[acs symposium series] green polymer chemistry: biocatalysis and materials ii volume 1144 ||...

Chapter 14

Synthesis of Poly-(R)-3 Hydroxyoctanoate(PHO) and Its Graphene Nanocomposites

Ahmed Abdala,*,1 John Barrett,2 and Friedrich Srienc2

1Department of Chemical Engineering, The Petroleum Institute,Abu Dhabi, United Arab Emirates

Permanent Address: Department of Chemical Engineering and PetroleumRefining, Faculty of Petroleum and Mining Engineering,

Suez University, Suez, Egypt2Department of Chemical Engineering and Materials Science,University of Minnesota, Minneapolis, Minnesota 55455 and

BioTechnology Institute, University of Minnesota,St. Paul, Minnesota 55108*E-mail: [email protected]

Polyhydroxyalkanoates are a popular class of bioplasticsvalued for their rapid biodegradadion, biocompatibility,and renewable feedstocks. While there are already a fewcommercial applications for these biopolymers, a greaterdiversity of properties is needed to compete with petroleumbased polymers. In this chapter, we report the synthesis andcharacterization of polyhdroxyoctanoate and its nanocompositewith thermally reduced graphene. The results indicate theincorporation of graphene into the PHO matrix leads to a smallupshift in the glass transition, enhance the thermal stability, and~600% increase in modulus. Electrical percolation between 0.5and 1 vol.% TRG was obtained.

© 2013 American Chemical Society

Dow

nloa

ded

by U

NIV

SO

UT

HE

RN

MIS

SISS

IPPI

on

Nov

embe

r 9,

201

4 | h

ttp://

pubs

.acs

.org

P

ublic

atio

n D

ate

(Web

): N

ovem

ber

22, 2

013

| doi

: 10.

1021

/bk-

2013

-114

4.ch

014

In Green Polymer Chemistry: Biocatalysis and Materials II; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Introduction

Polyhydrxyalkanotes (PHAs) are polyesters that can be biologicallysynthesized by microbial cultivation or in other biological systems (1).They become an important class of biopolymers due to their renewablesources, biodegradation and applications in tissue engineering because of theirbiocompatbility (2). Structurally, the PHA backbone is comprised of 3-carbonrepeat units with oxo-ester linkages. The attachment of various aliphatic andaromatic moieties stemming from the 3-carbon postion of each monomer impartsa range of material properties. Polyhydroxybutyrate (PHB), which possessesa single methyl group at the 3-carbon postion, is a stiff thermoplastic with ahigh melting temperature and is by far the most commonly used form of PHA.In contrast, medium chain-length PHA, (PHAmcl) contains longer aliphaticappendages (3-11 carbons) at the 3-carbon position which enhance elasticitybut reduce polymer strength and melting temperature (3). Among PHAmcl,poly(3-hydroxyoctanoate) (PHO), which is a heteropolymer composed of C6, C8,and C10 monomers, is significantly more amorphous and flexible than PHB (4, 5).Thus, new methods to increase the strength and melting temperature of PHAmclhave significant potential for stimulating commercial proliferation of thesematerials and encouraging development of the larger natural products industry.Therefore, Nanocomposites of biopolymers with nano-fillers such as carbonnanotubes or clay, offer a significant potential for their increased utilization, asa result of the improvements in mechanical and thermal properties. There are afew publications that reports the production and characterization nanocompositesof PHB with nanofillers such as clay/layered silicate (6, 7) and carbon nanotubes(8, 9).

Two new carbon allotropes, carbon nanotubes (10) and graphene (11), havereceived much attention as nanofillers because of their extraordinary mechanical,thermal, and electrical properties. With Young’s modulus of 1 TPa and ultimatestrength of 130 GPa, graphene is the stiffest and strongest material ever measured(12). The incorporation of graphene into polymer matrices is expected to resultin significant enhancement of the thermal, mechanical, electrical, and barrierproperties (13).

In this study, we develop a series of PHO-graphene nanocomposites withdifferent graphene loading using solvent mixing in chloroform. Graphenedispersion in the PHO matrix is examined using TEM and the effect of grapheneloading on the mechanical, thermal, and electrical properties are discussed.

