Influence of Nanoparticles on the Rheological Behaviour and ...of the ﬂow induced crystallization of iPP from the melt during and immediately after the application of strong shear

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2174

Influence of Nanoparticles on the RheologicalBehaviour and Initial Stages of Crystal Growthin Linear Polyethylene

Nilesh Patil, Luigi Balzano, Giuseppe Portale, Sanjay Rastogi*

We demonstrate the role of broad molecular weight distribution (MWD) polyethylene (PE), inthe presence of nanoparticles of different aspect ratios and binding efficiency with thepolymer, in the formation of shish-kebab structures under a shear protocol using time-resolved small-angle X-ray scattering (SAXS). The results indicate a scattered intensity inthe form of streaks at the equator while maxima in the meridian confirm the presence of anoriented structure in the polymer. The SAXS facilitated the probing of the steady growth of theobtained shish-kebab structures at an isothermal crystallization temperature (136 8C). Thestudy reveals the influence of nanoparticles (single walled carbon nanotubes (SWNTs) andzirconia) in chain orientation. The presence of nanoparticles promotes the high degree oforientation, where shish is formed along the flow direction and kebab perpendicular to it. Ahigher degree of chain orientation is observed in the presence of SWNTs compared to zirconiananoparticles. The SWNTs present in a small concentration (< 0.6wt.-%) are aligned in the flowdirection, which leads to an increase in shish length as estimated from Ruland’s streakanalysis. The stable shishes in the early stages of crystallization suppress the nucleationbarrier for further crystallization. Compared to the polymer without nanoparticles the shishlength increases in the presence of zirconia, however, the increase in shish length is muchmore pronounced in the presence of SWNTs compared to zirconia nanoparticles. The nano-particles favor the orientation fraction as deduced from the integrated intensity of scatteringat the equator and meridian in the patterns.Absence of a plateau in the low frequency regionof the polymer–SWNT composites suggests thenon-existence of network formation. Neverthe-less, comparing the storage modulus at twodifferent temperatures (142 and 160 8C), suggestsa strong temperature dependence and differencein adsorption energy of the two nanoparticles.

N. Patil, S. RastogiDepartment of Materials, Loughborough University,Leicestershire LE11 3TU, UKE-mail: [email protected]. RastogiDepartment of Chemical Engineering and Chemistry, TechnischeUniversiteit Eindhoven, P.O. Box 513, 5600 MB Eindhoven, TheNetherlands

L. Balzano, G. Portale, S. RastogiDutch Polymer Institute, P.O. Box 902, 5600 AX Eindhoven, TheNetherlandsL. BalzanoDepartment of Mechanical Engineering, Technische UniversiteitEindhoven, P.O. Box 513, 5600 MB Eindhoven, The NetherlandsG. PortaleDUBBLE, CRG/ESRF, Netherlands Organization for ScientificResearch (NWO), c/o ESRF BP 220, 38043, Grenoble Cedex, France

Macromol. Chem. Phys. 2009, 210, 2174–2187

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/macp.200900364

Influence of Nanoparticles on the Rheological Behaviour and . . .

Introduction

The resultant morphology of a viscoelastic melt is strongly

dependent on themolecular characteristics and the applied

processing conditions. The morphology thus obtained

influences the mechanical properties of the polymer, thus

it has always been a quest to develop knowledge of the

structure–property relationship. Because of their simplicity

and commercial viability, linear polyethylenes (PEs) and

poly(propylene)s are semicrystalline polymers that have

been investigated for some time.Oneof the frequentlyused

characterization tools to follow structure development is

time-resolved X-ray scattering, where the intensity at the

low angle region arises due to electron density fluctuation

with the organization of chains in the region of 1 to 100nm.

When the polymer melt is subjected to flow, highly

anisotropic structures, similar to shish-kebab, are formed.

To gain insight into the development of shish-kebab

structures a series of studies have been performed. In all

these studies the quest has been to understand the initial

stages of shish formation. The structure formation is an

outstanding issue because little information is available

concerning the early stages of crystallization as a result of

different parameters such as effect of average molecular

weight (Mw) and molecular weight distribution (MWD),

meltmemory, shear rate, and temperature.What follows is

a brief overview of some of the salient findings reported in

the literature.

The shish-kebab morphology in polymers was first

reported by Pennings et al.,[1–3] who, during the fractiona-

tion process of a high molar mass PE came across such a

unique structural formation. Binsbergen, [4] studied the

crystallization in isotactic poly(propylene) (iPP) from melt.

Using cross polarized optical microscopy under flow

conditions, Binsbergen observed birefringence arising as

a result of chain orientationwhile cooling after application

of shear at 180 8C. Keller et al.,[5] proposed a hypothesis onthe chain orientation above the criticalmolarmassM�. In a

sheared polymer melt with a particular polydispersity at a

given temperature, the chains longer thanM� can remain in

an extended state and orient after deformationwhile short

chainswould relax back to forma randomcoil state as their

relaxation times are short. Using rheological studies,[6–9] in

the past, many scientists have reported the effect of shear

rate and quenching depth on crystallization under shear

flow.

