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A Spectroscopic Approach for StructuralCharacterization of Polypropylene/Clay Nanocomposite
Saikat Banerjee,1 Mangala Joshi,2 Anup K. Ghosh1
1Centre for Polymer Science and Engineering, Indian Institute of Technology, New Delhi 110016, India
2Department of Textile Technology, Indian Institute of Technology, New Delhi 110016, India
This study focuses on the degree of dispersion andstructural development of organomodified MMT clay(OMMT) during processing of polypropylene clay nano-composites using both conventional and nonconven-tional characterization techniques. PP-g-MA and Cloi-site 15A were melt blended with three different gradesof PP separately in a micro-twin screw compounder atselected screw speed and temperature. The clay wasmodified with fluorescent dyes and the adsorbed dyecontent in the clay gallery was estimated by using UV-spectrophotometric method. The effects of residencetime and molecular weight of the PP matrix on the claydispersion were studied. The extent of dispersion andexfoliation of the clay in polymer matrix determinedfrom the torque versus time data obtained from micro-compounder. It was further supported by XRD, SEM,TEM, and DSC analysis. Offline dielectric and fluores-cence spectrophotometric studies were also carriedout. Changes in dielectric constant and dielectric losswith both frequency and temperature yielded quantita-tive information about the extent of clay exfoliation andintercalation in the polymer matrix. It was observed thatwith an increase in MFI (decrease in molecular weight)and mixing time, the extent of clay dispersion and exfoli-ation were also improved due to easy diffusion of poly-mer chains inside clay gallery. POLYM. COMPOS., 31:2007–2016, 2010. ª 2010 Society of Plastics Engineers
INTRODUCTION
Many of the desirable properties of polymer nanocom-
posites are related to the quality of the dispersion, includ-
ing polymer intercalation into the clay galleries and/or
exfoliation (delamination) into individual clay platelets [1,
2]. Measuring the amount of exfoliation that occurs dur-
ing compounding of clay with the polymer resin is a pre-
requisite to establishing a knowledge base of nanocompo-
site material properties and their functional relationship to
the extent of exfoliation. Simultaneous rheo-X ray studies
show that the best level of the dispersion requires a bal-
ance between the diffusion of polymer chains in interlayer
spacing and mechanical shearing to break up the clay tac-
toids. According to the various authors, the medium ma-
trix viscosity and the two-step processing (low shear fol-
lowed by high shear) gave the optimum results [3–6].
Another important issue is to control the degree of
clay dispersion. Transmission electron microscopy (TEM)
and X-ray scattering provide a qualitative local structural
characterization. As a result, an extensive imaging is
required to ensure a representative view of the whole ma-
terial. Rheological [7, 8] and mechanical tests [9] probe
the bulk of the nanocomposite material and sense the
changes in the dispersion process of clay agglomerates on
a large scale. New methods still need to be developed to
complement these nanocomposite characterization techni-
ques, especially methods that quantify the degree of nano-
dispersion in the bulk polymer.
Twin screw extruders and micro-compounders are the
most common compounding machines for making nano-
composites. The torque rheometry is an effective tool for
predicting the processing characteristics of thermoplastic
polymers [10]. It provides continuous monitoring of tor-
que and temperature data during compounding which is a
measure of processability [11].
The photo-functions of optical probes in intercalation
compounds have been a topic of great interest [12]. Cati-
onic fluorescent probes adsorb strongly on clay mineral
surfaces and their intermolecular interactions and surface
organizations are complex functions of the concentration,
stoichiometry, and charge density. In case of other optical
methods, the probing light must transmit through the mate-
rial, but fluorescence measurements can easily be carried
out by excitation and detection from one side only [13].
Dye-aggregate formation is common and their type is con-
trolled by the density of negatively charged sites at the clay
surface [14–16]. This study aims to obtain fluorescence
Correspondence to: Anup K. Ghosh; e-mail: anupkghosh@gmail.com
Contract grant sponsors: Department of Science and Technology, Gov-
ernment of India, Reliance Industries Ltd India.