Experimental

PHO Synthesis

PHOwas produced via a fed-batch biosynthesis using the wild-type organism,Pseudomonas oleovorans . Initially, one fresh colony was selected and inoculatedinto a test tube containing 5-mL of LB medium (10 g Tryptone, 5 g Yeast ExtractPowder, and 5 g NaCl in 1L of water) and grown overnight at 30°C. The culturewas then transferred into a 2-L baffled flaskwith 500-mL of LBmedium+ 1% (v/v)

200

Dow

nloa

ded

by U

NIV

SO

UT

HE

RN

MIS

SISS

IPPI

on

Nov

embe

r 9,

201

4 | h

ttp://

pubs

.acs

.org

P

ublic

atio

n D

ate

(Web

): N

ovem

ber

22, 2

013

| doi

: 10.

1021

/bk-

2013

-114

4.ch

014


alkane for an additional 16 hours at 30°C with shaking at 250 RPM. This 500-mLculture was used to inoculate a 10-L bioreactor containing 5-L of E medium +2% (v/v) alkane. E medium consisted of 1.1 g (NH4)2HPO4, 5.8 g K2HPO4, 3.7g KHPO4, 0.25 g MgSO4•7H2O, and 1mL of trace metals in 1 L of water. Tracemetals consisted of 2.78 g FeSO4•7H2O, 1.98 gMnCl2•4H2O, 2.81 gCoSO4•7H2O,0.17 g CuCl2•2H2O, 0.29 g ZnSO4•7H2O, 1.67 g CaCl2•2H2O, 1M 1 mL in 1 Lof water. Airflow and agitation were adjusted to maintain dissolved oxygen inthe culture above 40%. During biosynthesis carbon dioxide evolution rate (CER)of the culture was monitored via mass spectroscopy. A sharp decline in CERindicated depletion of the carbon source, at which time more alkane was addedto maintain growth. Batches were harvested at 50 hrs. The resulting cell pelletwas lyophilized to dry the cells. PHO within the pellet was extracted in boilingchloroform using a Soxhlet apparatus for 16 hrs. Dissolved polymer was thenprecipitated with excess methanol, 8:1 v/v. The solvent mixture was decanted andresidual solvent was evaporated under ambient conditions until the polymer is dry.

Purified PHO were analyzed via gas chromatography fitted with a flameionization detector (GC-17A, Shimadzu) using a DB-WAX column (ID0.32 mm, 0.5 µm film thickness) (Agilent Technologies). Prior to injection,polyhydroxyalkanoic acids were converted to 3-hydroxyalkanoic propylesters by the method of propanolyis (14). Quantitative determination the ofdifferent PHAs was made by comparison to standards synthesized from purified3-hydroxyalkanoic acids (Sigma)

Graphene Production and Characterization

Thermally reduced graphene (TRG) is produced following the thermalexfoliation method (15, 16). In this method, natural flake graphite (-10 mesh,99.9%, Alfa Aesar) is oxidized using Staudenmaier method (17) using a mixtureof H2SO4 (95-97%, J.T. Bakers) and HNO3 (68%, J.T. Bakers), and Potassiumchlorate (Fisher Scientific). The produced graphite oxide (GO) is washed with 5%HCl (37%, Reidel-de Haen), until no sulfate ions are detected then it repeatedlywashed with water till no chloride ions are detected and dried in a vacuumovernight. GO was exfoliated by rapid heating at 1000 °C in a tube furnace(Barnstead Thermolyne) under flow of nitrogen for 30 s.

XRD (X’Pert PRO MPD diffractometer, PANalytical) was used to test theoxidation of graphite and the complete exfoliation of graphite oxide. XRDscan between 5-35° was conducted at a scan rate of 0.02°/sec with instrumentparameters of 40 kV voltage, 20 A intensity and 1.5406 Å CuKα radiation. TEMimages were obtained using FEI Tecnai G20 TEM.

Fabrication and Characterization of the Nanocomposites

PHO/graphene nanocomposites with 1, 2, 5 wt.% (0.5, 1, and 2.5 vol.%)graphene were prepared by the following procedure. First, 1 g of polymer wasdissolved in 20 mL of chloroform using a vortex tube mixer. Periodic incubationof the mixture in an 80° water bath was used to promote dissolution. Second,graphene powder was dispersed in chloroform at a concentration of 0.5 mg/L. To

201

Dow

nloa

ded

by U

NIV

SO

UT

HE

RN

MIS

SISS

IPPI

on

Nov

embe

r 9,

201

4 | h

ttp://

pubs

.acs

.org

P

ublic

atio

n D

ate

(Web

): N

ovem

ber

22, 2

013

| doi

: 10.