Janeschitz-Kriegl and co-workers,[10–12] reported that

oriented nuclei, which are formed at high shear rates at

temperatures close to the equilibrium melting point, are

practically stable at temperatures where spherulites melt.

Nevertheless, theapplicationof shear rates at temperatures

below the melting point of the spherulites leads to stable

precursors because of longer relaxation times as compared

to deformation time. As a result, the long lasting deforma-


� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

tion under low stress leads to the sameprecursors as that of

short term deformation under high stress.

Schultz et al.,[13] studied the mechanism of the early

stages of structural development under flow in polymer

melt spinning using in situ simultaneous X-ray scattering

(small-angle X-ray scattering (SAXS)/wide-angle X-ray

scattering (WAXS)). They reported the presence of shish-

kebab structures in the PE melt and further verified such

structures by SAXSwithout anydetectionbyWAXS. Samon

etal.[14] further reported that theonset of crystallization is a

result of chain orientation and does not depend on chain

chemistry or specific undercooling.

Muthukumar and co-workers,[15] with the help of

molecular dynamics simulation studies, showed that the

emergence of shish-kebabs is related to the discontinuous

coil–stretch transition of isolated chains. They demon-

strated the presence of stretched and coiled conformations

at agivenflowrate. The stretch chains crystallize into shish,

the coil chains form single chain lamellae and then adsorb

to the shish constituting kebabs. Hsiao et al.,[16] studied a

sheared PE blend that contained 2wt.-% of ultrahigh

molecular weight (UHMW) PE and 98wt.-% of a low

molecular weight PE copolymer matrix. The scanning

electron micrographs of a solvent-extracted sheared PE

revealed the presence of shish-kebab structures with

multiple shish. They stated that the disentanglements of

UHMW PE in the blend, if any, were extremely low and

further considered the hypothesis that the ‘entangled

thread’ upon stretching will show a straight section with

short thread lengths aligned parallel to each other and the

remaining parts of the thread will stay entangled or form

globular sections. The multiple shishes originate from the

stretched chain sections and the kebabs originate from the

coiled chain section following the diffusion controlled

crystallization process.

Yang et al.,[17] investigated the influence of high-

molecular-weight chains on flow-induced crystallization

precursor formation using a bimodal PE blend. They

reported the higher degree of crystal orientation because

of the role of long chains in the enhancement of shear-

induced precursor formation. Zuo et al.[18] reported the

melting and re-formation of a shish-kebab precursor

structure in a sheared PE bimodal blend. The results

indicated that shish-kebab re-formation is directly related

to the relaxation behavior of stretched chain segments

confined in a topologically deformed entanglement net-

work.

Oginoetal.[19] studied theeffectof anUHMWcomponent

in the formation of shish-kebab structures. They followed

the crystallization of PE blends of low molecular weight

(LMW) and UHMW components using time-resolved

depolarized scattering (DPLS) techniques. The studies

showed evident streaks at the equator of the two-

dimensional patterns, which suggested the role of the

www.mcp-journal.de 2175

N. Patil, L. Balzano, G. Portale, S. Rastogi

2176

UHMW component. They argued that the critical concen-

tration is two to three times larger than the overlap

concentration of the chain (C�Rg), which indicated the role of

entanglements of UHMW PE chains in the formation of

oriented structures.

Keum et al.,[20] demonstrated the nucleation and

growth behaviour of twisted kebabs from a shear-induced

scaffold in entangled PE melts. They reported that the low

shear rate generates a lower shish density which enhances

the kebab twisting. Matsuba et al.[21] studied the role of

UHMW components on shish-kebab formation. The inves-

tigations revealed the significant effect of crystallization

rate and the relaxation rate of the UHMW component on

the shish-kebab formation process. Kimata et al.[22] used

small-angleneutron scattering to showthat long chains are

not over-represented in the shish relative to their con-

centration in the material as a whole. They concluded that

the molecular deformation of short and medium chains

in the shish were greater than that of long chains, and

there were enhanced fluctuations in the local molar

mass distribution because of shear or a difference in the

concentration of short and medium chains in the shish

relative to the bulk.

Balzano et al.[23] used specially synthesized linear

high density PE with a bimodal molar mass distribution

to demonstrate the presence of extended chains arising

because of the high molar mass component in the

suspension of a polymer melt while shearing above

but close to the equilibrium melting temperature (Tm¼141.2 8C). They found amatch between the dissolution timescale of the extended chains and the time scale for the

reptation of the HMW chains, which suggested the role of

HMWPE in the formation of flow induced precursors (FIPs).

Studies also indicated that the needle-like FIPs crystallized

with an orthorhombic lattice to result in only crystalline

shishes.Kumaraswamyetal.[24,25] showed that theoriented

crystals are formed as a result of a value exceeding the

critical shear rate and shearing time. They claimed the

presence of oriented nuclei under shear in themelt because

of the distribution of relaxation time. Balzano et al.[26]

recently studied precursor formation in the early stages

of the flow induced crystallization of iPP from the melt

during and immediately after the application of strong

shear. They reported the nucleation of crystalline

structures already during shear, which further directed

the development in structure and morphology. Recently,

Winey and co-workers[27] showed the formation of shish-

kebab structures as result of fiber spinning in single-walled

carbon nanotube (SWNT)–PE composites. They demon-

strated thehigher chainorientationof thePE in thepresence

of the SWNTs and stated that the SWNTs nucleate PE crystal

growth and accelerated the crystallization rate.