DOI 10.1002/pc.20998
Published online in Wiley Online Library (wileyonlinelibrary.com).
VVC 2010 Society of Plastics Engineers
POLYMER COMPOSITES—-2010
spectra of methylene blue (Fig. 1a) as a function of proc-
essing parameters and the resultant clay structures.
Although the data on dielectric response of polymer
are available [17–19], studies on the dielectric properties
of polymer-clay nanocomposites are rare [20]. When a
polymer either in solid or in melt form is subjected to an
external electrical field, the bound charges are displaced
and dipoles are oriented as a function of frequency of
applied external electric field and temperature [21]. This
behavior can be described by the complex relative permit-
tivity e* where, e* ¼ e0 2 ie’’. The real part e0, is associ-
ated with the polarization or capacitance of the material
and the imaginary part e’’, the dielectric loss, is associated
with the conductance. Dielectric constant is a measure of
the charge retention capacity of a medium. Materials sus-
ceptible to dielectric loss convert electric energy into
heat. It is recently reported by Wang et al. [20] that
dielectric constant at low frequencies gets completely sup-
pressed due to the intercalation of clay.
The aim of this article is to study the influence of
processing parameters on the degree of dispersion of clay
stacks in polymer matrix with respect to the rheological,
structural, and optical properties and to develop an
approach for the rapid analysis of the progress of disper-
sion [22]. An attempt has been made to study the degree
of dispersion of clay via intercalation/exfoliation in poly-
propylene/clay nanocomposites using conventional techni-
ques such as XRD, SEM, TEM as well as nonconven-
tional techniques such as dielectric and fluorescent techni-
ques. The effect of various material parameters such as
melt flow index of the polymers, use of compatibilizer
(PP-g-MA) and process parameters on degree of clay dis-
persion has been investigated systematically.
EXPERIMENTAL
Materials
The materials used for this study are three selected
commercial grades of polypropylene (PP) (REPOL H020
EG, REPOL H110 MA, REPOL H350 FG) having MFI
of 2, 11, and 35, respectively, obtained from Reliance
Industries, Mumbai. Maleic anhydride grafted polypropyl-
ene (PP-g-MA) (Fusabond PMD 511D) from DuPont was
used as a compatibilizer. The organoclay (Cloisite 15A),
modified with octadecylamine (Fig. 1b), was supplied by
Southern Clay. Methylene blue (MB) was procured from
Merck India Chemicals. The structural formula of the dye
is given in Fig. 1a. The aromatic moiety of MB contains
N and S atoms and the dimethylamino groups are
attached to the aromatic unit. The aromatic moiety is pla-
nar and the molecule is positively charged.
Modification of Organoclay With Dye
The MB dye and Cloisite 15A were taken to achieve
complete ion exchange. MB was dissolved in measured
95% hot ethanol and to this solution the organoclay was
added continuously with rapid stirring. The volume was
adjusted with distilled water and heated at 708C for 1 hr
FIG. 1. Structure of: (a) Methylene blue dye and (b) organic modifier
of Cloisite 15A (HT: hydrogenated tallow, �65% C18, �30% C16,
�5% C14; Anion: Cl2).
FIG. 2. Dyed clay and the filtrate.
FIG. 3. DSM-MICRO5: conical corotating twin screw micro-
compounder.
2008 POLYMER COMPOSITES—-2010 DOI 10.1002/pc
for secondary ion exchange. The slurry was allowed to
stand in a dark place for 3/4 days to have equilibrium and
then filtered. The residue was washed thoroughly with
50% hot ethanol until the filtrate was colourless. Finally,
the modified clay was dried in vacuum oven at 808C for
24 hrs and ground in a ball mill. The filtrate (Fig. 2) was
collected and the amount of dye exchanged to clay gallery
was estimated by using UV-spectrophotometric method.
Particle Size Analysis
The average particle size of the organoclay and dyed
clay was determined by using BECKMAN COULTER
Delsa NanoC Particle Analyzer. A stable dispersion of 0.1
wt% clay in aqueous medium was prepared by constant
stirring and kept for a long time. Average particle diame-
ters of 8.73 lm and 3.56 lm were found for Cloisite 15A
and dyed clay, respectively.
