenbreakthroughsinthefield.
Greenandrenewablepolymerssuchaspolylactide(PLLA),polyglycolicacids,andpolyaminoacids
have been developed and widely applied in food packaging research. Among them, PLLA is the
materialwiththebestapplicabilityandisrecognizedasamaterialthatcancontributetothegreen
material world of the 21st century [1]. The use of green, renewable, and degradable materials in
rePLLAcingconventionalpetroleumPLLAsticsisaneffectivemeanstofurtherensurefoodsafety,
environmentalprotection,andresourceconservation; itisalsothetrendoftechnicaldevelopment
andthegoalofscientists. Inorganicnanomaterial-PLLAcompositefilmscanpotentiallybedeveloped
asnext-generationfood-packagingmaterials.ToapplyPLLAinfoodpackaging,thewaterandgas
barrierpropertiesofPLLAmustbeenhanced,especiallyformodifiedatmospherepackagingused
inmeatpacking,whichrequiresamoreeffectivegasbarrierthanregularfood.Recently,inorganic
nanoparticlesandnanowireshavebeenappliedtomodifymaterialssuchasPLLAandsolvetheir
barrierissues.Additionalresearchandreviewarticles[2–6]havereportedtheuseofnanomaterialssuch
aszincoxide(ZnO),titaniumdioxide,hydroxyapatite,silicate,silvercomplexes,smectite,nanoclays,
and carbon nanotubes. The modification approaches include solution blending, melt blending,
andcoating. Theresultshaveindicatedthattheuseofnanomaterialsyieldseffectiveimprovements
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toorsolutionsforwatervaporandgasbarriers,aswellastheheatresistanceofthePLLAmaterials.
SomenanomaterialssuchasZnO-PLLAnanocompositematerialsmayevenimproveanti-UVand
antibacterialpropertiesandshowcharacteristicsforactivefoodpackaging[7–10]. Therehavebeen
manystudiesonapplicationsofinorganicnanomaterial-PLLAcompositefilmsinpackagingofdifferent
typesoffood. Manyscientists,basedonthesestudies,areoptimisticthatinorganicnanomaterial-PLLA
compositefilmsarethenewgenerationofgreenfoodpackagingmaterial. Inrecentyears,graphene
and GO have been utilized in PLLA nanocomposites, and it is found that expanded graphite or
GOcanincreasethecrystallinityandtheheat-resistanttemperatureofthenanocomposite[11–15].
Fengetal. developedaversatileroutebygraftingpolymerontheGOtoenhancethepropertiesof
nanocomposites[16,17]. Inourstudy,wegraftedPLLAonthegrapheneoxide(GO)-ZnOcompounds
topreparenanomaterialgrapheneoxide(GO)-ZnO-poly(L-lactide),andtomodifyPLLAusingthis
nanomaterial,whichisstructurallysimilartoPLLAbutissurface-modifiedbylow-molecular-weight
PLLA.Ahigh-barriernano-PLLAfilmmaterialwasprepared,providinganewparadigmandanew
methodtosolveacriticalissueencounteredintheapplicationsofnano-PLLAcompositematerialsin
foodpackaging,aswellasafoundationwiththeoreticalsupportforstudiesonnext-generationgreen
foodpackagingmaterials.
2. MaterialsandMethods
2.1. Materials
2.1.1. PreparationofRawMaterials
GOwasprovidedbySuzhouCarbonAbundanceGrapheneScienceandTechnologyCo.,Ltd.
(Suzhou, China) with the following specifications: purity of approximately 99 wt %, thickness of
0.6–1.0nm,diameterof0.5–5µm,in1–2layers,andaspecificsurfaceareaof1000–1217m2/g. L-lactide
wasprovidedbyPURAC.Grade4032D,fromNatureWorks(Minnetonka,MN,USA)wasemployed.
2.1.2. PreparationofGO-ZnO-PLLA
The GO-ZnO was prepared according to the literature [18]. Graphite oxide was dispersed in
ethanol (2 mg/1 mL) and sonication for 1 h under ambient conditions. Subsequently, 0.880 g of
zincacetate(ZnC H O ·2H O)wasdissolvedintothemixturewhilestirring.Then,apredetermined
4 6 4 2
amount of NaOH solution was added to the mixture and pH of the solution was adjusted to 10,
afterbeingstirredfor30min. Themixturewasthentransferredtoa100mLroundbottomflaskand
heated to 140 ◦C under N2 atmosphere for 24 h. The prepared composites were then centrifuged
andwashedbydistilledwaterforseveraltimes.Theproductwasdriedinavacuumovenfor24h
at 60 ◦C. A simple chemical approach was employed for the GO-ZnO nanocomposite in ethanol
medium,asshowninScheme1a. In1000-mLround-bottomflasks,800mLof1,4-dioxanewasadded
alongwith0.4gofGO-ZnO(GO-ZnOwaspreparedaccordingtotheliterature). Themixturewas
sonicatedfor1.5htoformanevenlydispersedbrownsuspension. Thesuspensionwasinfusedwith
80gofL-lactide,stirred,heatedunderreflux,dissolveduntilclear,andslightlycooled. Thesolvent
wasevaporatedunderatmosphericpressureat120◦C.Stannousoctoate(0.08g)wasaddedtothe
solution,followedbyatemperatureincreaseto140◦Cfora1hpolymerizationreaction. Reagents
weredeterminedtobepolymerizedifthemixturecouldnotbestirred. Thereactiontimewasextended
for1h,andthemixturewasthencooledtoroomtemperature. Then,chloroformwasusedtodissolve
thepolymerizationproducts. Atroomtemperaturefor12h,theinsolublesubstanceswereremovedby
centrifugation. Thefiltrateofhomogeneousphasewasprecipitatedbyexcessiveethanoltoachieve
solid. The solid were filtered and dried under vacuum at 60 ◦C for 8 h. The product gained was
GO-ZnO-PLLA. The reaction equation is shown in Scheme 1b. As a contrast, PLA and GO-ZnO
arealsodissolvedinchloroform,andatroomtemperaturefor12h,theinsolublesubstanceswere
removedbycentrifugation. Thefiltrateofhomogeneousphasewasprecipitatedbyexcessiveethanol
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was not GO-ZnO/PLLA blend compound, it was PLA. Filtration after centrifugation was dried by
toachievesolid. Thesolidwerefilteredanddriedundervacuumat60◦Cfor8h. Theproductgained
freeze-drying method to obtain GO-ZnO.
wasnotGO-ZnO/PLLAblendcompound,itwasPLA.Filtrationaftercentrifugationwasdriedby
freeze-dryingmethodtoobtainGO-ZnO.

SScchheemmee 11.. SSyynntthheettiicc rroouuttee ooff ((aa)) GGOO--ZZnnOO aanndd ((bb)) GGOO--ZZnnOO--PPLLLLAA..
