hybrid organic-inorganic poss (polyhedral oligomeric silsesquioxane)/polypropylene nanocomposite...

Fibers and Polymers 2010, Vol.11, No.8, 1137-1145

1137

Hybrid Organic-Inorganic POSS (Polyhedral Oligomeric silsesquioxane)/

Polypropylene Nanocomposite Filaments

B. S. Butola*, Mangala Joshi, and Sachin Kumar

Department of Textile Technology, Indian Institute of Technology, New Delhi 110016, India

(Received April 8, 2010; Revised August 5, 2010; Accepted August 9, 2010)

Abstract: Octamethyl-POSS and Octaphenyl-POSS reinforced polypropylene nanocomposite monofilaments were preparedby melt blending route. It was observed that incorporation of Octamethyl-POSS and Octaphenyl-POSS in polypropyleneshow improvement in mechanical as well as thermal properties. Octaphenyl POSS/PP nanocomposites show significantincrease in thermal stability even at very low concentration as compared to neat polymer matrix. An increase of 100 and140 oC was observed in thermal degradation temperature at 5 wt% loss and maximum degradation over neat PP filamentsrespectively at low OP-POSS loadings (<5 wt%). Both Octamethyl-POSS and Octaphenyl-POSS act as lubricating agentsfacilitating drawing which results in improvement in orientation as well as mechanical properties.

Keywords: POSS (polyhedral oligomeric silsesquioxane), Polypropylene, Nanocomposite filaments, Octamethyl-POSS,

Octaphenyl-POSS

Introduction

Engineering applications of polymers often exhibit major

problems due to low stiffness, low strength and low thermal

stability of the available polymers. Therefore, new technology

needs to be developed to meet new requirements, such as

high use temperature, high resistance to oxidation, reduced

flammability and improved mechanical properties [1].

Hybrid organic-inorganic polymer nanocomposites based

on polyhedral oligomeric silsesquioxane (POSS) nanoparticles,

provide a unique approach to the development of polymeric

nanoengineered materials and find many application from

food packaging to high performance materials including

medical textiles, polymer coating, high temperature lubricants

or catalysts, sensors and self healing polymers [2]. POSS

belongs to a wide family of silsesquioxanes, having general

formula (RSiO1.5)n where R is hydrogen or an organic group,

such as alkyl, aryl or any of their derivatives. POSS molecules

can be easily functionalized by chemically altering the R

substituent groups thus having the potentiality of undergoing

copolymerization or grafting reactions. Several polymeric

systems have been studied incorporating POSS cages both in

thermoplastic matrices, such as styryl-based polymers,

esters, acrylates, olefins, and in thermosets, mainly in epoxy

resins. POSS molecules can be incorporated into polymer

systems through melt blending, solution blending, grafting

or copolymerization [3-13].

The presence of the thermally robust POSS moiety has

been found to drastically modify the polymer thermal

properties and allowing the tailoring of the polymer glass

transition temperature by tuning of POSS concentration and

type. Moreover, incorporation of POSS molecules was

responsible for improvement of the mechanical properties as

well as reductions in flammability and heat evolution during

combustion [14].

Polypropylene (PP) has found extensive application as a

technical textile material in various engineering and high

technology applications such as marine ropes, spacecrafts,

sport goods, composites etc. High tenacity, high modulus

polypropylene monofilaments are generally prepared by

drawing polypropylene filaments at very high draw ratios,

high temperatures and at very slow strain rates [15]. There

have been attempts to produce PP nanocomposite filaments

with enhanced mechanical and thermal properties by using

nanoclay and other nanofillers [16].

However, there are no reports about the incorporation of

POSS and its effect on polypropylene filaments. This study

has been undertaken to study the effect of different POSS

architectures on thermal and other properties of PP

monofilaments which are finding increasing use in engineering

applications.

