Study of the Molecular Interactions between Functionalized Carbon

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A Study of the Molecular Interactions between Functionalized Carbon Nanotubes and Chitosan Dalyndha Aztatzi-Pluma, Edgar Omar Castrejón-González, Armando Almendarez-Camarillo, Juan F. J. Alvarado, and Yareli Duran-Morales J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08136 • Publication Date (Web): 15 Jan 2016 Downloaded from http://pubs.acs.org on January 22, 2016

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TheJournalofPhysicalChemistry

A Study of the Molecular Interactions between Functionalized Carbon Nanotubes and Chitosan D. Aztatzi-Pluma, Edgar O. Castrejón-González, ∗ A. Almendarez-Camarillo, Juan F.J. Alvarado, and Y. Durán-Morales Departamento de Ingeniería Química, Instituto Tecnológico de Celaya. Celaya, Gto. 38010, México. E-mail: [email protected]

Phone: +52 (461)611 7575. Fax: +52 (461)611 7575

∗ To

whom correspondence should be addressed

1 ACSParagonPlusEnvironment


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Abstract Molecular Dynamics (MD) simulations were performed to calculate the interaction between chitosan at different degrees of deacetylation (DD) and carbon nanotubes (CNTs) functionalized with either amine (-NH2 ) or carboxylic (-COOH) groups. The objective was to elucidate the effect of the CNT functionalization type and the different DD of chitosan on the mechanical properties of the nanocomposite. For a certain DD, where the glucosamine and acetyl-glucosamine units are uniformly distributed along the chitosan chain, MD simulations showed that this molecule depicts a large contacting superficial area that allows the interaction with the functionalized CNT. It was also found that the attractive interaction between a 50% DD chitosan and the -NH2 functionalized CNT (CNT-NH2 ) was the strongest among the different deacetylated cases under study. For the 50% DD case, a wrapping effect of the CS chain around the CNT-NH2 structure was displayed which was attributed to hydrogen bonds formation between the amine groups in the CNT and the -OH and -NH2 groups in the chitosan molecule. Composite films of chitosan, reinforced with multi-wall carbon nanotubes (MWCNT) functionalized with either -NH2 (MWCNT-NH2 ) or -COOH (MWCNT-COOH) were prepared to measure their Young’s module. The experiments showed that the films reinforced with MWCNT-NH2 exhibited larger Young’s modules than those functionalized with -COOH groups. The above was due, likely, to the strong interaction between the amino functionalized CNTs and the polymeric matrix. The simulation results were in agreement with the experimental data.

Introduction The reinforcement of natural and synthetic polymers using carbon nanotubes has been studied using both experimental and computational techniques. 1–8 The CNTs, as reinforcing agents, have been used mainly because their mechanical, chemical and electrical properties enhance the properties of the polymeric materials. 9 However, the CNTs form stable aggregates due to emerging van der Waals type interactions and these aggregates are quite difficult to disperse into the polymeric


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matrix. 6,7,10–15 The dispersion of CNTs into polymeric matrices can be achieved by several procedures. 16–18 One of these, widely used, is the functionalization of the CNTs with active chemical groups compatible with the polymeric matrix. 19,20 Chen and co-workers 21 prepared a nanocomposite by dispersing MWCNT functionalized with amine into a polymeric Nylon-6 matrix. Their measurements showed that the nanocomposite Young’s module improved notably. They observed also, from electron microscopy (TEM and SEM), a homogeneous dispersion of the MWCNTs into the polymeric matrix with a strong interfacial adhesion of the MWCNTs to the polymeric matrix. Experimental work 1,2,9 has showed that compatibility between some polymeric matrices and CNTs is enhanced by functionalizating the CNTs. However, it has been difficult to elucidate, at molecular level, the way the interactions between functional groups work to promote the dispersion of CNTs into the polymeric matrices. The objective of this work is to investigate this process by approaching the issue using MD simulations. A number of authors have employed MD simulations to determine the interactions between single-wall carbon nanotubes (SWCNT) and: polyethylene/polypropylene/polystyrene/polyaniline, 22 polyphenylacetylene, 23 and polyethylene. 24,25 In all cases, it was found that the CNT chirality has a definitive influence on the interaction with the polymer. The common conclusion was that the armchair type CNT promote strong interactions with the polymer molecules. Chen and coworkers 23 chemically modified CNT surfaces with fluoride -F, amine -NH2 and carboxyl -COOH groups and found that the interaction energies between functionalized CNTs and polyethylene (PE) were dependent on the chemical group attached to the CNT surface. The CNT-COOH arrangement depicted the strongest attractive interaction with the PE chains. Simulations using the ReaxFF 26 force field model were performed by Zaminpayma and Mirabbaszadeh 27 to simulate the adhesion of conjugated polymers with SWCNT in vacuum. The study revealed that the interaction between the SWCNT and the polymers was not influenced by the temperature, but it was strongly dependent on the SWCNT radius and chirality. The interaction increased with the radius and the chirality angle. The conclusion was that armchair SWCNT with



