Fabrication of GrapheneâPolymer Nanocomposites Through Ionic

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Fabrication of Graphene-Polymer Nanocomposites Through Ionic Polymerization Kausala Mylvaganam, and Liangchi Zhang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b05333 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 19, 2016

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Fabrication of Graphene-Polymer Nanocomposites Through Ionic Polymerization Kausala Mylvaganam a*, Liangchi Zhang b* Laboratory for Precision and Nano Processing Technologies The University of New South Wales, NSW 2052, Australia a

[email protected], [email protected]

ABSTRACT: The extraordinary properties of graphene nanosheets (GNS) and high performance of polymer-based composites have stimulated extensive research in the realm of polymer nanocomposites. This work examines the mechanisms and approach for the production of GNS-polymer composites by first principle ab initio calculations. The results show that GNS functionalized with anionic/cationic moieties can initiate anionic/cationic polymerization reactions, leading to chemically bonded GNS-polymer composites via the established anionic/cationic polymerization schemes. These outcomes deliver a solid theoretical basis for fabricating strong polymer nanocomposites.

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1.

Introduction

Traditional polymer-based composites contain a significant quantity (~60 vol%) of fibres dispersed in a polymer matrix, in which the fibres are used to reinforce the material. Usually glass, carbon or aramid fibres are used with epoxy polymers or polyesters and the resulting fibre-reinforced polymers are used in aerospace, automotive and marine industries. On the other hand polymer nanocomposites with very low loadings of nanofillers (~2 vol%) such as carbon nanotubes (CNTs) 1-3 and graphite nanoplatelets ,4, 5 provide much stronger multifunctional materials by optimizing the interface chemistry and the dispersion of nanofillers. The use of CNTs as nanofillers attracted considerable attention due to their outstanding mechanical, electrical and thermal properties as well as their high aspect ratio and surface area. Although introducing CNTs to polymer matrices improves the properties of the resulting polymer composites, the use of CNTs in nanocomposites have been limited to some extent due to the challenges in processing and dispersing CNTs. The interaction characteristics between CNTs and the polymer matrix significantly influence the performance of the nanocomposites.

Expanded graphite (EG) has also been explored as nano-fillers for manufacturing conductive polymer nanocomposites. For example, Zheng et al. 6 made expanded graphite by oxidation of natural graphite flakes, followed by thermal expansion at 600 ºC. Then they soaked the expanded graphite with styrene (St) and acrylonitrile (AN) monomers using 2,2´-azobis(isobutyronitrile) initiator to produce poly(St-co-AN)/EG composite. They found that the composite had well-connected graphite network with high electrical conductivities. However, as EG composed of many graphene sheets held together by van


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der Waals forces, and as such most of the sheets are not available to effectively interact with the polymer matrix, the performance would be typically limited. Ramanathan et al. 4 used sonication to break the EG into thinner graphitic nanoplatelets, and high-speed shearing methods to disperse the platelets within a solution of poly(methyl methacrylate) (PMMA) and thereby obtained a 30 ºC increase in glass transition temperature (Tg) at 1 to 5 wt% loading of graphitic nanoplatelets.

The above results of Ramanathan et al. clearly shows that a complete exfoliation of graphite into GNS and their dispersion in polymer solution would result in nanocomposites with better properties. It was shown that only a 0.05 wt% of functionalized graphene sheet (produced by rapid thermal expansion of completely oxidized graphite oxide) in PMMA gave an improvement in Tg, modulus, ultimate strength and thermal stability. 7 The above improvement was explained by the better interaction of functionalized GNS with polymer.

In the “grafting-from” and “grafting-to” approaches of making graphene-polymer composites, reactions are conducted between preformed functionalized polymer chains and functional groups on graphene oxide. Whereas in the in-situ polymerization technique, functionalized GNS can be incorporated to initiate the polymerization process. This means polymer chains can be grown on the GNS which can lead to a much stronger interaction between GNS and polymer.

