Investigation of Filler-Matrix Interactions and Confinement Effect in

Chapter 6

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Investigation of Filler-Matrix Interactions and Confinement Effect in Polymer Nanocomposites K. P. Pramoda,*,1 K. Y. Mya,1 T. T. Lin,1 J. Wang,1 and C. B. He1,2 1Institute

of Materials Research and Engineering (IMRE), (A*STAR) Agency for Science, Technology and Research, 3 Research Link, Singapore 117602 2Department of Materials Science and Engineering, National University of Singapore, 7 Engineering Drive 1, Singapore 117574 *E-mail: [email protected]

Although nanocomposites are known for decades, the molecular interactions under the confined environments of such composites are not well understood. In this article we report the influence of two types of fillers, viz., graphite (G) and graphene oxide (GO) nanosheets, on polyimide (PI) matrix with varying filler content. Solid state NMR and thermo-mechanical analysis were deployed to characterize the morphology of hybrid materials and the interactions between the polymer and the filler. CP-MAS and relaxation experiments were performed to probe the interaction between inorganic and organic phases on a molecular level and to elucidate polymer dynamics. NMR relaxation measurements further helped in elucidating the polymer dynamics and were supplemented by thermo-mechanical relaxation. From the relaxation data it is clear that GO interacts better than G with the polyimide matrix. This is further confirmed by the higher modulus observed for PI-GO nanocomposites than for PI-G.

Introduction Nanocomposites are a newly emerged class of composite materials that incorporate relatively small percentages of nanometer-sized filler particles. Nanocomposites with carbon as filler materials exhibit improved electrical, © 2011 American Chemical Society In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


thermal and mechanical properties (1, 2). Among the various other carbon additives, graphite is much more attractive to researchers as it is comparable to carbon nanotube (CNT) in properties. Moreover, it has a layered structure (similar to clay) constituted by a large number of graphene sheets held by van der Waals forces (3, 4). Therefore, pretreatment and modification of graphite is necessary to achieve the nanosheet distribution of graphene. Surface treatment of graphene makes it more compatible to polymer matrix which helps good dispersion in a polymer matrix, resulting in nanocomposites with excellent mechanical and electrical properties. Polymer nanocomposites find applications in automobile, food packaging and biomedical industries due to improvements in thermal, physical and gas permeability properties of the polymers attributed to the nanoscale filler materials. For silicates, these enhancements are achieved with less than 10% wt addition of the exfoliated nanoscale dispersion of 1 nm thick layers, with diameters between 20 and 500 nm. This is in stark contrast to conventional polymer fillers, such as talc, mica, silica, and carbon black, which require high concentrations (>30 wt %) and may cause deterioration of fracture toughness and in processibility. Daniel et al (5) reported a 15% improvement in elastic modulus of 1 wt % expanded graphite (EG)-epoxy nanocomposite over pure epoxy and attributed it to the in-situ formation of graphite nanosheets as well as to their uniform dispersion and exfoliation in the epoxy matrix. High density polyethylene reinforced with EG and untreated graphite by a melt compounding process also shows an improvement in the electrical and mechanical properties of nanocomposite (6). Poly(methyl methacrylate)-EG composites prepared by in-situ polymerization exhibits better mechanical and electrical properties than those prepared using solution blending methods (7). Although epoxy and thermoplastic nanocomposites with graphite/ graphene oxide nanosheet fillers have been studied, to our knowledge, there have been no reports available on polyimide (PI)-based graphite nanocomposites. Polyimides (PIs) are widely used in defense and aerospace applications as well as in the electronics industry for a variety of applications because of their high temperature resistance, low dielectric constant, inertness to solvent, and long-term stability (8). Polyimide nanocomposites with various fillers such as CNT, clay, and POSS (Poly Oligomer SilSesquioxane) have been studied (9–12). In this article we report on the influence of two types of fillers, namely, graphite (G) and graphene oxide (GO) nanosheets, on polyimide matrix with varying filler content. Solid state nuclear magnetic resonance (SSNMR) spectroscopy and thermo-mechanical analysis were deployed to characterize the morphology of hybrid materials and the interaction between the polymer and the filler. The chemical oxidations of graphite to graphite oxide are confirmed by XRD. The interaction between filler and the polymer matrix is further studied by XPS analysis. In addition, dynamic mechanical thermal analysis (DMTA), and thermomechanical analysis (TMA) are applied to characterize the glass transition temperature (Tg), storage modulus (E′), and coefficient of thermal expansion (CTE).

