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Polymer 52 (2011) 3633e3638

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Polymer

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Polyethylene/carbon nanotube nano hybrid shish-kebab obtained by solventevaporation and thin-film crystallization

Lingyu Li a, Wenda Wang a, Eric D. Laird a, Christopher Y. Li a,*, Matthieu Defaux b, Dimitri A. Ivanov b,*

aDepartment of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, USAb Institut de Sciences des Matériaux de Mulhouse-IS2M, CNRS LRC 7228, Jean Starcky, 15, F-68057 Mulhouse, France

a r t i c l e i n f o

Article history:Received 15 March 2011Received in revised form30 April 2011Accepted 3 May 2011Available online 23 May 2011

Keywords:Polymer crystallizationPolymer single crystalCarbon nanotube

* Corresponding authors. Tel.: þ1 215 895 2083; faE-mail addresses: Chrisli@drexel.edu (C.Y. Li), Dimit

0032-3861/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.polymer.2011.05.006

a b s t r a c t

Crystallization of polymers on carbon nanotubes (CNTs) has resulted in a novel nano hybrid shish kebab(NHSK) structure, within which CNTs serve as the nucleation sites (shish) and polymer lamellar crystalsform the kebabs. Previously reported NHSK structures were obtained by solution crystallization, bulkcrystallization and physical vapor deposition methods. Herein we report a simple, rapid, yet effectiveapproach to produce NHSK materials using solvent evaporation and thin film crystallization. Poly-ethylene (PE) was used as the model polymer. PE solution was drop cast on CNT-coated carbon films, andupon solvent evaporation, PE crystallized onto/near CNTs, following the template of the latter and NHSKstructure was then formed. The final morphology was found to result from the competition betweenheterogeneous nucleation and homogeneous nucleation of PE. The formation of NHSK also stronglydepends on the structure of CNTs as well as the molecular weight of PE. This work shows a facile methodto form NHSK and to study CNT-induced crystallization under nonequilibrium conditions.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon nanotubes (CNTs) have attracted great attention due totheir extraordinary mechanical, electrical and optical properties[1e4]. Numerous potential and practical applications of CNTshave been reported in the fields of emission tips [5e7], electronicdevices [8,9], sensors [10e13], and energy storage [14]. Oneintriguing direction is to pattern soft matter such as polymers,surfactants, as well as liquid crystals on CNT surface. Occasionally,periodic functionalization/patterning soft matter on CNTs canbe achieved. Czerw et al. demonstrated regular organization ofpoly(propionylethylenimine-co-ethylenimine) on CNTs using scan-ning tunneling microscopy (STM) [15]. Single-stranded DNAs havebeen bound to CNTs, resulting in periodic helical wrapping on thesurface of CNTs [16,17]. Surfactants such as sodium dodecyl sulfatehave been found to form uniform patterns on CNTs [18,19]. Kruseet al. carefully investigated the structure of the CNTs functionalizedby Bingel reaction [20]. Using STM, they were able to identifyperiodic patterns of functional groups on individual CNT surface.

More recently, CNT-induced polymer crystallization has beenreported [21e23]. It was demonstrated that in polymer solutioncontaining residue amounts of CNTs, upon lowing the solution

x: þ1 215 895 6760.ri.Ivanov@uha.fr (D.A. Ivanov).

All rights reserved.

temperature, certain crystalline polymers (such as polyethylene(PE) and Nylon 66) can quasi-periodically grow on CNTs. Theresultant peculiar morphology is similar to the conventionalpolymer shish kebab structures formed in an extensional field andwas thus named as “nano hybrid shish kebabs” (NHSK, Scheme 1).The formation mechanism was demonstrated as soft epitaxy [22].This hybrid structure is of great interest because, (1) the lamellarcrystals serve as spacers which can prevent CNT aggregation;(2) the polymer lamellae can be used as sacrificial phase forpatterning a third material on CNTs in the interval region; and(3) various functional groups can be introduced to the CNT vicinityvia the polymer crystal and the degree of functionality can be easilycontrolled by changing the lamellar size [24,25]. It has also beendemonstrated that diameter of CNTs and nanoparticle dimension-ality have profound effects on polymer crystallization [24,25].NHSK structure can also form in bulk systems of polymer nano-composites, which have been demonstrated by our group [26,27],and others [28e31]. An alternative method, polymer physical vapordeposition (PVD), was also employed to form NHSK with CNTs[32,33]. PE was vacuum degraded into oligomers, which crystal-lized into rod-shaped crystals with a distinct orientation that isperpendicular to the CNTaxis. The resultant NHSK is 2-dimensional(2D) due to the thin film nature of the sample [32,33]. A 2-stepformation mechanism of the 2D NHSK was proposed and tested.In this article, we report using a controlled solvent evaporationcrystallization technique to prepare polymer/CNT NHSKs. We

Scheme 1. Schematic representation of forming nano hybrid shish kebabs viacontrolled crystallization.

