Progress in Polymer Science 37 (2012) 1425â€“ 1455-EQUIPO 1 (1)

Progress in Polymer Science 37 (2012) 1425 1455

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Progress in Polymer Science

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Realizing the enhancement of interfacial interaction in semicrystallinepolymer/ller composites via interfacial crystallization

Nanying Ning, Sirui Fu, Wei Zhang, Feng Chen, Ke Wang , Hua Deng,Qin Zhang, Qiang Fu

College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, PR China

a r t i c l e i n f o

Article history:Received 11 August 2011Received in revised form17 December 2011Accepted 21 December 2011Available online 27 December 2011

Keywords:Interfacial crystallizationHybrid crystalCrystalline meInterfacial inteInterfacial enhMechanical pr

a b s t r a c t

Polymer/ller composites have been widely used in various areas. One of the keys toachieve the high performance of these composites is good interfacial interaction betweenpolymer matrix and ller. As a relatively new approach, the possibility to enhance poly-mer/ller interfacial interaction via crystallization of polymer on the surface of llers, i.e.,interfacial crystallization, is summarized and discussed in this paper. Interfacial crystal-lization has attracted tremendous interest in the past several decades, and some uniquehybrid crystalline structures have been observed, including hybrid shishkebab and hybridshishcalabash structures in which the ller served as the shish and crystalline polymer

Abbreviationsb, thickness ofmal expansionglucitol; Df , bide; DPIM, dyninfrared spectrultrahigh-modgraphite; HSCshear strengthcritical effectivweight PE; LMdisulde; MWhexylthiophenPE, polyethylelate); PHBV, ppolypropylenesupercritical CSWNT, single wmodulus carboX-ray diffractipolymer stren

Correspon Correspon

E-mail add

0079-6700/$ doi:10.1016/j.line structurechanismractionancementoperties

as the kebab/calabash. Thus, the manipulation of the interfacial crystallization architectureoffers a potential highly effective route to achieve strong polymer/ller interaction. Thisreview is based on the latest development of interfacial crystallization in polymer/llercomposites and will be organized as follows. The structural/morphological features ofvarious interfacial crystallization fashions are described rst. Subsequently, various inu-ences on the nal structure/morphology of hybrid crystallization and the nucleation and/orgrowth mechanisms of crystallization behaviors at polymer/ller interface are reviewed.

: 2d, two dimensional; AD-MWNTs, multi wall carbon nanotubes synthesized by arc discharge method; AFM, atomic force microscopy; polymer coating layer; CF, carbon ber; CNF, carbon nanober; CNT, carbon nanotube; CNTs, carbon nanotubes; CTE, coefcient of ther-; CVD, chemical vapor deposition; CVD-MWNTs, multi wall carbon nanotubes synthesized by CVD method; DBS, 1,3:2,4-dibenzylideneer diameter; DMA, dynamic mechanical analysis; DMAc, N,N-dimethyl acetamide; DMF, N,N-dimethyl formamide; DMSO, dimethyl sulfox-amic packing injection molding technology; DSC, differential scanning calorimetry; Fmax, maximum pullout force; FTIR, Fourier-transformoscopy; GF, glass ber; GONPs, graphite oxide nanoplatelets; HDPE, high density polyethylene; HDT, thermal distortion temperature; HMCF,ulus carbon ber; HMW-PE, high molecular weight PE; HMW-PP, high molecular weight polypropylene; HOPG, highly oriented pyrolytic, hybrid shishcalabash; HSK, hybrid shishkebab; H-T equation, Halpin-Tsai equation; HTCF, high-tenacity carbon ber; IFSS, interfacial; IMCF, intermediate-modulus carbon ber; iPP, isotactic polypropylene; K-BrBz, potassium 4-bromobenzoate; lc/D, critical aspect ratio; lc,e length; lemb, ber embedded length; LLDPE, linear low density polyethylene; lm, mean fragment length of ber; LMW-PE, low molecularW-PP, low molecular weight polypropylene; MAPP, maleic anhydride grafted polypropylene; MD, molecular dynamics; MoS2, molybdenumNT, multi wall carbon nanotube; MWNTs, multi wall carbon nanotubes; NF, natural ber; NHSK, nanohybrid shishkebab; P3HT, poly (3-e); PA, polyamide; PA-12, polyamide-12; PA-6, polyamide-6; PAN, polyacrylonitrile; PBT, polybutylece terephthalate; PCL, polycaprolactone;ne; PE-b-PEO, polyethylene-b-poly ethylene oxide; PEEK, poly (ether ether ketone); PEO, poly ethylene oxide; PET, poly(ethylene tereptha-oly(hydroxybutyrate-co-hydroxyvalerate); PLLA, poly(l-lactide); PP, polypropylene; PPDT, poly (p-phenylene terephthalamide); Pp-g-MA,

grafted maleic anhydride; PPS, poly (phenylene sulde); PVA, poly(vinyl alcohol); PVDF, poly(vinylidene uoride); rf , ber radius; SC CO2,O2; SEM, scanning electron microscopy; SMCW, SiO2MgOCaO whisker; sPP, syndiotactic polypropylene; sPS, syndiotactic polystyrene;all carbon nanotube; SWNTs, single wall carbon nanotubes; TC, transcrystallinity; TEM, transmission electron microscopy; UHMCF, ultrahighn ber; UHMWPE, ultrahigh molecular weight polyethylene; Vf , ber volume fraction; VGCF, vapor grown carbon bers; WAXD, wide-angleon; 0, orientation efciency factor of ber; l , length efciency factor of ber; c, composite strength; f , ber tensile strength; m, basalgth; s, shear strength at the edge of the interfacial layer region; i , interfacial shear strength.ding author. Tel.: +86 28 85461795.ding author.resses: [email protected] (K. Wang), [email protected] (Q. Fu).

see front matter 2011 Elsevier Ltd. All rights reserved.progpolymsci.2011.12.005

1426 N. Ning et al. / Progress in Polymer Science 37 (2012) 1425 1455

Then recent studies on interfacial crystallization induced interfacial enhancement ascer-tained by different research methodologies are addressed, including a comparative analysisto highlight the positive role of interfacial crystallization on the resultant mechanical rein-forcement. Finally, a conclusion, including future perspectives, is presented.

2011 Elsevier Ltd. All rights reserved.

Contents

1. Introd2. Hybri

2.1. 2.2. 2.3. 2.4.

3. The fa3.1.

3.2.

3.3. 4. Forma

4.1. 4.2. 4.3.

5. Interf5.1. 5.2. 5.3.

6. ConclAcknoRefer

1. Introdu

It is weamong the been widelautomobileof polymerous applicadispersion polymer mhigh-perfora great effobetween postrengthenistrategies: of the llematrix [8], (ing polyme[1114]. Inble means tin composimer and lenhanced iby the cryThese llermer lamelluction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1426d crystalline structures in semicrystalline polymer composites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1427Transcrystallinity (TC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1428Hybrid shishkebab (HSK) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1430Hybrid shishcalabash structure (HSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1432Other hybrid crystalline structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1432

ctors controlling the hybrid crystalline structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1434Effect of llers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14343.1.1. Surface chemical and physical characteristic of llers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14343.1.2. Geometry and size of llers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1435Effect of polymer matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14353.2.1. Molecular weight . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14353.2.2. Molecular chain structure and conformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14363.2.3. Functional groups of polymer matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1436Effect of external eld . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1436tion mechanisms of hybrid crystalline structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1438Epitaxy and soft epitaxy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1438Chemisorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1439Stress or strain induced interfacial crystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1439

acial and mechanical enhancement induced by interfacial crystallization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1440Interfacial enhancement of crystalline interface estimated by different research strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1440Analysis and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1446Mechanical properties of composites containing special interfacial crystallization structures . . . . . . . . . . . . . . . . . . . . . . . . . . . 1447

usions and nal remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1449wledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1451

ences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1451

ction

ll known that polymer/ller composites aremost important materials in industry, and havey used in chemical engineering, sports goods,s, aerospace and weapons, etc. Millions of tons/ller composites are consumed in numer-tions every year. In addition to homogeneousof llers, strong interfacial adhesion betweenatrix and ller is essentially crucial to obtainmance polymer/ller composites. In the past,rt has been made to enhance the interactionlymer and llers, with most attention on theng of polymer/ller interface focused on four(1) chemical or physical surface modicationr [17], (2) functionalization of the polymer3) adding compatibilizer [9,10], and (4) prepar-r composites via in situ polymerization methodterfacial crystallization offers another possi-o enhance polymer/ller interfacial interactionte systems composed of semicrystalline poly-ler with high aspect ratio. That is through thenteraction between polymer and ller caused

crystalline superstructures are generally denoted as hybridcrystalline structure or hybrid crystal, in contrast to con-ventional supermolecular crystalline structures consistingof only polymer species (Fig. 1).

