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Polymer Single Crystals in Nanoparticle-Containing Hybrid Systems

Eric D. Laird, Bin Dong, Wenda Wang, Tian Zhou, Shan Cheng and Christopher Y. Li*Department of Materials Science and Engineering, Drexel University, Philadelphia, PA, USA

Keywords

Carbon nanotubes; Crystal engineering; Nanomaterials; Nanoparticles; Polymer; Polymer crystal-lization; Polymer single crystals

Introduction

The past decade witnessed a fast growth of research on nanoparticles because of their fascinatingmechanical, electrical, and optical properties (Murray et al. 2000; El-Sayed 2001; Daniel and Astruc2004). Polymer science has been interacting with nanoparticle research in two ways: (1) variouspolymers have been used to form polymer brushes on the surface of nanoparticles in order tostabilize the latter for processing purposes (Daniel and Astruc 2004). (2) A variety of nanoparticleshave been used to blend with polymers (or block copolymers) to form value-added nanocomposites(Bockstaller et al. 2005; Balazs et al. 2006; Winey and Vaia 2007). Although crystalline polymer/nanoparticle nanocomposites have been investigated by a few groups, little attention was paid topolymer single crystal (PSC)-nanoparticle hybrids (Li 2009).

Polymer crystallization was first reported in 1938, when Storks described gutta-percha(trans-polyisoprene) single crystals obtained by casting thin films from dilute chloroform solutionand suggested a possible chain folding mechanism in polymer crystallization (Storks 1938). Hiswork was however overlooked till 1957, when Till, Keller, and Fisher independently reported thegrowth and identification of single crystals of linear polyethylene (PE) (Fisher 1957; Keller 1957;Till 1957). Since then, a library of beautiful polymer single crystals has been established. After55 years of extensive investigation, there are still active debates on numerous issues including thevery fundamentals of the crystallization mechanism as well as metastability of folded versusextended-chain crystals (Muthukumar 2007). Nevertheless, it is generally agreed that polymersingle crystals can be considered as markers of the corresponding crystallization process, allowingone to unambiguously determine crystal structures and chain conformation using techniques such aselectron diffraction (Lotz and Cheng 2005). Yet, the main focus of semicrystalline polymers hasbeen on bulk systems because they are more relevant to the majority of practical applications.

A typical polymer lamella is approximately 10 nm thick and a few nanometers to a fewmicrometers wide, while nanoparticles by definition have at least one dimension less than100 nm; the similar size of these two seemingly different objects ensures an interesting marriagebetween the fields of polymer single crystals and nanoparticles. The ability to form regular chainpacking suggests that crystalline polymers may be able to offer much more than simply passivatingnanoparticles as in the case of polymer brushes. In this entry, we will discuss how one-dimensionalnanofiber/nanotube induces polymer crystallization and how two-dimensional polymer lamellaeguide nanoparticle assembly.

*Email: [email protected]

Encyclopedia of Polymers and CompositesDOI 10.1007/978-3-642-37179-0_26-1# Springer-Verlag Berlin Heidelberg 2013

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Nanohybrid Shish Kebabs: Nanotube-Induced Polymer Crystallization

Nanohybrid Shish Kebabs Formation on CNTPolymer/carbon nanotube (CNT) nanocomposites (PCNs) have attracted great attention in recentyears (Thostenson et al. 2001, 2005; Coleman et al. 2006; Moniruzzaman andWiney 2006; Schaeferand Justice 2007; Green et al. 2009; Grady 2012; Ning et al. 2012). Due to the high aspect ratio ofCNT, percolation thresholds can be reached at very lowCNTcontents in PCNs, leading to a dramaticproperty enhancement. It has been observed that CNT can induce crystallization of numerouspolymers (Moniruzzaman and Winey 2006; Li et al. 2009b; Grady 2012; Ning et al. 2012).However, in a PCN, CNTs are coated with a thick layer of polymers and the polymer/CNT interfaceis not clearly revealed. On the other hand, diluents/solvents can be used to get controlled conditionsfor PSC with regular crystal structures (Li et al. 2005, 2006a). Just as with composites cooled fromthe melt state, CNTs can be used to nucleate PSCs in dilute and semidilute solutions as well. Theprecise control offered by dilution allows CNTs to crystallize polymer at temperatures above thehomogeneous nucleation temperature. In some cases, this results in transcrystallinity, and in othercases with lower polymer concentrations, single crystals are grown from the CNT sidewall, as inFig. 1. This is different from transcrystallinity insofar as the extent and direction of crystal growthare not limited by another impinging polymer crystal growth front. In such cases, a uniquemorphology is produced with a periodic or semi-periodic pattern of PSC-decorated CNTs (seeFig. 1). There are several techniques that can be used to produce this effect, including isothermal andnonisothermal solution crystallization (Li et al. 2005, 2006a; Uehara et al. 2007) and introduction ofsupercritical CO2(Zheng et al. 2010). The result in either case is termed a nanohybrid shish kebab(NHSK) structure and was first demonstrated using PE and Nylon 6,6 PSC decorations onMWCNTs (Li et al. 2005; Kodjie et al. 2006; Li et al. 2006a). The NHSK term comes from theclassical shish kebab structures observed in flow-induced polymer crystallization (Pennings andKiel 1965). On the other hand, no extensional/shear flow is needed to produce the NHSK structure,although there is a striking visual similarity. It is believed that the NHSK is formed as a result ofaffinity between the polymer chain and the CNT sidewall. As the polymer attempts to fold, the

Fig. 1 First published description of the nanohybrid shish kebab structure showing (a) SEM and (b) TEM image ofMWCNT used to template PE single crystals. (c) Early schematic depiction of PE single crystals grown on a SWCNT(From Ref. Li et al. 2005)


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crystal orientation is directed by the geometric confinement of the narrow-diameter CNT axis. Thiscauses the lamella to grow perpendicularly. As the local concentration of polymer is exhausted byincorporation into the lamella, it prevents crystal nucleation from areas of the CNT between existingkebabs. Thus, a semi-periodic structure is achieved. A similar structure has also been achieved byusing other methods such as physical vapor deposition (Li et al. 2006b, 2007), initiated chemicalvapor deposition (Laird et al. 2013), thin film crystallization (Li et al. 2011), etc.