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increasing radius is the best nanotube type to be used as reinforcement. In the reinforcement of polymeric matrices with CNTs a wrapping effect of the polymeric chains around the CNTs is observed. This effect modify, as expected, the properties of the nanocomposites. 28 Tallury and Pasquinelli 29 used MD simulations to study the wrapping of flexible polymeric chains around SWCNTs. Their results indicated that, depending on the composition and chemical structure of the polymer, there exists a fluctuating preference of the polymeric chains to wrap the CNTs. If the intra chain interactions dominate, then the chains display an agglomerate conformation and no wrapping effect is present. Poly-caprolactam and Nylon-6 are polymers with high wrapping response and large absolute interaction energies. The reinforcement of polymers with CNT has allowed extending their applications widely. Chitosan (CS) is a biopolymer with notable chemical and biological properties; it is biocompatible, biodegradable and no toxic. 30 CS is formed by a large sequence of glucosamine (GlcN) and nacetyl glucosamine (GlcNA) units; Figure 1. The number of GlcN units is determined by the degree of deacetylation and this parameter impacts on the biopolymer’s chemical and mechanical properties. Reinforcing chitosan with CNTs to enhance its mechanical properties has been a long time research topic. It has been experimentally observed that the dispersion of the CNTs into the polymeric matrix depends on its DD; 31 causing even a variation of its mechanical properties. The understanding, at molecular level, of the CS-CNT interactions, is an important issue that must be elucidated.


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O HO

OH O

NH O

O HO

CH 3

O

NH 2

OH

[ −GlcN − GlcNA −]n

n

Figure 1: Schematic representation of the repetitive unit of chitosan. GlcN and GlcNA refer to N-Glucosamine and N-Acetyl-Glucosamine, respectively. Skovstrup and co-workers 32 studied the flexibility of chitosan employing MD simulations and concluded that the GlcN and GlcNA building blocks distribution impacts on flexibility. They found that the most flexible chain conformation is obtained when the GlcN-GlcNA linkage dominates along the CS chain because this sequence promotes -OH group interactions. Saha and coworkers 33 reported that the interaction energies between CNTs and polymers with similar molecular weight but different structure, are ruled by the number of repetitive monomeric units along the chain but it is independent of the molecular weight. Sandoval and co-workers, 34 using MD simulations, determined the effect of -NH 2 and -CH2 OH functional groups in blends of CS with poly-vinyl alcohol (PVA) and poly-2-hydroxy ethyl methacrylate (P2HEM). Their results showed the effect of polymer concentration on the interactions of CS’s functional groups. At low concentrations of PVA and P2HEM, there is a preferential interaction with the hydroxymethyl (-CH 2 OH) group of CS and as the concentration of the polymers increases the interactions with the amino groups also augments. The results showed consistency with experimental FTIR data. Jawalkar and co-workers 35 additionally showed that miscibility of the CS/PVA blend is attributed to hydrogen bonds formed by the -NH 2 and -CH2 OH on the CS chain and the -OH group on the PVA. Dispersion of CNTs into polymeric matrices affects the mechanical properties of the nanocom-