L. Li et al. / Polymer 52 (2011) 3633e36383634

demonstrate that NHSK can be formed by drop-casting PE solu-tion onto CNTs. The final hybrid structure critically depends uponthe solution concentration, polymer molecular weight and thestructure of CNTs. Thermal annealing could dramatically improvethe uniformity of the NHSK.

2. Experimental section

2.1. Materials

Purified HiPco single-walled CNTs (SWNTs) were purchasedfrom Carbon Nanotechnologies Inc. (CNI Lot # P0272). Multi-walledCNTs (MWNTs) were purchased from Aldrich and washed with2.4 M nitric acid for 0.5 h. The resulting MWNTs were thencentrifuged, collected and dried in a vacuum oven. Linear PE(MFI¼ 12 g/10 min), PE oligomer (number averagemolecularweight,Mn w 800 g/mol) and 1,2-dichlorobeneze (CHROMASOLV�> 99%)(DCB) were purchased from Aldrich and used as received. ForMWNT modification, concentrated H2SO4/HNO3 Mixture (3:1) wasused for oxidation, which generated numerous surface carboxylgroups. The carboxyl groups reacted with amine at highertemperature in the next amidation reaction. For the amidationreaction, the oxidized MWNTs were dispersed in octadecylamineand maintained at 180e190 �C under nitrogen for 24 h.

2.2. Sample preparation

CNTs were first dispersed in DCB (0.01 wt %) by ultrasonication(250W, 44� 6 kHz) forw 1 h and then dispersed on carbon-coatedglass or transmission electron microscopy (TEM) grids using spincoating. A small drop of CNT/DCB solutionwas first placed on a TEMgrid. The spin speed and timewere 3000 rpm and 60 s, respectively.PE was dissolved in DCB (0.05 wt % and 0.02 wt %) at 120 �C. TheseCNTs dispersed on TEM grids or carbon coated cover glass werethen coated by small amounts of PE/DCB solution. Upon gradualevaporation of DCB at room temperature, a thin layer of PE filmwasformed on the TEM grid. To enhance the contrast, most TEM girdswere shadowed with Pt/Pd before TEM observation.

For the PVD experiments, a small drop of CNT/DCB solution wasfirst spincoated on a carbon-coated glass slide. The spin speed andtime were 3000 rpm and 60 s, respectively. The carbon glass slidewas then coated by PE vapor using the PVD method. A distance of15 cm between the substrate and the basket in the vacuum evapo-rator was chosen. The vacuumwas controlled to be 10�4e10�5 Torr.

2.3. Characterization

TEM experiments were carried out using a JEOL JEM2100 TEMoperated at an acceleration voltage of 120 kV. Thermal annealingwas conducted using a hot stage under N2 purge as needed. Ramanspectra were acquired with a Raman micro spectrometer(Renishaw 1000) using an Ar ion laser (514.5 nm) in back-scatteringgeometry. A 50� objective with a spot size of w 1 mm in diameterwas used. A low laser power density was used on the sample (laserpower< 2 mW) to avoid laser heating of the tubes. Atomic ForceMicroscopy (AFM) experiments were carried out with a NanoscopeIV Multimode AFM (Veeco Metrology Group) in tapping mode,which is most suitable for soft materials imaging. Tapping mode Siprobes from Nanosensors were used (PPP-NCL, resonant frequency172e191 kHz, spring constant 33e47 N/m). An AFM JVH piezo-scanner was equipped with a heating stage making it possible toperform in-situ variable-temperature measurements. Previously,AFM proved its efficiency in studies of morphology and thermalreorganization of solution-grown single crystals of linear PE andultra-long n-alkanes [34e36].

