SENIOR EDITORS

ISSN 0887-6266

VOLUME 47 • NUMBER 24 • DECEMBER 15, 2009

Articles published online in Wiley InterScience,10 November 2009 through 11 November 2009

JOURNAL OF

POLYMER SCIENCEP A R T • B

PolymerPhysics

GREGORY B. McKENNA SPIROS H. ANASTASIADIS

EDITORS

JOHN R. DUTCHER HIROSHI WATANABEJO

URN

AL OF PO

LYMER SCIEN

CEPART B:PO

LYMER PH

YSICSVol.47 • N

o.24 • Decem

ber 15,2009 pp.2429–2600W

iley

Guest EditorsVenkat Ganesan and Christopher L. Soles

The American Physical Society Division of Polymer Physics

Special Issue

cover_C1-C4-Spread 11/11/09 11:26 PM Page 1

VIEWPOINT

Polymer Single Crystal Meets Nanoparticles

CHRISTOPHER Y. LI

Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvannia 19104

Received 22 June 2009; revised 14 July 2009; accepted 15 July 2009DOI: 10.1002/polb.21855Published online in Wiley InterScience (www.interscience.wiley.com).

Keywords: carbon nanotubes; crystallization; nanocomposites; nanoparticles; polymer crystallization; singlecrystal

INTRODUCTION

In 1938, Storks reported gutta-percha (trans-polyiso-prene) single crystals obtained by casting thin filmsfrom dilute chloroform solution, and suggested a possi-ble chain folding mechanism in polymer crystalliza-tion.1 His work was however overlooked until 1957,when Till, Keller, and Fisher independently reportedthe growth and identification of single crystals of linearpolyethylene (PE).2–4 Since then, a library of beautifulpolymer single crystals has been established.5 The ele-gant work of Till, Keller, and Fisher laid the foundationof the field of crystalline polymers. In 2007, a sympo-sium was held in Boston ACS meeting on ‘‘50 years ofpolymer single crystals—a look back, current discov-eries, and future opportunities.’’6 Scientists around theworld gathered and discussed exciting developments ofthis relatively matured, yet dynamic field. After 50years of extensive investigation, there are still activedebates on numerous issues including the very funda-mentals of the crystallization mechanism as well asmetastability of folded versus extended chain crys-tals.7–10 Nevertheless, it is generally agreed that poly-mer single crystals can be considered as markers of thecorresponding crystallization process, allowing one tounambiguously determine crystal structures and chainconformation using techniques such as electron diffrac-tion.11 Yet, the main focus of semicrystalline polymershas been on bulk systems because they are more ‘‘rele-vant’’ to practical applications. Questions have

also been frequently raised on the future researchdirections in crystalline polymers and polymer singlecrystals. In this essay, we would like to demonstratethat the newly developed field of nanoparticles offersabundant opportunities for, and imposes numerouschallenges to the field of polymer physics and polymersingle crystals.

The past decade witnessed a fast growth of researchon nanoparticles because of their fascinating mechani-cal, electrical and optical properties.12–14 On the basisof their dimensionality, nanoparticles can be dividedinto 2-D (nanoplates/nanosheets), 1-D (nanotubes/nano-wires/nanorods) and 0-D (often referred to as nanopar-ticles as well) objects. Polymer science has been inter-acting with nanoparticles in two ways: (1) various poly-mers have been used to form polymer brushes on thesurface of nanoparticles to stabilize the latter for pro-cessing purposes.14 (2) A variety of nanoparticles havebeen used to blend with polymers (or block copolymers)to form value-added nanocomposites.15–17 Althoughcrystalline polymer/nanoparticle nanocomposites havebeen investigated by a few groups, little attention waspaid to polymer single crystal/nanoparticle hybrids.18 Atypical polymer lamella is �10 nm thick and a fewnanometers to a few micrometers wide while nanopar-ticles by definition have at least one dimension lessthan 100 nm; the similar size of these two seeminglydifferent objects ensures an interesting marriagebetween the fields of polymer single crystals and nano-particles. The ability to form regular chain packing sug-gests that crystalline polymers may be able to offermuch more than simply passivating nanoparticles as inthe case of polymer brushes. In the following

Journal of Polymer Science: Part B: Polymer Physics, Vol. 47, 2436–2440 (2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: C. Y. Li (E-mail: chrisli@drexel.edu)