Size-Dependent Soft EpitaxyThere are two possible factors that control NHSK growth: the epitaxial growth of PE on CNT andgeometric confinement. Epitaxial growth of PE on the surface of highly ordered pyrolytic graphite(HOPG) dictates the PE chain direction or the h001i of the PE crystal to be parallel with 21 10

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the underlying graphite (Cheng et al. 2012). However, because of their small diameters, CNTs can beconsidered as rigid macromolecules; polymer chains prefer to align along the tube axis regardless ofthe lattice matching between the polymer chain and the graphitic sheet, rendering a geometricconfinement on polymer chains. This mechanism can be attributed to “soft epitaxy”: strict latticematching is not required, while a cooperative orientation of the polymer chains and the CNT axes isneeded (Li et al. 2006a). The growth mechanism for CNT-induced PE crystallization involves both,and it is size dependent. On the surface of the fiber/tube with a diameter much larger than thepolymer size, the polymer behaves as if it were on a flat surface; epitaxy is the main growthmechanism. When the fiber/tube diameter is decreased to the order of the polymer size, asa polymer starts to crystallize onto this surface, geometric confinement is the major factor and thepolymer chains are exclusively parallel to the CNT axis, disregarding the CNT chirality. Asa consequence, the PE crystal lamellae should be perpendicular to the CNT axis and orthogonalorientation is obtained. Selective area electron diffraction (SAED) experiments have also confirmedthis orientation. Reduced graphene oxide (RGO) and graphene represent the extreme limit ofa radius equal to infinity: such two-dimensional surfaces nucleate PE crystals to grow in virtuallyany orientation (Cheng et al. 2012). It should be noted that any compatible CNT-crystalline polymer-weak solvent system is a candidate for NHSK formation, but many polymers crystallize in a helicalconformation, limiting their contact with the CNTsidewall. Thus, chains may simply wrap the CNTsurface or form poor crystals if the epitaxial relationship is unfavorable.

Block Copolymer-Decorated CNTsPeriodically functionalized CNTs can directly lead to controlled two-dimensional or three-dimensional CNT suprastructures, which is an essential step toward building future CNT-basednanodevices. Very few reports have addressed periodic functionalization/patterning on CNTs. Fromthe CNT surface patterning viewpoint, all the reported periodic patterns, including the previouslydiscussed NHSKs, suffer from limitations such as the locality of the pattern and/or poor periodicity.To this end, we recently used low-molecular-weight polyethylene-b-poly(ethylene oxide)(PE-b-PEO) block copolymers (BCPs) to achieve uniform, periodic patterns on CNTs (Wanget al. 2009b, 2012).

A single-walled CNT (SWCNT)/dichlorobenzene solution was dropcast on a carbon-coatednickel grid and dried at ambient temperature. The fractionated PE-b-PEO (molecular weight1,700 g/mol; 50 wt% PE) was dissolved in chloroform and the solution was then spin-coated ontothe SWCNT-loaded grid. The sample was stained with ruthenium tetroxide (RuO4) to enhance thecontrast. Figure 2a shows a TEM image of the resultant BCP/SWCNT hybrid. Numerous elongated“wormlike” structures with dark and bright stripes are clearly seen. These stripes are phase-separatedBCPs; the dark stripes are PEO, while the bright ones are PE blocks. The consistent orientations of


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the adjacent stripes and the aspect ratio of this unique wormlike morphology indicate that the axis ofthe underneath SWCNT is perpendicular to the stripes, as shown in Fig. 2b. Observing this regularpattern on CNTs at ~12 nm scale is intriguing and the formation of this unique structure is related tothe interplay between BCP phase separation and CNT-induced polymer crystallization. BCPs areknown to be able to phase separate into ordered microstructures at ~10–100 nm scale. As they aredissolved in solvents, BCPs can be considered as macromolecular surfactants. In a CNT/BCPsystem, if one segment of the BCP is crystalline and is able to form single crystals on CNTs, theBCP phase separation and the CNT-induced crystallization should affect each other. Depending onthe BCP/CNT/solvent interaction parameters, a few scenarios are possible: (1) BCPs form micelles(or other aggregates), which separate from CNTs; (2) BCPs form micelles wrapping around CNTs;and (3) one segment of the BCP crystallizes on CNTs, leading to the CNT-induced BCP phaseseparation. The morphology of the BCP/SWCNT hybrid in Fig. 2 clearly indicates that the phaseseparation of PE-b-PEO is directed by the underneath SWCNTs, suggesting that scenario 3 is thedominant physical process in the present system.

It was shown in the previous discussion that in PE NHSK, PE single crystals can be controlled togrow on CNTs and the growth mechanism is the size-dependent soft epitaxy. In BCP/SWCNThybrids, upon crystallization, PE chains aligned parallel to the SWCNT axis forming the brightstripes. The observed alternating stripes are thus perpendicular to the SWCNT axis. Compared withthe crystal patterns formed in the CNT-induced homopolymer crystallization, the present alternatingpattern formed by the BCP is far more uniform: the period of the alternating pattern is 11.9� 0.9 nm.The width of the bright stripes along the CNTaxes is 5.9� 0.7 nm. Comparing this number with theextended chain length of the PE block suggests that each PE domain is made of one layer ofinterdigitated extended PE chains.

