influence of nanoparticles on the rheological behaviour and ...of the flow induced crystallization...
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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
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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-
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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
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N. Patil, L. Balzano, G. Portale, S. Rastogi
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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,
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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
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Influence of Nanoparticles on the Rheological Behaviour and . . .
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.
Macromol. Chem. Phys. 2009, 210, 2174–2187
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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
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N. Patil, L. Balzano, G. Portale, S. Rastogi
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.
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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
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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.
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Influence of Nanoparticles on the Rheological Behaviour and . . .
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
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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
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N. Patil, L. Balzano, G. Portale, S. Rastogi
Figure 7. 2D SAXS patterns of broad MWD PE in the presence of SWNTs at 136 8C at selected times after application of shear.
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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.
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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
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Influence of Nanoparticles on the Rheological Behaviour and . . .
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.
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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)
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N. Patil, L. Balzano, G. Portale, S. Rastogi
Figure 10. SAXS patterns of the polymer melt in the presence of different concentrations of nanoparticles taken at 60 8C.
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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.
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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
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Influence of Nanoparticles on the Rheological Behaviour and . . .
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.
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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
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N. Patil, L. Balzano, G. Portale, S. Rastogi
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).
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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
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Influence of Nanoparticles on the Rheological Behaviour and . . .
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
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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
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N. Patil, L. Balzano, G. Portale, S. Rastogi
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.
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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
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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
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Influence of Nanoparticles on the Rheological Behaviour and . . .
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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