106 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

Experimental Section


Materials and Synthesis of Polyimide Composites Biphenyltetracarboxylic dianhydride, phenylene diamine, and N-methyl-2pyrrolidinone (NMP) were purchased from Sigma-Aldrich. All the monomers were purified before use. Graphite, donated generously by Asbury Carbons (Asbury, NJ, 3775 grade) was used as received and also oxidized by a modified Hummer’s method (13). The poly(amic acid) (PAA) precursor solution was prepared through polycondensation reaction between dianhydride and diamine in NMP. The resulting PAA solutions were controlled to have solid contents of 10-15wt%. In a separate flask, the filler (G/GO) was first uniformly dispersed in NMP using an ultrasonic bath to obtain their respective nanosheet. After that, the required amount of G-NMP/GO-NMP was added to the PAA solution and stirred continuously. The solution was then cast on a glass plate using automatic film applicator and thermally treated at 100°C for 1 h, 200°C for 1 h, and 300°C for 4 h, respectively. The films were then removed from the glass substrate with the aid of warm water and dried at 50°C in a vacuum oven for 2 days. The thickness of the films was ~100 µm. Hereafter, the PI composite film with G filler was denoted as “PI-G” and the film with GO filler “PI-GO.”

Characterization X-ray diffraction (XRD) patterns of the samples were recorded by using a Bruker GADDS diffractometer, with an area detector operating under 40 kV and 40mA, using CuKα radiation (λ = 0.154 18 nm). X-ray photoelectron spectroscopy (XPS) data were recorded using a VG Scientific EscaLab 220 IXL spectrometer with an Al Kα X-ray source (hν = 1 486.6 eV). The spectra were corrected for carbon shift binding energy, Cs = 284.5 eV in order to determine the accurate binding energies. SSNMR measurements were carried out with Varian 400 MHz (9.4 T) wide bore spectrometer and the samples were spun at the magic angle at the rate of 15 kHz using a 4 mm double resonance probe. The spectrometer was operated at resonance frequencies of 100 and 400 MHz to obtain 13C and 1H spectra, respectively. The spectra were referenced using tetramethylsilane (TMS). Thermomechanical behavior of the composites was analyzed with the help of dynamic mechanical thermal analyzer (DMTA) and thermo mechanical analyzer (TMA) from TA Instruments. The modeling and simulation on these complexes were done using Materials Studio software and DMol3 (14). The binding energy was calculated using the density functional theory (DFT) calculation with the following equation:



Results and Discussion Chemically oxidized graphite (G) is known as graphene oxide (GO) which forms a stable dispersion in water or other organic solvents leading to GO nanosheets. The extent of oxidation and the surface structure of GO are the key parameters to improve their compatibility with the polymer. The oxidation of graphite may affect its chemical structure and was confirmed by XRD and FTIR (15). Figure 1 represents the XRD pattern of G and GO together with its polymer composites. A strong peak is observed at 2θ = 26.6° for G and a scattering signal at 2θ = 9° for GO. The peak at 26.6° represents the intact crystal lattice in G and the peak at 9° for GO corresponds to a layer spacing of 10 Å (16). The absence of the peak at 26.6° (2θ) in GO implies that the crystal lattice structure of G has been disrupted. On the other hand, a small peak that appears at 26.6° (2θ) in the PI-G composites shows the intercalation of graphite in the PI matrix. However, the absence of the peak at 9° (2θ) that is the characteristic of GO in the PI-GO composites indicates that the packing of the GO has been entirely disrupted by PI-chains.

Figure 1. XRD spectra for G and GO along with the neat PI, PI-G and PI-GO systems, respectively. The interaction between filler and the PI matrix plays a critical role in the property enhancement in the nanocomposites and therefore it is studied by XPS and SSNMR analyses. XPS provides information on the specific interaction in polymer nanocomposites as the binding energy (BE) of a core-level electron depends on its chemical environment (17). SSNMR reveals the structure, conformation and dynamics of polymer chains in the composites, and thereby the interactions between the polymer and the filler. However, there have been only a few papers on the application of SSNMR to the structure and dynamics of organic-inorganic hybrid nanomaterials. VanderHart’s research group at 108 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.


NIST has studied Nylon-6/clay nanocomposites, including clay dispersion, crystal stratification, and stability of organic modifiers, using 1H T1 relaxation times. Nanodispersion in polystyrene-montmorillonite nanocomposites was also investigated by Gilman and coworkers (18–20). Figure 2 shows the 1H NMR spectra of the PI nanocomposites. It is observed that the peak shifts upfield from 7.73 ppm in neat PI to 7.16 ppm along with the presence of shoulder peaks (3.01 and 0.84 ppm) in PI-GO system. It is clearly indicated that the interaction between filler and the matrix depends on the structure and relaxation of polymer chains. In addition, the incorporation of GO shows a better intensified peak compared to G, indicating better compatibility between the filler and the matrix in PI-GO system.