This uniformly patterned hybrid structure represents a unique nanoscale complex architecture.Since PE is hydrophobic and PEO is hydrophilic, this system features a hybrid structure onindividual CNTs with alternating amphiphilicity at a ~12 nm period. The domain size can becontrolled by changing the BCP molecular weight. This structure is also similar to the multicom-partmental BCP structures, which can be used as the drug delivery system with the capability todeliver multiple drugs. Furthermore, the PEO domains can be modified with various functional

PEO

PE

ba

Fig. 2 The alternating pattern of PE-b-PEO formed on SWCNTs with a 12 nm period imaged using transmission electronmicroscopy. The dark and bright stripes represent the PEO and PE domains, respectively (From Ref. Li et al. 2009)


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groups, so that the periodic functionalization of CNTsmay be achieved. To this end, thiol-terminatedPE-b-PEO has been synthesized. Following the same experimental procedure, SWCNTs wereperiodically decorated by the BCP with thiol groups present in the PEO stripes. The resultantproduct was then incubated with a 5 nm gold colloid, and gold nanoparticle (AuNP)-decoratedBCP/SWCNT hybrid was obtained. The AuNPs were patterned periodically along the SWCNTs,and the period is the same as that of the BCP/SWCNT hybrid. This periodic AuNP/SWCNT hybridis of great interest in the areas of nanoelectronics and single-electron devices.

NHSK PaperPE single crystals in NHSKs enable controllable spacing between individual nanotubes and thus anexpanded form of buckypaper can be produced (Laird et al. 2012). To produce freestanding NHSKfilms, solutions of NHSK were vacuum-deposited over PTFE membranes and then rinsed withmethanol and allowed to dry. SWCNT loading contents including 13, 20, 25, 52, and 70 wt% filmswere produced, corresponding to approximately 8.3, 13, 17, 40, and 59 vol% ignoring the porevolume fraction of the film. These samples were compared to classical SWCNT buckypaper.“NHSK buckypaper” had thickness of 5 ~ 15 mm. It was found that these films had weak CNTorientation in the plane of the film. Waviness of the orientation of NHSK deposited from solutionwas attributed to entanglements of the quasi-three-dimensional structure which formed cloudlikeaggregates in solution. Figure 3 shows the top view of NHSK paper. It was noted that there was near-complete exfoliation of individual CNTs. In SWCNT buckypaper, CNTs typically bundle together.A noteworthy aspect of the NHSK paper was its high apparent pore volume. BET surface areaanalysis confirmed that pores averaging ~30 nm pervaded the 25 CNT wt% film, with porosityaccounting for ~50 % of the material. Electrical conductivity was affected in an unusual way by thepresence of the NHSK “spacers.” It was noted that, despite the fact that films should theoreticallyhave been far above the percolation threshold of the CNT loading fraction for a SWCNT/PEcomposite, conductivity scaled a power law exponent of 2 instead of 1. This unusual behaviorwas believed to be due to the way in which polymer was replaced by conductive filler in thecomposite film. In NHSK composites, this replacement is not isotropic at all: conductive filler isadded at the expense of flat, nearly circular edge-on lamellae.

NHSK paper has a unique structure: PSCs are oriented with their lamellae directed outward fromthe film surface. The surface also has furrows, mimicking the disorganized CNT alignment withinthe film. A hierarchical roughness is therefore created: at the submicron scale, inter-NHSK createsan undulating pattern on the surface. Within this pattern, intra-NHSK has a high-aspect-rationanoscale roughness due to the perpendicularly oriented kebabs. This unusual roughness pattern

Fig. 3 Top view of PE NHSK paper


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significantly affected the wetting properties of NHSK paper (Fig. 4). The sessile drop in Fig. 4 shows25 wt% SWCNT NHSK demonstrating their superhydrophobicity characteristic at a static contactangle of 152.3�, which was lost after annealing. Throughout the range of different SWCNT loadings,contact angle first increases then decreases. At lower PE contents, the nanoscale surface roughnessdecreases. Consequently, the droplet is able to wet more of the sub-microscale topology, and thecontact angle decreases toward the intrinsic contact angle of the bare SWCNT buckypaper,measured to be 82�. The contact angle of SWCNT paper reaches a maximum at 25 wt% SWCNTcontent. Above this loading fraction, the PE single crystals begin to collapse. Roughness againdecreases and the droplet contact angle begins to decrease toward the intrinsic contact angle of PEsingle crystals, ~94� (Schonhorn and Frank 1966). More interestingly, it was found that waterdroplets as large as 5 mL could be tilted to any angle without roll-off and even suspended upsidedown from superhydrophobic NHSK films. Several surfaces have been shown to exhibit this type ofstatically superhydrophobic but dynamically non-sliding behavior, including red rose petals (Fenget al. 2008). The cause of this curious wetting property has been shown to be the dual length-scaleroughness (Jeong et al. 2006; Guo and Liu 2007; Zhao et al. 2007; Caps et al. 2009; Jeonget al. 2009). Droplets deposited on dual-roughness surfaces have hydrophobic wetting characteris-tics that are dependent on the aspect ratio of the asperity over which the water droplet sits. High-aspect-ratio asperities might have Cassie-Baxter wetting behavior, that is, the droplet will remainsuspended above the feature without wetting it. Hydrophobic surfaces that have rough surfacefeatures with shallower profiles might be able to be wetted by droplets (Wenzel wetting behavior). Itwas determined that NHSK films have Cassie-Baxter wetting at the nanoscale andWenzel wetting atthe sub-microscale. Therefore, a droplet could deform to fill the sub-microscale inter-NHSKmorphology, but would still be suspended above the NHSK film by virtue of its inability to wetthe nanoscale intra-NHSK features. Extending the functionality of NHSK films via an additionalcoating step using a low surface energy material (Yang et al. 2010; Deng et al. 2012) has been foundto further enhance this repulsive behavior.