1021

/bk-

2013

-114

4.ch

014


promote the dispersion of graphene sheets, sonication was applied to the mixtureusing a electrode sonicator (Misonix 3000) at a power density of 1.5-3W/mL. Thegraphene dispersion was added to PHO solution and stirrer for 1 hr. This mixturewas then poured into a petri dish and evaporated on a hotplate at 55°C. Films weredried overnight to remove excess chloroform. For mechanical, rheological, andsurface resistance measurements specimens were prepared from the dry compositesamples by hot press (Tetrahedron, MTP-10) at 100°C and 1.0 MPa for 5 minutes.5 cm x 5 cm square with a thickness of 0.5mmwas prepared and cut into rectangles(5 cm x 5 cm x 0.5 mm) for (1.25 cm x 0.5 mm) disks for electrical conductivity.All samples were aged at room temperature for more than 72 hrs prior to testing.

Differential scanning calorimeter (DSC) (Netzsch 204 F1 Phoenix) andthermogravimetric analyzer (TGA) (Netzsch STA 409 PC) were employed toinvestigate thermal properties of PHO and its graphene nanocomposite. Themechanical properties of the pure and composite samples were measured usingdynamic mechanical analyzer (TA Instruments, RSAIII) operating in a tensionmode at an extension rate of 5 mm/min. Surface resistance measurements weretaken from circular disks, 10 x 0.5 mm, using an 11-point probe (Prostat Corp.,PRF-914B probe with PRS-801 meter). For each sample, 4 readings werecollected, two from each side of the film. Graphene morphology and graphenedispersion into the PHO matrix is analyzed with TEM (FEI Tecnai G20 TEM).80-100 nm thick composite films for TEM imaging were prepared at -80°C usingan ultramicrotome (Leica, EM UC6).

Results and Discussion

Preparation of Purified PHO

The biosynthesis route for PHO in Pseudomonas oleovorans is shown inFigure 1. Briefly, octane in the media is consumed by the microorganism whileundergoing a string of enzymatic conversions to produce a biologically activefatty acyl-CoA molecule.

This intermediate molecule is degraded via successive fatty acid β-oxidationcycle in which two carbons are removed to produce the central metabolite,acetyl-CoA, and the corresponding n-2 fatty acyl-CoA. In Pseudomonasoleovorans, one of the intermediates of β -oxidation, trans-2-enoyl-CoA isconverted via a 3-R-enoyl-CoA hydratase, PhaJ, to the monomer species,3-hydoxyalkanoyl-CoA. The cyclical nature of β-oxidation results in a distributionof different monomers. This variation in the monomer pool combined with thepromiscuous nature of the PhaC polymerase results in a random co-polymercomprised of several unique but structurally similar monomers. The synthesizedsample is a random polymer of 3-hydroxyoctanoate (3HB), 3-hydroxyhexanoate(3HH), 3-hydroxydecanoate (3HD), and 3-hydroxybutyrate (3HB) withcomposition of 91.4, 7, 1.2, and 0.4 mol%, respectively.

202

Dow

nloa

ded

by U

NIV

SO

UT

HE

RN

MIS

SISS

IPPI

on

Nov

embe

r 9,

201

4 | h

ttp://

pubs

.acs

.org

P

ublic

atio

n D

ate

(Web

): N

ovem

ber

22, 2

013

| doi

: 10.

1021

/bk-

2013

-114

4.ch

014


Figure 1. Metabolic pathway for PHO synthesis in Pseudomonas oleovorans.

Production and Characterization of TRG

Oxidation of graphite leads to the introduction of polar oxygen functionalitieson the surface of GO and change in carbon hyperdization to a mixture of sp2 andsp3 carbon. This leads to the expansion of the interlayer inter spacing from thegraphite 3.35 Å (002 peak at 2θ = 26.5) to 7.8 Å (2θ = 11.4) as indicated by theXRD patterns for graphite andGO, Figure 2-a. In contrast, TRG diffraction patternshows no noticeable diffraction peaks confirming the complete exfoliation of GOand production of TRG. TEM image of TRG (Figure 1-b) shows very thin andlarge (micron size) graphene sheets with wrinkled structure. The dark areas on theTEM micrograph represent the edges of folded or overlapped sheets.

203

Dow

nloa

ded

by U

NIV

SO

UT

HE

RN

MIS

SISS

IPPI

on

Nov

embe

r 9,

201

4 | h

ttp://

pubs

.acs

.org

P

ublic

atio

n D

ate

(Web

): N

ovem

ber

22, 2

013

| doi

: 10.