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posites. A good dispersion gives rise to strong interactions with the polymer; facilitating the distribution of stresses. 36 Rungrotmongkol and co-workers, 37 using MD simulations, concluded that dispersion and solubility of CNTs increase with the CS concentration. They also observed that acetyl groups in the GlcNA units depict stronger interactions with the aromatic rings of CNTs than those with the amine groups in the GlcN units. Wang and co-workers 38 prepared CS/MWCNTs nanocomposites where the MWCNTs were functionalized with carboxylic and hydroxyl groups. They concluded that strong interactions between these groups and the CS matrix are formed, giving rise to a homogeneous dispersion of the CNTs into the CS matrix. The mechanical properties of the CS/MWCNTs nanocomposite were dramatically improved compared to those of neat CS. Computational studies have allowed a better understanding about the macroscopic behavior of nanocomposites. 37 There exist a large number of research papers discussing several issues related to these materials. For example: the impact and role of the chemical groups on the CNT surface, 23,24 the wrapping effect of the polymeric chains around the CNTs 29 and the influence of the deacetylation on the mechanical properties of CS. 1 However, there have not been reported studies discussing how the interactions at molecular level are related to mechanical properties of these nanocomposites. The understanding of interfacial interaction between CNT and CS is crucial to explain the mechanism of how the mechanical properties are improved. 18,22,23,25 Some authors have demonstrated that chemical modification of CNTs can improve the load transfer between CNTs and polymer when nanocomposites are being produced. 1,39 Therefore the main objective of this work is to identify the way the atomic interactions between CS and functionalized CNT affect the mechanical properties of the nanocomposites; particularly the Young’s modulus. In this direction, MD simulations are employed to analyze the interactions between CNTs functionalized with carboxylic and amine groups and CS with different DD, ignoring the effect of the solvent bacause the aim is studying only the CS/CNT interactions. On the other hand, CS/MWCNTs nanocomposite films were prepared with MWCNT functionalized with amine and carboxylic groups.


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Methodology Molecular Model Chitosan One chain formed by 20 (GlcN, GlcNA) units with different DD was considered. The chain, for the sake of simplicity, is relatively short compared to real CS chains. However, it represents a generic CS molecule. 37,40 The Figure 2 shows the distribution of the GlcN and GlcNA units along the chain. In this figure the DD considered were: a) 0% (pure chitin), b) 10%, c) 20%, d) 40%, e) 50%, f) 75%, g) 85%, and h) 100% (pure chitosan). Every chain sequence was generated using the Medea 2.15.9 41 software.

[ −GlcNA −]20 b) [ GlcNA ] − GlcN − [ GlcNA ] − GlcN − [ GlcNA ] 6 6 6 c) ⎡ − [ GlcNA ] − GlcN − [ GlcNA ] − ⎤ ⎣ 2 2 ⎦4 d) [ −GlcNA − GlcN − GlcNA − GlcN − GlcNA − ] 4 e) [ −GlcNA − GlcN − ]10 f) [ −GlcN − GlcNA − GlcN − GlcN − ] 5 g) [ GlcN ] − GlcNA − [ GlcN ] − GlcNA − [ GlcN ] − GlcNA − [ GlcN ] 4 4 5 4 h) [ −GlcN − ] 20 a)

Figure 2: CS monomer sequences considered in the simulations for the following DD: a) 0%, b) 10%, c) 20%, d) 40%, e) 50%, f) 75%, g) 85%, and h) 100%.

Carbon Nanotube Three different models of SWCNTs were considered for the simulations. All of them were armchair type (10,10) because, according reported data, 22,23,25 this chirality depicted the best interacting conditions with polymeric matrices. Figure 3a shows a scheme of the first model, which is formed by 400 carbon atoms with sp 2 hybridization. The unsaturation effect at the extremes of the CNT was damped by adding 40 hydrogen atoms at the ends of the CNT. The bond length between carbon atoms (C-C) is 1.42 Å, meanwhile the bond length between hydrogen and carbon atoms 7 ACSParagonPlusEnvironment


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(C-H) is 1.14 Å. The second model, depicted in Figure 3b, is an armchair CNT (10,10) functionalized with carboxylic groups (-COOH). The CNT’s surface was modified by randomly adding, -COOH functional groups at sites were the hybridization of carbon changed from sp 2 to sp3 . The amount of atoms corresponding to -COOH groups represented only 5% of the total. Finally, the third model, depicted in Figure 3c, is similar to the second one, the only difference is that -COOH groups were substituted by amine groups (-NH 2 ).

a) Pristine

b) With carboxilic groups (-COOH)

c) With amine groups (-NH2)

Figure 3: CNT Molecular structures for a) Prystine, b) Functionalized with carboxylic groups (-COOH), and c) Functionalized with amino groups (-NH2 ). Gray, white, blue and red spheres represent carbon, hydrogen, nitrogen, and oxygen atoms, respectively.


