beckert 12

Sulfur-Functionalized Graphenes as Macro-Chain-Transfer and RAFTAgents for Producing Graphene Polymer Brushes and PolystyreneNanocompositesFabian Beckert, Christian Friedrich, Ralf Thomann, and Rolf Mulhaupt*

Freiburg Materials Research Center, FMF and Institute for Macromolecular Chemistry of the University of Freiburg,Stefan-Meier-Str. 31, D-79104 Freiburg, Germany

ABSTRACT: Novel families of polystyrene carbon nanocomposites andgraphene brushes were prepared by means of free radical styrene graftingusing sulfur-functionalized graphene (S-FG) as macro-chain-transferagent. Two strategies were examined for growing polystyrene chainsonto graphite oxide (GO), stearylamine-modied GO (Stearyl-GO), andthermally reduced GO (TRGO): (i) chain transfer with novel thiol-functionalized graphenes and (ii) RAFT-mediated polymerization usingdithiourethane-, dithioester- and dithiocarbonate-functionalized graphe-nes. Novel thiol-functionalized graphenes were obtained from FG eitherby esterication with 3-mercaptopropionic acid or by reaction withpropylene sulde. The styrene graft polymerization was initiated either with AIBN at 65 C or by thermal styrene self-initiation at130 C. The graphene content, varying between 0 and 5.2 wt %, was determined by means of thermogravimetric analysis (TGA).Upon hydrolytic cleavage of the dithiocarbonate linker, the stability of the S-FG dispersion in polystyrene was lost, and thenonbonded S-FG was separated quantitatively from polystyrene by solvent extraction. During melt processing the graphenebrushes self-assembled to produce nanoribbons and skeleton-like carbon superstructures, as determined by means oftransmission electron microscopy (TEM). Both grafting eectiveness and superstructure formation were monitored by means ofmelt rheology.

INTRODUCTIONGraphenes represent one-carbon-atom thick and micrometer-wide two-dimensional carbon macromolecules consisting of ahoneycomb-like hexagonal array of sp2-bonded carbonatoms.1,2 They exhibit extraordinary property combinationssuch as very high stiness and strength, high electron mobilityat room temperature, tunable band gap, high thermalconductivity, optical transparency, excellent thermal andenvironmental stability, high abrasion resistance, and veryeective absorption of UV and IR radiation.3,4 A wide range ofgraphene applications are currently being envisioned, includingadvanced functional materials such as transparent electrodes asITO substitute,5 printable electronics,6 sensors,7 membranes,8

ultrathin carbon lms,9 catalyst supports,10 coatings,11 andpolymeric graphene nanocomposites for advanced engineeringplastics.12,13 The incorporation of small amounts of graphenesinto polymers can aord substantial matrix reinforcement,improved damage tolerance, electrical and thermal conductivity,electromagnetic shielding, corrosion protection, abrasionresistance, ame retardancy, and even barrier resistance againstpermeation of gases and uids. It is well-known that thenanocomposite properties strongly depend upon eectivegraphene dispersion, good interfacial adhesion, and supra-molecular graphene self-assembly producing skeleton-likecarbon superstructures. In order to control both dispersionand self-assembly during melt compounding, it is an important

research objective to attach polymers to graphenes by means ofcovalent bond formation.Several grafting-from and grafting-o routes have been

explored using either graphite or functionalized graphenes asintermediates for the preparation of graphene brushes andsheet-coil carbon polymers.14 Although direct grafting ofgraphite is very attractive and cost-eective, few examples havebeen successful because grafting must be accompanied byintercalation and eective exfoliation of single graphenes fromgraphite during the grafting process. The use of alkali metalgraphite intercalates as macroinitiators for anionic styrenegrafting,15 mechanochemical production of polystyrene gra-phene nanocomposites,16,17 and electrochemical graphiteactivation in ionic liquids18 have been reported. Mostapproaches employ functionalized graphenes (FG) which areeasy to disperse in various organic media. Moreover, thefunctional groups of FG are readily modied to producemacroinitiators, macromonomers, and macro-chain-transferagents. The key intermediate is graphite oxide (GO), derivedfrom graphite which is intercalated in sulfuric acid and oxidizedwith strong oxidizing agents such as potassium permanganateor sodium chlorate.1921 Upon chemical or thermal reduction,GO is converted into FG containing hydroxyl, phenoxy,