1021

/bk-

2013

-114

4.ch

014


Figure 2. a) XRD patterns of pure graphite, GO and TRG. b) TEM of TRG.

Morphology PHO-TRG Nanocomposites

The dispersion of TRG in PHO is examined using TEM. As shown in Figure3, although TRG is homogeneously distributed in the PHO matrix, it is not verywell dispersed into the matrix as evidence by the presence of dark areas thatrepresent the edges of stacked graphene layers. The high resolution image in theright indicates that TRG maintained its wrinkled structure while imbedded intothe matrix.

Figure 3. TEM images of PHO-TRG composite with 1% TRG.

Thermal Properties

The nonoxidative thermal degradation of PHO and its TRG nanocomposites isstudied using TGA and the results are shown in Figure 4. The pure and compositesamples are stable up to 275°C as they show no significant weight loss belowthat temperature. Above 275°C, the pure and composite sample undergoes a slowdegradation up to 295°C. Above 295°C , the pure samples rapidly degrade to

204

Dow

nloa

ded

by U

NIV

SO

UT

HE

RN

MIS

SISS

IPPI

on

Nov

embe

r 9,

201

4 | h

ttp://

pubs

.acs

.org

P

ublic

atio

n D

ate

(Web

): N

ovem

ber

22, 2

013

| doi

: 10.

1021

/bk-

2013

-114

4.ch

014


almost zero weight over a narrow range of about 30°C . The degradation rate of thecomposite samples is lower than that of the pure sample suggesting TRG inhibitsthe nonoxidative thermal degradation of PHO.

Table 1. Effect of TRG loading on thermal transitions of PHO

TRG(vol%)

Tg(°C)

Tm(°C)

ΔHm(J/g)

T90%(°C)

T50%(°C)

0 -41.9 53.7 15.5 297.1 309.8

0.5 -38.1 55.7 13.9 299.3 311.7

1 -38.6 55.5 9.0 300.5 315.6

2.5 -39.0 54.1 12.2 299.5 314.5

The effects of TRG loading on the thermal transitions of PHO are alsoprovided in Table 1. The addition of TRG increases the glass transition of PHO bya few degrees. The increase in the glass transition can be attributed to restrainingthe motion of PHO chains by the TRG sheets.

Figure 4. TGA thermograms of PHO and its TRG nanocomposites with 0, 0.5,1, and 2.5 vol.% TRG.

The crystallization behavior of biodegradable polymers is an importantparameter because it significantly affects not only the crystalline structure andmorphology but also the final physical properties and biodegradability of thepolymer. Table 1 provides the melt temperature and heat of melting for the purePHO and the composite samples. The melt temperature increases by 2°C withthe addition of 0.5 vol.% TRG. A further increase in the TRG loading does notincrease the melt temperature. In contrary, the melt temperature decreases whenTRG loading increases from 1 to 2.5 vol.%. This behavior could be attributed to

205

Dow

nloa

ded

by U

NIV

SO

UT

HE

RN

MIS

SISS

IPPI

on

Nov

embe

r 9,

201

4 | h

ttp://

pubs

.acs

.org

P

ublic

atio

n D

ate

(Web

): N

ovem

ber

22, 2

013

| doi

: 10.

1021

/bk-

2013

-114

4.ch

014


the agglomeration of TRG sheets at higher loading. On the other hand, the heat ofmelting and consequently the %crystallinity decreases with the addition of TRG.This decrease in crystallinity suggests that the incorporation of TRG decreasesthe nucleation rate of PHO.

Mechanical Properties

The mechanical properties of pure PHO and its TRG nanocomposite havebeen studied using DMA. The addition of TRG significantly increases the stiffnessof PHO, reduces the elongation at break, and have no significant effect of theultimate strength as shown in Figure 5 and table 2.

Figure 5. Stress-strain curves of PHO and its TRG nanocomposites with differentTRG loading, vol.%.

Table 2. Mechanical properties of PHO-TRG nanocomposites

TRG(vol%)

Modulus(MPa)

Strength(MPa)

Elongationat break(%)

0 4.5 6.4 425

0.5 7.2 7.0 346

1 10.9 5.6 205

2.5 31.0 6.7 105

206

Dow

nloa

ded

by U

NIV

SO

UT

HE

RN

MIS

SISS

IPPI

on

Nov

embe

r 9,

201

4 | h

ttp://

pubs

.acs

.org

P

ublic

atio

n D

ate

(Web

): N

ovem

ber

22, 2

013

| doi

: 10.