Nanocomposite Processing
The melt mixing was done in a DSM-MICRO5 conical
corotating twin screw microcompounder (Fig. 3). The proc-
essing temperature was kept at 2008C to ensure proper vis-
cosity for the mixing while at the same time minimizing
degradation of both the polymer and dye. Before melt proc-
essing, all the components were dried at 808C for 24 hours
in a vacuum oven. The rotational screw speed was set at
100 rpm PP (11 MFI), PP-g-MA and dyed clay were com-
pounded for 4, 8, 12, and 16 mins, time intervals as sepa-
rate batches. In all the batches, the optimized concentration
of PP-g-MA and dyed organoclay were added in 3.5 and
4% by weight, respectively. To study the effect of matrix
molecular weight on clay dispersion and exfoliation, poly-
propylene grades having MFI of 2, 11, and 35 were com-
pounded for 8 min with same concentration of PP-g-MA
compatibilizer and dyed clay as mentioned earlier.
The compounded strands were chopped and compres-
sion molded at 2008C under a load of 10,000 lbs in a Car-
ver Inc Laboratory Press to prepare samples for fluores-
cence, dielectric and XRD analysis. Cold water was used
to bring the platens to room temperature while full hold-
ing pressure was maintained.
Characterization Techniques
Structural characterization (degree of delamination and
dispersion) was carried out using X-ray diffraction
(XRD), scanning electron microscopy (SEM), TEM, fluo-
rescence spectrophotometry, and dielectric techniques.
Wide angle X-ray diffraction (WAXD) patterns for com-
pression molded samples were recorded by using Cu Karadiation (40 kV, 30 mA) generated by an X-ray diffrac-
tometer (X’Pert PRO); corresponding data were scanned in
the reflection mode over a 2y angle of 22108 to character-
ize the inter layer spacing (d-spacing) of the dyed MMT af-
ter compounding. The scanning rate was 0.018/sec.
FIG. 4. FTIR spectra of: (a) Methylene blue, (b) Cloisite 15A, and (c)
MB treated Cloisite 15A.
FIG. 5. XRD patterns of: (a) cloisite 15A and (b) dyed cloisite 15A.
[Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
TABLE 1. XRD results for nanocomposites having different MFI grades of polypropylene (2, 11, and 35 MFI) with different mixing time (4, 8, 12,
and 16 mins).
Sample Dyed Clay
PP/Clay (4 wt %) nanocomposites
(8 minutes mixing) Mixing time (PP having 11 MFI grade)
PP(2 MFI) PP(11 MFI) PP(35 MFI) 4 mins 8 mins 12 mins 16 mins
2y-value (8) 2.93 2.67 2.60 2.59 2.83 2.60 2.50 a
d-spacing (A) 30.08 33.48 33.94 34.12 31.21 33.94 35.31 A
a No peaks were obtained.
DOI 10.1002/pc POLYMER COMPOSITES—-2010 2009
Scanning electron microscopy (SEM) was used to char-
acterize the morphology of the nanocomposites. Extruded
polymer strands were immersed in liquid nitrogen for
some time and a brittle fracture was performed. The frac-
tured surfaces were mounted on a sample holder and sil-
ver sputtered after proper drying. The samples were
scanned under Zeiss EVO 50 microscope at an acceler-
ated voltage of 20 kV and 6.5 k magnification.
The dispersion and exfoliation of the OMMT platelets
in the blend was studied by means of TEM. Freshly cut
glass knives with cutting edge of 458 were used to get the
cryosections of 50 nm thickness by using a Leica Ultracut
UCT microtome. JEOL-2100 electron microscope (Tokyo,
Japan) having LaB6 filament and operating at an acceler-
ating voltage of 200 kV was utilized to obtain the bright
field images of the cryo-microtomed samples.