Received: July 4, 2012Revised: August 13, 2012Published: August 30, 2012

Article

pubs.acs.org/Macromolecules

2012 American Chemical Society 7083 dx.doi.org/10.1021/ma301379z | Macromolecules 2012, 45, 70837090

carboxyl, and epoxy groups. During the thermal reduction, theFG oxygen content is governed by the reduction temper-ature.22,23 The strongly hydrophilic GO, which is dicult todisperse in styrene, is rendered hydrophobic when hydroxylgroups are converted with phenyl isocyanate into organophilicphenylurethane groups.24 In an alternative GO modication,the GO epoxy groups are reacted with stearylamine to formorganophilic and easy-to-disperse amphiphilic stearylamine-modied GO (Stearyl-GO), which is partially reduced duringthis amine addition reaction.25 Ruo and co-workers haveprepared novel GO-based macroinitiators for initiating the freeradical graft polymerization on the FG. The hydroxyl groups ofGO are esteried with -bromoisobutyryl bromide to producegraphene macroinitiators for styrene grafting via atom transferradical polymerization (ATRP).26,27 Novel acrylic FG macro-monomers are obtained when GO hydroxyl groups areesteried with methacryloyl chloride.28 As a function of theFG acrylate functionality, the FG acrylate content and theacrylate conversion during polymerization, grafting, and cross-linking could take place simultaneously.Chain-transfer agents containing dithioester, dithiourethane,

and di- or trithiocarbonate moieties are used extensively inreversible additionfragmentation chain transfer (RAFT) andcontrolled radical graft polymerization for tailoring segmentedof polymers and core/shell particles.29 Most FG-based macro-chain-transfer agents have been derived from GO. Sanderson etal. reported the esterication of the GO hydroxyl groups withdodecyl isobutyric acid trithiocarbonate. Incorporation of the n-dodecyl chains renders the GO chain-transfer agent hydro-philic, thus improving compatibility with organic solvents andmonomers commonly used in controlled radical polymer-ization. This GO-RAFT system enables the styrene mini-emulsion polymerization producing coreshell particles.30Another GO-RAFT macro-chain-transfer agents has beendesigned by Kang et al. for growing poly(N-vinylcarbazole)onto GO sheets.31 Han and Qi produced GO-RAFT systems ina multistep synthesis via esterication of the GO carboxylic acidgroups to dithioesters. The resulting GO-RAFT macro-chain-transfer agent was used in methacrylamide polymerization.32,33

Zhao and co-workers reported on the combination of RAFTpolymerization with click chemistry to attach poly(N-isopropylacrylamide, PNIPAM) brushes onto reduced GOsurfaces, thus rendering graphene brushes thermoresponsive.34

In another work, the group of Zhao presented a simultaneouscoupling reaction and RAFT process to graft dierent polymersfrom GO. A RAFT-active trithiocarbonate group terminatedwith a trimethoxysilyl moiety was coupled to GO locatedhydroxyl groups. After cleavage of the polymers by aminolysisor HF treatment, narrow weight distributions were found.35

Although thiols are well-known as very powerful chain transferagents in free radical polymerization and calculations on thiol-functionalized graphenes have been reported by Denis,36 to thebest of our knowledge FG thiols have not been used as FGmacro-chain-transfer agents.Here we report on a very versatile process for the preparation

of sulfur-functionalized Stearyl-GO and TRGO useful as macro-chain-transfer and RAFT agents for the free radical grafting ofpolystyrene chains onto FG. Melt rheology of conventionalpolystyrene/FG melt compounds was compared with that ofpolystyrene grafted onto S-FG in order to examine theinuence of grafting eectiveness on the dispersion andstructure formation, resulting from supramolecular assemblyof FG during melt compounding.