However, how the shish structure is influenced in the

presenceofnanofillers isnotwell investigated. In thiswork,



we have investigated the role of a broad molecular weight

distribution in the crystallization process of PE using time-

resolved X-ray scattering techniques to monitor the

development of shish-kebab formation from the early

stages of crystallization. We further demonstrated the role

of nanoparticles, such as SWNTs and zirconia, in the

formation of highly anisotropic structures. For PE, while

SWNTs provide a good epitaxy matching and high aspect

ratio, the presence of spherical zirconia particles provide a

very high surface-to-volume ratio with an aspect ratio of

nearly one. This study intends to provide insight into the

structure development under shear in the formation of

shish-kebab structures.

Experimental Part

In this studywemade use of PE with a broadMWD (Mw ¼246200g �mol�1, Mn ¼10100 g �mol�1, and a polydispersity index ofMw=Mn ¼ 24.2)whereMw andMn areweight-averageandnumber-average molecular weights, respectively. The broad molar mass

polymer was kindly provided by Dow Benelux B.V., The Nether-

lands. SWNTs were obtained from Udinym Inc., USA. An aqueous

suspension of nanosized yttria-stabilized zirconia particles was

obtained from MEL Chemicals, U.K. The suspensions of different

nanoparticleswereuniformlysprayedonthepowderofPE (withan

average particle size of 100mm) in a similar way as reported

elsewhere. [28,29] The particle size of spherical zirconia nanoparti-

cles used in our studies is 20nm. The individual SWNT diameters

were in the range of �0.8–1.2nm and individual SWNT lengthsranged between �100–1000nm.

Two-dimensional (2D) time-resolved SAXSmeasurements were

performed using the DUBBLE/BM26B beamline at the European

Synchrotron Radiation Facility (ESRF), Grenoble, France. A 2D gas-

filled detector with a resolution of 512�512 pixels and260mm� 260mm pixel size was employed to detect the time-resolved SAXS patterns for the shear experiments. The sample-to-

detector distance for SAXS measurements was 6.057 m, respec-

tively. The beamline consisted of a vacuum chamber in between

the sample and detector to reduce the scattering and absorption

from air as shown in Figure 1. The wavelength of the synchrotron

radiation used in the SAXS experiments was 1.24 Å. An acquisition

timeof 10 swasused to acquire imageswith a dead timeof 0.5 s for

data transfer following the correction of intensity of the primary

beam, sample thickness, and absorption needed between adjacent

images. The images were integrated to determine the scattered

intensity (I) asa functionof scatteringvector (q). Therangeof length

of the scattering vector q in the SAXSmeasurementswas 0.001–0.5

nm�1, where q is given by q¼4psinu/l, where 2u is the scatteringangle.

The samples in the form of flat disks of 400mm thickness were

obtained by compression molding. All the samples were mixed

with Irganox 1010 to avoid possible degradation. The flat disk-like

samples were mounted between two plates of Linkam shear cell

CSS 450. The quartz windows of the shear cell were replaced by

kaptonwindows toobtainthedesiredscattering.Theshear cellwas

calibrated with a tolerance range of 30mm to achieve the

maximum contact between plates of the sample while shearing.

DOI: 10.1002/macp.200900364


Figure 1. SAXS setup in the DUBBLE/BM26 beam line used for our experiments. The sample to detector distance was 6.057m. The wholesetup was connected online to controllers in the hutch of BM26B.

A sample of 400mm thickness was compressed in the shear cell up

to 200mm in themelt to ensure the correct application of shear on

the sample during the experiment. The temperature and applied

shear (refer to Figure 2) for the shear cell was as follows:

1)Heating the sample from room temperature to 160 8Cat a rateof30 8C �min�12)Holding the sampleat160 8C in themelt for5minto remove the melt history. 3) Cooling the sample at the rate of

10 8C �min�1 to the isothermal crystallization temperature of136 8C and soon thereafter applying the shear (100 s�1 for 1 s). 4)Maintaining the isothermal crystallization temperature (136 8C)for 10min during the experiment to follow the structure

Figure 2. The schematic of the thermal and flow history applied inthe present study. Shear rate of 100 s�1 for 1 s is applied at anisothermal crystallization temperature of 136 8C to study thegrowth of structures. Here, T1 represents the temperature inthe melt, i.e., 160 8C for 5min to remove the melt history. T2represents the isothermal crystallization temperature of 136 8C,and T3 is 60 8C.



development under shear. 5) Cooling the sample to room

temperature at the rate of 10 8C �min�1 to monitor the crystal-lization process while cooling.

Samples were subjected to steady shear at the isothermal

crystallization temperature of 136 8C to follow the structuredevelopment for 10min. The shear rate was 100 s�1 for 1 s, that

is, a strain of 100%was implemented. The steady 2D SAXSpatterns

were continuously taken for the whole profile of the thermal and

flow history.