Fig. 4 Characteristics of NHSK paper, composed of 25 wt% SWCNT. The materials are flexible and workable,advantages over the relatively brittle SWCNT buckypaper. Superhydrophobicity is achieved by a mechanism similarto the “petal effect.” Annealing of the NHSK films results in a relatively smooth composite where PE has completelywetted the surface (From Ref. Laird et al. 2012)


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Nanoparticle-Decorated Polymer Single Crystals

Metal nanoparticles formed in situ on the surface of inorganic and polymer crystals by vapordeposition have been used to reveal crystal defect structures since 1958 (Bassett 1958; Li 2009).Recently, we have demonstrated that preformed nanoparticles can also be immobilized onto thesurface of polymer single crystals via coupling with the polymer chain ends (Li and Li 2007; Wanget al. 2008). AuNP and thiol-terminated poly(ethylene oxide) (HS-PEO) were used as a modelsystem. As the chain ends are different from the rest of the polymer backbone, given the rightcrystallization conditions, they are excluded onto the surface of the lamellar crystals. Judiciouslyselected nanoparticles can then be bound onto the single crystal surface via chemisorption(Scheme 1a, b, c). If we consider a polymer lamella as a thin sheet of paper, lamella with functionalgroups on the surface can then be regarded as a nanoscale sticky tape (double- or single-sided,depending on the polymer chemistry) that can immobilize different nanoparticles. The implicationof this process is twofold: on one hand, nanoparticles can be used as a decoration tool, revealing thelocation of chain ends on the polymer single crystals. These nanoparticle-decorated polymer singlecrystals are of great interest from an application standpoint (see following discussions). On the otherhand, the coupling reaction directly leads to partially functionalized nanoparticles, because only thesurface that is in contact with the single crystals is coupled with the polymer chains. The oppositeside of the nanoparticle can be further functionalized with other groups including different types ofpolymer, small molecules, nanoparticles, etc. Upon dissolving the single crystal, a rich collection oftailor-made nanoparticles and nanoparticle clusters can be obtained.

Polymer Single Crystal-Guided Nanoparticle PatterningFor AuNP immobilization study, the number average molecular weights of PEO used were 2 k and48.5 kg/mol, respectively. Large-size, square-shaped single crystals were obtained using the self-seeding method. Most of the thiol groups were excluded onto the crystal surface upon crystallizationas shown in Scheme 1b. These HS-PEO single crystals were incubated with tetraoctylammoniumbromide (TOAB)-protected AuNPs (~5 nm in diameter) for ~2 h, and AuNPswere then immobilizedonto the single-crystal surface due to the Au-S bond formation (Scheme 1c). Figure 5a shows theTEM images of AuNPs on the HS-PEO (48.5 k) single crystal (inset shows the entire single crystal).AuNPs can be clearly seen on the surface as the dark dots. The density of AuNP population isrelatively low and the NPs are randomly located. Much denser arrangement of AuNPs can be seen inthe case of the 2 k HS-PEO, as shown in Fig. 5b, where most of the single-crystal surface was

Scheme 1 Schematic representation of polymer single crystal-templated nanoparticle assembly (From Ref. 59)


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covered with AuNPs. The inset shows a 6 � 6 mm, square-shaped AuNP assemble and the size canbe easily controlled by crystallization time. The difference of the AuNP area density on 2 k and48.5 k PEO crystals can be attributed to the different thiol group density on the PEO single-crystalsurface as shown in Fig. 5c, d. For 48.5 k PEO, the polymer chain end density is much lower thanthat of 2 k PEO (~24 times less assuming similar to the lamellar thickness). Furthermore,low-molecular-weight PEO undergoes integral folding which renders most of the chain ends onthe crystal surface, while for relatively long-chain PEO, nonintegral folding occurs. In this case,despite the fact that thiol groups are different from the rest of the polymer chain, they could beembedded in the lamellar crystals as shown in Fig. 5c. This further reduces the thiol population onthe 48.5 k crystal surface. Therefore, PEO molecular weight can be used as a controlling factor totune the thiol (and thus AuNP) density on the PEO crystal surface. Following similar methods,magnetic, platinum, and semiconducting nanoparticles have been successfully immobilized on thePEO, polycaprolactone (PCL), and PE-b-PCL single-crystal surfaces (Li et al. 2008; Wanget al. 2010b).

The previously discussed adsorption process occurred after polymer single crystal formation. Wehave also demonstrated that much more complex nanoparticle patterns on these single crystals canbe obtained by in situ assembling NPs during polymer crystallization (Li et al. 2009a). In order toachieve in situ nanoparticle assembly, tri-n-octylphosphine oxide (TOPO)-protected AuNPs wereadded to the HS-PEO/pentyl acetate solution during the crystallization process. The arrow inScheme 2a denotes the delay time (td) of AuNP addition after the crystallization began. By varyingtd, very different morphologies were observed after approximately 24 h of crystal growth(Scheme 2). When AuNPs were added at the beginning of the crystallization, td ¼ 0 min, theyassembled into a square pattern at the center of the HS-PEO single crystal (denoted by the dash lines,Fig. 6a), leaving a “blank margin” with a width of ~0.8 mm around the crystal edges (Fig. 6a).Figure 6b shows the boundary of these two areas. This morphology is different from the previouslyreported AuNP sheets, where the entire surface of the single crystals was covered by AuNPs, as

Fig. 5 AuNPs on the surface of HS-PEO (a) 48.5 k and (b) 2 k single crystals (inset shows the entire single crystal, scalebar 2 mm). Schematic representation shows low thiol density on 48.5 k PEO single crystal (c) and relatively high thioldensity on 2 k PEO single crystals (d). Note that extended-chain single crystals are shown in (d); folding could occurdepending on MW and crystallization conditions (From Ref. Li and Li 2007)


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shown in Fig. 6c, d for comparison. When AuNPs were added at td ¼ 10 min, AuNP “frames” wereformed (Fig. 6e). In addition to the margin area around the crystal edges, there is a “blank” area of~0.3 � 0.3 mm at the center of the crystal as well. For td ¼ 20 and 30 min, AuNPs assembled intosimilar frame-like patterns, while the central blank squares became larger and larger (~1.2� 1.2 mmand 2.2� 2.2 mm, respectively), as shown in Fig. 6f, g. The center dark dots on these single crystalswere due to the larger thickness of the crystalline seeds generated by self-seeding technique.