The crystallization and melting were studied by using
Perkin–Elmer Pyris 6 DSC instrument. The samples were
heated from 35 to 2008C at the rate of 108C/min under
nitrogen atmosphere and kept at this temperature for 10
min before cooling down to assure that the materials
melted uniformly, to destroy any residual nuclei before
FIG. 6. Force-time curves: (a) mixture of PP (11 MFI) and dyed Cloisite 15A (4 wt %) and (b) PP (2MFI),
PP (11 MFI) and PP (35 MFI) with dyed clay (4 wt %).
FIG. 7. SEM microphotographs of PP (11 MFI)/PP-g-MA/dyed Cloisite 15A compound obtained among
mixing time of: (a) 4 min, (b) 8 min, (c) 12 min, and (d) 16 min.
2010 POLYMER COMPOSITES—-2010 DOI 10.1002/pc
cooling at the desired rate and to eliminate the thermal
history. The sample was then cooled down to room tem-
perature at a cooling rate of 58C/min.
The thermal stability of the methylene blue, organo
clay, and dyed clay was examined by using Perkin–Elmer
Pyris 6 TGA instrument. The samples were heated up to
8008C under nitrogen atmosphere.
The fluorescence spectrophotometric study of the com-
pression molded disc samples was carried out using Fluo-
rolog HORIBAJOBIN YVON spectrophotometer. Xenon-
arc lamp was the source. The samples were excited using
a ‘2,1’ slit at a wavelength of 400 nm.
Dielectric parameters, such as capacitance and dielec-
tric loss were measured by a Hewlett-Packard 4192A LF
Impedance Analyzer at various frequencies (0.1 to 10
MHz) at temperatures of 40 to 808C. Dielectric constants
(er) of the specimens were calculated by the equation:
C ¼ er e0 (A/d). e0 is vacuum permittivity and equals
8.85 3 10212 F/m. C is the capacitance, A is the elec-
trode area and d is the thickness of the specimen. The
samples were in circular film shape having a thickness of
35–60 lm and coated with silver.
RESULTS AND DISCUSSION
FTIR Analysis
The modification of the clay with methylene blue dye
is analyzed from the FTIR data shown in Fig. 4. Methyl-
ene blue shows a characteristic peak of tertiary and higher
amines at 3458 cm21. The peaks due to asymmetric and
symmetric alkyl C��H stretching come around 2919 and
2851 cm21, respectively. Peaks below 1000 cm21 appear
due to the C��H bending of the alkyl groups. In the spec-
tra of organically modified clay, a broad peak around
3400 cm21 may be due to the presence of hydrogen
bonded and nonbonded ��OH functional groups on the
layered silicate surface. The peaks below 1000 cm21 are
due to the C��H bending of the long chain hydrocarbon
(tallow) of the organic modifier. Both peaks of substituted
amine (3632 cm21) and ��OH (3433 cm21) in the spectra
are prominent. It also shows the characteristics C��H
stretching. The peak at 1603 cm21 (Fig. 4c) due to aro-
matic ring shows the presence of dye inside the clay. All
these results corroborate the modification of the clay with
the methylene blue dye.
XRD Analysis
Figure 5 shows the X-ray diffractograms of the organi-
cally modified Cloisite 15A and dyed clay. The basal
spacing (d100-plane) for Cloisite 15A is observed around
30.31 A (2y � 2.928), whereas for dyed clay it is around
30.08 A (2y � 2.938). This decrease in d-spacing is due
to the fact that, during secondary ion exchange some of
the bulkier organic modifiers with long chain tallow
inside the clay gallery are replaced by planer MB dye
molecules.
Table 1 shows the basal spacing of dyed clay having
values of 30.08, 33.94, and 34.12 A when compounded
with PP grades having MFI values of 2, 11, and 35,
respectively. It is evident from this data that, MFI vis-
FIG. 8. SEM microphotographs of PP-g-MA/dyed Cloisite 15A mixture with PP of grades: (a) 2 MFI, (b) 11 MFI, and (c) 35 MFI.
FIG. 9. TEM microphotographs of PP (11 MFI)/PP-g-MA/dyed Cloisite 15A obtained by mixing for: (a) 4 mins, (b) 12 mins, and (c) 16 mins.