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discussion, we shall focus on 1-D and 0-D nanopar-

ticles/crystalline polymer hybrids because a 2-D nano-

particle/polymer single crystal system can be consid-

ered as a special case of the classic problem of polymer

epitaxy on solid substrates, which has been extensively

studied.19

Nanohybrid Shish Kebabs—Nanotube-Induced PolymerCrystallization

Polymer/carbon nanotube (CNT) nanocomposites (PCN)have attracted great attention in recent years.20 Due tothe high aspect ratio of CNT, percolation thresholds canbe reached at very low CNT contents in PCNs, leadingto a dramatic property enhancement. It has beenobserved that CNT can induce crystallization of numer-ous polymers.21 However, in a PCN, CNTs are coatedwith a thick layer of polymers and the polymer/CNTinterface is not clearly revealed. Using solution crystal-lization, ‘‘clean’’ single crystals of PE and Nylon 6,6were successfully grown on the surface of CNTs and theresultant structure is remarkably similar to the classicpolymer shish kebabs formed in extensional fields (Fig.1).22 Hence nanohybrid shish kebab (NHSK) was coinedas the name for this unique hybrid structure.21,23–25

Apparently, CNT acts as the 1-D nucleation sites andpolymer lamellar crystals grow periodically on the CNTsurface. The inter-kebab distance (period) reveals theNHSK formation process, which involves the competi-tion of nucleation, growth, and polymer diffusion nearand/or at the polymer/CNT interface. Because CNTsexist in the system before crystallization, externalshear is not needed to form NHSK. This is contrary tothe formation process of classic shish kebabs, whereshear is necessary to induce the coil-stretch transition

of polymer chains to form a shish. Formation of NHSK

suggests that the possible dangling chains around the

shish are not necessary for forming the relatively peri-

odic kebab crystals. Furthermore, while helical wrap-

ping of polymer chains around the CNT is suggested for

many amorphous polymer/CNT systems, in CNT-

induced polymer crystallization, size-dependent soft

epitaxy is the governing mechanism of NHSK forma-

tion.24 As the diameter of CNT is small, soft epitaxy

favors the parallel chain orientation with the CNT axis.

However, since CNTs have a variety of chiralities, this

parallel packing shall be broken as the CNT diameter

increases and molecular epitaxy between CNT and the

polymer takes over; multiple orientation of polymer

chains on the CNT surface can be observed. We have

just started to learn from this unique hybrid structure

and numerous questions can be raised:

1. What is the role of CNT chirality on CNT-induced crystallization? As the lattice orienta-tion of the graphene sheets with respect to thetube axis varies for CNTs with different chiral-ities, lattice matching between the CNT and thepolymer crystals competes with parallel align-ment of the polymer chains and CNTs. ForCNTs with different chiralities, the competitionleads to subtle differences of the energy gainfrom the heterogeneous nucleation on CNTs.Hence, CNT-induced polymer crystallization ischirality dependent and NHSK can be formedat different crystallization conditions (such ascrystallization temperatures) when we keep thepolymers as the same and vary CNT chiralities.This can potentially lead to CNT separation, animportant yet challenging field of study. Viceversa, if one keeps the CNT chirality the same,

Figure 1. Schematic representation of three scenarios of the interplay betweennanoparticles and polymer single crystals. (a) Nanoplate-induced polymer crystalliza-tion, (b) nanohybrid shish kebabs, and (c) nanoparticle-decorated polymer singlecrystals.

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polymers can be fractionated since the NHSKformation conditions are different.

2. What is the critical diameter of the soft epitaxyto molecular epitaxy transition? As the diameterincreases, the crystal growth mechanismchanged from soft epitaxy to molecular epitaxy.It is challenging, yet insightful, to quantita-tively study the critical diameter that is associ-ated with this transition, and to articulate itsphysical meaning as well as its dependence onthe chemical structure and molecular weight ofpolymers.

3. Is helical conformation of a polymer chain im-portant in NHSK formation? So far, the mostsuccessful polymers that have been used toform NHSKs are PE and Nylon 6,6, both pos-sess a planar molecular conformation. As theconformation of polymer chains becomes helical,the enthalpy gain in the CNT-induced crystalli-zation decreases due to the reduced contactbetween the polymer chain and the CNT; thiscan be an obstacle for NHSK formation. Inter-estingly, NHSK of maleic anhydride-grafted iso-tactic polypropylene was reported, and thechain orientation was reported to be perpendic-ular to the CNT axis.26 Detailed research isneeded to correlate the molecular conformationof helical polymer chains and the correspondingNHSK structure.

4. What is the surface chemistry effect on NHSKformation? Tailoring CNTs surface using surfac-tants or polymers can be readily achieved. It isof great interest to investigate the surfacechemistry effect on CNT-induced crystallization.By coating CNT with a layer of small moleculesor polymer brushes, one can decouple the epi-taxy effect from the geometrical confinement ofCNTs, which helps understand the significanceof each factor on NHSK formation.