3. Results and discussions

3.1. CNT/PE NHSK via solution casting

SWNTs/DCB suspension was spin-cast on a carbon-coated TEMgrid. One drop of 0.05 wt% PE/DCB solution was cast onto the samegrid at room temperature. After solvent evaporation, PE crystallizedand lamellar crystals can be clearly seen in Fig. 1. The TEM micro-graph shows that CNTs are separated and coated with PE crystals,rendering the elongated complex morphology. Two differentcrystal morphologies are observed: edge-on lamellae (blackarrows) and the flat-on, conventional tooth-saw crystals (whitearrows). The edge-on lamellae are relatively periodically strungtogether by CNTs and the lamellae are perpendicular to the CNTaxis. These edge-on crystals were formed due to the nucleation ofCNTs and are the kebab crystals similar to those in NHSKs formed insolution crystallization. The PE chains in these crystals are parallelto the CNT axis, following a soft epitaxy mechanism. The flat-oncrystal, however, resembles PE dendrites grown from solution[22]. The formation of these two distinct structures of PE can beattributed to the sample preparation process. At the beginning, hotPE solution was cast on CNT film. As the solvent evaporates, solu-tion temperature decreases and concentration increases. These twotypes of crystals were formed at different stages of the samplepreparation process: the edge-on lamellae were formed earlierwhen the temperature was relatively high and concentration wasrelatively low while at the late stage, low temperature and higherconcentration favors the dendrite structure formation (see laterdiscussion). From the NHSK formation standpoint, the flat-oncrystals can be considered as the overgrowth of PE on NHSKs. Toavoid the overgrowth and to clearly view PE/CNT interface, lowerpolymer concentration is desirable. Fig. 1c showed a TEM image ofPE/CNT complex structure following similar preparation conditionexcept that the PE concentration was reduced from 0.05 to 0.02 wt%. It is evident that NHSK structure was formed and much lessdendrite overgrowth was observed. Inset of Fig. 1c shows a selectedarea electron diffraction (SAED) pattern from the circled area ofFig. 1c with a correct orientation. A pair of (002) diffraction can beclearly seen, and the (002) plane is parallel to CNT axis. This indi-cates that the PE chains in the edge-on crystal are parallel to theCNT axis. This orientation is also similar to that observed in nano-fiber shish kebabs [37,38].

The dramatic effect of PE concentration can also be clearlyviewed while using MWNT-ADs (synthesized using the arc

Fig. 1. TEM micrographs of PE-coated SWNTs after solvent evaporation. (a) and (b) are samples formed using 0.05 wt% PE/DCB solution while (c) represents samples from 0.02 wt%PE/DCB. The inset in (c) shows SAED pattern from the circled area with correct orientation. (b) is an enlarged image of (a). White arrows show PE flat-on dendrites while blackarrows show edge-on kebab crystals.

L. Li et al. / Polymer 52 (2011) 3633e3638 3635

discharge method) as the crystal growth template. TEM images inFig. 2a and b show the TEM micrographs of PE-coated MWNT-ADusing a polymer concentration of 0.05 and 0.02 wt%, respectively.The results are similar to that of the SWNTs: namely, for 0.05 wt%,both NHSK and overgrown dendrites were observed while onlypure NHSK structure was observed as the PE concentrationdecreased to 0.02 wt%. Note that, in Fig. 2a, the reentrant of PElamellae can be seen as described by Keith and the [200] directionof the crystal is perpendicular/oblique to the CNT axis [39].

3.2. CNT effect on NHSK structure

In order to study the CNT structure effect on NHSK formation,a variety of different CNTs were employed as shown in Table 1. Inaddition to the HiPco SWNT and MWNT-AD previously discussed,we further used three different MWNTs, namely MWNT-I, II and III,which were formed using CVD methods. They possess differentdiameters/surface chemistry as shown in Table 1. In particular,MWNT-III was modified with octadecylamide as discussed in theExperimental section.

The wall structures of the MWNTs are different and Fig. 3ashowed the Raman spectra of these CNTs using a laser excitationwavelength of 514.5 nm. For CNT’s Raman spectra, typically twobands, one at w1578 cm�1 (G band), which represents the highfrequency E2g Raman scattering mode of sp2-hybridized carbon

Fig. 2. TEM micrographs of PE-coated MWNT-AD after solvent evaporation. (a) and (b)are samples used 0.05 and 0.02wt% PE/DCB solution, respectively.

TS

material, and a disordered structure-induced band at w1352 cm�1

(D band), which may originate from the defects in the curvedgraphene sheets and/or CNT ends. The degree of graphitization isoften used as an indicator of the CNT disorder level and is charac-terized by the intensity ratio of the D and G bands (R¼ ID/IG). Fromthe figure, it is evident that SWNT and MWNT-AD have very low Dband intensity, and their Rs are 0.09 and 0.21 respectively; whilstMWNT (IeIII) have strong D band intensity and Rs are 1.0, 0.79 and1.22 respectively. For MWNT-III, because of the surface modifica-tion with octadecylamide, its D band splits, indicating destructionof graphene sheets. It is worth noting that the spectra resembledthose reported in the literature for CVD-grown CNT [40,41].