DOI 10.1002/pc POLYMER COMPOSITES—-2010 2011
a-vis molecular weight of the matrix polymer is an im-
portant factor for better dispersion and exfoliation of
clay during processing. Increase in MFI implies a
decrease in molecular weight which has been found to
increase the basal spacing. The presence of peaks also
shows that even after 8 min of mixing time, the clay is
not well dispersed and not exfoliated. It can be consid-
ered as a combination of intercalated and exfoliated
structure. As the molecular weight of the polymer
decreases, the tendency of molecular chains to entangle
with each other decreases. So the molecular chains eas-
ily diffuse inside the clay galleries, which results in bet-
ter exfoliation.
Table 1 also shows the effect of mixing time on clay
dispersion and exfoliation in PP with 11MFI matrix. The
basal spacing of the dyed clay was 30.08 A. After mixing
for 4 min, the spacing increased to 31.21 A. This increase
is due to intercalation of polymer chains into clay gal-
leries. With the increase in the mixing time the sharp
peaks were flattened and the d-spacing also increased. It
showed occurrence of exfoliation and the structure is con-
sidered to be a mixture of intercalated and exfoliated
structure. For samples prepared with 16 min of mixing
time, no prominent peak was observed which means that
the clay is mostly exfoliated.
Analysis of Data for Micro-compounder
A simple indicator of the extent of clay exfoliation
within polypropylene matrix is the steady-state torque
recorded during melt compounding. As shown in Fig. 6a,
the torque/force curve of PP became relatively stable after
14 min of mixing and showed only a slight downward
trend with time, indicating little change in the viscosity of
the sample. The maximum force was 1189 N and it came
down to an average of 955 N after 14 min. Maximum tor-
que indicates that the clay was in agglomerated condition
initially and it reduced in size with continuous application
of shear. The compound reached maximum exfoliation af-
ter 12 min, as there was no prominent change in torque/
force value beyond 14 min of mixing.
Figure 6b shows the effect of molecular weight of PP
matrix on clay dispersion. The initial maximum torque/
force is higher for the PP having higher molecular weight
(2 MFI) as the melt viscosity is higher. During mixing,
the higher MFI PP reaches to its lowest torque/force value
earlier than the others. This may be due to the fact that,
PP with lower molecular weight diffuses more into the
clay gallery. It results in greater extent of intercalation
and exfoliation.
Morphological Analysis
SEM Analysis. Figures 7 and 8 show the SEM micro-
graphs of PP/clay nanocomposites obtained by varying
mixing time and MFI, respectively. The morphologies
were found to be almost similar for sample collected after
4 min (Fig. 7a) and 8 min (Fig. 7b). The clays are quite
agglomerated and not well dispersed. After 12 min of
mixing, the size of nanoclay aggregates reduced signifi-
cantly (Fig. 7c). Figure 7d shows the clay to be highly
dispersed in the matrix after 16 min.
The effect of molecular weight on extent of clay dis-
persion can be seen from Fig. 8a–c. As seen in the figure,
the clays are not well dispersed in the PP matrix having
higher molecular weight (MFI of 2). As the molecular
FIG. 10. TEM microphotographs of PP-g-MA/dyed Cloisite 15A mixture with: (a) 35 MFI, (b) 11 MFI, and (c) 2 MFI of PP.
TABLE 2. Crystallization and melting behavior of the composites with different mixing time.
Sample Tc (8C) Tc (onset) (8C) [Tc (onset) – Tc] (8C) Tm (8C) DHc (J/g) Xc (%)
PP (11 MFI) 115.3 120.4 5.1 165.2 120.4 33.8
PNC/4 mins 118.9 123.9 5.0 165.9 100.4 36.1
PNC/8 mins 119.1 124.0 4.9 167.2 90.8 34.2
PNC/12 mins 119.4 124.2 4.8 166.0 97.8 35.4
PNC/16 mins 121.5 125.5 4.0 166.4 109.4 42.9
DHm (fully crystalline PP) ¼ 209 J/g.