For instance, Zhu and Tour

8

functionalized graphene nanoribbons with anionic (p-

sulfobenzene-diazonium) and cationic (p-aminobenzene-diazonium) moieties to make GNR



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thin film transistor semiconductor devices. In this work, GNS are first functionalized using commonly used anionic and cationic initiators, namely iso-butyl lithium and proton (strong protic acid having a weak nucleophilic counterion) respectively. Then the functionalized GNS are used as initiators to grow polymer chains from monomer units. These mechanisms are explored and the feasibility of growing polymer chains on the functionalized GNS, by anionic and cationic polymerization reactions are demonstrated.

This paper is organized as follows: Section 2 presents the methodology including the basis set and functional used in performing the density functional theory calculations. Section 3 presents the optimized structures of the molecular species involved in the functionalization, initiation and propagation of the anionic and cationic polymerization reactions together with a detailed discussion on the feasibility of forming chemically bonded GNS-polymer composites. Section 4 presents the conclusions of this study.

2.

Computational Method

GNS were modelled by a sector of a 2D-graphene nanosheet having 36 carbon atoms with hydrogen atoms added to the dangling edge carbon atoms. The resulting model C36H16 was used to study the in-situ ionic polymerization reactions. Anionic polymerization was examined using the simplest chiral epoxide (1, 2-propylene oxide) monomer and the cationic polymerization was examined using isobutene monomer. To study the anionic polymerization reaction, first, the graphene nanosheet model, C36H16, was functionalized with an isobutyl anion (C4H9−) and the resulting anion functionalized GNS, [GNS- C4H9] −


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was used as an initiator to initiate the polymerization of epoxide. Thus the reaction of an isobutyl anion with GNS and the subsequent reaction of [GNS- C4H9]− with 1, 2-propylene oxide were examined for the initiation step. The succeeding reaction of [GNS-isobutylpropylene oxide] − with another epoxide monomer was examined for the propagation step. To study the cationic polymerization reaction, first the graphene nanosheet model, C36H16, was functionalized using a proton and the resulting cation functionalized GNS, [GNS- H]+ was used as an initiator to initiate the polymerization of isobutene. Thus the reaction of H+ with GNS and the subsequent reaction of the newly formed [GNS-H]+ with isobutene were examined for the initiation step. The succeeding reaction of [GNS-H-isobutyl]+ with another isobutene monomer was examined for the propagation step.

The calculations were performed using density functional theory (DFT). The geometry of the reactants and products of every step were fully optimized with a 6-31G** basis set 9 and hybrid exchange correlation functional B3LYP. 10-13 To include the solvation energies that are important in the study of ionic reactions, the energies of all the molecular species were also calculated in solution. The solution was modelled by the polarizable continuum model (PCM) using the integral equation formalism variant (IEFPCM) via the “SCRF” keyword in Gaussian 09. This method creates the solute cavity via a set of overlapping spheres placed around each solute atom using universal force field radii. The atomic charge distributions were analysed by the Mulliken method. For reactants and products having an odd number of electrons, the unrestricted formalism, DFT(UB3LYP)/6-31G**, was used.



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All computations were carried out using the ab initio quantum chemistry package, Gaussian 09. 14

3.

Results and Discussion

A.

Anionic functionalization of GNS and subsequent reactions with 1,2-propylene oxide

We had revealed mechanisms of various chemical reactions 15-19 through first principle calculations using the functional and basis set as specified in Section 2. Anionic functionalization of GNS was examined by studying the reaction between C36H16 and isobutyl lithium as detailed in Ref. 20 . Then the formation of polymer composite was examined using the anion functionalized GNS as an initiator and the chiral epoxide (1,2propylene oxide) as a monomer through DFT calculations. The computed gas-phase and solution-phase energies of the molecular species studied in the anionic functionalization of GNS and subsequent reactions with 1,2-propylene oxide are summarized in Table 1. The structural details of the molecular species are given in the supporting information.