EXPERIMENTAL SECTIONMaterials. All synthesis chemicals were purchased from Sigma-

Aldrich, Acros Organics, or Merck. THF (dried over CaH2), toluene(dried over potassium), and styrene were freshly distilled prior to use.All the other solvents and chemicals were used without furtherpurication. TRGO was obtained by thermal reduction of GO at 400C.37

Instrumentation. The sulfur content of the modied graphenewas determined with a VarioEL elemental analyzer from Elementar-analysensysteme GmbH. FT-IR spectra were measured, using KBrtablets containing the sample. With a Vektor 22 from Bruker 32 scanswith a resolution of 2 cm1 were recorded. The thermal degradationwas analyzed by means of thermogravimetric analysis (STA 409 fromNetzsch), 50650 C at 10 K/min under nitrogen. The glasstransition temperatures (Tg) were measured by dierential scanningcalorimetry. The samples (510 mg) were heated and cooled in threecycles in an EXSTAR DSC 6200 R from Seiko Instruments Inc.Molecular weight and polydispersity were analyzed using a polymersolution (4 mg/mL in CHCl3) with a PSS-SVDB column at 30 Cusing a RI K-2301 and a 1200 UV detector. Rheological measurementswere carried out with an ARES rheometer from Rheometric Scientic.The rheology specimens (diameter 25 mm) were measured using aplate/plate geometry. The frequency was varied from 0.1 to 100 rad/susing a deformation which was consistent with the linear range. Thetemperature was varied within the range of 135255 C. Transmissionelectron microscopy (TEM) images were made with a Zeiss/LEO 913W at 120 kV. The samples were microtomed at room temperature andimmobilized on Cu grids.Synthesis. Graphite Oxide (GO). GO was synthesized by a

modied Hummers method.21 In a typical synthesis, graphite (60.0 g)and NaNO3 (30.0 g) were dispersed in concentrated H2SO4 (1400mL) and stirred at ambient temperature for 12 h. The mixture wascooled to 0 C, and KMnO4 (180.0 g) was added during 2 h. Afterstirring for another 5 h at ambient temperature the dispersion wasdumped into ice water (2800 mL) before a H2O2 solution (5%, 300mL) was added. The raw GO was washed with water for a severaltimes and afterward was dried in vacuum (400 mbar) at 40 C. Theresulting GO (C = 52.8%, H = 2.4%, O = 44.8%) was used with ahydroxyl functionality of 26.7 mmol/g (the oxygen content of 44.8%was set as hydroxyl groups only).

Stearyl-GO. GO (5.0 g) was dispersed in NMP (200 mL). Afterheating to 70 C stearylamine (15.0 g) was added and the mixture wasstirred for 3 h at 70 C. The product was ltered o from hot solutionand washed with hot acetone (3 200 mL) for purication. Theobtained Stearyl-GO (content of stearylamine was 1.79 mmol/g) wasdried in a vacuum (10 mbar, 60 C).

FG Dithiocarbonates (FG-DTC). The deprotonation of FG hydroxylgroups followed by reaction with carbon disulde and benzyl bromidewas performed in dierent solvents varied as a function of the FGtype: NMP (100 mL) for GO, THF (100 mL) for Stearyl-GO, andtoluene (100 mL) for TRGO. After dispersion of the respectivematerial (1.0 g, 26.7 mmol), dierent bases (26.7 mmol) at ambienttemperature to deprotonate the hydroxyl groups were added. Aftersuccessively adding CS2 (1.6 mL, 26.7 mmol) and benzyl bromide(BnBr, 3.2 mL, 26.7 mmol), the resulting reaction mixture was stirredat 70 C for 2 h. After ltration and washing with acetone (3 100mL) the S-FG was dried in vacuum (10 mbar, 60 C) for 2 days.

FG Dithioester (FG-DTE). FG-DTE was prepared in NMP (200mL) containing dispersed GO (2.0 g, 53.4 mmol). An aryl Grignardsolution (53.4 mmol) in THF (40 mL) was reacted with CS2 (3.2 mL,53.4 mmol) and added to the GO dispersion. The reaction mixturewas stirred at 65 C for 45 min. After addition of ethanol (3 mL) theproduct was ltered o, washed with acetone (3 80 mL), and driedin vacuum (10 mbar, 40 C).

Thiol-Functional FG. Thiol-functionalized FG was derived fromStearyl-GO (1.0 g, 26.7 mmol), which was dispersed in CH2Cl2 (100mL). Then 3-mercaptopropionic acid (2.3 mL, 26.7 mmol), DMAP(1.6 g, 13.4 mmol), and DCC (23.0 mmol) were added. The mixturewas stirred at ambient temperature for 4 days. After ltration and

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washing with acetone (100 mL) and CH2Cl2 (2 100 mL) theproduct was dried in vacuum (10 mbar, 40 C).