2012 POLYMER COMPOSITES—-2010 DOI 10.1002/pc
weight decreases, the polymer chains become easily avail-
able for diffusion in the clay gallery. Hereby resulting in
better dispersion and exfoliation of the clay (Fig. 8c).
TEM Analysis. The information on the morphology of
the nanocomposites was obtained with TEM [23]. The
images shown in Fig. 9 illustrate the effects of the mixing
time on the development of the nanostructure. The white
part corresponds to the polymer matrix phase whereas the
black lines correspond to OMMT layers. In the sample
compounded for 4 min (Fig. 9a), the TEM picture indi-
cates presence of maximum agglomerates of clay par-
ticles. Figure 9b and c compares two specimens that ex-
hibit a heterogeneous i.e. combination of intercalated-
exfoliated structure formed by both stacks and individual
platelets. All the clay particles seem to be well exfoliated
along with a few intercalated stacks. Sample compounded
for 16 min shows better exfoliation than sample com-
pounded for 12 min. So, from the TEM micrographs it
may be concluded that, the clay platelets become more
uniformly dispersed as mixing time increases and for
microcompounding of this system, 12 min is the optimum
mixing time for better dispersion and exfoliation of the
clays in the polypropylene matrix.
Figure 10a–c shows the effect of molecular weight on
clay dispersion in polymer matrix. In case of higher mo-
lecular weight (MFI of 2) PP, the extent of intercalation
and exfoliation is low. The parallel black clay layers
show the presence intercalated clay tactoids (Fig. 10a).
As the molecular weight decreases (increase in MFI),
more polymer chains are able to diffuse in the clay gal-
leries. It results in the peeling off of the individual clay
layers in the polymer matrix (Fig. 10b and c).
Thermal Analysis
Crystallization of polypropylene with nanoclay has
been studied extensively [24–26]. Generally, the nano-
scale particulates act as nucleating agents, facilitating the
heterogeneous crystallization process. Table 2 shows an
increase in both crystallization temperature and onset tem-
perature for crystallization with increase in mixing time.
Moreover, during cooling the crystallization starts earlier.
This is due to the fact that, the well dispersed particulates
lead to an increased number of sites available for nuclea-
tion, therefore enhancing the crystallization rate and alter-
ing the kinetics and geometry of crystal growth (see Fig.
11). As the mixing time increase, the clays experience
more shear which results in exfoliated state in the com-
pound. This change in clay morphology from intercalation
to exfoliation exposes more surfaces to interact with the
polymer chains which lead to an earlier crystallization.
The presence of exfoliated clay not only influences the
degree of crystallinity, but also the rate of crystallization.
A decrease in [Tc(onset)-Tc] value with mixing time (Table
2) also supports the increase in rate of crystallization.
Due to restrictions in polymer chain mobility through
association with exfoliated platelets, a significant reduc-
tion in the degree of crystallinity and an increase in melt-
ing point with mixing time are observed (Table 2).
Effect of molecular weight (or, MFI) on the crystalliza-
tion behaviour is shown in Table 3. Figure 12 shows a
change in the crystallization temperature (Tc) with MFI.
With an increase in MFI (decrease in molecular weight)
Tc changes from 118 to 1208C. Decrease in molecular
weight results in better diffusion as well as dispersion and
exfoliation which results in an earlier initiation of crystal-
lization. The dependence of exfoliation on molecular
weight is also supported by the change in the crystalliza-
tion rate indicated by the difference of onset of crystalli-
zation temperature [Tc(onset)] and crystallization tempera-
ture [Tc] value.
Spectroscopic Analysis
Fluorescence Spectrophotometric Analysis. During
melt mixing, the methylene blue dispersed in PP matrix
(see Fig. 13) exhibits emissions at 439 and 468 nm when
excited at 400 nm. Comparing with the spectra of MB
FIG. 11. Crystallization (cooling) curves of the PNC (4 wt % clay)
with different mixing times: (a) PP (11 MFI), (b) 4 min, (c) 8 min, (d)
12 min, and (e) 16 min.