Experimental

Materials

Octamethyl POSS (OM-POSS) was supplied by Hybrid-

plastics as 10 wt% masterbatch in PP. Octaphenyl-POSS

(OP-POSS) was supplied by Sigma Aldrich and Polypropylene

(Grade-H350FG and Melt Flow Index=36 (230 oC, 2.16 kg))

was supplied by Reliance Industries, Jamnagar, India. The

molecular structure of OM-POSS and OP-POSS is given in

Figure 1 and 2.

Preparation of POSS/PP Nanocomposite Monofilaments

Master Batch Preparation of OP-POSS/PP Nanocomposite

OP-POSS was dispersed in acetone by ultrasonication for

10 min and then PP chips were poured in the POSS

dispersion. After thoroughly mixing, OP-POSS/PP mixture

was dried in hot air oven for 6 hr at 70 oC followed by drying

in vacuum oven for 4 hr at 70 oC. Master batch of OP-POSS*Corresponding author: [email protected]

DOI 10.1007/s12221-010-1137-y

1138 Fibers and Polymers 2010, Vol.11, No.8 B. S. Butola et al.

in PP was prepared in a DSM Twin screw extruder usingtemperatures 180, 190, 200, and 210 in different zones for4 min mixing time at 150 rpm. The master batch was usedfor preparation of OP-POSS/PP nanocomposites monofilamentsby melt spinning.

Melt Spinning

The spinning of OM-POSS/PP and OP-POSS/PP nano-composite monofilaments was carried out on a laboratorysingle screw extruder (BETOL-1820) by varying POSSconcentration from 0.1 to 10 % by weight. The codes usedfor describing nanocomposite fibres with various POSSconcentration are given in Table 1.

The diameter and the length of spinneret orifice were 0.5and 2 mm respectively. Extruded POSS/PP monofilamentswere quenched in ice cooled water (8±2 oC) to freeze thestructure in smectic phase (a phase possessing orderintermediate between the amorphous and crystalline statecan be obtained by quench cooling and is referred to as the‘smetic phase’. In the as Spun PP fibre the crystallites maybe in the paracrystallite Smectic form [17]. When drawnand/or annealed they move towards the more stable α form)of PP. Spinning temperature of different heating zone was190, 200, 210, and 220 oC. Single screw extruder speed was6 RPM and take up roller speed was 12 m/min.

Drawing

Two stage drawing of POSS/PP filaments was done on anindigenously built drawing machine to improve themolecular orientation and stabilization of the PP filament

structure to get better mechanical properties. The filamentswere drawn upto the maximum limit drawing just beforefilament weakening due to void formation and stresswhitening started. The final diameter of the compositefilaments was approximately 0.2 mm, corresponding to alinear density of 250 denier. The drawing parameters aregiven in Table 2.

Characterization Techniques

X-ray Diffraction

The determination of volume fraction crystallinity andorientation was carried out by Wide angle X-ray Diffraction(WAXD) of the samples on a Philips X-ray diffractometer(Model-X’PERT PRO, Panalytical, Netherland) with Nickel-filtered CuKα (1.54 Å) as the radiation source. Thediffractometer was operated at 40 kV and 30 mA inreflection mode with angle 2θ ranging from 5 to 35 o at ascanning rate of 2 o/min. All the samples were prepared bycutting the drawn monfilaments into fine powder form. Thedegree of crystallinity of various samples was evaluatedfrom the X-ray diffraction pattern by separating thecrystalline and amorphous portions under the diffractionpattern using the following expression.

Degree of crystallinity (%) = {Acr

/(Acr

+Aam

)} × 100

Where, Acr

is the area under crystalline peak and Aam

is thearea under amorphous halo.

Crystallite Thickness

The average lateral crystalline thickness is estimated fromthe broadening observed in the WAXD pattern recorded for

Table 1. Codes used for describing nanocomposite monofilaments

S. No. Code % POSS concentration

1 P0 0

2 P01 0.1

3 P02 0.25

4 P05 0.5

5 P1 1.0

6 P2 2.0

7 P5 5.0

8 P7 7.0

9 P10 10.0

Table 2. Drawing process conditions during drawing

Drawing roller speed (RPM) Temperature of heater (oC)

First roller 2 First heating zone 60±5

Second roller 8 Second heating zone 130±5

Third roller 20

Figure 1. Molecular structure of octamethyl POSS.