Table 1: Energies of the molecular species involved in the anionic reaction. Molecular species

E (gas-phase)/Eh

E (solution)/Eh

(B3LYP/6-31G**)

(B3LYP/6-31G**)

GNS (C36H16)

-1381.624804

-1381.633073

C4H9−

-157.761028

-157.854270

[C36H16-C4H9]− (i.e. [af-GNS]−)

-1539.495561

-1539.558996

1,2-propylene oxide (i.e. epoxide)

-193.116992

-193.120999


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[af-GNS-H2C‒CH(CH3)O] −

-1732.598126

-1732.683168

[af-GNS-H2C‒CH(CH3)O-H2C‒CH(CH3)O] −

-1925.757368

-1925.841722

Anionic functionalization of GNS: C36H16 + C4H9−

[C36H16-C4H9]−

(1)

Figure 1. DFT(B3LYP)/6-31G** optimized geometry of isobutyl anion functionalized (at a basal plane carbon) GNS, [af-GNS] −.

The DFT(B3LYP)/6-31G** optimized geometry of the isobutyl anion functionalized GNS, [af-GNS]− is shown in Figure 1. The carbon atom of the GNS that is functionalized by the anion is highlighted in yellow. During this functionalization reaction, as the anion C4H9− approaches the graphene carbon atom and forms a bond with that carbon atom, the pi-bond at that C atom breaks and the neighbouring carbon atom gets a negative charge. This negative charge can be carried away via the conjugated pi bonds of the GNS. As shown in



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our previous paper, from the Mulliken charge distribution analysis it is clear that the negative charge is now distributed on several edge C atoms of the GNS 20. The difference in energy between the reactants and products shows that in the gas-phase, this functionalization reaction is exothermic. In solution, the exothermicity of the reaction is reduced to some extent, but the reaction is still exothermic and releases 44.96 kcal/mol.

To explore the mechanism involved in the formation of GNS-polymer nanocomposites, in this section, the in-situ anionic polymerization reaction is examined. In an anionic polymerization, the active centres are anions, and polymerization can be carried out through a carbanion active species such as C4H9−. Like any chain-growth polymerization reaction, the anionic polymerization takes place in three steps: chain initiation, chain propagation, and chain termination. Living polymerizations lack a formal termination pathway. Here in the formation of GNS-polymer nanocomposites, the chain initiation step involves two reactions. First, the anion initiator functionalizes the GNS as explained above and then the negatively charged functionalized GNS, [C36H16-C4H9]− reacts with a monomer molecule (e.g. 1,2-propylene oxide) and thereby initiates the polymerization reaction: O [af-GNS] + H2C — CH(CH3) −

[af-GNS-H2C‒CH(CH3)O] −

(2)

As explained above, in the functionalized GNS, the negative charge is carried away to the edge carbon atoms. When this functionalized GNS reacts with the monomer, 1,2-propylene oxide, it attacks the first carbon atom and the epoxide ring opens up as shown in Figure 2(a). Now the negative charge is mainly on the oxygen. The DFT(B3LYP)/6-31G** optimized geometry of the resultant anion, [af-GNS-H2C‒CH(CH3)O] − is shown on the


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right-hand side of Figure 2(a). During this reaction, there is a change in the bonding of the GNS carbon atom, as it bonds to the monomer. As a result, this GNS carbon atom becomes sp3 hybridized as highlighted in yellow. Some energy is absorbed during this change in bonding as well as for the breaking of one of the –C–O– bonds in the epoxide, and some energy is released when the new bond is formed. Calculations show that in the gas phase, the reaction is endothermic and absorbs 9.1 kcal/mol. In solution, the reaction becomes marginally exothermic and releases 1.99 kcal/mol. Thus the initiation step, which is a combination of reactions (1) and (2), is exothermic.