TABLE 3. Crystallization and melting behavior of the composites with different MFI.
Sample Tc (8C) Tc (onset) (8C) [Tc (onset) – Tc] (8C) Tm (8C) DHc (J/g) Xc (%)
PP (11 MFI) 115.3 120.4 5.1 165.2 120.4 33.8
PNC (2 MFI) 118.2 122.3 4.1 165.7 91.3 35.9
PNC (11 MFI) 118.8 123.9 5.1 166.2 94.4 35.4
PNC (35 MFI) 120.3 123.4 3.1 167.2 96.2 34.6
DOI 10.1002/pc POLYMER COMPOSITES—-2010 2013
modified clay (Fig. 13c), the peak observed at a wave-
length of 439 nm is indicated to be the peak of the dye
which is inside the clay gallery. The peak at 468 nm rep-
resents the peak corresponding to the dye in the polymer
matrix. It is prominent from Table 4, that with increase in
mixing time there is a decrease in intensity (I439) of the
spectra. This trend is due to a decrease in the dye concen-
tration inside the clay gallery. With mixing, more and
more polymer chains are expected to intercalate into the
gallery by partially replacing the dye molecules. This
reduction in concentration produces the intensity response
in decreasing order. On the other hand, the decrease in in-
tensity (I468) with time is a result of concentration
quenching. As the matrix is nonpolar in nature and con-
tains very less amount of polar PP-g-MA compatibilizer,
it becomes less compatible with the polar cationic dye
molecules. As a result, the replaced dye molecules inter-
act with each other to form dimer, oligomer type of
aggregates. This aggregation reduces the intensity of the
emission spectra (at I468) in the matrix known as concen-
tration quenching.
Similar type of result is also found with polypropylene
having different MFI values. A decrease in both the inten-
sities has been found with an increase in MFI. With
increase in MFI i.e. a decrease in molecular weight, the
polymer chains can easily diffuse and intercalate into the
gallery replacing the dye molecules. This results in a
decrease in intensity of peaks at both I439 and I468 (Table 4).
Dielectric Spectroscopic Analysis
The dielectric constant of a material is related to the
polarization. When an alternating current (AC) is applied
to the dielectric material, the polarization could be initi-
ated. The dependency of dielectric properties on fre-
quency can provide important information about the mate-
rial morphology. Electrical conduction and polarization
mechanisms like dipole, ionic, electronic and Maxwell-
Wagner contribute to the dielectric loss factor. The Max-
well-Wagner polarization arises from the accumulation of
charges in the interface between components like polymer
and clay in heterogeneous systems. The Maxwell-Wagner
polarization effect peaks at about 0.1 MHz frequency
[27], but in general, its contribution is very less compared
to that of ionic conductivity.
Figure 14 shows a decrease in dielectric constant with
increase in frequency, temperature and mixing time. The
significant decrease in dielectric constant and loss arise
from the intercalation and exfoliation of clay in the poly-
mer matrix. The dielectric constant at low frequencies
(0.1 MHz) of polypropylene has been completely sup-
pressed due to the intercalation of clay. At low frequen-
cies, the magnitude of the decrease of dielectric constant
is much larger as shown in Fig. 14. It might be under-
stood that the decrease of dielectric constant at low fre-
quencies is due to this nanoscopic-confinement effects
from layered-silicate inorganic hosts and can be ascribed
to the restriction of polymer chain movement by nano-
particles. The increase in the values of these dielectric
constants with mixing time is also reported in the litera-
ture [28]. Probably, this may be due to an accidental
inclusion of the imperfection like inhomogeneous disper-
sion and agglomeration of nano-fillers or impurities mixed
in during manufacturing processes. The difference in rela-
FIG. 13. Fluorescence spectra of polypropylene/clay nanocomposites with: (A) mixing time and (B) MFI.
FIG. 12. Crystallization (cooling) curves of the PNC (4 wt % clay and
8 mins mixing) with different MFI: (a) virgin PP (11 MFI), (b) PNC (2
MFI), (c) PNC (11 MFI), and (d) PNC (35 MFI).