Figure 2. 1H SS NMR spectra for neat PI, and 1 wt% of PI-G, PI-GO. Relaxation measurements on these samples can be informative as well. The T1 relaxation times for 1H and 13C are tabulated in Table 1. The T1 values for the composites are reduced compared to neat PI. The T1 values for both 1H and 13C are reduced in the following order PI > PI-G > PI-GO. This trend suggests strong filler-matrix interaction between PI chains and GO nanosheets compared to PI-G. This was further verified by thermomechanical properties such as modulus and CTE enhancement (discussed later) in PI-GO composites as compared to PI-G. The N 1s binding energies (BEs) for PI composites are displayed in the Table 1. As seen in the Table, the peak at 400.8 eV in neat PI is shifted by 0.8 eV for 1 wt% GO (400 eV) and the shift is only 0.4 eV in the case of 1 wt% G (400.4 eV). The magnitude of the changes in BEs of N 1s correlates well to the strength of interaction, indicating that the interaction between PI and GO is stronger than that in PI-G composite. The observed strong interaction in PI-GO system is possibly due to the presence of functional groups such as OH, COOH, and epoxy, which becomes the preferable site for the interaction with the linear chain packing of PIs. 109 In NMR Spectroscopy of Polymers: Innovative Strategies for Complex Macromolecules; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.

However, the observed decrease in the BE values are not prominent, mainly due to the lower sensitivity of N1s in the XPS measurement.

Table 1. N1s XPS and 1H SS NMR fit data along with their relaxation time


BE N1s XPS (eV)

T1 (s) 13C

1H

PI

400.8

38

1.5

PI-G-1%

400.4

32

1.0

PI-GO-1%

400.0

22

0.3

Thermomechanical properties of PI composites were determined by DMTA and TMA analyses. Figure 3 shows the DMTA curves for PI-G and PI-GO composites with 1 wt% of G and GO, respectively. The storage modulus (E′) increased with increasing filler (G/GO) content as seen in Figure 3. Table 2 summarizes the storage modulus (E′) at 100°C along with α and β transition temperature (Tα and Tβ). Tα or Tg ~ 340°C; the presence of filler did not affect much the Tg of the PI matrix. The observed modulus enhancement was greater with GO addition as compared to PI-G system. A ca. 38% improvement in E′ for 1 wt% of G loading was observed versus an increase of ca. 106% with GO loading for PI-GO composites. The extent of increase in E′ was greater in the PI-GO system, which is possibly due to the better interaction between the GO and the PI, leading to both physical and chemical cross-linking between filler and the PI matrix, as evidenced by the 1H-NMR spectra. In comparison to our system, PI-clay composites (10) showed a modulus increase of ~42% with 2 wt% clay loading relative to that of neat PI. Similarly, Zhu et al (11) also reported that the modulus of the PI-multi-walled carbon nanotube (MWCNT) composites exhibited 40% increase with 5 wt% of MWCNTs content.

Table 2. Storage modulus, (E′@100°C), Tβ, Tα(Tg), and CTE values Tβ (°C)

Tα(Tg) (°C)

E′ (GPa) @ 100°C

CTE (100-250°C) ppm/°C

PI

191

341

8.41

15.70

PI-G-1%

187

340

11.6

3.98

PI-GO-1%

186

344

17.3

1.67



Figure 3. Temperature dependant storage moduli and tangent delta for the neat PI and its composites.

The influences of G/GO on the CTE of the PI composites were also investigated, and the results are shown in Table 2. It was found that the CTE decreased when G and GO were incorporated into the PI matrix. The CTE of the PI composite films is significantly reduced by about 75% and 90%, for 1% loading of G and GO, respectively. Since CTE depends on the rigidity of PI-chain and the interaction between PI and the fillers, the observed decreases in the CTE for PI-G and PI-GO composite systems suggested that the interaction between PI-chain and the G/GO nanosheet were strong which hindered the motion of the PI-chain. On the other hand, the relatively lower CTE observed in PI-GO when compared with PI-G could be attributed to the strong interaction between the GO with PI-chain, which has been confirmed by XPS and NMR studies. Furthermore, our DFT calculation also suggests that the binding energy for the composite PI-GO was found to be 10.7 kcal/mol with an orbital cutoff 3.7 Å while it was only 3.7 kcal/mol for PI-G with same value of cutoff (i.e. 3.7 Å). The theoretical calculations also supported our experimental findings; i.e., the interactions between the PI-chains and the GO nanosheets are much stronger than that in the PI-G.



Conclusions Spectroscopic investigation was carried out to determine the relaxation behavior of PI composites. The relaxation times obtained from NMR analysis indicate that the interaction between PI chain and GO nanosheet is stronger than that in PI-G composites. In agreement, the DFT calculations revealed that the chemical compatibility achieved between PI and filler were mainly with GO rather than G. Dynamic mechanical thermal analysis shows that the incorporation of GO nanosheets significantly enhance the storage moduli of PI nanocomposites compared to that of G nanosheets. The moduli improved ~106% with only 1 wt% GO nanosheets while it is only ~38% with G nanosheets. The extent of thermal expansion coefficient drop in the case of GO incorporation is greater than that of G.

Acknowledgments The authors thank IMRE and Agency for Science, Technology, and Research (A*STAR), Singapore for financial support.

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Investigation of Filler-Matrix Interactions and Confinement Effect in

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