Dynamic rheological properties were investigated on an ARES

Rheometer. The oscillatory shear mode using a parallel plate

geometry with a diameter of 25mm at 142 and 160 8C under anitrogen atmosphere was adopted for the measurements. A

frequency sweep that ranged between 0.01 and 100 rad � s�1 at alow strain rate of 2.0% was applied. The samples were placed

between the preheated plates and were allowed to equilibrate for

approximately 5min prior to each run. The compression molded

samples inside the rheometerwere cooled from142 8C to followtheevolution of the storagemodulus G0 with a constant strain of 2.0%

and a frequency of 10 rad � s�1. A slow cooling rate of 0.1 8C �min�1

was used in all the cases for precise monitoring of the onset of

crystallization.

Results and Discussion

The Role of a Broad Molecular Weight Distribution onthe Formation of Shear-Induced Structures

2D SAXS patterns of PE with a broad molecular weight

distributionat selected timesafter theapplicationof steady

shear (100 s�1 for 1 s) at 136 8C are presented in Figure 3.After shear the sample was left at the isothermal

temperature of 136 8C for ten minutes. The steady shearin thepolymermelt resulted in the formationofbroadweak



Figure 3. 2D SAXS patterns of sheared broad MWD PE at the isothermal crystallization temperature of 136 8C 600 s after an application of ashear rate of 100 s�1 for 1 s. The growth of the kebab starts at 100 s and can be noticed in the image. The patterns show the steady growth ofshishes and kebabs at the isothermal crystallization temperature.

2178

equatorial scattering which, with time, strengthened into

streak-like scattering. FromFigure 3, it is evident that at the

initial stages thebroadweak scattering along the equator is

constant for sometime (refer to thepattern takenafter50 s).

After 100 s of applied shear the development of scattering

along the meridian occurs. In general, the appearance of

streak-like scattering in the equator is attributed to the

presence of a shish-like structure. Considering variations in

the intensity along the equator at the initial stages such a

structure can bemetastable, where the chains are oriented

alongtheflowdirection.At the later stage theappearanceof

maxima in the meridian is related to the formation of a

kebab-like structure because of lamellar stacking that

results in outward growth of chains perpendicular to the

central core. The results indicate that the applied strong

shearing condition results in the formationof a shish-kebab

morphology and enhanced crystallization process in the

broad molecular weight distribution polymer. The growth

of shish-kebab structures is enhanced in this flow protocol,

Figure 4. The selected 2D SAXS patterns at different temperatures whcrystallization at 136 8C for 600 s to follow the structure developme



an observation in agreement with earlier reported find-

ings.[26] Similar to other studies our results also suggest the

role of a critical shear rate below which the streak-like

scattering in the equator doesn’t occur. If it doeshappen it is

beyond the detection limit of the applied experimental

conditions and the presence could be realized by the

appearance of the intensity along the meridian during

formation of the kebabs.

The SAXS intensity and the flow-induced oriented

crystals tend to increase as a function of time. The time-

resolved 2D-SAXS patterns give the real-time structure

development in a sheared polymer melt: 1) After the

application of shear, the increase in the streak-like

scattering with time along the equator suggests growth

of shish. 2) The increase in intensity along the meridian

with time at the isothermal temperature suggests the

growth of kebabs after 100 s. These findings are in

accordance with literature data,[30] where at least in the

initial stages of the applied shear the high molar mass

ile cooling the polymer melt to room temperature after isothermalnt.

DOI: 10.1002/macp.200900364


Figure 5. SAXS intensity built up as a function of time in themeridian and equator at the temperature of isothermal crystal-lization (136 8C) after application of shear (100 s�1 for 1 s) obtainedfrom SAXS. The intensity in the initial stages is more in theequator compared to meridian.

Figure 6. Evolution of total intensity scattered after the appli-cation of shear at the isothermal temperature of 136 8C.

componentpresent in thepolymermelt facilitates theshish

formation. The chain orientation changes the crystal-

lization kinetics by providing the nucleation sites rooting

the oriented lamellae to grow radially perpendicular to the

chain axis.

The 2D SAXS patterns, shown in Figure 4, are taken at

selected temperatures while cooling the polymer melt to

room temperature after following the structure develop-

ment process at the isothermal temperature, 136 8C.With adecrease in temperature, the scattering in the meridian

broadens and the intensity increases. The kebab grows and

ideally the meridional scattering separates out and

becomes transformed into distinctive lobes at low tem-

peratures. From the SAXS patterns it can be noticed that

such a process of nucleation and transformation of

meridional scattering into the formation of lobes starts

at 126.9 8C. The persistence of anisotropic intensity oncooling suggests the near absence of spherulites in the

shish-kebab dominated morphology. The increase in

intensity suggests avolume increase in the electrondensity

fluctuation because of a re-organization process of chain

segments that occurs on cooling within the lamellae.

