fabrication and characterization of mwcnt/thermoplastic microsphere nanocomposite foams

September 25, 2008 14:10 00523

International Journal of NanoscienceVol. 7, Nos. 2 & 3 (2008) 161–169c© World Scientific Publishing Company

FABRICATION AND CHARACTERIZATION OFMWCNT/THERMOPLASTIC MICROSPHERE

NANOCOMPOSITE FOAMS

VIJAYA K. RANGARI∗, MOHAMMAD I. JEELANI,YUANXIN ZHOU and SHAIK JEELANI

Tuskegee Center for Advanced Materials (T-CAM)Tuskegee University, Tuskegee, AL 36088, USA

∗[email protected]

Revised 30 April 2008

A novel sonochemical method is developed to coat multi-walled carbon nanotubes (MWCNTs)on expandable thermoplastic (acrylonitrile and methylacrylonitrile polymer) microspheres(Expancel). These polymeric thermoplastic microspheres were further fabricated in a form offoam panels using a compression molding technique. The test coupons were cut precisely fromthe as-prepared panels and characterized by scanning electron microscopy (SEM), X-ray diffrac-tion, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The SEMstudies have shown that the MWCNTs are well dispersed over the entire volume of the matrixwith minimal agglomeration. The foam cell structures are well-ordered, spherical in shape, anduniform in size. The TGA and DSC analyses indicate that the nanocomposite foam samplesare thermally more stable than the corresponding neat foam samples. The compression testshave been carried out for both nanocomposite and neat foam samples. These test results showa significant increase in compressive strength and modulus for nanocomposite foam samplesas compared to the neat foam samples. These enhancements in compressive properties havebeen observed repeatedly for multiple batches. The details of synthesis procedure, thermal andmechanical characterization are presented in this paper.

Keywords : Microspheres; nanocomposite; sonochemical.

1. Introduction

Polymeric foam materials are widely used in manyindustrial applications for their properties of lightweight, excellent strength to weight ratio, superiorinsulating abilities, energy-absorbing properties,low thermal conductivity, high sound absorption,and large compressive strains. The main applica-tions include sandwich structures, airframes, trans-portation vehicles, boat hulls, radar systems, andspace structures.1−4 High-performance structuralfoam materials are fabricated using a blowing agent(surfactants, hydrocarbons) in liquid polymers to

expand and form rigid, low-density foams. Some ofthe leading thermoplastic foams made in this wayare polymethacrylimide (PMI) and partly cross-linked polyvinyl chloride (PVC), with trade namesRohacell,2 Divinycell,1 and Expancel.5 Hollow ther-moplastic microspheres are produced under thetrade name of Expancel R©. These microspheresare small, spherical plastic particles consisting ofa polymer shell encapsulating hydrocarbon gas.When the gas inside the shell is heated, its pres-sure increases and the thermoplastic shell softens,resulting in a dramatic increase in the volume of the

161

Int.

J. N

anos

ci. 2

008.

07:1

61-1

69. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by U

NIV

ER

SIT

Y O

F Q

UE

EN

SLA

ND

on

05/1

4/13

. For

per

sona

l use

onl

y.

162 V. K. Rangari et al.

microspheres.6 Researchers are using these micro-spheres for various applications, such as car protec-tion, corrosion resistance, acoustic insulation, bodyfillers and marine hobby putties, underbody coat-ings, and synthetic wood.5,7 Kim and his coworkers8

developed closed-cell silicon oxycarbide foams withcell densities greater than 109 cells/cm3 and cellssmaller than 30 µm were obtained from a pre-ceramic polymer using expandable microspheres.Vaikhanski and Nutt9 also studied the reinforce-ment of microspheres in PVC with the aramid fibersand reported the improved mechanical properties.

Recently there has been much interest inimproving physical, mechanical, thermal and chem-ical properties of polymeric materials by usingnanoparticles as filler materials. Nanoparticlesembedded in polymer matrix have attractedincreasing attention because of the unique proper-ties displayed by nanoparticles. Due to the nanome-ter size of these particles, their physicochemicalcharacteristics differ significantly from those ofmolecular and bulk materials.10,11 Nanoparticle–polymer nanocomposites synergistically combinethe properties of both the host polymer matrix andthe discrete nanoparticles therein. Such nanocom-posite materials are expected to have novel ther-mal and mechanical properties.12,13 In the presentstudy, the morphological and mechanical proper-ties of thermoplastic polymeric foam materials rein-forced with MWCNTs as fillers are reported.