1021

/bk-

2013

-114

4.ch

014


The addition of TRG resulted in a very significant increase in the modulus(600% at 2.5 vol.%). This is even more impressive if we recall that the compositesamples has a lower crystallinity compared to the pure sample as discussed earlier.This level of enhancement is significantly higher than the reported for grapheneand graphene oxide composite with glassy polymers such as PA (18), PMMA (19),and PCL (20) but similar to that of elastomeric polymers such as natural rubber(21), PDMS (21), and TPU (22). Although, Figure 5 shows no appreciable changein the ultimate strength, we claim that TRG enhances the strength of PHO justenough to overcome the decrease in strength due to the lower crystallinity of thecomposite samples. The main drawback of the addition of TRG is the reductionin the elongation at break. Nevertheless, the composite samples remain highlyflexible with a minimum elongation at break of over 100%.

Electrical Properties

The greatest advantage of carbon nanofillers and graphene in particularversus other nanofillers is the ability to increase the electrical conductivity ofnonconductive polymers and the measured electrical resistivity of PHO and thenanocomposite samples provides an example of such ability. The required TRGloading to produce electrically conductive PHO, the electrical percolation, isslightly above 0.5 vol.%. This low percolation limit is an indication of a gooddispersion of TRG into the matrix. The observed percolation limit is similarto that solution processed polyethylene-TRG (23) but lower than that of polarpolymer-TRG composites (22). A further increase in the loading of TRG greatlyincreases the electrical conductivity (decreases resistivity) as shown in Figure 6.Compared to the resistivity of the pure polymer of 1.3x103 MΩ.m, a resistivity ofless than 5 Ω.m is obtained with 2.5 vol.% loading of TRG.

Figure 6. Effect of TRG loading on the electrical resistance of PHO-TRGnanocomposites.

207

Dow

nloa

ded

by U

NIV

SO

UT

HE

RN

MIS

SISS

IPPI

on

Nov

embe

r 9,

201

4 | h

ttp://

pubs

.acs

.org

P

ublic

atio

n D

ate

(Web

): N

ovem

ber

22, 2

013

| doi

: 10.

1021

/bk-

2013

-114

4.ch

014


Conclusions

We have successfully produced PHO, TRG, and their nanocomposites. Thesolvent blended PHO-TRG nanocomposite with TRG loading of 0.5, 1, and2.5 vol.% exhibited enhanced thermal, mechanical, and electrical propertiescompared to the pure PHO. The addition of TRG resulted in a slight upshift inthe glass transition, increase in the melt temperature, and decrease in crystallinty.Regardless of this decrease in crystallinity, the mechanical properties of thecomposite sample revealed a striking 600% increase in modulus with 2.5 vol.%TRG while maintaining elongation at break above 100%. Electrically conductivePHO samples can be made with the addition of slightly more than 0.5 vol% TRGand a very low resistivity of less than 5 Ω.m is obtained with 2.5 vol.% TRG.These promising results would increase the applications of medium chain PHAs.

Acknowledgments

Financial support from the Abu Dhabi-Minnesota Institute for ResearchExcellence (ADMIRE) is acknowledged. The authors also thank Dr. MariosKatsiotis at the Department of Chemical Engineering, the Petroleum Institute forthe TEM work and Mr. Daniel Rouse, the university of Minnesota, for his role inthe synthesis of PHO.

References

1. Jackson, J. K.; Srienc, F. Effects of recombinant modulation of the phbCABoperon copy number on PHB synthesis rates in Ralstonia eutropha. J.Biotechnol. 1999, 68 (1), 49–60.

2. Chen, G. Q.; Wu, Q. The application of polyhydroxyalkanoates as tissueengineering materials. Biomaterials 2005, 26 (33), 6565–6578.

3. Barrett, J. S. F.; Srienc, F. Green Chemistry for the Production ofBiodegradable, Biorenewable, Biocompatible, and Polymers; John Wiley &Sons, Inc.: New York, 2011.

4. Foster, L. J. R.; et al. Biosynthesis and characterization of deuteratedpolyhydroxyoctanoate. Biomacromolecules 2006, 7 (4), 1344–1349.