The propylene sulde functionalization of TRGO (1.0 g, 26.7mmol) was carried out in toluene (100 mL) and the functionalizationof Stearyl-GO (1.0 g, 26.7 mmol) in THF (70 mL). After dispersionand deprotonation with dierent bases (23.9 mmol) at ambienttemperature propylene sulde (2.1 mL, 26.7 mmol) was added. Themixture was stirred at 60 C for 3 h. After ltration and washing withacetone (3 100 mL) the product was dried in vacuum (10 mbar, 40C).

Polystyrene Grafting of S-FG (General Procedure). Typicalprocedure for AIBN-initiated grafting: Various amounts of the S-FGwere dispersed in styrene (freshly distilled, 9.1 g, 10 mL) bysonication. In some cases NMP was added as solution mediator. Thenvarious amounts of AIBN were added, and the mixture was degassedby three freezepumpthaw cycles. The polymerization was carriedout at 65 C in a nitrogen atmosphere. Then the reaction mixture wasdiluted with CHCl3 (100 mL) and centrifuged (8500 rpm, 1 h). Thesupernatant was precipitated in methanol (800 mL), and theresulting powder was dried in vacuum (10 mbar, 70 C).

In a typical procedure for the thermal self-initiated grafting, S-FG(500 mg, 5.5 wt %) was dispersed in styrene (freshly distilled, 9.1 g, 10mL) by sonication. After degassing the mixture with three freezepumpthaw cycles the polymerization was carried out at 130 C for17 h under a nitrogen atmosphere. Then the reaction mixture wasdiluted with CHCl3 (100 mL), followed by centrifugation (8500rpm, 1 h). The supernatant was precipitated in methanol (800 mL),and the gained composite was dried in vacuum (10 mbar, 70 C). Theproperties of the polystyrene grafted S-FG are summarized in Table 2.

RESULTS AND DISCUSSIONPreparation of Sulfur-Functionalized Graphenes as

Macro-Chain-Transfer Agents. Sulfur-functionalized graphe-nes (S-FG) containing thiol, dithioester, dithiourethane, anddithiocarbonate groups were prepared from graphite oxide(GO), stearylamine-modied GO (Stearyl-GO), and thermallyreduced graphite oxide (TRGO). In contrast to the hydrophilicGO (45.9% carbon content), Stearyl-GO with a carbon contentof 72 wt % and a stearyl functionality of 1.79 mmol/g was easy

Scheme 1. Synthetic Strategies toward S-FG and S-FG Brushes

Table 1. Properties of Dierent S-FG Types

sample functional group graphene type base S-contenta [mol/kg]

GO-DTC-1

-O-CS-SCH2C6H5 GO

N(iPr)2Et 0.05GO-DTC-2 KOH 0.12GO-DTC-3 NEt3 0.18GO-DTC-4 NaH 0.27Stearyl-GO-RAFT-1

-O-CS-SCH2C6H5-NH-CS-SCH2C6H5

Stearyl-GO

N(iPr)2Et 0.18Stearyl-GO-RAFT-2 NaH 0.40Stearyl-GO-RAFT-3 LDA 0.54Stearyl-GO-RAFT-4 n-BuLi 1.02GO-DTE-5 -S-CS-C6H5 GO

0.22GO-DTE-6 -S-CS-CH2-C6H5-C12H25 0.29Stearyl-GO-ES-1 -O-CO-(CH2)2-SH Stearyl-GO

(2.04)b

Stearyl-GO-S-2 -O-CH2-CH(SH)-CH3 NEt3 0.24TRGO-S -O-CO-(CH2)2-SH TRGO LiN(iPr2) 0.43

aS content was calculated using the sulfur content, obtained from elemental analysis. bHigh S content owing to side product impurities.