2. Experimental

Expancel-092-DU-120, the unexpanded thermoplas-tic polymeric powder (particle sizes 28–38 µm), wasreceived from Expancel Inc. Multi-walled carbonnanotubes (MWCNT-10–20 nm in diameter and0.5–20 µm in length) were purchased from Nano-structured and Amorphous materials. The experi-mental procedure for the coating of MWCNTs onthermoplastic microspheres and the fabrication ofMWCNTs/thermoplastic microsphere foam panelsare as follows: Expancel polymeric powder and aknown weight percentage of MWCNT were dis-persed in n-hexane using a high intensity ultrasonichorn (Ti-horn, 20 kHz, and 100 W/cm2) at roomtemperature for 1 h. The mixture was then driedin a vacuum for 12 h and the remaining n-hexanewas removed by heating the sample at 60◦C for1 h. The dry MWCNTs/thermoplastic microspheremixture was transferred to a rectangular aluminummold (4′′×4′′×1/2′′) and was uniformly spread over

Table 1. Density of the as-prepared neat andnanocomposite foams.

Material Density kg/m3

Neat foam 1541% nanocomposite foam 1532% nanocomposite foam 151

the entire volume of the mold. The mold was thenheated to ∼ 190◦C at a heating rate of 10◦C/min for30 min under a pressure of 15 atm using a MTP-14programmable compression molding equipment.The as-prepared samples were cut precisely andused for morphological thermal and mechanicaltesting. The densities of as-prepared foam samplesof MWCNTs/microspheres (nanocomposite) andthermoplastic microspheres only (neat) were mea-sured and presented in Table 1.5 The as preparedfoam samples without MWCNT are ∼ 154 kg/m3

and with MWCNT are ∼ 153 kg/m3. The smallervariations in densities of nanocomposite and neatfoam samples are due to the density differences inas-received microspheres and MWCNTs.

Thermo gravimetric analysis (TGA) of variousspecimens was carried out under nitrogen gas atmo-sphere on a Mettler Toledo TGA/SDTA 851 appa-ratus. The samples were cut into small pieces of10–20 mg weight using a surgical blade. The TGAmeasurements were carried out from 30◦C to 800◦Cat a heating rate of 10◦C/min. Differential scan-ning calorimetry (DSC) experiments were carriedout using a Mettler Toledo DSC 822 from 30◦C to400◦C at a heating rate of 10◦C/min under nitro-gen atmosphere. X-ray diffraction (XRD) measure-ments were carried out using a Rigaku D/MAX2200 X-ray Diffractometer with Cu Kα radiation(λ = 1.541 A) with a scanning speed of 1◦/minand operating at 40 kV and 30 mA. Sample size forX-ray tests was maintained at 17.5 mm (length),13.5 mm (width), and 1.7 mm (thickness).

The morphological analysis was carried outusing JEOL-JSM 5800 Scanning electron micro-scopy (SEM). The samples were cut into smallpieces and placed onto a double-sided carbon tapeand coated with gold/palladium to prevent chargebuildup by electrons absorbed by the specimen.

In order to investigate the quasi-static com-pression response, the specimens were testedin the thickness direction using Zwick/RoellMaterial Testing Machine. The ASTM C365-57test standard was followed for this quasi-static

September 25, 2008 14:10 00523

Int.

J. N

anos

ci. 2

008.

07:1

61-1

69. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by U

NIV

ER

SIT

Y O

F Q

UE

EN

SLA

ND

on

05/1

4/13

. For

per

sona

l use

onl

y.

Fabrication and Characterization of MWCNT/Thermoplastic Microsphere Nanocomposite Foams 163

compression test and the size of the specimen is12.7 mm× 25 mm× 25 mm. The test is carried outin a displacement control at a cross-head speed1.27 mm/min by using flat platens. In order tomaintain evenly distributed compressive loading,each specimen was sanded and polished with highaccuracy so that the opposite faces were parallelto each other. A software TestXpert was used todevelop a program which controls the test condi-tions and records both the load and crosshead dis-placement data. The load-deflection data recordedby the data acquisition system was converted toa stress–strain curve after dividing load by speci-men cross-sectional area and deflection by specimenthickness.

3. Results and Discussion

Thermo gravimetric analysis (TGA) measurementswere carried out to obtain information on thethermal stability of the neat and nanocompositefoam samples. The TGA results of foam materialsincluding neat and the nanocomposite foams areshown in Fig. 1 and results are presented in Table 2.

100 200 300 400 500 600 700 800

30405060708090

100110

We

igh

t %

Temperature (°C)

(a)

100 200 300 400 500 600 700 800

405060708090

100

We

igh

t %

Temperature (°C)

(b)

100 200 300 400 500 600 700 800

405060708090

100110

Weig

ht

%

Temperature (°C)

(c)

Fig. 1. TGA results of (a) neat foam, (b) 1% MWCNT nanocomposite foam, and (c) 2% MWCNT nanocomposite foam.