Of interest is how AuNPs assembled into these frame patterns. The single crystal with a AuNPframe can be divided into three regions as shown in Fig. 6g. Figure 6h shows the enlarged image ofthe boundary between regions 1 and 2. AuNPs selectively gathered in region 2. The lack of AuNP inregion 3 suggests complete consumption of AuNP at the late stage of the crystal growth. This wasvisually confirmed by the color change of the crystallizing solution from wine red to clear. Thequestion then is why AuNPs were immobilized onto region 2, not 1? AuNPs used in our experimentwere mainly coated with TOPO, which can be readily replaced by HS-PEO. As these TOPO-AuNPsare added to the crystallizing HS-PEO, they can undergo place exchange reaction with eitherHS-PEO single crystals or uncrystallized HS-PEO molecules in solution. The increased size ofregion 1 at longer td indicates that regions 1 and 2 were formed before and after AuNP addition,

Scheme 2 (a) The experimental procedure of using the self-seeding method to grow HS-PEO single crystals. In order topattern NPs during the growth of the single crystals, AuNP was added at different td during HS-PEO crystallization. (b)Growth mechanism of AuNP frames. (i) A HS-PEO lamellar single crystal, which has already formed prior to theaddition of gold colloid. The PEO chains are parallel to the lamellar normal and the thiol groups are on the crystalsurface. As gold colloid is added to the HS-PEO solution, AuNP-PEO conjugates are formed via the place exchangereaction. The AuNP-PEO conjugates crystallize around the already formed single crystals, generating AuNP frames (ii).After these conjugates are exhausted, PEO continues to crystallize around the AuNP frames, forming a blank margin(iii). 1, 2, and 3 in (iii) denote the three distinct regions of the pattern (From Ref. Li et al. 2009a)


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respectively. The lack of AuNPs in region 1 suggests that most TOPO-AuNPs reacted withuncrystallized HS-PEO molecules, leading to the formation of PEO-AuNP conjugates, as shownin Scheme 2b. Because of the steric hindrance imposed by the attached PEO chains, thesePEO-AuNP conjugates could not adsorb onto the already formed single crystals (region 1) underour experimental condition. Nevertheless, these PEO-AuNP conjugates possess improved ability tocrystallize, because the increased local concentration of PEO around AuNP facilitated polymercrystallization. Since PEO molecules were tethered to AuNPs, the latter were brought to the single-crystal surface as PEO crystallized, forming the AuNP frames. The proposed growth mechanism forAuNP frames is detailed in Scheme 2b.

Polymer Single Crystal-Nanoparticle ConjugatesPSC-NP conjugates provide a unique preassembled system which can broaden nanoparticle applica-tions. To this end, we recently demonstrated the utilization of PSCs as magnetically recyclable supportfor NP catalysts. Small NPs would be ideal candidates for surface catalysis, but because of their highspecific surface energy, they are subject to aggregation which undermines their efficacy. On the otherhand, large NPs have reduced specific surface energy and lower surface area/volume ratio, likewisemaking them relatively inefficient for this application. In recent work, we used PSCs to isolate catalystNPs as a means to take advantage of their high specific surface energy without having to contend withNP aggregation. As a proof of concept, we selected platinum nanoparticles (PtNPs) as the catalyticcomponent and magnetic nanoparticles (Fe3O4NPs) as the magnetically responsive materials (Donget al. 2012a). In order to attach both nanoparticles, the support is composed of PSCs with two differenttypes of surface groups, i.e., SH- and hydroxyl- (–OH). In this way, –SH can attach to a variety of metalsurfaces, such as platinum, gold, silver, copper, etc., while –OH can bond to iron oxide. a-Hydroxy-o-thiol-terminated polycaprolactone (HO-PCL-SH) with a molecular weight of 13.2 kg/mol wassynthesized according to the literature (Hedfors et al. 2005). Uniformly sized OH-PEO-SH singlecrystals were obtained using the self-seeding method outlined in Scheme 2a. The as-fabricatedHO-PCL-SH single crystal has a hexagon shape. It is 13 mm long and 5 mm wide. The thickness was

gfe

1

23

h

1 1 1 100

a b

1 100 2

d

100

c

Growth direction

Growth direction

Fig. 6 TEM images of AuNP patterns formed on HS-PEO single crystals by varying td. Gold colloid was added toHS-PEO solution at td¼ (a) 0 min, (e) 10 min, (f) 20 min, and (g) 30 min after HS-PEO began to crystallize. (b) and (h)show the enlarged images of (a) and (g), respectively. (c) and (d) show the AuNP sheets with full coverage obtained bymixing the filtered HS-PEO single crystals with AuNPs (From Ref. Li et al. 2009a)


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determined by using atomic force microscopy (AFM) to be around 8 nm (Fig. 7). Since PSCs havea relatively constant thickness which is determined by the crystallization temperature, their surface area-to-volume ratio can be estimated to be ~2/t, where t is the thickness of the PSC, which is independent ofthe single-crystal lateral size. The surface area-to-volume ratio in the present case therefore can becalculated to be ~2.5 � 108 m�1 (equivalent to that of a nanosphere with a 12 nm radius!). In contrast,a sphere with a similar volume (~0.45 mm3) has only a surface area-to-volume ratio of about 6.3 �106 m�1, ~40 times less than what polymer single crystal can offer. This implies thin polymer lamellarcrystal could be an excellent candidate for catalyst support. To this end, after crystallization, the endfunctional groups (–OH and –SH) were exposed to the single crystal surface. HO-PCL-SH single crystalwas then mixed with a PtNP solution. After saturated adsorption, PtNP was immobilized on single-crystal surface through thiol-platinum bond. Iron oxide nanoparticles were then introduced to bond toOH groups on the PSC surface.