22..11..33.. PPrreeppaarraattiioonn ooff tthhee CCoommppoossiittee MMaatteerriiaall GGOO--ZZnnOO--PPLLLLAA//PPLLLLAA
SSaammpplleess ooff 44,, 88 aanndd 1122 gg ooff GGOO--ZZnnOO--PPLLLLAA wweerree oobbttaaiinneedd aanndd mmiixxeedd sseeppaarraatteellyy iinn 22000000 mmLL ooff
cchhlloorrooffoorrmm.. TThhee mmiixxttuurreess wweerree mmeecchhaanniiccaallllyy ssttiirrrreedd uunnttiill ddiissssoollvveedd ttoo yyiieelldd aa cclleeaarr ssoolluuttiioonn,, ffoolllloowweedd
bbyy 22 hho offu ultlrtarsaosnoniciactaitoino.nT. hTehem mixtiuxtruerwe awsaths etnhepnr epcirpeictiaptietdatbeyd 5b0y0 050m0L0 omfLa nohfy adnrhouysdreothuasn eotlhtaonyoile ltdo
gyriealyd- wgrhaiyti-swhhsiotliisdhs s.oTlhidess. oTlhides swoleidres dwreierde durniedde ruvnadceuru vmacautu6m0 ◦aCt 6fo0 r°C8 hfo.rT 8h eh.p Trohdeu pcrtosdguacintse dgawineerde
Gwoe-rZe nGOo--PZLnLOA-P/LPLLALA/PLcoLmAp coosmitepomsaittee rmiaaltsecroianltsa cinonintgai0n.i5n%g ,01.5%%,,a 1n%d,1 a.5n%d 1G.5O%-Z GnOO--ZPLnOLA-P.LLA.
2.1.4. PreparationoftheGO-ZnO-PLLA/PLLACompositeFilm
2.1.4. Preparation of the GO-ZnO-PLLA/PLLA Composite Film
The composite material obtained above of Go-ZnO-PLLA/PLLA, using the technology of
The composite material obtained above of Go-ZnO-PLLA/PLLA, using the technology of
streamingfilm.FlowfilmtestingmachinefromShanghaikechuangplasticproductsco.LTD(Shanghai,
streaming film. Flow film testing machine from Shanghai kechuang plastic products co. LTD
China),ModelsforLY-300. Thethicknessofthepreparationwas0.02mm. Filmcastingequipmentwas
(Shanghai, China), Models for LY-300. The thickness of the preparation was 0.02 mm. Film casting
employedattemperaturesof160,170,175,170,170,and165◦Cforzones1,2,3,4,5,and6,respectively,
equipment was employed at temperatures of 160, 170, 175, 170, 170, and 165 °C for zones 1, 2, 3, 4, 5,
atarotationspeedof100r/min.
and 6, respectively, at a rotation speed of 100 r/min.
3. ResultsandDiscussion
3. Results and Discussion
3.1. CharacterizationofGO-ZnO-PLLA
3.1. Characterization of GO-ZnO-PLLA
InourexperimentwiththepreparationofGO-ZnO-PLLA,thepolymerizationwascompleted.
In our experiment with the preparation of GO-ZnO-PLLA, the polymerization was completed.
Then chloroform was used to dissolve the polymerization products. After 12 h of placement,
Then chloroform was used to dissolve the polymerization products. After 12 h of placement, we
we found that the chloroform solution was homogeneous. No insoluble substance was found by
found that the chloroform solution was homogeneous. No insoluble substance was found by
centrifugation. Thefiltrateofhomogeneousphasewasprecipitatedbyexcessiveethanoltoachieve
centrifugation. The filtrate of homogeneous phase was precipitated by excessive ethanol to achieve
solidity. ThesolidswerefilteredanddriedundervacuumtoobtainGO-ZnO-PLLA.Inthecomparison
solidity. The solids were filtered and dried under vacuum to obtain GO-ZnO-PLLA. In the
experimentofpreparingGO-ZnO/PLLAblendcompound,wefoundthatthechloroformdissolved
comparison experiment of preparing GO-ZnO/PLLA blend compound, we found that the chloroform
in GO-ZnO/PLLA after 12 h statics was divided into two phases. Because GO-ZnO is insoluble
dissolved in GO-ZnO/PLLA after 12 h statics was divided into two phases. Because GO-ZnO is
in chloroform, it will be precipitated from the solution. After centrifuge separation, the insoluble
insoluble in chloroform, it will be precipitated from the solution. After centrifuge separation, the
substanceobtainedispureGO-ZnO.Thecompoundsdissolvedinchloroformwerefoundtobepure
insoluble substance obtained is pure GO-ZnO. The compounds dissolved in chloroform were found
to be pure PLA after the same process. The two experiments also showed that free GO-ZnO could be
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PLAafterthesameprocess. ThetwoexperimentsalsoshowedthatfreeGO-ZnOcouldbeseparated
separated from GO-ZnO-PLLA or PLLA by chloroform dissolution and centrifugation. If GO-
fromGO-ZnO-PLLAorPLLAbychloroformdissolutionandcentrifugation. IfGO-ZnO/PLLAblend
ZnO/PLLA blend compound is to be prepared, the GO-ZnO and PLA cannot be centrifuged after
compoundistobeprepared,theGO-ZnOandPLAcannotbecentrifugedaftermixinginchloroform,
mixing in chloroform, GO-ZnO/PLLA can be obtained by direct freeze-drying.
GO-ZnO/PLLAcanbeobtainedbydirectfreeze-drying.
To confirm that the GO-ZnO-PLLA has a PLA structure unit, we performed magnetic
To confirm that the GO-ZnO-PLLA has a PLA structure unit, we performed magnetic
characterization on the GO-ZnO-PLLA polymers. As seen in Figure 1, the characteristic peaks of
characterizationontheGO-ZnO-PLLApolymers. AsseeninFigure1,thecharacteristicpeaksofPLLA
PLLA occurred at 1.5 and 5.1 ppm, which is consistent with information reported in the literature
occurredat1.5and5.1ppm,whichisconsistentwithinformationreportedintheliterature[19–24].
[19–24]. These characteristic peaks arise from the protons of the methyl (a) and the methenyl (b)
Thesecharacteristicpeaksarisefromtheprotonsofthemethyl(a)andthemethenyl(b)groupsonthe
groups on the PLLA chain illustrated in the chemical structure in Figure 1. As seen in the nuclear
PLLAchainillustratedinthechemicalstructureinFigure1. Asseeninthenuclearmagneticresonance
magnetic resonance (NMR) spectrum, the hydroxyl group on the GO-ZnO surface should be
(NMR)spectrum,thehydroxylgroupontheGO-ZnOsurfaceshouldbesuccessfullytriggeredbythe
Lsu-lcaccetisdsefurlilnyg t-roigpgeenriendg bpyo ltyhme eLr-ilzaacttiidone .ring-opening polymerization.

FFiigguurree 11.. 440000 MMHHzz 11HH NNMMRR ssppeeccttrruumm ooff tthhee GGOO--ZZnnOO--PPLLLLAA ssaammppllee..