TGA results show that the neat and nanocom-posite foams disintegrate in three steps: first step(as shown in Table 2) corresponds to the loss oforganic vapor and the second major weight losscorresponds to the rupture of the microspheres,and finally the third weight loss corresponds to thedecomposition of the polymer itself. The residualcontent was estimated as ∼ 40–42% for all the sam-ples including neat and nanocomposite foams. Inthe present study, a 50% initial weight loss is consid-ered as the structural decomposition temperatureof the material. It is a common practice to consider50% weight loss as an indicator for structural desta-bilization as given in Refs. 14 and 15. As seen inthe Figs. 1(a) and 1(b), the neat foam is stable upto 418◦C whereas nanocomposite (1 wt% MWCNT)foam (as shown in Fig. 1(b)) is stable up to 441◦C.As the loading of MWCNT increased to 2 wt% asshown in Fig. 1(c), the decomposition temperature(429◦C) is lower than the 1wt% MWCNT foam butstill higher than the neat foam. The reason for thedecrease in decomposition temperature for 2 wt%MWCNT loading foam may be explained macro-scopically as a simple colligative thermodynamic

September 25, 2008 14:10 00523

Int.

J. N

anos

ci. 2

008.

07:1

61-1

69. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by U

NIV

ER

SIT

Y O

F Q

UE

EN

SLA

ND

on

05/1

4/13

. For

per

sona

l use

onl

y.


Table 2. TGA results of neat and nanocomposite foams.

First step Second step Third step Temperature at 50%Sample weight loss weight loss weight loss weight loss

Neat foam (a) 1%/208 33%/295 28%/408 4181%-MWCNT nanocomposite 1.2%/198 35%/293 24%/412 441

foam (b)2%-MWCNT-nanocomposite 1.5%/200 35%/293 23%/416 429

foam (c)

effect of an impurity on a bulk solution. As parti-cle loading increases, the resulting composites willbegin to see more and more particle-to-particleinteraction rather than the intended particle-to-polymer interaction. Particle-to-particle interac-tion will lead to agglomerated particles which canbe visualized as a simple colligative thermody-namic effect of an impurity on a bulk solution.Microscopically, it may be seen as the result ofthe perturbation that the particles introduce to thethree-dimensional structure of the polymer. Thisperturbation weakens the Van der Waals interactionbetween the polymer chains, affecting the stabilityof the polymer. We believe this perturbation beginsat a point when the number of particles reachesa threshold level.16,17 These results show that thenanocomposite (1 wt% MWCNT) foam increasedthe thermal stability by 23◦C. This increase maybe due to the strong interaction of MWCNT withpolymer microspheres.

The glass transition temperatures (Tgs) weremeasured using DSC curves as shown in Fig. 2. TheTgs were determined as the inflection points of theheat flow curves.18,19 The Tgs measured for neat

60 80 100 120 140 160

-0.20

-0.15

-0.10

-0.05

0.00

0.05

Heat

Flo

w (

W/g

)

Temperature (°C)

Fig. 2. DSC results of (a) neat foam, (b) 1% MWCNT nanocomposite foam, and (c) 2% MWCNT nanocomposite foam.

and nanocomposite foams is ∼ 110–113◦C. Theseresults are consistent with the manufacturer’s datasheet and show that there is no significant increasein cross-linking of the polymer in the presenceof MWCNT. The X-ray diffraction measurementswere carried out to study the crystalline behaviorof the nanocomposite foam.

Figure 3 shows the XRD patterns of the (a)neat foam, (b) pristine MWCNT, (c) nanocompos-ite (1 wt% MWCNTs/microspheres) foam, and (d)nanocomposite (2 wt% MWCNTs/microspheres)foam. Figure 3(a) only shows a wide amorphouspeak at 16.5 of 2θ degrees. Figure 3(b) shows a widepeak at 26.4 of 2θ degrees which corresponds tothe graphite (PDF No: 41-1487), while Figs. 3(c)and 3(d) show much sharper peak at 16.5 of 2θdegrees and full width at half maximum of peak at16.5 of 2θ degrees is ∼ 0.26 degree lesser than thatof the neat foam. This indicates that the nanocom-posite foam sample is more crystalline than the neatfoam sample. We also observe that the graphitepeak at 26.5 of 2θ degrees for nanocomposite asshown in Figs. 3(c) and 3(d) represents the pres-ence of the MWCNT.

September 25, 2008 14:10 00523

Int.

