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					Benjamin Dach                                                                      03/27/09
Second-year report


        Crosslinked Nanocomposite Networks from Polymer-Silica Hybrid
                                Nanoparticles

Background

     Organic–inorganic hybrid materials represent an important field of research in material

science as they have applications in optics, electronics, ionics, mechanics, energy,

environment, biology and medicine.1,2 This versatility arises from the benefits and desirable

properties of both constituents: the inorganic component is mechanically and thermally stable

and the organic component is flexible and functional. Silica nanoparticles dispersed within

polymer matrices by mechanical, ultrasonic, or solvent-aided mixing have been reported to

display increased mechanical properties3,4 like modulus, flexural, tensile, and impact strength

up to a certain silica content (2.5%).5 Thereafter, mechanical properties are decreased, while

fracture toughness6 and thermal stability7,8 are improved. Sulfonated polyimide-silica

nanocomposite membranes were shown to function as proton conducting solid electrolytes for

fuel cells.9 Additionally, fumed silica has been used to control the nanoparticle dispersion

within a polymer matrix to create ultrapermeable, reverse-selective nanocomposite

membranes that allow heavier vapors to permeate more easily than light gases.10 While

mechanically mixed nanocomposites have attracted much attention, covalently crosslinked

polymer-silica nanocomposite networks are a related class of new materials which may

exhibit completely novel properties. This work investigates the formation of polymer-silica

hybrid nanocomposite networks that are covalently crosslinked via the ‘click’ reaction.

(Scheme 1)

Research Objectives

   1) Covalently graft polystyrene and poly(tertbutyl acrylate) to silica nanoparticles.
   2) Characterize the polymer grafted nanoparticles by thermal and spectroscopic analyses.
   3) Create polymer-silica hybrid nanocomposite networks.
   4) Investigate the properties of the networks and the mechanisms of formation.

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Benjamin Dach                                                                                03/27/09
Second-year report




    Scheme 1. Nanocomposite network formation via Hüisgen 1,3-dipolar cycloaddition, a ‘click’ reaction.
Synthesis of Bromoundecyl-Modified Silica Nanoparticle

     6 g of dried silica and 45 mL of anhydrous toluene were combined in a flask under inert

atmosphere. This mixture was sonicated for 30 min and then placed into an oil bath at 80°C.

5.5 mL of 3-bromoundecyltrichlorosilane in 15 mL toluene was added dropwise and the

solution was stirred at 80°C for 24 h. The particles were recovered by centrifugation at 3000

rpm for 30 min, then redispersed in toluene and centrifuged. This cycle was repeated six times

to afford the modified particles. Finally, the residual volatiles were removed under vacuum.

Synthesis of Azide-Modified Silica Nanoparticle

     5 g of bromoundecyl-modified silica nanoparticles and a saturated solution of NaN3 (2 g

of NaN3 in 100 mL DMF) were added to a flask under inert atmosphere. This solution was

stirred at 80°C for 24 h. The particles were repeatedly washed with water and centrifuged to

recover the modified silica nanoparticles.

Synthesis of Polymer-Silica Hybrid Nanocomposite Networks

     Scheme 2 illustrates the synthetic route from bare silica nanoparticles to polymer-silica

nanocomposite networks. Heterobifunctional α-alkyne-ω-bromopoly(styrene) and α-alkyne-

trimethylsilyl-ω-azidopoly(tert-butyl acrylate) are covalently bound to the surfaces of the

modified nanoparticles via the ‘click’ reaction. The polymer-silica nanoparticles are then

‘clicked’ together into networks.




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Benjamin Dach                                                                                        03/27/09
Second-year report




                 Scheme 2. Synthetic route to PS-PtBA silica nanocomposite network formation




Figure 1. IR spectra of (a) 1 [black] (b) 2 [blue] (c) 3 [green] (d) SiO2-‘click’-SiO2 [red] (e) 4 [grey]


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Benjamin Dach                                                                                    03/27/09
Second-year report
Infrared Spectroscopy

      Figure 1 shows diffuse reflectance Fourier transform infrared spectra (DRIFTS) of bare

and modified silica.