Interfacial crystallization has attracted tremendousinterest in the past several decades, not only due toits crystallography interests, but also due to the factthat it may be a novel strategy to enhance interfa-cial adhesion and realize the full potential of llers toreinforce the mechanical performance of composites. Obvi-ously, the formation of an interfacial crystalline layer canoffer a good interfacial combination between ller andpolymer matrix. Since the origins of ller/polymer inter-action for other interfacial-connecting fashions, such asmacromolecular chain wrapping, covalent bonding andchain-grafting attachment are distinct or well identied[5,15,16], the positive roles played by these physical orchemical interfacial combinations on improving load trans-fer efciency are anticipated. The nature of ller/polymerlinking arising from interfacial crystalline structure is,however, difcult to elucidate clearly, and there is stilla question of whether interfacial crystallization can ulti-mately bring effective interfacial enhancement or not hasstallization of polymer on the ller surface. particles can act as nuclei and induce the poly-ae grow on the ller surface. These as-formed

been arguethe experimresearchersd for a long time. Obvious contradictions inental work have been reported by different

in the literature [1720]. For micrometer-scale

N. Ning et al. / Progress in Polymer Science 37 (2012) 1425 1455 1427

ller-reinfointerfacial ctive role on advances intalline/polyance of inteadhesion apolymer mmechanicalmolecular dtative data layer, whicthan that focompositessystemic anand mechaites via integeneral revin polymerpaper is baFig. 1. Schematic representation of various hybrid cry

rced composites, a number of studies show thatrystallization offers little effect or plays a nega-interfacial enhancement. However, most recent

nanometer-scale ller-reinforced semicrys-mer composites demonstrate that the appear-rfacial crystallization favors a strong interfacialnd high load transfer efciency betweenatrix and nanoller. The methodologies of

model tting analysis and computationalynamic simulation have provided some quanti-about the shear strength of crystalline interfaceh could be one order of magnitude higherr amorphous interface in nanoller-reinforced. To the best of our knowledge, there is nod detailed review on interfacial enhancementnical reinforcement of polymer/ller compos-rfacial crystallization, though there are severaliews related to the mechanical reinforcement/ller composites [2123]. Thus, this reviewsed on the latest developments of interfacial

crystallizatnized accortalline struof micro- ancrystalline anisms of (4) interfacinterfacial cspectives.

2. Hybrid cpolymer co

For semact as a nucor alter poltallization ra decrease more, varioin these costalline structures.

ion in polymer/ller composites and it is orga-ding to the following topics: (1) hybrid crys-ctures/morphologies achieved in the presenced/or nano-llers; (2) factors controlling hybrid

structures/morphologies; (3) formation mech-hybrid crystalline structures/morphologies;ial and mechanical enhancement induced byrystallization; (5) conclusions and future per-

rystalline structures in semicrystallinemposites

icrystalline polymer composites, the llers canleating agent and have the potential to induceymer crystallization. Thus, an increase in crys-ate and crystallization temperature as well asin crystal size is generally observed. Further-us hybrid crystalline structures may be formedmposites, as schematically shown in Fig. 1.


For polymer/spherical particle composites, the hybridspherulites with much smaller size than bulk spheruliteswill be formed due to the nucleation effect of spherical par-ticles. For polymer/lamellar ller composites, the hybridcrystalline the surfaceposites, the(HSK) or hythe brous on its surfaspherulitesdirection, tller can inHSK structuodically decperpendiculler can onlarge polymHSC structumer spheruhybrid crysticular inter

2.1. Transc

The earlcalled TC, wThe formatsity of activhinder the fthe crystal lar to the bpolypropyleite is showimage) [24to the long1952 by Jeattention, bcal methodpolymer mto occur atpolymers, s(PA) [35,36de) (PPS) poly(l-lactihigh aspect[35,4648],bers (NF) copper [52]Here, we was there is [54].

Among quite attramer matrixcost, good Therefore, gated durindifcult to escent crysGF is compl

(a) Theure in iP

fying Gng the samples to generate thermal stress, or exert-n external stress eld on polymer/GF interface duringallization, have been proposed and successfully uti-

to obtain a TC structure on GF surface, as summarizedeview paper [54]. In recent years, there were still somees focused on TC structure at polymer/GF interfacened by using the above methods [5557]. For exam-hou and coworkers [56] reported that TC appearedF surface when adding 10 wt% MAPP in a PP matriximultaneously modifying GF with a common silaneling agent or a di-block copolymer coupling agent.ever, TC could not be induced at the interface whenF were treated with a kind of tri-block copolymer

ling agent, even under fast cooling or shearing condi-The reason may be that the exible interlayer formede tri-block copolymer could relax even under a stress[56]. Furthermore, Zheng and coworkers [58] reportedthe acid-corroded GF exhibited anomalous nucleat-bility to induce ringed alpha nuclei, and beta form TC

be developed unexpectedly from these nuclei duringermal crystallization, which provides a new approachuce TC on a GF surface [58].mpared with GF, CF is very expensive, but it haststanding weight-to-strength ratio. In terms of thist, polymer/CF composites are considered as the mostctive material that could be produced in apprecia-uantities. In the past few decades, the interfacialructure in polymer/CF composites has been widelystructure will be polymer lamellae decorating of the ller. For polymer/brous-ller com-

transcrystallinity (TC) or hybrid shishkebabbrid shishcalabash (HSC) could be obtained. Ifller can initiate a high density of active nucleice, which can hinder the free radial growth of

and therefore force the lamellae to grow in onehe TC structure will be formed. If the brousitiate a medium density of active nuclei, anre with polymer crystal lamellae (kebab) peri-orating its surface and aligning approximatelylar to its long axis will be formed. If the brously initiate a few nuclei, which can develop intoer spherulites without hindrance, the peculiarre with brous ller serves as shish and poly-lites as calabash will be formed. Among thesetalline structures, TC, HSK and HSC are of par-est and have attracted tremendous attention.

rystallinity (TC)

iest reported hybrid crystalline structure is so-hich often emerges at polymer/ber interfaces.ion of this unique TC layer is due to a high den-e nuclei on the ber/substrate surface, whichull extension of spherulites and therefore forcegrowth in one direction, namely perpendicu-er/substrate. A typical TC structure in isotacticne (iPP)/carbon nanotube (CNT) bers compos-n in Fig. 2(a) (SEM image) and Fig. 2(b) (PLM]. It is quite clear that TC grows perpendicular

axis of the ber. Since it was rst reported innckel et al. [25], TC has attracted tremendousecause it may be an effective and economi-

to improve the interfacial adhesion betweenatrix and ber. To date, TC has been reported

the interface between several semicrystallineuch as polypropylene (PP) [2634], polyamide], polyethylene (PE) [37], poly (phenylene sul-[38], poly (ether ether ketone) (PEEK) [3941],de) (PLLA) [4245], etc., and various llers with

(length/diameter) ratio, such as glass bers (GF) carbon bers (CF) [49], aramid ber, natural[34], carbon nanotubes (CNTs) [50], talc [51],, aluminum [53], etc., as summarized in Table 1.ill briey summarize the recent studies on TC,already a comprehensive review on this topic

the llers cited previously, glass ber (GF) isctive as a reinforcement material for a poly-

due to its high mechanical properties, very lowheat resistance, and high electrical resistivity.TC on a GF surface has been widely investi-g the past few decades. Generally, it is very

induce TC on a GF surface under common, qui-tallization conditions, since the structure of theetely amorphous. Hence a few methods such as

Fig. 2.struct

modicooliing acrystlizedin a rstudiobtaiple, Zon Gand scoupHowthe Gcouption. by theld that ing acouldisothto ind

Coan ouaspecattrable qTC st SEM image and (b) PLM image of a typical transcrystallineP/CNT ber composite [24] ( 2008 Elsevier Ltd).

F surface by nucleating or coupling agents, fast


Table 1Summary of different types of llers that can induce TC structure in various crystalline polymers.

Filler* Polymer Refs.

GF iPP, PA 66, PA-6, PPS [35,38,4648,5558]CF 66, iPP,Aramid ber 6, PA-6, NFNFNFNF A, PP NF NF NF NFCNTs bers Talc Copper Aluminum

* GF, glass

investigatedfor PEEK bepolymer [54matrixes, suRegarding pthat the higbetter nucltenacity CFin these po[60,62]. In rstructure aton the interon the surfller [63,64

Furthermextremely become vemer materibeen focusber/polymFor exampgamma ortbers undeaxes of thedistributedab face wasFeldman et66 was gen49) ber uswhich the lan angle ofTC layer, relayer couldsame proce

Various attention aposites duecost, low dbiodegradaA large numtigation of cotton [77][82], wood

er co morexamped onJosephence fthat inth of al str[77].

ed wit inducted thation poly(hfacilit/hempth rate

indicandepees [86,Ts, w

ess anctive re strugood ix. Theer/CN

tion iPEEK, PA iPP, PA 6

Cellulose iPPCotton iPPFlax iPP, PLA Sisal HDPE, PLJute iPP, PLA Wood iPP Kenaf iPP Hemp PHBV

iPP iPP iPP iPP

ber; CF, carbon ber, NF, natural ber; CNT, carbon nanotube.