TToo ccoonnfifirrmm tthhaatt tthhee GGOO--ZZnnOO--PPLLLLAA hhaass aa GGOO--ZZnnOO ssttrruuccttuurree uunniitt,, wwee ppeerrffoorrmmeedd mmaaggnneettiicc
cchhaarraacctteerriizzaattiioonn oonn tthhee GGOO-Z-ZnnOO-P-PLLLLAA poploylmymeresr. sF.iFguigruer 2e s2hoshwosw thset hUeVU/VVis/ sVpisecstprae cotfr aPLoLfAP,L GLOA-,
GZnOO-Z-PnLOL-APL aLnAd GaOnd-ZnGOO/P-ZLnLOA/. TPhLeL UAV. /TVhise spUeVct/ruVmis osf pPeLcLtrAu,m GOo-fZnPOL-LPAL,LAG aOn-dZ nGOO--PZLnLOA/PLaLnAd
GwOas- ZonbOse/rvPeLdL Ain twhea sDoCbMse srvoleudtioinn, twhehiDleC thMat soofl uPtLioLnA,, wGOhi-lZentOha-PtLoLfAP LanLdA G, OG-OZ-nZOn/OP-LPLLAL Awaasn idn
GthOe -sZonluOt/ioPnL.L GAOw-ZansOin-PthLeLsAo lsuhtioown.s GvOer-yZ nbOro-aPdL LaAbsoshrpotwiosnv ewriythb rcooandtianbusoourpsltyio dnewcriethascionngt iinnuteonussiltyy
draencrgeeads ifnrgomin t2e2n0s ittoy 3ra3n0g nemd.f rOomn t2h2e0 ottoh3e3r 0hnamnd.,O PnLLthAe oanthde rGhOa-nZdn,OPL-PLLALAan sdhGowOs-Z tnhOe -aPbLsLorAptsihoonw ins
tthhee raabnsgoerp frtioomn 2in50t htoe 3r3a0n ngemf,r aonmd n25o0 abtoso3r3p0tinomn p,eaankd isn oobasbesrovrepdt iionn thpee 3a3k0 itsoo 8b0s0e nrvme drainngteh. eFu3r3t0hetor,
8P0L0LnAm, GraOn-gZen.OFu-PrtLhLeAr, PaLnLdA G,GOO-Z-ZnOnO/P-LPLLALA shaonwdsG Och-aZrnaOct/erPiLstLicA psehaokws sinc htahrea cwtearviset iclepnegathks riengtihone
wshaovreteler ntghtahn r2e5g0io nnmsh, owrhteirlet hnaon e2v5i0dennmt ,awbshoirleptnioone vinid tehnet habigshorepr twioanvien ltehneghthig rheegriowna vise slhenogwtnh. rIeng itohne
iasbsshoorpwtino.nI nsptehcetraubmso irnp ttihoen rsapnegcetr ufrmomin 2t2h0e troa n32g0e nfrmom, th2e2 0GtOo-3Z2n0On-mPL,LthAe sGhOow-Zsn aOb-sPoLrpLtAionsh wowiths
asbpseociraplt fioeantuwrietsh cshpaercaicatlefreisattiucsr efsorc hbaortahc PteLrLisAti,c sinfdoirrebcottlyh iPnLdLicAat,iinngd tihreactt ltyhei nPdLiLcaAti cnhgatihna wtaths egPraLfLteAd
cohnation twhea ssugrrfaafctee dofo GntOo-tZhneOsu [r2f5a]c. eofGO-ZnO[25].
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Figure 2. UV/Vis spectra of PLLA, GO-ZnO-PLLA, and GO-ZnO/PLLA.
FFiigguurere2 .2U. UVV//VVisiss sppeecctrtraao offP PLLLLAA,,G GOO-Z-ZnnOO-P-PLLLLAA,,a annddG GOO-Z-ZnnOO//PPLLLLAA..
3.2. GO-ZnO-PLLA Molecular Weight Measurement (Using GPC)
33..22.. GGOO--ZZnnOO--PPLLLLAA MMoolleeccuullaarr WWeeiigghhtt MMeeaassuurreemmeenntt ((UUssiinngg GGPPCC))
The molecular weight of the GO-ZnO-PLLA polymer was measured by gel permeation
TThhee mmoolleeccuullaarr wweeiigghhtt ooff tthhee GGOO--ZZnnOO--PPLLLLAA ppoollyymmeerr wwaass mmeeaassuurreedd bbyy ggeell ppeerrmmeeaattiioonn
chromatography (GPC) (Waters, Milford, MA, USA). The GO-ZnO-PLLA polymer had sharp,
cchhrroommaattooggrraapphhyy ((GGPPCC)) ((WWaatteerrss,, MMiillffoorrdd,, MMAA,, UUSSAA)).. TThhee GGOO--ZZnnOO--PPLLLLAA ppoollyymmeerr hhaadd sshhaarrpp,,
unimodal distributions, indicating that GO-ZnO had completely copolymerized with lactide and that
uunniimmooddaall ddiissttrriibbuuttiioonnss,, ininddicicaatitningg ththata tGGOO-Z-nZOnO hahda cdomcopmleptleeltye clyopcoolpyomlyemrizeerdiz ewditwh liathctildacet aidned athnadt
no lactide homopolymerization had occurred. The standard was polystyrene, and the mobile phase
tnhoa tlancotidlaec thiodmehopomolyompoelryizmaetiroizna htiaodn ohcacdurorcecdu. rTrehde .stTahnedsatradn wdaarsd pwolaysstpyorleynset,y arnende t,haen mdothbeilem pohbailsee
pwwhaaasss etteewttrraaahhsyytddetrrrooaffhuuyrraadnnro ((fTTuHHraFFn)).. (TTThhHeeF )nn.uuTmmhbbeeenrr uaamvveebrreaarggeea vmmerooallegecceuumllaarro lwweceeuiigglahhrtt w((MMeinng))h tooff( Mppoonll)yymmofeeprriiozzleeyddm ppeoorillzyye((LLd--
lactide)-grafted graphene oxide (GO-ZnO-PLLA) was 15.8 thousand, with a polydispersity of 1.10
plaocltyid(Le-)l-agcrtaidftee)d-g grraaftpehdengera opxhiednee (GoxOid-Zen(OGO-P-LZLnAO)- PwLaLs A1)5.w8 athso1u5s.8anthdo, uwsiatnhd a, wpoitlyhdaisppoelrysditiys poefr 1si.1ty0
o((FFfii1gg.uu10rree( F33i gaaunnrdde 3TTaaanbblldee T11a))b.. lTeThh1ee). rrTiinnhgge--rooippneegnn-oiinnpgge nppinoollgyymmpoeelrryiizmzaaettiirooiznna tooioff nssuuorrfffasaucceer faLLc--lelaacLctt-iilddaecet iwwdeaassw aaaccshhaiieecvhveeieddv,, eaadss,
indicated by the detection of only one peak in the GPC analysis. The molecular weight and
ainsdinicdaitceadt ebdyb tyheth ededteecteticotnio nofo fonolnyl yoonne eppeeaakk iinn tthhee GGPPCC aannaallyyssiiss.. TThhee mmoolleeccuullaarr wweeiigghhtt aanndd
polydispersity were narrow and even.
ppoollyyddiissppeerrssiittyy wweerree nnaarrrrooww aanndde evveenn..

Figure 3. Gel permeation chromatogram of GO-ZnO-PLLA molecular weight.
FFiigguurree 33.. GGeell ppeerrmmeeaattiioonn cchhrroommaattooggrraamm ooff GGOO--ZZnnOO--PPLLLLAA mmoolleeccuullaarr wweeiigghhtt..
Table 1. Results of gel permeation chromatogram analysis of GO-ZnO-PLLA molecular weight.