5. How to achieve more regular patterns on CNTs?Of interest is the periodic pattern of NHSKs.However, the pattern of a homopolymer NHSKis far from regular. To this end, introducingblock copolymers into this system can dramati-cally enhance the regularity of the patternthanks to the block copolymer self assemblyprocess. To form block copolymer NHSK, onehas to carefully choose block copolymers so thatCNT-induced polymer crystallization is the driv-ing force of the block copolymer phase separa-tion and highly regular patterns can beachieved.27 Studying competition between CNT-induced crystallization and block copolymer selfassembly on CNTs can then lead to profoundblock copolymer/CNT hybrid structures.

6. Can other types of ‘‘shish’’ structures be used?The rich configuration of CNT itself complicatesthe CNT-induced crystallization study. To sim-plify the situation, one might turn to the newly

available inorganic nanowires/nanorods, whichare single crystals with well-defined crystal fac-ets.28 Polymer nanofibers can also be used asthe shish to form NHSK. Forming shish-kebabson electron-spun polymer nanofibers might leadto controlled drug delivery systems as well asfiber mats with tailored porosity and functional-ity.29

The unique NHSK structure also offers a variety oftechnological advantages. First, upon decoration withpolymer single crystals, the surface area of CNT is dra-matically enhanced. Second, polymer chains can befunctionalized; hence a variety of functional groups canbe brought to the CNT surface for application purposes.Third, kebab crystals are solid, tunable spacers, whichcan physically prevent CNTs from agglomeration.These crystals can also serve as the mechanical anchorsthat prevent slip of CNTs from the matrix and enhancethe mechanical load transfer. Numerous applicationssuch as sensors, catalyst support, electrodes and nano-composites are feasible on the basis of this uniqueNHSK structures.

Nano Sticky Tape—Nanoparticle-Decorated PolymerSingle Crystals

The interplay between 0-D nanoparticles and polymersingle crystals is less obvious compared with the afore-mentioned 1-D nanotube/polymer system, althoughmetal nanoparticles that formed in situ on the surfaceof inorganic and polymer crystals by vapor depositionhave been used to reveal crystal defect structures since1958.30–32 Recently, we have demonstrated that pre-formed nanoparticles can also be immobilized onto thesurface of polymer single crystals via coupling with thepolymer chain ends (Fig. 1). Gold nanoparticles andthio-terminated poly(ethylene oxide) (HS-PEO) wereused as the model system.33,34 As the chain ends aredifferent from the rest of the polymer backbone, giventhe right crystallization condition, they are excludedonto the surface of the lamellar crystals. Judiciouslyselected nanoparticles can then be bound onto the sin-gle crystal surface via chemisorption. If we consider apolymer lamella as a thin sheet of paper, lamella withfunctional groups on the surface can then be regardedas a nanoscale sticky tape (double- or single-sided,depending on the polymer chemistry) that can immobi-lize different nanoparticles. The implication of this pro-cess is twofold: on one hand, nanoparticles can be usedas a decoration tool, revealing the location of chain endson the polymer single crystals. These nanoparticle-deco-rated polymer single crystals are of great interest froman application standpoint (see following discussions).On the other hand, the coupling reaction directly leadsto partially functionalized nanoparticles, because onlythe surface that is in contact with the single crystals iscoupled with the polymer chains. The opposite side of

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the nanoparticle can be further functionalized withother groups including different types of polymer, smallmolecules, nanoparticles, and so forth.33,34 Upon dis-solving the single crystal, a rich collection of tailor-made nanoparticles and nanoparticle clusters can beobtained. One example is the so-called Janus nanopar-ticle, whose name was first coined by de Gennes afterthe famous Roman God Janus.35 Janus particles pos-sess a noncentrosymmetric structure with a single coresurrounded by a compartmentalized corona. Theoreticalstudies have shown that the Janus nanoparticles canassembly into sophisticated structures and preciselyplacing nanoparticles in microscopic ensembles alsoleads to new properties due to the collective effect of(same/different) nanoparticles.36 While numerousmethods have been used to synthesize[100 nm Janusnanoparticles, it is rather challenging to achieve sub 20nm ones. Polymer single crystal templates provide ageneric way to achieve this goal. Self-assembled mono-layers (SAM) and micrometer-sized particles have alsobeen used to fabricate Janus nanoparticles. Comparedwith SAM, since polymer crystals are suspended in so-lution, the present approach can be easily scaled up. Tothis end, polymer single crystals can be considered as‘‘free-standing SAMs’’ and the rich knowledge welearned from SAM systems can help the design of ourcrystal/nanoparticle hybrids. Compared with microme-ter-sized particles, polymer single crystals are also ad-vantageous since dissolution of them directly leads tografted polymer brushes on the nanoparticles while inthe micrometer-sized particle case, the particles areonly used as protecting objects, which are eventuallyremoved. Furthermore, the profound morphology ofpolymer crystals also enables synthesis of a variety ofpolymer single crystal/nanoparticle hybrids. Note thatthe immobilization process is similar to that observedin polypeptide/nanoparticle and DNA/nanoparticle sys-tems.37,38