to disperse in styrene and most solvents used in free radicalpolymerization.38,39 Upon ash pyrolysis of GO at 400 C, GOdecomposes and the evolved gases such as CO and CO2 causeexpansion of GO, as reected by much higher BET surface areaof 348 m2/g and higher carbon content of 60.3% compared tothe pristine GO. The four basic synthetic strategies for sulfurfunctionalization of GO, Stearyl-GO, and TRGO are outlinedin Scheme 1.In pathway A (Scheme 1) the FG hydroxyl groups were

deprotonated and converted with carbon disulde into thedithiocarbonate salts, which are alkylated with benzyl bromideto aord the corresponding benzyldithiocarbonate groups. As itis apparent from Table 1, the S group content increasedmarkedly with increasing strength (pKb) of the bases. Highest Sgroup content was obtained when using strong bases such asNaH and BuLi for the deprotonation of FG hydroxyl groups. Incomparison to GO, the Stearyl-GO gave much higher Scontent. Most likely, this results from better compatibility aswell as the simultaneous reaction of the secondary amines withcarbon disulde, producing dithiocarbonates together withdithiourethanes on FG. In pathway B (Scheme 1), the phenyland p-dodecylbenzyl Grignard reagent were reacted withcarbon disulde to produce phenyl- and benzyldithiocarbox-ylates which were reacted with the epoxy groups of GO. Theresulting S content was rather low. Although the GO epoxygroups are known to react with nucleophiles,25 the epoxy groupcontent is much lower with respect to the hydroxyl groupcontent. Moreover, it should be noted that GO is an oxidativeagent which can oxidize dithiocarboxylates to produce detacheddisuldes, which are removed during purication steps. Thedetected S functionalities were much lower than expected fromthe oxygen content. This was due to the fact that the oxygencontent of 26.7 mol/kg does not represent the ratio of chemicalavailable hydroxyl groups, but rather all kinds of oxygen groupsbound on FG.All S-FG were analyzed by means of FT-IR spectroscopy

detecting the characteristic CS and CS vibration bandsbetween 1000 and 1300 cm1 40,41 (see Figure 1). Because the

GO, Stearyl-GO, and TRGO exhibit no absorption in this area,IR spectroscopy represents a powerful analytic tool conrmingthe formation of the dithiocarbonates and dithiourethanes onFG.In pathway C (Scheme 1) thiol groups were introduced by

DCC/DMAP-mediated esterication of the Stearyl-GOhydroxyl groups with 3-mercapropropionic acid, following

procedures reported in the literature31,42 using 3-mercaptopro-pionic acid (26.7 mmol, 1.0 equiv), DMAP (0.38 equiv), andDCC (0.86 equiv) in CH2Cl2. Although the ester formation isconrmed by FT-IR spectroscopy and high sulfur content of2.04 mol/kg (Stearyl-GO-ES-1) was achieved, purication andcomplete removal of dicyclohexyl urea side product were ratherdicult. Therefore, in pathway D (Scheme 1), an alternativeroute to thiol-FG was developed without such requiring tediouspurication. The hydroxyl groups of Stearyl-GO and TRGOwere deprotonated, and the resulting alcoholates were reactedwith propylene sulde to produce thiols via nucleophilicaddition and subsequent ring-opening of the thiiran ring.Similar to the results reported above, the stronger base lithiumdiisopropylamide (LDA) with respect to triethylamine gavehigher SH functionality, which was found to be 0.43 mol/kg.The thiol functionalization was conrmed by means of FT-IRspectroscopy (see Figure 2), showing typical CS vibrationmodes at 10001200 cm1. Only for Stearyl-GO-SH-2, the CS vibrations were overlapped by Stearyl-GO modes.

Thermogravimetric analysis (TGA) was used to compare thethermal stability of S-FG with respect to that of GO, TRGO,and Stearyl-GO. Upon heating to 650 C, S-FG based uponGO and TRGO show higher weight loss in comparison to theS-free graphenes, as expected when incorporating the thermallymore labile sulfur groups. The TRGO-based systems havemuch higher thermal stability with respect to GO and Stearyl-GO because TRGO was thermally reduced at 400 C, whereasGO is thermally reduced during the TGA measurement, asillustrated by the massive weight loss above 150 C. In the caseof Stearyl-GO the thermal degradation is more complex. Mostlikely, the modication with stearylamine and the subsequentfunctionalization are accompanied by partial chemical reductionof GO, which could account for the slightly higher residualmass of sulfur-modied Stearyl-GO at 650 C (see Figure 3).Grafting Polystyrene onto S-FG. As illustrated in Scheme