. For example, CF is often used to induce TCcause it shows good nucleation ability for this]. CF has also been used to induce TC for otherch as PA 66 [59], iPP [60], sPP [61] and sPS [62].olymer/CF composites, it is quite interestingh modulus CF (HMCF) generally shows mucheation ability than high strength CF and high. Therefore HMCF is usually used to induce TClymer composites, as reported in some studiesecent years, less work has been focused on TC

polymer/CF interface, with only a few studiesfacial TC structure of semicrystalline polymersace of vapor grown carbon bers (VGCF) as a].ore, aramid ber has the characteristic of

high tensile modulus/high strength, and thusry popular as a reinforcing ber for poly-als. A considerable amount of work has alsoed on the interfacial TC structure of aramider composites in the recent decade [29,6573].le, Assouline et al. [29] reported that TC ofhorhombic iPP could be induced on aramidr high pressure. It was determined that the c

gamma iPP lamellae (the growth axes) were radially about the ber and that the lamellar

randomly oriented on the ber surface [29]. al. [69,73], found that a double TC layer of PAerated on the surface of treated aramid (Kevlaring a saturated aqueous bromine solution, in

polymmuchFor einduction, differthan growthermber treatity torepornuclemer and PHBVgrowbulk,two istudi

CNstiffnattrafuturvery matrpolymattenamellar a* axis is nearly perpendicular and at about 12 to the ber in the outer and innerspectively. By comparison, only one regular TC

be induced by pristine aramid ber under thessing conditions.natural bers (NF) have received increasings reinforcement materials for polymer com-

to their various attractivities, such as lowensity, high specic strength and modulus,bility and derivation from renewable resources.ber of studies have been focused on the inves-TC structure in various NF (cellulose [7476],, ax ber [45,7880], sisal [37,44,81], jute

[34,43,83], kenaf [84], hemp [85]) reinforced

mer system(see Sectiothat a typiunder apprsis showedboth andber interfalso observcent meltinformed by In this caseall interfacigeometric ctube spacin sPP, sPS [3941,5962]PEEK [29,6573]

[7476][77][45,7880][37,44,81][34,43,83][82][84][85][24,50,93,94][51][52][53]

mposites. In recent years, NF has attracted attention due to its environmental benets.le, it was reported that TC of iPP could be

cellulose nanocrystal surfaces [74]. In addi- et al. reported that the interfacial free energyor nucleation of PP on cotton ber is smaller

the bulk PP, which favors the formation andTC. Furthermore, ber surface roughness andesses facilitated the growth of TC on cottonOn the other hand, sisal bers untreated orh alkali or silane all had a nucleating abil-e TC in PLLA matrix [44]. Hermida and Megaat hemp bers could act as a heterogeneousagent for a biodegradable semicrystalline poly-ydroxybutyrate-co-hydroxyvalerate) (PHBV)

ate the formation of TC structure at the ber interface [85]. They reported that the

of TC was the same as that of spherulites in theting that nucleation and crystal growth werendent processes, as has been reported in some87].ith large aspect ratio, extremely high strength,d exibility, have been considered as a highlyeinforcing agents in a polymer matrix forctural materials. Furthermore, CNTs shownucleation ability for many kinds of polymerrefore, the interfacial crystallization behavior ofTs nanocomposites has attracted tremendousn recent years. For many crystalline poly-

s, CNTs can induce a unique NHSK structure

n 2.2) [8892]. However, Zhang et al. foundcal TC of iPP could be induced by CNTs beropriate condition [24]. Microstructure analy-

that CNT ber could induce the growth of TC, and TC dominated at the iPP/CNTace region [24]. Furthermore, TC of iPP wased around the individual CNTs during quies-g crystallization of iPP/CNT nanocompositesinltrating iPP into nanotube aerogel bers., the -form TC of iPP dominated the over-al crystalline morphology, perhaps due to theonnement of CNTs with the very small inter-g (10100 nm) [93]. Moreover, in the work of


Fig. 3. (a) TEMselected-area explaining themacromolecu 2008 Ameri

Loos and coCNTs was oites, as showelectron difiPP lamellaindices of th(h k 0) ree2 2 0, of theaxis orientaiPP lamellaplane and was obviouother studimolecules wCNTs, leadigraphic c-aas schemation, there isare orientesurface, whfrom the CNbeen report image of a transcrystallinity structure of -iPP around individual MWNT in uelectron diffraction pattern and sketch indicating the (h k l)-indices of the Bragg-

possible nucleation mechanism of iPP on the surface of a CNT, iPP macromolecle in the sketch, dark rod represents a CNT) and form a nuclei with crystallographcan Chemical Society.

workers [50,94], TC of -iPP around individualbtained in ultrathin lms of iPP/CNTs compos-n in Fig. 3(a). The corresponding selected-area

fraction (ED) pattern of transcrystalline growne around CNTs and sketch indicating the (h k l)-e Bragg-reection are shown in Fig. 3(b). Only

ctions such as 0 4 0, 0 6 0, 1 1 0, 1 3 0, 2 0 0, and -phase were present, indicating that the c-tion (chain orientation) of the transcrystalliner crystal was perpendicular to the substrateperpendicular to the long axis of CNTs. Thissly different from the assumptions proposed ines, and the authors ascribed this to iPP macro-rapped around rather than aligned along the

ng to the formation of a nuclei with crystallo-xis perpendicular to the long axis of the CNT,ically shown in Fig. 3(c). However, in our opin-

another possibility: the molecular chains of iPPd parallel to the long axis of CNT near the CNTile they will twist by 90 at a distance awayT surface due to the self-epitaxy effect, as hased in many studies on iPP crystallization in the

presence ofwith molecbe too thin is too weakcarried out

2.2. Hybrid

The hybattractive aphology, So far, theous cases sphysical vaand thin-ization [10spinning [1talline poly[102,111] p11 [104], puoride) (P[113,114] ltrathin lms of iPP/MWNT composites, (b) correspondingreection; only (h k 0) reections are presented. (c) Sketchules initially are partly wrapped around the CNT (brighteric c-axis perpendicular to the long axis of the CNT [50].

bers [28,95,96]. Nevertheless, the iPP lamellaeular chains aligned along the CNT surface mayor too small, thus the corresponding ED pattern

to be detected. More careful study should beto better understand the formation mechanism.

shishkebab (HSK)

rid shishkebab (HSK) structure is anothernd widely investigated interfacial crystal mor-rst observed by Thierry et al. in 1990 [97].

HSK structure has been obtained in vari-uch as solution crystallization [89,92,98104],por deposition [90,105], solvent evaporationlm crystallization [106], in situ polymer-7,108], injection molding [109], and melt10], at the interface between various semicrys-mers such as PE [98], PE block copolymerolyamide-6 [112], nylon 66 [89,91], polyamide-oly(vinyl alcohol) (PVA) [92], poly(vinylideneVDF) [90], poly(butylene terephthalate) (PBT)and inorganic or organic llers such as


Table 2Summary of various crystalline polymer/ller systems that can form HSK structure using various preparation methods.

Filler Polymer Preparation methods Refs.

CNTs HDPE Solution crystallization [89,92,98104]CNTs ition [90,105]CNTs and thin-lm crystallization [106]CNTs n [107,109]CNTs [109]CNTs [110]CNTs llowed by extrusion-stretching [124]CNTs [202]CNTs on [102,110]CNTs on [89,91]CNTs on [92]CNTs on [104]CNTs ition [90]CNTs g [113,114]Whiskers d melt spinning [119121,128]Whiskers [132]DBS ber ition [118]Clay n [122]

CNTs [89,91,3:2,4-dibe[119121], For examplbehavior ofthat the polthe surfacestructure isserved as skebab [97nucleating of PLLA. It self-organizThese brilsof PLLA lampendicular [122] reporcould be forby in situ pshish and n

Recentlystructure wtal lamellaefrom manyical functiorst observcrystallizatias carbon nture is showthat inorgater ratio aclamellae (kaligning apthickness olamellae calization constructure cocopolymers(PE-b-PEO)to amphipha unique 2

66, PVDF and poly(l-lysine)) oligomer patterning onidual CNTs has been achieved using a facile physicalr deposition method by Lis group [90,105]. Recently,ple and rapid yet effective approach to produce the

structure in PE/CNTs system using solvent evap-on and thin-lm crystallization was reported [106].gi and coworkers [124] sprayed an aqueous solu-of SWNTs directly onto ne UHMWPE powder andmelt compounded the CNTs-coated UHMWPE andsion-stretched the composite into a sheet to obtaine NHSWPE coworkcompotainedHDPE Physical vapor deposHDPE Solvent evaporation HDPE In situ polymerizatioHDPE Injection moldingHDPE Melt spinning UHMWPE Melt compounding foLLDPE Injection molding PE block copolymer Solution crystallizatiNylon 66 Solution crystallizatiPVA Solution crystallizatiPA11 Solution crystallizatiPVDF Physical vapor deposPBT Compression moldinHDPE Injection molding anLLDPE Injection molding PE Physical vapor deposNylon-6 In situ polymerizatio

0,98,101,102,104106,109111,113,115117],nzylidene glucitol (DBS) ber [118], whiskerclay [122], etc., as summarized in Table 2.e, Thierry et al. investigated the crystallization

polyolen induced by DBS ber and observedyolen crystalline lamellae epitaxially grew on

of DBS ber [118]. This epitaxial crystalline actually HSK structure, in which DBS berhish and polyolen crystal lamellae formed the,118]. Bai et al. [123], reported a special kind ofagent was used to enhance the crystallizationwas found that the nucleating agent could beed into ne brils prior to PLLA crystallization.

could then serve as shish to induce the growthellar (kebab-like structure) approximately per-to the long axis of the brils. Maiti and Okamototed that a so-called shishkebab superstructuremed in nylon-6/clay nanocomposites preparedolymerization method, in which clay acted asylon-6 lamellae formed the kebabs., a novel nanohybrid shishkebab (NHSK)ith CNT acting as shish and polymer crys-

forming kebab has attracted much attention researchers due to its potential use in period-nalizing CNTs. This NHSK superstructure was

Nylonindivvapoa simNHSKoratiRastotion then extrua nUHMand nanobe obed by Li et al. in PE and Nylon 66 solutionon in the presence of SWNT, MWNT, as wellanober (CNF) [89,98]. A typical NHSK struc-n in Fig. 4, where it can be clearly observed

nic ller (CNT) with a high length to diame-ts as shish and induces polymer (PE) crystalebab) periodically decorating its surface andproximately perpendicular to its long axis. Thef the kebab and the periodicity of the polymern be readily controlled by varying crystal-ditions. It was also reported that the NHSKuld be obtained with selected crystalline block, such as polyethylene-b-poly(ethylene oxide), periodically decorated along CNTs, which ledilic, alternating patterns [111]. Furthermore,d NHSK structure with periodic polymer (PE,

Fig. 4. A typiction) in PE/MW 2006 Wiley K structure with SWNTs acting as shish andforming the kebab. More recently, Bucknallers [125,126] reported that CNT ber basedsites with aligned HSK nanostructures could

by inltrating CNT arrays into PE/p-xyleneal NHSK structure (TEM image and schematic representa-NT system [89].