The intensity build up during the crystallization process

under isothermal conditions (136 8C) after the applicationof shear is shown in Figure 5. It is evident that after

100 s the intensity that corresponds to the meridian

dominates, which suggests the development of kebabs in

the obtained patterns. The equatorial intensity dominates

in the early stages, which indicates the formation of

shishes (extended chains) because of chain alignment

or segmental orientation of molecular chains in a



preferred direction. The intensities tend to grow as a

function of time, consequently accelerating the crystal-

lization kinetics.

Theobservations inFigure5are further supplementedby

the evolution of scattered intensity as shown in Figure 6 at

the isothermal temperature. The presence of a hump at

higher q-values after 100 s suggests the growth of

meridional maxima in the 2D SAXS pattern. This hump is

attributed to the formation of kebab-like structures that

result because of the growth of lamellar stacks perpendi-

cular to the direction of the central core in the shish-kebab

morphology. The overall intensity tends to grow as a

function of time with the development of the oriented

structures. The strongmeridionalmaxima that occur in the

patterns suggest a high volume fraction of kebab-like

structures.

Crystallization in Polymer Melts of PE Having BroadMWD in the Presence of Nanoparticles

From the literature[23] and our studies it is evident that a

broad MWD plays an important role in the formation of

shish-kebab structures. We further investigated the effect

of nanoparticles on the development of this morphology.

The results indicate the acceleration of crystallization

kinetics after additionofnanoparticles to thepolymermelt.

The crystallization temperature shifts to higher tempera-

tures in the presence of nanoparticles. It is obvious from the

SAXS patterns that more X-rays scattering takes place

around the beam center in the presence of SWNTs because

of their high aspect ratios. The spherical shape of the

zirconia nanoparticles limits this excess of scattering to a

certain extent as the particle size of such zirconia

nanoparticles used in our studies is 20nm. However, in

either case, the extent of formation of oriented structure

increases with an increase in the concentration of



Figure 7. 2D SAXS patterns of broad MWD PE in the presence of SWNTs at 136 8C at selected times after application of shear.

2180

nanoparticles. The 2D SAXS patterns of polymer in the

presence of different concentrations of SWNTs and zirconia

nanoparticles taken at selected times are presented in

Figure 7 and Figure 8, respectively. The presence of

nanoparticles enhances the alignment of chain segments

Figure 8. 2D SAXS patterns of broad MWD PE in the presence of zircotaken after selected times during isothermal crystallization.



to a preferred direction under applied shearing conditions.

The growthof formation of kebabs occurs early in polymers

that contain a different amount of nanoparticles as

compared to the pure polymer melt, which suggests the

early onset of crystallization.

nia nanoparticles at 136 8C after application of shear. The images are

DOI: 10.1002/macp.200900364


FromFigure 8 it is apparent that scattering in thevicinity

of the beamstop in the presence of nanoparticles imposes

difficulty in probing the weak scattering arising at the

initial stages of shish formation. However, with time, the

evolution of intensity gives the desired information about

structure development under the preferred shearing

conditions. A smeared, isotropically distributed intensity

just before the shear (at t¼ 0s), which increases with theconcentration of nanoparticles, suggests the dispersion of

nanoparticles in the polymer matrix. After the application

of step shear, now the subtracted intensity from t¼ 0s,shows continuous increase in the intensity along the

equatorial and meridional direction. The increase in

Figure 9. The intensity development in the equator for polymermelts at isothermal crystallization after application of shear: a) atdifferent SWNT concentrations, and b) at different zirconia con-centrations. It is clear that an increase in the concentration ofnanoparticles leads to an increase in intensity at the equator,which suggests the growth of shishes.



intensity suggests the evolution of a shish-kebabmorphol-

ogy in the presence of nanoparticles.

The increasing concentration of nanoparticles enhances

the formation of shishes in the structure, thus ultimately

leading to formation of oriented kebabs. In all cases, streak-

like scattering occurs in the equator as a result of the

formation of multiple shishes in the polymer melt. Such

scattering at the equator is soon followedby themeridional

maxima, which leads to the formation of shish-kebab

structures.

The Figure 9 represents the development of equatorial

intensity that occurs after the application of shear (100 s�1

for 1 s, at the isothermal crystallization temperature of

136 8C) in the polymer melts that consist of differentconcentrations of nanoparticles: SWNTs and zirconia. The

presence of nanoparticles leads to an increase in the

intensities as compared to the pure polymer melt.

Compared to the presence of SWNTs in the polymer

matrix, the equatorial intensity in the pure polymer

grows as a function of time. In the presence of SWNTs,

near constant intensities that correspond to the polymer

melt indicates the stability of these flow-induced pre-

cursors. In the case of the polymer melt having

different concentrations of zirconia nanoparticles, the

integrated intensity that corresponds to equatorial

region increases as a function of time, which indicates

the possible growth of crystalline shishes as a function of

time.

Figure 10 shows the 2D SAXS patterns collected at 60 8Cfor different concentrations of nanoparticles. All patterns

show a partially oriented morphology, which is basically a

mixture of oriented and isotropic distribution of lamellae.

This indicates the existence of shish-kebab and spherulitic

structures.