Figure 7c shows the TEM image of the HO-PCL-SH single crystal surface after incubating inthe PtNP solution (zoom-out view of the single crystal is shown in Fig. 7c inset), which indicatesthat PtNPs have been immobilized on HO-PCL-SH single crystal surface through thiol-platinumbond. The areal number density of PtNPs is about 440/mm2, and the catalyst loading can beestimated to be ~20 % by weight. Figure 7d shows the TEM image of a HO-PCL-SH single crystalafter adsorbing two types of nanoparticles. The number density of PtNPs after Fe3O4NP

0 2. 00

–45n

m0

45nm

4. 00 6. 00µm1µm

50 nm50 nm

8. 00

a b

c d

Fig. 7 (a) Height mode AFM image of HO-PCL-SH single crystal. (b) Cross-sectional analysis showing the thicknessof HO-PCL-SH single crystal is around 8 nm. (c) and (d) show TEM images of 7 nm PtNP-coated and 7 nm PtNP/10 nmFe3O4NP-decorated HO-PCL-SH single crystals, respectively. Insets of (a), (c), and (d) show the zoom-out view of thecorresponding single crystals. Scale bar for all insets, 1 mm (From Ref. Dong et al. 2012a)


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attachment remains unchanged, indicating there is no PtNP detachment during the secondnanoparticle adsorption process.

Reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride has been frequently used asa model reaction to check the catalytic activity of metallic nanoparticles (Wunder et al. 2010).Figure 8a shows successive UV/vis spectra of 4-nitrophenol reduction solution at the presence ofHO-PCL-SH single-crystal-supported 7 nm PtNP (PSC/PtNP in abbreviation). The spectra weretaken at 5 min interval. The 4-nitrophenolate anion, which forms after adding sodium borohydride,shows a strong absorption peak at 400 nm. As time lapses, the peak intensity decreases. At the sametime, the peak at 300 nm, which corresponds to 4-aminophenol, increases. The appearance of theisosbestic point at 314 nm indicates that there is no side reaction. This reaction is first order in4-nitrophenol concentration after a delay time (Mei et al. 2005), which is the time required for thereactants to diffuse onto nanoparticle surface. The apparent rate constant was calculated in thiscase to be around 0.08 min�1, as shown in Fig. 8a inset. Figure 8b shows the UV/vis spectra of4-nitrophenol reaction when using HO-PCL-SH single-crystal-supported PtNP and Fe3O4NP(PSC/PtNP/Fe3O4NP in short) as catalyst. The amount of PtNP added into the reaction solutionwas kept the same as that of PSC/PtNP without Fe3O4NPs. The reduction completed in 40 min forFig. 8a and in 14 min for Fig. 8b. The apparent rate constant increased from 0.08 min�1 in Fig. 8a toabout 0.4 min�1 in Fig. 8b. Since the HO-PCL-SH-supported Fe3O4NP has negligible catalyticactivity, the five times increase in apparent rate constant must be derived from the interactionbetween PtNP and Fe3O4NP (Wang et al. 2009a). Moreover, we have compared the present rateconstant of PSC/PtNP with that of similar systems (Mahmoud et al. 2010), which was fabricated byfirst synthesizing nanoparticles and then immobilizing them onto polymer support. The catalyticactivity is characterized under the same sodium borohydride (30 mM) and 4-nitrophenol concen-tration (0.015 mM). The rate constant in our current system is estimated to be 0.01 s�1 m�2 L whennormalized by nanoparticle surface area and 0.08 s�1 g�1 L when normalized by the total weight ofcatalyst and support, which are 2.5 and 26 times higher than the reported literature value (Mahmoudet al. 2010), respectively. Furthermore, as evidenced in Fig. 7d, the surface of HO-PCL-SH singlecrystal can be heavily loaded with a dense layer of Fe3O4NPs. Such high loading of Fe3O4NPensures efficient nanoparticle recycling. All PSC/PtNP/Fe3O4NP can be quickly separated out to theside wall of the test tube under magnetic field, demonstrating that it is recyclable.We further evaluatethe catalytic behavior of our system after magnetic recycling. The reaction catalyzed by PSC/PtNP/Fe3O4NP was first monitored by UV/vis spectrometer until its completion. Then, the catalyst was

Fig. 8 UV spectra of nitrophenol reduction reaction for (a) PSC/PtNP during 40 min reaction with 5 min interval. (b)PSC/PtNP/Fe3O4NP during 14 min reaction with 2 min interval. Inset: linear relationship of ln (A/A0) as a function oftime (From Ref. Dong et al. 2012a)


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separated using a magnet, rinsed, and re-dispersed in deionized water for the next cycle of catalysis.The nanocatalyst can be magnetically recycled and reused for four times with only slight changes inapparent rate constant.

PSC-nanoparticle conjugates can also be used for surface-enhanced Raman spectroscopy (SERS)study (Dong et al. 2012b). We recently reported the fabrication of a novel polymer-single-crystal@-Au nanoparticle (PSC@AuNP) nanosandwich structure for SERS applications. The detailed fabri-cation procedure starts with solution growth of a 2D PSC with –SH end groups exposed on the PSCsurface. The PSC can then be used to immobilize 6 nm AuNPs through thiol-gold bonds by solutionmixing. Then, 6 nm-AuNP-decorated PSCs are immersed in 10mg/ml AuCl3 solution for electrolessdeposition. One hour deposition time is utilized to ensure the formation of a dense layer of AuNPsapproximately 50 nm in diameter on the PSC surface. The spontaneous AuNP formation at theabsence of a reducing agent may be attributed to the nanoparticle-induced autocatalytic growthmechanism. The as-fabricated substrate can then be used for SERS study. Figure 9a shows the

