A spectroscopic approach for structural characterization of polypropylene/clay nanocomposite

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: [email protected]

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.


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.


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.


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


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).


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


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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A spectroscopic approach for structural characterization of polypropylene/clay nanocomposite