(a)

(b)



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Figure 2. DFT(B3LYP)/6-31G** optimized geometries of the reactants and product of (a) chain initiation reaction, and (b) chain propagation reaction of anionic polymerization.

In the chain propagation step, the new anion generated by the above initiation reaction reacts with the monomer, 1,2-propylene oxide, and produces another new anion: [af-GNS-H2C‒CH(CH3)O] − + epoxide

[af-GNS-H2C‒CH(CH3)O-H2C‒CH(CH3)O] − (3)

The optimized geometry of the new anion is shown on the right-hand side of Figure 2(b). The difference in gas-phase energies between the reactants and the product show that this reaction is exothermic and produces 26.5 kcal/mol. In solution, the reaction is still exothermic and produces 23.57 kcal/mol. A Mulliken charge density analysis showed that the negative charge is localized to the free end of the growing polymer chain. The above reaction can propagate with more and more 1,2-propylene oxide monomers, resulting in a polymer chain bonded to [af-GNS] −. Hence the overall reaction can be highly exothermic.


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B.

Cationic functionalization of GNS and subsequent reaction with isobutene

Cationic functionalization of GNS was examined by studying the reaction between H+ and C36H16. (Required H+ can be produced from high concentration of strong protic acids having a weak nucleophilic counterion). Then the composite formation was examined using the proton functionalized GNS as an initiator and isobutene as a monomer by DFT calculations. The computed gas-phase and the solution-phase energies of the molecular species studied in the cationic functionalization of GNS and subsequent reactions with isobutene are summarized in Table 2. The optimized geometries of the molecular species are given in the supporting information.

Table 2: Energies of the molecular species involved in the cationic reaction. Molecular species

E (gas-phase)/Eh

E (solution)/Eh

(B3LYP/6-31G**)

(B3LYP/6-31G**)

C36H16 (i.e. GNS)

-1381.624804

-1381.633073

H+

0

-0.164564

[C36H16-H]+ (i.e. [cf-GNS]+)

-1381.970786

-1382.025560

(CH3)2-C=CH2 (i.e. isobutene)

-157.238844

-157.240043

[cf-GNS-CH2-C-(CH3)2]+

-1539.125609

-1539.173932

[cf-GNS-CH2-C-(CH3)2-CH2-C-(CH3)2]+

-1696.372410

-1696.424588

Cationic functionalization of GNS: C36H16 + H+

[C36H16‒H]+


(4)


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Figure 3. DFT(B3LYP)/6-31G** optimized geometry of basal plane [cf-GNS] +.

The DFT(B3LYP)/6-31G** optimized geometry of the basal plane cation functionalized GNS, [C36H16-H]+ is shown in Figure 3. The GNS carbon atom with which H+ reacted, and formed a −C−H bond, is highlighted in yellow. As the cationic initiator, H+ approaches one of the carbon atom of GNS, the pi-bond with that carbon atom breaks and gives the electrons to that C atom. As a result, the C atom next to it becomes positively charged and this positive charge is carried away to the other carbon atoms of the GNS through the conjugated pi bonds. As stated in our previous paper, Mulliken partial charge analysis showed that the positive charge is distributed over several carbon atoms of the GNS 20. The difference in energy between the product and the reactants showed that the basal plane cationic functionalization reaction is exothermic. In solution, the reaction exothermicity is reduced, but the reaction is still exothermic and releases 143.02 kcal/mol.


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Next, in the formation of GNS-polymer nanocomposites, the involvement of cation functionalized GNS is examined through the in-situ cationic polymerization reaction. In cationic polymerization, the active centres are cations and the polymerization can be carried out using a strong protic acid where the proton is the initiator. The types of monomers necessary for this reaction are limited to olefins with electron donating substituents and heterocycles. Primarily, cationic polymerization is used for the commercial preparation of polyisobutene and poly(N-vinylcarbazole). In this work the polymerization of isobutene is examined. In the chain initiation step, the cationic initiator first reacts with GNS and thereby GNS becomes functionalized as detailed above. Then the [cf-GNS]+ reacts with isobutene and initiates the polymer chain formation: [cf-GNS]+ + (CH3)2-C=CH2

[cf-GNS- CH2-C-(CH3)2]+

(a)

(b)


(5)


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Figure 4. DFT(B3LYP)/6-31G** optimized geometries of the reactants and product of the (a) chain initiation reaction between [cf-GNS]+ and (CH3)2-C=CH2,and (b) chain propagation reaction between [cf-GNS-CH2-C(CH3)2]+ and (CH3)2-C=CH2.