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optical image of large AuNP-decorated PCL-SH single crystal after electroless deposition (AuCl3treatment for 1 h), i.e., PSC@AuNP nanosandwich (which exhibits a band in the near IR). Due to theexistence of large AuNPs, PSC shows a bright gold color. Figure 9b shows a low-magnificationTEM image of this structure, the surface of which is decorated with densely packed AuNPs.Figure 9c shows the enlarged TEM image of Fig. 9b. The average size of AuNPs is approximately50 nm (Fig. 9d) based on the image analysis of Fig. 9c. Figure 9e is a high-resolution TEM imageshowing the lattice structure (Au(111)) of the edge of a large AuNP. Furthermore, AFMwas utilizedto monitor the changes in thickness. Figure 9f, g, h show pure PCL-SH single crystal has a thicknessof about 8 nm. After the deposition of 50 nm AuNP, the thickness is increased to around 110 nm,which is about two times the average diameter of AuNP plus the thickness of PCL-SH single crystal.This, combined with TEM analysis, demonstrates that the AuNPs have decorated both sides ofPCL-SH single crystal forming a PSC@AuNP nanosandwich structure.

The as-fabricated PSC@AuNP nanosandwich can be used as SERS substrates in two ways. Onthe one hand, it can be utilized by mixing the hybrid material with an analyte solution. On the otherhand, detection is also possible using the tip-enhanced Raman spectroscopy (TERS) configuration,where PSC@AuNP is placed onto the analyte surface, as will be described below. Raman signalsfrom the analyte lies under PSC@AuNP can be probed. As a proof of concept, we have used 4-aminothiophenol (4-ATP) as the probemolecule to evaluate the SERS effect in both cases. In the firstcase, before 4-ATP adsorption, the pristine PSC@AuNP had only negligible signals (Fig. 10a redcurve). When 4-ATP molecules were attached using solution mixing method, it exhibited strongSERS signals, as shown in Fig. 10a black curve. We performed a control experiment to study SERSof 4-ATP chemically adsorbed on 6 nm-AuNP-decorated PSC (the AuNP-decorated PSC prior toelectroless deposition of additional gold from AuCl solution); much weaker Raman signals wereobserved (Fig. 10b black curve). According to the vibration of C-S at 1,077 cm�1, the enhancementfactor (EF) for PSC@AuNP nanosandwich was calculated to be approximately 1 � 107, while the6 nm-AuNP-decorated PCL-SH single crystal had a relatively modest EF of about 2 � 105.

In the second case, PSC@AuNP was cast on self-assembled monolayer (SAM) of 4-ATPmolecules chemically adsorbed on Pt/Pd (80/20) surface. Figure 10c black curve shows the strongRaman spectrum obtained at this configuration. Without applying PSC@AuNP, 4-ATP SAMs onmetal surface had very weak Raman signals (Fig. 10c red curve). The EF (based on C-S vibration) iscalculated to be around 4 � 106. Because of their different sizes, not all AuNPs contacted the SAMsurface. The contact area should also be smaller than the footprint area of a nanoparticle; this EF istherefore an underestimation. It is worth noting that although the EF is the same order of magnitudeas that of TERS (up to 106), the overall Raman intensity is much stronger because each PSC@AuNPmimics thousands of TERS probes. Furthermore, due to the thinness and flexibility of PSC, thishybrid structure can coat onto non-flat surfaces, such as yeast cells. SERS of the latter can then beobtained adopting the similar TERS configuration. As can be seen from Fig. 10d inset,a PSC@AuNP nanosandwich can cover a single yeast cell. Strong SERS signals from amide,protein, etc. of yeast cell can be obtained (Fig. 10d black curve).

Toward Janus and Multicompartment NanoparticlesAs shown in Scheme 1d, e, f, after nanoparticle immobilization, centrosymmetry of the nanoparticleis broken and the top area can be further functionalized with various polymer brushes; novel Janusnanoparticles (JNPs) and multicompartment nanoparticles can therefore be synthesized (Wanget al. 2008, 2010a, c; Dong et al. 2011; Zhou et al. 2012). PEO-AuNP-poly(methyl methacrylate)(PMMA) and PCL-silica nanoparticle (SiNP)-poly(N-isopropylacryamide) (PNIPAM) have beensynthesized, and they show interesting assembly behaviors. For example, Janus AuNPs (J-AuNPs)


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with bicompartment PEO and PMMA brushes (PEO114-Au6-PMMA208) were synthesized. Thesubscript numbers of PEO and PMMA denote the degrees of polymerization of the polymers whilethat of Au denotes the diameter of the nanoparticle in nanometers. The polydispersity indices (PDIs)of PEO and PMMA are 1.03 and 1.25, respectively. The average number of PEO and PMMA chainsper AuNP is 32 and 26, respectively. The mass ratio of the PEO over PMMA is approximately 1:3.2.These AuNPs grafted with bicompartment polymer brushes provide an ideal NP system for self-assembly studies. Solubility tests showed that they readily dissolved in good solvents of PEO andPMMA, such as acetone, chloroform, tetrahydrofuran, and dimethylformamide. To investigate thedetailed solution assembly behavior, acetone and dioxane were chosen as model solvents. Upondissolving PEO114-Au6-PMMA208, the color of the acetone solution remained red over the entireobservation period (1 month), indicating that the particles were stable in acetone. However, thedioxane solution progressively changed from red to pale blue (Fig. 11a). Figure 11b shows theUV/Vis spectra of PEO114-Au6-PMMA208 in dioxane. The surface plasmon resonance (SPR)absorption band red shifted from 527 to 545 nm in ~50 min and remained at 545–550 nm afterward.