Fig. 4 shows the TEM micrographs of PE-coated MWNT-I, II, andIII following the same solution casting method using 0.02 wt% PE/DCB solution. As the CNT wall becamemore disordered, we noticedthat PE was not able to crystallize on CNTs. For MWNT (IeIII), mostof the CNT remain “naked” or coated with a thin layer of PE poly-mer. Large size lamellar crystals were not observed. It should benoted that using solution crystallization, we demonstrated thatMWNT-I could induce PE crystallization and lead to NHSKmorphology [22]. The lack of NHSK formation in the present case isthus due to the relatively fast solvent evaporation process: themore ordered SWNT/MWNT-AD could efficiently induced PE crys-tallization so that NHSK was formed even in the fast solution castprocess while MWNTs (IeIII) have poorly ordered wall structure,which makes them less efficient in initiating PE crystallization. Theslow nucleation process thus prevented NHSK formation under thepresent experimental condition. We also did not observe separatedPE single crystals, indicating that homogenous nucleation was notsufficient to induce large size PE single crystals during this fastsolvent evaporation process.

able 1ummary of CNTs used in the study.

Category Syntheticmethod

Vendor Outerdiameter

Length

SWCNT HiPCO CarbonNanotechnology Inc.

0.8e1.3 nm wMicron

MWNT-AD ArcDischarge

Sigma 5e15 nm 1e10 mm

MWNT-I CVD Sigma 3e10 nmMWNT-II CVD Nanostructured &

Amorphous Materials, Inc20e30 nm 0.5e2 mm

MWNT-IIIa CVD Nanostructured &Amorphous Materials, Inc

8e15 nm w2 mm

a Note: MWNT-III was surface modified with octadecylamide.

Raman Shift1100 1200 1300 1400 1500 1600 1700 1800

Inte

nsity

(a.u

)

SWCNT

MWNT-AD

MWNT-I

MWNT-II

MWNT-III

Fig. 3. Raman spectra of SWNT and MWNTs used in this study.

Fig. 5. TEM micrographs of PE-coated MWNT-AD before (left) and after (right)recrystallization.

L. Li et al. / Polymer 52 (2011) 3633e36383636

3.3. Annealing effect on NHSK structure

The PE crystal structure is by no means uniform in Figs. 1 and 2,which is due to the short crystallization time allowed in the samplepreparation process. Of interest is isothermal crystallization of PEon the CNTs in the thin film geometry. Upon melting PE in thesolution cast PE/CNT, will PE crystallize following a heterogeneousnucleation or homogenous nucleation mechanism? To this end, thesample shown in Fig. 2b was melted at 140 �C for 10 min and thenbrought to 70 �C for isothermal crystallization for w 3 h. Fig. 5shows the resultant structure. Regular NHSK was formed afterrecrystallization. The inter-kebab distance was dramaticallydecreased from w 50 nm to w 20 nm. The lamellae appear to bethinner, which is due to the nearly perfect orthogonal orientation ofthe lamellar surface with respect to the CNTs (hence more parallelto the electron beam during TEM experiments) compared with theNHSKs formed during solvent evaporation. In the solvent evapo-ration process, the capillary force generated by residue solvent nearthe vicinity of CNT and polymer lamellae could deform the latterfrom an ideal orthogonal morphology. The smaller inter kebabdistance could be due to the longer crystallization time andincreased polymer concentration near the CNT after melting thesample at 140 �C.

Thermal annealing can also be applied to PVD samples. We havepreviously demonstrated that upon vacuum decomposing PE intooligomers with Mn w 1000 g/mol, these PE oligomers deposit onCNT-coated carbon films and crystallize into two types ofmorphologies, flat-on island crystals (solid circle in Fig. 6a) andedge-on rod-like crystals strung together by CNTs (dotted circle inFig. 6a) [32]. The former was formed by self nucleation of PE while

Fig. 4. TEM micrographs of PE-coated MWNT-I (a), MWNT-II (b), and MWNT-III (c)after solvent evaporation.