TTaabbllee 11.. RReessuullttss ooff ggeell ppeerrmmeeaattiioonn cchhrroommaattooggrraamm aannaallyyssiiss ooff GGOO--ZZnnOO--PPLLLLAAm moolleeccuullaarrw weeiigghhtt..
Mn Mw MP Mz Mz + 1 Polydispersity (Mw/Mn) Mz/Mw
Mn Mw MP Mz Mz + 1 Polydispersity (Mw/Mn) Mz/Mw
Mn Mw MP Mz Mz+1 Polydispersity(Mw/Mn) Mz/Mw
15,784 17,362 17,409 18,912 20,584 1.099972 1.08925
15,784 17,362 17,409 18,912 20,584 1.099972 1.08925
15,784 17,362 17,409 18,912 20,584 1.099972 1.08925
3.3. X-ray Photoelectron Spectroscopy (XPS) Analysis of GO-ZnO-PLLA
33..33.. XX--rraayy PPhhoottooeelleeccttrroonn SSppeeccttrroossccooppyy ((XXPPSS)) AAnnaallyyssiiss ooff GGOO--ZZnnOO--PPLLLLAA
Figure 4 shows that the X-ray photoelectron spectroscopy (XPS) obtains spectra by radiating
FFiigguurree 44 sshhoowwss tthhaatt tthhee XX--rraayy pphhoottooeelleeccttrroonn ssppeeccttrroossccooppyy ((XXPPSS)) oobbttaaiinnss ssppeeccttrraa bbyy rraaddiiaattiinngg
samples using X-rays, which excite electrons from the inner shells or the valence shell, and then
ssaammpplleess uussiinngg XX--rraayyss,, wwhhiicchh eexxcciittee eelleeccttrroonnss ffrroomm tthhee iinnnneerr sshheellllss oorr tthhee vvaalleennccee sshheellll,, aanndd tthheenn
analyzing the energy of the emitted photoelectrons. Each element has its own characteristic spectrum,
analyzing the energy of the emitted photoelectrons. Each element has its own characteristic spectrum,
which allows qualitative analysis of sample elemental composition. Figure 4 illustrates the XPS
which allows qualitative analysis of sample elemental composition. Figure 4 illustrates the XPS
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analyzingtheenergyoftheemittedphotoelectrons. Eachelementhasitsowncharacteristicspectrum,
wsuhricvheya lslopwecstrquumal iotaf tGivOe-aZnnaOly-sPisLoLfAs.a mThpel epeelaekmse innt atlhceo fmigpuorsei twioenr.eF aigtturirbeu4teildlu tsot rtahtees etlheemXePnStss Zunrv, eOy,
sapnedc tCru; mtheorfeG wOe-rZen nOo- PcLhaLrAac.tTehriestpice apkesakins tfhore ofitghuerre ewleemreenattst,r ifbuurttehdert oprtohveinelge mtheen ptsreZsne,nOce, oanf dGOC;-
tZhnerOe-wPLerLeAn ococmhaproascitteer. iTsthice preesauklstsf osruogtgheesrteedle tmhaent tPsL,LfuArt whearsp sruocvciensgsftuhlelyp grersaeftnecde oonftGoO th-Ze nsuOr-fPaLceL Aof
cGomO-pZonsOite. . TheresultssuggestedthatPLLAwassuccessfullygraftedontothesurfaceofGO-ZnO.

FFiigguurree 44.. XXPPSS ((XX--rraayy PPhhoottooeelleeccttrroonn SSppeeccttrroossccooppyy)) ssuurrvveeyys sppeeccttrruummo offG GOO--ZZnnOO--PPLLLLAA..
3.4. Infrared (IR) Spectroscopy
3.4. Infrared(IR)Spectroscopy
Figure 5 shows the Fourier transform IR (FTIR) spectra of the polymers, in which the typical
Figure 5 shows the Fourier transform IR (FTIR) spectra of the polymers, in which the
polyester absorption peaks appeared. In addition, the PLLA, GO-ZnO, GO-ZnO/PLLA, and GO-
typical polyester absorption peaks appeared. In addition, the PLLA, GO-ZnO, GO-ZnO/PLLA,
ZnO-PLLA IR measurements detected changes in the chemical functional groups upon grafting, as
and GO-ZnO-PLLA IR measurements detected changes in the chemical functional groups upon
gsrhaofwtinng. ,Tahse spheoawksn a.tT 3h4e54p cemak−1s, a3t42304 5c4mc−1m, 3−414,13, 4a2n0dc 3m47−41 ,cm34−41 1b,ealonndg3 t4o7 t4hecm O−–1Hb setlroentcghitnogt hpeeaOk–s Hin
PLLA, GO-ZnO, GO-ZnO/PLLA, and GO-ZnO-PLLA, respectively. The peak at 1759 cm−1
stretchingpeaksinPLLA,GO-ZnO,GO-ZnO/PLLA,andGO-ZnO-PLLA,respectively. Thepeakat
1c7o5r9recsmpo−n1dcso rtore tshpeo sntdrestctohitnhge vsitbreratcthioinn gpevaibkr oaft iCon=Op eina kthoef eCs=teOr binonthde ine sttheer PbLoLnAd icnhathine (PFLigLuArec h5aa,idn)
([F2i6g,u2r7e].5 Aa, ds)ig[2n6if,2ic7a]n.tAlys isgtrnoifincgaenr tCly=sOtr ostnrgeetcrhCin=gO vsitbrertacthioinng pveiabkr aatipopnepareeadk aapt p1e7a5r9e dcmat−11 f7o5r9 GcmO−-Z1nfoOr-
PLLA, which was likely caused by the grafting of the PLLA molecular chains onto the surface of GO-
GO-ZnO-PLLA,whichwaslikelycausedbythegraftingofthePLLAmolecularchainsontothesurface
ZnO. Therefore, the hydroxyl groups on the surface of GO-ZnO initiated the ring-opening of the
of GO-ZnO. Therefore, the hydroxyl groups on the surface of GO-ZnO initiated the ring-opening
lactide and the esterification with carboxyl groups [26]. The GO-ZnO/PLLA blends were also tested
of the lactide and the esterification with carboxyl groups [26]. The GO-ZnO/PLLA blends were
by infrared contrast, and the GO-ZnO/PLLA curves of the blends were found to be different from
also tested by infrared contrast, and the GO-ZnO/PLLA curves of the blends were found to be
that of the copolymer (Figure 5d), it is consistent with the findings of Sun and He [28], in which GO-
differentfromthatofthecopolymer(Figure5d),itisconsistentwiththefindingsofSunandHe[28],
ZnO-PLLA was prepared by the ring-opening polymerization of lactide. Furthermore, the
inwhichGO-ZnO-PLLAwaspreparedbythering-openingpolymerizationoflactide. Furthermore,
tchheacrhacatrearcitsetricis ptiecapkesa okfs PoLfLPAL,L iAnc,liundcliundg itnhge tshteresttcrheitnchg ivnigbrvaitbiroant ioofn Co–fCCH–C3, Hthe, tbheendbeinngd ivnigbrvaitbiorant ioofn –
3
oCfH–C3, Han,da nthdet haesyamsymmemtreict ribcebnednindgin gvivbirbartiaotnio nofo f––CCHH3, ,aappppeeaareredd aatt 11009955,, 11118877,, aanndd 11445577c mcm−−11,,
3 3
respectively [29–33], in the spectrum of GO-ZnO-PLLA. The results thus show that the PLLA
respectively[29–33],inthespectrumofGO-ZnO-PLLA.TheresultsthusshowthatthePLLAmolecular
molecular chains were successfully grafted onto the surface of GO-ZnO.
chainsweresuccessfullygraftedontothesurfaceofGO-ZnO.