Figure 2. Molecular structure of octaphenyl POSS.

Hybrid POSS/Polypropylene Nanocomposite Filaments Fibers and Polymers 2010, Vol.11, No.8 1139

2θ range of 10-35 o, at a scanning rate of 2 o/min. The

integral breadth of the diffraction intensity arising from the

imperfection of crystallites is measured in terms of β1/2 (hkl).

Higher the value of β1/2 (hkl), lower is the crystalline

perfection.

Apparent crystalline size was determined according to

Scherrer’s equation:

Where, β is the half width of the diffraction peak in radian,

K is equal to 0.9, θ is the Bragg angle and λ is the wave

length of the X-rays. The values of D(hkl) for (110) reflection

were calculated.

Differential Scanning Calorimetry

DSC studies were carried out to study the melting

behaviour, crystallinity level and crystallization behaviour of

the nanocomposites. The instrument used was Perkin Elmer

Pyris-1 Differential Scanning Calorimeter in flowing nitrogen

atmosphere. The scans were obtained from 50-180 oC at a

heating rate of 10 oC/min with sample weight around 5 mg.

The onset, end and peak melting temperatures, as well as

heat of fusion (∆H) were calculated from the scans using an

inbuilt software. The crystallinity of the samples was

calculated using the following relationship:

Xdsc = ∆Hsample / 209

Where Xdsc = DSC crystallinity of the sample, ∆Hsample=

heat of fusion of the sample and 209 kJ/kg [18] is the heat of

fusion of 100 % crystalline PP.

Scanning Electron Microscopy (SEM)

The surface and cross-section of the filaments of selected

nanocomposites were studied on a scanning electron

microscope (SEM), ZEISS (EVO-50) manufactured by K.E

Developers Ltd., Cambridge, England. Samples were coated

with a thin layer of silver. The basic aim of the study was to

look at the coarse level of POSS aggregation and any

possible change in the morphology caused by presence of

POSS in PP matrix.

Thermogravimetric Analysis

Thermogravimetric Analysis was carried out on a Perkin

Elmer TGA-7 instrument under nitrogen atmosphere.

Samples were heated at a heating rate of 20 oC/minute from

50 to 900oC. The percentage weight loss of the samples as a

function of temperature was recorded.

Tensile Properties

Tensile testing of the sample filaments was carried out on

Instron 4301 tester (ASTM-D5035-90). Test settings used

for nanocomposite filaments were as follows: load cell:

1 kg, gauge length: 250 mm, strain rate: 300 mm/min.

Sonic Modulus

The sonic modulus is a dynamic modulus (Young’s

modulus) measured by the velocity of train of the sound

pulses through the fibre. The sonic modulus for a long thin

fibre is given by

E =ρC2

Where E is the sonic modulus, ρ is the density and C is the

sonic velocity. The velocity of wave depends upon the

degree of crystallinity and orientation of sample.

The dynamic modulus tester PPM-SR (H. M. Morgan Co.

Inc., Norwood, Mass, USA) was used for testing sonic

modulus of neat PP as well POSS/PP nanocomposite

filaments. The filament was clamped at one end, passed over

a pulley, kept taut by a reasonable weight to the other end

(for slight tension). The fibre scanner had two transducers

mounted on the instrument. The fixed transducer containing

a propagation meter supplied sound pulses of longitudinal

waves at a frequency of 5 KHz. The other transducer

(receiver transducer) was driven by a synchronized motor

and moved along the sample at a constant speed (2.5 inch/

min). The transit time was monitored directly as a function

of the distance between probes on an attached recorder. The

sonic velocity can be calculated from the reciprocal of the

slope (1/C) of the line plotted by the instrument.