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Computational Method Interactions between functionalized CNT and CS were quantified through MD simulations using the LAMMPS (Large-Scale Atomic/Molecular Massively Parallel Simulator) code. 42 The COMPASS (Condensed phase Optimized Molecular Potential for Atomistic Simulation Studies) force field, 43 as implemented in LAMMPS, was employed in all the simulations. Initial configuration and the force field parameters were set up from the Medea package. 41 For the initial configurations the CS chain was set at 24 Å away from the mass center of the CNT as suggested by Chen and co-workers. 23 Simulations were performed considering a NVT ensemble. The temperature, fixed at 300 K, was controlled with a Nosé-Hoover thermostat. 44,45 The calculation of long-range contributions to the potential energy was done via the Ewalds sums method. The cut distance for non-bonded interactions was fixed at 9.5 Å. The integration time to equilibrate the systems was fixed at 350 ps after which additional 100 ps were performed to generate results. The integration step in all cases was set at 0.1 fs and the interaction energy between the CS chain and the CNT was collected every 10 ps.

Experimental Methodology Materials Industrial degree MWCNT with external diameter between 30-50 nm and length between 5-15

μ m, low molecular weight chitosan (Sigma Aldrich), glacial acetic acid (99.85% purity), methanol (99.9% purity) ethanol (99.89% purity) acetic anhydride (99.89% purity), ammonium hydroxide, nitric acid of analytical grade supplied by Sigma Aldrich, iron (97% purity).

Functionalizations of the MWCNT with carboxyl groups (MWCNT-COOH) 250 mg of pristine MWCNTs were dissolved in 150 ml of a deionized aqueous solution of HNO 3 with a 1:1 (v/v) proportion. The system was set to reflux for 72 h at 140 ◦ C to produce defects on



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the MWCNT surfaces, due to the oxidative process. The resulting solution was filtered and washed with deionized water to eliminate the remaining acids. The washings were monitored until a pH of approximately 7.0 was reached. The filtrate was dried in an air oven at 80 ◦ C for 24 h. The results are similar to those reported by Datsyuk and co-workers (2008). 46

Functionalizations of the MWCNT with amino groups (MWCNT-NH2 ) 500 mg of pristine MWCNTs and 20 ml of an acid solution (H 2 SO4 /HNO3 = 10/9, vol/vol) were added into a three-necked flask and sonicated for 5 min. The reaction system was heated up to 60 ◦C

and stirred for 90 min. In order to stop the reaction, this mixture was diluted with 600 ml of

distilled water. The mixture was filtered, washed until pH 7, and dried at 60 ◦ C (MWCNT-NO2 ). After this, a mixture of 3 g of iron powder, 6 ml of acetic acid and 150 ml of distilled water was prepared. This mixture was heated up to 100 ◦ C and stirred for 20 min to activate the iron into ferrous acetate. Then 100 mg of MWCNT-NO2 were added; this mixture was sonicated for 20 min to promote a uniform dispersion. This mixture was refluxed at 300 ◦ C for 9 h and 9 rpm. At the end of the reaction the superficial layer of (superfluous) iron was removed employing a magnet. The mixture was filtered at vacuum using a millipore membrane of 0.22 mμ and washed out with a hydrochloric acid solution (0.01M) and distilled water until a pH of 7 was reached. Finally the product was dried at 80 ◦ C to remove solvents. This reaction has been reported by Wang and co-workers (2010). 47

Partial N-acetylation of Chitosan CS was swelled in a 2% (v/v) solution of acetic acid and stirred for 16 h to prepare a 3% (w/w) solution of CS. Ethanol was added to this viscous solution in an equivalent amount as that of the acetic acid. The variation in the acetylation was carried out by modifying the amount of acetic anhydride (CH3 CO)2 . The solution was poured into Petri dishes and dried for 78 h at ambient temperature to produce the films. After this, the films were washed; first with an ammonia/methanol/distillated water solution and later with deionized water to deprotonate the films. Finally the films were dried