Periodicals.


Fig. 5. (a) SEM isker) c2011, Elsevier sing (b) and (c) melt sp 2010 Ameri

solutions atemperaturers [107] opolyethylenular to MWprepared bXu and cowstructures iPVA/CNTs [tance of suthat work, solvent couthus the NHinterval bevarying a seent solventand supercr

The metstructure wphysical vathe NHSK sreadily extWith this iour group table industsuperstructrst time inusing both [109,110,11tion of NHSmechanicalenhancemeCNTs (whisstructure. T

2.3. Hybrid

In additof hybrid

us-sharulitesP/CF coly obseSK st

llae, wsentatis repation ture thus-shalop intnally

liar HSbrid cr

studieent frf actixtensirowth images of NHSK structure in PE/inorganic whisker (SiO2MgOCaO wh Ltd.); (b) and (c) SEM images of NHSK structure in PE/CNTs composites uinning method [110].

can Chemical Society.

nd controlling the isothermal crystallizatione and crystallization time. Seo and cowork-bserved a NHSK structure with high densitye (HDPE) crystal lamellae growing perpendic-NT surface in HDPE/MWNTs nanocomposites

y in situ polymerization method. Furthermore,orkers successfully obtained a typical NHSK

n PE/CNTs [100,101], PE-b-PEO/CNTs [102] and92] via solution crystallization with the assis-percritical CO2 (SC CO2) as an anti-solvent. Inthe solubility of the polymer matrix in theld be varied by the introduction of SC CO2,SK structure, the size of the lamellae, and the

tween them along the CNT was controlled byries of experimental conditions such as differ-s, polymer concentration, CNTs concentration,itical CO2 pressure.hods described above to obtain the NHSK super-ere primarily based on solution crystallization,por deposition, or in situ polymerization. While

brosphein iPclearthe Hlamerepreture nuclestrucbrodeveand pecuof hysomediffersity ofull etal gtructure was obtained, these methods are notendable for large-scale industrial processing.mpetus, signicant effort has been devoted ino obtain a hybrid crystalline structure via scal-rial processing method. For example, the NHSKure (see Fig. 5) has indeed been observed for the

PE/inorganic whisker and PE/CNTs compositesinjection molding and melt spinning methods9121,127,128]. In these situations, the forma-K structure could potentially bring signicant

reinforcement in these composites due to thent of interfacial adhesion between polymer andkers) caused by the formation of interfacial HSKhis is discussed in more detail in Section 5.

shishcalabash structure (HSC)

ion to TC and HSK, there exists another kind shishcalabash (HSC) structure, in which

only a few large separller withoin single structure wceramic ba PP gel sytalline morthe injectio(SMCW) co(LLDPE)/SM[129,132].

2.4. Other h

Besides crystalline llers, as reomposites obtained by injection molding [128] (Copyrightinjection molding [109] (2009 American Chemical Society)

ped llers served as shish and polymer served as calabash. A typical HSC structuremposite is shown in Fig. 6(a) [30]. It may berved that this HSC structure is different fromructure, for the polymer crystals in HSK arehile they are spherulites in HSC. A schematicion of the formation process of the HSC struc-resented in Fig. 6(b) [129]. In this case, thedensity is much less for the formation of HSCan that for the formation of HSK structure. Theped ller can only initiate a few nuclei, whicho large polymer spherulites without hindrance,guided by the brous-shaped ller, form theC structure. It should be noted that this kindystalline structure was called a TC structure ins [30,130]. However, we think that it is actuallyom TC, since in a TC structure, the high den-ve nuclei on the ber surface will hinder theon of spherulites and therefore force the crys-in one direction, while in the HSC structure,

nuclei are induced and can fully develop intoated polymer spherulites on the surface of aut hindrance. HSC structure has been observedberpolymer system [30]. In addition, a HSCith MAPP spherulites serving as calabash anders acting as shish has also been reported forstem [131]. Furthermore, this interfacial crys-phology has also been successfully obtained inn-molded bar of iPP/SiO2MgOCaO whiskermposites and linear low density polyethyleneCW composites, as shown in Fig. 6(c) and (d)

ybrid crystalline structures

TC, HSK and HSC, many other kinds of hybridstructure can be induced on various kinds ofported in a number of studies [133136]. For


Fig. 6. (a) A tyHSC structure 2009, Elsevier 2011 Wiley

example, Printerfacial cweight polymolecule roedge-on nwhereas it a monolaye[133]. In adpical HSC structure in iPP/CF composite [30] (Copyright 1999, Elsevier Ltd.); (b) [129] (Copyright 2009, Elsevier Ltd.); (c) and (d) HSC structure obtained in injectio

Ltd.) and (d) LLDPE/SMCW composites [132].Periodicals.

okhorow and Nitta recently reported that therystalline morphology of ultrahigh molecularethylene (UHMWPE) on mica was small, singled-like nanocrystallites and isolated block-typeanolamellae comprising several PE molecules,was isolated lamellae and lamellar domains ofr height of UHMWPE on the surface of graphitedition, Brinkmann et al. reported that P3HT

could be epsalt (potasshighly orienular netwooriented aloMoreover, facial crystpyrolytic ga schematic representation of the formation process of then-molded bar of (c) iPP/SMCW composites [129] (Copyright

itaxially grown on the surface of an aromaticium 4-bromobenzoate) (K-BrBz), which led toted and nanotextured P3HT lms with a reg-

rk of interconnected semicrystalline domainsng two preferential in-plane directions [134].

Tracz and coworkers reported that the inter-alline morphology of PE on highly orientedraphite (HOPG) and molybdenite (MoS2) was


similar to the fractured surface of extended-chain crystalsof PE crystallized under high pressure [137,138].

3. The factors controlling the hybrid crystallinestructure

Hybrid cites has beMany factocharacterisformation avarious kinand growthattracted trmost impor

3.1. Effect o

3.1.1. Surfallers

It is welphysical cheffect on thand can sigpolymer/lchemical stmation of hinterface. Ftigated thesilicate surfthat the silclay and ambonding, wonal lamella result, thegrown on ta so-called and nylon-6

Second, play a key rphology of investigatelonitrile (PAfor iPP matrcompositessmall (aboudisorientatiand almostthe iPP/CF i(>10 nm) grdisplayed aand resulteThe extremon iPP couldtal lattice sthe impactmation of Nobserved thboth CVD-Mlization, buwas not as

locations, CVD-MWNTs were only partially decorated by PEcrystal lamella. However, when using solvent evaporationand thin-lm crystallization, only AD-MWNTs could induceNHSK due to relatively fast solvent evaporation [106]. This

be duously, tructubrid crird, itof llation oWang een inoroetheas tholy(ete non-bers.

ffect oogenevior ofrface wi of iPegion . This inum ce rouy, pla

at the lso rep

faciliturth, od tois widlymerositesange tnterfacple, Lcatiorystallts shoed oncatioal growd thatrred foragon

surfac/PP comducedsotherP-treatcompation

increaated ed axfacial crface

ller rystalline structure in polymer/ller compos-en investigated for more than half a century.rs, such as surface chemical and physical

tics of the llers, molecular weight, chain con-nd functional groups of polymer matrix, andds of external elds will affect the formation

of interfacial crystalline structures, which haveemendous attention. Here we summarize thetant progress in this area.

f llers

ce chemical and physical characteristic of

l known that the intrinsic surface chemical andaracteristics of various llers have a signicante nucleation activity of a given polymer matrix,nicantly affect the interfacial crystallization oflers composites. First of all, the specic surfaceructure of ller can play a crucial role in the for-ybrid crystalline structure at the polymer/llersor example, Maiti and Okamoto [122] inves-

unique crystallization behavior controlled byaces in nylon-6/clay nanocomposites and foundicon hydride chemical bonds on the surface of

ino bonds of nylon-6 could form hydrogenhich facilitated the formation of a pseudohexag-ar packing of nylon-6 on both sides of clay. As

exclusive formation of the -phase of nylon-6he surface of clay particles was observed; andshishkebab structure with clay acting as shish

-phase lamella as kebab was obtained.the crystal lattice structure of the llers canole in the formation of interfacial crystal mor-polymer/ller composites. For example, Hobbsd the nucleation ability of two types of polyacry-N) based CF with different graphitic structuresix and the interfacial crystalline behavior of the

[139]. The results showed that CF composed oft 2.5 nm) graphitic nuclei with a high degree ofon acted as a very poor nucleating agent for iPP

no interfacial crystallites could be observed atnterface. However, CF composed of much largeraphitic planes with a high degree of orientation

strong nucleation ability for iPP crystallizationd in a thick TC layer along the whole ber axis.e variation of nucleation ability of these two CFs

be ascribed to the difference in graphite crys-tructure. Recently, Li et al. [105] investigateding of side-wall structure of CNTs on the for-HSK in the PE decorated CNTs situation. Theyat PE crystal rods could periodically decorateWNTs and AD-MWNTs using solution crystal-

t the PE crystal lamella on CVD-MWNTs surfaceuniform as those on the AD-MWNTs. In some

couldObvitice sof hy

Thness formple, betwtrauwherand pto thPET the eheterbehaCu sunuclecial rrangealumsurfaenergtion [77] acould

Fomethand of pocompto chthe iexammodithe cresulinducmodicrystcludeprefeMondberberbe inthe iMAPthan nucleWithuntretreatinterby susomee to the crystal lattice defect of the CVD-MWNT.these results demonstrated that the crystal lat-re of llers played a crucial role in the formationystalline structure.