2014 POLYMER COMPOSITES—-2010 DOI 10.1002/pc
tive permittivity (dielectric constant) at low frequency
(0.1 MHz) and high frequency (10 MHz) is called the
dielectric dispersion. The dispersion for the clay/polymer
nanocomposite is considerably larger than that for the
neat polymer. This is because the clay particles in the
resin introduce ionic species that contribute to conductiv-
ity and polarization above that which is present in the
neat resin. There is no distinguished change in dielectric
constant after 5 MHz frequency. The clay platelets remain
in parallel direction in its tactoid and intercalated form
initially and charges are accumulated on its surface. This
configuration of the clay is quite similar to the function
of a capacitor. With an increase in mixing time, the clay
platelets are exfoliated by losing parallel symmetry. This
change in clay morphology with extensive exfoliation is
well reflected by the decrease in the capacitance as well
as dielectric constant value with frequency. From the
above results, it may be concluded that the polarization of
dipole orientation is largely reduced due to the randomly
exfoliated and intercalated layer structures and it is
clearly reflected from the decreasing value of dielectric
constant on addition of clay with increase in frequency.
Interesting observation is—decrease in dielectric constant
with increase in mixing time at a particular frequency.
This is due to large scale interaction of clay and polymer,
as clay dispersion gets better and leads to restricted mo-
bility of dipoles. At higher frequency ([5 MHz), the time
available is much shorter than the relaxation time and
hence no polarization is effective, leading to decrease in
dielectric constant.
CONCLUSIONS
Melt compounding of polypropylene-clay nanocompo-
site having three MFI grades of polypropylene has been
carried out in a twin screw microcompounder. PP-g-MA
has been used as a compatibilizer and the MMT clay was
modified with fluorescent cationic methylene blue dye. It
is observed that with an increase in MFI (decrease in mo-
lecular weight) and mixing time, the extent of clay disper-
sion and exfoliation were also improved. It is due to the
fact that the molecules having lower molecular weight
can diffuse easily into the clay gallery causing an increase
in gallery spacing. The morphology studies using conven-
tional characterization techniques such as torque rheome-
try, XRD, SEM and TEM support this observation. Inter-
calation and exfoliation also depends on mixing time. It
was found that for this system, 14 min was the optimum
time for proper dispersion and complete exfoliation.
The results from conventional techniques were further
supported by thermal, fluorescence and dielectric analy-
sis. An increase in both crystallization temperature and
onset temperature for crystallization with mixing time
and MFI indicates better exfoliation. A significant
decrease in dielectric constant and dielectric loss in
lower range frequency seemed to be due to confinement
effect of intercalating clay in the nanocomposites, as bet-
ter intercalation and exfoliation occurred with increase
in mixing time and MFI of polypropylene. The fluores-
cence data of MB-PP-clay nanocomposites further cor-
roborate this phenomenon.
FIG. 14. The dielectric constant of polypropylene-clay nanocomposites: (a) at various frequencies under
808C and (b) at various temperatures under 10 MHz.
TABLE 4. Intensity at different wavelengths for polypropylene having different grades of MFI (2, 11, and 35 MFI) with different mixing times (4, 8,
12, and 16 mins).
Sample
PP/Clay (4%) nanocomposites (8 minutes mixing) Mixing time (PP having 11 MFI grade)
PP(2 MFI) PP(11MFI) PP(35MFI) 4 mins 8 mins 12 mins 16 mins
I439 (a.u.) 3 105 6.6 5.2 7.5 8.7 10.6 8.5 6.4
I468 (a.u.) 3 105 2.5 2.0 1.9 4.6 4.5 4.2 2.7
DOI 10.1002/pc POLYMER COMPOSITES—-2010 2015
ACKNOWLEDGMENTS
The authors thank Prof. Ratnamala Chatterjee and Dr.
Siddharth Pandey of Indian Institute of Technology, Delhi
for their cooperation in dielectric and fluorescence spec-
troscopic analysis.
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