5. Sanguanchaipaiwong, V.; et al. Biosynthesis of natural-synthetic hybridcopolymers: polyhydroxyoctanoate-diethylene glycol. Biomacromolecules2004, 5 (2), 643–649.

6. Botana, A.; et al. Effect of modified montmorillonite on biodegradable PHBnanocomposites. Appl. Clay Sci. 2010, 47 (3−4), 263–270.

7. Maiti, P.; Batt, C. A.; Giannelis, E. P. New biodegradablepolyhydroxybutyrate/layered silicate nanocomposites. Biomacromolecules2007, 8 (11), 3393–3400.

8. Xu, C.; Qiu, Z. Crystallization behavior and thermal property ofbiodegradable poly (3‐hydroxybutyrate)/multi‐walled carbon nanotubesnanocomposite. Polym. Adv. Technol. 2011, 22 (5), 538–544.

208

Dow

nloa

ded

by U

NIV

SO

UT

HE

RN

MIS

SISS

IPPI

on

Nov

embe

r 9,

201

4 | h

ttp://

pubs

.acs

.org

P

ublic

atio

n D

ate

(Web

): N

ovem

ber

22, 2

013

| doi

: 10.

1021

/bk-

2013

-114

4.ch

014


9. Yun, S. I.; et al. Mechanical properties of biodegradablepolyhydroxyalkanoates/single wall carbon nanotube nanocomposite films.Polym. Bull. 2008, 61 (2), 267–275.

10. Byrne, M. T.; Gun’ko, Y. K. Recent advances in research on carbonnanotube–polymer composites. Adv. Mater. 2010, 22 (15), 1672–1688.

11. Verdejo, R.; et al. Graphene filled polymer nanocomposites. J. Mater. Chem.2011, 21 (10), 3301–3310.

12. Lee, C.; et al. Measurement of the elastic properties and intrinsic strength ofmonolayer graphene. Science 2008, 321 (5887), 385–388.

13. Kim, H.; Abdala, A. A.; Macosko, C.W. Graphene/polymer nanocomposites.Macromolecules 2010, 43 (16), 6515–-6530.

14. Riis, V.; Mai, W. Gas chromatographic determination of poly- beta-hydroxybutyric acid in microbial biomass after hydrochloric acidpropanolysis. J. Chromatogr., A 1988, 445 (1), 285–289.

15. Prud’homme, R. K.; et al. Thermally Exfoliated Graphite Oxide; Trustees ofPrinceton University: Princeton, NJ, 2010; p 65.

16. McAllister, M. J.; et al. Single sheet functionalized graphene by oxidationand thermal expansion of graphite. Chem. Mater. 2007, 19 (18), 4396–4404.

17. Staudenmaier, L. Method for the preparation of graphitic acid. Ber. Dtsch.Chem. Ges. 1898, 31, 1481–1487.

18. Steurer, P.; et al. Functionalized graphenes and thermoplasticnanocomposites based upon expanded graphite oxide. Macromol. RapidCommun. 2009, 30 (4−5), 316–327.

19. Ramanathan, T.; et al. Functionalized graphene sheets for polymernanocomposites. Nat. Nanotechnol. 2008, 3 (6), 327–331.

20. Hua, L.; et al. A new poly (L-lactide)-grafted graphite oxide composite:facile synthesis, electrical properties and crystallization behaviors. Polym.Degrad. Stab. 2010, 95 (12), 2619–2627.

21. Ponnamma, D.; et al. Rubber Nanocomposites: Latest Trends and Concepts.In Advances in Elastomers II; Springer: New York, 2013; pp 69−107.

22. Kim, H.; Miura, Y.; Macosko, C.W. Graphene/polyurethane nanocompositesfor improved gas barrier and electrical conductivity. Chem. Mater. 2010, 22(11), 3441–3450.

23. Kim, H.; et al. Graphene/polyethylene nanocomposites: Effect ofpolyethylene functionalization and blending methods. Polymer 2011, 52(8), 1837–1846.

209

Dow

nloa

ded

by U

NIV

SO

UT

HE

RN

MIS

SISS

IPPI

on

Nov

embe

r 9,

201

4 | h

ttp://

pubs

.acs

.org

P

ublic

atio

n D

ate

(Web

): N

ovem

ber

22, 2

013

| doi

: 10.

1021

/bk-

2013

-114

4.ch

014


[acs symposium series] green polymer chemistry: biocatalysis and materials ii volume 1144 ||...

Documents