Fig. 10 (a) SERS of PSC@AuNP before (red curve) and after (black curve) 4-ATP molecule adsorption. Inset:schematic of detection adopting conventional configuration with yellow hexagon and dark dots representinggold-colored PSC@AuNP and 4-ATP molecules, respectively. (b) Raman spectrum from pure 4-ATP molecules (redcurve) and SERS from 4-ATP molecules chemically adsorbed on 6 nm-AuNP-coated PCL-SH single crystal (blackcurve). Inset: molecular structure of 4-ATP. (c) Raman spectrum of 4-ATP SAMs on Pt/Pd (80/20) surface (red curve)and SERS spectrum obtained after casting PSC@AuNP on top (black curve). Inset: schematic of detection adoptingTERS configuration. (d) Raman spectrum of yeast cell (red curve) and SERS spectrum obtained after castingPSC@AuNP on top (black curve). Inset: optical image of yeast cell before (left) and after (right) coating withPSC@AuNP, scale bar 2 mm (From Ref. Dong et al. 2012a)


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Both red shift of the SPR band and the color change of the solution suggest that nanoparticleassembly occurred, which caused delocalization of the surface plasmon of AuNPs.

TEM experiments were conducted to confirm particle assembly in dioxane. Figure 12 showsPEO114-Au6-PMMA208 in dioxane at different times. JNPs were well dispersed in dioxane at thebeginning. Small clusters of 2–7 JNPs can be seen for samples incubated for ~10 min (Fig. 12b). Thesize of the cluster continuously increased and the ensemble became anisotropic, wormlike aggre-gates. After 35 min, the wormlike aggregates appeared to be branched, approximately 0.1–1 mmlong and 50 nm wide (Fig. 2d). There are approximately 7–15 nanoparticles along the short axis ofthe wormlike aggregates. This structure is remarkably similar to the recently reported ensembleformed by Janus particles with a relatively large size, where compromise between hydrophobicinteraction and charge repulsion at a given ionic strength led to the formation of wormlike particlestrings (Hong et al. 2008). The wormlike aggregates are believed to be due to the bicompartmentpolymer brushes on these J-AuNPs. The argument is as follows: there are ~32 PEO and 26 PMMAchains per AuNP. Dioxane is a relatively poor solvent for PEO, so PEO is expected to collapseagainst the AuNP. Furthermore, our previous experiments suggest that PMMA covers approxi-mately ¾ of the AuNP surface, while PEO covers ¼, suggesting that the local concentration of PEOis much higher than that of PMMA. Therefore, PEO “patches” prefer to be confined inside theaggregates so that the overall free energy of the system can be minimized. Note that the assembly isalso reversible: as we add good solvent such as acetone into the system, the aggregates dissembleand the SPR band shifts back.

Note that nanoparticle chains can also be formed in symmetrically functionalized nanoparticles(Kang et al. 2005; Lin et al. 2005; Benkoski et al. 2007; Nie et al. 2007; Fukao et al. 2009; Mann2009; Srivastava and Kotov 2009). In order to demonstrate that the formation of the wormlike

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aggregates is because of the Janus feature of the nanoparticles, a control experiment was performed.We synthesized AuNPs symmetrically grafted with PEO/PMMAmixed brushes (S-AuNPs, PEO114/PMMA152-Au6). Compared with J-AuNP PEO114-Au6-PMMA208, S-AuNP PEO114/PMMA152-Au6 has the same AuNP size. The molecular weights, PDIs of the polymer brushes, and the overallpolymer grafting density are also comparable with those of the JNPs (Mn

PMMA ~ 15.2 kg/mol,PDIPMMA ¼ 1.19, Mn

PEO ~ 5 kg/mol, PDIPEO ¼ 1.03, grafting density ~ 1.2 chains/nm2). In thecontrol experiment, these S-AuNPs were dissolved in acetone and dioxane and the assemblybehavior was studied. The color as well as the SPR band position of the S-AuNP solution in acetoneor dioxane did not change as they did for J-AuNPs. Figure 11c shows the UV/Vis spectra andFig. 11d shows the plot of the SPR band positions of J-AuNPs and S-AuNPs in dioxane against time.The plasmon absorption band red shifted for J-AuNPs while it remained constant for S-AuNPs.

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Figure 12f shows that after 3 h incubation in dioxane, S-AuNPs were well dispersed and aggregationwas not observed. This clearly confirmed that the previous wormlike aggregate formation was due tothe asymmetric property of JNPs. For J-AuNPs, each particle has one PEO “patch.” Since PEO tendsto collapse in dioxane, the PEO patches of adjacent J-AuNPs tends to coalesce, leading to theformation of few particle clusters and wormlike aggregates. This is similar to the formationmechanism of wormlike micelles of block copolymers. However, for S-AuNPs, PEO and PMMAbrushes are uniformly distributed on the nanoparticle surface. Although PEO chains also collapse inthis case, because PMMA chains evenly spread on the entire particle surface and stabilizenanoparticles, the collapse of PEO chains does not lead to particle aggregation.

Conclusions

Combining polymer single crystals and nanoparticulates leads to new types of hybrid material. Not onlydo they offer novel properties, but also they provide opportunities for studying the fundamentals ofpolymer physics. The unique NHSK structure showed a variety of technological advantages includinghigh specific surface areas, good potential mechanical load transfer, controllable pore sizes for NHSKpaper, etc. From a technological standpoint, the functionalized single crystals are also of great interestbecause they are nano-“tapes”which can immobilize desired nanoparticles, viruses, and proteins. Nano-tapes can therefore be used in drug delivery, nanoparticle recycling, controlled nanoparticle synthesis,etc. The nanoparticle-coated polymer single crystals have a sandwich structure; they might findapplications in microelectromechanical systems (MEMS) and nanoelectromechanical systems(NEMS) as well as microfluidic applications. The tailor-made Janus nanoparticles can find applicationssuch asmultiple functional nanoparticle clusters for bioimaging and drug delivery, stabilization of liquid-liquid interfaces, and controlled assembled nanoparticles for nanoelectronics.

Acknowledgments

This work was supported by the National Science Foundation Grants DMR-0239415,DMI-0508407, and DMR-0804838 and the corresponding NSF special creativity award andCMMI-1100166. EDL was also supported by the NSF-IGERT DGE-0221664.

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