It isworthnoting that the increase in thescattering in the

meridian occurs as a result of nucleation and growth of

unoriented crystals and possible re-organization of chain

segments or oriented crystals within the lamellae, which

causes the increase in electron density.

Determination of Crystal Orientation: Along theEquatorial and Meridional Direction

Herman’sorientationparameter,[31] isused inour studies to

determine the orientation of PE with different concentra-

tions of nanoparticles. The detailed analysis of our SAXS

data gave the orientation of crystalline lamella as demon-

strated elsewhere.[32] Herman’s orientation parameter ( fh)

can be defined as:

fh ¼3½cos2 f� � 1ð Þ

2

� �(1)



Figure 10. SAXS patterns of the polymer melt in the presence of different concentrations of nanoparticles taken at 60 8C.

2182

where the mean square cosine of the azimuthal angle can

be approximated as:

Macrom

� 2009

cos2

f ¼

Rp=20

IðuÞ: cos2 u: sin udu

Rp=20

IðuÞ: sin udu(2)

The2DSAXSpatterns are analyzedby taking cake sizes in

specific ways as shown in Figure 11a. The patterns are

integrated azimuthally in an anticlockwise direction from

u¼ 08 at the equator to get a full intensity profilewithin thecakes. The values for the integration used in the calculation

for Herman’s orientation function are taken from the

intensity that corresponds to the azimuthal angle (u¼ 08) toazimuthal angle (u¼ 908). Analyzing thepattern in thiswaygives the values of Herman’s factor ( fh), i.e., fh¼ 1when thescattered intensity is concentrated only perpendicular to

the flow direction in the 2D SAXS patterns, fh¼ –0.5, whenthe scattered intensity is parallel to theflowdirection in the

2DSAXSpatterns, and fh¼ 0,whenthescattered intensity isdiffused (isotropic in nature) and spread across the pattern

Figure 12 shows the comparison of Herman’s orientation

factors obtained for the polymer melts in the presence of

different concentrations of nanoparticles through the

regressive analysis of SAXS data. Within 50 s, after the

appliedshear step, it isapparent that the intensityalong the

equator exists,whichgives apositivevalue to theHerman’s

orientation. When compared with the different nanopar-

ticles, it is evident that theorientation factor in thepresence

ofnanotubes ishigher than in thepresenceof zirconia.With

the increasing concentration of nanoparticles the equator-

ial orientation arising as a result of shish formation

increases, which suggests that the presence of nanoparti-

cles favours the shish formation. Shish formation further

promotes the kebab morphology, which leads to the

evolution of intensity along the meridian. As anticipated,

a greater amount of shish provides greater nuclei for

crystallization, thus the higher orientation along the

meridian. This becomes apparent in Figure 12 after nearly

100 s of applied shear.

ol. Chem. Phys. 2009, 210, 2174–2187

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

It is evident from the experiments and Figure 3, 7, 8,

and 12 that the rapid increase of intensity in the meridian

(kebabs) as a function of time because of accelerated

crystallization kinetics dominates the orientation factors.

In general, flow promotes the orientation factors, the fhincreaseswith the increase of nanoparticles in either of the

cases (SWNTs and zirconia). The early fall of orientation

factors in the presence of nanoparticles suggests the

possible role of nanoparticles in shifting the crystallization

process to a higher temperature (as is consistent with our

rheological studies). The SWNTs orient more polymer

crystals parallel to the flow direction in the early stages

as compared zirconia, which we attribute to the alignment

of the nanotubes parallel to the flow direction.

The influence of nanoparticles in the development of

oriented structures is further strengthened by the

quantitative analysis performed below. From 2D SAXS

patterns (refer to Figure 3, 7, and 8), it is obvious that the

intensity along the equator and meridian increases with

time. For this reason, we considered both the intensities

obtained in the equator and meridian for calculation of

the total orientation fraction acquired from experiments

in previous studies of our research group.[33] The half of

the 2D SAXS patterns that correspond to two quadrants

(i.e., from u¼ 08 to 1808) only is considered to yield theintensity value related to different regions. Thus the

intensity scattered due to oriented crystallites is con-

sidered to be the sum of the intensity obtained in the

equator and meridian.

Ioriented ¼ Iequator þ Imeridian (3)

where, Iequator and Imeridian represent the intensities related

to the corresponding regions and can be considered as the

intensity of oriented crystallites obtained by summation.

Thus, the fraction of oriented crystals (F) in the SAXS

patterns is:

F ¼ IorientedItotal

(4)

Similar to Figure 12, Figure 13 shows that with an

increasing amount of nanoparticles the overall orientation

DOI: 10.1002/macp.200900364


Figure 11. The SAXS data analysis performed for the estimation ofHerman’s orientation factor ( fh) using two dimensional patterns.a) Shows the cakes considered for calculation as defined in fourquadrants for complete azimuthal integration (u¼08 to 3608) toobtain the azimuthal distribution of integrated intensity, whereu¼08 and 908 represent the equator and meridian, respectively.The integrated intensity corresponding to u¼908 to 1808 isconsidered to obtain the orientation factor. The SAXS patternshown here is taken at 60 8C. b) The illustrated azimuthal distri-bution of neat PE as a function of intensity obtained from theSAXS data analysis at different times for an isothermal crystal-lization temperature of 136 8C. The distinctive peaks correspond-ing to the equator and meridian can be noticed in the intensitydistribution.