the latter formed because the PE oligomers’ chains follow theorientation of CNT axes (soft epitaxy), and were therefore named2D NHSKs. The chains’ axes are perpendicular to the axis of the rod-like PE oligomer crystals as schematically shown in Fig. 6c. Flat-onPE crystals are common while edge-on crystals are not. In order tofurther confirm the chain orientation, SAED experiments werecarried out as shown in Fig. 7. Similar to Fig. 1c, the SAED results onthe PVD samples also show a pair of (002) diffraction and theorientation of the diffraction spots suggests that the PE oligomerchains in these rod-like crystals are parallel to the CNT axis, whichis consistent with Fig. 6c. The PE/CNT film shown in Fig. 6a wasmelted at 120 �C, and then cooled to room temperature. During thisprocess, the PE oligomers should melt and recrystallize. Fig. 6bshows the resultant structure. Compared with Fig. 6a, themorphology of PE crystals dramatically changed and 2D NHSKdisappeared. Although many crystals in Fig. 6b are elongated inshape, thewidth of the elongated crystals is much larger than thosein 2D NHSKs. Furthermore, the orientation of the crystals cannot becorrelatedwith CNTaxis. Both factors suggest that the crystals wereformed via homogenous nucleation of PE instead of heterogeneousnucleation of CNT. The thickness of the crystals is w8e9 nm,indicating the Mn of the PE oligomers is w a few hundred g/mol,consistent with previous work [32].

Of interest is why CNTs can nucleate PE oligomer crystallizationduring the PVD process while PE homogenous nucleation occurredin recrystallization. Comparing these two processes, one apparentdifference is the PE concentration. In the PVD process, PE oligomerssequentially deposit on the substrate so that during the nucleationprocess, PE concentration is relatively low, hence heterogeneousnucleation occurred. In the recrystallization process, all the PEoligomers are on the substrate and the local concentration of PE ishigh. This significantly enhances homogenous nucleation, whichleads to flat-on PE lamellae. Note that the observation is alsodifferent from the previous PE/MWNT recrystallization experiment,inwhich case, NHSK were formed as shown in Fig. 5. The differencebetween Fig. 5 and Fig. 6b can be attributed to different PE Mns,which significantly influenceshomogenousnucleation rate. In Fig. 5,PEMn is higher than that of PE oligomers shown in Fig. 6, leading toa faster homogeneous nucleation in the later. This is because chainfolding does not take place in oligomer crystallization, while it isa necessary, and slow, step for highMn PE crystallization. The faster

Fig. 6. Tapping Mode AFM height images of PE/SWNTs formed by PVD. The scan size is 1.0� 1.0 mm2. (a) shows the as- formed structure while (b) shows the morphology developedafter melting the sample in (a) and cooled to room temperature. The scan size is 2.3� 2.3 mm2. (c) and (d) schematically illustrate the chain orientation of PE oligomers (arrows) andthe CNTs. PE chains are parallel to the CNT axis in (c) while they are perpendicular to the CNT axis in (d).

L. Li et al. / Polymer 52 (2011) 3633e3638 3637

homogenous nucleation therefore leads to flat-on PE lamellarcrystals. This argument can be further supported by conductingsolvent evaporation crystallization using low Mn PE. PE with an800 g/molMnwas dissolved in DCB and solution cast on CNTcoated

Fig. 7. TEM micrograph of PE/SWNTs formed by PVD. The inset shows a SAED patternfrom the circled area with correct orientation.

carbon film as previously discussed. Fig. 8 shows the resultantcrystal morphology: large PE lamellar crystals are formed while noNHSK was observed. The absence of NHSK can clearly be attributedto the fast homogenous nucleation of lowMn PE.

Fig. 8. TEM micrograph of PE (Mn w 800 g/mol) coated MWNT-AD. Thin flat-onlamellae of PE are visible. CNTs are coated with a thin layer of PE whereas NHSK isabsent.

L. Li et al. / Polymer 52 (2011) 3633e36383638

4. Conclusions

In summary, we have investigated controlled PE crystallizationduring solvent evaporation onto CNT-coated carbon films. We havedemonstrated that both flat-on thin lamellae and NHSK structurescan be formed by controlling the polymer concentration. Theformation of NHSK was attributed to CNT-induced PE crystalliza-tion, which follows a soft epitaxymechanism. The formation of flat-on lamellae was attributed to PE homogenous nucleation. Wedemonstrated that the regularity of NHSK structure formed in PE/MWNT-AD case can be improved by melting/recrystallizationwhile recrystallization of 2D NHSK formed by PVD led to flat-on PElamellae, which were not observably influenced by the CNT axis,and can be explained by faster homogeneous nucleation of PEoligomers. The formation of NHSK is also evidently dependent onCNT purity. CNTs with more surface defects were less efficient innucleating PE crystals, and NHSKs were not observed when CVDCNTs were used as polymer single crystal templates in the solventevaporation process.

Acknowledgments

This work was supported by the National Science FoundationGrants DMR-0804838. EDL gratefully acknowledges the support ofthe National Science Foundation IGERT DGE-0221664. MD grate-fully acknowledges financial support from the CNRS and Region ofAlsace (BDI bursary).

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