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for 24 h. 48–50

Films of nanostructured chitosan The MWCNT-NH2 (1 % w/w with respect to chitosan) were sonicated into a 2 % (v/v) acetic acid solution for 20 min. After sonication, chitosan was added and the solution was mixed during 16 h to produce a 3 % (w/w) chitosan solution. Once this was done the procedure described in the previous section was repeated to complete the nano structuration and acetylation processes. The nanostructured film with MWCNT-NH2 is called CS-MWCNT-NH2 . The samples of MWCNTCOOH and MWCNT were synthesized employing the previously mentioned procedure; the films obtained were identified as CS-MWCNT-COOH and CS-MWCNT respectively.

Characterization: Tension The tension measurements were performed on a Texturometer (Texture Analyzer TA-XT) according to ASTM D882-02 with a crosshead speed of 0.2 mm/sec at room temperature. The samples were cut in rectangular shape measuring their thickness, length and width (0.10 × 120 × 10 mm, respectively). Three replicas of each system were analyzed.

Results and discussion Experimental work and molecular simulations have shown both that intrachain and intermolecular interactions between the CS and the CNTs are dependent on the DD and the GlcNA and GlcN sequence on the CS chain. 3,37,40 As mentioned above, the CNTs can improve the mechanical properties of CS; however if an adequate dispersion of the CNTs into the polymeric matrix is not present these properties can be affected. 51–53 The CNT dispersion is determined, among other factors, by the affinity between the CNT and the CS. This is the reason why the CNTs are modified with anchor groups (NH2 and COOH) compatible with CS chains. MD simulations were employed to determine, at atomic level, the interactions between CNTs



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functionalized with either NH2 or COOH groups and CS at different DD. The results from the simulations were compared to experimental Young’s modulus data of CS films reinforced with functionalized CNTs. The dynamical behavior of the molecular arrangements, as determined from the MD simulations, was used to calculate the interaction energy, ΔE, between the CS and the CNT defined as: ΔE = Etotal − (ECNT + ECS )

(1)

were Etotal is the total potential energy of each system including CNT and CS, ECNT is the potential energy of isolated CNT and ECS is the potential energy of chitosan. In other words, the interaction energy is the difference between the system’s minimum energy and the energy at infinite distance between the CNT and the polymer. 22,23,27 The Figure 4 depicts the interaction energy for the whole set of studied systems. It is evident that the interaction between the CS and the CNT is influenced by the DD. If the interaction energy decreases then the attractive interaction between the CNT and the CS decreases also. For the pristine CNT, the intermolecular interactions are less influenced by the DD because there is no functional group on the CNT surface affecting drastically its interaction with the active groups of the CS chain, namely: hydroxyl, acetamide and primary amine.


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Figure 4: Interaction energy between chitosan at different DD and -- pristine carbon nanotube, -- CNT-NH2 , and -- CNT-COOH. Error bars are included in each symbol. Please note that the lowest value indicates the best interaction. When a CNT functionalized with either -COOH or -NH2 is considered, the interaction energy shows, depending on the DD, large fluctuations. The nanocomposites formed by 50 % DD CS and CNT-NH2 or CNT-COOH depict the largest attractive interaction (191 and 153 kcal/mol, in absolute scale, respectively) compared to the rest of the systems. This could be due to the GlcNA and GlcN uniform unit distribution along the CS chain, which allows a better interaction between the active groups of CS and the functional groups of the CNT. It can be observed that there is a larger attractive interaction between the CS and the CNT-NH2 , which indicates that the amine groups are more affine to CS because they tend to form hydrogen bonds with the -OH and -NH 2 groups in the CS molecule. For a DD > 50% the interaction energy between the CS and the CNT decreases. This behavior could be due to a lower compatibility between the amine groups on the CS chain and the functionalized CNT compared to that with acetyl groups. However, it has been also observed 30,32,40,54 that the distribution and configuration of the GlcNA and GlcN units impact considerably the chemical and mechanical properties of the nanocomposite and, as a consequence, the interaction energy as well. With the aim to understand how the GlcNA and GlcN distribution affects the interaction 13 ACSParagonPlusEnvironment