Results and Discussion

XRD Analysis of OM-POSS and OP-POSS/PP Nano-

composites

Figure 3 shows the diffraction pattern for PP and OM-

POSS/PP nanocomposite filaments and it is observed from

the shape of diffraction patterns that presence of POSS does

not interfere with the crystal pattern of PP. It can also be seen

that a peak corresponding to the dominant OM-POSS peak

just appears at 0.25 wt% POSS. Though upto 1 wt%

concentration of OM-POSS, the peak height increases

progressively, it is not pronounced. However, beyond 1 wt%

D hkl( )Kλ

βcosθ--------------=

Figure 3. XRD of different OM-POSS/PP nanocomposite filaments.


content, the peaks becomes well defined, pronounced, rather

sharp and grow progressively with increase in the content of

OM-POSS in PP. This means that OM-POSS molecules

exist as crystals in PP matrix at OM-POSS concentrations

just over 0.25 wt%. Zheng et al. [19] reported that POSS is

able to crystallize in nanocomposites even when it is a part

of the polymeric chain and there are restrictions on its

movement. Hence, it is obvious that POSS is able to

crystallize when it is dispersed physically in PP matrix and

there are no chemical linkages to restrict its movement. It

means upto 0.25 wt%, POSS is almost completely dispersible

in PP matrix and at higher concentrations POSS starts

agglomerating and crystallizing. Another observation that

can be made from the diffractograms is that the peaks are not

as sharp as neat POSS peaks. This means that the dispersed

OM-POSS crystals are not as perfect as neat OM-POSS

crystals. It can be inferred that, these crystallites are formed

from the dispersed OM-POSS particles and although the

process of melt mixing is able to disperse the OM-POSS at

nano/molecular level in PP matrix, eventually POSS manages

to crystallize when concentration levels increase beyond

0.25 wt%.

The size of the OM-POSS crystals was determined using

Scherrer’s equation and given in Table 3. It is clear from the

results that crystals from dispersed OM-POSS in nanocom-

posites are smaller than the neat OM-POSS crystals.

The corresponding X-ray diffraction patterns for OP-

POSS and OP-POSS/PP nanocomposite filaments are shown

in Figure 4. Here also the presence of OP-POSS does not

seem to affect the crystal pattern of PP matrix. Appearance

of characteristic peaks of OP-POSS in nanocomposite

filaments means OP-POSS too crystallizes in PP matrix.

However, it is quite clear that the tendency to crystallize for

OP-POSS in PP matrix is more subdued than the OM-POSS.

Not only the dominant peak heights are lower, these appear

at higher OP-POSS (2.0 wt%) concentration as compared to

OM-POSS/PP nanocomposites. This is to be expected since

the bulky phenyl groups attached to the POSS skeleton

hinder its mobility and hence the tendency to crystallize.

Differential Scanning Calorimetry of OP-POSS/PP

Nanocomposite

DSC is another investigative tool to obtain useful

information about the thermal transitions of polymers. The

Original DSC thermograms for OM-POSS/PP and OP-

POSS/PP composites are shown in Figure 5 and 6. The

values of melting temperatures for different POSS/PP

nanocomposites are given in Table 4. The values indicate

that there is no significant effect of OM-POSS and OP-

POSS molecules on the melting temperature of PP polymer

upto 5 wt%.

In the earlier section on analysis of X-ray diffraction

patterns for OM and OP-POSS/PP nanocomposites, it was

concluded that presence of either type of POSS does not

cause significant change either in the crystal structure or

crystal volume fraction of PP matrix. As the crystal pattern

remains largely unaffected, melting temperatures too are not

affected significantly.

Figure 4. XRD of different OP-POSS/PP nanocomposite filaments.

Figure 5. DSC thermograms of selected OM-POSS/PP

nanocomposites.