Materials 2018,11,323 7of17
Materials 2018, 11, x FOR PEER REVIEW 7 of 17
Materials 2018, 11, x FOR PEER REVIEW 7 of 17
.
Figure 5. Fourier transform IR (FTIR) spectra of (a) PLLA, (b) GO-ZnO, (c) GO-ZnO/PLLA, and (d) GO-
Figure 5. Fourier transform IR (FTIR) spectra of (a) PLLA, (b) GO-ZnO, (c) GO-ZnO/PLLA,
ZnO-PLLA.
and(d)GO-ZnO-PLLA.
3.5. Differential Sca.nning Calorimetry (DSC) of PLLA and Its Composites
3.5. DifferentialScanningCalorimetry(DSC)ofPLLAandItsComposites
Figure 6 shows the differential scanning calorimetry (DSC) of PLLA and its composites. The melt
Figure 5. Fourier transform IR (FTIR) spectra of (a) PLLA, (b) GO-ZnO, (c) GO-ZnO/PLLA, and (d) GO-
crystallization behavior was investigated by DSC measurements, DSC cooling scan thermogram of
FigurZenO6-sPhLoLAw. sthedifferentialscanningcalorimetry(DSC)ofPLLAanditscomposites. Themelt
PLLA (Mn¼ 20,000), PLLA, GO-ZnO-PLLA, and GO-ZnO/PLLA blend at the cooling rate of 10 C/min.
crystallizaAt ionneatb PeLhLaAv icorryswtalaliszesin ivn eas tbigroaatde dcrybsytalDlizSatCionm teemaspuerraetumree nratnsg,eD wSiCth cpoeaokl incrgystsacllaiznatitohne rmogram
3.5. Differential Scanning Calorimetry (DSC) of PLLA and Its Composites
of PLLA a(Mppnea1ri/n4g a2t 016,000 °0C). ,ThPeL cLryAsta,llGizaOti-oZn npeOak-P teLmLpAer,ataunred of GPLOL-AZ inn OGO/-PZLnOL/APLLbAle oncdcuras tatt 1h6e1 °cCo, oling rate
of 10 C/mFaingidnu .trhee A6 t esmhnopeweartast tuPhrLee LrdaAinffgeecr oreyfn cstrityaasll tslacilzlaizenasntiioinnng i csa ablbroorraoidma, dwethrcyilre (y DthsStea Cclr)ly ioszftaa PltlLiizoLantAio tanen tmedm pitpese rcraaottmuurpree oosfr ittahenes .gG TeOh-ew mitehlt peak
crystacrllyisztaaZtlliniozOna-PtiaLopLnpA be seahhrifaitnvs gitoo rat htwe1 ah6si0g ihn◦eCvr et.esTmtihgpaeerteactdruy rbesy taa tD l1l6iSz8C a° Ctmi oaennadsp uthereae tmkemetenpmtesr,ap tDeurrSeaC tr aucnrogeoel obinfecgPo LmscLeasAn n atihrnreoGrwmO. o-gZranmO /oPf LLA
The nonisothermal crystallization behavior of GO-ZnO/PLLA blend, which has the same
occurPsLaLtAc1 o(6Mm1pno¼◦sCi t2i,o0na,0 n0odf0 )G, tOPhL-eZLntAOe,m aGsp Othe-arZtan otOuf G-rPeOL-rLZaAnnO,g a-PenLdoL GfA,Oc wr-yZassn tpOael/rlPfioLzraLmtAeido b ntloe nicsodm baprt oathraeed wc,oitwohl hiGniOgle- rZatnthOee -oPfcL 1rL0yA sC. t/amlliinz.a tion
tempAer anteuaTrth eeP coLrfyLstAtha lelcizrGaytOsiotan-lZ ltienzmeOps e-PrinaLt uLar eA borfso PhaLdiLf tAsc rintyo sthtatehl lGeizOah-tZiigonnhO e/PtreLmtLeApme isrp aaetburorauett u 1r6rae1n °gaCet. A1w6s i8ctoh◦m Cppeaaarenkd d wcrtiythhse tnaetlealitmz aptioenra ture
appearing at 160 °C. The crystallization peak temperature of PLLA in GO-ZnO/PLLA occurs at 161 °C,
PLLA, the crystallization temperature of PLLA in the GO-ZnO/PLLA has not been increased. However,
rangebecomesnarrow.
and ththee t cermysptaelrliaztautiroen r taenmgpee roaft ucrrey ostf aPlLliLzAat iino nth ies G bOro-ZadnO, w-PhLiLleA t whea sc irnycsrteaalsleidza ttoi o16n8 t °eCm., psuegragteustrien go tfh tahte GO-
The nonisothermal crystallization behavior of GO-ZnO/PLLA blend, which has the same
ZnO-PthLeL GAO -sZhnifOts p tloa ttehleet sh iingchoerpr oteramtepde irnatotu trhee aPtL 1L6A8 m°Cat rainx dh atvhee at enmucpleeartaintug reef freacnt ogne tbheec cormysetasl lnizaartrioonw .
compositiToohfn eP LonLfAoGn iOins o-thZthen eGOrOm-aaZsln tOch-rPayLtsLtoaAfll.Gi BzaaOstie-odZn no nOb te-hhPea LcvoLimoArp ,aorwifs aoGns Obpe-etZwrnfeoOern/m PthLeeLd rAets oublctlseo nomdf G,p Oawr-hZeincwOh-i PthhLaLGsA O tahn-eZd nsaOm-Pe LLA.
TheccroymstpaGolslOiitz-iZaonntOi oo/Pnf LGtLeOAm -bZplenenOdra, aats un urthecaleota fotiPfn gGL eLOffAe-Zctni fnOout-nPhdLe iLnGA thO, ew- mZaensl tOp cer/yrsfPotaLrlmlLizAeadti oitsno a ocfbo GomOup-tZa1nre6O 1w-P◦iLtCLhA .GA isO sn-oZcton omOnl-ypP aLrLeAd. with
due to the individual GO-ZnO platelets in the PLLA matrix, but is also due to the tight bonding between
neatPThLeL cAry,stthalelizcartyiosnta tlelmizpaetiroatnurtee mof pPeLrLaAtu inre thoef GPOL-LZAnOin/PtLhLeAG isO a-bZountO 1/61P °LCL. AAsh caosmnpoartebde wenithi nnceraet ased.
the GO-ZnO platelets [25].