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energy, more simulations of nanocomposites constituted by CNT-NH 2 and CS with 50% DD, were performed. Table 1 depicts the potential energies of six GlcNA-GlcN distributions; where E total , ECS and ΔE are total, CS and difference energies, respectively; and STD DEV is the total energy standard deviation. The CNT-NH2 potential energy, ECNT , was constant and equal to 15, 092 kcal/mol. According these results the [-GlcNA-GlcN-]10 distribution displays the largest intrachain potential energy (750 kcal/mol) but the total energy is the smallest (15, 651 kcal/mol), which represents the best interfacial interaction of the CS/CNT-NH2 nanocomposite. Thus, the largest intra-chain energy means that the chain does not interact with itself, allowing the interaction area increasing with the CNT-NH2 surface. Table 1: Potential energies (kcal/mol) for different GlcNA-GlcN distributions for nanocomposites constituted by CNT-NH2 and CS at 50% DD. The total energy and the CS energy are represented by Etotal and ECS , respectively. ΔE is calculated from Equation 1. ΔE

Distribution

Etotal

ECS

[−GlcNA − GlcN−]10 Random 1 [−(−GlcNA−)2 − (−GlcN−)2 −]5 [−(−GlcNA−)5 − (−GlcN−)− ]2 Random 2 [−(−GlcNA−)10 − (−GlcN−)10 −]

15652 15663 15698 15700 15731 15757

751 -191 738 -167 734 -145 732 -143 725 -112 718 -86

STD. DEV. 7.44 7.09 6.76 6.48 6.05 7.50

The Figure 5 shows two snapshots of CS/CNT-NH2 with 75% DD. In Figure 5a an ordered [-GlcN-GlcNA-GlcN-GlcN-]5 distribution, labeled as CS1(75)/CNT-NH 2 , is displayed; while in Figure 5b a random distribution of GlcN-GlcNA units, labeled as CS2(75)/CNT-NH 2 , is depicted. The CS1(75)/CNT-NH2 , generates an interaction energy larger than that of the CS2(75)/CNT-NH2 distribution; with values of -39.5 and -117.8 kcal/mol, respectively. The interaction energy main contribution is the intra-chain energy; thus, the random distribution, CS2(75)/CNT-NH 2 , has the largest intra-chain energy, producing an extended chain conformation. However, the wrapping effect is not evident, as in the CS(50)/CNT-NH2 with the [-GlcNA-GlcN-]10 distribution case, due to a better intra-chain interaction in CS2(75)/CNT-NH2 .


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(a)

(b) CS1(75)/CNT-NH2

CS2(75)/CNT-NH2

Figure 5: Simulation snapshots at 450 ps for the nanocomposite formed by CNT-NH 2 and CS with 75% DD. (a) [-GlcN-GlcNA-GlcN-GlcN-]5 ordered distribution and (b) GlcNA-GlcN random distribution.

Figure 6 depicts nine CNT-CS configurations at 450 ps of simulation time for pristine CNT and CS at (a) 50, (b) 75 and (c) 85% DD; for CNT-NH2 and CS at (d) 50, (e) 75 and (f) 85% DD; and for CNT-COOH and CS at (g) 50, (h) 75 and (i) 85% DD. It is observed that depending on the DD and the functionalization type of the CNT, the CS chain can wrap the CNT, however ΔE is large. The ΔE magnitude indicates that there is not a good compatibility between the CS and the CNT and also that the intermolecular interaction is weak. Distances between atoms in the CS chain and those in the CNT were calculated observing that the attractive interaction between pristine CNT and CS is due to the alignment of the CS rings upon the π bonds in the CNT.



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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Figure 6: Molecular snapshots at 450 ps simulation time for CNT and CS at (a) 50, (b) 75 and (c) 85% DD; for CNT-NH2 and CS at (d) 50, (e) 75 and (f) 85% DD; and for CNT-COOH and CS at (g) 50, (h) 75 and (i) 85% DD. The best interaction is depicted in system (d).