1, the S-FG agents were added to the free radical polymer-ization of styrene. The free radical polymerization was initiatedeither by addition of the azoisobutyronitrile (AIBN) initiator at65 C or by means of the initiator-free thermal self-initiation ofstyrene polymerization at 130 C.43,44 The sample code (r)denotes AIBN initiation, whereas (t) marks the thermal self-initiation in the absence of initiator. In order to separate S-FGfrom polystyrene-grafted S-FG, the reaction mixture wasdiluted with chloroform and S-FG was removed by means ofa centrifuge, whereas the dispersion of grafted S-FG was much

Figure 1. FT-IR spectra of (a) GO (), (b) GODTC-4 (- - -), and(c) Stearyl-GO-RAFT-3 ().

Figure 2. FT-IR spectra of (a) Stearyl-GO (), (b) Stearyl-GO-S-2(), (c) TRGO (- - -), and (d) TRGO-S (- -).



less sensitive to sedimentation. Only the supernatant solutionwas precipitated in methanol in order to recover the S-FGpolystyrene nanocomposites. The graphene content wasdetermined as residual mass upon heating at 600 C inthermogravimetric analysis. The results are listed in Table 2.Because of the rather poor dispersibility of GODTC instyrene, the addition of N-methylpyrrolidinone (NMP) wasrequired in some cases. During radical polymerization, chaintransfer as well as additionfragmentation reactions enabledthe growth of polystyrene onto S-FG in those systems.Typically for GO-DTC the S-FG content was varied between

2.7 and 5.5 wt % using AIBN concentration varying between0.7 and 1.3 mol %. In comparison to FG-free polystyrene PS(r),prepared under similar conditions, all GO-DTC systemsincreased glass transition temperatures (Tg) ranging between103 and 107 C. This is in accord with observations in theliterature, reporting Tg increases up to 10 C in the case ofsuccessful grafting.27 Because of the covalent attachment ofpolystyrene to the ultrathin but micrometer-sized graphenes,molar mass determination by means of size exclusionchromatography (SEC) was not possible. The graphene-freereference PS(r) had molar mass of Mw = 41.0 kg/mol. Incontrast to GO-DTC, Stearyl-GO-RAFT was easily dispersed in

styrene and no NMP addition was required. The resultingpolystyrene nanocomposites exhibited Tg varying around 107C. No clear correlation was found between the ratio of styreneto macro-chain-transfer agent and the residual mass, detectedupon heating the puried samples to 650 C (Table 2).In order to examine the role of polystyrene grafting and

covalent attachment of PS on S-FG, the dithiocarbonate linkerswere cleaved by hydrolysis in the presence of aqueouspotassium hydroxide at 85 C, similar to procedures reportedpreviously for other polystyrene graft copolymers.45 As it isapparent from Figure 4, the cleavage of the linker destroys the

excellent stability of the S-FG dispersion in polystyrene, whichleads to a phase separation. The nonbonded S-FG is readilyremoved quantitatively from polystyrene by means of acentrifuge. Afterward, the detached, pristine polystyrene wasprecipitated, giving a clear white powder. The average Mw ofthe resulting polystyrene after linker cleavage was determinedto be 30.4 kg/mol with a polydispersity of 5.0.The thiol-functionalized macro-chain-transfer agents were

grafted exclusively by thermal self-initiation at 130 C. Afterpurication, the resulting residual masses of the puried

Figure 3. Thermal degradation, as measured by thermogravimetricanalysis (TGA), of S-FG based upon GO (), Stearyl-GO (), andTRGO (- - -).