HowPeLvLerA,,t thhee ccrryysstatalllilziaztaiotino tnemtepmerpateurraet oufr PeLoLfAP iLn LthAe GinOt-hZenOG/POL-LZAn Oha-sP nLoLt Abeewn ainscirneacsreeda.s Hedowtoev1e6r,8 ◦C.,
suggethseti cnrgystthalalitztahtieonG tOem-ZpnerOatuprlea toefl PeLtsLiAn cino rthpeo GraOte-ZdniOn-tPoLtLhAe wPLasL iAncrmeaasterdix toh 1a6v8e °aC.n, suucglgeeasttiinngg ethffaet cton
the GO-ZnO platelets incorporated into the PLLA matrix have a nucleating effect on the crystallization
thecrystallizationofPLLAintheGO-ZnO-PLLA.Basedonthecomparisonbetweentheresultsof
of PLLA in the GO-ZnO-PLLA. Based on the comparison between the results of GO-ZnO-PLLA and
GO-ZnO-PLLAandGO-ZnO/PLLAblend, anucleatingeffectfoundinthemeltcrystallizationof
GO-ZnO/PLLA blend, a nucleating effect found in the melt crystallization of GO-ZnO-PLLA is not only
GO-ZnO-PLLAisnotonlyduetotheindividualGO-ZnOplateletsinthePLLAmatrix,butisalsodue
due to the individual GO-ZnO platelets in the PLLA matrix, but is also due to the tight bonding between
tothetightbondingbetweentheGO-ZnOplatelets[25].
the GO-ZnO platelets [25].

Figure 6. Differential scanning calorimetry (DSC) curves of PLLA and PLLA and its composites.

Figure 6. Differential scanning calorimetry (DSC) curves of PLLA and PLLA and its composites.
Figure6.Differentialscanningcalorimetry(DSC)curvesofPLLAandPLLAanditscomposites.
Materials 2018,11,323 8of17
Materials 2018, 11, x FOR PEER REVIEW 8 of 17
33..66.. TThheerrmmooggrraavviimmeettrriicc AAnnaallyyssiiss ((TTGGAA))
AAss oobbsseerrvveedd iinn FFiigguurree 77,, tthhee mmaassss lloossss zzoonnee ffoorr GGOO--ZZnnOO wwaass 5500––220000 ◦°CC.. TThhee lloossss aatt 5500––110000 ◦°CC
wwaass aattttrriibbuutteedd ttoo tthhee pphhyyssicicaal levevapapoorartaitoino nofo wf wataetre frrofrmo mGOG Oof GofOG-ZOn-OZn. OTh.eT lhoessl oast s10a0t–120000– 2°C00 w◦aCs
watatrsibatuttreibdu ttoe dthteo dtheecodmecpoomsiptioosni toiof nthoef ftuhnecftuionnctailo gnraolugpros,u spusc,hs uacsh haysdhroyxdyrol xaynlda cnadrbcoaxrbyol xgyrolugrposu, posn,
othne tGheOG-ZOn-OZ nsOurfsaucref atcheatt hdaetcodmecpoomsepdo steod CtOo,C COO,2C, aOn2d, awnadtewr avtaeprovra [p3o4r,3[53]4. ,T35h]e. mThaessm loassss olof sGsOof-
GZnOO-Z-PnLOL-APL aLtA 22a0t–222800– 2°C80 c◦oCulcdo bueld dbiveiddeivdi dinetdo itnwtoo tswegomseegnmts.e nAtts .22A0t–225200 –°2C5,0 t◦hCe ,mthaessm loassss lwosass
watatrsibatuttreibdu ttoe dthteo dtheceodmecpoomsiptioosni toiof nreosfidreusaild ouxaylgoexny-gceonn-tcaoinnitnagin fiungncftuionnctailo gnraolugpros;u apts 2;5a0t–225800– 2°8C0, t◦hCe,
tmheasms alossssl owssaws aatstraitbtruitbeudt etdo ttoheth deedcoecmopmopsoitsiiotino nofo PfLPLLALA grgarfatfetded oonntoto GGoo--ZZnnOO [[3366]].. TThhee mmaassss lloossss ooff
PPLLLLAA mmoossttllyy ooccccuurrrreedd aatt 334400––338800 ◦°CC,, wwhhiillee tthhee mmaassss lloossss ooff GGOO--ZZnnOO--PPLLLLAA wwaass bbeettwweeeenn tthhoossee ooff
GGOO--ZZnnOO aanndd PPLLLLAA.. TThheessee rreessuullttss ssuuggggeesstteedd tthhaatt tthhee PPLLLLAA mmoolleeccuullaarr cchhaaiinnss wweerree ssuucccceessssffuullllyy ggrraafftteedd
oonnttoo tthhee GGOO--ZZnnOO ssuurrffaaccee.. BBaasseedd oonn tthhee lliitteerraattuurree aanndd aaccccoorrddiinngg ttoo tthhee TTGGAA ccuurrvveess,, tthhee ggrraaffttiinngg rraattiioo
ooff PPLLLLAA ttoo tthhee ssuurrffaaccee ooff GGOO--ZZnnOO wwaass aapppprrooxxiimmaatteellyy 6600..22 wwtt %% [[3377]] ffoorr tthhee inin--ssiittuu rriinngg--ooppeenniinngg
ppoollyymmeerriizzaattiioonn ooff tthhee lalacctitdidee. .ThTeh ewweigehigth ltoslso sosf oGfOG-ZOn-ZOn-POL-LPALL wAasw baestwbeetewn etehnosteh oosf eGoOf-GZnOO-Z annOd
aPnLdLAPL. LWAe. Walseoa plseorfpoerrmfoerdm ae dTGa TaGnaalnyasilsy soins oGnOG-ZOn-ZOn-POL-LPALL bAlebnldens,d asn,dan tdheth reesreusltusl tsshsohwoewde dthtahta at
asisginginfiicfiacnatn dtidffiefrfeernecnec eexeisxtiesdte cdomcopmapreadr etdo tthoet hcoepcoolpyomlyemr oefr GoOfG-ZOn-OZ-nPOL-LPAL.L TAh.eT rheseurletss uthltusst hshuoswsh tohwat
tthhaet PtLhLeAPL mLoAlemcuollaerc cuhlaarincsh waienrsew suecrecessusfcucellsys fgurlalfytegdr atoft ethdet osutrhfeacseu orffa GceOo-ZfGnOO.- ZnO.

Figure 7. TGA (Thermogravimetric Analysis) curve of (a) GO-ZnO, (b) PLLA, and (c) GO-ZnO-PLLA.
Figure7.TGA(ThermogravimetricAnalysis)curveof(a)GO-ZnO,(b)PLLA,and(c)GO-ZnO-PLLA.