Figure 12.Herman’s orientation factor ( fh) for the polymermelt inthe presence of nanoparticles. The orientation is dominatedparallel to the flow direction (shishes) in the initial stagesobtained from equatorial scattering, while the orientation isdominated by the growth of polymer crystals (kebabs) as afunction of time perpendicular to the flow as a result of mer-idional scattering.

Figure 13. The orientation fraction (F) estimated from the inten-sities obtained in the equator and meridian direction.

increases. For the amount of nanoparticles used, compared

to the zirconia, the SWNTs seem to bemore effective in the

development of oriented structures. What follows is the

quantitative analysis on the length of the shish structures

that could be realized in the presence of nanoparticles.



Analysis for Estimation of Length of Shishes UsingRuland’s Streak Method

The length of shishes present in the obtained shish-

kebab structures are estimated using Ruland’s streak

method.[34–38] The integrated width of the angular dis-

tribution of the scattered intensity Bobs is used to estimate

the true width of the orientation distribution Bf (misor-

ientation) and the average length (L) of the shishes aligned

in the direction of the c-axis. The azimuthally distributed



2184

scans of intensities at different q values are analyzed using

the Lorentz function to yield the average width of the

angular distribution. The width of the equatorial streaks in

the reciprocal space can be related to obtain the length of

the shish (L). The relationship between the L and Bf can be

approximated as:

Figatflucstawitwid

Macrom

� 2009

q2B2obs ¼1

L2þ q2B2f (5)

Figure 15. Average length of the shish as a function of differentconcentrations of nanoparticles. (*-represents the shish lengthin the presence of SWNTs, &-represents the shish length in thepresence of zirconia).

The relationship gives a linear plot obtained between

q2B2obsand q2. The shish length (L) and degree of misorienta-

tion (Bf) are determined by the linear least square fitting

applied to our data. In the relation, q¼ 4p sin u/l (where u isthe scattering angle, q is the scattering vector, and l is thewavelength). The length of the shish (L) is determined from

the intercept of the linear plot, while Bf represents the

misorientation parameter. The interpretation of ‘L’ for the

shish is considered to be orderly aligned in the direction

of c-axis.

Figure 14 shows the typical azimuthal distribution fitted

with a Lorentzian function to estimate the integration

width of the angular distribution (Bobs) required to obtain

the lengthof shish (L). Theobtained integrationwidthof the

angular distribution (Bobs) is plotted as a function of

scattering vector (q) based on Equation (5). The average

values of the obtained shish length areplotted as a function

of different concentration of nanoparticles (see Figure 15).

The average values of shish length are found to range

between190–250nm.Thepresenceof SWNTs favours shish

ure 14. Typical distribution of the azimuthal scattered intensitythe equator in the presence of streaks. The intensity usuallytuates due to the metastable nature of these FIPs in the intialges of crystallization. For analysis, the data points were fittedh a lorentzian function to obtain proper values of the integralth of the angular distribution (Bobs).

ol. Chem. Phys. 2009, 210, 2174–2187

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

formation, as evident by the rapid increase in shish length

at a low concentration as compared to that of zirconia

nanoparticles. In the presence of both nanoparticles, the

average length of the shish grows with an increase in the

concentration. The increase in the shish length further

supports the observations in Figure 12where it is observed

that the growth of kebabs occurs earlier in the presence of

nanotubes than in zirconia, and is concentration depen-

dent. The kebabs grow rapidly in the perpendicular

direction to the shish. In contrast to some studies[39] (where

the shrinking of shish length as a function of time at the

isothermal crystallization temperature is observed), our

studies showthe steadygrowthof shishwithin the rangeof

20–30nm under isothermal conditions as a function of

time. Another interesting feature is the values of mis-

orientation obtained from the Ruland’s streakmethod. The

degree ofmisorientation (Bf) decreaseswith the increase in

nanoparticle concentration. The average value of misor-

ientation of the shishes is lower in the presence of

nanoparticles, which indicates the better orientation in

the obtained shish-kebab structures. These results are in

agreement with the conclusions drawn from Figure 12.

Rheological Properties of PE Melt in Presence ofNanoparticles

Before analyzing Figure 16, here we recall some of the

salient findings reported in ref.[28,29]. Experiments per-

formed on PEs in the presence of nanotubes, where the PEs

have a molar mass greater than a million g �mol�1, show a

DOI: 10.1002/macp.200900364


Figure 17. Storage modulus G0 of PE in the presence of zirconiananoparticles as a function of frequency at two different tem-peratures: a) at 142 8C and b) at 160 8C.