has been well established that surface rough-ers also plays a very important role in thef interfacial crystallization structure. For exam-and Liu [30] observed an inverse relationduction time and nucleation rate for polyte-ylene (PTFE) ber and carbon ber systems;

is inverse relation was not applicable to Kevlarhylene terepthalate) (PET) bers. This was dueuniformity of surface roughness of Kevlar andFurthermore, Lin et al. [53] also investigatedf surface roughness of copper (Cu) sheet on theous nucleation and interfacial crystallization

iPP at the iPP/Cu interface, and observed thatith higher surface roughness will induce more

P, resulting in a thicker TC layer in the interfa-upon supercooling over a certain temperatureresult was consistent with the investigation ofand talc surfaces [52,53], which showed thatghness, instead of chemical factors or surfaceyed a dominant role in interfacial crystalliza-polymer/ber interface. Joseph and coworkersorted that the surface roughness of cotton berate the growth of TC.surface treatment of llers is an important

change the surface characteristics of llersely used to improve the interfacial adhesion/ller composites. For semicrystalline polymer, surface treatment of llers could also be usedhe nucleation ability of the llers, and controlial crystallization behavior of composites. Fori et al. [105] investigated the effect of surfacen of MWNT by octadecylamine (C18-MWNT) onization behavior at the PE/CNTs interface. Thewed that no interfacial PE crystallization was

the C18-MWNT surface, indicating that alkane-n of the MWNT surface prohibited the PE singleth on the CNTs. Therefore, the authors con-

a uniform, smooth, graphene-like surface wasr PE crystal formation on the surface of CNTs.

and coworkers [140] investigated the effect ofe treatments on interfacial crystallization of ax

posites. It was observed that TC of iPP could by both untreated and MAPP-treated ber atmal crystallization temperature of 134 C, buted ber composites showed lower TC densityosites with untreated bers due to the betterability of untreated ber on PP crystallization.sing crystallization temperature to 140 C, the

ber still induced TC of iPP whereas MAPP- ber did not. These results indicated that therystal morphology could be strongly inuencedtreatment of the llers. On the other hand, forwith very poor nucleation ability, appropriate


Fig. 7. (a) The em withPE crystal lam of abou 2006 Wiley

surface treaability of this the iPP/Gsome nucleiPP matrix. form of iPPbers were

Furthermroughness, mal expansnucleation crystallizatibeen well r

3.1.2. GeomThe abo

characteristinterfacial cposites. It isize of llertalline strucour previoudifferent dibetter nuclcial HDPE csurface of scrystal lamones. The ron the sma

arger f smal typical NHSK structure in PE/CNT (with a diameter of about 10 nm) systellae are randomly decorated on CNF surface in PE/CNF (with a diameter Periodicals.

tment is necessary to improve the nucleatione llers to polymer matrix. A typical example

the lity oF composite systems: GF is usually coated withation agents to enhance its nucleation ability forAs reported by Assouline et al. [141], TC of beta

could be induced on the GF surface when the coated with appropriate beta nucleating agent.ore, the surface topography structure, surfacethermal conductivity mismatch, and ther-ion mismatch could also favor heterogeneousand has a signicant inuence on interfacialon of polymer llers composites, which haseviewed in ref. [54].

etry and size of llersve describes the surface chemical and physicalics and surface modication of llers on therystallization behavior of polymer/ller com-s also well established that the geometry ands also have signicant inuence on hybrid crys-ture of polymer composites. As an example, ins work [128] for the same whisker but withameters, the small-diameter whisker showedeation ability. As a result, signicant interfa-rystal lamellae were epitaxially grown on themall-diameter whiskers, while only little HDPEellae decorated the surface of large-diametereason could be that the melt pressure exertedll-diameter whisker was larger, which led to

in a more don this whthe diametin the interFig. 7(a), theeter of abouperpendicularger diamthe interfacshows a var

3.2. Effect o

3.2.1. MoleThe mol

the molecualso affect tTherefore, inuenced For exampllowest molehighest nucincreasing mers becameof the beralmost no face. The ab PE single crystal lamellae perpendicular to CNTs axis; (b)t 300 nm) system [89].

wetting force. Therefore, the nucleation abil-l-diameter whiskers was increased, resulting

ense interfacial HDPE crystal lamellae grown

isker. Furthermore, Li et al. [89] reported thater of brous carbon ller played a major rolefacial crystallization morphology. As shown in

typical NHSK structure in PE/CNT (with a diam-t 10 nm) system with PE single crystal lamellaelar to CNTs axis were obtained. However, foreters of CNF (with a diameter of about 300 nm),ial PE crystal lamellae on the surface of CNFiety of orientations, as clearly shown in Fig. 7(b).

f polymer matrix

cular weightecular weight of a polymer matrix dominateslar chain mobility of the matrix, and thus canhe crystal growth at the polymer/ller interface.the interfacial crystallization is signicantlyby the molecular weight of the polymer matrix.e, Folkes and Hardwick [142] observed that thecular weight PP/single ber system showed theleation density and the densest TC layer. Witholecular weight of the PP matrix, the TC lay-

less uniform, and even absent on some parts. For the highest molecular weight PP system,TC layer was observed at the PP/ber inter-ove phenomenon could be explained as follows.


On the one hand, adsorption of the polymer moleculesonto the ber surface (nucleation) is primarily occurredby attachments of the molecule chain ends. Therefore, theprobability of an attachment increases as the molecularweight of tthe numbedevelopmethe chain mular weightand higher and the densingle GF/Pour previouon the formHDPE/SMCWHSK structon the surfa relativelyHowever, iweight PE (with PE cryof SMCW wdependencmolded HDour group [system can whereas nosystem. Theatively highmatrix.

3.2.2. MoleMolecul

matically inonto the suthus can almer/ller cperformed tion behaviand PEO) wconformatimethod. Foonto the SWunique NHSmation andHowever, inby a thin amtorted helicwith SWNTbeen report66 [89,91], tion in the zigzag confthe case of NHSK strucThis indicaformation dcrystallizatstructure acomes fromiPP and LLD

crystal structure was only observed in the injection-molded HDPE/whiskers composites, whereas a unique HSCinterfacial crystal structure with whiskers served as shishand polymer spherulites formed calabash was observed

e injecosites

Funce funtial to

of llefacial chemi

66 onactionallites,nterfaF systphous, wherssfullyobserve polamide ionedide cheogen bon of n

Effect o

is w can micrysics camatiotropice easi. In thxtensiallizates on ed in sroup h

on hy epita

comption mmic pah an osooledssfullyical pr

be obtion mation oes. Furich b

cted ajectiohe polymer decreases due to the increase ofr of chain ends. On the other hand, the lateralnt of the TC layer (crystal growth) depends onobility of the polymer matrix. For low molec-

polymer, the higher chain end concentrationchain mobility led to higher nucleation densityser TC layer. Similar results were reported in

P composites by Moon [143]. Furthermore, ins work [120] the effect of PE molecular weightation of HSK structure in injection-molded

composites was investigated. An obviousure, with PE crystal lamellae closely packedace of SMCW, was observed in samples with

low molecular weight PE (LMW-PE) matrix.n samples with a relatively high molecularHMW-PE) matrix, an incomplete HSK structurestal lamellae loosely decorated on the surfaceas observed. Moreover, the molecular weight

e of interfacial crystal structure in injection-PE/mica composites was also investigated in144]. The results also showed that a LMW-PEfacilitate the growth of TC layer on mica surface,

obvious TC layer could be detected in HMW-PEse results could also be ascribed to the compar-

chain mobility and more chain ends of LMW-PE

cular chain structure and conformationar chain structure and conformation could dra-uence the adsorption of polymer moleculesrface of llers and lateral crystal growth, andso affect the interfacial crystallization of poly-omposites. For example, Zheng and Xu [145]a comparison study of interfacial crystalliza-or of two types of semicrystalline polymers (PEith different molecular chain architectures andons on SWNTs using a solution crystallizationr PE, the molecular chains could easily adsorbNT surface and further crystallize to form a

K architecture, due to its planar zigzag confor- favorable physical interactions with SWNTs.

the case of PEO system, SWNTs were wrappedorphous polymer coating, because of the dis-al conformation and unfavorable interactions. Typically, the ordered NHSK structure hased in the cases of PE [98], nylon-6 [112], nylonand PVA [92]. For these, the chain conforma-crystal unit-cell of these polymers is either aormation or a planar zigzag conformation. IniPP with 31 helical conformation, TC instead ofture was formed on the surface of CNTs [50,94].ted that molecular chain structure and con-id have a remarkable inuence on interfacial

ion. More evidence for the importance of chainnd conformation on interfacial crystallization

our recent work on the crystallization of HDPE,PE as induced by whisker. The HSK interfacial

in thcomp

3.2.3.Th

potenface interthat nylonintercrysteral iiPP/GamortionssucceThis of thpolyamenthydrhydrlizati

3.3.