3.7. Physical Property Characterization
3.7. PhysicalPropertyCharacterization
To further characterize the dispersity of GO-ZnO and GO-ZnO-PLLA in solution before and
To further characterize the dispersity of GO-ZnO and GO-ZnO-PLLA in solution before and
after grafting, we performed dynamic light scattering (DLS) to test the particle size and distribution
aftergrafting,weperformeddynamiclightscattering(DLS)totesttheparticlesizeanddistribution
of the materials in solution. The solutions were all at a concentration of 1 mg/mL. The materials were
of the materials in solution. The solutions were all at a concentration of 1 mg/mL. The materials
dissolved and ultrasonicated for 2 h prior to a rapid analysis. The results of the analysis are illustrated
were dissolved and ultrasonicated for 2 h prior to a rapid analysis. The results of the analysis are
iilnlu Fsitgruatreed 8i. nBeFfiogruer egr8a.ftBinefgo, rtehge rpaaftritnicgl,e tdhieampaertteirc loefd GiaOm-ZetneOr owfaGsO 3-1Z.0n1O μwma isn 3H1.20O1 aµnmd i5n5.H56 Oμman idn
2
chloroform. The dispersity of GO-ZnO in chloroform was significantly better than that in water. The
55.56µminchloroform. ThedispersityofGO-ZnOinchloroformwassignificantlybetterthanthat
data suggested that GO-ZnO existed stably in water in the form of single-layer or few-layers [36].
in water. The data suggested that GO-ZnO existed stably in water in the form of single-layer or
There was a strong Van der Waals force and the movement of a large molecular chain, such as a bond
few-layers [36]. There was a strong Van der Waals force and the movement of a large molecular
or electrostatic attraction, between the chains between GO nanolayers, so the layers tended to
chain,suchasabondorelectrostaticattraction,betweenthechainsbetweenGOnanolayers,sothe
aggregate. Hence, the particle size and particle size distribution could reflect the dispersity of the
layers tendedto aggregate. Hence, the particlesize andparticle sizedistribution couldreflectthe
material in solvents [38]. The particle size of GO-ZnO-PLLA in water was 15.14 μm and gave a wide
dispersityofthematerialinsolvents[38]. TheparticlesizeofGO-ZnO-PLLAinwaterwas15.14µm
distribution of particle size. These results showed that after lactide ring-opening polymerization, the
andgaveawidedistributionofparticlesize. Theseresultsshowedthatafterlactidering-opening
hydrophobic PLLA was successfully grafted onto the surface of GO, which then turned GO-ZnO-
polymerization,thehydrophobicPLLAwassuccessfullygraftedontothesurfaceofGO,whichthen
PLLA into a hydrophobic material and reduced its dispersity in water. The particle size in chloroform
turnedGO-ZnO-PLLAintoahydrophobicmaterialandreduceditsdispersityinwater. Theparticle
was 10.57 μm, suggesting that the dispersity of GO-ZnO-PLLA in chloroform was far greater than
that in water. After grafting, GO changed from a hydrophilic material to a hydrophobic material.
Materials 2018,11,323 9of17
sizeinchloroformwas10.57µm,suggestingthatthedispersityofGO-ZnO-PLLAinchloroformwas
Materials 2018, 11, x FOR PEER REVIEW 9 of 17
Materials 2018, 11, x FOR PEER REVIEW 9 of 17
fargreaterthanthatinwater.Aftergrafting,GOchangedfromahydrophilicmaterialtoahydrophobic
mThautesr, iaitl .iTs hfuesa,siitbilse fteoa siinbcleretaosein cthreea sdeistpheerdsiitsyp eorfs iftyunocftfiounnactliizoenda liGzeOd-ZGnOO-Z-PnLOL-APL iLnA cihnlocrholfoorromfo rbmy
Thus, it is feasible to increase the dispersity of functionalized GO-ZnO-PLLA in chloroform by
btryigtgriegrginegri nlagctliacc taiccidac aidnhayndhryiddrei dveiav tihaet hheydhryodxryolx gyrlogurpou opn othnet hGeOG sOursfuarcfea.c e.
triggering lactic acid anhydride via the hydroxyl group on the GO surface.

FFFiiiggguuurrreee 888... LLLaaassseeerrr aaannnaaalllyyysssiiisss ooofff pppaaarrrtttiiicccllleee sssiiizzzeee aaannnddd ssshhhaaapppeee ooofff GGGOOO---ZZZnnnOOO aaannnddd GGGOOO---ZZZnnnOOO---PPPLLLLLLAAA...
3.8. Dispersions of GO-ZnO–PLLA and GO-ZnO/PLLA in Chloroform
33..88.. DDiissppeerrssiioonnss ooff GGOO--ZZnnOO––PPLLLLAA aanndd GGOO--ZZnnOO//PPLLLLAA iinn CChhlloorrooffoorrmm
To further confirm the solubility of GO-ZnO/PLLA and GO-ZnO-PLLA in chloroform, we
TToo fufurtrhthererc ocnofinrfmirmth etshoel usboilluitbyioliftyG Oof- ZGnOO/-ZPnLOLA/PaLnLdAG aOn-dZ nGOO-P-LZLnAO-iPnLcLhlAor oinfo rcmhl,owroefporremp,a rwede
prepared 0.5 mg/mL GO-ZnO/PLLA and GO-ZnO-PLLA solutions in chloroform. After the solutions
0p.r5empagr/edm 0L.5G mOg-/ZmnLO G/POL-ZLnAOa/nPdLLGAO a-Zndn OG-OP-LZLnAOs-PoLluLtAio nsosliunticohnlso rino fcohrlmor.oAfofrtmer. tAheftesro ltuhtei osonlsuwtioenres
were sonicated for 2 h and left standing for 12 h (Figure 9), the solubility was investigated. All the
swoenriec astoendicfaotred2 fhora 2n dh alenfdt sletaftn sdtianngdifnogr f1o2r h12 (hF i(gFuirgeur9e) ,9)t,h teheso sloulbuibliitlyityw wasasi ninvveesstitgigaatetedd.. AAllll tthhee
GO-ZnO/PLLA precipitated to the bottom of the chloroform after the solution was sonicated and left
GGOO--ZZnnOO//PPLLLLAA prperceicpiiptaittaetde dto ttohet hbeotbtoomtt oomf thoef cthhleorcohfolormro faofrtmer tahfete sroltuhteiosno wluatiso snonwicaastesdo annicda tleedft
for 12 h, whereas GO-ZnO-PLLA was still evenly dispersed in the chloroform. Similarly, when
afonrd 1l2e fthf, owrh12erhea,sw GheOr-eZansOG-OPL-ZLnAO w-PaLsL sAtilwl aevsesntillyl edvisepnelyrseddis pine rsthede icnhltohreofcohrlmor. oSfoimrmila.rSlyim, iwlahrelyn,
chloroform was used to dissolve GO-ZnO/PLLA and PLLA blends, 12 h later, GO-ZnO/PLLA was
wchhloenrocfhorlomro wfoarsm uwseads utos eddistsoodlviess GolOve-ZGnOO-/ZPnLOLA/P aLnLdA PaLnLdAP LbLleAndbsl,e n12d sh, 1l2atherl,a GteOr,-GZOnO-Z/PnLOL/AP LwLaAs
precipitated, because the dispersion of GO-ZnO/PLLA in chloroform is poor. These different behaviors
wpraescipprietactiepdit,a bteecda,ubseec tahues deitshpeerdsiiospne orfs iGoOn-oZfnGOO/P-LZLnAO /inP cLhLloAroinfocrmhl oisr opfooorrm. Tihsepsoe odri.ffTerheenste bdeihffaevrieonrst
indicate that the PLLA molecular chains grafted to the GO-ZnO/PLLA surface very strongly interacted
binedhiacvaitoer tshiantd tihcea tPeLtLhaAt mthoelPecLuLlAar mchoaleincsu lgarracftheadi ntos gthraef GteOd-tZonthOe/PGLOL-AZ nsuOr/faPcLeL vAersyu srtfraocnegvleyr yinstterroancgteldy
with the solvent, thus increasing the dispersion of GO-ZnO-PLLA in chloroform [37].
iwntitehr atchtee dsowlviethntt,h theusos livnecnret,atshinugs tihnec rdeiasspienrgsitohne odfi sGpOer-sZinonO-oPfLGLOA- Zinn cOh-lPoLroLfAormin [c3h7l]o. roform[37].