Combining polymer single crystals and nanopar-ticles provides a novel means for hybrid material designand synthesis. As a decoration tool, this method caneasily identify the areal density of the polymer chainends on the single crystal surface. For example, wehave observed that with increasing the molecularweight of HS-PEO, the areal density of the gold nano-particle on the HS-PEO single crystals decreases, indi-cating that less polymer chain ends are located on thecrystal surface.39 Judiciously selected nanoparticles canalso reveal unbalanced crystal surfaces where the twosurfaces of the lamella are different, an interesting casethat have been observed in Nylon 6,6, polyvinylidenefluoride, and so forth.11,40 Furthermore, the tailorednanoparticle structures offered by this approach raise anumber of intriguing scientific questions:

1. From the polymer crystallization standpoint,what is the effect of the tethered nanoparticleson polymer crystallization? There are two possi-

ble routes for the tethered chain to crystallize:(a) the chains nucleate by themselves, awayfrom the interface of the particle and the poly-mer (homogenous nucleation), and (b) the nucle-ation occurs at the interface (heterogeneousnucleation). For homogenous nucleation, in so-lution crystallization, end tethering leads toincreased concentration of polymers near thenanoparticle. This can facilitate the nucleationprocess. On the other hand, heavy nanoparticlesalso hinder the diffusion of the polymer chain.The crystallization behavior should be the com-promise between these two competing effects.Varying tethered chain length and tetheringdensity allows one to systematically investigatethis intriguing problem. For heterogeneousnucleation, what is the size and shape effect ofnanoparticle-induced nucleation? Similar to thecase of 1-D nucleation, the nucleation on 0-Dnanoparticles should also be size-dependent. Asthe size of the nanoparticle increases, it may bemore efficient in serving as the nucleation agentsince it provides a larger surface for polymersto grow on. The shape of the particle (spherical,vs. faceted) should also play an important rolebecause for spherical particles, the nucleationeffect could be hindered by the mismatch of thecurved surface and the parallel packing of poly-mer chains.

2. From the Janus nanoparticle self assemblypoint of view, does Janus nanoparticles withtwo types of polymer brushes on the oppositesides of the particle mimick triblock copoly-mers? One may consider this type of particlesas a new kind of triblock copolymers with thenanoparticle as the central ‘‘block’’ and the twopatches of polymer brushes as the other twoblocks. One can therefore control the volumefraction of each block, the miscibility of the twotypes of polymer brushes; profound phase struc-tures can be obtained. Nanoparticle should playa central role in both kinetics and thermody-namics of the structure formation process ofthese hybrid materials. The ‘‘Multiblock copoly-mers’’ can also be achieved by using blockcopolymers to form polymer bushes.

From a technological standpoint, these functional-ized single crystals are also of great interest becausethey are nano tapes which can immobilize desirednanoparticles, viruses, and proteins. Nano tapes cantherefore be used in drug delivery, nanoparticle recy-cling, controlled nanoparticle synthesis, and so forth.The nanoparticle coated polymer single crystals have asandwich structure; they might find applications inmicroelectromechanical systems (MEMS) and nanoelec-tromechanical systems (NEMS) as well as microfluidicapplications. The tailor made Janus nanoparticles can

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find applications such as multiple functional nanopar-ticle clusters for bioimaging and drug delivery, stabili-zation of liquid–liquid interfaces, and controlledassembled nanoparticles for nanoelectronics.

In summary, combining polymer single crystals andnanoparticles leads to a new type of hybrid materials.Not only do they offer novel properties, but also theyprovide unique opportunities for studying the funda-mentals of polymer physics.

I would like to thank the editor for the opportunity toshare our view on the research opportunities offered bycombining polymer single crystals with nanoparticles.This work was supported by the National Science Foun-dation Grants DMR-0804838 and CBET-0730738.

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