During this reaction, the [cf-GNS]+ attacks the electrophilic carbon atom (i.e., the 1st carbon atom) of isobutene. Now the 2nd carbon atom of isobutene becomes positive. The DFT(B3LYP)/6-31G** optimized geometry of the resultant cation [cf-GNS-CH2C(CH3)2]+ is shown on the right-hand side of Figure 4(a). During this reaction, there is a change in the bonding of the GNS carbon atom as it bonds to the monomer, isobutene. As a result, this GNS carbon atom becomes sp3 hybridized as highlighted in yellow. Some energy would be consumed for breaking the pi-bond of isobutene and for the change in the bonding of the GNS carbon atom. On the other hand, some energy will be released when the new bond is formed. The energy difference between the reactants and the products show that the reaction is endothermic and absorbs 52.72 kcal/mol. In solution, the reaction is still endothermic and absorbs 57.73 kcal mol. However, the overall initiation step, which involves reactions (4) and (5), is exothermic.


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In the propagation step, the cation [cf-GNS-CH2-C(CH3)2]+ generated in the initiation step reacts with a monomer as shown in the left-hand side of Figure 4(b) and generates a new cation. The optimized geometry of this new cation is shown on the right-hand side of Figure 4(b). During this reaction, the pi-bond of the new monomer, (CH3)2-C=CH2 is broken and a covalent bond is formed between [cf-GNS-CH2-C(CH3)2]+ and (CH3)2C=CH2. A Mulliken charge density analysis showed that the positive charge is now located at the free end of the growing polymer chain. This reaction is slightly exothermic and releases 4.99 kcal/mol. In solution, the reaction is still marginally exothermic and releases 6.66 kcal/mol. We have also studied the propagation reaction with a second monomer molecule and found that the reaction is exothermic and releases 21.56 kcal/mol in the gasphase and 18.31 kcal/mol in solution. The above propagation reaction can continue by reacting with more and more isobutene monomers leading to a polymer chain bonded to cation functionalized graphene nanosheet. Hence the overall reaction can be highly exothermic.

Clearly, several carbon atoms of the GNS can be functionalized at the same time either by anionic or cationic moieties. Hence, as demonstrated above, more and more polymer chains can grow on the GNS and a strong GNS-polymer composite can be produced. Although the solvation energy that is important in the study of ionic reactions contributes significantly (as much as 0.1 Hartree, i.e. 62.5 kcal/mol) to the energies of the individual ionic species as listed in Tables 1 and 2, it did not make a big change in the reaction energies. This is because both the reactant(s) and product(s) involve ionic species.



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4.

Conclusions

This article has provided a solid theoretical basis for the fabrication of chemically bonded GNS-polymer nanocomposites by first functionalizing the GNS . DFT calculations showed that the anion/cation functionalized GNS can function as initiators for the in situ polymerization of epoxy polymers/polyolefins and thereby the polymer chains are chemically bonded to GNS. Thus the established anionic and cationic polymerization schemes can be used to make strong GNS-polymer composites.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Tel: +61-2-9385 6548

*E-mail: [email protected]

Tel: +61-2-9385 6078

ACKNOWLEDGMENTS This research was undertaken with the assistance of resources provided at the NCI National Facility systems at the Australian National University and the Intersect Australia Ltd. through the National Computational Merit Allocation Scheme supported by the Australian Government.

REFERENCES


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