Figure 9. Dispersions of GO-ZnO-PLLA and GO-ZnO/PLLA in chloroform.
FFiigguurree 99.. DDiissppeerrssiioonnss ooff GGOO--ZZnnOO--PPLLLLAA aanndd GGOO--ZZnnOO//PPLLLLAA iinn cchhlloorrooffoorrmm..
3.9. Testing of Tensile Properties of PLLA and GO-ZnO-PLLA/PLLA Composite Films
33..99.. TTeessttiinngg ooff TTeennssiilleeP Prrooppeerrttieiesso offP PLLLLAAa annddG GOO-Z-ZnnOO-P-PLLLLAA/P/PLLLLAAC CoommppoossitieteF Filimlmss
Figure 10 and Table 2 show the changes in the tensile strength and the elongation at break of
FFiigguurree 1100a annddT aTbalbele2 s2h sohwowth ethceh acnhgaensgiens tihne tthene stielenssitlree nsgtrtehnagntdh tahnede ltohneg ealtoionngaattiobnre aakt borfePaLkL oAf
PLLA and GO-ZnO-PLLA/PLLA composite films. All film samples tested were 1.5 mm in thickness
aPnLdLAG Oan-Zdn GOO-P-ZLnLOA-/PPLLLLAA/PcLoLmAp coosmitepfiolsmites .fiAlmllsfi. lAmll sfailmmp slaemstpelsetse dtewsteedre w1.e5rem 1m.5 imnmth iicnk tnheicssknaensds
and 15 mm in width. The results of the tensile property test showed that for pure PLLA, the tensile
1a5nmd m15 imnwmi dinth w. Tidhteh.r eTshuelt sreosfutlhtse otefn tshiele tpenrospileer ptyrotepsetrsthyo twesetd shthoawtefodr tphuarte fPorL LpAur,et hPeLtLenAs,i ltehest rteenngsitlhe
strength was 68.12 MPa, and the elongation at break was 6.3%. With the addition of 0.5% GO-ZnO-
strength was 68.12 MPa, and the elongation at break was 6.3%. With the addition of 0.5% GO-ZnO-
PLLA, the tensile strength and elongation rate of PLLA increased to 72.94 MPa and 7.42%,
PLLA, the tensile strength and elongation rate of PLLA increased to 72.94 MPa and 7.42%,
respectively. With the addition of 1.5% of the composite material GO-ZnO-PLLA, the tensile strength
respectively. With the addition of 1.5% of the composite material GO-ZnO-PLLA, the tensile strength
and elongation rate increased to only 86.24 MPa and 9.8%, respectively. These results suggested the
and elongation rate increased to only 86.24 MPa and 9.8%, respectively. These results suggested the
Materials 2018,11,323 10of17
was 68.12 MPa, and the elongation at break was 6.3%. With the addition of 0.5% GO-ZnO-PLLA,
the tensile strength and elongation rate of PLLA increased to 72.94 MPa and 7.42%, respectively.
WMaittehritahlse 20a1d8d, 1i1ti, oxn FOoRf 1P.E5E%R oRfEtVhIeEWco mpositematerialGO-ZnO-PLLA,thetensilestrengthandelong10a toifo 1n7
rateincreasedtoonly86.24MPaand9.8%,respectively. TheseresultssuggestedtheGO-ZnO-PLLA
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sAmdadliltdioifnfaelrleyn, ctehse.re are some small differences.

Figure 10. The stress–strain curve of PLLA and GO-ZnO-PLLA/PLLA composite films.
Figure10.Thestress–straincurveofPLLAandGO-ZnO-PLLA/PLLAcompositefilms.
Table 2. Test of tensile properties of PLLA and GO-ZnO-PLLA/PLLA.
Table2.TestoftensilepropertiesofPLLAandGO-ZnO-PLLA/PLLA.
Elastic Modulus Elongation Tensile Stress at Tensile Strength
Sample Name ElasticModulus Elongationat TensileStressat TensileStrength
SampleName (MPa) at Break (%) Break (MPa) (MPa)
(MPa) Break(%) Break(MPa) (MPa)
PLLA 1356.6 6.3 68.12 68.12
PLLA 1356.6 6.3 68.12 68.12
(0.5%) (G0.5O%-)ZGnOO-Z-PnOL-LPALL/AP/LPLLALA 1410450.55.588 77..4422 6666.8.833 72.9742.94
(1.0%) (G1.0O%-)ZGnOO-Z-PnOL-LPALL/AP/LPLLALA 1616686.86.622 66..8866 7777.4.499 77.6757.65
(1.5%)GO-ZnO-PLLA/PLLA 1730.62 9.8 86.24 86.24
(1.5%) GO-ZnO-PLLA/PLLA 1730.62 9.8 86.24 86.24
3.10. SEMAnalysisofGO-ZnO-PLLA/PLLA
3.10. SEM Analysis of GO-ZnO-PLLA/PLLA
ToobservethedispersityofGO-ZnO-PLLAinthecompositematerialandthefracturesurface
To observe the dispersity of GO-ZnO-PLLA in the composite material and the fracture surface
morphology,weemployedscanningelectronmicroscopy(SEM)toexaminethefracturesurfacesof
morphology, we employed scanning electron microscopy (SEM) to examine the fracture surfaces of
PLLAandGO-g-PLLA/PLLAcompositefilms(Figure11). Figure11aillustratestheSEMphotoof
PLLA and GO-g-PLLA/PLLA composite films (Figure 11). Figure 11a illustrates the SEM photo of
fracturesurfacesofpurePLLAandGO-ZnO-PLLA/PLLAcompositefilms. AsseeninFigure11a,
fracture surfaces of pure PLLA and GO-ZnO-PLLA/PLLA composite films. As seen in Figure 11a, the
the fracture surface of PLLA was neat and smooth. Obvious bumps were not observed in large
fracture surface of PLLA was neat and smooth. Obvious bumps were not observed in large numbers.
numbers. ThefracturesurfacesofGO-ZnO-PLLA/PLLAcompositefilmsillustratedinFigure11b–d
The fracture surfaces of GO-ZnO-PLLA/PLLA composite films illustrated in Figure 11b–d were
wererougheranddenserthanthoseofPLLA,therebyrequiringhigherenergyatfracturethanpure
rougher and denser than those of PLLA, thereby requiring higher energy at fracture than pure PLLA.
PLLA.ThefracturetypewasalsodifferentfromthatofpurePLLA,whichshowedobviousbrittle
The fracture type was also different from that of pure PLLA, which showed obvious brittle fracture.
fracture. On the other hand, there was no apparent graphene clustering observed on the fracture
On the other hand, there was no apparent graphene clustering observed on the fracture surfaces,
indicating that GO-ZnO-PLLA was evenly distributed in PLLA. When 0.5 wt %, 1 wt % and 1.5 wt %
of GO-ZnO-PLLA were added to PLLA, it was clearly seen that the GO-ZnO-PLLA flakes were tightly
embedded in PLLA and showed strong interface adhesion with PLLA.