Thermogravimetric Analysis

      Thermogravimetric analysis (TGA) was performed on a TA Q50 instrument at a scan

rate of 10°C/min under nitrogen atmosphere. The presence and the amount of grafted

polymers on silica surfaces can be estimated by TGA.11 Figure 2 shows the weight loss as the

silica nanoparticles are heated to 800°C. For bare silica, the observed weight loss of 5.33% is

due to silanol group dehydroxylation. The alkyne, azide, PS, and PtBA modified

nanoparticles lost 8.95%, 12.48%, 22.58%, and 23.23% of their weights, respectively. These

results indicate that the bare silica particles were successfully modified, because the

successive increase in weight loss is related to the pyrolysis of organic compounds.

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                        Figure 2. TGA degradation curve of (a) 1 (b) 3 (c) 2 (d) 4 (e) 5
            (a)                           (b)                          (c)                        (d)




Figure 3. TEM images of (a) bare silica nanoparticles and (b), (c), (d) crosslinked nanoparticle aggregates


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Benjamin Dach                                                                            03/27/09
Second-year report
Dynamic Rheology

     The dynamic rheology of crosslinked samples is measured using a TA Instruments

AR2000 rheometer. A 60 mm diameter steel parallel-plate geometry with a 25 μm gap is used

and a 5% oscillatory strain is applied at the rate of 1 Hz on all samples. Dynamic rheology

measures the elastic (G′) and viscous (G″) moduli of a sample. For weakly flocculated

systems, the presence of particle structures introduces both viscous and elastic effects into the

rheological response. If the particles flocculate into a volume-filling network structure the

material is classified as a gel as it shows predominantly elastic properties12 with G′ > G″.

Figure 5 indicates that networks form in 20 - 30 minutes.

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                  Figure 5. Oscillatory strain measurements of a nanocomposite network




Future Work
    We will begin to investigate the self-organization properties and phase behavior of a

new class of matrix-free nanocomposites: nanoparticles decorated with block copolymer

brushes.




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Benjamin Dach                                                                    03/27/09
Second-year report

References
1. C. Sanchez, B. Juliàn, Ph. Belleville, M. Popall, J. Mater. Chem. 15, 3559 (2005).

2. P. Judeinstein, C. Sanchez, J. Mater. Chem. 6, 511 (1996).

3. Y. P. Zheng, R. C. Ning, Y. Zheng, J. Reinforced Plast. Compos. 24, 223 (2005).

4. K. W. Liang, G. Z. Li, H. Toghiani, J. H. Koo, C. U. Pittman, Chem. Mater. 18, 301
(2006).

5. P. Singh, A. Kaushik, and A. Kirandeep, J. Reinforced Plast. Compos. 25, 119 (2006).

6. G. Ragosta, M. Abbate, P. Musto, G. Scarinzi, L. Mascia, Polymer 46, 10506 (2005).

7. Q. Chen, R. W. Xu, J. Zhang, D. S. Yu, Macromol. Rapid Commun. 26, 1878 (2005).

8. T. M. Lee, C. C. M. Ma, J. Polym. Sci. Part A: Polym. Chem. 44, 757 (2006).

9. C. H. Lee, S. Y. Hwang, J. Y. Sohn, H. B. Park, J. Y. Kim, Y. M. Lee, Journal of Power
Sources 163, 339 – 348 (2006).

10. T. C. Merkel, B. D. Freeman, R. J. Spontak, Z. He, I. Pinnau, P. Meakin, A. J. Hill,
Science 296, 519 (2002).

11. J. Che, B. Luan, X. Yang, L. Lu, X. Wang, Mater. Lett. 59, 603 (2005).

12. C.W. Macosko, Rheology: Principles, Measurements and Applications, VCH, New York,
NY, 1994.




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