Table 2. Properties of the S-FG Graft Copolymers

samplea initial S-FG content [wt %/mmol Sfunct] yield [g/%] Tg (DSC) [C] residual mass (650 C) [%]

PS(r) 31.6/87 95.0 0.0PS-GO-DTC-2(r)b 1.1/0.048 8.3/91 104.8 0.5PS-GO-DTC-3(r)b 1.1/0.072 7.4/81 103.4 0.5PS-GO-DTC-1(r)b 2.8/0.013 4.7/21 105.4 2.6PS-GO-DTE-5(r) 5.5/0.110 8.8/97 107.2 5.0PS-GO-DTC-4(r) 5.5/0.135 6.5/71 100.6 2.9PS-GO-DTE-6(t) 5.5/0.290 6.6/73 106.5 1.5PS-Stearyl-GO-RAFT-1(r) 2.8/0.045 8.0/88 107.8 2.1PS-Stearyl-GO-RAFT-2(r) 11.0/0.100 8.0/88 108.2 1.0PS(t) 7.9/87 107.5 0.0PS-Stearyl-GO-RAFT-2(t) 5.5/0.399 7.8/86 107.4 2.0PS-Stearyl-GO-RAFT-4(t) 5.5/1.015 7.8/86 106.1 2.3PS-Stearyl-GO-RAFT-3(t) 5.5/0.541 7.1/78 105.9 3.1PS-Stearyl-GO-S-2(t) 5.5/0.243 7.5/82 106.2 5.2PS-Stearyl-GO-ES-1(t) 5.5/2.036 7.6/84 102.5 3.4PS-TRGO-S(t) 5.5/0.434 8.1/89 108.2 2.3

a(r): radical polymerization at 65 C (AIBN initiated); (t): radical polymerization at 130 C (styrene self-initiated). bPolymerization with the use ofNMP.

Figure 4. Image of PS-Stearyl-GO-RAFT(r) before (left) and after(right) hydrolysis of the linker.



nanocomposites were high and varied between 2.3% and 5.2%.This is an indication that grafting was highly eective. Withexception of PS-Stearyl-GO-SH-1(t), the resulting S-FGnanocomposites exhibited Tg around 107 C, as observedpreviously for the other S-FG brushes.Rheology and Morphology Development. In order to

examine the role of grafting on S-FG dispersions, Stearyl-GOwas melt blended, as well as solution blended, together withcommercial polystyrene (PS 158K, Mw = 2.6 10

5 g/mol, PDI= 2.5). The melt-compound was produced with a micro-compounder (Type 20000 from Daca Instruments) by

processing for 2 min at 220 C. The solution blend wasproduced by sonication of a Stearyl-GO/polystyrene dispersionin toluene followed by heating to 130 C for 15 h andprecipitating in methanol. As is apparent from Figure 5a, themelt blending of Stearyl-GO fails to produce ne dispersionstypical for the grafted samples.Melt rheology is a powerful tool to characterize network-like

superstructures such as formed by S-FG brushes in polystyrene.Therefore, the van GurpPalmen plot46 makes it easy to dierbetween conventional polymers, showing ideal viscousbehavior, and polymers with a particle network. Withoutcovalent attachment, the van GurpPalmen plot ( vs |G*|)conrms that the timetemperature superposition principleapplies and shows that the melt-blend rheology is matrixdominated. As a consequence, all values converge together intoone curve approaching 90 for small |G*| values. For networkformation a decreasing curve progression at small |G*| values isobserved, indicating the equilibration modulus |G*eq(T)|

Scheme 2. Cleavage of the Dithiocarbonate Linkers of the S-FG Brushes by Hydrolysis with KOH

Figure 5. TEM images (left) and related van GurpPalmen plots (T = 135255 C) of (a) PS 158K melt compound (2.5 wt % Stearyl-GO), (b) anin situ grafted PS-Stearyl-GO-RAFT-3(t) (3.1 wt %), and (c) PS-Stearyl-GO solution blend (2.5 wt % Stearyl-GO). Solid lines in the van GurpPalmen plot represents the (a) pristine PS-behavior and (b, c) the extrapolated equilibration modulus |G*eq(T)|.