J. N

anos

ci. 2

008.

07:1

61-1

69. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by U

NIV

ER

SIT

Y O

F Q

UE

EN

SLA

ND

on

05/1

4/13

. For

per

sona

l use

onl

y.


10 15 20 25 30 35 400

1000

2000

3000

4000

5000

6000

Re

lativ

e I

nte

nsi

ty

2θ degrees

Fig. 3. XRD results of (a) neat foam, (b) pristine MWCNT, (c) 1% MWCNT nanocomposite foam, and (d) 2% MWCNTnanocomposite foam.

0

500

1000

1500

2000

2500

3000

3500

4000

0 10 20 30 40 50 60 70 80

2% Nano-System1% Nano-SystemNeat Foam

Strain, %

Com

pre

ssio

n S

tress

, K

Pa

Fig. 4. Compressive stress–strain curves are neat, 1% MWCNT, and 2% MWCNT nanocomposite foams.

Table 3. Compression properties of various foam samples.

Compressive CompressionMaterial strength [MPa] Change [%] modulus [MPa] Change [%]

Neat foam 1.25 ± 0.050 — 30 ± 3 —1% nanocomposite foam 1.81 ± 0.040 +45 34.1 ± 4.5 +202% nanocomposite foam 1.89 ± 0.045 +51 40.8 ± 5.5 +40

Compression tests were carried out for neat andnanocomposite foams. Nanocomposite (1% and 2by wt%) and neat were tested. Stress–strain curvesfor neat and nanocomposites are shown in Fig. 4.Compression data is presented in Table 3. It isobserved in Table 3 that the compressive stress of

the 2wt% nanosystem is about 51 wt% higher thanthe neat sample. The 2 wt% nanosystem also shows40 wt% improvements in compressive modulus. Thisimprovement may be the result of increasing theinterfacial bond between the nanoparticles andpolymeric matrix.

September 25, 2008 14:10 00523

Int.

J. N

anos

ci. 2

008.

07:1

61-1

69. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by U

NIV

ER

SIT

Y O

F Q

UE

EN

SLA

ND

on

05/1

4/13

. For

per

sona

l use

onl

y.


(a) (b)

(c) (d)

(e) (f)

Fig. 5. SEM pictures of (a) as-received MWCNT, (b) as-received microspheres, (c) MWCNT-coated microspheres, (d) openlyexpanded microspheres, (e) openly expanded MWCNT-coated microspheres, and (f) dispersion of MWCNT on microspheres.

September 25, 2008 14:10 00523

Int.

J. N

anos

ci. 2

008.

07:1

61-1

69. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by U

NIV

ER

SIT

Y O

F Q

UE

EN

SLA

ND

on

05/1

4/13

. For

per

sona

l use

onl

y.


(a) (b)

(c) (d)

(e)

Fig. 6. SEM pictures of (a) neat foam, (b) 2% MWCNT nanocomposite foam, (c) neat foam after compression test, (d)nanocomposite foam after compression test, and (e) MWCNT dispersion on microspheres.

September 25, 2008 14:10 00523

Int.

J. N

anos

ci. 2

008.

07:1

61-1

69. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by U

NIV

ER

SIT

Y O

F Q

UE

EN

SLA

ND

on

05/1

4/13

. For

per

sona

l use

onl

y.


A detailed SEM analysis has been carriedout to understand the morphology of as-receivedMWCNT and microspheres. The extent of coatingof MWCNT on microspheres was also studied usingSEM analysis. Figure 5(a) shows the SEM pictureof as-received MWCNT. The MWCNTs are highlyagglomerated and the sizes measured from the SEMmicrograph are ∼ 10–20 nm in diameter and 0.5–20 µm in length. Figures 5(b) and 5(c) depict theas-received microspheres and the MWCNT coatedmicrospheres respectively. The particle sizes mea-sured from Fig. 5(b) are ∼ 20–40 µm. The micro-spheres are well separated and slightly inflatedspheres. Figure 5(c) shows that microspheres arenot destroyed by high power ultrasound, insteadthey are slightly expanded because of the heat gen-erated during the sonication process. The particlesizes measured are ∼ 25–50 µm. The effect of tem-perature on morphology of microspheres (coatedMWCNT and neat microspheres) was studied byheating (190◦C for 5min) microspheres openly ina glass beaker. Figures 5(d) and 5(e) clearly showthat expansions of microspheres are uniform andwell separated. The MWCNT-coated microspheresexpanded ∼ 5 µm larger than the neat microspheres.Finally Fig. 5(f) shows the uniform coating ofMWCNT on entire surface of microspheres.