When the interaction energy is low and the CS chain wraps around the functionalized CNT, the attractive interaction between the functionalized CNT and the CS augments. This is because there is a better compatibility between the functional groups of CS and the acidic groups of the CNT. It is observed from Figure 6i: CNT-COOH with 85% DD CS, that the CS chain does not wrap totally around the CNT because the intra-chain interactions are larger than the interactions with the CNT carboxylic groups. However, for the 50% DD CS with CNT-NH2 , Figure 6d, the 16 ACSParagonPlusEnvironment

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chain wraps totally around the CNT and the interaction energy is low (-191 kcal/mol), this is an indicative that CS with 50% DD has the higher interfacial interaction with CNT-NH 2 . This could produce an adequate dispersion of the CNT into the polymeric matrix. The calculated distances between the CNT-NH2 atoms and those of the 50% DD CS indicate that the most common pair interactions arise between the hydroxyl and amine groups on the CS chain and the amine groups on the functionalized CNT-NH2 . This is due to the hydrogen bonds (HB) formed between the -NH 2 groups on the CNTs and the -OH and -NH2 in the CS, as depicted schematically in Figure 7.

Hydrogen bond

Figure 7: Zoom of the molecular interaction between CNT-NH2 and Chitosan at 50% DD. Hydrogen bonds are clearly observed between amino groups from CNT-NH2 and hydrogen atoms from hidroxyl groups of the Chitosan molecule.

With the aim to give a deeper description, HB formed at the CS/CNT interface were monitored from the last simulation configurations. Eight systems were analyzed: Two CS distributions (CS1 and CS2) with 50 and 75% DD respectively, interacting with CNT-NH 2 and CNT-COOH. Two HB types were considered: strong and weak, according to Skovstrup and co-workers. 32 There is evidence 55 that the HB is promoted by the electronegativity difference between the hydrogen and the interacting atom. If the HB is formed between one atom which is more electronegative than hydrogen, then a larger attractive interaction is obtained. However, the total number of HB does not correspond necessarily to the best interaction (minimum energy).



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The systems with the best interaction are CS(50)/CNT-NH 2 , because the -NH2 groups promote the HB with the -OH groups from CS (H −O · · ·H). Although the CS/CNT-COOH nanocomposites also display HB of the H − O · · · H type, the number of HB (3 strong and 16 weak), and hence the interaction, is less than that of CS1(50)/CNT-NH2 (3 strong and 18 weak). However, one case that does not satisfy the above condition is the CS2(75)/CNT-NH 2 system, which possess a lower number of H − O · · · H bonds but a better interaction than that of the CS1(75)/CNT-COOH. This could be due to the additional energy from the HB formed on the CS chain. 32 Therefore, it can be concluded that the functionalized CNT with -NH2 groups promote HB formation at the CS/CNT interface, improving in this way, their interface interactions. For detailed information see Table S1, where the number of HB for the eight systems, which includes both strong and weak D − H · · · A bonds are showed. The Young’s modulus is depicted in Figure 8. Data include error bars, ranging between 0.009 and 0.024 GPa, which are small and difficult to visualize. MWCNT were used because they are more resistant to acid treatments than SWCNT. Thus, it is possible to achieve an adequate balance between their structural integrity and their properties. 6 Since the interaction under study occurs at the CS/CNT interface, the information for SWCNT generated from MD is suitable to be compared with that from experimentation. It is evident that the CS-MWCNT-NH 2 films present a larger stiffness than that for either the CS-MWCNT-COOH nanocomposite or the pure CS. The results from the MD simulations showed that a GlcNA-GlcN uniform unit distribution along the CS chain, generates a better affinity between the -OH and -NH2 groups of the CS and the -NH2 groups of the functionalized CNT. This in turn, promotes a better CNT dispersion into the polymeric matrix producing a better stress distribution in the nanocomposite. This phenomenon has been widely studied; 16,19,36,38 proving in all cases that the CNT dispersion in the polymeric matrix plays an important role on the final nanocomposite properties. However, it has not been recognized, at molecular level that the attractive interaction between the CS and the functionalized CNT is due to the hydrogen bonds formed between the acidic groups of the CNT and the -OH and -NH 2 groups of the CS, as it is shown from our MD calculations. Further, these interactions impact positively