Figure 16. Storage modulus G0 of PE–SWNT composites as afunction of frequency for at different temperatures: a) at142 8C and b) at 160 8C.

network formation. In rheological studies, such a network

formation is realizedat lowfrequencies,where themodulus

of the polymer–SWNTs network is much lower than the

networkarisingbecauseof entanglements.Oneof theother

findings reported in such studies is the decrease in viscosity

at a specific concentration of nanotubes (0.2%). One of the

explanations provided for the decrease in viscosity is the

selective adsorptionof ahighmolarmass component in the

polydisperse PE.[28,29]

Figure 16, shows the change in the storagemodulusG0 at

two different temperatures (T¼ 142 and 160 8C) fordifferent concentrations of SWNTs in PE. Considering the

lowmolarmass of PEused for our studies, unlikeUHMWPE,

in the presence of nanotubes no plateau at the low

frequency region is observed.However,with the increasing

amount of nanotubes, a parallel shift in the modulus as a

function of frequency becomes apparent. Similar to earlier

reportedfindings, thepresenceofnanotubes in thepolymer



matrix also shows a non-linear increase in viscosity with

the increasing amount of nanotubes. The non-linear

increase in viscosity, complemented by the parallel

shift in the modulus at different frequencies, shows

temperature dependence. That is, with the increase in

temperature from 142 to 160 8C, the effect of a drop inviscosity is suppressed.

Such a drop in viscosity is attributed to selective

adsorption of high molar mass chains to the dispersed

nanoparticles. Considering the similar molecular config-

uration of the nanotubes and PE chains, compared to the

zirconia andPEas shown inFigure 17, a lowerbarrier for the

selective adsorption between the nanotube and PE could be

anticipated. Such a possibility becomes apparent on

comparing the changes that occur in the viscosity with

the increase in concentration of zirconia nanoparticles in

the polymer matrix, where with an increase in the

concentration of zirconia a regular increase in themodulus



Figure 19. Evolution of the storage modulus, G0, during cooling(slow cooling rate of 0.1 8C �min�1) from 142 8C at constant strain(g ¼ 2.0%) and frequency (10 rad � s�1): a) PE in the presence ofSWNTs and b) PE in the presence of zirconia nanoparticles.

Figure 18. Complex viscosity as a function of nanoparticles at twodifferent temperatures. a) For PE–SWNT composites and b) ForPE–zirconia composites.

2186

occurs and no drop in viscosity is observed,[28,29,40–43] as

shown in Figure 18.

To gain more insight into the influence of the selective

adsorption crystallization behaviour on cooling from the PE

melt in the presence of nanoparticles, rheological studies

have been employed. From Figure 19 it is apparent that the

onset of crystallization shifts to higher values with an

increase in the amount of nanotubes from 0.1 to 0.6wt.-%.

Whereas the shift in the onset of crystallization in the

presence of zirconia is realized above 0.5wt.-% of the

dispersed particles in the polymer matrix. The change in

slope in the modulus build up with crystallization further

strengthens the pronounced effect of the nanoparticles.

Below 122 8C, because of the possibility of slippage, datacannot be reliable.

Conclusion

An X-ray synchrotron facility is utilized to investigate the

origin of shish-kebab structures in the presence of



nanoparticles in our studies. The results suggest the

increase in the amount of shish-kebab structures in

the presence of nanoparticles in the polymer matrix. The

intense scattering of intensity at the equator in the early

stages of crystallization in the form of streaks indicate the

presence of a shish, while a steady increase in intensity in

themeridiansuggests thegrowthofkebabs. Theestimation

of Herman’s orientation factor indicates a higher degree of

chain orientation in the presence of nanoparticles in the

early stages of crystallization. The chain orientation in the

presence of SWNTs is found to be more as compared to

zirconia nanoparticles in the polymer matrix. The orienta-

tion fractions are found to increase in the presence of

nanoparticles. Ruland’s streak analysis shows the increase

in shish length because of the presence of nanoparticles in

the polymer. The higher shish length in the presence of a

small concentration of SWNTs (between 0.1 to 0.6%) as

compared to zirconia nanoparticles in the polymer

DOI: 10.1002/macp.200900364


indicates the possible alignment of SWNTs in the flow

direction. The shish length increases with the amount of

nanoparticles. The higher shish length favours the growth

of kebabs in the later stages (> 100 s) of crystallization. The

rheological study provides insight into the selective

adsorption of polymer chains to the nanoparticles. A

non-linear increase in the viscosity is observed with an

increase in SWNTs loading in the polymer. The drop in the

viscosity in the presence of SWNTs is suppressed at a higher

temperature, whereas, a regular increase in the viscosity is

observedwithan increased zirconia loading in thepolymer.

The shift in the onset of crystallization is much more

pronounced in the presence of SWNTs (between0.1 to 0.6%)

as compared to zirconia in the polymer matrix, where it is

realized above 0.5%. The results conclusively demonstrate

the role of nanoparticles in crystallization.

Acknowledgements: The authors acknowledge the ESRF andNetherlandsOrganization of Scientific Research (NWO) for grantingbeam time. The authors thank Frederik Geomoets from PlasticResearch Division, Dow Benelux BV, for providing PE and GPC datafor the study.

Received: July 10, 2009; Published online: November 20, 2009;DOI: 10.1002/macp.200900364

Keywords: flow induced crystallization; nanoparticles; polymermelts; rheology; shish-kebabs

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