SEM analysis has been carried out to exam-ine the morphology and dispersion of MWCNT inexpancel foam before and after the compressiontest. Figure 6(a) shows that all the microspheresare expanded and typical sizes measured are about40–100 µm.

Expanded nanocomposite microspheres areshown in Fig. 6(b). These results show that themicrospheres are slightly larger (60–120 µm) thanthe neat microspheres and are more uniformlyexpanded. The reason for this may be due to thehigh thermal conductivity of MWCNT in localizedheating of the microspheres.

Figures 6(c) and 6(d) depict the SEM picturesof the neat and nanocomposite foams after com-pression test. These micrographs clearly show thatall the microspheres are ruptured after compres-sion. Neat foam showed more collapsed cells andcracks in cells than the nanocomposite foam aftercompression. To examine this, we have carried outthe SEM analysis at higher magnification as shownin Fig. 6(e). These results show that the MWCNTare uniformly coated on the expanded microspheres.This uniform coating with adhering of the MWCNT

on microsphere translated into the enhanced com-pression strength and modulus.

4. Conclusions

Sonochemical technique has been developed to coatuniformly MWCNT on expandable microspheres.Thermal analysis results indicate that nanophasedfoam materials are thermally (23◦C) more stable ascompared to the neat foam samples. Quasi-staticcompression test results indicate that there is a sig-nificant increase in compressive strength (45–51%)and modulus 20–40% as compared to the neat sys-tem. This sonochemical method can be extendedto other nanoparticle coatings on the polymericmicrospheres.

Acknowledgments

The authors would like to thank the NSF-PREM forfinancial support and Expancel Inc. for providingpolymeric sample.

References

1. DIAB Inc., Technical Information for DivinycellFoam (2002), http://www.diabgroup.com/europe/products/e prods 2.html.

2. A. G. Rohm, Technical Information for ROHACELLFoam (2002), http://www.nfgsales.com/rohm.htm.

3. Baltek Corp., Technical Information (2002), http://www.baltek.com/.

4. J. Marsh, Reinforced Plastics 46, 26 (2002).5. Expancel R© Microspheres Technical information for

Expancel foam (1980), http://www.expancel.com/.6. K. EIfving and B. Soderberg, Reinforced Plastics 40,

64 (1994).7. M. Tomalino and G. Bianchini, Prog. Organic Coat.

32, 17 (1997).8. Y. W. Kim, S. H. Kim, H. D. Kim and C. B. Park,

J. Mater. Sci. 39, 5647 (2004).9. L. Vaikhanski and S. R. Nutt, Composites: Part A

34, 1245 (2003).10. M. Antonietti and C. Goltner, Angew. Chem. Int.

Ed. Eng. 36, 910 (1997).11. G. Schmid, Clusters and Colloids (VCH, Weinheim,

1994).12. G. Schmid, Chem. Rev. 92, 1709 (1992).13. H. Hirai, H. Wakabayashi and M. Komiyama, Bull.

Chem. Soc. Jpn. 59, 367 (1986).14. R. V. Kumar, Yu. Koltypin and A. Gedanken,

J. Appl. Polym. Sci. 86, 160 (2002).

September 25, 2008 14:10 00523

Int.

J. N

anos

ci. 2

008.

07:1

61-1

69. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by U

NIV

ER

SIT

Y O

F Q

UE

EN

SLA

ND

on

05/1

4/13

. For

per

sona

l use

onl

y.


15. R. V. Kumar, Yu. Koltypin, Y. S. Cohen, D.Aurbach, O. Palchik, I. Felner and A. Gedanken,J. Mater. Chem. 10, 1125 (2000).

16. H. Mahfuz, R. V. Kumar, M. Islam and S. Jeelani,Composites Part A: Appl. Sci. Manufacturing 35,453 (2004).

17. N. Chisholm, H. Mahfuz, R. V. Kumar, A. Ashfaqand S. Jeelani, Composite Structures 67, 115 (2005).

18. A. G. Loera, F. Cara, M. Dumon and J. P. Pascault,Macromolecules 35, 6291 (2002).

19. V. Rangari, Y. Koltypin, Y. S. Cohen, D. Aurbach,O. Palchik, I. Felner and A. Gedanken, J. Mater.Chem. 10, 1125 (2000).

September 25, 2008 14:10 00523

Int.

J. N

anos

ci. 2

008.

07:1

61-1

69. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by U

NIV

ER

SIT

Y O

F Q

UE

EN

SLA

ND

on

05/1

4/13

. For

per

sona

l use

onl

y.

fabrication and characterization of mwcnt/thermoplastic microsphere nanocomposite foams

Documents