It eldsof sekinetdeforanisocan b[150]nal ecryststudiformour geldsotherllerinjecdynawhically csuccepractcouldinjecformpositin whlae athe intion-molded iPP/whiskers and LLDPE/whiskers [119,120,128,129,132].

tional groups of polymer matrixctional groups of a polymer matrix have the

form some special interactions with the sur-rs, and thus also have a signicant effect oncrystallization. For example, it was reportedsorption of the polar groups of nylon-6 and/orto the graphite surface could lead to the strongs between the macromolecules and the graphite

and thus contribute to nucleation and lat-cial crystallization [146,147]. Furthermore, forem, no interfacial TC could be formed on the

surface of GF under normal quiescent condi-eas for polyamide matrix, interfacial TC could be

induced on GF under the quiescent conditions.ation could also be due to the chemisorptionr chemical groups ( CO and NH ) of thematrix on GF surface [148]. Moreover, as already

above, the amino bonds of nylon-6 and siliconmical bonds on the surface of clay could formonding, which facilitated the interfacial crystal-ylon-6 on clay surface [122].

f external eld

ell known that external extension/shearingstrongly impact the crystallization behaviorstalline polymer. Generally, the crystallizationn be signicantly promoted with extensionaln or shearing ow [19,149], and some highly

crystal superstructures, such as shishkebab,ly obtained from the oriented polymer meltse case of polymer/ller composites, the exter-on/shearing eld can induce novel interfacialion superstructure, such as HSK and TC. Manyexternal shear eld induced TC have been per-ingle berpolymer systems [54]. Furthermore,as investigated the effect of shear or extensionbrid crystal structures (interfacial TC, HSK, or

xial crystalline structure) of polyolen/brous-osites via melt processing methods, such asolding and melt spinning. By using a so-calledcking injection molding technology (DPIM), incillatory shear eld was imposed on the gradu-

melt during the solidication stage, we have achieved a clear-cut TC of iPP on GF usingocessing approaches [55]. In contrast, no TCserved for the sample prepared via traditionalolding. Obviously, shear plays a key role in thef TC in the injection-molded bar of iPP/GF com-thermore, a direct formation of HSK structure,rous whiskers served as shish while PE lamel-

s the kebab, has been successfully obtained inn-molded bar of HDPE/whiskers composites via


Fig. 8. (a) Schstructure form[100]. 2007 Ameri

the above-msamples prcrystal lamethe whiskereld plays crystal stru[127], a hieorientationprepared bywere randoskin and ththe typical Nodically deThis could shearing etypical HSKin PE/CNTs spinning [1ematic of the anti-solvent process of SC CO2 and the growth of PE crystal lameed in the same PE concentration and MWNT concentration, but at different SC

can Chemical Society.

entioned DPIM technology. Nevertheless, forepared via common injection molding, the PElla were randomly decorated on the surface ofs [119]. This indicates again that external sheara key role in the formation of the interfacialcture. Moreover, in the case of PE/CNTs systemsrarchical structure with changed PE lamellar

along the thickness direction of molded bar DPIM technology was detected. The lamellaemly distributed on the surface of CNTs in thee core layers. However, in the sheared layer,HSK structure with disk-shaped lamellae peri-

corated along the CNTs axis was developed.be due to the combined effect of the externalld and the temperature eld. Furthermore, the

structure could also be successfully induced(SMCW) composites by stretching during melt10,121].

The prec(stretching)morphologcrystallizata key role iFurthermoreffect on thsolution syand coworkthis area [1tal structurcould be sutemperaturple, at the scrystals (kedecorated oincrease as to 100 C, asllae on CNTs; (b) and (c) TEM images of PE/MWNT NHSKCO2 conditions: (b) 120 C/9 MPa; (c) 100 C/9 MPa, for 3 h

eding describes the effect of external shearing elds on the formation of interfacial crystaly under melt processing conditions. For solutionion, the solvent and temperature usually playn the formation of interfacial crystal structure.e, supercritical CO2 (SC CO2) also has an obviouse formation of interfacial crystal morphology instems, as schematically shown in Fig. 8(a). Xuers have achieved some interesting results in00,101]. They found that the nanohybrid crys-e at polymer/CNT interface in solution systemsccessfully adjusted by changing the solvent,e and pressure of SC CO2 [100,101]. For exam-ame SC CO2 pressure, the average PE lamellarbab) diameters, thicknesses and periodicitiesn the surface of CNTs all showed an obviousthe SC CO2 temperature decreased from 120 C

shown in Fig. 8(b) and (c). This could be due to


the stronger anti-solvent effect of SC CO2 under relativelylower temperature. In this situation, more PE moleculescould be separated and absorbed onto the surface of CNTs,which favored the formation of larger lamella (kebab) size.On the othefrom 9 MPasolved in thpower of pbe depositekebab incCO2 pressurprecipitatioincrease thof more PE

Furthermvent on theFor PVDF/Soccurred oDMAc as thPVDF wrapDMSO as thtem, the resurface, wh[102]. This ctions betweand CNTs inof moleculainterfacial csolvent, whcoatings wethat the PE state in DMthe CNTs. Oactions betcould hinde

4. Formatistructure

It is welsurface of hybrid crysof interfacitigated. Nekind of hybFurthermoroping a uncrystalline mechanismation on thas follows.

4.1. Epitaxy

Many knucleating can inducedue to spectic of theseof polymerobserved fo

to be the formation mechanism of interfacial crystalliza-tion. As we know, epitaxy is most generally dened asthe oriented overgrowth of one phase (guest crystal) onthe surface

153]. Tnsionarystallding parried

then, l crysta

as gr, pitchlational unitof the

resultf grapE crys.oweve

as CNTation i

the epe curvallizatonshiper cr

are notanismnemenermortures smetricbed ofacial cxy, as

explaxy, a simens

a diamolymef polyxial reolymee 2d la, then ated ohip. Asce of crystalar to tholymeese kinresultmella ned, asmecha

is anos coatlobuler hand, with the pressure of SC CO2 increasing to 11 MPa, the amount and speed of CO2 dis-e p-xylene solvent increased and the solvent-xylene decreased. Therefore, more PE couldd on the CNT surface and the diameter of thereased. However, with further increasing thee to 13 MPa, both the amount and rate of the PEn were greatly increased and could obviouslye nucleation number, leading to the formationlamellae (kebab).ore, they also investigated the effect of sol-

formation of interfacial crystal structure [151].WNTs systems, no interfacial crystallizationn the surface of CNTs, when using DMF ande solvent. Nevertheless, the helical structure ofping on CNTs surface was obtained when usinge solvent. Moreover, for PE-b-PEO/CNTs sys-

gular NHSK structure could be formed on CNTsen DCB and p-xylene were used as the solventould be due to the favorable molecular interac-en the CH groups and systems for PE-b-PEO

DCB and p-xylene, to facilitate the adsorptionr chains on CNTs and the lateral nucleation andrystal growth. By contrast, using DMAc as theich is more selective for PEO, only thin polymerre observed on the CNTs, ascribed to the factblock was in poorly solvated, perhaps globularAc, suppressing absorption of PE segments onn the other hand, the unfavorable OH inter-ween PE-b-PEO molecules and CNTs in DMAcr the nucleation of PEO on CNTs.

on mechanisms of hybrid crystalline

l known that the enhanced nucleation on thellers plays a key role in the formation of varioustalline structures. Since the 1950s, the original crystallization has been extensively inves-vertheless, the formation mechanism for eachrid crystalline structure is still not quite clear.e, to date, little work has been focused on devel-iversal formation mechanism for these hybridstructures. Typically, the proposed formations are all associated with the enhanced nucle-e surface of the llers and may be summarized

and soft epitaxy

inds of llers demonstrate a heterogeneouseffect on semicrystalline polymer matrix and

lateral interfacial crystallization. This is oftenic surface physical and chemical characteris-

llers and polymers. Epitaxial crystallization matrix on the surface of these llers is oftenr many crystalline llers, and is widely believed

[152,dimethe cregarrst cSincefacia(such[133]ial recrystsion couldface othe P[133]

Hsuchtallizcase,to thcrystrelatipolymably mechconFurthstruca geoabsorinterepita

Toepitatwo dwiththe ption oepitathe pof thtionsnucletionssurfamer similthe pof thAs a tal laobtaitaxy thereneousubg of a crystal of another phase (host crystal)he epitaxial match could be due to a unit-celll match or crystal structure similarity betweenine ller and polymer matrix. Pioneering workolymer epitaxial crystallization on llers was

out in 1958 by Willems and Fischer [153,154].it has been widely reported that polymer inter-llization on the surface of various kinds of llersaphite [133,138,154], talc [51,155157], mica-based CF [139] and so on) obeyed the epitax-ship. For example, the dimension of graphite-cell lattice (2.46 A) is very close to the dimen-PE crystal cell along the c-axis (2.55 A), which

in the epitaxial crystallization of PE on the sur-hite, with the PE chain direction (the oftal) parallel to direction of graphite

r, for some llers with a very small diameter,s, the formation mechanism of interfacial crys-s soft epitaxy (geometric connement). In thisitaxial mechanism no longer exists. This is dueature of small-diameter CNTs. If the interfacialion of polymers on CNTs still obeys the epitaxial, the curved surface of CNTs would lead to curveystals with distorted lattice, which presum-

stable. Therefore, the interfacial crystallization of polymers on the surface of CNTs is geometrict (soft epitaxy) as proposed by Li et al. [89].e, for some llers unique surface topographyuch as edge planes or grooves, also results in

connement effect on the polymer moleculesn the surface, and the formation mechanism ofrystallization could also be concluded as soft

observed in many cases [148,158].in clearly the mechanism of epitaxy and softchematic representation is shown in Fig. 9. Forional (2d) lamellar llers (see Fig. 9(a)) or berseter much larger than radius of gyration (Rg) ofr (see Fig. 9(b)) [88], the interfacial crystalliza-mer matrix on such llers may strictly obey thelationship, as shown in Fig. 9(a) and (b). At rst,r molecular chains will adsorb onto the surfacemellar llers or bers due to physical interac-these molecular chains will be orientated andn the surface of llers obeying epitaxial rela-

a result, the various crystal orientations on thellers lead to different orientations of the poly-

lamellae. However, for llers with a diametere polymer Rg, such as CNTs, as shown in Fig. 9(c)r chains are exclusively parallel to the long axisds of llers due to the geometric connement., a novel NHSK structure with polymer crys-perpendicular to the long axis of such llers is

described in Section 2.2. Regarding the soft epi-nism in semicrystalline polymer/CNTs systems,ther possibility: CNTs rst induce a homoge-

ing of polymer chains on its surface with fews, then the polymer chains expand from these