Table 3. Crystal size of POSS in free and dispersed state (using

Scherrer’s equation)

Material Crystal size (nm)

Neat POSS 40

P10 17

P7 14

P5 14

P2 17

P1 18


Mechanical Properties of POSS/PP Nanocomposites

Tensile Properties

The raw stress strain curves of POSS/PP nanocomposite

monofilament are plotted in Figure 7 and 8. From the results

it can be observed that increase in the concentration of OM-

POSS and OP-POSS upto 0.5 and 1 wt% respectively results

in gradual increase of the tenacity of nanocomposite

monofilaments. Beyond these limits of concentrations,

tenacity starts to fall down. It is proposed that due to cage

like structure, POSS acts as a lubricant when it is dispersed

at molecular/nanolevel, thus helping in improving the

drawablity of filaments [20,21]. But above 0.5-1.0 wt%

POSS concentration, aggregation tendency of POSS molecules

(confirmed from X-ray diffraction) increases which results

in decrease in tenacity as well as drawablity of polymer. At

low concentrations, it reduces the viscosity during melt

spinning of polymer and thus improves the processablity of

polymers as reported by authors [21] in an earlier study on

Figure 6. DSC thermograms of selected OP-POSS/PP

nanocomposites.

Figure 7. Stress strain curves of OM-POSS/PP nanocomposites.

Figure 8. Stress strain curves of OP-POSS/PP nanocomposites.

Figure 9. Effect of POSS concentration on modulus of POSS/PP

nanocomposite monofilament.

Table 4. Melting temperature for POSS/PP nanocomposites from

DSC studies

POSS conc.

in PP (%)

Melting temperature

OM-POSS/PP OP-POSS/PP

0 165.0 165.0

0.1 165.5 165.2

0.25 165.1 165.9

0.5 164.5 165.6

1 164.8 165.6

2 164.8 165.9

5 164.8 166.1

7 165.0 167.8

10 165.8 165.0


HDPE-OM POSS system.

The effect of incorporation of POSS molecules on the

modulus of nanocomposite filaments is shown in Figure 9.

Again it is apparent that as in case of tensile strength,

Young’s modulus too shows a gradual increase with increase

in POSS content upto 0.5 to 1.0 wt%, for both nanocomposites,

and decreases at higher concentrations.

Sonic Modulus

The sonic modulus values of OM-POSS nanocomposite

monofilaments are shown in Figure 10. The results show that

sonic modulus values increase as OM-POSS concentration in

PP matrix increases upto 0.5 wt%, which means better

orientation due to better drawability. Beyond 0.5 wt%

concentration, the modulus values start to fall. The same

trend is shown in case of OP-POSS also but upto a value of

1 wt%.

These findings are consistent with the results of tensile

properties of nanocomposite monofilaments. In case of

tensile strength, it was observed that tenacity values of

nanocomposite monofilaments increase upto a concentration

of 0.5 wt% in case of OM-POSS and 1 wt% in case of OP-

POSS. Sonic waves travel faster in crystalline and oriented

structures as compared to amorphous and randomly oriented

structures.

Thermogravimetric Analysis

The results of thermogravimetric analysis of different

POSS/PP nanocomposites in terms of Temperature of

degradation for 5 % mass loss (Td) and Maximum temperature

Figure 10. Effect of POSS concentration on sonic modulus.

Table 6. Effect of POSS concentration on the maximum

temperature of degradation (Tmax) of POSS/PP nanocomposites

POSS concentration

in the nanocomposite (%)

(Tmax) for OM-

POSS composites

(Tmax) for OP-POSS

composites

0 266 266

0.1 310 327

0.2 334 352

1 344 364

2 352 367

5 329 370

Neat POSS 255 450

Figure 11. TGA curves for OM-POSS/PP nanocomposite.