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Figure 8: Young’s Modulus for chitosan films without reinforcement (Pure CS), for chitosan films with MWCNT-COOH reinforcement (CS-MWCNT-COOH) and chitosan films with MWCNTNH2 reinforcement (CS-MWCNT-NH2 ) at 54 (), 64 () and 76% () of DD in the chitosan molecules. The largest value is obtained for CS-MWCNT-NH 2 and 54% DD. on the magnitude of the measured Young’s modules. The degree of deacetylation of the CS has also a definite influence on the mechanical properties of the nanocomposites. It is observed, from Figure 8, that the material stiffness depends on the degree of deacetylation of the CS. The stiffness augments as the DD decreases because the nanocomposites loss flexibility as the amine groups are substituted by acetyl groups. This behavior is relevant and it was also observed from the MD simulations. The concordance between the experimental data and the simulation results leads us to conclude that, for a DD close to 50%, it is possible to achieve a better GlcN unit distribution along the CS chain; favoring with this, a larger presence of hydrogen bonds between the active groups of the CS and the CNT with low intrachain (molecular) interactions and promoting consecuently the CNT dispersion. In concordance with this, the Figure 8 shows that the Young’s modulus for the nanocomposites formed by 54% DD chitosan depicts the largest stiffness which, according the MD simulations, correspond to a larger attractive interaction. From Figure 8, the 54% and 64% DD points display low variation of the Young’s modulus for both MWCNT-COOH and MWCNT-NH2 . This is because both functional groups are affine to the 19 ACSParagonPlusEnvironment


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chitosan. However, the -NH2 groups promote a better dispersion of the CNTs into the polymeric matrix (as seen in Supporting Information) than the -COOH groups. The dispersion causes an increment in the nancocomposite stiffness. It is observed from our MD simulations that the amine groups promote the wrapping of the CS chain around the CNT-NH2 . This is because the -NH2 groups facilitate the hydrogen bond formation with the -OH and -NH 2 on the CS chain. Put in other words, the -NH2 groups in the CNTs increase the attractive interaction with CS due to the affinity of these groups with the polymeric matrix.

Conclusions Molecular Dynamics simulations at equilibrium were performed to determine the interaction energies between a functionalized carbon nanotube and a chitosan chain with different degrees of deacetylation. Nanocomposite films composed by chitosan reinforced with functionalized (-NH 2 and -COOH) carbon nanotubes were prepared and tension tests were performed to measure their Young’s modulus. The results obtained from molecular dynamics simulations give a clear description of the expected behavior of chitosan at macroscopic level. It was determined that the inter- and intramolecular interactions depend mainly on three factors: the GlcN and GlcNA unit distribution along the chitosan chain, the chemical groups involved to functionalize the carbon nanotubes, and the degree of deacetylation of the chitosan. These factors impact on the chemical and mechanical properties of the nanocomposites. It was determined also from the molecular dynamics simulations that the GlcN-GlcNA unit distribution along the chitosan chain and its degree of deacetylation can promote the intra-chain interactions, avoiding with this the wrapping of the chitosan around the carbon nanotube and turning the interaction energy to a low absolute value. Further, it was shown that the carbon nanotube functionalized with amine groups promotes a larger compatibility with the chitosan chain because both strong and weak hydrogen bond formation are favored.


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From both experimental and molecular simulation work, it was determined that nanocomposites formed by 50 % deacetylated chitosan and amine functionalized carbon nanotubes possess the strongest intermolecular interaction among the whole set of studied cases. This blend holds also the largest Young’s module. From these facts, the conclusion is proved that the carbon nanotubes are adequately dispersed into the polymeric matrix producing a nanocomposite with the best mechanical properties.

Acknowledgement The main author (EOCG) thanks the financial support of CONACYT and National Institute of Technology, México (TECNM).

Supporting Information Available COMPASS Force Field is described as well as both FT-IR and SEM results of functionalized MWCNT are included. In addition, both detailed information about the number of HB formed and values of Young’s modulus are showed. This material is available free of charge via the Internet at http://pubs.acs.org.

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Graphical TOC Entry Chitosan at 50% Degree of Deacetylation Hydrogen Bond

CNT functionalized with amino groups


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Study of the Molecular Interactions between Functionalized Carbon

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