Fig. 9. A schemmechanism; (diameter simi

subglobulesCNT axis, as

4.2. Chemis

The chegen and oxproposed ation mechamechanisminorganic chemisorptonto the inoactions betllers, and tcial crystallinduced intin Fig. 10. amino grouhydride grobonding, totate nucleatclay surfaceto the moleresult, the dclay is formatic representation of epitaxy and soft epitaxy: (a) the interfacial crystallization b) bers with a diameter much larger than the radius of gyration (Rg) of the plar to the radius of gyration (Rg) of the polymer obeying soft epitaxy mechanism.

and form lamellar crystals perpendicular to the reported in some studies [99,145].

orption

misorption of atomic hydrogen, carbon, nitro-ygen on the surface of llers has also beens a nucleating and interfacial crystalliza-nism [146,147]. This interfacial crystallization

generally works in polar polymers andllers system [146,148]. In this case, theion of the polar groups of a polymer matrixrganic llers surface could lead to strong inter-ween the macromolecules and the inorganichus facilitate the nucleation and lateral interfa-ization. A typical example of the chemisorptionerfacial crystallization is shown schematicallyIt may be seen that the chemisorption of theps of PA-6 onto the surface of clay (with siliconups) could result in a formation of hydrogen

stabilize the molecular orientation and facili-ion. As one molecular layer is nucleated on the, other molecules could form hydrogen bondscules already attached on the clay surface. As aiscrete lamellar structure on both sides of theed [122].

4.3. Stress o

Enhancefactor for thtures. The nmatrix caninterfacial Stress or stcould lead ta decrease otribute to tof llers anstructures. tems, TC waquiescent cthe ber. Hto the bepolymer meular chainssuccessfullymechanismis as followple. First, thunder the shnuclei compface. Denseprocess of two dimensional lamellar llers obeying epitaxyolymer obeying epitaxy mechanism; and (c) llers with a

r strain induced interfacial crystallization

d nucleation on the surface of llers is the keye formation of various hybrid crystalline struc-ucleation ability of llers on crystalline polymer

lead to heterogeneous nucleation and lateralcrystallization, as described in the preceding.rain generated at the polymer/ller interfaceo an orientation of macromolecular chains andf the nucleation barrier, and thus can also con-he enhancement of nucleation on the surfaced the formation of various hybrid crystallineFor example, in some single berpolymer sys-s not induced on the ber surface under normalonditions due to the poor nucleation activity ofowever, imposition of an external shear eldr/matrix interface by pulling the ber in thelt, facilitated the orientation of polymer molec-

along the long axis of ber, and TC could be induced on the ber surface. The formation

of shear induced hybrid crystalline structureing, taking the iPP/GF system as an exam-e iPP chains will align along the GF long axisearing effect, followed by the formation of rowosed of these aligned iPP chains on the GF sur-

active nuclei will then be induced following a


ced inte

homogeneothe growthof GF, to forhere that thfrom those ular chain branching slamellae (wand daughtboth the bally observewill twist bThis phenom[24,28,54,9

Furthermmation meinduced hydescribed imation mecinjection-mtechnologyof NHSK stthree stepstation of bofolded-chaiand/or formon CNTs sulamellae, an

The preinduced intnal stress o

or theonduc

also is on t

barrieFig. 10. A schematic representation of the chemisorption indu

us nucleation of these row nuclei, promoting of closely packed iPP crystals from both sidesm an interfacial TC structure. It should be notede homogeneous nucleation (self-epitaxy) of iPProw nuclei could result in the change of molec-

melt mal ccouldchainationdirection by 80. Therefore, a unique lamellartructure (cross-hatching) composed of parenthose c-axes are parallel to the ber surface)er lamellae (whose c-axes are perpendicular toer surface and the parent lamellae), is gener-d near the ber surface, whereas these lamellaey 90 at a distance away from the ber surface.enon has been often reported in the literature

5,96].ore, under real processing conditions, the for-

chanism of external shear or extension eldbrid crystalline structure is similar to thatn the preceding. A typical example is the for-hanism of shear induced NHSK structure in theolded bar of HDPE/CNTs composites via DPIM

[109]. In this case, the formation mechanismructure could be summarized in the following: (1) shear induced disentanglement and orien-th HDPE chains and CNTs, (2) formation of PEn lamellae directly nucleated on CNTs surfaceation of PE extended-chain shish rst directlyrface followed by nucleation of folded-chaind (3) the growth of PE kebabs.

ceding describes the external stress or strainerfacial crystallization mechanism. The inter-r strain caused by fast cooling the sample from

the ller suobserved intion mechato the abovinterfacial c

5. Interfacinduced by

5.1. Interfaestimated b

Micromto evaluatethe level oshear strension/bondinor critical of interfacirectly deteparametersand most proportionaability of thexperimentfacial enharfacial crystallization mechanism.

rmal expansion coefcient mismatch or ther-tivity mismatch between llers and polymersnduce the pre-alignment of polymer molecularhe llers surface, and thus decrease the nucle-r, enhance the nucleation of polymer matrix on

rface, and lateral interfacial crystallization, as

some cases [54,148]. In such cases, the forma-nism of hybrid crystalline structure is similare-mentioned external stress or strain inducedrystallization.

ial and mechanical enhancement interfacial crystallization

cial enhancement of crystalline interfacey different research strategies

echanical parameters are commonly invoked the magnitude of interfacial interaction andf load transfer efciency, such as interfacialgth (IFSS, i) [159161], interfacial adhe-g strength [162], critical effective length (lc)

aspect ratio (lc/D) [163,164]. The assessmental enhancement behaviors is directly or indi-rmined for these crucial micromechanical. Among them, the IFSS is most importantcommonly used because its value is strictlyl to the interfacial interaction and load transfere system. Meanwhile, there are a number ofal means to identify the occurrence of inter-ncement, roughly divided into ve strategies:


micromechanical tests, such as the pullout [165] andfragmentation tests [163]; mechanical model predictionusing the role of mixtures [166] or the Halpin-Tsai equa-tions [167]; computational molecular dynamic simulation[168]; micand in situtroscopy [1interfacial between la ller-reinfor a llermechanicaloptimum ra non-covamodulus ofthat of amlayer can bhence lead discussed inobserved oIFSS with th

The moby performshaped lleused, in whand the forAccording tpullout forca general foFmax is equa[159]:

Fmax = i 2

where rf isded length.or embeddGati and Wscrystallinedroplets, anobtained frin Fig. 11 tle impact the IFSS foraround 3 Mthat the fora reductionbeen achievmicrostructi by usingof transcrysthe tensile remarkablytural featurenhanced inscrystallinetest in othelower thaninterfacial afail under emore appro

[164,176,177]. For the ber fragmentation test, the classicalKelly-Tyson mode [178] is used to determine i:

i =f

2(lc/Df)(2)

e f isc the cent leh lm issuallybers. T

3Dff8lm

viousSS. In td iPPA-PP)

ragmeoved ffacial t

obtaiF-reinff sPP/Pa, ino the fo

differeen sPe layerlso obe inteof micnhanc

and Holymeicrod

oved be intearance

adhecrysta. [183]metricfacial asubstraaticallas shled thterface

TC, rew trandary lapresenr nanoical tessfully

fractuo attathe precharostructureproperty relation analysis [125]; spectroscopic analysis such as Raman spec-69]. As suggested by Coleman et al., the strongadhesion and highly efcient load transferler and polymer are critical requirements forforced composite are the most requirements-reinforced composite [170]. Therefore, the

behavior of the interface is essential foreinforcement. It is logically deduced that forlent bonding the mechanical strength and

an ordered crystalline interface is better thanorphous one, since the crystalline interfaceear larger shear stress before its failure andto higher load transfer efciency. However, as

the following, this assumption is not alwaysn assessing the critical parameters such as thee interfacial crystallization structure.st straightforward means to evaluate IFSS ising various micromechanical tests. For brous-rs, the single ber pullout test was frequentlyich a ber was embedded in a polymer matrixce to pullout the ber was measured [161].o a general force-balance rule, the maximume Fmax is related to the IFSS i via. According torce-balance rule, the maximum pullout force,l to the i, multiplying the ber embedded area

rflemb (1)

the ber radius and lemb is the ber embed- Therefore the linear slope of Fmax versus lembed area may be used to estimate i [165,171].agner [18] purposely changed the lemb in tran-

poly(caprolactone) (PCL) or amorphous PCLd performed the pullout test. The i valuesom the plots of Fmax against lemb as shownindicated that the presence of TC offers lit-on the load transfer ability of the interface, as

both transcrystalline and amorphous PCL wasPa. In addition, Folkes and Wong [172] foundmation of TC in the PP/GF composites leads to

in the IFSS. A somewhat different result hased by Wu et al. [173]. They manipulated theure of TC using PP as matrix, and measured

the pullout test. Although the highest IFSStalline interface was prominently lower thanstrength of pure PP, the value of IFSS changed, from 1 MPa to 5.5 MPa, by altering the struc-e of TC, implying a possibility that TC tailoringterfacial adhesion/bonding. The IFSS of PP tran-

interface was also measured using the pulloutr studies [174,175], to obtain values generally