Table 5. Effect of POSS concentration on the temperature of

degradation for 5 % mass loss (Td) of POSS/PP nanocomposites

POSS concentration in

the nanocomposite (%)

Td (oC) for OM-

POSS composites

Td (oC) for OP-POSS

composites

0 321 321

0.1 410 426

0.5 443 446

2 430 478

5 409 484

Neat POSS 449 550

Figure 12. TGA curves for OP-POSS/PP nanocomposites; (a) neat

PP, (b) 0.1 wt% POSS/PP, (c) 0.2 wt% POSS/PP, (d) 0.5 wt%

POSS/PP, (e) 1 wt% POSS/PP, (f) 2 wt% POSS/PP, (g) 5 wt%

POSS/PP, (h) 10 wt% POSS/PP, and (i) neat OP-POSS.


of degradation (Tmax) are given in Table 5 and 6 and original

TGA curves shown in Figure 11 and 12. From the results it

is clear that the values of both Td and Tmax for nanocomposite

monofilaments increase significantly with increase in POSS

concentration. In case of OP-POSS/PP nanocomposites, the

increase in Td and Tmax is faster as compared to OM-POSS

with similar loading of POSS. This is to be expected as neat

OP-POSS has higher thermal stability as compared to both

neat PP and the other OM-POSS molecules.

SEM Studies of OM-POSS/PP Nanocomposites

The results of SEM of various OM-POSS/PP nanocom-

posites are shown in Figure 13. The images were taken at

magnifications of 1000X and 10000X. At lower POSS

concentrations, the dispersion of POSS in the matrix is

largely uniform and only some isolated domains of POSS

aggregates are visible. It is clear from the images that the

aggregation tendency of OM-POSS increases with increasing

POSS concentration in PP. This is to be expected since

Figure 13. SEM images of OM-POSS/PP nanocomposites at different POSS concentrations (Scale bar is 10 µ for 1000 magnification and

1 µ for 10000 magnification).


POSS-POSS interaction is stronger than POSS-Polymer

interaction. OM-POSS has higher tendency of aggregation

as compared to OP-POSS as it starts agglomerating just

above 0.5 wt% whereas in case of OP-POSS, this tendency

was observed at higher concentrations (~2 wt%).

Figure 14 shows SEM images of various OP-POSS/PP

nanocomposites monofilaments. Like OM-POSS/PP monofila-

ments, here too the dispersion of POSS is uniform. With

increasing OP-POSS concentration in PP matrix, one can

observe appearance of POSS aggregates. However, as

compared to OM-POSS/PP nanocomposites, the dispersion

is more uniform and aggregate size is less. This again is in

conformity with X-ray diffraction results where it was

shown that OM-POSS has a higher tendency to aggregate as

compared to OP-POSS molecule.

Conclusion

Octa Methyl-POSS and Octa Phenyl-POSS reinforced

polypropylene nanocomposite monofilaments were prepared

by melt extrusion followed by drawing process.

The Morphological analysis of the monofilaments by X-

ray diffraction and SEM have shown that although POSS

molecules disperse uniformly in PP matrix, there is a

tendency to aggregate and crystallize at higher POSS

loadings (>1 wt%). Among OM-POSS and OP-POSS, the

latter showed lower aggregation tendency mainly due to its

bulky size which restricts its mobility. The aggregation

tendency becomes marked above 0.5 and 1.0 wt%

concentrations of OM-POSS and OP-POSS respectively in

PP polymer. Below these concentrations, POSS molecules

act as lubricant due to their almost spheroidal shape. This

facilitates drawing and improved orientation, which results

in better tensile strength, modulus and sonic modulus.

Presence of POSS did not adversely affect the crystallinity

or the crystal structure of PP polymer. Hence the melting

temperature as obtained from DSC was also not affected

significantly.

Figure 14. SEM images of OP-POSS/PP nanocomposites cross section at different POSS concentrations (Scale bar is 2 µ for all diagrams at

5000 magnification and for 5 % OP-POSS/PP at 20000 magnification, and 1 µ for remaining two diagrams at 20000 magnification).


Incorporation of either type of POSS in PP improved its

thermal stability as evidenced by increasing Td and Tmaxvalues. The improvement in these parameters was more

marked for OP-POSS as compared to OM-POSS mainly due

to intrinsically high thermal stability of neat OP-POSS over

OM-POSS.

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