founded by the platelets network. This distinct dierence inrheological behavior was found for the polystyrene nano-composites prepared by in situ polymerization and grafting,whereas the melt blend and some free radical polymerized S-FG composites showed a behavior similar to the pristinepolystyrene (Figure 5a).The stearyl-modied S-FG brushes showed the formation of

a temperature-dependent network-like structure expressed bythe decreasing curve type in Figure 5b. This is in accord withthe results of the TEM investigation that only the polystyrene-grafted graphenes exhibit network-like superstructures.To check the covalent attachment of polystyrene chains, the

results above were compared to the PS-Stearyl-GO solutionblend (see Figure 5c). Because of the homogeneous dispersion,the van GurpPalmen plot showed a temperature-independentparticle network formation by the decreasing behavior of forlittle values of |G*|. This observation is in accord with previousstudies on the very similar rheological behavior of organoclaynanosheets containing covalently and ionically attachedpolystyrene47 and polypropylene.48 In contrary to the reportedsystems, a thermal equilibration without shearing wasimpossible in case of the in situ grafted composites. Inaddition, an aggregation of the grafted particles due to theinstability of thiol-modied polymer ligands, as observed by Liand co-workers, who studied polymerthiol grafted goldnanoparticles,49 was not detected.Table 3 shows the results of the melt rheological measure-

ments, including the residual mass detected by the means of

thermogravimetric analysis and the qualitative particle networkformation. The melt and solution blends were synthetized withan initial Stearyl-GO content of 2.5 wt %.The data in Table 3 indicate that a network formation was

only observed in the cases of stearyl-modied S-FG. This isattributed to the substantially improved dispersion of Stearyl-GO with respect to GO, causing percolation and formation of anetwork-like superstructure. Poor dispersions did not permitexperimental verication of grafting for the other materialsbased upon S-FG. In our case, the results of melt rheologicalmeasurements enabled the supervision of a covalent attachmentbecause grafting leads to a highly improved graphenedispersibility causing percolation and the characteristic behaviorshowed in the van GurpPalmen plot.

CONCLUSIONIn conclusion, four synthetic strategies were developed forenabling the conversion of hydroxyl and amine groups of GO,Stearyl-GO, and TRGO into thiol, dithioester, dithiocarbonate,and dithiourethane groups useful as macro-chain-transfer agentfor growing polystyrene chains onto S-FG. According toelemental analysis, TGA, and FT-IR spectroscopy, strong basesaord very eective deprotonation of FG hydroxyl groupswhich can react with carbon disulde, followed by alkylation, toproduce RAFT reagents. In an alternative process, propylenesulde was added to FG alkoxides to produce FG-thiols. BothFG-RAFT and FG-thiols were highly eective as chain-transferagents in free radical polymerization, initiated either with AIBNinitiator or by means of thermal self-initiation at 130 C. Onlyin the presence of covalent attachment of S-FG to polystyrene,stable S-FG dispersions in polystyrene melts were obtained.Cleavage of the dithiocarbonate linkers in FG-RAFT systemsdestroyed the stability of S-FG dispersions and enabledquantitative removal of S-FG. To verify the covalent attach-ment of polystyrene chains to S-FG, rheological measurementsplotting as a function of |G*| in the van GurpPalmen plothave enabled the monitoring of a temperature-dependentnetwork-like structure development. Highly eective graftingand formation of FG brushes were exclusively observed forstearyl-modied GO. Both conventional melt and solutionblending of Stearyl-GO with polystyrene failed to produceparticle networks in the absence of covalent coupling.

AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected] authors declare no competing nancial interest.

ACKNOWLEDGMENTSThe authors gratefully acknowledge the nancial support by theFederal Ministry of Education and Research (BMBF) as part ofthe FUNgraphen project (project 03X0111A). The authors alsothank Dr. R. Feher for a lot of helpful discussions and thesupply of graphite.

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Table 3. Rheological Properties of Dierent S-FGPolystyrene Composites and Reference Systems

nanocomposite residual massa [%] network formation

PS-GO-DTC-4(r) 2.9 PS-GO-DTE-5(r) 5.0 PS-GO-DTE-6(t) 1.5 PS-Stearyl-GO-RAFT-2(r) 1.0 PS-Stearyl-GO-ES-1(t) 3.4 PS-TRGO-S(t) 2.3 PS-Stearyl-GO-RAFT-1(r) 2.1 PS-Stearyl-GO-RAFT-4(t) 2.3 PS-Stearyl-GO-RAFT-3(t) 3.1 PS-Stearyl-GO-S-2(t) 5.2 PS-Stearyl-GO melt blend 2.5b PS-Stearyl-GO solution blend 2.5b

aResidual mass detected by the means of thermogravimetric analysis.bMelt and solution blends contained an initial content of 2.5 wt %Stearyl-GO.



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