5 MPa. Nevertheless, for the cases of strongdhesion, bers could be easily occur broken andxternal force effect; the fragmentation test ispriate than the pullout test to estimate the IFSS

wherand lfragmlengtand uthe as:

i =

Obthe IFGF anPP (Mthe fimprinter(3) tothe CIFSS o4.3 Mdue tto thebetwtallinwas a

Thtype ical eChenthe pthe mimprtallinappefacialtranset alasymintericon dramface, reveaof inlae infor lobouncally

Fochansuccefacialetc. Twith tive m the ber tensile strength, Df the ber diameterritical effective length. Since the distribution ofngths is related to the lc, the mean fragment

expressed as Klc, where K is a correction factor assumed to be 3/4 for random orientation ofherefore an empirical equation can be derived

(3)

ly, precise values of f are crucial for calculatinghe study of Nagae et al., the interaction between

was enhanced by coating maleic anhybride- around GF, and the IFSS was measured usingntation test [179]. The IFSS was dramaticallyrom 6 MPa to 24 MPa after the formation ofranscrystalline layer. Wu et al. [61] utilized Eq.n the IFSS of interfacial transcrystalline layer inorced sPP and iPP composites, respectively. TheCF, 12.7 MPa, was three times for that of iPP/CF,dicating obviously higher interfacial adhesionrmation of sPP TC structure. This was attributedence in lamellar orientation behavior of TC layerP/CF and iPP/CF. The formation of transcrys-

coating on CF induced interfacial enhancementserved by other authors [180,181].rfacial adhesion/bonding strength is anotherromechanical parameter to assess the mechan-ement behavior of the crystalline interface.siao [17] measured the debonding forces of

r/ber interfaces with or without TC, by usingebonding test. The debonding force may bey 40% with the appearance of a transcrys-

rface. Karger-Kocsis [182] also found that the of transcrystalline layer enhanced the inter-sion strength. To study the case for which alline layer occurred on a layered substrate, Cho

utilized a cleavage mode test, such as, the double cantilever beam test, to estimate thedhesion strength between the TC of PP and sil-te. The interfacial adhesion strength increasedy with the perfection of transcrystalline inter-own in Fig. 12(a); a microstructural analysisat for high transcrystalline level the breakage

occurred across the brillation of iPP lamel-sulting in strong interfacial bonding, whereasscrystalline level cracking initiated at the weakyer between TC and spherulites, as schemati-ted in Fig. 12(b).meter-scale bers, such as CNTs, the microme-sts of pullout and fragmentation were also

utilized to estimate the IFSS [163,165], inter-re energy [184], and critical ber length [163]in the IFSS in the CNTs-reinforced compositesesence of interfacial crystallization, the predic-nics models such as the prediction models of


Fig 1997 Ameri

mechanics Halpin-Tsaia brous-shideal interfalm is longertake place rically preserule of mixt

c = 01where c, and polymfraction, 0length efcEq. (2) or Eber reinfo. 11. Plots of maximum pullout force versus ber embedded length for PCL transcan Chemical Society.

like the Cox-Krenchels rule of mixtures and the (H-T) equations have been considered [170]. Inaped ller-reinforced polymer composite, if ancial bonding exists and the mean length of ber

than the critical ber length lc the bers mayupture under tension deformation, as schemat-nted in Fig. 13(a). For such a circumstance, theures is expressed as:

f Vf (1 Vf) m (4)

f and m are the strength for composite, berer matrix, respectively, Vf is the ber volumeis the orientation efciency factor and l the

iency factor. The IFSS can be calculated throughq. (3) after f has been identied. For a shortrced composite for which interfacial interaction

is weak, thedebonding In this caseas:

c =(

ilmDf

From thite strengthof linear IFSS. For soites [185,18is extremepolymer shobserved, amixtures iscrystalline and quasi-amorphous droplets [18].

failure model is described as ber pullout andoccurs at the polymer/ber interface (Fig. 13(c)).

an alternative of the rule of mixtures is given

) Vf (1 Vf) m (5)

is equation, one may note that the compos- is determined by the IFSS, i, and the slope

tting of c versus Vf is closely related to theme special cases of CNTs-reinforced compos-6], the mechanical strength of interfacial layerly stronger than that of polymer bulk, and aeath coating around the pullout CNTs could bes schematically shown in Fig. 13(b). The rule of

further modied with taking into account the


Fig. 12. (a) Changes of interfacial adhesion strength with varying level of crystalline structure in the TC region; (b) schematic indicating that a crackpropagates across transcrystallinities or the boundary between transcrystallinities and spherulites [183]. 2003 American Chemical Society.

Fig. 13. Three failure modes in ber-enhanced polymer composite: (a) ber breakage/rupture; (b) debonding between interfacial coating layer and polymermatrix; (c) debonding at ber/polymer matrix interface.


Fig. 14. (a) Ex pull ouand a linear t 2004 Wiley

thickness o(s) at the the sum of

c =(1 + b

r

By usingcidated thelayer to meites. As shcrystallinityexact microdemonstratthe pulloutadding CNTting of c vcould be cawith Eq. (6the tensile also demoncial crystalmodulus bytion [167,18induces an as comparebonding antion of stiff perimental evidence of PVA crystalline layer coating on CNT surface afterting between composite strength, c, and CNT volume fraction, Vf [187].

Periodicals.f the polymer sheath, b, and the shear strengthedge of the interfacial layer region, where R isrf and b.

f

) [lm2rf

s (1 + b

R

)m

] Vf + m (6)

Eq. (6), Coleman et al. [187] theoretically elu- contribution of the interfacial crystallizationchanical strengthening in PVA/CNTs compos-own in Fig. 14(a), the linear increment of

with the CNT volume fraction [187] and thescopic analysis on the pullout CNT [167,187],ed that a thick crystalline layer coating around

CNTs. A dramatic enhancement achieved bys is presented in Fig. 14(b), and the linear t-ersus Vf is also included. Subsequently, the slculated by correlating the slope of c versus Vf). It was about 95 MPa, obviously higher thanstrength of bulk PVA, which is 81 MPa. Theystrated the positive inuences of the interfa-lization layer on the enhancement in Youngs

using the role of mixtures and/or the H-T equa-8]. The occurrence of interfacial crystallization

interfacial layer with high mechanical strength,d to bulk polymer, and enhances interfaciald polymerCNT stress transfer. Thus the forma-ordered crystalline coating around CNT indeed

plays a domtalline PVA

Other rinterfacial dynamic simlated the CNspecic che100 MPa, wof microbealso suggesCNT/polymder Waals iof thermal among othbonding ensuggested bputational Wei et al. [talline PE aPE adsorpthigher struof composilc for crystaites was coonly 3 MPafaces undefor a very ably increat test; (b) stressstrain curves of PVA/CNT nanocompositesinant role in the reinforcement of semicrys-/CNTs nanocomposites.esearchers have attempted to estimate theenhancement using computational molecular

ulations. Liao and coworkers [189,190] simu-T pullout from a CNTpolymer system withoutmical bonding; the IFSS achieved was abouthich is one order of magnitude higher than thatr (GF, CF) reinforced polymer composites. Theyted that if there is no chemical bonding, theer adhesion arises from (1) electrostatic and vannteractions, and (2) mismatch in the coefcientexpansion (CTE) between CNT and polymer,er factors. An extremely strong non-covalentergy existing between polymer and CNT wasy Panhuis et al. through a combination of com-simulation and experimental evidence [191].192] studied the interface between semicrys-nd CNT. Their results indicated that orderedion layers wrapped around the CNT and thectural order parameter favors the enhancementte modulus. A comparative analysis of IFSS andlline PE/CNT and amorphous PE/CNT compos-nducted by Frankland et al. [193]. The IFSS was

for both of crystalline and amorphous inter-r a non-bonded circumstance; whereas, evensmall amount cross-linking, the IFSS remark-sed to 30 MPa for the amorphous interface and


to 110 MPa for the crystalline interface. It should be notedthat the value of 110 MPa is about two times that for theIFSS, 47 MPa, in a non-crystalline copolymer/CNT compos-ite, measured directly through the pullout test [165].

A methodology that is reasonably straightforward todetermine whether any load is transferred to the CNT isin situ spectroscopic analysis of the CNTpolymer com-posite under tension. Fluorescence [194], Raman [169,195]and X-ray [196] spectroscopy are all suitable as candi-dates. In particular, for Raman spectroscopy, the G bandof CNT, located around 1620 cm1, is sensitive to stress inthe CNT. This peak position will shift to a lower wavenum-ber under tensile deformation, and the slope of this peakshift as a function of strain should be proportional tothe modulus of the composite [197] or the strength ofthe CNTpolymer interface [198,199]. These features havebeen utilized to indicate load transfer from polymer matrixto CNT. In situ Raman measurements conrmed that ef-cient load transfer indeed occurred through the interfaceof semicrystalline polymer and CNT in the PP/CNT [199]and PVA/CNT [200] composites. In addition, Wang et al.[196] utilized synchrotron X-ray spectroscopy to detectprominent structural changes during deformation in thesemicrystalline PAN/CNT composite bers. The occurrenceof structural change was attributed to the fact that CNTsenhance the load transferred to PAN crystals, implyingstrong inteCNT lamen

A numbecrystalline iller, and tperformancenhanced bAn examplative analytranscrystawas performand Youngare signic

Fig. 15. Stresstallized PP andLtd).

indicating the effective load transfer realized by transcrys-talline interface layer. In other work, the individual CNTlam

Progress in Polymer Science 37 (2012) 1425â€“ 1455-EQUIPO 1 (1)

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Transcript of Progress in Polymer Science 37 (2012) 1425â€“ 1455-EQUIPO 1 (1)